Article pubs.acs.org/Langmuir
Waterborne Colloidal Polymer/Silica Hybrid Dispersions and Their Assembly into Mesoporous Poly(melamine-formaldehyde) Xerogels Dana Schwarz†,‡ and Jens Weber*,‡ †
Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Science Park Golm, 14424 Potsdam, Germany Fachgruppe Chemie, Hochschule Zittau/Görlitz (University of Applied Science), Theodor-Körner-Allee 16, 02763 Zittau, Germany
‡
S Supporting Information *
ABSTRACT: The acid-catalyzed polycondensation of oligo(melamine-formaldehyde) in aqueous phase and in the presence of silica nanoparticles leads to a stable dispersion of coexisting silica and polymer nanoparticles. The dispersion can be processed into mesoporous xerogels (SBET ≈ 200 m2 g−1), whose porosity can be enhanced by etching of silica up to specific surface areas of >400 m2 g−1. The formation mechanism and the characteristics of the hybrid dispersion are crucial to the materials derived from it and analyzed in detail using a variety of experimental techniques (electron and force microscopy, light and X-ray scattering, ultracentrifugation, and spectroscopy). The transformation of the dispersion into xerogels by electrostatic destabilization is described. Furthermore, the obtained materials are characterized with regard to their porosity and morphology using microscopy and porosimetry. The impact of selected synthesis parameters on the obtained properties is discussed, and it was found (most interestingly) that stable porosity was only observed if silica nanoparticles were present within the dispersion.
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INTRODUCTION Micro- and mesoporous organic polymers, i.e., polymers having pore sizes below 2 or 50 nm, respectively, have received a great deal of attention during the past decade. They potentially combine the high porosities, which are well known from activated carbons or zeolites, with the light weight, the good processability, and the broad range of functionalities that are well known from polymer science, resulting in potential applications ranging from adsorption technology to biomedical and optoelectronic applications.1−9 Melamine-based meso- and microporous polymers are a subclass that have attracted quite some attention, as the high incorporation of amine functionalities in the polymer provides some benefits for applications in adsorption technology (targeting gases such as CO2 or micropollutants and heavy metal contamination). Therefore, within the last decades various synthetic concepts were developed.10−18 However, most of the above-mentioned concepts do suffer from upscaling issues. Some concepts involve heating of the reaction solution in DMSO in closed vessels up to 180 °C, resulting in mesoporous materials of very high porosities.15 Those materials do however contain up to 5 wt % of sulfur,15,17 indicating that the reaction involves DMSO decomposition. This could ultimately result in formation of toxic byproducts and an unwanted pressure build up. The heat management could also be an issue upon upscaling, especially for bulk approaches.10 Those problems need to be solved to discuss a material seriously for any application. Hence, a synthetic route to larger quantities of (ideally spherical) particles would be needed. We investigated possibilities to use dispersion polycondensation to access © XXXX American Chemical Society
mesoporous poly(melamine-formaldehyde) (PMF) resins using water as the continuous phase and oxalic acid as catalyst.19 Those attempts resulted in irregular-shaped large particles (which sedimentated quickly) that were based on smaller primary polymeric nanoparticles. The dispersion polycondensation route proved to be scalable, and (as a water-based reaction) toxicity issues and cleaning from unwanted side products or decomposition leftovers are easier to achieve and prevent. The observation of the small primary particles that seemed to be crucial to the formation of the porous PMF particles reported previously triggered the question whether such nanoparticles can also be obtained in a controlled manner, i.e., as a stable dispersion. Within this paper, results obtained by further studies of the dispersion polycondensation at varying synthesis conditions are presented. Indeed, we found that a stable colloidal dispersion of a hybrid PMF/silica material can be formed under certain conditions. The stable dispersion can be processed into mesoporous materials with porosities even increased to the formerly described materials. Herein, the formation and stabilization of the mixed colloidal dispersion is discussed. It was found that the routine does not follow a simple core−shell structure path or seeded polycondensation regime. A variety of analytical methods (electron and force microscopy, light and X-ray scattering, ultracentrifugation, and spectroscopy) was used to study the dispersion and its transformation into porous materials. Received: March 17, 2015 Revised: July 8, 2015
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DOI: 10.1021/acs.langmuir.5b00990 Langmuir XXXX, XXX, XXX−XXX
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Langmuir Scheme 1. Supposed Mechanism of the Formation of the Transparent, Concentrated Precursor Solution
