Control of Mechanical Stability of Hollow Silica Particles, and Its

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Control of Mechanical Stability of Hollow Silica Particles, and Its Measurement by Mercury Intrusion Porosimetry Jelena Lasio, Alan M Allgeier, Christopher D. Chan, J. David Londono, Ebrahim Najafi, and Francis J. Woerner Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00506 • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on May 1, 2017

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Control of Mechanical Stability of Hollow Silica Particles, and Its Measurement by Mercury Intrusion Porosimetry Jelena Lasio*†, Alan M. Allgeier*‡, Christopher D. Chan‡, J. David Londono‡, Ebrahim Najafi†, and Francis J. Woerner† †Chemours Titanium Technologies, Experimental Station, Wilmington DE, 19803 ‡DuPont Corporate Center for Analytical Sciences, E. I. DuPont De Nemours and Co., Experimental Station, Wilmington, DE 19803

Abstract

Hollow silica particles (HSPs) have become the focus of interest in many labs recently, due to their versatility, stemming from the ability to control their size and shape, as well as surface functionalization. Determining the mechanical stability of hollow particles is essential for their use, both in applications in which they need to retain their structure, as well as those in which they need to break down. We have synthesized a series of HSPs (231 nm inner diameter) with increasing wall thickness (7-25 nm), using a template approach. Their mechanical stability was measured using mercury intrusion porosimetry (MIP), which represents the novel application of the technique for these materials. The samples with complete shells break at progressively

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higher pressures, and samples with wall thickness ≥ 21 nm remain stable to the highest pressure applied (414 MPa). Other characterization methods, namely microscopy, gas adsorption, and small angle X-ray scattering shed light on the particles’ size parameters, as well as the porosity of the silica walls. By varying the amount of silica precursor used in the template coating step, we were able to produce hollow silicas with variable stability, thereby allowing for the control of their mechanical properties.

Introduction Technologies for hollow particles syntheses have generated a lot of interest in the scientific community, because of the variety of applications, including thermal insulation,1 catalysis,2 uses as photonic materials,3 light scatterers,4 support for enzymes,5 drug delivery vehicles,6-9 and contrast agents.10 There are several synthetic pathways to hollow particles; most commonly they are made through precursor deposition onto the surface of organic11 or inorganic12 template particles, oil droplets,13 or within vesicles.14 The templates are then removed either through dissolution,15 hydrolysis,16 or calcination.17 In the case of organic particles, the commonly used method is pH change-induced core swelling of the core-shell particles at temperatures higher than Tg of the shell material.18-20 Finally, they can be made through template-free methods.21, 22 Mechanical stability of the hollow spheres is important for their applications, be they designed to keep their shape, or disintegrate in cases where they are used as a scaffold. Determining their strength a priori is thus an important task, one that should be incorporated in the material design. The mechanical and elastic properties of hollow spheres can be determined through atomic force microscopy (AFM).23, 24 While this technique provides precise measurements of buckling pressure, AFM only interrogates individual particles, not the bulk of the sample. Further, the force applied, due to the nature of the experiment, is directional instead of isotropic. That may


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be an advantage for some applications, but for others it may be beneficial to measure the stability of the particles under isotropic pressure. Mercury intrusion porosimetry (MIP) has classically been used to characterize the pore volume distribution of samples, utilizing the Washburn equation.25,

26

In this technique a sample is

placed in a sealed penetrometer cell and placed under vacuum. Liquid mercury is admitted to the cell at increasing intervals of pressure and the volume of mercury intruding the sample pores is recorded at each interval. The Washburn analysis provides the diameter of the pores associated with each pressure interval, smaller pores requiring a greater pressure to effect intrusion and overcome mercury’s high surface tension. Beyond supplying pore volume distributions, MIP was utilized to provide information about the mechanical stability of aerogel / xerogel materials. Early work from DuPont described irreversible deformation of aerogels during MIP measurements and correlations of bulk modulus and volume reduction during the measurements.27

Further, Pirard et al. evaluated buckling transitions in aerogel / xerogel

materials under mercury intrusion.28-30 The technique has not been used in measuring buckling pressures of hollow spheres, however. We report here the first application of MIP in measuring the buckling pressure of hollow spheres. The studied materials were HSPs, whose wall thickness was progressively increased, thereby increasing their mechanical stability. The template approach used for their synthesis allowed control of both the size of the hollow and the wall thickness of the particles during the silica deposition step, providing samples with high uniformity.

