When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass - a solid lacking long-range order. Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers. We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye-Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature. We end by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.

https://iopscience.iop.org/article/10.1088/1361-6633/aa5284/pdf

Glass transition of polymers in bulk, confined geometries, and near interfaces

Three kinds of OH-terminated polylactides were synthesized by the ring-opening polymerization of lactide, with an alcohol such as dodecanol, glycerol, or pentaerythritol, in the presence of stannous octoate. Moreover, Cl-, NH2-, and COOH-terminated polylactides were synthesized from OH-terminated polylactides. The end groups of the polylactides were identified by 1H NMR and 13C NMR. According to thermal analysis, the cold crystallization temperatures of Cl-, NH2-, and COOH-terminated polylactides were higher than those of OH-terminated polylactides. The thermal stability of OH-terminated polylactides was poor, whereas NH2- and Cl-terminated polylactides were more resistant to thermal degradation. In a hydrolysis degradation test, the mass and molecular weight loss of COOH-terminated polylactides were high, whereas those of Cl- and NH2-terminated polylactides were much lower. These end-group effects were increased with an increasing number of chain arms. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 973–985, 2001

Synthesis and degradation of end‐group‐functionalized polylactide

The methods available for the preparation of star-branched polymers are described. After presentation of theoretical predictions of the effect of this type of branching on the properties of polymers, available experimental data are compared and critically discussed. Thermodynamic and visco-elastic properties at infinite dilution are first presented followed by visco-elastic properties in concentrated solution and the melt. Finally the effect of star-branching on T g of low molecular weight polymers is described.

Preparation and properties of star-branched polymers

Glass Transition Temperatures of Polymers

We measured dynamic shear moduli of poly(macromonomer)s of an increased backbone chain length and those bearing branch chains of increased branch length up to the critical molecular weight for the intermolecular chain entanglement, Mc, of a linear polystyrene as functions of frequency and temperature using a parallel-plate rheometer. The poly(macromonomer)s were prepared from ω-methacryloyloxyethyl polystyrene macromonomers (MA-PSt)s or ω-vinylbenzyl polystyrene macromonomers (VB-PSt)s. It was revealed that a great increase in the backbone length caused the poly(macromonomer)s to show a weak rubbery plateau region in the master curves of the storage modulus G‘, where the molecular weights of the branch chains were smaller than Mc. The plateau modulus was much lower than that observed for the linear polystyrenes. On the other hand, the poly(macromonomer)s with polystyrene branches are nearly equal to the Mc, and a very short backbone chain showed a clear rubbery plateau region. The glass transition tempera...

Bulk Properties of Poly(macromonomer)s of Increased Backbone and Branch Lengths

The glass transition temperatures of linear and four- and six-branched regular star polystyrenes are measured by penetration, differential scanning calorimetry, and a density gradient technique. The results of the three methods show that the glass transition temperature depends on the concentration of chain ends in the polymer sample. An attempt is made to assess the influence of the presence of the branch point on the glass transition temperature using a series of four-branched polystyrenes containing varying amounts of isoprene near the branch point. However, any effect of the central branch point on the glass transition temperature is obscured by plastification of polystyrene by polyisoprene.

P. M. Toporowski

Papers

The glass transition temperature of star-shaped polystyrenes