Chengyin
Liu‡
a,
Bo
Liu‡
a and
Mary B.
Chan-Park
*ab
aSchool of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore. E-mail: MBECHAN@ntu.edu.sg
bLee Kong Chian School of Medicine, Nanyang Technological University, Singapore
First published on 16th December 2016
Polyimides have attractive properties such as a strong interaction with 1D/2D carbon nanomaterials but their solubilities in common organic solvents are limited. We report a new synthesis route for triblock copolymers of polycaprolactone-polyimide-polycaprolactone (PCL-PI-PCL) via polycondensation followed by ring-opening polymerization. The prepared OH-PI-OH homopolymer precursors were reacted with two equivalents of stannous(II) octoate (Sn(Oct)2) to afford α,ω-dihydroxyl-terminated polyimide macroinitiators which can polymerize with ε-caprolactone to obtain the final triblock copolymers (PCL-PI-PCL). Four different molecular weights of PCL-PI-PCL triblock copolymers with different lengths of PCL and PI blocks were synthesized to assess the best composition for carbon nanotube dispersion in a low boiling organic solvent (tetrahydrofuran, THF). The polyimide block interacts strongly with single walled carbon nanotubes (SWNTs) through charge transfer, as shown by Raman spectroscopy, while the polycaprolactone block has a good solubility in THF. An optimised triblock copolymer disperses the carbon nanotubes in THF well even after standing for 1 h, while the PI homopolymer-dispersed SWNTs settled completely under the same conditions. We applied the new PCL-PI-PCL in SWNT-reinforced epoxy composites with the use of THF as the casting solvent. The optimised triblock copolymer-dispersed SWNTs (2 wt%) increased the tensile strength, modulus, and elongation at maximum stress by 74%, 35%, and 62% respectively compared to the neat resin blend. The new synthesis route of the triblock copolymer is amenable to the synthesis of diverse PI-based triblock copolymers with various desired functionalities for myriad applications, such as for carbon nanotube-reinforced epoxy-based composites, water-based antibacterial dispersions, etc.
Single walled carbon nanotubes (SWNTs) are high-performance nanomaterials being exploited for diverse applications such as polymer matrix composites, antibacterial nanomaterials,8 field-effect transistors,9,10 biosensors,11,12etc. Their intrinsic mechanical, electrical and optical properties are outstanding.13 However, realization of these properties in practice typically depends on good dispersion of SWNTs in common solvents such as THF, dichloromethane (DCM), acetonitrile, etc. The high surface area of nanotubes results in strong intertube van der Waals forces14 so that they easily aggregate into bundles. Nanotube dispersing agents ought to have strong affinity for the nanotubes as well as good solubilities in common solvents, which may entail contrasting compositional requirements for dispersing agents. Both covalent and noncovalent functionalization approaches have been reported.14–16 Covalent functionalizations can improve dispersion but typically destroy the long-range π conjugation of SWNTs.17 Noncovalent functionalizations are usually realized through physical adsorption, which does not disrupt the intrinsic structure of SWNTs, but the types of polymers and organic solvents that can be applied are still limited.18,19 Recently, conjugated and aromatic polymers/copolymers have attracted much attention as dispersing agents.20–22
In this paper, we prepared three polyimide homopolymers with different molecular weights (Table 1) and used them as centre blocks for synthesizing a series of polycaprolactone-block-polyimide-block-polycaprolactone (PCL-PI-PCL) triblock copolymers via a novel synthesis route (Scheme 1). ABA block copolymers were synthesized through addition of hydroxyl (–OH) active groups to the two ends of the centre PI blocks, followed by ring-opening polymerization of the PCL end blocks (Scheme 1).7,23 Three PI homopolymers with different molecular weights (labelled as HP1, HP2 and HP3, Table 1) and four triblock copolymers with these PI centre blocks were synthesized (labelled as TB1a, TB1b, TB2 and TB3, see Table 2). The (co)polymers were characterized with NMR, GPC and FTIR. SWNT dispersions, using PCL-PI-PCL triblock copolymers (TBs) and homopolyimide (HP1), were characterized with UV-VIS-NIR, AFM, TEM, and Raman spectroscopy. We found that the TB1a triblock copolymer most effectively dispersed SWNTs. The central PI block noncovalently adsorbed on carbon nanotubes while the two ends of PCL blocks provided good solubilities in THF solvent (a model low boiling point solvent). There appeared to be an optimum ratio of PI to PCL block length for the triblock (as with TB1a) to achieve excellent SWNT dispersion in THF solvent. Furthermore, epoxy composites reinforced with SWNTs dispersed by TB1a in THF were fabricated and the tensile properties were improved by incorporation of the SWNTs.
