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Microfluidic synthesis of polymer composites particles for sensoric application by Nikunjkumar Visaveliya 1. PMMA particle synthesis:

The concentration of surfactants during the emulsion polymerization affects both, the size of the formed PMMA nanoparticles and their electrical surface state. The strong effect of the CTAB concentration on particle size in the range between 0.1 µM and 1 mM is well illustrated in Figure 1. Particles of only 70 nm were obtained in case of higher CTAB concentration (Figure 1a), while larger spheres with a diameter of about 500 nm are formed in case of lower concentration of surfactant. Besides the surfactant concentration, the flow rate ratio between the aqueous and the monomer phase during the emulsion formation also has a certain effect on the mean particle size (Figure 1f). The average size of the PMMA nanoparticle can be tuned between 93 nm and 117 nm at the same CTAB concentration (0.1 mM) by changing the flow rate ratio. The effect could be explained by an enhancement of formation of smaller monomer droplets in the emulsification. A more homogenous initiation of polymerization in case of a higher flow rate ratio leads to a more efficient starting of polymerization and, therefore, to obtain greater number of smaller PMMA particles. On another side, when surfactant concentration is below CMC, the formed particles are assisted by homogeneous nucleation mechanism. During the homogeneous nucleation, the surfactant molecules adsorbed on monomer droplets may also desorbs out of the droplet surface, diffuse across the continuous phase and then adsorb on the expanding particle surface. Finally, when PMMA particles are formed after completion of polymerization process at elevated temperature, the surfactant molecules are adsorb on the surface. The density of the surfactant molecules increases gradually on the surface with increasing surfactant concentration in continuous aqueous phase. It is suppose that when higher concentration of surfactant present during the synthesis, there will be more suppression ability present for growing the particles in growth step. During the dynamic adsorption and desorption process of surfactant while polymerization at higher temperature, the addition of monomer molecules continuing until completion of polymerization process. Thus, the particle size is certainly smaller with higher density of surface charge in case of higher concentration of surfactant used.

The effect of surfactant on the charging state of the polymer nanoparticles was analyzed by zeta-potential measurements. It can be demonstrated that the addition of CTAB leads to the formation of positively charged PMMA particles. The zeta-potential increases with increasing concentration of surfactant (Figure 2a). It is nearly zero if the surfactant concentration is below 10 µM. However, it increases up to about +40 mV at a concentration of 1 mM CTAB. An increasing concentration ratio of surfactant to polymer material leads to an increase of the particle charge. This interpretation correlates well with the observation of a decreasing zeta potential if SDS is applied instead of CTAB in the emulsion polymerization (Figure 2b). A strong decrease of the zeta potential takes place at SDS concentrations above 10 µM and drops down to -20 mV in case of an emulsion polymerization using an aqueous solution containing 1 mM SDS. CTAB and SDS are amphiphilic molecules and hence, the hydrophobic aliphatic chains are attached with surface of

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PMMA particles and residing hydrophilic positive and negative charge outwards to the aqueous dispersion. The emulsion is stable for months only due to surface coverage of particles by surfactant. It is remarkable that regular nanoparticles are also formed at lower surfactant concentrations and in the absence of any surfactant. The formation of emulsion in the micro flow-through emulsification as well as the polymerization by an emulsion mechanism take place if neither CTAB nor SDS was applied or their concentration was low. Under these conditions, larger submicron polymer particles are formed and their surface charge is nearly zero or slight negative due to initiator creates mild negative charge on particle surface through radical formation mechanism. Due to unavailability of any protecting surfactants, particles become bigger in size, slightly deformed in spherical shapes and in aggregated form.

Figure 1. SEM images of PMMA nanoparticles prepared by micro flow emulsification and emulsion polymerization in presence of different concentration of CTAB in aqueous phase: (a) 1000 µM, (b) 100 µM, (c) 10 µM, and (d) 0.1 µM; (e) and (f) graphical representation of dependence of size of PMMA nanospheres from the surfactant concentration and aqueous to monomer flow rate ratio, respectively.

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Figure 2. Dependence of Zeta-potential of PMMA nanoparticles on surfactant concentration in the aqueous phase during the emulsion polymerization: (a) increasing potential with increasing concentration of a cationic surfactant (CTAB), (b) decreasing potential with increasing concentration of an anionic surfactant (SDS).

