Csaba Kósa

Relative rates of photooxidation of benzil to benzoyl peroxide in various polymer matrices

Benzil (BZ), 16 wt.-%, has been irradiated (>400 nm) at ambient temperature in aerated polystyrene (PS), poly(methyl methacrylate) (PMMA), polycarbonate (PC), polysulfone (PSF) and poly(vinyl chloride) (PVC) films. The rate of BZ consumption, leading primarily to the formation of benzoyl peroxide, was followed by infrared spectroscopy. The rate of reaction is sensitive to the nature of the glassy polymer matrix and decreases in the order: PS > PC > PVC > PSF > PMMA. The dependence of the rate of BZ consumption on the concentration of dissolved molecular oxygen in the polymer matrices is rather complex. A kinetic treatment, assuming steady-state conditions, leads to the conclusion that the sum of the rates of bimolecular formation of benzoyl peroxide and quenching of BZ triplets by molecular oxygen, (k ox + k q )[ 3 O 2 ], represents only 0.8-13% of the sum of the unimolecular rates of radiationless and radiative deactivation (including k p , the pseudo first-order rate constant for reaction of BZ triplets with the polymer), (k d + k ph + k p ), in the various polymer matrices.

Photo-crosslinking of polyethylene by mono- and diacetophenone derivatives and their precursors

The efficiency of photochemical crosslinking of low-density polyethylene (PE) films by molecules containing one or two acetophenone groups and their precursors have been correlated with the relative rates of carbonyl group depletion under equivalent irradiation conditions. The acetophenone derivatives are found to be equally or more efficient than benzophenone as crosslinkers of PE. Because some additives are not thermodynamically equilibrated in the PE matrices and crystallize on standing at 30 degrees C, doped films were irradiated usually as soon as possible after their preparation. However, others were aged to determine the influence of equilibration on the efficiency of crosslinking. Thus, the effect of crystallization of some of the crosslinker additives in the PE matrices and the competition between photochemical addition of the carbonyl groups to C-H or C=C bonds of the polymer were investigated. GC-MS and FTIR analyses of photoproducts from irradiation of alkanophenones bearing a gamma-hydrogen atom in PE films showed that (intramolecular) Norrish Type II splitting and Yang cyclization processes proceed efficiently, and are more rapid than subsequent intermolecular hydrogen abstraction reactions by the acetophenone photoproducts.