-
Synthesis of Butyl Acrylate–Styrene Block Copolymersin Emulsion
by Reversible Addition-FragmentationChain Transfer: Effect of
Surfactant Migration uponFilm Formation
MICHAEL J. MONTEIRO,1 MARIE SJÖBERG,2 JEROEN VAN DER VLIST,1
CHRISTIANNE M. GÖTTGENS1
1 Department of Polymer Chemistry and Coatings Technology,
Eindhoven University of Technology, P.O. Box 513,5600 MB Eindhoven,
The Netherlands
2 Ytkemiska Institutet–Institute for Surface Chemistry, 45
Drottning Kristinas Väg, P.O. Box 5607,114 86 Stockholm,
Sweden
Received 5 May 2000; accepted 31 August 2000
ABSTRACT: The synthesis of block copolymers in an
environmentally friendly mediumwas carried out in emulsion
polymerizations through the reversible addition-fragmen-tation
chain transfer process, using a transfer active xanthate (MADIX)
agent, underbatch and starved-feed conditions. First, ab initio
experiments were carried out toprepare a seed of PBA dormant chains
(i.e., poly(butyl acrylate) (PBA) polymer attachedwith a transfer
active xanthate). The M# n and polydispersity were predicted
accuratelywith numerical simulations and equations derived by
Müller from the method ofmoments. Those seeds were then used in a
second-stage polymerization under starved-feed and batch conditions
to prepare composite polymer colloids of block
PBA-co-poly(styrene). Under starved feed conditions, approximately
90% of total polymerconsisted of blocks, whereas under batch
conditions only 70% consisted of blocks, whichis proposed to be due
a higher entry efficiency and thus greater termination rate.
Thefilms of these latexes were examined by atomic force microscopy.
Surfactant migrationto the surface increased with an increase in
the amount of MADIX, resulting from acombination of a smaller
particle size and a lower average molecular weight. © 2000
JohnWiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4206–4217,
2000Keywords: block copolymers; RAFT; living; emulsion
polymerization; MADIX
INTRODUCTION
Emulsion free-radical polymerization has provento be an
environmentally friendly method for theproduction of a wide range
of polymers. The meth-od1,2 uses water as the continuous medium
inwhich monomer is polymerized in surfactant mi-
celles aggregates. Surfactant plays a dual role; itnot only
provides the locus of polymerization butalso stabilizes the polymer
particles. This resultsin stabilized latex particles dispersed in
an aque-ous medium. The main advantages of emulsionover
conventional bulk or solution polymeriza-tions are that
polymerizations reach very highconversions and thus contain low
amounts of re-sidual monomer, the heat produced by the
highlyexothermic reactions is dissipated by the water,and the
processing of the high polymer solidscontent latex is easy owing to
its low viscosity.
Correspondence to: M. Monteiro (E-mail:
[email protected])Journal of Polymer Science: Part A: Polymer
Chemistry, Vol. 38, 4206–4217 (2000)© 2000 John Wiley & Sons,
Inc.
4206
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The practical problem is that the virtues of emul-sion
polymerization to produce high solids con-tent, high molar mass,
and hard polymer prod-ucts can be detrimental to film properties.2
Inaddition, the presence of additives in the recipesuch as
surfactant, initiator, buffer, and othercomponents can create
inhomogeneous films. Forexample, surfactant can migrate to the
surfaceand concentrate into pockets during film forma-tion.3 This
can affect the gloss of the coating andhardness of the film, reduce
adhesion, and in-crease water sensitivity.4,5
In recent years there has been a great deal ofindustrial
interest in composite latex particles,which uses the virtues of
emulsion polymeriza-tion to tailor-make film properties for
specific enduses. This process has created a new class ofmaterials
and allows one to design a large num-ber of structures from a wide
variety of mono-mers. Composite particles are usually preparedby a
series of consecutive emulsion polymeriza-tion steps. In general,
ab initio emulsion polymer-izations are first carried out to form a
latex orseed latex consisting of the chosen polymer. Theseseeds are
then swollen with the second monomerand polymerized either under
batch or semibatchconditions. Semibatch processes are usually
car-ried out under starved-feed conditions, such thatthe feed rate
is equal to or less than the intrinsicpolymerization rate. These
two processes can pro-duce a myriad of latex morphologies
dependingupon the experimental conditions,6,7 for
example,core-shell, raspberry, sandwich-like, and occludeddomains.
