Effects of Polymer Architecture on the Self-Assembly and Physical Properties of Graft Copolymers

Abstract

The molecular architecture of polymers influences the chemical and physical properties as well as their self-assembly in both solution and the bulk. Recent advances in controlled polymerization have enabled the synthesis of complex architectures, allowing for advanced materials design. Graft polymers are a class of nonlinear architectures featuring polymeric side chains attached to a polymer back bone. The properties of graft polymers can be tuned using the chemistry of the backbone and side chains, grafting density (i.e., the number of grafts per repeat unit), distribution of grafts, and molecular weight of the backbone and side chains. The self-assembly of two systems of graft polymers were investigated: diblock polymers in the bulk consisting of a highly grafted bottlebrush block attached to a linear coil, and sparsely grafted comb polymers in solution.The phase behavior of five series of norbornene-based coil-block-bottlebrush copolymers was studied to determine the effect of architectural asymmetry on network phase formation. Network phases, such as the double gyroid (GYR) are used in various applications from separations membranes to photonic material. However, linear diblock copolymers typically form these phases in small compositional windows, which limits their use. In an effort expand the GYR window, coil-brush polymers with high conformational asymmetry were synthesized with the coil block as poly(norbornene-exo,exo-dimethyl ester) and the bottlebrush block with oligomeric poly(ethylene-alt-propylene) side chains, using grafting-through living ring-opening metathesis polymerization. By changing the volume fraction of the coil block and the side chain length of the brush block, the effect of architectural asymmetry between the two blocks on the morphology at various temperatures was studied with small-angle X-ray scattering (SAXS) with the aim of linking polymer molecular parameters with structure formation. Structure formation was confirmed with transmission electron microscopy (TEM). Results show that increasing the side chain length of the brush block does not significantly change the gyroid compositional window in the phase diagram, but it does shift the GYR window to higher compositions of the coil block and lower total molecular weights. To investigate the low grafting regime in solution, graft copolymers with a methylcellulose (MC) backbone and poly(N-isopropylacrylamide) (PNIPAm) side chains were synthesized via a grafting-to method. Methylcellulose (MC) is a widely used commercial polymer that thermoreversibly gels to form a fibrillar network upon heating. We use PNIPAm-grafted MC to expand our understanding on how grafting affects the self-assembly of MC into fibrils. The aqueous solution properties at low concentration were studied using static and dynamic light scattering to show that grafting improved the solvent quality and increased coil size. Cryogenic-TEM and SAXS were used to observe and quantify changes in fibril structure. At high temperature the PNIPAm-grafted polymers show a decrease in fibril length with increasing grafting density, until fibril formation is suppressed.