Interior Dissection on Domain-Dependent Birefringence Types of Poly

Dec 30, 2016 - The mechanisms and correlations between the rotation of Maltese cross axis, optical birefringence types, and lamellar reassembly induce...
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Interior Dissection on Domain-Dependent Birefringence Types of Poly(3-hydroxybutyrate) Spherulites in Blends Eamor M. Woo,* Wan-Ting Tsai, and Graecia Lugito Department of Chemical Engineering, National Cheng Kung University, Tainan, 701-01, Taiwan S Supporting Information *

ABSTRACT: The mechanisms and correlations between the rotation of Maltese cross axis, optical birefringence types, and lamellar reassembly induced by phase-separation domains, in poly(3-hydroxybutyrate) (PHB)/poly(1,4-butylene adipate) (PBA). In this work, a novel approach via interior dissection into spherulites in correlations with the top-surface morphology of the PHB/PBA (50/50) blend crystallized at a series of Tc’s with various domain sizes. The PHB/PBA blend is used as the model system in comparison to alternative blends of PHB with other polyesters of various extents of phase-separated domains. Domains of optimal sizes were found to be essential in lamellar reassembly, which strongly affects the birefringence types of spherulites. Via the proven approaches, this work expounds how domain sizes and lamellar reorientation in partially miscible blends cause changes in the birefringence types of PHB spherulites, while, in contrast, complete miscibility or extreme phase separation with large domains does not.

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INTRODUCTION

Generally, at the same crystallization temperature (Tc), thermodynamic equilibrium usually dictates that neat polymers crystallize into spherulites of only a single birefringence type; however, blends of two polymers in some circumstances may crystallize into spherulites of two different birefringence types. Wang et al.12 blended syndiotactic polystyrene (sPS) with atactic polystyrene (aPS), and found that the sPS/aPS blend may display simultaneously positive- and negative-type spherulites when crystallized at a certain Tc, although neat sPS can only pack into positive-type spherulites. They also reported that the positive-type sPS spherulites grow twice as fast as the negative ones. After solvent etching out the amorphous aPS from the crystallized sPS/aPS blend, the transmission electron microscopy (TEM) and scanning electron microscopy (SEM) results revealed that the lamellae in positive-type sPS spherulites are packed in the radial direction; by contrast, the lamellae in the negative-type spherulites are mostly in the tangential directions. Such differences in the lamellae orientations are responsible for the two times difference in the spherulites growth rates and the different optical birefringence types in POM micrographs. Reversed behavior of lamellae orientation−birefringence correlation was reported by Han et al.,13 who investigated blends of poly(ethylene oxide) (PEO) with amorphous poly(methyl methacrylate) (PMMA). The blend displays negative-type spherulites if crystallized at higher Tc’s but positive-type ones if crystallized at lower Tc’s. Further using tetrahydrofuran (THF) to etch out the amorphous PMMA

Neat poly(3-hydroxybutyrate) (PHB) crystallizes at any Tc’s into positive-type birefringence spherulites. Nevertheless, as it is blended with other polymers and PHB in the blends may crystallize into either positive or negative type birefringence or intermediate types (i.e., unusual types) in-between positive and negative. The mechanisms behind such peculiar changes of optical birefringence in the PHB spherulites have remained to be puzzling. Crystalline morphology and lamellar assembly in polymer spherulites have been widely investigated in the literature1−6 using a variety of microscopy. When viewed under a polarized light microscopy, the polymer spherulite, in 2D thin film sample, appears as an optically birefringent circular crystalline entity with two perpendicular refractive indices: radial (nr) and tangential (nt). A Maltese-cross extinction usually appears in spherulite as a result of polarizer and analyzer mutual cancelation. Occasionally, the Maltese-cross is not vertically−horizontally aligned, but rotate a θ-angle with respect to the original position, then the spherulite is further classified as an unusual-type.7−11 A negative-type spherulite is defined as one that has nr < nt, which is oppositely distinguished from a positive type with nr > nt (i.e., nr − nt = positive). In classical textbooks, a general concept of the optical ellipsoid is commonly used to explain the three types of optical birefringence: long axis of the ellipsoid in radial direction corresponds to a positive type; long-axis in tangential to a negative type; long axis at an angle of θ to radial direction leads to an unusual type. With a tint plate, it is even more convenient to classify the spherulites as positive-type or negative-type.12−15 The differences among them have been amply discussed in the relevant literature cited above. © XXXX American Chemical Society

Received: October 24, 2016 Revised: December 6, 2016

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positive-type and unusual types. They found that the heat of fusion (ΔHm), the degree of crystallinity (xc), and melting point (Tm) for usual-type PBT spherulites are higher than those of the unusual ones. Later, Tsuji et al.11 using TEM confirmed that all three kinds of spherulites (positive, negative, and unusual-types) are made up of only the α-modification crystal lattices, but the growth for positive type, negative type, and unusual type are along the [010]*, [1−11]*, and [2−10]* directions in the reciprocal lattice, respectively. This result suggests that the dominant lamellae are oriented to different directions in the PBT spherulites of different optical types. There are ample literature reports that have found unusual-type spherulites in other polymers containing a terephthalate group, such as poly(pentamethylene terephthalate) (PPT),16 poly(hexamethylene terephthalate) (PHT),17 poly(heptamethylene terephthalate) (PHepT),18 poly(octamethylene terephthalate) (POT),19 or poly(nonamethylene terephthalate) (PNT),20 etc. Although the birefringence types (negative, unusual, and positive) are much easier to be observed under polarized microscopy with a tint plate and classified among the ringless Maltese-cross spherulites, these birefringence types are also displayed in ring-banded spherulites with notable alternating birefringent rings. It is worthy to note here that poly(ethylene adipate) (PEA), exhibits double-ring-banded spherulites with alternating birefringence patterns when crystallized at Tc = 28 °C.21,22 Interior dissection was performed to analyze the crystal lamellae in the double ring-banded spherulite of neat poly(ethylene adipate) (PEA). The alternating positive− negative birefringence (or orange−blue interference color bands with first order tint plate) in PEA spherulites is delicately understood to correspond to the interior crystal lamellae assembled as a corrugate-board structure of mutually perpendicular radial-tangential repetitions, where the positivering is related to a tangential-lamellae layer and the negativering is related to a radial-lamellae layer. Such interior dissection on ring-banded PEA spherulites has cast two pieces of critical evidence: (1) the lamellae in ring bands may be discrete and not continuous, and (2) the birefringence types (negative, unusual, and positive) in spherulites can be subtly related to the interior lamellae assembly modes of spherulites. Previous investigation on PHB blended with an amorphous polymer14 revealed that phase behavior of the PHB/amorphous polymer blend at various Tc’s might induce the originally positive-birefringence PHB spherulites to change to an opposite type of birefringence. This work aimed to further advance this critical scientific frontier of inherent interpretations with universal scopes on how and why polymer spherulites may display intriguingly various birefringence types (negative, unusual, to positive) and gradual rotation of Maltese axis with respect to blending composition, Tc, and phase behavior, and domain sizes, etc. that have been imposed during the growth formation of the spherulites. With crystalline/crystalline blends, such as PHB/PBA, PHB/poly(trimethylene adipate) (PTA), or PHB/PEA, the effect of superimposed crystallization of the second component on the optical birefringence of PHB spherulites needs further analyses. PHB displays various extents of phase behavior with homologous series of other semicrystalline polyesters [PBA, PEA, or PTA] from full homogeneity, partial miscibility, to phase separation leading to various domain sizes in the crystallized PHB spherulites. This work extends the previous investigations from a PHB-amorphous system14 as governed by Tc effects to several PHB−crystalline systems to explore in broader scopes of possible influences

