Influence of Plasma Processes on the Molecular Orientation of Polymers
W. Michaeli1, Ch. Hopmann1, H. Behm1, F. von Fragstein1, K. Bahroun1, W. Dorscheid1
1
Institute of Plastics Processing (IKV) at RWTH Aachen University, Aachen, Germany
Abstract:
Plasma processes constantly gain importance in the field of plastics processing. They are influenced by process
parameters on the one hand and by the substrate itself on the other hand. The properties of polymers vary
considerably depending on their processing conditions, their history and outer influences.
To define the influence of a plasma treatment on the process-induced inner properties, sheet-shaped
polypropylene samples are injection moulded. Parts are produced while varying mass temperature, and mould
temperature and injection speed. As a result the inner properties vary due to the different cooling conditions.
Samples are extracted close to and far from the gate. The samples are treated in a nitrogen plasma. In order to
generate a profile of the molecular orientation 20 µm-thin-sections are cut from the sample sheet and analysed by
means of FT-IR spectroscopy.
The findings indicate a decrease in molecular orientation as a result of the plasma treatment. This effect gets
more distinctive the more cooling conditions vary from equilibrium conditions. Those orientations are more likely
to relax during a plasma process, because of a temperature rise of the polymer. This effect is not limited only to
the surface exposed to the plasma, but also influences the bulk material.
Keywords: Polymer, Plasma-Substrate-Interaction, Molecular Orientation, Treatment
1. Introduction
In plastics processing plasmas are often used to
modify surface properties of thermoplastic parts.
There is a wide range of surface modification
possibilities. Next to cleaning, etching and activation
processes the deposition of highly functional
polymer films is possible. Cleaning and activation
processes are used e.g. to enhance the printability or
wettability [1, 2]. Etching can be used to implement
micro structures or reduce adhesion for example of
bulk goods. Deposition processes are used in order
to create scratch resistant surfaces, reduce friction or
to enhance permeation barriers [3-5]. The used
plasma processes are rather complex and influenced
by a great number of parameters. Next to the
boundary conditions, the properties of the polymer
itself are important in order to gain satisfying results.
The polymer properties vary considerably depending
on their processing conditions, their history and
outer influences. Therefore the influence of plasma
processes, for instance molecular orientation and
crystallinity, is exemplarily investigated for semicrystalline thermoplastics. This research is
embedded within the collaborated research project
SFB-TR 87 “Pulsed high power plasmas for the
synthesis of nanostructured functional layers”
aiming at fundamentally describing the process
chain from the production of a thermoplastic part to
the finished barrier coated polymer.
2. Theoretical Background
The injection moulding process is a discontinuous
process. Polymer pallets are fed into a plasticising
unit in which they are melted by heat transfer and
friction. The conditioned melt is injected into a
temperature controlled moulding cavity within a few
seconds. Next to the geometry of the part, the inner
properties such as molecular orientation, residual
stress, crystallinity and morphology are influenced
by the process parameters like mass temperature,
mould temperature and injection speed [6].
Furthermore the inner properties are highly
anisotropic and so are the surface properties.
The backbone of most polymers is a chain of carbon
atoms. These polymer chains are usually not straight
but form an entangled bulk. With increasing
temperature, chain mobility rises.
Polypropylene is a semi-crystalline material. While
cooling from the melt, a partial alignment of
molecule chains can take place. With the parallel
molecules
varying mass temperature, mould temperature and
injection speed according to Table 1.
Because of the material’s flow and solidification
processes a molecular orientation takes place during
the mould filling phase. By the melt front, a
biaxially stretched “membrane” of polymer material
with high viscosity is deposited at the mould wall;
there it solidifies instantly. The molecular
orientation is a result of the velocity profile and the
resulting shear rate. An orientation profile forms
over the cross section as well as the length of the
thermoplastic part with an orientation maximum at
the surface. The overall molecular orientation
decreases over the material’s flow path. A second
maximum near the surface is the result of shear
effects between frozen surface layer and flowing
melt (Figure 1) [6].
