Polymer network systems Subject

The generalized concept for producing composite structures capable of controlled additive release properties involves 1) solution or dispersion of additives in reactive monomer/polymer systems, 2) subjecting the additive/monomer-polymer solution dispersion to radiation, and 3) formation of a crosslinked polymer network which encapsulates the specific agent (Figure 14). Typical monomer and crosslinking oligomers utilized in these types of studies are shown in Table III. The effects of... 

The term "supramolecular isomerism" was first used by Zaworotko to describe distinct forms of highly related coordination polymer materials. This is complicated by the observation that supramolecular isomerism for a given network system is commonly combined with a variation in guest solvent molecules within the extended structure. Variation of guest molecules within a framework does not, of course, define new supramolecular isomers of the framework if the latter is unchanged. In a recent review, Zaworotko et stated that supramolecular isomerism is closely related to the well-documented subject of polymorphism in crystalline solids." Zaworotko defined supramolecular isomerism in this context as "the existence of more than one type of network superstructure for the same molecular building blocks" and related the phenomenon "to structural isomerism at the molecular level."... 

The extension of Rouse s approach from linear chains to other polymer systems is quite straightforward and leads eventually to the concept of generalized Gaussian structures (GGS), which are the subject of this review. In the framework of the GGS approach, a polymer system is modeled as a collection of beads (subject to viscous friction), coimected to each other by means of elastic springs in a system-spedfic way. Initially, the GGS concept was inspired by the study of cross-linked polymer networks however, its applications have turned out to cover large classes of substances, such as dendritic polymers, hybrid polymers, and hierarchically-built structmes. 

Polymer networks which can memorize the orientational order of the nematic liquid crystal environment where they are assembled [71], [72], [73], [74] are particularly attractive because of their potential for a variety of electrooptic technologies. We postpone this subject to the last section and here concentrate our attention on the ordering and structures of these composite materials. These systems have many physical properties analogous to liquid crystals confined to different submicrometer-sized cavities [75], [76] and random porous matrices [77], [78], Large surface-to-volume ratios enable a strong influence of the polymer network on nematic ordering in the liquid crystalline solvent and thus govern optical properties of the composites. 

Figure 26.7 shows the chemical structures of an NLO chromophore (APAN) and an epoxy-based polymer (BPAZO) where NLO moieties are attached to the backbone [81]. Both the dye and the polymer are functionalized with thermally cross-linkable acryioyl groups. As the dye-doped polymer is subjected to heat as part of the simultaneous poling/curing process, the inter- and intramolecular cross-linking reactions occur simultaneously (Fig. 26.8). The 7g of the cross-linked polymer-dye network is lower than that of the undoped polymer network because of the plasticizing effect of the dissolved dye. However, the temporal stability at 100°C of the polymer-dye network is better than that of the undoped polymer network (Fig. 26.9) as a direct result of the increased cross-linking density in the cross-linked guest-host system. Therefore, the addition of the thermally cross-linkable NLO dye not only increases the...

In the preceding section, we have examined the frictional properties of semi-dilute solutions and gels and the analogy between the two systems. In this section, our interest is focused on the dynamics of concentration fluctuations of the polymer. Let us consider first the case of a gel. The polymer network fluctuating around its equilibrium position is subjected to two driving forces The osmotic force tends to equalize the concentration and the elastic force tends to keep the network in its position. The fluctuations are damped by the frictional force between the polymer network and the solvent. 

Silicone adhesives are generally applied in a liquid and uncured state. It is therefore the physical and chemical properties of the polymers, or more precisely of the polymer formulation, that guide the various processes leading to the formation of the cured silicone network. The choice of the cure system can be guided by a variety of parameters that includes cure time and temperature, rheological properties in relation with the application process, substrates, the environment the adhesive joints will be subjected to and its subsequent durability, and of course, cost. 

Because of the interaction of the two complicated and not well-understood fields, turbulent flow and non-Newtonian fluids, understanding of DR mechanism(s) is still quite limited. Cates and coworkers (for example, Refs. " ) and a number of other investigators have done theoretical studies of the dynamics of self-assemblies of worm-like micelles. Because these so-called living polymers are subject to reversible scission and recombination, their relaxation behavior differs from reptating polymer chains. An additional form of stress relaxation is provided by continuous breaking and repair of the micellar chains. Thus, stress relaxation in micellar networks occurs through a combination of reptation and breaking. For rapid scission kinetics, linear viscoelastic (Maxwell) behavior is predicted and is observed for some surfactant systems at low frequencies. In many cationic surfactant systems, however, the observed behavior in Cole-Cole plots does not fit the Maxwell model. 

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