- Email: [email protected]
Microemulsion photopolymerization of methacrylates stabilized with sodium dodecyl sulfate and poly(N-acetylethylenimine) macromonomers
European Polymer Journal 38 (2002) 73±78
www.elsevier.com/locate/europolj
Microemulsion photopolymerization of methacrylates stabilized with sodium dodecyl sulfate and poly(N-acetylethylenimine) macromonomers b G. David a, F. Ozer , B.C. Simionescu a, H. Zareie b, E. Pisßkin b,* b
a Department of Macromolecules, ``Gh. Asachi'' Technical University, 6600 Jassy, Romania Chemical Engineering Department, and Bioengineering Division, Faculty of Engineering, Hacettepe University, 06532 Beytepe, Ankara, Turkey
Received 9 February 2001; received in revised form 20 May 2001; accepted 11 June 2001
Abstract Methyl methacrylate and butyl methacrylate were polymerized in oil-in-water microemulsions that were stabilized by sodium dodecyl sulphate (SDS). A poly(N-acetylethylenimine) (PNAEI) macromer was also included in the recipe, as a cosurfactant and a comonomer. Polymerizations were initiated by UV-irradiation. The average diameters of latex particles, obtained by STM, were in the range of 17±200 nm. The experimental data evidenced that the particle size was mainly dependent on the SDS/PNAEI ratio. Polymerization yields were around 75±85%. The synthesized copolymers have viscosity average molecular weights in the range of 2:1±2:4 106 and glass transition temperatures of 38.0±43.5°C, lower than those obtained without using PNAEI. The investigation by means of FTIR and 1 H-NMR techniques revealed that PNAEI was incorporated into the nanoparticles. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Microemulsion polymerization; Nanoparticles; Methyl methacrylate and butyl methacrylate; Poly(N-acetylethylenimine); Sodium dodecyl sulfate; UV-radiation
1. Introduction The concept of polymerization in microemulsions appeared only in the early eighties [1±3]. Polymerization in microemulsions allows the synthesis of ultra®ne latex particles within the size range of 10±100 nm and with narrow size distributions [4±9]. In contrast to the opaque and milky conventional emulsions and miniemulsions, microemulsions are isotropic, optically transparent or translucent and thermodynamically stable. Several groups have studied microemulsion polymerization. Styrene and methyl methacrylate (MMA) are typical monomers that have been polymerized in ternary
*
Corresponding author. E-mail address: [email protected] (E. Pisßkin).
oil-in-water microemulsions [10±27]. Gan and coworkers produced poly(methyl methacrylate) (PMMA) in ternary microemulsions using the cationic surfactants stearyl trimethylammonium chloride (STAC), cetyl trimethyl ammonium bromide (CTAB) and dodecyl trimethylammonium bromide (DTAB) with either a water-soluble or an oil-soluble initiator and found that the longer the hydrophobic chain length of the surfactant, the smaller the latex particles [23]. Kaler and coworkers also prepared small latexes from styrene and several dierent methacrylic esters using cationic surfactants [12,19,24±26]. Larpent and Tadros have found optimum mixtures of non-ionic surfactants (mixtures of nonylphenol±modi®ed oligo(ethylene oxide)) to form microemulsions of MMA and of styrene in water, and produced small latex particles at various surfactantto-monomer ratios using ascorbic acid/hydrogen peroxide as a redox initiator [27]. Microlatex particles with
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 1 6 2 - 8
74
G. David et al. / European Polymer Journal 38 (2002) 73±78
an average size of about 6±8 nm (with surfactant shell) were produced by using metal ion supported surfactants (metallosurfactants) [28]. Antonietti et al. have used hydrophobically modi®ed poly(ethylene oxide) [29]. Oligomers of amphipathic monomers such as commercially available sulfonated alkyl-poly(ethylene oxide) have also been examined [30]. Recently, Capek has thoroughly studied the features of the photopolymerization of alkyl methacrylates and methacryloyl-terminated polyoxyethylene macromonomers (POE-MA) in microemulsions in the presence of high amounts of SDS [31]. It was concluded that the addition of polyoxyethylene macromonomers in the system decreased the polymerization rate and the molecular weight and slightly increased the particle size. This behavior was mainly attributed to the transformation of the electrostatic to electrosteric stabilization mechanism. In our previous studies, we have polymerized methyl, ethyl, butyl methacrylates and hydroxyethyl methacrylate in microemulsion systems, and investigated their ®lm forming abilities [32±35]. In the present study we included a hydrophilic poly(N-acetylethylenimine) (PNAEI) macromer with cinnamoyl polymerizable end group, as an active cosurfactant in the formulations [36]. The polymerizations were realized by initiation with UV-radiation. The choice of the macromer was accomplished considering the biocompatibility of PNAEI and the opportunities to functionalize the surface of the obtained nanoparticles, oered by such a ``self-surfacting'' system [37]. Moreover, according to recent reports of Mura and Riess optimum conditions for ®lm formation could be achieved when the hydrophilic stabilizing chains from the surface of the polymer particles are miscible with the matrix polymer [38]. The compatibility of PNAEI with commodity polymers is well known [39]. 2. Experimental 2.1. Materials The monomers, MMA and butyl methacrylate (BMA) (Fluka, USA) were commercial grade, and were treated with an aqueous solution of NaOH (10%) to remove the inhibitor. Distilled/deionized water was used in all experiments. The emulsi®er was the reagent-grade sodium dodecyl sulfate (SDS) (Fluka, USA). The macromer with cinnamoyl end-group (Mn :2190, functionalization:100%, from 1 H-NMR data) was synthesized by the quenching of the oxazolinium living species in the 2-methyl-2-oxazoline polymerization with cinnamic acid in the presence of a macromolecular proton scavenger, i.e. poly(vinylpyridine-co-divinyl benzene) [36]. The chemical formula of this macromer, namely PNAEI is given below.
