New insights into rubber–clay nanocomposites by AFM imaging
Introduction
It has been well recognized that the extent of dispersion of nanoclays strongly influences the properties of the polymer/layered-silicate nanocomposites [1], [2], [3], [4], [5], [6]. The filler dispersion depends on the nature of matrix as well as nanoclay. In a true nanocomposite, the clay layers must be uniformly dispersed and exfoliated in the polymer matrix, rather than being aggregated as tactoids. But in most of the cases, a nanocomposite having both exfoliated and intercalated morphology is obtained.
Two complementary characterization techniques have been extensively used in the investigation of morphology of polymer–clay nanocomposites, namely X-ray diffraction (XRD) and transmission electron microscopy (TEM). However, certain disadvantages of TEM such as radiation damage and tedious sample preparation are also now well known. In contrast to the conventional electron microscopy, AFM does not require a conductive coating or staining. Hence, AFM analysis of the nanocomposites can be a good alternative to electron microscopy, without any limitations regarding contrast and resolution. The development of this technique has helped to image surface topography on a nanometric scale [7]. Besides, this multifunctional technique is suitable to understand the nanofiller dispersion in the matrix along with surface topography. In addition to the normal topographical imaging, AFM can also measure fundamental properties of sample surfaces, e.g., local adhesive or elastic properties on a nanometric scale. We have earlier reported from this laboratory various AFM investigations on carbon black and silica reinforced polymers [8], [9].
Interestingly, in the literature there have been only a few reports on the morphology of rubber–clay nanocomposites by atomic force microscopy (AFM) [10], [11], [12], [13], [14]. Literature survey shows that only the morphology of a few nanocomposites was investigated using AFM so far [15], [16], [17], [18], [19], [20]. The surface topography and quantitative surface analysis including the nano-range forces present in the composites have not been studied.
Out of three modes of AFM, tapping mode (TMAFM) is the most suitable for soft rubber samples. In this mode, short intermittent tip–sample contact reduces lateral forces, which minimizes sample damage during scanning [21]. In our earlier preliminary report, the morphology of rubber–clay based nanocomposites was investigated by both TEM and AFM [12]. The studies showed a good correlation between TEM and AFM results. But quantitative analysis cannot be done extensively with TEM.
Hence, in the present paper, for the first time, attempts have been made to study the morphology of rubber–clay nanocomposites, nanofiller distribution, interfacial region, contact and adhesion forces using AFM. Fluoroelastomer/clay nanocomposite has been taken as a representative system. The preparation and properties of these nanocomposites, reported elsewhere, indicated outstanding mechanical and dynamic mechanical properties, generally required for space application where this rubber is extensively used [22]. AFM has been used as an effective tool to analyze the morphology qualitatively and also to quantify the surface features of the nanocomposite systems. The scanning probe microscope (SPM) data of virgin rubber film and the nanocomposites were evaluated qualitatively and quantitatively using power spectral density to envisage the effect of different nanoclays on the morphology of the resulting nanocomposites. The interfacial region between rubber and filler has been traced with this tool. Also, the adhesion force at different positions – on the filler particle, interface region and on the matrix – has been determined using contact mode AFM. The latter information was also not reported on the rubber–clay system.
Section snippets
Materials used
Viton B-50 [a terpolymer of vinylidene fluoride (VF2), hexafluoropropylene (HFP) and tetrafluoroethylene (TFE), density 1850 kg m−3 at 25 °C, 68% F, Mooney Viscosity, ML 1 + 10 at 120 °C = 39] was procured from DuPont Dow Elastomers, Freeport, Texas, USA. Nanoclays namely Cloisite NA+ (NA) and Cloisite 20A (20A) were obtained from Southern Clay Products, Gonzales, Texas, USA. Methyl ethyl ketone was supplied by Nice Chemicals Pvt. Ltd, Cochin, India.
Preparation of rubber–clay nanocomposites
The rubber was first dissolved in methyl ethyl ketone
Particle size and nanofiller distribution
The phase images of the neat rubber, the unmodified and the modified clay loaded samples (FB5, FB5CNA4, FB5C20A4) are illustrated in Fig. 2a–c. In the case of the clay filled samples, some distinct white bright features are observed, which are absent in the unfilled sample. This suggests that the filler appears as white bright features in the grey rubber matrix. In the tapping mode, the measurement of the difference between the phase angle of the excitation signal and the phase angle of the
Conclusions
The present study utilized AFM as an investigating tool to observe the morphology of the fluoroelastomer-clay nanocomposites, the dispersion of the nanoclays in the rubber matrix, interface thickness, and interaction forces. The phase images of the filled nanocomposites revealed the presence of clay fillers as the bright features in the dark rubber matrix. Smaller particle size of the unmodified clay than that of the modified clay was apparent. The results obtained from section analysis and
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