Nanosize lipophilic polyacrylate particles and their photoinduced flocculation in hydrocarbon solvents

Article abstract of DOI:10.1021/jp972540x

The synthesis, characterization, and properties of colloidal nanostructures in hydrocarbon media are reported. These sterically stabilized structures are unusual in that the stabilizing arms are attached to the particle by photoactive groups, and the arms

Full text of DOI:10.1021/jp972540x

J. Phys. Chem. A 1998, 102, 5349-5355
5349
Nanosize Lipophilic Polyacrylate Particles and Their Photoinduced Flocculation in
Hydrocarbon Solvents†
Zhaoqing Liu and Mitchell A. Winnik*
Department of Chemistry, UniVersity of Toronto, 80 St. George St., Toronto, Canada M5S 3H6
Tze-Chi Jao
Ethyl Petroleum AdditiVes, Inc., 500 Spring St., Richmond, Virginia 23218-2189
Joachim Ro1sch
BASF Corporation, ZKP/H-B1, D-67056 Ludwigshafen, Germany
ReceiVed: August 25, 1997; In Final Form: December 9, 1997
The synthesis, characterization, and properties of colloidal nanostructures in hydrocarbon media are reported.
These sterically stabilized structures are unusual in that the stabilizing arms are attached to the particle by
photoactive groups, and the arms can be cleaved through irradiation, leading to photoinduced flocculation.
In the first step of the synthesis, cross-linked poly(butyl methacrylate) (PBMA) latex particles with controlled
functionality (-COOH and -OH groups) were prepared by emulsion polymerization. The purified PBMA
latex was neutralized with Cs2CO3 and then transferred to an organic medium (dimethylformamide), where
it was converted to lipophilic nanoparticles through chemical modification with 2-bromo derivatives of long-
chain 1-phenylalkyl ketones or nitrobenzyl ether derivatives of polyisobutylene. The particles can be dispersed
in hydrocarbon solvents. Irradiation leads to flocculation, except that in the presence of hydrocarbon-soluble
polymers bearing amino groups, the unprotected particles can be intercepted and flocculation inhibited. These
particles serve as an interesting model for precursors to sludge formation in motor oil.
Introduction
of the process in which small particles form and then aggregate
to form sludge. It has been suggested that the initially formed
In this paper we consider models for the formation of sludge
in gasoline-engine motor oil.1 Sludge is an unwanted precipitate
that interferes with oil circulation in engines and forms deposits
and varnishes on engine parts.2 Its formation is most severe
under the low-to-moderate temperature engine conditions that
occur when the engine is run in stop-and-go traffic. These
problems can eventually lead to engine failure. Sludge origi-
nates from incomplete combustion of fuel.3 The partially
oxidized byproducts find their way into the crankcase and
degrade via so-called “nitration-oxidation” processes into
products containing polar functional groups, which ultimately
react to form substances that are insoluble in oil.4 Although
investigators in the lubrication field have invested considerable
effort trying to understand the sludge-formation mechanism, the
sludge-formation process remains largely a mystery.5 Neverthe-
less, considerable progress has been made in the development
of additives for motor oil that retard the formation of sludge.
Some of these additives (dispersants) are designed to intercept
hypothetical intermediates that are precursors to sludge.6 Many
of these additives are very effective, and modern formulations
greatly slow down sludge buildup.
aggregates are cross-linked resin particles bearing carboxylic
acid, alcohol, and carbonyl functional groups5b and that the resin
is formed via aldol-type condensation of aldehyde and ketone
groups in the molecules present in the partially oxidized fuel.7
We consider the possibility that one could prepare and char-
acterize substances that would serve as model intermediates in
the sludge-formation process. Under the right circumstances,
one could study the flocculation and precipitation of this material
and also use these model sludge precursors as a test platform
to assess the effectiveness and the mechanism of action of the
functional polymers used as dispersants. Dispersants are
designed to capture sludge precursors while they are of colloidal
dimensions, hopefully less than 100 nm in size, by binding to
the surface of the aggregates and providing steric stabilization.
Our model is built on the idea that the partial-oxidation
products of fuel create molecules that contain aldehydes,
ketones, and carboxylic acid groups. Through a series of
condensation reactions, these molecules become larger in size
until their solubility limit is exceeded. The entities formed at
this point are the colloidal precursors to sludge formation that
we wish to model. These species aggregate, and these associ-
ated structures become larger in size until a macroscopic
precipitate forms. Our model intermediate must then be a
particle of colloidal size (target diameter of 50 nm) with polar
functional groups at its surface. Since the active functional
groups at the aggregate surface are unknown, the synthetic
strategy for preparing the model nanoparticles must have enough
versatility to allow control of functionality within a reasonable
The lack of understanding of the sludge-formation process,
and our limited knowledge of the function of these additives,
hinders the development of new products. Here, we attempt to
create a model system to provide insights into the mechanism
† This paper is dedicated, with our best wishes, to the 60th birthday of
Bob Liu, Doug Neckers, Jack Saltiel, Nick Turro, Pete Wagner, and Dave
Whitten.