2.2. Microemulsion polymerization The polymerizations were realized in an oil-in-water (o/w) system. In a typical procedure, a selected amount of SDS (three dierent values: 0.01, 0.006, and 0.002 g/ ml), 0.002 g/ml (or 0.004 g/ml) macromer (given above) and 1.25 g of the comonomer mixture (MMA/BMA ratio:50/50, by weight) were added to 50 ml distilled water, in a 100 ml Pyrex glass reactor, dotted with magnetic stirrer. Prior to polymerization, the reaction mixture was stirred at room temperature for about 10 min. It was then stored at 4°C for about 24 h to reach equilibrium. Polymerization was performed at 25°C, under inert atmosphere (nitrogen), with stirring, by using a high pressure mercury UV lamp of 300 W, disposed at 15 cm distance from the reactor. Polymerization duration was of 4 h. The latex particles were cleaned by washing with methanol and water several times to remove the surfactant. 2.3. Characterization The particle size of the nanoparticles was measured by a novel technique, scanning tunneling microscopy (STM), which is described in detail elsewhere [40]. For STM imaging, the latex samples (5 ll containing about 0.1 mg particles per ml) were deposited onto freshly cleaved highly oriented pyrolitic graphite (HOPG), and dried at room temperature. Then, the STM images were taken at 2 V sample bias and a tunneling current of a 20 pA. Etched tips of Pt/Ir (80:20) wires (0.5 mm in diameter, Digital Instruments, Santa Barbara, CA) were used. Prior to use, the tips were washed in acetone. The number average diameter (Dn ), the weight-average diameter (Dw ) and size distribution index (SDI) were evaluated from the micrographs by using the following relations: Dn Dw
X X
.X
Ni .X Ni D3i Ni D4i
Ni Di
SDI Dw =Dn The polymerization yield was obtained by extraction of copolymer from the latex particles by chloroform, and by weighting the solid phase after complete removal of the unconverted monomers by a controlled drying.
G. David et al. / European Polymer Journal 38 (2002) 73±78
75
Fig. 1. Typical STM micrographs of nanoparticles produced.
Viscosity measurements were used to obtain average molecular weights of the polymers produced in this study. Viscosities of the polymer solutions with dierent concentrations (0.1±2.0 g/100 ml) in chloroform were measured with an Ubbelhode capillary viscometer at 25.0° 0:1°C. The following Mark±Houwink equation was used to calculate the viscosity average molecular weights: g KMva The ``K'' and ``a'' values for PMMA and PBMA are 5:5 10 5 and 4:9 10 5 and 0.79 and 0.78, respectively [41]. For the copolymers under study the average values (K: 5:2 10 5 ; and a: 0.785) were used. Thermal transitions were obtained by using a dierential scanning calorimeter (DSC) (Shimadzu, Model DSC-50, Japan). Nitrogen was used as the sweeping gas. Samples (5±10 mg) were heated at a scan rate of 10°C/ min from 25°C to 300°C followed by rapid cooling. FTIR spectra of the nanoparticles were obtained by using an FTIR spectrophotometer (Schimadzu, Model: FTIR-8000 series, Japan) with KBr (IR grade)-nanoparticle mixtures in powder form. Polymer samples were dissolved in CDCl3 , and 1 HNMR spectra were recorded in a NMR spectrometer (Brucker AC 250, USA) working at 500 MHz at room temperature. The sample concentration in CDCl3 was 1% (w/v). The internal standard was tetramethylsilane. 3. Results and discussion In this study we attempted to copolymerize MMA and BMA in microemulsions in the presence of both SDS, as the main surfactant, and also a cosurfactant, i.e. a PNAEI macromer. This new stabilizing system, not described in our previous study, was used with the aim
to reach an improved quality of the resulted nanoparticles, considering their possible use in biomedical area [32]. As we know, the use of polyethylenimine derivatives in the stabilization of microemulsion polymerization systems was not reported until now. Making use of the optical peculiarities of the microemulsion systems the thermal scission of chemical initiators was replaced by initiation by UV-irradiation. For all the investigated polymerization systems stable latexes were obtained. In most cases, prior the polymerization, the medium was a transparent liquid, while after polymerization the latex obtained was bluish and clear. No precipitation was observed in about two months storage period. In order to obtain the average particle size, STM was used. Typical micrographs are shown in Fig. 1. Average diameters of the nanoparticles were calculated by evaluating micrographs containing approximately 300±500 particles. Table 1 gives the average particle size of the nanoparticles, which is one of the most interesting results observed in this study. As expected [4], a decrease in SDS concentration determined a signi®cant increase in the average size of the nanoparticles. A SDS/monomer mass ratio lower than about 1.0 shifts the polymerization from microemulsion to miniemulsion or even to classical emulsion limits (at lower values). Note that in order to obtain stable latexes the amount of SDS/monomer in microemulsion formulations should be around 1±3, which is quite high [4±7,10]. This high concentration of SDS is considered as one of the main drawbacks of microemulsion polymerization, due to the diculties to remove the residual SDS from the nanoparticles surface. The eect of including PNAEI in the formulations was very pronounced. First of all it allowed a very signi®cant decrease in the average size of the nanoparticles. It was possible to obtain values of the average diameter of copolymer particles as low as 10±12 nm, which cannot
76
G. David et al. / European Polymer Journal 38 (2002) 73±78
Table 1 The average particle size of the P(MMA/BMA)±PNAEI nanoparticles Experiment no.
