Langmuir 2006, 22, 2795-2801
2795
Octadecyl Maleamic Acid Salt As a Microreactor for Its Copolymerization Reaction with Butyl Acrylate in Aqueous Medium L. J. Milton Gaspar and Geetha Baskar* Industrial Chemistry Laboratory, Central Leather Research Institute, Adyar, Chennai 600020, India ReceiVed September 15, 2005. In Final Form: January 10, 2006 The hydrogelator, octadecyl maleamic acid salt (ODMAS), has been shown to perform as a microreactor in copolymerization reaction with butyl acrylate in aqueous medium thereby providing functionalized latex. The evidence for occurrence of controlled polymerization reaction inside the microreactor is drawn from the composition and the polydispersity index of the copolymers. The copolymers generated under microreactor conditions or in other words, from emulsion phase provided by the hydrogelator exhibit significant incorporation of ODMAS with narrow polydispersity index. For example, a copolymer with ODMAS as high as 0.62 m and polydispersity index at 1.39 could be achieved. On the contrary, the solution copolymerization reactions in THF resulted in low yield of polymers with molecular weight at 103 order and polydispersity index in range of 2.53-2.91. The particle size distribution of the latexes remains almost invariant at 74 ( 4 nm, over the concentration range of 0.12-0.62m with standard deviation (σ) of 0.12-0.22. The surface area/molecule of ODMAS on the latex particle has been estimated to be 0.21 nm2/molecule. The polymerized latexes exhibit zeta potential at 64 ( 3 mV and surface tension in range of 42.8-47.9 mN m-1 respectively. This is indicative of coverage of latex with ODMAS.
Introduction The role of emulsifier in emulsion polymerization is multifold, like stabilization of starting emulsion, particle nucleation and growth, providing controlled conditions to generate particles with narrow polydispersity, and stabilization of the final latex.1 Polymerizable surfactants provide conditions for emulsion polymerization reaction in absence of a free surfactant and are free from drawbacks generally associated with simple surfactants, for example migration of surfactants with aging. The organized emulsion phase provided by the polymerizable surfactants can be considered similar to microreactors promoting polymerization reaction under controlled conditions. A variety of polymerizable surfactants bearing polymerizable groups such as acryloyl, styrenyl, and maleyl have been investigated in an aqueous emulsion polymerization reaction.2-6 Gan and co-workers reported emulsifier free emulsion polymerization of styrene in the presence of ionic comonomer which produced stable polystyrene latexes of particle size in the range of 30-250 nm with high polystyrene content.7 During the past few years surfactants derived from maleic acid have been shown to be of high interest because of simple synthetic meholdogies and promising results in emulsion polymerization.8-11 They form a special class of surfactants since * Corresponding author. Fax: 91-44-24911589 Tel.: 91-44-24911386/ 24911108. E-mail:
[email protected]. (1) Hansen, F. K. In Polymer Latexes; Daniel, E. S., Sodom, E. A., El Aasser, M. S., Eds.; ACS Symposium Series 492; American Chemical Society: Washington, DC, 1992; p 12. (2) Joynes, D.; Sherrington, D. C. Polymer 1995, 37, 1453. (3) Greene, B. W.; Sheetz, D. P.; Filer, T. D. J. Colloid Interface Sci. 1970, 32, 90. (4) Yokota, K.; Ichihara, A.; Shinke, H. Industrial application of surfactants III; RSC: London, 1992; p 29. (5) Urqiola, M. B.; Dimonie, V. L.; Sudol, E. D.; El-Asser, M. S. J. Polym. Sci. Part A: Polym. Chem. 1992, 30, 2631. (6) Holmberg, K. Prog. Org. Coatings 1992, 20, 325. (7) Xu, X. J.; Siow, K. S.; Wong, M. K.; Gan, L. M. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 1634. (8) Schoonbrood, H. A. S.; Unzue, M. J.; Amalvy, J. I.; Asua, J. M. J. Polym. Sci. Part A: Polym. Chem. 1997, 35, 2561. (9) Zicmanis, A.; Hamide, T.; Graillat, C.; Monnet, C.; Abele, S.; Guyot, A. Colloid Polym. Sci. 1997, 275, 1. (10) Guyot, A.; Goux, A. J. Appl. Polym. Sci. 1997, 65, 2289.
