Polymers molecular shape

Plastics and Resins. Plastics and resin materials are high molecular weight polymers which at some stage in their manufacture can be shaped or otherwise processed by appHcation of heat and pressure. Some 40—50 basic types of plastics and resins are available commercially, but HteraHy thousands of different mixtures (compounds) are made by the addition of plasticizers, fillers, extenders, stabilizers, coloring agents, etc. 

As the temperature is decreased, free-volume is lost. If the molecular shape or cross-linking prevent crystallisation, then the liquid structure is retained, and free-volume is not all lost immediately (Fig. 22.8c). As with the melt, flow can still occur, though naturally it is more difficult, so the viscosity increases. As the polymer is cooled further, more free volume is lost. There comes a point at which the volume, though sufficient to contain the molecules, is too small to allow them to move and rearrange. All the free volume is gone, and the curve of specific volume flattens out (Fig. 22.8c). This is the glass transition temperature, T . Below this temperature the polymer is a glass. 

Plastics are long chain, high molecular weight polymers which can be formed under application of heat and pressure into desired shapes and contours by a variety of processing techniques. Plastic resins are classified either as thermoplastics, if the material can be reprocessed under heat and pressure, or as thermosets if the material, once formed, cannot be reprocessed. 

Two types of well defined branched polymers are acessible anionically star-shaped polymers and comb-like polymers87 88). Such macromolecules are used to investigate the effect of branching on the properties, 4n solution as well as in the the bulk. Starshaped macromolecules contain a known number of identical chains which are linked at one end to a central nodule. The size of the latter should be small with respect to the overall molecular dimensions. Comb-like polymers comprise a linear backbone of given length fitted with a known number of randomly distributed branches of well defined size. They are similar to graft copolymers, except that backbone and branches are of identical chemical nature and do not exhibit repulsions. 

The purpose of this review is to show how anionic polymerization techniques have successfully contributed to the synthesis of a great variety of tailor-made polymer species Homopolymers of controlled molecular weight, co-functional polymers including macromonomers, cyclic macromolecules, star-shaped polymers and model networks, block copolymers and graft copolymers. 

This relatively small catalog of molecular shapes accounts for a remarkable number of molecules. Even complicated molecules such as proteins and other polymers have shapes that can be traced back to these relatively simple templates. The overall shape of a large molecule is a composite of the shapes associated with its inner atoms. The shape around each inner atom is determined by steric numbers and the number of lone pairs. 

A molecular-size construction kit (159,160, and 161) (Fig. 85) for the assembly of carborane-containing grid-shaped polymers has been developed by Michl and co-workers.164 By synthesizing firmly connected, regular two-dimensional grid-shaped... 

The complete curve for the response of an uncross-linked polymer at a fixed temperature, depicted here, covers so many decades of time that it has only been measured at a single temperature on a very few low-molecular-weight polymers. The experimental results seen in the literature are actually a composite of data taken at several temperatures over a limited time scale. The effect of a temperature rise is to translate the main transition in the curve of Figure 5A to the left, toward shorter time, with essentially no change in shape. 

Thus, dendrimers exhibit a unique combination of (a) high molecular weights, typical for classical macromolecular substances, (b) molecular shapes, similar to idealized spherical particles and (c) nanoscopic sizes that are larger than those of low molecular weight compounds but smaller than those of typical macromolecules. As such, they provide unique rheological systems that are between typical chain-type polymers and suspensions of spherical particles. Notably, such systems have not been available for rheological study before, nor are there yet analytical theories of dense fluids of spherical particles that are successful in predicting useful numerical results. 

Shundo, A., Nakashima, R., Fukui, M., Takafuji, M., Nagaoka, S., and Ihara, H., Enhancement of molecular-shape selectivity in high-performance liquid chromatography through multi-anchoring of comb-shaped polymer on silica, J. Chromatogr. A, 1119, 115,2006. 

Plasticizers. Compounds added to high molecular weight polymers to give them flexibility, softness, and stretch. Plasticizers can be added mechanically at the compounding or shaping stage or chemically by copolymerization. For example, dioctyl phthalate is mechanically added to PVC vinyl acetate is copolymerized with PVC. Plasticizer content can vary from 5—40%. 

When a polymer is extruded through an orifice such as a capillary die, a phenomenon called die swell is often observed. In this case, as the polymer exits the cylindrical die, the diameter of the extrudate increases to a diameter larger than the diameter of the capillary die, as shown in Fig. 3.9. That is, it increases in diameter as a function of the time after the polymer exits the die. Newtonian materials or pure power law materials would not exhibit this strong of a time-dependent response. Instead they may exhibit an instantaneous small increase in diameter, but no substantial time-dependent effect will be observed. The time-dependent die swell is an example of the polymer s viscoelastic response. From a simplified viewpoint the undisturbed polymer molecules are forced to change shape as they move from the large area of the upstream piston cylinder into the capillary. For short times in the capillary, the molecules remember their previous molecular shape and structure and try to return to that structure after they exit the die. If the time is substantially longer than the relaxation time of the polymer, then the molecules assume a new configuration in the capillary and there will be less die swell. 

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