Accepted Manuscript Multiple emulsions as soft templates for the synthesis of multifunctional silicone porous particles Neus Vilanova, Yury V. Kolen’ko, Conxita Solans, Carlos Rodríguez-Abreu PII: DOI: Reference:

S0021-9797(14)00655-9 http://dx.doi.org/10.1016/j.jcis.2014.09.006 YJCIS 19814

To appear in:

Journal of Colloid and Interface Science

Received Date: Accepted Date:

25 June 2014 8 September 2014

Please cite this article as: N. Vilanova, Y.V. Kolen’ko, C. Solans, C. Rodríguez-Abreu, Multiple emulsions as soft templates for the synthesis of multifunctional silicone porous particles, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.09.006

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Multiple

emulsions

as

soft

templates

for

the

synthesis

of

multifunctional silicone porous particles Neus Vilanovaa,1,Yury V. Kolen’kob, Conxita Solansa, Carlos Rodríguez-Abreub* a

Institute for Advanced Chemistry of Catalonia, Consejo Superior de Investigaciones

Científicas (IQAC-CSIC) and CIBER de Bioingeniería, Biomateriales y Nanomedicina, Jordi Girona 18-26, 08034 Barcelona, Spain b

International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga s/n, 4715-

330 Braga, Portugal 1

Present address: Institute for Complex Molecular Systems, Technische Universiteit

Eindhoven, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands * Corresponding author. International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga s/n, 4715-330 Braga, Portugal. Tel. (351) 253 140 112; Fax (351) 253 140 119 E-Mail addresses: [email protected] (N. Vilanova); [email protected] (Y. V. Kolen’ko); [email protected] (C. Solans); [email protected] (C. RodríguezAbreu) Multiple emulsion templating is a versatile strategy for the synthesis of porous particles. The present work addresses the synthesis of multifunctional poly(dimethysiloxane) porous particles using multiple water-in-oil-in-water emulsions as soft templates with an oil phase constituted by a crosslinkable poly(dimethylsiloxane) (PDMS) oil. Herewith, the impact of the viscosity of PDMS oil (i.e., molecular weight) on the properties of both the emulsion 1

templates and the resulting particles was evaluated. The viscosity of PDMS oil has a strong effect on the size and polydispersity of the emulsion templates as well as on the mechanical properties of the derived particles. The elastic modulus can be tuned by mixing PDMS oils of different viscosities to form bimodal crosslinked networks. Iron oxide nanoparticles can be readily incorporated into the emulsion templates to provide additional functionalities to the silicone particles, such as magnetic separation or magnetic hyperthermia. The synthesized composite magnetic particles were found to be useful as recoverable absorbent materials (e.g., for oil spills) by taking advantage of their high buoyancy and high hydrophobicity. Keywords: multiple emulsions, porous particles, silicone, mechanical properties, swelling, magnetic properties. 1. Introduction Multiple emulsions, either water-in-oil-in-water (W1/O/W2) or oil-in-water-in-oil (O1/W/O2), have been extensively used to produce polymer porous particles. The templating process basically consists on first, the formation of the emulsion followed by the hardening of the intermediate phase. Thus, the type of the emulsion employed, W1/O/W2 or O1/W/O2, will be dictated by the solubility of the polymer precursor, oil- [1,2] or water-soluble [3,4], respectively. The polymer precursor can be either a crosslinkable polymer or a preformed polymer dissolved in a solvent, which is further evaporated to eventually precipitate the polymer. In the former case, the resulting particles closely resemble the emulsion template [1], whereas in the latter case the particles are much smaller than the template size [5]. Nevertheless, in both cases it is important to control the stability and 2

morphology of the template, as they will govern these of the emulsion-derived particles. Within all the emulsification methods, multiple emulsions are usually prepared by the two-step bulk procedure; firstly, a primary emulsion (W1/O or O1/W emulsion) is prepared and subsequently emulsified into the external phase (W2 or O2, respectively) [6]. Multiple emulsions can be kinetically stabilized by surfactants; for a W1/O/W2, normally a hydrophobic surfactant stabilizes the inner aqueous droplets, while a hydrophilic surfactant does that for the oil globules, and the reverse configuration for O1/W/O2 emulsions [6]. Morphology of the multiple emulsions, namely inner droplet and globule size, can be controlled through formulation parameters, such as the concentration of the surfactants and the relative volume fraction of the phases [1]. Besides these general parameters, it has been demonstrated that, similarly to simple emulsions [7,8], the ratio between the viscosity of the dispersed phase (here the primary emulsion) and the continuous phase (the external phase) has also a crucial impact, as it governs the way of how the globules (multiple drops) will be ruptured during the second emulsification step [9,10]. Multiple emulsion templating is thus, an attractive and versatile approach to create encapsulating systems with different morphologies and with a wide variety of different polymers. On the other hand, the choice of the material is also crucial for ensuring the intended properties to the final system, for instance the ability of the polymer to response upon the exposure to specific stimuli such as temperature, light, change in the chemical environment or an external stress. Among all the polymers, silicones are particularly attractive polymers due to their intrinsic properties. They present low glass transition temperatures, thermal and chemical stability, flexibility and high permeability to organic solvents and gases. Moreover, the low intermolecular

3

interactions and the chemical characteristics of some additional side groups give rise to some other remarkable and desirable properties such as a low surface free energy, low density or high hydrophobicity [11]. When these side groups are reactive, silicones can be crosslinked, leading to the formation of three dimensional networks whose mechanical properties can be easily tuned by controlling the degree of crosslinking [12,13] or the structure of the network. The latter strategy basically consists of a) varying the structure of the crosslinkable polymer, for instance, lengthening the chain of the precursor [14], or b) forming bimodal networks, that is, mixing crosslinkable silicones with long and short chains [15]. Understanding and controlling the reactions involved in the crosslinking process is, therefore, of great interest to regulate the mechanical properties of the final material. Besides the inherent properties of silicones, other features can also be readily added to the material by embedding functional fillers into the network [16-18]. The exceptional properties of crosslinked silicones and their versatility in terms of mechanical behavior, makes them suitable for a wide range of applications, such as waterproof coatings [19,20], membranes [21], microfluidic devices [22] and functional bulk materials [16,18]. Nonetheless, reports on the fabrication of particulate or porous silicone materials from soft templates are scarce [1,23]. Mainly because it is difficult to control the emulsification process due to the high viscosity of most common silicone precursors [14,24]. Hence, the successful preparation of silicone porous particles would open new interesting potential applications in other promising fields, such as encapsulating systems for drug delivery, protection of sensitive chemicals or as confined reaction vessels. Moreover, the possibility to tune the mechanical

