Phase Separation and Lamellae Assembly below UCST in Poly(l

Mar 28, 2012 - When the PLLA/PBA (50/50) blend is cooled to crystallize below UCST, phase-separated domains appear prior to crystallization of PLLA, r...
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Phase Separation and Lamellae Assembly below UCST in Poly(L-lactic acid)/Poly(1,4-butylene adipate) Blend Induced by Crystallization Siti Nurkhamidah†,‡ and Eamor M. Woo*,† †

Department of Chemical Engineering, National Cheng Kung University, Tainan 701-01, Taiwan Department of Chemical Engineering, Faculty of Industrial Technology, Sepuluh Nopember Institute of Technology, Kampus ITS Sukolilo, Surabaya 60111, Indonesia

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ABSTRACT: The crystalline/crystalline poly(L-lactic acid)/ poly(1,4-butylene adipate) (PLLA/PBA) blend system exhibits upper critical solution temperature (UCST) behavior below the melting temperature of PLLA. When the PLLA/ PBA (50/50) blend is cooled to crystallize below UCST, phase-separated domains appear prior to crystallization of PLLA, resulting in PLLA spherulites being overlapped with the phase-separated domains. However, phase-separated domains only appear in blends when they are crystallized after being subjected to maximum melting temperature (Tmax) for a short melting time (Δtmax < 3 min); oppositely, the phase domains in blends become invisible when held for longer times (>5 min). When PLLA/PBA (50/50) blend is melted for short Δtmax, the chain entanglement density is less; thus, the PBA chains are easy to be mutually expelled from the PLLA chains during crystallization of PLLA, resulting in PLLA-rich and PBA-rich separate domains. Apparently, phase separation in the PLLA/PBA blends is initiated by the crystallization of PLLA below UCST. Fractured surfaces of bulk blend samples show crystals of a polygonal shape, which is due to impingement between neighboring spherulites growing in 3D interior. For short Δtmax, the polygonal crystal is porous with two different sizes. Small and large pore sizes correspond to the PLLA-rich and PBA-rich domains, respectively. The formation of porous-structure lamellae in the PBA-rich domains is due to combined factors: (a) preformed PLLA spherulites acting as a growth template of PBA, leading to preferential oriention of PBA along PLLA lamellae, and (2) densification of PBA upon crystallization at ambient temperature. Detailed analyses were focused on the lamellar assembly leading to various pore sizes in the phase domains and dependence of the assembly patterns and pore sizes on the parameters governing the phase separation.

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INTRODUCTION Lamellar assembly in crystalline/crystalline blends may be influenced by mutual crystals of two constituents in the blends. Furthermore, if phase separation takes place either during, prior to, or after crystallization of one of the two constituents, the lamellar patterns are governed by two competing processes. Upper critical solution temperature (UCST) behavior, though much rarer than the lower critical solution temperature (LCST) phenomenon, can be occasionally observed above the melting points of the crystalline constituents, such as in the blend systems of isotactic polypropylene/ethylene−propylene (iPP/ EPR) random copolymer,1 poly(ε-caprolactone)/polystyrene (PCL/PS),2,3 poly(ethylene-co-hexene)/poly(ethylene-co-butene) (PEH/PEB), 4,5 polystyrene/poly(α-methylstyrene) (PS/PαMS), 6,7 polystyrene/poly(4-methylstyrene) (PS/ P4MS),8 and poly(methyl methacrylate) (PMMA)/polyester blend systems.9 However, there are even fewer blend systems that may exhibit UCST below the melting points of one or both constituents. When amorphous/crystalline polymer blends are quenched from a homogeneous state to a temperature below the UCST that is located below the melting point, liquid−liquid © 2012 American Chemical Society

phase separation (LLPS) and crystallization occur spontaneously,10 and the final blend morphology can be determined by kinetic competition between the crystallization and phase separation processes and if crystallization is too dominant and much faster than phase separation, phase domains may be hardly observed.11 Recently, a blend of relatively low weightaverage molecular weight (Mw) poly(L-lactic acid) (PLLA) with poly(butylene adipate) (PBA) is proven to exhibit UCST behavior below the melting temperature (Tm) of PLLA and above PBA’s Tm.12 Simultaneous LLPS and crystallization in blends of semicrystalline/amorphous polymers has been less studied because LLPS cannot be determined easily due to its competition with crystallization. Some semicrystalline/semicrystalline blend systems, such as poly(vinylidene fluoride)/poly(1,4-butylene adipate) (PVDF/PBA), also exhibit a complicated behavior.13−16 The phase diagram of this blend system exhibits a Received: February 10, 2012 Revised: March 22, 2012 Published: March 28, 2012 3094

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Figure 1. OM graphs showing phase reversibility upon cooling → heating → cooling cycles for PLLA/PBA (50/50) blend.

