Synthesis and Rheological Characterization of Latexes ...

1

Synthesis and Rheological Characterization of Latexes

Stabilized by Methacrylic Acid Containing

Macromonomers

Ingeborg Schreur-Piet,a Alexander M. van Herk,a,b Jozua Laven,a and Johan P.A. Heutsa*

aDepartment of Chemical Engineering & Chemistry, Eindhoven University of Technology, PO

Box 513, 5600 MB Eindhoven, The Netherlands

bInstitute of Chemical and Engineering Sciences, Agency for Science, Technology and

Research, 1 Pesek Road, Jurong Island, Singapore 627833

*Author for correspondence: [email protected]

KEYWORDS: Emulsion polymerization, reactive surfactant, latex rheology, catalytic chain

transfer

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ABSTRACT

A range of copolymers of methacrylic acid (MAA) macromonomers prepared by cobalt-

catalyzed chain transfer and methyl methacrylate (MMA) and/or butyl acrylate (BA) was

synthesized and used as stabilizer in the emulsion polymerization of MMA. Although clear

differences were observed in polymerization rates using the different MAAx-MMAy

stabilizers, these differences were not as clearly reflected in the particle sizes, nor in the rates

per particle. However, a clear difference between these systems and those stabilized by

MAAx-BAy was observed. The latter systems were all characterized by much smaller particle

sizes and corresponding higher rates of polymerization. In addition, the molar masses in the

latter systems were all significantly larger than those obtained in the MAAx-MMAy stabilized

system, in which the stabilizers act as "sulfur-free" RAFT agents. Interestingly, the prepared

latexes showed a range of appearances varying from "milky" to "gel-like" depending on the

used stabilizer. The MAAx-BAy stabilized latexes had in general a lower viscosity and a

significantly smaller (if any) yield stress than the MAAx-MMAy stabilized latexes, and in the

latter case the rheological behavior was found to depend on the block lengths in and

concentration of the stabilizer.

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1 INTRODUCTION

Functional polymer latexes prepared by emulsion polymerization have found applications in

many fields, including water-borne coatings, adhesives and in biomedical applications.1-4 In

emulsion polymerization surfactants control the colloidal stability of the latex, the particle size

and surface functionality of the formed particles and have a large influence on the rheology of

the latex.5,6 If surfactants are only physically bound to the surface of the latex particles, the

surfactants can migrate towards the film interface upon drying and may have a negative effect

on final film properties like water sensitivity, wettability, gloss, adhesion and blocking.7-11

Hence, preferably surfactants are used which are chemically bound to the surface of latex

particles.12-16 In order to avoid the chemically bound surfactants from being buried inside the

latex particles or from forming water-soluble polymer chains that may cause bridging

flocculation, an ideal reactive surfactant should not be too reactive at the start of the emulsion

polymerization, but at the end of the emulsion polymerization all surfactant should have

reacted to obtain a stable latex.13,17 Reactive surfactants containing a propenyl end-group

display the right reactivity,14,15 and methacrylic oligomers containing these end-groups (called

macromonomers in the remainder of this paper) are readily prepared via cobalt(II)-mediated

catalytic chain transfer polymerization (CCTP).18-20 It is known that in a copolymerization

these macromonomers act as addition-fragmentation chain transfer (AFCT) agents with

methacrylates and that the copolymerization results in block copolymers. In a

copolymerization with acrylates (and styrene) the mechanism is more complex and ultimately

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leads to graft copolymers.18-23 In earlier work we synthesized methacrylic acid (MAA)

macromonomers via CCTP and used these directly in an emulsion polymerization to form in

situ amphiphilic copolymers,24 in a mechanism similar to what is commonly known as

polymerization induced self-assembly (PISA).25-28 We observed that only in cases in which

these water-soluble MAA macromonomers were sufficiently quickly converted into

amphiphilic copolymers stable latexes could be produced. This was the case for emulsion

polymerizations of the (hydrophobic) monomers butyl acrylate (BA) and butyl methacrylate

(BMA), but in the case of (the more hydrophilic) methyl methacrylate (MMA) the reaction

with the MAA macromonomer was probably too slow and no stable latexes could be

produced. In order to circumvent this problem, we decided to separate the phases of

stabilizer formation and emulsion polymerization in the current study. Hence we pre-

polymerized the MAA macromonomer with MMA and BA to yield the amphiphilic

copolymers M and B, respectively, and used these as stabilizers in the emulsion

polymerization of MMA.

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R

COOBu

v

COOH

yCOOH

BuOOC

R'

z

BuOOC

Hx(B)

The different structures of M (MAAx-MMAy) and B (MAAx-BAy) reflect the fact that

methacrylates and acrylates react differently with the methacrylic macromonomers. 18,22 These

different architectures (but similar overall compositions) lead to potentially different

stabilizing and resulting rheological properties and these properties are discussed in the final

part of this paper.

2 EXPERIMENTAL SECTION

2.1 Materials

All monomers, MMA, BA and t-BMA were obtained from Sigma-Aldrich (99 %). The

monomers were passed over a column of inhibitor remover (Aldrich) to remove the inhibitor.

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N,N'-azobis(isobutyronitrile) (AIBN, Merck) was recrystallized from methanol. The bis-

methanol complex of cobaltoxime boron fluoride (COBF) was prepared as described

previously29 (chain transfer constant CT for MMA in bulk = 34·103 at 60 °C). Toluene (AR,

Biosolve), dichloromethane (AR, Biosolve), tetrahydrofuran (stabilized with BHT, Biosolve)

and trifluoro acetic acid (95%, Aldrich) were used as received. Potassium persulfate (KPS, p.a.)

and sodium carbonate (dehydrated, p.a.) were purchased from Merck and used as received.

