Studies of surface structures and compositions of polyethylene and polypropylene by IR+visible sum frequency vibrational spectroscopy

IR+visible sum-frequency generation (SFG) was used to obtain surface vibrational spectra of polyethylene and polypropylene. It was found that low density and ultrahigh-molecular-weight polyethylene have different surface structures. The latter has a surface mainly composed of orderly packed gauche conformers while the former has a surface with a more random distribution of polymer chains. The SFG spectra also show that surface structures of atactic and isotactic polypropylenes are different. Segregation of additives at surfaces of industrial polyethylene and polypropylene, that cannot be detected by IR and Raman spectroscopies, can be readily observed by SFG.