Thermal Analysis of Polymeric Materials

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Sect. 1.2) has a less extended chain because of the more compact cis-configuration of the double bond. To arrive at an extended chain, the translational repeat has to be doubled, as illustrated in the sketch in Fig. 5.28. Chains A and B have the methyl groups substituted on carbon atoms 3 and 5, but have their chain axes reversed. Chains C and D have the methyl groups substituted on carbon atoms 2 and 6, and again, the action of the screw axis in the ab-plane turns the chain in going from C to D. These four possible arrangements of the same chain lead to disorder in the crystals, as in the vinyl helices of Fig. 5.22. The most frequent disorder occurs when pairs A-B and C-D do not alternate regularly.

A coordination number of four is demonstrated by isotactic poly(o-methylstyrene) crystals, as shown in Fig. 5.29. The neighboring 2*4/1 helixes are created by the indicated glide planes c and are of opposite handedness, but isoclined (drawn are the up chains). Many of the vinyl helices with larger side-groups have 2*4/1 helices in their crystal structures (see also Fig. 5.14).

Fig. 5.29

The polyamides or nylons represent a different type of polymer crystals. Besides the question of packing of the chain, a major energetic advantage is possible by proper placement of the hydrogen bonds N H O=C . More energy is gained by making one H-bond than by several dispersion bonds. In addition, the H-bonds are directive, severely limiting their placement in the crystal. The crystal structure of nylon 6,6, represented in Fig. 5.30, is characterized by H-bonded sheets in the ac crystal plane. Depending on how these sheets are stacked, the or crystal form results. Characteristic is also the placement of the CH2-groups. The crystal is a compromise between the packing of paraffin sequences and amide groups. The crystal structure of nylon 7,7 in Fig. 5.31 can be characterized by tilted H-bonds relative to the chain axis. This aligns the CH2-groups horizontally with the indicated mirror planes. Both

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Fig. 5.30

Fig. 5.31

the nylon 6,6 and nylon 7,7 can be looked upon as polyethylenes with inserted amide groups. Due to the effect of the amide groups, the polyethylene substructure is not orthorhombic in packing, but rather triclinic and monoclinic, respectively.

Following the chains of the nylons, one can observe, that in nylon 6,6 and nylon 7,7 there is no difference in direction with respect to the encountered structures. This

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type of structure is called nonpolar. The nylons a, on the other hand, are polar as in nylon 6, see Fig. 1.18 as well as Fig. 3.10. In one direction one always encounters first the NH-group, in the other, first the CO-group. It is possible to classify the nylons according to this polarity and the even or odd number of chain atoms in the a and b sequences. The seven possible combinations distribute themselves in preference for either triclinic or monoclinic crystal structures, as shown above (even, polar, and nonpolar chains prefer triclinic crystals).

Nylon 2 or polyglycine is the homopolymer of the simplest poly(amino acid). It is, thus, also related to the proteins, which are copolymers of the 20 naturally occurring amino acids (see, for example, Sect. 3.1 and Fig. 3.1). Polyglycine is the parent substance of fibrous proteins. The structure of its crystal form II is shown in Fig. 5.32. It is trigonal with closely packed 3*3/1, polar helices containing intermolecular hydrogen bonds. Polyglycine II crystals, thus, are different from the scheme of nylons of Figs. 5.30 and 5.31, and do not assume the intramolecular -helix of Fig. 5.16, prominent in proteins. The undistorted, planar, intermolecular pleated- sheet-structure of Fig. 5.16 is only seen in polyglycine I crystals. Such exceptions are common in first members of homologous series where the influence of the paraffin chains is secondary to the stronger polar and hydrogen bonds. Note the rather high density and packing fraction, compared to the other polymer crystals.

Fig. 5.32

Figure 5.33 illustrates the same change in crystal structure from the first to latter compounds in a homologous series of polyesters. Polyglycolide contains a three-atom planar zig-zag chain, different from all known polyethylene structures. Neighboring chains in the ac planes form sheets of molecules in the same direction, while neighboring sheets have opposite directions. Noteworthy is the high packing density possible by alignment of the ester groups. In polycaprolactam the CH2-sequences are

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Fig. 5.33

close to polyethylene, as shown in the sketch of Fig. 5.33. The total length of the chain is, however, somewhat shorter than expected for a full planar zig-zag. It is caused by a tilt of the ester group.

5.1.10 Isomorphism

The crystal in Fig. 5.34 illustrates the concept of isomorphism, the possibility that two different motifs fit into the same crystal. For polymers isomorphism of repeating units is most important. Isomorphism of complete chains is seldom possible. Poly(vinyl fluoride) that fits into a crystal of poly(vinylidene fluoride) provides a rare exception. The repeating-unit isomorphism can be separated into three types: Type 1 occurs when both homopolymers have the same crystal structure, and a smooth change of the lattice parameters occurs on changing the concentration. An example of type 1 repeatingunit isomorphism is poly(vinylidene fluoride-co-vinyl fluoride). Type 2 is also called isodimorphism and occurs if the homopolymers have different crystal structures. A change in structure occurs at an intermediate concentration. Type 3 occurs if one homopolymer does not crystallize by itself, but participates in the crystal of the other.

