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Revisiting the Synthesis of a Well-Known Comb-Graft Copolymer Stabilizer and Its Application to the Dispersion Polymerization of Poly(methyl methacrylate) in Organic Media Mark T. Elsesser†,‡ and Andrew D. Hollingsworth*,† †

Center for Soft Matter Research, Department of Physics, New York University, 4 Washington Place, New York, New York 10003, United States, and ‡Department of Chemical Engineering, University of California, Santa Barbara, California 93106, United States Received September 1, 2010. Revised Manuscript Received October 14, 2010

Polymeric stabilizers are an essential ingredient for the dispersion polymerization of poly(methyl methacrylate) (PMMA) in nonpolar media. In this contribution, we focus on the synthesis of an amphipathic copolymer consisting of pendant poly(12-hydroxystearic acid) (PHS) chains grafted to an insoluble PMMA backbone. This type of steric stabilizer is well established and capable of producing spherically shaped, monodisperse PMMA colloids. Unfortunately, the comb-graft copolymer is not available commercially; furthermore, the multistep synthesis of the desired stabilizer has proven challenging to reproduce. We discuss the practical matter of preparing PHS-graft-PMMA, and report specific techniques developed over several years in our lab. Gel permeation chromatography, mass spectroscopy, and end group analysis of the stabilizer and the precursor macromonomer reveal important, previously unreported details about the chemical synthesis. Our protocol is reproducible and resulted in the production of low polydispersity PMMA particles.

Introduction In a nonaqueous dispersion polymerization, the monomer is dissolved in the organic media and is used to produce an insoluble polymer colloid that can be stabilized by the adsorption of an amphipathic polymer.1,2 A desirable stabilizer architecture consists of randomly incorporated side chains having an affinity for the dispersion medium, which are chemically attached to an anchor polymer. In this arrangement, the insoluble group, that is, the anchor, tends to associate with the particle surface via physical adsorption or covalent bond formation. These “comb-graft” copolymers are effective steric stabilizers and a necessary ingredient in the synthesis of nearly monodisperse colloidal spheres. Our preparation of poly(methyl methacrylate) (PMMA) particles utilizes a specific graft copolymer dispersant, poly(12-hydroxystearic acid)-graft-poly(methyl methacrylate) (PHS-g-PMMA). It was developed, along with other nonaqueous dispersion (NAD) stabilizers, in the 1960s by the surface coatings industry.3 Most physical property studies of NADs were done on PMMA colloids stabilized by PHS-g-PMMA graft copolymer.4-7 The development of such well-characterized dispersions prompted many academic studies of the structure, dynamics and phase behavior of more concentrated suspensions due to a nearly hard-sphere-like *To whom correspondence should be addressed. E-mail: andrewdh@ nyu.edu. (1) Barrett, K. Dispersion Polymerization in Organic Media; John Wiley and Sons, Inc.: London, 1975. (2) Walbridge, D. In Comprehensive Polymer Science; Pergamon Press, 1989; Vol. 4, pp 243-260. (3) Dowbenko, R.; Hart, D. Ind. Eng. Chem. Prod. Res. Dev. 1973, 12, 14–28. (4) Osmond, D.; Walbridge, D. J. Polym. Sci., Part C 1970, 381–391. (5) Doroszkowski, A.; Lambourne, R. J. Polym. Sci., Part C 1971, 253–264. (6) Barsted, S.; Nowakowska, L.; Wagstaff, I.; Walbridge, D. Trans. Faraday Soc. 1971, 67, 3598–3603. (7) Doroszkowski, A.; Lambourne, R. Faraday Discuss. 1978, 65, 252–263. (8) Cebula, D.; Goodwin, J.; Ottewill, R.; Jenkin, G.; Tabony, J. Colloid Polym. Sci. 1983, 261, 555–564. (9) Pusey, P.; VanMegen, W. Nature 1986, 320, 340–342.

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pair potential.8-12 A key feature of this model colloidal system is its low size polydispersity. Recipes for the preparation of PHS-g-PMMA are indicated in only a few references.1,13 Using this stabilizer for nonaqueous dispersion polymerization, Antl et al.13 described a single-step PMMA synthesis characterized by a remarkable increase in particle diameter with increasing monomer concentration. Their stabilizer recipe was developed from patent literature formulations,1 which have received minor modifications over time. These include the use of an alkylsulfonic catalyst during the esterification reaction,13 replacement of the aliphatic hydrocarbon solvent with toluene,14 and the purification of the 12-hydroxystearic acid (12-HSA) monomer by a recrystallization process.15 In the final step, methacrylic acid (MA) can be substituted for glycidyl methacrylate (GMA),15 precluding chemical attachment of the graft copolymer to PMMA particle surfaces. These procedural methods generally lack the detail necessary to make the stabilizer in a consistent fashion; consequently, PHS-gPMMA suffers a persistent reputation for being challenging to reproduce. Furthermore, a practicable material of this type has never been available commercially, which has motivated the development of various “replacement” copolymers. Recently, block and random copolymers of methyl methacrylate (MMA) and octadecyl acrylate were prepared by atom transfer radical polymerization (ATRP).16 These comb-stabilizers were then used to produce monodisperse PMMA particles in a mixture of hexane (10) Goodwin, J.; Ottewill, R. J. Chem. Soc., Faraday Trans. 1991, 87, 357–369. (11) Zhu, J.; Li, M.; Rogers, R.; Meyer, W.; Ottewill, R.; Russel, W.; Chaikin, P. Nature 1997, 387, 883–885. (12) Bryant, G.; Williams, S.; Qian, L.; Snook, I.; Perez, E.; Pincet, F. Phys. Rev. E 2002, 66, 060501–060504. (13) Antl, L.; Goodwin, J.; Hill, R.; Ottewill, R.; Owens, S.; Papworth, S.; Waters, J. Colloids Surf. 1986, 17, 67–78. (14) Pathmamanoharan, C.; Groot, K.; Dhont, J. Colloid Polym. Sci. 1997, 275, 897–901. (15) Hu, H.; Larson, R. Langmuir 2004, 20, 7436–7443. (16) Harris, H. V.; Holder, S. J. Polymer 2006, 47, 5701–5706.

