Sonication-Induced Modification of Carbon Nanotubes

materials

Article Sonication-Induced Modiﬁcation of Carbon Nanotubes: Effect on the Rheological and Thermo-Oxidative Behaviour of Polymer-Based Nanocomposites

Rossella Arrigo 1,2,* ID , Rosalia Teresi 1, Cristian Gambarotti 3, Filippo Parisi 4 ID , Giuseppe Lazzara 4 ID and Nadka Tzankova Dintcheva 1

1 Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, dei Materiali, Università degli Studi di Palermo, Viale delle Scienze, Ed. 6, 90128 Palermo, Italy; [email protected] (R.T.); [email protected] (N.T.D.) 2 Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Viale T. Michel 5, 15121 Alessandria, Italy 3 Dipartimento di Chimica, Materiali ed Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano, Italy; [email protected] 4 Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Viale delle Scienze, Ed. 17, 90128 Palermo, Italy; ﬁ[email protected] (F.P.); [email protected] (G.L.) * Correspondence: [email protected]; Tel.: +39-013-122-9363

Received: 25 January 2018; Accepted: 1 March 2018; Published: 5 March 2018

Abstract: The aim of this work is the investigation of the effect of ultrasound treatment on the structural characteristics of carbon nanotubes (CNTs) and the consequent inﬂuence that the shortening induced by sonication exerts on the morphology, rheological behaviour and thermo-oxidative resistance of ultra-high molecular weight polyethylene (UHMWPE)-based nanocomposites. First, CNTs have been subjected to sonication for different time intervals and the performed spectroscopic and morphological analyses reveal that a dramatic decrease of the CNT’s original length occurs with increased sonication time. The reduction of the initial length of CNTs strongly affects the nanocomposite rheological behaviour, which progressively changes from solid-like to liquid-like as the CNT sonication time increases. The study of the thermo-oxidative behaviour of the investigated nanocomposites reveals that the CNT sonication has a detrimental effect on the thermo-oxidative stability of nanocomposites, especially for long exposure times. The worsening of the thermo-oxidative resistance of sonicated CNT-containing nanocomposites could be attributed to the lower thermal conductivity of low-aspect-ratio CNTs, which causes the increase of the local temperature at the polymer/nanoﬁllers interphase, with the consequent acceleration of the degradative phenomena.

Keywords: UHMWPE; CNTs; sonication; nanocomposites; rheology; thermo-oxidative stability

1. Introduction The outstanding electrical, magnetic and mechanical properties of CNTs have motivated a flurry of interest in exploiting their potential as nanofillers in polymer-based nanocomposites [1,2]. Indeed, several studies reported in the literature show that adding CNTs to a polymer matrix causes a reinforcement of mechanical properties [3] and an enhancement of thermal and electrical conductivity [4,5]. Furthermore, the introduction of CNTs in polymeric matrices can enlarge the polymer application field, since this strategy allows the formulation of technologically relevant

Materials 2018, 11, 383; doi:10.3390/ma11030383 www.mdpi.com/journal/materials Materials 2018, 11, 383 2 of 14 nanostructured materials suitable for advanced applications, such as energy storage [6], electronics [7], catalysis [8] and sensors [9]. The final properties of CNT-containing nanocomposites strongly depend on the extent of the CNT dispersion within the polymeric matrix and on the quality of the polymer/nanofiller interfacial region. Due to their high surface-to-volume ratio, CNTs tend to interact through Van der Waals forces, forming agglomerates and aggregates that compromise the obtainment of a homogeneous distribution of CNTs thorough the host matrix and, hence, a proper transfer of the CNT properties to the polymer [10]. To overcome this drawback, significant efforts have been devoted to the modification of the external surface of CNTs in order to enhance their ability to disperse in polymers or to form stable suspensions in both organic and inorganic solvents. The functionalization of CNT surfaces can be achieved by exploiting several methods, including covalent linkage [11] and physical absorption [12], and, besides ensuring improved dispersability of the CNTs in polymers, can provide them with new functionalities. The intrinsic properties of CNTs themselves—such as surface quality [13], waviness and, above all, their aspect ratio [14]—are also of paramount importance to obtain nanostructured polymer-based materials with enhanced final properties compared to the corresponding neat matrices. Indeed, the high aspect ratio of CNTs is associated with a high surface-to-volume ratio, that helps promote the CNT superior properties to the polymer matrix [15,16]. Therefore, the improvement of the final properties of CNT-containing nanocomposites is in part attributed to the nanoparticles’ high aspect ratio, and many studies have shown enhanced physical properties of polymer nanocomposites with increasing nanofiller aspect ratio [17,18]. Kim et al. studied the effect of CNT geometry and dimensions on the thermal conductivity of polymer-based nanocomposites, showing that an effective control of this property can be achieved through the adjustment of the nanoparticle length. The dependence of the thermal conductivity of CNTs based nanocomposites on the nanofiller aspect ratio can be explained considering that a higher aspect ratio favors the formation of a percolation network, allowing the conduction of phonons over a long distance, without any transition from particle to particle [19]. The aspect ratio of CNTs also plays a fundamental role in determining the percolation threshold in polymer-based nanocomposites, that is, the critical concentration of CNTs needed to form interconnected networks of nanofillers within the polymer matrix. When the CNT concentration reaches the percolation threshold, the nanocomposite electrical conductivity (electrical percolation threshold) or storage modulus (rheological percolation threshold) suddenly increases, due to the formation of a semi-3D network of nanofillers within the host polymer matrix [20,21]. As reported in the literature, an increase of the CNT aspect ratio reduces the critical concentration for the formation of a percolated network and nanoparticles with a very high aspect ratio can form interconnected networks at very low volume fractions [22]. Cipriano et al. [23] studied the effect of the CNT aspect ratio on the rheological behavior of polystyrene-based nanocomposites, showing that CNTs with a higher aspect ratio bring about a more pronounced enhancement of both the storage modulus and viscosity compared to that of nanocomposite-containing CNTs with a lower aspect ratio. Similar results have been found by Potschke et al. in polycarbonate/CNT nanocomposites for which, at a given CNT concentration, the complex viscosity progressively increases with an increase in the CNT aspect ratio [24]. Ultrasonic treatment is widely used to disperse CNTs in solution, to functionalize CNTs or to formulate nanocomposites and nanomaterials [25,26]. The effects of sonication on CNTs derive from the shear forces coming from cavitation phenomena that are able to break the CNT aggregates formed due to the Van der Waals interactions between the CNTs themselves [27]. However, the cavitation phenomena can cause the formation of structural defects on the CNT’s surface and, eventually, their breakage, with modification of initial CNT length and aspect ratio [28]. The aim of this work is to evaluate the effect of ultrasonic treatment on the CNT aspect ratio and how this issue affects the morphology and the rheological properties of polymer-based nanocomposites, as well as their thermo-oxidative resistance. Despite qualitative and quantitative studies on reducing CNT length by sonication being present in the scientific literature, in many cases this has not been Materials 2018, 11, 383 3 of 14 accompanied by investigating how the alteration of the CNT’s structure and aspect ratio influences the rheological behaviour and, above all, the thermo-oxidative stability of CNT-containing nanocomposites. Specifically, in this work, CNTs were subjected to a sonication treatment for different time intervals and the thus modified nanofillers were used to formulate ultra-high molecular weight polyethylene (UHMWPE)-based nanocomposites. The rheological behavior and the thermo-oxidative resistance of formulated nanocomposites have been deeply evaluated and discussed, considering the effect of the sonication time on the initial dimensions of CNTs.

