Uniaxial Drawing and Mechanical Properties of Poly[(R)-3

Biomacromolecules 2004, 5, 1557-1566

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Uniaxial Drawing and Mechanical Properties of Poly[(R)-3-hydroxybutyrate]/Poly(L-lactic acid) Blends Jun Wuk Park,† Yoshiharu Doi,†,‡ and Tadahisa Iwata*,† Polymer Chemistry Laboratory, RIKEN Institute, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan, and Department of Innovative and Engineered Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8501, Japan Received February 16, 2004; Revised Manuscript Received April 2, 2004

Blends of poly(L-lactic acid) (PLLA) with two kinds of poly[(R)-3-hydroxybutyrate] (PHB) having different molecular weights, commercial-grade bacterial PHB (bacterial-PHB) and ultrahigh molecular weight PHB (UHMW-PHB), were prepared by the solvent-casting method and uniaxially drawn at two drawing temperatures, around PHB’s Tg (2 °C) for PHB-rich blends and around PLLA’s Tg (60 °C) for PLLA-rich blends. Differential scanning calorimetry analysis showed that this system was immiscible over the entire composition range. Mechanical properties of all of the samples were improved in proportion to the draw ratio. Although PLLA domains in bacterial-PHB-rich blends remained almost unstretched during cold drawing, a good interfacial adhesion between two polymers and the reinforcing role of PLLA components led to enhanced mechanical properties proportionally to the PLLA content at the same draw ratio. On the contrary, in the case of UHMW-PHB-rich blends, the minor component PLLA was found to be also oriented by cold drawing in ice water due to an increase in the interfacial entanglements caused by the very long chain length of the matrix polymer. As a result, their mechanical properties were considerably improved with increasing PLLA content compared with the bacterial-PHB system. Scanning electron microscopy observations on the surface and cross-section revealed that a layered structure with uniformly oriented microporous in the interior was obtained by selectively removal of PLLA component after simple alkaline treatment. Introduction Poly[(R)-3-hydroxybutyrate] (PHB) has attracted much attention as an environmentally degradable thermoplastic. However, its brittleness has been a drawback to enlarge its application. To overcome this difficulty, extensive studies have been performed in terms of copolymerization with other 3-hydroxyalkanoates,1,2 blending,3,4 and processing.5-8 The previous work in our research group has demonstrated that PHB films prepared by cold drawing and/or a two-step drawing procedure had enhanced flexibility as well as high strength stemmed from the formation of a zigzag conformation of tie molecular chains and network structure formed by fibril and lamellar crystals.7,8 In this work, we attempted to apply this cold drawing procedure to PHB blends with poly(L-lactic acid) (PLLA) that is also widely known as one of the most promising biocompatible aliphatic polyesters. Concerning PHB/PLLA blends, several works have been reported in terms of the miscibility, crystallization, and melting behavior.9,10 According to these works, PHB was miscible with low molecular weight PLLA (below Mw ) 18 000) in the melt over the whole composition range, whereas the PHB blends with high molecular weight PLLA showed biphasic separation. As a similar result, Ohkoshi et * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +82-48-467-9586. Fax: +81-48-462-4667. † RIKEN Institute. ‡ Tokyo Institute of Technology.

al. published a report on the miscibility and solid-state structure of PLLA blends with atactic-PHB with different molecular weights ranged from Mw ) 9400 to 140 000, showing that the PLLA component is miscible with only atactic-PHB with low molecular weight.11 These results suggest that the miscibility between these polymers is strongly dependent on molecular weight of the second component. In addition, the blending of PHB with PLLA,12 poly(D,L-lactic acid) (PDLLA),13 and poly(L-lactide-coglycolide) (PLLAGA)14 has also been reported in terms of the miscibility and thermal and mechanical properties. However, little work on the uniaxial drawing of this system has been reported in the literature. For other blend systems, many works have been reported on their drawing behavior. There are two categories: one is on miscible systems and the other on immiscible systems. In the case of the former, several works were focused on the effect of a minor component on the strain-induced orientation and orientation relaxation of a major component, which was strongly dependent on their interaction.15-19 On the contrary, drawing behaviors of the immiscible blend are widely known to be influenced by interfacial properties between two polymers. Particularly, entangled structure at interface is a very important factor controlling overall mechanical properties. The component being a continuous phase in an immiscible mixture orients mostly higher degree than the isolated component,20 whereas in miscible blends,

