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Effect of stereocomplex crystallite as a nucleating agent on the isothermal crystallization behavior of poly(L-lactic acid).

Today most plastics are derived from non-renewable crude oil and natural gas resources. While some plastics are being recycled and reused, most of them are disposed of in landfills due to end-use contamination. Poly(L-lactic acid) (PLLA) is a disposable and biodegradable plastic that has attracted more and more attention in the past decade. (1,2) However, PLLA has a limitation for commercial applications because its crystallization rate is slower than many other thermoplastics. As a consequence, physical nucleating agents, which act as nuclei in the crystallization process, must be added in order to improve the crystallization behavior and heat resistance of PLLA. (3) So far, many efforts have focused on the influence of nucleating agents on crystallization of PLLA, (3,4,5) while limited studies have been done on the effect of nucleating agents on the conformational change in crystallization. (6,7)

A 50:50 blend can form a stereocomplex, which is a complex between PLLA and poly(D-lactic acid) (PDLA). (8,9) It was reported in 1987 that PLLA and poly(D-lactide) crystallize into a 1/1 stereocomplex that has a 50[degrees]C higher melting point than the enantiomeric components. (10) To understand the crystal structure of the components, it is very helpful to clarify the packing of the helices in the stereocomplex, which causes its higher stability. The complexation of PDLA and PLLA leads to a stabilization of the helices, which in turn causes increased stability. (11)

However, the high production cost of PDLA is the bottleneck in the production of stereocomplexed PLA materials. On the other hand, increasing the crystallinity by the addition of nucleators to PLLA is effective for the production of low-cost PLA-based materials with high thermal stability. Asymmetric blends can include both homopolymer and stereocomplex crystallization. Researchers have intensively studied the effects of various parameters, including the blending ratio, molecular weight, optical purity, and blending conditions on the stereocomplexation between PLLA and PDLA. (12,13,14) Brochu et al. (12) investigated the crystallization behavior in asymmetric PLLA/PDLA blends and demonstrated that the stereocomplexation occurs with as little as 10 wt% PDLA. Schmidt et al. (15) also reported crystallization behavior in asymmetric blends of PLLA/low molecular-weight PDLA. Both researchers stressed the roll of a stereocomplex as a nucleating agent. At a temperature between Tm of PLLA and PDLA homo crystals and that of a stereocomplex, only the stereocomplex exists and is embedded in the molten mixture of PLLA and PDLA.

The aim of this report is to clarify the mechanism of PLLA crystallization under existence of a stereocomplex by thermal analysis, X-ray analysis, and optical measurement. We are going to show in this study the peculiar PDLA concentration dependence of PLLA crystallization, which might arise from the condition for the formation of stereocomplex crystallites. The mechanism for the acceleration of PLLA crystallization is discussed on the basis of a detailed analysis of the stereocomplex crystallite structure in the molten state of PLLA, in the isothermal crystallization kinetics with a small amount of PDLA content and low molecular-weight PDLA.

Experimental

The PLLA used in this study was obtained from commercial sources. The PLLA (NatureWorks 3001 D) utilized in this study was supplied by NatureWorks LLC (USA). The poly(D-lactide) used in this study was obtained from Musashino Co. (Japan). The PDLA was synthesized through ring-opening polymerization of the poly(D-lactide). Ring-opening polymerization of the poly(D-lactide)was performed in bulk at 180[degrees]C for one hour using stannous octoate and 1-octanol as polymerization catalyst and initiator, respectively. (16) The resulting polymer, PDLA, was used without fractionation, following purification by the cooling method in water. The characteristics and sources of the polymer samples used in this study are shown in Table 1.

The PLLA and PDLA were dried in a vacuum oven at 80[degrees]C for twelve hours before use. The polymer blends were premixed and extruded in a twin extruder under nitrogen gas (LID 40, screw diameter 25 mm, screw speed 300 RPM, operation temperature 160-190[degrees]C). The blend compositions are given in Table 2.

