Narrow MMD calibration

An SEC system can be calibrated relatively simply by plotting the elution volume (or time) of the peak maxima of a series of calibrants with polydispersities (ratio of to M ) of less than 1.1, against their molecular masses. The use of such narrow MMD calibrants means that sensible calibrations will be obtained regardless of which molecular mass average is used for the plot in practice, the manufacturers of these calibrants now usually supply a peak molecular mass to be used in the calibration. How this calibration is defined will be considered later. 

The main problem associated with using narrow-MMD calibrants is the restricted range of polymer types available. Poly(styrene) calibrants are the main type used, and a very wide range of molecular masses is available. A small number of other polymer types is obtainable, but the molecular mass range is often rather limited. Poly(ethylene oxide) and poly(ethylene glycol) calibrants deserve special mention, because they can frequently be employed where poly(styrene) behaves anomalously, for instance in polar solvents such as dimethylformamide. Some of the narrow-distribution polymer calibrants available are listed in Table 3.1 and Appendix 2. 

The narrow molecular mass distributions are usually obtained by polymerizing these materials directly to obtain a narrow distribution (i.e. by ionic polymerization techniques), rather than by using any fractionation techniques. As these polymerization techniques are limited in their applicability, there are technical as well as commercial reasons for the limited range of polymer types for which narrow MMD calibrants are available. The manufacturers of these narrow-distribution calibrants will undoubtedly seek to extend the number of chemical types and the molecular mass range available, but commercial considerations will inevitably limit this expansion. 

Clearly, the characterization of a broad-MMD calibrant requires a considerable amount of effort and expertise, and they have been mainly used where there are problems in producing narrow-MMD calibrants. In practice, the main applications of this approach have been the use of NBS Standard Sample 706 (polystyrene) and, in particular, NBS Standard Sample 1475 (linear polyethylene). SEC has been a very valuable technique for examining the MMDs of polyethylene, but as there has only been limited success in producing narrow-MMD polyethylene calibrants, NBS 1475 has been widely used for calibrating high-temperature SEC systems used for this application. Unfortunately, the molecular mass range of NBS 1475 is rather low and is not really appropriate for many samples. Barlow et al [5] used NBS 1475 and extended the calibration range with additional polymers (see Chapter 4, section 4.5.1.1). 

Most commercially available data handling systems which have SEC software written for them assume that the calibration will be carried out using a series of narrow MMD calibrants. The molecular masses are always used in the logarithmic form, and the calibration is usually established either as a linear relationship between the log molecular mass and the elution time, or by using a polynomial relationship. Different systems permit the use of various orders of polynomial, but in practice a linear fit or a third-order polynomial are the most commonly encountered. 

As explained in Sections 16.3.4, 6.4.1, and 16.4.2, SEC is a nonabsolute method, which needs calibration. The most popular calibration materials are narrow molar mass distribution polystyrenes (PS). Their molar mass averages are determined by the classical absolute methods—or by SEC applying either the absolute detection or the previously calibrated equipment. The latter approach may bring about the transfer and even the augmentation of errors. Therefore, it is recommended to apply exclusively the certified well-characterized materials for calibrations. These are often called PS calibration standards and are readily available from numerous companies in the molar mass range from about 600 to over 30,000,000g moL. Their prices are reasonable and on average (much) lower than the cost of other narrow MMD polymers. Other available homopolymer calibration materials include various poly(acrylate)s and poly(methacrylate)s. They are, similar to PS, synthesized by anionic polymerization. Some calibration materials are prepared by the methods of preparative fractionation, for example, poly(isobutylene)s and poly(vinylchloride)s. 

A quantitative analysis of the degradation of the sample substantially requires the use of an on-line absolute detector such as light scattering (SEC-LS). In this case, the on-line LS detector directly measures, without calibration, the molar mass of the sample. The characterization of a series of narrow MMD standards with increasing and known molar mass evidences the degradation of the sample. The simple direct comparison of the known molar mass of the standards and the... 

Conventional SEC is the classical method for measuring the MMD of synthetic or natural polymers. Conventional SEC essentially means the use of an on-line single concentration detector - differential refractometer (DRI) or UV. In such SEC systems, the molar mass is calculated by means of a direct or universal calibration applying a set of appropriate narrow and/or broad MMD standards. Unfortunately, classical narrow MMD standards for aqueous SEC mobile phases such as pullulan, poly(ethylene oxide), poly (ethylene glycole), and dextran are not suitable for characterization of HA. The listed SEC standards are neutral molecules existing in solution as flexible random coils, and some of... 

