Fundamentals of chemical incorporation of ionic monomers onto polymer colloids: paving the way for surfactant-free waterborne dispersions

Abstract

In this article, the fundamentals of the chemical incorporation of a pH and temperature insensitive ionic monomer (sodium styrene sulfonate, NaSS) onto polymer particles was investigated in an attempt to go beyond the current technology for production of a waterborne polymer dispersion, which is based on the use of surfactants to stabilize the dispersion. The success of this approach requires the chemical incorporation of NaSS onto the polymer particles and minimizing at the same time the amount of water soluble polymer. It was found that the chemical incorporation of NaSS can be improved by increasing the concentration of the comonomer in the aqueous phase, whereas the functionality of the comonomer did not play any significant role. Strategies to maximize incorporation of NaSS were proposed.

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Introduction

Waterborne dispersed polymers are commonly synthesized by emulsion polymerization using surfactants to provide colloidal stability. However, surfactants are deleterious in applications as their migration during film formation leads to lower gloss (migration to the air-film interface), poorer adhesion (migration to the substrate–film interface) and higher water uptake (formation of aggregates within the film).¹ Therefore, there is a need to develop emulsion polymers free of migratory surfactants. Several alternatives have been explored. Polymerizable surfactants (surfmers) are surfactants containing a double bond that during polymerization can be incorporated into the polymer backbone² and hence migration is avoided.³ The main drawback of the surfmers is that they are system dependent in the sense that it is necessary to adapt the reactivity of their double bond to that of the monomer system used, because too reactive surfactants become buried within the particles during polymerization and the slow reactive ones do not get attached to the polymer. Strategies for the optimal use of surfmers have been proposed,^4,5 but still the functionality of the surfmer has to be adapted to the particular monomer system. Polymeric surfactants that strongly adsorb on the surface of the particle show a limited migration during film formation³ and therefore limit the problems caused by the conventional surfactants. However, strong adsorption makes them inefficient for particle nucleation.⁶ Functional monomers and initiators containing ionic groups such as carboxylate,^7,8 sulfate,^8,9 amino,¹⁰ phosphate,¹¹ phosphonate¹² and sulfonate^13–15 are used in emulsion polymerization formulations to minimize the amount of surfactant used, and also to impart special properties such as better adhesion to substrates and biological tissues and better mechanical properties.^16,17

In this context, it is appealing to use functional monomers as the sole source of stabilizing moieties. However, some of the functional groups present practical limitations. For instance, the stabilization efficiency of carboxylic acids is dependent on the pH. The pH of the reaction medium should be well above the pK_a value of the acid (which for commonly used carboxylics such as acrylic and methacrylic acids is around 4.5) to ensure the ionic state. This is a problem because the pH of a polymerization initiated with persulfates (likely the most common initiators) is about pH = 3. On the other hand, the phosphate and sulfate groups are subjected to hydrolysis. Amines provide cationic stabilization, which have limited applications as most of the natural surfaces are anionic and hence destabilize the cationically stabilized particles. Sulfonate that combine a very low pK_a with high stability over a wide range of temperature and pH, is the most attractive functional group.¹⁸ Sodium styrene sulfonate (NaSS) is particularly interesting because it is commercially available and combines a very low pK_a (pK_a = 1)¹⁹ with a styrenic double bond that reacts well with many of the monomers commonly used in emulsion polymerization.

However, the use of NaSS in emulsion polymerization is not straightforward because in order to provide stability to the polymer dispersions and to avoid the negative effects of the migratory hydrophilic species on the application properties,³ NaSS should be chemically incorporated onto the polymer particles and the fraction that remains in the aqueous phase as water-soluble polymer should be minimized. Considering that, in the aqueous emulsion, NaSS is located in the aqueous phase and that its homopolymer is a water soluble polyelectrolyte, the only way to achieve these goals is to copolymerize NaSS with more hydrophobic monomers.

