Molecular Weight Dependence of Glassy Dynamics in Linear Polymers Revisited
Molecular Weight Dependence of Glassy Dynamics in Linear Polymers Revisited
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DOI：
10.1021/ma8016794
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发表时间：
2008-12-09
期刊：
影响因子：
5.5
通讯作者：
Roessler, E. A.
中科院分区：
文献类型：
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作者：
Hintermeyer, J.;Herrmann, A.;Roessler, E. A.
测量了一系列聚二甲基硅氧烷和聚苯乙烯的介电光谱，覆盖了包括单体极限在内的大分子量范围。从光谱分析中提取了段动力学的时间常数tau(alpha)(T)，从而确定了玻璃化转变温度T-g(M)和tau(alpha)(M)以及参数D(M)和T-0(M)，分别为Vogel-Fulcher-Tammann方程的Delta(M) = T-g -T -0。此外，我们分析了脆性m(m)，这里也包括聚丁二烯的数据。事实证明，所有的量都表现出作为M的函数的特殊性，而这些特殊性是不能用一般的智慧来解释的。T-g(M)以及T-0(M)数据揭示了与三种不同制度的不连续M依赖关系，当将我们的结果与流变学和核磁共振数据进行比较时，这可能变得合理。我们将这三种机制归因于简单的液体、劳斯和纠缠动力学，即T-g(M)和T-0(M)轨迹在劳斯单位M- r和纠缠分子量M-e处显示扭结。脆性m(m)在m -e处只显示一个扭结。这一事实可以用δ (M)的行为来解释，δ (M)在简单液体状态下随着M的增加而增加，但在MR时已经饱和；对于指数前因子tau(0)(M)也观察到类似的行为。我们得出结论，聚合物动力学特别改变玻璃动力学。
Dielectric spectra of a series of polydimethylsiloxanes and polystyrenes were measured covering a wide range of molecular weights M including the monomeric limit. From spectral analysis the time constants tau(alpha)(T) of the segmental dynamics are extracted, allowing determination of the glass-transition temperatures T-g(M) and tau(alpha)(M) as well as the parameters D(M) and T-0(M), respectively, Delta(M) = T-g - T-0, of the Vogel-Fulcher-Tammann equation. In addition, we analyzed fragility m(M) including here also data on polybutadiene. It turns out that all the quantities show particularities as a function of M which are not explained by common wisdom. T-g(M) as well as T-0(M) data reveal a noncontinuous M dependence with three distinct regimes which may become plausible when comparing our results with rheological and NMR data. We attribute the three regimes to simple liquid, Rouse, and entanglement dynamics, i.e., T-g(M) and T-0(M) traces show kinks at the Rouse unit M-R and entanglement molecular weight M-e. Fragility m(M) shows only one kink at M-e. This fact can be explained by the behavior of Delta(M), which increases with M in the simple liquid regime but saturates already at MR; a similar behavior is observed for the exponential prefactor tau(0)(M). We conclude that polymer dynamics specifically modify glassy dynamics.