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Synthesis and characterization of a novel Poly(methyl methacrylate) Composites using Copper-4, 4'- Trimethylenedipyridine Metal-Organic Framework as Fillers
h [electronic resource] /
by Shisi Liu.
[Tampa, Fla] :
b University of South Florida,
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Thesis (M.S.)--University of South Florida, 2009.
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ABSTRACT: A novel Poly (methyl methacrylate) Composites using Copper-4, 4'- Trimethylenedipyridine Metal-Organic Framework as Fillers (CTMOF) had been synthesized and analyzed. The CTMOF structure had been characterized by X-ray crystallography. The thermal and mechanical properties of CTMOF-PMMA composites had been examined via optical microscopy, differential scanning calorimetry (DSC), microhardness, and dynamic mechanical thermal analysis (DMTA). The results showed the increase of Glass transition temperatures and the improvement of mechanical properties of the PMMA composites as the concentration of CBMOF loading increased.
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t USF Electronic Theses and Dissertations.
Synthesis and Characterization of a Novel Poly(methyl methacrylate) Composites using Copper-4, 4Trimethylenedipyridine Me tal-Organic Framework as Fillers By Shisi Liu A Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Chemistry College of Arts & Science University of South Florida Chair person: David Rabson, Ph.D. Major Professor: Ju lie P. Harmon, Ph.D. Carmen Valdez Gauthier, Ph.D. Abdul Malik, Ph.D. Date of Approval: April 9, 2009 Keywords: PMMA composites, metal-orga nic framework, mechan ical properties. Copyright 2009, Shisi Liu
DEDICATION This work is dedicated to my dear mom and dad.
ACKNOWLEDGEMENTS First of all, I would like to thank Dr. Ca rmen Valdez Gauthier. She spent a lot of efforts in this thesis research and has been like my co-advisor. Thank you very much for the invaluable efforts, support and guidance. I especially would like to thank my major professor, Dr. Julie P. Harmon. Thank you very much for all the guidance, support and encouragement. Also I would like to thank my committee me mbers: Dr. Carmen Valdez Gauthier, Dr. Abdul Malik and Dr. David Rabson. Thank you for all the generous help. I would like to acknowledge Justin Massing, Dr. Carmen Valdez Gauthier, Dr. Lukasz Wojtas, and Dr. Michael J. Zaworot ko for their contribution to the MOF crystal project. At last, I would like to thank all my lab partners and friends: Ramakanth Ananthoji, Roger Bass, Kevin Clifford, Brent Hilker, Parul Jain, Mu Seong Kim, Bernard Knudsen, Krystal McCann, Paul Tate, C hunyan Wang. Thanks for all the support and help!
i TABLE OF CONTENTS LIST OF TABLES v LIST OF FIGURES vi ABBREVATION ix ABSTRACT x CHAPTER ONE INTRODUCTION 1.1 Metal-Organic Frameworks 1 1.2 Polymer Composites 5 1.3 Metal-containing Polymer Composites 6 CHAPTER TWO AN OVERVIEW OF POLYMER SCIENCE AND INSTRUMENTATION THEORY 2.1 Introduction 8 2.2 An overview of polymer scie nce and polymer synthesis 8 2.3 Microhardness 13 2.4 Differential Scanning Calorimetry (DSC) 16 2.5 Dynamic Mechanical Analysis (DMA) 19 CHAPTER THREE SYNTHESIS AND C HARACTERIZATION OF COPPER-4, 4TRIMETHYLENEDIPYRIDINE M ETAL-ORGANIC FRAMEWORK 3.1 Introduction 26 3.2 Experimental 27 3.2.1 Synthesis of Copper-4,4trimethyl enedipyridine metal organic framework (CTMOF) 27 3.2.2 X-ray Crystallography of CTMOF 28 3.2.3 Differential Scanning Calorimetry 31 3.3 Results and Discussion 32 3.3.1 Discussion of the x-ray structure 32 3.3.2 Other Characterizations-DSC 34 CHAPTER FOUR SYNTHESIS AND C HARACTERIZATION OF COPPER-4, 4TRIMETHYLENEDIPYRIDINE META L-ORGANIC FRAMEWORK-PMMA COMPOSITES 4.1 Introduction 36 4.2 Experimental 37
ii 4.2.1 Synthesis of CTMOF-PMMA composites 37 4.2.2 Optical Microscopy 38 4.2.3 Differential Scanning Calorimetry 39 4.2.4 Dynamic Mechanical Analysis 39 4.2.5 Microhardness 39 4.3 Results and Discussion 40 4.3.1 Optical Microscopy 40 4.3.2 DSC 41 4.3.3 Microhardness 46 4.3.4 Dynamic Mechanical Analysis 49 CHAPTER FIVE CONCLUSION AND FUTURE WORK 62 REFERENCES 64 ABOUT THE AUTHOR End Page
iii LIST OF TABLES Table 3.1 Crystal data and structur e refinement for compound CTMOF. 30 Table 3.2 Selected Bond Distances () and Angles (deg) for compound CTMOF. 31 Table 4.1 Glass transition temperat ures of the PMMA composites. 42 Table 4.2 The eight Vickers hardness measurements and their deviations for each PMMA composite sample. 47 Table 4.3 Glass transition temperatur e and Vickers hardness number of the PMMA composites. 47 Table 4.4 DMA data: Storage Modul us values at 90 Hz and -100 oC, -50 oC, 0 oC and 50oC. 56 Table 4.5 Comparison of ac tivation energies of the transition for the PMMA composites as determined from DMA. 61
iv LIST OF FIGURES Figure 1.1 The rapid growth of cita tions with the word coordination polymers in titles or abstracts from 1990 to 2005. 2 Figure 1.2 Some typical examples of organic ligands used in MOFs. 3 Figure 1.3 Node-and-spacer styles of MOFs: a) 0D nanoball; b) 1D zigzag chain; c) 1D helix; d) 1D ladder; e) 2D bilayers; f) 2D square grid; g) 2D honeycomb; h) 3D (10,3)-a net; i) 3D diamondoid net; j) 3D primitive cubic net; k) 3D NbO net (Wang, 2006). 4 Figure 1.4 Vertex-linked Polygons or Polyhedra (VLPP) styles of MOFs: a) 0D nanoball; b) 3D (10,3)-a net; c) 3D diamondoid net; d) 3D primitive cubic net; e) 3D NbO net (Wang, 2006). 5 Figure 2.1 The Vickers indentation (http://www.hardnesstesters.co m/microhardness-tester.htm). 14 Figure 2.2 The Leica Vicker Microhardness Tester. 14 Figure 2.3 Several possible transitions of polymer materi als characterized by DSC (TA Instruments DSC Brochure 2004). 17 Figure 2.4 The structure of heat flux DSC cell (TA Instruments 1998). 18 Figure 2.5 Sealed DSC sample pan (TA Instruments 1998). 19 Figure 2.6 100% elastic re sponse and 100% viscous re sponse of material (TA Instruments). 21 Figure 2.7 Viscoelastic response of polymer materials (Foreman, 1997). 21 Figure 2.8 A conceptual example of stored energy E, and lost energy E (Perkin Elmer Instruments PETech-90). 22 Figure 2.9 Modulus relationships of viscoelastic materials (TA Instruments DMA 2980 2002). 23