Published on Web 11/08/2010

DOI: 10.1021/la1034917

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and dodecane. Klein et al.17 succeeded in preparing PMMA particles with minimal polydispersity by dispersion polymerization in hexanes using commercially available methacryloxypropylterminated, polydimethylsiloxane (PDMS) stabilizers. Dispersants based on ATRP or synthetic silicone chemistry18-21 are relatively new and additional research will be needed to elucidate the properties of the attached layer (e.g., steric barrier thickness, average surface spacing of the solvated groups) and the interparticle pair potential in both types of stabilizer. The PHS-gPMMA stabilizer, on the other hand, has an extensive 40-year history of characterization, testing, and use in fundamental research studies. To date, the synthesis of PDMS-stabilized PMMA is limited to 26% solids.17 Particle suspensions utilizing PHS-gPMMA can be produced with 50-56% monomer, with or without mechanical agitation. To the best of our knowledge, seeded dispersion polymerization in nonpolar media-our route to coreshell PMMA particles-has not been demonstrated using the reactive silicones. The main goal of this paper is to review the PHS-g-PMMA synthesis step-by-step, pointing out important procedural details that can lead to improved results. Although the multistep process requires 2-3 days, it was successfully scaled up to yield approximately 1 L of high quality, 40% solids polymer solution. In the next section, the purification of the hydroxystearic acid monomer is discussed, along with suggested reagent handling guidelines. Analytical techniques, such as gel permeation chromatography (GPC), mass spectroscopy, and acid number determination were essential to the interpretation of results. The polydispersity in PMMA particle size was found to be directly related to the molecular weight distribution of the polymeric stabilizer. The paper closes with a discussion of the results and a comparison between different batches of the stabilizer.

Experimental Section Materials. 12-Hydroxystearic acid (technical grade) was purchased from Alfa Aesar and used as received. Another 12-HSA (90þ%, tech.) was obtained from Acros Organics without further purification. Toluene (HPLC grade) was purchased from Fisher Scientific and dried through molecular sieves and activated alumina (PureSolv, Innovative Technology, Newburyport, MA). Methanesulfonic acid (>99.5%), glycidyl methacrylate (97%), and 4-tert-butylcatechol (97%) were obtained from Sigma Aldrich. N,N-Dimethylethanolamine (99%) and N,N-dimethyldodecylamine (pract., 95%) were purchased from Acros Organics. Methyl methacrylate (99%) from Sigma Aldrich was passed through a packed column of inhibitor-remover (Sigma Aldrich) to separate monomethyl ether hydroquinone prior to use. Azo-bisisobutyronitrile (AIBN) from Aldrich was recrystallized at 0 °C from histological grade methanol. The ethyl and butyl acetates (certified ACS) were purchased from Fisher Scientific. Poly(12-hydroxystearic acid) (Hypermer LP1, neat) from Uniqema (now Croda) was generously donated by Dr. Martin Murray of ICI Strategic Technology Group. The stabilizers X190-442 (ICI Paints Division) and ICI NAD nonaqueous dispersant were kindly provided by Dr. Neil Williams and Dr. Richard Buscall of ICI Paints. X190-442 is a PHS-g-PMMA graft- copolymer of weight ratio 10:9:0.2 PHS-GMA adduct/ PMMA/AA (AA = acrylic acid).22 The stabilizer R7698-13 (17) Klein, S.; Manoharan, V.; Pine, D.; Lange, F. Colloid Polym. Sci. 2003, 282, 7–13. (18) Dawkins, J.; Taylor, G. Polymer 1979, 20, 599–604. (19) Croucher, M.; Milkie, T. Faraday Discuss. 1983, 76, 261–276. (20) Pelton, R.; Osterroth, A.; Brook, M. J. Colloid Interface Sci. 1990, 137, 120–127. (21) Pelton, R.; Osterroth, A.; Brook, M. J. Colloid Interface Sci. 1991, 147, 523–530. (22) Williams, N. ICI Paints, private communication, 1995.

17990 DOI: 10.1021/la1034917

(prepared ca. 1986) was provided by Dr. Julian Waters (ICI Paints) and represents another PHS-g-PMMA graft copolymer. It contains 5% (w/w) GMA in the anchor polymer.

Characterization. GPC Analysis of PHS-g-PMMA Stabilizers. GPC analyses were carried out using an Agilent 1200 series isocratic pump coupled to a Wyatt ReX Optilab 658 nm refractive index detector with methylene chloride as the eluant and a flow rate of 1 mL/min on an American Polymer Standards column set (guard column plus two columns in series: AM Gel 500/10, 500 A˚ pore size and AM Gel Linear/10 mixed bed from American Polymer Standards Corp.). All GPC measurements were calibrated using linear polystyrene standards and carried out at room temperature. Mw, Mn, and Mp represent the weight average, number average, and peak molecular weights, respectively. The polydispersity index is denoted PDI and defined as Mw/Mn. A linear PMMA standard was used to verify that the column calibration was accurate (Aldrich secondary standard, Mw = 102 600; Mn = 48 300). Other GPC measurements were made at the University of Michigan (Waters 150 GPC system with Wyatt Optilab DSP refractive index detector) and Princeton University (Waters 717 autoinjector, a Waters 515 pump working at 1 mL/min, coupled with a Waters 410 differential refractometer equipped with a Precision Detectors light scattering detector). Tetrahydrofuran was used as the eluant in both laboratories. At Princeton, three 60 cm Phenogel (Phenomenex) linear mixed bed columns with a 7.8 mm ID were set up in series. The molecular weight range was 100 to 106 daltons (Da), and the columns were maintained at constant temperature with an Eppendorf CH-460 column heater. MALDI TOF. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy of the PHS samples was performed on a Hewlett-Packard G2025A (Princeton University) and operated in linear mode with an external calibration. Polymer samples were prepared by mixing 5:1 v/v ratios of the matrix material, indoleacrylic acid, (40 mg/mL in toluene) to polymer (10 mg/mL in toluene). Spectra were acquired using a laser power of 4-5 μJ. The mass-to-charge ratio, m/z, of an ion is proportional to the square of its drift time which is measured. Acid Value. Acid values were determined by the AOCS Method Te 1a-6423 with methanol-toluene (1:3, v/v) substituted for methanol to increase the solubility of the fatty acids and polymeric fatty esters during titration. A potassium hydroxide volumetric standard, 0.0998 N solution in methanol (Aldrich) was used as the titrant. The analyte solutions, 1-2% (w/w), were prepared at room temperature. No evidence of cloudiness or turbidity was observed. All acid values were determined in duplicate; average values are reported in units of mg KOH/g product.

