Strain compatibilization

Strain compatibilization at low, steady state stress was considered by Lyngaae-J0rgensen [1985] ... 

Strain compatibilization Strain demixing Strain hardening, SH Strain softening, SS... 

The strain compatibilization of LCST blends was found to be similar [4] to that reported by Lyngaae-Jmgensen [237] for the styrene-butadiene-styrene (SBS) block copolymer that separates with the upper critical solution temperature, UCST. Thus, low, steady state shearing shifted the spinodal temperature Ts(y)—Ts(0) < 29 °C. 

With increasing strain the pure PP exhibits a moderate monotonous broadening. The MMT samples start with a slight homogenization of the stacks up to e >= 0.07 that is followed by a distinct loss of uniformity when the materials are above the yield. At high strain, compatibilization (PP-i-lcMMT, PP-i-hcMMT) even further attenuates the distortion introduced by MMT. The asymmetry of hca(r i = 0, r3> is not considered and cannot be quantified from a peak fit that is based on a second-order polynomial only. 

The strength can also be increased by using about the same amounts of the two polymers so that they form two continuous phases. Here, both phases can assist the blend to be strong. Another approach is to use compatibilizers. Compatibilizers are materials that help bind together the phases allowing stress or strain to be shared between the two phases. Many compatibilizers are block copolymers where one block is derived from polymers of one phase and the second block is composed of units derived from polymers of the second phase. The two blocks get locked into the structures of the like phases and thus serve to connect the two phases. 

Graft copolymers are also used as compatibilizers to tie together different phases. HIPS contains PS grafted onto polybutadiene backbones. This allows stress or strain to be transferred from the PS to the polybutadiene phase transferring energy that might break the brittle PS to the more flexible polybutadiene phase. That is why HIPS is stronger than PS itself. 

Figure 5.15. MFC can be obtained from incompatible polymer blends by extrusion and orientation (the fibrillization step) followed by thermal treatment at a temperature between the melting points of the two components at constant strain (the isotropization step). The block copolymers formed during the isotropization (in the case of condensation polymers) play the role of a self-compatibilizer. Prolonged annealing transforms the matrix into a block and thereafter into a random copolymer (a) an MFC on the macro level, (b) an MFC on the micro (molecular) level (Fakirov Evstatiev, 1994).

Blends of immiscible polymers exhibit a coarse and unstable phase morphology with poor interfacial adhesion. The ultimate properties of these blends are often poorer than those of either component. The poor mechanical properties can be improved with a small amount of an interfacial agent that lowers interfacial tension in the melt and enhances interfacial adhesion in the solid. High-strain properties, such as strength, tensile elongation, and impact strength, especially benefit from compatibilization (I, 2). 

High-strain properties of polymer blends, such as strength, tensile elongation, and impact strength, benefit from compatibilization. These... 

Figure 1. Effect of 5 wt% compatibilizer on the stress-strain curve of a blend with equal volumes of LLDPE and PS.

The general shape of the stress-strain curve is the same for all the blends with Q-series and Kraton compatibilizers. This shape is described by two tangent lines drawn from the initial elastic region and the plastic region, respectively. The intersection of the lines is defined as the yield point and is described by a yield stress (ay) and an apparent yield strain (ey). The stress (a) and strain (e) in the plastic region are related by... 

Figure 4. Schematic of the stress-strain behavior of a compatibilized blend with good adhesion.

Assuming further that the matrix has the same effective yield strain as LLDPE (e° = 2.4%), the yield strain of the compatibilized blends is given by... 

Measured Calculated Yielding Compatibilizer Yield Stress, Modulusa Yield Strain, Yield Strain, Angle,... 

Fracture Stress and Strain. Yielding and plastic deformation in the schematic representation of tensile deformation were associated with microfibrillation at the interface and stretching of the microfibrils. Because this representation was assumed to apply to both the core-shell and interconnected-interface models of compatibilization, the constrained-yielding approach was used without specific reference to the microstructure of the interface. In extending the discussion to fracture, however, it is useful to consider the interfacial-deformation mechanisms. Tensile deformation culminated in catastrophic fracture when the microfibrillated interface failed. This was inferred from the quasi-brittle fracture behavior of the uncompatibilized blend with VPS of 0.5, which indicated that the reduced load-bearing cross section after interfacial debonding could not support plastic deformation. Accordingly, the ultimate properties of the compatibilized blend depended on interfacial char-... 

Microfibrils in the blend compatibilized with Kraton G probably formed by drawing of the rubbery shell of the core-shell particle. Important factors would have been the amount of rubber in the shell, the strength of the rubber, and the strength of adhesion to LLDPE. All these factors may have contributed in some degree to the high fracture stress and strain of the blend with Kraton G. The amount of compatibilizer in the shell differed for the various Kratons the thicker coating was certainly one of the reasons Kraton G gave better properties to the compatibilized blend than the Kraton D compatibiliz-ers. 

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