Formation of Mesomorphic Polymorph, Thermal-Induced Phase

Article Cite This: Cryst. Growth Des. 2019, 19, 166−176

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Formation of Mesomorphic Polymorph, Thermal-Induced Phase Transition, and Crystalline Structure-Dependent Degradable and Mechanical Properties of Poly(p‑dioxanone) Ying Zheng,† Jian Zhou,† Fanfan Du,† Yongzhong Bao,† Guorong Shan,† Liang Zhang,‡ Hongbo Dong,‡ and Pengju Pan*,†

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State Key Laboratory of Chemical Engineering, College of Biological and Chemical Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China ‡ Tianjin Dongnan Hengsheng Medical Technology Limited Co., 80 Haiyun Street, Economic-Technological Development Area, Tianjin 300450, China S Supporting Information *

ABSTRACT: Polymorphic crystalline structure has been a key factor determining the mechanical property and degradation behavior of biodegradable polymers. Herein, we report on the polymorphic crystalline structure, phase transition, and their effects on the mechanical property and degradation behavior of poly(p-dioxanone) (PPDO), a typical biodegradable and bioabsorbable semicrystalline polymer for biomedical applications. Both the polymorphic crystalline structure and crystallization kinetics of PPDO depend on crystallization temperature (Tc) drastically. Melting enthalpy and degree of crystallinity of PPDO first decrease and then increase with increasing Tc. PPDO forms a new mesomorphic polymorph (denoted as the α′-form) during crystallization at low Tc (≤10 °C), in contrast to the common α crystals generated at high Tc (≥60 °C). The α′ crystals have weaker interchain interactions and a shorter long period than the common α crystals, indicating the looser chain packing of α′ crystals. The α′ crystals are metastable, and they transform into the thermodynamically stable α crystals during heating. The α′-form PPDO possesses a slower degradation rate, higher flexibility, but lower strength and modulus than its α counterpart. This would provide a feasible way to tailor the degradation and mechanical properties of PPDO by varying crystal modification.

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INTRODUCTION Poly(p-dioxanone) (PPDO) is a synthetic aliphatic poly(ether ester) that can be prepared by the ring-opening polymerization (ROP) of 1,4-dioxan-2-one, also called as p-dioxanone (PDO). Chemical structures of PPDO and PDO are illustrated in Scheme 1. PPDO is a semicrystalline polymer and has been

As the mechanical property and biodegradation behavior of semicrystalline biodegradable polymers depend strongly on solid-state structure, both the structure and amount of noncrystalline and crystalline phases are key factors in determining the properties of biodegradable polymers. So far, the crystallization kinetics, spherulitic morphology, and crystal structure of PPDO have been studied by several research groups.3−8 As reported by Furuhashi et al.,3 the solution-grown crystal of PPDO has an orthorhombic unit cell with a P212121 space group and the lattice parameters of a = 0.970, b = 0.742, and c (chain axis) = 0.682 nm. Two molecular chains with antiparallel arrangement are involved in each unit cell. Gesti ́ et al.4 have proposed a similar crystal structure for PPDO, having the lattice parameters of a = 0.970 nm, b = 0.751 nm, and c (chain axis) = 0.650 nm. On the other hand, PPDO forms the ring-banded spherulites in melt crystallization; the band spacing changes with varying the crystallization temperature

Scheme 1. Chemical Structure of PDO and PPDO

widely used as the biomedical materials such as surgical suture, drug delivery system, and bone and tissue fixation devices, because of its outstanding biodegradability, biocompatibility, bioabsorbability, and good mechanical properties.1,2 Compared to the other biodegradable polymers such as poly(glycolic acid) (PGA) and poly(L-lactic acid) (PLLA) used for surgical sutures, PPDO has balanced flexibility, strength, and suitable degradation rate, due to the presence of ether bonds and lower ester group concentration.2 © 2018 American Chemical Society

Received: August 17, 2018 Revised: November 21, 2018 Published: December 5, 2018 166

