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ABSTRACT The physical aging behavior, time‐dependent densification, of thin polystyrene (PS) films supported on silicon are investigated using ellipsometry for a large range of molecular weights (MWs) fromM_w = 97 to 10,100 kg mol⁻¹. We report an unexpected MW dependence to the physical aging rate ofh < 80‐nm thick films not present in bulk films, where samples made from ultra‐high MWs ≥ 6500 kg mol⁻¹exhibit on average a 45% faster aging response at an aging temperature of 40 °C compared with equivalent films made from (merely) high MWs ≤ 3500 kg mol⁻¹. This MW‐dependent difference in physical aging response indicates that the breadth of the gradient in dynamics originating from the free surface in these thin films is diminished for films of ultra‐high MW PS. In contrast, measures of the film‐average glass transition temperatureT_g(h) and effective average film density (molecular packing) show no corresponding change for the same range of film thicknesses, suggesting physical aging may be more sensitive to differences in dynamical gradients. These results contribute to growing literature reports signaling that chain connectivity and entropy play a subtle, but important role in how glassy dynamics are propagated from interfaces. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2019,57, 1224–1238 

Abstract Rubber toughening of glassy polystyrene (PS) has been manufactured commercially for decades as high impact polystyrene, where rubbery poly‐butadiene (PB) inclusions are added to modify the PS matrix response to deformation and impact. In this study, measurements of the local glass transition temperatureT_g(z) of PS next to PB rubber are presented, expanding the previous data to a polymer with a much lowerT_gvalue (PBT_g^bulk= −96 °C). After accounting for a small molecule additive present in the commercial PB sample that would otherwise migrate over to the PS domain causing plasticization, it is found that theT_g(z) profile in PS next to PB is consistent with previous results. It is also demonstrated that these broad and asymmetric experimentally observedT_g(z) profiles are not caused by the migration of low molecular weight chains across the interface by comparing samples made with two different poly(n‐butyl methacrylate) molecular weights. 

Chain adsorption to nanofiller interfaces creating bound layers has become central to understanding property changes in polymer nanocomposites. We determine the impact different kinds of adsorbed layers can have on the local glass transition temperature Tg of polymer matrices in a model film system using a localized fluorescence method. This work compares the adsorption and desorption of adsorbed layers grown in solution with the solution washing characteristics of adsorbed layers formed in the melt, leveraging knowledge about polymer adsorption in solution to infer the structure of adsorbed layers formed in the melt. In the limit of zero concentration after a long time in solution, we find that both kinds of adsorbed layers reach the same limiting adsorbed amount h∞(c → 0) ≈ 1 nm, appearing to evolve to the same thermodynamic equilibrium state of a near monolayer of surface coverage. We propose that melt annealing leads to a coarsening of polymer segment–surface contacts, increasing the length of trains and shrinking loops and tails, slowing the subsequent kinetics of these adsorbed chains in solution. Considering how the pyrene-labeled chains intermix with the adsorbed layer enables us to discriminate between the impact of tails, loops, and trains as threading of loops takes longer. We find that large fluffy loops, tails, and trains have little to no impact on the local Tg. A large 30 K increase in local Tg is observed for 30-min solvent washed well-annealed films at long intermixing times that we attribute to the threading of small tight loops. 

Polymers are increasingly being used in applications with nanostructured morphologies where almost all polymer molecules are within a few tens to hundreds of nanometers from some interface. From nearly three decades of study on polymers in simplified nanoconfined systems such as thin films, we have come to understand property changes in these systems as arising from interfacial effects where local dynamical perturbations are propagated deeper into the material. This review provides a summary of local glass transition temperature T g changes near interfaces, comparing across different types of interfaces: free surface, substrate, liquid, and polymer–polymer. Local versus film-average properties in thin films are discussed, making connections to other related property changes, while highlighting several historically important studies. By experimental necessity, most studies are on high enough molecule weight chains to be well entangled, although aspects that connect to lower molecule weight materials are described. Emphasis is made to identify observations and open questions that have yet to be fully understood such as the evidence of long-ranged interfacial effects, finite domain size, interfacial breadth, and chain connectivity. 

Recent studies suggest chain adsorption in the melt may be responsible for a number of property changes in thin films by making correlations between 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. This procedure, frequently called “Guiselin's experiment”, refers to the thought experiment proposed in a 1992 theoretical treatment by Guiselin that assumed chain segments in contact with the surface are irreversibly adsorbed whereby unadsorbed chains could be washed away by solvent without disturbing the adsorbed substrate contact points in the melt. In the present work, we review this recent literature, identifying and experimentally testing a common protocol for forming adsorbed layers h ads ( t ) from solvent washing melt films. We find h ads ( t ) curves to be far less reproducible and reliable than implied in the literature, strongly dependent on solvent washing and substrate cleaning conditions, and annealing at elevated temperatures is unnecessary as densification of films sitting at room temperature makes the glassy film harder to wash off, leaving behind h ads of comparable thickness. 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. Preformed adsorbed layers of different thicknesses h ads are shown to not affect the average glass transition temperature or physical aging of 30 nm thick films. In summary, a number of open questions and implications are discussed related to thin films and polymer nanocomposites.