Investigation of MWCNT addition into poly-dimethylsiloxane-based coatings

Abstract

The purpose of this work is to investigate the effects on the properties of poly-dimethylsiloxane-based coatings with the incorporation of multi-walled carbon nanotubes (MWCNT). For this reason, two types of MWCNT were examined (pristine and functionalised with carboxyl groups). An optimised dispersion method was employed to synthesise coatings with different wt-% concentrations of CNT, using different solvents. The morphology of the nanocomposite coatings was studied under optical and scanning electron microscopy. Moreover, their hydrophobic/hydrophilic behaviour was investigated by contact angle measurements. Furthermore, nanoindentation and nanoscratch tests were conducted to evaluate the nanomechanical properties. The results revealed an optimal threshold concentration of 0.1 wt-% in CNT that combines both acceptable dispersibility and mechanical enhancement of the composite coating. It was proven that the increase in CNT content deteriorates the surface and mechanical properties of the coatings.

Keywords

Coatings Contact angle Dispersion Hydrophobicity Multi-walled carbon nanotubes Nanomechanical properties Poly-dimethylsiloxane

Introduction

The trigger for this study was the need for the development of more efficient antifouling paints, in order to face the crucial problem of fouling in maritime industry, which presents high economic and environmental impacts. Till now, different types of antifouling coatings are used to prevent biofouling.¹ However, due to the stricter environmental regulations, the recent technological innovations focus on the development of non-toxic fouling release (FR) coatings, which exploit the physicochemical properties of different surfaces, either to prevent the organisms from settling onto the surface or to reduce the adhesion on it.² Adhesion is traditionally related to the hydrophobicity of the surface and the free surface energy, but is also influenced by other parameters such as surface roughness and mechanical properties (i.e. elastic modulus, surface hardness, elasticity, etc.).³

Silicon-based coatings are the most commonly used commercial FR coatings.⁴ They present the lowest adhesion with fouling organisms,¹ because of their soft surface and their low surface free energy; however their main disadvantage can be attributed to the weak mechanical properties they demonstrate. The innovative technologies used in silicon-based FR coatings include the addition of nanofillers,^{5, 6} the conversion of the silicon matrix with polyurethane,^{7, 8} epoxy⁹ and fluorinated segments and of course, the use of biocides.¹⁰ Carbon nanotubes (CNT) have been considered as promising fillers for silicon-based coatings due to their exceptional mechanical properties. The addition of CNT, even in low amounts, results not only in the improvement of the surface and mechanical properties, but can also reduce the adhesion strength of fouling organisms due to the obtained nanostructure surface modification.^{5, 6, 11}

The aim of this work is to improve the mechanical properties of the poly-dimethylsiloxane (PDMS)-based coatings, as well as their hydrophobic behaviour, through the incorporation of multi-walled CNT (MWCNT), with potential applications in maritime industry. The dispersion process as well as the surface and mechanical properties of the coatings were investigated for several different concentrations of the nanofillers, in order to conclude to the ideal one.

Experimental details

Materials and reagents

The studied coatings were based on PDMS (Sylgard 184, Dow Corning). This silicon elastomer is supplied as a two-part component; base material (Part A) and curing agent (Part B), to be mixed in a ratio 10:1 by weight. After the mixing, polymerisation via hydrosilylation occurs, and a cross-linked polymer is received as a final product. MWCNT, pristine and functionalised with carboxyl groups (MWCNT-COOH), purchased by Nanothinx S.A., Greece, were tested as possible nanofillers. Both materials had average purity of 97%, external diameter of 15–35 nm and an average length of at least 10 µm. Moreover, commercial dispersants, purchased by BYK-CHEMIE GmbH, Germany, were used in order to facilitate the dispersion of the carbon material into the silicon resin. Three different dispersants were tested: (i) a solution of a block copolymer with basic, pigment-affinic groups (Disperbyk 2150), ii) a hyperbranched polyester (Disperbyk 2152) and (iii) a high-molecular-weight copolymer with affinic groups (BYK 9077). Finally, a variety of solvents (Wilckens Paints S.A., Greece) were examined for the dispersion of MWCNT,¹² as it is described in the paragraph below.

