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Polyelectrolyte stabilised latices part 1, preparation
Coltoids and Surfaces. 3 (1981) 119-129 Etsevier Scientific Publis&ing Company,
POLYEUZCTROLYTE PART
STABILISED
119
LATICES
1,PREPARATION
TERENCB ICI Ltd.. dotn)
Amsterdam- Printedin Belgium
CORNER Corpomte
Laboratory.
P.0.
Box
II.
The Heath, Runcorn.
(Receiwd Uarch 1 lth. X980; accepted in final form December
Cheshire (United
King-
29th. 1980)
AEJ.STRACT Polystyrene (PST) latex dtppekarrs have been prepared by the disperaiott poZyme&ation of atyreine fn aIcobd/water mirtureo contaMng paCyektralyfes_ The latex pactictes are stabilteed by potydectrofyta mokutes which are irreversibly bound to them, The Infhaence of styrene concentration, potyekctrotyte type, mokufar weight and coucentrtitIon. Jcahol/wate; ratio and EMator concentration on the ulthnate partkcle s&e and surface charge density have been investrgated- U&g palyacryk acid (PAA), latkes with partI& diameters In the size range 03-2.2 #rn and with surface charge densities in the range WI--260 pC cm-* have been prepared.
lEiTRODUCTION
Aqueous dispersions are commonly stabilised by the use of p&electrolytes, indeed naturalIy occurAng polyelectrolytis are pexhaps the oldest type or’ stabiliser- However, a fundamental study of the mechankm by which poIyebcttotytis stabilise dispersions is lacking. This is perhaps eurprising but is most probably attributabIe to the lack of a simple made1 for study. Ideally, monodisper~ spherical non-deformable particclesstabUsed by covalently banded, terminally anchored polyelectrolyte molecules are required for which the particles and the polyelectrolyte can be characterised separately. With such particles the influence of various parameters, such as the typz and the mokcalar weight of the pofyelectrolyte, the type and concentration of added eIeckolytes and other poIymeIs etc., on the stabiIify and on other properties of the dispersions could be investigated. The work described in this paper was initiated in an attempt to provide latex dispersions which would satis& most if not ELII of these criteria and which wouId enable the mecha.nismby which poIyeIectroIytes stabiIise dispersions to be inve:rtigated. The poIyme&ation method to be described is a further example of aqueous dispersion polymerisation [l, 21. It was originally investigaw because it WZB feIt that an aqueous dispersion polymerisation method, analogous to the nonaqueous dispersion (NAD) polymetiation of methy methacrylate (MMA) in 016tX622/81/0090-00O0~$2_S0
Q
1981 Ekevier
Scic?ntific Publishing
Company
120
aiiphatic hydrocarbons [3] using degraded rubher as a stab-r, might be possible. Thus, the components of a NAD polymerisation wepe repiaced a% foilows: MMA by styrene; Gphatic hy&ocarbon by aqueous alcohol; degraded rubber by polyelzckolytes. The infiuence of some of the experimental parameters on the ultimate particle size and surface charge density of latex dispersions prepared by this aqueous dispersion polymerisation method has been investigated, EXPEREUENTAL
Styrene was treatid with 10% aqueous N&H to remove inhibitor, dried over anhydrous MgS04 and then distilled under reduced nitrogen pressure. 2,2’Azobis-(2-me~ylpropionit~te) (ADIB) was rt-crystallisedfrom diatilIeclethanol to remove any peroxide Iminnities and the water was double distilled, Waterdamped benzoyl peroxide (BzP) of reag&ntme was used as supplied by BDH Ltd., the quantities of EzP being corrected by assuming a water contint uf 25% w/w. The polyacrylic acids (PAA’s) weye supplied by Allied Colloids Ltd, of Bradford, Engiand. They were designate4 E6, E7, ES, El1 and El3 corresponding to moleculsrrweights of 3,580; 27,c186;76,000; 230,Oc)Oand 1.0 X l@ respectiveiy. other polyelectra~ytcs were prepared by normal methods.
Ldex prepardion The pofy&ctro1yte-stabilised iatices were prepared by the polymerisation of styrene monomer in a solution of the potyelectrolyte in an alcohol-water mixture adjusted so that sufficient alcohol was present to solubilise the atyren~ The agitation speed was fixed at 360 revolutions per min for ah experiments although it was later found not to be critical. A Qpical exampIe of the method is as foiiowx, A sofution of 5 g PAA (E13) in 600 cm3 ethanol and 280 cm3 watir was charged into the reaction fiask, set in a water bath at 78°C. Nitrogen was bubbhd through the sofution for 2 h after which 0.2 g BzP was added and the resultant solution allowed to degas for 8-12 h. After this time a solution of 1 g BzP in 50 cm3 styrene and 20 cm’ e&anol was charged into the reaction flask. Polymerisation was a!Iowed to proceed for 8 h at 78°C luider a stream of nitrogen to give a PST IateH with a mean particie diameter of 0.6 Mm and a cuefficient of variation of
121
Clean-up procedure
Typically 20-50 g of Iatex wem subjected to a series of centrifugationlredlape18ioncycies in 900 cm3 of 1: 1 voIume ratio alcohol: water mixtures (3 times) folIowedby centrifugation&diapersian in water (6 times). This procedure was ahown to be mo~le than adequate for the remtiv81of unraacted styrene, poIyeIecfxoIyte and initiator residues etc., by the analysis of the supernatants obtained aftc&each successivecenkifugation. Bfecfron
microscopy
The modal diameters of Iatices were determined by measuring the diameters of pa_ticIes on electron micrograph negatives obtained by convention& pr’r cedures. UnIessstatedotherwise, between 20~300 particles were measured for each latex in order to determine the mean and the standard deviation from the mean. The coefficient of variation \ras then c&dated. Error3 in particle size wen+ estimated to be < k 10%. L&ices prepared using PAA and poIy(metiacryk acid) were found to have dmoat monorlisperre particle size distributions. Coefficients of varIaSon were always found to be < 10% md quiti often they were
Pig.