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Scheme 1 Synthesis of the PCL-PI-PCL triblock copolymer: (a) dry NMP, 25 °C, 10 h; (b) dry p-xylene, reflux, 6 h; (c) Sn(Oct)2, dry NMP, 25 °C, 6 h (d) CL, 100 °C, 24 h. |
Sample | Design [I]![]() ![]() |
Initiator (Mn (Da)) | TB measured Mn (Da)/PDIb | Actual repeating PI units (m)c | Actual repeating CL units (n)c | Actual Ratios of n![]() ![]() ![]() ![]() |
---|---|---|---|---|---|---|
a Molar ratio of polyimide macroinitiator and caprolactone monomers for ring-opening polymerization. b Determined from GPC using polystyrene standards as references. c Calculated from the molecular weight of polymers and single units. | ||||||
TB1a | 1![]() ![]() |
HP1 (6000) | 13![]() |
9 | 60 | 3![]() ![]() ![]() ![]() |
TB1b | 1![]() ![]() |
HP1 (6000) | 19![]() |
9 | 120 | 6![]() ![]() ![]() ![]() |
TB2 | 1![]() ![]() |
HP2 (14![]() |
17![]() |
21 | 24 | 1![]() ![]() ![]() ![]() |
TB3 | 1![]() ![]() |
HP3 (35![]() |
39![]() |
51 | 28 | 3![]() ![]() ![]() ![]() |
The as-prepared OH-PI-OH (HP1 to HP3) can easily react with Sn(Oct)2 to yield an α,ω-OSn(Oct) terminated PI initiator, which can undergo ring-opening polymerization with cyclic ester monomers, such as ε-caprolactone, lactones, carbonates, etc. We prepared four PCL-PI-PCL triblock copolymers, labelled as TB1a, TB1b, TB2 and TB3, from the HP1 to HP3 center blocks (Table 2). The four triblock copolymers were characterized by NMR and GPC. For example, successful synthesis of the triblock copolymer TB1a was confirmed with 1H NMR and FTIR. The chemical shifts at 4.00, 2.30, 1.56 and 1.32 ppm, which were absent in homopolymer spectra, were assigned to the PCL block (Fig. S1†). The FTIR spectrum of the triblock copolymer had bands around 2800–3000 cm−1 (C–H stretching) which can be assigned to the characteristic absorption bands of PCL components. Furthermore, the characteristic bands of polyimides around 1730 cm−1 (CO stretching), 1360 cm−1 (C–N stretching) and 1200 cm−1 (aromatic–OH stretching) were observed in both HP1 and TB1a (Fig. S2†). The syntheses of the other PI homopolymers (HP2 and HP3) and PCL-PI-PCL triblock copolymers (TB1b, TB2, and TB3) were also confirmed by 1H NMR (Fig. S3 to S7†).
Fig. S11† shows the GPC curves of OH-PI-OH homopolymers (HP1, HP2, and HP3) and also the PCL-PI-PCL triblock copolymers (TB1a, TB1b, TB2, and TB3). The molecular weights (Mn) of PI and PCL-PI-PCL are summarized in Tables 1 and 2, respectively. The molecular weights of all the triblock copolymers exceeded those of the corresponding OH-PI-OH polymers. For example, Mn of the homopolymer HP1 was 6000 Da, but that of TB1a (the triblock made from HP1) was 13200 Da. The PDI of the triblock remained narrow (1.2 to 1.8, Table 2), indicating that the PCL-PI-PCL was a true triblock copolymer and not a mixture.
The solubilities of these polymers in common solvents are shown in Tables S1 and S2.† TB1a and TB1b were soluble in more solvents because they had short polyimide center blocks; TB3 was soluble in only a few solvents because of the high molecular weight of its polyimide block.