from the crystallized PEO/PMMA blend for SEM characterization, they found that long fibrillary crystals in the radial direction are in the negative-type PEO spherulites. By contrast, the crystals lamellae in the positive-type PEO spherulites are mostly aligned as alternating concentric ring patterns that are perpendicular to the radial direction (i.e., crystals are in the tangential direction). They proposed that if phase separation is weak, then the PEO lamellae are normally grown in the radial direction within spherulites to have the normal negative-type birefringence. In those analyses, they examined only the top surface of thin-film sample but did not probe the interior lamellae of these spherulites of different birefringent types. An earlier pioneering study14 has pointed out some preliminary clues that crystallization-induced phase separation in PHB blended with amorphous poly(methyl acrylate) (PMA) produces pebble-like PMA domains, which are responsible for the birefringence types of spherulites in crystallized blends with respect to composition and temperature. Neat PHB spherulites are of a positive-type birefringence; PHB/PMA blend with various degrees of phase separation revert the original positivetype birefringence gradually to unusual-, and finally negativetype. This transition in the PHB/PMA blend (going from original positive to negative) is reversely opposite to that in the PEO/PMMA blend (going from original negative to positive types). To further understand the true mechanisms, not only the top surfaces of crystallized blends but also the interiors of phase structures have to be examined. It should be noted that phase separation in blends is not a prerequisite for the temperature dependence of birefringence types of polymer spherulites. Birefringence types of spherulites in blends of two polymers may be influenced by the interplay of crystallization and phase separation; nevertheless, spherulites in neat polymers crystallized from a homogeneous molten phase may also exhibit various types of birefringence in spherulites. Poly(ethylene adipate) (PEA), as an example, is known to display positivetype, negative-type, or ring-banded spherulites with regard to Tc. Its miscible blend with amorphous phenoxy may also display similar temperature-dependence of birefringence. Without dissecting the interior and only by observing the top-surface morphology in thin-films of poly(ethylene adipate) (PEA) spherulites in PEA/phenoxy blend crystallized at various Tc’s (0−40 °C) using atomic-force microscopy (AFM), Lugito and Woo15 expounded in details the lamellar assembly responsible for the positive- and negative-type birefringence in PEA crystallized at various Tc’s. Negatively birefringent PEA spherulites (orange color in vertical quadrants) are typified with edge-on lamellae (mixed with a smaller fraction of flat-on platelets), which are arranged mostly in the radial direction. In contrast, positive-birefringence PEA spherulites (blue color in vertical quadrants) are typified with edge-on lamellae which bent or coiled from radial to tangential direction. Regarding the unusual Maltese-cross spherulites in polymers, one of the most noticeable examples is poly(butylene terephthalate) (PBT).7−10 TEM and SEM characterizations on PBT spherulites have revealed that the differences between the usual-positive-type and unusual-type spherulites are the inclination of lamellae plates in spherulites. Usual-positive-type spherulites have lamellae packed mainly in the radial or tangential directions. If the lamellae plates in spherulites are oriented at a slant angle with respect to the radial or tangential directions, the assembly leads to the unusual type. Ludwig and Eyerer9 in 1988 using SEM and DSC to study top-surface morphology and thermal behavior of PBT spherulites of usual B

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structure in the fractured and top surfaces, respectively. Samples were coated with gold vapor deposition using vacuum sputtering (2 mA, 20 × 30 s) prior to SEM characterization. Differential scanning calorimeter (DSC Diamond, PerkinElmer Corp., USA), equipped with an intracooler, was used to measured thermal behavior and crystallization kinetics. During the thermal treatments, a continuous flow rate of nitrogen in the DSC chamber was maintained to prevent sample degradation. Inside the DSC chamber, each sample was melted at Tmax = 190 °C for 2 min and then was rapidly quenched to a certain crystallization temperature for complete crystallization of PHB before reheated to 210 °C with heating rates of 20 °C minute−1 to study the melting behaviors. Wide-angle X-ray (WAXD) 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 Tc’s, were prepared and characterized. For analyzing the lamellar structures, small-angle X-ray scattering (Bruker SAXS Gmbh, Karlsruhe, Germany) measurements were performed at the beamline IμS. X-ray radiation source of energy of 50 kV and a detector VANTEC-2000 were used to collect 2D SAXS patterns. The distance from the sample to the detector was 3542 mm. The scattering vector, q (q = 4π/λ[sin θ]), with scattering angle θ, in these patterns, was calibrated with silver behenate. After the background subtraction and data reduction, 1D SAXS profiles were obtained with relative intensity (Iq2) distributions as a function of q.

from phase behavior and postcrystallization of the second component in blends, in addition to the Tc effects. The polymer spherulite optical birefringence is governed more by the majority interior crystal assembly than the superficial topsurface lamellar morphology; thus, characterization only on the top surfaces of spherulites may miss some critical clues for interpretations of mechanisms. This study explored it in details of the birefringence types (neg., unusual, and positive types) as influenced by Tc, phase morphology, and postcrystallization of second semicrystalline components in the blends by dissecting the interior of lamellae in the PHB spherulites to provide better interpretation of mechanisms of rotation of the birefringence types. Via such a novel approach on interior lamellae and phase domains, the crystal assembly under influence of various phase behavior in the crystallized blends and Tc dependence of the changes in birefringence types of PHB spherulites were analyzed.