Results and Discussion Synthesis of PHS-g-PMMA. To prepare the PHS-gPMMA stabilizer, we performed a multistep synthesis beginning with the self-condensation of the monomer 12-hydroxystearic acid (Stage I). The resulting polymeric material was reacted through its terminal carboxyl group with GMA to produce a macromonomer (Stage II). The PHS-GMA adduct (PHSA) was subsequently copolymerized with MMA and a minority portion of GMA (Stage III). Acrylic acid or methacrylic acid can be substituted for GMA in this step. The resulting graft copolymer was used without purification as a steric stabilizer in the dispersion polymerization of PMMA particles. Typically, we produced 110 g batches of the 40% comb-stabilizer solution. The PHS produced in Stage I using technical grade 12-HSA (Alfa Aesar) was denoted PHS-1 and PHS-2. Separately, we tested a high purity 12-HSA (Acros); that reaction product was designated (23) AOCS Official Method Te 1a-64, Official Methods and Recommended Practices of the AOCS, 5th ed.; AOCS Press: Champaign, IL, 1997.

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Figure 1. Polycondensation of 12-HSA: propagation (left) and termination (right) reactions. Technical grade 12-HSA contains ∼15% stearic acid impurity which limits chain length. The water of esterification must be removed to produce any significant amount of PHS. Methanesulfonic acid was used to catalyze the reaction.

PHS-3. Following Stage II, the macromonomer solutions PHSA1 and PHSA-2 were split into equal portions, one of which was retained and stored at 2-3 °C for later experiments. The PHS-gPMMA graft stabilizers made in Stage III were analyzed using GPC and evaluated in the dispersion polymerization of PMMA. Next, we describe and discuss the specifics of the three-step synthesis. Stage I. The acid-catalyzed polymerization of 12-HSA is outlined in Figure 1. The self-condensation reaction between carboxylic acid and alcohol groups produces a low molecular weight polyester that becomes the soluble component of the stabilizer. The synthesis can be accomplished using sulfonic acids,13,24 tin(II) chloride,25 or with no catalyst present.1,26 This is a reversible reaction, reaching chemical equilibrium when both products and reactants are present. The catalyst affects both the forward (esterification) and reverse (hydrolysis) reactions, and equilibrium is shifted by removing one of the products. An elegant separation technique involves adding an entrainer such as an aromatic hydrocarbon to remove the water as a minimum boiling azeotrope. We used toluene which forms a heteroazeotrope with water at 85 °C (20.2% water, 79.8% toluene). Polymerization in a neat monomer reaction phase is also possible24 to prevent intramolecular condensation, that is, lactone formation, which can occur in dilute solution. The polycondensation reaction follows second-order kinetics for ∼85% of the reaction.24 Catalyst type and loading, the amount of entrainer, as well as polymerization temperature and time have been studied systematically by Wang et al.25 Alternatively, the esterification may be done irreversibly via the acid chloride using phosphorus trichloride to produce monodisperse oligomers.7 The first approach is more direct and results in a constant degree of branching and polydisperse chain length. Monomer purity is regarded as an important factor in the synthesis of PHS. Reactant impurities include fatty acids, mainly stearic, which can react with hydroxyl-containing molecules causing chain termination during the polyesterification as indicated in Figure 1.1,27 The presence of stearic acid (SA) also affects the molecular weight distribution of the PHS, which will ultimately (24) Bawn, C.; Huglin, M. Polymer 1962, 3, 257–262. (25) Wang, Y.; Eli, W.; Nueraimaiti, A.; Liu, Y. Ind. Eng. Chem. Res. 2009, 48, 3749–3754. (26) Kargupta, K.; Rai, P.; Kumar, A. J. Appl. Polym. Sci. 1993, 49, 1309–1329. (27) Walbridge, D.; Waters, J. Discuss. Faraday Soc. 1966, 294–300.

