Comonomer Content

Stereocomplex Crystals of Poly(L-lactide)/Poly(o-lactide) 

The PLLA/PDLA stereocomplex, which is another crystal modification of PLA, was first discovered by Ikada and coworkers [80]. Its structure and physical properties have been studied using a number of different techniques, including infrared spectroscopy [82], optical microscopy [83], calorimetry [84], and X-ray diffraction [85]. Recent reviews by Tsuji and Fukushima et al. summarize the main properties of the stereo complex [86, 87]. 

Besides blending equimolar amounts of PLLA and PDLA, a number of different PLA architectures, able to form a stereocomplex, have been synthesized, including 

One of the main drawbacks of PLA is its slow crystallization rate, which largely limits the actual range of possible replacement of nonbiodegradable and noncompostable polymers. As L-lactic acid is usually the main component in the commercial PLA grades, the minor D-lactic acid units act as a noncrystallizable comonomers that reduce the crystallization rate. Crystal polymorphism of PLA is also affected by comonomer concentration, which in turn affects material properties. 

Sawyer, D.J. (2003) Bioprocessing—no longer a field of dreams. Macromol. Symp., 201, 271-81. 

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The comonomer content of copolymers may be estimated by nmr or by controlled solvolysis of the copolymer followed by quantitative chromatographic analysis of the residues. 

Acryhcs and modacryhcs are also useflil industrial fibers. Fibers low in comonomer content, such as Dolan 10 and Du Font s PAN Type A, have exceptional resistance to chemicals and very good dimensional stabihty under hot—wet conditions. These fibers are useflil in industrial filters, battery separators, asbestos fiber replacement, hospital cubical curtains, office room dividers, uniform fabrics, and carbon fiber precursors. The exceUent resistance of acryhc fibers to sunlight also makes them highly suitable for outdoor use. Typical apphcations include modacryhcs, awnings, sandbags, tents, tarpauhns, covers for boats and swimming pools, cabanas, and duck for outdoor furniture (59). 

The relaxatioa temperature appears to iacrease with increa sing HFP coateat. Relaxatioa iavolves 5—13 of the chaia carboa atoms. Besides a and y relaxations, one other dielectric relaxation was observed below —150° C, which did not vary ia temperature or ia magnitude with comonomer content or copolymer density (55). The a relaxation (also called Glass 1) is a high temperature transition (157°C) andy relaxation (Glass 11) (internal friction maxima) occurs between —5 and 29°C. 

Most commercial processes involve copolymerization of ethylene with the acid comonomer followed by partial neutralization, using appropriate metal compounds. The copolymerization step is best carried out in a weU-stirred autoclave with continuous feeds of all ingredients and the free-radical initiator, under substantially constant environment conditions (22—24). Owing to the relatively high reactivity of the acid comonomer, it is desirable to provide rapid end-over-end mixing, and the comonomer content of the feed is much lower than that of the copolymer product. Temperatures of 150—280°C and pressures well in excess of 100 MPa (1000 atm) are maintained. Modifications on the basic process described above have been described (25,26). When specific properties such as increased stiffness are required, nonrandom copolymers may be preferred. An additional comonomer, however, may be introduced to decrease crystallinity (10,27). 

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. 

Although the reaction rate of ethylene and various copolymers differs substantially, the reaction constants can be estabUshed by using an arbitrary value of 1 for ethylene (5). Thus, a value of 0.1 would indicate that the comonomer reacts at 10 times the rate of ethylene. However, the wide range of reaction rates can present problems not only in determining the comonomer content of the final product but also in producing a homogeneous product (4,6). 

In cases where the copolymers have substantially lower glass-transition temperatures, the modulus decreases with increasing comonomer content. This results from a drop in crystallinity and in glass-transition temperature. The loss in modulus in these systems is therefore accompanied by an improvement in low temperature performance. However, at low acrylate levels (< 10 wt %), T increases with comonomer content. The brittle points in this range may therefore be higher than that of PVDC. 

