Physisorbed Material

The surface chemical groups present on the adherend surfaces consist of species chemically bonded to the surface. A real surface exposed to the environment contains not only the surface chemical groups but also physisorbed water, carbon monoxide and carbon dioxide molecules. The amount adsorbed depends on the surface free energy of the adherend and can range in thickness from portions of a monolayer to multilayers of material. 

The polymeric reinforcing fibers while having surfaces which are less hydrophilic, can absorb water in bulk to about a percent by weight. This amount of material on a 

Vacuum degassing AU-fiber Vacuum degassing AS-fiber 

---

Prior to determination of an isotherm, all physisorbed material has to be removed from the surface of the adsorbent. This is best achieved by exposure of the surface to high vacuum, the exact conditions required (temperature and residual pressure) being dependent on the particular gas-solid system. In routine determinations of surface area it is generally advisable not to remove any chemisorbed species which may be present thus, the hydroxylated oxides are usually outgassed at 1S0°C. Microporous adsorbents such as zeolites or active carbons however require higher temperatures (350-400 C, say) for complete removal of physisorbed material from their narrowest pores. An outgassing period of 6-10 hours (e.g. overnight) is usually sufficient to reduce the residual pressure to 10 Torr. 

We undertook to investigate the reaction of olefins with oxide-free carbon surfaces in the hopes that chemisorption would occur in ways which could be related to the known chemistry of aromatic free radicals and dehydroaromatics, and that suitable manipulation might provide routes to homogeneously functionalized surfaces. High surface-area carbon fibers were heated to approximately 1000°C under vacuum to remove the surface oxides (evolution of H2O, C02, and CO). The samples were cooled to room temperature and exposed to vapors of various different substrates. The quantity of substrate adsorbed was determined and corrected for the quantity of physisorbed material which could be pumped off at room temperature. Some typical results are reported in Table II. In certain cases the reactivity towards oxygen was redetermined after exposure to the organic substrate. 

In one of the few relevant studies, Nakatsuka and co-workers [46] examined calcium carbonate and clay fillers coated with y-MPS by dry blending (from water/alcohol solution, however). They found that a considerable proportion of silane was actually lost from the calcium carbonate system, due to evaporation during drying. Physisorbed material was found on both fillers, but while that on calcium carbonate was of very low molecular weight, that on clay was of much higher molecular weight. 

They also found that pre-treating the calcium carbonate with phosphoric acid resulted in the formation of higher molecular weight physisorbed material and enhanced the coupling agent effect, as assessed in a vulcanised elastomer. 

The effect of the physisorbed material on reinforcement is much more difficult to predict and will probably depend very mnch on the system. In some cases this material will dissolve in the matrix and play little role in the interphase. In other cases it may penetrate a little way only and during processing and cure may react with the chemisorbed silane (through silane condensation or its reactive functionality), with itself, and with the matrix polymer. Under favourable circumstances then it could lead to an extensive interphase of an interpenetrating network type, with bonding to both filler and matrix. More often, however, reaction may only occnr with the matrix and not with the chemisorbed layers. This will give poor results. In the few stndies where the physisorbed material has been removed before use, properties have indeed been improved [65, 66]. 

Case B. E =A kJmol and E = 2 kJ mol Here one sees an interesting phenomenon. At the start of the isotherm there occurs some physisorption. However, with increased pressure the localized adsorption becomes greater, displacing some of the physisorbed material, thus produe-ing the first step that is seen. The second step is due almost entirely to the localized adsorption. With the near completion of the first layer, this is followed by the onset of the final physisorption. 

Prior to the determination of an adsorption isotherm all previously physisorbed material must be removed from the adsorbent surface. This is achieved by either outgassing or flushing the adsorbent with an inert gas at elevated temperature. To obtain reproducible isotherms it is necessary to control the outgassing conditions to within limits which depend on the nature of the system. For this purpose it is often useful to undertake a preliminary temperature programmed gravimetric study. ... 

---