10.1021/bm049905l CCC: $27.50 © 2004 American Chemical Society Published on Web 05/04/2004

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it has a lower orientation than in miscible state due to a poorer interconnectivity of the chains.21 PHB/PLLA blends in this work are expected to be immiscible because we selected high molecular weight polymers. Particularly for PHB, two kinds of polymers with different molecular weights, commercial-grade bacterial PHB (bacterial-PHB) and ultrahigh molecular weight PHB (UHMW-PHB), were selected in order to investigate the effect of molecular weight on the drawing behavior. In the latter case, recently our research group has successfully produced polymers with millions of molecular weights under specific fermentation conditions22 and reported on its thermal, mechanical properties, and orientation under hot drawing and cold drawing.23,24 In this work, we investigated the drawing behavior of PHB/PLLA blends during uniaxially cold drawing in terms of their orientation and morphology and discussed the relationship with their mechanical properties comparing two blend systems, bacterial- and UHMW-PHB blends. Furthermore, we reported the layered structure with oriented microporous in the interior obtained by the removal of PLLA domains after simple alkaline treatment. Experimental Section Materials. Bacterial-PHB (Mw ) 5.9 × 105, Mw/Mn ) 2.6) was supplied by Monsanto Co., Japan. Ultrahigh molecular weight PHB (Mw ) 5.3 × 106, Mw/Mn ) 1.7) was biosynthesized from glucose by recombinant Escherichia coli XL-1 blue (pSYL105) bearing Ralstonia eutropha H16 PHB biosynthesis phbCAB genes by the method reported previously.22 Poly(L-lactic acid) (Mw ) 3.5 × 105, Mw/Mn ) 1.6) was supplied by Shimadzu Co., Japan. All of the polymers were reprecipitated using chloroform as a solvent and n-hexane (or methanol for PLLA) as a nonsolvent. The molecular weights were measured by GPC using chloroform as an eluant.23 Preparation of Blends and Cold Drawing. The blends with four compositions, 9/1, 7/3, 5/5, and 3/7 (PHB/PLLA, w/w), were prepared by a solvent-casting method using chloroform as a cosolvent. Since PHB samples with high crystallinity (especially UHMW-PHB) did not completely dissolve in chloroform at room temperature, all of the homogeneous solutions were prepared in a high-pressure bottle at 100 °C, subsequently poured into glass Petri dishes, and kept at room temperature for 48 h to allow gradual evaporation of the solvent. The as-cast films were dried again under vacuum for at least 48 h to thoroughly remove the solvent. We carried out a preliminary drawing test in order to determine drawing temperature for each sample. The PHBrich blends were impossible to stretch at temperatures above PHB’s Tg because the stress-relaxation occurred too rapidly. On the contrary, PLLA-rich blends were not stretchable at temperatures below PLLA’s Tg because the PLLA component is in a frozen state at this temperature. These results indicate that physical properties of immiscible blends are strongly dependent on those of the major component forming the continuous phase. Therefore, we selected around the Tg of a major component as a drawing temperature, i.e., around

Park et al.

PHB’s Tg for PHB-rich blends (Neat PHB, 9/1, 7/3) and around PLLA’s Tg for PLLA-rich blends (5/5, 3/7, Neat PLLA). The samples for drawing were cut into 30 mm × 10 mm strips, hot-pressed between two Teflon sheets at 200 °C, and then rapidly quenched in ice water. The sample thickness was controlled to 0.1 mm. The melt-quenched samples were gripped to a portable drawing device designed for stretching up to a maximum of 30 times by hand-operation. The initial gauge length and draw rate were 10 mm and approximately 50 mm/min, respectively. In the case of the PHB-rich blends, the drawing procedure was carried out simultaneously with quenching in ice water. The actual temperature of the ice water was approximately 2 °C. On the other hand, PLLArich blend samples were immediately placed into a heating oven after gripping to the drawing device, subsequently kept for 1 min to be thermally stabilized, and then stretched to given draw ratio. All of the drawn samples were thermally annealed under tension in a heating oven at 100 °C for 2 h to increase the crystallization. This annealing condition was chosen with reference to our previous works.7 All of the drawn samples were aged for more than 3 days at room temperature. DSC Measurements. Thermal characteristics of the blends were measured using a Perkin-Elmer DSC Pyris 1. Sealed aluminum sample pans containing 4-6 mg of materials were used in all experiments. At the beginning of each experiment, the sample was heated to 200 °C, and maintained for 3 min so as to eliminate its thermal history, and then rapidly cooled to -50 °C. The actual measurement was recorded during the second heating from -50 to +200 °C at a heating rate of 10 °C/min. X-ray Analysis. Two-dimensional Laue images were obtained using an X-ray diffractometer (RINT UltraX 18, Rigaku, Japan) equipped with an imaging plate (BAS-SR 127, Fuji-Film Co. Japan). The X-ray source was Ni-filtered Cu KR radiation (λ ) 0.154 nm) generated at 40 kV and 110 mA. The distance from the sample to the imaging plate was about 40 mm and the exposure time was 2 h. To identify a preferred orientation of crystallites, the degree of orientation was calculated from the full width at half-maximum in the azimuthal direction of the diffracted peaks by the following equation: degree of orientation (%) )

