NRT 16: Hetero-structured Polymer Nanoparticles for Toner Materials

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1 NRT-16, Quarterly report, Mar2009-May2009, Page 1 of 9 NRT 16: Hetero-structured Polymer Nanoparticles for Toner Materials Aasheesh Srivastava and Galen D. Stucky Background and Motivation: The commercial toner particles are usually treated with external additives to achieve optimal performance. ne important additive is paraffin wax that promotes particle flow and fusion to the paper. However, in current toner materials, the wax is randomly mixed during toner preparation. This often results in nonuniform distribution of the wax, and can also result in its complete phase separation. This phase-separation can also occur with time on storage and results in compromised toner performance. We attempt to circumvent these problems in the current toners by a novel design of the toner particles wherein the wax is incorporated within the toner particle. A schematic of our proposed design for such toner particles is shown in Fig. 1. We propose a core-shell structure of the particles with the core made up of a low T g polymer entrapping the wax, and the shell is made from a mixture of high T g polymer and adhesion enhancing ingredients. This allows for an efficient use of ingredients and would prevent the phase-separation problems by design. The low T g core and the high T g shell will also improve flowability as well as mechanical and chemical stability of particles. Shell of high T g polymer and adhesion enhancers Core of wax entrapped in low T g polymer Fig. 1. Schematic of the novel toner particles where the core is made up of wax entrapped in a low T g polymer and the shell is made up of a high T g polymer and other ingredients that allow for enhanced adhesion to the paper.

2 NRT-16, Quarterly report, Mar2009-May2009, Page 2 of 9 Chemicals used: H 2 N + + NH 2 Cl - N N Cl - NH 2 H 2 N 2,2 -azobis(2-methylpropionamidine)dihydrochloride (V-50) (Initiator) S Ethyleneglycoldimethacrylate (EGDMA) (Crosslinker) S S Dibenzyl trithiocarbonate (RAFT reagent) N N-vinyl pyrrolidone (Adhesion promoter), n-butyl acrylate (low T g monomer), Styrene (high T g monomer) Brij 97 (C 18 H 35 -(CH 2 CH 2 ) n -H, n 10) (Surfactant) Heat, Add initiator Add other monomer Nanodrops of monomer + wax stabilized by surfactant Nanoparticles of wax entrapped in polymer nanosphere Shell of high T g polymer around core of waxpolymer nanosphere Fig. 2. Synthetic scheme for preparing the core-shell nanoparticles shown in Fig. 1.

3 NRT-16, Quarterly report, Mar2009-May2009, Page 3 of 9 1. Synthesis design and choice of surfactant: To achieve the design shown in Fig. 1, microemulsion polymerization of the monomers was attempted. This involved use of a surfactant to form the nano-droplets of a mixture of low T g monomer and wax in water. These hydrophobic droplets stabilized by surfactant were polymerized using a water soluble azo-based initiator. The resulting polymer nanoparticles are then used to template the formation of the high T g polymer shell by the use of living polymerization. This is schematically illustrated in Fig. 2. Surfactant plays an important role in achieving good, uniform emulsification of the monomer and wax. We employed a non-ionic surfactant, Brij 97 (C 18 H 35 -(CH 2 CH 2 ) n - H, n 10). Brij 97 was chosen due to the presence of long hydrophobic and hydrophilic segments in the form of an oleyl chain and polyoxyethylene groups, respectively. The hydrophobic part helps in solubilizing the wax efficiently while the long oxyethylene segment confers good water-solubility. It is known to have very low critical micellar concentration (cmc) of ~ 20 µm or 0.03 wt.%, compared to 1.2 mm for Sodium dodecylbenzenesulfonate (SDBS). We could successfully employ 1 wt.% solution of Brij 97 for preparation of these polymer nanoparticles, compared to ~10 wt.% of SDBS routinely used in commercial synthesis. 1.1 Paraffin wax butyl acrylate core and styrene, ethyleneglycol dimethylacrylate and N-vinylpyrrolidone shell: Employing the synthetic scheme in Fig. 2, paraffin wax (10 wt.%, M. P. ~ 75 C) and n-butyl acrylate were heated together to form a uniform solution. In another vessel, 10 ml of 1 wt.% Brij 97 was heated at 80 C. The hot butyl acrylate-wax mixture (total 500 µl) was poured into the surfactant solution resulting in slight uniform turbidity. After stirring this mixture under N 2 purge for 30 min, an aqueous

4 NRT-16, Quarterly report, Mar2009-May2009, Page 4 of 9 solution of 2,2 -azobis(2-methylpropionamidine) dihydrochloride (V-50) was added to this. Continued heating resulted in an increase in the turbidity, as the polymerization of the nano-droplets ensued. A mixture of styrene (2 ml), N-vinylpyrrolidine (0.2 ml) and ethylene glycol dimethacrylate was prepared in the mean time. After an hour of the V-50 addition, the styrene mixture was added to the reaction vessel in a dropwise fashion. This resulted in a substantially increased turbidity. The reaction mixture was heated/ stirred for 12 more hours. The resulting milky mixture was centrifuged to isolate a white solid that was suspended in water by sonication for electron microscopy analysis. Fig. 3. SEM (left) and TEM (right) of the polymer nanoparticles prepared in absence of RAFT reagent. Particles are around 200 nm but did not show core-shell morphology. Electron microscopy of these particles indicated the presence of particles about nm in diameter. TEM, however, indicates that only a few particles had the core-shell morphology. These particles exhibited a tendency to clump together on drying. Aqueous dispersions, however, were stable and light scattering indicated a particle size of ~ 96 nm. 1.2 Living polymerization for preparing core-shell nanoparticles: To achieve a true core-shell hetero-structure in the polymer nanoparticles, we used living polymerization technique known as Reversible Addition Fragmentation Chain Transfer or RAFT. This

