Isothermal Crystallization Kinetics of Poly(Lactic acid)/montmorillonite Nanocomposities

Mei -Jan Chu1,2, Tzong-Ming Wu1 1.Department of Materials Science and Engineering National Chung Hsing University, Taichung, Taiwan

2. Plastic Industry Development Center, Taichung, Taiwan

Abstract

The poly(lactic acid) (PLA)/montmorillonite (MMT) nanocomposites were prepared by melt blending the PLA with commercial organically modified MMT, CloisiteR30B. The effect of MMT content on the crystallization behavior of PLA/MMT nanocomposites was investigated using the DSC, POM, and XRD. The isothermal crystallization the of organically-modified MMT induced more heterogeneous nucleation effectively increased the crystallization rate of PLA and decreased the activation energy.

Itroduction

Poly(lactic acid) (PLA) is a linear aliphatic polyester synthesized from lactic acid monomer that obtained from renewable resources, such as corn and potato [1]. In addition, PLA is an environment-friendly biodegradable polymer in contrast to products from petroleum feedstocks. The physical properties, mechanical properties and degradation rate of PLA strongly depend on the structure, morphology and crystallinity of the polymer. In recent years, PLA nanocomposites have attracted interests because the addition of layered silicates can enhanced the physical properties and crystallization behavior when compared to those of polymer matrix. In this research, we used commercial organically modified montmorillonite (MMT) as a filler to fabricate PLA/MMT nanocomposites using melt compounding. The crystallization behavior of PLA/MMT nanocomposites was investigated by XRD, POM and DSC. The isothermal crystallization kinetics and melting behavior of PLA/MMT nanpcomposities will be discussed [2-5], especially the parameters of crystallization such as the lateral-surface and fold-surface energy as well as the activation energy.

Experimental Section

The pellets of poly(lactic acid) were obtained from Chi-Da Corporation (Taichung, Taiwan). Commercial montmorillonite, CloisiteR30B supplied by Southern Clay was used without further treatment. The PLA/MMT nanocomposites were produced by mixing various amount of commercial MMT using melt-blending at 190 in the brabender for 10 min.℃ X-ray diffraction scans of these specimens were obtained using a Rigaru diffractometer in the reflection mode with NiⅢ -filtered Cu Kα radiation. The isothermal crystallization kinetics of these samples was performed using a Perkin Elmer Pyris 1 differential scanning calorimetey (DSC) calibrated with indium, and all experiments

were carried out under a nitrogen atmosphere. All specimens were weighed about 10 mg. The samples for the isothermal crystallization experiments were heated room temperature to 190℃, where they were held for 3 min to eliminate the residual nuclei that might act as seed crystals. Then, the melt was cooled to the crystallization temperature in the range of 85~125℃ at a cooling rate of 5℃/min. The melting behavior of isothermally crystallized samples were reheated to 190 at a℃ heating rate of 10 /min℃ . Spherulitic measurements were carried out using a Zeiss optical microscope with crossed polarizer equipped with a Mettler FP-82 hot stage.

Results and Discussion

Figure 1 shows the X-ray diffraction datd of PLA/MMT nanocomposites. From curve in Figure 1, there is a strong diffraction peak at 2θ= 4.88o corresponding to the inter layer distance of 18.1 Ǻ. All diffraction data for nanocompostes show no intense reflection at the lower angle area, indicating the exfoliated feature for nanocomposites.

2 3 4 5 6 7 8 9 10

(b)

(e)

(d)

(c)

(a)

Rel

activ

e In

tens

ity

2θ

Figure 1. Typical WAXD patterns of (a)MMT and (b) PLA (c) 1wt % PLA/MMT (d) 3wt% PLA/

MMT (e) 5wt % PLA/ MMT nanocomposituies. Figue 2 shows the typical DSC curves of PLA and PLA/MMT nanocomposites isothermally crystallized at Tc = 105℃.These results indicate that the completion of crystallization of PLA/MMT nanocomposites takes a short time as compares to that of pure PLA matrix. Similar Tc-dependent crystallization behaviors were also observed at various Tcs.

