The water solubility of the modified cellulose ether is affected by temperature. Generally speaking, most cellulose ethers are soluble in water at low temperatures. When the temperature rises, their solubility gradually becomes poor and eventually becomes insoluble. Lower Critical Solution Temperature (LCST: Lower Critical Solution Temperature) is an important parameter to characterize the solubility change of cellulose ether when the temperature changes, that is, above the lower critical solution temperature, cellulose ether is insoluble in water.
The heating of aqueous methylcellulose solutions has been studied and the mechanism of the change in solubility has been explained. As mentioned above, when the solution of methylcellulose is at low temperature, the macromolecules are surrounded by water molecules to form a cage structure. The heat applied by the temperature rise will break the hydrogen bond between the water molecule and the MC molecule, the cage-like supramolecular structure will be destroyed, and the water molecule will be released from the binding of the hydrogen bond to become a free water molecule, while the methyl The hydrophobic methyl group on the cellulose macromolecular chain is exposed, which makes it possible to prepare and study the hydrophobic association of hydroxypropyl methylcellulose thermally induced hydrogel. If the methyl groups on the same molecular chain are hydrophobically bonded, this intramolecular interaction will make the entire molecule appear coiled. However, the increase in temperature will intensify the motion of the chain segment, the hydrophobic interaction in the molecule will be unstable, and the molecular chain will change from a coiled state to an extended state. At this time, the hydrophobic interaction between molecules begins to dominate. When the temperature gradually rises, more and more hydrogen bonds are broken, and more and more cellulose ether molecules are separated from the cage structure, and the macromolecules that are closer to each other gather together through hydrophobic interactions to form A hydrophobic aggregate. With a further increase in temperature, eventually all hydrogen bonds are broken, and its hydrophobic association reaches a maximum, increasing the number and size of hydrophobic aggregates. During this process, methylcellulose becomes progressively insoluble and eventually completely insoluble in water. When the temperature rises to the point where a three-dimensional network structure is formed between macromolecules, it appears to form a gel macroscopically.
Jun Gao and George Haidar et al studied the temperature effect of hydroxypropyl cellulose aqueous solution by means of light scattering, and proposed that the lower critical solution temperature of hydroxypropyl cellulose is about 410C. At a temperature lower than 390C, the single molecular chain of hydroxypropyl cellulose is in a randomly coiled state, and the hydrodynamic radius distribution of the molecules is wide, and there is no aggregation between macromolecules. When the temperature is increased to 390C, the hydrophobic interaction between the molecular chains becomes stronger, the macromolecules aggregate, and the water solubility of the polymer becomes poor. However, at this temperature, only a small part of hydroxypropyl cellulose molecules form some loose aggregates containing only a few molecular chains, while most of the molecules are still in the state of dispersed single chains. When the temperature rises to 400C, more macromolecules participate in the formation of aggregates, and the solubility becomes worse and worse, but at this time, some molecules are still in the state of single chains. When the temperature is in the range of 410C-440C, due to the strong hydrophobic effect at higher temperatures, more molecules gather to form larger and denser nanoparticles with a relatively uniform distribution. Elevations become larger and denser. The formation of these hydrophobic aggregates leads to the formation of regions of high and low concentration of polymer in solution, a so-called microscopic phase separation.
It should be pointed out that the nanoparticle aggregates are in a kinetically stable state, not a thermodynamically stable state. This is because although the initial cage structure has been destroyed, there is still a strong hydrogen bond between the hydrophilic hydroxyl group and the water molecule, which prevents hydrophobic groups such as methyl and hydroxypropyl from combination between. The nanoparticle aggregates reached a dynamic equilibrium and stable state under the joint influence of the two effects.
In addition, the study also found that the heating rate also has an impact on the formation of aggregated particles. At a faster heating rate, the aggregation of molecular chains is faster, and the size of the formed nanoparticles is smaller; and when the heating rate is slower, the macromolecules have more opportunities to form larger-sized nanoparticle aggregates.
Post time: Apr-17-2023