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polymer nanocomposite dispersion by mini-emulsion polymerization
Journal of Colloid and Interface Science 363 (2011) 129–136
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Synthesis and characterization of cellulose whiskers/polymer nanocomposite dispersion by mini-emulsion polymerization Ayman Ben Mabrouk a, Manuel Rei Vilar b, Albert Magnin c, Mohamed Naceur Belgacem d, Sami Boufi a,⇑ a
Laboratoire Sciences des Matériaux et Environnement, LMSE, University of Sfax, BP 802-3018 Sfax, Tunisia Univ. Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 75013 Paris, France c Grenoble Institute of Technology, The International School of Paper, Print Media and Biomaterials (Pagora), UMR CNRS 5518, BP 65, 38402 Saint Martin d’Hères Cedex, France d Laboratoire de Rhéologie, Grenoble-INP, UJF Grenoble 1, UMR CNRS 5520, BP 53, 38041 Grenoble Cedex 9, France b
a r t i c l e
i n f o
Article history: Received 10 May 2011 Accepted 18 July 2011 Available online 27 July 2011 Keywords: Nanocomposites Dispersion Colloids Cellulose
a b s t r a c t Stable film-forming nanocomposite particles with diameters ranging from 120 to 300 nm, based on polybutylmethacrylate (PBMA) and cellulose whiskers in water dispersions, were successfully synthesized in one step through mini-emulsion polymerization. The nanocomposite dispersion with a solid content of 25 wt.% and up to 5 wt.% of nanofiller loading was prepared by in situ polymerization, in the presence of the whiskers using dodecylpyridinium chloride (DPC), as a cationic surfactant, and 2,2azobis(isobutyronitrile) (AIBN), as initiator. The electrostatic interaction between the positively charged droplets and negatively charged whiskers ensured the anchoring of the nanofiller around the polymer particles. The ensuing dispersions were characterized by Dynamic Light Scattering (DLS), f-Potential Measurements, and Field-Emission Scanning Electron Microscopy (FE-SEM). After the film formation process, the nanocomposite film exhibits a high transparency, denoting the good dispersion of the whiskers throughout the matrix. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Cellulose nanofibres are one of the most promising reinforcements in nanocomposites based on organic polymer matrices [1,2]. In fact, taking into account the high storage modulus of this nanofiller in the range of 50–100 GPa [3] and its high breaking strength allied with their aspect ratio ranging between 10 and 100 [2], it is to expect a huge reinforcing effect once the nanofiller will be incorporated into a polymer matrices. Furthermore, the capacity of the nanofiller to bring on a percolated network holdup through strong hydrogen interactions between adjacent nanofibres resulted in a spectacular enhancement in the storage modulus, as well as in the breaking strength, even at filler levels lower than 5 wt.% [4,5]. The high transparency degree of the nanocomposite based on cellulose nano-sized filler is another feature that should be highlighted [6,7]. Actually, as long as a nanofiller of width less than one-tenth of the visible light wavelength is embedded into a transparent polymer matrix, the optical properties of the material should not be altered, providing the nanofibres do not aggregate. However, even if the high reinforcing potential of cellulose nanofiller is a well-established feature, as highlighted by ⇑ Corresponding author. Address: Laboratoire LMSE, University of Sfax, BP 1171 3000 Sfax, Tunisia. Fax: +216 74274437. E-mail address: sami.boufi@fss.rnu.tn (S. Boufi). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.07.050
numerous reports [8,9], this property is tributary of the good dispersion of the nanofiller within the host polymer matrix during the nanocomposite processing. In fact, owing to their nano-sized scale and their high surface hydroxyl group concentration, cellulose whiskers have a strong tendency to self-associate, leading to irreversible aggregation during the mixing process, which impedes any percolation to occur and decreases significantly the specific surface, thus losing a part of their beneficial effect [10]. Bearing in mind that cellulose whiskers are well stable in diluted aqueous suspensions, namely for those carrying sulphate or carboxyl anions, the mixing of aqueous polymer solutions or latex with cellulose whisker suspensions and the subsequent film casting remains the major method to conceive the nanocomposites [11,12]. The dispersion of the nanofibres in a high polar organic media such as dimethylformamide, dimethylsulphoxide or N-methylpyrrolidine [13], or in organic solvent after the chemical surface modification, involving the chemistry of the hydroxyl groups, has also been explored. Silylation [14], grafting of PEO [15] and acylation [16], for instance, have shown to improve the whiskers dispersion in organic solvents. Grafting of the whiskers surface is also expected to open up a new strategy to achieve a better dispersion of the nanofiller into the polymer matrix [17]. To the best of our knowledge, no attempts have been reported about the melt mixing behaviour of the grafted whiskers with thermoplastic polymer. However, when the surface modification succeeds to bring about
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the whiskers dispersion in low-polarity media, it usually reduces the inter-whiskers interactions, thus limiting the formation of load-bearing percolating network within the host polymer matrix. In situ polymerization in the presence of cellulose whiskers could be envisaged as another alternative way to produce tailored nanocomposites with a controlled degree of covalent linkage between the matrix and the nanofiller, as well as a good dispersion degree. For this purpose, in situ heterophase polymerization in aqueous media is preferred for different reasons: (i) There is no need to remove the whiskers from water in which they are well dispersed. This is a great advantage if one considers that once in the dry form, cellulose whiskers aggregate irreversibly. (ii) Using water as a dispersion medium is attractive for environmental considerations. (iii) The approach offers great advantage (in terms of process simplicity) over solution or bulk polymerization to tailor the nanocomposites’ morphology. In a previous work, nanocomposite dispersions with a solid content up to 30% and cellulose whiskers loading ranging from 1% to 6% were successfully prepared via mini-emulsion polymerization in the presence of anionic surfactant and methacryloxypropyl triethoxysilane (MPS), as a coupling agent [18]. It was shown that the amount of the added MPS greatly affects the reinforcing extent of the whiskers [19,20]. In the same vein, the present work introduces a new approach to prepare stable nanocomposite dispersions with a strong binding between the polymer particles and the whiskers. The nanocomposite dispersion was prepared by mini-emulsion polymerization using cationic surfactant and negatively charged whiskers. The idea is to emulsify the monomer containing the initiator in water in the presence of the whiskers suspension and the cationic surfactant. During the emulsification process, fine droplets of monomer stabilized by the cationic surfactant, onto which the whiskers could be anchored, are formed. Then, the polymerization will be induced by raising the temperature, which yields in situ free radical formation. The resulting reinforced emulsion could be transformed into nanocomposites by simple film-casting process. From the colloidal point of view, the proposed system requires the achievement of two major challenges. First, the heterophase in situ polymerization of monomer in the presence of cellulose whiskers and a cationic surfactant is not straightforward to implement. Indeed, given the anionic charges borne by the whiskers and the cationic ones associated with the surfactant, different events could occur. The most critical one concerns the colloidal stability of the system and is associated with the adsorption of cationic surfactant on the cellulose nanocrystal surface, leading to the flocculation, aggregation of the cellulose whiskers and coagulation of the polymer particles induced by the cellulose whiskers. Second, the miniemulsion polymerization was selected to guarantee the stability of the colloidal system in such a way that the polymerization exclusively occurs within the droplet. Moreover, this choice prevents homogeneous and micellar nucleation during the nucleation process. Such a specific mechanism is favoured by the addition of a co-stabilizer (or hydrophobic agent) that hinders Ostwald ripening and the diffusion of the monomer from one droplet to another. A detailed description of mini-emulsion polymerization mechanism has been the subject of several reviews [21,22]. Thus, the main purpose of this paper is to prepare water-based colloidally stable mini-emulsion of nanocomposites made of nanosized cellulose-reinforcing elements and organic polymer matrix, using a one-pot process without needing any isolation or purification stage. To the best of our knowledge, such a system has never been reported in the literature.
