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The synthesis and characterization of conformationally asymmetric diblock copolymers
Abstract
The phase separation behavior of three conformationally asymmetric diblock copolymer systems has been investigated: poly(butadiene-b-α-methylbenzyl isocyanide), poly(styrene-b-1,3-cyclohexadiene), and poly(styrene- b-carbodiimide). Kuhn length differences between the two blocks of each of these three systems provide rough estimates of their conformational asymmetry and classify these three as intermediate in asymmetry, compared to other literature systems. This dissertation considers their behavior within the context of theoretical predictions and reported experimental investigations. Effects of chirality, crystallinity, and liquid crystallinity are all considered in the interpretation of these conformationally asymmetric copolymers. Although the rigid poly(α-methylbenzyl isocyanide) block is a cholesteric liquid crystal, when linked to a flexible segment its liquid crystallinity cannot be retained. Frustration present in this system could explain the poorly ordered morphologies observed for all of the polyisocyanide diblocks studied. Only the polymer with the highest volume fraction of polyisocyanide displayed evidence of a lamellar type morphology. The crystallinity of these polyisocyanide diblocks; introduced added complexity. In contrast, poly(1,3-cyclohexadiene) does not possess a liquid crystalline phase and is amorphous. The remarkable core-shell cylinder morphologies for poly(styrene-b-carbodiimide)s might result from conformational asymmetry in the absence of mesophase formation or crystallization. The poly(styrene-b-carbodiimide) systems studied here are most similar to the highly asymmetric systems discussed in the literature. Poly(di-n-hexyl carbodiimide) homopolymers are nematic liquid crystals; however, the phase separation behavior of poly(styrene- b-di-n-hexyl carbodiimide) does not match that expected for a diblock containing a liquid crystalline, worm-like segment. The difference between the lamellar morphology observed here and the zigzag and wavy lamellae reported in the literature might be explained by the comparatively poor crystallinity of the polycarbodiimide block. The three diblocks examined here canvas an intermediate conformational asymmetry range hitherto unexplored. Comparison of these morphologies with those reported in systems with higher and lower asymmetries reveals significant differences. In particular, the core-shell cylinder morphology is unique and has not been previously observed in any other diblock system. The fact that these results are not adequately explained by any of the current asymmetry theories indicates that, even with the increased theoretical and experimental efforts of recent years, the physics of conformationally asymmetric materials is still not well understood.
Subject Area
Polymers|Plastics
Recommended Citation
David, Jennifer Lynn, "The synthesis and characterization of conformationally asymmetric diblock copolymers" (1999). Doctoral Dissertations Available from Proquest. AAI9932305.
https://scholarworks.umass.edu/dissertations/AAI9932305