180° - Wh × 100 180°

(1)

where Wh is the full width at half-maximum in the azimuthal direction of the diffraction. SEM Observation. For cross-section observations, the sample was immersed and kept in liquid nitrogen for a few minute and then cut by a razor blade in this state to prevent deformation of the cross-section by cutting. Surface-etching was performed by immersing the samples in 1 wt % NaOH aqueous solution at 40 °C to remove PLLA component selectively in the surface. The treatment time was controlled from 12 to 24 h depending on the draw ratio. For the highly oriented sample, the longer treatment time was required to provide good contrast. The treated samples were washed with distilled water and dried thoroughly under vacuum. After

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Cold Drawing of PHB/PLLA Blends Table 1. Thermal Characteristics of PHB/PLLA Blends

system bacterial-PHB blends

UHMW-PHB blends

PLLA

PLLA content (wt %)

Tga (°C)

0 10 30 50 70 0 10 30 50 70 100

0.7 1.4 0.9 0.3 -1.7 0.6 0.6 1.0 0.1 -0.6 57.0

1st exothermal peak Tc ∆Hc ∆H′cb (°C) (J/g) (J/g) 49.4 52.0 52.7 53.5 47.6 46.8 46.5 48.2 46.1 45.6

25.7 22.6 18.8 12.6 5.1 26.9 23.6 18.7 13.2 8.1

25.7 25.1 26.8 25.2 17.0 26.9 26.2 26.7 26.4 27.0

2nd exothermal peak Tc ∆ Hc ∆H′cb (°C) (J/g) (J/g)

111.5 110.8 116.5

113.0 115.6 123.7

3.6 5.8 13.2

4.9 12.0 20.6

12.0 11.6 18.9

9.8 17.1 20.6

endothermal peaks TPHBc TPLLAd ∆Hfe (°C) (°C) (J/g) 164.7 167.3 164.9 165.2 177.4 177.0 177.7 175.6 174.8

175.7 174.7 169.1

168.3 167.7 170.3

38.4 30.2 25.0 22.1 17.2 50.5 48.2 41.8 30.9 23.5 21.8

a Glass transition temperature of PHB component except for neat PLLA. b Calculated in consideration of a weight fraction of each component in blend. Melting point of the PHB component in the blend. A major peak maximum was taken in the case of double peaks. d Melting point of the PLLA component in the blend. e Calculated from total endothermal area regardless of the number of peaks.

c

the samples were coated with Au, scanning electron microscopy (SEM) images were taken using a JEOL JSM-6330F microscope operated at an acceleration voltage of 5 kV. Mechanical Properties. The tensile strength, Young’s modulus, and elongation at break were determined using a tensile testing machine (EZtest, Shimadzu Co, Japan) at a crosshead speed of 20 mm/min. The gauge length and the sample width were 10 mm and approximately 3 mm, respectively. The average of three measured values was taken for each sample. Results and Discussion Thermal Properties of PHB/PLLA Blends. DSC thermograms of bacterial- and UHMW-PHB/PLLA blends are shown in Figure 1, parts a and b, respectively, and their characteristic values are listed in Table 1. Neat PLLA showed a broad crystallization peak at 100-140 °C and a melting peak at about 170 °C. For neat bacterial-PHB, a sharp exothermal peak and melting peak were observed at 50 and 165 °C, respectively, indicating that PHB crystallizes more rapidly at a lower temperature as compared with PLLA. Although the glass transition of UHMW-PHB took place at the same temperature as bacterial one, its crystallization peak appeared at a slightly lower temperature. However, the melting point (177 °C) and heat of fusion of UHMW-PHB were considerably higher than those of bacterial-PHB, which is strongly connected with enhanced crystal perfection caused by ultrahigh molecular weight. In general, an increase of chain length lowers the number of the chain ends acting as a defect within crystals. As can be seen in Figure 1, the glass transition region of PLLA around 55∼60 °C was very close to the position of the exothermal peak of PHB. Because of this, we could not confirm whether the PLLA’s glass transition occurred or not in the blend samples. However, the glass transition temperature Tg of the PHB component in both systems remained almost unchanged over the whole range of the weight fraction of the PLLA component. In addition, there was no other evidence for these systems to be miscible, such as remarkable changes of crystallization behavior and melting point depres-

