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Synthesis and properties of poly(3-n-dodecylthiophene) modified thermally expandable microspheres
European Polymer Journal 49 (2013) 1503–1509
Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Synthesis and properties of poly(3-n-dodecylthiophene) modified thermally expandable microspheres George Vamvounis a,b,⇑,1, Magnus Jonsson a,c, Eva Malmström a, Anders Hult a a
Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane 4072, Australia c Akzo Nobel Pulp and Performance Chemicals AB, Box 13000, SE-850 13 Sundsvall, Sweden b
a r t i c l e
i n f o
Article history: Received 7 September 2012 Received in revised form 27 November 2012 Accepted 15 January 2013 Available online 13 February 2013 Keywords: Thermally expandable microspheres Polythiophene Conjugated polymers Graft-copolymers
a b s t r a c t Functionalization of thermally expandable microspheres (TEMs) with a conjugated polymer is explored. These functionalized thermally expandable microspheres were prepared by grafting 3-n-dodecylthiophene via oxidative polymerization from a thiophene modified TEM. The thiophene modified TEM to 3-n-dodecylthiophene ratio was varied during grafting and the resulting poly(3-n-dodecylthiophene) grafted TEM were characterized by FT-IR spectroscopy, Thermal Gravimetric Analysis, Scanning Electron Microscopy, ThermoMechanical Analysis and X-ray diffraction. The particles were approximately 30 lm in diameter and upon heating, the functionalized microspheres expanded up to 50 times. This expansion property was related to the poly(3-n-dodecylthiophene) content, where the increase in poly(3-n-dodecylthiophene) on microsphere decreased the thermal expansion. The X-ray diffraction shows a sharpening of the poly(3-n-dodecylthiophene) (1 0 0) diffraction peak upon expansion. Grafting conjugated polymers to thermally expandable microspheres provides robust functional photo- and electro-active TEMs. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction Thermally expandable microspheres (TEMs) are 10– 50 lm core/shell particles with a thermoplastic shell encapsulating an inert low boiling hydrocarbon [1]. Upon heating these TEMs above the glass transition temperature of the polymeric shell (e.g. polyacrylonitrile), they expand with an increasing volume of 40–60 times due to the large internal pressure caused by an encapsulated hydrocarbon. Because of these thermal expansion properties, TEMs have been used in products, such as paper board, printing ink and thermoplastic materials [2]. Studies based on the preparation [3,4], properties [5–7], and manufacture of microfluidic device components [8,9] have been performed to
⇑ Corresponding author at: School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane 4072, Australia. Tel.: +61 449897325. E-mail address: [email protected] (G. Vamvounis). 1 Currently at The University of Queensland.
further exploit the unique thermal expansion properties of TEMs. Herein, the modification of TEMs with a functional conjugated polymer is explored. One convenient method to tailor the properties of materials is by surface immobilization. However, the absence of functional groups to graft-to or -from the TEM combined with the harsh polymerization conditions limit this method. To overcome this obstacle in TEMs, poly(acrylonitrileco-methacrylonitrile-co-2-hydoxyethyl methacrylate) based TEMs were prepared [10], where we have successfully grafted poly(glycidyl methacrylate) via ARGET ATRP, enabling tailored TEM surface properties [10]. That is, since the hydroxyl group is susceptible to react with an acid halide, such as the ATRP initiator 2-bromoisobutyryl bromide, polymerization was possible without destroying the TEM, provided the appropriate temperature and solvent are used [10]. One interesting class of functional groups are conjugated polymers because of their optical (light absorbing and light emitting) and electronic (charge transport)
0014-3057/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.01.010
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properties. Therefore, these polymers have been useful as the active component in various microelectronic, microfluidic and sensory type applications [11] and could be an additional tool in TEM applications. For example, Armes and co-workers obtained cleaner expansion of TEMs with infrared irradiation by coating polypyrrole on the TEMs [12]. Grafting of conjugated polymers from the TEMs would be an additional advancement for preparing robust covalently-bound functionalized TEMs. Therefore, functionalization of TEMs with a conjugated polymer, poly(3-ndodecylthiophene) (P3DT), via a ‘‘grafting-from’’ approach is explored in this paper, and the properties of the grafted polymer before and after thermal expansion are examined. This is, to the best of the authors’ knowledge, the first example of grafting functional conjugated polymer brushes from thermally expandable microspheres. 2. Experimental
was stirred for 1 h. This reaction was quenched slowly in methanol in the presence of triethylamine. The poly(3-ndodecylthiophene) functionalized TEMs (TEM:P3DT) were collected with a Büchner funnel. The resulting TEMs were washed with copious amounts of methanol (to remove FeCl3 and triethylamine residues) and chloroform (to remove free poly(3-n-dodecylthiophene)). Various weight ratio’s of TEM-co-thiophene ester and 3-dodecylthiophene were used; for instance, 1.5 g of TEM-co-thiophene ester and 0.15 g of 3-dodecylthiophene produced TEM:P3DT (10:1). The P3DT functionalized TEMs were found to be insoluble in typical P3DT solvents such as chloroform, dichloromethane, toluene and in typical TEM solvents such as DMSO and DMF, regardless of the poly(3-n-dodecylthiophene) loading. FT-IR (neat): TEM:P3DT (10:1), 3700 (MgOH2), 3350 (OH), 2960 (CAH), 2900 (CAH), 2860 (CAH), 2250 (C„N), 1740, 1630, 1460 (C@C), 1400, 1170 cm1. TEM:P3DT (1:5), 3320 (AOH), 2940 (CAH), 2830 (CAH), 1590, 1470 (C@C), 827 (CAH), 721 cm1.
