Immiscible polymer blends mechanical behavior

It should be noted that the Doi and Ohta theory predicts oifly an enhancement of viscosity, the so called emulsion-hke behavior that results in positive deviation from the log-additivity rule, PDB. However, the theory does not have a mechanism that may generate an opposite behavior that may result in a negative deviation from the log-additivity rule, NDB. The latter deviation has been reported for the viscosity vs. concentration dependencies of PET/PA-66 blends [Utracki et ah, 1982]. The NDB deviation was introduced into the viscosity-concentration dependence of immiscible polymer blends in the form of interlayer slip caused by steady-state shearing at large strains that modify the morphology [Utracki, 1991]. 

Generalized dynamic mechanical data for miscible polymer blends and immiscible polymer blends are illustrated in Fig. 5.4 for tan S and Fig. 5.5 for log modulus ( or G ). Generahzed tan S and log modulus data are also illustrated for the cases of partial miscibility and micro-heterogeneous behavior. The immiscible blends exhibit the glass transition temperatures of the unblended components. The partially miscible blends exhibit TgS shifted inward to reflect the increase (or decrease) in the Tg due to the incorporation of minor concentrations of the other polymer constituent as expected from the phase diagram. 

Studies from our group and elsewhere have shown that uncompatibiUzed immiscible polymer blends provide synergy of mechanical properties when the processing and compositional parameters are near optimum values I Such blends are said to be mechanic y grafted due to the intimate mechanical contact between the phases, nearly repUcating the behavior of chemically grafted or bonded stractures. Leclair and Favis observed impact strength improvement in uncompatibilized PC/HDPE when PC was functioned as the dispersed phase in a HDPE matrix . 

The major problem in the reprocessing of commingled plastics, and therefore of utmost importance for PAB s, is the immiscibility of most polymers so that poor mechanical properties develop when they are blended together. Ways to enhance the mechanical behavior of these mixtures include adding a compatibilizer, adding a mbbery phase to impact modify the blend, or adding a reinforcement to restore mechanical properties. 

One exception to the above mentioned trend in physical properties of immiscible blends is in the utilization of liquid crystal polymers as a reinforcement for a more flexible thermoplastic polymer. The fact that LCP s can act as reinforcing agents in a blend has led some workers to model the mechanical behavior of the blends using theories of composites. Thus, Dutta et al. (1990) showed that the moduli values of highly drawn melts that contain liquid crystal polymers can be treated effectively by a simple rule of mixtures. That is, the modulus of a blend is given by... 

Block polymers and polymer blends deserve now a great intere because of their multiphase character and their related properties. The thermodynamic immiscibility of the polymeric partners gives rise indeed to a phase separation, the extent of which controls the detailed morphology of the solid and ultimately its mechanical behavior. The advent of thermoplastic elastomers and high impact resins (HIPS or ABS type) illustrates the importance of the industrial developments that this type of materials can provide. In selective solvents, and depending on molecular structure, concentration and temperature, block polymers form micelles which influence the rheological behavior and control the morphology of the material. 

Basic problems associated with the equilibrium and interfacial behavior of polymers, compatibilization of immiscible components, phase structure development, and the methods of its investigation are described herein. Special attention is paid to mechanical properties of heterogeneous blends and their prediction. Commercially important types of polymer blends as well as the recycling of commingled plastic waste are briefly discussed. 

Rheology is a part of continuum mechanics that assumes continuity, homogeneity and isotropy. In multiphase systems, there is a discontinuity of material properties across the interface, a concentration gradient, and inter-dependence between the flow field and morphology. The flow behavior of blends is complex, caused by viscoelasticity of the phases, the viscosity ratio, A (that varies over a wide range), as well as diverse and variable morphology. To understand the flow behavior of polymer blends, it is beneficial to refer to simpler models — for miscible blends to solutions and mixtures of fractions, while for immiscible systems to emulsions, block copolymers, and suspensions [1,24]. 

The viscoelastic properties of polymer blends determined by dynamic mechanical analysis to yield E, E" and tand has been reviewed in Section 5.2. The modulus-temperature behavior of polymer blends is a strong function of the phase behavior. In Fig. 6.2, the generalized modulus-temperature behavior of miscible versus immiscible blends is compared for the case of two amorphous polymers with different glass transition temperatures. The phase separated blend exhibits a modulus plateau between the TgS of the components with the plateau position dependent upon the composition. The miscible blends show single Tg behavior, with the Tg position dependent upon the composition. 

There have been many investigations of the mechanical properties of binary blends. If the two polymers are miscible, the mechanical behavior of the blend is generally intermediate between the two components. If the structures of the two polymers are different and they are immiscible, the mechanical behavior is usually found to be significantly decreased. 

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