Phase-Separation Induced Lamellar Re-Assembly and Spherulite

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Phase-Separation Induced Lamellar Re-Assembly and Spherulite Optical Birefringence Reversion Graecia Lugito, Chun-Yen Yang, and Eamor M. Woo* Department of Chemical Engineering, National Cheng Kung University No. 1, University Road, Tainan, 701-01, Taiwan S Supporting Information *

ABSTRACT: By using atomic-force and scanning electron microscopies (AFM, SEM), wide-angle and small-angle X-ray diffraction (WAXD and SAXS), and thermal and infrared spectroscopy characterization, mechanisms and correlations between lamellar reassembly, phase separation domains, and birefringence patterns in crystallized spherulites in poly(hydroxyl butyrate) (PHB) were probed in detail. Crystallization-induced phase separation in PHB blended with amorphous poly(methyl acrylate) (PMA) produces pebble-like PMA domains, which influence the birefringence types (positive vs negative types) of spherulites in crystallized blends with respect to composition and temperature. Extents of crystallization-induced phase separation were dependent on temperature and composition; for a fixed PHB/PMA (60/40) blend composition the phase separation extent was inversely proportional to temperature of crystallization (Tc). Correspondingly, depending on the extents of phase separation (counted as quantitative ratio of pebble-like PMA domains divided by entire area of the blend samples), the Maltese cross in spherulite patterns of PHB in the blend may rotate from the original positive-type spherulites at Tc = 90 °C to negative-type spherulites at Tc = 60 °C. The SAXS analyses showed the thickness variation of PHB lamellae (or PHB + amorphous polymer long periods) with Tc was not an influencing factor, but that the formation of grains in the PMA domains from the crystallizationinduced phase separation could majorly influence the optical birefringence. Correlations among the phase domains, lamellae assembly, transitions from positive-type, unusual-type, and negative-types of crystallized spherulites in polymers have been expounded upon.

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INTRODUCTION Spherulites of polymers can be packed in diversified patterns with many influencing factors. There have been numerous investigations in the literature dealing with positive- and negative-type spherulites.1−7 Alternately, there are also studies on investigating the differences between the so-called usual vs unusual spherulites.8−17 Some previous studies in the literature related to the birefringence types of spherulites in neat and blend of polymers are presented as summary comments in Table 1. As shown in Table 1 summary for comparison of various investigations in the literature, sPS, when blended with amorphous polymers, is capable of displaying positive- and negative-types of spherulites at a fixed Tc; by contrast, polymers such as PEO, PEA, and PHA are able to display negative- or positive-type spherulites at different Tc ranges. The spherulites’ birefringence variation with Tc in PEO/PMMA, PEA/phenoxy, and PHA/PVME blends points out a critical observation that the birefringence is governed by the combination of lamellar direction (radial or tangential), lamellar orientation (edge-on or flat-on), and also tilt/bend/twist angle of the lamellae. By comparison, ring bands in spherulites are composed of alternating edge-on−flat-on lamellae or radially−tangentially © 2014 American Chemical Society

arranged lamellae in periodical stacks in circumferential alteration. This study did not aim for mechanisms of bands in spherulites; thus, how the lamellar direction/orientation changing alternately to form banding patterns is not the focus of this study. Regarding the unusual Maltese cross spherulites in polymers, one of the most noticeable examples is poly(butylene terephthalate) (PBT)8−13 or its blends with other polymers.14−16 TEM and SEM characterization on PBT revealed that the differences between the usual (+-type Maltese cross) and unusual (×-type Maltese cross) spherulites are the inclination of lamellae plates. If lamellae are mainly in radial or tangential directions, the spherulites are “usual”. If lamellae plates are oriented at a slant angles with respect to radial or tangential directions, then unusual spherulites are resulted. A general concept of optical ellipsoid is used here to explain these three types of optical birefringence: long-axis of the ellipsoid in the radial direction corresponds to a positive type; long-axis in a tangential direction corresponds to a negative type; long-axis at Received: June 4, 2014 Revised: July 28, 2014 Published: August 7, 2014 5624

