Thermal Analysis of Polymeric Materials

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complicated. Three positions could be identified in the NMR spectrum. Again, above the disordering transition only one type of carbon exists. The differences in entropies of transition listed in Fig. 5.141 are linked to defects indicated by the two conformations of the bonds before and after the CH2-groups a, b, and c. The specific jumpmotion is discussed next.

5.5.4 Conformational Disorder

From the entropies of disordering in Fig. 5.141, one notes that cyclododecane gains almost no disorder in the low-temperature transition, while cyclotetraeicosane may develop 11 to 18 conformational isomers [calculated from 127.7/(7 to 12), based on Fig. 2.103]. The conclusion reached is that cyclododecane may undergo rotoreptation, a rotation with simultaneous change of conformational isomers. This motion leads to little disorder, particularly in a jump-like motion. Schematically, this jump process is shown in Fig. 5.142 for cyclodoeicosane. One can treat this rotoreptation similat to the jump-rotation, seen in cyclopropane and benzene [46].

Fig. 5.142

The cyclotetraeicosane, in contrast, has a sufficiently large conformational entropy-gain at Td to support a defect motion, utilizing most of the bonds, as illustrated in Fig. 5.142. The marked carbon atom changes in the shown concerted process by one position. Continuation of the process allows interchange of all carbon atoms, as required by the NMR results in Fig. 5.141.

A small molecule that exhibits many mesophases and may serve as a model for molecular motion is N,N -bis(4-n-octyloxybenzal)-1,4-phenylenediamine, OOBPD, mentioned already in Sect. 2.5. An overview of its heat capacity and transitions is given in Fig. 2.108. Figure 5.143 represents the same transitions on a more expanded

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on cooling. This continuous increase in order after the initial, reversible formation of the nematic liquid crystal, N, is typical for the highly mobile liquid crystals. The entropy change at the transition is summarized in Fig. 5.145. The four additional smectic liquid crystals, called H', G', I, and C have a lamellar structure and the mesogens are nanophase-separated from the appendages, although considerable translational motion is still possible. Note that the heat capacity of the smectic state is very close to liquid-like. In contrast to the nematic phase, changes in order occur only close to the transitions. Some of the transitions between the smectic polymorphs are very small and account only for small changes in the molecular order.

Fig. 5.145

Figure 5.143 shows also the transition to the K3 and K1 phases which were originally identified as crystals. The entropies of transition on going from K1 to the melt amount, when added, to only 59 J K 1 mol 1, as listed in Fig. 5.145, close to the gain expected for positional and orientational disordering of the mesogen alone, leaving all CH2- and CH3-groups largely mobile and disordered. The transition from K3 to K1, on the other hand, is only 47 J K 1 mol 1, also not enough to order the 16 flexible groups which should take 152 J K 1 mol 1. The crystal K3 must, thus, still have considerable conformational disorder and be a condis phase. Solid-state NMR could prove this interpretation of the calorimetry of OOBPD.

There is no further transition on cooling the K3 condis crystal of OOBPD. The shaded area in Fig. 5.143 accounts for some of the difference between expected and measured entropy assessed in Fig. 5.145. The total entropy accounting agrees well with the assessment of the motion by NMR. The overall interpretation of the sequential mesophases of OOBPD is then as follows: At Ti the melt changes on cooling reversibly to a nematic phase with some gain of orientational order. Four smectic mesophases are produced on further cooling. These have a lamellar structure

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Calorimetry detects an exothermic heat effect of about 600 J mol 1. The glass transition of the CD glass must occur at about 350 K. The mesophase of polypropylene is not stable under any known condition, it appears only as an intermediate on crystallization, similar to the mesophases found in the fibrous poly(ethylene terephthalate) of Fig. 5.72. Often these metastable mesophases determine the mechanical properties of the materials as demonstrated with Figs. 5.113–115.

Soap-like molecules are examples of condis crystals of small molecules. This is illustrated on the case of the homologous series of thallium carboxylates, TlX, in Fig. 5.147. These soaps are amphiphilic molecules and form lamellar, smectic liquid crystals without the help of a mesogen by ordering on both sides of the interface between the polar and nonpolar parts of the nanophase-separated molecules. The only regular increases of entropy with chain-length n occur, when adding all transition

Fig. 5.147

entropies, up to the neat phase. For n = 22 this leads to 120 J K 1 mol 1, still too low to account for the entropy of isotropization expected when melting a completely ordered crystal. From the data of Fig. 2.103 one would expect 185 300 J K 1 mol 1. The deficits of the total entropies of transition may have three reasons: 1) Some disorder remains in the state, erroneously called a crystal phase. 2) Some disordering may occur on heating before reaching the transition regions. Such disordering outside the sharp transition regions has been found for other condis crystals of small molecules such as OOBPD in Fig. 5.145. 3) Some order may be left in the melt. The different polymorphs are labeled condis crystals since they have different degrees of conformational mobility. On isotropization of the liquid crystalline phase only little of the orientational mobility is gained.

