Calorimetric Characterization of the Heterogeneities Produced by the Radiation-Induced Cross-Linking Polymerization of Aromatic Diacrylates

The structural heterogeneities in networks produced by UV- and by electron beam-initiated polymerization of aromatic diacrylates were unambiguously revealed by A FM imaging and studied by means of temperature-modulated differential scanning calorimetry. Two main second-order thermodynamic transitions were observed during the analysis of diacrylate networks exhibiting a fractional degree of conversion ranging from 0.1 to 0.8. The bimodal distribution of transition temperatures observed as fused peaks in the diagrams representing the differential quantity associated with the reversing heat capacity, dC(p,rev)/dT, was satisfactorily resolved by a two-component numerical fitting method allowing a quantitative exploitation of the data in terms of Gaussian contributions with particular central relaxation temperature and peak width affected by the gradual polymerization of the sample. The dual relaxation phenomenon was assigned to a biphasic-like structure consisting of clusters with a high cross-link density formed in continuous domain of lower monomer conversion. The evolution of the calorimetric data in the samples affords detailed information on the gradual formation of the network, with the initial broadening and final narrowing of the transition peak at high temperature related to the densely cross-linked clusters, whereas the low-temperature transition vanishes in intensity with an increasing broadening of the associated transition range. The analyses carried out in this work did not point out any influence of the UV- or EB-induced initiation mechanism of the time scale and thermal conditions of the polymerization process on the calorimetric features of the observed heterogeneities.