Crystallization Kinetics and Morphology of Novel Miscible Crystalline

Jul 5, 2013 - Bing Na , Jie Zhu , Ruihua Lv , Yunhui Ju , Renping Tian , and Bibo Chen. Macromolecules 2014 47 (1), 347-352. Abstract | Full Text HTML...
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Crystallization Kinetics and Morphology of Novel Miscible Crystalline/Amorphous Polymer Blends of Biodegradable Poly(butylene succinate-co-butylene carbonate) and Poly(vinyl phenol) 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: Biodegradable poly(butylene succinate-co-butylene carbonate) (PBSC) and poly(vinyl phenol) (PVPh) may form novel miscible crystalline/amorphous polymer blends that exhibit a single composition-dependent glass transition temperature and a depression of equilibrium melting point of PBSC in the blends. Relative to neat PBSC, blending with PVPh has not only suppressed the nonisothermal melt crystallization behavior of PBSC at the same cooling rate but also reduced both the overall isothermal melt crystallization rates and spherulitic growth rates of PBSC in the blends at the same crystallization temperature; however, the crystallization mechanism does not change for either neat PBSC or the PBSC/PVPh blends. Neat PBSC and its blends show the same crystal structure. The apparent increase in both long period and amorphous layer thickness values indicates that amorphous PVPh resides in the interlamellar region of PBSC spherulites in the blends.

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INTRODUCTION From both academic and industrial viewpoints, biodegradable polyesters have recently received considerable attention.1−5 Till now, several kinds of biodegradable polymers, including poly(3hydroxybutyrate) (PHB), poly(L-lactide) (PLLA), poly(εcaprolactone) (PCL), and poly(butylene succinate) (PBS) have been investigated extensively, because they may find various application fields as commercially available polymers.2−5 As a random copolymer related to biodegradable PBS and poly(butylene carbonate) (PBC), poly(butylene succinate-co-butylene carbonate) (PBSC) is also a biodegradable crystalline thermoplastic, which is a kind of poly(ester carbonate) (PEC). Because it has similar physical properties to polypropylene and polyethylene, PBSC has attracted some attention from the viewpoint of industrial application and has recently been developed by Mitsubishi Gas Chemical Co., Japan; however, it should be noted that only a few works have dealt with PBSC, relative to PHB, PLLA, PCL, and PBS. As a biodegradable polymer, PBSC may degrade completely and rapidly with a high yield of cell growth.6 PBSC has the same crystal structure as PBS, indicating that the butylene carbonate unit does not crystallize in the copolymer; moreover, it shows double melting behavior or one major melting endotherm with a shoulder depending on crystallization temperature, which may be well explained by the melting, recrystallization, and remelting model.7 It is well-known that polymer blending is a simple and economical way to develop new materials with desired physical properties.2,8 As far as PBSC is concerned, polymer blending is also used to modify its physical properties for extending its practical application fields.9−12 For example, Ikehara and co-workers9,10 prepared fully biodegradable PBSC/ PLLA blends via a solution and casting method and investigated the miscibility and spherulitic morphology of © 2013 American Chemical Society

PBSC/PLLA blends in detail. According to their study, PBSC and PLLA form typical miscible crystalline/crystalline polymer blends; furthermore, the two components may form interpenetrated spherulites depending on crystallization conditions.9,10 Lee et al.11,12 prepared PBSC/cellulose acetate butyrate (CAB) blends via a thermal compounding method and investigated the miscibility, crystallization behavior, and mechanical, thermal, and rheological properties. They found that PBSC was partially miscible with CAB, and increasing the CAB component reduced the overall crystallization rates of PBSC in the PBSC/CAB blends, relative to neat PBSC.11,12 Poly(vinyl phenol) (PVPh) is an amorphous polymer with high glass transition temperature. As a proton donor, PVPh may usually have hydrogen-bonding interactions with protonacceptor polymers, which is of great help to improve the miscibility of polymer blends.13−19 For example, PVPh may form miscible crystalline/amorphous polymer blends with some biodegradable polymers, such as PHB, poly(hydroxyvalerate) (PHV), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx), PLLA, PCL, poly(ethylene sebacate) (PESeb), etc.; moreover, the miscibility between PVPh and these biodegradable polymers should be attributed to the formation of hydrogen bonds between the hydroxyl group of PVPh and the carbonyl group of the partners.13−19 To our knowledge, PBSC/PVPh blends have not been reported so far in the literature; however, from the viewpoint of their specific intermolecular hydrogen bonding interactions, PBSC and PVPh are very likely to form miscible polymer Received: Revised: Accepted: Published: 10198

