Article pubs.acs.org/IECR
Carboxylic Acid-Functionalized Butyl Rubber: Synthesis, Characterization, and Physical Properties Matthew J. McEachran,† John F. Trant,† Inderpreet Sran,‡ John R. de Bruyn,§ and Elizabeth R. Gillies*,†,‡ †
Department of Chemistry, ‡Department of Chemical and Biochemical Engineering, and §Department of Physics and Astronomy, The University of Western Ontario, 1151 Richmond Street, London, Canada, N6A 5B7 S Supporting Information *
ABSTRACT: Polyisobutylene (PIB) and other PIB-based materials are of significant interest for a vast array of applications, but chemical modification is often required to obtain the desired properties. Described here are two new approaches for the preparation of carboxylic acid-functionalized PIB. The ring opening of cyclic anhydrides from an allylic alcohol derivative of butyl rubber and the atom transfer radical polymerization of tert-butyl methacrylate from a rubber derivative both ultimately afford carboxylated materials. These materials displayed significantly enhanced adhesion to stainless steel, as well as increased ultimate tensile strength and Young’s modulus in comparison to unmodified rubbers. Rheological studies suggested that they exhibit a greater degree of cross-linking-type behavior than the parent butyl rubber. Combined, these studies suggest that the properties of PIB can be readily tuned through synthetic modifications of the backbone, even at low mole percent, and that carboxylic acid moieties can impart desirable properties for various applications.
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INTRODUCTION Polyisobutylene (PIB)-based materials are widely used commercially in a diverse array of products such as automobile tires, sporting equipment, adhesive sealants, viscosity modifiers, chewing gum, and drug eluting stents.1,2 The widespread use of these polymers can be attributed to their favorable properties including exceptional thermal and chemical stability, impermeability to gas and water, high damping, and high elasticity. While the saturated hydrocarbon backbone is responsible for many of PIB’s advantageous properties, it also limits the ability to modify and tune the properties of the polymer. For this reason, many applications of PIB have involved the introduction of chemical functionalities to the PIB backbone or termini, or the incorporation of PIB into block copolymers. For example, isobutylene can be copolymerized with small amounts (i.e., < 8 mol %) of isoprene to produce a random copolymer commonly referred to as butyl rubber. These sites of unsaturation on the isoprene can be used to cross-link the rubber, providing the mechanical properties required for many applications. In the Taxus vascular stent, a linear triblock copolymer of polystyrene (PS)−PIB−PS (SIBS) is used in the drug-eluting coating.3,4 The PS blocks impart thermoplastic properties to the rubber, not only allowing it to behave as a cross-linked rubber at physiological temperature, but also making it readily processable at higher temperatures or in solution. There are also many other examples involving the functionalization of PIB for its incorporation into polymer networks5−8 as well as linear,9−13 star,14−16 miktoarm,17−19 graft,20−25 and comb26,27 copolymers. With these architectures, interesting functions such as stimuli-responsive network swelling,5,7 templating of inorganic materials,9,19,28 cell encapsulation,29 controlled drug release, 30 and protein patterning and resistance20,22 have been achieved. Of the various chemical functionalities that can be introduced to PIB, carboxylic acid moieties are of particular interest. First, © XXXX American Chemical Society
they provide versatile functional handles that can be used for further functionalization with a wide range of nucleophiles such as amines and alcohols. Second, they provide sites for ion-pair or hydrogen-bond-mediated aggregation within the rubber, allowing for the alteration of properties such as mechanical strength and rheological behavior.31−33 Third, carboxylic acid moieties can enhance adhesion to metal surfaces.34−36 This has the potential to reduce delamination from vascular stents, a challenge that has been previously reported for the SIBS material in the Taxus stent.3 There are several examples involving the functionalization of the PIB terminus with carboxylic acid moieties,37,38 as well as the polymerization of t-butyl acrylate and t-butyl methacrylate from atom transfer radical polymerization (ATRP) initiation sites at the terminus or in the middle of the PIB chain, followed by acidic or thermal deprotection to provide carboxylic acid moieties.12,17,18,39 Conetworks have been formed by the polymerization of ethoxyethyl-protected methyl methacrylate with methacrylate-terminated PIB, followed by acidic deprotection.40 There are also examples involving the introduction of pendant carboxylic acids along the backbone of butyl rubber via the grafting of maleic anhydride, followed by the anhydride ring-opening with amines or alcohols.41−44 However, there are still relatively few studies concerning the effects of these carboxylic acid moieties on the properties of these modified materials.42,43,45,46 We have recently reported a simple and highly efficient epoxidation/elimination sequence to provide access to butyl rubber derivatives having allylic alcohol moieties along the polymer backbone.20,20,21 These hydroxyl groups were activated Received: February 2, 2015 Revised: April 8, 2015 Accepted: April 13, 2015
