ELECTROSTATICALLY ACTIVE AEROGELS FOR AIR FILTRATION AND

SYNTHESIS OF SYNDIOTATIC POLYSTYRENE AEROGEL MICROPARTICLES

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirement for the Degree

Master of Science

Shuxin Ji

August, 2017

ELECTROSTATICALLY ACTIVE AEROGELS FOR AIR FILTRATION AND

SYNTHESIS OF SYNDIOTATIC POLYSTYRENE AEROGEL MICROPARTICLES

Shuxin Ji

Thesis

Approved: Accepted:

______Advisor Department Chair Dr. Sadhan C. Jana Dr. Sadhan C. Jana

______Committee Member Dean of College Dr. Alamgir Karim Dr. Eric J. Amis

______Committee Member Dean of the Graduate School Dr. Xiong Gong Dr. Chand Midha

______Date

ii

ABSTRACT

Syndiotactic polystyrene (sPS) aerogels present low bulk density, high surface area, and high porosity. This work focused on functionalization of sPS aerogels for the purposes of high efficiency airborne nanoparticle filtration and conversion into microparticles using oil-in-water emulsion technique.

In the first part, filtration performance of electrostatically active sPS aerogels was investigated. sPS gel was made in advance and soaked into a hydrolyzed TEOS solution for 24 hours. TEOS in sPS gel was then allowed to condense under a basic condition.

(3,3,3-trifluoropropyl) silane was used to modify silica and to introduce electrostatic charge into the aerogel sample. The resultant aerogels had high filtration efficiency (up to 99.999%) with at most 15% permeability loss compared to unmodified sPS aerogels with filtration efficiency of 98.889%. Scanning electron microscope images showed that silica, due to low concentration, only formed clusters attached to styrene polymer strands instead of forming a continuous network. The morphology of silica component explained the small loss of permeability. The strong IR peaks of Si-O-Si bonds were found in modified samples.

The intensity of C-F bond peak had a positive relationship with SiF3 concentration. The modified specimens showed significant electrostatic charge. Nitrogen and carbon dioxide adsorption isotherms were used to measure the surface area and pore size distribution.

Surface area increased with the addition of SiF3, as well as mesopores, which could be the consequence of extra condensation reaction involving SiF3.

In the second part, sPS aerogel microparticles were made via an oil-in-water emulsion process. This work sought to explore the feasibility of generation aerogel

iii

microparticles using a thermos-reversible mechanism. Several good solvents of sPS were used as the dispersed phase and for the final sPS solvent selection, a mixture of chloroform and toluene was found to be optimum. The effect of various emulsion process on the particle size distribution was investigated. The SEM images showed a dense skin layer covering the microparticles, which contributed to more mesopores in the microparticles.

The internal part of the microparticles was similar to that of the monolithic aerogels. There was no obvious difference in surface area between the aerogel monolith and the aerogel microparticles.

iv

ACKNOWLEDGEMENTS

I would like to express my appreciation to Prof. Sadhan C. Jana for his guidance to my research. His wisdom, patience and insight of research helped me all the way through my study.

v

TABLE OF CONTENTS

Page

LIST OF TABLES ……….………………………………….……………….………..…ix

LIST OF FIGURES………………………………………………………………………..x

CHAPTER

I. INTRODUCTION ...... 1

II. BACKGROUND ...... 4

2.1. Overview of aerogel ...... 4

2.2. Syndiotactic polystyrene (sPS) aerogels ...... 8

2.3. Silica aerogels ...... 12

III. MATERIALS AND METHODS ...... 17

3.1. Materials and preparation methods ...... 17

3.1.1. Syndiotactic polystyrene aerogels ...... 17

3.1.2. Silica aerogels ...... 17

3.1.3. Supercritical drying ...... 18

3.2. Characterization methods ...... 21

3.2.1. Bulk density, skeletal density, porosity, and total pore volume ...... 21

3.2.2. Permeability ...... 22

3.2.3. Filtration efficiency ...... 24

3.2.4. Surface area, pore size, and pore size distribution ...... 25

3.2.5. Structures and chemical composition...... 27

vi

3.2.6. Static charge ...... 28

IV. AIRBORNE NANOPARTICLE FILTRATION WITH ELECTROSTATICALLY

ACTIVE AEROGELS ...... 29

4.1. Introduction ...... 29

4.2. Experimental ...... 31

4.2.1. Materials ...... 31

4.2.2. Preparation of modified sPS aerogels ...... 31

4.2.3. Characterization ...... 32

4.3. Results and discussions ...... 34

4.3.1. Silica component in sPS aerogel ...... 35

4.3.2. Filtration performance ...... 43

4.3.3. Internal surface area and pore size distribution ...... 45

4.4 Conclusions ...... 49

V. SYNDIOTACTIC POLYSTYRENE AEROGEL MICROPARTICLES ...... 50

5.1. Introduction ...... 50

5.2. Experimental ...... 51

5.2.1. Materials ...... 51

5.2.2. Preparation of sPS aerogel microparticles ...... 52

5.2.3. Characterization ...... 52

5.3. Results and discussions ...... 53

5.3.1. Gelation time in different solvents ...... 53

5.3.2. Effect of emulsion process on sPS gel particle size distribution ...... 55

5.3.3. Morphology of sPS aerogel microparticles ...... 56 vii

5.3.4. Shrinkage calculation ...... 58

5.3.5. Internal surface area and pore size distribution ...... 60

5.4. Conclusions ...... 61

VI. OVERALL SUMMARY ...... 62

REFERENCES ...... 64

APPENDICES ...... 72

A.1. Viscosity and surface tension of polyvinyl alcohol aqueous ...... 72

A.2. Size distribution of sPS gel microparticles prepared under different emulsion

process ...... 73

viii

LIST OF TABLES

Table Page

2.1 Properties of sPS aerogels ...... 12

2.2 Properties of silica aerogels ...... 16

3.1 Critical conditions of common solvents ...... 20

4.1 Sample designation ...... 34

4.2 Bulk density, skeletal density, surface area, porosity, permeability (k), filtration efficiency (E), and static charge of samples prepared in the work ...... 35

4.2 Weight fraction of silica in hybrid materials calculated from TGA data ...... 39

4.3 Mesopore volume calculated from nitrogen adsorption data and carbon dioxide adsorption data ...... 48

5.1 Gelation time of sPS in different solvents ...... 54

5.2 Effect of stirring speed, surfactant concentration, and dispersed to continuous phase volume ratio on sPS gel particle size ...... 56

5.3 Aerogel microparticles preparation conditions ...... 57

5.4 Shrinkage of aerogel monolith and microparticle due to drying...... 59

ix

LIST OF FIGURES

Figure Page

2.1 Novel aerogels ...... 5

2.2 Schematic diagram of freeze-drying ...... 7

2.3 Diagram showing the relationship between density and enthalpy of different state of matter ...... 7

2.4 Chain conformation of sPS ...... 9

2.5 Toluene forming co-crystals with δ-form sPS chain ...... 10

2.6 Image of sPS aerogel ...... 12

2.7 Hydrolysis and condensation of TEOS ...... 13

2.8 The effect of pH value on morphology of silica network ...... 14

2.9 Bead-necklace structure of silica aerogel ...... 14

3.1 Force balance in a capillary tube ...... 18

3.2 Phase diagram of carbon dioxide ...... 19

3.3 Supercritical dryer used in this project...... 21

3.4 Schematic of Frazier test assembly...... 23

3.5 Frazier tester ...... 23

3.6 Typical permeability plot ...... 24

3.7 BET model of multilayer adsorption ...... 25

4.1 SEM images of sPS and modified aerogel samples ...... 37

4.3 T25S1 sample before and after buring ...... 40

x

4.4 Different response of T25S1 sample and sPS002 sample to electrostatically charged glass slide ...... 41

4.5 FT-IR spectra for sPS002, T25, T25S1, T25S2, and T25S3 aerogels ...... 42

4.6 Filtration efficiency of each sample as a function of SiF3 to TEOS ratio ...... 43

4.7 Permeability of each sample as a function of SiF3 to TEOS ratio ...... 44

4.8 N2 adsorption isotherms ...... 46

4.9 Pore size distribution from N2 adsorption ...... 47

5.1 SEM images of aerogel microparticles ...... 58

5.2 Particle size distribution of sPS gel and aerogel microparticles ...... 59

5.3 N2 adsorption isotherm and pore size distribution of sPS aerogel monolith and microparticles ...... 60

xi

CHAPTER I

INTRODUCTION

Aerogels are a class of materials well known for their excellent characteristics, such as ultralow density (down to 0.16 mg/cm3),1 high specific surface area (more than 1000 m2/g),2 high porosity (up to 99.99%),1 and low thermal conductivity (about 20 mW/m∙K).3

In 1931, Kistler first reported shrinkage-free silica aerogel using supercritical drying.4

According to Kistler, an aerogel is “a gel in which the liquid has been replaced by air, with very moderate shrinkage of the solid network”.4 Since then, many kinds of aerogels have been developed, including metallic, inorganic, and organic aerogels. However, varied materials and different processing methods went beyond that concept. For example, carbon nanotube aerogels were made from multi-walled nanotubes by chemical vapor deposition.1

Currently, an aerogel is not necessarily made from a gel but matter with solid, gel-like network structure and filled with air. The diversity of materials and processing methods make properties of aerogels adjustable, and find great applications for aerogels, such as in filtration,5-7 thermal insulation,3, 8 drug delivery,9-10 energy storage,11-13 and waste treatment.14-16 Research on modification of aerogels for different applications is ongoing.

Syndiotactic polystyrene (sPS) aerogel was first discovered in 2005 by Daniel et al.17

Like other aerogels, sPS aerogels have high specific surface area (300-500 m2/g), and high porosity (up to 97%). Unlike other aerogels, sPS aerogels have a physically linked three- dimensional network, instead of chemically bonded networks as in silica aerogel. When sPS crystallizes, intermolecular physical linking occurs at crystalline junction zones. It has been confirmed that the crystalline phase in aerogels is δ-form, which is believed to be very

1

suitable for capture of small molecules.18 The micropores formed by benzene rings on neighboring polymer chains is effective in the absorption of volatile organic chemicals

(VOC) from air and water, even when these volatiles are present at very low concentrations.19

Although sPS aerogels have many excellent properties, some characteristics such as hydrophobicity, inferior mechanical strength,20-21 and low chemical activity,22 still limit the applications of sPS aerogels. Many efforts have been dedicated to improving these properties in sPS aerogels. One such effort is a hybrid aerogel, prepared by Wang et al. using sPS and silica.20 The sPS gel was soaked in a highly concentrated, hydrolyzed TEOS solution, allowing TEOS to diffuse into sPS macropores. The resultant gel was then transferred into a basic solution where condensation of TEOS took place. In this hybrid aerogel, silica was found crosslinked within the networks formed by sPS strands. The compression modulus increased from 1.7 MPa to 24 MPa while there was no obvious increase in bulk density. Wang, Zhang and Jana reported functionalized sPS by introducing sulfonic acid groups into the polymer backbones.22 In this research, the sulfonating agent was prepared by combining acetic acid and sulfuric acid in a molar ratio of 3:1. The agent was then allowed to react with sPS-chloroform solution at 70 °C, and the sulfonated sPS was precipitated out in deionized water. Aerogels made from the resultant polymer absorbed as much as 80% of its weight in moisture in 3 hours. The hydrophobicity of the functionalized aerogel was altered due to polar sulfonic acid groups. Aniline was polymerized on the surface of this new gel, which greatly expanded the potential of sPS aerogels.

