Water Sensitivity of Latex-Based Films - Industrial & Engineering

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Ind. Eng. Chem. Res. 2003, 42, 456-464

Water Sensitivity of Latex-Based Films Lauren N. Butler, Christopher M. Fellows* and Robert G. Gilbert Key Centre for Polymer Colloids, School of Chemistry, University of Sydney, NSW 2006, Australia

A series of polymer latices based on methyl methacrylate (MMA) and butyl acrylate (BA) incorporating electrosteric stabilizers containing acrylic acid (AA) and methacrylic acid (MAA) was prepared, and the water sensitivity of films formed from these latices was investigated by immersion tests. A number of variables were considered, including the concentration of the hydrophilic monomer, the presence/absence of a free-radical inhibitor, and the use of various chaser systems to remove residual monomer. Water sensitivity did not increase proportionally to the amount of hydrophilic monomer in the feed and was not directly related to the amount or molecular weight of ungrafted polyelectrolyte or to the length of the grafted polyelectrolyte chains. A model is advanced suggesting that the primary role of the electrosteric stabilizer in reducing water resistance is connected with its ability to prevent coherence of the polymer particles to form a continuous film. This ability should be related to a poor capacity to stabilize water-in-oil emulsions. Introduction A common method of stabilizing latices used for surface coatings is the incorporation of electrosteric stabilizers, formed in situ by copolymerization of an ionizable hydrophilic surfactant with the hydrophobic monomer(s) comprising the bulk of the emulsion polymerization feed. These stabilizers provide colloidal stability, sites for adhesion of binder particles to hydrophilic fillers such as the calcium carbonate used in carpet backing,1 and much better freeze-thaw stability than conventional surfactants. Common hydrophilic monomers used are acrylic acid, methacrylic acid, and itaconic acid. The focus of the current investigation is the effect of the composition and means of production of the electrosteric stabilizer on the sensitivity of the binder to water. The mass of water absorbed by a cast binder film is the primary means used to quantify water sensitivity in this work. In classical emulsion polymerization, radicals are generated in the aqueous phase and add to monomer present in the aqueous phase until the radical oligomer reaches a length (z) with significant surface activity; this species then enters a hydrophobic particle, if such is present in the system, and begins polymerization in the hydrophobic phase. This description of the process has been placed on a quantitative basis in the MaxwellMorrison equation.2 It is evident that this process of entry is complicated by the addition of a hydrophilic monomer; more monomer units overall are required before the growing radical becomes surface-active, leading to the formation of hydrophilic chains of significant length embedded in the surface of the polymer particles. This is the accepted mechanism for the generation of electrosterically stabilized latices. However, the longer the time spent by the oligomeric radical in the aqueous phase, the greater the probability that it will be terminated. This balance between entry, growth, and termination determines the amount and size of both the * To whom correspondence should be addressed. E-mail: [email protected]. Phone: 61 2 9351 3751. Fax: 61 2 9351 8651.

grafted chains and the free polyelectrolyte formed in a polymerization. The postulate is made here that the oligomeric radical becomes sufficiently hydrophobic to be surface-active when it achieves a length of z hydrophobic monomer units, where z is the degree of polymerization required for entry in the homopolymerization of the hydrophobic monomer. The rationale for this statement is that the positive entropic contribution due to the loss of the structure imposed on the water by any shorter hydrophobic sequence should be insufficient to compensate for the loss of entropy on adsorption of the polymer chain, i.e., a hydrophobic sequence that is too short to be desolvated on its own account does not become more easily desolvable when incorporated into a largely hydrophilic polymer chain. There is experimental evidence that a lower degree of hydrophobicity might be sufficient.3 Experimental Section Preparation of Seed Latex. The monomers methyl methacrylate (MMA), butyl acrylate (BA), and styrene sulfonic acid were used as purchased (Sigma-Aldrich). Potassium persulfate (KPS, Merck), Aerosol MA (ethylhexyl sulfosuccinate, Cytec), and sodium carbonate (Univar) were used as received as initiator, surfactant, and buffer, respectively. The water used was Milli-RO water, passed through a single ion-exchange column. The poly(BA-co-MMA) seed was prepared using a 1.3-L computer-controlled reactor (Moore Products) with a high-shear impeller. The initial charge was purged with nitrogen for 30 min while the temperature was brought up to 60 °C, after which initiator was added. The reaction time was measured from the commencement of visible nucleation (indicated by an exotherm or a blue hue), 2-8 min after addition of initiator. The recipe used (Table 1) is designed such that the initial charge gives all particle nucleation and further polymerization occurs within these in situ seed particles. Monomer feed 1 was added using a Waters 501 HPLC pump and feed 2 using a syringe pump (Kent Instruments YA-32). This seed latex was then dialyzed for 4-5 weeks using dialysis membranes of 25-Å pore size

10.1021/ie020611v CCC: $25.00 © 2003 American Chemical Society Published on Web 12/20/2002

Ind. Eng. Chem. Res., Vol. 42, No. 3, 2003 457 Table 1. Preparation of Seed Latexa charge butyl acrylate methyl methacrylate styrene sulfonic acid potassium persulfate sodium carbonate water aerosol MA a

initiator

28.14 28.88

feed

feed 2

211.1 211.3 5.62 1.23

1.03 564.80 16.47

27.07

35.19

All values are in grams.