v Figure 2.10 TA instrument DMA 2980 (TA Instruments DMA 2980, 2002, Deleware). 24 Figure 2.11 DMA tension film cl amp (TA Instruments DMA 2980, 2002, Delaware). 25 Figure 3.1 (a) The Vial-in-Vial met hod of synthesis (Gauthier, 2007) and (b) The optical microscopy image of purple crystal. 28 Figure 3.2 Polymeric chain in CTMOF (Wojtas, 2009). 32 Figure 3.3 Coordination and atom numbering scheme for CTMOF (Wojtas, 2009). 33 Figure 3.4 Packing in CTMOF. View along polymeric chains. Hydrogen atoms were omitted for clarity (Wojtas, 2009). 33 Figure 3.5 Close packing of polym eric chains (Wojtas, 2009). 34 Figure 3.6 The melting temperature of MOF crystal obtained from DSC. 35 Figure 4.1 Branson Sonifier 450. 38 Figure 4.2 Comparison of Discs of (a) the 0.5% CTMOF-PMMA composite and (b) Pure PMMA. 40 Figure 4.3 Comparison of optical mi croscope images of (a) the 0.5% CTMOF-PMMA composite and (b) Pure PMMA. 41 Figure 4.4 DSC data: Glass transition temperature, T g of pure PMMA. 42 Figure 4.5 DSC data: Glass transition temperature, Tg, of 0.05% CTMOF-PMMA composite. 43 Figure 4.6 DSC data: Glass transition temperature, Tg, of 0.1% CTMOFPMMA Composite 44 Figure 4.7 DSC data: Glass transition temperature, Tg, of 0.5% CTMOFPMMA composite. 45 Figure 4.8 Direct re lationships between Tg and HV for the CTMOFPMMA composites. 48 Figure 4.9 DMA data: Storage Modulus, E vs. temperature for pure PMMA. 49
vi Figure 4.10 DMA data: Storage Modulus, E vs. temperature for 0.05% CTMOF-PMMA composite. 50 Figure 4.11 DMA data: Storage Modulus, E vs. temperature for 0.1% CTMOF-PMMA composite. 51 Figure 4.12 DMA data: Storage Modulus, E vs. temperature for 0.5% CTMOF-PMMA composite. 52 Figure 4.13 Storage Modulus, E, vs. temperature at 90 Hz for the 0%, 0.05%, 0.1% and 0.5% CTMOF-PMMA composite. 53 Figure 4.14 Loss Modulus, E , vs. temperature for the neat PMMA and 0.05% CTMOF-PMMA composite at 90 Hz. 55 Figure 4.15 DMA data: Loss Modulus, E vs. temperature for pure PMMA. 57 Figure 4.16 DMA data: Arrhenius plot of transition for pure PMMA. 57 Figure 4.17 DMA data: Loss Modulus, E vs. temperature for 0.05% CTMOF-PMMA composite. 58 Figure 4.18 DMA data: Arrhenius plot of transition for 0.05% CTMOFPMMA composite. 58 Figure 4.19 DMA data: Loss Modulus, E vs. temperature for 0.1% CTMOF PMMA composite. 59 Figure 4.20 DMA data: Arrhenius plot of transition for 0.1% CTMOFPMMA composite. 59 Figure 4.21 DMA data: Loss Modulus, E vs. temperature for 0.5% CTMOF-PMMA composite. 60 Figure 4.22 DMA data: Arrhenius plot of transition for 0.5% CTMOFPMMA composite. 60
vii LIST OF ABBREVIATIONS Activation energy Ea Cohesive energy density CED Copper-4, 4-trimethylened ipyridine Metal-organic framework CTMOF Differential Scanning Calorimetry DSC Dynamic Mechanical Analysis DMA Glass Transition Temperature Tg Loss Modulus E Methyl methacrylate MMA Poly (methyl methacrylate) PMMA Storage Modulus E' Vickers Hardness Number HV
viii Synthesis and Characterization of a Novel Poly(methyl methacrylate) Composites using Copper-4, 4Trimethylenedipyridine Me tal-Organic Framework as Fillers Shisi Liu ABSTRACT A novel Poly (methyl methacrylate ) Composites using Copper-4, 4Trimethylenedipyridine Metal-Organic Fr amework as Fillers (CTMOF) had been synthesized and analyzed. The CTMOF stru cture had been characterized by X-ray crystallography. The thermal and mechanical properties of CTMOF-PMMA composites had been examined via optical microscopy, differential scanning calorimetry (DSC), microhardness, and dynamic mechanical therma l analysis (DMTA). The results showed the increase of Glass transition temperatur es and the improvement of mechanical properties of the PMMA composites as the concentration of CBMOF loading increased.
1 CHAPTER ONE INTRODUCTION 1.1 Metal-Organic Frameworks Metal-organic frameworks (MOFs), al so called coordination polymers or supramolecular structures, are compounds with backbones constructed from metal ions and ligands to create zero-, one-, two-, a nd three-dimensional structures. MOFs are inorganic-organic hybrid compounds unlike traditionally organic polymers (Blake, Champness, Hubberstey, Li, Withersby & Sc hroder, 1999; Eddaoudi et al., 2001; Evans & Lin, 2002; Kitagawa, Kitaura & Noro, 2004). During the early 1960s, the first publication on MOFs was reported by Tomic (T omic, 1965) who studied the formation of MOFs from the reaction of 1,5-Dihydroxynapht halene-2,6-dicarboxylic acid (1,5-N-2,6) with Zn, Ni, Al, and Fe+3. The area did not blossom until the late 1980s when Robson reported the novel node-and-spacer approach (Moulton & Zaworotko, 2001) incorporating both transition metal ions of well-defined coordination geometries and rod-like organic ligands in the design of framework materials (Wang, 2006). This area continued to grow rapidly as advantageous char acteristics of these compounds were discovered. The number of MOF compounds showed a rapid growth fr om the late-1990s to present (Figure 1.1). The advantages of MOFs include: high poros ities of nanometer-si zed spaces in them, high designibility and regularity of the fram ework, high thermal and mechanical stability, and their world record surface areas (Hag rman, Hagrman & Zubieta, 1999; Moulton & Zaworotko, 2001; Mueller, Schubert, Teich, Pu etter, Schierle-Arndt & Pastre, 2006;
2 Yaghi, O'Keeffe, Ockwig, Chae, Eddaoudi & Kim, 2003). These advantages revealed their potential applica tions in catalysis, gas purifica tion, gas separation, and gas storage (Chui, Lo, Charmant, Orpen & Williams, 1999; Eddaoudi, Li & Yaghi, 2000; Matsuda et al., 2005; Seo et al., 2000; Wu, Hu, Zhang & Lin, 2005). Figure 1.1 The rapid growth of citations with the word coordination polymers in titles or abstracts from 1990 to 2005 (Wang, 2006). In Robsons node-and-spacer model (Robson, 2008), the MOF compounds are constructed from organic ligand spacers and metal cation nodes to form diverse dimensional shapes. Since the organic ligands and the metals centers have different geometric binding sites, this type of compound could be pr e-designed to different well-defined configurations.