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EXPERIMENTAL SECTION

Materials and Preparation. Poly(3-hydroxybutyrate) (PHB) was obtained from Polysciences, Inc. (USA), with a molecular weight (Mw) 55 800 g mol−1, polydispersity index (PDI) 1.45, glass transition temperature (Tg) 1.5 °C, and melting temperature (Tm) 172 °C. Poly(butylene adipate) (PBA) was supplied by Aldrich Chemical Co., Inc. (USA), with Mw = 12 000 g mol−1, PDI = 1.3, Tg = −68 °C, Tm = 54 °C. Blend of PHB with PBA was the main subject of study in this work. Thin-film PHB/PBA blend samples (8−15 μm in thickness) were prepared by dissolving two polymers in common solvent of chloroform (∼4 wt %), well stirred, and cast on glass slides. Then, the cast films on glass were air-dried on a hot plate kept at 45 °C for 24 h to remove the solvent. Other polyesters, poly(trimethylene adipate) (PTA) or poly(ethylene adipate) (PEA), were also used for blending with PHB for comparative studies. PTA was obtained from Scientific Polymer Products (SP2), Inc. (USA), with Mw = 8900 g/mol, PDI = 1.28, Tg = −60 °C, Tm = 38 °C. PEA was from Sigma-Aldrich Inc. (USA), with Mw = 10 000 g/mol, PDI = 1.30, Tg = −50 °C, Tm = 45 °C. Poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) (PHBV) from Aldrich Chemical Co. Inc. (USA), with 12 wt % HV, Mw = 39 000 g mol−1, PDI = 1.48, Tg = −10 °C, Tm = 146 °C was blended with PBA also for a comparative study. The PHB/polyester blend samples were crystallized via a stepwise crystallization, where PHB would crystallize first at Tc’s = 45−110 °C and PBA (or other polyesters) would postcrystallize (second step) on the prior Tc-crystallized PHB lamellae as templates at ambient temperature (∼28 °C). Thermal treatments on PHB/PBA blend (or similarly on other blend systems) were as follows. Sample films were heated to Tmax (190 °C) for 2 min to melt prior nuclei, quickly replaced to a hot stage set at a predesignated Tc for full crystallization. For evaluating the effect of melt time at Tmax, longer times at tmax (10 or 15 min) were also used. After full crystallization at Tc’s, a set of samples were fractured into halves and characterized with no etching treatments; for comparison, another set of samples were fractured and immersed into N,N-dimethylformamide (DMF) to etch out the PBA component from PHB lamellae (as PHB is not soluble in DMF). For blends of PHB with other polyesters (for comparisons), preparation and thermal treatments on samples were similar. The etched samples were cleaned by deionized water to strip off DMF and were dried properly in a vacuum oven, prior to characterization. Apparatus and Procedures. 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 with procedures described in the previous section. Scanning electron microscopy (SEM, FEI Quanta-400 F) characterizations were applied on the fractured surfaces and top surfaces of PHB/PBA blend samples crystallized at Tc’s for revealing the lamellar

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RESULTS AND DISCUSSION Transition of Birefringence Types Induced by Phase Separation. Neat PHB spherulites display only the characteristic positive-type birefringence at all Tc’s from 45 to 110 °C as shown in Figure S-1. Within the higher Tc range (90−110 °C), ring-band patterns appear along with the positive birefringence. For PHB/PBA blend, however, the birefringence type of the PHB spherulites depends on the temperature of crystallization (Tc’s) as shown in Figure S-2; although at Tc’s = 45−110 °C, only PHB could crystallize while PBA remains molten and amorphous. Thus, the birefringence types revealed in POM graphs obtained at these various Tc reflect only the crystalline PHB spherulites without any contribution from the molten PBA. For a summary comparison of the observed changes of birefringence types with Tc, Figure 1 shows schemes of birefringence patterns of neat PHB (scheme a) in comparison with PHB/PBA (50/50) blend (scheme b). Spherulites of neat PHB are always of positive birefringence; in contrast, the PHB/ PBA (50/50) blend display a systematic transition of birefringence types from negative type at low Tc’s (45 °C) to unusual type at intermediate Tc, then finally to positive type at high Tc (90−110 °C). It should be noted that the transition of birefringence of PHB spherulites with Tc from negative to positive type is via a gradual change of “unusual type” and not abruptly from negative to positive (or vice versa). The unusual type exists in a narrow range between Tc = 50−60 °C. Within this range, the axis of the Maltese cross was found to rotate gradually with either increase or decrease of Tc. Nevertheless, for Tc equal to or lower than 50 °C, the PHB spherulites remain to be of negative type; for Tc equal to or higher than 60 °C, the PHB spherulites remain to be of a positive type and do not change with respect to Tc. The Tc dependence of the optical birefringence of PHB spherulites suggests that the interior lamellar structures (orientations, thickness, phase domains, etc.) in spherulites of the PHB/PBA blend might differ with a respect to crystallization temperature. In the following C

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°C (positive type) aiming to probe the possible mechanisms responsible for the noted transition or reversion of the birefringence patterns in PHB spherulites. Lamellar structures in spherulites might differ with respect to film thickness, in addition to Tc, melt time at Tmax, constituents in the blend, etc. Effect of film thickness was thus examined. Two different levels of film thickness were examined and compared: thin films of 1−2 μm vs thick films of 25−35 μm. In the Supporting Information, Figure S-3 shows POM micrographs for PHB/PBA (50/50) blend films crystallized at Tc = 45, 55, and 90 °C, respectively. Micrographs at the upper row are for the film thickness of 1−2 μm, and the graphs at the lower row are for thicker blend films of 25−35 μm. Apparently, the crystallized PHB spherulites at Tc = 45, 55, and 90 °C, in either thin or thick blend films have the same birefringence types at same Tc, negative, unusual, and positive-type, respectively. Note that owing to the thin films, the PHB/PBA blend (50/50) shows extremely large spherulites (up to millimeters in diameter) when crystallized at Tc’s; nevertheless, the birefringence patterns of spherulites at the same Tc are the same for thin or thick blend films. Effects of longer melt time at T max on spherulite birefringence were assessed. In some blends, it is known that phase behavior and spherulite patterns may be influenced by melt time held at Tmax prior to quenching to crystallization, such as poly(L-lactic acid) (PLLA)/PBA blend.23 For the PHB/ PBA blend, one also needs to evaluate the effect of melt time at Tmax. Figure 2 shows POM micrographs for PHB/PBA (50/50) blend, after being melt at Tmax for 2, 10, or 15 min, crystallized at various Tc’s. Figure 2a shows results of PHB/PBA blend kept at Tmax for 2 min, then crystallized at Tc = 45, 55, and 90 °C,

Figure 1. Morphological birefringence diagrams of (a) neat PHB and (b) PHB/PBA (50/50) blend at specified Tc’s where first-step crystallization of PHB took place.

discussion, we focus on spherulites crystallized at three representative Tc = 45 (negative), 55 (unusual type), and 90

Figure 2. POM graphs of PHB/PBA (50/50) blend melted at different melting times (tmax) at Tmax = (a) 2, (b) 10, and (c) 15 min and then successively quenched to be crystallized at various Tcs. D

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Figure 3. WAXD intensity of (a) neat PHB and (b) PHB/PBA (50/50) blend crystallized at various Tcs and then successively crystallized at room temperature.

