Network structure of spontaneously forming physically cross-link hydrogel composed of two-water soluble phospholipid polymers

Hydrogels containing 2-methacryloyloxyethyl phosphorylcholine (MPC) moieties were prepared from aqueous solutions with water-soluble poly[MPC-co-methacrylic acid (MA)] (PMA) and poly[MPC-co-n-butyl methacrylate (BMA)] (PMB). We had found that the hydrogel would swell under acidic pH and dissociate under neutral and alkaline conditions. Investigating these properties together with the complex modulus and the calculating cross-link molecular weight, we tried to determine how the network is constructed. The hydrogel showed a low swelling and a low elastic modulus E as the PMA feed ratio increases. However, a slow dissociation behavior was shown at the same time. This is due to the formation of the intramolecular cross-links caused by hydrogen bonds provided by carboxyl groups inside the hydrogel, which does not contribute to the swelling or elastic property. However, they had affected the ionization of the carboxyl groups. The average cross-link molecular weight decreased with the higher PMA feed ratio, indicating that the network is actually denser. The calculated cross-link molecular weight for the high PMA feed ratio is low, but the number of cross-links is also low, indicating that there are many intramolecular cross-link networks, but fewer valid entanglements for high mechanical strength. Therefore, denser networks are formed for higher the PMA feed ratio, but they do not function as cross-link junctions. This could be confirmed by Fourier transform infrared spectroscopy (FTIR). When the release behavior of the loaded drugs was tested, it showed that the drugs with molecular weights higher than the cross-link molecular weight would be slower than that of drugs with lower molecular weights.