S1089-5639(97)02540-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/19/1998

5350 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Liu et al.
range. Finally, we must be able to control the colloidal stability
of the particles so that we can induce flocculation at will.
Dispersion stability is the central issue in all technologies in
which colloids play a role. In applications ranging from the
synthesis of commodity polymers, such as styrene-butadiene
by emulsion polymerization, to paints and inks, to medical
diagnostics, the system must meet the seemingly contradictory
demands of remaining colloidally stable during transport and
storage, and flocculating or otherwise separating from the fluid
medium during some key step during its application or process-
ing. Here, we consider the novel idea of photoinduced floc-
culation of sterically stabilized colloidal dispersions. To achieve
this goal, the flexible chains that provide colloidal stability are
anchored by groups that are susceptible to photofragmentation.
Since these types of particles are often referred to as “hairy”
latex, we can think of the flocculation-inducing step as a
“photochemical haircut”. We conceptualize the problem in the
following:
combined organic phases were dried over Na2SO4 and evapo-
rated to yield an oil, which was recrystallized from hexane. Yield
) 3.25 g (74%); mp ) 51.5-52 °C. 1H NMR (CDCl3, 200
MHz): δ 8.08-7.24 (5H, m, Ar-H), 5.14 (1H, m, R-H), 2.15
(2H, m, â-H), 1.32 (28H, br. s, (-CH2-)n), 0.90 (3H, t, J ) 7
Hz, CH3). FTIR (νmax): 2818, 2847, 1682 (CdO), 1466, 1258,
700, 521 cm-1. MS m/e (relative intensity): 424 (5.6, M+),
422 (5.9, M+), 343 (14.0), 200 (8.4), 198 (8.7), 120 (27.1), 105
(100), 77 (19.1). Calculated mass for C24H39BrO: 422.2184.
Found: 422.2165. UV-vis (THF, 6.0 × 10-5 M) λmax (ꢀ ×
10-3): 286 (0.31, sh), 254 (10.0) nm.
Preparation of 1-Phenyl-2-bromododecanone (2). This
was prepared in the same way as 1-phenyl-2-bromooctadec-
anone. The crude product was chromatographed on silica gel.
The pure product was eluted from the column with 10% diethyl
ether in hexanes after a small amount of impurities was washed
out by hexanes. The solid product, which would be liquefied
upon warming the flask in one’s hands, was obtained after
vacuum-drying. Yield ) 85%; mp ) 24-25 °C. 1H NMR
(CDCl3, 200 MHz): δ 8.08-7.63 (2H, m, Ar-H), 7.60-7.38
(2H, m, Ar-H), 5.12 (1H, t, J ) 7 Hz, R-H), 2.15 (2H, m,
â-H), 1.32 (14H, s, (-CH2-)n), 0.86 (3H, t, J ) 7 Hz, CH3).
MS m/e (relative intensity): 340 (0.7, M+), 338 (0.7, M+), 259
(17.9), 200 (15.4), 198 (15.6), 133 (9.4), 120 (7.9), 106 (16.0),
105 (100), 77 (23.5). Calculated mass for C18H27BrO: 338.1245.
Found: 338.1235. IR (KBr) νmax: 2933, 2867, 1691 (CdO),
1455, 1356, 1258, 705 cm-1. UV-vis (45:55 THF/cyclohex-
ane) λmax (ꢀ × 10-3): 254 (8.91), 286 (3.14, sh) nm.
Preparation of (1′-Phenylcarbonyl)heptadecanyl Isobu-
tyrate (4). Isobutyric acid (3.8211 g, 43.37 mmol) was mixed
with Cs2CO3 (7.0648 g, 21.68 mmol) in 10 mL of water. The
final solution was freeze-dried to yield a solid, which was
subsequently dried in a vacuum oven for 2 h at 80 °C. The
dried cesium isobutyrate (3, 0.1105 g, 0.50 mmol) and 1-phenyl-
2-bromooctadecanone (0.2111 g, 0.50 mmol) were mixed in 5
mL of DMF. The mixture was stirred overnight. By TLC
(silica gel, 1:10 diethyl ether/hexane), no 2-bromooctadecano-
phenone was left after an overnight reaction. The final mixture
was diluted with water and extracted with ether (3 × 10 mL).
The combined organic phase was dried over Na2SO4 and
evaporated to yield a white solid. The solid was further purified
by silica-gel chromatography. The product was eluted with 1:10
diethyl ether/hexane. Yield ) 0.18 g (83%); mp ) 39-40 °C.