Figure 1. DSC thermograms of (a) bacterial-PHB/PLLA blends and (b) UHMW-PHB/PLLA blends obtained from the 2nd heating run with a heating speed of 10 °C/min.

sion. For this reason, we concluded that all of the blends were immiscible systems, which is consistent with previous results.9-11 For the blend samples as shown in Figure 1, two cold crystallization peaks corresponding to PHB and PLLA components were observed for the blend samples, suggesting that this blend can be classified as a semicrystalline/ semicrystalline system. To investigate the change of the crystallization behavior of each component by the blend ratio, enthalpies of each peak before and after compensation for

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its weight fraction are listed in Table 1. In the case of UHMW-PHB blends, the exothermal peak and its enthalpy of the PHB component were not influenced at all by the blend composition. For bacterial-PHB blends, the exothermal peak of the PHB component shifted slightly to higher temperature with increasing PLLA content. In contrast, the crystallization behavior of the PLLA component became considerably influenced by the blend ratio. The exothermal peak shifted considerably to lower temperature with increasing PHB weight fraction. In addition, the enthalpy corrected with the weight fraction dramatically decreased, particularly when the PLLA content was lower than 50 wt %, and eventually dropped to zero. At lower than 10 and 30 wt % PLLA for bacterial- and UHMW-PHB systems, respectively, any trace for melting as well as the exotherm of PLLA component were not detected in the DSC scans. In immiscible blends, crystallization of a minor component could be restricted by effect of the small domain size and/or confined chain mobility at the interface under favorable interaction with a matrix. Considering that this disturbance of crystallization was remarkably observed only for the PLLA component, prior crystallization of the PHB component seems to seriously restrict the mobility of PLLA chains at the interface. In particular, the very long chain length of UHMW-PHB is expected to be more effective in confining the PLLA chains by higher entanglement density at the interface, which explains the fact that the crystallization enthalpy of the PLLA component in the UHMW-PHB system was lower than in the bacterial-PHB system. Since the melting endotherms of two components overlapped each other in position, we could obtain only the total value of the heat of fusion calculated from the entire endothermic area regardless of the number of peaks. As shown in Table 1, the total value of the blends decreased inversely proportional to the weight fraction of the PLLA component having a relatively low crystallinity. Drawing Behavior. Both bacterial- and UHMW-PHB could have a stretched-to-draw ratio (DR) ) 10 in ice water. On the contrary, neat PLLA was possible to be drawn up to DR ) 7 at 60 °C. In the case of PHB-rich blends, the maximum draw ratio decreased with increasing PLLA content. The blends with 7/3 and 5/5 composition could be stretched up to only DR ) 5. However, the maximum draw ratio of the PLLA-rich blend with 3/7 composition was the same as that of neat PLLA. A remarkable feature during the stretching process was that the strain-induced whitening occurred for PHB-rich blends, which could be explained in terms of a heterogeneous deformation. In particular, this appearance was the most conspicuous for the bacterial-PHB/PLLA 7/3 blend. In addition, this sample underwent less necking during stretching, and consequently, its width after drawing was larger than that of other samples. This appearance seems to be also closely related to the heterogeneous deformation. The deformation in immiscible blends is not homogeneous when one phase is above and the other is below its own glass transition temperature. As a result, micro-voids are produced and enlarged at the interface of two components. Many works on this micro-voiding of immiscible blends, particularly in

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Figure 2. 2D WAXD photographs of (a) bacterial-PHB at DR ) 10, (b) bacterial-PHB at DR ) 5, (c) 7/3 blend at DR ) 5, (d) 5/5 blend at DR ) 5, (e) 3/7 blend at DR ) 5, and (f) neat PLLA at DR ) 5.

