Supercritical carbon dioxide-soluble polyhedral oligomeric silsesquioxane (POSS) nanocages and polymer surface modification

J. of Supercritical Fluids 73 (2013) 171–177

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Supercritical carbon dioxide-soluble polyhedral oligomeric silsesquioxane (POSS) nanocages and polymer surface modification Cerag Dilek ∗ Department of Chemical Engineering, Middle East Technical University, Ankara, 06800, Turkey

a r t i c l e

i n f o

Article history: Received 3 August 2012 Received in revised form 27 October 2012 Accepted 27 October 2012 Keywords: Silica cage Nanoparticle Polymer processing Surface modification Cloud point

a b s t r a c t In this work, a functionalized polyhedral oligomeric silsesquioxane (POSS) has been investigated for its solubility in supercritical carbon dioxide for the first time in literature. POSS nanocages, which can be functionalized with a wide variety of organic substituents, are most commonly studied as nanofillers in polymer nanocomposites and coatings. Solubility of trifluoropropyl POSS in supercritical carbon dioxide has been determined by cloud-point measurements performed in a high-pressure view cell. At temperature and pressure ranges of 308–323 K and 8.3–14.8 MPa, these fluorinated organic–inorganic hybrid nanocage structures exhibit solubility up to 4.4% by weight, which is promising for green material processing applications using the environmentally benign solvent. Solubility of CO2 -philic POSS decreases with increasing temperature, while the solubility isotherms at two different temperatures converge. Choosing the processing conditions from the performed solubility studies, trifluoropropyl POSS–supercritical carbon dioxide system has been applied in high-pressure surface modification of a high-molecular weight, rigid poly(methyl methacrylate) (PMMA). Scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis of the processed PMMA sheet show that the functionalized nanoparticles were deposited on the PMMA surface, forming a uniform coating of POSS aggregates. This work proves that functionalized POSS with CO2 -philic groups can be solubilized in supercritical CO2 , which might allow them to be applied in a plethora of materials modification processes using supercritical carbon dioxide. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Functionalized nanoparticles have been used in polymer processing to enhance thermal, mechanical and barrier properties of polymers. Polymeric nanocomposites with functionalized nanoparticles are prepared mostly by melt-phase compounding, solution blending or in situ polymerization. However, these techniques are associated with a number of processing and environmental problems such as limited nanoparticle dispersion, thermal instability of nanoparticles at high temperatures, volatile organic compound (VOC) emissions of solvents, and limited applicability and scalability of processes [1–4]. In order to engineer, design and tailor nanocomposites for high-technology applications, precise morphology control is an essential challenge. While homogeneous and efficient dispersion of nanoparticles is crucial to obtain high-aspect ratio materials such as thin films and fibers, the nanoparticle agglomeration and poor dispersion issues have become major obstacles in commercialization of nanocomposite polymeric fibers and films. To modify polymeric surfaces with

∗ Corresponding author. Tel.: +90 532 543 7933; fax: +90 312 210 2600. E-mail address: [email protected] 0896-8446/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.supflu.2012.10.012

nanostructures, chemical and physical vapor deposition and sol–gel processes have been widely used [5,6]. These techniques, however, can often result in associated environmental concerns due to VOC emissions. While controlling the size and morphology of the particles deposited on the polymeric surface is vital, in solution based techniques due to capillary forces and high-surface tensions, local structural and concentration differences can occur, causing nonuniformity of the nanoparticle shell on the polymeric surface. Non-toxic, non-flammable, inexpensive, and abundant supercritical carbon dioxide (scCO2 ) has been used in materials processing with nanoparticles. scCO2 can plasticize a wide range of polymers including poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonates (PC), and poly(lactic acid) (PLA), by dissolving in the polymer matrix, decreasing their Tg and allowing their processability [7–11]. In recent studies, scCO2 -assisted conventional techniques such as melt extrusion, solvent dissolution or in situ polymerization was used in preparation of polymer nanocomposites reinforced with surface-modified nanoparticles [12–22]. In some of these studies, significant improvement in mechanical properties such as storage modulus and yield stress of the nanocomposites were obtained, which was attributed to better particle dispersion achieved in the polymer matrix with scCO2 treatment, compared to nanocomposites prepared with

