(381i) Molecularly-Templated Reaction for Forming Poly(dimethyl siloxane)/Graphene Oxide Composite Elastomers

Ha, H., University of Minnesota
Ha, K., Keimyung University
Ellison, C. J., University of Minnesota
Graphene oxide (GO) is a carbonaceous material consisting of various oxidative functional groups, such as carboxylic acids, hydroxyls, and epoxides. Owing to the inherent oxidative functionality and high surface area of GO, our group has recently investigated [1] many methods to crosslink polymers in the presence of a small amount of GO, using it as a multifunctional crosslinker. Among many possible chemical routes proposed in the past few years, research implementing polymers with secondary amide groups to form highly elastic composites is practically important for many applications. A short list of important materials containing secondary amide groups include DNA, amino acids, nylons, etc. However, a major challenge emerges from the fact that secondary amide groups are less reactive than other functional groups, for example, primary amines, due to their stable resonance structure. Therefore, introducing a catalytic functional group that can enhance the chemical reaction may introduce a new platform to synthesize highly crosslinked, robust polymer composites. Such catalytic functional groups could simply assist in co-locating reactive functional groups to assist chemical reaction. Herein, this catalytic function is served by pyrene groups that form π-π interactions with the basal plane of GO.

This study [2] explores the molecularly-templated reaction of pyrene-terminated telechelic poly(dimethyl siloxane) (PDMS) with GO. The polymer was terminated with pyrene ends connected to the main polymer chain through a secondary amide linkage. These pyrene ends form dynamic π-π interactions with GO surfaces, which reduce intermolecular spacing between amide groups near epoxides on the surface of GO, thereby templating the reaction. The key focus of this research was to generate a new method to crosslink polymers containing secondary amide groups with GO via an epoxy ring-opening reaction, and investigate the critical role of the pyrene end group participating as a catalyst during the crosslinking reaction. This chemical crosslinking reaction was confirmed and supported by infrared spectroscopy, oscillatory shear rheology, gel content, swelling ratio, and mechanical property measurements. We have also tuned the concentration of the secondary amide groups by modulating the molecular weight of the polymer main chain, and were able to prepare elastomers that are highly crosslinked (e.g., up to 96 wt % non-dissolving gel) but highly extensible (e.g., extensional strains of more than 200%) as well. For comparison, methoxy-terminated telechelic PDMS with the same secondary amide linkages and molecular weight was synthesized. As expected, this control material did not exhibit appreciable crosslinking with GO, illustrating the importance of pyrene end groups. Considering the crosslinked nature of the composites and generality of the proposed reaction, we believe this molecular-templating capability could introduce a wide range of cost-effective engineered composite elastomers beyond PDMS.

[1] Gas Permeation and Selectivity of Poly(dimethylsiloxane)/Graphene Oxide Composite Elastomer Membranes, H. Ha, J. Park, S. Ando, C.B. Kim, K. Nagai, B.D. Freeman and C.J. Ellison, Journal of Membrane Science, 518, 131-140 (Jun. 2016). http://dx.doi.org/10.1016/j.memsci.2016.06.028

[2] Molecularly-Templated Reaction for Forming Poly(dimethyl siloxane)-Graphene Oxide Composite Elastomers, H. Ha, K. Ha, and C.J. Ellison, under review.