4

properties of the network would also be interesting in applications when particles are subjected to external forces [13]. In a previous work [1], silicone porous particles with various morphologies were formed using W1/O/W2 emulsions as templates; the effect of several formulation parameters on emulsion and particle properties was determined. Herein, we expand that previous study by using silicone precursors of different chain lengths with the aim of tuning the mechanical properties of the porous particles. Considering that the longer the polymer chain, the higher the molecular weight and the viscosity, formulation studies were required to optimize the viscosity ratio between the primary W1/O emulsion and the external aqueous phase (W2), to ensure the production of stable templates, and as a consequence, emulsionderived particles with a well-controlled morphology. The swelling capacity of the resulting particles as a function of the starting poly(dimethylsiloxane) oil was also determined. To add multiple functionalities, magnetic nanoparticles were also incorporated inside the pores of the particles; such composites were found to be effective as oil absorbents, showing also magnetic hyperthermia response. 2. Experimental section 2.1 Materials Crosslinkable linear vinyldimethylsiloxy-terminated polydimethylsiloxanes (PDMS, supplied by ABCR, Germany) with kinematic viscosities (υ) of 0.7, 100 and 200 cSt (abbreviated as PDMS0.7, PDMS100 and PDMS200, respectively) were used as silicone precursors. The molecular weights (MW) of PDMS0.7, PDMS100 and PDMS200 were 186, 6000 and 9400 g mol-1, respectively. The crosslinker trimethylsilyl-terminated

poly(dimethylsiloxane-co-methyl

hydrosiloxane)

(with 5

50%mol of methylsiloxane) with υ=12 cSt (MW= 950g mol-1) was obtained from Sigma-Aldrich (USA). A Pt-based complex with the general formula platinum (0)-1,3divinyl-1,1,3,3-tetramethyl-disiloxane from Sigma-Aldrich (USA), was used as a 9.7wt% solution in chloroform to catalyze the crosslinking reaction. A branched silicone-based surfactant with polyglycerin groups, commercialized with the name KF6104 (HLB=3-4), was a gift from Shin-Etsu (Japan). The sorbitan alkylethoxylated surfactant with the commercial name Tween 80 (HLB=15) was supplied by Merck (Germany). These surfactants were used as hydrophobic and hydrophilic surfactants, respectively. Heptane, 1-butanol and Pyronin Y, from Sigma-Aldrich (USA), were used as swelling, spreading and hydrophilic fluorescent agents, respectively. FeCl2⋅4H2O and FeCl3⋅6H2O, both with a purity of 99%, aqueous ammonia solution (28–30%) and poly(acrylic acid sodium salt) (MW≈5100 g mol-1) from SigmaAldrich, were used for the synthesis of the iron oxide nanoparticles. 2.2 Methods Synthesis of iron oxide nanoparticles Iron oxide nanoparticles coated by poly(acrylic acid) (PAA), labeled as NP@PAA, were synthesized via a hydrothermal method. In a typical synthesis 0.398 g of FeCl2⋅4H2O and 0.946 g of FeCl3⋅6H2O were dissolved in 15 mL of MilliQ water. The solution was placed inside a 40 mL PTFE vessel with 7 mL of aqueous ammonia solution, and then 0.4 g of poly(acrylic acid sodium salt) dissolved in a 7 mL of water was added. The PTFE vessel was capped and placed into a stainless steel autoclave. The autoclave was sealed and kept at 180°C for 6 h. The PAA-coated Fe3O4 NPs were collected using centrifugation and washed twice with MilliQ water (redispersion

6

followed by centrifugation) before being centrifuged at 3000 rpm for 10 min to remove any insoluble solids. The resultant colloidal NP@PAA were stored in water. NP@PAA had an average particle size of about 10 nm (estimated by transmission electron microscopy). Rheology measurements Rheological properties of the intermediate oil phase and the external aqueous phase were determined on an AR-G2 Rheometer (TA instruments) using concentric cylinders (gap = 4000 µm). Prior to the measurement, samples placed in the cylinder were allowed to equilibrate at 70ºC (the crosslinking temperature) for 5 minutes. The rheological behavior was determined in the shear rate range 0.1 - 1000 s-1. All measurements were performed in duplicate. Formation of multiple emulsions Multiple W1/O/W2 emulsions were prepared following the two-step procedure. First, the primary W1/O emulsion was prepared by dispersing the inner aqueous phase (W1) into the oil phase (O) in a tube of 1.5 cm of diameter using an UltraturraxTM homogenizer (stick S25N-10G, IKA) at 13500 rpm for 3 minutes (total weight = 0.52g). Afterwards, the multiple W1/O/W2 emulsion was formed by dispersing the primary emulsion into the external aqueous phase (W2) (total weight = 2.08 g) placed in a vial of 2.7 cm in diameter. In this second emulsification step, the system was gently stirred by a magnet (diameter = 5 mm; length = 2.5 cm) at 1000 rpm in order to avoid the rupture of the oil globules. The formation and evolution of all multiple emulsions were followed by means of optical microscopy (ReichartPolyvar 2, Leica-Microsystems). All measurements were performed in duplicate. The size of the globules and inner droplets

7

was directly assessed from the optical microscopy pictures. Reported sizes are an average over approximately 1000 measurements. Preparation of porous particles Porous particles were prepared using the multiple W1/O/W2 emulsions as templates. This was done by heating the emulsions up to 70ºC to initiate the crosslinking reaction between the vinyl-terminated precursors and the crosslinker agent in the intermediate oil phase. The dispersion was kept at this temperature and under stirring for 90 min for emulsion templates made with PDMS0.7 and for 24 h for templates containing PDMS100 or PDMS200 to ensure a complete crosslinking, given that the rate of the hydrosilylation reaction depends on the type and the molecular structure of the precursor [12]. Afterwards, the obtained solid was washed 3 times with acetone using an ultrasound bath, and finally dried at 60ºC for 24 h. All experiments were performed in duplicate. Hybrid silicone particles encapsulating magnetic nanoparticles within the pores were prepared by substituting the inner aqueous phase of the templates for aqueous dispersions of pre-synthesized NP@PAA. SEM and SEM-EDS measurements The emulsion-derived particles were characterized by scanning electron microscopy (SEM, Hitachi TM-1000). Particle and pore sizes were directly assessed from SEM images (average over 1000 and 500 measurements, respectively, for each experiment). Chemical elemental composition of hybrid particles was determined using a field emission SEM coupled to an Energy-Dispersive Spectrometer (SEM-EDS, QUANTA 650 FEG, FEI). Determination of the swelling capacity 8

