Isothermal Crystallization Kinetics of Poly(3-hydroxybutyrate)/Layered
Double Hydroxide Nanocomposites
Tzong-Ming Wu, Sung-Fu Hsu, Chien-Shiun Liao
Department of Materials Science and Engineering, National Chung Hsing University
250 Kuo Kuang Road, Taichung, Taiwan 402
E-mail: [email protected]
ABSTRACT
Poly(3-hydroxybutyrate) (PHB)/layered double hydroxides (LDH) nanocomposites were fabricated by mixing PHB and
PEOPA-modified LDH (PMLDH) in solution. These results obtained from X-ray diffraction (XRD) and transmission electron
microscopy (TEM) for PHB/PMLDH nanocomposites reveal that the PMLDHs are randomly distributed into the PHB matrix.
Effect of PMLDH on the isothermal crystallization behavior of PHB was studied by differential scanning calorimetry (DSC)
and polarized optical microscopy (POM). These results demonstrate that the loading of 1 and 2 wt% of PMLDH content into
PHB matrix induced more heterogeneous nucleation in the crystallization process extensively increasing the crystallization
rate and reducing the activation energy. The addition of more PMLDH content into the PHB perhaps inhibit the transport of
the PHB molecule chains, resulting in a decrease of the PHB chain packing during crystallization, thus increasing the
activation energy. Mechanical properties of the fabricated PHB/PMLDH nanocomposites show significant improvements in
the storage modulus when compared to that of neat PHB.
Introduction
The biodegradable polymer, poly(3-hydroxybutyrate) (PHB), can be accumulated using a variety of bacteria as a reserve
energy source [1-2]. In addition to renewable resources, PHB is a thermoplastic material with mechanical and physical
properties close to those of isotactic polypropylene [3]. Therefore PHB has attracted much attention in various applications,
such as medicine, packages, biomedical implants and agriculture [4]. Nevertheless, the thermal instability and brittleness of
PHB significantly limit its application.
Organic/inorganic nanocomposites containing high strength and modulus of inorganic material can enhance the
mechanical and thermal properties of polymers. Polymer/layered silicate nanocomposites often reveal a significant
improvement of physical and/or chemical properties relative to the pure polymer matrix and have received extensive
attentions. For example, several investigations of PHB/MMT [5] and (PHB/HV)/MMT nanocomposites [6-7] have recently
reported in the literature and their results show remarkable enhancement in the physical properties of PHB by adding a
small amount of MMT into the original polymer matrix.
Recently a new emerging class of inorganic layered material, layered double hydroxides (LDH), has been developed and
considered as promising host-guest materials [8]. The crystal structure of LDH consists of brucite-like layers and the
exchangeable interlayer anions, in which the LDHs can be intercalated with both inorganic and organic anions [9-10].
In this study, the PHB/PMLDH nanocomposites have been successfully prepared by the insertion of PHB polymer chains
into the interlayer spacing of PMLDH. The aim of this work is to clarify the isothermal crystallization kinetics of PHB and
PHB/PMLDH nanocomposites by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The
laternal-surface and fold-surface energy as well as the activation energy of isothermal crystallization for of PHB and
PHB/PMLDH nanocomposites have also been determined.
Experimental
The poly(3-hydroxybutyrate) (PHB) obtained from Aldrich Chemical Co. and purified with precipitated method by adding
n-hexane into the PHB/chloroform solution. The Mg-Al layered double hydroxides (LDH) with the ratio of Mg/Al = 3 was
performed by aqueous co-precipitation and thermal crystallization method [11]. The organic-modified LDH (PMLDH) was
prepared using poly(ethyleneglycol) phosphonates (PEOPA) grafted to the surface of LDH and has been described
previously [12]. The PHB/PMLDH nanocomposites were prepared by refluxing method in chloroform solution [13].
