Molecular Engineering of Polyphosphazenes and ... - ACS Publications

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Molecular Engineering of Polyphosphazenes and SWNT Hybrids with Potential Applications as Electronic Materials Yi Ren,†,‡ Zhongjing Li,† and Harry R. Allcock*,† †

Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States School of Physical Science and Technology, Shanghai Technical University, Shanghai 201210, P. R. China

‡

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S Supporting Information *

ABSTRACT: Polymer/single-walled carbon nanotube (SWNT) hybrids are promising candidates in applications such as flexible and stretchable electronics. In this contribution, we have examined structure−property relationships for constructing new polyphosphazene−SWNT hybrids. UV−vis and Raman spectroscopy studies revealed that the unique PN backbone enables strong intermolecular donor−acceptor interactions between the polymer and SNWTs. Furthermore, the polymeric backbone and the environment at the Pcenters collectively play important roles in the formation of the hybrids. For polymers with shorter alkoxy substituents, the donor−acceptor interactions between the PN backbone and SWNTs play a crucial role in stabilizing the hybrid complexes, but for polymers with longer alkoxy substituents, the CH−π interactions and steric hindrance between the alkyl side chains and SWNTs counterbalance each other and control the stability of the hybrid complexes. Furthermore, the presence of fluorine and oxygen atoms is detrimental to the stability of the hybrid complexes. New cross-linkable polyphosphazenes with anthracene side units were also synthesized. When photo-cross-linked, these polyphosphazene/SWNT hybrids showed elastomeric characteristics and electronic properties that are promising for future applications.

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assist the solid-state structural developments of SWNTs.46−48 For example, aryl polymers, such as polyfluorenes and polythiophenes, interact with SWNTs via strong intermolecular π−π interactions, thus significantly improving the dispersibility of SWNTs in solution.46−48 Furthermore, finetuning of their backbones and side groups allowed the organic polymers to selectively interact with different types of SWNTs to give either semiconducting or metallic species.1−3,49−53 Such strategies have ultimately improved the processability and functionalities of SWNTs in organic electronics. The improved properties shown by aromatic organic polymer/SWNT hybrids raise the question of whether inorganic backbone polymers like polyphosphazenes might provide an access route to high performance polymer/SWNT hybrid materials. Compared to their organic counterparts, inorganic backbone polymers have only rarely been used to build hybrid materials with SWNTs.54−56 In one important study of inorganic backbone polymer/SWNT hybrids, Naito and co-workers explored the possibility of using polysilanes to interact with SWNTs. Although lacking aromatic substituents, the alkyl-substituted polysilanes were found to complex with SWNTs, perhaps via subtle but efficient CH−π interactions between the alkyl side chains of polysilanes and the πbackbones of the SWNTs. Moreover, these systematic studies

INTRODUCTION Complexes of polymers with single-walled carbon nanotubes (SWNTs) are of growing interest for their role in hybrid materials including stretchable electronics.1−5 Polyphosphazenes have special properties that arise from their unsaturated inorganic backbone and the wide range of different side group that can be linked to the polymer chain. Since the first reports of stable and soluble polyphosphazenes in the 1960s this field has expanded dramatically.6−9 Actual and potential applications of polyphosphazenes have been found in numerous areas, such as elastomeric materials,10−14 optoelectronic polymers,15−24 and biomaterials.25−35 Recently, we reported that chemical or physical cross-linking techniques significantly enhance the elastomeric characteristics of these materials12−14 This is a useful feature for aerospace, automotive, arctic, and oil drilling engineering and also for the development of stretchable electronic applications. Thus, achieving a combination of the properties of polyphosphazenes with SWNTs is an appealing prospect. Recently, SWNTs have been incorporated into organic polymers, and this has opened a new platform for designing electronic materials. SWNTs have excellent electronic properties,36−38 strong mechanical strength,39−41 and intriguing optical properties.42−44 However, these characteristics can be maximized in useful electronic devices only when homogeneous dispersions and well-organized morphologies of SWNTs can be obtained.1−3,45 To this end, organic polymers have improved the dispersibility of SWNTs in solution and also © XXXX American Chemical Society

Received: April 16, 2018 Revised: June 12, 2018

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DOI: 10.1021/acs.macromol.8b00779 Macromolecules XXXX, XXX, XXX−XXX