they cannot homopolymerize, but readily copolymerize with other monomers. The high reactivity of maleic surfactants in emulsion polymerization has been demonstrated.12,13 Surfactants from maleamic acids are highly significant especially in view of their stability to alkaline hydrolysis14 and scope for conversion into imide compounds exhibiting photo and thermal resist properties similar to alkyl acrylamide copolymers.15 The organized assemblies of polymerizable surfactants such as vesicles and lyotropic mesophases open up the possibility of synthesizing polymeric materials with unique ordering, shapes, and properties. A recent review by Solans et al. established the gel phase as a versatile reaction medium for the preparation of low-density polymeric materials such as solid foam and aerogels.16 Jung et al.17 reported the copolymerization of hydrophobic monomers in vesicular medium. The vesicular membranes formed by the polymerizable amphiphile provides a template to prepare different polymeric architectures. The reverse hexagonal phases formed by the anionic surfactant bis(2-ethyl hexyl)sulfosuccinate sodium salt (AOT) have been utilized for the copolymerization of divinyl benzene (DVB) and styrene.18 Polymer layers consisting of aligned strings of polymer beads with diameters of about 100 nm are achieved by this process. Recently James et al. demonstrated the use of an organic gel formed by a metalligand complex as a template in the preparation of macroporous polymethyl methacrylate.19 The method of polymerization of monomers which are capable of forming gels is a novel route to the preparation of stable polymeric gels as demonstrated in (11) Unzue, M. J.; Schoonbrood, H. A. S.; Asua, J. M.; Goni, A. M.; Sherrington, D. C.; Stahler, K.; Goebel, K. H.; Tauer, K.; Sjoberg, M.; Holmberg, K. J. Appl. Polym. Sci. 1997, 66, 1803. (12) Schoonbrood, H. A. S.; Unzue´, M. J.; Beck, O.; Asua, J. M.; Gon˜i, A. M. Sherrington, D. C. Macromolecules 1997, 30, 6024. (13) Schoonbrood, H. A. S.; Asua, J. M. Macromolecules 1997, 30, 6034. (14) Abele, S.; Graillat, C.; Zicmanis, A.; Guyot, A. Polym. AdV. Technol. 1999, 10, 301. (15) Guo, Y.; Feng, F.; Miyashita, Macromolecules 1999, 32, 1115. (16) Solans, C.; Esquena, J.; Azemar, N. Curr. Opin. Colloid Interface Sci. 2003, 8, 126. (17) Jung, M.; Hubert, D. H. W.; Bomans, P. H. H.; Fredrick, P.; Van Herk, A. M.; German, A. L. AdV. Mater. 2000, 12, 210. (18) Hentze, H. P.; Kaler, E. W. Chem. Mater. 2002, 15, 708. (19) Wei, Q.; James, S. L. Chem. Commun. 2005, 1555.
10.1021/la052518s CCC: $33.50 © 2006 American Chemical Society Published on Web 02/09/2006
2796 Langmuir, Vol. 22, No. 6, 2006 Scheme 1. Structure of N-Octadecyl Maleamic Acid (ODMA)
the polymerization of a gel forming amino acid based urea derivatives.20 We observed that the polymerizable surfactant N-octadecyl maleamic acid (ODMAS; Scheme 1) derived from maleic anhydride has been able to gelate water when converted to sodium salt. The hydrogelation phenomenon exhibited by ODMAS has been reported recently by us.21 It is significant to investigate the scope of ODMAS as a polymerizable surfactant in emulsion copolymerization reaction. Here, we present the emulsion polymerization of butyl acrylate (BA) using the gel forming polymerizable surfactant ODMAS in thermal as well as in photochemical conditions. The solution copolymerization of BA and ODMAS has been performed to compare the characteristics of polymerization reaction in nonaqueous medium and to understand unique characteristics of emulsion polymerization reaction. Experimental Section Materials. Octadecylamine and maleic anhydride used in synthesis of N-octadecyl maleamic acid were from s.d.fine Chemicals Ltd, India. N-Octadecyl maleamic acid (ODMA) was synthesized using the reported procedure elsewhere.14 The monomer n-butyl acrylate (BA), 98%, from Aldrich was used as such. Potassium persulfate, 98%, was obtained from s.d.fine Chemicals Ltd, India. 