Isothermal crystallization behavior of PHB and PHB/PMLDH nanocomposites was obtained using a Perkin-Elmer PYRIS
Diamond DSC and the measurement was conducted under a nitrogen atmosphere. For isothermal crystallization, the
samples were heated to melt at 190 oC and held for 3 min to remove the residual crystals. Then the specimens were quickly
cooled to the proposed crystallization temperatures (Tcs) in the range of 110~130 oC. In order to evaluate the effect of
crystallization temperatures on the isothermal crystallization behavior, all specimens were reheated from Tcs to 190 oC at a
o
-1
rate of 10 C·min . Spherulitic morphologies were performed using a Zeiss polarizing microscopy equipped with a Mettler
o
FP-82 hot stage. The samples were heated to melt at 190 C for 3 min on the hot stage to remove the previous thermal
history, and then rapidly cooled to the proposed Tcs. The spherulite size and density of nuclei of PHB and PHB/PMLDH
nanocomposites recorded at the proposed Tc for various times. X-ray diffraction scans of these specimens were carried out
with a Rigaku III diffractometer equipped with Ni-filtered CuKα radiation. Transmission electron microscopy was obtained
with a JOEL transmission electron microscope using an acceleration voltage of 120 keV. Ultrathin section of PHB/PMLDH
nanocomposites with a thickness of approximate 50 nm was prepared with a Reichert ultramicrotome. DMA experiments
o
were performed on a Perkin-Elmer instrument DMA 7e apparatus in the range of -50-100 C. The test samples size was 18
× 0.5 × 0.04 mm3 and the collected data were reproducible.
Theory
The isothermal crystallization kinetics of PHB and PHB/PMLDH nanocomposites can be analyzed by using the classical
Avrami equation [14]:
1 − X t= exp(−kt n )
(1)
where Xt is the relative crystallinity at time t; k and n are the Avrami parameters, depending on the nucleation and growth
mechanisms of spherulites. Equation (1) can be rewritten into
(2)
ln[− ln (1 − X t )] = ln k + n ln t
The k and n values can be determined from the intercept and slope of plots of ln [-ln (1-Xt)] versus ln t. The activation
energy (E) can be obtained using the following Arrhenius plot:
(3)
1 / n(ln k ) = ln k 0 − E / RTc
o
where k0 is a temperature-independent factor and R is the gas constant. The equilibrium melting temperature (Tm ) can be
determined from the crossing point of the Tm = Tc line with the extrapolation of Tm as a function of Tc by applying the
Hoffman and Week’s equation [15],
o
(4)
Tm =T m (1 − 1 / γ ) + (Tc / γ )
where γ is a factor that depends on the final laminar thickness. The regime theory of crystalline growth can be used to
analyze the parameter of crystallization kinetics according to the Hoffman-Lauritzen equation [16]:
ln G +
⎡ − Kg ⎤
U*
= ln G0 − ⎢
⎥
R (Tc − T∞ )
⎣ fTc ΔT ⎦
(5)
where G0 is a pre-exponential term, U* is the diffusional activation energy for the transport of crystallizable segments at the
liquid-solid interface and was assumed to be 4.12 kcal/mol, T∞ is the hypothetical temperature below which viscous flow
ceases and is universally equal to Tg – 51.6 K and f = 2Tc/(Tmo + Tc) is a correction factor that accounts for the change of
0
ΔHf with the temperature. The linear crystal growth (G) is obtained from the data of POM. The nucleation constant (Kg)
contains contributions from the surface free energies of the lamellar crystals and it can be expressed using the equation
[17],
Kg =
mb0 σσ eTm0
KΔH 0f
(6)
where b0 is the distance between two adjacent fold planes and is equal to 0.58 nm according to previous report [18]; m is a
parameter that depends on the regime of crystallization, which is 4 in regimes I and III and 2 in regime II; K is the Boltzmann
constant; σ and σe are the lateral and folding surface free energy; ΔHf0 is the enthalpy of fusion of the perfect crystalline
8
3
polymer and is equal to 1.85× 10 J/m . The Kg and G0 values can be determined from the slope and intercept of the plots of
ln G + U*/[R (Tc -T∞)] versus 1 / [f Tc ΔT]. The derived Kgs can be used to calculate the folding surface free energy, σe, for
the polymer as the lateral surface free energy, σ, was estimated by the Thomas-Staveley equation [19]:
(7)
σ = αΔH 0f (a0b0 )1/ 2
where α is an empirically determined constant and is equal to 0.25 for high-melting polyesters [20]; (a0b0)
cross-sectional area of a chain of PHB crystal [18].
