Crystalline and Amorphous Polymers

FIGURE 4.1 Depiction of (a) Three polymer chains randomly oriented and (b) a depiction of the end-to-end distance, r , for a single polymer. Also shown in figure (b) is the radius of gyration, Tg , which represents the average distance from the center of gravity to each monomer unit. 

Strong dipole interactions promote crystallinity and, other things being equal, raise the crystalline melting temperature. 

References 1-5 contain extensive discussions of the techniques used to study crystallinity in polymers as well as reviews of research results. 

The polymers used in injection moulding can be divided into three main classes, depending on their structure and properties. These classes are amorphous thermoplastics semi-crystalline thermoplastics rubbers. 

The amorphous and semi-crystalline categories are considered first. 

Let us start with a list of solid polymer commonly processed by injection moulding. 

Acrylic (perspex) (PMMA) - transparent, fairly hard, brittle 

Polyethylene (PE) - translucent, tough, leathery, quite soft 

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Some interesting differences are noted between amorphous and crystalline polymers when glass fibre reinforcement is incorporated into the polymer. In Figure 9.2 (ref. 10) it will be seen that incorporation of glass fibre has a minimal effect on the heat deflection temperature of amorphous polymers (polystyrene,... 

The chains that make up a polymer can adopt several distinct physical phases the principal ones are rubbery amorphous, glassy amorphous, and crystalline. Polymers do not crystallize in the classic sense portions of adjacent chains organize to form small crystalline phases surrounded by an amorphous matrix. Thus, in many polymers the crystalline and amorphous phases co-exist in a semicrystalline state. 

Some of the properties and differences that amorphous and crystalline polymers exhibit are ... 

Many polymer-salt complexes based on PEO can be obtained as crystalline or amorphous phases depending on the composition, temperature and method of preparation. The crystalline polymer-salt complexes invariably exhibit inferior conductivity to the amorphous complexes above their glass transition temperatures, where segments of the polymer are in rapid motion. This indicates the importance of polymer segmental motion in ion transport. The high conductivity of the amorphous phase is vividly seen in the temperature-dependent conductivity of poly(ethylene oxide) complexes of metal salts. Fig. 5.3, for which a metastable amorphous phase can be prepared and compared with the corresponding crystalline material (Stainer, Hardy, Whitmore and Shriver, 1984). For systems where the amorphous and crystalline polymer-salt coexist, NMR also indicates that ion transport occurs predominantly in the amorphous phase. An early observation by Armand and later confirmed by others was that the... 

Softening as a result of micro-Brownian motion occurs in amorphous and crystalline polymers, even if they are crosslinked. However, there are characteristic differences in the temperature-dependence of mechanical properties like hardness, elastic modulus, or mechanic strength when different classes of polymers change into the molten state. In amorphous, non-crosslinked polymers, raise of temperature to values above results in a decrease of viscosity until the material starts to flow. Parallel to this softening the elastic modulus and the strength decrease (see Fig. 1.9). 

The measurements of Young s modulus in dependence of the temperature (dynamic-mechanical measurements, see Sect. 2.3.5.2) and the differential thermal analysis (DTA or DSC) are the most frequently used methods for determination of the glass transition temperature. In Table 2.10 are listed and values for several amorphous and crystalline polymers. 

Polymerization of propylene oxide-a-d was carried out by the EtZnNBu ZnEt catalyst in benzene solution in the presence of varying amounts of added water at 70° C, and was terminated after 7 days. The microstructure of the crude polymer was determined by the 1H-NMR method and the yields of amorphous and crystalline polymers were determined by a fractionation method (Fig. 16). When the amount of added water was increased up to 0.3 mole per mole of catalyst, the yield of crystalline polymer increased remarkably, whereas that of amorphous one remained nearly constant, and the isotactic dyad content (I) increased remarkably while syndiotactic one (S) remained almost constant. Thus, the striking parallel was observed between the yield of crystalline polymer and the isotactic dyad content, and between the yield of amorphous polymer and the syndiotactic dyad content. It is therefore concluded that water contributes more remarkably to the formation... 

Whatever the method of control may be it seems evident that it is by no means completely efficient for the solid polymer is always a rather small fraction of the total. Price believes, presumably because any optical activity tends to be concentrated in the crystalline phase, that the amorphous and crystalline polymers are products of two different reactions, one in solution and the other heterogeneous (27). While this view is not impossible, it could be argued that solution reaction might be expected to lead to either inversion or retention of configuration and, hence, optically active polymer. Furthermore, some reports suggest rather strongly that the distinction between the two types of polymer is a rather arbitrary one based in part on polymer symmetry and in part on molecular weight. 