The swelling capacity of the polymer particles was determined using heptane as a swelling agent. Firstly, different sets of particles were dispersed in heptane for different times (up to 4 hours). Afterwards, each set of swollen particles were placed in a glass slide and observed by optical microscopy. As the heptane evaporated, particles shrunk. Particle diameter was measured along the deswelling process, until a constant value was reached (dried state). The swelling capacity was calculated by the volume difference of a particle between its swollen and dried state. The obtained values showed that the equilibrium was attained after 1 hour, as particles reached the maximum swelling capacity. The given values of swelling are averages over 10 measurements of particles dispersed in heptane for 1 hour. Contact angle measurements To evaluate hydrophobicity, we prepared films with the synthesized silicone particles, Firstly, silicone particles were dispersed in a small volume of 1-butanol, which acted as spreading agent. Several droplets of the dispersion were deposited onto a glass slide. After 24 hours at room temperature, a compact film was obtained after evaporation of the solvent. Water contact angles of those films were measured by the sessile drop method using a Drop Shape Analysis System DSA100 (KRÜSS) using water droplets of 3μL. Each given value is an average of 10 measurements at different locations of three different films. Mechanical properties of silicone monoliths For the measurement of mechanical properties of different silicone matrices, PDMS monoliths (diameter = 14 mm; thickness = 12 mm) were prepared. The precursor mixture for each monolith had the same composition as the intermediate oil phase of the corresponding multiple emulsion. The crosslinking reaction for the monoliths was 9

carried out following the same procedure described above. The elastic moduli were determined by performing compression assays using a MT-LQ dynamometer (Stable MicroSystems). Measurements were performed at a crosshead speed of 0.2 mm s-1 until an indentation displacement of 4 mm. The elastic moduli were calculated from the linear zone of the loading curve. All measurements were performed in triplicate. Confocal microscopy A Zeiss LSM780 confocal microscope was used to observe particles loaded with Pyronin Y, a hydrophilic fluorescent dye. Magnetic measurements Magnetization of both NP@PAA and the hybrid silicone particles, as a function of the applied magnetic field, was measured using a VSM (MicroSense LLC). Measurements were carried at 300 K and varying the magnetic field from -20 up to 20 kOe. Hyperthermia measurements were conducted in a DM100 instrument (NanoscaleBiomagnetics, Spain). Samples were placed in 2 mL vials and measured in the powder (dry) form. 3. Results and discussion 3.1 Formation of multiple emulsions with different silicone precursors In the present article, the formulation used to prepare multiple W1/O/W2 emulsions was based on our previous work [1]; the only difference is the type of silicone precursor (PDMS) used herewith. Table 1 summarizes this general composition. 10

Table 1. General composition of the multiple W1/O/W2 emulsion. PDMSX indicates the silicone precursor used, where X denotes the viscosity of the precursor: 0.7, 100, 200 cSt or a mixture of them. Phases/Pseudo-phases Inner aqueous phase (W1) Primary emulsion

W1/O

Multiple emulsion

W1/O/W2

Intermediate oil phase (O) Primary W1/O emulsion External aqueous phase (W2)

Components wt% Water 4 KF-6104 18 Crosslinker 52 PDMSX 22 Catalyst solution 4 Those of W1/O 20 5wt% Tween 80 80 aqueous solution

The intermediate oil phase consisted of the hydrophobic surfactant KF-6104, the catalyst solution, the crosslinker and the silicone precursor: PDMS0.7, PDMS100, PDMS200 or a blend of a highly viscous precursor (PDMS100 or PDMS200) with the low viscous PDMS0.7. Both PDMS100/PDMS0.7 and PDMS200/PDMS0.7 weight ratios were varied from 90/10 to 60/40. It is obvious that by changing the viscosity of the PDMS, the rheological properties of the intermediate oil phase will be also modified; leading to diverse types of primary W1/O emulsions and derived multiple emulsions. Therefore, the viscosity of the intermediate oil phases composed by different PDMS was measured (Table 2). All samples behaved as Newtonian liquids in the studied shear rate range (1- 1000 s-1) and, as expected, the viscosity of the intermediate oil phase increased by increasing the viscosity of the PDMS used. Furthermore, it was also observed that the addition of PDMS0.7 (even at low concentrations) to highly viscous PDMS reduced the viscosity of the whole oil phase. Hence, it was confirmed that the use of different PDMS substantially modifies the viscosity of the intermediate oil phase.

11

Table 2. Viscosities at 70ºC of intermediate oil phases (ηo) prepared with different types of PDMS. See the composition of the whole intermediate oil phase in Table 1. The viscosity ratio between the intermediate phase and the external phase (ηo/ηW2) is also indicated. Silicone precursor ηO (mPa·s) PDMS0.7 15.3 PDMS100 33.3 PDMS100/PDMS0.7 (90/10)* 29.5 PDMS100/PDMS0.7 (80/20)* 23.5 PDMS100/PDMS0.7 (70/30)* 20.6 PDMS100/PDMS0.7 (60/40)* 20.2 PDMS200 38.7 PDMS200/PDMS0.7 (90/10)* 31.2 PDMS200/PDMS0.7 (80/20)* 27.5 PDMS200/PDMS0.7 (70/30)* 22.2 PDMS200/PDMS0.7 (60/40)* 21.5 *weight ratio

ηO/ηW2 3.0 6.5 5.7 4.6 4.0 3.9 7.3 6.1 5.4 4.3 4.2

Despite the high viscosity of some of the oil phases, primary emulsions could be prepared in all cases. Optical microscopy observations of primary W1/O emulsions showed that the size of the water droplets decreased from 2.8±0.9 μm down to less than 1 μm (not measurable by optical microscopy) by increasing the viscosity of the intermediate oil phase from 15.3 up to 38.7 mPa·s, as shown Figure 1. It is well known that the applied stress and the viscosity ratio between the dispersed phase and the continuous phase are important parameters in determining the size and polydispersity of an emulsion, as both are related with the mechanism of droplet rupturing [7,8]. Therefore, such reduction in the droplet size was attributed to the higher shear stress induced by highly viscous oil phases during the first emulsification step. Moreover, visual observations revealed that, as expected, by increasing the viscosity of the PDMS, the viscosity of the primary W1/O emulsion also increased.

