Article pubs.acs.org/Macromolecules
Superior Thermostability and Hydrophobicity of Poly(vinylidene fluoride-co-fluoroalkyl 2‑trifluoromethacrylate) Mohan N. Wadekar, Yogesh R. Patil, and Bruno Ameduri* Institut Charles Gerhardt, Ingénierie et Architectures Macromoléculaires, UMR 5253, Ecole Nationale Supérieure de Chimie de Montpellier, 8, rue de l’Ecole Normale, F-34296 Montpellier, France S Supporting Information *
ABSTRACT: A 2-trifluoromethacrylate monomer containing a C6F13 side chain, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl 2-(trifluoromethyl)acrylate (MAF-C6F13) was synthesized and copolymerized with vinylidene fluoride (VDF). First, the preparation of MAF-C6F13 was achieved by two different routes: (i) transesterification of tert-butyl 2-trifluoromethacrylate (MAF-tBu) with C6F13C2H4OH (55% yield with ∼95% purity) and (ii) esterification of 2trifluoromethacryloyl chloride with the same fluorinated alcohol, which gave higher yield (82%) and purity (99%). The radical copolymerization of MAFC6F13 with VDF was initiated by three different systems (two peroxypivalates and one highly branched fluorinated persistent radical, PPFR), a chain transfer agent (CF3I), at various polymerization temperatures (65−90 °C) and in three types of solvents. The relative comonomer incorporation and number-average molecular weights (Mns) of the resulting random poly(VDF-co-MAF-C6F13) copolymers were assessed by NMR spectroscopy. Both features alongwith polymerization yields were influenced by the polymerization temperatures and the nature of solvents. The Mn values were further affected by the type of initiators and the use of chain transfer agent. When dimethyl carbonate was used as the solvent, lower (2−4 mol %) MAF-C6F13 comonomer was incorporated compared to those reactions that involved 1,1,1,3,3-pentafluorobutane (5−33 mol %). Surprisingly, Mns of these copolymers increased with polymerization temperatures when the reactions were initiated by PPFR. Thus, the copolymerization carried out at 65 °C and initiated by PPFR led to a Mn of 33 000 g mol−1, while those achieved at 90 °C yielded a Mn of 61 700 g mol−1. The thermostability of the obtained copolymers under air (the temperature for 10% weight loss, Td10% > 340 °C) was significantly improved compared to that of VDF copolymers containing 2-trifluoromethacrylic acid or MAF-tBu. Surface hydrophobicity of these copolymers was superior than that of PVDF homopolymer. For example, poly(VDF-co-MAF-C6F13) copolymer of 59,000 g mol−1 with 2 mol % MAF-C6F13 only, displayed a higher static water contact angle (114°) than that of PVDF homopolymer (ca. 94°).
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Polyvinylidene fluoride (PVDF)5 and vinylidene fluoride (VDF)-containing copolymers5−7 have attracted significant attention in recent years due their applications in coatings, piezoelectric devices, binders and separators for Li ion batteries, and membranes for water treatment. 5 Among various comonomers, 2-trifluoromethyl acrylic acid (MAF) and its derivatives8 are highly attractive fluorinated comonomers to tune the properties of PVDF, and to insert various functionalities into polymeric chains. As these monomers are electron deficient, they undergo anionic homopolymerization.9 The presence of bulky or strongly electron-withdrawing groups (such as −CF3 and −COOR) prevents MAF derivatives from homopolymerization by free radical method. However, they can
INTRODUCTION Fluoropolymers are niche specialty polymer materials with distinctive physicochemical properties such as high thermochemical stability, low friction coefficient, high weather resistance, low refractive index, and water absorption, attributed to the exceptionally strong C−F bonds and weak intermolecular van der Walls interactions. Hence, they are used in a variety of critical technological applications1−4 and at extreme conditions. However, fluorine-containing homopolymers are often difficult to process, and insoluble in common organic solvents, the latter property preventing molecular weight characterization. To overcome these limitations, fluorinated monomers can be copolymerized with appropriate comonomers to create copolymers with tuned properties, better solubility, processability, and to insert functionality for further designing original architectures (block, alternated, graft, gradient) and to allow cross-linkability. © 2013 American Chemical Society
Received: October 18, 2013 Revised: December 15, 2013 Published: December 23, 2013 13
dx.doi.org/10.1021/ma402147a | Macromolecules 2014, 47, 13−25
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NMR. In the details of NMR characterization, s, d, t, q, and m stand for singlet, doublet, triplet, quintet, and multiplet, respectively. Differential Scanning Calorimetry (DSC). DSC measurements were conducted using a Perkin-Elmer Pyris 1 apparatus. Scans were recorded at heating (as well as cooling) rates of 10 °C·min−1 from −80 to +200 °C. All scans were performed under the flow of N2. A separate scan was carried out for the assessment of the Tg, by first annealing the samples at 200 °C for 3 min, then quenching them to −60 °C and heating at 20 °C·min−1. This was achieved for a better determination of Tg values, which was defined as the inflection point in the heat capacity jump. The sample sizes were about 15−20 mg. Thermogravimetric Analyses (TGA). TGA analyses were performed on a TGA 51 apparatus from TA Instruments, under air flow, at the heating rate of 20 °C·min−1 from room temperature up to a maximum of 580 °C. The sample sizes varied between 8 to 12 mg. Determination of the Water Contact Angles (WCAs). WCA measurements were carried out using a Contact Angle System from OCA-Data Physics. The uniform thin films of synthesized copolymers were prepared by melting the polymer powder on glass plate using hot plate. The water sessile drop method was used for the static contact angle (CA) measurements at 20 °C. The probe liquid was water and the average CA value was determined from five different drops of 20 μL deposited on the same sample. 