Mesophase-Mediated Crystallization of Poly(l-lactide): Deterministic

Publication Date (Web): September 27, 2016 ... the CO2-induced mesophase with extremely high nucleation density underwent disordering–reorganization...
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Mesophase-Mediated Crystallization of Poly(L‑lactide): Deterministic Pathways to Nanostructured Morphology and Superstructure Control Qiaofeng Lan* and Yong Li Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences (CAS), Ningbo 315201, China S Supporting Information *

ABSTRACT: The effect of the CO2-induced mesophase on the isothermal crystallization of poly(L-lactide) (PLLA) was investigated by infrared (IR) spectroscopy and microscopy. It was found that the crystallization rate of PLLA was significantly enhanced by the CO2-induced mesophase, showing that the crystallization was completed even in a short period of 100−101 s. Compared with the directly crystallized samples that showed typical spherulites with lamellae, the crystallization via CO2-induced mesophase led to nonspherulitic (granular or featureless) morphologies consisting of nanorods, whereas the polymorphic behavior remained unaffected by the initial state, resulting in crystallized PLLA containing identical polymorphs of uniquely different nanostrutured morphologies and superstructures. The IR imaging results indicated that the formation of the equilibrium crystal was preceded by the formation of various metastable intermediate phases, including mesomorphic phase, preordering, and metastable crystal, all of which continuously evolved with time. The nucleation process proceeded via a similar pathway. In contrast to the negligible contribution of mesophase to the nucleation in direct crystallization, the CO2-induced mesophase with extremely high nucleation density underwent disordering−reorganization into the preordering, thereby providing a tremendous number of active nucleation sites for enhancing crystallization and serving as building blocks for nanorods. Importantly, the present results highlight the decisive role of mesophase in directing the nanostructure and superstructure and support a multistep process for the crystallization (including nucleation and crystal growth) of PLLA, validating the Ostwald step rule, providing mechanistic insights into the crystallization of polymers. about 2 min,9,26 which further increases with increasing D-unit concentration in the main chain; for example, for PLLA having 3−4% D-units, their minimum half-times of crystallization exceed 20 min.9,26 As a consequence, under application-relevant cooling conditions of PLLA, slow cooling usually results in bigsize spherulites, whereas fast cooling typically leads to inhibition of crystallization and thus formation of glassy state,26 thereby restricting its extensive applications. Therefore, to overcome this, considerable attention has been devoted to improve the crystallization kinetics of PLLA.9,10 In view of the well-known fact that the crystallization process consists of two major events, i.e., nucleation and crystal growth, the overall crystallization rate of PLLA can be typically enhanced by adding heterogeneous nucleators such as talc and graphene fillers, which provide a large number of nucleation sites and thus pronounced nucleating effect for enhancing the crystallization of PLLA.9,27,28 The addition of plasticizers is also usually used to increase the crystal growth rate by promoting the mobility.9,29 The combination of both

1. INTRODUCTION Biodegradable poly(L-lactide) (PLLA), an increasingly important biobased polymer that is derived from renewable resources, has attracted great interest due to its attractive merits such as good biocompatibility, promising processability, and favorable mechanical properties, which in turn makes it promising for numerous commercial applications in medical field, tissue engineering, and packaging materials.1−12 Similar to other semicrystalline polymers, the macroscopic physical properties of the PLLA material are dependent on the relevant microscopic parameters of crystalline structure/morphology,10−24 such as crystallinity,18−20 polymorphisms,21,22 and lamellae orientation.23,24 For instance, higher degree of crystallinity endows PLLA products with enhanced mechanical strength and thermal stability,20 whereas specific polymorph form offers them better mechanical properties.21,22 Despite the considerable practical importance and many advantages, however, one of the major limitations of PLLA has to be addressed is its low crystallization ability.9 Compared to those polymers having fast crystallization rates such as classical semicrystalline polyethylene, which possesses very short halftime of crystallization of 10−2−100 s at moderate supercoolings,25 PLLA has minimum half-time of crystallization of © XXXX American Chemical Society