Solvent selection

The compatibility between solvents and PDMS is an important parameter to be considered, especially for microfluidic applications (e.g. microreactors for organic reactions).¹² Another crucial issue, for which this compatibility is important, is when PDMS finds use as an additive in solvent-borne formulations.¹³ In principle, PDMS is used at low levels to ease the application of coatings and in the meanwhile, its surface properties enable the coating to wet easily the substrate, giving a smooth appearance. Because PDMS behaves in this case as performance-enhancing additive, it should be compatible with the solvents used in such formulations.⁴ For this reason, the most commonly used solvents for antifouling coatings systems¹³ were chosen to be studied in respect of their dielectric constant (ε_r):¹² deionised water (H₂O, ε_r = 80 at 20°C), ethanol (EtOH, ε_r = 24.5 at 20°C), 2-propanol (IPA, ε_r = 18.3 at 20°C), methyl-isobutyl-ketone (MIBK, ε_r = 13.1 at 20°C) and xylene (XY, ε_r = 2.2 at 20°C). Five mixtures of PDMS with each solvent were created in a ratio 4:1. The mixing was performed in an ultrasonic bath filled with tap water, in an operational frequency of 37.5 kHz. The ultrasonic bath was programmed for 5-min degassing and 1-h ultrasonication in room temperature. The solvents' evaporation rate from the mixtures was determined by measuring the weight loss in time intervals of 30 min after placing the mixtures in a water bath at 50°C.Τhe results from the compatibility test between the different solvents and PDMS are demonstrated in Fig. 1. The incomplete solvent evaporation during the first 10 h can be attributed to the creation of hydrogen bonds between the solvents molecules and the polymer chains of PDMS.

The solvent selection depends also on the dispersibility of CNT into the different solvents. As it is reported by Liu and Choi,¹⁴ the direct mixing of CNT in powder form with PDMS does not create the appropriate local shear force to lead to sufficient dispersion of CNT inside the polymer matrix, especially for high concentrations. For this reason, in this study, we attempted to obtain an acceptable dispersion of MWCNT in a preferable solvent at first and afterwards, to incorporate the mixture with PDMS, removing totally the solvent at the end. Dispersion in the chosen solvents (concentration of 1% w/v in MWCNT) was achieved though ultrasonication. The samples were placed in test tubes that were immerged in the ultrasonic bath. The dispersibility of MWCNT (pristine and functionalised) in the different solvents is tabulated in Table 1. As it can be seen from the dispersion testing, pristine MWCNT are not capable to present stable dispersions. In all cases, settling occurs in just 1 h after the ultrasonication. On the other hand, MWCNT-COOH seem to have a different behaviour. Specifically, it has been detected that dispersibility increases proportionally with the increase of the dielectric constant of the solvents. It is obvious that surface treatment with carboxyl groups is required in order to achieve stable dispersions of MWCNT in polar solvents. At this point, it should be mentioned that according to literature,^14-16 carboxyl functionalised MWCNT present better dispersion in polymers than the pristine ones, due to the formation of covalent bonds that prevent nanotubes from agglomerating and forming bundles. The functionalisation may also provide a strong covalent chemical bonding between MWCNT and the polymer matrix, which can lead to the enhancement of the coherence of the composite.¹⁶

Table 1

Dispersion assessment of pristine and functionalised MWCNT in different solvents

Solvent	ε_r/20°C	Pristine MWCNT/t₀	Pristine MWCNT/t = 1 h	MWCNT-COOH/t₀	MWCNT-COOH/t = 1 h
XY	2.2
IPA	18.3
EtOH	24.5
H₂O	80
MIBK	13.1

Taking into account the aforementioned requirements, it may be remarked that IPA can be regarded as the most suitable solvent, as it was homogeneously mixed with PDMS and evaporated faster than any other solvent. Generally, it is desirable to minimise the required time for the solvent evaporation and to dry the solution as soon as possible, in order to avoid re-aggregation of nanotubes, as the concentration of CNT rise.¹⁴ In parallel, IPA presented an acceptable dispersion of MWCNT, in terms of stability during time, thus selected as the preferable solvent for continuing the experiments.

