Crystal structures, atactic poly

Poly(vinyl acetate) is an atactic material and is amorphous. Whilst the structure of poly(vinyl alcohol) is also atactic the polymer exhibits crystallinity and has essentially the same crystal lattice as polyethylene. This is because the hydroxyl groups are small enough to fit into the lattice without disrupting it. 

Syndiotactic polymers, as we have seen above, are stereoregular and so are crystallizable. They, however, do not have the same mechanical properties as isotactic polymers, because the different configurations affect the crystal structures of the polymers. Most highly stereoregular polymers of commercial importance are isotactic, and relatively few syndiotactic polymers are made. Atactic polymers, on the other hand, are usually completely amorphous unless the side group is so small or so polar as to permit some crystallinity. Thus, while atactic poly(vinyl acetate) has never been crystallized, poly(vinyl alcohol), which is derived from it and is also atactic, has been found to crystallize. 

Properties. Poly(trifluorochloroethylene), -f-Cp2—CFC1 , is more susceptible to chemical attack than poly(tetrafluoroethylene), due to the C—Cl bond. The larger size of the chlorine atom, presumably in a compound with an atactic configuration, leads to a less tightly packed crystal structure, and hence to a lower melting point (220°C) and better solubility as compared to poly(tetrafluoroethylene). The glass-transition temperature is 50 C other transition temperatures have not been observed. If other... 

Wide-angle X-ray diffraction (WAXD) scattering has been used widely in studies of the characterisation of crystal structure of polymers (poly(ethylene-2,6-naphthalate) [53], atactic polypropylene [14], poly(co-pentadecalactone) [54], and PP homopolymer [55]). 

In 1948, C, W. Bunn(8) briefly reported the crystal structure of atactic poly(vinyl... 

Figure I. Crystal structure models of atactic poly(vinyl alcohol) proposed by (a) Bunn( ) and (b) Sakurada et al. W Broken lines show hydrogen bonds. (Reproduced with permission from reference 7. Copyright 1997, Wiley-InterScience)...

Neutron structure analyses were successfully carried out on two crystalline polymers, atactic poly(vinyl alcohol) and polyethylene-d4. Neutron structure analysis supports Bunn s model for atactic poly(vinyl alcohol). Three peaks found on the difference synthesis were assigned to the hydrogen atoms to be associated with the intra molecular hydrogen bond in an isotactic sequence of atactic polymer and the intermolecular hydrogen bonds. Neutron diffraction study clarified the detailed crystal structure of polyethylene, in which the azimuthal angle

molecular plane with respect to the b-axis is 45° within the accuracy of the standard deviation 1°. Furthermore, translational and librational motions of the molecule were estimated. The nature of the static disorder in polyethylene was also clarified. 

Atactic poly(vinyl alcohol) has an irregular chain structure but the hydroxyl groups are small, and this allows good packing of chains in the crystal. This polymer may crystallize to some extent. It can probably be quenched to a fully amorphous glassy polymer. 

Atactic poly(vinyl acetate) has an irregular chain structure and does not crystallize. 

When Rj is different from R2 in Formula (1.1) the carbon atom is asymmetric and may have d or 7 forms. If all the asymmetric carbon atoms have either d or 7 forms, the polymer chain is said to be isotactic. If these carbon atoms are instead alternating d and 7 , the polymer chain is said to be syndiotactic. If the d and 7 assignments are random along the chain, it is said to be atactic [8,18-20] (see Fig. 1.1). Isotactic polypropylene, poly(butene-l) and poly(4-methylpentene-l) are commercially available. Both isotactic and syndiotactic polypropylene and polystyrene have been synthesized, subjected to extensive investigation. The two isomeric polymers have different crystal structures and their atactic forms do not crystallize. Isotactic and syndiotactic polymers were originally developed by Natta and his coworkers [18, 19] at Milan Polytechnic and Montecatini. In recent years, there has been interest in producing polyolefins with controlled intermediate tacticities [20]. 

If the polymer host is structurally constrained from crystallizing, as is the case in atactic poly(propylene oxide) (PPO), the solutions of salts are also amorphous. Quite large concentrations of salt remain dissolved at low temperatures without crystallization occurring. Many conductivity studies have been made on solutions of salts in this polymer, but the conductivities obtained are lower than those obtained for amorphous varieties of PEO, and the range of salts dissolved is more restricted because of the steric hindrance to solvation. Also, steric hindrance to the backbone C—C bond rotation increases compared with the value for PEO. 

An interesting set of binary mixtures are those between polymers of different stereo structures. Among others the crystallization kinetics of isotactic poly(styrene)-atactic poly(styrene), (25,54,55) syndiotactic poly(styrene)-atactic poly(styrene), (56) isotactic poly(propylene)-atactic poly(propylene) (55,57) and blends of isotactic and atactic poly(3-hydroxy butyrate),(58,59) and poly(D-lactic acid) with poly(L-lactic acid) (60) have been studied. In analyzing the kinetics of such blends the distinction still needs to be made as to whether the components are miscible in all proportions, partially miscible, or immiscible at all concentrations. Surprising as it may seem, this is an important consideration for these kinds of mixtures. 

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