Average molecular weights determination

For a polymer with a molecular weight distribution the proper molecular weight average to use in equations (T) and (lO) can be determined using the following considerations. In the case of a heterogeneous polymer it has been shown that (7 19.20). 

Molecular weights were determined and the results for the weight average molecular weight are shown in Figure 12 for conversions of 30% and 50%. It is noted that molecular weight decreases with increasing (Nj e)e until just before the turbulence occurs. 

In principle, MALDI-TOF (MALDI-Time Of Flight) analysis allows for the determination of the complete polymer mass distributions and, from that, the calculation of various molecular weight averages like Mn and Mw and the... 

With this chapter we have tried to provide an overview of experimental techniques for determining molecular weight averages and molecular weight distributions. All methods discussed here have their specific advantages and weaknesses and differ very much in their complexity. The choice of the best method strongly depends on polymer properties, the information needed for a particular purpose, and on the available resources. Yet another aspect in the analytical characterization of polymers may be speed. 

The viscosity average molecular weight is determined through the use of the Mark-Houwink-Sakurada equation [3] using solution viscosity ... 

A. Materials. Polysulphone resin P-1700 and P-3500 were obtained from Union Carbide. Their weight average molecular weight, was determined by ultracentrifuge ... 

The sequential U test was performed by using several pairs of polymer samples. First example is shown in Figure 1. The sample mixture is a combination of NBS 706 (98.5 %) and PS 200000 (1.5 %). Two normalized chromatograms, NBS 706 (A) and the mixture (B), are nearly the same and molecular weight averages (the mean from three determinations) calculated are almost identical. The results of the sequential U test is shown in Figure 2. After the fourth pair of runs, all the values of were found to be located in the... 

M (t) and M (t) are the true or experimentally determined molecular weight averages. The "Method of Molecular Weight Averages" was included in the ASTM Standard Method, D3536-76, to correct for instrument spreading effects. 

Table I illustrates the molecular weight averages found from universal calibration (narrow standards) for four aspen lignins and two quinonemethide-derived polymers using the Unical software. The poly-dispersities were the same within the experimental errors for all lignin samples. In contrast, the polydispersities found for the quinonemethide-derived polymers by universal calibration were near 1.1. The molecular weight averages found for the four acetylated lignins studied by universal calibration were substantially larger than those determined from previous work using conventional GPC (e.g., AESE and BM lignins from refs. 7 and 12 had approximately one-third those values found by HPSEC-DV in the present study).

Vapor Phase Osmometry. A Wescan Model 233 vapor phase osmometer was used to obtain number average molecular weights. The lignin solutions were made up with HPLC grade tetrahydrofuran (THF) and shaken manually until the solutions were clear. The experiments were conducted at 30°C. Number average molecular weights were determined by multistandard calibration (41), a procedure found to greatly enhance reproducibility and accuracy of the results. Experiments were conducted immediately after sample preparation and three days later. 

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