Synthesis and characterization of polyHIPE composites containing halloysite nanotubes

2.1 Materials

Styrene (monomer, Merck, Darmstadt, Germany), divinylbenzene (cross-linker, technical grade, 80%, Aldrich Chemistry, Steinheim, Germany), Sp (bio-sorbent, Egert Natural Products Ltd. Co., İzmir, Turkey), Span 80 (non-ionic surfactant, Aldrich Chemistry, Steinheim, Germany), and calcium chloride dihydrate, CaCl₂‧2H₂O (Tekkim Chemistry Ltd. Co., Bursa, Turkey) were used without further purification. 2,2′-Azobisisobutyronitrile, AIBN (initiator, 98%) was purchased from Aldrich Chemistry (Steinheim, Germany) and used after purification by ethanol. HL nanotube (Tabanköy-Balıkesir, Turkey) was kindly provided from ESAN Eczacıbaşı Industrial Raw Materials Company (İstanbul, Turkey). Deionized water was used in all experiments.

2.2 Immobilization of spirulina on halloysite nanotubes by physical adsorption

Immobilization of Sp on HL nanotubes was performed according to the procedure given below, which was also reported in our previous study (28). The solutions of Sp (0.1 g) and HL (2 g) prepared with 300 ml and 200 ml of deionized water, respectively; were stirred at 50°C for 1 h. Then, 100 ml deionized water was added to the total solution after mixing the Sp solution with the HL solution. The final solution was stirred vigorously at 50°C for 4 h. The modified nanotubes named as “SpHL” were precipitated via centrifugation. SpHL particles were dried in a vacuum oven at 50°C for 48 h and washed with deionized water.

2.3 Preparation of HIPEs and polyHIPE composites

The polyHIPE composites were prepared by polymerization of HIPEs, which have 80 vol% internal phase (water) and 20 vol% external phase (monomer phase). The monomer phase (90 vol% styrene and 10 vol% divinylbenzene) was stirred with Span 80 (2–5 vol%, with respect to the organic phase) until the homogeneous solution was obtained. Then, SpHL (0.25, 0.50, 0.75 and 1.00 wt%; with respect to the organic phase) and AIBN (1 mol%) were added to the monomer phase, respectively. Then, the stirring process was continued vigorously for 10 min. While the CaCl₂‧2H₂O solution (0.7938 g in 20 ml deionized water) as the internal phase was dropped into the organic phase slowly, the obtained total emulsion was stirred simultaneously. The formation of homogeneous HIPE was followed with polymerization after transferring emulsion into a polyethylene centrifuge tube in a preheated oven at 70°C for 24 h. The polymer composites were dried in a vacuum oven at 40°C for 24 h after purifying them with ethanol in a soxhlet extraction. The neat porous polymer without nanotubes was also prepared according to the procedure described above.

PolyHIPEs synthesized in presence of SpHL were named as X-SpHL (Y sur.) where X and Y indicate HL weight percent and surfactant (Span-80) volume percent, respectively.

2.4 Characterization

X-ray diffraction (XRD) measurements of HL and SpHL were performed by a Rigaku D/Max 2200 Ultimat diffractometer (Rigaku, Tokyo, Japan) with CuKα radiation (λ=1.54 Å), operating at 40 kV and 40 mA with a scanning rate of 2°/min. Morphological characterization of HL, Sp, SpHL nanotubes and polyHIPE composites were examined by using an ESEM-FEG/EDAX Philips XL-30 microscope (Philips, Eindhoven, The Netherlands). The XRD results and SEM images of HL nanotubes and morphological characterizations of the polyHIPE composites having 3 vol% surfactant were reported in the supplementary material.

Specific surface area of the polyHIPE composites were investigated with a Micromeritics Gemini VII 2390t Surface area and a Porosity Analyzer (Micromeritics Instrument Corporation, Norcross, USA) by using the Brunauer–Emmet–Teller (BET) adsorption model. Furthermore, average interconnected pore diameters, average cavity diameters and cavity size distributions were determined by using high magnification SEM images. From each SEM image, over 100 measurements were done for the cavity size calculations and the average value was corrected with a statistical correction factor (2/3^1/2) to account for irregular cutting of the samples (29). Average interconnected pore diameters were also calculated fom SEM images by taking over 60 measurements from each image.

Thermogravimetric analyses (TGA) of the HL, SpHL as well as polyHIPE composites were contucted with a Seiko TG/DTA 6300 thermal analysis system instrument (Seiko Instruments, Tokyo, Japan) under nitrogen flow with a heating rate of 10°C/min. The TGA thermograms of the HL, SpHL and polyHIPE composites were given and discussed in the supplementary material.

