Outputs

Publications

J.W. Gilman, S. Lomakin, T. Kashiwagi, D.L. VanderHart, and V. Nagy, Characterization of Flame Retarded Polymer Combustion Chars by Solid-State ¹³C and ²⁹Si NMR and EPR, American Chemical Society Polymer Preprints, 38, 802 (1997).

N. Terashima, R.H. Atalla and D.L. VanderHart, Solid State NMR of Specifically ¹³C-Enriched Lignin in Wheat Straw, Phytochemistry, in press.

D.L. VanderHart and G.C. Campbell, Off-Resonance Proton Decoupling On-Resonance and Near Resonance: A Close Look at ¹³C CPMAS Linewidths in Solids for Rigid, Strongly-Coupled Carbons Under CW Proton Decoupling, Journal of Magnetic Resonance, submitted.

E. PJrez, M.L. Cerrada and D.L. VanderHart, Rapid Determination of Comonomer Content, Crystallinity, and Long Spacing by Multiple Pulse Proton NMR in Ethylene-Vinyl-Alcohol Copolymers, J. Polym. Sci. Polym. Phys. Ed., submitted.

N. Terashima, J. HafrJn, U. Westermark and D.L. VanderHart, Structure of Lignin in Ginkgo Wood Determined by a Combination of Specific ¹³C-Enrichment Technique and Solid State NMR Spectroscopy, Proceedings of the 9th International Symposium on Wood and Pulping Chemistry, in press.

J.W. Gilman, S. Lomakin, T. Kashiwagi, D.L. VanderHart, and V. Nagy, Characterization of Flame Retarded Polymer Combustion Chars by Solid-State ¹³C and ²⁹Si NMR and EPR, Fire and Materials Journal, submitted.

X. Hu, S. Kumar, M.B. Polk and D.L. VanderHart, Tetramethylbiphenyl Substituted Poly(benzobisthiazole) 2. Synthesis and Characterization of Polymers, J. Polym. Sci. Polym. Chem., submitted.

X. Hu, S. Kumar, M.B. Polk and D.L. VanderHart, Poly(Benzobisthiazole) with Tetramethylbiphenyl Moiety in the Main Chain, American Chemical Society Polymer Preprints, submitted.

Presentations

D.L. VanderHart, Aspects of ¹³C NMR of Solid Polymers: Crystallinity of Cellulose and its Derivatives, Partitioning of Defects in Semicrystalline Polymers; and Spectral Resolution in Strongly-Coupled Systems, Institute for Chemical Research, University of Kyoto, Japan (Uji campus), October 1996.

D.L. VanderHart, Comments on the Interpretation of Proton Spin Diffusion Data in Polymers and on ¹³C Resolution in CPMAS Spectra of Strongly Coupled Carbons, Chemistry Department, University of Kyoto, Japan, October 1996.

D.L. VanderHart, Characterization of Heterogeneities in Solid Polymers and Polymer Blends Using Proton Spin Diffusion, 10th Symposium of the Society of Solid State NMR for Materials, Kyoto, Japan, October 1996.

D.L. VanderHart, Characterization of Heterogeneities in Solid Polymers by Solid State NMR, Department of Polymer Chemistry, Tokyo Institute of Technology, Japan, November 1996.

D.L. VanderHart, Characterization of a Decorated Polymer Blend by Solid State NMR, Department of Polymer Science and Technology, Superior Counsel for Scientific Investigation, Madrid, Spain, November 1996.

D.L. VanderHart, Contributions to CPMAS Linewidths in Hydrocarbons for Strongly-Coupled Carbons Using CW Proton Irradiation: Polyethylene, 38th Experimental NMR Conference, Orlando, FL, March 1997.

D.L. VanderHart, Contributions to CPMAS Linewidths in Hydrocarbons for Strongly-Coupled Carbons Using CW Proton Irradiation: Polyethylene, Gordon Conference on Magnetic Resonance, Henniker, NH, June 1997.

Characterization of Polymer Morphology by Microscopy Techniques

C. L. Jackson

Objectives

The objectives are to utilize optical, electron and scanning force microscopy to study the morphology of polymeric materials and to investigate the potential of new microscopy techniques, such as cryo-TEM and TEM with image processing, to solve problems in polymer morphology.

Technical Description

The characterization of polymer morphology is important to better understand the relationship between structure and properties in many materials. Techniques used include transmission electron microscopy (TEM), scanning electron microscopy (SEM), as well as optical and atomic force microscopy. The combination of microscopy techniques with other methods such as scattering, diffraction or spectroscopic methods gives a more complete description of the structure of complex blends, composites or other multiphase materials. Many types of materials have been studied to date, including polymer blends, block copolymers, liquid crystalline polymers, dendrimers and organic-inorganic hybrid materials.

External Collaborations

Dr. Henri Chanzy of CERMAV and CNRS in Grenoble, France.

Dr. Frank Booy of the Laboratory of Structural Biology, NIAMS, NIH, Bethesda, MD.

Dr. Kenncorwin Gardner, DuPont Company, Wilmington, DE

Dr. Jonathan Coddington, National Museum of Natural History, Smithsonian, Washington D. C.

Professor Reimund Stadler, University of Bayreuth, Bayreuth, Germany

Dr. Donald Tomalia, Michigan Molecular Institute, Midland, MI

Accomplishments

• The characterization of dendrimer molecules of poly(amidoamine) (PAMAM) was achieved for generations ten (G-10) down to G-5, ranging in size from 14.7 nm to 4.5 nm, respectively, using biological-type TEM staining methods. For G-10 dendrimer, cryoelectron microscopy was also possible, where the molecules were imaged in vitrified solvent at very low temperature. This technique avoids both drying and staining of the particles and may give a more accurate representation of the size and shape of dendrimers in solution. For G-10, the sizes measured for both methods were similar, but the shapes observed by the cryo technique were more polyhedral than strictly spherical, as seen for the positively stained specimens. This work was done in collaboration with Dr. Frank Booy (NIH) and the samples were obtained from Dr. Donald Tomalia (MMI).
• A spatial ordering of the micelles consisting of graft copolymer of polystyrene (PS) and poly(methacrylic acid)(PMAA) was observed in TEM images of vitrified aqueous solutions using cryo-electron microscopy. Furthermore, the micelles appear to be connected by "arms" hypothesized to consist of the PMAA corona. The observed structure differs from disordered morphology seen using conventional drying and staining methods, suggesting that the latter perturbs the structure. This work was done in collaboration with C. C. Han (NIST) and C. L. Gettinger (3M, previously at NIST).
• Semi-crystalline block copolymers of polystyrene (PS), polycaprolactone (PCL), and polybutadiene (PB) were crystallized from solution to form single crystals of PCL with the amorphous block forming a thin layer on the crystal surfaces. The morphology and electron diffraction patterns were obtained by TEM and the shape and thickness of the crystals were measured by atomic force microscopy (AFM) in the non-contact mode. This work was done in collaboration with Prof. R. Stadler (University of Bayreuth) and future studies of the relative block lengths of the amorphous and crystalline blocks and a comparison with the morphology in bulk specimens will be made to better understand the unique properties of semi-crystalline block copolymers of this type.
• In collaboration with K. Gardner (DuPont) and J. Coddington (Smithsonian), the morphology of spider silk from the brown recluse family, which produces a ribbon-like morphology because of a unique slit-die spinneret, has been characterized. TEM and electron diffraction data in combination with SEM and X-ray diffraction data from DuPont has been obtained to characterize the crystal structure. In addition, AFM was used to measure the thickness of the silk ribbons. This work is stimulated by DuPont=s interest in artificial silk produced by genetic engineering.
• TEM was used to help characterize the morphology in polymer/clay nanocomposite materials and associated char materials being studied in the Fire Science Division at NIST (with Jeff Gilman, BFRL) for their revolutionary flame retardant behavior. A comparison of the original delaminated nylon-6/clay (5%) nanocomposite to the char of the material suggested that the delaminated structure subsequently collapses during combustion and returns to a layered clay structure that resembles the original material.

 

Outputs

Publications

C.L. Jackson, L. Sung and C.C. Han, Evolution of Phase Morphology in Compatibilized Polymer Blends at Constant Quench Depths: Complimentary Studies by Light Scattering and Transmission Electron Microscopy, Polym. Eng. Sci., 37, 1 (1997).

B.J. Bauer, D-W. Liu, C.L. Jackson and J.D. Barnes, Epoxy/SiO₂ Interpenetrating Polymer Networks, Chapter 17, in IPNs Around the World: Science and Engineering, S. C. Kim and L. H. Sperling, Eds., John Wiley & Sons, Ltd., p. 279 (1997).

