Conformational Change Restricted Selectivity in the Surface Sulfonation of Polypropylene with Sulfuric Acid

Surface sulfonation of solid polypropylene (PP) with hot concentrated H 2 SO 4 has been studied by diffuse reflectance Fourier-transformed infrared (DRIFT) and X-ray photoelectron (XP) spectroscopies. Both measurements indicated that the sulfonation of PP followed by slight oxidation and the formation of C C bonds is enhanced with both increasing reaction temperature and time. The time dependence of H 2 O penetration into sulfonated PP mesh sheets was analyzed using a rearranged Washburn's equation, providing a quantitative indicator of surface hydrophilicity. A positive linear correlation was obtained between the hydrophilicity and the degree of sulfonation. The charge distribution calculated for models of SO 3 and PP molecules by the PM3 molecular orbital (MO) method suggested that the sulfonation of PP proceeds via electrophilic addition of SO 3 . It was further predicted from the MO calculations for models of sulfonated PP molecules that side CH 3 groups are more susceptible to sulfonation than C atoms of the main chain due to a drastic conformational change of the main chain in the latter case. The heat of reaction in each pathway (Δ E ) was estimated to be Δ E (side CH 3 groups) = −63.5 kJ (unit mol) -1, Δ E (C(CH 3 )SO 3 H) = −56.8 kJ (unit mol) -1, and Δ E (CH(SO 3 H)) = −55.8 kJ (unit mol) -1, respectively. The selectivity in the sulfonation of PP termed “the conformational change restricted selectivity (CCRS)” was shown to increase with a decrease in the reaction temperature by analyzing the DRIFT signals around 600 cm - 1 assigned to the deformation of S−O bonds and the stretching vibration of S−C bonds.