We evaluated the different TBs (TB1a, TB1b, TB2 and TB3) for dispersing SWNTs in THF (Fig. 2). The nanotube concentrations of the supernatants of various TB/SWNT dispersions subjected to various settling regimes, i.e. specifically immediately after sonication, or sonication followed by centrifugation at 6000 rpm or 10000 rpm each for 10 min, were evaluated by UV-VIS-NIR and the Beer–Lambert law. For all the TBs, some SWNTs settled at the bottom after centrifugation at 6000 rpm or 10
000 rpm, while some still remained in the solvent (Fig. 2). After centrifugation at the higher speed (10
000 rpm), TB1a-dispersed SWNT solution had the highest nanotube concentration (Fig. 2(a)), indicating that TB1a was the most effective in dispersing SWNTs. TB2-dispersed SWNT solution was almost clear and the color of TB1b-dispersed SWNT solution was light after centrifugation (Fig. 2(b)), indicating that most SWNTs have settled at the bottom. Comparing the four TB dispersants, the length of PCL should be moderate; too long (TB1b) or too short PCL block (TB2) impaired the dispersion; a longer PI block while keeping the PCL block length constant was helpful for the nanotube dispersion, which was why TB3 was better than TB2 in dispersing the nanotubes. In order to confirm this statement, more TBs with different lengths of PCL blocks were made (TB1c, TB1d and TB1e, see Table S3†). They were all initiated by the same polyimide (HP1) and their molecular weights are summarized together with TB1a and TB1b in Table S3.† The synthesis of TB1c, TB1d and TB1e was confirmed by 1H NMR spectra (Fig. S8–S10†) and GPC curves (Fig. S12†). The molecular weights of the newly synthesized TB1c and TB1d are 9400 and 11
500 daltons, whereas the TB1e molecular weight is 16
600 daltons, an intermediate between TB1a and TB1b. The concentrations of SWNTs in THF dispersed by different TBs are calculated by using UV-Vis-NIR curves and are shown in Fig. S14.† Immediately after sonication, the concentrations of SWNTs are all around 0.2 mg mL−1, while after centrifugation at 6000 rpm and 10
000 rpm, the concentration of SWNTs using TB1a and TB1e dropped to the least compared to the other 3 ratios, indicating that a better dispersant is achieved by optimizing the ratio of PI to PCL block chain lengths to be around 1
:
3. Considering these various factors (different PCL block length ratios and PI lengths), TB1a was among the best TBs to effectively disperse SWNTs in THF. So we choose TB1a as a dispersant to further evaluate the dispersion of SWNTs and make some composites.
We also compared the efficacy of the polyimide homopolymer (HP1) center block versus the TB1a triblock copolymer in suspending SWNTs in THF after centrifugation, by comparing the SWNT content in solutions of HP1/SWNT versus that of TB1a/SWNT (Fig. 3(a)). Immediately after sonication, both TB1a/SWNT and HP1/SWNT dispersion had nearly the same nanotube concentration ∼50 mg L−1. After centrifugation, the nanotube concentrations in the supernatants of TB1a/SWNT decreased to 33 and 28 mg L−1 for 6000 rpm and 10000 rpm respectively, which were much higher than the corresponding values of HP1/SWNT (6 and 3 mg L−1) after similar centrifugation regimes (Fig. 3(a)), indicating that the dispersion of SWNTs in THF was greatly improved by incorporation of PCL end blocks in the copolymers. We also compared the settling of SWNTs upon standing after 1 h using HP1 (control) versus TB1a as dispersing agents; Fig. 3(b) and (c) show the photographs of SWNTs dispersed in THF after standing for 1 h and immediately after sonication respectively. After sonication, both the solutions were black (Fig. 3(b)). But after standing for 1 h, the SWNTs dispersed in HP1 solution mostly settled, while no obvious aggregates were observed in the TB1a/SWNT solution, indicating that homogeneous dispersion that was stable on standing was achieved with the TB1a triblock polymer (Fig. 3(c)). Because the solubility of PCL in THF was very good (Table S2†), it can improve the solubility of SWNTs in the solvent and help to form a more stable SWNT solution.
To further examine the nanotube dispersion and morphology, TEM and AFM images of the dispersed SWNTs were obtained. The HP1-dispersed SWNTs, which were used as the control, formed bundles with large diameters of about 30 nm (Fig. 4(a)), while the majority of the TB1a-dispersed SWNTs was observed in smaller bundles with diameters of around 5 nm (Fig. 4(b)i). At a lower magnification (×8000) (Fig. 4(b)ii), it can be seen that the carbon nanotubes were wrapped with the PCL-PI-PCL polymer (TB1a) and also intercalated by the polymers, to which the superior dispersion and long-term stability of the SWNT solution may be attributed. The ability of the triblock copolymer to disperse SWNTs into smaller bundles and individual tubes was also manifested by AFM (Fig. 5). The measured height of the SWNTs was below 5 nm, and some were 1–2 nm, suggesting that the SWNTs were dispersed as small bundles and individual tubes.
