High molecular weight n-alkanes

In the crystallization of C246H494 from a dilute solution in toluene (0.14% w/w) two minima are observed in the crystallization rate-temperature plot.(7) One occurs in the vicinity of the transition from the once-folded to extended form. The other, observed at a lower temperature, occixrs in the vicinity in the transition from the twice-folded form to the once-folded structure. It has been reported that the primary nucleation and growth rates also follow a pattern similar to that illustrated 

In analyzing and interpreting the results shown in Fig. 13.1 it is important that the structures that form initially from solution, or the melt, be clearly identified. In addition, it is essential that any changes in structure that occur during the course of the crystallization be accounted for in the analysis. If these concerns are not 

To obtain some insight into the process or processes that lead to these exotherms, the crystalUzation at a given temperature was interrupted at different times. For example, the crystalUzation was stopped at a time between the two exotherms, at a time corresponding to the maximum of the second exotherm, and after the second exotherm was completed. After each of these time intervals the crystals were rapidly melted from the crystalUzation temperature, T. The melting endotherms resulting from these interrupted experiments are given in Fig. 13.3.(18) This procedure also allows for the construction of a crystalUzation isotherm and an analysis of the kinetics, see below. The dissolution endotherms of Fig. 13.3 have been identified 

A concise summary of the time required for the complete transformation from the folded to extended forms, as a function of crystallization temperatures, is given in Fig. 13.4a.(18a) Here the data are for CiegHssg crystallizing from a 4% toluene solution. It is evident that the transformation is very rapid at crystallization temperatures greater than 72 °C. At the highest temperatures the formation of folded crystals and their transformation to extended form is almost simultaneous. In general, it is 

With this background with regard to the origin of the endothermic peaks in the thermogram the crystallization kinetics can be analyzed. It needs to be recognized that within the data to be analyzed there are two distinctly different, and separate, kinetic processes involved. One is the formation from the melt, or solution, of a distinct crystallite structure, such as the extended form, once-folded, twice-folded, etc. The other involves the isothermal transformation from one form to another. When the two are intermixed a very complex situation results. At this point the kinetics of the formation of the distinct forms from the melt (without any further structural changes) will be treated. The consequences when the two kinetic processes are intermixed will also be considered. 

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The overall crystallization kinetics of the high molecular weight n-alkanes yields superposable, classical isotherms of the Avrami type. An example is given in Fig. 9.73, where ln(l - X(t)) is plotted against In t for Ci92H3g6.(278) The studied temperature interval of 118-124 °C encompasses the time scale of about 0.25-200 min for the detection of crystallinity. Low frequency Raman acoustical mode (LAM) studies indicate that extended chain crystallites are formed at all temperatures in... 

The temperature dependence of the overall crystallization rate is of particular interest in terms of the nucleation processes involved. The log of the crystallization rates for the high molecular weight n-alkanes C168H338, C192H386 and C240H482 are plotted against the crystallization temperature in Fig. 9.75.(280) In this example. 

The availability of the high molecular weight n-alkanes, with their uniformity of chain length, has produced some new and interesting experimental results. Not unexpectedly several different interpretations have resulted. Most importantly the results serve as a base, or reference, for the crystallization of low molecular weight polymer fractions, the subject of the next section. 

The conclusion that the value of (Te is independent of whether or not lamellar crystalUtes are formed is similar to the conclusion reached in analyzing both the growth and overall crystallization rates of high molecular weight n-alkanes (see Sects. 9.14.1 and 9.14.2). In these instances, as well as with low molecular weight fractions of linear polyethylene, the same interfacial free energy for nucleation is involved, irrespective of whether extended or folded chain crystaUites are formed. It becomes clear that it is not necessary to postulate that a nucleus is composed of regular folded chains in order to form lamellar-Uke crystaUites. 

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