Probing the Dynamics of Water in Chitosan Polymer Films by Dielectric Spectroscopy P. Murugaraj, D. E. Mainwaring, D. C. Tonkin, M. Al Kobaisi School of Applied Sciences, College of Science, Engineering and Health, Royal Melbourne Institute of Technology, Melbourne 3001, Australia Received 2 August 2010; accepted 9 August 2010 DOI 10.1002/app.33163 Published online 23 November 2010 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: Chitosan biopolymers are increasingly being used in advanced biomedical applications, where aqueous interactions profoundly influence their physical properties and also their in vivo biomolecular and cellular activity. Here, hydration of chitosan films is studied by dielectric spectroscopy in a conventional constraining plate configuration and compared with free standing films. Film hydration proceeds by an initial water uptake followed by a spontaneous dehydration (deswelling) even in saturated atmospheres. At water contents above a critical value,  9.5 wt % a dielectric loss resonance peak (b wet ) arises from relaxation of evolving chitosan–water complexes, below this value insufficient interchain space for oscilla- tion of these complexes prevents b wet appearing. The b wet frequency was related to water content by a power law with the frequency changing by  3 orders of magnitude. Importantly the scaling exponents (slopes) differed signifi- cantly for unconstrained (free standing) and volume con- strained films indicating the effect of internal stresses in constrained films. Both dielectric and conductivity behav- ior were influenced by internal constraining stresses affect- ing both oscillatory motion and charge mobility. In biomedical devices, biopolymers may be free standing, surface adhered, or enclosed structures imposing different internal stresses on polymer chains and the mobility of their segments. Dielectric spectroscopy can examine these influences on dielectric and electrical characteristics, which play a critical role in biomolecular interactions. V C 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1307–1315, 2011 Key words: biopolymers; chitosan; dielectric properties; spontaneous deswelling; conducting polymers INTRODUCTION The poly(aminosaccharide) chitosan, (1!4)-2-amino- 2-deoxy-(D-glucose), is the N-deacetylated derivative of natural chitin. Chitosan shares the b-(1! 4) linked glucopyranose residues with cellulose. Next to cellulose, chitin represents the second most abundant polysaccharide biopolymer. Chitosan is increasingly being used in advanced biomedical applications, such as drug delivery, tissue engi- neering scaffolds, wound dressings, skin graft tem- plates, hemostatic agents, and hemodialysis mem- branes as well as general advanced technologies such as ionic conductors. As a biomaterial, chito- san has excellent mechanical strength, biocompati- bility, and nontoxicity. 1,2 Recently, a novel etched composite scaffold of chitosan-gelatin reinforced by a poly(lactic-co-glycolic acid) (PLGA) mem- brane has been explored for its cell growth and mechanical stress relaxation. 2 The dielectric properties of polysaccharides, carry- ing two hydroxyl groups and one methylol group, have been widely investigated in the past. 3,4 Dielec- tric relaxation spectroscopy probes the chain dynam- ics in polymers, separating contributions from differ- ent molecular groups of a repeating unit with respect to the rates of their orientation dynamics. 5 Many modes of relaxation in dielectric spectros- copy have been reported for polysaccharides and their derivatives to occur over wide ranges of fre- quency and temperature. a relaxation represents the major dielectric relaxation in materials with perma- nent dipoles where its origin lies in segmental motions. Whereas b relaxation is the main process found in all polysaccharides at low temperatures (135  C to þ20  C) being related to segmental motion of the chains via the glucosidic bond, and therefore, it corresponds to local chain dynamics appearing just above 10 6 Hz. Thus, a and b relaxations have different molecular length scales although interre- lated because the same dipoles usually contribute to both processes, hence b wet relaxations merge at high temperature (and high frequency) to give rise to an ab process. 6,7 Polysaccharides containing low levels of residual water show an additional relaxation Correspondence to: P. Murugaraj (pandiyan.murugaraj@ rmit.edu.au). Contract grant sponsors: Australian Cooperative Research Centre for Polymers (CRC-P) (Chitosan Chemistry Program). Journal of Applied Polymer Science, Vol. 120, 1307–1315 (2011) V C 2010 Wiley Periodicals, Inc.