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washing with aqueous NaOH (1M),10 resulting in coarse PMF xerogel pieces, which were washed with water, ethanol, and acetone and subject for further analysis after drying at 90 °C. It should be noted at this stage that the formation of xerogels with the properties discussed below is well reproducible if the synthesis protocol is followed exactly. A drastic change of the protocol (e.g., temperature, even within the first stages of the synthesis of the dispersion) could lead to xerogels, which are not porous. Those finer details are part of ongoing research. Analytical Methods. Elemental Analysis (EA) were done with a varioMicro cube elemental analysis instrument from Elementar Analysensysteme GmbH. Thermogravimetric analyses (TGA) experiments were accomplished on a Netzsch TG209-F1 Iris apparatus at a heating rate of 10 K/min under synthetic air. Fourier transform infrared spectra (FTIR) were collected using a Nicolet iS5 from Thermo Fisher Scientific FTIR spectrometer; samples were measured in the solid state using the ATR technique or KBr pellet technique. 13 C NMR spectra (nuclear magnetic resonance spectroscopy) were acquired on a Bruker DPX-400 operating at 400.1 MHz using D2O as solvent. Scanning electron microscopy (SEM) measurements of dried and purified materials were carried out using either a LEO 1550-Gemini electron microscope (acceleration voltage 3 kV) or an Environmental SEM Quanta FEG 600 (FEI Co., acceleration voltage 5 kV). HR-SEM measurements were carried out on a Jeol JSM-7500 F microscope with an acceleration voltage from 1 up to 30 kV. The samples were coated with platinum. Transmission electron microscopy (TEM) imaging was performed using a Zeiss EM 912Ω (acceleration voltage 120 kV) using the diluted (×10) crude dispersion, which was deposited on the grid dropwise. Atomic force microscopy (AFM) measurements were performed on a multimode AFM from Bruker Instruments. Typically, the reaction solution (5 μL) was placed on mica by spin coating with a rotation speed of 3.2 × 1000 rpm. The coated mica was measured in tapping
MATERIALS AND METHODS
Materials. Oligomeric poly(melamine-co-formaldehyde), methylated (nominal molecular weight, Mn = 430 g/mol, 84 wt % solution in 1-butanol), silica nanoparticles (Ludox HS40, 40 wt % silica in water, nominal diameter 12 nm), ortho-phosphoric acid (85%, Merck), and NaOH (98.0% pellets) were purchased from Sigma-Aldrich. Ethanol (absolute) was purchased from VWR chemicals. Deionized water was taken from a Milli-Q facility with a water quality of 18 MΩ cm−1 and used in all experiments. General Synthesis of the Precursor Solution. To obtain the precursor solution, 2.5 g of oligomeric poly(melamine-co-formaldehyde) was dissolved in 2 mL of abs. ethanol, resulting in a less viscous fluid. A 0.5 mL amount of acid (typically ortho-phosphoric acid, 85%) was added to the solution under stirring. Finally, 4.0 g of aqueous Ludox HS40 dispersion was added, leading to an intermediate precipitate (see Scheme 1). The mixture became transparent again after stirring. General Synthesis of the PM−Silica Hybrid Colloidal Sol. The precursor solution is mixed with 100 mL of water at 40 °C (roundbottom flask, equipped with a reflux condenser) and kept at 40 °C for 2 h, followed by heating to 80 °C with magnetic stirring for 20 h by using an oil bath. The nominal silica concentration of the resulting dispersion is ∼15 g·L−1; the PMF concentration (calculation based on the monomer) is ∼20 g·L−1. Synthesis of the Pure PMF. For comparative reasons, another dispersion was prepared in the way described above with the exception that NO Ludox HS40 (silica particles) was added. Therefore, a dispersion made out of pure PMF was obtained that way. Gel Formation and Purification Procedure. The formed gel was compacted by centrifugation at 8000 rpm over a period of 5 min, leading to the separation of a solvent-swollen solid from excess solvent. The excess solvent was removed, and the coagulated particles were washed with ethanol various times. The white hybrid material was dried and finally cured at 90 °C in the oven. Within the drying process, the compacted block underwent further shrinking to a single monolithic block (xerogel). Template removal was achieved by B
DOI: 10.1021/acs.langmuir.5b00990 Langmuir XXXX, XXX, XXX−XXX
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Langmuir mode with normal tapping mode tips and a resonance frequency of 300 kHz. Nitrogen adsorption/desorption experiments were conducted at 77.4 K with a Quadrasorb SI/MP machine or an Autosorb AS-1 MP instrument (both Quantachrome Instruments). Carbon dioxide adsorption/desorption experiments were performed at various temperatures (e.g., 273, 283, 293, and 303 K) using an Autosorb AS-1 MP instrument from Quantachrome instruments. Before measurement, the samples were degassed for 20 h at 373 K under vacuum. The surface characterization parameters (surface area, pore size distribution, pore volume) were determined by means of different methods within the Quantachrome program AS1win. Small angle X-ray scattering (SAXS) measurements of dry powders were performed using a rotating anode machine (Nonius, Cu Kα radiation, 0.154 nm) with a two-dimensional MARCCD detector. One-dimensional profiles were obtained by azimuthally averaging using the FIT2D software. SAXS measurements of dispersions were carried out using a Nanostar (Bruker AXS). Dynamic light scattering (DLS) was performed at 25 °C using an ALV/CGS-3 compact goniometer system, equipped with a 22 mW HeNe laser, typically at a fixed scattering angle of 90°. Analytical ultracentrifugation (AUC) was performed using a XL-I centrifuge (Beckman-Coulter). Streaming potential measurements were done with a Zetasizer Nano from Malvern. A 15 mL amount of the reaction mixture was used for the investigation. The zeta potential was measured toward higher pH values by the addition of KOH and with HCl to the lower pH values. Both curves (pH directions) were taken separately from each other to keep the ionic strength in the solution as low as possible.
