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Morphological origins of the nonlinear mechanical properties of semicrystalline ionomers.

机译：半结晶离聚物非线性力学性能的形态学起源。

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The extraordinary mechanical properties of ethylene/methacrylic acid (E/MAA) ionomers are a result of their fascinating and rich nanometer-scale morphology, which includes polyethylene crystals, amorphous polymer segments and ionic aggregates. The goal of this dissertation is to provide a framework for understanding the unique contribution of each structural motif to key nonlinear mechanical properties such as the yield stress (sigma y), ultimate stress/strain and toughness. We investigate these properties through a combination of complementary experimental techniques, including mechanical testing, calorimetry and X-ray scattering.;We begin by developing a model that relates polyethylene crystal thickness and incomplete amorphous phase relaxation mechanisms to sigma y, measured at various temperatures and strain rates. Taking advantage of the inverse correspondence between temperature and rate on the yield stress response, we devise an algorithm for the creation of sigma y master curves, thus expanding our predictive capabilities far beyond the typical experimentally accessible rate window.;Comparison of the master curves for the E/MAA copolymers shows that sigma y is highly dependent upon the glass transition temperature (Tg), which increases with MAA content. For high MAA-content copolymers, Tg can exceed room temperature. The resulting vitrification of the amorphous phase results in an increase in the yield stress, despite a reduction in the degree of crystallinity.;In E/MAA ionomers, partial neutralization of the MAA groups results in a several-fold increase in sigmay relative to the base resin. Here, nanoscale phase separation results in both ion-rich aggregates and ion-poor domains, with widely separated relaxation rates. The inability of the amorphous phase immediately surrounding the ionic aggregates to relax, except at extremely low strain rates, greatly increases the yield stress of the ionomers.;Moving to higher strains, we study the connection between the second yield shoulder---an anomalous hump in the plot of stress versus strain---and delayed permanent deformation. Mechanical testing and X-ray scattering show that this shoulder indicates the initiation of polyethylene crystal fracture, which results in crystalline fragments of reduced lateral extent. Connections formed between these fragments prevent the material from retracting to the initial sample dimensions once the load is removed, though greater recovery can be achieved by heating or melting the specimen. Only at strains beyond the end of the second yield hump does chain disentanglement begin, leading to truly unrecoverable deformation.;Finally, we review the effects of neutralization and test rate on the ultimate properties of E/MAA ionomers. We find that neutralization simultaneously increases the stress at break while decreasing the strain at break. The result is that the toughness, evaluated as the area under the tensile curve, does not change dramatically with ionomerization. This surprising result does not agree with high-speed tensile impact results, suggesting a strong rate-dependence of the toughness.
机译：乙烯/甲基丙烯酸（E / MAA）离聚物的非凡机械性能是其引人入胜且丰富的纳米级形态的结果，其中包括聚乙烯晶体，无定形聚合物链段和离子聚集体。本文的目的是为理解每个结构基元对诸如屈服应力（σy），极限应力/应变和韧性等关键非线性机械性能的独特贡献提供一个框架。我们通过互补的实验技术（包括机械测试，量热法和X射线散射）相结合的方法研究了这些性能。我们首先开发一个模型，该模型将聚乙烯晶体厚度和不完全非晶相弛豫机制与σγ关联，并在各种温度和温度下进行测量。应变率。利用温度和速率对屈服应力响应的反比关系，我们设计了一种用于创建sigma y主曲线的算法，从而将我们的预测能力扩展到远远超出典型的实验可访问速率窗口。 E / MAA共聚物表明，σ高度依赖于玻璃化转变温度（Tg），该温度随MAA含量的增加而增加。对于高MAA含量的共聚物，Tg可能超过室温。尽管结晶度降低，非晶相的玻璃化也会导致屈服应力的增加。在E / MAA离聚物中，MAA基团的部分中和相对于MAA基团的σ升高几倍。基础树脂。在这里，纳米级相分离导致富离子聚集体和贫离子域，且弛豫率相差很大。除了在极低的应变速率下，无定形相不能立即围绕离子聚集体松弛，这极大地增加了离聚物的屈服应力。向更高应变的方向，我们研究了第二个屈服肩之间的联系-一个反常现象应力与应变曲线图出现驼峰-以及延迟的永久变形。机械测试和X射线散射表明，该肩部表明聚乙烯晶体破裂的开始，这导致横向范围减小的晶体碎片。一旦去除负载，这些碎片之间形成的连接可防止材料缩回到初始样品尺寸，尽管通过加热或熔化样品可以实现更大的回收率。只有在第二屈服峰结束之后的应变下，链解缠才开始，从而导致真正不可恢复的变形。最后，我们审查了中和作用和测试速率对E / MAA离聚物最终性能的影响。我们发现中和同时增加了断裂应力，同时减小了断裂应变。结果是，韧性（以拉伸曲线下的面积评估）不会因离子化而显着变化。该令人惊讶的结果与高速拉伸冲击结果不一致，表明韧性的强速率依赖性。

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