Department of Physics, Indian Institute of Technology Palakkad, Kanjikode, Kerala 678623, India.
Soft Materials Laboratory, Department of Physics and Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India.
J Phys Chem B. 2021 Nov 18;125(45):12617-12626. doi: 10.1021/acs.jpcb.1c07903. Epub 2021 Nov 3.
We present the water vapor-induced swelling and the emergence of a penetrant-induced glass-like transition in the substrate-supported glassy chitosan thin films. The time evolution of the film thickness under different levels of relative humidity conditions is measured in real-time using a spectroscopic ellipsometer equipped with a humidity cell. In a dry film, the network of chitosan chains is in a glassy state, and upon exposure to water vapor, initially, the film swells by Fickian diffusion of water molecules, which triggers the structural relaxations of the chains. Under higher humidity conditions, a relatively slower evolution of thickness succeeds the initial rapid swelling due to the non-Fickian sorption of water molecules. The swelling characteristics of the polymer films are accounted for by considering the diffusion-relaxation mechanism of chains in the presence of smaller penetrant molecules. The penetrant-induced glass-like transition (), where the polymer film isothermally transits from a glassy to a rubbery state, is determined for pristine and cross-linked chitosan films. is determined from the abrupt change in the rate of swelling observed upon increasing the relative humidity. Chemical crosslinking has an evident influence on the penetrant-induced glass-like transition of the chitosan films. was found to rise sharply for stiffer films with higher cross-linking density.
我们展示了水蒸气诱导的溶胀和基底支撑的玻璃态壳聚糖薄膜中渗透诱导的玻璃化转变。使用配备湿度单元的光谱椭偏仪实时测量不同相对湿度条件下薄膜厚度的时间演化。在干燥的薄膜中，壳聚糖链的网络处于玻璃态，暴露于水蒸气后，薄膜首先通过水分子的菲克扩散溶胀，这引发了链的结构松弛。在较高的湿度条件下，由于水分子的非菲克吸附，厚度的演化会出现相对较慢的初始快速溶胀。通过考虑存在较小渗透分子时链的扩散-松弛机制，解释了聚合物薄膜的溶胀特性。确定了未交联和交联壳聚糖薄膜的渗透诱导玻璃化转变（）。通过增加相对湿度观察到溶胀速率的突然变化来确定。化学交联对壳聚糖薄膜的渗透诱导玻璃化转变有明显的影响。对于交联密度更高的刚性薄膜，发现上升得非常快。
