Negative Energy Elasticity in a Rubberlike Gel
Negative Energy Elasticity in a Rubberlike Gel
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DOI：
10.1103/physrevx.11.011045
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发表时间：
2021-03-05
影响因子：
12.5
通讯作者：
Sakai, Takamasa
中科院分区：
文献类型：
--
作者：
Yoshikawa, Yuki;Sakumichi, Naoyuki;Sakai, Takamasa
橡胶弹性是热力学第二定律中熵力的原型;对天然橡胶和合成橡胶的大量实验和理论研究表明，弹性主要源于形变的熵变。类似地，在含有大量溶剂的聚合物凝胶中，也已经假定作为一种弹性模量的剪切模量（刚性模量）G近似等于熵贡献G（S），但是这还有待于实验验证。在这项研究中，我们测量的剪切模量G的温度依赖性的橡胶状（超弹性）聚合物凝胶，其聚合物的体积分数是至多0.1。结果，我们发现能量贡献G（E）= G-G（S）可以是显著的负值，达到剪切模量G的两倍（即，垂直杆G（E）垂直杆类似或等于2G），尽管稳定材料的剪切模量通常必然为正。我们进一步认为，能量贡献G（E）是由一个消失的温度，这是一个通用的函数的归一化聚合物浓度，和G（E）消失时，溶剂被删除。我们的研究结果突出了橡胶弹性和凝胶弹性之间的本质区别（以前认为是相同的），并将凝胶弹性的既定领域推向一个新的方向。
Rubber elasticity is the archetype of the entropic force emerging from the second law of thermodynamics; numerous experimental and theoretical studies on natural and synthetic rubbers have shown that the elasticity originates mostly from entropy change with deformation. Similarly, in polymer gels containing a large amount of solvent, it has also been postulated that the shear modulus (the modulus of rigidity) G, which is a kind of modulus of elasticity, is approximately equivalent to the entropy contribution G(S), but this has yet to be verified experimentally. In this study, we measure the temperature dependence of the shear modulus G in a rubberlike (hyperelastic) polymer gel whose polymer volume fraction is at most 0.1. As a result, we find that the energy contribution G(E) = G - G(S) can be a significant negative value, reaching up to double the shear modulus G (i.e., vertical bar G(E)vertical bar similar or equal to 2G), although the shear modulus of stable materials is generally bound to be positive. We further argue that the energy contribution G(E) is governed by a vanishing temperature that is a universal function of the normalized polymer concentration, and G(E) vanishes when the solvent is removed. Our findings highlight the essential difference between rubber elasticity and gel elasticity (which were previously thought to be the same) and push the established field of gel elasticity into a new direction.