2.1. Materials 2.3. Characterization Hydroxyl functionalized TEMs (75 wt.% polymer (acrylonitrile (59 mole%), methacrylonitrile (33 mole%), 2-hydroxyethylmethacrylate (8 mole%)), 21 wt.% isopentane, 3 wt.% magnesium hydroxide, 1 wt.% water) were kindly provided by Expancel. 3-dodecylthiophene was synthesized according to Ref. [13], 2-(thiophen-3-yl)acetic acid (P98%, Fluka), DMF (99.94%, VWR), CH2Cl2 (analysis grade, GTF), CHCl3 (HPLC grade, Lab-Scan), 4-dimethylaminopyridine (99%, Acros Organics), triethylamine (>99%, VWR), ferric chloride (>98%, Merck), oxalyl chloride (98%, Aldrich). 2-(thiophen-3-yl)acetyl chloride was synthesized as previously reported [14]. 2.2. Synthesis 2.2.1. TEM-co-thiophene ester (TEM:T) TEMs (1.00 g, 0.01 mmole of OH), triethylamine (0.90 g, 0.89 mmole) and a catalytic amount of dimethylaminopyridine were dispersed into 8 mL of dry dichloromethane in a 50 mL round bottom flask under vigorous stirring at room temperature. 2-(Thiophen-3-yl)acetyl chloride (1.1 g, 0.07 mmole) was added over 15 min and left to react for 1 h. The reaction was quenched with excess methanol and filtered. The TEMs were washed with copious amounts of methylene chloride and methanol to remove excess reactants and bases. After drying the title compound at room temperature under reduced pressure, afforded approximately 1.00 g of product. 1H NMR (DMSO-d6), ppm: 1.61, 1.20, 2.09, 3.13, 3.32, 3.65, 4.05, 4.82, 5.68 (sh, s), 7.04 (br, s, Th-4), 7.29 (br, s, Th-2), 7.41 (br, s, Th-5). FT-IR (neat): 3700 (MgOH), 3350 (AOH), 2960 (CAH), 2900 (CAH), 2860 (CAH), 2250 (C„N), 1740, 1630, 1460 (C@C), 1400, 1170 cm1. 2.2.2. TEM-co-poly(3-n-dodecylthiophene) (TEM:P3DT) In a typical reaction, TEM-co-thiophene ester and 3-dodecylthiophene were dispersed in chloroform, where upon 1.5 mole equivalents (to 3-dodecylthiophene) of anhydrous ferric chloride was added. This reaction mixture
1
H and 13C NMR were recorded on a Bruker AM 400. Chloroform-d or DMSO-d6 containing 1% (v/v) TMS, was used as solvent. Thermal gravimetric analysis (TGA) was conducted on a Mettler Toledo TGA/SDTA851e to determine blowing agent content as well as to monitor polymer decomposition. All samples were dried prior to analysis in order to exclude as much moisture and residual monomers as possible. Drying was conducted with care, so that the hydrocarbon content was not perturbed, and therefore, the moisture was not completely removed. The water content was approximately 0.5–1 wt.%. The dried samples were heated from 30 to 650 °C at 20 °C min1, 2 min at 650 °C under N2 atmosphere, followed by 8 min at 650 °C in air. The particle morphology was studied using a Philips SEM XL 20 scanning electron microscope (SEM). All samples were coated with a thin layer of gold prior to analysis using a BAL-TEC SCD 005 sputter-coater (0.1–0.01 mBar, 230 s at 35 mA). Thermal mechanical analysis (TMA) was conducted on a Mettler Toledo TMA/SDTA841e to determine the expansion properties of the TEMs. The samples (0.5 mg) were heated from 30 to 250 °C at 20 °C min1 under N2 atmosphere with a 0.06 N load. Fourier transform infrared (FT-IR) analysis was recorded using a Nicolet Nexus 470 equipped with an attenuated total reflection (ATR) diamond ZnSe crystal (DurasamplerIR from SensIR Technologies). X-ray diffraction measurements were obtained at room temperature using a PANalytical X’ Pert-Pro diffractometer in a Bragg–Brentano geometry. The radiation source was Cu Ka of wavelength k = 0.154 nm. 3. Results and discussion 3.1. Synthesis Scheme 1 illustrates the synthetic route towards the P3DT-functionalized TEMs (TEM-P3HT). The TEMs employed had a hydroxy functionalized surface, and were
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TEM i
OH
S
O O
ii
S
O
O
C12H25
O
Cl
S
S S C12H25 Scheme 1. Solid-state functionalization of TEMs with poly(3-n-dodecylthiophene). (i) 3-Thiophene acid chloride, triethylamine, 4-N,N-dimethylamino pyridine, CH2Cl2 and (ii) 3-n-dodecylthiophene, FeCl3, CHCl3.