The differential scanning calorimeter (DSC) measurements of polyHIPE composites were done by using a DSC Q200 (TA Instruments, New Castle, DE, USA) instrument under nitrogen flow with a heating rate of 10°C/min.

Dye adsorption properties of polyHIPE composites were determined by using Ultraviolet/Visible (UV/VIS) analysis. The polyHIPE composites were immersed into a solution of cationic dye (Nile Blue, 1.10⁻⁵m) at a pH of 5–5.5. Absorbance value measurements during the adsorption process were done by using a UV-Vis spectrophotometer (T80+ UV/VIS Spectrophotometer, double beam optical system, PG Instruments Ltd., Leicester, UK) at the wavelengths of 635 nm. Dye adsorption capacities of polyHIPE composites were found according to the absorbance value of the remaining dye in aqueous solution. The amount of dye (mg) adsorbed by per g of polyHIPE composites was calculated with the following equation:

where C₀ and C_t are the initial and final concentrations of dye solution (mg/l) at each time intervals, V is the volume of the dye solution (l) and W is the weight of the polyHIPE composite used in the adsorption process (g).

3.1 Stability of HIPEs and synthesis of polyHIPE composites

Emulsions are known to be kinetically unstable systems. In this respect, use of a surfactant is required to provide the emulsion stability for long periods of time. For this purpose, conventional emulsifiers and surface modified nanoparticles are also used to prepare water-in-oil (w/o) and oil-in-water (o/w) emulsions. Herein, styrene/divinylbenzene based concentrated w/o emulsions, which were used as templates for polyHIPE composites, were stabilized by low amounts of surfactant (Span 80) in the presence of SpHL nanotubes. The compositions of HIPEs were given in Table 1. As shown in Table 1, in repeated experiments, surfactant concentrations and nanoparticle loading amounts were varied to investigate their effect on emulsion stability.

The stabilities of all the resultant emulsions prepared by using either Span 80 alone or using nanoparticles and Span 80 together were evaluated by observing occurrence of phase separation. For this purpose, the obtained emulsions were transferred into transparent polyethylene tubes and their stabilities at room temperature and polymerization temperature (70°C) were followed (Table1). Neat HIPEs were prepared in the absence of the nanotube to determine the minimum required Span 80 amount in terms of maintaining stabilization. A 2, 3 or 5 vol% of Span 80 were used for the preparation of neat HIPEs. The stability tests indicated that the emulsions having only 2 or 3 vol% of Span 80 were phase separated immediately at room temperature just after their preparation. On the other hand, the emulsions having 5 vol% Span 80 and also the ones with use of Span 80 <5 vol% together with various amount of SpHL nanotubes exhibited long-term stability (>5 weeks) at room temperature. Moreover, it was observed that emulsions which were stable at room temperature also preserved their stability during the polymerization reaction at 70°C. As a result, the minimum amount of surfactant needed for preparing neat polyHIPE in the absence of nanotubes was found to be 5 vol% with respect to volume of the continuous organic phase. On the other hand, by usage of the modified nanotubes in the range of 0.25–1.00 wt%, the necessary amount of Span 80 for efficient stabilization of HIPEs was reduced to 2 vol%. This result can be attributed to the contribution of the SpHL nanotube acting as a co-stabilizer-like material in the emulsion stability. It was most probably achieved with the help of the functional groups of Sp such as amino, hydroxyl, carboxyl and sulfate (25), (26) as well as its organic moieties. This reveals that the modified nanotube (SpHL) play a crucial role for providing the stabilization.

The morphology, adsorption and thermal studies were conducted with polyHIPE composites having 2 vol% surfactant and the results are discussed in the following sections accordingly.

3.2 Morphological and surface characterization of neat polyHIPE and polyHIPE composites

Figure 1 shows SEM images of neat polyHIPE which was synthesized in the absence of SpHL nanotubes and in the presence of 5 vol% conventional surfactant. As can be seen from the Figure 1, neat porous polymer has an open-cellular structure with pore interconnections. The effects of modified nanotubes on morphological properties of polyHIPEs synthesized with the minimum surfactant amount (2 vol% with respect to the organic phase) were also evaluated by SEM analyses (Figures 2–4). On the other hand, morphologies of the polyHIPE composites synthesized in presence of 3 vol% surfactant concentration are also discussed in the supplementary material file.