3. Syntheses. Synthesis of perfluoro-3-ethyl-2,4-dimethyl-3-pentyl persistent radical (PPFR). The details about the synthesis and thermal decomposition study of PPFR are reported elsewhere.24 3.1. Synthesis of 3,3,4,4,5,5,6,6,7,7,8,8,8-Tridecafluorooctyl 2(Trifluoromethyl)acrylate (MAF-C6F13). Preparation of MAF-C6F13 was carried out employing two different strategies. 3.1.1. Synthesis of MAF-C 6 F 13 by Transesterification of C6F13C2H2OH on MAF-tBu. A 50 mL round bottomed flask containing MAF-tBu (16.08 g, 82 mmol), C6F13C2H2OH, (32.04 g, 88 mmol), and 97% H2SO4 (0.971 g, 2 wt %) was heated to 110 °C for 20 h. After the first 3 h of the reaction, a partial vacuum of ca. 300 mbar was applied. At the end of the reaction, all the unreacted materials and byproducts were evaporated. The total product mixture was purified first by distillation (50 °C, 0.015 mbar) and then by column chromatography (SiO2, pentane:diethyl ether = 9.9:0.1). 95% pure MAF-C6F13 (21.9 g, 55% yield) was obtained as a colorless liquid. 3.1.2. Synthesis of MAF-C6F13 by Esterification of C6F13C2H2OH with MAF-COCl. i. Synthesis of 2-(Trifluoromethyl)acryloyl Chloride (MAF-COCl). MAF was converted to MAF-COCl as described previously25 using thionyl chloride as the reagent. In a typical procedure, MAF (20.01 g, 143 mmol) and SOCl2 (19.63 g, 165 mmol) were refluxed in a 50 mL round bottomed flask fitted with a vertical condenser. On the top of the condenser was attached an oil bubbler to monitor the evolution of the byproducts (HCl and SO2 gases). The heating was stopped when no more gas bubbles were detected. Fractional distillation of the crude materials gave pure MAF-COCl as a colorless liquid in 63% yield (bp = 90 °C/atmospheric pressure). The 1 H and 19F NMR spectra of the product were in good agreement to those from reference25. ii. Synthesis of MAF-C6F13. A 25 mL two neck round-bottom flask was fitted with rubber septum and charged with the above fluorinated alcohol (4.901 g, 12 mmol) and pyridine (1.102 g, 14 mmol) in 8 mL of dichloromethane. The flask was cooled to −40 °C using chloroform-liquid N2 mixture with N2 gas passing through the reaction mixture. MAF-COCl (2.060 g, 13 mmol) was added slowly for 30 min to the reaction mixture maintaining the flask at −40 °C. It was further stirred for 3 h at the same temperature and later at room temperature for 12 h. At the end, 2 mL methanol was added to the stirring mixture. The reaction mixture was extracted three times with 10 mL dilute HCl. The solvent was removed from the organic layer. Distillation of the crude material afforded MAF-C6F13 (3.0 g, 82%) as a colorless liquid, bp = 49 °C/0.011 mbar. 1 H NMR (400 MHz, CDCl3, Figure S1, Supporting Information): 2.53 (t, 3JHF = 18.1 Hz, 3JHH = 6.1 Hz, 2H, CH2C(CF3)(CO2CH2CH2C6F13), 4.54 (t, 3JHH = 6.25 Hz, 2H, CH2C(CF3)(CO2CH 2CH2C6F13), 6.45 and 6.72 (2 s, 2H, CH 2C(CF3)(CO2CH2CH2C6F13). 19F NMR (376 MHz, CDCl3, Figure S2):
only be radically copolymerized with electron donating comonomers like vinyl ethers10,11 or norbornene.12 Our group has carried out the copolymerization of MAF (or other MAF derivatives) with VDF13,14 and studied the properties of resulting copolymers. They have potential applications for nanocomposites,15 fuel cell membranes,16 and electrodes for Li ion batteries.17 Thus, MAF derivatives are classic comonomers to incorporate functionalities and tune the properties of PVDF. However, the thermal stability of poly(VDF-co-MAF) and poly(VDF-co-MAF-tBu) copolymers are not satisfactory due to the decarboxylation and isobutylene elimination above 150 °C, respectively.8,18,19 Thus, to create higher thermostable PVDF copolymers, a novel variant of MAF derivatives, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl 2-(trifluoromethyl)acrylate (MAF-C6F13) was developed. The synthesis of homologous monomer, 2,2,3,3,4,4,5,5,6,6,7,7,7-tridecafluoroheptyl 2-(trifluoromethyl)acrylate was claimed previously.20,21 Further, it has been copolymerized with various hydrogenated as well as fluorinated comonomers including VDF.22 However, its detailed characterization, polymerizability, and copolymer properties were not clearly disclosed and therefore deeper investigation is required. Further, due to the presence of C6F13 side-chain on MAF-C6F13, it is worth exploring the opportunity to improve hydrophobicity of the resulting copolymers.23 In addition, we have recently reported a method of molecular weight determination by means of 19F NMR spectroscopy,18 involving the end-group analysis of CF3 labeled chain ends in the copolymers when a branched perfluorinated persistent radical (PPFR) was used as the radical initiator that generates • CF3 above 60 °C.24 This method could be vastly applicable for poly(VDF-co-MAF-C6F13) copolymers as well. Thus, the main objective of our research aims at developing thermostable and hydrophobic VDF copolymers based on MAF-C6F13. In addition, the influence of polymerization conditions (such as the temperatures, nature of initiators, and choice of solvents) onto the molecular weights, thermal as well as surface properties of the resulting copolymers, and the comonomer incorporation deserve to be carefully examined. At the end, the conditions to synthesize poly(VDF-co-MAF-C6F13) copolymers with desired properties are discussed.