Synthesis

Nanocomposite coatings of PDMS/MWCNT were prepared at different MWCNT-COOH concentrations (0.05, 0.10 and 0.20 wt-%). The preparation method includes the dispersion of MWCNT-COOH in IPA and the addition of the produced dispersion into the PDMS matrix (Part A), followed by mechanical stirring at 1200 rpm and 50°C, until the solvent evaporation. Curing agent (Part B) was then added and stirred for 20 min at 1200 rpm. In order to improve the dispersion of MWCNT-COOH in the polymer matrix, the dispersants described in Section ‘Materials and reagents’ were tested. Each dispersant was added in Part A prior the addition of MWCNT-COOH, according to their technical specifications. After the mixture of the two parts, the resulting coatings were deposited on aluminium plates (9.5 cm × 3.5 cm × 1 mm) and glass plates (6 cm × 2 cm × 5 mm) using a manual film applicator.

Characterisation techniques

The thickness of the coatings was measured with a coating thickness gauge, Dualscope MPO (Fisher). The investigation of MWCNT-COOH dispersion in the PDMS structure was performed using optical microscopy (Mighty Scope 500× Digital Microscope, Aven) and scanning electron microscopy (SEM, PHILIPS Quanta Inspect (FEI Company) with tungsten filament, 25 kV). The static contact angle experiments were conducted by depositing a drop of deionised water on the sample (a total of five measurements were conducted for statistical reasons on each specimen), and the results were calculated using Young's equation.

The mechanical behaviour of the coatings was investigated via nanoindentation and nanoscratch tests. The tests were performed with HysitronTriboLab^® Nanomechanical Test Instrument, which allows the application of loads from 1 to 30 000 µN and records the displacement as a function of applied loads with a high load resolution (1 nN) and a high displacement resolution (0.04 nm). In all nanoindentation tests a total of 10 indents with a spacing of 50 µm were averaged for statistical purposes, to determine the mean hardness (H) and elastic modulus (E) values. The measurements were conducted in a clean area environment with 45% humidity and 23°C ambient temperature. In order to operate under closed loop load or displacement control, feedback control option was used. All nanoindentation measurements were performed with the standard three-sided pyramidal Berkovich probe, which has an average radius curvature of about 100 nm.¹⁷ Based on the half-space elastic deformation theory, H and E values were extracted from the experimental data (load–displacement curves) using the Oliver–Pharr method.¹⁸ The nanoscratch experimental procedure included the scratch of the surface in 10 µm path for 50 s, at a maximum imposed load of 50 µN. To reach the maximum load and simultaneous scratch of the surface, a period of 20 s covering a distance of 3 µm was necessary. After the scratch, the surface was scanned for 30 s, with a very low load (∼0.1 µN) to identify any plastic deformations (five experiments for each sample were conducted for statistical reasons).

Results and discussion

Morphology

For the conduction of accurate examinations, i.e. nanomechanical testing, it is necessary to outline the exact film thickness of the coatings. Figure 2 illustrates the thickness profile of the coatings in contour graphs. It may be remarked that the use of a manual applicator does not always create a uniform surface. Slight variation in the thickness of the upper and lower edge of the sample can be attributed to the deposition process. For this reason, all the examinations were conducted in the intermediate area of the samples, to avoid the edge effects. It should be noted that the applied wet film thickness was 200 µm; however, the obtained dry film thickness was less than half (∼100 µm), due to the evaporation of the solvent and the remaining of the solid part of the coating only.

