Formation of high melting crystal in a thermotropic aromatic copolyester
Formation of high melting crystal in a thermotropic aromatic copolyester
复制标题
热致芳香族共聚酯中高熔点晶体的形成
DOI：
10.1021/ma00186a022
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发表时间：
1988
期刊：
影响因子：
--
通讯作者：
H. Winter
中科院分区：
文献类型：
--
作者：
Y. G. Lin;H. Winter
对于热致液晶聚合物（LCP），Celanese公司的VECTRA A900，在熔融退火过程中观察到高熔点晶体的形成。当聚合物加热到290 ℃，即刚好高于其约280 ℃的标称DSC熔融温度时，它迅速熔融，显示出约103 Pa的低复数模量。然而，在此相同温度下退火过程中，熔体缓慢结晶，并经历了液体/固体的转变，如所示的由3个数量级的复杂模量在200分钟内逐渐增加。对于DSC和X-射线研究，一个离散的一组样品制备与退火时间增加在290 ℃和随后冷却到室温。这些样品的广角X射线散射图案显示出d= 3.8 A的额外反射环。在DSC第一次加热扫描中，退火样品显示出两个分离的吸热峰。第一个峰在280 ℃左右，第二个峰在退火温度以上20-30 K.未退火的样品只有第一个峰。随着退火时间的增加，第一峰衰减，第二峰增长并从305 ℃移动到315 ℃。发现复合模量的增长与第二吸热峰下的面积成正比。所有这些观察结果表明，一个高熔点的晶体具有更高的程度有序逐渐形成。高熔点晶体的形成是热可逆的。在被加热到高于DSC第二吸热峰的温度之后，退火的样品返回到它们在退火之前的初始状态，再次显示出低的复数模量和单个吸热峰。结晶的速率取决于退火温度和样品的机械和热历史。较高的退火温度导致较慢的结晶。在290 ℃下，拉伸样品表现出加速结晶.值得注意的是，预先加热到320 ℃的样品在290 ℃时没有明显的结晶现象。这表明，结晶过程是一个形核长大过程，在预热到320 ℃时，晶核熔化。
The formation of a high melting crystal during melt annealing is observed for a thermotropic liquid crystalline polymer (LCP), VECTRA A900 of Celanese Corp. When the polymer is heatedto 290 C, ie just above its nominal DSC melting temperature of about 280 C, it melts rapidly, displaying a low complex modulus of about 103 Pa. However, during annealing at this same temperature, the nematic melt slowly crystallizes and undergoes a liquid/solid transition, as shown by a gradual increase in the complex modulus by 3 orders of magnitude in 200 min. For DSC and X-ray studies, a discrete set of samples were prepared with increasing timeof annealing at 290 C and subsequentcooling to room temperature. Wide-angle X-ray scattering patterns of these samples shows an additional reflection ring with d= 3.8 A. In the DSC first heating scan, the annealed samples display two separated endothermic peaks. The first peak is about 280 C; the second peak is 20-30 K above the annealing temperature. The unannealed samples have only the first peak. As the annealing time increases, the first peak decays and the second peak grows and moves from 305 to 315 C. The growthof the complex modulus is found to be proportional to the area under the second endothermic peak. All these observations indicate a gradual formationof a high melting crystal with a higher degree of order. The formationof the high melting crystal is thermoreversible. After being heated to a temperature above the DSC second endothermic peak, the annealed samples return to their initial state before annealing, displaying once again low complex modulus and a single endothermic peak. The rate of the crystallization depends on annealing temperature and on mechanical and thermal histories of the samples. Higher annealing temperature leads to slower crystallization. At 290 C, stretched samples exhibit accelerated crystallization. It is interesting tonote that samples which have been subjected to a prior heatingto 320 C do not show significant crystallization at 290 C. This indicates that the crystallization is a nucleation and growth process and that the nuclei melt during the preheatingto 320 C.