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Graduate School
2009
Synthesis and characterization of a novel
Poly(methyl methacrylate) Composites using
Copper-4, 4'- Trimethylenedipyridine MetalOrganic Framework as Fillers
Shisi Liu
University of South Florida
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Synthesis and Characterization of a Novel Poly(methyl methacrylate) Composites using
Copper-4, 4’- Trimethylenedipyridine Metal-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: Julie P. Harmon, Ph.D.
Carmen Valdez Gauthier, Ph.D.
Abdul Malik, Ph.D.
Date of Approval:
April 9, 2009
Keywords: PMMA composites, metal-organic framework, mechanical 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. Carmen 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 members: 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. Zaworotko 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, Chunyan Wang. Thanks for all the support and
help!
TABLE OF CONTENTS
LIST OF TABLES
v
LIST OF FIGURES
vi
ABBREVATION
ix
ABSTRACT
x
CHAPTER ONE INTRODUCTION
1.1 Metal-Organic Frameworks
1.2 Polymer Composites
1.3 Metal-containing Polymer Composites
1
5
6
CHAPTER TWO AN OVERVIEW OF POLYMER SCIENCE AND
INSTRUMENTATION THEORY
2.1 Introduction
2.2 An overview of polymer science and polymer synthesis
2.3 Microhardness
2.4 Differential Scanning Calorimetry (DSC)
2.5 Dynamic Mechanical Analysis (DMA)
8
8
13
16
19
CHAPTER THREE SYNTHESIS AND CHARACTERIZATION OF COPPER-4, 4’TRIMETHYLENEDIPYRIDINE METAL-ORGANIC FRAMEWORK
3.1 Introduction
26
3.2 Experimental
27
3.2.1 Synthesis of Copper-4,4’trimethylenedipyridine 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 CHARACTERIZATION OF COPPER-4, 4’TRIMETHYLENEDIPYRIDINE METAL-ORGANIC FRAMEWORK-PMMA
COMPOSITES
4.1 Introduction
36
4.2 Experimental
37
i
4.2.1 Synthesis of CTMOF-PMMA composites
4.2.2 Optical Microscopy
4.2.3 Differential Scanning Calorimetry
4.2.4 Dynamic Mechanical Analysis
4.2.5 Microhardness
4.3 Results and Discussion
4.3.1 Optical Microscopy
4.3.2 DSC
4.3.3 Microhardness
4.3.4 Dynamic Mechanical Analysis
37
38
39
39
39
40
40
41
46
49
CHAPTER FIVE CONCLUSION AND FUTURE WORK
62
REFERENCES
64
ABOUT THE AUTHOR
End Page
ii
LIST OF TABLES
Table 3.1
Crystal data and structure refinement for compound CTMOF.
30
Table 3.2
Selected Bond Distances (Å) and Angles (deg) for compound
CTMOF.
31
Table 4.1
Glass transition temperatures 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 temperature and Vickers hardness number of
the PMMA composites.
47
Table 4.4
DMA data: Storage Modulus values at 90 Hz and -100 oC, -50
o
C, 0 oC and 50oC.
56
Table 4.5
Comparison of activation energies of the β transition for the
PMMA composites as determined from DMA.
61
iii
LIST OF FIGURES
Figure 1.1
The rapid growth of citations 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.com/microhardness-tester.htm).
14
Figure 2.2
The Leica Vicker Microhardness Tester.
14
Figure 2.3
Several possible transitions of polymer materials 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 response and 100% viscous response 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
iv
Figure 2.10
TA instrument DMA 2980 (TA Instruments DMA 2980,
2002, Deleware).
24
Figure 2.11
DMA tension film clamp (TA Instruments DMA 2980, 2002,
Delaware).
25
Figure 3.1
(a) The Vial-in-Vial method 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 polymeric 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 microscope images of (a) the 0.5%
CTMOF-PMMA composite and (b) Pure PMMA.
41
Figure 4.4
DSC data: Glass transition temperature, Tg, 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 relationships between Tg and HV for the CTMOFPMMA composites.
48
Figure 4.9
DMA data: Storage Modulus, E’, vs. temperature for pure
PMMA.
49
v
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 β 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 β 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 β 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 β transition for 0.5% CTMOFPMMA composite.
