Elucidating Structure-Property Relationships of Vapor-Deposited Polymer Thin Films

Abstract

The utilization of polymer thin films has seen tremendous growth with the advancements in nanotechnologies, such as energy conversion devices, microelectronics, pharmaceutical formulations, and separation membranes. Of critical importance is to understand how thin-film structure and properties are altered by processing routes. In this dissertation, our purpose is to advance the understanding of processing-structure-property relationships of vapor-deposited polymer thin films.

Physical vapor deposition (PVD) has been known to commercially produce atomic and molecular films. Nonetheless, traditional PVD techniques are not applicable to polymers due to material degradation. Uniquely, we overcome this limitation with an emerging PVD process called matrix-assisted pulsed laser evaporation (MAPLE). MAPLE is viable for making intact polymer thin films, therefore is believed to bring insights into understanding how vapor deposition can guide polymer crystallization and glass formation.

In the current work, we investigated the ability of MAPLE to gain morphological control for semi-crystalline polymer thin films. Two key parameters were studied: substrate temperature and polymer-substrate interactions. During the slow additive growth of poly(ethylene oxide) atop silicon substrates—a system with strong hindrance for crystallization—changes in substrate temperature was found effective to tune crystal thickness, orientation, the extent of crystallinity, and melting temperature of the polymer crystal.

For specific polymer-substrate pairs, such as linear polyethylene (PE) grown atop an epitaxial substrate of graphene, we demonstrated that MAPLE could exploit epitaxial crystallization to reach a highly oriented and defect-less crystal structure. Polyethylene crystals with a near-equilibrium melting temperature were consequently achieved. We discussed the advantage of the slow additive growth for guiding polymer crystallization in a well-controlled environment.

In addition, we presented MAPLE as an effective strategy to enhance mixing for binary polymeric systems. When PE formed a strongly immiscible blend with poly(methyl methacrylate) (PMMA), MAPLE led to phase separation at a size scale of 100 nm, which was an order of magnitude smaller than the phase separation scale after conventional processing.

Finally, for glassy thin films of pure PMMA supported on silicon substrates, we revealed a remarkable deviation from the standard glassy properties for the MAPLE-deposited glasses. Nanocalorimetric measurements reflected a lower thermodynamic state relative to the standard PMMA films, which was associated with enhanced segmental dynamics.