Table 2. Preparation of Electrosteric Stabilizera dialyzed seed (44% solids) butyl acrylate methyl methacrylate acrylic or methacrylic acid potassium persulfate sodium carbonate aerosol MA water a

charge

initiator

200.0 55.6 60.0 1.0-13.0 0.16 4.24 140.76

0.58 40.62

All values are in grams.

(Selby-Biolab) against distilled water until the conductivity of the distilled water remained constant. Second-Stage Polymerization of BA, MMA, and AA/MAA. Electrosteric stabilizers were incorporated into the polymer particles by seeded emulsion polymerization of either acrylic acid (Sumaika) or methacrylic acid (Aldrich) with BA and MMA onto the dialyzed seed. The polymerizations (Table 2) were carried out under positive nitrogen pressure in a 500-mL glass reactor equipped with a semicircular blade turbine agitator and a thermometer. The initial reactor charge was purged with nitrogen for 30 min to remove dissolved oxygen while the temperature was brought up to 60 °C. The resultant latex was dialyzed for 4-5 weeks using dialysis membranes of 25-Å pore size (Selby-Biolab) against distilled water until the conductivity of the distilled water remained constant and no odor of residual monomer remained. To compare properties of second-stage latices with those of an unmodified standard of equivalent size and properties, the same procedure was also carried out omitting the hydrophilic monomer but incorporating all other ingredients of Table 2. In initial studies, the hydrophilic monomers were used as purchased, and their concentrations (weight percent of total monomer) varied between 1 and 12%. To investigate the effect of the inhibitors present in the monomers on poly(AA)-based electrosteric stabilizer formation, a further series of seeded polymerizations was prepared with purified monomers. The monoethyl ether of hydroquinone (MEHQ) inhibitor in the monomers BA and MMA was removed using an MEHQ removal column (Sigma-Aldrich catalog number 30,6312). Acrylic acid was purified by a freeze-thaw method.4 Variations in the latex properties with synthesis temperature and initiator concentration were investigated by repeating the 4 wt % AA recipe at temperatures of 40, 70, and 80 °C and then at 60 °C with 0.320 and 1.757 g of KPS. To determine the effect of varying the feed profile on the products, the recipe of Table 1 was used with the exception that the AA (5 g, equivalent to 4 wt %) and KPS solution (0.641 g in 40 g of water) were introduced into the reaction mixture using a Matex syringe pump (Kent Scientific Corporation). Three feed rates were

used: 1.0 mL min-1 AA and 8.0 mL min-1 KPS solution, 0.5 mL min-1 AA and 4.0 mL min-1 KPS solution, and 0.1 mL min-1 AA and 0.8 mL min-1 KPS solution. Characterization. Latex viscosities were determined by adjusting 60 g of polymer latex to various pH values using 1:100 dilutions of concentrated hydrochloric acid (Sigma-Aldrich) and ammonia (Sigma-Aldrich). The absolute viscosity was measured using a Brookfield viscometer (model DV-II+) with an LV-1 spindle at 100 rpm at 20 °C. Surface charge density measurements were carried out by diluting latices to approximately 1% solids content, exchanging using a mixed-bed resin (Dowex M3, Sigma-Aldrich) for 16 h at a resin-to-solids ratio of 5:1, and then carrying out conductometric titration with dilutions of 0.01 M sodium hydroxide standard solutions (Sigma-Aldrich).5 Critical coagulation coefficients (CCCs) were measured by rapid mixing of 1.5 mL of latex dispersion at pH 9.0 with 1.5 mL of electrolyte solution (NaCl) in a UV-vis spectrometer cell by syringe injection. Variation of the turbidity with time was followed using a Cary 4.0 UV-vis spectrometer at a wavelength of 520 nm and CCC calculated from the initial slopes of these curves using an established method.6 Particle diameter was determined using capillary hydrodynamic fractionation (CHDF; Matec instrument with cartridge C560 at a wavelength of 220 nm) and was found to be 110 nm. Photon correlation spectroscopy (PCS; Malvern instrument 4700C with a 488-nm 150mW argon laser, Omnichrome 543-AP) was used to determine the Z-average hydrodynamic diameter (calculated by cumulants analysis) to estimate the increase in particle radius due to the water-swollen polymer layer. To obtain the aqueous-phase polymer, 10 g of latex and 20 g of 0.04 M sodium hydroxide solution (SigmaAldrich) were centrifuged using a Beckman L8-M UltraCentrifuge at 40 000 rpm for 90 min. The serum was collected and dried, after which the residual solids were redispersed in Milli-Q water and filtered. The serum extract was analyzed by gel permeation chromatography (Waters) using Ultrahydrogel linear and UHG250 columns with a UMRI refractive index detector. The eluent used was a mixture of 0.25 M sodium nitrate and 0.01 M sodium dihydrogenphosphate, adjusted to a pH of 7.0 with sodium hydroxide solution. Poly(ethylene oxide) and poly(ethylene glycol) standards (Polymer Labs) were used for calibration. The samples were prepared by dissolving the extracts in the eluent at concentrations of ∼3 mg of sample per 1 mL of eluent. The samples were filtered using a 0.05-µm filter. The analyses were performed using Polymer Lab software with a relative “molecular weight” calculated using the Mark-Houwink parameters of the standards (K ) 3.47 × 10-4, a ) 0.7) and of poly(acrylic acid) (K ) 1.74 × 10-5, a ) 0.909).7 Sorption data were obtained by preparing latex films on panels of soda glass (16 cm × 12 cm), cleaned with acetone. The films were applied with a 0.003-in. doctor blade/applicator and dried in a fan-forced oven at 60 °C for 24 h. Panels were weighed and placed in a water bath at room temperature for periods of about 14 days or until the weight gain of the film was above 100% of the original film mass, whichever came first. At variable intervals, the samples were weighed as rapidly as practicable after careful blotting of the surface liquid.

