Formation and Volume Phase Transition of Hydroxypropyl Cellulose Microgels in Salt Solution

Abstract

Surfactant-free hydroxypropyl cellulose (HPC) microgels have been synthesized in salt solution. The average size and size distributions of microgel particles have been correlated with polymer concentration, salt concentration, and reaction temperature using light scattering techniques. It is found that the low critical solution temperature of HPC polymer chains decreases upon the increase of sodium chloride concentration. In a narrow sodium chloride concentration range from 1.3 to 1.4 M, HPC chains can self-associate into colloidal particles at room temperature. These particles remained stable in solution for weeks without changing their sizes. The microgel particles were then obtained in situ by bonding self-associated HPC chains at 23 °C using divinyl sulfone as a cross-linker. The volume phase transition of the resultant HPC microgels has been studied as a function of temperature at various salt concentrations. A theoretical model based on Flory-Huggins free energy consideration has been used to explain the experimental results.

Spectral time moment analysis of microgel deswelling. Effect of the heating rate

“…This forces the solution to separate into two phases; one of which is made from poorly soluble metastable aggregates of self-associated chains. 16,17 The phase transition is reversible: lowering solution temperature below T c brings back the isotropic, one phase system. 18 HPC:water solutions also exhibit a lyotropic phase transition which starts at ambient temperature in the concentration range 360-420 g/L.…”

“…HPC chains can be chemically cross-linked into stable microgel nanoparticles above the critical temperature T c 4,16 or even at room temperature by effectively lowering T c with salt addition 17,28 consequently lowering the magnitude of steric interactions. In fact, the synthesized microgels undergo a reversible temperature dependent volume-phase transition at the temperature T v which is somewhat higher than T c .…”

“…29,30 Many controllable parameters including polymer and salt concentration, cross-linking density, chemical make up, and synthesis protocol affect microgel size, loading capacity and swelling/deswelling ability. 17 Other parameters of microgel systems may also be adjusted to effectively load medicines into the structure, target diseased tissue (e.g. cancer), and release the payload upon predetermined environmental conditions (such as high temperature or pH).…”

“…Xia et al 17 formed 1 MDa HPC microgels using the addition reaction of ref. 16, but at an ambient temperature.…”

“…13 Reference 28 meticulously examined the highly nonexponential light scattering spectra of microgels synthesized using the recipe of ref. 17. Instead of relying on the cumulant analysis used by Xia et al, 17 ref.…”