Langmuir 2009, 25, 873-880
873
Maleimide Immobilized on a PE Surface: Preparation, Characterization and Application as a Free-Radical Photoinitiator Rafael Silva, Edvani C. Muniz, and Adley F. Rubira* Grupo de Materiais Polime´ricos e Compo´sitos, Departamento de Quı´mica, UniVersidade Estadual de Maringa´, AVenida Colombo 5790, CEP: 87020-900 - Maringa´, PR, Brazil ReceiVed July 18, 2008. ReVised Manuscript ReceiVed September 25, 2008 Maleimide groups are synthesized on the surface of PE films by means of two different routes, from oxidized and anhydride-grafted PE films. Maleimide groups covalently bonded to the surface of PE film were used as photopolymerization initiators of acrylic acid (AA) and glycidyl methacrylate (GMA). The formation of poly (acrylic acid) (PAA) and poly (glycidyl methacrylate) (PGMA) covalently bonded to the modified PE film surface by the photopolymerization process was demonstrated by ATR-FTIR, gravimetry, and SEM results. The thickness of the polymer layers formed in the polymerization depends on the irradiation time. The PAA layer formed in the polymerization is thinner than 250 nm, whereas that of PGMA in some case is thicker than 3 µm.
Introduction The development of anchored thin polymer layers covalently bonded to the surface of solid materials promises to improve the features of substrate materials by changing the interface behavior. A strong interaction between the thin polymer layer and the substrate material achieved by covalent bonding ensures the stability of the system to solvent exposure and heating to temperatures above either the glass transition or melting point.1-4 Methods of polymerization initiation on a substrate material surface, called “grafting-from” methods, are promising approaches for the preparation of covalently attached polymer monolayers.5-7 Anionic, cationic, living, and free-radical polymerization have been successfully used to synthesize polymer layers tethered to the surface of solid substrates. The degree of grafting is proportional to the density of the initiator on the substrate surface in addition to other factors such as the polymerization time, monomer, concentration and UV intensity. Brushlike layers are obtained when the average distance between the initiator groups on the substrate surface is shorter than the gyration radii of the attached polymer. The polymer chain is oriented perpendicularly to the substrate surface in brushlike layers due to the excluded volume effect.8 Free-radical polymerization is a convenient technique because the reaction is insensitive to moisture and tolerates a large variety of organic functional groups. Free-radical initiators, similar in structure to 2,2′-azobisisobutyronitrile (AIBN), immobilized on the surface of polymeric materials have been used to synthesize grafted polymer films.9-12 Study results show that the grafting* To whom correspondence should be addressed. E-mail: afrubira@ uem.br. Phone: +55 44 3261 4332. Fax: +55 44 3261 4125. (1) Kingshott, P.; Wei, J.; Ravn, D. B.; Gadegaard, N.; Gram, L. Langmuir 2003, 19, 6912–6921. (2) Iyer, K. S.; Luzinov, I. Macromolecules 2004, 37, 9538–9545. (3) Iyer, K. S.; Zdyrko, B.; Malz, H.; Pionteck, J.; Luzinov, I. Macromolecules 2003, 36, 6519–6526. (4) Walters, K. B.; Hirt, D. E. Polymer 2006, 47, 6567–6574. (5) Prucker, O.; Naumann, C. A.; Ru¨he, J.; Knoll, W.; Frank, C. W. J. Am. Chem. Soc. 1999, 121, 8766–8770. (6) Zhang, C.; Luo, N.; Hirt, D. E. Langmuir 2006, 22, 6851–6857. (7) Luo, N.; Hutchison, J. B.; Anseth, K. S.; Bowman, C. N. Macromolecules 2002, 35, 2487–2493. (8) Harris, B. P.; Metters, A. T. Macromolecules 2006, 39, 2764–2772. (9) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592–601. (10) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 602–613. (11) Prucker, O.; Ru¨he, J. Langmuir 1998, 14, 6893–6898.
