Research Areas

Nanotechnology 2005 - 2007

Researcher: Luiz Henrique Capparelli Mattoso

Objective of Labex Nanotechnology

To prospect the opportunities of collaboration between the Agricultural Research Service ARS/USDA and Embrapa and to establish initial cooperation linkages involving these institution to exploit applications of nanoscience and nanotechnology and possibly other areas that can contribute to the development of agribusiness.


Projects


Development of Nanocomposites from renewable sources
Institution participating: USDA/ARS/WRRC/BCE, Embrapa Cotton, DEMA-UFSCar.


Objective


To extract and characterize nanofibers from renewable sources and to develop agro- based nanocomposites.


Results

Electrically conductive cellulose nanofibrils were obtained by the in situ polymerization of aniline onto CNF practically without precipitates or aggregates of PANI. The Voc profile demonstrated basically the same trend as that generally observed during the chemical oxidative polymerization of aniline, i.e., the initial increase of the potential from ~0.5 V (versus SCE) to 0.8 V characteristic of the formation of the pernigraniline intermediate oxidation state, followed by the decrease of the Voc to ~0.43 V due to the formation of the polyaniline in the emeraldine oxidation state. However the reaction was catalyzed by the presence of the CNF. The polymer was preferably polymerized onto the CNF surface, with no evidence of polymer precipitate, as it is also the case for other fibers, when the polymerization is carried out under diluted conditions as those used in the present work. Free-standing shinning films could be obtained by casting from aqueous solution. Parent PANI is unable to form films from aqueous solution since it is usually produced in the form of large aggregates obtained from the precipitation of the polymer during its synthesis. Importantly, the use of CNF in highly stable aqueous solution lead to the possibility of obtaining stable suspensions of electrically conductive PANI coated CNF, in both doped and dedoped states (Figure 5). Moreover, conductivity values even higher should be obtained for higher PANI content. The dopant, as well as the preparation conditions of the CNF played a key role on the stability of the suspension of nanocomposite. Usually much more stable solution were obtained by using as-prepared suspension of CNF. The re- dispersion of the freeze dried CNF lead to aggregation and then precipitation after few minutes. Additionally, the most stable suspension was obtained by adding DBSA, as dopant.

The films obtained appeared homogeneous, both macroscopically as well as microscopically, as it can be seen in Figure 6. One can observe the CNF with average diameter in the range of 15-20 nm in both images. The monitoring of the reaction by the measurement of the Voc potential as well as the color changes observed during the synthesis corroborates the in situ polymerization of aniline onto the nanofibrils Additionally these nanocomposites could be produced using different dopants for polyaniline, namely (HCl), toluenesulfonic acid, (TSA) sulphonated lignin acid (SLA), and dodecylbenzenesulfonic acid (DBSA) leading to electrical conductivity in the range of 10-2 to 10-4 S/cm, the best results being obtained with TSA and HCl.
We should emphasize that these new nanocomposites exhibit promising potential for uses in antistatic applications such as paints for instance, as well as applications in the development of electrically conductive nanocomposites with better mechanical properties due to the high mechanical strength of the CNF or pressure sensors.


Continued Collaboration

Nanocomposites using these cellulose nanofibers are being investigated in Brazil with gluten in collaboration with the nanotechnology network established at Embrapa. A Brazilian pos-doc (E. Medeiros - CNPDIA) funded by ARS headquarters is currently working at WRRC/ARS on new uses of glycerol as well as these cellulose nanofibers in nanocomposites.
A researcher from Embrapa (M. Rosa – CNPAT) is planned to come to work on nanocomposites made from latex, starch and cellulose nanofibers in collaboration with the nanotechnology network established at Embrapa.
A proposal was submitted to FAPESP for a pos-doc fellowship to investigate different sources of cotton developed by Embrapa Cotton in Brazil to select the best ones for the extraction of cellulose nanofibers and application in nanocomposites.


Preparation of Biopolymer Films Using Nanotechnology: Institution participating: USDA/ARS/WRRC/PFR, Embrapa Labex-USA.

Objective

To prepare and test the use of nanoparticles of chitosan on biopolymer films.


Results

Nanoparticles were obtained by polymerization of methacrylic acid (MAA) in CS solution in two-step process. In the first step, chitosan was dissolved in MAA aqueous solution (0.5 v:v-%) for 12 h under magnetic stirring. The CS concentrations used in synthesis were 0.2, 0.5, and 0.8 (wt %). In the second step, 0.2 mmol of K2S2O8 was added in the solution with continued stirring. Then, the polymerization was carried out at 70 0C under magnetic stirring. This system was kept at 70 0C for 1 h leading to the formation of CS–PMAA nanoparticles which was then cooled in an ice bath.
Nanoparticles were successfully produced through polymerization of methacrylic acid in the presence of chitosan solution, as show the results from Table 1. It can be observed that the size of CS-PMAA nanoparticles decrease with increase in the content of chitosan within the feeding solution used. It should be pointed out that the possibility of producing nanoparticles with controlled particle size is rightly desirable, in order to be able to optimize, for instance, the final properties of nanoparticles reinforced polymer films.