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EXPERIMENTAL SECTION
1. Materials. All reagents were used as received unless it is stated. 2-Trifluoromethacrylic acid (MAF) and tert-butyl 2-trifluoromethacrylate (MAF-tBu) were kindly provided by Tosoh F-Tech Company (Shunan, Japan). 1,1-Difluoroethylene (vinylidene fluoride, VDF) and 1,1,1,3,3-pentafluorobutane (HFC-245fa, Solkane 365mfc or C4H5F5) were generously supplied by Solvay S.A. (Tavaux, France and Brussels, Belgium). Acetonitrile was purchased from Fisher Scientific and distilled over calcium hydride. Acetone-d6 (purity >99.8%) and CDCl3 (99% pure) used for NMR spectroscopy were supplied by Euroiso-top (Grenoble, France). Initiators, tert-butyl peroxypivalate (TBPPi), and tert-amyl peroxypivalate (TAPPi), 65% and 75% solutions in isodecane, respectively, were purchased from from Akzo Nobel, Compiègne, France. Commercially available PVDF homopolymer was obtained from Solvay (Solef grade). 2. Characterization. Nuclear Magnetic Resonance (NMR). The NMR spectra were recorded on Bruker AC 400 instruments, using deuterated chloroform or acetone as the solvents while tetramethylsilane TMS and CFCl3 were the references for 1H and 19F nuclei, respectively. Coupling constants and chemical shifts are given in Hertz (Hz) and parts per million (ppm), respectively. The experimental conditions for recording 1H [or 19F] NMR spectra were as follows: angle 90° [or 30°], acquisition time 4.5 s [or 0.7 s], pulse delay 2 s [or 5 s], number of scans 36 [or 64], and a pulse width of 5 μs for 19F 14
dx.doi.org/10.1021/ma402147a | Macromolecules 2014, 47, 13−25
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Table 1. Experimental Conditions of the Radical Copolymerization of VDF and MAF-C6F13* VDF (mol %) expt. no.
initiator (temp, °C)
P1 P2 P3 P4 P5 P6 P7 P8 P9 P10
TBPPi (65) TBPPi (65) TAPPi (55) PPFR (65) PPFR (75) PPFR (90) PPFR (90) PPFR (65) PPFR (90) PPFR/CF3I (90) PPFR/CF3I (90)
P11
% initiator
feed
copolymers
yield (%)
Mn (g mol‑1) by 1H/19F NMR
onset melting (°C)
melting enthalpy (J g‑1)
Tg (°C)
Td(10%) (°C)
1.0 1.0 1.0 0.9 0.9 0.9 0.9 0.9 0.9 0.3/1
94 88 94 94 94 94 94 94 94 88
93 67 97 95 95 78 83 96 98 69
73 73 35 36 50 57 67 63 63 38
6200 17 400 n.p.b 32 900 37 400 61 700 59 000 41 400 53 600 22 000
143 129 126 153 156 156 150 149 156 none
−24.2 −5.0 −30.5 −13.7 −19.7 −25.5 −22.3 −27.0 −32.0 none
8 19 13 21 29 26 22 9 20 21
352 355 349 352 353 369 366 355 376 346
0.9/5
94
95
63
115
−22.9
14
285
4100
*
Solvent mixture for the copolymerization was C4H5F5/acetonitrile/deionized water in 47.5:47.5:5.0 ratio, except (a) for P7, C4H5F5, and (b) for P8 and P9, dimethyl carbonate were used as the solvents. Polymerization times for P1−P3, P4 (and P8), and P5 were 20, 60, and 24 h, respectively, whereas for P6, P7, and P9−P11, the reaction time was 18 h. bn.p. stands for “not possible” to be determined by 1H NMR spectroscopy. Relative comonomer percentage for P3 was obtained from 19F NMR spectrum. −66.42 (s, 3F, CH2C(CF3)(CO2CH2CH2C6F13), −81.54 (t, 3JFF = 10.03 Hz, 3F, CH2C(CF3)(CO2C2H4CF2(CF2)3CF2CF3), −114.18 (m, 3JFF, 3JFH = 13.19 Hz, 2F, CH2C(CF3)(CO2C2H4CF2(CF2)3CF2CF3), −122.31 to −124.07 (s, 6F, CH2C(CF3)(CO2C2H4CF2(CF2)3CF2CF3), −126.73 (s, 2F, CH2C(CF3)(CO2C2H4CF2(CF2)3CF2CF3). 3.2. Radical Copolymerization of VDF with MAF-C6F13. All copolymerizations of VDF with MAF-C6F13 were carried out in a 100 mL Hastelloy autoclave, Parr system (Hastelloy HC 276) equipped with a manometer, a mechanical Hastelloy anchor, a rupture disk (3000 PSI), and inlet and outlet valves. An electronic device regulated and controlled both stirring and heating of the autoclave. Prior to reaction, the autoclave was pressurized with 30 bar of nitrogen for 1 h to check for leaks. The autoclave was then conditioned for the reaction with the nitrogen/vacuum cycle (10−2 mbar) to remove any traces of oxygen. The liquid and dissolved solid phases were introduced under vacuum via a funnel tightly connected to the autoclave, and then, the gases were transferred by cooling the reactor to below −50 °C by acetone/liquid N2 bath. 3.2.1. Radical Copolymerization of VDF with MAF-C6F13 Initiated by tert-Butyl Peroxypivalate (TBPPi) (P1 and P2) and tert-Amyl Peroxypivalate (TAPPi) (P3). Typical Procedure for P1. In a glass round-bottom flask were placed 29 mL of 1,1,1,3,3-pentafluorobutane (C4H5F5), 29 mL of acetonitrile and 5 mL water. The solvent mixture was bubbled