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.