C.L. Jackson, H.D. Chanzy, F.P. Booy, D.A. Tomalia and E.J. Amis, Characterization of Dendrimer Molecules by Staining and Cryoelectron Microscopy Techniques, American Chemical Society PMSE Preprints, 77, 222 (1997).

C.L. Jackson, H.D. Chanzy, C.C. Han, V. Balsamo, R. von Gyldenfeldt and R. Stadler,

Surface Topology and Organization of Single Crystals from Polystyrene-Polycaprolactone (PS-b-PCL) and PS-Polybutadiene-PCL (PS-b-PB-b-PCL) Block Copolymers, American Chemical Society PMSE Preprints, 77, 650 (1997).

C.L. Jackson and B.J. Bauer, Synthesis and Characterization of Organic-Inorganic Interpenetrating Polymer Networks, Society of Plastics Engineers ANTEC >97, 1908 (1997).

C.L. Jackson and G.B. McKenna, Vitrification and Crystallization of Organic Liquids Confined to Nanoscale Pores, Chem. Mater., 8, 2128 (1996).

Presentations

C.L. Jackson, Characterization of Dendrimer Molecules by Staining and Cryoelectron Microscopy Techniques, American Chemical Society, Las Vegas, NV, September 1997.

C.L. Jackson, Surface Topology and Organization of Single Crystals from Polystyrene-Polycaprolactone (PS-b-PCL) and PS-Polybutadiene-PCL (PS-b-PB-b-PCL) Block Copolymers, American Chemical Society, Las Vegas, NV, September 1997.

C.L. Jackson, Characterization of Polymer Micellar Suspensions and Dendrimer Solutions by Cryo-electron Microscopy (poster) and Morphology Evolution and Phase Separation Kinetics in a Polymer Blend with Diblock Copolymer Additive, NIST-ERATO Joint Meeting on AMulticomponent Polymers and Polyelectrolytes@, Gaithersburg, MD, June 1997.

C.L. Jackson, Synthesis and Characterization of Organic-Inorganic Interpenetrating Polymer Networks, Society of Plastics Engineers, Toronto, Canada, April 1997.

C.L. Jackson, Morphology and Phase Separation Kinetics in a Ternary Polymer Blend, Lehigh University, Bethlehem, PA, November 1996.

Structural Characterization of Polymers by Small Angle X-Ray Scattering

J.D. Barnes and T.J. Prosa

Objectives

The objectives are to maintain a state-of-the-art Small Angle X-ray Scattering (SAXS) Facility that incorporates improvements in analysis methods and instrumentation to expand the use of SAXS as a technique for characterizing materials and processes. The SAXS Facility is promoted as a user facility for outside industrial clients and to support research within the Polymers Division.

Technical Description

By maintaining a state-of-the-art SAXS facility, Polymers Division researchers are able to perform in-house research relevant to ongoing programs as well as collaborative research with both industrial and academic scientists. The facility has a two-dimensional area detector capability and flexible sample handling for oblique incidence scattering, elevated temperature measurements, liquid samples, and uniaxial deformation. Test and analysis capabilities are constantly upgraded and implemented as the need or opportunity arise.

External Collaborations

The SAXS facility was used during the year for projects with outside users that fall into three categories. Collaborative research that is expected to lead to technical publications are currently underway with Mobil Chemical Co R&D Laboratory (SAXS and Wide Angle X-ray Diffraction, WAXD, orientation texture in HDPE films) and with Air Products Company (microstructure of high performance polyurethanes). Proprietary measurements in which the industrial client is interested in protecting sensitive data were performed by W. R. Grace and Mobil Chemical Co. Finally, the facility attracts collaborators from the academic community; staff from the University of Maryland study processing methods for producing oriented block copolymers as starting materials for nano-engineered polymer products.

NIST Internal Collaborations

Extensive measurements on dendrimers were carried out with the Polymer Blends and Processing Group. Studies on SAXS from several glassy polymers and, particularly semicrystalline polymers below their T_g, demonstrated that the scattering at low q differs from that which is to be expected from a liquid. Although the appearance of scattering from inclusions on the micrometer distance scale is not surprising, the observation of an additional component exhibiting a q^-2.5 power law dependence is, however, anomalous. A new study pursuing an understanding of such results is in its early stages. The Polymers Divisions 4-circle wide-angle x-ray diffractometer is aiding this work by its ability to characterize orientation in the amorphous component of these materials.

NIST has joined four industrial laboratories in the Participating Research Team (PRT) for the Advanced Polymer Beamline at Brookhaven National Laboratory. Data taken during the commissioning phase demonstrated wavevector range and resolution much superior to laboratory based facilities. When fully implemented, this beamline will provide real-time simultaneous small and wide angle x-ray scattering with high incident flux and multiple area detectors for dynamic measurements. Beam time available through PRT membership will complement our in-house SAXS facilities and significantly enhance capabilities available for internal research and industrial collaborations. In addition, the complementarity of SAXS and SANS has played an important role in the dendrimer work and in work on metal uptake by bacteria carried out with the NIST Center for Neutron Research.

Planned Outcomes

NIST researchers and external users from industry, government and academic laboratories will have available a state-of-the-art small angle x-ray facility operated by experts who can assist in collection and analysis of data.

Accomplishments

• Provided Proprietary usage beam time to Mobil Chemical Co. R&D Laboratories and W. R Grace Co.
• Upgraded SAXS area detector to provide improved electronic stability and to gain better resolution.
• Demonstrated, in collaboration with University of Maryland researchers, that shear using a channel die when crystallizing EtP/Et/EtP triblock copolymers (EtP = ethylene - propylene copolymer, E = ethylene) produces two distinct kinds of orientation that depend on cooling history.
• Provided support to dendrimers project within Polymer Blends and Processing Group.

Outputs

Publications

B.J. Bauer, D-W Liu, C.L. Jackson, and J.D. Barnes, Epoxy/SiO2 Interpenetrating Networks, Chapter 17 in IPN's Around the World: Science and Engineering, S. C. Kim and G. L. Sperling, Eds, John Wiley and Sons, Ltd., p. 279 (1997).

J.D. Barnes, G.B. McKenna, B.G. Landes, R.A. Bubeck and D.A. Banks, Morphology of Syndiotactic Polystyrene as Examined by Small Angle X-Ray Scattering, Polymer Science and Engineering, 37, 1480 (1997).

R. Hu, W.S. Lambert, and J.D. Barnes, Structure-Property Correlation in Stretched LLDPE Film Using Oblique Incidence Small Angle X-ray Scattering, Polymer Science and Engineering, 37, 1475 (1997).

Presentation

J.D. Barnes, G.B. McKenna, B.G. Landes, R.A Bubeck and D.A Banks, Morphology of Syndiotactic Polystyrene as Examined by Small Angle X-Ray Scattering, Materials Research Society, Boston, MA, December 1996.

Descriptive Measures for Small-Angle Scattering (SAS)

J. D. Barnes and T. J. Prosa

Objective

The objective is to improve the usability, precision and accuracy of SAS data. The development of software tools for reducing SAS data and for characterizing the performance of SAS measuring systems are the primary focus.

Technical Description

Experience with the user community for the NIST Digital SAXS Camera demonstrated that a lack of straightforward techniques for interpreting data from SAS patterns in Areal space@ terms is a barrier to the wider acceptance of this methodology, particularly in industry. This project seeks to devise software tools that will make this translation easier for inexperienced users of SAS methods.

Computer simulations and studies on model scattering systems are used in combination to develop and validate appropriate analytical tools. The simulations generate Avirtual@ SAS experiments whose results can be used as test cases for the development of improved algorithms for data reduction. The virtual SAS experiments also help to assess the importance of instrumental factors, interfering scattering sources, and noise. Scattering experiments on the model systems provide the relevant experimental data.

During FY 1997 three principal lines of investigation were pursued: (1) a careful study of the precision of SAS data from both real and virtual experiments, (2) methods for modeling data from isolated spherical polymer molecules in dilute solution, and (3) characterization of the scattering from oriented dispersed particulate scatterers in the form of pores in commercial track-etched membranes (rod-like particles).

External Collaborations

This project is closely linked to the worldwide SAS community through the Commission on Small-Angle Scattering of the International Union of Crystallography and the Small Angle Scattering Special Interest Group of the American Crystallographic Association.

Accomplishments

• A statistical procedure was developed to characterize uncontrolled systematic errors and noise effects on Small Angle Scattering (SAS) data. Modeling studies and real data demonstrate that adequate characterization of the precision of SAS data is not possible unless both noise and systematic error are presented to the end user. This finding will provide important input into the development of standardized data reduction and data transport tools for the SAS community.
• Monte Carlo methods have been shown to be an effective means of generating Avirtual@ SAS experiments for testing data reduction and model fitting procedures as well as for characterizing instrumental effects and counting noise.
• The SAS Worldwide web site (http://www.nist.gov/sas) has grown to include over 300 subscribers from the international SAS community as reflected in the subscriber list of its associated list server.

