Dynamic π-Conjugated Polymer Ionic Networks - Macromolecules

Article pubs.acs.org/Macromolecules

Dynamic π‑Conjugated Polymer Ionic Networks Shekhar Shinde, Jenna L. Sartucci, Dorothy K. Jones, and Nagarjuna Gavvalapalli* Department of Chemistry and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, D.C. 20057, United States S Supporting Information *

ABSTRACT: The elastic nature of π-conjugated polymer gels makes it difficult to process them using traditional coating techniques and inkjet printing, and as injectable gels, thereby limiting the use of preprocessing and storing the polymers as gels. Taking cues from the easy processability of reversible and thixotropic non-π-conjugated polymer gels, we report herein for the first time a simple strategy to obtain reversible and thixotropic π-conjugated polymer ionic network (π-PIN) gels. Reversible and thixotropic π-PIN gels are generated by synergistically combining the intriguing properties of π-conjugated polymers with the dynamic properties of ionically cross-linked networks.

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INTRODUCTION π-Conjugated polymer gels combine the advantages of both πconjugated polymers and gels and therefore are useful in many applications including organic electronics, energy conversion/ storage, bioelectronics, and as synthetic tissues.1−15 For example, π-conjugated polymer gels generate a stable and three-dimensional polymer network prior to coating, which avoids the cumbersome postprocessing morphology control step in organic electronics device fabrication process.11−13,16,17 π-Conjugated polymer gels are also used in bioelectronics devices as electrodes or implantable scaffolds.1,3,6 Unfortunately, the elastic nature of these gels makes it difficult to process them using traditional coating techniques and inkjet printing, and as injectable gels, thereby limiting the use of preprocessing and storing the polymers as gels.6,11−13,18−20 In contrast, non-π-conjugated polymer gels and low molecular weight gels are injectable and readily processed through conventional coating techniques due to the reversible and thixotropic nature of these gels.21−28 The reversible and thixotropic gels exhibit gel−sol transition at high shear strain amplitude and vice versa due to reversible physical cross-links between polymer chains. Injectable chitosan or gelatin-graf tpolyaniline polymer networks were generated using the dynamic nature of Schiff base.29,30 Although the development of thixotropic and reversible physically cross-linked gels is considerably well advanced in the case of non-π-conjugated polymers,21−28,31−33 there are no reports in the case of πconjugated polymers to the best of our knowledge.

Scheme 1. Synergistic Combination of the Intriguing Properties of π-Conjugated Polymers and Ionically CrossLinked Networks To Generate Dynamic π-PINs

electronic)34−37 and ionic networks (reversibility, self-healing capability, stimuli responsive, and tunable rheology),38−41 the generated π-PINs also pave the way for the development of self-healable and stimuli responsive electronic materials. We have chosen ionic interactions because their interaction strength and reversibility are controllable and tunable.38−40,42,43 The key challenge39,40,42−44 in generating dynamic π-PIN gels lies in identifying appropriate cross-linkers that exhibit strong

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RESULTS AND DISCUSSION Herein we use ionic cross-linking of π-conjugated polymers as a simple strategy to generate reversible and thixotropic πconjugated polymer ionic network (π-PIN) gels (Scheme 1). Since the proposed strategy synergistically combines the intriguing properties of π-conjugated polymers (optical and © XXXX American Chemical Society

Received: September 5, 2017

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DOI: 10.1021/acs.macromol.7b01896 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