Controlled radical polymerization (CRP) offers an elegant way of incorporating NaSS to the polymer particles^20–24 by forming amphiphilic block copolymers polymerizing first the NaSS and then a second more hydrophobic monomer. The block copolymer is then used as surfactant in the subsequent emulsion polymerization process where the main (co)polymer is produced. Although this method ensures the virtually complete incorporation of NaSS to the polymer particles, the CRP agent strongly affects the microstructure of the main (co)polymer (frequently lowering its molecular weight), and hence limiting the achievable range of properties that for most applications (e.g. adhesives;^25–27 coatings¹) require the broad molecular weight distributions and the high molecular weights obtained by free radical emulsion polymerization.

In free radical emulsion polymerization, the incorporation of the NaSS to the polymer particles by copolymerization with a more hydrophobic monomer is expected to be largely controlled by the nature of the comonomer. The choice of the comonomer is not obvious as it requires a compromise between two conflicting characteristics. On one hand, a certain water solubility of the comonomer is needed to be present in the aqueous phase in order to copolymerize with NaSS. On the other, hydrophobicity is needed to reduce the water solubility of the copolymer formed. Another important point is the reactivity of the comonomer with NaSS. In this regard, literature is not of much help as most of the works reported are limited to the use of NaSS in the emulsion polymerization of styrene^{13,18,28–30} and when other comonomers are used, the incorporation of NaSS to the polymer particles was not studied.³¹

In this work, for the first time, the effect of the functionality and water solubility of the comonomer on the chemical incorporation of NaSS onto the polymer particles was investigated using styrene as well as methacrylates and acrylates with different alkyl chain length. Seeded batch emulsion polymerization was used to eliminate the possible effect of the comonomer on particle nucleation. A nonionic surfactant was used to prepare a seed devoid of charges. tert-Butyl hydroperoxide/ascorbic acid redox pair initiator that forms noncharged hydrophobic radicals in the aqueous phase was utilized in all the reactions.

Experimental

Materials

The monomers, 2-hydroxy ethyl acrylate (2HEA, purity 96%, 200–650 ppm MEHQ, Sigma Aldrich), butyl acrylate (BA, purity 99.5%, 10–20 ppm MEHQ, Quimidroga), butyl methacrylate (BMA, purity 99%, 10 ppm MEHQ, Sigma Aldrich), ethyl acrylate (EA, purity 99%, 10–20 ppm MEHQ, Acros), ethyl methacrylate (EMA, purity 99%, 15–20 ppm MEHQ, Sigma Aldrich), methyl acrylate (MA, purity 99%, ≤100 ppm MEHQ, Sigma Aldrich), methyl methacrylate (MMA, purity 99.9%, 45–55 ppm MEHQ, Quimidroga), styrene (S, purity 99.7%, 10–20 ppm TBC, Quimidroga), sodium p-styrene sulfonate (NaSS, purity ≥ 90%, Sigma Aldrich) and the initiators tert-butyl hydroperoxide (TBHP, 70 wt% aqueous solution, Luperox Sigma Aldrich) and ascorbic acid (AsAc, purity ≥ 99%, Acros) were used without further purification. The nonionic emulsifier Disponil A3065 was provided by BASF. Hydroquinone (HQ, purity 99%, Panreac) and dimethyl formamide (DMF, chromatography grade, Fisher) were used as received. Deionized water was used throughout the work.

Polymerizations

Seed synthesis. Poly(butyl methacrylate) seeds were synthesized in batch using the recipe given in Table 1. Butyl methacrylate (BMA) was used due to a number of reasons. First of all, BMA has low water solubility and hence formation of free oligomer in the aqueous phase was minimized. The reaction temperature (55 °C) was above the glass transition temperature (T_g) of poly(BMA) which is 20 °C. Therefore, the conversion was not limited by the glass effect which is observed in the case of high T_g polymers such as polystyrene. Moreover, T_g of the poly(BMA) is low enough to provide good film forming capabilities at room temperature for coating applications.