Received: July 5, 2016 Revised: September 15, 2016

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a previous work, we reported the highly enhanced mesophase formation in PLLA by readily accessible low-pressure CO2 at low temperatures (e.g., 2 MPa and 0 °C),38 allowing one to tune the mesophase fraction and thus degree of ordering at a controllable level, but the effect of mesophase on the isothermal crystallization of PLLA has not been revealed yet. Moreover, although the crystallization behavior of PLLA has been studied extensively,9 the microscopic understanding of the crystallization process is still rather incomplete. The present work aims to further investigate the effect of CO2-induced mesophase on the crystallization at elevated temperatures and the resulting crystalline morphology of PLLA in terms of CO2 treatment time by Fourier transform infrared (FTIR) spectroscopy, polarized optical microscopy (POM), and atomic force microscopy (AFM). Furthermore, using FTIR spectroscopic imaging, we show that different intermediate phases distribute along the direction of spherulitic growth in direct crystallization, revealing a signature of the pathway of crystal growth being mediated by transient metastable phases including mesophase. This evidence leads us to propose a multistage model for the nucleation and crystal growth of PLLA, which can also be used to account for the present experimental findings of crystallization kinetics, nanostructure morphology, and polymorphic behavior in differently crystallized PLLA. More importantly, this work aims in particular to provide mechanistic insights into the role of metastable mesomorphic phase or mesophase in the crystallization (i.e., nucleation and growth) of polymers.

the nanofillers and plasticizers represents another effective approach to facilitate the crystallization by simultaneously providing heterogeneous nucleator and chain mobility promoter.9,30,31 Additionally, for unmodified and thus homogeneous PLLA system, it has been reported that the physical aging at high supercoolings/low temperatures around the glass transition temperature (Tg) can lead to the formation of the locally ordered structures,32−37 which accelerates the subsequent cold crystallization at elevated temperatures. Nevertheless, the ability to obtain high fraction of such structural ordering in PLLA is still very limited due to its kinetically slow nature of the crystallization,37,38 resulting in limited enhancement in crystallizability. Moreover, it is believed that this local structural ordering is a kind of mesomorphic phase or mesophase,37,38 which may act as nucleation sites. Differing from PLLA, for polymers having fast crystallization rates such as isotactic polypropylene (iPP), polyamide 11 (PA 11), and poly(butylene naphthalate) (PBN), the mesophase formation can be much more easily achieved by rapid quenching/cooling the isotropic melt to low temperatures around Tg.39−49 Moreover, on the basis of the formation of mesophase and appropriate postprocessing conditions, their microstructures and the ultimate properties can be easily tailored,45−49 making them promising for great potential for specific applications. As for PLLA, the formation of mesophase can be promoted to some extent by blending or copolymerization,50−52 which, in turn, inevitably leads to some negative aspects such as incompatibility and deterioration of homogeneity. As such, because of difficulties in obtaining controlled formation of mesophase, the knowledge about their effects on the crystallization and thus the microstructure and crystalline morphology has not been established for PLLA; thus, the ability to tune the ultimate properties of PLLA through the mesophase pathway is largely restricted. On the other hand, from a perspective of molecular mechanism, the mesomorphic phase in the crystallization of polymers has been an interesting subject and attracted increased attention.53−59 For example, it has been proposed that the growth and thus the formation of the lamellar crystallites in polymer crystallization is a multistage process passing over intermediate metastable structures.54−57 Furthermore, some reports have suggested that the crystallization is mediated by the formation of metastable intermediate phases and preordering.57−63 These multiple steps of crystallization seem to contradict the classical expectations;64 much more experimental evidence is therefore required. PLLA can crystallize into α, α′ (δ), α″, β, γ, and ε polymorphs under various crystallization conditions (e.g., thermal and solvent treatments).22,65−76 Generally, the most common pure α and α′ forms can be developed by the normal thermal crystallization at high (above 120 °C) and low (below 100 °C) crystallization temperatures, respectively.51,73−75 Previous works indicate that this polymorphic behavior PLLA is not affected much by the molecular weight;73 however, the critical temperature for forming pure α or α′ form can be greatly reduced by blending miscible noncrystalline poly(D,Llactide) (PDLLA).75 Additionally, it is generally believed that the α′ polymorph is metastable to some extent.75−78 Despite these advancements, however, the mechanism of this polymorphic behavior in PLLA is not fully understood. The above reports elucidate the crucial role of the metastable mesomorphic phase or mesophase in understanding and regulating the crystallization of polymers including PLLA. In

