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Self-assembly of polyhedral oligomeric silsesquioxane structures through ion exchange
Materials Science & Engineering B 243 (2019) 38–46
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Self-assembly of polyhedral oligomeric silsesquioxane structures through ion exchange
T
Caroline Luvisona, Cesar H. Wankea, Sidnei Mourab, Giovanna Manchadoc, María C.M. Fariasa, ⁎ Otávio Bianchia, a
Postgraduate Program in Materials Science and Engineering (PGMAT), Universidade de Caxias do Sul (UCS), Caxias do Sul, Brazil Laboratory of Natural and Synthetics Products, Universidade de Ca xias do Sul, Caxias do Sul, Brazil c Centro de Tecnologias Estratégicas do Nordeste (CETENE), Recife, Brazil b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silsesquioxane Ion exchange Macroions Self-assembly
In this study, we synthesized 3-chloroammoniumpropyl polyhedral oligomeric silsesquioxanes (CAP-POSS) by acid hydrolysis and condensation. Partial ion exchange resulted in the formation of supramolecular structures through electrostatic interactions. The interactions between POSS macroions were led by multiple hydrogen bonds that form supramolecular structures. After ion exchange, the structures of the CAP-POSS reduce the symmetry by the exchange of counterions and associate to form rigid transparent films at room temperature. The amount of chloride modulated the size and structure of macroions aggregates in the supramolecular films. Regardless of the amount of chloride removed, the films were formed of mass-fractal like structures with sizes ranging from 80 to 28 nm. The surface energy was proportional to the free amine content, and UV–vis was a light transmission characteristic. Therefore, the use of partial ion exchange represents an alternative to design of supramolecular POSS structures with properties tunable through multiple hydrogen bonds.
1. Introduction Polyhedral oligomeric silsesquioxanes (POSS) have a synergistic combination of organic and inorganic materials features. POSS are molecules approximately 1─3 nm in size, composed of an inorganic silica-like core (Si8O12) surrounded by organic groups. POSS have a range of applications, such as in coatings, optical devices, dielectric materials, liquid crystal displays, biomedical applications, drug and gene delivery systems and polymer nanocomposites, among others [1,2]. Octafunctional cubic cages, (RSiO3/2)8, called T8, are the most studied class, where eight substituent groups (R) are attached to each of the silicon atoms at the cage corners. Normally, R is pendant groups, such as phenyl, cyclopentyl, isobutyl, methyl or others [3–6]. The solubility of these compounds in water or organic solvents is directly related with these pendant groups [5]. In the same line, POSS can be found in the liquid [7] or solid [1] state at room temperature, which is a function of the type of substituent groups. The ability of POSS to create supramolecular structures through aggregation or compound crystallization has been highlighted [8–11]. This process occurs through intermolecular interactions such as hydrogen bonding, dipole-dipole and van der Waals, which are substituent-dependent. Self-assembly capability is a key advantage of POSS ⁎
nanostructured materials along with their chemical versatility [12]. The self-assembly process of the structures is induced by electrostatic interactions, with interaction strengths ranging widely from a few to several tens of kilojoules per mole [13]. Kaneko and coworkers produced a series of hybrid structures from the acid hydrolysis condensation of aminopropyltrimethoxysilane. They found that cage-like oligosilsesquioxane (POSS) and rod-like (ladderlike) polysilsesquioxanes were selectively and quantitatively prepared by changing the type of acid catalyst employed [14–16]. The conversion degree and POSS structure are dependents of the acid pKa and reaction time [17]. When POSS is prepared using HCl in open system (distillation) between 60─70 °C for 2 h, the formation of lamellar structures is observed. However, when the reaction is made using superacids such as CF3SO3H the formation of cubic structures is reported [18]. The formation of cubic structures also depends on the relative speed of hydrolysis and condensation [19]. It is observed that when the formation reactions of the POSS are conducted at higher temperatures (∼100 °C) in HCl there is no tendency to form cube structures (T8) [15–17]. The self-assembly of the POSS can occur in solution [20] or bulk [9], and melt crystallization normally occurs in several steps. The growth mechanism of POSS phase transformation in the bulk changes from
Corresponding author. E-mail address: [email protected] (O. Bianchi).
https://doi.org/10.1016/j.mseb.2019.03.019 Received 26 March 2018; Received in revised form 12 February 2019; Accepted 22 March 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental
three-dimensional to two-dimensional like axialite, with an activation energy of ∼170 kJ/mol. The symmetrical 3D dimensions of POSS structures results in a low under cooling degree ΔTc0 , which is related to the thermodynamic motive force for the nucleation process [9]. In solution, species with different charges tend to cluster forming structures such as blackberry [20]. Shih and coworkers [13] have succeeded in the formation of supramolecular structures by hydrogen interactions between octakis (dimethylsiloxy) silsesquioxane and tetrakis (nicotinoxymethyl)methane. The POSS self-association feature allows for obtaining numerous types of structures such as gels and films [11,21]. Wu and Kuo [21] observed similar behavior in the study of the self-assembly process of heteronucleobase-multifunctionalized POSS through multiple complementary hydrogen bonding. They also obtained transparent films from blends of octakis ((vinylbenzyltriazolyl) adenine-siloxy methyl) silsesquioxane with octakis (vinylbenzylthymine-siloxy) silsesquioxane. The transparent films formed by POSS blends had a single glass transition, indicating possible miscibility. The specific interactions between chemical groups are responsible for miscibility behavior in many systems, such as polymer blends [22]. Octa(γ-chloroammoniumpropyl) polyhedral oligomeric silsesquioxanes have also been used for self-assembled structures. These cages with functional amino groups show good solubility in water and are usually utilized as the building blocks of molecular assembly, drug controlled release materials and DNA delivery [23–26]. Liu and co-authors [27] obtained a lamellar microstructure by an ion-exchange reaction in an aqueous system using octa(γ-chloroammoniumpropyl) polyhedral oligomeric silsesquioxanes (OCAPPOSS) modified by dodecyl benzene sulfonic acid sodium salt, DBSS (anionic surfactant). The resulting SEM and TEM images and SAXS data suggest that the hybrid has a two-dimensional multilayered framework. The amount of DBSS organic chains attached to the cage has an impact on the spacing between the layers. Macroions can form blackberry-type structures that can be accurately and reversibly tuned by simply changing the solvent content [28]. POSS structures have the capacity to form blackberry-type structures (∼100 nm) from POSS with positive and negative charges (NH3+ and O−) stabilized with the counterions chlorine and tetramethylammonium [20]. Supramolecular interactions of these structures may be an alternative for obtaining nanostructures via nanochannel reactive chemistry, as obtained by block copolymers [10]. Due to its small size, ionic POSS can be an alternative to the stabilization of various biological macromolecules [29]. Thus, cationic POSS (e.g. ammonium-POSS) has received considerable attention because it can form a complex with anionic molecules with potential applications in DNA/gene delivery, drug delivery and DNA/protein detection [24,30–32]. Therefore, an understanding of the phenomenon of selfassembly is of fundamental importance for many future applications. In this work, we investigated the self-assembly of POSS structures as a result of partial ion exchange of 3-chloroammoniumpropyl POSS (CAP-POSS). These self-assembled supramolecular POSS structures were characterized using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and high resolution mass spectrometry (HRMS). Additionally, the supramolecular film formed by the POSS network was characterized by scanning and transmission electron microscopy (SEM and TEM), wide-angle X-ray diffraction (WAXD), synchrotron small-angle X-ray scattering (SAXS), contact angle and UV–vis measurements. Because CAP-POSS has ammonium groups around the core portion and Cl− anions as counterions, an anion-exchange property can be expected. Thus, it is possible to obtain POSS tunable structures by partial ion exchange and establishing a relationship between ion exchange conditions and design of supramolecular structures.
2.1. Materials 3-aminopropyltrimethoxysilane (H2N(CH2)3Si(OCH3)3) 98.9%, CAS 13822-56-5, was purchased from Wandachem (China). Solvents such as methanol ACS (MeOH), ethanol ACS (EtOH), hydrochloric acid (37.5%) and amberlite resin IRA-420 were purchased from Dinâmica Química Contemporânea Ltda (Brazil). All reagents and solvents were used without further purification. 2.1.1. Synthesis of 3-chloroammoniumpropyl POSS (CAP-POSS) The synthesis of CAP-POSS was adapted from Feher and Wyndham [33]. Briefly, 3-aminopropyltrimethoxysilane (15.41 g, 0.086 mol) and HCl (27 mL) were diluted in EtOH (360 mL) and the solution was mechanically stirred for 21 days at room temperature. The product (∼50 wt%) was a microcrystalline solid powder, which was separated by decantation and washed with MeOH (3 × 20 mL). 2.1.2. Preparation of the transparent POSS films The self-assembly POSS structures were prepared by the ionic exchange using anionic resin (amberlite IRA 420, CAS 63181-94-2) in solution. 1 g of CAP-POSS was added in a vial containing 10 g of anionic resin and homogenized with 20 mL of methanol at 25 °C. The effect of the exchange time (0.0, 0.5, 2.0, 12.0 and 48.0 h) was evaluated and monitored using titration (Mohr method) and FTIR analysis. The transparent films were obtained by solution casting (solvent evaporation), and were dried in a vacuum oven at 50 °C for 48 h. 2.1.3. Chemical characterization Fourier transform infrared spectroscopy (FTIR) measurements were performed using a Spectrum 400 (PerkinElmer) infrared apparatus in transmission mode. The samples (before and after ionic exchange) were dried for 4 h at 120 °C and mixed with potassium bromide (KBr), maintaining a ratio (w/w) of 1%. The measurements consisted of 32 scans with a resolution of 2 cm−1 in the range of 4000 cm−1 and 450 cm−1. For the mass analysis, a POSS solution (0.0001 g) was prepared in 10 mL of acetonitrile (Tedia®, USA) and ultrapure water (grade MilliQ®) 1:1 (v/v) with the addition of 0.1 wt% formic acid (positive mode). This solution was infused directly into the APCI source using a syringe pump (Harvard Apparatus) (180 μL·min−1). The APCI(+)-MS and tandem APCI(+)-MS/MS were acquired using a hybrid high-resolution and high accuracy (5 μL·L−1) microTOF (Q-TOF) mass spectrometer (Bruker® Scientific) under the following conditions: capillary and cone voltages were set to +3500 V and +40 V, respectively, with a de-solvation temperature of 100 °C. For APCI(+)-MS/MS, the energy for the collision-induced dissociations (CID) was 40 eV. For data acquisition and processing, QTOF-control data analysis software (Bruker® Scientific) was used. The data were collected in the m/z range of 70–2000 at a speed of two scans per second, providing a resolution of 50,000 (FWHM) at m/z 200. No important ions were observed below m/z 100 or above m/z 1400, therefore the spectrum is showed on this range. Solid state 29Si NMR spectra were obtained on an Agilent Technologies model 500/54 Annual Refill spectrometer. A frequency of 500 Hz was used, with a relaxation time of 1 s, and 12,000 scans (11.7 T, rotor 4 mm (CP/MAS)). Proton nuclear magnetic resonance spectra (1H NMR) were obtained at 300 MHz on Bruker Fourier 300 MHz spectrometer at room temperature. Spectra were recorded in D2O solutions (5 mg of compound in 0.5 mL) using 5 mm quartz tubes. For acquisition, the pulse sequence ZG30 was used with 1 s of relaxation time (rt). The chemical shifts are reported in ppm, referenced to residual H2O (4.70 ppm). Coupling constants (J) are reported in Hertz. The program TopSpin (Bruker Biospin®) was used for data collection and processing: spectral width 39
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Fig. 1. Analysis by HRMS in Q-TOF with APCI ionization in positive mode; in A] full mode and expansion in B] from m/z 550 until 1280.
