Fundamental Characteristics of Polymeric Materials

Resistance, Conductance. In some applications the plastics resistance or its reciprocal, the conductance is of functional significance, eg, in electronic circuits for which high resistance or impedance is of operational importance. More often resistance is used to measure indirectly the influence of other factors, such as moisture, which may affect electric strength. Resistance measurements are also useful in studying resin cure, determining the presence of impurities, and investigating the fundamental characteristics of polymeric materials. An extensive review of the theoretical aspects of conductivity is given in Ref. 41. 

Hence, the ability to understand and characterize the polymerization reaction behavior is extremely important and fundamental in attempts to develop improved dental materials. The following sections will discuss the general characteristics of polymerization reactions with particular emphasis on the complexities occurring in the high crosslinking regime. The effects of the reactions conditions, like oxygen and sample thickness, on the polymerization rate will also be included. 

A fundamental question in rheology is how those phenomena can be understood from the microscopic characteristic of the materials, i.e., their structure and the type of interaction. In later chapters, we shall discuss this in detail for polymeric liquids. In this section, we shall give a general base for developing microscopic theory for the mechanical properties of suspensions and polymer solutions. 

Isotropic polymeric systems as well as particulate systems might also show time-dependent moduli after cessation of flow. As long as the shear does not induce structure growth, the moduli always increase with time after flow. An increase of the moduli upon cessation of flow has also been reported for thermotropic PLCs (18) as well as for lyotropic solutions of hydroxy propyl cellulose in water (19) and in acetic add (20). The possibility of changing in either direction seems to be characteristic for mesomorphic materials. A fundamental theory for describing complex moduli does not exist for such materials. The present results, combined with the information about optical relaxation mentioned above, could be explained on the basis of reorientation of domains or defects. The different domains orient differently, even randomly, at rest whereas flow causes an overall orientation. Depending on the molecular interaction the flow could then cause an increase or decrease in moduli as recently suggested by Larson (21). 

From a material s point of view polymeric optical fibers are selected first of all on the basis of their optical characteristics. Table 4.4 summarizes the relationships between various physical parameters and fundamental optical properties. In recent years, Lorentz relationships between the refractive index and molecular parameters have been derived to guide experimentalists in selecting acceptable polymers. Representative optical characteristics of transparent polymers are reviewed in Table 4.5. The polymers CR-39, PMMA, and PC are... 

Many polymerizations are performed in heterogeneous media, so that the final polymer product is obtained as a particulate material. The characteristics of the final particle size distribution (PSD) of the product maybe of fundamental importance for many applications. 

The JKR theory, similar to the Hertz theory, is a continuum theory in which two elastic semi-infinite bodies are in a non-conforming contact. Recently, the contact of layered solids has been addressed within the framework of the JKR theory. In a fundamental study, Sridhar et al. [32] analyzed the adhesion of elastic layers used in the SFA and compared it with the JKR analysis for a homogeneous isotropic half-space. As mentioned previously and depicted in Fig. 5, in SFA thin films of mica or polymeric materials ( i, /ji) are put on an adhesive layer Ej, I12) coated onto quartz cylinders ( 3, /i3). Sridhar et al. followed two separate approaches. In the first approach, based on finite element analysis, it is assumed that the thickness of the layers and their individual elastic constants are known in advance, a case which is rare. The adhesion characteristics, including the pull-off force are shown to depend not only on the adhesion energy, but also on the ratios of elastic moduli and the layers thickness. In the second approach, a procedure is proposed for calibrating the apparatus in situ to find the effective modulus e as a function of contact radius a. In this approach, it is necessary to measure the load, contact area... 

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