poly carbonate

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Morphology, miscibility and mechanical properties of PMMA/PC blends
Manasvi Dixita
; Vishal Mathura
; Sandhya Guptaa
; Mahesh Babooa
; Kananbala Sharmaa
; N. S. Saxenaa
a
Semiconductor and Polymer Science Laboratory, Department of Physics, University of Rajasthan,
Jaipur-302 055, India
Online publication date: 12 January 2010
To cite this Article Dixit, Manasvi , Mathur, Vishal , Gupta, Sandhya , Baboo, Mahesh , Sharma, Kananbala and Saxena, N.
S.(2009) 'Morphology, miscibility and mechanical properties of PMMA/PC blends', Phase Transitions, 82: 12, 866 — 878
To link to this Article: DOI: 10.1080/01411590903478304
URL: http://dx.doi.org/10.1080/01411590903478304
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2. Phase Transitions
Vol. 82, No. 12, December 2009, 866–878
Morphology, miscibility and mechanical properties
of PMMA/PC blends
Manasvi Dixit*, Vishal Mathur, Sandhya Gupta, Mahesh Baboo,
Kananbala Sharma and N.S. Saxena
Semiconductor and Polymer Science Laboratory, Department of Physics, University of
Rajasthan, Jaipur-302 055, India
(Received 28 July 2009; final version received 11 November 2009)
This study deals with some results on morphology, miscibility and mechanical
properties for polymethyl methacrylate/polycarbonate (PMMA/PC) polymer
blends prepared by solution casting method at different concentration between
0 and 100 wt%. Dynamic storage modulus and tan were measured in a
temperature range from 30 to 180
C using dynamical mechanical analyzer
(DMA). The value of the storage modulus was found to increase with the
addition of the PC in the matrix. Transition temperature of pure PMMA and
pure PC is found to be 83.8 and 150
C, respectively. The result shows that the two
polymers are miscible for whole concentration of PC in PMMA. The distribution
of the phases in the blends was studied through scanning electron microscopy
(SEM). Also the mechanical properties like elongation at break and fracture
energy of the PMMA/PC blends increase with the increase in concentration of
PC in PMMA.
Keywords: poly (methyl methacrylate); polycarbonate; glass transition tempera-
ture; dynamic mechanical analyzer; scanning electron microscopy; mechanical
properties
1. Introduction
Polymer blends are the mixture of two or more polymers that can either mix completely on
a molecular scale or form two-phase structure. Polymer blends exhibit a new combination
of properties of component and depend strongly on the morphology of the blended
materials. By blending polymers, new materials can be developed that combine physical
and mechanical properties of their component, depending on the composition and level of
compatibility.
Blending of polymers may result in either compatible (miscible) or incompatible
(immiscible) system. A miscible polymer blend means single-phase system [1]. Miscibility
or compatibility is an important criterion for obtaining a synergistic effect from the blend
with appropriate composition [2,3]. Most polymers are immiscible or incompatible.
The reasons for incompatibility are high interfacial tension and consequently poor
interface adhesion [4].
*Corresponding author. Email: manasvi.spsl@gmail.com
ISSN 0141–1594 print/ISSN 1029–0338 online
ß 2009 Taylor Francis
DOI: 10.1080/01411590903478304
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3. The morphology of the miscible, immiscible and partially miscible polymer blends is
distinct from each other. In an immiscible blend, two phases are present: (1) discrete
phase (domain) which is lower in concentration and (2) continuous phase, which is higher
in concentration. The miscible polymer blends may form completely miscible blend
at a different concentration. The two phases in partially miscible blends may not have
a well-defined boundary. Each component of the blend penetrates the other phase at a
molecular level. The molecular mixing that occurs at the interface of a partially miscible
two-phase blend can stabilize the domains and improve interfacial adhesion.
