Behavior of Polymeric Systems

It seems desirable at this point to familiarize the reader with some concrete examples of the viscoelastic phenomena defined in the preceding chapter, and to provide an idea of their character as exhibited by various types of polymeric systems. Linear viscoelastic behavior in shear will be illustrated in considerable detail, with a few additional examples of bulk viscoelastic behavior and nonlinear phenomena. The examples are accompanied by some qualitative remarks about molecular interpretation, anticipating Chapters 9 and 10 where molecular theories will be discussed more quantitatively. 

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With both styrene and vinylpyridine, the autoacceleration index decreases as the reaction temperature rises. This effect can be considered normal behavior of polymerizing systems in which the gel effect is operative. As the temperature rises, the termination step, which involves the interaction of two polymeric chains in a highly viscous medium, increases in rate, and the over-all reaction tends to become normal. Ultimately, the stationary-state conditions may eventually apply. 

The differences in the polymerization kinetics and colloidal behavior of polymerization systems based on monomers of different polarity may be illustrated (Bakaeva et al., 1966 Yeliseyeva and Bakaeva, 1968) by the polymerization of the model monomers, methyl acrylate and butyl methacrylate, at various concentrations of sodium alkylsulfonate (C,5H3 S03Na). The fact that the solubility of the monomers in water differs by two orders of magnitude (5.2 and 0,08/ , respectively) was used as a criterion of polarity. An additional advantage to comparing these two monomers is that their polymers have rather close glass transition temperatures which is important for coalescence of particles at later stages of polymerization. 

Summarizing the described results, it may be concluded that when using the emulsifiers described above, the colloidal behavior of polymerization systems is mainly determined by monomer polarity. Latexes of polar... 

The behavior of polymeric systems under compression (high pressures) has also been studied insufficiently. Till now the equilibrium under normal pressure has been mainly considered. The experiments on the formation of the ordered state of polyethylene under the pressure above 3-4 kbar revealed that there is much to be done in this field to extend the concept to polymers in general (see ). 

G. Smets, Photochromic behavior of polymeric systems and related phenomena, PureAppl. Chem. 30,1-24(1972). 

Just a few years ago, the limitations of solubility parameter calculations and measurements discussed above were serious impediments to modeling the phasic and interfacial behaviors of polymeric systems. The coming of age of atomistic simulation methods over the last few years has improved this situation dramatically. As discussed in Section 5.A.3, whenever accuracy is important in calculating the phasic or the interfacial behavior of a system, it is nowadays strongly preferable to use atomistic simulations employing modem force fields of the highest available quality instead of solubility parameters in order to estimate the Flory-Huggins interaction parameters (%) between the system components as input for further calculations. 

Although theories of the Rouse-Bueche-Zimm type have been very successful in rationalizing the behavior of polymeric systems from a molecular point of view, another class of theories is presently commanding the most attention. These theories treat the motion of polymer molecules in terms of reptation, a reptile-like diffusive motion of each polymer molecule through a matrix formed by its neighbors. To a considerable extent, this new approach has overcome some of the most important shortcomings of the normal-mode theories, which... 

Recognizing that polymer-polymer systems exhibit phase behavior similar to - that of other condensed phase systems, it should be possible to prepare multiphase polymeric materials of unique structure by phase transition. The purpose of this work is to show that much of the theoretical and experimental analysis of phase transitions in metallurgical and ceramic systems can provide an interpretation of the behavior of polymeric systems. In addition, there are several features of polymeric systems which distinguish them from low molecular weight systems. 

Equation 7.42 well describes the flow behavior of polymeric systems, and it was found useful for polymer blends. It should be stressed that Equations 7.41-7.43 described the flow behavior of fluids without yield stress or thixotropicity. 

A study of the solid state behavior of polymeric systems is important because of the engineering applications of polymeric materials. These applications stem from their physical properties in the solid phase, which in turn are a natural consequence of the unique molecular structure of polymer molecules. 

Other theories within the general framework of the free-volume concept have been advanced. They include the works of Kumins and Roteman (1961), Bueche (1953), Barrer (1957), DiBenedetto (1963), DiBenedetto and Paul (1964), Wilkens and Long (Wilkens, 1957 Wilkens and Long, 1957), and Vasenin (1960). Part of the reason that these theories are not so popular lies in the fact that their predictions of D c) were no better than those obtained by Fujita and Doolittle. In addition, most of these other theories concentrated on the temperature dependence of the intrinsic mobility, which is less important compared to its concentration dependence. In spite of the predictive limitations of the free-volume theory, it is applicable at the widest concentration range, and certainly it is the best theory to use for modeling diffusion limited behavior of polymerization systems. 

Computer modeling can clearly help in the understanding of the deformation and yield behavior of polymeric systems by giving an insight into the individual molecular, indeed atomic, movements that occur. However, the simulations are typically run over a few tens of picoseconds at most and in a volume of a few cubic nanometers—such scales of time and dimensions cannot fiilly capture the processes involved in yield at the present state of development. 

The MC simulation of phase behavior of polymeric systems has led to many interesting insights, such as the finding that in two-dimensional polymer blends where chains cannot... 

The accurate description of the thermodynamic behaviors of polymeric systems provides benchmarks to phenomenological theories. Scaling analysis... 

TSD hat also been carried out to study the effect of cron linking [228], stereo- and structural isomerism [242], doping [218], and co-polymerialion [219] on the relaxation behavior of polymeric systems. TSD has also been carried out on several other organic and inorganic materiaU [231-233,235,243,259,260]. a. Faeton Affecting TSD Spectra... 

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