the PHB β-form crystal (planar-zigzag conformation). Gazzano et al.24 proposed that the different intensity ratios of diffraction planes are attributed to orientation differences in the PHB lamellae. For the PHB/PBA blend, as both components are crystalline, more care is needed for interpreting the WAXD peaks in the PHB/PBA blend. In addition to the assigned PHB peaks, for PBA, it is quite well-known that the diffraction signals at 2θ = 20.8° and 24.2° represent (110) and (020) planes, respectively, of β-form PBA crystals. Neat PHB crystal is known to have an orthorhombic lattice. For PHB, the intensity of (020) and (040) diffractions, as well as intensity for the β-form crystal, increase with Tc increase. Oppositely, the intensity of the (110) diffraction decreases with Tc increase. However, for the PHB/PBA (50/50) blend, the intensity increase of the (020) diffraction and decrease of the (110) diffraction with respect to Tc differ from those in the neat PHB. Intensity ratios at different Tc’s were thus calculated. At Tc = 45 or 55 °C, the neat PHB or PHB/PBA blend both show the ratio of I(020)/ I(110) = 1.0. However, at Tc = 90 °C, neat PHB shows a ratio of I(020)/I(110) = 1.6 in comparison to the PHB/PBA (50/50) blend’s ratio I(020)/I(110) = 4.0, suggesting likely stronger crystal orientation in the PHB spherulites of the crystallized PHB/PBA blend. Additional information from the WAXD result is that the diffraction peaks in the PHB/PBA blend samples appear to be sharper (narrower full width at half-maximum) than those in neat PHB indicating that the existence of PBA promotes the formation of larger accumulation of PHB crystallites. Note that there is no difference in the diffraction angles of the crystal lattices, other than the noted intensity ratio differences, suggesting that the birefringence reversion of the PHB spherulites in PHB/PBA blend is not attributed to different crystal lattice forms. DSC analysis was performed on PHB/PBA blend crystallized at various Tc’s for comparisons. Numerical values of DSC thermal events are listed in Table 1 for ΔHm,PHB and Tm,PHB. In general, Table 1 shows that the heats of fusion (ΔHm,PHB) and melting temperature (Tm) do not differ much among the negative-, unusual-, and positive-type PHB spherulites. The approximate 10% increment of PHB enthalpy of fusion in the

developed negative-, unusual-, and positive-types of birefringence of spherulites, respectively. The melt time at Tmax was increased to 10 min, and the results of PHB birefringence at various Tc’s are shown in Figure 2b. At Tc = 45 °C, the PHB spherulites in the PHB/PBA blend still retain the negative-type birefringence just like that of the same blend with melt time of 2 min at Tmax. Nevertheless, at Tc = 55 °C, the PHB spherulites display positive-type birefringence, which is entirely different from the same blend with 2 min at Tmax. For Tc = 90 °C, the PHB spherulites are of positive-type birefringence. This is to say, as the PHB/PBA blend is held at Tmax for a longer time (from 2 to 10 min), the birefringence reversion from negative to positive occurs at much lower Tc’s. With the melt time at Tmax further being lengthened to 15 min, the PHB crystallized at all three Tc’s (low-45 °C, medium 55 °C, and high 90 °C) are all of a positive type and no negative-type spherulites are present in any of blend samples crystallized at these three Tc’s ranging from low to high. (see Figure 2c). Apparently, not only Tc, but also melt time (tmax), can influence the birefringence types of PHB spherulite crystallized from the PHB/PBA blend. It may be speculated that both Tc and tmax can in different degrees govern the phase behavior and domain sizes as the PHB/PBA blend is brought from Tmax to Tc to grow crystals; in the meantime, phase separation takes place to display different extents/shapes of phase domains. From such assumptions, aims of probing the mechanisms of birefringence transition/reversion were directed to certain checkpoints. Thermal Behavior and Crystal Analysis of Lamellae Crystallized at Various Tc’s. WAXD measurements can reveal the crystal lattice dimensions in the PHB/PBA blend with positive or negative-type of birefringence. Figure 3 shows WAXD diffractograms for neat PHB and PHB/PBA (50/50) crystallized at various Tc’s. The literature results for neat PHB24−27 or PBA28−32 have provided some basic information. For PHB, the peaks at 2θ = 13.6°, 17.2°, 21.5−23°, 25.7°, and 27.4° are signals for PHB α-form crystals’ (helical-chain conformation) (020), (110), (101), and (111), (130), or (040) planes. A low-intensity peak at 2θ = 20° corresponds to E

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blends decrease with increasing Tc’s. For neat PHB, q = 1.12 for lower Tc (Tc = 45 °C), which gradually decreases to q = 0.78 for higher Tc (Tc = 90 °C). For the PHB/PBA (50/50) blend, q = 0.98 for samples crystallized at Tc = 45 °C to q = 0.74 for samples crystallized at Tc = 90 °C. From the Bragg’s law (long period = 2π/qmax), it is clear that both neat PHB and PHB/ PBA blend have longer long-period (L) with increasing Tc. This is quite typical for lamellae in most polymer spherulites. As the long period (L) is composed of two different contributions (crystalline and amorphous), the respective thickness (crystalline-phase thickness, lc) or (amorphous-phase thickness, la) has to be analyzed via one-dimensional correlation function. Parts a′ and b′ of Figure 4 show the correlation between (L), (la), and (lc = L − la) for neat PHB and PHB/PBA (50/50) blend, respectively. The numerical values are listed in Table 2. The

Table 1. Enthalpy Changes and Melting Peaks of Neat PHB and PHB/PBA (50/50) Blend Crystallized at Various Tc Neat PHB Tc (°C) ΔH(J/g PHB) Tm(°C)

45 89.4 174.9

50 89.0 173.2 PHB/PBA

55 60 91.5 86.8 174.5 173.1 (50/50)

70 87.5 172.6

90 86.6 173.8

Tc (°C) ΔH(J/g PHB) Tm(°C)

45 99.3 171

50 96.5 171.2

55 102.1 171.4

70 97.3 171.5 165.2

90 100.6 172.1 166.2

60 96.4 171.4 165.0

blend obtained from DSC characterization also supports that PBA promotes PHB crystallization in the blend. For neat PHB, the melting peak is at Tm,PHB = 173 °C (P1). With PBA blended into PHB, the melting peak is slightly lowered to Tm = 171 °C (P1). The heat of melting (ΔHm,PHB) slightly increases with the inclusion of PBA in the blend (after counting the weight fraction of PHB in the blend). For the PHB/PBA (50/50) blend, the locations of melting peaks do not differ much among the negative-, unusual- and positive-type PHB spherulites. In addition to the WAXD and thermal analysis as discussed, SAXS measurements were performed for comparing the lamellae dimensions in PHB/PBA that showed positive- vs negative-types of birefringence spherulites. Figure 4 shows the original SAXS profiles of neat PHB and PHB/PBA (50/50) crystallized at various Tc’s and calculated long-period lamellar thickness, respectively. Lorentz-corrected SAXS profiles in Figure 4a,b show that the peak positions of neat PHB or its

