Hydrogels prepared from polysiloxane chains by end linking them with trifunctional silanes containing hydrophilic groups

A strongly hydrophobic polymer was converted into a hydrogel by introducing hydrophilic side chains of sufficient lengths and amounts to overcome the hydrophobicity but sufficiently well dispersed to avoid disadvantages such as loss of the transparency required in applications. Specifically, poly(dimethylsiloxane) (PDMS) hydrogels were successfully prepared by end linking a combination of long and short chains to give the bimodal distributions of network chain lengths that generally give unusually good mechanical properties. The end linkers were chosen using trialkoxylsilanes R′Si(OR)3 having R′ side chains that are hydrophilic of variable lengths and of sufficient hydrophilicity to produce the desired hydrogels. The first trialkoxysilane was N-(triethoxysilylpropyl)-O-polyethylene oxide urethane (S1) with 4-6 units of ethylene glycol, and the second was [methyoxy(polyethyleneoxy)propyl]-trimethoxysilane (S2) with 6-9 units of ethylene glycol, and they were used to end link hydroxyl-terminated PDMS chains in standard room-temperature condensation reactions. It was possible to introduce the hydrophilic side chains into the hydrophobic networks without discernible phase separation. These linear side chains increase equilibrium water contents, from 0 to 11.2 wt% in the first series and from 0 to 29.8 wt% in the second. Longer hydrophilic chains clearly migrated to the surfaces of the resulting PDMS hydrogels to give reduction in static contact angles from 105° to 40° for the first series, and to 80° for the second. The longer hydrophilic chains were found to give larger decreases in the contact angles and larger equilibrium water contents. The mechanical properties demonstrated that Young's moduli of the hydrogels did not change upon introduction of the S1 hydrophilic cross linker, but did decrease from the presence of the S2. The tensile strength and elongation at break were relatively insensitive to the amounts of either of the hydrophilic groups.