The morphologies are controlled by boththermodynamic and kinetic
factors, and in somecases kinetics of polymerization can lock an
unfa-vorable thermodynamic morphology into place.The composite of
soft and hard polymer [e.g.,polybutyl acrylate (PBS) and
poly(styrene)(PSTY)] represents one environmentally
friendlystrategy for the production of latex coatings thatcompete
with solvent-based systems.8 In effect,the soft polymer forms the
continuous phase en-gulfing the hard particles, allowing good film
for-mation. The hard phase therefore provides themechanical
strength and block resistance, result-ing in film properties close
to those produced bysolvent-based systems. The appearance of
thefilm depends on the size of the hard domains, andcan be made
transparent if the hard domains aresufficiently small.
With the advent of controlled radical polymer-ization,9
synthesizing polymers with tailor-mademolecular weight
distributions and polydispersi-
ties is achievable. More importantly, controlledpolymer
architectures such as block, triblock,branched, and even star can
now be made.10–13
This extends the making of latex composites andthus polymer
properties to a range of possibilitiesfar greater than previously
envisaged.
The reversible addition-fragmentation chaintransfer
(RAFT)10,14–16 offers a promising methodto produce polymer
architectures through emul-sion polymerization. The technique
relies on asequence of addition-fragmentation events, asshown in
Scheme 1. The first step involves thechain transfer of active
species, Pi
•, to the RAFTagent (1), which then undergoes fragmentation
toreinitiate polymerization. The active moiety fromthe RAFT agent
is now attached to the polymericchain end, making it a dormant
species (2). Oncethe RAFT agent is consumed, equilibrium is
es-tablished between active (Pi
•) and dormant spe-cies (2), and continues until all monomer is
con-sumed, resulting in a living growth of chains. Thetechnique can
be used for a wide range of mono-mers and reaction conditions, and
offers a versa-tile controlled free-radical route. MADIX
agents(e.g., Scheme 1) are a small category of RAFT-type compounds,
and have been reported to havechain transfer constants, Ctr, close
to 1.
This work presents the preparation of blockcopolymers of butyl
acrylate and styrene in emul-sion using the MADIX agent,
[1-(O-ethylxanthyl)-ethyl]benzene.17 The methodology was first to
po-lymerize BA with MADIX by conventional ab in-itio emulsion
polymerization, and then through asecond-stage polymerize styrene
to prepare blockcopolymers either under batch or
starved-feedconditions. The polymer was characterized bydouble
detection [i.e., ultraviolet (UV) and differ-ential refractive
index detector (DRI)] size exclu-sion chromatography. Atomic force
microscopy(AFM) was then used to study the migration ofsurfactant
after film formation of these latexes.
EXPERIMENTAL
Ab Initio BA Emulsion Polymerizations
Water (700 g), sodium dodecyl sulfate (SDS; 4.3 g)and NaHCO3
(0.095 g) were added in a glass-reactor vessel and stirred (100
rpm) at 70 °Cwhile argon was bubbled through the mixture.After 1 h,
BA (300 g) and MADIX agent (0.5 or1.5 g) were added and the
stirring rate increasedto 250 rpm. When the temperature reached 70
°C
SYNTHESIS OF BLOCK COPOLYMERS USING RAFT 4207
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again, sodium persulfate (SPS; 0.248 g in water)was added. In
the first hour samples were takenevery 5 min, and after that every
half hour. Thereaction was stopped after 5 h and the final
solidscontent was 30%.
STY Emulsion Polymerizations Using PBA Seeds
Batch
Water (353 g), SDS (0.21 g), and PBA seed latexes(500 g at 15%
solids) prepared above were addedin a glass reactor vessel and
stirred (100 rpm).Styrene (150 g) was added dropwise and the
mix-ture was stirred overnight to swell. The temper-ature was then
increased to 70 °C, argon wasbubbled through the mixture and the
stirring ratewas increased to 250 rpm. After 1 h SPS (0.22 g
inwater) was added. In the first hour samples were
taken every 5 min, and after that every half hour.The reaction
was stopped after 5 h and the finalsolids content was 30%.