5 NRT-16, Quarterly report, Mar2009-May2009, Page 5 of 9 utilized a minute amount of sulfur-based RAFT reagent to impart livingness to the polymerization, so that the core acts as the true template for the shell. Dibenzyl trithiocarbonate was used as the RAFT reagent. A small amount of RAFT reagent was mixed with the n-butyl acrylate and wax mixture. This mixture was heated and added to a pre-heated 1 wt. % Brij 97 solution. Following similar protocol as discussed above, V-50 was added to this and the mixture heated for an hour. Finally, the styrene/ NVP/ EGDMA mixture was added to this. The reaction was allowed to proceed for another 12 h, product centrifuged, and analyzed by electron microscopy. Fig. 3. SEM of the polymer spheres synthesized by RAFT methodology. Left: Low magnification image shows a large collection of nanospheres. Right: High magnification image showing close-up of the particles. Particle diameters are nm. Fig. 4. Internal structure of the polymer nanoparticles synthesized by RAFT methodology using TEM. A core-shell kind of hetero-structure was visible in some particles.

6 NRT-16, Quarterly report, Mar2009-May2009, Page 6 of 9 The resulting sample exhibited a particle size in the range of nm in SEM (Fig 3). The internal structure of these nanoparticles probed by TEM (Fig. 4) elicited a core-shell structure in some particles. We believe that many wax nanoparticles (a few tens of nanometer in diameter) are entrapped in the interior of these particles by the poly-butyl acrylate chains in the first stage of reaction. This composite nanoparticle is then coated with the styrene based polymer on the surface in the second stage of polymerization. This was probed by dynamic light scattering that indicated that the initial particles are about 107 nm and they increase in the diameter to about 128 nm after the second step. The zetapotential of these particles was found to be ~ +40 mv. The positively charged initiator (V-50) used in the synthesis might confer the positive charge to the resulting particles. 2. Exploration of a plant-based wax to replace the paraffin wax: Candellila wax was investigated as a potential replacement for paraffin. There are multiple reasons that make Candellila wax as a promising candidate for this purpose. A few are as follows: Plant-based wax, hence potentially more eco-friendly Melting point ~ 71 C, close to paraffin wax employed in the current process Used as lubricant and adhesive in some processes Has low coefficient of expansion, is very durable and does not crack 2.1 Synthesis of polymer nanoparticles using Candellila wax: Exploiting similar procedure as one used for paraffin wax, polymer nanoparticles synthesis was attempted using Candellila wax. TEM (Fig. 5) shows that the resulting polymer particles have a networked internal structure and the particles are about nm across. SEM images of the as-prepared sample showed that the individual polymer nanospheres are about 150

7 NRT-16, Quarterly report, Mar2009-May2009, Page 7 of 9 nm across (Fig. 6, top left). These nanoparticles assemble into micron-sized spheres upon drying (Fig. 6, top right and bottom panel). Fig. 5. TEM image showing the internal structure of Candellila wax containing polymer nanospheres. The de-focused image on right shows a networked internal structure. Fig. 6. SEM images of Candellila wax-containing particles. The magnification of the images decreases clockwise from top left. Arrows on top right image indicate nanoparticles assembling into microspheres a couple of microns across.

8 NRT-16, Quarterly report, Mar2009-May2009, Page 8 of 9 3. Conclusions and Future Directions The experiments conducted in the previous quarter elicit a scalable methodology for preparing the polymer nanospheres incorporating waxes within them. We also show the requirement of living polymerization to achieve hetero-structured polymer nanospheres. To replace the paraffin wax, a plant (Candellila)-based wax was also investigated. Brij 97 was found to be a superior wax for achieving polymer nanospheres in nm range for both the waxes. In case of Candellila wax, aggregation of the polymer nanospheres to microspheres was also found. In the future following directions will be taken: ther uncharged polymerization initiators such as azo-based initiators like 2, 2 - azobis(isobutyronitrile) (AIBN) or Benzoyl peroxide, as well as aqueous initiator system consisting of Fe(III)-H 2 2. ur initial experiments (done in the first week of June) show that both of these initiators work well. The synthesis of paraffin wax-core polymer nanoparticles using benzoyl peroxide is shown in Fig. 7. The particle sizes are in the range of nm. Figure 7. SEM (left) and TEM (right) of the polymer nanospheres prepared using benzoyl peroxide as the initiator.

9 NRT-16, Quarterly report, Mar2009-May2009, Page 9 of 9 N-vinylpyrrolidone groups will be used to anchor other molecules like chitosan to the polymer nanospheres by imine formation (Fig. 8). The multiple hydrogenbonding sites in chitosan, and structural similarity between the glucosamine groups and the cellulose of paper should improve the adhesion properties considerably. A variation to the hetero-structure (Fig. 9), with the wax is embedded in the shell of the nanoparticles rather than the core will be undertaken to achieve the wax to function as the fusing agent and at the same time minimizing phase-separation. H H H NH 2 H H NH 2 H H n N H N Polymer Sphere Figure 8. Anchoring chitosan molecules to the polymer spheres for improved adhesion by imine formation between amines on chitosan and the carbonyl group on poly-n-vinyl pyrrolidine. Wax-containing shell of high T g polymer Core of low T g polymer Figure 9. A variation of hetero-structure with wax on the outer shell of the particles.