Single-phase polymer materials

The pyroelectric coefficients of the polymer-ceramic composites are large oompared to polymers, and the relative permittivities are small compared to ceramics (64j. Therefore tte figures of merit are enhanced over conventional single-phase polymer materials. (In certain pyroelectric systems a useful figure of merit to p/c, where p to the pyroelectric coefficient and c is the electric permittivity [16]). 

Intermolecular forces also play an important role in determining the compatibility of two or more polymers in a polymer blend or polymer alloy. Although the distinction between a polymer blend and a polymer alloy is still the subject of some debate, we will use the convention that a polymer alloy is a single-phase, homogeneous material (much as for a metal), whereas a blend has two or more distinct phases as a result of polymer-polymer immiscibility (cf. Section 2.3.3). In general, polymers are... 

The integration of all these five factors will be important in relation to a deeper understanding of processing behaviour. All that has been said so far applies to a single-phase polymer, although it must be remembered that commercial polymers can have broad molecular mass distributions and there is scope for phase separation effects to be present even within a single species material. Polymer blends suspensions and foams all add to the additional complexity of the problem. 

In addition, the polymer blends segment of the plastics industry increases at about three times faster than the whole plastics industry. Blending has been recognized as the most versatile, economic method to produce materials able to satisfy complex demands for performance. By the year 2000 the world market for polymer blends is expected to reach 51 million tons per annum, worth well over US 200 billion. The tendency is to offer blends that can be treated as any other resin on the market hence their processability must closely match that of single-phase polymer, but offer a much greater range of performance possibilities. 

The third group, Styrosorb 2, represents nanoporous single-phase polymers derived from spherical beads of gel-type styrene copolymers with largely 0.7% DVB, post-crosslinked in swollen state with monochlorodimethyl ether. The size of the micropores is approximately 10—30 A, and the apparent specific surface area reaches very large values of 1000—1900 m /g, which is comparable to the range of the best activated carbons. On the other hand, the pore volume of these materials is rather small, 0.2—0.3 cm /g. 

The performance of a material or a structure also depends on local variations due to processing conditions, such as cooling rate, shear stress, and melt-flow paths, which result in orientation and residual stress. Multiphase materials can exhibit even greater local variations in material properties than those observed in single phase polymers. A structure produced by a given process may possess significant morphological differences from test specimens. Ultimately, it may be necessary to test the impact resistance of components. 

Polymers in the category of engineering resins and plastics may be classified in several ways. They may be thermoplastic or thermoset. They may be crystalline or amorphous and they may be single phase or multiphase systems. This would allow for eight types of materials except that thermosets, because of their irregular cross-linked structure, are never crystalline. Single phase polymers do not have discemable second phase structures of different chemical composition. Thus homopolymers and random copolymers are single phase polymers, even if they are semicrystalline and so contain amorphous and... 

Much information can be obtained by microscopy of crystalline thermoplastics, whereas microstructural study of single phase amorphous materials is not usually of much practical interest. This is why most microscopy studies of single phase polymers relate to crystalline materials, and amorphous polymers are mostly described in multiphase systems. 

A convenient basis for all that follows is obtained by looking at the modulus of a polymer material as a function of temperature. To obtain a basic representation, we consider the ideal case of a linear (unbranched) polymer chain of homogeneous chemical composition in a single phase bulk material which is amorphous (non-crystaUine). For all such materials, when modulus is plotted... 

Dielectric data typically obtained for characterization of polymeric materials involve e and e" versus temperature or frequency. Generalized data for an imblended polymer or a single phase polymer blend are illustrated in Fig. 5.13. 

Ferroelectric Ceramic—Polymer Composites. The motivation for the development of composite ferroelectric materials arose from the need for a combination of desirable properties that often caimot be obtained in single-phase materials. For example, in an electromechanical transducer, the piezoelectric sensitivity might be maximized and the density minimized to obtain a good acoustic matching with water, and the transducer made mechanically flexible to conform to a curved surface (see COMPOSITE MATERIALS, CERAMiC-MATRix). 

It is necessary to consider the micro-mechanical processes of polymer glasses and elastomers separately as their mechanical properties are so different. In addition, cross-linking profoundly affects the deformation processes in glasses but very little is known about the micro-mechanical processe.s that occur in single phase cross-linked glasses so the latter materials will not be discussed further. 

The most widely used method for preparing extended-chain crystals involves solid-phase polymerization when the monomer exists as a single crystal. The polymerization of single crystals of the monomer permits the preparation of a polymer material with a maximum orientation a polymeric single crystal composed of fully extended macromolecules. Such polymer crystals are needle-shaped22. ... 

Polymer blends have been categorized as (1) compatible, exhibiting only a single Tg, (2) mechanically compatible, exhibiting the Tg values of each component but with superior mechanical properties, and (3) incompatible, exhibiting the unenhanced properties of phase-separated materials (8). Based on the mechanical properties, it has been suggested that PCL-cellulose acetate butyrate blends are compatible (8). Dynamic mechanical measurements of the Tg of PCL-polylactic acid blends indicate that the compatability may depend on the ratios employed (65). Both of these blends have been used to control the permeability of delivery systems (vide infra). 

Most polymers that have been of interest as membrane materials for gas or vapor separations are amorphous and have a single phase structure. Such polymers are converted into membranes that have a very thin dense layer or skin since pores or defects severely compromise selectivity. Permeation through this dense layer, which ideally is defect free, occurs by a solution-diffusion mechanism, which can lead to useful levels of selectivity. Each component in the gas or vapor feed dissolves in the membrane polymer at its upstream surface, much like gases dissolve in liquids, then diffuse through the polymer layer along a concentration gradient to the opposite surface where they evaporate into the downstream gas phase. In ideal cases, the sorption and diffusion process of one gas component does not alter that of another component, that is, the species permeate independently. 

Polymer blends and block-copolymers have been considered as membrane materials as mentioned later. If the components are miscible and a single-phase... 

The miscibilities of the components in polymer blends is often ascertained by the measurement of the material s glass transition temperature (Tg). The mixing of two polymers with no mutual interactions usually results in the mixture having two separate phases each with their own distinct glass transition temperature. However, when the two components do interact to form a single phase mixture, their glass transitions combine and there will be the emergence of only one transition temperature that is linearly dependent on composition [118]. 

The volumes of the polymer material of the packaging and food simulant are given by VP and VF, the contact surface area between the two phases is given by A and the layer thickness of the polymer (for single sided contact) is given by dP... 

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