Scheme 1. Proposed Initiation Mechanism for Maleimides18
from technique by free-radical polymerization may be used to prepare polymer layers with controlled high graft densities. Maleimide groups are a special class of vinyl monomers with strong electron-accepting ability due to the two carbonyl groups attached to the CdC bond. Maleimides have promising characteristics in photopolymerization, being able to act as initiators (Scheme 1).13-18 Maleimides may be excited by UV radiation. Excited maleimide initiates free-radical polymerization through a hydrogen-atom abstraction process. The presence of hydrogenatom donors is essential to initiate polymerization. In the presence of hydrogen atom donors, radicals are produced through an electron-transfer/proton-transfer reaction sequence. Both direct abstraction and electron/proton transfer result in the formation of two radicals capable of initiating polymerization: a succinimide radical on the maleimide residue, and a radical on the hydrogen atom donor. In the absence of hydrogen-atom donor species, hydrogen-atom abstraction may occur by an intermolecular process.19-23 The immobilization of maleimide groups on the surface of solid materials may be used to initiate the polymerization of (12) Biesalski, M.; Ru¨he, J. Macromolecules 1999, 32, 2309–2316. (13) Hoyle, C. E.; Viswanathan, K.; Clark, S. C.; Miller, C. W.; Nguyen, C.; Jo¨nsson, S.; Shao, L. Macromolecules 1999, 32, 2793–2795. (14) Nguyen, C. K.; Hoyle, C. E.; Lee, T. Y.; Jo¨nsson, S. Eur. Polym. J. 2007, 43, 172–177. (15) Senyurt, A. F.; Hoyle, C. E. Eur. Polym. J. 2006, 42, 3133–3139. (16) Andersson, H.; Gedde, U. W.; Hult, A. Macromolecules 1996, 29, 1649– 1654. (17) Oishi, T.; Kagawa, K.; Nagata, H. Polymer 1997, 38, 1461–1469. (18) Miller, C. W.; Jo¨nsson, S.; Hoyle, C. E.; Viswanathan, K.; Valente, E. J. J. Phys. Chem. B 2001, 105, 2707–2717. (19) Wang, Y.; Yang, W. Langmuir 2004, 20, 6225–6231. (20) Rahane, S. B.; Metters, T.; Kilbey, S. M. II. Macromolecules 2006, 39, 8987–8991. (21) Kohli, P.; Scranton, A. B.; Blanchard, G. J. Macromolecules 1998, 31, 5681–5689. (22) Wang, H.; Wei, J.; Jiang, X.; Yin, J. Polymer 2006, 47, 4967–4975. (23) Hoyle, C. E.; Clark, S. C.; Jo¨nsson, S.; Shimose, M. Polymer 1997, 38, 5695–5697.
10.1021/la802309u CCC: $40.75 2009 American Chemical Society Published on Web 12/18/2008
874 Langmuir, Vol. 25, No. 2, 2009
acrylic monomers and obtain a covalently bonded surface polymer layer. Maleimide photoinitiators are advantageous for the fabrication of these interfacial layers because they are lightmediated, which permits polymerization to be carried out at room temperature with ready spatial and temporal control over layer growth. The present work reports the unpublished synthesis of maleimide groups on the surface of low-density polyethylene and the photopolymerization of two different acrylic monomers, acrylic acid, and glycidyl methacrylate from surface-immobilized maleimide groups. Polymer thin layer grafted covalently to solid substrates are of great importance owing to their potential applications in food packaging, lithography, microelectronics, and design of corrosion-resistant and biocompatible materials.24 Of the various methods by which polymer brushes can be made, the grafting from approach using controlled (free) radical polymerizations has become perhaps the most widely practiced.25