Table 2. SAXS Long Period (L), Crystalline-Phase Thickness (lc), and Amorphous-Phase Thickness (la) in Neat PHB and PHB/PBA (50/50) Blend [Heating to 70 °C] Tc (°C)

L (nm)

lc (nm)

la (nm)

neat PHB 45 55 90 45 55 90

5.50 3.10 5.80 3.32 7.40 4.51 PHB/PBA (50/50) 6.20 3.59 6.30 3.67 8.0 4.90

2.40 2.48 2.89 2.61 2.63 3.10

lamellar thickness values for neat PHB or PHB/PBA (50/50) blend show that the long period (L), crystalline lamellae thickness (lc), and amorphous layer thickness (la) all increase similarly with Tc increase. By comparing the SAXS data for the neat PHB and PHB/PBA blend (50/50), the rates of increase of L, lc, and la are similar, suggesting that absence or inclusion of PBA in PHB spherulites has a similar effect. These SAXS results are similar and in line with the lamellar analyses expounded earlier for PHB blends with an amorphous polymer.14 The WAXD and SAXS results collectively show that the birefringence reversion in the PHB/PBA spherulites crystallized at different Tc’s is not related to either crystal lattice forms or lamella thickness. Thus, alternative causes for the birefringence reversion should be probed. Lamellae and Phase Morphology in Correlation with Optical Birefringence. As discussed, neither lamellae thickness nor crystal lattices could explain the birefringence reversion in PHB/PBA blend with respect to Tc, alternative causes should be probed. Morphological evidence of PHB lamellae was pursued using AFM or SEM. For SEM characterization, thicker films (∼30 μm) of PHB/PBA blend were used. Note that film thickness in the range of 10−40 μm has been verified earlier to have similar spherulites morphology and birefringence behavior with respect to Tc. PHB/PBA blend films were crystallized at Tc = 90 °C and fractured for exposing the interiors, then were characterized using SEM; and the results are shown in Figure 5. Figure 5a shows SEM micrographs for as-crystallized (Tc = 90 °C) PHB/PBA (50/ 50) films that were fractured but unetched. Apparently, many pebble-like domains are dispersed in the fractured interiors of the crystallized blend. One needs to know if the dispersed domains belong to the PHB or PBA components. Etching with DMF on samples was performed. DMF is known to discretely

Figure 4. SAXS profiles of Lorentz-corrected intensity (top) and SAXS long period (L); crystalline-phase thickness (lc); and amorphous-phase thickness (la) (bottom) of (a, a′) neat PHB and (b, b′) PHB/PBA (50/50) blend crystallized at various Tcs [heating to 70 °C]. F

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PHB, the PHB lamellae can be preferentially oriented in going around or circumcising the existing pebble-like PBA domains. Furthermore, DMF-etched PHB/PBA blend samples crystallized at other Tc’s (45−100 °C) were similarly analyzed using SEM. To sum up, Figure 6a−d show pebble-like domains virtually in interiors (fractured surfaces) of all PHB/PBA samples crystallized at all Tc’s. The differences are on the size and density of domains, which vary with Tc’s. At lower Tc’s = 45 or 55 °C (Figure 6a,b), the crevices (or holes), representing the previous PBA domains, are tighter and smaller. At higher Tc’s = 90 or 100 °C (Figure 6c,d), the crevices in the crystallized/ etched samples are larger. In addition to the interiors, the top surfaces of PHB/PBA blend samples crystallized at various Tc’s were examined. A similar trend of variation of domains sizes with respect to Tc was also observed on the top surfaces of blend samples. Figure 6a′−d′ shows SEM micrographs for top surfaces of PHB/PBA (50/50) blend crystallized at various Tc’s (45−100 °C). At lower Tc = 45 °C (Figure 6a′), the size of crevices ranges from 0.1 to 0.9 μm. At intermediate Tc = 55 °C (Figure 6b′), the crevice size ranges 0.1−1.2 μm; at higher Tc = 90 °C (Figure 6c′), the crevice size is significantly larger to be 0.2−1.5 μm. At the highest Tc = 100 °C (Figure 6d′), the crevice size is the largest in the range of 0.2−2.0 μm. In general, the compactness of the crevices (representing the PBA domains) in the blend samples systematically changes with the increase of Tc, being from tighter compactness at lower Tc’s to a looser/larger structure at higher Tc’s. Temperature dependence of domain structures in immiscible or partially miscible blends is well-known in classical textbooks of polymer blends. The compactness and domain size of PBA in the crystallizing PHB/PBA blend at various Tc’s thus impose different extents of orientation preferences of PHB lamellae in packing into a spherulite, which ultimately leads to gradual shifting of birefringence types as observed. To sum up, Figure 7 shows a quantitative relationship between the size distributions of the sizes of PBA domains (etched out to become voids) in PHB/PBA (50/50) blend with crystallization temperature. At lower Tc’s (45−50 °C), the compactness of small-size PBA domains has the strongest tendency to reorient the PHB lamellae (by going around or circumcising the domains), leading to an opposite birefringence

Figure 5. SEM micrographs of fractured surfaces of PHB/PBA (50/ 50) blend fully crystallized at Tc = 90 °C and then successively crystallized at room temperature for (a) unetched samples and (b) DMF-etched samples.

dissolve only PBA but not PHB. Figure 5b shows the SEM micrographs for DMF-etched PHB/PBA blend. The holes correspond to the previous pebble-like domains, which are determined to be of the PBA component that has been etched out by DMF to leave crevices in the samples. Obviously, in the crystallized PHB/PBA blend, the PBA component forms the phase-separated discrete domains while the PHB component forms continuous phase where lamellae are located. Owing to the presence of PBA discrete domains during crystallization of

Figure 6. SEM micrographs of fractured interiors (top) and top surface (bottom) of DMF-etched PHB/PBA (50/50) blend crystallized at various Tc’s, (a, a′) 45, (b, b′) 55, (c, c′) 90, and (d, d′) 100 °C, and then successively crystallized at ambient (room temperature). G

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Figure 7. Relationship between the size of domains (crevice voids) and crystallization temperature (Tc) in PHB/PBA (50/50) blend.