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influence the steric layer thickness and surface concentration.5-7 Hu and Larson15 have indicated that the recrystallization of technical grade 12-HSA was necessary to achieve an optimal degree of polymerization. By melting the hydroxy acid and withdrawing part of liquid during a subsequent cooling process, the authors claim to have increased the monomer purity from 70-80% to 85-90% (w/w) as determined by melting point analysis.28 We note that in most studies involving PMMA particle synthesis, commercial hydroxy acids were used as received and found to be sufficient.1,5,13,14,27,29,30 To determine whether purification is actually necessary, we tested two grades of commercial 12HSA-one technical, the other highly purified-without further refinement as will be presented later in this section. Next, we discuss the assessment of 12-HSA purity. Melting point analysis and acid value determination are convenient characterization methods31 that were used to evaluate the various commercial 12-HSAs listed in Table 1. The melting points listed here are slightly lower than the pure 12-HSA material. All the acid values measured were lower than calculated values, 186.7 and 197.2 mg KOH/g for pure 12-HSA and SA, respectively. The purest of these exhibited the highest melting points and acid values, whereas the technical grade flake showed the greatest deviation. The melting point data are consistent with stearic (mp = 70 °C) and other fatty acid contamination which would tend to produce lower values. The presence of a small portion of oligomeric acids, that is, esters of a secondary alcohol and fatty acids,32 is also possible because this type of impurity would tend to reduce the acid value. We observed that the acid numbers of synthetic mixtures of the (Acros) hydroxystearic and stearic acids linearly interpolated between their component values. The certified assay values were determined using 1H NMR, elemental analysis, or chromatographic methods, each of which should yield reliable information. A detailed discussion of 12-hydroxystearic acid composition is included in the Supporting Information. Azeotropic distillation was necessary to achieve a significant degree of esterification. The Dean-Stark apparatus34 used in our studies included vacuum jacketing around the receiver neck to prevent precondensation. In a typical reaction, 47.9 g of 12-hydroxystearic acid, 8.6 g of toluene, and 0.1 g (∼68 μL) of methanesulfonic acid were charged to a 250 mL round-bottom flask. The reactor was fitted with an immersion thermometer to monitor the reactor temperature. A water jacketed condenser was connected to a circulation bath (set point = 15 °C). The receiver leg was filled with ∼20 mL of toluene, and the reactor assembly was purged with nitrogen using a Schlenk line. For the synthesis of PHS-1 and PHS-2, the oil bath temperature was set to 110 °C and melting was observed at 77-78 °C. Within a few minutes, the solid mass had melted completely and the temperature began increasing slowly. The catalyst, methanesulfonic acid, was added at this time and the viscous solution was stirred with a Teflon-coated magnet. The oil bath temperature was gradually increased to 150 °C over a 30 min period with initial boiling at 133 °C. As the forward reaction proceeded, the reactor contents slowly increased to 140-142 °C with controlled reflux and remained in this temperature range overnight. Walbridge1 ran this reaction at 90% nonvolatiles and a higher reflux temperature. We found that when the toluene mass was lower than 15% of (28) Hu, H. Dept. of Chem. Eng., Univ of Michigan, private communication, 2003. (29) Waite, F. J. Oil Colour Chem. Assoc. 1971, 54, 342–350. (30) Cairns, R.; Ottewill, R.; Osmond, D.; Wagstaff, I. J. Colloid Interface Sci. 1976, 54, 45–51. (31) Bell, S.; Taub, A. J. Am. Pharm. Assoc. 1942, 31, 75–81. (32) Modak, S.; Kane, J. J. Am. Oil Chem. Soc. 1965, 42, 428–432. (33) Redlick, M. Sigma-Aldrich Corp., private communication, 2005. (34) Dean, E.; Stark, D. J. Ind. Eng. Chem. 1920, 12, 486–490.

DOI: 10.1021/la1034917

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Table 1. Melting Points (Manufacturer’s COA) and Acid Values (Measured) of Various 12-Hydroxystearic Acids. The Aldrich Product Was Purified from the Technical Grade via Multiple Crystallizations and Darco Decolorizing Carbon.33 manufacturer a

grade/assay

tech/g70% Fluka Alfa Aesara technical grade Alfa Aesar tech/85% Alfa Aesar 95% Alfa Aesar 95% Acros tech/90þ% Aldrich 99% a Discontinued.

form

cat. no.

lot no.

certificate of analysis (%)

melting point (°C)

acid no. (mg KOH/g)

flake flake flake flake flake white powder white flaky powder

56440 15263 A17347 44810 44810 20493 219967

J28C11 E8050A H14U033 D26M16 A0102453 19519JO

n/a n/a 83.7 95.0 96.9 99.4 99.7

75-80 77.5 72-74 72-74 77.5 80.7 not reported

166.1 170.3 168.5 166.4 179.1 182.1

Figure 2. Rate of water removal in the polycondensation reaction. The data represent two monomer purities; technical 12-HSA corresponds to batch PHS-1; the higher purity 12-PHS produced batch PHS-3. The dashed line represents the theoretical amount of water collected assuming all 12-HSA is converted to pentamers.

the reactor charge, the water of esterification was difficult to remove and the polymerization rate decreased. Throughout the period, a water layer accumulated in the receiver leg, with a slightly turbid organic phase directly above it. At 16 h, approximately 1 mL toluene was added to the reactor to maintain fluidity and refluxing. By 20 h, the boiling had diminished significantly and the total volume of water in the receiver leg was ∼2 mL, not including small droplets trapped on glass surfaces elsewhere within the separator. The acid values of the final products were 30.9 and 33.1 mg KOH/g for PHS-1 and PHS-2, respectively. Centrifuging the reactor products at 2500 rpm for 30 min helped clarify the polymer solutions. These acid values correspond to an 84-85% extent of reaction,24 with 96.1 g/mol for the catalyst molecular weight. The synthesis of PHS-3 followed the same procedure, except for the substitution of the high purity 12-HSA. Figure 2 compares the rate of water removal for the two grades of 12-HSA tested, corresponding to PHS-1 and PHS-3. The calculated water removal for a pentamer of 12-HSA is 2.30 mL as indicated by the dashed line; the maximum volume assuming no chain termination is 2.87 mL. The high purity material required longer time, that is, 40 h to collect 2.3 mL of water, mainly because the reactor was 4 to 5° cooler than the previous runs. This behavior was due, in turn, to an increased level of toluene used to maintain fluidity. At 24 h, 2 mL water had been collected from batch PHS-3; however, the acid value was 39.8 mg KOH/g, about 30% higher than typical values. An additional 9 h at a 150° reaction temperature, in combination with a second dose of catalyst yielded ∼0.3 mL water. The acid value decreased to 14.3 mg KOH/g, indicating a high degree of esterification (97% extent of reaction). 17992 DOI: 10.1021/la1034917