Starved Feed
Water (353 g), PBA seed latexes (500 g at 15%solids) prepared
above, and SDS (0.21 g) wereadded in a glass reactor vessel and
stirred at 100rpm. Then, 85 g of styrene was added such thatthe
majority of monomer was swollen into theparticles of the seed
latex. The mixture wasstirred overnight. The temperature was then
in-creased to 70 °C, argon was bubbled through themixture and the
stirring rate was increased to250 rpm. After 1 h SPS (0.22 g in
water) wasadded. The remaining styrene was then addedwith a
dosimate at a rate of 0.2 mL/min. In the
Scheme 1. Mechanism for RAFT.
4208 MONTEIRO ET AL.
-
first hour samples were taken every 5 min, andafter that every
half hour. The reaction wasstopped after 5 h and final solids were
30%.
AFM Methods
Atomic force microscopy is a powerful techniquefor determining
the morphology and topographyof polymer films. The latex samples
were appliedon thin sheets of mica with an applicator 60 mmthick.
AFM was thereafter performed on theselatex films under three
different conditions: (1)after drying at room temperature under
ambientconditions for 24 h; (2) after treatment of (1),heating at
60 °C in 50% relative humidity for19 h; and (3) after treatment of
(2), washing withwater, and drying to remove the remaining
water.
The AFM instrument used was Nanoscope IIIaMultimode from Digital
Instruments, and bothtopographic imaging (tapping mode) and
phaseimaging were used for all samples.
Gel Permeation Chromatography (GPC) Analysis
We carried out GPC analysis using a Waters Cor-poration Model
510 pump, Waters Model WISP712 autoinjector, Model 410 refractive
index de-tector, and Model 486 UV detector (at 254 nm).The columns
used were a PLgel guard 5-mm 503 7.5-mm precolumn, followed by two
PLgelmixed-C 10-mm 300 3 7.5-mm columns (40 °C) inseries.
Tetrahydrofuran (THF) was used as eluent(flow rate 1.0 mL/min) and
calibration was doneusing polystyrene standards (M 5 580 to 7.13
106). Data acquisition was performed usingWaters Millennium 32 (v.
3.05) software.
Light Scattering
Dynamic light-scattering measurements of PBAlatex particle sizes
were carried out using aMalvern 4700 multiangle light scatterer
withPCS for Windows.
RESULTS AND DISCUSSION
Ab Initio Emulsion Polymerization of BAwith MADIX
Ab initio experiments of BA and MADIX werecarried out at 70 °C
in the presence of SPS initi-ator, SDS as surfactant, and deionized
water. TheMADIX agent used was [1-(O-ethylxanthyl)ethyl]-
benzene, and has a water saturation concentra-tion of 0.4 mmol
dm23, which is an order of mag-nitude less than styrene. The effect
of increasedMADIX concentration led to a decrease in the rateof
polymerization (Fig. 1), with an inhibition timefor all reactions
between 2 and 5 min. Bulk andsolution experiments did not show
retardation.Therefore, it is believed that exit, through trans-fer
to the MADIX agent and fragmentation toproduce R• (Scheme 1),
resulted in retardationthrough termination of R• either with
radicals inthe aqueous phase or reentry into a particle al-ready
containing a growing chain. Ctr for thisMADIX agent is close to 1.5
for BA,18 whichmeans that MADIX agent will be consumed at aslightly
faster rate than monomer. The implica-tion of this is that exit due
to transfer to MADIXwill retard polymerization over most of the
con-version range. Further support for this argumentis from the
decrease in average particle size withincreasing MADIX
concentration (Table I). Thedecrease corresponds to a greater
number of mi-celles being nucleated in interval I owing to exitand
reentry into another micelle.
