Review of the Surface Chemical Properties of Polymer Latices.
By Paul A. Steward.
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Contents:
1 Introduction.
2 Surface Chemistry.
3 References. See lower frame. (Note you may be able to drag the dividing bar to make the lower frame larger and easier to read.)
Review of the Surface Chemical Properties of Polymer Latices.
1. Introduction.
The control and characterisation of the surface properties of a latex are often required in order that the properties of the latex are understood for its prospective use: be it the formation of a film, as a coating; or for use as a model colloid [Kamel et al. (1981) and Vanderhoff (1980)]. Because of the small size of a latex particle, the extremely high surface area to volume ratio means that the particle's properties are very much dictated by the surface properties. The complete characterisation of a latex obviously requires the determination of the surface charge density, and identification of the source of that charge, due to the fact that this plays such an important function in the colloidal properties of a latex and its stability [Ottewill (1990) and Theodoor & Overbeek (1982)].
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2. Surface Chemistry.
Factors affecting the surface chemistry of a polymer latex particle include the various reactants of the polymerisation recipe such as the initiator type, monomer (and comonomer) functionality, and surfactant type, etc. For example, in surfactant-free polymerisations, the surface properties of the latex are mainly dependent on the initiator type: Goodwin et al. (1978) list a number of common initiators and their possible functional groups. After preparation, the surface chemistry may be changed by the cleaning method employed, or simply from prolonged storage.The ideal model colloid would contain only one type of charged end group. However, in the case of an aqueous, surfactant-free latex, polymerised, for example, from non-polar styrene monomer using potassium persulphate (K2S2O8) as the initiator, it is to be expected that the reaction would follow the typical steps for a free radical reaction: namely initiation, propagation and termination, eg:
The termination step would therefore be expected to lead to the formation of two sulphate endgroups per polymer chain (if chain transfer, etc., is neglected), ie, the initiator is the source of the charged endgroups. If chain transfer is considered, then other endgroups (eg, carboxyl groups) might also be included and, hence, the number of sulphate groups, in practice, is normally less than two [Vanderhoff et al. (1970)]. Grancio and Williams (1970) core-shell theory would imply that these endgroups would reside at the particle surface. However, endgroups buried within the particles may also account for the fact that less than two sulphate groups per chain are detected.
In an aqueous environment, hydroxyl radicals [Kolthoff & Miller (1951)] are possible which are able to initiate polymerisation, leading to the formation of hydroxyl endgroups:
Acidic hydrolysis of sulphate endgroups may also lead to the formation of hydroxyl endgroups [Vanderhoff (1980)], and oxidation of such hydroxyl groups may possibly be the source of the carboxyl endgroups sometimes found during the characterisation of persulphate initiated polymerisations:
The hydroxyl radicals may also prove to be a source of oxygen, by the following disproportionation reaction:
The oxidising behaviour of persulphate initiator can be eliminated by the use of different initiators such as azo compounds (ie, At-N=N-Ar) that decompose forming nitrogen rather than oxygen, eg, a diazo-amino compound:
Vanderhoff found that allowing a persulphate-initiated latex to age at ambient temperature, with the sulphate groups in the H+ form, at low pH (as might result from the polymerisation being un-buffered, and due to bisulphate formation), led to the appearance of carboxyl groups, whilst storage at 90° C (363 K) led to the formation of hydroxyl endgroups.
There is, however, uncertainty as to whether such carboxyl groups should be present, and if so, from what source: eg, chain transfer with buffer, various oxidative side reactions, cleaning method, etc. [Hearn et al. (1981)].