Dispersion analysis of MWCNT-COOH/PDMS nanocomposites

Optical microscopy (×500) was used to evaluate the dispersion and the possible formation of aggregates (Fig. 3). Samples of PDMS with 0.05 wt-% in MWCNT-COOH and PDMS with 0.1 wt-% MWCNT-COOH exhibited uniform dispersion and 50 µm maximum diameter of agglomerates; thus, 0.1 wt-% in MWCNT-COOH was selected as a threshold concentration, as the further increase in the MWCNT-COOH content resulted in increased size of agglomerates. It was observed that the addition of the dispersant Disperbyk 2152 resulted in a more uniform dispersion of MWCNT-COOH in the polymer matrix, possibly, due to the increase of the adhesion forces between the aminic groups that the dispersant contains, toward MWCNT-COOH, which led to their deflocculation. Comparing these results with similar in literature, we conclude that the formation of agglomerates cannot be avoided. However, the functionalisation of MWCNT seems necessary in order to improve the dispersion.^{14, 15}

Scanning electron microscopy images provided information about the surface morphology of the samples (Fig. 4). It can be observed that their surface does not depict any cracks, grooves or uncovered areas, but be that as it may, it is not homogeneous. Moreover, the addition of MWCNT-COOH resulted in a more heterogeneous surface; this phenomenon was more intense for the case of PDMS 0.2 wt-% in MWCNT-COOH.

Hydrophobicity

A PDMS-based coating is regarded hydrophobic with contact angles ranging from 90° to 120°.¹⁹ The contact angle of deionised water on PDMS was calculated to be 98.8° ± 3°.¹⁹ Considering the obtained results, as presented in Fig. 5, it may be remarked that no significant deviations were observed for the MWCNT containing samples; only the sample containing 0.1 wt-% MWCNT-COOH exhibited increased hydrophobic behaviour (99.5° ± 3°), that is in common with literature. Specifically, Martinelli et al.²⁰ found that a composite sample of PDMS with 0.1 wt-% MWCNT-COOH presented a small increase (∼4°) in the contact angle with deionised water.

However, samples containing commercial dispersants presented a lower contact angle compared to PDMS with 0.1 wt-% MWCNT-COOH. The addition of dispersants may improve the dispersion of MWCNT in the polymer matrix, but influences the surface properties of the coating. PDMS/D.2150 and PDMS/D.9077 showed lower contact angle in comparison with PDMS/D.2152, including the same concentration in MWCNT-COOH.

Mechanical properties

Load–unload curves are presented in Fig. 6, in which the elastoplastic behaviour of the sample and the adhesion between the tip and the surface was observed. The yield point in Fig. 6 b represents the transition from the elastic to the elastic–plastic region.²¹ Limited increase was presented in the sample with 0.1 wt-% MWCNT-COOH when compared to bare PDMS, whereas PDMS/D.2152 with 0.1 wt-% MWCNT-COOH was similar to PDMS with 0.05 wt-% MWCNT-COOH. The addition of CNT increased the plastic deformation (a loop is evident between load and unload curves), indicating higher energy stored in the material during the nanoindentation experiment.²¹ The sample PDMS with 0.5 wt-% MWCNT-COOH exhibited 0.95% decrease in the work of adhesion compared to the bare PDMS coating (Fig. 6 c). The addition of Disperbyk 2152 contributed in the reduction of the work of adhesion by 94%, compared to PDMS with 0.1 wt-% MWCNT-COOH. The change in the adhesion behaviour is attributed to the dispersion of CNT in the structure of the material.^22-24

The differences in nanomechanical properties can be attributed to the size effect, the microstructure or density of the cross-linking and the dispersion of the CNT inside the polymer matrix.^{25, 26} For displacement up to 500 nm, the calculated values of H and E were increased with the addition of MWCNT (Fig. 7). As the tip penetrated the sample, the calculated values matched these of bulk PDMS, which are in agreement with literature (H ≈ 2.761 ± 0.153 MPa²⁷ and E ≈ 6.75 ± 0.125 MPa).²⁸ The highest values of H and E are revealed for PDMS/D.2152 with 0.1 wt-% MWCNT-COOH, which is the threshold concentration. The higher values of the nanomechanical properties are a result of the homogeneous dispersion of the MWCNT. As for the samples without dispersants, PDMS with 0.05 wt-% MWCNT-COOH exhibited improved surface properties.

