未硫化天然橡胶应变诱导结晶对其分子运动的影响

Effects of Strain Induced Crystallization on the Molecular Dynamics of Unvulcanized Natural Rubber

天然橡胶的应变诱导结晶(SIC)行为被认为是导致其产生优异力学性能的主要原因,而分子动力学作为联系聚合物微观结构与宏观性能的桥梁,研究SIC行为对于其分子运动能力的影响有助于更深层次地理解天然橡胶优异性能的来源。文中采用同步辐射二维广角X射线衍射探究了未硫化天然橡胶(NR)的应变诱导结晶行为,结合宽频介电松弛谱研究了天然橡胶中聚异戊二烯橡胶烃分子在不同应变下的分子运动能力,包括链段松弛模式(SM)和整链松弛模式(NM),并进一步探究了单轴拉伸过程中天然橡胶分子动力学与力学性能之间的关系。结果表明,在常温下以0.01 s-1的应变速率进行单轴拉伸时,NR在应变3.0附近开始出现结晶,在结晶前后,NR中的分子运动能力发生了明显的改变。结晶前,随应变增大分子链逐渐沿拉伸方向取向,SM对应的介电强度(Δε)和松弛时间(τHN)分别增大和缩短,同时SM和NM对应的理想玻璃化转变温度(T0)都出现明显的上升;结晶后,结晶度随应变增大而提高,晶体限制了SM和NM松弛,因此,二者对应的Δε随应变提高逐渐减小,τHN随应变提高而延长。进一步地,结晶前NR橡胶烃分子链取向使链段运动加快有利于分子链排入晶格;结晶后由于晶体对链段运动及整链运动的限制作用,导致松弛时间增长,由此使NR的力学性能得到明显提高。

The superior mechanical performance of natural rubber （NR） has been thought to originate from its strain induced crystallization （SIC） behavior, and molecular dynamics builds the bridge between microcosmic structure and macroscopic property, so a study on how SIC behavior affects chain dynamics is important to further understand how SIC behavior affect its mechanical property. In this paper, the SIC behaviors of unvulcanized natural rubber were studied by in situ synchrotron wide angle x ray diffraction and the dielectric relaxation behaviors at different strain ratios by broadband dielectric relaxation spectroscopy. Through the research on the two dielectric relaxation modes of NR： segmental mode （SM） and normal mode （NM）, we can get a deeper understand on the relationships between the evolution of chain conformation and mechanical performance. The results demonstrate that when stretched at the strain rate of 0.01 s^-1, unvulcanized natural rubber begins to SIC at the strain ratio about 3.0, and its dielectric relaxation behavior changes before and after SIC. Before SIC, the molecular chains orient with increasing strain ratio, so the dielectric strength Δε and relaxation time τHN of SM increase and decrease respectively. Meanwhile, the ideal glass transition temperature T0 of SM and NM both increase. After SIC, the crystallinity increases with strain ratio and the crystals restrict the chain dynamics of NR, thus both for SM and NM, the dielectric strength Δε and relaxation time τHN decrease and increase with strain ratio, respectively.