PNAEI Conc. (g/ml)
SDS Conc. (g/ml)
SDS/Monomer weight ratio
Dn (nm)
Dw (nm)
SDI
1 2 3 4 5
0.002 0.002 0.002 0.002 0.004
0.010 0.006 0.002 0.0 0.002
0.40 0.24 0.08 0.0 0.08
17 27 50 200 40
20.5 30 56 225 45
1.20 1.11 1.12 1.12 1.12
Table 2 Polymerization yields and viscosity average molecular weights Experiment no.
Polymerization yield (%)
Mv (106 )
Tg (°C)
1 2 3 4 5
85 82 78 75 85
2.3 2.1 2.2 2.4 2.3
38.0 39.0 43.0 43.5 40.0
be reached in the presence of SDS only. In addition, a SDS/monomer mass ratio lower than 1.0 does not allow microemulsion formation, but it was possible here due to the cosurfactant eect of PNAEI (see Table 1). Without SDS (Experiment no. 4) the particles were in the emulsion polymerization range. Doubling the PNAEI concentration caused again a drop in the average size down to microemulsion range (compare Experiments 3 and 5, Table 1). The particle size distribution, shown as SDI, was quite narrow, i.e., about 1.1± 1.2. Table 2 gives the viscosity average molecular weights, monomer conversions, and glass transition temperatures of the copolymers produced. The obtained polymerization yields were in the range of 75±85%. In addition to other parameters, the type of initiator is an eective factor of in¯uence on polymerization yield in microemulsions. It has been reported that yields are smaller than 70% in the unbuered microemulsion polymerizations stabilized with cationic surfactants (DTAB, CTAB etc.) and initiated with potassium persulphate (KPS) [6,13,14,21]. Polymerization yields in the microemulsions stabilized with anionic surfactants, such as SDS, may reach higher values (more than 75%) even with KPS [10,14,21,24]. Using organic phase soluble initiators (e.g., 2,20 -azobis isobutyronitrile, AIBN), with cationic or anionic surfactants one can achieve much higher polymerization yields, even upto 95% [13,14,21,24, 26,35]. In this study we observed that polymerization yield decreases with an increase in PNAEI/SDS ratio. This negative eect may be explained by considering the reactivity of the macromer, which is lower as compared to that of MMA or BMA. The formation of water-soluble
oligomers in an important amount, a known peculiarity of the polymerization systems using hydrophilic polymeric materials for stabilization, is another possibility [42]. Table 2 also gives the viscosity average molecular weights of the produced copolymers. Molecular weights were about 2:1±2:4 106 , not very dierent from each other. Methacrylate copolymers with lower molecular weights (0:63±1:75 106 ) were obtained when CTAB was used as surfactant and KPS as the initiator [32]. However, when AIBN was used instead of KPS, higher molecular weights (1:89±2:3 106 ) [35] were obtained. Several investigators have reported Mn values usually larger than 106 for PMMA [14,19], poly(butylmethacrylate) [21], and poly(hexylmethacrylate) [43]. In emulsion and microemulsion polymerization, the average molar masses and the average mass distributions are controlled by chain transfer to monomer [44]. The evaluation by calculus, considering the chain transfer to monomers, shows that the average-number molar masses for emulsion-made and microemulsion-made polymethacrylates should be in the range of 106 g/mol. Thermal transitions were analyzed by DSC. All the copolymers produced showed only glass transition temperatures (Tg ) (no melting points), which means that they were all amorphous. The poly(BMA-co-MMA) 50/ 50 produced without using PNAEI exhibited a Tg value about 50°C [32]. However, as shown in Table 2, the Tg values of the similar copolymers synthesized in the presence of PNAEI were lower, being situated in the range of 38±43.5°C. This decrease in the glass transition temperature may be attributed to a plasticizing eect of the PNAEI incorporated into the copolymers. Fig. 2 gives two FTIR spectra, one for the MMA/ BMA copolymer (prepared without using PNAEI), and the other obtained by including PNAEI in the polymerization recipe. Both spectra are very similar. The C± H symmetric and asymmetric stretching peaks, C±O absorption peak, C@O carbonyl peak, appear at about 2800±2900, 1500, 1000±1100, and 1730 cm 1 , respectively. There was only one dierence, a new signal at 1630 cm 1 , speci®c for the carbonyl group from the tertiary amide, however not so evident as expected. This peak was more intense when no SDS was used (Experiment 4). SDS is an acidic compound, which probably
G. David et al. / European Polymer Journal 38 (2002) 73±78
Fig. 2. FTIR spectra of the copolymers: (A) P(MMA/BMA) (without PNAEI); and (B) P(MMA/BMA) (with PNAEI).
77
caused partial hydrolyzation of the PNAEI sequence to polyethylenimine, having as eect the lowering of the intensity of the signal for the amidic carbonyl group. Fig. 3 gives two 1 H-NMR spectra, one for the MMA/ BMA copolymer (prepared without PNAEI), and the other by including PNAEI in the polymerization recipe. Both spectra are very similar. The following typical peaks were observed on the graphs: O±CH3 at 3.67 ppm (``a'' representing MMA), ±O±CH2 ± at d 3:52 ppm (``b'' representing BMA), CH3 ±C± at 0.88 ppm (``c'' representing both MMA and BMA), and ±CH2 ± at 1.12±1.83 ppm (``d'' representing both MMA and BMA). There was a new peak identi®ed at d 1:92, assigned to the CO±CH3 ± group from PNAEI sequences (Fig. 3B, ``e''). The signal for the ±CH2 ±N groups is most probably superposed with that of MMA, at 3.61 ppm. The MMA/BMA ratio in the synthesised copolymers was the same. Thus the presence of the macromer did not aect their relative reactivity. In conclusion it was proved that mixtures of anionic surfactants and PNAEI macromers with cinnamoyl endgroups as cosurfactants can be used as ecient stabilizing systems in the microemulsion polymerization. The ability in particle surface control by functionalization make such materials useful in a variety of applications (receptive binding, biosensors etc).