2

This research was focused on further functionalizing sPS aerogels. The research was divided into two parts.

In Chapter IV, the results of investigation on filtration performance of electrostatically active aerogels are presented. sPS gel was made in advance and soaked into a hydrolyzed TEOS solution for 24 hours. TEOS in sPS gel was then allowed to condense in a basic condition. The amount of silica was strictly controlled so that the silica could only form clusters on the surface of polystyrene strands. (3,3,3-trifluoropropyl) silane was used to modify silica and to introduce electrostatic charge into the sample due to polar C-F bonds. Compared with unmodified sPS aerogels or sPS aerogels incorporated with only silica,6 these samples had significant improvement in terms of filtration efficiency without compromising permeability. The highest filtration efficiency was

99.995% with a corresponding permeability of 2.04×10-10 m2. The introduction of electrostatic force was proved to be effective.

In chapter V, sPS microparticles were prepared using an oil-in-water emulsion. This was done to see the practicality of thermo-reversible gelation in emulsion system and to combine the advantages of microparticles and sPS aerogels. The effect of emulsion processing on gel particle size distribution was investigated. Several conventional properties, such as intrinsic surface area, pore size and pore size distribution, were measured and compared with those of monolithic sPS aerogels.

3

CHAPTER II

BACKGROUND

2.1. Overview of aerogel

Aerogels are a class of materials made from gels, in which the solvent component is replaced by air. Aerogels have many excellent properties, such as ultralow density (down to 0.16 mg/cm3),1 high specific surface area (more than 1000 m2/g),2 high porosity (up to

99.99%),1 and low thermal conductivity (about 20 mW/m∙K).3 Aerogels can serve as a drug holder9-10, 23-24 or catalyst support25-26 due to its high porosity with various size of pores.

Continuous and tortuous macropores make it a promising air filter.5-7 Smaller pores (<50 nm) enable aerogels to store energy11-13 and treat waste.14-16

In 1931, Kistler discovered the first aerogel.4 His intention was to remove the solvent from a gel without causing any shrinkage. A supercritical ethanol drying technology was applied to prevent pores from collapsing due to surface tension.

However, this method of preparing aerogels is expensive and time-consuming, making this promising material barely studied in the following 40 years.27-29 It was not until late 1960’s that there were breakthroughs. In 1968, Teichner et al. reported the replacement of water glass with organic precursor (tetramethyorthosilicate), which significantly reduced both time and cost, while making the reaction more controllable.28

Due to this contribution, the mechanism of silica gel formation was resolved. The development of carbon dioxide supercritical drying in 1985 provided more convenience.29

The supercritical condition of carbon dioxide (31.1 °C, 7.4 MPa) is more convenient

4

compared to those of alcohols. while saving time in drying. Although all gels are still prepared in organic solvents and this method requires another step of solvent exchange with carbon dioxide before drying, the time of drying is dramatically reduced from 2-3 days to 8-10 hours.

Benefiting from decades of exploration, increasingly more research was carried out to study different methods of functionalizing aerogels.30-31

Figure 2.1 (a) Hydrophobic carbon fiber aerogels; (b) An ultralight carbon nanotube aerogel standing on a flower (c) A flexible, mechanically strong silica aerogel made from methyltrimethoxysilane; (d) A flexible silica aerogel thermal insulator sheet protecting hands from 2000 °C flame. Reprinted with permission from Ref. 30-33.1

5

The ways of making desired aerogels include varying preparation methods,1, 32-33 reactants,9, 24, 34 catalysts,35-36 and drying conditions.37-38 In 2013, a kind of new material called ‘ultra-fly aerogel’ (UFA) was prepared by Gao et al. via “sol-cryo” method.1

Graphene oxide and carbon nanotube aqueous dispersions were mixed and stirred for 2 hours before being poured into a mold. The prepared material was then freeze-dried and chemically reduced by hydrazine to get the UFA, the density of which was 0.16 mg/cm3, making this material the lightest ever created. Zhao and co-workers used cellulose from bamboo pulp to prepare a new kind of aerogel for drug delivery.24 The drug-loading capacity of pristine cellulose is limited.23 Therefore, polyethylenimine was used as a graft agent to increase the capacity. A highly porous aerogel with many functional groups was obtained. The drug-loading capacity was extraordinarily increased, reaching a maximum of 287.39 mg/g at pH=3. Polyimide aerogels were first synthesized by Rhine et al. as reported in a US patent in 2006.39 Their mechanical properties were weak due to low crosslinking density. Kawagishi et al. used 1,3,5-tris(4-aminophenyl)benzene (TAB), a trifunctional amine, as crosslinker.35 Furthermore, Shen et al. replaced TAB with 2,4,6- tris(4-aminophenyl) pyridine (TAPP) to achieve better dimensional stability because TAPP is stiffer than TAB.36 Although supercritical drying is vital in the processing of aerogels, freeze-drying was instead applied in some special cases. Cai et al. prepared cellulose microparticles via a process of spraying, freezing, and drying.38 As shown in Fig. 2.2, aqueous cellulose was sprayed from a nozzle directly into a liquid nitrogen tank. Water in the particles was frozen immediately so that the 3-D structure was maintained.

6

Figure 2.2 Schematic diagram of freeze-drying. Reprinted with permission from Ref. 38. 2

Some believe the aerogel is not only a useful material, but also a new state of matter, for aerogels fill the density and enthalpy gap between liquid and gas.40

Figure 2.3 Diagram showing the relationship between density and enthalpy of different state of matter. Reprinted with permission from Ref. 27.3 7

The first international symposiums on aerogels (ISA) was held in 1985, which was the first international conference concentrating on aerogels and their development.

Aerogels are now moving towards a new era, bringing more functions to daily life.

2.2. Syndiotactic polystyrene (sPS) aerogels

Many kinds of organic aerogels, such as resorcinol-formaldehyde41 and melamine- formaldehyde aerogels,42 were discovered since Kistler first reported silica aerogels in

1931.4 Similar to silica aerogels, these aerogels are all fabricated from gels and the gels are formed in a solution by chemical reactions. As a result, the gel network is based on chemical bonding.

However, thermo-reversible gels provide another way to make organic aerogel. The gel network was bonded by physical crosslinks at crystalline junctions. In 2006, Daniel and co-workers first reported syndiotactic polystyrene aerogel.17 Syndiotactic polystyrene can be synthesized with the help of Ziegler-Natta catalyst or metallocene to achieve high stereoregularity,43 which contributes to the ability of forming semi-crystals. Different processing or solvents lead to two different crystalline lattices where polymer chains have planar zigzag conformation.44-45 These two kinds of crystals are usually known as the α- form and the β-form. A unique s(2/1)2 trans-trans-gauche-gauche helix, usually designated as δ-form, is obtained from solvent induced crystallization. This conformation leads to a polymer-solvent chelate.17

8

Figure 2.4 Chain conformation of sPS. Left is planar zigzag conformation, and right is s(2/1)2 trans-trans-gauche-gauche helical conformation.4

There is interest in how solvents influence the gelation behavior of sPS.44, 46 For example, in sPS- benzene or toluene solution whose concentration is 0.02 g/mL (0.02 gram of sPS per milliliter of solvent), gelation is completed in several minutes. However, gelation takes about 6 hours to accomplish in tetrahydrofuran for the same concentration.

In chloroform, there is no sign of gelation after 3 days in ambient condition at this concentration. Adding a small amount of toluene in chloroform helps accelerate gelation process,46 which is known as ‘solvent-induced crystallization’.

Many investigations have been conducted on solvent induced crystallization of sPS.

Solvents used in the system include chloroform, tetrahydrofuran, benzene, toluene, carbon tetrachloride, and xylene.44, 47-50 It is believed that solvents play an important role in 9

determining the chain structure in gels.44 All gels formed in poor solvents have trans-planar zigzag conformation whereas the situation may differ in good solvents. At lower concentration of sPS in good solvents, the s(2/1)2 trans-trans-gauche-gauche helical structure or the δ-from is thermodynamically favored while higher concentration results in trans-planar zigzag conformation.51-52

Figure 2.5 Toluene forming co-crystals with δ-form sPS chain. Solvent molecules are housed in the cavities created by adjacent phenyl groups in helical structure. Reprinted with permission from Ref. 44.5

The δ-form attracts most attention among all structures due to its special solvent polymer interaction. Light scattering and IR technology indicate that solvent molecules are housed in the cavities created by adjacent phenyl groups in helical structure.53-55 These cavities are considered to be origins of micropores in sPS aerogels.44 One kind of solvent 10

molecule can be replaced by another kind. However, the replacement happens so fast as if solvent molecules are always housed in cavities created by the phenyl groups.44 It was found that molecular chains remained rigid in sPS/benzene gels when temperature exceeded the sPS crystalline melting point, which suggests that the interaction between phenyl groups in sPS and solvent molecules minimizes the free energy and provides stability.56 Furthermore, the interaction between the solvent and phenyl groups makes it impossible for polymer chains to fold during cooling process, but form a fibrous structure instead.56 The space between fibers becomes macropores after sPS gels are converted into aerogels.

To make an sPS gel, polymer pellets are dissolved in a good solvent at high temperature (at least 100 °C) and then cooled. During cooling process, sPS molecules start packing and form crystals until the entire polymer phase forms the gel network, while the solvent-forming chelates with the phenyl groups minimize the energy and stabilize the system. The detailed preparation steps of sPS aerogels adopted in this work will be explained in Chapter III.

An image and properties of a typical sPS aerogel are presented in Fig. 2.6 and Table

2.1, respectively.

11

Figure 2.6 Image of sPS aerogel (polymer concentration is 0.05 g/mL in tetrahydrofuran).6

Property Value

Bulk density 0.032 g/cm3

Skeletal density 1.056 g/cm3

Internal surface area 379 m2/g

Porosity 97%

Compression modulus 1.7 MPa

Table 2.1 Properties of sPS aerogels with concentration of 0.05 g/mL prepared in tetrahydrofuran and supercritical dried in carbon dioxide.201

2.3. Silica aerogels

Silica aerogel is the first kind of aerogels ever made. In 1931, Kistler prepared silica aerogels with water glass via acid-catalyzed reaction and exchanged solvent with ethanol 12

for supercritical drying.4 However, this method discovered by Kistler was so expensive and time-consuming that new publications on silica aerogel were rarely seen in the following 40 years.27 This situation did not change until the replacement of water glass systems with organic precursor by Teichner’s group in 196828 and the development of carbon dioxide supercritical drying.29

The use of organic precursor, or tetramethyorthosilicate (TMOS)28 and tetraethylorthosilicate (TEOS)57 specifically, is more rapid and controllable compared to water glass. The reaction in aqueous ethanol follows steps shown in Figure 2.7 below, using TEOS as an example.57

Figure 2.7 Hydrolysis and condensation of TEOS. Reprinted with permission from Ref.

