Bengt Wittgren

Fast molecular mass and size characterization of polysaccharides using asymmetrical flow field-flow fractionation-multiangle light scattering

Asymmetrical flow field-flow fractionation (asymmetrical flow FFF), connected on-line to multi-angle light scattering detection (MALS) was shown here to be an efficient method for size characterization of pullulan standards and dextrans ranging from 20 000 up to 2 000 000 in molecular mass. The characterization of molecular mass and the molecular mass distribution of these polysaccharides is often complex and may require different methods. Using asymmetrical flow FFF-MALS, information was obtained not only about molecular mass and molecular mass distribution but also about hydrodynamic size as well as radius of gyration and conformation. The analysis time was very short, often below 5 min. It was shown that the pullulan standards have a narrow molecular mass distribution compared to the more polydisperse dextrans. Obtained molecular masses and distributions were in good agreement with data from the manufacturer. The dextrans, especially at high molecular mass, were found to have a more compact structure than the pullulans in both water and 0.1 M NaCl.

Investigation of critical polymer properties for polymer release and swelling of HPMC matrix tablets.

Four different HPMC batches were characterized to investigate properties related to critical functionality for their use in hydrophilic matrix tablets. In this study, the HPMC batches were chemically characterized and correlated to the behaviour of pure HPMC tablets. Parameters such as the molecular weight, viscosity, intrinsic viscosity and radius of gyration were kept in a rather limited range, which resulted in a weak correlation to polymer release and degree of swelling. The hydrophilic/hydrophobic character of the HPMC samples was elucidated by the degree of substitution and by the clouding behaviour, where an increased hydrophilicity increased the tablet swelling. This phenomenon was interpreted in a refined model for water transport into HPMC tablets. A five times slower polymer release and a considerably larger degree of swelling were found for one batch of HPMC tablets compared to the others, although the characterized average polymer parameters were in the same range. However, the conformation plot displayed a fraction with compact aggregates. In conclusion, the existence of aggregates in aqueous solution seems to perturb the functionality of HPMC tablets and it seems important to understand and characterize these aggregates to fully predict the polymer release and swelling of HPMC tablets.

Effects of flow-rates and sample concentration on the molar mass characterisation of modified celluloses using asymmetrical flow field-flow fractionation–multi-angle light scattering

Abstract Modified celluloses are an important group of polymers used in many applications, such as in the food and drug industry. Thus, their physicochemical properties are of considerable interest and need to be characterised carefully. In this study, asymmetrical flow field-flow fractionation (asymmetrical flow FFF) connected on-line to a multi-angle light scattering (MALS) detector was used to study the molar mass and molar mass distribution of three different hydroxypropylmethylcelluloses (HPMCs). The influence of the flow-rates and the sample concentration on the results obtained was found to be significant, emphasising the importance of optimising the experimental conditions so as to obtain reliable information about the polymer system. With the use of appropriate conditions, flow FFF–MALS was found to be a suitable method for the characterisation of these complex samples. The weight-average molar mass ranged from 132 000 g/mol to 309 000 g/mol. The z-average radius of gyration was found to be high relative to the molar mass, ranging from 58 nm to 73 nm, suggesting an expanded structure. This was also confirmed by double logarithmic plots of the molar mass versus the radius of gyration, the slope being approximately 0.7 for the two high molar mass samples.

The effect of chemical heterogeneity of HPMC on polymer release from matrix tablets.

Polymer release from hydrophilic matrix tablets, composed of hydroxypropyl methylcellulose, was studied for seven different polymer batches. A time difference of more than 80h between fully dissolved tablets was noticed although the batches were of the same pharmaceutical substituent (USP 2208) and viscosity (100 cps) grade. To find the functionality related parameters for polymer release from hydrophilic matrix tablets the polymer samples were characterised according to size and chemical composition. The size of the polymers was characterised by size-exclusion chromatography with multi-angle light scattering and refractive index detection. The average amount of substituents was measured with nuclear magnetic resonance and the distribution of the substituents along the cellulose chain was determined with high-performance anion-exchange chromatography with pulsed amperometric detection after acid and enzymatic hydrolysis. The results indicated that other types of interactions apart from entanglements were present between the polymer chains, which seemed to affect the polymer release. Most importantly, this study has shown a correlation between the polymer release and the substituent pattern, where the samples with slow release also were more heterogeneously substituted along the polymer chain. From this we can conclude that polymer release is very sensitive to alterations in chemical composition.

Conformational change and aggregation of κ‐carrageenan studied by flow field‐flow fractionation and multiangle light scattering

The relatively novel combination of flow field-flow fractionation (FFF) and multiangle light scattering (MALS) was employed to study a nondegraded κ-carrageenan in different 0.1M salt solutions. The applicability of the technique was tested, and the effects of salt type and salt composition on the molar mass and radius of gyration were studied. A conformational ordering was induced at room temperature by switching the solvent from 0.1M NaCl (coil form) to 0.1M NaI (helix form). An approximate doubling of the average molar mass and an increase in radius of gyration was then observed, in agreement with results obtained previously using size exclusion chromatography–MALS. This increase in size was attributed to conformational ordering and to the formation of double helices. Severe aggregation was observed above 40% CsI in the 0.1M mixed salt solution of CsI and NaI. This was ascribed to the association of helices into large aggregates. For these large associates, having molar masses of several millions, a reversal of the elution order in flow FFF was detected.

