Unique Fractional Crystallization of Poly(l-lactide) - ACS Publications

Jun 5, 2017 - ABSTRACT: The crystallization behaviors and microstructures of poly(L-2-hydroxyl-3-methylbutanoic acid)/poly(L-lactide) blends [P(L-...
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Unique Fractional Crystallization of Poly(L‑lactide)/Poly(L‑2-hydroxyl3-methylbutanoic acid) Blend Dongdong Zhou,†,‡ Shaoyong Huang,† Jingru Sun,† Xinchao Bian,† Gao Li,*,† and Xuesi Chen*,† †

Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: The crystallization behaviors and microstructures of poly(L-2-hydroxyl-3-methylbutanoic acid)/poly(L-lactide) blends [P(L2H3MB)/PLLA] were investigated by OM, DSC, SAXS, in situ temperature-dependent WAXD, and in situ synchrotron WAXS. The blends exhibited a homogeneous state at 250 °C. In the cooling process, P(L-2H3MB), with higher melting temperature, crystallized first at 166.7 °C and drove the formation of the phase separation and microstructure. And the amorphous P(L-2H3MB) and PLLA were excluded into the interlamellar and interfibrillar regions of the former P(L-2H3MB) crystallites. For P(L-2H3MB)/PLLA (5/5), the amorphous P(L2H3MB) in the interfibrillar regions continued to form crystallites sequentially at 134.6 °C, which clearly confirmed the fractional crystallization of P(L-2H3MB). In our knowledge, this unique fractional crystallization has not been reported yet, of the component in miscible blend, which crystallized first under little confinement. This work provided a new viewpoint and understanding of the relationship between fractional crystallization and microstructures in crystalline/crystalline blends.

1. INTRODUCTION Recently, more and more attention has been attracted by the investigations of crystalline/crystalline blends, since they could present various morphologies and crystallization behaviors.1−15 Several unusual crystallization behaviors have been found in special systems, for instance, the fractional crystallization.14,15 It was characterized by several exothermal peaks in the nonisothermal crystallization process.16,17 The phenomenon of the factional crystallization was often observed in the microphaseseparated block copolymers and immiscible blends.18−20 And the crystalline components were confined in different isolated domains, which may contain different types of heterogeneities and induce the nucleation and crystallization under different supercoolings. In some cases, the homogeneous nucleation may also occur because the isolated domains did not boast any heterogeneities. It is more interesting that the fractional crystallization could also be observed in some melt-miscible blends. The confined and fractional crystallization of PEO in the poly(butylene succinate)/poly(ethylene oxide) (PBS/PEO) were investigated by He et al.15 PBS and PEO were miscible at the melt state. The crystallization of PBS was first performed at different temperatures and drove the phase separation of the blend. The component PEO with lower melting temperature (Tm) formed the domains in different regions (interlamellar, interfibrillar, and interspherulitic), which would induce the crystallization of PEO at different specific temperatures. In general, the appearance of fractional crystallization was always © XXXX American Chemical Society

ascribed to the confined component in different domains, regardless of the miscibility of the blends. Poly(L-2-hydroxyl-3-methylbutanoic acid) [P(L-2H3MB)], with similar side-chain structure of poly(L-lactide) (PLA), was focused by several researchers.21−25 The synthesis of P(L2H3MB) could be originated from L-valine, which suggested that P(L-2H3MB) also boasted biodegradability and biocompatibility.21,22 Tm of P(L-2H3MB) was reported to reach ∼230 °C, which was extremely higher than that of PLA (ca. 180 °C). And the crystallization and solid-state of P(L-2H3MB) were investigated by Marubayashi.23 The results revealed that its crystallinity was ∼60% as crystallized at 155−180 °C, and the equilibrium melting temperature reached 240 °C. It was more interesting that P(L-2H3MB) could form the homostereocomplex and hetero-stereocomplex with poly(D-2hydroxyl-3-methylbutanoic acid) and poly(D -2-hydroxylbutanoic acid), respectively.24,25 And Tm of the homostereocomplex and hetero-stereocomplex increased further. Nevertheless, the chemical and physical properties of P(L2H3MB)/PLLA blend were rarely reported. P(L-2H3MB) and PLLA are both semicrystalline polymers. The crystallization behavior and morphology of crystalline/ crystalline blend are complex and interesting because they are Received: April 25, 2017 Revised: May 24, 2017