Table 1 Recipe used to prepare the poly(BMA) seed

Ingredient	Amount (g)	Introduced as
BMA	150.0	Initial charge
A3065 (active matter)	5.3
H2O	1356.0
TBHP	1.5	Shot
H2O	10.8
AsAc	1.5	Feed
H2O	58.5

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A 2 L jacketed glass reactor equipped with reflux condenser, N₂ inlet, temperature probe, feeding inlet and stainless steel 8-bladed frame-type agitator rotating at 200 rpm was used. The procedure was as follows: butyl methacrylate, Disponil A3065 emulsifier and water were sonicated at 80% duty cycle (0.8 on, 0.2 off) for 10 minutes while magnetically stirred at 500 rpm. The dispersion was transferred into the reactor and heated to 55 °C while being stirred under N₂ flow (12 cm³ min⁻¹). After 20 minutes, TBHP was introduced as a shot and the aqueous solution of AsAc (2.5 wt%) was fed at a rate of 1 g min⁻¹ for 60 minutes. Then, the reaction mixture was allowed to react for 2 additional hours. Complete conversion of BMA was confirmed by ¹H-NMR spectra of the final latex. Therefore, additional monomer removal steps or post-polymerization were not required.

The reason to utilize nonionic emulsifier in the seed synthesis is twofold. Firstly, it does not have charges that can interfere with the characterization of final lattices. In addition, since desorption of nonionic emulsifiers from the seed particles is very slow,⁶ the probability of secondary particle nucleation due to the presence of emulsifier in the aqueous phase is very low. The temperature of the reactions was kept at 55 °C to make sure not to exceed the cloud point of the nonionic emulsifier (75–79 °C, 1 wt% in 10 wt% aqueous solution of NaCl).

Seeded batch reactions. Seeded batch emulsion polymerizations were performed in 0.5 L reactor equipped with a reflux condenser, N₂ inlet, temperature probe, feeding inlet and stainless steel three-bladed Ekato MIG impeller. The comonomers used in this study and their water solubility are presented in Table 2. A representative recipe (based on MMA) is given in Table 3. In each reaction, the number of moles of comonomer was kept constant (141 mmol). In the reaction with two comonomers (BA and 2HEA), the amounts were 134 and 7.05 mmol, respectively. Because of the different molecular weights of the comonomers, the solids contents ranged from 13.6 wt% to 15.6 wt%. The poly(BMA) seed latex, the comonomer, the aqueous solution of NaSS and TBHP (this initiator is stable at room temperature) were charged into the reactor and allowed to swell overnight at 130 rpm at room temperature. To prevent the formation of monomer droplets, the amount of monomer was kept below the swelling capacity of the seed which for these monomers is at least 60% by volume (monomer/seed: 60/40).³⁴ After checking that there was no conversion of the monomers during the swelling, the stirring rate and the temperature were increased to 200 rpm and 55 °C, under N₂ purge. After 15 minutes, the aqueous solution of AsAc was fed at a rate of 0.4 g min⁻¹ for 1 hour.

Table 2 Water solubilities of the comonomers^a

	Comonomer	Solubility in water (mM) at 25 °C (ref. 32)
a ∞: miscible with water.
Methacrylates	Methyl methacrylate (MMA)	150
	Ethyl methacrylate (EMA)	45
	Butyl methacrylate (BMA)	4
Acrylates	2-Hydroxy ethyl acrylate (2HEA)	∞^33
	Methyl acrylate (MA)	650
	Ethyl acrylate (EA)	150
	Butyl acrylate (BA)	11
Styrene	Styrene (S)	3.5

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Table 3 Representative recipe for seeded batch reactions with different comonomers

Ingredient	Amount (g)	Introduced as
a 141 mmoles of comonomer is used in each reaction.
Seed latex	200	Initial charge
Comonomer (MMA)	14.1^a
NaSS	0.48
TBHP	0.14
H2O	3.8
AsAc	0.13	Feed
H2O	24

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The reason to use TBHP/AsAc redox couple as initiator is as follows. AsAc is water soluble, whereas TBHP partitions between the aqueous and the organic phase. The tert-butoxy radicals that are formed in the aqueous phase are nonionic and hydrophobic. Therefore, they do not contribute to the hydrophilicity of the oligomers formed by reaction with NaSS and the fraction of the comonomer soluble in the aqueous phase.