2. Experimental section 2.1. Materials Butylmethacrylate (BMA, from Aldrich, 99 wt.%) was distillated in vacuum and kept refrigerated until use. 2,2-Azobis(isobutyronitrile) (AIBN), sodium dioctylsulfano succinate (DOSS), hexadecane (HD) and dodecylpyridinium chloride (DPC) are commercial products supplied by Aldrich and used without further purification. Distilled water was used for all the polymerization and treatment processes. The critical micelle concentration (cmc) of DPC in distilled water at 25 °C determined by conductometry was found to be 1.2 102 L1. 2.2. Whisker preparation The method used to prepare the alfa whiskers was reported elsewhere [18]. Based on TEM analysis, the whiskers derived from alfa fibres displayed a needle-like structure with an average length L and width d being estimated to be around L = 150 ± 20 and d = 10 ± 2 nm, respectively. 2.3. Mini-emulsion preparation and polymerization The mini-emulsion typical composition is reported in Table 1. The emulsion was prepared mechanically starting from an oil phase containing HD, AIBN and the monomers mixture and 15 mL of water containing DPC. Then, the cellulose whiskers suspension was drop-by-drop added to the previously prepared emulsion, under sonication during 3 min, using ultrasonic homogenizer (Sonics Vibracel Model CV33) at a duty cycle of 70 wt.%. During the sonication process, the reactor was immersed into an ice bath. Upon completion of the sonication, the resulting mini-emulsion was polymerized at 70 °C for 3 h under continuous mechanical stirring with a half-moon shaped Teflon stirrer at 300 rpm and an inert nitrogen stream. 2.4. Calculations of polymer particle whiskers number The number of polymer particles per mL of dispersion was estimated as follows:
Np ¼
mp
ð1Þ
qp 43 pR3p
where p is the density of PBM at 25 °C (1.1 g cm3), mp is the mass of polymer per mL of the dispersion and Rp their hydrodynamic radius. Assuming that the cellulose whiskers have a rod-like shape with a square cross section, their number per mL of the nanocomposite dispersion could be estimated by:
Nw ¼
%W mp
ð2Þ
qw pd2 L
Table 1 Main constituent and their relative quantities (w/w, %) for the mini-emulsion polymerization of butylmethacrylate in the presence of cellulose whiskers. Component
Added (g)
% With respect to monomer
Butylmethacrylate Hexadecane AIBN Cellulose whiskers Dodecylpyridinium chloride Water
7.00 0.35 0.15 0–0.45 0.20 30.0–29.5
100.0 5.0 2.0 0–6.0 3.0
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where %W is the wt.% content of whiskers with respect to polymer, qw is the volume weigh of cellulose (in g cm3), d and L are the width and length of the whiskers (L = 150 ± 20 and d = 10 ± 2 nm), respectively. 2.5. Particle size determination The particle diameters were measured at 25 °C, using a Malvern Nano-Zetasizer ZS Instrument at a fixed scattering angle of 173°. The dispersion was diluted to about 5 wt.% with distilled water before starting the measurements. Dynamic Light Scattering (DLS) measurements give a Z-average size, which gives the mean diameter of the particles. Each measurement was repeated in triplicate, and the obtained values averaged to obtain the mean particle size. The apparent hydrodynamic diameter was calculated according to the Stokes–Einstein equation:
dh ¼
kT 3pgDapp
ð3Þ
where Dapp is the apparent diffusion coefficient and g the solution viscosity. 2.6. Polymerization kinetics Instantaneous and global polymerization conversions were determined by gravimetry. The solid content at a given polymerization time, Sc(t), was determined, and the conversion rate, %C(t), calculated according to:
%C ¼
ScðtÞ 100 Scð1Þ
ð4Þ
where Sc(t) is the solid content for a time t and (Sc(1)) for a complete polymerization reaction, respectively 2.7. Field-Emission Scanning Electron Microscopy (FE-SEM) A Weiss scanning electron microscope, operated at an accelerating voltage between 0.75 and 2 kV, was used to capture secondary electrons images of the nanofibrillated surface. A drop of the diluted nanocomposite dispersion (with a solid content of 0.1– 0.3 wt.%) was deposited on a surface of silicon wafer and freeze dried at 20 °C to remove water, so avoiding particle coalescence. Then, the sample was coated with a thin carbon layer applied by sputtering with a thickness limited to 3–4 nm.
merization, converting the monomer droplet into polymer particles. After several preliminary runs in which the amount as well as the structure of the surfactant were changed, we have succeeded the preparation of a stable dispersion without any trace of coagulum. It was shown that in the presence of DPC, a minimum of 3 wt.% surfactant based on monomer content is necessary to ensure stable 25 wt.% solid content dispersion. This system was chosen to implement the nanocomposite dispersion synthesis, as it ensures a compromise between a minimum amount of water to disperse the whiskers suspension without the excessive build-up of its viscosity and the film-forming aptitude of the dispersion after water evaporation and particles coalescence.
3.1. Evolution of particle size during polymerization Relatively monodisperse particles with an average particle size of 100 nm were obtained in the absence of the whiskers. However, when polymerization was carried out in the presence of the whiskers, a continuous growth in the particle size was observed as shown in Fig. 1. For instance, the mean particle size increased from 110 up to 265 nm, when the whiskers loading increased from 1 to 5 wt.%. Over 5 wt.%, it was necessary to increase the amount of the DPC to 6 wt.% (based on the monomer content) to prevent coagulation. In these conditions, the ensuing dispersion was completely stable, reaching a mean particle size between 290 and 350 nm at a filler content of 6 and 7 wt.%, respectively. The analysis of the particle size distribution reveals to be monomodal for dispersions with whiskers content lower than 2 wt.%, whereas a bimodal distribution is observed up to 3 wt.% of whiskers content. The first particles’ population size emerged around 110–140 nm and the second one around 500–600 nm (Fig. 2). The question now to answer is how the presence of the whiskers up to 3% brings about a bimodal distribution with one population at 110 nm and the second one around 500–600 nm. We infer that such behaviour arises from the partial aggregation of the monomer droplet following the adsorption of cellulose whiskers on their surface. Indeed, due to their negative surface charges, once the cellulose whiskers are added to the monomer emulsion, they will spontaneously adsorbed on the monomer droplet positively charged. Adsorption is driven by the electrostatic interaction between the negative sites on the cellulose whiskers and the headon cationic groups of the surfactant located around the monomer
350.0
2.8. f-Potential Measurement
3. Results and discussion The main advantage of the mini-emulsion polymerization relies on the predominant nucleation in the submicronic monomer droplet with the consequence that the polymer particles are the replica of the mini-emulsion droplets. This feature is favoured by the addition of the hydrophobic agent that hinders Ostwald ripening and the diffusion of the monomer from a droplet to another [21]. Accordingly, if the monomer droplets were generated in the presence of the cellulose whiskers and a cationic surfactant, the negatively charged whiskers could be immobilized on the positively charged droplet. Then, heating the dispersion will trigger the poly-
300.0 250.0
Size (nm)
The f-potentials were measured at 25 °C using a laser Doppler electrophoresis apparatus (Malvern Nano-Zetasizer ZS, UK). The sample consistency in water was set to 0.01% (w/v). The measurements were performed three times for each sample.
200.0 150.0 100.0 50.0 0.0 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
nanowhiskers content (%) Fig. 1. Evolution of the particle size, as a function of the nanowhiskers content (with respect to the polymer) in the dispersion. The solid content of the nanocomposite dispersion is equal to 25 wt.%.
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droplet. The resulting reduction in the droplet (or nucleated polymer particles) charge should lead to a partial aggregation, which decreases the surface-to-volume ratio, achieving a minimum surface charge, sufficient to ensure the colloidal stability. This hypothesis is reinforced by the f-potential analysis and the fact that the bimodal distribution in the presence of up to 3% whiskers appeared prior to the polymerization set off (activation), as soon as the emulsion is formed. This is clearly revealed by DLS measurements (cf. Fig. 3) that show the size distribution of the droplet before polymerization. It is worthy to note the small upward shift in the droplet size compared to the corresponding polymer particle size. Presumably, this may be due to the surfactant desorption and the monomer dissolution in the aqueous phase induced by the dilution operation before implementing the DLS measurement. Even though we have succeeded to reduce this effect by adding an aqueous solution of the cationic surfactant at the same concentration as that adopted for the polymerization, it was impossible to completely avoid the occurrence of such a phenomenon. Through this analysis, the following conclusion can be drawn: (i) the size distribution of the polymer lattice is a replica of the original monomer droplets arising from the mini-emulsification process, (ii) the droplets are the main locus of polymerization and (iii) the particle aggregation during the polymerization is minimized under the synthesis conditions. It is also worth noting that if we proceed by mixing ex situ the cationic lattice dispersion and the suspension of whiskers separately, instantaneous flocculation occurs, leading to the formation
Intensity (%)
20.0
0%
15.0
1%
2%
10.0
5% 4%
5.0 3%
0.0 10
100
1000
10000
Particle size (nm)
of coarse lumps. Further dilution of the cellulose whiskers down to 0.2% or inversion of the addition order (i.e. the cationic polymer dispersion added to the cellulose nanowhisker or vice versa). That is to say, the emulsification process in the presence of the whole component of the system, namely the monomer, the cationic surfactant and the whiskers, followed by the polymerization is essential in obtaining stable nanocomposite dispersion.
