Chains, molecular

Figure Bl.22.3. RAIRS data in the C-H stretching region from two different self-assembled monolayers, namely, from a monolayer of dioctadecyldisulfide (ODS) on gold (bottom), and from a monolayer of octadecyltrichlorosilane (OTS) on silicon (top). Although the RAIRS surface selection rules for non-metallic substrates are more complex than those which apply to metals, they can still be used to detemiine adsorption geometries. The spectra shown here were, in fact, analysed to yield the tilt (a) and twist (p) angles of the molecular chains in each case with respect to the surface plane (the resulting values are also given in the figure) [40].

Rosenbluth M N and Rosenbluth A W 1995 Monte Carlo calculation of the average extension of molecular chains J. Ohem. Phys. 23 356-9... 

Rosenbluth M N and A W Rosenbluth 1955. Monte Carlo Calculation of the Average Extension Molecular Chains. Journal of Chemical Physics 23 356-359. 

Branching occurs to some extent and can be controlled. Minimum branching results in a high-density polyethylene because of its closely packed molecular chains. More branching gives a less compact solid known as low-density polyethylene. 

The c axis corresponds to both the short axis of the crystal and the axis along the molecular chain. The observed repeat distance in the c direction is what would be expected between successive substituents on a fully extended hydrocarbon chain with normal bond lengths and angles (see Sec. 1.2). 

The molecular chain folding is the origin of the Maltese cross which identifies the spherulite under crossed Polaroids. The Maltese cross is known to arise from a spherical array of birefringent particles through the following considerations ... 

Terephthahc acid (TA) or dimethyl terephthalate [120-61 -6] (DMT) reacts with ethyleae glycol (2G) to form bis(2-hydroxyethyl) terephthalate [959-26-2] (BHET) which is coadeasatioa polymerized to PET with the elimination of 2G. Moltea polymer is extmded through a die (spinneret) forming filaments that are solidified by air cooling. Combinations of stress, strain, and thermal treatments are appHed to the filaments to orient and crystallize the molecular chains. These steps develop the fiber properties required for specific uses. The two general physical forms of PET fibers are continuous filament and cut staple. 

Flow processes iaside the spinneret are governed by shear viscosity and shear rate. PET is a non-Newtonian elastic fluid. Spinning filament tension and molecular orientation depend on polymer temperature and viscosity, spinneret capillary diameter and length, spin speed, rate of filament cooling, inertia, and air drag (69,70). These variables combine to attenuate the fiber and orient and sometimes crystallize the molecular chains (71). 

Prior to deposition on a moving belt or screen, the molten polymer threads from a spinnerette must be attenuated to orient the molecular chains of the fibers in order to increase fiber strength and decrease extendibiUty. This is accompHshed by hauling the plastic fibers off immediately after they have exited the spinnerette. In practice this is done by accelerating the fibers either mechanically (18) or pneumatically (17,19,20). In most processes, the fibers are pneumatically accelerated in multiple filament bundles however, other arrangements have been described wherein a linearly aligned row(s) of individual filaments is pneumatically accelerated (21,22). 

Chain Structure. LLDPE resins are copolymers of ethylene and a-olefins with low a-olefin contents. Molecular chains of LLDPE contain units derived both from ethylene, —CH2—CH2—, and from the a-olefin, —CH2—CHR—, where R is C2H for ethylene—1-butene copolymers, for... 

Optical properties also provide useful stmcture information about the fiber. The orientation of the molecular chains of a fiber can be estimated from differences in the refractive indexes measured with the optical microscope, using light polarized in the parallel and perpendicular directions relative to the fiber axis (46,47). The difference of the principal refractive indexes is called the birefringence, which is illustrated with typical fiber examples as foUows. Birefringence is used to monitor the orientation of nylon filament in melt spinning (48). 

Fig. 8. Formula of ceUulose (a) the central part of the molecular chain, and (b) end groups of the molecule denoting the carbon numbering scheme where...

Orientation sometimes leads to lower permeabiHty values (better barrier properties). Orientation can iacrease packing density, which lowers the diffusion coefficient D it can also iacrease the difficulty of hopping or diffusiag ia a direction perpendicular to the film. In the latter case, movement ia general may be fast, but movement through the film is limited. However, mere stretching does not always lead to orientation of the molecular chains. In fact, stretching can lead to void formation, which iacreases permeabiHty. 

Fig. 3. A cross-section of a nearly square cellulose microfibril, with the individual molecular chains shown as rectangles. Also shown are the one- and two-chain unit cells of la and ip. This view of the microfibril is parallel to the long axis. The chains are arranged so that the edges of the crystal correspond...

However, fine grinding of iasoluble dietary fiber such as bran reduces WHC. In general, branched polysaccharides are more soluble than are linear polysaccharides because close packing of molecular chains is precluded. WHC is strongly kifluenced by the pentosan components of cell-waU dietary fiber and varies with the stmcture and source of these hemiceUuloses. 

An important subdivision within the thermoplastic group of materials is related to whether they have a crystalline (ordered) or an amorphous (random) structure. In practice, of course, it is not possible for a moulded plastic to have a completely crystalline structure due to the complex physical nature of the molecular chains (see Appendix A). Some plastics, such as polyethylene and nylon, can achieve a high degree of crystallinity but they are probably more accurately described as partially crystalline or semi-crystalline. Other plastics such as acrylic and polystyrene are always amorphous. The presence of crystallinity in those plastics capable of crystallising is very dependent on their thermal history and hence on the processing conditions used to produce the moulded article. In turn, the mechanical properties of the moulding are very sensitive to whether or not the plastic possesses crystallinity. 

Liquid crystal polymers (LCP) are a recent arrival on the plastics materials scene. They have outstanding dimensional stability, high strength, stiffness, toughness and chemical resistance all combined with ease of processing. LCPs are based on thermoplastic aromatic polyesters and they have a highly ordered structure even in the molten state. When these materials are subjected to stress the molecular chains slide over one another but the ordered structure is retained. It is the retention of the highly crystalline structure which imparts the exceptional properties to LCPs. 

Thermal Properties. Before considering conventional thermal properties such as conductivity it is appropriate to consi r briefly the effect of temperature on the mechanical properties of plastics. It was stated earlier that the properties of plastics are markedly temperature dependent. This is as a result of their molecular structure. Consider first an amorphous plastic in which the molecular chains have a random configuration. Inside the material, even though it is not possible to view them, we loiow that the molecules are in a state of continual motion. As the material is heated up the molecules receive more energy and there is an increase in their relative movement. This makes the material more flexible. Conversely if the material is cooled down then molecular mobility decreases and the material becomes stiffer. 

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