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Polymer Science and Engineering: The Shifting Research Frontiers
being to tailor orientational and transitional states to the specific end use desired. Emerging problems include achieving sufficient density of active species to produce materials with competitive figures of merit (i.e., dipole concentration for nonlinear optical applications) and balancing mesogenicity effects with high and stable use temperatures. Clearly, the introduction of mesogenicity into polymers opens vast possibilities for molecular design, which may ultimately lead to the creation of materials with highly specific and unique property sets.
Films, Membranes, and Coatings
Polymers are used in many applications in which their main function is to regulate the migration of small molecules or ions from one region to another. Examples include containers whose walls must keep oxygen outside or carbon dioxide and water inside; coatings that protect substrates from water, oxygen, and salts; packaging films to protect foodstuffs from contamination, oxidation, or dehydration; so-called "smart packages," which allow vegetables to respire by balancing both oxygen and carbon dioxide transmission so that they remain fresh for long storage or shipping times; thin films for controlled delivery of drugs, fertilizers, herbicides, and so on; and ultrathin membranes for separation of fluid mixtures. These diverse functions can be achieved partly because the permeability to small molecules via a solution-diffusion mechanism can be varied over enormous ranges by manipulation of the molecular and physical structure of the polymer. The polymer that has the lowest known permeability to gases is bonedry poly(vinyl alcohol), while the recently discovered poly(trimethylsilyl propyne) is the most permeable polymer known to date. The span between these limits for oxygen gas is a factor of 1010. A variety of factors, including free volume, intermolecular forces, chain stiffness, and mobility, act together to cause this enormous range of transport behavior. Recent experimental work has provided a great deal of insight, while attempts to simulate the diffusional process using molecular mechanics are at a very primitive stage. There is clearly a need for guidance in molecular design of polymers for each of the types of applications described in more detail below. In addition, innovations in processing are needed.
As shown earlier, packaging applications currently consume roughly one-third of the production of thermoplastic polymers for fabrication of a wide array of rigid and flexible package designs (see Figure 3.2). These packages must have a variety of attributes, but one of the most important is to keep contaminants, especially oxygen, out, while critical contents such as carbon dioxide, flavors, and moisture are kept inside. Metals and glass are usually almost perfect barriers, whereas polymers always have a finite permeability, which can limit