Thermoresponsive polymers are of great interest as switchable smart materials. poly(N-isopropylacrylamide) (pNIPAM), an important thermoresponsive polymer, has been widely studied but the question of its reversibility (hysteresis) may limit its potential. Herein, we aimed to utilize well-defined micelles with thermoresponsive coronas and tunable aggregation numbers (Nagg) to study the effects of altering micellar structure on hysteresis. To this end, initial efforts focused on micelles composed of a thermoresponsive pNIPAM corona and a non-responsive core block (poly(n-butyl acrylate-co-N,N-dimethylacrylamide)). The block copolymers were synthesized using RAFT polymerization to yield a series of micelles with varying core compositions, which led to tunable Nagg values. Thermoresponsive behavior at the molecular and macroscopic level was probed using complimentary analyses. Our results showed that the degree of hysteresis increased as a function of core hydrophobicity (Fig. 1, LHS), which was attributed to differences in core hydration. Similar micellar series were then used to investigate different thermoresponsive coronas in order to find structure-property relationships behind the origins of hysteresis. Using linear pNIPAM and poly(N,N-diethylacrylamide) (pDEAm) coronas, the effect of hydrogen bonding ability on hysteresis was studied (Fig. 1, LHS). By comparing these to brush-like poly(diethylene glycol monomethyl ether methacrylate) (pDEGMA) and poly(oligo(ethylene glycol) monomethyl ether methacrylate) (pOEGMA) coronas, which showed irreversible transitions at high chain density, the effects of chain architecture and Tg was elucidated (Fig. 1, RHS). These results highlight the need for consideration of the effect that self-assembly plays on the responsive behavior of particles when compared with free unimers in solution.

Probing the causes of thermal hysteresis using tunable Nagg micelles with linear and brush-like thermoresponsive coronas

253rd National Meeting (2017)

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