The determination of glass transition temperature (softening temperature), Tg, is
important in polymer science, since processing temperature and final application tem-
perature are highly dependent on it. Polymers are often used in applications that involve
stress. Before using polymers in load-bearing applications, it is essential to study the effect
of stress on them. In determining the fabrication and possible practical application,
the mechanical properties of polymers, their strength, rigidity and ductility are of vital
importance. The mechanical properties of polymer blends are determined by the char-
acteristics of the components, their interaction, composition and structure. The additivity
of properties is assumed for homogeneous blends, while the appearance of a minimum is
expected in the case of immiscibility [5,6]. Partially miscible blends often yield properties
with a maximum and minimum as a function of composition [7].
Poly(methyl methacrylate) (PMMA) exhibits excellent mechanical properties and good
performance at various processing conditions. It is an amorphous polymer having high
optical properties, good chemical resistance and high tensile strength [8]. But the use of
PMMA presents some practical disadvantages, e.g. brittleness, low elongation at break
and high water absorption. To circumvent these drawbacks, many efforts have been made
through copolymerization and polymer blending. Among a number of PMMA blends
studied for miscibility, the mixture of PMMA and polycarbonate (PC) is one of the most
investigated polymeric systems. This may be attributed to the excellent properties of PC,
including outstanding ductility and low water absorption.
There have been many applications concerning PMMA/PC blends [9,10]. However,
only a few applications have focused on thermophysical properties. The objective of the
current work is to investigate the mechanical and morphological behaviour of PMMA/PC
blends. It is also aimed to study the compatibility of these blends.
2. Experimental
2.1. Materials
PMMA (Alfa Essar) in powder form and PC (Goodfellow) were used as received.
Tetrahydrofuran (THF) was used as solvent. Figure 1 shows the repeating units of PMMA
and PC polymers.
2.2. Preparation of blends
The films of PMMA and PC blends were prepared by solution casting technique.
The blend of PMMA/PC had compositions of 100/0, 75/25, 50/50, 25/75 and 0/100 by
weight. The solutions of PC and PMMA (in THF) were first prepared at various polymer
compositions at 50
C temperature. The polymer solutions were then mixed with
continuous stirring with the help of magnetic stirrer so as to get a better mixture of two
polymer solutions. This mixture was subsequently cast onto glass Petri dishes to form
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4. transparent films with a thickness of 0.1 mm. The cast films were dried under ambient
conditions for 24 h and then placed in a vacuum oven to remove residual solvent for
24 h [11].
2.3. Measurements
The surface morphology of the polymeric blends was analysed using scanning electron
microscopy (SEM). The films were fractured in liquid nitrogen and coated with gold.
These blended films were examined at 10 kV using the SEM instrument of Quanta Fe-200
model.
The mechanical properties and the glass transition temperature of the polymer blends
were measured by DMA of Tritec2000 make. In this instrument, a force is applied to
a sample and the amplitude and phase of the resultant displacement are measured.
The sinusoidal stress that is applied to the sample generates a sinusoidal strain or
displacement. By measuring both the amplitude of the deformation at the peak of sine
wave and the lag between the stress and strain sine waves, quantities like the modulus,
viscosity and the damping can be calculated. The details of DMA have been discussed
elsewhere [12,13].
The temperature scan was performed from 30 to 180
C temperatures at a heating rate/
ramp rate of 2
C min1
and stress–strain scan was performed at 30
C temperature with
constant load (10 N) in tension mode. Frequency of oscillation was fixed at 1 Hz and strain
amplitude 0.01 mm within the linear viscoelastic region.
For the confirmation of DMA results of miscibility, differential scanning calorimetry
(DSC) measurements were also performed on Rigaku DSC 8230. All Tg measurements
were made at a heating rate 10
C min1
within the range of 30–170
C. The Tg values were
taken at the peak of the glass transition region.