91% for low loading to 20% for high loading) where the greater the 3-n-dodecylthiophene content resulted in lower yields, indicating that free P3DT was formed, which was removed during the washing step. The P3DT functionalized TEMs were analyzed by infrared spectroscopy before (Fig. 2a) and after (Fig. 2b) thermal expansion. In the non-expanded state of the TEM:P3DT, the height of the peak attributed of the nitrile moiety (2250 cm1) is dependant on the P3DT content; where TEM:P3DT (10:1) shows a larger peak than TEM:P3DT (1:5). Furthermore, the CAH peaks (2800–3000 cm1) dramatically increase in intensity and became sharper with higher the P3DT loading, similar to the pristine P3DT. Upon thermal expansion of the TEM:P3DT’s, the nitrile peak increases in relative intensity, even at high loading of the P3DT. The appearance of the nitrile peak indicates that the P3DT film either decreases in thickness and/or isolation of P3DT to expose small areas of uncoated TEM:P3DT. Thermal gravimetric analysis of the TEMs and P3DT was performed, Fig. 3. The P3DT shows a minor decomposition at 300 °C (<5%) and a large decomposition at approximately 500 °C (75%). The parent TEMs undergo several thermal weight loss transitions; namely, 145 °C (4%) (minor loss of blowing agent during expansion), 190 °C (23%) (loss of blowing agent during TEM collapse) and 320 °C (55%) (degradation of the polyacrylonitrile component). Since minimal degradation occurs in P3DT at 320 °C, the amount of P3DT grafted onto the TEM was estimated at
kindly provided by Akzo Nobel Pulp and Performance Chemicals AB. Thiophene functionalized TEMs (TEM:T) were obtained after careful reaction with a thiophene acid chloride under basic conditions. Evidence for covalent attachment of the thiophene moiety is seen in the 1H NMR in DMSO-d6, Fig. 1, it can be seen that four new peaks occur, corresponding to the thiophene moiety (5.7 ppm for the a-protons on the aliphatic carbon, 7.0, 7.4 and 7.5 ppm for the aromatic protons). To prepare the polymer brushes, oxidatitive polymerization was explored as it was successful by Carter et al. in preparing a high density of polythiophene brushes on a modified silicon wafer [15]. In doing so, grafting of 3-n-dodecylthiophene onto the thiophene containing TEMs was performed in the presence of Iron (III) Chloride, to form P3DT brushes on the TEMs. After polymerizing for 1 h at room temperature, the reaction was quenched by slowly adding the reaction mixture to a methanol:triethylamine solution. The P3DT functionalized TEMs were collected by filtering and were washed with copious methanol:triethylamine solution (to remove iron impurities) and chloroform (to remove free P3DT), followed by drying under vacuum at room temperature, which resulted in a red-brown colored powder or hardbrittle material. The TEM:P3DT were found to be insoluble in dimethylformamide, dimethylacetamide, dimethyl sulfoxide, chloroform, dichloromethane, THF at elevated temperatures (up to boiling), regardless of the P3DT loading. The product yields depended on the P3DT content, (from
8
8
7.5
7
7
6.5
6
6
5
5.5
4
3
2
1
0
PPM Fig. 1. 1H NMR of TEMs (bottom) and thiophene modified TEMs (top) in DMSO-d6. Inset is an expanded view which shows the proton resonances of the thiophene moiety.
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Fig. 2. Infrared spectra of TEM, TEM:T, P3DT-grafted TEMs and P3DT before (a) and after (b) expansion.
Fig. 3. TGA analysis of TEM (j), TEM:P3DT (10:1) (), TEM:P3DT (1:5) (s) and P3DT ().
400 °C to be 10% for TEM:P3DT (10:1) and 70% for TEM:P3DT (1:5).
Fig. 4 illustrates the scanning electron microscope images (SEM) of TEM:T, TEM:P3DT (10:1) and TEM:P3DT
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Fig. 4. SEM images of (a and b) TEM:T, (c and d) TEM:P3DT (10:1), (e and f) TEM:P3DT (1:5) at 100 (a, c, and e) and 2000 (b, d, and f) magnifications.
(1:5) at two different magnifications. Upon reacting with the thiophene acid chloride, it is clear that the TEMs maintain their shape (Fig. 4a and b). By grafting a small amount of 3-dodecylthiophene from the thiophene functionalized TEM, the granular consistency was maintained (Fig. 4c) and a flaky surface seemed to occur. With a high loading of P3DT, the TEM granules are dispersed within P3DT (Fig. 4e). From Fig. 4f, it can be clearly seen that the texture of the microsphere is different due to the P3DT grafted from the TEMs. 3.2. Properties Thermo-mechanical analysis (TMA) of the TEMs was analyzed as a function of P3DT loading, as illustrated in
Fig. 5. The TEMs follow a typical pattern, where the onset of expansion occurs at approximately 125 °C until a maximum value at 180 °C. Above 180 °C, loss of the blowing agent occurs which results in a collapse in the TEMs. The loss of blowing agent is evident in the TGA, where a weight loss occurs at approximately 180 °C. From the TMA analysis, it is clear that the expansion properties decrease as the P3DT content increases, which is expected since pristine P3DT has a low thermal expansion property. For a clear demonstration of the expansion properties of these P3DT grafted TEMs, 20 mg of the modified TEMs were placed into two vials of the same size, where one vial was heated for 45 min at 150 °C and the other vial was not, Fig. 6. It is clear that the expansion property of the TEM:P3DT (10:1) is dramatic and similar to the parent TEM, while the
Fig. 5. Thermo-mechanical analysis of TEM (j), TEM:T (4), TEM:P3DT (10:1) () TEM:P3DT (1:5) (s).
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Fig. 6. A picture of 20 mg of the parent TEMs (a and b), TEM:P3DT (10:1) (c and d) and TEM:P3DT (1:5) (e and f), before (a, c, and e) and after (b, d, and f) heating at 150 °C for 45 min.