Outputs

Publications

J.D. Barnes and T.J. Prosa, Statistics in Area Detector Data Reduction, Advances in X-Ray Analysis, submitted.

Presentations

T.J. Prosa and J.D. Barnes, Parameterization of Morphology in Small-Angle Scattering, Fall Meeting of the Materials Research Society, Boston, MA, December 1996.

J.D. Barnes and T.J. Prosa, Statistics in Area Detector Data Reduction, Amer. Crystallographic Assn, St. Louis, MO, July 1997.

T.J. Prosa and J.D. Barnes, Descriptive Measures for Small Angle Scattering: Track-etched Membranes, St. Louis, MO, July 1997.

J.D. Barnes and T.J. Prosa, Statistics in Area Detector Data Reduction, Denver X-Ray Conference, Steamboat Springs, CO, August 1997.

Other

Dr. J.D. Barnes organized three sessions on Small-Angle Scattering at the annual meeting of the Amer. Crystallographic Assn. held July 20-24 in St. Louis, Mo.

Nonlinear Viscoelasticity of Solid Polymers

G.B. McKenna, D.M. Colucci and P.A. O=Connell^1
1University of Leeds, U.K.

Objectives

The objectives are to develop measurement methods, to characterize the nonlinear viscoelastic response of solid polymers and to evaluate the range of validity of material clock models for the prediction of the behavior of solid polymers.

Planned Outcomes

• A comprehensive materials data base for the nonlinear viscoelastic properties of a single glassy polymer in multiple deformation geometries (tension, torsion, compression) in stress relaxation conditions is to be created. The data are to be available for researchers to evaluate different constitutive models of polymer glasses.
• Establish the range of validity of reduced time based nonlinear viscoelastic constitutive laws to the behavior of solid, glassy polymers.
• Provide fundamental knowledge of the nonlinear viscoelastic response of glassy polymers.

 

Task 1. Time-Temperature, Time-Aging Time and Time-Strain Superposition below the Glass Transition

G.B. McKenna, P.A. O'Connell

Technical Description

Reduced time or material clock models of polymer behavior have been proposed as potentially powerful methods of introducing nonlinear behavior into the constitutive description of polymeric solids. Such models are a conceptual extension of the time-temperature and time-aging time superposition principles to the mechanical domain. However, there are few systematic studies that evaluate the validity of such concepts. Torsional stress relaxation measurements are used to characterize and compare time-temperature, time-aging time and time-strain superposition responses of a polycarbonate glass in the temperature range from 30 EC to 135 EC; that is, below the glass transition temperature.

External Collaborations

This work is being performed using the polycarbonate grade that is being used in the GM/GE ATP project and the results of this study are reported periodically to the appropriate technical team members at GM and GE.

Accomplishments

• Showed that time-temperature superposition is a valid descriptor for the viscoelastic response below the glass transition temperature in polycarbonate. The KWW (Kohlrausch-Williams-Watts) stretched exponential function often used in liquid and glassy state relaxation, however, does not successfully describe the master curve.
• The first ever comparison between time-strain and time-temperature superposition master curves was made. The stress relaxation master curves obtained from time-strain superposition are not the same as those obtained from time-temperature superposition. This severely limits the use of the material clock class of nonlinear constitutive equations based on time-strain superposition in materials modeling.
• The results from aging experiments on polycarbonate demonstrate that the conventional expectation of a singularity in the viscosity at approximately 50 EC below T_g is not observed. This result is in agreement with recent theoretical developments in these laboratories and dramatically alters the conventional view of molecular mobility as the glass transition is traversed.

Outputs

Publications

J.-J. Pesce and G.B. McKenna, Prediction of the Sub-Yield Extension and Compression Responses of Glassy Polycarbonate from Torsional Measurements, J. Rheology, in press.

P.A. O'Connell and G.B. McKenna, Large Deformation Response of Polycarbonate: Time-Temperature and Time-Aging Time Superposition, in Polycarbonate Science and Technology, ed. by D.G. LeGrand and J.T. Bendler, Marcel Dekker, New York, in press.

P.A. O'Connell and G.B. McKenna, Large Deformation Response of Polycarbonate: Time-Temperature, Time-Aging Time and Time-Strain Superposition, Polymer Engineering and Science, in press.

P.A. O'Connell, C.R. Schultheisz and G.B. McKenna, The Physics of Glassy Polycarbonate: Superposability and Volume Recovery, The Physics of Glassy Polymers, ed. By A. Hill and M. Tant, American Chemical Society books, 1998, in press.

G.B. McKenna, Torque and Normal Force Measurements in Viscoelastic Solids, Proceedings, 10^th International Conference on Deformation, Yield and Fracture of Polymers, Institute of Materials, Chameleon Press, London, 79-82, 1997.

P.A. O'Connell and G.B. McKenna, Reduced Time Concepts and the Sub-T_g Response of A Polycarbonate Glass, Proceedings, 10^th International Conference on Deformation, Yield and Fracture of Polymers, Institute of Materials, Chameleon Press, London, 415-418, 1997.

P.A. O'Connell and G.B. McKenna, Time-Temperature Superposition at Equilibrium in Polycarbonate below T_g, Proc. NATAS 25^th Annual Conference, September 1997, in press.

Presentations

G.B. McKenna, Solid Mechanics in Glasses: Time, Temperature and Deformation Effects, Institute of Materials Industries, Conference on >Oriented Polymers and Composites,= Boucherville, Canada, October 1996.

G.B. McKenna, Torque and Normal Force Measurements in the Study of the Nonlinear Response of Solid Polymers, Society of Engineering Sciences Annual Meeting, Tempe, AZ, October 1996.

P.A. O'Connell and G.B. McKenna, Time-Strain Superposition in Polymer Glasses, American Physical Society March Meeting, Kansas City, MO, March 1997.

G.B. McKenna, Torque and Normal Force Measurements in Viscoelastic Solids, Churchill Conference on Deformation, Yield and Fracture of Polymers, Cambridge, England, April 1997.

P.A. O'Connell and G.B. McKenna, Reduced Time Concepts and the Sub-Tg Response of a Polycarbonate Glass, Churchill Conference on Deformation, Yield and Fracture of Polymers, Cambridge, England, April 1997.

G.B. McKenna, Viscoelastic Behavior of Glass Forming Systems: Mechanical, Volumetric and Enthalpic Responses, Institut Charles Sadron, Strasbourg, France, April 1997.

G.B. McKenna, Time Dependent Properties of Polymers: Physical Aging, Viscoelasticity and their Implications for Long Term Use, EMPA (Swiss Federal Materials Testing Laboratory), Dhbendorf, Switzerland, April 1997.

G.B. McKenna, Viscoelastic Behavior of Glass Forming Systems: Mechanical, Volumetric and Enthalpic Responses, Polymers Division, NIST, Gaithersburg, MD, May 1997.

G.B. McKenna, Time Dependent Processes in Polymeric Materials: Viscoelasticity, Aging and Failure, Institute for Mechanics and Materials, Summer School, The Mechanics-Materials Linkage, Catholic University of America, Washington, DC, August 1997.

Task 2. Volumetric Behavior for Stress Relaxation Experiments in Tension and Compression of Polycarbonate below the Glass Transition Temperature

G.B. McKenna, D.M. Colucci and P.A. O=Connell

Technical Description

Few experiments are performed in stress relaxation conditions in which the volumetric response is measured. And while there is increasing evidence that the nonlinear viscoelastic response of polymeric glasses is not a simple function of the volume, it is essential that the volumetric response in such conditions be determined in order to separate the relative contributions of dilatational and deviatoric stresses to molecular mobility--i.e., nonlinear viscoelasticity. The technical approach is measurement of stress and volumetric responses during stress relaxation conditions in polycarbonate in tension and compression in the nonlinear viscoelastic, sub-yield regime. Biaxial extensometry is performed, using a modified lateral extensometer, on cylindrical samples to measure the lateral strain during uniaxial stress relaxation experiments. Experiments were performed in both tension and compression on samples that had been quenched from above the glass transition temperature to below it and aged for 24 hours at the testing temperature (22 EC) prior to testing. Stress and lateral strain measurements at constant axial strains were made for times to 3000 s. Two grades of polycarbonate were tested to investigate the possibility that the volumetric responses might be different because one sample yielded and the other fractured during tensile loading.

