Preliminary Studies Of Molecular Simulation On The Liquid Crystal Behaviors And Glass Transition Temperatures Of N-benzyl Chitosan Derivatives

The structure unit of chitosan contains-NH₂ and-OH,it can be a variety of chemical modification,such a wealth of chemical modification methods provide a very good condition for the study of the liquid crystalline property of chitosan and its derivatives.Researchs use it to study the impact of molecular weight,degree of substitution,substituent size and other structural factors to the lyotropic liquid crystal behavior of chitosan and its derivatives have been reported,but the study of the influence of the position of functional groups in substituents on liquid crystalline behavior of N-benzyl chitosan derivatives has not yet been reported.In this paper,we change the position of functional group in substituents and synthesis a series of chitosan derivatives to study the influence of it on the lyotropic liquid crystal property. We use molecular simulation to study the the relationship between structure and properties of chitosan derivatives.According to the composition of the polymer chain structure to predict the properties of the polymer,it has great significance on how to choose,modify and design polymer products in industrial applications.In this paper,we use chitosan and aryl-aldehyde reaction to synthesis the N-benzyl chitosan derivatives,characterize the structure of them through NMR, infrared spectroscopy,ultraviolet spectroscopy and fluorescence spectroscopy.Using polarized light microscopy method and refractive index method to determin the liquid crystalline critical concentration of them.Using molecular simulation method to simulate the inter-molecular and intra-molecular hydrogen bonding interaction with molecular dynamics simulation,study the relationship between the hydrogen bonding interaction and liquid crystalline critical concentration.By molecular simulation,we calculate the amorphous polymer unit's running average specific volume at different temperatures,and then draw V-T curve to obtain the glass transition temperature(Tg) of N-benzyl chitosan derivatives,compared with the experimental value of Tg which measured by DSC.By the molecular mechanics and molecular dynamics simulation of DOBOB dendrimer grafted chitosan liquid crystalline columnar self-assembly molecules,we got the optimized structure and measured the diameter of their column. We also want to study the relationship between the structure of dendrimers and the diameter of the columns.By the molecular dynamics simulation of bowlic molecule CTV grafted polymer molecular self-assembly structure,analysis the reason why CTV small molecule's mesophase belong to hexagonal columnar phase(Φ_h) but the grafted polymer not belong to that.We synthesized four N-benzyl chitosan derivatives in experiment successfully, the number and position of the functional groups in the substituents of which were different.Their degree of N-substitution werw close,which were 0.72,0.62,0.71 and 0.68 respectively.The lyotropic cholesteic liquid crystalline phase was observed in the formic acid solutions of the four chitosan derivatives.Their critical concentration (w/w) were measured to be 28%,37%,24%and 27%respectively,by means of both polarized optical microscopy and refractive index method.These values were all much higher than chitosan itself(12%),because the very strong hydrogen bonding interactions of chitosan were broken by the substituent groups.The molecular simulation approach was used to compare the intra-molecular hydrogen bonding interaction and the inter-molecular hydrogen bonding interaction of these four derivatives respectively.It can be seen that if the derivative have a stronger intra-molecular hydrogen bonding(such as NOCS) or a stronger inter-molecular hydrogen bonding(such as NPCS and NMPCS),the rigidity of the molecular chain is significantly improved,the arrangment and orientation of the molecular chain are more regular,as a results,the values of the critical concentration decrease.These two factors require just only one.On the contrary,if both of the two interactions are weaker(such as NMCS),the rigidity of the molecular chain is smaller;the value of the critical concentration becomes evidently higher(37%).For this system,the influence of the strength of the hydrogen bonding on the liquid crystalline critical concentration is definitive.The Tg predicted by molecular dynamics simulation (NOCS:134℃,NMCS:130℃,NPCS:135℃,NMPCS:139℃) is similar with the experimental value(NOCS:131℃,NMCS:129℃,NPCS:138℃,NMPCS:143℃) measureed by DSC,so we think the molecular simulation is a valuable method to predict the Tg of polymers.By molecular simulation measured the diameter of liquid crystalline self-assembly columnar molecules:DOBOB-CS:4.8nm,DOVOB-CS: 5.8nm,DOB-CS:3.1nm,BOB-CS:4.2nm,the diameter of this column can be self-assembled into hexagonal columnar phase(Φ_h).we can see DOBOB flexible elements in the alkyl chain contribute largest column diameter from the simulation results.We made a molecular dynamics simulation of the methyl acrylate-based polymer which contained the bowlic CTV as side group.Result show that if the "bowl" connect the main chain with short alkyl chain just like(CH₂)₂,the bowl's volume is too large leading the main chain to apparent bending,loss the symmetry of the molecular chain;when the "bowl" connect the main chain with long alkyl chain just like(CH₂)₁₁,Polymer main chain is more extended,though there is still some bending,but the side of the base of the "bowl" has been able to self-assembled into a column,However,in experiments BPM did not have a columnar phase,the liquid crystalline property was also lose.It means that there need to connect a longer chain to come to the realization of the main chain stretching and CTV bowl self-assembly,it also needs more experiments to further prove.We used the molecular simulation to simulate the chitosan chain with mechanics and dynamics method to predict its properties.The experimental results are used to verfify the simulation results.After the experiment verified,the molecular simulation method has a certain degree of reliability,can be used to guide laboratory synthesis,it can predict the properties of some chitosan derivatives that not synthesized yet.Use it to Molecular structure design,can reduce test cost and improve the efficiency of scientific research.