1H NMR (CDCl3, 200 MHz): δ 7.90-7.40 (5H, m, Ar-H),
5.82 (1H, dd, J ) 8.2, 6.3 Hz, R′-H), 2.65 (1H, sept., J ) 6.5
Hz, (CH3)2CHC(O)O-), 1.90-1.75 (2H, m, â′-H), 1.60-1.20
(34H, m, (CH3)2CHC(O)O- and -CHCH2(CH2)14CH3), 0.87
(3H, t, J ) 7.0 Hz, -CHCH2(CH2)14CH3). FTIR νmax (cm-1):
2919, 2850, 1744 (CdO), 1691 (CdO), 1471, 1190, 1159, 699
cm-1. MS m/e (relative intensity): 431 (2, M+), 408 (2), 342
(5), 206 (6.5), 176 (12.0), 105 (100), 77 (15.7), 71 (85.2).
Calculated mass for C28H46O3: 430.3447. Found: 430.3439.
UV-vis (THF, 7.0 × 10-5 M) λ (ꢀ × 10-3): 270 (0.36), 244
(10.7) nm.
Experimental Section
Instrumentation. 1H NMR spectra were recorded on a
Gemini 200-MHz spectrometer at room temperature, and
chemical shifts are quoted in ppm. UV-vis spectra were
recorded on a Hewlett-Packard 8452A diode-array UV-vis
spectrophotometer. FTIR spectra were obtained on a Perkin-
Elmer Spectrum 1000 spectrometer. High-resolution mass
spectra were recorded on a VG 70-250S double-focusing high-
resolution mass spectrometer at 70 eV. Measurements of the
particle size and size distribution were performed on a Brookhaven
model 90 dynamic light-scattering particle sizer. Photolysis was
carried out in a Rayonet photochemical reactor equipped with
3000 Å 12 RPR lamps (Southern New England Ultraviolet Co.).
Gel permeation chromatography (GPC) was performed on a
Varian 5000 liquid chromatograph equipped with a Waters 480
tunable UV-vis absorbance detector and Waters differential
refractometer R401 detector. Two microstyragel columns (500
and 10 000 Å, American Polymer Standards Corporation) in
series were used with THF as solvent. Molecular weight was
calibrated with PMMA standards (Polymer Laboratories Ltd.).
Materials. Methyl methacrylate (MMA), butyl methacrylate,
ethylene glycol dimethacrylate (EGDMA), 2-hydroxyethyl
methacrylate (HEMA) and methacrylic acid (MAA), purchased
from Aldrich, were distilled under nitrogen and stored at -10
°C. Spectral grade solvents were used for spectroscopic and
photochemical studies. All other chemicals were obtained from
Aldrich and used without further purification.
Preparation of Particles. The latex particles were prepared
by batch emulsion polymerization in deionized water. Typical
recipes are shown in Table 1. The monomers, surfactant, and
Na2CO3 were mixed with water. The mixture was degassed
by nitrogen bubbling for at least 30 min. The mixture was
heated to 75 °C while being stirred mechanically, and then the
initiator (K2S2O8) solution (1-2 mL) was added. Stirring was
continued for another 2-3 h under nitrogen. The final latex
was cleaned by ion exchange three times by stirring the resin
(5-10 g, Bio-Rad, AG-501-X8, washed subsequently with hot
Preparation of 1-Phenyl-2-bromooctadecanone (1). To a
stirring solution of 1-phenyloctadecanone (Aldrich, 3.450 g,
10.00 mmol) in 15 mL of glacial acid, bromine (1.7 g, 10.63
mmol) in 5 mL of glacial acid was added dropwise over 1 h.
The final mixture was poured over water, and the excess of
bromine was eliminated by adding Na2SO3 until a colorless
solution was observed. The product was extracted with dichlo-
romethane (3 × 100 mL) from the aqueous phase. The

Polyacrylates and Their Flocculation
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5351
TABLE 1: Recipe for the Preparation of Polyacrylate Latex
Particles
Photoinduced Flocculation. Samples with concentrations
of 0.1-10 mg/mL of polymer particles were prepared as
dispersions. If required, other additives such as PIB-NH2 were
added to the solution before irradiation. The final dispersions
were transparent, and size measurements by DLS indicated no
aggregation or coalescence of particles. Each sample was sealed
in a Pyrex cell, irradiated in a Rayonet photochemical reactor,
and monitored by dynamic light scattering and/or visible
absorption at 600 or 650 nm as a function of irradiation time.
Note that in the case of PBMA particles, the dispersion contains
a certain amount (ca. 5%) of unreacted 1-phenyl-2-bromo-
octadecanone.
PBMA/CO2H/OH
PMMA/CO2H/OH
water (mL)
acrylate (g)
SDS (g)
NaHCO3 (g)
KPS (g)
150
6.04 (BMA)
0.46
0.50
0.10
0.30
0.39
43
50
7.0 (MMA)
0.448
0.229
0.0906
0.448
0.406
61
EGDMA (g)
MAA (g)
latex sizea (nm)
a Measured by dynamic light scattering.
deionized water at 80 °C, methanol, and water) for every 100
mL of latex each time. The cleaned latex was neutralized to
pH 7.2 with Cs2CO3 and freeze-dried to yield the dry particles.
