Simulation study of semi‐crystalline polymer interphases

Abstract

The study of structure and properties of semi‐crystalline polymer inter‐phases is important to explain and extend polymer applications. In this region, polymer chains exist in three distinct populations: tie chains that bridge the two crystals, chain folds and chain ends. The distribution of these populations influences the properties of the interphase. We have developed off‐lattice Monte Carlo simulations of constrained interphases of semi‐crystalline polymers which utilize robust off‐lattice moves. A united atom model with polyethylene‐like interactions and with freely rotating bonds is used to mimic the prototypical flexible chain structure. These simulations capture the limiting distributions of tight and loose chain folds and of tie chains within the metastable phase. The dissipation in order and density between the crystal and amorphous regions has been studied, and results for freely rotating chains indicate that the characteristic decay of anisotropy occurs in a length scale of ca. 10 Å. Simulation results for the effect of system size and molecular weight for freely rotating chains have also been investigated.

Atomistic Simulation of a Thermoplastic Polyurethane and Micromechanical Modeling

“…this value is very similar to the average interfacial thickness calculated for semicrystalline polyethylene 17,84,85 ( ) The probability distributions of tails, loops and bridges in the noncrystalline region of TPU68 as functions of their number of constituent united atoms, Nb, are shown in Figure 5. It has been previously seen that for Nb > 100, these distributions decay exponentially, indicative of a most probable distribution of segment lengths, as defined by Krishna Pant et al 86 Additionally, internal equilibration between the populations of loops and bridges is confirmed by their overlap for Nb > 100.…”

Quasi-continuous melting of model polymer monolayers prompts reinterpretation of polymer melting

“…Conformations of chain segments emerging from the crystalline layer will be highly restricted due to the lack of lateral space, referred to as “overcrowding” at the crystal-melt interface (Fig. 5a ) 32 , 33 . For mathematical expediency, in our model this is taken into account by assuming that the first t /2 groups of the melted ends, coming out of the crystalline layer, have the energy of the melt but the entropy of the ordered chain.…”

“…In the following we compare melting in 2D and 3D systems of chain molecules and examine why no significant premelting occurs in the bulk. For perfectly aligned chains exiting a 3D crystal to instantly adopt random segment orientation, their effective cross-section must increase by a factor g of 2–3 according to theory 32 , 33 and 2.3 according to experiment 34 . Therefore, in order for a polymer lamella to grow large laterally, a fraction (1 − g −1 ) of all chains must either end at the interface, or reenter the crystal through a sharp fold to make room for the escaping chains.…”

Systematic coarse‐graining of semicrystalline polyethylene

“…Molecular simulations, such as molecular dynamics (MD), are powerful tools for exploring structure–property relationships in materials. However, the limited time and length scales generally accessible to molecular modeling methods make it challenging to study multiphase systems, particularly polymers, where even generating a realistic starting configuration can be difficult . As a result, coarse‐grained (CG) models, that is, where groups of atoms are represented by interaction sites referred to as beads or super atoms, are becoming increasingly popular for modeling molecular systems .…”

“…However, the limited time and length scales generally accessible to molecular modeling methods make it challenging to study multiphase systems, particularly polymers, where even generating a realistic starting configuration can be difficult. [6][7][8][9] As a result, coarse-grained (CG) models, that is, where groups of atoms are represented by interaction sites referred to as beads or super atoms, are becoming increasingly popular for modeling molecular systems. [10][11][12][13] Few CG models, however, have been rigorously developed to model semicrystalline polymers, in which amorphous and crystalline phases coexist.…”