Thermo-active polymer nanocomposites: a spectroscopic study

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Thermo-active polymer nanocomposites: a spectroscopic study A. Douglas Wintera, Eduardo Lariosb, Cherno Jayec, Daniel A. Fischerc, Mária Omastovád, Eva M. Campo*a,b aSchool

of Electronic Engineering, Bangor University, Bangor LL57 1UT, United Kingdom, of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249, United States, School of Materials Science & Engineering, cMeasurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States, dPolymer Institute, Slovak Academy of Sciences, Bratislava 84541, Slovak Republic

bDepartment

ABSTRACT Photo- and thermo-mechanical actuation behaviour in specific polymer-carbon nanotube composites has been observed in recent years and studied at the macroscale. These systems may prove to be suitable components for a wide range of applications, from MOEMs and nanotechnology to neuroscience and tissue engineering. Absence of a unified model for actuation behaviour at a molecular level is hindering development of such smart materials. We observed thermomechanical actuation of ethylene-vinyl acetate | carbon nanotube composites through in situ near-edge X-ray absorption fine structure spectroscopy to correlate spectral trends with macroscopic observations. This paper presents spectra of composites and constituents at room temperature to identify resonances in a building block model, followed by spectra acquired during thermo-actuation. Effects of strain-induced filler alignment are also addressed. Spectral resonances associated with C=C and C=O groups underwent synchronised intensity variations during excitation, and were used to propose a conformational model of actuation based on carbon nanotube torsion. Future actuation studies on other active polymer nanocomposites will verify the universality of the proposed model. Keywords: Thermo-active, smart behaviour, NEXAFS spectroscopy, MWCNT, polarised Raman spectroscopy, EVA

1.

INTRODUCTION

Polymer-carbon nanotube composites are mostly fabricated as passive systems, with the goal of translating the extraordinary mechanical, thermal, electronic and optical properties of nanofillers to the host polymer matrix.1-4 In recent years, certain polymer nanocomposites have shown ‘smart’ or active behaviour. These systems respond to external thermal or optical stimuli producing a mechanical output.5-7 Such composites could clearly be used in a wide range of applications, from ultrafast optical switches in MOEMS to artificial muscles for human implants. Smart behaviour of polymer nanocomposites has only been studied at the macroscale, however, and the development and deployment of smart devices based on these smart composites is hindered by a lack of understanding of the underlying mechanisms. Although there have been tentative explanations proposed, there is currently no unified molecular model for this behaviour.7-9 In this study we have investigated actuation of photo- and thermo-active ethylene-vinyl acetate (EVA) – multiwall carbon nanotube (MWCNT) composites using in situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to correlate macroscopic observations of smart behaviour with spectral trends. NEXAFS spectroscopy is an excellent tool for the study of nanocomposites as it offers information about chemical environment, orbital hybridisation and

Nanoengineering: Fabrication, Properties, Optics, and Devices XI, edited by Eva M. Campo, Elizabeth A. Dobisz, Louay A. Eldada, Proc. of SPIE Vol. 9170, 917003 · © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2064904 Proc. of SPIE Vol. 9170 917003-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 12/10/2014 Terms of Use: http://spiedl.org/terms

bonding, and molecular conformation.10-13 We also address the effects of strain on filler alignment, assessed through polarised Raman spectroscopy, and subsequently the impact of alignment on actuation. A strong coupling between filler alignment and direction of actuation has been previously reported, making alignment a critical aspect to discuss within molecular actuation mechanisms.14 Following a discussion of room temperature NEXAFS spectra of composites and individual constituents linking spectral resonances to electronic transitions, and addressing CNT alignment, this paper analyses spectral trends during actuation, and subsequently offers a conformational model of actuation based on CNT torsion.

2.

