Crystallinity in polymers

1 Copolymerization. Block copolymers can form domain structures. For example in the thermoplastic, elastomeric polyester/poly-THF (spandex), the polyester segment (which is made from terephthalic acid and butane-1, 4-diol) is crystalline. 

Random copolymers do not crystallize—this is a common technique for making elastomers (section 1.13). Thus, in ethylene/propylene rubbers (EP rubbers) and ethylene/vinyl acetate (EVA) the crystallinity of the PE is destroyed. A 50/50 copolymerization of the following polyacrylates yields an amorphous copolymer, 180°C. 

2 Stereoregularity. Unless a chain is predominantly isotactic or (much more rarely) syndiotactic, it usually cannot be fitted well into its neighbours to crystallize. Thus atactic PS, PP and PMMA are amorphous whereas isotactic PS and PP and syndiotactic PMMA are crystalline. Exceptions are —CH2—CH(X)— where X = F and OH. In these examples the F group is small enough to form a PE-type lattice and with X = OH, hydrogen bonding plays a role. 

Many crystalline polymers are translucent or opaque compared with transparent amorphous polymers (e.g. PS, PMMA). This is because the refractive index of the crystalline region is often different from the amorphous region. However, in the case of poly (4-methylpent-l-ene), the crystalline size can be made small by nucleation (by copolymerization with a-olefins) 

As stated above, the factors influencing Tg are similar to the ability of the polymer to crystallize. These factors also affect The value of is a guide to the temperature of fabrication (this is usually performed at temperatures 30-50° above —temperatures higher than this tend to 

Problem 2.14 Derive an equation relating the degree of crystallinity of a semicrystalline polymer to the sample density and densities of the crystalline and amorphous components. 

Let Vc = total volume of the crystalline components Va = total volume of the amorphous components  

V = total volume of the specimen me, nta, and m are the corresponding masses and pc, Pa, and p are the corresponding densities. Then 

In terms of speci c volumes (reciprocal density), Vc and Va, of the crystalline and amorphous components, respectively, Eiq. (P2.14.6) becomes (Young and Lovell, 1990)  

Problem 2.15 Estimate the fraction of crystalline material in a sample of polyethylene of density 0.983 g/cm.  

Volume of unit cell = 7.41x4.94x2.55x10 cm  

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The essential requirement for crystallinity in polymers is some sort of stereoregularity. This is not to say that the entire collection of macromolecules... 

It has proved difficult to decide which of these two theories of polymer crystallisation is correct, since both are consistent with the observed effects of crystallinity in polymers. These effects include increased density, increased stiffness, and higher softening point. However, the balance of opinion among those working with crystalline polymers favours the latter theory, based on lamellae formed by the folding of single molecules. 

A. Ciferri (Ed.), Liquid Crystallinity in Polymers Principles and Fundamental Properties, VCH Publishers, New York, 1991. 

Figure 1.60 Schematic illustration of extended chain crystallinity in polymers (a) polyethylene and (b) polypropylene.

Ciferri. A. Liquid Crystallinity in Polymers Principles and Fundamental Properties. 

Price, F. P. Calculation of degree of crystallinity in polymers from density measurements. J. Phys. Chem. 19, 973 (1951). 

The several definitions of the fraction crystallinity (xc) are presented in Table 19.1. A critical discussion of meaning and measurement of crystallinity in polymers was given by Kavesh and Schultz (1969). 

Ciferri A (ed) (1991) Liquid crystallinity in polymers Principles and fundamental properties. VCH, New York... 

As with small-molecule materials, liquid crystallinity in polymers generally occurs at conditions intermediate between those for which isotropic liquid and crystalline solid states... 

The most direct evidence of the crystallinity in polymers is provided by x-ray diffraction studies. The x-ray patterns of many crystalline polymers show both sharp features associated with regions of three-dimensional order, and more diffuse features characteristic of molecularly disordered substances like liquids. The occurrence of both types of feature is evidence that ordered regions (called crystallites) and disordered regions coexist in most crystalline polymers. X-ray scattering and electron microscopy have shown that the crystallites are made up of lamellae which are built-up of folded polymer chains as explained below. 

Some polymers manifest liquid crystalline ordering, which does not have the full long-range three-dimensional periodicity of crystallinity but is far more ordered than amorphicity. Since many excellent books and articles have been published on such polymers and the author does not have much that is new to add to this background information, very little will be said about polymer liquid crystallinity in this book. Van Krevelen [3] has reviewed liquid crystallinity in polymers in a readable manner and discussed its effects on properties for which quantitative structure-property relationships are available. Adams et al [41] have published a valuable compendium of articles covering the theory, synthesis, physical chemistry, processing and properties of liquid crystalline polymers. Woodward [42] has discussed and illustrated liquid crystallinity in polymers with many beautiful micrographs. 

Peterlin, A. in Flow-Induced Crystallination in Polymer Systems Miller, R. L., Ed. Gordon and Breach Science Publishers New York 1979 pp 1-29. 

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