Unusual Fractional Crystallization Behavior of Novel Crystalline

Nov 19, 2014 - Novel crystalline/crystalline polymer blends of biodegradable poly(ethylene suberate) (PESub) and biocompatible poly(ethylene oxide) (P...
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Unusual Fractional Crystallization Behavior of Novel Crystalline/ Crystalline Polymer Blends of Poly(ethylene suberate) and Poly(ethylene oxide) with Similar Melting Points Mengting Weng and Zhaobin Qiu* State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: Novel crystalline/crystalline polymer blends of biodegradable poly(ethylene suberate) (PESub) and biocompatible poly(ethylene oxide) (PEO) were prepared through a solution and casting method. The basic thermal properties, including both glass transition temperature and melting point, of both of the components are very close to each other. The two components are miscible as no obvious phase separation can be detected, forming novel miscible polymer blends of two crystalline polymers. PESub/PEO blends show two or three crystallization exotherms at different supercoolings. The complex crystallization behaviors are attributed to the occurrence of the fractional crystallization of the minor component of the blends. Depending on the blend composition, the major component of the blend crystallizes first and the minor component crystallizes later. During the crystallization of the major component, the amorphous minor component is completely included the interlamellar region of the major component or most of the amorphous minor component is expelled out of the interlamellar region while only a few is incorporated between the lamellae of the major component. In both cases, the fractional crystallization of the minor component occurs at large supercooling because of the confinement effect of the lamellae of the major component. In the present work, the fractional crystallization of PESub or PEO may occur at large supercooling when its content is low. PESub/PEO blends may be the first model that the fractional crystallization of each component occurs at small supercooling when its content is low; thus, they provide a rare system to study the unique crystalline morphology and crystallization behavior of miscible crystalline/ crystalline polymer blends. Such research is very interesting and challenging for a better understanding of the crystalline morphology and crystallization behavior of crystalline polymer blends from both academic and practical viewpoints.

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INTRODUCTION It is more interesting and difficult to study the crystalline morphology and crystallization behaviors of miscible crystalline/crystalline polymer blends than those of amorphous/ crystalline systems because both of the components may crystallize separately or simultaneously, providing various possibilities for a better study and understanding of crystalline polymer blends.1−24 Some unique crystalline morphologies have only been observed in these special systems, such as the formation of interpenetrating spherulites and the growth of concentric spherulites.13−17 In addition to the interesting morphology, the miscible pairs of two crystalline polymers may also show some complex crystallization behaviors. One of the complex crystallization behaviors is the stereocomlex formation. For instance, poly(L-lactide) and poly(D-lactide) may form a typical stereocomplex, which should be ascribed to the formation of CH3···OC hydrogen bonding, thereby significantly enhancing the thermal stability.18−20 The other of the complex crystallization behaviors is the occurrence of the fractional crystallization of one component at its low content in the miscible blends of two crystalline polymers.21−24 © XXXX American Chemical Society

The fractional crystallization behaviors are characterized by a number of crystallization exotherms at different temperatures, when semicrystalline polymers are crystallized nonisothermally from the melt at constant cooling rate. The fractional crystallization is usually attributed to the crystallizations of a number of domains at specific supercoolings.21−28 The different heterogeneities in a series of different isolated domains are able to induce the nucleation and crystallization at different temperatures. In some cases, there are not any active heterogeneities in some of the isolated domains, thereby resulting the occurrence of the crystallization of these domains via a homogeneous nucleation at a large supercooling. The fractional crystallization behaviors have often been found in some immiscible crystalline polymer blends or in some microphase-separated block copolymers, when the crystallizable component or block is the minor phase.25−28 Received: September 30, 2014 Revised: November 2, 2014