3.1.1. f-Potential analysis Given the key role of the surface charge in controlling the surfactant–whiskers, as well as the monomer–whiskers interactions, f-potentials of cellulose whiskers dispersed in water were investigated. The evolution of the f-potential of a cellulose whiskers dispersion as a function of the amount of the added DCP is shown in Fig. 4. Initially, the whiskers display a high negative f-potential around 40 mV that imparts enough electrostatic stabilization to prevent whisker agglomeration in diluted aqueous solution (viz., <1–2 wt.%) and ensure the suspension stability during storing, albeit their tendency to ‘‘self-assemble’’ into large regions of cholesteric ordering above a critical concentration. The origin of negative charges on the surface of the whiskers obtained by hydrolysis of the fibre in H2SO4 is well documented [23]. Addition of DPC molecules induce an abrupt rise in f-potential to 20 mV, as the level of the surfactant attains about 50 wt.% (respectively to the whiskers content) followed by a continuous rise in the f-potential until it reaches a value close to zero. Then, this parameter undergoes an inversion going up to a positive value with further addition of DCP. The increase in the f-potential is expected and results from the DCP adsorption on the surface negative sites of the whiskers through an exchange mechanism driven by electrostatic interaction. As the f-potential gets close to zero (isoelectric point, iep), the whiskers suspension flocculates. To avoid such an artefact, it was necessary to apply an intense sonication before the f-Potential Measurement. The amount of DPC added to reach the iep lies between 2 and 2.3 times the amount of whisker, from which the whiskers surface negatively charged can be evaluated to be about 800 lmol of SO 4 anions per g of whiskers. The f-potential inversion observed as the DPC is added over the iep is an indication of further adsorption of the surfactant even after the entire negative sites on the surface are consumed. Presumably, this phenomenon results from the hydrophobic interaction between the long alkyl chains of DPC leading to the formation of a second layer with the surfactant head groups facing towards the solution. This phenom-
Fig. 2. Evolution of size distribution of the nanocomposite lattice particles at different whiskers contents.
20.0 10.0
20.0
z-potential (mV)
Intensity (%)
2% 4% 0% 10.0
3%
1%
100
1000
-10.0 -20.0
Polymerization conditions
-30.0 -40.0
0.0 10
0.0
10000
Particle size (nm) Fig. 3. Size distribution of the mini-emulsion droplets prior to the polymerization, as a function of the whiskers content.
-50.0 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0 10.0 11.0
CDTMA (mmol/l) Fig. 4. f-Potential of the whiskers, vs. the amount of the added surfactant (DCP) at pH 7–8. The X-scale corresponds to the weight ratio DCP/whiskers.
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60.0
ζ -potential (mv)
50.0 40.0 30.0 20.0 10.0 0.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Nanowhiskers content (%) Fig. 5. Evolution of the f-potential of the nanocomposite dispersion as a function of the whiskers content.
enon was already evidenced in the case of oxidized cellulose fibres [24]. Evolution of the f-potential of the nanocomposite dispersion according to the whiskers content is shown in Fig. 5. In the absence of the cellulose whiskers, the polymer particles displayed a high positive f-potential around +57 mV for the dispersion prepared with 3 wt.% DPC. The value decreases down to +38 mV for the dispersion prepared in the presence of 1 wt.% cellulose whiskers and is maintained around +30 mV as their loading go up to 5 wt.%. In the presence of 6 and 7 wt.% whiskers, the f-potential reached about +35 mV with an amount of 6 wt.% DPC. In all instances, the values of the f-potential higher than +25 mV accounts for the colloidal stability of the nanocomposite dispersion ensured by the means of electrostatic repulsion among the charged polymer particles. Furthermore, the maintenance of the f-potential around +30 mV provides further supports to the above hypothesis accord-
133
ing to which the partial aggregation of the monomer droplet observed when the content of the whiskers exceeds 3% allows the monomer droplet to acquire enough surface charges to ensure electrostatic stabilization. Indeed, as the amount of added surfactant is at the same level viz., 1%, the rise in the cellulose whiskers will lead to a consumption of a fraction of the cationic surfactant involved in the interaction with the anchored whiskers. The resulting deficiency in the surface charge coverage of the monomer droplet with non-binding surfactant will bring about the partial agglomeration of the monomer droplet to reduce the interfacial area and keep the density of the positive surface charge enough to ensure stabilization. This also explains why the f-potential drops notably from +57 mV for polymer dispersion synthesized in the absence of the cellulose whiskers down to 38 mV as the polymerization was carried in the presence of 1% whiskers. Indeed, as the particle size did not change notably at this level, the anchoring of the whiskers around the monomer droplet will consume a part the surfactant around the droplet leading to a decrease in their surface charge. However, the later remained enough for adequate stabilization, and no partial agglomeration is observed. From the above analysis, we conclude that the nanocomposite dispersion particles are overall positively charged. Furthermore, at the level of the DPC added to implement the polymerization, the cellulose whiskers remained negatively charged. This provides a useful indication about the synthetic routes involved in the formation of PBMA/whiskers colloidal nanocomposites. During the mixing process and the shearing/sonication action to break down the monomer droplets, the surfactant molecules accumulated on their surface and then the whiskers will be attached on them, through the cationic surfactant head groups acting as an anchoring site. Once the temperature is raised to 70 °C, the polymerization is triggered converting each monomer droplet to polymer particles while keeping the nanofibres attached to the nucleated droplet. A schematic illustration of the synthetic procedures is depicted in Fig. 6. This mechanism will be further supported by SEM observation.
Fig. 6. Schematic representation of the synthetic routes involved in the formation of nanowhiskers/polymer colloidal nanocomposites.
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lulose whiskers were monitored using gravimetry by determining the quantity of monomer conversion as a function of time. Results showed in Fig. 7 revealed a quite fast rate with a final conversion reaching 100 wt.% after 60 min. However, albeit a lower number of polymer particles formed, one can note that the polymerization appears to be faster in the presence of the whiskers. This result seems unexpected if one considers that in mini-emulsion polymerization, the monomer droplet is the principle locus of particle nucleation. In turn, the higher the monomer droplet number, the faster the conversion kinetic. In the present case, no convincing explanation could be given.
Conversion (%)
100.0 80.0
0% 1% 2%
60.0
3% 4%
40.0 20.0 0.0 0
10
20
30
40
50
60
70
80
90
100
Time (min) Fig. 7. Monomer conversion vs. time (min) for the mini-emulsion polymerizations of BMA carried out in the presence of different contents of whiskers (see Table 1).
3.1.2. Conversion rate The overall rates of polymerization for the mini-emulsion polymerizations carried out in the presence of increased content of cel-
3.1.3. SEM observations In order to analyse the morphology of the nanocomposite dispersion and to get an accurate idea about the localization of the whiskers, FE-SEM observations were made on different samples. Many preliminary trials were necessary before succeeding to distinguish the whiskers with enough resolution. For this purpose, the following conditions were adopted. First of all, a drop of the nanocomposite dispersion (with a solid content about 1– 0.5 wt.%) was deposited on the surface of a silicon wafer capable of ensuring electron conduction during the analysis. To enhance the electrical conductivity at the surface, samples were coated with
Whiskers l= 150 nm L=11 nm 150 nm
370 nm
520 nm
Fig. 8. FE-SEM image of nanocomposite dispersions with (a) 4% and (b) 5% nanowhiskers content.
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tained by DLS measurements showing bimodal distribution. From Fig. 8, exhibiting FE-SEM images of the nanocomposite dispersion with 4 and 5 wt.% whiskers, the presence of the whiskers strongly adhering to the polymer particles on the particles surface is clearly evidenced, even if some of them were not fully individualized. However, the covering of the polymer particles’ surface by the whiskers seems to be rather heterogeneous. Furthermore, the number of whiskers per polymer particles (Table 2) is also heterogeneous, especially for the large particles bearing the whiskers. This may be due to the method of whiskers addition leading to an uneven distribution of the nanofiller around the monomer droplet. Work is in progress to analyse the effect of addition mode on the whiskers coverage around the polymer particles. Up to 3 wt.%, the size of the polymer particles is not entirely uniform. In fact, the large ones with a size between 400 and 600 nm seem to be more covered with whiskers. It could be assumed that these particles correspond to the second population noted by DLS measurements, which were related to partial particle aggregation during the stage of droplet generation or in the early stage of the polymerization.
