Crystallinity, polymer crystallites

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... 

Figure 3.6). This theory known as the fringed mieelle theory or fringed crystallite theory helped to explain many properties of crystalline polymers but it was difficult to explain the formation of certain larger structures such as spherulites which could possess a diameter as large as 0.1 mm. 

Figure 3.6. Two-dimensional representation of molecules in a crystalline polymer according to the fringed micelle theory showing ordered regions (crystallites) embedded in an amorphous matrix.

Because of its regularity it would be expected that the polymer would be capable of crystallisation. In practice, however, the X-ray pattern characteristics of crystalline polymer is absent in conventionally fabricated samples. On the other hand films which have been prepared by slow evaporation from solvent or by heating for several days at 180°C do exhibit both haziness and the characteristic X-ray diagram. The amount of crystallisation and the size of the crystallite structures decrease with an increase in the molecular weight of... 

Possible morphologies of partially crystalline polymers are shown in Fig. 18. Figure 18a depicts the case of small crystallites that act as physical crosslinks between polymeric chains, thus connecting those chains into a 3-dimensional network. In the case depicted in Fig. 18b, the material forms ribbon-shaped or needle-shaped crystalline regions in which different segments of a large number of chains are incorporated. This could explain the low degree of crystallinity at the LST as detected for the iPP system [80]. 

Fig. 18a, b. Possible morphologies of partially crystalline polymers. Small crystallites act as crosslinks (a) large ribbon-shaped or needle-shaped crystalline regions connect a large number of polymeric chains (b)... 

Asymmetrical Peaks are rarely found in WAXS from polymers, but they are ubiquitous in the MAXS of liquid crystalline polymers. For asymmetrical peaks in isotropic patterns it is best to determine the peak position from the maximum of the peak, if peak asymmetry is a result of linear or planar disorder. Linear disorder means that the crystals are more or less one-dimensional (a tower of unit cells). Planar disorder means that the crystallites are made from only very few layers of unit cells (cf. Guinier [6] Chap. 7). 

The temperature dependence of the compliance and the stress relaxation modulus of crystalline polymers well above Tf is greater than that of cross-linked polymers, but in the glass-to-rubber transition region the temperature dependence is less than for an amorphous polymer. A factor in this large temperature dependence at T >> TK is the decrease in the degree of Crystallinity with temperature. Other factors arc the reciystallization of strained crystallites ipto unstrained ones and the rotation of crystallites to relieve the applied stress (38). All of these effects occur more rapidly as the temperature is raised. 

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... 

These concepts have to be correlated with the inherent helical form of the starch molecule, and attempts which have been made to determine the orientation of the portions of the molecules in the crystallites will be dealt with later (in the Section on x-ray diffraction studies—see p. 376). In this connection, the work on the orientation of synthetic crystalline polymers into spherulites containing helically arranged molecules23 may be important. 

X-ray powder diffractometry is widely used to determine the degree of crystallinity of pharmaceuticals. X-ray diffractometric methods were originally developed for determining the degree of crystallinity of polymers. Many polymers exhibit properties associated with both crystalline (e.g., evolution of latent heat on cooling from the melt) and noncrystalline (e.g., diffuse x-ray pattern) materials. This behavior can be explained by the two-state model, according to which polymeric materials consist of small but perfect crystalline regions (crystallites) that are embedded within a continuous matrix [25]. The x-ray methods implicitly assume the two-state model of crystallinity. 

Crystalline polymers exhibit the following basic properties They are opaque as long as the size of the crystallites or spherulites, respectively, lies above the wavelength of light. Their solubility is restricted to few organic solvents at elevated temperature. The following crystalline polymers have attained technical importance as thermoplastic materials polyethylene, polypropylene, aliphatic polyamides, aliphatic/aromatic polyamides, aliphatic/aromatic polyesters, poly-oxymethylene, polytetrafluoroethylene, poly(phenylene sulfide), poly(arylene ether ketone)s. 

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