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Wide-angle X-ray diffraction (WAXD) experiments were performed with a Rigaku X-ray diffractometer RINT2100 at 40 kV and 200 mA at 5°/min. Small-angle X-ray scattering (SAXS) measurements were carried out on a NanoStar X-ray diffractometer (Bruker AXS Inc.) with Cu Kα radiation at 40 kV and 650 μA. Samples for WAXD and SAXS experiments were first pressed into films with thickness of around 1 mm on a hot stage at 200 °C and then transferred into a vacuum oven at 79 °C for 3 days. Both the WAXD and SAXS experiments were performed at room temperature.

blends. The aim of this research is to get a better understanding of the miscibility and crystallization behavior of biodegradable polymer blends, because it affects not only the physical properties but also the biodegradation of polymer blends.13−21 In this work, PBSC/PVPh blends were chosen as the miscible crystalline/amorphous polymer blends model system; moreover, the miscibility, nonisothermal melt crystallization behavior, overall isothermal melt crystallization kinetics, spherulitic morphology and growth, crystal structure, and microstructure of PBSC/PVPh blends were investigated in detail with various techniques.

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RESULTS AND DISCUSSION Miscibility Study of PBSC/PVPh Blends. As introduced in the Experimental Section, the miscibility of PBSC/PVPh blends was studied first with DSC. Figure 1 illustrates the DSC

EXPERIMENTAL SECTION Materials. PBSC (Mw = 2.0 × 105 g/mol) used in this study has a carbonate content of 10 mol % and was kindly provided by Mitsubishi Gas Chemical Co., Japan. PVPh (Mw = 2.0 × 104 g/mol) was purchased from Sigma−Aldrich (Shanghai) Trading Co., Ltd. Both samples were used as received. PBSC/PVPh blends were prepared with N,N-dimethylformamide (DMF) as a solvent at around 50 °C. The solution of both polymers (0.0125 g/mL) was cast on glass plates at 50 °C. The solvent was allowed to evaporate in a controlled air stream for 1 day, and the resulting films were further dried in vacuum at 50 °C for 3 days to remove DMF completely. In this way, blends were prepared with various compositions of 100/0, 80/ 20, 60/40, 40/60, 20/80, and 0/100 weight ratio, where the first number refers to PBSC. Characterizations. Thermal analysis was performed on a TA Instruments differential scanning calorimeter (DSC) Q100 with a Universal Analysis 2000 under nitrogen atmosphere. The glass transition temperature (Tg) and melting temperature (Tm) of PBSC/PVPh blends were measured at 20 °C/min for the melt-quenched samples. The samples were first annealed at 200 °C for 3 min to erase any thermal history and then quenched to −80 °C quickly. Both the nonisothermal melt crystallization behavior and overall isothermal melt crystallization kinetics of PBSC/PVPh blends were also studied with DSC under different conditions and compared with those of neat PBSC. For the nonisothermal melt crystallization behavior study, the samples were studied with DSC at a cooling rate of 5 °C/min from the crystal-free melt. For the overall isothermal melt crystallization kinetics study, the samples were first annealed at 200 °C for 3 min to erase any thermal history, cooled quickly to the chosen crystallization temperature (Tc) at 60 °C/min, and then maintained at Tc until the crystallization was completed. After isothermal melt crystallization, the samples were heated to the melt again at 20 °C/min (if not otherwise specified) to study the subsequent melting behavior of PBSC for estimation of equilibrium melting point. In the present work, only neat PBSC, 80/20, and 60/40 were investigated for the crystallization studies, because PBSC could not crystallize or crystallized too slowly in the PBSC/PVPh blends when the PVPh content was greater than 60 wt %. The spherulitic morphology and growth rates of PBSC/ PVPh blends were investigated by a polarized optical microscope (POM) (Olympus BX51) equipped with a firstorder retardation plate and a temperature controller (Linkam THMS 600). The samples were first annealed at 200 °C for 3 min to erase any thermal history and then cooled to the desired Tc at 60 °C/min. The spherulitic growth rate (G) was determined by measuring radius (R) against crystallization time (t), that is, G = dR/dt.