TEM:P3DT (1:5) still maintained significant expansion properties. X-ray diffraction of the P3DT grafted to the TEMs is depicted in Fig. 7a. The parent TEMs displayed three diffraction peaks; namely, 2h = 16.59, 27, 38.2 corresponding to 5.34, 3.2 and 2.37 Å, respectively. These peaks are typical to polyacylonitrile based polymers due to equatorial reflections [16]. Pristine P3DT displays two typical reflections; namely, 2h = 3.5, 25 corresponding to 26 and 3.8 Å. These reflections are due to the (1 0 0) and (0 1 0) planes along the polythiophene backbone. The X-ray diffraction of the TEM:T resulted in a similar pattern as the parent TEM, indicating that the crystalline nature is maintained. Upon grafting P3DT from the TEM:T, a significant broadening of the diffraction patterns occur at low loadings, while at a high loading, virtually no reflections similar to the original
TEM:T are present. Furthermore, the appearance of a small peak at 2h = 3.7° which is assigned to the (1 0 0) reflection of the P3DT. Since the TEM structure is clearly maintained (according to the SEM analysis) and the expansion properties is maintained (according to the TMA analysis), it is inferred that the strong decrease in the X-ray diffraction of the P3DT modified TEM is due to the decrease in the TEM content, rather than any differences in the crystallinity of the polyacrylonitrile component. Upon heating these TEMs at 150 °C, a significant change in the XRD pattern is observed (Fig. 7b). For the parent TEM, there is a decrease in the intensity of the polyacrylonitrile equatorial reflections, followed by the appearance of two new peaks at 2h 44° and 51°, which is attributed to reflections from the sample holder. The reflections originating from the sample holder, even though it is 1 cm away from the focal
Fig. 7. X-ray diffraction of TEM (j), TEM:T (4), TEM:P3DT (10:1) () and TEM:P3DT (1:5) (s) before (a) and after (b) thermal expansion.
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Fig. 8. Comparison of XRD of TEM:P3DT (1:5) as a TEM (d) and the expanded TEM (s).
point, are observed due to the low density of the expanded TEM. The TEM:P3DT (10:1) sample shows a very similar XRD pattern to the parent TEM. Interestingly, the high loading TEM:P3DT (1:5) sample shows peaks corresponding to the sample holder and P3DT (2h 3.4°), since the density is greater because of the higher non-expanding P3DT content. Fig. 8 compares the (1 0 0) reflection of TEM:P3DT (1:5) before and after expansion. An increase in the polymer crystallite size from 0.06 to 0.10 nm, estimated with the Scherrer’s equation, was observed upon annealing. The difference in polymer crystallites is likely due a combination of thermal annealing and stretching of the P3DT caused by the thermal expansion of the TEM. Thermal annealing of P3DT is known to influence its structural ordering, as evidence by X-ray diffraction. For instance, Gobsch and coworkers have monitored the X-ray diffraction of poly(3-n-hexylthiophene)/[6,6]-phenyl C61 butyric acid methyl ester composites and found that the cystallinity increased after their film was heated [17]. Stretching, on the other hand, may induce order and orientation in conjugated polymers [18]. 4. Conclusions Preparation of poly(3-n-dodecylthiophene) functionalized TEMs was demonstrated for the first time without degradation of the TEMs. The thermal expansion properties were dependant on the P3DT content, where greater P3DT content resulted in lower expansion. The X-ray diffraction clearly showed an increase in the crystallinity of the P3DT, indicating the polymer morphology remained in its crystalline state after thermal expansion. This methodology towards functionalized TEMs is a versatile route towards TEMs with tailored optical and electrical properties. Acknowledgments This work was supported by the KAMI foundation, Swedish Research Council and Akzo Nobel. Mrs. Taina Tollgren and Mrs. Annika Persson are thanked for performing the SEM and TMA measurements. GV is an Australian Research Fellow supported by the Australian Research Council (Grant DP1095404).