60
vi
LIST OF ABBREVIATIONS
Activation energy
Ea
Cohesive energy density
CED
Copper-4, 4’-trimethylenedipyridine Metal-organic
framework
Differential Scanning Calorimetry
CTMOF
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
vii
Synthesis and Characterization of a Novel Poly(methyl methacrylate) Composites using
Copper-4, 4’- Trimethylenedipyridine Metal-Organic Framework as Fillers
Shisi Liu
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.
viii
CHAPTER ONE INTRODUCTION
1.1 Metal-Organic Frameworks
Metal-organic frameworks (MOFs), also called coordination polymers or
supramolecular structures, are compounds with backbones constructed from metal ions
and ligands to create zero-, one-, two-, and three-dimensional structures. MOFs are
inorganic-organic hybrid compounds unlike traditionally organic polymers (Blake,
Champness, Hubberstey, Li, Withersby & Schroder, 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 (Tomic, 1965) who studied the formation of
MOFs from the reaction of 1,5-Dihydroxynaphthalene-2,6-dicarboxylic acid (1,5-N-2,6)
with Zn, Ni, Al, and Fe+3. The area did not blossom until the late 1980’s 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 characteristics of these compounds were discovered. The number
of MOF compounds showed a rapid growth from the late-1990s to present (Figure 1.1).
The advantages of MOFs include: high porosities of nanometer-sized spaces in them,
high designibility and regularity of the framework, high thermal and mechanical stability,
and their world record surface areas (Hagrman, Hagrman & Zubieta, 1999; Moulton &
Zaworotko, 2001; Mueller, Schubert, Teich, Puetter, Schierle-Arndt & Pastre, 2006;
1
Yaghi, O'Keeffe, Ockwig, Chae, Eddaoudi & Kim, 2003). These advantages revealed
their potential applications in catalysis, gas purification, 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 Robson’s “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 pre-designed to different well-defined configurations.
2
Figure 1.2 (Wang, 2006) shows some typical examples 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 exist. The first is the “nodeand-spacer” approach (Robson, 2008; Wells, 1977; 1984) in which the compound
frameworks consist of simple dimensional 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. Another 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,
3
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 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).
1.2 Polymer Composites
As early as the mid-20th century, the research 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 characterization of polymer composites in order to achieve
advantageous properties such as increased stiffness, strength, dimensional stability,
modified electrical, optical and magnetic properties, and reduced cost (Clayton,
Gerasimov, Cinke, Meyyappan & Harmon, 2004; Varga, Feher, Filipcsei & Zrinyi, 2003;
Wilson et al., 2004). Many commercial polymeric materials are composites, for example
polyblends and ABS materials, filled poly (vinyl chloride) materials used in floor tile and
wire coatings, filled thermosetting resins, and glass or graphic-fiber-filled plastics
5
(Nielsen & Landel, 1994). Polymer composite materials is defined as materials consisting
of two or more components and containing two or more phases (Nielsen & 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, such 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 the interface between the phases. Many techniques
have been developed to improve the interfacial adhesion and particle-fillers dispersion in
polymer matrices, such as in situ polymerization and melt blending (Mohomed, 2006;
Park & Jana, 2003; Rong, Jing, Li & Sheng, 2001; Tatro, Clayton, Muisener, Rao &
Harmon, 2004; Xiong, Wu, Zhou & You, 2002). Each 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 applications. On the other hand, the melt blending is a
mature technique widely used in large scale composite production but is often
accompanied by filler agglomeration.
1.3 Metal-containing Polymer Composites
Since the late 20th century, functionalized polymer materials with advanced
properties have been the target. One of the principal trends in composites is to
6
incorporate metal-ligand supramolecular structures into traditional carbon-based
polymers. These metal-containing hybrid polymer materials have the potential to change
physical, electronic, optical, and catalytic 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 states or facilitate electron flow (Williams,
Boydston & Bielawski, 2007). For example, such polymers can catalyze carbon-carbon
bond-forming reactions by oxidative insertions and reductive eliminations. Thus they can
be employed as recoverable catalysts in industry. 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 chains or the side chains of the carbon-based
polymers (Pefkianakis, Tzanetos & Kallitsis, 2008). This type of materials 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 composites by using MOF compounds as fillers and
organic polymers as matrices. This method is used in this study.
