Glass transition as a key to identifying solid phases

Abstract

AbstractThe earlier suggested definition, “a solid is a condensed phase at a temperature below its glass transition,” has developed into the key to the understanding of macromolecular materials. Its importance and implications for the understanding of not only polymers but all materials are reviewed in this article. A macromolecular sample can be a system consisting of a single amorphous phase or be a molecularly coupled multiphase system of different degrees of order and metastability in its subsystems. It is well known that liquid crystals, plastic crystals, and conformationally disordered crystals are still ordered above the glass transition, but recently even the monoclinic crystals of poly (oxyethylene) have been shown to have an additional glass transition below the melting temperature. This supports the inference that in the delineation of the solid state, the glass transitions take preference to melting transitions. The main tools for recognizing the different equilibrium and nonequilibrium phases are differential scanning calorimetry and temperature‐modulated differential scanning calorimetry. Such calorimetry yields quantitative heat capacities to be interpreted in terms of molecular motion and latent heats. Separated into reversible and irreversible contributions, the entropy changes connected to ordering and disordering can be evaluated. The value of the recognition of phase transitions lies in the fact that materials must be pliable for being manufactured into a final shape but often need high modulus and strength in their applications. Near the glass transition, forming is easy, whereas below the glass‐transition temperature, a high modulus is reached for properly oriented polymer molecules. By the identification of the glass transitions of materials with different states of order, a better understanding of the solid state of soft materials is reached. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 105: 49–59, 2007