In addition to MIP

measurements, the samples were characterized using small-angle X-ray scattering (SAXS), nitrogen pore volume distribution (PVD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

The MIP technique allows for measurement of


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buckling pressure on the bulk of the sample rather than individual particles, which may provide a better overview of the mechanical stability of the sample as a whole. In the sample series studied here, the buckling pressure increased with increased wall thickness. At the same time, a decrease in their porosity was observed. The measurements also allowed for calculation of Young’s modulus of the materials.31 Experimental Section Materials Styrene was purchased from Alfa Aesar, and its inhibitor was removed prior to polymerization by an inhibitor removal disposable column for t-butyl catechol (Aldrich, product number: 306320). Tetraethoxysilane (TEOS), polyvinylpyrrolidone (average Mw 10,000, PVP), and ethanol were obtained from Aldrich.

2,2'-Azobis(2-methylpropionamidine)dihydrochloride

(AIBA) was obtained from Wako. Ammonium hydroxide was obtained from Fisher Scientific. They were used without any further purification. Polystyrene (PS) particle synthesis: To a 250 mL three-neck round bottom flask, equipped with a mechanical stirrer, thermometer, and a reflux condenser, was added styrene (13.0 mL, 113.5 mmol), PVP (500 mg), and 100 mL of degassed water. The resulting mixture was stirred at room temperature for 15 min. The mixture was degassed by bubbling nitrogen for 20 min. To the reaction was then added a degassed solution of 2,2-azobis(2-methylpropionamidine) hydrochloride, AIBA (300 mg, 1.1 mmol) in 20.0 mL water, and the reaction was heated to 70 oC overnight. Particle size analysis of the resulting suspension via dynamic light scattering revealed particle size of 241 nm (d10=174 nm, d50=241 nm, d90=328 nm. d10 represents a size at which 10% of the sample volume is below


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the d10 value. d50 and d90 represent the particle sizes where 50% and 90% of the sample volume fall below those values, respectively). Core shell particle synthesis: To a 1L Erlenmeyer flask was added 100.0 mL of PS suspension (8.9 wt%) in water, followed by 700.0 mL of EtOH, and 20.0 mL of aq. NH4OH. The flask was placed in a sonicating bath (Branson 3510R-MTH), to which was added 10.0 mL of TEOS via a syringe pump, at a 0.1 mL/min rate. The resulting suspension was left sonicating for 2h, and the suspension was concentrated in vacuo by removing ethanol. The core-shell particle suspension was centrifuged, and the precipitate washed twice with ethanol and air-dried overnight, yielding 11.8g of white solid. Hollow silica particle synthesis: The core-shell material (5.00g) was calcined in a tube furnace under static air at 500 oC (r.t.500 at 1 oC/min, then 5 h at 500 oC, to generate hollow silica particles (1.05g). Characterization Specimen Preparation for Electron Microscopy. SEM samples were prepared by taking the dry hollow silica spheres and dusting them onto a double-sided adhesive carbon tape. The hollow spheres were imaged over a 134 second single pass acquisition using a JEOL 7600F field emission scanning electron microscope operated at 15 kV. To prepare TEM samples, the hollow silica spheres were dispersed in EtOH under sonication. The solutions were then drop-cast onto lacy or holey carbon grids, which were left to dry overnight. The measurements were carried out by a JEOL 2011 LaB6 electron microscope at 200 kV acceleration voltage at 25,000 X magnification.