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an angle of θ to radial direction leads to a unusual type. In addition to the differences in orientation angles of the lamellae plates, the DSC results for these two lamellae types are also different. The heat of fusion (ΔHm) for usual spherulites is larger than that of the unusual spherulites, which is attributed to the usual-type lamellae have a greater crystallinity. The usualtype lamellae also have a higher Tm than the unusual ones. There are ample literature reports that also found unusual-type spherulites in polymers containing terephthalate group, such as poly(pentamethylene terephthalate) (PPT), 1 8 poly(hexamethylene terephthalate) (PHT),19 poly(heptamethylene terephthalate) (PHepT),18,20 poly(octamethylene terephthalate) (POT),21 poly(nonamethylene terephthalate) (PNT),22,23 etc. Neat PHB only exhibits positive-type spherulites, which is just the opposite of neat poly(L-lactic acid) (PLLA) that displays only negative-type spherulites when crystallized at any Tc. Introduction of miscible or partially miscible amorphous diluents may induce birefringence changes, and crystal lamellae alteration, in either of these polymers. Preliminary studies in this laboratory showed that poly(3-hydroxybutyrate) (PHB) blended with amorphous poly(methyl acrylate) (PMA) displayed various extents of phase separation, especially when induced by crystallization. Clarity point and phase boundary for PHB/PMA blends as a function of composition are shown in Figure S-1, Supporting Information. The PHB/PMA blend exhibits UCST behavior: two phases (immiscibility) are observed at temperatures below the phase boundary, while only one phase (miscibility) is observed above the boundary. As an initiating study, blend of PHB/PMA served as an ideal model for investigating effects of diluents, Tc, and phase separation on the birefringence patterns of spherulites in the blend. By utilizing the powerful analytical capacity of atomicforce microscopy (AFM) for revealing the minute microstructures in the lamellae, further understanding could be advanced to expound the mechanisms in positive-, negativetype, and usual- vs unusual-types spherulites. Furthermore, interior dissection on inner lamellar assembly was probed by using scanning electron microscopy (SEM) for providing views not just limited on the top surfaces but also into the interior lamellae responsible for these various birefringence patterns in spherulites.

sPS/aPS

PEO/PMMA

PEA vs PEA/phenoxy

PHA/PVME

2

3

5

6, 7

Neat syndiotatic polystyrene (sPS) displays only positive-type spherulites, while sPS/poly(vinyl methyl ether) (PVME) blend is able to display both positive- and negative-types of spherulites when crystallized at a fixed Tc. The negative-type spherulite in sPS/PVME blend is attributed to heterogeneous nucleation induced by PVME. Both positive- and negative-types spherulites coexist in the sPS/aPS blend. After aPS is etched out by amyl acetate, the lamellar stacks of sPS in positive-type spherulites are revealed in radial orientation, but those in the negative-type spherulites are in tangential arrangement, due to the presence of polarizable phenyl substituents as the side branches of sPS. Negative spherulites of poly(ethylene oxide) (PEO) observed at higher level of phase separation (higher Tc) have more closely bundled and fiber-like lamellae in radial direction; while in positive spherulites observed at lower level of phase separation (lower Tc), the lamellae are in tangential directions. Birefringence reversion has nothing to do with the types of crystal lattices (lattice geometries), however, it is related to the lamellae assembly and orientation. The negative-type spherulites in poly(ethylene adipate) (PEA)/phenoxy blend are composed of mainly lamellae that are oriented mainly in the radial direction; while, the positive-type spherulites are composed of lamellae which are periodically or sporadically bended in tangential direction or coiled into “o” or “8” shapes. The positive-type spherulites in poly(hexamethylene adipate) (PHA)/PVME blend are composed of mainly flat-on lamellae with slightly tilted upward or downward during its growth in radial direction, while negative-type spherulites are composed of a good fraction (though not 100%) of lamellae being reoriented to edge-on, still in radial direction arrangement. sPS vs sPS/PVME 1

materials ref

Table 1. Literature Studies Related to the Birefringence Types of Polymer Spherulites

summaries

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EXPERIMENTAL SECTION

Materials and Preparation. Poly(3-hydroxybutyrate) (PHB) was obtained from Polysciences, Inc. (USA), with Mw = 71 300 g mol−1, polydispersity index PDI = 1.29, Tg = −2.2 °C, and Tm = 171.3 °C. Poly(methyl acrylate) (PMA) was supplied by Aldrich Chemical Co., Inc. (USA), with Mw = 59 800 g mol−1, PDI = 1.53, Tg = 7.3 °C. PMA was specifically chosen to mix with PHB for investigating the spherulite patterns, because our preliminary findings already disclosed that PMA was partially miscible with PHB and phase domains were present in the blend upon crystallization. Thin-film PHB/PMA blend samples (cast with 10−14 μm in thickness) were prepared by dissolving two polymers in a common solvent of chloroform (∼4 wt %), well stirred, and cast on glass slides and KBr pellets. Then, the cast films on glass and KBr pellets were air-dried on a hot plate adjusted to 45 °C for 24 h to remove the solvent. Thermal treatments on PHB/PMA blend were as follows. Sample films on glass substrates or KBr pellets were heated to Tmax = 190 °C (near but slightly below the clarity points as shown in Figure S-1) and held for 2 min, quickly replaced to a hot stage set at a predesignated Tc for full crystallization. After full crystallization, a set of blend samples was etched using p-dioxane to strip off the PMA component from PHB lamellae. Another set of blend samples were as-crystallized with 5625