7
CHAPTER TWO AN OVERVIEW OF POLYMER SCIENCE AND
INSTRUMENTATION THEORY
2.1 Introduction
This chapter introduces the basic knowledge and techniques applied in this study.
A brief overview of polymer science and polymer synthesis is discussed. The theories
and operations of microhardness and two thermal analysis techniques used in this
research for characterization of the polymer 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 consisting of molecular repeating units
connected by covalent bonds. The properties of a polymer do not change significantly
when one or several repeating units are added 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, and natural rubber; 2) modified natural
polymer materials, or derivatives of natural polymers (Seymour, 1987), such as rayon,
cellulose acetate, and modified starch; and 3) synthetic polymer materials, such as
8
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 ancient 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; proteins, 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 could no longer satisfy human’s requirement, the
processing and modification methods for natural polymers were developed in the 19th
century (Dai, Zhang & Jiang, 2005). In 1839, Goodyear 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 real 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). Glyceryl phthalate resins, poly (2,3dimethylbutadiene), ethyl cellulose and urea-formaldehyde resins had also been
synthesized in the first decades of this century (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 were presented at this era.
9
Polymer science is a material subfield studying polymer structures, properties,
synthesis, processing, and applications. It includes 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 investigates polymer configurations, properties,
characterizations, and the interrelationships between structure and property. Polymer
engineering connects polymer science and 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 reaction through which monomers covalently
bond to form polymers (Odian, 2004). There are several different ways to classify
polymers or polymerizations. During the development of polymer science, two types of
classifications have been widely used. One classification based on polymer structures
divides polymers into condensation and addition polymers. Another one based on the
mechanisms of polymerization processes divides polymerization into step and chain
polymerizations. In most situations, these two sets of terms are 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 formation of Nylon 6/6, a extensively used fiber and
10
plastic (Odian, 2004). As shown in Equation 2.1, hexamethylene diamine reacts with
adipic acid to produce poly (hexamethylene adipamide) or nylon 6/6.
n HO—R—OH +
n HO2C—R’—CO2H
→
Equation 2.1 The step polymerization of nylon 6/6, where R = (CH2)6 and R’ = (CH2)4.
Addition or chain polymerizations are the reactions producing various polymers
without the loss of small molecules. This type 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 methacrylate). The poly (methyl methacrylate) is used as
polymer matrix in this study.
Equation 2.2 The polymerization of polyethylene.
11
Equation 2.3 The chain polymerization of poly (methyl methacrylate).
In chain polymerization, usually 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 thesis, 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 radical and reacts as reactive center (Fernandes,
2005; McConnell, Barton, LaPack & DesJardin, 2002).
Equation 2.4 Thermal initiation of Vazo 52 (Mohomed, 2006).
12
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 microhardness 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 & Fakirov, 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 hardness. 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 indenters, such as a steel ball (Brinell test), diamond cone
(Grodzinski test) and diamond pyramid (Berkovich, Knoop and Vickers tests) (Mohomed,
2006).
13
Figure 2.1 The Vickers indentation (http://www.hardnesstesters.com/microhardnesstester.htm).
Figure 2.2 The Leica Vicker Microhardness Tester.
14
Microhardness testing usually involves measurements 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 Microhardness Tester (VMHT) MOT equipped
with a square Vicker indenter was used (Figure 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 held for 6-30 s, and then removed. The Vicker
hardness number HV, expressed in megapascals (MPa), was determined via Equation 2.6.
α
sin
F
F
HV =
= 2 F 22 = 1854.4 2
A
d
d
Equation 2.6
Where F is the applied force in newtons, A is the surface area of the imprint in square
millimeters, α is the angle, and d is the average 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 glass transition temperature, Tg, generally increases with
increasing cohesive energy density (CED) as shown in equation 2.7:
2δ 2
Tg =
+C1
mR
Equation 2.7
15
where δ2 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
material’s HV value (Mohomed, Abourahma, Zaworotko & Harmon, 2005a). Therefore,
the positive proportion between the Tg and HV is established by relating 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 function 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 chemical 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 crystallization time & temperature, percent
crystallinity, heats of fusion and reaction, specific heat, oxidative stability, rate of cure,
degree of cure, reaction kinetics, purity, and thermal stability (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 advantages, including fast analysis time
(usually less than 30 minutes), easy sample preparation, applicability to both solids and
liquids, wide temperature range, and excellent quantitative capability.
