Crystallization Kinetics of Poly(1,4-butylene adipate) - American

Oct 3, 2014 - ABSTRACT: Poly(1,4-butylene adipate) (PBA), poly(L-lactic acid) (PLLA), and poly(D-lactic acid) (PDLA) are biodegradable...
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Crystallization Kinetics of Poly(1,4-butylene adipate) with Stereocomplexed Poly(lactic acid) Serving as a Nucleation Agent Yi-An Chen and Tzong-Ming Wu* Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung, Taiwan 402, Republic of China ABSTRACT: Poly(1,4-butylene adipate) (PBA), poly(L-lactic acid) (PLLA), and poly(D-lactic acid) (PDLA) are biodegradable and environmentally friendly polymers. PLLA- and PDLA-formed stereocomplex crystallites (SCs) by solvent-casting served as nucleating agents of PBA. The isothermal crystallization kinetics and morphology of PBA and 0.2−3 wt % PBA/SC composites were studied by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. The crystallization rate of neat PBA and PBA/SC composites increased as the crystallization temperature increased. With the addition of 3 wt % SC, the half-time for crystallization of the PBA/SC composite decreased by 25−47% for isothermal crystallization at 38−44 °C compared with that of neat PBA. In the presence of SC, the number of PBA nuclei increased, and the spherulite size decreased substantially. Therefore, the addition of SC to PBA induces heterogeneous nucleation. The product of the surface free energies of the PBA/SC composites is considerably higher than that of PBA because of the presence of amorphous PLA chains that limit the PBA chain flexibility, thereby leading to an increase in the surface free energy.

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cyclodextrin,20,21 orotic acid,22,23 and uracil,24,25 have attracted extensive attention for their processing into objects for medical applications. For example, the incorporation of biocompatible and biodegradable uracil into PBA was investigated as a nucleation agent for accelerating the crystallization of PBA.25 Polylactide (PLA) is one of the biodegradable polyesters attracting considerable attention for its potential applications in medical devices.26,27 Three types of isomeric forms have been reported in PLA homopolymers, namely, PLLA, PDLA, and poly(racemic lactic acid) (PDLLA), which display an extensive variety of properties. The stereocomplexation of PLA occurs when PLLA and PDLA are blended, based on the interactions of stereoselective van der Waals forces to form stereocomplex crystallites (SCs). The SC melting temperature (Tm) is 50 °C higher than that of PLLA and PDLA.28−30 Several authors have also found that the crystallization rate of PLLA and poly(3hydroxybutyrate) increases with PDLA addition because of the nucleation effect of the SC.18,28,30−33 The crystallization behavior of PBA with nucleation agents, such as uracil and carbon nanotubes, and the addition of common reagents have been investigated extensively.1,22,34,35 However, the effect of the SC on the crystallization behavior of PBA has not yet been discussed. In this study, we investigate the effect of the SC on the crystallization behavior and morphology of isothermally crystallized PBA.

INTRODUCTION In the last 2 decades, biodegradable polymers, such as poly(1,4butylene adipate) (PBA), poly(L-lactic acid) (PLLA), and poly(D-lactic acid) (PDLA), have attracted considerable attention because of their potential application in fields related to environmental protection and physical health maintenance.1 PBA is a typical synthetic aliphatic biodegradable polyester, and its crystalline structure was first studied by Fuller and Erickson in the 1930s.2,3 Later, Minke and Blackwell studied the morphology and structure of PBA single crystals and reported that PBA could crystallize in the α- or β-form crystal under different crystallization temperatures.4,5 The α-form crystal belongs to a monoclinic unit cell with dimensions of a = 6.73 Å, b = 8.00 Å, c = 14.20 Å, and β = 45.5°. The β-form crystal contains an orthorhombic unit cell with dimensions of a = 5.05 Å, b = 7.36 Å, and c = 14.67 Å.4,5 The crystallization temperature (Tc) reported by Gan et al. was a key factor in determining the crystalline structure of PBA.6,7 According to their study, pure β-form crystals were formed below 29 °C, a mixture of both α- and β-form crystals coexisted between 29 and 31 °C, and pure α-form crystals developed at crystallization temperatures above 31 °C. The α form is thermodynamically stable because its equilibrium melting point is higher than that for the β-form crystals.7 Upon annealing at elevated temperatures, the metastable β-form crystals of PBA might be able to transform into the α form.6−13 The initiation and transformation of polymorphic crystals in PBA induced by thermal treatment have been widely studied.6−17 However, practical applications of PBA have been limited because of its softness and slow crystallization rate. The ability to overcome the aforementioned problems in a useful and convenient manner is of considerable interest.1 In general, the addition of a nucleation agent can reduce the free energy required for nucleus formation and results in an increase in the polymer crystallization rate.18 More recently, nucleation agents with biocompatible and nontoxic properties, such as lignin,19 α© 2014 American Chemical Society

