About the influence of different external fields on the swelling kinetics of PMAA hydrogels

Abstract

Hydrogels are 3D cross-linked structures that, due to their unique physical and chemical properties and possible applications, are the subject of intensive research in recent decades. In this work, the effects of external physical fields (ultrasonic (US) and microwave (MW)) on the isothermal swelling kinetics of poly(methacrylic acid) hydrogel were examined. A sample of poly(methacrylic acid) (PMAA) xerogel (PMMAX) was synthesized by the process of cross-linking free radical polymerization, which is described in detail in the paper [1]. The basic primary structural properties of PMMA-X including the equilibrium swelling degree in distilled water (SDeq), the density of xerogel (ρxg), the average molar mass between the crosslinks of a network (Mc), the degree of crosslinking (ρc) and the distance between the macromolecular chains (ξ), were determined and calculated. The isothermal kinetic swelling curves were determined by the tee-bag method at a temperature range of T= 293-323K in the presence of thermal, ultrasonic, and microwave fields. The isothermal kinetic curves of swelling were fitted with the Peppas’ kinetic model [2]. The values of the isothermal kinetic parameters of the Peppas model (k, n) were calculated. Based on the temperature dependence of the parameter using the Arrhenius equation, all values of the activation energy (Ea) and the pre-exponential factor (lnA) of the swelling process in the presence of different physical fields were calculated. The obtained results indicate that the presence of US and MW fields leads to an increase in the values of k, n, Ea, and compared to the corresponding values during conventional heating. The US field relatively slightly increases the value of SDeq, while the MW field significantly reduces the value of SDeq. The values of Ea and lnA at different physical fields are interrelated by a correlation relationship (compensation effect) which is given by the relation: lnAf=0.326+0.406Ea, f The existence of a compensation effect [3] is an indication of the unique mechanism of activation of the polymer network at different physical fields for swelling and of the quantum nature of Ea. The influence of US and MW fields on the diffusion of water molecules through the polymer network, activation of the polymer network for swelling, and relaxation of the polymer network was analyzed.