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CHAPTER TWO
Polymer surface Modification, Plasma Surface Treatment and
Plasma Polymerisation
2.1 Polymer Surface Modification
2.1.2
Corona Discharge
Corona discharge is widely used in surface modification of polymers for
printing and adhesion. A corona discharge is produced when air is ionised by
high electric field.4 Surface treatment by corona discharge is a process to
make active groups on the surface of materials and to increase their adhesive
strength by the collision of electrons against the surface of the material. The
electrons are generated by application of high tension and high frequency
voltage between two electrodes in air. This process has been used in areas of
printing5 and painting as the pre-treatment, particularly for the surface
modification of polyolefins (PP, PE, ect.) and fluoroplastics.
2.1.3 Ion Beam Deposition
Ion beam deposition uses an energetic, broad beam ion source carefully
focused on a grounded metallic or dielectric sputtering target. Material
sputtered from the target deposits on a nearby substrate to create a film. Ion
beam deposition yields excellent control and reproducibility of film thickness
and properties. There are several ion beam-processing techniques, which
may be used for surface modification, each having its own advantages and
disadvantages. The techniques include ion beam implantation, ion beam
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assisted deposition, ion beam texturing and ion beam polishing and
sharpening technologies. Ion beams have been used to texturize polymer
surfaces, especially flouropolymers, to increase adhesion.6,7 Ion beam
implantation in particular offers a number of advantages over other
techniques; because it is known to facilitate both chemical and structural
modification of the near surface volume of a material without the creation of a
defined interface between modified and unmodified volume. In so doing, a
gradual transition in chemistry, structure and properties is produced, avoiding
the possibility of interfacial delamination.8
2. 1.4 Other Surface Treatment Methods
Besides the methods mentioned above, there are still many other methods.
Flame treatment has been used commonly in the polymer industries to
improve the adhesive characteristics of surfaces, or more particularly to
enhance ink permanence on polymer surfaces.9 Photon irradiation should be
mentioned, which includes modification by ultraviolet (UV) and infrared (IR)
lasers to treat very small and localised areas. Ultraviolet irradiation of typically
wavelength between 250 and 400nm produces photons that result in
activation of polymer surfaces.10
2.2 Plasma Surface Treatment
Plasma is an ionised medium consisting of electrons, ions, and neutral
particles that are in constant interactions.11 The term plasma was introduced
by Irving Langmuir in his studies of electrified gases in vacuum tubes.4 There
are equilibrium (thermal) and non-equilibrium (non-thermal) plasmas. Low-
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pressure plasma as discussed here is non-equilibrium with an electron energy
distribution close to a thermal distribution of 10000 Kelvin, while the energy
distribution of ions and neutrals corresponds to a thermal distribution of about
300 Kelvin.12
Polymer surfaces can be treated or modified by the plasmas of a gas such as
argon, hydrogen, oxygen, nitrogen, ammonia, ect which does not lead to the
deposition of material but chemically modifies the surface. 13 The most
common application of plasma treatment is surface cleaning or etching to
increasing surface wettability, surface energy alteration as well as surface
preparation for bonding by removing organic and inorganic materials that
prohibits desired bond strengths without affecting the bulk properties and
more recently, to improve cell attachment in tissue culture studies. 4 It can also
be applied to a wide range of materials including metals, polymers, plastics,
and biomaterials.
2.2.1 Plasma Deposition
Plasma deposition can be divided into two groups: plasma enhanced
chemical vapour deposition and sputter deposition. In plasma enhanced
chemical vapour deposition, the plasma is used to help stimulate a reaction
on the substrate surface of two or more species from the gas phase. The
plasma helps break down the parent molecule and allows the reaction to
occur at lower temperatures than chemical vapour deposition. The major
advantage of plasma enhanced chemical vapour deposition is its lower
pressure capability with respect to other systems as chemical vapour
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deposition. For example, while deposition temperatures of 700-900oC are
required for silicon deposition in chemical vapour deposition, temperatures in
the range of 250-350oC are sufficient in plasma enhanced chemical vapour
deposition systems. Sputter deposition, also known as physical vapour
deposition is usually carried out in diode plasma systems known as
magnetrons, in which the cathode is sputtered by ion bombardment and emits
atoms or molecules, which are then deposited on a wafer to form thin films. 14
2. 3 Plasma Polymerisation
Plasma polymerisation is a vacuum coating process in which monomer gases
are guided into a reaction chamber10 either alone or in combination with an
activator gas,
which
itself
does not necessarily participate in
the
polymerisation reaction (e.g. argon, helium, nitrogen, ect.). The plasma is
excited and sustained by the application of electromagnetic radiation, under a
process pressure of less than 1 mbar. The monomer molecules are activated
in the plasma phase and bombard the substrate surface leading to the
dissociation of bonds at the interface, surface etching and chemical reaction
between the active sites at the surface and the reactive monomer species in
the plasma. These processes can be complicated by ablation and
polymerisation mechanism taking place simultaneously at the treated surface.