Experimental Section Materials. Low-density polyethylene (LDPE) films with thickness between 150 and 250 µm were supplied by Poliolefinas Co. (Brazil). The films were cut to 2 × 1 cm and washed with acetone for 24 h in Soxhlet. Additional compounds used here were sulfuric acid (Mallinckrodt, 97.5%), maleic anhydride (Vetec, 99%), benzoyl peroxide (Riedel-de Haen, 98%), glycidyl methacrylate (Aldrich, 97%), ethylenediamine (Acros, 99%), acrylic acid (99.5%), potassium dichromate (Nuclear, 99%), nitric acid (Synth, 66%), acetic anhydride (Vetec, 97%), chloroform (Merck, 99%) and tethahydrofuran (Acros, 99.99%), which were used as received. Diethyl ether (Fmaia, 98%) was distilled from sodium metal. Chemical Oxidation with Chromic Acid (1). LDPE films were submersed for 5 min in a chromic acid solution prepared with 5 g of potassium dichromate (K2Cr2O7) in 20 g (∼21 mL) of concentrated sulfuric acid (H2SO4) at 70 °C. The films were removed from the solution with tweezers and rinsed three times in distilled water, immersed in nitric acid at 50 °C for 10 min to remove inorganic residues formed by the oxidation process, and rinsed in distilled water extensively. Maleic Anhydride Grafting (2). The radical grafting process was carried out in a mixture of 80 mL acetic anhydride and 4 g maleic anhydride. LDPE films were placed in this solution and the temperature was maintained constant at 100 °C. The amount of 0.22 g of benzoyl peroxide (BPO) was added to the mixture as a free-radical source. LDPE films stayed in solution for 6 h. In the sequence, the films were removed from the solution, immersed in acetone for 12 h, and rinsed in water 3 times. Ethylenediamine Anchoring on Carboxylic Acid-Functionalized Surface (3). Oxidized PE films were immersed in a solution with 2 g of phosphorus pentachloride (PCl5) and 30 mL of dry diethyl ether in nitrogen atmosphere for 2 h at room temperature (∼25 °C). Subsequently, the films were removed from the PCl5 solution and immersed in ethylenediamine (20 mL) for 1 h at room temperature (∼25 °C). Then, the film was removed from the solution and washed with acetone. Afterward, the films were put in a Soxhlet extractor for 10 h (∼50 cycles) with water to eliminate the ethylenediamine not bonded to the oxidized PE film surface. Ethylenediamine Anchoring on Anhydride-Functionalized Surface (4). Anhydride maleic-grafted PE films were immersed in 50 mL of ethylenediamine for 5 h at 100 °C. Then the films were removed from the solution and washed with acetone. Afterward, the films were put in a Soxhlet extractor for 10 h (∼50 cycles) with water to eliminate the ethylenediamine not bonded to the grafted PE film. (24) Silva, R.; Muniz, E. C.; Rubira, A. F. Polymer 2008, 49, 4066–4075. (25) Rahane, S. B.; Metters, A. T.; Kilbey, S. M. Macromolecules 2006, 39, 8987–8991.
SilVa et al. Reaction of Maleic Anhydride with the Amine-Functionalized Surface (5). Samples of amine-functionalized films were immersed in a solution of 2 g of maleic anhydride and 30 mL of chloroform. The system was maintained under reflux for 24 h. Maleimide Formation through Cyclization Reaction (6). After the reaction of maleic anhydride with the amine-functionalized surface, the samples were maintained immersed in 30 mL of acetic anhydride and 0.1 g of sodium acetate at 60 °C for 24 h to perform the cyclization reaction. The chemical reactions proposed to prepare supported maleimide can be seen in Scheme 2. PAA and PGMA Polymerization Initiated by Maleimides Immobilized on a PE Surface. The maleimide-functionalized films were put in a quartz cuvette with an internal width of 0.1 cm. The cuvette containing the maleimide-functionalized film was filled with the monomer solution and closed. Monomer solutions of acrylic acid (AA) and glycidyl methacrylate (GMA) were prepared with tetrahydrofuran (THF). The AA solution was prepared in the concentration of 10% (v/v) (1.39 mol/L) and the GMA solution was prepared in the concentration of 20% (v/v) (1.14 mol/L). The cuvette with the maleimide-functionalized polymeric film and the monomer solution was exposed to polychromatic radiation provided by a high pressure 250 W mercury lamp without the glass bulb. The cuvette was placed 40 cm away from the lamp. After the photopolymerization process, the PAA-modified films were washed in an ultrasonic bath with water and the PGMA-modified films were washed with acetone to remove the monomers and materials physically adsorbed. Characterization. The chemical characterization was performed by Attenuated Total Reflection Fourier Transform Infrared (ATRFTIR) in a Bomem model MB-100 apparatus with a Pike MIRacle ATR accessory at an incident angle of 45° and a ZnSe crystal and nitrogen purging. Scanning electron microscopy (SEM) on SHIMADZU SS-550 was used for morphological characterization. The thicknesses of the polymer layers formed in the photopolymerization process were estimated by the increase in substrate mass with the equation:
Layer Thickness )
∆w Ad
(1)
where ∆w is the increment in the substrate mass in grams (g) in the photopolymerization process. ∆w was obtained by weighing the sample in an analytical balance AA 200 DS (sensitivity 0.02 mg, Denver Instrument Co.), and A is the film surface area. It was determined by measuring the dimensions of the substrate (a rectangle area, side x height) with a caliper (accuracy of 0.05 mm) considering the film perfectly flat. The density of the polymers formed in the photopolymerization process is represented by d. The values of density used were 1.250 g/cm3 for poly (acrylic acid) and 1.080 g/cm3 for poly (glycidyl methacrylate). After the photopolymerization process, the samples were washed in an ultrasonic bath with each suitable solvent to remove the monomers and materials that were physically adsorbed. The washed samples were dried with a N2 flow and maintained under a vacuum atmosphere (vacuum desiccator) during a week prior to the final weight determination.
Results and Discussion PE Functionalization Procedure. The PE surface functionalization was accomplished by two different methods. In the first method, the PE surface was chemically oxidized with chromic acid.26-32 Chromic acid is generated in chromate or dichromate (26) Rasmussen, J. R.; Stedronsky, E. R.; Whitesides, G. M. J. Am. Chem. Soc. 1977, 99, 4736–4745. (27) Rasmussen, J. R.; Bergbreiter, D. E.; Whitesides, G. M. J. Am. Chem. Soc. 1977, 99, 4746–4756. (28) HolmesFarley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides, G. M. Langmuir 1985, 1, 725–740. (29) HolmesFarley, S. R.; Whitesides, G. M. Langmuir 1987, 3, 62–76. (30) HolmesFarley, S. R.; Reamey, R. H.; Nuzzo, R.; McCarthy, T. J.; Whitesides, G. M. Langmuir 1987, 3, 799–815.
Maleimide Immobilized on Polyethylene Surface
Langmuir, Vol. 25, No. 2, 2009 875
Scheme 2. Schematic Representation of the Chemical Reaction Proposed for the Synthesis of Supported Maleimide Groups on a PE Surface
salt acidic solution and it is a powerful oxidant capable of oxidizing hydrocarbons. The formation of alcohols, aldehydes, ketones, and carboxylic acids is expected in the oxidation of hydrocarbons. However, the selective formation of carboxylic acid occurs in the oxidation of PE with chromic acid. In the ATR-FTIR spectrum of the oxidized PE, Figure 1, it can be verified a signal at 1713 cm-1 attributed to the carbonyl stretching absorption band not present in the virgin PE spectrum. This signal shifts almost entirely to 1565 cm-1 when oxidized PE is treated with 1 mol/L NaOH at 50 °C for 2 h. The signal at 1565 cm-1 is due to the carbonyl stretching absorption band of the carboxylate. The little signal remaining at 1713 cm-1 may be attributed to the presence of aldehyde and ketone groups. The presence of alcohol groups cannot be verified by the absence of hydroxyl vibration modes. The second functionalization method used was radicalar grafting of maleic anhydride on PE. Maleic anhydride attaches to the PE surface in the form of succinic anhydride, as shown in Scheme 1.33,34 The anhydride-functionalized surface was (31) HolmesFarley, S. R.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 921–931. (32) HolmesFarley, S. R.; Whitesides, G. M Langmuir 1986, 2, 266–281. (33) Heinen, W.; Rosenmoller, C. H.; Wenzel, C. B.; deGroot, H. G. M.; Lugtenburg, J.; vanDuin, M. Macromolecules 1996, 29, 1151–1157.