3 Figure 1.2 (Wang, 2006) shows some typical exam ples of organic ligands used in MOFs, including linear, angular, trigonal, and tetrahedral shapes. Figure 1.2 Some typical examples of organic ligands used in MOFs (Wang, 2006). In the MOFs design principles two main strategies exis t. The first is the nodeand-spacer approach (Robson, 2008; We lls, 1977; 1984) in which the compound frameworks consist of simple dimensiona l dots and lines. As shown in Figure 1.3, various network architectures are directly constructed from those topological dots and lines which are the building units. A nother synthetic approach is called Vertexlinked Polygons or Polyhedra (VLPP) (Bourne, Lu, Mondal, Moulton & Zaworotko, 2001; Lu, Mondal, Moulton & Zaworotko, 2001; Moulton, Lu, Mondal & Zaworotko, 2001; Wang,
4 Kravtsov & Zaworotko, 2005). In this approach the particular shapes of building unit construct the geometric network of MOFs (Figure 1.4). Figure 1.3 Node-and-spacer styles of MOFs: a) 0D nanoball; b) 1D zigzag chain; c) 1D helix; d) 1D ladder; e) 2D b ilayers; f) 2D square grid; g) 2D honeycomb; h) 3D (10,3)-a net; i) 3D diamondoid net; j) 3D primitiv e cubic net; k) 3D NbO net (Wang, 2006).
5 Figure 1.4 Vertex-linked Polygons or Polyhe dra (VLPP) styles of MOFs: a) 0D nanoball; b) 3D (10,3)-a net; c) 3D diamondoi d net; d) 3D primitive cubic net; e) 3D NbO net (Wang, 2006). 1.2 Polymer Composites As early as the mid-20th century, the rese arch focus in the area of traditional carbon-based homogeneous polymers has shifted to specialty materials with advanced properties (Kusy, 1986; Whittell & Manners, 2007). Scientists devoted tremendous efforts to synthesis and char acterization of polymer com posites in order to achieve advantageous properties such as increased stiffness, strength, di mensional stability, modified electrical, optical and magnetic properties, and reduced cost (Clayton, Gerasimov, Cinke, Meyyappan & Harmon, 2004; Va rga, Feher, Filipcsei & Zrinyi, 2003; Wilson et al., 2004). Many commercial polymeric materials are composites, for example polyblends and ABS materials, fi lled poly (vinyl chloride) mate rials used in floor tile and wire coatings, filled thermosetting resins, and glass or graphic-fiber-filled plastics
6 (Nielsen & Landel, 1994). Polymer composite materials is defined as materials consisting of two or more components and containing tw o or more phases (Nie lsen & Landel, 1994). There are three types of polymer composites: (1) Composites using discrete particles as fillers. These composites have a continuous polymer matrix phase and a discontinuous filler phase. (2) Composites using fibers as fillers. (3) Skeletal composites which have continuous filler and matrix phases, su ch as polymer filled open-cell foams. This study is concentrated on the first type of composite material. The properties of composite materials are influenced by many factors. One of the most important factors is the nature of th e interface between the phases. Many techniques have been developed to improve the interfacial adhesion and particle-f illers dispersion in polymer matrices, such as in situ polymerization and melt blending (Mohomed, 2006; Park & Jana, 2003; Rong, Jing, Li & She ng, 2001; Tatro, Clayton, Muisener, Rao & Harmon, 2004; Xiong, Wu, Zhou & You, 2002). E ach technique has its virtues and drawbacks. For instance, in situ ultrasonic polymerization (Mohomed, 2006) can achieve a much more uniform dispersion of fillers than melt blending. However, this technique is difficult to scale up for industrial applicati ons. On the other hand, the melt blending is a mature technique widely used in large scale composite pro duction but is often accompanied by filler agglomeration. 1.3 Metal-containing Po lymer Composites Since the late 20th century, functionalized polymer materials with advanced properties have been the target. One of th e principal trends in composites is to
7 incorporate metal-ligand supramolecular stru ctures into traditional carbon-based polymers. These metal-containing hybrid polymer materials have the potential to change physical, electronic, optical, and cata lytic properties of organic polymers to achieve higher performance or utility. The transition metal centers within the polymer matrix have the ability to change the oxidation st ates or facilitate electron flow (Williams, Boydston & Bielawski, 2007). For exampl e, such polymers can catalyze carbon-carbon bond-forming reactions by oxidative insertions and reductive eliminations. Thus they can be employed as recoverable catalysts in i ndustry. Furthermore, the metal-containing polymers are able to be attached with small functional molecules, solids with 2D or 3D extended structures, or biological materials because of the presence of the metal centers and ligand binding sites. A synthetic approach to create these special materials is to place metal binding sites in either the main chai ns or the side chains of the carbon-based polymers (Pefkianakis, Tzanetos & Kallitsis, 20 08). This type of mate rials is termed as organometallic polymers and was first reported as early as 50 years ago with free radical polymerization of vinyl ferrocene (Arimoto & Haven, 1955). An alternative approach is to create polymer compos ites by using MOF compounds as fillers and organic polymers as matrices. This method is used in this study.
8 CHAPTER TWO AN OVERVIEW OF POLYMER SCIENCE AND INSTRUMENTATION THEORY 2.1 Introduction This chapter introduces the basic knowle dge and techniques applied in this study. A brief overview of polymer science and polym er synthesis is discussed. The theories and operations of microhardne ss and two thermal analysis techniques used in this research for characterization of the polyme r materials are demonstrated in the later sections. These backgrounds may help one to better understand the data collected in this study. 2.2 An overview of polymer science and polymer synthesis A polymer is defined as a compound c onsisting of molecular repeating units connected by covalent bonds. The propertie s of a polymer do not change significantly when one or several repeating units are a dded to the polymer molecule (Gedde, 1995). The name polymer is derived from the Greek words poly meaning many and meros meaning parts (Seymour, 1971). Polymer can be divided into three main types of materials: 1) natural polymer materials, such as wood, hemp, cotton, silk, animal skin and horn, cellulose, protein, bitumen, lacquer, a nd natural rubber; 2) modified natural polymer materials, or derivatives of na tural polymers (Seymour, 1987), such as rayon, cellulose acetate, and modified starch; and 3) synthetic polymer materials, such as
9 polyvinyl chloride resin (PVC ), poly(methyl methacrylate) (PMMA), and polybutadiene rubber. As early as several thousands years ago, natural polymer materials were used and processed by humans (Seymour, 1971). In an cient times, natural polymers such as wood, animal skin, horn, and bitumen were used for transportation, tools, and shelter; wood and other plant fibers were made into paper; pr oteins, cellulose, and starch were foodstuffs; silk, cotton and flax were used for making cloth; and even amber was used for jewel (Seymour, 1971). As natural polymers coul d no longer satisfy humans requirement, the processing and modification methods for na tural polymers were developed in the 19th century (Dai, Zhang & Jiang, 2005). In 1839, G oodyear first invented vulcanized rubber by adding sulfur to natural rubber (Seymour 1971). Later in 1846, cellulose nitrate was produced by Schonbein. In 1872, the first re al synthetic polymer was gained by Baekeland through the condensation reaction of phenol and formaldehyde. There were several commercially available plastics in 1900, such as amber, bitumens, shellac, guttapercha, and ebonite (Seymour, 1971). Gl yceryl phthalate resins, poly (2,3dimethylbutadiene), ethyl cellulose and ur ea-formaldehyde resins had also been synthesized in the first decades of this centu ry (Seymour, 1971). However, little attention was devoted to these natural polymeric materials until 1930s. Pioneer scientists Staudinger, Carothers, Mark and many others recognized the true structure of polymers, and the first textbook was published in 1932 (Dai et al., 2005; Seymour, 1971). The modern concepts of polymer we re presented at this era.