2,2′Azobisisobutyronitrile (AIBN), 98%, was obtained from Aldrich, USA. Preparation of the Gel. In a typical example, to 1 g of ODMA, aq. NaOH solution was slowly added dropwise with constant stirring till the system reached the pH of 10.5. The ODMAS concentration was adjusted to 5%. The viscous solution was heated to 65 °C with constant mixing and then cooled to get the homogeneous gel. Emulsion Polymerization. Thermal Polymerization. To the 5% aqueous gel of ODMAS in a polymerization tube was added BA. The molar ratio of ODMAS and BA in the feed was varied from 0.12:0.88 m to 0.70:0.30 m. About 1% K2S2O8 initiator was added (on the weight of monomers), and the polymerization was carried out at a temperature of 80 °C for a period of 6 h with constant stirring under inert atmosphere. Photopolymerization. Photopolymerization reactions were performed on a Heber multilamp photoreactor model HML-compactLP-MP-812, 8W medium-pressure UV lamp under inert atmosphere. The emulsions taken in the polymerization tube were irradiated with radiation of λ ) 365 nm at 30 °C for a period of 6 h. About 0.1% initiator, K2S2O8 was employed. Solution Copolymerization. The selected identical feed compositions of ODMA and BA used in emulsion polymerization were dissolved in tetrahydrofuran, pH raised to 10.5 adding few drops of 20% NaOH solution and taken in a polymerization tube. The pH of the solution was measured using a digital pH meter, from Electronic Corporation of India Ltd. About 1% and 0.1% AIBN was added for thermal and photopolymerization reactions, respectively. All reactions were carried out for 24 h under nitrogen atmosphere. The percentage yield of polymers from emulsion and solution polymerization methods was estimated from gravimetry. The polymers were isolated by nonsolvent precipitation method using methanol as a precipitating solvent. The polymers were repeatedly (20) Wang, G.; Hamilton, A. D. Chem. Euro. J. 2002, 8, 1954. (21) Gaspar, L. J. M.; Baskar, G. Chem. Commun. 2005, 3603.
Milton Gaspar and Baskar washed with water and methanol to remove the unreacted monomers. The purified polymers were dried under vacuum until a constant weight was obtained. Characterization of the Latexes. The particle size and zeta potential measurements were performed for the latexes on a Malvern particle size analyzer 1000HS/3000HS at a fixed scattering angle of 90°. The latex samples for analysis were freed of excess surfactant and cosurfactant by dialysis using low molecular weight cut off membranes. The dialysis was performed for about a week, with a repeated change of water. The samples were filtered using 0.45 µm filter paper before measurements. The surface tension of the latexes before and after polymerization was measured on Nima dynamic static tensiometer 9005. The interfacial area was calculated from Np and Dw using the following equation: Interfacial area ) Np[4π(Dw/2)2]
(1)
where Dw is the diameter of the particle and Np is the number of particle per liter of the latex calculated using the reported equation.22 Characterization of the Copolymers. 1H NMR measurements for the characterization of the copolymers were performed on a JEOL ECA 500 (500 MHz) NMR spectrometer in CDCl3 solvent using tetramethylsilane as an internal standard. Molecular weight estimations of the polymers were performed on a JASCO GPC chromatograph model MX-2080-31 fitted with PL gel 5µm Mixed-C columns, 300 × 7.5 mm, in tetrahydrofuran with a flow rate at 1 mL min-1 at 30 °C using a refractive index (RI) detector. The composition of the copolymers was estimated using the 1H NMR spectroscopic method. The copolymer composition was also confirmed by the titration method where the alkalinity due to -COONa was estimated by titrating the copolymer solution in THF with standard aqueous HCl. Differential scanning calorimetric measurements on the polymers were made on a NETZSCH DSC200PC differential scanning calorimeter with the heating rate of 5 °C per minute. The thermogravimetric analysis of the copolymers was done using a NETZSCH STA 409PC thermogravimetric analyzer with the heating rate of 10 °C/min.