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EXPERIMENTAL SECTION Materials and Preparation. PLLA (4032D) and PDLA (1010) were supplied by Wei Mon Corp., Taipei, Taiwan. The weight-average molecular weights of PLLA and PDLA Received: Revised: Accepted: Published: 16689

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determined using gel permeation chromatography were 132000 and 64800 g/mol, respectively. PBA was purchased from Aldrich Co. with a weight-average molecular weight of 12000 g/mol. Dichloromethane was acquired from Mallinckrodt Baker, Inc., and used as received without further purification. Deionized water was used throughout all experiments. Fabrication of PBA/SC Composites. PLLA and PDLA were separately dissolved in dichloromethane for 3 h. Various compositions (40, 50, and 60 wt %) of PDLA and PLLA were prepared through a solution-mixing process for 12 h. The 1 g/ 20 mL solution of binary blends was solvent-cast onto a glass Petri dish at 40 °C and dried in a vacuum oven at 40 °C for 24 h. The stereocomplex crystallites (SCs) of PLLA/PDLA can be obtained. The PBA/SC composites with 0.2, 1, and 3 wt % loading content of SCs were prepared by a solution-mixing process. The solution was then cast on an aluminum plate at 40 °C and dried in a vacuum oven at 40 °C for 3 h. In order to understand the structural behavior of the SC in the following differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) experiments, the solvent-cast SC was melted at 200 °C for 1.5 min and then isothermally crystallized at 40 °C, which is the crystallization temperature used for the following DSC and POM experiments. The melting temperature selected below that of the SC at 200 °C was very effective for enhancing the nucleating effect of the SC on PLLA crystallization.36 DSC. Thermal analysis of the PBA/SC composites was performed using a PerkinElmer Pyris Diamond differential scanning calorimeter. An indium standard was performed for calibration, and all experiments were executed under a nitrogen atmosphere. The specimens weighing in the range of 5−6 mg were heated to a premelting temperature (Tmax) of 200 °C at a heating rate of 10 °C/min and held for 1.5 min to erase the thermal history. Subsequently, the samples were rapidly cooled to the proposed crystallization temperatures (Tcs) at a cooling rate of 100 °C/min and held there for a period of time until the crystallization was complete. The proposed Tcs values were selected in the range between 38 and 44 °C. The exothermal traces were recorded for data analysis. X-ray Diffraction (XRD). The XRD scan of each sample was operated on a Bruker D8 diffractometer equipped with nickel-filtered Cu Kα radiation in the reflection mode. The scan ranges of the specimens were collected from 2θ = 2° to 40° with a scan rate of 1°/min. The baseline of diffraction data was taken just as a straight line in the 2θ range from 5 to 30°. The relative crystallinity (Xc) was calculated using Xc = Ac/(Ac + Aa), where Ac and Aa represent the crystalline area under the diffraction peak at 11.5° and the corresponding amorphous halo, respectively. The crystalline diffractions and amorphous component have been separated with a fitting program after subtraction from the baseline. POM. POM was performed with a Zeiss optical microscope equipped with crossed polarizers. The crystallization process of the PBA/SC composite was heated to melting at Tmax = 200 °C for 1.5 min on a Mettler FP-82 hot stage to eliminate the previous thermal history. Subsequently, the samples were then cooled quickly to the proposed Tcs. POM data were recorded at the proposed Tcs for various times.

Figure 1. XRD data of PDLA/PLLA ratios of (a) 40/60, (b) 50/50, and (c) 60/40 prepared after melting at 200 °C for 1.5 min and then isothermal crystallization at 40 °C.