A

DOI: 10.1021/acs.macromol.7b00855 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis Route of P(L-2H3MB) from L-Valine

Table 1. Summary of the Molecular Weight (Mn), Dispersity (Đ), and Basic Thermal Properties of Neat P(L-2H3MB) and PLLA

a

code

Mn/(104 g/mol)a

Đa

Tc/°Cb

ΔHc/(J/g)b

Tm/°Cb

ΔHm/(J/g)b

P(L-2H3MB) PLLA

3.90 5.34

1.13 1.36

173.3 112.1

37.3 38.2

228.7 177.2/169.7

39.5 45.1

Determined by GPC. bCalculated from DSC results in Figure 1. cooling and second heating processes at 10 °C/min were recorded. The temperature ranged from 250 to 0 °C for neat P(L-2H3MB) and P(L-2H3MB)/PLLA blends. While for neat PLLA, the temperature ranged from 200 to 0 °C. The heating flow and temperature were corrected by pure indium to ensure the reliability of the DSC tests. In-situ temperature-dependent wide-angle X-ray diffraction (WAXD) measurements were conducted at a Bruker D8 Advance X-ray diffractometer. The incident X-ray beam was Cu Kα radiation (1.54 Å). The samples, neat PLLA, neat P(L-2H3MB), P(L-2H3MB) (7/3), and P(L-2H3MB)/PLLA (3/7), were preheated to the melt state before measurement and then cooled to the desired temperature at a rate of 10 °C/min. The 2θ angle ranged from 10° to 30° with a rate of 2°/min. In situ synchrotron wide-angle X-ray scattering (WAXS) measurement was performed at Beijing Synchrotron Radiation Facility (BSRF, 44 Beijing, China), using 1W2A (λ = 1.54 Å) as the incident X-ray beam. The sample, P(L-2H3MB)/PLLA (5/5) blend, was heated to 250 °C and then cooled to 0 °C at 10 °C/min. The SAXS measurement was conducted at Bruker NanoStar-U using Cu Kα radiation (1.54 Å) under room temperature. The samples were preheated to the melt state, cooled to the desired temperature, and then quenched into the liquid nitrogen immediately.