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2. Experimental 2.1. Materials

Fig. 1. Polyhedral oligomeric silsesquioxane structure with R functional groups.

conventional techniques [14–16,19]. scCO2 has also been used in order to deposit metal nanoparticles on various substrates including metal oxides, silica, carbon aerogel, or polymers. One common application is to deposit metal nanoparticle precursors with CO2 -philic groups on a substrate, and to form the elemental metal nanoparticle by reduction of the deposited precursor [23]. Metal nanoparticles have also been deposited on surfaces by supercritical CO2 used as a benign antisolvent, which expands the organic solvent-nanoparticle suspensions, causing the nanoparticles to precipitate on the substrates [24,25]. The objective of our studies on nanoparticles is to determine the scCO2 -soluble nanoparticles, and use them in environmentally benign modification of materials with scCO2 . Polyhedral oligomeric silsesquioxanes (POSS) are among common nanofillers used to enhance thermal and mechanical properties, flame retardancy, and oxidation resistance of engineering plastics [26–32]. With (RSiO1.5 )n formula, where n is generally 8, they have cage structures made of silicon and oxygen (Fig. 1). A wide variety of organic functional groups can be attached to the silicones, which give opportunity to design custom-tailored POSS with the desired chemical and physicochemical properties. In this study, these functional groups are chosen as CO2 -philic moieties, which can allow POSS to solubilize in scCO2 . In case they are soluble in this benign solvent, functionalized nanocage structures can be applied in a plethora of green processing of advanced materials with the supercritical fluid. The solubility of a component in scCO2 is attributed to the component’s ability to participate in electrostatic or Lewis-acid base interactions with CO2 [33–37]. Although being a lowdielectric fluid, CO2 can exhibit Lewis acid–base interactions due to its quadrupole moment, which also allows CO2 to exhibit dipole–quadropole or quadropole–quadropole interactions with certain types of functional groups. Fluorinated hydrocarbons are one of the most common CO2 -philic groups incorporated to compounds to increase their solubility in scCO2 significantly. Through high-pressure NMR and theoretical studies, high solubility of most fluorinated compounds was attributed to interactions between the quadrupole moment of CO2 and the dipoles of fluoroalkanes [38]. On the other hand, more recent ab initio calculations and highpressure NMR studies suggest that the fluorine atoms act as weak Lewis base sites, while CO2 acts as a weak Lewis acid in CO2 fluorocarbon interactions [39,40]. As the entropic contribution, incorporation of highly electronegative fluoroalkane side chains with weak van der Waals forces can increase the free volume of the compound, decrease its cohesive energy density and surface tension, enhancing its solubility in scCO2 [41,42]. In this work, the solubility of trifluoropropyl POSS with CO2 -philic fluoroalkanes, in scCO2 has been studied for the first time, and polymer surface modification with the scCO2 -soluble nanomaterials and carbon dioxide has been demonstrated.