Figure 1. DSC heating traces of neat PBSC, neat PVPh, and their blends at 20 °C/min for the melt-quenched samples.

heating traces of neat PBSC, neat PVPh, and their blends for the melt-quenched samples. Figure 1 shows that neat PBSC is a semicrystalline polymer with a Tg of −35.4 °C and a Tm of 106.9 °C, while neat PVPh is an amorphous polymer with a high Tg of 134.2 °C. To show the glass transition regions more clearly, Figure S1 (Supporting Information) shows enlarged DSC traces for neat PBSC, neat PVPh, and their blends. The PBSC/PVPh blends exhibit a single Tg between those of neat PBSC and neat PVPh, which increases with increasing weight fraction of PVPh in the blends; therefore, PBSC is miscible with PVPh over the entire range of blend compositions, forming novel miscible crystalline/amorphous polymer blends. In addition, with increasing weight fraction of PVPh, Tm of PBSC shifts gradually downward to low temperature range in the blends; moreover, it cannot be detected when the PVPh content is 80 wt % and above, suggesting that PBSC cannot crystallize any more after blending with such a high content of amorphous PVPh. Relative to neat PBSC, the depression of Tm of PBSC with increasing PVPh composition in the PBSC/ PVPh blends indicates again that PBSC and PVPh form miscible crystalline/amorphous polymer blends, which may arise from their specific interaction, that is, hydrogen bonding between the two components. Figure 2 displays the variation trends of Tm and Tg with weight fraction of PVPh for the PBSC/PVPh blends. From Figure 2, a slight depression of Tm of PBSC is found with an increase in the weight fraction of PVPh up to 60 wt % in the PBSC/PVPh blends, relative to neat PBSC, indicative of the miscibility between PBSC and PVPh. In addition, the wellknown Gordon−Taylor equation was applied to fit the 10199

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Figure 2. Variation trends of Tm and Tg of PBSC/PVPh blends with weight fraction of PVPh (the dashed line is based on the Gordon− Taylor equation).

Figure 3. Nonisothermal melt crystallization of neat PBSC and PBSC/ PVPh blends at a cooling rate of 5 °C/min.

blends, the normalized ΔHc values were 73.9 and 38.5 J/g for 80/20 and 60/40 blends, respectively. The final degree of crystallinity values of the crystallizable component PBSC are comparable for neat PBSC and 80/20 blend, which are significantly greater than that of 60/40 blend. In brief, increasing the weight fraction of PVPh in the blends shifts Tp downward to low temperature range, relative to neat PBSC, indicating that the nonisothermal melt crystallization behavior of PBSC has been suppressed in the miscible crystalline/ amorphous blends after blending with high Tg component PVPh. The overall isothermal melt crystallization kinetics was further studied by DSC for neat PBSC at different Tc values ranging from 75 to 83 °C and for its blends at different Tc values ranging from 73 to 81 °C. The well-known Avrami equation was used to analyze the overall isothermal melt crystallization kinetics of neat PBSC and PBSC/PVPh blends. According to the Avrami equation, the relative degree of crystallinity (Xt) develops with crystallization time as follows:

measured Tg values against the weight fraction of PVPh. Based on the Gordon−Taylor equation, the Tg value of the blends may be predicted as follows: Tg =

W1Tg1 + q(1 − W1)Tg2 W1 + q(1 − W1)

(1)

where W1 is the weight fraction of PBSC; Tg1 and Tg2 refer to the Tg values of neat PBSC and neat PVPh, respectively; and q is the Gordon−Taylor parameter, which can be related to the strength of interaction between the components in miscible polymer blends.22 For polymer blends, a lower value of q usually indicates a poorer interaction between the two components.22 Figure 2 clearly shows that the Gordon−Taylor equation may fit the measured Tg values of the PBSC/PVPh blends very well with a q value of 0.25; moreover, such a lower q value also indicates a considerably weak specific interaction between the two components, thereby resulting in a negative deviation of the Tg curve. In brief, the miscibility between PBSC and PVPh has been confirmed over the whole range blend compositions, as evidenced by both the depression of Tm of PBSC and the single composition-dependent Tg of the blends. Nonisothermal Melt Crystallization Behavior and Overall Isothermal Melt Crystallization Kinetics of Neat PBSC and PBSC/PVPh Blends. In the previous section, the miscibility of PBSC and PVPh was evidenced by the single composition-dependent Tg over the entire range of blend compositions. In this section, the nonisothermal melt crystallization behavior and overall isothermal melt crystallization kinetics of neat PBSC and its blends were studied by DSC at different crystallization conditions. Figure 3 shows the nonisothermal melt crystallization behavior of neat PBSC and its blends with PVPh at a cooling rate of 5 °C/min. As shown in Figure 3, both neat PBSC and the PBSC/PVPh blends crystallized during the cooling process from the melt at 5 °C/ min, and the crystallization exotherms of PBSC shift gradually downward to low temperature range with increasing weight fraction of PVPh in the blends, relative to neat PBSC. Neat PBSC shows a nonisothermal melt crystallization peak temperature (Tp) at 65.3 °C, with a crystallization enthalpy (ΔHc) of 69.3 J/g. The 80/20 blend exhibits Tp at 61.1 °C with ΔHc of 59.1 J/g, while for the 60/40 blend, Tp shifts downward to 58.8 °C, and ΔHc decreases to 23.1 J/g. When they were normalized with respect to the composition of PBSC in the