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crystalline polymers. The Tm differences are over 50 °C for PBS/PEO and PBS/PBA blends and over 100 °C for PVDF/ PBA blends. As a result, the high Tm component always crystallizes first in the blends, prior to the crystallization of the low Tm component, even when its content was low. Thus, an open and interesting question still exists whether the fractional crystallization of each component will occur in a miscible pair of two crystalline polymers. To our knowledge, such miscible crystalline/crystalline polymer blends have not been reported so far. It is expected that the factional crystallization behaviors of each component at its low content are most likely to be found in a miscible pairs with similar thermal properties, especially melting points. In this work, we studied the crystallization behaviors of novel crystalline/crystalline polymer blends consisting of biodegradable poly(ethylene suberate) (PESub) and biocompatible PEO. PESub and PEO show the similar basic thermal properties, including glass transition temperature (Tg) and Tm. We first studied the miscibility of PESub/PEO blends and found they were miscible over the whole blend compositions. After investigating the effect of blend composition on the nonisothermal crystallization behaviors, we found that the fractional crystallization behaviors of both PESub and PEO occurred at their low contents in the blends. In comparisons to the other systems, the new aspects of this work are that PESub/PEO blends are to our knowledge probably the first system, exhibiting the fractional crystallization behaviors of each component at its low content and providing a rare system to study the unique crystalline morphology and crystallization behavior of miscible crystalline/crystalline polymer blends.

It is interesting to note that several miscible pairs of two crystalline polymers can also exhibit the fractional crystallization behaviors, such as poly(butylene succinate) (PBS)/ poly(ethylene oxide) (PEO), PBS/poly(butylene adipate) (PBA), and poly(vinylidene fluoride) (PVDF)/PBA blends.21−24 In these systems, the confined and fractional crystallization behaviors of low melting point (Tm) component PEO or PBA occurred in their miscible crystalline polymer blends with high Tm component PBS or PVDF when the PEO or PBA content was low. The low Tm component may be expelled into the interlamellar, interfibrillar, and interspherulitic region of high Tm component, when the crystallization of high Tm component occurred. The distribution of the amorphous low Tm component is of great importance for the occurrence of fractional crystallization. Figure 1 depicts the schematic drawing

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EXPERIMENTAL SECTION

PESub (Mw = 3.56 × 104 g/mol, Mn = 1.51 × 104 g/mol) was synthesized by our laboratory. PEO with hydroxyl end groups (Mw = 4.35 × 104 g/mol, Mn = 1.40 × 104 g/mol) was bought from SigmaAldrich Company, Shanghai, China. Figure 2 displays the chemical structures of PESub and PEO.

Figure 1. Schematic drawing of fractional crystallization behaviors under different conditions: (a) all of the amorphous content of low Tm component is completely incorporated between the lamellae of high Tm component, and (b) a some of the amorphous content of the low Tm component is restricted in the interlamellar region and most of that is expelled out of the interlamellar region (red: high Tm component or the component crystallizes first; blue: the low Tm component or the component crystallizes later).

of the occurrence of fractional crystallization behaviors under different conditions, which provides the detailed information on crystalline and amorphous regions formed by which material and their proposed spatial arrangement. When all of the amorphous content of low Tm component is completely incorporated between the lamellae of high Tm component, the fractional crystallization will occur at a large supercooling because of the confinement of the lamellae of the high T m component and the less number of active heterogeneities (Figure 1a). When some of the amorphous content of the low Tm component is restricted in the interlamellar region and most of that is expelled out of the interlamellar region, the fractional crystallization will also occur, exhibiting two crystallization exotherms at low and high temperatures, respectively (Figure 1b). In other words, most of the component out of the interlamellar region (interfibrillar and interspherulitic region) crystallizes first at small supercooling, and some of the component in the interlamellar region crystallizes later at large supercooling, thereby exhibiting two crystallization exotherms at different supercoolings. However, it should be noted that only the fractional crystallization of one component, i.e., low Tm component, has been found for the above-mentioned miscible systems of two

Figure 2. Chemical structures of PESub and PEO.

In this work, a solution and casting method was used to prepare PESub/PEO blends over the entire compositions (100/0, 85/15, 70/ 30, 50/50, 30/70, 15/85, and 0/100 in weight ratio of PESub and PEO) with chloroform as a mutual solvent. The miscibility and crystallization process of PESub/PEO blends were observed by an Olympus BX51 polarized optical microscope (POM), which was equipped with a Linkam THMS 600 temperature controller and a first order retardation plate. The fracture surfaces of PESub/PEO blends were examined with a Hitachi S-4700 scanning electron microscope (SEM). Thermal properties were studied under a nitrogen atmosphere with a TA Instruments differential scanning calorimeter (DSC) Q100. The crystal structure and microstructure studies were performed with a Rigaku X-ray diffractometer and a NanoStar X-ray diffractometer (Bruker AXS Inc.), respectively. B

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Figure 3. (a) Nonisothermal melt crystallization exotherms, (b) subsequent melting endotherms, and (c) glass transitions of PESub/PEO blends.

