Controlled Crystallization in Novel Block Copolymers

Princeton University, Princeton NJ

Investigators

Abstract

This award will investigate the ability of block copolymers to direct polymer crystallization. Diblock and triblock copolymers with defect-free crystallizable blocks will test the prediction of an equilibrium degree of chain folding in crystalline-amorphous block copolymers, which contrasts sharply with the kinetically-induced chain folding typically exhibited by homopolymers. Control of crystallite thickness through crystalline and amorphous block lengths or the addition of a nonvolatile diluent will yield materials with narrow and tunable melting ranges. Diluted triblocks having crystallizable endblocks will represent a new type of thermoplastic elastomer gel with low modulus and good dimensional stability-desirable candidates for large-deformation actuators. In the second aspect of this work, novel block copolymers synthesized by combining ring-opening metathesis and anionic polymerizations will address the long-standing issue of cocrystallization between linear and short-branched polyethylenes. These polymers will contain blocks of linear polyethylene, PE, and poly(ethylene-co-butene), PEB, separated by an amorphous block of variable length. When this "linker" run is short, the polymers are essentially PE-PEB diblocks, which are expected to show an interesting crystal heterogeneity. When the linker run is of medium length (long enough to microphase-separate from the other blocks), then the PE and PEB blocks are tethered to amorphous domains, favoring PE/PEB cocrystallization. Finally, when the linker run is the majority component, the mixed PE + PEB chains will be segregated within discrete microdomains, where they will crystallize only at deep undercoolings and with extensive cocrystallization. This provides a route to the preparation of PE/PEB cocrystals for systematic examination of their structure and properties. Crystallization of polymers has a profound impact on their properties, and underpins many of their applications. At size scales ranging from nanometers to microns, a typical polymer's crystalline structure depends strongly upon details of method used to crystallize it. In this work, we propose to robustly control the crystallization process, and hence the structure, through molecular features built into the polymers-the so-called "block architecture"-during synthesis. The proposed work will provide an integrated research and educational experience for two graduate students and approximately six undergraduates, who will be able to disseminate results from their work both internally and externally. The PI and the graduate students will continue their active work in support of K-6 science education in Mercer County, both through the development of new educational materials and through teacher training.