type (negative). At high Tc’s (80−90 °C), the PBA domains are larger and more loosely dispersed in the crystallized PHB/PBA blend. Thus, the PHB lamellae are less likely to be reoriented by these looser domains, leading to that PHB lamellae retain the original birefringence type (positive). At intermediate Tc’s, the effect of PBA domains is expected to be average of these two tendencies, thus leading to an unusual birefringence in the PHB spherulites (intermediate between the positive and negative types). Neat PHB crystallized at any Tc = 45−90 °C is positive-type, where PHB lamellae are mostly packed in orientation for positive-type birefringence and not influenced by Tc; in contrast, the birefringence type in PHB/PBA blend depends on Tc. Owing to the blend’s partial miscibility between PBA and PHB, there is some amount of PBA chains dispersed in the intralamellae regions. In addition, the POM graphs already show that PBA domains are also distributed within the PHB spherulites, suggesting that PBA as discrete domains can interrupt the PHB lamellae. Schemes for lamellae orientations in birefringent spherulites are illustrated in Figure 8, which shows collective effects of PBA phase domains present interfering with the PHB lamellae. Such interference between the PBA domains and PHB lamellae depends on Tc, which eventually causes optical birefringence changes. Figure 8a shows the PBA domains of an optimal size distribution, being tiny but numerous (at lower Tc’s), have the highest tendency to reorient the PHB lamellae to deviate from the original packing. In such case, the PHB spherulites show negative-type birefringence (lower Tc’s = 45−50 °C). Figure 8b illustrates that as the PBA domains are gradually larger but of a fewer number (at intermediate Tc’s), the PHB lamellae are less influenced, and thus, the spherulites assume a transitional unusual type optical birefringence. Most of the PHB lamellae are partially reoriented to make a θ angle with the radial directions (as arrow-marked in Figure 8b). Figure 8c shows the most extreme case where the PBA domains are the largest and the numbers of domains are the least (at high Tc’s = 80−90 °C). In such condition, most of the PHB lamellae are undisturbed by phase domains and still packed in original orientation, retaining the typical positive-type birefringence of neat PHB. Effect of PBA Postcrystallization on PHB Spherulite Birefringence. The PHB/PBA blend contains two components that both are crystalline, with PHB crystallizing at Tc = 45−100 °C and PBA crystallizing at 20−33 °C, respectively.

Figure 8. Schemes of the relationship between phase-domain size and optical birefringence with three possible PHB lamellar orientations in spherulites: (a) negative type, (b) unusual type, and (c) positive type.

When held at Tc = 45−100 °C (first-stage), only PHB can crystallize but PBA remains molten, as discussed in previous sections. If the blend is further cooled to ambient temperature (25−28 °C), PBA in the blend can sequentially crystallize (second stage). A question then arises: will the ambientcrystallized PBA crystals influence the birefringence type of the PHB/PBA blend? The evidence is discussed as following. Figure S-4 shows POM micrographs for PHB/PBA (50/50) blend crystallized at Tc = 45−100 °C [graphs on left column], then cooled to ambient (28 °C) [graphs on right column]. For the blend crystallized in first-stage at high Tc = 90, 100, 110 °C, the birefringence of the spherulites, as discussed earlier, is all positive-type. However, interestingly, upon cooling to the ambient (28 °C) to allow PBA crystals to develop, these highTc crystallized positive-type spherulites become a negative type. For a more detailed analysis, the PHB spherulites were observed at three key stages: (a) Tc’s where only PHB crystallizes, (b) cooling at the ambient where PBA crystallizes in the second stage, and (c) reheating to 70 °C from the ambient to melt PBA. Figure 9 shows (a) POM graphs of PHB/PBA (50/50) blend: (i) crystallized first at three Tc’s (45, 55, 90 °C) for PHB crystallization, then (ii) successively cooled/crystallized at ambient for PBA crystallization, and (iii) finally reheated to 70 °C to melt the PBA crystals (PHB lamellae remain unmelted). Parts i and ii of Figure 9a show that for the PHB/PBA blend crystallized at low or intermediate Tc’s, the birefringence types are negative and unusual, respectively, and the PBA phase domains are less distinct with smaller sizes. Later crystallization of PBA on the PHB spherulites does not alter the original birefringence type. This is likely so because the negative-type or unusual-type PHB lamellae are more effective in serving as templates for PBA chains to pack into the original orientations of PHB lamellae, thus leading to unaltered birefringence types. By comparison, for PHB/PBA blend crystallized in the first stage at a higher Tc’s (90 °C as an example here), the second-stage PBA crystallization upon cooling to ambient apparently reverses the original positive H

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Figure 9. (a) POM graphs of PHB/PBA (50/50) blend: (i) crystallized first at Tc (45, 55, 90 °C), (ii) successively cooled/crystallized at room temperature, and (iii) reheated to 70 °C to melt the PBA crystals; birefringence variations diagram for (b) blend crystallized at Tc and cooled at ambient temperature, as well as after (c) reheating to 70 °C to melt PBA crystals.

birefringence type could be returned to the original one. Figure 9a-(iii) shows the Tc-crystallized and ambient-cooled PHB/ PBA blend, upon heating to 70 °C to melt the second-stage PBA crystals, retain the original birefringence of the PHB spherulites crystallized at Tc’s. The result indicates that the PBA crystals could be discretely removed without altering the

type to negative type. The positive-type PHB lamellae are less effective as templates for guiding PBA crystals; in addition, the larger extent PBA domains allow fuller-scale PBA crystals (negative type) to cover up on the PHB lamellae (positive type) to reverse the birefringence type. We could prove that upon removal of PBA crystals from the PHB spherulites, the I

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spherulites (positive), the birefringence patterns may be compounded. Phase-Separation-Induced Lamellar Distortion/Reorientation. As discussed, phase separation during crystallization and resulted domain size was supposed to be one of governing factors in reverse of the optical birefringence in crystallized spherulites in the blend. Overall, the comparisons of PHB/PBA blend to three other blend systems, PHB/PEA, PHB/PTA, and PHBV/PBA blends, could provide a critical piece of information for correlating the birefringence reversion with phase behavior (miscibility, partial miscibility, or immiscibility). First of all, the phase behavior (miscibility, partial miscibility, or immiscibility) of the PHB/PBA blend was investigated using thermal analysis. In the Supporting Information, Figure S-5a,b shows results of thermal analysis on rapidly quenched PHB/ PBA blend of various compositions for revealing Tg transitions in the blend. From the broadened, yet composition-dependent, Tg transitions, we confirmed that partial miscibility existed in the PHB/PBA blend, and such partial miscibility could display a limited extent of phase separation during crystallization. In addition, to reveal the melting behavior of the PBA crystals in separated domains of the blend, Figure S-5c shows DSC traces in the PBA crystals’ melting range (40−60 °C) for the PHB/ PBA (50/50) blend crystallized at Tc’s = 45−100 °C, then cooled to ambient for secondary crystallization. The DSC result shows the typical double melting peaks in PBA with P1 = 54.3 °C (higher peak) and P2 = 49.1 °C (lower peak). The PBA melting enthalpy ΔHm,PBA remains constant, regardless of variation in Tc’s. The PBA crystals, being molten at first-stage Tc’s and crystallized only at second-stage ambient temperature, are mostly confined in the discrete phase-separated domains, and thus the PBA melting behavior is not influenced by the first-stage PHB crystallization at Tc = 45−90 °C. Depending on the sizes and numbers of PBA phase-separated domains in PHB/PBA blend, the optical birefringence of PHB spherulites may change gradually from the original positive-type to negative-type. During the second-step PBA crystallization at room temperature following first-step PHB crystallization at Tc, the optical birefringence of PHB spherulites is altered from positive-type to become negative-type. It has been proved earlier that the PHB/PBA blends exhibit UCST with clarity points located at 178−180 °C for all compositions.33 As an example, Figure S-6 (Supporting Information) shows the in situ OM observation on PHB/ PBA (50/50) blend upon heating (rate 2 °C min−1) and quenching to crystallization temperature (Tc = 45 °C) as well as POM observation upon holding at Tc. The as-cast sample is filled with tiny spherulites; upon heating, the blend showed a gradual transition from heterogeneous (170 °C) to homogeneous phase (molten state at 190 °C) supported by the OM images. The blend remained in homogeneous phase even after quenching to 45 °C. Upon holding at 45 °C for a few seconds, PHB nuclei appeared, continued by the formation of phase domains. Further growth of PHB crystals generated microbubbles within the spherulites; a few tiny bubbles even merged together forming a larger bubble while the spherulite grew bigger. It is very likely that the crystals formation induced phase separation in the blends. However, further detail in situ experiment on the initial crystallization process is indeed necessary to justify the process, which would be further considered in the future study. Phase behavior and optical birefringence behavior of PHB/ PBA blend (partially miscible) were further compared with

