Universal calibration analysis

Electrostatic and adsorption effects conspire to make aqueous GPC more likely to be nonideal than organic solvent GPC. Thus, universal calibration is often not obeyed in aqueous systems. Elence, it is much more critical that the standard chosen for calibration share with the polymer being analyzed chemical characteristics that affect these interactions. Because standards that meet this criterion are often not available, it is prudent to include in each analysis set a sample of a secondary standard of the same composition and molecular weight as the sample. Thus, changes in the chromatography of the analyte relative to the standards will be detected. 

The discussion and experimental approach presented here rely on the principles and use of universal calibration for SEC analysis. For a review of this method the reader is referred to several useful articles (1, 3,8,9 ). 

Branching in the polymer chain affects the relationship between retention and molecular weight.83 Universal calibration has been used with some success in branched polymers, but there are also pitfalls. Viscosimetry84-91 and other instrumental methods have proved to be useful. A computer simulation of the effects of branching on hydrodynamic volume and the detailed effects observable in GPC is available in the literature.92 93 In copolymer analysis, retention may be different for block and random copolymers, so universal calibration may be difficult. However, a UV-VIS detector, followed by a low-angle light-scattering (LALLS) detector and a differential... 

Novolac molecular weights were measured in THF at 35°C by high pressure size exclusion chromatography using a Waters Model 510 pump (flow rate=1.0 ml/min), 401 differential viscometer detector and a set of Dupont PSM 60 silanized columns. A universal calibration curve was obtained with a kit of 10 narrow molecular weight distribution, linear polystyrene standards from Toya Soda Company. Data acquisition and analysis were performed on an AT T 6312 computer using ASYST Unical 3.02 software supplied with the Viscotek instrument. 

Other analysis methods dependent on multiple detectors can be implemented using this automated system. Two methods under development are the use of a continuous viscometer detector with a refractive index detector to yield absolute molecular weight and branching, utilizing the universal calibration curve concept (4), and the use of a UV or IR detector with the refractive index detector to measure compositional distribution as a function of molecular weight. 

Universal Calibration. One of the goals of this work was to evaluate the applicability of the universal calibration technique (/3) to SEC analysis of these multi-arm macromolecules. This technique assumes a unique calibration relationship... 

In order to determine g as a function of molecular weight, one approach is to use universal calibration with SEC analysis of molecular-weight polydisperse samples (31-33). For a multiarm (MA) branched material, the intrinsic viscosity of polymer eluting in v is ... 

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). 

The determination that the low MW, acetylated aspen lignins examined actually fit universal calibration, however, must be deferred to future studies comparing these data to results from LALLS and sedimentation equilibrium analysis (if possible). 

As a result of the dependence of universal calibration on column elution behavior (i.e., anomalous behavior due to adsorption or exclusion), the contribution of the polymer core and shell components (33,34) to hydrodynamic behavior must be fully understood if competent analysis of block copolymers and branched heteropolymers is to be made. It is hoped that with the advent of appropriate MW, composition, and branched polymer standards, the limits of fit of universal calibration to biopolymers such as lignin can be judged. 

If the Mark-Houwink coefficients are not available, a universal calibration curve is established using polystyrene calibration standards and the SEC-vis-cometer combination. The basic steps involved in the MMD analysis are summarized in Fig. 11. First, the universal calibration curve of the SEC separation system has to be established by using narrow molar mass standards as indicated by the top arrow pointing to the right. Once the universal calibration curve is established, the procedure can then be reversed, by going from right to left following the bottom arrow, to obtain the molar mass calibration curve of any unknown... 

Both FFF and SEC require careful control of the temperature for universal calibration. For SEC and Fl-FFF, this means controlling the temperature of the room or of the channel/column. For Th-FFF, it is important to maintain the specified cold-wall temperature, Tc. Fortunately, the temperature at the center of gravity of a component is independent of the field strength in Th-FFF, so that universal calibration constants do not change when AT is tuned to optimize the analysis of a particular range in M, provided Tc is held constant. 

Absolute MWD can be measured using light scattering or viscometry combined with universal calibration. Compositional drift over the MWD of a polymer can be measured using a UV spectrophotometer and a differential refractive index detector. The increase in the available information also expands the complexity of data analysis. We discuss some of the concerns regarding data analysis that arise in multidetector SEC. 

Table II contains the results of the analysis of star-branched copolymers (Mw, Mn, and polydispersity d) by both the viscometric coupling (V) and the light-scattering coupling (L). The numerical values are in agreement, especially for the values. These results confirm that the universal calibration is perfectly valid for branched molecules, even for a high degree of long-chain branching.

The application of dual detection [UV and refractive index (RI)] to the SEC analysis of polystyrene-poly(methyl methacrylate) (PS-PMMA) has already been studied in this laboratory (2). Both MWD and CCD were determined using a methodology outlined by Runyon et al. (3). This approach relies on SEC column calibration with narrow polydis-persity standards for each of the homopolymers as well as a measure of the detector response factors for each homopolymer to produce a copolymer MWD. In the case of PS and PMMA this is feasible, but in other block copolymer systems the availability of suitable molecular weight standards may be more limited. In addition, this procedure does rely on true SEC and is not valid for block copolymers for which the universal calibration does not hold true for both blocks in a given solvent system. 

XRF analysis was used for the determination of ruthenium, tin, sodium and chlorine content. Solid catalysts samples were examined by automatic sequence RTG spectrometer (ARE 9400 XP). Elements loadings were evaluated by UNIQUANT analyzer. The program used a universal calibration method. 

Recently, Veith and Cohen (3) analyzed nylon 6 in TFE using the universal calibration method and peak retention data from narrow PMMA fractions. Silanized silica columns obviate the solvent incompatibility problem of TFE with styrene-based packings and give reproducible results. The accuracy of the calculated nylon 6 molecular weights was cross-checked with an independent end-group analysis (for M ) and intrinsic viscosity measurement (forMJ. 

The intrinsic viscosity and Mark-Houwink constants of standards can be determined from a static capillary viscometer or an on-line viscometer detector in an SEC system. If the intrinsic viscosity is to be used for constructing a universal calibration curve, it is important to use the identical conditions in performing the SEC analysis and the intrinsic viscosity measurement. A Mark-Houwink plot for five PAM standards and one PAA standard is shown in Figure 4. The intrinsic viscosity of PAM may decrease with time and becomes constant after about one week. It is recommended that the PAM solution be analyzed while still fresh. 

---