Thermoplastic elastomers phase separation, thermodynamics

Commercial BCs are prepared from monomers that, upon polymerization, yield immiscible macromolecular blocks, one rigid and the other flexible, that separate into a two-phase system with rigid and soft domains. The concentration and molecular weights provide control of the size of the separated domains, thus the morphology and the interconnection between the domains. The existence of a dispersed rigid phase in an elastomeric matrix is responsible for its thermoplastic elastomer behavior. For symmetric block copolymers, Leibler (1980) showed that a sufficient condition for microphase separation is (XabN) = 10.5, where Xab is the binary thermodynamic interaction parameter and N is the degree of BC polymerization (Folkes 1985). 

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. 

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-... 

An important and desirable feature of polyurethanes and polyureas is the phase separation of the small isocyanate/chain extender blocks, which is possible provided the thermodynamics is favorable this means a high enough concentration and chain length of hard blocks. These hard blocks act as physical crosslinks at a temperature lower than their melting point, and thermoplastic elastomers (including elastic fibers) can therefore be obtained. 

Styrenic block copolymers and their compounds have been in widespread commercial use for many years, with many applications. With the latest technology, they have become particularly interesting as impact modifiers for plastics, both thermoplastics and thermosets. Most polymers are thermodynamically incompatible with others polymers and mixtures tend to separate into two phases, even when they are part of the same molecule, as in block copolymers. Poly(styrene-P-elastomer-P-styrene) copolymers, in which the elastomer is the main constituent, give a structure in which the polystyrene end-segments form separate spherical regions ( domains ) dispersed in a continuous phase. 

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