Preparation of a Surfactant-Free Poly(Butyl Methacrylate) Polymer Latex.
By Paul A. Steward.
This page contains characters formatted with superscripts & subscripts. It should therefore be viewed with a browser capable of showing such. Chemicals should only be handled by qualified persons. It is advised that chemical hazard data is consulted before proceeding with the method. Waste materials should be disposed of responsibly and in accordance with any local regulations. The author accepts no responsibility for any consequences arising from this method.
Contents:
1 Introduction.
2 Preparation.
2.1 Apparatus and Materials.
2.2 Cleaning of Glassware.
2.3 Preparation of the Monomer.
2.3.1 Testing for Quinol.
2.4 Preparation of Initiator.
3 Polymerisation Method.
4 Polymerisation Reaction Characterisation.
4.1 Percentage Conversion of Monomer to Polymer.
4.2 Determination of Latex Particle Size.
5 References..
Preparation of a Surfactant-Free Poly(Butyl Methacrylate) Polymer Latex.
1. Introduction.
This page describes the method of preparation of a surfactant-free poly(butyl methacrylate) (PBMA) latex. The method described could be used for an undergraduate project, etc., with the progress of the polymerisation easily characterised with respect to percentage conversion of monomer to polymer by sampling during the course of the reaction. (Full conversion being achieved in 4 to 5 hours at the temperature of 50° C described below.) If the apparatus to measure particle size is available, then particle growth can also be monitored, and from this data, the particle number density can be calculated.The latex is prepared by the process of free radical "emulsion" polymerisation, and the final product, the PBMA latex, is monodisperse and film forming at temperatures above 35 to 40° C. The resultant films makes an ideal subject for electron microscopy, or permeation studies. If it is intended to characterise the latex surface (eg, charge density; Zeta potential, etc.), then obviously all glass should be cleaned as thoroughly as possible, and all materials should be as pure as possible.
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2. Preparation.
2.1 Apparatus and Materials.
The polymerisation recipe requires the following materials to prepare 450 ml of latex of ca 10% polymer solids content:
- Clean (ie, twice distilled) water: 400 g;
- BMA monomer: 45 g;
- potassium persulphate initiator: 0.85 g;
- buffer (eg, potassium hydrogen carbonate): ca 0.2 g, to give pH 7 - 8;
- nitrogen gas (oxygen free), to maintain an oxygen free atmosphere during the reaction.
The reaction itself should be performed in a fume hood, and requires the following apparatus:
(NB. further standard laboratory apparatus is required for preparation of the materials, or for sampling the reaction.)
- constant temperature water bath;
- three-necked round bottomed 1 dm3 flask (or, if it is intended to sample the reaction, then a four necked flask will simplify the task!) with ground glass joints;
- cold water reflux condenser + associated plumbing;
- fine capillary nitrogen bleed + associated tubing;
- PTFE paddle-type stirrer, + motor;
- thermometer.
2.2 Cleaning of Glassware.
All glassware should initially be thoroughly cleaned using organic solvent such as toluene, acetone or butan-2-one. (Since the reaction is performed in the absence of surfactant, it is advisable not to use detergents for cleaning apparatus: especially if it is intended to characterise the surface chemistry of the latex.) After drying, the glassware should be rinsed with either distilled water, or analytical grade water, until the conductivity of the rinsing solution approximates of the clean water (eg, 1.5 µS cm-1, allowing for dissolved CO2).
2.3 Preparation of the Monomer.
The monomers, as supplied, typically contain an inhibitor such as quinol (hydroquinone) to prevent polymerisation during transportation and storage. This must be removed before the polymerisation.It is usual practice to distil the monomer under reduced pressure, and in a nitrogen atmosphere: discarding the first and last ca 5% of distillate. This procedure will remove the inhibitor and any polymer.
In the case of BMA and heavier monomers, however, distillation can be difficult due to the high molecular weight of the monomer. A further method to remove inhibitor, more quickly than by distillation, is by a process of repeated washing [Riddle 1954] of the monomer with a caustic (1%)/carbonate (25%) solution (by vigorous shaking in a separating funnel). (NB. The carbonate simply acts to further decrease the solubility of the monomer in the aqueous solution.) Since the monomer and caustic solution are immiscible, the water forms the lower layer in a separating funnel and can be run off. The monomer can then be given a final rinse of clean water, and allowed to stand overnight (at 5° C in a fridge, to prevent polymerisation) to allow a final separation. (Drying of the monomer is not entirely necessary because of the low aqueous solubility of the monomer.)
2.3.1 Testing for Quinol.
The quantity of quinol inhibitor in BMA can be determined spectrophotometrically using a method obtained from BDH Chemicals.The absorbance of a solution of BMA (10%) in methanol is measured at 295 nm against a methanol blank, in a 1 cm quartz cell. The concentration (parts per million) of quinol is then given by:
After washing BMA monomer using four parts of caustic solution to one part monomer, and distillation, the concentration of quinol in BDH supplied monomer decreased from ca 99.5(±0.2) ppm to 2.7(±1.4) ppm. Washing alone caused a similar decrease, but would not have removed any polymer that may have been present. 