2.2 Synthesis of Copolymer Stabilizers

First, a range of t-BMA-macromonomers with number-average degrees of polymerization,

DPn, of 4, 6 and 12 (as determined by 1H-NMR) were prepared as described previously.24

These macromonomers were then reacted with MMA or BA to give t-BMAx-MMAy and t-

BMAx-BAy "pre-cursor" copolymers, respectively, which were subsequently treated with

trifluoro acetic acid to yield the corresponding MAAx-MMAy and MAAx-BAy copolymer

stabilizers. As a typical example of the followed procedure we report here the synthesis of

MAA12-MMA2; for the exact quantities used for all other copolymer stabilizers we refer to the

SI. First, 4.0 g of MMA (0.04 mol) was added to a 100 mL round bottom flask containing a

magnetic stirring bar. After the addition of 22 mg AIBN (0.13·10-3 mol), the flask was sealed

airtight with a septum and the solution was deoxygenated by purging with nitrogen in an ice

bath for 30 minutes. Simultaneously 45 g of a toluene solution containing t-BMA

macromonomer (44 wt% macromonomer, DPn = 12) was deoxygenated in a separate 100 mL

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round bottom flask. Subsequently, the macromonomer solution was added to the

monomer/initiator solution, heated to 60°C and left to react at this temperature for 16 h under

continuous stirring. The obtained t-BMA12-MMA2 macromonomer was isolated by

evaporation of toluene and residual monomer under reduced pressure, and dried in a vacuum

oven at 60 °C for 24 h (overall yield = 90%). Subsequently 24 g t-BMA12-MMA2 was added to a

100 mL round-bottom flask containing 50 mL dichloromethane, stirred until the polymer

dissolved completely after which 20 mL trifluoro acetic acid (0.3 mol) was added and stirred

at room temperature for 48 h. Dichloromethane and excess trifluoro acetic acid were removed

by evaporation under reduced pressure and the resulting MAA12-MMA2 polymer was dried

in a vacuum oven at 60°C for 2 days (quantitative yield).

2.3 Emulsion Polymerization

Emulsion polymerizations were carried out in batch. All experiments were carried out under

argon in a jacketed glass reactor (250 mL), thermostatted at 60 °C and equipped with a

mechanical four bladed turbine stirrer. The monomer conversions during reaction were

determined gravimetrically.

First, all ingredients except for the initiator solution were charged into the reactor, the reaction

mixture stirred at 350 rpm, purged with argon for 30 minutes and subsequently heated to 60

°C. Five minutes after reaching the desired temperature, the aqueous KPS solution was added

with a syringe to initiate the polymerization (for composition, see Table 1).

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Table 1. Recipe of a batch emulsion polymerization at T = 60 °C

Ingredient Amount

Water 120 g

Na2CO3 0.4 g (0.02 M)

Polymeric stabilizer Variable; 1.5 g (5 wt%a) is standard level

MMA 30 g (solids content = 20%)

K2S2O8 (KPS) 0.08 g (0.25 wt%a = 2.5·10-3 M)

a wt% = weight percentage relative to monomer (= g/100 g of monomer).

2.4 Analysis

Nuclear Magnetic Resonance

1H NMR spectra were recorded on a Varian MercuryVx spectrometer at 400 MHz chloroform-

d1, methanol-d4, DMSO-d6, THF-d8 and tetramethylsilane were used as solvents and internal

standard, respectively. All NMR results were obtained after solvent suppression of D2Oδ=4.8.

MALDI-ToF MS

MALDI-ToF MS spectra were measured on a PerSeptive Biosystems Voyager-DE STR

MALDI-ToF MS spectrometer equipped with 2 m flight tubes for linear mode and 3 m flight

tubes for reflector mode and a 337 nm nitrogen laser (3 ns pulse). All mass spectra were

obtained with an accelerating potential of 20 kV in positive ion and reflector mode with

delayed extraction. Data were processed with Voyager software. Simulations were performed

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with the MaldiAnalysis software by Staal and Willemse.30,31 As matrix 2,4,6-

trihydroxyacetophenone (80 mg/mL THF or methanol) was used and diammonium hydrogen

citrate (5 mg/mL THF or methanol) as cationating agent. Acrylate containing polymer samples

were dissolved in THF and methacrylic acid containing polymer samples were dissolved in

methanol at concentrations of 5 mg/mL solvent. Analyte solutions were prepared by mixing

solutions of matrix, salt and polymer at a 4:1:4 volume ratio. Subsequently, a spot of 0.30 µL of

such a mixture was put on the sample plate and dried at room temperature.

Dynamic Light Scattering

Dynamic light scattering (DLS) measurements were performed on a Nanotrac Ultra (Microtrac

systems). The used laser was a gallium-aluminum-arsenide semiconductor diode laser with a

wavelength of 780 nm and a power of 3-5 mW. Angle of incident-to-scattered light is 180°

(backscatter). This technique uses the Brownian motion of the molecules. The number-average

diameters (Dn) and the polydispersity indices (DPI) were determined using the cumulant

algorithm according to international standards ISO22412 and ISO13321).32,33

Size Exclusion Chromatography

Size exclusion chromatography (SEC) was carried out using a Waters Alliance system

equipped with a Waters 2695 separation module, a Waters 2414 refractive index detector (40

°C), a Waters 2487 dual UV absorbance detector, a PSS SDV 5 µm bead size guard column

followed by two PSS SDV 5 µm bead size linearXL columns in series (300 * 8 mm) at 40°C.

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THF with 1 v/v-% acetic acid was used as eluent at a flow rate of 1.0 mL min-1. The system

was calibrated with polystyrene standards (Polymer Laboratories, Mn = 580 - 7.1·106 g mol-1).

