步进扫描差示扫描量热法研究不同链结构的聚乙烯类聚烯烃热力学特性

New Thermal Features of Ethylene-based Polyolefins with Different Chain Architectures as Investigated by Step Scan Differential Scanning Calorimetry Method

采用步进扫描差示扫描量热法研究了几种具有不同链结构的聚乙烯类聚烯烃的热力学性质.结果表明,不同链结构的聚烯烃在热力学平衡熔融态下,其绝对比热容的温度依赖性在实验误差范围内几乎相同;而无论链结构的变化如何,在极低温度下稳定结晶态的热容也非常接近.但在熔融温度范围内,比热容对不同聚烯烃的链结构非常敏感,这是由于在远低于聚烯烃主熔化峰温度范围内,聚乙烯晶体结构中的不同支链的影响.在时间-热流的步进扫描曲线中,具有长支链结构的聚烯烃表现出独特的熔融-再结晶行为,这可能是由于交联点之间较长的受约束链段的运动在熔融过程中得到释放并重排所致.

Several selected commercial polyethylene-based polyolefins with different chain architectures were studied by step scan differential scanning calorimetry(DSC) method. It is found that the temperature dependence of heat capacity is the same for all the polyolefins with different chain architectures in equilibrium liquid state;the heat capacity in stable crystalline state at very low temperature is also very close regardless the variation of chain architectures. While in melting temperature range the heat capacity is very sensitive to the chain architectures, which is ascribed as the branching chains in the crystalline structures of polyethylene far below the main melting peak temperature range of polyolefins. In the step scan DSC of time-heat flow curves, low density polyethylene(LDPE) showed unique melting-recrystallizing behavior, possibly due to the relief of restrained segments between crosslinking points during melting process.