Co-unit content

Figure 6. Plot of linewidth, against temperature at 67.9 MHz for ethylene-butene-1 random copolymers. Co-unit contents d), 10 ( ), 7 (0), 21 (O), 26.

The homopolymers obtained with pure l- or pure D-feed are referred to as polyfL-lactic acid) PLLA) and poly(D-lactic acid) (PDLA), respectively. However, commercial PLA grades are usually based on an L-rich mixture as the majority of bacteria used in fermentation processes mainly produce L-lactic acid and typically comprise a minimum of 1-2% D units [14]. The presence of both L-lactic and D-lactic units in the polymer chain makes PLA a random copolymer as such, its properties are affected by the co-unit content [16,17]. 

Despite the inherent initial attractiveness of the fringed micelle concept, these type structures cemnot be the general characteristic for gelation accompanying crystallization of either homopolymers or copolymers. These structures certainly are not required for the gelation of copolymers despite the attractiveness of the concepts that led to this postulate.6 ll 12 The essential fact that has been established here is that gelation can be associated with lamellar crystallites, and the associated super molecular structure despite a branching or co-unit content of as much as 2 mole percent. 

The very important question then arises, as to what happens when the co-unit concentration is increased further. It is well known that for crystallization from the pure melt an increase in co-unit content lowers the melting temperature and level of crystallinity of random type copolymers. The level of crystallinity can become very small and eventually vanishes.24.25 Ye are then concerned with the question as to whether gels can form at high co-unit content and if so, are the crystallites still lamellar in character. The companion question is whether the gel mechanism remains that of an overlapping super-molecular structure for higher co-unit copolymers. 

The shift of the melting hne increases linearly with the co-unit content x, in agreement with Raoult s law... 

Figure 23.5 demonstrated for sPP with related copol5Tners the independence of the crystal thickness from the co-unit content. Figure 23.7 showed for PEc014 the effect of two different diluents, namely of n-hexadecane and of methylanthracene [17]. The resnlts demonstrated that the effect of diluents can be different A dissolntion of methylanthracene leaves the crystalUzation hne unchanged, prodneing only a shift in the melting line, but the dissolution of n-hexadecane results in shifts of both the melting- as well as the crystallization line. The thermodynamic scheme provides an understanding, and the two different situations are dealt with in Fig. 23.14. Effects depend on whether or...

A different type of pseudo-phase diagram based on the liquidus, and involving ethylene, is found in ethylene-vinyl alcohol random type copolymers.(96) These copolymers are prepared by the saponification of ethylene-vinyl acetate copolymers. Since the latter are in random sequence distribution the ethylene-vinyl alcohol copolymers have the same distribution. However, the crystallinity levels and melting points between the two are quite different. The level of crystallinity of the ethylene-vinyl acetate copolymer decreases continuously with co-unit content, as was illustrated in Fig. 5.11. The crystalline phase remains pure for this copolymer. The copolymer becomes completely noncrystalline at ambient temperature, when the co-unit content reaches about 20 mol %. The ethylene-vinyl alcohol copolymer, on the other hand, gives quite different results as is shown in Fig. 5.13.(96) This rather unusual diagram for a random type copolymer requires a more detailed... 

Lamellar-type crystallites are widely recognized and universally accepted as the characteristic mode for bulk crystallization of homopolymers. Surprisingly, copolymers up to a relatively high co-unit content also form lamellar crystallites [200, 201]. The visual observation of lamellae, or even the occasional viewing of defined sectors within lamellae, is not a license to describe the interfacial structure,... 

Experiments were carried out on a syndiotactic polypropylene [4] and two samples of syndiotactic poly(propene-co-octene) [5], synthesized by S. Jiingling in the Institute of Macromolecular Chemistry of our university using a metallocene catalyst. The chemical properties of the two samples are given in Table 1 (co-unit contents were determined by NMR spectroscopy). 

EVA copolymers and LLDPE copolymers follow the same relationship of modulus as a function of increasing total branch content and decreasing degree of crystallinity, hardness, and yield stress. The tensile impact and puncture strength increase with co-unit content. The VA content improves the ESCR relative to LDPE. 

A knowledge of effects of the molar mass or, for statistical copolymers, of the co-unit content, is also of importance. Some of these relationships are remarkably simple, as demonstrated by the following examples. 

Analysis of the lattice parameters has been a very popular method of determining the purity of the crystal lattice. Extensive work in this area has been carried out with polyethylene-based copoly-mers. " The usual assumption that is made is that the expansion of the lattice reflects the inclusion of the co-unit. However, Bunn has pointed out that this is not necessarily a unique interpretation. The crystallite thicknesses of these copolymers are relatively small, of the order of 100 A or less depending on co-unit content. With such thin crystallites and a thick interfacial region the strain that develops could easily cause the lattice expansion. Hence, the analysis of the lattice parameters does not necessarily yield definitive information with respect to the problem at hand. 

Droscher et al prepared chemically similar poly(oxymethylene)-based copolymers (with 1,3-dioxalane, 1,3-dioxane, 1,3-dioxepane and 1,3,6-trioxocane as respective comonomers) which were of extended crystal form and presumed to be completely crystalline. For this series of copolymers, the melting temperature-composition relation at low co-unit content depended on the chemical nature of the comonomer used. The characteristic curve observed by Inoue " was not found. It was presumed that an appreciable concentration of co-units enter the lattice. An interesting feature developed at higher co-unit concentration. Depending on the co-unit type, the melting temperatures became invariant with composition, while the normal poly(oxymethylene) unit cell was maintained. 

Hachiboshi et al were able to crystallize random copolymers of ethylene terephthalate/ ethylene isophthalate over the complete composition range. The wide-angle X-ray patterns of these copolymers change systematically with co-unit content and it is concluded that the two units can cocrystallize and can form a new unit cell. A complete phase diagram, involving both the solidus and liquidus composition, was determined and is illustrated in Figure 23. The solidus was determined by... 

Figure 35 Effect of co-unit content on the thickness of crystals of hydrogenated polybutadiene grown from dilute solution in xylene and from the melt. (From Ref. 107.)...

The crystallization rate is also dependent on the co-unit content at a fixed molecular weight. This point is illustrated in Fig. 10.4 for random ethylene copolymers, where the co-unit concentration varies, but the molecular weight is held fixed at about M = 5 X10 . (3) The major features of copolymer crystallization are observed again. However, these features are accentuated with increasing co-unit content. For... 

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