Trifluoropropyl POSS, a white-powder solid (Hybridplastics) with a molecular formula of (C3 H4 F3 )n (SiO1.5 )n with n = 8 was used in the solubility experiments without further purification. Carbon dioxide (99.998%) was obtained from Messer Aligaz. Trifluoropropyl POSS was characterized with DSC (Seteram, DSC 131) and Atomic Absorption Spectrophotometer (Shimadzu, AA6300), to detect its melting point, which was found to be 462 ± 3 K, and residual calcium content, which was found to be 600 ± 14 ppm. Highly cross-linked, very rigid, and high impact resistance PMMA sheets were obtained from Goodfellow, and had a number average molecular weight of 600,000 g/mol, with polydispersity of 4.5. Its ambient-pressure glass transition temperature was measured with DSC as 406 K. The morphology and micro-structural features of the PMMA samples were characterized by field emission scanning electron microscope (FE-SEM, FEI Quanta FEG 450) operated at 20 kV, while Si and F dispersion of trifluoropropyl POSS domains were detected by energy dispersive solid state spectroscopy (EDS), semi-quantitative elemental mapping. Polymer surfaces were sputter-coated with gold, and directly imaged in the electron microscope. 2.2. Experimental set-up Both the solubility measurements of trifluoropropyl POSS in scCO2 and polymer surface deposition experiments were performed in a high-pressure, jacketed vessel, with two sapphire windows (Fig. 2). The temperature of the high-pressure vessel was controlled with a water circulating heater (Cole-Parmer Polystat Circulating Bath). A Teledyne ISCO pump (model 260D), was used to charge a measured amount of liquid CO2 into the vessel. The temperature of the pump was also controlled with a water circulating heater (Cole-Parmer Polystat Circulating Bath) to keep the charged CO2 at constant temperature. The temperature of the ISCO pump reservoir was controlled within ±0.2 K, and its pressure was measured to within ±0.05 MPa. A thermocouple along with a meter (Omega Engineering, GTMQSS-062G-6 and DP462) and a pressure transducer along with a strain meter (Omega Engineering, PX41006KGV and DP25B-D-230) were used to measure the temperature and pressure of the contents of the vessel. Temperature and

Fig. 2. Experimental set-up for cloud and bubble point measurements and surface deposition experiments.

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12

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Table 1 The cloud point pressures of trifluoropropyl POSS–CO2 binary system at 308 K and 323 K, and the corresponding carbon dioxide densities at the cloud point temperature and pressures.

10

Trifluoropropyl POSS mol fraction

Pressure (MPa)

CO2 density

308 K 0.0017 0.0017 0.0017 0.0087 0.0245 0.0246 0.0348 0.0440

0.00006 0.00006 0.00006 0.00033 0.00093 0.00093 0.00133 0.00169

8.22 8.26 8.37 8.62 9.50 9.34 9.72 10.73

0.5606 0.5726 0.5970 0.6322 0.6940 0.6860 0.7039 0.7382

323 K 0.0017 0.0038 0.0126 0.0126 0.0254 0.0427

0.00006 0.00014 0.00096 0.00096 0.00047 0.00164

11.58 12.00 13.04 13.07 14.24 14.81

0.5581 0.5869 0.6398 0.6410 0.6810 0.6963

Trifluoropropyl POSS weight fraction

P (MPa)

8

6

4

2

0 0

0.2

0.4

0.6

0.8

1

Mole Fracon of CO2 Fig. 3. Bubble points of ethanol–carbon dioxide binary system at 333 K. () Kartal et al., 333 K [44], () Galicia-Luna et al., 333.75 K [45], () Joung et al., 333.40 K [46], (×) Secuianu et al., 333.15 K [47], (*) Galicia-Luna et al., 333.82 K [45], (+) Suzuki et al., 333.4 K [48], (䊉) This work, 333 K.

pressure measurements are within an accuracy of ±0.5 K and ±0.25% full scale, respectively. 2.3. Experimental procedure 2.3.1. Solubility experiments The high-pressure vessel was loaded with a weighed amount of trifluoropropyl POSS (±0.1 mg), and was sealed. The cell connected to the syringe pump was repeatedly flushed with CO2 , and evacuated to purge air from the system. The maximum pressure of CO2 attained during flushing was 0.3 MPa. Next, the temperature of the cell was brought to the desired value, and liquid CO2 was charged into the cell at constant syringe pump temperature and pressure. The mass of the liquid CO2 charged was determined from the density of carbon dioxide at the loading temperature and pressure, and the total volume of CO2 delivered to the cell, which was calculated from the initial and final volume recorded from the digital display of the pump. Density of carbon dioxide was determined from the NIST Chemistry WebBook [43]. The cloud point of trifluoropropyl POSS–CO2 system, which was