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Poly(L-lactide) (PLLA) (M̅ w = 1.6 × 105, M̅ w/M̅ n = 1.37) was kindly supplied by Corbion Purac (The Netherlands). The glass transition temperature (Tg) and the peak melting temperature (Tm) of the PLLA sample are equal to ca. 60 and 177 °C, respectively. CO2 with a purity of 99.995% was supplied by Shanghai Tomoe Gas Co., Ltd., China. Dichloromethane (CH2Cl2) (AR grade) was used as received. All melt-quenched PLLA films at ca. 5 μm (unless otherwise specified) were prepared by casting CH2Cl2 solution (10 mg/mL of PLLA) onto substrates of cover glass (ca. 130 μm thick) according to our previous report.38 2.2. CO2 Treatment and Thermal Annealing. In order to obtain PLLA mesophase with different fraction, the melt-quenched PLLA films were treated by CO2 at a fixed pressure of 2 MPa and a fixed treatment temperature (Tt) of 0 °C for a certain time according to our previous work.38 The treated films were kept under dry atmosphere for 24 h prior to further thermal treatment and measurements. Note that the fraction of mesophase and its degree of ordering formed at a fixed condition (2 MPa and Tt = 0 °C) are solely dependent on the treatment time (tt). Here, the influence of the mesophase and its related structural features on the subsequent crystallization of PLLA at elevated temperatures (under atmospheric pressure) was discussed in terms of tt for brevity. For the thermal annealing, all CO2-treated film samples on substrates were rapidly heated to a desired temperature (Ta) at atmospheric pressure and held for a certain period of time (ta), followed by quenching to 0 °C. The temperature−time profile for the CO2 treatment and/or the subsequent thermal annealing under atmospheric pressure is shown in Figure S1. 2.3. Measurements. The Fourier transform infrared (FTIR) spectroscopic analysis of melt-quenched and treated/annealed PLLA samples was performed on a Thermo-Nicolet 6700 FTIR spectrometer. Transmission scans were conducted between wavenumbers of 4000 and 400 cm−1 at a resolution of 2 cm−1 and scan numbers at 64. B

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Macromolecules The IR spectroscopic imaging measurement was performed on a Cary 600 series FTIR spectrometer (Agilent Technologies) equipped with a 32 × 32 pixel focal plane array (FPA) detector. Image data (transmission mode) were collected between 4000 and 700 cm−1 at 4 cm−1 spectral resolution and scan numbers at 128 using 5.5 μm pixel resolution. The area of the sample (ca. 20 μm thick) was imaged over 176 μm × 176 μm. Acquisition of an image consisting of 1024 spectra took ca. 10 min. Before FTIR and FTIR imaging measurements, the free-standing PLLA films were obtained by floating the samples off the glass substrate onto the surface of water and then were dried thoroughly in a vacuum at room temperature. The crystalline superstructure was observed on an Axioskop 40A Pol POM (Carl Zeiss, Germany) and recorded with a connected digital camera. The nanoscopic surface morphology of the films on substrates of cover glass was obtained at room temperature using an atomic force microscope (AFM) (Dimension 3100 V, Veeco, US). Both height and phase images were recorded simultaneously using the retrace signal. Silicon tips with a resonance frequency of 300 kHz and a spring constant number of 40 N/m were used, and the scan rate ranged from 0.3 to 3 Hz (corresponding to scan size of 5 × 5 to 0.5 × 0.5 μm2) with the scanning density of 512 lines/frame. Wide-angle X-ray diffraction (WAXD) analysis was performed on a Bruker D8 Advance diffractometer with Cu Kα radiation (40 kV and 40 mA) and a wavelength of 0.154 18 nm. The scanning 2θ angle range was from 5° to 30° with a scanning rate of 4°/min. Differential scanning calorimetry (DSC) measurement was carried out on a TA Instruments Q-2000 DSC at a heating rate of 3 °C/min under a nitrogen flow of 50 mL/min.