The reaction precursor mixture described above is defined as the successful “standard” precursor solution that can give a stable dispersion. Later, we will discuss the impact of varying acid and silica/MF content in more detail. However, before examining the impact of variations on the system, we wish to discuss the polymer/silica hybrid dispersion obtained from such standard recipe in detail. By adding the precursor mixture, consisting of 2.5 g of MF oligomer and 1.6 g of SiO2 and 0.5 mL of H3PO4 (85%), to 100 mL of water at 40 °C, the solution turned milky, turbid again, presumably because of liquid−liquid phase separation (see Supporting Information and DLS results below). The water solubility of methylol groups on the oligomer is very poor due to the possibility of hydrogen bonding. With decreasing amount of methylol groups on the oligomer the water solubility increases, resulting in the almost transparent mixture after ∼10 min. The mixture was kept at 40 °C for 2 h, after which it is heated to 80 °C for 20 h. Finally, an opaque to bluish colloidal dispersion was obtained, which was stable for at least several months at room temperature. While MF solutions/dispersions at higher pH (neutral to basic) are known to be stable only for a limited time,21−23 the existence of stable, colloidal PMF dispersions at low pH was known for quite some time.24 To the best of our knowledge, there have however been no reports on stable colloidal PMF dispersions that contain also silica nanoparticles. As the properties of the obtained porous materials also depend strongly on the finer synthesis details, the dispersion characteristics will be analyzed in more detail at this stage. Spectroscopic Analysis of the Reaction. The reaction procedure has been studied with 13C NMR (see Figure 1) using a dispersion mixture based on D2O as solvent.
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RESULTS AND DISCUSSION Preparation of the PMF−Silica Colloidal Sol. The synthesis starts with the preparation of a concentrated precursor solution containing both PMF precursors and silica particles, which were used previously for the bulk polycondensation process.10 The oligomeric, partly methylated MF species (84 wt % solution in butanol with the abbreviation MF oligomer) was first diluted with ethanol to decrease the viscosity, and concentrated phosphoric acid was added subsequently. Phosphoric acid was used initially, as it gave good results in the synthesis of bulk mesoporous PMF materials previously. The latter step, which is important to obtain water-soluble MF oligomers (as a consequence of the hydrolysis of the methylated methylol gropus), initiates the release of some methanol, and an acidic, clear solution is obtained. After adding the aqueous Ludox HS40 solution (pH = 9) to the acidic MF oligomer solution in ethanol, the mixture formed a white precipitate. It can be suspected that the resulting pH of the solution is close to the isoelectric point of silica, which results in initial electrostatic destabilization. The neutralized SiO2 particles agglomerated and probably formed hydrogen bonds among each other. However, after stirring the mixture for a very short time, it turned transparent again, indicating the formation of a homogeneous dispersion of silica nanoparticles and MF oligomers. Interestingly, most often such stabilization was found only for reactions with inorganic acids at sufficient concentration (see below) and not for most organic acids at low concentration, e.g., not for oxalic acid.19 We suspect that phosphoric acid could get adsorbed on the silica nanoparticles based on hydrogen-bonding interactions,20 thereby changing the electrostatic stabilization regime of the particles, which could lead to stabilization. Additionally, MF oligomers could also adsorb on the silica surface, which would also induce some additional steric stabilization. Scheme 1 summarizes the initial steps, which led to the clear and transparent precursor solution.