References [1] Morehouse DSJ, Tetreault RJ, Dow Chemical Co.; 1964. [2] www.expancel.com. [3] Jonsson M, Nordin O, Malmström E, Hammer C. Suspension polymerization of thermally expandable core/shell particles. Polymer 2006;47(10):3315–24. [4] Kawaguchi Y, Oishi T. Synthesis and properties of thermoplastic expandable microspheres: the relation between crosslinking density and expandable property. J Appl Polym Sci 2004;93(1): 505–12. [5] Ahmad M. Flexible vinyl resiliency property enhancement with hollow thermoplastic microspheres. J Vinyl Addit Technol 2001;7(3):156–61. [6] Han M, Zhao K. Effect of volume fraction and temperature on dielectric relaxation spectroscopy of suspensions of PS/PANI composite microspheres. J Phys Chem C 2008;112(49):19412–22. [7] Nishiyama Y, Uto N, Sato C, Sakurai H. Dismantlement behavior and strength of dismantlable adhesive including thermally expansive particles. Int J Adhes Adhes 2003;23(5):377–82. [8] (a) Andersson H, Griss P, Stemme G. Expandable microspheres– surface immobilization techniques. Sens Actuators B: Chem 2002;84(2–3):290–5; (b) Griss P, Andersson H, Stemme G. Expandable microspheres for the handling of liquids. Lab Chip 2002;2(2):117–20. [9] Loverich JJ, Kanno I, Kotera H. Concepts for a new class of all-polymer micropumps. Lab Chip 2006;6(9):1147–54. [10] Jonsson M, Nyström D, Nordin O, Malmström E. Surface modification of thermally expandable microspheres by grafting poly(glycidyl methacrylate) using ARGET ATRP. Eur Polym J 2009;45(8):2374–82. [11] Heeger AJ, Sariciftci NS, Namdas EB. Semiconducting and metallic polymers. Oxford University Press; 2010. [12] Schmid A, Sutton LR, Armes SP, Bain PS, Manfrè G. Synthesis and evaluation of polypyrrole-coated thermally-expandable microspheres: an improved approach to reversible adhesion. Soft Matter 2009;5(2):407–12. [13] Bert de Boer PF, Hutten V, Ouali L, Grayer V, Hadziioannou G. Amphiphilic, regioregular polythiophenes. Macromolecules 2002;35(18):6883–92. [14] Jang SY, Clark M, Sotzing GA. Poly(terthiophene) networks via electrochemical crosslinking of terthiophene derivatized norbornylene monomers and polymers. PMSE Prepr 2002;86: 205–6. [15] Hagberg EC, Carter KR. Advances in nanocontact molding for the patterning of polythiophene. Polym. Prepr. 2005;46:356–7. [16] Ko TH, Ting HY, Lin CH. Thermal stabilization of polyacrylonitrile fibers. J Appl Polym Sci 1988;35(3):631–40. [17] Erb T, Zhokhavets U, Gobsch G, Raleva S, Stühn B, Schilinsky P, et al. Correlation between structural and optical properties of composite polymer/fullerene films for organic solar cells. Adv Funct Mater 2005;15(7):1193–6. [18] Kijima M, Mukai S, Ohmura K, Shirakawa H, Kyotani M. Synthesis of conjugated polymer with carbynoid structure by dehydrochlorination of chlorinated polyacetylene film. Synth Met 1999;101(1–3): 59–60.
Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Synthesis and properties of poly(3-n-dodecylthiophene) modified thermally expandable microspheres George Vamvounis a,b,⇑,1, Magnus Jonsson a,c, Eva Malmström a, Anders Hult a a
Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane 4072, Australia c Akzo Nobel Pulp and Performance Chemicals AB, Box 13000, SE-850 13 Sundsvall, Sweden b
a r t i c l e
i n f o
Article history: Received 7 September 2012 Received in revised form 27 November 2012 Accepted 15 January 2013 Available online 13 February 2013 Keywords: Thermally expandable microspheres Polythiophene Conjugated polymers Graft-copolymers
a b s t r a c t Functionalization of thermally expandable microspheres (TEMs) with a conjugated polymer is explored. These functionalized thermally expandable microspheres were prepared by grafting 3-n-dodecylthiophene via oxidative polymerization from a thiophene modified TEM. The thiophene modified TEM to 3-n-dodecylthiophene ratio was varied during grafting and the resulting poly(3-n-dodecylthiophene) grafted TEM were characterized by FT-IR spectroscopy, Thermal Gravimetric Analysis, Scanning Electron Microscopy, ThermoMechanical Analysis and X-ray diffraction. The particles were approximately 30 lm in diameter and upon heating, the functionalized microspheres expanded up to 50 times. This expansion property was related to the poly(3-n-dodecylthiophene) content, where the increase in poly(3-n-dodecylthiophene) on microsphere decreased the thermal expansion. The X-ray diffraction shows a sharpening of the poly(3-n-dodecylthiophene) (1 0 0) diffraction peak upon expansion. Grafting conjugated polymers to thermally expandable microspheres provides robust functional photo- and electro-active TEMs. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction Thermally expandable microspheres (TEMs) are 10– 50 lm core/shell particles with a thermoplastic shell encapsulating an inert low boiling hydrocarbon [1]. Upon heating these TEMs above the glass transition temperature of the polymeric shell (e.g. polyacrylonitrile), they expand with an increasing volume of 40–60 times due to the large internal pressure caused by an encapsulated hydrocarbon. Because of these thermal expansion properties, TEMs have been used in products, such as paper board, printing ink and thermoplastic materials [2]. Studies based on the preparation [3,4], properties [5–7], and manufacture of microfluidic device components [8,9] have been performed to
⇑ Corresponding author at: School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane 4072, Australia. Tel.: +61 449897325. E-mail address: [email protected] (G. Vamvounis). 1 Currently at The University of Queensland.
