Pmma Carbon Nanotube Nanocomposite Foams for Energy Dissipation Applications Kristin Kynard

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2011 Pmma Carbon Nanotube Nanocomposite Foams for Energy Dissipation Applications Kristin Kynard

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FAMU-FSU COLLEGE OF ENGINEERING

PMMA CARBON NANOTUBE NANOCOMPOSITE FOAMS FOR ENERGY DISSIPATION

APPLICATIONS

KRISTIN KYNARD

A Thesis submitted to the Department of Industrial and Manufacturing Engineering in partial fulfillment of the requirements for the degree of Master of Science

Degree Awarded: Summer Semester, 2011

The members of the committee approve the thesis of Kristin Kynard defended on June 30, 2011.

______Changchun Zeng Professor Directing Thesis

______Okenwa Okoli Committee Member

______Arda Vanli Committee Member

Approved:

______Chuck Zhang, Chair, Department of Industrial and Manufacturing Engineering

______John Collier, Interim Dean, FAMU-FSU College of Engineering

The Graduate School has verified and approved the above-named committee members. ii

For the many blessings God has bestowed upon me: to my mother, Margaret for encouraging to continue; to my sister, Charita for the constant guidance and advice; and my wonderful friends for pushing me and supporting me through all the ups and downs…On to the next chapter

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ACKNOWLEDGEMENTS

I would to acknowledge all of the wonderful people who have provided guidance and motivation during the journey to complete this research. First, I would to thank my committee members. To Dr. Changchun Zeng, my major professor, thank you for your constant guidance and patience during this process. I would also like to thank Dr. Okenwa Okoli and Dr. Arda Vanli for their support and guidance by serving on my committee members. I appreciate the entire faculty and staff of the Department of Industrial and Manufacturing Engineering and High Performance Materials Institute (HPMI) for their support. In particular, I would like recognize Dr. Jin-Gyu Park, Mr. Jerry Horne, Mr. Jim Horne, Mr. Frank Allen and Ms. Judy Gardner for their continued assistance with many aspects of my research. I would like to express my appreciation to National Nuclear Security Administration (NNSA) for providing the financial support for this work. Moreover, I would like to thank those who helped me with various aspects of this project. I would especially like to thank Dr. Zhenhua Chen for help with the foaming procedure and testing.. In addition, thank you to Dr. Jhunu Chatterjee for training me on TA Instruments for mechanical property testing and William Garrett Sullivan for providing training on the Shimadzu AGS-J. Thank you to Nemat Hossieny, Wei Wei (Thomas) Yang, Quyet Do, Chase Knight, Tarik Dickens, Micah McCrary Dennis, and Lambert Parker for training and helping fabricate my nanocomposite and nanocomposite foams. Finally, I would like to show my gratitude to my friends and colleagues at Florida State University, as well as my family for their unwavering support.

TABLE OF CONTENTS

LIST OF TABLES ...... VII LIST OF FIGURES ...... VIII ABSTRACT ...... X

CHAPTER ONE ...... 1 INTRODUCTION...... 1 1.1 BACKGROUND ...... 1 1.2 PROBLEM STATEMENT ...... 1 1.3 OBJECTIVES ...... 2

CHAPTER TWO ...... 3 LITERATURE REVIEW ...... 3 2.1 POLYMER NANOCOMPOSITES ...... 3 2.1.1 CARBON NANOTUBES ...... 4 2.1.1.1 STRUCTURE AND MORPHOLOGY OF CARBON NANOTUBES ..... 4 2.1.1.2 PROPERTIES OF CNTS ...... 6 2.1.2 CARBON NANOTUBE REINFORCED NANOCOMPOSITES ...... 7 2.1.3 PROCESSING METHODS OF POLYMER NANOCOMPOSITES ...... 8 2.1.3.1 SOLUTION BLENDING ...... 9 2.1.3.2 MELT BLENDING ...... 9 2.1.3.3 IN SITU POLYMERIZATION ...... 10 2.2 POLYMER NANOCOMPOSITE FOAMS ...... 10 2.2.1 FUNDAMENTAL PROCESSES OF FOAMING ...... 11 2.2.2 CELL NUCLEATION AND GROWTH...... 11 2.2.3 PHYSICAL FOAMING BY CO2 ...... 14 2.2.4 RETROGRADE VITRIFICATION ...... 15 2.2.5 FOAMING TECHNIQUES: BATCH FOAMING AND CONTINUOUS FOAMING ...... 16 2.2.6 EFFECT OF NANOPARTICLES ON FOAM MORPHOLOGY AND PROPERTIES ...... 17 2.3 COMPRESSIVE AND ENERGY DISSIPATION PROPERTIES OF POLYMER FOAM ...... 17 2.3.1 GENERAL BEHAVIOR OF POLYMER FOAM UNDER COMPRESSION .... 18 2.3.2 TOUGHNESS AS INDICATION OF ENERGY DISSIPATION CAPABILITIES ...... 19 2.3.3 COMPRESSIVE AND ENERGY ABSORBING PROPERTIES OF POLYMER AND POLYMER CNT FOAM ...... 21

CHAPTER THREE ...... 26 EXPERIMENTAL APPROACH ...... 26 3.1 MATERIALS AND RESOURCES ...... 26

3.2 POLYMER NANOCOMPOSITE SYNTHESIS ...... 26 3.3 FOAMING PROCESS ...... 28 3.4 CHARACTERIZATION AND TESTING ...... 28

CHAPTER FOUR ...... 30 RESULTS AND DISCUSSION ...... 30 4.1 NANOCOMPOSITE SYNTHESIS ...... 30 4.2 FOAMABILITY OF POLYMER NANOCOMPOSITES ...... 31 4.2.1 EXPANSION RATIO ...... 31 4.3 MORPHOLOGY OF POLYMER NANOCOMPOSITE FOAMS...... 36 4.3.1 EFFECT OF CNT CONCENTRATION ON FOAM MORPHOLOGY ...... 36 4.3.2 EFFECT OF PROCESSING CONDITIONS ON FOAM MORPHOLOGY ...... 38 4.4 COMPRESSIVE AND ENERGY ABSORBING PROPERTIES OF POLYMER FOAMS ...... 40

CHAPTER FIVE ...... 43 CONCLUSION AND RECOMMENDATIONS ...... 43 5.1 CONCLUSION ...... 43 5.2 RECOMMENDATIONS ...... 43

REFERENCES ...... 45 BIOGRAPHICAL SKETCH ...... 51

LIST OF TABLES

2.1 COMPARISON OF CARBON NANOTUBE PROPERTIES TO TYPICAL FILLERS...... 6

2.2 FACTORS WHICH AFFECT FOAM PROCESSING AT DIFFERENT STAGES ...... 11

2.3 MECHANICAL PROPERTIES OF PU AND PU NANOCOMPOSITE FOAMS ...... 22

4.1 PHOTOOS OF BULK AND FOAMED SAMPLES ...... 32

4.2 EXPANSION RATIOS OF PMMA AND PMMA NANOCOMPOSITE FOAMS AT DIFFERENT FOAMING TIMES AND FOAMING TEMPERATURES ...... 32

4.3 ENERGY ABSORPTION OF PMMA NANOCOMPOSITE FOAMS ...... 40

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LIST OF FIGURES

2.1 ILLUSTRATION OF VARIOUS NANOPARTICLES ...... 3

2.2 ILLUSTRATION OF THE CHIRAL VECTOR AND CHIRAL ANGLE FROM A NANOTUBE ...... 5

2.3 ILLUSTRATION OF THE DIFFERENT SWNT CONFIGURATIONS: ARMCHAIR, ZIG-ZAG, AND CHIRAL ...... 5

2.4 GRAPH OF THE RELATIVE MODULUS (E/EM) VS. FILLER CONTENT FOR PA6 REINFORCED WITH MWNT, ORGANOCLAY, AND SILICA NANOPARTICLES ...... 7

2.5 GRAPH OF THE RELATIVE MODULUS (E/EM) VS. FILLER CONTENT FOR PMMA REINFORCED WITH SWNT, ORGANOCLAY, AND ALUMINA NANOPARTICLES ..8

2.6 SCHEMATIC OF HOMOGENEOUS BUBBLE NUCLEATION ...... 12

2.7 SCHEMATIC OF THE HETEROGENEOUS NUCLEATION THEORY ...... 13

2.8 SCHEMATIC OF NUCLEATING PARTICLE INTERACTION OF GAS AND POLYMER ...... 14

2.9 P–T PHASE DIAGRAM OF FLUID: (1) T, TRIPLE POINT; (2) C, CRITICAL POINT ..15

2.10 TG OF PMMA-CO2 PLOTTED AGAINST GAS PRESSURE. , 2001 WORK, , 2000 WORK. THE CURVE REPRESENTS THE RETROGRADE TREND ...... 16

