Jordan Journal of Chemistry Vol. 1 No.2, 2006, pp. 155-170 JJC Miscibility, Crystallinity and Morphology of Polymer Blends of Polyamide-6/ Poly (β-hydroxybutyrate) Aiman Eid Al-Rawajfeh a *, Hasan A. Al-Salah b , Ismael Al-Rhael c a Tafila Technical Univesity, Department of Chemical Engineering,P.O. Box 179, Tafila 66110, JORDAN, b Mu’tah University, Department of Chemistry, P.O. Box 7, Al-Karak, JORDAN c Mu’tah University, Department of Physics., P.O. Box 7, Al-Karak, JORDAN Received on Feb. 26, 2006 Accepted on Nov. 30, 2006 Abstract Polyamide-6 (PA6) with bacterial poly ( β -hydroxybutyrate) (PHB) are typical polyamide and polyester, respectively. PA6 is known to be high-strength engineering thermoplastic. Although it is ductile at room temperature, it becomes brittle under severe conditions such as high strain rates and/or low temperatures. This is due to the low crack propagation resistance of polyamides. PHB is a biodegradable and biocompatible thermoplastic polymer of high melting temperature (180 o C ) and crystallinity. PHB has attracted much attention as an environmentally degradable resin to be used for agricultural, marine and medical applications. One of the limitations of PHB for these applications is its brittleness and narrow processing window. Blending of friendly environmental biopolymers with synthetic polymers has proven to be a suitable tool to produce novel materials with combined characteristics in having both improved application properties and low cost advantages in material performance. In this study, the segmental interaction parameters, crystallinity, miscibility and morphology of polymer blends (PB) of PA6 and PHB have been studied at different weight fractions and different crystallization temperatures. The experimental approaches utilized are Differential Scanning Calorimetry (DSC), Fourier Transform Infrared (FTIR) and Polarized Optical Microscopy (POM). The interaction parameters were calculated using the Nishi-Wang equation, which is based on the Flory-Huggins theory. The values of interaction parameters 12 χ were negative for all blend compositions suggesting that 12 χ depends on the volume fraction ( Φ ) of the polymer. Significant upward shifts of O C= ν and NH ν are observed with increasing the voume fraction 2 Φ of PHB which reflect the miscibility of the blend systems. Polymer blends show spherulitic morphology and the spherulites exhibit a banded structure. The band spacing decreases with increasing PHB content. Keywords: PA6; PHB; Crystallinity; Miscibility; Hydrogen bonding; Interaction parameters; Morphology. * Corresponding Author: E-mail: [email protected]
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Jordan Journal of Chemistry Vol. 1 No.2, 2006, pp. 155-170
JJC
Miscibility, Crystallinity and Morphology of Polymer Blends of Polyamide-6/ Poly (β-hydroxybutyrate)
Aiman Eid Al-Rawajfeha*, Hasan A. Al-Salahb, Ismael Al-Rhaelc
a Tafila Technical Univesity, Department of Chemical Engineering,P.O. Box 179, Tafila 66110, JORDAN, b Mu’tah University, Department of Chemistry, P.O. Box 7, Al-Karak, JORDAN c Mu’tah University, Department of Physics., P.O. Box 7, Al-Karak, JORDAN Received on Feb. 26, 2006 Accepted on Nov. 30, 2006
Abstract
Polyamide-6 (PA6) with bacterial poly ( β -hydroxybutyrate) (PHB) are typical polyamide
and polyester, respectively. PA6 is known to be high-strength engineering thermoplastic.
Although it is ductile at room temperature, it becomes brittle under severe conditions such as
high strain rates and/or low temperatures. This is due to the low crack propagation resistance of
polyamides. PHB is a biodegradable and biocompatible thermoplastic polymer of high melting
temperature (180 o C ) and crystallinity. PHB has attracted much attention as an environmentally
degradable resin to be used for agricultural, marine and medical applications. One of the
limitations of PHB for these applications is its brittleness and narrow processing window.
Blending of friendly environmental biopolymers with synthetic polymers has proven to be a
suitable tool to produce novel materials with combined characteristics in having both improved
application properties and low cost advantages in material performance. In this study, the
segmental interaction parameters, crystallinity, miscibility and morphology of polymer blends
(PB) of PA6 and PHB have been studied at different weight fractions and different crystallization
temperatures. The experimental approaches utilized are Differential Scanning Calorimetry
(DSC), Fourier Transform Infrared (FTIR) and Polarized Optical Microscopy (POM). The
interaction parameters were calculated using the Nishi-Wang equation, which is based on the
Flory-Huggins theory. The values of interaction parameters 12χ were negative for all blend
compositions suggesting that 12χ depends on the volume fraction (Φ ) of the polymer.