1 − X t = exp( −kt n)

(2)

where k is the crystallization rate constant involving both nucleation and growth rate parameters and n is the Avrami exponent that depends on the nature of nucleation and growth geometry of crystals.23,24 For example, Figure 4 shows the Avrami plots of neat PBSC and an 80/20 blend crystallized at different Tc values. It is obvious from Figure 4 that a series of almost parallel straight lines are found for the two samples, regardless of T c , suggesting that the isothermal melt crystallization process may be well described by the Avrami method. Similar results were also found for the 60/40 blend; for brevity, they are not shown here. Accordingly, the n and k values were obtained from the slopes and intercepts of the Avrami plots, respectively, which are summarized in Table 1. The average n value is around 2.2 for neat PBSC and PBSC/PVPh blends, indicating that the crystallization may correspond to three-dimensional truncated sphere growth with athermal nucleation;25 moreover, the crystallization mechanism does not change, regardless of Tc and PVPh composition. The unit of k is minutes−n and n is not a constant in this work; therefore, it is better not to compare directly the overall crystallization rates from the k values. Therefore, the crystallization half-life (t0.5) is introduced for the discussion of crystallization kinetics of neat PBSC and PBSC/ PVPh blends; t0.5 means the time required to achieve 50% of 10200

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Figure 4. Avrami plots of (a) 100/0 and (b) 80/20 PBSC/PVPh blends.

Table 1. Overall Isothermal Melt Crystallization Kinetics Parameters of Neat PBSC and Blended PBSC/PVPh 100/0 Tc (°C) 75 77 79 81 83

n 2.5 2.3 2.3 2.3 2.6

80/20 −n

k (min ) 3.85 2.75 1.86 1.09 5.27

× × × × ×

−1

10 10−1 10−1 10−1 10−2

−1

1/t0.5 (min ) 0.79 0.67 0.57 0.44 0.37

Tc (°C) 73 75 77 79 81

n 2.5 1.9 2.0 1.9 2.0

60/40 −n

k (min ) 2.96 2.01 1.58 1.30 7.45

× × × × ×

−1

1/t0.5 (min )

−1

10 10−1 10−1 10−1 10−2

0.71 0.53 0.48 0.42 0.33

Tc (°C) 73 75 77 79 81

n 1.9 1.8 1.9 2.0 1.9

k (min−n) 2.34 1.87 1.46 9.00 7.22

× × × × ×

−1

10 10−1 10−1 10−2 10−2

1/t0.5 (min−1) 0.56 0.48 0.44 0.35 0.31

Figure 5. (a) Melting behavior of an 80/20 PBSC/PVPh blend isothermally crystallized at various Tc values from the melt, (b) effect of heating rate on subsequent melting behavior after crystallization at 77 °C from the melt, and (c) Hoffman−Weeks plot for estimation of equilibrium melting point of an 80/20 blend.

t0.5 =

the final crystallinity of the samples. From the Avrami equation, t0.5 can be calculated as follows: 10201

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠

(3)

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Figure 6. Series of POM images showing the spherulitic morphology of (a) 100/0, (b) 80/20, and (c) 60/40 PBSC/PVPh crystallized at 75 °C.

