Absolute Molecular Weight Distributions Research Articles

Determination of absolute polydispersity and molecular weight distribution of high molecular weight polymers from dynamic light scattering

Intensity correlation function obtained from dynamic light scattering (DLS) experiments has been theoretically analyzed within the framework of the Schulz molecular weight distribution. The correlation function has been expanded about the mean molecular weight instead of the average linewidth as is normally done in cumulant analysis. This allows the explicit determination of polydispersity (Z). In addition to this, it has been shown that the molecular weight exponent of diffusion coefficient ν, occurring in the Mark–Houwink equation DM−ν and the average molecular weight M̄, can in principle be determined from DLS data. Finally, from the knowledge of Z and M̄ it is possible to construct the Schulz molecular weight distribution function. Experiments performed on three model systems, polystyrene in cyclohexane at 35 °C (θ system), polystyrene in toluene at 20 °C (good system), and latex beads in a deionized aqueous medium (hard spheres system), provide excellent matching between the experimental data and results obtained using our proposed method of analysis of DLS studies.

N‐methylation of wholly aromatic polyamides for size‐exclusion chromatography

AbstractWholly aromatic polyamides, including poly(1,4‐phenyleneisophthalamide) and poly(1,4‐phenyleneterephthalamide) are N‐methylated to render them soluble in N,N‐dimethylformamide, a common size‐exclusion chromatography eluent. The procedure N‐methylates 50–55% of the total amide linkages in these two examples, permitting reproducible measurement of their absolute molecular weight distributions using differential viscometry detection. There is no observable degradation in molecular weight resulting from the N‐methylation, provided the excess methylating reagent is destroyed shortly after completion of the derivatization by quenching with potassium acetate. The validity of the molecular weight data obtained by the method is confirmed by light‐scattering measurements on derivatized polymer and by comparison to the molecular weight of underivatized polymer that can be approximated from the intrinsic viscosity in concentrated sulfuric acid. The method is applicable to a variety of wholly aromatic polyamide structures. Examples are given.

Comparison of different methods for determination of molecular weight and molecular weight distribution of alginates

The scope of the present work was to assess and compare different methods to determine molecular weight (MW) averages and molecular weight distributions (MWD) of two samples of sodium alginate of different composition. Weight average ( M w ) and number average ( M n ) molecular weights were determined by totally integrated laser light scattering (LLS), both at wide angles (WA) and at a low angle (LA), and membrane osmometry, respectively. The two alginates were fractionated by preparative and analytical size exclusion chromatography. Absolute molecular weight distribution curves were obtained from SEC, using an on-line LA-LLS detector coupled with an RI concentration detector. Relative MWD curves were determined from ‘universal’ calibration of the SEC data. M w , M v , and M n were calculated from the ‘universal’ calibration curve and [η] j values of the fractions. Good correlation between molecular weight averages obtained from the different methods was observed. Mark-Houwink-Sakurada parameters, a and K, for the two different alginate samples have been determined.

Size Exclusion Chromatography of Quaternized Poly(vinylpyrrolidone-co-dimethyl Amino Ethyl Methacrylate) Copolymer (PVPDMAEMA): Absolute Molecular Weights and Molecular Weight Distributions by Low Angle Laser Light Scattering and Universal Calibration

Abstract In 0.1M tris. pH 7 buffer containing 0.5M LiNO3, PVPDMAEMA (which exists as a cationic polyelectrolyte) was found to elute based on hydrodynamic volume on Waters Ultrahydrogel columns. Within the exclusion limit of the column set an accurate weight average molecular weight of PVPDMAEMA can be obtained by universal calibration which is in very good agreement with the SEC/LALLS result.