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RESULTS AND DISCUSSION Basic Thermal Properties of PESub and PEO. The basic thermal properties of PESub and PEO were first investigated with DSC. Figure 3 illustrates the crystallization exotherms, melting endotherms, and enlarged glass transitions of both of the neat components. Figure 3a shows that PESub and PEO were first crystallized at a cooling rate of 10 °C/min from the crystal-free melt. PESub exhibits a nonisothermal melt crystallization temperature (Tcc) of 30.9 °C, while PEO shows a Tcc of 42.6 °C. In addition, PESub has a crystallization enthalpy (ΔHcc) value of 67.3 J/g, and PEO shows a ΔHcc value of 144.9 J/g. Figure 3b displays the subsequent melting behaviors at 10 °C/min for PESub and PEO. PEO displays a single melting endotherm at 64.1 °C. PESub shows two melting endotherms at 51.5 and 62.2 °C. Such double melting behaviors are often found for some biodegradable polymers and are usually explained by the melting, recrystallization, and remelting mechanism.29,30 In addition, PESub exhibits a heat of fusion (ΔHm) value of 69.6 J/g, and PEO has a ΔHm of 153.3 J/g. The crystallinity (Xc) values were determined to be about 52% for PESub and 80% for PEO, when 133 and 196.6 J/g were used to represent the heat of fusion values of 100% crystalline PESub and PEO, respectively,31 indicating that both of the components are highly crystalline polymers. PEO shows a greater crystallinity than PESub, suggesting a stronger crystallizability of the former than that of the latter. Figure 3c illustrates the glass transitions of PESub and PEO. PESub exhibits a Tg of −48.1 °C, while PEO shows a Tg of −50.9 °C. The Tg values are very close to each other for the two components. For comparison, Table 1 lists all the basic thermal properties for both of the components. Both PESub and PEO

Table 1. Summary of Basic Thermal Properties of PESub and PEO samples

Tg (°C)

Tcc (°C)

ΔHcc (J/g)

Tm (°C)

ΔHm (J/g)

Xc (%)

PESub PEO

−48.1 −50.9

30.9 42.6

67.3 144.9

51.5/62.2 64.1

69.6 153.3

52 80

exhibit the similar Tg and Tm values, thereby leading to a challenge in studying the miscibility, crystallization behavior, and crystalline morphology of PESub/PEO blends. However, because of their similar basic thermal properties, they may also exhibit some unique crystallization behavior and crystalline morphology, which will be reported in the following sections. Miscibility Study of PESub/PEO Blends. It is well-known that a single composition dependent Tg indicates the miscibility of polymer blends, when the difference in their Tg values is over 20 °C; however, in the present work, the miscibility of PESub/ PEO blends cannot be evidenced using this method because of their almost the same Tg values. We confirmed the miscibility by observing the homogeneous melt of PESub/PEO blends because phase separation would occur if they were immiscible. Figure 4 shows an optical microscopy (OM) image of a 50/50 blend, exhibiting the homogeneous melt. For other samples with different blend compositions, they display the similar results and are not shown here for brevity. In addition, SEM was further used to support the miscibility of PESub/PEO blends. Figure S1 in the Supporting Information illustrates a typical SEM image of the surface of a 50/50 blend. The samples for the SEM observations were prepared as follows. The films of the blends were first annealed at 100 °C for 3 min and then fractured into liquid nitrogen. As shown in Figure S1, no C