the field of rubber-filled toughened plastics, have been carried out in terms of their formation mechanisms such as crazing, internal cavitation, and debonding between the matrix and dispersed particles.25-27 This will be discussed in detail in the morphology section. Two-Dimensional WAXD Measurements. Laue-camera images of bacterial- and UHMW-PHB/PLLA blend films are presented in Figures 2 and 3, respectively. Both neat PHB and PLLA at high draw ratio showed highly oriented patterns of only the R-form crystal, which was identified by previous reports.28-31 The crystal structure of PHB (R-form) has been known to be orthorhombic with lattice parameters of a ) 0.576 nm, b ) 1.320 nm, and c ) 0.596 nm with its chain conformation in the left-handed 2/1 helix.28,29 According to Orts et al., when the R-crystal was further cold-drawn, a new polymorph was observed as β-modification having a twisted planar zigzag conformation, suggesting the β-crystal was formed upon stretching from the amorphous regions between orthorhombic R-form lamellae.30 PLLA has also undergone the crystal transformation from the R-form to the β-form depending on the drawing conditions.31 However, in this work, the crystal modification (β-form) of PLLA as well as the PHB component was not detected in all of the samples including the blends, which is due to the different beginning state of the sample before drawing. The initial state of our samples was expected to become completely amorphous by melting and quenching process.

Cold Drawing of PHB/PLLA Blends


Figure 4. Plots of crystalline orientations as a function of the blend ratio for (a) bacterial-PHB/PLLA and (b) UHMW-PHB/PLLA blends, calculated from the full width at half-maximum in azimuthally scan of the 2D WAXD image using the (020) reflection for the PHB component and (110) and (200) reflections for PLLA component.

Figure 3. 2D WAXD photographs of (a) UHMW-PHB at DR ) 10, (b) UHMW-PHB at DR ) 5, (c) 7/3 blend at DR ) 5, (d) 5/5 blend at DR ) 5, and (e) 3/7 blend at DR ) 5, and (f) neat PLLA at DR ) 5.

As compared with bacterial-PHB blends, UHMW-PHB blends showed more intense and higher oriented patterns at the same draw ratio. In particular, at the 7/3 blend ratio, the two systems exhibited contrary patterns. For bacterial-PHB/ PLLA blends in Figure 2c, the main reflection by (110) and (200) planes of the PLLA crystal appeared as a weak Debye-Scherrer ring and the (203) reflection was not detected, indicating that the PLLA component was not oriented at all. On the contrary, no ring pattern of the main reflection of the PLLA component was observed for the UHMW-PHB system in Figure 3c. We thought that this reflection was concentrated on the equator by orientation, as a result, overlapped with arc-shaped (110) reflection of the PHB crystal. However, four weak arcs of the (203) reflection of the PLLA component were clearly observed in Figure 3c, which is a positive evidence to prove that the PLLA component was also oriented to a considerable extent in UHMW-PHB systems. This is a surprising appearance because even pure PLLA was impossible to be elongated in ice water (2 °C). The reason will be discussed together with morphology results. For PLLA-rich blends drawn at 60 °C, the Laue patterns revealed that both PHB and PLLA crystals were oriented simultaneously by stretching. To evaluate the effect of the blend compositions and the drawing conditions on the crystal orientation of each component quantitatively, the degree of

orientation was calculated from the full width at halfmaximum in the azimuthal direction by using the (020) reflection for the PHB component and (110) and (200) reflections for the PLLA component and exhibited in Figure 4. As mentioned above, (110) and (200) reflections for the PLLA crystal were overlapped with the (110) reflection of the PHB crystal. However, when the PLLA content was 50 wt % or more, the PLLA reflection became very sharp and intense compared to the (110) reflection of the PHB component, which made it possible to extract the PLLA constituent. On the other hand, for PHB-rich blends, we could not obtain the degree of orientation for PLLA component because of relatively weak PLLA reflections. As can be observed in Figure 4, neat PLLA drawn at 60 °C showed a high orientation (above 95%) even at DR ) 3. In contrast, the degree of orientation of bacterial-PHB (Figure 4a) changed a great extent depending on the draw ratio. In particular, neat UHMW-PHB (Figure 4b) exhibited larger values than the bacterial one peculiarly at a low draw ratio, which is due to an increase of chain entanglements caused by its very long chain length. Generally, the chain entanglement acts as a transient cross-link restricting the chain slippage during deformation, which is closely related to the stress relaxation.32 Consequently, the larger number of chain entanglements results in higher orientation during drawing. In the case of the PHB-rich blends drawn in ice water, the crystalline orientation of PHB tends to be reduced by blending with PLLA except at DR ) 3. Especially, bacterial-PHB blends showed more significant drops in orientation than UHMW-PHB blends, suggesting that the isolated

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Figure 6. SEM micrographs of undrawn UHMW-PHB/PLLA blends after alkaline-treatment at various blend ratios of (a) 9/1, (b) 7/3, (c) 5/5, and (d) 3/7. Figure 5. Plots of total crystallinity as a function of the draw ratio (λ) for (a) bacterial-PHB/PLLA and (b) UHMW-PHB/PLLA blends, calculated from equatorial scans of the 2D WAXD image.

