High performance composites containing perfluoropolyethers-functionalized carbon-based nanoparticles: Rheological behavior and wettability

High performance composites containing perfluoropolyethers-functionalized carbon-based nanoparticles: Rheological behavior and wettability

Composites Part B 95 (2016) 29e39 Contents lists available at ScienceDirect Composites Part B journal homepage: www.elsevier.com/locate/compositesb ...

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Composites Part B 95 (2016) 29e39

Contents lists available at ScienceDirect

Composites Part B journal homepage: www.elsevier.com/locate/compositesb

High performance composites containing perfluoropolyethersfunctionalized carbon-based nanoparticles: Rheological behavior and wettability Nadka Tzankova Dintcheva a, b, *, Elisabetta Morici a, b, Rossella Arrigo a, b, Giulia Zerillo a, Valeria Marona c, d, Maurizio Sansotera c, d, Luca Magagnin c, d, Walter Navarrini c, d  di Palermo, Viale delle Scienze, Ed. 6, 90128 Palermo, Italy Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, dei Materiali, Universita Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (UdR-Palermo), via G. Giusti, 9, 50121 Firenze, Italy Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, Via Mancinelli, 7, 20131 Milano, Italy d Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (UdR-PoliMi), via G. Giusti, 9, 50121 Firenze, Italy a

b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 February 2016 Received in revised form 14 March 2016 Accepted 30 March 2016 Available online 5 April 2016

Ultra High Molecular Weight Polyethylene (UHMWPE) based composites filled with carbon nanotubes (CNTs) and carbon black (CB) modified by perfluoropolyethers (PFPE) have been formulated. All composites show a segregated morphology with nanofillers selectively localized at the polymer particle eparticle interface. The composites rheological properties have been deeply investigated: the PFPE functionalities linked on CNTs facilitate the semi-3D nanofillers network formation in the composites that show a solid-like behaviour even at lower investigated filler contents, reaching the rheological percolation threshold at lower nanofiller content than bare CNTs filled composites. For composites containing CB, the presence of PFPE is useful to prevent the CB re-aggregation but does not significantly affect the rheological behaviour. Finally, the UHMWPE-based composites containing CNTs-PFPE and CB-PFPE show tunable wettability, due to the inherent hydrophobicity of PFPE chains. © 2016 Elsevier Ltd. All rights reserved.

Keywords: A. Polymer-matrix composites (PMCs) A. Nano-structures B. Rheological properties B. Surface properties Perfluoropolyethers nanofillers

1. Introduction The development of innovative multi-functional hybrid carbonbased nanoparticles and their polymer composites have gained a great interest among researchers in the last two decades. Carbonaceous structure, such as carbon nanotubes (CNTs) and carbon black (CB), appear to be good candidates for the formulation of advanced polymer-based composites. In particular, adding carbonaceous nanofillers to a polymeric matrix can improve significantly the mechanical, electrical and magnetic properties [1,2], as well as can be considered an useful approach to provide new functionalities to the polymeric composites [3,4]. Concerning CNTs addition to a polymeric matrix, an effective strategy to develop multifunctional materials is the functionalization and/or modification of the CNTs surface with different functional groups carrying

* Corresponding author. Dipartimento di Ingegneria Civile, Ambientale, Aero di Palermo, Viale delle Scienze, Ed. 6, 90128 spaziale, dei Materiali, Universita Palermo, Italy. Tel.: þ39 091 2386 3704; fax: þ39 091 2386 0841. E-mail address: [email protected] (N.T. Dintcheva). http://dx.doi.org/10.1016/j.compositesb.2016.03.095 1359-8368/© 2016 Elsevier Ltd. All rights reserved.

