Degrees of Crystallinity

The degree of crystallinity of a polymer reflects the relative amount of crystalline regions and of amorphous regions. This amount can be expressed on a volume or a mass basis. The degree of crystallinity is most accurately determined by X-ray scattering. In practice, this is a tedious operation and is rarely performed. 

The approximate crystallinity of a polymer sample can be calculated from density measurements. Based on the two-phase model of polymer behavior, the mass or volume fraction of crystallinity can be calculated by measuring the density of a polymer sample, if the densities of the amorphous material and of pure crystals are known, as indicated by the following equations  

The density of a polymer sample can be determined by the density gradient method, as described by ASTM D1505. In this method, solutions A and B are prepared with densities in the range of interest. Solution A, with the lowest density, and solution B, with the highest density, are combined in a glass tube to form a vertical column of liquid in which the density varies linearly from the bottom to the top. The column is calibrated with glass beads of known density. Plastic samples are dropped in the column and will rest at the level corresponding to their density. The density of the plastic is calculated from the position of the sample compared to that of the calibration beads. 

The approximate percent crystallinity of a polymer can also be calculated from measurements of the heat of fusion (see Section 3.11.5) made using differential scanning calorimetry. If the heat of fusion for a pure crystalline sample of the polymer is known, the mass percent crystallinity can be determined by dividing the heat of fusion of the sample by the heat of fusion of 100% crystalline polymer, and multiplying by 100%. 

A number of important polymer properties depend on the morphology of the polymer. As crystallinity increases  

There are essentially two main etching techniques (a) vapour etching for only a few seconds and (b) controlled, isothermal treatment with a liquid solvent for a considerably longer time (several hours). A solvent-etching temperature (Tj) is selected on the basis of the melting point of the segregated species 

Scanning electron microscopy of fracture surfaces may provide useful information about the super-molecular structure, although artificial structures have been reported. These pseudospherulites arise from fractures initiated at spots in front of the main propagating fracture front. These early fractures propagate in a radial manner outwards from the initiation spots and create a spherulite-like topography which can very easily be mistaken for true spherulites. 

Low-magnification transmission electron microscopy of chlorosulphonated polyethylene samples also provides information about the supermolecular structure. 

The crystallinity, mass crystallinity or volume crystallinity is the mass or volume fraction of the sample in the crystalline state. It is assumed that only two components exist in the semicrystalline polymer. This postulate may be expressed in the statement that any intensive property ((/ ) is an additative function with contributions from the two components present  

The most fundamental and direct method of determining the degree of crystallinity is X-ray diffraction. It is based on the principle that the total coherent scattering from N atoms is independent of the state of aggregation. This statement leads to the 

The density method is very convenient, because the only measurement required is that of the density of a polymer sample. It suffers from some uncertainties in the assignments of crystalline and amorphous density values. An average crystallinity is estimated as if the polymer consisted of a mixture of perfectly crystalline and completely amorphous regions. The weight fraction of material in the crystalline state Wc is estimated assuming that the volumes of the crystalline and amorphous phases are additive  

Each of the methods cited yields a measure of average crystallinity, which is really only defined operationally and in which the polymer is assumed artificially to consist of a mixture of perfectly ordered and completely disordered segments. In reality, there will be a continuous spectrum of structures with various degrees of order in the solid material. Average crystallinities determined by the different techniques cannot always be expected to agree very closely, because each method measures a different manifestation of the structural regularities in the solid polymer. 

A polymer with a regular structure can attain a higher degree of crystallinity than one that incorporates branches, configurational variations, or other features that cannot be fitted into crystallites. Thus linear polyethylene can be induced to crystallize to a greater extent than the branched polymer. However, the degree of crystallinity and the mechanical properties of a particular crystallizable sample depend not only on the polymer structure but also on the conditions under which crystallization has occurred. 

Crystallization cannot take place at temperatures below Tg, and is therefore always at a higher temperature than Tg. The presence of a crystalline phase in a polymer extends its range of mechanical usefulness compared to strictly amorphous versions of the same species. In general, an increased degree of crystallinity also reduces the solubility of the material and increases its rigidity. The absolute level of crystallinity that a polymer sample can achieve depends on its structure, but the actual degree of crystallinity, which is almost always less than this maximum value, will also reflect the crystallization conditions. 

Because of the complex manner in which the lamellae grow and interact, the crystalline content of bulk samples can vary significantly. Determination of the degree of crystallinity is a very useful indicator of the morphology of the 

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Because x-rays are particularly penetrating, they are very usefiil in probing solids, but are not as well suited for the analysis of surfaces. X-ray diffraction (XRD) methods are nevertheless used routinely in the characterization of powders and of supported catalysts to extract infomration about the degree of crystallinity and the nature and crystallographic phases of oxides, nitrides and carbides [, ]. Particle size and dispersion data are often acquired with XRD as well. 