higher Tm and PBA, an aliphatic polyester with relatively low Tm are ideally suited for such investigations. In this study, phase behavior of PLLA/PBA blends upon melt crystallization was investigated. PLLA and PBA are both semicrystalline polymers and both exhibit polymorphism behavior.22−25 Upon heating and cooling, PLLA/PBA blends may exhibit complex combination of UCST, phase domains, dual crystalline spherulites, polymorphic crystal cells, etc. Phase behavior of blends with UCST may be influenced by some subtle factors to lead to distinct crystalline/crystalline phase separation upon cooling from UCST. Morphology of resultant phase separation and crystallization-induced lamellar assembly was analyzed to reveal influences from these two competing processes. Both morphologies of phase domains and lamellar structures in the domains might be influenced by the competing processes of these two kinds of kinetics in crystalline/crystalline blends of two biodedegradable polymers, as modeled by PLLA/PBA mixtures showing UCST, which constituted the objectives of this study.

single glass transition temperature over the entire composition range; two distinct melting temperatures and a thermally reversible lower critical solution temperature (LCST) are observed. By holding at temperature above the melting point of the lower melt constituent, a semicrystalline/semicrystalline blend turns into a semicrystalline/amorphous one. A few other binary systems such as poly(ethylene terephthalate)/poly(ether imide) (PET/PEI)17,18 and poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA)19,20 also reportedly exhibit UCST below the respective melting points and upon cooling, and their kinetics of phase separation is coupled with crystallization. In PET/PEI blend, the drastic increase in nucleation or a significant reduction in spherulite size due to the coupling of LLPS with crystallization has been observed.17 A similar morphological feature is observed in poly(εcaprolactone)/poly(ethyl glycol) (PCL/PEG) by Chuang et al.,21 who have reported that when PCL/PEG blend crystallized at 48 °C (below the melting temperature of PCL = 62 °C), LLPS occurs prior to crystallization. They have suggested that this result is thermodynamically favored due to the fact that PCL supercooled below the melting temperature is short of free energy to overcome the energy barrier of the formation of stable nuclei. Thus, the spontaneous spinodal decomposition usually precedes the polymeric crystallization. For this reason, the liquid−liquid phase boundary can still affect the crystalline morphology during a kinetic process, although the final crystal−liquid equilibrium structure below the melting temperature is eventually determined by thermodynamics. Depending on structures of polymers, PBA exhibit different phase behavior in blends with other polymers. For examples, PBA is thermodynamically miscible with PVDF with LCST, but PBA blended with poly(3-chloropropyl methacrylate) (PCPMA) or poly(2-iodoethyl methacrylate) (PIEMA) is miscible. On the other hand, phase behavior of blend of PBA with PLLA, both being polyesters, is not so straightforward. Blends of two polymers of not too different structures often show UCST behavior. Biodegradable polyesters are increasingly modified with other polyesters for enhancing their properties of balancing biodegradability. For these reasons, it is critical to understand the phase behavior and correlation between phase separation and crystallization for better patterning the crystalline morphology and for enhancing properties. Poly(Llactic acid) (PLLA), a biodegradable polyesters with relatively

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EXPERIMENTAL SECTION

Materials and Preparation. Poly(L-lactic acid) (PLLA) was purchased from Polysciences, Inc., with a relatively low weight-average molecular weight. The PLLA material was characterized in-house using gel permeation chromatography (GPC, Waters) to reveal Mw = 11 000 g/mol (PDI = 1.1), which differs from the supplier-provided data using solution viscosity measurement. For this grade of PLLA, Tg = 45.3 °C and Tm = 155 °C. Poly(1,4-butylene adipate) (PBA) was purchased from Aldrich Co. with weight-average molecular weight of 12 000 g/mol, PDI = 1.30, Tg = −68 °C, Tm = 54 °C, and Td = 250 °C. PLLA of a low-Mw grade was chosen for blending with PBA for balancing the phase behavior and faster crystallization rate of PLLA in the blends. Samples of PLLA/PBA blends were prepared by solution casting using chloroform as solvent with concentration of 4 wt %. A drop of solution of the polymer was deposited and uniformly spread on a micro glass slide at 45 °C, and the solvent was allowed to fully evaporate in an atmosphere. The dried film on the micro glass slide was crystallized with top cover glass. Samples were heated on a hot stage to a maximum melting temperature (Tmax = 190 °C) and held for different times (Δtmax) ranging from 1 to 10 min for erasing the prior residual crystals or nuclei, then rapidly removed to another hot stage preset at a designated isothermal Tc = 110 °C, and subsequently cooled to ambient. Apparatus and Procedures. A polarized optical microscope (POM, Nikon Optiphot-2), equipped with a digital camera charge3095

