(393a) Understanding the Modification of Local Glass Transition Dynamics By Surface Bound Chains

The perturbed material properties observed in polymer thin films and nanocomposites is primarily caused by interfacial interactions. While early work studying the impact of interfacial perturbations in nanoconfined geometries predominantly investigated enthalpic effects such as hydrogen bonding or interfacial energy, recent studies have focused on the role of surface adhered chains such as grafted and adsorbed polymers. Such surface modification by the attachment of polymer chains is a cornerstone of tailoring interfacial phenomena, and has been well studied in polymer solutions. In contrast, the impact of surface bound chains in polymer melts and glasses have been considerably less studied and are complicated by the difficulty of accessing the adsorbed chains. Efforts to investigate adsorbed layers in melt films and polymer nanocomposites frequently rely on solvent washing to expose such near-surface, “bound layer” chains.

This presentation compares and contrasts our group’s recent experimental results investigating local perturbations to the glass transition temperature T_g near polymer interfaces. In particular, we compare the importance of chain interpenetration at polymer-polymer interfaces [1-3] with the impact of end-grafted [4] and adsorbed [5-7] chains to silica interfaces. Our work on polymer-polymer interfaces has shown that the annealing and interpenetration of polymer chains across the interface is central to creating broad glass transition temperature profiles coupling the dynamics across the interface [2,3]. We have also recently demonstrated that the local glass transition temperature profile T_g(z) as a function of distance z from the interface in polystyrene (PS) next to silica substrates with end-grafted chains is the same to within experimental error as that observed for PS next to a much higher T_g polymer, polysulfone (PSF), suggesting some common phenomenon [2,4].

Our more recent work has tried to understand the role of adsorbed chains by comparing adsorbed layers obtained from the solvent washing of melt annealed films with the well-studied case of adsorbed layers formed in solution [5-7]. We find that the residual thickness of the bound layer of adhered chains remaining from the solvent rinsing of melt films is entirely determined by the solvent washing conditions used [6]. With extended time in solution both types of adsorbed layers, melt annealed and solution grown, are found to reach the same limiting value in adsorbed amount, equivalent to that grown in very dilute solutions. Only the time needed to reach this final limiting value varies, reflecting the prior treatment of the adsorbed layer. Our observations are consistent with surface diffusion and exchange measurements of adsorbed chains in solution demonstrating that surface bound chains are highly mobile, even for the case of strong adsorption. We compare how these different populations of surface bound chains alter the local glass transition temperature of neighboring polymer chains in bulk films, using these local T_g measurements to infer the adsorbed layer structure formed during melt annealing [7].

[1] R. R. Baglay and C. B. Roth, J. Chem. Phys. 2015, 143, 111101.

[2] R. R. Baglay and C. B. Roth, J. Chem. Phys. 2017, 146, 203307.

[3] Y. J. Gagnon and C. B. Roth, ACS Macro Letters 2020, 9, 1625-1631.

[4] X. Huang and C. B. Roth, ACS Macro Letters 2018, 7, 269-274.

[5] M. F. Thees and C. B. Roth, J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1224-1238.

[6] M. F. Thees, J. A. McGuire, and C. B. Roth, Soft Matter 2020, 16, 5366-5387.

[7] M. F. Thees, X. Huang, and C. B. Roth, 2021, in preparation.