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Figure 1. Water uptake of poly(AA)-containing latices prepared with (2) 3.1, (O) 4.4, (b) 6.1, (]) 8.8, and ([) 12.9% AA in feed.

Figure 3. Surface charge density of latices containing (a) poly(AA) and (b) poly(MAA). Figure 2. Water uptake of poly(MAA)-containing latices prepared with (2) 2.5, (b) 6.2, (]) 8.3, and ([) 11.2% MAA in feed.

For all water sorption data presented, each curve contains data points from independent swelling experiments on separate samples of binder film. To determine the amount of water-extractable material in the latices, each was adjusted to pH ) 9.0 with ammonia (Sigma-Aldrich) and dried for 24 h at 60 °C. Precisely 4.0 g of the dry film was weighed into a cellulose extraction thimble (Bonnet equipment), and the top of the thimble was plugged with paper tissue. The thimble was placed in the Soxhlet equipment, and its contents were extracted with 120 mL of deionized water for 24 h. The extract was transferred to a container and water-evaporated to dryness. A blank thimble was run concurrently with the latices, and the water-extractable weight percentage was calculated from the difference between the weights of the extracted material from the sample and the blanks, divided by the initial weight of the sample. Results and Discussion The sorption results for films made from latices prepared with AA in the presence of inhibitor are shown in Figure 1. The relationship between the amount of AA in the formulation and the water sensitivity as measured by this method is clearly nonlinear. Below 6 wt % AA, there were no significant differences in water sorption between formulations, whereas above 6 wt %, the affinity of the films for water was such that they swelled relatively rapidly and lost their structural integrity. By contrast, relatively little water was taken up by films containing MAA (Figure 2), and all films maintained structural integrity over the duration of the experiment. As a less-hydrophilic monomer, it appears

Table 3. Particle Size wt % AA

DW (nm)

polydispersity

wt % MAA

DW (nm)

polydispersity

seed 1.5 3.1 4.4 6.1 8.1 12.9

108.8 152.5 153.4 155.5 153.3 146.7 184.6

1.14 1.10 1.09 1.10 1.11 1.30 1.51

seed 1.1 2.3 5.0 6.2 8.3 11.2

117.6 143.3 139.9 139.2 139.5 140.2 140.2

1.14 1.11 1.14 1.14 1.14 1.13 1.13

reasonable that MAA would contribute to a lesser extent to the water sensitivity of the films. Surface charge densities for these latices are shown below (Figure 3). Note that the “surface” charge density for poly(AA)-containing latices rises monotonically to physically unreasonable levels of approximately 5 Å2 per electronic charge, suggesting that the surface charge might be distributed throughout three dimensions in a charged polymer chain, whereas the surface charge density of poly(MAA) plateaus at 8 µC/cm2, equivalent to approximately 200 Å2 per electronic charge. The surface charge plateau found in polymerizations with MAA suggests that the majority of the MAA in these latices might be within the interior of the polymer particles and sequestered from the OH- titrant in the aqueous phase. The weight-average diameter, DW, and polydispersity of each latex prepared above were determined by CHDF (Table 3), and incorporation of the layer of electrosteric stabilizer was found not to have a significant effect on the diameter of the latex particles. An increase was found only for the latex prepared with the highest AA concentration, most likely as a result of the presence of a population of higher-diameter particles arising from coagulation during the polymerization (note the much higher polydispersity for this system). The apparent reduction in DW for the latex prepared with 8.1 wt %

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Figure 5. Gel-permeation chromatographs of extracted polyelectrolyte from poly(AA)-containing latices.

Figure 4. Viscosity of latices as a function of pH: (a) poly(AA)containing latices prepared with (2) 3.1, (b) 6.1, (]) 8.8, and ([) 12.9% AA in feed; (b) poly(MAA)-containing latices prepared with (2) 1.1, (0) 2.5, (O) 5.0, (b) 6.2, (]) 8.3, and ([) 11.2% MAA in feed. Table 4. Amount of Extracted Polyelectrolyte wt % AA

