利用单分子光学探针测量幂律分布的聚合物动力学

Single molecule optical-probes measured power law distribution of polymer dynamics

研究聚合物薄膜纳米尺度的动力学特性对于高性能材料的制备具有重要的意义.本文利用尼罗红单分子作为光学探针吸附在聚丙烯酸甲酯(PMA)聚合物链上,研究该聚合物薄膜的动力学特性.通过单分子散焦宽场荧光成像显微镜技术测量了单分子随PMA聚合物链转动弛豫的三维再取向特性,当环境温度高于PMA的玻璃点温度19 K时,发现处于PMA聚合物薄膜中的单分子光学探针的转动态和非转动态的持续时间概率密度服从指数截止的幂律分布.研究结果表明该温度下PMA聚合物薄膜的纳米环境动力学仍存在空间和时间异构性.

The optical signals of single molecules provide information about structures and dynamic behaviors of their nanoscale environments, and eliminations of space and time averaging effect. These are particularly useful whenever complex structures or dynamic behaviors are present, especially in polymers. The single molecules absorbed onto polymer chains rotate with rotational relaxation of polymer chains. Thus, we can measure the dynamic properties of polymer thin films by measuring the rotational properties of single molecules. Here, we use single Nile Red（NR） dye molecules as nanoprobes to measure polymer dynamic behaviors of poly（methyl acrylate）（PMA） polymer film. The polymer films are prepared on cleaned glass coverslips by spin-coating 1.0 wt.% solution of PMA containing ～10^-9mol/L NR molecules in toluene. Defocused wide-field fluorescence microscopy is used to measure the three-dimensional molecular rotational diffusion of single NR molecules in PMA polymer thin film. The local environmental change driven by heterogeneous dynamics of the polymer can be probed by parallel imaging of several molecules. It is found that at Tg＋19 K, rotations of NR single molecules in different nano-areas are in two different ways, i.e., rotational way（rotational molecules account for ～83%） and non-rotaional way（non-rotational molecules occupy ～17%）. The rotational molecules include the single molecules of intermittent rotation with a short time and a long time. The different rotational patterns indicate that there is still a spatial and temporal heterogeneity of dynamics in PMA polymer film at a temperature of Tg＋19 K. The autocorrelation function C（t） of angular change of dipole orientation of NR single molecules is calculated to reveal the property of polymer dynamics. The decay of C（t） can be fitted by Kohlrausch-Williams-Watt stretched exponential function. The averaged timescale of rotational diffusion τc for 183 rotational NR single molecules indicates that the timescale of polymer dynamics at 300 K is ～3 s. In order to investigate the temporal heterogeneity of PMA polymer dynamics, we define a threshold to separate the single molecular rotation into two parts： rotational state and nonrotational state. According to the statistics of duration time of rotational state and non-rotational state, we can obtain the probability densities of duration time of rotational states and non-rotational states of the single molecules. The probability densities obey a truncated power law, which indicates that there are still the behaviors of trapping and self-trapping in PMA polymer chains at Tg＋19 K. The researches of spatial and temporal heterogeneity of dynamics of PMA polymers in nano-environment have great significance for preparing the high performance materials.