network strength, a series of difunctional 2° and 3° amines with increasing alkyl group substituents length/size on the amine are synthesized (see Supporting Information) and used as crosslinkers (Figure 1b). For all studies, the ratio of the carboxylic acid to amine functional groups is used to represent the composition of the complex. As the small molecule NMR studies confirm the complex formation (Figures S1−S6, Tables S1 and S2), we next proceeded to synthesize the π-PINs. The π-PINs were synthesized by dissolving 3 wt % PTCOOH in DMAc followed by the addition of amine solutions of increasing concentration to generate π-PIN complexes of varying composition (1:0.2, 1:0.4, 1:0.75, and 1:1) at room temperature (Figure 2a,b). The resultant solution is either a homogeneous or heterogeneous mixture (precipitate) depending on the amine. The bulky amines (3°nPr and 3°iPr) resulted in soluble complexes due to weak ionic interactions. The heterogeneous complex, which is a result of kinetically trapped network formation, is heated (3° amines at 100 °C and 2° amines at 110 °C) and cooled back to room temperature to thermally dissociate and re-form the ionic bonds. The π-PINs of amines with lean ionic head groups (2°Me and 2°Et) were not soluble even at the higher temperature (110 °C) for the studied amine composition range due to the strong, irreversible ionic bonds. The 1:1 complexes of the rest of the amines became soluble upon heating due to the reversible ionic bonds and turned into gels upon cooling to room temperature. As the amine composition is reduced from 1:1 to 1:0.2, the 2° amines (2°nPr, 2°iPr, 2°nBu, and 2°tBu) resulted in weak gels (vide inf ra) whereas the 3°Me and 3°Et amines formed soluble complexes, indicating that the 2° amines exhibit stronger ionic interactions compared to the 3° amines. Thus, as the size of the amines increases, the morphology of the π-PINs changes from a precipitate to a gel to a soluble complex. Importantly, increasing the number of substituents on the nitrogen (2°Me to 3°Me) is more effective than increasing the substituent chain length in 2° amines (2°Me to 2°Et) in controlling the

yet reversible ionic interactions with the polymer. Thus, we have systematically varied the cross-linker ionic headgroup size and studied its effect on the network morphology, assembly, and the thermal, optical, and rheological properties of the πPINs. Since the dynamic properties of the network depend strongly on the charged ionic centers and less on the nature of the polymer repeat unit, this strategy is widely applicable to a plethora of π-conjugated polymers. As a proof of concept, we have generated reversible and thixotropic π-PIN gels of poly [3(6-carboxyhexyl)thiophene-2,5-diyl] (PTCOOH) by ionically cross-linking it using diamine cross-linker. The π-PINs are synthesized by reacting the PTCOOH with diamines in dimethylacetamide (DMAc) solvent. A proton transfer from the carboxylic acid of the PTCOOH to the amine generates a carboxylate polyanion and an ammonium dication cross-linker, thereby resulting in ionically cross-linked networks (Figure 1a). We chose PTCOOH for this study because the

Figure 1. (a) Synthesis of π-PINs. (b) Chemical structures of the diamine cross-linkers used in this study.

optical and electronic properties of poly(3-alkylthiophene)based systems are well studied.45−48 PTCOOH of molecular weight 51 kDa was purchased from Rieke Metals. In order to control the ionic interaction strength and hence the π-PIN

Figure 2. (a) Photographs of π-PINs at 1:1 composition. (b) Phase diagram of the complexes. (c) ATR-IR spectra of the dried 1:1 π-PINs; % of COO− formed is also included in the image. B

DOI: 10.1021/acs.macromol.7b01896 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. (a) Strain sweep of the 2°nBu gels. (b) Storage moduli of the complexes. (c) Amine composition dependent storage moduli of the 2°nBu gels. (d) Dynamic strain amplitude cyclic test of the 2°nBu gel. (e) Shear rate dependent viscosity of 2°nBu gel. (f) Preloaded 2°nBu gel in a syringe. (g) Freely flowing gel through the syringe needle. (h) Rapid gel formation capability of the sample after injection.

ca. 5% strain, suggesting that beyond this strain the π-PIN is disrupted and the polymer chains are free to flow. The G′ (taken from the plateau modulus of the strain sweep experiments) of the 2° amines (ca. 65−100 Pa) is 2−3 times higher than that of the 3° amines (ca. 30 Pa) even though they have a lower cross-linker concentration (only 20 mol %) (Figure 3b). The G′ of the 2°nBu π-PIN increases with increasing amine ratio (1:0.2 to 1:1), suggesting an increase in the ionic cross-linking density at the higher amine composition (Figure 3c). G′ for the 2°nBu π-PIN (ca. 12 000 Pa) is 400 times higher than that of the 3°Me π-PIN (ca. 30 Pa) at 1:1 composition, confirming that the 2° amines exhibit higher ionic interaction strength than the 3° amines. The PTCOOH polymer and the control PTCOOH−amine complex with monofunctional 2°nBu amine are viscous in nature and do not have a LVE region. The π-PIN-gels of 2° (1:0.4) and 3° amines (1:1) were subjected to repeated dynamic strain amplitude cyclic tests (at 0.1 and 100% strain) to determine the reversibility and thixotropic nature of the π-PINs (Figure 3d and Figure S10).27,52 The value of the G′ dropped and is less than G″ at high strain, which is indicative of the disruption of the ionic network, indicating that the gel has transitioned to a sol. Importantly, when the strain was reduced to a lower value, the G′ rapidly recovers to the initial value, indicating that the ionic networks are re-formed and the π-PINs transition back to a gel. The G′ and G″ values are reversible at the low and high strains for more than five cycles, thereby confirming that the proposed π-PINs are reversible and thixotropic. The viscosity of the gel reduces as the shear rate increases, confirming the shear thinning nature of gel (Figure 3e and Figure S11). The π-PIN gels are also easily injectable through the syringe needle, and they turn into gel immediately after the injection (Figure 3f− h). The repeated strain sweep and shear thinning experiments do not show hysteresis, indicating that the ionic network is re-