3. Results and discussion
The most commonly used method for establishing miscibility or partial miscibility in
polymer blends is through determination of the glass transition (or transitions) in the
blend versus those of the unblended constituents [3]. A miscible polymer blend will exhibit
a single composition-dependent glass transition located between those of two pure
components. With cases of limited miscibility, two separate transitions between those
of the constituents may result, depicting a component 1-rich phase and a component
2-rich phase. The difference between the glass temperature of the partially mixed
phase and that of the corresponding pure component gives information about the level
Figure 1. Repeating units of poly methyl methacrylate and polycarbonate.
868 M. Dixit et al.
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5. of partial miscibility. The measurement of Tg as a function of the composition gives an
idea about miscibility of the system. However, when a polymer–polymer system
demonstrates a single Tg, it does not definitely mean that mixing has occurred on a
molecular scale. The materials showing one Tg may also be considered where the two
separate phases have been distributed uniformly at the micro-level. In this article, the
miscibility of PMMA/PC blends has been first discussed in terms of the appearance of
single Tg using DMA.
3.1. Glass transitions and blend miscibility
Figure 2 show the plots of tan versus temperature for pure PMMA, PC and selected
blends. The pure PMMA sample shows a Tg at 83.8
C. However, Tg for conventional
PMMA is around 105
C as reported in the literature [14]. This shift in the value of Tg
could be explained in the following manner. Normally during the casting and drying
procedures, the majority of THF solution is removed from the samples. However, a
significant amount of THF is still present in these films, which shifts the transition to a
lower temperature due to plasticization [15]. The pure PC shows the transition at 150.4
C
which is agreed with the literature values [14].
A plot (Figure 3(a)) of storage modulus, loss modulus and tan as a function of
temperature of pure PMMA sample has been provided as a representative case for the
better understanding of the mechanism taking place in the material. From Figure 3(a) it is
observed that the value of storage modulus is higher than the loss modulus upto 75
C
temperature and beyond this temperature in the glass transition temperature region, loss
modulus is greater than storage modulus. Since the difference in the values of storage
Figure 2. Variation of tan of PMMA, PC and PMMA/PC blends with temperature.
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6. modulus and loss modulus is very small in the glass transition region (75–95
C), this region
has been shown separately in Figure 3(b).
From Figure 3(b), it is observed that in the vicinity of glass transition temperature, the
values of loss modulus are higher than storage modulus. This can be explained on the basis
of structural changes occurring in the material when the material is subjected to higher
temperatures.
The storage modulus is related to loss modulus through the following relation:
Damping factor ðtan Þ ¼
Loss modulus
Storage modulus
Figure 3.(a). Variation of storage modulus (E0
), loss modulus (E00
) and tan with temperature of
PMMA polymer. (b) Variation storage modulus and loss modulus with temperature in the glass
transition region (75–95
C).
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7. Here the loss modulus represents the viscous flow of material whereas the storage
modulus represents the elastic flow. In the glass transition temperature measurements, as
the temperature increases, the chains present in the polymer matrix stretch and hence chain
lengthening takes place. The chain lengthening causes an increase in viscosity of the
material. In the vicinity of the glass transition temperature, the material becomes viscous
and at this temperature viscous flow of material dominates its elastic flow (as evident from
Figure 3(b)). Therefore, in the region of glass transition temperature the loss modulus of
material becomes higher than the storage modulus. This aspect has also been exhibited
through the plot of variation of storage modulus with loss modulus (Figure 4) of pure
PMMA. It is observed from this figure that in the glass transition region loss modulus is
higher than storage modulus; hence value of tan is greater than 1 in the glass transition
region.
The result (Figure 2) shows that all blends exhibited only single Tg, which is between
the Tg’s of both blend components. This single glass transition temperature indicates
miscibility between two polymers, suggesting that PC is compatible with PMMA. Similar
Figure 4. Variation of storage modulus with loss modulus of PMMA polymer.
Table 1. Glass transition temperature of PMMA, PC and
their blends.
Sample
Glass transition
temperature (Tg)
100PMMA 83.8
75PMMA/25PC 129.55
50PMMA/50PC 133.21
25PMMA/75PC 149.40
100PC 150.5
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8. result has also been predicted by other researchers [16,17]. The values of the glass
transition temperature of PMMA/PC blends are given in Table 1.