wt % MAA

added

extracted

added

extracted

1.5 3.1 4.4 6.1 8.1 12.9

0.2-0.4 0.4-0.5 0.4-0.5 1.6-1.8 2.6 3.7-3.9

1.1 2.3 5.0 6.2 8.3 11.2

0.1-0.4 0.2-0.5 0.1-0.5 0.1-0.4 0.1-0.4 0.4

AA, accompanied by an increase in polydispersity, supports the possibility of increasing coagulation in high-AA systems. Marked increases in latex viscosity were seen only for those systems that also showed a high degree of water sensitivity (Figure 4); these were also the systems where a polydisperse particle size distribution was obtained. Although changes in particle size can affect viscosity, the magnitude of the effects seen suggests that changes were also occurring in the continuous phase, i.e., a greater amount of higher-molecular-weight polyelectrolyte was being generated in these systems. Changes in the particle hydrodynamic radius due to the incorporation of a grafted layer were estimated using PCS for latices prepared with 4.4 and 12.9% AA in the second-stage polymerization. The method outlined by Santos et al.8 was followed, and grafted layer thicknesses of 4 and 26 nm, respectively, were obtained. Although no sharp increase in the amount of extracted polyelectrolyte was seen for the latices that absorbed the most water (Table 4), it is possible that simultaneous increases in molecular weight might account for the substantially increased viscosity observed. To gauge the contribution of increasing hydrophilic polymer molecular weight on the observed viscosities,

the extracted polymers were examined by GPC. Detector response is shown as a function of elution time in Figure 5. It can be seen that the polymers that were apparently of the highest molecular weight were those for which high latex viscosities and high water sensitivities were observed. It should be noted that the smallest chains are lost in the initial dialysis of the latex and do not contribute to the distributions given. More polyelectrolyte, as well as higher-molecular-weight polyelectrolyte, is evidently formed at the high-AA feeds where poor water resistance is found. However, it is unclear whether the water sensitivity is a result of the presence of the polyelectrolyte or a reflection of changes in the grafted electrosteric stabilizer that accompany the growth in aqueous-phase polymer. The predicted lengths of both the polyelectrolyte and the electrosteric stabilizer can be estimated by considering the average degree of polymerization (DP) required to attain a sufficiently hydrophobic sequence for chain entry. When a chain grows to a size where it contains such a sequence, it will cease aqueous-phase propagation, so this DP should give the maximum (average) chain length attainable in the aqueous phase; the DP of the majority of aqueous-phase polymer chains should be significantly less than this value. The required degree of polymerization can be estimated as follows. As a first (terminal model) approximation for a copolymerization of two monomers, the probability of a given comonomer unit A being in a sequence of n comonomers of type A is given by

pAA(1 - pAA)n-1

(1)

where pAA is the probability of a polymer radical ending in A adding monomer A

pAA )

rA[A] rA[A] + [B]

(2)

where rA is the reactivity ratio of monomer A. If A is the hydrophobic monomer, then by summing the probabilities for all sequence lengths up to n ) z and rescaling the probability of finding a sequence n ) z in a chain as 1, the average number of monomer units A in an entering chain can be determined. The degree of polymerization of this chain can then be found from the terminal model expression for the ratio of comonomers in a copolymer.9 This chain length should be the average length of the grafted chains and the free polyelectrolyte, assuming that termination processes are relatively minor; if termination in the aqueous phase is significant,

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Table 5. Predicted and Experimental Degrees of Polymerization %AA bulk isopentanol Q-e experimental

z)4 z)3 MMA/AA z)4 z)3 MMA/AA z)4 z)3 MMA/AA

1.5 33 12 1.47 23 9 1.87 67 22 0.91 82-84

3.1 100 31 0.72 62 21 0.87 270 66 0.44 65-110

then the molecular weight of polymer in the aqueous phase should be reduced. The assumption that only two comonomers need be considered in this system is a reflection of the relative solubilities in water of MMA (0.15 M at 50 °C10) and BA (0.0064 M at 50 °C11). Accurate reactivity ratios for MMA and AA in water do not exist, and values obtained from the Q-e scheme (rAA ) 1.096, rMMA ) 0.869),12 in bulk (rAA ) 0.60-0.74, rMMA ) 1.38-1.44)13 and in the polar protic solvent isopentanol (rAA ) 0.62, rMMA ) 1.97)14 were used. Under conditions where the acrylic acid is not hydrolyzed, the difference between the latter values and the true reactivity ratios in water is not likely to be pronounced. The aqueous-phase concentration of MMA was taken as 0.15 M, which will be accurate only in the initial stages of the polymerization. Initially, the AA concentration will decline relative to the MMA concentration as long as the latter concentration is maintained, but as the reaction proceeds and the global concentration of MMA falls, the quoted reactivity ratios suggest that the aqueous phase late in the reaction will become AA-rich. Predicted sequence lengths, polyelectrolyte compositions, and estimated experimental degrees of polymerization (DP) are shown in Table 5. These figures will be applicable only at relatively low conversions, and the assumption of uniform Mark-Houwink-Sakurada parameters over this range of likely compositions is unphysical, so the DP values are purely suggestive. The Mark-Houwink approximation is poorest below DP ≈ 100, which can explain the greater divergence between the experimental and predicted values at low concentrations of AA. Reasonable agreement between measured and estimated sequence lengths can be obtained. The values of z giving the best fit are unexpectedly low, and it is possible that the majority of chain entry occurs when the aqueous-phase concentration of AA relative to MMA is already significantly reduced. A discussion of the most likely value of z is unprofitable given the uncertainties discussed. The consistently larger experimental DP values at low concentrations of AA most likely reflect a loss of low-molecular-weight polyelectrolyte in the preliminary dialysis of the latices. The critical coagulation coefficients determined indicate the a high degree of stability was imparted by the electrosteric stabilizers under all conditions (Table 6). The CCC values correlate well with the surface charge density; it should be noted that only where a CCC of >2500 mM NaCl is obtained for all pH values is the latex considered to be suitable for use as a binder in paint systems. Inhibitor. Although industrial emulsion polymerizations are normally carried out without removal of inhibitor, the reactions that these species can undergo can cause numerous complications, and it is customary to remove them in academic studies of polymerizations.