influenced by not only the phase separation but also the interplay between the components. For the normal PLLA blends,1−5,9,26−32 such as PLLA/PEG, PLLA/PBS, and PLLA/ PCL blends, PLLA always crystallizes first under little confinement as crystallized from the melt state. But for the P(L-2H3MB)/PLLA blend, Tm of P(L-2H3MB) is greatly higher than that of PLLA, which would induce PLLA to crystallize under the confinement of P(L-2H3MB) crystallites. Furthermore, the miscibility between P(L-2H3MB) and PLLA is not clear. In the present study, P(L-2H3MB)/PLLA blends were prepared by the solution-casting method. The miscibility, crystallization behavior, crystal structure, and morphology of P(L-2H3MB)/PLLA were investigated by OM, DSC, SAXS, in situ temperature-dependent WAXD, and in situ synchrotron WAXS.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis and Sample Preparation. L-Valine, sodium nitrite (NaNO2), and p-toluenesulfonic acid (TsOH) were purchased from Aladdin and used as received. Isopropanol and stannous octoate [Sn(Oct)2] were bought from Aldrich. L-Lactide was obtained from Zhejiang Hisun Biomaterials Co., Ltd. (China), which was used after recrystallized three times in dried ethyl acetate. PLLA was synthesized by the ring-opening polymerization of L-lactide catalyzed by Sn(Oct)2 in the presence of isopropanol.33,34 The synthesis of cycle dimers of L-2-hydroxy-3-methylbutanoic acid and P(L-2H3MB) are both shown in Scheme 1. PLLA and P(L-2H3MB) products were purified by precipitation, using ethanol as the precipitant. The products were dried to constant weight at vacuum under 60 °C. P(L-2H3MB)/PLLA blends were prepared by the solution-casting method at room temperature. The dried P(L-2H3MB) and PLLA were separately dissolved in dichloromethane (CH2Cl2) at a concentration of 10.0 g/dL, and the weight ratios were set as 7/3, 5/5, and 3/7. The solutions were mixed together completely under vigorous stirring for 3 h. Then the mixed solution was poured into a Petri plate which was placed horizontally and volatilized freely at the room temperature. The sample was dried absolutely prior to use. 2.2. Characterization. 1H NMR spectra were recorded on a Bruker AV 300 MHz spectrometer. The solvent was chloroform-d (CDCl3). The spectra of L-2H3MB, cycle-dimers of L-2H3MB, and P(L-2H3MB) are presented in Figure S1. Gel permeation chromatography (GPC, Waters Instrument, USA) was employed to determine the molecular weights (Mn) and dispersity (Đ) of P(L-2H3MB) and PLLA. The GPC measurements were carried out in chloroform at 35 °C, and the results are shown in Table 1. The thermal properties of neat PLLA, neat P(L-2H3MB), and P(L2H3MB)/PLLA blends were investigated by the differential scanning calorimeter (DSC, Q100, TA Instruments, USA) under nitrogen flow (50 mL/min). The samples were preheated to the melt state, and the

3. RESULTS AND DISCUSSION 3.1. Basic Thermal Properties of Neat P(L-2H3MB) and PLLA. Figure 1 illustrates the DSC cooling and second heating thermograms of neat P(L-2H3MB) and neat PLLA, and the results are also listed in Table 1. The samples were preheated to the melt state. The cooling and heating rates were set as 10 °C/ min. Both neat P(L-2H3MB) and PLLA presented one crystallization peak at 173.3 and 112.1 °C, respectively. The crystallization enthalpies (ΔHc) of P(L-2H3MB) and PLLA were 37.3 and 38.2 J/g. In the second heating scans, the melting temperatures (Tm) of P(L-2H3MB) and PLLA were observed at 228.7 and 177.2 °C. And the fusion enthalpy (ΔHm) of P(L2H3MB) and PLLA reached 39.5 and 45.1 J/g. In addition, double melting behavior of PLLA was observed, which could be ascribed to the melt-recrystallization of PLLA crystallites. The melt-recrystallization may also result in the difference between ΔHc and ΔHm of PLLA. It is known that ΔHm for 100% crystallized PLLA is 93.6 J/g, but this value for P(L-2H3MB) has not been reported yet. As a result, ΔHc and ΔHm were employed to stand for the crystallinity of PLLA and P(L2H3MB) in the following. In general, Tm and Tc of P(L2H3MB) were higher than those of PLLA, thereby leading that P(L-2H3MB) crystallized first under little confinement as cooled from the melt state in P(L-2H3MB)/PLLA blends. Therefore, P(L-2H3MB)/PLLA blends may exhibit complex crystallization and melting behaviors. B

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Figure 1. DSC profiles of neat P(L-2H3MB) and PLLA: (a) cooling scans; (b) second heating scans.

Figure 2. DSC thermograms of P(L-2H3MB)/PLLA blends with different weight ratios at 10 °C/min: (a) the cooling scans; (b) the second heating scans.