2.11 STRESS-STRAIN CURVE OF RIGID FOAM IN TYPICAL THREE STAGES UNDER COMPRESSION ...... 18

2.12 BEHAVIOR OF MATERIAL BASED ON TYPE OF AREA UNDER THE STRESS- STRAIN CURVE ...... 20

2.13 COMPARISON OF TENSILE MODULUS OF PS AND PS/CNFS NANOCOMPOSITE FOAMS ...... 23

2.14 COMPARISON OF COMPRESSIVE MODULUS OF PS AND PS/CNFS NANOCOMPOSITE FOAMS ...... 23

2.15 PLOTS OF MECHANICAL PROPERTY RESULTS FOR PMMA AND PMMA/MWNT NANOCOMPOSITES (A) STRESS-STRAIN CURVE, (B) YOUNG’S MODULUS AND (C) COMPRESSIVE STRENGTH ...... 24

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2.16 GRAPH OF THE ABSORPTION COEFFIECIENT AS A FUNCTION OF FREQUENCY FOR PU AND PU NANOCOMPOSITE FOAMS ...... 25

3.1 STEP-BY-STEP OF POLYMER NANOCOMPOSITE SYNTHESIS ...... 26

3.2 (A) THE COMPRESSION MOLDING SET-UP, (B) DETAILED VIEW OF THE COMPRESSION MOLD SET-UP AND (C) EXAMPLES OF MOLD AND PREPARED SAMPLES ...... 27

3.3 BATCH FOAMING SET-UP ...... 28

3.4 SHIMADZU AGS-J MECHANICAL TESTING EQUIPMENT WITH COMPRESSION CLAMP ...... 29

4.1 PMMA CNT SUSPENSION OF 1.0WT% CONCENTRATION ...... 30

4.2 SEM IMAGES OF PMMA CNT NANOCOMPOSITES WITH (A) 0.5%, (B) 1.0%, AND (C) 2.0% CNT ...... 31

4.3 COMPARISON OF EXPANSION RATIO OF PMMA AND PMMA NANOCOMPOSITE FOAMS AT DIFFERENT FOAMING TIMES: (A) 30 SECONDS, (B) 60 SECONDS, AND (C) 90 SECONDS ...... 34

4.4 EFFECT OF WOOD FLOUR CONTENT ON THE EXPANSION RATIO OF PLA FOAMS ...... 35

4.5 MORPHOLOGY OF PMMA NANOCOMPOSITE FOAMED AT 100°C FOR 60 SECONDS: (A) PMMA, (B) PMMA/0.5% CNT, AND (C) PMMA/2.0% CNT ...... 37

4.6 SEM OF a PMMA/2.0% CNT NANOCOMPOSITE. AGGREGATES ARE OBSERVED ...... 38

4.7 MORPHOLOGY OF PMMA AND 0.5WT% NANOCOMPOSITE FOAMS PREPARED AT 835 PSI AND DIFFERENT FOAMING TEMPERATURE AND TIME ...... 39

4.8 TOUGHNESS MEASUREMENT OF THE ENERGY ABSORBED IN PMMA NANOCOMPOSITE FOAMS OF DIFFERENT CNT CONCENTRATION AT DIFFERENT FOAMING TEMPERATURES: (A) 80°C, (B) 100°C, (C) 120°C ...... 41

5.1 LOSS MODULUS OF CNT FILM VERSUS BASELINE EPOXY COMPARED TO TEST FREQUENCY ...... 44

ABSTRACT

Nanomaterials have attracted a great deal of research efforts due to the potential unprecedented properties these materials may provide. Carbon nanotubes (CNTs) are of particular interest because of their exceptional mechanical, thermal and electrical properties. The purpose of this research is to develop poly (methyl methacrylate) (PMMA) carbon nanotubes (CNTs) nanocomposite foams with improved energy dissipation capabilities (toughness). PMMA CNTs nanocomposites were first synthesized by anti-solvent precipitation process (ASP). Nanocomposites with different CNTs concentrations were prepared. The dispersion of the CNTs in the polymer matrix was observed by scanning electron microscopy (SEM). Nanocomposite foams were prepared by a batch process using carbon dioxide as the foaming agent. The foaming

was conducted from the retrograde phase that enabled high CO2 solubility and facilitated formation of foams of high bubble density and small bubble size. The effects of foaming temperature, foaming time and CNTs concentration on the foam expansion ratio was investigated. The morphology of the prepared foams was studied by SEM. The compressive properties of the foams were measured and toughness determined. The nanocomposite foams with 0.5% CNT show improvement in energy absorbing capabilities. Upon further increasing CNT concentration, the capability decreases. Further analysis revealed that this was due to the non-uniform foam morphology in those nanocomposite foams. This in turn resulted in from the mixed nucleation mechanisms because of the insufficient CNT dispersion when foamed from the retrograde phase. Enhancement of CNT dispersion in the matrix is needed in order to improve the uniformity of the foams and realize the potential of these materials.

CHAPTER ONE INTRODUCTION

1.1 Background

Currently, polymeric foams are extensively used in various applications in the automotive, construction, aerospace and many others due to their lightweight, and good thermal and acoustic insulation capabilities. However, polymer foams have lower mechanical properties than bulk polymers [1, 2]. Throughout the years, a need to reinforce the foam materials being produced for durability, sustainability, and longevity has plagued industry. The interest in carbon nanotubes has grown due the great property potential. Carbon nanotubes possess exceptional structural, mechanical, electrical properties in comparison to typical carbon fillers. CNTs are extensively used as fillers because of the exceptional stiffness and strength, and remarkable high aspect ratio and electrical property to enhance the properties present in the materials being reinforced[3]. Polymer nanocomposite foams have great potential for applications in many industries. Interest in these foams gravitates towards the improvement of foam properties by adding small amounts of nanoparticles to create strong, lightweight materials. Interesting experimental research reveals that the combination of functionalized nanoparticles mixed in nanocomposites

fabricated using carbon dioxide (CO2) is introducing a new category of lightweight, low cost, and multifunctional materials. The purpose of this research is to develop polymer nanocomposite foams with high energy absorption capabilities produced through eco-friendly processes and materials.

1.2 Problem Statement

Research involving energy dissipation or energy absorption is important because it affects many areas of everyday life. Energy dissipation is becoming an important feature in today’s industries. This feature deals with a materials ability to withstand impact and act as a barrier against negative forces. By alleviating potential for vibrations within a structure, it allows for

better performance, safety, and reliability that the components can withstand stress. This research is looking to develop a material that possesses high energy dissipation capabilities with high stiffness load bearing properties using polymer nanocomposite foams.

1.3 Objectives

In this research, Poly (methyl methacrylate) (PMMA) carbon nanotube nanocomposite foams was developed as an energy absorbing material using an environmentally benign processing technique. This will allow for longer lasting, tougher products. An increased interest in improving energy dissipating capabilities for products in many industries such as construction, sporting goods, and aerospace is important due to the lower costs in production and raw materials potential. In addition, improving the energy absorbing efficiency within materials will decrease the need for frequent maintenance. The main objective of the study is to prepare PMMA/CNT nanocomposite foams using an environmentally friendly process and study the compressive properties and energy absorbing capabilities of the nanomaterials. To achieve these, the following tasks will be conducted: 1. Synthesize PMMA/CNT nanocomposite 2. Prepare nanocomposite foam using carbon dioxide 3. Study the effect of CNT on foam morphology and properties 4. Study the effect of processing conditions on morphology and properties 5. Investigate the compressive properties and energy dissipation properties of the nanomaterials

CHAPTER TWO LITERATURE REVIEW

2.1 Polymer Nanocomposites

Polymer nanocomposites are becoming important materials for the defense [4], construction, automotive, sporting goods, aerospace, and prosthetics industries because of the structural[5], mechanical[6-12], physical[5, 7, 13], thermal[10, 14, 15], and electrical[13, 15-19] property enhancements compared to pure polymers. The nanocomposite are composed of fillers of nanoscale size, typically nanoparticles such platelets, nanofibers, or nanotubes; the nanoparticles facilitate reinforcement of the weak polymer matrix. Industry is particularly interested in the development of polymer nanocomposite due to the potential of improving the properties by adding small amounts of nanoparticles to create strong, lightweight materials. The definition of a polymer nanocomposites are composed a polymer where one segment, the filler, has nanoscale dimensions. The various types of nanoparticles are shown in Figure 2.1[2, 20]. Platelet nanoparticles are one dimensional, flat-like structure, whereas the most popular nanoparticles are of two dimensional structures of nanofibers and nanotubes. Nanofibers and nanotubes are elongated structures, unlike the third multi-dimensional structures of spherical structures like silica crystals which represent one, two and three dimensional structures on the nanometer scale. Most research is based on the two dimensional carbon nanofibers (CNFs) and carbon nanotubes (CNTs) because of the processing capabilities.

