Linear polymers classes

In all of the examples given so far in this chapter the product of polymerisation has been a long chain molecule, a linear polymer. With such materials it should be possible for the molecules to slide past each other under shear forces above a certain temperature such that the molecules have enough energy to overcome the intermolecular attractions. In other words above a certain temperature the material is capable of flow, i.e. it is essentially plastic, whereas below this temperature it is to all intents and purposes a solid. Such materials are referred to as thermoplastics and today these may be considered to be the most important class of plastics material commercially available. 

However both classes, nucleic acids and proteins, are linear polymers in which the linear (nucleobase or amino acid) sequence encodes the three-dimensional structure and function of the polymer. 

The term molecule has been purposely avoided in the above discourse for the reason that it cannot be applied appropriately to a most important class of polymers, namely, those composed of indefinitely large network structures. Even In the case of linear polymers, which are composed of finite (although in many cases exceedingly large) molecules, the continuity of structure may deserve greater emphasis than the individuality of the molecules. This is particularly true insofar as properties of the bulk polymers are concerned. 

The process proceeds through the reaction of pairs of functional groups which combine to yield the urethane interunit linkage. From the standpoint of both the mechanism and the structure type produced, inclusion of this example with the condensation class clearly is desirable. Later in this chapter other examples will be cited of polymers formed by processes which must be regarded as addition polymerizations, but which possess within the polymer chain recurrent functional groups susceptible to hydrolysis. This situation arises most frequently where a cyclic compound consisting of one or more structural units may be converted to a polymer which is nominally identical with one obtained by intermolecular condensation of a bifunctional monomer e.g., lactide may be converted to a linear polymer... 

Carbosilanes, defined this narrowly, as a class, monomers, cyclic and polycyclic oligomers and linear polymers, with emphasis on the cyclic and polycyclic systems, have been discussed in detail in an excellent recent book by Fritz and Matern [2]. 

Type 2 gels are essentially infinite molecular weight molecules Their three-dimensional macroscopic networks comprise structural components that are covalently linked through multifunctional units. This is a very broad class that includes linear polymers that have been chemically or radiochemically cross-linked into a permanent structure as well as networks that have been built up by the step or chain polymerization of difunctional and multifunctional monomers. 

It is now recognized that a continuum of architecture and properties, which begins with the classical branched polymers, resides between these two classes. Typical branched structures such as starch or high pressures polyethylene are characterized by more than two terminal groups per molecule, possessing substantially smaller hydrodynamic volumes and different intrinsic viscosities compared to linear polymers, yet they often exhibit unexpected segmental expansion near the theta state . 

Polymers fall into one of two major classes, thermoplastics, and thermosets. Despite the fact that thermosets have been around much longer, thermoplastics make up about 80% of the industry output. Thermoplastics are linear polymers that can be resoftened a number of times, usually by applying heat and pressure. They can be dissolved in solvents (suitable for that purpose). That s not true for thermosets once they re set. After they re formed or cured (by heat and/or pressure), these cross-linked three-dimensional polymers become nonmelting and insoluble. Thermosets actually decompose under heat before they melt. 

There are basically two major classes of linear polymers (1) addition and (2) condensation, and most polymers can be divided into these two groups. We will consider these types of polymers in turn because their kinetics and reactors are quite different. 

Amphiphilic polymers studied thus far for this purpose can be broadly classified into amphiphilic block copol5mers and amphiphilic homopolymers. We will discuss both of these types of linear polymer architectures. Another interesting class of polymeric amphiphiles is based on branched architectures, known as dendrimers. The most interesting aspect of dendrimers is that their molecular weight and polydisper-sity can be precisely controlled hence, these systems have the potential to be moved... 

The inset to Fig. 6 exhibits as depending sensitively on the polymer class, but relatively weakly on molar mass. The temperature variation of Sc T)/sl is roughly linear for small 6T near Tq, consistent with the empirical VFTH equation, as noted earlier. On the other hand, this dependence becomes roughly quadratic in 6T at higher temperatures, where Sc achieves a maximum s at 7a-Attention in this chapter is primarily restricted to the broad temperature range (To 7 7a), vdiere a decrease of Sq vith T is expected to correspond to an... 

Dendronized polymers are a class of polymers produced by the combination of linear polymers and dendritic molecules as side chain pendant moieties [67-69],... 

A supplementary form of linear polymers is those of the diene class. In this case, the repeating unit with one side group from which these polymer types are derived is shown in Figure 2.36 [197]. 

An increase in heat deflection temperature of some thermoplastic polymers can be achieved by the addition of polyfunctional aromatic cyanates (BPA/DC in particular) and trimerization catalysts. A rigid network is formed as a resul t of the cyanate trimerization. The polymer material consists of a linear polymer and a crosslinked network and belongs to the class of semi-IPNs (semi-interpenetrating Polymer Networks) the corresponding classification is given in [34-37]. 

In addition, some cross-linked resins having essentially the same structure as linear polymers belonging to the above list will be considered. All available data relative to each class of polymers will be reported, and critically evaluated. 

Nylon 66. The word nylon was established as a generic name for polyamides, one class of the new high molecular weight linear polymers. The first of these, and the one still produced in the largest volume, was nylon 66 or polyhexamethylene adipamide. Numbers are used following the word nylon to indicate the number of carbon atoms contributed by the diamine and dicarboxylic acid constituents, in this case hexamethylenediamine and adipic acid, respectively. 

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