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DOI: 10.1021/acs.iecr.5b00421 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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reduced pressure. The product was purified by precipitation (2:1 acetone/toluene) and then dried under high vacuum to provide 9.1 g of polymer 4d-l as an off-white amorphous solid in 90% yield. Tg = −61 °C. 1H NMR (400 MHz, CDCl3): δppm 5.29 (br s, 1H), 5.12 (s, 1H), 4.95 (s, 1H), 4.20−4.40 (m, 4H), 1.42 (s, 145H), 1.12 (s, 431H). IR (KBr pellet): 1230, 1365, 1390, 1475, 1733, 1758, 2974 cm−1. SEC (CHCl3): Mw = 309 kg/mol, PDI = 2.5. Synthesis of Polymer 4d-h. Epoxidized butyl rubber derivative 1-h22 (10 g, 12 mmol of epoxide) was dissolved in 350 mL of anhydrous toluene. To this solution was added 1 equiv of HCl (1.0 mL, 12 mmol), and the reaction mixture was stirred at room temperature for 1 h. Because of solubility issues, 2-h was not isolated. Instead, the HCl was neutralized with sodium carbonate and then the solution was dried with MgSO4. The mixture was centrifuged and the solution of 2-h was decanted from the MgSO4. The solution was then heated to 70 °C, and 20 equiv of NEt3 (33.7 mL, 242 mmol) were added followed by 2 equiv of DMAP (3.1 g, 24.2 mmol). A solution of diglycolic anhydride (10 equiv, 14.0 g, 121 mmol) dissolved in toluene was then added, and the reaction mixture was stirred at 70 °C overnight. The product was isolated and purified as described above for 4d-l to provide 8.6 g of the polymer as a white amorphous solid in 86% yield over the two steps. Tg= −53 °C. 1H NMR (400 MHz, CDCl3): δppm 5.29 (brs, 1H), 5.12 (s, 1H), 4.95 (br, 1H), 4.20−4.40 (m, 4H), 1.42 (s, 69H), 1.12 (s, 209H). IR (thin film from CH2Cl2): 1230, 1365, 1390, 1475, 1733, 1758, 2974 cm−1. SEC data could not be obtained because of interactions of the carboxylic acid groups with the column. Synthesis of Macroinitiator 5. Allylic alcohol functionalized polymer 2-l21 (10 g, 3.9 mmol hydroxyl) was dissolved in 350 mL of anhydrous toluene. The solution was heated to 70 °C. NEt3 (7.6 mL, 55 mmol) was added followed by 2bromoisobutyryl bromide (7.0 mL, 55 mmol). The solution was stirred for 16 h at 70 °C then the reaction mixture was washed with distilled water three times before being concentrated under reduced pressure. The product was further purified by precipitation (2:1 acetone/toluene) and then dried under high vacuum to provide 7.8 g of 5 as a white amorphous solid in 77% yield. 1H NMR (400 MHz, CDCl3): δppm 5.22− 5.15 (m, 2H), 4.92 (s, 1H), 1.97 (s, 6H), 1.42 (s, 290H), 1.12 (s, 913H). IR (thin film from CH2Cl2): 1232, 1367, 1390, 1479, 1736, 2977 cm−1. SEC (THF): Mw = 397 kg/mol, PDI = 2.8. Synthesis of Graft Copolymer 8a. A predried Schlenk tube purged with nitrogen and equipped with a magnetic stir bar was charged with CuBr (0.19 g, 0.13 mmol), 1,1,4,7,10,10hexamethyltriethylenetetramine (HMTETA) (71 μL, 0.26 mmol), deoxygenated tert-butyl methacrylate (tBMA) (0.32 mL, 2.0 mmol), and anhydrous toluene. The tube was then sealed with a rubber septum and degassed through three freeze−pump−thaw cycles. Immediately following the addition of the macroinitiator 5 (0.36 g, 0.13 mmol of initiation sites) as a degassed, anhydrous toluene solution, the reaction mixture was completely sealed and warmed to 70 °C for 12 h. After cooling, the solution was exposed to atmosphere and concentrated under reduced pressure. The crude solution was precipitated into acetone and the resulting solid was extensively washed with water, then dried in vacuo to provide 0.45 g of graft copolymer 8a as an amorphous white solid. 1H NMR (400 MHz, CDCl3): δppm 5.16 (br s, 2H), 4.92 (s, 1H), 2.10−1.60 (m, 45H), 1.60−1.35 (m, 598H), 1.20−0.85 (m, 1244H). IR
and reacted with alcohol and amine functionalized poly(ethylene oxide) (PEO) to provide butyl rubber-PEO graft copolymers. They have also been further derivatized to promote elimination to exo-dienes, which can undergo Diels− Alder reactions to prepare graft copolymers with or without carboxylic acid moieties.44 Here, we present two different approaches for the elaboration of the allylic alcohol moieties to introduce pendant carboxylic acid moieties via either the ring opening of cyclic anhydrides or the introduction of poly(carboxylic acid)s. The latter approach involves the conjugation of α-bromoisobutyryl groups as initiators for ATRP, followed by a grafting-from polymerization of tert-butyl methacrylate (tBMA), and finally deprotection to provide butyl rubberpoly(methacrylic acid) graft copolymers. Combined, these two approaches allow the carboxylic acid loading to be readily tuned. We also explore the effects of the carboxylic acid moieties on the adhesion of the resulting materials to stainless steel surfaces, as well as their tensile and rheological properties.