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traces at 5 °C/min for neat PESub, neat PEO, and their blends. For each neat component, it only exhibits a single crystallization exotherm. Neat PESub shows a Tcc of 35.6 °C with a ΔHcc of 70.6 J/g, and neat PEO has a Tcc of 44.6 °C with a ΔHcc of 159 J/g. When both of the neat components were crystallized from the melt, a cooling rate of 5 °C/min is more efficient than a cooling rate of 10 °C/min to favor more perfect crystallization, thereby resulting in greater Tcc and ΔHcc values. For the blends, they show complex crystallization behaviors, presenting two or three crystallization exotherms. Depending on the blend composition, one component crystallizes first, followed by the crystallization of the other component. For the 85/15 blend, two crystallization exotherms are observed at 35.9 and −9.4 °C, corresponding to the crystallization of PESub and PEO, respectively. For the 70/30 blend, three crystallization exotherms are found at 33.8, 24.4, and −7.7 °C, respectively, when it was crystallized from the high to low temperature range. As the major component, PESub crystallizes first at 33.8 °C and PEO crystallizes later, presenting two crystallization exotherms at 24.4 and −7.7 °C, respectively. For the 50/50 blend, it displays two crystallization exotherms at 32.3 and 27.6 °C, respectively. In previous section, PEO is found to be easier to crystallize than PESub; therefore, PEO crystallizes first, presenting a crystallization exotherm at 32.3 °C, and PESub crystallizes later, showing a crystallization exotherm at 27.6 °C. For the 30/70 blend, PEO crystallizes first and then PESub crystallizes later, presenting two crystallization exotherms at 40.3 and 20.1 °C, respectively. For the 15/85 blend, the crystallization exotherm at 43.9 °C should arise from the crystallization of PEO, while the crystallization exotherm at 0.3

Figure 4. Optical micrograph showing the homogeneous melt of a 50/ 50 blend at 100 °C.

obvious phase separation between the two components was observed, suggesting again that a homogeneous phase really exists between PESub and PEO. Other samples display the similar morphological results, which are omitted for simplicity. As a result, PESub and PEO form novel miscible crystalline/ crystalline polymer blends with similar basic thermal properties. Fractional Crystallization Behaviors of PESub/PEO Blends. In previous sections, we studied the basic thermal properties and phase behaviors of PESub and PEO and found they may form novel miscible crystalline/crystalline polymer blends. In this section, we further investigated the nonisothermal crystallization behaviors of PESub/PEO blends from the crystal-free melt. Figure 5a illustrates the DSC cooling

Figure 5. (a) Crystallization exotherms from the melt of PESub/PEO blends, (b) enlarged DSC thermograms of some samples, and (c) subsequent melting endotherms of PESub/PEO blends. D

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Table 2. Summary of Thermal Properties of PESub/PEO Blends

a

samples

100/0

85/15

70/30

50/50

30/70

15/85

0/100

Tcc (°C) ΔHcc (J/g) Tm (°C) ΔHm (J/g)

35.6a 70.6a 61.8a/53.9a 71.8a

35.9a/−9.4b 61.4a/16.5b 59.4c 83.1c

33.8a/24.4b/−7.7b 103.2c 59.7c 106.9c

32.3b/27.6a 107.4c 60.7c 107.8c

40.3b/20.1a 109.2b/19.0a 62.4c 128.4c

43.9b/0.3a 151.5c 63.9c 157.2c

44.6b 159b 66.0b 166.5b

From the contribution of PESub. bFrom the contribution of PEO. cFrom the contribution of PESub and PEO.