16
Figure 2.3 Several possible transitions of polymer materials characterized 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 instrument 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). Figure 2.4 shows the structure of a heat flux
DSC cell. The sample and the reference pan 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 reference pans and their temperatures are
measured by area thermocouples under the platforms. The Ohm’s Law, expressed as
Equation 2.8, is used to measure the heat flow difference.
17
dQ ΔT
=
dt RD
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 the 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.
18
In this study, a TA instruments 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 analysis (DMA), or dynamic mechanical thermal analysis
(DMTA), is a technique used to study and characterize the viscoelastic properties of
materials. In DMA, an oscillating minor sinusoidal force is applied to a sample as a
function of time and temperature, and the material’s response to that force is analyzed
19
(Menard, 1999). The force loaded to sample is named stress and is marked by the Greek
letter, σ. The deformation with which a material responds is strain, or γ. The deformation
can be determined by amplitude and phase shift. The phase shift angle, or time lag,
between the stress and strain is marked as δ. 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 δ = 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 (δ = 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 δ 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 material’s response. The unrecovered the strain represents the
energy dissipated or viscous portion of the material’s 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).
20
Figure 2.6 100% elastic response and 100% viscous response of material (TA
Instruments).
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).
The complex modulus is defined as stress over strain, marked as E* or G* (shown
in Figure 2.9). E* measures a material’s 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 δ), 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 measuring conditions and the history of materials
(Ehrenstein et al., 2004).
22
Figure 2.9 Modulus relationships of viscoelastic materials (TA Instruments DMA 2980
2002).
Three experimental testing modes can be applied in DMA: dynamic multifrequency oscillatory mode, creep mode (or transient test mode), and stress relaxation
mode [TA Instruments DMA 2980 2002]. In dynamic multi-frequency oscillatory test, an
oscillatory (sinusoidal) strain (or stress) is applied to the material and the responding
stress (or strain) is measured. Storage modulus, loss modulus, and other data will be
collected as a function of time, temperature 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 such as creep compliance and the recoverable
compliance will be collected. In a stress relaxation 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 relaxation modulus and the sample recovery will
be collected versus time as the strain releases.
23
In this study, a TA instrument DMA 2980 (Figure 2.10) was employed to
examine the viscoelastic behavior of the samples. 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 time, frequency and temperature. The tension
film clamp was applied, as shown in Figure 2.11. Several calibrations are performed
before the sample testing, including temperature, instrument, position, and clamp
calibrations.
Figure 2.10 TA instrument DMA 2980 (TA Instruments DMA 2980, 2002, Deleware).
24
Figure 2.11 DMA tension film clamp (TA Instruments DMA 2980, 2002, Delaware).
25
CHAPTER THREE SYNTHESIS AND CHARACTERIZATION OF
COPPER-4, 4’-TRIMETHYLENEDIPYRIDINE 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) (Pefkianakis 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. Gauthier’s
group concentrated on design and synthesis of MOFs using copper (II) as metal centers
(Cherenfant, West & Gauthier, 2006). One MOF synthesized by this group, Copper-4, 4’trimethylenedipyridine, was chosen as the polymer filler in this project. This overall
polymer composite project is a joint collaboration between Dr. Gauthier’s lab and Dr.
Harmon’s lab. The MOF used as polymer filler, Copper-4, 4’-trimethylenedipyridine,
was synthesized by Justin Massing from Dr. Gauthier’s 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. Zaworotko’s group. The melting temperature 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.
26
The experiment procedures and results were demonstrated in this chapter in order to
better understand the structure and the properties of the filler. In the 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-4,4’trimethylenedipyridine metal organic framework (CTMOF)
Copper(II) nitrate hemipentahydrate (0.20 g, 0.86 mmol) and 1,3adamantanedicarboxylic acid (0.192 g, 0.86 mmol) were dissolved in 10 ml of methanol
and placed in a small vial, which was placed inside a larger vial (shown in Figure 3.1).
The copper/adamantane dicarboxylic acid solution 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 larger vial, completing submerging the small
reaction vial. After one week of slow diffusion purple and green crystals were isolated.
