Simultaneously increasing the hydrophobicity and interfacial adhesion of carbon fibres: a simple pathway to install passive functionality into composites.

Chantelle L. Arnold, Daniel J. Eyckens, Linden Servinis, Mark D. Nave, Huaying Yin, Ross K. W. Marceau, Jean Pinson, Baris Demir, Tiffany R. Walsh, Luke C. Henderson.

XPS spectra of modified Carbon Fibers

C1s (eV)

Unmodified CF

284.7 88.8%

Tailing of the C1s peak

Surface 1 285.5 63.5%

287.2 9.2%

289.0 5.1%

Surface 2 285.1 56.1%

286.5 16.2%

289.0 4.6%

Surface 3 285.0 44.4%

286.7 8.6%

289.9 3.1%

292.0 4.9%

294.3 0.6%

Surface 4 285.0 43.8%

286.3 17. 6%

288.9 4.2%

291.8 3.9%

294.2 0.5%

Surface 5 284.6 10.3%

285.8 4.5%

286.9 4.0%

289.1 2.2%

291.7a) 17.2%

293.8a) 3.4%

Assignment CH2 Aromatic CH

C-O C(=O)OH CF2a) CF3

a)

a) see references1,2, 3,4

N1s O1s F1s

Unmodified CF

400.1 1.5%

532.6 9.8%

Surface 1 400.1 1.3%

402.1 0.3%

405.8 1.4%

532.6 18.7%

685.8 0.5%

Surface 2 399.9 2.3%

401.7 1.0%

406.1 2.1%

533.0 17.7%

Surface 3 400.2 1.0%

402.4 1.1%

406.0 1.7%

532.6 11.9%

689.3 22.8%

Surface 4 400.2 3.0%

402.4 0.75%

406.2 1.2%

532.6 15.2%

689.0 9.4%

Surface 5 400.7 1.2%

532.6 2.2%

689.0 55.0%

Assignment Contamination +Azo bonds a)

N+c) NO2 Contamination, NO2

Perfluoro group

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2019

+Reduction NO2

b)

a) see ref5, b) reduction of the nitro group under the XPS beam, see ref6, c) some NPF6+ entrapped in the

film.

Figure S1. XPS spectra of A) bare carbon fibers (C1s), B) Surface 1 (N1S), C) Surface 5 (F1s), D) Surface 5

(C1s)

Bare carbon fiber: the thin (1.2 eV FWMH) C1s peak at 284.6 eV is characteristic of the carbon

fiber, tailing of this peak on the high energy side is due to the numerous oxidized groups (C-O,

C=O, C(=O)OH) present on the surface of the fibers (Figure S1 A), these groups are also

responsible for the O1s peak.7 The surface also includes some nitrogen

4: The C1s peak at 285.0 eV corresponds to the aromatic groups attached the surface, a

tentative deconvolution with a peak at 284.6 eV as in the bare carbon fiber indicates that it

should be quite small. If the 284.6 eV is shadowed by the organic film, the thickness of the

later should in the 5-10 nm range. The two peaks at higher energy (286.7, 289.0 eV) can be

0

35000

282287292

Cou

nts

8000

10000

12000

396.00401.00406.00

Cou

nts

0

100000

280285290295

Binding energy (eV)Binding energy (eV)

Binding energy (eV)

0

250000

680685690695

Cou

nts

Binding energy (eV)

Cou

nts

A B

CD

assigned to oxidized species, but also C-N (at 286 eV8) and C-N=N (formation of azo bonds

inside the film5). The presence of the nitro group is attested by the N1s peak at 405.8 eV. The

peak at 402 eV corresponds to some residual NBu4+ cation from the supporting electrolyte

while the anion PF6- is observed as a small F1s peak (Figure S1 B). Despite extensive rinsing

some supporting electrolyte remains entrapped in the film.

5: The reduction of the nitro group is only partial (~ 50%) it is known that some nitro groups

are electrochemically silent and are not reduced by electrochemistry.9 The reduction is

performed in acidic medium that results ins some protonated amines at 401.7 eV.