4.4 200 53 0.51 110 35 0.60 620 130 0.31 94-190

6.1 420 93 0.37 220 59 0.43 1500 250 0.23 130-230

8.1 860 160 0.28 430 97 0.32 3500 460 0.17 190-240

12.9 3300 440 0.17 1500 250 0.20 16 000 1400 0.11 450-480

Figure 6. Water uptake of poly(AA)-containing latices prepared without MEHQ inhibitor and with (0) 1.3, (2) 3.0, (O) 4.2, (b) 6.3, (]) 8.0, and ([) 12.6% AA in feed. Table 6. CCC Determination for Latices Containing Electrosteric Stabilizers at pH Values of 6.0 and 9.0 CCC (mM NaCl) 1.1 2.3 5.0 % MAA 6.2 8.3 11.2 1.5 3.1 4.4 % AA 6.1 8.1 12.9 no hydrophilic monomer

pH ) 6.0

pH ) 9.0

447 586 1199 1340 1549 >2500 392 1749 >2500 >2500 >2500 >2500 730

740 1000 2539 >2500 >2500 >2500 744 2075 >2500 >2500 >2500 >2500 778

Possible differences arising in the present systems from this variable were investigated by repeating all measurements carried out on poly(AA)-containing latices above on a series of latices prepared in the absence of any inhibitor. The primary inhibitor species present in these systems is the monoethyl ether of hydroquinone (MEHQ). It has been reported that MEHQ is an effective inhibitor of acrylic acid polymerization only in the presence of oxygen; once oxygen is exhausted, any remaining MEHQ will in all probability act as a chaintransfer agent.15 In the absence of oxygen, MEHQ is believed to continue to retard polymerization as a result of the high abstractability of its phenolic hydrogen: growing polymer radicals react with this functional group, generating a persistent oxygen-centered radical that is unlikely to reinitiate propagation.16 This mechanism is likely to lead to a greater amount of aqueousphase polymer of lower molecular weight. The latices prepared with monomers from which MEHQ had been removed tended to absorb more water than latices containing MEHQ (Figure 6), suggesting, in light of the mechanisms above, that increasing amounts of (lowmolecular-weight) polyelectrolyte do not significantly impair the water resistance of these films.

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Figure 7. Viscosity of latices prepared without MEHQ inhibitor and with (2) 3.0, (O) 4.2, (b) 6.3, (]) 8.0, and ([) 12.6% AA in feed as a function of pH.

Figure 8. Viscosity of 4% AA latices prepared at a range of temperatures as a function of pH. (O) 40, (2) 60, ([) 70, and (0) 80 °C.

Table 7. Particle Size of Latices Prepared with MEHQ-Free AA wt % AA

DW (nm)

polydispersity

seed 1.3 3.0 4.2 6.3 8.0 12.6

108.8 140.9 140.8 141.4 143.4 142.1 143.0

1.14 1.13 1.14 1.14 1.10 1.20 1.32

Table 8. Amount of Extracted Polyelectrolyte from Latices Prepared with MEHQ-Free AA no inhibitor

inhibitor

% in feed

% extracted

% in feed

% extracted

1.3 3.0 4.2 6.3 8.0 12.6

0.2 0.0-0.2 0.3-0.4 1.5-1.6 1.9 2.9-3.0

1.5 3.1 4.4 6.1 8.1 12.9

0.3-0.4 0.4-0.5 0.4-0.5 1.5-1.8 2.6 3.7-3.9

Surface charge density measurements were nearly identical for the two series, whereas particle diameters were generally lower for polymers prepared in the absence of inhibitor, although increasing polydispersity at higher concentrations of AA was again seen (Table 7), implying a degree of coagulation in these reactions. Both intrinsic viscosity (Figure 7) and GPC suggested that the polyelectrolyte formed in the absence of MEHQ had an increased molecular weight, although the amount of extracted electrolyte in the absence of MEHQ was significantly smaller (Table 8). These results support the hypothesis that MEHQ affects water absorption chiefly through its function as a chain-transfer agent and that the observed increase in water absorption in the absence of MEHQ is related either to an increase in high-molecular-weight polyelectrolyte or to an increase in the average size of grafted chains on the surface of the latex particles. Temperature. One means of controlling emulsion polymerizations is to change the reaction temperature, a variable that has a marked effect on numerous factors in the polymerization. The difficulty with controlling the polymerization in this way is the great number of variables that will be changed by a change in temperature: for typical systems, the monomer concentration in the aqueous phase and degree of polymerization required for entry will fall somewhat, the viscosity will fall, and a change in reactivity ratios toward rA ) rB ) 1 will occur (with all changes affecting entry by less than an order of magnitude over a 10 °C increment). There

