ScienceWeek

SYMPOSIUM: POLYMERS, LIQUID CRYSTALS, AND COLLOIDS

1. INTRODUCTION

ON SELF-ORGANIZING POLYMERS

Despite much recent publicity concerning potential applications of new inorganic materials in nanotechnology and optoelectronics, many chemists believe that self-organizing organic polymers hold the greatest promise for future important discoveries and applications. Writing in a recent review, Paul Calvert of the University of Arizona (US) pointed out that "the known polymers are only a small set from a vast array of possible chain molecules. In fact, we are mostly interested in small, simple chain units and don't really expect behavior outside the envelope we know. Sometimes, such as with Kevlar, we are surprised."

Kevlar (poly-para-phenylene terephthalamide), the miracle polymer that came into existence 30 years ago, is five times stronger per unit weight than steel. It is used in bullet-proof vests, belts for radial tires, cables, reinforced composites for aircraft panels and boat hulls, flame-resistant garments, and sports equipment. Chemically, Kevlar is an "aramid polymer", which means an aromatic nylon (a nylon containing benzene rings). It is also a "liquid-crystalline" polymer.

Liquid crystals can be considered a 4th phase of matter, a state qualitatively different from the ordinary 3 phases, gas, liquid, and solid. Liquid crystals flow like a liquid, but there is order in at least one dimension in the arrangement of the molecules. "Nematic crystals" are liquid crystals with long molecules all aligned in the same direction. "Cholesteric" and "smectic" liquid crystals have molecules arranged in distinct layers: in cholesteric crystals, the axes of the molecules are parallel to the plane of the layers; in smectic crystals, the axes of the molecules are perpendicular to the plane of the layers. A liquid-crystal polymer is a polymer with a self-organized liquid crystal structure that combines strength with lightness.

The term "polymer" derives from the Greek "polymeros" and means "many parts". The individual units of polymers are called "monomers", and most common polymers are composed of regular repetitions of one or more monomers. There is no formal restriction on the composition of a polymer: for example, asbestos is an inorganic polymer. Although polymeric substances have been known and used for thousands of years, the first polymeric materials to attract recorded scientific interest were silk and cobwebs: in 1665 Robert Hooke (1635-1703) suggested that the products of the silkworm and spider could be imitated by drawing a suitable glue-like substance out into a thread. This is essentially the process used today in industry to manufacture synthetic polymer fibers such as rayon.

The explosive growth of the plastics industry in the 19th and 20th centuries was based on two developments: the discovery of the principles of polymer chemistry and the extraction of the monomers required for polymer synthesis as products of petroleum refining. It was not until the 1920s that it was understood that the various plastics being produced in industry consisted of linear molecular chains rather than disorderly conglomerates of small molecules.

A simple linear polymer is a chain molecule composed of monomers with two reactive sites (bifunctional monomers), with monofunctional terminal units. If more than one bifunctional monomer is present, the chain is known as a "copolymer". A copolymer in which a number of units of the same monomer are located adjacent to one another (in "blocks" of monomers) is called a "block copolymer". A "diblock copolymer" is composed of two types of monomers (e.g., A and B), and may be depicted thus: AAAAAABBBBBAAAAAABBBBBAAAAAAA.

CHEMISTRY OF COLLOIDS

Chemists in the 19th century recognized the existence of systems with unusual behaviors: for example, hydrated alumina, starch, dextrin, and gelatin. Wolfgang Ostwald (1883-1943) grouped these substances into a separate category and coined the term "colloids" (meaning "glue-like"). Thomas Graham (1805-1869) had demonstrated that the constituent entities had low mobility, and he had been convinced they were aggregates of smaller molecules. Colloids could be metal particles, such as the gold colloids made by Michael Faraday (1791-1867); they could also be oxides or organic systems.

The early concept of colloids was vague, at first even counterproductive, and colloid science delayed the birth of polymer science. Rubber vulcanization, for example, was claimed not to be a chemical reaction (now called "crosslinking") but rather a modification of an aggregated state. Ultimately, due primarily to Hermann Staudinger (1881-1965), polymers were clearly identified and removed from the colloid group.

But colloids are clearly important: white paint, for example, is a colloid based on titanium oxide and water. Most foods, cosmetics, and inks consist of colloidal structures. Colloids comprise soap micelles, emulsions, suspensions of solids, (with sizes from microns to nanometers), and latex particles whose dimensions can be tuned in composition and size. A common feature of colloids is that the particles are floating in a solvent; at low enough concentrations, a colloid can flow freely, which is of considerable practical importance.

Adapted from: P-G. de Gennes: Nature 2001 412:385

ON COLLOIDS

A colloid is basically a system of particles 1 to 1000 nanometers in diameter dispersed in another phase, and such systems, particularly systems of electrically charged colloids, have important practical significance and are also of considerable theoretical interest. The existence of long-range attractive (as opposed to the expected repulsive) electrostatic forces between particles of like charge is one of the current major controversies of colloid science. The established classical theory of colloidal interactions predicts that an isolated pair of like-charged colloidal spheres in an electrolyte should experience a purely repulsive screened electrostatic (coulombic) interaction. Direct measurements of such interactions have shown quantitative agreement with the classical theory, but recent experiments have provided evidence that the effective interparticle potential can have a long-range attractive component in more concentrated suspensions and for particles confined by charged glass walls. This long range attraction in concentrated systems is apparently due to multi-body interactions. Theoretical explanations have been proposed but remain the subject of controversy.

Note: In this context, "screening" is a reduction of the effective electric field at a point, the reduction due to the space charge of ambient charged particles of sign opposite to the source of the field.

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