THE SPATIAL DISTRIBUTION OF LOCAL GLASS TRANSITION TEMPERATURES IN NANOSTRUCTURED BLOCK COPOLYMERS

Abstract

This dissertation demonstrates the use of fluorescence labeling and spectroscopy as a characterization tool which provides component- and location-specific measurements of the glass transition temperature (Tg), a fundamental polymer property, in a model nanostructured diblock copolymer—a polymer chain comprised of two chemically dissimilar homopolymers bound by a single covalent bond. Nanostructured diblock copolymers are self-assembled periodic materials with domains rich in either block, confined by the domain interface. The parameters governing the domain Tg—namely, the bulk Tg of the homopolymers, the domain period, and the degree of segmental mixing between the blocks—can be systematically varied by choosing the block chemistries and total chain length. This combination of features offers a wide parameter space for property optimization.

Until now, no characterization tool has demonstrated the capability of providing spatially-resolved Tg measurements across the domain structure, information that is crucial for optimizing the end-use properties. The length scales associated with the domain period, O(10 - 100 nm), pose a significant barrier towards performing such measurements. Here, this barrier was overcome by selectively incorporating a fluorescent monomer into the diblock copolymer chain during synthesis, thereby controlling the location where Tg was measured in the self-assembled structure.

In diblock copolymers of poly(n-butyl methacrylate-b-methyl methacrylate), PBMA-PMMA, which self-assemble into a lamellar morphology, a strong gradient in Tg of the higher-Tg PMMA block—42 K over 4 nm—was mapped with nanometer resolution. These measurements also revealed a strongly asymmetric influence of the domain interface on Tg, with a much smaller dynamic gradient being observed for the lower-Tg PBMA block.

Tg was characterized for a series of fluorescently-labeled homopolymers dilutely blended into an unlabeled diblock copolymer matrix. Unlike in the neat diblock copolymer, the labeled PMMA chains were not attached to the domain interface. A comparison of the Tg depression of the homopolymers in the blends to equivalent labeled diblock copolymers reveal ~ 5 K and 10 K contributions due to nanoscale confinement and block attachment to the interface, respectively.