Pyramidal polymer crystals

In the lamellar or pyramidal, single crystals having the very small extension, about 100 A, in the chain direction and no limitation for lateral extension, the lateral van der Waals forces in a polymer crystal are about... 

Most polymer crystals will exhibit facets and some like polyoxymethylene form hollow pyramids. The occurrence of the hollow pyramid is for similar reasons to that proposed for polyethylene and is a direct consequence of the constraints on the chain folding. The smooth surfaces observed for many crystal systems are evidence of regular chain folding, but are not proof that this occurs. 

For many polymers, the single crystals are not simple flat structures. The crystals often occur in the form of hollow pyramids, which collapse on drying. If the polymer solution is slightly more concentrated, or if the crystallization rate is increased, the polymers will crystallize in the form of various twins, spirals, and dendritic structures, which are multilayered (see Figure 6.11) (34). These latter form a preliminary basis for understanding polymer crystallization from bulk systems. 

The first direct observation of the namre of polymer crystallinity resulted from the growth of single crystals from dilute solution. Either by cooling or by evaporation of solvent, thin pyramidal or plate-like polymer crystals (lamellae) were precipitated from dilute solutions... 

Figure 4.11 Electron micrographs of polyethylene crystals, (a) Dark-field illumination shows crystals to have a hollow pyramid structure. (Reprinted with permission from P. H. Geil, Polymer Single Crystals, Interscience, New York, 1963.) (b) Transmission micrograph in which contrast is enhanced by shadow casting [Reprinted with permission from D. H. Reneker and P. H. Geil, /. Appl. Phys. 31 1916 (I960).]...

Fig. 3. Polymer single crystals (a) flat lamellae and (b) pyramidal lamellae. Two concepts of chain re-entry are illustrated (6).

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

Many polymers form more complex single crystals when crystalhzed from dilute solution including hollow pyramids that often collapse on drying. As the polymer concentration increases, other structures occur, including twins, spirals, and multilayer dendritic structures with the main structure being spherulites. 

Violet phosphorus (Hittorf s phosphorus) is a complex three-dimensional polymer in which each P atom has a pyramidal arrangement of three bonds linking it to neighboring P atoms to form a series of interconnected tubes, as shown in Fig. 15.3.3. These tubes lie parallel to each other, forming double layers, and in the crystal stmcture one layer has its tubes packed at right angles to those in adjacent layers. 

Reaction of a methanolic solution of copper(II) acetate and enantiomerically pure (/ )/(S)-methyl( )-4ethyl-2-oxazolidinylidene)cyanoacetate 64 leads to the coordinatively unsaturated CVsymmetric intermediates (R,Rj-65 and (S,S)-65, which are sterically shielded at one side by two ethyl groups. Therefore, in contrast to the 2D- and 3D-coordination polymers, coordination of (R,R)/(S,S)-65 with only one cyano donor is possible, resulting in the formation of polymers (/>)-oc1[Cu(Li )2] (P-66) and (M)-J[Cu(L5)2] (M)-66) (Scheme 24) ([166, 169, 170] for other chiral lD-coordination polymers of our group, see [171, 172]). The X-ray crystal structure analysis of polymer (P)-66 clearly proves a well-ordered infinite onedimensional architecture. The central copper atoms in (P)- [Cu(L )2] CP-66) are almost tetragonal-pyramidally coordinated, and in contrast to the 2D- and... 

Finally the oligomers form a crystalline nucleus and the reaction becomes now exothermic due to the added heat of crystallization during simultaneous or successive polymerization and crystallization. The crystals of polyoxymethylene observed are hexagonal with the polymer chain peuallel to the c-axis (2,82). They increase with time in lateral dimensions as well as in thickness, taking a p5mmidal shape 82). The addition of new oligomers on the lateral surface would explain an autocatalytic acceleration of the reaction with time as well as the pyramidal shape 82). 

Polyphosphazenes. Single crystals of N P Cl. were grown on a variety of alkali halide crystals. The resuitant morphology of the trimer and polymer was identical and of 2 distinct types pyramidal or square shaped crystals, and rodlike crystals. All crystals were oriented in the substrate s <110> directions independent of method of crystallization. Representative films from vapor and solution depositions are shown in figures 2 and 3. 

---