specific properties. Numerous approaches are reported in literature: (i) defect functionalization [5], (ii) covalent [6] and (iii) noncovalent [7,8] functionalization, and (iv) functionalization through click chemistry [9]. Among other approaches, the fluorination is particularly appreciated to form fluorine-containing nanotubes, which resulted more efficient in polymer reinforcement as compared with pristine CNTs [10e14]. Fluorinated CNTs can be also used as fundamental precursors for energy storage and conversion applications [15e17]. Moreover, the fluorination of carbonaceous substrates such as CNTs and CB is a key point to confer specific characteristics to the fillers, such as low surface energy, high chemical stability, UV resistance and to improve composite materials processability [18e21]. The functionalization of CNTs and CB by using perfluoropolyethers (PFPE), which are well known as highly performing fluorinated polymers suitable for coatings, membranes, solvents and lubricants for harsh environment, can be considered as an attractive alternative procedure to the fluorination [18,22]. In this process the covalent linkage of fluorine-containing groups occurs selectively on the carbonaceous outer surface and the bulk properties of the nanoparticles remain unchanged. In particular, the functionalization of carbonaceous structures with

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PFPE peroxide represents a suitable technique to modify CNTs and CB surface as well as their morphological properties [18,23]. The thermolysis of PFPE peroxides generates carbon-centred perfluorinated free radicals, which bond to the unsaturated moieties on the carbonaceous surfaces of CNTs and CB, which act as radical scavengers [24,25]. In this work, composite materials based on Ultra High Molecular Weight Polyethylene (UHMWPE) matrix filled with multi-walled carbon nanotubes (CNTs) and carbon black (CB) modified by perfluoropolyethers (PFPE) have been produced by hot compaction and their rheological properties have been compared to that of composite materials reinforced with pristine CNTs and CB. Linear stress relaxation and frequency sweep tests have been performed, in order to assess the influence of perfluorinated alkyl chains on the linear viscoelastic properties of UHMWPE-based composites. Moreover, surface analyses have been performed to correlate the effect of PFPE-modified CNTs and CB incorporation into UHMWPE with the concomitant changes in the composites wettability. 2. Experimental part 2.1. Materials UHMWPE was a commercial grade polymer purchased by SigmaeAldrich, with weight-average molecular weight 3÷6 MDa, softening point T ¼ 136  C (Vicat, ASTM D 1525B), melting point Tm ¼ 138  C (determinate by DSC) and density 0.94 g/mL at 25  C. CNTs (average diameter: 9.5 nm, average length: 1.5 mm, Product code: 7000) were purchased from Nanocyl, S.A. (Belgium) and were produced by a CVD process. Cabot Vulcan® XC72R was used in this work as high conductive and commercially available graphitic CB (particle size 30 nm) [26]. PFPE peroxide (Fomblin® Z PFPE peroxide) was kindly given by Solvay Specialty Polymers. It was prepared industrially through the oxidative photopolymerization of tetrafluoroethylene (TFE) [24]. The general chemical structure of PFPE peroxide is as follows: TO(CF2CF2O)m(CF2O)n(O)vT0, where T, T0 are trifluoromethyl, CF3, or acyl fluorides like C(O)F and CF2C(O)F, as terminal groups. The specific chemical characteristics of the PFPE peroxide used in this study are as follows: average molecular weight (AMW) around 29,000 amu, ratio between perfluoroethylene oxide, CF2CF2O, and perfluoromethylene oxide, CF2O, units 1.15, peroxidic content of 1.3 wt.% and equivalent molecular weight (EMW) around 1200 g/eq. CF3OCFClCF2Cl (b.p. 40e41  C) was used as inert fluorinated solvent during the chemical treatments for PFPE functionalization of the carbonaceous fillers.