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polyphosphazenes allowed a study of both the steric and electronic influences of the side groups on the structures and properties of the polyphosphazene/SWNT hybrids. Preparation and Characterization of Polyphosphazene/SWNT Hybrids. We first investigated whether the polyphosphazene synthetic precursor P-Cl would complex with SWNTs in solution. To prepare the hybrid, we mixed 40 mg of P-Cl and 1 mg of SWNTs in 10 mL of THF. These mixtures were subjected to bath sonication for 2 h. Then the mixture was centrifuged at 5000 rpm for 2 h. The top 2/3 supernatant was carefully collected and characterized. It was found that the SWNTs can be dispersed in P-Cl solutions (Figure 1a inset, picture of P-Cl/SWNT). Since

uncovered chemical interactions in which the polymer backbone and alkyl side chain structures of polysilanes play significant roles in the complexation with SWNTs. It was found that only random-coiled and flexible polysilanes complex with SWNTs in solution. It was believed that the flexible, random-coiled polysilanes easily adopt a conformation that fits the surface curvatures of SWNTs, while semiflexible polysilanes with more rigid backbones do not.54,55 Although these studies have shed light on building inorganic backbone polymer/SWNT hybrids, the detailed structure−property relationships of these species are still not clear and are often varied system by system. For example, the presence of phenyl groups in polyacetylenes was found to be beneficial for the formation of the polyacetylene/SWNT complexes in solution,57,58 while phenyl groups in polysilanes did not stabilize the formation of polysilane/SWNT complexes in solution.55 Therefore, more detailed studies are desirable in order to fully understand inorganic polymer/SWNT hybrids. As an important class of inorganic backbone polymers, the polyphosphazenes with their diverse chemical structures provide a good platform to investigate inorganic backbone polymer/SWNT hybrids. With their intriguing elastomeric and biocompatible characteristics, the polyphosphazenes should be exceptional candidates for studying new SWNT hybrids for stretchable electronic and bioelectronic applications. Although functionalized multiwalled carbon nanotubes (MWNTs) and polyphosphazenes have been employed to construct hybrid materials, chemical modification of the functionalized MWNTs does not reveal the intrinsic interactions between the polymers and the carbon nanotubes.59 In the current study, we have systematically investigated the structure−property design rules for producing new polyphosphazene/SWNT hybrid materials. As a proof of concept, we also explored the potential applications of the hybrid materials for stretchable electronics.

Figure 1. (a) TEM and (b) AFM images of P-Cl/SWNT hybrid. (c) Photographs of P-C2,3,4,6/SWNT hybrids in THF. (d) 1H NMR spectra of P-C6 (bottom) and P-C6/SWNT (top).

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RESULTS AND DISCUSSION Synthesis and Characterization of Polyphosphazenes. To systematically investigate the structure−property relationships of polyphosphazene/SWNT hybrids, we synthesized several series of polyphosphazenes (Chart 1) with

SWNTs alone cannot be dispersed in THF under the same condition, it appears that P-Cl and SWNTs must interact with each other and form hybrid complexes in THF. In the stable dispersed solution, the soluble P-Cl not only assists dispersion of SWNTs but also prevents self-aggregation of SWNTs by wrapping around their surface. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) images (Figure 1a,b) further confirmed that the polymers did indeed wrap around the SWNTs. The diameter of the P-Cl/SWNTs in the TEM images was ca. 5 nm, suggesting that 3−4 individual SWNTs were encapsulated. Previous studies of polymer/SWNT systems have revealed that strong intermolecular interactions such as π−π and CH−π interactions between the polymers and SWNTs are beneficial for constructing hybrid complexes.1−5,46−49 Complex formation between P-Cl and SWNTs suggests that other types of intermolecular interactions may play an important role in the formation of P-Cl/SWNT complexes. The earlier studies revealed that intermolecular charge transfer occurs between conjugated materials and SWNTs via intermolecular donor− acceptor interactions.62−64 Recent studies also found that the polar nitrile groups of polyacrylonitrile adopt parallel “prone” configurations toward SWNTs, thus allowing intermolecular charge transfer to occur between SWNTs and the polymers.65 Given the proposed polar zwitterion structure of the PN backbone of P-Cl, it appears that similar donor−acceptor interactions between the inorganic P-Cl and SWNTs are responsible for stabilizing the hybrid complex.