1. Eleclranmicragwaphs of polyacrylic acidstabilised polystyrene tatices.
The amount of PA G in sotution or associated with particles in latex dispersions was determined by conductometric titration. A Wayne-Kerr autabaIance universal bridge 8642 with a dip type ceII wss used to measure conductivity and the meter readout was fed to a chari;recorder. Titrant, usually 0.1M NaOH, wetsadtIed cont.inuousSyat a suitable rate by a Radiometer ABU 13 automaEc br-&tte, The titrations were carried out with a Radiometer TTA
122
60 titration assembly which provided stirring, atmosphere con-1 fUJities arId convenient thermostatting. Titrations were carried out at 20°C under a nitrogen atmosphere with continuous stirring, ensuring that the s&&ion or Iatex dispersion was d-d under nitrogen for at least 20 min beforehand. The end pointi were read dkctly from the recorder trace. Since bath the ou&te and the recorder were driven at constant speed the recorder trace was readily calibrated in tears of vohme of titrent added. The cocductivity was not determined absoMeIy since arbitrary vafues wem sufficient for end point determination, RESULTS
PAA strlbU&edtutices Infftrence of s@nme concentmtin OR uttimute part&S2 size. Several series of experiments were c-lied out in which the concentration of stidrenewas varied whilst all other experimental parameters we= held cona&u~t.It was observed that the uftima& particle diameters were found to increaseas the styxene concentration was increased above a lower limit, Mow which nil la&x was produced, and pn upper Iimft above which a prt;yrene swollen gel was obtaIned_ The resuIts obttlined using solutions of 5 g El2 in 600 cm3 ethanol, 280 cm’ water and conUning I g BzP are given iu Tabb 1 tug&her with those for soIutionr c*f 0.5 g E7 in 60 cm’ ethanol, 6 cm3 water containing 0.1 g BzP. AU of the potymerisations were carried out at 78°C. The experiment designated 37/E was terminated after 1 h when a sticky precipitite was produced. All of the other experimenti gave cunvrzrsiansof 100% within bhe 8 h allowed, TABLE
1
Lata designatiooa
PM
37/A 341C 34lB 34/A 31/B 37/B
Ei El
17/B 6/B
El3 El3 El3 El3
6/A 57
El E7 E7
styreae cam.
PaeiEIJ diameter
(mote dm”l
km1
0.008 o.am
No latex
0.240 0.470 0.640 4.200
0.06
0.12 0.23 0.59
0.6 0.7 0.8 1.6 SwoUea gel 0.18 0.24 0.40 0.60
123
Influence qf PAA concentration on ultimate partide s&e. A series of experiments was carried out using solutions of E? in 60 cm’ ethanol, 25 cm3 water and 5.0 cm3 styrene containing 0.2 g BzP, The results are presented in Fig. 2. They show that in the absence of PAA no latex particles are produced and that as the concentration of PAA is increased between 0.1-1090, the particle size obtained appears to pass through a maximumsat -4.0%.
Fig, 2. The effect of PAA conazentrat~on i~Ig paRtiCk diaineteM to be aCCWdt0 t0
on *
particle size_ Error bars were constructed
assum-
10%.
fnfluence of PAA molecuhr zue&ht on ultimate sire, A series of experiments was carried out in which 5 cm3 styrene were poiymerised in solutions of 0.6 g PAA in 60 cu3 ethanol, 26 cm3 water and 0.1 g BzP at 78°C for 6 h. lt was observed that fatices were produced with particIe diameters of about 1.0 pm regardless of the PAA molecuk weight. Influence of alcahol: wuter ratio and alcohol type on ultimate size, Several series of experiments were carried out with PAA’s of various molecular weight and using various styrene concentrations in which the influence of the ethanolto-water ratio on the ultimate particle diameter of latices was investigated. In all cases it was observed that an increase in the ratio of ethanol to water caused an increase in the uItimate particle size of the latices. This effect is shown in Table 2 which contains the results obtained from a series of experiments in which 5 cm3 styrene were polymerised in solutions of ethanol and water containing 0.5 g E7 and 0.1 g BzP at 78°C for 6 h. The effect of replacing ethanol by methanol was investigated by carrying out pairs of experiments at 60°C for 8 h under identical conditions apart from the akohol used. In all cases it was
124 TABLE
2
Dependence
of
particle diameter an ethanol:water
Lacy
VoIume
Vohme
PartLIe
designation
of eihanot (cm’)
ofwater [cm’)
diameter Irm)
31/B
so
35/A
60
25
0.8
61
so
27 28
0.6 0.5
68
50
6
ratIo
1.5
absented that the use of methanol resulted in Iatex dispersions whose ultimati particte size was W-60% as the co-solvent.
of the ultimate size of latices obtained with ethanol
Influence of initiator type and concentfutha an particle diame&er_ Several experiments were caxrkd out in which the concentration of BzP ~18svaried and in which the Bzk? was replaced by ADIB. En tile absence of initiator no latex was produced and no signtEicant dependence of particle size on BzP concenktion in the range 0_06-0.6% was obmrved. When BzP was replaced by AlJIB, nn increase in the ultimate parti& diameter of between IO-2596 was obtained. Dispersii~~ polymer&ration uersus fnoh Con poZymertitiorr. A disperaian was carried out by polymerising 60 cm3 StyTene,5W cm3 ethanol, 280 cm3 water, 6 g E7 and 1.0 g BzP at 78°C. During the polymerisation, aliquok of the dispmion were removed, quenched by addition of hydroquinone soIution and analyzed In order to obtain particle size tlrtd conversion data as a function of time. In a separate experiment, 50 cm3 sfyrene were palymerised in eolution in a mixture of 315 cm3 ethanol and 460 crn3 toluene containing 1.0 g 0zP at 78%. The ratio of tohem fo ethanol was chosen so pO~ym0Ii!Sdtiion
that sufficient etianol was present to provide d simiIar ethanol conCentratian to that used in the dispersion polymerisation whilst ensuring that the PST produced remained in solution, During the solution polymer&&ion, aliquots were removed, quenched and analyad to obtain conversion versus time data,
The resuk of bo*h of these experiments are given in Fig. 3. In the dispersion polymerisation, it was found that the rate of change in both the size of growing lateat particks
and in the ;lercentage conversion of styrene to PST accelerated after an initial induction period, It was obsenrecl that the rate of conversion of styrene to PST during this induction period of the diaper&on polymerisation was quite similar to that measured in the equivalent soIution polymerisation. PST htew
dispemfons
stabilized by olherpolyelecvolytea
PST latex dk
pensionsstabilized by poly(methap ylic acid) can easily be prepared by the dispizmion poIym&sation method described above. By altetig the experimental
126
Pig. 3. Perwntage conversiun (*I and padide size (a) as a furwfol: of timtt far a dispersion patyme&atIon and pementage conversion as L\function of time far a sotutian polymerisation (a}.