EXPERIMENTAL

2.1 Composite synthesis: A dispersion of MWCNT and PyChol dispersant (weight ratio 1:5) in chloroform was sonicated for 1h under magnetic stirring with a Hielscher 400S sonicator at an amplitude of 20% and a duty cycle of 100%. 10g of EVA was then added, and the solution magnetically stirred for several hours. Following oven drying, composite films were compression-moded for 15 min under a pressure of 2.4MPa at 80°C. A prestrain of 50% was applied at 50°C for 20 min using a custom-made stretching apparatus, and the strained composite subsequently cooled in ice water fix the CNT orientation. Two EVA-MWCNT (0.7wt%) were prepared following this procedure: one with no prestrain and on with 50% prestrain. Pristine PyChol, EVA and methanol-dispersed CNTs were also used for comparison. 2.2 Aberration-corrected Transmission Electron Microscopy: Untreated MWCNTs were characterised using a JEOL JEM-2010F field-emission operated at 200kV and a JEOL JEM-ARM200F electron microscope. STEM images were simultaneously recorded in both the high-angle annular dark-field and bright-field modes at 80kV. Probe correction was performed with a CEOS corrector obtaining a 12-fold Ronchigram with a flat area of ~40mrad. Images were registered with a condenser lens aperture of 30µm (convergence angle 25mrad), and the HAADF collection angle ranged from 45 to 180mrad. 2.3 NEXAFS spectroscopy: Carbon k-edge spectra were collected at U7A (NSLS, BNL) in partial electron yield mode using a horizontally polarized beam. A toroidal spherical grating monochromator with 600 lines/mm and slits opening of 30 µm x 30 µm provided an energy resolution of ~0.1eV. An electron floodgun operating at 60µA mitigated surface charging. For in-situ temperature studies, samples were secured on a tantalum metal plate inserted on a customized heating stage. The stage angle was controlled by goniometer, and the temperature was controlled by a voltage supply. Temperature oscillations were observed as ±1.5°C of the set point. Spectra were acquired at 55° by sequentially running macros that controlled stage coordinates and specified energy parameters. The temperature sequence was 30, 35, 38, 42, and 48°C.

3.

RESULTS AND DISCUSSION

NEXAFS spectra of composites (0 and 50% strains) and individual constituents (pristine EVA, MWCNT, PyChol dispersant) were acquired at room temperature with a 55° incidence angle (Figure 1). Each feature may be attributed to an electronic transition (Table 1).11 Both PyChol dispersant and MWCNTs show a π* (C=C) resonance at 285eV, from pyrene groups and graphitic walls respectively, with PyChol showing an additional π* (C=C) resonance at 284.2eV. A similar feature of considerably lower intensity is observed in EVA, attributed to surficial carbonaceous impurities.

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3.0

C5" 2.0

o Ú 0)

m 1.5

r

a 1.0

0.5

0.0

285

290

295

300

Photon energy (eV)

Figure 1. NEXAFS spectrum of unstrained composite consists of linear combination of PyChol, MWCNT and pristine EVA spectra. Intensity variations observed in prestrained composite, attributed to CNT alignment. All spectra acquired at room temperature.

All constituents share a common resonance at 297.7eV attributed to σ* (C-H) transitions. π* (C=O) transitions at 289.1eV arise from PyChol, but more predominantly from EVA. Both σ* (C-H) and π* (C=O) resonances are also present in MWCNTs, arising from impurities in outer walls (Figure 2). Finally, all systems show σ* (C-C) transitions, centred at slightly varying energies all above 290eV. In this analysis we considered π* (C=C) at 295eV and π* (C=O) at 298.1eV as effective ‘fingerprints’ for MWCNT and EVA respectively.11, 12

Figure 2. TEM images of MWCNTs confirm the presence of impurities appended to outer walls.

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Table 1. NEXAFS resonances are attributed to particular electronic transitions.

1

Energy (eV)

Transition

284.2

1s→π* (C=C)1

285.0

1s→π* (C=C)2

287.7

1s→σ* (C-H)

289.1

1s→π* (C=O)

292.0

1s→σ* (C-C)

Emission from cholesteryl group in PyChol

2Emission

from CNTs and pyrene group in PyChol

NEXAFS spectrum of unstrained composite is clearly a linear combination of EVA, MWCNT and PyChol. Interestingly, spectral variations are observed in prestrained composite: a decrease in π* (C=C) intensity and an increase in π* (C=O). As the chemistry does not change as the sample is strained, these differences are attributed to conformational effects.11 Indeed, CNTs are expected to align with applied strain, and assuming an efficient polymer adhesion, chains would respond to CNT alignment as well. Actuation behaviour was studied in situ by acquiring NEXAFS spectra of composites whilst temperature increased from 30°C to 48°C. A seemingly coupled intensity interplay between π* (C=C) and π* (C=O) resonances is observed (Figure 3). As mentioned, these features identify filler and polymer respectively. Additionally, the trend is more pronounced in prestrained composite, coherent with macroscopic observations.7

prestrained

unstrained

1['C_0

48 48 °C

+

38 °C 35 °C

7C C=C

Room temperature

284

288 Photon energy (eV)

Figure 3. In situ temperature-resolved NEXAFS spectra reveals synchronous intensity interplay of π* C=C and π* C=O resonances for unstrained and prestrained composites.