The molecular weight distribution (MWD) isimportant when
considering the amount of con-trol or living behavior a system
obeys. Figure 2shows the number-average molecular weight(M# n) and
polydispersity (PD) of PBA prepared attwo concentrations of MADIX
agent. The M# n de-creased from 1.6 3 105 to 5 3 104 when theMADIX
agent concentration was tripled. The PDat low conversions started
at approximately 2 anddecreased to 1.9 and 1.6 for MADIX
concentra-tions of 2 and 6 mM, respectively. Growth of dor-mant
chains (that is, living behavior) was seen by
Figure 1. Conversion-time plots of ab initio
emulsionpolymerizations of BA at 70 °C with (a) no MADIX (■);(b) 2
mM of MADIX (F); and (c) 6 mM of MADIX (Œ).The concentrations of
MADIX are with respect to totalreaction volume.
SYNTHESIS OF BLOCK COPOLYMERS USING RAFT 4209
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the linear increase in M# n over the whole conver-sion range,
where the rate of increase was greaterfor the higher MADIX
concentration. These re-sults are consistent with predictions using
theexperimentally found Ctr (; 1.5).
18 The equationsused to predict the M# n and PD were derived
fromthe method of moments. Equations 1 and 2 takeinto account the
effect of residual MADIX agentdue to its low Ctr
19 and were only valid duringinterval III, in which droplets
were no longerpresent. During interval II the ratio of monomerto
MADIX being transported was equal to theinitial ratio, g, which is
in accord with the two-films theory.23 Therefore, g was constant in
theparticles until all MADIX agent had been con-sumed in the
droplets. During this time, M# n and
PD also remained constant at the starting valuesof eqs 1 and 2.
The fractional conversion for thechangeover from interval II to III
was approxi-mately 0.3, after which eqs 1 and 2 became valid.
M# n <gx
1 2 ~1 2 a!~1 2 x!b M0 (1)
PD 51
gx 11x F2 1 b 2 1a 2 b ~2 2 x!G
22a~1 2 a!~b2 2 a2!x2 @1 2 ~1 2 x!
11b/a# (2)
where g 5 [M]0/[RAFT]0, M0 is molar mass ofmonomer, x is
fractional conversion, a 5 [Pn
•]/[RAFT], and b 5 Ctr.
Second-Stage Emulsion Polymerization of Styrenein BA/MADIX
Seed
The latexes produced from the previous experi-ments were used as
the seeds, in which styrenewas further batch or starved-fed in a
second-stagepolymerization to give a final polymer ratio of BAto
STY 5 1. The PBA seed prepared with MADIXat the 2-mM and 6-mM
concentrations (denotedhere as PBA-MAD and PBA-MAD3,
respectively)consisted of poly(butyl
acrylate)-(thiocarbonyl)sulfanyl compounds. These dormant chains
canundergo further RAFT reactions with polystyrylradicals to
produce block copolymers. Figure 3shows that the rate of
polymerization (beyondexperimental error) for styrene in PBA-MAD
andPBA-MAD3 under batch conditions was not
Table I. Average Particle Size and Nc for Ab InitioBA
Polymerizations at 70 °C in the Presence ofMADIX Agent
[MADIX]a
(mol dm23)Particle Sizeb
(nm) Ncc
0 98 1.5 3 1017
2 3 1023 58 7.3 3 1017
6 3 1023 38 2.6 3 1018
a MADIX concentration is calculated from the total reac-tion
volume.
b Particle size are based on number average as determinedby
dynamic light scattering.
c Nc 5 number of particles per unit volume of water,
herecalculated for the second-stage recipes with styrene at
15%solids using the particle sizes above.
Figure 2. Ab initio emulsions of BA polymerized at70 °C in the
presence of MADIX. (a) M# n at MADIXconcentrations of 2 mM (}) and
6 mM (Œ); and (b) PD atMADIX concentrations of 2 mM (■) and 6 mM
(F).Lines are theoretical prediction using eq 1 for M# n andeq 2
for PD.
Figure 3. Conversion-time plots of second-stage po-lymerization
of STY at 70 °C using as the PBA seedlatexes. (a) PBA prepared with
no MADIX (Œ); (b) PBA-MADIX (2 mM) (F); (c) PBA-MADIX (6 mM)
(■).