3. Results and discussion
4.799 Hz, number of points 16.384.
3.1. Chemical characterization 2.1.4. Thermal, morphological, structural and surface characterizations The thermal analysis of the CAP-POSS and samples after ion exchange was performed on a Shimadzu TGA-50 thermogravimetric analyzer operated at a scan rate of 10 °C·min−1 (nitrogen atmosphere) from 50 to 800 °C. The sample mass used in all procedures ∼10 mg. The crystalline structure of CAP-POSS at different ionic exchange times was characterized by means of wide angle X-ray diffraction (WAXD) in a Shimadzu XRD-6000 diffractometer using Cu Kα radiation (λ = 1.5405 Å). The angle of incidence θ was fixed at 6°, 2θ ranging of 3° to 40° with steps of 0.05°. The WAXD experiments were performed using POSS powders. TEM images were taken using a FEI Tecnai G2 Spirit TWIN microscope operated at an accelerating voltage of 100 kV. POSS/isopropyl alcohol (0.1 mg·mL−1) suspensions were deposited onto a holey carbon grid (300 mesh). The excess solution was quickly wicked away with a piece of filter paper and the sample was dried. Transverse sections were taken and the topological structure of the films was assessed using Shimadzu SSX-550 and MIRA3 Tescan microscopes. All samples were sputter-coated with gold before imaging. Static contact angle measurements were performed at room temperature using distilled water and hexadecane in a Krüss model DSA30. In this work, the sessile drop method was used, wherein the drop volume was 2.0 μL. The value of the contact angle was calculated by using Drop Shape AnalysisTM software and the surface free energy was estimated by the Owens-Wendt method [34]. The structural parameters of the hybrid material films were evaluated by SAXS experiments (samples were 3 mm in diameter and 0.5 mm thick) using the SAXS2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS). The X-rays were monitored with a photomultiplier and detected on a marCCD 165 (8 × 8 binning). Sampledetector distances of 500 and 2000 mm were used. The wavelength of the incident X-ray beam (λ) was 0.155 nm and generated scattering wave vectors (q) from 0.08 to 5.2 nm−1. A silver behenate (AgBeh) standard was used to calibrate the diffracting angle. The transmittance of light of the self-assembled POSS films was evaluated using a Thermo Scientific Evolution 60 UV–vis spectroscope with a wavelength range of 200–900 nm.
CAP-POSS was synthesized by acid hydrolytic condensation of (3aminopropyl) trimethoxysilane in an EtOH medium (HCl) and had a yield of ∼50 wt%. Feher and Wyndham [33] obtained a yield of ∼30 wt% for H2N(CH2)3Si(OCH2CH3)3 in MeOH, while Gravel et al. [35] obtained in yield about 40 wt%. We obtained a higher yield due to the displacement of the reaction equilibrium toward the hydrolysis products. Thus, the hydrolysis and condensation step occurred more rapidly, increasing the yield [19]. The product was indicated by the FTIR (Fig. S1) σ Si-O-Si band at 1100 cm−1, which is commonly found with POSS cage structures [36,37]. The bands at 2960 cm−1 and 2830 cm−1 are assigned to axial δ C-H and H-C-H, respectively. σ and δ N-H (NH3+) were also observed at 3030 cm−1, 2000 cm−1, 1600 cm−1, 1500 cm−1 and 800 cm−1 (data shown in the Supplementary files). Free σ OH groups were observed in the region of ∼3440 cm−1. At 650 cm−1 [38], a more intense band of -NH is observed as a function of the exchange time. After ion exchange, peaks were observed at 0.5 ppm (SiCH2), 1.4 ppm (CH2NH2), 1.6 ppm (SiCH2CH2), 2.8 ppm (CH2NH) and 5.8 ppm (CH2NH2), related to aminopropyl (show in Supplementary files), as reported by Feher and Wyndham [33]. Due to the electronegativity of chlorine, the signals observed in CAP-POSS at 0.7 ppm (SiCH2), 1.7 ppm (SiCH2CH2) and 2.9 ppm (CH2NH) were shifted when compared to POSS after ion exchange (Fig. S2). The Δδ change was related to the association generated between the POSS-POSS particles by partial ion exchange, resulting in multiple hydrogen bond interactions. Thus, the greater the value of Δδ compared to CAP-POSS, the higher the interaction force. Similar behaviors have been observed in previous studies [8,21]. In the same line, the 29Si NMR of CAP-POSS showed a 29Si signal at ∼68 ppm, which is related to the formation of fully condensed T3 structures (shown in Fig. S3 in the Supplementary files). This result is in agreement with the literature [18,35,36,39]. HRMS has been used as a powerful tool for the identification of complex compounds and mixtures by determination of exact mass, isotopic ratio and fragmentation pathway [40,41]. Experimentally, we have tested different sources of ionization such as electrospray (ESI) and atmospheric pressure chemical ionization (APCI), in both positive and negative modes. In this line, in accordance with the POSS chemical functions, APCI in positive mode was the most suitable method for the 40
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Fig. 2. Analysis by HRMS in Q-TOF with APCI ionization in positive mode; in A] expansion from m/z 870 until 890 with information about exact mass and isotopic ratio; and in B] MS-MS mode for ion m/z 881.
specific interactions due to the difference in charge between -OH and -NH3+ groups, which resulted in the aggregation of the POSS particles. The weight loss curves (Supplementary files, Fig. S5) showed similar behavior for all samples and two thermal events were observed. The maximum decomposition temperatures were obtained from the mass loss derivative (DTG) curve. The values of the weight loss and residual weight are summarized in Table 2. The first weight loss in the TG curves corresponds to the elimination of chloride ions and amine radicals, while the second weight loss relates to the degradation of another portion of the radicals linked to the POSS cage, according to stoichiometric calculations for T8 structures. The samples had a constant residual mass (dark appearance) at temperatures above 700 °C, which is attributed to the fraction of the cage and remnants of the remaining organic groups [45,46]. The residual weight amount differences between the samples are related to the initial chloride quantity. For CAP-POSS, the sum of the residual mass (49.1 wt%) and the amount of chloride ions determined by titration (24.7 wt%) results in 73.8 wt%. Applying this same procedure to the other samples, we obtained a value of ∼70 wt%. Therefore, the residual weight difference between the samples at 700 °C is due to the amount of chlorine that was removed by partial ion exchange. The absence of weight loss events below 300 °C, due to the high SiO2 content present in these materials, suggests excellent thermal stability. This finding has motivated studies in which POSS-NH2 is used to improve the thermal stability of materials [20,35,47,48]. Fig. 3 shows the FTIR spectra of the region of hydrogen bond interactions and stretches of the OH and NH groups for the samples after ion exchange. The presence of broad absorption bands can be observed in the range of 3000–3600 cm−1. In CAP-POSS, as described above, the broad band at
confirmation of the molecular ions, as shown in Fig. 1. Thus, we identified a molecular ion with m/z 881.2907 relative to T8 structures, m/z 1119.3680 relative to T10 and non-target molecules such as 11 and 9 aminopropyl silane adducts with H2O addition and/or OH losses. The starting products 3-aminopropyl) trimethoxysilane were also visible. A chemical structure confirmation can be performed using the exact mass and the fit of the isotopic ratio, as shown in Fig. 2 and Table 1. The results for the molecular formula C24H65N8O12Si8+, with a mass error of 4.2 ppm and fit of 3.2 msig, are in agreement with what was expected for Q-TOF/APCI (error ≥ 10 ppm) [41–43]. Furthermore, the molecule fragmentation (MS-MS mode) showed amine silane and the loss of NH2 in series, which is in agreement with previous reports [33].
3.2. Effect of ion exchange on the self-association of POSS structures Ionic exchange resulted in the appearance of bands characteristic of NH2 vibrations at 1400 cm−1 and 650 cm−1 [44]. The NH3+ (1600 cm−1) band was observed in all samples after ion exchange [15,38]. The results show that ion exchange occurred partially (shown in the Supplementary files). CAP-POSS has a theoretical value of 24.20 wt% of chloride ions in its composition. The experimental value found by titration was 24.70 ± 0.90 wt%. After ion exchange, the number of chloride ions was reduced proportionally with the exchange time to 8.40 wt% ( ± 0.40) for 0.5 h of exchange time, 7.66 wt% ( ± 0.46) for 2 h, 4.66 wt% ( ± 0.27) for 12 h and 3.87 wt% ( ± 0.43) for 48 h. The anion exchange resulted in partial exchange of chlorine counterions by hydroxyl groups: R″OH + Cl− ↔ R″Cl + OH−, where R″ represents the anion exchange resin. The anions of CAP-POSS were partially replaced by OH− ions. Thus, partial ion exchange led to Table 1 Main ions identified by HRMS in APCI(+). Entry 1 2 3 4 5 6
Ion +
[M + 3Si(OH)2(CH2)3NH2 + H] [M + 2Si(OH)2(CH2)3NH2 + H]+ [M + Si(OH)2(CH2)3NH2 + H]+ [M + 2 H2O + H]+ [M + H2O + H]+ [M + H]+
Elementary
Experimental m/z
Theorical m/z
Erro (ppm)
Isotopic fit
C33H94N11O18Si11 C30H83N10O16Si10 C27H74N9O14Si9 C27H74N9O14Si9 C24H67N8O13Si8 C24H65N8O12Si8
1240.4104 1119.3680 1000.3319 917.2710 899.3004 881.2907
1240.4230 1119.3676 1000.3273 917.3087 899.2982 881.2876
9.5 0.5 4.5 – 2.3 4.2
– 1.1 3.2 – 8.1 3.3
41
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physical crosslinking due to interactions between open and closed cages or through the formation of hydroxide ions resulting from the reaction of water with the free amine (multiple hydrogen bonding, according to Fig. 3) and the remaining chlorine. The films formed from the POSS-CAP, with different ion exchange times, showed differences in their characteristics, which could be observed in the SEM images and EDX results (see Supplementary files, Fig. S6). EDX analysis of the surface of the films indicated the presence of carbon, nitrogen and silicon, i.e. the elements present in CAP-POSS, and gold, from the coating of the samples via sputtering. In the films formed after 0.5 h and 2 h, chlorine was also detected, indicating that these small particles were CAP- POSS, remaining after the ion exchange process. Fig. 6 shows the WAXD diffractograms of the samples (POSS-CAP) before and after ion exchange (2 h and 12 h). For the POSS-CAP sample, peaks at q = 9, 15, 18 nm−1 and a broadening peak at 31 nm−1 were observed. After the exchange process, broad peaks at q = 9, 15 and 30 nm−1 were observed. The peak at q = 9 nm−1 was observed in T8 POSS structures [9,20]. According to Waddon and Coughlin [49], POSS structures can be considered as spheres that pack hexagonally. The size of the substituent groups results in a difference in the lattice parameters. Thus, the apparent center-to-center distance of the POSS T8 structures after ion exchange is little changed, remaining at ∼0.448 nm, according to Braggs Law. In a study done by Luvison et al. [50], liquid POSS samples presented similar results, where two extended peaks were observed and attributed, one at the distance between the centers of the hybrid structures, i.e. the diameters, and the other at the average distance between the organic groups attached to the polyhedron. This result is similar to that found through computer simulations for T8 POSS structures [51]. After the removal of chloride ions, the solid particles of POSS selfassociated to form supramolecular structures [9]. This was evidenced by the broadening of the peaks for the CAP-POSS samples, as these structures had low symmetry and long-range order reduction. The formation of POSS structures with symmetry reduction seems to be the reason for the formation of transparent films [18,52]. After partial ionic exchange occurs the formation the non-organized POSS structures
Table 2 Weight loss and residual weight determined by TG. Sample
1ª Weight loss (250–430 °C) (%)/Tp#
2ª Weight loss (430–670 °C) (%)/Tp#
* Residual weight (wt.%)
CAP-POSS 2h 12 h
37.2/351 18.5/347 12.5/386
13.17/496 18.3/486 18.9/500
49.1 62.2 65.8
* At 700 °C. # Maximum decomposition temperature (°C).