Potentiometric Titrations

Potentiometric titrations were conducted with a Scott Titronic Titration System with a Scott

pHG201 pH glass electrode. A 0.0099 M aqueous NaOH solution was used as titrant. The

precise concentration of NaOH was obtained by titrating with a sodium oxalate solution of a

known concentration. The titrations were performed at room temperature, in a titration vessel

filled with an exactly measured quantity (ca. 10 mL) of a sample, under continuous stirring.

The titration was carried out by adding titrant in doses of 0.2-0.04 mL. A 10 s time interval

was allowed between two subsequent doses to ensure that the equilibrium of the reaction was

reached.

For the determination of the carboxylic acid content in the water phase the latex was

centrifuged at 40,000 rpm for 3 h and a part of the clear top layer was used for titration. For

the determination of the carboxylic acid content at the surface of the particles the latex was

diluted to a solid content of 5 wt%, and titrated with the NaOH solution; the earlier obtained

amount of acid in the water phase was subtracted to obtain the amount of acid on the particles

surface. The amount of carboxylic acid buried inside the particles was determined by means

of a mass balance, i.e., deduced from the initial amount of carboxylic monomer introduced

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and the determined amounts of carboxylic groups on the surface of the particles and in the

water phase.34-37

Zeta Potential Analysis and Critical Micelle Concentration Measurements

Zeta potential measurements using Laser Doppler Micro-electrophoresis were performed on a

Zetasizer Nano ZS (Malvern Instruments). An electric field was applied to a highly diluted

dispersion of latex particles, which then move with a velocity related to their zeta potential.

The velocity was measured using PALS (Phase Analysis Light Scattering); the used laser was a

4 mW He-Ne laser with a wavelength of 633 nm. The critical micelle concentrations (CMC)

were determined from the intensity of the scattered light as a function of the concentration of

the copolymer. All copolymers were dissolved in an aqueous Na2CO3 solution (4·10-2 M) at

pH = 10; dilutions were made with the same Na2CO3 solution.38,39

Rheology

Rheological measurements were performed at 20 °C in the auto optimization mode using the

Smoluchowski model, on an Anton Paar Physica MCR 301 Rheometer with the concentric

cylinder CC27 system (inner and outer diameter are 26.66 mm and 28.92 mm, respectively).

For each latex first dynamic measurements were performed twice with a constant frequency

of 6.3 rad/s and a strain of 0.001. Before and between measurements the latex was “stirred” at

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a rate of 100 s-1 for 100 s. Then steady state viscosity measurements were performed using

different shear rates between 0.001 and 100 s-1 (for these experiments we report the viscosities

after the viscosities reached a constant value). The linear viscoelastic region was determined

by performing a strain sweep experiment at angular frequency of 6.3 rad/s. A strain of 0.001

was chosen because this strain was sufficiently high to effectively suppress noise and small

enough to stay in the linear region (see SI).

3 RESULTS AND DISCUSSION

3.1 Synthesis and Characterization of Copolymer Stabilizers

A range of amphiphilic copolymers of MMA and BA with MAA blocks were synthesized from

t-BMA macromonomers produced via CCTP and characterized. A summary of these results is

given in Table 2 (for details, see SI). The copolymer compositions were determined by 1H

NMR. Since MAAx-MMAy is a block copolymer with a vinylic end group, the intensity of the

signal of the vinyl end group could also be used to determine the (number average) molar

mass (Mn). In the case of MAAx-BAy, the end groups are less well-defined and therefore Mn

was estimated from SEC.

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Table 2. Characteristics of Copolymer Stabilizersa

Sample codeb fMc Mn

d

(g/mol)

Ðe FMf CMCg

(M)

MAA12-MMA2 0.22 1.3·103 2.7 0.16 3.2·10-3

MAA12-MMA6 0.42 1.6·103 6.1 0.32 5.4·10-5

MAA12-MMA15 0.59 2.6·103 3.9 0.56 1.6·10-5

MAA4-BA6 0.54 2.3·103 1.9 0.60 3.3·10-3

MAA6-BA6 0.54 5.5·103 2.2 0.51 2.2·10-3

MAA12-BA2 0.20 2.1·103 1.3 0.19 1.1·10-3

MAA12-BA6 0.40 3.2·103 1.7 0.47 2.0·10-3

MAA12-BA14 0.60 1.3·104 2.1 0.61 3.1·10-4

a Copolymers of MAAx-MMAy and MAAx-BAy synthesized from t-BMAx macromonomer DPnNMR = 4,

6 resp. 12. b The sample code reflects the average copolymer composition. For MAAx-MMAy, x = number of MAA units and y = number of MMA units in the copolymer; for MAAx-BAy, x : y = number of MAA units : BA units in the copolymer. In both cases, x and y are estimated from FM using DPn = 4, 6 resp. 12 for the MAAx block. c fM = mole fraction of the comonomer MMA or BA in the feed (wrt to all monomer units - all MAA units in the MAA-macromonomes are counted separately). d Number-average molar mass estimated via 1H NMR (for MAAx-MMAy, determined from chain length of MAAx and FM) and SEC (for MAAx-BA)y. e Dispersity of t-BMAx-MMAy and t-BMAx-BAy copolymers determined by SEC. f FM = mole fraction of comonomer MMA or BA in the copolymer, estimated from 1H NMR (see S.I.); standard error ≈ 0.04, all NMR results were obtained after solvent suppression of D2Oδ=4.8; g At 25 °C, standard error ca. 5%.

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Critical micelle concentrations (CMC) of the copolymers were determined by DLS and from

Table 2 it can be seen that, as expected, the CMC of the copolymers are all lower than that of

sodium dodecyl sulfate (CMC = 9.0·10-3 M at 25°C),40,41 a commonly used surfactant in

emulsion polymerization. For the MAAx-MMAy copolymers the CMC clearly decreases with

increasing MMA block length (at fixed MAA block length), but for the MAAx-BAy copolymers

no clear trend is observed, which may be caused by the fact that these copolymers consist of a

more complex mixture of structures as discussed below.