Modification of the Particles by 1-Phenyl-2-bromoal-
kanone. The same procedure was employed for the modifica-
tion of the PMMA and PBMA particles in the Cs+ salt form
(polymer/OH/COOCs) with 1-phenyl-2-bromooctadecanone and
1-phenyl-2-bromodecanone. As an example, a freeze-dried
sample of particles (PBMA/OH/COOCs, 2.4 g) was suspended
in 20 mL of DMF and stirred with 1-phenyl-2-bromooctadec-
anone (0.48 g) for 4 days at room temperature. The reaction
mixture was concentrated to 10 mL at 40 °C under vacuum
and diluted to 30 mL with THF. CsBr was removed by stirring
the diluted mixture with an ion-exchange resin (Bio-Rad, AG-
501-X8, previously washed sequentially with hot deionized
water at 80 °C, THF, and DMF). The ion-exchange treatment
was repeated three times. The cleaned dispersion was almost
transparent. The particles, which were in a mixture of THF
and DMF, can be air-dried to yield a glassy material that can
be redispersed in THF or cyclohexane. The same material can
be obtained as a white powder by freeze-drying the dispersion
in cyclohexane. GPC showed 70% of the chromophore absorp-
tion was retained on the particles. The purification of PMMA
particles was carried out by precipitating the particles from THF
by adding excess hexane, and the procedure was repeated at
least three times.
Modification of Particles by PIB-NH2. This transforma-
tion begins with functionalization of amino-end-capped poly-
isobutylene (PIB-NH2) with a photoremovable chromophore
and then attachment of the polymer to the particles. Step 1 is
the reaction of PIB-NH2 with 4-bromomethyl-3-nitrobenzoic
acid. PIB-NH2 (1.045 g of 50% in tetradecane, MW ) 1100,
0.642 mmol NH2/g PIB-NH2, a gift from Dr. Ro¨sch at BASF,
Ludwigshafen) was mixed with 4-bromomethyl-3-nitrobenzoic
acid14 (0.1497 g, 0.576 mmol) and dicyclohexylcarbodiimide
(0.1100 g, 0.582 mmol) in 10 mL of dry CH2Cl2. The mixture
was warmed slightly to give a clear solution and then allowed
to stand for 2 days. Needle crystals of dicyclohexylurea formed
and were removed by filtration. The filtrate was evaporated to
give an oil. Step 2 is the attachment of (PIB-NH-C(O))-
C6H3(4-bromomethyl)(3-nitro) to PMMA particles. The above-
obtained oil was redissolved in 15 mL of 1:2 THF/DMF together
with PMMA/OH/CO2Cs (1 g, 0.62 mmol CO2Cs(H)/g). The
PMMA particles were stirred in this solution for 2 days at room
temperature. Cesium bromide was removed by stirring with
an ion-exchange resin. The filtrate was dried under vacuum
and redispersed in THF. A pure solid sample was obtained by
precipitating the particles from THF dispersions using hexane.
UV-vis (THF, 0.488 mg/mL) λ (ꢀ × 103): 286 (0.46), 240
(1.78). On the basis of the maximum molar absorbance of
4-bromomethyl-3-nitrobenzoic acid at 292 nm in THF (ꢀ ) 1.81
× 103), the chromophore content of the particles was estimated
as 518 µmol/g polymer.
Results and Discussion
We describe a methodology to prepare nanometer-sized
particles, bearing functional groups near or at the surface, that
form stable dispersions in aliphatic-hydrocarbon media. Steric
stabilization is provided by relatively short aliphatic chains (n-
C9H19-, n-C15H31-) or by longer polymer chains (polyisobu-
tylene, PIB, MW ) 1100). These are connected to the particles
by means of photoactive groups in such a way that irradiation
cleaves the chains from the particles. In this way, flocculation
can be induced photochemically. The particles are first prepared
in water by emulsion polymerization. Because they are cross-
linked, the particles prepared in water can be dried and
transferred to an organic solvent like tetrahydrofuran (THF) or
dimethyl formamide, where the particles disperse in the form
of solvent-swollen microgels.8
One of the interesting features of the systems we have
prepared is that their colloidal stability is not dependent only
on the presence of the aliphatic or PIB chains. Colloidal stability
also depends on the interaction between the polymers in the
particle and the solvent. These effects can be quite subtle but
turn out to be useful in the purification of the particles. The
materials we have prepared do not disperse well in hexane or
pentane. They precipitate in these solvents and can be recovered
by filtering or centrifugation. Some of the particles disperse
well in cyclohexane and in cyclohexane/hexadecane mixtures.
Other particles can be dispersed in mixtures of cyclohexane with
more polar solvents such as THF. These represent conditions
where useful photochemical experiments can be carried out.
