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UCLEOBASE-COTAIIG POLY(n-BUTYL ACRYLATE) AS OVEL PRESSURE SESITIVE ADHESIVES Shijing Cheng, Virginia Tech, Department of Chemistry (0212), Blacksburg, VA Gozde Ozturk, Virginia Tech, Department of Chemistry
UCLEOBASE-COTAIIG POLY(n-BUTYL ACRYLATE) AS OVEL PRESSURE SESITIVE ADHESIVES Shijing Cheng, Virginia Tech, Department of Chemistry (0212), Blacksburg, VA Gozde Ozturk, Virginia Tech, Department of Chemistry (0212), Blacksburg, VA Timothy E. Long, Virginia Tech, Department of Chemistry (0212), Blacksburg, VA Abstract The presence of hydrogen bonding enhances melt viscosity and cohesive strength of polymers in adhesive applications. In this study, functionalization of poly(n-butyl acrylate) with hydrogen bonding units was investigated to enhance adhesive properties. Self-complementary thymine and adenine, which are nucleobases in DA, were utilized as the hydrogen bonding units. Hydrogen bonding groups along the polymer backbone was achieved with copolymerization of adenine and thymine containing monomers. These monomers were synthesized through -alkylation of 4-chloromethyl styrene with adenine or thymine. 1 H MR and mass spectroscopy confirmed the structure of adenine and thymine functionalized monomers. Copolymerization reactions resulted in poly(n-butyl acrylate) with different amounts of adenine or thymine functionalities. Melt viscosity of the adenine or thymine functionalized poly(n-butyl acrylate) and its non-hydrogen bonding analogue was studied at temperatures lower and higher than 80 o C, which corresponds to the dissociation temperature of hydrogen bonds. Moreover, peel testing measurements were performed to investigate the influence of complementary hydrogen bonding on adhesive performance. Introduction Pressure-sensitive adhesives (PSAs) are commonly used in a variety of applications due to their ease of application and versatility. Pressure-sensitive adhesives mainly comprising styrene-based monomer and acrylate monomer received significant attention, due to good light resistance of acrylate monomers. O H O H H Figure 1. Hydrogen bonds in Watson-Crick paired thymine (left) and adenine (right) as contrived to be present in mixtures of the random copolymers investigated in this work. In recent years, 1 there has been interest exhibited in polymers containing polar, ionic, or other entities capable of intermolecular association. Researchers have concluded that the hydrogen bonds contribute to the increase in viscosity and are responsible for interfacial interactions. In our system, thymine and adenine, which are nucleobases in DA, were utilized as complementary hydrogen bonding units. These 163 nucleobase pairs have a structural unit (Figure 1), which combines several hydrogen bonds in a single functional unit. Macromolecules containing complementary multiple hydrogen bonding units based on nucleic acid heterocyclic bases have received significant attention in recent years. 2-8 The use of multiple hydrogen bonding sites in conjunction with conventional oligomers and polymers has resulted in themoreversible mechanical and rheological properties, which are tunable based on the strength of the hydrogen bond. Adhesive compositions comprising a high molecular weight n-butyl acrylate polymer component and a low content of nucleobases modified styrene are disclosed. The objective of this research is to elucidate the influences of hydrogen bonding association of nucleobase-functionalized acrylic pressure-sensitive adhesives for melt applications. Experimental Materials. n-butyl acrylate (99%) was purchased from Aldrich and purified using an alumina column followed distillation from calcium hydride. AIB was purchased from Aldrich and used without further purification. 9-(4-vinylbenzyl)adenine and 1-(4-vinylbenzyl)thymine were synthesized according to the previous literature. 10 Synthesis of nucleobase containing random copolymer. 