57. 7

The above reactions show a strong pH dependence. Cihlář showed that the rate of hydrolysis step decreases quickly with an increase of pH.58 The lowest condensation rate is obtained in the pH range of 1-2.3. A weak basic condition is desired for condensation reactions. Stolarski and co-workers further pointed out the tetraethoxysilane concentration in alcohol should be 15-25 wt% and the ratio of water and oxyethylene group should be

1:1 for optimum condition. 59 13

Figure 2.8 The effect of pH value on morphology of silica network. Reprinted with permission from Ref. 61. 8

Figure 2.9 Bead-necklace structure of silica aerogel. There are two levels of particles in the structure. The secondary particle, also known as the ‘bead’, is formed by aggregation of primary particles. Micropores exist between primary particles while mesopores are formed by secondary particles. Reprinted with permission from Ref. 27.9

The pH value also plays a role in determining the structure of silica network.60 Under acidic conditions, hydrolysis occurs at a higher rate than condensation reaction, resulting 14

in formation of Si-O-Si chains in early stages and subsequent branching and crosslinking during aging. However, in basic conditions, condensation is faster than hydrolysis, which causes TEOS to form spherical particles and eventually crosslink to produce a bead- necklace structure.61

Weak mechanical strength62-63 and inherent hydrophilicity64-66 have limited the applications of silica aerogel. Many investigations were conducted to improve the performance of silica aerogel. Hexamethyldisilazane was used to alter the hydrophilicity of silica aerogels by Yokogawa et al.64 Silica gels were soaked in hexamethyldisilazane solution for 3 hours at 65-68 °C with stirring to get rid of the -OH groups on silica surface.

The modified samples showed significantly higher degrees of hydrophobicity and moisture resistance compared to pure silica aerogels, while density and other properties were maintained. Researchers have focused on the neck region of silica particles for reinforcement of mechanical properties.67-69 In this region, neighboring silica beads connect with each other (See Fig. 2.9). The word ‘neck’ vividly shows the weak nature of this region. Leventis et al. reported the usage of isocyanate reacted with hydroxyl groups on the secondary silica particles.62 Duan and Jana showed significant improvement in compression modulus by reacting silica component with polyhedral oligomeric silsesquioxane.2

To make silica aerogel, TEOS, ethanol, and water were mixed in a certain ratio in acidic condition. Excess amount of ammonium hydroxide solution with pH=9 was then added into the mixture to condense the hydrolyzed TEOS. The detailed preparation steps of silica aerogels will be explained in chapter III.

15

Typical properties of a silica aerogel are shown below.

Property Value

Bulk density 0.003-0.35 g/cm3

Internal surface area 600-1,000 m2/g

Mean pore diameter ~20 nm

Primary particle diameter 2-5 nm

Refractive index 1.0-1.08

Thermal conductivity 0.005-0.03 W/mK

Dielectric constant ~1.1

Sound velocity 100 m/sec

Table 2.2 Properties of silica aerogels.602

16

CHAPTER III

MATERIALS AND METHODS

3.1. Materials and preparation methods

3.1.1. Syndiotactic polystyrene aerogels

The preparation of sPS aerogels consists of two steps, namely the thermo-reversible gelation and the supercritical drying. sPS pellets and its good solvent were added into a vial and screw-capped. The mixture was heated to at least 100 °C and magnetically stirred for 30 minutes to get a uniform solution. The solution was then poured into a disc shape mold with a diameter of 30 millimeters and allowed to age for 6 hours for complete gelation.

The sPS gel was then carefully taken out of the mold and immersed into ethanol and washed for 6 times. After the solvent in gel was completely replaced by ethanol, the gel was supercritically dried in carbon dioxide at 10.3 MPa and 50 °C

3.1.2. Silica aerogels

Preparation of silica gel is based on a hydrolysis-condensation method. For this purpose, two different solutions were prepared. In solution I, tetraethyl orthosilicate, ethanol, and deionized water in a certain molar ratio were added into a beaker. A little of hydrochloric acid was required to adjust the solution pH to 2 for better hydrolysis. The mixture was magnetically stirred for at least 3 hours. Solution II contained ethanol, deionized water, and ammonium hydroxide. The amount of ammonium hydroxide was calculated so that the solution pH was 9. Then, these two solutions were injected into a

17

mold with a diameter of 30 millimeters, in a volume ratio of 1:1. The gelation took one day.

The gels were washed 6 times with ethanol and subsequently supercritically dried in carbon dioxide.

3.1.3. Supercritical drying

There is surface tension at the interface of liquid and air and the surface tension tend to form a minimum surface of the liquid. Shown in Fig.3.1 are four forces at the contact point in a capillary tube, which are adhesive force (FAd), solid-air tension force (FS-A), liquid-air tension force (FL-A), and solid-liquid tension force (FS-L). Both the vertical and horizontal forces must cancel at the contact point due to force balance.

퐹푆−퐿 + 퐹푆−퐴 + 퐹퐿−퐴푐표푠휃 = 0 (1)

퐹퐴푑 + 퐹퐿−퐴푠푖푛휃 = 0 (2)

Figure 3.1 Force balance in a capillary tube. 10 18

When the liquid evaporates, liquid-air tension force (FL-A) continues to drag the surrounding walls, causing distortion and shrinkage if the wall is not strong enough.

However, in supercritical state, the distinction between liquid and air disappears. Hence, there is no surface tension and the structure can be recovered without shrinkage. This can be explained from the phase diagram. Fig.3.2 shows three possible alternate paths from the liquid phase to the gas phase. Path 1 does not work as previously discussed. Path 2 avoids the triple point using freeze drying method, which avoids crossing liquid-air boundary directly. Unfortunately, crossing the solid-air boundary still could contribute to some disruption on the structure and the material could even crack into powders in some extreme cases. Path 3, on the other hand, does not cross any phase boundary, instead passing through the supercritical region. As a result, surface tension effect is eliminated, as well as shrinkage.

Figure 3.2 Phase diagram of carbon dioxide. 11 19

Solvent Critical temperature (℃) Critical pressure (MPa)

Methanol 240 7.9

Ethanol 243 6.3

Acetone 235 4.7

Water 374 22.1

Carbon dioxide 31 7.3

Cyclohexane 279 4.07

Table 3.1 Critical conditions of common solvents. 3

Table 3.1 shows critical points of commonly used solvents. Obviously, carbon dioxide is an ideal solvent for supercritical drying because of its convenient operating condition, low price, and low toxicity.

Before supercritical drying, the solvent within the sample should be completely replaced by acetone or ethanol. The sample is then placed into the chamber of the dryer and the solvent is exchanged six times with carbon dioxide. Carbon dioxide is then heated to supercritical state and kept for an hour. Finally, carbon dioxide is released before taking out the sample.

20

Figure 3.3 Supercritical dryer used in this project. 12

3.2. Characterization methods

3.2.1. Bulk density, skeletal density, porosity, and total pore volume

Two kinds of densities were used in this reseach, namely the bulk density (ρb) and skeletal density (ρs).

Bulk density was obtained from the ratio of mass and volume. The sample was treated as a cylinder for the convenience in calculation of specimen volume.

Skeletal density was obtained from Accupyc 1340 helium pycnometer

(Micromeritics, Norcross, GA). Samples were cut into small pieces and placed in a sample holder. 25 test cycles were carried out automatically by the pycnometer to minimize the error as much as possible. An average value was taken as a result.

21

Porosity (Π) and total pore volume (Vtotal) were calculated from those two densities based on equations (3) and (4).

휌 훱 = (1 − 푏) ×100% (3) 휌푠

1 1 푉푡표푡푎푙 = − (4) 휌푏 휌푠

Total pore volume also equals the sum of volume of micropore, mesopore, and macropore.

3.2.2. Permeability

Permeability was measured by using Frazier tester (Frazier Precision Instrument,

Hagerstown, MD) based on Darcy’s law, shown in equation (5)

퐴 ∆푃 푄 = 푘× × (5) 휇 퐿

In this equation,

Q is the volumetric flow rate, k is the permeability constant, A is the cross-sectional area, μ is the viscosity of air, ΔP is the pressure drop cause by sample, and L is the sample thickness.

The cross-sectional area, air viscosity, and the sample thickness were all treated as constants during tests. Therefore, as per Darcy’s law, Q and ΔP should exhibit a linear relationship with k as the slope of Q vs. ΔP plot. The testing assembly is schematically

22

shown in Fig. 3.4. A sample holder was used to fix the specimen and to prevent leakage due to lateral air flow. All gaps between the sample, sample holder, and bottom plate were sealed with vacuum grease. The entire assembly was then transferred to Frazier tester (see

Fig. 3.5) to get at least 6 pairs of volumetric flow rate and pressure drop data for generating a Q vs. ΔP plot.

Figure 3.4 Schematic of testing assembly. 13

Figure 3.5 (a) Schematic of Frazier tester and (b) actual image of Frazier tester. 14

23

10000

9000

8000

/s)

3 (m

7 7000

- y = 7.7973x - 5788.8

10 × 6000

Flow Flow rate 5000

4000

3000 1200 1300 1400 1500 1600 1700 1800 1900 2000 Pressure drop (Pa)

Figure 3.6 Typical flow rate vs. ΔP plot. The permeability is calculated from the slope.15

3.2.3. Filtration efficiency

Filtration efficiency indicates the ability of capturing particles passing through the filter media. Hence, filtration efficiency (E) is defined in equation (6), where NA is the number of particles before filtration and NB is the number of particles penetrating the filter media:

푁 −푁 퐸 = 퐴 퐵 ×100% (6) 푁퐴

In this work, a TSI-8130 filter tester (TSI Inc., Shoreview, MN) was used to measure filtration efficiency by generating sodium chloride nanoparticles and counting the amount passing the sample. The diameter of sodium chloride particles was between 25-150 nm with an average of 75 nm. The sample was placed in a sample holder and supported by a 24

metal mesh. An O-ring was used to cover the gap the between sample and the sample holder, preventing nanoparticles from passing through. The tester is able to measure the efficiency up to 99.999% and each sample was tested for 10 times to get an average value of the efficiency.

3.2.4. Surface area, pore size, and pore size distribution

Figure 3.7 BET model of multilayer adsorption.16

Surface area was obtained from nitrogen sorption isotherms (TriStar II,

Micromeritics, Norcross, GA) using Brunauer–Emmett–Teller (BET) method.70 BET theory is an extension of Langmuir theory,71 which describes the behavior of single layer molecule adsorption, to multilayer adsorption. It is based on the following three hypotheses:

1. Gas molecules are adsorbed on a solid surface;

2. There are no interactions between each layer;

3. Langmuir theory is still applied to each layer.

The core equation of BET theory is shown below. 25

1 푐−1 푝0 1 푝 = ∙ + (7) 푣( 0−1) 푣 푐 푝 푣 푐 푝 푚 푚

where p and p0 are the equilibrium pressure and saturated pressure, respectively, v is the volume of adsorbed gas, vm is the volume of monolayer adsorbed gas in Langmuir theory, and c is the BET constant, as further described in equation 8 below.

퐸1−퐸퐿 푐 = 푒 푅푇 (8)

where E1 is the heat of adsorption of first layer molecule and EL is the heat of adsorption of second and other layers of molecule.

1 푝0 Equation 7 shows a linear relationship between 푝 and , and the plot of them 푣( 0−1) 푝 푝

푝 is called the BET isotherm. The linear relationship only holds between 0.05< 0<0.35. The 푝 volume of monolayer adsorbed gas (vm) and the BET constant (c) can be calculated from the slope k and y-axis intercept y0, as in equations (9) and (10), respectively.

1 푣푚 = (9) 푘+푦0

푘 푐 = 1 + (10) 푦0

The total surface area can be easily calculated from the volume of monolayer adsorbed gas using Eq. 11, in which V is the molar volume of the adsorbate gas, s0 is the cross-sectional area of one adsorbed gas molecule, and N is Avogadro's number.

푣 푁 푆 = 푚 푠 (11) 푡표푡푎푙 푉 0

The specific surface area is given by Eq. 12, where m is the mass of the sample.