3.2. Crystallization Behavior of P(L-2H3MB)/PLLA Blends. The crystallization and melting behaviors of P(L2H3MB)/PLLA blends with different weight ratios were studied by DSC, and the cooling and second heating profiles are presented in Figure 2. The locations of crystallization and melting peaks and their corresponding enthalpy are exhibited in Table 2. The samples were preheated to 250 °C to eliminate the thermal histories. In Figure 2a, as the weight ratio was 7/3, P(L-2H3MB)/PLLA blend presented two crystallization peaks at 167.0 and 115.0 °C, respectively. In the above section, the crystallization temperature (Tc) of neat P(L-2H3MB) and PLLA appeared at 173.3 and 112.1 °C. Thus, it could be confirmed that the crystallization peak at 167.0 °C was assigned to P(L-2H3MB) crystallization, and the peak at 115.0 °C belonged to PLLA. Similar results were observed when the weight ratios was 3/7. But for P(L-2H3MB)/PLLA (5/5) blend, three exothermal peaks were observed at 166.7, 134.6, and 88.6 °C, labeled as P1, P2, and P3, respectively. The first extensive peak P1 at 166.7 °C could corresponded to the crystallization of P(L-2H3MB), and the exothermal peak P3 at 88.6 °C was assigned to the PLLA crystallization. In previous works, the phenomenon of several crystallization peaks at different supercoolings was called as fractional crystallization. But how to clarify the appearance of peaks P2 at 134.6 °C? It cannot be ascribed accurately just due to the results of DSC.

In the second heating process (shown in Figure 2b), only two endothermal peaks appeared in the profiles of P(L2H3MB)/PLLA (7/3) and (3/7), which could be ascribed to the melting of P(L-2H3MB) and PLLA, and Tms of P(L2H3MB) and PLLA remained almost constant. But as the weight ratio was 5/5, the melting peak of PLLA appeared at 156.1 °C, which was lower than that of PLLA in P(L-2H3MB)/ PLLA (7/3) and (3/7) blends. In addition, an additional cold crystallization peak was observed at 98.0 °C, and the cold crystallization enthalpy (ΔHcc) was calculated as 10.6 J/g. It should be emphasized that the difference between ΔHc and ΔHm of PLLA was ca. 12.2 J/g, which was close to ΔHcc. At the same time, ΔHc of P(L-2H3MB) was almost the same as its ΔHm. Thus, we suggested that this cold crystallization may be assigned to PLLA. In order to discuss the crystallinities of P(L-2H3MB) and PLLA in P(L-2H3MB)/PLLA blends in details, ΔHc and ΔHm were corrected by their weight fraction, which are shown in Table S1. It was seen that the corrected crystallization and melting enthalpy (ΔHc′ and ΔHm′) of P(L-2H3MB) and PLLA were all lower than neat P(L-2H3MB) and PLLA, regardless of the weight ratios, which indicated the crystallization of P(L2H3MB) and PLLA were both suppressed in the blends. As the content of P(L-2H3MB) decreased, the corrected crystallization and melting enthalpy (ΔHc′ and ΔHm′) of P(L-2H3MB) decreased slightly. But ΔHc′ of PLLA was just 9.2 J/g in P(LC

DOI: 10.1021/acs.macromol.7b00855 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 2. Basic Thermal Properties of P(L-2H3MB)/PLLA Blends with Different Ratios

a

samples

Tc/°C

ΔHc/(J/g)

Tm/°C

ΔHm/(J/g)

7/3 5/5d 3/7

170.4a/108.4b 166.7a/134.6c/88.6b 167.7a/110.3b

19.8a/8.8b 13.1a/1.2c/4.6b 7.4a/25.7b

225.8a/175.2b 224.1a/156.1b 225.9a/175.2b

22.0a/10.1b 15.4a/16.8b 8.3a/27.1b

Contributed by P(L-2H3MB). bContributed by PLLA. cCannot be ascribed accurately. dCold crystallization: 98.0 °C (Tcc) and 10.6 J/g (ΔHcc).