further exploit the unique thermal expansion properties of TEMs. Herein, the modification of TEMs with a functional conjugated polymer is explored. One convenient method to tailor the properties of materials is by surface immobilization. However, the absence of functional groups to graft-to or -from the TEM combined with the harsh polymerization conditions limit this method. To overcome this obstacle in TEMs, poly(acrylonitrileco-methacrylonitrile-co-2-hydoxyethyl methacrylate) based TEMs were prepared [10], where we have successfully grafted poly(glycidyl methacrylate) via ARGET ATRP, enabling tailored TEM surface properties [10]. That is, since the hydroxyl group is susceptible to react with an acid halide, such as the ATRP initiator 2-bromoisobutyryl bromide, polymerization was possible without destroying the TEM, provided the appropriate temperature and solvent are used [10]. One interesting class of functional groups are conjugated polymers because of their optical (light absorbing and light emitting) and electronic (charge transport)
0014-3057/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.01.010
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properties. Therefore, these polymers have been useful as the active component in various microelectronic, microfluidic and sensory type applications [11] and could be an additional tool in TEM applications. For example, Armes and co-workers obtained cleaner expansion of TEMs with infrared irradiation by coating polypyrrole on the TEMs [12]. Grafting of conjugated polymers from the TEMs would be an additional advancement for preparing robust covalently-bound functionalized TEMs. Therefore, functionalization of TEMs with a conjugated polymer, poly(3-ndodecylthiophene) (P3DT), via a ‘‘grafting-from’’ approach is explored in this paper, and the properties of the grafted polymer before and after thermal expansion are examined. This is, to the best of the authors’ knowledge, the first example of grafting functional conjugated polymer brushes from thermally expandable microspheres. 2. Experimental
was stirred for 1 h. This reaction was quenched slowly in methanol in the presence of triethylamine. The poly(3-ndodecylthiophene) functionalized TEMs (TEM:P3DT) were collected with a Büchner funnel. The resulting TEMs were washed with copious amounts of methanol (to remove FeCl3 and triethylamine residues) and chloroform (to remove free poly(3-n-dodecylthiophene)). Various weight ratio’s of TEM-co-thiophene ester and 3-dodecylthiophene were used; for instance, 1.5 g of TEM-co-thiophene ester and 0.15 g of 3-dodecylthiophene produced TEM:P3DT (10:1). The P3DT functionalized TEMs were found to be insoluble in typical P3DT solvents such as chloroform, dichloromethane, toluene and in typical TEM solvents such as DMSO and DMF, regardless of the poly(3-n-dodecylthiophene) loading. FT-IR (neat): TEM:P3DT (10:1), 3700 (MgOH2), 3350 (OH), 2960 (CAH), 2900 (CAH), 2860 (CAH), 2250 (C„N), 1740, 1630, 1460 (C@C), 1400, 1170 cm1. TEM:P3DT (1:5), 3320 (AOH), 2940 (CAH), 2830 (CAH), 1590, 1470 (C@C), 827 (CAH), 721 cm1.
2.1. Materials 2.3. Characterization Hydroxyl functionalized TEMs (75 wt.% polymer (acrylonitrile (59 mole%), methacrylonitrile (33 mole%), 2-hydroxyethylmethacrylate (8 mole%)), 21 wt.% isopentane, 3 wt.% magnesium hydroxide, 1 wt.% water) were kindly provided by Expancel. 3-dodecylthiophene was synthesized according to Ref. [13], 2-(thiophen-3-yl)acetic acid (P98%, Fluka), DMF (99.94%, VWR), CH2Cl2 (analysis grade, GTF), CHCl3 (HPLC grade, Lab-Scan), 4-dimethylaminopyridine (99%, Acros Organics), triethylamine (>99%, VWR), ferric chloride (>98%, Merck), oxalyl chloride (98%, Aldrich). 2-(thiophen-3-yl)acetyl chloride was synthesized as previously reported [14]. 2.2. Synthesis 2.2.1. TEM-co-thiophene ester (TEM:T) TEMs (1.00 g, 0.01 mmole of OH), triethylamine (0.90 g, 0.89 mmole) and a catalytic amount of dimethylaminopyridine were dispersed into 8 mL of dry dichloromethane in a 50 mL round bottom flask under vigorous stirring at room temperature. 2-(Thiophen-3-yl)acetyl chloride (1.1 g, 0.07 mmole) was added over 15 min and left to react for 1 h. The reaction was quenched with excess methanol and filtered. The TEMs were washed with copious amounts of methylene chloride and methanol to remove excess reactants and bases. After drying the title compound at room temperature under reduced pressure, afforded approximately 1.00 g of product. 1H NMR (DMSO-d6), ppm: 1.61, 1.20, 2.09, 3.13, 3.32, 3.65, 4.05, 4.82, 5.68 (sh, s), 7.04 (br, s, Th-4), 7.29 (br, s, Th-2), 7.41 (br, s, Th-5). FT-IR (neat): 3700 (MgOH), 3350 (AOH), 2960 (CAH), 2900 (CAH), 2860 (CAH), 2250 (C„N), 1740, 1630, 1460 (C@C), 1400, 1170 cm1. 2.2.2. TEM-co-poly(3-n-dodecylthiophene) (TEM:P3DT) In a typical reaction, TEM-co-thiophene ester and 3-dodecylthiophene were dispersed in chloroform, where upon 1.5 mole equivalents (to 3-dodecylthiophene) of anhydrous ferric chloride was added. This reaction mixture
1
H and 13C NMR were recorded on a Bruker AM 400. Chloroform-d or DMSO-d6 containing 1% (v/v) TMS, was used as solvent. Thermal gravimetric analysis (TGA) was conducted on a Mettler Toledo TGA/SDTA851e to determine blowing agent content as well as to monitor polymer decomposition. All samples were dried prior to analysis in order to exclude as much moisture and residual monomers as possible. Drying was conducted with care, so that the hydrocarbon content was not perturbed, and therefore, the moisture was not completely removed. The water content was approximately 0.5–1 wt.%. The dried samples were heated from 30 to 650 °C at 20 °C min1, 2 min at 650 °C under N2 atmosphere, followed by 8 min at 650 °C in air. The particle morphology was studied using a Philips SEM XL 20 scanning electron microscope (SEM). All samples were coated with a thin layer of gold prior to analysis using a BAL-TEC SCD 005 sputter-coater (0.1–0.01 mBar, 230 s at 35 mA). Thermal mechanical analysis (TMA) was conducted on a Mettler Toledo TMA/SDTA841e to determine the expansion properties of the TEMs. The samples (0.5 mg) were heated from 30 to 250 °C at 20 °C min1 under N2 atmosphere with a 0.06 N load. Fourier transform infrared (FT-IR) analysis was recorded using a Nicolet Nexus 470 equipped with an attenuated total reflection (ATR) diamond ZnSe crystal (DurasamplerIR from SensIR Technologies). X-ray diffraction measurements were obtained at room temperature using a PANalytical X’ Pert-Pro diffractometer in a Bragg–Brentano geometry. The radiation source was Cu Ka of wavelength k = 0.154 nm. 3. Results and discussion 3.1. Synthesis Scheme 1 illustrates the synthetic route towards the P3DT-functionalized TEMs (TEM-P3HT). The TEMs employed had a hydroxy functionalized surface, and were
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TEM i
OH
S
O O
ii
S
O
O
C12H25
O
Cl
S
S S C12H25 Scheme 1. Solid-state functionalization of TEMs with poly(3-n-dodecylthiophene). (i) 3-Thiophene acid chloride, triethylamine, 4-N,N-dimethylamino pyridine, CH2Cl2 and (ii) 3-n-dodecylthiophene, FeCl3, CHCl3.
