Rubbery domains

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

Attractive interactions are also the reason for the self-assembly of PS-fo-PB-fo-PMMA at the interface of poly(styrene-co-acrylonitrile), SAN, and poly(2,6-dimethylphenylene ether), PPE. In this blend, PS and PPE are miscible on one side and PMMA and SAN are miscible on the other one, with negative / parameters. This blend, in which the rubbery domain is located at the interface between SAN/PMMA and PPE/PS, was originally prepared by coprecipitation of all components from a common solution [195]. From a processing point of view, in this system the difficulty was to get the dispersion of PPE in SAN via melt mixing of SAN, PPE and the triblock terpolymer. 

Figure 10.1 Time-temperature map. Shape of main boundaries for linear or network polymers. (I) Glassy brittle domain B, ductile-brittle transition. (II) Glassy ductile domain G, glass transition. (Ill) Rubbery domain. The location of the boundaries depends on the polymer structure but their shape is always the same. Typical limits for coordinates are 0-700 K for temperature and 10-3 s. (fast impact) to 1010 s e.g., 30 years static loading in civil engineering or building structures. Fpr dynamic loading, t would be the reciprocal of frequency. For monotone loading, it could be the reciprocal of strain rate s = dl/ Idt.

For epoxy networks modified by liquid reactive rubbers, it is not so easy to discuss these parameters separately, because they are interdependent. For example, an increase in the acrylonitrile content of the carboxy-termi-nated butadiene acrylonitrile rubber (CTBN) induces a size reduction of the rubbery domains but also a higher miscibility with the epoxy-rich phase, leading to a higher amount remaining dissolved in the matrix at the end of cure (Chapter 8). It is not possible to separate the influence of these two effects on toughness. 

If the hard blocks are longer than the soft ones, such as in SBS with a high styrene content, the hard phase will be continuous, and the rubbery phase is present as domains (see Figure 9.6). In such a case SBS behaves as a high-impact PS. Another example of this type is a PP/EP block copolymer tails of EP (random copolymer of ethylene and propylene) on the PP chains segregate into rubbery domains in the PP matrix, which improve the impact strength. 

The presence of hard and soft domains in segmented polyurethanes also has been confirmed by experimental results using pulsed NMR and low-frequency dielectric measurements. Assink (55) recently has shown that the nuclear-magnetic, free-induction decay of these thermoplastic elastomers consists of a fast Gaussian component attributable to the glassy hard domains and a slow exponential component associated with the rubbery domains. Furthermore, the NMR technique also can be used to determine the relative amounts of material in each domain. 

Epoxy resins are toughened with functionally terminated liquid polymers. Small rubbery domains of a definite size and shape are formed in situ during cure. The selectivity and reactivity of the functional groups of liquid polymers are critical for the in situ formation of rubber particles. 

We demonstrated previously that the elastic character of the rubbery domain is an important factor (4) in toughening epoxy resins. However the moduli of these liquid polymers when cured with a stoichiometric amount of epoxy resin (Epon 828) all have similar values (2.8-4.1 MPa at 300-500% elongation). 

The initial solubility of a reactive liquid polymer in epoxy resins before cure is a critical condition not only for chemical reactions but also for the in situ formation of rubbery domains. A good initial solubility... 

In all block copolymers investigated, the principal morphological wave length of the rubbery domains, i.e. sizes or mean spacings, is too short to concentrate the stress in a large enough volume element to nucleate crazes in the majority phase of PS. Furthermore, with the exception of perhaps the pure KRO-3 Resin with lamellar morphology, the combination of thermal stresses and the levels of applied stress are also insufficient to cavitate the rubbery domains without any further assist from more macro stress concentrations. Hence the available evidence indicates that in these polymers, crazes initiate entirely from surface stress concentrations. 

Once crazes have developed, however, their growth is governed by the repeated cavitation of the rubbery domains, and the developments of Section 2.3 apply. The velocity of the craze is given by... 

Figure 12.14 Transmission electron micrographs (varying magnification as indicated by scale bars) of (a) TPO-0 (Owt% clay), (b) TPO-1 (0.6 wt% clay), (c) TPO-3 (2.3 wt% clay), and (d) TPO-6 (5.6 wt% clay). EPR-rich rubbery domains (elliptically shaped) surrounded by clay platelets (dark rodlike structures) are clearly seen. (From Reference 42 with permission from John Wiley Sons, Inc.)...

Figure 19 Transmission electron micrographs of thin sections cut from ABS materials. The rubbery domains appear darker, (a) An ABS material prepared by bulk polymerization showing the characteristic salami-like morphology of the toughening particl which contain several SAN domains within the rubber, (b) ABS materials prepared by emulsion polymerization showing cote-shell partides (i) widi and (ii) widiout subinclusions. (Reproduced with permission from ref. 29.)...

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