Thermal Analysis of Polymeric Materials

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Fig. 6.77

Fig. 6.78

This short summary of microscopic observations of annealing reveals that there are at least two processes that must be considered when interpreting the macroscopic annealing experiments of thermal analysis: (1) A crystal perfection within the original crystal morphology. (2) A change in crystal shape towards equilibrium, which may involve several stages of crystal thickening.

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The macroscopic thermal analysis, furthermore, proves that it is also common for melting of the original crystals to be followed by recrystallization to a more stable morphology. Finally, some crystals may have different crystal structures at high temperatures and, thus, show polymorphic transitions on annealing. Annealing of polymers to a more stable state as shown in Fig. 6.79, simply expressed by a decrease in free enthalpy, may thus be a rather complicated process (compare to Sect. 2.4.2).

Fig. 6.79

On discussing the melting of polyethylene in Sect. 6.2.1, the annealing of samples on heating was observed in curves A and B of Fig. 6.22 to cause a decrease in melting temperature with heating rate. In case the kinetics of rearrangement of the crystals could be outrun, a constant zero-entropy-production melting temperature was obtained. At the same time, the slower heating rate experiments give some information about the annealing process. Besides using fast heating to minimize reorganization, it is possible to stop the thickening of crystal lamellae by immobilizing the amorphous phase. Two examples are shown in Fig. 6.80, one for nylon 6, the other for polyethylene. For nylon 6 the NH-groups of the amorphous nylon (see Fig. 1.18) are methylmethoxylated to N CH2 O CH3, so that they cannot be moved into the crystal. Figure 6.80 implies that after 4 h, the single, heating-rate independent, sharp melting peak is related to the zero-entropy-production melting. On longer reaction, this peak broadens because of reaction with the crystals. In addition, the melting point seen after 4 h has also increased by about 3 K due to the permanent change of the molecules, or possibly due to a change of the initially present -crystals to the higher melting -crystals (see also Sect. 6.2.6). For immobilizing the amorphous polyethylene, cross-linking by irradiation has been used, as also shown in Fig. 6.80. The irradiation caused a similar effect as fast heating shown in curves A and B in Fig. 6.22.

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extension, either on crystallization, or on subsequent annealing at higher temperatures. For polyethylene, long-time annealing at temperatures above about 400 K can reach thicknesses of over 100 nm. Full extension of the polyethylene crystals, however, needs the intervention of the hexagonal condis phase (see Fig. 5.156).

Of the polyamides, nylon 6 is perhaps best studied. Figure 6.82 shows the similarity of the melting of solution-grown crystals of nylon 6 lamellae with polyethylene in Fig. 6.22. The initial crystal thickness was only 5 6 nm, accounting for the larger drop in melting temperature. The melt-crystallized nylon 6 is initially about 10 nm in fold length and leads to the correspondingly higher zero-entropy- production melting temperature. On annealing, the perfection gained during slow

Fig. 6.82

heating leads on faster heating to a small amount of superheating. The curves in Fig. 6.82 show only the temperature of the last (major) melting peak. As many as four lower-temperature melting peaks can be observed, as are illustrated in the summary in Fig. 6.83. The form has a somewhat higher melting temperature ( ) than the form ( ). The horizontal appearance of lines 1 and 2 are an indication of reorganization of the initially less-perfect crystals, as in Fig. 6.82 for the crystals. Lines 3 and 4 are closer to zero-entropy-production melting. Line 5, finally, refers to the “annealing peak.” It corresponds to small crystals that form between prior grown crystals. They melt somewhat above the crystallization or annealing temperature. As seen in Fig. 8.62, most melting temperatures shift when changing the heating rate. On reorganization, the peak sizes will also change. Sometimes, an attempt is made to extrapolate the assumed zero-entropy-production melting points (lines 3 and 4) to the point of intersection with the line that marks the position of Tm = Tc to estimate the equilibrium melting temperature, Tmo. This is only safe on using the zero-entropy- production melting temperatures, on avoiding superheating of crystals grown at higher

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Fig. 6.85

Fig. 6.86

Tm = Tc. Complications arise when crystal polymorphs are possible, as seen in poly(vinylidene fluoride). The polymorphs must be identified by X-ray diffraction, as described in Appendix 15. Also, the appearance of the melting peaks closer to the zero-entropy-production condition is characteristic for different polymers and may depend on the heating rate.

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Fig. 6.89

by recrystallization. The larger crystals which do not melt can still anneal and the newly grown material is enlarging the remaining crystals and also anneal. The polymer crystals undergo Ostwald ripening1. Of interest is the larger rates of recrystallization when compared to melt crystallization at the same temperature in the right graph of Fig. 6.88.

The amount of recrystallization for poly(ethylene terephthalate), PET, in Fig. 6.89 increases with annealing temperature, as in polyethylene. At 520 K, the thin film of PET could be melted almost completely by fast heating. Thin films were chosen for quick temperature equilibration at constant temperature. The analysis of crystallinity after quenching to room temperature showed that relatively quick melting is followed by recrystallization. Above 518 K practically all crystals melt first. This temperature is at the beginning of the melting peak of a standard DSC trace, as given in Fig. 3.92. A qualitative correspondence exists between the dilatometry of 1960 and the analysis of the instantaneous, complete melting of thin films pressed against a hot surface at 518 K [54]. The completion of melting was identified by the nature of the viscous flow at the high temperature and by dilatometry and DSC after quick quenching between cold plates. In the same research isothermal primary melt crystallization was shown to be followed by insertion of secondary lamellar stacks and continuous crystal perfection by time-resolved small-angle X-ray scattering and DSC at different heating rates. The major high-melting peak at 525 K was shown not to be due to the originally grown crystals, but was changed due to recrystallization and annealing.

1 F. Wilhelm Ostwald (1853–1932), Professor at the Umiversity of Leipzig, Germany (1887–1906). Organized physical chemistry as a major discipline of science, Nobel Prize in 1909 (work on equilibria and reaction rates). The rule states that initial, small crystals disappear in time and energetically favorable, large ones appear [Z Phys Chem 22: 289 (1897)].