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identical conditions but left unetched for comparison. Samples of PHB/PMA blend cast as thin films on glass slides were fractured into halves; p-dioxane was then sprayed on the samples to wash off the PMA constituent slowly from the blend. This procedure was repeated several times to ensure that PMA was stripped off thoroughly; then the etched-samples were dried properly in a vacuum oven, prior to characterization. Characterization. A polarized optical microscope (POM, Nikon Optiphot-2), equipped with a Nikon Digital Sight (DS)-U1 camera control system and a microscopic hot stage (Linkam THMS-600 with T95 temperature programmer), was used for the preliminary confirmation of different birefringence spherulites in samples prepared by crystallization at the procedures described in the previous section. Atomic-force microscopy (AFM, diCaliber, Veeco Corp., Santa Barbara, CA) investigations were made in an intermittent tapping mode with a silicon tip (fo = 70 kHz, r = 10 nm) installed. The largest scan range was 150 × 150 μm, and the scan was kept at 0.4 Hz for the overview scan and zoom-in regions. Thin films of PHB/PMA blend were deposited on glass slides without cover, with an open face for AFM characterization. Scanning electron microscopy (SEM, FEI Quanta-400 F) characterizations were applied on the fractured surfaces and top surfaces of bulk-form or thin-film PHB/PMA blend samples crystallized at Tcs for revealing the lamellar structure in the fracture and top free surfaces, respectively. Samples were coated with gold vapor deposition using vacuum sputtering (2 mA, 10 × 60 s) prior to SEM characterization. Fourier-transform infrared spectroscopy (FTIR, Nicolet Magna560) was used for investigating possible molecular interactions between the constituents. Spectra were obtained at 4 cm−1 resolution and averages of spectra were obtained from at least 64 scans (for enhancing signals) in the standard wavenumber range 500−3500 cm−1. Samples for IR measurements were cast as thin films with uniform thickness directly on KBr pellets and crystallized at Tcs then cooled to ambient temperature. X-ray instrument (Shimadzu XRD-6000) with a Cu Kα radiation (30 kV and 40 mA) was used to determine the crystal lattice structures. The scanning angle (2θ) covered a range between 10° and 30° with a rate of 2° per min. PHB/PMA blend in thin-film samples, respectively crystallized at specified T c, were prepared and characterized.

Figure 1. POM graphs of neat PHB crystallized at various Tcs: (a) 50 °C, (b) 60 °C, (c) 70 °C, (d) 80 °C, and (e) 90 °C.

axes of Maltese cross rotate clockwise of 90° in angle). Figure 2 shows POM graphs for PHB/PMA (60/40) blend samples crystallized at a range of Tc from 60 to 90 °C. For Tc = 60 °C, the crystallized spherulites display the usual-negative-type

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RESULTS AND DISCUSSION Figure 1 shows POM graphs for neat PHB crystallized at Tc = 50−90 °C. Apparently, for neat PHB, Tc only influences the number of nucleation sites and spherulite sizes, but does not change the birefringence-type of PHB spherulites, which remains as usual-positive-type (blue color in the odd quadrants and orange color in the even quadrants with positive-type Maltese cross extinction) regardless of Tc. Blend samples of PHB with PMA were similarly characterized for comparing with the morphology results for neat PHB. Upon blending with 10 or 20 wt % PMA into PHB, however, the crystallized samples do not display any alteration in the birefringence-types of crystallized spherulites, as shown in the Supporting Information (Figure S-2 and Figure S-3). At Tc = 50−90 °C, the crystallized PHB/PMA blend with PMA ≤ 20 wt % still displays the usual positive-type spherulites regardless of Tc variation. There is a critical concentration of PMA in the blend for inducing changes in spherulite birefringence types. The critical concentration for changes in the spherulites birefringence is located at roughly 30 wt % PMA in the PHB/PMA blend. For PMA ≥ 30 wt % in the blend, the Maltese cross axes of spherulites in the blend were seen to rotate with respect to temperature of crystallization (Tc). For simplicity, PHB/PMA (60/40) was chosen as a representative composition showing the Maltese cross rotation from the negative-type spherulite at Tc = 60 °C to the positive-type spherulite at Tc = 90 °C (i.e.,