3440 cm−1 corresponds to the stretching of OH. After ion exchange, two unresolved bands can be seen in this region: one at 3350 cm−1, which corresponds to the hydrogen bonds of OH (OH⋯OH, self-association) groups and another at 3130 cm−1. These results show that the appearance of these bands is directly related to the formation of supramolecular structures by multiple hydrogen bonding interactions. These results are in agreement with the literatures [13,15,16,38]. POSS nanoparticles can self-associate through electrostatic attractions, forming blackberry-like structures [20,49]. Therefore, CAP-POSS, through electrostatic interactions, forms agglomerates with irregular characteristics (Fig. 4a). Nonetheless, after ionic exchange, particles form with a diameter of approximately 100 nm. The blackberry structures that arise through electrostatic interactions between polyhedrons can be self-grouped to form transparent films, as shown in Fig. 4. Supramolecular hybrid films, deposited on glass slides by a coating technique, were fragile at room temperature, breaking into pieces under the application of an external force; they were also water soluble and optically transparent (Fig. 5a). Fig. 5b shows the SEM cross-section of the film deposited onto a glass slide. For a more detailed evaluation of the morphology of the films, due to the nanometric size of the POSS particles, FEG-SEM (field emission gun scanning electronic microscopy) was used. Fig. 5c shows the surface micrograph of a CAP-POSS film after 12 h of ion exchange, obtained at a magnification of 1,000,000×. It was observed that the film presented a homogeneous surface morphology, formed by granular structures, about 10 nm in size. The formation of the film, as mentioned above, can be described as resulting from supramolecular interactions, which form a kind of electrostatic
Fig. 3. Extended FTIR spectrum for the 3700–2500 cm−1 and 900–500 cm−1 regions of the CAP-POSS after different ion exchange times (0.5, 2, 12 and 48 h). 42
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where D denotes the fractal dimension, r is the radius of the particles, q is the scattering vector and ξ is a cut-off distance or characteristic size [57,58]. In the form factor, P (q), A (constant independent of scattering of primary particles) is related to the scattered intensity, I(q), according to Eq. (3):
P (q) =
because the ionic rays of the counterions are different (Cl− > OH− [53]). Fig. 7 shows the small angle X-ray scattering curves of the samples before and after ion exchange. The curves of I(q) vs. q were displaced by an arbitrary factor for better visualization and comparison between the curves of each condition. For the q range of 0.08–1 nm−1, exponential decay was observed for all curves, which is typical of systems with fractal structures [51]. In this q range, the system is described by Porod’s law [54,55]. Subsequently, a peak was observed at about 4.4 nm−1. As in the X-ray diffraction results, if a relation is applied to Bragg’s law (d = 2π / qmax ), it is possible to estimate the average size of the structures. Since POSS has a certain sphericity, in the case of T8 type structures, this size is the mean diameter of the primary particle that forms the aggregate. Thus, it was concluded that, regardless of the exchange time, the particle sizes remained practically the same, i.e. around 1.4 nm in diameter. The total scattering of the sample (I (q)) is composed of a portion associated with one form factor (P(q)) and a structure factor (S(q)), given by [56]: (1)
Teixeira [56] developed a relation for the structure factor for structures that present as scattered fractals (Eq. (2)):
S (q) = 1 +
D 1 Γ(D − 1) sen (D − 1) tan−1 (qξ ) (qr )2 (1 + 1/ q2ξ 2) D2− 1
(3)
The experimental data were fitted using a non-linear regression method with the Levenberg-Marquardt algorithm [59]. Table 3 shows the parameters of the fit of the SAXS curves according to the application of Eq. (1). All curves showed of determination coefficient (R2) above 0.99. For all fits, a particle was considered with an average diameter of 1.4 nm, as found in the SAXS results for the Bragg region. Regarding the parameters found, the values of the constant A (Eq. (3)) were between 4.3 × 10−5 and 3.8 × 10−4 and were associated with only one constant. In general, this constant depends on the number of irradiated particles [60]. The fractal dimension found for this system was 3.0, which corresponds to a mass-fractal whose objects have a self-similarity regime in a short distance domain, which depends on the correlation size, ξ. In this type of structure, the particles are associated so as to form small defined branches and 3D lattices [61,62]. In sol-gel reactions, in general, the formed silica particles agglomerate to form mass fractals. This has been observed in several systems, such as nanocomposites of polymers [63] and silica [57,58]. The values of the correlation size decreased with the ion exchange time. That is, as charge variation occurs, the aggregated particles approach each other by virtue of the groups that can form electrostatic interactions (as shown in the FTIR and NMR results). In addition to the role of counterion attraction between POSS particles, macroion behavior depends on the nature of the ionic domains. The ionic group character determines the ionic association-driven process, as observed by Hawker et al. [64], who studied hydrogel formation by ABA triblock copolyelectrolytes functionalized with ionic groups. Polyelectrolyte solutions with opposite charges were mixed for coacervate cross-linking and the formation of a network structure. They found that mixing solutions that bore weaker ionic groups (i.e., ammonium and carboxylate ions) resulted in a viscous fluid. The results revealed the effect of polymer ionic strength on ionic association-driven crosslinking processes. In our study, partial ion exchange resulted in the formation of rigid films, due to the nature of multiple hydrogen bonding. Based on the data presented so far it is possible to infer that correlation distance (ξ) between the particles changed due to the multiple hydrogen bonds formed after ionic exchange. This displacement was due to the partial removal of chloride from the POSS particles [16,17]. Consequently, ionic exchange resulted in the formation of self-assembled POSS films, whose structural characteristics are directly related to the electrostatic charge difference in the CAP-POSS structure. The contact angle analyses were performed for CAP-POSS after 2 and 12 h of ion exchange. The films showed a high degree of hydrophilicity (see Table 4). For the sample after 12 h of ion exchange, greater hydrophilicity was when compared to samples taken after 2 h. In this case, the material properties were associated with the free amine content. The surface tension results for the self-associated POSS films (Owens-Wendt equation [34]) showed a higher surface tension after 12 h than after 2 h. The polarity of the sample is directly related to the surface tension [65]. Therefore, the higher the number of free amines, the more polar the material will be, thus presenting a higher surface tension. However, if a decrease in the contact angle favors the wettability of the film by water and n-hexadecane, then the particles with more ionic exchange time are wetted by the acid, favoring reactions between them. The UV-vis spectra of the film showed two different behaviors regarding high transmittance in the ultraviolet (UV) and visible regions
Fig. 4. TEM micrography of the (a) POSS-CAP and (b) CAP-POSS after 12 h of ion exchange.
I (q) = S (q). P (q)
A (1 + q2r 2)2
(2) 43
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Fig. 5. (a) Photograph of the film where its transparency is evident. (b) SEM cross section of (a). (c) FEG-SEM micrograph of the CAP-POSS film surface after 12 h of ion exchange.
Fig. 6. Diffractograms of CAP-POSS and CAP-POSS after 2 and 12 h of ion exchange.
(Fig. 8). It can be observed that the films after 2 h and 12 h of ionic exchange presented absorption in the UV region and were partially transparent to visible light. As mentioned above, these films were formed of aggregates of POSS (blackberry structures with a diameter of ∼100 nm) and were structures with low symmetry and long-range order reduction (Fig. 6). The structures formed by these aggregates act as a light scattering centers and can absorb light in a specific wavelength range [66,67], which explains the absorption in the UV region and the partial transparency in the visible region. Thus, at lower particle correlation distances the higher the transmittance due to the more significant amount of multiple hydrogen bonds promoted by the partial ion exchange.
Fig. 7. SAXS profiles for the samples with different ionic exchange times. The solid line represents the fit of Eq. (1).
which was characterized by NMR, HRMS, IR, WAXD, SAXS, SEM, TEM, TG and UV-vis analyses. The partial removal of chloride ions resulted in the formation of POSS structures with different charges and counterion sizes. Due to the in charge change in POSS, the particles form aggregates (blackberry structures capable of forming rigid films) approach each other and interact, forming partially transparent self-assembly films. The films formed by partial ion exchange were associated with multiple hydrogen bonds, which resulted in structures with less spatial regularity due to de difference in counterion sizes (according to XRD). Thus, the removed chlorides modulated the amount of charge and therefore the correlation size of the aggregate particles in the film. The surface energy was proportional to the free amine content, as well as
4. Conclusions In this work, we investigated the self-assembly of POSS structures by partial ion exchange of 3-chloroammoniumpropyl POSS (CAP-POSS), 44
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Table 3 Parameters of the fit of the SAXS curves according to the application of Eq. (1). Sample
A
Fractal dimension (D)
Correlation size ξ (nm)
Radius of the primary particle γ (nm)
R2
CAP-POSS 0.5 h 2h 12 h 48 h
3.8 × 10−4 1.5 × 10−5 3.5 × 10−5 4.3 × 10−5 1.1 × 10−5
3.0 3.0 3.0 3.0 3.0
80.0 37.4 32.0 30.0 28.0
0.7 0.7 0.7 0.7 0.7
0.9996 0.9997 0.9998 0.9998 0.9998
Table 4 Contact angle and surface tension for POSS films. Sample
2h 12 h
[2] [3] [4]
Contact angle (°)
Surface tension (mN/m)
Water
n-Hexadecane
Polar
Dispersive
Total
73 ± 4 63 ± 2
27 ± 2 35 ± 2
24.7 22.8
6.1 16.2
30.8 39.0
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
[20] [21] [22] [23] [24] [25] [26]
Fig. 8. UV-Vis spectra of films after ionic exchange.