3. RESULTS AND DISCUSSION 3.1. Mesophase Greatly Enhanced Crystallization of PLLA Revealed by FTIR. We first investigated the effect of the mesophase in terms of tt and the annealing temperature (Ta) on the cold crystallization/ordering by using FTIR. Melt-quenched PLLA films were treated in CO2 for tt = 101−104 s and then subjected to Ta = 130 °C to allow the isothermal annealing for cold ordering for ta = 30 s, followed by quenching to 0 °C to disrupt the crystallization. If the ta is too long, then the amount of the resultant structure would not be a function of the tt because of the completed crystallization. As such, a short ta = 30 s was selected in order to effectively monitor the differences of the fraction of the resultant crystalline structure. Figure 1a shows the FTIR spectra for the annealed film samples in the region of 970−850 cm−1, which is sensitive to the formation of the structural ordering in PLLA. Specifically, the bands around 918 and 922 cm−1 are relevant to the helical chain conformation of mesophase and crystal, respectively, whereas the band at 956 cm−1 is assigned to the amorphous phase.37,38,79,80 For the untreated sample (i.e., tt = 0 s, ta = 30 s), the mesomorphic/crystalline 918/922 cm−1 band in the IR spectrum shows very weak intensity and is almost similar to that of melt-quenched one (i.e., tt = 0 s, ta = 0 s), implying that no apparent ordered structure was formed due to the insufficient crystallization time of ta = 30 s. However, upon annealing for the same period of time (30 s), notable changes in the IR spectra are observed for various treated samples. Even for tt = 15 s, the absorbance of the band at 956 cm−1 (characteristics of amorphous phase) decreases, whereas a new band appears at around 922 cm−1 (crystalline phase). Meanwhile, the band initially observed for tt = 0 s shifts to the higher frequency to 872 cm−1 and becomes sharper. These observations indicate that obvious crystalline phase was produced despite the short treatment time (tt = 15 s) and annealing time (ta = 30 s). With further increasing tt, judging from the concurrent enhancement/reduction in intensity of the

Figure 1. FTIR spectra (a) in the wavenumber range of 970−850 cm−1 and the calculated fraction (b) of structural ordering for PLLA films that treated under 2 MPa of CO2 at Tt = 0 °C for different tt and then thermally annealed at Ta = 130 °C for ta = 30 s. Dashed line is shown to guide the eye.

922/956 cm−1 band, respectively, it is clear that the amount of the resultant ordered phase in annealed PLLA increases with increasing original mesophase fraction. In order to quantitatively understand the effect of mesophase fraction on the overall crystallization rate of PLLA, the fraction of the overall structural ordering (Xordering) including crystalline phase (Xcrystal) and/or mesomorphic phase as a function of tt is calculated from the FTIR spectra in Figure 1a according to the equation (see Figure S2 for details).37,38 The results are displayed in Figure 1b. Upon annealing at Ta = 130 °C for ta = 30 s, the obtained Xordering of the untreated sample (i.e., tt = 0 s) is still very low (ca. 10%). However, it is seen that the Xordering was significantly enhanced, even for the sample that initially CO2-treated for tt = 15 s, having Xcrystal = 37%. With increasing tt, Xordering gradually increases and then reaches a constant value (ca. 70%) when the tt is comparable to ca. 4−5 min. When considering the same Ta and ta, the increased Xcrystal is clearly related to the tt, namely the fraction of the mesophase. Moreover, Xcrystal approaches a maximum plateau when tt exceeds ca. 5 min, indicating that only ta = 30 s is a sufficiently long time to complete the crystallization of the samples treated in CO2 (Tt = 0 °C/2 MPa) for tt = 5 min. This result means that the kinetics of crystallization of PLLA with mesophase was dramatically enhanced, significantly shortening the time for completing the crystallization. Therefore, this result also points to the profound nucleating effect of the mesophase, particularly in the samples with longer tt, which was confirmed by further experiment of much shorter ta = 2 s. C

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Macromolecules Figure 2 shows the FTIR spectra and corresponding Xordering for the annealed (ta = 2 s) PLLA samples that were initially

Figure 3. Calculated fraction of structural ordering for PLLA films that treated under 2 MPa of CO2 at Tt = 0 °C for different tt as indicated and then thermally annealed at Ta = 80−135 °C for ta = 1 min.

