Bonding, secondary

The weaker secondary bonding mechanisms generally play a lesser role in the presence of primary bonds such as providing the attractive forces between covalently bonded chains in a polymer. However, they provide the cohesive forces that hold materials together when no primary bonds are present. All of these secondary bonds involve dipole-dipole interactions. 

Both polyethylene fiber and wax are essentially -CH -ethylene fiber differ from wax  

Natural fibers can have very long chains but polymers used for synthetic fibers typically have shorter chain lengths. Why  

Consider a itylon 6,6 fiber having the following distribution of polymer 

20 chains of degree of polymerization 20 120 chains of degree of polymerization 60 200 chains of degree of polymerization 80 100 chains of degree of polymerization 100 30 chains of degree of polymerization 500 

Write the chemical structures of polyethylene, polypropylene, nylon 6, nylon 6,6, polyester, polyaciylonitrile, and Kevlai fibers. 

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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. 

Only 20—40% of the HNO is converted ia the reactor to nitroparaffins. The remaining HNO produces mainly nitrogen oxides (and mainly NO) and acts primarily as an oxidising agent. Conversions of HNO to nitroparaffins are up to about 20% when methane is nitrated. Conversions are, however, often ia the 36—40% range for nitrations of propane and / -butane. These differences ia HNO conversions are explained by the types of C—H bonds ia the paraffins. Only primary C—H bonds exist ia methane and ethane. In propane and / -butane, both primary and secondary C—H bonds exist. Secondary C—H bonds are considerably weaker than primary C—H bonds. The kinetics of reaction 6 (a desired reaction for production of nitroparaffins) are hence considerably higher for both propane and / -butane as compared to methane and ethane. Experimental results also iadicate for propane nitration that more 2-nitropropane [79-46-9] is produced than 1-nitropropane [108-03-2]. Obviously the hydroxyl radical attacks the secondary bonds preferentially even though there are more primary bonds than secondary bonds. 

Secondary Bonding. The atoms in a polymer molecule are held together by primary covalent bonds. Linear and branched chains are held together by secondary bonds hydrogen bonds, dipole interactions, and dispersion or van der Waal s forces. By copolymerization with minor amounts of acryhc (CH2=CHCOOH) or methacrylic acid followed by neutralization, ionic bonding can also be introduced between chains. Such polymers are known as ionomers (qv). 

Secondary bonds are considerably weaker than the primary covalent bonds. When a linear or branched polymer is heated, the dissociation energies of the secondary bonds are exceeded long before the primary covalent bonds are broken, freeing up the individual chains to flow under stress. When the material is cooled, the secondary bonds reform. Thus, linear and branched polymers are generally thermoplastic. On the other hand, cross-links contain primary covalent bonds like those that bond the atoms in the main chains. When a cross-linked polymer is heated sufficiently, these primary covalent bonds fail randomly, and the material degrades. Therefore, cross-linked polymers are thermosets. There are a few exceptions such as cellulose and polyacrylonitrile. Though linear, these polymers are not thermoplastic because the extensive secondary bonds make up for in quantity what they lack in quahty. 

Similarly, polymers dissolve when a solvent penetrates the mass and replaces the interchain secondary bonds with chain-solvent secondary bonds, separating the individual chains. This cannot happen when the chains are held together by primary covalent cross-links. Thus, linear and branched polymers dissolve in appropriate solvents, whereas cross-linked polymers are insoluble, although they may be swelled considerably by absorbed solvent. 

Secondary bonds - Van der Waals and hydrogen bonds, which are both relatively weak (they melt between 100 and 500 K). 

Although much weaker than primary bonds, secondary bonds are still very important. They provide the links between polymer molecules in polyethylene (and other polymers) which make them solids. Without them, water would boil at -80 C, and life as we know it on earth would not exist. 

It is because these primary and secondary bonds can form that matter condenses from the gaseous state to give liquids and solids. Five distinct condensed states of matter,... 