In Fig. 8 the H/E and H³/E² ratios are presented. The H/E ratio has been previously used to rank materials in respect to their wear resistance,²⁹ whereas H³/E² describes the amount of elasticity exhibited by a coating; i.e. high (low) H³/E² values indicate an elastic (plastic) behaviour of the coating.²⁹ In Fig. 8, it is observed that the surface of the coating presents different results from the bulk part of the sample; PDMS/D.2152 with 0.1 wt-% MWCNT-COOH reveals the higher values. As for the samples without dispersants, the values for depths up to 500 nm are verified from previous researches.³⁰ Furthermore, the H³/E² ratio revealed a similar trend: PDMS/D.2152 with 0.1 wt-% MWCNT-COOH presented an improved elastic behaviour. Αs for the rest of the composite samples, it was observed that, in a surface region of 300 nm, higher values of H³/E² were calculated, indicating an improved elastic behaviour in respect to bare PDMS. For higher displacements, H/E and H³/E² ratios tended to reach constant values, mainly indicating the bulk properties of PDMS. Based on the above, the coatings surface revealed higher resistance to wear, probably due to the presence of MWCNT, setting these coatings possible candidates for marine applications.

The effect of different loading rates was also studied (Fig. 9). Loading rate is an important variable that influences particularly the response of soft materials, on imposed loads. Changing the loading rate can significantly alter the slope of the load–unload curves, thereby affecting the computable H and E. In addition, different loading rates can affect the maximum recorded depth; i.e. samples exhibiting strong time-dependent behaviour reveal lower (greater) maximum indentation depths for higher (lower) loading rates.³¹ Three different loading rates (10, 20 and 50 µN s⁻¹, at maximum imposed load of 100 µN) were tested. Bare PDMS, PDMS with 0.05 wt-% MWCNT-COOH and PDMS with 0.1 wt-% MWCNT-COOH did not exhibit time-dependent behaviour, unlike PDMS with 0.2 wt-% MWCNT-COOH, in which the heterogeneous dispersion affected the time-dependent behaviour of the sample.

In Fig. 10 the calculated H and E values are presented, for the different loading rates tested. Regarding bare PDMS, the increase in the loading rate revealed an intense reduction on the H values and a lesser effect on the E values. The reduction comes up to 18% for the H and 9% for the E, for 10 and 50 µN s⁻¹, respectively. Samples with 0.05% and 0.1 wt-% MWCNT-COOH revealed mild changes, presenting an increasing trend as the loading rate increased. PDMS with 0.2 wt-% MWCNT-COOH exhibited lower values of H and E, due to the greater time-dependent behaviour, confirming the previously calculated values.

Due to its viscoelastic nature, PDMS exhibits creep phenomenon;³² i.e. during the holding time in maximum load, a continuous increase in the indentation depth is observed. Figure 11 presents this increase in the indentation depth for a specific holding time (5 s) and a maximum load (100 µN) at different loading rates (10, 20 and 50 µN). The main goal is to control the extent to which the increase in the loading rate and the addition of MWCNT affects the creep phenomenon. It was observed that increasing the loading rate led to a more intense creep. PDMS with 0.1 wt-% MWCNT-COOH presented the lowest creep deformation, as an effect of the homogeneous dispersion. On the other hand, PDMS with 0.2 wt-% MWCNT-COOH presented the highest creep deformation, especially in higher loading rates, due to the extensive plastic deformation occurred under the indenter, the heterogeneous dispersion of MWCNT that caused aggregates, the tube–tube slip phenomena and the alteration in the structure of the bulk sample.

Figure 12 depicts the progress of the creep displacement for a fixed loading rate (10 µN s⁻¹) and holding time (50 s) at the maximum load (100 µN). The holding time was chosen so as to determine the point at which the increase of the indentation depth stops. In soft matter, long holding times (>120 s) are avoided, as the local grade of temperature which is developed due to the stress process, also called thermal drift, affects the development of the creep phenomenon.³³ The uniform dispersion of CNT in PDMS prevents, to some extent, the mobility of the polymeric chains; the extremely favourable CH–p electron interactions between the methyl groups of PDMS and the aromatic ring of MWCNT possibly reduce the mobility of the chains, resulting in a significant reduction of the viscoelastic behaviour of the coating.³⁴

Finally, the presence of MWCNT hinders the scratch of the surface. PDMS with 0.1 wt-% MWCNT-COOH revealed the lowest coefficient of friction (CoF, µ), thus being the most appropriate coating for marine applications due to the low wear that will undergo during operation (Fig. 13). The low percentage of MWCNT (0.05 wt-%), the use of Disperbyk 2152 for the coating containing 0.1 wt-% MWCNT-COOH and the addition of the higher percentage of MWCNT-COOH (0.2 wt-%) led to the recording of higher values of CoF compared to bare PDMS.

