The local dynamics and the conformational properties of polyisoprene next to a smooth graphite surface constructed by graphene layers are studied by a multiscale methodology. First, fully atomistic molecular dynamics simulations of oligomers next to the surface are performed. Subsequently, Monte Carlo simulations of a systematically derived coarse-grained model generate numerous uncorrelated structures for polymer systems. A new reverse backmapping strategy is presented that reintroduces atomistic detail. Finally, multiple extensive fully atomistic simulations with large systems of long macromolecules are employed to examine local dynamics in proximity to graphite. Polyisoprene repeat units arrange close to a parallel configuration with chains exhibiting a distribution of contact lengths. Efficient Monte Carlo algorithms with the coarse-grain model are capable of sampling these distributions for any molecular weight in quantitative agreement with predictions from atomistic models. Furthermore, molecular dynamics simulations with well-equilibrated systems at all length-scales support an increased dynamic heterogeneity that is emerging from both intermolecular interactions with the flat surface and intramolecular cooperativity. This study provides a detailed comprehensive picture of polyisoprene on a flat surface and consists of an effort to characterize such systems in atomistic detail.

The effects of interphase topology, entanglements, and chain dynamics on the mechanical response of semicrystalline polyethylene have been examined using atomistic simulations. In particular, the prevalence of the cavitation and melting/recrystallization mechanisms for yield and plastic flow were found to depend on both topological and dynamical properties of the molecular segments in the semicrystalline interphase. First, two different protocols were used during preparation of the interphase ensemble to modulate the distribution of (i) loops, bridges, and tails and (ii) entanglements within the noncrystalline domain. A protocol denoted "step-wise cooling" produced structures having a large fraction of long, entangled segments that yielded by the melting/recrystallization mechanism about 50% of the time. By contrast, the protocol denoted "instantaneous quench" produced structures that yielded by melting/recrystallization about 73% of the time. Second, two different united atom force fields, PYS and TraPPE-UA, that exhibit nearly identical topological characteristics of the noncrystalline domain but different mobilities were used to study the effect of chain dynamics on yield mechanisms. At the slower strain rate used in this work, yield and plastic flow proceeded exclusively via cavitation for the model using the TraPPE-UA force field, whereas both cavitation and melting/recrystallization were observed for the model using the PYS force field. The greater prevalence of melting/recrystallization in the latter case is attributed to faster chain-sliding dynamics in the crystalline domain. The dependences of the yield mechanism on topology and dynamics are found to be related.

The vibrational spectra of semicrystalline polyethylene and its distinct domains were investigated using molecular dynamics (MD) simulations. A method for the vibrational analysis of the domains within the lamellar stack model of semicrystalline polymers has been developed and demonstrated on semicrystalline polyethylene using force fields having either united atom (UA) or explicit atom (EA) detail. In the UA description, the calculated vibrational spectra were found to differ from the observed skeletal vibrations of polyethylene with the force field used in this work. Therefore, a modified UA force field with different stretching and bending force constants is proposed, which was found to reproduce the observed frequencies of the skeletal vibrations. In the EA description, the vibrational spectra of semicrystalline polyethylene were in satisfactory agreement with typical infrared and Raman signatures of polyethylene melts and crystals. Experimental interpretations regarding the assignment of peaks in the Raman spectra to components of semicrystalline polyethylene were examined. The spectrum of the interphase domain obtained using the EA model was found to be adequately reproduced by a superposition of the spectra of the crystalline and amorphous domains, at variance with experimental observation. The lack of a distinct interphase spectrum in the EA model was attributed to the absence of the CH 2 bending peak associated with the orthorhombic phase, despite confirming an orthorhombic crystal structure in the crystalline domain.