Single phase polymers

A comparison of pit sizes in PC disks in Fig. 5.59 shows ICAFM images of audio CD, DVD, and high density DVD surfaces with submicrometer sized pits [216]. A stained audio CD was also examined by ICAFM (Fig. 5.60), which showed various regions to exhibit surface damage that provided a direct correlation of playability with morphology [216]. 

Crystalline polymers are more correctly termed semicrystalline as their measured densities differ from those obtained for perfect mated- 

The morphology of molded articles depends on the chemical composition of the polymer, the process variables, and the mold geometry. Standard molded tensile bars are discussed here for simplicity, but the principles are the same for any molding, although the nature of the specific flow field must be taken into account. The relationships between process conditions, microstructure, and mechanical properties of an injection molded thermoplastic have been reviewed [236, 290, 291]. Semicrystalline moldings and extrudates are most often 

An example of the multilayered structures common in polyacetals is shown in the polarized light micrographs (Fig. 5.62) that depict a uniformly nucleated crystalline structure formed due to mold filling and variations in the 

The topic of multiphase polymers is vast with books, reviews [224-227, 229, 230, 235, 302-311], and hundreds of research papers describing the processes, morphologies, properties, and applications of these important materials. The field of multiphase polymers has been driven by the realization that wholly new molecules are not always required for new applications and that blends can provide a rapid and economical means of development. 

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A need for single-phase polymers in some applications, e.g., polytetrafluoroethylene, VHMWPE, light-sensitive polymers, etc. 

Earlier work on chain extended HPLs has shown that these derivatives produce uniform (i.e., single phase) polymers with Tg varying in accordance with the Gordon-Taylor relationship (12). Polyurethanes from chain-extended HPLs were found to be rubber-like at room temperature with modulus declining as lignin content is reduced (8). Star-like structure determines functionality, Tg, viscosity, and several other properties that influence utility as polymer segment. 

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. 

Polymer-solvent mixtures can be separated and the polymer recovered from solution at the lower critical solution temperature (LCST). This is the temperature at which the miscible polymer-solvent mixture separates into a polymer-rich phase and a solvent-rich phase. LCST phenomena are related to the chemical nature of the mixture components, the molecular weight of the mixture components, especially the polymer, and the critical temperature and critical pressure of the solvent (Allen and Baker, 1965). As the single-phase polymer solution is isobarically heated to conditions near the critical point of the solvent, the polymer and solvent thermally expand at different rates. This means their free volumes change at different rates (Patterson, 1969). The thermal expansion of the solvent is much greater than that of the polymer. Near its critical point, the solvent has expanded so much that it is no longer able to solubilize the polymer. Hence, the polymer falls out of solution. If the molecular weight of the polymer is on the order of 10 a polymer-solvent LCST can occur within about 20-30°C of the solvent s critical temperature. If the molecular weight of the polymer is closer to 10, the LCST phase... 

The elastic creep compliance for PMMA/ PS = 16/84 behaved regularly, similar to what has been observed for single-phase polymers. However, when the composition was reversed,... 

For the better understanding of blend morphologies, the fundamental mechanisms of morphology development are discussed, viz. the liquid-solid phase transition (crystallization), the liquid-liquid phase separation e.g., spinodal decomposition under non-isoquench depth), as well as the complex mechanism of the morphology generation that results from the competition between these two transitions. The effects of chemical reactions and flow fields on morphology development have also been discussed. Finally, several evidences of a local structure in single-phase polymer-polymer mixtures are presented. 

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 traditional views on plasticization of polymers are based on results of studying the single phase polymer systems and microheterogeneous polymers which have the same chemical stmctures of hard and soft phases. 

Incorporation of multifunctional POSS into polymer systems has been investigated with different polymers [6,62-66]. In these cases, single-phase polymer networks with POSS molecularly dispersed are often formed. POSS acts as a polyhedral cross-link. But no definite effect of POSS on network properties has been established. Both a decrease [64,65] and no change in Tg [6] were reported. The rubbery modulus increases due to a high crosslink density, and thermal stability increases with POSS content. 

Similar observations were reported for PMMA/PS blends (Gramespacher and Meissner 1995). The elastic creep compliance for PMMA/PS = 16/84 behaved regularly, similar to what has been observed for single-phase polymers. However, when the composition was reversed, i.e., PMMA/PS = 84/16, the recovery creep compliance showed a maximum at which the recovery direction was reversed. The authors attributed the dissymmetry of behavior to different retardation times of the blend components. 

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. 

Finally, it is important to note that our focus in this chapter and throughout the text is on single-phase polymers with flexible backbones. The rheology of polymers with rigid elements in the backbone, which are often liquid crystalline in the melt, is quite different from that described here. Immiscible blends and filled polymers may have behavior similar to that of the flexible melts, but there are often important differences. Structured fluids are discussed briefly in Chapter 13. 

Equation (44) is the well-known WLF equation. Universal values of the various physical parameters in Eq. (44) lead to Cf = 17.44 and C = 51.26 K [10]. These are of the same order of magnitude as Cf and C obtained empirically (34 and 80 K for PMMA, for example), and indeed, time-temperature superposition has been found to work well for a wide range of single-phase polymers, with the proviso that it begins to break down for the relatively fast vibrational modes characteristic of the glassy state [12]. Moreover, although superposition may work for T Tg, at temperatures above about 7 + 50 K the shift factors tend to show an Arrhenius dependence rather than following the WLF equation. 

In the following sections, the experimental results which have been found in various studies of single-phase polymer flow in 1-D porous media will be discussed. Results will be referred to the convection-dispersion equation outlined above as a model for the flow. However, when there are deviations from this, the appropriate equations/models will be developed. In addition to discussing the macroscopic fit of the generalised convection-dispersion model for polymer transport in porous media, some aspects of the microscopic or physical basis of the phenomena under consideration will also be discussed. 

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]). 

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. 

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