Impact of Bonding and Chain Connectivity on the Local Glass Transition of Polymers

Numerous advanced applications such as polymer nanocomposites, ionomers, and vitrimers involve polymers with chains tethered to nanoparticle surfaces or aggregated into clusters resulting in renewed interest in how chain connectivity alters the glass transition. This talk summarizes a series of recent studies from our group that interrogates how the glass transition temperature Tg(z) is locally altered near planar interfaces with different kinds of surface bound chains, and discusses what can be concluded about the underlying mechanism of bonding and chain connectivity on the glass transition in polymers. Using pyrene fluorescence, we have found that end-grafted chains impart large +45 K increases in local Tg of intermixed homopolymer chains independent of grafting density across the mushroom-to-brush transition regime [Merrill, et al., ACS Macro Lett 2023, 12, 1-7; Huang, et al., ACS Macro Lett 2018, 7, 269-274]. The system is completely athermal with chemically identical homopolymer and grafted chains on energetically neutral substrates meaning the Tg increase is entirely topological in origin. In contrast, adsorbed chains do not impart the same increase in local Tg, i.e. tails do not behave the same as end-grafted chains [Thees, et al., J Chem Phys 2024, 160, 044908]. Surprisingly, the range of Tg(z) perturbation is largely independent of grafted chain length, having similar scaling to the Tg(M) dependence, but extends far beyond the range of the grafted chains, out to 5-10 x 2 Rg.