PLLA phase frozen in ice water might restrict the orientation of PHB chains during stretching. On the contrary, since the PLLA phase is also stretched by drawing in UHMW-PHB systems, the existence of PLLA may have a relatively less influence on the orientation of the PHB matrix. For PLLArich blends including the 5/5 blend ratio drawn at 60 °C, the orientation of the PHB component in two blend systems showed a nearly similar degree of orientation to those in PHB-rich blends. In addition, there is no significant drop in crystal orientation of the PLLA component with increasing the weight fraction of PHB. To investigate how the change of crystallinity is influenced by the draw ratio, the overall crystallinity was obtained from Laue images. In this system, it was impossible to obtain the exact crystallinity of each component, because not only main X-ray diffractions but also melting peaks in DSC of the respective components overlapped each other as mentioned earlier. So we could obtain only overall crystallinity from equatorial scans of the Laue images using a peak separation method by which the amorphous halo and crystalline portion were deconvoluted, which is displayed in Figure 5. Undrawn PLLA-rich blends possessed relatively low crystallinity, but it dramatically increased at DR ) 3 and leveled off to about 60% with an increasing draw ratio. On the contrary, both bacterial- and UHMW-PHB even in the undrawn state showed a relatively high crystallinity of approximately 50%. The crystallinity of bacterial-PHB-rich blends started to increase from DR ) 5, whereas, UHMW-PHB-rich blends showed a steady increase from DR ) 3. These tendencies seem to be strongly connected to changes of their molecular orientation, because the molecular orientation causes enhanced mobility and chain alignment of molecular chains leading to an increase of crystallinity.

Morphology. Figure 6 presents SEM micrographs for the surface of undrawn UHMW-PHB/PLLA blends after surface treatment with alkaline solution. The undrawn blends showed a particle-dispersed morphology caused by phase separation over the entire composition range as described in the DSC results. The PLLA component was found to be selectively removed by short-time strong alkaline treatment regardless of its phase, isolated or continuous phases. The PLLA domain size was smaller than 1 µm at the 9/1 blend composition (Figure 6a) and became larger to several micrometers at 50 wt % PLLA content. Contrary to our expectation for co-continuous morphology at the 5/5 composition, the PLLA component lay in an isolated phase to 50 wt % content. At 70% PLLA content as shown in Figure 6d, the phase became inversed; that is, PLLA constituted the matrix phase, and PHB formed globular domains with the diameter of 1-2 µm. The bacterial-PHB blend system also exhibited biphasic morphology over the whole composition range, of which photographs were not presented here because of similarities to those of the UHMW-PHB system. Micrographs for bacterial- and UHMW-PHB blends drawn to DR ) 5 are presented in Figures 7 and 8, respectively, where left and right rows represent the surface and cross-section of each sample, respectively. When the two systems are compared, they showed different morphologies at the 7/3 blend ratio. For the bacterial-PHB system at this composition, the SEM micrographs in Figure 7, parts a and a′, revealed that globular shaped PLLA domains (2-4 µm) surrounded by the PHB matrix remained unstretched by drawing, whereas the PHB phase was highly extended connecting between PLLA domains. There was, however, no interfacial breakage between the PLLA domain and the PHB matrix, indicating that interfacial adhesion between two polymers seemed to be excellent. As described earlier, this sample showed prominent stress-whitening and less necking during the drawing process. As an evidence for these



Figure 7. SEM micrographs of bacterial-PHB/PLLA blends drawn to DR ) 5 after alkaline-treatment at various blend ratios of (a) 7/3, (b) 5/5, and (c) 3/7. Left and right rows represent the surface and cross-section of each sample, respectively.