Table 2 Polymer particles size (D), calculated number of polymer particle (Np) and nanowhiskers (Nw). %Whiskers 0
1
2
3
4
5
D Np Nw Nw/Np
110 3.0 1014 1.4 1014 0.5
140 1.7 1014 2.8 1014 2.0
185 7.5 1013 4.2 1014 6.0
250 3.0 1013 4.4 1014 15.0
265 2.6 1013 5.6 1014 22.0
101 4.6 1014 0.0 0.0
135
a thin carbon layer applied by sputtering with a thickness being limited to 3–4 nm. Finally, the accelerating voltage was maintained at a relatively low range (0.5–1 kV), in order to prevent the heating of the polymer particles, thus avoiding their melting which could induce the damage of the whiskers during the analysis. The FESEM observation of the nanocomposite dispersion showed spherical polymer particles with diameters ranging from 100 to 130 nm for nanocomposite dispersion with whiskers content of 0–2 wt.%, respectively and from 120 to 600 nm for the dispersion with higher whiskers content. These values are consistent with the data ob-
(a)
(b) 0%
100.0
1% Transmittance (%)
90.0
2% 3%
80.0 70.0
4%
60.0 50.0
40.0 400
500
600
700
800
Wavelength (nm)
(c) Fig. 9. Digital photographs of (a) the nanocomposite dispersion at different nanowhiskers content and after a duration of 7 months. (b) of the nanocomposite films at different nanowhiskers content (mean thickness 200 lm) and (c) the corresponding transmittance spectra recorded in the visible wavelength range.
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3.1.4. Film transparency The visual observation of the nanocomposite dispersion at different whiskers content reveals a milk-like dispersion giving rise to a transparent film after water evaporation (Fig. 9a and b). The photograph of the nanocomposite dispersion taken after a period of 7 months, subsequent to the reaction, does not present any coagulum or phase separation, indicating the long-term stability of these nanocomposites. To access the optical transparency of the nanocomposite, films thick in the range of 200–300 lm were prepared in a Teflon mould, and their transmittance was measured in the visible domain (see Fig. 9c). To avoid the effect of the variation of the film thickness, the film transmittance was normalized to a 200 lm-thick film using the Beer–Lambert law. The film transmittance remains higher than 70% up to a whiskers content of 4 wt.%, which indicates a rather good dispersion of the nanofiller within the film. However, the progressive downward shift of the transmittance implies the inevitable clustering of the whiskers, as their content goes up. The high aptitude of cellulose whiskers to self-associate, namely over a critical level, makes impossible to entirely prevent this phenomenon, although the present approach (in situ polymerization) contributed to immobilize the whiskers distribution prior to the polymerization initiation. These data are in good agreement with the FE-SEM observations, showing that the whiskers are not fully individualized at 4 wt.% content. Besides an academic point of view, one could ask about the utility of the in situ polymerization comparatively to the mixing approach, which is the current route in processing nanocomposite based on cellulose nanofillers. In fact, the approach here presented offers two main advantages. First, it enables the formation of nanocomposite particles in only one reaction step using conventional component for heterophase polymerization. This could simplify the potential applications in the field of adhesive or coating. Indeed, when the mixing process is adopted, a prior dispersion of a dilute cellulose nanofiller by sonication or intensive shearing is prerequisite to ensure an homogenous dispersion throughout the matrix. However, this is not very practical to implement since it needs a special device for the dispersion, the sonicator. Moreover, it brings about a dilution effect leading to a longer film formation process. Second, one has the possibility to tailor the binding degree of the nanofiller to the polymer particles, which in turn will affect the reinforcing potential of the nanofiller, as well as its dispersion degree. Furthermore, the synthesis of polymer/cellulose nanocrystal nanocomposites via heterophase polymerization in water as a dispersion medium is also attractive for environmental consideration. Work is in progress to investigate the mechanical properties of the ensuing nanocomposite film and to compare the reinforcing potential of the cellulose nanocrystal relatively to that obtained with the mixing process. 4. Conclusion Colloidally stable nanocomposite dispersions with a solid content of 25 wt.% and up to 5 wt.% of whiskers loading were efficiently produced via one-step mini-emulsion polymerization. The dispersions obtained were stable for several months, with whiskers being attached around the polymer particles through electrostatic interactions between the cationic head groups of the
surfactant and the negatively charged site on the nanofiller surface. Results showed a continuous increase in the particle size as the nanofiller loading augmented. This phenomenon is explained by the partial aggregation of the monomer droplets resulting from the addition of the whiskers to the emulsion, which decreases the surface-to-volume ratio and achieves a minimum surface coverage necessary to achieve the colloidal stability of the system. The nanocomposite films were prepared by film casting, and their optical properties remained higher than 70% in the presence of up to 4 wt.% of whiskers, indicating a good dispersion of the nanofiller within the film. To the best of our knowledge, this paper reports for the first time the existence of the interactions between the cationic charges of the (monomer or polymer) matrix particles and the anionic sites borne by cellulose nanofiller. The resulting colloidally stable filmforming nanocomposite mini-emulsion is a simple ready-for-use formulation, potentially useful for the development of waterborne adhesive or coating without volatile organic compounds (VOC). The adequate choice of the matrix and the whiskers content could perform materials with the required hardness and strength. Acknowledgments The authors owe an utmost thank for the NATO (Grant NATO062010-CBP.MD. CLG983981), the PHC-UTIQUE (Grant 11/G 1115) and the Tunisian Ministry of Higher Education and Scientific Research for their financial support. References [1] S.J. Eichhorn, A. Dufresne, M. Aranguren, N.E. Marcovich, J.R. Capadona, S.J. Rowan, C. Weder, W. Thielemans, M. Roman, S. Renneckar, W. Gindl, S. Veigel, J. Keckes, H. Yano, K. Abe, M. Nogi, A.N. Nakagaito, A. Mangalam, J. Simonsen, A.S. Benight, A. Bismarck, L.A. Berglund, T. Peijs, J. Mater. Sci. 45 (2010) 1. [2] A. Dufresne, Molecules 15 (2010) 4111. [3] A. Sturcova, G.R. Davies, S.J. Eichhorn, Biomacromolecules 6 (2005) 1055. [4] A. Dufresne, J. Nanosci. Nanotechnol. 6 (2006) 322. [5] J.R. Capadona, K. Shanmuganathan, D.J. Tyler, S.J. Rowan, C. Weder, Science 319 (2008) 1370. [6] S. Ifuku, M. Nogi, K. Abe, K. Handa, F. Nakatsubo, H. Yano, Biomacromolecules 8 (2007) 1973. [7] H. Yano, J. Sugiyama, A.N. Nakagaito, M. Nogi, T. Matsuura, M. Hikita, K. Handa, Adv. Mater. 17 (2005) 153. [8] G. Siqueira, J. Bras, A. Dufresne, Polymer 2 (2010) 765. [9] M.A.S. Azizi Samir, F. Alloin, A. Dufresne, Biomacromolecules 6 (2005) 612. [10] N. Ljungberg, C. Bonini, F. Bortolussi, C. Boisson, L. Heux, J.Y. Cavaillé, Biomacromolecules 6 (2005) 2732. [11] A. Bendahou, Y. Habibi, H. Kaddami, A. Dufresne, J. Biobased Mater. Bioenergy 3 (2009) 81. [12] A. Bendahou, H. Kaddami, A. Dufresne, Eur. Polym. J. 46 (2010) 609. [13] O. Van den Berg, J.R. Capadona, C. Weder, Biomacromolecules 8 (2007) 1353. [14] C. Gousse, H. Chanzy, G. Excoffier, L. Soubeyrand, E. Fleury, Polymer 43 (2002) 2645. [15] J. Araki, M. Wada, S. Kuga, Langmuir 17 (2001) 21. [16] H.H. Yuan, Y. Nishiyama, M. Wada, S. Kuga, Biomacromolecules 7 (2006) 696. [17] Y. Habibi, A.L. Goffin, N. Schiltz, E. Duquesne, P. Dubois, A. Dufresne, J. Mater. Chem. 18 (2008) 5002. [18] A. Ben Mabrouk, W. Thielemans, A. Dufresne, S. Boufi, J. Appl. Polym. Sci. 114 (2009) 2946. [19] A. Ben Mabrouk, H. Kaddami, A. Magnin, M.N. Belgacem, A. Dufresne, S. Boufi, Polym. Eng. Sci. 62 (2011) 62. [20] A. Ben Mabrouk, A. Magnin, M.N. Belgacem, S. Boufi, Compos. Sci. Technol. 71 (2011) 818. [21] K. Landfester, Angew. Chem. Int. 48 (2009) 4488. [22] J.M. Asua, Prog. Polym. Sci. 1283 (2002) 1283. [23] V. Favier, H. Chanzy, J.Y. Cavaille, Macromolecules 28 (1995) 6365. [24] S. Alila, S. Boufi, N. Belgacem, D. Beneventi, Langmuir 21 (2005) 8106.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis and characterization of cellulose whiskers/polymer nanocomposite dispersion by mini-emulsion polymerization Ayman Ben Mabrouk a, Manuel Rei Vilar b, Albert Magnin c, Mohamed Naceur Belgacem d, Sami Boufi a,⇑ a
