Controlled Preparation Of Polysaccharide-Based Physical Hydrogels And Their Multi-Responsive Behaviors

The characteristics of methyl cellulose (MC), sodium alginate (SA) and polyvinylalcohol (PVA) were employed to prepare MC-and SA-based physical hydrogel. Thestimuli-responsibility of the hydrogels was investigated.Firstly, esterification reaction between methyl cellulose and maleic anhydride wascarried out to incoporate the anionic carboxylic group onto MC backbones.Thestructure of carboxylated methyl cellulose (MMC) was verified with FTIR, XRD, andUV spectroscopy. The analyses results indicated the structure of MMC depended onits carboxyl group percentages. MMC/PVA hydrogel which prepared byfreezing-thawing cycles not only exhibited temperature-sensitivity, but also showedpH-sensitivity because of the existed anionic groups. The Rhodamine B encapsulatedMMC/PVA hydrogels exhibited different release behaviors in simulated body fluid(pH=7.4) and simulated gastric fluid (pH=1.2), which was in accordance with thepH-sensitivity of MMC/PVA hydrogels.Reversible addition-fragmentation chain transfer (RAFT) polymerization and Clickchemistry were adopted to systhesis methyl cellulose covalently bonded withwell-defined poly(vinly acetate)(PVAc), and represented as MCVAc. Gel permeationchromatography (GPC) ananlysis results indicated the molecular weight distributionof PVAc side chains was narrow (PDI<1.3). FTIR and1HNMR spectra were utilizedto verify the structure of MCVAc. Amphiphilic MCVAc self-assemblyed and formedmicrogels in aqueous solution. The morphology of MCVAc microspheres wasobserved with transmission electron microscope (TEM). It was found that the sizedistribution of MCVAc microgels was relied on the chain length of PVAc. Theaverage size of microgels for the side chains with number average molecular of49312,32513and18185g/mol were105.6,95.9and75.87nm respectively.Sodium alginate-based physical hydrogel was also examined in this paper. in orderto overcome the low-strength and easily disruption of Ca(II) ions crosslinkedSA-based hydrogels, the radical graft copolymerization was applied to introduce PVA gelator onto the backbones of SA. The structure of copolymer SA-g-PVA was verifiedwith FTIR, XRD, and UV spectroscopy. SA-g-PVA hydrogels exhibited pH-andionic strength-sensitivity, which were controlled by the content of PVA contained inSA-g-PVA.The length of PVA side chains was a key factor affected the characteristic of thecovalently bonded product of SA and PVA (SAVA). In viewing of this, we adoptd theRAFT polymerization and Click chemistry combination strategy to prepare SAVAhydrogel with well-defined side chains. Thermogravimetric analysis (TGA) andX-ray diffraction (XRD) analysis results indicated the physical crosslinked networkwas controlled by the length of side chains. The longer the chains was, the morecomplicated the crosslinked network was, which led to the crystallization capacitydeclined and heat-resistance increased. It was found that SAVA hydrogel exhibitedpH-sensitive swelling, which relied on the length of PVA chains as well.In a word, a series of physical hydrogels behaved multi-responsive properties wereprepared by taking advantage of the inherent properties of MC, SA and PVA such asthe temperature-sentitive sol-gel phrase transition, reversibleprotonation-deprotonation of anionic polyelectrolyte and physically cross-linkingunder mild conditions.