[27]
the UV-vis light transmission characteristics. The current work expands our exploration of macroionic behavior initiated by the partial ion exchange. This approach may contribute to the design of new nanostructures such as nanochannels or the stabilization of compounds via electrostatic mechanisms.
[28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
Acknowledgements The authors thank CAPES and FAPERGS for scholarships to Caroline Luvison and Cesar H. Wanke. CNPq – National Council for Scientific and Technological Development, Brazil for financial support (Grant 407546/2018-9). The authors also thank the Brazilian Synchrotron Light Laboratory (LNLS) by the use of its facilities (SAXS2 beamline) and Greice Helen Suzin for help in synthesis experiments.
[38] [39] [40] [41]
Appendix A. Supplementary data
[42]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.mseb.2019.03.019.
[43] [44]
References
[45] [46]
[1] K. Pielichowski, J. Njuguna, B. Janowski, J. Pielichowski, Polyhedral Oligomeric Silsesquioxanes (POSS)-Containing Nanohybrid Polymers, Supramolecular
[47]
45
Polymers Polymeric Betains Oligomers, Springer-Verlag, Berlin, Berlin, 2006, pp. 225–296. H. Zhou, Q. Ye, J. Xu, Mater. Chem. Front. 1 (2017) 212–230. D. Gnanasekaran, K. Madhavan, B.S.R. Reddy, J. Sci. Ind. Res. 68 (2009) 437–464. G.Z. Li, L.C. Wang, H.L. Ni, C.U. Pittman, J. Inorg. Organomet. Polym. 11 (2001) 123–154. J.D.N. Martins, R.V.B.D. Oliveira, Sci. Ind. 1 (2013) 1–5. O. Bianchi, L.G. Barbosa, G. Machado, L.B. Canto, R.S. Mauler, R.V.B. Oliveira, J. Appl. Polym. Sci. 128 (2013) 811–827. S.H. Phillips, T.S. Haddad, S.J. Tomczak, Curr. Opin. Solid State Mater. Sci. 8 (2004) 21–29. J.-H. Wang, O. Altukhov, C.-C. Cheng, F.-C. Chang, S.-W. Kuo, Soft Matter 9 (2013) 5196–5206. O. Bianchi, J.N. Martins, C. Luvison, S.G. Echeverrigaray, C. Dal Castel, R.V.B. Oliveira, J. Non-Cryst. Solids 394–395 (2014) 29–35. J. Rao, H. Ma, J. Baettig, S. Woo, M.C. Stuparu, J. Bang, A. Khan, Soft Matter 10 (2014) 5755–5762. E. Carbonell, L.A. Bivona, L. Fusaro, C. Aprile, Inorg. Chem. 56 (2017) 6393–6403. M. Sánchez-Soto, D.A. Schiraldi, S. Illescas, Eur. Polym. J. 45 (2009) 341–352. R.-S. Shih, C.-H. Lu, S.-W. Kuo, F.-C. Chang, J. Phys. Chem. C 114 (2010) 12855–12862. K. Imai, Y. Kaneko, Inorg. Chem. 56 (2017) 4133–4140. Y. Kaneko, N. Iyi, T. Matsumoto, K. Fujii, K. Kurashima, T. Fujita, J. Mater. Chem. 13 (2003) 2058–2060. Y. Kaneko, N. Iyi, K. Kurashima, T. Matsumoto, T. Fujita, K. Kitamura, Chem. Mater. 16 (2004) 3417–3423. Y. Kaneko, H. Toyodome, M. Shoiriki, N. Iyi, Int. J. Polym. Sci. 2012 (2012) 14. T. Tokunaga, M. Shoiriki, T. Mizumo, Y. Kaneko, J. Mater. Chem. C 2 (2014) 2496–2501. C.J. Brinker, G.W. Scherer, CHAPTER 3 – hydrolysis and condensation II: silicates, in: C.J. Brinker, G.W. Scherer (Eds.), Sol-Gel Science, Academic Press, San Diego, 1990, pp. 96–233. J. Zhou, P. Yin, L. Hu, F. Haso, T. Liu, Eur. J. Inorg. Chem. 2014 (2014) 4593–4599. Y.-C. Wu, S.-W. Kuo, J. Mater. Chem. 22 (2012) 2982–2991. M.M. Coleman, P.C. Painter, J.F. Graf, Specific Interactions and the Miscibility of Polymer Blends, Taylor & Francis, 1995. T. Cassagneau, F. Caruso, J. Am. Chem. Soc. 124 (2002) 8172–8180. C. McCusker, J.B. Carroll, V.M. Rotello, Chem. Commun. (2005) 996–998. T.L. Kaneshiro, X. Wang, Z.-R. Lu, Mol. Pharm. 4 (2007) 759–768. R.Y. Kannan, H.J. Salacinski, J. De Groot, I. Clatworthy, L. Bozec, M. Horton, P.E. Butler, A.M. Seifalian, Biomacromolecules 7 (2005) 215–223. L. Liu, Y. Hu, L. Song, H. Chen, S. Nazare, T.R. Hull, Mater. Lett. 61 (2007) 1077–1081. J.M. Pigga, J.A. Teprovich, R.A. Flowers, M.R. Antonio, T. Liu, Langmuir 26 (2010) 9449–9456. A. Proust, R. Thouvenot, P. Gouzerh, Chem. Commun. (2008) 1837–1852. Q.-C. Zou, Q.-J. Yan, G.-W. Song, S.-L. Zhang, L.-M. Wu, Biosens. Bioelectron. 22 (2007) 1461–1465. L. Cui, L. Zhu, Langmuir 22 (2006) 5982–5985. L. Cui, D. Chen, L. Zhu, ACS Nano 2 (2008) 921–927. F.J. Feher, K.D. Wyndham, Chem. Commun. (1998) 323–324. D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741–1747. M.-C. Gravel, C. Zhang, M. Dinderman, R.M. Laine, 1999. H. Liu, W. Zhang, S. Zheng, Polymer 46 (2005) 157–165. C. Ni, G. Ni, S. Zhang, X. Liu, M. Chen, L. Liu, Colloid Polym. Sci. 288 (2010) 469–477. Y. Kaneko, N. Iyi, T. Matsumoto, K. Kitamura, Polymer 46 (2005) 1828–1833. D.R. do Carmo, L.L. Paim, D.R. Silvestrini, A.C. de Sa, U.D.O. Bicalho, N.R. Stradiotto, Int. J. Electrochem. Sci. 6 (2011) 1175. W.E. Wallace, C.M. Guttman, J.M. Antonucci, J. Am. Soc. Mass Spectrom. 10 (1999) 224–230. S.E. Anderson, A. Somogyi, T.S. Haddad, E.B. Coughlin, G. Gadodia, D.F. Marten, J. Ray, M.T. Bowers, Int. J. Mass Spectrom. 292 (2010) 38–47. A.M. Knolhoff, J.H. Callahan, T.R. Croley, J. Am. Soc. Mass Spectrom. 25 (2014) 1285–1294. A.W.T. Bristow, K.S. Webb, J. Am. Soc. Mass Spectrom. 14 (2003) 1086–1098. L.K. Neudachina, N.V. Lakiza, A.V. Pestov, V.A. Osipova, L.V. Adamova, E.M. Gorbunova, Glass Phys. Chem. 37 (2011) 537–544. A. Fina, D. Tabuani, F. Carniato, A. Frache, E. Boccaleri, G. Camino, Thermochim. Acta 440 (2006) 36–42. O. Bianchi, G.B. Repenning, L.B. Canto, R.S. Mauler, R.V.B. Oliveira, Polym. Test. 32 (2013) 794–801. T. Seçkin, S. Köytepe, H.İ. Adıgüzel, Mater. Chem. Phys. 112 (2008) 1040–1046.
Materials Science & Engineering B 243 (2019) 38–46
C. Luvison, et al.
[60] R. Besselink, T.M. Stawski, H.L. Castricum, J.E. ten Elshof, J. Colloid Interface Sci. 404 (2013) 24–35. [61] D.W. Marquardt, J. Soc. Ind. Appl. Math. 11 (1963) 431–441. [62] K. Levenberg, Q. Appl. Math. 2 (1944) 164–168. [63] D.R. Vollet, D.A. Donatti, A. Ibañez Ruiz, J. Non-Cryst. Solids 288 (2001) 81–87. [64] S. Mihara, R.N. Datta, W.K. Dierkes, J.W.M. Noordermeer, N. Amino, Y. Ishikawa, S. Nishitsuji, M. Takenaka, Rubber Chem. Technol. 87 (2014) 348–359. [65] D. Jiang, L. Liu, J. Long, L. Xing, Y. Huang, Z. Wu, X. Yan, Z. Guo, Compos. Sci. Technol. 100 (2014) 158–165. [66] C.F. Bohren, D.R. Huffman, Particles Small Compared with the Wavelength, Absorption and Scattering of Light by Small Particles, Wiley-VCH Verlag GmbH, 2007, pp. 130–157. [67] C.F. Bohren, D.R. Huffman, Absorption and Scattering by a Sphere Absorption and Scattering of Light by Small Particles, Wiley-VCH Verlag GmbH, 2007, pp. 82–129.
[48] T. Seckin, A. Gültek, S. Köytepe, Turk. J. Chem. 29 (2005) 49–60. [49] A.A. Verhoeff, M.L. Kistler, A. Bhatt, J. Pigga, J. Groenewold, M. Klokkenburg, S. Veen, S. Roy, T. Liu, W.K. Kegel, Phys. Rev. Lett. 99 (2007) 066104. [50] I. Penso, E.A. Cechinatto, G. Machado, C. Luvison, C.H. Wanke, O. Bianchi, M.R.F. Soares, J. Non-Cryst. Solids 428 (2015) 82–89. [51] A.J. Waddon, E.B. Coughlin, Chem. Mater. 15 (2003) 4555–4561. [52] H. Araki, K. Naka, Macromolecules 44 (2011) 6039–6045. [53] Y. Marcus, Chem. Rev. 88 (1988) 1475–1498. [54] C. Luvison, M.C.M. Farias, O. Bianchi, Sci. Ind. 3 (2014) 19–25. [55] A. Striolo, C. McCabe, P.T. Cummings, Macromolecules 38 (2005) 8950–8959. [56] K. Sinkó, V. Torma, A. Kovács, J. Non-Cryst. Solids 354 (2008) 5466–5474. [57] O. Glatter, O. Kratky, Small Angle X-ray Scattering, Academic Press, 1982. [58] L. Feigin, D.I. Svergun, Structure Analysis by Small-Angle Xray and Neutron Scattering, Springer, 1987. [59] J. Teixeira, J. Appl. Crystallogr. 21 (1988) 781–785.