References
Fig. 3. 1 H-NMR spectra of the copolymers: (A) P(MMA/ BMA) (without PNAEI); and (B) P(MMA/BMA) (with PNAEI).
[1] Atik SS, Thomas JK. J Am Chem Soc 1981;103:4279. [2] Johnson PL, Gulari E. J Polym Sci Polym Chem Ed 1984;22:3967. [3] Jayakrishnan A, Shah DA. J Polym Sci Polym Lett Ed 1984;22:31. [4] Candau F. Recent advances in polymeric dispersions, vol. I. Navarra, Spain: Nato ASI Publ.; 23 June±5 July, 1996. [5] Guo JS, El-Aasser MS, Vanderho JW. J Polym Sci: Part A: Polym Chem 1989;27:691. [6] Guo JS, Sudol ED, Vanderho JW, El-Aasser MS. J Polym Sci Polym Chem Ed 1992;30:691. [7] Guo JS, Sudol ED, Vanderho JW, El-Aasser MS. J Polym Sci Polym Chem Ed 1992;30:703. [8] Antonietti M, Lohmann S, Bremser W. Prog Colloid Polym Sci 1992;89:62. [9] Antonietti M, Basten R, Lohmann S. Macromol Chem Phys 1995;196:441. [10] Perez-Luna VH, Puig JE, Castano VM, Rodriguez BE, Murthy AK, Kaler EW. Langmuir 1990;6:1040. [11] Texter J, Oppenheimer L, Minter JR. Polym Bull 1992;27:487. [12] Full AP, Puig JE, Gron LU, Kaler EW, Minter JR, Mourey TH, Texter J. Macromolecules 1992;25:5157. [13] Puig JE, Perez-Luna VH, Perez-Gonzales M, Macias ER, Rodriguez BE, Kaler EW. Colloid Polym Sci 1993;271:114. [14] Rodriguez-Guadarrama LA, Mendizabal E, Puig JE, Kaler EW. J Appl Polym Sci 1993;48:775. [15] Gan LM, Chew CH, Lee KC. Polymer 1993;34:3064.
78
G. David et al. / European Polymer Journal 38 (2002) 73±78
[16] Gan LM, Chew CH, Ng SC, Loh SE. Langmuir 1993;9:2799. [17] Ferrick MR, Murtagh J, Thomas JK. Macromolecules 1989;22:1515. [18] Antonietti M, Lohmann S, Van Niel C. Macromolecules 1992;25:1139. [19] Bleger F, Murthy AK, Pla F, Kaler EW. Macromolecules 1994;27:2559. [20] Capek I, Potisk P. J Polym Sci: Part A: Polym Chem 1995;33:1675. [21] Escalante JI, Rodriguez-Guadarrama LA, Mendizabal E, Puig JE, Lopez RG, Katime I. J Appl Polym Sci 1996;62:1313. [22] Santanu R, Devi S. J Appl Polym Sci 1996;62:1509. [23] Gan LM, Lee KC, Chew CH, Tok ES, Nig SC. J Polym Sci Part A: Polym Chem 1995;33:1161. [24] Rodriguez-Guadarrama LA, Mendizabal E, Puig JE, Kaler EW. J Appl Polym Sci 1993;48:2559. [25] Full AP, Kaler EW. Langmuir 1994;10:2929. [26] Full AP, Kaler EW, Arellano J, Puig JE. Macromolecules 1996;29:2764. [27] Larpent C, Tadros TF. Colloid Polym Sci 1991;269: 1171. [28] Antonietti M, Nestl T. Macromol Rapid Commun 1994;15:111.
[29] Antonietti M, Bremser W, Schmidt M. Macromolecules 1990;23:3796. [30] Antonietti M. Macromol Chem Phys 1995;196:441. [31] Capek I. Eur Polym J 2000;36:255. [32] Ozer F, Besßkardesß O, Pisßkin E. J Appl Polym Sci 2000;78:569. [33] Ozer F, PhD Thesis, Hacettepe University, Chemical Engineering Department, Ankara, Turkey, 2000. J Colloid Interface [34] Arda E, Ozer F, Pisßkin E, Pekcan O. Sci 2000;233:271. [35] Ozer F, Besßkardesß O, Pisßkin E. J Appl Polym Sci, in press. [36] David G, Ioanid A. J Appl Polym Sci 2001:2191. [37] Kobayashi S. Prog Polym Sci 1990;15:751. [38] Mura J-L, Riess G. Polym Adv Technol 1994;6:497. [39] Kobayashi S, Kaku M, Saegusa T. Macromolecules 1988;21:334. [40] Kocßum C, Zareie H, Ozer F, Pisßkin E. Colloid Polym Sci 2000;278:587. [41] Brandrup J, Immergut EH. Polymer handbook. 3rd ed. VII/8-12, II/153-251, V/77-79, VI/209-277. New York: Wiley; 1989. [42] Pichot C. Polym Adv Technol 1995;6:427. [43] Morgan, et al. Macromolecules 1997;30:1897. [44] Gilbert RG. Emulsion polymerization. New York: Academic Press; 1995.