10 Polymer science is a material subfield studying polymer structures, properties, synthesis, processing, and a pplications. It incl udes three main branches: polymer chemistry, polymer physics and polymer engineering. Polymer chemistry works on the design, synthesis and property modification of polymer materials. Its purpose is to provide new materials and compounds. Polymer physics is the fundamental basis of the polymer structure theory. It investigat es polymer configurations, properties, characterizations, and the interrelationship s between structure and property. Polymer engineering connects polymer science a nd polymer industry. It studies polymer manufacture and processing methods. In the following paragraph, the different polymerization mechanisms will be introduced, emphasizing on free radical polymerization. Polymerization is defined as the reac tion through which monomers covalently bond to form polymers (Odian, 2004). There ar e several different ways to classify polymers or polymerizations. During the develo pment of polymer science, two types of classifications have been widely used. On e classification based on polymer structures divides polymers into condensation and a ddition polymers. Another one based on the mechanisms of polymerization processes di vides polymerization into step and chain polymerizations. In most situations, these two sets of terms ar e interchangeable. Condensation or step polymerizations are the reactions producing various polymers with the elimination of some small molecules such as water. One example of this type of polymerization is the formati on of Nylon 6/6, a extensively used fiber and
11 plastic (Odian, 2004). As shown in Equation 2.1, hexamethylene di amine reacts with adipic acid to produce poly (hexamet hylene adipamide) or nylon 6/6. n HOROH + n HO2CRCO2H Equation 2.1 The step polymerization of nylon 6/6, where R = (CH2)6 and R = (CH2)4. Addition or chain polymerizations are th e reactions producing various polymers without the loss of small molecules. This t ype of polymers has the same repeating units as their corresponding monomers. The majority of these monomers are vinyl monomers. Equation 2.2 and 2.3 give two examples of this polymerization: the formation of polyethylene and poly (methyl me thacrylate). The poly (methyl methacrylate) is used as polymer matrix in this study. Equation 2.2 The polymerization of polyethylene.
12 Equation 2.3 The chain polymerization of poly (methyl methacrylate). In chain polymerization, usua lly an initiator is used to produce an initiator species R* with a reactive center (Odian, 2004). The reactive center may be a free radical, a cation, or a anion. In this thes is, the free radical initiator 2,2, -azobis(2,4-dimethylpentane nitrile) (Vazo 52, DuPont), is applied. As shown in Equation 2.4, the initiator decomposes into the cyanoalkyl free radica l and reacts as reactive center (Fernandes, 2005; McConnell, Barton, LaPack & DesJardin, 2002). Equation 2.4 Thermal initiation of Vazo 52 (Mohomed, 2006).
13 2.3 Microhardness The hardness is a measure of a material s resistance to surface deformation against indentation (Calleja & Fakirov, 2000) One of the important properties of polymer materials is creep. So the microhard ness of polymer surface is a time-dependent test and the dwell time must be specified (Calleja & Fakirov, 2000). As described in Equation 2.5 (Calleja & Faki rov, 2000), the microhardness of a polymer material is inversely proportional to dwell time, t. H0 is a coefficient depending on temperature and loading stress, and k is a constant. H = H0 t-k Equation 2.5 There are three main categories of hardness measurements: scratch hardness, static indentation hardness, and dynamic hardne ss. In this research, static indentation hardness is employed. The static indentation hardness method involves the formation of a permanent indentation pressed via an indenter in the surface of the material (Figure 2.1). The hardness is determined by the load force and the size of the indentation formed. There are several different test methods used for static indentation hardness. Different tests employ different indenter s, such as a steel ball (B rinell test), diamond cone (Grodzinski test) and diam ond pyramid (Berkovich, Knoop a nd Vickers tests) (Mohomed, 2006).
14 Figure 2.1 The Vickers indentation (http: //www.hardnesstesters.com/microhardnesstester.htm). Figure 2.2 The Leica Vicker Microhardness Tester.
15 Microhardness testing usually involves m easurements with force loads ranges from 1 N to 30 N (Calleja & Fakirov, 2000). In this study, the static indentation hardness testing was performed. A Leica Vicker Mi crohardness Tester (VMHT) MOT equipped with a square Vicker indenter was used (F igure 2.2). The indenter is a diamond square pyramid and the angles between non-adjacent faces of the pyramid are 136o. The force applied to sample is 5 N and is usually he ld for 6-30 s, and then removed. The Vicker hardness number HV, expressed in megapascal s (MPa), was determined via Equation 2.6. HV = 2 24 1854 2 sin 2 d F d F A F Equation 2.6 Where F is the applied force in newtons, A is the surface area of the imprint in square millimeters, is the angle, and d is the averag e diagonal length of the imprint in millimeters. The length of the imprint is measured with a microscope equipped with a filar eyepiece. The microhardness of a polymer material is a complex time-dependent property related to viscoelastic behavior (Calleja & Fakirov, 2000; Mohomed, 2006). As described by Gedde (Gedde, 1995), the gla ss transition temperature, Tg, generally increases with increasing cohesive energy density (CED) as shown in equation 2.7: Tg = mR22+ C1 Equation 2.7
16 where 2 is the CED, m is a parameter that describes the internal mobility of the groups in a single chain, R is the gas constant and C1 is a constant. CED is also a main factor in a materials HV value (Mohomed, Abourahma, Zaworotko & Harmon, 2005a). Therefore, the positive proportion between the Tg and HV is established by re lating these two factors. 2.4 Differential Scanning Calorimetry (DSC) Differential Scanning Calorimetry (DSC ) measures the heat-flow difference between a sample and a reference pan as a f unction of time and temperature. (Ehrenstein, Riedel & Trawiel, 2004). The enthalpy change, or heat flow, marks the internal energy change in a sample undergoing a physical or ch emical transition. The heat flow recorded is associated with transitions in materials as a function of temperature and time (Thomas, Kiwit & Kerner, 1998). DSC provides not only qualitative information about material transitions such as the glass transition temperatures ( Tg) and melting temperature ( Tm), but also quantitative properties like cr ystallization time & temperature, percent crystallinity, heats of fusion and reaction, specific heat, oxidative stability, rate of cure, degree of cure, reaction kine tics, purity, and thermal stab ility (Thomas et al., 1998). Figure 2.3 shows several possible transitions of polymer materials characterized by DSC (Mohomed, 2006). Samples of varying compositions such as films, fibers, powders, gels, solutions and composites can be analyzed via DSC. DSC is the most commonly used thermal analysis technique and it has many a dvantages, including fast analysis time (usually less than 30 minutes), easy sample preparat ion, applicability to both solids and liquids, wide temperature range, and excellent quantitative capability.
17 Figure 2.3 Several possible transitions of pol ymer materials char acterized by DSC (TA Instruments DSC Brochure 2004). There are two types of DSC methods: heat-flux DSC and power-compensation DSC. In this research, a heat-flux DSC in strument was employed. In the heat-flux DSC cell, the sample and the reference pans are heated or cooled by a certain temperature control program (Ehrenstein et al., 2004). Fi gure 2.4 shows the structure of a heat flux DSC cell. The sample and the reference pa n stay on the raised platforms made of constantan alloy. The platforms transfer heat to the sample and reference pans. The heatflow difference between the sample and re ference pans and their temperatures are measured by area thermocouples under the pl atforms. The Ohms Law, expressed as Equation 2.8, is used to measur e the heat flow difference.
18 DR T dt dQ Equation 2.8 where dQ/dt is the heat flow, T is the temperature difference between reference and sample pans and RD is the thermal resistance of th e constantan disc (Mohomed, 2006). Data are graphed as heat flow versus temperature. Figure 2.4 The structure of heat flux DSC cell (TA Instruments 1998). The thermal properties of polymers can be affected by processing. In order to remove the former thermal history, Tg & Tm are taken only from the second run cycle. That means the sample is initially heated to above its Tg or Tm, cooled below Tg or Tm and then heated again.
