我室博士后孟令蒲和博士生陈晓伟在李良彬研究员的指导下，利用在线X射线散射记录各向同性高密度聚乙烯-增塑剂共混膜拉伸过程的结构演化，拉伸温度范围设定为包含工业生产的纵拉温度，结果发表在European Polymer Journal（2018, 101: 358-367）期刊上。
观察到小角散射二维图从环、四点、六点到最终双小叶的转变。根据WAXS数据，观察到结晶度下降到最低点后单调增加，由此证明在拉伸过程中发生熔融再结晶。根据不同方位角WAXS衍射环的峰位移，获得了沿不同方向取向片晶的三向应力。发现了不同方向片晶所承受应力和其熔融开始应变有非常强的关联：片晶应力达到临界值，晶体在小应变即开始发生熔融；而当此应力小于临界值，片晶开始发生熔融的应变增加。不同方位角片晶所承受应力不同，造成了其破坏的先后顺序，是四点SAXS图案的出现的结构根源。通过热力学分析，比较了体系弹性能和相变驱动力，验证了片晶应力诱导熔融的物理本质是弹性能驱动晶体向无定形的转变。
以上研究工作得到国家自然科学基金（51227801, 51325301, 51633009）的资助。
不同方位角下晶体破坏先后和应力的关系
文章链接：http://www.sciencedirect.com/science/article/pii/S0014305717320050
Structure evolution of slightly oriented polyethylene (PE)-plasticizer film is recorded with small- and wide-angle X-ray scattering (SAXS/WAXD) during stretching at temperature from 80 oC to 100 oC. An interesting evolution of SAXS patterns from anisotropic ring, four-point, six-point, two point is observed, which is validated to be resulted from melting-recrystallization. With peak shift of WAXD pattern, triaxial local stresses exerted on lamellae at different azimuthal angles are acquired. As consequence, lamellae melting is revealed to be stress induced process, where stress exerted along chain direction of lamellae and stress perpendicular to chain direction are the criteria at azimuthal angle from 0o to 60o and that from 70o to 90o, respectively. Due to the heterogeneous stress distribution of the system, the appearance of four-point pattern stems from the sequence of lamellae melting at different azimuthal angles. Through thermodynamic analysis, the physical essence of stress induced melting of lamellae is validated to be elastic energy driven crystal to amorphous phase transition.