Preparation And Phase Separation Of Polyvinyl Alcohol Grafted With Short Alkyl Chains

Polyvinyl alcohol（PVA）is a massive chemical that combines the advantages of good biocompatibility and microbial degradation.To improve the water resistance and processability of polyvinyl alcohol,PVA needs to be modified with appandent alkyl side chain.The bulk properties of PVA derivatives strongly depend on the molecular structures and the high-order condensed structures formed during processing.Systematically regulating the molecular structure of polyvinyl alcohol derivatives and their evolution to condensed structure is important to clarify the structure-property relationship.Therefore,the main research contents of this thesis can be divided into three sections as follows:（1）A series of PVA derivatives with different carbon chain lengths are prepared by post-modification method,and the influence of preparation conditions on the molecular structure of the products is studied;（2）These PVA derivatives are formed into films by non-solvent-induced phase separation,and the molecular structure and phase separation morphology,as well as viscoelasticity are studied;finally,（3）Polyvinyl butyral（PVB）,which is commonly used in industry,is selected as the research object.We dissolve it in a toluene-ethanol mixed solution,and then conduct phase separation studies The research work is specified as follows:（1）We obtain polyvinyl alcohol derivatives with a range of different hydroxyl substitution rates by some specific operations.These operations includes grafting short alkane chains with different carbon numbers on PVA by Mitsunobu reaction or acetal reaction,and controlling the degree of substitution（DS）by controlling the reaction conditions such as the feeding ratio,temperatures,and reaction times,and so on.The carbon number of the alkyl side chain of this series of derivatives ranges form 3 to 8,and the degree of substitution is fixed to be 0.5.（2）By controlling the length of the short alkyl side chain（Cn）,a series of alkyl side chain modified hydrogels（ASHs）with various mechanical properties are constructed,the Cn dependence of the microstructure of ASHs is investigated and the effect on the phase separation process and morphology is revealed.We investigate the important role of the polymer-rich phase in the dynamics,focusing on the stretch rate and temperature dependence of large and small deformation properties.The results show that the domains of the polymer-rich phase thicken because the hydrophobic interactions are intensified as the length of the side chains increases.Rheology shows that the longer alkyl side chains,the higher the glassy transition temperature and the slower kinetics.In addition,by correlating the stretching rate and temperature dependence of large and small deformation properties,we find when the tensile test temperature of the hydrogel is close to the glassy transition temperature,or in the case of an intermediate frequency state,the stretching rate is matched to the relaxation time,the hydrogel become tough regardless of the length of the side chains.（3）On the basis of the above research,we further study the dissolution of polyvinyl butyral（PVB）,a class of short alkyl chain modified polyvinyl alcohol,which is widely used in multilayer ceramic capacitors（MLCC）binders.We clarify the influence mechanism of factors such as the relative humidity of the film-forming atmosphere on the PVB conformation in the solution and the microphase separation structure of the condensed membrane.We find that ethanol and toluene are good and poor solvents for PVB,respectivley,but the best results are achieved when they are mixed.In addition,film formation at higher humidity causes defects,but the defects do not affect the effective modulus and thermodynamic properties of PVB films.In summary,this thesis investigates the polyvinyl alcohol modified with hydrophobic short alkyl side chains,and explores the effect of the side chain length on its phase separation process,kinetics and mechanical properties.Furthermore,as a specific example of short alkyl side chain modified PVA series,the dissovation behavior of PVB also is investigated.