4210 MONTEIRO ET AL.
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greatly affected by the concentration of the poly-meric RAFT
agent in the seed. It should be notedthat considerable
polymerization occurred beforethe addition of the initiator when
heating (over 30min) from room temperature to the reaction
tem-perature of 70 °C, a consequence presumably dueto the presence
of residual initiator unreacted inthe preparation of the seed.
The GPC traces of STY polymerized in a secondstage using PBA-MAD
as the seed under batchand starved-feed conditions are shown in
Figure4(a,b), respectively. A double-detection GPC wasused to
determine the amounts of blocks formed.The differential refractive
index (DRI) detectorshowed the combined MWD of all polymers,whereas
the UV detector (at 254 nm) showed onlychains containing PSTY,
either as homo- or blockcopolymers. All DRI chromatograms were
nor-malized with respect to the UV chromatogram,with a fitting
parameter for the fraction of blockformation ( fPBA-co-PSTY).
20 If all PBA-MAD is con-
verted to blocks, the UV will be identical to andoverlap the DRI
signal (that is, fPBA-co-PSTY 5 1).Under batch conditions [Fig.
4(a)], the DRI andUV signals do not overlap. The fPBA-co-PSTY
usedto obtain the DRI chromatogram was 0.7, whichmeans that
approximately 70% of PBA-MAD wasconverted to blocks. Under
starved-feed condi-tions the overlap between the two signals
wasmuch better, where fPBA-co-PSTY used was 0.9. Asmall percentage
(,10%) of dead PBA will beformed through termination when making
thePBA-MAD seed.
Similar results were also found when PBA-MAD3 was used as the
seed. Figure 5(a) showsthe GPC chromatograms for batch conditions,
andFigure 5(b) for starved-feed conditions. Again,starved-feed
conditions produced the best resultsto obtain block copolymers (
fPBA-co-PSTY used was0.9) compared with fPBA-co-PSTY of 0.7 for
batchconditions.
The difference in the amount of blocks betweenbatch and starved
feed could be due to secondary
Figure 5. GPC w(Log Mw)] curves with UV and DRIdetection of
second-stage emulsion polymerization ofSTY using PBA-MADIX (6 mM):
(a) under batch con-ditions; (b) under starved-feed conditions. The
DRIchromatograms were normalized with respect to theUV
chromatogram, with a fitting parameter for thefraction of block
formation ( fPBA-co-PSTY).
Figure 4. GPC w (Log Mw)] curves with UV and DRIdetection of
second-stage emulsion polymerization ofSTY using PBA-MADIX (2 mM):
(a) under batch con-ditions; (b) under starved-feed conditions. The
DRIchromatograms were normalized with respect to theUV
chromatogram, with a fitting parameter for thefraction of block
formation ( fPBA-co-PSTY).
SYNTHESIS OF BLOCK COPOLYMERS USING RAFT 4211
-
particle formation and/or excess radicals, produc-ing
homopolymer of polystyrene. Secondary par-ticle formation is highly
unlikely in our systemsbecause the number of particles per unit
volumewas far greater (Table I) than the catastrophicregion of ; 1
3 1014 found for seeded STY poly-merizations.21 The most probable
difference be-tween batch and starved feed is the lower
entryefficiency of radicals into the particles22 forstarved-feed
conditions, which lowers the amountof termination and consequently
the amount ofSTY homopolymer.
AFM of Films
Polybutyl Acrylate Latexes
The topographic image and phase image of PBA(prepared without
MADIX) after drying for 1 dayat room temperature are shown in
Figure 6. Fromthe phase image we acquired information aboutregions
of different viscoelastic properties at thesurface: dark contrast
regions correspond to softdomains, whereas regions with light
contrast cor-respond to hard domains. In Figure 6(b) we de-tected
flakes of a phase with light contrast on topof a darker phase. The
darker phase probablyconsisted of the soft PBA polymer, and the
lightphase was probably a second phase that migratedto the surface
during drying.
After heating the sample at 60 °C (Fig. 7), weobserved that the
amount of the light secondphase at the surface increased. This
indicatedthat there was more and faster migration of thesecond
phase at elevated temperatures, owing toincreased mobility of the
polymer chains in thePBA phase at higher temperatures. After
thesample was washed with water (Fig. 8) the secondwhite phase was
removed almost completely fromthe surface and there were holes left
in the film.
