Physical Categories of Polymers

The chains that make up a polymer can adopt several distinct physical phases the principal ones are rubbery amorphous, glassy amorphous, and crystalline. Polymers do not crystallize in the classic sense portions of adjacent chains organize to form small crystalline phases surrounded by an amorphous matrix. Thus, in many polymers the crystalline and amorphous phases co-exist in a semicrystalline state. 

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The crosslinking reaction is an extremely important one from the commercial standpoint. Crosslinked plastics are increasingly used as engineering materials because of their excellent stability toward elevated temperatures and physical stress. They are dimensionally stable under a wide variety of conditions due to their rigid network structure. Such polymers will not flow when heated and are termed thermosetting polymers or simply thermosets. More than 10 billion pounds of thermosets are produced annually in the United States. Plastics that soften and flow when heated, that is, uncrosslinked plastics, are called thermoplastics. Most of the polymers produced by chain polymerization are thermoplastics. Elastomers are a category of polymers produced by chain polymerization that are crosslinked (Sec. 1-3), but the crosslinking reactions are different from those described here (Sec. 9-2). 

Physical characteristics of polymers are dependant on their molecular weights, molecular shapes and structures. The physical properties of greatest interest to conservation professionals are those used to determine plastics reaction to outside stimuli and environment, their mechanical behaviour and their response to heat. These properties described here under the categories density, electrical, mechanical and thermal. 

Responsive polymer bmshes are a category of polymer bmshes that can change the physical and chemical properties with the environmental stimulus, such as pH, temperature, light, " and salt concentration. Such polymer bmshes have potential applications in the field of biotechnology, medical analysis, and dmg delivery systems. 

Linear crystalline polymers always contain a fraction of amorphous material. For this reason they are usually considered biphasic systems. They show the typical transitions of amorphous polymers (glass and secondary) but also the common transitions of crystalline polymers (polymorphic, order-disorder, melting). Mechanical and physical properties of this category of polymers depend on morphology and amorphous/crystalline ratio, but also on the molecular mobility of the amorphous phase. 

One of the points made in Schwenz and Moore was that the physical chemistry laboratory should better reflect the range of activities found in current physical chemistry research. This is reflected in part by the inclusion of modem instrumentation and computational methods, as noted extensively above, but also by the choice of topics. A number of experiments developed since Schwenz and Moore reflect these current topics. Some are devoted to modem materials, an extremely active research area, that I have broadly construed to include semiconductors, nanoparticles, self-assembled monolayers and other supramolecular systems, liquid crystals, and polymers. Others are devoted to physical chemistry of biological systems. I should point out here, that with rare exceptions, I have not included experiments for the biophysical chemistry laboratory in this latter category, primarily because the topics of many of these experiments fall out of the range of a typical physical chemistry laboratory or lecture syllabus. Systems of environmental interest were well represented as well. 

Once a polymer is fuUy saturated, the physical tests described above can be conducted with confidence. Naturally, minimizing the evaporation of water should be considered. The one exception in this new category of testing is flow of water through the foam. This is not covered in the standard but will be very important for some applications, particularly in environmental remediation. If the intent is to build a biofilter or a continuous flow enzyme reactor, we must know the hydrodynamic properties of the materials we produce. Since polyurethanes are rarely used in these environments, the flow of water even through a reticulated foam is not described by the manufacturers. Furthermore, if we are to make composites of reticulated foams, the amount of polymer grafted to the surface will have a dominating effect on the flow of water. In a later chapter, we will describe our work in this area. 

So many kinds of polymers exist that scientists have developed ways of categorizing them to make it easier to study and describe them. Polymers formed by addition or condensation reactions, for example, are placed in the same category because they are formed by a common chemical reaction and, in many cases, have common physical and chemical properties. Similarly, thermoplastic and thermosetting polymers are grouped together primarily because of their behaviors when exposed to heat, and, hence, applications for which they are likely to he most suitable. 

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