Clay-Containing Polymeric Nanocomposites Volume 1

458 Pages·2004·5.1 MB·English

by  L A Utracki

Clay-Containing
Polymeric
Nanocomposites
Volume 1
L.A. Utracki

Clay-Containing
Polymeric
Nanocomposites
Volume 1
L.A. Utracki
C. Vasile
Rapra Technology Limited
Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom
Telephone: [phone] Fax: [phone]
http://www.rapra.net
First Published in 2004 by
Rapra Technology Limited
Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK
©2004, Rapra Technology Limited
All rights reserved. Except as permitted under current legislation no part
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have been overlooked.
ISBN: 1-85957-437-8
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Preamble
Preamble
L.A. Utracki
During the last few years terms like nanomaterials, nanocomposites and
nanosystems have become fashionable. It seems that anything with ‘nano’ attached
to it has nearly a magical effect – not so much on performance as on expectations.
There is an extensive worldwide effort to introduce nanotechnology for the
production of materials with specific functional characteristics, e.g.,
semiconducting, electromagnetic, optical, etc. New magneto-resistance materials
with nanometre-scale spin-flip mean free path of electrons have been
commercialised. The National Science Foundation (NSF) has solicited
collaborative research proposals in the area of nanoscale science and engineering,
including: nanoscale biosystems; nanoscale structures; novel phenomena and
control; nanoscale devices and system architecture; nanosystems-specific software;
nanoscale processes; multi-phenomena modelling and simulation at the nanoscale
level; studies on societal implications of nanoscale science and engineering, etc.
Nanostructures are of interest to many technologies. The potential of precise
control of impurities and defects in a crystal and the ability to integrate perfect
inorganic and organic nanostructures may lead to a new generation of advanced
materials. To electronics, they offer quantum devices (resonant tunnelling
transistors; single electron transistors; cellular automata based on quantum dots)
and new processor architectures. To catalysis, they form the templates for catalytic
activity, zeolite pores, etc. In biology, nanostructures are components of the
mitochondrion, the chloroplast, and the ribosome.
The advances in the synthesis and fabrication of isolated nanostructures range
from colloidal synthesis of nanocrystals to the growth of epitaxial quantum dots.
The techniques of molecular biology have made a wide range of biological
nanostructures readily available through cloning and overexpression in bacterial
production systems. Furthermore, work has begun on the use of self-assembly
techniques to prepare complex and designed spatial arrangements of
nanostructures. Techniques derived from microlithography in microelectronics
(viz. photo, X-ray, and e-beam lithography) offer the potential to economically
generate new types of 3D-structures. In short, there is a great potential for the
wide use of nanotechnology for functional materials and devices.
The central theme of this book is the use of nanotechnology for the
development of new structural polymeric systems, the polymeric nanocomposites
(PNCs), and particularly the clay-containing polymeric nanocomposites (CPNCs).
These mass produced materials are dispersions of inorganic, nanoscale platelets
in a polymeric matrix. Economics preclude the use of most of the manufacturing
methods developed for ceramic and metallic nanocomposites. The key to the
success of the CPNC industry is to provide new materials significantly
outperforming the old ones at a marginal incremental cost increase per unit
volume. Considering that at present the nanoclay is a natural mineral with well-
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Clay-Containing Polymeric Nanocomposites
recognised variability of composition, the secondary concerns focus on the
consistency of performance, not only at the batch-to-batch level, but also on a
long-term basis.
Nanostructures are intermediate in size between molecular and micron-size
systems, such as blends and composites. There is no doubt that structures with
architecture controlled on the molecular level may lead to refined properties or
even new sets of performance characteristics. Chemists have known this for
centuries, viz. more recent developments in nanosize structures such as fullerenes,
buckytubes, dendrimers and complex block copolymers. The unexpected
behaviour of adsorbed monolayers of organic molecules on a high-energy crystal
surface was discovered many decades ago. In the meantime the advances of
microscopy reached atomic-scale, providing images of crystalline unit cells and
bioactive macromolecules. Thus, it is legitimate to ask what, if anything, is so
different about the nanomaterials that warrants the distinction.
It is known that within the nanometre scale such properties as the melting
temperature, the remanence of a magnet, and the band gap of a semiconductor
depend upon the size of the component crystals. Furthermore, it has been shown
that the mechanical properties of metallic alloys hyperbolically increase with the
reduction of domain size.
