Nucleobase Self-Assembly in Supramolecular Adhesives

Article pubs.acs.org/Macromolecules

Nucleobase Self-Assembly in Supramolecular Adhesives Shijing Cheng, Mingqiang Zhang, Ninad Dixit, Robert B. Moore, and Timothy E. Long* Department of Chemistry, Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, Virginia 24061-0344, United States S Supporting Information *

ABSTRACT: Novel acrylic monomers functionalized with nucleobase-containing units (adenine and thymine) were prepared upon aza-Michael addition and successfully copolymerized with nbutyl acrylate. At a content of 7 mol %, adenine-containing units self-assembled into needle-like microstructures within amorphous polymer matrices as shown with atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD); thymine-containing units did not aggregate into distinct morphologies even to 30 mol %. Upon blending, thymine- and adenine-containing statistical copolymers associated into a thermodynamically stable complex, which was physically cross-linked through adenine−thymine base pairing. The molar fractions of the nucleobase monomer, nucleobase stacking interactions, and complementary hydrogen bonding principally influenced self-assembly. Additionally, the nucleobasefunctionalized polyacrylates exhibited tunable adhesive and cohesive strength.

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INTRODUCTION The use of noncovalent interactions as a tool for the synthesis of well-defined supramolecular structures is paramount in current polymer science.1−5 Our motivation is to impart structural features of biomacromolecules with extension to emerging adhesive technologies, e.g., the unique molecular recognition of complementary nucleobases, the stimuliresponsiveness of multiple hydrogen bonding, and the selfassembly of cooperative multiple noncovalent interactions. Hydrogen-bonding based molecular recognition plays a crucial role in the functions of biopolymers. Recognition based upon nucleobase interactions, such as molecularly imprinted polymers6,7 and nucleobase-templated polymerization,8 also enables applications in catalysis, separations, biomedicine, chemo/biosensors, and information storage. The design of synthetic polymers bearing nucleobase recognition sites is often challenging and requires multiple synthetic steps due to susceptibility of multiple substitutions. Nucleobase units were introduced using postpolymerization modification,9,10 and protection of the nucleobase monomers was necessary to obtain high conversions.11,12 Recent developments in controlled metathesis and radical polymerization techniques have expanded our ability to create well-defined polymers with high tolerance to hydrogen bonding and ionic functionalities. Sleiman et al. utilized ring-opening metathesis polymerization to synthesize adenine-containing block copolymers, which self-assembled into rod morphologies.13 van Hest et al. synthesized thymine-functionalized block copolymers using atom transfer radical polymerization (ATRP) of thymine methacrylate monomers from a poly(oxyethylene) macroinitiator.14 Lutz et al. polymerized styrenic nucleobase monomers using ATRP.15,16 Inaki et al. synthesized a wide © 2012 American Chemical Society

range of nucleobase-functionalized statistical and homopolymers and block copolymers using ring-opening cationic and anionic polymerization of acyclic derivatives of the nucleobases.17 Our group previously reported the high tolerance of nitroxide-mediated polymerization to nucleobase functionalities,18 and we prepared styrenic adenine- and thyminecontaining ABA triblock copolymers using a novel difunctional alkoxyamine initiator.19,20 However, most bioreceptors and nucleobase-functionalized synthetic polymers for molecular recognition have either relatively rigid structures or sterically congested polymer backbones,21 such as vinyl,22 styrene,16,23 and methacrylate.14,15,24 Examples of receptors having relatively flexible structures and spacers are relatively very few.25,26 Meijer et al. recently reported the increase of the polymer backbone polarity significantly decreased the stability of benzene-1,3,5tricarboxamides aggregates.27 The molecular recognition process is highly dependent on both the recognition groups and the nature of these spacers. In our present work, we constructed flexible alkyl chain spacers for recognition sites, which were compared with the styrene analogues to evaluate the influence of structural modifications on nucleobase self-assembly using AFM and SAXS. The novel acrylic nucleobase monomers were synthesized using a one-step aza-Michael addition without functional group protection. Michael addition is an efficient and mild reaction that is suitable for introducing highly functional groups, biopolymers (e.g., proteins), or bioactive species (e.g., drugs) into polymers.28 The novel monomers were homopolyReceived: September 19, 2011 Revised: December 21, 2011 Published: January 6, 2012 805

