Chemical structure and glass transition temperature of non-ionic cellulose ethers

Abstract

Glass transitions of several non-ionic cellulose ethers differing in molecular mass and nature and amount of substituents were analyzed (as compressed probes) by differential scanning calorimetry (DSC), modulated temperature differential scanning calorimetry (TMDSC@®), and oscillatory rheometry. In general, the low energy transitions accompanying the Tg of methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), and hydroxypropylcelluloses of low (L-HPC) or medium-high (HPC) degree of substitution were difficult to characterize using DSC. Non-reversing heat flow signals obtained in TMDSC experiments were more sensitive. However, the best resolution was obtained using oscillatory rheometry since these cellulose ethers undergo considerable changes in their storage and loss moduli when reaching the Tg. Oscillatory rheometry also appears as a useful technique to characterize the viscoelastic behavior and thermal stability of pharmaceutical tablets. Tg values followed the order HPC (105°C)<HPMC (170-198°C)<MC (184-197°C)<L-HPC (220°C). For HPMCs, the Tg increases as the methoxyl/hydroxypropoxyl content ratio decreases. The results indicate that Tg depends strongly on the structure of the cellulose ethers. In general, increasing the degree of substitution of cellulosic hydroxyls, the hydrogen bonding network of cellulose decreases (especially when the substituents cannot form hydrogen bonds) and, in consequence, Tg also decreases.