6: Both the NO2 (406.0 eV) and the perfluoro groups (689.3 eV) are present on the surface.

The CF2 and CF3 groups appear at 292.0, 294.3 eV, the experimental ratio CF2/CF3 = 8.2 is

somewhat higher than the expected value of 5. The two CH2 should appear at 284.4 and 285.6

eV4 , they are included in the aromatic and oxidized carbons at 285.0 and 286.3 eV. The ratio

N1s (400.2 eV)/CF2(291.8eV) = 0.25 indicates that the ratio number of nitrophenyl groups

/number of perfluoro chain is ~1.3.

7: The reduction is more efficient than with 5, 82% of the nitro groups are reduced.

8: The main contribution of the carbon C1s peak is located at 284.6 eV that correspond to the

carbon fiber and the CH2 group bonded to the fiber. One observes a high percentage of F1s

(Figure S1 C) and the CF2 and CF3 contributions to the C1s peak with the expected ratio of 5

(Figure S1 D).

IR spectra Carbon Fibers

Figure S2 IRATR spectrum of bare carbon fibers

0

0.001

1200130014001500160017001800

Wavenumber cm-1

Ab

sorb

an

ce

Oxidized

groups

Aromatic

groups

Bare carbon fibers present three main peaks at 1758, 1694 and 1655 cm-1 that can be assigned

respectively to lactones, carboxylic acids and ketones10,11. Aromatic signals are observed in

the 1600-1500 cm-1 range (Figure S2).

References for XPS and IR Section

1 H. Lu, D. Zeysing, M. Kind, A. Terfort, M. Zharnikov, J. Phys. Chem. C 2013, 117, 18967-

18979.

2 A. Shaporenko, P. Cyganik, M. Buck, A. Ulman, M.Zharnikov, Langmuir 2005, 21, 8204-

8213.

3 F. Laffineur, Z. Mekhalif, L. Tristani, J. Delhalle, J. Mater. Chem. 2005, 15, 5054-5062.

4 D. Hetemi, F. Kanoufi, C. Combellas, J. Pinson, F. I. Podvorica, Langmuir 2014, 30, 13907-

13913

5 P. Doppelt, G. Hallais, J. Pinson, F. Podvorica, S. Verneyre, Chem. Mater. 2007, 19, 4570-4575 6 A. Adenier, E. Cabet-Deliry, A. Chaussé, S. Griveau, F. Mercier, J. Pinson, C. Vautrin-Ul

Chem. Mater. 2005, 17, 491-501.

7 E. Desimoni, G. I. Casella, A. Morone, A. M. Salvi, Surf Interface Anal., 1990, 15, 627-634

8 N. Graf, E. Yegen, T. Gross, A. Lippitz, W. Weigel, S. Krakert, A. Terfort, W. E.S. Unger, Surf.

Sci. 2009, 603, 2849–2860

9 S. S. C. Yu, E. S. Q. Tan, R. T. Jane, A. J. Downard, Langmuir 2007, 23, 11074-11082.

10 Socrates, G. Infrared and Raman characteristic group frequencies, 3rd ed.; John Wiley &

Sons, 2008.

11 C. Sellitti, J. L. Koenig, H. Ishida, Carbon, 1990, 28, 221-228.

Separation of water and chloroform using a fibre plug.

Finally, to highlight the potential non-traditional uses of these fibres, perfluoroalkyl functionalized

fibres (Surface 5) were taken, and sought to determine if these could be used as a means to separate

mixtures of water and organic solvent. To do this, a plug of these fibres was placed into a Pasteur

pipette and a mixture of water (blue) and chloroform (green) was poured on top. For the untreated

fibres both phases quickly passed through the fibre plug giving no discrimination between the aqueous

and organic phases. When repeating this with treated fibres (Figure 7b), the chloroform phase quickly

passed through the fibre plug, while the aqueous phase remained on top, with the image presented

below being taken 1 hour after initial filtration.