12

a) 4

b)

droplet size (μ m)

3

50 µm 2

c) 1

0 10

15

20 ηο

25

50 µm

(mPa·s)

Figure 1. a) Dependence of the droplet size of primary emulsions with viscosity of the oil phase (ηo). Dashed line is only a visual guide. Above ηo = 30 mPa·s, the droplet size was below one micrometer, not measurable by the microscope. Optical microscopy images of primary W1/O emulsions prepared using oil phases with viscosities of b) 15.3 and c) 38.7 mPa·s (see compositions in Table 2). Regardless the viscosity of the primary emulsion, formation of multiple W1/O/W2 emulsions was always possible. However, differences concerning globule size and polydispersity were noticeable. During the second emulsification step, as the external aqueous phase (W2) and the applied shear rate were the same in all tests, the applied stress was identical in all the systems. Hence, the only parameter modified was the ηW1/O/ηW2 ratio, which led to different globule rupturing conditions. The rheological properties of a simple emulsion are mainly controlled by those of the continuous phase, for this reason in the present work, and for the sake of simplicity, the viscosity of the primary W1/O emulsion was assumed to be similar to that of the intermediate oil phase (ηW1/O ~ ηO). Therefore, considering that the viscosity of the external aqueous phase (ηW2) was 5.15 mPa·s, the ηO/ηW2 viscosity ratios were calculated for each system (see 13

Table 2). The influence of the ηO/ηW2 ratio on globule size and polydispersity is evidenced in Figure 2a. As a general trend, considerably smaller multiple globules were formed by reducing the ηO/ηW2 ratio, namely, the viscosity of the PDMS. Basically, under the same applied stress, the lower the ηO, the easier to break the big globules in smaller ones. For ηO/ηW2>5.7, big and polydisperse globules were obtained. In contrast, systems with viscosity ratios below 5.7, showed almost a constant globule size but slightly smaller than that for systems with ηO/ηW2>5.7 and the polydispersity was remarkably reduced down to a 20%. As a representative example, Figures 2b-c clearly show that decreasing ηO/ηW2 ratio from 6.5 (Figure 2b) down to 3.9 (Figure 2c) favors the formation of smaller and less polydisperse multiple oil globules.

a) 280 240 200

globule size (μ m)

b)

PDMS0.7 PDMS100 mixtures of PDMS100 + PDMS0.7 PDMS200 mixtures of PDMS200 + PDMS0.7

160 120

c)

80 40 0 1

2

3

4 η ο /η w

5

6

7

8

2

Figure 2. a) Dependence of globule size with viscosity ratio between the intermediate oil phase and the external aqueous phase (ηO/ηW2) for emulsions containing PDMS0.7, PDMS100, PDMS200 or mixtures of them (see composition of the mixtures in Table 2). Optical micrographs of multiple emulsions prepared using b) PDMS100 only (ηO/ηW2=6.5) or c) a mixture of PDMS100/PDMS0.7 (weight ratio of 60/40, ηO/ηW2=3.9).

14

3.2 Preparation and characterization of silicone particles The preparation of silicone porous particles was carried out by crosslinking the intermediate oil phase of multiple emulsions. Multiple emulsions prepared with PDMS100 or PDMS200 only, showed globule flocculation and release of the inner droplets during the crosslinking reaction, resulting then in amorphous solids rather than porous particles (such an outcome was more evident when using PDMS200). The stability of multiple emulsions during the crosslinking reaction was gradually enhanced by adding PDMS0.7 to the system, given that the flocculation of the globules and the release of the inner droplets were substantially reduced, allowing to obtain porous silicone particles. As the only parameter varied was the nature of the PDMS mixture, the improvement of the emulsion stability due to different interfacial properties of the silicone oils cannot be completely excluded. Observations under optical microscopy revealed that the multiple structure was preserved after 24 h of reaction, regardless of the concentration of PDMS0.7 in the system. Optical micrographs in Figure 3a-d show that the multiple structure is still visible after the crosslinking reaction for all compositions. At that point, all samples showed a similar appearance under the optical microscope, however, they were found to be different after the washing and drying process. Surprisingly, the porous structure of the particles disappeared in the case of using PDMS100/PDMS0.7 weight ratios of 90/10 and 80/20 or a PDMS200/PDMS0.7 weight ratio of 80/20 as evidenced by SEM observations. For intermediate concentrations of PDMS0.7 the porous structure was partially disrupted. However, when a weight ratio of 60/40 was used (for both PDMS100 and PDMS200), the porous structure remained after washing and drying. As a representative example of the collapsing of the pores as a function of the PDMS mixture composition for the system PDMS100, SEM images of broken particles are shown in Figure 3 e-h. The rise in the 15

polymer viscosity is due to an increase in its molecular weight. Thus, highly viscous PDMS, such as PDMS100 or PDMS200, present longer polymer chains than the low viscous PDMS0.7, which once crosslinked, form silicone networks with a lower elastic modulus (bigger mesh size). Consequently, the observed pore collapse is most likely due to the low elastic modulus of such networks, which presumably could not overcome the capillary forces created within the pores [25,26] rather than to the emulsion stability. The addition of PDMS0.7 not only reduced the viscosity of the system as described above, but also allowed to form bimodal networks with smaller mesh sizes, hence with a greater elastic modulus enable to retain the porous structure. Namely, the short chains of the PDMS0.7 provide strength to the bimodal silicone matrix.