Partially fluorinated paraffins are another interesting group of small-molecule model-compounds for mesophase polymers and shown in Fig. 5.148. The condis

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Nematic liquid crystals are found for DDA-12, as illustrated in Fig. 5.127, and for OOBPD, as illustrated in Figs. 2.108 and 5.143–145. In these cases the aromatic groups are sufficiently long and rigid to serve as mesogens. As required for nematic liquid crystals, the two different segments are soluble in the LC and isotropic phases. Both compounds form condis crystals on cooling with nanophase-separated mesogens and flexible segments. The MBPE and PEIM compounds are at the borderline to liquid crystals, while HPX forms only two condis polymorphs on cooling from the melt. A few of the special properties of MBPE, PEIM and HPX are illustrated next on hand of experimental data.

The isotropization enthalpy of the homologous series of MBPEs is compared in Fig. 5.150 with DDA with flexible spacers. One notes immediately that the aromatic sequence of MBPE, in contrast to DDA, does not seem to contribute to the ordering

Fig. 5.150

transition. A DSC analysis of MBPE-9 is shown in Fig. 5.151. The high-temperature ordering does not supercool. The DSC curves of MBPE-9 on the right side of Fig. 5.151 reveal that on annealing between the two exotherms on the left, an even more stable phase is generated using up the entire ordered phase formed on cooling and melting at 350 K, only little above the ordering temperature. The more stable phase is identified by the higher melting temperature of about 360 K. One cannot assume that a once-grown, single-phase liquid crystal will revert back to a metastable, amorphous phase and then change to a semicrystalline, two-phase structure where only the crystal phase is more stable. It is much more likely that the ordering is an example of Ostwald’s law of successive states. A semicrystalline, less stable condis phase 1 forms first at 340 K, and not a liquid crystal. This condis crystals of type 1 convert on annealing to a more stable condis phase 2 with similar crystallinity as 1 in a process similar to PP in Fig. 5.146, where the condis phase converts into a stable crystal.

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In the melt, there is practically no condis spectrum. At the two lower temperatures most types of carbon atoms contribute comparably to both spectra, suggesting a partial crystallinity, not observed by liquid crystals. The details of the different spectra reveal that the order of the MBPE-9 observed on cooling between the two exotherms is realized by partial change of the CH2-sequences to trans conformations at the four bonds marked by * in Fig. 5.152. In the more stable room-temperature phase, melting at 360 K, the methyl-substituted phenylene ring also participates in creating increased order without a further increase in the order of the CH2-sequences. The observed transition entropies by DSC could be linked to the transition of polyethylene from the melt to the condis phase, and of polyparaxylylene from the melt to its crystal phase. The model that has been proposed for this crystallization with little or no nucleation involves a local nanophase-separation or cluster formation between the methylene spacers and the phenylene groups. The interface that develops between the two nanophases keeps the flexible spacers in a more extended conformation and also introduces some orientational order to the aromatic nanophase. No or only little supercooling should then be necessary for the MBPEs to order, while locally undergoing a spinodal phase separation (see Chap. 7).

Finally, quantitative thermal analysis of MBPE-9 was carried out for the mesophase present at low temperature. It was used as an example mesophase in Sect. 2.3.10 in Fig. 2.68. At low temperature the heat capacity agrees with a vibrationonly motion. This is followed at higher temperature by three major events: 1) a glass transition, 2) a slow unfreezing of the condis crystals (shaded area) as in the OOBPD of Fig. 5.143, and 3) the isotropization endotherm. A comparison of the increase in heat capacity at the glass transition and the heat of isotropization reveals a rigidamorphous fraction, i.e., an amorphous fraction that does not become mobile at the glass transition described in Sect. 6.1. At room temperature the composition is 40% melt, 17% of a rigid-amorphous phase, and 43% CD glass.

The fourth example of molecules with short, alternating rigid and flexible segments in Fig. 5.149 is a series of poly(ester imide)s, PEIMs. It was concluded from a number of investigations that the PEIMs always form layered, supermolecular structures, in both, crystalline and glassy states. According to the discussion above, the nanophase-separated, layered structures could result either from the formation of both, a smectic LC, driven by the mesogen orientation and the phase separation, or only by the phase-separation of the segments, due to their amphiphilic character, without any LC formation. In the latter case, the layer structure and its molecular order in the immediate vicinity of the interface is based on the need to maintain minimal interfacial energy, no additional ordering is involved. The molecular orientational order that might exist in one or both of the phase volumes is, then, introduced by the geometric restrictions caused by the interface, and not by the intervention of orientational ordering of the mesogen. Small changes of the aromatic segment of the PEIMs by reversing of the carbonyl groups led to a loss of the amphiphilic character due to the shortening the rigid sequence. In this case, stable, nematic, single-phase LC mesophases are observed. Furthermore, by increasing the length of the mesogenic group in PEIMs by the addition of an ethyne group, -CH=CH-, to the mesogen between phenylene and carboxyl groups, a third set of PEIMs results, but now with stable, two-phase, thermotropic, smectic-LC mesophases.