Chart 1. Chemical Structures of Polyphosphazenes for Building Hybrid Materials

different types of side groups. These include poly(dichlorophosphazene) (P-Cl), alkoxy derivatives (P-Cn), an alkyl ether derivative (P-EEM), fluoroalkoxy derivatives (PTFE and P-OFP), and phenoxy derivatives (P-Ph and P-FPh). The polymers were synthesized by techniques reported previously.60,61 All the polymers used here were characterized by 1H and 31P NMR techniques, which are consistent with results from earlier work.60,61 The diverse substituents in the B

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decrease in the quantities of dispersed SWNTs in the hybrids. Although the butoxy and hexoxy groups of P-C4,6 are bulkier than the alkoxy groups in P-C2,3, we suggest that the longer groups of P-C4,6 offer stronger CH−π interactions with SWNTs compared to P-C2,3,4. Such strong CH−π interactions overcome the steric effects of C4,6, ultimately resulting in a net enhanced intermolecular interaction between P-C4,6 and SWNTs. Additionally, even with these longer side groups the SWNTs are not too distant from the P-C4,6 backbones, such that the intermolecular donor−acceptor interactions remain strong between P-C4,6 and SWNTs. However, further increases in the length of the alkoxy groups to octoxy and dodecyloxy in P-C8 and P-C12 prevent the SWNTs from a close approach to the PN backbone, and this seriously weakens the intermolecular donor−acceptor interactions between the P-C8,12 backbone and the SWNTs. Even so, the polyphosphazenes with longer alkoxy C4,6,8,12 chains disperse larger quantities of SWNTs than those of polyphosphazenes with the Cl group or the C2,3 side chains. As discussed above, multiple intermolecular interactions, such as donor−acceptor interactions, CH−π interactions, and steric hindrance, play important roles in hybrid formation between the polyphosphazenes and SWNTs. These studies suggest that the donor−acceptor interactions between the polymers and SWNTs play the dominant role in stabilizing the hybrids when the alkoxy chains are short (n = 2, 3), while CH−π interactions between alkoxy groups and SWNTs are the dominating influence for stabilizing the hybrids when the alkoxy chains are long (n = 4, 6, 8, 12). The steric hindrance of alkoxy group increases gradually as the length of alkoxy chain increases. At the point where the length of the alkoxy group is longer than six carbon atoms, steric hindrance starts to dramatically weaken the donor−acceptor interactions between the polymer and SWNTs, thus resulting in a decreased stability of the hybrid complexes. We also investigated the electronic effects of the polyphosphazene side groups on hybrid complex formation. P-Ph with its aromatic phenyl groups (Chart 1) also formed stable complexes with SWNTs in THF. P-Ph was able to disperse more SWNTs in THF solution than P-Cl (Figure 2b), suggesting that the presence of phenyl groups enhances the stability of the hybrids. This is very different from the polysilane/SWNT systems where phenyl groups in the polysilane did not stabilize hybrid complex formation.55 In the polysilane derivative, the phenyl groups connect directly to the Si center, while the phenyl groups in P-Ph connect to the P-center through oxygen atoms. The additional oxygen atoms not only spatially extend the phenyl groups but also provide phenyl groups with more fluxional freedom compared to those in polysilanes. Therefore, P-Ph with its more flexible and extended phenyl groups should readily adopt the conformation needed to accommodate the curved backbone of the SWNTs. The influence of fluorine on these systems was examined by using P-TFE bearing trifluoroethoxy groups (Chart 1) to complex with SWNTs. Unlike P-C2, P-TFE with the almost identical side-chain length, P-TFE did not disperse SWNTs under the same conditions. Increases in the concentration of P-TFE (200 mg/10 mL), or different solvents (CHCl3, toluene, CH2Cl2, and Et2O), did not lead to the formation of stable hybrid complexes. Like P-TFE, P-OFP with its fluoroaryloxy groups did not disperse SWNTs in THF. It appears that these fluorinated polymers do not provide the CH−π interactions that are found in P-Cn’s. Moreover, P-FPh

