Observations by DSC on bound water structure in some physically crosslinked hydrogels

Linear, compositionally homogeneous copolymers containing 23–62 wt% N‐vinyl‐2‐pyrrolidone (VP) were prepared by a semi‐continuous copolymerisation with methyl methacrylate (MMA) in the absence of chemical crosslinker. Films cast from solution in chloroform were swollen in water at various temperatures (T) within the range 283–343 K. The total water content (11–80 wt%) of the resultant hydrogels at swelling equilibrium, as well as its subdivision into freezing and non‐freezing water components, were measured and found to be dependent on copolymer composition and T. The network structure of these hydrogels is considered to arise from a combination of normal VP–water interactions with physical crosslinking due to hydrophobic MMA sequences, and the novel, albeit only semi‐quantitative, findings are discussed on this basis.