Composition distributions

The methods described above allow the determination of the average composition of the SAN copolymer. However, the polymers formed are rather complex mixtures containing both a range of molecular weights and a range of chemical compositions. Since the average composition of the polymer does not uniquely define the expected properties of the mixture, several techniques have been developed to measure the breadths of the molecular weight and composition distributions. 

The composition distribution of SAN copolymers can be determined by liquid chromatography. An effective method is to precipitate the polymer on a column, and then successively elute fractions that differ compositionally by increasing the polarity of the solvent [13-18]. Both normal- and reversed-phase chromatography can be used. 

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Over a period of about 50 years, the science of polymer chemistry has developed a comprehensive means of polymer characterization techniques. In the case of PE, these parameters include the composition, molecular weight, and compositional distribution. The composition of ethylene copolymers is usually measured by C-nmr, H-nmr, or in techniques. 

The compositional distribution of ethylene copolymers represents relative contributions of macromolecules with different comonomer contents to a given resin. Compositional distributions of PE resins, however, are measured either by temperature-rising elution fractionation (tref) or, semiquantitatively, by differential scanning calorimetry (dsc). Table 2 shows some correlations between the commercially used PE characterization parameters and the stmctural properties of ethylene polymers used in polymer chemistry. 

The projected equHibrium melting point of completely linear PE is 146—147°C (5) its highest actual melting point is 133—138°C. In the case of ethylene copolymers with a uniform compositional distribution, the melting point decreases almost linearly with copolymer composition for instance. 

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. 

Table 3. Properties of Commercial LLDPE Film of Resins with Nonuniform Compositional Distribution ...

Most Kaminsky catalysts contain only one type of active center. They produce ethylene—a-olefin copolymers with uniform compositional distributions and quite narrow MWDs which, at their limit, can be characterized by M.Jratios of about 2.0 and MFR of about 15. These features of the catalysts determine their first appHcations in the specialty resin area, to be used in the synthesis of either uniformly branched VLDPE resins or completely amorphous PE plastomers. Kaminsky catalysts have been gradually replacing Ziegler catalysts in the manufacture of certain commodity LLDPE products. They also faciUtate the copolymerization of ethylene with cycHc dienes such as cyclopentene and norhornene (33,34). These copolymers are compositionaHy uniform and can be used as LLDPE resins with special properties. Ethylene—norhornene copolymers are resistant to chemicals and heat, have high glass transitions, and very high transparency which makes them suitable for polymer optical fibers (34). 

During copolymerization, one monomer may add to the copolymer more rapidly than the other. Except for the unusual case of equal reactivity ratios, batch reactions carried to completion yield polymers of broad composition distribution. More often than not, this is an undesirable result. 

The maximum rates of crystallisation of the more common crystalline copolymers occur at 80—120°C. In many cases, these copolymers have broad composition distributions containing both fractions of high VDC content that crystallise rapidly and other fractions that do not crystallise at all. Poly(vinyhdene chloride) probably crystallises at a maximum rate at 140—150°C, but the process is difficult to foUow because of severe polymer degradation. The copolymers may remain amorphous for a considerable period of time if quenched to room temperature. The induction time before the onset of crystallisation depends on both the type and amount of comonomer PVDC crystallises within minutes at 25°C. 

Acetone, methyl ethyl ketone, methyl isobutyl ketone, dimethylformamide, ethyl acetate, and tetrahydrofuran are solvents for vinyhdene chloride polymers used in lacquer coatings methyl ethyl ketone and tetrahydrofuran are most extensively employed. Toluene is used as a diluent for either. Lacquers prepared at 10—20 wt % polymer sohds in a solvent blend of two parts ketone and one part toluene have a viscosity of 20—1000 mPa-s (=cP). Lacquers can be prepared from polymers of very high vinyhdene chloride content in tetrahydrofuran—toluene mixtures and stored at room temperature. Methyl ethyl ketone lacquers must be prepared and maintained at 60—70°C or the lacquer forms a sohd gel. It is critical in the manufacture of polymers for a lacquer apphcation to maintain a fairly narrow compositional distribution in the polymer to achieve good dissolution properties. 

Heterogeneous copolymerization of acrylamide causes redistribution comonomers between phases I and II. This leads to a change of copolymer composition in phases I and II. As a result, the values of ri and change. This accounts for anomalous widening of the experimental composition distribution curves as compared with theoretical curves. 

