Nonuniformity, copolymers

A similar procedure can be employed with a LALLS detector to measure of the whole sample without using a concentration detector (28,29), but this procedure is not applicable to mixtures or nonuniform copolymers for reasons already mentioned. 

Some of the difficulties mentioned in analyses of nonuniform copolymers can be circumvented by use of the average, which can be estimated from the GPC chromatogram and the intrinsic viscosity of the polymer without calibration for their components. It may be useful when is used to characterize... 

Eig. 1. Melting curves (dsc) of two ethylene—1-hexene copolymers produced in a gas-phase process one with a uniform branching distribution (1-hexene content 2.5 mol %) and another with a nonuniform branching distribution (1-hexene content 2.8 mol %). 

Content of Ot-Olefin. An increase in the a-olefin content of a copolymer results in a decrease of both crystallinity and density, accompanied by a significant reduction of the polymer mechanical modulus (stiffness). Eor example, the modulus values of ethylene—1-butene copolymers with a nonuniform compositional distribution decrease as shown in Table 2 (6). A similar dependence exists for ethylene—1-octene copolymers with uniform branching distribution (7), even though all such materials are, in general, much more elastic (see Table 2). An increase in the a-olefin content in the copolymers also results in a decrease of their tensile strength but a small increase in the elongation at break (8). These two dependencies, however, are not as pronounced as that for the resin modulus. 

Branchings Uniformity. Comparison of uniformly and nonuniformly branched ethylene—1-butene copolymers of the same density (Table 4) shows that uniformly branched resins are much more elastic, their tensile modulus is lower, and their strain recovery is neady complete. 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

Advanced computational models are also developed to understand the formation of polymer microstructure and polymer morphology. Nonuniform compositional distribution in olefin copolymers can affect the chain solubility of highly crystalline polymers. When such compositional nonuniformity is present, hydrodynamic volume distribution measured by size exclusion chromatography does not match the exact copolymer molecular weight distribution. Therefore, it is necessary to calculate the hydrodynamic volume distribution from a copolymer kinetic model and to relate it to the copolymer molecular weight distribution. The finite molecular weight moment techniques that were developed for free radical homo- and co-polymerization processes can be used for such calculations [1,14,15]. 

Two classes of LLDPE resins are on the market. One has a predominantly uniform compositional distribution (uniform branching distribution) that is, all copolymer molecules in these resins have approxnnately the same composition. Most commercially produced LLDPE resins, in contrast, have pronounced nonuniform branching distributions there are significant differences in copolymer compositions among different macromolecules in a given resin. 

So far no direct attempt to test Eq. (17) experimentally has been described in the relevant literature. Clearly, such testing involves a number of methodological difficulties, in particular, the necessity for independent experimental determination of the functions of the compositional and molecular weight nonuniformity of the separated fractions and of the initial copolymer.. In fact, cross-fractionation is perhaps the only possible direct method of experimental determining of the compositional and molecular weight nonuniformity of the copolymers the algorithm for calculation of the nonuniformity by cross-fractionation was suggested in 58). 

This behavior supports the hypothesis formulated by Heidingsfeld (9) that the CPVC obtained by heterogeneous unswollen processes besides having molecules whose chlorine atoms are nonuniformly distributed (block copolymers), might also contain, in a different quantity, unaltered PVC. This product behaves similarly to a mixture of the two components (highly chlorinated CPVC and PVC), contrary to the CPVC specimens obtained with the swollen processes which should have a relatively more uniform distribution of the chlorine content on the macromolecules. 

The toughness parameter q is defined by Equation 13. The effect of nonuniform polymerization is shown for a 50/50 copolymer in which the power feed profile involved a decreasing ethyl acrylate concentration (0 1), with x = 1 (linear). (( ) power feed, q = 0.19)... 

The most important structural features of amorphous SAN copolymers are the weight fraction (h an) of acrylonitrile and the molecular weight distribution (MWD). These features control the solid-state properties and fabrication performance. Also important are the type and level of conjugated chromopores and the monomer sequence distribution. These features control the visual appearance of the SAN copolymer. The chromophores may introduce unwanted yellowness. A nonuniform sequence distribution may cause unwanted haze from phase separation. 

Copolymer composition is a third area of contrast. Copolymer composition in a batch reactor tends to change with time. The first polymer formed is rich in the more reactive monomer and the final polymer contains more of the least reactive monomer. This drift in composition can lead to polymer particles with nonuniform composition in the radial dimension. The copolymer product formed in a single CSTR, however, should be relatively uniform in composition if the reactor is operated at steady state. If several CSTRs are connected in series, polymers of several different... 

Other examples from the research group of Koenig (2000-2002) include details of the dissolution behavior of polymers in different solvents (partly with the addition of nonsolvents) or a mixture of solvents. From these investigations, a nonuniform dissolution of the polymer at the interface could be observed for several solvent systems [7-11]. Further results of diffusion measurements were presented in 2002 by Rafferty and Koenig, and later in 2005 by Bobiak and Koenig [12, 13], in which the diffusion of nicotine into an ethylene-vinyl acetate copolymer was analyzed. 

The synthesis of block copolymers PO-EO with a terminal poly[EO] block is practically impossible, cloudy polyols always being formed, with a very low ethoxylation rate. The formation of cloudy polyols is because of an unfavourable (nonuniform) distribution of... 

One significant result of the nonuniform distribution of PET and HBA segments in the copolyester chains is the low HDT as shown in Table 5.10. The HDT for the copolymer containing 60 mol-% HBA (PET/60-HBA) was only 64 °C which was about the same as that of PET. Obviously this low value was attributed to the presence of PET-rich phase. Another result is the presence of highly melting particles which may come out unmelted during process leading to structural defects and lower properties. 

The high tendency to homopolymerization of the 4-acetoxybenzoic acid relative to transesterification is responsible for the nonuniform distribution. In order to obtain copolymers with HBA units randomly distributed along the chains, the monomer 4-acetoxybenzoic acid may be added in portions rather than in a singla batch. Unitika (Suenaga and Okada, 1989) advanced the production process so that the HBA and PET moieties are uniformly distributed in the copolymers. With the advanced process the resins ( Rodrun LC-5000 ) showed almost 100% solubility in a hot (150 °C-160 °C) 50/50 mixed solvent of tetrachloroethane and phenol, while that produced by the old (Jackson and Kuhfuss, 1973) method has an insoluble residue of about 30 wt-%. The 13C-NMR spectra also showed that... 

A second motivation for using a monomer-feed semibatch procedure is to control copolymer composition and/or particle morphology. Delayed feed of part of the more reactive monomer can be used to eliminate or reduce the composition drift of the copolymer. The delayed feed of a comonomer mixture when the reactor is operated in the monomer-starved regime can also be used to prevent copolymer composition drift. Such operations will produce polymer particles with more uniform morphology. Different monomer addition schemes can be employed to control nonuniform particle morphology (see papers by Bassett et al. in General References 7 and 9). 

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