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Advancing Materials Research
be improved blends (polymer alloys) that are also readily manufacturable and have a predictable lifetime. Much has been accomplished on blends, but a lot remains to be done. Lightweight batteries and high-quality aspheric lenses will become available. Clearly, camera manufacturers have already made advances here for their current models of small cameras. Corrosion resistance will be improved. The possibility of self-assembling polymeric chips may be farfetched but cannot be ignored. Practical polymers from direct biological conversion of inexpensive feedstocks (e.g., waste) are clearly possible—polyhydroxybutyrate, which in many respects rivals polyethylene in its physical properties, is currently obtained in this manner. Escherichia coli and some other bacteria can manufacture good, very pure polymers of known molecular weight and molecular weight distribution. This, of course, is what an important part of biotechnology is all about.
Blends: Block Copolymers and Polymer Alloys
Two extremely important subjects illustrate succinctly the current technology, current knowledge, and directions of future advances. First, there are blends, which may be either block copolymers (two or more chemically different polymer chains connected together by a chemical bond) or polymer alloys (two or more chemically different polymers that are mechanically mixed).
The manner of blending the two different polymers strongly affects the resultant properties—for example, the important property of impact strength (as mentioned earlier). Figure 22 shows this for blends of thermoplastic polystyrene (S) and polybutadiene (B). With mechanical mixtures (simple blends), impact strengths show little or no improvement. However, if the proper kind of block copolymer is made with chemical linkages between the two types of chains, superior impact strengths are achieved. The different types of copolymer that can be made are listed in Table 2 together with approximate tensile strengths. One can see that most of the possible copolymers offer little improvement in properties. However, with one of the possible triblock copolymers (S-B-S) there is a marked increase in tensile strength as well as in impact strength (Figure 22). The reason for this behavior is shown in Figure 23, in which the morphologies of S-B diblock and S-B-S and B-S-B triblock polymers are represented. In this figure, the butadiene (B) portion is shown by the solid lines and the styrene (S) portion by the broken lines. In each case there is an aggregation of one species (the styrene). Only in the case of the S-B-S block copolymer, however, does this aggregation create a three-dimensional network, which in turn leads to the improved toughness and tensile strength. This is another example of the effect of molecular architecture on properties, in this case on the important property of strength.