Effects of molecular weight of polyvinylpyrrolidone on the glass transition and crystallization of co-lyophilized sucrose

The purpose of the present study was to investigate the effects of molecula r weight (MW) of polyvinylpyrrolidone (PVP) on glass transition and crystal lization of sucrose. Thus. sucrose was co-lyophilized with 2.5 and 5.0% w/w PVP of different molecular weights. which were characterized using gel per meation chromatography. Freeze drying was carried out for 48 h at a shelf t emperature of - 40 degreesC and a pressure of about 36 Pa. The samples were then dried in a vacuum oven at 24 degreesC for 12 h before drying for a fu rther 12 h at 40 degreesC. Differential scanning calorimetry (DSC) was empl oyed to measure the glass transition temperature (T-g). dynamic crystalliza tion temperature (T-c) and isothermal crystallization induction lime (t(c)) at 85 degreesC of sucrose. Isothermal water vapour sorption of each sample was also measured at different relative humidities. ir, values of sucrose varied from 48.3 +/- 0.8 degreesC for freeze-dried (FD) sucrose alone to 58 .8 +/- 0.8 degreesC for the mixture containing 5.0% PVP of nominal MW 300 K . PVP increased sucrose T-g significantly (ANOVA P < 0.05). Although there was no significant difference (P > 0.05) in T-g of the mixtures containing 2.5% w/w PVP of different MW. samples with 5.0% PVP of MW 300 K produced a significantly higher (P < 0.05) T, than the other mixtures. All mixtures we re shown to possess higher (P < 0.01) T, than FD sucrose alone, which exhib ited a T, of approximately 85 degreesC. PVP of MW 300 K consistently induce d a significantly (P < 0.05) higher T, of sucrose than PVP of smaller MW. I ncreasing PVP concentration from 2.5 to 5.0% also resulted in a substantial increase in sucrose T-c. Using isothermal water vapour absorption, sucrose t(c) was found to increase up to over 10 times when it was co-lyophilized with 2.5%, PVP. the actual value of t(c) being dependent upon the MW of the PVP. For example. PVP of MW 300 K resulted in a sucrose t(c) at 85 degrees C (89.1-95.6 min), which was approximately seven times higher than that of 2.5% PVP of MW 24 or 40 K. A longer t(c) of sucrose was also observed for m ixtures containing PVP of MW 300 K than when sucrose was mixed with PVP of smaller MW. Thus the effect of PVP on sucrose T-g. T-c and t(c) was found t o be dependent upon MW. PVP of higher MW was more efficient in inhibiting s ucrose crystallization and by stabilizing glassy structures of the sugar, t hese polymers may improve the stability of co-lyophilized proteins and pept ides. (C) 2001 Elsevier Science B.V. All rights reserved.