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EXPERIMENTAL SECTION General Procedures and Materials. Two forms of butyl rubber, one with 2 mol % isoprene, Mw = 400 kg/mol, PDI = 2.8 and the other having 7 mol % isoprene, Mw = 1050 kg/mol, PDI = 3.3 were provided by LANXESS, Inc. (London, Canada). Polymer 2-l was prepared as previously reported.21 Solvents were purchased from Caledon Laboratories (Caledon, Ontario). All other chemicals were purchased from either Sigma-Aldrich or Alfa Aesar and were used without further purification unless otherwise noted. Dry toluene was obtained from an Innovative Technology (Newburyport, USA) solvent purification system based on aluminum oxide columns. Dichloromethane, pyridine, and triethylamine were freshly distilled from CaH2 prior to use. 1H NMR spectra were obtained in CDCl3 at 400 or 600 MHz on Varian Inova instruments. NMR chemical shifts (δ) are reported in ppm and are calibrated against residual solvent signals of CHCl3 (δ 7.26). Infrared spectra were obtained of films from CH2Cl2 on NaCl plates or as KBr pellets using a Bruker Tensor 27 instrument. Size exclusion chromatography (SEC) was performed in CHCl3 or tetrahydrofuran (THF) with a flow rate of 1 mL/min at 25 °C using an SEC instrument equipped with a Viscotek Max VE2001 solvent module and a Viscotek VE3580 RI detector operating at 30 °C. The stationary phase employed two PolyPore columns (300 mm × 7.5 mm, Agilent) connected in series equipped with a Polypore guard column (50 mm × 7.5 mm). Molecular weight calibration was carried out using polystyrene standards. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed on a Mettler Toledo DSC 822e instrument. For DSC, the heating/ cooling rate was 10 °C/min between −120 and +150 °C. The glass transition temperature (Tg) was obtained from the second heating cycle. Synthesis of Polymer 4d-l. Allylic alcohol functionalized polymer 2-l21 (10 g, 3.9 mmol of hydroxyl groups) was dissolved in 350 mL of anhydrous toluene in a nitrogen purged flask containing a magnetic stir bar. The solution was heated with stirring to 70 °C, then triethylamine (NEt3) (11 mL, 78 mmol) and 4-(dimethylamino)pyridine (DMAP) (0.99 g, 7.8 mmol) were added. Finally, a solution of diglycolic anhydride (4.5 g, 39 mmol) dissolved in anhydrous toluene (30 mL) was added, and the reaction mixture was stirred at 70 °C for 16 h. After being cooled, the product was washed with distilled water and then twice with 6 M HCl before being concentrated under B
DOI: 10.1021/acs.iecr.5b00421 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Scheme 1. Synthesis of Butyl Rubber with Pendant Carboxylic Acid Moieties
(KBr pellet): 1253, 1367, 1392, 1483, 1728, 2981 cm−1. SEC (THF): Mw = 195 kg/mol, PDI = 2.3. Synthesis of Graft Copolymers 8b,c. These polymers were prepared by the same procedure described above for 8a, but varying the equivalents of tBMA (50 equiv for 8b and 100 equiv for 8c). 8b was purified by precipitation into acetone and 8c was purified by precipitation into methanol. 1H NMR spectra are included in the Supporting Information. Synthesis of Graft Copolymer 9a. Graft copolymer 8a (0.89 g) was dissolved in CH2Cl2 (2.6 mL) in a round-bottom flask equipped with a magnetic stir-bar. A large excess of trifluoroacetic acid (TFA) (2.6 mL) was then added, and the reaction mixture was stirred for 24 h at ambient temperature. After the mixture was concentrated under reduced pressure, residual TFA was removed through azeotroping with toluene and the product was dried in vacuo to provide 800 mg of 9a as a brown amorphous solid. The material was found to be insoluble in all standard NMR solvents and was consequently only characterized by IR spectroscopy. IR (thin film on NaCl plate): 1203, 1280, 1390, 1450, 1488, 1701, 2603, 2972, 3471 cm−1. Adhesion Testing. A Monsanto Tel-Tak model TT-1 was used to determine the adhesion of rubber samples to a stainless steel substrate surface. The adhesion test procedure was based on ASTM D-429 Method A. The compound was sheeted from a two-roll mill and cut into 5″ × 3″ sample sheets. The sheets were then pressed into a 12.7 cm × 7.6 cm mold containing square woven fabric using a 15 pound weight for 5 min at 100 °C. The mold was backed by Mylar on one side and Teflon on the other in order to preserve the integrity of the sample surfaces. Stainless