The purple crystals (CTMOF) were characterized by single x-ray crystallography and
differential scanning calorimetry (DSC).
27
(a)
(b)
Figure 3.1 (a) The Vial-in-Vial method of synthesis (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 collected using Bruker-AXS SMART-APEX
CCD diffractometer (MoKα ,λ = 0.71073 Å). Indexing was performed using SMARTv5.625 (Bruker-AXS, Data Collection Software. Madison, Wisconsin, USA, 2001).
Frames were integrated with SaintPlus 6.01 (Bruker-AXS, SAINT-V6.28A, Data
Reduction Software. Madison, Wisconsin, USA, 2001) software package. Absorption
correction was performed by multi-scan method implemented in SADABS (Sheldrick, G.
M., Program for Empirical Absorption Correction. University of Gottingen, Germany,
1996). The structure was solved using SHELXS-97 and refined using SHELXL-97
contained in SHELXTL v6.10 (Sheldrick, G. M., Bruker-AXS Madison, Wisconsin, USA.
2000) and WinGX v1.70.01 (Farrugia, 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
28
included in the refinement process using riding model. Crystal data and refinement
conditions are shown in Table 3.1 Geometrical parameters are shown in Table 3.2.
29
Table 3.1 Crystal data and structure refinement for compound CTMOF (Wojtas, 2009).
Empirical formula
Cu•2NO3•2(C13H14N2)•C6H4Cl2•1.7CH3OH
Formula weight
392.36
Temperature
100(2) K
Crystal system, space group
Triclinic, P-1
Unit cell dimensions
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)
Volume
1779(2) A^3
Z, Calculated density
2, 1.461 Mg/m^3
Absorption coefficient
0.822 mm^-1
Theta range for data collection (MoKα)
1.96 to 25.62 deg.
Reflections collected / observed / unique
9227 / 6463 [R(int) = 0.0594]
Completeness to theta = 28.31
96.0 %
Refinement method
Full-matrix least-squares on F^2
Goodness-of-fit on F^2
0.985
Final R indices [I>2sigma(I)]
R1 = 0.0799, wR2 = 0.1762
Largest diff. peak and hole
0.855 and -0.701 e.A^-3
30
Table 3.2 Selected Bond Distances (Å) and Angles (deg) for compound CTMOF (Wojtas,
2009).
Cu(1)-N(21)
2.004(5)
Cu(1)-N(1)#1
2.010(5)
Cu(1)-N(2)
2.014(5)
Cu(1)-N(22)#2
2.019(5)
Cu(1)-O(63)
2.210(5)
N(21)-Cu(1)-N(1)#1
91.1(2)
N(21)-Cu(1)-N(2)
164.7(2)
N(2)-Cu(1)-N(22)#2
90.9(2)
N(21)-Cu(1)-O(63)
109.1(2)
N(1)#1-Cu(1)-O(63)
95.2(2)
N(2)-Cu(1)-O(63)
86.2(2)
N(22)#2-Cu(1)-O(63)
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.
31
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 pyramidal 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 percentage of trigonal distortion as defined in (Hathaway, 1987)
equals 6.5%. In the structure the charge is balanced through nitrate anions. Polymeric
chains are closely packed forming the layers [Figure 3.4 & Figure 3.5]. Space between
the layers is occupied by 1, 2-dichlorobenzene 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 Waals and weak hydrogen bonds forming a 1-D
coordination polymer type of structure.
Figure 3.2 Polymeric chain in CTMOF (Wojtas, 2009).
32
Figure 3.3 Coordination and atom numbering scheme for CTMOF (Wojtas, 2009).
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 polymeric chains (Wojtas, 2009).
3.3.2 Other Characterizations-DSC
The melting temperature of this MOF crystal was obtained from the DSC curve,
as shown in Figure 3.6. Tm is 207.16 oC.
34
Figure 3.6 The melting temperature of MOF crystal obtained from DSC.