paramebrs in the manner described for PAA, latex dispersions with particle sizes in the range 0.2-1.0 pm have been prepared, However, the dispersion polymetiation method has a much wider rage of applicability than simple po,ly(ca~boxytic acids). Other poIyelectrolytes which hwe been used successfully as stabilisers by the author are listed in Table 3 v rhich records the results of experiments carried out by polymerking 5 cm’ styrene in the presence of TABLE
3
Polystyrene latices prepared in Lhe presence of other polyetectrolytes PolyelecGofyte
Par Ltcle
diameter bm) PoIy(methacryIIc acid) Poty(ethylene 6uIphonate) Poly(atynne aufphonate) Paty(2-acrylamida-2-methyll-propane nrlphonlc acid) Poly(4-vinyl pyridinium n-butyfchkride) UgnosurphoMtes Gelatin Gum arab?c
Poly(N-vinyl pyrroIfdone) Poly(rthyXene imine) Poly (2 and 4 vinyl pyridinq) -.
1.0 1.4 0-s 0.3 0.8 0.3 1.0
0.2 0.6 0.6 0.5
126
0.5g of the po1yelectroIyteand 0.1 g BzP in 50 cm3 ethanot and 25 *?rn)water at 78°C for 8 h. The particle size da& given in Table 3 were determined by counting 0nIy W-100 particles. t’onductometitc CitmCkz d&a for PM sfaabilisedbtkes. A number of rigorously purified PAA stabilised Iatices with narrow particle size distributions i.e. with coemciente of variation of < 5%, were titrated with N&H using conductance to determine the endpoints. From the titres, the number of titratable carboxyl groups present in each latex was determined. Using surface areas calculated from particle sizes determined by electron microscopy, the surface charge densities of the la&es were c&uIated. These data wem then used to cakuWe the area per carboxyl group, the weight of PAA per unit surface area of latex i.e. the adsorption density and the surface atea per adsorbd m&c& of PAA. Resuhs expressed in these ways wz given in Table 4 fur several PAA latex disperrions. The results are typicai of those ob-lahled for PAA &biked latex disper;lfons prepared by the aqueous dispersion pofymerisation method. Indeed, very similar results have also been obtained for poly(methacrylic acid) stabilised latex dispersions. TABLE
4
Analysis of conductometric titration dataobtained dbpersions PAA
24 67 72 621B WA
EB E7
R7 RI1 El3 El3
Farticle radius tin-0 0.26 0.16 0.16 0.20 0.1.' 0.20
8urface charge
ClEW3ity [rlC cll-‘) 100
LX 116 12B 244 159
fnorsome PAA stablllsed PST la&x
Athrptkn density (mc m-=1
Surface area per mokute
of PAA Inma)
-_ 0.76 0.87 am a9ci 1.80 1.20
8 62 68 400 910 1400
Dl!XXJSSION
The detailed mechanism by which la&r particles are produced and stabWed during the aqueous dispersion polymerisations described above cannot be deduced from the limited dr,‘cahii!: have been obtained. However, the influence of the vtious experimental parameters on the ultimate particle sizes of Iafs~ dispersions crtnbe readily explained in terms of a mechanivm analogous to that which operates in the NAD polymerisation of MMA u&g degraded rubher as a stabiliser precursor [3J - Zn such NAD polymer&&ions, it has been shown that tie fhsf prodrlcts are small amounts of low-mokcular weight PMLMA together
127
with a quantity of rubbwg-PMMA graft copolymer, The next step in the palymerisation is thought to be the precipitation of PMMA followed by i& stabilisation by the adsorption of the graft copolymer which has been generated in situ. The formation of the ~:.%ftcopolymer is dependent upon fhe generation of reactive free radical sites on the degraded rubber moIecuIes by hydrogen abstraction or by radical addition to double bonds. Such in situ fonnatiun of graft copolymer or stabiliser in NAD poIymerisations. even when using preformed stabiliser, is very difficult to eliminate entireIy because of the ease with which random grafting can occur especially with peroxide-type free radical initiators. It therefore seems reasonable to postulate that the PST dispersions which have been prepared during the present work are stabilised by PAA-g-PST graft copolymer which has been formed in situ. Certainly it is not possible to distinguish between a typical NAD polymerization of MMA using daraded rubber and of styrene uslnngPAA in alcohol-water mixtures by visual observation. The onset of opalescence as particle nucleation begins follow& by a gradual change to a milky whiti latex is identical in both instances_ Thus, it is likely that in the early stages of the aqueous dispersion poIymeris&ions PST and small amounts of PAA-g-Pm graft copolymer are formed. Then, as the PST precipitates due to the medium having been designed to be zt solvent for styrene but a non-solvent for PST, aggregates of precipitated, monomer-swollen PST will form onto which PAA-g-PST gmft copolymer wili adsorb. Colloidally stable PST particles produced in this way will then continue to grow until the supply of styrer 3 monomer is exhausted. In the present work an increaa! in the styrene concentration, the ratio of alcohol to water, and the use of ethano: innsteadof methanol cause latices with increased particle size, but of sindar size distribution, to be produced than would have been produced under otherwise identical conditions, Such variation of these experimental parameters results in an increase in the solvency of the polymerisation medium towards PST. ConsequentIy the tendency of any PAAg-PST graft copolymer pressnt in the polymerisation reaction medium to either associate with growing PST domingparticle formation or to anchor to pa&i&s which have already been produced, will be reduced. This in turn would be expect& to cause the stabtiimtion of a smaI.Iernumber Gf particles which will consequently grclw to a larger size than would otherwise have been produced. TIie repracement of BzP by AD16 and of PAA by PMA aIso results in -c;he formation of Iatex partictes with Iarger particle sizes than are obtinrd under otherwise identical conditions. These changes can both be expected to ause a reduction in the amount of PAA-g-PST graft copolymer produced in a given time since the likelihood of random grafting will be reduced. Again the practical outcome is expected to be the formation of fewer pazticfes which then grow to a larger size than they arrouldotherwise have done. No change in particle s&z wzs observed when either the PAA mokcular weight or the concentration of BzP was varied. Such results are to be expected ii the proposed polymerisation mechanism is operating, The effL& of varying the PAA concentration is ambiguous. The particIe size was obsenred to increase
