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The wonderful relationship between polymers and solvents: how to affect solubility in different temperature ranges?
In polymer science, solubility is the basis for many important applications, especially in areas such as textiles, medical and materials science. The behavior of polymer solutions changes as a function of temperature, a phenomenon that becomes even more apparent when discussing the lower critical solution temperature (LCST). LCST is an important parameter indicating the temperature at which the components of a mixture are completely miscible in all proportions. Once the temperature exceeds this critical point, local insolubility will occur.
Phase behavior in polymer solutions is an important property in the development and design of most polymer-related processes.
Some polymers exhibit complete miscibility in aqueous solutions. For such polymers, such as poly(N-isopropylacrylamide), the phase change usually occurs at 32°C (90°F), but in practice The phase change temperature of may deviate by 5 to 10°C depending on polymer concentration, molar mass of the chain, and other factors. This shows that the structural features of the polymer and its additives, such as salts or proteins, can significantly change the cloud point temperature, or LCST.
Physical factors make LCST unique, mainly due to the entropy change factor of the mix.
Therefore, this is an anomalous value because normally entropy drives mixing, as the mixing process increases the volume available for each component.Below the LCST, mixing is spontaneous, meaning that the free energy change (ΔG) is negative, while above the LCST this value becomes positive.
Theoretically, the LCST model can be described by a lattice fluid model. This model is an extension of Flory-Huggins solution theory, taking into account density and compressibility effects. The latest extension of Flory-Huggins theory allows the observation of LCST phenomena simply by considering the geometric correlation and correlation interactions between solute and solvent.
There are also many ways to predict LCST. The first type of method is proposed based on experimental data and has a fixed theoretical background, which requires the adjustment of unknown parameters. The other is to use empirical equations to relate LCST through physical and chemical properties (such as density, critical properties). However, this method cannot obtain the required data in some cases.
Recently, Liu and Zhong proposed a linear model based on molecular connection index. This method shows good prediction ability, and it is hoped that some important data can be obtained through calculation before experiments. In addition, the existing QSPR (Quantified Structure Activity/Property Relationship) model can effectively reduce the cost of trial and error, allowing researchers to make relatively reliable predictions of the LCST of polymer solutions before actual synthesis, which has great implications for material design. great significance.
Currently, more than 70 nonionic polymers have shown LCST behavior in aqueous solutions, which is a great inspiration for the design of new polymers.
As science advances, the relationship between polymers and solvents will continue to receive attention. Researchers continue to explore new polymer systems and their solubility behaviors, and more applications may be tied to these research results in the future. So, how can we use this knowledge to design better materials in future scientific research?
