Three-dimensional molecules polymers

The commonly used resins in the manufacture of decorative and industrial laminates ate thermosetting materials. Thermosets ate polymers that form cross-linked networks during processing. These three-dimensional molecules ate of essentially infinite size. Theoretically, the entire cured piece could be one giant molecule. The types of thermosets commonly used in laminates ate phenoHcs, amino resins (melamines), polyesters, and epoxies. 

Lignin is a polymer built up of phenylpropane units substituted with hydroxy and methoxy groups, see Figure 48. Lignin is a three-dimensional molecule like a 3-D matrix, see Figure 48 [65]. 

As shown originally by Malcolm Dole, polyethylene molecules may be cross-linked when subjected to high-energy radiation. These three-dimensional network polymers may be represented by the structure shown in Figure 1.5. 

A key challenge is figuring out how to get less exotic polymers to self-assemble. Physical scientists are a long way from being able to control polymer syntheses so that the self-assembly of complicated, three-dimensional molecules is possible. This is a very active area of research, one that promises eventually to result in interesting new scientific understanding, new technological tools, and new materials with unique properties. 

A three-dimensional crystalline polymer often can be described as a fringed micelle (chains packed as a sheaf of grain) or as a folded chain. Regions where the polymer chains exist in an ordered array are called crystalline domains. These crystalline domains in polymers are typically smaller than crystalline portions of smaller molecules. Furthermore, imperfections in polymer crystalline domains are more frequent, and one polymer chain may reside both within a crystalline domain and within amorphous regions. These connective chains are responsible for the toughness of a polymer. Sharp boundaries between the ordered (crystalline) and disordered (amorphous) portions are the exception but do occur in some instances such as with certain proteins, poly(vinyl alcohol), and certain cellulosic materials. Highly crystalline polymers exhibit... 

A second motif encountered in tetrapyridylporphyiin systems is typified by inclusion compounds with wet methanol and water that produce three-dimensional coordination polymers. Ttv/ni-pyridyl substituents on a Zn(TPyP) were ob.served to axially ligate the metal centers of adjacent porphyrin moieties generating a polymeric chain in one dimension. Cross-linking in a second dimension occurs when the original porphyrin molecule is coordinated by two pyridyl moieties from two additional porphyrin molecules... 

A further approach to the determination of the molecular weight of coal invokes the concept of the time-dependent response of bituminous coals to constant stress and presents indications that these coals are cross-linked, three-dimensional molecules. There have also been attempts to apply the Flory-Huggins theory to coal but there is some question about the validity of such an approach. But coal is most unlikely to be a simple polymer netwoik its heterogeneity, its mineral content (e.g., clays might act as fillers in the polymer inunobilizing any layers), and other factors indicate that a simple application of polymer theory is not justified. If some of the problems of the application of polymer theory to coals could be resolved, the method could provide a valuable aid to chemical methods of molecular weight determination. 

First, we will consider the phenomena connected with the transformation interval of some materials with three-dimensional molecules (glass, asphalt), turniii later to a chain polymer (sulphur), where the phenomena are much more complicated. 

Crosslinked or three-dimensional network polymers - Crosslinked polymers may be represented as a three-dimensional network structure. They are crosslinked, that is they consist of insoluble and infusible three-dimensional giant molecules. Examples are thermosetting polymers such as crosslinked rubbers, resins, and so on. 

---