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Scheme 1. Michael Addition of Adenine and Thymine with 1,4-Butanediol Diacrylate

an inert atmosphere (argon) for 1 h and then treated with 1,4butanediol diacrylate (3.0 mL, 15.9 mmol). The reaction solution turned clear after stirring at room temperature for 24 h. Then, it was poured into water (150 mL) and washed with hexane to remove excess 1,4-butanediol diacrylate. The water layer was extracted with dichloromethane (3 × 20 mL). The combined extracts were dried over MgSO4, filtered, and concentrated in a vacuum evaporator to remove all the solvents. The evaporation residue was separated using flash column chromatography with CHCl3−MeOH (20:1) on silica gel. Evaporation of the total eluent resulted in a white solid of 1.82 g with an overall yield of 71%; melting at 70 °C. 1H NMR (400 MHz, d6-DMSO): 1.54−1.68 (m, 4H, Hb+b′), 1.71 (s, 3H, He), 2.66 (t, 2H, J = 6.0 Hz, Hc), 3.83 (t, 2H, J = 5.2 Hz, Hd), 3.97−4.13 (m, 4H, Ha+a′), 5.91 (d, 1H, J = 11.2 Hz, Hg1), 6.14 (dd, 1H, J1 = 8.4 Hz, J2 = 16.8 Hz, Hh), 6.30 (d, 1H, J = 6.8 Hz, Hg2), 7.48 (s, 1H, Hf), 11.23 (br, 1H, Hi). 13 C NMR (100 MHz, d6-DMSO): 12.4, 25.1, 33.2, 44.2, 64.1, 64.2, 108.6, 128.8, 131.9, 142.2, 151.2, 164.7, 165.9, 171.2. HRMS (ES+): m/z calcd for [M + H+] 324.13 g/mol; found 324.14 g/mol. Synthesis of Acrylic Nucleobase-Containing Copolymers and Hydrogen-Bonding Analogues. Poly(acrylic adenine-co-nBA), poly(acrylic thymine-co-nBA), and poly(AA-co-nBA) and poly(VP-conBA) were synthesized using solution free radical polymerization with AIBN as the initiator. A typical synthesis was carried out as follows. Acrylic adenine (1.0 g, 3.0 mmol) was weighed into a 100 mL roundbottomed flask containing a magnetic stir bar. The flask was sealed with a rubber septum and purged with N2 for 20 min. Then anhydrous DMSO (35 mL, 10 wt % of solid) was added to the flask to dissolve acrylic adenine monomers. Purified n-butyl acrylate (4.0 mL, 28.0 mmol) was syringed into the reaction flask. Lastly, AIBN (10 mg, 0.15 mol %) was dissolved in 5 mL of anhydrous DMSO and flushed with dry nitrogen for 10 min and was then syringed into the reaction mixture. The reaction flask was placed in an oil bath at 60 °C for 24 h with constant stirring. After polymerization, DMSO was distilled off under reduced pressure. The product was redissolved in THF (25 mL) and precipitated into a methanol and water mixture (800 mL). The final products were dried at 30 °C under reduced pressure (0.5 mmHg) for 24 h. The product contained 10 mol % acrylic adenine and 90 mol % nBA, with a yield of 82%. Synthesis of Styrenic and Acrylic Nucleobase-Containing Homopolymers. Styrenic and acrylic adenine- and thyminecontaining homopolymers were synthesized using conventional free radical polymerization in DMF or DMSO solution with AIBN as the initiator. A representative example of the synthesis was conducted as follows. Acrylic adenine (4.0 g, 12.0 mmol), AIBN (10 mg, 0.6 mmol, 0.25 wt %), and anhydrous DMSO (30 mL) were placed into a 100 mL round-bottomed flask. After the flask was sealed and purged with N2 for 20 min, it was placed in an oil bath at 60 °C for 24 h with constant stirring. After polymerization, the polymer solution was precipitated into diethyl ether (800 mL). The final products were dried at 30 °C under reduced pressure (0.5 mmHg) for 24 h. Yield: 90%

merized or copolymerized with n-butyl acrylate using conventional free radical polymerization. The hydrogen-bonding polymers provided a reversible network with strongly temperature-dependent viscosity, and self-healing performance may impact industrial interest in coatings and hot melt adhesives. Peel strengths and shear strengths of nucleobase-containing polyacrylates were significantly higher than acrylic acid- and 4vinylpyridine-based polyacrylate analogues at similar hydrogenbonding molar concentrations.