steel surfaces were cleaned with ethanol, while the Teflon and Mylar were wiped down with ethanol directly prior to testing. All surfaces were cut into test strips measuring 6.35 mm × 50.8 mm. The rubber specimen was placed face up into the bottom of the sample holder of the TelTak apparatus, and the protective Mylar layer was removed. The stainless steel substrate surface was placed into the top sample holder above the specimen. The surfaces were moved into contact with one another, and a built-in timer set to 60 s was automatically activated. A contact pressure of 32 psi with a 450 g weight was applied. Following the 60 s contact time, the specimen and substrate surfaces were separated from one another at a speed of 2.54 cm per minute. The force required to separate the specimen from the surface was measured using a calibrated force gauge with a capacity of 2270 g and a built-in indicator for the maximum value. Tests were carried out in triplicate, and the mean values ± standard deviation are reported. Tensile Testing. To prepare samples for tensile testing, 1.5 g of polymer was compressed into a 0.3 mm thick flat sheet using a hydraulic hot press (Carver Hydraulic Unit model no. 3851 OC). Samples 60 mm × 5 mm in size were cut from this
sheet. The tensile test was performed at ambient (22 ± 1 °C) temperature using an Instron 3365 series universal testing machine with a 1-kN load cell at an extension rate of 400 mm/ min, in accordance with ASTM D882-12.47 Load and extension were calibrated prior to the test. To prevent slippage of the samples from the clamps of the testing machine, 10 mm of material was inserted into each clamp, leaving an effective sample length of 40 mm. At least six trials were performed for each polymer. Rheology. Rheological measurements were performed using a TA Instruments AR-1500ex stress-controlled rheometer with a 25 mm-diameter parallel-plate tool. Sandpaper was glued to both plates to prevent slip. Circular samples 25 mm in diameter were cut from a sheet prepared by compressing 1.5 g of polymer into a flat sheet using the hydraulic hot press. The sample thickness was measured at three different places, and was approximately 0.5 mm for all samples. The sample was then placed in the rheometer, the gap between the plates was set to the lowest of these measurements, and the sample was annealed at 100 °C for 1 h. Small-angle oscillatory shear measurements were carried out on each sample. Oscillatory tests were performed at angular frequencies between 0.1 and 100 rad/s with the oscillating stress amplitude controlled at 100 Pa, which we confirmed was in the linear viscoelastic regime for our materials. All rheological measurements were done at 37 °C. The data were averaged over at least three trials for each polymer.
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RESULTS AND DISCUSSION The starting material for the synthesis was a previously reported epoxidized butyl rubber derivative.21,22 This material was prepared from butyl rubber containing either low (2 mol %) or high (7 mol %) isoprene content by reaction of the isoprene units with m-chloroperoxybenzoic acid in toluene, providing derivatives containing either 2 mol % (1-l) or 7 mol % (1-h) epoxidized monomers. As previously reported, 1-l was reacted with aqueous HCl in toluene to provide the allylic alcohol derivative 2-l (Scheme 1).21 For the initial synthetic studies, the reactivity of 2-l with various cyclic anhydrides (3a−d) was investigated. As shown in Scheme 1, the reaction of 2-l with excess maleic anhydride 3a in the presence of triethylamine (NEt3) and 4(dimethylamino)pyridine (DMAP), standard conditions for the reaction of alcohols with anhydrides,48,49 resulted only in the recovery of the starting material. The saturated, and thus more reactive succinic anhydride, 3b, under the same conditions, provided only minimal conversion to the desired carboxylic acid, 4b-l. The more reactive pentadioic anhydride 3c provided approximately 50% conversion to acid 4c-l, while finally the electron deficient diglycolic anhydride 3d provided a quantitative conversion of all allylic alcohols to the correspondC
DOI: 10.1021/acs.iecr.5b00421 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research ing acid 4d-l based on 1H NMR spectroscopy (Figure 1 and Supporting Information). Even if the excess of anhydride was