In addition to the end group analysis, MALDI-TOF mass spectroscopy and GPC were used to evaluate the reaction products PHS-1 and PHS-3 (see Supporting Information). The mass spectroscopy indicated that chain terminating side reactions, resulting in stearate-capped PHS, were suppressed using the high purity monomer. The corresponding molecular weight distributions showed that PHS-3 was more than two times larger than PHS-1. Although the GPC overestimated the molecular weights, we combined the number average values from end group analysis (1820 and 3920 Da) with the PDIs obtained from GPC and estimated the weight averages to be 3540 and 10120 Da. PHS-3 exhibited a significantly higher molecular weight as compared with values reported in the literature, producing an average of 13 (versus 5-6) ester linkages. It is likely that the longer chains will (1) pack together differently on a particle surface and (2), modify the adsorbed layer thickness, which may differ from the linear length of the polyester. A “16-mer hydroxystearic acid” obtained by fractional precipitation was thought to have adsorbed to PMMA particles in a coiled fashion, producing only a 5-7 nm thick steric layer.7 The stabilizer synthesis is relatively insensitive to the relative portions of the anchor and the soluble components,1 which implies that PHS-3 can be used in the standard recipe. The number average molecular weights obtained by Antl et al.13 and Pathmamanoharan et al.14 are comparable with our PHS-1 and PHS-2 results. We have concluded that technical grade 12-HSA yields satisfactory PHS without the need to recrystallize the monomer, or resort to higher purity levels. Ideally, a 5:1 monomer to SA mixture (i.e., 83% purity) would yield 6-mers of molecular weight 1700 Da, assuming equal reactivities and no other side reactions. However, the presence of secondary products and other impurities complicates the establishment of a direct and linear relationship between 12-HSA purity and the melting point, for example. Since many technical grade products do not include a certified assay value, we recommend that the acid number be determined for each batch. On the basis of our results, values falling into the 166-170 mg KOH/g range should be acceptable for PHSg-PMMA stabilizer preparation. Note that the cost for refining technical grade material to 99.7% purity is reflected in a 500-fold increase in the product pricing. For this reason, the blending of pure 12-HSA and SA to adjust the polyester molecular weight is impractical. Stage II. The second step in the PHS-g-PMMA stabilizer synthesis involved coupling GMA to the PHS produced in Stage I via a base-catalyzed, nucleophilic addition reaction. Antl et al.13 indicated that it is essential to use the GMA monomer in a “fresh state”. One reason is because the monomer is susceptible to oxiranering-opening reactions, for example, nucleophilic attack by water. Infrared spectroscopy (FTIR) was used to determine whether any significant epoxide ring hydration had occurred. None was detected as indicated by the strong absorption band at 915 cm-1 corresponding to the epoxide group. However, one bottle of GMA tested exhibited a relatively weak absorption signal corresponding Langmuir 2010, 26(23), 17989–17996

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Table 2. Ingredients Used in the Preparation of PHS-Glycidyl Methacrylate Adduct, Denoted PHSA-1. An Extra 18 g of Toluene Was Needed to Fill the Receiver Leg reagent

mass (g)

Reaction Flask poly(12-hydroxystearic acid) solution PHS-1, 80.8% w/w toluene

49.51 18.02

Separate Beakera GMA 5.18 tert-butyl catechol 0.073 N,N-dimethyldodecylamine (95%) 0.20 toluene 10.08 a Reagents mixed in a separate beaker in order to dissolve the inhibitor.

to the alkenyl (CdC) stretch at 1640 cm-1. The peak intensity, relative to the strong carbonyl (CdO) stretch at 1730 cm-1, was about half of that expected when compared with the supplier’s spectra. The apparent deficiency of polymerizable double bonds could impact the subsequent formation of the graft copolymer (Stage III), as well as the stabilizer performance in dispersion polymerization. To ensure the integrity of this difunctional monomer, we purchased small quantities of GMA, for example, 5 g glass bottles, and stored it at 2-3 °C for no more than 6 months. The commercial product is stabilized with hydroquinone monomethyl ether to scavenge free radicals and contains two chiral impurities: (R)glycidol (1%) and (()-epichlorohydrin (∼0.2%). Since these chemical compounds were present at low concentrations, we used the GMA as received. N,N-dimethyldodecylamine was used to catalyze the condensation reaction. Because the amine is very hygroscopic, this material was obtained in small 5 g bottles, which were kept padded with nitrogen. The 250 mL round-bottom flask reactor and the Dean-Stark apparatus were left intact for the Stage II reaction after draining the water and toluene layers from the receiver leg. To clarify the PHS solution, 18 g of toluene was added to the reactor contents. The flask was reheated causing the condensing toluene to refill the receiver and carry over any water trapped in the solution. After 1 h, the reactor was cooled to 80 °C and a mixture of the monomer, inhibitor, catalyst, and additional toluene was added to the warm, transparent polymer solution. Table 2 indicates the amounts used. The molar ratio of GMA to -COOH was 1.5, which is sufficient to offset potential epoxide hydrolysis reactions. Therefore, any small amount of water dissolved in the toluene should not significantly affect the conversion of terminal carboxyl groups to methacrylate residues. The solvent mixture was refluxed for a period of 7 h under a nitrogen atmosphere. The reactor temperature was maintained at 142-143 °C with the oil bath temperature set to 155 °C. PHSA-1 and PHSA-2 (from PHS-2) appeared brown in color and was transparent; there was no evidence of poly(glycidyl methacrylate). The polymer solutions contained about 65% nonvolatiles. They were diluted with toluene to 50% solids to maintain fluidity during cooling. We computed acid values of 0.54 and 0.45 mg KOH/g for PHSA-1 and PHSA-2, respectively. These numbers indicate that the GMA was successfully attached to the terminal acid groups. End point detection using the phenolphthalein indicator was less obvious due to the weak color change using 0.01N KOH in methanol. PHSA reaction products stored at 2-3 °C had essentially the same acid value 2 years later. Langmuir 2010, 26(23), 17989–17996

Figure 3. Formation of the comb-graft copolymer stabilizer was accomplished by reacting the macromonomer (PHSA) with MMA and GMA. The resulting amphipathic graft copolymer (PHSg-PMMA) is shown schematically and contains randomly distributed PHS molecules along the primarily PMMA anchor chain.

