Polymer crystallites

Polymer crystals consist of well defined arrays of aligned chain segments from adjacent molecules. Polymer crystals (with only a few exceptions) are too small for us to see wth the naked eye, or even wth an optical microscope. Given their small size, we normally refer to polymer crystals as crystallites . 

The dimensions of crystallites vary tvidely some measure only a few nanometers in any direction, while others, known as lameUae are platelets with lateral dimensions of several tens of nanometers and thicknesses of a few nanometers. The chain axes in lamellae typically span the thickness of the crystallite. Hth reference to the unit cell illustrated in Fig. 7.2 a), the c direction corresponds to the thickness of the crystallite. 

At the surface of crystallites, polymer chains can adopt one of four configurations, as shown in Fig. 7.4. They can  

The lamellae within a semicrystalline polymer are often aligned vith one another. On a local scale, neighboring lamellae tend to be stacked, so that their lateral planes are parallel, as shown in Fig. 7.6. On a longer scale, lamellae can arrange themselves into extended stacks, known as cylindrites , or radially in three dimensions, to form spherulites , as illustrated schematically in Fig. 7.7 a) and b) respectively. 

The properties of a semicrystalline polymer are controUed by its degree of crystallinity, the alignment of crystallites relative to one another, the number and type of links between the crystallites and amorphous regions, and the overall orientation of molecules within the material. 

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Polymer crystallization is usually initiated by nucleation. The rate of primary nucleation depends exponentially on the free-energy barrier for the formation of a critical crystal nucleus [ 110]. If we assume that a polymer crystallite is a cylinder with a thickness l and a radius R, then the free-energy cost associated with the formation of such a crystallite in the liquid phase can be expressed as... 

Figure 7 Chain folding in a polymer crystallite. The number of re-enttrant folds. per unit surface area would be much higher than sketched here,.

The volume inside the semicrystalline polymers can be divided between the crystallized and amorphous parts of the polymer. The crystalline part usually forms a complicated network in the matrix of the amorphous polymer. A visualization of a single-polymer crystallite done [111] by the Atomic Force Microscopy (AFM) is shown in Fig. 9. The most common morphology observable in the semicrystalline polymer is that of a spherulitic microstructure [112], where the crystalline lamellae grows more or less radially from the central nucleus in all directions. The different crystal lamellae can nucleate separately... 

Figure 9. Visualization of a single-polymer crystallite done by atomic force microscopy [111].

The unconventional applications of SEC usually produce estimated values of various characteristics, which are valuable for further analyses. These embrace assessment of theta conditions for given polymer (mixed solvent-eluent composition and temperature Section 16.2.2), second virial coefficients A2 [109], coefficients of preferential solvation of macromolecules in mixed solvents (eluents) [40], as well as estimation of pore size distribution within porous bodies (inverse SEC) [136-140] and rates of diffusion of macromolecules within porous bodies. Some semiquantitative information on polymer samples can be obtained from the SEC results indirectly, for example, the assessment of the polymer stereoregularity from the stability of macromolecular aggregates (PVC [140]), of the segment lengths in polymer crystallites after their controlled partial degradation [141], and of the enthalpic interactions between unlike polymers in solution (in eluent) [142], as well as between polymer and column packing [123,143]. 

The determination of crystal structure in synthetic polymers is often made difficult by the lack of resolution in the diffraction data. The diffuseness of the reflections observed in most x-ray fiber patterns results from the small size and imperfect lattice nature of the polymer crystallites. Resolution of individual reflections is also made difficult from misorientation of the crystallites about the fiber axis. This lack of resolution leads to poor accuracy in measurement of peak positions. In particular, this lack of accuracy makes determination of layer line heights difficult with a corresponding loss of significant figures in evaluation of the repeat distance for the molecular conformation. In the case of helical conformations, the repeat distance may be of considerable length or, as we shall show, indeterminate and, in effect, nonperiodic. This evaluation requires high accuracy in measurements of layer line heights. 

In semi-crystalline polymers, crystallites may act as effective crosslinks, which gives rise to rubbery properties above T. In systems with long chains, the presence of crystallites... 

The crystalline melting point, Tm is (theoretically) the highest temperature at which polymer crystallites can exist. Normally, crystallites in a polymer melt in a certain temperature range. 

A strain energy in the polymer crystallites caused by molecular displacement during polymerization is observed. In some cases, this is due to the fact that the polymer crystallites show interference color patterns under a polarizing microscope20. Such an accumulated strain energy is characterized as a distinctive feature of polymer property. This type of strain energy may play an important role in the crystal growth of natural polymers. 

During the photopolymerization the produced polymer crystallites exhibit a high degree of orientation with respect to the monomer single crystal (199,205,208). 

In semicrystalline polymers, crystallites and the macrostructures that they contribute to form, provide an added dimension to the consequences of imposed deformation. 

Fig. 8 Theoretical calculation of the effect of lamellar thickness on the melting point of a polymer crystallite using the Thompson-Gibbs equation. (From Ref 1) (View this art in color at www.dekker.com.)...

Identity of the angles of diffraction maximums and equality of their hemispheres show that introduction of XLY does not change interplane distances of polymer crystallites and their sizes. 

Sorption in Semicrystalline Polymers. The work of several investigators has concluded that the chain packing in polymer crystallites is such that it precludes the dissolution of even small gas molecules (16-23). 

The diffraction peaks obtained with a perfect crystal are in theory expected to be infinitely sharp. The finite widths of the observed diffraction peaks as seen in Figure 3.2 reflect the fact that crystallites in semicrystalline polymers are not perfect, and the analysis of the line widths can tell us about the nature and degree of imperfection in the polymer crystal lattices and the size of the polymer crystallites if they are small. 

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