Effect of UV light scattering during photopolymerization on UV spectroscopy measurements

A photopolymerization reaction involving 2-hydroxyethylmethacrylate (HEMA), a hydroxytelechelic polybutadiene (HTPB) and 2,2-dimethyl-2-hydroxyacetophenone (Darocur 1173) as photoinitiator has been investigated by Real Time UV and Real Time IR spectroscopies. For this system, RTUV analyses are completely opposite to the expected curves, namely a decrease in the sample absorbance (photobleaching phenomenon). This apparent contradiction has been explained by a scattering phenomenon inside the sample, which can be linked to the formation of sub-structures that can create local fluctuations of refractive index during the polymerization. A four-flux theory, which takes into account the polymer absorption and scattering properties, has allowed us to describe the UV light intensity variation inside the sample.