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coupled device (CCD) and a microscopic hot stage (Linkam THMS600 with TP-92 temperature programmer), was used for characterizing optical homogeneity and crystalline morphology of the blends. Atomic force microscopy (AFM, diCaliber, Veeco Corp., Santa Barbara, CA) investigations were made in intermittent tapping mode with a silicon tip (f 0 = 70 kHz, r = 10 nm) installed. The largest scan range was 150 × 150 μm but could be zoomed-in to 5 × 5 μm, and the scan was kept at 0.5 Hz for zoom regions (5 × 5 μm). Thin films were deposited on substrates of glass slides, with an open face for AFM characterization. AFM samples, sandwiched between two glass slides for fully crystallization, were quenched into liquid nitrogen for easy removal of the top cover. AFM measurements were also carried out to determine the height difference between the inside and outside areas of phase domain in PLLA/PBA (50/50) blend. Samples of solution-cast films and bulk forms of PLLA/PBA blends were examined using a scanning electron microscope (FEI Quanta400F, SEM) for revealing the lamellar pattern in the surface of spherulites and morphology in the interior of bulk sample. The top surface or fractured surface of sample is coated with gold by vacuumsputtering prior to SEM characterization. PLLA/PBA (50/50) blend samples in thicker bulk forms were characterized using wide-angle X-ray instrument (Shimadzu/XRD6000) with a Cu Kα radiation (30 kV and 40 mA). The scanning angle (2θ) covered a range between 10° and 30° at a rate of 2°/min. Melt viscosity of PLLA/PBA blend was measured by using Thermo Scientific HAAKE RheoStress 600 instrument in oscillation time curve mode with shear stress and frequency of 0.001 Pa and 0.1 Hz, respectively. Viscosity of this blend was measured at temperature of 190 ± 0.5 °C (Tmax) as a function of time.

PLLA was fully crystallized and the PLLA spherulites were seen to overlap with the crystal-induced phase domains. Such transitions are depicted by in-situ POM graphs. Figure 2 shows

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RESULTS AND DISCUSSION Figure 1 shows OM graphs of phase reversibility upon cooling → heating → cooling cycles for PLLA/PBA (50/50) blend. As reported in a previous work,12 the PLLA/PBA blend system exhibits UCST behavior below the melting temperature of PLLA. When samples were heated from ambient temperature (as cast sample) to higher temperature above melting temperature of PBA and PLLA, the PLLA/PBA blend showed homogeneous phase (one phase). However, after being cooled to a lower temperature, below PLLA melting temperature but above PBA melting temperature, the blend system transformed from the homogeneous phase to a heterogeneous phase (twophase domains). After reaching the homogeneous phase at higher temperatures (i.e., 190 °C), the PLLA/PBA (50/50) blend could be returned to a heterogeneous phase at 110 °C upon cooling to ambient. However, after being heated back again to higher temperatures, the heterogeneous phase became homogeneous phase again at 128 °C. This temperature of clarity point (homogeneous state) is defined as temperature transition from heterogeneous to homogeneous phase upon heating. Reversibility in the cycle is seen as the heterogeneous phase can be obtained again after the sample is cooled from 128 °C to lower temperature, 110 °C. It is quite unusual that UCST is located below the melting points of the two constituents (below PLLA’s Tm and above PBA’s Tm). That is, above PLLA’s Tm only a single molten mixture of PLLA/PBA exists, between Tm and UCST, the crystal and amorphous phases coexist in equilibrium; further down below UCST (but still above PBA melting), a PLLA crystal phase coexists with the amorphous PLLA and PBA domains in the blend within this temperature window. The UCST phase diagram with clarity points for PLLA/PBA blends as a function of composition has been shown in the previous study.12 For the PLLA/PBA (50/50) blend melted at 190 °C for 2 min and then crystallized at 110 °C, phase separation occurred in the early stage of initiation of crystallization. Eventually,

Figure 2. (A) OM graph showing phase separation for PLLA/PBA (50/50) blend. (B) POM graph showing PLLA spherulites overlapped with the phase domains. Graphs A and B taken at Tc = 110 °C.

