Low-temperature plasma modification of carbon nanofillers for improved performance of advanced rubber composites
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In modern polymer industry, there still is a room for new generations of fillers capable of enhancing the performance of composite materials. Currently, much effort is being put into a process of improving mechanical properties of elastomer materials. In this work, multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GnPs) were modified with silane, titanate, or zirconate using plasma treatment, in order to apply them as fillers for styrene/butadiene rubber. Following its modification, filler surface was analyzed: Surface free energy (SFE) was measured with tensiometry, and micromorphology and chemical composition were studied with scanning electron microscopy–energy-dispersive spectroscopy (SEM–EDS), while elemental composition and bonding were assessed with X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Low-temperature oxygen plasma activation of MWCNT fillers leads to a significant increase in the SFE polar component, with the same effect being much weaker for GnP fillers. Grafting silanes, zirconates, and titanates on activated filler surface results in a decrease in SFE polar component—surface oxygen-containing active groups react with silane/zirconate/titanate molecules. Fillers modified in this way exhibit different micromorphology and surface chemical composition what is revealed with the SEM–EDS, ToF-SIMS, and XPS techniques. As the ultimate step, either MWCNT or GnP rubber nanocomposites were manufactured using the modified fillers with their mechanical properties and cross-link density being studied. Filler modification resulted in substantial changes both in composite performance, and in its cross-linking density. In the case of modified filler containing composites, improved tensile strength and elongation at break were observed.
KeywordsNanocomposites Polymer Carbon nanotubes Plasma modification Graphene nanoplatelets Surface modification Rubber
Although the filler most commonly used in rubber technology is carbon black, in this dynamically developing industry, there is still a lot of room for new generations of carbon fillers that can improve performance or introduce novel functions to rubber vulcanizates. Due to their extraordinary properties and thanks to current prices that are much more acceptable than those of the last decade, such advanced powder fillers as carbon nanotubes (CNT) or graphene nanoplatelets (GnP) are more and more often applied in the rubber industry. Since their discovery, these forms of carbon have focused scientific interest of research workers representing different fields . Due to their unique mechanical, electrical, and thermal properties, CNTs and GnPs find themselves among the most promising candidates for fillers used in advanced rubber composites . Unfortunately, as it has been quickly exposed, there is a major problem with their satisfactory dispersion in a rubber matrix.
A manufacture of high-quality rubber mixes requires certain degree of compatibility between the polymer matrix and the filler. It can only be achieved when, during the mixing process, filler particles remain in effective phase contact with rubber macromolecules. On the other hand, in order to improve a strengthening effect of the filler, a formation of large secondary filler structures should be avoided [3, 4]—especially when nanoparticles such as CNTs or GnPs are considered.
Modern polymer composite materials must be characterized with high performance, particularly in such demanding applications as aerospace or surface transportation (aircraft fuselages, vehicle tires). In order to meet these requirements, chemical industry keeps experimenting with ever more novel filler materials, capable of improving properties of the composites. There has been a real revolution in the polymer industry filler market taking place for the last two decades which, to a large extent, is due to an introduction of the so-called coupling agents. They are chemicals capable of compatibilizing surfaces of two phases of interest, namely filler surface and that of the polymer matrix, often substantially differing with their physical and chemical properties. The first coupling agents to be introduced in a mass production about 50 years ago were organosilane connections used to modify fiberglass. Approximately 20 years ago, organosilanes were first applied in the production of polymer composites filled with mineral fillers, such as kaolin, wollastonite, or mica. In this way, materials of an increased rigidity and improved dynamic properties were obtained. The aim of functionalized silane is an addition of its molecules to surface hydroxyl groups as a consequence of their hydrolysis. In this way, the organic part of silane is claimed to be exposed to the contact with polymer matrix, therefore improving phase compatibility, filler dispersion in the matrix and, consequently, properties of the composite.