Thermal properties of the blends were examined with a DSC (DSC Q200, TA Instruments) in nitrogen gas. The apparatus was calibrated with pure indium, lead, and zinc standards at various scanning rates. A fresh sample was used for each experimental measurement, in order to minimize thermal degradation. About 5 mg of sample was placed in an aluminum pan. To investigate the overall kinetics of the isothermal crystallization, the samples were heated from 30[degrees]C to 190[degrees]C at a rate of 20[degrees]C/min, and melted at 190[degrees]C for two minutes, then cooled at a rate of 80[degrees]C/min to the desired temperature and allowed to crystallize.

The growth rates and morphology of the sample spherulites were determined by a polarized optical microscope (LV100 POL, Nikon) equipped with a hot stage. A polymer film was interposed between two micro cover glasses. Isothermal crystallization behavior at 100, 120, and 140[degrees]C, after quenching from 190[degrees]C, was observed. The sealed samples were heated on the hot stage from room temperature to 190[degrees]C and held at this temperature for five minutes before quenching to 100, 120, and 140[degrees]C. The growth of the spherulites was recorded during solidification by taking photomicrographs at appropriate intervals of time using a video camera. Spherulite radii were measured with image analysis software (Image pro). Dry nitrogen gas was purged throughout the hot stage during all measurements and thermal treatments. The data of the spherulite growth rates (G) were obtained in isothermal conditions from the slope of the radius vs. the PDLA content plot.

Wide-angle X-ray diffraction (WAXD) patterns were obtained at room temperature with a Multi-Purpose Attachment X-ray Diffractometer (Rigaku D/Max-2500). Monochromatized CuK[alpha] radiation (-=1.542 [Angstrom])was used as an incident X-ray beam. The diffraction angles reported for [alpha]-aluminum oxide ([alpha]-[Al.sub.2][O.sub.3]) were used as a standard; the angles of the diffraction patterns were corrected with three diffraction angles of [alpha]-[Al.sub.2][O.sub.3] for the CuK[alpha] radiation. After isothermal crystallization treatment, the morphology of the films was studied with a Zeiss polarizing microscope.

Results and Discussion

Thermal analysis

Figure 1 shows the isothermal crystallization kinetics by crystallization induction time of PLLA with the different contents of PDLA at 100, 120, and 140[degrees]C. The crystallization induction time increased with an increase of isothermal crystallization temperature. This seems to have been caused by lower mobility of free PDLA chains, which did not form stereocomplex crystallites. The crystallization induction time of the blend with 4 wt% PDLA is much shorter than the others irrespective of the isothermal crystallization temperature. In addition, the crystallization peak area shows a similar tendency. This means that the crystallization kinetics of the blend with 4 wt% PDLA are much faster than others, due to the nucleation effect of stereocomplex crystallites.

Optical analysis

Figure 2 shows the polarized optical micrographs of the spherulites grown at 140[degrees]C observed in the blends that contained PDLA content, after being quenched from 190[degrees]C. The number of the spherulites of blends increased significantly. It is clear that the blends with PDLA content have a higher number of nucleation sites. In particular, the number and size of PLLA spherulites of blends with 4 and 10 wt% PDLA was significantly higher than the other blends, because the network structure disturbs the chain mobility of PLLA, and the amorphous PDLA chains in the blends interrupt the crystal growth of PLLA. These nucleation sites are stereocomplex crystallites with 31 helix in conformation and surrounded by a PLLA crystalline phase. These photographs clearly indicate the role of the stereocomplex as a nucleating agent, and the crystallization of a pure PLLA was initiated in instantaneous homogeneous nucleation.

In order to show the effect of the addition of PDLA on the isothermal crystallization behavior, the diameter of the PLLA spherulite against time is plotted in Figure 3. When the blends were cooled to 140[degrees]C from 190[degrees]C, the spherulite diameter increased steadily, except in the blend with 25 wt% PDLA. Figure 4 shows the spherulite growth rate in the blends against the PDLA content. When the blends were cooled from 190[degrees]C, the spherulite growth rate was almost independent of or slightly greater with the PDLA content. The spherulite growth rate of the PLLA blends with 4 and 10 wt% of PDLA was higher than the other blends.

