Origin and influence of water-induced chain relaxation phenomena in chitosan biopolymers

Chain dynamics, probed by dielectric spectroscopy, provide a route to further understanding of the molecular interactions induced by hydration, degree of crosslinking, and microstructural changes occurring on swelling of biopolymers such as chitosan, which is becoming a focus for biomedical engineering and therapeutic delivery. The basis of the ß-wet relaxation peak is established as segmental chain relaxation between chitosan water bridges and related to its hysteresis induced by microstructural changes during wetting and dewetting cycles. Linear expansion probes the hysteresis arising from bridging water interactions during the hydration-dehydration paths which is also shown in the resultant ionic conductivity. ß-wet relaxation and ionic conductivity exhibit identical hysteresis behavior with both degrees of chemical crosslinking and water contents. X-ray diffraction shows that the degree of crosslinking and hydration also influences the degree of disorder of the polymer chains changing both the crystalline phase fraction and lattice dimensions. These molecular interactions provide power law behavior between ß-wet relaxation dynamics and ionic mobility over five orders of magnitude for all degrees of chemical crosslinking and water bridging which is independent of the significant hysteresis in these properties indicative of scaling behavior within the noncrystalline gel phase.