20.5°, and 23.5° are attributed to the SC crystal.37,38 There are no diffraction peaks at 2θ = 14.3°, 16.3°, 18.6°, and 22.0° corresponding to the α-form crystal of PLLA and PDLA.39,40 In addition, the degree of crystallinity is estimated to be approximately 58.3%, 61.6%, and 64.4% for PDLA/PLLA mass ratios of 40/60, 50/50, and 60/40, respectively. The sample with the highest degree of crystallinity was chosen as the nucleating agent for PBA crystallization. Isothermal Crystallization Behavior of PBA and Its Composites. The overall isothermal crystallization kinetics of neat PBA and its composites were studied via DSC in the crystallization temperature range of 38−44 °C. Because samples were crystallized above 31 °C, the α-form crystal of neat PBA and its composites were obtained. The relative composite crystallinity (Xt) indicates the influence of additives on the composite material crystallization behavior. The relative crystallinity at any given time is determined from the DSC curves based on the integrated area of the exothermic peak from t = 0 to t divided by the entire area of the crystallization exothermic curve. Xt =

t dH(t ) dt dt ∞ dH(t ) dt dt 0

∫0 ∫

=

ΔHt ΔH0

(1)

where dH(t)/dt is the ratio of the heat exotherm. Figure 2 shows the plots of relative crystallinity versus crystallization time for neat PBA and its composites. All results reveal that the crystallization time is reduced as Tc decreases, suggesting that the crystallization rate increases at lower Tc. The crystallization time at the same Tc is shorter for the PBA/SC composites compared with that of neat PBA. For example, the crystallization completes within 44 min for neat PBA at 44 °C, while it is only approximately 12 min for the 3 wt % PBA/ SC composite at the same Tc. Thus, the incorporation of the SC into the PBA matrix has accelerated the isothermal melt crystallization of PBA. The isothermal crystallization kinetics is determined through the Avrami equation, which is shown as follows:41,42

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1 − X t = exp( −kt n)

RESULTS AND DISCUSSION Figure 1 shows the XRD data of the PLLA/PDLA SCs after melting at 200 °C for 1.5 min and then isothermal crystallization at 40 °C. The diffraction peaks at 2θ = 11.5°,

(2)

where n is the Avrami exponent and k is the crystallization rate constant. The values of n and k can be calculated from the linear form of eq 2 as follows: 16690

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Figure 2. Plots of relative crystallinity versus crystallization time for neat PBA and its composites.

ln[− ln(1 − X t )] = n ln t + ln k

(3)

t1/2 =

The Avrami plots of PBA and PBA/SC composites are shown in Figure 3. All curves are approximately analogous to each other, suggesting that the crystallization mechanism of PBA and PBA/SC composites at different Tcs remains the same. The Avrami parameters of n and k were estimated from the slopes and intercepts of the curves. For comparison, the n and k values are obtained for PBA and PBA/SC composites and summarized in Table 1. The n values of neat PBA at given Tcs range from 2.65 to 2.92. The nonintegral n values may occur because of the presence of crystalline branching and two-stage crystal growth.43 In general, a value of n close to 3 may represent an athermal nucleation process followed by a three-dimensional crystal growth and homogeneous nucleation mechanism. However, the obtained Avrami exponent n values are between 3.29 and 3.76 for PBA/SC composites at various Tcs, indicating a crystallization mode of heterogeneous nucleation for the isothermal melt crystallization process.44−46 Similar results were reported for biodegradable polymers/nucleating agent composites, where the Avrami exponent n values are in the range of 3.1−5.2.44,45 In addition, the k values of the PBA/SC composites are strongly dependent on the SC content. When crystallized at the same temperature, the k values of the PBA/ SC composites increased with increasing SC content. These results are attributed to the heterogeneous nucleation of the SC on the crystallization behavior of PBA. The time of half-crystallization (t1/2) can be calculated from eq 4.

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠

(4)

The data of t1/2 are also listed in Table 1. The t1/2 values of PBA and its composites decrease with decreasing Tc. Meanwhile, for samples crystallized at the same Tc, the t1/2 values decrease with increasing SC content. For example, the t1/2 values of PBA decrease significantly from 21.99 to 5.68 min in composites with 3 wt % SC content when crystallized at 44 °C. This indicates that the incorporation of the SC content into PBA significantly enhances the crystallization rate of PBA under the same isothermal crystallization conditions. POM was used to compare the crystal morphologies of PBA and PBA/SC composites. Figure 4 shows the POM graphs of PBA and its composites isothermally melt-crystallized at 40 °C. For the PBA and PBA/SC samples, typical Maltese cross spherulites are observed. The spherulite size of the PBA/SC composite decreases and the number of spherulites increases with increasing loading of the SC. The addition of the SC induces nucleation sites for PBA crystallization and results in the restricted growth of PBA spherulites. The results reveal that the incorporation of the SC into PBA could induce heterogeneous nucleation. The increasing number of spherulites suggests that the nucleation density of the PBA/SC composites has been enhanced probably because of the SC that acts as a nucleating agent. These results are in agreement with the DSC data. Growth Behavior of PBA and PBA/SC Composites. The Hoffman and Week’s equation is used to determine the equilibrium melting temperature (Tm°) of PBA and PBA/SC 16691

