On the characterization principles of some technically important water soluble non-ionic cellulose derivatives

Size-exclusion chromatography of ultrahigh molecular weight methylcellulose ethers and hydroxypropyl methylcellulose ethers for reliable molecular weight distribution characterization

Size-exclusion chromatography (SEC) coupled with multi-angle laser light scattering (MALLS) and differential refractive index (DRI) detectors was employed for determination of the molecular weight distributions (MWD) of methylcellulose ethers (MC) and hydroxypropyl methylcellulose ethers (HPMC) having weight-average molecular weights (Mw) ranging from 20 to more than 1,000kg/mol. In comparison to previous work involving right-angle light scattering (RALS) and a viscometer for MWD characterization of MC and HPMC, MALLS yields more reliable molecular weight for materials having weight-average molecular weights (Mw) exceeding about 300kg/mol. A non-ideal SEC separation was observed for cellulose ethers with Mw>800kg/mol, and was manifested by upward divergence of logM vs. elution volume (EV) at larger elution volume at typical SEC flow rate such as 1.0mL/min. As such, the number-average molecular weight (Mn) determined for the sample was erroneously large and polydispersity (Mw/Mn) was erroneously small. This non-ideality resulting in the late elution of high molecular weight chains could be due to the elongation of polymer chains when experimental conditions yield Deborah numbers (De) exceeding 0.5. Non-idealities were eliminated when sufficiently low flow rates were used. Thus, using carefully selected experimental conditions, SEC coupled with MALLS and DRI can provide reliable MWD characterization of MC and HPMC covering the entire ranges of compositions and molecular weights of commercial interest.

Gelation studies of a cellulose-based biohydrogel: the influence of pH, temperature and sterilization

The present paper investigates the rheological properties of silated hydroxypropylmethylcellulose (Si-HPMC) biohydrogel used for biomaterials and tissue engineering applications. The general property of this modified cellulose ether is the occurrence of self-hardening due to silanol condensation subsequent to a decrease in pH (from 12.4 to nearly 7.4). The behavior of unsterilized and sterilized Si-HPMC solutions in diluted and concentrated domains is first described and compared. In addition, the influence of physiological parameters such as pH and temperature on the rate of the gelation process is studied. In dilute solution, the intrinsic viscosity ([eta]) of different pre-steam sterilization Si-HPMC solutions indicates that macromolecular chains occupy a larger hydrodynamic volume than the post-steam sterilization Si-HPMC solutions. Although the unsterilized Si-HPMC solutions demonstrate no detectable influence of pH upon the rheological behavior, a decrease in the limiting viscosities (eta(0)) of solutions with increasing pH is observed following steam sterilization. This effect can be explained by the formation of intra- and intermolecular associations during the sterilization stage originating from the temperature-induced phase separation. The formation of Si-HPMC hydrogels from injectable aqueous solution is studied after neutralization by different acid buffers leading to various final pHs. Gelation time (t(gel)) decreases when pH increases (t(gel) varies from 872 to 11s at pH 7.4 and 11.8, respectively). The same effect is observed by increasing the temperature from 20 to 45 degrees C. This is a consequence of the synergistic effect of the increased reaction rate and acid buffer diffusion. pH and temperature are important parameters in the gelation process and their influence is a key factor in controlling gelation time. By adapting the gel parameters one could propose hydrogels with cross-linking properties adapted to clinical applications by controlling the amount of pH of neutralization and temperature.

Macromolecular geometries determined with field-flow fractionation and their impact on the overlap concentration

In this paper we aim to understand the size/conformation relationship in waxy barley starch, a polydisperse and ultrahigh molar mass biomacromolecule. Characterizations are performed with asymmetrical flow field-flow fractionation (AsFlFFF). Furthermore, we study the effect of homogenization on the molar mass, rms radius (r rms) and hydrodynamic radius (r h). For the untreated sample, the macromolecules are elongated objects with low apparent density. As a result of homogenization, molar mass, and r rms decrease, while r h remains unaffected. The process also induces an increase, and scaling with size, of apparent density as well as changes in conformation, represented qualitatively by r rms/ r h. Finally, results from AsFlFFF are compared with viscosimetry and discussed in terms of concentration and close-packing in relation to macromolecular shape and conformation. Hence, the results show that AsFlFFF and our novel methodology enable the determination of several physical properties with high relevance for the solution behavior of polydisperse macromolecules.

Determination of molecular parameters of hydroxyethyl and hydroxypropyl celluloses by chromatography with dual detection

Hydroxyethylcellulose (HEC) and hydroxypropylcellulose (HPC) were studied by means of size exclusion chromatography with dual detection, i.e. employing simultaneously a refractive index (concentration sensitive) and a multiangle light scattering (molecular weight sensitive) detectors. The eluent was water and water solutions containing different concentrations of ionic salts. Molecular weight distributions and averages, coefficients of the scaling law of molecular dimensions and unperturbed dimensions were thus obtained from a single polydisperse sample of each polymer. Measurements were performed at 25 degrees C and the anomalous chromatographic behaviour, due to a combination of ion and size exclusion mechanisms, found when using pure water as eluent is transformed into a size exclusion mechanism by the addition of ionic salts. However, the two polymers behave on a different way in presence of salts. Thus, HEC, which is of low degree of substitution (DS), is close to theta conditions in the aqueous salt solutions (i.e. the q exponent of the scaling law has a value close to 0.5), whereas in the case of HPC the addition of salt improves the quality of the solvent up to a value of q around 0.6. Unperturbed dimensions are also calculated for both celluloses.

