Determinants of Polymer Crystallinity

Regardless of the precise picture of order and disorder in polymers, the prime consideration that should be emphasized is that polymers have a tendency to crystallize. The extent of this crystallization tendency plays a most significant role in the practical ways in which polymers are used. This is a consequence of the large effect of crystallinity on the thermal, mechanical, 

Polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate) show very poor crystallization tendencies. Loss of structural simplicity (compared to polyethylene) results in a marked decrease in the tendency toward crystallization. Fluorocarbon polymers such as poly(vinyl fluoride), poly(vinylidene fluoride), and polytetrafluoroethylene are exceptions. These polymers show considerable crystallinity since the small size of fluorine does not preclude packing into a crystal lattice. Crystallization is also aided by the high secondary attractive forces. High secondary attractive forces coupled with symmetry account for the presence of significant crystallinity in poly(vinylidene chloride). Symmetry alone without significant polarity, as in polyisobutylene, is insufficient for the development of crystallinity. (The effect of stereoregularity of polymer structure on crystallinity is postponed to Sec. 8-2a.) 

Polymers with rigid, cyclic structures in the polymer chain, as in cellulose and poly(ethy-leneterephthalate), are difficult to crystallize. Moderate crystallization does occur in these cases, as a result of the polar polymer chains. Additional crystallization can be induced by mechanical stretching. Cellulose is interesting in that native cellulose in the form of cotton is much more crystalline than cellulose that is obtained by precipitation of cellulose from 

Chain flexibility also effects the ability of a polymer to crystallize. Excessive flexibility in a polymer chain as in polysiloxanes and natural rubber leads to an inability of the chains to pack. The chain conformations required for packing cannot be maintained because of the high flexibility of the chains. The flexibility in the cases of the polysiloxanes and natural rubber is due to the bulky Si—O and rxv-olelin groups, respectively. Such polymers remain as almost completely amorphous materials, which, however, show the important property of elastic behavior. 

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The development of modern microcomputers and associated instrumentation enables the automatical of a nunber of IGC techniques. Automation is desirable because often 50 to 100 separate injections of very small volumes of probes are required over a period of time as the temperature of the GC is slowly increased, for example in the determination of transition temperatures or crystallinity. This paper will discuss the determinations of polymer crystallinity and the surface area of polymer-coated particles using automated instrumentation. 

A common application of DSC is the determination of the weight fraction of crystalline material in semicrystalline polymers. The method is based on the measurement of the polymer sample s heat of fusion, AHf, and the plausible assumption that this quantity is proportional to the crystalline content. If by some process of extrapolation the heat of fusion, AHf, of a hypothetical 100% crystalline sample is known, then the weight fraction of crystallinity is AHf/AHJ (155). The determination of polymer crystallinity has been reviewed by Gray (156) and Dole (157, 158). 

Another method for the determination of polymer crystallinity was discussed by Duswalt (159). It is based on the ability of the instrument to cool a molten sample rapidly and reproducibJy to a reselected temperature where isothermal crystallization is allowed to occur. A number of crystallization curves for polyethylene obtained isothermalJy at different, preset crystallization temperatures are shown in Figure 7.57. Differences in polymer crystallizability that may be caused by branching, nucleation, or molecular weight effects can be observed. The sensitivity and speed of the method allow pellet-to-pellet variations in a lot of polymer to be examined. 

Special interest has been focused on the determination of polymer crystallinity via gas chromatography because it is one of the few methods which does not require X-ray diffraction for standardization. 

Bergmann K. Determination of polymer crystallinity from proton solid-echo NMR measurements. Polym Bull 1981 5 355. 

In this chapter we study the characteristics that determine the crystallinity of polymers, crystalline morphology, and the factors affecting the crystallization and melting of polymers. We describe the amorphous state, focusing on the glass transition, a fundamental property for defining the mechanical behavior of polymers. The entire description refers exclusively to synthetic polymers. 

The determination of the crystallinity of polymer stationary phases is based on the differential solubility of the K>lute in crystalline and amorjdious domains. In effect the solute senses only the amorphous regions, leading to an increa in retention volume with decreasing crystallinity. 

As materials polymers are almost always used as solids . A structural and dynamic characterization of the polymers in question is necessary in order to understand the relations between properties and structure and, on the basis of these relations, to design new polymer materials. As is well known, the X-ray diffraction method has contributed to the structural determination of polymers with high crystallinity. However, most polymers have low crystallinity and so structural information about the noncrystalline region, which is the major component, cannot be obtained by X-ray studies. Therefore, the X-ray diffraction method has a limitation for the structural analysis of such systems. Further, it can be said that chain segments in the noncrystalline region are sometimes in a mobile state, so that the X-ray difl action method provides no structural or dynamical information. On the other hand, the solid state NMR method provides information about the structure and dynamics of a sample irrespective of whether the region studied is crystalline or noncrystalline. 

