Noreen L. Thomas

A theory of case II diffusion

Abstract A theory is proposed to explain the transport behaviour of organic penetrants in glassy polymers in terms of two basic parameters: the diffusivity of the penetrant, D, and the viscous flow rate of the glassy polymer, 1 η 0 . The rate controlling process for transport in these systems is considered to be diffusion of solvent down an activity gradient coupled with time-dependent mechanical deformation of the polymer glass in response to the swelling stress. The theory combines these two factors and is able to predict a wide range of observed transport phenomena from Fickian diffusion kinetics at one extreme to so-called Case II and Super-Case II behaviour at the other. The existence of a sharp front separating swollen and unpenetrated polymer is shown to result from the concentration dependence of the viscous flow rate.

Transport of methanol in poly(methyl methacrylate)

Abstract The transport kinetics of liquid methanol in PMMA sheet (Perspex) have been studied over the temperature range 23° to 63°C. At the lower temperatures the transport is typical Case II; the methanol penetrating the polymer behind a sharp front which moves at constant velocity. For higher temperatures the concentration of methanol at the front and the front velocity both decrease with increasing penetration. These factors, which are no doubt related, combine to give mass absorption kinetics in which the exponent of time is no longer unity (Case II) but approaches 0.5, the value typical of Fickian diffusion. Iodine has been added to the methanol to make the positions of the penetration fronts readily visible. The iodine does not affect either the rate of penetration or the equilibrium absorption. Measurements of colour density profiles on thin cross-sections using a microdensitometer give a reliable indication of the true methanol profile. The rate at which the methanol penetrates the glassy polymer is proportional to its concentration at the advancing fronts. The apparent activation energy for methanol penetration for constant concentration at the fronts is 25 kcal/mol.

A deformation model for Case II diffusion

Abstract Case II diffusion in glassy polymers is considered in terms of a deformation model. The process is controlled by the mechanical response of the glass just ahead of the sharp front to an osmostic swelling stress. The important factor governing the establishment of the sharp solvent front is taken to be the concentration dependence of the creep response time. These ideas are set in the context of existing models for diffusion in glassy polymers, and experimental evidence is presented to support the deformation mechanism.

The retarding action of sugars on cement hydration

Abstract Sugars retard the hydration of Portland cement. The effectiveness of different sugars is compared from studies of solution analysis, calorimetry, calcium binding ability and alkaline stability. The best retarders, sucrose and raffinose, have a remarkable ability to solubilize cement constituents and in particular give rise to dramatic increases in the amount of silica in solution. However, 13C and 29Si N.M.R. do not reveal the existence of sucrose-silicate complexes. The retarding action of sugars is explained in terms of adsorption onto and poisoning of hydrate surfaces.

Flocculation of latex by water-soluble polymers: Experimental confirmation of a nonbridging, nonadsorptive, volume-restriction mechanism

Abstract The reversible flocculation of polymeric latices by a water-soluble polymer such as hydroxyethyl cellulose thickener does not necessarily occur by the oft-invoked adsorptive or “bridging” mechanism. On the contrary, a mechanism wherein attractive forces between latex particles are induced by nonadsorbed soluble polymer appears most applicable to the systems examined. The critical experimental result is that very low levels of surfactant not only displace adsorbed soluble polymer from the latex particle surface, but simultaneously reduce the concentration of soluble polymer required to effect flocculation. Effects of latex polymer composition, latex concentration, soluble polymer molecular weight, and serum electrolyte are explored. Rheological implications of the flocculation model are briefly considered.

Diffusion mechanics of the system PMMA-methanol

Abstract Case II diffusion of penetrants in polymer sheet is associated with constrained swelling of the material and the generation of internal stresses. The work reported here is based on data from the model system PMMA-methanol. Specimen dimensions have been measured during swelling and these are related to the mechanics of the diffusion process. In particular the discontinuous reduction in sheet specimen thickness when the fronts meet has been compared with behaviour predicted from a simple model of Case II diffusion. The fact that there is a quantitative discrepancy indicates that the specimen shape change, as the fronts meet, is also accompanied by an increase in the degree of ‘equilibrium’ solvent uptake as the hydrostatic stress component on the swollen layers is relaxed. This increase is accounted for in terms of thermodynamic equations incorporating the work of deformation of a rubber network and it is apparent that a measurement of the volume change in a swelling polymer, when approaching Case II fronts meet, can give a measure of the Flory-Huggins interaction parameter x. At elevated temperatures a substantial concentration gradient develops behind the advancing fronts, which complicates the prediction of the discontinuous shape change observed at 42°C, 52°C and 62°C. The generation and partial relaxation of molecular orientations behind the advancing Case II fronts has been observed by optical birefringence. The technique has also been used to study the development of biaxial tensile stresses in the surface layers during desorption of partially swollen specimens. These stresses readily exceed that necessary for crazing, and a sorption-desorption cycle has been shown to induce such mechanical damage in the absence of any external or prior internal stress.

The barrier properties of paint coatings

The aim of this article is to outline the theory of diffusion of small molecules through polymer films and to describe some techniques for the measurement of water vapour, oxygen and carbon dioxide permeation through paint coatings. The effect of polymer type and pigmentation on permeability will be discussed, together with some practical applications where permeability is an important factor governing performance

The effect of lead nitrate on the early hydration of portland cement

Abstract Solution analysis, calorimetry and electron microscopy have been used to study the retarding effect of Pb(NO 3 ) 2 admixtures on the early stages of hydration of Portland cement. Analyses of cement filtrates show rapid precipitation of basic lead compounds incorporating nitrate and sulphate. The precipitation is accompanied by an increase in the early heat liberation followed by a longer term retardation. Microscopy shows that the precipitate is largely in colloidal gelatinous form and coats the surfaces of the cement grains. The protective effect of these coatings is clearly responsible for the inhibition of hydration.

On the architecture and function of cuttlefish bone

The internal shell of the cuttlefish, which acts as a rigid buoyancy tank, is structured to combine high compressive strength — since it must withstand the external hydrostatic pressure — with minimum weighT. The micro-architecture of cuttlebone has been examined by electron microscopy and the relevance of the structure to the mechanical duties required of the shellin vivo are briefly discussed. The inorganic calcareous structure is associated with an organic component which may act as a template for mineralization.

Studies of the growth of “silicate gardens” and related phenomena

Various growth morphologies obtained in “silicate gardens” are described and the influence of salt type and silicate concentration are examined. It is demonstrated that these growths develop by an osmotic mechanism based on the semipermeable properties of the silicate gel membrane precipitated between the metal ion and the silicate. The osmotic effect is not confined to silicates but also includes aluminates and ferrocyanides — and in general, it probably applies to systems where a continuous colloidal gel membrane is precipitated between two aqueous solutions of differing compositions. The implications in relation to certain practical problems (i.e. the hydration of Portland cement, the corrosion of metals in an aqueous environment) are considered.