Figure 1. 13C NMR spectra of the dispersion reaction mixture at 26, 50, 70, and 90 °C (from bottom to top), D2O. Measurement time: 1.5 h for each spectrum.
The first measurement of the freshly prepared solution was done at room temerature (26 °C). Subsequently, the precursor solution was heated up further to follow the temperaturedependent reaction procedure. The spectra at temperatures of 26, 50, 70, and 90 °C were acquired within 1.5 h each. Among every temperature step, there was a heating and calibration time of ∼15 min. It should be noted that the NMR spectra were collected using normal mode, although the monitored species was colloidal particles; hence, there were limitations for resolution of signals from moieties confined in the growing C
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Langmuir networks. Sharp signals can be attributed to low molecular weight species, which were free to move. The spectra feature a variety of signals; most of them are attributed to solvents (e.g., butanol, methanol). Main interest is given to the region between 40 and 100 ppm, due to the significance for functional groups. The following discussion is largely based on the assignments given by Tomita and Ono.25 The precursor mixture did not show any signals within the range of 73−80 ppm, indicating that no methyl ethers of methylol groups were left. Further comparison of the spectra acquired at 26 and 90 °C show some significant changes, indicative of the ongoing reaction. Most prominently, broad signals at 64 and 71 ppm (attributed to methylol groups) vanish. The appearance of signals attributable to methylene bridges (see Scheme 1) is hard to discover, as the signal for methylene bridges between secondary amines (−NH−CH2− NH−) is expected at 48 ppm, which overlaps with methanol signals. Methylene bridges between a tertiary and a secondary amine (−N(CH2−)−CH2−NH−) are expected around 53 ppm. There is a faint hint of such signal within the spectra acquired at 90 °C, though this is not fully convincing. Peaks observed at 82 and 89 ppm can be assigned to various methylene glycol species, which are observed as byproducts. In summary, NMR gives evidence that the oligomeric MF species were consumed and start to condense quite fast, as they are absent at high temperature. IR analysis of the final materials (obtained after processing to xerogels and silica etching, see below) gives also clear indication that PMF resin has been formed successfully. The spectrum (see Supporting Information) shows the archetypical bands of PMF resins (triazine ring bending at 810 cm−1, −NH bend and/or CN ring vibration at ∼1550 cm−1, methylene bending at ∼1460 and 1330 cm−1, methylene stretch at 2950 cm−1) along with a broad band at ∼3350 cm−1, which is most probably related to the presence of secondary NH moieties.19 Characterization of the Nanoparticle Dispersion. The obtained dispersion is central to any material obtained from its processing and requires thorough characterization. The particles in solution were investigated by means of TEM, AFM, AUC, zeta-potential measurements, SAXS, and DLS. All experiments described in this section are based (if not stated otherwise) on dispersions prepared at cPMF ≈ 20 g·L−1, cSiO2 ≈ 15 g·L−1, and a concentration of phosphoric acid of cacid ≈ 0.07 M within the final dispersion (corresponding to 0.5 mL of H3PO4 (85%) used for the initial sol).10 Initial experiments were conducted using transmission electron microscopy (TEM). For comparative reasons, TEM images of the aged silica nanoparticle dispersion are displayed together with the PMF−silica hybrid dispersion and the PMF dispersion obtained in the absence of silica particles in Figure 2. The silica nanoparticles can be clearly identified, and it is obvious that they are surrounded/encapsulated within a soft PMF matrix (Figure 2a and 2b). TEM imaging of the pure PMF dispersion does not give evidence of well-defined and isolated PMF nanoparticles. Finally, the impact of a reduction of the MF content within the dispersion was analyzed. TEM images obtained at reduced MF content do still show strongly interacting and mixed SiO2/PMF aggregates. However, the average size of the aggregates seems smaller (see Supporting Information for additional micrographs). In summary, TEM can prove that the silica nanoparticles “survive” the synthesis protocol, although they seem to have grown a little bit (average diameter estimated from TEM ≈ 14.5 nm). On the other hand,
Figure 2. Transmission electron micrographs of the PMF/silica dispersion (a and b) and of the dispersions obtained either in the absence of silica (c) or in the absence of MF (d).
