Crystal Structure of the a Form

3 MORPHOLOGY OF THE ZIGZAG FORMS 9.3.1 Crystal Structure of the a Form 

X-ray diffraction has been widely used in the recognition and quantification of the different polymorphs in SPS. 

Guerra et al. [4,5] first gave the nomenclature of a and p crystalline forms and studied their formation in detail. Discriminating the zigzag forms, they 

To quantify the fractions of the zigzag crystals, a procedure was suggested for evaluating, from the X-ray patterns, the relative amounts of the two crystalline forms a and P [4]. The 20 region, 10-15° was considered and a baseline between the two intensity minimal located at 20 = 10.8° and 20 = 14.8° was drawn. The areas of the two peaks located at 20 = 11.6° (a form) and 20 = 12.2° (P form) was measured, and the percentage of the a form in the crystaUine fraction was evaluated by the approximate relation (9.1)  

In the successive model of De Rosa, the three triplets are also shifted by c/3 along the c-axis [60]. The one triplet a from structure was analyzed by Greis et al. and by De Rosa as a statistical assembly of the three triplets existing in the more stable a phase. 

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Sasaki S. and Asakura, T. (2003) Helix distortion and crystal structure of the a-form of poly(L-lactide). 

The crystal structure of the A -form of penta-decanoic acid (von Sydow, 1954) is quite similar to the A-form and the form A-super. It has the same triclinic chain packing (thus these forms can be called )8-forms). The angle of tilt is between 66 and 71° which is in agreement with that of the A-form (67°). An interesting difference, however, is that the A -form is the usual lipid bilayer structure. 

Figure 8.26 Structure of the lauric acid crystal form A-super and the relations with the A-form, after von Sydow (1956) and Lomer (1965). If the structure unit is equivalent to the two molecules denoted 1 and 2, the crystal structure of the A-form is obtained.

De Rosa C, Guerra G, Petraccone V, Corradini P (1991) Crystal Structure of the a-Form of Syndiotactic Polystyrene. Polymer Journal 23 1435-1442... 

PLLA can crystallize into three forms (a, 3, and y), generally referred to as polymorphism. Polymorphism in materials science refers to the existence of more than one form of crystalline structure in a solid material with the same chemical composition [84]. The crystal structure of the a-form has been investigated by several research groups using XRD and electron diffraction techniques [65-74]. The a-structure with a 7A of 185°C is more stable than the 3-structure with a 7A of... 

Figure 9.3 Schematic representation of the crystal structure of the a" form derived by (a) Greis et al. [1] and (b) Corradini et al. Reprinted from [59], with permission from Elsevier.

Cartier et al. [7] re-evaluated the crystal structure of the a" form, pointing out the frustrated character of all the previously proposed models. As matter... 

De Rosa, C., Guerra, G., Petraccone, V., Corradini, P. Crystal structure of the a-form of syndiotactic polystyrene. Polymer Journal (Tokyo, Japan), 23(12), 1435-1442, (1991). 

Fig. 9.5. Crystal structures of the a form (a) and the p form (b) of iPP. Full (RH) and open (LH) triangles indicate 3i helical iPP chains of different handedness. The labels A and B designate the inequivalent sites. Numerals at the triangle vertices indicate heights of methyl groups above a plane perpendicular to the c axis in twelfths of c. The circle at the triangle vertices in (a) corresponds to methyl carbons, and the cross-hatched and stippled pairs of circles correspond to the enmeshed A-site methyls. (Reproduced with permission from Ref. [30]. 1987 Butterworth-Heinemann, Ltd.)...

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