Chemical Engineering Science 59 (2004) 2159–2170 www.elsevier.com/locate/ces Dierential water sorption studies on Kevlar TM 49 and as-polymerised poly (p-phenylene terephthalamide): adsorption and desorption isotherms Damian A. Mooney * , J.M. Don MacElroy Department of Chemical & Biochemical Engineering, University College Dublin, Beleld, Dublin 4, Ireland Received 15 August 2003; received in revised form 24 February 2004; accepted 25 February 2004 Abstract We have conducted dierential water vapour sorption experiments on Kevlar TM 49 at 30 ◦ C over a series of water vapour pressures from 0 to 90% of saturation, and on the as-polymerised form of the material at 30 ◦ C, 45 ◦ C and 60 ◦ C over a series of water vapour pressures of 0–60%, 0–25% and 0–15%, respectively. The equilibrium isotherms obtained for both samples show a distinct hysteresis-type behaviour. For Kevlar TM 49, the hysteresis loop can be divided into two regions, namely above 30%, which is indicative of the presence of microvoids, and below 30%, which suggests inclusion of water into the intimate structure of the surface layer of the polymer crystallites, in a process known as intercalation. ? 2004 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Desorption; Polymers; Kevlar TM ; Hysteresis; Intercalation 1. Introduction Investigations regarding the sorption and transport prop- erties of water in polymeric materials are of particular value in situations where the material may be in contact with a humid environment. For example, in load-bearing applica- tions, important information may be gleaned about the eect that atmospheric conditions, particularly humidity, may have on both the long-term and short-term mechanical prop- erties of the material. In situations where isolation from the environment is required, for example, microelectronic en- capsulation, such studies provide information on the ability of the material to act as a vapour barrier. On a more funda- mental level, such investigations can also act as a sensitive probe of the underlying microstructure of the material. Be- cause of their complicated microstructure and the intrinsic mobility of their chains, both penetrant sorption and trans- port behaviour in solid polymers is concomitantly complex (Neogi, 1996). The sorbing species not only experiences ∗ Corresponding author. Tel.: +353(0)1-716-1827; fax.: +353(0)1-716-1177. E-mail address: damian.mooney@ucd.ie (D.A. Mooney). a highly convoluted and structured environment, but also evolution in time. Polymers can also exhibit a wide degree of crystallinity ranging from totally amorphous to almost completely crystalline states which, as gases/vapours are considered essentially insoluble in the crystalline regions of a semi-crystalline polymer, can act so as to reduce the rate of sorption and diusion. In addition, the polymer itself also undergoes pseudo-second-order phase transitions, most particularly the glass transition (T g ) which has a signicant eect on the diusive process (Errde, 1991). Onepolymerofparticularinterest,intermsofitsmoisture sorption behaviour, is poly (p-phenylene terephthalamide) (PPTA), a rigid-chain aromatic polyamide, which is better knownunderoneofitscommercialnames:Kevlar TM .PPTA was rst synthesized by S.L. Kwolek, a Du Pont research scientist, in 1965 (Kwolek, 1972, 1974), and consists of alternatingpara-linkedandamidelinkages(seeFig. 1).Inits bril form, the rod-like macromolecules are well orientated in the axial direction and essentially fully extended as a result of the high-energy barrier (80 kJ mol -1 ) associated with rotation about the N(H)–C(O) bonds, which have a partial double-bond characteristic due to de-localisation of charge.RotationabouttheAryl–N(H)andAryl–C(O)bond is less restricted (8 kJ mol -1 ), but there is still a preference 0009-2509/$-see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.02.011