Mechanical degradation and changes in conformation of hydrophobically modified starch

In this paper, we study the mechanical degradation and changes in conformation of a branched ultrahigh molar mass biomacromolecule, hydrophobically modified starch, as caused by high-pressure homogenization. The characterization was performed with asymmetrical flow field-flow fractionation (AsFlFFF) with multiangle light scattering (MALS) and refractive index detection. The starch which had been chemically modified with octenyl succinate anhydride (OSA) proved to be very large and polydisperse. Upon high-pressure homogenization, the molar mass and rms radius (r(rms)) decreased, and the extent of these changes was related to the turbulent flow conditions during homogenization. The treatment also induced an increase and scaling with size in the apparent density of the macromolecules. To further study the changes in conformation, it was necessary to calculate the hydrodynamic radii (r(h)). This can be determined numerically from the elution times in the analysis and the flow conditions in the AsFlFFF channel. The results showed that the treatment can cause a dramatic decrease in the quotient between r(rms) and r(h), suggesting major conformational changes. These results together could be interpreted as degradation and "crumpling" of the macromolecule, which would give a decrease in r(rms) and an increase in apparent density, together with a "fraying" of more outer parts of the macromolecule, which could give rise to the increase in r(h).

Characterization of Chemical Substitution of Hydroxypropyl Cellulose Using Enzymatic Degradation

The distribution of substituents along the polymer backbone will have a strong influence on the properties of modified cellulose. Endoglucanases were used to degrade a series of hydroxypropyl cellulose (HPC) derivatives with a high degree of substitution. The HPCs were characterized with cloud-point analysis prior to degradation. The extent of enzymatic degradation was determined with size-exclusion chromatography with online multi-angle light scattering and refractive index detection and also with high-pH anion exchange chromatography with pulsed amperometric detection. To further characterize the formed products, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was employed for analysis of short-chained oligosaccharides. The different endoglucanases showed varying degradation capability depending on structure of the active site. The highly substituted HPCs had different susceptibility to degradation by the endoglucanases. The results show a difference in substituent distribution between HPCs, which would explain the differing cloud-point behaviors. Increased number of regions with low substitution could be correlated with lower polymer cloud point. The study shows the usefulness of enzymatic degradation to study the distribution of substituents in soluble biopolymer derivates.

Molecular mass distribution analysis of ethyl(hydroxyethyl)cellulose by size-exclusion chromatography with dual light-scattering and refractometric detection

Dual low-angle light scattering and refractometric detection coupled to size-exclusion chromatography provided proof for the presence of a low amount of stable aggregates/particles in ethyl(hydroxyethyl)cellulose. Unlike the correct size-exclusion chromatographic behavior of the parent polysaccharide itself, the aggregates exhibit variable size-dependent weak retention as a function of flow-rate and of ionic strength of the aqueous mobile phase. Therefore, determination of the molecular mass of non-aggregated polymer is possible in aqueous mobile phase containing 0.1 M NaCl under conditions at which aggregates are completely adsorbed on the column packing irrespective of the flow-rate used. Flow-rate and ionic strength-dependent variations of aggregate behavior as well as model size-exclusion experiments with latex particles indicate that they partly carry a minute charge and have a compact structure. Their weak retention under the separation conditions used suggests a difference in their surface chemistry when compared with the dissolved polymer coils which exhibit a correct size-exclusion behavior.

Amphiphilic association of ibuprofen and two nonionic cellulose derivatives in aqueous solution

The aqueous interaction of the sodium salt of ibuprofen with the cellulose ethers ethyl hydroxyethyl cellulose, EHEC, and hydroxypropyl methyl cellulose, HPMC, has been investigated in the concentration range 0-500 mM ibuprofen and 0.1-1% (w/w) polymer, by cloud point, capillary viscometry, equilibrium dialysis, and fluorescence probe techniques. Ibuprofen forms micelles in pure water, with the critical micelle concentration, cmc, at 180 mM. A combination of time-resolved and static fluorescence quenching shows that micelle-like ibuprofen aggregates are formed in the solution. The average aggregation number of pure ibuprofen micelles in water is about 40. In the presence of EHEC or HPMC the aggregation numbers decrease. The interaction of ibuprofen with cellulose ethers is similar to the normally accepted model for polymer-surfactant interaction, although more complex. Ibuprofen adsorbs to the polymer in the form of mixed polymer-drug micelles, noncooperatively up to cmc and cooperatively when cmc is passed. The interaction starts below 50 mM ibuprofen as monitored by the fluorescent probes pyrene and 1,3-di(1-pyrenyl)propane, P3P, with a maximum in microviscosity below cmc, corresponding to polymer-dense mixed micelles. The study illustrates the importance of a precise apprehension of the aggregation behavior as a background for transport studies in drug-polymer systems.

