Vibrational Analysis of Semicrystalline Polyethylene Using Molecular Dynamics Simulation

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 CH2 bending peak associated with the orthorhombic phase, despite confirming an orthorhombic crystal structure in the crystalline domain.