Laboratoire Sciences des Matériaux et Environnement, LMSE, University of Sfax, BP 802-3018 Sfax, Tunisia Univ. Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 75013 Paris, France c Grenoble Institute of Technology, The International School of Paper, Print Media and Biomaterials (Pagora), UMR CNRS 5518, BP 65, 38402 Saint Martin d’Hères Cedex, France d Laboratoire de Rhéologie, Grenoble-INP, UJF Grenoble 1, UMR CNRS 5520, BP 53, 38041 Grenoble Cedex 9, France b
a r t i c l e
i n f o
Article history: Received 10 May 2011 Accepted 18 July 2011 Available online 27 July 2011 Keywords: Nanocomposites Dispersion Colloids Cellulose
a b s t r a c t Stable film-forming nanocomposite particles with diameters ranging from 120 to 300 nm, based on polybutylmethacrylate (PBMA) and cellulose whiskers in water dispersions, were successfully synthesized in one step through mini-emulsion polymerization. The nanocomposite dispersion with a solid content of 25 wt.% and up to 5 wt.% of nanofiller loading was prepared by in situ polymerization, in the presence of the whiskers using dodecylpyridinium chloride (DPC), as a cationic surfactant, and 2,2azobis(isobutyronitrile) (AIBN), as initiator. The electrostatic interaction between the positively charged droplets and negatively charged whiskers ensured the anchoring of the nanofiller around the polymer particles. The ensuing dispersions were characterized by Dynamic Light Scattering (DLS), f-Potential Measurements, and Field-Emission Scanning Electron Microscopy (FE-SEM). After the film formation process, the nanocomposite film exhibits a high transparency, denoting the good dispersion of the whiskers throughout the matrix. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Cellulose nanofibres are one of the most promising reinforcements in nanocomposites based on organic polymer matrices [1,2]. In fact, taking into account the high storage modulus of this nanofiller in the range of 50–100 GPa [3] and its high breaking strength allied with their aspect ratio ranging between 10 and 100 [2], it is to expect a huge reinforcing effect once the nanofiller will be incorporated into a polymer matrices. Furthermore, the capacity of the nanofiller to bring on a percolated network holdup through strong hydrogen interactions between adjacent nanofibres resulted in a spectacular enhancement in the storage modulus, as well as in the breaking strength, even at filler levels lower than 5 wt.% [4,5]. The high transparency degree of the nanocomposite based on cellulose nano-sized filler is another feature that should be highlighted [6,7]. Actually, as long as a nanofiller of width less than one-tenth of the visible light wavelength is embedded into a transparent polymer matrix, the optical properties of the material should not be altered, providing the nanofibres do not aggregate. However, even if the high reinforcing potential of cellulose nanofiller is a well-established feature, as highlighted by ⇑ Corresponding author. Address: Laboratoire LMSE, University of Sfax, BP 1171 3000 Sfax, Tunisia. Fax: +216 74274437. E-mail address: sami.boufi@fss.rnu.tn (S. Boufi). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.07.050
numerous reports [8,9], this property is tributary of the good dispersion of the nanofiller within the host polymer matrix during the nanocomposite processing. In fact, owing to their nano-sized scale and their high surface hydroxyl group concentration, cellulose whiskers have a strong tendency to self-associate, leading to irreversible aggregation during the mixing process, which impedes any percolation to occur and decreases significantly the specific surface, thus losing a part of their beneficial effect [10]. Bearing in mind that cellulose whiskers are well stable in diluted aqueous suspensions, namely for those carrying sulphate or carboxyl anions, the mixing of aqueous polymer solutions or latex with cellulose whisker suspensions and the subsequent film casting remains the major method to conceive the nanocomposites [11,12]. The dispersion of the nanofibres in a high polar organic media such as dimethylformamide, dimethylsulphoxide or N-methylpyrrolidine [13], or in organic solvent after the chemical surface modification, involving the chemistry of the hydroxyl groups, has also been explored. Silylation [14], grafting of PEO [15] and acylation [16], for instance, have shown to improve the whiskers dispersion in organic solvents. Grafting of the whiskers surface is also expected to open up a new strategy to achieve a better dispersion of the nanofiller into the polymer matrix [17]. To the best of our knowledge, no attempts have been reported about the melt mixing behaviour of the grafted whiskers with thermoplastic polymer. However, when the surface modification succeeds to bring about
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the whiskers dispersion in low-polarity media, it usually reduces the inter-whiskers interactions, thus limiting the formation of load-bearing percolating network within the host polymer matrix. In situ polymerization in the presence of cellulose whiskers could be envisaged as another alternative way to produce tailored nanocomposites with a controlled degree of covalent linkage between the matrix and the nanofiller, as well as a good dispersion degree. For this purpose, in situ heterophase polymerization in aqueous media is preferred for different reasons: (i) There is no need to remove the whiskers from water in which they are well dispersed. This is a great advantage if one considers that once in the dry form, cellulose whiskers aggregate irreversibly. (ii) Using water as a dispersion medium is attractive for environmental considerations. (iii) The approach offers great advantage (in terms of process simplicity) over solution or bulk polymerization to tailor the nanocomposites’ morphology. In a previous work, nanocomposite dispersions with a solid content up to 30% and cellulose whiskers loading ranging from 1% to 6% were successfully prepared via mini-emulsion polymerization in the presence of anionic surfactant and methacryloxypropyl triethoxysilane (MPS), as a coupling agent [18]. It was shown that the amount of the added MPS greatly affects the reinforcing extent of the whiskers [19,20]. In the same vein, the present work introduces a new approach to prepare stable nanocomposite dispersions with a strong binding between the polymer particles and the whiskers. The nanocomposite dispersion was prepared by mini-emulsion polymerization using cationic surfactant and negatively charged whiskers. The idea is to emulsify the monomer containing the initiator in water in the presence of the whiskers suspension and the cationic surfactant. During the emulsification process, fine droplets of monomer stabilized by the cationic surfactant, onto which the whiskers could be anchored, are formed. Then, the polymerization will be induced by raising the temperature, which yields in situ free radical formation. The resulting reinforced emulsion could be transformed into nanocomposites by simple film-casting process. From the colloidal point of view, the proposed system requires the achievement of two major challenges. First, the heterophase in situ polymerization of monomer in the presence of cellulose whiskers and a cationic surfactant is not straightforward to implement. Indeed, given the anionic charges borne by the whiskers and the cationic ones associated with the surfactant, different events could occur. The most critical one concerns the colloidal stability of the system and is associated with the adsorption of cationic surfactant on the cellulose nanocrystal surface, leading to the flocculation, aggregation of the cellulose whiskers and coagulation of the polymer particles induced by the cellulose whiskers. Second, the miniemulsion polymerization was selected to guarantee the stability of the colloidal system in such a way that the polymerization exclusively occurs within the droplet. Moreover, this choice prevents homogeneous and micellar nucleation during the nucleation process. Such a specific mechanism is favoured by the addition of a co-stabilizer (or hydrophobic agent) that hinders Ostwald ripening and the diffusion of the monomer from one droplet to another. A detailed description of mini-emulsion polymerization mechanism has been the subject of several reviews [21,22]. Thus, the main purpose of this paper is to prepare water-based colloidally stable mini-emulsion of nanocomposites made of nanosized cellulose-reinforcing elements and organic polymer matrix, using a one-pot process without needing any isolation or purification stage. To the best of our knowledge, such a system has never been reported in the literature.
