Despite the decade-long study of the effect of nanoconfinement on the glass-transition temperature (T(g)) of amorphous materials, the quest to probe the distribution of T(g)s in nanoconfined glass formers has remained unfulfilled. Here the distribution of T(g)s across polystyrene films has been obtained by a fluorescence/multilayer method, revealing that the enhancement of dynamics at a surface affects T(g) several tens of nanometres into the film. The extent to which dynamics smoothly transition from enhanced to bulk states depends strongly on nanoconfinement. When polymer films are sufficiently thin that a reduction in thickness leads to a reduction in overall T(g), the surface-layer T(g) actually increases with a reduction in overall thickness, whereas the substrate-layer T(g) decreases. These results indicate that the gradient in T(g) dynamics is not abrupt, and that the size of a cooperatively rearranging region is much smaller than the distance over which interfacial effects propagate.

We analyzed the glassy-state structural relaxation of polymers near surfaces and interfaces by monitoring fluorescence in multilayer films. Relative to that of bulk, the rate of structural relaxation of poly(methyl methacrylate) is reduced by a factor of 2 at a free surface and by a factor of 15 at a silica substrate interface; the latter exhibits a nearly complete arresting of relaxation. The distribution in relaxation rates extends more than 100 nanometers into the film interior, a distance greater than that over which surfaces and interfaces affect the glass transition temperature.

Owing to the improvement of properties including conductivity, toughness and permeability, polymer nanocomposites are slated for applications ranging from membranes to fuel cells. The enhancement of polymer properties by the addition of inorganic nanoparticles is a complex function of interfacial interactions, interfacial area and the distribution of inter-nanofiller distances. The latter two factors depend on nanofiller dispersion, making it difficult to develop a fundamental understanding of their effects on nanocomposite properties. Here, we design model poly(methyl methacrylate)-silica and poly(2-vinyl pyridine)-silica nanocomposites consisting of polymer films confined between silica slides. We compare the dependence of the glass-transition temperature (Tg) and physical ageing on the interlayer distance in model nanocomposites with the dependence of silica nanoparticle content in real nanocomposites. We show that model nanocomposites provide a simple way to gain insight into the effect of interparticle spacing on Tg and to predict the approximate ageing response of real nanocomposites.

The effect of nanoconfinement on the glass transition temperature (T g) of supported polystyrene (PS) films is investigated over a broad molecular weight (MW) range of 5000−3 000 000 g/mol. Polystyrene MW is shown to have no significant impact on the film thickness dependence of T g − T g,bulk. In contrast, a small modification to the repeat unit structure of PS has a dramatic impact on the T g-nanoconfinement effect. The strength of the thickness dependence of T g is greater for poly(4-methylstyrene) (P4MS) than for PS and yet much greater for poly(4-tert-butylstyrene) (PTBS). The T g reduction for PTBS is 47 K below T g,bulk for a 25 nm thick film, with the onset thickness for confinement effects in PTBS being 300−400 nm. Measurements of the size of cooperatively rearranging regions, ξCRR , in bulk polymer systems at T g reveal that PS MW has no significant effect on ξCRR unless PS is oligomeric or nearly oligomeric. However, changes to repeat unit structure and diluent addition affect ξCRR values, but not in a manner that yields an obvious correlation with the T g-nanoconfinement effect.

Vitrimers form a promising class of dynamic polymer networks, but they have an Achilles’ heel: elastomeric vitrimers exhibit significant creep under conditions where permanently cross-linked, elastomeric networks exhibit little or no creep. We demonstrate that vitrimers can be designed with strongly suppressed creep and excellent reprocessability by incorporating a substantial yet subcritical fraction of permanent cross-links. This critical fraction of permanent cross-links, which has little or no detrimental effect on reprocessability, is defined by the gelation point of only permanent cross-links leading to a percolated permanent network. Via a modification of classic Flory–Stockmayer theory, we have developed a simple theory that quantitatively predicts an approximate limiting fraction. To test our theory, we designed vitrimers with controlled fractions of permanent cross-links based on thiol–epoxy click chemistry. We characterized the rubbery plateau modulus before and after reprocessing as well as stress relaxation of our original vitrimers. Our experimental results strongly support our theoretical prediction: as long as the fraction of permanent cross-links is insufficient to form a percolated permanent network, the vitrimer can be reprocessed with full recovery of cross-link density. In particular, with a predicted limiting fraction of 50 mol %, a vitrimer system designed with 40 mol % permanent cross-links achieved full property recovery associated with cross-link density after reprocessing as well as 65–71% creep reduction (for both original and reprocessed samples) relative to a similar vitrimer without permanent cross-links. In contrast, a system with 60 mol % permanent cross-links could not be reprocessed into a well-consolidated sample, nor did it recover full cross-link density; it failed by breaking at early stages of creep tests. The ability to predict an approximate limiting fraction of permanent cross-links leading to enhanced creep resistance and full reprocessability represents an important advance in the science and design of vitrimers.

The spin coating of thin (> 200 nm thick) and ultrathin (< 200 nm thick) polymer films is examined in several solvents of varying volatility over a broad range of polymer solution concentrations and spin speeds. Experimentally measured film thicknesses are compared with a simple model proposed by Bornside, Macosko, and Scriven, which predicts film thickness based on the initial properties of the polymer solution, solvent, and spin speed. This model is found to predict film thickness values within 10% over the entire range of conditions explored, which gave film thicknesses from 10 nm to 33 μ:m. The model underpredicts film thickness for cases in which a very volatile solvent is used or the initial concentration of polymer is high, while overpredicting film thickness for cases in which a low volatility solvent is used or the initial polymer concentration is very low. These deviations are a consequence of how the model decouples fluid flow and solvent evaporation.

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By selective placement of fluorescent dyes, we have measured the glass transition temperature (T g ) of individual layers within supported bilayer films of different polymers to determine the extent to which strong free-surface effects and substrate interactions are mediated by a narrow interface between immiscible polymers. We have discovered that the impact a free surface has on T g within an ultrathin PS layer is extremely sensitive to the polymer species used in the underlayer. The large T g reduction of ∼32 K relative to bulk T g observed for a 14 nm thick surface layer of polystyrene (PS) supported on bulk PS is virtually eliminated when a 14 nm thick surface layer of PS is placed on an underlayer of poly(methyl methacrylate) or poly(2-vinylpyridine) (P2VP), even of bulk thickness. Thus, the cooperative segmental mobility associated with the T g of the PS freesurface layer is greatly hindered by the narrow, several-nanometer-wide interfacial region formed with the dissimilar polymer underlayer. This indicates that the dynamics of nanoscale layers can be strongly modified by adjacent layers or domains of unlike polymers via propagation of effects across an interfacial layer of cooperatively rearranging regions containing segments of the two immiscible polymers, which has important implications for multilayer films and nanostructured blends. Conversely, the T g of an ultrathin P2VP film is unaffected by the presence of a PS capping layer, indicating that strong attractive interactions of P2VP with hydroxyl groups on the surface of the silica substrate dominate over a much weaker free-surface effect in P2VP.