°C should originate from the crystallization of PESub. The crystallization exotherms at low temperature range are not very clear to observe for the 85/15, 70/30, and 15/85 samples. The enlarged DSC thermograms are shown in Figure 5b, from which the crystallization exotherms may be clearly observed. In particular, two crystallization exotherms from PEO are present in Figure 5b for the 70/30 sample, which are lower than the main crystallization exotherm from PESub at high temperature. Table 2 lists the Tcc and ΔHcc values for both of the neat components and their blends. It should be emphasized that the crystallization exotherms from the two components cannot be separated completely in the case of 70/30, 50/50, and 15/85 samples; therefore, the ΔHcc values arise from the contribution of both of the components. With increasing the content of the other component, Tcc and ΔHcc of each component usually decrease, indicating a suppressed crystallization by the other component. Figure 5c illustrates the subsequent melting behaviors of all the samples after the nonisothermal melt crystallization. All the other samples exhibit only one melting endotherm, except that two melting endotherms are present for neat PESub. The aforementioned crystallization study reveals that it should arise from the contribution of both of the components, although only one melting endotherm is observed for the blends. All the Tm and ΔHm values are also listed in Table 2, which decrease gradually with decreasing the PEO content. In the above section, we extensively studied the nonisothermal melt crystallization behaviors of PESub/PEO blends with DSC and found they were mainly influenced by the blend composition. Depending on the blend composition, different crystallization behaviors would occur, which would further affect the number, location, and order of crystallization exotherms. The crystal structures and microstructures were further investigated with wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) for the samples after finishing the nonisothermal melt crystallization at 5 °C/min to around 20 °C from the crystal-free melt. Such studies will be helpful for a better understanding of the complex crystallization behaviors of PESub/PEO blends. Figure 6 illustrates the WAXD patterns of PESub/PEO blends and the two neat components. PESub and PEO are highly crystalline polymers, each presenting their own characteristic diffraction peaks. For the blends with compositions ranging from 70/30 and 30/70, they involve the diffraction peaks from both of the neat components, indicating that cocrystallization does not occur in the blends. As shown in Figure 5a and Table 2, the crystallization of each component may occur under this crystallization condition. For the 85/15 and 15/85 samples, they show different WAXD patterns. In the case of 85/15 sample, it only presents the diffraction peaks from PESub and does not show any diffraction peaks from PEO. The WAXD pattern of 85/15 is the similar as that of neat PESub, indicating again that only the crystallization of the major component PESub occurred and the minor component PEO should be still

Figure 6. WAXD patterns of PESub/PEO blends.

in the amorphous state. According to the previous DSC study shown in Figure 5a, the crystallization of PEO cannot occur until crystallization temperature is further cooled to around −10 °C. In the case of 15/85 sample, its WAXD pattern consists of the diffraction peaks from both of the components, indicating that not only the major component PEO may crystallize but also the minor component PESub may crystallize before Tcc of 0.3 °C is reached. Under the nonisothermal melt crystallization at 5 °C/min, PEO crystallizes first and PESub crystallizes later. On one hand, the crystallization of some of PESub may occur at relatively high temperature range, thereby the crystallization exotherm of PESub occurring and overlapping with that of PEO. On the other hand, some of PESub can only crystallize at low temperature range, thereby exhibiting a crystallization exotherm at −0.3 °C. Therefore, when temperature was only cooled to around 20 °C, the crystallization of PESub may also occur, presenting the typical diffraction peaks of PESub in the WAXD pattern. The microstructures of some samples were further studied by SAXS. Figure 7 illustrates the SAXS profiles. Neat PESub

