The above properties are also observed in polymers when they cooled below the glass transition temperature. Structure of B 2 O 3 Glass. Although there is no long-range crystalline order, there is a short-range coordinational order. Each boron atom is among three oxygen atoms. Each oxygen atom is coordinated with two boron atoms.

Order and disorder in polymers Polymers are high molecular weight compounds. No change in the molecular structure when materials crystallize. The molecular structure of polymers can be changed when mechanical forces are applied. Polymer molecules are normally long chains Under stress, polyethylene molecule may be pulled out nearly straight such as a fiber of cotton boll These variations lead to additional types of disorder.

Molecular weights Low molecular weight compounds are gasses and high molecular compounds are solids.

Molecular size is a function of the degree of polymerization, n

Average molecular weight

Polydispersity index = 1 in monodisperse (only one size of molecules is present.

Conformational disorder The length of polyethylene with n = 500 = 154 nm, because –C—C- = nm, 1000 bond = 154 nm. This is wrong because: the bond angle is 120 o The length = (154 nm) sin 60 =135 nm

Conformational disorder Butane: the distance a—d = 0.4 nm the distance a---d` < 0.3 nm The molecular length changes randomly between these two limits

Polyethylene When n = 500; the end-to-end length = 135 nm The minimum end-to-end length would be a fraction of a nanometer (when the molecule is twisted or kinked to bring the two ends in contact.

The root mean square length

Doubling the degree of polymerization of a polyvinyl increases the mean end-to-end length by 40%. Twisting and coiling arising from bond rotation = conformational entropy, or disorder. Rotation is limited to single C-C bonds; however, it is important above the glass-transition temperature, Tg Many polymers may be stretched from their kinked conformation to give high strains without changing the interatomic distances. Rubbers possess this characteristic, so they develop high strains at relatively low stresses. Rubber is visibly elastic because it has a very low elastic modulus

Configurational variants Molecules may possess more than one configuration = isomers. Transformation between isomers involve bond breaking and bond formation. Isomers are different in their physical properties.

Geometric isomerism Natural rubber = polyisoprene