HPMC Hydrogel Formation Mechanisms Unveiled by the Evaluation of the Activation Energy.

Saray Perez-Robles,Claudia Carotenuto,Mario Minale

doi:10.3390/polym14030635

Saray Perez-Robles, Claudia Carotenuto + Show 1 more

Open Access

https://doi.org/10.3390/polym14030635

Copy DOI

Journal: Polymers	Publication Date: Feb 7, 2022
Citations: 9	License type: CC BY 4.0

Affiliation: University of Campania "Luigi Vanvitelli"

Abstract

Aqueous solutions of hydroxypropyl methylcellulose (HPMC) show inverse thermoreversible gelation, i.e., they respond to small temperature variations exhibiting sol–gel transition during heating, and reversibly gel–sol transition during cooling. According to the pertinent literature on HPMC aqueous systems, at room temperature, the loss modulus (G”) is higher than the storage modulus (G’). During the heating ramp, the viscoelastic response follows a peculiar path: initially, G” and G’ smoothly decrease, then drop to a minimum and finally increase. Eventually, G’ overcomes G”, indicating the gel formation. A recent explanation of this behaviour considers a two-step mechanism: first, phase separation occurs, then fibrils form from a polymer-rich phase and entangle, leading to a three-dimensional network. Based on this, our research focuses on the rheological analysis of the different steps of the sol–gel transition of an HPMC aqueous solution. We perform different viscoelastic tests: thermal ramps, time sweeps, and frequency sweeps at selected characteristic temperatures. We couple classical analysis of the SAOS experiments with an innovative approach based on the evaluation of the activation energy (Ea), made possible by the instrument intrinsic temperature oscillations around the target value. Results show that Ea can be a valid tool that contributes to further clarifying the peculiar microstructural evolution occurring in this kind of thermoreversible gel.

Highlights

Aqueous solutions of hydroxypropyl methylcellulose (HPMC) show inverse thermoreversible gelation, i.e., they respond to small temperature variations exhibiting sol–gel transition during heating, and reversibly gel–sol transition during cooling
Our research focuses on the rheological analysis of the different steps of the sol–gel transition of an HPMC aqueous solution
The partial substitution of hydroxyl groups with small substituents, such as, e.g., methyl [15,16,17], ethyl [18,19], hydroxypropyl [20,21], and carboxyl [22,23], turns cellulose into water-soluble derivatives because the substituents hinder the formation of the intra- and intermolecular hydrogen bonds, allowing the hydration of the anhydroglucose units (AGU) units

Summary

Introduction

The most abundant organic polymer available on Earth, is a polysaccharide formed by a linear chain of β (1→4) linked D-anhydroglucose units (AGU) [13]. The partial substitution of hydroxyl groups with small substituents, such as, e.g., methyl [15,16,17], ethyl [18,19], hydroxypropyl [20,21], and carboxyl [22,23], turns cellulose into water-soluble derivatives because the substituents hinder the formation of the intra- and intermolecular hydrogen bonds, allowing the hydration of the AGU units. The substitution of the hydroxyl groups makes the polymer amphiphilic and might provide the property of undergoing thermoreversible gelation on temperature variations that would allow expansion of its use [24]. Thermoreversible gelation of cellulose derivatives aqueous solutions must be understood as sol–gel transition, to form a hydrogel when the temperature is increased and can be reversed by simple cooling [25]

Methods

Results

Conclusion