2. Experimental section 2.1. Materials Butylmethacrylate (BMA, from Aldrich, 99 wt.%) was distillated in vacuum and kept refrigerated until use. 2,2-Azobis(isobutyronitrile) (AIBN), sodium dioctylsulfano succinate (DOSS), hexadecane (HD) and dodecylpyridinium chloride (DPC) are commercial products supplied by Aldrich and used without further purification. Distilled water was used for all the polymerization and treatment processes. The critical micelle concentration (cmc) of DPC in distilled water at 25 °C determined by conductometry was found to be 1.2 102 L1. 2.2. Whisker preparation The method used to prepare the alfa whiskers was reported elsewhere [18]. Based on TEM analysis, the whiskers derived from alfa fibres displayed a needle-like structure with an average length L and width d being estimated to be around L = 150 ± 20 and d = 10 ± 2 nm, respectively. 2.3. Mini-emulsion preparation and polymerization The mini-emulsion typical composition is reported in Table 1. The emulsion was prepared mechanically starting from an oil phase containing HD, AIBN and the monomers mixture and 15 mL of water containing DPC. Then, the cellulose whiskers suspension was drop-by-drop added to the previously prepared emulsion, under sonication during 3 min, using ultrasonic homogenizer (Sonics Vibracel Model CV33) at a duty cycle of 70 wt.%. During the sonication process, the reactor was immersed into an ice bath. Upon completion of the sonication, the resulting mini-emulsion was polymerized at 70 °C for 3 h under continuous mechanical stirring with a half-moon shaped Teflon stirrer at 300 rpm and an inert nitrogen stream. 2.4. Calculations of polymer particle whiskers number The number of polymer particles per mL of dispersion was estimated as follows:
Np ¼
mp
ð1Þ
qp 43 pR3p
where p is the density of PBM at 25 °C (1.1 g cm3), mp is the mass of polymer per mL of the dispersion and Rp their hydrodynamic radius. Assuming that the cellulose whiskers have a rod-like shape with a square cross section, their number per mL of the nanocomposite dispersion could be estimated by:
Nw ¼
%W mp
ð2Þ
qw pd2 L
Table 1 Main constituent and their relative quantities (w/w, %) for the mini-emulsion polymerization of butylmethacrylate in the presence of cellulose whiskers. Component
Added (g)
% With respect to monomer
Butylmethacrylate Hexadecane AIBN Cellulose whiskers Dodecylpyridinium chloride Water
7.00 0.35 0.15 0–0.45 0.20 30.0–29.5
100.0 5.0 2.0 0–6.0 3.0
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where %W is the wt.% content of whiskers with respect to polymer, qw is the volume weigh of cellulose (in g cm3), d and L are the width and length of the whiskers (L = 150 ± 20 and d = 10 ± 2 nm), respectively. 2.5. Particle size determination The particle diameters were measured at 25 °C, using a Malvern Nano-Zetasizer ZS Instrument at a fixed scattering angle of 173°. The dispersion was diluted to about 5 wt.% with distilled water before starting the measurements. Dynamic Light Scattering (DLS) measurements give a Z-average size, which gives the mean diameter of the particles. Each measurement was repeated in triplicate, and the obtained values averaged to obtain the mean particle size. The apparent hydrodynamic diameter was calculated according to the Stokes–Einstein equation:
dh ¼
kT 3pgDapp
ð3Þ
where Dapp is the apparent diffusion coefficient and g the solution viscosity. 2.6. Polymerization kinetics Instantaneous and global polymerization conversions were determined by gravimetry. The solid content at a given polymerization time, Sc(t), was determined, and the conversion rate, %C(t), calculated according to:
%C ¼
ScðtÞ 100 Scð1Þ
ð4Þ
where Sc(t) is the solid content for a time t and (Sc(1)) for a complete polymerization reaction, respectively 2.7. Field-Emission Scanning Electron Microscopy (FE-SEM) A Weiss scanning electron microscope, operated at an accelerating voltage between 0.75 and 2 kV, was used to capture secondary electrons images of the nanofibrillated surface. A drop of the diluted nanocomposite dispersion (with a solid content of 0.1– 0.3 wt.%) was deposited on a surface of silicon wafer and freeze dried at 20 °C to remove water, so avoiding particle coalescence. Then, the sample was coated with a thin carbon layer applied by sputtering with a thickness limited to 3–4 nm.
merization, converting the monomer droplet into polymer particles. After several preliminary runs in which the amount as well as the structure of the surfactant were changed, we have succeeded the preparation of a stable dispersion without any trace of coagulum. It was shown that in the presence of DPC, a minimum of 3 wt.% surfactant based on monomer content is necessary to ensure stable 25 wt.% solid content dispersion. This system was chosen to implement the nanocomposite dispersion synthesis, as it ensures a compromise between a minimum amount of water to disperse the whiskers suspension without the excessive build-up of its viscosity and the film-forming aptitude of the dispersion after water evaporation and particles coalescence.
3.1. Evolution of particle size during polymerization Relatively monodisperse particles with an average particle size of 100 nm were obtained in the absence of the whiskers. However, when polymerization was carried out in the presence of the whiskers, a continuous growth in the particle size was observed as shown in Fig. 1. For instance, the mean particle size increased from 110 up to 265 nm, when the whiskers loading increased from 1 to 5 wt.%. Over 5 wt.%, it was necessary to increase the amount of the DPC to 6 wt.% (based on the monomer content) to prevent coagulation. In these conditions, the ensuing dispersion was completely stable, reaching a mean particle size between 290 and 350 nm at a filler content of 6 and 7 wt.%, respectively. The analysis of the particle size distribution reveals to be monomodal for dispersions with whiskers content lower than 2 wt.%, whereas a bimodal distribution is observed up to 3 wt.% of whiskers content. The first particles’ population size emerged around 110–140 nm and the second one around 500–600 nm (Fig. 2). The question now to answer is how the presence of the whiskers up to 3% brings about a bimodal distribution with one population at 110 nm and the second one around 500–600 nm. We infer that such behaviour arises from the partial aggregation of the monomer droplet following the adsorption of cellulose whiskers on their surface. Indeed, due to their negative surface charges, once the cellulose whiskers are added to the monomer emulsion, they will spontaneously adsorbed on the monomer droplet positively charged. Adsorption is driven by the electrostatic interaction between the negative sites on the cellulose whiskers and the headon cationic groups of the surfactant located around the monomer
350.0
2.8. f-Potential Measurement
3. Results and discussion The main advantage of the mini-emulsion polymerization relies on the predominant nucleation in the submicronic monomer droplet with the consequence that the polymer particles are the replica of the mini-emulsion droplets. This feature is favoured by the addition of the hydrophobic agent that hinders Ostwald ripening and the diffusion of the monomer from a droplet to another [21]. Accordingly, if the monomer droplets were generated in the presence of the cellulose whiskers and a cationic surfactant, the negatively charged whiskers could be immobilized on the positively charged droplet. Then, heating the dispersion will trigger the poly-
300.0 250.0
Size (nm)
The f-potentials were measured at 25 °C using a laser Doppler electrophoresis apparatus (Malvern Nano-Zetasizer ZS, UK). The sample consistency in water was set to 0.01% (w/v). The measurements were performed three times for each sample.
200.0 150.0 100.0 50.0 0.0 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
nanowhiskers content (%) Fig. 1. Evolution of the particle size, as a function of the nanowhiskers content (with respect to the polymer) in the dispersion. The solid content of the nanocomposite dispersion is equal to 25 wt.%.
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droplet. The resulting reduction in the droplet (or nucleated polymer particles) charge should lead to a partial aggregation, which decreases the surface-to-volume ratio, achieving a minimum surface charge, sufficient to ensure the colloidal stability. This hypothesis is reinforced by the f-potential analysis and the fact that the bimodal distribution in the presence of up to 3% whiskers appeared prior to the polymerization set off (activation), as soon as the emulsion is formed. This is clearly revealed by DLS measurements (cf. Fig. 3) that show the size distribution of the droplet before polymerization. It is worthy to note the small upward shift in the droplet size compared to the corresponding polymer particle size. Presumably, this may be due to the surfactant desorption and the monomer dissolution in the aqueous phase induced by the dilution operation before implementing the DLS measurement. Even though we have succeeded to reduce this effect by adding an aqueous solution of the cationic surfactant at the same concentration as that adopted for the polymerization, it was impossible to completely avoid the occurrence of such a phenomenon. Through this analysis, the following conclusion can be drawn: (i) the size distribution of the polymer lattice is a replica of the original monomer droplets arising from the mini-emulsification process, (ii) the droplets are the main locus of polymerization and (iii) the particle aggregation during the polymerization is minimized under the synthesis conditions. It is also worth noting that if we proceed by mixing ex situ the cationic lattice dispersion and the suspension of whiskers separately, instantaneous flocculation occurs, leading to the formation
Intensity (%)
20.0
0%
15.0
1%
2%
10.0
5% 4%
5.0 3%
0.0 10
100
1000
10000
Particle size (nm)
of coarse lumps. Further dilution of the cellulose whiskers down to 0.2% or inversion of the addition order (i.e. the cationic polymer dispersion added to the cellulose nanowhisker or vice versa). That is to say, the emulsification process in the presence of the whole component of the system, namely the monomer, the cationic surfactant and the whiskers, followed by the polymerization is essential in obtaining stable nanocomposite dispersion.
