Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge, CB3 0AS, UK.
Soft Matter. 2018 Oct 31;14(42):8475-8482. doi: 10.1039/c8sm01468j.
We study the viscoelastic response of amorphous polymers using theory and simulations. By accounting for internal stresses and considering instantaneous normal modes (INMs) within athermal non-affine theory, we make parameter-free predictions of the dynamic viscoelastic moduli obtained in coarse-grained simulations of polymer glasses at non-zero temperatures. The theoretical results show very good correspondence with rheology data collected from molecular dynamics simulations over five orders of magnitude in frequency, with some instabilities that accumulate in the low-frequency part on approach to the glass transition. These results provide evidence that the mechanical glass transition itself is continuous and thus represents a crossover rather than a true phase transition. The relatively sharp drop of the low-frequency storage modulus across the glass transition temperature can be explained mechanistically within the proposed theory: the proliferation of low-eigenfrequency vibrational excitations (boson peak and nearly-zero energy excitations) is directly responsible for the rapid growth of a negative non-affine contribution to the storage modulus.
我们使用理论和模拟研究非晶态聚合物的粘弹性响应。通过考虑内部应力和在非定域非弹性理论中考虑瞬时正常模式 (INM)，我们对在非零温度下进行的聚合物玻璃的粗粒化模拟中获得的动态粘弹性模量进行了无参数预测。理论结果与从分子动力学模拟中收集的流变学数据非常吻合，在五个数量级的频率范围内，在接近玻璃化转变时，低频部分会出现一些不稳定性。这些结果表明，力学玻璃化转变本身是连续的，因此代表了一种交叉而不是真正的相变。在所提出的理论中，可以从力学上解释低频储能模量在玻璃化转变温度下的急剧下降：低本征频率振动激发（玻色峰和近零能量激发）的扩散直接导致对储能模量的负非定域贡献的快速增长。
