Hydrogels improvements through freeze-casting and anti-freezing additives

Freeze-casting is a widely applied processing route for numerous kinds of materials, including gels. As obtained pores are a «negative» of the frozen solvent, improvements in porosity control can be achieved by anti-freezing macromolecules addition. Photopolymerization coupled to freeze-casting was herein investigated for the syntheses of poly(hydroxyethylmethacrylate)/(acrylic acid) (pHEMA/AA) hydrogels, with and without the addition of polyvinylalcohol (PVA) as anti-freezing additive. PVA was characterized in terms of ice recrystallization inhibition property and by Differential Scanning Calorimetry (DSC) to evaluate its thermal hysteresis. An optimal PVA concentration was considered to be 0.8 mg/mL. Gels at different water contents (from 10 to 50%) were prepared in the following sets: (i) blanks at r. t., (ii) with/without PVA isotropically at -5 °C, (iii) with/without PVA anisotropically by liquid nitrogen immersion at 6 mm/s velocity. Samples were then characterized by means of thermogravimetric analysis (TGA), DSC and swelling tests in neutral, acidic and basic conditions. Freeze-casting results to produce a higher degradation temperature of pHEMA/AA gels (around 370 °C) compared to literature data, possibly due to high levels of cross-linking and interconnectivity. PVA addition did not affect significantly the gels’ swelling while improving their free water content. Polydispersity of freeze-cast samples resulted smaller than references probably because the polymerization at low T, allowed the formation of more controlled porosity. Since the existence of cis and trans HEMA conformers in liquid phase has been scertained through spectroscopic and computational studies, molecular dynamics simulations on these conformers in aqueous solution have been performed. The simulations have allowed to obtain information useful in studying properties of HEMA-based polymers.