More detailed structural information about the stabilizers was obtained by MALDI analysis

and the spectra for the MAAx-MMAy (specifically of MAA12-MMA15) and MAAx-BAy

(specifically of MAA6-BA6) stabilizers are shown Figures 1 and 2, respectively. In order to aid

the interpretation of these spectra, simplified reaction mechanisms for the two

copolymerizations of the t-BMA macromonomer precursors are shown in Schemes 1 and 2,

respectively. The copolymerization of the t-BMA macromonomer with MMA (Scheme 1)

proceeds via the addition-fragmentation chain transfer process first described by Moad and

co-workers23 and recently coined "sulfur-free RAFT" by Haddleton and coworkers.42,43 The

main product of this reaction is a block copolymer with propenyl end group (pM2), with

potential other products being macromonomers pM1', pM2' and pM3, and possible

termination (by combination) products that we lumped into the same overall structure pM4.

This is indeed observed in the MALDI spectra in Figure 1, where the simulated spectrum of

M2 (i.e., the MAA-containing macromonomer that results from hydrolysis of the precursor

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pM2) in Figure 1c shows excellent agreement with the experimental spectrum shown in Figure

1b. The majority of the peaks in Figure 1 can be assigned to M2. Also peaks of M2’ are

observed, but no significant amounts of macromonomer M3 or termination products M4. (See

S.I. for more details).

Scheme 1. Simplified overview of the reaction mechanism for copolymerization and possible

products of t-BMA macromonomer and MMA, R and R’ = H or C(CH3)2CN. The labels pMX (with X

= 1 - 4) are used to indicate the precursor molecules for stabilizers MX (with X = 1 - 4) in Figure 1.

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H

COOtBMA COOtBMA

x

H

COOtBMA COOMe

x

COOtBMA

y

H

COOtBMA COOtBMA

x-1

H

COOtBMA COOMe

x

COOMe

y-1

COOMe

y-1

COOMe

MMA

AIBN

CN COOMe

y

COOtBMACN

MMA

RR'

COOtBMA COOMe

x yH

COOMe

y

COOMe

Other final products

pM1'

H

COOMe

y

COOtBMA

pM2' pM4

pM2

pM1

pM3

pMX MXHydrolysis

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Figure 1. MALDI-ToF MS spectrum of MAA12-MMA15; (a) overview, red rectangle shows the

range of the enlarged spectrum in (b); (b) enlarged spectrum for m/z = 970 — 1120 g/mol

(same labeling as in Scheme 1) and (c) simulation of MAAx-MMAy macromonomer (M2) with

Na+ as cationating agent, and DAC-THAP as salt-matrix combination.

The spectrum of the MAAx-BAy (Figure 2) is much more complicated than that of MAAx-

MMAy shown in Figure 1 and is virtually impossible to interpret without considering the

mechanism for the copolymerization of t-BMA macromonomer with BA shown in Scheme 2

(again we used pBX to indicate the precursor to stabilizer BX).18,22 When a radical with a BA

terminal unit reacts with an t-BMA macromonomer (pB1 in Scheme 2) then it will first

undergo an addition-fragmentation step that results in a macromonomer containing a BA

penultimate group (e.g., pB2, pB3 or pB5). This macromonomer with a BA penultimate group

will now react as a true comonomer when a BA radical adds to its double bond and a graft is

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formed. The resulting propagating branched radicals will then stop growing by reaction with

t-BMA macromonomer pB1, which results in a macromonomer pB5, or by termination leading

to pB6 (all with a range of possible end groups and different degrees of branching). The

MALDI spectrum in Figure 2 indeed shows peaks for both B5 and B6. (See SI for a more

detailed analysis).

Scheme 2. Simplified overview of the reaction mechanism for copolymerization and

possible products of t-BMA macromonomer and BA, R and R’ = H or C(CH3)2CN (= IBN).

The labels pBX (with X = 1 - 4) are used to indicate the precursor molecules for stabilizers BX (with X

= 1 - 4) in Figure 2.

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H

COOtBMA COOtBMA

x

R

COOtBMA COOBu

x

COOBu

y-1

BA

H

COOtBMA COOtBMA

x-1

R

COOBu

v

COOtBMA

yBuOOC

CN

R

COOtBMA COOBu

x

COOtBMA

y

pB2

pB1

R

COOBu

v

COOtBMA

yCOOtBMA

BuOOC

z

COOBu COOBu

COOtBMA

RxHx

BA+

in terms of mass

Possible end products

R

COOBu

v

COOtBMA

yCOOtBMA

BuOOC

z'

COOBu

R1

Hx

IBN

COOBu COOtBMA

y'

pB3

pB5

IBNR

COOBu

y'

pB4 pB6

R

COOBu

v

COOtBMA

yCOOtBMA

BuOOC

z'

BuOOC

Hx

COOH

=

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Figure 2. (a) MALDI–ToF MS spectrum of MAA6-BA6, m/z = 650 — 2650 g/mol, used salt-

matrix combination: DAC-THAP, red rectangle shows the range of the enlarged spectrum in

(b); (b) Enlarged spectrum m/z = 950 — 1100 g/mol.

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3.2 Emulsion Polymerization of Methyl Methacrylate

Batch emulsion polymerizations of MMA have been performed with MAAx-MMAy and

MAAx-BAy copolymeric stabilizers. For all polymerizations the kinetics were studied and the

final latexes were characterized.

In Figure 3 the conversion vs. time curves of the emulsion polymerizations with the different

stabilizers always at 5 wt% are shown and it is clear that all display typical ab initio emulsion

polymerization behavior.

