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We describe a method for determining the thermal and elastic properties of the noncrystalline phase of semicrystalline polymers (the "interphase") by Monte Carlo simulations. The method is applied to an interphase composed of freely rotating polyethylene-like chains, studied previously by Balijepalli and Rutledge (J. Chem. Phys. 1998, 109, 6523). The isochoric and isobaric heat capacities, Gru ¨neisen coefficients, thermal expansion coefficients, and elastic stiffnesses are reported. The interphase exhibits material properties comparable to that of the corresponding melt, with significant contributions of both enthalpic and entropic origins. Through judicious selection of Monte Carlo moves, we approximate the rate sensitivity of the stiffness normal to the crystal surface, C 33, on time scales that are long and short, respectively, relative to the characteristic time scale of the crystalline Rc relaxation.

We present the first theoretical estimates for thermoelastic properties of the noncrystalline domain (the "interlamellar phase") of semicrystalline polyethylene obtained by Monte Carlo simulations. The interlamellar phase is prescribed to be thermodynamically metastable, with the constraints that it have an average density less than that of the crystal and that it be bounded by two static crystalline lamellae oriented with the {201} crystal plane parallel to the interface. Polyethylene was modeled using a realistic united atom force field with inclusion of torsional contributions, and the results are compared to those of prior studies that used a freely rotating chain model. Parallel tempering between 350 and 450 K was used to simulate several isochoric/isothermal ensembles simultaneously and efficiently, from which the heat capacity, thermal expansion coefficients, Gru ¨neisen coefficients, and the elastic stiffness tensor were determined at atmospheric pressure. The noncrystalline interlamellar phase exhibits properties intermediate between that of the semicrystalline solid and the amorphous melt.

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Thermoplastic polyurethanes (TPUs) are useful materials for numerous applications due in part to their outstanding resilience and ability to dissipate energy under large mechanical deformation. However, the mechanistic understanding of the origins of these mechanical properties at the molecular level remains elusive, largely due to the complex, heterogeneous structure of these materials, which arises from the segregation of chemically distinct segments into hard and soft domains. In this work, molecular simulations are used to identify the mechanism of mechanical response under large tensile deformation of a common thermoplastic polyurethane comprising 4,4′-diphenyl­methane diisocyanate and n-butanediol (hard segment) and poly­(tetramethylene oxide) (soft segment), with atomic resolution. The simulation employs a lamellar stack model constructed using the Interphase Monte Carlo method established previously for semicrystalline polymers, which models the interfacial zone between hard and soft domains with thermodynamically rigorous distributions of bridges, loops, and tails. Molecular-level mechanisms responsible for yield, toughening, and the Mullins effect are reported. We have found several distinct mechanisms for yield and plastic flow, which we categorize as (i) cavitation, (ii) chain pull-out, (iii) localized melting with shear band formation, and (iv) block slip. The activity of these mechanisms depends on the topology of chains in the soft domain and the direction of loading (e.g., parallel or perpendicular to the interface). Further insights regarding toughening mechanisms and the Mullins effect are obtained from cyclic loading, where mechanisms ii to iv were found to be irreversible and account for the superior resilience and dissipation at large tensile strains in thermoplastic polyurethanes.

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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.

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