Results and Discussion Emulsion Polymerization. The hydrogelation phenomenon is exhibited by ODMAS solution at a concentration between 0.75 and 10%. The appropriate concentration to perform the copolymerization reaction needs to be identified. We have designed copolymerization reactions considering some of the important issues such as solubility of comonomers, stability of latex during and after polymerization reaction, incorporation of comonomers, and yield of the copolymer product. In line with this, we identified 5% ODMAS hydrogel to perform the copolymerization reaction. In these reactions, BA an oil soluble monomer was employed. The 5% gel phase was chosen due to its tendency to form stable emulsion at reasonable monomer concentrations in the range of 5.8-17.5% after emulsification. We have established that 5% ODMAS hydrogel transforms into free flowing emulsion phase with addition of BA in the concentration range of 0.30-0.88 m. It is significant to observe that the addition of BA at a concentration of 0.88 m BA was added. Although promotion of copolymerization reaction in the gel phase cannot be ruled out, the reaction using such phase is expected to be very different in comparison to emulsion phase. It is more appropriate and useful to investigate copolymerization behavior employing a similar phase. We have therefore chosen to investigate, in detail, the copolymerization reaction in emulsion phase employing BA over the identified concentration range of 0.30-0.88 m (Table 1). The reactions were carried out under (1) (22) Xu, X. J.; Goh, H. L.; Siow, K. S.; Gan, L. M. Langmuir 2001, 17, 6077.
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Table 1. (A) Characteristics of the Copolymers, 5% ODMAS Gel, Thermal, Initiator, 1% K2S2O8, Temp. 80 °C (B) Characteristics of the Copolymers, 5% ODMAS Gel, Photo λ ) 365 nm, Initiator, 0.1% K2S2O8, Temp. 30 °C copolymer composition feed (m) (m) yield system ODMAS BA ODMAS BA (%) ODET1 ODET2 ODET3 ODET4 ODET5 ODET6 ODET7 ODET8
0.12 0.15 0.20 0.26 0.40 0.50 0.60 0.70
0.88 0.85 0.80 0.74 0.60 0.50 0.40 0.30
ODEP1 ODEP2 ODEP3 ODEP4 ODEP5 ODEP6 ODEP7 ODEP8
0.12 0.15 0.20 0.26 0.40 0.50 0.60 0.70
0.88 0.85 0.80 0.74 0.60 0.50 0.40 0.30
(A) 0.12 0.15 0.18 0.24 0.35 0.44 0.52 0.62 (B) 0.12 0.14 0.18 0.24 0.35 0.44 0.52 0.60
Mn
Mw/Mn
0.88 0.85 0.82 0.76 0.65 0.56 0.48 0.38
96 95 95 94 71 66 54 47
1.13 × 106 1.21 × 106 1.25 × 106 1.18 × 106 8.37 × 103 6.56 × 103 5.23 × 103 5.10 × 103
1.42 1.45 1.41 1.36 1.27 1.38 1.41 1.39
0.88 0.86 0.82 0.76 0.65 0.56 0.48 0.40
96 95 95 93 76 69 58 53
1.18 × 106 1.25 × 106 1.15 × 106 1.11 × 106 8.72 × 103 6.87 × 103 5.76 × 103 4.83 × 103
1.47 1.42 1.44 1.38 1.32 1.36 1.39 1.42
thermal conditions at a temperature of 80 °C and (2) photo conditions using radiation of λ ) 365 nm at room temperature using K2S2O8 initiator in order to investigate the effects of polymerization reaction characteristics. The concentration of initiator was maintained constant at 1% for thermal polymerization and 0.1% for photopolymerization reactions for all compositions. All reactions were performed under nitrogen atmosphere, and therefore, inhibition due to oxygen from air is ruled out. The polymerization reactions were carried out for a maximum period of 6 h, the minimum period identified after several experiments so as to get reasonable yield at about 50%. The polymers have been isolated using methanol as a precipitating solvent. Repeated washings in methanol and water were performed to ensure complete removal of unreacted monomer of BA and ODMAS. The isolated polymers were characterized using 1H and 13C NMR and FT-IR spectroscopic techniques. The absence of a peak due to CHdCH protons at δ ) 5.5-6.5 ppm in 1H NMR and CdC at δ ) 120-135 ppm in 13C NMR confirms the complete removal of unreacted monomer and also the occurrence of polymerization due to addition across double bond. The presence of peaks due to methyl groups of ODMAS at δ ) 0.85 ppm and BA at δ ) 0.96 ppm demonstrates the incorporation of both the monomers in the polymer chain. The peak at δ ) 3.5 ppm due to R-CH2 of ODMAS further confirms the incorporation of ODMAS in the polymer chain. The peak at 1590 cm-1 in the FT-IR spectrum of the copolymer shows that ODMAS is in the form of sodium salt. The composition of the copolymers has been estimated using the 1H NMR spectroscopic method. The peaks at δ ) 0.85 ppm due to methyl protons of ODMAS and δ ) 0.96 ppm due to methyl protons of BA were chosen for the estimation of copolymer composition. The copolymer composition was confirmed by the titration method wherein the alkalinity due to -COONa of ODMAS component was estimated. Table 1, parts A and B, presents molecular weight, polydispersity, composition, and yield of the series of copolymers from thermal and photopolymerization performed using K2S2O8 initiator. It is interesting to note that copolymers with a high molecular weight of the order 106 could be generated with incorporation of BA at level of 0.76-0.88m from the investigated emulsion polymerization reaction. On the other hand, those copolymers consisting of 0.38-0.65m of BA exhibit low