Figure 2. POM graphs of PHB/PMA (60/40) blend crystallized at various Tcs: (a) 60 °C, (b) 65 °C, (c) 70 °C, (d) 75 °C, (e) 80 °C, and (f) 90 °C. 5626

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birefringence, which is completely opposite to the birefringence-type of neat PHB crystallized at the same Tc. However, when Tc imposed on the PHB/PMA blend to crystallize is increased gradually from 60 to 90 °C, the Maltese cross axes of the spherulites starts to rotate, from negative- back to positivetype. The degree of rotation is roughly proportional to the temperature of crystallization imposed on the blend samples. On average, for Tc = 60−75 °C, the cross-axis rotation degrees are averaged at 4° angle for every degree Celcius increase in Tc. For the range of Tc = 75−90 °C, the Maltese cross axis rotation angle degrees are 2.2° for every degree Celsius increase in Tc; for Tc = 90 °C or higher, the rotation of Maltese cross axis stops and the spherulites remain as the usual positive-type birefringence. Thus, the above results have shown that the Tc effects on birefringence are dependent on a critical content in the PHB/ PMA blend. This fact suggests that the mechanism driving the birefringence changes of the PHB spherulites in PHB/PMA blend may be dependent on the blend composition, which influences the phase behavior of the mixtures. Phase separation can lead to formation of discrete PMA domains in the crystallizing mixture, which in turn affects the PHB crystal packing and orientation in PHB/PMA blend and leads to alteration in the birefringence pattern with rotating Maltese cross. For a quantitative measure of Tc’s effect on axes rotation, the rotation (of the Maltese cross of the spherulite) angle versus Tc was plotted and shown in Figure 3. Generally, at low Tc (≤60

Figure 4. AFM phase images (8 μm × 8 μm) of PHB/PMA (60/40) blend crystallized at (a) 60 °C, (b) 65 °C, (c) 80 °C, and (d) 90 °C. White arrow represents the radial direction of each sample.

the AFM at greater resolution, the sample shows that there are numerous dot-like domains almost uniformly distributing throughout the viewing range. The diameter of each domain is less than 1 μm. Surrounding the dot-domains are apparently flat, with no special crystal patterns/arrangements. By comparison, for the sample crystallized at Tc = 65 °C (Figure 4b), the earlier POM result (Figure 2b) shows that it exhibits an axis rotation of ca. 18° angle from the original negative-type birefringence to become the “unusual” Maltese cross type (neither negative nor positive). The only difference is that at this higher Tc (65 °C), percentage area covered by dot-like domains is less than that crystallized at 60 °C. With the increasing Tc to 80 °C, in POM (Figure 2e) the spherulite shows larger rotation to nearly a positive-type spherulite and in AFM (Figure 4c) the number of domains is much fewer, more sporadically, and much less uniformly distributed within the spherulites. At Tc = 90 °C, the spherulite Maltese cross has rotated a full 90° angle (in comparison to those crystallized at 60 °C) and become a neatly positive-type spherulite (Figure 2f). The corresponding AFM image (Figure 4d) shows that the dot-like domains completely disappear from the spherulites and only flat-on lamellae stack on each other to form a terrace-like microstructure. The film thickness of most samples of the PHB/PMA blend was kept at 12 μm; however, for SEM characterization, the film thickness was increased three times to 35 μm for viewing on the cross section of fractured samples. The objective was to explore the interiors of the crystallized blends. It should be mentioned that the film thickness had no effect on altering the behavior of spherulite axis rotation with respect to crystallization, which had been confirmed by preparing and characterizing POM on thicker samples. That is to say, the morphology of cross section of 35 μm blend samples represents that of films of 12 μm. As a matter of fact, the axis rotation in spherulites with Tc had been proven to be true for films of thickness of 35 μm down to as

Figure 3. Quantitative plots of the rotation angles (of Maltese cross of the spherulite) vs temperature of crystallization (Tc) for four compositions of PHB/PMA blend: 70/30, 60/40, 50/50, and 40/60 as indicated on the graph.