Previous studies have demonstrated that CH−π interactions between the alkyl side chains of polymers and SWNTs occur by allowing flexible alkyl side chains to wrap around SWNTs, to significantly stabilize the hybrid complexes in solution.1,3,47,48 We examined whether P-C2,3,4,6 with alkoxy side chains could complex with SWNTs under the same conditions. Thus, P-C2,3,4,6 and SWNTs formed stable hybrid complexes in THF, evidenced by the dark color of the stable dispersed solutions after centrifugation, and the fact that they are colored to varying degrees indicates that the SWNTs are dispersed to different extents. Figure 1d shows the 1H NMR spectra of P-C6 and the P-C6/SWNTs complex in THF solution. After complexing with SWNTs, the proton peaks of P-C6 became very broad. The similar broadening of proton peaks has been observed in other SWNT hybrids and has been attributed to the strong interactions between alkoxy side chains and the π-backbone of SWNTs.66 Substituent Effects of Polyphosphazenes on Hybrid Formation. We investigated whether the length of the alkoxy groups of the polyphosphazenes affects hybrid formation. In control experiments, 20 mg of the corresponding P-Cn derivatives (n = 2, 3, 4, 6, 8, 12) were mixed with 1 mg of SWNTs in 5 mL of THF. These mixtures were subjected to bath sonication for 2 h. Then the mixtures were centrifuged at 5000 rpm for 2 h. The top 2/3 supernatant was carefully collected and was subjected to further characterization. Figure 2a shows the absorption spectra of the P-Cn/SWNT

Figure 2. (a) Absorption spectra of polyphosphazene/SWNT hybrids in THF. (b) Absorbance at 1008 nm extracted from the spectra of polyphosphazene/SWNT hybrids in THF.

complexes in THF. These spectra show multiple optical transitions from SWNTs, which further confirm the presence of SWNTs in the hybrid complexes. Similar multiple optical transitions of SWNTs were also observed in both small molecule/SWNT and polymer/SWNT hybrids.1−5,67,68 Plotting the absorption intensity of SWNTs at 1008 nm as a function of the length of the alkoxy side groups was used to track the approximate quantities of dispersed SWNTs in the hybrids. Within the series of P-Cl to P-C3, a decrease in quantities of dispersed SWNTs in the hybrids was detected with increasing length of the alkoxy side groups. Since steric hindrance is detrimental to complex formation in polysilane/SWNT systems,54,55 it appears that the ethoxy and propoxy groups weaken the donor−acceptor interactions between P-C2,3 and SWNTs, thus resulting in a decrease in the quantities of dispersed SWNTs. Moreover, ethoxy and propoxy groups are too short to provide efficient CH−π interactions. However, from P-C3 and P-C4 to P-C6 an increase in the quantities of dispersed SWNTs in the hybrids was detected with increasing length of alkoxy groups. Further increases in the length of the alkoxy groups from P-C6 and P-C8 to P-C12 resulted in a C

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high-molecular-weight macromolecules thoroughly wrap the SWNTs are such high-frequency shifts observed with the RBMs.72 According to the previous studies,73,74 we tentatively attribute such shifts to intermolecular electron transfer from the SWNTs to the polymers through the donor−acceptor interactions mentioned in the previous section. Collectively, these findings suggest the presence of strong intermolecular interactions and electron transfer between SWNTs and P-Cn. Based on the results discussed above, the polarized PN backbone and the alkoxy groups play important roles in the formation of P-Cn/SWNT hybrids, which is consistent with the UV−vis results discussed above. Preparation and Characterization of Cross-Linkable Polyphosphazenes/SWNT Hybrids. In recent years, SWNTs have been used to enhance the electrical and mechanical properties of organic polymeric materials. A significant increase in tensile modulus and yield strength of polymers has been achieved by randomly dispersing SWNTs or multiwalled carbon nanotubes (MWNTs) in polymers.75,76 The easy accessibilitiy of polyphosphazene/SWNT hybrids, opens the possibility of applying them to stretchable electronics. P-C6 and P-C8 disperse more SWNTs than the other polymers and exhibit low glass transition temperatures (Figure S1). Therefore, P-C6 and P-C8 appeared to be good candidates for stretchable hybrid materials by introducing cross-linkable functional groups. Thus, a new series of polyphosphazenes AN-C6 and AN-C8 were synthesized with photo-cross-linkable anthracene moieties (Figure 4a). The

with its perfluorophenyl groups does not form complexes with SWNTs under the same conditions. Previous studies have revealed that the presence of one bromine atom or one iodine atom in the octyl side chains of polyfluorene increases the dispersibility of SWNTs compared to non-halogenated counterparts. This was attributed to the electronegative and polarizable halogen influence on the stabilities of the SWNT hybrids.69 We hypothesize that unlike the single halogen effect in the polyfluorenes, the less polarizable character of multiple fluorine atoms may also negatively affect interactions between the polymer with fluorinated side chains and SWNTs compared to the polyphosphazenes without fluorinated side groups and SWNTs. Similar fluorine atom effects on SWNTs dispersion have been observed in the polysilane/SWNTs system.55 In the present work it appears that the fluorine effect is a general characteristic in both of these inorganic polymer/ SWNT systems. The P-EEM polymer with methoxy−ethoxy− ethoxy side groups also forms complexes with SWNTs under the same conditions. However, compared to P-C6,8, P-EEM with a similar length of side chains dispersed fewer SWNTs in THF (Figure 2b), which suggests that the presence of the oxygen atoms is detrimental to the formation of hybrid complexes. Thus, we hypothesize that the EEM side chain having fewer −CH2− groups results in weaker CH−π interactions compared to its alkoxy counterparts having a similar length. Influence of Polyphosphazenes on SWNTs in the Hybrids. Raman spectroscopy experiments were carried out to study the interactions between the polyphosphazenes and SWNTs (Figure 3a,b). Since P-Cn derivatives can easily form