Conclusions

Silicone-based coatings (PDMS) filled with different wt-% of MWCNT were prepared. A threshold concentration was revealed, beyond which the increase in content of CNT results in deterioration of the surface and mechanical properties of the coating (increase of nanotubes content leads to agglomeration). The use of Disperbyk 2152 reduces the agglomeration tendency and significantly contributes to the uniform dispersion of the nanotubes in the polymer matrix. Consequently, it was observed that the agglomerates size reduction and the uniform distribution of CNT in the PDMS matrix enhance the properties (surface and mechanical) of the composite. Nevertheless, it was determined that the addition of MWCNT slightly affected the hydrophobicity of the surface. The threshold concentration (0.1 wt-%) demonstrated minimally increased contact angle, indicating improvement of the hydrophobicity of the surface of the material. The mechanical properties of the bulk material remain primarily unchangeable, regardless of the incorporation of CNT. The results revealed a significant change in the surface region (0–500 nm), in which the composite coatings exhibited increased H, E and wear resistance as well as higher elastic response to imposed loads compared to PDMS (low friction coefficient values were confirmed also by nanoscratch testing).

Footnotes

Acknowledgements

This research of E. P. Koumoulos, A.-F. A. Trompeta, I. A. Kartsonakis and C. A. Charitidis was founded by the Hellenic and European Regional Development Funds under the Action ‘Synergasia (Cooperation) 2011: Partnerships of Production and Research Institutions in Focused Research and Technology Sectors’, of the Project ‘Novel Self-Healing Eco-friendly Coatings with Antifouling and Anticorrosion Properties for Maritime Applications (MARIPAINTS)’.

References

Lejars

, Margaillan

and Bressy

: ‘Fouling release coatings: a nontoxic alternative to biocidal antifouling coatings’, Chem. Rev. , 2012, 112, (8), 4347–4390. doi: 10.1021/cr200350v

Dafforn

K. A.

, Lewis

J. A.

and Johnston

E. L.

: ‘Antifouling strategies: history and regulation, ecological impacts and mitigation’, Mar. Pollut. Bull. , 2011, 62, (3), 453–465. doi: 10.1016/j.marpolbul.2011.01.012

Almeida

, Diamantino

T. C.

and de Sousa

: ‘Marine paints: the particular case of antifouling paints’, Prog. Org. Coat. , 2007, 59, (1), 2–20. doi: 10.1016/j.porgcoat.2007.01.017

Witucki

G. L.

: ‘The evolution of silicon-based technology in coatings’, 2003, Midland, MI, Dow Corning Corporation.

Beigbeder

, Degee

, Conlan

S. L.

, Mutton

R. J.

, Clare

A. S.

, Pettitt

M. E.

, Callow

M. E.

, Callow

J. A.

and Dubois

: ‘Preparation and characterisation of silicone-based coatings filled with carbon nanotubes and natural sepiolite and their application as marine fouling-release coatings’, Biofouling , 2008, 24, (4), 291–302. doi: 10.1080/08927010802162885

Carl

, Poole

A. J.

, Vucko

M. J.

, Williams

M. R.

, Whalan

and de Nys

: ‘Enhancing the efficacy of fouling-release coatings against fouling by Mytilus galloprovincialis using nanofillers’, Biofouling , 2012, 28, (10), 1077–1091. doi: 10.1080/08927014.2012.728588

Fang

, Zhou

and Wu

: ‘Microphase separation behavior on the surfaces of PEG–MDI–PDMS multiblock copolymer coatings’, Appl. Surf. Sci. , 2006, 253, (5), 2978–2983. doi: 10.1016/j.apsusc.2006.06.044

and Luo

: ‘Synthesis and characterization of siloxane-modified two-component waterborne polyurethane’, Prog. Org. Coat. , 2013, 76, (11), 1522–1526. doi: 10.1016/j.porgcoat.2013.06.007

Rath

S. K.

, Chavan

J. G.

, Sasane

, Jagannath, Patri

, Samui

A. B.

and Chakraborty

B. C.

: ‘Two component silicone modified epoxy foul release coatings: effect of modulus, surface energy and surface restructuring on pseudobarnacle and macrofouling behavior’, Appl. Surf. Sci. , 2010, 256, (8), 2440–2446. doi: 10.1016/j.apsusc.2009.10.084

10.

