Morphology microheterogeneous

Fig. 17. The development of a single broad transition spanning the transition range of both polyrners can be accomplished through extensive but incomplete mixing on the 100 A scale, resulting in a microheterogeneous morphology.

Another important aspect of developing the microheterogeneous morphology is the kinetics of phase separation. If the polymers gel before phase separation takes place, then the crosslinks will restrict phase... 

Most of the IPNs and SINs so formed are phase separated, as are most polymer blends. For IPNs and SINs, however, if gelation of both networks precedes phase separation, the crosslinks will tend to hold the phases together and limit their sizes, for.ming a microheterogeneous morphology. However, if phase separation precedes gelation, then the crosslinks will tend to keep the phases apart, and gross phase separation will occur with concomitant loss of physical and mechanical properties. 

Thermoplastic elastomeric behavior requires that the block copolymer develop a microheterogeneous two-phase network morphology. Theory predicts that microphase separation will occur at shorter block lengths as the polarity difference between the A and B blocks increases. This prediction is borne out as the block lengths required for the polyether-polyurethane, polyester-polyurethane, and polyether-polyester multiblock copolymers to exhibit thermoplastic elastomeric behavior are considerably shorter than for the styrene-diene-styrene triblock copolymers. 

Multicomponent polymeric materials with microheterogeneous mophologies include a number of polymer blends and block copolymers, however, an especially easy way to bring about the desired morphology is through interpenetrating polymer networks. Several papers in the symposium are concerned with IPN s and related materials. 

Phase segregation and formation of a microheterogeneous structure, if it occurs, affects the kinetics of curing. Polymerization reactions leading to the formation of crosslinked structures are not reactions of isolated macromolecules and, therefore, cannot be considered without taking into account the morphology of the reactive system. 

Crosslinking occurs by a physical process in block copolymers which possess a microheterogeneous, two-phase network morphology. The two important types of block copolymers are the triblock, ABA, and multiblock (AB) copolymers (III and IV, respectively) where A and B... 

During simultaneous polymerization of organic epoxy-amine and silica-siloxane systems, the microphase separation takes place. Relative rates of structure growth resulting in gelation of the hybrid and of microphase separation determine the morphology of the microheterogeneous hybrid. 

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