with nitrogen for 10 min and 0.902 g (1 mol %) of tertbutyl peroxypivalate (TBPPi) and 7.292 g (15.0 mmol) of 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl 2-(trifluoromethyl)acrylate (MAF-C6F13) were added. This mixture was transferred to 100 mL clean Hastelloy autoclave under vacuum. The autoclave was cooled using liquid N2/acetone mixture and subjected to three freeze thaw cycles to remove the traces of oxygen. Vinylidene fluoride (15.0 g, 234.0 mmol) was filled into the autoclave while it was around −50 °C. Then, the autoclave was gradually brought to 65 °C, while stirring mechanically. A maximum pressure (Pmax) of 20 bar was reached. The polymerization was continued for 20 h and at the end of the reaction, the final pressure (Pmin) was 8 bar. The autoclave was weighed, chilled using ice for 30 min, and then unreacted VDF (5.5 g) was released. A white powder and a yellowish liquid were obtained on opening the autoclave. The liquid phase was evaporated using vacuum and the polymer was redissolved in minimum quantity of acetone. The total product mixture was precipitated from chilled pentane, filtered, and then dried under vacuum (10−2 bar) at 50 °C for 12 h. Then 16 g (70 wt %) of yellowish powder were recovered and characterized by 1H and 19F NMR spectroscopy (run P1, Table 1). 1 H NMR (400 MHz, acetone-d6, δ ppm of P1, Figure S3): 0.86− 1.12, 1.21 (m, tBu of (CH3)3C(O)CH2CF2− polymer chain end
formed by TBPPi fragmentation); 1.8 (t, 3JHF = 15 Hz, −CF2−CF2− CH3 end-group created due to the transfer of proton from solvent or polymer); 2.34 (t, 3JHF = 12.4 Hz, −CF2−CH2−CH2−CF2− reverse VDF−VDF dyad addition); 2.65 to 3.1 (m, −CH2−CF2−CH2−CF2−, CH2 of VDF, normal VDF−VDF dyad addition), (−CH2−CF2− CH2−C(CF3)(CO2CH2CH2C6F13)) protons on MAF-C6F13 in main chain) and (−CH2−CH2−CF2(CF2)4CF3); 4.55 (s, −CH2−CF2− CH2−C(CF3)(CO2CH2CH2C6F13)). 19 F NMR (376 MHz, acetone-d6, δ ppm of P1, Figure S12): −67.9 to −69.1 (−CH2C(CF3)(COOCH2CH2C6F13)), CF3 of MAF-C6F13); −80.7 to −81.7 (−CH2C(CF3)(COOCH2CH2(CF2)5CF3), CF3 end of C6F13 chain on MAF-C6F13); −91.5 to −93.5 (−CH2CF2CH2CF2− normal addition or head-to-tail VDF−VDF dyads); −94.0 to −96.1 (−CH2CF2CH2C(CF3)(COOCH2CH2C6F13)) CF2 on VDF attached to MAF-C6F13 or CF2 on VDF- C6F13 ester dyads); −107.9 (s, −CH 2 CF 2 CF 2 CH 3 ); −112.8 (−CH 2 C(CF 3 )(COOCH 2 CH 2 CF2(CF2)3CF2CF3)), −113.4, −113.8, −114.8, and −116.1 (multiple peaks, CH2−CF2−CF2−CH2 reverse addition of VDF or head to head VDF−VDF dyads, and −CH2CF2CF2CH3, CF2 on the normal VDF unit adjacent to −CF2CH3 end-group); −121.7, −122.7, and −123.5 (−CH 2 C(CF 3 )(COOCH 2 CH 2 CF 2 (CF 2 ) 3 CF 2 CF 3 )); −126.1 (−CH2C(CF3)(COOCH2CH2CF2 (CF2)3CF2CF3)). For runs P2 and P3, the reaction conditions were similar as those for P1 as noted in Table 1. 1H and 19F NMR spectra of P2 and P3 display similar signals as those of P1. 3.2.2. The Typical Copolymerization Initiated by Perfluoro-3Ethyl-2,4-dimethyl-3-Pentyl Persistent Radical (PPFR) (Runs P4 to P9). This is described in the Supporting Information. 1H NMR (400 MHz, acetone-d6) δ (ppm) of P4 (Figure S6 in the Supporting Information): 1.8 (t, 3JHF = 15 Hz, −CF2−CF2−CH3 end-group created due to the transfer of proton from solvent); 2.35 (t, 3JHF = 12.4 Hz, −CF2−CH2−CH2−CF2− reverse VDF−VDF dyad addition); 2.7 to 3.1 (m, −CH2−CF2−CH2−CF2− of VDF, normal VDF−VDF dyad addition), (−CH2−CH2−CF2(CF2)4CF3, protons on the carbon attached to C6F13 chain on MAF-C6F13), and (−CH2−CF2−CH2C(CF3)(CO2CH2CH2C6F13)); 3.25 (m, CF3−CH2−CF2−, regioselective addition of ·CF3 radical onto CH2 of VDF); 4.56 (s, −CH2− CF2−CH2−C(CF3)CO2CH2CH2C6F13). 19 F NMR (376 MHz, acetone-d6) δ (ppm) of P4 (Figure S15 in the Supporting Information): −61.2 (q, 3JHF = 4JFF = 10 Hz CF3 chain end in CF3CH2CF2−); −68.4 to −69.0 (broad s, −CH2C(CF3)COOCH2CH2C6F13); −80.7 to −81.7 (broad s, −CH2C(CF3)(COOCH2CH2(CF2)5CF3)); −91.5 (broad s, −CH2CF2CH2CF2− normal addition or head-to-tail VDF−VDF dyads); −94.5 to −95.7 (m, −CH 2 CF 2 CH 2 C(CF 3 )(COOCH 2 CH 2 C 6 F 13 )); −107.8 (s, −CH2CF2CF2CH3); −113.4 (broad s, −CH2C(CF3)15
dx.doi.org/10.1021/ma402147a | Macromolecules 2014, 47, 13−25
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Scheme 1. Reaction Pathways to Synthesize MAF-C6F13a
a Key: (i) 2 wt % concentrated H2SO4, reflux, 300 mbar after 3 h of the reaction, 20 h; yield = 55%; (ii) SOCl2, reflux, 22 h; yield = 60%; (iii) pyridine, CH2Cl2, −20 °C, 4 h; yield = 82%.