The carbon-based filler (weighted sample of around 6.0 g) and a solution of PFPE peroxide in fluorinated solvent (120 ml) were introduced in a glass reactor. Two samples of PFPE-modified CNTs (CNTs-PFPE50 and CNTs-PFPE100) and one of PFPE-modified CB (CB-PFPE) were prepared: the amounts of PFPE peroxide in the fluorinated solutions were 50 and 100 wt.% of CNTs weight in samples CNTs-PFPE50 and CNTs-PFPE100, respectively; in the sample CB-PFPE the PFPE peroxide was 60 wt.% of CB weight. At first, the reaction mixture was heated at 40  C until the complete evaporation of the solvent. Thereafter, the PFPE peroxide was thermally decomposed starting from 150  C to 195  C, by increasing the temperature stepwise at the rate of 15  C/h, and then heated at 200  C for 4 h. At the end of the thermal treatment, the solid residue was rinsed three times with 50 ml of pure fluorinated solvent (CF3OCFClCF2Cl) and three times with 50 ml of deionized water. The sample was finally dried under vacuum (0.01 mmHg) at 200  C for 6 h. The acyl fluoride end-groups generated during the thermal decomposition of the PFPE peroxide were hydrolyzed to carboxylic acid during the rinsing with water and then decarboxylated during the final drying under vacuum. The carbon-based fillers were weighted before and at the end of the entire procedure for their chemical treatment with peroxidic PFPE in order to evaluate the percentage weight gain due to the bonded portion of PFPE chains. The surface composition, the mass fraction due to linked PFPE, the specific surface area and the water contact angle value of bare and PFPE-modified CNTs and CB used as carbon-based fillers in this work are resumed in Table 1 [18,19,25]. It is worth noticing that PFPE-modified CNTs fillers have a higher PFPE mass fraction with respect to PFPE-functionalized CB fillers, although the latter show a higher F/C, because PFPE chains are more homogeneously dispersed on the CB surface and in a larger volume than PFPEmodified CNTs. 2.3. Composite materials preparation Pristine CNTs, two samples of PFPE-modified CNTs with different degrees of functionalization (CNTs-PFPE50 and CNTsPFPE100), bare CB and a PFPE-modified CB (CB-PFPE) (see Sections 2.1 and 2.2 for details) were considered as fillers for the preparation of composites. The UHMWPE powder and different loadings (i.e. 0.5, 1, 3, 5 wt.%) of fillers were mixed under magnetic stirring at room temperature until the formation of a homogeneous black powder. The resulting powders were then hot compacted at 200  C for 5 min under a pressure of 1500 psi to get thin films (thickness less than 100 mm) for the subsequent analyses. Pure UHMWPE was subjected to the same procedure for comparison.

2.2. Chemical treatment of carbon-based fillers with PFPE peroxide 2.4. Characterizations The functionalization of CNTs and CB by using PFPE peroxide was accurately studied and documented before by Sansotera et al. [18,19].

Rheological tests were performed by using a strain-controlled rotational rheometer (ARES G2 by TA Instruments) in parallel

Table 1 Surface composition (at%) measured by XPS, mass fraction due to linked PFPE measured by mass balances, BET surface area and static contact angle measurements with water of CNTs and CB fillers before and after treatment with PFPE peroxide. Sample

MWCNTs (CNTs) CNTs-PFPE50 CNTs-PFPE100 CB CB-PFPE a b

Amount [at%] F

O

C

S

Cl

e 4.2 6.5 e 12.2

1.3 2.4 3.1 1.2 5.0

98.7 93.4 90.5 98.4 82.4

e e e 0.4 0.4

e e e e e

Mass fraction due to linked PFPE obtained by mass balances. The droplet is not stable and is absorbed in few seconds into the bare CNTs or CB.

PFPE mass fractiona [wt.%]

Surface area [m2/g]

Static contact angle

e 12.5% 15.0% e 5.9%

389 308 280 262 107

n.s.b 159 168 n.s.b 174

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Fig. 1. Optical observations (80) of UHMWPE-based composites containing bare CNTs, CNTs-PFPE50 and CNTs-PFPE100 at different weight loadings.

Fig. 2. TEM observations of UHMWPE-based composites with CNTs and CNTs-PFPE50 (3 wt.%).

