Heterogeneity in copolymers

Copolymers present a double problem in GPC it is difficult to define an appropriate calibration method and compositional heterogeneity can alter detector responses. For these reasons quantitative studies of copolymers are rare although Janca et al. have published an extensive study of the behaviour of vinyl acetate-vinyl chloride copolymers. Elgert and Wohlschiess have used a combination of refractive index and ultraviolet detection to study compositional heterogeneity in copolymers of a-methyl styrene with butadiene. Studies of ethylene-propylene copolymers have been reported by Osawa and Inaba. ... 

Provder T, Whited M, Huddleston D, Kuo CY (1997) Characterization of compositional heterogeneity in copolymers and coatings systems by GPC/FTIR. Prog Org Coating... 

There is increasing interest in copolymer systems, which, due to their chemical heterogeneity, may require very complex eluent systems in order to dissolve the sample and ensure that the separation ensues hy a pure size exclusion mechanism. In these examples, the PLgel is also compatible with eluent systems containing mixed solvents of different polarity (including water as a cosolvent up to 10% hy volume) and in organic solvents modified with acids or bases (e.g., acetic or formic acid, triethanolamine) as it is stable in the pH range of 1-14. 

The existence of an azeotropic composition has some practical significance. By conducting a polymerization with the monomer feed ratio equal to the azeotropic composition, a high conversion batch copolymer can be prepared that has no compositional heterogeneity caused by drift in copolymer composition with conversion. Thus, the complex incremental addition protocols that arc otherwise required to achieve this end, are unnecessary. Composition equations and conditions for azeotropic compositions in ternary and quaternary eopolymerizations have also been defined.211,21... 

As introduced previously, type 2 ABC triblock copolymer micelles are formed by triblock copolymers containing an insoluble A block while the B and C blocks are soluble in the considered solvent. The insoluble blocks can be located either between the two soluble blocks (BAC structure) or at one end of the triblock (ABC or ACB structures). Micelles of the latter type were discussed above for, e.g., PS-P2VP-PEO pH-responsive micelles and are indeed considered as core-shell-corona, onion, or three-layer structures since the heterogeneity in the micellar corona is observed in the radial direction (Fig. 18). Micelles formed by BAC triblock copolymers are different from the previous case because they can give rise in principle to a heterogenous corona in the lateral dimension (Fig. 18). This could induce the formation of noncentrosymmetric micelles as discussed in Sect. 7.3. 

The alternating tendency of the copolymers is advantageous in that the polymerizations can be carried out to high conversions with little or no compositional drift. For random copolymerizations in which there is preferential incorporation of one monomer due to a mismatch in reactivity ratios, the compositional variations with conversion can be substantial. Such compositional heterogeneities in resist materials can lead to severe problems during image development. 

Most copolymers are heterogeneous in both molecular weight and composition. The latter of these arises from the mechanism of the copolymerisation (particularly at high conversion) and individual copolymer molecules differ slightly in their value of WA. Solutions of heterogeneous copolymers constitute multicomponent systems... 

Examples have been reported for several systems161,166 some results are listed in Table 16. In summary, polymer mixtures can be treated as copolymers of extreme heterogeneity in chemical composition. Under suitable conditions the techniques used in analysing data of LS from copolymer solutions can be applied to yield with good accuracy the molecular weights of components of the binary polymer mixtures. 

Quantitative NMR analysis may also be applied to water-soluble polymers or copolymers in some cases. Polymers usually have large molecular weights and are heterogeneous in size. However, the proton NMR signals of some polymers... 

In the commercial use of copolymerization it is usually desirable to obtain a copolymer with as narrow a distribution of compositions as possible, since polymer properties (and therefore utilization) are often highly dependent on copolymer composition [Athey, 1978]. Two approaches are simultaneously used to minimize heterogeneity in the copolymer composition. One is the choice of comonomers. Choosing a pair of monomers whose copolymerization behavior is such that F is not too different from f is highly desirable as long as that copolymer has the desired properties. The other approach is to maintain the feed composition approximately constant by the batchwise or continuous addition of the more reactive monomer. The control necessary in maintaining f constant depends on the extent to which the copolymer composition differs from the feed. 

Thus, the main problem is how the triple sum in Eq. (C.3) can be evaluated. At first sight, this problem looks formidable. In the following, the techniques of evaluation are described in some detail, starting with the simplest cases of monodisperse homopolymers and proceeding step by step to the more complex molecules of branched copolymers, which are highly polydisperse in molecular weight and heterogeneous in composition. 

Heterogeneity, as in polyblends, has also been observed in random copolymers. F. Kollinsky and G. Markert found phase separation in binary mixtures of copolymers of methyl methacrylate and butyl acrylate. C. Kraus and K. W. Rollmann discovered heterogeneity in blends of random copolymers of butadiene and styrene if they differ by more than 20% in composition. 

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