Second-order Phase Transition Behavior behind Polymer Glass Transition

Glass transition has similarity to the second-order phase transition in temperature dependent changes in entropy, non-Arrhenius viscosity, and heat capacity of glass forming materials. However, it has primarily been considered to be not phase transition. Recent single-molecule spectroscopy developments prompted re-investigating glass transition at the nanometer scale probing resolution, showing that glass transition includes phenomena similar to the second-order phase transition. They are characterized by microscopic collective polymer motion and discontinuous changes in temperature dependent relaxation times, the latter of which resembles the critical slowing down of second-order phase transitions, within a temperature window above the polymer calorimetric glass transition temperature. Simultaneous collective motion and critical slowing down occurrences disclose that the second-order phase transition hides behind polymer glass transition.

Environmentally Sensitive Luminescence Reveals Spatial Confinement, Dynamics and Their Molecular Weight Dependence in a Polymer Glass

Polymer glasses have an irregular structure. Among the causes for such complexity are the chemically distinct chain end-groups that are the most abundant irregularities in any linear polymer. In this work we demonstrate that chain end induced defects allow polymer glasses to create confined environments capable of hosting small emissive molecules. Using environmentally sensitive luminescent complexes we show that the size of these confinements depends on molecular weight and can dramatically affect the photoluminescence of free or covalently bound emissive complexes. We confirm the impact of chain end confinement on the bulk glass transition in poly(methyl acrylate) and show that commonly observed T_g changes induced by the chain ends should have a structural origin. Finally, we demonstrate that size and placement of luminescent molecular probes in pMA can dramatically affect the probe luminescence and its temperature dependence suggesting that polymer glass is a highly irregular and complex environment marking its difference with conventional small molecule solvents. Considering the ubiquity of luminescent glassy materials, our work lays down a blueprint for designing them with structural considerations in mind, ones where packing density and chain end size are key factors.