Characterisation of the thermal properties of ethylcellulose using differential scanning and quasi-isothermal calorimetric approaches


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Abstract

The thermal behaviour of the ethylcellulose (EC), a polymer that is widely used in pharmaceutical dosage forms, has been investigated with a view to study the glass transition and higher temperature thermal events as well as to develop new approaches to characterise this complex polymer system. Samples of EC powder were studied using conventional and modulated temperature differential scanning calorimetry (MTDSC), quasi-isothermal (Qi-)MTDSC and hot stage microscopy (HSM) with simultaneous transmitted light intensity measurements. The Tg was noted at circa 128–130 °C, with an accompanying baseline drift associated with a temperature dependent heat capacity change. A higher temperature combined endo/exothermic event was noted at 170–190 °C. TGA studies indicated that the exotherm was associated with oxidative degradation, with the accompanying DSC data being highly dependent on the sample encapsulation method used. The endotherm was found to be kinetically hindered, as demonstrated using Qi-MTDSC; the technique also indicated that there was little evidence for reversing processes through this transition. HSM studies indicated birefringence for the sample at low temperatures which disappeared as the material temperature approached Tg but reappeared on further heating, again disappearing at circa 180 °C. Light intensity scans produced a profile similar to that seen for the DSC studies. It is proposed that the sample contains microcrystals composed of unsubstituted segments of the cellulose backbone. The implications of these findings for the understanding of the pharmaceutical behaviour and thermal characterisation of ethylcellulose are discussed.

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