Similar observations are made when organic solvents such as methanol versus water are dropped onto

the fibre surface (video in ESI). The former is rapidly taken up by the fibres, while the latter is unable

to penetrate the fibre bundle. This is confirmed in the observations made regarding interfacial

adhesion, as the fibres are hydrophobic yet organo-philic, as there is only one carbon distinguishing

water from methanol.

Additional SEM images at 15,000x magnification

The SEM images were taken using a Jeol JSM 7800F FEG-SEM and was prepared by placing the

fibres on double sided carbon tape. The conductivity of the fibres was sufficient for sputtering

not to be required.

Pristine

Surface 1

Surface 2

Surface 3

Surface 4

Surface 5

SEM of Fibre Fractures Highlighting Wet-Out

To complement the videos taken of fibre wet-out SEM of fibres after being sheared from epoxy

were taken to show encapsulation of the resin. Here, the resin is seen surrounding the fibre

clearly still adhering to the surface. In several instances the resin can be seen to fill the

longditudinal striations along the fibre axis showing that these fibres are indeed easily

infiltrated by the epoxy resin.

Modelling Additional Methodology

General Simulation Details: In this work we used the all-atom DREIDING force-field2 to describe the

interactions between bare/grafted fibre substrate and polymer matrix. Periodic boundary conditions

were implemented in all three dimensions. The Nosé-Hoover thermostat 3-4 and barostat 3, 5 were used

to control the temperature and pressure, respectively (unless otherwise stated). The cut-off distance

for long-range interactions was set to 12 Å and the particle-particle-particle-mesh solver was used 6.

Throughout this study we used time-step of 1 fs, using the LAMMPS (lammps.sandia.gov) simulation

software package7.

Our polymer model consisted of EPON-862 (diglycidyl ether of bisphenol F) and DETDA (diethyl

toluenediamine) in a ratio of 2:1. Following our previous works, 8-9 we used the activated form of the

EPON-862, where the epoxide bond was opened and the epoxide carbon atoms were saturated with

hydrogen atoms. This modified EPON-862 molecule herein referred to as EPON. Reactive atomic sites

of these two molecules were cross-linked in the presence of the grafted fibre surface to form a 3D

network. Five different surfaces were modelled. Surfaces 1 and 3 were modelled to be reactive with

respect to chemical cross-link formation with the polymer molecules. Surfaces 2, 4 and 5 were

modelled as unreactive towards the formation of chemical cross-link bonds with the polymer

molecules. A graphite slab, comprising three graphene layers with an initial interlayer spacing of 3.348

Å was used as an approximation of the carbon fibre surface. Each graphene layer contained 7200

carbon atoms, and the lateral dimensions of our graphite slab (and thus our simulation cell) were 127.8

x 147.6 Å. Each carbon atom of the substrate carried a zero partial charge.

Partial Charge Calculations: The DREIDING force-field did not include partial atomic charges, and these

must be calculated for all the atoms present in the system. To accomplish this, we followed our

previously-published computational procedure 8, 10, such that the initial structures of the EPON and

DETDA molecules, and functional groups, were generated using the AVOGADRO software 11 and

subjected to geometry optimisation using the Generalized Amber force field (GAFF) 12. We geometry

optimised these structures until the energy difference between two successive configurations was

smaller than 1 x 10-8 kJ/mol. Following this, we predicted the partial atomic charges using our

previously-reported procedure 8, 10. The partial atomic charges for the EPON and DETA molecules were

reported in our previous study 10. As for the SGM, Table S1 reports the partial atomic charges for the

unique atomic sites on each functional group, whose labels were provided in Figure S3.

Carbon Fibre Surface Functionalisation: We generated our initial simulation cell for surface grafting of

the substrate by placing the graphite slab in the centre of the simulation cell while filling the remaining

space with the SGMs. A total of 50 molecules were grafted at random locations on the exterior surfaces

of the carbon fibre substrate (50 molecules per surface of the graphite slab, giving a total of 100 grafted

molecules in total). This resulted in a grafting density of 1 grafted molecule for every ~4 nm2 of

available carbon fibre surface.