A modern group of coupling agents is comprised of titanates and zirconates. They exhibit numerous advantages over silanes—they are more stable, contain more functional groups able to react with polymer matrix, and may be used practically with any type of a filler [5, 6]. Today, rubbers and composite materials manufactured with the use of titanate and zirconate coupling agents constitute a group of irreplaceable construction materials, mainly due to their excellent dynamic properties and ability to accept cyclic nondestructive deformations of high magnitudes. Phase boundary interactions between a polymer matrix and a filler have been constituting a subject of scientific interest for several decades. Nevertheless, it was not until 20 years ago that advanced surface modification methods appeared in the subject literature [7, 8]. The majority of reports concerned “wet” chemical modification of the fillers or other components of a composite. As far as such environment-friendly (practically producing no waste) methods as their low-temperature plasma treatment are concerned, there are hardly any publications found in the literature [9, 10].
To meet modern polymer industry requirements, surface free energy of the filler particles needs to be modified. Energetic compatibility of the rubber–filler system is a key to the success—too high SFE of the filler can adversely affect a distribution and intensify an agglomeration of its particles. In the case of carbon fillers, a potential to modify the SFE dispersive part is limited, so changes of rubber–filler interaction can rather be achieved by an adjustment of the polar component. One of the most often used methods of an activation of a carbon filler surface is comprised of its chemical solvent treatment. It generates covalently bonded functional groups . As an effect of the treatment, the number of structural defects on the filler surface (e.g., CNTs sidewalls) increases, and various oxygen-containing functional groups are formed. The process is effective, but has a few serious drawbacks with the most severe being an application of large amounts of environmentally hazardous chemicals (usually a mix of sulfuric, nitric, and hydrochloric acid). Apart from oxidation, also halogenation, amination, alkylation, and an introduction of phenolic fragments to filler structure are being examined as wet modification techniques [12, 13, 14].
There are a number of reports in the literature dealing with a modification of multiwall carbon nanotubes (MWCNTs) for their application in nonpolar polymer matrix based composite materials [15, 16]. The composites prepared in this way are characterized with extended strength, higher rigidity and often modified electrical properties . Today, more and more frequent attempts to modify filler surface with alternative methods are being made. One such method is low-temperature plasma treatment. Since it is a gas-phase technique, it has no drawbacks characteristic for the solvent-based methods. An application of plasma techniques is particularly promising from the point of view of a modern polymer technology, especially due to environment protection issues—plasma treatment does not generate waste, and it is fast and economically effective.
Low-temperature plasma can be generated with an electrical discharge taking place in a low-pressure reactor chamber. The discharge is maintained in the presence of a gas, such as Ar, O2, H2, N2, acetylene, methane, or simply air. Depending on the type of the gas used and the process operational parameters, plasma treatment can be applied either for cleaning the material surface (so-called micro-sandblasting), for grafting functional groups via reaction of the surface with ionized gas particles, or for thin-film formation on that surface. For different materials and purposes, a variety of generator frequencies can be applied—beginning from audio frequency up to microwave range.
- Stage 1
Plasma irradiation, expansion of amorphous carbon layer, and beginning of its elimination occur, and oxygen concentration increases.
- Stage 2
Ion bombardment causes creation of vacancies and interstitials (sp3 structure is much weaker than sp2), and oxygen concentration increases.
- Stage 3
Ion bombardment continues until amorphous layer is totally peeled from carbon nanotubes, and oxygen concentration increases.
- Stage 4
Amorphous carbon is oxidized and ultimately eliminated when material is treated for a long enough period of time.
Our previous studies have shown that in the case of carbon nanotubes low-temperature plasma treatment introduces changes to surface free energy and its components . In the past years, several attempts were made to adapt low-temperature plasma to the modification of filler surface [20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35]. However, there is general lack of work reporting an application of this technique to plasma grafting of coupling agents on the surface of MWCNT filler. The present work reports results of low-temperature plasma modification of two types of carbon filler, namely multiwall carbon nanotubes (MWCNTs) and graphene nanoplatelets (GnPs), that are later used to manufacture a styrene/butadiene rubber vulcanizate. The modification process is performed in two stages: First, filler surface activation is carried out in oxygen plasma, and it is then followed by grafting silane, titanate, and zirconate functions on that surface. This approach is powered by new possibilities and perspectives for applications of CNT containing composites, among others in the modern tire industry.
KER 1500 styrene–butadiene rubber manufactured by Synthos, Poland, was used as rubber matrix.