For the subsequent treatment a microwave driven
low pressure plasma process is used. The reactor is
shown in Figure 2.
alignment and/or folding
crystallites arise [7].
orientation in flow direction
gate
of
near the gate
-1
0
the
0
Figure 1. Orientation
(schematically) [3]
1 -1
pattern
0
along
1 -1
the
0
flow
1
path
3. Experimental Details
In the present case, sheet-shaped samples with the
dimensions 118 x 118 x 4 mm³ are injection
moulded. The used material is a polypropylene
(PP 505 P by Sabic, Sittard, Netherlands).
injection
moulding
parameter
C (cold)
mass
temp.
[°C]
210
mould
temp.
[°C]
20
injection
speed
[cm/s]
40
M (moderate)
240
50
25
W (warm)
270
80
10
V
substrat shutter
with sample
vacuum
pumps
plasma
reactor
massflow
controller
M
plasma
spectrometer
microwave generator
process gases
Figure 2. Low pressure plasma reactor
far from the gate
1 -1
pressure transducer
ventilation valve
Table 1. Injection moulding parameters for the fabrication of
polypropylene sheet-shaped samples
To apprehend the influence of a plasma process on
the substrate, samples are injection moulded with
three different process parameter combinations
The microwaves are generated with a frequency of
2.45 GHz and form an electromagnetic field inside
the rectangular waveguide. A quartz-glass-tube with
a diameter of 35 mm is incorporated in the
waveguide. The process gas nitrogen (N2, purity:
5.0, Linde AG, Wiesbaden, Germany) is released
into the glass-tube and excited to a plasma. Using a
downstream setup, the excited plasma is transported
due to the gas flow to the substrate. The temperature
of the substrate during the plasma treatment is about
80 °C. The substrate is placed inside a substrate
shutter. The shutter is closed at the beginning of the
process and opened 20 seconds after plasma ignition.
The used process parameters are listed in Table 2.
process parameter
treatment time
process pressure
gas flow of N2
microwave power
value
60 s
20 Pa
50 sccm
600 W
Table 2. Process parameters for the plasma treatment of
polypropylene
In order to identify the profile of molecular
orientation over the substrates thickness, 20 µm thinsections are cut out from the sample sheets with the
help of a microtome and analysed. Since the surface
properties vary over the flow path of the polymer
during the process, samples are taken near the gate
bar gate
y
z
IR ray
flow path
y
x
4 mm
D
near the gate
far from gate
mapping measurement
Figure 3. Sample location
To characterise the samples, a Fourier transformed
infrared spectroscopy (FT-IR microscope, Nexus
870 by Thermo Nicolet, Waltham, MA, USA) is
used. With this method it is possible to analyse the
samples chemistry, molecular orientation and
crystallinity.
4. Results & Discussion
Polypropylene primarily contains CH3- and CH2groups. Therefore, IR-spectra mainly consist of CH3and CH2-vibrations. For qualitative validation of the
orientation polarised FT-IR spectra are recorded
with the polariser being aligned parallel and
perpendicular to the flow path, i.e. the orientation
axis, of the polymer. The dichroic ratio is given by
the ratio of the particular absorbance measured each
wise parallel and perpendicular [8]. The ratio
correlates to the molecular orientation.
The spectra show an absorbance at 841 cm-1 that is
caused by methyl rocking modes and stretching of
the carbon atoms from the backbone (Figure 4) [9].
0.10
841 cm-1
This region corresponds to the isotactic
conformation of the polypropylene and the
crystalline parts respectively. It is used to
characterise the orientation of crystalline regions
[10, 11].
Figure 5 shows the orientation profile near the gate.
Comparing the three injection moulding parameter
sets, it is obvious, that a decreasing injection speed
and an increasing melt and mould temperature result
in lower dichroic ratios at the surface. On the left
hand side the untreated surface is shown, the treated
surface is on the right side.
untreated
surface
near the gate
2.0
1.5
1.0
0.5
0
Figure 5. Orientation distribution near the gate
Especially near the gate, parameter W has a
distinctly smaller dichroic ratio than C. The variation
between parameter C and M is not as definite. The
parameter variation does not influence the bulk
material significantly. The molecular orientation
decreases over the flow path of the polymer
(Figure 6). The gradient between the ratio in the bulk
material and the surface is less steep near the gate.