This behavior was previously found for othertypes of latex
films,3,4 and it has been explainedas migration of the surfactant
used in the emul-sion polymerization process to the surface. It
isprobable that also in this case we had migrationof the surfactant
(SDS) to the surface of the film,as this was the only
low-molecular-weight sub-stance present in the formulation in
relativelylarge amounts (ca. 1.4 wt % of the monomer con-tent).
However, it is possible that short oligomersof the polymers also
can migrate to the surface.
PBA-PSTY Latexes
The topographic images of the PBA-PSTY latexesdisplayed high
surface roughness of the films andno additional information about
the morphologyof the films could be obtained from these
images;therefore, only phase images of these latexes willbe shown
here. PBA-PSTY composite latexeswere studied with AFM. All samples
gave two
Figure 6. AFM images of PBA dried at room temperature under
ambient conditionsfor 24 h: (a) topographic image, size 15 3 15 mm;
(b) phase image, size 15 3 15 mm.
4212 MONTEIRO ET AL.
-
glass-transition temperatures (Tg’s), of 251.4 °Cand 96.9 °C,
which correspond to the Tg’s of PBAand PSTY, respectively. The
latexes were synthe-sized under both starved-feed and batch
condi-tions. However, there were no large differences inthe AFM
images comparing the starved and thebatch types of latexes.
The AFM images of the latexes with and with-out MADIX differed
in some respects; however,this will be explained in detail later,
and we willstart with the general AFM appearance of thePBA-PSTY
latexes under different conditions.
As an example, we have chosen to show theAFM images of PBA-PSTY
(batch) without
Figure 7. AFM images of PBA heated at 60 °C in 50% relative
humidity for 19 h:(a) topographic image, size 15 3 15 mm; (b) phase
image, size 15 3 15 mm.
Figure 8. AFM images of PBA heated at 60 °C in 50% relative
humidity for 19 h andthereafter washed with water and dried: (a)
topographic image, size 15 3 15 mm;(b) phase image, size 15 3 15
mm.
SYNTHESIS OF BLOCK COPOLYMERS USING RAFT 4213
-
MADIX. Phase images of this sample after dryingat room
temperature for 24 h are displayed inFigure 9. In the 2 3 2-mm
image more or lessspherical domains of light contrast embedded in
adarker phase can be observed. These spherical
domains probably consisted of the hard PSTYphase and the dark
continuous phase was proba-bly the soft PBA phase. The diameter of
the PSTYdomains was approximately 100 nm; however, it
Figure 9. AFM images of PBA-PSTY (batch) dried at room
temperature under am-bient conditions for 24 h: (a) phase image,
size 2 3 2 mm; (b) phase image, size 15 315 mm.
Figure 10. AFM phase image of PBA-PSTY (batch)heated at 60 °C in
50% relative humidity for 19 h, size15 3 15 mm.
Figure 11. AFM phase image of PBA-PSTY (batch)heated at 60 °C in
50% relative humidity for 19 h andthereafter washed with water and
dried, size 15 315 mm.
4214 MONTEIRO ET AL.
-
is not possible to say whether the whole PSTYdomain was visible
at the surface or whether onlythe top of the domains was visible.
Furthermore,while “islands” 0.5–1 mm in size were observed inthe 15
3 15-mm phase image of the same sample[Fig. 9(b)]. These islands
were consequently muchlarger than the PSTY domains and probably
con-sisted of a second phase.
When the sample was heated to 60 °C, thewhite second phase
covered more than 50% of thesurface; however, in the areas of the
surface thatwere not covered with the second phase the STYdomains
were still visible (Fig. 10). Finally, whenthe surface was washed
with water after heating,the second phase was removed completely
and theSTY domains were again exposed over the whole
Figure 12. AFM phase images of PBA-PSTY (batch) and
PBA-PSTY-MADIX (batch),size 2 3 2 mm: (a) PBA-PSTY (batch) dried at
room temperature under ambientconditions for 24 h; (b)
PBA-PSTY-MADIX (batch) dried at room temperature underambient
conditions for 24 h; (c) PBA-PSTY (batch) heated at 60 °C in 50%
relativehumidity for 19 h and thereafter washed with water and
dried; (d) PBA-PSTY-MADIX(batch) heated at 60 °C in 50% relative
humidity for 19 h and thereafter washed withwater and dried.