DOI: 10.1021/acs.cgd.8b01246 Cryst. Growth Des. 2019, 19, 166−176

Crystal Growth & Design

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(Tc).9 Crystallization of PPDO also influenced the selfassembled structures of its copolymers in solution.10,11 Polymorphism has been a general behavior of semicrystalline polymers. Most of the semicrystalline polymers are able to form different polymorphs or crystal modifications by varying the crystallization or processing conditions. In the case of biodegradable polymers, their different crystal modifications usually exhibit distinct physical properties such as the thermal, mechanical, barrier, and degradation properties; this provides an effective way to tune the physical performances of the resulting materials. For example, in the case of PLLA, the αform displays higher modulus, heat deformation property, creep resistance, and barrier to water vapor than its α′ counterpart.12,13 The β-form PLLA possesses higher modulus and tensile strength than its α counterpart; the modulus and tensile strength of PLLA increase with increasing the β-form fraction.14 In the case of poly(butylene adipate) (PBA) that can form α and β polymorphs by varying Tc,15,16 the degradation rate of α-form PBA is faster than that of the βform.17 In the case of poly(3-hydroxybutyrate) [P(3HB)], the tensile strength and modulus of P(3HB) fiber increase, but its degradation rate decreases with the improvement of β-form fraction.18,19 As for the poly(vinylidene fluoride) (PVDF), the imperfection of the solid−solid crystalline phase transition from the paraelectric to ferroelectric phase plays a major role in its dielectric properties.20 Therefore, elucidating the relationships between polymorphic crystalline structure, crystallization condition, and physical properties is of fundamental importance for tuning the physical properties of semicrystalline polymers in processing. In spite of the recent progress on crystallization kinetics and crystalline structure of PPDO, the dependence of polymorphic crystalline structure and physical properties of PPDO on crystallization conditions is still unclear. To date, just a single polymorph has been reported for PPDO.3,4 In this work, we have systematically studied the polymorphic crystalline structure, phase transition, alkali catalyzed hydrolysis, and mechanical property of PPDO crystallized under different conditions. It is found that a new mesomorphic polymorph (assigned as α′-form) of PPDO is generated during crystallization at a low Tc (≤10 °C). The structural features (including lattice dimension, intermolecular interactions, and chain packing) and thermal-induced phase transition of α′form PPDO were investigated and compared with the common α-form PPDO. The relationships between polymorphic crystalline structure and alkali catalyzed hydrolysis, and mechanical property of PPDO were elucidated.

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Differential Scanning Calorimetry (DSC). Crystallization and melting behavior of PPDO were measured on a Netzsch 214 Polyma DSC (Netzsch, Germany) equipped with an IC70 intracooler under the nitrogen gas flow (40 mL/min). Thermal procedures for the isothermal crystallization of PPDO are illustrated in Figure S2. For the isothermal melt crystallization, after melting at 140 °C for 2 min, the sample (8−10 mg) was cooled to the desired Tc’s (0−90 °C) at a fast cooling rate of 100 °C/min and held at this temperature for enough time to crystallize. After crystallization, the sample was heated to 140 °C at a heating rate of 10 °C/min to observe the melting behavior. For the isothermal cold crystallization, after melting at 140 °C for 2 min, the sample was cooled to −70 °C at a fast cooling rate of 100 °C/min to prevent the crystallization during cooling. It was then fast heated to the desired Tc’s (0−90 °C) at 100 °C/min and held at this temperature to crystallize. After crystallization, the sample was heated to 140 °C at a heating rate of 10 °C/min to observe the melting behavior. Wide Angle X-ray Diffraction (WAXD) and Small Angle Xray Scattering (SAXS). WAXD and SAXS measurements were performed on the beamline BL16B1 of Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of X-ray is 0.124 nm. WAXD and SAXS patterns were collected by using a Rayonix SX-165 CCD detector (Rayonix, Illinois, USA). The sample-to-detector distances were 0.16 and 2.0 m in WAXD and SAXS analyses, respectively. The isothermally crystallized PPDOs used for WAXD and SAXS analyses were prepared according to the thermal procedures of isothermal crystallization (Figure S2). For the temperature-variable WAXD and SAXS analyses, the sample was sandwiched by polyimide films and heated from 35 to 120 °C at 10 °C/min on a Linkam THMS600 hot stage. WAXD and SAXS patterns were recorded with a temperature interval of 5 °C. The collection time of each WAXD or SAXS pattern was 15 s. For the nonstretched samples, the two-dimensional patterns were converted to the one-dimensional data by integration via a Fit2D software. FTIR Spectroscopy. FTIR spectra were measured on a NICOLET iS50 FTIR spectrometer (Thermo Scientific, USA) equipped with a MCT detector in the transmission mode. PPDO was sandwiched by two ZnSe slides, and it was melted at 140 °C for 2 min before being cooled to the desired Tc’s for isothermal crystallization. For investigating the Tc-dependent crystalline structure, the samples crystallized at different Tc’s were measured at 25 °C to eliminate the temperature effect on FTIR spectra. For the temperature-variable FTIR analysis, PPDO crystallized at 5 or 90 °C was heated from 25 to 120 °C at a heating rate of 5 °C/min in an Instec HCS402 hot stage (Instec Inc., Colorado, USA). The spectra were recorded with a 2.5 °C interval during heating. The spectra were collected with 32 scans and a resolution of 2 cm−1. Uniaxial Tensile Test. Tensile tests were performed on a SUNS UTM2503 instrument under a strain rate of 20 mm/min at ambient temperature. The dumbbell specimen with a length of 35 mm, crosssection width of 3.0 mm, and thickness of ∼0.4 mm was cut from the crystallized PPDO film. Seven replicated measurements were performed for each sample and the averaged results were used. Young’s modulus was calculated from the slope of stress−strain plot in the elastic region (strain range: 2−4%). Alkali Catalyzed Hydrolysis. PPDO film sample (thickness ∼0.4 mm) with a predetermined weight was immersed in the plastic tube containing 10 mL NaOH solution (0.02 mol/L). The tube was incubated in a water bath at 37 °C under moderate shaking. After a certain period, the sample was taken out and washed thoroughly by deionized water; it was then dried in a vacuum oven at 30 °C until a constant weight was obtained. The degradation ratio was calculated from the weight loss. Three replicated measurements were performed for each sample, and the averaged degradation ratio was used. Scanning Electron Micrographs (SEM). SEM images were measured on an SU-8010 SEM instrument (Hitachi, Japan) using an accelerated voltage of 3.0 kV.

