Characteristic size of molecular dynamics in polymers probed by dielectric probes of variable length
Characteristic size of molecular dynamics in polymers probed by dielectric probes of variable length
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DOI：
10.1016/j.jnoncrysol.2005.03.063
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发表时间：
2005-09
影响因子：
3.5
通讯作者：
O. V. D. Berg;M. Wübbenhorst;S. Picken;W. Jager
中科院分区：
文献类型：
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作者：
O. V. D. Berg;M. Wübbenhorst;S. Picken;W. Jager
最近，我们引入了一种刚性棒型生色团E-4，4 ′-N，N-二丁基氨基硝基二苯乙烯（DBANS），它可以作为介电和荧光探针，通过介电弛豫光谱（DRS）研究非极性聚合物的分子动力学。这项工作的重点是对特定的探针响应的探针的大小和浓度的影响。为此目的，使用一系列具有0.6和2nm之间的核长度（l）的探针分子以0.1至1.0重量%的浓度“掺杂”聚苯乙烯。对于所有的探针，发现在整个探针浓度范围内的固有α-弛豫的选择性放大是线性的。此外，两个明显的尺寸效应的探针动力学被揭示。在高温下，探针的弛豫时间服从τ π 1 ~ 3，证实了旋转扩散是聚合物链段动力学和介电探针响应之间的联系机制。最短的探针揭示了一个额外的Arkenius型（β-）松弛，这是由于大的角度探针波动涉及离散跳跃。它的唯一出现表明了一个临界的较低的探针尺寸（1纳米至0.6纳米），低于该尺寸，介电探针也作为一个“自由体积探针”，如[公式：见正文]依赖性中明显的异常所示。最后，试图将探针浓度对玻璃化转变温度（塑化效应）的影响与合作动力学的固有长度尺度联系起来。基于一个简单的局部增塑模型，我们已经确定了一个下限为5 nm的顺序。
Recently we introduced a rigid-rod type chromophore, E-4,4′-N,N-dibutylamino-nitro-stilbene (DBANS), that acts as a dielectric and fluorescent probe, and enables the study of molecular dynamics in apolar polymers by dielectric relaxation spectroscopy (DRS). This work focuses on the effect of the probe size and concentration on the specific probe response. For this purpose, a series of probe molecules with core lengths (l) between 0.6 and 2nm was used to ‘dope’ polystyrene at concentrations from 0.1 to 1.0wt%. For all probes, a selective amplification of the intrinsic α-relaxation was found that was linear within the whole probe concentration range. Moreover, two clear size effects on the probe dynamics were revealed. At high temperatures, the probe relaxation time obeys τ∝l3, which confirmed rotational diffusion as the linking mechanism between the polymeric segmental dynamics and dielectric probe response. The shortest probe revealed an additional Arrhenius-type (β∗) relaxation that was attributed to large angular probe fluctuations involving discrete jumps. Its exclusive occurrence indicates a critical lower probe size (l∗∼0.6nm) below which the dielectric probe acts also as a ‘free-volume probe’ as indicated by the pronounced anormality in the [Formula: see text] dependence. Finally, an attempt was made to relate probe concentration effects on the glass transition temperature (plasticizing effect) to the intrinsic length scale of cooperative dynamics ξ. Based on a simple model for local plasticization we have determined a lower bound for ξ in the order of 5nm.