Preparation and Properties of the Functional Nanopar-
ticles. Particle Synthesis. Dispersions of nanometer-sized
polymer particles can be prepared either in aqueous media by
emulsion polymerization9 or in organic media by dispersion
polymerization.10 Alternative approaches involve polymeriza-
tion in microemulsions, by which one can prepare dispersions
of nanosize particles in aqueous11 and nonaqueous12 phases. The
microemulsion approach is effective for producing very small
particles (i.e., 10 nm in diameter), but the method requires large
amounts of surfactant. Here, we take an indirect method to
the synthesis of nanometer-sized particles for dispersion in
aliphatic-hydrocarbon media. We prepare the base particle by
emulsion polymerization in water, with sufficient bismethacry-
late in the monomer component to yield fully cross-linked
particles. In emulsion polymerization, the size of the particles
can be controlled through the amount of surfactant added. If
anionic surfactant is employed, the particles can be purified,
and the surfactant removed, by stirring the dispersion with an
ion-exchange resin. This strategy offers considerable flexibility,
since it is possible to control the location of functional groups
in the system through seeded, multistage emulsion polymeri-
zation. Here, we find that batch polymerization, in which all
the monomers are introduced into the initial reaction mixture,

5352 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Liu et al.
SCHEME 1
SCHEME 2
provides particles in which almost all of the functional groups
are accessible and presumed to be near the surface of the
particles.
We prepared both PMMA and PBMA particles by batch
emulsion polymerization, using ethylene glycol dimethacrylate
as a cross-linking agent and methacrylic acid and 2-hydroxyethyl
methacrylate as comonomers. In this way, the particles prepared
contained both carboxylic acid and primary-alcohol functional
groups. We refer to these materials as PMMA/OH/CO2H and
PBMA/OH/CO2H, respectively. Two typical recipes for the
emulsion polymerization reactions are given in Table 1. The
latex particles obtained had diameters ranging from 40 to 60
nm with narrow polydispersity as measured by dynamic light
scattering (DLS). The dispersions of cross-linked latex were
purified by ion exchange, neutralized to pH 7.2 with Cs2CO3,
and freeze-dried for further chemical modification. The Cs+
salt of latex particles can be dispersed homogeneously in DMF
but not in THF. Attempts to disperse these powders as
individual particles in CH2Cl2, benzene, toluene, and aliphatic-
hydrocarbon solvents also failed. In contrast, freeze-dried latex
particles in the acid form (freeze-dried after purification by ion
exchange) can be redispersed in organic solvents such as DMF
and THF. The particles are swollen by these solvents, and their
diameters increase by approximately 30-40% as determined
by DLS over their dimensions in water.
isobutyrate, was obtained in 83% isolated yield after reaction
in DMF at room temperature overnight.
In this way, the cesium carboxylate salts of the particles were
converted to esters by nucleophilic reaction with 1-phenyl-2-
bromododecanone and with 1-phenyl-2-bromooctadecanone
(Scheme 1). The phenyldodecane derivatives (n ) 9 in Scheme
1) gave particles that could not be dispersed in aliphatic-
hydrocarbon media but could be dispersed if a polar solvent
like THF was added to the solvent. Better and more robust
dispersions of both types of particles were obtained by reaction
with 1-phenyl-2-bromooctadecanone (n ) 15, Scheme 1). We
took a similar approach in preparing particles stabilized with
polyisobutylene chains (Scheme 2). Here, PIB-NH2 was first
reacted with 4-bromomethyl-2-nitrobenzoic acid14 to form an
ester containing the reactive bromomethyl group, which sub-
sequently was reacted with dispersed polymer/OH/CO2Cs in the
same way as with the bromoalkanones. In each case, the
dangling aliphatic chains were attached to the particles by
photocleavable groups, either an R-carboxy phenyl ketone or a
nitrobenzyl ester group. The particles were analyzed by UV
Particle Modification. Chemical modifications were carried
out to obtain particles that have enhanced dispersibility in
hydrocarbon solvents and that can be flocculated upon UV
irradiation. The synthesis procedures are based upon the
following reaction
1
and H NMR spectroscopy in order to determine the chro-
mophore content. The microgels formed in THF could also be
analyzed by gel permeation chromatography (GPC) to determine
whether all UV-absorbing entities were attached to the polymer.
As shown in Table 2, not all carboxylic acid groups on the
particles react. We assume that the unreactive acid groups are
buried inside the particle cores and are less reactive because of
steric factors.
Dispersibility of Modified Particles in Hydrocarbon SolVents.