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plate geometry (plate diameter 25 mm). Linear stress relaxation measurements were carried out submitting the samples to a single step strain g0 ¼ 1%, and the shear stress evolution during time s(t) was measured to obtain the relaxation modulus G(t) ¼ s(t)/g0. Besides, complex viscosity (h*) and storage modulus (G0 ) were measured performing frequency sweep tests at T ¼ 200  C from 102e102 rad/s at a maximum strain of 5%. As proved by preliminary strain sweep experiments, such amplitude is low enough to be in the linear viscoelastic regime. Optical microscopy on the thin film surface has been carried out using Leica Microscope in reflection mode and magnification at 80. Transmission Electron Microscopy (TEM) observations were performed at Centro Grandi Apparecchiature e UninetLab, University of Palermo. The samples were mounted on the Lacey carbon films on 300 mesh copper grids and then observed by JEOL JEM2100 under accelerated voltage of 200 kV. The calorimetric data were evaluated by differential scanning calorimetry (DSC), using a PerkineElmer DSC7 calorimeter, at scanning rate of 10  C/min. Microanalysis of the nanocomposites surface was carried out by using an Environmental Scanning Electron Microscope (ESEM) Zeiss EVO 50 EP, equipped with energy-dispersive X-ray spectrometer (EDS) Oxford Inca Energy 200. Static contact angles were measured for all the samples with an FTA 1000 instrument using deionized water, in order to correlate the effect of PFPE-modified CNTs (CNT-PFPE50 and CNTs-PFPE100) and CB (CB-PFPE) incorporation into UHMWPE with the concomitant changes in their hydrophilic properties. 3. Results and discussion 3.1. Assessment of the properties of CNTs-containing composites 3.1.1. Morphology and rheological behaviour In Figs. 1 and 2 optical and TEM observations of UHMWPE composites containing bare CNTs and PFPE-modified CNTs are displayed. The processing conditions affect the composite morphology; as known the hot-compaction method, used for the composite formulation, leads to a segregated morphology formation consisting of CNTs-rich channels surrounding polymer-rich islands. Indeed, the CNTs cover the surface of the UHMWPE particles and, after hot pressing, the nanofillers remain in the interfacial region between the polymer particles. As displayed in Fig. 1, the composites with bare CNTs show really an undefined segregated morphology, since the CNTs are partially at boundaries between the polymeric islands. Differently, it seems that the presence of PFPE chains promotes the formation of nanofiller-rich channels within the polymeric phase and the morphology of UHMWPE-based composites with CNTs-PFPE50 and CNTs-PFPE100 appears very similar. TEM observations confirm the formation of nanofiller pathways around the polymeric phases ascribable to a beneficial effect of PFPE (Fig. 2), in agreement with results reported in literature for fluorinated CNTs [27,28]. Moreover, a confirmation of the achievement of the segregate morphology comes from EDS surface analysis of the composites. Indeed, the composition of surface of PFPE-functionalized CNTs containing composites appears inhomogeneous: the surface exhibits fluorine-rich regions, corresponding to CNTs channels, dispersed in the UHMWPE matrix. In particular, in the fluorinated islands and in the polymeric matrix, carbon and fluorine contents were quantified, and F/C ratios were calculated and discussed above. Rheology offers a useful mean to assess the level of nanofiller dispersion and distribution in polymer-based composites directly in the molten state. The rheological properties of particle-filled polymeric materials are sensitive to the structure, particle size,

Fig. 3. Linear stress relaxation moduli for UHMWPE-based composites containing (a) CNTs, (b) CNTs-PFPE50 and (c) CNTs-PFPE100 in comparison to unfilled UHMWPE at 200  C.