These complicating factors influence not only the middle composition and composition distribution curves of copolymers, but also the kinetic parameters of copolymerization and the molecular weight of copolymers. An understanding of these complicating factors makes it possible to regulate the prosesses of copolymerization and to obtain copolymers with different characteristics and, therefore, with various properties. 

Polymers and copolymers of acrylamide (obtained by copolymerization or postreaction of polyacrylamide) with different values of the molecular weight, composition, distribution of molecular weight and compositions, linear and cross-linked have different functions and are used in many fields. The main functions and applications of acrylamide polymers are shown in Table 4. 

Narrow composition distribution Absence of high comonomer fraction ease processesing and comonomer effective in reducing m.p. sharp m.p. Heat seal layers in composites films... 

In early work, while compositional heterogeneity was recognized and could be predicted, it was difficult to measure. Now, methods such as GPC combined with NMR and/or MALDI,238 239 GPC coupled with FTIR240 and two dimensional HPLC or GPC241"245 can provide a direct measure of the composition distribution. 

One reactant is charged to the reactor in small increments to control the composition distribution of the product. Vinyl copolymerizations discussed in Chapter 13 are typical examples. Incremental addition may also be used to control the reaction exotherm. 

More recent inventions are the metallocene catalysts based on zirconium. They offer more uniform catalyst activity and can give a relatively narrow molecular weight distribution. More importantly, they offer better control over structure and copolymer composition distributions. 

In a batch reactor, the relative monomer concentrations will change with time because the two monomers react at different rates. For polymerizations with a short chain life, the change in monomer concentration results in a copolymer composition distribution where polymer molecules formed early in the batch will have a different composition from molecules formed late in the batch. For living polymers, the drift in monomer composition causes a corresponding change down the growing chain. This phenomenon can be used advantageously to produce tapered block copolymers. 

Example 13.7 A 50/50 (molar) mixture of st5Tene and acrylonitrile is batch polymerized by free-radical kinetics until 80% molar conversion of the monomers is achieved. Determine the copolymer composition distribution. 

FIGURE 13.6 Copolymer composition distribution resulting from the batch polymerization of styrene and acrylonitrile. 

The properties of a polymer depend not only on its gross chemical composition but also on its molecular weight distribution, copolymer composition distribution, branch length distribution, and so on. The same monomer(s) can be converted to widely differing polymers depending on the polymerization mechanism and reactor type. This is an example of product by process, and no single product is best for all applications. Thus, there are several commercial varieties each of polyethylene, polystyrene, and polyvinyl chloride that are made by distinctly different processes. 

Continuous-flow stirred tank reactors are widely used for free-radical polymerizations. They have two main advantages the solvent or monomer can be boiled to remove the heat of polymerization, and fairly narrow molecular weight and copolymer composition distributions can be achieved. Stirred tanks or... 

Copolymerizations. The uniform chemical environment of a CSTR makes it ideally suited for the production of copolymers. If the assumption of perfect mixing is justified, there will be no macroscopic composition distribution due to monomer drift, but the mixing time must remain short upon scaleup. See Sections 1.5 and 4.4. A real stirred tank or loop reactor will more closely... 

The variance of compositional distribution for Instantaneously formed polymers is negligibly small. 

For long linear chains the second condition is supported by the Stockmayer bivariate distribution (8,9) which shows the bivariate distribution of chain length and composition is the product of both distributions, and the compositional distribution is given by the normal distribution whose variance is inversely proportional to chain length. 

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]. 

Gel Permeation Chromatography (CPC) is often the source of molecular wei t averages used in polymerization kinetic modelling Q.,2). Kinetic models also r uire measurement of molecular weight distribution, conversion to polymer, composition of monomers in a copolymerization rea tion mixture, copolymer composition distribution, and sequence length distribution. The GPC chromatogram often reflects these properties (3,. ... 

In analysis of homopolymers the critical interpretation problems are calibration of retention time for molecular weight and allowance for the imperfect re >lution of the GPC. In copolymer analysis these interpretation problems remain but are ven added dimensions by the simultaneous presence of molecular weight distribution, copolymer composition distribution and monomer sequence length distribution. Since, the GPC usu y separates on the basis of "molecular size" in solution and not on the basB of any one of these particular properties, this means that at any retention time there can be distributions of all three. The usual GPC chromatogram then represents a r onse to the concentration of some avera of e h of these properties at each retention time. 

Figure 17. Theoretical normalized copolymer composition distributions (ti 0.56, Tf = 0.40 letters correspond to sample in Table V)...

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