(A) OM graph showing phase separation for PLLA/PBA (50/ 50) blend in the early stage of crystallization and (B) POM and OM graphs showing PLLA spherulites overlapped with the phase domains. Size of phase domains increases with increasing annealing time at Tc. As shown earlier in Figure 1 for the blend at T = 110 °C, the size of phase domain is small and irregular, wormlike structure. With increasing annealing time (a few seconds), those wormlike structure aggregates and form larger size of phase domain which is dominated by sphere shapes with diameter ≥15 μm (Figure 2A). After achieving equilibrium state, the size of phase domain is constant with increasing annealing time at Tc and PLLA crystals crystallized. PLLA forms ring-banded spherulites as shown in Figure 2B. PLLA has been reported shows ring-banded spherulites at Tc ranging from 125 to 130 °C.26 Apparently, the crystallization temperature of PLLA to form ring-banded spherulites decreases by adding PBA as same as PLLA blended with PHB.27 The OM 3096

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density decreases, which leads to large size of spherulites. Not only nucleation density but also spherulites growth rate decreases with the increase of Δtmax as shown in Figure 3B. The growth rate of PLLA spherulites for Δtmax = 2 and 10 min is 1.65 and 1.18 μm/s, respectively. For short and long Δtmax, the difference is not only in the existence of phase domain, nuclei density, size, and growth of spherulites but also on regularity of PLLA ring-banded spherulites. Irregular ringbanded spherulites are associated with PLLA spherulites crystallized by quenching from Tmax for short Δtmax (2 min). However, for blends subjected to long tmax and similarly crystallized, the ring bands become regular owing to more severe impingement between spherulites. Those phenomena can be explained based on the number of entanglement density (ve) during the melting process. The number of entanglement density in blends usually increases with increasing melting time held at maximum melting temperature.28,29 Figure 4 shows a scheme for illustrating the effect of Δtmax on the formation of phase separation in the PLLA/PBA (50/50) blend system. For short Δtmax, the number of entanglement density is less or almost zero; thus, PBA chains are easy to be expelled from PLLA chains during crystallization of PLLA, resulting in PLLA-rich and PBA-rich domains and exhibiting crystallization of PLLA spherulites overlap with phase-separated domains as shown in Figure 2B. In contrast, for long Δtmax, there is no phase-separated domain because the number of entanglement density is more; thus, most of the PBA chains are entangled with PLLA chains which lead to larger spherulites size with slower growth rate. Morphology of crystallized blend samples by quenching from Tmax (190 °C) for Δtmax ranging from 1 to 10 min was monitored in situ with time evolution by POM (not shown here for brevity). For shorter Δtmax (1 and 2 min), phase domains in crystallized blends with size ≥15 μm appear, and the PLLA spherulites are small and irregular. Tiny phase domains and regular ring-banded spherulites are resulted for intermediate Δtmax = 5 min. In contrast, there are no phase

graph in Figure 2B shows the phase-separated domain clearly, and those domains overlap with ring-banded spherulites of PLLA. Figure 3 shows the holding time at maximum melting temperature (Δtmax) is the influencing factor for initiation of

Figure 3. (A) Effect of Δtmax to the formation of phase domains in PLLA/PBA (50/50) blend system. (B) Growth rates of PLLA/PBA (50/50) blend crystallized at 110 °C at different Δtmax: (●) 2 min and (■) 10 min.

phase domains in PLLA/PBA (50/50) blend system upon crystallization. Phase-separated domains only appear in the short Δtmax (2 min). With increasing Δtmax, the nucleation

Figure 4. Scheme showing the effect of Δtmax on formation of phase separation in PLLA/PBA (50/50) blend system. 3097

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changes in the blend system. The stable with minor increase of melt viscosity indicates (1) no thermal degradation and (2) minor increase of chain entanglement density.30 In-situ growth of PBA at ambient temperature in the presence of PLLA ring-banded spherulites is shown in Figure 6.

domains, and the size of PLLA ring bands becomes greater in spacing and more regular for longer Δtmax (7 and 10 min). In the early discussion, with increasing Δtmax, the number of entanglement density increases which leads to large size of spherulites. From the thermogravimetry analisys (TGA), the degradation temperature of PLLA and PBA in nitrogen was determined to be 310 and 250 °C, respectively. TGA results proved no degradation at Tmax = 190 °C. Evidence of thermal stability at 190 °C could be used to rule out possibility of decreasing molecular weight due to thermal degradation. The original Mw’s (11 000 and 12 000 g/mol) of these two polymers, albeit not particular high, could be expected to generate sufficient number of chains entanglement to influence the phase behavior. The classical polymer theory points out that a polymer generally should have Mw = 25 000 g/mol to be an onset of sufficient entanglement for good mechanical properties of polymers. It should be emphasized that it is not the mechanical properties but phase separate kinetic process of the blend that is the issue here. That is, it does not take extremely high Mw to achieve the onset of chain entanglement for influencing the kinetics of phase separation, which ultimately influences the solidified morphology when later cooled to crystallize. To further show evidence whether the number of entanglement density or thermal degradation is correlated or attributed to the phase behavior in the PLLA/PBA (50/50) blend at Tmax, the melt viscosity of this blend as a function of melting time (from 0 up to 60 min) was measured, as shown in Figure 5. The result shows that the complex melt viscosity of

Figure 6. POM images observing in-situ growth of PBA at ambient temperature.