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Table 2 Low frequency slope values of G0 (u) modulus trends as a function of the nanofillers content. wt.%

UHMWPE/CNTs

UHMWPE/CNTs-PFPE50

UHMWPE/CNTs-PFPE100

0 0.5 1 3 5

1.98 1.72 1.63 0.71 0.55

1.98 0.60 0.50 0.48 0.38

1.98 0.57 0.40 0.37 0.35

shape and interface characteristics of the dispersed phase. First of all, transient stress relaxation measurements, obtained at low strain in order to assure linear viscoelastic regime, have been carried out and G(t) trends of all CNTs-containing composites in comparison to that of unfilled UHMWPE are reported in Fig. 3. As expected, the unfilled UHMWPE and CNTs-containing composites show different relaxation kinetics; particularly, the matrix relaxes more quickly than the composites in which longer times to recover their relaxed configuration after the strain imposition are required. Indeed, the modulus G(t) of the unfilled UHMWPE drop down ~3400 s after the imposition of strain, while the nanofilled samples behave as a pseudosolid-like material for times as long as ~6500 s. Although G(t) values for samples containing 0.5 and 1 wt.% CNTs are higher than that of the pristine UHMWPE (Fig. 3-a), these two composite samples relax like a liquid at higher times ~6500 and 7500 s, respectively. Differently, the composites containing 0.5 and 1 wt.% of both CNTs-PFPE50 and CNTs-PFPE100 behave as a pseudosolid-like material and show relaxation phenomena similar to those observed for composites containing large amount of nanofillers, i.e. at 3 and 5 wt.% (Fig. 3-b and 3-c). It is worth noticing that the composites carrying PFPE-modified CNTs at highest nanofiller content (i.e. 5 wt.%) show a lower G(t) than that containing bare CNTs, because of the lubricant action due to the large amount of perfluorinated alkyl chains. The linear viscoelastic behaviour of the composites detected through stress relaxation measurements can be understood considering the motion of two distinct populations of dynamic species within the composites: a matrix portion, which is unaffected by the presence of carbon nanotubes and its relaxation process is homopolymer-like; the second dynamic class, which behaviour is governed by nanofiller semi-3D network dynamics, is responsible for the pseudosolid-like features observed at long relaxation times. Similar hypothesis has been proposed by La Mantia et al. [29] in order to understand the linear viscoelastic behaviour of polyethylene-based clay-reinforced hybrid materials. Therefore, the presence of carbon nanotubes emerges when the homopolymer-like fraction relaxes faster than the pseudosolid-like one. These obtained results agree with the results coming from low amplitude dynamic frequency sweep tests measurements, carried out in order to evaluate the shear rheological behaviour of the composites. In Fig. 4 the storage modulus as a function of the frequency of bare and PFPE-functionalized CNTs containing UHMWPE composites in comparison to that of neat UHMWPE is reported. The last shows typical terminal behaviour of homopolymers, i.e. G0 a u2 in the low frequency region, indicating a liquid-like rheological behaviour. For the composites containing bare and PFPEfunctionalized CNTs, G0 tends to become nearly independent from the frequency as the nanofillers content increases, confirming the transition from liquid-like to solid-like viscoelastic behaviour. In particular, in Table 2, the calculated slopes of the G0 trends at low

Fig. 4. Storage modulus, G0 , of neat UHMWPE and UHMWPE-based composites containing (a) CNTs, (b) CNTs-PFPE50 and (c) CNTs-PFPE100 as a function of the frequency.