1-(4-vinylbenzyl)thymine(1-VBT) or 9-vinylbenzyl adenine 12 (9-VBA, 77 mg, 0.31 mmol) and AIB were weighed into a 100-mL round-bottomed, flask with a magnetic stirbar. The flask was sealed with a rubber septum and purged with 2 for 20 min. Purified n-butyl acrylate and DMSO were syringed into the flask. Finally the reaction mixture was stirred at 60 C for 24 h. After the polymerization, reaction solution was precipitated into methanol/water mixture. Figure 2. Synthesis of nucleobase-containing polymers Preparation of hydrogen bonding blends. Blends of adenine- and thymine-containing PnBA were prepared using solution casting. Chloroform solutions containing a 5 wt% polymer mixture were stirred overnight and cast on a Teflon dish. The solution was allowed to evaporate slowly at room temperature for 1 day. The blend films were then dried at 50 C for 2 days. 164 Instrumentation. Size exclusion chromatography (SEC) was performed using a Waters size exclusion chromatograph. The instrument was equipped with an autosampler, three 5!m PLgel Mixed-C columns, a Waters 2410 refractive index (RI) detector operating at 880 nm, a Wyatt Technologies minidaw multi-angle laser light scattering (MALLS) detector operating at 690 nm, and a Viscotek 270 viscosity detector at 40 C at a flow rate of 1 ml/min in THF. Reported molecular weights are based on absolute measurements using the MALLS detector. 1 H MR spectra were collected in d-dmso on a Varian 400 MHz spectrometer at 23 C. Melt rheology measurements were conducted on a TA Instruments G2 Rheometer in parallelplate geometry with a diameter of 8mm and separation 1mm. Strain amplitude was 1%. Thermal analysis using a TA instruments Differential scanning calorimetry (DSC) determined Tg at a heating rate of 20 C/min under a nitrogen flush. Glass transition temperatures were measured as the midpoint of the transition. FTIR spectroscopic analysis was performed using an ASI ReactIR 1000 attenuated total reflectance (ATR) spectrometer. Results The most important molecular parameters determining polymer melt rheology are molecular weight, molecular weight distribution, and the amount of long chain branching. The synthetic strategy followed in the study avoided the complications of molecular weight effects through simply incorporating adenine and thymine into the same polymer backbones with close M w. The molecular weights and thermal properties are summarized in Table 1. In addition, poly(n-butyl acrylate) was selected as the reference material with no interchain effects, therefore hydrogen bonding was investigated through the nucleobase-functionalized polymers. Table 1. Molecular weights and molecular weight distributions for nucleobase-containing polynba. Sample M w M w /M n HB Sites(mol%) b Tg( C) Thymine-Containing PnBA 250 K Adenine-Containing PnBA 240 K PnBA 227 K Rheological characterization ln*l (Pa.s) blends at 1 Hz thymine-polymer at 1 Hz adenine-polymer at 1 Hz Temperature ( o C) Figure 3. Temperature sweep of melt viscosity measurements for adenineand thymine-containing polymer blends. To understand the dissociation of nucleobase pairs in poly(n-butyl acrylate), the melt rheological characterization of adenine- and thymine-functionalized poly(n-butyl acrylate) and their hydrogen bonding blends was performed over a temperature range that spans both associated and dissociated states of hydrogen bonding groups (Figure 3). The thermal dissociation of the hydrogen bonding groups may occur at temperatures from 60 to 120 o C. At 60 o C, the complex viscosity of hydrogen bonding blends was more than 10 times higher than that of adenine- and thymine-functionalized polymer. Upon heating, the melt viscosities of all three samples exhibited similar transitions, which decreased dramatically as the temperature approached 70 o C. In addition, the complex viscosity of thymine-pnba are higher than that of adenine-pnba, suggesting the hydrogen bonding association within thymine groups are stronger than that of adenine. The polymer blends favor hydrogen-bonding interactions due to a relatively low dielectric constant. An apparent increase in melt viscosity of the hydrogen bonding blends was observed at all the measured temperatures. 166 Variable temperature FT-IR characterization Figure 4. Variable temperature FT-IR spectra of the hydrogen bonding blends of adenine-containing PnBA (12.2 mol% 9-VBA) and thymine-containing PnBA (11.8 mol% 1-VBT). In addition to melt rheological studies, FT-IR analyses at various temperatures were conducted to probe the dissociation of hydrogen bonding units in polymer blends (Figure 4). For the hydrogen bonding blends at 35 o C, three peaks exhibited in the lower region ( cm -1 ) of the spectra: one weak peak at 1699 cm -1, which have been tentatively associated with carbonyl groups from thymine and a peak at 1637 cm -1 corresponding to H 2 scissors vibration of adenine. 