128
with ircreasing FAA concentration up to 4,076 which is apposite to what would be expected on the basis of the proposed mechanism. However, at 10.4% PAA a considerabh reduction in partick size was obsemed. Further support fc.t the hypothesis that the mechanism which 0~ &es in the aqueous dispersion polymerisations is analogous to that which ope&es in NAD polymerisatirlns comes from a comparison of the changes in particle size and rate of conversion with time in the >wo types of polympzisationsThe;f are very simiIar indeed and it has also been observed that the initial rate of conversion in the aqueous dispersion polymerisation is very similar to that in the equiv&nt solution poXyyraerisation, a result which is obtained when NAD polymerisations are compared to their equivalent solution polymeristions. Furthermore, maIysis of PST produced in an aqueous dkpr?lsionpolymerisation and in the equivalent solution polymerisation gave average molecular weighte of 76,000 and 11,000 respectively. Again it is to be expected that a much higher-molecular-weight polymer would be obt&ned from a dispersion polymer&&ion as compared to that from an equivalent solution poIwmisation. The finat piece of evidence comes from an a&yais of the data sbtaincd by conductometric titration of c!eaned-up latex dispersions. The datolahow that purified latex dispersions posz3esstotal titx&dbIe charges In the region of 80-260 PC cm”, These values are significvltly kger than those obtained far conr*entionaIcharge stabilised I&ices prepared by persulphati initiators, even when an ionisable ca-monomer is utilised. [See, for example, refs 4-q These levels of titr&abb charge are equivalent to about 0.5-2.0 mg of PAA per m* of PST surface, values Rhich are comparable to experimental adsorption densities of non-ionic waterGoIubk palyme= on PST la&es IS]. From the experimentally determiaed surface charge densities of the Iatices, the wea of SWface per mokcule of PAA can easily be obtained. Values obtain*1 for several PAA-stabilkd I&ices are given in Table 6 together with values for the rms radius of gyration (?)H and the KMSend-to-end distance (?)fi for PAA’s in solution in l,Q&oxane, a B solvent for FAA [9J The everage spwing of the l
TABLE
5
A comparison
of
PAA rms dimensions
with the average spacing of FAA makuks
latex particles L;iter designation
PM
molecular weight (x
24
67 72
on PST
10-q 1.0 3.0 3.0
Area of
surface per motecute of PAA
8 62 58
Average spacing per PAA
moIeccI?e(run)
(OnA’)
2.8 7.2
rms vahs (F’,l*
(E*jN
(nm)
(nm)
3.9 10.9
1.6 4,s
&El
iao.0 23.0
910 400
7.6 30.1 20.0
10.9 66.5 31.9
45 27.2 13.0
6/A
lOO.il
1400
37*4
66.6
27.2
12!! .
PAA molecuIes on the surface of the latex particles can be estimated by simply taking the square roof of the area per PAA molecule. In their work on NAD polymerisation Osmond and Walbridge [3] determined the surface coverages for Iatex dispersions in hydrticarbon solvents stabilised by poly(laury1 me& acrylate). They found that the average spacing of the stabtiising polymer ~8s independent of particle size and that for high moXecuIarweight, Iinear polymers, anchored at random points along their length, the average spacings of the stabibsing polymer were quite similar to the rms dimensions of the polymer in solution. In the present work insufficient data have been obtained to be able to draw any conclusions on the effect of particle size on the spacing of PAA molecuIes. However, an examination of the data in Table 6 shows that the average spacings of PAA moXecuXes are similar to the rms dimensions of the PAA molecules in solution. The fact that the average spacings of PAA molecules anchored on Iatex particles are nignificantly smaller than th3 rms dimensions of the PAA in soIution is indicative of the PAA on the latex particle being in the form of a ctosely packed three-dimensional layer. The stability of some of the PAA-stabXsed latices in the presence of added electrolyte ia now being studied by determining the critical ffacculation temperatures for the Iatices at various degrees of neutralisation and in etectroIyte aofutions of various mokuity. Initial results show that fully neutralised latices remain stable in saturated sodium chloride solutions. Details of the coiloid stability, the elecfxophcretic mobiLity and of other physical properties of thes la&es will be reported elsewhere. ACKNQWLEDCEMENTS
The author wishes to thank A, Price for carrying out much of the experimental work which is reported here and the Management of ICI for permission to publish thb paper. REFERENCES F.K.R. L&In-On, B. Vincent aad F.A_ Waite, ColIoidal Dispersions and MicelIar Behaviaur, AC.8. Symp. Ser., 8 (1875) -~65C. CowelI, F.K.R. L&In-On and 8. Vincent. tins. Faraday Sot., 74 (1978) 337. D.J. Walbridge. in K.&l. Barrett (Ed.), Dispersion Palymerisation in Organic Media, Wiley, New York. 1975, p- 80. R.H, OttewfIl and T. Walker, Trans. Faraday Sot., 70 (1974) 9 17. A. Katera, K. Furusawa aad Y. Ta7reda. Kollaid 2.Z Polym., 39 (1970) 677. A. Homola and R-0. Jsmes, J. Colloid Interface Sci., 69 (1977) 123. R.W. Greene, D.P. She&z and T.D. Filer, J. (=ol!aid Interface Sci., 32 (1970) 99. M.J. Harvey, Th.F. Tadros and B. Vincent, J. Cobid Interface Sci., 65 (1976) 440. 3. Brandmp and E-II. Immergu:, Polymer Handhook. Wiley-Interscience, New Yark, 1966, Ch. IV, p. 63.