Copolymers of acrylonitrile [107-13-1] are used in extmsion and molding appHcations. Commercially important comonomers for barrier appHcations include styrene and methyl acrylate. As the comonomer content is increased, the permeabiUties increase as shown in Figure 3. These copolymers are not moisture-sensitive. Table 7 contains descriptions of three high nitrile barrier polymers. Barex and Cycopac resins are mbber-modified to improve the mechanical properties. 

Metallocene catalysts produce high-comonomer content products, such as polypropylene block... 

Chen H., Guest M.I., Chum S., Hiltner A., and BaerE. Classification of ethylene-styrene interpolymers based on comonomer content, J. Appl. Polym. Sci., 70, 109, 1998. 

Crystallization of the same sequence length—such that, independent of comonomer content, a minimal sequence length always exists that is capable of crystallization at room temperature but this fraction decreases as comonomer content increases. 

Change in crystal structure above a certain comonomer content—such that the new crystal stmcture could accommodate comonomer units in the crystal. It is possible that on adding increased amounts of comonmer, the crystal stmcture changes in the commonly observed monoclinic (PP) to a hexagonal stmcture. 

Theoretical calculations were also conducted on the influence of/-functional initiators on DB in SCVCP [72]. In the semi-batch system, DB is only sHghtly affected by the presence of polyinitiator and is mostly governed by the comonomer content. The calculations are also applied to polymerizations from surface-bound initiators (see later). 

Relatively small changes in comonomer content can result in significant changes in physical or chemical properties. Polymer resin manufacturers exploit such relationships to control the properties of their products. The composition of a copolymer controls properties such as stiffness, heat distortion temperature, printability, and solvent resistance. For example, polypropylene homopolymer is brittle at temperatures below approximately 0 °C however, when a few percent ethylene is incorporated into the polymer backbone, the embrittlement temperature of the resulting copolymer is reduced by 20 °C or more. 

Short chain branches are frequently introduced into polymers by copolymerization. The chemical structure of the comonomer controls the type and length of the short chain branch. The polymerization catalyst, reaction conditions, and comonomer content in the reaction medium determine the probability of finding a branch at any particular location along a chain. Comonomers, and hence the short chain branches derived from them, can be introduced at random or as blocks. 

The properties of ethylene-vinyl acetate copolymers vary widely with their ester content. At the lowest levels of vinyl acetate, they have physical properties that are similar to those of low density polyethylene. As the comonomer content increases, the material becomes less crystalline and more elastic. Copolymers made with the highest comonomer levels contain no measurable crystallinity. The resulting products are tough, flexible, and clear. The ester... 

Basic definitions Let X be some property of a polymer chain such as the degree of polymerization, molar mass, radius of gyration, or comonomer content of a copolymer, etc. In general, the polymer is heterogeneous with respect to X, which can assume discrete values X,. We now define for molecules with X = X,-. 

Fig. 14 Snapshots of random copolymers with variable comonomer mole fractions at the reduced temperature of 1 in the cooling process of Fig. 12. a-f Comonomer contents of 0, 0.06, 0.12, 0.24, 0.36, and 0.44, respectively. Polymer bonds are drawn in cylinders and the bonds containing comonomers are shown in double thickness [52]...

The copolymer composition produced by these two catalysts can be estimated using the Mayo-Lewis equation [38] and these values of i and r2. Figure 10 depicts the hypothetical comonomer content in the polymer (F2) as a function of the mole fraction of comonomer in the reactor (f2). The good incorporator produces a material with higher F2 as f2 increases. In contrast, the composition from the poor incorporator is relatively flat across a broad range and increases only at very high values of/2. The F2 required to render the copolymer amorphous is comonomer-dependent for 1-octene, this value is near 0.19. In this hypothetical system, the good incorporator produces that composition at f2 = 0.57, at which the poor incorporator incorporates very little comonomer (F2 = 0.01). 

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