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frequencies are reported. Usually, suddenly decrease of the slope value of G0 suggests that the rheological percolation threshold is achieved, highlighting the formation of semi-3D CNTs network, accountable for the solid-like viscoelastic behaviour. For composites containing bare CNTs the percolated network formation seems occurs for nanofiller content >1 wt.%. For composites containing PFPE-functionalized CNTs, the low frequency slope of G0 approach to zero since 0.5 wt.% of both CNTs-PFPE50 and CNTs-PFPE100. The presence of PFPE alkyl chains on CNTs, hence, brings about the semi-3D CNTs network formation and the rheological behaviour modification at lower nanofillers content, decreasing the percolation threshold. Fig. 5 displays the viscosity values at three different frequencies, i.e. (a) u ¼ 0.01, (b) u ¼ 0.1 and (c) u ¼ 10 rad/s, versus nanofiller concentration for all investigated samples (full viscosity curves are available as Supplementary information, Fig. A). The three frequencies have been chosen considering that the low frequencies behaviour is sensitive to the nanofiller dispersion and distribution, while the high frequencies behaviour reveals the melt state dynamics of polymer chains. Consistently with the addition of solid particles into a polymeric matrix, as the pristine CNTs filling increase, the viscosity values at all investigated frequencies rise. The shear rheological behaviour of PFPE-functionalized CNTs containing composites is more complex. In particular, at low frequencies (u ¼ 0.01e0.1 rad/s), where the nanofiller distribution is determinant for melt behaviour, a significant increase of the viscosity values for PFPE-functionalized CNTs based composites up to 3 wt.% was recorded. These findings can be understood considering a beneficial effect of the surface perfluorinated chains on the nanofiller distribution. Besides, at low frequencies, both UHMWPEbased composites with CNTs-PFPE50 and CNTs-PFPE100 at 5 wt.% loading show a slight decrease of the viscosity values, although these values are higher than those of bare CNTs-containing composite (Fig. 5-a and 5-b). Moreover, at high frequency, i.e. at u ¼ 10 rad/s, both CNTs-PFPE50 and CNTs-PFPE100-containing composites, at all investigated loadings, show viscosity values

Fig. 5. Values of the complex viscosity of UHMWPE-based nanocomposites as a function of the CNTs contents at different frequencies: (a) 0.01 rad/s, (b) 0.1 rad/s and (c) 10 rad/s.

Fig. 6. Differential scanning calorimetry traces of second heating scan of pristine UHMWPE and UHMWPE-based CNTs-containing nanocomposites.

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Table 3 Melt temperature (Tm), crystallinity degree (cc) of second heating scan, contact angle measurement and EDS F/C ratio of pristine UHMWPE and MWCNTs-containing UHMWPEbased nanocomposites. Polymer

Filler

Filler content [wt.%]

Tm [ C]

cc a [%]

Static contact angle

F/C ratiob

UHMWPE

e CNTs CNTs-PFPE50 CNTs-PFPE100 CNTs CNTs-PFPE50 CNTs-PFPE100

e 3 3 3 5 5 5

129.0 129.1 129.1 129.3 129.0 129.1 129.0

71.8 57.5 55.8 55.9 51.3 51.7 50.1

81 87 91 95 93 102 109

e e 0.044 0.054 e 0.072 0.086

a b

Crystallinity degree calculated considering DH100% ¼ 200 J/g for 100% crystalline UHMWPE. Measured by EDS analysis.

lower than those of UHMWPE/CNTs composites (Fig. 5-c). The viscosity trends can be explained considering two different contrasting effects of PFPE-containing CNTs composites: first, the beneficial effect of PFPE chains on the nanotubes distribution;

second, the lubricant action of fluorinated PFPE chains linked to CNTs. Therefore, the well known lubricant action of PFPE chains governs the shear rheological behaviour of the composites at highest CNTs loading and, especially, it plays a determinant role at

Fig. 7. Optical observations (80) of UHMWPE-based nanocomposites containing native CB and CB-PFPE at different weight loadings.

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Fig. 8. TEM observations of UHMWPE/CB and UHMWPE/CB-PFPE samples.

Fig. 9. Linear stress relaxation moduli for (a) CB and (b) CB-PFPE-containing composites in comparison to unfilled UHMWPE at 200  C.

Fig. 10. Storage modulus, G0 , of neat UHMWPE and UHMWPE-based composites containing (a) CB and (b) CB-PFPE as a function of the frequencies.

