Thermally Induced Association and Dissociation of Methylcellulose in Aqueous Solutions

Abstract

Aqueous solutions of a methylcellulose, ranging from 0.30 to 2.49 wt %, were studied by means of micro differential scanning calorimetry (micro DSC) and rheology. The effects of polymer concentration on the thermodynamic properties of these solutions were examined through a heating process and a following cooling process at a fixed rate of 1 °C/min. Upon heating, an endothermic peak was observed at about 63 °C, which was independent of polymer concentration. The total energy defined by the endothermic peak area was found to be a linear function of polymer concentration. On the other hand, when samples were cooling from about 90 °C, a broad exothermic peak appeared at about 33 °C, and the peak height and its broadness increased with polymer concentration. A shoulder was observed above the peak temperature of 33 °C, and the shoulder became more prominent with increasing polymer concentration to eventually appear as a second peak at about 40 °C. The thermal analysis results clearly show that the association of methylcellulose molecules in water is thermorevesible but the dissociation occurred at much lower temperatures than the association temperatures. The viscoelastic properties of these solutions correlated excellently with the results obtained from the micro thermal analysis. Thermodynamic mechanisms involved in the association and the dissociation are proposed.

Thermoresponsive hydrogels in biomedical applications

“…Substitution of the hydroxyl groups on cellulose with more hydrophobic units as methyl or hydroxypropyl groups renders the originally insoluble cellulose water soluble [11]. Methylcellulose (MC) is a cellulose derivative that has been extensively investigated for biomedical applications.…”

“…Methylcellulose (MC) is a cellulose derivative that has been extensively investigated for biomedical applications. It has thermoreversible gelation properties in aqueous solutions, gelling at temperatures in the range of 60-80°C and turning into a solution upon cooling [11,20]. Liu et al [21] have grafted methylcellulose with the synthetic N-isopropylacrylamide (NiPAAm), combining the thermogelling properties of both materials.…”

“…In contrast, hydrogels that are formed upon cooling of a polymer solution have an upper critical solution temperature (UCST) [10]. The sol-gel transition of thermosensitive hydrogels can be experimentally verified by a number of techniques such as spectroscopy [11][12][13], differential scanning calorimetry (DSC) [11,12] and rheology [11].…”

“…The entropy increases due to water-water associations which are the governing interactions in the system. This phenomenon is the so-called hydrophobic effect [11,14,15]. Polymer micelle packing [16] and coil to helix transition causing network formation [17] are examples of the conformational changes that take place at the critical solution temperature.…”

Transmittance and Reflectance Studies of Thermotropic Material for a Novel Building Integrated Concentrating Photovoltaic (BICPV) ‘Smart Window’ System

“…HPC has previously been considered as a suitable biopolymer for thermotropic glazing both independently [15,16] and as part of an interpolymer complex [17][18][19]. In addition, HPC is water-soluble due to the substitution of reactive hydroxyl groups on the cellulose structure with hydrophobic hydroxypropyl groups which results in weakened intermolecular hydrogen bonding within the cellulose [20]. HPC chains are dissolved within water at temperatures below the T s due to a sheath of tightly bound water molecules surrounding HPC chains via hydrogen bonding resulting in HPC chains being uncoiled and separated.…”