www.elsevier.com/locate/europolj
Microemulsion photopolymerization of methacrylates stabilized with sodium dodecyl sulfate and poly(N-acetylethylenimine) macromonomers b G. David a, F. Ozer , B.C. Simionescu a, H. Zareie b, E. Pisßkin b,* b
a Department of Macromolecules, ``Gh. Asachi'' Technical University, 6600 Jassy, Romania Chemical Engineering Department, and Bioengineering Division, Faculty of Engineering, Hacettepe University, 06532 Beytepe, Ankara, Turkey
Received 9 February 2001; received in revised form 20 May 2001; accepted 11 June 2001
Abstract Methyl methacrylate and butyl methacrylate were polymerized in oil-in-water microemulsions that were stabilized by sodium dodecyl sulphate (SDS). A poly(N-acetylethylenimine) (PNAEI) macromer was also included in the recipe, as a cosurfactant and a comonomer. Polymerizations were initiated by UV-irradiation. The average diameters of latex particles, obtained by STM, were in the range of 17±200 nm. The experimental data evidenced that the particle size was mainly dependent on the SDS/PNAEI ratio. Polymerization yields were around 75±85%. The synthesized copolymers have viscosity average molecular weights in the range of 2:1±2:4 106 and glass transition temperatures of 38.0±43.5°C, lower than those obtained without using PNAEI. The investigation by means of FTIR and 1 H-NMR techniques revealed that PNAEI was incorporated into the nanoparticles. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Microemulsion polymerization; Nanoparticles; Methyl methacrylate and butyl methacrylate; Poly(N-acetylethylenimine); Sodium dodecyl sulfate; UV-radiation
1. Introduction The concept of polymerization in microemulsions appeared only in the early eighties [1±3]. Polymerization in microemulsions allows the synthesis of ultra®ne latex particles within the size range of 10±100 nm and with narrow size distributions [4±9]. In contrast to the opaque and milky conventional emulsions and miniemulsions, microemulsions are isotropic, optically transparent or translucent and thermodynamically stable. Several groups have studied microemulsion polymerization. Styrene and methyl methacrylate (MMA) are typical monomers that have been polymerized in ternary
*
Corresponding author. E-mail address: [email protected] (E. Pisßkin).
oil-in-water microemulsions [10±27]. Gan and coworkers produced poly(methyl methacrylate) (PMMA) in ternary microemulsions using the cationic surfactants stearyl trimethylammonium chloride (STAC), cetyl trimethyl ammonium bromide (CTAB) and dodecyl trimethylammonium bromide (DTAB) with either a water-soluble or an oil-soluble initiator and found that the longer the hydrophobic chain length of the surfactant, the smaller the latex particles [23]. Kaler and coworkers also prepared small latexes from styrene and several dierent methacrylic esters using cationic surfactants [12,19,24±26]. Larpent and Tadros have found optimum mixtures of non-ionic surfactants (mixtures of nonylphenol±modi®ed oligo(ethylene oxide)) to form microemulsions of MMA and of styrene in water, and produced small latex particles at various surfactantto-monomer ratios using ascorbic acid/hydrogen peroxide as a redox initiator [27]. Microlatex particles with
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 1 6 2 - 8
74
G. David et al. / European Polymer Journal 38 (2002) 73±78
an average size of about 6±8 nm (with surfactant shell) were produced by using metal ion supported surfactants (metallosurfactants) [28]. Antonietti et al. have used hydrophobically modi®ed poly(ethylene oxide) [29]. Oligomers of amphipathic monomers such as commercially available sulfonated alkyl-poly(ethylene oxide) have also been examined [30]. Recently, Capek has thoroughly studied the features of the photopolymerization of alkyl methacrylates and methacryloyl-terminated polyoxyethylene macromonomers (POE-MA) in microemulsions in the presence of high amounts of SDS [31]. It was concluded that the addition of polyoxyethylene macromonomers in the system decreased the polymerization rate and the molecular weight and slightly increased the particle size. This behavior was mainly attributed to the transformation of the electrostatic to electrosteric stabilization mechanism. In our previous studies, we have polymerized methyl, ethyl, butyl methacrylates and hydroxyethyl methacrylate in microemulsion systems, and investigated their ®lm forming abilities [32±35]. In the present study we included a hydrophilic poly(N-acetylethylenimine) (PNAEI) macromer with cinnamoyl polymerizable end group, as an active cosurfactant in the formulations [36]. The polymerizations were realized by initiation with UV-radiation. The choice of the macromer was accomplished considering the biocompatibility of PNAEI and the opportunities to functionalize the surface of the obtained nanoparticles, oered by such a ``self-surfacting'' system [37]. Moreover, according to recent reports of Mura and Riess optimum conditions for ®lm formation could be achieved when the hydrophilic stabilizing chains from the surface of the polymer particles are miscible with the matrix polymer [38]. The compatibility of PNAEI with commodity polymers is well known [39]. 2. Experimental 2.1. Materials The monomers, MMA and butyl methacrylate (BMA) (Fluka, USA) were commercial grade, and were treated with an aqueous solution of NaOH (10%) to remove the inhibitor. Distilled/deionized water was used in all experiments. The emulsi®er was the reagent-grade sodium dodecyl sulfate (SDS) (Fluka, USA). The macromer with cinnamoyl end-group (Mn :2190, functionalization:100%, from 1 H-NMR data) was synthesized by the quenching of the oxazolinium living species in the 2-methyl-2-oxazoline polymerization with cinnamic acid in the presence of a macromolecular proton scavenger, i.e. poly(vinylpyridine-co-divinyl benzene) [36]. The chemical formula of this macromer, namely PNAEI is given below.