consistent with the introduction of carboxylic acid moieties via ester linkages (Supporting Information). Chloroform performed better than THF for SEC of 4d-l, and a weight-average molecular weight (Mw) of 309 kg/mol and a polydispersity index (PDI) of 2.5 relative to polystyrene standards was measured under these conditions. This was a modest decrease relative to the starting butyl rubber, which had an Mw of 400 kg/mol and a PDI of 2.8. This might be attributable to some interactions of the carboxylic acid moieties with the column rather than due to any degradation of the backbone. Because of column interactions, meaningful SEC results could not be obtained for 4d-h. TGA was performed on both 4d-l and 4d-h. The onset degradation temperatures (To) were found to be 349 and 315 °C, respectively, in comparison with 368 °C for the starting 2 mol % isoprene rubber and 352 °C for the starting 7 mol % isoprene rubber. This indicates that the incorporation of the carboxylic acid and/or the diglycolic acid linker moieties lowers the thermal stability of the rubber to some extent. However, the materials are still stable up to more than 200 °C, which would be suitable for many applications. DSC revealed that polymer 4d-l had a glass transition temperature (Tg) of −61 °C, in comparison to −70 to −63 °C reported for butyl rubber.50,51 A further increase to −53 °C was observed for polymer 4d-h with the higher carboxylic acid content. It is likely that the increase in intermolecular hydrogen-bonding and dipole−dipole interactions due to the introduction of the polar pendant groups restricts the segmental motion of the polymer, resulting in this increase in Tg. Because of the propensity of isoprene to undergo chain transfer reactions, the synthesis of butyl rubber with much higher isoprene content is a significant challenge.52 Therefore, an alternative route to increase the quantity of pendant carboxylic acids is to polymerize poly(carboxylic acid)s from the butyl rubber backbone. ATRP of polystyrene from PIB-cop-methylstyrene-co-p-bromostyrene was performed using the benzylic halides as ATRP initiation sites to prepare graft copolymers.23 Allyl halides, sec-benzylic chlorides, and αbromoisobutyryl groups on the terminus of linear PIB and other sites on PIB and its copolymers have also been used for ATRP.12,16,18,53−55 In the present report, the allylic alcohol moieties on 2-l were used to introduce α-bromoisobutyryl groups via reaction with α-bromoisobutyryl bromide in the presence of NEt3 to afford the ATRP macroinitiator 5 (Scheme 2).
Figure 1. Downfield region of the 1H NMR spectra of (a) polymer 2-l and (b) polymer 4d-l showing conversion of the allylic alcohol to the carboxylic acid derivative.
dropped to 10 equiv, quantitative conversion was obtained. To achieve a higher carboxylic acid content, the reaction was also performed on the material derived from the 7 mol % isoprene rubber. In this case, the epoxide 1-h was directly converted to 4d-h without isolating the alcohol intermediate. In addition to NMR spectroscopy, the products were also characterized by infrared (IR) spectroscopy. The appearance of two peaks in the carbonyl region at 1728 and 1748 cm−1 was
Scheme 2. Synthesis of PIB with Pendant Poly(Carboxylic Acid)s via ATRP Grafting
D
DOI: 10.1021/acs.iecr.5b00421 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research ATRP of tBMA (6) from macroinitiator 5 was performed under standard conditions using CuBr with 1,1,3,5,7,7hexamethylenetetramine (7) as a ligand. The equivalents of tBMA were varied from 15 to 100 per initiation site to give graft copolymers (8a−c) with varying composition (Table 1). The Table 1. Synthesis and Characterization of Poly(tbutylmethacrylate)-butyl Rubber Graft Copolymers with the Degree of Polymerization Determined Using both TGA and NMR degree of polymerization polymer
equiv of methacrylate
wt % labile t-butyl group (TGA)
TGA
NMR
8a 8b 8c
15 50 100
9 19 33
5 17 89
6 17 54
Figure 3. Size exclusion chromatograms of polymers 8a−c.
Table 2. Molecular Weight Data for the Graft Polymerization As Determined by SEC Analysis in THF
resulting copolymers were purified by precipitation into acetone and characterized by 1H NMR and IR spectroscopies, TGA and SEC. IR spectra clearly show the characteristic carbonyl stretch of the methacrylate at 1718 cm−1 (Figure 2).
