Secondary bond forces

Solution Properties. Typically, if a polymer is soluble ia a solvent, it is soluble ia all proportions. As solvent evaporates from the solution, no phase separation or precipitation occurs. The solution viscosity iacreases continually until a coherent film is formed. The film is held together by molecular entanglements and secondary bonding forces. The solubiUty of the acrylate polymers is affected by the nature of the side group. Polymers that contain short side chaias are relatively polar and are soluble ia polar solvents such as ketones, esters, or ether alcohols. As the side chaia iacreases ia length the polymers are less polar and dissolve ia relatively nonpolar solvents, such as aromatic or aUphatic hydrocarbons. 

The size and shape of polymers are intimately connected to their properties. The shape of polymers is also intimately connected to the size of the various units that comprise the macromolecule and the various primary and secondary bonding forces that are present within the chain and between chains. This chapter covers the basic components that influence polymer shape or morphology. 

Perfluorination techniques have been developed for the conversion of many hydrocarbons to their perflnorinated counterparts. Flnorocarbons are chemically inert becanse of their kinetically nmeactive carbon skeletons and have been considered as blood substitntes becanse of their high oxygen solnbUity. The thermal stability and low secondary bond forces of flnorocarbons have contributed to their use as greases, lubricants, and vapor-phase heat transfer reagents. 

Molecular solids like polymers present greater problems since the creation of surfaces may involve the severence of primary (intra-molecular) or secondary (inter-molecular) bonds, or, more likely, both simultaneously. In thermoplastics it is possible to envisage molecular pull-out in which no molecules are broken but are simply separated from one another against the frictional secondary bonding forces. 

In the glassy state (below Tg) the critical stress required to plastically deform the amorphous molecular network (H) involves displacement of bundles of chain segments against the local restraints of secondary bond forces and internal rotations. The intrinsic stiffness of these polymers below Tg leads to H values which are 3-4... 

Figure 2-16 Stabilization of conformations by secondary bonding forces.

A more vital application is to discern how reinforcement surface treatments improve adhesion to thermoplastic matrices. Since the nonreactive nature of thermoplastics normally precludes interfacial covalent bond formation, secondary bonding forces, such as London dispersion interactions and Lewis add/base interactions, may play a major role in these drcumstances. These secondary binding forces are subject to surface energetics analysis. 

The first two ways are operating with physical, secondary bonding forces, the last two methods of compatibilization are reactive ways. Radiation processing is capable to form chemical bonds between the components, promising higher efficiency. 

Ethyl to hexyl substituents tend to lower the tendency for crystallization because their major contribution is to increase the average distance between chains and thus decrease the contributions of secondary bonding forces. If the substituents become longer (from 12 to 18 carbon atoms) and remain linear, a new phenomenon occurs—the tendency of the side chains to form crystalline domains of their own. 

A. Secondary Bonding Forces (Cohesive Energy Density)... 

A. SECONDARY BONDING FORCES (COHESIVE ENERGY DENSITY)... 

A quantitative measure of the magnitude of secondary bonding forces is the cohesive energy density (CED), which is the total energy per unit volume needed to separate all intermolecular contacts and is given by ... 

Secondary bonding forces, as we saw earlier, are responsible for intermolecular bonding in polymers. You will recall also that these forces are effective only at very short molecular distances. Therefore, to maximize the effect of these forces in the process of aggregation of molecules to form a crystalline solid mass, the molecules must come as close together as possible. The tendency for a polymer to crystallize, therefore, depends on the magnitude of the inherent intermolecular bonding forces as well as its structural features. Let us now discuss these in further detail. 

Recall again that secondary bonding forces are effective only over short molecular distances. Therefore, any structural feature that tends to increase the distance between polymer chains decreases the cohesive energy density and hence reduces Tg. This effect has aheady been clearly demonstrated in the polyacrylate series where the increased distance between chains due to the size of the all[Pg.114]

Intermolecular bonding — Since secondary bonding forces are responsible for intermolecular bonding, polymer molecules with specific groups that promote enhanced intermolecular interaction and whose structural features lead to identity periods are more crystallizable. 

The extent to which polymer molecules will crystallize depends on their structures and on the magnitudes of the secondary bonds forces among the polymer chains the greater the structural regularity and symmetry of the polymer molecule and the stronger the secondary forces, the greater the tendency toward crystallization. We give a few examples ... 

The viscoelastic deformation is characterized by time-, temperature-, and velocity-dependent deformation processes. Relatively low levels of hardness and strength, high plasticity, low thermal conductivity, and high thermal expansion are effects of the weak secondary bonding forces between the macromolecules and their coiled structures. 

The intermolecular bonding forces are highly dependent on temperature and bond length and are as a rule less than 12 kJ mol The ratio of the secondary bonding force Bs to the primary bonding force Bp is approx. 

The external plasticizing method is applied most frequently to polyvinyl chloride (PVC). The distance between the macromolecules of the plastics is increased by incorporation of plasticizer molecules [aromatic plasticizers, e.g., tricresyl phosphate (TCP) and aliphatic plasticizers, e.g., dioctyl phthalate (DOP) or dioctyl sebacate (DOS)]. This reduces the secondary bonding forces and increases chain segment mobility. Solvents (including water in polyamides) may also act by inward diffusion in a manner similar to plasticizers [11]. 

Physical properties of polymers are influenced by the sizes of the molecules and by the nature of the primary and secondary bond forces. They are also influenced by the amount of symmetry or uniformity in molecular structures, and by arrangements of the macromolecules into amorphous or crystalline domains. This affects melting temperatures, solubilities, melt and solution viscosities, tensile strengths, elongation, flexibility, etc. ... 

Due to the large sizes of the polymer molecules, the secondary bond forces assume much greater roles in influencing physical properties than they do in small organic molecules. These secondary bond forces are van der Waal forces and hydrogen bonding. 

In summary, polymeric materials exhibit rubber elasticity if they satisfy three requirements (a) the polymer must be composed of long-chain molecules, (b) the secondary bond forces between molecules must be weak, and (c) there must be some occasional interlocking of the molecules along the chain lengths to form three-dimensional networks. Should the interlocking arrangements be absent, then elastomers lack memory, or have a fading memory and are not capable of completely reversible elastic deformations. 

What are the secondary bond forces that influence the physical properties of macromolecules ... 

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