Figure 3. (a) Conversion vs. time curve of the emulsion polymerization of MMA containing 5 wt% of stabilizer, other conditions as in Table 1: (a) MAAx-MMAy copolymers: () MAA12-

MMA2; () MAA12-MMA6 and () MAA12MMA15. (b) MAAx-BAy copolymers: () MAA4-BA6; () MAA6-BA6; () MAA12-BA2; () MAA12-BA6 and () MAA12-BA14.

In Figure 3a the conversion-time curves for the emulsion polymerization of MMA with

MAA12-MMAy stabilizers are shown. The curves show clearly that the overall polymerization

rate increases with an increasing hydrophobic block length of the stabilizer. This would

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suggest that with increasing hydrophobic block length a larger number of particles (Np) is

obtained, but this is not supported by the measured particle sizes and derived particle

numbers as listed in Table 3 (it should be noted here that the polymerization using MAA12-

MMA2 showed significant coagulation - 7% - which was removed before further analyses).

We cannot discern an obvious trend in these data and we expect the polymerization behavior

to be more complicated than what can be captured by simple emulsion polymerization theory.

What can, however, be concluded from Figure 3a is that nucleation is fastest for the stabilizer

with the largest hydrophobic block and this is conceivably explained by the fact that these

stabilizers more easily form (block copolymer) micelles that act as a "seed" for the

polymerization. The results for other concentrations of the stabilizers not shown graphically

are summarized in Table 3 and when considering the concentration dependencies within a

series of the same stabilizer molecule, some clearer trends appear. In general (with the

exception of the lowest concentration of MAA12-MMA2), particles sizes decrease (and Np

increases) with increasing stabilizer concentration, as expected for conventional surfactants.

Rates per particle are relatively constant and experimental uncertainty precludes us from

drawing any definite conclusions. However, it is interesting to note that the systems with the

smallest particles also have the lowest rates per particle.

The conversion vs time data for a range of MAAx-BAy stabilized emulsion polymerizations are

shown in Figure 3b and the first thing that one notices is that all these polymerizations are

significantly faster than the MAAx-MMAy stabilized polymerizations (Figure 3a) with the

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same stabilizer contents (all 5 wt% - with similar molar quantities of hydrophilic MAA units).

Comparing these two systems we may conclude that this large difference in rate is caused by

different particle numbers, as the obtained particle sizes in the MAAx-BAy systems are

significantly smaller than those obtained in the MAAx-MMAy systems (see Table 3). This in

turn suggests that the stabilizing properties of the MAAx-BAy molecules are better than those

of MAAx-MMAy.44 Within this series of MAAx-BAy experiments, no clear trends are

discernable in Figure 3b and Table 3. The only result that "sticks out" are those for MAA12-

BA2, which is the most hydrophilic stabilizer (and it should be noted here that MAA12-BA2 is

an "average composition" denoting a range of different molecules, including those with only 1

or 0 BA units). This system is significantly slower than all the others and the particle sizes are

much larger, which in retrospect, is not unexpected. Rates per particle are similar for all these

systems, but are all smaller than those for the MAAx-MMAy systems. When considering all

the rates per particle summarized in Table 3, then the data suggest that the rate per particle

increases with increasing particle size, which would be consistent with decreasing exit and

termination rates.45

Table 3. Characteristics of the Synthesized Latexes

Stabilizer CSa

(mM)

CMAAb

(mM)

Dnc

(nm)

dX/dtd

(min-1)

Npe

(m-3)

rppf

(m3/min)

Mng

(kg/mol

)

Mpg

(kg/mol

)

Ðg

MAA12-MMA2 5.2 62 162 ± 14 7·10-3 8·1019 9·10-23 212 830 2.5

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7.0 83 177 ± 19 1·10-2 6·1019 2·10-22 65 620 9.110* 118 162 ± 20 9·10-3 6·1019 2·10-22 70 370 5.819 230 120 ± 9 5·10-3 8·1019 6·10-23 15 140 9.0

5.0 60 157 ± 14 1·10-2 1·1020 1·10-22 111 420 4.3MAA12-MMA6

7.7* 92 141 ± 25 1·10-2 1·1020 1·10-22 46 230 6.3

4.9* 59 161 ± 15 2·10-2 5·1019 3·10-22 116 1070 6.5MAA12-

MMA15

7.7 92 138 ± 17 1·10-2 1·1020 8·10-23 56 450 9.3MAA4-BA6 5.8* 51 83 ± 22 3·10-2 7·1020 5·10-23 >104 h

MAA6-BA6 2.4* 61 69 ± 17 4·10-2 1·1021 3·10-23 >104 h

MAA12-BA2 6.2* 124 118 ± 9 2·10-2 2·1020 8·10-23 >104 h

MAA12-BA6 4.1* 88 84 ± 12 4·10-2 7·1020 6·10-23 >104 h

MAA12-BA14 1.0* 55 83 ± 13 3·10-2 7·1020 5·10-23 >104 h

a Concentration of MAAx-MMAy or MAAx-BAy stabilizer (* corresponds to 5 wt% with respect to

overall monomer content), determined from used mass and Mn; b Overall concentration of MAA units, determined from the used mass and the weight fraction of MAA in the stabilizer; c Number-average diameter and standard deviation determined by SEM, particle count > 100; d Overall conversion rate between 20 and 60% conversion; e Number of particles per m3 water; f rate per particle = (dX/dt)/Np; g Number-average and peak molar mass and dispersity determined by SEC and reported against polystyrene standards; h estimates, exclusion limit reached.

