Role of Topological Repulsive Forces in Polymer Crystallization

在半结晶聚合物中，结晶相与非结晶相共存，但后者的性质至今还不清楚。本文提出了一个新的假设，即在熔体结晶过程中，聚合物间的缠结从结晶相中排出，聚集到非结晶相中，使得非晶相的自由能增加，并且在结晶驱动力与自由能的增加相平衡的点处结晶停止。为了证明这一假设，我们对纠缠高度凝聚的无定形聚合物进行了计算机模拟。作为这种系统的模型，永久纠缠的环状聚合物，这通常被称为链状网络，被认为是。在保持体系的缠结状态、聚合物数目和元素浓度不变的情况下，改变体系的缠结浓度，研究了体系的热力学和力学性质与聚合物结晶的关系。结果表明：1）数值计算了非晶相元素的化学势随纠缠浓度的变化关系，表明化学势来自于“局域结”之间的排斥作用; 2）利用化学势和文献中的结晶熵，得到了结晶温度与结晶相比例之间的关系，3）在非晶相中，高分子链呈现出一种特殊的构象，这种构象在整体尺度上是正常的，但在局部高度伸展。

In semi-crystalline polymers, ciystalline phase coexists with amorphous phase, but the nature of the latter has not yet been known well In this work, we propose a new hyposesis that, in crystallizing from melts, entanglements among polymers are expelled form the crystalline phase and accumulated into the amorphous phase, se that the free energy of the amorphous phase increases and ciystallization stops at point where driving force of ciystallization balances the increase of the free energy. To prove the hyposesis, we have performed computer simulations of amorphous polymers in which entanglements are highly condensed. As a model of such systems, permanently entangled cyclic polymers, which are usually called catena networks, are considered. Changing the concentration of entanglements, while keeping the entanglement state, the polymer number and the element concentration of the systems, the thermodynamic and mechanical properties of the systems are studied in relation to the crystallization of the polymers. The following are found.1)The chemical potential of elements in the amorphous phase is computed numerically as function of the concentration of entanglements, and it is shown that the chemical potential comes from repulsive interaction among "local-knots".2)With use of this chemical potential and the entropy of crystallization found in the literature, we have found a relation between the crystalline temperature and the ratio of crystalline phase, which explain well the known experimental results.3)In the amorphous phase, polymer chains take an unusual conformation, which is normal in global scale, but highly extended locally.