birefringence types of PHB lamellae in the spherulites. The presence of PBA crystals in the second-stage crystallization did not alter the PHB lamellae packed at Tc’s. The ambient-cooling leads PBA to simply cover up on the prior PHB crystals, and the PBA crystals do not vary much in the thermal behavior, regardless PBA crystals being packed on the positive-type or negative-type PHB lamellae. Summary schemes in Figure 9b illustrate the final birefringence patterns of PHB/PBA (50/50) in first-stage crystallization at Tc’s (where only PHB crystallizes) and cooled to ambient for second-stage crystallization (where PBA crystallizes). For the blend crystallized at low Tc’s, the birefringence of spherulites remain as the original one upon cooling to ambient; that is, the second-stage crystallization of PBA does not alter the original birefringence (negative or unusual) types of PHB ringless spherulites. By contrast, for the blend crystallized at high Tc’s (positive-type PHB spherulites), the second-stage crystallization of PBA imposes reversion from positive to negative (which is the PBA’s birefringence type). Apparently, the ring-banded and positive-type PHB lamellae (at high Tc’s) in the first-stage crystallization are much easier to be covered by the negative-type PBA lamellae in the second-stage crystallization. The PBA crystals are of negative-type birefringence, and its crystallization on the positive-type PHB lamellae can overwhelm the PHB’s birefringence to display the negative type birefringence of the PBA crystals. The original ring-band patterns of PHB spherulites (at Tc = 90−110 °C) are also highly disrupted into ringless disorder with PBA postcrystallization. Subsequently, Figure 9c shows schemes illustrating that as the ambient-cooled PHB/PBA blend was reheated to 70 °C (PBA being a molten liquid, but PHB remaining as crystals), the birefringence type is immediately reverted back to be same as the blend held at respective Tc’s. It is worthy to compare the birefringence behavior of the crystalline/crystalline PHB/PBA blend in this study with the crystalline/amorphous PHB/poly(methyl acrylate) (PMA) blend system in an earlier study.14 Both systems display similar temperature-dependence of birefringence types with respect to Tc, with negative type at low Tc; unusual type at intermediate Tc, and positive type at high Tc’s. The similarity in these two blend systems is that both are phase-separated with partial miscibility and medium sizes of domains. This fact means that if these blends were either fully immiscible or completely miscible, then, they would not have exhibited the temperature-dependence transition or reversion of the birefringence types. Among the similarity, there are some subtle differences between these two blends. For a PHB/amorphous polymer blend (such as PHB/PMA), the birefringence type is fixed once crystallized at Tc and will not change upon cooling to ambient temperature. By contrast, for the crystalline/crystalline PHB/ PBA blend, ambient-cooling of second-stage PBA crystallization may or may not alter the birefringence types of PHB. Postcrystallization of PBA crystals on the PHB lamellae imposes a reversion from positive-type optical birefringence to negative type. For the negative- or unusual-type PHB, postcrystallization of PBA does not change the birefringence type of PHB; by contrast, for the ring-banded and positive-type PHB spherulites crystallized at high Tc’s (90, 100, 110 °C), postcrystallized PBA crystals are able to revert the PHB spherulite birefringence. In short, as the second crystallizable polymer component that has an opposite birefringence type (negative) in the blend further packs on the original PHB J

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dependent transition of the birefringence types. Figure 10 shows POM micrographs of partially miscible PHB/PTA (50/

those of three other blends: PHBV/PBA (miscible), PHB/PEA (immiscible), and PHB/PTA (partially miscible). PHB, upon being blended with other polyesters such as PEA, PTA, and PBA, respectively, displayed a varying trend of phase behavior in the blends. Thus, the transition of birefringence behavior in spherulites of the model PHB/PBA (50/50) blend was compared to blends of PHB with other polyesters, such as poly(ethylene adipate) (PEA) (immiscible with obvious phaseseparation domains), or poly(trimethylene adipate) (PTA) (partially miscible with PHB with small PTA domains). Note that PEA and PTA differ from PBA only by the number of methylene units in the main chains. It has been proved in the literature that PHB in blending with aliphatic polyesters displays a systematic variation trend from miscibility to partial miscibility and finally to immiscibility, depending on the structures of polyesters.33 PHB is known to be partially miscible (with small phase domains) with poly(trimethylene adipate) (PTA), similar to the partial miscibility in PHB with PBA, but PHB is immiscible with PEA with a UCST (immiscible at low temperatures, but becoming miscible at elevated temperatures) only at a high temperature.33 Alternatively, if PHB was replaced with poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) (PHBV) in blending with PBA, the PHBV/PBA blend has been known to be fully miscible, which differs from the partial miscibility in PHB/PBA. Poly(3-hydroxy butyrate-co-3-hydroxy valerate) (PHBV) is a copolymer based on PHB, thus, has a similar crystalline structure and same birefringence type (positive) as PHB; nevertheless, PHBV/PBA blend, unlike PHB/PBA, is a fully miscible blend. Thus, the fully miscible PHBV/PBA blend serves as a very informative comparison with the partially miscible PHB/PBA blend in terms of Tc-dependent birefringence types. PHBV was used to blended with PBA at 50/50 blending ratio and crystallized at the same Tc range (Tc = 45− 90 °C). Figure S-7 (Supporting Infomation) shows that the PHBV spherulites in the miscible PHBV/PBA blend crystallized at Tc = 45−70 °C (Figure S-7a−e) display only positivetype birefringence at all Tc’s. At Tc = 90 °C (Figure S-7f), the spherulites are difficult to judge, but no apparent change in the birefringence type is seen. There are no visual domains in the miscible PHBA/PBA blend, which is unlike the partial miscibility in the PHB/PBA blend. The result shows clearly that the birefringence types in the fully miscible PHBV/PBA blend remain positive at all Tc’s. On the other extreme of blend miscibility is immiscibility in blends. Figure S-8 shows POM results that the PHB/PEA (50/50) blend is completely immiscible with apparent large spherical domains. At Tc = 45 or 50 °C (Figures S-8a,b), the PHB spherulites are small and overly crowded with the birefringence types difficult to be judged; yet at Tc = 55 °C or higher (Figures S-8c−f), all spherulites are of a positive type. That is, for the completely immiscible PHB/PEA blend, it appears that the birefringence of PHB spherulites remains the same positive type at all Tc’s−just like the fully miscible PHBV/PBA blend, whose birefringence type remains to be positive and is not influenced by Tc’s at all. Between the full miscibility and complete immiscibility exists a range of “partial miscibility” in the PHB/polyester blends. The PHB/PTA (50/50) blend, known to be partially miscible− with its phase behavior similar to the PHB/PBA blend, was crystallized at various Tc’s and similarly characterized. Thus, again, the partially miscible PHB/PTA blend serves as a very informative comparison with the similarly partially miscible PHB/PBA blend in terms of evaluating the behavior of Tc-