It is customary to define nanocomposites in terms of the size of the dispersed
particles and the specific behaviour they engender. Thus, at least one dimension
of these particles must be less than 10 nm. Since these particles are usually
crystalline, the size and high surface energy leads to high surface area to volume
ratio and strong orientational forces that may lead to high packing densities and
quantum behaviour (explored as electronic, magnetic or optic elements in
microelectronics technology). In the most popular nanofiller in the CPNC industry,
montmorillonite, over 40% of atoms rest on the surface – the clay lamellae should
be treated either as giant inorganic molecules or at least as hybrids occupying the
grey zone between molecules and particles. This is not mere semantics, but has
profound consequences as far as the fundamentals of CPNC are concerned, viz.
miscibility or flow behaviour.
This book summarises the pertinent developments in the area of the science
and technology of clay-reinforced polymeric nanocomposites. There are several
reasons for using clays, viz. availability, cost, and aspect ratio. The theory and
experiments show that to maximise the benefits of nanotechnology the clay must
be fully decomposed into individual crystalline lamellae (exfoliated) and these
must be uniformly dispersed in a given matrix material. Furthermore, considering
the large aspect ratio of clay platelets, their orientation must be controlled – for
some applications perfect alignment is desirable, whereas in others isotropicity
of reinforcement is essential. For example, aligning clay lamellae perpendicular
to the flux direction may increase barrier properties by a factor of 100, whereas
orienting them in the flux direction will hardly change the barrier properties
over those of the matrix. Considering that the relaxation time of standard clay
platelets (aspect ratio of 200 to 300) is of the order of one hour and that their
dimensions are of the nanometre scale, the dispersion and orientation of clay
during polymer processing is challenging.
The main difficulties for CPNC technology rest in the hygroscopic character
of clay and strong solid-solid interactions. It is a relatively simple task to disperse
clay platelets or lamellae (i.e., to exfoliate them) in water or in water-soluble,
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Preamble
polar monomers or oligomers (e.g., amino acids or glycols). However, preparation
of CPNC in a hydrophobic, non-polar high molecular weight polymer, e.g., a
polyolefin or polystyrene, is difficult. The most sensible way to approach the
problem is to consider the process of preparation of CPNCs as blending two
highly immiscible ingredients, i.e., from the perspective of polymer blending and
compatibilisation. As in polymer blends here also one is obliged to ensure good
interaction between the two antagonistic components: hygroscopic clay and
hydrophobic polymer.
The clay of preference is montmorillonite (MMT) with micron-sized particles
formed by stacks of three-layer sandwiches: a layer of Mg and Al oxides inbetween
the silicate layers. These sandwiches of 0.96 nm thickness and an average diameter
of about 100 to 500 nm are the desired reinforcing entities for CPNC. The chemical
constitution of the MMT unit cell offers three types of reactive sites: anions on
the silicate surface, hydroxyl (–OH) groups, and (few) cations on the narrow
edges. Historically, compatibilisation of clay involved forming an ionic bond
between the clay surface and organophilic onium cations, especially quaternary
ammonium ones. The advantage of this is that the chemical reaction not only
changes the hydrophilic clay character into hydrophobic, but also it causes the
clay particles to expand, i.e., to intercalate as a first step to the total dispersion of
the clay platelets, i.e., to exfoliation. The disadvantage is that this chemical
equilibrium process is diffusion controlled hence it may require an excess of
intercalatant and it may take a long time to complete!
‘Compatibilisation’ with at least partial utilisation of the –OH groups has
been carried out using their ability to react with epoxy or acid anhydride groups.
However, since the –OH groups are mostly located on the peripheries of clay
particles, there are few of them readily accessible and the reaction does not
necessarily lead to intercalation/exfoliation. Since the solid-solid interactions are
100 times stronger than liquid-liquid ones, it is imperative that there is good
miscibility between the pre-intercalated clay and the polymeric matrix – if not,
even the initially exfoliated clay platelets may reassemble during processing.
The most reasonable strategy is to prepare exfoliated CPNC using multiple
steps, for example:
1. Swelling sodium montmorillonite in warm water which causes the interlayer
spacing to expand from the initially dry state of 0.96 to about 1.3 nm.
2. Intercalation with cations suitable for the envisaged CPNC organophilic
molecules, onium or Lewis-base types that increase the interlayer spacing to
about 3 to 4 nm and will improve miscibility between the clay and the matrix.