2.4. Preparation of Initiator.
Potassium persulphate (K2S2O8) initiator is recrystallised (typically 24 hours prior to the polymerisation) from a saturated solution of cold water, and dried.
3. Polymerisation Method.
The reaction vessel, fitted with stirrer, condenser, and nitrogen bleed should be sited in the constant temperature bath (itself inside a fume hood) at 50° C. The vessel is initially charged with most of the water to be used (eg, 400 g) and buffered to pH 7 - 8 with potassium hydrogen carbonate. The remaining 50 g of water is held aside to dissolve the initiator.The vessel should be continuously stirred at ca 250 rpm, using the PTFE paddle stirrer, mounted through the central neck of the flask at a height just above the bottom. Following the addition of the 400 g water, nitrogen should be bubbled through the water, and the water allowed to equilibrate to the temperature of the bath. (The temperature can be monitored using a thermometer or thermistor inserted into the vessel through a spare neck of the flask.) The monomer (45 g) is next added to the water, after having been outgassed with nitrogen, and the vessel again left to equilibrate. The nitrogen flow should be continued at a reduced rate (just sufficient to maintain the N2 atmosphere) at this point, so as to prevent evaporation of the monomer.
Finally, the initiator (0.85 g) should be dissolved in the remaining 50 g water, and again outgassed, before being added to the vessel. This point of addition is taken as time zero for the reaction. At this time, the nitrogen bleed should be exchanged for a shorter version whose end remains above the level of the reactants, to prevent it becoming blocked.
Typical reaction time is 5 hours for full conversion. (It is not necessary to proceed to full conversion if the experiment is simply to demonstrate the procedure or monitor reaction rate/percentage conversion, etc.)
4. Polymerisation Reaction Characterisation.
4.1. Percentage Conversion of Monomer to Polymer.
Percentage conversion of monomer to polymer is determined (via percentage solids calculations) gravimetrically by sampling the polymerisation mixture at intervals during the course of the reaction. This provides useful information as to the extent of the reaction, and the time to completion.Samples should be withdrawn from the vessel by means of a pipette inserted through an auxiliary neck of the flask, thereby allowing the N2 atmosphere to be maintained by means of the positive pressure from the bleed. The stirrer should be stopped at ca 2 minutes before sample extraction, to allow phase separation and prevent monomer being removed from the reaction vessel. The pipette should be inserted under the monomer layer, and into the aqueous layer to remove the sample.
On removal from the reaction vessel, the samples should be sealed in specimen bottle, and immediately placed in an ice bath to quench the reaction. (Alternatively, the reaction can be quenched by adding a known amount of quinol. Both techniques being found to be equally effective.)
After cooling, the sample is placed in a dish of known weight, and weighed. (Let this wet weight minus the container weight = WW, the wet sample weight. ) The dish (+ sample) is then placed in an oven at 80° C and left until constant weight is recorded. (Let the dry sample weight, DW, be equal to this weight minus the container weight.) The percentage solids of the sample and, hence, the reaction is then given by:
From the known initial weight of monomer used, the final theoretical weight of polymer, at 100% conversion, can be determined allowing the percentage conversion to be calculated throughout the reaction by sampling at various intervals. 
4.2. Determination of Latex Particle Size.
Latex particle sizing requires the availability of a technique such as, for example, transmission electron microscopy (TEM), or photon correlation spectroscopy (PCS). If these are available to you, then you will obviously be aware of their respective advantages/disadvantages. If not, there is little point discussing them here, other than to say they provide a means to follow the increase in particle diameter as a function of reaction time.It should be noted that the particles will probably be swollen with monomer until the reaction is completed, and this will be included in the particle size, unless the latex samples are cleaned, for example, by steam stripping. For this reason, it is possible that the particles may decrease in size as the reaction nears completion, since the polymer is more dense than the monomer. (Ie, when BMA polymerises to PBMA, the density changes from 0.889 to 1.055 g cm-3.)
If the initial mass of monomer is known, then the final mass of polymer at 100% conversion is assumed equal to this mass. From a knowledge of the density, the final theoretical volume of polymer can therefore be determined, as can the volume of a single particle (assumed to be, and is, spherical) if the particle diameter is known. Hence, the particle number density can also be calculated.
Adapted from: Modification of the Permeability of Polymer Latex Films., Nottingham Trent University PhD Thesis, 1995.
Copyright © Paul Steward, 1995.
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3. References.
Riddle E.H., in Monomeric Acrylic Esters, Analytical Methods, Pub. Rheinhold, Ch. 7, 203-221, 1954. Return to Text.BDH Chemicals Ltd., Poole, Dorset, UK. Return to Text.
Adapted from: Modification of the Permeability of Polymer Latex Films., Nottingham Trent University PhD Thesis, 1995.
Copyright © Paul Steward 1995.Created by Dr. Paul A. Steward.
Last Revised: Monday, June 08, 1998 12:17 PM
Email: paul.steward@initium.demon.co.uk
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