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Particle Size Measurement To measure polymer template particle size, aqueous PS suspension samples (aprox. 50 µL) were diluted to 3 mL with DI water and tested using Malvern Zetasizer Nano S. He Pycnometry Samples were analyzed by helium pycnometry on a Micromeritics AccuPyc 1340, wherein outgassed samples (room temperature overnight in a vacuum oven) were weighed to a calibrated sample cup, and the sample volume was determined via displacement with helium at room temperature and approximately 20 psig. The equilibration rate threshold was 0.005 psi/min. A minimum of five measurements were conducted and the average value reported. The mass was divided by the average volume measurement to obtain density. Mercury Intrusion Porosimetry Mercury intrusion porosimetry was conducted on the samples to assess porosity in the 0.003 – 400 µm region. Outgassed samples were charged to the calibrated penetrometer cell of the Micromeritics AutoPore III. The penetrometer was sealed, installed in the instrument and placed under vacuum. Mercury was admitted to the penetrometer at gradually increasing pressure from 0 – 414MPa and intrusion was recorded (64 data points). To assess sample stability toward compression, a series of mercury intrusion / extrusion experiments (cycling experiments) were conducted on the instrument to evaluate reproducibility and hysteresis. Pore Volume Distributions by Nitrogen Adsorption Pore volume distributions in the region 2-100 nm were determined by nitrogen adsorption using the Micromeritics ASAP 2420. Samples were pretreated in vacuum at 50-150 °C for 12 h to remove adsorbed moisture, then transferred to the instrument, evacuated and cooled to 77 K in


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a liquid nitrogen dewar. Nitrogen gas was admitted to the sample cell in doses from 50 Torr to 755 Torr and back to 50 Torr, while adsorption was recorded via differential pressure measurements. The sequence typically involved 44 steps in the pressure table. Adsorption data were analyzed by the BET equation32 to determine total surface area, by the BJH model33 to determine pore volume distribution and the t-plot model to determine micropore volume and surface area.26 Small Angle X-ray Scattering As core diameter and shell thickness differed by about an order of magnitude, small angle Xray scattering measurements were performed on two instruments to cover a broad enough range of length scales to measure both sizes. A rotating anode source from Rigaku with a three pinhole instrument, and fitted with a 2048 x 2048 Vantek-2000 2D detector from Bruker, was used in two configurations (140 and 34 cm) to cover a scattering vector magnitude range of 0.1< (q=4πsinθ/λ) < 9 nm-1, where 2θ is the scattering angle and λ is the X-ray CuKα wavelength (0.15 nm). A second instrument running at the same wavelength but fitted with a fixed-anode Xray tube of the Coolidge type, was used to cover the range 0.0003 < q < 0.02 Å-1. The second instrument consisted of a double crystal camera of the Bonse Hart type from Rigaku. Data from the three instrumental geometries were reduced, desmeared, azimuthally averaged and combined by using well-established procedures.34 A publicly available suite of routines35 was used to fit the combined data to the form factor for a polydisperse spherical particle with a core-shell structure. In this model, the ratio R(core)/R(core+shell) is held constant. Hard sphere structure factor contributions were included to account for inter-particle interference effects. Due to the very high resolution of the Bonse-Hart instrument compared to the pinhole instrument, the core and shell dimensions could not be fitted together. The parameters for the core were fixed during


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the fitting of the shell and the fitting range was accordingly set to the high angle portion of the data. Similarly, shell parameters were fixed and the fitting range set to the low angle portion during the fitting of the core size. Figure S13 shows an example of the two fits performed on a single set of data. Each fit is shown within its appropriate range. Results and Discussion Hollow silica particles were made through a template approach, using monodispersed, positively-charged polystyrene particles36 as template (Scheme 1).37 Polystyrene particles (PS) were made through emulsifier-free emulsion polymerization protocol.