In Table 3 also the molar masses of the produced PMMA are shown and two things clearly

stand out: the PMMA latexes stabilized by MAAx-BAy are characterized by much higher

molar masses than those stabilized by MAAx-MMAy, and in the latter systems, a higher

stabilizer concentration leads to lower molar masses. In Figure 4, the stabilizer concentration

dependence of the molar mass distributions for a range of latexes is shown, in Figure 4a for

the latexes stabilized by MAA12-MMA2 and in Figure 4b for those stabilized by MAAx-BAy.

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Figure 4. Molar mass distributions of latexes made different stabilizers: (a) MAA12-MMA2 at different concentrations (Cs); (b) Different MAAx-BAy stabilizers at a concentration of 5 wt%. Data are reported against polystyrene standards.

The observed differences in molar mass distributions are easily explained by the differences in

molecular structure of the used stabilizers M and B, i.e., M is representative for the MAAx-

MMAy series and B for the MAAx-BAy series. In the emulsion polymerization of MMA, M

will act as an addition-fragmentation chain transfer agent,28 and as with any other chain

transfer agent, this will lead to a molecular weight reduction. B on the other hand will largely

behave as an unreactive surfactant (except for the small fraction of the molecules that still

contains a propenyl endgroup, i.e., B1, B2, B3 and B5) and high molar masses, typical of

emulsion polymerization are expected. The unreacted stabilizers are visible in the molar mass

distribution as the low molar mass peak in Figure 4b, whereas the small "bumps" in some of

the molar mass distributions around M = 106 g/mol may be indicative of some chain transfer

going on (because of the presence of a small amount of molecules with propenyl endgroups).

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We conclude this section by stating that both the MAAx-MMAy and MAAx-BAy series of

stabilizers are suitable for emulsion polymerization, be it that the former series in general

leads to slower polymerizations, larger particles and lower molar masses than the latter series.

Since the used stabilizers are not fully homogeneous in terms of molar mass and chemical

composition distributions, it is difficult to identify clear trends within each series of

experiments.

3.3 Latex Characterization

In the previous section we discussed the use of a range of short MAA-containing copolymers

as stabilizers in the emulsion polymerization of MMA and concluded that all these

copolymers in principle resulted in stable latexes. We have not, however, commented yet on

the appearance of these latexes, which varied from very liquid, "milky", to very viscous, "gel-

like". It is well known that the rheological behavior of a latex is determined by the volume

fraction and particle size of the polymer particles, and by the amount and type of stabilizer

used.46 Thus, some differences in rheological behavior are expected, but we did not anticipate

the differences we observed.

To facilitate the further discussion, we subdivide the latex appearance into three types which

are schematically shown in Figure 5. Type I is a colloidally stable, liquid-like, latex (low

viscosity, containing no visible sediment/coagulum), type II is a flocculated latex which may

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contain some redispersible sediment and type III is a highly viscous latex with internal

network structure (gelated).

I II III

Figure 5. Schematic representation of the three types of appearances of the latexes produced in this work: I) fluid-like latex; II) viscous latex with reversible flocculation; III) highly viscous, gelated latex.

In Table 4 the appearances of the final latexes with corresponding stabilizers and

concentrations are listed in combination with the zeta potentials and the distributions of the

stabilizer in different phases of the latex (water, surface or inside particle). What is

immediately clear from these data is that the MAAx-MMAy-stabilized latexes (L1-L8) are in

general more viscous (mainly type II) than those stabilized by MAAx-BAy (L9-L13, mainly

type I). This difference is conceivably explained47,48 by the fact that in general (except for L11)

the amount of water-soluble molecules of stabilizer B is lower than the amount of water-

soluble molecules of stabilizer M, as shown by the titration results (i.e., the values for CMAA,w

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in Table 4). Notable exceptions are L4 (type I) and L11 (type II), but these results are not

inexplicable. L4 is the latex with the highest MAA12-MMA2 concentration, which has a very

high CMAA,w, but also the highest number of stabilizing groups per m2 surface area (n"MAA,s).

L11 is the latex with the best water-soluble stabilizer, MAAx-BAy (which contains a large

fraction of molecules with only one or even no BA groups, see SI) and therefore results in a

high CMAA,w and the lowest n"MAA,s. Additionally, it is important to stress here that all the

copolymer stabilizers in this study have relatively broad molar mass and chemical

composition distributions, so preferential adsorption of only certain molecules is highly likely

and this complicates the exact interpretation of our results.

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Table 4 Characteristics and MAA Distributions of the Synthesized Latexes

Latex StabilizerCS

a

(mM)

CMAAb

(mM)

Typec d

(mV)

pHe CMAA,we

(mM)nMAA,s

f

(mmol)

n'MAA,sg

(mmol/part)

n"MAA,sh

(mmol/m2)

nSi

(1/part)

MAA,pj

(%)

L1 5.2 62 III -56 8.1 3.6 0.5 5·10-17 8·10-8 3103 87

L2 7.0 83 II -56 7.2 13.6 0.5 1·10-16 9·10-8 5103 78

L3 10* 118 II -52 6.2 46.8 1.0 1·10-16 2·10-7 5103 54

L4

MAA12-MMA2

19 230 I -55 5.3 121.9 4.8 5·10-16 8·10-7 2104 29

L5 5.0 59 II -50 7.0 4.2 2.7 2·10-16 4·10-7 1104 55

L6MAA12-MMA6

7.7* 92 II -62 7.0 16.2 1.2 7·10-17 1·10-7 4103 71

L7 4.9* 59 II -55 8.2 1.8 0.06 6·10-18 1·10-8 3102 96

L8MAA12-MMA15

7.7 92 I/II -58 7.0 10.8 7.5 7·10-16 1·10-6 4103 19

L9 MAA4-BA6 5.8* 51 I -45 8.0 1.5 1.0 1·10-17 6·10-8 5102 82

L10 MAA6-BA6 2.4* 61 I -49 8.4 0.6 0.7 5·10-18 4·10-8 3102 90

L11 MAA12-BA2 6.2* 124 II -55 6.5 22.0 1.2 7·10-18 1·10-8 4102 68

L12 MAA12-BA6 4.1* 88 I -47 8.8 0.6 0.3 5·10-18 2·10-8 3102 96

L13 MAA12-BA14 1.0* 55 I -43 8.1 0.1 0.09 2·10-18 6·10-9 1102 98

a Concentration of MAAx-MMAy or MAAx-BAy stabilizer (* corresponds to 5 wt% with respect to overall monomer content), b Overall concentration of MAA