Figure 2.1: Illustration of various nanoparticles 3

2.1.1 Carbon Nanotubes

The discovery of carbon nanotubes has been a breakthrough in research since the discovery by Ijima in the 90’s[21, 22]. Carbon nanotubes (CNTs) possess great potential for changing the landscape of current technology. Due to the extraordinary capabilities of these nanoparticles, major improvements to properties of common materials are widely being researched. CNT have great electrical, mechanical, chemical, thermal, and physical properties which combined with impressive properties in certain polymers could create new materials that are of lightweight and cost effective.

2.1.1.1 Structure and Morphology of Carbon Nanotubes Carbon nanotubes are sheets of graphite rolled into tubes or cylinders [7, 23]. Based on the number of rolled layers of graphitic sheets determines the type of CNT. There are two types of nanotube structures: single-walled carbon nanotubes (SWCNTs) and multi-walled CNTs (MWCNTs). SWCNTs are formed by rolling a sheet of graphene into a cylinder along an (m,n) lattice vector in the graphene plane. The (m,n) indices determine the diameter and chirality, which are key parameters of a nanotube. The atomic structure of the CNT is based on the chirality, or helocity which is the chiral vector and angle. The chiral vector determines the number of rolls along the carbon bonds of the CNT hexagonal lattice. The chirality of the carbon nanotube has major implication on the properties of the nanotubes. Figure 2.2[24] depicts the chiral vector and angle of a nanotube. The tube chirality is known to have a strong impact on the electronic properties of carbon nanotube causing them to be either metallic or semiconductor. A SWCNTs consists of a single rolled sheet of graphite, whereas multiple concentric layers of graphite form a multi-walled carbon nanotube[25]. MWCNTs are concentric SWCNTs which can be of varying hexagonal rings and diameters. These features account for some of difficulty researchers have characterizing MWCNTs compared to SWCNTs.

Figure 2.2: Illustration of the chiral vector and chiral angle from a nanotube

SWCNTs have three variations or configurations: armchair, zig-zag, and chiral as shown in Figure 2.3[26]. As the figure shows, the chiral angle increases from zig-zag, chiral to armchair. These variations are determined by the direction of twist in the nanotube at the chiral angle. The differences between these SWCNT are based on the diameter of the nanotube and the chirality.

Figure 2.3: Illustration of the different SWNT configurations: armchair, zig-zag, and chiral

2.1.1.2 Properties of CNTs

As previously mentioned, carbon nanotubes have impressive properties. Carbon nanotubes exceptional properties are a result of their structure. Based on the atomic arrangement or the way in which the graphitic sheets are formed, the dimensions of the tubes and the morphology determine the property makeup of the specific nanotube. Through theoretical and experimental research, it has been found that carbon nanotubes have extremely high elastic modulus, greater than 1 TPa, which is comparable to the that of a diamond at 1.2 TPa, as well as strength 10 to 100 times stronger than steel at a fraction of the weight [7]. Although there has been amazing results, the relationship between theoretical and experimental testing is lacking. To better depict impressiveness of CNTs, Table 2.1[27] shows how CNT properties compare to other conventional high strength materials such as steel and Kevlar.

Table 2.1: Comparison of CNT Properties to Typical High Strength Materials

Fillers Modulus Tensile Aspect Density Strength Ratio (gm/cm3) (S/cm) (W/m-°K) (GPa) SWNT ~1 TPa ~50 to 500 ≥1000 1.33 (Bundles ) 1750 to 3 x 104 5800 MWNT ~0.5 to 1.2 11 to 63 2 105 >3000 GPa Carbon Up to 800 3.5 ~200 to 1.75 500 to 2000 Fibers GPa 1000 2 x 104 Steel 210 GPa 1.3 <100 7.8 5.8 x 104 54 Kevlar 60 GPa 3.6 <100 1.44 Low 0.04 Graphite 1 TPa 50 3000 (in plane) (in plane) (in plane) :Electrical Conductivity, :Thermal Conductivity

In addition to the exceptional mechanical properties, carbon nanotubes have extraordinary thermal and electric properties. Carbon nanotubes have been found to be thermally stable up to 2800°C in vacuum, along with increased thermal conductivity two times that of a diamond and 1000 times better electric current carrying capacity than copper wires [7].

2.1.2 Carbon Nanotube Reinforced Nanocomposites

The interest in polymer nanocomposites has grown tremendously branching across both thermoset and thermoplastic polymers such as epoxy[4, 12, 16, 28-30], polycarbonate (PC)[31, 32], polystyrene(PS)[18], polyethylene (PE)[33], and poly (methyl methacrylate) (PMMA)[9, 10, 18, 34, 35] to name a few. The attention surrounding nanocomposites stems from the enhanced property capabilities. These enhancements are directly related to several factors such as the properties of the polymer matrix, properties and distribution of the fillers, as well as the synthesis and processing methods [5]. The different types of fillers mentioned in the previous section each have different influences on the behavior of the nanocomposite outcome. Figure 2.4[5] depicts the change in relative modulus of Nylon-6 (PA6) loaded with organoclay, MWNT, and silica nanoparticles[5]. The relative modulus (E/Em) represents the Young’s modulus of the nanocomposite over the matrix. The graphs shows that at lower concentrations of MWNT greatly improved the modulus of PA6 while although a greater concentration of silica nanoparticles were used, increased strength could be observed.

Figure 2.4: Graph of the Relative modulus (E/Em) vs. filler content for PA6 reinforced with MWNT, organoclay, and silica nanoparticles

A similar experiment was performed using the polymer PMMA by reinforcing it with SWNT, organoclay and alumina were the results are displayed in Figure 2.5[5] to observe the influence on the polymer behavior. While the alumina nanoparticles toughened the PMMA

7 comparable to the silica for the PA6, the SWNT and organoclay stiffened the PMMA as the filler content increased [5].

Figure 2.5: Graph of the Relative modulus (E/Em) vs. filler content for PMMA reinforced with SWNT, organoclay, and alumina nanoparticles

2.1.3 Processing Methods of Polymer Nanocomposites

The main issue of fabricating polymer nanocomposite is establishing a good dispersion of CNTs throughout the polymer matrix. This is a difficult task due to the nanotubes tendency to cluster in bundles. Good dispersion of the CNTs is important to realize the exceptional properties of the nanoparticles in the nanocomposites. There are multiple characterization techniques used to evaluate the effectiveness of dispersion. For instance, Raman spectroscopy, Scanning Electron Microscopy (SEM) imaging, and Transmission Electron Microscopy (TEM) are just a few tools. Good processing methods will lead to good dispersion of the CNTs in the polymer matrix. Good dispersion based on the processing technique chosen can optimize the appropriate properties desired for a specific study. The most common methods for nanocomposite processing include solution blending, melt blending and in situ polymerization.

2.1.3.1 Solution Blending The typical method for fabricating polymer nanocomposite is solution blending. Solution blending is the methods of dispersing the CNTs in a selected solvent, combining the CNTs with the polymer and precipitating the solvent from the nanocomposite[13]. The common problem with most processing methods is proper dispersion of the CNTs in solvents. CNTs typically agglomerate or cluster together during processing or after processing is complete causing non- uniform dispersion within the polymer matrix. To address this, typically solution blending is done by sonication or tip sonication which uses sound waves to separate CNT clusters in liquid solvents. Once the sonication is complete, the combined mixture of polymer and CNTs can be rapidly precipitated using a high shear mixture to separate the nanocomposite from the solvent. Choosing a good solvent system is important to the separation of the CNTs due to the weak van der Waals interactions where the polymer chains are able to coalesce with the nanoparticles [2]. Although solution blending is a common synthesis method, it can be costly due to the large amount of solvent required to complete the process. Nonetheless, this method had shown to be effective for many polymer nanocomposite systems including polyimide (PI) [36], polyethylene oxide (PEO)[2, 37], and high density polyethylene (HDPE) [17].

