Structured polymer thin films: self-assembly and forced-assembly

Abstract

Polymers have become indispensable because they can be chemically and physically tailored to meet diverse application needs, and are easily shaped into various forms like thin films. Polymer films naturally introduce film thickness and adjoining interfaces as factors that can be manipulated to achieve specific properties, such as internal film structure, optical transparency, barrier properties, and mechanical flexibility. The focus of this thesis is on fabrication and characterization of polymer/filler composite and functional network-forming block copolymer (BCPs) thin films, utilizing both forced-assembly and self-assembly techniques. Forced-assembly imposes external confinement as a means of inducing or directing the organization of system subunits whereas self-assembly is a spontaneous organization process. Polymer/filler composites integrate particles made of carbonaceous or inorganic materials into polymer matrices which often enhances mechanical and barrier properties compared to neat polymers. Carbonaceous graphene oxide (GO) nanoplatelets as fillers have attracted considerable attention due to their easy production from common graphite. Furthermore, the variety of oxygen-containing functional groups present on GO, such as hydroxyl, epoxy, and carboxyl groups, offer chemical handles for reactions and secondary bonding. Our group previously developed crosslinked polydimethylsiloxane (PDMS)/GO composite thin film membranes formed using a forced-assembly solution-casting method which induced the orientation of GO fillers, resulting in remarkable gas barrier and separation properties at GO concentrations around 5 wt% and higher. This thesis explores the effect of GO sheet size; larger GO sheets (LGO) were synthesized using the modified Hummers' method that were additionally fractionated into smaller GO sheets (SGO) through ultrasonication. At low GO concentrations (2-5 wt%), SGO composites exhibited more than three times higher gas permeabilities due to a shorter and less tortuous diffusion pathway but showed similar CO2/N2 gas selectivity compared to LGO composites. At higher GO concentrations (8 wt%), SGO composites demonstrated comparable gas transport properties to LGO composites, with significantly reduced gas permeability (> 99.9% compared to neat PDMS) and a twofold enhancement in CO2/N2 gas selectivity. In comparison to many other polymer/GO composites, PDMS/GO composites explored here possessed comparable or superior CO2/N2 gas selectivity using simpler materials and processes, suggesting significant potential for carbon capture applications.

Considering the limited availability of commercialized amine functional polymers, we also expanded this research to explore the potential use of commercially available maleic anhydride (MA)-containing polymers, such as SEBS-g-MA (a polystyrene-poly(ethylene/butylene)-polystyrene triblock copolymer grafted with MA). We fabricated SEBS-g-MA/GO composites that were partially crosslinked by the reaction of MA and GO, and the resulting composites showed a remarkable 138% improvement in tensile strength and a significant 100% increase in modulus. Surprisingly, despite the incorporation of rigid fillers, the composites retained their elongation at break (~530%). The preservation of ductility in the composites can be attributed to either the partial chemical crosslinking between SEBS-g-MA and GO or a partially-developed non-equilibrium SEBS morphology with physical crosslinks formed by glassy polystyrene domains.

The second major focus of this thesis is on the fabrication of functional network-forming block copolymer (BCP) thin films through a combined forced-assembly and self-assembly process. The self-assembled double gyroid (DG) network structure is of interest because of its unique three-dimensional triply-continuous structure. Our investigation focused on examining the self-assembly behavior of acetyl-terminated DG-forming polystyrene-block-poly(D,L-lactide) (SL-G) thin films. We discovered that surface-relief terraces form within (211)-oriented SL-G thin films when the film thickness is incommensurate with an integer multiple of the d-spacing of (211) planes (d211). In contrast, no terrace formation occurs when the film thickness matches integer multiples of d211. In addition, we explored the effect of surface fields (i.e., differing surface energies of adjoining thin film interfaces) on the orientation of SL-G thin films by synthesizing and applying various surface energy modifying layers (SMLs) at each interface of SL-G thin films. The results demonstrated a transition in the SL-G unit cell orientation from (211)-oriented to (110)-oriented as the applied surface field at the bottom interface became less polar. This study contributes to our understanding of methods for controlling domain orientation and surface smoothness in BCP thin films. These features are expected to be important as BCP films could be used as templates for producing filtration membranes, photonic crystals, and optical metamaterials.