Vinyl polymers crystal structures

The fully extended planar zigzag trcms conformation) is the minimum energy conformation for an isolated section of polyethylene or paraffin hydrocarbon. The energy of the trans conformation is about 800 cal/mol less than that of the gauche form. Consequently, the trans form is favored in polymer crystal structures. Typical polymers that exhibit this trans form include polyethylene, poly(vinyl alcohol), syndiotactic forms of poly(vinyl chloride) and poly(l, 2-butadiene), most polyamides, and cellulose. Note that trans conformation is different from the trans configuration discussed in Section IV.A. 

The exact make-up and architecture of the polymer backbone will determine the ability of the polymer to crystallize. Figure 15.7 shows the different types of microstructure that can be obtained for a vinyl polymer. Isotactic and syndiotactic structures are considered stereospecific polymers, and their highly regular backbone structure allows them to crystallize. The atactic form is irregular and would produce an amorphous material. This nature of the polymer microstructure can be controlled by using different synthetic methods. As will be discussed below, the Ziegler-Natta catalysts are capable of controlling the microstructure to produce different types of stereospecific polymers. 

A new process from the paint industry may have application for the suspension coating of pesticide crystals [25]. Low molecular weight water-soluble vinyl polymer chains are synthesized and the process stopped with a terminal vinyl group on each polymer chain. Hydrophobic acrylic monomers are then idded to create the hydrophobic strongly adsorbing backbone polymer. As each water-soluble polymer terminal vinyl group reacts with the growing hydrophobic backbone polymer it becomes inserted like a tooth on a comb. Indeed, these polymer structures are referred to as comb polymers. 

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. 

Un-cross-linked semicrystalline poly vinyl alcohol) hydrogels were prepared by solvolysis of the corresponding vinyl trifluoroacetate polymers and copolymers. The relationships between polymer crystallinity, hydrogel structure, and mechanical properties in the subject hydrogels were examined. Evidence was presented that comonomers acted to disrupt crystal structure and increase water content. The effects of copolymer structure on surface characteristics important to biomedical applications were examined, and the importance of hydrogel nonionic character was demonstrated through protein binding studies. 

Tough, leatherlike polymers are limited for use in the immediate vicinity of their Tg. Such behavior is observed in vinyl chloride-based plastics, which are used as substitutes for leather in automobile seat covers, travel luggage, and ladies handbags. Highly crystalline fiber-forming polymers must be used at temperatures substantially below (about 100°C), since changes in crystal structure can occur as T , is... 

The crystal in Fig. 5.34 illustrates the concept of isomorphism, the possibility that two different motifs fit into the same crystal. For polymers isomorphism of repeating units is most important. Isomorphism of complete chains is seldom possible. Polyfvinyl fluoride) that fits into a crystal of poly(vinylidene fluoride) provides a rare exception. The repeating-unit isomorphism can be separated into three types Type 1 occurs when both homopolymers have die same crystal structure, and a smooth change of the lattice parameters occurs on changing the concentration. An example of type 1 repeating-unit isomorphism is poly(vinylidene fluoride-co-vinyl fluoride). Type 2 is also called isodimorphism and occurs if the homopolymers have different crystal structures. A change in structure occurs at an intermediate concentration. Type 3 occurs if one homopolymer does not crystallize by itself, but participates in the crystal of the other. 

It is known that the physical properties of a polymer depend not only on the type of monomer(s) comprising it, but also on the secondary and tertiary structures, i.e., the stereochemistry of the linkage, the chain length and its distribution, its ability to crystallize or remain amorphous under various conditions, and the shape or distribution of the shapes of the chain in the crystalline and amorphous states. Through advances in polymer chemistry, in most cases polymers can be designed with specific properties. Control of the microstructure, e.g., the tacticity and molecular weight distribution of vinyl polymers, has been the focus of a number of papers in the last two decades. 

In vinyl polymers (Structure (111), Chapter 1) the molecules in the crystal form helices. The way in which this is achieved by rotation from the planar zig-... 

Polymers that cannot crystallize usually have some irregularity in their structure. Examples include the atactic vinyl polymers and statistical copolymers. 

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