2.1.3.2 Melt Blending Other than solution blending, nanoparticles can be dispersed directly within the polymer material by melt blending. Unlike the previous method, melt blending does not require solvents, thus less expensive. In order for the melt blending technique to work, high temperatures and high shear forces are needed to disperse nanotubes in polymers. However, control of the shear force is essential to not damage the nanoparticles like breaking carbon fibers[2] degrading its properties. Compared to the more common solution blending technique, there have been reports that melt blending is less efficient in dispersing nanotubes in polymer matrices. There have been some effectual examples of melting blending including Jin et al. MWCNT-PMMA composites by melt blending and produced increased storage modulus and glass transition temperature [6, 18]. Melt blending is compatible with polymers which are difficult to dissolve in typical solvents such as polystyrene[2].

2.1.3.3 In situ Polymerization The in situ polymerization involves dispersing CNTs in a monomer followed by polymerization of the solution. In comparison to solution blending, in situ polymerization uses little or no solvent. In addition, this is the only feasible method of processing for thermoset polymers due to its inability to be remolded once cured. The low viscosity of monomer is beneficial for mixing and better dispersion of nanotubes making in situ polymerization an attractive route for nanocomposite synthesis. Generally the process is more complicated and more difficult to implement.

2.2 Polymer Nanocomposite Foams

Foams are defined as materials containing gaseous voids surrounded by a denser matrix, which is usually a liquid or solid[2].. Foams are generally applied in areas for insulating and as barriers to prevent noise and vibrations to get through. Foams are produced from various polymers such as polycarbonate (PC)[38], polystyrene (PS) [39, 40], polyurethane (PU)[41-43] and many others. The polymer poly (methyl methacrylate) will be the main polymer of interest for this research. Classification of polymeric foams depends on the composition, cell morphology, cell structure of open or closed foams, and physical properties of rigid versus flexible foams. In closed cell foams, the cells are separated by cell walls, whereas in open cell foams the cells are interconnected. Typically, rigid foams are utilized for areas of insulation and cushion purposes in automobile and construction industries because of the higher strength and rigidity. On the other hand, flexible foams are in more personal uses in home goods such as furniture and bedding or fashion in shock absorbing in shoes and clothing. Research in uses for polymer nanocomposites foams has taken to new heights in recent decades. The applications of various polymer nanocomposites range from construction projects of bridges and buildings to medical devices and materials such as tissue engineering. Interest in these materials arises because the improved property potential of combining polymer foams with nanoparticles, including enhanced mechanical, electrical and thermal properties.

2.2.1 Fundamental Processes of Foaming

There are two main foaming techniques for processing polymer foams: physical foaming and reactive or chemical foaming. Physical foaming involves the phase separation of the initially dissolved blowing agent from the polymer matrix and formation of porous structure. On the other hand, in chemical foaming, the blowing agent is generated by chemical reactions. Physical foaming is used in the study. An environmentally benign blowing agent, carbon dioxide is used. The basics of physical foaming involve the creation of bubbles and controlling the growth within a material matrix. The foaming procedure involves the following steps: (1) dissolution of foaming agent in the polymer matrix, (2) bubble nucleation, and (3) bubble growth and stabilization.

2.2.2 Cell Nucleation and Growth

Important steps in foaming process are bubble nucleation and growth. The nucleation step involves generation of a new bubble phase from the homogeneous polymer-gas mixture[2]. The growth step is where the bubble nucleus grows into the bubble. The bubble density is greatly affected by the nucleation step. These steps are affected by many factors especially the type of foaming conditions used as seen in Table 2.2[45].

Table 2.2: Factors which Affect Foam Processing at Different Stages

Parameter Implementation Expansion Stabilization Temperature Solubility Volatility Solidification Viscosity Surface Tension Permeability Reactivity Viscosity Diffusivity Interaction Parameter Pressure Solubility Shear hear Solidification − Viscosity − Surface Tension − Homogenization Nucleation Shear Solubility − Nucleation Solidification Dissolution Growth Dispersion Cell Distribution

The foam nucleation can be adequately described by classical nucleation theory. The classic nucleation theory revolves around molecules of the polymer overcome an energy barrier and cluster to form the nuclei of the bubble phase. When the size of the nuclei is less than the critical radius, as the size of the nuclei increase, an increase in free energy occurs. However, if the nuclei size exceeds the critical size, a decrease in free energy occurs as the bubble continues to grow and stable nuclei are created. The formation of bubbles and the nucleation processes generally centers around this classic nucleation theory. The critical bubble radius to cross the threshold shown in Figure 2.6[45] shows the change of energy as the nuclei size increases.

Figure 2.6: Schematic of Homogeneous Bubble Nucleation

Based on the classical nucleation theory[46], the steady state nucleation rate for homogeneous nucleation , N0, is given by [2, 47]: ≈ ∆G ’ ∆ crit ÷ N0 = C f00 exp∆− ÷ (1) « kBT ◊ where Gcrit is the free energy critical nucleus formation, kB is the Boltzmann factor, T is the absolute temperature, C0 is the number of gas molecules dissolved per unit volume of the primary phase, and f0 is the kinetic pre-exponential factor which is loosely dependent on temperature [2]. Nanoparticles are effective nucleation agents, and their effect can be explained by heterogeneous nucleation theory. The existence of the nanoparticles reduces the required

activation energy needed to create a stable nucleus. Figure 2.7[45] shows the reduction in free energy works in heterogeneous nucleation.

Homogeneous Nucleation

Nucleus Radius

Free Energy G*homo G*hetero Heterogeneous Nucleation

Figure 2.7: Schematic of the Heterogeneous Nucleation Theory

The nucleation rate for heterogeneous nucleation is given by: ≈ ∆G het ’ ∆ crit ÷ N1 = C f11 exp∆− ÷ (2) « kBT ◊ where f1 is the frequency factor of gas molecules joining the nucleus and C1 is the concentration

het of heterogeneous nucleation sites. The ∆Gcrit is derived by the following Equation (3) where f() is the heterogeneity factor shown in the configuration of Figure 2.8[2] used to derive the

het heterogeneous nucleation equation from the homogeneous equation[45]. The ∆Gcrit is critical to the importance of using nanoparticles because it changes the activation energy term to account for the increase in nucleation sites created in heterogeneous nucleation. ∆G = ∆Ghet f (θ ) crit crit (3)

Gas Polymer

Nucleating Particle Surface

Figure 2.8: Schematic of nucleating particle interaction of gas and polymer

2.2.3 Physical Foaming by CO2

The use of carbon dioxide (CO2) for foaming processes is highly desirable due its versatility and positive impact on the environment compared to the toxic CFCs. CO2 faces a few issues as a potential blowing agent. First, CO2 has low solubility in most polymers. In addition,

CO2 has a high diffusion rate in polymer as well as high gas thermal conductivity compared to HCFCs [2]. With these challenges, the addition of nanoparticles can be a great asset. The

nanoparticles provide an energy barrier to slow down the CO2 diffusion rate and boost nucleation efficiency. Moreover, surface of the nanoparticles improve the interaction within the gas. Supercritical fluids are a processing fluid where the pressure and temperature reach beyond the critical level as seen Figure 2.9[48]. This fluid has been successfully used as solvents, or antisolvents or plasticizers in polymer processes: e.g. polymer modification, polymer composites, polymer blending, microcellular foaming, and particle production, and in polymer synthesis[48]. Supercritical fluid works as a good solvent under supercritical conditions. The interest in scCO2 is gaining for processing polymers due to the physical properties, as well its low cost, non-flammability and non-toxicity. In addition, the processing conditions are relatively low and obtainable at a critical temperature of 30.85°C and critical pressure of 7.38 MPa.

Figure 2.9: P–T phase diagram of fluid: (1) T, triple point; (2) C, critical point

2.2.4 Retrograde Vitrification A few polymers – PMMA, poly (ethyl methacrylate) (PEMA), acrylonitrile-butadiene- styrene copolymer (ABS), and syndiotactic PMMA (sPMMA) [49-51] exhibit a unique thermodynamic behavior, retrograde vitrification. In the retrograde phase, the solubility in PMMA is exceptionally high. Furthermore, the rubbery state ensures possible foamability. Both will be beneficial for producing foams with high cell density and smaller cell size, which is expected to enhance energy absorbing capabilities. Most researchers investigate to understand the relationship formed between the temperature and pressure applied in a polymer-gas system, especially the glass transition temperature, Tg, and gas pressure, p, respectively. Typically, the

Tg-p system has a linear correlation. For the polymer-CO2 system mentioned earlier, they exhibit two Tgs and polymers are in glassy state between the two Tgs and rubbery outside the Tgs window. The Tg represented in Figure 2.10[50] demonstrates retrograde behavior. For example, a PMMA-CO2 process begins at 0°C at constant 40 atm (~4.05 MPa) pressure, which is consequently heated and transitions to the glass state at 10°C. At this stage it is at the Tg,l which represents the transition of the polymer from the rubber to glass phase in retrograde behavior while heating. As the process continues, the sample will reach its second glass transition temperature, Tg,h at 70°C as it changes from the glass state back to rubber. This unique behavior characterizes the behavior occurs under retrograde vitrification. This behavior can be measured through creep compliance[49, 50], high pressure thermal analysis and diffusion coefficient methods to determine retrograde behavior [50].

