THE AMORPHOUS STATE

As the length of the side chain increase, however, melting points decrease and are accompanied by increases in flexibility [7] until the length of the side chains reached six carbons. At that point, the minimum takes an upturn and there is an increase in the melting points and decrease in flexibility. This phenomenon is believed to be due to crystallization of the side-chains [9]. 

Alkyl substituents on the polymers of a-olefins that are on the a-carbon yield polymers with the highest melting points. Isomers substituted on the p-carbon, however, if symmetrical, yield polymers with lower melting points. Unsymmetrical substitutions on the p-carbon, on the other hand, tend to lower the melting points further. Additional drop in the melting points result from substitutions on the y-carbon or further out on the side-chains. Terminal branching yields rubbery polymers [10]. 

Copolymers melt at lower temperatures than do homopolymers of the individual monomers. By increasing the amount of a comonomer the melting point decreases down to a minimum (this could perhaps be compared to a eutectic) and then rises again. 

The bulk state, sometimes called the condensed or solid state, includes both amorphous and crystalline polymers. As opposed to polymer solutions, generally there is no solvent present. This state comprises polymers as ordinarily observed, such as plastics, elastomers, fibers, adhesives, and coatings. 

A few definitions are in order. Depending on temperature and structure, amorphous polymers exhibit widely different physical and mechanical behavior patterns. At low temperatures, amorphous polymers are glassy, hard, and brittle. As the temperature is raised, they go through the glass-rubber transition. The glass transition temperature (Tg) is defined as the temperature at which the polymer softens because of the onset of long-range coordinated molecular motion. This is the subject of Chapter 8. 

Above Tg, cross-linked amorphous polymers exhibit rubber elasticity. An example is styrene-butadiene rubber (SBR), widely used in materials ranging 

Introduction to Physical Polymer Science, by L.H. Sperling ISBN 0-471-70606-X Copyright 2006 by John Wiley Sons, Inc. 

Polymers that cannot crystallize usually have some irregularity in their structure. Examples include the atactic vinyl polymers and statistical copolymers. 

Thus far, we have focused on solids that have a well-ordered crystalline structure. It is now time to turn our attention to some examples of amorphous solids. We already discussed the synthesis of amorphous metals those obtained through fast nonequilibrium conditions. However, there is a more pervasive class of 

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Cryoinnnobilization procedures tiiat lead to vitrification (immobilization of the specimen water in the amorphous state) are the sole methods of preserving the interactions of the cell constituents, because the liquid character of the specimen water is retained (reviewed in [25]). 

This last interpretation makes P(0) the same as the fraction of a sample in the amorphous state. It is conventional to focus on the fraction crystallized 6 therefore the fraction amorphous is I - 6 and... 

Crystallinity is low the pendent allyl group contributes to the amorphous state of these polymers. Propylene oxide homopolymer itself has not been developed commercially because it cannot be cross-baked by current methods (18). The copolymerization of PO with unsaturated epoxide monomers gives vulcanizable products (19,20). In ECH—PO—AGE, poly(ptopylene oxide- o-epichlorohydrin- o-abyl glycidyl ether) [25213-15-4] (5), and PO—AGE, poly(propylene oxide-i o-abyl glycidyl ether) [25104-27-2] (6), the molar composition of PO ranges from approximately 65 to 90%. 

Poly(ethylene terephthalate) film is produced by quenching extruded film to the amorphous state and then reheating and stretching the sheet approximately three-fold in each direction at 80-100°C. In a two-stage process machine direction stretching induces 10-14% crystallinity and this is raised to 20-25% by... 

Amorphous stereotactic polymers can crystallise, in which condition neighbouring chains are parallel. Because of the unavoidable chain entanglement in the amorphous state, only modest alignment of amorphous polymer chains is usually feasible, and moreover complete crystallisation is impossible under most circumstances, and thus many polymers are semi-crystalline. It is this feature, semicrystallinity, which distinguished polymers most sharply from other kinds of materials. Crystallisation can be from solution or from the melt, to form spherulites, or alternatively (as in a rubber or in high-strength fibres) it can be induced by mechanical means. This last is another crucial difference between polymers and other materials. Unit cells in crystals are much smaller than polymer chain lengths, which leads to a unique structural feature which is further discussed below. 

Low shrinkage - all thermoplastics are processed in the amorphous state. On solidification, the random... 

Supercooling The rapid cooling of a normally crystalline plastic through its crystallization temperature, so it does not get a chance to crystallize and it remains in the amorphous state. 

Fig. 12. Free energy vs. temperature for FCC-cttrve /, for ECC-curve II and for the amorphous state (melt)-cur-ve III. Points 1 and 2 represent the melting temperatures of FCC and ECC, respectively, point 3 separates the ranges of the predominant existence of FCC and ECC (see text)...

The amorphous state frequently passes spontaneously into the crystalline state (plastic sulphur, devitrification of glass, Gore s amorphous antimony). 

Blends of enzymatically synthesized poly(bisphenol-A) and poly(p-r-butylphenol) with poly(e-CL) were examined. FT-IR analysis showed the expected strong intermolecular hydrogen-bonding interaction between the phenolic polymer with poly(e-CL). A single 7 was observed for the blend, and the value increased as a function of the polymer content, indicating their good miscibility in the amorphous state. In the blend of enzymatically synthesized poly(4,4 -oxybisphenol) with poly(e-CL), both polymers were miscible in the amorphous phase also. The crystallinity of poly(e-CL) decreased by poly(4,4 -oxybisphenol). 

Solid polymers can adopt a wide variety of structures, all of which are derived from the three basic states rubbery amorphous, glassy amorphous, and crystalline. Either of the amorphous states can exist in a pure form. However, crystallinity only occurs in conjunction with one of the amorphous states, to form a semicrystalline structure. 

Miyazaki et al. [43] reported that a sol-gel-derived Ta2Os gel shows hydroxyapatite-forming ability in SBF even in the amorphous state. Therefore, PTM0-Ta205 nano-hybrids prepared by a sol-gel method can be expected to form hydroxyapatite in SBF without any treatment. PTMO-CaO-Ta2Os nano-hybrids were prepared by... 

Fig. 3.11. Schematic two-dimension representation of the structure of cristobalite (a crystalline form of Si02) and of vitreous Si02. Si atoms are represented by full circles, oxygen by open circles. A, B and C represent three cases of double possible equilibrium positions for the atoms of the material in the amorphous state A, transversal displacement B, longitudinal displacement C, small-angle rotation of the Si04 tetrahedron. 

Researchers who have focused more on understanding cause-effect relationships in solution processing have given attention to film drying and pyrolysis behavior, densification processes, and nucleation and growth into the desired crystalline state. Both thermodynamic and kinetic factors associated with the phase transformation from the amorphous state to the crystalline state have been considered.11 119 Control of these factors can lead to improvements in the ability to influence the microstructure. It is noted that in the previous sentence, influence has been carefully chosen, since the ability to manipulate the factors that govern the nature of the phase transformation to the extent that full control of the microstructure is possible remains to be demonstrated. However, trends in characteristics such as film orientation and columnar versus uniaxial grains have certainly already been achieved.120... 

In addition to the role of the physical characteristics of the amorphous state on nucleation and growth, as indicated by Eq. 11, surface energies also play a role. The role of these properties on film orientation and microstruc-... 

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