Significant upward shifts of OC=ν and NHν are observed with increasing the voume fraction
2Φ of PHB which reflect the miscibility of the blend systems. Polymer blends show spherulitic
morphology and the spherulites exhibit a banded structure. The band spacing decreases with
as shown in Figure 2. The (1-λ)% values increase with increasing PHB content up to
50%. Such a result is probably due to the trapped fraction of PA6 in the interlamellar
amorphous regions of PHB spherulites which is not allowed to crystallize. The
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crystallinity percent of PA6 depends on the crystallization temperatures. For example,
the crystallinity percent of PA6 at 150 oC is doubled compared to ambient conditions.
Figure 2: Crystallinity percent, ( )λ−1 %, versus Φ2 of PA6/PHB at ambient and
150 Co .
Melting Point Depression
The equilibrium melting points, omBT , values for individual blends can be
determined by the method of Hoffman and Weeks [43]:
omB
CmB T
TT •
−+=γγ11 , (6)
where γ is the ratio of the initial and the final lamellar thickness, and CT is the
isothermal crystallization temperature.
The mT of PA6, crystallized isothermally from its mixtures with PHB, are plotted
versus CT (Figure 3) and extrapolated to line where Cm TT = to obtain the equilibrium
melting points omBT (or o
mT ) of an infinitely large crystals. A good linear correlation
between mT and CT was observed. Because only mT values need to be determined,
this technique has been the method of choice for estimating blend equilibrium melting
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point. The values of omT obtained by this method clearly indicate that the addition of
PHB cause a significant depression in the omT .
Figure 3: Hoffman-Weeks plot of PA6/PHB blends.
Flory-Huggins Interaction Parameter
Thermodynamics predicts that the decrease in chemical potential of the
crystallizable polymer is due to the presence of the miscible amorphous polymer,
resulting in a decease in the melting point. This has been modeled by Nishi and Wang [44], which is based on the Flory-Huggins Theory to obtain polymer-polymer interaction
parameter 12χ . Blend melting points are frequently used to estimate the magnitude of
the polymer-polymer interactions at omBT . When the entropy of mixing is negative:
2212
2
1 )1(11Φ−=
−
∆− χo
mo
mB
of
TTRVVH
, (7)
where omT and o
mBT are the equilibrium melting point of the crystallizable component
in the pure state and the blend, respectively, R is the universal gas constant, iV is
the molar volume of the respective component, ofH∆ is the heat of fusion per mole of
repeat unit, and 2Φ is the volume fraction of second component in the blend. If values
of omT and o
mBT are obtained, a suitable graphical procedure using Equation 7
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facilitates an estimation of 12χ (i.e. Blendχ ). A plot of the left-hand side of Equation 7
versus the square of the PHB volume fraction 22 )1( Φ− in the blend, Figure 4, can be
used to estimate the PA6/PHB interaction parameters 12χ . This should give a straight
line passing through the origin if 12χ is independent of the composition and the
melting point depression is not influenced by morphological effects.
Figure 4: Left-hand side of Equation (7) versus 22 )1( Φ− of PA6/PHB blends.
In order to calculate the Left-Hand Side of Equation 7 the parameter values
used are as follows: ofH∆ =1268 J/mol.monomer PHB [25], 1V =104.24 cm3/mol, 2V =
75 cm3/mol, and R is the universal gas constant (8.314 J/mol.K). The values Vi are
calculated by using the amorphous density of PA6 (1.084 gcm-3), and
PHB (1.15 gcm-3), respectively.
In Table 1, the values of 12χ were found to be negative for all investigated
compositions, as PHB content increases the 12χ become more negative, which
explains the depression of mT with increasing PHB content. This strongly indicates
that in the melt, at omT , PA6 and PHB are compatible. The negative values of 12χ that
indicate miscibility, is due to the hydrogen bonding between PA6 segments and PHB.
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Table 1: Values of 1Φ , omT and 12χ of the blends.
respectively [4]. The values of ijχ and 'ijχ are shown in Table 2. It can be noticed that
the correlation constants (R2 ) are very high. The physical basis of such a large positive
interaction between CH2/COO segments can be understood and justified easily within
the context of the PHB chemical nature which does not include any of possible
interactions (i.e. no self-association). The positive values of ACχ and 'ACχ means that
there are very good repulsion between PHB segments which is favorable to the net
interaction parameter 12χ . The repulsion effect of each CH2 of PHB and CONH of
polyamide is very small comparing to its repulsion in the mother polymer which is
consistent with net interaction of CH2 /CONH segments.