0 5 10 15 20

PLLA 1wt% PLLA/MMT 3wt% PLLA/MMT 5wt% PLLA/MMT

Hea

t Flo

w (e

ndot

herm

up)

Time (min)

Figure 2. DSC thermograms of isothermal crystallization at 105℃ for (a)PLA and (b) 1wt%

PLA/MMT (c) 3wt% PLA/MMT (d) 5wt% PLA/MMT nanocomposities. Figure 3 shows the plot of log 1/t1/2 versus crystallization temperature in which t1/2 is defined as the time required to reach 50% crystallinity . It is clear that the curve can be divied into two categories as Tc increases. For PLA and PLA/MMT nanocomposities, 1/t1/2 increases as Tc increases from 85℃ to 110℃ and 105℃ and then decreases as Tc continuously increases. This result indicates the rate of crystallization is dominated by the increase of chain mobility as the Tc below 110℃ for PLA and 105℃ for PLA/MMT nanocomposites, respectively reach this critical temperature, the crystallization rate is controlled by the nucleation of PLA spherulites.

90 100 110 120-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

log

(1/t 1/

2)

TC (oC)

PLLA PLLA/1wt% MMT PLLA/3wt% MMT PLLA/5wt% MMT

105oC110oC

Figure 3. The half-time of crystallization log (t1/2) vs Tc for (a)PLA (b) 1wt% PLA/MMT(c) 3wt%

PLA MMT (d) 5wt% PLA MMT nanocomposities. The Avrami equation is widely used to describe the isothermal crystallization of polymer and is shown below[6]:

])(exp[1 nt tkX −=− (1)

where the Xt is the time-dependent volume fraction of crystallinity, k is the crystallization constant (min -1), and n is the Avrami exponent. Taking the double logarithm of eq (1) gives

tnkX t lnln)]1ln(ln[ +=−− (2) Figure 4 presents polts of ln [-ln (1-Xt )] vs ln t for the neat PLA and the nanocomposites at different Tcs. The plot of ln [-ln (1-Xt )] vs ln t should be linear and the parameters k and n can be determined by curve fitting the experimental data with slope n and intercept ln k. The crystallization parameters k, n and t1/2 for PLA and PLA/MMT nanocomposites are summarized in Table 1.

0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4 Neat PLA

85 90 95100105110

∇ 115 120 125

ln(-

ln(1

-Xt)

)

ln(t-tstart)/min

0.8 1.2 1.6 2.0 2.4 2.8 3.2-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4 3wt% PLA/MMT

85 90 95100105110

∇ 115 120 125

ln(-l

n(1-

Xt))

ln(t-tstart)/min

Figure 4 Avrami analysis for PLA and 3wt% PLA/MMT nanocomposites isothermally crystallized at

various Tcs.

Table 1. Values of t1/2 , n, and k at various Tcs for PLA and PLA/MMT nanocomposites

Tc( )℃ n k t1/2 (min) Ea (KJ/mole) Tc( )℃ n k t1/2 (min) Ea

(KJ/mole)PLA

85 3.25 1.8E-4 12.78 96.6 110 3.04 1.9E-3 7.02 -219.0 90 3.06 3.3E-4 12.15 115 2.98 1.2E-3 8.45 95 2.92 1.2E-3 8.81 120 2.62 4.7E-4 16.2 100 2.94 1.5E-3 8.15 125 2.64 2.0E-4 21.95 105 2.81 2.7E-3 7.2

1wt % PLA/MMT 85 3.52 3.4E-4 8.74 89.9 105 2.99 8.9E-3 4.3 -214.4 90 3.30 1.2E-3 6.89 110 2.96 4.0E-3 5.7 95 3.20 2.7E-3 5.63 115 2.85 2.5E-3 7.21 100 2.99 6.4E-3 4.81 120 2.90 1.2E-3 9.07

125 2.74 3.7E-4 15.7 3wt % PLA/MMT

85 3.62 4.3E-4 7.69 90 105 3.04 1.1E-2 3.86 -215.0 90 3.23 1.9E-3 6.19 110 2.96 4.2E-3 5.56 95 3.07 5.9E-3 4.73 115 2.97 2.6E-3 7.09 100 3.05 8.6E-3 4.23 120 2.81 1.5E-3 8.88