19 In this study, a TA instrume nts DSC 2910 was used to obtain Tg & Tm of the samples. Before the measurement, the baseline calibration was performed. Approximately 4-10 mg sample was sealed in a sample aluminum pan. The empty sample pan should have an identical mass to the reference pan. The DSC cell was heated under nitrogen gas to maintain an inert atmosphere The colleted data were analyzed in TA instrument software Universal Analysis 2000. Figure 2.5 Sealed DSC sample pan (TA Instruments 1998). 2.5 Dynamic Mechanical Analysis (DMA) Dynamic mechanical analys is (DMA), or dynamic mech anical thermal analysis (DMTA), is a technique used to study and ch aracterize the viscoelastic properties of materials. In DMA, an oscillating minor si nusoidal force is applied to a sample as a function of time and temperature, and the mate rials response to that force is analyzed
20 (Menard, 1999). The force loaded to sample is named stress and is marked by the Greek letter, The deformation with which a ma terial responds is strain, or The deformation can be determined by amplitude and phase sh ift. The phase shift angle, or time lag, between the stress and strain is marked as When a 100% elastic material is subjected to a stress within its Hookean limit, it will deform in an in-phase sine strain (Figure 2.6). This means there is no time lag and = 0o (Mohomed, 2006). The material will turn back to its original shape when the stress is removed. When a 100% viscous material is subjected to a stress it will deform in an out of phase sine strain ( = 90o). The material will not turn back to its original shape when the stress is removed. However, most polymer materials are not 100% elastic (ideal solids) or 100% viscous (ideal liquids) but a combination of both (Ferry, 1970). This property is called viscoelastic and ranges from 0o to 90o as shown in Figure 2.7. A viscoelastic material will respond a timedependent deformation when it is subjected to a stress. When the stress is removed the material will partially recover. The recovered strain represents the energy stored or the elastic portion of the materi als response. The unrecovered the strain represents the energy dissipated or viscous portion of the ma terials response. A conceptual example is shown in Figure 2.8. A tennis ball will not bounce back to the same height where it drops. The height where the ball bounces back denotes the energy stored or the elastic part of the material and the difference between the original and bounce back height denotes the energy lost or the viscous part (TA Instruments DMA 2980 2002).
21 Figure 2.6 100% elastic res ponse and 100% viscous res ponse of material (TA Instruments). Figure 2.7 Viscoelastic response of polymer materials (Foreman, 1997).
22 Figure 2.8 A conceptual example of stored energy E, and lost energy E (Perkin Elmer Instruments PETech-90). The complex modulus is defined as stress over strain, marked as E* or G* (shown in Figure 2.9). E* measures a materials stiffness and is dependent on the temperature and the applied stress (Menard, 1999). The complex modulus (E*) consists of the storage modulus E (the real part) and the loss modulus E (the imaginary part). E* = E + iE E examines the ability of the material to return or store energy, and E examines the ability to dissipate or lose energy. Tan delta (tan ), which is the ratio of the loss modulus to the storage modulus, is called damping. The magnitude of these moduli depends critically on the sweep frequency, the measuri ng conditions and the hi story of materials (Ehrenstein et al., 2004).
23 Figure 2.9 Modulus relationships of viscoelastic materi als (TA Instruments DMA 2980 2002). Three experimental testing modes can be applied in DMA: dynamic multifrequency oscillatory mode, cr eep mode (or transient test mode), and stress relaxation mode [TA Instruments DMA 2980 2002]. In dynami c multi-frequency oscillatory test, an oscillatory (sinusoidal) strain (or stress) is applied to the material and the responding stress (or strain) is measured. Storage m odulus, loss modulus, and other data will be collected as a function of time, temperat ure and frequency (Mohomed, 2006). In a creep mode test, the sample material is subjected to a constant stress and the responded strain is measured as a function of time. The data su ch as creep compliance and the recoverable compliance will be collected. In a stress relaxa tion test, the sample material is subjected to an instantaneous strain and the stress applied to keep that strain is measured as a function of time. The data such as stress re laxation modulus and the sample recovery will be collected versus time as the strain releases.
24 In this study, a TA instrument DMA 2980 (Figure 2.10) was employed to examine the viscoelastic behavior of the sa mples. The DMA operates at a temperature range from -150 oC to 500 oC and within the frequency range of 0.1 Hz to 100 Hz. The DMA was run under the dynamic multi-frequency oscillatory mode. The modulus and tan delta data were obtained as a function of tim e, frequency and temperature. The tension film clamp was applied, as shown in Fi gure 2.11. Several calibrations are performed before the sample testing, including te mperature, instrument, position, and clamp calibrations. Figure 2.10 TA instrument DMA 2980 (TA Instruments DMA 2980, 2002, Deleware).
25 Figure 2.11 DMA tension film clamp (T A Instruments DMA 2980, 2002, Delaware).
26 CHAPTER THREE SYNTHESIS AND CHARACTERIZATION OF COPPER-4, 4-TRIMETHYLENED IPYRIDINE METAL-ORGANIC FRAMEWORK 3.1 Introduction In the MOFs synthesis, the metal centers are usually transition metals cations such as Fe(II), Cu(II), Os(II), Ir(II), and Ru(II) (Pef kianakis et al., 2008). Among the various transition-metal ions, copper (II) is ideal for the design of metal-ligand complexes because it is inexpensive and has the potential to adopt a flexible coordination sphere (Legendre, Mauro, de Oliveira & Gambardella 2008; Mauro et al., 2004). Dr. Gauthiers group concentrated on design and synthesis of MOFs using copper (I I) as metal centers (Cherenfant, West & Gauthi er, 2006). One MOF synthesized by this group, Copper-4, 4trimethylenedipyridine, was chosen as the po lymer filler in this project. This overall polymer composite project is a joint collabo ration between Dr. Gauthiers lab and Dr. Harmons lab. The MOF used as polymer fille r, Copper-4, 4-trimethylenedipyridine, was synthesized by Justin Massing from Dr. Ga uthiers lab. The author also performed the synthesis of several batches of MOFs under the guidance of Dr. Gauthier. The structure of this MOF was analyzed via X-ray crystallography by Dr. Lukasz Wojtas from Dr. Zaworotkos group. The melting temp erature of this MOF was analyzed via DSC by the author. After this MOF compound was synthesized and characterized, it was chosen as polymer filler to modify the thermal and mechanical properties of polymer.
27 The experiment procedures and results were demonstrated in this chapter in order to better understand the structure and the properties of the filler. In th e later chapter, the polymer composites using this MOF crystal as filler was characterized and compared with the control polymer. 3.2 Experimental 3.2.1 Synthesis of Copper-4,4trimethylenedi pyridine metal organic framework (CTMOF) Copper(II) nitrate hemipentahydrate (0.20 g, 0.86 mmol) and 1,3adamantanedicarboxylic acid (0.192 g, 0.86 mmol) we re dissolved in 10 ml of methanol and placed in a small vial, which was placed in side a larger vial (shown in Figure 3.1). The copper/adamantane dicarboxylic acid solu tion was then layered with 1 ml of anhydrous 1, 2-dichlorobenzene. A 15 ml solution of 4,4-trimethylenedipyridine (0.49 g, 2.5 mmol) in methanol was added to the la rger vial, completing submerging the small reaction vial. After one week of slow diffusi on purple and green crystals were isolated. The purple crystals (CTMOF) were charac terized by single x-ra y crystallography and differential scanning calo rimetry (DSC).
