Polystyrene broad standards

Polystyrene standards of known MW used in this study (Pressure Chemical Company and National Bureau of Standards) were narrow MWD, with MW values ranging from 4000 to 1.8 million, except for one broad standard,... 

The broad-standard linear calibration curves for polystyrene in THF in Figure 1 and 4 are used to illustrate the K and a calculations as follows. The particular calibration curves are found to correspond to the following calibration equations ... 

The PCP (Aldrich, catalogue no. 18,168-4, lot no. 03) had no listed molecular weight data. A broad MMD polystyrene (PS) standard (National Bureau of Standards) had a light scattering fl = 257,800 (SRM 706, R7R =2.1). Six of the narrow MMD PS standards (Pressure ChemTcaT Co.) had nominal molecular weights... 

One possibility is that although averages for polystyrene standards require correction, those for PMMA would not According to symmetrical axial dispersion theory (5) the correction depends upon both the slope of the calibration curve (different for each polymer type) and the variance of the chromatogram of a truly monodisperse sample. Furthermore, the calibration curve to be utilized can be obtained from a broad standard as well as from monodisperse samples. The broad standard method may itself incorporate some axial dispersion correction depending upon how the standard was characterized. 

Standard Reference Material 706, Polystyrene (Broad Molecular-Weight Distribution)... 

Figure 1 presents the conformation plot for log(r ) versus log(M) as obtained from a MALS measurement for the polystyrene broad linear standard NIST706 in toluene. Superimposed thereon is a plot of the calculated log [17] as a function of log(M) for the same sample. From the latter plot, the MHS coefficients may be deduced by inspection of the slope and intercept to yield a = 0.77 and K 0.008. 

GPC s of the diree resins are shown in Figure 7 and are calibrated relative to linear polystyrene standards. All of the resin samples are low molecular weight with relatively broad polydiqiersities. Given the inappropriate nature of the linear polystyrene GPC standards for SSQ derivatives, e data in Figure 7 are usefiil for relative comparison only. 

In this stage of the investigation, poly(methyl methacrylates) (PMMAs) were selected as the polymeric probes of intermediate polarity. Polymers of medium broad molar mass distribution and of low tacticity (14) were a gift of Dr. W. Wunderlich of Rohm Co., Darmstadt, Germany. Their molar masses ranged from 1.6 X 10" to 6.13 X 10 g-mol. For some comparative tests, narrow polystyrene standards from Pressure Co. (Pittsburgh, PA) were used. 

Two polystyrene standards were used to demonstrate the proposed calibration method. One narrow-and one broad-MWD sample were purposely selected to show that the method works for both narrow and broad... 

Figure 20 shows an example of its use. As expected, the GPC 2 chromatogram of a fraction of a monodisperse standard (obtained by sampling it with GPC 2 at its peak) is narrower than the whole standard which in turn is narrower than chromatograms of slices of broad polystyrene distributions. In Figure 20, two examples of the latter show the difference obtained by improving resolution in GPC 1. 

In principle, one could solve equations (la), (2a) and (3a) for K, a and. Unfortunately, and [n] are often highly correlated and it is recommended that only one of these data be used per standard. A practical procedure is to estimate using narrow MWD polystyrene standards leaving two unknowns, K and a. To illustrate the method, suppose Mjj and M data are available for the single broad MWD standard. Dividing equation (2a) by (la) eliminates a and one is left with a single-variable search for 3. 

Figure 2. Intrinsic viscosity-molecular weight data for polystyrene and polyvinyl chloride measured by osmometry and by SEC using broad MWD standard calibration (polystyrene (m) M = 7,06 X 10 in THF/1% PPG (O) [ ] =...

Dow 1683 polystyrene standard was utilized as a broad MWD standard in the linear calibration method due to its distribution symmetry and particular lack of significant tail at the low end of its MWD. 

Six commercial narrow UWD polystyrene standards ( Applied Research Laboratories Limited, England ) and five broad MWD 2-polybuta-diene fractions were used to calibrate an ARL 950 GPG instrument with silica bead packed columns. The ARL polystyrene standards were also used to calibrate a number of homeMnade SEC column units packed with silica beads or styrene-divinylbenzena copolymer beads, Hie average molecular weights of these samples are listed in Table I. Tetrahydrofuran or toluene was used as eluents for these columns. 

FIGURE 3.17 Three-dimensional plot of scattering intensity as a function of scattering angle and elution volume for a broad molecular weight distribution (MWD) polystyrene (PS) (NITS standard reference 706). (Courtesy of Wyatt Technology Corporation, Santa Barbara, CA 93117 wyatt wyatt.com. With permission.)... 

There is so-me scatter in the computed Mar k-Houw i nk constants from the broad polystyrene standards of Table lib. However, the average of the four samples is very close to that found from the set of narrow standards. 

Figure 1. Master universal calibration curve obtained with a Beckman /z-Spherogel column system in THF. The fit of narrow MWD standards, polystyrene star polymers, and a single broad MWD standard (calculated with Unical 2.71 software) are shown.

Values for system peak parameters were found using a narrow distribution polystyrene standard (PS68K) before calculating MWD data for the lignin samples from universal calibration. To check software and instrument operation, several narrow MWD polystyrene and one broad MWD polymethylmethacrylate standards were treated as unknown samples and subjected to analysis with the universal calibration curve assembled from all polymer standards files. It was found that the MWD could be estimated for the recalculated polymer standards with errors between 5 and 10% of the original value indicated by the supplier of the standard (e.g., Mw for PS11K and Mw and M for PMMA17K-6). 

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