| Flow-induced crystallization is a general phenomenon during the processing of semicrystalline polymers,which plays an important role in the material properties of polymer products,and thus attracts intensive attention in both the academic research and the technological applications.Shear-induced crystallization is a kind of flow-induced crystallization,one of the most notable behaviors associated with shear-induced crystallization is the formation of shish-kebab crystals.As a highly oriented structure,shish-kebab crystals can significantly improve the stiffness,strength and other properties of products.Hence,the investigations on the formation mechanism of shish-kebab is not only important for understanding the shear-induced crystallization,but also expected to be helpful to improve the properties of industrial production.In this paper,the modulation of shish-kebab crystals formation in bimodal PE under shear by changing shear temperature,the influence of short chain branch on formation of shear induced crystals in bimodal PE at high and low shear temperatures were investigated by using in-situ smallangle X-ray scattering(SAXS)and wide-angle X-ray diffraction(WAXD)techniques.The main results are as following:1.The influence of different shear temperatures on shish-kebab crystals formation due to coilstretch transition and stretched network in bimodal PE under shear was studied.The SAXS results showed that more shish-kebab crystals formed at high shear temperatures of 139 and 136 °C,and less shish-kebab but more partially oriented lamellar crystals formed at low shear temperatures of127 and 124 °C.The WAXD results revealed that the crystallinity was low as 5 % and the orientation held high value 0.50~0.80 at 139 and 136 °C,while at 127 and 124 °C,the crystallinity was high as 40 % and the orientation held low value 0.15~0.40.During isothermal crystallization temperature of 121 °C,bimodal PE with different temperature sheared exhibited lower crystal orientation but higher crystallization due to the coexistence of kebab and partially oriented lamellar crystals.The results also indicated that the formation mechanism of shish-kebab crystals came from the CST concept at high shear temperature and agreed well with the stretched network at low shear temperature.These evidences clearly indicated that the modulation of shish-kebab crystalsformation owing to different mechanisms by adjusting the entanglement only changing shear temperature was achieved.2.Shish-kebab crystals formation at high shear temperature and low shear rate during shearinduced crystallization in bimodal PE with different content of short chain branch under shear was studied.The results indicate that shish crystals form at different shear temperatures of 139 and136 °C,subsequently kebab crystals and oriented lamellae form at the isothermal crystallization temperature of 121 °C.The presence of short chain branch significantly promotes the formation of shish-kebab crystals owing to the increase of the relaxation time,but overmuch short chain branch impedes the formation of shish-kebab crystals and form more oriented lamellae.3.Shish-kebab crystals formation at low shear temperature and low shear rate during shearinduced crystallization in bimodal PE with different content of short chain branch under shear was studied.The SAXS results showed that there is more shish-kebab but less partially oriented lamellar crystals formed in the sample with short chain branch,which is different from those at high shear temperature.The WAXD results revealed that during shear,the crystallinity was highest as 40 % in the sample without short chain branch,lower as 30 % and 25 % in sample with 0.11 %and 0.30 % content of short chain branch,respectively.The orientation held low value 0.10~0.50 in all samples.The results indicated that the formation mechanism of shish-kebab crystals agreed well with the stretched network at low shear temperatures.These evidences clearly indicated that even though the existence of short chain branch impeded the crystallization ability,the formation of shish-kebab crystals was promoted at low shear temperatures.In summary,the modulation of shish-kebab crystals formation owing to different mechanisms by adjusting the entanglement only changing shear temperature was achieved.we further investigated the influence of short chain branch on the formation of shish-kebab crystals during shear-induced crystallization at different shear temperature.We discovered that short chain branch impedes the crystallization.Meanwhile,the existence of short chain branch can promote the formation of shish-kebab crystals at different shear temperatures,but overmuch short chain branch impedes the formation of shish-kebab crystals at high shear temperatures. |
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