Hexagonal subcell packing

It has been reported extensively that fats solidify in more than one crystalline type (2-23). Triglycerides exhibit three main crystal types—ot, p, and p—with increasing degrees of stability and melting point. The molecular conformations and packings in the crystal of each polymorph have been reported. In the ot form, the fatty acid chain axes of the triglyceride are randomly oriented and the ot form reveals a freedom of molecular motion with the most loosely packed hexagonal subcell structure. 

The C -form was found to have the carbon atoms located in a distorted way with regard to the planar zig-zag conformation. Also the dimensions of the orthorhombic subcell indicates an intermediate arrangement towards hexagonal chain packing. This is certainly a consequence of the existence of the C -form in a narrow temperature range close to the melting point. Similar disorder effects in the C-form close to the melting point were discussed in Section 8.1. 

Subcell Structure Subcell structure defines a lateral packing mode of the hydrocarbon chains (2, 15, 16). Three typical subcefl structures are shown in Figure 3. The a, p, and p forms have hexagonal (H), orthorhombic perpendicular (Oj ), and triclinic parallel (T//) subcell structures, respectively (2). 

The lozenge habit characteristic of orthorhombic polyethylene is paralleled by hexagons of polyoxymethylene (14) and i-polystyrene (15), squares of i-poly(4-methylpentene-1) (16), and, very unusually, triangles of poly(L-hydroxyproline) (17), in each case corresponding to the symmetry of the chain packing or subcell. 

The hydrocarbon chain-packing modes are usually described by means of a subcell, which gives the symmetry relations between equivalent positions in one chain and its neighbors [9,10]. Four types of subcells have been identified (1) The planes contain parallel hydrocarbon chains, (2) the chains are perpendicular to each other, (3) the chain axes are crossed, and (4) the chains are packed in a hexagonal lattice. By lateral repetition of the subcell, the entire structure of the chain region is obtained. Structural analysis of normal paraffins with more than nine carbon atoms in the chain revealed mainly four possible distinct crystal structures hexagonal, triclinic, monoclinic, and orthorhombic [11]. 

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