°C), the spherulites are negative-type; at higher Tc (≥90 °C), the Maltese cross of the spherulites rotates with the increasing Tc to eventually a positive type. For the purpose of showing the correlations, the position of Maltese cross axes in the negativetype spherulite was fixed as “0°” (zero degree), and the degree of rotation of the axes was measured as clockwise (from initial negative-type to final positive-type). Figure 4 shows AFM morphology for the representative PHB/PMA (60/40) blend crystallized at various Tc’s. The topsurface morphology was presented as phase image AFM (8 μm × 8 μm). Figure 4a shows the phase-image result for sample crystallized at Tc = 60 °C which is shown as a negative-type spherulite in earlier POM (Figure 2a) result. Corresponding to 5627

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low as 2 μm. Figure 5 shows SEM graphs for the cross sections of PHB/PMA (60/40) blend crystallized at Tc = 60 °C. Figure

these pebble-like phase domains were of amorphous PMA aggregation and not PHB crystals. Figure 6 shows SEM graphs for the fractured cross section of PHB/PMA (60/40) blend crystallized at Tc = 70, 80, and 90 °C

Figure 5. SEM micrographs of fractured surfaces of PHB/PMA (60/ 40) blend fully crystallized at 60 °C: (a) unetched-sample and (b) pdioxane-etched-sample [thickness = 35 μm].

5a is the SEM graph for the cross section of the unetchedsample of the blend. The arrow marks the exact location of pebble-like spherical domains (larger than those dot-like domains in thinner samples, with size ranges from a few nanometers to 5 μm). The large size range may be associated with the fact that these spherical pebbles-like domains might have been imbedded in the blend matrix to various extents, leading to results that some domains appear to be larger than the others. In comparison to the top surface, the domain underneath the surface (i.e., bulk interior) have a slightly larger fraction. Like icebergs floating on the ocean, some larger portions of the domains were buried underneath the top surface. The SEM characterization was on the fractured cross sections, and was able to reveal what was buried underneath. One major issue may be related to answer whether the pebble-like domains were crystals (PHB) or amorphous aggregations (PMA). To answer this critical question, one technique of preferential etching was used. PHB could not dissolve in p-dioxane, but PMA is easily dissolved in it. Therefore, p-dioxane was used to etch the crystallized-samples of the blend. Figure 5b shows now the pebbles disappear in the etched-sample; instead, many pin-holes (spherical cavities) are visible. These cavities in the etched-sample happen to be in the same range of sizes as the original pebble-like domains in the unetched-sample. Thus, we could easily reach a conclusion that

Figure 6. SEM micrographs of fractured surfaces of unetched PHB/ PMA (60/40) blend crystallized at: (a) 70 °C, (b) 80 °C, and (c) 90 °C [thickness = 35 μm].

[thickness = 35 μm]. The morphology for these samples clearly demonstrates that the fractions of the pebble-like domains are dependent on Tc at which for the PHB/PMA blend was crystallized. Pebble-like domains here refer to the amorphous PMA phase which was discretely rejected into isolated pebblelike islands observed on the fracture surface of crystallized bulk samples. As the PHB/PMA blend was held at lower Tc (70−80 °C), the isolated pebble-like domains of PMA within the PHB crystal lamellae are still apparent; however, PMA domains in PHB crystal become less isolated or more assimilated when the blend is crystallized at higher Tc. These discrete domains 5628

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disappear or diminish significantly and become a more homogeneous (less phase-separated) phase in the PHB/PMA (60/40) blend if crystallized at 90 °C. Thus, the isolated pebble-like domains were hardly distinguished or seen in the fracture surface of PHB/PMA blend sample crystallized at 90 °C. Quantitative estimations of area ratios of dot-like domains in the entire viewing ranges on the samples were performed. From the AFM results shown earlier in Figure 4, the areas of the dotlike domains (summed over all visible dots) divided by the entire viewing area of the sample were calculated and the results are shown in Figure 7. The plots show that for PHB/

Figure 8. Dependence of rotation angle of Maltese cross axis of crystallized spherulites and area fraction of dot-like phase domains in entire blend.

Lamellae structure of PHB and their variation in PHB/PMA blend crystallized at various Tc may be related to their thermal behavior. Thermal behavior of PHB crystals in PHB/PMA (60/ 40) blend crystallized at various Tc was then examined by using DSC and the results of PHB/PMA were compared to that for neat PHB in Figure 9. Figure 9a shows DSC traces of neat PHB crystallized at 60−90 °C. From the traces for neat crystallized PHB, double melting is clearly seen, with a quite prominent first peak (P1 = 162 °C), followed by a second peak (P2 = 172 °C), which is a behavior quite well-known to be associated with either a dual-lamellae distribution or melting-recrystalllization of thinner lamellae into thicker ones. Such multiple melting

Figure 7. Dependence between the area fraction of dot-like domains (divided by entire area) and crystallization temperature (Tc).