91% for low loading to 20% for high loading) where the greater the 3-n-dodecylthiophene content resulted in lower yields, indicating that free P3DT was formed, which was removed during the washing step. The P3DT functionalized TEMs were analyzed by infrared spectroscopy before (Fig. 2a) and after (Fig. 2b) thermal expansion. In the non-expanded state of the TEM:P3DT, the height of the peak attributed of the nitrile moiety (2250 cm1) is dependant on the P3DT content; where TEM:P3DT (10:1) shows a larger peak than TEM:P3DT (1:5). Furthermore, the CAH peaks (2800–3000 cm1) dramatically increase in intensity and became sharper with higher the P3DT loading, similar to the pristine P3DT. Upon thermal expansion of the TEM:P3DT’s, the nitrile peak increases in relative intensity, even at high loading of the P3DT. The appearance of the nitrile peak indicates that the P3DT film either decreases in thickness and/or isolation of P3DT to expose small areas of uncoated TEM:P3DT. Thermal gravimetric analysis of the TEMs and P3DT was performed, Fig. 3. The P3DT shows a minor decomposition at 300 °C (<5%) and a large decomposition at approximately 500 °C (75%). The parent TEMs undergo several thermal weight loss transitions; namely, 145 °C (4%) (minor loss of blowing agent during expansion), 190 °C (23%) (loss of blowing agent during TEM collapse) and 320 °C (55%) (degradation of the polyacrylonitrile component). Since minimal degradation occurs in P3DT at 320 °C, the amount of P3DT grafted onto the TEM was estimated at
kindly provided by Akzo Nobel Pulp and Performance Chemicals AB. Thiophene functionalized TEMs (TEM:T) were obtained after careful reaction with a thiophene acid chloride under basic conditions. Evidence for covalent attachment of the thiophene moiety is seen in the 1H NMR in DMSO-d6, Fig. 1, it can be seen that four new peaks occur, corresponding to the thiophene moiety (5.7 ppm for the a-protons on the aliphatic carbon, 7.0, 7.4 and 7.5 ppm for the aromatic protons). To prepare the polymer brushes, oxidatitive polymerization was explored as it was successful by Carter et al. in preparing a high density of polythiophene brushes on a modified silicon wafer [15]. In doing so, grafting of 3-n-dodecylthiophene onto the thiophene containing TEMs was performed in the presence of Iron (III) Chloride, to form P3DT brushes on the TEMs. After polymerizing for 1 h at room temperature, the reaction was quenched by slowly adding the reaction mixture to a methanol:triethylamine solution. The P3DT functionalized TEMs were collected by filtering and were washed with copious methanol:triethylamine solution (to remove iron impurities) and chloroform (to remove free P3DT), followed by drying under vacuum at room temperature, which resulted in a red-brown colored powder or hardbrittle material. The TEM:P3DT were found to be insoluble in dimethylformamide, dimethylacetamide, dimethyl sulfoxide, chloroform, dichloromethane, THF at elevated temperatures (up to boiling), regardless of the P3DT loading. The product yields depended on the P3DT content, (from
8
8
7.5
7
7
6.5
6
6
5
5.5
4
3
2
1
0
PPM Fig. 1. 1H NMR of TEMs (bottom) and thiophene modified TEMs (top) in DMSO-d6. Inset is an expanded view which shows the proton resonances of the thiophene moiety.
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Fig. 2. Infrared spectra of TEM, TEM:T, P3DT-grafted TEMs and P3DT before (a) and after (b) expansion.
Fig. 3. TGA analysis of TEM (j), TEM:P3DT (10:1) (), TEM:P3DT (1:5) (s) and P3DT ().
400 °C to be 10% for TEM:P3DT (10:1) and 70% for TEM:P3DT (1:5).
Fig. 4 illustrates the scanning electron microscope images (SEM) of TEM:T, TEM:P3DT (10:1) and TEM:P3DT
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Fig. 4. SEM images of (a and b) TEM:T, (c and d) TEM:P3DT (10:1), (e and f) TEM:P3DT (1:5) at 100 (a, c, and e) and 2000 (b, d, and f) magnifications.
