Thermal expansion of free volume in "classic" and regulated dimethacrylates: photocured directly and via a mask to study pillar formation.

The temperature dependence of free volume in dimethacrylates (poly2M), cured by direct irradiation (poly2M-A) or via a mask (poly2M-B), and in a thiol-based 2M sample (poly2M-co-EDDT), was investigated by positron annihilation lifetime spectroscopy (PALS) and dilatometry (DIL) to study the influence of thiol regulation on the microstructure via free volume characteristics. It was found that the free volume fraction as determined from experimental data by using the standard spherical approach for the hole shapes showed systematic differences from the analogous quantity as evaluated from the lattice-hole theory. Much better results were obtained for cylindrical holes, which expand 'anisotropically' in poly2M samples and 'isotropically' in the poly2M-co-EDDT resin. In addition, the hydrogen bond changes and the conversion of monomers in cured samples studied by near infra-red spectroscopy (NIR) revealed spectrum-structure correlations for the final cured thermosets.

Shape and Temperature Expansion of Free Volume Holes in Some Cured Polybutadiene-Polyisoprene Rubber Blends.

The free volume fraction of a macromolecular structure can be assessed theoretically by using a suitable model; however, it can also be evaluated from experimental data obtained from dilatometry and positron annihilation lifetime spectra. In this second case, a regular geometry of the sub-nanometric cavities forming the free volume has to be assumed, although in fact they are irregularly shaped. The most popular approach is to guess spherical holes, which implies an isotropic growth of these last with temperature. In this work, we compared the free volume fraction, as obtained from experiments in a set of polybutadiene and polyisoprene cured rubbers and their blends, with the analogous quantity expected by using the lattice-hole model. The results allowed us to obtain insights on the approximate shape of the holes. Indeed, a cylindrical flattened geometry of the cavities produced a better agreement with the theory than the spherical shape. Furthermore, the best fit was obtained for holes that expanded preferentially in the radial direction, with a consequent decrease of the aspect ratio with temperature.