2. Ag microrods synthesis:

In the small conical flask, 5 ml ethylene glycol is being heated at 160 °C for 1 hour with continuously stirring. Meanwhile prepare 3 ml 0.25 M AgNO3 solution in ethylene glycol, 3 ml 0.35 M PVP (repeating unit concentration, M. wt. 25 kDa) solution in ethylene glycol and 1 ml 0.1 M ascorbic acid solution of ethylene glycol. After 1 hour heating of ethylene glycol, there is an of 60 µL of AgNO3 solution followed by immediate addition of 60 µL of PVP solution, and then 20 µL of ascorbic acid solution of ethylene glycol. By this manner, add all reactants within 7 minutes (standard procedure). Ascorbic acid is a driving force to form Ag microrods, so when 0.1M AA was used, the reaction would complete within 10-15 minute, but reaction mixture has been heated for 40 minutes. (During different experiment for obtaining different aspect ratio of Ag microrods, the concentrations of ascorbic acid were changed from 0.01 M to 1 M). For characterizing the product in SEM and UV spectrometry, prior to take small amount of reaction mixture from reaction at 1, 3, 5, 7, 10, 15, 30 and 40 minutes. After reaction has been complete, apply the washing and centrifugation process repeatedly until unreacted impurities and ethylene glycol are completely removed from the Ag microrods product. Put one drop of Ag microrods suspension on the silicon wafers and measure in the SEM. AA plays key role during this process. Therefore, to observe the effect of AA on the Ag microrods product, it added differently into the reaction medium in different experiments (previous, simultaneous, or later).

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Figure 3. SEM images of size controlled Ag microrods synthesized by adding different concentration of 1 ml AA at 20 different time interval along with 3 ml of each, 0.25M AgNO3 and 0.35M PVP (repeating unit conc.) in heated EG at 160 °C: (a) 0.1 M AA, simultaneous drop-wise addition of AgNO3, PVP and AA together within 7 minutes by keeping ratio of addition 3:3:1 (60:60:20 µL) of

AgNO3:PVP:AA in sequences, (b) 0.1M AA, 25 first successively addition of 600 µl of each AgNO3 and PVP, then 200 µl of AA, all addition within 7 minutes simultaneously, and (c) and (d) 0.3 M AA, simultaneously drop-wise addition with addition ratio 3:3:1 (60:60:20 µ1L) of AgNO3:PVP:AA within 7 minutes and 4 minutes respectively. Table 1. Effect of different concentration of 1ml AA on the formation of Ag microrods by keeping same molar concentration for 3ml of each, 0.25M AgNO3 and 0.35M PVP (repeating unit concentration). Conc. of 1ml

AA

Product Lengths Diameter

0.01M < 5% Ag microrods and rest 95% Ag

colloidal particles

< 4 µm 150 nm

0.05M ~25% Ag microrods and rest ~75% Ag

colloidal particles

~8 µm 200-250

nm

0.08M >35% long and ~30% short (~3µm) Ag

microrods respectively , rest ~35% Ag

colloidal particles

~15 µm ~250 nm

0.1M 95% Ag microrods and rest 5% Ag

colloidal particles

20-25

µm

250 nm

0.2M 90% Ag microrods and rest 10% Ag

colloidal particles

15-20

µm

350 nm

0.3M > 85% Ag microrods and rest ~15% Ag

colloidal particles

~5 µm 400-500

nm

0.5M All are distorted shaped Ag microrods ~4 µm >500 nm

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1.0M All are distorted shaped Ag microrods ~3-4

