Atomistic Simulation of the Structure and Mechanics of a Semicrystalline Polyether

Abstract

We report the use of atomistic simulation to study semicrystalline poly­(tetramethylene oxide) (PTMO), which is one of the major components of thermoplastic polyurethanes. This work reports the first application of an Interphase Monte Carlo model previously developed for polyethylene to a more complex chemistry involving heteroatoms, about which much less is known experimentally. The interface between the crystalline and amorphous domains of PTMO has been modeled in detail, complete with the equilibrium distributions of tails, loops and bridges. In doing so, a criterion has been established for selecting the relevant interface between domains, and a methodology developed that identifies the energetically most favorable interface in a heterogeneous material. A representative sample of configurations was then simulated by molecular dynamics, and analysis of deformation to small strains at different strain rates is described. Estimation of the full stiffness matrix of semicrystalline PTMO is reported for the first time.

Simulation of the structure and mechanics of crystalline 4,4′-diphenylmethane diisocyanate (MDI) with n-butanediol (BDO) as chain extender

“…This conformation imparts increased flexibility to the MDI/BDO crystal, which can then be translated to decreased stiffness along the chain direction. Contrary to that, the two lateral stiffnesses of the MDI/BDO crystal, E 1 and E 2 , along with the corresponding shear moduli, G i , i=1,2,3, are larger than the ones of crystalline PTMO [53], which is a consequence of the dense packing in the MDI/BDO crystal lattice opposing shear or lateral deformation.…”

“…It also shows the anisotropic nature of crystalline MDI/BDO. The stiffness along the chain direction (E 3 ) of crystalline MDI/BDO is almost 7 times larger than the stiffnesses in the other two principal directions, yet substantially smaller than the stiffness of crystalline polyethylene (PE) (E 3 ~ 283 GPa) [54] and PTMO (E 3 ~ 67 GPa) [53] when stretched along their chain directions. This is probably a consequence of the bent zigzag molecular conformation of the MDI/BDO chains in the chain direction (see Figure 1), which allows for easier extensibility compared to the linear all-trans conformations of PE and PTMO.…”

“…As was done previously for simulations of crystalline and semicrystalline poly(tetramethylene oxide) (PTMO) [49], simple strain deformations were performed using a non-steady-state, non-equilibrium form of molecular dynamics in which each crystalline specimen was deformed by changing one component of the strain tensor, at a constant strain rate ε̇ = 5 x 10 6 s -1 , up to a true strain of ε = 0.1, while the other components were held constant at their equilibrium values. The full elastic stiffness tensor C was then extracted from the slopes of the stress-strain curves at small strain [50,51].…”

“…The elastic moduli of crystalline MDI/BDO were finally expressed as the inverses of the diagonal elements of the compliance tensor [52] In addition to the simple strain deformations, uniaxial extension was also considered. In this type of loading condition, the system was deformed in one of the three principal directions, while allowing the two lateral dimensions to change in response to the barostat in those directions [53], so that a constant stress at about 1 atm was maintained. These types of deformation yielded independent estimates of the elastic (Young's) moduli, E i , and isothermal compressibility, β T , directly, and served as checks on the numerical accuracy of the first deformation method (simple strain), which is itself complete.…”

Ab initio and classical atomistic modelling of structure and defects in crystalline orthorhombic polyethylene: Twin boundaries, slip interfaces, and nature of barriers

“…These force fields display different levels of atomic resolution and are commonly used for polymer modelling. Although there is a multitude of all-atom potentials available in the literature, the former was chosen for the present paper based on its common use, its simplicity and relatively low computational cost, while the latter was chosen because it has been used extensively for investigating the mechanical properties of amorphous and semi-crystalline polymers [32][33][34][35][36][37][38][39][40][41][42].…”