Effect of the molecular weight of polyethylene glycol (PEG) on the properties of chitosan-PEG-poly(N-isopropylacrylamide) hydrogels.

Our prior study has shown that polyethylene glycol (PEG) played a crucial role in improving the properties of the physically crosslinked chitosan-PEG-poly(N-isopropylacrylamide) (PNIPAAm) hydrogels. In this paper, we further investigated the effect of the molecular weight (MW) of PEG on the properties of the chitosan-based physical hydrogels. Fourier Transform Infrared Spectroscopy (FTIR) study showed that the interaction between PEG and other components in the physically crosslinked hydrogels became stronger as the MW of PEG increased. The wide angle X-ray diffraction (WAXD) study indicated that the crystallinity of the physical hydrogels decreased with an increase in the MW of PEG. The thermal study using differential scanning calorimetry (DSC) revealed the crystallizability of the physical hydrogels first reduced with an increase in the MW of PEG, but slightly increased thereafter with a further increase in the MW of PEG. The swelling test showed the water uptake capability of the physical hydrogels increased with an increase in the MW of PEG. The results obtained by scanning electron microscope (SEM) found that the morphological changes of the physical hydrogels with MW of PEG were consistent with the results of swelling and thermal properties; and, contrary to pure PNIPAAm hydrogels which showed a compact and dense network structure at a temperature (37 degrees C) above its LCST, the physical chitosan-PEG-PNIPAAm hydrogels exhibited porous network structure at 37 degrees C instead. The mechanical property of the physical hydrogels was initially increased with an increase in PEG MW, but deteriorated with a further increase in PEG MW. Therefore, the MW of PEG played a key role in controlling the property of the chitosan-based physical hydrogels.