From Figure 2, it is also observed that the intensity of the tan peak decreases to a
great extent with the increase of PC content in blend composition. This decrease in
intensity is due to the restriction of the main chain mobility in the polymer blends [18].
The location of the transition peaks is composition dependent. The increase in PC content
in the PMMA/PC blends leads to an increase in the glass transition temperature values.
This miscibility is due to the n–p complex formation between the ester group of PMMA
and the phenyl ring of PC [9].
Figure 5 shows a DSC thermogram for 75PMMA/25PC, 50PMMA/50PC and
25PMMA/75PC blends. The glass transition temperature is found to be 128, 136 and
145
C for 75PMMA/25PC, 50PMMA/50PC and 25PMMA/75PC, respectively. The small
difference in the values of Tg obtained from DMA and DSC is due to the difference in the
procedure used for the determination of Tg. A single glass transition temperature in all
the blends, which is also observed in DMA results, is a strong evidence of miscibility of
these blends.
3.2. Storage modulus
The variation of storage modulus versus temperature for pure PMMA, PC and selected
blends are shown in Figure 6.
All the plots of storage modulus show a small decrement initially with temperature in all
samples. As the temperature further increases, modulus shows a sharp decrement and then
attains a constant value at higher temperatures. This is due to the fact that the molecules
may be considered as a collection of mobile segments that have degree of free movement.
At lower temperature, the molecules of the solid material have lower kinetic energies and,
due to the fact that their oscillations about mean position are small, the material is
tightly compressed. In this state, therefore, the lack of free volume restricts the possibility of
Figure 5. DSC thermogram of 75PMMA/25PC, 50PMMA/50PC and 25PMMA/75PC blends.
872 M. Dixit et al.
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9. motion in various directions and hence they are unable to respond to the application of load/
stress to which the sample is subjected [19]. This gives a high value of modulus (more
stiffness).
At higher temperature a sharp decrement in the storage modulus has been observed.
This is attributed to the fact that at higher temperature, kinetic energy of the chain
segments of polymer matrix increases which in turn increases the oscillation of chain
molecules about their mean position. The increased oscillations augment the free volume
in the polymer matrix, which results in the transition from glassy state to rubbery state and
ultimately decreases the compactness of the polymer. Due to this phase transition, a sharp
reduction in storage modulus is observed for all the samples.
The pure polymers PMMA and PC have the values of storage modulus 1.278 and
1.198 GPa at 30
C, respectively. These values show an increase when PC is added in
PMMA and the storage modulus increases to a maximum value of 1.826 GPa for 75 wt%
of PC in PMMA (Figure 6).
The variation of storage modulus with composition of the PMMA/PC blends at room
temperature is shown in Figure 7. It can be seen from this figure that the modulus of
blends exhibits a maximum at some intermediate blend composition. This shows a
synergistic behaviour of the blend films. The positive deviation from linear additivity in the
mechanical properties is observed only when the two polymeric phases are distributed
uniformly. It shows that the quality of the blend depends on the concentration [20–22].
3.3. Morphological observation
Morphological examination by means of SEM was performed on the fractured surfaces of
the blends and the micrographs are shown in Figure 8. As can be seen in these SEM
micrographs, the fractured surface of the PMMA/PC blends showed very fine phase
morphology. The boundary between the PMMA and PC phase cannot be
Figure 6. Variation of storage modulus of PMMA, PC and PMMA/PC blends with temperature.
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10. clearly observed. This observation suggested a fine dispersion and homogeneous
incorporation of PC into the PMMA. In the miscible phase region only one phase can
be observed [23]. The result means that the PC is compatible with PMMA in the
morphological sense. This is entirely consistent with results of mechanical testing.