Characterizations

The conversion of volatile monomers was determined gravimetrically. Approximately 1 mL latex samples were withdrawn from the reactor and transferred into aluminum cups having 1–2 drops of aqueous solution of hydroquinone (1 wt% in water). The aliquots in the cups were kept for 2–3 hours in a fume hood, and then dried in oven at 60 °C overnight.

The conversion of NaSS for the final latexes was determined by ¹H-NMR. ¹H-NMR spectra were recorded in a Bruker Avance-400 instrument. NaSS conversion was determined from the ratio of the vinyl hydrogen doublets of NaSS and the single amide peak of the DMF used as reference (see ESI for details†).

Z-Average particle diameter was measured at 25 °C and 173° backscatter angle by using dynamic light scattering (DLS, Malvern ZetaSizer Nano-S instrument equipped with a 633 nm red laser). Before the analysis, the samples withdrawn from the reactor were diluted 1000 times with deionized water to prevent multiple scattering. The Z-average particle diameters measured were used to calculate the evolution of number of particles during the reactions.

Particle size distributions were obtained by using capillary hydrodynamic fractionation in a CHDF-2000 instrument (Matec Applied Sciences). The samples were run at a carrier rate of 1.4 mL min⁻¹ at 35 °C (1/4 GR 500 carrier from Matec).

The amount of coagulum was calculated based on the total solids content of the latex. The final latex was filtered through a mesh to collect the coagulum. After the discharge of the latex from the reactor, the coagulum around the nitrogen tube, thermocouple, agitator and the reactor wall was also carefully taken. The coagula were combined, washed several times with water, dried in oven at 60 °C and weighed.

The gel fractions were determined by Soxhlet extraction with THF for 16 hours.

The molecular weight distributions of the soluble polymer were measured in a GPC system equipped with three columns in series (Styragel HR2, HR4 and HR6) and a refractive index detector (Waters 2410). THF was used as mobile phase at 1 mL min⁻¹ flow rate at 35 °C. PS standards were used for calibration.

The incorporation of NaSS was calculated by titration of the dialyzed and ion exchanged latexes. The latexes were diluted to 5% solids content and dialyzed against ultrapure water by using Spectra-Por®⁴ membranes (M_w cut-off 12 000–14 000 Da) until constant conductivity. The dialyzed latexes were passed through a Dowex Marathon MSC cation exchange resin in order to convert Na⁺ of the sulfonate groups into titratable H⁺ form. After ion exchange, the latexes were titrated conductometrically against 0.01 M NaOH. The yield of NaSS incorporation was calculated as yield = n/NaSS_total where n is number of moles of NaOH used in the titration until the end point and NaSS_total is the total number of moles of NaSS in the formulation.

The latexes with 2HEA, MA and BA were also dialyzed by using Spectra-Por®⁶ membranes with a higher cut-off (50 000 Da) to investigate whether the M_w cut-off of the membrane had an effect on the cleaning of latexes. No effect of the membrane cut-off on the NaSS incorporated was observed.

Results and discussion

Complete conversion of BMA was obtained at the end of seed synthesis. The seed had an average particle size of 141 nm (Fig. 1) and displayed a relatively narrow particle size distribution with a coefficient of variation (CV) of 13.5% (monodisperse particles have CV less than 10%).³⁵ Table 4 also includes the molecular weights of the poly(BMA).

Fig. 1 Particle size distribution (by CHDF) of the poly(BMA) seed.

Table 4 Characteristics of the seed

dparticle (nm) by CHDF	Coag. (%)	Insoluble polymer (gel) wt%	Mn	Mw	PDI
141 ± 19.0	6.6	—	198 000	424 000	2.1

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Fig. 2 presents the evolution of the number of particles (N_p, calculated from DLS measurements) for the polymerizations carried out with different comonomers. It can be seen that after the initial drop, N_p remained relatively constant for all the comonomers, except for 2HEA. The initial drop indicates that the nonionic surfactant contained in the seed was not enough to stabilize the monomer swollen particles. The in situ formation of copolymers containing NaSS provided stabilization to the polymer particles.