molecular weight of 4.83-8.72 × 103. However, it is significant to observe that all the copolymers irrespective of composition and molecular weight exhibit narrow polydispersity of 1.39(0.08. It could be argued that polydispersity of copolymers are underestimated in view of the possibility of aggregation among ionic groups. However, in molecular weight estimations using GPC, THF was the preferred solvent. Under the conditions of GPC measurements, the likelihood of aggregation among polymeric particles is ruled out. A close examination of emulsion copolymerization in terms of % yield and copolymer composition (Table 1) shows an interesting information. Under thermal conditions employing K2S2O8 as the initiator, it could be seen that the pattern of incorporation of comonomers follows the feed compositions (Table 1A). The incorporation of ODMAS as high as 87-100% of the feed composition could be achieved over the entire range investigated. However, the incorporation of ODMAS progressively decreases from 100 to 92% on increasing the concentration from 0.12 to 0.26 m. With a further increase in concentration up to 0.70 m, the incorporation of ODMAS is maintained almost at 87-88%. Also it is interesting to observe that the yield of copolymers, maintained at 94-96% for the first four sets of copolymers (ODET1 to ODET4), drops close to about 47% with an increase in incorporation of ODMAS, which reaches a maximum value of 0.62 m. It is well-known that incorporation of comonomers in a polymeric chain as well as kinetics of polymerization are dictated by various factors such as conditions of polymerization reaction and reactivity of the comonomers. The observed behavior of small variations e12% between feed and copolymer composition demonstrates good reactivity of comonomers under the conditions investigated. However, the decrease in yield and molecular weight of the copolymer with the increase in incorporation of ODMAS beyond 0.24 m suggest a lower polymerization propagation rate and promotion of the termination reaction. In view of the occurrence of the polymerization reaction under controlled conditions in the emulsion phase, and the tendency of ODMAS to promote gel-like structures in aqueous medium, under conditions of feed compostion of ODMAS g0.40 m, considerable enhancement in microviscosity of the emulsion phase could be anticipated. This is expected to influence diffusion characteristics of growing polymer chain and might account for reduction in propagation reaction kinetics and promotion of the termination reaction. Under conditions of photopolymerization reaction using the same initiator, K2S2O8 the similar pattern of incorporation of ODMAS in the range of 87-100%, and decrease in molecular weight of polymer from 106-103 with incorporation of ODMAS g 0.24 m was observed (Table 1B). This indicates that varying conditions of polymerization between thermal and photo do not affect monomer reactivity or the polymerization reaction characteristics. The significant and interesting feature of this emulsion polymerization reaction of BA in the presence of ODMAS comonomer is the generation of high molecular weight copolymers with narrow polydispersity index in the range of 1.271.45. The GPC traces of the copolymers ODET1 and ODET4 are presented in Figures 1a and 2a, respectively. It is inferred that the gel forming polymerizable surfactant ODMAS provides controlled conditions, promoting formation of polymers with narrow polydispersity. Of course, it is to be noted that the investigated emulsion polymerization reaction promotes narrow dispersed polymers irrespective of composition of the comonomers in the feed and their incorporation in the copolymer chain. To substantiate this, copolymerization of ODMAS and BA was
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Table 2. (A) Characteristics of Solution Polymerization, Thermal, Solvent: THF, Initiator 1% AIBN, 24 h (B) Characteristics of Solution Polymerization, Photo λ ) 365 nm, Solvent: THF, Initiator 0.1% AIBN, 24 h
system
feed (m) ODMAS BA
ODST1 ODST3 ODST4 ODST5 ODST8
0.12 0.20 0.26 0.40 0.70
0.88 0.80 0.74 0.60 0.30
ODSP1 ODSP3 ODSP4 ODSP5 ODSP8
0.12 0.20 0.26 0.40 0.70
0.88 0.80 0.74 0.60 0.30
copolymer composition (m) ODMAS BA (A) 0.09 0.15 0.19 0.28 0.49 (B) 0.12 0.18 0.24 0.34 0.59
yield (%) Mn/103 Mw/Mn
0.91 0.85 0.81 0.72 0.51
57 54 50 44 37
19.36 18.23 4.54 4.45 4.15
2.91 2.68 2.53 2.61 2.63
0.88 0.82 0.76 0.66 0.41
74 66 62 56 51
5.10 4.82 4.54 4.21 3.89
2.56 2.68 2.88 2.81 2.67