Figure 7-2. Physical properties of amorphous and crystalline polymers as a function of temperature. (Allcock, Harry, and Fred Lampe. 1990. Contemporary Polymer Chemistry. 2nd ed. By permission of Pearson Education, Inc., Upper Saddle River, NJ.)...

An increase in molecular weight leads to significant improvements in the fatigue life of both amorphous and crystalline polymers. This improvement is attributed primarily to an increase in craze stability and to an improvement in the fibril density, contributing to greater resistance to liquid transport. As the fraction of low-molecular-weight molecules cannot contribute to ESCR, it is found that for similar molecular weights narrow MWDs are clearly superior. 

There can be no doubt as to the importance of plane strain conditions for the fracture of plastics especially where sharp notches and thick sections are concerned. Such conditions nearly always lead to brittle or semi-brittle fracture. Vincent has shown that the notch sensitivity in a braod range of amorphous and crystalline polymers is increased as the testing temperature is lowered and the loading rate is increased. Before fracture occurs, amorphous plastics often craze under these conditions. The complex questions of craze initiation, propagation and transformation into a crack have been treated extensively for amorphous polymers in the first three chapters of this book (see also The problem becomes more complicated when... 

Elinck and coworkers were the first to report on the discontinuous nature of DCG, while the Lehigh University research team under the direction of Hertzberg and Manson, discovered the generality of the DCG process in both amorphous and crystalline polymers. Recently, Doll, Konczol and Schinker have used optical interferometry to provide the greatest insist as to the micromechanical processes that underlie the DCG mechanism. 

In between these two extremes of amorphous and crystalline polymers there is a wide spectrum of polymeric materials with different degrees of... 

This phenomenon was observed with polymers some years ago [177—183]. The more recent investigations are due to Buben and Nikolskii [183]. These workers measured the emission from many amorphous and crystalline polymers and observed correlations between the temperature corresponding to the glow peak maxima and the structural transition temperatures of the materials. A detailed study of polyethylene thermoluminescence was made by Charlesby and Partridge [184]. The glow curve obtained after irradiation in vacuo possesses three peaks, a, j3 and y, whose luminescence intensities are proportional to the irradiation dose for doses below 5 x 104 rads. The maxima occurs, respectively, at —110, —65 and —27°C, when the total... 

The unordered (amorphous) state of aggregation in which the polymer chains also assume random conformations represents one extreme in the physical state of the polymer. This is the state that exists in such amorphous states as solution, melts, or some solids, the randomness being induced by thermal fluctuations. The other extreme is the case where the molecules are able to pack closely in perfect parallel alignment as is found in those polymers that exhibit fibrous behavior— that is, in those possessing a high degree of crystallinity and crystal orientation. In between these two extremes of amorphous and crystalline polymers there is a wide spectrum of polymeric materials with different degrees of crystallinity and amorphous character. These are called semicrystalline. 

A miscible blend of amorphous and crystalline polymers usually means a single phase in the melt and a neat crystalline phase with a mixed amorphous region in the sohd. Because of chain folding during crystallization, the crystal lamellae are formed. Their radical growth usually lead to the formation of spheniUtes [Nadkami and Jog, 1991]. 

Utracki, L. A., PVT of amorphous and crystalline polymers and their nanocomposites, Polym. Degrad Stab., 95, 411 21 (2010). 

Utracki, L. A., Compressibility and thermal expansion coefficients of nanocomposites with amorphous and crystalline polymer matrix. Ear. Polym. J., 45, accepted (2009e). 

Tensile Properties Similar to polyethylene, the stress-strain curve of JSR RB has a yield point. Above the yield point, the stress-strain curve continues to increase with elongation, then breaks. This kind of stress-strain curve is similar to EVA and indicates a characteristic property lying somewhere between amorphous and crystalline polymers. The dynamic properties of JSR RB can be improved by stretch-... 

The densities of amorphous and crystalline polymers can differ by up to 15% (Table 5-3). Polymers with unsubstituted monomeric units, such as poly(ethylene) and nylon 6,6, for example, show the greatest difference in density. These chains crystallize in an dAVtrans conformation with particularly close packing of molecular chains. In helix-forming macromolecules with large substituents, such as it-poly(styrene), for example, the packing is, by contrast, less efficient. 

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