a) 90/10

b) 80/20

c) 70/30

d) 60/40

e) 90/10

f) 80/20

g) 70/30

h) 60/40

50 µm

30 µm

30 µm

30 µm

Figure 3. a-d) Optical microscopy images of completely crosslinked samples made with different weight ratios of PDMS100/PDMS0.7, where the inner multiple structure is visible in all cases. e-h) SEM images of broken particles of the same samples but after being washed and dried, where the collapse of the pores is visible in some cases. In order to get more evidences about the effect of the molecular weight of the PDMS on the mechanical properties of the final network, the elastic moduli of some silicone monoliths were determined. These monoliths were prepared with the same precursor compositions as the intermediate (O) phase of their emulsions counterparts. 16

The obtained values are listed in Table 3. From the obtained values it can be confirmed that the elastic modulus of the material effectively decreases with the molecular weight of the PDMS, and what is more important, the mixture of different types of PDMS renders networks with intermediate elastic moduli values, which allowed to obtain porous particles as discussed above. Table 3. Elastic moduli (E) of silicone monoliths prepared using different types of precursors. Siliconeprecursor PDMS0.7 PDMS100 PDMS200 PDMS100/PDMS0.7 (60/40)* PDMS200/PDMS0.7 (60/40)* *weight ratio

E (kPa) 428.41±27.25 2.33±0.04 0.95±0.14 108.73±1.17 81.41±15.31

Concerning the characteristics of the particles with a well-defined morphology, the measured particle and pore sizes for each system are represented in Figure 4. Pore size decreased when using mixtures of PDMS100 or PDMS200 with PDMS0.7 (Figure 4a), because the viscosity of the intermediate oil phases in the multiple emulsions was higher (Table 1), and consequently they exerted a higher shear stress on the inner aqueous phase (W1) during the first emulsification step. Conversely, particle size slightly increased (Figure 4b), as highly viscous primary emulsions are prone to form bigger globules (Figure 2a). Polydispersity was in all cases near 25% for particles and 43% for pores. Particles were 15-20% smaller than the original emulsion templates, mainly as a result of the crosslinking and drying process, but the observed trends were the same, thus confirming the templating effect.

17

4

a) Pore size (μ m)

3

2

1 Inner droplet size Pore size

0 PDMS0.7

PDMS100/PDMS0.7 PDMS200/PDMS0.7 (60/40) (60/40)

PDMS

b)

80

Particle size (μ m)

70 60 50 40 30 20 Globule size Particle size

10 0 PDMS0.7

PDMS100/PDMS0.7 PDMS200/PDMS0.7 (60/40) (60/40)

PDMS

Figure 4. a) Pore and b) particle sizes of samples synthesized with different silicone precursors: PDMS0.7 and a mixture of PDMS100 or PDMS200 with PDMS0.7 in a weight ratio of 60/40. For comparison, the size of the inner droplets and the globules of the original templates are also shown. Dashed lines are only visual guides. Swelling experiments were also performed with the obtained silicone particles. Heptane was chosen as a model swelling agent [27,28]. Swelling originates from the ability of a polymer chain to extend when the organic solvent penetrates into the matrix network. Thus, the swelling capacity of a material provides information about its 18

elasticity and consequently about the structure of the matrix as well. Particles made by PDMS0.7 and PDMS100 resulted in swelling capacities of 54% and 470%, respectively. Such an increase in the swelling capacity is a consequence of the longer distance between the entanglement points within the matrix and hence the greater extensibility of the PDMS100 long chains. Note that such a high elasticity of PDMS100 was responsible for the collapse of the porous structure of the particles (Figure 3e, Table 3). Figure 5 shows the swelling capacity of particles made with bimodal networks (PDMS100 or PDMS200 mixed with PDMS0.7). For comparison, the swelling capacity of particles made with PDMS0.7 only is also plotted as a black line. As already described, the addition of PDMS0.7 promoted an enhancement of the stability of the porous structure upon drying and as expected, it also modified the swelling capacity. Figure 5 confirms that the swelling capacity tends to diminish by increasing the concentration of PDMS0.7, given by the limited extensibility of PDMS0.7 chains as compared to that of the long chains. Despite of the decrease in the swelling capacity, all bimodal networks swelled much more than that made of PDMS0.7 only (black line). Hence, the attractive point of such bimodal networks stems from the synergetic effect provided by the stiffness of the short chains and the elasticity of the long chains. It should be noted that such swelling ability of the particles is a combination of both, their mechanical properties (measured on monoliths) and their porosity. The similarity in the swelling capacity of bimodal networks made with PDMS100/PDMS0.7 and PDMS200/PDMS0.7 at a weight ratio of 60/40, and considering that they have similar porosities as well, agrees fairly well with the similar elastic modulus found for their corresponding monolith counterparts (Table 3). Note that the swollen particles went back to their original shape after removing the heptane, without being disrupted, exemplifying the reversibility of the process.

19

600 PDMS100 PDMS200 100µm

100µm

% Swelling

400

200

0 100/0

80/20

70/30

60/40

PDMSX:PDMS0.7

Figure 5. Swelling capacity of particles made with PDMS100 or PDMS200 mixed with PDMS0.7 at different weight ratios. The black line denotes the swelling capacity of particles made with PDMS0.7 only. Inset: optical images of swollen (left) and dried (right) particles made with a PDMS200/PDMS0.7 weight ratio of 60/40. Swelling also allows encapsulation within the particles. To prove such concept, dried silicone particles were dispersed in a solution of Pyronin Y in ethanol for 1 hour. Ethanol swelled the network and facilitated the diffusion of the dye into the pores. Subsequently, the ethanol solution was removed and the swollen particles were washed with water. Using water instead of organic solvents for washing diminishes dye losses, as their swelling is avoided because the particles are hydrophobic [1]. The confocal microscopy image shown in Figure 6 confirms that such swelling approach is an effective strategy for encapsulation.