The absolute molecular weight distribution of hydroxypropylated lignins

Mesure par GPC/LALLS et GPC/DV et en utilisant l'osmometrie en phase vapeur pour obtenir une Mn de reference. les resultats de GPC/LALLS sont compliques par l'absorbance, la fluorescence et la necessite d'une correction de polarisation. Les resultats montrent la fiabilite de la mesure par GPC/DV et l'existence d'une association moleculaire fonction du temps

Average molecular weight and molecular weight distribution of agarose and agarose-type polysaccharides

A procedure to determine the absolute weight-average molecular weight and molecular weight distribution of agarose and agarose-type polysaccharides by aqueous size-exclusion chromatography coupled with low-angle laser light scattering is described. The molecular weights of the majority of the commercial samples investigated were between 80 000 and 140 000 with a polydispersity lower than 1·7. In contrast, most of the laboratory-extracted agarose-type polysaccharides had lower molecular weights.

Effects of dextran polydispersity on red blood cell aggregation

B 512 F dextran fractions of different polydispersities were prepared by fractionnal precipitation and preparative elution chromatography. By combination of low angle laser light scattering (LALLS) and gel permeation chromatography (GPC), absolute average molecular weights (MWs) and molecular weight distribution functions (MWDs) were determined. Static and thermodynamical properties in terms of polymer dimensions and second virial coefficient of dilute solutions of dextran in water have been investigated. The results indicate that dextran macromolecules in water are rather compact and impenetrable coils. Measuring the disaggregation shear stress of dextran-induced red blood cell aggregates by laser light reflectometry, the macromolecular bridging energy was shown to depend upon dextran sample polydispersity. This reflects the weak and reversible character of red blood cell aggregation by dextran chains in physiological saline solution.

Molecular weight distribution of carrageenans by size exclusion chromatography and low angle laser light scattering

A procedure to determine the absolute weight average molecular weight and molecular weight distribution of carrageenans by high pressure aqueous size exclusion chromatography coupled with low angle laser light scattering is described. Experimental parameters are successively discussed, particular attention being focused on the absence of shear degradation during elution. The distribution curves were highly reproducible in time and weight average molecular weights integrated along the chromatogram were in good agreement with static light scattering results. A large difference in the molecular weight range between native (food-grade) and acidic degraded carrageenan samples was observed. Weight average molecular weights were found to be in good correlation with viscosity values, for degraded as well as undegraded products. It is also shown that the method described can help people using carrageenans in pharmacological studies by providing information on the real molecular weight distribution of the products they are employing.

Determination of the absolute molecular weight of a styrene–butyl acrylate emulsion copolymer by low‐angle laser light scattering (LALLS) and GPC/LALLS

AbstractThe application of low‐angle laser light scattering (LALLS) and combined GPC/LALLS for the measurement of absolute molecular weight distribution of a styrene–butylacrylate (30/70) emulsion copolymer is discussed. From the static light scattering measurements in four different solvents, i.e., toluene, tetrahydrofuran (THF), methyl ethyl ketone (MEK), and dimethylformamide (DMF), the true weight average molecular weight (Mw) and heterogeneity parameters are determined. The apparent Mw obtained from the static measurement in THF was in good agreement with the Mw determined from the multiple solvent analysis, suggesting the validity of using THF as the mobile phase in the combined GPC/LALLS analysis.

Absolute molecular weight and molecular weight distribution of guar by size exclusion chromatography and low-angle laser light scattering

A procedure to determine the absolute weight-average molecular weight (M w ) and molecular weight distribution (MWD) of guar by aqueous size exclusion chromatography coupled with low angle laser scattering is described. It is shown that for a rigorously purified sample of guar solution the values for M w and MWD are 2·2×10 6 and 1·9 respectively. The effect of sample preparation and purification on these molecular parameters are discussed. Limitations and challenges in the aqueous size exclusion chromatography of complex water soluble polymers such as guar are also explored.