Negative interactions between CONH of PA6 with COO of PHB can be clearly
observed. This result supports the net value of 12χ (i.e. miscibility), which can be
attributed to the intermolecular hydrogen bonding of these segments. It can be
understood within the context of the polymer chemical nature of containing the
hydrophilic functionality. A supporting result can be obtained by FTIR vibration
frequency of amide, and hydroxyl units which will be discussed in the next section. The
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negative and therefore favorable interaction between the methylene and amide units
ABχ means that there is still interaction between them, which is not easy to qualify.
When a negative value of PA6 methylene and amide units arises, an equal counter
current effect of negative interaction of PHB methylene segment with the same amide
segment arises simultaneously, also it is attributable to overcoming this interaction of
such segments by the effect of the strong interaction of CONH/COO and the repulsion
of PHB segments (i.e. CH2/COO). The interaction of CH2/CONH of PA6/PHB is nearly
1/6 of the repulsion of CH2/COO, on the other hand, the interaction of CH2/CONH of 'ABχ are small and positive which are more logical than the values of ABχ which mean
high repulsion between the segments in the blend.
Table 2: Segmental and corrected interaction parameters ijχ and 'ijχ of PA6/PHB
R2 0.9679 0.9679 * Calculated using Ellis equations [3-5]. ** This work
The value of 12χ for any polymer blend is usually small and representative of
the overall interaction of two different polymer molecules [4]. In this type of assumption,
where the individual mers are chemically distinct by its molar volume fraction (x and y)
and the volume fraction Φ from which each segment becomes, the values of ijχ and
'ijχ may be extreme. If the individual mers become less distinct, then the individual
segmental interaction parameters will be closer to that of the original polymer
molecule 12χ .
For all these values of ijχ and 'ijχ , the qualitative picture does not change at all;
only the quantitative values change such that the role of 12χ in such approach is
simply to give an idea about the segmental interaction parameter ijχ of each segment,
as interaction or repulsion, and to support the FTIR results of affected vibration modes
of that segment.
Fourier Transform Infrared (FTIR) Spectroscopy
Polyamides are self-associating polymers through hydrogen bond, while
polyester (such as: PHB) is an example of so called adduct or inter-association group
which associate only with the other component. IR spectroscopy has proved to be an
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excellent tool to study the hydrogen-bonding behavior in polyamides and polyamide
blends. If the blend is immiscible, the absorption spectrum of the blend will be the sum
of those for the components. If the blend is miscible because of the specific
interactions, then differences will be noted in the spectrum of the blend relative to the
sum of those for the components. The FTIR investigation of a miscible blend will not
only reveal the presence of such an interaction, but will provide information on which
groups are involved [47]. No detailed information can be derived from the order-
sensitive absorption band in the 500 to 1200 cm-1 wavenumber region, while significant
changes can be observed for the intensity, wavenumber position and shape of the
OC=ν and NHν absorption bands, as consequence of change in the hydrogen
bonding state (Figure 5). The peak at ca. 1613 cm-1 is due to the free carbonyl, and the
peak at 1532 cm-1 arises from the carbonyls that are hydrogen bound. As PHB content
increases the two peaks split to two sharper peaks and lower the area at higher
frequency, until both disappear. The peaks of free association and bound NH arise at
3337 and 3250 cm-1, respectively, they remain unchanged up to (60/40) PA6/PHB, and
then become shifted and sharper at 50/50, 40/80, and 20/80 fractions, and the sharp
peak of PHB at 3438 cm-1 appears and does not change.
Figure 5: FTIR spectra of PA6/PHB blend system: (A) NH stretching band, (B) shifts in
C=O frequency.
The vibration of IR spectra at the NH and C=O stretching band reflect the
hydrogen bonding condition of amide groups [48] and the hydrogen bond formation
generally shifts to lower frequently due to the N-H and C=O stretching modes. This is
an important source of information, since hydrogen bonding is a strong intermolecular
interaction and should affect the miscibility. In the presence of PHB slight downward
167
frequency shift of N-H and significant upward frequency shift of C=O stretching
absorptions had occurred on blending. The chemical structure of the polymers
supports the possibility of formation hydrogen bonds between amide group with each
of the ester and hydroxyl groups that should lead to miscible blends. The occurrence of
a hydrogen bonding interaction perturbs the C=O, NH and OH groups by changing its
electronic environment, and provide a favorable enthalpic contribution to mixing. The
enthalpy of hydrogen bond formation is always negative when a self-associating
polymer molecule is a component of a mixture; a positive contribution to the change in
enthalpy arises from the breaking of the hydrogen bonds of the self-associating
molecules [47].