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Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Self-assembly of polyhedral oligomeric silsesquioxane structures through ion exchange
T
Caroline Luvisona, Cesar H. Wankea, Sidnei Mourab, Giovanna Manchadoc, María C.M. Fariasa, ⁎ Otávio Bianchia, a
Postgraduate Program in Materials Science and Engineering (PGMAT), Universidade de Caxias do Sul (UCS), Caxias do Sul, Brazil Laboratory of Natural and Synthetics Products, Universidade de Ca xias do Sul, Caxias do Sul, Brazil c Centro de Tecnologias Estratégicas do Nordeste (CETENE), Recife, Brazil b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silsesquioxane Ion exchange Macroions Self-assembly
In this study, we synthesized 3-chloroammoniumpropyl polyhedral oligomeric silsesquioxanes (CAP-POSS) by acid hydrolysis and condensation. Partial ion exchange resulted in the formation of supramolecular structures through electrostatic interactions. The interactions between POSS macroions were led by multiple hydrogen bonds that form supramolecular structures. After ion exchange, the structures of the CAP-POSS reduce the symmetry by the exchange of counterions and associate to form rigid transparent films at room temperature. The amount of chloride modulated the size and structure of macroions aggregates in the supramolecular films. Regardless of the amount of chloride removed, the films were formed of mass-fractal like structures with sizes ranging from 80 to 28 nm. The surface energy was proportional to the free amine content, and UV–vis was a light transmission characteristic. Therefore, the use of partial ion exchange represents an alternative to design of supramolecular POSS structures with properties tunable through multiple hydrogen bonds.
1. Introduction Polyhedral oligomeric silsesquioxanes (POSS) have a synergistic combination of organic and inorganic materials features. POSS are molecules approximately 1─3 nm in size, composed of an inorganic silica-like core (Si8O12) surrounded by organic groups. POSS have a range of applications, such as in coatings, optical devices, dielectric materials, liquid crystal displays, biomedical applications, drug and gene delivery systems and polymer nanocomposites, among others [1,2]. Octafunctional cubic cages, (RSiO3/2)8, called T8, are the most studied class, where eight substituent groups (R) are attached to each of the silicon atoms at the cage corners. Normally, R is pendant groups, such as phenyl, cyclopentyl, isobutyl, methyl or others [3–6]. The solubility of these compounds in water or organic solvents is directly related with these pendant groups [5]. In the same line, POSS can be found in the liquid [7] or solid [1] state at room temperature, which is a function of the type of substituent groups. The ability of POSS to create supramolecular structures through aggregation or compound crystallization has been highlighted [8–11]. This process occurs through intermolecular interactions such as hydrogen bonding, dipole-dipole and van der Waals, which are substituent-dependent. Self-assembly capability is a key advantage of POSS ⁎
nanostructured materials along with their chemical versatility [12]. The self-assembly process of the structures is induced by electrostatic interactions, with interaction strengths ranging widely from a few to several tens of kilojoules per mole [13]. Kaneko and coworkers produced a series of hybrid structures from the acid hydrolysis condensation of aminopropyltrimethoxysilane. They found that cage-like oligosilsesquioxane (POSS) and rod-like (ladderlike) polysilsesquioxanes were selectively and quantitatively prepared by changing the type of acid catalyst employed [14–16]. The conversion degree and POSS structure are dependents of the acid pKa and reaction time [17]. When POSS is prepared using HCl in open system (distillation) between 60─70 °C for 2 h, the formation of lamellar structures is observed. However, when the reaction is made using superacids such as CF3SO3H the formation of cubic structures is reported [18]. The formation of cubic structures also depends on the relative speed of hydrolysis and condensation [19]. It is observed that when the formation reactions of the POSS are conducted at higher temperatures (∼100 °C) in HCl there is no tendency to form cube structures (T8) [15–17]. The self-assembly of the POSS can occur in solution [20] or bulk [9], and melt crystallization normally occurs in several steps. The growth mechanism of POSS phase transformation in the bulk changes from
Corresponding author. E-mail address: [email protected] (O. Bianchi).
https://doi.org/10.1016/j.mseb.2019.03.019 Received 26 March 2018; Received in revised form 12 February 2019; Accepted 22 March 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental
three-dimensional to two-dimensional like axialite, with an activation energy of ∼170 kJ/mol. The symmetrical 3D dimensions of POSS structures results in a low under cooling degree ΔTc0 , which is related to the thermodynamic motive force for the nucleation process [9]. In solution, species with different charges tend to cluster forming structures such as blackberry [20]. Shih and coworkers [13] have succeeded in the formation of supramolecular structures by hydrogen interactions between octakis (dimethylsiloxy) silsesquioxane and tetrakis (nicotinoxymethyl)methane. The POSS self-association feature allows for obtaining numerous types of structures such as gels and films [11,21]. Wu and Kuo [21] observed similar behavior in the study of the self-assembly process of heteronucleobase-multifunctionalized POSS through multiple complementary hydrogen bonding. They also obtained transparent films from blends of octakis ((vinylbenzyltriazolyl) adenine-siloxy methyl) silsesquioxane with octakis (vinylbenzylthymine-siloxy) silsesquioxane. The transparent films formed by POSS blends had a single glass transition, indicating possible miscibility. The specific interactions between chemical groups are responsible for miscibility behavior in many systems, such as polymer blends [22]. Octa(γ-chloroammoniumpropyl) polyhedral oligomeric silsesquioxanes have also been used for self-assembled structures. These cages with functional amino groups show good solubility in water and are usually utilized as the building blocks of molecular assembly, drug controlled release materials and DNA delivery [23–26]. Liu and co-authors [27] obtained a lamellar microstructure by an ion-exchange reaction in an aqueous system using octa(γ-chloroammoniumpropyl) polyhedral oligomeric silsesquioxanes (OCAPPOSS) modified by dodecyl benzene sulfonic acid sodium salt, DBSS (anionic surfactant). The resulting SEM and TEM images and SAXS data suggest that the hybrid has a two-dimensional multilayered framework. The amount of DBSS organic chains attached to the cage has an impact on the spacing between the layers. Macroions can form blackberry-type structures that can be accurately and reversibly tuned by simply changing the solvent content [28]. POSS structures have the capacity to form blackberry-type structures (∼100 nm) from POSS with positive and negative charges (NH3+ and O−) stabilized with the counterions chlorine and tetramethylammonium [20]. Supramolecular interactions of these structures may be an alternative for obtaining nanostructures via nanochannel reactive chemistry, as obtained by block copolymers [10]. Due to its small size, ionic POSS can be an alternative to the stabilization of various biological macromolecules [29]. Thus, cationic POSS (e.g. ammonium-POSS) has received considerable attention because it can form a complex with anionic molecules with potential applications in DNA/gene delivery, drug delivery and DNA/protein detection [24,30–32]. Therefore, an understanding of the phenomenon of selfassembly is of fundamental importance for many future applications. In this work, we investigated the self-assembly of POSS structures as a result of partial ion exchange of 3-chloroammoniumpropyl POSS (CAP-POSS). These self-assembled supramolecular POSS structures were characterized using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and high resolution mass spectrometry (HRMS). Additionally, the supramolecular film formed by the POSS network was characterized by scanning and transmission electron microscopy (SEM and TEM), wide-angle X-ray diffraction (WAXD), synchrotron small-angle X-ray scattering (SAXS), contact angle and UV–vis measurements. Because CAP-POSS has ammonium groups around the core portion and Cl− anions as counterions, an anion-exchange property can be expected. Thus, it is possible to obtain POSS tunable structures by partial ion exchange and establishing a relationship between ion exchange conditions and design of supramolecular structures.
2.1. Materials 3-aminopropyltrimethoxysilane (H2N(CH2)3Si(OCH3)3) 98.9%, CAS 13822-56-5, was purchased from Wandachem (China). Solvents such as methanol ACS (MeOH), ethanol ACS (EtOH), hydrochloric acid (37.5%) and amberlite resin IRA-420 were purchased from Dinâmica Química Contemporânea Ltda (Brazil). All reagents and solvents were used without further purification. 2.1.1. Synthesis of 3-chloroammoniumpropyl POSS (CAP-POSS) The synthesis of CAP-POSS was adapted from Feher and Wyndham [33]. Briefly, 3-aminopropyltrimethoxysilane (15.41 g, 0.086 mol) and HCl (27 mL) were diluted in EtOH (360 mL) and the solution was mechanically stirred for 21 days at room temperature. The product (∼50 wt%) was a microcrystalline solid powder, which was separated by decantation and washed with MeOH (3 × 20 mL). 2.1.2. Preparation of the transparent POSS films The self-assembly POSS structures were prepared by the ionic exchange using anionic resin (amberlite IRA 420, CAS 63181-94-2) in solution. 1 g of CAP-POSS was added in a vial containing 10 g of anionic resin and homogenized with 20 mL of methanol at 25 °C. The effect of the exchange time (0.0, 0.5, 2.0, 12.0 and 48.0 h) was evaluated and monitored using titration (Mohr method) and FTIR analysis. The transparent films were obtained by solution casting (solvent evaporation), and were dried in a vacuum oven at 50 °C for 48 h. 2.1.3. Chemical characterization Fourier transform infrared spectroscopy (FTIR) measurements were performed using a Spectrum 400 (PerkinElmer) infrared apparatus in transmission mode. The samples (before and after ionic exchange) were dried for 4 h at 120 °C and mixed with potassium bromide (KBr), maintaining a ratio (w/w) of 1%. The measurements consisted of 32 scans with a resolution of 2 cm−1 in the range of 4000 cm−1 and 450 cm−1. For the mass analysis, a POSS solution (0.0001 g) was prepared in 10 mL of acetonitrile (Tedia®, USA) and ultrapure water (grade MilliQ®) 1:1 (v/v) with the addition of 0.1 wt% formic acid (positive mode). This solution was infused directly into the APCI source using a syringe pump (Harvard Apparatus) (180 μL·min−1). The APCI(+)-MS and tandem APCI(+)-MS/MS were acquired using a hybrid high-resolution and high accuracy (5 μL·L−1) microTOF (Q-TOF) mass spectrometer (Bruker® Scientific) under the following conditions: capillary and cone voltages were set to +3500 V and +40 V, respectively, with a de-solvation temperature of 100 °C. For APCI(+)-MS/MS, the energy for the collision-induced dissociations (CID) was 40 eV. For data acquisition and processing, QTOF-control data analysis software (Bruker® Scientific) was used. The data were collected in the m/z range of 70–2000 at a speed of two scans per second, providing a resolution of 50,000 (FWHM) at m/z 200. No important ions were observed below m/z 100 or above m/z 1400, therefore the spectrum is showed on this range. Solid state 29Si NMR spectra were obtained on an Agilent Technologies model 500/54 Annual Refill spectrometer. A frequency of 500 Hz was used, with a relaxation time of 1 s, and 12,000 scans (11.7 T, rotor 4 mm (CP/MAS)). Proton nuclear magnetic resonance spectra (1H NMR) were obtained at 300 MHz on Bruker Fourier 300 MHz spectrometer at room temperature. Spectra were recorded in D2O solutions (5 mg of compound in 0.5 mL) using 5 mm quartz tubes. For acquisition, the pulse sequence ZG30 was used with 1 s of relaxation time (rt). The chemical shifts are reported in ppm, referenced to residual H2O (4.70 ppm). Coupling constants (J) are reported in Hertz. The program TopSpin (Bruker Biospin®) was used for data collection and processing: spectral width 39
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Fig. 1. Analysis by HRMS in Q-TOF with APCI ionization in positive mode; in A] full mode and expansion in B] from m/z 550 until 1280.