Scheme 2. Radical Copolymerization of VDF with MAF-C6F13 with or without Any Chain Transfer Agent CF3Ia
R can represent various functionalities such as −OC(CH3)3, −C(CH3)3, −C(CH3)2C2H5, or −CF3, whereas R′ can be −CF3, −C(CH3)3, −CH3, or −I.
a
(COOCH 2 CH 2 CF 2 (CF 2 ) 5 CF 2 CF 3 )); −113.8 (s, −CH 2 CF 2 − CF2CH2−, reverse addition of VDF or head to head VDF−VDF dyads); −114.8 (s, −CH2CF2CF2CH3); −116.1 (s, −CH2CF2− CF2CH2−, reverse addition of VDF or head to head VDF−VDF dyads); −121.7, −122.7, −123.5, and −126.1 as mentioned above. As expected, higher maximum pressure values (Pmax) were observed for the reactions at higher temperature. Thus, for the polymerizations at 90 °C, Pmax was in the 25−32 bar range. The decrease in the reactor pressure (ΔP = Pmax − Pmin) was found to be in the range of 5−18 bar, as shown in Table S1. Typically for the reactions with lower yield (15 mol %), high reaction temperature (90 °C) in C4H5F5 as the solvent, and high MAFC6F13 mol % (12 mol %) in feed have been required. In summary, molar percentages of VDF incorporated in the copolymers were high (>90 mol %) when the reactions were carried out with a high VDF mol % (94 mol %) in feed and at low reaction temperatures (55−75 °C). On the other hand, to achieve high mol % (>15 mol %) incorporation of MAF-C6F13, high reaction temperature (90 °C) in C4H5F5 and MAF-C6F13 mol % in feed higher than 12% was required. 4.2. Conditions Affecting the Yields of Copolymer. It is noteworthy that the obtained yields for both P1 and P2 copolymers are the highest among all the reactions, which confirm the efficiency of TBPPi initiator in radical copolymerizations of VDF.26,32 However, when TAPPi was used as the initiator at 55 °C, the yields obtained for this reaction were comparatively lower (35%) than those of other reactions. In addition they seem to be increasing with temperature for P4− P6 series. This is expected due to the increase in the reaction rate by faster decomposition of PPFR41 at 90 °C than it is at 65 or 75 °C and for higher VDF pressure in the autoclave at higher temperature. Interestingly, P7, P8, and P9 copolymers led to higher polymerization yields compared to other runs. The synthesis of P7 was carried out in C4H5F5 unlike those of P4− P6. Since C4H5F5 is a fluorinated solvent, it is expected to be more soluble with growing polymeric chains. However, the process is a precipitant copolymerization, as evidenced by the presence of a powder in the liquid medium. Hence, a deeper study deserves to be conducted to fully understand the state of the medium. The absence of acetonitrile as the solvent in P7 copolymerization further minimizes the chances of chain transfer reactions, thus increasing the molecular weights and the overall yields of resulting copolymers. P8 and P9 copolymers, were obtained in high yields (ca. 63%) which is in agreement with the results obtained by Asandei et al.41 for the homopolymerization of VDF catalyzed by Mn2(CO)10 (as activator of C6F13I) under light, in dimethylcarbonate. As discussed above, the actual phase behavior of the polymerization mixture is complex and might have an impact on the yield, especially since these reactions were performed at different temperatures (and thus at various pressures) in the presence of different solvents. Yield of P10 was surprisingly lower (38 mol %) than that of P11 and to other polymerization reactions initiated by PPFR. 20
dx.doi.org/10.1021/ma402147a | Macromolecules 2014, 47, 13−25
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P1, P5, P6, P8, P9 and, P11 copolymers exhibit two melting peaks (at 160 and 167 °C, Figure S23) in the second heating scan, whereas those of P2, P4, and P7 copolymers show a single melting peak. Broad and weak melting transition peaks were observed for P10 copolymer. In contrast, DSC thermogram of P3 copolymer displays three broad melting peaks (centered at 135, 145, and 155 °C, Figure S23) attributed to the melting of PVDF domains within the copolymeric chains due to the statistical distribution of the MAF-C 6 F 13 comonomer. Similarly, melting temperatures of α and β crystalline forms of PVDF are known to occur at different temperatures.5,6 Thus, it can be proposed that PVDF domains between two MAF-C6F13 units are absent or too short in P10 copolymer which thus displays an amorphous phase mainly, as for poly(VDF-co-HFP) copolymers.6 On the other hand, P6 copolymer consists of an amorphous fraction due to the presence of MAF-C6F13 (22 mol %) and also significant amount of crystalline part attributed to the presence of PVDF domains. 5.2. Thermal Stability of Poly(VDF-co-MAF-C6F13) Copolymers. TGA experiments were carried out under air at 20 °C min−1 heating rate (see Figure S24). Except for P11 that has a quite low Mn value (4100 g mol−1), all copolymers exhibit suitable thermostability (up to around 350 °C). The decomposition occurs in two stages. The first one begins at ca. 400 °C while the second one starts after 450 °C. Although the products of the decomposition were not analyzed, it can be argued that the first stage arises from the decomposition of ester group in MAF-C6F13, while the second one comes from the degradation of PVDF segments. The decomposition of PVDF homopolymer50 occurs mainly via two mechanisms, by chain stripping or “unzipping” (or depolymerization) and formation of polyenic structures by dehydrofluorination.50b,c In addition, the decomposition temperatures [10% weight loss (Td(10%))] for P1 and P2 copolymers (ca. 353 °C) are not very different. As for P3 sample that contains 3 mol % of MAFC6F13, a lower thermostability (Td(10%) = 349 °C) is noted, which is expected to arise from a slightly lower molecular weight polymer. Interestingly, Td(10%) increases from P4 to P7 copolymers (from 352 to 369 °C). For P4 and P5, Td(10%) was ca. 350 °C, in the similar range as that of P1 to P3, while for P6, P7, and P9, Td(10%) values were 369, 366, and 376 °C, respectively. The increased thermal stability for these copolymers can be associated with their high Mn values (ca. 54−61 kg mol−1). Thus, change in the polymerization medium contributed to small changes in the molecular weights and thermal properties of the obtained copolymers. Further, P9 exhibits a slightly better thermal stability attributed to the lower MAF-C6F13 incorporation compared to that in P6 and P7 and higher Mn values than that of P8 (Td10% = 355 °C). Td(10%) of P10 is similar to those of P1-P5 (ca. 350 °C). Further, with the similar Mn values, P6 and P7 copolymers (that contain ca. 20 mol % MAF-C6F13) were slightly less stable than P9 copolymer which has a lower MAF-C6F13 content (2 mol %). From P4 to P6, the copolymer fraction that decomposed in the second stage seems to be increasing compared to that in the first one (Figure 3). Surprisingly, this effect is observed even for P6 that contains 22 mol % of MAF-C6F13. The second stage of decomposition for P4−P6 copolymers matched well with the decomposition signal obtained for PVDF homopolymer as shown in Figure 3. These results suggest that with increasing
observed even in P10 copolymer synthesis where the PPFR’s concentration was three times lower than those in other polymerizations. P11 copolymer’s Mn value is the lowest (4100 g mol−1) as expected because of both higher PPFR amount used and the presence of CF3I. No specific relationships were observed between the obtained yields (or comonomer conversions) of the copolymers with their calculated Mn values. Similarly, it is difficult to comment on the effect of the phase behavior of the polymerization mixture on Mn of growing copolymer chains. Thus, the synthesis of poly(VDF-co-MAFC6F13) copolymer with suitable Mn and containing reasonable amount of MAF-C6F13 can be achieved at 90 °C with 0.9 mol % PPFR. 5. Thermal Properties of Poly(VDF-co-MAF-C6F13) Copolymers. The thermal properties of P1−P11 copolymers were investigated by means of DSC and TGA. 5.1. Assessments of Glass Transition and Melting Temperatures by DSC Experiments. DSC scans for all copolymers were made from −60 to +200 °C. Typical DSC thermograms are displayed in the Supporting Information (Figure S23) and enabled to determine glass transition (Tg) and melting (Tm) temperatures of the copolymers, as listed in Table 1. Tg values in the whole series of poly(VDF-co-MAF-C6F13) copolymers were found to be in the 8 to 29 °C range rather than ca. −40 °C, characteristic of the Tg value of PVDF homopolymer. As expected, the Tg also showed slight variation due to changing comonomers incorporation and change in Mn (Figure S23). Higher Tg values for all copolymers compared to that of PVDF homopolymer is assigned to the restriction in the bond rotations of VDF-MAF-C6F13 alternating dyads (attributed to the presences of both −CF3 and −CO2C2H4C6F13 groups) compared to that of VDF−VDF dyads in polymeric chains. PVDF homopolymers initiated by PPFR (with Mn in the range of 3000−6000 g mol−1) have melting temperatures and ΔHm values in the ranges of 140 to 150 °C and −50 to −60 kJ g−1, respectively.45,46 For 100% crystalline PVDF (α form), ΔHm is 104.6 kJ g−1 47−49 and a high melting point (ca. 170 °C).6 High MAF-C6F13 incorporation along with lower Mn (P2 copolymer) induced a lower onset melting temperature (129 °C) and lower melting enthalpy (ΔHm) value of 5.0 kJ g−1, compared to those of P1, which starts melting at ca. 143 °C with ΔHm = −24.2 kJ g−1. As for P3 copolymer that contains 3 mol % of MAF-C6F13, the onset melting temperature is 126 °C. As expected, higher ΔHm value (−30.5 kJ g−1) for such a copolymer among all the copolymers can be attributed to the low percentage of MAF-C6F13 incorporation in the polymeric chains. However, lower melting temperature onset is also related to low molecular weight copolymers and explains why the onsets of melting temperatures in P4−P9 copolymers series were higher (ca. 150−157 °C) than those of the P1−P3 series (126−143 °C). Melting enthalpies (ΔHm) increased from P4 to P6 copolymers (−13.7 to −25.5 kJ g−1, respectively). ΔHm value estimated for P7 copolymer is −22.3 kJ g−1 similar to that of P6. In the cases of P8−P9 copolymers, lower MAF-C6F13 incorporation was reflected into high ΔHm values (−27.0 and −32.2 kJ g−1 for P8 and P9 samples, respectively). These changes in the thermal properties of poly(VDF-co-MAF-C6F13) copolymers are attributed to both the higher molecular weights, and modification in crystallinity due to comonomer incorporation.47 The high Mn value (53,600 g mol−1) and low comonomer incorporation (2 mol %) for P9 induced a high melting enthalpy value (−32.0 kJ.g−1). DSC thermograms of 21
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Figure 4. TGA thermograms of PVDF homopolymer and various poly(VDF-co-MAF derivative) copolymers. PVDF homopolymer Solef, Mn >400 000 g mol−1, Td(10%) = 451 °C. Sample P2, (Table 1), (Mn = 17 400 g mol−1, VDF in copolymer 67 mol %, Td(10%) = 355 °C). Poly(VDF-co-MAF), (Mn = 21,000 g mol−1, VDF in copolymer 54 mol %, Td(10%) = 233 °C). Poly(VDF-co-MAF-tBu), (Mn = 49 900 g mol−1, VDF in copolymer 49 mol %, Td(10%) = 176 °C). Heating rate of 20 °C min−1 under an air flow.