- When it comes to fillers, the following carbon forms were applied:
either multiwalled carbon nanotubes (MWCNT) of 98% purity made by Cheaptubes (USA),
or graphene nanoplatellets (GnP) Grade 3.95% purity manufactured by Cheaptubes (USA).
Low-temperature plasma treatment
An activation of the filler surface was conducted in oxygen plasma, with process operational parameters being discharge power and its duration time. Chamber pressure amounted to 30 Pa and the process gas flow rate was maintained at the level of 30 sccm. Following oxygen plasma activation of the fillers (8–48 min), one of the silane/titanate/zirconate surface modifiers was introduced (in a form of vapor) into a reactor chamber for 30 min with its flow rate remaining at 20 sccm.
Preparation of rubber mixes
A composition of the rubber mixes studied
An assessment of surface free energy
Changes of the surface free energy (SFE), as well as those of its polar and dispersive components were measured with the help of a KRÜSS GmbH (Germany), model K100 MKII tensiometer. The values of contact angle were determined using both polar (water, methanol, ethanol) and nonpolar (n-hexane, n-heptane) liquids. As the final step of the assessment, magnitudes of SFE and its components were calculated by applying the Owens–Wendt–Rabel–Kaeble formalism .
The parameters of vulcanization such as maximum (Mmax) and minimum (Mmin) torque, a difference between minimum and maximum torque (ΔM), and vulcanization time (t90) were determined with the help of a Monsanto (USA) model R100 rheometer, at the temperature of 160 °C, in accordance with the ISO 3417 standard.
Mechanical properties of rubber vulcanizates
Mechanical properties of the vulcanizates were determined using a Zwick 1435 (Germany) universal mechanical testing machine. The tests were carried out on “dumbbell”-shaped 2-mm-thick and 4-mm-wide specimens, in accordance with the ISO 37 standard. In particular, elongation at break (Eb), stress at 100% elongation (M100), and tensile strength (TS) were determined.
Cross-link density of rubber vulcanizates
Micromechanical properties (nanoindentation)
The micromechanical properties of rubber vulcanizates were determined with the help of Micromaterials Ltd. (UK), model NanoTest600 apparatus equipped with a Berkovich indenter. The following were the measurement parameters: max depth of 3500 nm, unloading speed dP/dT = 0.05 mN/s. Measurements were carried out under controlled atmosphere with its temperature equal 25 ± 2 °C and relative humidity of 60 ± 5%. The calculation procedure was based on the Olivier and Pharr method .
Time-of-flight secondary ion mass spectrometry (SIMS)
ToF-SIMS measurements were performed using an ION-TOF GmbH (Germany) model TOF-SIMS IV instrument equipped with a 25-kV pulsed Bi+ primary ion gun operating in a static mode. The filler samples were pressed into the indium plate and attached to the sample holder using a double-sided tape. For each sample, three spectra collected at various locations were recorded. The analyzed area corresponded to a square of 500 μm × 500 μm size. A pulsed electron flood gun was used for the charge compensation.
Scanning electron microscopy (SEM)
Filler microstructure in a rubber matrix was studied with a Zeiss (Germany) model Auriga scanning electron microscope. A secondary electron signal was analyzed, as well as signals from energy-selective backscattered detector (ESB) and energy-dispersive detector (EDS). Accelerating voltage applied was 10 keV.
Filler dispersion in rubber composite
An analysis of filler dispersion in the rubber composite was performed with the Montech (USA) model DisperTester 3000 apparatus working in accordance with the ISO 11345:2006 standard. The instrument’s precision telecentric optical system makes use of reflected light for obtaining high-resolution reflective images of the sample surface. Data processing was carried out with the help of MonDispersion software, delivered with the instrument. Magnification applied was × 1000, with all the images referring to an area of 250 × 150 µm.
X-ray photoelectron spectroscopy (XPS)
The XPS measurements were carried out with the help of a Kratos AXIS Ultra XPS spectrometer using monochromatic Al Kα X-rays source of an excitation energy of 1486.6 eV. The spectra were obtained from an area of 300 µm × 700 µm. The power of anode was set at 150 W, and the hemispherical electron energy analyzer was operated at a pass energy 20 eV for all the high-resolution measurements. All measurements were performed with the use of a charge neutralizer, and every spectrum recording was repeated 10 times in order to increase its signal-to-noise ratio. Evaluation of XPS data was conducted using a Kratos Vision 2 software.