3.1.1. f-Potential analysis Given the key role of the surface charge in controlling the surfactant–whiskers, as well as the monomer–whiskers interactions, f-potentials of cellulose whiskers dispersed in water were investigated. The evolution of the f-potential of a cellulose whiskers dispersion as a function of the amount of the added DCP is shown in Fig. 4. Initially, the whiskers display a high negative f-potential around 40 mV that imparts enough electrostatic stabilization to prevent whisker agglomeration in diluted aqueous solution (viz., <1–2 wt.%) and ensure the suspension stability during storing, albeit their tendency to ‘‘self-assemble’’ into large regions of cholesteric ordering above a critical concentration. The origin of negative charges on the surface of the whiskers obtained by hydrolysis of the fibre in H2SO4 is well documented [23]. Addition of DPC molecules induce an abrupt rise in f-potential to 20 mV, as the level of the surfactant attains about 50 wt.% (respectively to the whiskers content) followed by a continuous rise in the f-potential until it reaches a value close to zero. Then, this parameter undergoes an inversion going up to a positive value with further addition of DCP. The increase in the f-potential is expected and results from the DCP adsorption on the surface negative sites of the whiskers through an exchange mechanism driven by electrostatic interaction. As the f-potential gets close to zero (isoelectric point, iep), the whiskers suspension flocculates. To avoid such an artefact, it was necessary to apply an intense sonication before the f-Potential Measurement. The amount of DPC added to reach the iep lies between 2 and 2.3 times the amount of whisker, from which the whiskers surface negatively charged can be evaluated to be about 800 lmol of SO 4 anions per g of whiskers. The f-potential inversion observed as the DPC is added over the iep is an indication of further adsorption of the surfactant even after the entire negative sites on the surface are consumed. Presumably, this phenomenon results from the hydrophobic interaction between the long alkyl chains of DPC leading to the formation of a second layer with the surfactant head groups facing towards the solution. This phenom-
Fig. 2. Evolution of size distribution of the nanocomposite lattice particles at different whiskers contents.
20.0 10.0
20.0
z-potential (mV)
Intensity (%)
2% 4% 0% 10.0
3%
1%
100
1000
-10.0 -20.0
Polymerization conditions
-30.0 -40.0
0.0 10
0.0
10000
Particle size (nm) Fig. 3. Size distribution of the mini-emulsion droplets prior to the polymerization, as a function of the whiskers content.
-50.0 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0 10.0 11.0
CDTMA (mmol/l) Fig. 4. f-Potential of the whiskers, vs. the amount of the added surfactant (DCP) at pH 7–8. The X-scale corresponds to the weight ratio DCP/whiskers.
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60.0
ζ -potential (mv)
50.0 40.0 30.0 20.0 10.0 0.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Nanowhiskers content (%) Fig. 5. Evolution of the f-potential of the nanocomposite dispersion as a function of the whiskers content.
enon was already evidenced in the case of oxidized cellulose fibres [24]. Evolution of the f-potential of the nanocomposite dispersion according to the whiskers content is shown in Fig. 5. In the absence of the cellulose whiskers, the polymer particles displayed a high positive f-potential around +57 mV for the dispersion prepared with 3 wt.% DPC. The value decreases down to +38 mV for the dispersion prepared in the presence of 1 wt.% cellulose whiskers and is maintained around +30 mV as their loading go up to 5 wt.%. In the presence of 6 and 7 wt.% whiskers, the f-potential reached about +35 mV with an amount of 6 wt.% DPC. In all instances, the values of the f-potential higher than +25 mV accounts for the colloidal stability of the nanocomposite dispersion ensured by the means of electrostatic repulsion among the charged polymer particles. Furthermore, the maintenance of the f-potential around +30 mV provides further supports to the above hypothesis accord-
133
ing to which the partial aggregation of the monomer droplet observed when the content of the whiskers exceeds 3% allows the monomer droplet to acquire enough surface charges to ensure electrostatic stabilization. Indeed, as the amount of added surfactant is at the same level viz., 1%, the rise in the cellulose whiskers will lead to a consumption of a fraction of the cationic surfactant involved in the interaction with the anchored whiskers. The resulting deficiency in the surface charge coverage of the monomer droplet with non-binding surfactant will bring about the partial agglomeration of the monomer droplet to reduce the interfacial area and keep the density of the positive surface charge enough to ensure stabilization. This also explains why the f-potential drops notably from +57 mV for polymer dispersion synthesized in the absence of the cellulose whiskers down to 38 mV as the polymerization was carried in the presence of 1% whiskers. Indeed, as the particle size did not change notably at this level, the anchoring of the whiskers around the monomer droplet will consume a part the surfactant around the droplet leading to a decrease in their surface charge. However, the later remained enough for adequate stabilization, and no partial agglomeration is observed. From the above analysis, we conclude that the nanocomposite dispersion particles are overall positively charged. Furthermore, at the level of the DPC added to implement the polymerization, the cellulose whiskers remained negatively charged. This provides a useful indication about the synthetic routes involved in the formation of PBMA/whiskers colloidal nanocomposites. During the mixing process and the shearing/sonication action to break down the monomer droplets, the surfactant molecules accumulated on their surface and then the whiskers will be attached on them, through the cationic surfactant head groups acting as an anchoring site. Once the temperature is raised to 70 °C, the polymerization is triggered converting each monomer droplet to polymer particles while keeping the nanofibres attached to the nucleated droplet. A schematic illustration of the synthetic procedures is depicted in Fig. 6. This mechanism will be further supported by SEM observation.
Fig. 6. Schematic representation of the synthetic routes involved in the formation of nanowhiskers/polymer colloidal nanocomposites.
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lulose whiskers were monitored using gravimetry by determining the quantity of monomer conversion as a function of time. Results showed in Fig. 7 revealed a quite fast rate with a final conversion reaching 100 wt.% after 60 min. However, albeit a lower number of polymer particles formed, one can note that the polymerization appears to be faster in the presence of the whiskers. This result seems unexpected if one considers that in mini-emulsion polymerization, the monomer droplet is the principle locus of particle nucleation. In turn, the higher the monomer droplet number, the faster the conversion kinetic. In the present case, no convincing explanation could be given.
Conversion (%)
100.0 80.0
0% 1% 2%
60.0
3% 4%
40.0 20.0 0.0 0
10
20
30
40
50
60
70
80
90
100
Time (min) Fig. 7. Monomer conversion vs. time (min) for the mini-emulsion polymerizations of BMA carried out in the presence of different contents of whiskers (see Table 1).
3.1.2. Conversion rate The overall rates of polymerization for the mini-emulsion polymerizations carried out in the presence of increased content of cel-
3.1.3. SEM observations In order to analyse the morphology of the nanocomposite dispersion and to get an accurate idea about the localization of the whiskers, FE-SEM observations were made on different samples. Many preliminary trials were necessary before succeeding to distinguish the whiskers with enough resolution. For this purpose, the following conditions were adopted. First of all, a drop of the nanocomposite dispersion (with a solid content about 1– 0.5 wt.%) was deposited on the surface of a silicon wafer capable of ensuring electron conduction during the analysis. To enhance the electrical conductivity at the surface, samples were coated with
Whiskers l= 150 nm L=11 nm 150 nm
370 nm
520 nm
Fig. 8. FE-SEM image of nanocomposite dispersions with (a) 4% and (b) 5% nanowhiskers content.
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tained by DLS measurements showing bimodal distribution. From Fig. 8, exhibiting FE-SEM images of the nanocomposite dispersion with 4 and 5 wt.% whiskers, the presence of the whiskers strongly adhering to the polymer particles on the particles surface is clearly evidenced, even if some of them were not fully individualized. However, the covering of the polymer particles’ surface by the whiskers seems to be rather heterogeneous. Furthermore, the number of whiskers per polymer particles (Table 2) is also heterogeneous, especially for the large particles bearing the whiskers. This may be due to the method of whiskers addition leading to an uneven distribution of the nanofiller around the monomer droplet. Work is in progress to analyse the effect of addition mode on the whiskers coverage around the polymer particles. Up to 3 wt.%, the size of the polymer particles is not entirely uniform. In fact, the large ones with a size between 400 and 600 nm seem to be more covered with whiskers. It could be assumed that these particles correspond to the second population noted by DLS measurements, which were related to partial particle aggregation during the stage of droplet generation or in the early stage of the polymerization.