Figure 7. Lorentz-corrected SAXS profiles of some samples. E

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exhibits a scattering peak when q is around 0.557 nm−1, while neat PEO displays two scattering peaks at low and high q values of 0.23 and 0.50 nm−1, respectively, suggesting that both of the components are crystalline polymers. For the 85/15 blend, it only displays one scattering peak at a lower q value of 0.528 nm−1, indicating again that only the crystallization of PESub occurred. In this case, PEO cannot crystallize and must be in the amorphous phase. Because PESub and PEO are miscible, the 85/15 sample is composed of the PESub crystalline phase and the homogeneous amorphous phase from both of the components. The long period values are calculated to be 11.26 and 11.95 nm for neat PESub and 85/15 sample, respectively. The increase in the long period values indicates that amorphous PEO should mainly reside in the interlamellar region of PESub. For the 15/85 sample, it still shows two scattering peaks. One is at a low q value of 0.258 nm−1, which should be attributed to the crystallization of PEO. The other is at a high q value of 0.628 nm−1, which may originate from the mixture of the crystals of PEO and PESub. The SAXS profile of the 15/85 blend reveals that some of PESub may crystallize at relative high temperature under this crystallization condition. Finally, let us discuss the complex crystallization behaviors of PESub/PEO blends with similar thermal properties on the basis of the DSC, WAXD, and SAXS results. When the PEO content is as low as 15%, PESub crystallizes first, and PEO is completely expelled into the interlamellar region of PESub with a thickness of only several nanometers after the crystallization of PESub (Figure 1a). As PESub and PEO are miscible, the amorphous phase consists of all PEO and amorphous PESub. The lamellae of PESub separate the amorphous phase into a number of amorphous layers; therefore, the crystallization of PEO is confined in an isolated layer. The lamellae of PESub make it difficult for the spreading of the crystallization of PEO in one amorphous layer to another layer. The number of active heterogeneities is lower than that of the amorphous layer, thereby resulting in the occurrence of the fractional crystallization of PEO with a ΔHcc of 16.5 J/g at low temperature during the DSC cooling traces as shown in Figure 5b. Before an extreme supercooling is reached, PEO is incorporated in the amorphous state and free from crystallization; therefore, its diffraction peaks and scattering peak cannot be observed in the WAXD and SAXS profiles. Similar results were also reported in the PBS/PEO blends.21 With further increasing the PEO content to 30%, most of the amorphous PEO is expelled out of the interlamellar region of PESub, while some of that is still between the lamellae of PESub after the crystallization of PESub at a high temperature of 33.8 °C during the cooling process (Figure 1b). Therefore, two crystallization exotherms are found for the crystallization of PEO. One is at small supercooling (24.4 °C), which is from the amorphous PEO out of the interlamellar region and is induced by the heterogeneous nucleation. The other is at large supercooling (−7.7 °C), which is from the PEO component between the lamellae of PESub and is mainly induced by the less active heterogeneities, thereby resulting in the fractional crystallization of PEO at low temperature. As shown in Figure 5b, the crystallization exotherm of PESub at 33.8 °C and the two crystallization exotherms of PEO at 24.4 and −7.7 °C are overlapped with each other, exhibiting a whole large crystallization exotherm with a total ΔHcc of 103.2 J/g. As PESub is the major component, the crystallization from PESub may contribute more to the total ΔHcc value than PEO. Accordingly, it is easy to understand that PEO shows two

crystallization exotherms at small and large supercoolings for the 30/70 sample from the DSC cooling traces; moreover, its diffraction peaks can be observed from the WAXD patterns, even when only a small supercooling is reached. In the case of 50/50 and 30/70 samples, PEO crystallizes first and PESub crystallizes later because the crystallizability of the former is stronger than the latter. As a result, the PESub component is expelled out of the interlamellar region of PEO. The crystallization of each component is induced by the heterogeneous nucleation at small supercooling, similar to those of the neat components. Therefore, the DSC cooling traces presents two crystallization exotherms at relatively high temperature range, corresponding to the crystallization of each component, and the WAXD patterns involve the characteristic diffraction peaks from both the components, even only a small supercooling is reached. With further increasing the PEO content to 85%, PEO becomes the major component, while PESub is the minor component. Upon cooling from the melt, PEO crystallizes first and PESub crystallizes later. During the crystallization of PEO, some of the amorphous PESub is expelled out of the interlamellar region of PEO, while some of that is between the lamellae of PEO (Figure 1b). Upon further cooling to low temperature range, the crystallization of the amorphous PESub out of the interlamellar region is first induced by the active heterogeneities at small supercoolings; however, it does not illustrate a separate crystallization exotherm and overlaps with that of PEO. At large supercooling, the crystallization of the amorphous PESub in the interlamellar region is then induced by the less active heterogeneities; therefore, the factional crystallization of PESub occurs at around 0 °C, exhibiting a small crystallization exotherm in the DSC cooling traces. As shown in Figure 5b, the crystallization exotherm from PEO at 43.9 °C and the crystallizations from PESub at small supercooling and at large supercooling of 0.3 °C are overlapped each other, exhibiting a large crystallization exotherm with a total ΔHcc of 151.5 J/g. Since PEO is the major component, the crystallization from PEO contributes most of the ΔHcc value. The results from the WAXD and SAXS profiles may also provide further evidence to these complex crystallization behaviors. It is expected that POM would be extremely helpful to support the above-mentioned conclusions. Figure 8 illustrates the crystallization process of a 30/70 sample, which was nonisothermally crystallized from the crystal-free melt at a cooling rate of 2.5 °C/min. For a convenient observation, a slow cooling rate of 2.5 °C/min was used in this research, since a cooling rate of 5 °C/min was too fast to follow the morphology variation of the PESub/PEO blends. As shown in Figure 8a, two different spherulites were observed when the temperature was cooled to about 40.9 °C. The large spherulite is from the PEO component, while the small one is from the PESub component. The PEO spherulite is compact, while the PESub spherulite is coarse, exhibiting ring bands. With further lowering crystallization temperature to 40.6 °C, the PEO spherulite was about to contact the PESub spherulite (Figure 8b). When the temperature was further cooled to 40.3 °C, the large PEO spherulite finally penetrated the ring-banded PESub spherulite (Figure 8c), indicating the formation of the typical interpenetrating spherulites, which are often observed in miscible crystalline/crystalline polymer blends.13−17 Figures 8b,c also illustrate two additional PEO spherulites appeared and continued to grow freely during the crystallization process. The F

