Spatial Heterogeneity Accompanying Gel Formation of Poly(N-isopropylacrylamide) Aqueous Solution at a Temperature below Cloud Point
Spatial Heterogeneity Accompanying Gel Formation of Poly(N-isopropylacrylamide) Aqueous Solution at a Temperature below Cloud Point
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DOI：
10.1021/acs.macromol.0c02292
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发表时间：
2020-12-22
期刊：
影响因子：
5.5
通讯作者：
Tanaka, Keiji
中科院分区：
文献类型：
--
作者：
Kogo, Takuro;Shundo, Atsuomi;Tanaka, Keiji
聚n -异丙基丙烯酰胺（PNIPAM）是一种典型的热响应性聚合物，其水溶液在类似于304 K的温度高于云点（T-cp）时发生相分离，在各种生物医学应用中发挥着重要作用。因此，为了进一步促进PNIPAM作为功能材料的使用，有必要更好地了解其相行为，特别是在略低于Tq的温度下。为此，我们通过粒子跟踪技术研究了浓度为10 wt %的无策略PNIPAM溶液的局部流变特性，其中探针粒子在溶液中的热运动被跟踪，结合傅里叶变换红外光谱、小角度x射线散射测量、荧光光谱和共聚焦激光扫描显微镜。在远低于T-cp的温度下，溶液呈现单相均相特性。PNIPAM水溶液在低于T-cp （302 K）的温度下不受干扰后，形成物理凝胶。通过改变探针颗粒的大小进行跟踪测量，我们发现所得凝胶在大约50-100纳米的长度范围内，其流变性能具有空间异质性。PNIPAM链中疏水珍珠的形成促进了溶液的凝胶化，形成了具有非均质性的网络结。在这里获得的知识应该有助于理解和控制PNIPAM溶液的相行为，从而导致热响应功能材料的进一步发展。
Poly(N-isopropylacrylamide) (PNIPAM) is a representative thermoresponsive polymer, and its aqueous solution becomes phase-separated at a temperature higher than the cloud point (T-cp) at similar to 304 K, which plays an important role in various biomedical applications. Thus, to further promote the use of PNIPAM as a functional material, it is necessary to gain a better understanding of its phase behavior especially at a temperature just below Tq,. To this end, we examined the local rheological properties of atactic PNIPAM solutions with a concentration of 10 wt % by a particle tracking technique, in which the thermal motion of probe particles in the solution was tracked, in conjunction with Fourier-transform infrared spectroscopy, small-angle X-ray scattering measurements, fluorescence spectroscopy, and confocal laser scanning microscopy. At temperatures far below T-cp, the solution exhibited one-phase homogeneous characteristics. After the PNIPAM aqueous solution was left undisturbed at a temperature just below T-cp (302 K), it formed a physical gel. Changing the size of probe particles for the tracking measurements, we found that the resultant gel was spatially heterogeneous in terms of its rheological properties at a length scale of approximately 50-100 nm. The gelation of the solution promoted by the formation of hydrophobic pearls in PNIPAM chains led to the formation of network junctions with heterogeneity. The knowledge obtained here should be useful for understanding and controlling the phase behavior of the PNIPAM solution, thereby leading to the further development of thermoresponsive functional materials.