EXPERIMENTAL SECTION

Materials. PPDO was obtained from Tianjin Dongnan Hengsheng Medical Technology Limited Co. (China) and used as received. Number-average molecular weight (Mn) and polydispersity (PDI) of PPDO are 30 kg/mol and 2.2, respectively. As shown in the thermogravimetric analysis (TGA) curve (Figure S1), the onset degradation temperature of PPDO is higher than 200 °C, and the thermal degradation rate is the fastest at 310 °C. Therefore, the thermal degradation of PPDO is neglectable in our crystallization and melting processes, in which the used highest temperature is 140 °C. Measurements. Gel Permeation Chromatography (GPC). Molecular weight and PDI of PPDO were measured on a Waters 1515 GPC instrument using CHCl3 as the eluent (flow rate: 1.0 mL/min) at 35 °C. Molecular weight was calibrated with the polystyrene standards. 167



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Figure 1. DSC results of PPDO: (a) DSC curves of PPDO recorded in isothermal melt crystallization at different Tc’s; (b) DSC heating curves of PPDO melt-crystallized at different Tc’s; (c) t1/2−Tc plot; (d) plot of Xc and ΔHm with Tc. The asterisk and arrow in panel b indicate the annealing peak and exotherm peak (Pexo), respectively.

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RESULTS AND DISCUSSION Crystalline Kinetics and Melting Behavior. Panels a and b of Figure 1 show the DSC curves of PPDO recorded in isothermal melt crystallization at different Tc’s (0−90 °C) and subsequent heating scans, respectively. As shown in Figure 1a, the crystallization peak is narrowest at 50 °C and becomes broader when Tc is lower or higher than 50 °C. The change of crystallization rate with Tc was quantitatively evaluated by crystallization half-time (t1/2) and depicted in Figure 1c, which was calculated by Avrami equation.21 The t1/2 shows a minimum at ∼50 °C; it is enhanced with increasing Tc to the melting temperature (Tm) side or decreasing Tc to the Tg side, due to the difficulty of crystal nucleation and chain diffusion in crystallization, respectively. As shown in Figure 1b, a small endotherm (indicated by the asterisk), so-called ‘‘annealing peak”,22,23 is observed at ca. 10− 20 °C above Tc. PPDO shows different melting behavior after crystallization at different Tc’s. Three typical melting curves were observed with varying Tc. A small exotherm (Pexo), indicated by the arrow in Figure 1b, is present prior to the dominant melting endotherm when Tc ≤ 60 °C. Double melting endotherms are observed in the DSC heating curves at Tc = 70−80 °C. However, a single melting peak appears for the PPDO crystallized at Tc = 90 °C, because of the formation of

thicker lamellar crystals at small supercooling. The coldcrystallized PPDOs show the similar Tc-dependent melting behavior (Figure S4). The multiple melting behavior can be attributed to three mechanisms: different crystal modifications, different lamellae population, and melt-recrystallization.24 The melting mechanism cannot be recognized solely from these DSC results; we would like to discuss this in the following section with a combination of the temperature-variable WAXD and FTIR results. Melting enthalpy (ΔHm) of PPDO was calculated from the DSC melting curve and plotted as a function of Tc in Figure 1d. Interestingly, ΔHm first decreases and then increases with increasing Tc, showing a minimum at Tc = ∼20 °C. This is different from the Tc-dependent ΔHm results of common polymers, whose ΔHm’s generally increase with Tc, due to the formation of more regular crystals at high Tc. The Tcdependent ΔHm of PPDO is similar to that of PLLA25 and would be associated with the change of crystal modification at different Tc’s and the thermal-induced phase transition upon heating, as discussed in the next section with combination of WAXD and FTIR results. The degree of crystallinity (Xc) was calculated by Xc = ΔHm/ΔHm0, in which ΔHm0 is the melting enthalpy of PPDO with infinite crystalline lamellae thickness (ΔHm0 = 141 J/g).26 As shown in the following part, PPDO forms different crystal modifications at high and low Tc’s. It is 168