units, determined from the used mass and the weight fraction of MAA in the stabilizer; c Appearance of latex, see text and Figure 5 for definition; d -potential

in diluted solutions, pH ≈ 7; e pH of final latex; f Concentration of MAA in the water phase determined by titration; g Amount of MAA on particle surface

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expressed in mmoles determined by titration; h Amount of MAA units on the particle surface in mmol per particle; i Amount of MAA units on the particle

surface in mmol per m2; j Number of stabilizer molecules on surface per particle, ignoring any preferential adsorption; k Percentage of added MAA units

buried inside the particles.

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The data in Table 4 also show that, in this study, the measured -potential (of the highly

diluted latex) is not necessarily a good reflection of colloidal stability in the latex: in

general, the MAAx-BAy stabilized latexes are characterized by a somewhat smaller

negative -potential, but exhibit a more fluid-like, flocculate-free, behavior. We also

cannot observe any direct correlation between the -potential and the number of MAA

units on the surface of a particle (n"MAA,s), but for most MAAx-BAy stabilized latexes this

number is lower than those observed for the MAAx-MMAy stabilized latexes. What is also

interesting to note is the fact that a very large fraction of the surfactant molecules is buried

inside the particles (MAA,p denotes the fraction of all MAA units buried inside the particle)

irrespective of the type of used stabilizer. Although for the series of MAAx-MMAy

stabilized latexes the trend appears to be that the buried fraction (MAA,p) decreases with

increasing surfactant concentration (CS or CMAA), this trend is less clear when considering

the absolute number of moles that is buried (e.g., for the series L1-L4, the number of buried

MAA units is relatively constant : ~ 7 mmol). With similar amounts of surfactant buried

inside the particles, the observed (and expected) trends of increasing aqueous phase

concentrations and adsorbed polymer on the interface with increasing surfactant

concentration are then fully consistent.

In order to get a more quantitative insight into the appearances and the rheological

properties of the prepared latexes we measured the steady state viscosities as a function of

the shear rate and probed the mechanical microstructure by measuring the dynamic

moduli G' and G'' for a period of 1000 s at a fixed frequency (i.e., we performed a dynamic

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time sweep) with an amplitude small enough to not disturb any possible structures. In

order to check the recoverability of the structure after such time sweep, the sample was

stirred (using a shear rate of 100 s-1 for a duration of 100 s), after which another dynamic

time sweep was recorded. In Figure 6, the results are shown for L2, L6 and L8, as an

example for the influence of the length of the MMA block on the rheological properties.

Figure 6. Rheology of latexes stabilized with 7-8 mM MAAx-MMAy copolymers. (a) Steady state viscosity; (b) Storage modulus G' (symbols) and loss modulus G" (lines) as function of time; strain = 0.001, ω = 6.3 rad/s; at 1000 s the sample was agitated using a shear rate of 100 s-1 for 100 s. () MAA12-MMA2 (L2), () MAA12-MMA6 (L6) and () MAA12-MMA15 (L8). Values of G' ~ 0 are plotted as 10-2 Pa.

In Figure 6a a comparison of the flow curves is made for latexes stabilized with 7-8 mM

MAAx-MMAy copolymer with different MMA block lengths (L2, L6 and L8). All latexes

show shear thinning behavior and at high shear rates the viscosities of the latexes tend to

level off, suggesting that internal structures are no longer broken up by increasing shear

rates. The relatively high viscosities at which the levelling off take place, however,

suggest that large agglomerates or flocs with strong interactions still exist.46 At lower

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shear rates the viscosities increase, and approach a slope of -1 in double-log plot,

indicative of plastic behavior; this means that large agglomerates stick to each other and

form a soft solid with yield stress values from ~ 0.001 Pa (for L8) to ~ 0.2 Pa (for L2). In

Figure 6b the results are shown of two sequential time sweeps (duration = 1000 s, = 6.3

rad/s and strain = 0.001), separated by a period of 100 s with stirring at 100 s-1. The three

latexes show all a different behavior. The latex stabilized with MAA12-MMA2 (L2)

displays a solid like behavior (G' > G'') and the initial increase in G' is indicative of build

up of some internal structure over time. Stirring breaks up this structure, but after stirring

the structure is restored, albeit at a lower modulus level, which suggests that the mobility

of the flocs or particles is still high. In the case of MAA12-MMA6 the latex (L6), which is

more fluid-like (G" > G') also some internal structure is built up in time, and this structure

is immediately restored after stirring at the same viscosity level. G'' values give a yield

stress of ~ 0.1 Pa, which implies that network formation is minor. For the latex stabilized

with MAA12-MMA15 (L8), which also is more fluid-like (G'' > G'), it can be seen that the

internal network is destroyed after stirring after the first measurement and not built up

anymore (G' 0). The results shown in Figure 6 suggest that there is a significant effect of

hydrophobic block length on the final viscosity and stability of the latex. If we compare

these results with the results in Table 4 for the amount of MAA on the surface (nMAA,s) we

can conclude that higher amounts of MAA units on the surface of the particles give less

network formation and a lower viscosity of the latex, so at a concentration of 8 mM

MAA12-MMA15 appears to be a better dispersant than MAA12-MMA6 and MAA12-MMA2,

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but the relatively high viscosities at high shear suggest that still higher concentrations are

required for a good colloidal stabilization.