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EXPERIMENTAL SECTION

Materials. n-Butyl acrylate (nBA, 99+%), acrylic acid (AA, 99+%), and 4-vinylpyridine (VP, 99+%) were purchased from Aldrich and passed through neutral alumina columns before use. α,α′-Azobis(isobutyronitrile) (AIBN, Fluka, 99%) was recrystallized from methanol. 1,4-Butanediol diacrylate (Alfa Aesar, 99%) was used without further purification. Adenine (A, 99%), thymine (T, 99%), triethylamine (TEA, 99%), potassium carbonate (99%), and 2,6-di-tertbutyl-4-methylphenol (BHT, 99%) were purchased from Aldrich and used without further purification. Hexane (HPLC grade), chloroform (CHCl3, HPLC), tetrahydrofuran (THF, HPLC grade), N,N-dimethyl sulfoxide (DMSO, HPLC grade), and N,N-dimethylformamide (DMF, HPLC grade, anhydrous) were purchased from Fisher Scientific and used as received. 9-(4-Vinylbenzyl)adenine and 1-(4-vinylbenzyl)thymine were synthesized according to the previous literature.29 Synthesis of 4-((3-(Adenin-9-yl)propanoyl)oxy)butyl Acrylate. A suspension of adenine (1.0 g, 11.9 mmol), BHT (60 mg), and K2CO3 (40.0 mg, 0.29 mmol) in DMSO (20 mL) was stirred at 50 °C for 1 h and followed with adding 1,4-butanediol diacrylate (3 mL, 15.9 mmol). After the reaction mixture was stirred at 50 °C for 5 h, it was poured into water (150 mL) and washed with hexane to remove excess 1,4-butanediol diacrylate. Then, the water layer was extracted with dichloromethane (3 × 20 mL). The combined extracts were dried over MgSO4, filtered, and concentrated in a vacuum evaporator to remove all the solvents. The evaporation residue was separated using chromatography with CHCl3−MeOH (20:1) on silica gel. A trace amount of N-7 adduct were eluent first. Evaporation of the remained eluent gave a white solid of 1.30 g with an overall yield of 53%, melting at 117 °C. 1H NMR (400 MHz, d6-DMSO): 1.45−1.64 (m, 4H, Hb+b′), 2.93 (t, 2H, J = 6.8 Hz, Hc), 3.95−4.10 (m, 4H, Ha+a′), 4.35 (t, 2H, J = 6.8 Hz, Hd), 5.91 (d, 1H, J = 10.4 Hz, Hg1), 6.13 (dd, 1H, J1 = 10.4 Hz, J2 = 17.2 Hz, Hh), 6.29 (d, 1H, J = 17.2 Hz, Hg2), 7.18 (bs, 2H, Hi), 8.07 (s, 1H, He), 8.11 (s, 1H, Hf). 13C NMR (100 MHz, d6DMSO): 25.1, 34.0, 64.0, 64.2, 128.7, 131.9, 141.3, 152.8, 156.4, 165.9, 171.1. HRMS (ES+): m/z calcd for [M + H+] 334.14 g/mol; found 334.15 g/mol. Synthesis of 4-((3-(Thymin-l-yl)propanoyl)oxy)butyl Acrylate. A suspension of thymine (1.00 g; 7.93 mmol) and BHT (60.0 mg) in DMF (20 mL) was treated with triethylamine (0.22 mL, 1.58 mmol) at room temperature. The reaction mixture was stirred under 806

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Scheme 2. Copolymerization of n-Butyl Acrylate and 4-(3-Adenine-9-yl-propanoyloxy)butyl Acrylate or 4-((3-(Thymin-lyl)propanoyl)oxy)butyl Acrylate

Polymer Complexes Preparation. A 5 wt % solution of adeninecontaining polyacrylate copolymer in CHCl3 was mixed with a 5 wt % CHCl3 solution of thymine-containing polyacrylate copolymer and stirred for 6−8 h. Then, CHCl3 was left to evaporate slowly at room temperature for 48 h and annealed under reduced pressured at 100 °C for 2 days. Instrumentation. See the Supporting Information.

