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Effect of Aliphatic Chain Length on the Stress–Strain Response of Semiaromatic Polyamide Crystals
Macromolecules ( IF 5.5 ) Pub Date : 2022-06-08 , DOI: 10.1021/acs.macromol.2c00081
Quanpeng Yang 1 , Wenjun Li 2 , Spencer T. Stober 2 , Adam B. Burns 2 , Manesh Gopinadhan 2 , Ashlie Martini 1
Affiliation  

Reactive molecular dynamics simulations were used to model poly(p-phenylene terephthalamide) and related aromatic–aliphatic polyamides derived from p-phenylenediamine and aliphatic diacids with different numbers of carbon atoms in the aliphatic chain (5, 6, 7, or 8). Tensile strain was applied to each polymer crystal in the chain direction, and the mechanical response was characterized. All the polymers with aliphatic segments exhibited strain hardening, transitioning from an initial (low-strain) linear regime to a second (high-strain) linear regime. The modulus at high strain was similar for all polymers, but the modulus calculated at low strain decreased with increasing aliphatic chain length. The decrease in the low-strain modulus with increasing chain length was explained by the observation that polymers with longer aliphatic chains were wavier (i.e., deviated more from the fully extended conformation) in the quiescent state such that they could accommodate low strain without deforming covalent bonds. Extension of wavy chains occurred through an intrachain process for all polymers, quantified by the bond dihedral angles. In addition, for polymers with an even number of non-aromatic carbons, the strain response involved slip between chains within the hydrogen-bonded sheets. The ultimate stress of the polymers exhibited an odd–even effect (even polymers were weaker) which was explained by differences in hydrogen bonding and ring–ring coplanarity prior to failure; polymers with an even number of carbon atoms had less favorable H-bonding and poorer ring alignment. The results revealed direct correlations between aliphatic chain length, intra- and interchain interactions, and the mechanical properties of polyamide crystals.

中文翻译:

脂肪族链长对半芳族聚酰胺晶体应力-应变响应的影响

反应性分子动力学模拟用于模拟聚(苯二甲酰胺)和衍生自p的相关芳族 - 脂肪族聚酰胺-苯二胺和脂肪链中具有不同碳原子数(5、6、7 或 8)的脂肪族二酸。在链方向上对每个聚合物晶体施加拉伸应变,并表征机械响应。所有具有脂肪族链段的聚合物都表现出应变硬化,从初始(低应变)线性状态转变为第二(高应变)线性状态。所有聚合物在高应变下的模量相似,但在低应变下计算的模量随着脂肪链长度的增加而降低。低应变模量随着链长的增加而降低的原因是观察到具有较长脂肪链的聚合物更松散(即,在静止状态下更多地偏离完全延伸的构象),以便它们可以适应低应变而不会使共价键变形。波浪链的延伸通过所有聚合物的链内过程发生,由键二面角量化。此外,对于具有偶数个非芳族碳的聚合物,应变响应涉及氢键片内链之间的滑动。聚合物的极限应力表现出奇偶效应(偶数聚合物较弱),这可以通过失效前氢键和环-环共面性的差异来解释;具有偶数个碳原子的聚合物具有不太有利的氢键和较差的环排列。结果揭示了脂肪链长度、链内和链间相互作用之间的直接相关性,
更新日期:2022-06-08
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