26

푆 푆 = 푡표푡푎푙 (12) 퐵퐸푇 푚

Pore size and pore size distribution are measured using Barrett-Joyner-Halenda (BJH) method. The BJH mode uses the Kelvin equation which describes the relationship between pore size and condensation pressure required to fill the pores. The actual pore size is the sum of the pore sizes calculated from Kelvin equation and the molecular thickness which can be obtained from Halsey equation.72 Therefore, the pore size distribution is obtained from Kelvin equation and Halsey equation with the ratio of the equilibrium pressure and saturated pressure. However, the adsorption of N2 depends largely on the mesopores while the adsorption of CO2 shows the number of micropores more accurately. So, the adsorption data of these two adsorbates should be combined to obtain a complete distribution.

The sample should be cut into small pieces and weighed before loading into a BET sample tube. 12 hours of degassing is required to get rid of residual molecules. The test was carried out in liquid nitrogen (77 K) and 79 data points were recorded automatically for a BET isotherm. Internal surface area, pore size, and BET isotherm were generated by the software automatically. Pore size distribution requires special data fitting using data from both N2 and CO2 adsorption.

3.2.5. Structures and chemical composition

A scanning electron microscope (SEM) was used to examine the internal structure of the sample. A thin, small piece of sample was cut and attached to an aluminum stub using carbon tape. The sample was then coated with silver to increase the conductivity.

Images were taken at an accelerating voltage of 2 kV.

27

A transmitted light microscope (Olympus SZ-PT, Tokyo, Japan) was used to investigate gel particles as well as aerogel particles. Images were taken under the magnification of 10× to 100× and an open source software called ImageJ was used to measure particle diameter.

Fourier transform infrared spectroscopy (Nicolet IS50 FT-IR, Thermoscientific,

Hudson, NH) was used for analyzing chemical bonds. This test was mainly for confirming the existence of SiF3 groups in aerogel specimens. Background was collected before scanning samples. Each sample was scanned 32 times to get an average spectrum.

Thermogravimetric analysis (TGA 2050, TA Instrument, New Castle, DE) measured the weight ratio of silica component in the incorporated aerogels discussed in chapter IV.

About 10 mg of sample was placed on a platinum pan and heated to 800 °C at a rate of

10 °C/min under nitrogen atmosphere.

3.2.6. Static charge

Static charge was measured by an electrostatic field meter (Simco FMX-003,

Hatfield, PA) based on Coulomb's law. The meter was placed one inch away from and perpendicular to the sample. Ten readings were taken from different spots on each sample to determine an average value.

28

CHAPTER IV

AIRBORNE NANOPARTICLE FILTRATION WITH ELECTROSTATICALLY

ACTIVE AEROGELS

4.1. Introduction

It is known that air pollution is a hazard to human health. Many researchers pointed out that airborne particulate matter can lead to breathing difficulties, weakened immune systems, and even lung cancers and cardiopulmonary diseases. 73-75 In 2012, the death of approximate by 7 million people is related to air pollution according to World Health

Organization (WHO).76 The effects of fine and ultrafine airborne particles are even stronger because they can penetrate human body and 50% of them will stay in the alveoli.73 The pathogens residing on the surfaces of the particles could bring more complex biological effects.

One common way to solve the problem above is to remove particles from air streams with a filter. The high efficiency particulate absorption (HEPA) filters, a class of widely used commercial filters available in the market fabricated from fiber mats, have a good efficiency over 99.95% for removing particles with a diameter of around 0.3 μm.77-79

However, removal of particles smaller than 200 nm is a big challenge for HEPAs, although particles with this dimension are more harmful to human as discussed above.

There are four basic mechanisms governing the filtration of airborne particles, which are direct interception, inertial impaction, diffusional deposition, and electrostatic attraction. Direct interception and inertial impaction are responsible for filtration

29

mechanism in HEPA. When particles go with air flow and pass close to the filter medium, they may be intercepted by the filter. In the case of inertial impaction, particles deviate from the streamlines due to much inertia and are captured by the filter. Apparently, these two mechanisms work for large particles. In contrast, diffusional deposition accounts for particles smaller than 0.1 μm. Particles with this size are constantly hit by the air molecules, which deviate the particles from air streamlines and cause them to hit the filter surface.

Different from other mechanisms, electrostatic attraction does not favor a certain particle size. Attraction happens between the charged particles and the oppositely charged filter medium.

Kim and Jana reported a novel use of syndiotactic polystyrene (sPS) aerogels to filter airborne nanoparticles.5 Syndiotactic polystyrene aerogels have large surface area (300-

400 m2/g) and high porosity (up to 97%) with various pore sizes. These features ensure good filtration performance of sPS aerogel. In their work, they showed very high filtration efficiency (>99.95%) when filtering sodium chloride nanoparticles of mean size 75 nm.

While they were able to achieve high filtration efficiencies, this came at the expense of low permeability. In this chapter, as the effects of incorporation of a material element that could generate electrostatic charge inside sPS aerogel monolith were evaluated as means to maintaining high permeability, while providing higher filtration efficiencies. In this context, direct modification of sPS poses a challenge due to its inherent low reactivity.80 In this work, sPS/silica hybrid aerogel monoliths were first fabricated and the -CF3 groups were introduced to the silica component through the reaction of dimethoxy-methyl (3,3,3- trifluoropropyl) silane (denoted as SiF3) with the residual -OH groups from the silica component of the hybrid aerogel. It was envisioned that the attached -CF3 groups would 30

improve the filtration performance of the treated aerogel through the generation of electrostatic charges, which in turn would increase the filtration efficiency through electrostatic attraction. The concentration of TEOS was kept at lower level compared to the work of Wang and Jana to ensure high levels of air permeability.20 Note that Wang and

Jana grew co-continuous networks of silica inside the macropores of sPS aerogel monolith.20 Such co-continuous networks are detrimental to air permeability.

4.2. Experimental

4.2.1. Materials

Tetraethoxysilane (TEOS), ethanol, and ammonium hydroxide were purchased from

Sigma Aldrich (St. Louis, MO). Syndiotactic polystyrene pellets with 98% syndiotacticity and molecular weight 300,000 g/mol were bought from Scientific Polymer Products Inc.

(New York, NY). Tetrahydrofuran (THF) was obtained from Fisher Scientific and dimethoxy-methyl (3,3,3-trifluoropropyl) silane (SiF3) was purchased from Gelest Inc.

(Morrisville, PA).

4.2.2. Preparation of modified sPS aerogels

The preparation of the aerogel cannot be achieved by mixing the silanes and sPS in a solution, as the mechanisms of formation of sPS and silica gels are different. Silica gels are made by chemical cross-linking,28, 57, 81 while sPS gels undergo a thermo-reversible gelation process17 (see Chapter II). A two-step method, which allows silica to polymerize in a sPS matrix was used.20

31

First, sPS (0.1000 g) and THF (5 mL) were added into a vial to make an sPS solution, where the solid concentration was 0.02 g/mL. The mixture was heated to 100 °C and stirred at 400 rpm for at least 20 minutes. After completely dissolving the sPS pellets, the solution was transferred into a disk mold and allowed to gel at room temperature. The gel was then aged for 6 hours before taking out of the mold. The gel was then washed with ethanol for

4 times to remove residual THF from the specimen.

The sPS gels were then soaked in a solution of TEOS, ethanol, and deionized water

(DIW). The TEOS solution was prepared in advance in molar ratio of

TEOS:EtOH:DIW=1:x:5. The value of x=62 and 80, so that the concentration of TEOS in each solution was 0.25 and 0.20 mol/L, respectively. pH of the solution was adjusted to a value of 2 to achieve a maximum hydrolysis rate of TEOS by adding nitric acid.58 The entire system was allowed to stand for 24 hours to let hydrolyzed TEOS diffuse into the pores of sPS gel structures. Subsequently, sPS gels were transferred to an empty container in which excess amounts of ammonium hydroxide solution with pH=9 was added drop by drop. The sPS gels were immersed in this basic solution for another 24 hours during which the condensation of hydrolyzed TEOS took place to produce silica particles. The hybrid gels were then moved to the SiF3 solution and allowed to age for 3 days. The resultant gels were washed with ethanol for 6 times and finally supercritically dried using carbon dioxide at 10.3 MPa and 50 °C.

4.2.3. Characterization

The bulk density (ρb) of aerogel was calculated from the ratio of weight to volume.

The skeleton density (ρs) was obtained from Accupyc 1340 helium pycnometer

32

(Micromeritics, Norcross, GA). The samples were completely dried before testing to ensure that all moisture was removed.

Permeability was measured by Frazier tester (Frazier Precision Instrument,

Hagerstown, MD). Filtration efficiency was determined from TSI-8130 filter tester (TSI

Inc., Shoreview, MN), which generated sodium chloride nanoparticles with an average diameter of 75 nm.

Static charge was measured by an electrostatic field meter (Simco FMX-003,

Hatfield, PA). The device has the ability of detecting static charges up to ±20 kV. Ten readings were made on each sample and an average value was taken as a result.

Pore size distribution and surface area were obtained from nitrogen sorption isotherms (TriStar II, Micromeritics, Norcross, GA) based on Brunauer-Emmett-Teller

(BET) method. The samples were cut into small pieces and weighed before loading into a

BET sample tube. About 12 hours of degassing was required to get rid of residual molecules. The test was carried out in liquid nitrogen (77 K) and 79 data points were recorded automatically for a BET isotherm. Internal surface area, pore size, and BET isotherm were calculated by the software automatically. Pore size distribution required special data fitting using the data from both N2 and CO2 adsorption.

Fourier transform infrared spectroscopy (Nicolet IS50 FT-IR, Thermoscientific,

Hudson, NH) was used for analyzing chemical bonds. This test was conducted mainly to confirm the presence of SiF3 groups in the fabricated hybrid aerogel.

Thermogravimetric analysis (TGA 2050, TA Instrument, New Castle, DE) was used to determine the weight ratio of silica component in the incorporated aerogels. About 10 33

mg of sample was placed on a platinum pan and heated to 800 °C at a rate of 10 °C/min under nitrogen atmosphere.

4.3. Results and discussions

Samples designation is introduced in Table 4.1.

TEOS sPS concentration concentration SiF to TEOS ratio Sample (g/mL) 3 (mol/L)

0 0 sPS002

1:1 T20S1

1:2 T20S2 0.20 1:3 T20S3

0.02 0 T20

1:1 T25S1

1:2 T25S2 0.25 1:3 T25S3

0 T25

Table 4.1 Sample designation.4

34

Bulk Skeletal Surface k Static Porosity Sample density density area (×10-10 E (%) charge (%) (g/cm3) (g/cm3) (m2/g) m2) (kV) 0.030 1.067 2.23 98.889 sPS002 330±11 97.21% - ±0.001 ±0.022 ±0.36 ±0.998

0.039 1.310 2.07 99.956 -0.93 T20S1 372±9 97.02% ±0.003 ±0.010 ±0.15 ±0.034 ±0.03

0.038 1.293 2.03 99.847 -0.72 T20S2 367±15 97.07% ±0.004 ±0.014 ±0.11 ±0.077 ±0.03

0.037 1.293 2.16 99.237 -0.68 T20S3 341±12 97.13% ±0.002 ±0.025 ±0.17 ±0.053 ±0.01

0.036 1.297 2.23 99.098 T20 337±18 97.21% - ±0.001 ±0.015 ±0.21 ±0.129

0.043 1.346 2.04 99.999 -1.04 T25S1 387±13 96.80% ±0.003 ±0.012 ±0.08 ±0.000 ±0.03

0.042 1.353 1.90 99.957 -0.82 T25S2 399±20 96.83% ±0.003 ±0.030 ±0.31 ±0.007 ±0.02

0.040 1.348 2.20 99.880 -0.70 T25S3 351±8 97.02% ±0.001 ±0.017 ±0.24 ±0.028 ±0.01

0.040 1.348 2.18 99.510 T25 344±11 97.04% - ±0.002 ±0.041 ±0.15 ±0.124

Table 4.2 Bulk density, skeletal density, surface area, porosity, permeability (k), filtration efficiency (E), and static charge of each sample.5

As can be seen in Table 4.2, the bulk and skeletal density increased about 0.01 g/cm3 and 0.03 g/cm3 compared to sPS monolith due to addition of silica. Additionally, porosity change is insignificant, which can be visually explained from SEM images shown in Fig

4.1.

