Characterization And Application Of Temperature Responsiveness Polyvinyl Alcohol Derivatives

Polyvinyl alcohol?PVA?hydrogel is an intriguing hydrogel,which has been investigated as a promising substitute for repairing diseased or damaged articular cartilages,meniscuses and tendons due to its excellent biocompatibility,low toxicity and water absorption abilities.However,the poor mechanical and water-retention properties of pure PVA hydrogels have hindered their applications.There are two effective ways to solve these problems.One is to integrate PVA hydrogel with other materials to form composite hydrogels,and the other is chemical modification.In this paper,we obtained a hydrogel with a temperature-responsive,self-repairing property,a polyvinyl alcohol derivative that can be adjusted to a gelatinization temperature by pH and polymer concentration by post-synthesis modification and introduction of borax crosslinks;The resulting hydrogel has relatively poor mechanical properties and its tensile stress is in the KPa range,which greatly limits its application.On this basis,we obtained a highly stretchable,tough but hard,self-healing hydrogel by grafting a short alkyl carbon chain and hydrothermal treatment.The enhancement of the hydrogel toughness relies on the hydrophobic cluster toughening mechanism,which has multiple strengths.More specifically,the work of this paper includes the following two parts:First,commercialized polyvinyl alcohol was used as the grafting object,and N,N-Diethylethylenediamine?DEEDA?was obtained by post-synthesis modification method on the polyvinyl alcohol side linking branch to obtain PVA-DEEDA.The new dynamic thermal gel system PVA-DEEDA-Borax was directly synthesized by adding borax to the PVA-DEEDA solution.¹¹B nuclear magnetic resonance?NMR?results showed that cross-linking occurred between PVA-DEEDA and borax.The gelatinization temperature at different concentrations and different pH was investigated using the vial inversion method.The PVA-DEEDA-Borax hydrogel was found to have a highly adjustable gel-sol transition temperature(T_sol-gel)with an adjustable range of 30 to 65 ?.The self-healing properties were studied using a rotational rheometer,and dynamic step strain amplitude tests of 1%and500%for 4 cycles were performed.After four cycles of testing,the hydrogel remained intact and almost maintained its initial mechanical properties.PVA-DEEDA-Borax is injected into40 ? water to form a gel and effectively capture the target load.Carbon dioxide can easily cause the release of the payload.Second,the-OH group on the polyvinyl alcohol?PVA?was partially substituted with n-hexylamine?C6?to obtain PVA-C6,which was subjected to solvent exchange and hydrothermal treatment to obtain PVA-C6-gel.The relative intensity of the XRD characteristic peak of PVA-C6-gel at 2?=4.1^o is significantly higher than that of PVA-C6-as,and the enhanced XRD peak indicates an increase in the average number of alkyl chains involved in the cross-linking.PVA-C6-gel did not exhibit a clear melting behavior by differential scanning calorimetry?DSC?testing.The increase in the number of hexyl clusters after hydrothermal treatment effectively makes the hydrogel hard and tough.The fracture stress??_f?of PVA-C6-gel is 3.9±0.3 MPa,the strain at break??_f?is 333±17%,and the elastic modulus?E?is as high as 21.7±0.2 MPa at normal strain rate(0.17 s^-1<1/?_e)has a high fracture energy of 32±1.1 kJ m^-2,and PVA-C6-gel exhibits good durability against fatigue damage at 50%strain.The average activation energy E_a of PVA-C6-gel is 230 kJ/mol?93k_BT?.This value is lower than the dissociation energy of the covalent bond?350 kJ/mol,140k_BT?,but much higher than the single hydrophobic association energy composed of two hexyl alkyl chains?10 k_BT?.Therefore,there must be dynamic alkyl chain cluster formation during the synthesis of PVA-C6-gel.The self-healing behavior of PVA-C6-gel was studied by cyclic loading-unloading tensile test.Finally,we also demonstrated that PVA-C6-gel has good chemical stability and self-healing properties,as well as shape memory properties.