2. Materials and Methods

2.1. Materials UHMWPE is a commercial grade product purchased by Sigma-Aldrich (Saint Louis, MO, USA) in the form of a white powder. Its main properties are: average molecular weight Mw = 3 ÷ 6 MDa, softening point T = 136 ◦C (Vicat, ASTM D 1525B), melting point Tm = 138 ◦C, degree of crystallinity about 48.1% (estimated through Differential Scanning Calorimetry measurement considering a heat of fusion for perfectly crystalline polyethylene ∆Hf = 288.84 J/g), and density $ = 0.94 g/mL at 25 ◦C. The average molecular weight was detected by means of intrinsic viscosity [η] measurements according to ASTM D4020-05 using a Lauda Pro-line PV15 viscometer. The UHMWPE was dissolved in decahydronaphtalene (reagent grade Sigma-Aldrich) at 150 ◦C and maintained under magnetic stirring for one hour. The viscosimetric average molecular weight, estimated from three independent measures of [η] using the Margolies equation, Mv = 5.37 × 104 [η]1.37, was Mv =∼ 4.9 × 106 g/mol. Multiwalled carbon nanotubes, CNTs, were prepared by the typical chemical vapor deposition (CVD) protocol, using ethylene as the carbon source [29]. The purification was performed with 50% aqueous sulfuric acid, obtaining carbon nanotubes with outer diameters ranging between 14 and 20 nm, inner diameters in the range of 2 to 5 nm, length 1 ÷ 10 µm, and purity >98 wt %. After the purification process, the content of carboxylic groups was estimated to be about 0.5% by means of XPS analysis and acid/base titration. All chemicals and reagents were used as received, without further purification.

2.2. Carbon Nanotubes (CNTs) Sonication Six identical samples of 0.2 g of CNTs were placed in 50 mL one-neck round bottom ﬂasks, then 20 mL of distilled water were added to each one. The ﬂasks were placed in an ultrasonic bath (water volume 2.5 L, power 260 W) taking care to have the water level higher than the level of the suspension. The six reaction mixtures were sonicated at room temperature for different time intervals, particularly, 15, 30, 60, 90 and 120 min. The ultrasound treated CNTs were labeled as CNT-X, where X is the sonication time.

2.3. Nanocomposite Preparation The UHMWPE powder and 1 wt % of untreated CNTs and sonicated CNTs were manually mixed at room temperature and the resulting powder was homogenized by thorough grinding in a porcelain mortar to a visually homogeneous state. Afterwards, the composite powder was kept under magnetic stirring for about 12 h, until the achievement of a homogeneous black powder. The blends were then hot compacted at 210 ◦C for 5 min and under a pressure of 1500 psi to get thin ﬁlms (thickness less than 100 µm) for the subsequent analyses. The neat UHMWPE was subjected to the same procedure for comparison.

2.4. Characterization To assess the surface quality of CNTs, micro-Raman spectroscopy was performed at room temperature through a Renishaw (Gloucestershire, UK) Invia Raman Microscope equipped with Materials 2018, 11, 383 4 of 14 a 532 nm Nd:YAG laser excitation and 100 mW power. Non-confocal measurements were carried out in the range 3200–100 cm−1 with a spectral resolution between 0.5 and 1 cm−1. Transmission electron microscopy (TEM) analyses were performed on a homogeneous dispersion of the CNTs in 2% aqueous sodium dodecyl sulfate (SDS, supplied by Sigma-Aldrich) using a Philips (Amsterdam, The Netherlands) CM 200 field emission gun microscope operating at an accelerating voltage of 200 kV, with the aim of inspecting the morphology of treated CNTs. A Gatan (Pleasanton, CA, USA) US 1000 CCD camera was used and 2048 × 2048 pixels images with 256 grey levels were recorded. For the specimen preparation, a few drops of the water solutions were deposited on 200 mesh lacey carbon-coated copper grid and air-dried for several hours before analysis. In order to evaluate the dimensions of the ultrasound-treated CNTs, dynamic light scattering measurements on the high diluted CNT dispersions (10−3 w/w %) were carried out using a Zetasizer NANO-ZS (Malvern Instruments, Malvern, UK). A 5 mM sodium dodecyl sulphate (SDS) aqueous solution was used to improve the CNTs’ colloidal stability. The field-time autocorrelation functions were analysed by inverse Laplace transformation (ILT) that provide the distribution of the decay rates (Γ) for the diffusive modes. For the translational motion, the collective diffusion coefficient (D) given by D = G/q2, where q is the scattering vector (4πnλ − 1 sin(θ/2); with n being the water refractive index, λ the wavelength (632.8 nm), and θ the scattering angle (173◦)). The distribution of the apparent hydrodynamic diameters (dH) was calculated by using the Stokes−Einstein equation. The rheological behavior of formulated nanocomposites was studied through rheological tests conducted by means of a strain-controlled rotational rheometer (ARES G2 by TA Instruments, New Castle, DE, USA) in parallel plate geometry (plate diameter 25 mm). The complex viscosity (η*) was measured performing frequency sweep tests at T = 210 ◦C from 10−2 to 102 rad/s at a maximum strain of 2%. As proved by preliminary strain sweep experiments, such an amplitude is low enough to be in the linear viscoelastic regime. Besides, linear stress relaxation measurements were carried out, submitting the samples to a single step strain γ0 = 1%, and the shear stress evolution during time σ(t) was measured to obtain the relaxation modulus G(t) = σ(t)/γ0. As regards the reproducibility of the obtained results, it was satisfactory (±5%). The nanocomposite morphology was inspected using optical microscopy (Leica (Torino, Italy) Microscope) in reflection mode at a magnification of 20×. Images were acquired on the surface of the nanocomposite films. A Fourier transform infrared spectrometer (FTIR) (Spectrum Two FTIR spectrometer, Perkin Elmer, Waltham, MA, USA) was used to record the infrared spectra. FT-IR analyses were carried out to infer the advance of the degradation phenomena on nanocomposite films. Specifically, the samples were ◦ first treated in an air oven at T = 120 C, which is a temperature lower than the Tm of the polymer but high enough to accelerate the degradation processes. Then, FT-IR spectra were collected performing 16 scans between 4000 and 500 cm−1 on samples subjected to thermo-oxidation for different exposure times. The carbonyl index (CI) was calculated as the ratio between the carbonyl absorption area (1850–1600 cm−1) and the area of a reference peak at about 2019 cm−1.

