Rastogi S, Kurelec L, Lemstra PJ
Eindhoven Polymer Laboratories/The Dutch Polymer Institute, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
Macromolecules. 1998 Jul 28;31(15):5022-31. doi: 10.1021/ma980261h.
Polymorphism is a well-established phenomenon in crystalline materials and is important for pharmaceutical and polymeric materials. In our study concerning the processability of polymers, we came across an unusual observation related to polymorphism induced by pressure. The experimental observation is that polyethylene crystals transform from the stable orthorhombic crystal into a transient hexagonal phase. The occurrence of a transient hexagonal phase is shown to be dependent on the polymer crystal size; smaller crystals transform into the transient hexagonal phase at temperatures and pressures much below the thermodynamic critical point Qo, which is located at P = 3.6 kbar and T = 230 degreesC. The crystal size dependence in the phase transition was investigated by in situ X-ray studies in the unirradiated and irradiated solution-crystallized films. Since the chain mobility is rather high in the hexagonal phase, sintering has been attempted via this transient phase using ultrahigh molecular weight polyethylene (UHMW-PE) as a model system. UHMW-PE is an intractable polymer due to its high molar mass but possesses excellent abrasion resistance properties. For this reason it is used as an inlay in demanding applications such as artificial hip and knee joints. The service life of UHMW-PE in these artificial joints, however, is limited due to the poor processing characteristics notably during sintering, and often a second operation is needed to replace the UHMW-PE interface. Sintering via the transient hexagonal phase could provide a solution for this important problem which concerns an increasing number of people.
多晶型现象在晶体材料中是一种已被充分证实的现象，对药物和聚合物材料很重要。在我们关于聚合物加工性能的研究中，我们遇到了一个与压力诱导的多晶型有关的不寻常观察结果。实验观察到聚乙烯晶体从稳定的正交晶型转变为瞬态六方相。结果表明，瞬态六方相的出现取决于聚合物晶体尺寸；较小的晶体在远低于位于P = 3.6千巴和T = 230℃的热力学临界点Qo的温度和压力下转变为瞬态六方相。通过对未辐照和辐照的溶液结晶薄膜进行原位X射线研究，研究了相变过程中晶体尺寸的依赖性。由于六方相中链的流动性相当高，因此尝试以超高分子量聚乙烯（UHMW-PE）为模型体系，通过这个瞬态相进行烧结。UHMW-PE由于其高摩尔质量而难以加工，但具有优异的耐磨性能。因此，它被用作人工髋关节和膝关节等苛刻应用中的镶嵌物。然而，由于加工特性差，特别是在烧结过程中，UHMW-PE在这些人工关节中的使用寿命有限，通常需要二次手术来更换UHMW-PE界面。通过瞬态六方相进行烧结可以为这个涉及越来越多人的重要问题提供一个解决方案。
