Problems With Copolymers

The composition of a copolymer determines its misdbihty characteristics. PMMA has limited miscibility with polystyrene or polyacrylonitrile individually. However, addition of PMMA to a random copolymer of styrene and acrylonitrile (SAN) can produce a miscible blend at certain compositions. The aromatic styrene group of styrene and polar nitrile group of acrylonitrile of the random copolymer are partially miscible with PMMA. This will be discussed further in Chapter 6. 

Copolymers with the same structural units but different compositions may be immiscible with themselves. This has been studied notably with styrene-acrylonitrile copolymers. 

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The problem with copolymers is also caused by the complexity of the dn/dc values. There are three factors to be considered ... 

Some cast (unoriented) polypropylene film is produced. Its clarity and heat sealabiUty make it ideal for textile packaging and overwrap. The use of copolymers with ethylene improves low temperature impact, which is the primary problem with unoriented PP film. Orientation improves the clarity and stiffness of polypropylene film, and dramatically increases low temperature impact strength. BOPP film, however, is not readily heat-sealed and so is coextmded or coated with resins with lower melting points than the polypropylene shrinkage temperature. These layers may also provide improved barrier properties. 

In most of the studies discussed above, except for the meta-linked diamines, when the aromatic content (dianhydride and diamine chain extender), of the copolymers were increased above a certain level, the materials became insoluble and infusible 153, i79, lsi) solution to this problem with minimum sacrifice in the thermal properties of the products has been the synthesis of siloxane-amide-imides183). In this approach pyromellitic acid chloride has been utilized instead of PMDA or BTDA and the copolymers were synthesized in two steps. The first step, which involved the formation of (siloxane-amide-amic acid) intermediate was conducted at low temperatures (0-25 °C) in THF/DMAC solution. After purification of this intermediate thin films were cast on stainless steel or glass plates and imidization was obtained in high temperature ovens between 100 and 300 °C following a similar procedure that was discussed for siloxane-imide copolymers. Copolymers obtained showed good solubility in various polar solvents. DSC studies indicated the formation of two-phase morphologies. Thermogravimetric analysis showed that the thermal stability of these siloxane-amide-imide systems were comparable to those of siloxane-imide copolymers 183>. 

Micelle formation of our block copolymers in fluorinated solvents indicates that these polymers might act as stabilizers or surfactants in a number of stabilization problems with high technological impact, e.g., the surface between standard polymers and media with very low cohesion energy such as short-chain hydrocarbons (isopentane, butane, propane), fluorinated solvents (hexafluoroben-zene, perfluoro(methylcyclohexane), perfluorohexane) and supercritical C02. As... 

With this continued growth in variety of plastics materials, selection and matching of optimum combinations between materials and applications becomes a major problem. With at least 50 types of commercial plastics already available, each in a variety of copolymers, molecular weights, and different manufacturers, there are already too many for rational manual choice of the optimum material for any specific application and the situation is growing worse at an alarming rate. 

There are many problems associated with copolymer analysis which require a whole variety of techniques for solution. Isotopic tracers can play their own small part by establishing the exact composition in terms of relative amounts of the monomeric constituents. Consider the following examples ... 

The coordination polymerisation of carbonyl monomers by their carbonyl group concerns mostly acetaldehyde, trichloroacetaldehyde, propionaldehyde and butyraldehyde. One basic problem with all polyaldehydes, and especially with polyketones, is not the polymerisation itself but the stabilization of resulting polymers (or copolymers) against thermal degradation. 

Alkylene 4,4 -Biphenyldicarboxylate/PTME 4,4 -Biphenyldicarbox-ylate Copolymers. Tetramethylene 4,4 -biphenyldicarboxylate/PTME 4,4 -biphenyldicarboxylate copolymers containing 20 and 30% tetra-methylene 4,4 -biphenyldicarboxylate were prepared without incident (Table VI). Attempts to prepare similar copolymers containing 40 and 50% tetramethylene 4,4 -biphenyldicarboxylate led to problems with phase separation in the melt during the copolymerizations. 