morphology of the complex. The gels are stable at room temperature for over 2 weeks. The control complexes with monofunctional amines (Figure S7 and Figure 2b) resulted in soluble complexes (analogous difunctional amines resulted in gels), confirming that the diamines are needed to generate the cross-linked π-PINs. The ATR-IR spectra of the dried 1:1 π-PINs confirm the formation of the acid−amine complex in π-PINs (Figure 2c). All the 2° and 3° amine complexes showed peaks at both 1700 and 1560 cm−1, indicating that there are both complexed and uncomplexed −COOH groups along the polymer backbone. The extent of ionic complexation is determined from the ratio of the area49,50 of the carboxylate anion peak (COO−) to the total carbonyl peak (COOH + COO−) (Table S3). In general, 2° amines showed a higher percentage (between 59% and 70%) of carboxylate anion than the 3° amines (between 29% and 43%), indicating that the π-PINs with 2° amines exhibit higher cross-linking density. The thermal, rheological, and optical properties of the πPINs were studied to determine the strength and reversible nature of the ionic interactions and the thixotropic nature of the π-PIN gels. The π-PIN gels of 1:0.4 (2° amine) and 1:1 (3° amine) compositions were used for the rheological studies. The strain sweep experiments of the gel samples clearly showed a linear viscoelastic regime (LVE) below ca. 1% strain and within this regime the storage modulus (G′) is larger than that of loss modulus (G″), suggesting that the π-PINs are elastic gels (Figure 3a and Figure S8). With increase in the applied strain, the G′ decreases continuously whereas the G″ increases, reaches a maximum, and then decreases. A similar rheological response in G″ is also observed in the hydrophobically modified polymers, and such a behavior is attributed to the straininduced imbalance between the rate of formation and dissociation of the cross-links within the network.28,51 The crossover of G′ and G″, i.e., the gel to sol transition, occurs at C

DOI: 10.1021/acs.macromol.7b01896 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. DSC traces of the 1:1 gels: (a) heating and (b) cooling cycles; peak maximum are shown on the traces. (c) Temperature-dependent UV− vis spectra of the 2°nBu π-PIN. (d) Normalized time-dependent absorbance of 2°nBu π-PIN at 445 nm. (e) Plot to determine the activation energy using eq 1 in the Supporting Information.

the PTCOOH spectrum, indicating that the ionic interactions are weak and do not induce any change in conformation of the polymer backbone. The 2°nBu complex spectrum is thermally reversible between aggregated and uncomplexed state (Figure 4c) and hence is useful to determine the activation energy for ionic bond dissociation. The time-dependent change in the absorbance at 445 nm at different elevated temperatures was recorded for three 2° amine π-PINs, and the Ea was determined using eq 161,62 (Figure 4d; Figures S24 and S25, see Supporting Information). An Arrhenius-type analysis of the data yielded a linear plot (Figure 4e) wherein the slope corresponds to the Ea. The Ea’s for the ionic bond dissociation of 2° amine−polymer complexes are in between 40 and 50 kJ/mol (Table S5), suggesting that the substituents do not play a significant role on bond dissociation. The Ea’s of the proposed π-PINs are slightly lower than the phosphonium and poly(acrylic acid) based ionic network melts.8a We speculate that the presence of solvent in the π-PINs lowers the activation energy for bond dissociation. Since the 3° amines did not show any spectral changes upon complexation, we are exploring other techniques to determine the Ea. The low-angle and powder XRD spectra of all the dried πPINs are similar to the reported PTCOOH spectra with lamellar assembly49,63 but with a higher d-spacing (Δd spacing = 0.4 nm; Figure 5, Figure S26, Tables S6 and S7). For comparison, the 2°nPr gel sample was also recorded and is found to be similar to that of the dried π-PIN, indicating that the polymer assembly occurs during the ionic complexation and gelation. The additional ∼0.4 nm spacing corresponds to the amine cross-linker that is tilted between the polymer layers (Figure 5b). Thus, the ionic cross-linking of the PTCOOH generates π-PINs with a lamellar assembly, which is advantageous for charge transport and hence the electronic applications. Currently, we are exploring the electronic properties of the π-PINs.

