Polymer-gel formation and reformation on irradiation of tertiary-butyl acrylate

• Gamma-ray polymerization of tertiary-butyl acrylate; dose and dose-rate dependences of monomer conversion.
• The radiolytic production of a rigid, UV/vis transparent organic gel.
• Primary radiolytic processes relevant to the preparation and application of a radio-fluorogenic polymer-gel.
• Reformation of an organic polymer gel by swelling subsequent to removal of monomer by evacuation.

The purpose of the present research was to provide a radiation-chemical basis for the use of tertiary-butyl acrylate gels in radio-fluorogenic dose-imaging applications (Warman et al., 2011a, Warman et al., 2011b, Warman et al., 2013a and Warman et al., 2013b). The radiation-induced polymerization of tertiary-butyl acrylate (TBA) results in the formation of a transparent gel with an optical density lower than 0.1 cm−1 from 600 nm down to 315 nm. The fractional monomer-to-polymer conversion, CM, determined gravimetrically, increases super-linearly with dose, D Gy. Up to CM≈40%, and over the dose rate range D′=3.5 to 49 cGy s−1, the dose dependence is given by CM=[1+ACM]KD/√D′ with K=1.43×10−3 Gy−0.5 s−0.5 and A=0.70. For D′=3.5 cGy s−1 the average polymer size is estimated to be 1.2×105 monomer units or 17 megadalton. For CM≥10% the gel is quasi-rigid, displaying little tendency to flow on a timescale of an hour or more. After removal of monomer by evacuation, the gel can be reformed by adding a volume of monomer to the remaining polymer equal to that removed and allowing this to swell for several days. The dose and dose rate dependence of radiation-induced monomer conversion in the reformed gel show no evidence of a discontinuity caused by the intervening evacuation and reformation procedures.