Differential molecular weight distribution determination

Reliable determination of the differential molecular weight distribution by means of the dasscal precipitation or extraction procedures is not possible. In the e duation of the fractionation data problems are encountered that can, in fact, only be solved satisfactorily if the distribution is known beforehand. 

The scales on the [G co)/Glif versus (1/ ) plot can now be shifted to yield the final estimate of the cumulative or differential molecular weight distribution. Tuminello et al. [20, 21 ] used this technique to determine the MWD of several fluoropolymers that could not be analyzed using GPC due to their insolubility. 

Its actual molecular weight, as obtained from gel permeation chromatography, is My, = 1263 M = 1136. (29) Its melting teir ier-ature, under the conditions of the NMR measurements, was 108.8°C as determined by differential calorimetry. The low density (branched) polyethylenes studied here were commercial varieties whose molecular weights, distribution and side group concentrations have been reported. (30) The ethylene-butene-1 copolymers were a gift from the Exxon Chemical Corporation. 

L is obtained from the first point of inflection on the low-molecular-weight side of the major peak of the molecular weight distribution plot. In practice this is most easily determined from the maxima in a differential plot of the molecular weight distribution, chi (log M)/d log M versus log M, the dotted plot in Fig. 3-12. For this polymerization it is easy to detect not only L. but also Li, L3, and Lj. The detection of several Lt values acts as an internal check on the PLP-SEC method. For Fig. 3-12, the kp values calculated from L, La, and L are within 2% of each other the value from L4 is within 6-7% of the other values. In practice, closeness between kp values from L and L2 are considered sufficient to validate the PLP-SEC method. 

The use of multidetector GPC greatly increases the power of SEC, particularly in the case of copolymers. For copolymers of styrene-maleic anhydride (SMA), not only can the molecular weight distribution be determined, using a differential refractive index (DRI) detector, but also the compositional information of SMA (styrene content or acid number) by combining chromatograms from DRI and UV detectors [9]. 

This study describes the application of differential vis-cometry as a GPC detector to the problem of determining molecular weight distributions of acetylated hardwood lignins in tetrahydrofuran. Molecular weight distributions of ball-milled, organosolv, alkali-extracted/mild acid hydrolyzed, and alkali-extracted/steam exploded aspen lignins were estimated using universal calibration. 

For compounds bearing acrylate or methacrylate groups, the copolymer compositions were almost the same as the monomer feed compositions, and the molecular weights were nearly identical to that of poly(methyl methacrylate) (PMMA) synthesized as a control under the same reaction conditions. In addition, the dye-bearing repeat units were present uniformly in all molecular weights, as seen by comparing GPC molecular weight distribution curves determined by differential refractometry and by visible absorbance detection at the X of the... 

The latexes were cleaned by ion exchange and serum replacement, and the number and type of surface groups were determined by conductometric titration. The molecular weight distributions of the polymers were determined by gel permeation chromatography. The stability of the latexes to added electrolyte was determined by spectrophotometry. The compositional distribution was determined by dynamic mechanical spectroscopy (Rheovibron) and differential scanning calorimetry, and the sequence distribution by C13 nuclear magnetic resonance. 

Gel permeation chromatography (GPC) was used to determine molecular weights and molecular weight distributions, Mw/Mn, of polymer samples. The GPC consisted of an Alliance 2690 pump equipped with a Wyatt Rex Differential Refractive Index Detector and utilized three Polymer Labs PL Mixed B GPC Columns at 70°C (DMF) or 40°C (THF) at a flow rate of 1.0 mL/min. The molecular weights were calculated relative to the retention times of polystyrene and polyethylene oxide standards for DMF or polystyrene and polytetrahydrofuran standards for THF using Empower software. 

Size Exclusion Chromatography. The molecular weight distribution of each sample was determined by size exclusion chromatography (SEC). A volume of 150 jxL of each sample at a concentration of 1-2 mg/mL was injected onto a set of four 30-cm PLgel (cross-linked polystyrene) columns (with porosities of 10 , 10 , 10, and 10 A) housed in a Waters 150C liquid chromatograph at 40 °C. The mobile phase was tetrahydrofuran, and the concentration detector was a differential refrac-tometer. 

Characterization. Molecular weights and molecular weight distributions of polymer samples were determined using a Waters GPC with a differential refractometer... 

---