SYNTHESIS OF BLOCK COPOLYMERS USING RAFT 4215
-
surface (Fig. 11). As for the PBA samples, webelieve that the
second white phase observed ontop of the film was surfactant that
migrated to thesurface. This is plausible because the surfactantwas
the only low-molecular-weight componentthat was soluble in water
and present in the for-mulation in relatively large amounts.
When comparing the samples with and withoutMADIX, differences in
both the size of the PSTYdomains and the amount of surfactant that
mi-grated at 60 °C were detected. As mentioned pre-viously, the
size of the PSTY domains was 100 nmfor PBA-PSTY without MADIX after
drying inroom temperature [Fig. 12(a)]. The correspondingsamples
with MADIX had PSTY domains with asize only half this value (40–50
nm) [Fig. 12(b)].However, when these samples were heated andwashed,
we observed that the PSTY domains ofthe latex with MADIX became
larger, more mono-disperse, and more close-packed at the
surface[Fig. 12(d)], whereas the size and shape of thePSTY domains
in the sample without MADIXwere almost unchanged [Fig. 12(c)]. It
is not prob-able that the PSTY domains grew in size uponheating; it
is more likely that the PSTY domainsrose to the surface and
therefore became morevisible in the AFM images when the sample
washeated. This could be because the soft PBA phasemoved down into
the bulk of the film, filling outcavities and voids left in the
film after film forma-tion of the latex particles. This process was
accel-erated when the temperature increased, and thusthe mobility
of the PBA phase increased. More-over, the PBA-MADIX seed had a
lower averagemolecular weight than the PBA seed withoutMADIX (1 3
105 compared with 7 3 106), and thePBA-MADIX chains were therefore
expected to bemore mobile than the PBA chains.
Consequently,movement of PSTY domains to the surface of thefilm
would be facilitated when MADIX waspresent.
We also observed that more surfactant mi-grated to the surface
for PBA-PSTY with MADIXthan without, after heating the samples to
60 °C.Again, the difference in molecular weight (andthus mobility)
of the PBA polymer probablycaused this difference in migrated
surfactant.However, in this study we investigated the sam-ples at
only a relatively short time scale (1–2days). After longer times
(weeks to months) theamount of migrated surfactant might become
thesame for the two samples.
CONCLUSION
Block copolymers were prepared in emulsionthrough the RAFT
process using the MADIXagent under batch and starved-feed
conditions.First, ab initio experiments were carried out toprepare
a seed of PBA dormant chains (i.e., PBApolymer attached with a
transfer-active xan-thate). Those seeds were then used in a
second-stage polymerization under starved-feed andbatch conditions
to prepare composite polymercolloids of block PBA-co-PSTY. Under
starved-feed conditions approximately 90% of total poly-mer
consisted of blocks, whereas under batch con-ditions only 70%
consisted of blocks, which isproposed to be due a higher entry
efficiency andthus greater termination rate. The films of
thoselatexes were examined by AFM. Surfactant mi-gration to the
surface increased with an increasedamount of MADIX, resulting from
a combinationof smaller particle size and lower
molecularweight.
The authors gratefully acknowledge the support of theEuropean
Community within the Brite-EuRam pro-gram (Contract no.
BRPR-CT-0510).
REFERENCES AND NOTES
1. Blackley, D. C. Emulsion Polymerisation; AppliedScience:
London, 1975.
2. Gilbert, R. G. Emulsion Polymerization: A Mecha-nistic
Approach; Academic Press: London, 1995.
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SYNTHESIS OF BLOCK COPOLYMERS USING RAFT 4217
INTRODUCTIONEXPERIMENTALScheme 1.
RESULTS AND DISCUSSIONFigure 1.Table I.Figure 2.Figure 3.Figure
4.Figure 5.Figure 6.Figure 7.Figure 8.Figure 9.Figure 10.Figure
11.Figure 12.
CONCLUSIONREFERENCES AND NOTES