28 (a) (b) Figure 3.1 (a) The Vial-in-Vial method of synt hesis (Gauthier, 2007) and (b) The optical microscopy image of purple crystal. 3.2.2 X-ray Crystallography of CTMOF The X-ray diffraction data were colle cted using Bruker-AXS SMART-APEX CCD diffractometer (MoK = 0.71073 ). Indexing was performed using SMARTv5.625 (Bruker-AXS, Data Collection Softwa re. Madison, Wisconsin, USA, 2001). Frames were integrated with SaintP lus 6.01 (Bruker-AXS, SAINT-V6.28A, Data Reduction Software. Madison, Wisconsin, US A, 2001) software package. Absorption correction was performed by multi-scan method implemented in SADABS (Sheldrick, G. M., Program for Empirical Absorption Corr ection. University of Gottingen, Germany, 1996). The structure was solved using SHELXS-97 and refined using SHELXL-97 contained in SHELXTL v6.10 (She ldrick, G. M., Bruker-AXS Madison, Wisconsin, USA. 2000) and WinGX v1.70.01 (Farru gia, 1999; Sheldrick, 1990; 2008) programs packages. All non-hydrogen atoms, except disordered methanol/nitrate species, were refined anisotropically. Hydrogen atoms were placed in geometrically calculated positions and
29 included in the refinement process using ri ding model. Crystal data and refinement conditions are shown in Table 3.1 Geometri cal parameters are shown in Table 3.2.
30 Table 3.1 Crystal data and structure refi nement for compound CTMOF (Wojtas, 2009). Empirical formula Formula weight Temperature Crystal system, space group Unit cell dimensions Volume Z, Calculated density Absorption coefficient Theta range for data collection (MoK) Reflections collected / observed / unique Completeness to theta = 28.31 Refinement method Goodness-of-fit on F^2 Final R indices [I>2sigma(I)] Largest diff. peak and hole Cu 2NO32(C13H14N2)C6H4Cl21.7CH3OH 392.36 100(2) K Triclinic, P-1 a = 10.369(7) A alpha = 102.839(13) b = 12.645(9) A beta = 92.253(14) c = 14.307(9) A gamma = 102.395(11) 1779(2) A^3 2, 1.461 Mg/m^3 0.822 mm^-1 1.96 to 25.62 deg. 9227 / 6463 [R(int) = 0.0594] 96.0 % Full-matrix least-squares on F^2 0.985 R1 = 0.0799, wR2 = 0.1762 0.855 and -0.701 e.A^-3
31 Table 3.2 Selected Bond Distances () and Angles (deg) for compound CTMOF (Wojtas, 2009). Cu(1)-N(21) Cu(1)-N(1)#1 Cu(1)-N(2) Cu(1)-N(22)#2 Cu(1)-O(63) N(21)-Cu(1)-N(1)#1 N(21)-Cu(1)-N(2) N(2)-Cu(1)-N(22)#2 N(21)-Cu(1)-O(63) N(1)#1-Cu(1)-O(63) N(2)-Cu(1)-O(63) N(22)#2-Cu(1)-O(63) 2.004(5) 2.010(5) 2.014(5) 2.019(5) 2.210(5) 91.1(2) 164.7(2) 90.9(2) 109.1(2) 95.2(2) 86.2(2) 93.9(2) Symmetry transformations used to generate equivalent atoms: #1 -x+1,-y,-z+1 #2 -x+2,-y+1,-z 3.2.3 Differential scanning calorimetry (DSC). The Melting Temperatures (Tm) of this MOF crystal were recorded with a TA Instrument DSC 2920. Approximately 4-10 mg samples were sealed in aluminum pan. The DSC cell was heated under nitrogen gas using a ramp rate of 10oC/min from 30 oC to 250oC.
32 3.3 Result & Discussion 3.3.1 Discussion of the x-ray structure The crystal structure of CTMOF consists of polymeric chains with Cu(II) ions bridged by two 4,4-trimethylenedipyridine molecules [Figure 3.2 & Figure 3.3]. Cu(II) ions adopt slightly distorted square pyr amidal geometry where each Cu(II) ion is coordinated by four N atoms of 4,4-trimethylenedipyridine and one O atom of nitrate anion [Figure 3.3]. The percenta ge of trigonal distortion as defined in (Hathaway, 1987) equals 6.5%. In the structure the charge is balanced thr ough nitrate anions. Polymeric chains are closely packed forming the la yers [Figure 3.4 & Figure 3.5]. Space between the layers is occupied by 1, 2-dichloroben zene and methanol molecules as well as by nitrate anions interacting through weak and moderate hydrogen bonds. Chains and layers are held together through van der W aals and weak hydrogen bonds forming a 1-D coordination polymer type of structure. Figure 3.2 Polymeric chain in CTMOF (Wojtas, 2009).
33 Figure 3.3 Coordination and atom number ing scheme for CTMOF (Wojtas, 2009). Figure 3.4 Packing in CTMOF. View along polymeric chains. Hydrogen atoms were omitted for clarity (Wojtas, 2009).
34 Figure 3.5 Close packing of polym eric chains (Wojtas, 2009). 3.3.2 Other Characterizations-DSC The melting temperature of this MOF crys tal was obtained from the DSC curve, as shown in Figure 3.6. Tm is 207.16 oC.
35 Figure 3.6 The melting temperature of MOF crystal obtained from DSC.
36 CHAPTER FOUR SYNTHESIS AND CHARACTERIZATION OF COPPER-4, 4TRIMETHYLENE DIPYRIDINE METAL-ORGANIC FRAMEWORK-PMMA COMPOSITES 4.1 Introduction Poly(methyl methacrylate) (PMMA) is a tough, highly tran sparent plastic material with excellent resistance to the outdoor e nvironment such as ultraviolet radiation and weathering (Dorman & Cavette, 2002). It is pr esently one of the ol dest and one of the most widely used polymers because of its idea l properties. In this part of study, the MOF compound synthesized previously by Dr. Gauthiers group, copper-4,4trimethylenedipyridine metal-organic framewor k or CTMOF, was used as polymer fillers. It was characterized via X -ray Crystallography By Dr. Lukasz Wojtas and its crystal structure consists of infinite polymeric chains with Cu(II) ions bridged by two 4,4trimethylenedipyridine molecules. In this chapter, CTMOF-P MMA composites were synthesized by in situ polymerization and then charac terized via optical microscopy, differential scanning calori metry (DSC), microhardness, and dynamic mechanical analysis (DMA). The results are demonstr ated and compared to the control PMMA.
37 4.2 Experimental 4.2.1 Synthesis of CTMOF-PMMA composites Methyl methacrylate (MMA) monomer wa s purchased from Sigma-Aldrich and de-inhibited of the monomethyl ether hydr oquionone (MEHQ) inhibitor by a column. 0.2 wt% of the free radical initiator 2,2-azobi s (2,4-dimethylvaleroni trile) (Vazo52, Dupont) was added to the monomer. The CTMOFs were dispersed throughout the monomer matrix via in situ polymerization. The CTMOFs were originally dispersed in MMA monomer using a magnetic stir bar for 5 hrs a nd later sonicated using a Branson Sonifier 450 (Figure 4.1 Branson Sonifier 450) under ni trogen gas until the mixture became viscous. Following sonication, the mixt ure was cured in an oven at 65 oC for 6 hrs. CTMOFs wt% of 0%, 0.05%, 0.1% and 0.5% of the PMMA composites were created.
