Dimethacrylate or divinyl-functionalized acetal-based crosslinkers were synthesized as building elements of acid-sensitive crosslinked hydrogels. Each crosslinker was prepared under catalytic acidic conditions with different functional groups installed at the acetal position. The hydrophilicity of the crosslinkers was tuned to control acidic-hydrolysis rate. We report the synthesis of hydroxyethyl dimethacrylatefunctionalized dimethyl ketal (CL1), meta-or para-methoxybenzaldehyde based acetals (CL2m and CL2p), poly(ethylene glycol) dimethacrylate-functionalized dimethyl ketal-based crosslinker (CL3), and divinyl-functionalized meta-methoxybenzaldehyde-based acetal crosslinker (V-CL2m). An examination of acetal hydrolysis kinetics of the monomers was performed in aqueous buffer solutions using 1 H NMR (proton nuclear magnetic resonance) and UV-Vis (ultraviolet-visible) spectroscopy at various pH ranges.The hydrolysis rates were strongly dependent on the structure of the acetal. Network films containing CL2m were prepared by thermally initiated polymerization with either hydroxyethylmethacrylate (HEMA) or methylmethacrylate (MMA). A study of the hydrolysis kinetics of these crosslinked films was performed using GC-MS (gas chromatography and mass spectroscopy) to understand the effect of monomer hydrophilicity, crosslinking density, and polymerization mechanism at different pHs. The crosslinked films composed of the hydrophilic monomer, HEMA, show faster hydrolysis than those containing more hydrophobic monomers (e.g. MMA). The hydrolysis rate decreases as the crosslinking density increases. In the case of thiol-ene networks formed by reacting pentaerythritol tetrakis(3mercaptopropionate) and V-CL2m, each repeating unit is composed of an acid-degradable acetal-moiety.Hydrolysis of the thio-ene network films results in depolymerization into two lower molecular weight components, pentaerythritol tetrakis(3-(6-hydroxyhexylthio)propanoate) and meta-methoxybenzaldehyde.

Thin hydrogel films containing the acid sensitive cross-linker 2,5-dimethyl-2,5-hexanediol dimethacrylate (DHDMA) were synthesized as part of a larger project to create protective layers against chemical and biological (CB) threats. In order to operate efficiently as fabric coatings, the mechanical and moisture vapor transport properties of these materials must be understood. The hydrogels were composed primarily of poly­(ethylene glycol) dimethacrylate (PEGDMA) and 2-hydroxyethyl methacrylate (HEMA) and were prepared by free radical cross-linking polymerization. The influence of different compositions of DHDMA, PEGDMA, and HEMA on the moisture vapor transmission rate (MVTR), mechanical properties, and swelling properties of PEGDMA/DHDMA/HEMA copolymer hydrogels has been investigated. The characterization by Fourier transform infrared (FTIR) spectroscopy confirmed the presence of all monomers used in the gel structure. The hydrogels showed high MVTR, which are comparable to the widely accepted breathable membrane of Gore-Tex fabric and expanded polytetrafluoroethylene (ePTFE). The swelling data indicated that the equilibrium water content depends on the composition of hydrogels. It was found that addition of HEMA into the PEGDMA-based hydrogels was very helpful to improve the swelling properties but decrease in water contact angle. Rheological study of hydrogels showed that the hydrogels are very stiff and their stiffness increases with increasing PEGDMA content in the hydrogels.