Carbon Nanotube Adsorptive Materials Derived from Acid Degradable

Article pubs.acs.org/Macromolecules

Carbon Nanotube Adsorptive Materials Derived from Acid Degradable Poly(acetals) Andrew M. Spring,† Leandro A. Estrada,†,§ Svetlana V. Vasilyeva,‡ Andrew G. Rinzler,‡ and John R. Reynolds†,§,* §

School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States † The George and Josephine Butler Polymer Chemistry Laboratories, Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida 32611, United States ‡ Department of Physics, University of Florida, Gainesville, Florida 32611, United States S Supporting Information *

ABSTRACT: In this paper, we report the synthesis and characterization of a well-controlled and acid degradable poly(acetal) which can adsorb strongly to the surface of carbon nanotubes (CNTs). These polymers, generated via acyclic diene metathesis (ADMET), incorporate pendant pyrene groups that are well-known to associate strongly to CNTs by noncovalent interactions. Films and solutions of the polymer were degraded through the controlled addition of aqueous hydrochloric acid in ethanol. The polymer’s ability to adsorb to and be removed from a CNT film was also evaluated.

T

he high electrical conductivity, wide spectral range of transparency, nanoporosity, and flexibility render carbon nanotube (CNT) thin films (10−100 nm) attractive as surrogates of semiconducting oxides for electrode fabrication.1 For practical use in applications, solution processability is desirable as this is amenable to the plastic substrates required for roll-to-roll printing. The avoidance of high vacuum systems furthermore reduces costs significantly. Key requirements for the preparation of high quality CNT films is that stable inks must be formulated with a high degree of nanotube dispersion, the CNTs themselves must be of high quality, and the dispersion aids should be easily removed after coating. Pyrene is known to nondestructively assemble onto the surface of CNTs and has been used to furnish polymers that disperse CNTs.2−11 Polyacetals, on the other hand, are examples of materials that degrade under acidic conditions and have been well investigated for biological applications,12 and fragrances.13 Therefore, we aimed to produce a material which by the combination of these two functionalities should yield a polymer that is both able to strongly bind to a CNT film noncovalently in common nonhalogenated organic solvents and be degraded and removed from the CNT film in a controlled and manageable fashion. This concept is illustrated in Figure 1. Figure 1A presents a tailored monomer composed of reactive terminals, internal acetal units, and pyrene all linked by alkyl chains. Upon polymerization, the CNT adsorptive material (Figure 1B) is created; exposure to a thin film of CNTs should favor the formation of a polymer/CNT complex (Figure 1C). © 2014 American Chemical Society

Figure 1. Illustration of useful design parameters for a monomer unit that enables the preparation of CNT/polymer complex and its subsequent disintegration by weak acid.

After treatment with a weak acid solution, the polymer backbone should be disintegrated in a well-controlled manner and be easily removed by washing with a common organic solvent. Residual pyrene molecules furnished with terminal aldehyde groups should be left behind, adhering to the surface of the CNT. (Figure 1D). Received: January 10, 2014 Revised: March 25, 2014 Published: April 2, 2014 2556

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Scheme 1. Synthesis of 1-(6,6-Bis(hex-5-en-1-yloxy)hexyl)pyrene (7) from Hexan-1,6-diol (1)a

a Key: (i) 48% HBr/H2O, toluene, reflux, 72 h. (ii) 2,3-Dihydropyran, Fe(ClO4)3, ether, room temperature, 1.5 h. (iii) (a) Mg, ICH2CH2I, (3), ether, reflux, 2 h; (b) 1-bromopyrene, Ni(dppe)Cl2, ether, reflux, 2 h. (iv) Fe(ClO4)3, 1:1 toluene/MeOH, 50 °C, 12 h. (v) (a) DMSO, (CO)2Cl2, −78 °C, DCM, 2 h; (b) Et3N, −78 °C → room temperature, 30 min. (vi) 5-hexen-1-ol, HSO3−SiO2, THF, reflux, 24 h.