The properties of the modified particle are listed in Table 2. If
the pendant aliphatic chains were solely responsible for particle
stabilization, we would anticipate that the particles would form
stable dispersions in any solvent that is a good solvent for these
aliphatic chains. Under these circumstances, the dispersibility
of the particles would not depend on the core material, whether
in which the cesium salt of a carboxylic acid reacts with an
R-bromoketone to form an R-ketoester. SN2 reactions of cesium
carboxylate salts with aliphatic bromides often take place with
nearly quantitative yield and are relatively fast (k ≈ 103 L mol-1
s-1).13 For example, the model compound 4, the reaction
product of 1-phenyl-2-bromooctadecanone (1) with cesium
TABLE 2: Typical Properties of Cross-Linked Polymeric Particles
PMMA/OH/CO2H
PBMA/OH/CO2H
original latex (nm)a
cross-linkerb (wt %)
total CO2H (µmol/g)b
total OH (µmol/g)b
chromophore type
chromophore (µmol/g)
modified particle sizea (nm)
CPCg
61
7.6
61
7.6
620
216
60
4.4
43
4.2
620
216
PC12
473d
564
300
PC12
274f
959
315
PC18
329f
c
c
c
c
PIB-NH2
518e
79 (THF)c
71(1:2 THF/cy)c
1:2 THF/cyc
92 (THF)c
62 (cy)c
45:55 THF/cyc
1:10 THF/cyc
1:3 C16/cyc
a Particle sizes were measured by dynamic light scattering. b Calculated based on the emulsion polymerization recipe. c PC12, 1-phenyldodecanone
derivative; PC18, 1-phenyloctadecanone derivative; PIB-NH2, polyisobutylene derivative; THF, tetrahydrofuran; cy, cyclohexane; C16, hexadecane.
d Estimated by UV spectroscopy on a purified sample based on ꢀmax ) 10 700 at 244 nm of (1′-phenylcarbonyl)heptadecanyl isobutyrate (4) in
THF. e Estimated by UV spectroscopy based on ꢀmax ) 1810 at 292 nm of 4-bromomethyl-2-nitrobenzoic acid. f Estimated by GPC according to
UV absorption at 241 nm, eluent THF with microstyrogel columns. g Limiting solvent composition yielding stable dispersions, the cloud-point
composition.

Polyacrylates and Their Flocculation
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5353
SCHEME 3
it was PMMA, PBMA, or even silica. Here, we find significant
differences between PMMA and PBMA particles, consistent
with the idea that solvent swelling of the core plays a role in
the stabilization process. Particles swollen by solvent have
smaller Hamaker constants (weaker attractive dispersive interac-
tions) and smaller surface tensions than dense particles.15 We
also note differences due to the length and type of aliphatic
substituent attached to the carboxyl groups. For example, the
longer PIB chains are more effective steric stabilizers than the
dangling ends of the octadecanol derivatives.
Figure 1. Gel permeation chromatograms of PBMA/OH/CO2PC18
before (upper pair of curves) and after (lower pair of curves) 10-min
photolysis (UV, >300 nm) in THF: GPC column, 500 and 10 000 Å;
solvent, THF; UV detector at 249 nm (thick curves); refractive index
(thin curves).
For all of these dispersions, during addition of nonsolvent,
one reaches a critical solvent composition (cloud-point composi-
tion, CPC) at which flocculation occurs.5 For example, PBMA
particles modified with 1-phenyl-2-bromododecanone (PBMA/
OH/CO2PC12) can be dispersed in 1:10 THF/cyclohexane while
PMMA with the same modification can only be dispersed in
45:55 THF/cyclohexane. In Table 2 we list these limiting
solvent compositions. For the same PMMA particles at roughly
equal chromophore content, those stabilized with PIB are more
easily dispersed in hydrocarbon-rich solvents than those with
the 1-phenyldodecanone chromophore. The CPC of the PIB-
stabilized PMMA particles is 1:2 THF/cyclohexane, whereas
for the 1-phenyldodecanone derivative, the limiting dispersibility
is 45:55 THF/cyclohexane. This behavior is consistent with
the greater chain length of the PIB (MW ) 1100) compared
with that of the alkyl ketone derivative.
SCHEME 4
When carrying out these reactions, we were able to follow
the effectiveness of the photoreaction by gel permeation
chromatography (GPC). It is not widely appreciated that one
can often analyze or even purify colloidal dispersions by column
chromatography. Here, we use GPC to monitor the change in
chromophore content of the particles after the irradiation.
Consider the example of PBMA/OH/CO2H modified with
1-phenyl-2-bromooctadecanone (PBMA/OH/CO2PC18) irradi-
ated in THF in order to prevent the flocculation during
irradiation. In Figure 1, we show the GPC chromatogram of
the sample before irradiation, using dual detectors. The swollen
particles elute at a retention volume corresponding to a
molecular weight (based upon linear PMMA standards) of 1.1
× 107 (Mw/Mn ) 1.1), as detected by UV absorption at 249 nm
and refractive index (RI). After irradiation for about 10 min in
THF, the position of the particles in the GPC trace remains
unchanged as monitored by the RI detector, but the UV
absorption for the particle peak is reduced almost to zero. The
loss of UV absorption from the particles indicates that the
photodissociation has occurred. The chromophore-containing
entity has been severed from the particles, and the reaction has
gone nearly to completion.