POLYEUZCTROLYTE PART
STABILISED
119
LATICES
1,PREPARATION
TERENCB ICI Ltd.. dotn)
Amsterdam- Printedin Belgium
CORNER Corpomte
Laboratory.
P.0.
Box
II.
The Heath, Runcorn.
(Receiwd Uarch 1 lth. X980; accepted in final form December
Cheshire (United
King-
29th. 1980)
AEJ.STRACT Polystyrene (PST) latex dtppekarrs have been prepared by the disperaiott poZyme&ation of atyreine fn aIcobd/water mirtureo contaMng paCyektralyfes_ The latex pactictes are stabilteed by potydectrofyta mokutes which are irreversibly bound to them, The Infhaence of styrene concentration, potyekctrotyte type, mokufar weight and coucentrtitIon. Jcahol/wate; ratio and EMator concentration on the ulthnate partkcle s&e and surface charge density have been investrgated- U&g palyacryk acid (PAA), latkes with partI& diameters In the size range 03-2.2 #rn and with surface charge densities in the range WI--260 pC cm-* have been prepared.
lEiTRODUCTION
Aqueous dispersions are commonly stabilised by the use of p&electrolytes, indeed naturalIy occurAng polyelectrolytis are pexhaps the oldest type or’ stabiliser- However, a fundamental study of the mechankm by which poIyebcttotytis stabilise dispersions is lacking. This is perhaps eurprising but is most probably attributabIe to the lack of a simple made1 for study. Ideally, monodisper~ spherical non-deformable particclesstabUsed by covalently banded, terminally anchored polyelectrolyte molecules are required for which the particles and the polyelectrolyte can be characterised separately. With such particles the influence of various parameters, such as the typz and the mokcalar weight of the pofyelectrolyte, the type and concentration of added eIeckolytes and other poIymeIs etc., on the stabiIify and on other properties of the dispersions could be investigated. The work described in this paper was initiated in an attempt to provide latex dispersions which would satis& most if not ELII of these criteria and which wouId enable the mecha.nismby which poIyeIectroIytes stabiIise dispersions to be inve:rtigated. The poIyme&ation method to be described is a further example of aqueous dispersion polymerisation [l, 21. It was originally investigaw because it WZB feIt that an aqueous dispersion polymerisation method, analogous to the nonaqueous dispersion (NAD) polymetiation of methy methacrylate (MMA) in 016tX622/81/0090-00O0~$2_S0
Q
1981 Ekevier
Scic?ntific Publishing
Company
120
aiiphatic hydrocarbons [3] using degraded rubher as a stab-r, might be possible. Thus, the components of a NAD polymerisation wepe repiaced a% foilows: MMA by styrene; Gphatic hy&ocarbon by aqueous alcohol; degraded rubber by polyelzckolytes. The infiuence of some of the experimental parameters on the ultimate particle size and surface charge density of latex dispersions prepared by this aqueous dispersion polymerisation method has been investigated, EXPEREUENTAL
Styrene was treatid with 10% aqueous N&H to remove inhibitor, dried over anhydrous MgS04 and then distilled under reduced nitrogen pressure. 2,2’Azobis-(2-me~ylpropionit~te) (ADIB) was rt-crystallisedfrom diatilIeclethanol to remove any peroxide Iminnities and the water was double distilled, Waterdamped benzoyl peroxide (BzP) of reag&ntme was used as supplied by BDH Ltd., the quantities of EzP being corrected by assuming a water contint uf 25% w/w. The polyacrylic acids (PAA’s) weye supplied by Allied Colloids Ltd, of Bradford, Engiand. They were designate4 E6, E7, ES, El1 and El3 corresponding to moleculsrrweights of 3,580; 27,c186;76,000; 230,Oc)Oand 1.0 X l@ respectiveiy. other polyelectra~ytcs were prepared by normal methods.
Ldex prepardion The pofy&ctro1yte-stabilised iatices were prepared by the polymerisation of styrene monomer in a solution of the potyelectrolyte in an alcohol-water mixture adjusted so that sufficient alcohol was present to solubilise the atyren~ The agitation speed was fixed at 360 revolutions per min for ah experiments although it was later found not to be critical. A Qpical exampIe of the method is as foiiowx, A sofution of 5 g PAA (E13) in 600 cm3 ethanol and 280 cm3 watir was charged into the reaction fiask, set in a water bath at 78°C. Nitrogen was bubbhd through the sofution for 2 h after which 0.2 g BzP was added and the resultant solution allowed to degas for 8-12 h. After this time a solution of 1 g BzP in 50 cm3 styrene and 20 cm’ e&anol was charged into the reaction flask. Polymerisation was a!Iowed to proceed for 8 h at 78°C luider a stream of nitrogen to give a PST IateH with a mean particie diameter of 0.6 Mm and a cuefficient of variation of
121
Clean-up procedure
Typically 20-50 g of Iatex wem subjected to a series of centrifugationlredlape18ioncycies in 900 cm3 of 1: 1 voIume ratio alcohol: water mixtures (3 times) folIowedby centrifugation&diapersian in water (6 times). This procedure was ahown to be mo~le than adequate for the remtiv81of unraacted styrene, poIyeIecfxoIyte and initiator residues etc., by the analysis of the supernatants obtained aftc&each successivecenkifugation. Bfecfron
microscopy
The modal diameters of Iatices were determined by measuring the diameters of pa_ticIes on electron micrograph negatives obtained by convention& pr’r cedures. UnIessstatedotherwise, between 20~300 particles were measured for each latex in order to determine the mean and the standard deviation from the mean. The coefficient of variation \ras then c&dated. Error3 in particle size wen+ estimated to be < k 10%. L&ices prepared using PAA and poIy(metiacryk acid) were found to have dmoat monorlisperre particle size distributions. Coefficients of varIaSon were always found to be < 10% md quiti often they were
Pig.