Acyclic diene metathesis (ADMET)14 offers a versatile solution for the preparation of novel pyrene containing polymers, as it involves the polycondensation of linear α,ωdienes.15,16 Potentially, a large variety of functional groups can be incorporated amid two terminal alkenes thus enabling access to a large gamut of α,ω-dienes that permit the synthesis of a diverse range of macromolecular architectures. Apart from the ADMET reaction,17 polyacetals have been prepared by other methods.18−21 ADMET, however, offers the advantage of avoiding the random nature of branching and permitting control of the molecular weight through the regulation of the release of ethylene. In this manuscript, we report the preparation of a linear unsaturated polyacetal via ADMET. This polymer has pendant pyrenes, which are able to strongly associate with a CNT thin film thus forming a polymer/CNT complex. The polymer is engineered to be degradable under mild acidic conditions and be easily removed from the CNT film. The decomposition of the polyacetal was induced by the addition of a dilute acid, with the process being carefully monitored using GPC, NMR and IR. 1-(6,6-Bis(hex-5-en-1-yloxy)hexyl)pyrene (7), whose structure is presented in Scheme 1, includes all the aforementioned parameters and was prepared through the modification of established procedures.17 Details of its preparation may be found in the Supporting Information. This α,ω-diene can be synthesized in high yields via the condensation of 6-(pyren-1yl)hexanal (6) and 5-hexen-1-ol using an SO3H-functionalized silica catalyst.22 The precursor (6) was synthesized from inexpensive hexan-1,6-diol (1). The monomer (7) was purified using preparative HPLC so as to ensure success in the ADMET polycondensation step. Analytical HPLC and 1H/13C NMR analyses confirmed the required purity, with all integrals yielding the expected values. Key 1H-NMR environments are presented in Figure 2A, confirming the successful preparation of the material. The triplet belonging to the terminal olefinic protons, marked as (H2), can be observed at 5.85 ppm and the internal olefinic protons (H1) appear as a complex multiplet at

Figure 2. 1H NMR spectra of (A) 1-(6,6-bis(hex-5-en-1-yloxy)hexyl)pyrene monomer (7) and (B) polyacetal (8).

5.00 ppm, respectively. The acetal proton environment (H3) generates a triplet at 4.47 ppm. Monomer identity and purity was additionally confirmed by elemental analysis and highresolution mass spectrometry (see section S2 in the Supporting Information). The monomer (7) was dried thoroughly under vacuum, prior to addition of 1 mol % of the Grubbs first generation initiator ((Cy3P)2(Cl)2RuCHPh)). The resulting mixture was then stirred under vacuum at 45 °C for 4 days. This ruthenium benzylidine is favored to the Schrock system,23 as it is known to be tolerant to residual aldehydes that may deactivate the Schrock catalyst. A visual representation of this reaction is presented in Scheme 2. Mechanical stirring is required to obtain access to high molecular weight materials under solvent-free conditions. After polymerization, the remaining active catalyst was quenched by the addition of a solution of ethyl vinyl ether in 1,2dichloroethane. Once the mixture was stirred for an hour, the polymer solution was precipitated into nonacidic methanol with the majority of the polymer collecting around the side of the vessel as a viscous off-white gum. After the unsaturated polyacetal was isolated, the material was characterized by GPC, NMR, and FTIR. 1H and 13C NMR revealed that the material 2557

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Scheme 2. Synthesis of Polyacetal (8) and Its Degradation under Acidic Conditionsa

a

Key: (vi) ((Cy3P)2(Cl)2RuCHPh)), P = 100 μmHg, 45 °C, 4 days. (vii) 1 M HCl(aq), EtOH, room temperature.

