Stress polyblends

At the present time it is generally accepted that the toughening effect is associated with the crazing behaviour.Because of the presence of the low-modulus rubber particles most of the loading caused when a polyblend is subject to mechanical stress is taken up by the rigid phase (at least up to the moment of... 

When a craze occurs around a rubber droplet the droplet is stressed not only in a direction parallel to the applied stress but also in the plane of the craze perpendicular to the applied stress (see Figure 3.9). Such a triaxial stress leading to dilation of the particle would be resisted by the high bulk modulus of the rubber, which would thus become load bearing. The fracture initiation stress of a polyblend should not therefore be substantially different from that of a glass. 

The mechanical properties of two-phase polymeric systems, such as block and graft polymers and polyblends, are discussed in detail in Chapter 7. However, the creep and stress-relaxation behavior of these materials will be examined at this point. Most of the systems of practical interest consist of a combination of a rubbery phase and a rigid phase. In many cases the rigid phase is polystyrene since such materials are tough, yet low in price. 

N. Polyblends, block, and graft polymers IJ. Brittle Fracture and Stress Concentrators... 

The superiority of the internally plasticized copolymer is clearly evident if we plot volume resistivity against a measure of plasticization such as tensile stress at 100% elongation (Figure 1). Here we can see that the best way to increase softness and flexibility, without loss of volume resistivity, is by copolymerization with 2-ethvlhexvl acrylate. Polyblending with nitrile rubber provides only little advantage... 

Dynamic and Stress-Optical Properties of Polyblends of Butadiene—Styrene Copolymers Differing in Composition... 

Stress Birefringence. Confirmation of the multiphase nature of the polyblends described may be obtained by stress-optical measurements. Since this technique apparently has not been used before to demonstrate incompatibility in polyblends, a brief description is given of the rationale behind the method. 

Here n is the average refractive index, k is Boltzman s constant, and T is absolute temperature (13). If a polyblend were to form a homogeneous network, the stress would be distributed equally between network chains of different composition. Assuming that the size of the statistical segments of the component polymers remains unaffected by the mixing process, the stress-optical coefficient would simply be additive by composition. Since the stress-optical coefficient of butadiene-styrene copolymers, at constant vinyl content, is a linear function of composition (Figure 9), a homogeneous blend of such polymers would be expected to exhibit the same stress-optical coefficient as a copolymer of the same styrene content. Actually, all blends examined show an elevation of Ka which increases with the breadth of the composition distribution (Table III). Such an elevation can be justified if the blends have a two- or multiphase domain structure in which the phases differ in modulus. If we consider the domains to be coupled either in series or in parallel (the true situation will be intermediate), then it is easily shown that... 

The basic issue confronting the designer of polymer blend systems is how to guarantee good stress transfer between the components of the multicomponent system. Only in this way can the component s physical properties be efficiently used to give blends with the desired properties. One approach is to find blend systems that form miscible amorphous phases. In polyblends of this type, the various components have the thermodynamic potential for being mixed at the molecular level and the interactions between unlike components are quite strong. Since these systems form only one miscible amorphous phase, interphase stress transfer is not an issue and the physical properties of miscible blends approach and frequently exceed those expected for a random copolymer comprised of the same chemical constituents. 

When the polymer components in a blend are less miscible, phase separation will form larger domains with weaker interfacial bonding between them. The interfaces will therefore fail under stress and properties of polyblends are thus likely to be poorer than for either of the polymers in the blend. U-shaped property curves (Figure 4.40c) thus provide a strong indication of immiscibility. In most cases they also signify practical incompatibility, and hence lack of practical utility. 

Attention will now be turned to two important experimental aspects of glass transition behavior the temperature dependence of the modulus, and stress-relaxation studies. This will be followed by a brief discussion of mathematical models that describe polyblend glass transition behavior. 

Figure 2.8. Stress-relaxation data for a PVAc/PMMA 50/50 polyblend. The PMMA portion was prepared from a lightly crosslinked latex to suppress flow at high temperatures. Numbers at right are temperatures in °C. (Takayanagi et ai, 1963.)...

Figure 3.18. Tensile stress-strain curves at 23°C for (a) 70/30 SAN resin and (b) a mechanical blend of 70/30 SAN with 25 wt % NBR. Although the final breaking stress is lower, the polyblend requires much more work to break than the simple SAN copolymer. (Bergen, 1968.)...

In multiphase polyblends, a critical factor is the interface between the phases. If the two polymers reject each other and separate into phases, they are likely to reject each other at the interface as well. Such a weak interface will fail under stress, and most properties will suffer. Thus, most polymer blends are practically incompatible. Yef most snccessful commercial polyblends are multiphase systems. This means that there mnst he a mechanism to strengthen the interface. 

Based on the assumptions made during the derivation of Eq. (8.24), it is imperative to choose Mn(T, 0) > MF1(T, < >2). In the case of filled systems, this condition is naturally satisfied when the polymeric matrix is taken as the reference medium. For polyblends, a deliberate choice of the reference medium would have to be made so that the MFI value of the reference medium would be greater than that of the multiphase system. Eq. (8.84) predicts that a plot of 1/log Otm versus 1/ >2 should be linear, and the propriety of this model has been examined quantitatively in the light of the reported experimental data. Existing viscosity data in the literature available for all multicomponent systems is in the form of viscosity versus shear rate or shear stress versus shear rate curves. In each case, the data are transformed into specific MFI values using Eqs. (4.7) and (4.8) and the specified load condition for each system based on ASTM D1238. 

Low-modulus elastomers may be added to compati-bilize multi-phase polyblends whose weak interfaces suffer from brittle failure. The elastomer forms a soft rubbery interphase which cushions mechanical or thermal stress and thus reduces brittleness. 

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