Photosynthetic membranes IV: kinetic investigation of photoinitiated grafting and photo-cross-linking of an epoxydiacrylate resin onto cellulose

Photochemical grafting onto cellulose and successive photo-cross-linking of 2.00 - 37.00 mg cm-2 of an epoxydiacrylate prepolymer based on glycidyl ether of bisphenol A, containing 25 wt./wt.% trimethylolpropane triacrylate, were investiated kinetically at 30 °C in the presence of 1,2-diphenyl-2,2-dimethoxyethanone as a photoinitiator, with the weight concentration ratio of photoinitiator to prepolymer varied between 0.070 and 1.115. Irradiations were carried out polychromatically, in air or under a stream of nitrogen, with incident radiation of flux I ranging from 2.1 × 10-8 to 20.5 × 10-8 einsteins s-1 cm-2. Graft yields as a function of irradiation time were determined gravimetrically: two consecutive kinetic processes were observed which had rates linearly dependent on I. The quantum yields Φ1 of the first constant-rate period showed a linear dependence on photoinitiator concentration, while the quantum yields Φ2 for the second constant-rate process showed an inverse dependence on n/S (moles of prepolymer initially deposited per unit apparent cellulose surface). Kinetic analysis of the degree of unsaturation by multiple internal reflection IR spectroscopy was carried out in parallel and the results indicated that a structure with one pendant acrylate group per monomer unit may be assigned to the grafted polymer. At longer irradiation times, further decrease in unsaturation resulting from photo-cross-linking was observed which had a rate linearly dependent on I0.5, suggesting a bimolecular termination for this process. In the light of this information, the mechanism of photografting of poly-functional acrylate monomers onto polymeric surfaces is discussed.