1. Eleclranmicragwaphs of polyacrylic acidstabilised polystyrene tatices.
The amount of PA G in sotution or associated with particles in latex dispersions was determined by conductometric titration. A Wayne-Kerr autabaIance universal bridge 8642 with a dip type ceII wss used to measure conductivity and the meter readout was fed to a chari;recorder. Titrant, usually 0.1M NaOH, wetsadtIed cont.inuousSyat a suitable rate by a Radiometer ABU 13 automaEc br-&tte, The titrations were carried out with a Radiometer TTA
122
60 titration assembly which provided stirring, atmosphere con-1 fUJities arId convenient thermostatting. Titrations were carried out at 20°C under a nitrogen atmosphere with continuous stirring, ensuring that the s&&ion or Iatex dispersion was d-d under nitrogen for at least 20 min beforehand. The end pointi were read dkctly from the recorder trace. Since bath the ou&te and the recorder were driven at constant speed the recorder trace was readily calibrated in tears of vohme of titrent added. The cocductivity was not determined absoMeIy since arbitrary vafues wem sufficient for end point determination, RESULTS
PAA strlbU&edtutices Infftrence of s@nme concentmtin OR uttimute part&S2 size. Several series of experiments were c-lied out in which the concentration of stidrenewas varied whilst all other experimental parameters we= held cona&u~t.It was observed that the uftima& particle diameters were found to increaseas the styxene concentration was increased above a lower limit, Mow which nil la&x was produced, and pn upper Iimft above which a prt;yrene swollen gel was obtaIned_ The resuIts obttlined using solutions of 5 g El2 in 600 cm3 ethanol, 280 cm’ water and conUning I g BzP are given iu Tabb 1 tug&her with those for soIutionr c*f 0.5 g E7 in 60 cm’ ethanol, 6 cm3 water containing 0.1 g BzP. AU of the potymerisations were carried out at 78°C. The experiment designated 37/E was terminated after 1 h when a sticky precipitite was produced. All of the other experimenti gave cunvrzrsiansof 100% within bhe 8 h allowed, TABLE
1
Lata designatiooa
PM
37/A 341C 34lB 34/A 31/B 37/B
Ei El
17/B 6/B
El3 El3 El3 El3
6/A 57
El E7 E7
styreae cam.
PaeiEIJ diameter
(mote dm”l
km1
0.008 o.am
No latex
0.240 0.470 0.640 4.200
0.06
0.12 0.23 0.59
0.6 0.7 0.8 1.6 SwoUea gel 0.18 0.24 0.40 0.60
123
Influence qf PAA concentration on ultimate partide s&e. A series of experiments was carried out using solutions of E? in 60 cm’ ethanol, 25 cm3 water and 5.0 cm3 styrene containing 0.2 g BzP, The results are presented in Fig. 2. They show that in the absence of PAA no latex particles are produced and that as the concentration of PAA is increased between 0.1-1090, the particle size obtained appears to pass through a maximumsat -4.0%.
Fig, 2. The effect of PAA conazentrat~on i~Ig paRtiCk diaineteM to be aCCWdt0 t0
on *
particle size_ Error bars were constructed
assum-
10%.
fnfluence of PAA molecuhr zue&ht on ultimate sire, A series of experiments was carried out in which 5 cm3 styrene were poiymerised in solutions of 0.6 g PAA in 60 cu3 ethanol, 26 cm3 water and 0.1 g BzP at 78°C for 6 h. lt was observed that fatices were produced with particIe diameters of about 1.0 pm regardless of the PAA molecuk weight. Influence of alcahol: wuter ratio and alcohol type on ultimate size, Several series of experiments were carried out with PAA’s of various molecular weight and using various styrene concentrations in which the influence of the ethanolto-water ratio on the ultimate particle diameter of latices was investigated. In all cases it was observed that an increase in the ratio of ethanol to water caused an increase in the uItimate particle size of the latices. This effect is shown in Table 2 which contains the results obtained from a series of experiments in which 5 cm3 styrene were polymerised in solutions of ethanol and water containing 0.5 g E7 and 0.1 g BzP at 78°C for 6 h. The effect of replacing ethanol by methanol was investigated by carrying out pairs of experiments at 60°C for 8 h under identical conditions apart from the akohol used. In all cases it was
124 TABLE
2
Dependence
of
particle diameter an ethanol:water
Lacy
VoIume
Vohme
PartLIe
designation
of eihanot (cm’)
ofwater [cm’)
diameter Irm)
31/B
so
35/A
60
25
0.8
61
so
27 28
0.6 0.5
68
50
6
ratIo
1.5
absented that the use of methanol resulted in Iatex dispersions whose ultimati particte size was W-60% as the co-solvent.
of the ultimate size of latices obtained with ethanol
Influence of initiator type and concentfutha an particle diame&er_ Several experiments were caxrkd out in which the concentration of BzP ~18svaried and in which the Bzk? was replaced by ADIB. En tile absence of initiator no latex was produced and no signtEicant dependence of particle size on BzP concenktion in the range 0_06-0.6% was obmrved. When BzP was replaced by AlJIB, nn increase in the ultimate parti& diameter of between IO-2596 was obtained. Dispersii~~ polymer&ration uersus fnoh Con poZymertitiorr. A disperaian was carried out by polymerising 60 cm3 StyTene,5W cm3 ethanol, 280 cm3 water, 6 g E7 and 1.0 g BzP at 78°C. During the polymerisation, aliquok of the dispmion were removed, quenched by addition of hydroquinone soIution and analyzed In order to obtain particle size tlrtd conversion data as a function of time. In a separate experiment, 50 cm3 sfyrene were palymerised in eolution in a mixture of 315 cm3 ethanol and 460 crn3 toluene containing 1.0 g 0zP at 78%. The ratio of tohem fo ethanol was chosen so pO~ym0Ii!Sdtiion
that sufficient etianol was present to provide d simiIar ethanol conCentratian to that used in the dispersion polymerisation whilst ensuring that the PST produced remained in solution, During the solution polymer&&ion, aliquots were removed, quenched and analyad to obtain conversion versus time data,
The resuk of bo*h of these experiments are given in Fig. 3. In the dispersion polymerisation, it was found that the rate of change in both the size of growing lateat particks
and in the ;lercentage conversion of styrene to PST accelerated after an initial induction period, It was obsenrecl that the rate of conversion of styrene to PST during this induction period of the diaper&on polymerisation was quite similar to that measured in the equivalent soIution polymerisation. PST htew
dispemfons
stabilized by olherpolyelecvolytea
PST latex dk
pensionsstabilized by poly(methap ylic acid) can easily be prepared by the dispizmion poIym&sation method described above. By altetig the experimental
126
Pig. 3. Perwntage conversiun (*I and padide size (a) as a furwfol: of timtt far a dispersion patyme&atIon and pementage conversion as L\function of time far a sotutian polymerisation (a}.