20

Figure 6. Confocal image of silicone porous particles made by a mixture of PDMS200/PDMS0.7 at a weight ratio of 60/40 and loaded with Pyronin Y using the swelling approach. Films with the synthesized particles were prepared as described in the experimental section. SEM images of the films confirmed that silicone particles were homogeneously and densely packed on the surface of the glass (see Supporting Information, Figure S1.a). The water contact angles exhibited by the films were between 127º and 129º, regardless of the type of PDMS. These angles were greater than those reported for silicone materials with a smooth surface, typically between 110º and 116º [29,30]. Such difference in the contact angle is connected with the different morphology of the surface. The roughness created by the particles gives rise to the formation of air pockets (see Supporting Information, Figure S1.b), which are responsible of such increase in the contact angle [31]. Therefore, the results revealed that the assembly of silicone particles onto a surface creates roughness that increases hydrophobicity, which could be of practical interest for specific applications, e.g., as coatings. 3.3 Synthesis, characterization and applications of hybrid silicone porous particles

21

To provide additional functionalities to the silicone particles, magnetic nanoparticles (NP@PAA) were encapsulated into the pores. To prove the encapsulation capacity of the emulsion templates, aqueous dispersions of NP@PAA at different concentrations ranging from 0.8 to 17wt% were used as inner aqueous phase. The intermediate oil phase was composed by a PDMS200/PDMS0.7 weight ratio of 60/40. The formation of the multiple emulsion templates, as well as the crosslinking process was followed by optical microscopy. When dispersions above 15wt% of NP@PAA were used, some coalescence events between the inner droplets were detected, leading to globules with bigger inner droplets (see Supporting Information, Figure S2.a). Perhaps this is a result of the interaction between the hydrophilic capping of NP@PAA and the hydrophobic surfactant that stabilizes the inner aqueous droplets. Despite of the detected coalescence, in all the cases, silicone particles with a well-defined porous structure were obtained (see Supporting Information, Figure S2.b). Figure 7 shows SEM images of pores from particles obtained using a 17wt% NP@PAA dispersion as inner aqueous phase. The different texture that can be distinguished on the wall of the pores suggested that NP@PAA were mainly located within the pores (Figure 7a). The elemental analysis confirmed the presence of NP@PAA inside the silicone particles. In particular, the EDS spectrum of the area marked in the SEM image in Figure 7b showed the expected three peaks of Fe at 0.7, 6.4 and 7.1 keV (Supporting Information, Figure S3). In addition, Fe EDS mapping also confirmed that the NP@PAA were mostly located on the pore walls (inset in Figure 7b).

22

a)

b)

9µm

Figure 7. SEM images of pores of hybrid silicone particles: a) the arrow points the presence of magnetic nanoparticles located on the pore wall, b) selected area where the elemental analysis was performed together with the Fe elemental mapping (inset). As expected, due to presence of the magnetic nanoparticles, hybrid silicone particles were instantaneously attracted by an external NdFeB magnet (inset Figure 8). Magnetic measurements of hybrid samples were also performed to verify whether the NP@PAA preserved their magnetic properties after being encapsulated within the pores. Figure 8 shows the magnetization as a function of the applied field (M(H)) for raw NP@PAA and hybrid particles. The absence of hysteresis in the plot in Figure 8b revealed that hybrid particles are superparamagnetic, exhibiting a saturation magnetization (Ms) of 0.27 emu g-1 (of sample).

23

a) M (emu/gFe3O4)

60 40 20 0

-30

-20

-10

0

10

20

30

H (KOe)

-20 -40 -60 0.3

b) M (emu/g sample) 0.2 0.1 0.0 -20

-10

0 -0.1 -0.2

10

20

30

H (KOe) magnet

-30

-0.3

Figure 8. M(H) data at 300 K of raw (a) NP@PAA and (b) hybrid silicone particles with 0.9wt% of NP@PAA (prepared from a template with a 17wt% NP@PAA dispersion as inner aqueous phase). Inset: hybrid particles attracted by an external permanent magnet. The magnetic hyperthermia response of the composite particles was measured. Magnetic heating increases with the applied field and frequency (Figure 9a, b). Particles can be heated up ca. 30 ºC in 10 min at 869 kHz and 200 G (Figure 9a). Namely, starting from room temperature, particles can reach about 50 ºC. As the magnetic nanoparticles are superparamagnetic, fixed onto the wall of the polymer matrix and 24

measured in the dry state, there are neither hysteresis losses nor Brownian relaxation, therefore the main heating mechanism in the present case is Néel relaxation [32]. The magnetic hyperthermia response can be used, for example, to induce the release of encapsulated substances at high temperatures, as revealed in our previous paper [1].

a)

35

869 869 kHz kHz 688 688 kHz kHz 544 kHz kHz 544

30

ΔΤ (ºC)

25 20 15 10 5 0 0

100

200

300

400

500

600

400

500

600

time (s)

b)

35

200 200GG 180 180GG 160G 160 G

30

Δ T (ºC)

25 20 15 10 5 0 0

100

200

300

time (s)

Figure 9. Magnetic hyperthermia curves for dry composite particles at (a) 869 kHz and different fields and (b) 200 G and different frequencies. The concentration of NP@PAA in the composite particles estimated from the emulsion formulation is 0.9 wt%. A possible environmental application of the as-synthesized hybrid magnetic particles was tested. The high swelling capacity of the particles made by a mixture of 25

PDMS200/PDMS0.7 (weight ratio 60/40), together with their added magnetic functionality, suggested a practical application of the material as a novel absorbent. To prove qualitatively this application, an oil spill was simulated. Heptane colored with Black Sudan HB was added to a Petri dish containing water. Heptane is shown floating on the water surface in Figure 10a. Then, a small amount of hybrid particles was added. Owing to their low density, particles floated and remained on the water surface, mixing up with heptane (Figure 10b). Particles absorbed the heptane in few seconds due to their enhanced swelling/absorption capacity, as evidenced by the disappearance of the purple spill in Figure 10c, and consequently, particles became colored. Further, the hybrid particles could be easily collected, even in the swollen state, by an external permanent magnet, living behind nearly pure water (Figure 10d).

Figure 10. Removal of an oil spill with hybrid silicone particles: 0.5 mL of dyed heptane was poured onto a water surface (a), then 50 mg of hybrid particles were added (b). The organic solvent was absorbed by the particles (c), and finally, swollen particles were removed with the help of a permanent magnet, leaving almost solvent-free clean water (d).