Now we move on to consider the analysis of copolymers. There are usually two things we would like to know. First, the composition of the copolymer and, second, some measure of sequence distributions. Again, in the early years, before the advent of commercial NMR instruments, infrared spectroscopy was the most widely used tool. The problem with the technique is that it requires that the spectrum contain bands that can be unambiguously assigned to specific functional groups, as in the (transmission) spectrum of an acrylonitrile/methyl methacrylate copolymer shown in Figure 7-43 (you can tell this is a really old spectrum, not only because it is plotted in transmission, but also because the frequency scale is in microns). 

Although there are numerous references to the emulsion polymerization of vinyl ferrocene, they all appear to emanate from a single source (j4). These workers polymerized vinyl ferrocene alone, and with styrene, methyl methacrylate, and chloroprene. No characterization was reported other than elemental analysis. The molding temperatures reported (150 - 200 C) correspond to the Tg range indicated by Pittman ( ) for similar copolymers. The initiation system was preferably azobisisobutyronltrile, although potassium persulfate was also used. Organic peroxides were contraindicated, due to oxidation problems with the ferrocene moiety. 

In the grafting onto fibers or Aims, the homopolymer occluded in the grafting product is generally removed by extraction with a selective solvent TWs method is in itself relatively simple and hence the most widely used, though time-consuming. However, it is pointed out that an preciable amount of homopolymer is furthermore extracted if the unreacted substrate polymer has been previoudy removed. According to Staimett and collaborators the conventional extraction method is inadequate to obtain the pure graft copolymer free of homopolymers, unless both homopolymers are subjected to a repeated, alternate extraction. We have also revealed that the alternate extraction is necessary for separation in various cases . However, there are serious problems with respect to the extractability of homopolymers. 

Poly( inyl chloride), poly(acrylonitrile) and the high acrylonitrile copolymers have presented the major problems with respect to reducing the residual monomer content to extremely low levels. These are in the glas state tmder the conditions where monomer removal must be carried out. In principle, the temperature should be raised above the glass temperature to facilitate monomer removal. In practice, however, the systems are usually lattices or slurries of suspension polymer and coagulation could become a problem. In any case, both the rubbery and glassy states must be considered in any discussion of the monomer removal problem. The basic principles of the transport of gases in both situations have been presented briefly and with appropriate literature references in the introductory section of this review. 

With regard to carbon fiber stabilization. Table 1 and Figure 3 clearly show that the vinyl halides depress the melting point considerably less than more voluminous monomers such as methyl acrylate, methyl methacrylate or vinyl acetate, at comparable molar level. It can be concluded that fiber fusion will be less of a problem with the former, as compared with the latter. Figure 4 illustrates this point for copolymers AN/VBr as compared to AN/VA. The melting points are calculated using Eq. (2),... 

One potential problem with conventional free-radical copolymerization is that the reactivity ratios of the two monomers tend to be different from one another [6]. On one hand this leads to non-random sequences of the monomers on a single chain (usually the product of the reactivity ratios is less than one so that there is a tendency to form alternating sequences) and, on the other, to substantial composition drift if the polymerization is carried out in bulk to high conversions. Random copolymers with a range of compositions as a result of composition drift may however be useful in practice, allowing a compositionally graded interface to be formed. 

It can be seen that most containers and almost aU film materials arc potentially includwi in the 6 categories. There are several cautionary points to be observed. Except for poly(ethylene terephthalate), PET, containers arc not the predominant use for any one polymer. For poly (vinyl chloride), PVC, consumer-indentifiable containers are less than 10% of the total. TTie identification on products is not always very prominently displayed. It can take some effort to ascertain that a given grocery bag is either No. 2 (HDPE), high-density polyethylene, or No. 4 (LDPE), low-density polyethylene. And there is a problem with composite products. Even the symbol on the base of a 2-liter soda bottle is somewhat ambiguous. The symbol is No. 1 (PETE). While it is true that the clear bottle is, indeed, PET, the base (placwi over the round bottom of the bottle) is itself No. 2 (HDPE). Also, the bottle usually will contain small amounts of paper (label) and poly(vinyl acetate) (adhesive) in addition to a metal cap with a copolymer liner. 

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