3. Results and Discussion

3.1. Assessment of the Effect of Ultrasonic Treatment on the CNT Dimensions First of all, to assess the effect of sonication on the morphology and dimensions of CNTs, preliminary analyses of untreated CNTs and CNTs subjected to the sonication treatment for different time intervals were carried out. More specifically, to evaluate the effect of the treatment on the CNTs’ defect content, Raman analyses were performed. Figure1a reports the spectra obtained for untreated CNTs and CNT-120. In the Raman spectra, two prominent peaks were detected: the D-band located at 1340 cm−1 and the G-band centered at about 1580 cm−1. The first is known as defect-induced disorder mode, and refers to the carbon atoms on the CNT surface having sp3 hybridization; the second is the so-called tangential mode and is related to the graphitic structure of the CNT surface [30]. In Figure1a, Materials 2018, 11, 383 5 of 14 each spectrum has been normalized such that the G-band intensity is constant. It is clearly visible that Materialsthe intensity 2018, 11 of, x the D-band is more pronounced for the CNTs subjected to the maximum sonication5 of 14 time than for the untreated CNTs, indicating damage to the CNT structure and the formation of lattice the formation of lattice defects due to the sonication treatment. The unusual high intensity of the D- defects due to the sonication treatment. The unusual high intensity of the D-band for the untreated band for the untreated CNTs can be explained considering that the purification process can cause the CNTs can be explained considering that the purification process can cause the modification of the modification of the CNT surface, with the anchoring of oxygen-containing groups. To qualitatively CNT surface, with the anchoring of oxygen-containing groups. To qualitatively evaluate the content of evaluate the content of structural defects, the ratio ID/IG between the intensities of the two Raman structural defects, the ratio ID/IG between the intensities of the two Raman bands has been calculated bands has been calculated for all CNTs and the obtained values are reported in the inset in Figure 1a. for all CNTs and the obtained values are reported in the inset in Figure1a. Indeed, it is known from Indeed, it is known from the literature that this ratio accounts for the concentration of defect site content the literature that this ratio accounts for the concentration of defect site content on CNT structures [31]; on CNT structures [31]; specifically, a higher value of the ratio means that more defects are formed. specifically, a higher value of the ratio means that more defects are formed.

1600

sonication ID/IG D band time [min] 00.96 Gband 1200 15 1.05 30 1.1 60 1.12 90 1.13 120 1.2 800 Intensity, cps

400

CNTs-0 CNTs-120 0 1200 1300 1400 1500 1600 1700 1800 Raman shift, cm-1 (a) (b)

Figure 1. (a) Raman spectra of untreated CNTs and CNTs subjected to sonication for 120 min (in the Figure 1. (a) Raman spectra of untreated CNTs and CNTs subjected to sonication for 120 min (in the inset the values of ID/IG ratio of all CNTs samples are listed); (b) representative TEM micrographs of inset the values of ID/IG ratio of all CNTs samples are listed); (b) representative TEM micrographs of investigatedinvestigated CNTs. CNTs.

The ID/IG ratio is 0.96 for the untreated CNTs progressively increased with the increasing sonicationThe I Dtime,/IG untilratio it is reaches 0.96 for a value the untreated of 1.20 for CNTs the CNT-120. progressively This feature increased suggests with that, the increasingdue to the sonicationsonication treatment, time, until progressive it reaches damage a value to of the 1.20 CN forTs’ the graphitic CNT-120. structure This featureoccurs, with suggests the formation that, due ofto thestructural sonication defects treatment, on the progressiveCNT surface, damage the co toncentration the CNTs’ of graphitic which structurerises with occurs, the increasing with the sonicationformation oftime. structural defects on the CNT surface, the concentration of which rises with the increasing sonicationTo qualitatively time. evaluate the morphological modifications of CNT structures associated with the defectTo formation qualitatively on sonication evaluate the treatment, morphological a visual modiﬁcations inspection using of CNT TEM structures was carried associated out. In with Figure the 1b,defect representative formation on TEM sonication micrographs treatment, of auntreate visual inspectiond CNTs and using CNTs TEM wassubjected carried to out. sonication In Figure for1b, differentrepresentative time TEMintervals micrographs are reported. of untreated For the CNTs sonicated and CNTs CNTs, subjected alterations to sonication of the for nanoparticles’ different time structuralintervals are integrity reported. can For be thenoticed, sonicated with CNTs, the appearance alterations ofof the waviness nanoparticles’ and presence structural of integrityamorphous can carbonbe noticed, on the with nanoparticle the appearance surface, of wavinesswhile untrea andted presence CNTs show of amorphous a well-ordered carbon graphitic on the nanoparticle structure. surface,Defect while formation untreated on CNTs showinduced a well-ordered by the sonication graphitic treatment structure. can lead to nanoparticle scission. To verifyDefect the formation effect of onthe CNTs sonication induced on by the the CN sonicationTs’ length, treatment the distribution can lead to of nanoparticle the hydrodynamic scission. diameterTo verify of the the effect untreated of the and sonication sonicated on theCNTs CNTs’ was length,evaluated the by distribution dynamic light of the scattering hydrodynamic (DLS) measurements.diameter of the The untreated average and apparent sonicated hydrodynamic CNTs was evaluated diameters by are dynamic related lightto the scattering characteristic (DLS) lengthsmeasurements. of the anisotropic The average particles, apparent as hydrodynamicwell as to the shell diameters hydration are related and the to aggregation the characteristic phenomena. lengths Itof is the known anisotropic from the particles, literature as that well the as toapparent the shell hydrodynamic hydration and diameter the aggregation for anisotropic phenomena. particles It is is aknown direct frommeasure the literature of the translational that the apparent diffusion hydrodynamic behaviour diameter of the scattering for anisotropic objects particles and it is acan direct be correlatedmeasure of to the the translational CNTs’ actual diffusion characteristic behaviour sizes of the[32–34]. scattering In particular, objects and for ita cancylinder, be correlated the value to theof the hydrodynamic diameter can be calculated by considering that [35] 2L Dh = (1) 2s0.198.24/s12/s2 where L is the cylinder length, and s = ln(L/r) where r is the radius of the cylinder.