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Figure 3. Avrami plots of neat PBA and PBA/SC composites with 0.2−3 wt % SC.

to an extrapolation to the infinite lamellar thickness, and the determined Tm° is approximately 59 °C for PBA. According to a previous investigation, the Tm° value of PBA is 61.3 °C.49 The values of Tm° for PBA/SC composites are close to one other, indicating that the arrangement of crystalline PBA in the PBA/ SC composites is similar to that of neat PBA. The regime theory developed by Lauritzen and Hoffman50 is applied to analyze the crystal growth data of PBA and PBA/SC composites and thus to obtain the thermodynamic parameters related to the crystallization process. The linear growth rate (G) as a function of the temperature is given as follows:

Table 1. Kinetic Parameters of Neat PBA and Its Composites Isothermally Melt-Crystallized at Tc = 38−44 °C sample PBA

0.2 wt % PBA/SC

1 wt % PBA/SC

3 wt % PBA/SC

Tc (°C)

n

k (min−n)

t1/2 (min)

38 40 42 44 38 40 42 44 38 40 42 44 38 40 42 44

2.65 2.78 2.92 2.87 3.23 3.27 3.75 3.72 3.55 3.72 3.51 3.42 3.64 3.40 3.52 3.76

3.33 × 10−2 4.61 × 10−3 5.20 × 10−4 9.85 × 10−5 3.67 × 10−2 8.22 × 10−3 2.83 × 10−4 3.35 × 10−5 7.25 × 10−2 1.10 × 10−2 8.12 × 10−3 4.02 × 10−4 1.65 × 10−1 4.31 × 10−2 1.44 × 10−2 1.02 × 10−3

3.14 6.04 11.72 21.99 2.48 3.88 8.02 14.48 1.89 3.05 3.54 8.82 1.48 2.27 3.00 5.68

⎡ −U * ⎤ ⎡ −K g ⎤ G = G0 exp⎢ ⎥ exp⎢ ⎥ ⎣ R(Tc − T∞) ⎦ ⎣ fTcΔT ⎦

where G0 is a preexponential term, U* is the diffusional activation energy, Kg is a nucleation constant, T∞ is the hypothetical temperature below which viscous flow ceases, and f = 2Tc/(Tm° + Tc) is a correction factor. Kg contains contributions from the surface free energies, and it can be determined from the following equation:

composites.47,48 Tm° obtained from the crossing point of the Tm = Tc line with extrapolation of Tm as a function of Tc is calculated from eq 5: ⎛ 1⎞ T Tm = Tm°⎜1 − ⎟ + c γ⎠ γ ⎝

(6)

Kg =

4bσσeTm° βk ΔHf °

(7)

where β is a parameter that depends on the regime of crystallization, b is the distance between two adjacent chainfolding planes, σ and σe are the lateral and fold surface free energies, respectively, and k is the Boltzmann constant. The parameter β applied in this equation is 1 in regimes I and III and 2 in regime II. The overall crystallization rate should be

(5)

where γ is a factor depending on the final laminar thickness. This procedure of Hoffman and Week’s equation corresponds 16692

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Figure 4. Optical micrographs of the spherulitic morphology of (a) neat PBA, (b) 0.2 wt % PBA/SC, (c) 1 wt % PBA/SC, and (d) 3 wt % PBA/SC after complete crystallization at 40 °C.

interpreted in terms of a combination of nucleation and growth phenomena, while U* describes the activation energy of crystal growth and Kg represents the free energy necessary to form a nucleus of critical size. Hoffman and Weeks47 found that T∞ = Tg − 30 K and U* = 6300 J/mol by fitting the crystallization rate data for various polymers. The value of G can be estimated from the POM data as a function of Tc for PBA and PBA/SC composites. Figure 5 shows the linear growth rates of PBA and PBA/SC composites at different Tcs values. Increasing SC content results in a decrease in the PBA/SC composite growth rate. Figure 6 shows the plots of [ln(G) + U*/[R(Tc − T∞)] versus 1/[f TcΔT] for PBA and PBA/SC composites. The Kg values obtained from the slope of Figure 6 are listed in Table 2. The values of b and ΔHf° are 8.00 Å and 149.29 J/g, respectively.51 In order to define the crystallization regimes at the selected Tcs values, the Lauritzen Z test is usually applied and shown as follows:52 ⎛ L ⎞2 ⎛ −X ⎞ Z ≈ 103⎜ ⎟ exp⎜ ⎟ ⎝ 2a0 ⎠ ⎝ TcΔT ⎠