Fig. 2 Time evolution of the number of particles (N_p) in the seeded batch emulsion copolymerizations of NaSS with different comonomers.

The significant drop in the number of particles in case of 2HEA in comparison to the other comonomers suggests that the copolymer 2HEA/NaSS formed was too hydrophilic and did not adsorb on the polymer particles. Consequently, the nonionic surfactant contained in the seed was the only surfactant available and it was not able to stabilize the particles. It is worth pointing out that although the calculation of N_p from measurements of the particle size should be taken with precaution because N_p is inversely proportional to the third power of the measured size, and hence relatively small errors in the measurement lead to larger errors in N_p, the low N_p values obtained for 2HEA are far beyond the experimental errors, namely they are due to the characteristics of 2HEA.

The evolutions of the conversion of the comonomers are given in Fig. 3. They exhibited typical behavior of batch processes. The conversions of MA, EA and BA started very soon, but inhibition periods of ca. 30 minutes were observed for the methacrylates. The polymerization rates in all presented cases have shown significant differences up to ca. 30% comonomers conversion, thus, all the comparison presented below refer to this point. Acrylate monomers have higher homopropagation rates than methacrylates.^36,37 The slow polymerization rate of styrene was due to its low propagation rate coefficient.³⁸

Fig. 3 Time evolution of the conversion of the comonomers for the seeded batch emulsion copolymerization with NaSS.

With the exception of 2HEA, within each family, the water solubility of the comonomer did not significantly affect the polymerization rate measured at 50% conversion. The low polymerization rate of 2HEA was due to both the lower number of particles (Fig. 2) and its lower concentration in the main locus of polymerization (polymer particles) which is a consequence of its high water solubility. This effect can also be seen in BA/2HEA reaction, where 5 mol% of the hydrophobic monomer (BA) was replaced with the hydrophilic one (2HEA) leading to a decrease of the polymerization rate. Fig. 3 shows that full conversion was achieved for both MMA and S, even though the polymerization temperature was lower than the T_g of these polymers and under these conditions glass effect is expected. Polymerization in both the aqueous phase and the hydroplasticized NaSS-rich outer layer of the particles may be the reason for the complete conversion of these monomers.

The characteristics of the latexes are summarized in Table 5. The theoretical particle sizes were calculated based on the assumption of constant number of particles and agreed well with experimental sizes for most of the latexes, indicating no secondary nucleation and colloidally stable systems. This was supported by the monomodal and relatively narrow particle size distributions (Fig. 4). On the contrary, in case of 2HEA, the experimental value diverged significantly from the theoretical one. The large particle size and a very broad particle size distribution (Fig. 4 inset) showed that the system was less stable.

Table 5 Characteristics of the final latexes obtained by seeded batch emulsion copolymerization of NaSS and different comonomers

Monomer	dparticle theoretical (nm)	dparticle by CHDF (nm)	Coag. (%)	NaSS conversion (%)	Insoluble polymer (gel) wt%	Mn	Mw	PDI
a Below detection limit.
MMA	189	187.8 ± 23.4	None	89.9	^a	134 400	475 000	3.5
EMA	195	195.6 ± 22.2		86.5	^a	136 000	473 000	3.5
BMA	206	200.7 ± 24.3		77.3	^a	137 000	480 000	3.5
2HEA	189	475.5 ± 120.3	1	100	46.7	197 000	656 800	3.3
MA	183	192.6 ± 26.5	1.4	97.0	13.9	175 700	670 500	3.8
EA	190	193.6 ± 25.3	None	90.0	11.9	146 400	440 000	3.0
BA	202	210.6 ± 26.0		83.0	35.6	193 000	662 000	3.4
BA/2HEA (19/1)	201	193.1 ± 21.5		99.9	35.7	185 600	464 000	2.5
S	192	197.3 ± 24.3	1	72.4	^a	80 200	238 000	3.0

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Fig. 4 Particle size distributions (by CHDF) of the final latexes obtained by seeded batch emulsion copolymerization of NaSS with different comonomers (inset plot is for 2HEA).