PBMA particles functionalized with 1-phenyloctadecanone
(PBMA/OH/CO2PC18) can be completely dispersed in cyclo-
hexane, in methylcyclohexane, and in mixtures of these with
linear aliphatic-hydrocarbon solvents, while those modified with
1-phenyldodecanone (PBMA/OH/CO2PC12) can only be dis-
persed in cyclohexane by adding a small amount of THF (Table
2).
Photochemistry. R-Carboxyphenyl ketones are very pho-
toreactive. The phenylalkyl ketone function has been used in
the synthesis as a photoremovable protective group for car-
boxylic acids.16 The reaction is thought to proceed through
homolytic cleavage of the COO-CH bond followed by hydrogen-
atom transfer to regenerate the original carboxylic acid. The
reaction occurs with a high quantum yield so that the formation
of carboxylic acid is the major reaction pathway.16 However,
other types of reactions occur that do not form the COOH group.
For example, the Norrish type II reaction may occur before the
bond to the ester group is broken.17 Even in this case, the long
aliphatic chains are cleaved, and this reaction should also be
effective for our purposes. In the photoreaction as shown in
Scheme 3, the particles lose their steric-stabilizing chains and
develop hydrophilic surfaces. This should lead to particle
flocculation.
The o-nitrobenzyl group has been used as a photoremovable
protective group for carboxylic acids, alcohols, and other
functional groups.16 The reaction follows the path shown in
Scheme 4 for the example of PMMA/OH/CO2PIB. Hydrogen
abstraction of a benzylic hydrogen by the nitro group occurs
upon photoexcitation. Subsequent rearrangement of the inter-
mediate diradical leads to cleavage of the benzylic-oxygen bond
and the formation of an orthonitrosoaldehyde. In our system,
the course of the photoreaction can be followed by loss of UV
absorption by the PMMA particles.
Photoinduced Flocculation of Dispersions. PMMA and
PBMA particles, derivatized with either 1-phenyldodecanone
or 1-phenyloctadecanone, flocculate when dispersions in media
close to the CPC are irradiated with UV light. There are several
ways to monitor the photoinduced flocculation. One approach
uses dynamic light scattering, which provides a measure of the
effective size of the aggregates formed as the photolysis
proceeds. We do not have equipment to monitor light scattering
during the irradiation, so the measurements involved removing
the sample from the photoreactor and quickly carrying out a
dynamic light-scattering measurement. A typical result is shown
in Figure 2. Flocculation of the PBMA/OH/CO2PC18 at its CPC
(1:3 hexadecane/cyclohexane) occurred after about 3 min of
irradiation. The polymer particles eventually precipitated out

5354 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Liu et al.
Figure 4. Photochemical bleaching of the chromophore from PBMA/
OH/CO2PC18 (3.7 mg/mL) in 80:20 cyclohexane/hexadecane in the
presence of 15 mg/mL of PIB-NH2 as stabilizer. The UV absorbance
was monitored at 249 nm. The vertical dashed line at ca. 8 min indicates
where flocculation would be visible to the eye in the absence of PIB-
NH2.
Figure 2. Effective size of the particles measured by dynamic light
scattering as a function of irradiation time. Samples were irradiated in
a Rayonet reactor (λ > 300 nm). The sample was PBMA/OH/PC18 (4
mg/mL) in 25:75 hexadecane/cyclohexane. PIB-NH2 is polyisobutylene
(MW ≈ 1000) with a primary amino terminal group. The flocculation
can also be detected visually.
through, for example, reaction of the aldehyde to form an acetal
or hemiacetal with the hydroxyl groups of the PMMA particles,
or that the nitroso and aldehyde groups might form hydrogen
bonds with the COOH groups on the particle surface.
Polymeric Dispersants. Polymeric dispersants are hydro-
carbon-soluble polymers introduced into a system with the
express intent of preventing flocculation of any colloidal material
that may form. One of the long-range goals of this research is
to develop the particle and particle-surface chemistry so that
photoinduced flocculation can be used as a test for the
effectiveness of commercial dispersants. A convenient model
dispersant, based on ideas described in the previous paragraph,
might be PIB-NH2. This polymer can interact with the COOH
groups formed through irradiation at the particle surface.
We saw above that dispersions of PBMA/OH/CO2PC18 in
cyclohexane and in cyclohexane/hexadecane mixtures flocculate
when irradiated with UV light. Here, we examine the effective-
ness of PIB-NH2 as a model dispersant in suppressing the
flocculation. Figure 2 shows the effective size of the aggregates
formed, as detected by dynamic light scattering, for irradiation
of a PBMA/OH/CO2PC18 dispersion (4 mg/mL in 25:75
cyclohexane/hexadecane) in the absence and presence of PIB-
NH2. In the absence of PIB-NH2, flocculation occurs after
only about 3 min of irradiation. In the presence of very small
amounts (0.5 wt %) of this polymer, flocculation is somewhat
retarded. If a larger amount (e.g., 1.4 wt %) of PIB-NH2 is
present, no flocculation occurs. Other short-chain aliphatic
amines such as butylamine do not suppress flocculation, even
at higher concentrations. In the presence of PIB-NH2, pho-
tobleaching occurs as expected. Figure 4 shows the decrease
of the absorption of the PBMA/OH/CO2PC18 particles in 4:1
cyclohexane/hexadecane at 249 nm. Without PIB-NH2, this
dispersion will begin to flocculate after 4 min of irradiation,
roughly after 20% of the total chromophore has reacted. In
the presence of the PIB-NH2, the solution remains transparent
and the absorbance can be monitored until the reaction goes to
completion.