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Fig. 4 TEM images of dispersed SWNTs with (a) HP1 (control) (×50![]() ![]() |
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Fig. 5 AFM image of TB1a-dispersed SWNTs deposited on silicon wafer by drop-casting dilute solution (0.01 mg mL−1). |
To confirm the strong interaction between the synthesized polymer and carbon nanotubes, 785 nm laser Raman spectra of pristine SWNTs and different polymer (PCL, HP1 and TB1a) dispersed SWNTs were obtained (Fig. 6). It was known that covalent functionalization can introduce defects into SWNTs, leading to an increased intensity of the D band (∼1330 cm−1).24,25 On non-covalent functionalization with the various polymers (PCL, HP1 and TB1a), the intensity ratios of the D band to the G band (ID/IG) of the various dispersions were not larger than that of unfunctionalized SWNTs (Fig. 6(a)), corroborating that the nanotube graphene structure was well preserved. However, the G+ band showed varying degrees of shift. For all three polymers treated with SWNTs, the G+ band shifted from 1592.5 cm−1 to 1589.6 cm−1, indicating n-doping due to charge transfer (Fig. 6(a)).26 The electron-donating behaviour of PCL was due to the oxygen and alkyl chain on it while that from PI was due to the electrons of the aromatic rings. The G+ peak of HP1/SWNT, PCL/SWNT and TB1a/SWNT shifted by 3.6 cm−1, 2.5 cm−1, and 2.9 cm−1 respectively so that the shift of TB1a/SWNT was intermediate between HP1/SWNT and PCL/SWNT. From the Raman shift of HP1 and TB1a, the polyimide (either by itself or in the triblock copolymers) had a strong interaction with the nanotubes and was an effective way to increase the interaction between the polymer dispersants and carbon nanotubes. The Raman radial breathing mode (RBM) peaks also became broader due to the interaction between the polymer and SWNTs,27 and the change of the shape was more obvious in HP- and TB-dispersed carbon nanotubes (Fig. 6(b)i and iii) (the RBM frequency was related to the radius of carbon nanotubes, so the up-shifts of RMB peaks were attributed to the polymer wrapping28).
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Fig. 6 Raman spectra (785 nm laser) of polymer-dispersed SWNTs: (i) TB1a/SWNT; (ii) PCL/SWNT; (iii) HP1/SWNT and (iv) pristine SWNTs (ID/IG = (i) 0.122; (ii) 0.127; (iii) 0.117; (iv) 0.173). |
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Fig. 7 Visual appearance of the TB1a/epoxy/SWNT composite film (a) 2 wt% SWNTs, (b) 4 wt% SWNTs, and (c) 6 wt% SWNTs. |
SWNT loading (wt%) | Strength (MPa) (increase%) | Modulus (MPa) (increase%) | Elongation at maximum stress (%) |
---|---|---|---|
0 | 35.3 ± 6.6 | 1709.9 ± 191.7 | 3.40 ± 1.16 |
2 | 61.4 ± 2.2 (74) | 2307.0 ± 136.3 (35) | 5.50 ± 1.04 |
4 | 48.5 ± 3.2 (37) | 2318.5 ± 184.7 (36) | 4.24 ± 0.81 |
6 | 45.2 ± 4.3 (28) | 2431.1 ± 107.6 (42) | 3.46 ± 0.48 |
Fracture surfaces of the composite films after tensile testing are shown in Fig. 9. There were some pull-outs observed for the 2% SWNT fractured surface but they were not too extensive. The SWNTs were well dispersed in the composite films and no obvious agglomeration was observed, indicating that the PCL-PI-PCL triblock (TB1a) was an effective SWNT dispersant for epoxy composites.
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Fig. 9 FE-SEM images of the fracture surface of TB1a/epoxy/SWNT with 2 wt% SWNT loading at (a) low magnification (×5000) and (b) high magnification (×20![]() |
0.2 mg mL−1 solutions of TBs/SWNT (mass ratio of polymer and SWNTs = 2:
1) in THF were prepared by tip sonication (150 W, 60%) for 30 min (1 and 1 second on/off) and the absorption at 700 nm was measured immediately and after centrifugation at 6000 rpm for 10 min and 10
000 rpm for 10 min. A 0.05 mg mL−1 solution of TB1a/SWNT (mass ratio of polymer and SWNTs = 2
:
1) in THF was prepared by tip sonication (150 W, 60%) for 30 min (1 and 1 second on/off) and the absorption at 700 nm was measured immediately and after centrifugation at 6000 rpm for 10 min and 10
000 rpm for 10 min. Comparison measurements were made of HP1/SWNT in THF at the same concentration and centrifuge preparation.
Footnotes |
† Electronic supplementary information (ESI) available: Fig. S1–S14 and Tables S1–S3. See DOI: 10.1039/c6py01933a |
‡ These two authors contributed equally. |
This journal is © The Royal Society of Chemistry 2017 |