No coagulum was observed in most of the cases, and only for the more water soluble comonomers (MA and 2HEA) as well as for the slowest comonomer (S) a small amount of coagulum was obtained. For the MA and 2HEA, the reason might be the formation of water soluble polymer. On the other hand, the coagulation observed in the reaction of S may be related to the low conversion of NaSS that resulted in less stabilized particles.

This implies that the nonionic emulsifier of the seed particles was not able to stabilize the particles. In order to demonstrate this, a seeded batch polymerization of MMA without NaSS was performed and a huge amount of coagulation (26%) was observed.

The fraction of polymer insoluble in THF (commonly called gel) was only significant for acrylates. These monomers suffer intermolecular chain transfer to polymer that coupled with termination by combination leads to the formation of large macromolecules that may be insoluble in good solvents.³⁹ In this context, it is worth pointing out that it has recently been confirmed that the termination of secondary acrylate radicals occurs by combination.⁴⁰ If this is correct, the rate coefficient for intermolecular chain transfer to polymer should be higher than the previously estimated values.³⁹

Taking into account that the insoluble fractions were based on the total amount of polymer (i.e., including the seed which represented about 50 wt% of the final polymer), and that the poly(BMA) of the seed does not contain abstractable hydrogens and hence it cannot suffer transfer to polymer, the insoluble fractions found for 2HEA, BA and BA/2HEA were very high. In particular, almost all of the poly(2HEA) was not soluble in THF, likely due to the formation of H-bonds and the incorporation of NaSS in the copolymer. For BA, about 71% of the poly(BA) formed was not soluble in THF. This is substantially higher than what is usually found for batch emulsion polymerization (54 wt% (ref. 41)). The reason may be that in the present case, hydrophobic tert-butoxy radicals able to enter rapidly in the polymer particles were generated. These oxygen centered radicals are very efficient in abstracting hydrogen from the polymer backbone, which leads to the formation of long branches and eventually to gel. The lower gel fraction found for MA and EA was likely due to the higher water solubility of these monomers favored the polymerization of the tert-butoxy radicals with monomer yielding carbon-centered radicals which are less efficient for hydrogen abstraction. Neither methacrylates nor styrene suffer any significant transfer to polymer, and hence no gel was formed.

Table 5 also shows that, within each comonomer family, NaSS conversion increased with the water solubility of the comonomer, which indicates that copolymerization was beneficial for NaSS conversion. The main reason is that the cross propagation rate coefficient of (meth)acrylate radical with NaSS was much higher than the rate coefficient for homopropagation of NaSS (see ESI†). A second reason is that the critical length for both precipitation of the oligoradicals in the aqueous phase and their adsorption on the polymer particles increased with the hydrophilicity of the comonomer. On the other hand, the type of double bond (acrylic/methacrylic/styrene) had no effect on the NaSS conversion.

The results of the NaSS incorporation are given in Table 6. It can be seen that incorporation increased with the water solubility of the comonomer and that as in the case of NaSS conversion, the type of double bond had no effect on the incorporation. However, comparison between the NaSS conversion (Table 5) and the NaSS chemically incorporated to the polymer particles (Table 6) shows that a significant part of the NaSS reacted was not strongly attached to the polymer particles.

Table 6 Effect of the comonomer type on the incorporation of NaSS into the polymer particles

Monomer	Yield of NaSS incorporation (%)
MMA	75.5
EMA	57.2
BMA	53.8
2HEA	99.7
MA	86.9
EA	77.6
BA	48.7
BA/2HEA (19/1)	78.9
S	42.3

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Fig. 5 presents the fraction of the NaSS reacted that was incorporated onto the polymer particles. It can be seen that this fraction also increases with the water solubility of the comonomer suggesting that in the interplay between hydrophobicity of the comonomer and its content in the NaSS copolymer, the content is the main factor controlling the incorporation of the copolymer to the polymer particles.