0.5−360 min; see Figure 11b) show a characteristic peak at 918 cm−1. In addition, previous work indicates that the mesophase fraction increases with increasing tt, accompanied by an increase in the rigid amorphous fraction (RAF), which not only help to kinetically stabilize the mesophase but also tend to devitrify upon extending time or increasing temperature.38 Thus, as compared to the unannealed samples containing much more mesophase of ca. 12−15%, it can be concluded from Figure 2 that the annealed samples of tt = 0.5−2 min contain a smaller fraction (ca. 6−13%) of an intermediate ordering between mesomorphic and crystalline phase, implying a structural reorganization process of fast disordering and subsequent incomplete ordering due to the extremely short ta = 2 s. On the other hand, as shown in Figure 3, the relatively low Ta = 80−90 °C had low mobility and tendency to disorder the RAF of mesophase in ta = 1 min, resulting in low Xordering. Therefore, the enhanced crystallization observed here is assumed to be directly correlated with the ordering degree of the mesophase acting as nuclei, which will be discussed in detail in section 3.4. 3.2. Superstructure and Nanostructured Morphology of Differently Crystallized PLLA. The significant enhancement in the crystallization of PLLA is accompanied by unusual changes in the crystalline morphology. Melt-quenched PLLA films had been subjected to CO2 treatment for various tt and then rapidly heated to desired Ta (i.e., 130 and 90 °C) for isothermal crystallization for ta = 1 h, followed by quenching. Figure 4 shows the effect of tt on the superstructure of PLLA films isothermally crystallized at 130 °C as observed by POM. The result of directly crystallized sample is also shown for comparison. As shown in Figure 4a, it shows the spherulitic texture with the typical characteristic birefringence pattern and the size being larger than about 1000 μm, indicating a rather low nucleation density due to the low supercooling. However, the initial CO2-induced mesophase greatly affected the crystalline superstructure. As shown in Figures 4b−d, the final morphology of these samples shows a granular appearance with a dimension of about 2−3 μm. With increasing tt, as shown in Figures 4e,f for samples of tt = 5 min and tt = 10 min, the initially featureless POM pattern (not shown) of the CO2treated PLLA films keeps essentially unchanged after the completion of crystallization upon annealing, indicating that the crystals formed at elevated temperature have very tiny size and show the absence of birefringence pattern. It is worth pointing out that the results shown in Figure 1 confirm that these featureless samples (Figures 4e,f) were definitely crystallized.

Figure 2. FTIR spectra (a) in the wavenumber range of 970−850 cm−1 and the calculated fraction (b) of structural ordering for PLLA films that treated under 2 MPa of CO2 at Tt = 0 °C for different tt and then thermally annealed at Ta = 130 °C for ta = 2 s.

CO2-treated for various tt. As shown in Figure 2a, with increasing tt from 0.5 min to 5 min, mesomorphic band with low absorbance around 918 cm−1 slightly shifts to the higher frequency, and then to crystalline 922 cm−1 with greatly enhanced intensity, whereas the absorbance of amorphous 956 cm−1 decreases. Additionally, the Xordering increases with increasing tt (Figure 2b), showing low Xordering value for tt = 0.5−2 min while approaching a constant value (ca. 60%) for large tt = 45−360 min. These results indicate that ta = 2 s was too short to trigger apparent crystallization in the samples of tt = 0.5−2 min as opposed to the case of ta = 30 s (Figure 1b). In contrast, a relatively high Xcrystal of ca. 60% was obtained for samples of tt = 45−360 min, confirming the unprecedented nucleating ability of the mesophase formed by CO2 treatment, in particular for relatively long tt. Figure 3 (for the FTIR spectra, see Figure S3) shows the Xordering for the isothermally annealed (Ta = 80−135 °C, ta = 1 min) PLLA samples that were initially CO2-treated for tt = 30 s and tt = 0 s (i.e., melt-quenched). Upon annealing, particularly at higher Ta = 95−135 °C, the samples of tt = 30 s show much higher Xcrystal of ca. 55−60% than those (less than ca. 20%) of tt = 0 s. Consequently, the above results collectively highlight the important role that the initially CO2-induced mesophase plays in facilitating the crystallization of PLLA. Furthermore, it should be noted that all the treated (Tt = 0 °C) but unannealed PLLA samples (i.e., mesophase, e.g., tt = D

