Measurement of the thermoviscoelastic behavior of glass-forming liquids in the nanometer size range offers the possibility of increased understanding of the fundamental nature of the glass-transition phenomenon itself. We present results from use of a previously unknown method for characterizing the rheological response of nanometer-thick polymer films. The method relies on the imaging capabilities of the atomic force microscope and the reduction in size of the classical bubble inflation method of measuring the biaxial creep response of ultrathin polymer films. Creep compliance as a function of time and temperature was measured in the linear viscoelastic regime for films of poly(vinyl acetate) at a thickness of 27.5 nanometers. Although little evidence for a change in the glass temperature is found, the material exhibits previously unobserved stiffening in the rubbery response regime.

We have examined the linear rheological responses of a series of well-defined, dense, regularly branched brush polymers. These narrow molecular weight distribution brush polymers had polynorobornene backbones with degrees of polymerization (DP) of 200, 400, and 800 and polylactide side chains with molecular weight of 1.4 kDa, 4.4 kDa, and 8.7 kDa. The master curves for these brush polymers were obtained by time temperature superposition (TTS) of the dynamic moduli over the range from the glassy region to the terminal flow region. Similar to other long chain branched polymers, these densely branched brush polymers show a sequence of relaxation. Subsequent to the glassy relaxation, two different relaxation processes can be observed for samples with the high molecular weight (4.4 and 8.7 kDa) side chains, corresponding to the relaxation of the side chains and the brush polymer backbone. Influenced by the large volume fraction of high molecular weight side chains, these brush polymers are unentangled. The lowest plateau observed in the dynamic response is not the rubbery entanglement plateau but is instead associated with the steady state recoverable compliance. Side chain properties affect the rheological responses of these densely branched brush polymers and determine their glassy behaviors.

Fossil amber offers the opportunity to investigate the dynamics of glass-forming materials far below the nominal glass transition temperature. This is important in the context of classical theory, as well as some new theories that challenge the idea of an 'ideal' glass transition. Here we report results from calorimetric and stress relaxation experiments using a 20-millionyear-old Dominican amber. By performing the stress relaxation experiments in a step-wise fashion, we measured the relaxation time at each temperature and, above the fictive temperature of this 20-million-year-old glass, this is an upper bound to the equilibrium relaxation time. The results deviate dramatically from the expectation of classical theory and are consistent with some modern ideas, in which the diverging timescale signature of complex fluids disappears below the glass transition temperature.

Narrow fractions of polystyrene molecules in the form of uncatenated rings (cycles) were synthesized by reacting bifunctional living linear precursors with an appropriate coupling agent at very low concentrations. The cyclic molecules were separated from the simultaneously formed linear polycondensates by fractionate precipitation. The molecular weights of the cycles ranged from 11100 to 185000, thus encompassing the critical molecular weight for entanglements in linear polystyrene. The ring-like nature of these fractions has been investigated by a variety of techniques, including the limiting viscosity number in a good and in a solvent as well as neutron scattering in deuteriated cyclohexane. These measurements, part of which are reported here in some detail, display a gratifying agreement with the theoretical predictions reported earlier for uncatenated cyclic polymers. The zero-shear melt viscosities of these cyclic fractions and several others prepared by Fetters and Hostetter to extend the molecular weight range to 390000 were measured over a wide range of temperatures and compared with the viscosities of linear polystyrenes of similar molecular weights. Above the critical molecular weight for entanglement coupling, no major differences were found between the temperature dependence or the molecular weight dependence of the cyclic polymers and those of their linear counterparts. For the same molecular weight, however, the cycles exhibit somewhat lower melt viscosity values than do the linear molecules. The results are critically compared with those reported by Roovers on similar polystyrene cycles and briefly discussed in terms of recent molecular theories based on snake-like motion (reptation) of chains along a curvilinear tube formed by the constraints of the surrounding entangled matrix.

The phenomenology of the glass transition and the associated behavior in the near liquid and glassy states are detailed, including the cooling rate dependence of the glass transition, Kovacs’ three signatures of structural recovery, and enthalpy overshoots. Dynamics in the liquid regime just above T g and the associated temperature dependences are also covered since this behavior is important to understanding the glassy dynamics. The current models of structural recovery and their shortcomings are presented. A number of important unanswered questions are discussed, including how the relaxation time in the glassy state depends on structure, the relationship between the evolution of different properties, the resolution of the Kauzmann paradox, and the behavior of the equilibrium relaxation time below T g. New experimental approaches are needed to make breakthroughs, such as two that are described: one involving 20 Ma amber to test whether the Vogel temperature dependence continues for the equilibrium state below T g and another involving an ideal polymer/pentamer mixture to obtain the entropy of the liquid far below T K in a test of the Kauzmann paradox. An unexplored regime of glassy behavior, characterized by ultrastability, high density, and low fictive temperature, is identified, and experiments to understand the material behavior in this region are motivated.