用X射线光子相关光谱研究偶氮聚合物分子层中的慢动力学
Slow dynamics in an azopolymer molecular layer studied by x-ray photon correlation spectroscopy
摘要：
报道了一种光敏偶氮聚合物LB膜的X射线光子相关谱（XPCS）实验结果。在不同的温度和动量转移（q）和不同的光照条件下（黑暗，紫外线或可见光）的时间关联函数已被测量。关联函数可以用弛豫时间与q^-1成比例的Kohlrausch-Williams-Watts（KWW）形式很好地描述，在其他系统中，这已经用间歇重排来解释[L. Cipelletti等人，物理修订版信函84，2275-2278（2000）]或弹性介质内的随机偶极相互作用[J. P. Bouchaud和E.皮塔尔J.E6，231-236（2001）]。特征弛豫时间遵循众所周知的Vogel-Fulcher-Tammann定律，该定律描述了该聚合物的体积粘度的温度依赖性。UV光扰动加速的松弛动力学，在定性协议与流化效果的UV光扰动先前观察到的表面流变仪，并用于驱动系统的平衡。瞬态动力学的特征在于，由两次关联函数的方差chi。chi的一个明显峰值出现在一个明确的时间τ_C，它与q^-1和老化时间成比例，其方式与以前在胶体悬浮液中报道的类似[O. Dauchot等人，物理修订版信函95，265701（2005）]。从精确的分析的关联函数，我们可以证明一个温度依赖的交叉从KWW压缩到简单的指数行为，这是修改的流化由于侧链偶氮苯基团的顺反异构化的光泵。
英文摘要：
We report the results of X-ray photon correlation spectroscopy (XPCS) experiments on Langmuir Blodgett multilayers of a photosensitive azo-polymer. Time correlation functions have been measured at different temperatures and momentum transfers (q) and under different illumination conditions (darkness, UV or visible). The correlation functions are well described by the Kohlrausch-Williams-Watts (KWW) form with relaxation times that are proportional to q^-1, which in other systems have been explained in terms of intermittent rearrangements [L. Cipelletti et al., Phys. Rev. Lett. 84, 2275-2278 (2000)] or random dipolar interactions within an elastic medium [J.- P. Bouchaud and E. Pitard, Eur. Phys. J. E 6, 231-236 (2001)]. The characteristic relaxation times follow the well known Vogel-Fulcher-Tammann law describing the temperature dependence of the bulk viscosity of this polymer. UV photoperturbation accelerates the relaxation dynamics, in qualitative agreement with the fluidification effect of UV photo-perturbation previously observed by surface rheometry, and is used to drive the system out of equilibrium. Transient dynamics is characterized, by the variance chi of the two-times correlation functions. A clear peak in chi appears at a well defined time tau_C which scales with q^-1 and with the ageing time, in a similar fashion as previously reported in colloidal suspensions [O. Dauchot et al., Phys. Rev. Lett. 95, 265701 (2005)]. From an accurate analysis of the correlation functions we could demonstrate a temperature dependent cross-over from KWW compressed to simple exponential behaviour, which is modified by the fluidification due to the optical pumping of the cis-trans isomerisation of the side-chain azobenzene group.
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