Figure 3. (a) D-/G-band region and (b) RBM region in Raman spectra of P-Cn/SWNT hybrids (ex@514 nm) in the solid state.

hybrid complexes, we focused on the P-Cn/SWNT system for more detailed studies. Like the uncomplexed SWNTs, Raman spectra of all P-Cn/SWNT hybrids show typical D and G bands from SWNTs,70 further confirming the presence of the nanotubes in the hybrids. Raman spectra of the P-Cn/SWNT hybrids show smaller D-band/G-band intensity ratios compared to those of the uncomplexed SWNTs. Since high D-band intensity is correlated to chemical integrity (defects and other disorders) of SWNTs,70 the observation of small Dband/G-band intensity ratios in P-Cn/SWNT hybrids suggests that P-Cn’s behave as “pre-purifying agents” being able to remove impurities like amorphous carbon from the raw SWNTs. In the radical breathing mode (RBM) region of P-Cn/ SWNT hybrids (Figure 3b), the peaks from SWNTs between 250 and 350 cm−1 shifted to higher wavenumbers compared to those of uncomplexed SWNTs. Generally, these RBMs do not show appreciable shifts in noncovalent coassemblies of πconjugated molecules with SWNTs.71 Only in cases where

Figure 4. (a) Synthesis protocols of AN-C6,C8. (b) Aromatic region of 1H NMR of AN-C6 (top) and AN-C8 (bottom). (c) Full region of 1 H NMR spectra of AN-C6 (top) and AN-C8 (bottom). D

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induced crystallinity enhancement has also been observed in previous studies of polypropylenes.79−82 To prepare cross-linked hybrid elastomers, AN-C6/SWNTs and AN-C8/SWNTs hybrid films were cross-linked by irradiation with 365 nm light. The cross-linked films were then subjected to mechanical tests. The cross-linked AN-C6,8/ SWNTs showed excellent elastomeric characteristics (Figure 6a). Figure 6b shows the stress−strain curves of AN-C6/

target polymers were obtained via two-step nucleophilic subsitutions on the phosphorus atoms (see details in the Supporting Information). AN-C6 and AN-C8 were purified by dialysis against THF/MeOH for 5 days followed by precipitation three times in methanol. 1H NMR spectra of AN-C6 and AN-C8 are shown in Figure 4b,c. Based on the 1H NMR spectra, both AN-C6 and AN-C8 contain 1 mol % of the anthracene moiety. On the basis of GPC data, it was estimated that the Mws of AN-C6 and AN-C8 are 790 and 740 kDa, respectively, and the dispersity (Đ) of AN-C6 and AN-C8 is 2.2 and 2.5, respectively. The photo-cross-linking reactions of neat AN-C6 and ANC8 were evaluated for both solution and thin film systems. Figures 5a and 5b show the UV−vis spectra changes of AN-C6

Figure 6. (a) Photographs of a stretched UV-exposed AN-C8/SWNT film under 400% strain. (b) Stress−strain curves of the polymers. (c) Cyclic stress−strain curves of a UV-exposed AN-C6/SWNTs film. (d) Cyclic stress−strain curves of a UV-exposed AN-C8/SWNT film. Figure 5. (a) UV−vis spectral change of AN-C6 (1 × 10−2 M) in THF solution under irradiation by 365 nm light. (b) UV−vis spectra change of AN-C6 thin film under irradiation with 365 nm light. (c) Time dependence UV−vis change film under irradiation of 365 nm light. (d) UV−vis spectra of AN-C6/SWNT (solid line) and AN-C8/ SWNT (dashed line) in THF.