PBA does not crystallize at Tc = 110 °C but only crystallizes at temperature near or below ambient temperature (30 °C or lower). In the early stage of crystallization at this temperature, some part of PBA crystallizes as indicated by circle dashed line and finally fully crystallized in the whole sample. The pattern of PBA crystals cannot be distinguished; apparently, crystallization of PBA follows the pattern of PLLA ring-banded spherulites. The later-stage PBA crystallization on or within PLLA does not disturb or change the ring-band pattern of PLLA, but it only enhances the optical birefringence of PLLA ring-banded spherulites. PBA does not crystallize only inside of phaseseparated domains but also extents to the whole crystalline PLLA domains. As shown in the previous scheme in Figure 4, there are two separated phases, PLLA-rich and PBA-rich phases, for shorter Δtmax. Thus, it can be expected that PBA crystallizes in the whole PLLA spherulites and not only inside the phase-separated domains. AFM analysis was carried out to investigate the detailed morphology of phase domains in the crystallized PLLA/PBA (50/50) blend. Figure 7 shows AFM images of PLLA/PBA (50/50) blend melt-crystallized at 110 °C by quenching from Tmax = 190 °C for 2 min. After being fully crystallized, the sample was quenched into liquid nitrogen to remove the top cover easily and then scanned by using AFM. The phase domain is indicated by black arrow in Figure 7A. Figure 7B shows zoom-in of square-marked region in Figure 7A on the phase domains to see more clearly where the ridge region of spherulites overlaps with phase domains. Morphology of the

Figure 5. Melt viscosity of PLLA/PBA (50/50) blend as a function of time at Tmax = 190 °C.

PLLA/PBA (50/50) blend remains almost stable but slightly increases with longer time at Tmax = 190 °C. It should be commented here that the measured complex viscosity of the PLLA/PBA (50/50) blend is relatively low at 190 °C. This is quite expected because (1) PLLA is mixed with PBA at 50 wt %, (2) the complex viscosity of polymers typically follows the Arrhenius relationship [η* = η*° exp(−E/RT)], and Tmax = 190 °C is substantially above PLLA’s Tm = 155 °C and much above PBT’s Tm = 54 °C. Additionally and more importantly, this melt-viscosity result also shows that there is no thermal degradation during polymer melt holding at Tmax = 190 °C, as thermal degradation, if taking place, would have rendered chain cessation and thus a decrease in the viscosity. Thus, both TGA and viscosity measurements are in mutual agreement, proving none or very little degree of thermally induced chemical 3098

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phase domain have some height difference. By further measuring the height profiles of these two areas, it was found that the outside area of phase domain is about 40 nm higher than the inside area of phase domain. The AFM phase image (Figure 7D) shows that the lamellar patterns of these two different areas are the same. Obviously, the inside and outside areas of phase domain are only different in height, but the lamellar pattern remains unchanged. This result shows that the phase domains do not affect the morphology of PLLA spherulites in PLLA/PBA blend. During crystallization, the PLLA crystals penetrate into the phase-separated domain, resulting in PLLA ring-banded spherulites overlapping with the phase-separated domains. Apparently, PLLA ring-banded spherulites serve as the growth template for PBA crystals to later grow onto them. The stronger birefringence of the PLLA/ PBA (50/50) sample crystallized at ambient temperature after being quenched from Tc is apparently resulted from the additional PBA crystal birefringence imposed onto the PLLA spherulites. Phase domains have been shown to appear in thin-film PLLA/PBA blend samples. PLLA/PBA bulk samples were also characterized by monitoring the thermal behavior using hot stage. First, more details of phase separation morphology of fractured samples were analyzed by using SEM. Figure 8 shows SEM graphs of PLLA/PBA (50/50) bulk samples melt crystallized at 110 °C by quenching from Tmax = 190 °C for 2 min: (A) top surface, (B) fractured surface, (C) zoom-in of fractured surface in graph (B), and (D) underneath the top surface in correlation with fractured surface. As shown in SEM graph (A), the morphology of top surface is ringless spherulites, or at most, it can be called a ringed spherulite with only one wide band (50 μm wide). The size of the spherulites in bulk blend is smaller; apparently, about 100 μm in diameter; however, by contrast, the spherulites size in thin films is much

Figure 7. AFM images of PLLA/PBA (50/50) blend melt-crystallized at 110 °C by quenching from Tmax = 190 °C for 2 min. (A) Height image at 150 × 150 μm, (B) zoom-in of square-marked regions in images (A), (C) further zoom-in of squared region in (B), and (D) phase image of (C).

area inside and outside of phase domain is the same as shown in Figure 7C,D. The boundary between interior and exterior of phase domain is marked by a dashed line. The difference between inside and outside area of phase domain is the brightness, reflecting the differing height of the regions in AFM height images. It means that the inside and outside areas of the