polymer
equiv
Mw (kg/mol)
Mn (kg/mol)
PDI
5 8a 8b 8c
0 15 50 100
400 200 280 570
140 86 96 180
2.8 2.3 3.0 3.6
adventitious radicals during the polymerization.56,57 8a and 8b, with lower molecular weight poly(tBMA) arms could be precipitated in acetone, in which the contaminating poly(tBMA) homopolymer is soluble; however, the higher poly(tBMA) content of 8c made the graft copolymer partially soluble in acetone, necessitating precipitation in methanol along with the equally insoluble poly(tBMA) homopolymer. The presence of homopolymer contamination in 8c means that the degree of polymerization calculated from TGA and NMR results would be slightly overestimated. Table 2 also shows that the apparent molecular weights of 8a and 8b decreased relative to that of the macroinitiator 5 (Mw = 400 kg/mol) upon grafting of the poly(tBMA) arms. This can likely be attributed to the different chemical composition as well as the branched architecture of the graft copolymers rather than due to any cleavage of the backbone.24 However, the Mw did increase systematically with increasing ptBMA chain length, reaching an Mw of 570 kg/mol at the highest degree of polymerization in 8c. With the graft copolymers in hand, deprotection of the tertbutyl groups was performed to unmask the poly(carboxylic acid). Thermolysis, an approach previously used to cleave the tert-butyl esters on various PIB-poly(t-butyl acrylate) copolymers18,46,55 failed to remove a significant fraction of the tertbutyl groups. This may result from the impermeability of PIB to gases, which would prevent the escape of the reaction product isobutylene. In the previously reported cases the lower PIB molecular weight and the presence of other blocks such as polystyrene in the structures would have increased their permeability. After determining that the butyl rubber backbone was stable to trifluoroacetic acid (TFA), standard tert-butyl ester deprotection conditions involving TFA/CH2Cl2 were used to remove the protecting groups from 8a to prepare 9a (Scheme 2).58 Unfortunately, following the removal of the solvent, the resulting copolymer was found to be insoluble in all standard NMR solvents including chloroform-d, benzene-d6,
Figure 2. IR spectra of acid-treated polymer 9a (dotted line) superimposed on that of its parent ester 8a (solid line).
On the basis of NMR spectroscopy, assuming that all initiation sites on macroinitiator 5 initiated polymerization, the degree of polymerization ranged from 5 to 55 (Table 1, Supporting Information, Table S1). To support this analysis, TGA was also used to calculate the degree of polymerization as the tert-butyl groups are cleaved thermally at ∼210 °C, before the onset of degradation of the polymer backbone. This provides an additional means of calculating the degree of polymerization. As shown in Table 1, these values were in good agreement with those obtained by NMR spectroscopy for 8a and 8b, though for 8c the NMR approach underestimates the value relative to the TGA calculation. SEC traces are shown in Figure 3 and the Mw, numberaverage molecular weights (Mn) and PDIs for the copolymers are shown in Table 2. Most notably, the SEC traces of copolymers 8a and 8b are monomodal while 8c has a small side peak attributable to the presence of a small amount of homopolymer, which was not successfully removed during purification. It is likely that this homopolymer arises due to either a small degree of chain transfer or the presence of E
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isoprene content gave a separation force of 30.3 psi, very similar to that of the carboxylic acid functionalized 4d series.52 Importantly, our results indicate that significantly enhanced adhesion can be achieved even at very low (2 mol %) carboxylic acid content. Tensile Testing. Tensile strength is very important for predicting the structural integrity and strength of the material and determining its performance in load-bearing applications. Tensile testing of samples 1-l/h, 2-l/h, and 4d-l/h was performed using the standard ASTM D882-12 protocol.47 Representative stress−strain curves are shown in Figure 4, and
methanol-d4, acetone-d6, and DMSO-d6. Consequently IR spectroscopy was the primary method used for the characterization of polyacid 9a, and although not conclusive, it suggests that the desired transformation occurred. The spectrum of 8a is shown in Figure 2, along with that of tert-butyl protected 9a for comparison. The carbonyl absorbance in 8a broadens and shifts to shorter wavelengths, a result consistent with the spectroscopic data previously observed by Storey and co-workers in their deprotection of similar tert-butyl acrylates.59 Further evidence for the deprotection is provided by the red shift of the C−O stretch at 1100 cm−1, which is consistent with the conversion of an ester to a carboxylic acid. This spectroscopic result, along with the change in the physical properties of the material that made it insoluble in all examined solvents, is strong evidence for the success of this synthetic process. However, because the difficulty in manipulating the poly(carboxylic acid) functionalized rubber limits its future use as a biomedical material, we did not further explore the physical properties of this material. Conseqeuntly, physical characterization of the pendant carboxylic acid material 4d, described above, was the focus of further studies. Measurement of Adhesion. The adhesion of rubber to stainless steel is critical for its use in vascular stent coatings, as well as for a vast array of other potential coating applications as weak adhesion can result in delamination. Unmodified butyl rubber is known to show only moderate adhesion to stainless steel.60 It is therefore of interest to determine whether the introduction of carboxylic acid moieties enhances the rubber’s adhesivity. In addition, the adhesivity of the various synthetic intermediates, including the epoxide- and hydroxyl-functionalized rubber derivatives, was also studied. To measure adhesion, a butyl rubber sample was pressed between two stainless steel plates, and the force required to separate the plates was measured. As shown in Table 3, all of
Figure 4. Representative stress−strain curves for functionalized rubber derivatives.