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Figure 2. WAXD results of PPDO melt-crystallized at different Tc’s: (a) 1D WAXD patterns; (b) 2D WAXD of oriented PPDOs initially crystallized at 5 and 90 °C; (c) 1D WAXD integrated along the equatorial direction for the oriented PPDOs initially crystallized at 5 and 90 °C. Stretching direction is the meridian direction. The hkl Miller indexes were assigned according to the crystal structure of α-form PPDO.

diffraction peak around 2θ = 17.8° changes little with varying Tc (Figure 2a). Second, apart from the three major diffractions at 2θ = 17.8°, 19.2°, and 23.4°, many small diffractions are observed for PPDO crystallized at Tc = 90 °C; however, these small diffractions are absent for PPDO crystallized at Tc = 0 °C (Figure S5). These WAXD results resemble those of PPDOs cold-crystallized at various Tc’s (Figure S6). The melt-crystallized PPDOs are isotropic and contain various sizes of crystallites randomly oriented. Different crystallite sizes may result in the different breadths and positions of diffraction peaks. In order to exclude the effect of different crystal orientation on the breadth and position of diffraction peaks, WAXD patterns of oriented PPDOs initially crystallized at different Tc’s were analyzed. The oriented PPDOs were prepared by melt crystallization at Tc = 5 and 90 °C; and then stretched at 20 °C with the strains of 750% and 230%, respectively. Panels b and c of Figure 2 shows the 2D and 1D WAXD patterns of oriented PPDOs crystallized at Tc = 5 and 90 °C. As shown in Figure 2b, the (210) and (020) diffractions of oriented PPDO initially crystallized at 90 °C are concentrated in the direction perpendicular to the stretching force. However, the diffraction arcs overlaps for the oriented PPDO initially crystallized at 5 °C. As shown in Figure 2a,c, WAXD results of oriented PPDOs are similar to those of the isotropic PPDO films crystallized at the same Tc. Three major diffraction peaks, i.e., (210), (020), and (310) diffractions, are

assumed that the different crystal modifications of PPDOs have the similar ΔHm0; the same ΔHm0 value was used in the calculation of Xc’s of PPDOs crystallized at various Tc’s. Such assumption is frequently used in determining the Xc of polymorphic polymers (e.g., PLLA,27,28 and PBA29). It has been also reported that the different crystal modifications of polymorphic polymers (e.g., isostatic propylene,30 nylon-631) have the very similar ΔHm0 values. As shown in Figure 1d, the variation of Xc with Tc has the same tendency as that of ΔHm. Temperature-Dependent Formation of Polymorphic Crystals. Crystalline structure of PPDO crystallized at different Tc’s (0−90 °C) was investigated by WAXD and FTIR spectroscopy. Figure 2a shows the WAXD patterns of PPDOs melt-crystallized at different Tc’s. As reported previously, PPDO has three characteristic diffraction peaks at 2θ = 17.8°, 19.2°, and 23.4° (λ = 1.24 Å) at high Tc (e.g., 100 °C), corresponding to the diffractions of (210), (020), and (310) planes, respectively.3 As seen in Figure 2a, crystalline structure of PPDO is strongly influenced by Tc; WAXD patterns of PPDOs crystallized at high and low Tc’s are different. First, the diffraction peak of PPDO around 2θ = 19.2° cannot be clearly seen at low Tc (≤10 °C), but it becomes obvious at high Tc. The diffraction peak around 2θ = 19.2° shifts toward high 2θ with increasing Tc at Tc ≥ 20 °C; the peak shift is more obvious when Tc = 20−50 °C, as indicated by the arrow in Figure 2a. However, the position of 169



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Figure 3. FTIR spectra in different wavenumber regions for PPDO melt-crystallized at different Tc’s: (a) ν(CO) region; (b) ν(CH2) region.