In a separate reaction, we attempted to couple GMA to PHS-3. The measured acid value of 4.3 mg KOH/g indicated that the attachment was not as efficient as PHSA-1 or PHSA-2. The cloudy appearance of the polymer solution and gradual sedimentation of polymeric solids indicated that some poly(glycidyl methacrylate) may have been produced. GPC analysis was used to verify this hypothesis. We suspect that the reactivity of GMA with PHS-3 was greatly diminished due to the lower concentration of carboxyl groups. Kinetic studies26 corroborated this result. Stage III. In the last step of the stabilizer synthesis, a graft copolymer (PHS-g-PMMA) was produced by reacting the PHSA macromonomer, prepared in Stage II, with MMA and GMA. The incorporation of GMA in the anchor group allows the adsorbed stabilizer to become chemically attached to the particle surfaces through the reaction between epoxide and the carboxylic groups in the disperse polymer. The semicontinuous process is described briefly in Antl et al.13 The copolymerization reaction is outlined in Figure 3 and the reactants and solvents used are listed in Table 3. A syringe pump was used to meter the monomer solution into a refluxing mixture of butyl and ethyl acetate in a 250 mL roundbottom flask. This feeding technique is employed to ensure uniform copolymer composition, so as to optimize the yield of effective dispersant.1 A standard glass dripping funnel can also be used. Approximately 66 mL of the premixed solution containing PHSA, GMA, MMA, and AIBN were loaded into a 100 mL glass syringe and the flow rate was set to 0.37 mL/min. The acetates were charged to the round-bottom flask. The monomer mixture was metered continuously over a period of 3 h. Reflux conditions (110 °C oil bath) were maintained for an additional 6 h during which time two small additions of AIBN, 0.15 g each, were made at intervals of 2 h. We dissolved the AIBN in a small amount of acetate mixture, injecting the liquid through a side neck. This solution was prepared by dissolving 284 mg of AIBN in 4.5 mL of acetates solution. DOI: 10.1021/la1034917

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Table 3. Ingredients Used in the Preparation of PHS-g-PMMA Stabilizer Solution reagent

mass (g)

macromonomer/toluene solution PHSA-1, 50% (w/w) toluene GMA MMA AIBN ethyl acetate butyl acetate

31.04 9.20 2.05 18.32 0.307 13.94 6.97

Following the reaction, the PHS-g-PMMA product was diluted to ∼40% by weight with the acetates mixture. The graft copolymer should be diluted to a maximum of 40% solids before cooling to prevent gelation by micellar aggregation effects.1 In one reaction, the polymer appeared to have gelled because the concentration exceeded 60% solids following Stage III. GPC analysis of the subsequently diluted polymer showed an unusually high molecular weight fraction (>106 Da) that may have represented the aggregated polymer molecules. Kargupta et al.26 noted that graft copolymer precipitation from the solvent resulted in irreversible stabilizer aggregation. Nevertheless, a technique to extract low molecular weight polymer from PHS-g-PMMA by precipitation in methanol has been described.35,36 Following solvent separation and drying, the solid stabilizer fraction was redissolved in warm dodecane, a nonsolvent for the PMMA anchor, to form a slightly turbid dispersion.35 Using n-heptane heated to 100 °C, the purified polymer appeared to dissolve completely.36 Oxirane cleavage of the epoxy-containing stabilizer anchor molecule upon exposure to methanol is possible (see for example Lin et al.37). Consequently, any subsequent attempt to chemically attach the stabilizer to the particle surface would not occur. Analysis of PHS-g-PMMA. In the previous section, the synthesis of the PHS-g-PMMA copolymer was discussed in detail. The next step involves its qualification as a stabilizing agent in dispersion polymerization. GPC analysis was used to determine the average molecular weight and molecular weight distribution of each stabilizer produced. We also evaluated the stabilizers by simply using them to prepare PMMA colloids and analyzing the scanning electron microscopy (SEM) images of the resulting particles. Our main objective was to identify stabilizers capable of producing stable, spherical shapes with low size polydispersity. In addition to the graft copolymers produced in our lab, we tested three nonaqueous dispersants from ICI Paints, two of which were commercial products, and one that was made specifically for scientific research. The latter has been used successfully for particle synthesis by several academic groups over the past 25 years. The GPC results pertaining to these stabilizers, as well as a brief compositional description, are located in the Supporting Information section. A PHS-g-PMMA stabilizer prepared by the Solomon group at the University of Michigan was also analyzed. Table 4 lists the number and weight average molecular weights, as well as the PDI, of all stabilizers evaluated, along with the weight ratios of their constituent monomers. The ICI products were used in single-stage dispersion polymerizations at 49% monomer (w/w) concentration, and 5% (w/w) stabilizer (based on monomer). The particle synthesis followed the well-established method published by Antl et al.13 The batch size was 20 g and carried out in a stirred, 100 mL round-bottom (35) Papworth, S. Ph.D. Thesis, University of Bristol, Bristol, UK, 1993. (36) Lee, K.; Winnik, M.; Jao, T. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 2333–2344. (37) Lin, B.; Yang, L.; Dai, H.; Yi, A. J. Am. Oil Chem. Soc. 2008, 85, 113–117.

