Temperature and molecular bonding force

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. ... 

When polymers are exposed to liquids, the main forms of degradation are swelling and dissolution. With swelling, the liquid or solute diffuses into and is absorbed within the polymer the small solute molecules fit into and occupy positions among the polymer molecules. Thus the macromolecules are forced apart such that the specimen expands or swells. Fmthermore, this increase in chain separation results in a reduction of the secondary inter-molecular bonding forces as a consequence, the material becomes softer and more ductile. The liquid solute also lowers the glass transition temperature and, if depressed below the ambient temperature, causes a once-strong material to become rubbery and weak. 

Methyl ethyl ether is a gas at room temperature (boiling point = 8 °C), but 1-propanol, shown in Figure 11-13. is a liquid (boiling point = 97 °C). The compounds have the same molecular formula, C3 Hg O, and each has a chain of four inner atoms, C—O—C—C and O—C—C—C. Consequently, the electron clouds of these two molecules are about the same size, and their dispersion forces are comparable. Each molecule has an s p -hybridized oxygen atom with two polar single bonds, so their dipolar forces should be similar. The very different boiling points of 1-propanol and methyl ethyl ether make it clear that dispersion and dipolar forces do not reveal the entire story of intermolecular attractions. 

Zou S, Schonherr H, Vancso GJ. Force spectroscopy of quadmple H-bonded dimers by AFM dynamic bond rupture and molecular time-temperature superposition. J Am Chem Soc 2005b 127 11230-11231. 

Dispersion is a fascinating phenomenon. It is sufficiently strong that even tlie dimer of He is found to have one bound vibrational state (Luo et al. 1993 witli a vibrationally averaged bond length of 55 A it is a remarkable member of the molecular bestiary). Even for molecules with fairly large permanent electric moments in the gas phase, dispersion is the dominant force favoring condensation to the liquid state at favorable temperatures and pressures (Reichardt 1990). 

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