Thermal Analysis of Polymeric Materials

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inspection of the Lissajous figures reveals a number of details. The crystallizations with the amplitudes of A = 2.0 and 3.0 K start at the same temperature after having reached close to steady state on completion of the prior melting. The melting, in contrast, begins earlier for the larger modulation amplitudes. On crystallization at constant frequency, but different amplitudes, the once grown crystals have more time to perfect in the case of A = 2.0 K and reach, thus, a higher melting temperature than in the case of A = 3.0 K. The TMDSC is thus a tool to study the shape of the growthrate curves in Fig. 3.76.

Figure 6.39 illustrates the melting of a high-molar-mass poly(oxyethylene). Its crystal morphology is that of folded chain crystals (see Figs. 5.49 and 5.55). The behavior is similar to the poly(ethylene terephthalate), detailed in Figs. 3.92 and 4.136–139. The reversing melting peak is on the high-temperature side of the irreversible melting peak and the reversing amplitude decreases with increasing modulation time without reaching the expected thermodynamic heat capacity.

Fig. 6.39

The analysis of the melting of poly(oxyethylene) disclosed that for a small fraction of low molar mass poly(oxyethylene) below a molar mass of about 1,000 Da, melting and crystallization could be reversible. On poor crystallization, however, the reversing melting peak is not reversible, since melting and crystallization does not occur at the same temperature. Such processes which could be reversed, but either were not reversible or had not been analyzed with respect of their reversibility are customarily called reversing (see also Sect. 4.4). Crystal perfection is common in polymers, but for the low molar mass fractions, it is also important to consider the phase diagram, as discussed in Chap. 7 for polyethylene. The increase of reversing melting of POE1500 with time may also have a contribution from slow diffusion of the proper chain length molecules to the proper crystal, as seen in Fig. 6.25 for polyethylene.

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Polyterephthalates. The molecular motion and their connection to the thermal parameters for the three most common members of the homologous series of polyterephthalates is summarized in Fig. 6.40. The number of vibrations and the derived -temperatures allows the calculation of a vibrational heat capacity of the solid state, as outlined in Sect. 2.3.7. The changes within the -temperatures are practically within the error limit. The specific heat capacities of the polyterephthalates are, as a result, also almost the same. The transition parameters are extrapolated to the equilibrium crystals and the fully amorphous glasses. Their values show regular changes with chemical structure. All thermal properties are next related to the vibrational baselines computed from the parameters of Fig. 6.40.

Fig. 6.40

Poly(ethylene terephthalate), PET, has always been the standard polymer for thermal analysis and many thermal analyses are illustrated with help of its properties. A basic DSC trace is shown in Fig. 5.116. The first measurement of its heat capacity by adiabatic calorimetry with the calorimeter of Fig. 4.33 was published in 1956 [34], and more extensive data sets are now available in the ATHAS Datas Bank (see Appendix 1). The PET is easily quenched from the melt to an amorphous glass, and on heating shows then a narrow glass transition range, described in Sect4.4.6 (see Figs. 4.129–133). It is also a textbook example for the cold crystallization, which follows the glass transition on heating (see Sect. 3.5.5 and also Figs 4.122, 4.138, and 4.139). On heating from room temperature, the samples of different thermal histories display at higher temperature varying degrees of recrystallization and one or more melting peaks. The details of the recrystallization of PET are discussed in Sect. 6.2.3, and the effects of the crystallinity on its glass transition in Sect. 6.3.4. The analysis of drawn films of PET, is summarized in Sect. 6.2.6. Typical TMDSC data in the time-domain are shown in Fig. 4.122 and 4.138, and analyzed in Fig. 4.139 [1].

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Finally, detailed results on quasi-isothermal TMDSC are compared to standard DSC in Fig. 3.92, and for two thermal histories, results are given in Figs. 4.136. The approach of quasi-isothermal data to reversibility in local equilibria is seen in Fig. 6.41. These results are compared to the calculated heat capacities in Fig. 4.137.

Fig. 6.41

Although it is known from solid-state NMR that PET crystals at 430 K can undergo 180° flips of their phenyl rings and interchanges between the trans-gauche conformational isomers in the O CH2 CH2 O chain segments [35], this has not yet been connected to increases in Cp. The reasons for this missing information are that symmetric ring flips do not change the disorder (entropy) of the crystal and, thus, do not show in Cp (Sect. 5.5.3). Also, the calculations for Cp of solid PET are mainly based on glassy, not crystalline samples and small errors in the Cv-to-Cp conversion may conceal increases in the crystalline Cp due to defect contributions (see Sect. 2.31). Finally, the possible defects occur in a large repeating unit and are more difficult to measure than for polyethylene, where the changes are easily detectable (see Fig. 2.65). Thus, it is not possible to separate with certainty the initial reversible increase in Cp into latent heats of transitions and contributions from conformational mobility.