Figure 9. Water uptake of 4% AA-containing latices prepared at a range of temperatures. The second-stage conversion in all these reactions was >97%. (O) 40, ([) 70, and (0) 80 °C.

will be higher rates of propagation and termination and, most importantly, of initiator dissociation (activation energy of 135.14 kJ mol-1 for KPS,17cf. 20-50 kJ mol-1 for propagation and termination of typical monomers). This will lead to a greatly increasing radical flux. A good first-order approximation of the effect of increasing temperature is therefore a reduced rate of entry, giving greater amounts of polyelectrolyte of reduced molecular weight. The 4% AA feed recipe in the presence of MEHQ was repeated at a range of temperatures to test this idea. Measured viscosities (Figure 8) and GPC results qualitatively support the hypothesis that smaller chains are obtained at higher temperatures; no conclusions can be drawn from the amount of polyelectrolyte extracted, as very little extracted product was obtained at all times. No clear trends in particle size were determined for this series of latices, making any conclusions on variation in grafted chain length purely speculative. Poorer water resistance is seen for latices prepared at 70 and 80 °C (Figure 9), but this poor water resistance is not clearly correlated with polyelectrolyte molecular weight (which decreases at higher temperatures), amount of polyelectrolyte (which remains roughly constant and low for all latices investigated), or viscosity of the latices (which is lowest for the high-temperature systems). It is possible that the electrosteric stabilizers formed in these systems, although hydrophilic at the temperatures at which they are synthesized, are much poorer stabilizers at lower temperatures as a result of their greater content of hydrophobic monomer units. It is also likely that these chains are also significantly shorter than those formed at lower temperatures. Poorer

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Figure 10. Water uptake of 4% AA-containing latices prepared at varying initiator concentration. The second-stage conversion in all reactions was greater than 98%. (0) 20, ([) 14.3, (2) 11, and (O) 7.3 mM KPS.

stabilization will then lead to the formation of a film with poor particle packing, giving many voids and poor resistance to water. Further work is clearly required to characterize the electrosteric stabilizers as a function of temperature to place this hypothesis on a firmer basis. Initiator Concentration. Varying initiator concentration provides a more controlled means of changing the radical flux than varying the reaction temperature. The increased radical flux should increase termination in the aqueous phase, giving a larger amount of lowermolecular-weight polyelectrolyte, but as the amount of polyelectrolyte remains small in absolute terms, this should not have a significant impact on the population of grafted chains. Water sorption curves for a series of latices prepared at 4% AA in the second-stage feed with different concentrations of initiator are given in Figure 10. Little change in water sensitivity can be seen, and no clear trend from low to high KPS concentration. Viscosity decreases with increasing initiator concentration, consistent with the formation of shorter chains at increased radical flux. However, more low-molecularweight polyelectrolyte is also seen at higher KPS concentration. The GPC traces are clearly bimodal in several cases, suggesting that a proportion of PAA is formed late in the reaction after composition drift has reduced the probability of entry to near nil. As was seen for the series of latices prepared at different temperatures, in all cases, only a small amount of polyelectrolyte was extracted, and this was constant within experimental error. Soxhlet extraction of all of these latices suggested that, in all cases, AA was quantitatively incorporated into the electrosteric stabilizer. The lack of any change in water sensitivity with changing initiator concentration suggests that the nature of these grafted chains, which should remain relatively constant under these conditions, is a more important factor in determining water sensitivity than changes to the polyelectrolyte. Feed Regime. Feeding in one or more components of a copolymerization reaction is a common industrial practice to reduce the amount of composition drift. In the absence of monomer droplets, reactivity ratios suggest that the aqueous phase will be progressively depleted in MMA as the copolymerization proceeds; where these droplets are present, they will maintain the aqueous-phase concentration of MMA, and AA will be depleted as the polymerization proceeds. The effect of the feed regime on the characteristics of the electrosteric

Figure 11. Water uptake 4% AA-containing latices prepared at different feed rates: (O) batch, (2) 0.1, ([) 0.5, and (0) 1.0 mL min-1. Table 9. Surface Charge Density of Poly(AA)-Containing Latices monomer mixture feed rate (mL min-1)

KPS feed rate (mL min-1)

surface charge density (µC/cm2)

none 1.0 0.5 0.1

8.0 4.1 0.8

28 43 42 62

temperature (°C)

surface charge density (µC/cm2)

40 60 70 80

31 40 41 39

[KPS] (mM)

surface charge density (µC/cm2)

7.3 11.0 14.6 20.0

28 34 41 37

stabilizer was investigated by considering a number of feed profiles. Feeding in the monomer should give a lower concentration of MMA in the aqueous phase at all times, retarding entry and giving more AA-rich polyelectrolyte. The most PAA-rich polyelectrolyte, and the longest grafted chains, should theoretically be formed with the slowest feed rate. In contrast to variations in temperature and initiator concentration, in which no significant change in surface charge density was found, variations in feed rate had a pronounced effect on this parameter (Table 9). The greatest surface charge densities were obtained at the slowest feed rates where the longest, most AA-rich chains are expected. Water sorption data for these latices are given in Figure 11. The highest feed rate gives the most water absorption, whereas the batch systems absorb the least water at all times. The viscosity and GPC elution times of extracted polyelectrolyte were greatest at the lowest feed rates, consistent with a primarily AA-rich aqueous phase. As initiator is being fed in as well, the lower the feed rate, the more MMA should be proportionally incorporated into each grafted chain and the more the AA concentration in the aqueous phase should rise. These results indicate the need for higher-level characterization and monitoring/prediction of the aqueousphase polymer composition; if the dimensions of the

