They include describing the basic thermodynamics and kinetics of formation of the mesomorphic phase, the nature of defects (disclinations), the chain conformations and intermolecular packing in mesomorphic states, the dependence on processing history, and heterogeneities at large spatial scales. In particular, molecular descriptions are required that can explain and predict the influence of monomer structure, copolymer sequence, and interactions between chains.

Polymer Blends

Multicomponent polymer systems, such as polymer mixtures (blends), provide a new approach to the development of novel materials. These materials enable properties to be tailored, without resorting to costly new synthetic routes and also without the problems of proof of environmental and health safety entailed in new syntheses. Many important polymer properties, such as toughness, impact strength, heat and solvent resistance, and fatigue, have thus been improved significantly by blending polymers with different properties, and this has enabled many new commercial applications. Despite these advances, questions remain unanswered concerning predictions of miscibility and phase separation, control of morphology, and the structures of the interfaces between domains of different phases. Unlike small molecules, the mixing of two polymers is accompanied by only a small gain in entropy, and hence miscibility is more the exception than the rule. Because many of these materials must be processed in the liquid state, it is important to understand those features governing the phase diagrams of polymer liquid mixtures, that is, those factors determining whether the system is homogeneous or phase separated at a given temperature, pressure, composition, and so on. Traditionally, theories of this phase behavior have been based on simple models, but such models have not been fully satisfactory in relating the thermodynamic behavior of liquid polymer mixtures to the detailed chemical structures and interactions of their constituents; this is necessary for the molecular design of novel composite materials.

Newly emerging theories are establishing the much-needed link between phase behavior and molecular driving forces, through generalization of the classic lattice model of polymer solutions and through generalization, to polymers, of integral equation methods that have traditionally been applied to small-molecule liquids. A growing body of theoretical and experimental information suggests an alteration of chain dimensions in polymer blends, and further investigation is desirable. More difficult problems are those dealing with composite systems with mixtures of different phases, such as mixtures of crystalline with amorphous, liquid crystalline (stiff chains) with amorphous (flexible chains), and so on. For example, a challenging problem occurs when both amorphous phase separation and crystallization occur in the same system. There is coupling of the kinetics of the two processes, whereby the resulting morphology and



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