Table 1. Molecular Characterization of Adenine- and Thymine-Containing Poly(n-butyl acrylate) Copolymers and Complementary Hydrogen-Bonding Polymer Analoguesa sample name

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A-4

RESULTS AND DISCUSSION Synthesis of Nucleobase Functional Polyacrylates. Synthesis of acrylic nucleobase-containing polymers involved acrylic monomer synthesis using Michael addition (Scheme 1) and direct radical polymerization of functional acrylic adenine and thymine monomers. Synthesis of nucleobase styrene29 and methacrylate14 monomers has been reported in the literature. However, the corresponding homopolymers were glassy at room temperature and insoluble in polar aprotic organic solvents, which strongly disfavored molecular recognition.16 We targeted acrylic nucleobase monomers possessing alkyl chain spacers, which potentially promote the solubility in a variety of organic solvents. The introduction of nucleobases through regioselective substitution is crucial due to the presence of more than one nucleophilic sites on respective nucleobases. Previously, our group successfully attached nucleobases onto polystyrene10 and poly(D,L-lactide)30 chain ends through Michael addition. The base-catalyzed Michael addition is thermodynamically controlled, which affords N1-substitubted thymine and N9-substituted adenine as the major products.31 The reaction conditions, such as solvent, temperature, and the amount of base catalyst, were optimized and the regioselectivity was achieved in a satisfactory yield. The reaction was initially heterogeneous and became homogeneous with time due to enhanced solubility of the final products. In addition to product solubility, excess diacrylate suppressed undesirable difunctionalization. Conventional free radical copolymerization of acrylic nucleobase monomers with n-butyl acrylate was performed homogeneously in DMF or DMSO, and the polymers were isolated in high yields (Scheme 2). The nucleobase concentration in the copolymer was calculated from the ratio of 1H NMR resonances integration at 7.8−8.0 and 3.6−4.4 ppm (Figures S3(2) and S4(2)), which corresponded to the chemical shift of −CH (Hf) in nucleobase and −O−CH2− (Ha+a′+a″) in both n-butyl acrylate and acrylic nucleobase monomers. Table 1 lists the monomer feed ratios and the corresponding copolymer compositions. The copolymer composition matched with the feed ratio well possibly due to the similar radical intermediate stability of nBA and acrylic

A-7 A-16 A-100 T-10 T-20 T-25 T-32 T-100 StA100 StT100 VP-10 AA-4

M1 acrylic adenine acrylic adenine acrylic adenine acrylic adenine acrylic thymine acrylic thymine acrylic thymine acrylic thymine acrylic thymine styrenic adenine styrenic thymine [VP] [AA]

[M1] in feed (mol %)

[M1] in polymerb (mol %)

Mwc (g/mol)

Mw/Mnc

Tg (°C)

5

4

95K

2.09

−41

10

7

125K

1.64

−25

19

16

279K

2.39

0

100

100

NA

NA

65

10

10

122K

2.40

−30

20

20

155K

2.40

−14

29

25

164K

2.73

1

40

32

NA

NA

19

100

100

NA

NA

43

100

100

−

−

204

100

100

−

−

168

10 4

10 10

70.1K 64.1K

2.33 1.85

−42 −37

a NA: polymers were difficult to filter; −: polymers were insoluble. b1H NMR. cSEC: 0.01 M LiBr/DMF.

nucleobase monomers. SEC results revealed relatively broad molecular weight distributions, which were attributed to conventional radical polymerization and strong hydrogen bonding. Thermal Transitions. To understand the association of nucleobase-containing units, we studied the thermal and dynamic mechanical properties of adenine- and thyminefunctionalized polynBA and their complexes. DSC analysis of acrylic adenine- and thymine-containing copolymers all showed a single Tg (Figure S5), suggesting a statistical copolymerization of n-butyl acrylate and acrylic nucleobase monomers. Figure 1 shows the influence of nucleobase content on the Tg’s of the copolymers. A linear increase of Tg was observed with an increase of the mole fraction of adenine and thymine with positive deviation from the Fox equation. Such deviation was attributed to the incorporation of strong hydrogen-bond units.32−34 The more significant Tg deviation for adenine807

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Infrared Spectroscopy. Infrared spectroscopy (IR) is well suited for monitoring supramolecular association and provides information on ordering processes.35 Supramolecular assembly is temperature dependent, and reversibility is typically observed.25,36 Therefore, we performed variable temperature FT-IR on the adenine- and thymine-containing polymers in order to investigate the kinetics of supramolecular ordering. A slow heating and cooling rate of 1 °C/min was used to suppress kinetic (nonequilibrium) effects. Figure 3 shows the scale-

