Stiff polymer molecules

The PET polymer structure can also be generated from the reaction of ethylene glycol and dimethyl terephthalate, with methyl alcohol as the byproduct. A few producers still use this route. The aromatic rings coupled with short aliphatic chains are responsible for a relatively stiff polymer molecule, as compared with more aliphatic structures such as polyolefin or polyamide. The lack of segment mobility in the polymer chains results in relatively high thermal stability, as will be discussed later. 

In the limit ofL/Lp 1, eqn [158] becomes R = (2Lp) (L/2Lp)/6 for a Gaussian chain, while in the limit of L/fp< 1, R =L I12 for a thin rod. Rh can also be theoretically calculated for the wormlike chain. Static and dynamic light scattering experiments demonstrate that the model can successfidly describe stiff polymer molecules to extraa their characteristic quantities. Many calculations have been reported to theoretically evaluate Rg and Rh for rigid particles of various shapes. Table 2 lists Pig], Rg, and Rh for some rigid particles with a weU-defined shape. For a spherical uniform particle of radius R, we calculate Rg by eqn [30] as... 

The various mechanical properties of polyamides may be traced in many instances to the possibility of intermolecular hydrogen bonding between the polymer molecules and to the relatively stiff chains these substances possess. The latter, in turn, may be understood by considering still another equilibrium, this one among resonance structures along the chain backbone ... 

In the ordered smectic or nematic phase, the rigid rods are arranged in parallel arrays that allow for close packing. The nematic phase is the most common type found with synthetic polymer molecules. The molecules long axes are parallel, but there is no layering. Aromatic polymer chains that have stiff ester or amide linkages are ideal. 

The softening behaviour of a thermoplastic material depends to a large extent on the flexibility of the chain and the ability to crystallise. Significant cross-linking of a reasonably stiff-chained polymer will lead to material that is unlikely to soften below its decomposition temperature. Intermediate to the linear and cross-linked polymers are various ladder polymers in which the polymer molecule consists of a pair of more-or-less parallel chains bridged in a manner analogous to the rungs of a ladder. 

The conformations adopted by polyelectrolytes under different conditions in aqueous solution have been the subject of much study. It is known, for example, that at low charge densities or at high ionic strengths polyelectrolytes have more or less randomly coiled conformations. As neutralization proceeds, with concomitant increase in charge density, so the polyelectrolyte chain uncoils due to electrostatic repulsion. Eventually at full neutralization such molecules have conformations that are essentially rod-like (Kitano et al., 1980). This rod-like conformation for poly(acrylic acid) neutralized with sodium hydroxide in aqueous solution is not due to an increase in stiffness of the polymer, but to an increase in the so-called excluded volume, i.e. that region around an individual polymer molecule that cannot be entered by another molecule. The excluded volume itself increases due to an increase in electrostatic charge density (Kitano et al., 1980). 

The focus of this chapter is on an intermediate class of models, a picture of which is shown in Fig. 1. The polymer molecule is a string of beads that interact via simple site-site interaction potentials. The simplest model is the freely jointed hard-sphere chain model where each molecule consists of a pearl necklace of tangent hard spheres of diameter a. There are no additional bending or torsional potentials. The next level of complexity is when a stiffness is introduced that is a function of the bond angle. In the semiflexible chain model, each molecule consists of a string of hard spheres with an additional bending potential, EB = kBTe( 1 + cos 0), where kB is Boltzmann s constant, T is... 

Linear polymers, polystyrene and cellulose triacetate exhibit differences in hydrodynamic behavior in solution. Cellulose and its derivatives are known to have highly extended and stiff chain molecules below a Dp of about 300, but as the Dp Increases above 300 the chain tends to assume the character of a random coll (27,28). The assumption that hydrodynamic volume control fractionation in GPC may not be true for polystyrene and cellulose triacetate, though it has been found satisfactory for non-polar polymers in good solvents (29). 

The diffusion of larger organic vapor molecules is related to absorption. The rate of diffusion is dependent on the size and shape of the diffusate molecules, their interaction with the polymer molecules, and the size, shape, and stiffness of the polymer chains. The rate of diffusion is directly related to the polymer chain flexibility and inversely related to the size of the diffusate molecules. 

The free volume, i.e., the volume not occupied by the polymer molecules, is similar for polymers at the Tg and increases as the temperature is increased. More-mobile short chains have lower entropy values and hence lower Tm values, while less-mobile stiff chains have higher entropy values and higher Tm values. 

Plasticization is the process in which the plasticizer molecules neutralize the secondary valence bonds, known as van der Waal s force between the polymer molecules. It increases the mobility of the polymer chains and reduces the crystallinity. These phenomena become evident in reduced modulus or stiffness, increased elongation and flexibility, and lowering of the brittle or softening temperature of the plasticized product. The effect of plasticizers on polymers is the subject of the first chapter by E. H. Immergut and H. F. Mark. 

V. N. Tzvetkov, Stiff-Chain Polymer Molecules, Nauka, Leningrad, 1986. 

---