Results and discussion
Plasma activation of filler surface
Both types of fillers, namely multiwalled carbon nanotubes and graphene nanoplatellets, were subjected to oxygen plasma activation of their surface. The activation efficiency was then assessed with the measurements of surface free energy as well as both its components: dispersive and polar. As far as filler containing vulcanizates are concerned, mechanical properties were used as a measure of their performance.
Mechanical properties of rubber composites containing activated carbon fillers
Micromechanical properties of the rubber vulcanizates containing oxygen plasma-activated carbon fillers
Plastic deformation [μm]
2.6 ± 0.1
2.9 ± 0.3
3.1 ± 0.1
2.6 ± 0.2
4.5 ± 0.1
2.2 ± 0.2
3.2 ± 0.1
2.6 ± 0.4
4.1 ± 0.1
2.3 ± 0.2
2.3 ± 0.1
3.1 ± 0.5
3.1 ± 0.1
2.6 ± 0.2
5.7 ± 0.1
1.9 ± 0.1
An important factor affecting mechanical properties of the composites appears to be filler microstructure, which changes for oxidized MWCNTs—expanded material is less bundled , and it can be incorporated more uniformly into the rubber matrix. Apparently, an energetic effect may also play a significant role—if modified filler exhibits SFE values higher than or close to that of polymer matrix (29–33 mJ/m2 for SBR), than the expected interfacial interactions should be more intensive.
Activated filler modification with silane derivatives
For further studies, 48-min GnP and 32-min MWCNT oxygen plasma-activated samples were selected, as materials characterized by the highest amounts of surface active centers and oxygen-containing groups. The second step of a filler modification was comprised of the exposure of its activated surface to the discharge maintained in the vapors of one of the two different silane derivatives, either vinyltrimethoxysilane or mercaptotrimethoxysilane. It was expected that the hydroxyl groups formed after silane hydrolysis would react with active polar groups present on filler’s surface, thus exposing vinyl (VTMS) or thiol (MTMS) functions to interactions with the rubber macromolecule. These interactions, together with material surface cleaning effect, were anticipated to increase the compatibility of filler–matrix system and additionally to be involved in the cross-linking process.
In the above figure, one can observe the mass contrast (Fig. 12b, d) between carbon filler and silicon originating from VTMS-grafted moieties, with bright areas indicating the presence of Si atoms. It is clear from the figure that silicon is not present in the reference sample (Fig. 12b), while there is a lot of signal originating from this element in the case of VTMS-grafted MWCNTs (Fig. 12d). This finding confirms the effectiveness of vinyltrimethoxysilane grafting on MWCNT surfaces. In Fig. 12e, f, one can see images of a single nanotube with a VTMS particle grafted.
XPS data of surface elemental composition of nanotube samples
Type of modifier
Atomic composition of the films [at.%]
83.8 ± 1.60
16.2 ± 0.6
60.1 ± 1.09
29.3 ± 0.1
10.6 ± 0.1
In the sample modified with VTMS, a surface content of silicon amounts to ca. 10.6 at.%. As seen in Table 3, non-modified nanotubes contain substantially lower amount of oxygen on their surface than the surface concentration of this element recorded for the filler modified with vinyltrimetoxysilane and amounting to 29.3 at.%. This result is also higher than oxygen content in a VTMS molecule, which is a consequence of high silicon affinity toward oxygen on the one hand, and a substantial fragmentation of vinyltrimetoxysilane under plasma conditions, on the other.
Contribution of particular components of the O 1s, C 1s and Si 2p core-level XPS spectra of plain and modified CNT
Modified with VTMS
Contribution of particular components [%]
Contribution of particular components [%]
The highest intensity is shown by the maximum at 284.6 eV corresponding to C=C (sp2) bonds. The maximum at 285.4 eV represents C–C (sp3) and/or C–H bond. A contribution of the sp2 phase is 4/5 times larger than that of the sp3 fraction, with this proportion being characteristic for carbon nanotubes. The nanotube surface also contains oxygen atoms present either in a form of C–O bonds corresponding to the 286.4 eV band or in carbonyl moieties recorded at the binding energy of 287.9 eV. The lowest intensity band at 290.0 eV is attributed to the presence of carboxyl groups. The presence of oxygen-containing functions is confirmed by the O 1s core-level spectrum which contains a single band at 532.7 eV that corresponds to C=O and –OH chemical bonds.
