Résumé : |
The differences between the properties of crystalline organic materials of low molecular weight and the more indefinable class of materials referred to by Graham in 1861 as ‘colloids’ has long engaged the attention of chemists. This class includes natural substances such as gum acacia, which in solution are unable to pass through a semi-permeable membrane. Rubber is also included among this class of material.
The idea that the distinguishing feature of colloids was that they had a much higher molecular weight than crystalline substances came fairly slowly. Until the work of Raoult, who developed the cryoscopic method of estimating molecular weight, and Van’t Hoff, who enunciated the solution laws, it was difficult to estimate even approximately the polymeric state of materials. It also seems that in the nineteenth century there was little idea that a colloid could consist, not of a product of fixed molecular weight, but of molecules of a broad band of molecular weights with essentially the same repeat units in each.
Vague ideas of partial valence unfortunately derived from inorganic chemistry and a preoccupation with the idea of ring formation persisted until after 1920. In addition chemists did not realise that a process such as ozonisation virtually destroyed a polymer as such, and the molecular weight of the ozonide, for example of rubber, had no bearing on the original molecular weight.
The theory that polymers are built up of chain formulae was vigorously advocated by Staudinger from 1920 onwards. He extended this in 1929 to the idea of a three-dimensional network copolymer to account for the insolubility and infusibility of many synthetic polymers, for by that time technology had by far outstripped theory. Continuing the historical outline, mention must be made of Carothers, who from 1929 began a classical series of experiments which indicated that polymers of definite structure could be obtained by the use of classical organic chemical reactions, the properties of the polymer being controlled by the starting compounds [2]. Whilst this was based on research in condensation compounds (see Section 1.2) the principles hold good for addition polymers.
The last four decades have seen major advances in the characterisation of polymers. Apart from increased sophistication in methods of measuring molecular weight, such as the cryoscopic and vapour pressure methods, almost the whole range of the spectrum has been called into service to elucidate polymer structure. Ultraviolet and visible spectroscopy, infrared spectroscopy, Raman and emission spectroscopy, photon correlation spectroscopy, nuclear magnetic resonance and electron spin resonance all play a part in our understanding of the structure of polymers ; X-ray diffraction and small-angle X-ray scattering have been used with solid polymers. Thermal behaviour in its various aspects, including differential thermal analysis and high-temperature pyrolysis followed by gas–liquid chromatography, has also been of considerable value. Other separation methods include size exclusion and hydrodynamic chromatography. Electron microscopy is of special interest with particles formed in emulsion polymerisation. Thermal and gravimetric analysis give useful information in many cases. There are a number of standard works that can be consulted. |