Figure 2.10: Tg of PMMA-CO2 plotted against gas pressure. , 2001 work[50], , 2000 work. The curve represents the retrograde trend

2.2.5 Foaming Techniques: Batch Foaming and Continuous Foaming

There are multiple methods of producing foams such as injection molding foaming, which is a non-continuous method. Yet the most common method of non-continuous method is the batch foaming method. Batch foaming process[2] is done by placing the polymer nanocomposite samples in a pressurized vessel and saturating the samples with a blowing agent under specified conditions of temperature and pressure. During the batch foaming process, by raising the temperature above the glass transition temperature, Tg, of the polymer nanocomposite, or using pressure drop after the release of the blowing agent will cause the cell to nucleate and grow. The cell morphology will be set once the samples are cooled below the Ts. On the other hand, if the temperature is lowered below the Tg, the blowing agent will not be able to saturate the specimen and the cells will not nucleate or grow. In addition to non-continuous foaming, continuous foaming is used through the extrusion method. The extrusion foaming process is most common technique because the improved nucleation rate. Extrusion foaming is performed by injecting a foaming gas into an extrusion barrel and combined with the polymer nanocomposite. Unlike batch foaming, the pressure is the main factor of nucleation. Once the combined foaming agent/nanocomposite solution passes through the die a rapid pressure drop occurs which triggers the cell nucleation. Expansion of the foams continues until the temperature of the vessel drops below the Tg.

2.2.6 Effect of Nanoparticles on Foam Morphology and Properties

Polymer nanocomposite foams are the future of lightweight, cost friendly materials. The components used in the fabrication play an important role on the foam morphology and a major influence on the properties of these foams. Many studies have found that the dispersion of the nanoparticles within a host has a major effect on the morphology and properties, as well as the quality. For the dispersion, the number and size of bubbles for nucleation is controlled by the concentration of the foaming agent, hence the uniformity of the cell structure and the cell density are confined to certain methods of mixing the foaming agents and polymer [2]. Nanoparticles are important to foam morphology in achieving good dispersion within the polymer in creation of nucleation sites and foam quality. Nanoparticles cause the cell size of the nanocomposites to reduce because the available gas for bubble growth is lowered as the nucleation process proceeds. In addition, the presence of nanoparticles stimulate a congregation of gas on the polymer-particle interface and increase the potential of nucleating sites [2]. The addition of nanoparticles with a delicate dispersion throughout the polymer matrix facilitates the development of nuclei for the gas phase. Research of nanoparticles fillers is due to the exceptional property enhancement possibilities in polymer matrixes. Multiple properties such as electrical conductivity, thermal stability, mechanical enhancement and strength, and barrier performance can potentially be improved by the high aspect ratio and surface area of the nanoparticles used to fortify the polymers. 2.3 Compressive and Energy Dissipation Properties of Polymer Foam

The study of polymer nanocomposite foams is of increasing interest due to improved property potential, particularly mechanical properties. With polymer nanocomposite foams, the addition of fillers leads to increased nucleation sites. In addition, the fillers aid in reducing the density is important; however it typically hurts the mechanical property [2, 41, 52, 53]. Thus, research is important to bridge the gap between achieving lower densities and lightweight materials with high strength mechanical properties. Carbon nanotubes exhibit wonderful qualities which can enhance polymer matrices to create nanocomposite foams with impressive properties, particularly in the areas of compression and energy absorption.

2.3.1 General Behavior of Polymer Foam under Compression

The typical behavior of polymer foam while compressed is similar to the behavior in tension except for the force being pressed inward while the sample is deformed. Under compression, the compressive stress is computed by: F σ = (4) A0 The compressive strain is calculated by:

l − l ∆l ε = i 0 = (5) l0 l0 where li is the instantaneous length and l0 is the original length before force is applied. Strain can also be computed as d ε = (6) l0 where, d, is the displacement divided by l0, the original length. Performing compression tests occur to show the degree of deformation attributed to the stress to produce a stress-strain curve. The stress-strain curve will depict the material’s tolerance and reaction to deformation. Typical polymer foams exhibit three stages throughout compression: elastic deformation in region I, plateau in region II, and densification in region III as depicted in Figure 2.11 [39].

Figure 2.11: Stress-Strain curve of rigid foam in typical three stages under compression

Region I represent the linear elasticity of the polymer foam which occurs at the smaller strain range. During this phase, there is a comparative relationship between the stress and strain applied to the material. In this phase, the material remains in a recoverable In Region II; the sample begins to plateau and starts the plastic deformation where the transformation becomes permanent followed by the densification. Densification occurs in region III when the material can no longer deform and the cell walls collapse. During this phase, there will be an observable rapid increase in the strain with increased load.

2.3.2 Toughness as Indication of Energy Dissipation Capabilities

Research involving energy dissipation or energy absorption is important because it affects many areas of everyday life. This feature deals with a materials’ ability to withstand impact and act as a barrier against negative forces. In addition, materials need to provide a barrier to prevent vibrations and noise from penetrating other areas. Materials of high dissipating capabilities can be used in many applications such as controlling vibration and noise[54], improving crashworthiness[55], and high impact resistivity. For example, structural materials need to be strong and tough enough to protect forces of nature such as earthquakes and hurricanes. The ability of a product, tool, building, or automobile to withstand damage or protect its consumer from potential harm is crucial. For these products, the need to suppress vibrations and noise, as well as sustain the impact of outside forces from causing catastrophic damage is key. This research is looking to develop a material to handle energy dissipation within the matrix of the polymer nanocomposite. Current products on the market use an external approach such as damping tapes to handle energy absorption. Currently, energy dissipating tapes on the market used as an external bandage to be placed on imperfections. Damping tapes work well to a certain extent. This type of remedy is a temporary fix because often times, the tapes need to be replaced due to debonding[54]. These tapes often require frequent repair and replacement, and are restricted to certain weight and volume limits. Development of energy absorbing materials must also account for the limitations to the performance at high temperatures, low thermal conductivity, poor reliability, compactness issues, and high weight penalty[12]. There is extensive research into the potential applications and uses of polymer nanocomposite. L. Sun et al.[55] focuses on materials with added nanoparticles as well with emphasis on how the nanoparticles affect the stiffness and toughness of a material while improving the mechanical properties and vibrational damping. In addition, 19 the article reviews the mechanisms which cause energy absorption and what influences the changes or improvements with a polymer nanocomposite. These improvements show how the internal placement of nanoparticles can increase the performance of materials for future use in construction, automobiles, sporting goods, and prosthetics. Toughness is a mechanical property with many definitions. This mechanical term can be defined as a material’s ability to oppose fracture due to defects or more commonly used as a material’s capacity to absorb energy and plastically deform[56]. Plastic deformation is the permanent deformation of a material past the yield point and transitions from elastic region to plateau. The behavior of the material as the load is applied within this elastic phase determines the strength of the material; this shows whether the material will be stronger or tougher as seen in Figure 2.12[57]. When the material achieves a steeper rise in strain at higher stress the material is a stronger material, yet brittle. On the other hand, a tougher and more ductile material deforms at longer strains at lower stress.

Figure 2.12: Behavior of material based on type of area under the stress-strain curve

The shaded portion of Figure 2.12 depicts the area under the stress-strain curve. This area is a measurement of the energy absorbed per unit volume of a sample under compression. The area under the stress-strain curve for the energy absorbed can be calculated by equation (7)[57]:

U 0 = —σdε (7) 0 There has been extensive research using many different polymers for nanocomposite, especially the use of poly (methyl methacrylate) (PMMA) nanotube nanocomposite focused on the study of the usefulness of the polymer as a good matrix material[9]. PMMA is a brittle material, and a good candidate to be considered for reinforcement. PMMA is a popular choice as a matrix material based on affordability, ease of use and processing ability, as well as the commercial availability. Gorga et al.[9] worked on improving the mechanical properties of PMMA nanocomposite prepared by melt processing by extrusion. This research, like previous property investigations showed mixed improvements in the matrix materials. Gorga et. al[9] reported a 170% improvement in tensile toughness of 1wt% MWNT PMMA nanocomposite compared to oriented PMMA.