Accomplishments

• Discovered an apparent Aimplosion@ event in glassy polycarbonate subjected to tensile and compressive deformations. Implosion means that upon tensile deformations, while the sample increases in volume initially, after long relaxation times at strains in the order of 0.03 to 0.04 the sample density becomes greater than that of the original, undeformed, polymer. In compression, implosion is manifested by a decrease in volume followed by a continuing densification. The two polycarbonates showed some implosion upon compression. However, the polycarbonate with the greatest propensity to implode in tension yielded while that which showed negligible tensile implosion fractured at strain of about 0.035. The results suggest that the factors leading to implosion may be related to the propensity of a material to yield or to fracture.

 

Outputs

Publications

D.M. Colucci, P.A. O'Connell and G.B. McKenna, Stress Relaxation Experiments in Polycarbonate: A Comparison of Volume Changes for Two Commercial Grades, Polymer Engineering and Science, in press.

Presentations

G.B. McKenna, Mechanical Measurements in Viscoelastic Solids: Time-scale and Volume Phenomena, Chalmers Institute of Technology, Gothenburg, Sweden, April 1997.

G.B. McKenna, Stress and Strain Induced Mobility Changes in Glassy Polymers: Some Thoughts about Reduced Time in Constitutive Modeling, International Discussion Meeting on Relaxations in Complex Systems, Vigo, Spain, July 1997.

Physical Aging and Structural Recovery in Polymers

G.B. McKenna, C.R. Schultheisz, J.J. Filliben¹, D.M. Colucci, P.A. O=Connell², M.L. Cerrada³, S.L. Simon, A. Lee⁵, D.B. Curliss⁶, K.B. Bowman⁶, J.D. Russell⁶

¹Statistical Engineering Division, NIST

²University of Leeds, U.K.

³Instituto de Ciencia Y Technologia de Polimeros, Madrid, Spain

⁴University of Pittsburgh, Pittsburgh, PA

⁵Michigan State University, East Lansing, MI

⁶U.S. Air Force, Wright Laboratory Materials Directorate, Dayton, OH

Objective

The objective is to use existing methods and develop, where appropriate, new methods to measure kinetics of glass formation. The kinetics are related to the underlying microstructure of the glass through appropriate physical models.

Planned Outcomes

• Establish whether or not there exists an isochoric glass transition through experimentation with a glassy polycarbonate.
• Determine the material parameters in the Tool-Narayanaswamy-Moynihan-Kovacs-Aklonis-Hutchinson (TNM-KAHR) model of structural recovery for the polycarbonate being used in the GM/GE ATP project on thermoplastic engineering design.
• Determine the physical aging response of poly(ethylene naphthalate) in both semi-crystalline and amorphous states and provide results to Eastman Kodak Company.
• Establish a research material for the purpose of a round robin investigation on the relative time-scales required for glassy properties, such as relaxation response and volume or enthalpy recovery, to attain equilibrium.
• Provide a new analysis of the Kovacs enhanced set of structural recovery data for poly(vinyl acetate) and establish whether or not the data support the controversial J-effective paradox and expansion gap which are often used as bench marks for structural recovery models.

 

Task 1. Isobaric and Isochoric Responses of Polymers

G.B. McKenna, D.M. Colucci, J.J. Filiben, A. Lee, D.B. Curliss, K.B. Bowman, and J.D. Russell

Technical Description

The technical approach is to perform pressure, volume, temperature (PVT) experiments in which both the isobaric and isochoric paths into the glassy state are followed. The fragility of glass forming liquids along both isochoric and isobaric paths is analyzed and compared to the experimental results. PVT experiments can be performed in two ways. First, in the classical manner, the sample is brought to the liquid state, pressurized and then cooled under constant pressure. The volume is measured as a function of temperature and an isobaric glass transition is obtained. Conversely, the material can be brought into the liquid state, pressurized to a given volume and then cooled while the pressure is changed to maintain constant sample volume. The pressure is then recorded as a function of temperature and a break in the P-T curve is interpreted as an isochoric glass transition. Polycarbonate was used and studied over a range of pressures from 10 MPa to 200 MPa and temperatures from 50 EC to 250 EC. Experiments at six constant specific volume conditions from (0.8275 to 0.8550) cc/g were also performed. The results were analyzed in the context of the Fox-Flory free volume model.

The temperature dependence of the viscosity of glass forming liquids is often analyzed in terms of the Angell plot. Here, the logarithm of the viscosity (or of the shift factor) is plotted vs T_g/T to yield an Arrhenius plot that is reduced by the distance one is above the glass transition temperature T_g . Strong deviations of the response (Vogel or WLF-type behavior) are interpreted as a fragile liquid behavior, while Arrhenius behavior is interpreted as strong liquid response. To bring new insight into the meaning of fragility an examination was made of the dynamic (mechanical or dielectric) data from the literature for 14 polymers for which there existed both the dynamic (mechanical or dielectric) and PVT data above the glass transition. Cross-plotting the response both isobarically and isochorically yielded a comparison of the fragility of the materials as a function of path.

Accomplishments

• An isochoric glass transition was observed in dilatometric experiments on polycarbonate. This discovery strongly brings into question the validity of free volume models of the glass transition that are commonly used to describe thermal behavior of solid polymers during processing.
• Thermodynamic analysis of the PVT surfaces of isochoric vs isobaric glass forming systems showed that the glass formation pressure, temperature and volume points are independent of the path. However, the isochoric glass has a lower specific volume than does the isobaric glass.
• Showed that the strength of the isochoric glass transition as measured by, e.g., the change in the coefficient of thermal expansion or bulk modulus at T_g, is weaker for the isochoric glass than for the isobaric glass.
• Showed that the isochoric liquid is less fragile than is the isobaric liquid.

Outputs

Publications

D.M. Colucci, G.B. McKenna, J.J. Filliben, A. Lee, D.B. Curliss, K.B. Bowman and J.D. Russell, Isochoric and Isobaric Glass Formation: Similarities and Differences, J. Polym. Sci., Polym. Phys. Ed., 35, 1561 (1997).

D.M. Colucci and G.B. McKenna, Fragility of Polymeric Liquids: Correlations between Thermodynamic and Dynamic Properties, in Structure and Dynamics of Glasses and Glass Formers, ed. by C.A. Angell, T. Egami, J. Kieffer, K.L. Ngai and U. Nienhaus, MRS Symposium Proceedings, Vol. 455, 1997.

D.M. Colucci and G.B. McKenna, Measurement of an Isochoric Glass Transition, Proceedings, 10^th International Conference on Deformation, Yield and Fracture of Polymers, Institute of Materials, Chameleon Press, London, 411-414, 1997.

Presentations

D.M. Colucci and G.B. McKenna, Path Dependence of the Fragility of Polymeric Glass Formers, American Physical Society March Meeting, Kansas City, MO, March 1997.

D.M. Colucci and G.B. McKenna, Measurement of an Isochoric Glass Transition, Churchill Conference on Deformation, Yield and Fracture of Polymers, Cambridge, England, April 1997.

Task 2. Dilatometric Investigation of the Structural Recovery of Polymer Glasses

G.B. McKenna, C.R. Schultheisz, M. Vangel¹, S.D. Leigh¹, A. Rushkin¹, C. Straupe² and B. Lotz²

¹Statistical Engineering Division, NIST

²Institute Charles Sadron, Strasbourg, France

Objectives

The objectives are: i. Determine the Tool-Narayanaswamy-Moynihan-Kovacs-Aklonis-Hutchinson (TNM-KAHR) parameters for the polycarbonate glass used by the GM/GE Thermoplastic Engineering Design project supported by the ATP. ii. Create an international round robin with the same polycarbonate to investigate the glassy and approach to glassy behavior using multiple measurement techniques for glassy dynamics. iii. Determine the time-scales for mechanical and volume responses in the model epoxy being studied in the NIST torsional dilatometer at temperatures further below the glass transition than obtained previously. iv. Analyze the Kovacs volume recovery data for poly(vinyl acetate) using both his published and unpublished data to establish whether or not the Struik criticisms of the data as insufficiently accurate to support the existence of the J-effective paradox and expansion gap.

Technical Description

Up and down temperature jump experiments are performed in both classical dilatometers and the NIST torsional dilatometer for measurement of the structural (volume) recovery response in asymmetry of approach conditions. In the case of classical dilatometers, experiments are performed at temperatures as high as 150 EC on polycarbonate. The data are analyzed using the TNM-KAHR model of glassy kinetics. Material parameters for the models are obtained and compared with those in the literature for enthalpy recovery. The parameters are also to be provided to the technical contacts in the GM/GE ATP project. In the case of the torsional dilatometry, mechanical measurements are also made and the time-scales for the structural recovery as determined by volume and the evolution of the mechanical properties (physical aging) are compared. This work extends work performed previously on the same type of epoxy, but at temperatures nearer to the glass transition. Here, because the temperatures are farther below the glass transition temperature experimental times can be in excess of two months simply to equilibrate the samples volumetrically.