Figure 3. First derivative plot of weight percentage with respect to the temperature of PVDF homopolymer, Solef, Mn > 400 000 g mol−1, P4−P6 copolymers from Table 1. The TGA experiments were carried out at 20 °C under an air flow.
Mn values, the fraction of PVDF segments within the copolymeric chains increases for P4−P6 series and they are complementary to increasing melting enthalpy values for the same copolymers. Comparing P2 and P10 that possess 67 and 69 mol % of VDF and have 17.4 and 22 kg mol−1 molecular weights, respectively, it is noted that P2 exhibits a higher thermal stability (Td(10%) = 355 °C) than P10 (Td(10%) = 346 °C). It is assumed that the depolymerization of P10 starts by HI elimination from VDF-I end-groups (since CF2−I exhibits a weak bond dissociation energy,51 ca. 226 J mol−1), followed by “unzipping” (or depolymerization) of the polymeric chains.50 The second stage of the decomposition in case of both copolymer samples was very small as displayed by the first derivative of weight percent loss plot in Figure S26. This suggests the presence of a small fraction of PVDF segments within the copolymeric chains of these samples. The presence of single stage of degradation of P10 copolymer (Mn = 21 000 g mol−1) in TGA thermogram is nicely corroborated with the absence of a melting temperature in DSC scan. On the other hand, P6 copolymer (Mn = 63 700 g mol−1) displays a two stage-decomposition as depicted in the first derivative plot of weight percent with respect to temperature (Figure 3) and a clear melting signal in DSC scan (Figure S23a). 6. Comparison of the Thermostability of Poly(VDF-coMAF-C6F13) Copolymers with Those of Copolymers Based on VDF and MAF Derivatives. Figure 4 displays the TGA thermograms under air flow of PVDF homopolymer and poly(VDF-co-MAF-C6F13) copolymers that contain three 2-trifluoromethyl acrylate derivatives: 2-trifluoromethacrylic acid (MAF), tert-butyl trifluoromethacrylate (MAF-tBu) and MAF-C6F13. PVDF homopolymer exhibits the highest thermostability (Td10% = 451 °C) attributed to high Mn (>400 000 g mol −1 ) and the absence of weak points (e.g., ester functionality) due to comonomer in the polymer chains, in contrast to poly(VDF-co-MAF-C6F13) copolymers. Though these copolymers have different Mns, those based on MAFC6F13 show superior thermal stability over those of the copolymers based on MAF or MAF-tBu. It can be clearly seen that P2 copolymer with (Mn = 17,400 g mol−1) and 33 mol % MAF-C6F13 is more stable than both poly(VDF-co-
MAF) with Mn = 21 000 g mol−1 and 46 mol % incorporated MAF and poly(VDF-co-MAF-tBu) with Mn = 49 900 g mol−1 and 51 mol % incorporated MAF-tBu. Both the latter copolymers start to decompose from 130 °C releasing isobutylene and CO2,19 whereas the decomposition of P2 occurs above 290−300 °C. The first wt % derivatives versus the temperature of three poly(VDF-co-MAF-monomer) copolymers are plotted in Figure S27. Three stages were observed in the thermal degradation of poly(VDF-co-MAF-tBu): initial stage starts at ca. 130 °C, the second above 250 °C, and further degradation of main chain occurs above 300 °C. Poly(VDF-coMAF) copolymers also undergo two-stage thermal degradation. The second stage of this degradation (above ca. 250 °C) is similar to that of poly(VDF-co-MAF-tBu), arising from the decomposition of carboxylic acid functionality. In contrast, C6F13 group in P2, seems to behave as a protective shield to prevent its early thermal oxidation. Onset of degradation for poly(VDF-co-MAF-C6F13) copolymers is above 250 °C, i.e. almost 100 °C higher than those of VDF copolymers that contain MAF or MAF-tBu. Thus, the presence of C2H4C6F13 fluorocarbon chain stabilizes the ester group on MAF-C6F13 and improves the thermostability of the resulting copolymers. 7. Kinetics of Thermal Degradation of Poly(VDF-coMAF-C6F13) Copolymers. To better understand the kinetics of thermal degradation, copolymer P6 was allowed to decompose in TGA in air at four constant heating rates (Figure 5) and the activation energies of the decomposition of that copolymer were estimated by Flynn and Wall’s method.53 The accuracy of this method is satisfactory due to use of multiple heating rates, in contrast to Friedman method, where only a single heating rate was used to determine the activation energy.54 The decomposition of P6 copolymer occurs in two stages as depicted by the plot of the first derivative of weight loss versus the temperature (Figure S26). The activation energies at various weight percentage losses of P6 copolymer during TGA experiment were calculated using eq 6. 22
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topography of these copolymers is expected to be different than that of PVDF homopolymer due to the presence of a C6F13 side-chain, which is known to influence the hydro- and oleophobicity of the polymeric materials.56 Figure 7 and Figure
Figure 7. Water droplets (∼20 μL) deposited on uniform thin films of polymer to determine static water contact angle (WCA) measurements at 20 °C of a) PVDF homopolymer Solef, Mn > 400 000 g mol−1 (94°), P1 (7 mol % MAF-C6F13, 112°), P4 (6 mol % MAFC6F13, 108°), P9 (2 mol % MAF-C6F13, 114°), and P10 (31 mol % MAF-C6F13, 114°) copolymers.