was free from any monomer and catalyst residues. The 1H NMR spectra presented in Figure 2B displays a broadening of peaks associated with polymerization relative to those of the monomer; furthermore, the internal olefin protons (H1) can be observed as a large singlet integrating to two protons at 5.42 ppm. A similar polymer17 presents a splitting of the olefin signals attributed to the presence of both cis and trans conformations. Such signals are unobserved in the case of polyacetal (8), possibly obscured by those from the pyrene and linking groups. The singlet at 4.45 ppm can be attributed to the acetal proton (H3) and the signals at 5.00 ppm, attributed to terminal alkenes (H2) of the monomer (7), are absent in the polymer spectrum. Polymer molecular weights were on average ∼10 kDa, with a maximum case of 27 kDa as determined by GPC against polystyrene standards. The highest molecular weight materials were found to be insoluble in both THF and Chloroform. Degradation experiments were performed on the polymers (Mn = 9.3 kDa) both in THF solution and when air brushsprayed onto a substrate as a film from this solution. The expected results are presented in Scheme 2, where the acid promotes the dissociation of the polyacetal into a saturated diol (9) and 6-pyren-1-ylhexanal (6). The substrates were either a clean microscope glass slide or a glass slide with a predeposited CNT film (60 nm thick), as prepared by a filtration procedure.1 In the case of the solution degradation experiments a sample of the polyacetal (8) (500 mg) was dissolved in THF (50 mL), whereupon a small volume of 1 M aqueous HCl (1 mL) was added to the solution. From the vigorously stirred mixture, small aliquots were taken via syringe on each 12th minute and quenched by adding water and DCM; upon mixing, the organic layer was isolated and analyzed. NMR, GPC and IR characterization was performed on each aliquot to determine the rate of polymer degradation and the species that were being evolved from the backbone. Figure 3 shows a collection of 1H NMR spectra of four of the aliquots from the initial as-formed polymer state, to a reaction time of 96 min. The dynamic changes occurring are exemplified by an increase in the area attributed to the aldehyde signal at 9.77 ppm (★), a reduction in the area attributed to the acetal singlet at 4.39 ppm (+) and an increase in the area of the multiplet at 2.44 ppm (⧫) associated with the linker chain environments of the molecular aldehyde and diol. Extending the reaction time from the initial state to 96 min increases the area under the aldehyde peak at 9.77 ppm relative to that of the internal olefin singlet at 5.35 ppm. Concurrently, the area attributed to the acetal singlet at 4.39 ppm, relative to the internal olefin signal in the initial state, also decreases. Finally, the area of the multiplet at 2.44, assigned to the small molecule aldehyde and diol, increases after 96 min as well. This illustrates that, in solution, the degradation is relatively slow and well

Figure 3. Evolution of proton resonance signals during acid-promoted degradation of polyacetal (8) followed by 1H NMR.

controlled thus permitting the identification of the evolved species. In support of the 1H NMR data, the evolution of Mn vs time presented in Figure 4 shows the degradation pattern of the polyacetal (8). After the first 12 min exposed to the acid, the molecular weight drops from 9.3 kDa to 6.7 kDa and subsequently to 5.1, 4.7, 3.7, 3.3, 2.9, 2.7, 2.5, and 2.2 kDa. Figure S3.1A (Supporting Information) shows all GPC curves superimposed onto one graph. Features of note are the shifting of the curve maxima from left to right on the x-axis,

Figure 4. Reduction in Mn of polyacetal (8) over time upon exposure to acid. 2558