formed once the mechanical stimulus is removed (Figures S9 and S12). The low shear strain amplitude onset and the rapid recovery of the ionic complex from a sol to a gel state make the π-PINs processable by conventional coating techniques and inkjet printing and as injectable gels. G′ is higher than the G″ across the studied frequency range and showed no crossover (Figure S13), further confirming that the π-PINs behave as gels. The thermoreversible nature of the gels is also confirmed visually by heating and cooling the gels in a vial (Figure S14). The network to liquid transition temperature (Tnl) of π-PIN gel samples is an indication of the strength53,54 of the ionic interactions and is determined using DSC. All the π-PIN gels show a broad endothermic and an exothermic transition during the heating and cooling cycles, respectively (Figure 4a,b and Table S4), which correspond to the network to liquid (Tnl) and liquid to network transitions (Tln). Moreover, both the endoand exothermic transitions are observed in the latter two cycles, indicating that the transitions are thermoreversible in nature (Figures S15−S20). The higher Tnl of the 2° amine gels (between 100 and 125 °C) than the 3° amine gels (ca. 80 °C) indicates that the 2° amines form more thermally stable π-PINs (stronger ionic networks) compared to the 3° amines. The optical properties of π-conjugated polymers are sensitive to complexation and hence are useful to elucidate the complex formation process.55−60 The π-PINs at 1:1 composition ([PTCOOH] = 95 μM and [diamine] = 47.5 μM in DMAc) do not show significant spectral change compared to PTCOOH (Figure S21), which may be due to the low fraction of complexation. So, the UV−vis absorption spectra of the πPINs at higher composition (1:50) were recorded (Figures S22 and S23). The 2° amine π-PINs showed a red-shift in the absorption maximum from 465 to 535 nm (Δλ = 70 nm) and a low-energy vibronic transition at 600 nm. On the basis of the observations for poly(3-hexylthiophene)s, we attribute the redshift in the case of the 2° amines to the planarization and aggregation of the polythiophene45−48 backbone due to ionic cross-linking. The 3° amine π-PINs spectra resembled that of D

DOI: 10.1021/acs.macromol.7b01896 Macromolecules XXXX, XXX, XXX−XXX

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CONCLUSION To summarize, we have shown ionic cross-linking of πconjugated polymers as a simple strategy to generate reversible and thixotropic π-PIN gels. The cross-linker ionic headgroup size is found to play a key role in controlling the thermal, optical, and rheological properties of the π-PINs. The low shear strain amplitude onset and the immediate recovery of the ionic complex from a sol to a gel state make the π-PINs processable by conventional coating techniques and inkjet printing and as injectable gels. Also, the ionic cross-linking generates a lamellar assembly of the polymer chains, which is advantageous for electronic applications. Because of the dynamic nature of the ionic cross-linking, the generated π-PINs also pave the way for the development of self-healable and stimuli responsive electronic materials. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01896. Synthetic, experimental, and characterization details (PDF)

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Figure 5. (a) Low-angle-XRD spectra of the dried 1:1 π-PINs and 2° nPr 1:1 π-PIN gel. (b) Schematic cartoon of the lamellar assembly of π-PINs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (N.G.). ORCID

Nagarjuna Gavvalapalli: 0000-0002-2812-1694 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.G. gratefully acknowledges the start-up funds from Georgetown University. We thank Prof. Peter Olmsted and Prof. Dan Blair for rheology discussions. E

DOI: 10.1021/acs.macromol.7b01896 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01896 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01896 Macromolecules XXXX, XXX, XXX−XXX