0.04
0.03
0.02
0.01
-0.01
2800
2300
1800
wavenumber [cm-1]
Figure 4. Example of an ATR spectrum
1300
800
bulk
treated
surface
3.0
far from the gate
dichroic ratio [-]
0.05
CH3
3300
W
1000 2000 3000 4000
measurement position [μm]
untreated
surface
absorbance [-]
810
CH2
3800
M
0.09
0.06
910
C
0.0
0.07
1010
treated
surface
2.5
0.08
CH2, CH3
bulk
3.0
dichroic ratio [-]
and far from the gate of the injection moulded
samples (Figure 3).
2.5
2.0
1.5
1.0
0.5
C
M
W
0.0
0
1000 2000 3000 4000
measurement position [μm]
Figure 6. Orientation distribution far from the gate
Figure 7 shows the orientation distribution of an
untreated and one treated in the described way.
dichroic ratio [-]
3.0
untreated
surface
bulk
treated
surface
near the gate
2.5
2.0
1.5
1.0
untreated
0.5
60 s treated
0.0
0
1000 2000 3000 4000
measurement position [μm]
Figure 7. Orientation distribution of treated and untreated
samples near the gate (process M)
The interesting question is how the plasma treatment
influences the polymer part. We can clearly see a
change in the part’s molecular orientation. Since the
orientation is always connected with the part’s
properties, a plasma treatment might result in a
geometry change (warpage) due to the relaxation of
the polymer. In order to minimize the changes in
inner part properties the temperature should always
be as low as possible during the process.
4. Conclusion
2.5
Plasma treatment influences the molecular
orientation of a polypropylene sample. The level of
this effect depends on the process parameters. Due
to a locally varying cooling behaviour of the
polymer the influence of the plasma treatment
differs over the flow path of the polymer melt. The
area near the gate is more sensitive to a plasma
treatment. The material freezes far from equilibrium
in this area. A subsequent plasma process allows
chains to relax into a more favourable situation.
Near the gate the whole sample is affected by the
treatment, while far from the gate, only the surface
and surface near areas are influenced. The relaxation
process gets less distinct over the flow path. When
treating a polymer part, it is always exposed to
particle bombardment, radiation (e.g. UV radiation)
and heat. Not investigated was the influence of the
UV radiation induced during the plasma process.
Further studies are necessary regarding this concern.
2.0
5. Acknowledgements
The orientation distribution is in good correlation
with the schematically displayed orientation in
Figure 1. The dichroic ratio is highest near the
surfaces and lowest in the middle of the sample. It
slightly increases from 0 to 200 µm before
decreasing. A plasma treatment of 60 s affects the
whole sample and not only the surface. A decrease
in molecular orientation can be seen throughout the
sample. The cause of this behaviour is the heat
induced due to the plasma process. At sample
temperatures of about 80 °C chains relax into a more
favourable situation.
Interestingly the influence of the plasma treatment
far from the gate is different from the area near the
gate (Figure 8).
3.0
untreated
surface
bulk
treated
surface
far from the gate
dichroic ratio [-]
in the dichroic ratio. The molecular orientation
inside the bulk is after the plasma treatment near and
far from the gate almost the same. The orientation
values near and at the surface approach each other.
1.5
1.0
untreated
0.5
60 s treated
0.0
0
1000 2000 3000 4000
measurement position [μm]
The depicted research has been funded by the
Deutschen Forschungsgemeinschaft (DFG / German
Research Foundation) as part of the Collaborative
Research Centre SFB-TR 87.
References
Figure 8. Orientation distribution of treated and untreated
samples far from the gate (process M)
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The orientation at the surface seems to increase
while it decreases a few µm inside the bulk. The
centre of the sample shows no mentionable change
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