2.2. Microemulsion polymerization The polymerizations were realized in an oil-in-water (o/w) system. In a typical procedure, a selected amount of SDS (three dierent values: 0.01, 0.006, and 0.002 g/ ml), 0.002 g/ml (or 0.004 g/ml) macromer (given above) and 1.25 g of the comonomer mixture (MMA/BMA ratio:50/50, by weight) were added to 50 ml distilled water, in a 100 ml Pyrex glass reactor, dotted with magnetic stirrer. Prior to polymerization, the reaction mixture was stirred at room temperature for about 10 min. It was then stored at 4°C for about 24 h to reach equilibrium. Polymerization was performed at 25°C, under inert atmosphere (nitrogen), with stirring, by using a high pressure mercury UV lamp of 300 W, disposed at 15 cm distance from the reactor. Polymerization duration was of 4 h. The latex particles were cleaned by washing with methanol and water several times to remove the surfactant. 2.3. Characterization The particle size of the nanoparticles was measured by a novel technique, scanning tunneling microscopy (STM), which is described in detail elsewhere [40]. For STM imaging, the latex samples (5 ll containing about 0.1 mg particles per ml) were deposited onto freshly cleaved highly oriented pyrolitic graphite (HOPG), and dried at room temperature. Then, the STM images were taken at 2 V sample bias and a tunneling current of a 20 pA. Etched tips of Pt/Ir (80:20) wires (0.5 mm in diameter, Digital Instruments, Santa Barbara, CA) were used. Prior to use, the tips were washed in acetone. The number average diameter (Dn ), the weight-average diameter (Dw ) and size distribution index (SDI) were evaluated from the micrographs by using the following relations: Dn Dw
X X
.X
Ni .X Ni D3i Ni D4i
Ni Di
SDI Dw =Dn The polymerization yield was obtained by extraction of copolymer from the latex particles by chloroform, and by weighting the solid phase after complete removal of the unconverted monomers by a controlled drying.
G. David et al. / European Polymer Journal 38 (2002) 73±78
75
Fig. 1. Typical STM micrographs of nanoparticles produced.
Viscosity measurements were used to obtain average molecular weights of the polymers produced in this study. Viscosities of the polymer solutions with dierent concentrations (0.1±2.0 g/100 ml) in chloroform were measured with an Ubbelhode capillary viscometer at 25.0° 0:1°C. The following Mark±Houwink equation was used to calculate the viscosity average molecular weights: g KMva The ``K'' and ``a'' values for PMMA and PBMA are 5:5 10 5 and 4:9 10 5 and 0.79 and 0.78, respectively [41]. For the copolymers under study the average values (K: 5:2 10 5 ; and a: 0.785) were used. Thermal transitions were obtained by using a dierential scanning calorimeter (DSC) (Shimadzu, Model DSC-50, Japan). Nitrogen was used as the sweeping gas. Samples (5±10 mg) were heated at a scan rate of 10°C/ min from 25°C to 300°C followed by rapid cooling. FTIR spectra of the nanoparticles were obtained by using an FTIR spectrophotometer (Schimadzu, Model: FTIR-8000 series, Japan) with KBr (IR grade)-nanoparticle mixtures in powder form. Polymer samples were dissolved in CDCl3 , and 1 HNMR spectra were recorded in a NMR spectrometer (Brucker AC 250, USA) working at 500 MHz at room temperature. The sample concentration in CDCl3 was 1% (w/v). The internal standard was tetramethylsilane. 3. Results and discussion In this study we attempted to copolymerize MMA and BMA in microemulsions in the presence of both SDS, as the main surfactant, and also a cosurfactant, i.e. a PNAEI macromer. This new stabilizing system, not described in our previous study, was used with the aim
to reach an improved quality of the resulted nanoparticles, considering their possible use in biomedical area [32]. As we know, the use of polyethylenimine derivatives in the stabilization of microemulsion polymerization systems was not reported until now. Making use of the optical peculiarities of the microemulsion systems the thermal scission of chemical initiators was replaced by initiation by UV-irradiation. For all the investigated polymerization systems stable latexes were obtained. In most cases, prior the polymerization, the medium was a transparent liquid, while after polymerization the latex obtained was bluish and clear. No precipitation was observed in about two months storage period. In order to obtain the average particle size, STM was used. Typical micrographs are shown in Fig. 1. Average diameters of the nanoparticles were calculated by evaluating micrographs containing approximately 300±500 particles. Table 1 gives the average particle size of the nanoparticles, which is one of the most interesting results observed in this study. As expected [4], a decrease in SDS concentration determined a signi®cant increase in the average size of the nanoparticles. A SDS/monomer mass ratio lower than about 1.0 shifts the polymerization from microemulsion to miniemulsion or even to classical emulsion limits (at lower values). Note that in order to obtain stable latexes the amount of SDS/monomer in microemulsion formulations should be around 1±3, which is quite high [4±7,10]. This high concentration of SDS is considered as one of the main drawbacks of microemulsion polymerization, due to the diculties to remove the residual SDS from the nanoparticles surface. The eect of including PNAEI in the formulations was very pronounced. First of all it allowed a very signi®cant decrease in the average size of the nanoparticles. It was possible to obtain values of the average diameter of copolymer particles as low as 10±12 nm, which cannot
76
G. David et al. / European Polymer Journal 38 (2002) 73±78
Table 1 The average particle size of the P(MMA/BMA)±PNAEI nanoparticles Experiment no.
