Separation of polymers

For preparative purposes batch fractionation is often employed. Although fractional crystallization may be included in a list of batch fractionation methods, we shall consider only those methods based on the phase separation of polymer solutions fractional precipitation and coacervate extraction. The general principles for these methods were presented in the last section. In this section we shall develop these ideas more fully with the objective of obtaining a more narrow distribution of molecular weights from a polydisperse system. Note that the final product of fractionation still contains a distribution of chain lengths however, the ratio M /M is smaller than for the unfractionated sample. 

Thermodynamics and kinetics of phase separation of polymer mixtures have benefited greatly from theories of spinodal decomposition and of classical nucleation. In fact, the best documented tests of the theory of spinodal decomposition have been performed on polymer mixtures. 

For SEC separations of polymers, column efficiency is better characterized by specific resolution, R,p, and efficiency, T, than by theoretical plate count. Peak resolution, R is calculated according to (7) ... 

Restricted access phases are another approach to exploiting the differences in characteristics of analytes. Large analytes are excluded from an internal surface on which an adsorptive stationary phase is present. A herbicide analysis for Metsulfuron methyl, Bentazone, Bromoxynil, methylchlorophenoxy acid, and Mecoprop in the presence of humic acid was performed on restricted access reversed phase media.52 The cytostatic compound epirubicin and its metabolites were separated from plasma using a Pinkerton GFF II column.53 Gradient separations of polymers on reversed phase and on normal phase represent an alternative to gel permeation chromatography. Polyesters of noncrystalline materials were separated on a variety of such phases.54... 

More recently, the same author [41] has described polymer analysis (polymer microstructure, copolymer composition, molecular weight distribution, functional groups, fractionation) together with polymer/additive analysis (separation of polymer and additives, identification of additives, volatiles and catalyst residues) the monograph provides a single source of information on polymer/additive analysis techniques up to 1980. Crompton described practical analytical methods for the determination of classes of additives (by functionality antioxidants, stabilisers, antiozonants, plasticisers, pigments, flame retardants, accelerators, etc.). Mitchell... 

Polymer/additive analysis then usually proceeds by separation of polymer and additives (cf. Scheme 2.12) using one out of many solvent extraction techniques (cf. Chapter 3). After extraction the residue is pressed into a thin film to verify that all extractables have been removed. UV spectroscopy is used for verification of the presence of components with a chromophoric moiety (phenolic antioxidants and/or UV absorbers) and IR spectroscopy to verify the absence of IR bands extraneous to the polymer. The XRF results before and after extraction are compared, especially when the elemental analysis does not comply with the preliminary indications of the nature of the additive package. This may occur for example in PA6/PA6.6 blends where... 

Several qualifying features for polymer extract analysis are summarised in Table 2.11. Quantitative separation of polymer and (thermolabile and/or volatile) additives without decomposition of the analyte(s) is difficult for thermoplasts, but even more difficult for... 

Unfortunately, extraction procedures are often elaborate and labour intensive since many of the polymer matrices are poorly soluble or insoluble. For this reason, substantial efforts have been directed towards additive analysis without prior separation from the polymer. Chapter 9 deals with direct methods in which such separation of polymer and additive can be omitted. Yet, this direct protocol still requires sample pretreatment (dissolution) of the polymer/additive system as before. 

There is no need for perfect separation in the eluting peak MALDI can improve the resolution of chromatography. Semi on-line SEC-MALDI-ToFMS makes allowance for the separation of polymer, oligomers and additives. Ion suppression has been noticed for... 

Analysis after separation of polymer and additive (in solution). 

Staal et al. [15] have described the dissolution of polycarbonate and polysulfone in THF and precipitation on to a Cig guard column. Separation of polymer and additives was achieved using gradient elution from water-THF (50 50 vol. %) as nonsolvent to 100 % THF. The additives elute first, followed by the oligomers and polymer. 

For diffusion NMR, which is another approach to the separation of polymer and additive signals by application of field gradients, see Section 5.4.1.1. 

Ultrafiltration has been used for the separation of dendritic polymeric supports in multi-step syntheses as well as for the separation of dendritic polymer-sup-ported reagents [4, 21]. However, this technique has most frequently been employed for the separation of polymer-supported catalysts (see Section 7.5) [18]. In the latter case, continuous flow UF-systems, so-called membrane reactors, were used for homogeneous catalysis, with catalysts complexed to dendritic ligands [23-27]. A critical issue for dendritic catalysts is the retention of the catalyst by the membrane (Fig. 7.2b, see also Section 7.5). 

Now, the separation of polymers from the reaction mixtnre containing the vinyl additive indicates that the snbstrate prodnces a radical at the intermediate stage. The radical prodnced adds to a probe molecnle and forms an addnct with the vinyl monomer, that is, initiates the monomer polymerization. Sometimes, however, the polymerization does not start bnt the reaction yields a low-molecnlar-weight individnal snbstance containing fragments of substrate, an added monomer, and reactant. To illnstrate, let ns consider the following reaction of perfluoroalkyl iodide (snbstrate) with nitropropenide salt (reactant) in the presence of a monomer probe (vinyl acetate, methylmethacrylate, and styrene) (Feiring 1983) ... 

In conclusion, the enthalpic partition processes in the columns for polymer HPLC substantially differ from the adsorption processes. Enthalpic partition can be employed for the separation of polymers of the low-to-medium polarity in combination with the alkyl bonded phases on silica gels. The extent of the enthalpic partition and consequently also of the polymer retention is controlled primarily by the thermodynamic quality of eluent toward separated species and by the extent of the bonded phase solvation. 

The first papers describing an LC LC separations of polymer blends due to the barrier effects were published in the mid-1990s [226,227] and the explanation of separation principle was originally presented in 1996 [231]. The review of both LC LC procedures and systems, as well as their first applications can be found in Ref. [28]. 

As will be shown later, the former three mechanisms mentioned above are applicable to TLC separation of polymers. From the standpoint of TLC applied to polymer separation, the partition mechanism may be better expressed by phase-separation or precipitation mechanism, as will be explained in Section II.3. It should be noted that all these mechanisms are generally present during a chromatographic separation. Therefore, one mechanism should be made to be predominant for a given separation aim. This can, in principle, be done by properly selecting the developer and adsorbent. However, such a selection is the major problem in application of TLC, especially, to polymer separation, and the following three sections will be devoted to describing the rules that have been established to solve this problem. 

Thin-layer chromatography (TLC) is a common laboratory technique for separating complex mixtures of solutes, usually by an adsorbtion mechanism. Several laboratories have applied the technique to the separation of polymer fractions and characterization of polymer molecular weight distributions. This work reviews the experimental results and theoretical approaches to the fractionation mechanisms. 

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