Figure 1. Synthesis of hollow silica particles. ))) indicates sonication during TEOS deposition. Silica deposition onto PS particles was done using tetraethoxysilane (TEOS), in a modified version of the Stöber process.38 Sonication, through use of a sonicating bath during silica deposition ensured reproducible results. Core/shell particles were then calcined at 500 oC to generate hollow silica particles (Figure 1). The wall thickness of the hollow particles, and consequently, their porosity, could be easily controlled by the amount of TEOS used in the coating process (Figure 1, Table 1). Unsurprisingly, the more TEOS used as a silica precursor, the thicker the walls of the resulting hollow particles were. The coating process did not generate any filled, secondary silica particles, as observed by SEM (Figure 2). Increasing the wall thickness led to a decrease in specific surface area and specific pore volume, as measured by nitrogen adsorption (Table 1). The generated isotherms are Type II isotherms reflective of low pore volume in the mesopore range, 2-50 nm (Figure 3). T-plot analyses reveal that the thinner


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wall sample 1a has larger micropore area than the thicker wall sample 1b (Table 1). Consequently, the porosity and surface area decreased with increasing wall thickness (Table 1). Measured surface areas were larger than the calculated surface areas of model spheres with smooth surfaces (79 vs 28 for sample 1a, for example), the disparity being reflective of the porous wall. In the case of the thicker walled samples, the difference is less dramatic indicating that additional silica deposition gave rise to low porosity structures and that the external surface dominates the measured BET value. It seems unlikely that condensation of nitrogen in the hollow void can be resolved from condensation in the bulk. Notably, negligible hysteresis was observed in the isotherm; hysteresis would be expected for a large pore separated from the bulk gas by a narrow pore throat. When the densities of the two samples were measured by helium pycnometry, however, the resulting numbers (2.17 and 2.15 g/cm3, respectively, Table 1) were closer to the value for amorphous silica (2.31 g/cm3), than the calculated density for hollow particles of the same size (0.82 and 1.07 g/cm3, respectively). This indicates that helium was certainly able to enter the central hollow of both types of particles, even in the case of the sample with thicker walls. The measured density values were lower than the reported silica density, implying that the wall contained some closed, inaccessible pores or, perhaps, residual carbon, coming from incomplete calcination of the template particles.

1a

1b


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Figure 2. SEM of hollow silica particles with 246 nm diameter of the hollow and different wall thicknesses: 1a) 18-22 nm, and 1b) 28-30 nm (Bar =100nm). Particle size and wall thickness measured by SAXS. Sample

Wall

BET

surface External

Thickness

area

surface

[nm]

(measured)

(calculated),

[m2/g]

assuming

BJH area Volume [cm3/g]

Pore Density (He Pycnometry) [cm3/g]

smooth surface [m2/g] 1a

18-22

79

28

0.144

2.17

1b

28-30

24

18

0.074

2.15

Table 1. Particle parameters for materials in Figure 2.

140

120

Volume Adsorbed (cc/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1a

100

1b

80

60

40

20

0 0

0.2

0.4 0.6 0.8 Relative Pressure (P/Po)

1


1.2

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1000 900 800 Pore Volume (10-6 cc/g*Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1a

700

1b

600 500 400 300 200 100 0 0

200

400

600

800

1000

1200

1400

Pore Width (Å)

Figure 3. (top) Nitrogen (77 K) adsorption isotherms for samples 1a and 1b; (bottom) Pore size distributions using BJH Method for 1a and 1b HSP samples. In order to test mechanical stability of the hollow particles, we subjected them to mercury intrusion porosimetry.26 The volume of sample plus unfilled pores (Vsp) as a function of mercury pressure is shown in Figure 4.