PNAEI Conc. (g/ml)
SDS Conc. (g/ml)
SDS/Monomer weight ratio
Dn (nm)
Dw (nm)
SDI
1 2 3 4 5
0.002 0.002 0.002 0.002 0.004
0.010 0.006 0.002 0.0 0.002
0.40 0.24 0.08 0.0 0.08
17 27 50 200 40
20.5 30 56 225 45
1.20 1.11 1.12 1.12 1.12
Table 2 Polymerization yields and viscosity average molecular weights Experiment no.
Polymerization yield (%)
Mv (106 )
Tg (°C)
1 2 3 4 5
85 82 78 75 85
2.3 2.1 2.2 2.4 2.3
38.0 39.0 43.0 43.5 40.0
be reached in the presence of SDS only. In addition, a SDS/monomer mass ratio lower than 1.0 does not allow microemulsion formation, but it was possible here due to the cosurfactant eect of PNAEI (see Table 1). Without SDS (Experiment no. 4) the particles were in the emulsion polymerization range. Doubling the PNAEI concentration caused again a drop in the average size down to microemulsion range (compare Experiments 3 and 5, Table 1). The particle size distribution, shown as SDI, was quite narrow, i.e., about 1.1± 1.2. Table 2 gives the viscosity average molecular weights, monomer conversions, and glass transition temperatures of the copolymers produced. The obtained polymerization yields were in the range of 75±85%. In addition to other parameters, the type of initiator is an eective factor of in¯uence on polymerization yield in microemulsions. It has been reported that yields are smaller than 70% in the unbuered microemulsion polymerizations stabilized with cationic surfactants (DTAB, CTAB etc.) and initiated with potassium persulphate (KPS) [6,13,14,21]. Polymerization yields in the microemulsions stabilized with anionic surfactants, such as SDS, may reach higher values (more than 75%) even with KPS [10,14,21,24]. Using organic phase soluble initiators (e.g., 2,20 -azobis isobutyronitrile, AIBN), with cationic or anionic surfactants one can achieve much higher polymerization yields, even upto 95% [13,14,21,24, 26,35]. In this study we observed that polymerization yield decreases with an increase in PNAEI/SDS ratio. This negative eect may be explained by considering the reactivity of the macromer, which is lower as compared to that of MMA or BMA. The formation of water-soluble
oligomers in an important amount, a known peculiarity of the polymerization systems using hydrophilic polymeric materials for stabilization, is another possibility [42]. Table 2 also gives the viscosity average molecular weights of the produced copolymers. Molecular weights were about 2:1±2:4 106 , not very dierent from each other. Methacrylate copolymers with lower molecular weights (0:63±1:75 106 ) were obtained when CTAB was used as surfactant and KPS as the initiator [32]. However, when AIBN was used instead of KPS, higher molecular weights (1:89±2:3 106 ) [35] were obtained. Several investigators have reported Mn values usually larger than 106 for PMMA [14,19], poly(butylmethacrylate) [21], and poly(hexylmethacrylate) [43]. In emulsion and microemulsion polymerization, the average molar masses and the average mass distributions are controlled by chain transfer to monomer [44]. The evaluation by calculus, considering the chain transfer to monomers, shows that the average-number molar masses for emulsion-made and microemulsion-made polymethacrylates should be in the range of 106 g/mol. Thermal transitions were analyzed by DSC. All the copolymers produced showed only glass transition temperatures (Tg ) (no melting points), which means that they were all amorphous. The poly(BMA-co-MMA) 50/ 50 produced without using PNAEI exhibited a Tg value about 50°C [32]. However, as shown in Table 2, the Tg values of the similar copolymers synthesized in the presence of PNAEI were lower, being situated in the range of 38±43.5°C. This decrease in the glass transition temperature may be attributed to a plasticizing eect of the PNAEI incorporated into the copolymers. Fig. 2 gives two FTIR spectra, one for the MMA/ BMA copolymer (prepared without using PNAEI), and the other obtained by including PNAEI in the polymerization recipe. Both spectra are very similar. The C± H symmetric and asymmetric stretching peaks, C±O absorption peak, C@O carbonyl peak, appear at about 2800±2900, 1500, 1000±1100, and 1730 cm 1 , respectively. There was only one dierence, a new signal at 1630 cm 1 , speci®c for the carbonyl group from the tertiary amide, however not so evident as expected. This peak was more intense when no SDS was used (Experiment 4). SDS is an acidic compound, which probably
G. David et al. / European Polymer Journal 38 (2002) 73±78
Fig. 2. FTIR spectra of the copolymers: (A) P(MMA/BMA) (without PNAEI); and (B) P(MMA/BMA) (with PNAEI).
77
caused partial hydrolyzation of the PNAEI sequence to polyethylenimine, having as eect the lowering of the intensity of the signal for the amidic carbonyl group. Fig. 3 gives two 1 H-NMR spectra, one for the MMA/ BMA copolymer (prepared without PNAEI), and the other by including PNAEI in the polymerization recipe. Both spectra are very similar. The following typical peaks were observed on the graphs: O±CH3 at 3.67 ppm (``a'' representing MMA), ±O±CH2 ± at d 3:52 ppm (``b'' representing BMA), CH3 ±C± at 0.88 ppm (``c'' representing both MMA and BMA), and ±CH2 ± at 1.12±1.83 ppm (``d'' representing both MMA and BMA). There was a new peak identi®ed at d 1:92, assigned to the CO±CH3 ± group from PNAEI sequences (Fig. 3B, ``e''). The signal for the ±CH2 ±N groups is most probably superposed with that of MMA, at 3.61 ppm. The MMA/BMA ratio in the synthesised copolymers was the same. Thus the presence of the macromer did not aect their relative reactivity. In conclusion it was proved that mixtures of anionic surfactants and PNAEI macromers with cinnamoyl endgroups as cosurfactants can be used as ecient stabilizing systems in the microemulsion polymerization. The ability in particle surface control by functionalization make such materials useful in a variety of applications (receptive binding, biosensors etc).