performed through solution polymerization method employing same feed composition and AIBN initiator in thermal and photopolymerization reactions. It is to be noted that K2S2O8 initiator was not employed in solution polymerization reaction in view of its poor solubility in THF. The characteristics of the polymers obtained from solution polymerization method performed under thermal and photopolymerization conditions using 1% and 0.1% AIBN in THF are presented in Table 2, parts A and B. The solution photopolymerization reaction bears a strong resemblance to emulsion polymerization reaction especially in terms of reactivity of the monomers. This is observed from incorporation of ODMAS to an extent of 84-100% of feed composition, similar to those observed in emulsion polymerization reaction. On the contrary, solution polymerization reaction under thermal conditions resulted in comparatively less incorporation of ODMAS in the copolymer as observed from % incorporation of ODMAS in range of 7078% (Table 2A). This suggests that the reactivity of ODMAS and BA are influenced by conditions of solution polymerization reaction. Such differences in reactivity might arise due to various factors such as initiation kinetics, proximity of reacting species and nature of growing polymer chain. However, it is significant to observe that solution polymerization reactions generate polymers with low molecular weight of the order of 103 with an high polydispersity index of 2.53-2.91, irrespective of feed composition. GPC traces of the copolymers ODST1 and ODST4 from solution polymerization reaction are presented in Figures 1b and 2b in comparison with those from emulsion polymerization. The solution polymerization reactions both under photo and thermal conditions do not provide the benefits of controlled reaction conditions observed with emulsion polymerization and thus accounting for high polydispersity index. Furthermore, the % yield of copolymers in solution is consistently much lower compared to emulsion polymerization.
Characteristics of the Latexes. The latexes from emulsion polymerization reaction retained their phase structures throughout the polymerization reaction. The latexes were dialyzed using low molecular weight cut off membranes. The dialysis was performed for about a week with change of water every 24 h so as to ensure complete removal of monomers. The aqueous polymerized latexes after dialysis have been characterized for particle size distribution, zeta potential and surface tension. The characteristics of polymer latexes from thermal polymerization reaction performed using K2S2O8 initiator are presented in Table 3. The particle size distribution of the latexes from all the systems with ODMAS component in range of 0.12-0.30 m remains almost invariant at 74 ( 4 nm with standard deviation (σ) of 0.12-0.22. At the first instance, σ is indicative of narrow polydispersity in particle size distribution supporting the controlled conditions provided by the emulsion phase structures. The small changes in particle size distribution among various latexes indicate that the presence of ODMAS as low as 0.12 m, as in composition ODET1 is able to provide efficient interfacial coverage for the copolymer containing a very high proportion of BA (0.88 m). The surface area/molecule of ODMAS on the latex particle has been estimated to be 0.21 nm2/molecule (Table 3). The surface area of ODMAS, close to the value of a saturated C18 acid, e.g., stearic acid (surface area ∼ 0.20 Å2/molecule), is indicative of close packed structures of ODMAS on the surface of copolymer particle. Considering the closed packed structures of ODMAS provide surface area of 0.21 nm2/molecule, surface area values less than this could only be explained on the basis of adsorption of an estimated fraction (f) of ODMAS component on the latex particle. The fraction (f) is estimated from the ratio of surface area/molecule of ODMAS calculated for the ODMAS component of the copolymer to that providing complete surface coverage. It is observed from Table 3 that f decreases systematically from 1.0 to 0.058 with an increase in ODMAS component. It is significant to note the ratio (R), indicative of ratio of BA and adsorbed ODMAS remains at about 10 ( 3% (Table 3). These values provide an important parameter in terms of molar range for adsorption of ODMAS on the latex particle surface. The increase in the ODMAS component does not contribute fully to its adsorption at the interface and fraction (f) of ODMAS adsorbed on the surface is governed by this ratio. The remaining fraction (1 - f) of ODMAS must be buried inside the latex particle. Considering that ODMAS is a part of copolymer chain as established from copolymer composition, f and 1 - f in this case would essentially mean the conformation of the copolymer chain contributing to retention of ODMAS segment on the latex particle interface and toward the interior of the latex particle. The physical adsorption of the ODMAS is ruled out for the same reasons of copolymer structure and composition. Such a model could also explain other characteristics such as stability and zeta potential values of latexes as discussed below.
