Direct Evidence for Percolation of Immobilized Polymer Layer around Nanoparticles Accounting for Sol-Gel Transition in Fumed Silica Dispersions
Direct Evidence for Percolation of Immobilized Polymer Layer around Nanoparticles Accounting for Sol-Gel Transition in Fumed Silica Dispersions
复制标题
纳米颗粒周围固定聚合物层渗透解释气相二氧化硅分散体中溶胶-凝胶转变的直接证据
DOI：
10.1021/acs.langmuir.5b03899
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发表时间：
2015
期刊：
影响因子：
3.9
通讯作者：
Zheng Qiang
中科院分区：
文献类型：
--
作者：
Zheng Zhong;Song Yihu;Yang Ruiquan;Zheng Qiang
固定化的聚合物部分已被声称是至关重要的溶胶-凝胶转变通常在纳米粒子分散体中观察到的，但仍然是一个争论的问题，关于机制和预测的难度。在这里，我们研究了亲水性和疏水性气相二氧化硅（FS）纳米粒子的表面附近的三官能度聚醚多元醇（PPG）的固定层结构，以揭示表面化学对分散体的分子动力学和溶胶-凝胶转变的作用。采用调制式差示扫描量热法，测定了玻璃化转变比热容和冷结晶焓。与形成完全固定化（玻璃状）层的疏水FS相比，我们发现亲水FS固定化更多PPG，形成部分固定化外层，其不能在内部玻璃状层旁边结晶。通过将玻璃层的厚度与纳米颗粒之间的最小间距的一半相关联，我们直接证明了该层沿着负责溶胶-凝胶转变的最近邻纳米颗粒的渗透。使用有效的体积分数，包括玻璃层，我们成功地构建主曲线的相对粘度的亲水性和疏水性FS分散体，指向一个共同的溶胶-凝胶转变机制介导的表面化学。
Immobilized polymer fractions have been claimed to be of vital importance for sol–gel transitions generally observed in nanoparticle dispersions but remain a matter of debate regarding mechanism and difficulty for prediction. Here we investigate the immobilized layer structures of trifunctionality polyether polyol (PPG) near the surfaces of hydrophilic and hydrophobic fumed silica (FS) nanoparticles to reveal the role of surface chemistry on the molecular dynamics and sol–gel transitions of the dispersions. Using modulated differential scanning calorimetry, we measure the specific heat capacity during glass transition and the enthalpy during cold-crystallization. Comparing with hydrophobic FS that forms a fully immobilized (glassy) layer, we find that hydrophilic FS immobilizes more PPG, forming a partially immobilized outer layer being unable to crystallize next to the inner glassy layer. By correlating the thickness of the glassy layer with half of the minimum spacing between nanoparticles, we directly evidence the percolation of this layer along the nearest neighbor nanoparticles responsible for the sol–gel transition. Using effective volume fraction including the glassy layer, we successfully construct master curves of relative viscosity of both hydrophilic and hydrophobic FS dispersions, pointing to a common sol–gel transition mechanism mediated by the surface chemistry.