Chakraborty Priyadarshi, Das Sujoy, Mondal Sanjoy, Bairi Partha, Nandi Arun K
Polymer Science Unit, Indian Association for the Cultivation of Science , Jadavpur, Kolkata 700 032, India.
Langmuir. 2016 Feb 23;32(7):1871-80. doi: 10.1021/acs.langmuir.5b04714. Epub 2016 Feb 10.
In the quest to engineer the nanofibrillar morphology of folic acid (F) gel, poly(4-vinylpyridine-co-styrene) (PVPS) is judiciously integrated as a polymeric additive because of its potential to form H-bonding and π-stacking with F. The hybrid gels are designated as F-PVPSx gels, where x denotes the amount of PVPS (mg) added in 2 mL of F gel (0.3%, w/v). The assistance of PVPS in the gelation of F is manifested from the drop in critical gelation concentration and increased fiber diameter and branching of F-PVPSx gels compared to that of F gel. PVPS induces a magnificent improvement of mechanical properties: a 500 times increase of storage modulus and ∼62 times increase of yield stress in the F-PVPS5 gel compared to the F gel. The complex modulus also increases with increasing PVPS concentration with a maximum in F-PVPS5 gel. Creep recovery experiments suggest PVPS induced elasticity in the otherwise viscous F gel. The fluorescence intensity of F-PVPSx gels at first increases with increasing PVPS concentration showing maxima at F-PVPS5 gel and then slowly decreases. Gelation is monitored by time-dependent fluorescence spectroscopy, and it is observed that F and F-PVPSx gels exhibit perfectly opposite trend; the former shows a sigmoidal decrease in fluorescence intensity during gelation, but the latter shows a sigmoidal increase. The gelation rate constants calculated from Avrami treatment on the time-dependent fluorescence data manifest that PVPS effectively enhances the gelation rate showing a maximum for F-PVPS5 gel. The hybrid gel exhibit 5 orders increase of dc conductivity than that of F-gel showing semiconducting nature in the current-voltage plot. The Nyquist plot in impedance spectra of F-PVPS5 xerogel exhibit a depressed semicircle with a spike at lower frequency region, and the equivalent circuit represents a complex combination of resistance-capacitance circuits attributed to the hybrid morphology of the gel fibers.
在构建叶酸（F）凝胶的纳米纤维形态的过程中，聚（4-乙烯基吡啶-共-苯乙烯）（PVPS）被明智地用作聚合物添加剂，因为它有与F形成氢键和π堆积的潜力。这种混合凝胶被命名为F-PVPSx凝胶，其中x表示在2 mL F凝胶（0.3%，w/v）中添加的PVPS的量（mg）。与F凝胶相比，F-PVPSx凝胶的临界凝胶浓度降低、纤维直径增加且分支增多，这表明PVPS有助于F的凝胶化。PVPS极大地改善了机械性能：与F凝胶相比，F-PVPS5凝胶的储能模量增加了500倍，屈服应力增加了约62倍。复数模量也随着PVPS浓度的增加而增加，在F-PVPS5凝胶中达到最大值。蠕变恢复实验表明，PVPS在原本粘性的F凝胶中诱导了弹性。F-PVPSx凝胶的荧光强度起初随着PVPS浓度的增加而增加，在F-PVPS5凝胶中达到最大值，然后缓慢下降。通过时间相关荧光光谱监测凝胶化过程，观察到F凝胶和F-PVPSx凝胶呈现出完全相反的趋势；前者在凝胶化过程中荧光强度呈S形下降，而后者呈S形增加。根据对时间相关荧光数据进行的阿弗拉米处理计算出的凝胶化速率常数表明，PVPS有效地提高了凝胶化速率，在F-PVPS5凝胶中达到最大值。与F凝胶相比，这种混合凝胶的直流电导率增加了5个数量级，在电流-电压图中显示出半导体性质。F-PVPS5干凝胶阻抗谱中的奈奎斯特图呈现出一个压低的半圆，在较低频率区域有一个尖峰，等效电路代表了归因于凝胶纤维混合形态的电阻-电容电路的复杂组合。