Figure 7. Rheology of MAA12-MMA2 stabilized latexes of pMMA (a) Steady state viscosity; (b) Storage modulus G' (symbol) and loss modulus G'' (dotted line) as function of time, strain = 0.001, ω = 6.3 rad/s. Different concentrations of MAA12-MMA2 stabilizer: () 5 mM (L1), () 7 mM (L2), () 10 mM (L3), () 19 mM (L4).

The effect of stabilizer concentration on latex rheology is shown in Figure 7 for the MAA12-

MMA2 systems (L1-L4). The results of the steady state viscosity measurements are shown

in Figure 7a. The viscosity of the latex stabilized with 19 mM MAA12-MMA2 (L4) was so

low that it could not be determined at low shear rates. Plastic behavior is observed for the

other latexes, comparable to what is observed in Figure 6a. The high viscosities at high

shear rates are indicative of large agglomerates with strong interactions, which is

supported by the dynamic time sweep results shown in Figure 7b. All latexes behave like

soft solids (G' > G") and the behavior of L1 to L3 is consistent with a space-filled

flocculated packing where the flocs break up the bonds with their neighboring flocs when

sheared, but on rest immediately restore these bonds.

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Figure 8. Rheology of latex stabilized with 5 wt% MAA-BA: (a) Steady state viscosity; (b) Storage modulus G' (symbols) and loss modulus G" (dotted lines) as a function of time; strain = 0.001, ω = 6.3 rad/s; () MAA4-BA6 (L9); () MAA6-BA6 (L10); () MAA12-BA2 (L11); () MAA12-BA6 (L12);() MAA12-BA14 (L13).

Finally, in Figure 8 the results of the steady state viscosity and the results of the dynamic

time sweep experiments for the latexes stabilized with MAAx-BAy copolymers are shown.

The low shear rate data for the low viscous systems were left out because they were below

the equipment sensitivity limit. From Figure 8a can be seen that none of these latexes show

noticeable plastic behavior (slope -1 in a double-log plot), with the exception of L13

(stabilized with MAA12-BA14). From the Newtonian plateau at around 0.005 Pa·s in the

shear rate range of 0.1-100 s-1 it can be concluded that these non-plastic latexes are well-

stabilized, non-aggregating dispersions. L13 clearly shows a slight plasticity with a yield

value of about 0.2 Pa. We consider this as indication that the colloidal stability may be not

perfect. At shear rates > 100 s-1 an increase of the viscosity is observed due to Taylor

turbulences.49,50 The results of the oscillatory deformation tests shown in Figure 8b further

confirm that the extent of agglomeration at rest is very limited. The elastic component of

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the dynamic modulus is for all latexes larger than the viscous component (G' > G"). The

loss moduli (G'') give a value of around 0.1 Pa and in combination with a low viscosity

these latexes could be suitable to be used in a coating formulation.47

When comparing the appearances of latexes stabilized with MAAx-MMAy and MAAx-BAy

it can be seen that the latter are all characterized by a low viscosity and a low yield stress

(types I or II), whereas the former show more flocculation, higher viscosities and higher

yield stresses (types II or III). These differences in appearances and rheological behavior

can partly be explained by the final distribution of the stabilizers over the different phases

in the polymer latex. Due to the more hydrophilic nature of the MAAx-MMAy

copolymeric stabilizers, more molecules may dissolve in the water phase thus increasing

the viscosity of the solution. These molecules may form aggregates, be present on the

surface of the polymer particles thus increasing the effective volume fraction of particles,

or form links between particles and thus form an internal network which can be destroyed

at higher shear rates.47,48 These effects will be less in the case of the more hydrophobic

MAAx-BAy stabilizers.

4 CONCLUSIONS

Copolymers of MMA or BA with MAA macromonomers prepared via catalytic chain

transfer polymerization were prepared, characterized and used as efficient stabilizers in

the emulsion polymerization of MMA. The MAAx-MMAy copolymers (M) have a blocky

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structure and act as (reversible) chain transfer agents in the emulsion polymerization. The

MAAx-BAy copolymers (B) have a more complex structure, which on average resembles

more that of a graft copolymer, and are (in general) unreactive in the emulsion

polymerization. This difference in behavior manifests itself in the polymerization kinetics

(B leads to smaller particles and much faster rates), molar masses of the emulsion polymer

(M leads to significantly reduced molar masses) and distribution of the stabilizer

molecules over the three different locations, i.e., the water phase, the particle surface and

the particle interior. In the case of M, more molecules are located in the water phase, and

this may be the cause for the significant difference in appearances of the B- and M-

stabilized latexes. The former latexes are all characterized by a low viscosity and no

significant aggregation (low yield stresses), whereas the latter latexes are characterized by

higher viscosities, stronger aggregation and yield stresses, which depend on block length

and concentration.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at

DOI:

xxxx

Details related to characteristics (NMR, MALDI-ToF-MS, SEC, CMC, DLS) of

synthesized macromonomers and latexes (pdf).

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xxxx.pdf

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]

ORCID

Johan PA Heuts: 0000-0002-9505-8242

Ingeborg Schreur-Piet: 0000-0002-2079-4333

Alex M van Herk: 0000-0001-9398-5408

Jozua Laven: 0000-0002-1860-8677

Funding

This work was partially funded by the Stichting Emulsion Polymerization

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

We gratefully acknowledge financial support by the Stichting Emulsion Polymerization.

Electron microscopy was performed at the Center for Multiscale Electron Microscopy at

Eindhoven University of Technology.

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Synthesis and Rheological Characterization of Latexes ...

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