O) (1825−1675 cm−1) and ν(CH2) (3080−2780 cm−1) regions. FTIR spectra of crystallized PPDOs are also influenced by Tc. As shown in Figure 3a, the crystallized PPDO exhibits dual absorption peaks at 1748 and 1734 cm−1, while the amorphous PPDO shows a broad absorption at 1754 cm−1 and a small shoulder at 1734 cm−1 in the ν(CO) region. The absorptions at 1748 and 1734 cm−1 are assigned to the ν(CO) vibration mode of PPDO chains; these bands become sharper and more split at high Tc. Relative to the 1748 cm−1 band, the intensity of 1734 cm−1 band increases remarkably with increasing Tc. As shown in Figure 3b, the crystallized PPDO mainly shows three absorptions at 2958, 2926, and 2880 cm−1 in the ν(CH2) region, compared to the broad absorption peak of amorphous PPDO. Additionally, the splitting bands at 2988 and 2979 cm−1 are present in the PPDOs crystallized at high Tc (≥60 °C) but absent in those crystallized at low Tc (≤10 °C). The splitting bands of 2988 and 2979 cm−1 become more pronounced with an increase of Tc. These FTIR results further demonstrate that the crystalline structures of PPDOs crystallized at high and low Tc’s are different. The band splitting has been widely observed in the crystallized polyesters that have relative stronger interchain interactions inside the crystalline phase.32,33,35,40 The crystallization-induced band splitting is usually caused by the correlation field splitting, owing to the interchain interaction between the adjacent molecular chains packed in crystal unit cell.41 To confirm the origin of band splitting, we analyzed the FTIR spectra of α and α′-form PPDOs during cooling from 90 (or 40 °C) to −100 °C, as shown in Figures S7 and S8. Cooling the polymer crystals would cause the contraction of crystal lattice, tighter chain packing, and shorter interchain distance. FTIR spectra of α′-form PPDO change little with cooling (Figure S7); however, the splitting bands (e.g., 2988, 1452, and 1376 cm−1) of α-form PPDO become more distinct with cooling (Figure S8). This indicates the weaker interchain interactions of α′ crystals than those of the common α ones. Since the interchain interactions are correlated with the interchain distance, the presence of interchain interactions of α-form PPDO would imply the denser chain packing within crystal lattice. The shift of (020) diffraction to a large angle with increasing Tc also supports this conclusion. According to the FTIR results of other polyesters,32,35,40 it is speculated that

observed for the oriented PPDO crystallized at 90 °C. However, the oriented PPDO crystallized at 5 °C only shows a broad diffraction peak (Figure 2c). These WAXD results signify that the crystal modifications of PPDOs crystallized at high and low Tc’s are different. WAXD pattern of PPDO crystallized at high Tc (≥60 °C) is in agreement with that of common PPDO reported in the literature;3 therefore, we assign the polymorph of PPDO crystallized at high Tc as the α-form. Considering the similarity in the strong diffraction peak at 2θ = 17.8°, PPDOs crystallized at low and high Tc’s may have the similar unit cell structure but different structural order concerning the packing and conformation of polymer chains. Accordingly, the limit model (limiting disordered modification) and mesomorphic form, named as the α′-form, is used to designate to the crystal modification of PPDO developed at low Tc (≤10 °C). Such designation has been widely used to define the mesomorphic polymorph of semicrystalline polymers such as PLLA,25,32−35 syndiotactic polystyrene,36 isotactic, and syndiotactic polypropylene (iPP, sPP).37,38 The sharpness and resolution of WAXD patterns can reflect the structural order of crystals. The presence of broad and lessresolved diffraction peak in α′-form PPDO would imply its less ordered structure associated with the chain packing and conformation. In the case of PPDO, the limiting disordered modification, α′-form, may have a similar unit cell structure but dissimilar degree of structural order from the common α-form. Because of the broad and less-resolved WAXD peaks presented in the PPDO crystallized at low Tc (≤10 °C), it is difficult to give an accurate identification on the unit cell structure (e.g., type, dimension) and chain conformation of α′-form PPDO based on the present WAXD results. More detailed structural characterization and calculation are required for determining the crystal structure of α′-form PPDO. Figure 3 shows the FTIR spectra in the CO and CH2 stretching [ν(CO), ν(CH2)] regions of PPDOs meltcrystallized at different Tc’s (5−90 °C), in which the spectrum of amorphous PPDO collected in the molten state (140 °C) is included for comparison. FTIR bands of amorphous and crystalline PPDOs were assigned and summarized in Table S1.39 Obviously, FTIR spectra of PPDO are sensitive to the crystallization process; the amorphous and crystalline PPDOs exhibit much different FTIR spectra, especially in the ν(C 170



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Figure 4. SAXS results of PPDO melt-crystallized at different Tc’s: (a) Lorentz-corrected SAXS profile; (b) changes of LP, Lc, and La with Tc.