Table 3. Adhesive Properties of Butyl Rubber Derivatives sample
separation force (psi)
sample
separation force (psi)
PIB-l 1-l 2-l 4d-l
14.6 ± 0.3 22 ± 2 29.6 ± 0.6 33 ± 1
PIB-h 1-h 2-h 4d-h
12.1 ± 0.2 25 ± 2 20 ± 2 28 ± 3
the Young’s modulus, ultimate tensile strength, and elongation at break are summarized in Table 4. The Young’s moduli of the Table 4. Tensile Properties of the Polymers
the functionalized rubber derivatives exhibited stronger adhesion than the parent butyl rubbers. As expected, the carboxylic acid functionalized rubber had the highest adhesivity, likely due to the ability of the carboxylic acid moieties to undergo specific ligand−metal interactions at the stainless steel surface. The hydroxyl-functionalized rubber also exhibited good adhesivity for similar reasons. On the basis of the expected interactions with the metal surface, it was anticipated that the materials derived from the butyl rubber with the higher isoprene content (PIB-h, 1-h, 2-h, 4d-h) would exhibit higher adhesivity than the analogous materials prepared from the lower isoprene content rubber (PIB-l, 1-l, 2-l, 4d-l). This was not the case, however. The lower apparent adhesion of 2-h and 4d-h can possibly be attributed to the properties of the materials, as the test resulted in the fracture of the material itself during the test, rather than delamination from the metal surface. These properties will be explored further through mechanical and rheological studies described below. For comparison, a comparable experiment on a phosphoniumfunctionalized rubber prepared from butyl rubber with 6.5%
sample
Young’s modulus (MPa)
ultimate tensile strength (MPa)
elongation at break (%)
PIB-l 1-l 2-l 4d-l PIB-h 1-h 2-h 4d-h
0.6 ± 0.2 0.4 ± 0.1 0.43 ± 0.07 0.35 ± 0.08 0.59 ± 0.02 0.41 ± 0.07 0.8 ± 0.3 3.1 ± 0.8
0.23 ± 0.01 0.20 ± 0.03 0.32 ± 0.05 1.7 ± 0.3 0.8 ± 0.2 0.25 ± 0.04 0.42 ± 0.05 3.5 ± 1
770 ± 70 800 ± 300 1600 ± 600* 600 ± 100 430 ± 30 800 ± 100 600 ± 100 150 ± 30
*
Some samples failed to break at the maximum elongation of 2000%. Data presented are the mean of six measurements per polymer, and uncertainties are the standard deviations.
butyl rubber with 2 mol % isoprene (PIB-l) and its derivatives 1-l, 2-l, and 4d-l, were all similar, in the range of 0.3−0.6 MPa. The tensile strength of 1-l was the same as that of the original butyl rubber. The tensile strength was slightly higher for the hydroxyl-functionalized polymer 2-l, and increased significantly for the carboxylic-acid-functionalized polymer 4d-l. The observed increase in strength may result from ionomeric or hydrogen-bond-mediated cross-linking involving the carboxylic F
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materials. In Figure 6, the elastic and viscous moduli at ω = 1 rad/s are shown for all of the materials. In all cases, G′ ≫ G″, but the values of the moduli vary by an order of magnitude, depending on functionalization. At this frequency, both unmodified butyl rubbers, PIB-l and PIB-h, have elastic moduli slightly less than 105 Pa and viscous moduli about a factor of 3 smaller. The epoxide derivative 1-h has moduli similar to its parent unmodified rubber at ω = 1 rad/s, and indeed, over the full frequency range studied. The carboxylic acid and hydroxyl derivatives 4d-h and 2-h, which have the ability to form hydrogen bonds, have substantially lower moduli but qualitatively similar frequency dependence. Although the moduli of PIB-l and PIB-h are similar, the materials derived from PIB-l have much lower viscous and elastic moduli than the corresponding derivatives of PIB-h. In general, the hydrophobic derivatives have higher moduli than the hydrophilic derivatives, with the carboxylic acid derivative 4d-l having the lowest value over most of the frequency range studied. The ratio of the viscous and elastic moduli G″/G′ is equal to tan δ, where δ is the phase angle between the applied stress and the measured strain; tan δ is less than 1 for predominantly elastic materials, while it is greater than 1 for viscous materials. Figure 7 shows tan δ for the polymers studied here. Tan δ is less than 1 in all cases and at all frequencies studied, reflecting the fact that these materials are all primarily elastic. For a given polymer, the moduli are expected to become equal to each other and cross over at some low frequency ωc, with the reciprocal of the crossover frequency being a measure of the slowest relaxation time in the system. Although ωc is below the frequency range we investigated, the approach to the crossover is indicated by a rise in tan δ as the frequency is lowered. Our data suggest that ωc is highest and, correspondingly, the polymer relaxation time is shortest, for polymer 2-l. The relatively high values of tan δ we observed at low ω for 2-l and 2-h and their fairly low moduli are consistent with our qualitative observation that these materials are softer and behave more like weak gels than like rubbers. On the other hand, the carboxylated polymers 4-l and 4-h show no increase in tan δ at low ω, suggesting that the relaxation time in these materials is much longer. This suggests that the carboxylic acid groups in these materials do indeed form a significant number of cross-links that restrict the dynamics of the polymer molecules.