paramebrs in the manner described for PAA, latex dispersions with particle sizes in the range 0.2-1.0 pm have been prepared, However, the dispersion polymetiation method has a much wider rage of applicability than simple po,ly(ca~boxytic acids). Other poIyelectrolytes which hwe been used successfully as stabilisers by the author are listed in Table 3 v rhich records the results of experiments carried out by polymerking 5 cm’ styrene in the presence of TABLE
3
Polystyrene latices prepared in Lhe presence of other polyetectrolytes PolyelecGofyte
Par Ltcle
diameter bm) PoIy(methacryIIc acid) Poty(ethylene 6uIphonate) Poly(atynne aufphonate) Paty(2-acrylamida-2-methyll-propane nrlphonlc acid) Poly(4-vinyl pyridinium n-butyfchkride) UgnosurphoMtes Gelatin Gum arab?c
Poly(N-vinyl pyrroIfdone) Poly(rthyXene imine) Poly (2 and 4 vinyl pyridinq) -.
1.0 1.4 0-s 0.3 0.8 0.3 1.0
0.2 0.6 0.6 0.5
126
0.5g of the po1yelectroIyteand 0.1 g BzP in 50 cm3 ethanot and 25 *?rn)water at 78°C for 8 h. The particle size da& given in Table 3 were determined by counting 0nIy W-100 particles. t’onductometitc CitmCkz d&a for PM sfaabilisedbtkes. A number of rigorously purified PAA stabilised Iatices with narrow particle size distributions i.e. with coemciente of variation of < 5%, were titrated with N&H using conductance to determine the endpoints. From the titres, the number of titratable carboxyl groups present in each latex was determined. Using surface areas calculated from particle sizes determined by electron microscopy, the surface charge densities of the la&es were c&uIated. These data wem then used to cakuWe the area per carboxyl group, the weight of PAA per unit surface area of latex i.e. the adsorption density and the surface atea per adsorbd m&c& of PAA. Resuhs expressed in these ways wz given in Table 4 fur several PAA latex disperrions. The results are typicai of those ob-lahled for PAA &biked latex disper;lfons prepared by the aqueous dispersion pofymerisation method. Indeed, very similar results have also been obtained for poly(methacrylic acid) stabilised latex dispersions. TABLE
4
Analysis of conductometric titration dataobtained dbpersions PAA
24 67 72 621B WA
EB E7
R7 RI1 El3 El3
Farticle radius tin-0 0.26 0.16 0.16 0.20 0.1.' 0.20
8urface charge
ClEW3ity [rlC cll-‘) 100
LX 116 12B 244 159
fnorsome PAA stablllsed PST la&x
Athrptkn density (mc m-=1
Surface area per mokute
of PAA Inma)
-_ 0.76 0.87 am a9ci 1.80 1.20
8 62 68 400 910 1400
Dl!XXJSSION
The detailed mechanism by which la&r particles are produced and stabWed during the aqueous dispersion polymerisations described above cannot be deduced from the limited dr,‘cahii!: have been obtained. However, the influence of the vtious experimental parameters on the ultimate particle sizes of Iafs~ dispersions crtnbe readily explained in terms of a mechanivm analogous to that which operates in the NAD polymerisation of MMA u&g degraded rubher as a stabiliser precursor [3J - Zn such NAD polymer&&ions, it has been shown that tie fhsf prodrlcts are small amounts of low-mokcular weight PMLMA together
127
with a quantity of rubbwg-PMMA graft copolymer, The next step in the palymerisation is thought to be the precipitation of PMMA followed by i& stabilisation by the adsorption of the graft copolymer which has been generated in situ. The formation of the ~:.%ftcopolymer is dependent upon fhe generation of reactive free radical sites on the degraded rubber moIecuIes by hydrogen abstraction or by radical addition to double bonds. Such in situ fonnatiun of graft copolymer or stabiliser in NAD poIymerisations. even when using preformed stabiliser, is very difficult to eliminate entireIy because of the ease with which random grafting can occur especially with peroxide-type free radical initiators. It therefore seems reasonable to postulate that the PST dispersions which have been prepared during the present work are stabilised by PAA-g-PST graft copolymer which has been formed in situ. Certainly it is not possible to distinguish between a typical NAD polymerization of MMA using daraded rubber and of styrene uslnngPAA in alcohol-water mixtures by visual observation. The onset of opalescence as particle nucleation begins follow& by a gradual change to a milky whiti latex is identical in both instances_ Thus, it is likely that in the early stages of the aqueous dispersion poIymeris&ions PST and small amounts of PAA-g-Pm graft copolymer are formed. Then, as the PST precipitates due to the medium having been designed to be zt solvent for styrene but a non-solvent for PST, aggregates of precipitated, monomer-swollen PST will form onto which PAA-g-PST gmft copolymer wili adsorb. Colloidally stable PST particles produced in this way will then continue to grow until the supply of styrer 3 monomer is exhausted. In the present work an increaa! in the styrene concentration, the ratio of alcohol to water, and the use of ethano: innsteadof methanol cause latices with increased particle size, but of sindar size distribution, to be produced than would have been produced under otherwise identical conditions, Such variation of these experimental parameters results in an increase in the solvency of the polymerisation medium towards PST. ConsequentIy the tendency of any PAAg-PST graft copolymer pressnt in the polymerisation reaction medium to either associate with growing PST domingparticle formation or to anchor to pa&i&s which have already been produced, will be reduced. This in turn would be expect& to cause the stabtiimtion of a smaI.Iernumber Gf particles which will consequently grclw to a larger size than would otherwise have been produced. TIie repracement of BzP by AD16 and of PAA by PMA aIso results in -c;he formation of Iatex partictes with Iarger particle sizes than are obtinrd under otherwise identical conditions. These changes can both be expected to ause a reduction in the amount of PAA-g-PST graft copolymer produced in a given time since the likelihood of random grafting will be reduced. Again the practical outcome is expected to be the formation of fewer pazticfes which then grow to a larger size than they arrouldotherwise have done. No change in particle s&z wzs observed when either the PAA mokcular weight or the concentration of BzP was varied. Such results are to be expected ii the proposed polymerisation mechanism is operating, The effL& of varying the PAA concentration is ambiguous. The particIe size was obsenred to increase
