Surface Segregation of Methyl Side Branches Monitored by Sum Frequency Generation (SFG) Vibrational Spectroscopy for a Series of Random Poly(ethylene-<i>co</i>-propylene) Copolymers

The surface composition and surface chain conformation of a series of random aspecific poly(ethylene-co-propylene) rubber copolymers (aEPR) was quantified by sum frequency generation surface vibrational spectroscopy (SFG). All of the copolymers are found to preferentially orient side-branch methyl groups out of the surface. As the ethylene content of the copolymer increases, the number of methyl groups contributing to the sum frequency signal decreases. However, the percentage of methyl groups oriented out of the surface, relative to the bulk concentration of methyl groups, increases. This surface excess of oriented methyl groups is proposed to be a result of decreased steric hindrances between adjacent methyl groups in ethylene-rich copolymers. Additionally, analysis of the CH2 bands in the SFG spectra suggests that the CH2 units at the surface become more oriented toward the surface normal and adopt a trans configuration as the ethylene content increases.