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Author

Di ZengFollow

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2019

Month Degree Awarded

February

First Advisor

Ryan C. Hayward

Subject Categories

Polymer Science

Abstract

Microphase separated copolymers with nano-scale morphologies are critically important in designing next generation materials. Cocontinuous nanoscale structures, in which domains of multiple different phases each simultaneously percolate in three dimensions, provide opportunities to synergistically combine properties of the constituent polymers in a wide variety of contexts. While cocontinuous nanostructures are fabricated through equilibrium self-assembly of block or graft copolymers and kinetically trapped phase separation of polymer blends or crosslinked copolymer networks, their formation is highly sensitive to changes in chemical details, synthesis and/or processing conditions, bringing practical challenges to generalization to multiple systems.

In this dissertation, we focus on transforming the design of cocontinuous morphologies from complicated protocols to general and robust principles by pre-designing telechelic polymers with well-defined end functionality, molecular weight and polydispersity. Relying on end-linking of telechelic polystyrene (PS) and poly (D,L - Lactide) (PLA) with a multi-functional crosslinker, randomly end-linked copolymer networks (RECNs) are synthesized and thoroughly characterized. Particularly, for the first time we are able to map the phase diagram of symmetric (Mn, AMn, B) RECNs, highlighting the critical microphase separation transition (6 < (χN)critical < 12), above which disordered cocontinuous nanostructures span over 30 vol% and morphologies with dispersed domains reside on either side. The critical impacts of chemical parameters (strand length Mn, strand asymmetry, strand dispersity Đ, junction functionality) are further evaluated to influence the microphase separated structures. While maintaining cocontinuity, uniaxial stretching of PS/PLA RECNs above the glass transition temperatures introduces controlled orientation through a two-step process (domain stretching and domain rotation), which is found to provide substantial improvements in yield strength, toughness, and stiffness for bulk materials at room temperature. Nanoporous materials with interconnected porous structures are then fabricated by selective removal of the easily degradable PLA domains.

Lastly, linear and branched multi-block copolymers (MBCs) with various block length are fabricated using step polymerization of telechelic PS and PLA. In addition to their ability to form cocontinuous morphologies within microphase separated and non-crosslinked MBCs, their solubility is dramatically improved in comparison to crosslinked copolymers.

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