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

Abstract

The temperature dependence of free volume in dimethacrylates (poly2M), cured by direct irradiation (poly2M-A) or via mask (poly2M-B), and in thiol-based 2M sample (poly2M-co-EDDT) was investigated by positron annihilation lifetime...

Effect of Crosslink Density on Thermal Aging of Bio-Based Rigid Low-Density Closed-Cell Polyurethane Foams

“…It is very challenging to isolate this effect from the contribution of the number density of holes to I o Ps . In addition, the material constant of proportionality between I o Ps and the number density of holes is usually unknown for a given polymer, and its precise evaluation is complicated by the need of determination of free volume fractions using dilatometric measurements with subsequent comparison with PALS data and theoretical models. , …”

“…In addition, the material constant of proportionality between I oPs and the number density of holes is usually unknown for a given polymer, and its precise evaluation is complicated by the need of determination of free volume fractions using dilatometric measurements with subsequent comparison with PALS data and theoretical models. 45,46 Thus, estimation of the fractional free volume (FFV) using PALS measurements in the samples is complicated, mainly due to several factors related to the interpretation of relative I o-Ps intensities, as mentioned above. In addition, the approximation of the real cavity volume by an idealized spherical volume introduces another error in the effort to accurately determine the FFV.…”

Microstructural study of different thick dimethacrylate-based samples using different amounts of photoinitiator

“…Comparison of the same thickness but different amounts of PI, the final cured poly2M samples with 0.5 mol% of PI show a larger shift of the 1 st peak to a higher wavenumber and the higher intensity of the 3 rd peak compared to the poly2M sample with 0.1 mol% of PI. Both findings, weakening of the H bonding interactions in the network structure and the for-Table 1 The characteristic parameters of the sigmoidal function which describe the time evolution of the local free volume of the investigated samples: time at the inflection point (t ip ) of the sigmoid function, free volume of holes V h,ip at t ip and V h,2 of the final cured 2M samples and the full width at half maximum of the derivative of the sigmoid function (FWHM) Paper Polymer Chemistry mation of NH free for the 2M samples with 0.5 mol% of PI could be correlated 14 with the reduced V h,ip and V h,2 (Table 1), reflecting an increased internal microstructural shrinkage and highly dense network structure given by the bulk density (ρ s ). PALS dataevaluation with the dispersion.…”

“…1 Chemical structures of the two dimethacrylates (UDMA and D 3 MA) forming 2M and the photoinitiator (TPO-L). 14 Fig. 2 The sandwich arrangement of the PALS apparatus consisting of the blue silicon ring (2 mm and 5 mm), radioactive source 22 Na (yellow circle) and the reference Al.…”

“…The entire process of photopolymerization is a very complex phenomenon that has been intensively studied using various techniques such as photo-DSC, 7 real-time infrared spectroscopy (RT-IR), 8,9 coupled real-time near-infrared (NIR) spectroscopy 10 with photorheometry providing information about double bond conversion (DBC) and shrinkage stress 11,12 and dielectric spectroscopy (DS). 13,14 A less commonly used technique for the study of photopolymerization is positron annihilation lifetime spectroscopy (PALS). Polymer research mainly uses the orthopositronium probe, which is a triplet form of an electron-positron bound state with a lifetime in the nanosecond range that depends strongly on the local electron density at the site of annihilation.…”