Self-Association of Hydroxypropylcellulose in Water

Abstract

The association of hydroxypropylcellulose (HPC) chains above their lower critical solution temperature (LCST, ∼41 °C) leads to metastable nanosphere aggregates instead of precipitation. Laser light scattering (LLS) measurements revealed that the size of the aggregates is dependent not only on temperature and HPC concentration but also on heating history. There existed a narrow temperature range from ∼41 to ∼44 °C in which narrowly distributed metastable nanospheres formed in a few minutes and lasted for a few days without changing their sizes and molar masses. Increasing temperature in this range and increasing HPC concentration resulted in larger and denser nanosphere aggregates. In situ cross-linking of self-associated HPC chains was performed in a LLS sample cuvette at 42 °C using divinyl sulfone as a cross-linker. Formation of HPC gel microspheres was monitored by LLS and demonstrated by allowing the gel microspheres to swell and shrink at temperatures below and above the LCST, respectively.

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

“…• C. [13][14][15][16] At room temperature (T ≤ T c ) water is a good solvent to HPC chains and solutions are homogeneous. Above T c hydrophobic interactions are favorable and water becomes a poor solvent for HPC chains.…”

“…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.…”

“…• C. Gao et al 16 induced selfassociation of microgels via an addition reaction of 100 kDa HPC and a crosslinker (no surfactant) in a basic environment (pH = 12-13) above T c . Xia et al 17 formed 1 MDa HPC microgels using the addition reaction of ref.…”

“…Xia et al 17 formed 1 MDa HPC microgels using the addition reaction of ref. 16, but at an ambient temperature. This was achieved by lowering T c from 41…”

“…What has not been explored in detail (to author's knowledge) is the role of the heating history on the HPC microgel deswelling. Gao et al 16 reported that the formation of metastable HPC aggregates in uncrosslinked polymer solutions depends on "how the system is brought to that temperature." They found that the metastable HPC clusters formed under multi-step heating to 42…”