2.3.3 Compressive and Energy Absorbing Properties of Polymer and Polymer CNT Foam

Liu et al.[58] determined that cell wall thickness and the cell density of polymer nanocomposite foams are factors in the foam mechanical properties. However, the cell size has no influence on the compressive behavior. This study examined the biodegradable polymer poly( -caprolactone) (PCL) produced by batch foaming. Based on experimental evidence, the relative compressive modulus of the PCL foam has a correlation to its cell density. As the cell density increases so does the relative compressive modulus. Subsequently, when the cell density decreases so did the relative compressive modulus. Bandarian et al.[43] studied the compressive behavior of PU nanocomposite foams. Table 2.3[43] shows a comparison of the PU nanocomposite foams calculated mechanical properties compared to pure PU foams. The compressive properties of the PU nanocomposite increased the compressive modulus and tensile strength with presence of the nanotubes.

Table 2.3: Mechanical Properties of PU and PU Nanocomposite Foams

Sample Tensile Tensile Compressive Compressive Modulus Strength (Pa) modulus (Pa) plateau (Pa) strength (kPa) PU 660± 13 104±8 125±12 3.2±0.2 PU/CNT-NH 750±8 129±2 142±15 3.7±0.2 PU/CNT-OH 970±8 154±11 176±6 4.6±0.4 PU/CNT-COOH 980±18 156±11 181±21 5.1±0.8

Blumental et al.[8] examined the influence temperature and strain rate have on the compressive behavior of PMMA and PC. This study found that the mechanical properties and behaviors of the PC and PMMA act differently. The PC polymer is limitedly influenced by the strain rate and temperature. The PMMA strength shows linear dependency on temperature and highly influenced by the strain rate. Blumenthal et al. shows the pure PMMA fails under compression at low strain rates and low temperatures (-197°C). Polymer nanocomposite foams have shown impressive property improvements compared to pure polymers. For instance, Lee et al. [2] investigation found that the tensile properties of extruded PS/CNFs nanocomposite foams showed improvement compared to pure PS. Figure 2.13[2] depicts the bulk foams compared to PS/CNFs nanocomposite at similar foam densities of 0.6-0.7 g/cm3. The pure PS showed a weight reduction of 37% thus lowering the tensile modulus by 40% (1.26-0.74 GPa). However, the addition of CNFs raised the tensile modulus with increasing filler concentration by 28% at1 wt% (0.74-0.94 GPa). The reduction in tensile modulus of the PS/CNFs foam is higher that the pure PS foams due to the low densities.

Figure 2.13: Comparison of Tensile modulus of PS and PS/CNFs nanocomposite foams

Furthermore, PS and PS/CNFs foams were fabricated by batch foaming process and the compressive properties were measured. The foams showed improved compressive properties in the presence of CNFs (1 wt% and 5 wt%) as seen in Figure 2.14[2]. The PS foams containing 5 wt% CNFs demonstrated an increase in compressive modulus and a 136% increase in the reduced modulus over the pure PS. Based on the observed results, potential to bridge the gap between the lightweight and high strength requirements is possible with use of nanoparticles reinforcing the polymer matrix.

Figure 2.14: Comparison of compressive modulus of PS and PS/CNFs nanocomposite foams

Chen L. et al.[59] investigated the compressive properties of neat PMMA and PMMA/MWNT. The results are shown in Figure 2.15[59], the nanocomposites show an increase in elastic modulus and compressive strength. The nanocomposites with 1% MWNT100, which are nanocomposites with oxidized MWNT with a large aspect ratio, improved by ~40% in Young’s modulus and ~14% in compressive strength. Yet, the addition of 1% MWNT20 (with smaller aspect ratio) only improved the compressive properties of the nanocomposite by ~8% for the Young’s modulus and ~7% in compressive strength.

(a) (b) (c)

Figure 2.15: Plots of mechanical property results for PMMA and PMMA/MWNT nanocomposites (a) Stress-strain curve, (b) Young’s modulus and (c) Compressive strength

The use of nanocomposite foams for energy absorption mechanism is relatively new alternative and research is at the beginning stages. There are many different forms of energy absorption where nanocomposite foams will be useful, from impact resistance to squelching noise and vibration. Bandarian et al.[43] prepared polyurethane (PU) foams and PU nanocomposite with various surface functionalization to determine acoustic absorption coefficient versus frequency as seen in Figure 2.16[43]. The pure PU foam exhibited a frequency range of 1000 to 2000 Hz which is common of PU foams. The addition of nanotubes increased the acoustic damping capabilities of the PU foams. The PU foams were prepared using different surface functional groups which affected the degree acoustic damping improvement. The PU/CNT-NH achieved a lower loss factor (tan ) compared to the PU/CNT-COOH and

PU/CNT-OH, which is attributed to the existence of micro-voids, which is the potential reason for the increase in acoustic coefficient. The micro-voids create paths for absorbing noise.

Figure 2.16: Graph of the absorption coefficient as a function of frequency for PU and PU nanocomposite foams

CHAPTER THREE EXPERIMENTAL APPROACH

3.1 Materials and Resources

The PMMA used in this research is from Evonik Industries. The multi-walled carbon nanotubes (MWCNTs) (NC3101) were purchased from Nanocyl, Belgium. The nanotubes have more than 95% carbon purity. The nanotubes were synthesized by catalytic carbon vapor deposition (CCVD) and then functionalized with COOH functional group. NC3101 have an average diameter of 9.5 nm and average length of 1.5 m. Dimethylformamide (DMF) supplied by EMD Chemicals was the solvent used in the processing of PMMA and CNTs. The blowing agent used was a bone dry CO2 (99% pure, Air Gas). Ethylene glycol supplied by (VWR International) was as the heating fluid for foaming.

3.2 Polymer Nanocomposite Synthesis

The polymer nanocomposite is prepared using the anti-solvent precipation method as shown in Figure 3.1. PMMA is dissolved in DMF under heating and magnetic stirring. Separately, CNTs are dispersed in DMF using ultrasonication for eight hours. The two mixtures are then combined, stirred for 30 minutes to ensure proper integration of the CNTs and PMMA. This is followed by another 30 minute sonication. The nanocomposite solution is then slowly added to deionized (DI) water under high shear mixing at 5000 rpm to precipitate the nanocomposite. The final suspension is placed on a hotplate and heated to separate the nanocomposite from the DMF and DI water.

Figure 3.1: Step-by-step of polymer nanocomposite synthesis 26

The nanocomposite is then placed in a vacuum oven to remove residual solvent. The dried nanocomposites are then compression molded using a hot press at 220°C to produce samples for the foaming process (Figure 3.2). Once the hot press reaches this temperature, it is compressed for three separate series of at three pressure levels: 3,000 lbs, 6,000 lbs, and 12,000lbs. The polymer plates are then laser cut into small samples for foam preparation.

(a) (b)

(c)

Figure 3.2: (a) The compression molding set-up, (b) Detailed view of the compression mold set- up and (c) Examples of mold and prepared samples

3.3 Foaming Process

The batch foaming method was used to prepare the foam. The process utilized CO2 as the blowing agent supplied to the system by a Teledyne ISCO high pressure syringe pump at 835 psi. The system (shown in Figure 3.3) was set at room temperature and ran for 48 hours during which CO2 was dissolved in the PMMA. Pressure was then released and samples are taken out and placed in a glycerin bath at different temperature and time to prepare the foams. The foams are then immersed in ice water mixture to freeze the foam morphology. The samples were free foamed at three different foaming temperatures of 80°C, 100°C and 120°C; as well as three different foaming times of 30 seconds, 60 seconds, and 90 seconds.

Figure 3.3: Batch foaming set-up

3.4 Characterization and Testing

Characterization of the morphology of the polymer nanocomposites and nanocomposite foams is performed using the JOEL (JSM-7401 F) scanning electron microscope (SEM) to observe the CNT dispersion in the nanocomposites and cell morphology of the nanocomposites

28 and foams. Samples were fractured in liquid nitrogen and then sputter coated with Au/Pd. The SEM accelerating voltage was set at 5 kV with a working distance between 6 mm to 7.8 mm. Shimadzu AGS-J tester (Figure 3.3) was used to measure the compressive properties. The samples were laser cut into squares of 5.5 mm x 5.5 mm dimensions. Due to the limitations of achievable sample size, sample dimensions do not follow ASTM standards. Nonetheless all samples used in the study have the same dimensions, thereby allowing the in-group investigation of the effect of processing conditions and CNT concentration on the foam properties. Compression tests were performed in static single cycle mode with 1 mm/min crosshead speed. Load and displacement data was recorded to obtain the stress-strain relationship. The area underneath the stress-strain curve (before the sharp rise of stress) is calculated and toughness determined.