3. Results and discussion
4.799 Hz, number of points 16.384.
3.1. Chemical characterization 2.1.4. Thermal, morphological, structural and surface characterizations The thermal analysis of the CAP-POSS and samples after ion exchange was performed on a Shimadzu TGA-50 thermogravimetric analyzer operated at a scan rate of 10 °C·min−1 (nitrogen atmosphere) from 50 to 800 °C. The sample mass used in all procedures ∼10 mg. The crystalline structure of CAP-POSS at different ionic exchange times was characterized by means of wide angle X-ray diffraction (WAXD) in a Shimadzu XRD-6000 diffractometer using Cu Kα radiation (λ = 1.5405 Å). The angle of incidence θ was fixed at 6°, 2θ ranging of 3° to 40° with steps of 0.05°. The WAXD experiments were performed using POSS powders. TEM images were taken using a FEI Tecnai G2 Spirit TWIN microscope operated at an accelerating voltage of 100 kV. POSS/isopropyl alcohol (0.1 mg·mL−1) suspensions were deposited onto a holey carbon grid (300 mesh). The excess solution was quickly wicked away with a piece of filter paper and the sample was dried. Transverse sections were taken and the topological structure of the films was assessed using Shimadzu SSX-550 and MIRA3 Tescan microscopes. All samples were sputter-coated with gold before imaging. Static contact angle measurements were performed at room temperature using distilled water and hexadecane in a Krüss model DSA30. In this work, the sessile drop method was used, wherein the drop volume was 2.0 μL. The value of the contact angle was calculated by using Drop Shape AnalysisTM software and the surface free energy was estimated by the Owens-Wendt method [34]. The structural parameters of the hybrid material films were evaluated by SAXS experiments (samples were 3 mm in diameter and 0.5 mm thick) using the SAXS2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS). The X-rays were monitored with a photomultiplier and detected on a marCCD 165 (8 × 8 binning). Sampledetector distances of 500 and 2000 mm were used. The wavelength of the incident X-ray beam (λ) was 0.155 nm and generated scattering wave vectors (q) from 0.08 to 5.2 nm−1. A silver behenate (AgBeh) standard was used to calibrate the diffracting angle. The transmittance of light of the self-assembled POSS films was evaluated using a Thermo Scientific Evolution 60 UV–vis spectroscope with a wavelength range of 200–900 nm.
CAP-POSS was synthesized by acid hydrolytic condensation of (3aminopropyl) trimethoxysilane in an EtOH medium (HCl) and had a yield of ∼50 wt%. Feher and Wyndham [33] obtained a yield of ∼30 wt% for H2N(CH2)3Si(OCH2CH3)3 in MeOH, while Gravel et al. [35] obtained in yield about 40 wt%. We obtained a higher yield due to the displacement of the reaction equilibrium toward the hydrolysis products. Thus, the hydrolysis and condensation step occurred more rapidly, increasing the yield [19]. The product was indicated by the FTIR (Fig. S1) σ Si-O-Si band at 1100 cm−1, which is commonly found with POSS cage structures [36,37]. The bands at 2960 cm−1 and 2830 cm−1 are assigned to axial δ C-H and H-C-H, respectively. σ and δ N-H (NH3+) were also observed at 3030 cm−1, 2000 cm−1, 1600 cm−1, 1500 cm−1 and 800 cm−1 (data shown in the Supplementary files). Free σ OH groups were observed in the region of ∼3440 cm−1. At 650 cm−1 [38], a more intense band of -NH is observed as a function of the exchange time. After ion exchange, peaks were observed at 0.5 ppm (SiCH2), 1.4 ppm (CH2NH2), 1.6 ppm (SiCH2CH2), 2.8 ppm (CH2NH) and 5.8 ppm (CH2NH2), related to aminopropyl (show in Supplementary files), as reported by Feher and Wyndham [33]. Due to the electronegativity of chlorine, the signals observed in CAP-POSS at 0.7 ppm (SiCH2), 1.7 ppm (SiCH2CH2) and 2.9 ppm (CH2NH) were shifted when compared to POSS after ion exchange (Fig. S2). The Δδ change was related to the association generated between the POSS-POSS particles by partial ion exchange, resulting in multiple hydrogen bond interactions. Thus, the greater the value of Δδ compared to CAP-POSS, the higher the interaction force. Similar behaviors have been observed in previous studies [8,21]. In the same line, the 29Si NMR of CAP-POSS showed a 29Si signal at ∼68 ppm, which is related to the formation of fully condensed T3 structures (shown in Fig. S3 in the Supplementary files). This result is in agreement with the literature [18,35,36,39]. HRMS has been used as a powerful tool for the identification of complex compounds and mixtures by determination of exact mass, isotopic ratio and fragmentation pathway [40,41]. Experimentally, we have tested different sources of ionization such as electrospray (ESI) and atmospheric pressure chemical ionization (APCI), in both positive and negative modes. In this line, in accordance with the POSS chemical functions, APCI in positive mode was the most suitable method for the 40
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Fig. 2. Analysis by HRMS in Q-TOF with APCI ionization in positive mode; in A] expansion from m/z 870 until 890 with information about exact mass and isotopic ratio; and in B] MS-MS mode for ion m/z 881.
specific interactions due to the difference in charge between -OH and -NH3+ groups, which resulted in the aggregation of the POSS particles. The weight loss curves (Supplementary files, Fig. S5) showed similar behavior for all samples and two thermal events were observed. The maximum decomposition temperatures were obtained from the mass loss derivative (DTG) curve. The values of the weight loss and residual weight are summarized in Table 2. The first weight loss in the TG curves corresponds to the elimination of chloride ions and amine radicals, while the second weight loss relates to the degradation of another portion of the radicals linked to the POSS cage, according to stoichiometric calculations for T8 structures. The samples had a constant residual mass (dark appearance) at temperatures above 700 °C, which is attributed to the fraction of the cage and remnants of the remaining organic groups [45,46]. The residual weight amount differences between the samples are related to the initial chloride quantity. For CAP-POSS, the sum of the residual mass (49.1 wt%) and the amount of chloride ions determined by titration (24.7 wt%) results in 73.8 wt%. Applying this same procedure to the other samples, we obtained a value of ∼70 wt%. Therefore, the residual weight difference between the samples at 700 °C is due to the amount of chlorine that was removed by partial ion exchange. The absence of weight loss events below 300 °C, due to the high SiO2 content present in these materials, suggests excellent thermal stability. This finding has motivated studies in which POSS-NH2 is used to improve the thermal stability of materials [20,35,47,48]. Fig. 3 shows the FTIR spectra of the region of hydrogen bond interactions and stretches of the OH and NH groups for the samples after ion exchange. The presence of broad absorption bands can be observed in the range of 3000–3600 cm−1. In CAP-POSS, as described above, the broad band at
confirmation of the molecular ions, as shown in Fig. 1. Thus, we identified a molecular ion with m/z 881.2907 relative to T8 structures, m/z 1119.3680 relative to T10 and non-target molecules such as 11 and 9 aminopropyl silane adducts with H2O addition and/or OH losses. The starting products 3-aminopropyl) trimethoxysilane were also visible. A chemical structure confirmation can be performed using the exact mass and the fit of the isotopic ratio, as shown in Fig. 2 and Table 1. The results for the molecular formula C24H65N8O12Si8+, with a mass error of 4.2 ppm and fit of 3.2 msig, are in agreement with what was expected for Q-TOF/APCI (error ≥ 10 ppm) [41–43]. Furthermore, the molecule fragmentation (MS-MS mode) showed amine silane and the loss of NH2 in series, which is in agreement with previous reports [33].
3.2. Effect of ion exchange on the self-association of POSS structures Ionic exchange resulted in the appearance of bands characteristic of NH2 vibrations at 1400 cm−1 and 650 cm−1 [44]. The NH3+ (1600 cm−1) band was observed in all samples after ion exchange [15,38]. The results show that ion exchange occurred partially (shown in the Supplementary files). CAP-POSS has a theoretical value of 24.20 wt% of chloride ions in its composition. The experimental value found by titration was 24.70 ± 0.90 wt%. After ion exchange, the number of chloride ions was reduced proportionally with the exchange time to 8.40 wt% ( ± 0.40) for 0.5 h of exchange time, 7.66 wt% ( ± 0.46) for 2 h, 4.66 wt% ( ± 0.27) for 12 h and 3.87 wt% ( ± 0.43) for 48 h. The anion exchange resulted in partial exchange of chlorine counterions by hydroxyl groups: R″OH + Cl− ↔ R″Cl + OH−, where R″ represents the anion exchange resin. The anions of CAP-POSS were partially replaced by OH− ions. Thus, partial ion exchange led to Table 1 Main ions identified by HRMS in APCI(+). Entry 1 2 3 4 5 6
Ion +
[M + 3Si(OH)2(CH2)3NH2 + H] [M + 2Si(OH)2(CH2)3NH2 + H]+ [M + Si(OH)2(CH2)3NH2 + H]+ [M + 2 H2O + H]+ [M + H2O + H]+ [M + H]+
Elementary
Experimental m/z
Theorical m/z
Erro (ppm)
Isotopic fit
C33H94N11O18Si11 C30H83N10O16Si10 C27H74N9O14Si9 C27H74N9O14Si9 C24H67N8O13Si8 C24H65N8O12Si8
1240.4104 1119.3680 1000.3319 917.2710 899.3004 881.2907
1240.4230 1119.3676 1000.3273 917.3087 899.2982 881.2876
9.5 0.5 4.5 – 2.3 4.2
– 1.1 3.2 – 8.1 3.3
41
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physical crosslinking due to interactions between open and closed cages or through the formation of hydroxide ions resulting from the reaction of water with the free amine (multiple hydrogen bonding, according to Fig. 3) and the remaining chlorine. The films formed from the POSS-CAP, with different ion exchange times, showed differences in their characteristics, which could be observed in the SEM images and EDX results (see Supplementary files, Fig. S6). EDX analysis of the surface of the films indicated the presence of carbon, nitrogen and silicon, i.e. the elements present in CAP-POSS, and gold, from the coating of the samples via sputtering. In the films formed after 0.5 h and 2 h, chlorine was also detected, indicating that these small particles were CAP- POSS, remaining after the ion exchange process. Fig. 6 shows the WAXD diffractograms of the samples (POSS-CAP) before and after ion exchange (2 h and 12 h). For the POSS-CAP sample, peaks at q = 9, 15, 18 nm−1 and a broadening peak at 31 nm−1 were observed. After the exchange process, broad peaks at q = 9, 15 and 30 nm−1 were observed. The peak at q = 9 nm−1 was observed in T8 POSS structures [9,20]. According to Waddon and Coughlin [49], POSS structures can be considered as spheres that pack hexagonally. The size of the substituent groups results in a difference in the lattice parameters. Thus, the apparent center-to-center distance of the POSS T8 structures after ion exchange is little changed, remaining at ∼0.448 nm, according to Braggs Law. In a study done by Luvison et al. [50], liquid POSS samples presented similar results, where two extended peaks were observed and attributed, one at the distance between the centers of the hybrid structures, i.e. the diameters, and the other at the average distance between the organic groups attached to the polyhedron. This result is similar to that found through computer simulations for T8 POSS structures [51]. After the removal of chloride ions, the solid particles of POSS selfassociated to form supramolecular structures [9]. This was evidenced by the broadening of the peaks for the CAP-POSS samples, as these structures had low symmetry and long-range order reduction. The formation of POSS structures with symmetry reduction seems to be the reason for the formation of transparent films [18,52]. After partial ionic exchange occurs the formation the non-organized POSS structures
Table 2 Weight loss and residual weight determined by TG. Sample
1ª Weight loss (250–430 °C) (%)/Tp#
2ª Weight loss (430–670 °C) (%)/Tp#
* Residual weight (wt.%)
CAP-POSS 2h 12 h
37.2/351 18.5/347 12.5/386
13.17/496 18.3/486 18.9/500
49.1 62.2 65.8
* At 700 °C. # Maximum decomposition temperature (°C).
