Impact of Surface Bound Chains

Surface bound chains such as grafted and adsorbed polymer chains are widely used for modifying interfacial interactions in polymer materials and nanocomposites, but the mechanisms by which the changes to material properties are conferred to the system are not well understood.

Optimizing the Grafting Density of Tethered Chains to Alter the Local Glass Transition Temperature of Polystyrene near Silica Substrates:   The Advantage of Mushrooms over Brushes
Xinru Huang and Connie B. Roth,
ACS Macro Letters2018, 7, 269-274.

In this first study on grafted chains by our group, we demonstrated that end-tethered polystyrene (PS) chains with a molecular weight Mw = 100 kg/mol increased the local glass transition temperature Tg(z) of PS matrices by several decades and persisted out to distances of z = 100-125 nm away from the substrate interface.  Surprisingly, the largest local Tg(z=0) increase next to the substrate interface of +50 K above the bulk Tg of PS occurred for a very low grafting density of σ = 0.011 chains/nm^2, within the mushroom-to-brush transition regime.  We believe the good interpenetration that can be obtained between the grafted and matrix chains at these low grafting densities is central to creating the large Tg increases observed.  These results are extremely puzzling because typically the glass transition in polymers is not associated with chain connectivity as packing frustration occurs at the segmental level.

End-Tethered Chains Increase the Local Glass Transition Temperature of Matrix Chains by 45 K Next to Solid Substrates Independent of Chain Length
James H. Merrill, Ruoyu Li, and Connie B. Roth, ACS Macro Letters2023, 12, 1-7

Following up on this work, we have shown that low grafting densities of end-grafted PS chains can substantially increase the local glass transition temperature Tg(z=0) of PS matrices next to silica susbstrates by +45 K, regardless of end-group grafting chemistry.  Surprisingly, the magnitude of the Tg increase is independent of grafted chain length from 9 kg/mol to 200 kg/mol and grafting density σ from 0.003 to 0.33 chains/nm^2 spanning the mushroom-to-brush transition regime.  The large tens-of-degree increase in local Tg(z=0) observed in this athermal system by only the immobilization of chain ends from covalent bonding suggests a chain connectivity mechanism that substantially increases the local activation energy required for cooperative rearrangements.

Ongoing work in our lab is investigating this unexpected phenomenon further by studying the local glass transition temperature Tg(z) next to the substrate interface as a function of tethered chain length and grafting density to identify the underlying factors controlling this behavior.

Review and Reproducibility of Forming Adsorbed Layers From Solvent Washing of Melt Annealed Films
Michael F. Thees, Jennifer A. McGuire, and Connie B. Roth,
Soft Matter2020, 16, 5366-5387.

It is well known that in solution, polymer chains naturally segregate and adsorb to interfaces.  In polymer melts, however, it is less clear the extent to which chains can similarly adsorb to interfaces.  The study of such adsorbed chains are complicated by the difficulty of accessing and identifying the surface bound 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. 

In this publication, we review recent literature efforts to quantify the residual adsorbed layer thickness h_ads(t) measured after a given solvent washing procedure as a function of annealing time t of the film at an elevated temperature prior to this solvent rinse, a procedure frequently called the “Guiselin’s experiment”.  We identify and experimentally test a common protocol for forming adsorbed layers h_ads(t) from solvent washing melt films, and find them to be far less reproducible and reliable than implied in the literature, strongly dependent on solvent washing and substrate cleaning conditions.  This review also summarizes literature understanding developed over several decades of study on polymer adsorption in solution, which experimentally demonstrated that polymer chains in solution are highly mobile, diffusing and exchanging on the surface even in the limit of strong adsorption, contradicting Guiselin’s assumption.  A number of open questions and implications are discussed related to thin films and polymer nanocomposites.

Ongoing work in our lab is studying how these different populations of surface bound chains alter the local glass transition temperature of neighboring polymer chains in bulk films, using these local Tg measurements to infer the adsorbed layer structure formed during melt annealing.

Experimental Study of Substrate Roughness on the Local Glass Transition of Polystyrene
Xinru Huang, Michael F. Thees, William B. Size, and Connie B. Roth,
Journal of Chemical Physics2020, 152, 244901.

Another form of surface treatment is to vary the roughness of the interface.  For decades, computer simulations have shown that increased interface roughness leads to slower dynamics.  In this study, we investigated this experimentally by creating silica substrates with increasing roughness using a hydrogen fluoride vapor treatment.  We found that the local glass transition temperature Tg near silica substrates increased by 10 K with increasing roughness, but only for extremely large roughness scales not normally associated with the glass transition, leaving the mechanism for this observed behavior uncertain.