1/2
was the
Results and discussion
The X-ray diffraction curves of LDH, PMLDH and PHB/PMLDH nanocomposites are shown in Figure 1. From the data of
LDH, there are three strong diffraction peaks at 2θ = 11.4°, 22.9° and 34.5°, indicating three orders of interlayer spacing
between LDHs. The interlayer distance of LDH is 7.8 A determined by the Bragg’s equation using the diffraction peak 2θ =
11.4°, corresponding to the distance of 003 plane. These diffraction peak intensities of PMLDHs are significantly decreased,
indicating that the ordered structures of LDHs are gradually disappeared. Nevertheless all X-ray diffraction data of
nanocomposites showed no d003-reflection in this region, thus suggesting no regular periodicity between the interlayer
distances. These results demonstrate that polymer chains of PHB could have been successfully distributed into the PMLDH
galleries. In order to clarify the dispersion of PMLDH in PHB, TEM was used to visually estimate the degree of exfoliation.
Figure 2 shows TEM micrographs of 5 wt% PHB/PMLDH nanocomposites, in which the gray areas represent the inorganic
layers of PMLDH randomly distributed in the PHB matrix (bright). From the TEM results, the dispersion of PMLDH in the
fabricated nanocomposites was well distributed in the polymer matrix. Therefore the results of X-ray diffraction and TEM
image demonstrated that most of the hydroxide layers are exfoliated in the PHB matrix.
Relative Intensity
d003
(f)
(e)
(d)
(c)
(b)
(a)
2
6
10
14
18
22
26
30
34
38
2θ (degrees)
Figure 1. X-ray diffraction data for (a) LDH, (b) PMLDH, (c) neat PHB, (d) 1 wt % PHB/PMLDH, (e) 2 wt % PHB/PMLDH and
(f) 5 wt % PHB/PMLDH nanocomposites.
Figure 2. TEM micrographs of 5 wt % PHB/PMLDH nanocomposite.
The isothermal crystallization kinetics of PHB and PHB/PMLDH nanocomposites are obtained using the Avrami equation.
The Avrami parameters k and n values of neat PHB and PHB/PMLDH nanocomposites obtained from the intercept and
slope of a plot of ln [-ln(1-Xt)] versus ln t are listed in TABLE 1. The n values of PHB and PHB/PMLDH nanocomposites are
in the range of 2.1~2.6 with increasing Tc. These results suggest that the addition of PMLDH into PHB did not change the
crystallization mechanism of PHB. In addition, the k value decreased with increasing Tc due to a gradual decrease in the
degree of supercooling. The k value of PHB/PMLDH nanocomposites increased as the PMLDH content increases from 1 to
2 wt %, indicating the presence of PMLDH induced more heterogeneous nucleation of PHB matrix and significantly
increased the crystallization rate of PHB polymer. However, the k value decreased as PMLDH increased to 5 wt %,
indicating more PMLDH probably causes more steric hindrance during the formation of PHB crystallization. Similar
phenomena have been observed in other nanocomposites [21-22].
The activation energy E for PHB and PHB/PMLDH nanocomposites obtained by Equation (3) is shown in Figure 3 and
summarized in TABLE 1. The E decreases with increasing PMLDH content from 1 to 2 wt %, indicating that the addition
PMLDH into PHB maybe induces more heterogeneous nucleation to decrease activation energy during crystallization. But
the addition of more PMLDH caused more steric hindrance also reduces the transport of polymer chains during
crystallization, hence the E of nanocomposites increased with increasing the PMLDH content from 2 wt % to 5 wt %.
TABLE 1. Values of n, k and E at various Tc for neat PHB and PHB/PMLDH nanocomposites.