continuously stirred with a magnetic stirrer, was measured by slow depressurization of the single-phase system, which had all of the trifluoropropyl POSS dissolved in scCO2 . During the depressurization, at the cloud point, the precipitation of trifluoropropyl POSS out of the single-phase solution induced cloudiness in the highpressure cell, which could be detected visually. At the onset of the precipitation of trifluoropropyl POSS, the system pressure at constant temperature and composition were recorded as the cloudpoint pressure of the system. This procedure was used to obtain the cloud-point data of trifluoropropyl POSS–CO2 binary system at various compositions at two different temperatures. In order to verify the accuracy of the solubility measurement technique, the bubble point pressures of ethanol–CO2 system was measured at 333 K, and the obtained data was compared with literature data. The bubble points of ethanol–CO2 system were determined by the same technique applied for the cloud-point measurements of trifluoropropyl POSS–CO2 system; i.e. slow depressurization of a single-phase binary system and observation of the phase separation. However, instead of the precipitation of a solid at the cloud point, a vapor-phase formation at the bubble point was observed during the phase separation. The data obtained in these runs agreed well with the literature data [44–48] as seen in Fig. 3.

0.05

0.75 0.03

CO2 Density

POSS Weight Fracon

0.80 0.04

0.02 0.01 0

5

7

9

11

13

15

17

P(MPa) Fig. 4. Pressure-concentration plot of trifluoropropyl POSS–CO2 binary system cloud points at 308 K and 323 K. () Binary system cloud points at 308 K. () The repeated cloud-point data at 308 K, 0.0017 and 0.245 wt. fr. (♦) Binary system cloud points at 323 K (*) The repeated cloud point data at 323 K, 0.0126 wt. fr. Estimated error bars are comparable to the size of the symbols.

0.70 0.65 0.60 0.55 0.50 0

0.01

0.02

0.03

0.04

0.05

Trifluoropropyl POSS weight fracon Fig. 5. scCO2 densities at the cloud points of trifluoropropyl POSS–CO2 system at various binary system trifluoropropyl POSS concentrations. () 308 K, () 323 K.

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Fig. 6. (a) The cross-sectional SEM image of the tensile fractured PMMA sheet processed with pure scCO2 , 2400× magnification.(b) The SEM image of the PMMA sheet surface processed with pure scCO2 , with 5000× magnification.

2.3.2. Trifluoropropyl POSS deposition on PMMA Trifluoropropyl POSS deposition on PMMA sheets was performed by exposing the sheets to scCO2 for a certain period of time, followed by depressurization of the system at a certain rate. The temperature and pressures at which the polymer sheets were exposed to trifluoropropyl POSS–CO2 high-pressure system were decided based on the solubility studies conducted in this work. The conditions were chosen such that the loaded trifluoropropyl POSS would dissolve in scCO2 completely, forming a single phase binary system. In a typical experiment, first, the high-pressure view-cell was loaded with a weighed amount of trifluoropropyl POSS and PMMA sheets. The cell was flushed with CO2 at 0.3 MPa, and evacuated repeatedly to purge air from the cell. After the cell was heated to the desired temperature, it was filled with liquid CO2 using the syringe pump. The volume of CO2 to be delivered from the pump was calculated based on the desired composition of the trifluoropropyl POSS–CO2 binary system, and the CO2 density at the loading temperature and pressure of the pump, which would be kept constant during loading. PMMA was exposed to trifluoropropyl POSS–scCO2 single phase binary system for 24 h at constant pressure and temperature of 17 MPa and 313 K, respectively. The POSS concentration of the POSS–CO2 binary system was 3.5 wt%. Finally, the system was depressurized at a flow rate of 0.17 MPa/min, and

Fig. 7. (a) The cross-sectional SEM image of the tensile fractured, trifluoropropyl POSS deposited PMMA sheet, processed with trifluoropropyl POSS–scCO2 system, 2400× magnification.(b) The surface SEM image of the trifluoropropyl POSS deposited PMMA sheet, processed with trifluoropropyl POSS–scCO2 system, 12,000× magnification.