Ellipsometry measurements can provide infomiation about the thickness, microroughness and dielectric ftinction of thin films. It can also provide infomiation on the depth profile of multilayer stmctiires non-destmctively, including the thickness, the composition and the degree of crystallinity of each layer [39]. The measurement of the various components of a complex multilayered film is illustrated m figure Bl.26.17 [40]. 

Between T j, and Tg, depending on the regularity of the polymer and on the experimental conditions, this domain may be anything from almost 100% crystalline to 100% amorphous. The amorphous fraction, whatever its abundance, behaves like a supercooled liquid in this region. The presence of a certain degree of crystallinity mimics the effect of crosslinking with respect to the mechanical behavior of a sample. 

As before, this quantity in relation to the degree of crystallinity is given by Eq. (4.22), so equating the latter to Eq. (4.27) gives... 

Density, mechanical, and thermal properties are significantly affected by the degree of crystallinity. These properties can be used to experimentally estimate the percent crystallinity, although no measure is completely adequate (48). The crystalline density of PET can be calculated theoretically from the crystalline stmcture to be 1.455 g/cm. The density of amorphous PET is estimated to be 1.33 g/cm as determined experimentally using rapidly quenched polymer. Assuming the fiber is composed of only perfect crystals or amorphous material, the percent crystallinity can be estimated and correlated to other properties. 

Poly(vinyl fluoride) [24981-14-4] (PVF) is a semicrystaltiae polymer with a planar, zig-zag configuration (50). The degree of crystallinity can vary significantly from 20—60% (51) and is thought to be primarily a function of defect stmctures. Wide-line nmr and x-ray diffraction studies show the unit cell to contain two monomer units and have the dimensions of a = 0.857 nm, b = 0.495 nm, and c = 0.252 nm (52). Similarity to the phase I crystal form of poly (vinytidene fluoride) suggests an orthorhombic crystal (53). 

The ease of sample handling makes Raman spectroscopy increasingly preferred. Like infrared spectroscopy, Raman scattering can be used to identify functional groups commonly found in polymers, including aromaticity, double bonds, and C bond H stretches. More commonly, the Raman spectmm is used to characterize the degree of crystallinity or the orientation of the polymer chains in such stmctures as tubes, fibers (qv), sheets, powders, and films... 

AppHcations of soHd-state nmr include measuring degrees of crystallinity, estimates of domain sizes and compatibiHty in mixed systems from relaxation time studies in the rotating frame, preferred orientation in Hquid crystalline domains, as weU as the opportunity to characterize samples for which suitable solvents are not available. This method is a primary tool in the study of high polymers, zeoHtes (see Molecular sieves), and other insoluble materials. 

Nylon. The high degree of crystallinity in nylon means that plasticization can occur only at very low levels. Plasticizers are used in nylon but are usually sulfonamide based since these are generally more compatible than phthalates. DEHP is 25 phr compatible other phthalates less so. Sulfonamides are compatible up to 50 phr. 

Tensile Properties. Tensile properties of nylon-6 and nylon-6,6 yams shown in Table 1 are a function of polymer molecular weight, fiber spinning speed, quenching rate, and draw ratio. The degree of crystallinity and crystal and amorphous orientation obtained by modifying elements of the melt-spinning process have been related to the tenacity of nylon fiber (23,27). 

It must be kept in mind that mechanical properties are influenced by factors other than the degree of crystallinity (molecular weight, in particular). 

Similarly, the random introduction by copolymerization of stericaHy incompatible repeating unit B into chains of crystalline A reduces the crystalline melting point and degree of crystallinity. If is reduced to T, crystals cannot form. Isotactic polypropylene and linear polyethylene homopolymers are each highly crystalline plastics. However, a random 65% ethylene—35% propylene copolymer of the two, poly(ethylene- (9-prop5lene) is a completely amorphous ethylene—propylene mbber (EPR). On the other hand, block copolymers of the two, poly(ethylene- -prop5iene) of the same overall composition, are highly crystalline. X-ray studies of these materials reveal both the polyethylene lattice and the isotactic polypropylene lattice, as the different blocks crystallize in thek own lattices. 

Stmctural order varies from x-ray indifferent (amorphous) to some degree of crystallinity. The latter product has been named pseudoboehmite or gelatinous boehmite. Its x-ray diffraction pattern shows broad bands that coincide with the strong reflections of the weU-crystallized boehmite. 

An important chemical finishing process for cotton fabrics is that of mercerization, which improves strength, luster, and dye receptivity. Mercerization iavolves brief exposure of the fabric under tension to concentrated (20—25 wt %) NaOH solution (14). In this treatment, the cotton fibers become more circular ia cross-section and smoother ia surface appearance, which iacreases their luster. At the molecular level, mercerization causes a decrease ia the degree of crystallinity and a transformation of the cellulose crystal form. These fine stmctural changes iacrease the moisture and dye absorption properties of the fiber. Biopolishing is a relatively new treatment of cotton fabrics, involving ceUulase enzymes, to produce special surface effects (15). 

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