Figure 3.4: Shimadzu AGS-J mechanical testing equipment with compression clamp

CHAPTER FOUR RESULTS AND DISCUSSION

4.1 Nanocomposite Synthesis

The PMMA and PMMA nanocomposite were synthesized by ASP. The nanocomposites were prepared using three concentrations of CNT, 0.5wt%, 1.0wt%, and 2.0wt%. CNTs agglomerate or cluster together due to the strong van der Waals interaction and large surface area. In order to achieve the impressive properties of the nanoparticles within the polymer, good dispersion of the nanoparticles in the polymer matrix must be achieved. Choosing a good solvent system is one factor which aids in achieving good dispersion. Thus, DMF was chosen because it was one of the best solvents for dispersing CNTs that also can dissolve PMMA. Furthermore, high power sonication was used to facilitate the CNT dispersion. The CNTs were well dispersed by the current method. Figure 4.1 shows a sample of CNT dispersion with DMF after an eight hour sonication. The stable dispersion of CNT was maintained after a long period of time.

Figure 4.1: PMMA CNT suspension of 1.0wt% concentration

The CNT dispersion is further characterized by SEM. Figure 4.2 shows the SEM micrographs of 0.5%, 1.0%, and 2.0% nanocomposites. The CNT is well dispersed in the polymer matrix.

(a)

(b)

(c)

Figure 4.2: SEM images of PMMA CNT nanocomposites with (a)0.5%, (b)1.0%, and (c)2.0% CNT

4.2 Foamability of Polymer Nanocomposites

4.2.1 Expansion Ratio

Four PMMA nanocomposites (0wt%, 0.5wt%, 1.0wt%, and 2.0wt%) are produced using a range of different processing conditions to examine their foamability. These nanocomposites were subjected to three foaming temperatures at 80°C, 100°C and 120°C. For each temperature,

31 the nanocomposites are foamed at three different foaming times of 30, 60, and 90 seconds. These parameters are used to study the effects of processing conditions and CNT concentration on the foamability of the PMMA nanocomposites. Table 4.1 shows the samples before and after being foamed at different conditions. The expansion ratio is determined by the measuring the sample dimensions before and after foaming. The results are shown in Table 4.2 and Figure 4.3.

Table 4.1: Photos of Bulk and Foamed Samples

Sample PMMA 0.5% 1.0% 2.0% Time 80°C 30 seconds

60 seconds

90 seconds

Time 100°C 30 seconds

60 seconds

90 seconds

Table 4.1: Photos of Bulk and Foamed Samples Continued Time 120°C 30 seconds

60 seconds

90 seconds

Table 4.2: Expansion Ratios of PMMA and PMMA Nanocomposite Foams at Different Foaming Times and Foaming Temperatures

Temperature Concentration 80 100 120 Time (seconds) 30 60 90 30 60 90 30 60 90 PMMA 3.1948 4.3745 3.7022 5.9927 7.6443 6.1932 6.0873 4.5699 3.7068 0.5wt% 3.3450 3.6734 3.6151 6.3853 7.8534 6.4864 3.7904 2.9117 2.5158 1.0wt% 3.1978 3.1300 3.9645 5.1336 6.0867 5.4563 3.2792 3.1103 2.5228 2.0wt% 2.8722 3.2931 3.3186 4.6746 4.6139 4.8562 3.5528 2.3711 2.5873

The data in the table shows the highest expansion ratio occurs when 0.5% CNT nanocomposite is foamed at 100°C for 60 seconds. As the concentration of nanoparticles increases, the expansion ratio of the foams decreases. The decrease in the expansion ratio occurs at all foaming temperatures.

7 6 5 4 PMMA 3 0.5wt% 2 1.0wt% Expansion Expansion Ratio 1 2.0wt% 0 80 100 120 Foaming Temperature (°C) (a)

9 8 7 6 5 PMMA 4 0.5wt% 3 1.0wt%

Expansion Expansion Ratio 2 2.0wt% 1 0 80 100 120 Foaming Temperature (°C) (b)

6 5

4 PMMA 3 0.5wt% 2 1.0wt% Expansion Expansion Ratio 2.0wt% 1

0 80 100 120 Foaming Temperature (°C) (c)

Figure 4.3: Comparison of Expansion Ratio of PMMA and PMMA nanocomposite foams at different foaming times: (a) 30 seconds, (b) 60 seconds, and (c) 90 seconds

The observations in this study are similar to the study presented by Matauna and Faruk [63] which studied the effect of wood flour on the foamability of poly(lactic acid) (PLA). The study found that the use of wood flour greatly reduced the expansion ratio of the PLA foams with increased concentration of the additives as seem in Figure 4.4[63]. The effect of foaming time is also studied. At lower foaming temperature, the expansion ratio increases slightly when foaming time increases. On the other hand, the expansion ratio decreases at higher foaming temperature with longer foaming time. This is because at high temperatures, the polymer viscosity (and rigidity) is low, while gas diffusion is high. This leads to rapid diffusion of gas in the foam cells out of the materials, and partial collapse of the porous structure.

Figure 4.4: Effect of the wood flour content on the expansion ratio of PLA foams

4.3 Morphology of Polymer Nanocomposite Foams

4.3.1 Effect of CNT concentration on Foam Morphology The effect of CNT on the foam morphology is investigated. Shown in Figure 4.5 are SEM micrographs showing the morphology of pure PMMA and PMMA CNT nanocomposite foam prepared under the same conditions. The pure PMMA foam generally exhibits relatively uniform cell size, whereas the PMMA nanocomposite foams display a bimodal cell size distribution. Small amount of large bubbles are inter-dispersed between large amount of small bubbles. This bimodal cell size occurrence resembles the observed findings in [64] for PMMA CNT nanocomposite foams prepared at higher temperature (120°C). Bimodal cell size distribution resulted from inadequate dispersion of the CNT in the polymer matrix and the presence of polymer and particle rich regions. In the particle abundant region, heterogeneous nucleation is favored and many bubbles grow simultaneously, therefore the growth of larger sized bubbles is less likely. On the other hand, in the polymer rich regions homogeneous nucleation takes place. Due to the high nucleating free energy, smaller amount of bubbles can nucleate, and each of them grows to a larger size. The nanocomposite preparation used in this study resulted in good dispersion of the CNT[64], yet a bimodal cell size distribution was observed in the nanocomposite foams produced through the retrograde phase. Although bimodal cell size distribution is similar to that in [64], the solubility and nucleation rate, as well as the cell density of the resulting bubbles differ. The foams cell density in the current study is three to four orders of magnitude higher than the previous work. The bimodal response can be attributed to the existence of inter-particle region and higher homogeneous nucleation rate in this region under the foaming conditions in this study. Thus, in the previous study it is possible to suppress the homogeneous nucleation under the foaming conditions (120°C at 13.8-16.5 MPa), due to lower gas solubility; in the current study, the high solubility and homogeneous nucleation leads to formation of bubbles in the inter-particle region. Suppression of homogeneous nucleation to achieve uniform cell size distribution would require further reduction of inter-particle distance.

(a)

(b)

(c)

Figure 4.5: Morphology of PMMA nanocomposite foamed at 100°C for 60 seconds: (a) PMMA, (b) PMMA/0.5% CNT, and (c) PMMA/2.0% CNT

The PMMA nanocomposite in this investigation exhibits good dispersion at higher concentrations as seen in Figure 4.6. Although there is good dispersion at increased concentration, there are still obvious CNT rich and polymer rich regions, which will cause a competition for cell nucleation. Moreover, cell nucleation could occur more than once. The retrograde behavior exists when the nanocomposite is saturated at a constant pressure and

37 temperature near the glass transition temperature. Nucleation might have taken place during the release of pressure before sample is foamed in the oil bath. Secondary foam takes place when the sample is heated, during which the bubbles formed earlier will grow into bigger cells.