Acton

Q. A.

: ‘Silicones – advances in research and application’; 2012, Atlanta, GA, ScholarlyEditions.

11.

Irani

, Jannesari

and Bastani

: ‘Effect of fluorination of multiwalled carbon nanotubes (MWCNTs) on the surface properties of fouling-release silicone/MWCNTs coatings’, Prog. Org. Coat. , 2013, 76, (2–3), 375–383. doi: 10.1016/j.porgcoat.2012.10.023

12.

Lee

J. N.

, Park

and Whitesides

G. M.

: ‘Solvent compatibility of poly(dimethylsiloxane)-based microfluidic devices’, Anal. Chem. , 2003, 75, (23), 6544–6554. doi: 10.1021/ac0346712

13.

Stoye

, Marwald

and Plehn

,: ‘Paints and coatings, 1. Introduction’, in ‘Ullmann's encyclopedia of industrial chemistry’, 2010, Weinheim, Wiley-VCH Verlag GmbH & Co. KGaA.

14.

Liu

C.-X.

and Choi

J.-W.

: ‘Improved dispersion of carbon nanotubes in polymers at high concentrations’, Nanomaterials , 2012, 2, (4), 329–347. doi: 10.3390/nano2040329

15.

Chua

T. P.

, Mariatti

, Azizan

and Rashid

A. A.

: ‘Effects of surface-functionalized multi-walled carbon nanotubes on the properties of poly(dimethyl siloxane) nanocomposites’, Compos. Sci. Technol. , 2010, 70, (4), 671–677. doi: 10.1016/j.compscitech.2009.12.023

16.

Khosla

and Gray

B. L.

: ‘Preparation, characterization and micromolding of multi-walled carbon nanotube polydimethylsiloxane conducting nanocomposite polymer’, Mater. Lett. , 2009, 63, (13–14), 1203–1206. doi: 10.1016/j.matlet.2009.02.043

17.

Charitidis

C. A.

, Koumoulos

E. P.

, Giorcelli

, Musso

, Jagadale

and Tagliaferro

: ‘Nanomechanical and tribological properties of carbon nanotube/polyvinyl butyral composites’, Polym. Compos. , 2013, 34, (11), 1950–1960. doi: 10.1002/pc.22602

18.

King

R. B.

: ‘Elastic analysis of some punch problems for a layered medium’, Int. J. Solids Struct. , 1987, 23, (12), 1657–1664. doi: 10.1016/0020-7683(87)90116-8

19.

Armani

, Liu

and Aluru

: ‘Re-configurable fluid circuits by PDMS elastomer micromachining’, Proc. 12^th IEEE Int. Conf. on ‘Micro electro mechanical systems (MEMS)’ Urbana, USA, February 1999, University of Illinois, 222–227.

20.

Martinelli

, Suffredini

, Galli

, Glisenti

, Pettitt

M. E.

, Callow

M. E.

, Callow

J. A.

, Williams

and Lyall

: ‘Amphiphilic block copolymer/poly(dimethylsiloxane) (PDMS) blends and nanocomposites for improved fouling-release’, Biofouling , 2011, 27, (5), 529–541. doi: 10.1080/08927014.2011.584972

21.

Charitidis

C. A.

and Koumoulos

E. P.

: ‘Nanomechanical properties and nanoscale deformation of PDMS nanocomposites’, Plast. Rubber Compos. , 2012, 41, (2), 88–93. doi: 10.1179/1743289810Y.0000000037

22.