Figure 5. Temperature-variable 1D WAXD patterns of PPDO during heating from 35 to 120 °C at 10 °C/min: (a) α′-form (Tc = 5 °C); (b) αform (Tc = 90 °C). The hkl Miller indexes were assigned according to the crystal structure of α-form PPDO.

the interchain interactions in α crystals of PPDO could be the dipolar interactions associated with the CH2 and CO groups of adjective chains. As shown in Figure 1d, PPDOs crystallized at different Tc’s exhibit the distinct Xc’s. Since the WAXD patterns and FTIR spectra of PPDOs shown in Figures 2a and 3 include the contributions of both amorphous and crystalline phases, they may be influenced by the Xc’s of used samples. To eliminate the effect of Xc, we improved and controlled the Xc of α′-form PPDO by stretching at 20 °C. The α-form PPDO (Xc = 56.5%) was prepared by melt crystallization at high Tc = 90 °C for 6 h. To prepare the α′-form PPDO with the similar Xc (55.5%), PPDO was first melt-crystallized at 5 °C for 12 h and then stretched at 20 °C with a strain of 750% to improve the Xc. Figures S9−S11 show the DSC heating curves, WAXD patterns, and FTIR spectra of α and α′-form PPDOs with the similar Xc’s. These results are similar to those of the α and α′form PPDOs prepared by direct crystallization at low and high Tc’s (Figures 2a and 3). Therefore, the differences of FTIR spectra and WAXD patterns between PPDOs crystallized at low and high Tc’s indeed result from the crystal modification, but not the crystallinity. Figure 4a depicts the SAXS profiles of PPDO meltcrystallized at different Tc’s. Long period (LP) was calculated

from the Bragg’s equation; i.e., LP = 2π/q*, where q* corresponds to the peak position of the Lorentz-corrected SAXS profiles (Iq2−q plot; I, scattering intensity; q, scattering vector). The scattering peak shifts toward smaller q with increasing Tc, indicating the enhancement of LP. This behavior is common in the polymer crystallization, due to the formation of thicker lamellae crystals at high Tc. As shown in Figure 4b, LP of PPDO increases from 7.3 to 9.1 nm as Tc increases from 0 to 90 °C. We have first tried to evaluate the thicknesses of crystalline (Lc) and amorphous (La) regions via the onedimensional electron density correlation function;42 however, the calculated Lc/(Lc + La) values deviate greatly from the Xc’s calculated from DSC. Therefore, the Lc and La were roughly estimated by Lc = LP × Xc and La = LP × (1 − Xc) in this work, as shown in Figure 4b. Lc and La vary little with Tc at Tc < 50 °C. However, Lc increases and La decreases with increasing Tc at Tc > 50 °C, indicating that the formation of thicker lamellar crystals at high Tc. Therefore, the α crystals have the larger lamellar thickness than their α′ counterparts. Heating-Induced Crystalline Structural Transition. In order to explore the thermal stability and phase behavior of different polymorphs of PPDO, the structural evolutions of α′and α-form PPDOs upon heating were investigated by the temperature-variable WAXD, FTIR, and SAXS analyses. Panels 171



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Figure 6. Temperature-variable FTIR results for α′-form PPDO (Tc = 5 °C) during heating from 25 to 120 °C at 5 °C/min: (a) spectra in the 1825−1675 cm−1 wavenumber range; (b) changes of peak height for 1734 and 1748 cm−1 bands. The intensities were normalized by the largest peak height of 1734 cm−1 band.

Figure 7. Temperature-variable SAXS results for α′-form PPDO (Tc = 5 °C) during heating from 35 to 120 °C at 10 °C/min: (a) SAXS pattern; (b) change of long period.

that no phase transition takes place in the heating process of αform PPDO before melting. Figure 6a shows the temperature-variable FTIR spectra of α′-form PPDO collected upon heating. The normalized intensity of 1734 and 1748 cm−1 bands (characteristic bands of crystalline phase) were evaluated and plotted as a function of temperature in Figure 6b. In the heating process of α′ crystals, the intensity of 1734 cm−1 band changes little below 60 °C but increases remarkably upon heating from 60 to 100 °C. The intensity of 1748 cm−1 band (characteristic band of both α′ and α crystals) also shows a slightly increase in the temperature range of 70−100 °C. These results further confirm the occurrence of α′-to-α phase transition during heating. However, the α-form PPDO exhibits much different changes in the temperature-variable FTIR analysis (Figure S12). As shown in Figure S12, the spectral shape and the 1734, 1748 cm−1 band intensities vary little during heating below 100 °C. For both the α′ and α-form PPDOs, the intensities of 1734 and 1748 cm−1 bands (characteristic bands of crystalline phase) decrease drastically with heating to above 100 °C, because of the crystal melting.

a and b of Figure 5 shows the temperature-variable WAXD patterns of α′ and α-form PPDOs collected in the heating process, respectively. As shown in Figure 5a, the diffraction angle of the peak around 2θ = 17.8° [(210) diffraction of α crystals] vary little upon heating from 35 to 105 °C. However, the diffraction intensity of the peak around 2θ = 19.2° [(020) diffraction of α crystals] becomes more obvious upon heating from 50 to 80 °C and gradually shifts toward larger angle upon heating from 50 to 90 °C, as indicated by the arrow in Figure 5a. The WAXD patterns observed at high temperature (90− 105 °C) are the same as those of α-form PPDO. The (210) and (020) diffraction peaks of the α-form disappear upon further heating to 120 °C, due to the crystal melting. On the basis of these WAXD results, it can be concluded that in the heating process of α′-form PPDO, the crystal modification is almost unchanged upon heating to 55 °C, but it gradually transforms into the α-form with further heating to 90 °C. The thermal-induced structural change of α crystals was also investigated for comparison, as shown in Figure 5b. The positions of (020) and (210) diffraction peaks are almost unvaried or slightly increase with heating to 100 °C, indicating 172