125 2.82 6.4E-4 11.99

5wt % PLA/MMT85 3.14 2.1E-3 6.36 85.4 105 3.09 1.9E-2 3.23 -194.6 90 3.27 3.0E-3 5.27 110 3.17 5.1E-3 4.72 95 3.04 6.8E-3 3.62 115 2.88 5.0E-3 5.49 100 3.13 8.9E-3 3.39 120 2.90 1.8E-3 7.74

125 2.85 6.5E-4 11.54 For PLA, the k value increases with increasing Tc to 110 and then ℃ decreases as Tc further increasing. The transition temperature for PLA/MMT nanocomposites is at Tc =105℃.The Avrami exponent n represents the dimensionality of the growth. All n values of PLA and PLA/MMT nanocomposites are close to 3. The n value is affected by many factors, such as the nucleation density and restriction of crystalline formation due to the presence of inorganic filler. In this case, the PLA and PLA/MMT show three-dimensional spherical growth. The crystallization rate parameter k can be approximately described by Arrhenius equation shown as follows[7]:

( )RT

Ekkn o

Δ−= lnln1 (3)

where k0 is a temperature-independent pre-exponential factor, ΔE is an activation energy, R is the universal gas constant. The activation energy can be determined from the slope of Arrhenius plots of 1/n(lnk) against 1/T for PLA and PLA/MMT nanocomposities. The activation energy also listed in Table 1 slightly decreases with increasing the content of MMT. The result indicates the addition of more MMT into the PLA matrix causes more heterogeneous nucleation. The equilibrium temperature Tm0 of the PLA and PLA/MMT nanocomposities can be determined by the plot of Tm versus Tc according to Hoffman and Week’s [8]equation

( ) ( )γγ //11 comm TTT +−= (4)

where γ is a factor depending on the final laminar thickness. It is assumed that γ = ℓ/ℓ* where ℓ and ℓ* are he thickness of a nature crystalline and of critical crystalline nucleus. Tm

0 can be determined from the crossing point of Tm=Tc line with the extrapolation of Tm as a function of Tc. This procedure is equivalent as extrapolation to infinite lamellar thickness and the extrapolated equilibrium melting points are the results listed in the Table 2.

Table 2. Value of Tm0, Kg, σσe for PLA and PLA/MMT nanocomposities The Tm

0 slightly decreases as the content of MMT increases. This phenomenon is probably due to the presence of more heterogeneous nucleation to reduce the perfection of PLA crystallite in PLA/MMT nanocomposities. The regime theory of crystal growth is applied to analyze the thermodynamic parameters related to the crystallization process. Therefore, the temperature dependence of the linear growth rate (G) is given as follows equ (5)[9]:

( ) ⎥⎦

⎤⎢⎣

⎡Δ

−⎥⎦

⎤⎢⎣

⎡−

−=

∞ TfTK

TTRUGG

c

g

c

expexp*

0 (5)

where G0 is a pre-exponential term, U* is the diffusion activation energy for transport of

PLA 1wt%MM T 3wt%MM T 5wt%MM TTm

0 (oC) 465.3 463.9 462.0 460.0 Kg (K2) ×105 1.73 1.69 1.64 1.63 σσe (J2/cm4) 3.96E-5 3.88E-5 3.78E-5 3.77E-5 σ (J/m2) 5.67E-3 5.67E-3 5.67E-3 5.67E-3

σe (J/m2) 6.99E-3 6.84E-3 6.67E-3 6.65E-3

crystallizable segment at the liquid-solid interface, T∞ is the hypothetical temperature below which viscous flow ceases, and f = 2Tc/(Tm

0 + Tc) is a correction factor that accounts for the change in the enthalpy of fusion of the perfect crystal(ΔHf

0) with the temperature. The nucleation constant (Kg) contains contributions from the surface free energies, and it can be obtained from eq (6)[10]:

0

00

f

meg HK

TmbKΔ

=σσ (6)

where b is the distance between two adjacent fold planes, σ and σe are the lateral and folding surface free energy, k is the Boltzmann constant, and m is a parameter which depends on the regime of crystallization. The parameter m use in Eq.6 is 4 in regimes I and III and 2 in regime II. The lateral surface energy (σ), can be determinal using the Thomas－Stavely equation (7)[11]. σ=0.1ΔHf

