Semicrystalline structure

Fibers are thin threads produced by extruding a molten polymer through small holes in a die, or spinneret. The fibers are then cooled and drawn out, which orients the crystallite regions along the axis of the fiber and adds considerable tensile strength (Figure 31.3). Nylon, Dacron, and polyethylene all have the semicrystalline structure necessary for drawing into oriented fibers. 

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. 

The Semicrystalline Structure of Amylopectin. Amylopectin crystallises according to a cluster molecule,25 as shown in Figure 8b. Within this structure there are crystalline and amorphous regions. An... 

Unlike starch, which is amorphous, cellulose is fermented with difficulty because of its semicrystalline structure. As a consequence, ethanol fermented from cellulose using the latest generation enzymes would still be more expensive... 

The melting of a crystalline-amorphous block copolymer of poly(tetrahydro-furan)-poly(isoprene) (PTHF-PI) was investigated using DSC by Ishikawa et al. (1991). They found a double melting peak, which was proposed to result from the semicrystalline structure of the crystalline PTHF layer, with less-ordered crystallites melting before those with well-ordered domains of chain-folded PTHF. Alternative explanations include fractionation of the polydisperse block copolymer or melting of crystals with different fold lengths. 

Cellulose is a polymer that meets these requirements as an adhesive. However, due to its semicrystalline structure, highly hydrogen-bonded cellulose cannot be dissolved easily in conventional solvents, and it cannot be melted before it burns. This is because the attractive forces and stability of crystal structures are greater than those that result from interaction between polymer and solvent. Hence, cellulose itself is not suitable for use as an adhesive. The same can be said of regenerated cellulose. In order to make cellulose soluble or meltable, the hydrogen bonds must be broken (i.e., cellulose molecules must be more flexible and possess high entropy, so that they can be separated easily). 

Crystalline polymers appear to be the most studied by ESR techniques. The model wiiich seems to emerge from these results is, in fact, a variant of a model proposed over twenty years ago by Cumberbirch and associates (Shirley Institute Memoirs) to explain the tenacity of wet raycm monofilaments. Briefly, Cumberbirch, et al propose a fringe-micelle structure in which the fringe r ons, swollen by water, are assumed to obey rubber elasticity theory. These fringe reglcms are, of course, the more accessble (to water), more disordered, r ons of the semicrystalline structure. 

Theoretical and experimental investigations dealing with the polyelectrolyte adsorption and the structure of polyelectrolyte liquid films have been carried out for more than twenty years in order to give a better understanding of emulsion stabilization mechanisms by polyelectrolytes on a microscopic scale. In this section, we describe the structure of liquid films composed of three kinds of polyelectrolytes the diblock amphiphilic polyelectrolytes, which lead to the formation of brushes, the homopolyelectrolytes, which form a semicrystalline structure within the films, and finally the amphiphilic random polyelectrolytes. Special attention is given to the charged monomer layer thickness at interfaces. 

Eunctionahzation of PO blends, as a rule, leads to an improved compatibihty of the components and to variations in the parameters describing their semicrystalline structure. 

PD8S. The large variation in the phase transition temperatures and other physical properties such as UV absorption as reported by different authors indicates a rich polymorphism of this polymer. Very recently, Chunwachirasiri et al. reported on five different crystalline or semicrystalline structures in quenched samples of PD8S, some of which occur only below the temperature range considered here. Our own DSC experiments on samples prepared in three different ways (denoted A-C) confirm that the phase behavior strongly depends on the conditions of sample preparation. 

Electron density function is defined as the number of electrons per unit volume and it is denoted as p r). Electron density function of a semicrystalline polymer consists of a series of alternating steps, positive Pi and negative P2, which represent phases, and flucmate around an average value p. If, for example, an r vector is passed along the semicrystalline structure (Fig. 19.5), it can be seen that p r) = /Oj if p r) > Pm-On the other hand, p(r) = P2 if p f) < Pm where p is a high density region (crystalline) and P2 is a low density region (amorphous). 

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