Figure 1. Relationship between acrylic adenine or thymine monomer contents and Tg.

containing polymers compared to thymine-containing polymers suggested stronger self-association of adenine units relative to thymine units at similar molar concentrations. The thermodynamic mechanical properties of adenine- and thymine-containing acrylate copolymers at small deformations were studied using DMA in tension mode (Figure 2). The

Figure 3. Variable temperature FT-IR spectra in the 1600−1700 cm−1 region for the complex of poly(nBA-co-acrylic adenine) and poly(nBAco-acrylic thymine).

expanded FT-IR spectra of the adenine− and thymine−acrylate polymer complex with a stoichiometry ratio of [A]:[T] = 1:1 at various temperatures. The broad band with a peak maximum at 1672 cm−1 corresponded to the stretching vibration of hydrogen bonded CO in thymine. The peaks at 1643 and 1600 cm−1 were attributed to the N−H bending/scissoring vibration of adenine. When temperature increased from 30 to 190 °C, the band at 1672 cm−1 disappeared, indicating the dissociation of hydrogen bonded CO in thymine. Meanwhile, the absorbance of hydrogen-bonded N−H bending vibration of adenine at 1643 cm−1 also decreased upon heating, and a new peak emerged at lower wavenumber, suggesting the disruption of hydrogen bonds. The new peak shifted toward lower wavenumbers with temperature increase, and the peak maximum centered around 1627 cm−1. In this case, the majority of complementary hydrogen bonds were disrupted. Variable temperature FT-IR study further confirmed hydrogen bonding at room temperature and its dissociation upon heating. Morphology. AFM enables imaging of surface texture and morphology of microphase-separated structures and is frequently applied to polymer films.37,38 AFM micrographs of acrylic adenine-containing copolymers revealed intriguing surface morphologies as shown in Figure 4. Polymers with 7 mol % of adenine microphase separated into needle-like hard structures. These structures of approximately 100 nm in length and less than 10 nm in width were randomly oriented in the soft polymer matrix. SAXS results (Figure 5) also revealed a broad peak near 0.90 nm−1 with a d spacing of ca. 7.0 nm, which corresponded to the average distance between needle-like structures observed using AFM. These structures percolated as the adenine content increased from 7 to 16 mol %. The broadness of the SAXS peak also suggested the disparate distribution of length scales. To further understand the internal structure of the nanoneedles, wide-angle X-ray diffraction studies (WAXD) were performed on the annealed film samples

Figure 2. DMA traces of poly(nBA-co-acrylic adenine) with 7 and 16 mol % of acrylic adenine and poly(nBA-co-acrylic thymine) with 25 and 32 mol % of acrylic thymine.

storage modulus (G′) of acrylic adenine-containing polymer with 7 mol % of adenine (A-7) exhibited a rubber plateau between 5 and 50 °C, followed with a subsequent sharp decrease of modulus and eventually flow at 57 °C. It was immediately obvious how the polymer properties significantly changed with low levels (mol %) of adenine units. With increasing adenine incorporation to 16 mol %, Tg increased; the plateau modulus was 2 orders of magnitude higher and the flow temperature increased to 85 °C. This change was attributed to the presence of associated adenine units, which reinforced the polymer matrices as physical cross-links. Additionally, the tan δ curves in DMA thermograms revealed broad glass transitions spanning 80 °C for A-7 and 60 °C for A-16. This slow relaxation was presumably due to the dynamic bonds that restricted mobility of polymer backbone. In comparison to adenine, thymine-containing polymers only showed a short and less well-defined rubbery plateau at even higher nucleobase concentration, suggesting less pronounced self-association. 808

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Figure 4. Tapping mode AFM phase images of thins films of poly(nBA-co-acrylic adenine) with (1) 7 mol % and (2) 16 mol % adenine, poly(nBAco-acrylic thymine) with (3) 25 mol %, and (4) 32 mol % thymine. The polymer complexes of (2) and (4) at (5) [A]/[T] = 2 and (6)[ A]/[T] = 4.