Plasma modification of carbon nanotubes with the organosilicon compound also introduces substantial changes of the content of different chemical moieties on their surface. Energetic conditions in plasma favor a fragmentation of the vinyltrimetoxysilane molecule, with its fragments being bound to the surface modification. XPS core-level C 1s spectrum of the filler modified with VTMS is presented in Fig. 16 As seen in the figure, the majority of chemical bonds found are also present on the non-modified surface, with the difference being quantitative. The amount of C=C bonds is decreased by 2 at.%, with a simultaneous substantial increase in the concentration of the sp3 bonding. Double C=C bonds are very functional in plasma—they easily break down with the active carbon atoms formed subsequently reacting with the molecular fragments of the modifier molecule. In this way, new C–C, and also C–O and C=O bonds are formed on the filler surface what is, indeed, reflected in its XPS core-level C 1s spectrum.
A presence of new chemical moieties formed in the process of plasma surface modification with VTMS is also revealed by the core-level O 1s spectra. In comparison with the oxygen spectrum of a non-modified nanotube surface, three new bands appear after modification. They are: C–OH band at 530.6 eV, –OxSi band at 532.4 eV, and Si–OH or Si–O–Si band at 533.8 eV. The presence of O–Si bonds unambiguously points to surface chemical modification of the nanotube filler.
On the basis of the results presented, one can state that surface plasma modification of carbon nanotubes with organosilicon compounds leads to a formation of new –O–Si bonds at the expense of C=C double bonds. What is more, no presence of direct silicon bonding with carbon was observed on these surfaces.
On the basis of the results acquired in this work and in accordance with the literature data , one can assume that the mechanism of surface modification of carbon fillers with the chemicals presented may be divided into two stages. Stage one comprises a formation of active sites on the carbon surface with the help of oxygen plasma. Under glow discharge conditions, plasma-formed activated oxygen atoms are bound to carbon surface with the foundation of carbonyl C=O and hydroxyl C–OH connections. Hydrogen content required for the hydroxyl groups originates either from surface terminal C–H bonds or from water adsorbed on that surface.
The second stage of the filler modification process involves the effect of plasma on the molecules of the modifying chemicals. Plasma energy is high enough to bring about a partial fragmentation of these compounds. It is quite reasonable to assume that the resulting fragments of the lowest molecular weight comprise: Si, Si–O, and C–C=C, C=O, C–O–C moieties as well as short aliphatic groups such as –CH3.
Silicon as one of the elements used in the present work is characterized with a substantial affinity toward oxygen. XPS spectra of the surfaces modified with its compounds indicate that this constituent is bound to the carbon matrix exclusively via oxygen atoms. As far as carbon-containing active fragments are concerned, they are bound to the surface as a consequence of a breakage of the matrix C=C double bonds which is specified by an increased concentration of the σ sp3 carbon–carbon bonds on the expense of π sp2 carbon–carbon bonds.
The results presented unambiguously show that the plasma chemical treatment of carbon filler surfaces with silanes, combined with oxygen plasma activation of these surfaces, constitutes an effective means of their modification and, therefore, of a manufacture of a new generation of carbon fillers for the elastomer industry.
As seen in the figure, an application of VTMS-modified MWCNTs significantly increases the modulus at 100% of elongation, as well as elongation at break and tensile strength of rubber vulcanizates. The effect is different for MTMS and VTMS modification. In the case of VTMS-modified MWCNTs, the 100% elongation modulus increases from 1.23 to 1.52 MPa, while MTMS modification of the filler particles makes this parameter grow only slightly, i.e., up to 1.32 MPa. It means that, in a course of polymer processing, grafted MTMS very likely plays a quite different role than VTMS, being involved in cross-linking the matrix in a specific way due to the presence of sulfur containing thiol groups. For all the samples containing modified filler, elongation at break is higher than that of the reference sample, and definitely the best performance has been observed for rubber vulcanizates containing VTMS-modified MWCNT fillers. Maximum tensile strength recorded is 9.52 MPa in this case, while that of the reference sample equals 6.87 MPa. Finally, the respective data for the vulcanizates containing oxygen plasma-activated and MTMS-modified MWCNT fillers amount to 8.63 MPa and to 8.80 MPa.