2H3MB)/PLLA (5/5), which was obviously lower than that in the other two blends. Combined with Tc and Tm of PLLA, it could confirmed that the crystallization of PLLA was further suppressed in the cooling process of P(L-2H3MB)/PLLA (5/ 5). Generally, P(L-2H3MB)/PLLA blends exhibit complex crystallization and melting behaviors depending on the blend compositions. Then WAXD and SAXS were employed to investigate the crystal structures and microstructures of P(L2H3MB)/PLLA blends, which was expect to help better understand their crystallization and melting behaviors. 3.3. Fractional Crystallization of P(L-2H3MB). In Situ Temperature-Independent WAXD of Neat PLLA and Neat P(L-2H3MB). For comparison, the crystal structures of neat PLLA and neat P(L-2H3MB) were investigated by in situ temperature-independent WAXD. Cu Kα radiation (1.54 Å) was used as the incident X-ray beam. Neat PLLA and neat P(L2H3MB) were preheated to the melt state before the measurement. Then the samples were cooled to the desired temperature at 10 °C/min. Figure 3a present the WAXD profiles of PLLA in the cooling process. It could be seen that the diffraction peaks started to appear when the temperature reached 140 °C. Then the intensity of diffraction peaks enhanced quickly. As the temperature was lower than 120 °C, several peaks were observed at 15.0°, 16.9°, 19.1°, and 22.5°. The appearance of 15.0° and 22.5° indicated the formation of PLLA α-form crystals.35,36 With the temperature decreasing, the location of these peaks remained constant, which suggested the crystalline structure of PLLA did not change anymore. In addition, it is known that the area of the typical diffraction peaks could stand for the crystallinity of crystalline polymers in the in situ WAXD measurement. Here, the area of diffraction peak at 16.9° was employed to study the crystallinity of PLLA, and the result is shown in Figure S2a. The peak area increased gradually from ca. 130 to 110 °C but did not increase further as the temperature was below 100 °C, which was associated well with the DSC result in Figure 1a. The in situ temperature-independent WAXD profiles of P(L2H3B) are exhibited in Figure 3b. Typical diffraction peaks were observed at 14.0°, 17.1°, 18.8°, 20.9°, and 24.0°, and a similar result had been reported by Marubayashi.23 The area of the peak at 14.0° was chosen to stand for the crystallinity of P(L-2H3MB), which is shown in Figure S2b. When the temperature was 180 °C, the peak at 14.0° appeared. Then the area of peak at 14.0° increased quickly, until the temperature reached 140 °C, which was generally in agreement with DSC result. Similar with PLLA, the location of the diffraction peaks of P(L-2H3MB) also did not change with the temperature. In general, the crystal structures of neat PLLA and neat P(L2H3MB) did not alter in the cooling process from the melt state. In Situ Synchrotron WAXS of P(L-2H3MB)/PLLA (5/5) Blend. In order to study the unique crystallization behaviors and crystalline structures of the P(L-2H3MB)/PLLA (5/5) blend, in situ synchrotron WAXS was performed. The sample

Figure 3. In situ temperature-independent WAXD profiles in the cooling process from the melt state: (a) neat PLLA; (b) neat P(L2H3MB).

was first heated to 250 °C and then cooled to 0 °C at a rate of 10 °C/min. The WAXS profiles in the cooling process are presented in Figure 4. In the above section, the characteristic diffraction peaks of neat P(L-2H3MB) were observed at 14.0°, 17.1°, 18.8°, 20.9°, and 24.0°. At the beginning of the cooling process, no obvious diffraction peak was observed, indicating that the amorphous state remained as the temperature above 190 °C. After the temperature was below 180 °C, several peaks appeared at 14.0°, 17.1°, 18.8°, and 20.9°. Since the melting temperature of PLLA (Tm,PLLA) was lower than 180 °C, all these diffraction peaks corresponded to P(L-2H3MB) crystallites. Moreover, the peak at 17.1° gradually shifted to 16.9°, while the locations of diffraction peaks at 14.0°, 18.8°, and 20.9° remained constant, as the temperature below 110 °C. According to WAXD results of neat PLLA, the diffraction peak at 16.9° must be contributed by PLLA crystallites. It was also D