Figure 8. SEM micrographs of UHMW-PHB/PLLA blends drawn to DR ) 5 after alkaline treatment at various blend ratios of (a) 7/3, (b) 5/5, and (c) 3/7. Left and right rows represent the surface and crosssection of each sample, respectively.

appearances, empty regions (so-called “cavity”) between extended PHB fibrils were distinctly observed in Figure 7a, which was caused by a heterogeneous necking process. For the continuous necking process, a sustaining supplement of the major component into the necking region must be required. However, in this case, frozen PLLA domains might play a role as a stoppage point to prevent the supplement of PHB chains. For this reason, necking appears to take place only locally between the PLLA domains, which gives rise to so many cavities. On the contrary, the traces of the PLLA phase removed by alkali treatment in the UHMW-PHB system were found to be not spherical but ellipsoidal or cylindrical in shape as shown in Figure 8a. The cross-section observation in Figure 8a′ also revealed that, although the PLLA traces in the bacterial-PHB system were globe-shaped in Figure 7a′, the traces in the UHMW-PHB system appeared to be distorted or nearly oval-shaped. These results can be taken as unquestionable evidences that the PLLA component was also stretched along the drawing direction even though the PLLA phase was in frozen state in ice water, which is keeping with X-ray results. Considering that the two systems differ only in molecular weight of the PHB component, this result seems to be attributed to increased entanglement density not only between PHB chains but also at the interface of the two polymers caused by very long chain length of UHMW-

PHB. Generally, the entanglement density at the interface controlling interfacial adhesion and toughness in an immiscible blend is known to depend on the intermolecular interaction and chain stiffness (or flexibility).33 If these factors are constant, the longer backbone chain could undoubtedly induce the higher entanglement density at the interface. Moreover, the increased entanglement density between similar chains results in restricting the stress relaxation of the matrix phase, which requires a higher loading stress to stretch this sample and allows the increased loaded stress to be transferred to a minor component. At the 5/5 blend ratio, two blend systems showed the same morphology having the fibrillar structure with a width of about 1-2 µm, where a discrete phase could not be perceptively distinguished from a continuous phase, suggesting that the particle-dispersed morphology might be changed to a co-continuous morphology by simultaneous deformation of both components during the drawing process. In addition, the microporous morphology in the cross-section after alkaline treatment (Figures 7b′ and 8b′) demonstrated that the fibrils of PLLA and PHB were arranged alternately forming a layered structure along the cross-section. Here it is a very surprising fact in the viewpoint of their applications that this layered structure having uniformly oriented microporous or hollows in the interior could be easily obtained by only simple alkaline treatment, which suggests new

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Figure 9. Schematic illustration of the drawing behaviors of immiscible PHB/PLLA blends.

potentialities for these materials to be applied to various fields such as drug delivery systems, scaffolds for tissue engineering, membranes, and microporous fibers. In the case of the PLLA-rich blend with a 3/7 composition, highly extended PHB phases to the stretching direction were clearly exposed by removing a continuous phase of the PLLA component, not only at the surface but also at the crosssection. The drawing behaviors of PHB/PLLA blends on the basis of these SEM observations are schematically illustrated in Figure 9. In summary, when the PHB/PLLA blends having a sea-island morphology were cold-drawn, the globular shaped PLLA domains surrounded by the PHB phase were not stretched at all in bacterial-PHB-rich blends. On the other hand, PLLA domains in UHMW-PHB-rich blends were also stretched along the stretching direction due to high entanglement density at the interface of two polymers which is attributable to the very long chain length of UHMWPHB. In the case of PLLA-rich blends containing more than 50 wt % PLLA content, the highly stretched PLLA component formed micro-fibrils by drawing and extended PHB component were located between the PLLA fibrils. Mechanical Properties. Mechanical properties of bacterial- and UHMW-PHB/PLLA blends as a function of the draw ratio (λ) are shown in Figures 10 and 11, respectively. Although the Young’s modulus and the elongation at break of neat PHB increased gradually with an increasing draw ratio, the tensile strength began to drastically increase at DR ) 5. UHMW-PHB exhibited superior values to the bacterial one at the same draw ratio. These tendencies are in good agreement with our previous results,7 except for some difference of the maximum values. In the case of neat PLLA, the tensile strength and modulus dramatically increased proportionally to the draw ratio even at DR ) 3, which is strongly connected and consistent with the behaviors of its orientation and crystallinity. Overall values of PLLA were considerably higher than those of neat PHB at the same draw ratio. With increasing draw ratio, the tensile properties of all of the blends were also improved similar to neat polymers. It is noteworthy that, even though they had a higher crystallinity, the elongation property was also enhanced by stretching. Considering that PHB is known to be brittle at