Influence of Substitution Pattern on Solution Behavior of Hydroxypropyl Methylcellulose

Industrially produced hydroxypropyl methylcellulose (HPMC) is a chemically heterogeneous material, and it is thus difficult to predict parameters related to function on the basis of the polymers average chemical values. In this study, the solution behavior of seven HPMC batches was correlated to the molecular weight, degree of substitution, and substituent pattern. The initial onset of phase separation, so-called clouding, generally followed an increased average molecular weight and degree of substitution. However, the slope of the clouding curve was affected by the substitution pattern, where the heterogeneously substituted batches had very shallow slopes. Further investigations showed that the appearance of a shallow slope of the clouding curve was a result of the formation of reversible polymer structures, formed as a result of the heterogeneous substituent pattern. These structures grew in size with temperature and concentration and resulted in an increase in the viscosity of the solutions at higher temperatures.

The effect of substitution pattern of HPMC on polymer release from matrix tablets.

The purpose of this study was to gain further understanding of how the substituent heterogeneity of hydroxypropyl methylcellulose, HPMC, affects the polymer release from hydrophilic matrix tablets. The hypothesis was that the heterogeneous substituent pattern facilitated hydrophobic interactions that increased the viscosity and therefore affected the release rate to a major extent. Polymer tablets were prepared from three heterogeneously substituted HPMC batches of the same substituent (2208) and viscosity (100 cps) grade. To elucidate the hypothesis, fractions of both the dissolved polymer and the tablet residue were collected from the dissolution bath and further characterised. The extensive characterisation showed that, although the dissolved bath fraction and the tablet residue had a similar average degree of substitution, the residue was more heterogeneously substituted. It was further revealed that the heterogeneous substituent pattern of the tablet residue facilitated the formation of soluble gel-like components already at room temperature, which increased the viscosity. The viscosity increased by 150% at temperatures correlated to the dissolution bath, and it was thus concluded that the gel-like components grew in size with temperature. Finally, much lower release rates were obtained by tablets composed of the residue compared to tablets composed of the bath fraction, which clarified the hypothesis.

Release of theophylline and carbamazepine from matrix tablets--consequences of HPMC chemical heterogeneity.

The release of theophylline and carbamazepine from matrix tablets composed of microcrystalline cellulose, lactose and hydroxypropyl methylcellulose (HPMC) was studied. The aim was to investigate the effect of different substituent heterogeneities of HPMC on the drug release from matrix tablets composed of either 35% or 45% HPMC. The release of the poorly soluble carbamazepine was considerably affected by the HPMC heterogeneity, and the time difference at 80% drug release was more than 12h between the formulations of different HPMC batches. This was explained by slower polymer erosion of the heterogeneous HPMC and the fact that carbamazepine was mainly released by erosion. In addition, results from magnetic resonance imaging showed that the rate of water transport into the tablets was similar. This explained the comparable results of the release of the sparingly soluble theophylline from the two formulations even though the polymer erosion and the swelling of the tablets were considerably different. Thus, it can be concluded that the drug release was highly affected by the substituent heterogeneity, especially in the case of carbamazepine, which was released mainly by erosion.

Model drug release from matrix tablets composed of HPMC with different substituent heterogeneity

The release of a model drug substance, methylparaben, was studied in matrix tablets composed of hydroxypropyl methylcellulose (HPMC) batches of the USP 2208 grade that had different chemical compositions. It was found that chemically heterogeneous HPMC batches with longer sections of low substituted regions and lower hydroxypropoxy content facilitated the formation of reversible gel structures at a temperature as low as 37°C. Most importantly, these structures were shown to affect the release of the drug from matrix tablets, where the drug release decreased with increased heterogeneity and a difference in T80 values of 7h was observed between the compositions. This could be explained by the much lower erosion rate of the heterogeneous HPMC batches, which decreased the drug release rate and also released the drug with a more diffusion based release mechanism compared to the less heterogeneous batches. It can therefore be concluded that the drug release from matrix tablets is very sensitive to variations in the chemical heterogeneity of HPMC.

Molar mass distribution of hydroxypropyl cellulose by size exclusion chromatography with dual light scattering and refractometric detection

Physico-chemical properties of cellulose derivatives are of considerable interest in many technical applications, for example, in the food and drug industry. Efficient and careful characterisation of these properties is thus highly desirable. In this study, two different size exclusion chromatography (SEC) systems, connected on-line either to a low-angle laser light scattering detector (LALLS) or to a multi-angle laser light scattering detector (MALLS), are employed for size characterisation of three batches of hydroxypropyl cellulose (HPC) from different manufacturers. All three samples turned out to have a weight average molar mass around 100,000 g/mol, but considerable differences concerning conformational properties were found. Two of the samples contained compact components, presumably aggregates of HPC, which were clearly detected by both SEC-systems. The third sample, obtained from another manufacturer, did not show any indication of aggregation. Both SEC-MALLS and SEC-LALLS are proven to be efficient techniques for characterisation of complex polysaccharides like HPC containing mixtures of solvated polymer chains, as well as micelle-like aggregates.