Crystalline Flexibility

It is an interesting problem how the solid structure of PVA films affects complex formation. It is well known that the solid structure of crystalline flexible polymers is not homogeneous but very complicated. 

The properties of the polysilanes, like those of the polyphosphazenes, depend greatly on the nature of the substituent groups. Polysilanes cover the entire range of properties from highly crystalline and insoluble, through partially crystalline, flexible solids, to glassy amorphous materials and rubbery elastomers. 

It often occurs that it is desirable to modify the properties of a homopolymer to achieve certain application-specific characteristics that are perhaps not possible by solely manipulating the polymer molecular weight or by chemical modification of the final producf. Perhaps one is interested in achieving properties that are intermediate to two homopolymers. Properties of interest could include crystallinity, flexibility, tensile strength, melting point, glass-transition temperature, and many others. 

Another point to be made in connection with Dalton s writings is the distinction between phases and substances (molecules). It will be shown later in the book that large molecules may reach the size of phases. On the other hand, phases that were initially thought to be macroscopic in size, may also be very small in form of microphases and nanophases, as will be detailed in Sect. 2.5. Sufficiently large molecules may then reside at the same time in more than one phase. Many of the special properties of crystalline flexible polymers, for example, are based on the smallness of their phases, and this will be a major item of discussion in the various chapters of the book. 

The structure of polymers determines the principle and their nsefulness as durable materials such as film, plastic, and rubbery forms. In polymeric materials, the chains are based on various factors such as crystallinity, flexibility, inflexibility, stiffness, and cross-link (Figure 3.1). Therefore, the chains are... 

Crystallinity. Generally, spider dragline and silkworm cocoon silks are considered semicrystalline materials having less crystalline flexible chains reinforced by strong stiff crystals (3). The orb web fibers are Composite Materials (qv) in the sense that they are composed of crystalline regions immersed in less crystalline regions, which have estimates of30-50% crystallinity (3,16). Earlier studies by X-ray diffraction analysis indicated 62-65% crystallinity in cocoon silk fibroin from the silkworm, 50-63% in wild-t5q)e silkworm cocoons, and lesser amounts in spider silk (17). 

PREPARATIONS OF BLOCK COPOLYMERS BY CHEMICAL REACTIONS ON LAMELLAS OF PARTIALLY CRYSTALLINE FLEXIBLE POLYMERS... 

The complicated segmental dynamics in highly crystalline flexible-chain polymers may be illustrated, for instance, by the results of DSC/NMR/X-ray diffraction studies performed for the well-defined HDPE samples with different crystallinities DSC. ranging from43% to 80% [15,209,213,214], as well as for POM [21,22,210],... 

P-relaxation in non- or low-crystalline flexible-chain polymers, and its dyntunic heterogeneity... 

The mclPHAs, compared to the sclPHAs, are polymers with lower crystallinity, flexible and soft and exhibit behavior like those of thermoplastic elastomers, with low tensile strengths and high elongations at break. Copolymers of sclPHA and mclPHA, with low contents of mclPHA units, have improved properties compared to brittle sclPHAs and exhibit properties similar to poly(propylene). 

Other authors have attributed the improved corrosion resistance with increasing Cr content with the increasing tendency of the oxide to become more disordered [69]. This would then suggest that an amoriDhous oxide film is more protective than a crystalline one, due to a bond and stmctural flexibility in amoriDhous films. 

Mesogenic diols, such as 4,4 -bis( CO-hydtoxyaLkoxy)biphenyls, ate used with 2,4-TDI or 1,4-diisocyanatobenzene (PPDI) to constmct Hquid crystalline polyurethanes (7). Partial replacement of the mesogenic diols by PTMG shows that the use of lower molecular weight flexible spacers form polymers that have a more stable mesophase and exhibit higher crystallinity (8). Another approach to Hquid crystal polyurethanes involves the attachment of cholesterol to the polyurethane chain utilizing the dual reactivity in 2,4-TDI (9). 

The two main amphibole asbestos fibers are amosite and crocidoHte, and both are hydrated siHcates of iron, magnesium, and sodium. The appearance of these fibers and of the corresponding nonfibrous amphiboles is shown in Figure 1. Although the macroscopic visual aspect of clusters of various types of asbestos fibers is similar, significant differences between chrysotile and amphiboles appear at the microscopic level. Under the electron microscope, chrysotile fibers are seen as clusters of fibrils, often entangled, suggesting loosely bonded, flexible fibrils (Fig. 2a). Amphibole fibers, on the other hand, usually appear as individual needles with a crystalline aspect (Fig. 2b). 

Syndiotactic Polybutadiene. Syndiotactic polybutadiene is a unique material that combines the properties of plastic and mbber. It melts at high (150—220°C) temperatures, depending on the degree of crystallinity in the sample, and it can be molded into thin films that are flexible and have high elongation. The unique feature of this plastic-like material is that it can be blended with natural mbber. 1,4-Polybutadiene and the resulting blends exhibit a compatible formulation that combines the properties of plastic and mbber. 

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