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Figure 8. Alkali catalyzed hydrolysis results of PPDO melt-crystallized at 10, 50, and 90 °C in NaOH solution (0.02 mol/L) for different periods: (a) degradation ratio (%); (b) SEM images of degraded sample. The numeral in panel a indicates the degradation rate (h−1).

(0.02 mol/L) at 37 °C for different periods. The degradation ratios of PPDO films increase linearly with the degradation time. Therefore, the slopes of degradation ratio vs degradation time curves were used to indicate the degradation rate. Interestingly, the degradation rate of PPDO exhibits a much unique dependence on Tc. The α′-form PPDO generated at low Tc (10 °C) undergoes slow degradation, having a degradation rate of 0.0011 h−1. However, the α-form PPDO crystallized at high Tc (90 °C) shows the fast degradation, whose degradation rate is 2.4 times of α′-form. The degradation rate of α′/α-mixed PPDO was 0.0021 h−1, which was between those of the α′- and α-form. Similar results are also demonstrated by the SEM results of degraded PPDO films (Figure 8b), in which less pores are observed on the surface of α′-form PPDO films crystallized at low Tc (10 °C). As shown in Figure 8b, the surface morphology of PPDO films changes greatly with Tc and degradation time. More holes are observed in the film surface as the Tc and degradation time increase. In general, the degradation rates of aliphatic polyesters drop with increasing Xc and lamellar thickness,43−46 because the amorphous phase can be more readily degraded than the crystalline phase. We proposed that the unique Tc-dependent degradation kinetics of PPDO could be attributed to two aspects. First, as indicated by POM results shown in Figure S15, PPDO forms the ring-banded spherulites in melt crystallization; the banded rings can be more clearly observed at high Tc. It is generally believed that the formation of banded spherulites is caused by the twisting or bending of crystalline lamellae along the growth direction.47,48 The lamellar twisting or bending would increase the contact area between the crystalline lamellae and degradation media, thus promoting the degradation of polymer. Second, the Tc-dependent degradation kinetics of PPDO is somewhat similar to those of PLLA.49 The alkali catalyzed hydrolysis of PPDO would proceed via the bulk erosion mechanism, since the degradation temperature is higher than Tg.50 Because the chain terminals of semicrystalline polymers tend to segregate into the amorphous regions, the amorphous region of α-form PPDO crystallized at high Tc would have a higher density of terminal carboxyl and hydroxyl groups, due to the smaller fraction of amorphous phase. The higher density of terminal carboxyl and hydroxyl groups can enhance the hydrophilicity of amorphous domain, thus improving the penetration and diffusion of water.49 This

Figure 7a depicts the temperature-dependent SAXS profiles of α′-form PPDO upon heating from 35 to 120 °C. A single scattering peak is observed, and the scattering peak gradually shifts toward lower q upon heating. Consequently, LP of PPDO enhances with heating, which is more obvious in the temperature range of 85−105 °C. LP of PPDO is 13.2 nm at 105 °C, which is 1.75 times of that measured at 35 °C (7.6 nm). The heating-induced increase of LP is ascribed to the heating-induced lamellar thickening and α′-to-α phase transition. As measured by polarized optical microscopy (POM), we did not see the discernible changes of spherulitic morphology upon heating α′-form PPDO (Figure S13). All these temperature-variable WAXD, FTIR, and SAXS results suggest that the α′ crystals of PPDO formed at low Tc are thermally metastable, and they transform into the thermodynamically stable α crystals upon heating. The thermal-induced α′-to-α crystal transition is irreversible and the transformed α crystals do not recover to their α′ crystals during cooling, as demonstrated by the in situ WAXD and FTIR results. Multiple melting behavior of PPDO can be explained with combination of the phase transition behavior. Pexo observed in the DSC heating curves (Figure 1b) of α′form PPDO generated at low Tc (≤10 °C) would originate from the α′-to-α phase transition; while the dual melting peaks of α-form PPDO formed at high Tc ≥ 70 °C is ascribed to the conventional melt-recrystallization mechanism.5 To discuss the phase transition mechanism, we evaluated the intensity change of WAXD and SAXS peaks for α′-form PPDO (Tc = 5 °C) upon heating from 35 to 120 °C from Figures 5a and 7a. As shown in Figure S14, no decrease of WAXD and SAXS peak intensities is observed during the phase transition below Tm. This indicates that the phase transition does not proceed via the melt-crystallization mechanism, in which WAXD and SAXS peak intensities will first decrease and then increase.3 Therefore, we speculate that the α′-to-α phase transition of PPDO proceeds via the solid-to-solid mechanism. Alkali Catalyzed Hydrolysis and Mechanical Property. Effects of crystalline structure and crystal modification on the alkali catalyzed hydrolysis and mechanical property of PPDO were further investigated. The α′-from, α′/α-mixed, and αform PPDOs used for alkali catalyzed hydrolysis and mechanical property tests were prepared by crystallizations at 10, 50, and 90 °C; they had the Xc’s of 44.5%, 44.2%, and 56.5%, respectively. Figure 8 shows the degradation ratios and SEM images of PPDOs after hydrolysis in NaOH solution 173