9 Vibrations at 1637 cm -1 shifted to lower region as temperature increased. At low temperatures, some of adenine units along polymer backbone have been frozen in a non-hydrogen-bonding state. Increase in temperature provides enough thermal energy in the system for these pendant groups to rearrange, allowing hydrogen bonding to be maximized. From the rheological data (Figure 3), the mechanical properties of hydrogen bonding blends decreased slowly above 70 o C, which appears to be a result of hydrogen bonding dissociation. 167 Figure 5. Variable temperature FT-IR spectra of the hydrogen bonding blends of adenineand thymine-containing PnBA. Hydrogen bonding reversibility was investigated using heat-cool cycles.(figure 5) Samples were heated from 30 to 120 o C and equilibrated at 120 o C for 5 minutes, then cooled to 30 o C. Upon heating, the weak peak at 1637 cm -1 shifted to the lower region about 20 cm -1 ; while in the cooling process, the peak at 1637 cm -1 returned to its position before heating, which indicated some degree of reversibility for hydrogen bonding. The strong peak at 1725 cm -1 associated with non-hydrogen bonded carbonyl groups was broadened from 30 to 120 o C, suggesting the increase of non-hydrogen bonded C=O and this broaden peak did not diminish in the cooling process. The results suggest the irreversibility of complementary hydrogen bonding in melt state. The specific recognition between adenine and thymine are not easily to achieve in bulk state, possibly due to limited diffusion of pendant groups. Conclusions Well-defined PnBA of a low percent (~10 mol%) of nucleobases (adenine and thymine) were synthesized to achieve complementary multiple-hydrogen-bonding DA base pair units. Blending of adenine- and thymine-containing PnBA resulted in higher melt viscosity compared to the corresponding non-hydrogen-bonded adenine- and thymine-pnba samples, confirming strong hydrogen-bonding associations between adenine and thymine hydrogen bonding units. Moreover, variable-temperature IR studies also demonstrated the reversibility of the hydrogen bonded structures. 168 References 1. Gower, M. D.; Shanks, R. A. Acrylic Acid Level and Adhesive Performance and Peel Master- Curves of Acrylic Pressure-Sensitive Adhesives. Journal of Polymer Science: Part B: Polymer Physics, 2006, 44, Sibesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J.; Lange, R. F. M.; Lowe, J. K. L.; Meijer, E. W. Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding. Science. 1997, 278, Mather, B. D.; Baker, M. B.; Beyer, F. L.; Green, M. D.; Berg, M. A.G.; Long, T. E. Supramolecular Triblock Copolymers Containing Complementary ucleobase Molecular Recognition. Macromolecules. 2007, 40, Mather, B.D.; Baker, M.B.; Beyer, F.L.; Berg, M.A.G.; Green, M.D.; Long, T.E. Supramolecular Triblock Copolymers Containing Complementary ucleobase Molecular Recognition. Macromolecules. 2007, 40, Elkins, C. L.; Park, T.; Mckee, M. G.; Long, T. E. Synthesis and Characterization of Poly(2- ethylhexyl methacrylate) Copolymers Containing Pendant, Self-Complementary Multiple-Hydrogen- Bonding Sites. Journal of Polymer Science: Part A: Polymer Chemistry, 2005, 43, Yamauchi, K; Kanomata, A.; Inoue, T.; Long, T. E. Thermoreversible Polyesters Consisting of Multiple Hydrogen Bonding (MHB). Macromolecules. 2004, 37, Yamauchi, K.; Lizotte, J. R.; Long, T. E. Synthesis and Characterization of ovel Complementary Multiple-Hydrogen Bonded (CMHB) Macromolecules via a Michael Addition. Macromolecules. 2002, 35, Yamauchi, K.; Lizotte, J. R.; Hercules, D. M.; Vergne, M. J.; Long, T. E. Combinations of Microphase Separation and Terminal Multiple Hydrogen Bonding in ovel Macromolecules. J. Am. Chem. Soc. 2002, 124, Toshio Itahara, T.; Uto, T.; Sunose, M.; Ueda, T. Molecular Assemblies of Bis- and Tris-adenine Derivatives. Journal of Molecular Structure 2002, 616, Acknowledgment The authors acknowledge the financial support of the Petroleum Research Fund (ACS-PRF AC7), which is administered by the American Chemical Society. 169 TECH 32 Technical Seminar Speaker ucleobase-containing Poly(n-butyl acrylate) as ovel Pressure Sensitive Adhesives Shijing Cheng, Virginia Tech Shijing Cheng is a doctoral student in organic polymer, Department of Chemistry, Virginia Tech. She earned a B.S. in chemistry from anjing University, China, in During her graduate research, she has published three times: Taking advantage of noncovalent interactions in the design of self-healing polymers, anoscale ionic aggregation of Phosphonium ionomers and Supramolecular assembly of carbon nanotubes on silicon substrates. She can be reached at 161
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