DOI: 10.1021/acs.macromol.7b00855 Macromolecules XXXX, XXX, XXX−XXX

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Corresponding to the second small crystallization peak (P2), the area of diffraction peak at 14.0° increased slightly as the temperature ranging from 150 to 130 °C, which suggested this exothermal peak should also belong to the secondary crystallization of P(L-2H3MB). The results clearly indicated that P(L-2H3MB), the component with higher melting temperature, crystallized at two different supercoolings, which was often called fractional crystallization. The in situ temperature-independent WAXD profiles of P(L2H3MB)/PLLA (7/3 and 3/7) blends are shown in Figure S3. The areas of diffraction peak at 14.0° of P(L-2H3MB)/PLLA (7/3 and 3/7) blends are plotted as a function of the temperature in Figure S4. As 7/3 and 3/7 blends were cooled from 250 °C, only one crystallization process was observed ranging from 180 to 140 °C. Combined with DSC cooling scans in Figure 2a, the fractional crystallization behavior was just absent in P(L-2H3MB)/PLLA (7/3 and 3/7) blends. Fractional crystallization behaviors had often been reported in many microphase separation block copolymers, immiscible blends, and miscible blends.14,15,17−20 The crystalline component, as the minor phase, was confined into different domains or microdomains, which would induce the crystallization under different supercoolings. Nevertheless, in this work, the fractional crystallization behavior of P(L-2H3MB) was observed, which boasted higher Tm and Tc, and crystallized first under little confinement. But it should be also noticed that the crystallization of P(L-2H3MB) could induce the phase separation and microstructures of P(L-2H3MB)/PLLA (5/5), which may be related with this ternary fractional crystallization. 3.4. Formation of the Microstructure in P(L-2H3MB)/ PLLA (5/5) Blend. It is well know that the morphology and crystallization of crystalline/crystalline polymer blends are not only influenced by the interaction between the crystallization of the components but also determined by the initial state of the blends. Therefore, the miscibility of P(L-2H3MB) and PLLA at the melt state was first investigated by SAXS and POM. The SAXS profile of P(L-2H3MB)/PLLA (5/5) blend at 250 °C is also shown in Figure 6 (the profiles, labeled as other temperatures, are discussed in the following), and the scattering peak was absent, which indicated the P(L-2H3MB)/PLLA (5/ 5) blend was a homogeneous state at 250 °C. In addition, there

Figure 4. In situ synchrotron WAXS profiles of P(L-2H3MB)/PLLA during cooling from 250 °C.

confirmed that the crystalline structure of P(L-2H3MB) in P(L2H3MB)/PLLA (5/5) blend did not alter with the temperature. For PLLA, it should be emphasized that the other diffraction peaks at 15.0°, 19.1°, and 22.5° were absent, which may be ascribed to the suppression of PLLA crystallization. In addition, the peak at 19.1° was too close to the peak at 18.8° of P(L-2H3MB) crystallites, which may also influence the detection of the peak at 19.1°. In the above section, the diffraction peaks of P(L-2H3MB) were confirmed to appear first, as the temperature was 180 °C. But PLLA formed crystallites until the temperature below 110 °C. So the exothermal peak P3 of P(L-2H3MB)/PLLA (5/5) blend in Figure 2a, ranging from 110 to 65 °C, was the crystallization peak of PLLA, and the exothermal peaks P1 and P2 were assigned to the crystallization of P(L-2H3MB). To further investigate the crystallization behaviors of P(L2H3MB)/PLLA (5/5) blend, the area of its diffraction peak at 14.0° as a function of temperature is shown in Figure 5. The peak appeared at 185 °C and enhanced very quickly until the temperature reached ca. 160 °C, which was associated with the main crystallization peak of P(L-2H3MB) (P1) in Figure 2a.

Figure 5. Area of diffraction peak at 14.0° as a function of the cooling temperature.