Upon increasing penetrometer pressure, mercury intrudes

interparticle space and pores of the solids and Vsp rapidly decreases. This decrease corresponds to interstitial spaces with diameter in the range 55-135 nm via the Washburn equation,25, 26 in reasonable agreement with micrographs. (see Figure 2 and supplementary material). For sample 1a, with thinner shell walls, the Vsp vs P curve showed an interesting profile after the initial reduction, Vsp remained constant until reaching 164.8MPa of pressure when it dropped abruptly and then, upon further increases in mercury pressure, did not change. After the extrusion of mercury, subsequent mercury intrusion / extrusion cycles reveal a much lower sample volume


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(decrease from 1.4 cm3/g to 0.6 cm3/g), and absence of any significant hysteresis. Sample 1b did not show the same profile, however. Thicker wall silica particles did not exhibit a decrease in Vsp aside from the initial drop that results from filling of the interstitial spaces. Vsp for 1b sample has a higher value at 400 MPa (1.04 cm3/g compared to 0.67 cm3/g for sample 1a) further suggesting that the particles were not crushed in the experiment as happened for the 1a material (In generating Vsp, interstitial spaces are included in the analysis as “pore spaces”). Following extrusion of mercury, subsequent mercury intrusion / extrusion cycles identically overlay the first cycle. The data suggest that at a certain yield pressure the hollow particles are crushed, and mercury then enters the central cavity, thereby reducing the overall volume of the sample.

3.0

1a

2.5

Vsp (cc/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1b

2.0

1.5

1.0

0.5 0

100

200

300

400

Pressure (MPa)

Figure 4.

Mercury porosimetry Vsp/P curves for samples in Figure 2, with average shell

thickness of 20 nm (1a, diamonds) and 29 nm (1b, squares).


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To systematically investigate the properties of HSPs, both their porosity as well as mechanical stability, we synthesized ten different hollow silica samples following the method in Figure 1. The experiments differed in the amount of TEOS used in the core/shell particle synthesis, which was incrementally increased by 10% along the sample series, and resulted in hollow particles of the same core size (231 nm), and increasingly thicker walls (Figure 5, Table 2). We analyzed the particles’ porosity with nitrogen adsorption and mechanical stability with mercury porosimetry. Unsurprisingly, with increased wall thickness, the porosity and specific surface area of the particles decreased. Sample

Core

Wall

BET

surface Surface

diameter

Thickness

area

(calculated),

Volume

[nm]

[nm]

(measured)

assuming

[cm3/g]

[m2/g]

smooth

area Pore

surface [m2/g] 2a

231.6

7

192

66

0.59

2b

231.8

10

154

47

0.57

2c

231.2

12

122

40

0.28

2d

231.2

14

94

35

0.21

2e

231.2

18

69

27

0.16

2f

231.6

18

65

27

0.17

2g

233.4

21

34

24

0.10

2h

233.0

23

28

22

0.08

2i

235.4

27

23

20

0.06

2j

234.2

25

23

21

0.06

Table 2. Particle parameters for materials in Figure 5. Particle size and wall thickness values


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obtained from SAXS. The smooth sphere surface area values were calculated from SAXS data.

2a

2b

2c

2d

2e

2f

2g

2h

2i

2j

Figure 5. SEM of hollow silica particles with increasingly thicker walls. TEM images are shown in Figure S14.


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In evaluating the nitrogen adsorption isotherms, one might question if the volume of the hollow cavity is recorded as pore volume. Based on the size parameters of the particles in the series, their calculated specific hollow volumes were plotted against the measured pore volumes (Figure 6). The linear fit has a slope of 2.75, rather than 1, indicating that the volumes of the hollow cavities were not recorded as pore volume. Instead, the recorded pore volumes are predominantly associated with very small diameter pores (average measured pore width is 14 nm), as could be present in the wall of the HSPs (Figure 7). Even though nitrogen undoubtedly enters the hollow center during the measurement, the BJH method employed to convert nitrogen isotherm data to pore volume distributions loses resolution above 100 - 150 nm pore diameter and microscopy and SAXS evidence suggest that all the hollow cavities in this study have diameters above 150 nm. 2.50

2.00 Model Hollow volume (cc/g)

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1.50

1.00

0.50

0.00 0.000

0.100

0.200

0.300 0.400 BJH PV (cc/g)


0.500

0.600

0.700

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Figure 6. Specific hollow volume vs measured BJH pore volume for samples in Figure 5.

