Solution phase behavior of stimuli-responsive block polymers in ionic liquids

Abstract

Stimuli-responsive polymer solutions have found applications from drug delivery to electrochemical devices. Fundamentally, these responsive properties arise from the interactions between the polymer and the solvent, and understanding these interactions allows for control over the response. This work has two focuses: the development of a light- and temperature-responsive block polymer/ionic liquid solution to enable light triggered ordering and disordering, and the combination of an ABC triblock terpolymer with ionic liquid to produce networks with fewer defects. Copolymerization of a temperature-responsive monomer, benzyl methacrylate (BzMA), with a light-responsive monomer, 4-phenylazophenyl methacrylate (AzoMA), afforded a polymer with a mixture of these properties. The isomerization of azobenzene from the nonpolar trans state to the polar cis state under UV light increases the solubility of the polymer in the ionic liquid EMI TFSI. We studied the effect of varying the ratios of the two monomers and the overall polymer molecular weight, and found that AzoMA has a significant absorbance in the UV region, leading to a sample thickness dependence on the phase separation behavior. We were able to obtain copolymers with light-dependent phase separation upon heating, with phase separation temperatures up to 30 ºC. Using this statistical copolymer as a block in a block polymer generated light-responsive block polymer solutions, where samples at concentrations of 40 wt% and below generated spherical ordering upon heating, and disordered upon cooling. The ordering and disordering could also be triggered using light, showing excellent reversibility. An ABC triblock terpolymer consisting of an insoluble A block, a soluble B block, and a temperature-responsive C block was combined with the ionic liquid EMI TFSI to generate an ion gel. When mixed at temperatures above the phase separation temperature of the C block, the polymer exists in solution as micelles with an A core. Cooling below the phase separation temperature of the C block causes aggregation, where A and C domains are crosslinked by the midblock. Due to the mutual insolubility of the two endblocks, there are no observed looping network defects. This enables higher mechanical integrity and lower necessary concentration of polymer as compared to the analogous ABA triblock.