Amorphous plastic region

A distinction should be made between solvent plasticizers and nonsolvent plasticizers. With an amorphous polymer, any plasticizer is a solvent plasticizer— i.e., under suitable conditions the polymer would eventually dissolve in the plasticizer. With a crystalline or semicrystalline polymer, there are some compounds which enter both the crystalline (ordered) and the amorphous (disordered) regions. These are true plasticizers-sometimes they are called primary plasticizers. If, on the other hand, only the amorphous regions are penetrated, the compound may be considered as a nonsolvent plasticizer, also known as a secondary plasticizer, or softener. Such softeners are used sometimes as diluents for the primary plasticizer. 

Fig. 5.21 Polymer feed temperatures are at or near Tmom. For common amorphous plastics, TV00m < Tg, and for semicrystalline T < Tmom < As disussed in the text, PED, through large solid-state irreversible deformations, makes the solid an active participant in the melting process, rapidly creating a molten state. The modulus of amorphous polymer is higher and less temperature dependent in the region T > rroom. Consequently, the magnitude of amorphous PED is larger and less temperature dependent when compared to semicrystalline PED.

Permeability of gases and vapours through plastic involves absorption, followed by diffusion, followed by evaporation and desorption from the other face. Permeability is greatest with amorphous plastics where diffusion occurs via the spaces between the moving mass of molecular chains. Crystalline plastics or those with crystalline regions present a greater barrier to diffusion. With thinner materials where pinholes or micropores occur, diffusion may occur via these small holes. Various other factors influence permeation, including ... 

Polymers can exhibit a number of different conformational changes with each change accompanied by differences in polymer properties. Two major transitions occur at Tg, which is associated with local, segmental chain mobility in the amorphous regions of a polymer, and the melting point (Tjj), which is associated with whole chain mobility. The Tn is called a first-order transition temperature, and Tg is often referred to as a second-order transition temperature. The values for Tjj are usually 33 to 100% greater than for Tg, and Tg values are typically low for elastomers and flexible polymers and nigher for hard amorphous plastics. 

Plastics can be classified according to the physical properties imparted to them by the way in which their individual chains are arranged. Thermoplastic polymers have both ordered crystalline regions and amorphous noncrystalline regions. Thermoplastic polymers are hard at room temperature, but soft enough to be molded when heated, because the individual chains can slip past one another at elevated temperatures. Thermoplastic polymers are the plastics we encounter most often in our daily lives—in combs, toys, switch plates, and telephone casings, for example. They are the plastics that are easily cracked. 

Basically, birefringence is the contribution to the total birefringence of two-phase materials, due to deformation of the electric field associated with a propagating ray of light at anisotropically shaped phase boundaries. The effect may also occur with isotropic particles in an isotropic medium if they dispersed with a preferred orientation. The magnitude of the effect depends on the refractive index difference between the two phases and the shape of the dispersed particles. In thermoplastic systems the two phases may be crystalline and amorphous regions, plastic matrix and microvoids, or plastic and filler. See amorphous plastic coefficient of optical stress compact disc crystalline plastic directional property, anisotropic ... 

Thus, each individual phase i contributes to the birefringence according to its volume fraction (f>i and birefringence Am. These different phases can, for example, be the amorphous and crystalline phases of partially crystalline polymers, aggregates in block copolymers, fillers, or plasticized regions. 

Linear macromolecules, either without substituents or with small ones regularly arranged, can juxtapose themselves at a microscopic level in uniform parallel aereas and form crystals. Plastics with crystalline regions always also contain more or less amorphous, unordered regions, for which reason they are termed semicrystalline. This macromolecular arrangement can be altered by a number of influences. 

In the plastic region the sorption of low molecular compounds markedly decreases with an increase of stress, because the density of the amorphous component rises in this case. 

In summary, the interdiffusion of polymer chains across a polymer/polymer interface requires the polymers (adhesive and substrate) to be mutually soluble and the macromolecules or chain segments to have sufficient mobility. These conditions are usually met in the autohesion of elastomers and in the solvent welding of compatible, amorphous plastics. In both these examples interdiffusion does appear to contribute significantly to the intrinsic adhesion. However, where the solubility parameters of the materials are not similar, or one polymer is highly crosslinked, crystalline or below its glass transition temperature, then interdiffusion is an unlikely mechanism of adhesion. In the case of polymer/metal interfaces it appears that interdiffusion can be induced and an interphase region created. But this effect enhances the interfacial adhesion by improving the adsorption of the polymeric material rather than by a classic diffusion mechanism. 

Pressure-area isotherms for many polymer films lack the well-defined phase regions shown in Fig. IV-16 such films give the appearance of being rather amorphous and plastic in nature. At low pressures, non-ideal-gas behavior is approached as seen in Fig. XV-1 for polyfmethyl acrylate) (PMA). The limiting slope is given by a viiial equation... 

The plasticizer content of a polymer may be increased by the suppression of crystallization in the polymer, but if crystallization subsequently occurs the plasticizer exudes. For highly crystalline resins, the small amounts of plasticizer allowable can change the nature of the small amorphous regions with a consequent overall change in properties. 

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