In 1964 Kovacs published a paper in which he analyzed structural recovery data in asymmetry of approach experiments using a parameter that he referred to as J-effective (J_eff) defined in terms of the volume departure from equilibrium * as J_eff=-1/* d*/dt. In plots of the log(1/ J_eff) vs * Kovacs observed an apparent paradox in that the values of J_eff did not converge to the same point as * approached zero, i.e., equilibrium--hence the equilibrium mobility of the structural recovery seemed path dependent. Also, the apparent paradox was accompanied by a spreading of the curves for J_eff in the up-jump experiments which has come to be known as the expansion gap. While it is currently accepted that the paradox itself does not exist because the curves will converge if the measurements are made closer to *=0(Kovacs= measurements went to

*. 2x10^-5), the existence of the expansion gap is still a subject of dispute. In particular, Struik in 1997 published an article claiming that the experimental uncertainties were too great to support the existence of the expansion gap. It is particularly important to resolve the issue now because recent models of structural recovery have claimed >success= specifically because the expansion gap is obtained. The original Kovacs 1964 data as well as unpublished data from his note books and more recent data obtained at the Institut Charles Sadron under his tutelage were subjected to a rigorous statistical analysis. The following hypothesis was tested: the value of J_eff as *610^-4 for a temperature jump from T_i to T₀ is significantly different from the value obtained for the temperature jump from T_j to T₀. The T_i=s can be either greater or less than T₀. If the hypothesis is rejected, the J_eff-paradox and expansion gap need to be rethought. If the hypothesis is accepted, then the argument that reproduction of the expansion gap is an important test of structural recovery models is strongly supported. The hypothesis is also being examined with new data from the polycarbonate dilatometry.

Accomplishments

• The TNM-KAHR parameters for polycarbonate used in the GM/GE ATP project have been obtained.
• An international round robin was initiated to study the thermoviscoelastic behavior of a research grade polycarbonate. The co-leader on the project is Prof. J.M. Hutchinson of the Aberdeen University in Scotland with eight confirmed participants and five tentative participants. This round robin will help to resolve the fundamental issues involved in the reports from various laboratories of different (or not) fictive temperatures for different processes in glass forming liquids.
• Torsional dilatometry experiments have been performed at temperatures as much as 15 EC below the nominal glass transition. Replicate experiments are underway.
• A new statistical analysis shows that, by accounting for the correlation of the errors in the Kovacs 1964 data, the expansion gap exists and is statistically significant at the 2 F level for values of */1x10^-4. This is well within the range of the observed expansion gap and makes the Kovacs observations valid for testing the behavior of structural recovery data.
• The volume dilatometry from polycarbonate also shows an expansion gap, though not as strongly as that observed in poly(vinyl acetate) by Kovacs.

Outputs

Publications

C.R. Schultheisz and G.B. McKenna, Volume Recovery and Physical Aging in Polycarbonate Following Temperature Jumps, Proceedings, 10^th International Conference on Deformation, Yield and Fracture of Polymers, Institute of Materials, Chameleon Press, London, 419-422, 1997.

C.R. Schultheisz and G.B. McKenna, Volume Recovery and Physical Aging in Glassy Polycarbonate Following Temperature Jumps, Proc. NATAS 25^th Annual Conference, September 1997, in press.

Presentations

C.R. Schultheisz and G.B. McKenna, Volume Recovery and Physical Aging in Glassy Polycarbonate Following Temperature Jumps, Society for Experimental Mechanics Spring Conference on Experimental and Applied Mechanics, Bellevue, WA, June 2-4, 1997.

C.R. Schultheisz and G.B. McKenna, Volume Recovery, Physical Aging and the Tau-Effective Paradox in Glassy Polycarbonate Following Temperature Jumps, Joint ASME/ASCE/SES Summer Meeting, McNU 97, Northwestern University, Evanston, IL, June 29 - July 2, 1997.

C.R. Schultheisz and G.B. McKenna, Volume Recovery, Physical Aging and the Tau-Effective Paradox in Glassy Polycarbonate Following Temperature Jumps, North American Thermal Analysis Society 25th Annual Conference, McLean, VA, September 1997.

C.R. Schultheisz and G.B. McKenna, Volume Recovery and Physical Aging in Polycarbonate and Epoxy, Society of Rheology Meeting, Galveston, TX, February 1997.

C.R. Schultheisz and G.B. McKenna, Volume Recovery and Physical Aging in Polycarbonate Following Temperature Jumps, 213th American Chemical Society National Meeting and Exposition, San Francisco, CA, April 1997.

C.R. Schultheisz and G.B. McKenna, Volume Recovery and Physical Aging in Polycarbonate, GE/GM ATP Semiannual Meeting, Buffalo, NY, October 1996.

C.R. Schultheisz and G.B. McKenna, Volume Recovery and Physical Aging in Glassy Polycarbonate Following Temperature Jumps, Churchill Conference on Deformation, Yield and Fracture of Polymers, Cambridge, England, April 1997.

B. Lotz, C. Straupe, G.B. McKenna, M. Vangel, A.L. Rushkin, and S.D. Leigh, The J-effective Paradox Revisited: An Extended Analysis of Kovacs= Original and Unpublished Volume Recovery Data on Poly(vinyl Acetate), International Discussion Meeting on Relaxations in Complex Systems, Vigo, Spain, July 1997.

G.B. McKenna, Physical Aging, Viscoelasticity, and Their Effects on the Performance of Polymers and Composites, Society of Plastics Engineers, Regional Technical Conference, Akron, OH, October 1996.

Task 3. The Physical Aging Response of Industrial Materials

G.B. McKenna, C.R. Schultheisz, J.M. O=Reilly¹, J. Greener¹, J.R. Gilmore¹ and M. Cerrada²

¹Eastman Kodak, Rochester, NY

²Instituto de Ciencia Y Technologia de Polimeros, Madrid, Spain

Technical Description

Classical time-aging time experiments are being performed to characterize the evolution of material properties at temperatures typical of industrial processing and annealing and subsequent use. The properties are important in determining the dimensional stability of the materials: e.g., amorphous and semi-crystalline films of poly(ethylene naphthalate) (PEN). While in many instances one has the option of choosing materials so that classical time-aging time and time temperature superposition principles apply, many industrial materials do not fall into this range of behaviors. Hence, semi-crystalline materials or amorphous glasses with large A$@ relaxations may not obey the classical time-aging time or time-temperature reduction principles. Currently amorphous and semi-crystalline PEN are studied. Classical analysis of the data cannot always be performed due to the presence of two large and overlapping transitions in PEN in the temperature and aging time ranges. Means are being sought to separate the mechanisms through both appropriate data analysis and by the use of new experimental tests sequences.

External Interactions

This work is being performed in collaboration with researchers at Eastman Kodak Company.

Accomplishments

• Demonstrated that for amorphous and semi-crystalline PEN there are two ranges of behavior: at low temperatures and short aging times, the creep and relaxation behaviors follow a power law behavior. Here the time-aging time and time-temperature superpositions appear to work. At long aging times and at low temperatures there appears a second mechanism which looks typically non-exponential, but is not a power law. Hence, time-aging time superposition is no longer valid. At intermediate temperatures, both mechanisms are present at all aging times. At high temperatures one only observes the non-exponential behavior and one finds again the time-aging time superposition. Detailed analysis of the data shows that, strictly speaking, simple temperature and aging time superposition principles do not apply to PEN and deconvolution of the two viscoelastic mechanisms needs to be performed.

Outputs

Publications

J. Beckmann, G.B. McKenna, B.G. Landes, D.H. Bank, and R.A. Bubeck, Physical Aging Kinetics of Syndiotactic Polystyrene as Determined from Creep Behavior, Polymer Engineering and Science, 37, 1459 (1997).

A. Lee and G.B. McKenna, Anomalous Aging in Two Phase Systems: Creep and Stress Relaxation Differences in Rubber-Toughened Epoxies, J. Polym. Sci., Polym. Phys. Ed., 35, 1167 (1997).

M.L. Cerrada and G.B. McKenna, Viscoelastic and Physical Aging Behavior in Semi-Crystalline PEN, Proc. NATAS 25^th Annual Conference, September 1997, 404-410.

Presentations

G.B. McKenna, Physical Effects in the Aging of Amorphous Resins, McNU >97, Northwestern University, July 1997.