acid moieties and, to a lesser extent, the hydroxyl groups. No significant trend was observed in the data for percent elongation at break. Similar trends were observed for the compunds 1-h, 2-h, and 4d-h, derived from butyl rubber with 7 mol % isoprene (PIBh), although the effects were more significant because of the higher density of functional groups. For example, the carboxylic acid derivative 4d-h, had a Young’s modulus of 3.10 MPa, 5 to 10 times higher than all of the other materials, and an ultimate tensile strength of 3.46 MPa, again higher than the other materials, including 4d-l. Elongation at break for 4d-h was significantly smaller than for any of the other materials, indicating that it is more brittle. This increase in brittleness supports the observations made in the adhesion tests. At this increased level of carboxylic acid loading, the brittleness of the material outweighs possible increases in adhesion and interactions with stainless steel surfaces. Overall, these results suggest that the introduction of chemical functionalities, and in particular carboxylic acid moieties, along the butyl rubber backbone results in significant changes in tensile properties, and that the magnitude of these changes depends on the degree of functionalization. While the carboxylic-acid-functionalized materials do not exhibit the same tensile properties as crosslinked rubber or SIBS,3,61 their properties are indicative of a physical or supramolecular cross-linking. Rheological Evaluation. Rheological measurements are helpful for determining the processing characteristics of materials. Butyl rubber exhibits a considerable amount of creep when subjected to stress over a long period.59,62,63 We hypothesized that the functionalization of PIB with carboxylic acid or other moieties might reduce its susceptibility to viscous flow, making it more useful for applications such as biomedical coatings. With this application in mind, the viscoelastic behavior of the materials was measured under small-amplitude oscillatory shear at 37 °C. The viscous and elastic moduli of two representative materials, PIB-h and 4d-h, are plotted as a function of angular frequency ω in Figure 5. Both the viscous modulus G″ and the elastic modulus G′ depend only weakly on frequency, and G′ is much larger than G″. This behavior is typical of rubbery
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CONCLUSIONS The synthesis of carboxylic acid functionalized butyl rubber was accomplished by two different approaches. The ring opening of diglycolic anhydride from allylic alcohol moieties along the polymer backbone afforded pendant carboxylic acids while ATRP of tBMA from a macroinitiator having bromoisobutyryl groups along the backbone, followed by cleavage of the t-butyl protecting groups provided poly(carboxylic acid) arms along the backbone. The degree of functionalization was controlled via the synthetic approach and by the isoprene content of the butyl rubber starting material. The properties of epoxide, hydroxyl, and carboxylic acid functionalized butyl rubbers were studied. All of the functionalized materials showed stronger adhesion to stainless steel than the unfunctionalized rubber, with the 2 mol % carboxylic-acid-functionalized rubber exhibiting the highest adhesivity. Carboxylic acid moieties also significantly increased the ultimate tensile strength of the high-isoprene-content polymer and the Young’s modulus of both the high- and low-isoprene-content materials. Rheological
Figure 5. Frequency dependence of the elastic and viscous moduli, G′ and G″, respectively, for two representative materials. The data points are averages over at least three trails, and error bars are standard deviations. G
DOI: 10.1021/acs.iecr.5b00421 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 6. (a) Elastic moduli of the polymers at ω = 1 rad/s; (b) viscous moduli of the polymers at ω = 1 rad/s.
Figure 7. tan δ as a function of frequency: (a) behavior of low isoprene content derivatives; (b) behavior of high isoprene derivatives.
measurements showed that functionalization of the butyl rubber tended to decrease both elastic and viscous moduli. The incorporation of carboxylic acids significantly decreased the ratio of the viscous to the elastic modulus, consistent with the carboxylic acid groups contributing to the formation of a cross-linked network of polymer molecules. Overall, many of the measured properties of these new materials may prove useful in new applications of butyl rubber, potentially including coatings for stents and other biomedical devices.
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ACKNOWLEDGMENTS
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REFERENCES
We thank LANXESS Inc. and the Natural Sciences and Engineering Research Council of Canada for funding this work, and LANXESS for providing all of the rubber materials. Aneta Borecki is thanked for assistance with the acquisition of the GPC data, and Ruiping Ge is thanked for assistance with rheology. Brianna Binder is thanked for performing the adhesion tests.
ASSOCIATED CONTENT
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S Supporting Information *
NMR and IR spectral data; thermal characterization data; additional rheological data; raw data for the calculation of the degree of polymerization for the graft copolymers. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. H
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