Wang Dengfei, Li Shiyan, Lu Ying, Wang Jian, Men Yongfeng
Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, China.
Daqing Petrochemical Research Center, Petrochemical Research Institute of PetroChina, Daqing 163714, China.
Polymers (Basel). 2023 Mar 30;15(7):1721. doi: 10.3390/polym15071721.
Homo-polymers of reasonable molecular weight relax very fast in the molten state. Starting from a semi-crystalline structure, when the homo-polymer is heated up to a temperature higher than its nominal melting temperature, it relaxes quickly into a homogenous molten state. The following crystallization temperature during cooling remains constant irrespective of the melt temperature. However, the situation is evidently different in copolymers. A phenomenon named the crystallization melt memory effect denotes an increased crystallization rate during cooling after a polymer was melted at different temperatures, which is often observed. The melt temperature can be even higher than the equilibrium melting temperature of the corresponding polymer crystals. In this work, we investigated such memory effect in a polyethylene random terpolymer with a small fraction of 1-octene and 1-hexene co-units using differential scanning calorimetry techniques. Both non-isothermal and isothermal protocols were employed. In non-isothermal tests, a purposely prepared sample with well defined thermal history (the sample has been first conditioned at 200 °C for 5 min to eliminate the thermal history and then cooled down to -50 °C) was melted at different temperatures, followed by a continuous cooling at a constant rate of 20 °C/min. Peak crystallization temperature during cooling was taken to represent the crystallization rate. Whereas, in isothermal tests, the same prepared sample with well defined thermal history was cooled to a certain crystallization temperature after being melted at different temperatures. Here, time to complete the isothermal crystallization was recorded. It was found that the results of isothermal tests allowed us to divide the melt temperature into four zones where the features of the crystallization half time change.
具有合理分子量的均聚物在熔融状态下松弛非常快。从半结晶结构开始，当均聚物被加热到高于其标称熔点的温度时，它会迅速松弛成均匀的熔融状态。冷却过程中的后续结晶温度保持恒定，与熔体温度无关。然而，共聚物的情况明显不同。一种名为结晶熔体记忆效应的现象是指聚合物在不同温度下熔融后冷却过程中结晶速率增加，这种现象经常被观察到。熔体温度甚至可以高于相应聚合物晶体的平衡熔点。在这项工作中，我们使用差示扫描量热法技术研究了一种含有少量1-辛烯和1-己烯共聚单元的聚乙烯无规三元共聚物中的这种记忆效应。采用了非等温法和等温法。在非等温测试中，将一个特意制备的具有明确热历史的样品（该样品首先在200°C下处理5分钟以消除热历史，然后冷却至-50°C）在不同温度下熔融，随后以20°C/分钟的恒定速率连续冷却。冷却过程中的峰值结晶温度被用来代表结晶速率。而在等温测试中，将相同的具有明确热历史的制备样品在不同温度下熔融后冷却到某个结晶温度。在此，记录完成等温结晶的时间。结果发现，等温测试的结果使我们能够将熔体温度分为四个区域，其中结晶半衰期的特征会发生变化。