M.L. Cerrada and G.B. McKenna, Viscoelastic and Physical Aging Behavior in Semi-Crystalline PEN, NATAS 25^th Annual Conference, September 1997.

Task 4. Effects of Structural Recovery on Modulated and Dynamic Thermal Measurements

G.B. McKenna and S.L. Simon

Technical Description

The technical approach consists of: i. Analyze the impact of structural recovery and thermal lag within a sample on the heat flow signal obtained in modulated differential scanning calorimetry experiments. ii. Compare predictions from the structural recovery model of Tool-Narayanaswamy-Moynihan with literature results showing an apparent dynamic heat capacity.

The Tool-Narayanaswamy-Moynihan (TNM) model of structural recovery under oscillatory thermal histories is solved to address two problems in the thermal analysis of glass forming materials. The first problem is the effect of structural recovery on the thermal signal obtained in modulated DSC experiments. This question is important because increasing numbers of laboratories are purchasing modulated DSC calorimeters for thermal analysis and one needs to fully understand this new technique. Hence, the TNM model is solved using material parameters for polystyrene (PS) and poly(vinyl chloride) (PVC) for typical thermal histories encountered in the thermal analysis laboratory. Using a Lissajous loop analysis in which heat flow is plotted vs the time derivative of temperature (HF vs dT/dt) it is shown that the response can be tremendously influenced by structural recovery and that a linear analysis will give erroneous results in these cases. Hence it is recommended that users of the modulated DSC avoid thermal histories in which there is a great deal of structural recovery (enthalpy overshoot). It was further shown, by performing an analysis that included sample geometry, that thermal lag in the samples due to simple thermal conductivity effects can cause anomalous phase lags in the thermal signals even for relatively small samples (thickness.0.2 cm).

For the dynamic heat spectroscopy analysis, it was shown that the apparent dynamic heat capacity obtained by Birge and Nagel for glycerol and by Donth and co-workers for poly(vinyl acetate) could be quantitatively predicted from the TNM model using material parameters obtained in the glass transition region. These calculations show that structural recovery in high fictive temperature glasses can explain the dynamic heat capacity of glass forming liquids. It also suggests, strongly, that dynamic heat spectroscopy measurements may be a powerful method to determine the structural recovery retardation function used in the TNM model in a way that is independent of the other material parameters. This is important because the model is highly nonlinear and the parameters are normally obtained by curve fitting results to, e.g., DSC measurements. Because the parameters in the model are not statistically independent having a means to determine by independent experiments one of the parameters removes some of the uncertainties in the model.

Accomplishments

• Demonstrated by model calculations that structural recovery in polymeric glasses can dramatically affect the heat flow signal in modulated DSC measurements. This finding is important to users of modulated DSC instruments as it helps them to avoid misinterpretation of data due to structural recovery induced artifacts.
• Demonstrated by model calculations of structural recovery that dynamic heat spectroscopy results are a simple manifestation of structural recovery in high fictive temperature glasses. This leads to a different interpretation of dynamic heat capacity from that previously used to interpret such experiments. This could have profound implications for the relationship between structural recovery and fluctuations in complex fluids.

Outputs

Publications

S.L. Simon and G.B. McKenna, Interpretation of the Dynamic Heat Capacity Observed in Glass-Forming Liquids, J. Chem. Phys., in press.

S.L. Simon and G.B. McKenna, The Effects of Structural Recovery and Thermal Lag in MDSC, Thermochimica Acta, in press.

S.L. Simon and G.B. McKenna, The Effects of Structural Recovery in Modulated DSC, Society Plastics Engineers, ANTEC, II, 2232 (1997).

S.L. Simon and G.B. McKenna, The Effects of Structural Recovery and Thermal Lag in Modulated DSC Measurements, Proc. NATAS 25^th Annual Conference, September 1997, 358-365.

G.B. McKenna and S.L. Simon, Interpretation of the Dynamic Heat Capacity Observed in Glass-Forming Liquids, Proc. NATAS 25^th Annual Conference, September 1997, 677-684

Presentations

G.B. McKenna and S.L. Simon, Dynamic Heat Capacity: Myth or Reality, Max Planck Instht fhr Polymerforschung, Main, Germany, April 1997.

S.L. Simon and G.B. McKenna, The Effects of Structural Recovery in Modulated DSC, Society Plastics Engineers, ANTEC, Toronto, CN, May 1997.

S.L. Simon and G.B. McKenna, The Effects of Structural Recovery and Thermal Lag in Modulated DSC Measurements, NATAS 25^th Annual Conference, McLean , VA, September 1997.

G.B. McKenna and S.L. Simon, Interpretation of the Dynamic Heat Capacity Observed in Glass-Forming Liquids, NATAS 25^th Annual Conference, McLean , VA, September 1997.

G.B. McKenna, The Glass Transition as a Kinetic Phenomenon: Implications for Thermal Analysis, NATAS 25^th Annual Conference, McLean , VA, September 1997.

Task 5. Physical Aging and Structural Recovery in Polymeric Glasses in Plasticizing Environments

G.B. McKenna and W.H. Han¹

¹Korean Government Fellowship for Overseas Doctoral Studies

Technical Description

The technical approach is to investigate the hypothesis that the chemical activity (or solvent concentration) can be treated in the same way as temperature as a means of treating structural recovery and physical aging in polymeric glasses. It is well known that small molecules such as H₂O and CO₂ act to depress the glass transition in polymers: hence they act as plasticizers. Furthermore, there is some evidence that concepts such as time-concentration superposition can account for much of the change in viscoelastic properties in plasticized polymers. However, there have been few works to explore how changes in concentration (as induced by a change in the activity of the surrounding environment) impact the structure of the glass. This is an issue of fundamental importance for both processing and long term performance in materials because the volume changes induced by changes in plasticizer content can easily be equivalent to hundreds of degrees of temperature change.

An investigation is made of the hypothesis that changes in glassy structure induced by changes in plasticizer concentration are equivalent to similar changes in temperature. In order to test the hypothesis an apparatus is under construction that permits the rapid change of relative humidity or supercritical carbon dioxide pressure over thin polymer films simultaneously tested for creep response, volume change and mass uptake. Because the measurement of volume change and mass uptake are the most difficult to automate, the focus is on the physical aging response in a model epoxy system subjected to isothermal relative humidity (RH) jumps and iso-RH temperature-jumps. Investigations have also begun using isothermal carbon dioxide pressure-jumps and iso-CO₂ pressure, temperature-jumps.

Accomplishments

• Made the first measurements demonstrating that RH-jump experiments manifest the >memory= effect typically seen in volume recovery experiments.
• Preliminary results show that RH-jump and T-jump experiments lead to the same response for the epoxy system considered. This may be because the moisture uptake is less than 2% by mass.
• Preliminary results show that jumps in the CO₂ pressure induce a different structure in a glassy polycarbonate than do temperature jumps to the same final condition. The creep response in the CO₂ pressure jump experiments exhibits a narrower distribution of retardation times than the same response in the temperature-jump experiments.

Outputs

Publications

W.H. Han and G.B. McKenna, The Physical Aging Response of Epoxy in Relative Humidity Jump Experiments, Proceedings NIST/Rome Laboratories Workshop on Moisture Effects in Microelectronics, NIST, Gaithersburg, MD, October 1996, in press.

W.H. Han and G.B. McKenna, Plasticizer Effects on Physical Aging of Epoxy, (1997) Society Plastics Engineers ANTEC, II, 1539.

W.H. Han and G.B. McKenna, Polymers and Small Molecules: Glass Transition and Aging Effects, Proc. NATAS 25^th Annual Conference, September 1997, 382-389.

Presentations

W.H. Han and G.B. McKenna, Physical Aging of Polycarbonate Films under High Pressure Gases, McNU >97, Northwestern University, July 1997.

W.H. Han and G.B. McKenna, Plasticizer Effects on Physical Aging of Epoxy, Society Plastics Engineers ANTEC, Toronto, Canada, May 1997.

W.H. Han and G.B. McKenna, Structural Recovery and Physical Aging after Relative Humidity Jumps: First Evidence of a Memory Effect, American Physical Society March Meeting, Kansas City, MO, March 1997.

W.H. Han and G.B. McKenna, The Physical Aging Response of Epoxy in Relative Humidity Jump Experiments, Rome Laboratories/NIST Workshop on Moisture in Electronic Packaging, NIST, Gaithersburg, MD, October 1996.

W.H. Han and G.B. McKenna, Polymers and Small Molecules: Glass Transition and Aging Effects, NATAS 25^th Annual Conference, McLean, VA, September 1997.

