彈性體之拉伸強度與韌性的最佳交聯密度: 交聯與纏結之間的競爭

Abstract

交聯通常可以提升高分子的機械性能，然而已有研究指出一些機械性能的表現會隨著交聯劑濃度的增加呈現非單調的變化，但這些行為背後的機制卻尚未被完全理解。因此，本研究使用耗散粒子動力學的模擬方法，探討交聯程度對交聯分支高分子系統之機械特性影響。除了探究應力-應變曲線外，亦透過測量凝膠含量、結晶度和微觀結構等微觀特性來深入了解彈性體的結構-性能關係。研究結果如預期所示，交聯程度的增加會導致結晶度降低，但是凝膠含量會提高。此外，隨著交聯劑濃度的增加，楊氏模數會上升，而斷裂伸長量會下降。不同的是，抗拉強度和韌性會隨交聯程度的增加，而呈現先增加後穩定下降的趨勢。交聯高分子的微觀結構特性可以透過平均鍵長(l¯b)以及高分子迴轉半徑(R¯g)來分析，隨著交聯劑濃度的增加，斷裂處的平均鍵長會一開始增加，但在最適濃度後會趨於飽和，而斷裂處的平均高分子迴轉半徑則會逐漸下降。這些研究結果顯示，抗拉強度和韌性的最大值是交聯和纏結之間競爭的結果。

Crosslinking generally enhances the mechanical properties of crosslinked polymers. However, some mechanical properties have been reported to vary non-monotonically with the concentration of crosslinkers, and the underlying mechanism for this behavior is not yet fully understood. In this work, the effect of the crosslinking degree on the mechanical characteristics of the crosslinked systems made of branched polymers are explored by dissipative particle dynamics simulations. In addition to the stress-strain curve, the microscopic characteristics of the elastomer, such as the gel content, crystallinity, and microstructure, are obtained to gain insights into the structure-property relationship. As expected, an increase in the degree of crosslinking leads to a decrease in the degree of crystallinity, but an increase in the gel content. Moreover, the Young’s modulus increases with the crosslinker concentration, while the elongation at break decreases. In contrast, the tensile strength and toughness initially increase and then steadily decrease as the crosslinking degree increases. The microstructural characteristics are analyzed based on average bond length (l¯b) and radius of gyration of polymers (R¯g). As the crosslinker concentration increases, l¯b at break grows initially but becomes saturated beyond the optimal concentration, while R¯g at break always decreases. Our findings suggest that the maxima in tensile strength and toughness are the consequence of the competition between crosslinking and entanglement.