additional noncovalent interactions, such as π−π stacking, which was more pronounced for adenine than the other pyrimidine or purine derivatives.41 In general, the extent of stacking decreased as purine−purine > purine−pyrimidine > pyrimidine−pyrimidine.42 In addition, Hawker and Kramer et al. synthesized random copolymers consisting of nBA with quadruple hydrogen-bonding side chains based on 2-ureido4[1H]-pyrimidinone (UPy).43 Although the UPy groups strongly dimerized, they did not π−π stack into ordered structures and microphase separation was not observed in this case. Meijer et al. studied the self-assembly process of ureidotriazine functionalized oligo(p-phenylenevinylene), and they found that the π-conjugated structures were crucial in guiding hydrogen-bonded molecules to further assemble into supramolecular structures.44 WAXD was performed on styrenic and acrylic nucleobasecontaining homopolymers to further elucidate the significance of structural flexibility on supramolecular self-assembly (Figure

Figure 5. SAXS of poly(nBA-co-acrylic adenine) with (1) 7 mol % and (2) 16 mol % of adenine, poly(nBA-co-acrylic thymine) with (3) 25 mol % and (4) 32 mol % of thymine, and the polymer complexes of (1) and (4) at (5) [A]/[T] = 2 and (6)[ A]/[T] = 4. For sake of clarity, data were shifted by arbitrary factors.

(Figure S6). The diffraction pattern of the adenine acrylatecontaining copolymers displayed an intense diffraction peak located at 2θ = 20°, which was assigned to the local chain packing (amorphous halo) of PnBA.39 The reflection peak at 2θ = 12.87° (d = 6.87 Å) corresponded to the distance between nucleobase stacks,25 which were held together with π−π interactions and, adenine−adenine hydrogen bonding from the FT-IR data. We interpreted these needle-like structures as onedimensional ordered stacks of associated adenine units with a parallel arrangement of the rings. The AFM micrograph of thymine acrylate copolymers did not show any obvious organized nanostructures even up to 32 mol % of thymine. Accordingly, the SAXS profiles of thyminecontaining copolymers only showed broad peaks with very low intensity, implying less pronounced self-association of thymine than adenine. This observation was similar to the self-assembly behavior of adenine- and thymine-terminated telechelic poly(tetrahydrofuran)s.25 The weak hydrogen-bonding self-dimerization constant of adenine and thymine (both less than 10 M−1 in CDCl3)40 suggested that the unique higher order selfassembly of adenine-containing polymers was driven through

Figure 6. X-ray (radiation source Cu Kα) diffraction diagram of (1) acrylic adenine homopolymer, (2) acrylic thymine homopolymer, (3) styrenic adenine homopolymer, and (4) styrenic thymine homopolymer. For sake of clarity, data were shifted by arbitrary factors. 809

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6). The diffraction pattern of acrylic adenine homopolymer displayed reflection peaks at 2θ = 12.9° (d = 6.87 Å) and 22.5° (d = 3.95 Å), which were assigned to associated adenine stacks.25 In contrast, thymine did not π−π stack strongly and thymine-containing homopolymers did not show distinct reflection peaks. Interestingly, a styrenic adenine homopolymer did not reveal reflection peaks either. The bulky phenyl rings potentially hindered the well-defined position between adenine−adenine π−π stacks. Moreover, the spacing between functional groups along a polymer chain, and the presence of bulky pendant groups can also significantly reduce the associated hydrogen bonding per unit volume, as a result of a so-called functional group accessibility effect. This effect is also considered to be the origin of steric crowding and shielding.45 Thus, the relatively strong π−π interactions of adenine as well as mobility and accessibility from alkyl chain spacers resulted in the unique well-defined nanostructures observed in acrylic adenine-containing copolymers. Variable temperature SAXS and WAXD experiments were conducted on A-16 to further investigate the influence of temperature on adenine self-assembly (Figures S7 and S8). The intensity of the broad Bragg peak near q = 0.90 nm−1 decreased monotonically with temperature until the peak was barely visible at 80 °C. The same trend was shown in WAXD profiles; i.e., the reflection of stacked adenine−adenine decreased in intensity with temperature increase and eventually disappeared above 80 °C, implying the dissociation of the needle-like nanostructures upon heating. From DMA analysis (Figure 2), the polymer film lost mechanical integrity at 80 °C and eventually flowed. These results were consistent with the proposed structural model of hard needle-like structures composed of π−π stacks of hydrogen-bonded adenine units, which reinforced the polymer matrix. Blending acrylic adenine- and thymine-containing copolymers resulted in a polymer complex with weak microphase separation as shown in AFM (Figure 4). The absence of distinctive needle-like nanostructures was presumably due to the A−T base pairs that possessed weaker π−π interactions than A−A42 and the lack of stereoregularity in polymer backbone. For adenine-containing polymers, adenine intramolecularly π−π packed with neighboring adenine. This supposition was reinforced with SAXS results of the polymer complexes, which revealed averaged, intermediate Bragg spacings and no evidence of multiple interdomain spacings (Figure 5). As the ratio of [A]/[T] decreased from 4:1 to 2:1, the scattering peak intensity decreased and the polymer complexes became more uniform. Kuo et al. also reported a similar behavior for triazine- and thymine-containing PMMA copolymer mixtures, and the degree of homogeneity of the polymer complex was relatively higher than the precursors.46 Hydrogen bonding suppressed the concentration fluctuations. Rheology. Melt rheological characterization was conducted on adenine- and thymine-containing polyacrylates and their complexes ([A]:[T] = 1:1). Master curves of storage modulus versus frequency were referenced to 20 °C, since the Tg’s of these polymers were lower than room temperature (Figure 7). It should also be noted that in the construction of the master curves vertical shift of the data was not needed, demonstrating that the plateau modulus GN0 was not a strong function of temperature. The master curve of the hydrogen-bonding complex exhibited an increased storage modulus and Tg with an extended terminal relaxation time by several orders of magnitude, which was attributed to complementary hydrogen

