Secondary molecular bonds

Except in larger molecules, hydrogen bonds are usually an intermolecular force, not a bonding mechanism within a single molecule. Intermolecular forces are also sometimes described as secondary molecular bonds . Hydrogen bonds will be described in Skill 1.3d... 

As commented earlier, the rate of loss of strength of a joint under environmental attack will usually be faster if a tensile or shear stress is present 145,126-130], albeit an externally applied stress or internal stresses induced by adhesive shrinkage (incurred during cure) or by adhesive swelling (due to water uptake) [131,132]. Such stresses render primary and secondary molecular bonds more susceptible to environmental attack and also probably increase the rate of diffusion and the solubility in the adhesive of the diffusing medium [133-135]. 

Systems such as the concentrated solution of the UHMWPE in paraffin oil (2-8% w/w) contain a three-dimensional molecular network in which the junction points are produced by secondary valence bonds which cause crystalline regions and by physical entanglements of different life times. Entanglements that are trapped between crystallites have, like the crystallites, essentially infinite life times. 

The extreme hardness and lack of formability of thermoset plastics is due to the extremely large number of primary valence bonds between the plastic s atoms. In an extreme case of crosslinking with covalent bonds practically no secondary valence bonds exist that can be loosened by increasing the temperature. This means that no thermoplastic processing of the polymer is possible. A thermoset plastic can be depicted as being a huge single molecule. The removal of these intra molecular bonds by increased tempera-... 

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. 

Proton NMR studies of N-methyl formamide (NMF) and NMA at high dilution in deuterated solvents have shown that the level of cis isomer of NMF is 8% in water, 10.3% in chloroform, 8.8% in benzene, and 9.2% in cyclohexane, while the level of cis-NMA (a model for the secondary peptide bond) is 1.5% in water and does not change very much in nonpolar solvents [18]. Ab initio molecular calculations suggest that the small difference in dipole moments in cis and trans forms explain the relative insensitivity of amides to solvent change, unlike esters [22,41], This may be explained by nearly identical free energies of solvation for the two isomers [18]. The energy difference between cis and trans isomers in aqueous solution (AG° = 2.5 kcal mol-1) accounts for the preferential trans conformation adopted by most peptide bonds. Similar results were obtained with nonproline tertiary amides [22]. 

On a molecular level, strain- stress measurements for polymers vary with crystallinity, chain entanglement and other structural factors. Crystalline polymers contain areas where molecular chains are closely packed. Weak secondary intermolecular bonds connect adjacent chain segments and inhibit inter-segmental motion. Amorphous areas contain a lower density of secondary... 

Since heterocoagulation is a stochastic process, great care needs to be taken not to end up with large fractal clusters or flocks of the two colloidal components. Driving forces to promote adhesion of inorganic nanoparticles onto the surface of polymer latex particles, or vice versa, can be based on a variety of forces, such as electrostatic attraction, hydrophobic interactions, and secondary molecular interactions such as (multiple) hydrogen bond interactions and specific molecular recognition (e.g. complementary proteins like avidin-biotin). 

Beyond electrostatic and hydrophobic forces, the heterocoagulation process could be controlled by secondary molecular interactions. We will briefly highlight with some examples the hydrogen bonding, ji-ti interactions, and speciflc molecular interactions obtained from complementary DNA strands, and biotin-avidin complexation. 

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