Network yielding mechanism

Compared with commercial high-impact PS (HIPS), the droplet structures exhibit improved toughness and are transparent. In such blends with PS content up to 40%, a size of the PS droplets below a critical characteristic value can be realized with the effect that the PS droplets can be strongly plastically deformed (see Fig. 3.69). The comparison with the network yielding mechanism (Section 52.6.2) reveals the similarity and the possibility to toughen materials other than PVC using this effect [6]. 

Examples of the synthesis of polysiloxane nanocomposites reported in the literature include Work by Ma et al (6) who modified montmorilIonite with short segments of PDMS and blended this into a polymer melt/solution to yield examples of fully exfoliated or intercalated PDMS/clay nanocomposites. Pan, Mark et al (7) synthesized well defined nano-fillers by reacting groups of four vinyl terminated POSS cages with a central siloxane core. These materials were subsequently chemically bonded into a PDMS network yielding a significant improvement in the mechanical properties of the polymer. 

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

A mechanistic argument yields this factor (1 — 2/f) in a simple way [24]. The N strands of the rubberlike network are not independent, since they are linked with their ends in the junctions. The sum of the forces operating at each junction has to cancel in order to guarantee the mechanical equilibrium of the network. The number C of junctions with functionality f is ... 

Table II shows data obtained by sequential hexanes and ethanol extractions of the desilylated networks. Attempts to extract the networks prior to desilylation failed because the samples swelled to such a great degree that they lost their mechanical integrity and disintegrated. The amount of hexanes extractable material is less than 3% in all cases which indicates high copolymerization yields. Ethanol extractables were not determined because the presence of the byproducts of the desilylation, which are in the ethanol, would have given artificially high readings.

Based on this physical view of the reaction dynamics, a very broad class of models can be constructed that yield qualitatively similar oscillations of the reaction probabilities. As shown in Fig. 40(b), a model based on Eckart barriers and constant non-adiabatic coupling to mimic H + D2, yields out-of-phase oscillations in Pr(0,0 — 0,j E) analogous to those observed in the full quantum scattering calculation. Note, however, that if the recoupling in the exit-channel is omitted (as shown in Fig. 40(b) with dashed lines) then oscillations disappear and Pr exhibits simple steps at the QBS energies. As the occurrence of the oscillation is quite insensitive to the details of the model, the interference of pathways through the network of QBS seems to provide a robust mechanism for the oscillating reaction probabilities. 

The first type, termed sequential IPN s, involves the preparation of a crosslinked polymer I, a subsequent swelling of monomer II components and polymerization of the monomer II in situ. The second type of synthesis yields materials known as simultaneous interpenetrating networks (SIN s), involves the mixing of all components in an early stage, followed by the formation of both networks via independent reactions proceeding in the same container (10,11). One network can be formed by a chain growth mechanism and the other by a step growth mechanism. 

The previous discussion has shown that the CIPS technique allows one to produce macroporous epoxy networks with either a narrow or bimodal size distribution. However, no indication has been given on the type of phase separation mechanism to yield these morphologies. As discussed earlier, the formation of a closed cell morphology can result either from a nucleation and growth mechanism or from spinodal decomposition. 

While the surface modification is not effective to suppress cavitation, Yee and coworkers performed an experiment to suppress the cavitation mechanically in a rubber-modified epoxy network. They applied hydrostatic pressure during mechanical testing of rubber toughened epoxies [160]. At pressures above BOSS MPa the rubber particles are unable to cavitate and consequently no massive shear yielding is observed, resulting in poor mechanical properties just like with the unmodified matrix. These experiments proved that cavitation is a necessary condition for effective toughening. 

Intramolecular reactions always accompany Intermolecular crossllnk-Ing. Their Intensity depends on the structure of the constituent units and very much on the reaction mechanism. Thus, if the network is built up by step reactions from low functionality components, cycllzatlon is relatively weak. On the contrary, chain vinyl-divinyl copolymerization yields highly cycllzed products just in the beginning of the polymerization especially if the concentration of the polyvinyl monomer is higher. This case will be briefly commented on later in this article. 

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