Materials 2018, 11, 383 6 of 14

CNTs’ actual characteristic sizes [32–34]. In particular, for a cylinder, the value of the hydrodynamic diameter can be calculated by considering that [35]

2L D = (1) h 2s − 0.19 − 8.24/s + 12/s2 Materials 2018, 11, x 6 of 14 where L is the cylinder length, and s = ln(L/r) where r is the radius of the cylinder. InIn FigureFigure2 a,2a, the the obtained obtained distribution distribution functions functions of of the the average average hydrodynamic hydrodynamic diameters diameters are are reported,reported, for for untreated untreated CNTs CNTs and and for CNTsfor CNTs subjected subjec toted sonication to sonication treatment treatment for different for different time intervals. time Theintervals. depicted The curves depicted clearly curves show clearly that the show average that hydrodynamicthe average hydrodynamic diameter of the diameter CNTs progressively of the CNTs decreasesprogressively with decreases increasing with sonication increasing time. sonication Figure2b time. reports Figure theaverage 2b reports hydrodynamic the average hydrodynamic diameter as a functiondiameter of as sonication a function time. of sonication time.

30 150 CNTs-0

CNTs-15 120 CNTs-30 20 CNTs-60 90 CNTs-90

CNTs-120 60 Frequency, % Frequency, 10 Hydrodinamic diameter, nm diameter, Hydrodinamic 30

0 0 10 100 1000 030609012015 Hydrodinamic diameter, nm Sonication time, min (a) (b)

Figure 2. (a) Distribution functions of apparent hydrodynamic diameters for untreated and sonicated Figure 2. (a) Distribution functions of apparent hydrodynamic diameters for untreated and sonicated CNTsCNTs andand (b(b)) apparent apparent hydrodynamic hydrodynamic diameter diameter of of CNTs CNTs as as a a function function of of sonication sonication time. time.

Before the sonication, the untreated CNTs show a distribution of hydrodynamic diameters rangingBefore between the sonication, 100 and the250 untreated nm centered CNTs at show 150 nm, a distribution that is, in ofthe hydrodynamic range of the value diameters reported ranging for betweenacid-treated 100 andmulti 250 walled nm centered CNT stable at 150 nm,dispersions that is, in(150 the range± 40 nm) of the [36]. value The reported CNT lengths, for acid-treated ranging ± multibetween walled 0.2 and CNT 1 stableμm, can dispersions be calculated (150 from40 Equation nm) [36]. (1) The considering CNT lengths, 14 nm ranging as the between external 0.2 radius and µ 1(fromm, canthe beTEM calculated results) and from hydrodyn Equationamic (1) consideringdiameters reported 14 nm as above. the external radius (from the TEM results)After and 15 hydrodynamic min of sonication, diameters the diameter reported drasticall above. y decreases, becoming about half its original size,After i.e., 72 15 minnm, ofand sonication, it continues the diameter to progressiv drasticallyely decrease decreases, with becoming the sonication about half time. its original Then, size, for i.e.,sonication 72 nm, andtimes it of continues 90 and 120 to progressively min, the size decrease of the CNTs with is the no sonication longer altered time. Then,and the for value sonication of the timeshydrodynamic of 90 and 120diameter min, the remains size of theunchanged. CNTs is no It longer is worth altered noting and the that value the of final the hydrodynamic value of the diameterhydrodynamic remains diameter unchanged. reached It isat worth the maximum noting that sonica thetion final time value is about of the 33 hydrodynamic nm, indicating diameter that the reachedapplied atsonication the maximum treatment sonication has a dramatic time is abouteffect on 33 nm,the CNTs’ indicating length that and, the thus, applied on their sonication aspect treatmentratio. Based has on a Equation dramatic (1), effect it is on possible the CNTs’ to estimate length that and, the thus, final on length their aspectdecreases ratio. by a Based factor on of Equationseven, considering (1), it is possible r unaltered to estimate and that the the hydrodynamic final length decreases radius by before a factor and of seven, after considering120 min of r unalteredultrasonication and the time. hydrodynamic radius before and after 120 min of ultrasonication time. 3.2. UHMWPE/CNT Nanocomposites: Rheology, Morphology and Thermo-Oxidation Behaviour 3.2. UHMWPE/CNT Nanocomposites: Rheology, Morphology and Thermo-Oxidation Behaviour Untreated and sonicated CNTs have been used as nanofillers in UHMWPE-based nanocomposites Untreated and sonicated CNTs have been used as nanofillers in UHMWPE-based with the aim of evaluating how the sonication-induced modification of a CNT’s aspect ratio affects the nanocomposites with the aim of evaluating how the sonication-induced modification of a CNT’s morphology of formulated nanocomposites and their thermo-oxidative resistance. aspect ratio affects the morphology of formulated nanocomposites and their thermo-oxidative resistance. Rheological analyses are a powerful tool for investigating the microstructure of polymer-based nanocomposites, since the dispersed nanoparticles are able to alter the polymer chain dynamics. Herein, we performed linear viscoelastic measurements to infer the influence of the different aspect ratios of CNTs subjected to sonication treatment on the relaxation processes of UHMWPE macromolecules. In Figure 3a, the trend of complex viscosity as a function of frequency for neat UHMWPE and all CNT-containing nanocomposites is reported. The complex viscosity of the neat polymer matrix exhibits a very short Newtonian plateau, only at the lowest investigated frequencies,