3440 cm−1 corresponds to the stretching of OH. After ion exchange, two unresolved bands can be seen in this region: one at 3350 cm−1, which corresponds to the hydrogen bonds of OH (OH⋯OH, self-association) groups and another at 3130 cm−1. These results show that the appearance of these bands is directly related to the formation of supramolecular structures by multiple hydrogen bonding interactions. These results are in agreement with the literatures [13,15,16,38]. POSS nanoparticles can self-associate through electrostatic attractions, forming blackberry-like structures [20,49]. Therefore, CAP-POSS, through electrostatic interactions, forms agglomerates with irregular characteristics (Fig. 4a). Nonetheless, after ionic exchange, particles form with a diameter of approximately 100 nm. The blackberry structures that arise through electrostatic interactions between polyhedrons can be self-grouped to form transparent films, as shown in Fig. 4. Supramolecular hybrid films, deposited on glass slides by a coating technique, were fragile at room temperature, breaking into pieces under the application of an external force; they were also water soluble and optically transparent (Fig. 5a). Fig. 5b shows the SEM cross-section of the film deposited onto a glass slide. For a more detailed evaluation of the morphology of the films, due to the nanometric size of the POSS particles, FEG-SEM (field emission gun scanning electronic microscopy) was used. Fig. 5c shows the surface micrograph of a CAP-POSS film after 12 h of ion exchange, obtained at a magnification of 1,000,000×. It was observed that the film presented a homogeneous surface morphology, formed by granular structures, about 10 nm in size. The formation of the film, as mentioned above, can be described as resulting from supramolecular interactions, which form a kind of electrostatic
Fig. 3. Extended FTIR spectrum for the 3700–2500 cm−1 and 900–500 cm−1 regions of the CAP-POSS after different ion exchange times (0.5, 2, 12 and 48 h). 42
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where D denotes the fractal dimension, r is the radius of the particles, q is the scattering vector and ξ is a cut-off distance or characteristic size [57,58]. In the form factor, P (q), A (constant independent of scattering of primary particles) is related to the scattered intensity, I(q), according to Eq. (3):
P (q) =
because the ionic rays of the counterions are different (Cl− > OH− [53]). Fig. 7 shows the small angle X-ray scattering curves of the samples before and after ion exchange. The curves of I(q) vs. q were displaced by an arbitrary factor for better visualization and comparison between the curves of each condition. For the q range of 0.08–1 nm−1, exponential decay was observed for all curves, which is typical of systems with fractal structures [51]. In this q range, the system is described by Porod’s law [54,55]. Subsequently, a peak was observed at about 4.4 nm−1. As in the X-ray diffraction results, if a relation is applied to Bragg’s law (d = 2π / qmax ), it is possible to estimate the average size of the structures. Since POSS has a certain sphericity, in the case of T8 type structures, this size is the mean diameter of the primary particle that forms the aggregate. Thus, it was concluded that, regardless of the exchange time, the particle sizes remained practically the same, i.e. around 1.4 nm in diameter. The total scattering of the sample (I (q)) is composed of a portion associated with one form factor (P(q)) and a structure factor (S(q)), given by [56]: (1)
Teixeira [56] developed a relation for the structure factor for structures that present as scattered fractals (Eq. (2)):
S (q) = 1 +
D 1 Γ(D − 1) sen (D − 1) tan−1 (qξ ) (qr )2 (1 + 1/ q2ξ 2) D2− 1
(3)
The experimental data were fitted using a non-linear regression method with the Levenberg-Marquardt algorithm [59]. Table 3 shows the parameters of the fit of the SAXS curves according to the application of Eq. (1). All curves showed of determination coefficient (R2) above 0.99. For all fits, a particle was considered with an average diameter of 1.4 nm, as found in the SAXS results for the Bragg region. Regarding the parameters found, the values of the constant A (Eq. (3)) were between 4.3 × 10−5 and 3.8 × 10−4 and were associated with only one constant. In general, this constant depends on the number of irradiated particles [60]. The fractal dimension found for this system was 3.0, which corresponds to a mass-fractal whose objects have a self-similarity regime in a short distance domain, which depends on the correlation size, ξ. In this type of structure, the particles are associated so as to form small defined branches and 3D lattices [61,62]. In sol-gel reactions, in general, the formed silica particles agglomerate to form mass fractals. This has been observed in several systems, such as nanocomposites of polymers [63] and silica [57,58]. The values of the correlation size decreased with the ion exchange time. That is, as charge variation occurs, the aggregated particles approach each other by virtue of the groups that can form electrostatic interactions (as shown in the FTIR and NMR results). In addition to the role of counterion attraction between POSS particles, macroion behavior depends on the nature of the ionic domains. The ionic group character determines the ionic association-driven process, as observed by Hawker et al. [64], who studied hydrogel formation by ABA triblock copolyelectrolytes functionalized with ionic groups. Polyelectrolyte solutions with opposite charges were mixed for coacervate cross-linking and the formation of a network structure. They found that mixing solutions that bore weaker ionic groups (i.e., ammonium and carboxylate ions) resulted in a viscous fluid. The results revealed the effect of polymer ionic strength on ionic association-driven crosslinking processes. In our study, partial ion exchange resulted in the formation of rigid films, due to the nature of multiple hydrogen bonding. Based on the data presented so far it is possible to infer that correlation distance (ξ) between the particles changed due to the multiple hydrogen bonds formed after ionic exchange. This displacement was due to the partial removal of chloride from the POSS particles [16,17]. Consequently, ionic exchange resulted in the formation of self-assembled POSS films, whose structural characteristics are directly related to the electrostatic charge difference in the CAP-POSS structure. The contact angle analyses were performed for CAP-POSS after 2 and 12 h of ion exchange. The films showed a high degree of hydrophilicity (see Table 4). For the sample after 12 h of ion exchange, greater hydrophilicity was when compared to samples taken after 2 h. In this case, the material properties were associated with the free amine content. The surface tension results for the self-associated POSS films (Owens-Wendt equation [34]) showed a higher surface tension after 12 h than after 2 h. The polarity of the sample is directly related to the surface tension [65]. Therefore, the higher the number of free amines, the more polar the material will be, thus presenting a higher surface tension. However, if a decrease in the contact angle favors the wettability of the film by water and n-hexadecane, then the particles with more ionic exchange time are wetted by the acid, favoring reactions between them. The UV-vis spectra of the film showed two different behaviors regarding high transmittance in the ultraviolet (UV) and visible regions
Fig. 4. TEM micrography of the (a) POSS-CAP and (b) CAP-POSS after 12 h of ion exchange.
I (q) = S (q). P (q)
A (1 + q2r 2)2
(2) 43
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Fig. 5. (a) Photograph of the film where its transparency is evident. (b) SEM cross section of (a). (c) FEG-SEM micrograph of the CAP-POSS film surface after 12 h of ion exchange.
Fig. 6. Diffractograms of CAP-POSS and CAP-POSS after 2 and 12 h of ion exchange.
(Fig. 8). It can be observed that the films after 2 h and 12 h of ionic exchange presented absorption in the UV region and were partially transparent to visible light. As mentioned above, these films were formed of aggregates of POSS (blackberry structures with a diameter of ∼100 nm) and were structures with low symmetry and long-range order reduction (Fig. 6). The structures formed by these aggregates act as a light scattering centers and can absorb light in a specific wavelength range [66,67], which explains the absorption in the UV region and the partial transparency in the visible region. Thus, at lower particle correlation distances the higher the transmittance due to the more significant amount of multiple hydrogen bonds promoted by the partial ion exchange.
Fig. 7. SAXS profiles for the samples with different ionic exchange times. The solid line represents the fit of Eq. (1).
which was characterized by NMR, HRMS, IR, WAXD, SAXS, SEM, TEM, TG and UV-vis analyses. The partial removal of chloride ions resulted in the formation of POSS structures with different charges and counterion sizes. Due to the in charge change in POSS, the particles form aggregates (blackberry structures capable of forming rigid films) approach each other and interact, forming partially transparent self-assembly films. The films formed by partial ion exchange were associated with multiple hydrogen bonds, which resulted in structures with less spatial regularity due to de difference in counterion sizes (according to XRD). Thus, the removed chlorides modulated the amount of charge and therefore the correlation size of the aggregate particles in the film. The surface energy was proportional to the free amine content, as well as
4. Conclusions In this work, we investigated the self-assembly of POSS structures by partial ion exchange of 3-chloroammoniumpropyl POSS (CAP-POSS), 44
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Table 3 Parameters of the fit of the SAXS curves according to the application of Eq. (1). Sample
A
Fractal dimension (D)
Correlation size ξ (nm)
Radius of the primary particle γ (nm)
R2
CAP-POSS 0.5 h 2h 12 h 48 h
3.8 × 10−4 1.5 × 10−5 3.5 × 10−5 4.3 × 10−5 1.1 × 10−5
3.0 3.0 3.0 3.0 3.0
80.0 37.4 32.0 30.0 28.0
0.7 0.7 0.7 0.7 0.7
0.9996 0.9997 0.9998 0.9998 0.9998
Table 4 Contact angle and surface tension for POSS films. Sample
2h 12 h
[2] [3] [4]
Contact angle (°)
Surface tension (mN/m)
Water
n-Hexadecane
Polar
Dispersive
Total
73 ± 4 63 ± 2
27 ± 2 35 ± 2
24.7 22.8
6.1 16.2
30.8 39.0
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
[20] [21] [22] [23] [24] [25] [26]
Fig. 8. UV-Vis spectra of films after ionic exchange.