characterized (Figure 2) by the presence of anhydride peaks in the ATR-FTIR spectrum at 1785 and 1720 cm-1 due to the asymmetric and symmetric carbonyl stretched modes, and the signals at 1184 and 918 cm-1 appear in the spectrum as a result of C-C(dO)-O-C(dO)-C stretching vibration modes of cyclic anhydrides. The reactivity of anhydride groups was tested by treating the sample with 1 mol/L NaOH at 50 °C for 2 h. The treatment of the maleic anhydride-functionalized film with a basic solution promotes the incomplete hydrolysis of the anhydride groups and the partial neutralization of the carboxylic acid formed in the hydrolysis, as shown in the ATR-FTIR spectrum (Figure 2) of the film after the treatment with the basic solution. The intensities of the bands at 1785, 1184, and 918 cm-1 attributed to anhydride peaks decreased, whereas the signal at 1720 cm-1 hardly changed due to the CdO stretching of the carboxylic acid from the anhydride hydrolysis overlapped with remaining symmetric carbonyl stretched mode signal of anhydride groups. At 1576 cm-1 appeared a more intense signal due the CdO stretching of the carboxylate from the neutralization of the carboxylic acid formed in the hydrolysis. The reaction of the anhydride groups with a concentrated NaOH solution is a (34) Russell, K. E. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 555–561.
876 Langmuir, Vol. 25, No. 2, 2009
SilVa et al.
Figure 1. ATR-FTIR spectra of (a) virgin PE, (b) oxidized PE, and (c) oxidized PE treated with basic solution.
Figure 3. ATR-FTIR spectra of (a) carboxylic acid-functionalized PE (PEoxi), (b) amine-functionalized PE (POE), (c) PE after reaction with maleic anhydride, and (d) maleimide-functionalized PE (POEMI).
Figure 2. ATR-FTIR spectra of (a) virgin PE, (b) PE grafted by radicalar grafting of maleic anhydride, and (c) PE grafted by radicalar grafting of maleic anhydride treated with basic solution.
Figure 4. ATR-FTIR spectra of (a) anhydride-functionalized PE (PEenx), (b) amine-functionalized PE (PEE), (c) after reaction with maleic anhydride PE, and (d) maleimide-functionalized PE (PEEMI).
quantitative reaction. The incomplete conversion of the anhydride with the basic solution is associated with the presence of anhydride groups in regions not accessible to the basic solution. The functionalization of the PE films by heterogeneous reaction generated a 3D spatial distribution of functional groups in the surface region.27 The concentration of the functional groups was maximum on the upper surface and decreased from the surface to the bulk, where the concentration approached zero. The penetration of the aqueous solution in contact with PE, a very hydrophobic polymer, into the polymer film depends on the concentration of the functional groups and it was limited to the upper surface region. Consequently, functional groups buried in the surface region with a low concentration of functional groups, a hydrophobic region, were not accessible to the aqueous solution. The groups not accessible to aqueous solution are sensitive to ATR-FTIR. Infrared radiation penetrates approximately one quarter of the wavelength of the observing light (1200 nm at 1700 cm-1).26 Ethylenediamine Anchoring on Carboxylic Acid-Functionalized Surface. The anchoring of ethylenediamine on the carboxylic acid- and anhydride surface-functionalized films was
used to obtain an amine-functionalized surface. To anchor ethylenediamine to the oxidized films, the carboxylic acid groups were first converted to acyl chloride groups through reaction with a PCl5 solution because acyl chloride is more reactive than carboxylic acid is in relation to amide formation. To minimize hydrolysis by ambient water vapor, the films treated with the PCl5 solution were immediately immersed in ethylenediamine under inert atmosphere. The ATR-FTIR spectrum of the film after treatment with ethylenediamine and submitted to extraction to remove the ethylenediamine not bonded to the film is shown in part b of Figure 3. It shows the decrease in the carboxylic acid signal at 1713 cm-1. The formation of the amine-functionalized surface may be verified by the presence of a broadband attributed to primary amine N-H stretching at 1587 cm-1 and amide group carbonyl stretching at 1637 cm-1. Ethylenediamine Anchoring on Anhydride-Functionalized Surface. The anhydride-functionalized surface was used without previous treatment to prepare the amine-functionalized surface by direct reaction of the anhydride groups with ethylenediamine. The reaction between a cyclic anhydride and a primary amine yields a product with both an amide and an amine salt. In the
Maleimide Immobilized on Polyethylene Surface
Figure 5. ATR-FTIR spectra of PAA photopolymerized on POMI in different irradiation times: (a) 15, (b) 30, (c) 45, and (d) 60 min.