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each neat component, PESub/PEO blends show two or three crystallization exotherms at different supercoolings because both of the components are able to crystallize. The complex crystallization behaviors are attributed to the occurrence of the fractional crystallization of the minor component of the blends. Depending on the blend composition, the major component crystallizes first and the minor component crystallizes later. During the crystallization of the major component, the amorphous minor component is completely included the interlamellar region of the major component or most of the amorphous minor component is expelled out of the interlamellar region while only a few is incorporated between the lamellae of the major component. In both cases, the fractional crystallization of the minor component occurs at large supercooling because of the confinement effect of the lamellae of the major component. In the middle blend compositions, the suppressed crystallization exotherm of each component is observed, like that of neat component, indicating the amorphous of one component is out of the interlamellar region of the other. In the present work, it is interesting to find that the fractional crystallization of PESub or PEO may occur at large supercooling when its content is low. PESub/PEO blends may be the first model that the fractional crystallization of each component occurs at small supercooling at its low content; thus, they provide a rare system to study the unique crystalline morphology and crystallization behavior of miscible crystalline/ crystalline polymer blends, which are different than those of common crystalline polymer blends. Such research is of great interest and importance for a better understanding of the crystalline morphology and crystallization behavior of crystalline polymer blends from both academic and practical viewpoints.

Figure 8. POM images showing the crystallization process of a 30/70 PESub/PEO blend at a cooling rate of 2.5 °C/min when temperature was lowered to (a) 40.9, (b) 40.6, and (c) 40.3 °C.

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ASSOCIATED CONTENT

S Supporting Information *

formation of the interpenetrating spherulites obviously indicates that both of the components crystallize separately; moreover, most of one component should mainly reside out of the interlamellar region (interfibrillar and interspherulitic region) of the other component (Figure 1b).13−17 In brief, the PESub/PEO blends present complex crystallization behaviors. Depending on the blend composition, the fractional crystallization behavior of each component may occur when its content is low. This is the first system that the fractional crystallization of both of the components may occur at large supercooling, providing a good model to study the complex crystalline morphology and crystallization behavior of miscible crystalline/crystalline polymer blends. Such study is still underway and will be reported in the forthcoming research in detail.

SEM image of a 50/50 blend, showing no obvious phase separation of the two components. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Fax +86-10-64413161 (Z.Q.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Miss Lu Tang and Mr. Shoutian Qiu for their kind and important contributions, Prof. Shouke Yan for his helpful discussion, the reviewers for their constructive suggestions, and the National Natural Science Foundation, China (51221002 and 51373020), for the financial support of this research.

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CONCLUSIONS Novel crystalline/crystalline polymer blends of biodegradable PESub and biocompatible PEO were prepared through a solution and casting method over the entire blend compositions. The basic thermal properties, including both glass transition temperature and melting point, of both of the components are very close to each other, with the crystallizability of PEO being stronger than that of PESub. The OM and SEM results demonstrate that no clear phase separation can be detected for the PESub/PEO blends; therefore, they form novel miscible polymer blends of two crystalline polymers. Unlike one crystallization exotherm for

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dx.doi.org/10.1021/ma502019x | Macromolecules XXXX, XXX, XXX−XXX