Materials 2018, 11, 383 7 of 14

Rheological analyses are a powerful tool for investigating the microstructure of polymer-based nanocomposites, since the dispersed nanoparticles are able to alter the polymer chain dynamics. Herein, we performed linear viscoelastic measurements to infer the influence of the different aspect ratios of CNTs subjected to sonication treatment on the relaxation processes of UHMWPE macromolecules. In Figure3a, the trend of complex viscosity as a function of frequency for neat UHMWPE and all CNT-containing nanocomposites is reported. The complex viscosity of the neat polymer matrix exhibits a very short Newtonian plateau, only at the lowest investigated frequencies, followed by a Materials 2018, 11, x 7 of 14 distinct power-law trend for the whole investigated frequency range. Such markedly non-Newtonian behaviourfollowed can by be a distinct attributed power-law to the verytrend high for the molecular whole investigated weight of frequency the matrix, range. whose Such macromolecules markedly requirenon-Newtonian long relaxation behaviour times can because be attr ofibuted the large to the content very high of molecu entanglements.lar weight Theof the adding matrix, of whose untreated andmacromolecules sonicated CNTs require brings long about rela anxation increase times ofbecause the complex of the larg viscositye content values of entanglements. in the whole The tested frequencyadding range,of untreated but significantand sonicated differences CNTs brings emerge about an when increase comparing of the complex the various viscosity nanocomposite values in samples.the whole Untreated tested frequency CNTs have range, a more but significan markedt effectdifferences on the emerge complex when viscosity comparing curve, the various causing an nanocomposite samples. Untreated CNTs have a more marked effect on the complex viscosity curve, important increase of the η* values with respect to those of neat matrix and the disappearance of the causing an important increase of the η* values with respect to those of neat matrix and the Newtonian plateau at low frequencies. Nanocomposites containing sonicated CNTs show complex disappearance of the Newtonian plateau at low frequencies. Nanocomposites containing sonicated viscosityCNTs values show complex higher thanviscosity those values of neat higher matrix, than those but the of neat effect matrix, of the but nanofillers the effect of is the progressively nanofillers less pronouncedis progressively with increasing less pronounced sonication with time. increasing The obtained sonication result time. can The be obtained explained result considering can be that usuallyexplained a more considering pronounced that increase usually ofa more the complexpronounced viscosity increase indicates of the complex an increasing viscosity importance indicates an of the tube–tubeincreasing interactions importance in of dictating the tube–tube the rheological interactions behaviour in dictating [ 37the]. rheological behaviour [37].

1.E+06 1.E+08 Increasing Increasing sonication time sonication time UHMWPE 1.E+07 UHMWPE/CNTs-0 UHMWPE/CNTs-15 UHMWPE/CNTs-30 UHMWPE/CNTs-60 1.E+05 UHMWPE UHMWPE/CNTs-0 1.E+06 UHMWPE/CNTs-90 UHMWPE/CNTs-120 UHMWPE/CNTs-15 UHMWPE/CNTs-30 Sample Slope value UHMWPE/CNTs-60 UHMWPE 2.04 UHMWPE/CNTs-90 1.E+05 UHMWPE/CNTs-0 0.31 UHMWPE/CNTs-120

Storage Modulus, Pa Modulus, Storage 1.E+04 UHMWPE/CNTs-15 0.65

Complex viscosity, Pa*sec viscosity, Complex UHMWPE/CNTs-30 0.70 UHMWPE/CNTs-60 1.00 1.E+04 UHMWPE/CNTs-90 1.30

1.E+03 1.E+03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 Frequency, rad/sec Frequency, rad/sec (a) (b)

Figure 3. (a) Complex viscosity and (b) Storage modulus (in the inset the values of the low frequency Figure 3. (a) Complex viscosity and (b) Storage modulus (in the inset the values of the low frequency G0 G′ slopes are reported) as a function of frequency for neat UHMWPE and all CNT-containing slopes are reported) as a function of frequency for neat UHMWPE and all CNT-containing nanocomposites. nanocomposites.

In ourIn our case, case, the the high high aspect aspect ratioratio ofof untreateduntreated CNTs CNTs favors favors the the establishment establishment of intertube of intertube interactionsinteractions and and hence hence adding adding the the nanofillers nanofillers caus causeses an an alteration alteration of ofthe thepolymer polymer chain chain dynamics, dynamics, resultingresulting in rheological in rheological behaviour behaviour different different to that to that of neat of neat UHMWPE. UHMWPE. As theAs sonicationthe sonication time time increases, the CNTsincreases, aspect the ratioCNTs diminishesaspect ratio anddiminishes the nanofillers and the nanofillers are not able are not to formable to interconnected form interconnected structures structures that modify the macromolecular dynamics. As a result, the effect of adding the nanofillers that modify the macromolecular dynamics. As a result, the effect of adding the nanofillers on the on the complex viscosity curve is progressively less significant as the sonication time increases, and η complexthe η* viscosity trend of the curve nanocomposites is progressively becomes less similar significant to that asof theneat sonicationmatrix. Similar time conclusions increases, can and bethe * trenddrawn of the looking nanocomposites at the trends becomes of storage similar modulus to thatas a offunction neat matrix. of frequency Similar reported conclusions in Figure can 3b. be As drawn lookingfar as at neat the UHMWPE trends of storageis concerned, modulus at low asfrequenc a functionies neat of matrix frequency exhibits reported the typical in Figure homopolymer-3b. As far as neatlike UHMWPE terminal behaviour is concerned, with atscaling low frequenciesproperties of approximately neat matrix exhibits G′ α ω2, see the values typical listed homopolymer-like in the inset terminalin Figure behaviour 3b. The addition with scaling of untreated properties and sonicated of approximately CNTs causes G 0anα increase ω2, see of values the G′ listedvalues in in thethe inset in Figurewhole3 b.investigated The addition frequency of untreated range; moreover, and sonicated for all CNTsinvestigated causes nanocomposites, an increase of the terminal G 0 values in the wholebehaviour investigated tends to disappear frequency and range; the dependence moreover, forof G all′ on investigated frequency at nanocomposites, low frequencies becomes the terminal behaviourweaker. tends However, to disappear as already and noticed the dependencein the analysis of of G the0 on complex frequency viscosity at low curves, frequencies a significant becomes effect of the CNT sonication time on the nanocomposites’ rheological behaviour can be noticed. For weaker. However, as already noticed in the analysis of the complex viscosity curves, a significant effect the nanocomposite containing untreated CNTs, the low frequency storage modulus is almost independent of frequency, suggesting a transition from liquid-like to solid-like rheological behaviour for this sample. This non-terminal behaviour can be attributed to the arrangement of CNTs in interconnected structures with the formation of a semi-3D network of nanoparticles that restrains the polymer relaxation process. Nanocomposites containing CNTs treated for low sonication times, i.e., CNT-15 and CNT-30, show similar solid-like rheological behaviour, although the values of the low

