剪切历史对尼龙熔体结晶行为的影响

采用差示扫描量热法研究了原始聚酰胺材料在经历不同的预剪切过程或应用不同设备进行剪切后其结晶行为的变化,结果表明,原始样品经过预剪切后,结晶温度升高5～10℃,半结晶时间降低到原始样品的一半.当聚酰胺材料经历很低的剪切时其结晶温度就有很大的提高,进一步提高剪切强度,结晶温度的增加趋势变缓.偏光显微镜观察表明有剪切历史的样品,球晶尺寸减小.作为对此,聚烯烃和聚酯材料的结晶对剪切历史不敏感,剪切与非剪切样品的结晶行为基本相同,据此推测聚酰胺分子间氢键可能是这种剪切记忆效应产生的原因.

THE EFFECTS OF SHEAR HISTORY ON CRYSTALLIZATION OF POLYAMIDES

Crystallization of polyamides shows memory of shear even after undergoing a melting process at high temperatures for a long period to erase their thermal history,which was seldom noticed in the past.Considerable change of crystallization behavior of polyamides took place in the samples pre-sheared in various kinds of processing or measuring equipments,such as MFR instrument,rheometer,mixer and extruder.The peak crystallization temperatures increased by 5 to 18 ℃ and the half crystallization time decreased to about a half compared to the sample without shear history.What's more,the most obvious change generally occurred at a very low shear level,beyond which the changes levelled off.Polarized light microscopy revealed the decrease of spherulite size.On the contrary,crystallization of the pre-sheared polyolefins and polyesters did not exhibit considerable difference from the unsheared samples.The hydrogen bonding was proposed to account for the memory of shear in polyamides.