0.0018 0.0016 0.0014

Increasing Shell Thickness

0.0012 dV/dw (cc / g* Å)

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2a

0.0010

2c 0.0008

2e 2g

0.0006

2i

0.0004 0.0002 0.0000 0

200

400

600

800

1000

Pore diameter (Å)

Figure 7. Differential Pore volume distribution for select samples from Figure 5. The complete data set is shown in Figure S1. In order to determine their mechanical stability, the hollow silica particles were tested in MIP experiments. The Vsp/P curves (Figure 8, Table 3), showed a similar pattern as those in Figure 4. The thinner particles showed the same Vsp/P profile as sample 1a in Figure 2 with volume decreasing after reaching a particular pressure. Given that the particles are fairly uniform in size and wall thickness, the onset of particle breakage occurs at a relatively narrow pressure range. Moreover, with increasing wall thickness along the series, the particle breakage occurred at progressively higher pressures, indicating the growing particle stability. When particles reached


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wall thickness of 21 nm and higher, the sharp decrease in volume did not occur over the range of pressure tested (up to 414 MPa), which showed that the particles with core size of 232 nm, and wall thickness of 21 nm or more were stable to pressures up to at least 414 MPa (60,000 psi). 6.0

5.0

4.0 2c Vsp (cc/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2e 3.0

2i

2.0

1.0

0.0 0

50

100

150

200

250

300

350

Pressure (MPa)

Figure 8. Mercury intrusion Vsp / P curves for select samples in Figure 5.


400

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Sample

Core diameter Shell

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thickness Onset

of

particles’

[nm]

[nm]

breaking pressure [MPa]

2a

231.6

7

82.4

2b

231.8

10

61.8

2c

231.2

12

82.4

2d

231.2

14

103.0

2e

231.2

18

123.6

2f

231.6

18

144.2

2g

233.4

21

>413.7

2h

233.0

23

>413.7

2i

235.4

27

>413.7

2j

234.2

25

>413.7

Table 3. Particles’ core and wall thickness and the onset of their breaking pressure in a mercury intrusion porosimetry experiment for samples in Figure 5. Plotting the onset of particles’ breakage vs the ratio of shell thickness to the radius of the particles allowed us to calculate the effective Young’s modulus of these materials, using the thin shell model39 (Figure 9): ℎ ଶ ܲ௖௥ = ∗൬ ൰ ඥ3ሺ1 − ߥ ଶ ሻ ܴ 2‫ܧ‬

where Pcr is the buckling pressure, E is Young’s modulus, ν is the Poisson’s ratio (0.17 for silica), and h and R are the wall thickness and the radius of the hollow cavity, respectively. From the slope of the curve, the Young’s modulus value was calculated to be 8.60 GPa. This value is very different from that reported for fused silica (76 Gpa)31 as well as for other, larger hollow silica particles, whose buckling pressure was measured by AFM.23 The difference in values is


