苯并噁嗪热固化过程中体积变化的研究

STUDY ON THE VOLUMETRIC CHANGE DURING RING- OPENING POLYMERIZATION OF BENZOXAZINES

采用不同的测试方法 ,对双酚A型苯并嗪和两种苯并嗪模型化合物热聚合体系固化过程中的体积变化从表观体积收缩率、密度 固化时间曲线、恒温固化收缩率以及密度 温度曲线几方面进行了研究 ,深入探讨了苯并嗪在固化过程中是否发生体积膨胀 ,并着重讨论了不同酚核结构苯并嗪开环聚合过程中体积变化的差异及影响因素 .结果表明 ,不同结构的苯并嗪均呈现出宏观体积膨胀效应 ,但在恒温固化过程中却均呈现体积收缩 ,且这种体积变化的大小与苯并嗪的分子结构。

Benzoxazine\|based phenolic resin has recently attracted a great deal of attention due to its versatile properties.One of its unique properties is volumetric expansion or near\|zero shrinkage upon polymerization.However,the cause for the expansion or near\|zero shrinkage is not well understood.In this paper,systematic investigation on the cure shrinkage of three benzoxazines based on bisphenol A,p\|cresol and 2,4\|dichlorophenol respectively has been made.This research has laid particular emphasis on the effect of the phenlic portion of the benzoxazine molecule on volumetric expansionl,wich is different from that of Ishida's work,namely,the effect of the amine moiety of the benzoxazine molecule on volumetric expansion.The cure reactions of these benzoxazines are carried out without the aid of catalysts above 140℃.By means of gravimetric and dilatometric methods etc.,the studies on volumetric shrinkage,density versus cure time plots,isothermal cure shrinkage and density versus temperature plots have been performed.The cure shrinkage of benzoxazine is demonstrated from the point of view of polymerization for the first time.The volumetric changes of benzoxazines during cure reaction process as well as their effects on volumetric expansion are discussed.The results of apparent volumetric shrinkage show that both mono\|benzoxazine and bis\|benzoxazine precursors exhibit apparent volumetric expansion after polymerization,that is,the densities of monomer are larger than those of polymers at room temperature.Moreover,the apparent volumetric expansion of crystalline monomer after polymerization is larger than that of amorphous monomer.The density versus cure time plots give another point of view of the volumetric expansion of benzoxazines at room temperature,that is,the higher the cure temperature is,the longer the cure time last,the larger the volumetric expansion of benzoxazines will be.The analysis of isothermal cure shrinkage reveals that these benzoxazines exhibit volumetric shrinkage while curing isothermally at elevated temperature.Furthermore,the isothermal cure shrinkages of bis\|benzoxazine precursors are smaller than those of monobenzoxazine precursors.Results of the density versus temperature plots depict the full view of these benzoxazines as they undergo thermal expansion,shrinkage by curing,and shrinkage by cooling from the cure temperature to room temperature,which is in consistent with the results of apparent volumetric shrinkage and isothermal cure shrinkage.The thermal expansion coefficient of benzoxazine polymer is much smaller than that of monomer.From the deviated point of the density versus temperature plots of benzoxazine monomers,we can predict the initial polymerization temperature of benzoxazines.To sum up,these three kinds of benzoxazines exhibit apparent volumetric expansion after polymerization and volumetric shrinkage while curing isothermallyl.The volumetric changes of different benzoxazines at different cure reaction temperatures as well as with different cure times are evidently distinctive.The larger the extent of polymerization is,the larger the volumetric expansion of benzoxazines will be.Therefore,this volumetric expansion phenomenon of benzoxazines is a consequence of the molecular packing rather than the ring\|opening polymerization mechanism and the dimension of benzoxazine is associated with its molecular structure,extent of polymerization and crosslinking density.