TEM cannot give a good insight into the formed PMF colloids, as those seem to aggregate and fuse upon drying of the dispersion. Further microscopic characterization of the dispersion was conducted using AFM, mostly after dilution of the dispersions by a factor 100−150. AFM imaging of the diluted hybrid dispersion gives strong evidence for the coexistence of silica particles and PMF colloids. PMF colloids can be identified as flattened particles of a height of typically less than 10 nm, which allows distinction from the SiO2 colloids that have a larger height (see Supporting Information). Additionally, AFM imaging of the pure PMF dispersion can resolve the individual PMF colloids after spin coating from diluted dispersion. The colloids have sizes in the range of a few tens of nanometers at the maximum. The flat structure indicates that the initial particles have some flexibility left and are not yet fully hardened. Additional AFM analysis was conducted for comparative reasons. Spin coating of the dispersion without dilution on the MICA grid results in complete coating, consisting of several layers, giving evidence about potential fields of application. Therefore, the dispersion shows a good coatability on negatively charged surfaces. These good coatability properties seem to origin from the PMF particles. Pure silica particles from the Ludox HS40 dispersion were also spin coated on the negatively charged grid but show clearly repellent character to the grid. This is expected as the Ludox HS40 dispersion has a pH of 9, where the particles are negatively charged, resulting in electrostatic repulsion. The surface charge of the MICA substrate was reversed by coating with positively charged poly(ethylene imine) (PEI) to study the aggregation behavior of the particles in further detail. The PMF−silica dispersion forms larger aggregates that are isolated from each other on the positively charged substrate, compared to the negatively charged MICA. D
DOI: 10.1021/acs.langmuir.5b00990 Langmuir XXXX, XXX, XXX−XXX
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Figure 3. (a) Distribution of sedimentation coefficients as obtained from analysis of diluted dispersions by AUC. (b) Distribution of apparent hydrodynamic radii as obtained from analysis of the dispersions by DLS (scattering angle 90°, unweighed distribution). (c) SAXS patterns of the aqueous dispersions (gray line indicates Porod power law: I(s) ≈ s−4 as a guide to the eye). Color code: blue, nonpolymerized MF oligomer; black, Ludox HS-40 dispersion; magenta, hybrid dispersion; green, pure MF dispersion.
(noncured but acidified MF oligomer), probably due to the fact that the density difference of the oligomers to the solvent mixture is too low and back-diffusion is too high. Dynamic light scattering experiments (at pH = 3) support the presence of slightly multimodal distributions (Figure 3). The bimodal distribution is obvious in the case of the pure MF dispersion, where two peaks are observed in the size distribution. DLS analysis usually favors the detection of large particles that have larger scattering power (intensity I scales with the radius R as follows I ≈ R6). Hence, it can be assumed that the larger particles (probably related to a few aggregates) detected for the pure PMF dispersion are present in a small number only, while the main fraction has a hydrodynamic radii of ∼8 nm. This would also explain the fact that AUC can detect only one species. Additionally, such size regime would be in accordance with the AFM analysis of the pure PMF dispersion. The hybrid dispersion does also show a bimodal distribution with peaks at dH ≈ 12 nm and a broad and much larger peak at dH ≈ 100 nm. The peak related to the smaller particles is of low intensity, which is again not to be confused with the total amount of this fraction. The scattering behavior of the starting compounds, namely, the HS-40 silica dispersion and the MF oligomer, were also analyzed for comparative reason. The silica particles give a single signal in the expected size range of 12−14 nm diameter, while the DLS analysis of the MF oligomer dispersion (nonacidified) gives a signal at sizes of ∼100−200 nm. This can be interpreted as droplets of the liquid−liquid phase-separated oligomer, which only becomes fully water soluble after acidification and release of the methyl ether groups (see experimental details). No evidence of smaller objects based on MF oligomer were found by DLS in accordance with results obtained by AUC and SAXS. Generally, the DLS results support the presence of small colloidal objects (isolated silica and/or PMF particles) in coexistence with the presumed larger aggregates composed of primary particles. Furthermore, the interaction/attraction of the negatively charged SiO2 and partly covalent cross-linked PMF particles forming larger aggregates is expected to be electrostatic. Those results are again in agreement with AUC and microscopic analysis. Further support for the multimodality and presence of colloidal objects comes from preliminary SAXS investigation of the dispersion (see Figure 3). The scattering pattern of the
Thus far, microscopy indicates that the dispersion is made of silica nanoparticles coexisting with PMF colloids. There seems to be an attractive interaction between the two species upon drying as indicated by the well-mixed appearance in TEM analysis but also in AFM analysis. It is however not clear whether the silica particles are also coated by MF oligomers within the dispersion state. Further analysis was conducted using scattering, ultracentrifugation, and electrokinetic measurements to directly analyze the material in the dispersion state without any drying steps involved. The zeta-potential−pH curve of the dispersed particles was measured in a range of 2.5 < pH < 9. The zeta potential of the hybrid dispersion was about ζ = +29 mV at pH = 2.5, indicating cationic stabilization of the particles under synthesis conditions. However, the dispersion of the pure PMF had a slightly higher zeta potential of about ζ = +39 mV