In the presence of PHB, the maximum peaks positions of OC=ν and NHν have
shifted toward lower wavenumbers as shown in Figure 5. The NH stretching band are
shown in Figure 5A and shifts in the C=O ( OC=∆ν ) are shown in Figure 5B. Shifts in
OC=ν and NHν indicate the miscibility of the blend system [7, 47, 49]. It can be
proposed that the formation of hydrogen bonding between C=O and NH groups is the
predominant, since the formation of hydrogen bonding is always favorable to the
miscibility. It was claimed [48] that non-combinatorial entropy change involves in the
strongly associating polymer blends, and that the favorable contribution of the entropy
can overcome the unfavorable enthalpy increase accompanied by the dissociation of
the hydrogen bonding. This non-combinatorial entropy mechanism of miscibility also
explains phase separation observed in the high-PA6-content blend systems.
Morphology
Polarized optical microscopy observations of the crystallization processes at
ambient temperature and 80°C, indicate that PA6 and PHB are able to crystallize
according to spherulitic morphology even in case of the 80/20 PA6/PHB blend (Figure
6). The spherulites exhibit a typical banded structure. In 80/20 blend the banded
spherulite of PHB superimposed on the spherulitic structure of PA6. Two phases are
observed in 60/40 blend, the bright region is PHB and the dark region is PA6. The
same observation is observed in 50/50 blend. In 40/60 and 20/80 blend systems,
miscible regions are observed, characterized by ringed spherulites and the band
spacing decreases with the composition as PHB content is increased.
The preliminary results at these crystallization temperatures show the existence
of wide variety of morphologies. These complicated morphologies will result from
competitive nucleation and growth processing for PA6 and PHB as expressed
previously. Beside that, which is consistent with depression of the equilibrium melting
temperature, the blend composition will also control the nature of the interlamellar
amorphous material and its composition. It is observed that each spherulite exhibits
extinction cross (a Maltese cross). The Maltese cross occurs when the principal
directions of the radial units fall parallel to those of the polarizing microscope.
168
Nucleation rate together with growth determine the size and number of spherulites:
slow nucleation and fast growth result in a small number of large spherulites, whereas
fast nucleation and slow growth result in a large number of small spherulites [50].
Figure 6: POM (X 100) of PA6/PHB blends at ambient temperature and 80oC.
The other significant observation of the blends is the appearing of cracks. It was
inferred that those cracks might contribute to the embrittlement of PHB. Two types of
cracks were identified, one running circumferentially around spherulites and the other
running radially through them. Barham et al [51] suggested that the circumferential
cracks may be caused by thermal stresses that occur during the cooling of the
spherulites from the high crystallization temperatures down to room temperature. They
show that the cracks are, in fact, due to differences in thermal expansion between the
PHB film and the constraining glass slides.
Conclusion Blending of friendly environmental biopolymers with synthetic polymers has
proven to be a suitable tool to produce novel materials with combined characteristics in
having both improved application properties and low cost advantages in material
performance. The melting and crystallization of Polyamide-6 (PA6) with bacterial poly
( β -hydroxybutyrate) (PHB) were studied at different weight fractions. In this blend
system clear depression of the omT was observed. The blend systems are found
miscible because of the negative values of 12χ at each composition. The values of
PA6/PHB interaction parameters up to 50/50 are slightly negative; so that those blend
169
systems can be considered as partially miscible such a result is strongly supported by
POM in which two phases are observed. The segmental interaction parameters ijχ
and 'ijχ are evaluated by fitting 12χ with iα and found to be consistent with the net
12χ . FTIR results strongly support the DSC results that give an idea about
intermolecular hydrogen bonding which is favorable to the free energy of mixing.
Ringed spherulites are formed in the mixture crystallized from the melt suggesting a
strong effect of PHB and PVA on the melting and crystallization of PA6.
Acknowledgement:
This work has partially been supported by the Ministry of Higher Education and
Scientific Research, Jordan. The valuable discussions with Prof. S. Khalil, Mu'tah
University, are greatly acknowledged.
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