µm

> 500 nm

3. Ag microrods-PMMA composites particles:

The heterogeneous assemblies of colloidal polymer particles and metal nano- and microstructures are versatile platform for the combined mechanical and conducting properties. And therefore, the assemblies of silver (Ag) microrods and flower-like zinc oxide (ZnO) microparticles with poly(methyl methacrylate) (PMMA) nanospheres are presented in order to prepare advanced composite materials. PMMA nanoparticles have been prepared by emulsion polymerization using a microfluidic preparation step in presence of cationic surfactant CTAB. The surface charge of PMMA particles determines the binding interaction strength with the inorganic constituents. Ag microrods and ZnO microparticles have been synthesized in batch and in continuous flow process, respectively. The assembling process can be explained for both types of nano-assemblies by a particle/particle binding process due to electrostatic interaction. The possibility of arranging PMMA nanospheres with regular distances on Ag microrods is related to slightly weaker binding force compared to PMMA nanospheres on ZnO microparticles. The observed binding pattern reveals a certain lateral mobility of the small polymer particles at the surface of the larger metal particle. In contrast, the stronger binding between the polymer spheres on the ZnO surface is obviously related to a fast fixation of the PMMA particles. The particle ratios in the nano-assemblies can be tuned over a wide range. The synthetic steps of the individual constituents, the relation between particle-particle ratio and binding capability through electrostatic interaction, and relative strength of surface charge for interaction are presented.

Figure 4. SEM images of binding of smaller sized PMMA nanoparticles on Ag microrods with different diameter: (a) 70 nm PMMA particles on Ag microrods of 150 nm diameter and a length of 5 µm, (b) 70 nm PMMA particles on Ag microrods of 900 nm diameter and a length of 8 µm, (c) 110 nm PMMA particles on Ag microrods of 150 nm diameter and a length of 5 µm, and (d) 110 nm PMMA particles on Ag

microrods of 900 nm diameter and a length of 8 µm.

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4. Dumbbell shape PMMA particles: effect of polyelectrolyte on shape:

Polymeric particles play a central role in various applications such as drug delivery, labelling, sensorics and in fundamental studies in microfluidics and nanotechnology. The shape of the particles strongly influences its functional properties. By the application of microreaction technology, we have synthesized PMMA particles with different aspect ratios in a one step synthesis via emulsion polymerisation. The lipophilic monomer phase is injected in to a stream of continuous flowing aqueous phase through a micro nozzle array fabricated by microlithographic techniques. The aqueous phase possesses various concentrations of different polyelectrolytes and it was found that the addition of polyelectrolytes is responsible for the non-spherical shape of the formed particles. A well reproducible generation of different types of polymer nanoparticles was achieved by addition of polyelectrolytes (PSS-co-PM, PSSS, PAES) of different molecular weights in a repeating unit concentration between 0.01 and 20 mM. The shape (elliptical, dumbbell and rod-like long chain) and size (length, width and aspect ratio) of particles can be tuned by variation of the flow rate ratios of aqueous and monomer phase, and by varying the concentration and type of polyelectrolytes in the aqueous phase. The diameter of particles can be tuned between 100 and 300 nm, the length between 150 and 1500 nm, and the aspect ratio between 1.2 and 7 by applying different reaction condition. It is supposed that the formation of non-spherical nanoparticles is caused by a spontaneous assembling of nanoparticles during the growth. The linear structure of nanoparticle assemblies could be due to a partial repulsion of the nuclei. A measurement of the zeta potentials reveals the negative excess charge on the particles surface. The effect of particle charge and the control of partial charge compensation for the spontaneous assembling during particle growth will be studied in further investigations.

Aqueous phase: Polyelectrolyte (PSSS-co-PMA) Monomer phase: MMA + EGDMA + AIBN Temp: 95 degree.

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Figure 5. The effect of different flow rate ratio on PMMA particles aggregation: (a) 50/1400 (monomer/aqueous), (b) 50/1000, (c) 50/500, (d) 50/300, (e) 50/150, (f) 100/300, (g) 100/220, (h) 100/150, and (i) 100/130; the concentration of PSS-co-PM in continuous flow is 0.09 mM. Scale bar is 300 nm. Table 2: Effect of flow rate ratio on the aspect ratio of particles. Flow rate ratio Length width Aspect ratio 50/1400 325 230 1.41 50/1000 400 210 1.9 50/500 425 225 1.88 50/300 450 240 1.87 50/150 460 240 1.91 100/300 635 240 2.64 100/220 800 240 3.33 100/150 900 240 3.75 100/130 1500 240 6.25

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5. Flower shaped PMMA particle- effect of PVP for shape divergent and aggregation:

6. PMMA-Ag nanoprism composites:

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7. Size-tuned Dye dopped PMMA nanoparticles:

8. Polyacrylamide gel particles: Work continues…..