STRUCTURE-PROPERTY RELATIONSHIPS IN CRYSTALLINE-AMORPHOUS BLOCK COPOLYMERS: SYNTHESIS, PHASE BEHAVIOR AND MECHANICAL PROPERTIES
Briber, Robert M
Sita, Lawrence R
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Block copolymers are composed of two or more polymer chains that are chemically distinctive and covalently bonded at the single point. Due to the incompatibility between blocks, the block copolymers undergo microphase separation generating long-range ordered microdomains. Considerable effort has been put on the understanding of the microdomain structure-mechanical properties relationships in block copolymers where both blocks are amorphous. Less research has been done on structure-property relationships in crystalline-amorphous block copolymers. In this thesis, the structure-property relationships in crystalline-amorphous block copolymers have been explored using two polymer systems, 1) well-defined isotactic-atactic-isotactic stereoblock (sbPP) and stereoirregular (irPP) polypropylene materials, 2) poly(1-hexene)-poly(methyl-1,3-cyclopentane) (PH-PMCP) block copolymers. The structure-property relationships in sbPP and irPP materials have been characterized using tensile testing, dynamic mechanical analysis (DMA) and rheology. The sbPP materials demonstrated higher tensile modulus exhibiting elongation ratios of more than 2,000 % while irPP materials showed lower tensile modulus with elongation of less than 100 %. Furthermore, sbPP materials exhibited more than 90 % of tensile recovery in contrast to 60-70 % tensile recovery for irPP materials. The discrete crystalline-amorphous-crystalline structure in sbPP materials provides both rigidity and flexibility. The morphologies of PH-PMCP materials have been investigated using in-situ small and wide angle X-ray scattering (SAXS/WAXS), rheology, atomic force microscopy (AFM), and transmission electron microscopy (TEM). The melting of the crystalline PMCP followed by the microphase separation in PH-PMCP results in a modulus transition after the PMCP melting. In particular, sphere-forming PH-PMCP block copolymers exhibit a drop of 2 orders of magnitude in the storage modulus at the PMCP melting temperature. The recovery in storage modulus within 2 min results from the low molecular weight and the incompatibility between PH and PMCP blocks. By tuning PMCP content, this modulus transition has been demonstrated in sphere, cylinder, double gyroid, and lamellae forming block copolymers. The viscoelastic response in sphere-forming PH-PMCP block copolymers indicate that after the PMCP crystal melt the molten PMCP blocks participate the formation of block copolymer microdomains and improve the microdomain ordering.