Materials 2018, 11, 383 8 of 14 of the CNT sonication time on the nanocomposites’ rheological behaviour can be noticed. For the nanocomposite containing untreated CNTs, the low frequency storage modulus is almost independent of frequency, suggesting a transition from liquid-like to solid-like rheological behaviour for this sample. This non-terminal behaviour can be attributed to the arrangement of CNTs in interconnected structures with the formation of a semi-3D network of nanoparticles that restrains the polymer relaxation process. Nanocomposites containing CNTs treated for low sonication times, i.e., CNT-15 and CNT-30, showMaterials similar 2018 solid-like, 11, x rheological behaviour, although the values of the low frequency slope8 of 14of the storage modulus are higher than that of CNT-0 containing nanocomposite. Otherwise, the rheological behaviourfrequency of the slope nanocomposites of the storage modulus containing are CNTs higher treated than that for of long CNT-0 times containing became progressivelynanocomposite. similar to thatOtherwise, of neat matrixthe rheological with the behaviour increase of of the the nanoco sonicationmposites time, containing indicating CNTs that treated the lowfor long aspect times ratio of the sonicatedbecame progressively CNTs does similar not allow to that the of CNTs neat matrix to physically with the bridgeincrease and of the to sonication form the percolationtime, indicating network. that the low aspect ratio of the sonicated CNTs does not allow the CNTs to physically bridge and to To sum up, as the sonication time increases, the aspect ratio of the CNTs diminishes due form the percolation network. to shortening, and this issue implies that the sonicated CNTs are no longer able to hinder the To sum up, as the sonication time increases, the aspect ratio of the CNTs diminishes due to polymershortening, chain and relaxation this issue processes. implies that To the confirm sonicate thisd CNTs feature, are no stress longer relaxation able to hinder measurements the polymer were performedchain relaxation and the processes. obtained To results confirm are this reported feature, stress in Figure relaxation4, where measurements the relaxation were performed modulus of investigatedand the obtained nanocomposites results are is comparedreported in toFigure that of4, UHMWPE.where the relaxation The modulus modulus G(t) of of investigated CNT-containing nanocompositesnanocomposites is higher is compared than that to of that neat of matrix UHMWPE. and, similarly The modulus to what G(t) was observedof CNT-containing through linear viscoelasticnanocomposites measurements, is higher than the that enhancement of neat matrix of and, G(t) similarly is more to pronounced what was observed for untreated through linear CNTs and progressivelyviscoelastic diminishes measurements, with the increasing enhancement CNT of sonicationG(t) is more times. pronounced Furthermore, for untreated while CNTs at short and times the stressprogressively relaxation diminishes behaviour with is increasing quite similar CNT for soni nanocompositescation times. Furthermor and unfillede, while matrix, at short at longtimes times the stress relaxation behaviour is quite similar for nanocomposites and unfilled matrix, at long times different relaxation kinetics can be observed. In particular, neat UHMWPE and nanocomposites different relaxation kinetics can be observed. In particular, neat UHMWPE and nanocomposites containing CNTs sonicated for long times relax like a liquid, indicating that the presence of treated containing CNTs sonicated for long times relax like a liquid, indicating that the presence of treated CNT-60,CNT-60, CNT-90 CNT-90 and and CNT-120 CNT-120 does does not affectnot affect the chainthe chain relaxation relaxation spectrum spectrum of UHMWPE.of UHMWPE. In contrast,In addingcontrast, untreated adding and untreated CNTs and sonicated CNTs sonicated for short for periods short periods significantly significantly modified modified the the polymer polymer chains’ relaxationchains’ and relaxation the nanocomposites and the nanocomposites showed ashowed pseudo-solid-like a pseudo-solid-like behaviour. behaviour. Indeed, Indeed, the stress the stress relaxes to an equilibriumrelaxes to an value equilibrium rather value than torather zero than and to the ze occurrencero and the occurrence of this phenomenon of this phenomenon can be attributed can be to the formationattributed to of the a three-dimensional formation of a three-dimens superstructureional superstructure of CNTs within of CNTs UHMWPE. within UHMWPE.

1.E+06

1.E+05 Increasing sonication time 1.E+04

1.E+03 UHMWPE UHMWPE/CNTs-0 G(t), Pa UHMWPE/CNTs-15 1.E+02 UHMWPE/CNTs-30 UHMWPE/CNTs-60 1.E+01 UHMWPE/CNTs-90 UHMWPE/CNTs-120

1.E+00 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04

Time, sec Figure 4. Stress relaxation behaviour of neat UHMWPE and all CNT-containing nanocomposites. Figure 4. Stress relaxation behaviour of neat UHMWPE and all CNT-containing nanocomposites. In order to gain insight into the state of dispersion of untreated and sonicated CNTs within polymerIn order matrixes, to gain morphological insight into theobservations state of dispersionthrough optical of untreated microscopy and were sonicated carried out CNTs and in within polymerFigure matrixes, 5 representative morphological optical micrographs observations are throughreported optical(the embedded microscopy CNTs were appear carried as the outdark and in Figurephase).5 representative It is evident opticalfrom the micrographs optical observations are reported that (thethe distribution embedded of CNTs both appearuntreated as theand dark phase).sonicated It is evident CNTs fromis quite the uniform optical observationsand homogeneous, that the although distribution in the of nanocomposite both untreated containing and sonicated CNTsuntreated is quite CNTs, uniform some and agglomerates homogeneous, can be although observed. in The the sonication nanocomposite treatment, containing thus, is beneficial untreated in CNTs, enhancing the extent of CNT dispersion. According to the literature, the shortening of CNTs some agglomerates can be observed. The sonication treatment, thus, is beneﬁcial in enhancing the facilitates their dispersion within the polymer matrix due to the decreasing degree of physical interaction between the CNTs themselves. Indeed, as the length of the CNTs diminishes, the contact area between nanofillers becomes lower and decreases the number of physical entanglements between nanotubes [38,39].