at comparable pH values. Pure silica particles had a much lower zeta potential between ζ = 0 and −10 mV at a pH of 2.5. In a first approximation, the measured zeta potential of a multicomponent mixture could be described as a sum of all charges, which would support our data. However, it is known that the exact behavior of the zeta potential of mixtures is not trivial, and a number of possible regimes can be imagined.26 We do not wish to speculate too much at this stage but can conclude that the PMF/silica hybrid dispersion is cationically stabilized and has an isoelectric point within the range of pH ≈ 7−9. Analytical ultracentrifugation (AUC) of the dispersion was used to study the sedimentation speed of the particles (Figure 3). Determination of particle sizes was not easily possible due to the partly complex solvent mixtures and unknown particle densities. However, analysis of the sedimentation speed gives (together with the DLS analysis) valuable insights into the dispersion characteristics. The hybrid dispersion shows a multimodal distribution of sedimentation coefficients with main peaks at 30 and 150 S, the latter peak accompanied by a broad shoulder at 300 S. Comparison to the sedimentation coefficients obtained for the pure PMF dispersion (s = 20 S) and the pure silica nanoparticles (s = 160 S) indicates that the hybrid dispersion is indeed made up of coexisting silica and PMF colloids. The broad shoulder could be related to partly agglomerated SiO2/PMF particles of larger size, which are of course expected to sediment faster. Finally, no sedimentation coefficient could be measured for the dispersed starting material E
DOI: 10.1021/acs.langmuir.5b00990 Langmuir XXXX, XXX, XXX−XXX
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Langmuir hybrid dispersion is dominated by the form factor of the silica spheres at high scattering vector s, while at low s some forward scattering is clearly observed which can be attributed to the larger aggregates. SAXS investigation of the dispersion prepared in the absence of silica (pure PMF) gives evidence of polydisperse colloidal objects with some weak forward scattering due to the larger aggregates. SAXS patterns of the starting materials (concentrated HS40 silica dispersion and pure MF oligomer) are displayed for comparison. The pure SiO2 particles offer a pattern well known from the literature,27 showing a correlation peak together with the classic form factor undulations of spherical particles, while the SAXS pattern of the pure MF oligomer indicates that no small colloidal objects of sizes < 100 nm are present in the oligomer solution/mixture. Impact of Reaction Conditions on the Dispersion Characteristics. The impact of the reaction conditions on the obtained dispersion and its characteristics was studied in some detail to gain a more general understanding on the formation mechanism and the limitations of the approach. The following points describe briefly our findings before a summary is given. (i) Lowering the MF concentration at fixed acid and water content by a factor of 5 resulted also in a stable bluishwhite dispersion. TEM imaging of the dispersion indicated again the presence of silica/PMF aggregates (see Supporting Information). The PMF bridges between the silica particles were however significantly smaller compared to the aggregates monitored at higher MF content, and pearl-necklace structures coexist with more compact aggregates. (ii) It was not possible to obtain a translucent dispersion if no PMF is present at all, i.e., if the silica particles are hydrothermally treated (aged) in the presence of ethanol and phosphoric acid alone. The resulting colloidal solution was transparent. Analysis of the size of the resulting silica particles by DLS indicated that the SiO2 particles expanded slightly to a size of 20 nm (see also TEM image Figure 2d)). Furthermore, it was not possible to precipitate those particles by addition of ethanol (which is in contrast to MF-containing dispersions, see below). (iii) Increasing or reducing the water content of the reaction mixture and resulting dispersion in a range from 25 to 200 mL (corresponding to PMF concentration ranging from ∼85 to 10 g·L−1) did not seem to have significant influence on the resulting dispersion. DLS and AFM measurements indicated for all samples a bi- or multimodal distribution with size ranges very much comparable to the standard dispersion. (iv) On the contrary, the change of the SiO2 concentration at fixed water and PMF content had a significant impact on the properties of the resulting dispersions. There was a clear trend that increasing SiO2 concentrations (from 8 to 30 g·L−1) resulted in the formation of larger aggregates (diameter range 50−250 nm) based on DLS investigations. (v) Likewise, the acid content has some influence on the properties of the dispersion. No stable dispersion was formed at very low phosphoric acid content (cacid = 0.005 M; please note cacid is calculated based on the volume of the final dispersion). Instead, large particles that sedimented easily were formed. Those were very much comparable (also with regard to their porosity, see
Supporting Information) to the particles obtained using oxalic acid at cacid ≈ 0.02 M in a previous study in terms of porosity, particle size, and network formation. Increasing the acid concentration to ∼0.03 M led to the formation of a dispersion with particles having average diameters of ∼250 nm. If the acid content was further increased to levels of 0.06 M and higher, smaller aggregates (as described above) are observed with sizes of dH ≈ 80−100 nm. Interestingly, exchanging the used acid from phosphoric acid to hydrochloric acid gave comparable results. No stable dispersion but larger particles that were prone to sedimentation could be obtained at low HCl (or H2SO4) concentrations of 0.005 M. A stable dispersion was obtained at higher hydrochloric acid concentrations of 0.06 M. However, high concentrations of H2SO4 did not result in stable dispersions but in large particles that sediment easily.
