Bio-based Polyalcohol Reinforced And Toughened Polyvinyl Alcohol And Its Molecular Mechanism

Since the middle of the 20th century,global plastic production has been increasing rapidly,which brings convenience to people but causes a large number of non-degradable plastic pollution.Therefore,the task of developing high-performance,biodegradable polymer materials to replace traditional non-degradable polymer materials is more and more urgent.Polyvinyl alcohol(PVA)has good biocompatibility,gas barrier and mechanical properties is an important renewable,biodegradable polymer material,which can be used in drug delivery,separation membrane,packaging materials,tissue engineering,aerospace materials,and many other fields.However,its application is still limited by its insufficient mechanical properties and slow natural degradation property.The purpose of this study is to design new polymer materials with high mechanical properties,high transparency,and biodegradability based on the novel hydrogen bond cross-linking strategy,and to study the mechanical mechanism of the hydrogen bond crosslinking strategy.In this paper,saccharide polyols of different sizes were selected to enhance the polyvinyl alcohol film.The main results are as follows:Firstly,three cyclic polysaccharide biobased polyol molecules(?,?,?-cyclodextrin)were selected to strengthen and toughen PVA.Among the three composite films prepared by the casting film method,PVA/?-cyclodextrin(PVA/?-CD)showed the highest tensile strength(136.5 MPa),which was increased by 21%compared with pure PVA,with fracture toughness of 82.1 MJ/m3,four times higher than pure PVA.UV-vis light tests and natural degradation tests showed that PVA/?-CD composite film had high visible light transparency and good natural degradation performance.For example,the mass loss rate of 5 wt%?-CD composite film increased by 68%compare with PVA after 60 days.Then,rheological,IR,DSC,DMA and other characterization methods were used to study the physical properties of the films in detail.The results showed that there was a strong multiple hydrogen bond interaction between ?-CD and PVA,which formed a physical crosslinking network,resulting in limited molecular chain motion,increased Tg,and improved the equivalent crosslinking density(ne)of the system.The grain size(L)of PVA is reduced,which provides higher strength,modulus and ductility for PVA/?-CD composite films.Secondly,two kinds of disaccharide polyols(maltose MAL and trehalose TH)were selected to reinforce and toughen PVA respectively.Also,mechanical tests were done on the two kinds of composite films prepared by the casting and drying method.Both composite films have more significant mechanical enhancement than PVA/?-CD,but the toughening effect is decreased.The PVA/MAL composite film shows higher tensile strength than PVA/TH.When the content of MAL is 0.5-5 wt%,the PVA/MAL system shows both improved strength and toughness,The strength is 149±2 MPa,and the toughness is also 2-4 times that of PVA.Compared with PVA,the tensile strength is increased by 51.8%,but the fracture toughness is significantly reduced.UV-vis and soil burial tests showed that the PVA/MAL composite film had excellent transparency and better natural degradation performance than PVA/?-CD.The 60 day natural degradation rate of MAL-5 was 90%higher than that of PVA.Then,FTIR,rheology,DMA,XRD,and other tests were used to study the mechanical strengthening mechanism of PVA/MAL system.The results show that there is a physical crosslinking network induced by multiple hydrogen bonds between MAL and PVA,which leads to the increase of glass transition temperature,the increase of the system crosslinking density,and the decrease of average PVA grain size(L).However,the smaller molecular size of MAL has a more significant plasticizing effect compared with ?-CD.and the plasticizing effect of MAL on the storage modulus and molecular chain motion is particularly obvious at high MAL content,which also shown as the significant decrease of TgDMA and ne,Young's modulus,the increase of the loss modulus and internal friction.In addition,the tensile strength and modulus of the composite films show a significant positive correlation with the ne and the relative hydrogen bond density of the system,while the fracture toughness has a negative correlation with L at a low MAL content.Finally,a simple monosaccharide biobased polyol small molecule inositol(IN)was selected to strengthen and toughen PVA.The prepared PVA/IN composite film can improve the mechanical strength and fracture toughness at the same time.In terms of reinforcement,the optimum IN content is 1 wt%,and the strength is 148.2 MPa(increased by 31%),the ductility was increased by 250%,the fracture toughness was 93.4 MJ/m3(4.6 times of pure PVA),and the maximum fracture toughness was 106 MJ/m3(5.2 times of PVA)when In content was 5 wt%.The results of UV-vis test and natural degradation experiment showed that the PVA/IN system still had excellent transparency and better natural degradation performance than pure PVA.Results show that multiple hydrogen bonding between PVA and IN form crosslinking network that creates a limitation on the molecular chain movement.Small size IN molecular has more significant plasticizing effect than MAL and ?-CD.Under the joint influence of the two,the Tg,ne of PVA/IN showed a trend of increased first then decreased,and L showed a trend of decreased first then increased.When IN content is 1 wt%,the tensile strength and modulus reached the highest,Tg and the relaxation activation energy(Ea)(121.7 KJ/mol)also reach maximum value.Meanwhile,the correlation length(?)(16 nm)in the concentrated solution,the diffusion coefficient(D)(10^-7.852 cm2.s-1)of the PVA molecular chain are the smallest in IN content of 1 wt%.It is seen that the molecular mechanism of hydrogen bond crosslinking strategy is the multiple hydrogen bond interaction between polyols and PVA,and the H-bond physical crosslinking method increases ne of PVA,which reduces the movement ability of the PVA chain,and provides additional resistance to stress and deformation for the material.These similar trends further confirm the positive correlation between tensile strength and modulus and ne,Ea,glass transition temperature(Tg),reciprocal of average grain size(1/L),reciprocal of diffusion coefficient(1/D).Finally,the semi-quantitative relationship between strengthening and toughening effects of composite films(??/?)and(??/?p)and ne and L is established:??/?0.93?ne/ne,p;??/?p ?12?ne/ne,p;??/?p ?20?L/Lp...