Cooling sequence lengths

Figure 7.30 illustrates the limits from equilibrium to cold crystallization. They are based on a computer calculation for three-dimensional cold crystallization. It is assumed in this case that sequences of less than three A-units do not crystallize, and that at least two out of the four neighboring chains must match the sequence of three repeating units to make the central sequence crystalline. If all four lateral neighbors are crystalline, the repeating unit is in the interior of the crystal, if only three are crystalline, it is on the surface, and if only two are crystalline, it is part of an edge. With this critical sequence length and two surface free energies, experimental data of poly(ethylene terephthalate-co-sebacate) can be discussed, as is shown in Fig. 7.31. The experimental phase diagram of the same copolymer, but after slower cooling, is displayed in Fig. 7.26.

Crystallization analysis fractionation (Crystaf) fractionates polymer chains according to differences in crystallizability. Crystaf can be used to fractionate polymers due to differences in chemical composition, comonomer sequence length, and tacticity. It may also respond to long-chain branching, provided that the polymer is branched enough to affect its crystallinity. The fractionation principle operative in Crystaf was discussed in the section on batch fractionation for the case of slowly cooling (or warming) solutions of semicrystalline polymers. 

Thermal fractionation employs a temperature routine (either programmed step cooling or a series of heating and cooling cycles) that is designed to produce a distribution of lamellar crystals whose sizes reflect the distribution of methyl sequence length (MSL) that are present in the ethylene/a-olefin copolymer [33]. The experiment is performed in a conventional DSC. 

Fig. 13 Cooling (solid line) and heating (dashed lines) curves of crystallinity and averaged length of crystallized sequences for slightly alternating copolymers with a comonomer mole fraction 0.24. The crystallized sequences are defined as the monomer sequences more than half of whose bonds are in crystalline states [124]...

Fuel is cooled to the test temperature through a predetermined temperature sequence and is then caused to flow through a length of copper tubing and filter screen similar to that found in full-scale fuel handling equipment. The rate at which the fuel flows and the volume... 

Initiation of DNA polymerisation always requires short lengths of DNA (about 20 bp long) called primers. In vivo the primers are made during replication by DNA polymerase, but in vitro they must be synthesised separately and added at this stage. This means that a short length of the sequence of the DNA must already be known. The DNA must be cooled to 40 °C to allow the primers to anneal to their complementary sequences on the separated DNA strands. 

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