Figure 4. Size distributions of hybrid dispersions (DLS, scattering angle 90°, apparent hydrodynamic diameter) at varying phosphoric acid (left) or silica (right) content.
At this stage, the presented and discussed results about the supposed formation and stabilization mechanism of the dispersions are summarized. (a) The addition of the precursor mixture to the water phase led to a liquid−liquid phase separation, which is indicated by the turbidity of the mixture straight after mixing and the DLS results obtained for a dispersion of the MF oligomer in water. (b) The initial turbidity is lost after ∼10 min, and a transparent mixture was formed (c) Heating to 80 °C for 20 h led to the formation of a white-bluish dispersion, made up of silica/PMF particles. The mixture was stable against precipitation even after cooling and storage for some months (at room temperature). On the basis of our results, we postulate that the growing of the colloidal PMF particles starts straight after heating to 80 °C. The growing PMF colloids were electrostatically stabilized at pH ≈ 3, i.e., at the conditions used for the synthesis, if phosphoric acid or hydrochloric acid at high concentration did catalyze the reaction. At this pH, the silica particles should bear only a very slight negative charge, which would not be sufficient for long-term stability, and it can be expected that the PMF colloids act as protective colloids. Intermolecular interactions such as hydrogen bonding between the PMF particle surface, the phosphoric acid, and the SiO2 surface hydroxyl groups might lead to further stabilization. On the contrary, a lower pH F
DOI: 10.1021/acs.langmuir.5b00990 Langmuir XXXX, XXX, XXX−XXX
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Scheme 2. Summary of the Proposed Reaction Mechanism (left) and Xerogel Formation by Electrostatic Destabilization and Compaction of the Obtained Dispersions (right)
It should be noted at this stage that the obtained stable dispersion can also be processed into porous materials using other routines (such as freeze drying, plain evaporation, coating etc.). Materials obtained by these ways are however subject for further investigations. Characterization of the Xerogels. The highly crosslinked structure of the PMF material renders it usually to a thermostable and solvent-resistant material. Hence, the obtained xerogels were first analyzed with regard to their chemical identity and thermal stability before their porosity and nanostructure will be analyzed in a subsequent section. IR analysis of the final materials (obtained after processing to xerogels and silica etching, see below) gives also a clear indication that PMF resin has been formed successfully. The spectrum (see Figure 1) shows the archetypical bands of PMF resins (triazine ring bending at 810 cm−1, −NH−bend and/or CN ring vibration at ∼1550 cm−1, methylene bending at ∼1460 and 1330 cm−1, methylene stretch at 2950 cm−1) along with a broad band at ∼3350 cm−1, which is most probably related to the presence of secondary NH moieties.19 The obtained FTIR spectra of the final gave evidence for the successful formation of PMF (see Supporting Information). Further analysis was done using elemental analysis (EA, see Supporting Information). The main interest was spent on the C/N content, which can give information about the predominant linkages between the triazine units. Values of C/N ≈ 0.85 were found for both xerogels, which were obtained either from the hybrid dispersion or from the pure MF dispersion. Such values were found previously19 and can be attributed to PMF materials which possess secondary amine moieties and are predominantly linked by methylene groups. Those results are very well in agreement with the interpretation of the FTIR data. Further analysis was conducted using thermogravimetric analysis (TGA, ESI). The hybrid xerogel shows a residue of about 69 wt % at 1000 °C, which can be related to the template (silica particles). Interestingly, the expected silica content at full conversion of the MF oligomers would be lower (∼45 wt %). Hence, it can be speculated that the formation and compaction of the xerogel comes along with some loss of PMF colloids to
is obtained by the addition of sulfuric acid at high concentration (pH ≈ 1.5−2). In this case, the silica is at the isoelectric point and electrostatic stabilization and attraction are minimized. Hence, the silica particles agglomerate and sediment. Sedimentation occurred also by using a low acid concentration (resulting in higher pH values of the dispersion) as the obtained particles were larger in size. Whether the potential ion-exchange and sulfate adsorption ability of PMF also play a role here (strong uptake and high selectivity of sulfate, unpublished results) cannot be clarified at this stage but might also be an important part of the puzzle. Additional proof for the electrostatic stabilization came from the fact that dialysis of the