Sample
o
Tc ( C)
n
k (×102·min-1)
110
2.1
36.6
115
2.2
7.6
120
2.3
1.4
125
2.5
0.2
130
2.4
0.05
110
2.2
57.9
115
2.3
24.9
120
2.4
4.1
125
2.4
0.6
130
2.5
0.07
E (kJ·mol-1)
Neat PHB
-165.6
1 wt % PHB/PMLDH
-170.9
2 wt % PHB/PMLDH
110
2.4
100.6
115
2.4
31.1
120
2.4
5.2
125
2.5
0.7
130
2.6
0.08
-183.1
5 wt % PHB/PMLDH
110
2.2
55.1
115
2.4
15.4
120
2.5
3.9
125
2.4
0.5
130
2.4
0.07
-167.4
0.5
0.0
-0.5
1/nlnk
-1.0
-1.5
-2.0
PHB
1 wt % PHB/PMLDH
2 wt % PHB/PMLDH
5 wt % PHB/PMLDH
-2.5
-3.0
-3.5
2.45
2.50
2.55
2.60
2.65
-1
1/Tc (K )
Figure 3. Arrhenius plots of 1/n(lnk) versus 1/Tc for neat PHB and PHB/PMLDH nanocomposites.
o
o
The equilibrium melting temperature (Tm ) was determined using the Equation (4). The Tm obtained from our experimental
o
data is close to that reported in the literature using the Tc in the range of 80~140 C [23] and is also listed in TABLE 2. The
value of Tmo decreases with the addition of PMLDH content from 2 to 5 wt %, which indicated the crystalline phase of 2 wt%
and 5 wt% PHB/PMLDH nanocomposites was less perfect than that of neat PHB. This phenomenon is probably due to the
presence of more heterogeneous nucleation induced by PMLDH, thus leading to small crystallite of PHB in PHB/PMLDH
nanocomposites.
TABLE 2. Values of Tm0, Kg G0, σ and σe for neat PHB and PHB/PMLDH nanocomposites.
Neat PHB
1 wt %
PHB/PMLDH
2 wt %
PHB/PMLDH
5 wt %
PHB/PMLDH
190.9
191.3
190.1
188.4
5.5
5.3
5.0
4.2
390.9
163.4
63.0
5.1
σ (erg·cm )
28.6
28.6
28.6
28.6
-2
σe (erg·cm )
45.6
44.0
41.4
35.4
0
Tm (oC)
2
-5
Kg (K ) ×10
-1
-12
G0 (μm·sec ) ×10
-2
Figure 4 shows polarized optical micrographs of the PHB and PHB/PMLDH nanocomposites isothermal crystallized at
o
o
115 C and 130 C. From this figure, the numbers of spherulites at the same Tc are increased as the PMLDH content
increases, indicating that the incorporation of PMLDH induced the heterogeneous nucleation in the PHB matrix. At the
same time, the nucleation density in PHB also increases with the existence of PMLDH. Nevertheless, the amounts of
spherulites for the same specimens are approximately identical as Tc increases, indicating that the crystallization
temperature is not a governing factor of the nucleation of PHB crystallization.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Figure 4. POM of (a) neat PHB, (b) 1 wt % PHB/PMLDH, (c) 2 wt % PHB/PMLDH and (d) 5 wt % PHB/PMLDH
nanocomposites and (e) neat PHB, (f) 1 wt % PHB/PMLDH, (g)2 wt % PHB/PMLDH and (h) 5 wt % PHB/PMLDH
nanocomposites isothermally crystallized at 115oC and 130 oC.
The linear growth rate of spherulite (G) of neat PHB and PHB/PMLDH nanocomposites was obtained as a function of Tcs.
The Kg and G0 values determined from the slope and intercept of the plots of ln G + U*/[R (Tc - T∞)] versus 1 / [f Tc ΔT] are
shown in Figure 5 and listed in TABLE 2. The G0 value of the PHB is much higher than those of the PHB/PMLDH
nanocomposites, suggesting the presence of PMLDH could inhibit the chain mobility of PHB segments. Thus the G0 value
decreases as PMLDH loading increases.
lnG+U*/R(Tc-T8 )
14
13
12
11
PHB
1 wt % PHB/PMLDH
2 wt % PHB/PMLDH
5 wt % PHB/PMLDH
10
9
3.4
3.6
3.8
4.0
4.2
5
4.4
4.6
4.8
-1
1/fTcΔT(x10 K )
Figure 5. Plots of ln(G) + U*/[ R (Tc-T∞)] versus 1 /[ fTcΔT] for neat PHB and PHB/PMLDH nanocomposites.
