Crystals with Irregular Motifs

The sketch below shows the position of the small solvent molecules, s, In the crystal 

Poly(styrene b/oc/c-1,4 buta llene Wock-styr(5ne) 

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

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 macro-molecular crystals, but common for atoms and small molecules (see Sect. 5.2.7). 

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The presence of an asymmetric chiral carbon atom in PLA leads to the existence of stereoisomers (Figure 11.11). Therefore, levorotatory (r), dextrorotatory (d), or racemic (dl) forms of PLA can be obtained. Stereoregular PLAs result from polymerization of optically pure lactides. Optically pure PLAs - poly(L-lactide), PLLA, and poly(D-lactide), PDLA-are therefore usually semicrystalline, while if there is an LL/DD motif distribution in PLA chains, then PLAs are stereo-irregular and can be regarded as copolymers whose crystallization ability decreases with optical purity [114, 115],... 

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