difficulties are caused by the highly viscous nature of polymer melts, which makes it hard to know whether the equilibrium shape has been obtained. It is particularly difficult to measure the interfacial tension between phases in a polymer melt. One promising approach involves measuring the retraction and break-up into droplets of a fiber of one phase in a matrix of another, but more attention should be devoted to this problem because the interfacial tension helps determine the morphology of melt-processed polymer multiphase blends.

The atomic force microscope (AFM) has already had an impact on the measurement of the surface topology of polymers. It seems likely, however, that the AFM can be suitably modified to allow measurement of very local surface properties as well as mechanical properties of polymers. One example is the lateral force microscope (LFM), an AFM modified so it can measure lateral forces as well as normal forces. With the LFM, it is possible to reveal the morphology of a phase-separated blend of hydrocarbon elastomers simply by scanning a microtomed surface. Another, more speculative, possibility would be the chemically sensitive AFM. In such an instrument, the tip would be modified to expose an outer surface of hydrogen bonding groups. By operating such an AFM in a "tapping" mode, it might be possible to distinguish hydrogen bonding regions of the surface from nonhydrogen bonding ones. Such further developments of the AFM and related instruments should have a large payoff in polymer surface research.

Characterization of Biopolymers

Biopolymers require many techniques other than those used for synthetic polymers. To characterize a biopolymer, the first steps are to purify and produce the material in quantity and learn its molecular sequence. The main methods for sequencing and purification include chromatography, gel electrophoresis, centrifugation, and dialysis. They will continue to play important roles. There are also new methods that promise to revolutionize how we obtain, purify, and understand the message encoded in biopolymer sequences. Capillary zone electrophoresis separates materials with very high resolution and works with extremely small volumes of material (in some cases, even the volume of a single biological cell!). Mass spectrometry, particularly in conjunction with electrospray and matrix-assisted laser desorption methods, can determine the sequences of informational polymers very rapidly and can detect subtle aspects of biologically important sequences. At present, there are limitations on the sizes of molecules that can be studied in this way, but the maximum size is growing as the technology evolves. Polymerase chain reaction (PCR) is a method for amplifying very small quantities of DNA. Because most molecules in the cell are in limited supply, this technology now opens the possibility of fishing out even the rarest of molecules and producing the appropriate DNA, RNA, or protein sequence in

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