Figure 3. Gel permeation chromatogram of PMMA/OH/CO2PIB-
NH2 before (upper pair of curves) and after (lower pair of curves) 30-
min photolysis (UV, >300 nm) in 2:1 cyclohexane/THF: GPC column,
500 and 10,000 Å; solvent, THF; UV detector at 249 nm (thick curves);
refractive index (thin curves).
of the solution, either upon further irradiation or if the sample
was allowed to stand. Increasing the cyclohexane content of
the continuous medium increased the irradiation time necessary
to flocculate the dispersion. More than 1 h of irradiation was
required to flocculate the particles in cyclohexane under the
same irradiation conditions.
Surprisingly, PMMA particles modified with PIB-NH2 did
not show any indication of flocculation upon UV irradiation.
When the PMMA particles modified with PIB (PMMA/OH/
CO2-PIB) were irradiated in THF dispersion, they showed a
behavior similar to that described above for the PBMA/OH/
CO2PC18 particles. When the reaction was followed by GPC,
as is shown in Figure 3, the UV absorption of the particles
decreased dramatically after UV irradiation. We emphasize the
point, however, that no photoinduced flocculation occurred even
in a solvent mixture at the CPC (1:2 THF/cyclohexane), whereas
the GPC analysis showed that the photochemical reaction of
the chromophore did occur in this system.
This observation is very important because it provides the
first indication that a free polymer in solution can act to stabilize
the carboxyl-bearing particles. For flocculation to occur, not
only must the chromophore-bearing chains be cleaved from the
particles but the “free” chains must not reassociate with the
particles. According the reaction shown in Scheme 4, the
detached PIB moiety has both nitroso and aldehyde groups. Both
are reactive species and are also hydrogen-bond acceptors. One
can imagine that the polymer might reassociate with particles
The antiflocculation properties of PIB-NH2 can be attributed
to its ability to associate with the particles by hydrogen bonding
or salt formation with the newly formed carboxylic acid groups.
PIB-NH2 not only blocks the carboxylic groups of the particles
from self-association but its long hydrocarbon chains provide
steric stabilization of the particles with hydrophilic surfaces
formed during the irradiation.

Polyacrylates and Their Flocculation
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5355
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1989, 97 (3), 515-536.
(5) (a) Murakami, Y.; Aihara, H. Effect of NOx and Unburned Gasoline
on Low Temperature Sludge Formation in Engine Oil. SAE Conference,
Detroit, MI, February 25 through March 1, 1991; Paper 910747. (b) Shaub,
H.; Kelemen, S. P. SAE Trans. 1992, 101, 984.
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Conclusions
PBMA and PMMA particles ca. 50 nm in diameter, which
form stable dispersions in aliphatic-hydrocarbon-containing
media, were prepared in a multistep process. Cross-linked
particles containing -COOH and -CH2OH groups were
prepared by batch emulsion polymerization in water. After
neutralization with CsCO3, the particles were dried and redis-
persed as microgels in DMF. At this point, hydrocarbon-soluble
steric-stabilizer groups were attached, using a photocleavable
chromophore as part of the attachment site. When these
particles were irradiated in hydrocarbon-containing media,
flocculation due to cleavage of the stabilizer arms from the
particles occurred. When the irradiation was carried out in the
presence of PIB-NH2, flocculation was suppressed. The
dispersant action of the polymer most likely occurs through
strong adsorption of the amino end onto the particle surface,
which after irradiation contains carboxylic acid groups. We
see these results as the first step in developing a system to model
precursors to sludge formation in motor oil and as a test platform
for examining the effectiveness of other dispersants in sup-
pressing flocculation of these precursors.
(7) Lee, K.-C.; Winnik, M. A.; Jao, T. C. J. Polym. Sci., Part A: Polym.
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(8) For a discusion of microgels, see the following. Napper, D. H.
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1983; pp 354, 374 ff.
(9) Blackley, D. C. Emulsion Polymerization; John Wiley & Sons: New
York, 1975.
Acknowledgment. The authors thank the Additives Division
of Texaco (now part of Ethyl Petroleum Additives, Inc.) and
NSERC Canada for their support of this research.
(10) Barrett, K. E. J., Ed. Dispersion Polymerization in Organic Media;
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