Molecular Dynamics Study on Glass Transitions in Atactic-Polypropylene Bulk and Freestanding Thin Films

Abstract

Molecular dynamics simulation has been adopted in investigation of different glass transition behaviors of the bulk and the freestanding thin films (about 8 nm) of atactic polypropylene (a-PP). For characterization of glass transition temperature (T(g)) of above systems, both the specific volume and the local conformational transition rate of the systems were examined. The T(g) characterization from the local conformational transition rate of polymer chains was recently developed. Our simulation results show that the films have lower T(g) than the bulk, and the descent of T(g) is in a range of 30 approximately 10 K. These are consistent with experiments. a-PP chain consists of meso-dyad and racemic-dyad, the above results are obtained from both dyads. Individual contribution of the stereoregular dyads to the conformational transition rate was further studied. It was revealed that T(g) obtained singly from the meso-dyads was almost the same with T(g) individually from the racemic-dyads, although the racemic-dyads have much lower transition rate and higher transition barrier than the meso-dyads. In the present study, the reason for the thin films having lower T(g) than the bulk is attributed to the novel behavior that the films have lower transition rate and higher barrier than the bulk when it is below T(g). Such behavior was discussed according to the "coupling rotation" of the dihedrals, which depends on the increase of free volume and the inhibition of kinetic energy.

Temperature Dependence of Polypropylene Configurations

“…In view of gratifying success of MD in many research fields and a high degree of similarity of the temperature dependence of the specific volume between experimental observations and simulation calculations, it has reason to believe that we can get insight into microscopic details about the microstructure and molecular motion by MD. In order to illustrate the justifiability of our simulation calculations, Figure gives the running average of specific volume obtained during dynamics simulations against simulation time at the several specified temperatures for a‐PP of this work, which are reasonable consistent with the reports in literature . Figure is the corresponding relationship between the average specific volume and temperature in the equilibrium state, when the fluctuation of the average values is negligible small.…”

Effect of chain structure on the glass transition temperature and viscoelastic property of cis‐1,4‐polybutadiene via molecular simulation

“…According to the free volume theory, the density or its reciprocal, specific volume of system will vary greatly when the system cools down from high to low temperature across the glass transition temperature, which is determined by finding the point of intersection of the fitted lines in the rubbery and glassy regions. This method has been adopted in many simulation researches . First, we focus on the influence of the cooling rate on the T g for the chain structure I, which is shown in Figure (a).…”

“…The change of T g seriously depends on the interfacial interaction and the mobility of the confined boundary . Furthermore, the freestanding thin films of atactic polypropylene have lower T g than the bulk, and the descent of T g is in a range of 10–30 K . This is due to the increase of free volume near the surface region of thin films .…”

Structural characteristics of a cooperatively rearranging region during the glass transition of a polymer system

“…42 By using the single-molecule uorescence microscopy, Wöll's group found that the temperature dependence of fraction of mobile molecules in polystyrene lms exhibited similar trends with that of PSM. [26][27][28][29][30][31][32] Similarly, we also observe the conformational changes during cooling in the current simulation. [26][27][28][29][30][31][32] Similarly, we also observe the conformational changes during cooling in the current simulation.…”

“…13,14 For instance, the structural characteristics of CRR are still unknown. [27][28][29][30][31][32] The increase of the fraction of transconformations may induce the formation of the short-range order in polymers. For colloidal system, Tanaka et al 23,24 and Yodh et al 25 detected the formation of some local ordered structures during cooling, and they argued that the development of the local ordered structures may be the origin of the slow dynamics.…”