Resolution polymer fractionation

Fig. 7. Dependence of soluble polymer fraction ( ) and intensity of C NMR signal collected under standard high-resolution conditions (A) of EPR containing 36% propylene, irradiated in vacuum by doses up to 2 MGy. The solid lines are drawn as a guide to the eye. Adapted firom ref. 93 with permission. Copyright 1992 Elsevier Science.

HPLC equipment dedicated to high-osmotic-pressure chromatography is used for the fractionation of narrow polymer fractions from broad distribution samples. This technique, which employs columns that are packed with a control pore glass of very narrow pore distribution, separates polymers by molecular weight as a function of osmotic pressure. When this approach is coupled with a fraction collector the technique can generate polymer fractions in significant quantities for further study by nuclear magnetic resonance, (NMR), FTIR, or other spectroscopic techniques. This technique can offer superior resolution to the previously mentioned preparative GPC. This technique has been applied to the characterization of both copolymers and homopolymers. 

To characterize the efficiency of polymer fractionation, it is essential to determine molecular weight distribution of the fractions. Good resolution within short times, combined with amenability for automation, are the main... 

The incidence of these defects is best determined by high resolution F nmr (111,112) infrared (113) and laser mass spectrometry (114) are alternative methods. Typical commercial polymers show 3—6 mol % defect content. Polymerization methods have a particularly strong effect on the sequence of these defects. In contrast to suspension polymerized PVDF, emulsion polymerized PVDF forms a higher fraction of head-to-head defects that are not followed by tail-to-tail addition (115,116). Crystallinity and other properties of PVDF or copolymers of VDF are influenced by these defect stmctures (117). 

Larger injection volumes, e.g., 2% of the total column volume, are sometimes advantageous in the preparative fractionation of polymers (33). More samples can be injected using larger injection volumes with a slight decrease in resolution. When the same amount of sample is injected with a smaller injection volume and a higher sample concentration, the resolution decreases more significantly. 

However, for quantities substantially less than this level, 7- to 10-mm i.d. analytical columns can often be used in a semipreparative mode. By repeatedly injecting 300 to 500 ju,l of up to 1% polymer, reasonable quantities of polymer can be isolated. An autosampler and automated fraction collector can be setup to perform such injections around the clock. Although the larger injections and higher concentrations will lead to a loss of resolution, in some situations the result is quite acceptable, with a considerable savings in time being realized over other means of trying to make the same fractionation. 

A criterion for selecting a right pore size to separate a given polydisperse polymer is provided here. To quantify how much the MW distribution narrows for the initial fraction, an exponent a is introduced (2). The exponent is defined by [PDI(0)] = PDI(l), where PDI(O) and PDI(l) are the polydispersity indices of the original sample and the initial fraction, respectively. A smaller a denotes a better resolution. If a = 0, the separation would produce a perfectly monodisperse fraction. Figure 23.7 shows a plot of a as a function of 2RJd (2). Results... 

Table 1 gives yields and molecular weights for the total polymer and for the low and high molecular weight fractions (LMWF and HMWF). Since the resolution of GPC traces into LMWF and HMWF was estimated subjectively the molecular weights for these fractions are merely indicators of trends. 

In particular, for copolymers this required an orthogonal coupling of one GPC to another to achieve the desired cro fractionation before application of dual detectors. This method is really a new polymer analysis member of a family of approaches developed in the literature which we are now terming "Orthogonal Chromatogr hy . It not only provides both a cro fractionation approach for copolymers and a new way of determining the GPC s "imperfect resolution" it also enables separation mechanisms previously reserved for the liquid chromatography of small molecules to be used for polymer analysis. 

As can be seen from Figure 2, pore permeation increases with ionic strength, but the curves are not linear and in particular show poor resolution at MW less thcui a million. Complete loss of resolution in this MW reuige is seen at 0.5 M NaCl reflecting, presumably total permeation. However the total permeated volume (as measured with NaCl) is significantly greater than the polymer elution voliame at 0.5 M NaCl. Such a volume difference could be explained if a fraction of the pores is inaccessible to even the lowest M.W. polymer investigated. 

Table 5.1 lists several heart-cut and comprehensive techniques. Heart-cut 2DLC is very common and has great application for the increased resolution of one or several components from the first dimension (Augenstein and Stickler, 1990 Majors, 1980 Pasch et al., 1992 and Dixon et al., 2006). Heart-cut 2DLC for the analysis of polymers is often referred to as cross-fractionation (Balke and Patel, 1980). Protein digest analysis with MS/MS identification has been called multidimensional protein identification technology or MUDPIT. This is described in detail in Chapter 11. 

The advent of high-resolution 13 C NMR allows the determination of tetrad, pentad, and even higher sequence distributions in many polymers [Bovey, 1972 Bovey and Mirau, 1996 Farina, 1987]. The tetrad distribution consists of the isotactic sequence mmm, the syndiotactic sequence rrr, and the heterotactic sequences mmr, rmr, mrm, rrm. The sum of the tetrad fractions is unity, and the following relationships exist ... 

Even if relatively new, HF FIFFF has been used to separate supramicrometer particles, proteins, water-soluble polymers, and synthetic organic-soluble polymers. Particle separation in HF FIFFF has recently been improved, reaching the level of efficiency normally achieved by conventional, rectangular FIFFF channels. With these channel-optimized HF FIFFF systems, separation speed and the resolution of nanosized particles have been increased. HF FIFFF has recently been examined as a means for off-line and on-line protein characterization by using the mass spectrometry (MS) through matrix-assisted laser desorption ionization time-of-flight mass spectrometry (M ALDl-TOF MS) and electrospray ionization (ESl)-TOF MS, as specific detectors. On-line HF FIFFF and ESl-TOF MS analysis has demonstrated the viability of fractionating proteins by HF FIFFF followed by direct analysis of the protein ions in MS [38]. 

Axial Dispersion Characterization. Use of THF in both instruments as a method of examining the fractionation situation led to the investigation of CX as a method of supplying polymer of extremely narrow molecular weight distribution for resolution characterization of the second instrument (7). To do this, a ccmmercially available narrow molecular weight distribution steuidard was injected into the first instrument and sampled at its peak by the second instrument. 

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