4.3.1. Silica component in sPS aerogel 35

The presence of silica component was vital to this project since the electrostatic force was introduced on the surfaces of silica component. SEM images of sPS002, T25, and

T25S1 aerogels are shown in Fig. 4.1. As can be seen in Fig. 4.1 (a), sPS aerogel monolith had fibrillar structures with the diameter of individual strands between ten to twenty nanometers. The macropores with diameter larger than 50 nm were abundant in this case.

In the case of silica incorporated specimens shown in Fig. 4.1 (b), the silica nodules are found attached to sPS strands. Many of them formed aggregates and clusters at the junctions of sPS strands. It is also noted that the silica components existed as discrete nodules instead of forming co-continuous network with sPS. No significant difference is found between Fig. 4.1 (b) and Fig. 4.1 (c), although the latter specimen is further modified

20 by SiF3. Wang and Jana reported hybrid sPS-silica aerogel with a co-continuous structure.

In their work, they used very high concentration of TEOS in solutions and the mass ratio of silica to sPS was varied from 0.97 to 4.2. Macropores of sPS network was partially filled with silica aerogel and the porosity decreased to 92%, which could have a negative influence on air permeability. However, in this work, TEOS concentration was much lower and the silica clusters did not occupy too much volume in the sPS macropores. Therefore, no significant decrease in porosity was observed.

36

Figure 4.1 SEM images of (a) sPS002 aerogel, (b) T25 aerogel, and (c) T25S1 aerogel.17 37

Figure 4.2 (a) TGA curves of each sample. (b) TGA curve of T25S1 sample.

A set of TGA curves is shown in Fig. 4.2 (a). Silicon dioxide has a decomposition temperature of ~1000 °C while sPS decomposes at a high temperature between 400 and

500 °C. Therefore, for every hybrid materials, a residual weight due to SiO2 was observed after all sPS underwent decomposition. Samples with only sPS showed residual weight of

38

zero at 450 °C. For example, the TGA curve of T25S1 sample (Fig. 4.2 b) shows small weight loss around 100 °C because of removal of moisture. A large reduction in weight appears between 400 and 500 °C attributed to the decomposition of sPS. After 600 °C, a plateau is seen indicating the residual weight of silica component in the sample. The mass ratio of silica listed in Table 4.2 was calculated from the TGA data.

Sample Weight fraction of silica (%)

sPS002 0

T20 23.7

T20S1 27.8

T20S2 25.0

T20S3 22.3

T25 38.4

T25S1 41.0

T25S2 37.1

T25S3 39.0

Table 4.2 Weight fraction of silica in hybrid materials calculated from TGA data.6

39

Although silica components were present as discrete particles in sPS aerogels, they formed aggregates when sPS was burnt off. Fig 4.3 shows how the T25S1 sample transformed into a smaller volume silica aggregate. The sample was heated to 800 °C at a rate of 10 °C/min in a furnace and then kept at 800 °C for 1 hour. After heating, the sPS part completely decomposed and left behind the silica part.

Figure 4.3 T25S1 sample before and after buring.18

40

The samples were found to be electrostatically active due to the incorporation of SiF3.

The static charge values of the various aerogels are listed in Table 4.1. Electrostatic charges were significantly developed in SiF3 modified samples while sPS and sPS incorporated with pristine silica samples did not show any reading from the charge detector. The charges varied from -0.68 kV to -1.04 kV in this work. The electrostatic property can also be checked visually by placing a piece of sample under a glass slide which was electrostatically charged by friction in advance. The modified sample (left) and pure sPS sample (right) in Fig 4.4 were placed on a desk. A piece of glass with electrostatic charge was brought closer to both samples. When the distance between glass and the sample was reduced to about 1 mm, the modified sample was attracted upwards, while sPS sample did not move. The sPS sample would not be attracted by the electrostatically charged slide even on contact, indicating the absence of electrostatic charge in such specimens.

Figure 4.4 Different response of T25S1 sample and sPS002 sample to electrostatically charged glass slide. 19

41

Figure 4.5 FT-IR spectra for sPS002, T25, T25S1, T25S2, and T25S3 aerogels. 20

Fig. 4.5 shows FT-IR spectra for sPS002 and T25 group samples. In the first row, due to sPS sample, the strong absorption peaks at 680 and 755 cm-1 indicate aromatic C-H deformation vibration. Aromatic C=C bonds stretching vibration is found at around 1450 and 1500 cm-1. The peaks at 2900 and 3025 cm-1 correspond to aromatic C-H stretching vibration. The significant peak at 1100 cm-1 in the other four spectra is indicative of a Si-

O-Si bond. The Si-OH absorption peaks at 960 cm-1 in the last three spectra are obviously weaker than that in T25 spectrum, indicating that many -OH groups reacted with SiF3. C-

F bond has two characteristic peaks. One is at 1262 cm-1, which is on the left shoulder of

Si-O-Si peak in the last three spectra. Another is at 800 cm-1, which overlaps the Si-O-Si stretching vibration absorption peak (790 cm-1). The intensity of these two peaks shows a positive relationship with SiF3 concentration.

42

4.3.2. Filtration performance

Figure 4.6 Filtration efficiency of each sample as a function of SiF3 to TEOS ratio.21

Fig 4.6 shows filtration efficiency as a function of SiF3 to TEOS ratio for each TEOS concentration. It is seen that the filtration efficiency increased dramatically with the increase of SiF3 ratio, e.g. T20S1, T25S2 and T25S1 shows efficiencies of 99.956%,

99.957%, and 99.999%, respectively. The results show significantly improved filtration efficiencies compared with sPS monolith, whose efficiency was recorded as 98.889%. The results show an improvement in filtration efficiency due to the presence of electrostatic forces. The higher the electrostatic charge, the higher was the efficiency. For example, of

43

the T20 group, specimen with SiF3/TEOS ratio of 1:3 had a static charge of -0.68±0.01 kV and efficiency of 99.237±0.053%. In contrast, the specimen obtained with SiF3/TEOS ratio of 1:1 showed electrostatic charge of -0.93±0.03 kV and efficiency of 99.956±0.034%.

Figure 4.7 Permeability of each sample as a function of SiF3 to TEOS ratio.22

The air permeability shown in Fig 4.7 falls in the range of 10-10 m2 and does not exhibit great loss with an increase of SiF3 or TEOS concentration in this work. The lowest permeability value of 1.90×10-10 was achieved in T25S2 sample, showing only a loss of

14.8% compared with sPS sample. These results are not surprising as the silica component were only found at the junctions of sPS strands as clusters, shown in the SEM images of

Figure 4.1. The main pathway for air transport in the aerogel is through the sPS macropores, 44

which are not significantly blocked by the silica clusters. Therefore, air permeability was not significantly reduced.

4.3.3. Internal surface area and pore size distribution

The internal surface area data are listed in Table 4.1. sPS aerogel shows a surface area of 320 m2/g while all modified samples show slightly larger surface area. It is not surprising that the addition of silica increased the surface area, as silica aerogels themselves have surface area up to 1000 m2/g. However, it is interesting to note that the surface area also increased with the addition of SiF3. We attribute this increase in surface area to the extra mesopores formed by the condensation of SiF3.

Nitrogen adsorption isotherms are presented in Fig. 4.8. Shown in Fig, 4.8 (1), (3) are the original curves generated by BET software while Fig, 4.8 (2), (4) are the shifted

3 curves for clarity. About 30 cm /g of N2 was absorbed by each sample when the relative pressure was close to 0, which indicates the existence of a small amount of micropores.

The narrow hysteresis loops found in each curve in the region of large p/p0 values suggest the presence of mesopores in the sample. The increased size of the hysteresis loops and the shifting of the loops towards the left indicates that the volume fraction of mesopores increased due to the modification step. Fig. 4.9 showed the pore size distribution of specimens in the regime under 100 nm. There was no obvious difference between sPS and modified sPS aerogels in micropores (< 2 nm). The curves of SiF3 modified samples started to depart from the sPS sample after pore diameter larger than 5 nm, and reached a peak at pore diameter of 11 nm, which is in the mesopore region. This is shown qualitatively in

Table 4.3. The values were calculated from nitrogen adsorption data and carbon dioxide

45

adsorption data. The mesopore volume in sPS aerogels was about 0.50261 cm3/g.

Mesopore volume had a slight decrease in T20 and T25 samples compared with sPS aerogels. There could be two reasons. The silica particles were discrete and disconnected by the sPS network, which could lead to a loosely packed silica structure and a reduction of the mesopore volume. And the data were presented in the unit of volume per mass, thus the extra mass could also be responsible for the decrease in mesopore volume, considering the skeletal densities of silica and sPS aerogels are about 1.80 g/cm3 and 1.07 g/cm3 respectively. The mesopore volume reached a highest value of 0.75169 cm3/g in this work in sample T25S2.

Figure 4.8 (1) and (3) are the original N2 adsorption isotherms, and (2) (4) are the shifted isotherms.23

46

Figure 4.9 Pore size distribution from N2 adsorption. Note that the volume (V) showed in y-axis is accumulative volume. 24 47

Sample Mesopore volume (cm3/g)

sPS002 0.50261

T20S1 0.66534

T20S2 0.59340

T20S3 0.55534

T20 0.35484

T25S1 0.72269

T25S2 0.75169

T25S3 0.66324

T25 0.44816

Table 4.3 Mesopore volume calculated from nitrogen adsorption data and carbon dioxide adsorption data.7

48

4.4 Conclusions

Silica particles modified by SiF3 were incorporated into sPS aerogels to produce an electrostatically active hybrid aerogel. The resultant aerogel showed excellent filtration performance. Discrete silica component spreading in sPS network had insignificant effect on porosity, which enabled high air permeability. The -CF3 group introduced on to surface of silica aerogel generated noticeable electrostatic charges, which attracted nanoparticles and improved filtration efficiency to a perfect value (up to 99.999%) with a small permeability loss (less than 10%). N2 and CO2 adsorption confirmed that both internal surface area and meospores were increased after modification.