PMA (60/40) blend samples crystallized at 60 °C (negativetype spherulite), the area of dot-like domains per unit sample area occupies the highest ratio (ca. 35%). At Tc = 60 °C, almost all PMA was rejected into isolated dot-like domains. With the increase of Tc on the blend samples, the area ratios occupied by the dot-like domains in the crystallized blend samples rapidly decrease. For the PHB/PMA (60/40) blend sample crystallized at 90 °C (spherulites are in positive-type birefringence), no dotlike domains are visible on the top film surfaces as viewed in AFM graphs; thus, the area ratio of pebble-domains is considered to be zero. Similarly, Figure 8 shows the dependence between the rotation angle of Maltese cross and the fraction of dot-like domains in the PHB/PMA blend (60/40) crystallized at various Tc. The axes of Maltese cross in PHB negative-type spherulites crystallized at 60 °C was taken as the comparison basis (zero degree). The dot-like domains corresponding to the negative-type spherulites occupy 33−38% of the entire area observed. With an increase in Tc and a decrease in the area ratios of the pebble-like domains, the axes of the Maltese cross in spherulites start to rotate (i.e., transition from negative type to positive type). For the blend crystallized at 90 °C (or higher), the Maltese cross of spherulite rotates a full 90° to become a positive-type spherulite. The plots show that the axis rotation angle increases almost linearly with the area ratio of the pebble-like domains in the entire blend samples. This correlation clearly proves that the presence of the isolatedpebble-like domains (of PMA aggregations) in PHB/PMA (60/ 40) blend do influence the spherulites’ Maltese cross rotation, leading to eventual reversion between birefringence types.

Figure 9. DSC traces of (A) neat PHB, and (B) PHB/PMA (60/40) blend crystallized at various Tc’s as indicated on the trace [heating rate = 20 °C/min]. 5629

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Table 2. Enthalpy changes of the first and second melting peaks of neat PHB and PHB/PMA (60/40) blend crystallized at various Tc, obtained from Gaussian peak fitting of DSC traces neat PHB

60PHB/40PMA

Tc (°C)

P1(ΔH, J/gPHB)

P2(ΔH, J/gPHB)

P1:P2

P1(ΔH, J/gPHB)

P2(ΔH, J/gPHB)

P1:P2

160 165 170 175 180 190

15.26 − 18.84 − 19.58 33.06

73.12 − 62.75 − 61.67 48.51

1:4.8 − 1:3.3 − 1:3.1 1:1.5

0 6.78 9.27 9.1 11.02 −

81.92 77.98 83.47 66.78 67.63 −

− 1:11.5 1:9.0 1:7.3 1:6.1 −

texts. The numerical values of these SAXS data (L, lc, la) for the lamellar dimensions and the linear crystallinity in the PHB/ PMA blend are summarized and listed in Table 3. By

peaks (P1 and P2) are typical in many polymers in DSC scanning. The former peak (P1) becomes larger whereas the latter peak (P2) becomes smaller with increasing Tc. For comparison between neat PHB and PHB/PMA blend, Figure 9b shows the DSC traces of PHB/PMA (60/40) samples crystallized at 60−80 °C (which show different crystalline morphology from that of the neat PHB). The melting behavior of crystallized PHB/PMA blend is found to be also different from that of neat PHB. Interestingly, P1 does not show up in a PHB/PMA (60/40) blend crystallized at 60 °C, which displays negative-type spherulites under POM. Although P1 appears in the blend of PHB/PMA (60/40) crystallized at 65 to 80 °C showing unusual-type spherulites, the P1 peak is comparably smaller than that of the neat PHB positive-type spherulites. The quantitiative values of melting enthlapies of P1 and P2 after normalization to the weight of only PHB and their comparisons are listed in Table 2. These thermal results indicate that the addition of amorphous PMA to some extent can impede the formation of thinner lamellae with lower melting temperatures; in the mean time, enhance the formation of thicjer lamellae with higher melting temperatures. Apparently, these lower-melting-temperature lamellae orientatation in PHB/PMA blend crystallized at low Tc (