Incoherent quasi-elastic neutron scattering study of chemical hydrogels based on poly(vinyl alcohol)
Introduction
Studies on the behaviour of water in hydrogels are receiving increasing attention in the scientific community because of the basic interest of this subject and the possibility of tailoring hydrogels as devices for controlled drugs delivery, for guided tissutal regeneration and responsive biomaterials [1], [2], [3]. One striking feature of these materials is their ability to retain water up to 99% of their total weight. The swelling phenomenon has been studied from the thermodynamic standpoint extensively; yet the microscopic, structural and dynamic behaviour of water and its relationship with the physical and chemical characteristics of the polymer network are issues that need to be further elucidated.
Recently, we have focussed on the synthesis and the characterization of a set of chemical hydrogels based on poly(vinyl alcohol) [4], [5].
Poly(vinyl alcohol) (PVA), is a synthetic biocompatible polymer already used in surgery as thread for stiches. The simultaneous presence of alkyl and hydroxyl moieties in its structure imparts to the polymer a dual behaviour toward water molecules in analogy with random uncharged polysaccharides, a class of natural polymers highly coordinating water, which are of great relevance to food science.
Changes in the dynamic features of water caged in the polymeric network with respect to bulk water can be diagnostic for forecasting the behaviour of a hydrogel and its responsiveness to the external parameters. It is known that PVA gives physical hydrogels by freeze-thaw cycles, and thorough studies of this system have been reported in the literature [6], [7]. Here we wish to report on a chemical hydrogel based on PVA using glutaraldehyde as cross-linker. One of the features of this type of gel is the thermal stability of the hydrogel and its good stability in acidic medium. A schematic representation of this polymeric network is given in Fig. 1.
Low resolution proton pulsed NMR on similar matrices has been studied in our laboratory with the indication of a strong interaction of water with the polymer chains. This effect determines a non-exponential magnetization decay in terms of both the spin–lattice and the spin–spin relaxation mechanism [5]. The analysis of the curves evidenced the presence of different water domains characterized by 2 or 3 time constants. This information, together with the possibility to carry out molecular dynamic simulations for the PVA network/water system with the computational tools available nowadays, prompted us to use the incoherent quasi-elastic neutron scattering technique in order to characterize at molecular level the diffusion processes with correlation times of some picoseconds and length scale values of few Ångstroms. In this context, with the high resolution backscattering spectrometer IRIS at the pulsed neutron source ISIS it was possible to obtain information on the solvent and to some extent on the polymer network as well.
In this study we aimed at structural and dynamic information of both the components present in the hydrogel, i.e. water and polymer network.
Section snippets
Materials
PVA Was a Sigma product (USA) used without further purification. The number average molecular weight, Mn, determined by membrane osmometry was 30 000 (g/mol). A deacetilation degree of 99% was evaluated by 13C NMR in D2O solution
Aqueous glutaraldehyde 25% (w/v) “microscopy grade” was a Sigma product. Reagent grade HCl (Carlo Erba, Italy) was used as a catalyst for the cross-linking reaction.
Milli-Q water (Millipore, USA) was used for the preparation of all the samples.
Methods
Determination of the number
Results and discussion
Large incoherent cross-section of proton and characteristic spatial and frequency ranges place incoherent QENS in a useful region for investigating the microscopic dynamics of water and of polymer chains in hydrogels. The scattering of neutrons is essentially governed by the presence of the protons of solvent as the main constituent of these systems is water. Other contributions arising from the protons of the second constituent, i.e. the polymer moiety, may be also detected depending on
Concluding remarks
In this work we have investigated the dynamic properties of a hydrogel of PVA cross-linked with glutaraldehyde. We found that water in this system is super cooled and the dynamic parameters are in close analogy to that found for water present in polysaccharidic hydrogels. We also made some estimation regarding the segmental dynamics of the polymer network at temperatures 330 and 350 K, showing that in favourable cases an incoherent QENS experiment can yield information simultaneously on both the
Acknowledgements
We thank Prof. A. Deriu for his constant suggestions and encouragement during all the steps of this work.
References (18)
- et al.
J. Contr. Rel.
(1992) - et al.
Physica B
(1992) - et al.
Biophys. Chem.
(1994) - et al.
Biophys. J.
(1999) - R.M. Ottenbrite, S.J. Huang, K. Park (Eds.), Hydrogels and Biodegradable Polymers for Bioapplications, American...
- O. Kramer (Ed.), Biological and Synthetic Polymer Networks, Elsevier, New York,...
- W. Burchard, S.B. Ross-Murphy (Eds.), Physical Networks, Polymers and Gels, Elsevier, New York,...
- et al.
J. Polym. Sci. Part A: Polym. Chem.
(1996) - et al.
J. Polym. Sci. Part B: Polym. Phys.
(1999)
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