The effect of pH on particle size is shown in Figure 7. One can observe that particle size increases from 99 to 218 with an increase in pH from 3.0 to 8.0. Possible reasons for this behavior are two. Firstly, also an increase in pH leads to an increase on the ionization degree and charge density of the PMAA molecules. As a consequence, the electrostatic repulsive forces of inter- and intra-PMAA molecules increased, resulting in an increase of the swelling degree of PMAA and therefore an increase of the mean size of CS–PMAA nanoparticles. Secondly, as the pH increases the solubility of, chitosan decreases, what may lead to aggregation of nanoparticles. Figure 8 shows that both aggregation and swelling effects are possible, although under the dried conditions under which the TEM were carried out an opposite effect to swelling was noticed, as expected, with particle size decreasing from 180 to around 70 nm.

TEM microphotographs of chitosan nanoparticle prepared using 0.5 of chitosan (wt%) at pH 4.0 are depicted in Figure 5. The TEM images indicate a spherical shape of chitosan nanoparticles and also confirm the de-swelling effect, showing a decrease from 82 nm to around 40 nm upon drying.

In summary, the CS–PMAA nanoparticles could be successfully prepared by polymerizing methacrylic acid into chitosan solution. The particle size is dependent on the chitosan concentration used in the synthesis. The nanoparticles solution obtained are also pH- sensitive, due to swelling and aggregation of the nanoparticles, which present a predominantly spherical shape. It is proposed that the carboxylic groups of PMAA are dissociated into COO- groups which complex with protonated amino groups of CS through ionic interaction to form a polyelectrolyte complex during the polymerization, which leads to the formation of nanoparticles. This system might find interesting applications in food packaging.


Continued Collaboration

A Brazilian Ph.D student (M. Moura – CNPDIA) funded by CNPq (Brazil) is currently working at WRRC/ARS on this project to test these chitosan nanoparticles in biodegradable films.
A researcher from Embrapa (H. Azeredo – CNPAT) is planned to come to work on the preparation of edible films for packaging in collaboration with the nanotechnology network established at Embrapa.


Characterization of Natural Rubber from New Brazilian Clones of Hevea brasiliensis


Institutions participating: ARS/WRRC/CIU-USDA, Embrapa-Labex-USA, IAC/Campinas, UC-Berkeley, and University of Nevada at Reno


Objective

To study the characteristics and properties of new series of Brazilian Hevea brasiliensis clones (series IAC 300 and 400 series) by new techniques to better understand their behavior and to correlate with the technological properties being studied in Brazil.


Results

Natural rubber latex from different IAC clones from Votuporanga city (IAC 40, 56, 300, 301, 302 and 303) and RRIM 600 clone (used as a control), and clones of IAC 400 series from Mococa city (IAC 405, 406, 410, 413, and 420) and RRIM 600 clone (used as a control), in both cases, were collected from IAC experimental plantations. The stabilization of the latex samples was made using a (1:1 v/v) solution of Tris (Tris(hydroxymethyl) aminomethane) - 15.76 g/L, with 20% of glycerol.
The clones were grown for ten years in the plateau region of São Paulo; the ecological conditions of which are summarized below:
Mococa city: 21o18’S, 47o01’W; altitude 665 m; mean annual temperature 24oC; mean annual rainfall 1,500 mm; Eutrustok soil, with good nutrient status and physical structure. Votuporanga city: 20o25’S, 49o50’W, altitude 450 m; mean temperature during growing season 32oC; mean annual rainfall 1,480 mm; Paleudalf soil, with average nutrient status and poor physical structure.
Particle size distributions were determined using a Horiba LA-900 Laser Light Scattering Particle Size Distribution Analyzer according to the manufacture’s instructions. The particle size output was analyzed with LAM-900 and DISP200 software, using at least five different measurement of each sample.
Morphology analysis of the rubber particles was recorded in a Hitachi S 4700 SEM at 2.5 kV. The samples were prepared according to Wood et al. methodology, glued in proper stubs and covered with gold in a Polaron E5100 sputter-coating unit.
The particle size distribution of rubber particles of IAC series clones from Mococa and from Votuporanga is shown in Figures 11 and 12, respectively. The distribution curves were normalized on the y-axis to facilitate comparison.
A wide unimodal particle size distribution ranging from 0.1 to 10 μm and presenting a similar shape was observed for rubber particles in latex for both, Mococa and Votuporanga samples.
Mococa clones presented, in general, a particle size average of 1.0 micron, including the control RRIM 660 with the exception of clone 410 which presented a higher average of about 1.7 μm, as shown in Table 1. The data indicated that the average size of the rubber particles varies within and among clones.
For Votuporanga, Table 3 shows the average of rubber particle size of IAC 300 series and the results for IAC 40 and IAC 56. It can be observed that IAC 40 and IAC 56 clones present larger average particle size than the average of IAC 300 series. The control, RRIM 600 clone, is the smallest of all clones, which can be an indication that a smaller particle size average may be a desirable feature, since the RRIM 600 is considered to be a high productivity clone which also presents a high performance on technological properties.