This means that, the reactive species in the plasma phase do not originate
from the monomer gas, but may be mixed with reactive species from
competitive ablation of the already deposited material.
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Plasma polymerisation takes place through several reaction steps. In the
initiation stage, free radicals and atoms are produced by collision of electrons
and ions with monomer molecules or by the dissociation of monomers
adsorbed on the surface of the sample. Actual formation of the polymeric
chain, which is the propagation step, can, takes place both in the gas phase
and on the substrate film. Termination can also take place in the gas phase or
at the polymer surface by similar processes as in the propagation step, but by
ending with a closed polymer chain.10
The properties of the layer can be adjusted over a wide range by selecting the
appropriate process parameters. Plasma polymerisation processes allows the
wettability and corrosion behaviour of inexpensive substrates to be tailored for
desired applications. There is also an improvement in the gluing, painting,
printing, corrosion resistance and bacterial growth characteristics of the
underlying substrate material.16
Apart from the properties just mentioned, plasma polymerised films can have
numerous advantages including excellent coating adhesion on almost all
substrates, chemical, mechanical and thermal stability and high barrier
effect.10
Functional groups such as hydroxyl, ketone and carboxylic acid groups have
been introduced to surfaces to adjust the surface free energy and to facilitate
chemical bonding between substrates and adhesives.17
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There are also other advantages of plasma polymerisation including the
following:
1. A broad range of functional groups can be introduced at the surface,
by variation of the gas that is used.
2. In general, polymerisation is fairly uniform over the whole substrate.
3. The polymerisation is limited to the top layer and does not affect the
bulk properties of the polymer.
4. Plasma polymerisation does not involve the use of solvents and may
be deposited on all kinds of substrates. (Polymer, metal, glass and
ceramic)
The main disadvantages of plasma polymerisation include the following:
1. A vacuum system is required which makes the cost of operation
higher.
2. Due to the complexity of plasma processes it is difficult to control the
chemical composition of the surface after polymerisation.
There are also some factors that affect the surface chemistry and structure of
the plasma polymers and these include the design or geometry of the reactor,
the input power, monomer flow rate as well as the substrate temperature. All
these factors should be investigated separately in order to find an optimal
process condition.
The cross-link densities of films produced by plasma polymerisation have
been shown to be higher than that of conventional polymers produced from
the same monomers.18 For example conventional polymerisation of
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polyethylene gives a liner polymer whiles plasma polymerised polyethylene
gives a highly cross-linked structured polymer (see fig. 2.1).
H
Conventional
polymerisation
H
C
C
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H n
H
Plasma polymerisation
CH2
CH
CH2
CH
CH2
CH
CH
CH
CH2
CH
CH
CH
CH
CH3
CH
CH
CH
CH
CH2
CH3
CH2
CH2
CH
Fig 2.1 Conventional polymerisation Vs Plasma polymerisation of
polyethylene.
2. 4
Plasma Surface Modification for Medical Application
Although many synthetic biomaterials have physical properties that meet
those of natural body tissue, they can often cause adverse physiological
reactions such as infection and inflammations. Through surface modification,
biocompatibility as well as biofunctionality can be achieved without changing
the bulk properties of the material. There are many ways by which to alter the
interaction of biomaterials with their physiological environments, of these;
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plasma surface modification provides device manufacturers with a flexible,
safe and environmentally friendly process that is extremely effective. The field
of biomedical applications needs polymers that, besides satisfying the
physical requirements of their application show biocompatibility during their
application in biological environment. Since biocompatibility involves the
interface between the device and the biological environment, surface
modification technique can be of great help to improve the biocompatibility of
surfaces without changing the bulk properties of the materials
There are several factors that must be considered when selecting a coating
material for medical application and surface modification process for a
particular application. For example, a modified surface for medical application
must generally exhibit biocompatibility and biostability in the presence of body
fluids and tissues. Surface modification must also be achieved without toxic
by-products that could be harmful to the patient or degrade the function of the
item being coated.19
2.5 Proteins on Surfaces
The immobilisation of proteins on surfaces is a first step in the lengthy
process of using active biological molecules in man-made nanodevices. The
interaction of proteins with solid surfaces is not only a fundamental
phenomenon but also a key to several important and novel applications.
