Thesis Title: Modification of the Permeability of Polymer Latex Films.By Paul A. Steward. |
A project performed in collaboration with the Chemical and Biological Defence Establishment (CBDE, now part of the Defence Science & Technology Laboratory), Porton Down, the research was of interest with the modern trend to move away from organic based solvents to 'greener' aqueous based products for use as coatings, etc.
The aims of the project
were to investigate latex-cast films with respect to:
Additionally, the films were tested for phenomena previously observed by other researchers in the field,
ie, asymmetric permeability properties arising from morphological or structural features (such as differing face morphologies or film stratification); and film formation/morphology was investigated with respect to particle coalescence. The surface functionality (see also review page on Latex Surface Chemistry) and particulate nature of a latex, as opposed to a polymer in solution, yields films that have a correspondingly
different, particulate, structure which affects the film's permeability properties.
In order to achieve the aforementioned aims, film permeability properties were investigated using methods such as:
- UV spectrophotometry, along with conductivity measurements and radio-labelled tracers, to monitor solute permeability;
- a modified British Standard method & apparatus to investigate gas permeability; and
- gravimetric measurements to measure water vapour permeability coefficients.
The project's bias towards a study of films for controlled or sustained release-type coatings was taken into account when selecting leachable additives for study. Types of additive considered included:
- soluble polymers such as hydroxypropyl methylcellulose (HPMC), and a soluble anionic copolymer latex: poly(methacrylic acid, ethyl acrylate);
- sucrose;
- salts;
- plasticiser;
- and surfactant.
Polymers used for film formation included a range of surfactant-free alkyl methacrylates:
- poly(butyl methacrylate), poly(amyl methacrylate), and poly(hexyl methacrylate), which were prepared as part of the project; and
- members of the commercially available Eudragit® range of methacrylate/acrylate copolymers.
The following text is the Abstract from the Thesis (with links added to enable graphical data to be viewed).
Thesis 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. (See 4-nitrophenol permeation graph.)
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 (see 4-nitrophenol permeability trend graph) 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, up to 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 (see electrolyte permeability graph). 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 in 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.
Copyright © Paul Steward 1995.
Chapters within the Thesis |
Section 1: Literature Reviews.
Chapter 1: Introduction and Background.
Chapter 2: Preparation and Properties of Polymers and Polymer Colloids.
Chapter 3: Latex and Polymer Characteristics.
Chapter 4: Latex Film Formation and Properties.
Chapter 5: Diffusion and Permeation in Polymer Films.
Chapter 6: Controlled Release Methods and Devices.
Section 2: Experimental.
Chapter 7: Procedures and Analysis of Results.
Section 3: Results and Discussion.
Chapter 8: Morphology and Permeability of Eudragit® Films.
Chapter 9: Morphology and Permeability of Surfactant-free Films.
Chapter 10: Sucrose as a Film Additive.
Chapter 11: Film Additives Other Than Sucrose.
Chapter 12: Functionalised Latex Films.
Section 4: Conclusions.
Chapter 13: Final Summary and Conclusions.Chapter 14: References.
Appendix A: Derivation of Formulae used in Analysis of Experimental Data.
Appendix B: Philips User Programming Software. (Programs used to control automated UV/vis spectrophotometer.)
Copyright © Paul Steward 1995.
Revised 05 June 1998 02:52:33 pm
paul.steward@initium.demon.co.uk
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NOTES:
As an example of a morphological feature, this electron micrograph shows sucrose (which was used as an example of a leachable additive based on its potential to be molecularly dispersed, its low molecular weight, and high water solubility) exuding from the upper surface of a commercially available copolymer of methyl methacrylate and ethyl acrylate. The exudations show a base diameter of ca 50 µm, which compares to a latex particle diameter of ca 163 nm (when measured by photon correlation spectroscopy). The orientation of this micrograph is such that the (freeze-fracture) cross-section of the film is seen to the left hand side of the frame, and the polymer-air interface of the film (as opposed to the polymer-casting substrate interface) is seen to the right hand side of the frame.
Such exudations of additives are typical of films in which the additive shows some incompatibility with the polymer, and in this case, the exudation process was found to be dependent on the humidity of the air around the films.
To see a larger scaled up EM of the above, Click Here. (18Kb JPEG file; typically < 1 minute to load.)
To see the upper face of a single exudation Click Here. (22Kb JPEG file; typically < 1 minute to load.)
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The following graph shows the 4-nitrophenol permeabilities of some of the polymer latex films under investigation. The equation of the plots is derived from Fick's First Law of Diffusion (see Fick's Laws), and is given by:
where: P = permeability coefficient; l = film thickness; V = Volume of chamber of permeability cell (ie, the experiment was performed with the film separating two chambers, of equal volume, of a cell: the permeant diffusing through the film from one chamber to the other from high concentration to low concentration); A = area of film exposed to permeant; t = time; C0 = initial permeant concentration; Ct = permeant concentration at time t. (The units of permeability are m2 hr-1.)
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The following graph shows the effects of the various film additives on the 4-nitrophenol permeability:
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This graph shows the increase in electrolyte permeability as a result of increasing levels of sucrose in a Eudragit NE30D film. Note that there is little increase in permeability before 25% initial sucrose load: the point after which the linear increase in 4-nitrophenol permeability tended to plateau.
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