窄分子量分布聚乙烯在纳米粒子表面的受限结晶特性

Polymer nanocomposites (PNC) are an emerging class of hybrid materials composed of hard nanoparticles or "fillers" dispersed in soft polymer matrices. Owing to the improvement of properties including conductivity, toughness and premeability, PNC are slated for application ranging from membranes to fuel cells. Nevertheless, the complex crystallization behaviors of polymers with high molecular weight and broad molecular weight distribution hinder the in-depth understanding of crystallization mechanism of PNC from molecular level, especially in the existance of external inorganic fillers. Therefore, the main challenge for establishing the correlations between structure and properties of crystalline polymer based PNC relys on elucidating two kinds of effects: surface or interface effect and confinement effect. Investigating the crystalline structure formation and phase transition of polymer matrices on the surface of nanoparticles is essential to predict and control the properties of PNC. The present project chooses polyethylene (PE) samples with a series of molecular weight and narrow molecular weight distribution and a series of monodisperse silica particles as PNC main components, aiming to study the confined crystallizaiton features of PE on the surface of nanosized silica. The size (curvature), surface characteristic (hydrophilic or hydrophobic) of silica as well as the chain length of PE will be considered as the key factors to study the surface inducing effect, crystallite nucleation and growth, phase transition behavior of polymers in silica nanocomposites. The phase transition behavior will be discussed in terms of a possible crossover from fullly extended molecule nuclei with shorter chain length to folded "bundle" nuclei with longer chain length of PE. The investigation on the crystallization behavior of PE in confined geometry will provide not only well-defined model systems for investigating crystallization mechanism of polymers, but also the theoretical aid for the design of inorganic particle-polymer nanocomposites with high performances.

在结晶性聚合物-纳米填料复合体系中存在两种效应--表面(界面)效应和空间受限效应，这两种效应决定了聚合物的结晶特性和复合材料的最终性能。如何清楚地认识纳米粒子表面聚合物的凝聚态结构及其与纳米粒子之间的相互作用本质，从而精确预测和控制聚合物纳米复合材料的结构和性能，成为制备纳米复合材料的主要挑战之一。本项目拟选用分子量分布窄的聚乙烯作为研究对象，分子量从1000到二十多万，重点研究高含量纳米二氧化硅粒子对聚乙烯结晶的诱导效应，探讨二氧化硅微球的尺寸（曲率半径）、表面特性（亲水或疏水）、聚乙烯链长等因素对复合体系结晶的影响，阐明界面效应以及空间受限效应对聚乙烯链段运动和晶体成核生长的影响规律，从而建立加入无机粒子填料后聚合物结晶时的成核生长和晶型转变模型，深入理解纳米无机粒子-聚合物复合体系的结构和性能关系。

阐明聚合物纳米复合材料的界面相互作用，对设计和改善聚合物纳米复合材料的性能有着重要的理论意义和应用价值。本项目系统研究了长链烷烃在SiO2微球表面的受限结晶行为，并将体系扩展到低分子量聚乙烯在SiO2微球表面的受限结晶行为，取得成果如下：（1）十八烷在SiO2微球表面的受限结晶行为。长链烷烃可以在SiO2表面形成稳定的表面结晶单分子层。一方面，长链烷烃与无定形SiO2表面的弱范德华力，不影响表面单分子层的稳定存在；另一方面，粗糙SiO2表面增强了界面区烷烃分子沿长轴方向的位移无序和分子链末端构象缺陷，有利于表面单分子层的稳定性；最后，SiO2微球较大的比表面积使复合物中参与表面结晶的烷烃占所有烷烃的比例大大增加。以上因素共同作用，最终导致C18/SiO2复合物中出现了特殊的双表面结晶现象。（2） 六十烷在SiO2微球表面的受限结晶行为。当n < 44时，SiO2表面的长链烷烃结晶有两个共同点：长链烷烃/SiO2复合物在高于本体液固相转变温度之上出现双表面结晶现象，并且旋转相稳定性增强。当44≤ n ≤60时，长链烷烃/SiO2复合物只有一个表面结晶峰，C60H122/SiO2复合物的表面结晶现象增强。且在低于液固相转变温度时不再出现新的固固相转变峰，二氧化硅网络对旋转相稳定性的影响消失。（3） 聚乙烯在SiO2微球表面的受限结晶行为。制备了三种不同表面性质的SiO2，研究了不同界面作用对聚乙烯（PE）受限结晶行为的影响。随着PE与SiO2之间界面作用的增强，PE分子链的运动能力和结晶能力逐渐下降。由于PE与SiO2之间的相互作用仅为范德华力，SiO2网络结构的形成与SiO2表面性质无关，粒子-粒子相互作用占主导因素。这种三维网络结构严重束缚了聚合物分子链的运动，导致PE/SiO2复合物结晶行为出现转折点。本项目达到了预期目标，发表学术论文3篇，包括Journal of Physical Chemistry B 2篇、Journal of Polymer Science: Polymer Physics 1篇。通过研究小分子和高分子在二氧化硅表面的不同结晶特点，建立起从长链烷烃到聚乙烯结晶跨越的桥梁。长链烷烃和低分子量聚乙烯的受限结晶研究不仅能够揭示这些物质受限结晶过程中的相态变化规律，也能够为进一步从分子尺度上研究聚合物特别是聚烯烃的结晶提供有益的启示。