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Figure 4. POM micrographs for PLLA films that treated under 2 MPa of CO2 at Tt = 0 °C for tt = 0.5−10 min (b−f) as indicated and then thermally annealed at Ta = 130 °C for ta = 1 h. For comparison, panel a shows the result for the sample that directly melt-crystallized at 130 °C (quickly cooled from 210 °C). The insets in panels b−d show highmagnification images.

Figure 5. AFM height images for PLLA films that treated under 2 MPa of CO2 at Tt = 0 °C for tt = 0.5−10 min (b−f) as indicated and then thermally annealed at Ta = 130 °C for ta = 1 h. For comparison, panel a shows the result for the sample that directly melt-crystallized at 130 °C (quickly cooled from 210 °C).

Further, POM analysis for the samples with much longer tt = 45 min and tt = 180 min discloses similar results (Figure S4). Therefore, the drastically decreased size of crystalline morphology strongly indicates that the crystal nucleation of PLLA was significantly enhanced by the mesophase caused by the previous CO2 treatment, even after a short time period of tt = 0.5 min, accounting for the greatly enhanced overall crystallization rate as shown in Figures 1−3. Particularly, when tt is equal to and longer than 5 min, the size of the superstructure of crystallized samples was much smaller than the resolution of an optical microscope, which is comparable on average to the wavelength of visible light (ca. 400−800 nm). To gain additional insights into the understanding the effect of crystallization pathway via the mesophase on the nanoscopic structure, the real surface morphologies of the differently crystallized PLLA samples were further investigated by AFM (Figure 5). As shown in Figure 5a, the directly crystallized sample shows classical crystalline lamellar stacks with a thickness of ca. 19 ± 2.7 nm (estimated from AFM height profiles). After being annealed, all the treated (tt = 0.5−10 min) PLLA films show completely different nanostructure morphology as compared with that of the untreated one (Figure 5a) and that of the unannealed mesophase samples (Figures S5a1−a8). Surprisingly, as shown in Figures 5b−f (for the higher magnification and/or representative phase images, see Figure S4), a kind of novel nanometer-scale rodlike morphology is evident in the treated samples. To the best of our knowledge, these directly observed nanorods are rarely reported for mesophase in other polymers such as iPP and PA 11.45−47 Moreover, judging from Figures 4b−d, it is clearly seen that the observed small irregular-shaped grains in the POM images (tt = 0.5−2 min) are definitely composed of a tremendous number of irregularly organized nanorods (Figures 5b−d), as exemplified by the sample treated for tt = 2 min as shown in Figure S6. In contrast, as for the longer tt = 5 min and tt = 10

min, their corresponding POM results of Figures 5e−f reveal featureless superstructure, indicating that the formed crystalline aggregates consisting of similar nanorods were not spatially organized into grains that have size larger than the resolution limit of optical microscopy. It is worth noting that the samples having shorter tt (0.5−2 min) should have relatively low viscosity (evidenced by Figures S5b1−b3) as compared to those with longer tt (5−10 min) and thus were especially prone to aggregate into larger granular assemblies (Figures 4b−d) upon sudden annealing at elevated temperature, leading to the observed similar rodlike nanostructure but different POM morphologies. Importantly, these randomly distributed nanorods are observed in all crystallized PLLA samples that initially treated even for only tt = 0.5 min, implying the distinctive role of the mesophase that greatly enhanced by previous CO2 treatment, which will be analyzed later. Furthermore, according to the measurements on about 100 nanorods, the crystallized samples initially treated for tt = 0.5 min, tt = 1 min, tt = 2 min, tt = 5 min, and tt = 10 min show average diameters (for the size distribution, see Figure S7) of 20.5 ± 3.5, 21 ± 3.9, 19.8 ± 2.8, 19.7 ± 2.7, and 19 ± 3.5 nm, respectively. These similar average diameters of the crystallized samples are nearly identical to that of the directly crystallized one. This similarity between the diameters of the nanorods and the thickness of the crystalline lamella, both of which are comparable to the long-spacing (i.e., the sum of the lamellar crystal thickness and amorphous layer thickness) obtained by X-ray scattering,81 can be reasonably expected because they are formed at an identical crystallization temperature of 130 °C. As such, despite the different crystallization pathways, all crystallized samples possess similar lamellae thickness, which is mainly dependent on the supercooling instead of the potential nuclei E