The polymers in Table III catalyzed by sodium-mercury show structures identical with sodium polybutadienes. Because mercury, alone, does not catalyze the polymerization, these results should be compared with previous work (2) using sodium hydride which gave similar results. Both of these sets of experiments show merely that the crystalline structure of the sodium metal, or some other constitutive property, is not the deciding factor in the determination of polymer microstructure. 

Quantitative analysis XRD is used for the determination of percent crystallinity in polymers, the composition of mixtures, mixed crystals, soils, and natural products. 

Another use of Raman spectroscopy for quantitative analysis is the determination of percent crystallinity in polymers. Both the frequency and intensity of peaks can shift on going from the amorphous to the semicrystalline state for polymers. The percent crystallinity can be calculated with the help of chemometrics software. 

Determination of the crystalline polymer fraction at different temperatures allows the construction of melting curves. Their form mirrors the morphology of the polymer determined by the thermal treatment it was subjected to. In the case of high density polyethylene the melting curves differ for rapid and slow cooling of the melt (Fig. 5.7, curves 1,2). Curve 3... 

The crystallinity of poly(CHD) is estimated by X-ray diffraction (XRD). Three major peaks (d = 5.28, 4.51, and 3.91 A) are observed in its XRD spectrum in most cases. Although differential scanning calorimetry (DSC) has been adopted to investigate Tjn, the degree of polymer crystallinity cannot be quantitatively determined because the heat of fusion (AH ) value (in J/g) of a poly(CHD) single crystal has not yet been determined. 

Nuclear magnetic resonance (NMR) line width studies of crystalline polymers are based on the work of Wilson and Pake [102], This method was, however, unsuccessful due to the rather arbitrary decomposition procedures used, which yielded a crystalline fraction that was not in agreement with crystallinity results obtained by the X-ray method. To overcome this difficulty Bergmann [103-105] decomposed the spectrum into three components and this resulted in an excellent agreement between NMR and X-ray crystallinities. Unfortunately, with this method it was not possible to prove the existence of the two amorphous components of the polymer examined. Also, the two amorphous mobilities could not be predicted theoretically. Bergmann [106] succeeded eventually, as discussed next, in improving the separation procedure by finding more suitable line widths for the crystalline and amorphous components of the polymer. In this procedure a new method was evolved for the determination of the crystalline component and of the amorphous component based on a distribution of correlation times, instead of the two discrete correlation times as used in earlier work [103-105],... 

In summary, it seems clear that particulate fillers can have significant nucleating effects in semi-crystalline polymers and that this may lead to effects on mechanical properties. Much work remains to be done to clarify this and provide a clear, coherent description of the effects involved, however. This would be greatly helped if simple techniques for determining polymer crystal structures in filled systems were available. It would also be highly desirable to have a better understanding of how the structure of polymer crystallinity affects composite properties. 

One of the most useful, and practical, concepts in the characterization of a semicrystalline polymer Is the degree of crystallinity. Mechanical and physical properties depend on morphology but are closely related to the degree of crystallinity. Many methods, including A"-ray diffractometry, density determination, thermal analysis, and spectroscopic techniques have been used, but the degree of crystallinity cannot be measured absolutely since it is defined by the method of measurement. This has led to the questioning of the meaning of polymer crystallinity. Exact comparability between the values determined by the different methods should not be expected because some assumptions are invariably made in each case. 

In order to remove the residual solvent from PHB films, the samples were kept for 2-3 h in vacuum at 80 °C. The completeness of solvent removal was checked by monitoring a decrease in intensity of the corresponding absorption bands of dioxane (873-876, and 2855 cm ) and chloroform (756, 3012-3040,2976-2992 cm ) in the IR spectra [12]. The degree of polymer crystallinity in the samples of both types was about 70%, as evidenced by the X-ray diffraction data [7, 8]. The weight-average molecular mass determined by viscosimetry was (310 26) 10 ... 

Figure 6.24 The experimental determination of the extent of polymer crystallinity using the density method.

The density of a crystalline phase must be determined indirecdy in polymers. Examination of crystalline polymers by X-ray difiEraction shows periodicity analogous to that found in simple crystals, but also reveals a background of diffuse scattering arising from the presence of disordered material. Electron and light microscopy fre-quendy show amorphous material between the crystalline aggr ates. The preferred method of determination of the crystalline density in the presence of this amorphous phase is by analysis of the X-ray diffraction pattern. 

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