Preparation And Properties Of Modified Polyvinyl Alcohol Materials Obtained Through Post-reaction Method

The post-reaction modification method can enhance the performance of the polymer materials,endow them with some new properties and broaden their application areas.Polyvinyl alcohol?PVA?,as one of the synthetic water-soluble polymer materials,is widely modified because of its excellent mechanical performance,gas barrier property,biocompatibility and high reactivity of hydroxyl groups on the molecular chains.In this paper,photo-responsive azobenzene compound and hydrophilic L-dopa was grafted to PVA by esterification.The product was fully characterized.And the modified PVA has the characteristics of photoisomerization and water solubility at low temperature.The main work is summarized as follows:1.Azobenzene-containing long-chain small molecules?CH₃-O-Az5-COCl,both the cis-and the trans-isomer are very stable?with five flexible methylene groups was prepared through diazotization,electrophilic addition,substitution and chloroformylation reaction.And the modified PVA with photo-responsiveness was obtained through grafting CH₃-O-Az5-COCl onto PVA macromolecular chains by esterification reaction.Fourier transform infrared?FTIR?,nuclear magnetic spectrum?1H NMR?,differential scanning calorimetry?DSC?,thermogravimetric analysis?TG?and ultraviolet-visible spectroscopy?UV-Vis?were used to characterize the composition,photo-responsiveness and thermal properties of azobenzene-containing small molecules and modified PVA.The UV-Vis results verified the photoresponsiveness of the modified polyvinyl alcohol.The DSC and TG results showed that the hydrogen bond between the molecules of polyvinyl alcohol was destroyed by introducing CH₃-O-Az5-COCl.And then,the movement of PVA molecular chain was promoted and the crystal structure was weakened.At the same time,new stronger hydrogen bond between azobenzene-containing small molecules and hydroxyl on PVA chains was formed.Therefore,the melting point of PVA was reduced while its thermal stability was improved.2.As we all know,PVA-1799 can only be dissolved in hot water?95??.When hydrophilic 3-hydroxy-L-tyrosine?L-dopa?was grafted onto PVA chains through one-step esterification,the modified PVA with water-soluble property at low temperature?10 ??was obtained.FTIR,1H NMR,DSC and TG were used to investigate the chemical structure and thermal properties of the ultimate product.The water solubility at low temperature of the modified PVA was confirmed by water solubility test.The DSC and TG results showed that the structure was transferred from crystalline to amorphous and the melting point of PVA was decreased by 80 ? due to introducing L-dopa.At the same time,the stronger hydrogen bond between the hydroxyl and carboxyl groups on the L-dopa and the hydroxyl groups on PVA was formed.And then,the hydroxyl groups on modified PVA would be removed at higher temperature.However,part decomposition occurred due to the oxidation of odihydroxybenzene.