Dynamic Surface Tension of Dilute Aqueous Solutions of Nonionic Cellulose Derivatives in Relation to Other Macromolecular Characterization Parameters

The dynamic surface tension of 10 nonionic water soluble cellulose derivatives with molecular weights (M(w)) in the range 100,000-1,300,000 has been measured by the pendant drop method at the water/air interface. The surface tension measured after 11.7 h, gamma*, was taken as an apparent steady state surface tension. The gamma* at the highest concentration, 500 ppm, gamma*(500), showed values from 63 to 37 mN/m, and the value of gamma*(500) depends on the hydrophilic/hydrophobic balance of the polymer. The aim of the present work was to discuss the dynamic surface tension in relation to some other characterization parameters, like M(w), diffusion coefficient, intrinsic viscosity ([eta]), cloud point, and degree of hydrophilic/hydrophobic substitution. Dynamic surface tension was shown in a concentration range of 2-10 ppm. The time to reach (gamma(H2O)-gamma*)/2, defined as t(1/2), was found to be inversely correlated both to the diffusion coefficient and to the square of the bulk concentration, which can be expected for a diffusion controlled process. The time t(1/2) showed an approximately linear relationship against [eta](1/3). The pendant drop was found to decrease in volume with time due to the high vapor pressure caused by the curvature of the drop (DeltaP = 2gamma/r(drop)). Copyright 1999 Academic Press.

Multivariate parameter evaluation of pharmaceutically important cellulose ethers

A set of nonionic cellulose ethers with varying hydrophobicity and molecular weight has been investigated by principal component analysis (PCA). Several experimental variables such as dynamic surface tension, diffusion coefficient, microviscosity as monitored by a fluorescence probe technique, and intrinsic viscosity are included in the analysis. The experimental variables and observations (polymer fractions) are analyzed in models with good predictive capacities. The apparent equilibrium surface tension correlates to the cloud point and to the critical aggregation concentration in the presence of surfactant. The microviscosity is shown to be a predictive parameter for the degree of hydrophobic substitution. The irreversible process of dynamic surface tension is dependent on the diffusion coefficient but to an even larger degree on the polymer concentration, which is well illustrated by the PCA models.

Thermodynamics of Interaction between Some Cellulose Ethers and SDS by Titration Microcalorimetry

The interaction between certain nonionic cellulose ethers (ethyl hydroxyethyl cellulose and hydroxypropyl methyl cellulose) and sodium dodecyl sulphate (SDS) has been investigated using isothermal titration microcalorimetry at temperatures between 25-50 degrees C. The observed heat flow curves have been interpreted in terms of a plausible mechanism of the interaction of the substituent groups with SDS monomers and clusters. The data have been related to changes occuring in the system at the macro- and microscopic levels with the addition of surfactants and with temperature. The process consists predominantly of polymer-surfactant interactions initially and surfactant-surfactant interactions at the later stages. A phenomenological model of the cooperative interaction (adsorption) process has been derived, and earlier published equilibrium binding data have been used to recover binding constants and Gibbs energy changes for this process. The adsorption enthalpies and entropies have been recovered along with the heat capacity change. The enthalpic cost of confining the nonpolar regions of the polymers in surfactant clusters is high, but the entropy gain from release of hydration shell water molecules as well as increased freedom of movement of these nonpolar regions in the clusters gives the process a strong entropic driving force. The process is entropy-driven initially and converts to being both enthalpy and entropy-driven at high SDS concentrations. An enthalpy-entropy compensation behavior is seen. Strongly negative heat capacity changes have been obtained resulting from the transfer of nonpolar groups from aqueous into nonpolar environments, as well as a reduction of conformational domains that the chains can populate. Changes in these two components cause the heat capacity change to become less negative at the higher binding levels. The system can be classified as exhibiting nonclassical hydrophobic binding at the later stages of binding. Copyright 1999 Academic Press.

Thermodynamics of Interaction between Some Cellulose Ethers and SDS by Titration Microcalorimetry

A titration calorimetric study of the interaction between nonionic cellulose ethers and ionic surfactant (SDS) has been extended to a larger number of polymers to explore the effect of variation of polymer hydrophobicity on the energetics of the process. "Hydrophobicity" as used here is an overall effect of the nature, degree, and number of substituents and is characterized by the cloud point and (aqueous) surface tension lowering abilities of the polymer. A direct correlation is found between the extent of "hydrophobicity" and the endo-enthalpic peak in the initial SDS concentration region of interaction. However, the overall mechanism of interaction is similar for all the polymers, being dominated by polymer-surfactant interactions initially and converting into a surfactant-surfactant interaction process at higher SDS concentrations. The importance of polymer characteristics thus becomes weaker at the later stages of the process. Differences between the polymers is also reduced by an increase of temperature, leading to a near overlap of observed enthalpy curves at 40 degrees C. The energetics of interaction are also mirrored by the isothermal surfactant binding curves and the changes in macroscopic and microviscosity of the system. Copyright 1999 Academic Press.