26

High buoyancy characteristics, high oil absorption, high hydrophobicity, and recyclability are attractive properties of this high-performance composite material to be used for the removal of oil spills, which is important for environmental applications. 4. Conclusions Based on our previous work [1], we have developed a facile multiple W1/O/W2 emulsion templating process to prepare silicone porous particles with tuned mechanical properties. This was achieved by using silicone precursors with different chain lengths (i.e., different viscosities). By taking advantage of the properties of each silicone precursor and their mixtures, the viscosity of the intermediate oil phase was successfully modified, leading to multiple W1/O/W2 emulsions with tailored characteristics. As expected, the viscosity ratio ηO/ηW2 is a primary parameter to control the size and polydispersity of emulsion globules [9,10]; the globule size and polydispersity decreased with the ηO/ηW2 ratio. Compression and swelling tests revealed that, similarly to bulk silicone materials [14,15], mechanical properties of the emulsion-derived particles can be tuned by changing the nature of the silicone precursor. The use of precursors with long polymer chains entails networks with increased elasticity, but also higher tendency to collapse. This collapse can be prevented by mixing highly viscous precursors (PDMS100 and PDMS200) with low viscous PDMS0.7 in a weight ratio of 60/40. It is interesting to emphasize that the resulting bimodal network provided sufficient mechanical strength to simultaneously exhibit a porous structure and a great swelling capacity. In addition, silicone particles showed ability to encapsulate hydrophilic fluorescent molecules and functional nanomaterials, such as magnetic nanoparticles. The observed effective removal of organic solvents from aqueous media

27

by our hybrid magnetic nanocomposite and the hyperthermia response can provide inspiration for the application of the described material for environmental uses. In summary, the present results demonstrate the versatility of multiple emulsion templating as a facile method to integrate inorganic-organic components into particulate, porous materials. The possibility of incorporating nanomaterials with other properties (e.g., optical, fluorescent) using different polymer matrices is a motivation for future work. Acknowledgements We thank Dr. M. Bañobre-López, Dr. F. L. Deepak, Dr. B. Espiña and Dr. A. Manich for assistance with VSM, SEM-EDS, confocal microscopy and mechanical properties testing, respectively. Ministerio de Economia y Competitividad (grant CTQ201129336-C03- 01/PPQ), Generalitat de Catalunya (grant 2009 SGR-961), and the European

Union's

Seventh

Framework

Programme

(FP7/2007-2013)

under

COOPERATION program NMP-theme (grant agreement n° 314212) are gratefully acknowledged for financial support. Dr. N. Vilanova also thanks CSIC for a JAEpredoctoral scholarship. [1] N. Vilanova, C. Solans, C. Rodríguez-Abreu, Langmuir 2013, 29 (49), 15414–15422 [2] Y. Y. Yang, T. S. Chung, N. P. Ng, Biomaterials 2001, 22 (3), 231-234 [3] M. H. Lee, S. G. Oh, S. K. Moon, S. Y. Bae, J. Colloid Interf. Sci. 2001, 240 (1), 8389 [4] Y. H. Cho, J. Park, J. Food Sci. 2003, 68 (2), 534-538 [5] S. E. Bae, J. S. Son, K. Park, D. K. Han, J. Control. Release. 2009, 133 (1), 37-43 28

[6] F. Leal-Calderón, V. Schmitt, J. Bibette, Emulsion Science, Basic principles, Second Edition, Springer-Verlag, New York, 2007, Chapter 6 [7] C. Mabille, V. Schmitt, P. Gorria, F. Leal Calderón, V. Faye, B. Deminière, J. Bibette, Langmuir 2000, 16 (2), 422-429 [8] C. Mabille, F. Leal-Calderón, J. Bibette, V. Schmitt, Europhys. Lett. 2003, 61 (5), 708-714 [9] C. Goubault, K. Pays, D. Olea, P. Gorria, J. Bibette, V. Schmitt, F. Leal-Calderón, Langmuir 2001, 17 (17), 5184-5188 [10] J. Weiss, G. Muschiolik, J. Disper. Sci. Technol. 2007, 28 (5), 703-716 [11] J. E. Mark, H. R. Allcock, R. West, Inorganic Polymers, Second Edition, Oxford University Press, New York, 2005 [12] A. C. C. Esteves, J. Brokken-Zijp, J. Laven, H. P. Huinink, N. J. W. Reuvers, M. P. Van, G. de With, Polymer 2009, 50 (16), 3955-3966 [13] N. Vilanova, C. Rodriguez, A. Fernandez-Nieves, C. Solans, ACS Appl. Mater. Interfaces 2013, 5 (11), 5247-5252 [14] C. Pina-Hernandez, J. S. Kim, L. J. Guo, P. F. Fu, Adv. Mater. 2007, 19 (9), 12221227 [15] G. D. Genesky, C. Cohen, Polymer 2010, 51 (18), 4152-4159 [16] I. Pastoriza-Santos, J. Pérez-Juste, G. Kickelbick, L. M. Liz-Marzán, J. Nanosci. Nanotechnol. 2006, 6 (2), 1-6

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[17] S. V. Patwardhan, V. P. Taori, M. Hassan, N. R. Agashe, J. E. Franklin, G. Beaucage, J. E. Mark, S. J. Clarson, Eur. Polym. J. 2006, 42 (1), 167-178 [18] C. E. Hoppe, C. Rodríguez, M. Lazzari, M. A. López-Quintela, C. Solans, Phys. Status Solidi A 2008, 205 (6), 1455-1459 [19] M. Zielecka, E. Bujnowska, Prog. Org. Coat. 2006, 55 (2), 160-167 [20] H. F. Hoefnagels, D. Wu, G. de With, W. Ming, Langmuir 2007, 23 (26), 1315813163 [21] N. Stafie, D. F. Stamatialis, M. Wessling, Sep. Purif. Technol. 2005, 45(3), 220231 [22] J. C. McDonald, G. M. Whitesides, Accounts Chem. Res. 2002, 35 (7), 491-499 [23] H. Wang, P. Chen, X. Zheng, J. Mat. Chem. 2004, 14 (10), 1648-1651 [24] Z. Nie, M. Seo, S. Xu, P. C. Lewis, M. Mok, E. Kumacheva, G. M. Whitesides, P. Garstecki, H. A. Stone, H. A. Microfluid. Nanofluid. 2008, 5 (5), 585-594 [25] K. A. Cavicchi, A. S. Zalusky, M. A. Hillmyer, T. P. Lodge, Macromol. Rapid Comm. 2004, 25 (6), 704-709 [26] V. Muralidharan, C-Y. Hui, Macromol. Rapid Comm. 2004, 25 (16), 1487-1490 [27] J. N. Lee, C. Park, G. M. Whitesides, Anal. Chem. 2003, 75 (23), 6544-6554 [28] A. Mahomed, D. W. Hukins, S. N. Kukureka, Med. Eng. Phys. 2010, 32 (4), 298303 [29] G. T. Roman, C. T. Culbertson, Langmuir 2006, 22 (9), 4445-4451 30