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Figure 9. Tensile properties of PPDO crystallized at different Tc’s: (a) stress−strain curve; (b) yield stress; (c) Young’s modulus; (d) breaking strain.

leads to the faster degradation of α-form PPDO than its α′ counterpart. Figure 9a shows the stress−strain curves of PPDO crystallized at different Tc’s measured by uniaxial tensile test. The yield stress (σy), Young’s modulus (E), and breaking strain (εb) of PPDOs were evaluated and plotted as a function of Tc in panels b−d of Figure 9, respectively. As shown in Figure 9a, the stretching behavior of PPDO is very peculiar; the tensile stress fluctuates during necking in the uniaxial tensile test, called as “stress oscillation”. Macroscopically, multiple necking and neck propagation are observed during stretching. This phenomenon has been observed in the other polymers such as poly(ethylene terephthalate) and sPP.51,52 Because of the multiple necking and neck propagation, alternating stripe zones appear in the specimen under stretching, as illustrated in Figure S16. Even though several mechanisms including the local heating caused by orienting elongation51 and the stretching-induced crystallization53 have been proposed to explain the stress oscillation, the accurate mechanism of stress oscillation is still under debate. Mechanical properties of PPDO are strongly influenced by the crystal modifications. The α′-form PPDO prepared by crystallization at low Tc (10 °C) is highly flexible and ductile; it has a large εb of 8.5, a σy of 19.9 MPa, and an E of 138.2 MPa. However, the α-form PPDO prepared by crystallization at high

Tc (90 °C) is rigid, having a small εb of 2.3, a large σy of 37.8 MPa, and a large E of 334.3 MPa. The σy, E, and εb of PPDOs can be tuned within the wide ranges (19.9−37.8 MPa for σy, 138.2−334.3 MPa for E, and 2.3−8.5 for εb) by varying Tc (Figure 9b−d). A similar trend is reported for PLLA crystallized at different Tc’s, due to the increase of Xc.54 The higher flexibility and ductility of α′-form PPDO can be ascribed to the lower Xc, less ordered, and looser chain packing inside the crystalline phase. Therefore, control over the crystal modification would be an effective approach to tailor the degradable behavior and mechanical properties of PPDO.

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CONCLUSION In conclusion, we have found the Tc-dependent polymorphic behavior of PPDO in this study. Both the crystalline structure and crystallization kinetics of PPDO are strongly influenced by Tc. PPDO shows the fastest crystallization rate at ∼50 °C in the isothermal melt crystallization. A new mesomorphic polymorph, denoted as the α′-form, is assigned for PPDO crystallized at low Tc (≤10 °C), in contrast to the common αform generated by crystallization at high Tc (≥60 °C). Melting enthalpy and degree of crystallinity of PPDO first drop then are enhanced as Tc increases. The α′ crystals possess weaker interchain interactions than the α ones, suggesting the looser chain packing of the former. The α′ crystals are less stable, and 174



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they transform into the thermodynamically stable α crystals upon heating. Degradation behavior and mechanical properties of PPDO are strongly correlated with its crystalline structure. The α′-form PPDO has the slower degradation rate and higher flexibility, but lower strength and modulus than its α counterpart. This study has elucidated the relationships between crystallization conditions and polymorphic crystalline structures of PPDO, and would provide a feasible method to tune the degradation and mechanical properties of PPDO through controlling the processing or crystallization conditions.

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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.cgd.8b01246. TGA, DSC thermal program, partial DSC, WAXD, FTIR, SAXS, POM data and images of stretched specimen of PPDO (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-571-87951334; e-mail [email protected]. ORCID

Guorong Shan: 0000-0001-5676-6310 Pengju Pan: 0000-0001-6924-5485 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of China (2016YFC1100801, 2016YFC1100805). WAXD and SAXS experiments were carried out on the beamline BL16B1 of SSRF, China.

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Formation of Mesomorphic Polymorph, Thermal-Induced Phase

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