Figure 8. SEM graphs of PLLA/PBA (50/50) blend bulk samples melt crystallized at 110 °C by quenching from Tmax = 190 °C for 2 min: (A) top surface, (B) fractured surface, (C) zoom-in of fractured surface in graph B, and (D) underneath the top surface in correlation with fractured surface. 3099

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Figure 9. SEM graphs of PLLA/PBA (50/50) blend bulk samples melt crystallized at 110 °C by quenching from Tmax = 190 °C for 10 min: (A) top surface in correlation with fractured surface, (B) zoom-in of graph A, (C) zoom-in of fractured surface in graph A, and (D) detailed morphology of fractured surface.

larger, about 300−400 μm in diameter. With 100 μm in diameter, it only forms center of spherulites and one band of ring-banded spherulites in the bulk PLLA/PBA blend sample. The number of nuclei in the bulk sample is greater; that may be the reason why it exhibits small size of spherulites. There is no phase-separated domain discernible on the top surface, but there are many voids/pores on the top surface, especially in the impingement region between two spherulites, forming interfaces of two crystal types. Actually, pores on the top surface of bulk sample also appear in the center or in the whole spherulite as shown by the arrow mark in SEM graph A. Owing to interspherulite impingement among the neighboring 3D spherulites, the fractured surface morphology of bulk PLLA/PBA blend samples exhibits polygonal shapes with rough cleaved lamellae. Apparently, the polygonal-shape boundary in the fractured surface of PLLA/PBA bulk sample is caused by the impingement between spherulites. When the growth rates between two spherulites are the same and the two primary nuclei appear simultaneously, a straight impingement line will be formed.31 In the 3D bulk forms, the impingement of spherulites with equal growth rate should form a planar surface instead of straight impingement line which is usually formed in 2-D. In the bulk sample, spherulites are easy to grow in 3D. Spherulites grow almost simultaneously and impinge to each other from any directions when they crystallize in 3D; thus, polygonal shape could be formed instead of a well-rounded sphere. Graph B shows the morphology of fractured surface in the bulk sample is heterogeneous, which is indicated by the presence of two types of surface pattern in the polygonal. These two types of surface pattern can be seen clearly in graph C. The detailed surface pattern of fractured surface is similar to a porous structure. For PLLA/PBA blend held with short Δtmax = 2 min, there are two different pore sizes on the top of polygonal crystals, small and large pores. Large pore size is minority, and

its shape is similar to the phase-separated domain that is observed in the surface of thin films, sphere shape. The correlation between pores and interiors of polygonal shape can be observed clearly in graph D. Pores only form in the surface of polygonal crystals (spherulites); they do not go through the interior of polygonal. As indicated by arrow in graph B, the interior of polygonal crystals is not porous. The length of pore is ca. 20 μm from the polygonal surface as shown in graph D. In comparison, the detailed morphology of PLLA/PBA (50/ 50) bulk sample for Δtmax = 10 min with no phase separation is shown in Figure 9. As shown in graph A, the morphology on the top and fractured surface is ring-banded spherulites and polygonal shape, respectively. The morphology on the top surface of bulk is in agreement with that in thin film, large and regular ring-banded spherulites. Zoom-in of the left side of graph A is shown in graph B. The detailed morphology inside the polygonal crystals can be seen clearly, and it does not show a porous structure. Graph C shows the surface of polygonal is similar to a porous structure. However, the detailed morphology of polygonal surface is not of a porous structure as shown in graph D. In addition, two additional PLLA/PBA blend compositions nearby 50/50 were examined. Figure 10 shows fractured surfaces of phase separation morphology of PLLA/PBA blend (40/60 and 60/40 in comparison to 50/50) crystallized at 110 °C by quenching from Tmax = 190 °C (2 min). The PLLA/PBA blend (40/60) composition shows a greater volume fraction of the PBA constituent than either (50/50) or (60/40) blend compositions and result in large pore size. However, with a less fraction of the PBA constituent, the PLLA/PBA (60/40) blend shows small pore size. Apparently, the size of porous structure depends on the fraction of PBA in the PLLA/PBA blend system. By comparing the morphology patterns of blends of various compositions, it is apparent that the large and small 3100