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corresponding to the reduction in molecular weight of the polymers and the presence of the monomeric aldehyde observed at a retention time of approximately 20 min (★). The latter peak also increases in intensity as its relative concentration in the sample becomes higher. The large peak at 22 min and collection of smaller peaks shown in Figure S3.1B, may be attributed to the diol (9) as well as a collection of similar species and residual solvents. FTIR was also employed to monitor the concentration of aldehyde in each of the aliquots (0−96 min). The spectra presented in Figure S3.2 illustrate the evolution of the carbonyl and the broad alcohol signals. UV−vis spectra were also obtained to note any differences between the as-formed polymer, and the degraded material after 12 and 96 min of contact with acid. All spectra presented in Figure S3.3 were identical to that of the reference 1-bromopyrene except for the loss of vibronic structure in the polymer case. This implies that the solution preserves the chromophoric structure upon contact with acid. The polymer was also dissolved in toluene and sprayed onto microscope glass slides creating five thin films of polyacetal (viz. P1−P5). Each slide was dried thoroughly and its UV−vis spectrum was recorded. To measure the degradation rate of the polymer films, each glass slide was transferred to a stirred bath of ethanol (50 mL) with the addition of a small quantity of aqueous 1 M HCl (1 mL). After treatment, each slide was again dried and the UV−vis absorption reacquired. The similar absorbance of the initial films presented in Figure S3.4 reflects their comparable thicknesses. After exposure to acid for 60 min (P1), 30 min (P2), 10 min (P3), 60 s (P4), and 30 s (P5), respectively, the absorption in all cases was reduced to baseline intensity indicating that the thin film degrades rapidly (within 30 s). A thicker film was prepared by multiple spraying from the same solution onto another glass slide, also, enabling the analysis of the species diffusing into the ethanol solution and those remaining on the glass slide. Upon degradation of the polymer, the local concentration of aldehyde (6) should increase as well as that corresponding to the alcohol (9) initially present on the polymer chain (Scheme 2). The film was treated as before, being placed in a stirred ethanol/acid solution for 24 h. The polymers were removed from the substrate by dissolving in deuterated chloroform, this solution was then used to measure the 1H NMR. Figure S3.5A, shows the 1H resonances of the material remaining on the glass postdegradation; this material is similar to that of the as-formed polymer, which was not observed in the thin film case (i.e., the degradation was incomplete). Figure S3.5B, on the other hand, shows the small molecule aldehyde released from the polymer film upon degradation: the presence of an aldehyde peak at 9.77 ppm and the lack of any internal olefin and acetal signals at 5.42 and 4.45 pm, respectively, illustrate this change. The degradation profile of the polyacetal polymer sprayed onto a predeposited 60 nm CNT film on a glass substrate was also investigated. The amount of polymer sprayed was roughly comparable to that sprayed onto the glass slides without CNTs. This experiment followed the same protocol as the degradation of the polymer deposited directly on the glass substrate yielding the absorbance profiles shown in Figure 5, recorded at the times indicated in the inset. In contrast to the rapid (30 s) depletion of the pyrene absorption to baseline intensity in the case of the polyacetal film directly on the glass, it was found that even after 340 min (>5 h) low intensity absorption signals

Figure 5. Reduction in pyrene absorbance after immersion of the polyacetal/CNT film on slide glass into the acid/ethanol bath for time periods of 30 s, 60 s and 5−340 min, respectively. Inset: Plot of absorbance vs time monitored at 350 nm.

could still be observed for the polyacetal/CNT film. The possible explanation for this behavior is 2-fold: (1) during spray-coating the polymer likely infiltrates the porous carbon nanotube film creating a diffusion barrier for the polymer decomposition products and (2) the pyrene units associate with CNT surfaces via strong π−π interactions that inhibit its dissociation from those surfaces. The experiment conducted with polyacetal film sprayed onto the pristine glass indicates the polymer’s rapid degradation suggesting that the residual UV− vis absorption in the nanotube case can be attributed to pyrenes associated with CNT surfaces. The high baseline absorbance reflects the possibility of cross-linking of the internal olefin with the CNT. In conclusion, ADMET has been successfully utilized in the preparation of a unique linear unsaturated polyacetal, incorporating pendant pyrene units which enable the polymer to associate strongly to CNTs. This polyacetal has been degraded in a manageable and predictable fashion as measured in thin films and solutions, with and without the presence of CNTs. The degradation of the polyacetal was carefully monitored by NMR, GPC, and IR through the release of a molecular aldehyde and formation of an alcohol group on the polymer. This and alternative degradable polymer systems are presently under investigation.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedures and characterization plus materials and instrumentation details, supporting figures showing 1H and 13C NMR spectra of novel compounds and 1H NMR spectra of the polymers, and spectroscopic and analytical data of polymers and GPC curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(J.R.R.) E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2559

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ACKNOWLEDGMENTS We acknowledge the technical contributions of Dr. Fengjun Deng on various experimental aspects of this work. We thank Ms. Natasha B. Terán and Mr. Alexander G. Pemba for the acquisition of GPC data, Ms. Natasha B. Terán and Mr. Justin A. Kerszulis for their assistance on the preparation of acetal monomer (7), and Mr. James J. Deininger for assistance in the purification of acetal monomer (7) via preparative HPLC. Financial support from Nanoholdings, LLC is gratefully acknowledged.

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ABBREVIATIONS ADMET,acyclic diene metathesis; GPC,gel permeation chromatography; M n ,number-average molecular weight; ĐM,molecular weight dispersity; DP,degree of polymerization; ppm,parts per million; OD,optical density; NMP,N-methyl-2pyrrolidone

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REFERENCES

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