Károly Süvegh

Effect of plasticizer on the dynamic surface tension and the free volume of Eudragit systems.

The purpose of the present study was to determine the changes of dynamic surface tension of aqueous Eudragit dispersions quantitatively and compare these data with the free volume of the free films formed from several dispersions of different dibutyl sebacate (DBS) concentrations. Eudragit L 30D and Eudragit RL 30D aqueous colloidal polymer dispersions and their cast free films were examined. The concentration of DBS varied in the dispersions from 0 to 20%. The dynamic surface tension of the dispersions were measured by the Du Nouy ring method while the free volume of cast films were determined by positron annihilation spectroscopy. The obtained results show that dynamic surface tension measurements indicate the white point (WP) of Eudragit dispersions by a significant standard deviation increase. This may suggest the applicability of dynamic surface tension measurements for the determination of the WP of polymeric dispersions. A decrease in the WP temperature of Eudragit dispersions with an increasing plasticizer concentration was observed up to the DBS concentration of 10% w/w. Above this concentration neither the WP of the polymer dispersion nor the free volume of the free films of the polymer were changed remarkably.

Evaluation of surface and microstructure of differently plasticized chitosan films

Surface and structural investigations of natural biopolymer (chitosan) films containing various conventionally applied hydrophilic plasticizers (glycerol and poly(ethylene glycol) 400) were performed and the results were compared, with the aim of acquiring new information concerning the formation of these plasticized films. The surface tests revealed that the water uptake, the water-binding properties (moisture content) and the polarity were higher for the film containing glycerol as plasticizer. Positronium lifetime measurements and NMR studies performed to evaluate the effects of the plasticizer on the polymer structure demonstrated relevant differences in the effects of the plasticizers. The influence of glycerol on the structure of the film formed was more intensive than that of PEG 400. It can be concluded that the surface properties of the films, which are very important for their storage and application, cannot be established exactly by means of structural tests. Both surface and structural tests must be performed before the formulation of this type of plasticized mucoadhesive films.

Dose effect in neutron-irradiated C60: a positron lifetime spectroscopy and DSC study

Abstract Polycrystalline C 60 samples irradiated with five doses of nuclear reactor neutrons stretching from 2.44 × 10 14 to 1.03 × 10 18 n/cm 2 were investigated by positron annihilation spectroscopy and DSC. The measurements indicated that the crystal structure of buckminsterfullerene is unaffected up to a dose of about 7.56 × 10 16 n/cm 2 . Above that dose the crystal lattice begins to collapse and at a dose of 1.03 × 10 18 n/cm 2 the structure becomes totally disordered.

Effects of Storage Conditions on the Free Volume of Polyvinylpyrrolidone: Comparison of Positron Lifetime Data with the Tensile Strength of Tablets

9 batches of 300 tablets were produced using a wet granulation procedure beginning with the mixing of 100 g Anhydrous Received February 17, 2000; accepted May 4, 2000 theophylline (Ph.Hg. VII., Hungaropharma, Budapest, Batch No. B-16263.95) and 10 ml of a 15% (w/v) Kollidon 25 aqueous

Tracking the physical aging of poly(ethylene oxide): A technical note

Summary and ConclusionsPhysical aging of 2 types of PEO could be tracked by the combination of PALS, DSC, and SEM methods. After storing the samples at 40°C±2°C and 75%±5% RH, a decrease in the o-Ps lifetime values and an increase in the melting enthalpies as a function of storage time indicated a reorientation of the polymer chains. The limitations of monitoring enthalpy relaxation confirm the importance of methods that track volume relaxation, such as PALS. Structural changes could be observed even after a short storage time (4 weeks), which highlights the effect of further investigations of the influence of physical aging on the properties of PEO-containing dosage forms.

Positronium as a tool to monitor changes of chemical structure

Abstract The applicability of positron annihilation spectroscopy for chemical structural problems is demonstrated by two examples: For the swelling dynamics of an amphiphilic polymer network, very quick structural changes were indicated by positronium lifetime parameters at very low swelling ratios and a hydration mechanism was proposed accordingly. For the series [Fe x Zn 1− x (propyltetrazole) 6 ](BF 4 ) 2 ( x =1, 0.6, 0.4, 0.1, 0), changes in the dynamic structure were revealed by positronium lifetime spectroscopy. For compounds of x >0, temperature-induced spin-crossover was detected.

Positron annihilation study of spin-crossover in [FexZn1−x(ptz)6](BF4)2 single crystals

Abstract Positron lifetime measurements were carried out on [Fe x Zn 1− x (Ptz) 6 ](BF 4 ) 2 single crystals in the temperature range of 72–300 K. Complementary Doppler broadening spectra were also recorded at room temperature. The probability of the formation of ortho-positronium (o-Ps) (lifetime 500–1400 ps) was found to be between 10–30%. It was established that the electron spin state transformation can be monitored by positron lifetime spectroscopy also in the solid state.

Prediction of the drug release stability of different polymeric matrix tablets containing metronidazole

The aim of the present study was to predict the structural changes of polymeric excipients in the course of storage causing undesired changes in drug release stability of tablets containing different polymers. Matrix tablets were formulated with metronidazole as a model drug, using polyvinylpyrrolidone and carbopol as matrix materials. Dissolution tests were carried out before and after storing the tablets under stress conditions for different time intervals. Parameters characterizing the release kinetics of matrix tablets, just as difference and similarity factors, were calculated to compare the release profiles as a function of storage time. FT-IR measurements were carried out to track the structural changes of the physical mixtures of metronidazole and polymers during storage. The changes of the characteristic peaks of the FT-IR spectra of metronidazole-polymer mixtures were in good correlation with the significant changes of release parameters of tablets. The latter was confirmed by ab initio calculations. The work showed that the combination of ab initio calculations with structural examinations could predict the possible instability of drug release and, thus, enables the screening of polymeric excipients of undesired physical stability.

Prediction of the stability of polymeric matrix tablets containing famotidine from the positron annihilation lifetime distributions of their physical mixtures

The aim of the present work was to elucidate the impact of the structural changes of polymeric excipients during the course of storage on the drug release stability of tablets containing different polymers. Matrix tablets were formulated with famotidine as a model drug, using polyvinylpyrrolidone and carbopol matrix. Dissolution tests were carried out before and after storing the tablets under stress conditions for different time intervals. Parameters characterizing the release kinetics of matrix tablets, just as difference and similarity factors, were calculated to compare the release profiles as a function of storage time. Positron annihilation lifetime measurements were carried out to track the structural changes of the physical mixtures containing polymers during the course of storage. The changes in the positron lifetime distribution curves of the famotidine-polymer mixtures were in good correlation with the significant changes of release parameters of tablets. Thus the method would be a valuable tool for the screening of possible destabilizing interactions in the preformulation phase.

Positron lifetime in supramolecular gamma- and delta-cyclodextrin- C60 and - C70 compounds

Abstract Temperature dependence of positron lifetime parameters have been studied in pure cyclodextrins and their supramolecular complexes with C 60 and C 70 molecules. It was found that while C 60 molecules are localized in molecular cavities in gamma-cyclodextrin, they are too small to be trapped by the larger cavities of delta-cyclodextrin. On the other hand, the larger C 70 molecules fill these larger voids completely. In cases where fullerene molecules are localized in molecular cavities, a low-temperature phase transition was observed. The activation energies for the phase transitions were calculated assuming a simple two-state annihilation model.