addition, the ratio of the area of Tm1 to that of Tm2 increases with an increase in Tc from 73 to 81 °C. Such melting behavior may be well explained by the melting, recrystallization, and remelting mechanism.7 Tm1 corresponds to the melting of crystals formed during the isothermal crystallization process, while Tm2 is attributed to the melting of crystals formed through recrystallization during the heating process. It is wellknown that the melting, recrystallization, and remelting mechanism may usually be evidenced by heating-rate dependence of the multiple melting behavior.28,29 Figure 5b shows the heating-rate dependence of the subsequent melting behavior of an 80/20 blend after crystallization at 77 °C. As shown in Figure 5b, Tm1 shifts upward from 91.6 to 93.1 °C with an increase in heating rate from 10 to 40 °C/min, while Tm2 is at around 103.4 °C and remains almost unchanged, regardless of heating rate. Therefore, Tm1 is used for analysis of the Hoffman−Weeks equation. Figure 5c shows the Hoffman− Weeks plots for the 80/20 blend, from which the value of Tmo was determined to be 117.1 °C. The Tmo values of neat PBSC and the 60/40 blend were also determined to be 125.4 and 114.4 °C, respectively, by the same method. The depression of Tmo confirms again that PBSC and PVPh are thermodynamically miscible. Spherulitic Morphology and Growth of PBSC/PVPh Blends. In this section, the effects of both Tc and PVPh content on the spherulitic morphology and growth of PBSC were further investigated in a wide Tc range, which may influence both the final physical properties and biodegradation of PBSC. For example, Figure 6 illustrates a series of POM images of neat PBSC and two blends crystallized at 75 °C. Both neat PBSC and PBSC/PVPh blends crystallized according to spherulitic growth. The number of PBSC spherulites is smaller in the PBSC/PVPh blends than in neat PBSC; moreover, the number of PBSC spherulites is smaller in a 60/40 blend than in an 80/20 blend. Such results indicate that blending with amorphous PVPh has reduced the nucleation density of PBSC spherulites in the PBSC/PVPh blends, relative to neat PBSC at the same Tc. It should be noted that the nucleation density of PBSC spherulites is affected by supercooling (ΔT = Tmo −Tc); therefore, it must be further dependent on the PVPh composition in the PBSC/PVPh blends, because an apparent depression of Tmo is found in the blends with increasing PVPh composition. Figure S2 (Supporting Information) presents POM images showing the spherulitic morphology of neat PBSC and its blends at the same supercooling of 45 °C. From Figure S2, the nucleation density values of PBSC spherulites are greater in the blends than in neat PBSC, because they were crystallized at lower Tc values relative to neat PBSC.

Consequently, the overall isothermal melt crystallization rate may be described by the reciprocal of t0.5, that is, 1/t0.5. Table 1 also lists all the 1/t0.5 values for neat PBSC and its blends crystallized at different Tc values. Table 1 clearly shows that the 1/t0.5 values decrease with increasing Tc for all three samples, suggesting that the overall isothermal melt crystallization rate becomes slower at higher Tc. However, at a given Tc, the 1/t0.5 values are greater in neat PBSC than in its blends; moreover, the 1/t0.5 values are greater in an 80/20 blend than in a 60/40 blend. Such results indicate that blending with amorphous PVPh has decelerated the isothermal melt crystallization process of PBSC in the blends, relative to neat PBSC. The reduction of overall isothermal melt crystallization rates with increasing PVPh content will be discussed in the following section. In brief, blending with PVPh does not change the crystallization mechanism but reduces the overall isothermal melt crystallization rates of PBSC in the blends. Melting Behavior and Depression of Equilibrium Melting Point of PBSC in PBSC/PVPh Blends. For crystalline/amorphous polymer blends, the depression of melting point of the crystalline polymer may provide further important information with regard to its miscibility, because immiscible blends or partially miscible blends usually do not show an apparent depression of melting point, while a significant melting point depression is often found for miscible polymer blends.13−19 The melting point of a crystalline polymer is influenced by many factors, which include both thermodynamic and morphological factors, such as crystalline lamellar thickness and the perfection of spherulites; therefore, the depression of equilibrium melting point (Tm°) is usually used to discuss the miscibility of polymer blends, because it may separate the morphological effect from the thermodynamic effect.26 Tmo can be derived from the well-known Hoffmann− Weeks equation: Tm = ηTc + (1 − η)Tm°