the PMMA samples were taken out of the high-pressure cell to be characterized. In order to observe if PMMA foamed by scCO2 under the conditions of the nanoparticle deposition process, some control tests were performed. In these runs, PMMA sheets were exposed to pure scCO2 at the same temperature and pressure for the same time period as the deposition experiments, while the system was depressurized at the same flow rate. 3. Results and discussion 3.1. Solubility of trifluoropropyl POSS in supercritical carbon dioxide Solubility of trifluoropropyl POSS in scCO2 is presented in Fig. 4, which shows the solubility isotherms plotted against the binary system pressure. The solubility data of trifluoropropyl POSS in scCO2 is also given in Table 1, which shows the cloud-point pressures measured at temperatures of 308 and 323 K, in a pressure range of 8 and

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Fig. 8. SEM-EDS spectrum of trifluoropropyl POSS domains deposited on the PMMA surface.

15 MPa. As seen in Fig. 4 and Table 1, some of the data were measured randomly twice or three times to prove the repeatability of the measurement technique. The exact experimental procedure as described earlier in the text was followed in the repeating experiments, which were conducted on different days. The reproducibility of the cloud-point pressures was obtained within an error margin of ±0.8 MPa. In Fig. 4, the binary system exists as a single phase above the solubility isotherm. The trifluoropropyl POSS solubility isotherms approach each other with increasing POSS concentration. The solubility of the hybrid nanoparticles decreases in scCO2 with increasing temperature, and to remain the system in single phase, a comparable solvent power is needed to be maintained by increasing the pressure. While the solubility of trifluoropropyl POSS in scCO2 is around 4.4% by weight at 10.68 MPa, it decreases to less than 0.2% when the temperature is increased to 323 K at constant pressure. In order to bring the system to single phase with all the trifluoropropyl POSS solubilized in scCO2 , the pressure has to be increased to around 14.8 MPa. In previously conducted solubility measurements of different solids in scCO2 , melting point depression of the solute was observed [49,50]; however, in this work, no phase change of solid into liquid occurred, which shows that trifluoropropyl POSS does not exhibit a melting point depression under the CO2 pressure and temperature ranges applied in this study. The solvent strength of CO2 directly depends on its density, which is adjustable by changing temperature and pressure. Fig. 5 shows the carbon dioxide density at the cloud-point pressures of the trifluoropropyl POSS–CO2 binary system at 308 and 323 K plotted against the trifluoropropyl POSS weight fraction in the binary system. The CO2 density values were obtained from the NIST Database as explained in the experimental procedure. The CO2 density, indicating its solvent power required to dissolve trifluoropropyl POSS does not change with temperature at constant solubility concentration up to about 4 wt%. Above this concentration, a slight increase in CO2 density is required to dissolve trifluoropropyl POSS when the temperature is decreased from 323 K to 308 K.

3.2. Deposition of trifluoropropyl POSS with supercritical carbon dioxide In order to demonstrate that trifluoropropyl POSS nanoparticles and scCO2 can be applied for modification of a polymer surface, the binary system was used for the first time to deposit the functionalized caged-nanoparticles on a high-molecular weight PMMA surface. As explained in the experimental procedure, the deposition was performed by exposing the polymer to the high-pressure, single-phase binary system, which was followed by depressurization of the medium. PMMA is a CO2 -philic polymer through its intermolecular Lewis acid–base type interactions, while a couple hundred thousand molecular weight types can be plasticized and swollen by CO2 [7,36,51]. In order to observe how scCO2 affected the polymer, PMMA sheets were exposed to pure CO2 under the conditions of the deposition study, and the same experimental procedure was followed to obtain the CO2 -processed polymers. The cross-sectional and surface SEM images of the processed PMMA sheets are given in Figs. 6 and 7, which show PMMA sheets processed with pure CO2 and with trifluoropropyl POSS–CO2 binary system, respectively. The cross-sectional SEM image of the polymer sheet processed by pure scCO2 in Fig. 6a shows that no porous structure was formed in the cross-section of the PMMA sheet, while Fig. 6b shows the macropores formed on the polymer surface with diameters between 0.8 and 2.5 ␮m. These figures indicate that scCO2 was absorbed in the surface layer of the polymer, and plasticized the surface forming a porous structure upon depressurization. Even though relatively low CO2 pressures around 4–6 MPa would lead to a remarkable decrease in PMMA glass transition temperature, a highly cross-linked structure of PMMA can slow the scCO2 sorption kinetics significantly, causing longer CO2 -exposure times for polymer to swell [52]. In this work, significantly slow scCO2 sorption kinetics might have been caused by highly cross-linked structure of the high-molecular weight PMMA, which might have restricted the CO2 diffusion into the polymer matrix and the specific molecular interactions between PMMA and CO2 due to the polymer’s decreased conformational flexibility.