Table 3. Characterization of the Latexes, Thermal, Initiator 1% K2S2O8
system
particle size (nm)
zeta potential (mV)
surface tension (mN m-1)
fraction of ODMAS adsorbed (f)
ODET1 ODET2 ODET3 ODET4 ODET5 ODET6 ODET7 ODET8
70 74 75 78 75 74 78 78
-61.9 -64.4 -65.6 -67.0 -63.2 -65.1 -66.5 -64.7
42.8 44.1 44.5 47.9 44.8 45.4 46.1 46.4
1.0 0.65 0.45 0.26 0.14 0.10 0.072 0.058
surface area of ODMAS/ molecule, nm2/ molecule [ODMAS] ) mole fraction × f
ratio of mole fraction ) BA/[ODMAS]
0.21 0.21 0.20 0.20 0.20 0.21 0.21 0.21
7.33 8.72 10.12 12.25 13.26 12.72 12.90 10.55
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Figure 1. GPC chromatogram of the copolymers (a) ODET1, from emulsion polymerization, (thermal) and (b) ODST1 from solution polymerization, (thermal).
Figure 2. GPC chromatogram of the copolymers (a) ODET4, from emulsion polymerization, (thermal) (b) ODST4 from solution polymerization, (thermal).
The adsorption of the reactive surfactant ODMAS on the surface of the particle is investigated from the zeta potential estimations, which gives an idea of the charge characteristics at the interface. All latexes exhibit negative zeta potential values in the range of -61.9 to -67.0 mV (Table 3). The zeta potential of neat ODMAS dispersion was -90.0 mV. The decrease in zeta potential from -90.0 to -61.9 mV for the polymerized latexes suggests that adsorption of ODMAS on the surface of the copolymer consisting of BA, a neutral monomer. In accordance with observation from particle size estimations of the latexes, the zeta potential values do not change significantly with the change in concentration of BA and ODMAS, suggesting complete coverage provided by fractional (f) adsorption of ODMAS on the particle surface in all compositions as discussed above. The surface tension of the polymerized latexes is estimated to be in the range of 42.8-47.9 mN m-1 (Table 3). It could be seen that the neat ODMAS dispersion exhibits surface tension around 44 mN m-1. The surface tension of the latexes close to that of neat ODMAS dispersion, further supports, complete coverage of ODMAS on the surface of the latex. All latexes remain stable for about 5 months. The stability of the latexes over a period of 5 months is established from visual appearance, transmittance at 600 nm, zeta potential, and surface tension measurements. Negligible changes in transmittance at 600 nm, zeta potential, and surface tension values after a period of 5 months indicate stability of latexes. This must be due to complete coverage provided by ODMAS, which prevents the coagulation of the particles. The coverage provided by the fraction (f) of ODMAS component maintainting the ratio (R) of BA/f ODMAS in range of 10 ( 3 is able to provide stability to the latex particle. The latexes obtained from photopolymerization using K2S2O8 initiator exhibited almost similar characteristics. Thermal Characteristics of the Copolymers of ODMAS and BA from Emulsion Polymerization Reaction. The thermal
characteristics of the copolymers of ODMAS and BA obtained from emulsion polymerization have been investigated from DSC and TG analysis. For this, three typical copolymers ODET1, ODET4, and ODET5 consisting of 0.12, 0.24, and 0.35 m ODMAS were chosen for the reason of wide coverage of copolymers. The thermal characteristics of copolymers have been investigated in comparison to PBA homopolymer in order to draw information on the influence of ODMAS component on the thermal behavior. The PBA homopolymer (mol. wt. 2.56 × 105) used in thermal studies was synthesized in this laboratory from emulsion polymerization method using SDS surfactant. From the DSC trace (Figure 3a), Tg of PBA is estimated to be -55 °C and those of copolymers (Figure 3 b-d) at -36.3 ( 0.6 °C. It is significant to note in the first place that Tg of the copolymer remains almost invariant with incorporation of ODMAS in the copolymer chain. Also, Tg of the copolymers are higher in comparison to the homopolymer, PBA. The higher Tg suggests greater rigidity of the copolymer chain. The comonomer, ODMAS, in the copolymer is expected to provide flexibility to the copolymer in view of C18 side chain. However, it is to be noted that ODMAS also consists of ionic functional group viz. COO-Na+ and this is expected to promote rigidity due to intermolecular association. The competitive effects of flexibility due to C18 chain and rigidity from ionic group are expected to decide on the ultimate mobility. In case of copolymers, it is observed from Tg values that the intermolecular interaction due to ionic site is predominant and this outweighs the flexibility provided by the C18 side chain thus accounting for higher Tg. In line with this, increase in Tg with ODMAS content of the copolymers could be anticipated. However, manipulation of opposite effects of rigidity and flexibility provided by ODMAS component in the copolymer chain might account for the negligible change in estimated Tg. All copolymers exhibit series of small endothermic transitions at different temperatures up to
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Figure 3. DSC traces of copolymers (a) PBA, (-100-500 °C) (b) ODET1, (c) ODET4, and (d) ODET5, from emulsion polymerization, (thermal) Temp. -50 to +500 °C.
