Modification of the permeability of polymer latex films

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Steward, P.A., 1995. Modification of the permeability of polymer latex films. PhD, Nottingham Trent University.

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Abstract

The modification of the aqueous solute permeability of methacrylic acid ester polymer latex-cast films resulting from the leaching of water soluble additives has been studied. The effects of the additives on the leached films' morphology are investigated with respect to the films' transport properties. The films' permeation properties to both gases and water vapour are also reported.

The solute permeability of surfactant-free, additive-free, homopolymer methacrylate latex films was found to be dependent on the hydrophobicity of the polymer: the longer the alkyl side-chain, the greater the permeability coefficient. In the case of a range of commercial (Eudragit) latices, the solute permeability was influenced by whether the films contained endogenous surfactant, or required a plasticiser; both of which underwent a degree of leaching.

The loading of the films with water soluble additives could be used to modify their permeability coefficients. Low levels of addition, being difficult to leach, yielded lower permeability coefficients than higher levels of addition which afforded greater porosity more quickly. The mechanism of solute permeant transport changed gradually from a predominantly Fickian solution-diffusion type mechanism in the additive-free film to a mechanism of predominantly convective transport through water filled channels.

Initially, the solute permeability coefficient increased linearly with increasing additive load since low levels of additive did not confer continuous porosity through the full thickness of the film. The increase in permeability was therefore attributed to increased void volume in the film allowing Increased polymer chain motion - plasticised by the progressive percolation of water into the film as the additive leached. The inability of such films to transport electrolyte showed that continuous porosity did not exist, and the linear increase in permeability Is considered to Indicate a gradual thinning of the barrier in the region of a leached channel, upto the point when the pore does span the full film thickness. Porosity was confirmed by the sudden ability of a film to exhibit transport of electrolyte at a certain level of additive addition. The porosity was, however, of an effective diameter similar to that of the solute permeants, and increased little, with increasing additive load, before the film became structurally weakened. The ability to retain porosity following additive leaching was dependent on the polymer either being kept below its glass transition temperature, or kept wet: the process of drying allowing a soft-polymer, porous, film to heal.

The water vapour permeability of additive containing films was little affected by the additive as a result of the film not swelling to the same extent as hi liquid water. Water (in filled channels) Inhibits diffusion to a lesser extent than does polymer such that whilst the diffusion of a solute permeant may be less Impeded if it can remain in water without having to dissolve in polymer, the same is not true for the water vapour (or gas) whose diffusion can be further impeded by the presence of an additive which is less permeable than the polymer.

Access to latex particle functionality was achieved with the use of leachable additives, where it was not possible in the additive-free film.

Item Type:

Thesis

Description:

A collaborative programme between The Chemical and Biological Defence Establishment, (M.o.D.), Porton Down, and The Nottingham Trent University.

Creators:

Steward, P.A.

Date:

1995

ISBN:

9781369312867

Identifiers: