El SEVIER Polrmer Vol. 37 No. 15, pp. 3405 3410, 1996 Crown copyright 1996 Published by Elsevier Science Ltd Printed in Great Britain 0032-3861 '96/$15.00 + 0.00 The effect of structure on gas solubility and gas induced dilation in a series of poly(urethane) elastomers B. J. Briscoe* and C. T. Kelly'l" Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BY, UK (Received 1 August 1995; revised 15 November 1995) This paper describes a study of the interaction of high pressure subcritical and supercritical carbon dioxide with a series of polylurethane) elastomers of differing hard segment content. The extent of gas absorbed by each poly(urethane) type and also the corresponding dilation of each poly(urethane) have been measured under gas pressures of up to ca. 23 MPa at 42C. The data indicates that, for this particular series of poly(urethanes), the extent of sorption and dilation are directly related to the structural properties of each polymer including the extent and characteristics of the soft phase component. Crown copyright ,~ 1996 Published by Elsevier Science Ltd. (Keywords: poly(urethane); dilation; solubility; carbon dioxide) INTRODUCTION The solubility of foreign molecules in a polymer may be defined, as the state of equilibrium between the concentrations of molecules outside and inside the polymer, in terms of a sorption coefficient. Generally, the sorption coefficient for elastomers is either constant or increases with gas pressure. Sorption is usually assumed to take place homogeneously throughout the whole of the amorphous" polymer phase. An increasing sorption coefficient with an increase in the extent of the sorption is generally attributed to the 'plasticization' and associated swelling of the amorphous component of the polymer, thus enhancing mobility and flexibility of the polymer chains. Hence, the extent of the gas sorption and the gas induced dilation will normally be directly interrelated, and also a function of the nature of the polymer system. The nature of these types of processes is invariably sensitive to changes in the polymer micro- structure on a scale comparable to the size of the penetrant. The morphology of the polymer may be such that it inhibits or promotes sorption due to: (a) geometric constraints, which affect the available free volume and its distribution; and (b) specific interactions between the gas and polymer molecules. In semi-crystalline polymers it has been rationalized that the cr~cstallites do not take part in the gas sorption process . Similarly, in a study of the diffusion of some simple gases (oxygen, carbon dioxide and hydrogen) through poly(urethane) elastomers at a gas pressure of 4.25 psi, McBride et al.: have reported that the extents of hard segment content in * To whom correspondence should be addressed t Present address: Department of Chemical Engineering, The Uni- versity of Queensland, St Lucia, Brisbane, Queensland 4072, Australia the polymer directly influence the rate of diffusion of the gas, and deduced that hard segment domains may be impermeable to an imbibed solvent. Poly(urethane)s provide an interesting subject material for studies of gas sorption and the related gas induced dilation. Poly(urethane) elastomer molecules are made up of 'soft* (mobile) polyol segments, which alternate along the polymer chain with highly polar, 'hard' (immobile) urethane segments. Hydrogen bonding inter- actions may occur between the hard segments, and this phenomenon, coupled with the fact that the soft and hard segments are incompatible, mean that an effective phase separation of the hard segments and soft segments may occur. Thus, the resulting structure of the poly(urethane) may be one where the relatively hard blocks separate into highly ordered regions or domains ('hard phase'), which may be para-crystalline, and are dispersed in a matrix of soft segments ('soft phase') 3. It may also be the case that some specific interactions may exist between the hard and soft segments, and that not all of the hard segments will be located within the hard domains. In addition, some molecules may exist entirely within one phase. In general, the co-existing phases are now thought not to be completely pure 4. Figure 1 shows a schematic diagram of the supposed structure of a typical poty(urethane). The degree of phase separation is not only a function of the system thermodynamics, but also depends a great deal on the size, length and content of the hard segments, and also on the extent of hydrogen bonds of each type, i.e. hard segment hard segment and hard segment-soft segment. This paper examines the effect of altering hard-segment content upon the extent of sorption of carbon dioxide and the corresponding dilation of the polymer, up to gas pressures of ca. 23MPa. It will be seen that carbon dioxide is a particularly good solvent for poly(urethane)s, and induces POLYMER Volume 37 Number 15 1996 3405