Measurement of the absolute molecular weight and molecular weight distribution of polyolefins using low‐angle laser light scattering

AbstractBy connecting a low‐angle laser light scattering photometer (LALLSP) on line with a high‐temperature gel permeation chromatograph (GPC), molecular weight distributions were determined for three polyethylene standards (two high density and one low density) without resorting to column calibration procedures. In addition, the absolute weight‐average molecular weight and second virial coefficients of four polyethylene standards (three high density and one low density) were obtained using low‐angle laser light scattering. The molecular weights measured were in excellent agreement with those specified by the National Bureau of Standards. For the low density material, a microgel fraction was detected by the on‐line LALLSP but not by the conventional GPC detector. This resulted in a z‐average molecular weight of over 3 × 106 daltons, an order of magnitude greater than that determined by conventional GPC techniques.

Solution properties of polymers by low angle laser light scattering photometry

Many of the solution properties of polymers can now be rapidly measured with a high degree of precision because of a recent innovation in Rayleigh light scattering photometry. Absolute molecular weight distribution and molecular weight averages of several polymers were readily obtained by on line coupling of the low angle laser light scattering photometer with a commercially available gel permeation chromatograph. Data on the characteristic ratios ( C N = <(r 2)> 0 nl 2 ) and the second virial coefficients as well as the Mark-Houwink constants for poly(bicycloheptene sulfone), P(MMA), and P(MMA/MAA) copolymers of different compositions in THF, CHCl 3, and cyclohexanone solvents were obtained. Some translational diffusion coefficient data of polymers in dilute solution measured by the dynamic Rayleigh light scattering technique is also presented.

Exclusion chromatography using porous glass. II. Application to hydrophilic polymers

AbstractA chromatograph employing five columns packed with porous glass of pore size 1250 Å to 75 Å provided peak retention volumes (VR) that were reproducible and essentially independent of sample size and flow rate when aqueous eluents were used. Calibration was carried out with a series of dextran fractions and polystyrene sulfonate samples, both of moderately narrow molecular weight distribution. The universal calibration method, based on hydrodynamic volume, was tested for four different polymer types. All four types produced a common curve within experimental error, which indicates that absolute molecular weight distributions may be derived from aqueous exclusion chromatography data for at least these polymer types. Additional study using a higher salt concentration produced hydrodynamic‐volume plots that superposed with those above. The use of the same set of porous glass columns with polystyrene standards in three different organic solvents produced calibration curves that agreed well with the aqueous curves after corrections were made for differences in available pore volumes.

Gel permeation and thin‐layer chromatographic characterization and solution properties of butadiene and styrene homopolymers and copolymers

AbstractGel permeation chromatographic (GPC) and thin‐layer chromatographic (TLC) studies of polystyrene, polybutadienes (BR), and their copolymers (SBR) have been carried out. GPC primarily separates them on the basis of molecular size, and TLC, on the basis of composition. Methods of obtaining absolute molecular weight distributions for BR and SBR based upon variations of the Strasbourg Universal Calibration procedure are described. In particular, [η]–M relationships in both the GPC solvent (THF) and in a second solvent (toluene) were used; in addition, results of statistical mechanical calculations for \documentclass{article}\pagestyle{empty}\begin{document}$\overline {s^2 }$\end{document} (based on the assumption of negligible steric hindrance and freely rotating bonds) were applied. An experimental comparison of these methods was carried out, and use of the [η]–M relationships for both solvents was found to give satisfactory results. The predictions of the statistical theory were too low. A detailed study of polymer–solvent–gel interaction in the GPC unit was made through investigation of ternary phase equilibrium in the (polystyrene)–THF–(polymer) system. The polymers studied included BR and SBR with varying styrene contents. Experimental techniques for TLC separations of BR, SBR, and polystyrene according to the composition are described.

Gel permeation chromatography: Data treatment

Since the advent of gel permeation chromatography, it has been difficult to compare molecular weight distributions of samples analyzed on different gel columns or under different experimental conditions. Corrections are described herein which convert gel permeation chromatographic curves to absolute molecular weight distributions. Good agreement of corrected molecular weight distributions for several samples analyzed on different column sets was obtained.