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The cross-sectional and surface images of PMMA processed with trifluoropropyl POSS–CO2 binary system are given in Fig. 7. These figures show that aggregates of trifluoropropyl POSS nanoparticles with particle size of 1–4 ␮m were deposited on the polymer surface, forming a uniform coating film with 22 ± 3 ␮m thickness. The lighter colored particles, which can be seen between the trifluoropropyl POSS aggregations in Fig. 7b, were detected as calcium impurities by the EDS-elemental analysis. Fig. 8 shows a spectrum obtained with SEM-EDS semi-quantitative elemental mapping of trifluoropropyl POSS deposited PMMA surface. The spectrum shows the peaks of fluorine, silicone and a trace amount of calcium impurity belonging to trifluoropropyl POSS domains deposited on the polymer sheet. Therefore, EDS analysis evidences coating of the polymer with the inorganic/organic hybrid cage nanostructures by scCO2 . The fluorine and silicone peaks of trifluoropropyl POSS were not observed in the semi-quantitative elemental mapping analysis performed on the cross-section of the surface deposited PMMA sheets, which also supports the fact that carbon dioxide did not diffuse into the high-molecular weight rigid PMMA, and therefore trifluoropropyl POSS was not carried into the polymer matrix.

4. Conclusions Polyhedral oligomeric silsesquioxanes (POSS) are cagestructured nanofillers commonly used in engineering plastics, to improve thermal, mechanical, flame retardancy and oxidation resistance properties of polymers. These organic–inorganic hybrid nanoparticles can be functionalized with CO2 -philic groups, which can allow them to solubilize in supercritical carbon dioxide (scCO2 ). In case POSS can be scCO2 soluble, they can be applied in various green materials processing, using the environmentally benign solvent. In this work, solubility of a POSS in scCO2 has been studied for the first time. The solid POSS was functionalized with fluoroalkyl groups, and its solubility in scCO2 was determined by measuring cloud points in a high-pressure visible cell. Trifluoropropyl POSS exhibits a remarkable solubility in scCO2 , which decreases with increasing temperature in the studied trifluoropropyl POSS concentration range. Its solubility in scCO2 is 4.4% by weight at 10.7 MPa and 308 K. When the temperature is increased from 308 K to 323 K, the pressure of the system is needed to be increased to 14.8 MPa to maintain the solubility concentration constant. The trifluoropropyl POSS–carbon dioxide binary system was used to deposit the nanocage hybrid particles on high-molecular weight, rigid PMMA sheets. PMMA sheets were exposed to the high-pressure, single-phase binary system, followed by depressurization of the medium. In order to observe how scCO2 affected this high-molecular weight, rigid polymer, control-PMMA sheets were exposed to pure CO2 . The SEM images show that pure CO2 plasticized the surface of PMMA, and formed a porous surface upon depressurization. However, the cross-section of the polymer sheet did not have a porous structure, indicating that scCO2 plasticized only the surface of the polymer. The crosssectional and surface SEM images of the polymer sheets processed with trifluoropropyl POSS–CO2 system show that trifluoropropyl POSS was deposited on the sheet as a few micrometer-sized aggregates, forming a film of 22 ± 3 ␮m thickness. The SEM-EDS spectrum showed the peaks of fluorine and silicone, evidencing trifluoropropyl POSS deposition on the polymer sheet by scCO2 . The SEM-EDS spectrum of the cross-section of the surface deposited PMMA sheets did not show any peaks of fluorine and silicone.

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