49

CHAPTER V

SYNDIOTACTIC POLYSTYRENE AEROGEL MICROPARTICLES

5.1. Introduction

Microparticles are particles with diameters between 0.1 and 100 μm.82 It is widely known that materials exhibit different properties when their dimensions are reduced to micro or nano scale. For example, colloidal particles experience less gravitational effects and more surface effects and interactions. Aerogel is a class of material with high surface area, high porosity, and ultra-low density.83-87 Since Kistler invented the first aerogel in

1931,4 it received much attention and development during recent decades.27 Many materials, such as resorcinol and formaldehyde,41 polyurea,62 polyimide,35, 39 and polybenzoxazine,88-89 have been successfully synthesized into monolith aerogels to meet various purposes, including thermal insulation,3, 8 drug delivery,9-10, 24 air filtration,5-6 energy storage,11-13 etc. Due to its unique structure and low solid content, surface tension force due to the evaporation of liquid in ambient conditions can cause irreversible and undesirable deformation of the gel network. Therefore, supercritical drying is required to eliminate the surface tension effects and prevent structural distortion.29 Given the inferior mechanical properties, fabrication of aerogel microparticles using mechanical methods has been proven unsuccessful.

Prior work on synthesis of aerogel microparticles mainly proposed two different preparation methods.34, 38, 90 Method 1 applies freeze drying technology. Micrometer-sized droplets of polymer solutions are first generated using an aerosol method. The generated 50

droplets are quickly frozen in a liquid nitrogen tank so that both the macro and micro structure can be preserved. The frozen particles are then subsequently dried to obtain aerogels.38 In method 2, micrometer-sized droplets were formed in an emulsion system.

Polymer solution is first dispersed into an immiscible liquid phase. The droplet size can be tuned by the surfactant concentration, stirring rate, stirrer shape, or the emulsion phase ratio.

The polymer solution droplets are then turned into gels via conventional gelation process.

Finally, these gel microparticles are supercritically dried to obtain aerogel microparticles.34,

90 Despite different processing methods, a common process flow is evident, whereby microparticles are first formed, gelation occurs, and drying is conducted to obtain aerogel microparticles.

In this work, we demonstrated a method of preparing sPS aerogel microparticles via oil-in-water emulsion. Several good solvents of sPS were used to obtain sPS solution. For the final sPS solvent selection, a mixture of chloroform and toluene was found to be optimum. The sPS solution was used as the dispersed phase in preparation of gel micro- particles. The sPS solution was then dispersed into the aqueous continuous phase, using

PVA as the surfactant. Making micrometer-sized aerogel particles based on chemical reactions via emulsion process was successful.90 The work seeks to explore the feasibility of generation aerogel microparticles from a system that undergoes thermo-reversible gelation.

5.2. Experimental

5.2.1. Materials

51

Syndiotactic polystyrene pellets with 98% syndiotacticity and 300,000 g/mol molecular weight were bought from Scientific Polymer Products Inc. (New York, NY).

Polyvinyl alcohol (Mw=90000 g/mol, 99+% hydrolyzed), chloroform, benzene, toluene, and acetone were obtained from Sigma Aldrich (St. Louis, MO).

5.2.2. Preparation of sPS aerogel microparticles

PVA solution was prepared first. PVA powder was added into 100 mL of water and placed in an oven overnight at 80 °C. 0.1 gram of sPS pellets and 2 mL of solvent were placed in a vial together with 8 mL of PVA solution. The vial was closed tightly and heated to 100°C with stirring at 600 rpm for at least 20 minutes in an oil bath. After complete dissolution of sPS pellets, the vial was transferred to another stirring plate under ambient condition. The mixture was stirred for 10 minutes until a stable emulsion was formed. The mixture was subsequently poured into an ice water bath and allowed to cure for one day to ensure complete gelation. The resultant sPS gel particles were filtered out and washed using acetone for 6 times prior to supercritical drying.

The supercritical drying step differed slightly from a monolithic gel sample. Gel particles were placed in a vial which was covered by a piece of Kimwipe® paper to enable fluid flow in and out of the vial.

5.2.3. Characterization

A scanning electron microscope (SEM, JEOL JSM5310) was used to examine the internal structure of the sample. Microparticles were attached to an aluminum stub using

52

carbon tape. The sample was then coated with silver to increase conductivity. Images were taken at an accelerating voltage of 2 kV.

A transmitted light microscope (Olympus SZ-PT, Tokyo, Japan) was used to investigate the shape and size of both gel and aerogel particles. A magnification of 10× was used and an open source software called ImageJ was used to measure the particle diameter.

Pore size distribution and surface area were obtained from nitrogen sorption isotherms (TriStar II, Micromeritics, Norcross, GA) based on Brunauer-Emmett-Teller

(BET) method. The monolithic samples were cut into small pieces and weighed before loading into a BET sample tube whereas particle samples were loosely enclosed with aluminum foil to prevent loss due to vacuum. Twelve hours of degassing was required to remove the residual molecules. The test was carried out in liquid nitrogen (77 K) and 79 data points were recorded automatically for a BET isotherm. Internal surface area, pore size, pore size distribution, and BET isotherm were generated by the software automatically.

5.3. Results and discussions

5.3.1. Gelation time in different solvents

The gelation time of sPS differs based on the nature of the solvents. sPS gelation is due to crystallization, during which solvent molecules and sPS polymer chains form co- crystals (See Chapter II).44, 48 Therefore, the ease of the solvent in forming co-crystals with sPS determines the final gelation time.

53

On one hand, sPS undergoes a thermo-reversible gelation, where sPS is dissolved in a good solvent at high temperature (at least 100 °C) and then gels during cooling. On the other hand, high temperature weakens the stability of an emulsion due to the increasing insolubility of the surfactant in the aqueous phase. Therefore, an acceptable gelation time under a certain cooling condition is important in this work. Gelation should not take place prior to the formation of a stable and homogenized emulsion. In addition, gelation time should not be too long, in order to reduce the frequency of coalescence of emulsion droplets.

The gelation times of sPS in various solvents are shown in Table 5.1.

Conventional Chloroform Benzene Toluene solvent

Gelation time > 6 hours 4 minutes 5 minutes

Chloroform: Benzene Chloroform: Benzene Chloroform: Benzene Hybrid solvent 7:3 8:2 9:1

Gelation time 15 minutes 20 minutes 59 minutes

Chloroform: Toluene Chloroform: Toluene Chloroform: Toluene Hybrid solvent 7:3 8:2 9:1

Gelation time 17 minutes 31 minutes 90 minutes

Table 5.1 Gelation time of sPS in different solvents. The concentration of sPS in each solution was 0.05 gram per milliliter of solvent. Hybrid solvents were prepared in respective volume ratio. Solutions were heated to 100 °C to dissolve sPS and then cooled down under ambient condition. The time at which solvents started to lose fluidity was taken as the gelation time.8 54

The gelation time of sPS in conventional solvents was either too long (e.g. >6 hours for chloroform) or too short (e.g. 4 minutes for benzene). In view of this, solvents were applied to tune the gelation time. The solvent mixtures, chloroform: benzene = 7:3 and 8:2 and chloroform: toluene = 7:3 and 8:2, both showed acceptable gelation times, in the window of 15-31 minutes.

5.3.2. Effect of emulsion process on sPS gel particle size distribution

A surfactant is used in an emulsion to prevent dispersed phase from coalescence. The effect of surfactant concentration on sPS gel particle size distribution was investigated. The mean particle diameter and standard deviation are listed in Table 5.2. It is interesting to see that the mean particle diameter reached a minimum (34±21 μm) at PVA concentration of

0.5 wt%. We attribute this to the competition of interfacial tension and viscosity. The viscosity almost linearly increased with the concentration while the surface tension reduction was slow after 0.5 wt% of PVA (See Appendix A1). Therefore, we believe that at lower than 0.5 wt% of PVA, surface tension was the dominant factor, and the mean diameter decreased with an increase of PVA concentration. After 0.5 wt% of PVA, the interfacial tension reduction cannot compensate the effect of higher viscosity, thus making viscosity a more dominant factor. Consequently, the mean diameter increased with an increase of PVA concentration.

The effect of stirring rate and the dispersed and continuous phase ratio was also investigated. A higher stirring rate translates into higher energy input into the system, which overcomes the interfacial tension and increases the interfacial area. This translates to smaller droplets. The phase ratio influenced the particle size dramatically, nearly

55

quadrupling the diameter when dispersed/continuous phase ratio was changed from 1:4 to

2:3.

The corresponding particle size distribution histogram and microscope images are presented in Appendix A2.

Surfactant concentration 0.1 wt% 0.3 wt% 0.5 wt% 0.7 wt% 0.9 wt%

Particle size (µm) 98±45 61±37 34±21 72±34 105±58

Stirring rate 800 rpm 1200 rpm 1600 rpm

Particle size (µm) 127±53 69±26 46±23

Dispersed/continuous 1:4 2:3 phase volume ratio

Particle size (µm) 61±37 233±99

Table 5.2 Effect of stirring speed, surfactant concentration, and dispersed to continuous phase volume ratio on sPS gel particle size. 9

5.3.3. Morphology of sPS aerogel microparticles

The aerogel microparticles formed aggregates during supercritical drying. Note that sPS is insoluble in liquid CO2 while PVA has solubility in liquid CO2. Individual particles

56

were scarce, but still present, as shown in Fig. 5.1. The microparticles discussed in this and following sections were made under the conditions listed in Table 5.3.

Surfactant 0.5 wt% PVA

Stirring rate 1200 rpm

Phase volume ratio Dispersed: Continuous = 1:4

Table 5.3 Aerogel microparticles preparation conditions. 10

As can be seen from Fig. 5.1 (a), the microparticles made in this work were nearly spherical, with some surface roughness. This roughness could result from shrinkage in the drying step due to incomplete solvent exchange with carbon dioxide. An enlarged view in

Fig. 5.1 (b) shows that the aerogel microparticles have fibrillar network structure similar to those observed for monolithic sPS aerogels. Some tiny particles with diameters around

200 nm are found on the fractured surface in Fig. 5.1 (c). They are believed to be undissolved due to surfactant not removed during the washing step. A close examination of the skin layer shows that the density of sPS strands is higher at the skin layer (Fig. 5.1

(c)). We attribute this to two-dimensional network formation at the surface during gelation.

A fraction of pores on the surface are mesopores (<50 nm), while the internal pores are mostly macropores (>50 nm).

57

Figure 5.1 SEM images of aerogel microparticles. (a) An individual aerogel microparticle.

(b) Surface of aerogel microparticles. (c) Cross-section of aerogel microparticles. 25

5.3.4. Shrinkage calculation

The shrinkage of both monolith and microparticles is calculated from Eq. 13, in which D is the diameter of aerogel and D0 is the diameter of the gel. In the case of microparticles, D and D0 represents respectively the mean value.

퐷 훿 = (1 − ) ×100% (13) 퐷0

58

The values of D and D0 of monolith were easily measured by a caliper while those of microparticles were statistically obtained from the corresponding particle size distribution. The calculated diameters are listed in Table 5.2. It is seen that the aerogel microparticles showed similar shrinkage values as the monoliths, although the microparticle data cannot be used literally due to the use of mean values of an ensemble of microparticles.

Figure 5.2 Particle size distribution of (a) sPS gel microparticles and (b) sPS aerogel microparticles. 26

Gel diameter (D0) Aerogel diameter (D) Shrinkage (δ)

Monolith 30.0 mm 29.1 mm 3.00%

Microparticle 34.6 μm (mean) 33.5 μm (mean) 3.18%

Table 5.4 Shrinkage of aerogel monolith and microparticle due to drying.11

59

5.3.5. Internal surface area and pore size distribution

Figure 5.3 N2 adsorption isotherm and pore size distribution of sPS aerogel monolith and microparticles.27

Surface area and pore size distribution were measured by N2 adsorption method. The surface area of sPS monolith and microparticles were 255 µm ±9 µm and 261 µm ±7 µm , respectively. It is not surprising to have close values due to similar internal structure as shown in Fig. 5.1. The N2 adsorption isotherms and pore size distribution of microparticles and monolith are presented in Fig. 5.3. Both isotherms show small amount of N2 adsorbed by samples at p/p0=0, which indicates the existence of micropores. Isotherms in the region of 0.9

monoliths, which was obtained from the combination of nitrogen and carbon dioxide adsorption data.

