Molecular Mobility of Polymers at the Melting Transition

Abstract

Melting of crystals is an archetypical first order phase transition. Albeit extensive efforts, the molecular origin of this process in polymers is still not clear. Experiments are complicated by the tremendous change in mechanical properties and the occurrence of parasitic phenomena masking the genuine material response. Here, we present an experimental procedure permitting to circumvent these issues by investigating the dielectric response of thin polymer films. Extensive measurements on several commercially available semicrystalline polymers allowed us to identify a genuine molecular process associated with the newly formed liquid phase. In line with recent observations of amorphous polymer melts, we show this mechanism�known as the slow Arrhenius process (SAP)�involves time scales longer than those characteristics of segmental mobility and has the same energy barrier of the flow of the melt.

The slow Arrhenius process in small organic molecules

“…It has recently been discovered that this class of fast equilibration mechanisms is coupled, at the molecular level, to the slow-Arrhenius process (SAP), 13 a mode whose molecular relaxation rate obeys a law of the type f SAP = f 0 exp(ÀE SAP /RT), where E SAP is a temperature invariant activation energy on the order of 100 kJ mol À1 (E40 k B T at room temperature). The SAP, active both in the liquid and glassy state, is strongly related to high-temperature flow 13,14 in which it shares the same thermal barrier. This condition allows us to discriminate between the SAP and the nearly Arrhenius relaxation mechanisms associated with the a-modes out of equilibrium, which usually exhibit much higher thermal barriers.…”

“…17 This undesired effect is, nevertheless, greatly suppressed by employing thin films, where the separation of the electrodes -meaning the thickness of the dielectric medium -is smaller than the Debye length. 13,14 Thin polymer films are commonly prepared via spincoating, 18 a fast fabrication technique permitting to obtain flat layers starting from dilute polymer solutions. Here, we extended this preparation method to a set of molecular glasses.…”

“…It appears as a peak in the dielectric loss, more easily detectable at temperatures above T g for amorphous systems, 13 and above the melting point for semicrystalline materials. 14 Despite its invariance in the macromolecular nature and sample size, 13 a direct observation of this mode in small molecules is lacking. Here, by employing broadband dielectric spectroscopy (BDS), we show that SAPs are present also in small molecules.…”

Fast Processing Affects the Relaxation of Polymers: The Slow Arrhenius Process Can Probe Stress at the Molecular Level

“…Differently than other processes slower than the α-relaxation at T > T g , − the thermal barrier of the SAP is temperature-independent and this quantity is sufficient to uniquely determine the experimental time scale of the process . In addition to dielectric spectroscopy, the SAP can also be studied via measurements of the elastic modulus and molecular dynamics simulations . Thanks to its weaker temperature dependence, the SAP is capable of efficiently driving glasses toward less energetic states with time scales much shorter than those otherwise necessary for macroscopic relaxations mediated by the α-modes.…”