Materials 2018, 11, 383 9 of 14 extent of CNT dispersion. According to the literature, the shortening of CNTs facilitates their dispersion within the polymer matrix due to the decreasing degree of physical interaction between the CNTs themselves. Indeed, as the length of the CNTs diminishes, the contact area between nanoﬁllers becomes lower and decreases the number of physical entanglements between nanotubes [38,39]. Materials 2018, 11, x 9 of 14

(a) (b)(c)

Figure 5. Representative optical micrographs of: (a) UHMWPE/CNT-0; (b) UHMWPE/CNT-60; Figure 5. Representative optical micrographs of: (a) UHMWPE/CNT-0; (b) UHMWPE/CNT-60; (c) UHMWPE/CNT-120. (c) UHMWPE/CNT-120. The effect of the sonication-induced shortening of CNTs and of the consequent modification of Thethe nanofiller effect of thedistribution sonication-induced on the thermo-oxidati shorteningve behaviour of CNTs of and UHMWPE of the consequent-based nanocomposites modification of the nanofillerwas investigated distribution through on the the analysis thermo-oxidative of the evolutio behaviourn time of FTIR of UHMWPE-based spectra collected on nanocomposites neat matrix was investigatedand nanocomposites through during the analysis an oxidative of the treatment evolution performed time of in FTIR an air spectra oven at collected 120 °C. In on Figure neat matrix6, the FTIR spectra for neat UHMWPE and all CNT-containing nanocomposites are shown. In and nanocomposites during an oxidative treatment performed in an air oven at 120 ◦C. In Figure6, particular, the progress of the degradative processes was followed through the monitoring of the the FTIR spectra for neat UHMWPE and all CNT-containing nanocomposites are shown. In particular, carbonyl index (CI). The CI refers to the area of the complex peak in the FTIR spectrum in the range the progress1850–1600 of cm the−1, degradative thus reflecting processes the formation was followedof carboxylic through acids, ketones, the monitoring esters and of lactones, the carbonyl which index −1 (CI). Theare the CI main refers products to the area of the of UHMWPE the complex thermo-oxida peak in thetion FTIR [40,41]. spectrum The calculated in the range CI values 1850–1600 for neat cm , thusmatrix reflecting and theall formulated formation nanocomposites of carboxylic acids, are repo ketones,rted in estersFigure and7, as lactones,a function which of the arethermo- the main productsoxidation of the time, UHMWPE along with thermo-oxidation the values of the induct [40,41ion]. Thetime, calculated defined as CIthe valuestime needed for neat for CI matrix reaches and all formulatedthe value nanocomposites of five [42]. are reported in Figure7, as a function of the thermo-oxidation time, along with the valuesLet us now of the consider induction the thermo-oxidative time, defined as beha theviour time of needed UHMWPE for CI and reaches nanocomposites the value at of short five [42]. Letoxidation us now times consider (i.e., until the thermo-oxidative72 h). Neat UHMWPE behaviour shows a of sudden UHMWPE increase and of nanocomposites carbonyl formation, at short indicating that the degradation of neat matrix begins in the early stage of the oxidative treatment. oxidation times (i.e., until 72 h). Neat UHMWPE shows a sudden increase of carbonyl formation, The nanocomposite containing untreated CNTs shows improved thermo-oxidative resistance, with indicating that the degradation of neat matrix begins in the early stage of the oxidative treatment. an increase of the induction time compared to that of neat matrix, and this result can be ascribed to The nanocompositethe well-documented containing radical scavenging untreated activity CNTs showsof CNTs. improved Indeed, it thermo-oxidativeis known from the literature resistance, that with an increaseCNTs, because of the inductionof their peculiar time electronic compared structure, to that ofare neat able matrix,to protect and polymers this result against can degradative be ascribed to the well-documentedphenomena, acting radical as antioxidants scavenging and activity interrupting of CNTs. the Indeed,chain propagation it is known during from thepolymer literature thatdegradation CNTs, because [43,44]. of As their far peculiaras the nanocomposites electronic structure, containing are sonicated able to CNTs protect are concerned, polymers the against degradativebuild-up phenomena,curves of CI reveal acting a marked as antioxidants influence of and the interruptingCNT sonication the time chain on the propagation oxidation rate; during polymerspecifically, degradation as a function [43,44]. of As the far sonication as the nanocomposites time, an increase containingof the stabilizing sonicated action CNTs of CNTs are can concerned, be the build-upobserved. curves According of CI to reveal the literature, a marked the influence radical ofscavenging the CNT activity sonication of CNTs time oncan the be oxidationamplified rate; through the increase in the number of structural defects on the CNT surface [45]. The last can be specifically, as a function of the sonication time, an increase of the stabilizing action of CNTs can be explained considering that the antioxidant feature of CNTs is attributed to the presence of acceptor- observed.like localized According states, to thewhich literature, results from the radicallattice def scavengingects on CNTs. activity As probed of CNTs through can be Raman amplified analysis, through the increasethe content in the of numberstructuralof defects structural progressively defects onincreases the CNT with surface the sonication [45]. The time; last therefore, can be explained the consideringamplified that antioxidant the antioxidant action of feature sonicated of CNTs CNTs is can attributed be attributed to the to presence their higher of acceptor-like amount of lattice localized states,defects which compared results from to untreated lattice defectsnanotubes. on CNTs.After 72 As h probedof thermo-oxidative through Raman treatment, analysis, an unexpected the content of structuralinversion defects of the progressively CI build-up increases curves occurs. with theThe sonication slope of the time; CI therefore,curves for theneat amplified UHMWPE antioxidant and actionnanocomposite-containing of sonicated CNTs can CNT-0 be attributed decreases, to indicating their higher a reduction amount of the lattice rate defects of formation compared of to untreatedoxidized nanotubes. species. In After contrast, 72 h ofthe thermo-oxidative nanocomposites containing treatment, sonicated an unexpected CNTs exhibit inversion a faster of the CI build-updegradation curves occurs.kinetic and The the slope growth of the of CIthe curves CI values for neatincreases UHMWPE as a function and nanocomposite-containing of the CNT sonication time. The obtained results could be explained considering that, as reported in the literature, the CNT-0 decreases, indicating a reduction of the rate of formation of oxidized species. In contrast, stabilizing action of CNTs can be compromised if local increases of the temperature at the the nanocompositespolymer/nanoparticle containing interface sonicated occur [46]. CNTs In our exhibit case, the a oxidation faster degradation treatment is kinetic carried-out and at the high growth of thetemperature CI values (i.e., increases 120 °C); as therefore, a function after ofa certai then CNT period sonication of exposure, time. nanocomposites The obtained are no results longer could be explainedable to dissipate considering the thermal that, energy as reported and the in local the te literature,mperature thestarts stabilizing to increase, action causing of in CNTs turn an can be acceleration of degradative phenomena.