From the peak positions of X-ray diffraction data shown in Figure 1, the neat PHB and PHB/PMLDH nanocomposites
contain the same crystalline structure. Therefore the crystalline parameter b0 of PHB/PMLDH nanocomposites can be
assumed to be the same as that of neat PHB. In order to further investigate the surface energy of PHB during the
crystallization process, the Lauritzen Z test was performed to determine the regime of selected Tc. Z test is defined as
Z ≈10 3 (
L 2
X
) exp(
)
Tc ΔT
2a 0
(8)
where a0 is the molecular width and L is the effective substrate length. The value of a0 reported in previous literature is 0.66
nm [18]. According to Z test, if the substitution of X = 2Kg into the test results in Z ≥ 1.0, regime II kinetics are followed. If
with X = Kg into the test results in Z ≤ 0.01, regime I or III kinetics are followed. As mentioned by Lauritzen and Hoffman [19],
this method is more suitable to identify the value of Kg and the inequalities for Z to calculate the values of L in regime I/III or
regime II and to determine whether the value is reasonable. Assuming Z ≥ 1.0 and substituting X = 2Kg into the test, L ≥
6
2.1×10 nm, which is obviously impossible. Assuming Z ≤ 0.01 and substituting X = Kg into the test, L ≤ 29.6 nm, and this
calculation is reasonable for PHB. According to previous publication [24], there was a transition for PHB from regime III to
regime II at the temperature around 130-140°C. Therefore, the present data of crystallization regime at 110°C≦Tc≦130°C
is attributed to be regime III, and m = 4 was adopted. Therefore the folding surface free energy (σe) for the PHB listed in
TABLE 2 can be calculated using the derived Kgs, as the lateral surface free energy (σ) was estimated by the
-2
Thomas-Staveley equation. The data of folding surface free energy for PHB is 45.6 erg·cm and agrees quite well with the
previous investigation [18]. The σe of PHB/PMLDH nanocomposites was lower than that of neat PHB, suggesting that the
addition of PMLDH into PHB may induce the heterogeneous nucleation of PHB crystallization and then decrease the
surface energy barrier for PHB crystallization.
DMA performed over a temperature range of -50 to 100°C was used to determine the mechanical properties of PHB and
PHB/PMLDH nanocomposites, in which the data of dynamic storage modulus (G’) are listed in TABLE 3. At -50°C, the G’
increased with increasing PMLDH loading from 1 to 2 wt %, indicating that the intercalated PMLDH into PHB matrix have
significant enhancement on the elastic properties of the PHB matrix. The enhancement of G’ of 2 wt % PHB/PMLDH
nanocomposite is about 88% higher than that of the neat PHB. Nevertheless, the G’ decreases with the loading of PMLDH
increases from 2 wt % to 5 wt %. These results suggested the reinforcement effects of PHB/PMLDH nanocomposite are
predominated by the loading of PMLDH and their interaction with the PHB matrix. As the addition of more PMLDH into
PHB, the PHB/PMLDH nanocomposites containing more organic PEOPA molecules with low mechanical properties may
reduce the storage modulus of material that results in the lower storage modulus in 5 wt % PHB/PMLDH nanocomposites.
TABLE 3. Dynamic storage modulus (G’) of neat PHB and PHB/PMLDH nanocomposites.
G’ (MPa)
- 50 oC
o
25 C
Neat PHB
1 wt %
PHB/PMLDH
2 wt %
PHB/PMLDH
5 wt %
PHB/PMLDH
1600
2730
3000
2020
700
1340
1450
890
Conclusion
The exfoliated PHB/PMLDH nanocomposite has been successfully prepared by mixing the PHB and PEOPA-modified LDH
(PMLDH) in solution. Mechanical properties of prepared PHB/PMLDH nanocomposites show significant improvements in
the storage modulus when compared to that of neat PHB. Isothermal crystallization results indicate that the loading of 1 and
2 wt% PMLDH content into PHB matrix induced more heterogeneous nucleation in the crystallization process extensively
increasing the crystallization rate and reducing the activation energy. The addition of more PMLDH content into the PHB
perhaps inhibit the transport of the PHB molecule chains, resulting in a decrease of the PHB chain packing during
crystallization, thus increasing the activation energy. The analysis of kinetic data suggests that PHB has the highest values
of σσe, indicating that the incorporation of PMLDH into PHB probably induces heterogeneous nucleation and then
decreases the surface energy barrier for PHB crystallization.
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