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hydrophilic section of the grafted polymers increase as the feed rate decreases, by analogy with the series in which the concentration of AA was varied, it would appear that the lowest feed rate, rather than the highest, should give the film with the highest affinity for water. The most likely conclusion is that the relation of water absorption to electrosteric stabilizer is not a simple matter of greater absorption at increased stabilizer size. When the feed rate data are considered together with the temperature data, two possibilities seem likely: (1) There is an optimum length for electrosteric stabilizer, below which poor particle packing has a negative impact on water resistance and above which the amount of hydrophilic functionality increases water sensitivity. (2) Both the size and composition of the electrosteric stabilizer are important, with a higher proportion of hydrophobic units giving worse water resistance. An attempt is made in the next section to rationalize this apparently counterintuitive hypothesis in terms of the influence of the electrosteric stabilizer on film formation. Electrosteric Stabilizer Surfactancy. It has been demonstrated that styrene-butyl acrylate latex stabilized by poly(acrylic acid) displays resistance to humidity only if annealed at temperatures such that the hydrophilic “shell” around the particles is fully disrupted;18 the results presented here suggest that this disruption will be dependent not only on temperature, but also on the dimensions and surfactancy of the stabilizer itself. A marked increase in water sensitivity is seen at a certain concentration of AA in the secondstage feed. The lengths of polyelectrolyte and grafted stabilizer chains both increase gradually, and there is no evidence for a sudden increase in the amount of free polyelectrolyte at the concentrations where water sensitivity becomes marked. Increased water sensitivity is also apparent when the electrosteric stabilizer is likely to contain a higher proportion of MMA units. One possible explanation for this effect lies in the behavior of the electrosteric stabilizer, considered as a polymeric surfactant, during the process of film formation. The latices considered in this study are all rubbery at the temperatures at which film formation takes place (Tg ≈ 5-10 °C, with film formation at 60 °C). Under these conditions, film drying in the presence of a surfactant cannot be considered simply as a process of concentrating polymer particles to give a lattice of closepacked spherical particles.19 Instead, the potential exists for phase inversion as the ratio of water to organic phase is reduced, giving a continuous polymer phase in which small droplets of water are dispersed. This effect is well documented for film formation from natural rubber latex.20 If this occurs, particle interpenetration will not be dependent on diffusion through a layer of grafted surfactant, and the formed film should show fewer mesoscopic voids and defects giving entry to water. That is, if the electrosteric stabilizer is capable of facilitating phase inversion, a more water-resistant film will be formed. A polymeric surfactant will be most effective in stabilizing a water-in-oil emulsion if the hydrophobic “head” portion is large in comparison to the hydrophilic “tail” portion (Figure 12). As the hydrophilic polymer length increases, it is reasonable to expect a relatively well-defined limit beyond which a water-in-oil dispersion cannot be stabilized. In addition, if the hydrophilic tail contains a concentration of hydrophobic units such

Figure 12. Stabilization of disperse water phase in continuous polymer phase by polymeric surfactants. (a) Small hydrophilic, large hydrophobic chain segments; (b) large hydrophilic, small hydrophobic chain segments; (c) surface-active hydrophilic, large hydrophobic chain segments.

that it is not well solvated as the ionic strength of the concentrating aqueous phase increases, it would be expected to adsorb to the particle/aqueous phase interface, giving a very large effective “tail” area. Appropriate dimensions and hydrophilicity of the stabilizer, therefore, would encourage phase inversion and drive the disruption of the hydrophilic shell around the coalescing particles necessary for polymer interdiffusion. This model is consistent with the data presented in this work, allowing for reasonable assumptions as to the composition of the electrosteric stabilizer chains. It is also consistent with published observations of the effect of poly(ethylene oxide) surfactants on polymer interdiffusion and film drying.21,22 Unfortunately, it was not possible in this work to prepare sufficient polyelectrolyte sample for characterization by NMR spectroscopy, which would provide solid evidence of the composition of the stabilizer. Conclusions Further modification of the latex syntheses presented here is required to adequately deconvolute the many factors contributing to water sensitivity of binder films incorporating electrosteric stabilizers. This work can be considered a preliminary survey of these factors, identifying possible lines of advance for any concerted effort to determine the impact of electrosteric stabilizers on water resistance. The significant conclusions that can be drawn from this work are as follows: (1) Water sensitivity is not a