Poly(trimethylene terephthalate), PTT, with an additional methylene group has higher molecular flexibility and an increased rate of crystallization. Its melting and glass transition temperatures are also lower relative to PET, as seen in Fig. 6.40. The thermodynamic characterization by standard DSC allows a detailed interpretation of the TMDSC analyses, as illustrated with Fig. 6.42 for a melt-cooled sample [36]. The glass transition of the semicrystalline PTT is a bout 20 K higher than for the amorphous PTT, but in addition to the broadening of the transition, there is no indication of a RAF above about 370 K. A reversing latent heat contribution shows 25 K above the upper end of the glass transition and reaches a maximum in the

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transition is sufficiently low to produce a reasonable enthalpy relaxation due to keeping the sample at room temperature before analysis. The enthalpy relaxation is largely irreversible. The cold-crystallization exotherm is fully irreversible and decreases for the slower TMDSC to about 335 K. In contrast to the melt-crystallized sample of Fig. 6.42 and the PET of Fig. 4.136, the cold crystallization produces a large RAF (33%) which completes its softening at about 370 K. The recrystallization of quenched PTT reaches a broad exotherm at about 460 K. Figure 6.44 shows that the reversing melting peak of the quenched sample is double the size of the meltcrystallized sample. Above 450 K both samples have a larger reversible Cp than the melt, i.e., not all of the increases above the vibrational Data Bank heat capacity of the

Fig. 6.44

solid can be due to conformational motion within the crystal, since the melt usually contains more conformational mobility than the crystal. When analyzing the reversing melting, in the time-domain one finds that the exotherm is larger than the endotherm before reversibility is approached. The remaining, locally reversible latent heat is similar for the samples of Figs. 6.43 and 6.44. As for PET in Fig. 4.139, the reversing melting peak of PTT with an underlying heating rate at low frequency is larger than the total melting peak by the standard DSC trace [36]. At higher frequencies the reversing melting peak decreases. This decreasing reversing heat capacity is also observed for other polymers. Measurements on PET were used as early evidence of the erroneous data that can be generated by TMDSC [37] when the conditions of steady state and stationarity are not obeyed. Model calculations and experiments by modulating the sample temperature externally with light suggest that with a high frequency, the reversing heat capacity should reach the level given by the quasiisothermal measurement. Data for polyethylene are illustrated in Fig. 6.31, for nylon 6 in Fig. 4.119, and for a polymeric mesophase in Fig. 5.154.

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of the RAF was used for this calculation. It is listed at the top of Fig. 6.47. This equation can account not only for the change in heat of fusion and heat capacities with temperature, but also for the change due to the glass transition of the RAF. The assumptions used are listed in Fig. 6.47. This approach allowed a full analysis of the quenched PBT, covering the whole temperature range between Tg and Tm.

Fig. 6.48

Polynaphthoate, specifically the poly(ethylene-2,6-naphthalene dicarboxylate)

(O CH2 CH2 O CO C10H6 CO )x, PEN, is a more rigid polymer than PET due to the replacement of the phenylene ring with the larger and asymmetric naphthylene

ring. The thermal properties were initially based on measurements by standard DSC as shown in Fig. 6.49 [39]. Quite similar to PET, PEN can be quenched to the amorphous state. In the amorphous state, on heating at 10 K min 1, it has a higher glass transition at 390 K, compared to 342 K for PET. At about 300 K, a small increase of the heat capacity may indicate some local motion of the naphthylene group, as was also reported by mechanical and dielectric measurements [40]. Similar to glassy polyethylene (Fig. 2.65), this motion gets excited over a wide temperature range. In contrast to PET, the jump or rotation about the axis of the aromatic group breaks symmetry, so that one expects that the crystals have an increase in Cp at higher temperature. The cold crystallization of PEN starts at a higher temperature than for PET and has a broader temperature range (460 510 K), with a maximum growth-rate at about 490 K. On continued heating, the exothermic cold crystallization changes directly into the melting endotherm with a peak at about 533 K.

Semicrystalline PEN increases in glass transition temperature with decreasing crystallization temperature and also develops an increasing RAF [39]. On crystallization above 490 K, the glass transition becomes constant at the value found for amorphous PEN, and the RAF decreases towards zero at somewhat higher crystalliza-

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Fig. 6.49

tion temperature. Above 440 460 K, the heat capacity increases beyond that of the values calculated for the given composition of RAFs and crystallinity. This may suggest a glass transition of the RAF, at the beginning of the melting region. Insufficient TMDSC data exist to identify the major reasons for this initial increase in heat capacity, which may also be due to an increase in conformational motion.

As is common for these aromatic polyesters, isothermal crystallization can produce as many as three melting peaks in standard DSC traces, as seen in the graph A of Fig. 6.49, which is an example of isothermal cold crystallizations after heating from the glassy state. The crystallized samples were then cooled before the DSC runs and show the melting peaks labeled 1, 1', 1''; 2, 2', 2"; ... . At the higher crystallization temperatures, the three peaks fuse into a single peak. The results on crystallization after cooling from the melt are shown in the graph B of Fig. 6.49. The data closely match the graph A for the cold crystallization. At higher crystallization temperatures, the melting peak temperatures increase beyond the horizontal line, and indicate the existence of an even higher equilibrium melting temperature. The squares in graph B represent the small annealing peaks which are caused by secondary growth or annealing at the crystallization temperature. The main melting peak, marked by 1'' 5", is practically constant up to 500 K and then increases. The constant level is due to rearrangement of the crystals grown at low temperatures.

The TMDSC traces shown in Fig. 6.50 are for a sample crystallized from the glassy state at 483 K for 30 min and analyzed under conditions of heating only [41]. The traces should be compared to Fig. 4.139 for PET. The three melting peaks are marked, analogously to Fig. 6.49, as A, A', and A'', although peak A is barely visible. The zero-entropy-production melting temperature of the original crystals could be identified at position X by fast heating. The exotherm in the heat-only, nonreversing heat-flow-rate curve is a safe indication of major recrystallization in the vicinity of X