(4)

where Tm is the apparent melting point, Tc is the crystallization temperature, and η may be regarded as a measure of the stability.27 As introduced in the Experimental Section, the subsequent melting behaviors of neat PBSC and PBSC/PVPh blends were studied by DSC after they finished isothermal crystallization at different Tc values. For instance, Figure 5a displays the melting behavior of an 80/20 blend isothermally crystallized at various Tc values ranging from 73 to 81 °C. It is obvious that two endothermic melting peaks are found, regardless of Tc. The lower endothermic peak (Tm1) shifts upward from 90.3 to 95.1 °C with an increase in Tc from 73 to 81 °C, while the higher endothermic peak (Tm2) is at around 103.5 °C and remains almost unchanged, irrespective of Tc. In 10202

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Figure 7 shows the variation of G with Tc for neat and blended PBSC. For both neat PBSC and the 80/20 blend, the

PVPh. Second, the PBSC chain concentration at the front of spherulites was diluted in miscible blends by the presence of amorphous PVPh. Third, the thermodynamic driving force required for the growth of PBSC spherulites was dropped by a depression of Tm° in the blends, relative to neat PBSC. Such results are often found in some miscible crystalline/amorphous blends.13−19 Crystal Structure and Microstructure Studies of PBSC/ PVPh Blends. Crystal structure and microstructure studies of neat and blended PBSC were further performed by WAXD and SAXS, respectively. The effect of PVPh composition on the crystal structure of PBSC/PVPh blends was investigated first with WAXD. Figure 8a shows the WAXD patterns of neat and blended PBSC, which were crystallized at 79 °C for 3 days. It is clear from Figure 8a that all the samples exhibit almost the same diffraction peaks at similar locations, regardless of the weight fraction of PVPh. Such results indicate that blending with PVPh does not modify the crystal structure of PBSC in PBSC/PVPh blends. As shown in Figure 8a, the three main diffraction peaks located at 19.48°, 21.69°, and 22.61° are attributed to (020), (021), and (110) planes, respectively.7 In addition, it is also clear from Figure 8a that the intensity of the diffraction peaks decreases obviously with increasing PVPh content, indicative of a reduced degree of crystallinity of PBSC in the blends. From the WAXD patterns shown in Figure 8a, the degree of crystallinity values were estimated to be 47% ± 2% for neat PBSC, 41% ± 2% for 80/20 blend, and 29% ± 2% for 60/40 blend. When they were normalized with respect to the composition of PBSC in the blends, the normalized degree of crystallinity values were around 50%, regardless of the weight fraction of PVPh. In brief, blending with amorphous PVPh does not modify the crystal structure of PBSC in the PBSC/PVPh blends. In the previous section, Figure 6 showed that PBSC spherulites filled in the whole space for the PBSC/PVPh blends, indicating that amorphous PVPh must reside inside the PBSC spherulites;30,31 however, it is of interest to study where it may reside primarily, that is, in the interlamellar or interfibrillar regions of PBSC spherulites. Therefore, the effect of PVPh composition on the microstructure of PBSC/PVPh blends was further studied with SAXS. Figure S3 (Supporting Information) shows the SAXS profiles of neat PBSC and its

Figure 7. Crystallization temperature dependence of spherulitic growth rates of neat and blended PBSC.

spherulitic growth rates decrease with increasing Tc, indicating a slower growth rate at smaller supercooling; however, for the 60/40 blend, the spherulitic growth rates first increase and then decrease with an increase in Tc from 55 to 83 °C, exhibiting a maximum value at around 63 °C. In other words, a bell-shaped spherulitic growth rate is found for the 60/40 blend in a wide Tc range, because blending with such a high amount of PVPh has reduced the nucleation and growth of PBSC spherulites apparently, thereby making the accurate measurement of growth rates of PBSC spherulites possible at low Tc values. From Figure 7, it is also found that the spherulitic growth rates are faster in neat PBSC than in the PBSC/PVPh blends at a given Tc; moreover, the spherulitic growth rates are faster in an 80/20 blend than in a 60/40 sample with increased PVPh composition. The aforementioned results indicate that both neat PBSC and PBSC/PVPh blends show similar variation trends of spherulitic growth rates as those of overall isothermal melt crystallization rates, which become slower with increasing PVPh composition. The reduction of spherulitic growth rates of PBSC in the blends may be attributed to the following three reasons. First, relative to neat PBSC, the mobility of PBSC in the blends was reduced after blending with high Tg component

Figure 8. (a) WAXD patterns and (b) one-dimensional correlation function curves for neat PBSC and its blends. 10203