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ascribed to the stretching vibration of −CH3 [ν(CH3)] and C− H [ν(CH)], the carbonyl stretching vibration [ν(CO)], and the C−O−C stretching, respectively,74,75 can be used to identify the polymorphs. As shown in Figure 7a, the two splitting bands of νas(CH3) at 3006 cm−1 and νs(CH3) at 2964 cm−1 ascribed to α form appear in the spectra for Ta higher than 110 °C but disappear in those for Ta lower than 105 °C. Similar trends can be found for the other two regions, e.g., the splitting bands of ν(CO) at 1749 cm−1, νas(C−O−C) + ras(CH3) at 1222 cm−1, and ν(C−CH3) at 1053 cm−1, as shown in Figures 7b and 7c. In addition, similar results are found for the PLLA samples initially CO2-treated for tt = 30 min, as shown in Figure S9. In combination with the WAXD data in Figure 6, these results of characteristic splitting bands indicate that α form was predominantly formed at high temperatures for both groups of tt = 30 s and tt = 30 min samples. Therefore, it is concluded that the polymorphic behavior was not influenced much by the presence of initial mesophase, irrespective of the tt and related ordering degree. That is pure α form and α′ form crystals are exclusively formed at high (above 120 °C) and low (below 100 °C) temperatures for PLLA with different tt, respectively; this was additionally evidenced by crystallization experiments at much higher and lower Ta for initially CO2treated PLLA with a wide tt range. Figure 8 shows the FTIR spectra and corresponding second derivative spectra for the annealed (Ta = 130 °C) PLLA samples initially CO2-treated for tt = 0.5−180 min. Their counterparts of melt-quenched sample (tt = 0) are also shown for comparison. All the characteristics splitting bands at 3006, 2964, 1749, 1222, and 1053 cm−1, attributed to α form, can be clearly observed. This indicates that α form crystals were exclusively developed in all crystallized samples including the melt-quenched one, as verified by the WAXD data (Figure S10). On the other hand, as shown in Figure S11, when compared with the spectra for PLLA sample directly coldcrystallized (tt = 0) at the lower Ta = 90 °C, which firmly enables the formation of pure α′ form, it is clear that pure α′ form was also formed at Ta = 90 °C for all samples with different tt. Therefore, the polymorphic behavior was not affected by the initial CO2-induced mesophase. Accordingly, on the basis of the POM and AFM results shown in Figures 4 and 5, the crystallized PLLA samples possessing identical crystalline polymorphs but largely different morphologies were obtained. 3.4. Mechanistic Understanding of Mesophase-Mediated Crystallization Pathways of PLLA. The crystallization rate of PLLA can be greatly enhanced by the presence of mesophase that initially induced by low pressure CO2 at low temperature (Tt = 0 °C). Interestingly, when compared with the directly crystallized sample that shows typical spherulite with lamellae, the crystallization via CO2-induced mesophase results in nonspherulitic superstructure consisting of nanorods. As such, crystallized PLLA samples having identical crystalline polymorphs of markedly different superstructure and nanostruture morphologies were achieved. Consequently, it is reasonably assumed that the mesophase plays a deterministic role in controlling the crystallization pathways of PLLA and thus the resultant crystalline morphology and structure. In the following, in combination with the crystallization pathway of directly crystallized PLLA, we will discuss the main findings using a framework based on the multistage model. IR spectroscopic imaging was employed to investigate the spatial heterogeneity of the structural (crystalline and conformational ordering) evolution of PLLA during the direct