Figure 6. SAXS profiles of P(L-2H3MB)/PLLA (5/5) blend cooled from 250 °C to different temperatures. E

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Macromolecules was also no phase separation in the OM image of P(L2H3MB)/PLLA (5/5) blend at 250 °C (Figure S5), which also suggested the good miscibility of P(L-2H3MB) and PLLA at the melt state. The microstructures of the P(L-2H3MB)/PLLA (5/5) blend were further investigated by SAXS, which is also illustrated in Figure 6. Before SAXS measurements, the samples were preheated to 250 °C to eliminate the thermal history and then cooled to different temperatures, which were performed by DSC. Then the samples were quenched into liquid nitrogen. The temperatures were chosen as 150, 115, and 0 °C, corresponding to the ending of P1, P2, and P3, respectively. As the temperature was cooled to 150 °C, only one scattering peak (q1) was observed at 0.34 nm−1, which indicated the formation of lamellar structure. And this scattering peak was ascribed to the first crystallization of P(L-2H3MB) (P1). With the temperature decreased, the intensities of peak 1 increased generally. When the temperature was 115 °C, a broad and weak peak (q2) appeared at 0.64 nm and enhanced as the temperature was 0 °C. The ratio of the locations of two peaks (q2/q1) was 1.88, revealing that peak q2 was not the higher ordering peak of peak q1.37 It suggested the scattering peak q2 may be accompanied by the secondary crystallization of P(L-2H3MB) and the crystallization of PLLA (exothermal peaks P2 and P3 in Figure 1). The microstructure parameters of the P(L-2H3MB)/PLLA (5/5) blend are shown in Table 3. The long period (L) was

Figure 7. Schematic model for the mechanism of fractional crystallization and microstructures of P(L-2H3MB)/PLLA blend.

regions of crystallites. Thus, in the cooling process, P(L2H3MB) crystallized first and drove the formation of the lamellar structure, shown as Figure 7a. In the interlamellar and interfibrillar regions of P(L-2H3MB) crystallites, amorphous P(L-2H3MB) and PLLA mixed together. As the temperature decreased sequentially, the secondary P(L-2H3MB) crystallization was observed by DSC and in situ synchrotron WAXS (Figures 2a, 4, and 5). In the above sections, L1 and lc,1 of the scattering peak q1 did not change with temperature. So it was confirmed that PLLA and the secondary P(L-2H3MB) crystallites formed in the interfibrillar regions of former P(L2H3MB) crystallites (as shown in Figure 7b,c). In the section 3.2, DSC result proved that the crystallization of PLLA in P(L2H3MB)/PLLA (5/5) blend was further suppressed, which may be ascribed to the confinement of first and secondary P(L2H3MB) crystallites. In addition, it should be noticed that the secondary P(L-2H3MB) crystallization may be influenced by the environment of amorphous P(L-2H3MB) and PLLA in the inerfibrillar regions, which greatly depended on the composition of P(L-2H3MB)/PLLA blends. As a result, the fractional crystallization of P(L-2H3MB) was not observed in P(L2H3MB)/PLLA (7/3 and 3/7) blends. Generally, for P(L2H3MB)/PLLA (5/5) blend, owing to the specific confinement of former P(L-2H3MB) crystallites, the amorphous P(L2H3MB) in the interfibrillar region crystallized under larger supercooling and induced the fractional crystallization of P(L2H3MB).

Table 3. Basic Structures Parameters of P(L-2H3MB)/PLLA (5/5) Blend after Cooled to Different Temperatures temp/°C

q1/nm−1

L1a/nm

L1b/nm

lc/nm

150 115 0

0.34 0.34 0.34

18.5 18.5 18.5

17.8 18.1 17.9

7.27 7.25 7.29

a

The long period (L) was determined by L = 2π/q. bL was determined by the correlation function.