Figure 1:

Low and high magnification SEM images of neat polyHIPE stabilized with 5 vol% conventional surfactant.

Figure 2:

Low and high magnification SEM images of polyHIPE composites synthesized using a constant surfactant concentration of 2 vol%: (A) 0.25-SpHL, (B) 0.50-SpHL.

Figure 3:

Low and high magnification SEM images of polyHIPE composites synthesized using a constant surfactant concentration of 2 vol%: (A) 0.75-SpHL, (B) 1.00-SpHL.

Figure 4:

HL nanotube dispersion at high magnification SEM images of (A) 0.25-SpHL, (B) 0.50-SpHL, (C) 0.75-SpHL, (D) 1.00-SpHL composites having 2 vol% of surfactant.

It is clear from Figures 2–4 that; all the polyHIPE composites have the cavities together with pore interconnections as is generally observed in conventional polyHIPE morphology. The polyHIPE composites were found to have open pore structures and partially or completely closed large cavities (Figures 2 and 3). This result is highly similar with the morphological properties of polyHIPE composites synthesized with constant 3 vol% surfactant concentration which were discussed in the supplementary material file. The formation of such hierarchical structures could be attributed to usage of nanoparticle and surfactant simultaneously for the stabilization of the HIPE system as reported in the literature (30).

From the high magnification SEM images (Figure 4), the distributed SpHL nanotubes on the pore walls as well as polymer balls within some pores can be clearly observed. The aglomeration of the modified nanotubes observed in Figure 4C and D can be ascribed to the interactions of the SpHL nanotubes with each other above 0.5 wt% loading degree. These interactions occur most probably due to abovementioned functional groups of Sp and/or HL nanotubes at their high amounts. Additionally, the presence of polymer balls within some pores of resultant polyHIPEs can be attributed to presence of a minor amount of mostly hydrophilic nanotubes which tend to form o/w type emulsions. The polymerization of these o/w type emulsions together with the main droplets of w/o emulsions may result in the polymer balls on the walls (Figure 4C and D). The formation of such polymer balls with a similar attribution was also reported in the use of modified titanium dioxide in the synthesis of polyHIPEs by Menner et al. (31).

On the other hand, use of the nanotube at the fixed amount and decrease in the surfactant concentration from 3 vol% to 2 vol% resulted in formation of relatively larger cavities (given in the supplementary material file). As compared to neat polyHIPE, the formation of larger cavities surrounded by interconnected smaller ones in polyHIPE composites may be attributed to the predominantly stabilized emulsion droplets by the modified nanotubes in place of the conventional surfactant (32).

Average cavity diameters of the materials were calculated from their corresponding SEM images and the cavity size distribution graph based on SEM calculations are presented in Table 2 and Figure 5, respectively. Depending on cavity size calculations it was found that the average cavity diameter of the neat polyHIPE (23.78 μm) was decreased to 11.16 μm via the 0.25-SpHL composite. On the other hand, with the addition of nanotubes the average pore interconnection diameter first decreased from 4.31 μm to 0.91 μm, and then increased to 3.26 μm with an increase in the amount of nanotubes (Table 2). According to the cavity size distribution graph presented in Figure 5, the sizes of cavities with a heterogeneous distribution were found to be mostly in the range of 0–50 μm for all the materials. The specific surface areas and average pore sizes of the porous polymers synthesized by using the lowest surfactant concentration of 2 vol% were tabulated in Table 2. The surface areas of the resultant polyHIPE composites were found not to change significantly in comparison with the neat polyHIPE sample. As can be seen from the Table 2, surface area of the 0.25-SpHL composite was found to be higher than that of neat polyHIPE and then it decreased with the increase in nanotube loading. While the surface area and the average interconnected pore diameter of the neat polyHIPE was 1.80 m²/g and 4.31 μm, respectively, these values were found to be 2.38 m²/g and 0.91 μm for the 0.25-SpHL composite. According to this result, the surface area of the 0.25-SpHL composite is ca 30% higher than that of neat polyHIPE. Considering the surface areas calculated by applying BET models for N₂ adsorption/desorption isotherms, the decrease of surface area is in the relation to the increase in the average interconnected pore diameters (Table 2). Moreover, the lower surface areas at higher nanotube amounts could also be explained by the particle aggregates embedded in polymer cavities (Figure 4).

Figure 5:

Cavity size distribution of the polyHIPEs.

