Structure and Property Control in Semicrystalline Polymers through Block Copolymerization, Macromolecular Architecture, and Tacticity

Abstract

This dissertation presents three examples of how the structure and properties of semicrystalline polymers can be controlled through careful design of key aspects of the polymer chain. Linear and 6-arm star block copolymer thermoplastic elastomers (TPEs) comprising crystalline, glassy, and rubbery blocks are prepared by anionic polymerization, chlorosilane coupling, and hydrogenation optimized for the preparation of well-defined structures. Two pentablock copolymers, with the block sequence crystalline-glassy-rubbery-glassy-crystalline, are evaluated and compared against triblocks with either crystalline or glassy end blocks. Judicious choices of block lengths yield disordered melts for both pentablocks, dramatically improving the processability compared to the more conventional microphase-separated glassy-rubbery-glassy triblock.

In the pentablocks, crystallization of the end blocks followed by aggregation and vitrification of the glassy blocks produces composite crystalline-glassy physical cross-links. The pentablocks are elastomeric with lower Young’s moduli and higher ultimate strengths than the crystalline-rubbery-crystalline triblock—both desirable qualities for a soft TPE—highlighting the strong influence of the crystal morphology on the mechanical performance. Compared to their linear counterparts, 6-arm star TPEs with the same block motifs exhibit similar phase behavior and equivalent or improved ultimate strength, recovery, and hysteresis. The covalent junction at the core of the star strengthens and accelerates the recovery of the network, but does not prevent plastic deformation of the crystallites.

At the other extreme, strongly segregated crystalline-rubbery diblock copolymers, in which crystallization is confined by the melt morphology, are investigated. Confined crystallites are forced to adopt a texture where the b-axis lies in the plane of the microdomains and the c-axis is tilted with respect to the lamellar normals; the tilt angle increases with increasing molecular weight. The lamellar spacing dilates substantially (by 15–30%) upon crystallization, but the maximum crystal thickness is limited by the rubbery domains.

Finally, tactic hydrogenated polynorbornenes (hPN) are examined. hPN is unique in that the isotactic (i), syndiotactic (s), and atactic (a) forms are all semicrystalline with distinct crystal structures. Additionally, ihPN shows evidence of polymorphism. Estimates of the c-axis dimensions of ihPN and shPN suggest slightly kinked chain conformations. The melting points of ihPN and shPN bracket that of ahPN.