17994 DOI: 10.1021/la1034917

flask at 80 °C. The SEM images presented in Figures 4A and 4B reveal a striking difference in particle size and size distribution. The use of the “benchmark” ICI-1 stabilizer resulted in a highly monodisperse suspension of PMMA spheres, with an average diameter of 1.68 μm. Following the same recipe, the commercial stabilizer, ICI-3 (X190-442), produced a very polydisperse colloidal dispersion. Image analysis of Figure 4B indicated that the particle size distribution was multimodal and peaked at 0.26, 0.45, and 0.74 μm diameter. The polydispersity, as measured by the ratio of the standard deviation to the mean of the particle size distribution (PSD), was more than 40%. (Samples for SEM image analysis were prepared by diluting the suspension with hexanes and depositing a 5 μL drop on a 300 mesh, Formvar-coated TEM grid mounted to a conductive carbon disk atop a pin-type specimen mount. The samples were coated with ∼3 nm iridium using an ion beam sputter. Microscopy was performed on a Philips XL-30 FEG-SEM at the Princeton Imaging and Analysis Center. To obtain meaningful statistics, at least 200-300 particles were measured.) The PHS-g-PMMA stabilizer synthesis should yield 80% active dispersant,38 which is preferentially absorbed by the particle surfaces during dispersion polymerization to provide colloidal stability as well as monodisperse particle size. By separating the inefficient low molecular weight polymer, Waite1 demonstrated that this stabilizer fraction produced a more polydisperse latex than the original dispersant. We attempted to purify the ICI-3 stabilizer by precipitating it in methanol, drying the solid material in a vacuum oven, and redissolving it in warm dodecane. The material was analyzed before and after processing. The GPC results revealed that the molecular weight distribution was modified in the lower MW range, that is, 102-104 Da, where the peak intensity diminished noticeably. However, the purified stabilizer failed to produce particles with a narrow size distribution. Lee et al.36 observed similar results using a PHS-g-PMMA stabilizer from ICI Canada. Clearly, ICI-3 is a stabilizer with an unusually broad molecular weight distribution that produced very polydisperse PMMA particles. The primary particle size was less than half the size of the PMMA particles made from ICI-1 stabilizer, with smaller particle sizes down to ∼100 nm diameter as can be seen in the SEM image (Figure 4B). The presence of a significant amount of uncoupled PHSA during the dispersion polymerization could help explain the results. If the PHSA macromonomer was reactive, it could copolymerize with the MMA and MA to produce sterically stabilized particles. However, the GPC results suggest that it did not completely couple with the anchor polymer. In that case, weak, single-point anchoring-due to the association between inactive methacrylate residues on the PHSA and complementary polar groups in the disperse polymer-could allow any Stage II product present to partially stabilize precursor particles. These colloidally unstable entities would otherwise undergo coagulation with stable particles thus increasing their average size. Although fatty acids such as stearic and oleic acid were determined to be ineffective stabilizers in dispersion polymerization,1 the PHSA macromonomer would be expected to have different adsorption characteristics as noted above. We did not observe any particle aggregation or flocculation following the dispersion polymerization using either ICI-1 or ICI-3. The GPC results of four stabilizers prepared in our lab or elsewhere are presented in Figure 5. Two of the GPC traces (STAB3 and STAB-4) are similar to the ICI-1 results, with a slightly larger peak molecular weight. These were prepared in separate facilities, four years apart, using different 12-HSA sources. (38) Fitch, R.; Kamath, Y. J. Indian Chem. Soc. 1972, 49, 1209–1220.

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Article Table 4. GPC Results for Various PHS-g-PMMA Stabilizers

stabilizer

composition (PHSA:MMA:GMA)

12-HSA source

Mn

Mw

PDI

10:9:1 not reported 6 040 22 020 3.64 ICI-1a 10:9:1 not reported 4 850 21 000 4.33 ICI-2b c d 10:9:0.2 X190-243 3 020 63 150 20.9 ICI-3 ICI-4 not reported not reported 4 160 75 820 18.2 STAB-1 10:9:1 Hypermer LP1 (PHS) 6 010 28 380 4.72 STAB-2 10:9:1 Alfa Aesar (technical) 8 450 30 920 3.66 e 10:9:1 Fluka (tech. grade) 8 250 31 410 3.81 STAB-3 10:9:1 Alfa Aesar (technical) 8 320 34 960 4.20 STAB-4f STAB-8 10:9:0.6 Alfa Aesar (technical) 5 700 29 020 5.09 a ICI notebook designation no. R7698/13. b ICI notebook designation no. R7205/17. c ICI product designation no. X190-442. d PHSA:MM:MAA. e U. Mich. stabilizer “Batch 4B” prepared Aug. 2005. f From PHSA-1.

Figure 5. Molecular weight distributions corresponding to various PHS-g-PMMA graft copolymers prepared in our lab or elsewhere. The shoulder appearing at 5000 Da indicates uncoupled PHSA. Figure 4. SEM micrographs of PMMA particles. (A) Highly monodisperse spheres prepared from ICI-1 stabilizer. The average particle diameter was 1.68 μm, with less than 3% polydispersity. (B) Polydisperse PMMA particles prepared from ICI-3 stabilizer. Scale bars are 5 μm.

STAB-1 and STAB-8 both exhibited nearly the same peak molecular weights; however, rather than the expected unimodal shape, these traces revealed a shoulder at about 5000 Da molecular weight. On the basis of the ICI stabilizer results, we believe this feature is indicative of some amount of uncoupled PHSA in the graft copolymer solution. The consequence of such a result should be a detectable increase in particle size polydispersity. To test this hypothesis, we used each of these stabilizers in dispersion polymerizations following the standard recipe,13 and then analyzed SEM images of the resulting particles. The monomer concentration was reduced from 49 to 43% to amplify any polydispersity. Table 5 presents the results of the dispersion polymerizations, including the average particle size, polydispersity, and PSD shape. Generally, PHS-g-PMMA stabilizers with a shoulderless GPC trace produced the narrowest size distributions. Stabilizers STAB-2, STAB-3, and STAB-4 resulted in polydispersity of 3.5 to 5% and average particle sizes in the 640 to 740 nm range. ICI-1 stabilizer had a slightly smaller average size and a 6.9% polydispersity. Here, we observed a hint of the characteristic shoulder in the GPC data (see Supporting Information). The stabilizers with a pronounced shoulder, STAB-1 and STAB-8, produced significantly smaller particles and broader size distributions. The polydispersity was 11.4 and 25.6%, respectively, Langmuir 2010, 26(23), 17989–17996

