ABSTRACT
Acrylic Polymer Characteriz;ption in Solid State and Solution 199
hydrogen bonding and dipolar bonding in the crystal. The length of a single polymer chain is far longer than the longest axis of the unit cell, • and the polymer chains contained within a unit cell actually enter through one face and leave through the opposite face [1]. For the following reasons this crystalline packing is not possible if the chain is atactic. First, it may not be possible for the chain to form the same regular structure as the corresponding isotactic or syndiotactic chain because steric hindrance between substituent groups on adjacent monomer units may occur when the backbone attempts to assume this structure. Secondly, even if a regular conformation is found, then there still will be hindrance between groups on adjacent chains when the chains pack to form the crystalline unit cell. Most commercial vinyl polymers such as polystyrene and polymethyl methacrylate are atactic and form amorphous glasses that have no long-range crystalline order. Fibers can be melt-spun from these glassy polymers, but they cannot be used for textile applications because of the lack of crystallinity. As soon as these fibers are heated near the glass transition temperature, which is approximately l00°C for the latter two polymers, they begin to shrink. When a crystalline phase is present the fiber shrinkage is minimal until it is heated near the melting point. There are, however, exceptions. Some atactic polymers are not truly random but possess a surprisingly high degree of order or pseudocrystallinity, and PAN falls into this interesting class of polymers. Common among these polymers is a substituent group capable of forming strong interactions through secondary bonding. Other examples of commercial importance are polyvinyl alcohol and polyvinyl chloride; commercial fibers have been spun from both.