3.4. Tensile properties of PMMA/PC
The stress–strain curve for polymeric films is obtained by applying a tensile force at a
uniform rate to a viscoelastic sample at a constant temperature. The stress–strain curve
profiles are strongly influenced by the polymer structure, molecular weight, molecular
weight distribution, chain branching, degree of crosslinking, chain orientation, extent of
crystallization, crystal structure, size and shape of crystal, processing conditions and
temperature [24]. The curve gives information about the Young’s modulus, yield point,
break point, elongation at break and the recovery behaviour of polymeric films.
Figure 9 shows the stress–strain curves of PC/PMMA blends. In all cases, at low strain,
there exists in the materials the so-called elastic energy, which is stored in the form of
strain energy of the chemical bonds prior to break. For greater strain, the stored energy is
termed plastic energy and in this region, the specimens exhibit ‘‘irreversible’’ plastic
deformations with increasing strain. In plastic flow, the material is undergoing a
rearrangement of its internal molecular or microscopic structure, in which atoms are being
moved to new equilibrium positions [25].
Figure 9(a) shows stress–strain behaviour for pure PMMA, a linear elastic polymer.
In PMMA, there is no yield point and the fracture is mainly caused by crazing.
Here, the stress increases linearly with strain (or very nearly linearly) until ultimate
mechanical failure is obtained while pure PC behaves as a ductile polymer undergoing
Figure 7. Variation of storage modulus with PC content in PMMA.
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11. yielding. Yielding behaviour shows deviations from linearity. This behaviour is also
noticed in the other blends (Figure 9, Table 2) under investigation.
The values of Young’s modulus, ultimate tensile strength, fracture energy and
elongation at break are summarized in Table 2.
From the values of Young’s modulus and ultimate tensile strength of PMMA/PC
blends, it is observed that there is a sharp increase in modulus and ultimate tensile strength
with the increase in content of PC. The elongation at the break of a polymer is usually
defined as the maximum strain reached during the stress–strain curve or the strain when
the sample breaks. Tensile strain of PMMA/PC blend is obviously improved by the
addition of PC. The total area under the stress–strain curve, which represents the fracture
energy, also increases with the PC content. This fracture energy is related to the toughness
of polymer. High fracture energy is for tough or ductile polymer and low fracture energy
is for brittle polymer. From Table 2, it is observed that PMMA is a brittle polymer and
PC is ductile in nature. The elongation at breaks and fracture energy of the PC/PMMA
blends increases after blending indicating a high level of compatibility between the two
polymers.
Figure 8. (a) SEM micrographs of 75PMMA/25 PC. (b) SEM micrographs of 50PMMA/50 PC.
(c) SEM micrographs of 25PMMA/75 PC.
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12. 4. Conclusion
The occurrence of single glass transition temperature in DMA and DSC is suggestive of
the fact that these blends (PMMA/PC) are good compatible blends. The SEM
measurements also confirm the same fact. The mechanical properties show that the
modulus and strength of PMMA/PC blends is dramatically influenced by composition and
follows a synergic behaviour. The higher value of mechanical properties for 75PMMA/
25PC indicates the better quality of this blend. It can be concluded that the increase in the
PC content in PMMA matrix reduces the mobility of main chain movements and enhances
the toughness of PMMA/PC blends.
Acknowledgements
One of the authors (Manasvi Dixit) is thankful to UGC and BRNS for providing financial assistance
during this work. Authors would also like to thank Ms Deepika for her help in various ways during
the course of this work.
Figure 9. Tensile stress–strain curve for PMMA/PC blends.
Table 2. Mechanical properties of PMMA, PC and their blends.
Sample
Young’s
modulus (GPa)
Ultimate
tensile strength
(MPa)
Elongation
at break (%) at
room temperature
Fracture
energy (103
J)
100PMMA 0.70 5.69 0.76 0.31
75PMMA/25PC 1.85 18 1.99 0.62
50PMMA/50PC 2.37 21.3 2.61 1.10
25PMMA/75PC 2.53 23 2.97 1.74
100PC 1.18 30.8 3.41 1.98
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