3. Reactive compatibilisation of the organoclay/matrix polymer system, which
results in stable exfoliation of the clay platelets (interlayer spacing larger
than 8.8 nm). This step may not be necessary for highly polar polymers, such
as water-soluble polymers (e.g., polyvinyl pyrrolidone or polyvinyl alcohol),
or even for polyamides, but it is crucial for polyolefins or styrenics.
4. Melt compounding the pre-intercalated clay with matrix polymer. An
alternative strategy is to disperse the product of step (2) in a monomer and
polymerise it. This method has been particularly successful when the
intercalatant used in step (2) can be incorporated into the macromolecular
chain, thus forming what has became known as ‘hairy clay platelets’ with
over one thousand macromolecules end-tethered to the clay surface.
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Clay-Containing Polymeric Nanocomposites
To provide condensed information on the essential elements of CPNC technology,
the book is divided into parts:
Part 1. Introduction – presents a general overview of nanocomposites with
polymeric as well as non-polymeric matrices.
Part 2. Basic elements of PNC technology – focuses on the general methods and
principles of polymeric nanocomposites. It starts with a brief description of PNC
comprising non-clay nanoparticles (e.g., carbon nanotubes, polyhedral oligomeric
silsesquioxanes (POSS), etc.), and then focuses on the clay-containing polymeric
nanocomposites. The individual elements of CPNC technology are discussed,
namely the general characteristics of clays, methods of purification, and the diverse
methods used for intercalation and exfoliation.
Part 3. Fundamental aspects – discusses the pertinent aspects of the
thermodynamics, thermal stability, rheology, crystallisation and mechanical
behaviour.
Part 4. Technology of CPNC – reviews the evolution of CPNC technology for
specific polymeric matrices, primarily using the patent literature. Thus, CPNCs
with individual polymer matrices are reviewed, starting in historical order with
polyamide (PA), polyolefin (PO) and other thermoplastics, then epoxies,
polyurethanes and other thermosets.
Part 5. Performance – discusses selected properties of CPNCs, viz. mechanical,
flame retardancy, and barrier.
Part 6. Closing remarks – summarises the information.
Part 7. Appendices – provides explanations of abbreviations, symbols, and
concepts used in the book.
Part 8. References – contains well over 1,000 references to open and patent
literature up to the beginning of 2004.
Polymeric nanotechnology is in statu nascendi. In consequence, there is a bit
of confusion and uncertainty about its value and the most suitable applications.
Hopefully, this book will help answer some of these questions and in a small way
accelerate wider introduction of this technology.
In 1953, after getting a chemical engineering degree and spending the
obligatory six-month stage as plant engineer, I started graduate studies in the
field of phase equilibria and flow of polymer solutions. To emphasise the
objectiveness of scientific writings it is expected to use the impersonal form.
However, after 50 years in the profession I wish to revert to a more personal
style, dedicating this latest creation (and, as Hermann Mark used to say, the
dearest) to Czeslawa, my Wife, Friend and Supporter of nearly as many years.
Curiously, this book was not planned, but rather it evolved in response to
questions, comments and stories told to me by my colleagues from the Americas,
Asia and Europe. There are too many people to whom I owe my thanks to list
them all, but I wish to express my thanks to a very special trio: to Robert Simha
who has been my mentor and brilliant star to follow for all these 40-odd years of
my post-doc’ing with him, to Osami Kamigaito for introducing me to the
fascinating world of clay-containing polymeric nanocomposites, and to my
colleague and friend of many years, Jørgen Lyngaae-Jørgensen.
