Photoinduced Reactions of Benzophenone in Biaxially Oriented Polypropylene

The photoinduced reactions of benzophenone (B) in biaxially oriented polypropylene (BOPP) were studied with nanosecond laser photolysis (N₂ laser, λ337.1 nm). The first observed transient was a triplet state ³B*. Decay of ³B* led to formation of a radical pair (RP) of BH^• and R^•, where R^• is a radical formed by hydrogen abstraction from BOPP (RH) by ³B*. We studied BOPP after the preheating for a short time in a temperature range 298-423 K, which is essentially lower than its melting point of 453 K. All measurements with not-heated and with preheated (annealed) BOPP were made at 298 K. A radical pair (RP) apparently decays as a contact pair ³[BH^•, R^•] in nonheated BOPP. A critical phenomenon takes place: dissociation of RP with a formation of free radicals in the polymer bulk is observed at preheating temperature T_crit ≈ 403 K and at a higher T. The physical process of heating and cooling of BOPP apparently resulted in the restructuring of crystallites, their agglomeration, shrinking of the distribution of crystallites according to their sizes in BOPP. Overall BOPP becomes softer which manifests itself in the radical kinetics. The decay kinetics of ³B* and RP in the cage fits well the first-order law. Rate constants were obtained. Radicals BH^•, which exit into the polymer bulk at temperatures of preheating T ≥ 403 K, decay by cross-termination according to the second-order law. A relatively high rate constant ∼10⁸ M^-1·s^-1 for this reaction was obtained due to diffusion of BH^• enclosed in the soft amorphous phase of BOPP. Properties of BOPP containing B were studied with ESR, DSC, IR, and WAXD.