The above results confirm that when the PDLA content of the blend was 4 and 10 wt%, the stereocomplex crystallites acted as a more effective homocrystallization nucleating agent.

X-ray analysis

In order to investigate or confirm the formation of homo PLLA crystallites, we carried out X-ray diffractometry of the melt-quenched films with different contents of PDLA prepared by annealing at 100[degrees]C for one day. In Figure 5, as PDLA contents increased from 0 to 2, 4, 6, 8, 10, and 25 wt%, the typical diffraction peak (I 6.8[degrees]) (16) of the homo PLLA crystallites was observed. Crystal size (in [Angstrom]) of blends with different contents of PDLA was 321,403, 421,419, 417, 414, and 397, respectively. The crystal size ([Angstrom]) of blends with 4 wt% of PDLA was bigger than the others. The above result is consistent with optical and thermal analysis results.

The mechanisms suggested by the above results of the stereocomplex formation and the effectiveness as a nucleating agent are schematically described in Figure 6. When the PDLA content was low (below 2 wt%), PDLA molecules were remotely isolated in the PLLA matrix, and each PDLA molecule was apart from a neighboring PDLA molecule.

Because of the strong interaction between the PLLA and PDLA molecules, PLLA and the isolated PDLA molecule may form a small and imperfect stereocomplex. The acceleration effect of PLLA crystallization is caused by stress-induced nucleation due to the existence of the homogeneously distributed stereocomplex crystallite network. (17)

Conclusion

Isothermal crystallization behaviors of PLLA blended with different contents of PDLA were studied. PDLA molecules added in PLLA formed stereocomplex crystallites in the PLLA matrix. Stereocomplex crystallites stayed unmelted at 190[degrees]C and embedded in the PLLA molten matrix. Resulting blends were molten viscous liquids even at a temperature between Tm of PLLA and that of the stereocomplex. Isothermal crystallization measurement at 100[degrees]C showed that the crystal radius growth rate decreased with an increase in the isothermal crystallization temperature. The spherulite growth rate had a peculiar PDLA concentration dependence. PLLA crystallization behavior might be affected by network structure and homogeneous dispersibility of stereocomplex crystal.

References

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(17.) Nelly, R., Takahiko, K., Go, M., Koji, N., Toshiji, K., Hiroshi, W., Hirotaka, O., Makoto, K., Arimitsu, U., Masatoshi, M., Katsuhiko, N., and Nobutaka, H., Macromolecules 42, 4739 (2009).

By H.S. Lee [1], E.H. Kim [1], and J.D. Kim [2]

[1] Honam Petrochemical Corp. (Daejeon, Republic of Korea)

[2] Korea Advanced Institute of Science and Technology (Daejeon, Republic of Korea)

Table 1: Characteristics of PLLA and PDLA Homopolymers

Sample   MI a)   Mw (g/mol)   [T.sub.m]
                              ([degrees]C)

PLLA      90      136,000     169.7
PDLA      119      40,000     177.4

Sample   [DELTA][H.sub.m]   HDT b)
         (J/g)              ([degrees]C)

PLLA     45.12              51.6
PDLA     50.00              53.9

a) MI: Melt Index (ASTM 1238)

b) HDT: Heat Distortion Temperature (ISO 75-1, 4.6 kg.f/[cm.sup.2])

Table 2: Blend Compositions in This Study

Sample code    PLLA (wt %)   PDLA (wt %)

PLLA              100             0
PL98D2             98             2
PL96D4             96             4
PL94D6             94             6
PL92D8             92             8
PL90D10            90            10
PL75D25            75            25
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Comment:Effect of stereocomplex crystallite as a nucleating agent on the isothermal crystallization behavior of poly(L-lactic acid).
Author:Lee, H.S.; Kim, E.H.; Kim, J.D.
Publication:Plastics Engineering
Article Type:Statistical data
Geographic Code:1USA
Date:Oct 1, 2013
Words:2197
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