Leszek Utracki
Montreal, March 2004
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Contents
Contents Volume 1
Preamble.................................................................................................... i
Part 1 Introduction
1.1 General............................................................................................... 1
1.2 NCs with Ceramic or Metallic Matrix.............................................. 2
1.2.1 Metallic Nanoparticles in Amorphous Matrix................................2
1.2.2 Magnetic Oxides in Silica Nanocomposites ....................................2
1.2.3 Optoelectronics...............................................................................3
1.2.4 Summary on Non-Polymeric NC ....................................................3
1.3 NCs with Polymeric Matrix.............................................................. 3
1.3.1 PNC Definitions..............................................................................6
1.3.2 Methods of Characterisation of CPNCs..........................................8
1.3.2.1 X-Ray Diffraction (XRD)..................................................8
1.3.2.2 Small Angle Neutron Scattering (SANS)..........................11
1.3.2.3 Transmission and Atomic Force Electron Microscopy
(TEM and AFM).............................................................14
1.3.2.4 Fourier Transform Infrared Spectroscopy (FTIR)............15
1.3.2.5 Nuclear Magnetic Resonance Spectroscopy (NMR)........16
1.3.2.6 Other Methods................................................................17
1.3.3 Determination of PNC Properties .................................................18
1.3.4 PNC Types and Methods of their Preparation ..............................18
1.3.5 PNCs of Commercial Interest........................................................18
1.3.6 Journals and Research Groups......................................................29
1.3.7 Historical Perspective....................................................................30
Part 2 Basic Elements of Polymeric Nanocomposite Technology
2.1 Nanoparticles of Interest to PNC Technology................................. 35
2.1.1 General..........................................................................................35
2.1.2 Layered Nanoparticles ..................................................................35
2.1.3 Fibrillar Nanoparticles..................................................................38
2.1.3.1 Carbon Nanotubes (CNTs).............................................38
2.1.3.1.1 Origin, Characteristics and Structure.................. 38
2.1.3.1.2 Computation of Potential CNT Properties.......... 41
2.1.3.1.3 Non-Polymeric Applications of CNTs................. 44
2.1.3.1.4 Sources................................................................. 46
2.1.3.1.5 PNC with CNTs for Electrical Conductivity ....... 46
2.1.3.1.6 Graphite .............................................................. 47
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Clay-Containing Polymeric Nanocomposites
2.1.3.1.7 PNC with CNTs – Thermoset Matrix ................. 48
2.1.3.1.8 PNC with CNTs – Thermoplastic Matrix............ 50
2.1.3.2 Rod-Like CdSe Nanocrystals...........................................54
2.1.3.3 Imogolite.........................................................................54
2.1.3.4 Vanadium Pentoxide, V O5.............................................54
2
2.1.3.5 Inorganic Nanotubes.......................................................55
2.1.4 Other Nanoparticles......................................................................56
2.1.4.1 Spherical or Nearly-Spherical Particles............................56
2.1.4.2 Sol-Gel Hybrids...............................................................56
2.1.4.3 Polyhedral Oligomeric Silsesquioxanes (POSS)...............58
2.1.4.3.1 Origin and Structure............................................ 58
2.1.4.3.2 Properties............................................................. 60
2.1.4.3.3 Sources................................................................. 66
2.1.4.3.4 Applications......................................................... 67
2.2 Clays ............................................................................................... 73
2.2.1 General Characteristics .................................................................73
2.2.2 Crystalline Clays ...........................................................................74
2.2.2.1 Kaolins............................................................................74
2.2.2.2 Serpentines......................................................................74
2.2.2.3 Illite Group (Micas).........................................................74
2.2.2.4 Chlorites and Vermiculites ..............................................76
2.2.2.5 Other Clays.....................................................................76
2.2.2.5.1 Glauconite........................................................... 76
2.2.2.5.2 Sepiolite, Palygorskite and Attapulgite................ 76
2.2.2.5.3 Mixed-Layer Clay Minerals................................. 76
2.2.2.6 Smectites or Phyllosilicates..............................................76
2.2.2.6.1 Bentonite ............................................................. 79
2.2.2.6.2 Montmorillonite (MMT)..................................... 80
2.2.3 Purification of Clay.......................................................................84
2.2.4 Reactions of Clays with Organic Substances.................................85
2.2.4.1 Clay in Aqueous Medium................................................90
2.2.4.1.1 General................................................................ 90
2.2.4.1.2 Reactions with Edge Cations............................... 91
2.2.4.1.3 Reactions with –OH Groups............................... 91
2.2.4.1.4 Reaction with the Silicilic Surface Anions ........... 91
2.2.4.1.5 Stabilisation by Polyelectrolytes .......................... 92
2.2.4.2 Clay Dispersion in Polar Organic Liquids.......................93
2.2.4.3 Absorption of Organic Molecules by Organoclay...........93
2.3 Intercalation of Clay ....................................................................... 97
2.3.1 Introduction..................................................................................97
2.3.2 Intercalation by Solvents and Solutions.......................................100
2.3.3 Intercalation by Organic Cations................................................102
2.3.4 Intercalation by Organic Liquids.................................................124
2.3.5 Intercalation by Monomers, Oligomers or Polymers...................126
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