Yang Man, Zhao Kongshuang
College of Chemistry, Beijing Normal University , Beijing 100875, China.
J Phys Chem B. 2015 Oct 15;119(41):13198-207. doi: 10.1021/acs.jpcb.5b05491. Epub 2015 Oct 1.
A new experimental result from dielectric spectroscopy of poly(N-isopropylacrylamide)/poly(acrylic acid) semi-interpenetrating polymer network (PNIPAM/PAA SIPN) microgel, which undergoes significant volume phase transition, is reported. Two significant dielectric relaxations were observed around 0.1-0.5 MHz and 1-5 MHz, respectively. The high-frequency relaxation is attributed to the migration of counterions tangentially and radially along the domain formed by linear PAA chains (counterion polarization). The temperature dependence of the domain size obtained from this relaxation shows that the SIPN microgel with higher content of PAA has better thermal response and swelling property. The low-frequency relaxation shows two separate mechanisms below and above the volume phase transition temperature (VPTT), which are dominated by different relaxation processes, respectively: micro-Brownian movement of solvated side groups of PNIPAM dominates when T < VPTT, while the interfacial polarization does when T > VPTT. A dielectric model was proposed to describe the collapsed microspheres suspension, from which the electrical parameters of microgel were calculated. The permittivity of microgel shows that a special ordered arrangement of water molecules is formed in microgel with less PAA. Thermodynamic parameters obtained from Eyring equation reveal that the difference in PAA content has a great influence on the thermodynamics of the phase transition process. Besides, it was found that the VPTT of the SIPN microgel was significantly increased compared with pure PNIPAM hydrogel microspheres. The essence of anomalous VPTT revealed by relaxation mechanism is the difference in composition content leading to different hydrophilic/hydrophobic and electrostatic interaction. Determining the reason for anomalous VPTT is of instructive significance to understand the volume phase transition of complex polymer materials.
报道了聚（N-异丙基丙烯酰胺）/聚（丙烯酸）半互穿聚合物网络（PNIPAM/PAA SIPN）微凝胶介电谱的一项新实验结果，该微凝胶会发生显著的体积相变。分别在0.1 - 0.5 MHz和1 - 5 MHz附近观察到两个显著的介电弛豫。高频弛豫归因于抗衡离子沿线性PAA链形成的区域切向和径向迁移（抗衡离子极化）。由该弛豫得到的区域尺寸的温度依赖性表明，PAA含量较高的SIPN微凝胶具有更好的热响应和溶胀性能。低频弛豫在体积相变温度（VPTT）以下和以上表现出两种不同的机制，分别由不同的弛豫过程主导：当T < VPTT时，PNIPAM溶剂化侧基的微布朗运动起主导作用，而当T > VPTT时，界面极化起主导作用。提出了一个介电模型来描述塌陷微球悬浮液，据此计算了微凝胶的电学参数。微凝胶的介电常数表明，在PAA含量较少的微凝胶中形成了水分子的特殊有序排列。从Eyring方程得到的热力学参数表明，PAA含量的差异对相变过程的热力学有很大影响。此外，发现与纯PNIPAM水凝胶微球相比，SIPN微凝胶的VPTT显著升高。由弛豫机制揭示的异常VPTT的本质是组成含量的差异导致不同的亲水/疏水和静电相互作用。确定异常VPTT的原因对于理解复杂聚合物材料的体积相变具有指导意义。