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high frequency, where the melt state dynamics of polymer chains are probed. 3.1.2. Thermal properties, surface composition and wettability The effect of carbon nanotubes presence on the matrix crystalline degree has been evaluated through calorimetric analysis performed on the UHMWPE/CNTs composites. The second heating scans of all investigated samples are reported in Fig. 6. The melting temperatures of UHMWPE-based composites remained unchanged in comparison to that of pristine UHMWPE, while the estimated crystalline degree, based on measured melting enthalpy, significantly changes because of the indistinct presence of bare or PFPEmodified CNTs at high concentrations (Table 3). In particular, the addition of 3 and 5 wt.% of CNTs (indifferently bare or PFPEmodified CNTs) decreases UHMWPE crystalline degree of about 22% and 28%, respectively. As known, the nanofillers affect the crystallinity degree of the host matrix in two contrasting ways acting as nucleating agents, increasing the matrix crystalline degree, or acting as disturbance for macromolecular motions, decreasing the total crystalline degree [30,31]. The obtained results clearly suggested that the latter behaviour is predominant, as the presence of all kinds of the employed CNTs reduces the crystalline degree of UHMWPE. As discussed before, the surfaces of UHMWPE/CNTs composites were analyzed by means of EDS and resulted heterogeneous, presenting fluorinated islands. The calculated F/C ratios are reported in Table 3: in PFPE-modified CNTs aggregates F/C ratio ranges from 0.044 in UHMWPE with 3 wt.% of CNTs-PFPE50 to 0.086 in UHMWPE with 5 wt.% of CNTs-PFPE100; in UHMWPE matrix and UHMWPE/CNTs composites the F/C ratios are zero, confirming obviously the absence of fluorine. Besides, the wettability of UHMWPE/CNTs composites at high nanofiller concentration (i.e. 3 and 5 wt.%) has been evaluated and the measured contact angles are reported in Table 3. The filling of UHMWPE with bare CNTs kept almost unaltered the neutral behaviour of the composites towards water: contact angle values ranges from 81 of the unfilled polymer to 87 and 93 of UHMWPE with 3 and 5 wt.% CNTs, respectively. The presence of CNTs-PFPE50 and CNTs-PFPE100 considerably decreases the wettability of the composites until the appearance of an evident hydrophobic behaviour due to the hydrophobic nature of PFPE chains: water contact angle values varied from 91 of UHMWPE with 3 wt.% CNTs-PFPE50 to 109 with 5 wt.% CNTs-PFPE100. 3.2. Assessment of the properties of CB-containing composites 3.2.1. Morphology and rheological behaviour The optical and TEM observations at different filler loadings showed that the morphology of the UHMWPE composites containing CB and CB-PFPE appeared very similar (Figs. 7 and 8). The formation of segregated morphology for the spherical CB particles can be noticed in both CB- and CB-PFPE-filled composites; in addition, CB-rich channels were detected in all the samples containing CB and their formation seems to be not significantly

Table 4 Low frequency slope values of G0 (u) modulus trends as a function of the nanofillers content. wt.%

UHMWPE/CB

UHMWPE/CB-PFPE

0 0.5 1 3 5

1.98 1.21 1.08 0.91 0.87

1.98 1.21 0.95 0.88 0.85

Fig. 11. Values of the complex viscosity of UHMWPE-based composites as a function of the CB contents at different frequencies: (a) 0.01 rad/s, (b) 0.1 rad/s and (c) 10 rad/s.

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relaxation times, which results in well distinct pseudosolid-like behaviour. Further confirmation about pseudosolid-like behaviour for CB- and CB-PFPE containing samples comes from the analysis of the trends of G0 vs u, see Fig. 10. Indeed, the low frequency slope of G0 tends to become frequency-independent and in Table 4 the values are reported. Besides, the shear rheological behaviour of UHMWPE/CB and UHMWPE/CB-PFPE is very similar (Figs. 10 and 11, and Fig. B in Supplementary information). It can be noticed that the values of storage modulus and complex viscosity at highest filler content are lower than that of the samples containing 3 wt.% probably due to some nanofiller re-aggregation phenomena, although this decrease is less pronounced for CB-PFPE containing samples, due to the presence of perfluorinated chains that hinders the CB agglomerates formation.