5.4. Conclusions

In this work, oil-in-water emulsion was used to prepare sPS aerogel microparticles.

The gelation time of sPS was tuned by changing the various volume ratios of the selected solvents. The resultant aerogel microparticles had a surface area value close to what sPS aerogel monolith had due to similar internal structure. The skin layers in microparticles significantly increased the mesopore content compared with the monolith.

61

OVERALL SUMMARY

This work focused on the functionalization of sPS aerogels on two accounts. The first part was devoted to making electrostatically active aerogels for air filtration by incorporation of silica particles carrying -SiF3 moieties. In the second part, sPS aerogel microparticles were made through oil-in-water emulsion system, and the effects of shape and size on internal structures and pore size distribution were examined.

In the first part, silica particles were incorporated in sPS gels using a classic two-step hydrolysis-condensation method. Gels were subsequently soaked into SiF3 solution for elimination of Si-OH groups and introduction of SiF3 groups in the aerogel structure.

Electrostatic charge was significantly established in the resultant aerogels. With the help of static force, the filtration performance was improved remarkably, achieving 99.999% filtration efficiency with a little permeability loss compared with unmodified sPS aerogels.

The SEM images showed silica nodules attached to sPS strands. Many of them formed aggregates and clusters at the junctions of these strands, instead of forming co-continuous network with sPS. The addition of silica as well as SiF3 increased the surface area and the amount of mesopores.

In the second part, sPS gel microparticles were prepared in an oil-in-water emulsion via thermo-reversible gelation. Various solvents, including conventional solvents and hybrid solvents, were used to attain a suitable gelation time. The effect of aqueous phase viscosity, stirring speed, and interfacial tension on the particle size distribution were evaluated. The N2 adsorption data showed that the surface area of sPS aerogel monolith and microparticles had no obvious difference, and confirmed similar internal structure.

62

However, the microparticles had more skin layers than the monolith, which contributed to an increase of the mesopore content.

63

REFERENCES

1. Sun, H.; Xu, Z.; Gao, C., Multifunctional, ultra-flyweight, synergistically assembled carbon aerogels. Adv Mater 2013, 25 (18), 2554-60. 2. Duan, Y. N.; Jana, S. C.; Lama, B.; Espe, M. P., Hydrophobic silica aerogels by silylation. J Non-Cryst Solids 2016, 437, 26-33. 3. Baetens, R.; Jelle, B. P.; Gustavsen, A., Aerogel insulation for building applications: A state-of-the-art review. Energ Buildings 2011, 43 (4), 761-769. 4. Kistler, S. S., Coherent expanded aerogels and jellies. Nature 1931, 127 (3211), 1. 5. Kim, S. J.; Chase, G.; Jana, S. C., Polymer aerogels for efficient removal of airborne nanoparticles. Sep Purif Technol 2015, 156, 803-808. 6. Kim, S. J.; Chase, G.; Jana, S. C., The role of mesopores in achieving high efficiency airborne nanoparticle filtration using aerogel monoliths. Sep Purif Technol 2016, 166, 48-54. 7. Raghavan, B., Nanofiber Filter Media for Air Filtration: Experimental and Modeling Study. The University of Akron, Dissertation Abstracts International 2010, 71- 10, 6376.; 225 p. 8. Wang, J.; Kuhn, J.; Lu, X., Monolithic Silica Aerogel Insulation Doped with Tio2 Powder and Ceramic Fibers. J Non-Cryst Solids 1995, 186, 296-300. 9. Abeer, M. M.; Amin, M. C. I. M.; Martin, C., A review of bacterial cellulose-based drug delivery systems: their biochemistry, current approaches and future prospects. J Pharm Pharmacol 2014, 66 (8), 1047-1061. 10. Smirnova, I.; Suttiruengwong, S.; Arlt, W., Feasibility study of hydrophilic and hydrophobic silica aerogels as drug delivery systems. J Non-Cryst Solids 2004, 350, 54-60. 11. Frackowiak, E.; Beguin, F., Carbon materials for the electrochemical storage of energy in capacitors. Carbon 2001, 39 (6), 937-950. 12. Fang, B. Z.; Binder, L., A modified activated carbon aerogel for high-energy storage in electric double layer capacitors. J Power Sources 2006, 163 (1), 616-622. 13. Mirzaeian, M.; Hall, P. J., Preparation of controlled porosity carbon aerogels for energy storage in rechargeable lithium oxygen batteries. Electrochim Acta 2009, 54 (28), 7444-7451. 64

14. Rana, P.; Mohan, N.; Rajagopal, C., Electrochemical removal of chromium from wastewater by using carbon aerogel electrodes. Water Res 2004, 38 (12), 2811-2820. 15. Ahmed, M. S.; Attia, Y. A., Aerogel Materials for Photocatalytic Detoxification of Cyanide Wastes in Water. J Non-Cryst Solids 1995, 186, 402-407. 16. Xu, P.; Drewes, J. E.; Heil, D.; Wang, G., Treatment of brackish produced water using carbon aerogel-based capacitive deionization technology. Water Res 2008, 42 (10- 11), 2605-2617. 17. Daniel, C.; Alfano, D.; Venditto, V.; Cardea, S.; Reverchon, E.; Larobina, D.; Mensitieri, G.; Guerra, G., Aerogels with a microporous crystalline host phase. Advanced Materials 2005, 17 (12), 1515-1518. 18. Sivakumar, M.; Yamamoto, Y.; Amutharani, D.; Tsujita, Y.; Yoshimizu, H.; Kinoshita, T., Study on delta-form complex in a syndiotactic polystyrene/organic molecules system, 1 - Preferential complexing behavior of xylene isomers. Macromol Rapid Comm 2002, 23 (1), 77-79. 19. Manfredi, C.; Del Nobile, M.; Mensitieri, G.; Guerra, G.; Rapacciuolo, M., Vapor sorption in emptied clathrate samples of syndiotactic polystyrene. Journal of Polymer Science Part B: Polymer Physics 1997, 35 (1), 133-140. 20. Wang, X.; Jana, S. C., Synergistic Hybrid Organic-Inorganic Aerogels. ACS Appl Mater Inter 2013, 5 (13), 6423-6429. 21. Wang, X.; Jana, S. C., Syndiotactic polystyrene aerogels containing multi-walled carbon nanotubes. Polymer 2013, 54 (2), 750-759. 22. Wang, X.; Zhang, H.; Jana, S. C., Sulfonated syndiotactic polystyrene aerogels: properties and applications. J Mater Chem A 2013, 1 (44), 13989-13999. 23. Mu, L.; Feng, S. S., A novel controlled release formulation for the anticancer drug paclitaxel (Taxol (R)): PLGA nanoparticles containing vitamin E TPGS. J Control Release 2003, 86 (1), 33-48. 24. Zhao, J. Q.; Lu, C. H.; He, X.; Zhang, X. F.; Zhang, W.; Zhang, X. M., Polyethylenimine-Grafted Cellulose Nanofibril Aerogels as Versatile Vehicles for Drug Delivery. ACS Appl Mater Inter 2015, 7 (4), 2607-2615.

65

25. Guilminot, E.; Fischer, F.; Chatenet, M.; Rigacci, A.; Berthon-Fabry, S.; Achard, P.; Chainet, E., Use of cellulose-based carbon aerogels as catalyst support for PEM fuel cell electrodes: Electrochemical characterization. J Power Sources 2007, 166 (1), 104-111. 26. Smirnova, A.; Dong, X.; Hara, H.; Vasiliev, A.; Sammes, N., Novel carbon aerogel- supported catalysts for PEM fuel cell application. Int J Hydrogen Energ 2005, 30 (2), 149- 158. 27. Du, A.; Zhou, B.; Zhang, Z.; Shen, J., A Special Material or a New State of Matter: A Review and Reconsideration of the Aerogel. Materials 2013, 6 (3), 941-968. 28. Nicolaon, G. A.; Teichner, S. J., On a New Process of Preparation of Silica Xerogels and Aerogels and Their Textural Properties. B Soc Chim Fr 1968, (5), 1900-&. 29. Tewari, P. H.; Hunt, A. J.; Lofftus, K. D., Ambient-Temperature Supercritical Drying of Transparent Silica Aerogels. Mater Lett 1985, 3 (9-10), 363-367. 30. Bi, H. C.; Yin, Z. Y.; Cao, X. H.; Xie, X.; Tan, C. L.; Huang, X.; Chen, B.; Chen, F. T.; Yang, Q. L.; Bu, X. Y.; Lu, X. H.; Sun, L. T.; Zhang, H., Carbon Fiber Aerogel Made from Raw Cotton: A Novel, Efficient and Recyclable Sorbent for Oils and Organic Solvents. Advanced Materials 2013, 25 (41), 5916-5921. 31. Flexible silica aerogel. http://www.aerogel.org/wp- content/uploads/2009/02/mtms-rao.jpg accessed on Apr 27, 2017. 32. Budunoglu, H.; Yildirim, A.; Guler, M. O.; Bayindir, M., Highly Transparent, Flexible, and Thermally Stable Superhydrophobic ORMOSIL Aerogel Thin Films. ACS Appl Mater Inter 2011, 3 (2), 539-545. 33. Mulders, N.; Chan, M. H. W., Heat-Capacity Measurements of He-3-He-4 Mixtures in Aerogel. Phys Rev Lett 1995, 75 (20), 3705-3708. 34. Alnaief, M.; Alzaitoun, M. A.; Garcia-Gonzalez, C. A.; Smirnova, I., Preparation of biodegradable nanoporous microspherical aerogel based on alginate. Carbohyd Polym 2011, 84 (3), 1011-1018. 35. Kawagishi, K.; Saito, H.; Furukawa, H.; Horie, K., Superior nanoporous polyimides via supercritical CO2 drying of jungle-gym-type polyimide gels. Macromol Rapid Comm 2007, 28 (1), 96-100.