Figure 10. Mechanical properties of bacterial-PHB/PLLA blends as a function of draw ratio: (a) tensile strength, (b) Young’s modulus, and (c) elongation at break.

room temperature caused by heterogeneous secondary crystallization,34,35 the high molecular orientation induced by drawing seems to restrain producing theses heterogeneities. In fact, our previous works demonstrated that the tensile properties of highly drawn PHB were preserved without deterioration after long-term storage at room temperature.7,23 In the case of PHB-rich blends stretched in ice water, the tensile strength and modulus values increased with PLLA content at the same draw ratio, even though the orientation was reduced by blending with PLLA and PLLA domains remaining unoriented as mentioned above. This result could be explained by considering the relatively rigid nature of dispersed PLLA domains and a good interfacial adhesion between PLLA domains and the PHB matrix; that is, PLLA acted as a reinforcing agent. In particular, UHMW-PHB blends exhibited more considerable enhancement in mechanical properties by introducing PLLA than bacterial-PHB ones, which was due to a higher orientation of the PHB component and stretched PLLA domains caused by increased interfacial entanglements as previously described.



Conclusion

Figure 11. Mechanical properties of UHMW-PHB/PLLA blends as a function of draw ratio: (a) tensile strength, (b) Young’s modulus, and (c) elongation at break.

In the case of PLLA-rich blends, the tensile strength and modulus could be expected to be remarkably deteriorated with increasing PHB content since PHBs possess a soft nature as compared with PLLA. In addition, the degree of orientation of the PLLA crystal was slightly decreased at the same draw ratio by blending with PHB as revealed in X-ray analysis. However, the remarkable drop was not observed in both systems except for the 5/5 bacterial-PHB/ PLLA blend. We thought that the parallel arrangement of PLLA fibrils and extended PHB domains along the drawing direction could prevent a significant deterioration of the physical properties. On the whole, the results of tensile tests suggest that these materials have a very wide physical property range from a soft (PHB-rich blends) to a very rigid nature (PLLA-rich blends) depending on their composition and draw ratio; that is, one can obtain desired properties by controlling the composition and draw ratio, which provides great possibilities that these materials can be applied to various uses not only in the field of biomaterials but also for a substitute of common plastics.

Blends with PHB and PLLA were prepared with various compositions, and their drawing behaviors were investigated in terms of orientation, morphology, and mechanical properties. The blends with PHB and PLLA were found to be immiscible systems, and they brought about a significant phase separation over the entire composition range. In bacterial-PHB-rich blends, the isolated PLLA phase being in the frozen state into ice water remained unstretched and disturbed the orientation of continuous PHB phase during cold drawing into ice water. Consequently, the degree of orientation of the PHB component tends to decrease by blending with PLLA. On the contrary, X-ray analysis and SEM observations clearly revealed that the PLLA component in the UHMW-PHB-rich system was also stretched along the drawing direction, which was attributed to increased entanglement density not only between PHB chains but also at the interface of two polymers caused by the very long chain length of UHMW-PHB. As a result, the existence of PLLA had a relatively less influence on the orientation of the PHB matrix. The mechanical property was enhanced in proportion to the PLLA content due to a good interfacial adhesion between two polymers and the reinforcing role of PLLA components. Particularly, the UHMW-PHB/PLLA system showed more considerable improvement due to the simultaneous orientation of PLLA domains. For PLLA-rich blends, WAXD and SEM observation revealed that not only the PLLA component but also the PHB component were highly oriented and arranged parallel to the stretching direction. Consequently, this parallel arrangement could prevent significant deterioration of physical properties, such as tensile strength and modulus, due to the relatively soft nature of the PHB component. Finally, these materials showed a wide physical property range depending on their composition and draw ratio. In addition, it was found that layered microporous or hollow structure could be simply obtained by selectively removing the PLLA component after alkaline treatment. These facts suggest that PHB/PLLA blends could be applied to various fields from biomaterials to a potential substitute for common plastics. Acknowledgment. We thank Emeritus Professor K. Okamura of Kyoto University for the English correction of our manuscript. This work has been supported by a Grantin-Aid for Young Scientists (A) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (No. 15685009) and by a SORST (Solution Oriented Research for Science and Technology) grant from Japan Science and Technology Corporation (JST). References and Notes (1) (2) (3) (4)

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BM049905L

Uniaxial Drawing and Mechanical Properties of Poly[(R)-3

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