Fig. 12. Differential scanning calorimetry traces of second heating scan of pristine UHMWPE and UHMWPE-based CB-containing composites.

affected by the PFPE presence onto outer CB surface. Moreover, at highest fillers content, especially in UHMWPE/CB sample, some reaggregation phenomena occurs. However, the effect of PFPE chains results more significant in UHMWPE composites containing PFPEmodified CNTs because PFPEs promoted the formation of fillerrich channels instead of an undefined segregated morphology due to bare CNTs, as discussed above. Concerning rheological analysis, G(t) trends of the CBcontaining composites in comparison to that of pristine UHMWPE as a function of time are reported in Fig. 9. Unfilled UHMWPE and CB-containing composites show different relaxation kinetics. In particular, the UHMWPE drop down ~3400 s after the imposition of strain, while both CB- and CB-PFPE-filled samples behave as a pseudosolid-like materials for times as long as ~7000 s. Furthermore, the values of modulus of UHMWPE/CB-PFPE samples are slightly higher than that of the UHMWPE/CB, suggesting a beneficial effect of the perfluorinated chains on the composites viscoelastic properties. The rheological behaviour of CB-containing UHMWPE composites is very similar to that discussed above for CNTs-containing composites. Even so, the stress relaxation phenomena of CB- and CB-PFPE-containing composites are mainly governed by the presence of dynamic species, having long

3.2.2. Thermal properties, surface composition and wettability The second heating scans coming from the thermal analysis of UHMWPE samples filled with CB and CB-PFPE in comparison to that of unfilled UHMWPE are reported in Fig. 12: the indistinct addition of CB or CB-PFPE does not affect the values of UHMWPE melting temperature but reduces the crystallinity degree of the matrix (see Table 5), suggesting that overall CB hinders the macromolecular motions and it does not exert any nucleating effect, similarly to CNTs. The surface composition of UHMWPE/CB composites was measured by EDS analysis and resulted rather homogeneous with aggregates of PFPE-modified CB dispersed in the UHMWPE matrix. Carbon and fluorine contents were measured on several spots and average F/C ratios were calculated (Table 5): F/C ratios varies from 0.029 in UHMWPE with 3 wt.% of CB-PFPE to 0.037 in UHMWPE with 5 wt.% of CB-PFPE. Thus, UHMWPE-based composites with CBPFPE showed a lower fluorine content than UHMWPE/CNTs composites with PFPE-modified nanofillers. The wettability of UHMWPE/CB composites was evaluated and the presence of CB increases the hydrophobicity of composites surface: the static contact angle passes from 81 on the unfilled UHMWPE to 92 and 105 on the composites with 3 and 5 wt.% of CB, respectively. The hydrophobic effect is exacerbated by the presence of PFPE onto CB surface: the static contact angle reaches 98 and 111 on the samples with 3 and 5 wt.% of CB-PFPE, respectively. 4. Conclusions The rheological and surface properties of composite materials based on UHMWPE and PFPE-modified CNTs and CB have been compared to that of the composites produced with native carbonaceous substrates. Overall rheological behaviour of UHMWPE/ CNTs-PFPE composites is significantly affected by the presence of the perfluorinated alkyl chains. Indeed, the PFPE functionalities linked onto the CNTs outer surface facilitate the semi-3D nanofillers

Table 5 Melt temperature (Tm), crystallinity degree (cc) of second heating scan, contact angle measurement and EDS F/C ratio of pristine UHMWPE and CB-containing UHMWPE-based composites. Polymer

Filler

Filler content [wt.%]

Tm [ C]

cc a [%]

Static contact angle

F/C ratiob

UHMWPE

e CB CB-PFPE CB CB-PFPE

e 3 3 5 5

129.0 129.7 129.7 129.0 129.0

71.8 55.0 54.6 50.7 50.2

81 92 98 105 111

e e 0.029 e 0.037

a b

Crystallinity degree calculated considering DH100% ¼ 200 J/g for 100% crystalline UHMWPE. Measured by EDS analysis.

N.T. Dintcheva et al. / Composites Part B 95 (2016) 29e39

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