References
Fig. 3. 1 H-NMR spectra of the copolymers: (A) P(MMA/ BMA) (without PNAEI); and (B) P(MMA/BMA) (with PNAEI).
[1] Atik SS, Thomas JK. J Am Chem Soc 1981;103:4279. [2] Johnson PL, Gulari E. J Polym Sci Polym Chem Ed 1984;22:3967. [3] Jayakrishnan A, Shah DA. J Polym Sci Polym Lett Ed 1984;22:31. [4] Candau F. Recent advances in polymeric dispersions, vol. I. Navarra, Spain: Nato ASI Publ.; 23 June±5 July, 1996. [5] Guo JS, El-Aasser MS, Vanderho JW. J Polym Sci: Part A: Polym Chem 1989;27:691. [6] Guo JS, Sudol ED, Vanderho JW, El-Aasser MS. J Polym Sci Polym Chem Ed 1992;30:691. [7] Guo JS, Sudol ED, Vanderho JW, El-Aasser MS. J Polym Sci Polym Chem Ed 1992;30:703. [8] Antonietti M, Lohmann S, Bremser W. Prog Colloid Polym Sci 1992;89:62. [9] Antonietti M, Basten R, Lohmann S. Macromol Chem Phys 1995;196:441. [10] Perez-Luna VH, Puig JE, Castano VM, Rodriguez BE, Murthy AK, Kaler EW. Langmuir 1990;6:1040. [11] Texter J, Oppenheimer L, Minter JR. Polym Bull 1992;27:487. [12] Full AP, Puig JE, Gron LU, Kaler EW, Minter JR, Mourey TH, Texter J. Macromolecules 1992;25:5157. [13] Puig JE, Perez-Luna VH, Perez-Gonzales M, Macias ER, Rodriguez BE, Kaler EW. Colloid Polym Sci 1993;271:114. [14] Rodriguez-Guadarrama LA, Mendizabal E, Puig JE, Kaler EW. J Appl Polym Sci 1993;48:775. [15] Gan LM, Chew CH, Lee KC. Polymer 1993;34:3064.
78
G. David et al. / European Polymer Journal 38 (2002) 73±78
[16] Gan LM, Chew CH, Ng SC, Loh SE. Langmuir 1993;9:2799. [17] Ferrick MR, Murtagh J, Thomas JK. Macromolecules 1989;22:1515. [18] Antonietti M, Lohmann S, Van Niel C. Macromolecules 1992;25:1139. [19] Bleger F, Murthy AK, Pla F, Kaler EW. Macromolecules 1994;27:2559. [20] Capek I, Potisk P. J Polym Sci: Part A: Polym Chem 1995;33:1675. [21] Escalante JI, Rodriguez-Guadarrama LA, Mendizabal E, Puig JE, Lopez RG, Katime I. J Appl Polym Sci 1996;62:1313. [22] Santanu R, Devi S. J Appl Polym Sci 1996;62:1509. [23] Gan LM, Lee KC, Chew CH, Tok ES, Nig SC. J Polym Sci Part A: Polym Chem 1995;33:1161. [24] Rodriguez-Guadarrama LA, Mendizabal E, Puig JE, Kaler EW. J Appl Polym Sci 1993;48:2559. [25] Full AP, Kaler EW. Langmuir 1994;10:2929. [26] Full AP, Kaler EW, Arellano J, Puig JE. Macromolecules 1996;29:2764. [27] Larpent C, Tadros TF. Colloid Polym Sci 1991;269: 1171. [28] Antonietti M, Nestl T. Macromol Rapid Commun 1994;15:111.
[29] Antonietti M, Bremser W, Schmidt M. Macromolecules 1990;23:3796. [30] Antonietti M. Macromol Chem Phys 1995;196:441. [31] Capek I. Eur Polym J 2000;36:255. [32] Ozer F, Besßkardesß O, Pisßkin E. J Appl Polym Sci 2000;78:569. [33] Ozer F, PhD Thesis, Hacettepe University, Chemical Engineering Department, Ankara, Turkey, 2000. J Colloid Interface [34] Arda E, Ozer F, Pisßkin E, Pekcan O. Sci 2000;233:271. [35] Ozer F, Besßkardesß O, Pisßkin E. J Appl Polym Sci, in press. [36] David G, Ioanid A. J Appl Polym Sci 2001:2191. [37] Kobayashi S. Prog Polym Sci 1990;15:751. [38] Mura J-L, Riess G. Polym Adv Technol 1994;6:497. [39] Kobayashi S, Kaku M, Saegusa T. Macromolecules 1988;21:334. [40] Kocßum C, Zareie H, Ozer F, Pisßkin E. Colloid Polym Sci 2000;278:587. [41] Brandrup J, Immergut EH. Polymer handbook. 3rd ed. VII/8-12, II/153-251, V/77-79, VI/209-277. New York: Wiley; 1989. [42] Pichot C. Polym Adv Technol 1995;6:427. [43] Morgan, et al. Macromolecules 1997;30:1897. [44] Gilbert RG. Emulsion polymerization. New York: Academic Press; 1995.