TEM/EDXS studies of phase separation in block and graft copolymers

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Virginia Polytechnic Institute and State University

The relationships between molecular parameters and microdomain formation of a variety of block- and graft-copolymers were studied by Transmission Electron Microscopy (TEM). Molecular variables included chemical composition: dimethyl-, fluoropropyl and diphenyl-siloxane , sulfone styrene, paramethylstyrene, t-butylstyrene, arylester and methyl methacrylate, as well as molecular weight and distribution. Effects of the kinetics of phase-separation were also determined . Thick (approximately lmm) films cast from solvent showed more complete phase separation than either thin (about 10nm) cast films or compression-molded specimens.

Spherical domains formed in alternating poly(ester/siloxanes), and phase mixing seemed to correlate with the solubility parameters of the three siloxane types. Shear-stresses during molding changed domain shapes and eliminated short-range ordering. In the PMMA-graft-dimethyl siloxane system, SK, 10K and 20K <Mn> siloxanes were incorporated at 16% and 45% by weight. At 16%, spherical siloxane domains formed in both thick- and thin-cast films. The domain sizes and interdomain distances scaled with siloxane molecular weight and total block molecular weight respectively to a 2/3 power law in excellent agreement with theoretical predictions for di- and triblock copolymers. Thin films cast from the 45% siloxane graft copolymers also showed spherical domains with sizes dependent on molecular weight. However, the thick films showed phase transitions from disordered bicontinuous (Mn = 5K) to lamellar (Mn = 10K) to cylindrical <Mn = 20K). Qualitative TEM/EDX analysis of other systems was used to identify oligomers, homopolymers, and contaminants, thus monitoring the effects of novel reaction conditions and work-up procedures.