取代基上官能团位置对苄基壳聚糖衍生物液晶性的影响

Influence of the Position of Functional Groups in Substituents on Liquid Crystalline Behavior of N-benzyl Chitosan Derivatives

合成了4种N-苄基化壳聚糖衍生物,即N-(2-羟基苄基)壳聚糖、N-(3-羟基苄基)壳聚糖、N-(4-羟基苄基)壳聚糖和N-(3-甲氧基-4-羟基苄基)壳聚糖(分别简写为NOCS、NMCS、NPCS和NMPCS),它们的取代基上官能团的数目和位置不同.N-取代度相近,分别为0.72,0.62,0.71和0.68.衍生物溶解在甲酸溶液中均呈现胆甾型溶致液晶相.用偏光显微镜法和折射率法测得各衍生物的液晶临界浓度w分别为28%,37%,24%和27%,均比纯壳聚糖的12%有很大提高,因为取代基的引入破坏了壳聚糖分子内与分子间非常强烈的氢键作用.通过分子模拟,对4种壳聚糖衍生物分别从分子内氢键作用与分子间氢键作用两个方面进行了比较.在分子内氢键方面,NOCS较强,NMCS、NPCS和NMPCS均较弱.在分子间氢键方面,NOCS较弱,NMCS居中,NPCS与NMPCS较强.将分子内或分子间氢键作用与临界浓度相联系,可见分子内氢键强(如NOCS)或者分子间氢键强(如NPCS,NMPCS),都显著地提高了分子链的刚性,分子链的排列与取向更加规整,因而降低了壳聚糖衍生物的液晶临界浓度,这两个因素只要具备一个即可.若两者都不强(如NMCS),则分子链的刚性较小,临界浓度明显较高(37%).对于该体系,氢键的强弱对液晶临界浓度有着决定性的影响.

Four N-benzyl chitosan derivatives, i. e. N-（2-hydroxyl-benzyl） chitosan （NOCS）, N-（3-hydroxyl-benzyl） chitosan （NMCS） ,N-（4-hydroxyl-benzyl）chitosan （NPCS） and N-（3-methoxyl-4-hydroxyl-benzyl）chitosan （NMPCS）,were synthesized,the number and position of the functional groups in the substituents of which were different. The degrees of NOCS, NMCS, NPCS and NMPCS were close to each other, 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. The critical concentrations of NOCS,NMCS, NPCS and NMPCS were measured to be 28% ,370% ,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. In the aspect of the intra-molecular hydrogen bonding, NOCS was stronger, but NMCS,NPCS and NMPCS were weaker. In the aspect of the intermolecular hydrogen bonding, NOCS was weaker, NMCS was middle,but NPCS and NMPCS were stronger. Relating the critical concentration with the hydrogen bonding interaction,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.