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is similar to that in fractured surface of poly(vinylidene fluoride) (PVDF) membranes.31,32 Large pore size in the PLLA/PBA blend system is also similar to that of porous chitosan scaffolds prepared by freeze-drying.34 Apparently, the crystalline/crystalline PLLA/PBA blend with UCST-phase separation can be considered as a potential candidate to fabricate into either polymer membranes or polymeric scaffolds. In summary, the PLLA/PBA blend exhibits UCST and a borderline phase behavior between miscibility and immiscibility, whose miscibility or phase separation is dependent on holding time at Tmax and Tc at which crystallization takes place. The PBA/PLLA blend is miscible when kept in melt above UCST but exhibits simultaneous phase separation prior to crystallization when cooled down and crystallized below UCST. With increasing annealing time at Tc, phase domain transforms from a wormlike structure to sphere with diameter ≥15 μm. The phase domains within a few seconds reach an equilibrium state, as indicated by phase domains stoping to grow. After that, PLLA crystallized and overlapped with those phase domains. Phase-separated domain only appears in PLLA/PBA blends held for short Δtmax due to the number of entanglement density for short Δtmax is less than that for long Δtmax. Thus, the PBA chains are easy to be rejected from the growth front during crystallization of PLLA and form a PBA-rich domain. Phase domains not only can be observed in the thin films but also in the bulk samples. Phase domains in the fractured surface of PLLA/PBA (50/50) bulk sample with short Δtmax appear with large pores. Fractured surface of bulk sample is polygonal with porous structure and the size of pore is affected by the fraction of PBA in the blend system. The regions of large and small pore sizes are the characteristic of the PBA-rich and PLLA-rich domains, respectively. Scheme for the formation of porous structures in the PLLA/PBA blend is shown in Figure 11. The scheme illustrates that most of PBA chains are rejected from the growth front of PLLA during crystallization of PLLA in the blend, which initially leads to formation of PLLA-rich and PBArich domains. When the PLLA crystal continues to grow, PLLA penetrates/overlaps with PBA-rich domain and amorphous liquid (molten) PBA segregates in the interlamellar region of PLLA. After being cooled to ambient temperature, PBA crystallizes on or within PLLA lamellae and PLLA crystal as its growth template. Thus, crystallization of PBA does not disturb or change the morphology of PLLA spherulites. Apparently, later crystallization of PBA at ambient enhances the crystalline birefringence of spherulites and the larger porous structure in the PBA-rich domain can be formed due to the crystal densification upon crystallization of PBA. The crystal cell types in the PLLA/PBA blend were verified, in addition to the spherulite morphology and pores created by lamellar assembly in the crystalline/crystalline PLLA/PBA blend. Both PLLA and PBA constituents in the blend have been known to have polymorphism behavior. To investigate the crystal cells in the blends, characterization using WAXD analysis was done. PLLA/PBA blends were crystallized by using two-step crystallizations, at 110 °C for 1 h to first crystallize PLLA constituent and then quenched to 28 or 35 °C, respectively, to crystallize the PBA constituent. Figure 12 shows WAXD patterns of PLLA/PBA (50/50) blend melted at 190 °C for different tmax (2 and 10 min), first crystallized at 110 °C and then crystallized at different Tc (28 and 35 °C). When crystallized at 28 and 35 °C, PBA is known to be packed in polymorphic α + β- or solely α-form crystals, respectively.25 When crystallized at Tc = 28 °C, PBA shows multiple

Figure 10. SEM graphs for fractured surfaces of PLLA/PBA blend (40/60 and 60/40 in comparison to 50/50) crystallized at 110 °C by quenching from Tmax = 190 °C for 2 min and then cooled to ambient (28 °C).

pore sizes in PLLA/PBA (50/50) blend with short Δtmax can be attributed to the separated domains of the PBA-rich and PLLArich constituents, respectively. By contrast, phase-separated domains in the fractured surface of PLLA/PBA of either 60/40 or 40/60 composition cannot be observed, which may be due to the size of phase-separated domain being too small to be discerned. Additional morphology characterization on thin film samples (instead of bulk) revealed that phase-separated domains with 1−2 μm in diameter can be observed in the thin films after being crystallized at 110 °C for PLLA/PBA (60/ 40) and (40/60) (for brevity, morphology results for thin films not shown here). The PBA lamellae in the domain assume all edge-on orientation (perpendicular to fracture plane), with weblike interconnection between lamellar plates. The pores in PBA-rich domains have micrometer-scale sizes, while those in the PLLA-rich domains are averaged in the hundreds of nanometers (submicrometers). The nano- or micrometer-size pores in the phase-separated domains of PLLA/PBA (50/50) blend are apparently created by solidification and densification of PBA lamellae upon crystallization at ambient temperature. Such pores and their lamellar patterns in the UCSTcrystallized crystalline/crystalline PBA/PLLA blend resemble the phase-separation patterns in many commercially available polymer membranes. Materials with a porous structure of controlled porosity sizes are usually used in membranes or scaffolds, which are fabricated by using thermally induced phase separation (TIPS) method.32−39 In the TIPS method,35 a solution of polymer in an appropriate solvent is cast in a mold at an elevated temperature and then quenched to temperature below the freezing point of the solvent; then, the sample is subsequently freeze-dried to produce a desired porous structure. PLLA is commonly used for biodegradable polymer membranes or porous biodegradable polymeric scaffolds.36−39 The blend of PLLA/PCL has been used to fabricate polymer membrane via TIPS by using mixed diluent, 1,4-dioxane and water.40 However, in the PLLA/PBA blend system, porous structure can be obtained from crystallization below UCST without using solvent as diluents. The polygonal shape with porous structure in the fractured surfaces of PLLA/PBA blend 3101

