Synthesis of poly(propylene-glycol-diacrylates) and properties of the photocured networks

A series of poly(propylene-glycol-diacrylates) (PPGDA) having molecular weights (MW) in the range 300–3,000 and an acrylic functionality near to two were synthesized by acrylation of the corresponding hydroxy-terminated oligomers with acrylic acid in the presence of p-toluensulphonic acid as a catalyst. The M¯n of the acrylated products was found slightly lower than that of the starting oligomers, indicating the occurrence of an acidic degradation reaction which does not influence the acrylic functionality. The acrylated oligomers were ultraviolet (UV) cured until a complete double bond disappearance was obtained: only in the presence of tripropylene-glycol-diacrylate (TPGDA) were small amounts of residual unsaturations revealed. Rubbery materials were usually obtained, with the exception of TPGDA. The properties of the cured PPGDA were investigated by means of differential scanning calorimetry, thermomechanical analysis, and dynamic mechanical thermal analysis. The Tg values were found to decrease by increasing the MW of the used oligomers, that is, by increasing the length of the chain between the two acrylic double bonds. A good agreement with the Nielsen equation was found. Moreover, the equilibrium swelling values in water were measured; the obtained values were interpreted in terms of the solubility parameters of the oligomers and of the crosslinking density of the networks. Finally, some mixtures of PPGDA oligomers with a typical epoxy-acrylate resin were UV cured; their properties confirm the high flexibilizing effect of the PPGDA oligomers.