Figure 5.34 indicates the repeating-unit isomorphism of atactic poly(vinyl alcohol). The OH group on every second chain-atom occurs randomly along the chain, in contrast to isotactic vinyl polymers. Two positions are available for the OH, i.e., in the actual crystal half of these positions remain filled by the smaller H. Only because the size difference between H and OH is sufficiently small, is such a compromise possible.

Figure 5.35 demonstrates the changes in the a-dimension of polyethylene copolymers with side-chain concentrations. The b- and c-dimensions are little affected. Although these results seem to indicate that all groups may be accommo-

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Fig. 5.34

Fig. 5.35

dated, the fact that larger side-groups have smaller effects suggests that the change in a is produced at the surface of the only nanometer-size crystals. True isomorphism and isodimorphism is limited to OH and CO. Figure 5.36 shows the change in lattice parameters of poly(4-methyl-1-pentene) with inclusion of other side chains (tetragonal crystals, helices 2*7/2, see Fig. 5.14). The crystal structure does not

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Fig. 5.36

change over the analyzed concentration range (isomorphism). The small influence of(CH2 )15 CH3 is explained by exclusion of most copolymer units. Poly(1-hexene) does not crystallize on its own, its isomorphism is thus of Type 3.

5.1.11 Crystals with Irregular Motifs

Very large and irregular motifs may also produce a regular, crystalline arrangement. The electron micrograph of the virus, seen in Fig. 2.102 is an example (triclinic, space group P1, with 21.4% water). Similarly, globular proteins may have a sufficiently compact shape to crystallize with the whole molecule being the motif (often orthorhombic and with 10 to 50% water). The motifs of these examples are of such sizes that the spaces between them are too large to be bridged by the van der Waals forces, and large enough to hold small molecules. In the biological molecules these spaces are usually filled with water. Much of the energetics of the crystal structure is then not supplied by packing of the motif itself, but by the interaction with the interstitial water. Similarly, fullerenes, which are still large when compared to the repeating units of many linear polymers, can cocrystallize with small solvent molecules placed in the interstices left between the large motifs. An example is drawn in Fig. 5.37 for C60. A comparison of this fullerene C60 with other allotropes of carbon is shown on Fig. 2.109.

Block copolymers are another group of molecules with a crystalline-like order with irregular motifs [8,9] (see also Sect. 5.5). Figure 5.38 is a schematic of the triblock copolymer poly(styrene-block-1,4-butadiene-block-styrene). The two chemically different segments are incompatible and try to segregate as much as possible. The stable structure that arises is a packing of spheres which collect the junction points of the different blocks at their surfaces. The sizes of the motifs are determined by the

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Fig. 5.37

Fig. 5.38

lengths of the sequences. The regular structure is caused by the drive to reach a minimum of the surface free energy, not from packing considerations. Some distance from the interface the polystyrene spheres as well as the 1,4-polybutadiene matrix have their normal, amorphous glassy or liquid homopolymer structure. A triblock copolymer oriented by drawing is illustrated by the electron micrographs reproduced

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5.2 Crystal Morphology

5.2.1. Crystal Habit

Crystal morphology refers to the external appearance or shape of a crystal (Gk.

"#, form, shape). It will be discussed at the level of the appearance of the crystal, its habit (L. habitus, appearance) and the conformation of the overall polymer chain within the crystal, the macroconformation of the molecule. A selection of basic habits is given in Fig. 5.41. These terms and their later expansions should be used in the identification of crystal shapes.

Fig. 5.41

It is not surprising that macromolecules can assume fibrous habits with the long chains aligned parallel to the fiber axis as described in Sect. 5.2.6, but frequently they are also lamellar, with the chain close to normal to the surface of the lamella, as discussed in Sect.5.2.4. The names fibrous and lamellar habits have in addition to an indication of the shape of the crystal a connotation of flexibility. The isometric habit, i.e., crystals with close to equal dimensions in all directions are rare for the macromolecular crystals, but common for atoms and small molecules (see Sect. 5.2.7).

5.2.2 Molecular Macroconformation

The macroconformation is a representation of the overall molecular shape, in contrast to the local conformations, discussed in Sect. 1.3.5 and describes the shape on a repeating unit scale (see also Appendix 14). In the description of crystals, the macroconformation must fit the crystal habit and it must be recognized that polymer crystals are usually small, so that the chain often extends beyond the crystal.