Figure 6. ATR-FTIR spectra of PAA photopolymerized on PEMI in different irradiation times: (a) 15, (b) 30, (c) 45, and (d) 60 min.
Langmuir, Vol. 25, No. 2, 2009 877
Figure 8. ATR-FTIR spectra of PGMA photopolymerized on POEMI in different irradiation times: (a) 15, (b) 30, (c) 45, (d) 60, and (e) 90 min.
Figure 9. ATR-FTIR spectra of PGMA photopolymerized on PEEMI in different irradiation times: (a) 15, (b) 30, (c) 45, (d) 60, and (e) 90 min.
Figure 7. Thickness of PAA layer as a function of irradiation time.
extraction process with water, the aminium ions are removed and the carboxylate are converted into carboxylic acid groups. In the ATR-FTIR spectrum of the film after extraction, part b of Figure 4, it may be seen that most anhydride groups were consumed in the reaction, which is evidenced by the absence of the peak at 1785 cm-1 produced by the asymmetric carbonyl
Figure 10. Thickness of PGMA layer as a function of irradiation time.
stretching of the anhydride groups. The amine-functionalized surface is characterized by the presence of N-H bending vibration of primary amine at 1586 cm-1. The amide formation is confirmed by the presence of the signal at 1667 cm-1 attributed to amide
878 Langmuir, Vol. 25, No. 2, 2009
SilVa et al.
Figure 11. SEM micrographs of (a) POEMI and of (b) PEEMI films.
Figure 12. SEM micrographs of (a) POEMI and (b) PEEMI films after the polymerization of PAA with 90 min of irradiation time.
carbonyl stretching. Carboxylate is converted to carboxylic acid and the amine salt formed in the reaction is removed during the extraction with water, which justifies the presence of a signal at 1720 cm-1 attributed to the carboxylic acid carbonyl stretching (part b of Figure 4). Reaction of Maleic Anhydride with the Amine-Functionalized Surface. Maleic anhydride may react with the aminefunctionalized surface by breaking the anhydride group and forming amide and carboxylic acid. The amine-functionalized surface was immersed in a solution of maleic anhydride in chloroform under reflux for 24 h. In the next step, the samples were washed several times with water to remove reaction residues. The accomplishment of this reaction may be verified by the changes in the intensity of the amine, amide, and carboxylic acid group signals. The ATR-FTIR spectra of the film after reaction with maleic anhydride are shown in part c of Figure 3 and in part c of Figure 4. Comparing these spectra with those of aminefunctionalized films, part b of Figure 3 and part b of Figure 4, it can be noted that there is a decrease in the signal at 1587 cm-1 attributed to the amine groups. This fact indicates that the amine groups were consumed during the reaction. Two new peaks appeared in the film ATR spectra after reaction, one at 1715 cm-1 and a shoulder at 1635 cm-1 attributed to carboxylic acid groups and amide, respectively. Maleimide Formation through Cyclization Reaction. Cyclization is a dehydration reaction that occurs at high temperature. It may occur at low temperature only by the action of a catalyst. The maleimide ring is formed in acetic anhydride using sodium acetate as a catalyst.35 The change in surface species after the cyclization promoted changes in the film ATR-IR spectra, part (35) Zhang, X.; Li, Z.-C.; Li, K.-B.; Lin, S.; Du, F.-S.; Li, F.-M. Prog. Polym. Sci. 2006, 31, 893–948.