Fig. 5 Fraction of the reacted NaSS that was incorporated into polymer particles.

Based on the results described above, the mechanisms leading to the incorporation of NaSS onto the polymer particle can be described as follows:

The tert-butoxyl radicals created from the initiator system reacted with the monomers dissolved in the aqueous phase. Direct entry into the particles may be reduced due to the high concentration of monomer in the aqueous phase and the relatively low solids content. The composition of the copolymer formed depended on the relative concentrations of comonomer and NaSS as well as on the reactivity ratios. However, the results in Fig. 5 and Table 6 show that [comonomer]_aq/[NaSS] was the main factor as methacrylates (r_NaSS = 1.9; r_methacrylate = 0.3) gave the same results as acrylates (r_NaSS = 2.8; r_acrylate = 0.1). Hydrophobicity of the oligoradicals depends on both comonomer content and hydrophobicity of the comonomer. It is expected that the higher the water solubility of the comonomer the more comonomer incorporates into the copolymer, but the contribution of each comonomer unit to the hydrophobicity of the oligoradicals will be lower. Data in Fig. 5 and Table 6 show that the higher amount of comonomer incorporated for the more hydrophilic comonomers compensated their lower hydrophobicity. The copolymer incorporated onto the polymer particles may contain active radicals and continue polymerizing in the particles becoming chemically linked or may be dead chains that adsorbed onto the polymer particles. These chains may suffer further grafting reactions (transfer to polymer in the case of acrylates). The copolymer rich in NaSS stays in the aqueous phase.

From a practical point of view, the fact that NaSS incorporation is controlled by the concentration of the comonomer in the aqueous phase may be a limitation because in most practical cases, the main monomers and their relative concentrations are given by the application sought. Therefore, it is interesting to investigate if chemical incorporation of NaSS can be improved by using small fractions of hydrophilic comonomers in the formulation. BA was chosen as a case study for the proof of concept finding that substitution of 5% of BA by 2HEA increased the incorporation of NaSS from 48.7% to 78.9%.

Conclusions

In this article, the fundamentals of the chemical incorporation of a pH and temperature insensitive ionic monomer (sodium styrene sulfonate, NaSS) to the polymer particles was investigated in an attempt to go beyond the current technology for production of waterborne polymer dispersion, which is based on the use of surfactants to stabilize the dispersion. The reason for avoiding surfactants is that they reduce desirable properties of the films cast from the dispersions such as gloss, adhesion and water resistance. pH and temperature insensitiveness are needed because the dispersions should be stable in the reactor (pH = 2–3, T ∼ 80 °C) and during the application (pH ∼ 7 and T ∼ 15–40 °C).

The necessary condition for the success of this approach is to chemically incorporate the NaSS onto the polymer particles minimizing at the same time the amount of water soluble polymer. To achieve this goal, NaSS was copolymerized with more hydrophobic monomers in the aqueous phase. Using monomers of different functionality (styrene, acrylates, methacrylates) and different water solubilities, it was found that the key factor controlling the chemical incorporation of NaSS was the concentration of the comonomer in the aqueous phase, whereas the functionality did not play any significant role. Of practical importance is the finding that chemical incorporation of NaSS to a rather hydrophobic dispersion can be boosted using small amounts of a relatively water soluble monomer in the formulation.

Acknowledgements

The authors would like to acknowledge the financial support provided by the Industrial Liaison Program of POLYMAT (Akzo Nobel, Arkema, Allnex, BASF, Foresa, Nuplex Resins, Stahl, Solvay, Synthomer, Vinavil and Wacker).

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Footnote

† Electronic supplementary information (ESI) available: Calculation of conversion of NaSS by ¹H-NMR. Reactivity ratios of comonomers with NaSS. See DOI: 10.1039/c6ra07486c

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