38 Figure 4.1 Branson Sonifier 450. 4.2.2 Optical Microscopy A 0.5% CTMOFs -PMMA composite and a pure PMMA sample were compress molded into 16.51 mm round films using a Carver Press equipped with a heating element. The stainless steel non-magnetic mirro r surface plates were used to achieve the smooth surfaces of the polymer samples. The press plates were pre-heated to 120oC. The composites were compression molded at this temperature for 10 min and then air cooled to room temperature. A Leica DMRX optical microscope was used to obtain images.
39 4.2.3 Differential scanning calorimetry (DSC) Glass transition temperatures (Tg) were recorded with a TA Instrument DSC 2920. Approximately 4-10 mg samples were sealed in aluminum pans. The DSC cell was heated under nitrogen gas using a ramp rate of 10oC/min from 30 oC to 140oC. The samples were air cooled to room temperature and heated again to 140oC. The Tg was determined from the second run in order to remove the thermal history. 4.2.4 Dynamic mechanical analysis (DMA) DMA samples were compress molded into 3261 mm rectangular films using the same technique for optical microscopy. A TA instrument DMA 2980 was used to obtain the mechanical data of the samples. Prior to measuring, the instrument, position, and clamp calibration were performed. The sa mples were tested under the tension film mode using a heating rate of 5oC/min from -150oC to 140oC and a scanning frequency range of 9-90 Hz with an amplitude of 10 microns. 4.2.5 Microhardness The optical microscopy samples were used for Microhardness testing. The Vicker hardness number (HV) for each sample was determined at room temperature with a Leica Vicker Microhardness Tester (VMHT) MOT e quipped with a square Vicker indenter. The values were taken from the average of ei ght indents. A load of 5N and a dwell time of 20s were used. Units were recorded in million pascal (MPa).
40 4.3 Result & Discussion 4.3.1 Optical Microscopy As shown in Fig. 4.2 & 4.3, the 0.5% CT MOF-PMMA sample is not optically clear, which indicates that the CTMOF is well dispersed & not soluble in the matrix. In order to ascertain any persistent intera ctions between CTMOF and PMMA, the 0.5% CTMOF-PMMA composite was immersed in acet one. The polymer matrix dissolved and the CTMOF appeared as particulate matter. This verified that there is no permanent interaction between CTMOF and polymer matrix. (a) (b) Figure 4.2 Comparison of Discs of (a) the 0.5% CTMOF-PMMA composite and (b) Pure PMMA.
41 (a) (b) Figure 4.3 Comparison of optical microscope images of (a) the 0.5% CTMOF-PMMA composite and (b) Pure PMMA. 4.3.2 DSC The glass transition temperature (Tg) is a rate-dependent temperature at which an amorphous polymer becomes soft and flexible on heating. It was observed that the Tg increases as the concentration of CTMOF increases. As showed in Table 4.1, the Tg of PMMA composite increases 4.8 oC as the CTMOF concentration increases from 0% to 0.5%. This trend in Tg suggests a decrease in the avai lable free volume as the CTMOF concentration increases. This can be explaine d as the large size of CTMOF increases the entanglement of the polymer chains and thus restricts their movement. This trend is consistent with Mohomeds work on na noball-poly (hydroxyethyl methacrylate) composites (Mohomed et al., 2005a; Mohome d, Gerasimov, Abourahma, Zaworotko & Harmon, 2005b): the Tg increases as the nanoball concen tration increases, which is a result from the maximum interactio n between the nanoball and polymer. Figure 4.4 Figure 4.7 are the original DSC plots from which the Tg are calculated.
42 Table 4.1 Glass transition temperat ures of the PMMA composites Sample Tg (oC) Neat PMMA 112.3 0.05% CTMOF-PMMA 113.6 0.1% CTMOF-PMMA 116.1 0.5% CTMOF-PMMA 117.1 Figure 4.4 DSC data: Glass transition temperature, Tg, of pure PMMA.
43 Figure 4.5 DSC data: Glass transition temperature, Tg, of 0.05% CTMOF-PMMA composite.
44 Figure 4.6 DSC data: Glass transition temperature, Tg, of 0.1% CTMOF-PMMA composite.
45 Figure 4.7 DSC data: Glass transition temperature, Tg, of 0.5% CTMOF-PMMA composite.
46 4.3.3 Microhardness The Vicker hardness numbers (HV) and their deviations for each composite material are shown in Table 4.2. HV number increases as the conc entration of CTMOF increases, which confirms the same trend as Tg. As described in the theory part of microhardness: Tg generally increases with increasi ng cohesive energy density (CED) and CED is also a main factor in a materials HV value (Mohomed et al., 2005a). As shown in Figure 4.8, materials with high HV number show high Tg values, which verify the direct relationships between Tg and HV. The increased resistance to the surface deformation may be due to the decreased free volume in the composite material resulting from the increased entanglement of the polymer chains.
47 Table 4.2 The eight Vickers hardness measurem ents and their devia tions for each PMMA composite sample (unit: MPa). Sample No. Neat PMMA 0.05% CTMOFPMMA 0.1% CTMOF PMMA 0.5% CTMOF PMMA 1 225.4 233.3 237.2 248.9 2 226.4 234.2 234.2 247.0 3 223.5 235.2 233.3 250.9 4 226.4 231.3 240.1 248.0 5 227.4 231.3 236.2 251.9 6 229.3 231.3 236.2 252.9 7 226.4 225.4 234.2 251.9 8 227.4 232.3 234.2 246.0 Average 226.7 231.9 235.9 251.1 Deviation 1.6 2.8 2.1 4.9 Table 4.3 Glass transition temperature and Vickers hardness number of the PMMA composites. Sample Tg (oC) Hardness Number, HV (MPa) Neat PMMA 112.3 226.7 1.6 0.05% CTMOF-PMMA 113.6 231.9 2.8 0.1% CTMOF-PMMA 116.1 235.9 2.1 0.5% CTMOF-PMMA 117.1 251.1 4.9
48 Figure 4.8 Direct relationships between Tg and HV for the CTMOF-PMMA composites. 220 225 230 235 240 245 250 255 260 112113114115116117118 Tg (C)HV (MPa)
49 4.3.4 Dynamic mechanical analysis (DMA) DMA is used to measure viscoelastic beha vior of materials. The viscoelastic property of polymers under applied stresses is a combination of a true elastic solid and a true liquid. The storage modulus E, or the elastic modulus, examines the ability of the material to return or store energy. E increases as the sweep frequency increases since the polymer chains need time to respond. Th e following figure 4.9 to figure 4.13 showed the storage modulus of CTMOF-PMMA composite samples. The E increases as the CTMOF loading increases. Figure 4.9 DMA data: Storage Modulus, E, vs. temperature for pure PMMA.