Silberzan

, Perutz

, Kramer

E. J.

and Chaudhury

M. K.

: ‘Study of the self-adhesion hysteresis of a siloxane elastomer using the IKR method’, Langmuir , 1994, 10, (7), 2466–2470. doi: 10.1021/la00019a073

23.

Choi

G. Y.

, Kang

J. F.

, Ulman

, Zurawsky

and Fleischer

: ‘Acid-base interaction in the adhesion between two solid surfaces’, Langmuir , 1999, 15, (26), 8783–8786. doi: 10.1021/la991222h

24.

Choi

G. Y.

, Kim

and Ulman

: ‘Adhesion hysteresis studies of extracted poly(dimethylsiloxane) using contact mechanics’, Langmuir , 1997, 13, (23), 6333–6338. doi: 10.1021/la970525d

25.

Pharr

G. M.

, Herbert

E. G.

and Gao

: ‘The indentation size effect: a critical examination of experimental observations and mechanistic interpretations’, Annu. Rev. Mater. Res. , 2010, 40, (1), 271–292. doi: 10.1146/annurev-matsci-070909-104456

26.

Shen

Y. X.

, Wei

P. J.

and Lin

J. F.

: ‘Initial and adhesive contact between a diamond indenter and polydimethylsiloxane’, Rev. Sci. Instrum. , 2008, 79, (9), 096106-1–096106-3. doi: 10.1063/1.2987692

27.

C.-L.

, Lin

H.-C.

, Hsu

J.-S.

, Yip

M.-C.

and Fang

: ‘Static and dynamic mechanical properties of polydimethylsiloxane/carbon nanotubes nanocomposites’, Thin Solid Films , 2009, 517, (17), 4895–4901. doi: 10.1016/j.tsf.2009.03.146

28.

Vast

, Carpentier

, Lallemand

, Colomer

J.-F.

, Van Tendeloo

, Fonseca

, Nagy

J. B.

, Mekhalif

and Delhalle

: ‘Multiwalled carbon nanotubes functionalized with 7-octenyltrichlorosilane and n-octyltrichlorosilane: dispersion in Sylgard®184 silicone and Young's modulus’, J. Mater. Sci. , 2009, 44, (13), 3476–3482. doi: 10.1007/s10853-009-3464-1

29.

Leyland

and Matthews

: ‘On the significance of the H/E ratio in wear control: a nanocomposite coating approach to optimised tribological behaviour’, Wear , 2000, 246, (1–2), 1–11. doi: 10.1016/S0043-1648(00)00488-9

30.

Koumoulos

E. P.

, Jagdale

, Kartsonakis

I. A.

, Giorcelli

, Tagliaferro

and Charitidis

C. A.

: ‘Carbon nanotube/polymer nanocomposites: a study on mechanical integrity through nanoindentation’, Polym. Compos. , 2015, 36, (8), 1432–1446. doi: 10.1002/pc.23049

31.

Oliveira

G. L.

, Costa

C. A.

, Teixeira

S. C. S.

and Costa

M. F.

: ‘The use of nano- and micro-instrumented indentation tests to evaluate viscoelastic behavior of poly(vinylidene fluoride) (PVDF)’, Polym. Test. , 2014, 34, 10–16. doi: 10.1016/j.polymertesting.2013.12.006

32.

Carrillo

, Gupta

, Balooch

, Marshall

S. J.

, Marshall

G. W.

, Pruitt

and Puttlitz

C. M.

: ‘Nanoindentation of polydimethylsiloxane elastomers: effect of crosslinking, work of adhesion, and fluid environment on elastic modulus’, J. Mater. Res. , 2011, 20, (10), 2820–2830. doi: 10.1557/JMR.2005.0354

33.

Fischer-Cripps

A. C.

: ‘Factors affecting nanoindentation, test data’, in ‘Nanoindentation’ part of the series ‘mechanical engineering series’, 61–82; 2002, New York, Springer.

34.

Beigbeder

, Linares

, Devalckenaere

, Degée

, Claes

, Beljonne

, Lazzaroni

and Dubois

: ‘CH-π interactions as the driving force for silicone-based nanocomposites with exceptional properties’, Adv. Mater. , 2008, 20, (5), 1003–1007. doi: 10.1002/adma.200701497