d of Figure 3 and part d of 4, such as the decrease in the shoulder of amide at 1635 cm-1 and the decrease in the signal of carboxylic acid at 1715 cm-1. The decrease in the signal in the amide groups at 1635 cm-1 is observed in the spectra of both samples; however, the decrease in the signal of the carboxylic acid in the region of 1715 cm-1 is not very clear due to the appearance of the maleimide group signal in the same region.36 PAA and PGMA Polymerization Initiated by SurfaceImmobilized Maleimides. Maleimide groups generated on the surface of PE films through two different routes were used as photopolymerization initiators. The modified films were placed in contact with the monomer solutions (PAA and PGMA) and irradiated. At the end of the irradiation process, the films were washed in ultrasonic bath with suitable solvents to remove the monomers and materials physically adsorbed on the surface completely. Figures 5 and 6 show ATR-FTIR spectra of films modified with maleimide groups and immersed in acrylic acid solution after different irradiation times. The signals at 1717, 1246, and 1166 cm-1 are characteristic of poly (acrylic acid) and increase as the irradiation time increases. The CdC vibration modes are characteristic of acrylic acid, and monomer is not present. No difference is observable between the films prepared by the different routes. In Figure 7, the thickness of the PAA layer formed on the surface of the modified films is plotted as a function of the irradiation time. The PAA layer thickness increases with the increase in irradiation time for both initiators; however, the increase in the PAA layer thickness as a function of irradiation time is more pronounced for the maleimide-functionalized surface prepared from the anhydride-functionalized surface. The polymerization with the initiator synthesized from the anhydride-
Maleimide Immobilized on Polyethylene Surface
Langmuir, Vol. 25, No. 2, 2009 879
Figure 13. SEM micrographs of (a and b) POEMI and (c and d) PEEMI films after the polymerization of PGMA with 90 min of irradiation time.
Figure 14. SEM micrographs of fractures of (a and b) POEMI film and of (c and d) PEEMI film after the polymerization of PGMA with 90 min of irradiation time.
functionalized surface generates a PAA layer thicker than that obtained with the initiator synthesized from the carboxylic acidfunctionalized surface for any irradiation time.
The ATR-IR spectra of the films modified with the initiator groups immersed in GMA solution after exposure to the irradiation source are shown in Figures 8 and 9. The amount of polymer
880 Langmuir, Vol. 25, No. 2, 2009
prepared on the surface of the functionalized films is higher for the PGMA polymerization than for the PAA polymerization, as demonstrated by the intensity of the characteristic PGMA signals, mainly at 1726 and 1141 cm-1. As in the polymerization of PAA, the intensity of the PGMA signals increases when the irradiation time is increased. After 60 min of irradiation, the signals present are attributed only to PGMA, indicating that the PGMA layer formed on the surface of the PE-modified film is thicker than the deep penetration of the ATR-IR system. The average value of deep penetration of the ATR-FTIR system estimated for the PE-modified films is approximately 1.2 µm. The thicknesses of the PGMA layers polymerized on the surface of the films modified in different irradiation times are shown in Figure 10. The results obtained from the ATR-IR spectra are confirmed. The photopolymerization of PGMA is more efficient than the photopolymerization of PAA. In high irradiation time, the PGMA layer was thicker than 1 µm, whereas the maximum value for PAA was less than 250 nm. This difference may be due to differences in the free-radical propagation rate constant, which has a direct influence on the degree of polymerization.37 The propagation rate constant of PAA should be smaller than that of PGMA due to the existence of strong intermolecular interaction, hydrogen bonds, among acrylic acid molecules, and the resulting decrease in the frequency factor (A).38 Another difference is the size of the radicals formed in the first few additions steps, which are longer in the PGMA polymerization than in the PAA polymerization. The activation energy of the first addition step is significantly higher than that of (long-chain) propagation.39 By comparing the results of both modified surfaces, a behavior similar to that observed for PAA photopolymerization is noticed. The polymerization promoted by the maleimide-functionalized surface synthesized from the anhydride-functionalized surface provided a thicker polymeric layer than that obtained with