Preparation of Composite Simulation Cell Prior to Cross-linking: Once we obtained our surface grafted

substrate, we placed this substrate at the bottom of an orthorhombic periodic cell, of which the

remaining space was randomly filled with 1600 EPON and 800 DETDA using the PACKMOL software

package 13. The initial dimensions of our simulation cell were 127.8 x 147.6 x 194 Å for surfaces 1, 2

and 5 and 127.8 x 147.6 x 200 Å for surface 3 and 4, where 192 Å and 200 Å are the dimension of the

cells perpendicular to the substrate surface. Following this, we geometry optimised this low-density

pre-composite sample (containing the liquid precursor mixture and the surface grafted substrate) via

the FIRE algorithm 14 for 50000 steps. Once the optimisation step was finalised, molecular dynamics

(MD) simulations were performed in the isothermal-isobaric ensemble (NpT ensemble), at 300 K and

1 atm, to ensure the liquid precursor mixture was at the appropriate density and was well mixed in the

presence of the surface functionalised substrate. More details regarding this procedure can be found

in our previous studies 8, 10.

Simulated Annealing Procedure: We reported in our previous studies 8, 10 that the presence of the

bare/functionalised fibre surface influenced the spatial distribution of the liquid polymer precursor.

Based on this, prior to forming chemical cross-link bonds between the monomer and the cross-linking

agent (also between the monomer and SGMs on surfaces 2 and 4) to obtain polymerised composite

sample, we ensured that the interfacial liquid structure was equilibrated by applying a simulated

annealing (SA) procedure as reported in our previous work, 8-9. In this procedure we heated the pre-

composite sample to high temperature (1000 K) and then cooled back to the cross-linking temperature

(500 K, as per our previous studies) 8, 10. This SA process utilised MD simulations in the canonical (NVT)

ensemble and the schedule necessitated equilibration at 500 K for 0.2 ns, heating to 1000 K over a

period of 0.2 ns, maintenance of 1000 K for a further 1.0 ns, followed by cooling to 500 K over 0.5 ns.

This entire SA schedule was repeated twice for each surface.

Surface Grafted Molecule Conformational Distribution Analysis: We characterised the

conformational distribution of the surface grafted molecules (SGMs) on the fibre surface in the

presence of the liquid precursor mixture. To achieve this, we calculated the vertical distance (which is

perpendicular to the fibre surface plane) between the distal nitrogen atoms of the nitro and amino

groups on surfaces 1, 2, 3 and 4, and between the distal carbon atoms of the CF3 group on surfaces 2,

4 and 5 and the centre of mass (COM) of the graphene layer to which the functional group was

covalently bonded. This calculation was done for every SGM in each trajectory frame following the

procedure below. We considered frames from the last SA cycle. Our SA cycle ended with a 200 ps NVT-

MD simulation at 500 K. We generated frames from this 200 ps interval, at a rate of 1 frame per ps,

which amounted to 200 frames that were used for our analyses. The probability distribution for each

fibre-SGM distance was histogrammed using these 200 frames with a bin width of 0.2 Å and provided

in Figure S4.

Dynamic in-situ Cross-linking Procedure: Upon the completion of the equilibration of pre-composite

sample, we formed new chemical cross-link bonds via our in-situ cross-linking procedure 8, 10. In this

procedure, the reactive atom pairs were dynamically detected and connected to form the covalent

cross-link bonds. We pre-determined a distance-based cut-off for cross-linking (initially set to 3 Å) and

used this to detect the reactive atomic sites in the EPON and DETDA molecules (as well as the reactive

sites in the EPON and reactive amino groups on surfaces 2 and 4) within this cut-off distance. After the

formation of new chemical bonds, the resultant structure was relaxed via our multi-step relaxation

protocol 8, 10. Once the desired degree of cross-linking (in this case, 78%) was achieved, this crosslinking

procedure was terminated, and the partly-polymerised composite samples were relaxed for 0.5 ns at

300 K via NpT MD simulations at 1 atm pressure. The details for this protocol are provided in our

previous works 8, 10.