Figure 4.6: SEM of a PMMA/2.0% CNT nanocomposite. Aggregates are observed

4.3.2 Effect of Processing Conditions on Foam Morphology The foam morphology of the PMMA nanocomposite foams showed a bimodal response with increasing CNT concentration. Further studies in the processing conditions on foam morphology were conducted. The pure PMMA and PMMA nanocomposites were saturated at a constant 835 psi at room temperature and foamed at different foaming times and foaming temperatures (Figure 4.7). The pure PMMA foam generally exhibits relatively uniform cell size. As the foaming temperature increases the cell size decreases. Within the same foaming temperature range, the cell size decreases at increased foaming time. As the foaming temperature increases from 80°C to 100°C, SEM of the pure PMMA foams show increase in cell size at 60 seconds foaming time. The 0.5% CNT nanocomposite foams prepared at different conditions display a bimodal cell size. This bimodal distribution displays more inter-dispersed large bubbles with increased foaming temperature and foaming times. The smaller bubble sizes are more apparent in the foams processed at 80°C and 30 seconds. The foam has a more uniformed pattern. Yet, all the foams show varying bubble growth of various size small, medium and large bubbles.

PMMA 80°C 30 seconds 60 seconds

100°C

0.5%

30 seconds 60 seconds

80°C

100°C

Figure 4.7: Morphology of PMMA and 0.5wt% nanocomposite foams prepared at 835 psi and different foaming temperature and time

4.4 Compressive and Energy Absorbing Properties of Polymer Foams

The compressive properties of PMMA and PMMA nanocomposite foams of different CNT concentrations (0wt%, 0.5wt%,1.0wt%, and 2.0wt%) were tested using the Shimadzu AGS-J in single compression mode at 1 mm/min crosshead speed. The area under the curve was calculated from the stress-strain curve to determine the energy absorption of the foams shown in Table 4.3 and Figure 4.8.

Table 4.3: Energy Absorption of PMMA Nanocomposite Foams

80°C 100°C 120°C Time 30 (seconds) Energy Absorbed I II III AVG I II III AVG I II III AVG (J/mm3) PMMA 10.38 10.03 11.08 10.50 6.37 6.11 6.53 6.33 6.00 5.54 6.25 5.93 0.5% 7.75 7.66 7.22 7.54 7.27 6.87 7.17 7.10 6.32 7.21 6.52 6.68 1.0% 9.25 7.40 8.07 8.24 6.38 7.24 6.97 6.86 7.58 6.87 6.20 6.88 2.0% 5.60 6.11 6.67 6.13 6.16 6.16 6.09 6.14 6.05 5.35 6.14 5.85 Time 60 (seconds) Energy Absorbed I II III AVG I II III AVG I II III AVG (J/mm3) PMMA 8.10 8.34 7.77 8.07 5.28 5.42 5.07 5.26 6.06 5.77 6.00 5.94 0.5% 8.83 8.19 7.99 8.34 7.48 7.52 7.06 7.35 6.75 7.01 6.88 1.0% 8.35 7.85 7.89 8.03 6.40 7.31 7.01 6.91 7.23 7.10 6.39 6.90 2.0% 5.10 5.07 4.98 5.05 5.19 7.14 6.76 6.36 5.57 5.24 5.92 5.58 Time 90 (seconds) Energy Absorbed I II III AVG I II III AVG I II III AVG (J/mm3) PMMA 9.44 10.10 9.71 9.75 4.41 5.33 5.25 5.00 6.65 6.34 6.97 6.65 0.5% 9.04 8.93 7.93 8.63 6.69 7.19 6.84 6.90 6.36 8.51 7.43 1.0% 8.87 6.29 7.09 7.42 6.18 5.78 5.93 5.96 4.28 5.57 4.83 4.89 2.0% 8.25 6.87 7.77 7.63 5.62 6.94 6.13 6.23 5.78 6.34 7.57 6.56

12 8 ) 3 ) 3 10 7 6 8 5 PMMA PMMA 6 4 0.50% 0.50% 4 3 1.00% 2 1.00% 2 2.00% 1 2.00% Energy Absorbed (J/m Energy Absorbed (J/mm 0 0 30 60 90 30 60 90 Foaming Time (seconds) Foaming Time (seconds) (a) (b)

9 ) 3 8 7 6 5 PMMA 4 0.50% 3 1.00% 2 1 2.00% Energy Absorbed (J/m 0 30 60 90 Foaming Time (seconds) (c)

Figure 4.8: Toughness measurement of the energy absorbed in PMMA nanocomposite Foams of different CNT concentration at different foaming temperatures: (a) 80°C, (b) 100°C, (c) 120°C

For the foam prepared at 80°C, the highest energy absorbing capability is observed in the pure PMMA foam. As the CNT concentration increase, the absorbed energy further decreases. The uniform cell morphology of the pure foams may contribute to the observed reduction. On the other hand, for foam prepared at 100°C and 120°C, with the addition of CNT, the energy absorption first increases then decreases. The 0.5% CNT nanocomposite foams have the highest energy absorbing capability. This may be attributed to the effect of CNT dispersion. As previously observed, the CNT dispersion is rather uniform in the 0.5% CNT nanocomposites. As the CNT concentration increases, the CNT dispersion deteriorates and aggregates is observed the 2% CNT nanocomposites. The poorer CNT dispersion causes less uniform cell morphology.

Furthermore they are stress concentrators inducing earlier failure. In fact, the 2% CNT nanocomposite foams are very brittle.

CHAPTER FIVE CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion

PMMA carbon nanotube nanocomposites were synthesized by anti-solvent precipitation. Good CNT dispersion was achieved. The nanocomposite foam were produced by the batch foaming processing using carbon dioxide at various CNT concentrations and processing conditions (foaming temperature and time). Bimodal cell size distribution is observed in the PMMA nanocomposite foams. At higher concentrations of CNTs, CNT rich and polymer rich regions were observed in the nanocomposites causing competition in the cell nucleation and bimodal cell size distribution. The effect of CNT concentration and processing conditions on foamability were studied. A decrease in expansion ratio is observed in nanocomposite foam, and is attributed to the increase in the matrix viscosity due the addition of the carbon nanotubes. To study the energy dissipation capabilities of the PMMA nanocomposite foams, compressive properties were measured. The toughness was obtained by integrating the stress-strain curve and used as an indication for energy absorbing capabilities. 0.5% CNT nanocomposite foam show improvement in energy absorption while the capabilities decrease when CNT concentration increase.

5.2 Recommendations

Based on the results of this study, future work is suggested to further explore the effects of CNTs and processing conditions on the foam morphology and properties of PMMA nanocomposite. Continuation of this work could focus on mold design to control foam uniformity and expansion. In addition, the use of a variety of CNTs with different surface chemistries, shape or structure to broaden the study of their effect on morphology. To study the energy dissipation properties under dynamic conditions, dynamic mechanical analysis (DMA) can be used. DMA will be used to characterize the storage and loss modulus, G’

43 and G”. The measurement of energy dissipation can be quantified by G”, where under sinusoidal load, the energy dissipated by a cycle of deformation per unit of volume as: (8) In the area of energy absorption it has been found that the interfacial sliding among the nanotubes in the polymer matrix is an important factor for improved mechanical damping. Suhr et al.[12] investigated the use of nanoparticles as potential aids in the mechanical damping in polymer materials. This testing showed improvement of the viscoelastic response with up to 1400% increase in the loss fraction compared to the epoxy baseline in Figure 5.1[12]. Based on the results, Suhr et al. was able to deduce that the interfacial slippage of the well dispersed nanotube-nanotube interfaces is a factor for the mechanical damping in conjunction to the energy dissipation[12]. It will be worthwhile to study the behavior of the nanocomposite foams using similar methodology.

Figure 5.1: Loss modulus of CNT film versus baseline epoxy compared to test frequency[12].

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BIOGRAPHICAL SKETCH

Kristin Kynard was born in Cleveland, Ohio to Margaret and Joseph Kynard on July 16, 1985. She spent her early life between Toledo and Cleveland, Ohio before her mother was transferred to Tampa, FL in 1986. In Tampa, she attended Cecile Essrig Elementary and Ben Hill Middle School before graduating with honors from Vivian Gaither High School in 2003. In the spring of 2009, Kristin Kynard completed her Bachelors degree in Industrial Engineering at The Florida State University. During her tenure at The Florida State University, she was a member of the Institute of Industrial Engineers and executive board member of the National Society of Black Engineers. Since August 2009, she has been working as a Graduate Assistant with the High Performance Materials Institute (HPMI) in the area polymer nanocomposite foam for energy dissipation applications. Under the direction of Professor Dr. Changchun Zeng, she obtained her Masters degree in the summer of 2011, also from the Department of Industrial Engineering at FAMU-FSU College of Engineering from The Florida State University.