Figure 4. TGA thermograms of copolymers (a) PBA, (b) ODET1, (c) ODET4, and (d) ODET5, from emulsion polymerization, (thermal) Temp. 30 to 700 °C.
about 120 °C. The small weight loss associated with these transitions as observed from TGA traces (Figure 4 b-d) suggest
that these transitions might arise from the melting of the flexible C18 chain. The DSC curves (Figure 3) show that onset temperature
Octadecyl Maleamic Acid Salt As a Microreactor
for the major transition occurs around 245-250 °C for the copolymers and that for PBA at 310 °C. The homopolymer PBA (Figure 4a) undergoes single stage decomposition in contrast to the copolymers exhibiting twostage decomposition as influenced by the copolymer composition. The onset temperature for 1st stage decomposition occurs around 254 °C for ODET1 (0.12 m ODMAS), which decreases down to 192 °C for ODET5 (0.35 m ODMAS). It is significant to note that weight loss to an extent of 34% is observed for ODET5 during 1st stage. These results suggest that the 1st sage decomposition process may be due to transformations associated with decarboxylation. It is observed that the copolymer ODET1 (Figure 4b) exhibits 52 and 88% weight loss at 400 and 485 °C, respectively, with the residual weight of 9.9% at 700 °C. The copolymer ODET5 (Figure 4d) exhibits 44 and 74% weight loss at 400 and 485 °C, respectively, with the residual weight of 23.1% at 700 °C. On the contrary, the homopolymer, PBA, exhibits weight loss of 52% at 401 °C, undergoing maximum weight loss of 93% at 485 °C with the residual weight of 6.2% at 700 °C. The trend of increase in residual weight with an increase in ODMAS component which reaches maximum value of 23% with ODET5 copolymer suggests scope for some chemical transformations promoted due to ODMAS component in addition to decomposition process. Of course detailed studies on the characterization of the compounds evolved at various stages of decomposition are required to confirm these decomposition processes.
Langmuir, Vol. 22, No. 6, 2006 2801
Conclusion The polymerizable surfactant derived from N-octadecyl maleamic acid exhibiting hydrogelation phenomenon has been shown to act as an organized reactor for controlled polymerization reactions generating functionalized polybutyl acrylate copolymers with a narrow molecular weight and size distribution. Depending on the concentration of ODMAS in the feed, high molecular weight polymers in the range of 1.11 to 1.25 × 106 were obtained. It is significant to observe that the copolymers irrespective of composition and molecular weight exhibit narrow polydispersity index of 1.39 ( 0.08. In contrast to emulsion polymerization, the solution copolymerization reactions resulted in oligomers with a broad distribution, (PDI ) 2.56-2.88) with an average molecular weight of order of 103. The polymerized latexes exhibit zeta potential and surface tension in range of -61.9 to -67.0 mV and 42.8 to 47.9 mN m-1, respectively, indicating complete surface coverage by ODMAS segments. The negligible changes in these values even after 5 months indicate the stability of the latexes. Acknowledgment. The authors thank Dr. T. Ramasami, Director, CLRI, India for his encouragement and permission to publish the work. The support of Dr. B. S. R. Reddy, Deputy Director, CLRI, India is acknowledged. L.J.M.G. thanks CSIR, India for fellowship. The authors wish to thank DST, India for the financial assistance through grant SR/S1/PC14/2004. LA052518S
