Aggregation of Modified Celluloses in Aqueous Solution: Transition from Methylcellulose to Hydroxypropylmethylcellulose Solution Properties Induced by a Low-Molecular-Weight Oxyethylene Additive

摘要

Temperature effects on the viscosity andnaggregation behavior of aqueous solutions of three differentncellulose ethersmethylcellulose (MC), hydroxypropylmethylcellulosen(HPMC), and ethyl(hydroxyethyl)cellulosen(EHEC)were investigated using viscosity and dynamicnlight scattering measurements as well as cryo-TEM. In allncases, increasing temperature reduces the solvent quality ofnwater, which induces aggregation. It was found that the aggregation rate followed the order EHEC > HPMC > MC, suggestingnthat cellulose ethers containing some bulky and partially hydrophilic substituents assemble into large aggregates more readly thannmethylcellulose. This finding is discussed in terms of the organization of the structures formed by the different cellulose ethers.nThe temperature-dependent association behavior of cellulose ethers was also investigated in a novel way by addingndiethyleneglycolmonobutylether (BDG) to methylcellulose aqueous solutions. When the concentration of BDG was at andnabove 5 wt %, methylcellulose adopted HPMC-like solution behavior. In particular, a transition temperature where the viscositynwas decreasing, prior to increasing at higher temperatures, appeared, and the aggregation rate increased. This observation isnrationalized by the ability of amphiphilic BDG to accumulate at nonpolar interfaces and thus also to associate with hydrophobicnregions of methylcellulose. In effect, BDG is suggested to act as a physisorbed hydrophilic and bulky substituent inducingnconstraints on aggregation similar to those of the chemically attached hydroxypropyl groups in HPMC and oligo(ethyleneoxide)nchains in EHEC.