Calculation of Interfacial Shear Stress: After the generation of polymerised composite samples, we

calculated the total potential energy (PE) change curves via non-equilibrium MD simulations at 300 K.

We performed pull-out simulations, in which the substrate was laterally pulled out of the simulation

cell with a constant pulling speed of 50 ms-1 for 80 ps, as the potential energy of the system was

registered. All atoms in the simulation cell were free to move at 300 K. The PE data were averaged over

every 400 MD time-steps. We displaced the substrate by a maximum distance of 40 Å (Figure S5).

Figure S3. Atomic labels for the unique atomic sites on the surfaces (CR’ is the carbon atom that will

be covalently connected to the fibre surface. The partial atomic charge of HR will be added to the

partial atomic charge of CR’ after the molecule is chemically attached to the surface (because one

hydrogen atom (HR) will be deleted)).

Table S1. Partial atomic charges (in esu) for the unique atomic sites of the surface grafted

molecules.

Figure S4. Probability distribution of the vertical site-surface distance between the distal nitrogen (on

surfaces 1, 2, 3, 4, 5) and carbon atoms (on surfaces 3, 4 and 5) of the surface grafted molecules and

the fibre surface, in the presence of liquid precursor mixture.

Figure S5. Total potential energy change curves as a function of substrate displacement for each

surface at 300 K.

References for Modelling Section

(1) Weibull, W. A statistical distribution function of wide applicability. J. Appl. Mech. 1951, 18, 293-

297.

(2) SL Mayo; Olafson, B. D.; Goddard, W. A. DREIDING, a generic force-field for molecular simulations.

J. Phys. Chem. 1990, (94), 8897-8909.

(3) Nosé, S. A unified formulation of the constant temperature molecular dynamics methods. J. Chem.

Phys. 1984, 81 (1), 511-519.

(4) Hoover, W. G. Canonical dynamics: Equilibrium phase-space distributions. Phys. Rev. A 1985, 31 (3),

1695-1697.

(5) Hoover, W. G. Constant-pressure equations of motion. Phys. Rev. A 1986, 34 (3), 2499-2500.

(6) Hockney, R. W.; Eastwood, J. W. Computer simulation using particles, Taylor & Francis, Inc.: 1988;

p 564.

(7) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995,

117 (1), 1-19.

(8) Demir, B.; Henderson, L. C.; Walsh, T. R. Design Rules for Enhanced Interfacial Shear Response in

Functionalized Carbon Fiber Epoxy Composites. ACS Appl. Mater. Interfaces 2017, (9), 11846-11857.

(9) Servinis, L.; Beggs, K. M.; Scheffler, C.; Wöfel, E.; Randall, J. D.; Gengenbach, T. R.; Demir, B.; Walsh,

T. R.; Doeven, E. H.; Francis, P. S.; Henderson, L. C. Electrochemical surface modification of carbon

fibres by grafting of amine, carboxylic and lipophilic amide groups. Carbon 2017, 118, 393-403.

(10) Demir, B.; Walsh, T. R. A robust and reproducible procedure for cross-linking thermoset polymers

using molecular simulation. Soft Matter 2016, 12 (8), 2453-2464.

(11) Hanwell, M. D.; Curtis, D. E.; Lonie, D. C.; Vandermeersch, T.; Zurek, E.; Hutchison, G. R. Avogadro:

an advanced semantic chemical editor, visualization, and analysis platform. J. Cheminform. 2012, 4 (1),

17.

(12) Wang, J.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. Development and testing of a

general amber force field. J. Comput. Chem. 2004, 25 (9).

(13) Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M. PACKMOL: A package for building initial

configurations for molecular dynamics simulations. J. Computat. Chem. 2009, 30 (13), 2157-2164.

(14) Bitzek, E.; Koskinen, P.; Gähler, F.; Moseler, M.; Gumbsch, P. Structural Relaxation Made Simple.

Phys. Rev. Lett. 2006, 97 (17), 170201.