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2.5.1
Proteins
Proteins are complex high molecular weight organic compounds that consist
of amino acids joined by peptide bonds. There are four distinct protein
structures, namely, primary, secondary, tertiary and quaternary structures.
The primary structure shows the sequence of amino acids in the polypeptide
chain with reference to the locations of any disulfide bonds. The primary
structure may be thought of as a complete description of all of the covalent
bonding in a polypeptide chain or protein. The Secondary structure is the
ordered arrangement or conformation of amino acids in localized regions of a
polypeptide or protein molecule. Hydrogen bonding plays an important role in
stabilizing these folding patterns. The two main secondary structures are the
alpha helix and the anti-parallel beta-pleated sheet. There are other periodic
conformations, but the  -helix and  -pleated sheet are the most stable. A
single polypeptide or protein may contain multiple secondary structures.
The tertiary structure of a polypeptide or protein is the three-dimensional
arrangement of the atoms within a single polypeptide chain. For a polypeptide
consisting of a single conformational folding pattern (e.g., an alpha helix only),
the secondary and tertiary structure may be one and the same. Also, for a
protein composed of a single polypeptide molecule, tertiary structure is the
highest level of structure that is attained.
Quaternary structure is used to describe proteins composed of multiple
subunits (multiple polypeptide molecules, each called a 'monomer'). The
arrangement of the monomers in the three-dimensional protein is the
quaternary structure.20, 21
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In addition to their structures, proteins may rearrange themselves in
performing their biological functions. In context of these functional
rearrangements, these tertiary or quaternary structures are usually referred to
as conformations and transitions between them are called conformational
changes. Proteins are essential to the structure and functions of all living cells
and viruses. Functions of proteins include the immune response and the
storage and transport of various ligands. They also serve as a source of
amino acids for organisms that do not synthesise those amino acids natively.
Proteins are one of the classes of biomolecules, alongside polysaccharides
and nucleic acids that make up the primary constituents of living things. They
are among the most actively studied molecule in biochemistry.
2.5.2 Protein- surface interactions
Protein adsorption and interaction at surfaces are involved in several
situations of technical and scientific interest. There are several diagnostic
methods based on the interaction between protein molecules at surfaces,
such as antibody-antigen binding reactions. Furthermore, an understanding of
the behaviour of protein molecules at surfaces is of great importance for the
development
of
new
types
of
biosensors.22
There
exist
several
thermodynamic models which attempt to explain certain features of protein
adsorption isotherms including interaction between the protein molecules
adsorbed on the surface. Because of the degree of uncertainty related to the
time needed for the equilibrium of the interfacial proteins, another approach is
often used in protein adsorption studies instead of measuring the adsorption
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isotherms, one resorts to the adsorption kinetics. There are a lot of processes
that takes place during protein adsorption and these include:
a. Transport towards the interface
b. Attachment at the interface
c. Structural rearrangements in the adsorbed state
d. Detachment from the interface and
e. Transport away from the interface
Each of these steps can in principle determine the overall rate of the
adsorption. It is, however evident from a variety of experimental studies that
protein adsorption is a highly dynamic phenomenon and also evidence of
protein- induced exchange reactions on surfaces whereby already adsorbed
protein molecules are exchanged with protein molecules from solution.
Fig. 2.2 Exchange reaction taking place on a surface- adsorbed protein being exchanged
with protein from solution
Proteins have specific sites called binding sites to which molecules and ions
bind. These sites exhibit chemical specifity and the strength of ligand-protein
binding is a property of the binding site known as affinity. There are also
solution conditions that affect the adsorption of proteins to surfaces and these
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are the pH, the ionic strength, ionic components and buffer type, the protein
type and the concentration. The nature of the surface is of course important
and plays a major role in the adsorption process.23
The present work helps to understand the various solution conditions that
affect protein adsorption to surfaces. That is, the effect that different protein
concentrations have on the amount of proteins adsorbed, how strongly
proteins are bound to plasma polymerised allyamine films after washing with
buffers of different pH and the role of film thickness in the protein adsorption.
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