50 Figure 4.10 DMA data: Storage Modulus, E, vs. temperature for 0.05% CTMOF-PMMA composite.
51 Figure 4.11 DMA data: Storage Modulus, E, vs. temperature for 0.1% CTMOF-PMMA composite.
52 Figure 4.12 DMA data: Storage Modulus, E, vs. temperature for 0.5% CTMOF-PMMA composite.
53 Figure 4.13 Storage Modulus, E, vs. temperat ure at 90 Hz for the 0%, 0.05%, 0.1% and 0.5% CTMOF-PMMA composite.
54 The loss modulus (E) is a measure of the ability of a material to dissipate mechanical energy by converting it into heat (Clayton et al., 2004; Harmon et al., 2002; Higgenbotham-Bertolucci, Gao & Ha rmon, 2001; Muisener et al., 2002). The absorption of mechanical energy is often related to the movements of molecular segments within the material. Neat PMMA exhibits three relaxations: , and The primary relaxation is transition which is referred to the movement of the PMMA main ch ain and it corresponds to the glass transition. The secondary relaxation, transition, refers to the rotation of the ester side group, and the relaxation results from the rotation of the methyl group attached to the main chain. The comparison of E vs. temperature between neat PMMA and 0.05% CTMOF-PMMA composite at 90 Hz is shown in Figure 4.14. The transition is barely visi ble in pure PMMA at -85 oC but not resolved for CTMOF-PMMA composites. The transition onset is not ed, but the samples softened upon heating and the full transition region was obscured. The hi gher glass transition temperature of the CTMOF-PMMA composite suggested that CTMOF hindered the movement of the PMMA chain and thus stiffened the material. This is also shown by the comparison of storage modulus vs. temperature between neat PMMA and 0.05% CTMOF-PMMA composite in Figure 4.13. The table 4.4 shows the storage modulus (E) values at 90 Hz and -100oC, -50oC, 0oC, 50oC for the 0%, 0.05%, 0.1% and 0.5% CTMOF-PMMA composites. At the same temperatures, the E increases as the concentration of CTMOF increases. However, the E decreases with increasing temperature for all samples. This is consistent with the viscoelastic properties of polymers. The mobility of polymer chains increases as the temperature increases which leads to the softening of the polymer material.
55 Figure 4.14 Loss Modulus, E, vs. temperature for the neat PMMA and 0.05% CTMOFPMMA composite at 90 Hz.
56 Table 4.4 DMA data: Storage Modulus values at 90 Hz and -100 oC, -50 oC, 0 oC and 50oC. Sample Storage Modulus at 90 Hz (MPa) -100 oC -50 oC 0 oC 50 oC Neat PMMA 5970 5273 4441 3266 0.05% CTMOF-PMMA 7032 6200 5134 3815 0.1% CTMOF-PMMA 7213 6334 5211 3820 0.5% CTMOF-PMMA 7502 6580 5467 3928
57 Figure 4.15 DMA data: Loss Modulus, E, vs. temperature for pure PMMA. Figure 4.16 DMA data: Arrhenius plot of transition for pure PMMA. Pure PMMAy = -8309.7x + 28.965 R2 = 0.9974 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0.00290.0030.00310.00320.00330.00340.00350.0036 1/T (1/K)ln Frequency Ea = 69.1 KJ/mol
58 Figure 4.17 DMA data: Loss Modulus, E, vs. temperature for 0.05% CTMOF-PMMA composite. Figure 4.18 DMA data: Arrhenius plot of transition for 0.05% CTMOF-PMMA composite. 0.05%crystal-PMMAy = -8764.8x + 30.507 R2 = 0.9903 0 1 2 3 4 5 0.00290.0030.0030.00310.00310.00320.00320.00330.0033 1/T (1/K)ln Frequency Ea=72.9 KJ/mol
59 Figure 4.19 DMA data: Loss Modulus, E, vs. temperature for 0.1% CTMOF-PMMA composite. Figure 4.20 DMA data: Arrhenius plot of transition for 0.1% CTMOF-PMMA composite. 0.1%crystal-PMMAy = -8845.2x + 31.295 R2 = 0.9992 0 1 2 3 4 5 0.0030.00310.00320.00330.00340.0035 1/T (1/K)ln Frequency Ea=73.5 KJ/mol
60 Figure 4.21 DMA data: Loss Modulus, E, vs. temperature for 0.5% CTMOF-PMMA composite. Figure 4.22 DMA data: Arrhenius plot of transition for 0.5% CTMOF-PMMA composite. 0.5%crystal-PMMAy = -10841x + 37.333 R2 = 0.9948 0 1 2 3 4 5 0.0030.00310.00320.00330.00340.0035 1/T (1/K)ln Frequency Ea=90.14 KJ/mol
61 The activation energies of transition were obtained by plotting 1/temperature at the maximum peak height against the natura l logarithm of the frequency (from Figure 4.15 to Figure 4.22) and are listed in Table 4.5. The activation energy for the relaxation of the composite material increases as th e concentration of CTMOF increases. This results from the decreased free volume in the PMMA matrix. The ester side group is hindered by CTMOF and thus need s more energy to rotate. Table 4.5 Comparison of activation energies of the transition for the PMMA composites as determined from DMA. Sample Activation Energy (KJ/mol) Neat PMMA 69.1 0.05% CTMOF-PMMA 72.9 0.1% CTMOF-PMMA 73.5 0.5% CTMOF-PMMA 90.14
62 CHAPTER FIVE CONCLUSI ON AND FUTURE WORK In this study, a novel copper-4,4-tri methylenedipyridine metal-organic framework (CTMOF)Poly(methyl methacryl ate) (PMMA) composites was synthesized by in situ polymerization. CTMOF fillers were previously synthesized by Massing from Dr. Gauthiers group via the vi al-in-vial method and were characterized by Dr. Wojtas from Dr. Zaworotkos group via X-ray crysta llography. The crystal structure of CTMOF consists of infinite polymeric chains with Cu(II) ions bridged by two 4,4trimethylenedipyridine molecules. Cu(II) ions a dopt a slightly distorted square pyramidal geometry where each Cu(II) ion is c oordinated by four N atoms of 4,4trimethylenedipyridine and one O atom of nitrate anion. CTMOF-PMMA composites were then characterized. The DSC data show s that the glass transition temperatures (Tg) of the composites increase as the CTMOF c oncentration increases, which was explained as the large size of CTMOF increases the en tanglement of the polymer chains and thus restricts their movement. In a microhardness st udy, the Vicker hardne ss numbers (HV) of the composites show the same trend as Tg, which may be due to the decreased free volume in the composite material. The storage modulus (E), loss modulus (E), and the activation energies of transition data obtained from th e Dynamic mechanical analysis (DMA) also suggested that CTMOF hindered the polymer chains and thus the polymer chains need more energy to move and dissipate the mechanical energy to heat. This initial
63 study suggests that these novel metal-organi c composites with enhanced mechanical properties warrant further study for use in dielec trics, sensors and elec tronic applications. The green MOF crystal we got from chapte r 3 still needs to be characterized to determine its structure and physical pr operties. The green MOF crystal-PMMA composites could be synthesized via in situ polymerization and be characterized by DSC, DMA, microhardness, and optical microscopy or SEM. The thermal properties and filler dispersion could be determined and compared to the CTMOF-PMMA composites. Since 1,3-adamantanedicarboxylic acid was present as a reactant, it is po ssible that the green MOF is constructed by a copper (II) metal center and 1,3-adamantanedicarboxylic acid ligand. This means the green crystal should be hydrophilic. Therefore, both the maximum and minimum interactions between the fillers and polymer matrices can be achieved by carefully selecting a hydrophi lic and a hydrophobic polymer matrices. The comparison of the physical properties of these two different composites will be made. Potentially, this will help to better understand how the proper ties of polymer compos ites are determined by the composite structure and the interface between the filler and matrix.
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ABOUT THE AUTHOR Shisi Liu received a Bachelor of Scien ce in Analytical Chemistry from Beijing University of Chemical Technology (China) in 2002. She worked as a lab technician at the Beijing Tianrui Nanomate rial Company in China until 2004. She entered the Master in Chemistry Program at the University of South Florida in spring 2005 and worked in the lab. of Dr. Julie P. Harmon.