Figure 7. Storage and loss modulus master curves of polyacrylates containing 7 mol % of adenine and 25 mol % of thymine, respectively, and their complex ([A]:[T] = 1:1) referenced to 20 °C showing the effect of complementary hydrogen bonding.

bonding and the formation of a higher apparent molar mass. Even at low frequencies (higher temperatures), the hydrogen bonding complex exhibited a G′ 10 times higher than the precursors, indicating that a low level of A−T nucleobase pairs remained even at high temperatures; this was expected because of the higher association constant of A−T hydrogen bonding than A−A or T−T hydrogen bonding. In comparison with the precursors, the tan δ peak of the polymer complex broadened and shifted to lower frequencies, and the corresponding plateau modulus was broader, suggesting weaker temperature dependence, possibly because the hydrogen-bonding nucleobase pair dissociates and re-forms in equilibrium prior to complete separation.43 Although distinct microphase separation driven by π−π interactions occurred in A-7, time−temperature superposition was satisfied in all cases, indicating that all governing relaxation process had similar temperature dependences. In addition, plotting the shift factors (aT) versus inverse temperature followed the Williams−Landel−Ferry (WLF) equation:

− c 0(T − T0) log aT = 0 1 c 2 + (T − T0)

(1)

where c10 and c20 are constants and T0 is the reference temperature. Values of the constants c10 and c20 were determined from the plot of the −(T − T0)/log aT against T − T0; the coefficient c10 was obtained from the reciprocal of the slope and the coefficient c20 from the intercept. The complexation of adenine and thymine led to an increase in c10 and a significant decrease in c20 (Table 2). Fractional free volume is an important Table 2. Rheological Characterization of Adenine- and Thymine-Containing Poly(n-butyl acrylate) Copolymers and Their Complex (T0 = 20 °C) sample

GN0 (MPa)

c10

c20

f0

adenine 7 mol % thymine 20 mol % complex

74 49 165

8.1 7.5 9.1

110 103 98

0.054 0.058 0.048

parameter that can be obtained from these fits.

f0 = 810

B 2.303c10

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where B is a constant usually assumed to be unity. The fractional free volume decreased for the polymer complex, which suggested strong complementary hydrogen bonding favored compact molecular packing. Hawker and Kramer et al. previously reported that TTS was satisfied for UPy-containing polyacrylates, but the shift factors followed an Arrhenius-type behavior since the dissociation of the polymer side-chain hydrogen bonding was a thermally activated process.43 In our case, the kinetic restriction at Tg < T < Tg + 100 °C for molecular motions was free-volume availability. Adhesion Measurements. Hydrogen-bonding associations provide strategies to increase polymer apparent molecular weight to inhibit creep and cohesive failure, while improving interfacial adhesion.47 To demonstrate the effectiveness of complementary multiple hydrogen bonding on polymer adhesion, we measured the peel strength (the ability to resist removal upon peeling) and shear resistance (the ability to resist flow when shear forces are applied). Additionally, a comparative study was performed on acrylic acid (AA)- and 4-vinylpyridine (VP)-based polymer analogues with similar molecular weights and hydrogen-bonding contents in order to qualitatively describe the influence of bond strength on polymer peel strength (Table 1). An ASTM-D3300 standard 90° peel testing method was adopted, and the results are depicted in Figure 8.