3.3 Dye adsorption capacities of the polyHIPE composites

Adsorption tests of the polyHIPEs synthesized with 2 vol% surfactant concentration were conducted by using a cationic dye, Nile Blue. It is known from the literature that HL nanotubes and Sp are also good candidates for adsorption applications in their pure forms (23), (24), (25), (26), (27), (28). The adsorption performances of the composites including both Sp and HL nanotubes were shown comparatively with neat porous polymer in Figure 6. Although the neat polyHIPE sample having open large cavities (Figure 1) was expected to permit diffusion of dye molecules, the results showed a low adsorption capacity for this material. The low adsorption capacity of neat polyHIPE (0.28 mg of dye per g of polymer) is possibly due to lack of any functional group in the polymer matrix which can attract the dye molecules.

Figure 6:

Nile Blue dye adsorption capacities of neat polyHIPE and SpHL polyHIPE composites.

On the other hand, the adsorption capacities of all the polyHIPE composites were found to be higher than neat polymer due to the increased selectivity arising from active groups of Sp such as hydroxyl, carboxylic, phosphate, amine, amide, and sulfate. Among the composites, 0.25-SpHL as the best adsorbent displayed an adsorption capacity of 1.02 mg/g which is about 264% higher than that of neat polyHIPE. The adsorption capacities of other polyHIPE composites, 0.50-SpHL, 0.75-SpHL and 1.00-SpHL were found to be 0.56, 0.80, and 0.63 mg/g, respectively. When the nanotube loading increased from 0.25 wt% to 0.5 wt%, the adsorption was found to decrease. It is interesting that 0.75 wt% clay loading resulted in higher dye adsorption again. The increments and decrements of dye adsorption capacities can be ascribed to pore collapse during the drying process of the synthesized polyHIPE composites. According to Jerabek et al. pores in rigid polymers may collapse due to the capillary forces generated in the menisci of liquid filling the pores. Accordingly, different small pore structures can be formed with the drying process (33). This situation could play a role in the adsorption behavior of the composites having different amounts of nanoparticles. In summary, all the polyHIPE composites showed higher adsorption rates at shorter times of the adsorption test than neat polyHIPE (Figure 6). Moreover, in the first 90 min of the adsorption test, neat polyHIPE has an adsorption value of 0.09 mg/g whereas this value increases to 0.78 mg/g by 0.25-SpHL which is almost a 765% increment.

3.4 Thermal behavior of polyHIPE composites

The DSC thermograms of neat polymer and polyHIPE composites are given in Figure 7. The Tg values were taken as the intersection of extrapolation of baseline and extrapolation of the inflection (34). As can be seen from the figure, Tg for all the polyHIPE composites were found to be increased in comparison with neat polyHIPE. The Tg value of neat polyHIPE is calculated as 114.14°C. It shifted to values of 119.39, 120.62, 122.18 and 121.82°C, which are close to each other, for 0.25-SpHL, 0.50-SpHL, 0.75-SpHL and 1.00-SpHL, respectively. This can be attributed to physical interactions between the nanotubes and the matrix molecules resulting in restrictions of polymer chains and their segmental motions.

Figure 7:

DSC curves of the neat polyHIPE and SpHL containing polyHIPE composites.

Thermal stabilities of neat polyHIPE and polyHIPE composites were investigated by thermogravimetric analyses (TGA) and their thermograms are presented in Figure 8. The neat polyHIPE exhibited a degradation onset temperature of 375.30°C, at which 10% weight loss occurs. As can be seen from the magnified inset figure, all the polyHIPE composites have higher onset degradation temperatures (379.60°C–385.30°C) compared to neat polymer. This result is also in good agreement with their DSC results (Figure 7). The highest onset degradation temperature (385.30°C) was achieved via the 0.25-SpHL composite. This enhancement can be due to the presence of a relatively high amount of open smaller cavities with smaller pore interconnections for 0.25-SpHL composite (Figure 2A) which inhibits heat transfer through the matrix. It can also be ascribed to comprehensive interactions between the nanotubes and polymer molecules because of the relatively better dispersion of the nanotubes in the same composition (Figure 4A). These maximized interactions can also contribute to difficulty in thermal motions of polymer chains attached to the nanotube surface.

Figure 8:

TGA thermograms of neat polyHIPE and SpHL polyHIPE composites.

As a result, we can safely state that use of SpHL nanotubes as a co-stabilizer-like material in the current study both enhances the emulsion stability and also reinforces the polymer matrix. The remarkable adsorption performances and enhanced thermal properties can be achieved by optimizing nanotube loading degree.