Materials 2018, 11, 383 10 of 14 compromised if local increases of the temperature at the polymer/nanoparticle interface occur [46]. In our case, the oxidation treatment is carried-out at high temperature (i.e., 120 ◦C); therefore, after a certain period of exposure, nanocomposites are no longer able to dissipate the thermal energy and the local temperatureMaterials 2018, 11 starts, x to increase, causing in turn an acceleration of degradative phenomena.10 of 14

UHMWPE/CNTs-15 Absorbance, a.u. Absorbance,

120h

0h 1800 1750 1700 1650

Figure 6. FTIR spectra collected during thermo-oxidative treatment for neat UHMWPE and CNT- Figure 6. FTIR spectra collected during thermo-oxidative treatment for neat UHMWPE and containing nanocomposites. CNT-containing nanocomposites. The worsening of the thermo-oxidative resistance of the nanocomposites as a function of the CNT sonication time could be ascribed to the lower thermal conductivity of shorter CNTs with

Materials 2018, 11, x 11 of 14

respect to that of long nanotubes. Indeed, it is well documented in the literature that CNTs with higher aspect ratios can provide a larger enhancement of nanocomposite thermal conductivity than CNTs with a lower aspect ratio [19,47]. The last because long CNTs are able to form a percolation network within the polymer matrix in a more efficient way than short CNTs, as already demonstrated Materialsthrough2018, 11rheological, 383 analyses. 11 of 14

UHMWPE UHMWPE/CNTs-0 100 UHMWPE/CNTs-15 UHMWPE/CNTs-30 UHMWPE/CNTs-60 UHMWPE/CNTs-90 75 UHMWPE/CNTs-120 Sample Induction time UHMWPE 25.5 UHMWPE/CNTs-0 34.5 50 UHMWPE/CNTs-15 36

Carbonyl Index Carbonyl UHMWPE/CNTs-30 39 UHMWPE/CNTs-60 46 UHMWPE/CNTs-90 49 25 UHMWPE/CNTs-120 50

5 0 0 24487296120 Thermo-oxidation time, h

FigureFigure 7. 7.Carbonyl Carbonyl index as as a afunction function of thermo-oxidation of thermo-oxidation time for time neat forUHMWPE neat UHMWPE and CNT- and containing nanocomposites (in the inset the values of the induction time are listed). CNT-containing nanocomposites (in the inset the values of the induction time are listed). 4. Conclusions The worsening of the thermo-oxidative resistance of the nanocomposites as a function of the CNT Nanocomposites based on UHMWPE and CNTs subjected to a sonication treatment for different sonicationtime intervals time could were beproduced ascribed with to the the aim lower of inve thermalstigating conductivity the influence of of shorter the sonication-induced CNTs with respect to thatshortening of long of nanotubes. the CNTs on Indeed, their rheological it is well behaviour documented and thermo-oxidative in the literature stability. that CNTs The effective with higher aspectreduction ratioscan of the provide length a largerof the enhancementoriginal CNTs ofwas nanocomposite probed through thermal accurate conductivity spectroscopic than and CNTs withmorphological a lower aspect analyses. ratio [19 The,47 ].study The of last the because nanocomp longosites’ CNTs rheological are able behaviour to form a revealed percolation that networkthe withinsonication the polymer treatment matrix strongly in a more affected efficient the way rheological than short CNTs,behaviour as already of the demonstrated CNT-containing through rheologicalnanocomposites, analyses. since the reduction of the nanofiller aspect ratio counteracts the formation of a percolation network within a polymer matrix; as a result, the rheological behaviour of 4. Conclusionsnanocomposites containing CNTs sonicated for long times is quite similar to that of neat matrix. The reduction of the aspect ratio in sonicated CNTs also dramatically affects the thermo-oxidative resistanceNanocomposites of UHMWPE-based based on UHMWPE nanocomposites and CNTsand results subjected in a progressive to a sonication deterioration treatment of the for long- different timeterm intervals stability were of producednanocomposites with thewith aim increasing of investigating sonication the times. influence The obtained of the sonication-induced results can be shorteningexplained of theinvoking CNTs the on low their thermal rheological conductivity behaviour of nanocomposites and thermo-oxidative containing short stability. CNTs, The that effective do reductionnot allows of the the lengthdissipation of theof accumulated original CNTs thermal was energy, probed with through a consequent accurate increase spectroscopic of the local and morphologicaltemperature analyses.and, thus, of the The oxidation study kinetics. of the nanocomposites’ rheological behaviour revealed that the sonication treatment strongly affected the rheological behaviour of the CNT-containing Author Contributions: All authors contributed equally to this work nanocomposites, since the reduction of the nanofiller aspect ratio counteracts the formation of a percolationConflicts network of Interest: within The authors a polymer declare matrix; no conflict as of a result,interest. the rheological behaviour of nanocomposites containing CNTs sonicated for long times is quite similar to that of neat matrix. The reduction References of the aspect ratio in sonicated CNTs also dramatically affects the thermo-oxidative resistance of UHMWPE-based1. Coleman, J.N.; nanocomposites Khan, U.; Blau, W.J.; and Gun’ko, results Y.K. in a Small progressive but strong: deterioration A review of the of mechanical the long-term properties stability of carbon nanotube-polymer composites. Carbon 2006, 44, 1624–1652, doi:10.1016/j.carbon.2006.02.038. of nanocomposites with increasing sonication times. The obtained results can be explained invoking the 2. Kaseem, M.; Hamad, K.; Deri, F.; Gun Ko, Y. A review on recent researches on polylactic acid/carbon low thermalnanotube conductivity composites. of Polym. nanocomposites Bull. 2017, 74, containing2921–2937, doi:10.1007/s00289-016-1861-6. short CNTs, that do not allows the dissipation of accumulated thermal energy, with a consequent increase of the local temperature and, thus, of the oxidation kinetics.

Author Contributions: All authors contributed equally to this work Conﬂicts of Interest: The authors declare no conﬂict of interest.

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