Elastomers or rubbers are almost-linear polymers with occasional cross-links in which, at room temperature, the secondary bonds have already melted. The cross-links provide the "memory" of the material so that it returns to its original shape on unloading. The common rubbers are all based on the single structure... 

Elastomers are a special sort of cross-linked polymer. First, they are really linear polymers with just a few cross-links - one every hundred or more monomer units - so that a molecule with a DP of 500 might have fewer than five cross-link points along its length. And second, the polymer has a glass temperature which is well below room temperature, so that (at room temperature) the secondary bonds have melted. Why these two features give an elastomer is explained later (Chapter 23). 

The glass temperature, T, you will remember, is the temperature at which the secondary bonds start to melt. Well below the polymer molecules pack tightly together, either in an amorphous tangle, or in poorly organised crystallites with amorphous... 

Fig. 23.2. A schematic of o linear-amorphous polymer, showing the strong covalent bonds (full lines) and the weak secondary bonds (dotted lines). When the polymer is loaded below Tg, it is the secondary bonds which stretch.

Here / is the fraction of stiff, covalent bonds (modulus Ej) and 1 - / is the fraction of weak, secondary bonds (modulus E2). The polymer modulus is... 

At yet higher temperatures (>1.4T ) the secondary bonds melt completely and even the entanglement points slip. This is the regime in which thermoplastics are moulded linear polymers become viscous liquids. The viscosity is always defined (and usually measured) in shear if a shear stress o produces a rate of shear 7 then the viscosity (Chapter 19) is... 

Secondary bonding between chains, e.g. hydrogen bonding. 

The atoms of a molecule are held together by primary bonds. The attractive forces which act between molecules are usually referred to as secondary bonds, secondary valence forces, intermolecular forces or van der Waals forces. 

Although the primary bonds are important when considering the chemical reactivity and thermal stability of polymers, it is the secondary bonds which are... 

The scale of the microscopic surface roughness is important to assure good mechanical interlocking and good durability. Although all roughness serves to increase the effective surface area of the adherend and therefore to increase the number of primary and secondary bonds with the adhesive/primer, surfaces with features on the order of tens of nanometers exhibit superior performance to those with features on the order of microns [9,14], Several factors contribute to this difference in performance. The larger-scale features are fewer in number... 

Chemical secondary bonding. Low-energy bonds, dipolar interactions, dispersion may all play an important role in the development of interfacial adhesion. 

S.2.2.2. Composite adherends. Composite adherends are bonded in both the cured and uncured states. Wherever possible the adhesive and all adherends are cured simultaneously to avoid the added cost of additional autoclave cure cycles. In many cases this is not practical due to part size and complexity. Cured parts can be bonded to uncured parts, which is known as cobonding, and fully cured parts can be bonded together, which is known as secondary bonding. Adhesives for composites are formulated to be compatible with matrix resins in either cured or uncured states. 

Sharp melting point - the regular close-packed structure results in most of the secondary bonds being broken down at the same time. 

Low chemical resistance - the more open random structure enables chemicals to penetrate deep into the material and to destroy many of the secondary bonds. 

A common interpretation of the interaction of chalcogens with nucleophiles considers donation of electron density from a lone pair on the donor atom into the o- (E-X) orbital (Figure 15.1). As the degree of covalency increases, a hypervalent three-centre four-electron bond is formed. Real systems fall somewhere between secondary interactions and hypervalent (three centre - four electron) bonds. The two extremes can be distinguished by the correlation of X-E and E D distances.In the hypervalent case both bond distances decrease simultaneously, whereas in the secondary bond the distances are anticorrelated. This concept has been applied in a study of selenoquinones 15.17 (R = Ph, Me) with short Se 0 contacts,for... 

Stiffness The same factors that influence thermal expansion dictate the stiffness of plastics. Thus in a TS the degree of cross-linking and amount of overall flexibility are important. As an example, in a TP its crystallinity and secondary bond s strength control its stiffness. 

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