As seen in the figure, elongation at break figures for rubber vulcanizates containing fillers modified with silane derivatives are higher than 800% (823% for MTMS and 867% for VTMS modification). The mean value of their tensile strength equals 6.54 MPa and 6.64 MPa, respectively. An improved performance is also observed for the sample containing oxygen plasma-activated graphene platelets solely. The above results suggests that the improvement achieved is mainly due to the material cleaning and exfoliation effects being observed as a result of plasma treatment of the filler particles.
It is clear from the above figures that the treatment significantly alters dispersion of carbon nanotubes and their distribution in the rubber composite. The most important difference between composites containing non-modified and modified MWCNTs is the amount of small agglomerates (< 5 µm), which are responsible for their strengthening effect. In the case of modified filler, one can also observe a smaller number of large destructive structures which are considered the weakest spots of a composite. In general, the details of microstructure presented well correspond to the results of mechanical testing that show a major improvement after VTMS grafting.
As far as GnP-filled rubber composites are concerned, an improvement in the filler dispersion following the treatment is less evident than in the case of MWCNT. There is definitely a higher amount of small agglomerates (< 3 µm) in the treated filler, for which a slight strengthening effect has also been observed during an analysis of their mechanical properties. However, in this case, it appears to be the spatial distribution of GnP agglomerates that is more important than the degree of their dispersion—it is much better in the case of the modified filler than that of a virgin filler. Finally, when compared to MWCNT containing samples, the GnP-filled composites comprise more large agglomerates resulting from well-known dispersion difficulties in rubber matrix during mixing process.
In the case of VTMS-modified carbon nanotubes, the enhancement of filler dispersion in the composite well corresponds to the results of its mechanical tests. Substantially, larger amounts of agglomerates of average size remaining below 1 µm have been observed for the composites based on modified MWCNTs than for those containing unmodified nanotubes, with the respective numbers amounting to 27.7% and 22.7%. The same concerns agglomerates of the size below 5 µm that are beneficial from the viewpoint of the composite structure. A reversed situation is observed in the case of larger agglomerates—their concentration is higher in a composite filled with unmodified nanotubes. The materials where the filler dispersion is better, i.e., those filled with VTMS-modified MWCNTs, are also characterized with higher magnitudes of tensile strength and modulus of elasticity. The same effect for composites filled with the GnPt filler is smaller, but still well observable—an increase in filler dispersion is followed by an improvement in their mechanical properties.
As revealed by the XPS analysis (Table 3), a modification of carbon nanotubes with VMTS introduces substantial surface concentration of silicon (over 10%). The results comprise an increase in network density of the composite and an improvement in the filler dispersion both leading to the reinforcement and stiffening of the material.
Activated filler modification with titanates and zirconates
The above data are not easy to interpret. Complicated reactions take place here, due to the branched structure of the respective compounds with multiple oxygen atoms that are thought to be potentially involved in the modification mechanism. It should be noted that the performance of the composites is also affected by the use of the fillers solely activated with the oxygen plasma. All the treatment processes performed resulted in the decrease in the SFE polar component below the value for the reference sample, what indicates a successful modification. The polar component differences between titanate- and zirconate-modified filler are very small. Apparently, oxygen atoms and unsaturated bonds are more likely involved in the modification process, with hydrocarbon parts of the compounds being exposed to further interactions with rubber macromolecules.
XPS data of surface elemental composition of nanotube samples
Type of modifier
Atomic composition of the films [at.%]
83.8 ± 1.60
16.2 ± 0.6
72.0 ± 1.34
25.5 ± 0.4
2.5 ± 0.1
Figure 24 shows examples of XPS survey spectra of the surfaces of: non-modified MWCNTs (a) and NZ33-modified nanotubes (b). A calibration of these spectra was made on the basis of a maximum corresponding to sp2 (C=C) bonds, present at the binding energy of 286.4 eV. In the case of zirconate-modified filler, a small maximum appears at the binding energy of 283.6 eV. This maximum corresponds to extra C=C bonds, very likely originating from the fragments of initial zirconate molecule. Bands characteristic for MWCVTs are described in one of the previous sections presenting results of filler modification with silanes. As far as NZ33-modified nanotubes are concerned, its spectrum contains the following extra bands corresponding to different core-level electrons of zirconium: Zr 3s at ca. 432.2 eV, Zr 3p1/2 at ca. 347.6 eV, Zr 3p3/2 at ca. 332.1 eV, Zr 3d 5/2 at ca. 182.5 eV, Zr 3d 3/2 at ca. 184.9 eV, Zr 4s at ca. 53.8 eV and Zr 4p at ca. 31.0 eV.