the maleimide-functionalized surface obtained from the carboxylic acid-functionalized surface. A higher density of maleimide groups on PEEMI than on POEMI is possible because of the difference in thickness of the polymeric layers, and the lower density of maleimide groups on POEMI may be attributed to the formation of the amine-functionalized surface in a two-step reaction, whereas the formation of the amine-functionalized surface from the anhydride-functionalized surface occurs in a single-step reaction. The SEM micrographs of the maleimide-functionalized PE films are presented in Figure 11. As can be seen, the surface of the films functionalized with maleimide groups is porous, and the pores were formed during the entire route proposed for the synthesis of the maleimide groups (Scheme 2). The polymerization of PAA changed the morphologies of the maleimide-functionalized films, as seen in Figure 12. The PAA formed on the POEMI film, part a of Figure 12, grew on the pore surfaces, causing a decrease in pore size and a change in pore format. The thickness of the PAA layer formed on the POEMI film with 90 min irradiation and determined gravimetrically was around only 50 nm. The thickness of the PAA layer and the SEM micrograph are in good agreement. As can be seen, the PAA (36) Ishida, H.; Ohba, S. Polymer 2005, 46, 5588–5595. (37) Gridnev, A. A.; Ittel, S. D. Macromolecules 1996, 29, 5864–5874. (38) Heuts, J. P. A.; Russell, G. T. Eur. Polym. J. 2006, 42, 3–20. (39) Thickett, S. C.; Gilbert, R. G. Polymer 2004, 45, 6993–6999.
SilVa et al.
layer is not enough to cover the POEMI film pores. The morphology of the PEEMI film after the polymerization of PAA can be visualized in part b of Figure 12. The PEEMI film pores are almost covered by the PAA layer formed in the photopolymerization with 90 min of irradiation. For this sample, the PAA thickness is around 225 nm, as determined by gravimetry. The polymerization of PGMA on maleimide-functionalized films is verified by the formation of uniform structures that covered the surface of the films entirely, as shown in Figure 13. The PGMA structure formed on the PEEMI film was more compact than the one formed on the POEMI film. The PGMA layers on the POEMI and PEEMI films, also formed in the photopolymerization in 90 min irradiation, were 1.5 and 3.4 µm thick, respectively; these values of thickness were obtained from the gravimetric analyses. In the Figure 14 are presented SEM images of fractures of POEMI and PEEMI films after the polymerization of PGMA. Through fractures of images of films it was possible to visualize the features of the PGMA layer on the PE films. The thickness of PGMA layers is around 4.0 µm for both samples POEMI and PEEMI. However, the PGMA layer polymerized on the PEEMI is more compact and regular than the PGMA layer polymerized on the POEMI. The thickness observed in the SEM images of fractures of films is in agreement with the values determined by gravimetric analyses only for the PEEMI films, 4.0 and 3.4 µm, respectively. For the POEMI films, a larger discrepancy occurs with a value of 1.5 µm, as determined by gravimetric analyses, and 4.0 µm observed by SEM images of the fractures of the film. The lower value obtained in the gravimetric analyses may be attributed mainly to irregularities in the PGMA layer (no compact layer) polymerized on the surface of the films.
Conclusions The synthesis of maleimide groups on the surface of PE was successfully accomplished by the two routes proposed in this work, from oxidized PE and anhydride-grafted PE films. The successful synthesis of maleimide groups on the surface of PE was confirmed by ATR-FTIR data and corroborated by the polymerization of acrylic monomers on the PE surface. The amount of polymers prepared in the photopolymerization increased with the increase in irradiation time. In the photopolymerization of PGMA, a larger amount of polymer was formed than in the photopolymerization of PAA for both samples (POEMI and PEEMI), as may be concluded by the analysis of ATRFTIR, gravimetry, and SEM results. The synthesis route from the anhydride-functionalized surface provided better results than did the route from the carboxylic acid-functionalized surface. The PAA and PGMA layers formed on PEEMI films were thicker than the layers formed on POEMI films. The surface morphologies of the PAA and PGMA layers formed on the surface of the PEEMI films were different when compared to those formed on POEMI films. Acknowledgment. The authors thank CNPq, CAPES, and Fundac¸a˜o Arauca´ria (Parana´ State’s, Brazil foundation that supports technological and scientific development) for the financial support. LA802309U
