Hydrophobically-modified PEG hydrogels with controllable hydrophilic/hydrophobic balance

This work reports on a novel method to synthesize hydrophobically-modified hydrogels by curing epoxy monomers with amines. The resulting networks contain hydrophilic poly(ethylene gly-col) (PEC) segments, poly(propylene glycol) (PPG) segments, and C18 alkyl segments. By varying the content of C18 segments, networks with different hydrophilic-lipophilic balance (HLB) are obtained. All networks show an amphiphilic behavior, swelling considerably both in organic solvents and in aqueous media. In the latter they display a thermosensitive behavior, which is highly affected by the network HLB and the pH of the solution. A decrease in HLB results in an Increment of the polymer weight content (wp) due to hydrophobic association. Furthermore, a reduction in HLB induces a remarkable increase in Initial modulus, elongation at break and tensile strength, especially when Wp becomes greater than about 10%. Low field nuclear magnetic resonance (LF-NMR) experiments evi- dence that, when HLB decreases, a sudden and considerable Increase in hydrogel heterogeneity takes place due to occurrence of extensive physical crosslinking. Available data suggest that in systems with wp ≳10% a continuous physical network superimposes to the pre-existing chemical network and leads to a sort of double network capable of considerably improving hydrogel toughness.