[30] M. Cretich, V. Sedini, F. Damin, G. di Carlo, C. Oldani, M. Chiari, Sensor. Actuat. B 2008, 132 (1), 258-264 [31] M. Krasowska, J. Zawala, K. Malysa, Adv. Colloid. Interfac. 2009, 147-148 (C), 155-169 [32] J. Rivas, M. Bañobre-López, Y. Piñeiro-Redondo, B. Rivas, M. A. López-Quintela, J. Magn. Magn. Mater. 2012, 324 (21), 3499-3502

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Figure captions Figure 1. a) . a)

Dependence of the droplet size of primary emulsions with viscosity of the

oil phase (ηo). Dashed line is only a visual guide. Above ηo = 30 mPa·s, the droplet size was below one micrometer, not measurable by the microscope. Optical microscopy images of primary W1/O emulsions prepared using oil phases with viscosities of b) 15.3 and c) 38.7 mPa·s (see compositions in Table 2). Figure 2. a) Dependence of globule size with viscosity ratio between the intermediate oil phase and the external aqueous phase (ηO/ηW2) for emulsions containing PDMS0.7, PDMS100, PDMS200 or mixtures of them (see composition of the mixtures in Table 2). Optical micrographs of multiple emulsions prepared using b) PDMS100 only (ηO/ηW2=6.5) or c) a mixture of PDMS100/PDMS0.7 (weight ratio of 60/40, ηO/ηW2=3.9). Figure 3. a-d) Optical microscopy images of completely crosslinked samples made with different weight ratios of PDMS100/PDMS0.7, where the inner multiple structure is visible in all cases. e-h) SEM images of broken particles of the same samples but after being washed and dried, where the collapse of the pores is visible in some cases. Figure 4. a) Pore and b) particle sizes of samples synthesized with different silicone precursors: PDMS0.7 and a mixture of PDMS100 or PDMS200 with PDMS0.7 in a weight ratio of 60/40. For comparison, the size of the inner droplets and the globules of the original templates are also shown. Dashed lines are only visual guides. Figure 5. Swelling capacity of particles made with PDMS100 or PDMS200 mixed with PDMS0.7 at different weight ratios. The black line denotes the swelling capacity of

32

particles made with PDMS0.7 only. Inset: optical images of swollen (left) and dried (right) particles made with a PDMS200/PDMS0.7 weight ratio of 60/40. Figure 6. Confocal image of silicone porous particles made by a mixture of PDMS200/PDMS0.7 at a weight ratio of 60/40 and loaded with Pyronin Y using the swelling approach. Figure 7. SEM images of pores of hybrid silicone particles: a) the arrow points the presence of magnetic nanoparticles located on the pore wall, b) selected area where the elemental analysis was performed together with the Fe elemental mapping (inset). Figure 8. M(H) data at 300 K of raw (a) NP@PAA and (b) hybrid silicone particles with 0.9wt% of NP@PAA (prepared from a template with a 17wt% NP@PAA dispersion as inner aqueous phase). Inset: hybrid particles collected by an external permanent magnet. Figure 9. Magnetic hyperthermia curves for dry composite particles at (a) 869 kHz and different fields and (b) 200 G and different frequencies. The concentration of NP@PAA in the composite particles estimated from the emulsion formulation is 0.9 wt%. Figure 10. Removal of an oil spill with hybrid silicone particles: 0.5 mL of dyed heptane was poured onto a water surface (a), then 50 mg of hybrid particles were added (b). The organic solvent was absorbed by the particles (c), and finally, swollen particles were removed with the help of a permanent magnet, leaving almost solvent-free clean water (d).

33

Table 1. General composition of the multiple W1/O/W2 emulsion. PDMSX indicates the silicone precursor used, where X denotes the viscosity of the precursor: 0.7, 100, 200 cSt or a mixture of them. Phases/Pseudo-phases Inner aqueous phase (W1) Primary emulsion

W1/O

Multiple emulsion

W1/O/W2

Intermediate oil phase (O) Primary W1/O emulsion External aqueous phase (W2)

Components wt% Water 4 KF-6104 18 Crosslinker 52 PDMSX 22 Catalyst solution 4 Those of W1/O 20 5wt% Tween 80 80 aqueous solution

Table 2. Viscosities at 70ºC of intermediate oil phases (ηo) prepared with different types of PDMS. See the composition of the whole intermediate oil phase in Table 1. The viscosity ratio between the intermediate phase and the external phase (ηo/ηW2) is also indicated. Silicone precursor ηO (mPa·s) PDMS0.7 15.3 PDMS100 33.3 PDMS100/PDMS0.7 (90/10)* 29.5 PDMS100/PDMS0.7 (80/20)* 23.5 PDMS100/PDMS0.7 (70/30)* 20.6 PDMS100/PDMS0.7 (60/40)* 20.2 PDMS200 38.7 PDMS200/PDMS0.7 (90/10)* 31.2 PDMS200/PDMS0.7 (80/20)* 27.5 PDMS200/PDMS0.7 (70/30)* 22.2 PDMS200/PDMS0.7 (60/40)* 21.5 *weight ratio

ηO/ηW2 3.0 6.5 5.7 4.6 4.0 3.9 7.3 6.1 5.4 4.3 4.2

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Table 3. Elastic moduli (E) of silicone monoliths prepared using different types of precursors. Silicone precursor PDMS0.7 PDMS100 PDMS200 PDMS100/PDMS0.7 (60/40)* PDMS200/PDMS0.7 (60/40)* *weight ratio

E (kPa) 428.41±27.25 2.33±0.04 0.95±0.14 108.73±1.17 81.41±15.31

35

Graphical abstract

Highlights: •

Silicone porous particles were obtained by multiple emulsion templating

•

Silicone precursors with different molecular weights were used

•

Each silicone precursor led to particles with different mechanical properties

•

Multifunctional particles were prepared incorporating magnetic nanoparticles

36