Figure 5. Growth rates of neat PBA and PBA/SC composites with 0.2−3 wt % SC spherulites as a function of the crystallization temperature.

be smaller than 0.052 nm, which is unrealistic. Assuming Z ≥ 1.0 and substituting X = 2Kg into the Z test, we obtain L ≥ 6.28 nm, which is acceptable for PBA. According to the Z test, the crystallization regime of PBA and PBA/SC composites is determined to be regime II. The parameters b and ΔHf° could be assumed to be the same as those of pure PBA because the additional SC content is low. The surface free energy (σσe) data of PBA and PBA/SC composites were obtained from eq 7, as summarized in Table 2. The σσe data of PBA are 123.40 erg2/cm4, and those of PBA/ SC composites are estimated to be approximately 124.04,

(8)

where L is the effective lamellar width and a0 is the width of the molecular chain in the crystal. If, with X = Kg, the test yields Z ≤ 0.01, regime I crystallization kinetics are followed. Regime II kinetics are followed if the substitution of X = 2Kg into the test yields Z ≥ 1.0. As pointed out by Lauritzen and Hoffman,50 it is more convenient to use the known value for Kg and the inequalities for Z to obtain the values of L in both regimes I and II and to estimate whether such values of L are realistic. If we presume Z ≤ 0.01 and substitute X = Kg into the Z test, L will 16693

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All data for the PBA/SC composites are almost identical with those for PBA. These results indicate that the incorporation of the SC into PBA does not change the crystal arrangement of PBA. Therefore, our postulation that PBA/SC composites have the same b values as those of the pure PBA matrix is acceptable.

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CONCLUSIONS A biodegradable PBA/SC composite has been prepared successfully via a solution-mixing process. The isothermal crystallization kinetics of neat PBA and its composite were studied by DSC at different crystallization temperatures and analyzed using the Avrami equation. With the addition of 3 wt % SC into PBA, the half-time for crystallization of the composite decreased by 25−47% for isothermal crystallization at 38−44 °C compared with that of neat PBA. A spherulitic morphology study confirmed the effect of the SC as a nucleating agent during PBA crystallization. Thus, the addition of the SC into PBA induces heterogeneous nucleation. The σσe data of the PBA/SC composites are much higher than those of PBA, probably because of the presence of amorphous PLA chains that limit the PBA chain flexibility and increase the end surface free energy, σe, leading to an increase in σσe. Finally, it was found that the addition of the SC does not change the PBA composite crystal structure.

Figure 6. Plot of ln G + U*/R(T − T∞) versus 1/( f TcΔT) of neat PBA and PBA/SC composites.

Table 2. Values of Tm°, Kg, and σσe at Various Tc Values for PBA and PBA/SC Composites

Tm° (K) Kg (K2) σσe (erg2/cm4)

PBA

0.2 wt % PBA/SC

1 wt % PBA/ SC

3 wt % PBA/ SC

331.45 3.16 × 104 123.40

332.12 3.18 × 104 124.04

332.78 3.50 × 104 136.75

332.88 3.53 × 104 137.28

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +886 4 2287 2482. Fax: +886 4 2285 7017. E-mail: [email protected].

136.75, and 137.28 erg2/cm4 for 0.2, 1, and 3 wt % loading of the SC, respectively. This result reveals that the addition of the SC into PBA causes an increase in σσe. Because the samples are heated to Tmax = 200 °C for 1.5 min and quenched immediately to the proposed Tc, the PLA chain state is amorphous. Therefore, the increase in σσe data probably occurs because of the presence of amorphous PLA, which limits the PBA molecule chain flexibility and increases the end surface free energy, σe, leading to an increase in σσe. To study the effect of the SC on the crystal structure of PBA, a XRD method was used to determine the crystalline structure of PBA and PBA/SC composites. Figure 7 shows the XRD spectra of neat PBA and PBA/SC composites, which were crystallized at 40 °C. Three diffraction peaks exist for neat PBA at 2θ = ∼21.69°, 22.41°, and 24.08°, which correspond to (010), (200), and (203) of the α-form crystal, respectively.53,54

Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 7. XRD data of (a) PBA, (b) 0.2 wt % PBA/SC, (c) 1 wt % PBA/SC, and (d) 3 wt % PBA/SC composites. 16694

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dx.doi.org/10.1021/ie503303u | Ind. Eng. Chem. Res. 2014, 53, 16689−16695