Rath Sangram K, Edatholath Saji S, Patro T Umasankar, Sudarshan Kathi, Sastry P U, Pujari Pradeep K, Harikrishnan G
Naval Materials Research Laboratory, Shil-Badlapur Road, Ambernath, Maharashtra 421506, India.
Department of Chemical Engineering, Indian Institute of Technology Kharagpur, West Bengal 721302, India.
Phys Chem Chem Phys. 2016 Jan 28;18(4):2682-9. doi: 10.1039/c5cp04432d.
We conducted transport studies of a common solvent (toluene) in its condensed state, through a model hard-soft segmented polyurethane-clay nanocomposite. The solvent diffusivity is observed to be non-monotonic in a functional relationship with a filler volume fraction. In stark contrast, both classical tortuous path theory based geometric calculations and free volume measurements suggest the normally expected monotonic decrease in diffusivity with increase in clay volume fraction. Large deviations between experimentally observed diffusivity coefficients and those theoretically estimated from geometric theory are also observed. However, the equilibrium swelling of a nanocomposite as indicated by the solubility coefficient did not change. To gain an insight into the solvent interaction behavior, we conducted a pre- and post swollen segmented phase analysis of pure polymers and nanocomposites. We find that in a nanocomposite, the solvent has to interact with a filler altered hard-soft segmented morphology. In the altered phase separated morphology, the spatial distribution of thermodynamically segmented hard blocks in the continuous soft matrix becomes a strong function of filler concentration. Upon solvent interaction, this spatial distribution gets reoriented due to sorption and de-clustering. The results indicate strong non-barrier influences of nanoscale fillers dispersed in phase segmented block co-polymers, affecting solvent diffusivity through them. Based on pre- and post swollen morphological observations, we postulate a possible mechanism for the non-monotonic behaviour of solvent transport for hard-soft segmented co-polymers, in which the thermodynamic phase separation is influenced by the filler.
我们通过一种硬软段相间的聚氨酯-粘土纳米复合材料模型，对一种常见溶剂（甲苯）在其凝聚态下进行了传输研究。观察到溶剂扩散率与填料体积分数呈非单调函数关系。与之形成鲜明对比的是，基于经典曲折路径理论的几何计算和自由体积测量均表明，随着粘土体积分数的增加，扩散率通常会呈单调下降。实验观测到的扩散系数与几何理论估算值之间也存在较大偏差。然而，由溶解度系数所表明的纳米复合材料的平衡溶胀并未改变。为深入了解溶剂相互作用行为，我们对纯聚合物和纳米复合材料进行了溶胀前后的相分离分析。我们发现，在纳米复合材料中，溶剂必须与因填料而改变的硬软段相间形态相互作用。在改变后的相分离形态中，连续软基质中热力学相分离硬段的空间分布成为填料浓度的强函数。溶剂相互作用时，这种空间分布会因吸附和解聚集而重新定向。结果表明，分散在相分离嵌段共聚物中的纳米级填料具有很强的无障碍影响，通过它们影响溶剂扩散率。基于溶胀前后的形态观察，我们推测了硬软段相间共聚物溶剂传输非单调行为的一种可能机制，其中热力学相分离受填料影响。