Rubber particles from all samples were predominantly spherical and varied in size as previously discussed. Both techniques laser light scattering and SEM, showed that the rubber particle size varies within and among the clones for the IAC series. The same basic results have been observed for all samples. Small rubber- particle diameters measured using laser light scattering differed from those observed using SEM, probably due to sensitivity of the instrument and differences in the preparation of samples required by these techniques. SEM results clearly show several small particles, which might have not been detected by the other technique, indicating the importance of the scanning electronic microscopy analysis.
In summary, a similar wide unimodal particle size distribution ranging from 0.1 to 10 μm was observed for rubber particles in latex for both, Mococa and Votuporanga samples. The average size of the rubber particles varies within and among clones. In general, the particle size average of clones from Votuporanga was bigger than those from Mococa. Rubber particles of IAC clones have predominantly the spherical form, consistent with other clones and rubber producing species.

Continued Collaboration

Biochemical properties measurements (Protein content, molecular weight, complex viscosity, particle size) are still being concluded at WRRC-CIU Albany-CA, and two new Brazilian pos-docs (M.A. Martins – CNPDIA, J.A. Malmonge – UNESP) are planned to come to work at this laboratory.


Synthesis of nanostructures of conducting polymers


Institution participating: this research was carried out through collaboration with Prof. Alan G. MacDiarmid from the University of Pennsylvania and Dr. Sanjeev Manohar from the University of Texas in Dallas.


Objective

To investigate electrochemical techniques to produce nanostructures of conducting polymers.

Results

It was highly interesting to observe that only by decreasing the potential used during the cyclic voltammetry, after the first scan to 0.9V it was possible to eliminate the presence of nanoparticles to then obtain a morphology which contains only polyaniline nanofibers with average diameters from 20 to 40 nm. Figures 14 and 15 show the effect of the potential interval on the morphology of the electropolymerized polyaniline.
A morphology consisting mainly of nanoparticles (Fig. 16) with diameters from 50 to 80 nm, can be also obtained by selecting appropriate electropolymerization conditions, in this case by decreasing the monomer concentration and the scan rate. We believe that during the electropolymerization of aniline the nucleation process and the growth kinetics are crucial in defining the morphology of the nanostructure obtained. Under certain conditions the concentration of aniline may play a key role in defining the formation of either nanoparticles or nanofibers of PANI. Figure 17 shows AFM images of bundles of PANI nanofibers containing some nanoparticles dispersed on it, which agreement with SEM results. These AFM images show that the nanofibers grow in a sinusoidal fashion. Figure 15b shows that the nanoparticles tend to agglomerate into interconnected networks until form branched network-like nanofibers. In addition, the results show that higher concentrations of aniline enhance the yield of nanofibers. It is important to note that the process of nanostructure formation seems to occur in two different steps, i.e. nucleation of nanoparticles of PANI on the naked electrode surface followed by the growth of PANI on the already deposited PANI modified surface. Therefore, the rate in which these processes occur might probably define the type and size of the nanostructure obtained. In addition, it should be stressed that nanofibers seems to be formed by joined nanoparticles.
The cyclic voltammograms of the electrochemical polymerization of aniline using an aqueous solution (1.0molL-1 HCl) containing 0.5molL-1 and 0.02molL-1 of aniline are shown in the Figs. 18 (a) and (b), respectively. The current peak intensities increase with the increase of the number of cycles, indicating the regular growth of the PANI nanostructures. There are two redox peaks in Fig. 19 (a), peaks A and B. In both Figs. 18 (a) and 18(b), the current peak A slightly shifts to the higher potential, which probably derives from thickening of PANI films during the electrochemical polymerization. At this peak potential, the radical cation may be generated upon oxidation (polaron state). The current peak A* is broader and more negative when compared with the current peak A. This peak is relevant to the reduction of the polaron state. The current peak B shifts to the lower potential, which means that autocatalytic polymerization occurs during electrolysis of aniline and the diradical dications may be generated, which is attributed to the further oxidation of PANI to the form of quinoid (bipolaron state). In figure 18 (b) there are three redox peaks. The redox peak G and G* are oxidation and reduction processes of the gold electrode. This means that small amount of polyaniline is deposited in the electrode, as the results of Figure 12 indicate. In the middle current peaks, i.e. current peaks M and M*, there exists some controversy on the literature. Some authors attributed them to the presence of a polymer containing phenazine rings, while other authors attributed them to the oxidation and reduction of degradation products. Despite this divergence, it is known that changes on the electropolymerization conditions leads to changes in the initial nucleation and growth steps of polyaniline, as shown in figures 14 and 15, which therefore might dictate the nanostructure of the polymer being formed, as well as lead to polymer with differences in chemical structure.

Continued Collaboration

The pos-doc who worked in this collaboration for Prof. Dr. Alan G. MacDiarmid for 4 years started to work in 2007 at the National Nanotechnology Laboratory at Embrapa with another Ph.D Student (M. Xavier) on this subject in collaboration with the nanotechnology network established at Embrapa.