forum of pharmaceutical sciences edition:14 location:Blankenberge date:14-15 may 2009
Recently, the graft copolymer of ethyleneglycol and vinylalcohol (PEV) has been found to be potentially useful in the formulation of solid dispersions of poorly soluble drugs. The polymer is hydrophilic, readily soluble in water, slightly surface-active and semi-crystalline. The purpose of the present study is to characterize the phase behavior of unprocessed PEV in order to fully exploit this polymer in the formulation of solid dispersions.
In order to investigate the phase behavior of PEV, modulated differential scanning calorimetry (MDSC), High Performance (HPer) DSC and X-ray powder diffraction (XRPD) were used. At first, the conditions for MDSC were determined. Special attention was payed to the modulated heat flow and the Lissajous plots of the modulated heat flow versus modulated heating rate. Reproducible and accurate results in MDSC are based on the assumption that the sample is able to follow the heating/cooling regime imposed on it. An underlying heating rate of 2.5°C/min, an amplitude of 0.40°C and a period of 40s was selected out of 48 tested conditions. This condition ensures enough cycles (>6) of the modulated heat flow during step changes and transitions in the total heat flow. Also the Lissajous figure showed an adequate elliptic form without too much phase lag on it.
Analysis of unprocessed PEV showed the presence of two glass transitions and two melting peaks, proving its semi-crystallinity. Investigation of PEG2000 and PVA(9500Da), the polymers that resemble the blocks in the co-polymer, showed that the first glass transition (Tg=-55°C) and first melting peak (Tm=15°C, ΔHm=ca.4J/g) correspond to those of PEG and the second glass transition (Tg=45°C) and second melting peak (Tm=212°C, ΔHm=ca.60J/g) correspond to those of PVA. XRPD analysis confirmed its semi-crystallinity and again PEV showed Bragg reflections comparable to those of PEG and PVA.
The polymer remained semi-crystalline after hot-melt extrusion and spray-drying. Controlled cooling at rates up to 500°C/min could delay, but not avoid crystallization. Plastification of the amorphous domain of PEG or PVA was dependent on the type and concentration of plasticizer.
The present results indicate the suitability of PEV from a material science perspective to be used as a carrier for solid dispersions.