Contribution of particular components of the O 1s, C 1s, and Zr 3d core-level XPS spectra of plain and modified CNT
Modified with NZ 33
Contribution of particular components [%]
Contribution of particular components [%]
Carbon modification of nanotubes with metal–organic NZ33 connection results in a formation of extra bonds on their surface. The most characteristic feature is comprised of a substantial decrease in C=C bond content, compared to a non-modified filler. The intensity of the sp3 band at the binding energy of 285.4 eV, on the other hand, increases for the NZ33-modified surface. Another band that shows a substantially (more than twice) increased intensity in the spectrum of zirconate-modified filler is the 286.3 eV band corresponding to C–O bonds. Finally, the presence of C=O bonds is attributed to the band at 287.9 eV.
The XPS O 1s core-level spectrum of MWCNT filler modified with NZ33 zirconate indicates that only 7% of oxygen is bound to zirconium. The vast majority of its atoms on the surface are present in a form of either C=O or –OH bonds, what is confirmed by the 529.6 eV and 532.1 eV bands, respectively. The notion that zirconium is bonded to oxygen finds its confirmation in the core-level Zr 3d spectrum, where the bands at 182.5 eV and at 184.9 eV correspond to the presence of Zr–O bonding. Low intensity maxima at 183.5 and at 185.9 eV, on the other hand, are attributed to Zr–OH connections.
Modification of the filler with NZ33 brings about a slightly different result than the treatment with VTMS does, very likely due to a larger size of a NZ33 molecule and its different affinity toward the cross-linking process. There is no increase of cross-link density observed in that case, probably due to steric hindrance, with an accompanying substantial upsurge of composite elasticity—both elongation at break and tensile strength could not be measured because of an overrunning of the measurement range. A conclusion can, therefore, be drawn that this form of modification also results with a growth of interactions at the polymer matrix–filler interphase.
The above differences might be explained with the modifier affecting the cross-linking process, and changing filler surface availability for the interactions with polymer macromolecules. The surface of carbon nanotubes is grafted with the compounds (or their fragments after decomposition in plasma) of a branched structure and relatively high volume. As a result, a rubber macromolecule is not able to closely interact with a filler surface with grafted species acting as spacers.
On the basis of the results presented above, the following conclusions may be formulated. First of all, the study has shown that it is possible to substantially increase surface free energy of carbon nanotubes with low-temperature oxygen plasma. When it comes to graphene nanoplatellets, however, it is much harder to achieve a similar level of modification. As an effect of the treatment, substantial changes in filler microstructure as well as chemical composition of its surface are observed. Therefore, modification of filler surface affects rubber–filler and filler–filler interactions which have a great impact on the properties of rubber composites.
As far as silane/titanate/zirconate plasma processing of the filler is concerned, it proves to be an effective tool for the functionalization of carbon nanotubes. In the case of graphene nanoplatellets the effect is much weaker due to the more stable structure and smaller amount of active polar and radical centers generated during the plasma activation stage. By involving the filler in a cross-linking process of the rubber composite, it is possible to achieve an improved composite performance. The effects of modification were confirmed with the ToF-SIMS, SEM–EDX, XPS as well as with swelling and mechanical tests.
The project was funded by the National Science Centre Poland (NCN) conferred on the basis of the Decision Number DEC-2012/05/B/ST8/02922.
MS conceived and designed the experiments; MS, AP, and TG performed the experiments; MS and DB analyzed the data; MS and TG contributed reagents/materials/analysis tools; MS wrote the paper, and HS was entirely responsible for its revised version.
Compliance with ethical standards
Conflict of interest
The authors declare no conflict of interest.
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