New Insight in the Structure-properties Relationship of Hydroxypropyl Cellulose

Abstract

Hydroxypropyl cellulose (HPC) is a semi-synthetic polymer that results from the hydroxypropylation of hydroxyl groups of the repeating glucose units that form the cellulose backbone. The grafting of hydroxypropyl substituents provides the molecule with good surface properties. The resulting water-soluble polymer has many applications, mainly in the pharmaceutical and food industries. Due to the flexibility of its manufacturing process, HPC can be obtained in different commercial grades, each characterized by its molecular weight and viscosity. It has already been reported in the past that differences in functionality can occur even between theoretically similar products. Some research groups had already drawn attention to the fact that compendial specifications provided by suppliers were too broad to allow an accurate description of the molecular state of the HPC. This results in some cases in the impossibility to guarantee the same functionality even for products of the same grade. This work studied different HPC of the same commercial grades and provides new insights into the influence of the structure on the techno-functional properties of HPC. Surface properties, rheology, and thermal behavior have been studied. Different complementary surface tension analysis methods were used to determine the surface properties. Equilibrium surface tension of 0.2% w:w HPC aqueous solutions varied from 40-44 mN/m (20°C). Differences in viscosity and cloud point were also observed. Nuclear Magnetic Resonance allowed to determine the degree of substitution and gain information about structure, which were later related to the differences in properties. The results show that the degrees of substitution and the heterogeneity of distribution of substituents along the polymer chain have a great impact on surface-properties, rheological, and thermal behavior of the samples. These results have great significance for applications in emulsions and foams.