determined by the equation L = 2π/q. When the temperature was 150 °C, the long period of scattering peak q1 (L1) was 18.5 nm. L1 did not changed anymore because the location of q1 remained constant, and L1 could also be obtained by the correlation K(z), which is shown in Figure S6. It was seen that L1 by K(z) was associated well with that by q1. The crystalline thickness (lc,1), corresponding to the scattering peak q1, could also be calculated by the correlation K(z) from Figure S6. And lc,1 was 7.27 nm, as the temperature was cooled to 150 °C. When the temperature became 115 and 0 °C, lc,1 was 7.25 and 7.29 nm, respectively. In general, the long period and crystalline thickness did not increase with the secondary crystallization of P(L-2H3MB) and the crystallization of PLLA. 3.5. Mechanism of Fractional Crystallization Behavior of P(L-2H3MB). In order to discuss the fractional crystallization and microstructures of P(L-2H3MB)/PLLA blend in details, a schematic model is illustrated in Figure 7. Both OM and SAXS results suggested that P(L-2H3MB)/PLLA (5/5) blend was homogeneous at 250 °C. In the previous study, the crystallization behaviors and microstructures of the miscible crystalline/crystalline blends have been investigated. Since the initial state was homogeneous, the component with higher Tm would crystallize first and drove the formation of phase separation and lamellar structures. And the amorphous component was excluded into the interlamellar and interfibrillar

4. CONCLUSION In the present work, the crystallization behaviors and microstructure of P(L-2H3MB)/PLLA blend were investigated by OM, DSC, SAXS, and in situ synchrotron WAXS. P(L2H3MB) with similar structure of PLLA was synthesized by the ring-opening polymerization in this work. The crystallization temperature and melting temperature of P(L-2H3MB) were higher than those of PLLA. The OM and SAXS results revealed that the P(L-2H3MB)/PLLA blend was miscible and homogeneous at 250 °C. During the cooling process of the P(L-2H3MB)/PLLA (5/5) blend, three crystallization peaks were observed at 166.7, 134.6, and 88.6 °C, which were induced by the unusual fractional crystallization of P(L2H3MB). As crystallized from the melt state, P(L-2H3MB) crystallized first and drove the formation of phase separation and lamellar structure. And the amorphous P(L-2H3MB) and PLLA were excluded into the interlamellar and interfibrillar F

DOI: 10.1021/acs.macromol.7b00855 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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regions of the former P(L-2H3MB) crystallites. In the next step, P(L-2H3MB) and PLLA continued to crystallize at the interfibrillar region, which was demonstrated by SAXS and in situ synchrotron WAXS. And this fractional crystallization greatly did not appear in P(L-2H3MB)/PLLA (7/3 and 3/7) blends, indicating that the fractional crystallization greatly depended on the composition of blends. Unlike the normal fractional crystallization under different confinements, P(L2H3MB), with higher Tm and Tc, crystallized first under little confinement. The fractional crystallization of P(L-2H3MB) was induced by the confinement from its former crystallites, which may provide a new understanding of the relationship between the fractional crystallization and microstructures of crystalline/ crystalline blends.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00855. 1 H NMR of L-2H3MB, cycle dimers, and P(L-2H3MB); in situ temperature-independent WAXD profiles of P(L2H3MB)/PLLA (7/3 and 3/7) blends; the area of peaks at 16.9° and 14.0° as a function of temperature in neat PLLA, neat P(L-2H3MB), and P(L-2H3MB)/PLLA (7/ 3 and 3/7) blends; the optical micrograph of P(L2H3MB)/PLLA blend at 250 °C; SAXS analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected], Fax +86-0431-85262667 (G.L.). *E-mail [email protected], Fax +86-0431-85262112 (X.C.). ORCID

Gao Li: 0000-0003-1171-9179 Xuesi Chen: 0000-0003-3542-9256 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from National Natural Science Foundation of China (No. 51373169, 51473166, 51403199, 51573178, and 51403089) and National High Technology Research and Development Program (“863” Program) of China (No. 2015AA034004).

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DOI: 10.1021/acs.macromol.7b00855 Macromolecules XXXX, XXX, XXX−XXX