(1:5) at two different magnifications. Upon reacting with the thiophene acid chloride, it is clear that the TEMs maintain their shape (Fig. 4a and b). By grafting a small amount of 3-dodecylthiophene from the thiophene functionalized TEM, the granular consistency was maintained (Fig. 4c) and a flaky surface seemed to occur. With a high loading of P3DT, the TEM granules are dispersed within P3DT (Fig. 4e). From Fig. 4f, it can be clearly seen that the texture of the microsphere is different due to the P3DT grafted from the TEMs. 3.2. Properties Thermo-mechanical analysis (TMA) of the TEMs was analyzed as a function of P3DT loading, as illustrated in
Fig. 5. The TEMs follow a typical pattern, where the onset of expansion occurs at approximately 125 °C until a maximum value at 180 °C. Above 180 °C, loss of the blowing agent occurs which results in a collapse in the TEMs. The loss of blowing agent is evident in the TGA, where a weight loss occurs at approximately 180 °C. From the TMA analysis, it is clear that the expansion properties decrease as the P3DT content increases, which is expected since pristine P3DT has a low thermal expansion property. For a clear demonstration of the expansion properties of these P3DT grafted TEMs, 20 mg of the modified TEMs were placed into two vials of the same size, where one vial was heated for 45 min at 150 °C and the other vial was not, Fig. 6. It is clear that the expansion property of the TEM:P3DT (10:1) is dramatic and similar to the parent TEM, while the
Fig. 5. Thermo-mechanical analysis of TEM (j), TEM:T (4), TEM:P3DT (10:1) () TEM:P3DT (1:5) (s).
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Fig. 6. A picture of 20 mg of the parent TEMs (a and b), TEM:P3DT (10:1) (c and d) and TEM:P3DT (1:5) (e and f), before (a, c, and e) and after (b, d, and f) heating at 150 °C for 45 min.
TEM:P3DT (1:5) still maintained significant expansion properties. X-ray diffraction of the P3DT grafted to the TEMs is depicted in Fig. 7a. The parent TEMs displayed three diffraction peaks; namely, 2h = 16.59, 27, 38.2 corresponding to 5.34, 3.2 and 2.37 Å, respectively. These peaks are typical to polyacylonitrile based polymers due to equatorial reflections [16]. Pristine P3DT displays two typical reflections; namely, 2h = 3.5, 25 corresponding to 26 and 3.8 Å. These reflections are due to the (1 0 0) and (0 1 0) planes along the polythiophene backbone. The X-ray diffraction of the TEM:T resulted in a similar pattern as the parent TEM, indicating that the crystalline nature is maintained. Upon grafting P3DT from the TEM:T, a significant broadening of the diffraction patterns occur at low loadings, while at a high loading, virtually no reflections similar to the original
TEM:T are present. Furthermore, the appearance of a small peak at 2h = 3.7° which is assigned to the (1 0 0) reflection of the P3DT. Since the TEM structure is clearly maintained (according to the SEM analysis) and the expansion properties is maintained (according to the TMA analysis), it is inferred that the strong decrease in the X-ray diffraction of the P3DT modified TEM is due to the decrease in the TEM content, rather than any differences in the crystallinity of the polyacrylonitrile component. Upon heating these TEMs at 150 °C, a significant change in the XRD pattern is observed (Fig. 7b). For the parent TEM, there is a decrease in the intensity of the polyacrylonitrile equatorial reflections, followed by the appearance of two new peaks at 2h 44° and 51°, which is attributed to reflections from the sample holder. The reflections originating from the sample holder, even though it is 1 cm away from the focal
Fig. 7. X-ray diffraction of TEM (j), TEM:T (4), TEM:P3DT (10:1) () and TEM:P3DT (1:5) (s) before (a) and after (b) thermal expansion.
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Fig. 8. Comparison of XRD of TEM:P3DT (1:5) as a TEM (d) and the expanded TEM (s).
point, are observed due to the low density of the expanded TEM. The TEM:P3DT (10:1) sample shows a very similar XRD pattern to the parent TEM. Interestingly, the high loading TEM:P3DT (1:5) sample shows peaks corresponding to the sample holder and P3DT (2h 3.4°), since the density is greater because of the higher non-expanding P3DT content. Fig. 8 compares the (1 0 0) reflection of TEM:P3DT (1:5) before and after expansion. An increase in the polymer crystallite size from 0.06 to 0.10 nm, estimated with the Scherrer’s equation, was observed upon annealing. The difference in polymer crystallites is likely due a combination of thermal annealing and stretching of the P3DT caused by the thermal expansion of the TEM. Thermal annealing of P3DT is known to influence its structural ordering, as evidence by X-ray diffraction. For instance, Gobsch and coworkers have monitored the X-ray diffraction of poly(3-n-hexylthiophene)/[6,6]-phenyl C61 butyric acid methyl ester composites and found that the cystallinity increased after their film was heated [17]. Stretching, on the other hand, may induce order and orientation in conjugated polymers [18]. 4. Conclusions Preparation of poly(3-n-dodecylthiophene) functionalized TEMs was demonstrated for the first time without degradation of the TEMs. The thermal expansion properties were dependant on the P3DT content, where greater P3DT content resulted in lower expansion. The X-ray diffraction clearly showed an increase in the crystallinity of the P3DT, indicating the polymer morphology remained in its crystalline state after thermal expansion. This methodology towards functionalized TEMs is a versatile route towards TEMs with tailored optical and electrical properties. Acknowledgments This work was supported by the KAMI foundation, Swedish Research Council and Akzo Nobel. Mrs. Taina Tollgren and Mrs. Annika Persson are thanked for performing the SEM and TMA measurements. GV is an Australian Research Fellow supported by the Australian Research Council (Grant DP1095404).
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