W.H. Han and G.B. McKenna, Physical Aging of Plasticized Polymer Glass, the 4th Sigma-Xi Postdoctoral Presentation, NIST, February 1997.

W.H. Han and G.B. McKenna, Plasticization Effects in Polymer Glasses: Concentration Jump Experiments, 68th Annual Meeting of the Society of Rheology, Galveston, Texas, February 1997.

Failure in Polymers and Adhesives

M.Y.M. Chiang, G.B. McKenna, M. Delin¹, H. Chai² and M. Fernandez-Garcia³

¹Chalmers University of Technology, Goteborg, Sweden

²Tel Aviv University, Israel

³Instituto de Ciencia Y Technologia de Polimeros, Madrid, Spain

Objectives

The objectives are to develop numerical models of adhesive behavior and validate these with experimental observation. The effect of aging and environment on the failure and pre-failure behavior of polymers and adhesives is investigated.

Planned Outcomes

• Demonstration by experiment and finite element analysis that mode II fracture can be described by a criterion in which the local strain at the crack tip determines failure.
• Development of test methods that impact on long term performance evaluation and predictability in polymers, composites and adhesives.
• Establish correlations between long term moisture exposure and the mechanical response of commercial adhesives to support ATP focus program on Composites.
• Validation of theoretical models of craze initiation and growth through use of structural recovery (physical aging) to vary the material=s viscoelastic response.

 

Task 1. Finite Element Analysis of Interfacial Crack Propagation Criterion Based on Local Shear (Near Tip Deformation and Fracture)

M.Y.M. Chiang and H. Chai

Technical Description

The majority of early strength analysis of lap shear joints either neglected stress variations across the bond or employ LEFM (linear elastic fracture mechanics) based approaches. More recently, small scale yielding solutions were produced for bi-material interface crack problems. Such solutions, however, are still insufficient to account for the very large deformations that occur in tough adhesives. Recent experiments from this laboratory show that the fracture process in shear loaded adhesive bonds is characterized by a stable crack propagation followed by catastrophic growth. During the stable growth, a variety of failure process may be activated at or ahead of the crack tip, the most predominant one being interfacial cracking. One of the more interesting aspects of this failure mode is that even though the deformation in the majority of the bond remains in the elastic range, very large shear strains (i.e over unity) may develop over a considerable distance ahead of the crack tip. The plastic zone in this case may be several orders of magnitude greater than the bond thickness while the relative shearing displacement across the bond is of the same size as the bond thickness. Therefore, the present study is carried out using a large strain, incremental plasticity finite element analysis. The analysis focuses on the interface straight ahead of the crack tip, where the fracture occurred.

Accomplishments

• Showed that the distribution of the engineering shear strain, (, along the interface conforms to the following relationship: ( = A  ^{- *} ,  = x /h where x is a horizontal coordinate axis originating from the crack tip, h is the bond thickness,  is the average shear strain in the bond (Aload@) and A and * are known numerical functions of  .
• Established that the distribution of the shear strain along the interface near the crack tip in mode II fracture conforms to a power law singularity under small as well as large scale yielding conditions. This changes the paradigm by which researchers model large deformation crack tip singularities. There was no prior solution to the large scale yielding problem in mode II fracture conditions.

Outputs

Publications

M.Y.M. Chiang, and H. Chai, Finite Element Analysis of Constrained Interfacial Crack Propagation Based on Local Shear, Part I - Near Tip Solution, Intl. J. Solids and Structures, in press.

H. Chai and M.Y.M. Chiang, Finite Element Analysis of Constrained Interfacial Crack Propagation Based on Local Shear Deformation, Part II - Fracture, Intl. J. Solids and Structures, in press.

S.Yang, M.Y.M. Chiang and F.G. Yuan, Analytical Forms of Higher-order Asymptotic Elastic-plastic Crack-tip Fields in a Linear Hardening Material under Antiplane Shear, Intl. J. of Fracture, 80, 59 (1996).

M.Y.M. Chiang and H. Chai, A Fracture Criterion Based on Local Shear Strain for Adhesive Bonds, Proceedings 20th Anniversary Meeting of the Adhesion Society, Hilton Head, SC, February 1997.

M.Y.M. Chiang, Finite Element Analysis and Experimental Monitoring of Strain Localization and Failure Process of Thin Adhesive Bonds, 42nd Intl. SAMPE Symposium, 1997.

A. Kuo, D. Newport, W. Yin, and M.Y.M. Chiang, Generalized Stress Intensity Factor Concept for Fatigue and Fracture Evaluations, ASME EEP-Vol.19, Advances in Electronic Packaging 1997, pp.1451-1460.

Presentations

M.Y.M. Chiang, An Intrinsic Fracture Criterion Based on Local Strain for Highly Constrained Interfacial Cracking, Society of Engineering Science, 33rd Annual Technical Meeting, Tempe, AZ, October 1996.

M.Y.M. Chiang, Finite Element Analysis and Experimental Monitoring of Strain Localization and Failure Processing of Thin Adhesive Bonds, Intl. Conferences on Polymer Characterization (POLYCHAR-5), Denton, TX, January 1997.

M.Y.M. Chiang, A Fracture Criterion Based on Local Shear Strain for Adhesive Bonds, 20th Anniversary Meeting of the Adhesion Society, Hilton Head, SC, February 1997.

M.Y.M.Chiang, Finite Element Analysis and Experimental Monitoring of Strain Localization and Failure Process of Thin Adhesive Bonds, 42^nd Intl. SAMPE Symposium/Exhibition, Anaheim, CA, May 1997.

M.Y.M. Chiang, Finite Element Analysis and Experimental Monitoring of Strain Localization and Failure Process of Thin Adhesive Bonds, Intel Corp., San Jose, CA, May 1997.

M.Y.M. Chiang, Finite Element Analysis and Experimental Monitoring of Strain Localization and Failure Process of Thin Adhesive Bonds, Hexcel Composites, Dublin, CA, May 1997.

M.Y.M. Chiang, Finite Element Analysis and Experimental Monitoring of Strain Localization and Failure Process of Thin Adhesive Bonds, AlliedSignal, Morristown, NJ, June 1997.

Task 2. Hygrothermal Aging of Commercial Adhesives

M.Y.M. Chiang and M. Fernandez-Garcia¹

¹Instituto de Ciencia Y Technologia de Polimeros, Madrid, Spain

Technical Description

The implementation and use of adhesive materials in structural and engineering designs require understanding of material performance under environmental conditions during long-term use. Also, increased use of adhesive bonding in structural components has renewed interest in the analysis of adhesive joints. A particular problem is effects of the diffusion of environmental agents into the adhesive layer. Diffusion of moisture or solvent into the adhesive can significantly change the state of stress field and hence the reliability of the bond joints. In this project, hygrothermal aging of commercial adhesives is studied in terms of effects on mechanical properties and failure behavior of highly constrained thin adhesive layers.

Most engineering analyses of adhesive materials either neglect the effect of the interlayer or assume that it behaves like the bulk material. However, recent experimental observations and finite element analyses show that the mechanical mass of the interlayer are greatly affected by its thickness. For example, it was found that the ultimate shear strain of a brittle epoxy increases from a bulk value (about 6%) to between 15% and 300% upon decreasing the bond thickness from 0.1 mm to 25 :m. In this work, the interaction of environmental conditions (dry and wet) and bond thickness on the stress-strain relation of the adhesive will also be studied in order to enhance our understanding of their effects. Experimental tests under simple shear conditions will be studied using the napkin ring specimen. The bond thickness is a primary variable which will be varied from several micrometers up to values large enough to expose the bulk behavior.

Accomplishments

• Developed method to make void free thin films appropriate to the commercial adhesives used in this study.

 

Task 3. Measurement of Craze Initiation and Growth in Polymers

G.B. McKenna and M. Delin¹

¹Chalmers University of Technology, Goteborg, Sweden

Technical Approach

This project examines the crazing process as one that depends very strongly on the viscoelastic response of the glassy polymer under study. It is known that temperature can have large effects on craze initiation and growth. In the current work the viscoelastic response is altered by changing both temperature and aging time, the aging time permitting isothermal changes in the >glassy structure=. Craze initiation and growth is studied using special fixtures to provide equibiaxial and uniaxial loading in creep conditions and uniaxial loading in stress relaxation conditions.

Accomplishments

• Modified existing equipment to make visual measurements of craze growth under uniaxial stress relaxation conditions.
• Discovered a transition in craze growth rate in stress relaxation conditions for a styrene-acrylonitrile copolymer. The logarithmic craze growth rate decreases by approximately a factor of five in going from below to above the transition aging time. If this is a universal behavior for all polymers it implies that the long term performance may be enhanced due to the aging process stabilizing the material against craze growth.