35
CHAPTER FOUR SYNTHESIS AND CHARACTERIZATION OF
COPPER-4, 4’- TRIMETHYLENEDIPYRIDINE METAL-ORGANIC
FRAMEWORK-PMMA COMPOSITES
4.1 Introduction
Poly(methyl methacrylate) (PMMA) is a tough, highly transparent plastic material
with excellent resistance to the outdoor environment such as ultraviolet radiation and
weathering (Dorman & Cavette, 2002). It is presently one of the oldest and one of the
most widely used polymers because of its ideal properties. In this part of study, the MOF
compound synthesized previously by Dr. Gauthier’s group, copper-4,4’trimethylenedipyridine metal-organic framework 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 4,4’trimethylenedipyridine molecules. In this chapter, CTMOF-PMMA composites were
synthesized by in situ polymerization and then characterized via optical microscopy,
differential scanning calorimetry (DSC), microhardness, and dynamic mechanical
analysis (DMA). The results are demonstrated and compared to the control PMMA.
36
4.2 Experimental
4.2.1 Synthesis of CTMOF-PMMA composites
Methyl methacrylate (MMA) monomer was purchased from Sigma-Aldrich and
de-inhibited of the monomethyl ether hydroquionone (MEHQ) inhibitor by a column. 0.2
wt% of the free radical initiator 2,2’-azobis (2,4-dimethylvaleronitrile) (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 and later sonicated using a Branson Sonifier
450 (Figure 4.1 Branson Sonifier 450) under nitrogen gas until the mixture became
viscous. Following sonication, the mixture 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.
37
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 16.5×1 mm round films using a Carver Press equipped with a heating
element. The stainless steel non-magnetic mirror 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.
38
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 32×6×1 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 samples 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 equipped with a square Vicker indenter.
The values were taken from the average of eight indents. A load of 5N and a dwell time
of 20s were used. Units were recorded in million pascal (MPa).
39
4.3 Result & Discussion
4.3.1 Optical Microscopy
As shown in Fig. 4.2 & 4.3, the 0.5% CTMOF-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 interactions between CTMOF and PMMA, the 0.5%
CTMOF-PMMA composite was immersed in acetone. 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.
40
(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 available free volume as the CTMOF
concentration increases. This can be explained as the large size of CTMOF increases the
entanglement of the polymer chains and thus restricts their movement. This trend is
consistent with Mohomed’s work on nanoball-poly (hydroxyethyl methacrylate)
composites (Mohomed et al., 2005a; Mohomed, Gerasimov, Abourahma, Zaworotko &
Harmon, 2005b): the Tg increases as the nanoball concentration increases, which is a
result from the maximum interaction between the nanoball and polymer. Figure 4.4 –
Figure 4.7 are the original DSC plots from which the Tg are calculated.
41
Table 4.1 Glass transition temperatures 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.
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% CTMOF-PMMA
composite.
44
Figure 4.7 DSC data: Glass transition temperature, Tg, of 0.5% CTMOF-PMMA
composite.
45
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 concentration of CTMOF
increases, which confirms the same trend as Tg. As described in the theory part of
microhardness: Tg generally increases with increasing cohesive energy density (CED) and
CED is also a main factor in a material’s 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.
46
Table 4.2 The eight Vickers hardness measurements and their deviations for each PMMA
composite sample (unit: MPa).
Sample
Neat PMMA
No.
0.05% CTMOF-
0.1% CTMOF -
0.5% CTMOF -
PMMA
PMMA
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
47
260
255
HV (MPa)
250
245
240
235
230
225
220
112
113
114
115
116
117
118
Tg (C)
Figure 4.8 Direct relationships between Tg and HV for the CTMOF-PMMA composites.
48
4.3.4 Dynamic mechanical analysis (DMA)
DMA is used to measure viscoelastic behavior 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. The 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.
49
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
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 & Harmon, 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: α, β, and γ. The primary relaxation is α
transition which is referred to the movement of the PMMA main chain and it corresponds
to the glass transition. The secondary relaxation, β transition, refers to the rotation of the
ester side group, and the γ 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 γ
transition is barely visible in pure PMMA at -85 oC but not resolved for CTMOF-PMMA
composites. The α transition onset is noted, but the samples softened upon heating and
the full transition region was obscured. The higher 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.
54
Figure 4.14 Loss Modulus, E”, vs. temperature for the neat PMMA and 0.05% CTMOFPMMA composite at 90 Hz.
55
Table 4.4 DMA data: Storage Modulus values at 90 Hz and -100 oC, -50 oC, 0 oC and
50oC.