66

36. Shen, D. X.; Liu, J. G.; Yang, H. X.; Yang, S. Y., Highly Thermally Resistant and Flexible Polyimide Aerogels Containing Rigid-rod Biphenyl, Benzimidazole, and Triphenylpyridine Moieties: Synthesis and Characterization. Chem Lett 2013, 42 (12), 1545-1547. 37. Sarawade, P. B.; Quang, D. V.; Hilonga, A.; Jeon, S. J.; Kim, H. T., Synthesis and characterization of micrometer-sized silica aerogel nanoporous beads. Mater Lett 2012, 81, 37-40. 38. Cai, H. L.; Sharma, S.; Liu, W. Y.; Mu, W.; Liu, W.; Zhang, X. D.; Deng, Y. L., Aerogel Microspheres from Natural Cellulose Nanofibrils and Their Application as Cell Culture Scaffold. Biomacromolecules 2014, 15 (7), 2540-2547. 39. Rhine, W.; Wang, J.; Begag, R., Polyimide aerogels, carbon aerogels, and metal carbide aerogels and methods of making same. Patent number: US7074880 B2: Jul 11, 2006. 40. Wagh, P. B.; Begag, R.; Pajonk, G. M.; Rao, A. V.; Haranath, D., Comparison of some physical properties of silica aerogel monoliths synthesized by different precursors. Mater Chem Phys 1999, 57 (3), 214-218. 41. Mulik, S.; Sotiriou-Leventis, C., Resorcinol–formaldehyde aerogels. In Aerogels Handbook, Springer: 2011; pp 215-234. 42. Pekala, R. W., Melamine-formaldehyde aerogels. Patent number: US5086085 A: Feb 4, 1992. 43. Ishihara, N.; Seimiya, T.; Kuramoto, M.; Uoi, M., Crystalline Syndiotactic Polystyrene. Macromolecules 1986, 19 (9), 2464-2465. 44. Daniel, C.; Menelle, A.; Brulet, A.; Guenet, J. M., Thermoreversible gelation of syndiotactic polystyrene in toluene and chloroform. Polymer 1997, 38 (16), 4193-4199. 45. Gowd, E. B.; Tashiro, K.; Ramesh, C., Structural phase transitions of syndiotactic polystyrene. Prog Polym Sci 2009, 34 (3), 280-315. 46. Wypych, G., Handbook of solvents. ChemTec Publishing: 2001. 47. Daniel, C.; Avallone, A.; Guerra, G., Syndiotactic polystyrene physical gels: Guest influence on structural order in molecular complex domains and gel transparency. Macromolecules 2006, 39 (22), 7578-7582.

67

48. Daniel, C.; Deluca, M. D.; Guenet, J. M.; Brulet, A.; Menelle, A., Thermoreversible gelation of syndiotactic polystyrene in benzene. Polymer 1996, 37 (7), 1273-1280. 49. Daniel, C.; Guerra, G., Thermal behavior of syndiotactic polystyrene/1,2- dichloroethane gels and stoichiometry of polymer-solvent compounds. Soft Mater 2004, 2 (1), 47-56. 50. Daniel, C.; Guerra, G.; Musto, P., Clathrate phase in syndiotactic polystyrene gels. Macromolecules 2002, 35 (6), 2243-2251. 51. Wang, X. Tailoring of pore structures and surface properties of syndiotactic polystyrene aerogels. Ph. D. Dissertation, University of Akron, 2013. 52. Rastogi, S.; Goossens, J. G. P.; Lemstra, P. J., An in situ SAXS/WAXS/Raman spectroscopy study on the phase behavior of syndiotactic polystyrene (sPS)/solvent systems: Compound formation and solvent (dis)ordering. Macromolecules 1998, 31 (9), 2983-2998. 53. Scherer, G. W., Aging and Drying of Gels. J Non-Cryst Solids 1988, 100 (1-3), 77- 92. 54. Poco, J. F.; Coronado, P. R.; Pekala, R. W.; Hrubesh, L. W., A rapid supercritical extraction process for the production of silica aerogels. Mat Res S C 1996, 431, 297-302. 55. Scherer, G. W.; Gross, J.; Hrubesh, L. W.; Coronado, P. R., Optimization of the rapid supercritical extraction process for aerogels. J Non-Cryst Solids 2002, 311 (3), 259- 272. 56. Guenet, J.-M., Factors influencing gelation versus crystallization in cooling polymer solutions. Trends in polymer science 1996, 4 (1), 6-11. 57. Russo, R. E.; Hunt, A. J., Comparison of Ethyl Versus Methyl Sol Gels for Silica Aerogels Using Polar Nephelometry. J Non-Cryst Solids 1986, 86 (1-2), 219-230. 58. Cihlar, J., Hydrolysis and Polycondensation of Ethyl Silicates .2. Hydrolysis and Polycondensation of Ets40 (Ethyl Silicate-40). Colloid Surface A 1993, 70 (3), 253-268. 59. Stolarski, M.; Walendziewski, J.; Steininger, M.; Pniak, B., Synthesis and characteristic of silica aerogels. Appl Catal a-Gen 1999, 177 (2), 139-148. 60. Dorcheh, A. S.; Abbasi, M. H., Silica aerogel; synthesis, properties and characterization. J Mater Process Tech 2008, 199 (1-3), 10-26.

68

61. Cushing, B. L.; Kolesnichenko, V. L.; O'Connor, C. J., Recent advances in the liquid-phase syntheses of inorganic nanoparticles. Chem Rev 2004, 104 (9), 3893-3946. 62. Ilhan, U. F.; Fabrizio, E. F.; McCorkle, L.; Scheiman, D. A.; Dass, A.; Palczer, A.; Meador, M. B.; Johnston, J. C.; Leventis, N., Hydrophobic monolithic aerogels by nanocasting polystyrene on amine-modified silica. J Mater Chem 2006, 16 (29), 3046-3054. 63. Nguyen, B. N.; Meador, M. A. B.; Medoro, A.; Arendt, V.; Randall, J.; McCorkle, L.; Shonkwiler, B., Elastic Behavior of Methyltrimethoxysilane Based Aerogels Reinforced with Tri-Isocyanate. ACS Appl Mater Inter 2010, 2 (5), 1430-1443. 64. Yokogawa, H.; Yokoyama, M., Hydrophobic Silica Aerogels. J Non-Cryst Solids 1995, 186, 23-29. 65. Ziegler, B.; Mronga, N.; Teich, F.; Herrmann, G., Hydrophobic silica aerogels. Patent number: US5738801 A: Apr 14, 1998. 66. Standeker, S.; Novak, Z.; Knez, Z., Adsorption of toxic organic compounds from water with hydrophobic silica aerogels. J Colloid Interf Sci 2007, 310 (2), 362-368. 67. Meador, M. A. B.; Capadona, L. A., Process for preparing polymer reinforced silica aerogels. Patent number: US8067478 B1: Nov 29, 2011. 68. Obrey, K. A.; Wilson, K. V.; Loy, D. A., Enhancing mechanical properties of silica aerogels. J Non-Cryst Solids 2011, 357 (19), 3435-3441. 69. Randall, J. P.; Meador, M. A. B.; Jana, S. C., Tailoring mechanical properties of aerogels for aerospace applications. ACS Appl Mater Inter 2011, 3 (3), 613-626. 70. Brunauer, S.; Emmett, P. H.; Teller, E., Adsorption of Gases in Multimolecular Layers. Journal of the American Chemical Society 1938, 60 (2), 309-319. 71. Langmuir, I., The adsorption of gases on plane surfaces of glass, mica and platinum. Journal of the American Chemical society 1918, 40 (9), 1361-1403. 72. Halsey, G., Physical adsorption on non‐uniform surfaces. The Journal of Chemical Physics 1948, 16 (10), 931-937. 73. Valavanidis, A.; Fiotakis, K.; Vlachogianni, T., Airborne Particulate Matter and Human Health: Toxicological Assessment and Importance of Size and Composition of Particles for Oxidative Damage and Carcinogenic Mechanisms. J Environ Sci Heal C 2008, 26 (4), 339-362.

69

74. Brook, R. D.; Franklin, B.; Cascio, W.; Hong, Y.; Howard, G.; Lipsett, M.; Luepker, R.; Mittleman, M.; Samet, J.; Smith, S. C., Air pollution and cardiovascular disease. Circulation 2004, 109 (21), 2655-2671. 75. Brook, R. D.; Rajagopalan, S.; Pope, C. A.; Brook, J. R.; Bhatnagar, A.; Diez-Roux, A. V.; Holguin, F.; Hong, Y.; Luepker, R. V.; Mittleman, M. A., Particulate matter air pollution and cardiovascular disease. Circulation 2010, 121 (21), 2331-2378. 76. WHO, G., Switzerland, WHO: 7 million premature deaths annually linked to air pollution. 2014. 77. Reisman, R. E.; Mauriello, P. M.; Davis, G. B.; Georgitis, J. W.; DeMasi, J. M., A double-blind study of the effectiveness of a high-efficiency particulate air (HEPA) filter in the treatment of patients with perennial allergic rhinitis and asthma. Journal of Allergy and Clinical Immunology 1990, 85 (6), 1050-1057. 78. Cheney, W. A.; Spurgin, W. P., Electrostatically enhanced HEPA filter. Patent number: US4781736 A: Nov 1, 1988. 79. Payet, S.; Boulaud, D.; Madelaine, G.; Renoux, A., Penetration and pressure drop of a HEPA filter during loading with submicron liquid particles. Journal of Aerosol Science 1992, 23 (7), 723-735. 80. Dulog, L.; David, K. H., Reactivity of polystyrene and model compounds toward tert‐butoxy radicals. Die Makromolekulare Chemie 1976, 177 (6), 1717-1724. 81. Coêlho, R. d. A. L.; Yamasaki, H.; Perez, E.; Carvalho Jr, L. B. d., The ue of polysiloxane/polyvinyl alcohol beads as solid phase in IgG anti-Toxocara canis detection using a recombinant antigen. Memórias do Instituto Oswaldo Cruz 2003, 98, 391-393. 82. Vert, M.; Doi, Y.; Hellwich, K. H.; Hess, M.; Hodge, P.; Kubisa, P.; Rinaudo, M.; Schue, F., Terminology for biorelated polymers and applications (IUPAC Recommendations 2012). Pure Appl Chem 2012, 84 (2), 377-408. 83. Zhang, G.; Dass, A.; Rawashdeh, A.-M. M.; Thomas, J.; Counsil, J. A.; Sotiriou- Leventis, C.; Fabrizio, E. F.; Ilhan, F.; Vassilaras, P.; Scheiman, D. A., Isocyanate- crosslinked silica aerogel monoliths: preparation and characterization. J Non-Cryst Solids 2004, 350, 152-164.

70

84. Pekala, R., Organic aerogels from the polycondensation of resorcinol with formaldehyde. Journal of Materials Science 1989, 24 (9), 3221-3227. 85. Ahmed, M.; Attia, Y., Multi-metal oxide aerogel for capture of pollution gases from air. Applied thermal engineering 1998, 18 (9), 787-797. 86. Fricke, J., Aerogels and their applications. J Non-Cryst Solids 1992, 147, 356-362. 87. Duan, Y. N.; Jana, S. C.; Lama, B.; Espe, M. P., Reinforcement of Silica Aerogels Using Silane-End-Capped Polyurethanes. Langmuir 2013, 29 (20), 6156-6165. 88. Gu, S.; Jana, S., Effects of Polybenzoxazine on Shape Memory Properties of Polyurethanes with Amorphous and Crystalline Soft Segments. Polymers 2014, 6 (4), 1008-1025. 89. Gu, S.; Li, Z.; Miyoshi, T.; Jana, S. C., Polybenzoxazine aerogels with controllable pore structures. RSC Advances 2015, 5 (34), 26801-26805. 90. Gu, S.; Zhai, C.; Jana, S. C., Aerogel Microparticles from Oil-in-Oil Emulsion Systems. Langmuir 2016, 32 (22), 5637-5645.

71

APPENDIX

A.1. Viscosity and surface tension of polyvinyl alcohol aqueous

Figure A.1 Viscosity and surface tension as a function of PVA concentration.

72

A.2. Size distribution of sPS gel microparticles prepared under different emulsion process

73

Figure A.2 Effect of surfactant concentration on particle size distribution. Surfactant concentrations in each sample are (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7, and (e) 0.9 wt%, respectively.

74

Figure A.3 Effect of stirring rate on particle size distribution. Stirring rates in each sample are (a) 800, (b) 1200, and (c) 1600 rpm, respectively.

75

Figure A.4 Effect of dispersed and continuous phase ratio on particle size distribution. The ratio is 1:4 in (a) and 2:3 in (b).

76