Figure 2.4 reports the values of the constant "a" for different macromolec-ular shapes. The values of a" are in the range 0.5-0.8 for random coils. For other shapes, such as rods, the "a" value is larger. The constants K and "a" are determined by obtaining (usually by SEC fractionation) a series of polymers (referred to as the calibrants) which possess different MM, a narrow MMD, and the same structure of the polymer to be analyzed. K and "a" have been tabulated for many polymer-solvent combinations. ... 

CALIBRATION AND DATA ANALYSIS Table 3.1 Narrow MMD polymer calibrants ... 

It is clear from the previous two sections that there is only a limited range of polymer types for which either narrow- or broad-MMD calibrants are available. Clearly it would be desirable to have a mathematical procedure which would allow for the difference in chemical type between the calibrants available and the samples of interest. 

In the opinion of the author, the biggest difficulty with the use of SEC-LALLS calibration is conceptual. It is not clear whether it is because the weight average is measured at each point or because the LALLS response is emphasized at the higher molecular masses, but the calculated MMDs for samples always appear narrower for SEC-LALLS than for SEC on its own. This would be acceptable if users of the techniques took due account of these differences, but the general preference for polymers with narrow MMDs appears to cloud some users interpretation of the results. 

Conti and Sorti [57] studied the relationship between molecular mass distribution and melt flow characteristics of poly (e-caprolactam). Samples of nylon-6 with different molecular masses and distributions were synthesized in order to check the predictions of flow theories. SEC was carried out in m-cresol at 112°C. Narrow-MMD polystyrenes were used for calibration. and were calculated from the chromatograms after correcting for skewness and... 

Empirical approach. In SEC, a calibration curve of log molecular mass (log M) V. retention volume (Fr) must first be constructed in order to calculate molecular mass averages of the polymers in question. It is usual to report the results as polystyrene equivalent molecular masses, using a calibration curve constructed with polystyrenes of known molecular masses having narrow MMD. For accurate estimation of copolymer molecular weight, one must use a calibration curve for the copolymer in question. Therefore, it is necessary and important to estimate molecular mass of the copolymer species eluted at retention volume i. 

The universal calibration curve was constructed with polystyrene having narrow MMD by using the equation... 

As mentioned in the previous paragraph, the calibration curve constructed by relating an average molar mass to the retention volume of a series of preferentially narrow MMD polymer samples can be used only for the determination of the MMD and average molar masses of an unknown sample of the same chemical composition and structure. The [rj] is the intrinsic viscosity determined experimentally. On the other hand, the use of the product M[rj],... 

In the case of propylene homopolymer, the universal calibration procedure established by Grubisic et al. [2] can be applied using anion-ically polymerized polystyrene molar mass standards with narrow MMD for TCB at 135°C, the following values of the parameters K and a in the equation ... 

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... 

The experimental dependence M = /(V), i.e. the classical SEC calibration curve usually obtained by using narrow standards, in such a case can be obtained directly without calibration from the on-line LS detector. By combining the experimental function M = /(V) and the concentration profile (from DRI), one can construct the complete MMD of the HA sample. The differential and cumulative MMD of a high molar mass HA sample (Mw = 652 kDa, D = 2.1) are shown in Fig. (10). Starting from the initial MMD, the molecular weight averages and dispersity index (Mn, Mw, Mz, and D) could be easily calculated using the appropriate definitions. 

The accuracy of the SEC/MALDI procedure has been demonstrated." A series of narrow distributed PMMA samples were injected into a SEC apparatus, and the elution volumes were recorded. A PMMA sample with a broad MMD (M = 13000 Ma, = 33000) obtained by free radical polymerization was injected in an SEC apparatus, and SEC fractions were collected." The MALDI-TOF mass spectra of the SEC fractions were recorded and the respective MM could be computed. Figure 10.14 reports the SEC calibration line obtained using the MALDI-TOF mass spectra of the SEC fractions and also the SEC calibration line obtained using the PMMA standards. " " The two lines are virtually superimposed, thus confirming the accuracy of this procedure, which makes use of MALDI-TOF mass spectra of SEC fractions. The M and... 

Figure 7.1 Conventional calibration in SEC. The calibration curve is constructed by measuring the elution volumes of a series of narrow standards, ideally with the same chemical structure as the analyte polymer. The elution profile of a polymer (top) can be transformed into an MMD (right).

---