acidic, stable dispersions to a neutral pH did result in destabilization and coagulation. Scheme 2 summarizes the process schematically. Preparation of Xerogels. As stated previously, the obtained hybrid dispersion is/was very stable, and no sedimentation was observed within several months. The addition of a hydrophilic organic solvent (ethanol) to the hybrid dispersion resulted however in the immediate formation of a turbid gel-like material. It can be expected that the changed solvent composition led to a loss of the electrostatic stabilization as a consequence of the changed solvent polarity and dielectric constant (resulting in compression of the electrical double layer). It was also possible to destabilize the pure PMF dispersion by addition of ethanol, providing further evidence for the stabilization mechanism discussed previously. However, this effect does depend strongly on the used acid. We could only recognize this effect with phosphoric acid. The formed gel was compacted by centrifugation. The excess solvent was removed, and the coagulated particles were washed with ethanol various times. (Please note: The precipitation process with ethanol is reversible. By adding water to the PMF precipitate, the particles could be easily redispersed in water.) The white hybrid material was dried and finally cured at 90 °C in the oven. Within the drying process, the compacted block underwent further shrinking to a single monolithic block (xerogel). The silica could be etched by washing with aqueous NaOH (1 M),10 resulting in coarse PMF xerogel pieces, which were subject to further analysis after purification. G
DOI: 10.1021/acs.langmuir.5b00990 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
shows agglomerated spherical particles that seem to be entrapped or connected by what is believed to be the polymeric matrix. The particles had predominant diameters of roughly 20 nm, a size that was also observed for pure silica particles that were aged without the presence of PMF. Whether there is some additional PMF coating on the surface of the particles cannot be deduced from the SEM images. Etching of the silica particles results in a material of less defined appearance. The porous PMF materials seem to be composed of irregularly shaped particles, which appear to have a broader size distribution covering sizes from 10 to 50 nm. This finding is in accordance with SAXS investigations of the etched and dried PMF material. Those give also evidence of a material made up from irregularly shaped colloidal particles (see Supporting Information). Further analysis of the porosity of the xerogels was made by cryogenic N2-adsorption/desorption experiments. The resulting N2-isotherms are given in Figure 6. Both the hybrid as well as the polymer show hysteresis loops as commonly observed for mesoporous materials.29 Specific surface areas were determined by the BET approach, and values of 232 and 421 m2 g−1 were calculated for the hybrid xerogel and the PMF xerogel, respectively. The pore size distribution (PSD, Figure 6) was calculated on the basis of quenched-solid density functional theory (QSDFT) using the adsorption branch of the isotherm.30 The hybrid material had a pore volume of 0.26 cm3 g−1, which could be increased to 0.68 cm3 g−1 after etching of the silica. Comparing the obtained PSDs, a new fraction of pores with pore radii of ∼8−9 nm at preservation of the other pore fractions was observed. The diameter of the pores created by the etching process (d ≈ 16−18 nm) corresponds well with the sizes of the silica particles that were observed by HRSEM, indicating that the etching process was successful. However, a rough calculation of the expected pore volume under the assumption that all silica particles have been transformed to pores results in an estimated pore volume of 1.3 cm3 g−1 after etching. Hence, it can be expected that some pores collapsed upon silica removal as the supporting framework was probably not strong enough. For comparative reasons, a xerogel was also prepared from the pure PMF dispersion, i.e., without the addition of any template. As shown above, the pure PMF dispersion consists
the liquid phase, resulting in an increased SiO2 content within the gel. The decomposition of the PMF xerogels was observed to happen within two steps, in agreement with previous reports.10,19,28 A first weight loss was additionally observed within the range from 50 to 150 °C, which may be caused by the desorption of the water from the surface and the pores of the materials. In the first decomposition step, a rapid weight loss is observed at about 350−400 °C, which is usually associated with a first breakdown of the network that goes along with the release of ammonia, HCN, and melamine along with the formation of some condensed products (melam type).28 A second, broader decomposition step sets in between 600 and 800 °C, resulting in almost full decomposition of the material. Residual masses of
