Elastomer blends thermodynamics

The majority of elastomer blends are phase separated, but of interest, as crosslinking can achieve mechanical compatibilization due to crosslinking between the phases as noted in Chapter 3. This compatibilization method can lead to unique and useful blends with a compromise in properties, offering useful commercial products as well illustrated by the applications in tire construction. The unsaturated hydrocarbon elastomers without polar functional groups are rarely miscible with each other, because no specific interactions are present to achieve the necessary thermodynamic driving force for miscibility. The few miscible examples noted generally exhibit matched solubility parameters. 

Tfrom thermodynamic considerations (2) supported by microscopic ob-servations (5), two different high molecular weight polymers when blended exist in a heterogeneous state. In the case of elastomers capable of crosslinking these separate phases may crosslink in the presence of one another. However, the question arises, does bonding exist across the interfaces ... 

Although two dissimilar elastomers—e.g., chlorinated butyl rubber and polybutadiene—may crosslink when in contact with one another, does bonding exist between the two interfaces Based upon thermodynamic theory as well as microscopic observations, we know that two such elastomers are not molecularly dispersed in a blend, so the diagnostic problem is one of considering two dispersed phases. 

SBM) as a compatibilizer. As a result of the particular thermodynamic interaction between the relevant blocks and the blend components, a discontinuous and nanoscale distribution of the elastomer at the interface, the so-called raspberry morphology, is observed (Fig. 15). Similar morphologies have also been observed when using triblock terpolymers with hydrogenated middle blocks (polystyrene-W<9ck-poly(ethylene-C0-butylene)-Wock-poly(methyl methacrylate), SEBM). It is this discontinuous interfacial coverage by the elastomer as compared to a continuous layer which allows one to minimize the loss in modulus and to ensure toughening of the PPE/SAN blend [69],... 

Intense commercial and academic interest in block copolymers developed during the 1960s and continues today. These materials attract the attention of industry because of their potential for application as thermoplastic elastomers, tough plastics, compatibilizing agents for polymer blends, agents for surface and interface mo dification, polymer micelles, etc. Academic interest arises, primarily, from the use of these materials as model copolymer systems where effects of thermodynamic incompatibility of the two (or more) components on properties in bulk and solution can be probed. The synthesis, characterization, and properties of classical linear block copolymers (AB diblocks, ABA triblocks, and segmented (AB)n systems) have been well documented in a number of books and reviews [1-7] and will not be discussed herein except for the sake of comparison. 

Miscible blends of elastomers differ from corresponding blends of thermoplastics in two important areas. First, the need for elastic properties requires elastomers to be high molecular weight. This reduces both the kinetic rate and the thermodynamic driving force for the interdiffusion and thus formation of a miscible single phase of dissimilar elastomers. Second, elastomers are plasticized in conventional compounding with process oils. The presence of plasticizers leads to both a higher free volume for the blend components and a decrease of the endothermal interactions. 

The extension of thermodynamics to a blend of elastomers has been discussed by Roland [4], Miscible blends are most commonly formed from elastomers with similar three-dimensional [7] solubility parameters. An example of this is blends from copolymer elastomers (e.g., ethylene-propylene or styrene-butadiene copolymers) from component polymers of different composition, microstructure, and molecular weights. When the forces between the components of the polymer blend are mostly entirely dispersive, miscibiUty is only achieved in neat polymers with a very close match in Hansen s three-dimensional solubility parameter [7]. 

The recent proliferation of metallocene-based polyolefins and polyolefin elastomers have gained their popularity owing to their density, cost, and ease of processabUity. PVC/POE blends have therefore been investigated as flexible PVC compounds. However, these blends are thermodynamically immiscible and needed suitable compatibiUzers such as the chlorinated polyethylenes (Eastman and Dadmun 2002). Since they are not miscible, POEs do not lower the PVC modulus sufficiently unless some plasticizer or a compatible elastomer such as EPE is also added. Commercially, some PVC/POE alloys are offered by TeknorApex under Flexalloy trade name with a shore A hardness 40-60 and brittle points down to —50 °C. They are claimed to have excellent low-temperature toughness, flexibility, compression set-resistance, and oil resistance. Suitable applications include automotive hoses, seals, gaskets, wire jacketing, etc. 

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. 

Polymer blends are a mixture of at least two polymers, their combination being supposed to lead to new materials with different properties. The classification of polymer blends into (1) immiscible polymer blends, (2) compatible polymer blends, and (3) miscible polymer blends is given by the thermodynamic properties of the resulting compound by means of the number of glass transition temperatures observed for the final product. To improve the compatibility between the blended polymers, some additives or fillers are used. To the same extent, rubber blends are mixtures of elastomers, which are usually combined to obtain an improved product, with properties derived from each individual component. 

NR is normally blended with ethylene-propylene-diene rubber (EPDM) to improve the ageing resistance of the former without losing its good mechanical properties. However, due to the difference in unsaturation level between these components, a mutual incompatibility can exist, which decreases the mechanical performance. In addition to the poor interfacial adhesion caused by the thermodynamic incompatibility, these blends usually present cure rate incompatibility because of the differences between the reactivity of the elastomers with the curing agents and/or differences in solubilities of the curatives in each phase. In the case of NR/EPDM blends, the curing system can be consumed by the vulcanization of the NR phase, which is more rapidly vulcanizable because of the higher unsaturation level. ... 

Poly(ethylene-octene) Copolymer/PLA Blends Poly(ethylene-octene) copolymer, a thermoplastic polyolefin elastomer (TPO), was melt blended with PLA at a ratio of 20/ 80 wt% [10]. The difference in polarities of the two polymers led to thermodynamic immiscibility and phase separation of the final blends as determined by the Molau test. Therefore, a copolymer of TPO-gra//-PLA (TPO-PLA synthesized via functionalization of TPO and MA with benzoyl peroxide (BPO), followed by esterification of the MA-functionalized TPO (TPO-MAH) with PLA using 4-dimethylaminopyridine (DMAP) as a catalyst) was introduced to improve the compatibility of the TPO/PLA blends. The use of 5 wt% TPO-... 

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