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not surprising, given that Young’s modulus depends on the size and shape of material, not just its chemical nature,40 and porosity as well as the presence of the central hollow impact the modulus of a material.23, 41, 42 Moreover, isotropic and directional forces will break the shells at different pressures, so they are not directly comparable. It is therefore important to keep final application of a material in mind when designing its mechanical properties, as well as the testing methods to measure its stability and Young’s modulus. When comparing the pressures where the onset of particles’ breaking takes place along the series (Table 3, Figure 9), the value for the thinnest wall particles, 2a, is higher than the next one in the series, sample 2b (particles with wall thicknesses of 7 and 10 nm, respectively). The reason for that lies likely in the increased porosity of the wall of the hollow particle for sample 2a vs 2b and others in the series. In examining SEM images of the samples (Figure 10), it is evident that the pore sizes of 2a are significantly larger than those of the subsequent samples in the series. Some of the pores, while evident in the SEM images, are not detected by the nitrogen adsorption tests, given that they are too large to be measured (BJH method loses resolution above 100 nm). It is, however, likely, that the presence of such pores provide intruding mercury easier entrance to the inner cavity, without the breaking event, thereby making particles with larger porosity seem more mechanically stable. With subsequent samples in the series (Figure 11), where more silica was used to build the wall, the shell is more complete, with fewer, if any large pores through which mercury can intrude without much resistance. For the subsequent five samples in the series, the buckling pressures increase and form a straight line when plotted against the ratio of shell and radius, allowing for calculation of Young’s modulus for these materials. The last four samples in the series did not break when exposed to the highest pressure attainable in the MIP experiment, so they could not be plotted in Figure 9.


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250

200

Pcr [MPa]

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150

100

50

0 0

0.005

0.01

0.015

0.02

0.025

(h/R)2

Figure 9. Correlation of crushing pressure to the shell thickness to radius ratio.

Summary and Conclusions The described study shows that the hollow silica particles’ parameters can be tightly controlled, through use of templates with defined sizes and conditions of silica deposition. The information about the wall structure can be obtained from simple nitrogen adsorption porosimetry and density measurements, and their mechanical stability can be measured through use of mercury intrusion porosimetry. MIP is a simple method for obtaining mechanical stability information of uniform hollow particle samples, as it provides the volume-pressure data on the bulk of the samples, not individual particles. In the current study, a series of hollow silica particles with inner diameter of 232 nm and wall thickness in the 7-25 nm range were tested with MIP, and it was shown that, for samples with complete shell structure, the mechanical stability increased steadily until wall thickness of 21 nm, when particles withstood the highest pressure exerted by the instrument.

In a MIP experiment, in contrast to AFM measurements of

mechanical robustness, the force applied is isotropic rather than unidirectional, which can be a


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useful alternative for those applications where hollow particles experience uniform pressures. There are limitations to the MIP method, however: in order to generate unambiguous crushing pressure data, the particles have to be relatively uniform. Further, there are pressure limits of the MIP method (414 MPa in this case); in the present series, the last four samples’ breaking pressures could not be determined, since they were stable to the highest pressure applied by the instrument. However, for particles whose stability falls below the MIP pressure limit, the present study describes a simple method for determination of their stability, with possible prediction of failure events under pressure.

Figure 10. SEM of samples 2a (left) and 2b (right). Incomplete shells are evident on the 2a sample, while mostly absent from the next sample in the series. More SEM images of the two samples are shown in Figure S15.

ASSOCIATED CONTENT Supporting Information


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The supporting information is available free of charge at the ACS Publications website at DOI: . Pore volume distributions for samples 2a-2j, helium pycnometry density measurements, MIP Vsp/P curves for samples 2a-2j, SAXS data for sample 2h, repeated BET and BJH data for two representative samples in Figure 6, TEMs of samples 2a-2j, and additional SEMs for samples 2a and 2b. AUTHOR INFORMATION Corresponding Authors *Email: [email protected]; [email protected] ORCID Jelena Lasio: 0000-0002-6317-2657 Alan M. Allgeier: 0000-0001-9122-2108 Chrishopher D. Chan: 0000-0001-8379-0615 Francis J. Woerner: 0000-0003-4818-098X ACKNOWLEDGEMENTS We are grateful to Dana Jones and Min Yang for PVD and MIP data collection, and Laura Clinger for SAXS data collection. REFERENCES 1.

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Table of Contents Image

Left: Vsp/P curves obtained during mercury porosimetry experiments for hollow silica particles with different wall thickness. The drop in volume indicates the breaking of hollow spheres, and the pressure at which different spheres break follows their wall thickness order. Right: SEM of one hollow silica sample used in the study.


Control of Mechanical Stability of Hollow Silica Particles, and Its

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