SWNTs and AN-C8/SWNTs hybrids. Compared to the neat P-C6 and P-C8 films and their un-cross-linked SWNT films, the cross-linked AN-C6/SWNTs and AN-C8/SWNTs films have a larger Young’s modulus and greater break strength. Figures 6c,d also show that the cross-linked AN-C6/SWNT and AN-C8/SWNT films have relative stable mechanical characteristics in the four-cycle tests. To explore the electrical properties, four-probe conductivity experiments were carried out on an AN-C6/SWNTs film as a proof of concept (Supporting Information). By applying mechanical shearing with a glass slide, we are able to generated aligned 1D structures in the AN-C6/SWNTs film (Figure 7a). The electrical conductivity of the AN-C6/SWNTs film is 0.042 S/m, which is lower than the electrical conductivity of neat SWNTs reported in the literature.83 The lower electrical conductivity could be due to the insulating character of the polyphosphazenes on the SWNTs surface, which negatively influences electric conductivity pathway between SWNTs. Four-probe conductivity measurements on the AN-C6/ SWNTs film were also carried out before and after mechanical stretching by applying 100% strain. It was found that the electrical resistance of the AN-C6/SWNTs film increased slightly after the first stretching but remained relatively stable thereafter (Figure S26). We also tested the in situ electrical characteristic under 30% cycic strain (Figure 7b,c), which shows relative stability of the change in the resistance. These preliminary electrical characterizations suggest that the polyphosphazene/SWNT hybrids have promising characteristics for stretchable electronics.

(1 × 10−2 M) in both THF solution and as a thin film during irradiation with 365 nm radiation as a function of time. The decrease in the absorption bands between 300 and 400 nm is consistent with the consumption of the anthracene moieties during the photo-cross-linking reaction between adjacent anthracene units.77,78 Changes of the absorption peak intensity at 386 nm as a function of time are shown in Figure 5c. The decreasing absorption peak intensity of AN-C6 in the solid state is much faster than that in THF solution, presumably due to the shorter distance between anthracene units in the solid state. Like AN-C6, species AN-C8 also showed an efficient anthracene photo-cross-linking reaction in the solid state (Figure 5c). Thus, both AN-C6 and AN-C8 can disperse SWNTs in THF. Their SWNTs hybrid absorption spectra are shown in Figure 5d. These spectra show multiple optical transitions from SWNTs, which confirm the presence of SWNTs in these hybrid complexes.1−5,67,68 We also prepared hybrid films of AN-C6/SWNT and AN-C8/SWNT. Compared to the X-ray diffraction patterns of the pristine polymer films, the hybrid polymers show a stronger diffraction intensity, thus suggesting that the presence of SWNTs enhances the crystallinity of the polymers (see Figure S24). A SWNTE

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Synthetic details and characterization of the polymers

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(PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.R.A.). ORCID

Harry R. Allcock: 0000-0002-8822-3457 Funding

This work was supported by the PSU Leadership and Evan Pugh Funds. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Professors John Asbury, Mark Maroncelli, and John Badding for the use of their instruments.

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Figure 7. (a) AFM image of aligned AN-C6/SWNT film. (b) Current−voltage characteristics of AN-C6/SWNT film at 0% strain and at 30% strain. (c) Relative change in the resistance of AN-C6/ SWNT film under 30% cyclic strains.

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CONCLUSIONS In this report, we describe a new series of inorganic polymer/ SNWT hybrid materials. The results shed light on the structure−property relationships for building polyphosphazene/SWNT hybrids. These studies show that the unique P N backbone of the polyphosphazenes enables strong intermolecular donor−acceptor interactions between the polymers and SWNTs. With alkoxy (C2−C12) side chains on the polymers, the donor−acceptor interactions, the CH−π interactions, and steric hindrance collectively affect the formation of the hybrids. The balanced donor−acceptor interactions, CH−π interactions, and steric hindrance of PC6 with its hexoxy side groups give rise to stronger intermolecular interactions with SWNTs than with the other polyphosphazenes. Furthermore, the presence of phenyl groups is beneficial for stabilizing the hybrid complexes. However, the presence of fluorine and oxygen atoms in the alkoxy side chains is detrimental. Introduction into the phosphazene of photo-cross-linkable anthracene moieties facilitates the construction of new hybrid elastomers with improved mechanical properties. Finally, the presence of aligned SWNTs allows the hybrid materials to maintain stable electrical characteristics during multiple mechanical stretching, which opens a door to the design of new stretchable electronic materials.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00779. F

DOI: 10.1021/acs.macromol.8b00779 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b00779 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b00779 Macromolecules XXXX, XXX, XXX−XXX