A unified picture of the local dynamics of poly(dimethylsiloxane) across the melting point
A unified picture of the local dynamics of poly(dimethylsiloxane) across the melting point
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DOI：
10.1021/ma034843x
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发表时间：
2003-11-18
期刊：
影响因子：
5.5
通讯作者：
Telling, MTF
中科院分区：
文献类型：
--
作者：
Arrighi, V;Gagliardi, S;Telling, MTF
用准弹性中子散射(QENS)研究了聚二甲基硅氧烷(PDMS)的局域动力学。在215K(即低于熔化温度，T-m约为235K)下，甲基重定向是QENS光谱的主要成分。CH3基团的动力学用由弹性分量和准弹性分量组成的模型函数来解释，后者由洛伦兹线的高斯分布给出。在T-m以上，考虑了两个过程：(A)甲基旋转和(B)链段运动。后者的活化能为14.6kJ/mol，与流变学数据吻合较好。此外，与后者一致的是，通过逆傅立叶变换计算的中间散射函数I(q，t)根据流变位移因子遵循时间-温度叠加。分段运动对散射函数I(q，t)的贡献用一个拉伸的指数函数(或其在频域中的傅立叶变换)来拟合。分段运动的拉伸指数β在频域和时间域均为0.61，远高于Rouse模型的0.5，但与实际考虑链条刚度的理论结果一致。对PDMS节段运动的QENS研究表明，实验数据符合Rouse模型，直到非常大的Q，远远超出了该模型的有效范围。我们认为甲基的旋转运动是造成这一观察结果的原因。
The local dynamics of poly(dimethylsiloxane) (PDMS) has been investigated by quasi-elastic neutron scattering (QENS). Methyl group reorientations dominate the QENS spectra up to 215 K (i.e., below the melting temperature, T-m approximate to 235 K). The dynamics of the CH3 groups is interpreted in terms of a model function consisting of elastic and quasi-elastic components, the latter given by a Gaussian distribution of Lorentzian lines. Above T-m, the QENS spectra are analyzed considering two processes: (a) the methyl group rotation and (b) the segmental motion. The activation energy for the latter is 14.6 kJ/mol, in excellent agreement with rheological data. Moreover, in agreement with the latter, the intermediate scattering function, I(Q,t), computed via the inverse Fourier transform, follows time-temperature superposition according to the rheological shift factor. The contribution of the segmental motion to the scattering function I(Q,t) was fitted with a stretched exponential function (or its Fourier transform in the frequency domain). The fitted stretching exponent beta for segmental motion is 0.61 in both frequency and time domain, much higher than 0.5 (Rouse model), but in agreement with theoretical results realistically accounting for the chain stiffness. QENS studies of segmental motion in PDMS had indicated that the experimental data followed the Rouse model up to a very large Q, well beyond the validity range of the model. We suggest that the rotational motion of the methyl groups is responsible for this observation.