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are slower in blends than in neat PBSC and become slower with increasing PVPh composition. Neat PBSC and PBSC/ PVPh blends show similar WAXD patterns, indicating that blending with amorphous PVPh does not modify the crystal structure. According to the SAXS results, the increase in both the long period and amorphous layer thickness values becomes apparently larger with increasing PVPh composition in the blends, while the increase in the crystal layer thickness values is considerably smaller, indicating that amorphous PVPh resides in the interlamellar region of PBSC spherulites in the blends.

blends. On the basis of Figure S3, Figure 8b illustrates the curves of normalized one-dimensional correlation function curves for both neat and blended PBSC, from which the values of long period (LP), crystal layer thickness (Lc), and amorphous layer thickness (La) were obtained.32 For neat PBSC, the values of LP, Lc, and La were determined to be 8.97, 3.46, and 5.51 nm, respectively. For an 80/20 blend, they were increased to 10.69, 4.03, and 6.63 nm, respectively. For a 60/40 sample, they were further increased to 13.72, 4.85, and 8.87 nm, respectively. From the above-mentioned results, all the LP, Lc, and La values of PBSC increase with increasing PVPh content for the PBSC/PVPh blends. It is clear that the increase in LP is obvious between neat PBSC and PBSC/PVPh blends. Relative to neat PBSC, the increase in LP is 1.72 nm for an 80/20 blend, while the increase in LP is 4.75 nm for a 60/40 sample. It is interesting to study the detailed contribution of Lc and La on the increase in LP, because LP is the sum of Lc and La. It is clear that the increase in La is large. Relative to neat PBSC, the increase in La is 1.12 nm for an 80/20 blend; moreover, the increase in La is 3.36 nm for a 60/40 sample. The significant increase in La suggests that amorphous PVPh resides in the interlamellar region of PBSC spherulites in PBSC/PVPh blends.31,32 On the contrary, the increase in Lc is considerably smaller. Relative to neat PBSC, the increase in Lc is only 0.57 nm for an 80/20 blend; moreover, the increase in Lc is 1.39 nm for a 60/40 sample.

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ASSOCIATED CONTENT

S Supporting Information *

Three figures showing enlarged DSC traces of glass transition regions of neat PBSC, neat PVPh, and blends; POM images of neat PBSC and blends crystallized at the same supercooling; and SAXS profiles for neat PBSC and blends. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-10-64413161. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this research was financially supported by the National Natural Science Foundation, China (Grant 51221002).

CONCLUSIONS Miscibility, nonisothermal melt crystallization behavior, overall isothermal melt crystallization kinetics, spherulitic morphology and growth, crystal structure, and microstructure of PBSC/ PVPh blends were investigated with DSC, POM, WAXD, and SAXS in detail. The miscibility of PBSC/PVPh blends was first evidenced by the single composition-dependent glass transition temperature over the entire range of blend compositions; furthermore, the measured glass transition temperature values of PBSC/PVPh blends may be well fitted by the Gordon− Taylor equation with a q value of 0.25. The nonisothermal melt crystallization peak temperature shifts downward to low temperature range with increasing PVPh composition in PBSC/PVPh blends, relative to neat PBSC, indicating that blending with amorphous PVPh has suppressed the nonisothermal melt crystallization behavior of PBSC. Overall isothermal melt crystallization rates become slower with increasing crystallization temperature for both neat PBSC and PBSC/PVPh blends; however, the crystallization mechanism of PBSC remains unchanged, regardless of crystallization temperature and PVPh composition in the blends. It is also found that overall isothermal crystallization rates are faster in neat PBSC than in PBSC/PVPh blends at a given crystallization temperature; furthermore, they are faster in an 80/20 blend than in a 60/40 blend. Double melting behaviors were found for both neat PBSC and PBSC/PVPh blends, which may be well explained by the melting, recrystallization, and remelting mechanism. Accordingly, the equilibrium melting point values were determined to be 125.4, 117.1, and 114.4 °C for neat PBSC, 80/20 blend, and 60/40 blend, respectively. The depression of equilibrium melting point confirms again that PBSC and PVPh are thermodynamically miscible. The spherulitic morphology and growth rates of neat PBSC and its blends were investigated in a wide crystallization temperature range. Similar to the variation trends of the overall isothermal melt crystallization rates, the spherulitic growth rates

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