[27]
the UV-vis light transmission characteristics. The current work expands our exploration of macroionic behavior initiated by the partial ion exchange. This approach may contribute to the design of new nanostructures such as nanochannels or the stabilization of compounds via electrostatic mechanisms.
[28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
Acknowledgements The authors thank CAPES and FAPERGS for scholarships to Caroline Luvison and Cesar H. Wanke. CNPq – National Council for Scientific and Technological Development, Brazil for financial support (Grant 407546/2018-9). The authors also thank the Brazilian Synchrotron Light Laboratory (LNLS) by the use of its facilities (SAXS2 beamline) and Greice Helen Suzin for help in synthesis experiments.
[38] [39] [40] [41]
Appendix A. Supplementary data
[42]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.mseb.2019.03.019.
[43] [44]
References
[45] [46]
[1] K. Pielichowski, J. Njuguna, B. Janowski, J. Pielichowski, Polyhedral Oligomeric Silsesquioxanes (POSS)-Containing Nanohybrid Polymers, Supramolecular
[47]
45
Polymers Polymeric Betains Oligomers, Springer-Verlag, Berlin, Berlin, 2006, pp. 225–296. H. Zhou, Q. Ye, J. Xu, Mater. Chem. Front. 1 (2017) 212–230. D. Gnanasekaran, K. Madhavan, B.S.R. Reddy, J. Sci. Ind. Res. 68 (2009) 437–464. G.Z. Li, L.C. Wang, H.L. Ni, C.U. Pittman, J. Inorg. Organomet. Polym. 11 (2001) 123–154. J.D.N. Martins, R.V.B.D. Oliveira, Sci. Ind. 1 (2013) 1–5. O. Bianchi, L.G. Barbosa, G. Machado, L.B. Canto, R.S. Mauler, R.V.B. Oliveira, J. Appl. Polym. Sci. 128 (2013) 811–827. S.H. Phillips, T.S. Haddad, S.J. Tomczak, Curr. Opin. Solid State Mater. Sci. 8 (2004) 21–29. J.-H. Wang, O. Altukhov, C.-C. Cheng, F.-C. Chang, S.-W. Kuo, Soft Matter 9 (2013) 5196–5206. O. Bianchi, J.N. Martins, C. Luvison, S.G. Echeverrigaray, C. Dal Castel, R.V.B. Oliveira, J. Non-Cryst. Solids 394–395 (2014) 29–35. J. Rao, H. Ma, J. Baettig, S. Woo, M.C. Stuparu, J. Bang, A. Khan, Soft Matter 10 (2014) 5755–5762. E. Carbonell, L.A. Bivona, L. Fusaro, C. Aprile, Inorg. Chem. 56 (2017) 6393–6403. M. Sánchez-Soto, D.A. Schiraldi, S. Illescas, Eur. Polym. J. 45 (2009) 341–352. R.-S. Shih, C.-H. Lu, S.-W. Kuo, F.-C. Chang, J. Phys. Chem. C 114 (2010) 12855–12862. K. Imai, Y. Kaneko, Inorg. Chem. 56 (2017) 4133–4140. Y. Kaneko, N. Iyi, T. Matsumoto, K. Fujii, K. Kurashima, T. Fujita, J. Mater. Chem. 13 (2003) 2058–2060. Y. Kaneko, N. Iyi, K. Kurashima, T. Matsumoto, T. Fujita, K. Kitamura, Chem. Mater. 16 (2004) 3417–3423. Y. Kaneko, H. Toyodome, M. Shoiriki, N. Iyi, Int. J. Polym. Sci. 2012 (2012) 14. T. Tokunaga, M. Shoiriki, T. Mizumo, Y. Kaneko, J. Mater. Chem. C 2 (2014) 2496–2501. C.J. Brinker, G.W. Scherer, CHAPTER 3 – hydrolysis and condensation II: silicates, in: C.J. Brinker, G.W. Scherer (Eds.), Sol-Gel Science, Academic Press, San Diego, 1990, pp. 96–233. J. Zhou, P. Yin, L. Hu, F. Haso, T. Liu, Eur. J. Inorg. Chem. 2014 (2014) 4593–4599. Y.-C. Wu, S.-W. Kuo, J. Mater. Chem. 22 (2012) 2982–2991. M.M. Coleman, P.C. Painter, J.F. Graf, Specific Interactions and the Miscibility of Polymer Blends, Taylor & Francis, 1995. T. Cassagneau, F. Caruso, J. Am. Chem. Soc. 124 (2002) 8172–8180. C. McCusker, J.B. Carroll, V.M. Rotello, Chem. Commun. (2005) 996–998. T.L. Kaneshiro, X. Wang, Z.-R. Lu, Mol. Pharm. 4 (2007) 759–768. R.Y. Kannan, H.J. Salacinski, J. De Groot, I. Clatworthy, L. Bozec, M. Horton, P.E. Butler, A.M. Seifalian, Biomacromolecules 7 (2005) 215–223. L. Liu, Y. Hu, L. Song, H. Chen, S. Nazare, T.R. Hull, Mater. Lett. 61 (2007) 1077–1081. J.M. Pigga, J.A. Teprovich, R.A. Flowers, M.R. Antonio, T. Liu, Langmuir 26 (2010) 9449–9456. A. Proust, R. Thouvenot, P. Gouzerh, Chem. Commun. (2008) 1837–1852. Q.-C. Zou, Q.-J. Yan, G.-W. Song, S.-L. Zhang, L.-M. Wu, Biosens. Bioelectron. 22 (2007) 1461–1465. L. Cui, L. Zhu, Langmuir 22 (2006) 5982–5985. L. Cui, D. Chen, L. Zhu, ACS Nano 2 (2008) 921–927. F.J. Feher, K.D. Wyndham, Chem. Commun. (1998) 323–324. D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741–1747. M.-C. Gravel, C. Zhang, M. Dinderman, R.M. Laine, 1999. H. Liu, W. Zhang, S. Zheng, Polymer 46 (2005) 157–165. C. Ni, G. Ni, S. Zhang, X. Liu, M. Chen, L. Liu, Colloid Polym. Sci. 288 (2010) 469–477. Y. Kaneko, N. Iyi, T. Matsumoto, K. Kitamura, Polymer 46 (2005) 1828–1833. D.R. do Carmo, L.L. Paim, D.R. Silvestrini, A.C. de Sa, U.D.O. Bicalho, N.R. Stradiotto, Int. J. Electrochem. Sci. 6 (2011) 1175. W.E. Wallace, C.M. Guttman, J.M. Antonucci, J. Am. Soc. Mass Spectrom. 10 (1999) 224–230. S.E. Anderson, A. Somogyi, T.S. Haddad, E.B. Coughlin, G. Gadodia, D.F. Marten, J. Ray, M.T. Bowers, Int. J. Mass Spectrom. 292 (2010) 38–47. A.M. Knolhoff, J.H. Callahan, T.R. Croley, J. Am. Soc. Mass Spectrom. 25 (2014) 1285–1294. A.W.T. Bristow, K.S. Webb, J. Am. Soc. Mass Spectrom. 14 (2003) 1086–1098. L.K. Neudachina, N.V. Lakiza, A.V. Pestov, V.A. Osipova, L.V. Adamova, E.M. Gorbunova, Glass Phys. Chem. 37 (2011) 537–544. A. Fina, D. Tabuani, F. Carniato, A. Frache, E. Boccaleri, G. Camino, Thermochim. Acta 440 (2006) 36–42. O. Bianchi, G.B. Repenning, L.B. Canto, R.S. Mauler, R.V.B. Oliveira, Polym. Test. 32 (2013) 794–801. T. Seçkin, S. Köytepe, H.İ. Adıgüzel, Mater. Chem. Phys. 112 (2008) 1040–1046.
Materials Science & Engineering B 243 (2019) 38–46
C. Luvison, et al.
[60] R. Besselink, T.M. Stawski, H.L. Castricum, J.E. ten Elshof, J. Colloid Interface Sci. 404 (2013) 24–35. [61] D.W. Marquardt, J. Soc. Ind. Appl. Math. 11 (1963) 431–441. [62] K. Levenberg, Q. Appl. Math. 2 (1944) 164–168. [63] D.R. Vollet, D.A. Donatti, A. Ibañez Ruiz, J. Non-Cryst. Solids 288 (2001) 81–87. [64] S. Mihara, R.N. Datta, W.K. Dierkes, J.W.M. Noordermeer, N. Amino, Y. Ishikawa, S. Nishitsuji, M. Takenaka, Rubber Chem. Technol. 87 (2014) 348–359. [65] D. Jiang, L. Liu, J. Long, L. Xing, Y. Huang, Z. Wu, X. Yan, Z. Guo, Compos. Sci. Technol. 100 (2014) 158–165. [66] C.F. Bohren, D.R. Huffman, Particles Small Compared with the Wavelength, Absorption and Scattering of Light by Small Particles, Wiley-VCH Verlag GmbH, 2007, pp. 130–157. [67] C.F. Bohren, D.R. Huffman, Absorption and Scattering by a Sphere Absorption and Scattering of Light by Small Particles, Wiley-VCH Verlag GmbH, 2007, pp. 82–129.
[48] T. Seckin, A. Gültek, S. Köytepe, Turk. J. Chem. 29 (2005) 49–60. [49] A.A. Verhoeff, M.L. Kistler, A. Bhatt, J. Pigga, J. Groenewold, M. Klokkenburg, S. Veen, S. Roy, T. Liu, W.K. Kegel, Phys. Rev. Lett. 99 (2007) 066104. [50] I. Penso, E.A. Cechinatto, G. Machado, C. Luvison, C.H. Wanke, O. Bianchi, M.R.F. Soares, J. Non-Cryst. Solids 428 (2015) 82–89. [51] A.J. Waddon, E.B. Coughlin, Chem. Mater. 15 (2003) 4555–4561. [52] H. Araki, K. Naka, Macromolecules 44 (2011) 6039–6045. [53] Y. Marcus, Chem. Rev. 88 (1988) 1475–1498. [54] C. Luvison, M.C.M. Farias, O. Bianchi, Sci. Ind. 3 (2014) 19–25. [55] A. Striolo, C. McCabe, P.T. Cummings, Macromolecules 38 (2005) 8950–8959. [56] K. Sinkó, V. Torma, A. Kovács, J. Non-Cryst. Solids 354 (2008) 5466–5474. [57] O. Glatter, O. Kratky, Small Angle X-ray Scattering, Academic Press, 1982. [58] L. Feigin, D.I. Svergun, Structure Analysis by Small-Angle Xray and Neutron Scattering, Springer, 1987. [59] J. Teixeira, J. Appl. Crystallogr. 21 (1988) 781–785.
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