Molecular cohesion distribution

The broad molecular weight distribution (MWD) and low cohesive strength of... 

Well-defined products from the chaotic turmoil, which is a chemical reaction, result from a balance between external thermodynamic factors and the internal molecular parameters of chemical potential, electron density and angular momentum. Each of the molecular products, finally separated from the reaction mixture, is a new equilibrium system that balances these internal factors. The composition depends on the chemical potential, the connectivity is determined by electron-density distribution and the shape depends on the alignment of vectors that quenches the orbital angular momentum. The chemical, or quantum, potential at an equilibrium level over the entire molecule, is a measure of the electronegativity of the molecule. This is the parameter that contributes to the activation barrier, should this molecule engage in further chemical activity. Molecular cohesion is a holistic function of the molecular quantum potential that involves all sub-molecular constituents on an equal basis. The practically useful concept of a chemical bond is undefined in such a holistic molecule. 

High molecular weight narrow molecular weight distribution increases cohesive bond strength... 

However, solvation is not the only mode of action taken by the solvent on chemical reactivity. Since chemical reactions typically are accompanied by changes in volume, even reactions with no alteration of charge distribution are sensitive to the solvent. The solvent dependence of a reaction where both reactants and products are neutral species ( neutral pathway) is often treated in terms of either of two solvent properties. The one is the cohesive energy density or cohesive pressure measuring the total molecular cohesion per unit volume,... 

Fig. 10. Maximum extension of the fibrils, Smax. as a function of reduced debonding rate, or Vdeb. for PnBA with 2.5% acrylic acid (upper panel) and for PnBA without acrylic acid (lower panel). T = 23°C, c = I s. , cohesive failure o, adhesive failure. Molecular weights and molecular weight distributions are identical for both polymers. Data from [17].

In order to understand why this is the case, it is useful to examine the results obtained with polymers with a narrow or very narrow molecular weight distribution [17,34,38]. As shown in Fig. 11 for a polyisobutylene, if the comparison is made for the same experimental conditions, reasonable tackiness is only obtained in a relatively narrow range of molecular weights. This can be understood in the following way the increase of M increases the viscosity, r), since t) a M , or, in terms of characteristic relaxation times, the terminal relaxation time, rj. In a tack experiment, this increase in viscosity leads to a larger cohesive strength of the... 

According to Kravelen,( l the fundamental characteristics of a polymer are the chemical structure and the molecular mass distribution pattern. The former includes the nature of the repeating units, end groups, composition of possible branches and cross-links, and defects in the structural sequence. The molecular mass distribution, which depends upon the synthesis method, provides information about the average molecular size and its irregularities. These characteristics are responsible, directly or indirectly, forthe polymer properties. They are directly responsible forthe cohesive force, packing density and potential crystallinity, and molecular mobility (with phase transitions). Indirectly, these properties control the morphology and relaxation phenomena (behavior of the polymer). 

Expression (9.15) gives the total electrostatic energy and not the cohesive energy of a molecular crystal. It ignores the quantum-mechanical nature of the charge distribution an electron cannot interact with itself, but just such a self-energy is included in the expression. 

The differences between interfacial and bulk molecular interaction energies are due mainly to the two-dimensional geometry of the surface and also to differences in interfacial structure and differences in magnitude of the molecular interactions at the interface, from those of the bulk. In principle, it would be possible to calculate the energy of cohesion between molecules within a single phase if the potential energy functions and the spatial distributions of all the atoms and molecules were known. Moreover, if the complete... 

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