剪切作用下纳米粘土对LDPE熔体粘弹响应的影响

THE INFLUENCES OF NANOCLAY ON VISCOELASTIC RESPONSE OF LOW DENSITY POLYETHYLENE MELTS UNDER SHEAR

以LDPE/EVA/纳米粘土复合体系为研究模型,考察了剪切作用下,分散良好的纳米粘土对聚合物基体熔体稳态及瞬态粘弹响应的影响.发现剪切作用下,纳米粘土增加了聚合物熔体粘弹特性对剪切速率、剪切应变及剪切作用史的依赖性,改变了相应的依赖关系.稳态剪切时,纳米粘土的加入使体系第一法向应力差(N1)在低剪切速率区变为负值,而在高剪切速率区N1与粘土的含量无关;同时就瞬态剪切应力及N1的应变依赖关系而言,复合体系明显不同于聚合物基体;预剪切对聚合物基体瞬态粘弹响应几乎没有影响,而当纳米粘土的加入量大于3wt%后,与未经预剪切的样品相比较,经预剪切的复合体系的瞬态剪切应力值、应力过冲程度以及稳态剪切应力值均明显下降,且预剪切前后复合体系达到稳态时其瞬态剪切应力差值随纳米粘土含量的增高而线性增加.此外,纳米粘土的添加对聚合物熔体受剪切作用的非线性粘弹响应存在影响.复合体系熔体呈现特异非线性粘弹响应,其缘由被认为是由于纳米粘土在聚合物基体中剥离分散,或聚合物分子链插层于粘土片层间,形成局部有序结构,受剪切作用而排列取向.

The viscoelastic responses for the melts of low density polyethylene （LDPE）/ethylene-vinyl acetate copolymer （EVA）/nanoclay composites containing different contents of nanoclay were studied. It is found that the viscoelasticity dependence of the melts on shear rate, shear strain and shear history is related with the dispersion and amount of nanoclay added. A negative primary normal force difference （ N1 ） appears when the composites are sheared at lower shear rate during steady flow, while N1 is independent of the nanoclay amount at higher shear rates. Moreover, compared with the polymer matrix, the composites exhibit different dependence of transient stress or N1 on the strain. The viscoelastic responses of polymer matrix are independent of the pre-shear that the samples are exposed to. However, for the composites containing 3 wt % or more nanoclay, the level of transient stress, stress overshoot and the steady stress observed in flow reversal experiments with pre-shear are found to be lower than those of the samples without pre-shear. With the increasing of nanoclay content, the differences of transient stress at steady state increase for the composites with and without pre-shear, and a positive N1 instead of a negative one appears for the composites subjected to pre-shear. It is believed that the layers of nanoclay are exfoliated or intercalated by polymer chains, resulting in the formation of directors in the polymer matrix. The reason for the composites melts showing abnormal nonlinear viscoelastic response is ascribed to alignment or orientation of the directors under shear stress.