with a repeat dispersion polymerization using STAB-1 that yielded 14.8% polydispersity. By scaling up this particle recipe 2.5 times, we obtained crystallizable 550 nm particles with 8.2% polydispersity. The small variability in PSD using STAB-1 may be related to the particle nucleation period, which appeared to depend upon the reactor size and how the initiator was added. All of these particular PSDs exhibited an appreciable negative skew and could be fitted with a Weibull distribution function. As discussed in the previous section, FTIR analysis of the GMA monomer corresponding to STAB-8 showed evidence of diminished CdC functionality. The ratio of absorbance for alkenyl stretching to that of carbonyl stretching (ACdC/AC=O = 0.184) was about 50% of what was expected (AC=C/AC=O = 0.381). This observation correlates with the larger PDI of STAB-8 and low molecular weight shoulder in the GPC chromatogram, as well as the particle size polydispersity. There appears to be a systematic trend with the GPC shoulder intensity and the degree of particle size polydispersity. Although we did not systematically check each batch, FTIR should be used to verify the integrity of the difunctional monomer, and ensure the coupling of PHSA to the anchor component. The commercial PHS (Hypermer LP1), with an acid value of 33.2 mg KOH/g, appeared to be a viable substitute for Stage I reaction products. The PMMA-2 and PMMA-4 results indicate good batch-to-batch reproducibility. Both were prepared with stabilizers using the same 12-HSA. We also observed that PHS-gPMMA stabilizer effectiveness was independent of the 12-HSA source. DOI: 10.1021/la1034917

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Elsesser and Hollingsworth Table 5. SEM Analysis of PMMA Dispersion Polymerization

particle dispersion

stabilizer

% monomer

av particle diameter (nm)

polydispersity

PSD shape

PMMA-ICI-1 PMMA-1A PMMA-1B PMMA-2 PMMA-3 PMMA-4 PMMA-8

ICI-1 STAB-1 STAB-1 STAB-2 STAB-3 STAB-4 STAB-8

43.0 42.9 42.9 42.6 43.1 43.0 42.0

491 422 449 642 641 743 336

0.069 0.114 0.148 0.050 0.048 0.035 0.256

unimodal neg. skew neg. skew unimodal unimodal unimodal neg. skew

article, the stabilizers prepared in this study are applied to the synthesis of cross-linked, core-shell PMMA colloids and cluster particles.39

Conclusions

Figure 6. PMMA particle diameter, measured from SEM images, as a function of monomer concentration. The two data sets correspond to different PHS-g-PMMA stabilizers. STAB-1 (open square) was prepared in our lab; ICI-1 stabilizer (open circle) was provided courtesy of ICI Paints. The line and open triangle symbol correspond to data from Antl et al.13.

Stabilizer Performance. Finally, we show the effect of monomer concentration on particle size for two different stabilizers STAB-1 and ICI-1. Figure 6 shows the results from two sets of experiments. Using from 41 to 49% (w/w) monomer, monodisperse PMMA particles were prepared following the standard recipe.13 The corresponding particle sizes ranged from 0.33 to 1.9 μm diameter and depended on the particular stabilizer used. These average values were determined from SEM images. The size polydispersity ranged from 3 to 11% and was generally smaller for the larger particle sizes. The results are similar to the trend reported by Antl et al.,13 and we have included their data in Figure 6 for comparison. Because no two PHS-g-PMMA stabilizers perform identically, the monomer concentration-particle size relationship needs to be determined for each graft copolymer, providing incentive to produce larger batches. Following Pathmamanoharan et al.,14 we scaled up the stabilizer synthesis and prepared approximately 1 L of the graft copolymer solution. The preliminary results showed that monodisperse PMMA particles (∼1.5 μm diameter) were produced using 48% monomer concentration. In a subsequent (39) Elsesser, M.; Hollingsworth, A.; Edmond, K.; Pine, D. Submitted for publication.

17996 DOI: 10.1021/la1034917

The paper addressed the synthesis of an amphipathic graft copolymer, poly(12-hydroxystearic acid)-g-poly(methyl methacrylate), which is regarded by many as being troublesome to make. Our results should provide the essential information needed to prepare the stabilizer and allow successful dispersion polymerization of PMMA particles in nonaqueous solvents. GPC and mass spectroscopy analysis of the poly(12-hydroxystearic acid) component showed that technical grade monomer can be used without purification. Several PHS-g-PMMA stabilizers were synthesized and tested. A unimodal PHS-g-PMMA molecular weight distribution was correlated to successful dispersion polymerization results, that is, stable, monodisperse PMMA spheres. The complete coupling of PHS to the PMMA anchor using glycidyl methacrylate monomer appears necessary to obtain low polydispersity in the PMMA dispersion polymerization described. Acknowledgment. This research was supported by awards to the NYU Center for Soft Matter Research from the MRSEC Program of the NSF, and by NSF support under Grant No. DMR-0706453. For preparation of STAB-3 (Batch 4B), GPC analysis, and organizing the 2005 PMMA particle synthesis workshop we thank L. Shereda, M. Kogan, and C. Dibble (University of Michigan Ph.D. students). For useful discussions and advice, we also thank Prof. A. van Blaaderen, Dr. C. van Kats, Dr. H. Pathmamanoharan, K.S. Lacina, (Utrecht University); Dr. H. Hu, (University of Michigan); Dr. D. Adamson (Princeton University); Dr. R. Buscall, Dr. M. Murray, Dr. N. Williams (ICI Plc, UK); Prof. P. Bartlett (University of Bristol); Dr. A. Schofield (University of Edinburgh); Dr. Julian Waters (ret., ICI Plc, UK); The MALDI-TOF was kindly done by Dr. D. Little (Princeton University). T. Pinon (NYU) assisted with the GPC analysis. Supporting Information Available: Discussion on 12hydroxystearic acid composition; MALDI-TOF results for PHS; GPC results for PHS and for stabilizers from ICI Paints. This material is available free of charge via the Internet at http:// pubs.acs.org.

Langmuir 2010, 26(23), 17989–17996