Tombari Elpidio, Johari G P
Istituto per i Processi Chimico-Fisici del CNR, via G. Moruzzi 1, 56124 Pisa, Italy.
Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada.
J Chem Phys. 2015 Mar 14;142(10):104501. doi: 10.1063/1.4913759.
To investigate whether a non-exponential relaxation always indicates 2-4 nm-size regions of dynamic heterogeneity, we studied the kinetic freezing and unfreezing of structural fluctuations involving the rotational modes in orientationally disordered crystal (ODIC) of levoglucosan by calorimetry. The heat capacity, Cp, of levoglucosan measured over the 203 K-463 K range shows that its low-temperature, orientationally ordered crystal (ORC) transforms to ODIC phase on heating, which then melts to a low viscosity liquid. On cooling, the melt transforms to the ODIC which then does not transform to the ORC. Instead, the ODIC supercools. Fluctuations resulting from hindered (random) rotations of levoglucosan molecules confined to the lattice sites and from their conformational changes become progressively slower on cooling and an orientational glass (O-G) forms showing the sigmoid shape decrease in Cp characteristic of structural arrest like that of a glass. On heating the O-G state, rotational fluctuations begin to contribute to Cp at To-g of 247.8 K and there is an overshoot in Cp and thermal hysteresis (characteristic of physical ageing) in the temperature range of 230-260 K. The non-exponential relaxation parameter, β(cal), determined by fitting the Cp data to a non-exponential, nonlinear model for relaxation of a glass is 0.60, which is similar to β(cal) found for polymers, molecular liquids, and metal-alloy melts in which Brownian diffusion occurs. Such β(cal) < 1 are seen to indicate 2-4 nm-size dynamically heterogeneous domains in an ultraviscous liquid near the glass formation, but its value of 0.60 for ODIC levoglucosan, in which Brownian diffusion does not occur, would not indicate such domains. Despite the lack of Brownian diffusion, we discuss these findings in the potential energy landscape paradigm. Levoglucosan melt, which is believed to vitrify and to stabilize a protein's disordered structure, did not supercool even at 200 K/min cooling rate. The findings have consequences for reports on the dielectric relaxation studies that indicated that levoglucosan melt supercools to form a structural glass of Tg of ∼245 K, and for computer simulation of its dynamics. Levoglucosan is the ninth ODIC that forms O-G. It does so more easily than the other eight.
为了研究非指数弛豫是否总是表明存在2 - 4纳米尺寸的动态异质性区域，我们通过量热法研究了左旋葡萄糖酐取向无序晶体（ODIC）中涉及旋转模式的结构涨落的动力学冻结和解冻过程。在203 K - 463 K范围内测量的左旋葡萄糖酐的热容Cp表明，其低温取向有序晶体（ORC）在加热时转变为ODIC相，然后熔化为低粘度液体。冷却时，熔体转变为ODIC，然后不再转变为ORC。相反，ODIC会过冷。左旋葡萄糖酐分子局限于晶格位置的受阻（随机）旋转及其构象变化所导致的涨落在冷却时逐渐变慢，形成一种取向玻璃（O - G），其Cp呈现出类似玻璃结构冻结的S形下降。加热O - G态时，在247.8 K的To - g温度下旋转涨落开始对Cp有贡献，并且在230 - 260 K温度范围内Cp会出现峰值以及热滞现象（这是物理老化的特征）。通过将Cp数据拟合到玻璃弛豫的非指数非线性模型确定的非指数弛豫参数β(cal)为0.60，这与在发生布朗扩散的聚合物、分子液体和金属合金熔体中发现的β(cal)相似。这样的β(cal) < 1被认为表明在接近玻璃形成的超粘性液体中存在2 - 4纳米尺寸的动态异质区域，但对于不发生布朗扩散的ODIC左旋葡萄糖酐，其β(cal)值为0.60并不表明存在这样的区域。尽管缺乏布朗扩散，我们仍在势能景观范式中讨论这些发现。据信能使蛋白质无序结构玻璃化并稳定化的左旋葡萄糖酐熔体，即使在200 K/min的冷却速率下也不会过冷。这些发现对关于介电弛豫研究的报告有影响，该报告表明左旋葡萄糖酐熔体过冷形成玻璃化转变温度约为245 K的结构玻璃，并且对其动力学的计算机模拟也有影响。左旋葡萄糖酐是第九种形成O - G的ODIC。它比其他八种更容易形成O - G。