Outputs

Publications

G.M. Gusler and G.B. McKenna, The Craze Initiation Response of a Polystyrene and a Styrene-Acrylonitrile Copolymer During Physical Aging, Polymer Engineering and Science, in press.

M. Delin and G.B. McKenna, Impact of Physical Aging on Craze Growth in a Styrene-Acrylonitrile Co-Polymer, Proc. NATAS 25^th Annual Conference, September 1997, in press.

Presentations

M. Delin, and G.B. McKenna, Impact of Physical Aging on the Craze Initiation and Propagation in Styrene Acrylonitrile Co-polymer, Fourth Annual SigmaXi Post-Doctoral Associate Poster Presentation, National Institute of Standards and Technology (NIST), Gaithersburg, MD, February 1997.

G.B. McKenna and M. Delin, Craze Propagation in Stress-relaxation Conditions for a

Styrene-acrylonitrile Copolymer, Poster Session of the Polymers Division, National Institute of Standards and Technology (NIST), Gaithersburg, MD, May 1997.

R.W. Rychwalski, J. Vernel, M. Delin, J. Kub<t, C. Klason, J.M. Hutchinson, A.V. Vannikov and V. PelRek, Effect of Physical Aging on The Dynamic - Mechanical, Enthalpic And

Non-linear Optical Mass of PVAc, 3rd International Discussion Meeting on Relaxations in Complex Systems, Vigo, Spain, June 30 -July 11, 1997.

M. Delin and G.B. McKenna, Impact of physical aging on craze growth in a styrene-acrylonitrile co-polymer, 25th Annual Conference of North American Thermal Analysis Society, McLean, VA, September 7-9, 1997.

Other

C.R. Schultheisz, W.G. McDonough, S. Kondagunta, C.L. Schutte, K.S. Macturk, M. McAuliffe, G.A. Holmes, D.L. Hunston, Durability of E-Glass/Epoxy Composites Immersed in Water, Society of Plastics Engineers Regional Technical Conference (SPE RETEC), Cuyahoga Falls, Ohio, October 28-29, 1996.

C.R. Schultheisz, W.G. McDonough, S. Kondagunta, C.L. Schutte, K.S. Macturk, M. McAuliffe, G.A. Holmes, D.L. Hunston, Durability of E-Glass/Epoxy Composites Immersed in Water, University of Delaware, Wilmington, DE, November 15, 1996.

C.R. Schultheisz, K. Liao, D.L. Hunston, Long-Term Durability of Composites in Infrastructure Applications, National Seminar on Advanced Composite Material Bridges, Arlington, Virginia, May 5-6, 1997.

C.R. Schultheisz, W.G. McDonough, S. Kondagunta, C.L. Schutte, K.S. Macturk, M. McAuliffe, G.A. Holmes and D.L. Hunston, Effect of MoisturMicro-Mechanics Measurement Technologies for Fiber-Polymer Interfaces, NIST, Gaithersburg, MD, May 28-30, 1997.

Rheological Characterization of Polymer Dynamics

G.B. McKenna, C.R. Schultheisz, V. Rouiller¹ and R. Zorn²

¹NIST Guest Research from Houdemont, France
²KFA, Julich, Germany

Objective

The objective is to develop rheological descriptions for polymeric materials that provide standard reference materials, produce critical data and provide defining tests of existing physical and phenomenological models for polymer dynamics.

Planned Outcomes

• Produce a new nonlinear fluid as a Standard Reference Material.
• Provide a good representation of the viscoelastic response of polyurethane based on a hybrid Valanis-Landel-BKZ model.
• Extension of hybrid viscoelastic model to foams in hydrostatic creep.

 

Task 1. Develop a Nonlinear Fluid Standard

G.B. McKenna and C.R. Schultheisz

Technical Description

Polymeric fluids, such as polymer melts and solutions, often do not follow the simple, Newtonian ideal in their flow behavior. Such fluids see wide application in everyday life (injection molding, paints and coatings, food products, etc.), and the ability to measure accurately and characterize their behavior is very important to optimizing their processing conditions. Since there are a number of commonly used methods to measure the flow behavior of polymers, the new Standard Reference Material will provide a means for comparing the performance of different instruments. The new fluid will be certified for the shear rate dependence of viscosity and first normal force difference over a temperature range of approximately 50 /C. The linear viscoelastic responses will also be certified along with the temperature dependence of the shift factors and the zero shear viscosity.

The new fluid supersedes the previous Standard Reference Material 1490 Nonlinear Fluid Standard, which was composed of a high molecular mass polyisobutylene, PIB, dissolved in n-hexadecane. This fluid was found to have two problems: first, it showed evidence of a loss of homogeneity on storage, and second, the working range of the fluid was limited by the crystallization of the n-hexadecane, which has a melting point of 18 EC. The approach taken to overcome these difficulties focuses on using a solution of high molecular mass polyisobutylene dissolved in low molecular mass (oligomeric) polyisobutylene. The material is produced by dissolving the two components in a third, highly volatile solvent, such as n-pentane or cyclohexane. By driving off the solvent, one thus obtains a homogeneous solution of the desired concentration of PIB in its oligomer. Initial experimentation shows that the approach leads to a solution with the appropriate range of viscosity and normal force response. However, the material produced shows some excessive tackiness which makes it difficult to handle. Further, there is some evidence that the oligomer itself may contain low molecular mass species that also evaporate, albeit more slowly than the solvent used to prepare the solution. This leads to somewhat less control over the final product than desired. Therefore, several alternatives are investigated including a different processing method for the current solution, the use of a higher molecular mass solvent that can be retained in a stable manner in the solution, and the use of a long-chain branched alkane (similar to the n-hexadecane but with a lower melting temperature) in place of the low molecular mass PIB.

The project goals also include a round robin test of the candidate Standard Reference Material involving instrument manufacturers and users in industry and at universities. The round robin will ensure that the chosen material mass are acceptable to the community, and will provide information regarding the lab-to-lab variability in the measurements. The Statistical Engineering Division at NIST will be involved in evaluating the results of the round robin.

External Collaborations

The following have agreed, in principle, to participate in the round robin.

Instrument Manufacturers

ATS RheoSystems Paar Physica

Bohlin Instruments Rheometric Scientific

Brookfield Engineering Laboratories TA Instruments

Capillary Rheometer TherMold Partners

Goettfert Vilastic Scientific

Haake

Industrial Users

DuPont National Starch and Chemical

Fluid Dynamics Procter and Gamble

 

University

David Boger University of Melbourne

Ralph Colby Pennsylvania State University

Francis Gadala-Maria University of South Carolina

William Graessley Princeton University

Chris Macosko University of Minnesota

Jaye Magda University of Utah

Gareth McKinley Harvard University

Skip Rochefort Oregon State University

Accomplishments

C A candidate material has been successfully produced, and preliminary measurements indicate that it has appropriate shear thinning and normal stress difference behavior.

 

C A new rheometer, redesigned in collaboration with NIST staff to improve normal force measurement capability, has been acquired.

 

C Industry and academic partners have been contacted for the purpose of running the round robin and interest in the new fluid and promise of participation in the round robin has been high.

 

Outputs

Presentation

C. R. Schultheisz and G.B. McKenna, A Nonlinear Fluid Standard Reference Material: Progress Report, 69th Annual Meeting of the Society of Rheology, Columbus, Ohio, October 23, 1997.

Task 2. Viscoelastic Constitutive Model for the Creep Behavior of Polyurethane Foams

G.B. McKenna and V. Rouiller

Technical Approach

The applicability of using a hybrid Valanis-Landel-BKZ type of constitutive equation as a robust means of describing the nonlinear viscoelastic response of polymers above the glass transition is explored. In the past this worked reasonably well for modeling the response of a poly(vinyl chloride) film material. Under sponsorship of the NSWC Carderock of the U.S. Navy, the same approach is applied to characterization of polyurethane rubber. In addition, the project aims to use the time and temperature dependent parts of the model to obtain an accurate description of the behavior of polyurethane foams used by the Navy. Experimentation requires tension and compression characterization of the neat resin over a range of deformations and temperatures. Comparison of the hybrid model predictions with both creep response of the rubber and the response in multi-step creep and stress relaxation experiments are being made. Experiments in hydrostatic compression of foams are planned and a foam model extension of the hybrid viscoelastic model will be used to predict the long term performance of the foams.

Accomplishments

C Built temperature and humidity control chamber for uniaxial test apparatus.

 

C Developed new software to drive the uniaxial test apparatus in multi-step creep and stress relaxation experiments.

 

C Demonstrated that a hybrid viscoelastic constitutive model can predict multi-step stress