of adenine were 11.8 and 19.1 N/in., respectively, whereas poly(AA-co-nBA) and poly(VP-co-nBA) containing 4 mol % of AA and 10 mol % of VP, respectively, showed peel strength below 5 N/in., which was 3−4 times lower. The enhanced wettability was possibly due to strong nucleobase pairing as well as the strong association between nucleobases and the stainless steel surface in comparison to the association between carboxylic acid (AA) and vinylpyridine (VP). Additionally, peel strengths of the adenine and thymine-containing polymer complexes were higher than their precursors. Dynamic shear measurement was performed on an Instron tensile tester to demonstrate the influence of complementary intermolecular hydrogen bonding, which potentially provided enhanced cohesion due to supramolecular structure. As shown in Figure 8, the shear strength of nucleobase-containing polymers is 3−4 times higher than the analogues, suggesting a significant improvement in adhesion at interface. The association of nucleobases led to an increase in apparent molecular weight and network structure with increased modulus and Tg. Because of stronger hydrogen bonding, blending adenine- and thymine-containing polymers further enhanced the shear strength. The optimum combination of cohesive and adhesive strengths was achieved, which contributed to high shear strength.

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CONCLUSION Novel acrylic adenine and thymine monomers were synthesized in decent yields through aza-Michael addition and then copolymerized with nBA using conventional radical polymerization. The resulted adenine-containing polyacrylate with only 7 mol % of adenine units self-assembled into ordered needlelike nanostructures. Theses needles approximately 100 nm in length and less than 10 nm in width were randomly oriented in the soft polymer matrix as shown from AFM. For thyminecontaining polyacrylates possessing up to 32 mol % of thymine units, both AFM and SAXS images were featureless. In addition, compared to adenine-containing polyacrylates, thymine-containing analogues with similar hydrogen-bonding molar concentrations exhibited lower glass transition temperatures and storage moduli at room temperature. The different self-assembly behavior was ascribed to the pronounced π−π stacking of adenine than thymine. Time−temperature superposition (TTS) was successfully applied to adenine- and thymine-containing polyacrylates and their polymer complex. The shift factors and SAXS results demonstrated a decrease in free volume and an increase in homogeneity of the polymer complex in comparison to the precursors. Additionally, nucleobase-containing hydrogen-bonding polymers possessed peel and shear strengths 3−4 times higher than the AA- and VP-based complementary polymer analogues.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details; Figures S1−S8. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Peel strength and shear strength of poly(nBA-co-acrylic adenine) and poly(nBA-co-acrylic thymine) in comparison to AA- and VP-based complementary hydrogen-bonding polymer analogues.

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All polymers with ≤10 mol % of hydrogen-bonding content failed cohesively. Peel strength of acrylic thymine- and adeninecontaining copolymers with 10 mol % of thymine and 4 mol %

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (540) 231-2480; Fax (540) 2318517. 811

dx.doi.org/10.1021/ma202122r | Macromolecules 2012, 45, 805−812

Macromolecules

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Article

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the Petroleum Research Fund (ACS-PRF 35190-AC7), which is administered by the American Chemical Society. Parts of this work were carried out using instruments in the Nanoscale Characterization and Fabrication Laboratory, a Virginia Tech facility operated by the Institute for Critical Technology and Applied Science (ICTAS). We also acknowledge funding from NSF (CHE-0722638) for the acquisition of our Agilent 6220 LC-TOF-MS. This material is partially based upon work supported by the National Science Foundation under Grant DMR-0923107. Additionally, this research was supported in part by the U.S. Army Research Laboratory and the U.S. Army Research Office under Contract/Grant W911NF-07-1-0452, Ionic Liquids in Electro-Active Devices Multidisciplinary University Research Initiative (ILEAD MURI).

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