-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
Sample Storage Modulus at 90 Hz
(MPa)
56
Figure 4.15 DMA data: Loss Modulus, E”, vs. temperature for pure PMMA.
ln Frequency
Pure PMMA
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0.0029
y = -8309.7x + 28.965
R2 = 0.9974
0.003
0.0031
Ea = 69.1 KJ/mol
0.0032
0.0033
0.0034
0.0035
1/T (1/K)
Figure 4.16 DMA data: Arrhenius plot of β transition for pure PMMA.
57
0.0036
Figure 4.17 DMA data: Loss Modulus, E”, vs. temperature for 0.05% CTMOF-PMMA
composite.
0.05%crystal-PMMA
ln Frequency
5
y = -8764.8x + 30.507
R2 = 0.9903
4
3
2
1
0
0.0029 0.003
0.003
Ea=72.9 KJ/mol
0.0031 0.0031 0.0032 0.0032 0.0033 0.0033
1/T (1/K)
Figure 4.18 DMA data: Arrhenius plot of β transition for 0.05% CTMOF-PMMA
composite.
58
Figure 4.19 DMA data: Loss Modulus, E”, vs. temperature for 0.1% CTMOF-PMMA
composite.
0.1%crystal-PMMA
y = -8845.2x + 31.295
R2 = 0.9992
5
ln Frequency
4
3
2
1
0
0.003
0.0031
Ea=73.5 KJ/mol
0.0032
0.0033
0.0034
0.0035
1/T (1/K)
Figure 4.20 DMA data: Arrhenius plot of β transition for 0.1% CTMOF-PMMA
composite.
59
Figure 4.21 DMA data: Loss Modulus, E”, vs. temperature for 0.5% CTMOF-PMMA
composite.
0.5% crystal-PMMA
5
y = -10841x + 37.333
R2 = 0.9948
ln Frequency
4
3
2
1
0
0.003
0.0031
Ea=90.14 KJ/mol
0.0032
0.0033
0.0034
0.0035
1/T (1/K)
Figure 4.22 DMA data: Arrhenius plot of β transition for 0.5% CTMOF-PMMA
composite.
60
The activation energies of β transition were obtained by plotting 1/temperature at
the maximum peak height against the natural logarithm of the frequency (from Figure
4.15 to Figure 4.22) and are listed in Table 4.5. The activation energy for the β relaxation
of the composite material increases as the 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 needs more energy to rotate.
Table 4.5 Comparison of activation energies of the β 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
61
CHAPTER FIVE CONCLUSION AND FUTURE WORK
In this study, a novel copper-4,4’-trimethylenedipyridine metal-organic
framework (CTMOF)- Poly(methyl methacrylate) (PMMA) composites was synthesized
by in situ polymerization. CTMOF fillers were previously synthesized by Massing from
Dr. Gauthier’s group via the vial-in-vial method and were characterized by Dr. Wojtas
from Dr. Zaworotko’s group via X-ray crystallography. The crystal structure of CTMOF
consists of infinite polymeric chains with Cu(II) ions bridged by two 4,4’trimethylenedipyridine molecules. Cu(II) ions adopt a slightly distorted square pyramidal
geometry where each Cu(II) ion is coordinated by four N atoms of 4,4’trimethylenedipyridine and one O atom of nitrate anion. CTMOF-PMMA composites
were then characterized. The DSC data shows that the glass transition temperatures (Tg)
of the composites increase as the CTMOF concentration increases, which was explained
as the large size of CTMOF increases the entanglement of the polymer chains and thus
restricts their movement. In a microhardness study, the Vicker hardness 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 β transition data obtained from the 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
62
study suggests that these novel metal-organic composites with enhanced mechanical
properties warrant further study for use in dielectrics, sensors and electronic applications.
The green MOF crystal we got from chapter 3 still needs to be characterized to
determine its structure and physical properties. 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 possible 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 hydrophilic 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 properties of polymer composites are determined
by the composite structure and the interface between the filler and matrix.
63
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ABOUT THE AUTHOR
Shisi Liu received a Bachelor of Science in Analytical Chemistry from Beijing
University of Chemical Technology (China) in 2002. She worked as a lab technician at
the Beijing Tianrui Nanomaterial 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.