Alamgir Karim, Jack F. Douglas, Li-Piin Sung1 and Charles C. Han

¹University of California, Santa Barbara, CA

Objectives

This study characterizes the interface between polymer phases in thin film coatings and investigates the kinetics of phase separation in polymer blends using state-of-the art neutron and x-ray reflection and optical and atomic force microscopy techniques. We investigate the interfacial activity of block copolymers in stabilizing the morphology of dispersed polymer phases. This project is aimed at quantifying phase separation and microphase separation of polymer blends and the interfacial activity of block copolymers in thin film coatings.

Accomplishments

Characterizing the interface between phase separated polymers in the form of thin film coatings is a difficult task due to the requirement of a high resolution technique to probe the interface. There is however significant interest in obtaining quantitative information on these systems. In addition, the interfacial activity of block copolymers in polymer blends is poorly understood. Issues related to changes in blend morphology, coalescence and changes in interfacial tension upon addition of block copolymer are technologically important in thin film coatings.

A combination of non- destructive high resolution techniques are used to profile depthwise the longitudinal distribution of phase separating polymer blends and microscopy techniques to characterize the transverse distribution. The depth profiling techniques include x- ray and neutron reflectivity while lateral scanning techniques include reflection optical microscopy and atomic force microscopy. The internal spatial distribution of blend components normal to the surface is obtained by specular neutron reflectivity. Phase separation is characterized by a "roughening transition" of the air surface measurable by x-ray reflectivity while the kinetics of phase separation is readily monitored by optical microscopy. Changes in blend film morphology as it affects surface composition is characterized by atomic force microscopy.

Phase separation was studied in thin films of two blend systems, one which exhibits a lower critical solution temperature (deuterated polystyrene/ polyvinylmethylether) (dPS/PVME) and another which exhibits an upper critical solution temperature (deuterated polystyrene/ polybutadiene) (dPS/PB). For both blend systems we utilize x-ray and neutron reflectivity and reflection optical microscopy as well as atomic force microscopy to examine phase separation in thin films of two component blends. Phase separation is found to be accompanied by topographical changes at the free polymer/air interface that reflect the underlying spinodal decomposition process within the film. This phenomenon was observed in systems where the same component segregated to the vacuum and substrate interfaces. In the dPS/PVME system, blend films with composition ranging between 0% and 100% dPS were investigated. The surface roughening phenomena was found to occur at all compositions. The magnitude of the interfacial tension relative to the surface tension is shown to set the size scale of the surface features. The observed structures are consistent with the aspect ratio estimated by balancing the reduction in free energy from decreasing the coexisting phase contact area with the excess free energy due to increasing the polymer- vacuum interfacial area.

The kinetics of phase separation in thin film blends of deuterated polystyrene (dPS) and polybutadiene (PB) on a silicon substrate was examined by optical microscopy of the free polymer-air interface. Phase separation within the film induces pattern formation at the free boundary as in the dPS/PVME. The characteristic scale R(t) for the intermediate and late stage phase separation patterns is determined by Fourier transformation of the optical data. Importantly, the thickness range of the films is chosen small enough (thickness, L < 1000 Å ) to suppress the development of surface directed concentration waves, but large enough so that a crossover from three-dimensional bulk-like to near two dimensional (thickness ~ 200 Å ) phase separation kinetics can be observed. The exponent n, characterizing the coarsening (R(t) ~ tⁿ) in the viscous hydrodynamic regime, is found to equal 1.0 and 0.44 in the 1000 Å and 200 Å films, respectively (Figure 2). These observations are consistent with a dimensional crossover in the late stage phase separation kinetics. At a still later stage in the phase separation process the bicontinuous surface pattern breaks up and the pattern scale "pins;" an effect which seems to be associated with the finite film thickness. The addition of even 1% of a diblock copolymer of PS-PB to the blend dramatically slows down the kinetics of the process. The mechanism for this strong effect remains to be further investigated.

Outputs

Publications
T. M. Slawecki, A. Karim, S. K. Kumar, T. P. Russell, S. K. Satija, C. C. Han, M. H. Rafailovich, R. M. Overney, Phase Separation in Thin Films of Polymer Mixtures, Phys. Rev. Lett., Submitted.

J. Dudowicz, K. F. Freed, J. F. Douglas, Modification of the Phase Stability of Polymer Blends by Diblock Copolymer Additives, Macromolecules, 28, 2276 (1995).

L. J. Norton, E. J. Kramer, F. S. Bates, M.D. Ghelsen, R. A. L. Jones, A. Karim, G. P. Felcher, R. Kleb, Neutron Reflectivity Study of Surface Segregation in an Isotopic Poly(ethylene propylene) Blend: Deviation from Mean Field Theory, Macromolecules, In Press.

M. Sikka, N. Singh, F. S. Bates, A. Karim, S. K. Satija, C. F. Majkrzak, Surface Segregation in Model Symmetric Polyolefin Diblock Copolymer Melts, J. Phys. II (France) 4, 2231 (1994).

P. Brant, A. Karim, J. F. Douglas, F. S. Bates, Surface Compositions of Blends of Amorphous and Crystallizable Polyethylene Blends as Measured by Static SIMS, Macromolecules, Submitted.

Presentations
A. Karim, C. C. Han, S. K. Satija, T. M. Slawecki, S. K. Kumar, T. P. Russell, M. H. Rafailovich, Phase Transitions in Thin Polymer Blend Films, Polymer Physics Gordon Conference, Newport, RI, August 1994.

A. Karim, C. C. Han, S. K. Satija, T. M. Slawecki, S. K. Kumar, T. P. Russell, M. H. Rafailovich, Phase Transitions in Thin Polymer Blend Films, Fall Meeting of American Chemical Society, Washington, D. C., August 1994.

A. Karim, C. C. Han, S. K. Satija, T. M. Slawecki, S. K. Kumar, T. P. Russell, M. H. Rafailovich, Phase Transitions in Thin Polymer Blend Films, Condensed Matter Symposia Series, Materials Science Division, Argonne National Laboratory, IL., September 1994.

A. Karim, C. C. Han, S. K. Satija, T. M. Slawecki, S. K. Kumar, T. P. Russell, M. H. Rafailovich, Phase Transitions in Thin Polymer Blend Films, Condensed Matter Symposia Series, Physics Department, University of Missouri, Columbia, MO, September 1994.

A. Karim, C. C. Han, S. K. Satija, T. M. Slawecki, S. K. Kumar, T. P. Russell, M. H. Rafailovich, Phase Transitions in Thin Polymer Blend Films, Polymer Surfaces and Interfaces meeting, Cape Cod, MA, October 1994.

A. Karim, C. C. Han, S. K. Satija, T. M. Slawecki, S. K. Kumar, T. P. Russell, M. H. Rafailovich, Phase Transitions in Thin Polymer Blend Films, Exxon Chemical Co., Baytown, TX, February 1995.