dx.doi.org/10.1021/ma300288v | Macromolecules 2012, 45, 3094−3103

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Article

of PLLA. When the PLLA/PBA (50/50) blend crystallized below UCST, phase-separated domains appear prior to crystallization of PLLA, resulting in the PLLA spherulites which are seen to overlap with the phase-separated domains. However, phase-separated domains only appear when the sample is melted for a short maximum melting time (Δtmax = 2 min). For longer Δtmax (10 min), there is no phase-separated domain. Apparently, when PLLA/PBA (50/50) blend is melted for short Δtmax, the number of entanglement density is less; thus, PBA chains are easy to be expelled from PLLA chains during crystallization of PLLA, resulting in PLLA-rich and PBArich domains. Because the entanglement density is less, the number of nucleation density is more, resulting in smaller size of spherulites, and the spherulites growth is fast. However, for long Δtmax, the number of entanglement density is more; thus, PBA chains remain entangle with PLLA chains during crystallization of PLLA, resulting in smaller nucleation density, larger size of spherulites, and slow growth rate of PLLA spherulite. Apparently, phase separation in the PLLA/PBA blends is initiated by the crystallization of PLLA. The laterstage crystallization of PBA at ambient temperature on or within PLLA does not disturb or change the earlier morphology of PLLA; however, it enhances the birefringence of PLLA spherulites. Apparently, PLLA spherulites is the growth template of the later-stage crystallization of PBA. The crystal cells of PLLA and PBA remain the same after blending and after thermal treatment at different Δtmax as shown by WAXD pattern of the PLLA/PBA blend. Interiors of bulk PLLA/PBA blends with crystallizationinduced phase separation below UCST were examined. Lamellar assembly in the phase domains exhibits interesting patterns depending on kinetic process of phase separation. Fractured surface of PLLA/PBA blend exhibits polygonal shape due to the interspherulite impingement among the neighboring 3D spherulites. The surface pattern of polygonal is porous structure, and pore size is affected by the fraction of PBA in PLLA/PBA blend. Large and small pores (cavities) can be obtained from PBA-rich and PLLA-rich domains, respectively. Porous structures are formed due to densification of PBA upon crystallization at ambient temperature. It has been shown that the PLLA/PBA blend exhibits dual-distribution porous structures in both domains; thus, this kind of blend system can be considered as potential candidate materials to fabricate membranes or scaffolds based on biodegradable polymers and their mixtures of controlled compositions.

Figure 11. Scheme of the formation of pore structure on the surface of fractured bulk sample in the PLLA/PBA blend system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Fax: +886 6 234-4496; Tel: +886 6 275-7575 ext 62670.

Figure 12. WAXD intensity profiles of PLLA/PBA (50/50) blend melted at 190 °C for different Δtmax (2 and 10 min), first crystallized at 110 °C and then crystallized at different Tc (28 and 35 °C).

Notes

The authors declare no competing financial interest.

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diffraction peaks, attributed to existence of α- or β-form crystal cells (polymorphism). For PLLA crystallized at 110 °C, its crystal cell is α-form. The facts of WAXD analysis reveals that the crystal cells of PLLA and PBA remain the same after blending and after thermal treatment at different Δtmax.

ACKNOWLEDGMENTS This work has been financially supported by basic research grants (NSC 99-2221-E-006-014-MY3) in three consecutive years from Taiwan’s National Science Council (NSC), to which the authors express their gratitude. Melt viscosity of PLLA/ PBA blend was measured with instrument and technical help provided by Mr. Chao-Shun Chang of R&D Center of Formosa Plastics Corp. (Kaohsiung, Taiwan), to which the authors express their sincerest thanks.

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CONCLUSION The PLLA/PBA blend system exhibits upper critical solution temperature (UCST) behavior below the melting temperature 3102

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(38) Goh, Y. Q.; Ooi, C. P. J. Mater. Sci.: Mater. Med. 2008, 19, 2445−2452. (39) Budyanto, L.; Goh, Y. Q.; Ooi, C. P. J. Mater. Sci.: Mater. Med. 2009, 20, 105−111. (40) Tanaka, T.; Tsuchiya, T.; Takahashi, H.; Taniguchi, M.; Lloyd, D. R. Desalination 2006, 193, 367−374.

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dx.doi.org/10.1021/ma300288v | Macromolecules 2012, 45, 3094−3103