Figure 10. POM graphs of PHB/PTA (50/50) blend crystallized at various Tcs: (a) 45, (b) 50, (c) 55, (d) 60, (e) 70, and (f) 90 °C. (g) Birefringence diagram of PHB/PTA (50/50) blend as a function of Tc.

50) blend at various Tc’s (45−90 °C) and their birefringence diagram as a function of Tc. The birefringence types and reversion in the PHB/PTA blend are very similar to those in the partially miscible PHB/PBA blend. The birefringence of the PHB spherulites in the PHB/PTA blend is of negative type at low Tc’s (Figures 10a,b), then to the unusual type (Figure 10c) at intermediate Tc, finally to the positive types at higher Tc’s (Figures 10d−f). Schematic for the birefringence types of PHB spherulites in the PHB/PTA (50/50) blend as a function of Tc are illustrated in the bottom graph of Figure 10g. The reversion of optical birefringence with respect to Tc’s in the PHB/PTA blend is exactly same as the PHB/PBA blend, suggesting that partial miscibility with a limited extent of phase separation (usually induced by crystallization) and optimal domain sizes are a prerequisite for birefringence reversion with respect to Tc. A critical finding can be obtained out of these comparative observations. Consistently, the results indicate that only the partially miscible PHB/PTA and PHB/PBA blends exhibit Tcdependent reversion of optical birefringence with respect to Tc’s. The two extremes of either large-scale immiscibility with large domains or complete miscibility with no phase domain K

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have exemplified the effects of phase domain sizes on possible lamellar reorientation.

leads to no reversion of birefringence types of the PHB spherulites in these blends. The PHBV/PBA is fully miscible with a homogeneous phase and no domains; the PHB/PEA blend is completely immiscible with grossly phase-separated large domains; both fully miscible PHBV/PBA and completely immiscible PHB/PEA blend systems do not display reversion of optical birefringence with respect to Tc’s. Large-scale immiscibility with large domains or complete miscibility with no phase domains leads to no reversion of birefringence types of the PHB spherulites in these blends. Thus, the PHB/PBA blend uniquely stands as an interesting model for probing the birefringence changes with respect to Tc’s. Apparently, neat PBA crystals are negative type in optical birefringence when crystallized at all Tc’s (20−35 °C); by contrast, neat PHB crystals are positive type in positive type at all Tc’s (45−110 °C). As PBA and PHB is blended as 50/50 blend, the birefringence types of spherulites in the PHB/PBA blend display interesting but complex phenomenon when crystallized stepwise in two temperature ranges. Within the range of Tc’s = 45−110 °C, the PHB spherulites in blend display a gradual transition of optical birefringence from negative- to positive type with respect to Tc’s. As the PHB/PBA blend (50/50) is crystallized at low or medium Tc’s (45, 55 °C), the optical birefringence of PHB spherulites in the blend are negative- or unusual-types. By contrast, as the PHB/PBA blend (50/50) is crystallized at high Tc’s (90−110 °C), the optical birefringence of PHB spherulites in the blend are positive and ring-banded. Crystallization of PHB in the partially miscible PHB/PBA blend at Tc = 45−110 °C induces different scales of phase separation of various PBA domain sizes, which act as barriers for the growing PHB lamellae to go around or circumcise the phase domains. Such phase-separated domains in the partially miscible blends can effectively deform the PHB lamellae to reorient to different directions (i.e, deflecting at θangle deviation from the original growth direction), leading to gradual but systematic transitions of the optical birefringence of PHB spherulites with respect to Tc’s.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02306. POM graphs, DSC thermograms, and in situ OM observations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.M.W.). ORCID

Graecia Lugito: 0000-0003-1785-8321 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. W.-T.T. (MS graduated student) conducted the experiments under guidance; G.L. (postdoc) searched references for background and produced and analyzed the data as well as helped writing part of the draft manuscript; E.M.W. (Professor) conceived the original research ideas, advised/ guided the experiments and analyses, wrote the main portion of the text, and further revised it for refinement and compactness. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by a basic research grant (MOST-105-2221-E-006-246-MY3) for three consecutive years from Taiwan’s Ministry of Science and Technology (MOST). This research was also partially supported by the Ministry of Education, Taiwan, R.O.C., in aim for the TopUniversity Project to National Cheng Kung University (NCKU).

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CONCLUSION This study has advanced the knowledge of how and why the optical birefringence of PHB spherulites may transform from original positive to negative type via gradual transitions of intermediate unusual types. By investigating a group of PHB blends with different polymers with various phase behavior (from fully miscible, to partial, to fully phase separated), this continuing work has in a greater scope proved that the phenomenon is not just incidental on one PHB blend system, but universal by investigating other blend systems that possess a wide range of phase behavior with different domain sizes upon crystallization. Only within a suitable range of phase-domain sizes can the effects of phase separation take place on inducing the rotation of Maltese cross axis from positive to negative types in the crystallized PHB spherulites. Birefringence reversion (from positive to negative type) in the polymer spherulites or gradual rotation of the Maltese cross axis does not occur in PHB blend systems that have either full miscibility (no phase domains) or extreme phase separation with grossly large domains. Only partial miscibility with an optimal range of domain sizes (usually induced by crystallization) in the PHB/ PBA blend and similarly in PHB/PTA blend is a prerequisite for birefringence reversion with respect to Tc. The results of interior dissection of the crystal assembly in PHB spherulites

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DOI: 10.1021/acs.macromol.6b02306 Macromolecules XXXX, XXX, XXX−XXX