0b (7) The σ value is 5.67E-3 J/m-2 for all PLA and PLA/MMT nanocomposities. The folding surface free energy (σe) for the PLA listed in TABLE 2 can be calculated using the derived Kgs. The data of folding surface free energy for PLA is 6.99 E-3 J/m-2 . The σe of PLA/MMT nanocomposites was lower than that of neat PLA. This is probably the reason that the introduction of MMT to PLA causes induce the heterogeneous nucleation of PLA crystallization and then decrease the surface energy barrier for PLA crystallization. Figure 5 shows the POM data of PLA at Tc =110℃ and 125℃. When PLA was crystallized from the melt to 125℃, the grown spherulites exhibit typical extinction crosses under the POM. As Tc decrease to 110℃, the size of spherulite becomes small.

Figure 5. POM images of PLA at isothermal crystallization from their melts (190℃) quench to

Tc (a) 125 and (b)℃ 110℃. Because the spherulite size of PLA is too small to estimate (figure 4), the half time of crystallization t1/2 instead of spherulitical growth rate G can be used. Therefore, Eq. (5) can be rewritten as follows:

( ) ⎥⎦

⎤⎢⎣

⎡Δ

−−=

−+

∞ TfTK

GTTR

Ut c

g

c0

*

2/1

ln1ln (8)

Hoffman et al. found T∞ = Tg-30K and U* =1500 cal/mol by fitting the crystallization rate data for various polymers with Eq.(8)

From the plots of ln(1/ t1/2) + U*/[R(Tc-T∞)] versus 1/[f TcΔT] for PLA and PLA/MMT, the Kg and G0 values can be obtained from the slop of curve and the data are listed in Table 2. The Kg value of PLA is slight higher than those of the PLA/MMT nanocomposites, suggesting the presence of MMT could inhibit the chain mobility of PLA segments. Thus the Kg value decreases as MMT loading increases. From the Figure 6 X-ray diffraction data the peak positions, the PLA and PLA/MMT nanocomposities contain the same α crystalline structure. The data of b of α crystalline form is 0.61 nm according to the lattice parameter of PLA and bulky enthalpy of fusion of perfect crystal ΔHf

0 is 93 J/g[4].

10 15 20 25 30 35 40

Rel

ativ

e In

tens

ity

2θ

(105)

(203)

(110) or (200)

(010)

(a)

(b)

(c)

(d)

Figure 6. Typical WAXD patterns as function of pure PLA (a)(b)and PLA/3wt % MMT

(c)(d)crystallized at 85℃and 110℃

In order to determine to which regime the data in the selected crystallization temperatures belong, the Lauritzen Z test is usually applied. Z is a quantity defined by

)exp()2

(10 2

0

3

TTcX

aLZ

Δ−≈ (9)

where L is the effective lamellar width and a 0 is the width of the molecular chain in the crystal. According to this test, regime I crystallization kinetics are followed if the substitution of X = Kg into the test results in Z 0.01. If regime II with X = 2K≦ g the test contains Z 1.0. ≧ As point out by Lauritzen and Hoffman, it is more convenient to use the known value Kg and the inequalities for Z to obtain the values of L in regimes I or regimes II and to estimate if such value of L is realistic. The α crystalline form of PLA is orthorhombic unit cell with parameters a = 1.06 nm, b =0.61 nm and c =2.88 nm [5]. The thickness of a monomolecular layer, b0, 0.61 nm and the chain width, a0 is 1.06 nm. Assuming Z 0.01 and substituting X = K≦ g into the Z-test, L will be smaller than 0.058 nm. This is clearly unrealistic. Assuming Z 1.0 and substituting X = 2K≧ g into the Z-test, L 5.01 nm and it is ≧reasonable for PLA[12]. The crystallization regime is determined to be regimes II.

Conclusion

The isothermal crystallization behavior of PLA and PLA/MMT nanocomposties shows two distinct characteristics in the range of Tc between 85 and 125 . The crystall℃ ization kinetics of PLLA and PLA/MMT nanocomposties are three-dimensional spherical growth and are belong to regime II crystallization behavior. The addition of MMT into PLA induced heterogeneous nucleation significantly increases the crystallization rate and decreases the Ea and surface energy barrier for PLA crystallization.

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