Industrial emulsion polymerisations commonly use redox initiation systems containing sulphites or metabisulphites (in addition to persulphates, etc.) which form sulphonate (HSO3°) radicals and, hence, sulphonate endgroups, in addition to sulphate end groups. McCarvill and Fitch (1978) prepared a sulphonated polystyrene latex, using a bisulphite/Fe(III) redox system alone, with no persulphate:
The final latex contained only strong acid sulphonic groups, and was therefore resistant to the hydrolysis/air oxidation of sulphate systems. Other redox systems include:
Persulphite/Sulphite:
Persulphite/Metabisulphite/Ferrous ion (metabisulphites are derived from bisulphites by the loss of one water; they act as a source of bisulphite ions by hydrolysis):
Hydrogen Peroxide/Ferrous ion:
Persulphate/Reducing agent R:
eg, R = thiosulphate:
or bisulphate:
In many instances, surface functionality is deliberately introduced by means of the reaction recipe, using a functional monomer [Upson (1985)], or by using techniques such as copolymerisation or "shot growth" type addition [Sakota & Okaya (1976)]. In his review of polymer colloids, Fitch (1985) notes that in order to augment functionality at the particle surface, the comonomer should be surface active and insoluble in both the polymer and water. Vijayendran (1979) investigated the carboxylation of polystyrene looking at the effect of a number of carboxylated acidic monomers (eg, itaconic acid (IA {ie, CH2=C(COOH)CH2COOH}), acrylic and methacrylic acids (AA {ie, H2C=CH2COOH} and MAA, respectively). It was found that the more hydrophobic acid (and, hence, the most soluble in styrene) concentrated at the particle core during the polymerisation, in the order MAA > AA > IA, citing ease of diffusion of the MAA into the styrene as the reason, but also noting the lower reactivity of the IA.
From the results of similar experiments investigating the mode of addition of a carboxylated monomer (in industrial-type reactors), Hoy (1979) discusses how the method of addition (be it in the aqueous feed, or the initial reactor charge) has little effect on where the carboxyl groups are finally located: which is on or near the particle surface.
Typical of the functional monomers are: (meth)acrylic acids, for the introduction of carboxyl groups [Sakota & Okaya (1976)]; styrene sulphonate for the introduction of strong acid groups [Kim et al. (1989)]; and tertiary amines such as, for example, CH3N(CH3)C(C2H5)=C(CH3)CONH2 (ie, N, N-dimethylaminoethyl methacrylamide), for the generation of quaternary ammonium salts [Upson (1985)]. Many of these functional monomers are capable of forming polyelectrolyte and, in addition to becoming part of the latex polymer, will be present in the aqueous phase, or weakly bound to the particle and, hence, labile.
Post polymerisation reactions may also lead be used to modify the surface functionality. Typical of this is the oxidation of hydroxyl endgroups to carboxyl groups - necessary to allow for their detection by titration (ie, during latex surface characterisation). However, surface species may also be grafted onto the particles [Ryan (1988)]. Pelton (1990) has reviews a number of reactions that are possible at the latex surface.
In addition to their role as steric stabilisers, surfactants and emulsifiers can also contribute to the stabilising surface charge density due to hydrophilic endgroups (again typically sulphate groups), which may be either physically or chemically bound to the latex particle's surface. Stone-Masui and Watillon (1975) determined that latices prepared in the presence of sodium alkyl sulphate or sulphonate surfactants contained only strong acid groups, after cleaning by ion exchange, whereas those latices prepared in the presence of potassium stearate or sodium laurate (ie, CH3(CH2)16COO-K+ or CH3(CH2)10COO-Na+, respectively) contained weak acid groups in addition to the strong acid groups. (It should be noted that characterisation should be performed immediately after cleaning since it has been shown that the number of strong acid sulphate groups decreases with time [Kamel et al. (1981)] due to hydrolysis or oxidation.)
Fitch and McCarvill (1978) also investigated the ability of surfactant to contribute to surface charge, finding sodium dodecyl sulphate and sodium dodecyl sulphonate (ie, CH3(CH2)11SO-4Na+ and CH3(CH2)11SO-3Na+, respectively) contributed considerably to the number of chemically bound sulphate and sulphonate groups, respectively. Citing the work of Brace (1978 PhD Thesis), who (using radioactively labelled surfactant) showed that the heating of a polymer to near its glass transition temperature (Tg) allowed the surfactant to desorb, Fitch reasoned that since similar treatment of his latices did not cause surfactant desorption, then it must be chemically bound.
Adapted from: Modification of the Permeability of Polymer Latex Films., Nottingham Trent University PhD Thesis, 1995.
Copyright © Paul Steward, 1995.
Created by Paul Steward.
Last Revised: Monday, June 08, 1998 12:12 PM
Email: paul.steward@initium.demon.co.uk
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