Further investigation of these trends is possible by plotting the relative intensity changes for both resonances, calculated here as (It – IRT) / IRT (Figure 4). These intensities have been corrected for the background carbonaceous impurities in EVA, whose NEXAFS intensity also increases with increasing temperature.11

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1.5

C =C Strained composite C =0 Strained composite

-

O C =C Unstrained composite

tá 1.0 m

O

C =0 Unstrained composite

a 35

30

45

40

Temperature ( °C)

Figure 4. Relative intensity changes of π* C=C and π* C=O resonances show linear trends.

Linear trends were fitted to the relative intensity changes, shown in Table 2. Table 2. Linear fits to relative intensity changes.

slope (m)

intercept (c)

Unstrained

π*(C=C) π*(C=O)

-0.013 -0.003

-0.37 -0.08

Prestrained

π*(C=C) π*(C=O)

-0.090 -0.009

-2.64 -0.27

Trends of intensity variation (TV) are coupled, as described by equation 1, by a factor of 10 in strained composites, and 5 in unstrained (x strained = 10; x unstrained = 5). (

)

= −

(

(1)

)

In addition, prestrained composite shows greater sensitivity to temperature (yC=C = 7 and yC=O = 4 in equation 2), consistent with macroscopic observations. (

)

=

(

)

(2)

We now propose a conformational model of actuation based on our spectral observations (Figure 5). The model is a 3D representation that assumes CNTs are lying preferentially on the xy plane and the beam is incident upon the z axis, with its polarisation vector along the x axis. Note that PyChol is not included in the discussion, as its role is purely as dispersant, and indeed similar behaviour has been observed in other systems in the absence of a chemical dispersant.5 Polymeric chains are shown to append along CNTs through CH-π interactions with C=O groups lying at a preferred angle to CNT vector. We propose that upon excitation, CNTs experience a torsion along existing kinks from growth, resulting in an isotropic distribution of C=O groups in EVA. This could explain the decrease in π* C=O intensity observed in NEXAFS.11

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Carbon

A

Oxygen (single bond) Oxygen (double bond)

Z

ft

Y

Figure 5. In our proposed model, carbon nanotubes experience a torsion along as-grown defects, disturbing the uniform alignment of C=O groups in EVA, and enhancing uniaxial symmetry.

Further, we propose this torsion would enhance uniaxial symmetry. In this scheme, π* C=C orbitals previously inaccessible to the incident beam (Figure 5, xz plane, highlighted in grey box) would be rendered accessible, explaining increased intensity in NEXAFS spectra.11 Finally, as this model considers only one CNT, a degree of alignment would be needed to translate the behaviour to the macroscale. This could explain the increasing macroscopic response as alignment increases, as well as our prior trend discussion.11, 14

4.

CONCLUSIONS

This work detailed the molecular study of smart behaviour in thermo-active EVA-MWCNT composites through in situ temperature-resolved NEXAFS spectroscopy. Resonances corresponding to CNTs (π* C=C) and EVA (π* C=O) showed systematic coupled intensity variations. It was shown that these transitions were twice as strongly coupled in prestrained composite, where greater filler alignment was observed. Active behaviour was modelled as torsion of CNTs upon irradiation, increasing uniaxial symmetry of CNT-polymer ensembles. The conformational model addresses also the importance of filler alignment, which would facilitate the translation of molecular actuation to the macroscale. Similar in situ studies of other polymer nanocomposites displaying active behaviour will test the validity and universality of the proposed model.

ACKNOWLEDGEMENTS We kindly acknowledge Professor Eugene Terentjev at Cambridge University, Faisal M. Alamgir at Georgia Institute of Technology and Conan Weiland at Synchrotron Research Inc. for insightful discussions. Research was carried out in part at the National Synchrotron Light Source at Brookhaven National Laboratory, which is supported by the U.S. Department of Energy under contract number DE-AC02-98CH10886. This project was partially founded by FP7VEGA

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2/0149/14 and NMP 22896 and by grants from the National Center for Research Resources (5 G12RR013646-12) and the National Institute on Minority Health and Health Disparities (G12MD007591) from the National Institutes of Health. We acknowledge the NSF for support with grants DMR-1103730, Alloys at the Nanoscale: The Case of Nanoparticles Second Phase and PREM: NSF PREM grant DMR-0934218.

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