128
with ircreasing FAA concentration up to 4,076 which is apposite to what would be expected on the basis of the proposed mechanism. However, at 10.4% PAA a considerabh reduction in partick size was obsemed. Further support fc.t the hypothesis that the mechanism which 0~ &es in the aqueous dispersion polymerisations is analogous to that which ope&es in NAD polymerisatirlns comes from a comparison of the changes in particle size and rate of conversion with time in the >wo types of polympzisationsThe;f are very simiIar indeed and it has also been observed that the initial rate of conversion in the aqueous dispersion polymerisation is very similar to that in the equiv&nt solution poXyyraerisation, a result which is obtained when NAD polymerisations are compared to their equivalent solution polymeristions. Furthermore, maIysis of PST produced in an aqueous dkpr?lsionpolymerisation and in the equivalent solution polymerisation gave average molecular weighte of 76,000 and 11,000 respectively. Again it is to be expected that a much higher-molecular-weight polymer would be obt&ned from a dispersion polymer&&ion as compared to that from an equivalent solution poIwmisation. The finat piece of evidence comes from an a&yais of the data sbtaincd by conductometric titration of c!eaned-up latex dispersions. The datolahow that purified latex dispersions posz3esstotal titx&dbIe charges In the region of 80-260 PC cm”, These values are significvltly kger than those obtained far conr*entionaIcharge stabilised I&ices prepared by persulphati initiators, even when an ionisable ca-monomer is utilised. [See, for example, refs 4-q These levels of titr&abb charge are equivalent to about 0.5-2.0 mg of PAA per m* of PST surface, values Rhich are comparable to experimental adsorption densities of non-ionic waterGoIubk palyme= on PST la&es IS]. From the experimentally determiaed surface charge densities of the Iatices, the wea of SWface per mokcule of PAA can easily be obtained. Values obtain*1 for several PAA-stabilkd I&ices are given in Table 6 together with values for the rms radius of gyration (?)H and the KMSend-to-end distance (?)fi for PAA’s in solution in l,Q&oxane, a B solvent for FAA [9J The everage spwing of the l
TABLE
5
A comparison
of
PAA rms dimensions
with the average spacing of FAA makuks
latex particles L;iter designation
PM
molecular weight (x
24
67 72
on PST
10-q 1.0 3.0 3.0
Area of
surface per motecute of PAA
8 62 58
Average spacing per PAA
moIeccI?e(run)
(OnA’)
2.8 7.2
rms vahs (F’,l*
(E*jN
(nm)
(nm)
3.9 10.9
1.6 4,s
&El
iao.0 23.0
910 400
7.6 30.1 20.0
10.9 66.5 31.9
45 27.2 13.0
6/A
lOO.il
1400
37*4
66.6
27.2
12!! .
PAA molecuIes on the surface of the latex particles can be estimated by simply taking the square roof of the area per PAA molecule. In their work on NAD polymerisation Osmond and Walbridge [3] determined the surface coverages for Iatex dispersions in hydrticarbon solvents stabilised by poly(laury1 me& acrylate). They found that the average spacing of the stabtiising polymer ~8s independent of particle size and that for high moXecuIarweight, Iinear polymers, anchored at random points along their length, the average spacings of the stabibsing polymer were quite similar to the rms dimensions of the polymer in solution. In the present work insufficient data have been obtained to be able to draw any conclusions on the effect of particle size on the spacing of PAA molecuIes. However, an examination of the data in Table 6 shows that the average spacings of PAA moXecuXes are similar to the rms dimensions of the PAA molecules in solution. The fact that the average spacings of PAA molecules anchored on Iatex particles are nignificantly smaller than th3 rms dimensions of the PAA in soIution is indicative of the PAA on the latex particle being in the form of a ctosely packed three-dimensional layer. The stability of some of the PAA-stabXsed latices in the presence of added electrolyte ia now being studied by determining the critical ffacculation temperatures for the Iatices at various degrees of neutralisation and in etectroIyte aofutions of various mokuity. Initial results show that fully neutralised latices remain stable in saturated sodium chloride solutions. Details of the coiloid stability, the elecfxophcretic mobiLity and of other physical properties of thes la&es will be reported elsewhere. ACKNQWLEDCEMENTS
The author wishes to thank A, Price for carrying out much of the experimental work which is reported here and the Management of ICI for permission to publish thb paper. REFERENCES F.K.R. L&In-On, B. Vincent aad F.A_ Waite, ColIoidal Dispersions and MicelIar Behaviaur, AC.8. Symp. Ser., 8 (1875) -~65C. CowelI, F.K.R. L&In-On and 8. Vincent. tins. Faraday Sot., 74 (1978) 337. D.J. Walbridge. in K.&l. Barrett (Ed.), Dispersion Palymerisation in Organic Media, Wiley, New York. 1975, p- 80. R.H, OttewfIl and T. Walker, Trans. Faraday Sot., 70 (1974) 9 17. A. Katera, K. Furusawa aad Y. Ta7reda. Kollaid 2.Z Polym., 39 (1970) 677. A. Homola and R-0. Jsmes, J. Colloid Interface Sci., 69 (1977) 123. R.W. Greene, D.P. She&z and T.D. Filer, J. (=ol!aid Interface Sci., 32 (1970) 99. M.J. Harvey, Th.F. Tadros and B. Vincent, J. Cobid Interface Sci., 65 (1976) 440. 3. Brandmp and E-II. Immergu:, Polymer Handhook. Wiley-Interscience, New Yark, 1966, Ch. IV, p. 63.