Table 2 Polymer particles size (D), calculated number of polymer particle (Np) and nanowhiskers (Nw). %Whiskers 0
1
2
3
4
5
D Np Nw Nw/Np
110 3.0 1014 1.4 1014 0.5
140 1.7 1014 2.8 1014 2.0
185 7.5 1013 4.2 1014 6.0
250 3.0 1013 4.4 1014 15.0
265 2.6 1013 5.6 1014 22.0
101 4.6 1014 0.0 0.0
135
a thin carbon layer applied by sputtering with a thickness being limited to 3–4 nm. Finally, the accelerating voltage was maintained at a relatively low range (0.5–1 kV), in order to prevent the heating of the polymer particles, thus avoiding their melting which could induce the damage of the whiskers during the analysis. The FESEM observation of the nanocomposite dispersion showed spherical polymer particles with diameters ranging from 100 to 130 nm for nanocomposite dispersion with whiskers content of 0–2 wt.%, respectively and from 120 to 600 nm for the dispersion with higher whiskers content. These values are consistent with the data ob-
(a)
(b) 0%
100.0
1% Transmittance (%)
90.0
2% 3%
80.0 70.0
4%
60.0 50.0
40.0 400
500
600
700
800
Wavelength (nm)
(c) Fig. 9. Digital photographs of (a) the nanocomposite dispersion at different nanowhiskers content and after a duration of 7 months. (b) of the nanocomposite films at different nanowhiskers content (mean thickness 200 lm) and (c) the corresponding transmittance spectra recorded in the visible wavelength range.
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3.1.4. Film transparency The visual observation of the nanocomposite dispersion at different whiskers content reveals a milk-like dispersion giving rise to a transparent film after water evaporation (Fig. 9a and b). The photograph of the nanocomposite dispersion taken after a period of 7 months, subsequent to the reaction, does not present any coagulum or phase separation, indicating the long-term stability of these nanocomposites. To access the optical transparency of the nanocomposite, films thick in the range of 200–300 lm were prepared in a Teflon mould, and their transmittance was measured in the visible domain (see Fig. 9c). To avoid the effect of the variation of the film thickness, the film transmittance was normalized to a 200 lm-thick film using the Beer–Lambert law. The film transmittance remains higher than 70% up to a whiskers content of 4 wt.%, which indicates a rather good dispersion of the nanofiller within the film. However, the progressive downward shift of the transmittance implies the inevitable clustering of the whiskers, as their content goes up. The high aptitude of cellulose whiskers to self-associate, namely over a critical level, makes impossible to entirely prevent this phenomenon, although the present approach (in situ polymerization) contributed to immobilize the whiskers distribution prior to the polymerization initiation. These data are in good agreement with the FE-SEM observations, showing that the whiskers are not fully individualized at 4 wt.% content. Besides an academic point of view, one could ask about the utility of the in situ polymerization comparatively to the mixing approach, which is the current route in processing nanocomposite based on cellulose nanofillers. In fact, the approach here presented offers two main advantages. First, it enables the formation of nanocomposite particles in only one reaction step using conventional component for heterophase polymerization. This could simplify the potential applications in the field of adhesive or coating. Indeed, when the mixing process is adopted, a prior dispersion of a dilute cellulose nanofiller by sonication or intensive shearing is prerequisite to ensure an homogenous dispersion throughout the matrix. However, this is not very practical to implement since it needs a special device for the dispersion, the sonicator. Moreover, it brings about a dilution effect leading to a longer film formation process. Second, one has the possibility to tailor the binding degree of the nanofiller to the polymer particles, which in turn will affect the reinforcing potential of the nanofiller, as well as its dispersion degree. Furthermore, the synthesis of polymer/cellulose nanocrystal nanocomposites via heterophase polymerization in water as a dispersion medium is also attractive for environmental consideration. Work is in progress to investigate the mechanical properties of the ensuing nanocomposite film and to compare the reinforcing potential of the cellulose nanocrystal relatively to that obtained with the mixing process. 4. Conclusion Colloidally stable nanocomposite dispersions with a solid content of 25 wt.% and up to 5 wt.% of whiskers loading were efficiently produced via one-step mini-emulsion polymerization. The dispersions obtained were stable for several months, with whiskers being attached around the polymer particles through electrostatic interactions between the cationic head groups of the
surfactant and the negatively charged site on the nanofiller surface. Results showed a continuous increase in the particle size as the nanofiller loading augmented. This phenomenon is explained by the partial aggregation of the monomer droplets resulting from the addition of the whiskers to the emulsion, which decreases the surface-to-volume ratio and achieves a minimum surface coverage necessary to achieve the colloidal stability of the system. The nanocomposite films were prepared by film casting, and their optical properties remained higher than 70% in the presence of up to 4 wt.% of whiskers, indicating a good dispersion of the nanofiller within the film. To the best of our knowledge, this paper reports for the first time the existence of the interactions between the cationic charges of the (monomer or polymer) matrix particles and the anionic sites borne by cellulose nanofiller. The resulting colloidally stable filmforming nanocomposite mini-emulsion is a simple ready-for-use formulation, potentially useful for the development of waterborne adhesive or coating without volatile organic compounds (VOC). The adequate choice of the matrix and the whiskers content could perform materials with the required hardness and strength. Acknowledgments The authors owe an utmost thank for the NATO (Grant NATO062010-CBP.MD. CLG983981), the PHC-UTIQUE (Grant 11/G 1115) and the Tunisian Ministry of Higher Education and Scientific Research for their financial support. References [1] S.J. Eichhorn, A. Dufresne, M. Aranguren, N.E. Marcovich, J.R. Capadona, S.J. Rowan, C. Weder, W. Thielemans, M. Roman, S. Renneckar, W. Gindl, S. Veigel, J. Keckes, H. Yano, K. Abe, M. Nogi, A.N. Nakagaito, A. Mangalam, J. Simonsen, A.S. Benight, A. Bismarck, L.A. Berglund, T. Peijs, J. Mater. Sci. 45 (2010) 1. [2] A. Dufresne, Molecules 15 (2010) 4111. [3] A. Sturcova, G.R. Davies, S.J. Eichhorn, Biomacromolecules 6 (2005) 1055. [4] A. Dufresne, J. Nanosci. Nanotechnol. 6 (2006) 322. [5] J.R. Capadona, K. Shanmuganathan, D.J. Tyler, S.J. Rowan, C. Weder, Science 319 (2008) 1370. [6] S. Ifuku, M. Nogi, K. Abe, K. Handa, F. Nakatsubo, H. Yano, Biomacromolecules 8 (2007) 1973. [7] H. Yano, J. Sugiyama, A.N. Nakagaito, M. Nogi, T. Matsuura, M. Hikita, K. Handa, Adv. Mater. 17 (2005) 153. [8] G. Siqueira, J. Bras, A. Dufresne, Polymer 2 (2010) 765. [9] M.A.S. Azizi Samir, F. Alloin, A. Dufresne, Biomacromolecules 6 (2005) 612. [10] N. Ljungberg, C. Bonini, F. Bortolussi, C. Boisson, L. Heux, J.Y. Cavaillé, Biomacromolecules 6 (2005) 2732. [11] A. Bendahou, Y. Habibi, H. Kaddami, A. Dufresne, J. Biobased Mater. Bioenergy 3 (2009) 81. [12] A. Bendahou, H. Kaddami, A. Dufresne, Eur. Polym. J. 46 (2010) 609. [13] O. Van den Berg, J.R. Capadona, C. Weder, Biomacromolecules 8 (2007) 1353. [14] C. Gousse, H. Chanzy, G. Excoffier, L. Soubeyrand, E. Fleury, Polymer 43 (2002) 2645. [15] J. Araki, M. Wada, S. Kuga, Langmuir 17 (2001) 21. [16] H.H. Yuan, Y. Nishiyama, M. Wada, S. Kuga, Biomacromolecules 7 (2006) 696. [17] Y. Habibi, A.L. Goffin, N. Schiltz, E. Duquesne, P. Dubois, A. Dufresne, J. Mater. Chem. 18 (2008) 5002. [18] A. Ben Mabrouk, W. Thielemans, A. Dufresne, S. Boufi, J. Appl. Polym. Sci. 114 (2009) 2946. [19] A. Ben Mabrouk, H. Kaddami, A. Magnin, M.N. Belgacem, A. Dufresne, S. Boufi, Polym. Eng. Sci. 62 (2011) 62. [20] A. Ben Mabrouk, A. Magnin, M.N. Belgacem, S. Boufi, Compos. Sci. Technol. 71 (2011) 818. [21] K. Landfester, Angew. Chem. Int. 48 (2009) 4488. [22] J.M. Asua, Prog. Polym. Sci. 1283 (2002) 1283. [23] V. Favier, H. Chanzy, J.Y. Cavaille, Macromolecules 28 (1995) 6365. [24] S. Alila, S. Boufi, N. Belgacem, D. Beneventi, Langmuir 21 (2005) 8106.