Figure 5. TGA thermograms under air flow of P6 (Table 1) at various heating rates: (a) 2.5, (b) 5.0, (c) 10, and (d) 20 °C min−1.
−R ⎡ d(log β) ⎤ E= ⎢ ⎥ b ⎣ d(1/T ) ⎦
S27 display the water contact angle measurements on the copolymer surface. The WCAs for all copolymers were observed to be in the range of 104 to 114° ± 3, greater than that of PVDF homopolymer 94° (or even 82°).57 The WCAs of particularly P8, P9, P10, and P11 copolymers were higher than those of the other copolymers. First two copolymers (containing 2−4 mol % MAF-C6F13) were synthesized in dimethyl carbonate solutions whereas P10 and P11 copolymers were produced by ITP. It has been previously reported that poly(4′-nonafluorobutylstyrene)s prepared by ITP display higher WCAs compared to those synthesized by conventional radical polymerization,58 that also confirms better surface properties of poly(fluoroacrylate)s by ATRP.59 However, a deeper study is required for the determination of surface energies and also on the molecular aggregation of C6F13 chains at the copolymer surface.
(6)
where E, R, T, β, and b are activation energy of copolymer decomposition, gas constant, temperature at constant conversion, heating rate, and a constant, respectively (b = 0.457 ± 3%).53 [d(log β)/d(1/T)] values are the slopes of the straight lines obtained by plotting log β versus 1/T for various heating rates as shown in Figure 6. The estimated average activation energies in the first and the second decomposition stages were 155 ± 5 and 159 ± 7 kJ mol−1, respectively. Li and Kim50a estimated the activation energy of PVDF homopolymer decomposition in the first stage as 219 ± 49 and 131 ± 48 kJ mol−1 in the second stage using the Friedman method.54,55 Similar slopes of the straight lines at various weight percentages of P6 copolymer (Figure 6) and comparable activation energies in both stages of decomposition imply that the degradation mechanism at various weight percentage losses in both stages is similar. 8. Surface Properties of Poly(VDF-co-MAF-C6F13) Copolymers by Static Water Contact Angle Measurements. Surface properties of poly(VDF-co-MAF-C6F13) copolymers were studied from static water contact angle (WCA) measurements using the sessile drop method. Surface hydrophobicity is a relevant property of fluoropolymers as much as thermal stability and other properties.1−7 Surface
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CONCLUSIONS A 2-trifluoromethacrylate monomer containing a C6F13 side chain (MAF-C6F13) was synthesized via two different routes. The purity and yield of this monomer was improved using the esterification of C6F13C2H4OH with 2-trifluoromethacryloyl chloride. MAF-C6F13 does not homopolymerize under radical conditions. However, its radical copolymerization with vinylidene fluoride (VDF) was successfully carried out at various polymerization conditions, and thus acts as a suitable partner of VDF. These conditions involved three different initiators (including a highly branched fluorinated persistent radical,
Figure 6. Plots of the logarithms of the heating rates versus reciprocal temperatures for the first stage of the decomposition of P6 at (a) 10%, (b) 20%, (c) 30%, and (d) 40% weight losses, respectively. The second stage of the decomposition is represented by (e) 60%, (f) 65%, and (g) 70% weight losses. 23
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PPFR), a chain transfer agent (CF3I) under iodine transfer copolymerization, at various polymerization temperatures (65− 90 °C) and using various solvents. For the polymerization carried out at 90 °C, a higher percentage of MAF-C6F13 was incorporated than for reactions achieved at 65 and 75 °C, when C4H5F5 was used as the solvent. Number-average molecular weights Mns and comonomer molar percentages in copolymers were affected by the polymerization temperature and nature of solvents. Mns of the copolymers initiated by PPFR were in the range 30.0−62.0 kg mol−1, and surprisingly, they increased with the polymerization temperatures. The effect of chain transfer agent is clearly observed as the decrease of Mns of the copolymers synthesized by ITP, for which −I was one of both end-groups. The thermostability of the resulting copolymers (>300 °C) was significantly improved compared to that of VDF copolymers with other existing MAF derivatives (10 h at 70 °C and, ∼50 h at 60 °C. All polymerization reactions were carried out for the respective times given in the caption of Table 1, keeping in mind the half-life time of PPFR. (43) Mekarbane, P. G.; Tabner, B. J. Macromolecules 1999, 32, 3620− 3625. (44) Ono, T.; Fukaya, H.; Hayashi, E.; Saida, H.; Abe, T.; Henderson, P. B.; Fernandez, R. E.; Scherer, K. V. J. Fluor. Chem. 1999, 97, 173− 182. (45) David, G.; Boyer, C.; Tonnar, J.; Ameduri, B.; LacroixDesmazes, P.; Boutevin, B. Chem. Rev. 2006, 106, 3936−3962. (46) Boschet, F.; Ono, T.; Ameduri, B. Macromol. Rapid Commun. 2012, 33, 302−308. (47) Nandi, A. K.; Mandelkern, L. J. Polym. Sci., Part B: Polym. Phys. 1991, 29, 1287−1297. (48) Durand, N.; Ameduri, B.; Boutevin, B. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 82−92 For low molecular weight PVDF homopolymers (telomers with Mn of 800 to 1,200 g mol−1) synthesized in this work, melting enthalpy values are 70−90 kJ 25
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