Confinement Effects on Chain Dynamics and Local Chain Order in Entangled Polymer Melts.
Confinement Effects on Chain Dynamics and Local Chain Order in Entangled Polymer Melts.
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DOI：
10.1021/ma1003248
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发表时间：
2010-05-25
期刊：
影响因子：
5.5
通讯作者：
Saalwächter K
中科院分区：
文献类型：
--
作者：
Ok S;Steinhart M;Serbescu A;Franz C;Vaca Chávez F;Saalwächter K
导论.纳米限制对聚合物结构和动力学的影响已经关注了超过15年，主要目的是为纳米级聚合物应用（如复合材料）提供物理理解。Tg的变化可能是研究最多的，但仍然有争议的问题，与明确和直观的预期迹象的情况下，强烈的吸收表面，但有时相互矛盾的趋势和缺乏理解的物理基础往往降低Tg接近自由或nonabsorbing接口的减速段动态。1-8人们可以预期，Tg的潜在变化，与节段性弛豫的时间尺度有关，也应该直接影响更大尺度的弛豫。然而，即使在Tg附近缓慢的机械松弛和低于Tg的老化过程似乎与观察到的Tg变化没有简单的关系，9，10并且可以在远超过回转半径（Rg）的长度尺度上持续存在的改性单体堆积中寻找原因，如在计算机模拟中看到的。2在更大的长度尺度上，全链构象的变化，再次在模拟中看到，2，4已经通过散射技术进行了评估，11-15大多数研究得出的结论是没有偏离高斯行为。然而，自集中预计将增加本地，因为链的随机行走接近中性界面上反映到自己。因此，链间缠结密度的降低被认为是受限链流动增强的原因。15-18相反，“粘性”表面或局部取向效应以及因此接近界面的片段的各向异性动力学被认为是弹性增强19或扩散系数降低的原因。20，21有趣的是，对于接近自由表面的链，也报告了扩散的减缓，22与其他人观察到的（有时只是短暂的）粘度降低形成明显对比。[15，17，18]因此，退火和非平衡效应可能发挥重要作用。7，18注意，在所有情况下，观察到的影响范围高达或甚至超过Rg。在这篇文章中，我们提出了第一个结果的各向异性程度的纠缠和几何约束链在两种不同类型的定义良好的模型纳米复合材料与弱相互作用的界面。我们解决的温度远高于Tg和时间尺度超过体纠缠时间τe，提供了一个分子基础上的观点所讨论的现象。我们使用最近建立的多量子（MQ）NMR技术，23-26，其通过单体平均的表观残余偶极耦合常数Dres探测局部链取向的时间依赖性程度。在给定的时间和温度下，短脉冲序列时间τDQ的实验强度取决于该量的平方;更准确地说，它与方向自相关函数成正比
Introduction. Effects of nanometric confinement on polymer structure and dynamics have been in the focus for more than 15 years, primarily with the aim of providing the physical understanding for nanoscale polymer applications such as in composite materials. Changes in Tg are probably the most researched, but still controversial issue, with clear and intuitively expected indications of slowed-down segmental dynamics for the case of strongly absorbing surfaces but sometimes conflicting trends and a lack of understanding of the physical basis for the often decreased Tg close to free or nonabsorbing interfaces. 1-8 One may expect that potential changes in Tg, being related to the time scale of segmental relaxation, should directly influence also larger scale relaxations. However, even slow mechanical relaxation around Tg and aging processes below Tg appear to bear no simple relation to the observed Tg changes, 9, 10 and the reason may be sought in a modified monomer packing persisting over length scales much beyond the radius of gyration (Rg), as seen in computer simulations. 2 At larger length scales, changes in the whole-chain conformation, as again seen in simulations, 2, 4 have been assessed by scattering techniques, 11-15 with most studies concluding no deviation from Gaussian behavior. Nevertheless, the selfconcentration is expected to be increased locally since the random walk of a chain close to a neutral interface is reflected onto itself. Consequently, a reduced interchain entanglement density has been discussed as the reason for enhanced flow of confined chains. 15-18 In contrast, a “sticky” surface or local orientation effects and thus anisotropic dynamics of the segments close to an interface are thought to be the reason for enhanced elasticity19 or reduced diffusion coefficients. 20, 21 Interestingly, a slowdown of diffusion has also been reported for chains close to a free surface, 22 in apparent contrast to the (sometimes only transiently) reduced viscosity observed by others. 15, 17, 18 Annealing and nonequilibrium effects may thus play an important role. 7, 18 Note that in all cases the effects were observed in a range up to or even beyond Rg. In this Communication, we present first results on the degree of anisotropy of segmental orientation fluctuations in entangled and geometrically confined chains in two different types of well-defined model nanocomposites with weakly interacting interfaces. We address temperatures far above Tg and time scales beyond the bulk entanglement time τe, providing a molecular-basis view of the discussed phenomena. We use a recently established multiple-quantum (MQ) NMR technique, 23-26 which probes the time-dependent degree of local chain orientation via the monomer-averaged apparent residual dipolar coupling constant, Dres. At a given time and temperature, the experimental intensity at short pulse-sequence times τDQ depends on the square of this quantity; more precisely, it is directly proportional to the orientation autocorrelation function
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DOI：
10.1209/0295-5075/27/1/011
发表时间：
1994-07-01
期刊：
EUROPHYSICS LETTERS
影响因子：
--
作者：
KEDDIE, JL;JONES, RAL;CORY, RA
通讯作者：
CORY, RA
影响因子：
56.9
作者：
HU, HW;GRANICK, S
通讯作者：
GRANICK, S
影响因子：
3.2
作者：
Li, AP;Muller, F;Gosele, U
通讯作者：
Gosele, U
影响因子：
56.9
作者：
Fakhraai, Z.;Forrest, J. A.
通讯作者：
Forrest, J. A.
影响因子：
8.6
作者：
Barbero, David R.;Steiner, Ullrich
通讯作者：
Steiner, Ullrich