Monoclinic phase in reactor powders of ultra-high-molecular-weight polyethylene and its changes during compaction and monolithizationстатьяИсследовательская статья
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Дата последнего поиска статьи во внешних источниках: 4 января 2018 г.
Местоположение издательства:Road Town, United Kingdom
Первая страница:620
Последняя страница:626
Аннотация:Reactor powders of ultra-high-molecular-weight polyethylene not subjected to any mechanical treatment after synthesis are shown to contain up to 50% monoclinic crystallites. During compaction, two-thirds of the monoclinic crystallites are transformed into the orthorhombic phase. The compaction is accompanied by the development of a new monoclinic phase; at 100-150 MPa, the content of this phase reaches its limiting level of ∼50%. At constant pressure, the content of the monoclinic phase increases in proportion to the logarithm of the treatment time. Upon monolithization, the monoclinic phase of the pellets is fully transformed into the orthorhombic phase. Monolithization is accompanied by the development of a new monoclinic phase; at ultimate plastic deformation ratios, the content of this phase is ∼20%. The resultant structure is not stable: after pressure release, the content of the monoclinic phase decreases. The rate of the transition from the monoclinic to the orthorhombic phase increases with increasing temperature, and, finally, the monoclinic phase fully disappears at 100°C. The development of the monoclinic phase is assumed to take place not in the process of compaction but immediately thereafter (once the pellet is released from pressure). During the action of pressure, bonds between reactor powder grains appear and prevent any volume relaxation after the pressure release. The stresses induced at the bonds are transferred to the crystallites; as a result, the orthorhombic phase is transformed into the monoclinic phase. The relaxation character of this process is due to the involvement of amorphous regions in the polymorphic transition.