Polymer network homogeneous

Even if completely homogeneous and disordered in the relaxed state, a real network differs from the ideal network, defined in Chapter I. Three types of network defects are commonly considered to be present in polymer networks unreacted functionalities, closed loops, and permanent chain entanglements. Within each group there are several possibilities dependent on the arrangement of chains the effect of defects on the elastic properties of the network is thus by no means simple, as has been stressed e.g. by Case (28). Several possible arrangements are shown in Fig. 1, where only nearest neighbour defect structures have been drawn. 

Other transitions such as degradation and phase separation may be also observed during the formation of the polymer network. Degradation is usually present when high temperatures are needed to get the maximum possible conversion. Phase separation may take place when the monomers are blended with a rubber or a thermoplastic, to generate rubber-modified or thermoplastic-modified polymer networks. In these cases, formulations are initially homogeneous but phase-separate during the polymerization reaction. This process is discussed in Chapter 8. 

The structure of precursors, the number of functional groups per precursor molecule, and the reaction path leading to the final network all play important roles in the final structure of the polymer network. Some thermosets can be considered homogeneous ideal networks relative to a reference state. It is usually the case when networks are prepared by step copolymerization of two monomers (epoxy-diamine or triol-diisocyanate reactions) at the stoichiometric ratio and at full conversion. 

This method applies for whatever polymer concentration. In practice, it is preferable to use high concentrations in order to increase the signal-to-noise ratio and therefore minimize counting time. However, it can also be applied to semidilute or even dilute solutions where Zimm plots are useful. It also applies not only to linear polymers but also to any form of chain architecture and to deuterated/protonated mixtures in non-solvent matrices such as polymer blends or polymer networks provided that changing the deuterated fraction does not change the homogeneous nature of the mixture (i.e., no change to the chain conformations and interactions). 

During precipitation polymerization, all ingredients are dissolved in a solvent (water) to form a homogeneous mixture in which initiation of polymerization takes place. The formed polymers are transformed into a collapsed state because the reaction temperature is far above VPTT (for example in the case of PNIPAAm) and become crosslinked by crosslinker molecules to form a colloidal polymer network or microgel. This technique has been widely used for the synthesis of thermosensitive PNIPAAm [30-35] and poly(/V-vinyl caprolactam) (PVCL) [36] microgels. 

The molecular model is confined to the case of weak adhesion (5 < 0) to ensure homogeneous contact [48, 65]. In this case, two sources contribute to the frictional stress of a gel elastic deformation of an adsorbing polymer chain Cei and the lubrication of the hydrated layer of the polymer network ffvis, which can be represented as follows (Fig. 12) ... 

Fig. 21 Scheme of nanocapsule formation. The template liposome with homogeneously distributed monomers is irradiated with UV light, resulting in the formation of a fortified liposome. After lipid removal, the 2D polymer network constitutes an intact hollow nanocapsule. Reprinted with permission from [112]. Copyright 2006, American Chemical Society... 

The microstructure of the green body as observed by SEM is shown in Fig. 4. After consolidation, powders in green body compact closely and homogeneously. However, the polymer network was not observed via SEM. The pore diameter distribution, obtained by Hg intrusion porosimetry, showed a monomodal distribution type. The relative density, porosity, and the median pore diameter of green samples were 57.67%, 35.02%, and 10.6nm, respectively. 

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