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direct function of the amount or molecular weight of ungrafted polyelectrolyte present. (2) Neither is water sensitivity a function solely of the length or amount of the grafted polyelectrolyte chains. (3) The value of z estimated is apparently less than for a poly(MMA) oligomeric radical. This might indicate that the dependence of oligomer surfactancy on composition is more complex, that the small amounts of BA in the aqueous phase might be playing a significant role, or that depletion of AA relative to MMA in the aqueous phase is important and the electrosteric stabilizer is largely formed once this has made an impact on feed composition. More accurate determination of aqueous-phase reactivity ratios and characterization of grafted chains are required to explore these ideas. It is suggested that both the chain length and composition of the electrosteric stabilizer are significant and that the primary role of this species in reducing water resistance is connected with its ability, or otherwise, to allow the binder particles to cohere to a continuous film. This ability should be related to the capacity of a stabilizer to stabilize water-in-oil emulsions. Clearly, the composition of the electrosteric stabilizer and aqueous-phase monomer requires further investigation; careful monitoring of conversion and composition and determination of aqueous-phase reactivity ratios are required. Acknowledgment The Key Centre for Polymer Colloids is established and supported by the Australian Research Council’s Key Centres Program. Dr. Butler’s Ph.D. studies were supported by the Australian Research Council and Wattyl Australia Pty. Ltd. Literature Cited (1) Blackley, D. C. Polymer Latices. 3. Applications of Latices; Chapman & Hall: London, 1997. (2) Maxwell, I. A.; Morrison, B. R.; Napper, D. H.; Gilbert, R. G. The entry of free radicals into latex particles in emulsion polymerization. Macromolecules 1991, 24, 1629-1640. (3) Wang, S.-H.; Poehlein, G. W. Studies of water-soluble oligomers formed in emulsion copolymerization. J. Appl. Polym. Sci. 1994, 51, 593-604. (4) Vorwerg, L.; Gilbert, R. G. Electrosteric stabilization with poly(acrylic acid) in emulsion polymerization: Effect on kinetics and secondary particle formation. Macromolecules 2000, 33, 66936703. (5) Cheong, I. W.; Kim, J. H. Effects of surface charge density on emulsion kinetics and secondary particle formation in emulsi-

fier-free seeded emulsion polymerization of methyl methacrylate. Colloid Polym. Sci. 1997, 275, 736-743. (6) Ottewill, R. H.; Shaw, J. N. Stability of monodisperse polystyrene latex dispersions of various sizes. Discuss. Faraday Soc. 1966, 42, 154-163. (7) Mccarthy, K. J.; Burkhardt, C. W.; Parazak, D. P. Properties of heterodisperse poly(acrylamide-co-sodium acrylate); MHS constants and dilute solution behavior of heterodsiperse p(AAm-coSA) in 0.5 M and 1 M NaCl. J. Appl. Polym. Sci. 1987, 33, 16831698. (8) Santos, R. M.; Forcada, J. Acetal-functionalized polymer particles useful for immunoassays. II. Surface and colloidal characterization. J. Polym. Sci. A: Polym. Chem. 1999, 37, 501511. (9) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (10) Capek, I.; Barton, J.; Orolinova, E. Emulsion polymerization of butyl acrylate. Chem. Zvesti 1984, 38, 802. (11) Lane, W. H. Determination of the solubility of styrene in water and of water in styrene. Ind. Eng. Chem., Anal. Ed. 1946, 18, 295-296. (12) Brandrup, J.; Immergut, E. H.; Grulke, E. A., Eds. Polymer Handbook, 4th ed.; Jon Wiley & Sons: New York, 1999. (13) Brar, A. S.; Arunan, E.; Kapur, G. S. Sequence determination in acrylic acid-methyl methacrylate copolymers by carbon13 and proton NMR spectroscopy. Polym. J. (Tokyo) 1989, 21, 689695. (14) Stahl, G. A. Influence of solvents on the copolymerization of acrylic acid. J. Polym. Sci., Polym. Lett. Ed. 1980, 18, 811814. (15) Levy, L. B. Inhibition of acrylic acid polymerization by phenothiazine and p-methoxyphenol. J. Polym. Sci. 1985, 23, 1505-1515. (16) Cutie, S. G.; Henton, D. E.; Powell, C.; Reim, R.; Smith, P. B.; Staples, T. L. The effects of MEHQ on the polymerization of acrylic acid in the preparation of superabsorbent gels. J. Appl. Polym. Sci. 1997, 64, 577-589. (17) Behrman, E. J.; Edwards, J. O. The thermal decomposition of peroxodisulfate ions. Rev. Inorg. Chem. 1980, 2, 179-206. (18) Joanicot, M.; Wong, K.; Cabane, B. Interdiffusion in Cellular Latex Films. Macromolecules 1996, 29, 4976-4984. (19) Winnik, M. A. Latex film formation. Curr. Opin. Colloid Interface Sci. 1997, 2, 192-199. (20) Blackley, D. C. Polymer Latices. 1. Fundamental Principles; Chapman & Hall: London, 1997. (21) Winnik, M. A. Influence of polar substituents at the latex surface on polymer interdiffusion rates in latex films. ACS Symp. Ser. 1996, 648, 51-63. (22) Cannon, L. A.; Pethrick, R. A. Influence of chain length in nonyl-phenol ethoxylate surfactants on the film formation behaviour of methyl methacrylate-2-ethylhexyl acrylate copolymer latexes: Part 1. Differential scanning calorimetry and atomic force microscopy. Polymer 2001, 43, 1223-1233.

Received for review August 2, 2002 Revised manuscript received November 25, 2002 Accepted November 26, 2002 IE020611V