that formed from the mesophase with different tt and thus ordering degree. As for the samples treated in CO2 and crystallized at Ta = 90 °C, the representative POM and AFM results are shown in Figure S8. Very similar to the case at Ta = 130 °C, unlike the spherulitic superstructure consisting of lamellae observed for the directly crystallized sample, granular or featureless morphology made up of nanorods can be found in the crystallized samples initially treated by CO2. It is additionally noted that the nanorods seem to be embedded in the surrounding amorphous matrix probably because of the low crystallinity arising from low crystallization temperature. Therefore, the above POM and AFM results (Figures 4 and 5) obtained on crystallized PLLA samples unambiguously reveal that different initial states (i.e., mesophase fraction) resulted in different crystallization pathways, leading to the formation of exactly different crystalline superstructures and nanostructure morphologies. 3.3. Polymorphic Behavior. The effect of CO2-enhanced mesophase fraction (in terms of tt) and cold ordering temperatures (i.e., Ta) on the polymorphic behavior of PLLA was further investigated by WAXD and FTIR. Figure 6 shows

Figure 6. WAXD patterns for PLLA samples that treated under 2 MPa of CO2 at Tt = 0 °C for tt = 30 s (a) and tt = 30 min (b) and then thermally annealed at Ta = 95−125 °C for ta = 1 h.

the WAXD patterns for the PLLA samples initially CO2-treated for tt = 30 s and tt = 30 min followed by isothermal annealing at Ta = 95−125 °C for ta = 1 h. With increasing Ta, notable differences in the WAXD patterns are observed for both groups of tt = 30 s and tt = 30 min samples. As shown in Figure 6a, for the tt = 30 s samples, with increasing Ta from 95−100 °C to 120−125 °C, the two main reflections observed at around 2θ = 16.6° and 19.0°, which are attributed to the characteristic reflections of 200/110 and 203 lattice planes,51 respectively, shift to higher 2θ, definitely pointing to the crystalline polymorphs changes from α′ form to α form. A very similar trend of peak shifting to the higher diffraction angle can be observed for the tt = 30 min samples, as shown in Figure 6b. FTIR results provide additional insight regarding the effects of tt and Ta on the polymorphic behavior. Figure 7 shows the FTIR spectra and corresponding second derivative (used to distinguish small and splitting bands) spectra for the PLLA samples initially CO2-treated for tt = 30 s followed by annealing at Ta = 95−125 °C for ta = 1 h. According to the literature, the 3050−2850, 1800−1700, and 1250−1000 cm−1 regions, mainly F

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Figure 7. FTIR and corresponding second derivative spectra in the wavenumber ranges of 3050−2850 (a), 1800−1720 (b), and 1250−1000 cm−1 (c) for PLLA samples that treated under 2 MPa of CO2 at Tt = 0 °C for tt = 30 s and then thermally annealed at Ta = 95−125 °C for ta = 1 h.

Figure 8. FTIR and corresponding second derivative spectra in the wavenumber ranges of 3050−2850 (a), 1800−1720 (b), and 1250−1000 cm−1 (c) for PLLA samples that treated under 2 MPa of CO2 at Tt = 0 °C for tt = 0.5−180 min as indicated and then thermally annealed at Ta = 130 °C for ta = 1 h.

mesophase and crystal, which is referred to as preordering. It is noted that the melt (position 31) shows absence of characteristic bands around the 918−922 cm−1 region. As shown in Figure 9b, the notable changes in peak positions and intensity of the characteristic bands can be clearly observed and are similar to that as shown in Figure 2a. That is, a mesomorphic band with low absorbance around 918 cm−1 slightly shifts to the higher frequency, and then to crystalline 922 cm−1 band with highly enhanced intensity, whereas the absorbance of amorphous 956 cm−1 decreases, indicating the presence of isotropic amorphous, preordering, and crystalline phases along the radial direction, which is confirmed by the

isothermal crystallization. Figure 9a shows the typical optical microscope (OM) image obtained on the quenched PLLA film (20 μm thick) initially crystallized at 130 °C for 30 min. The corresponding IR spectra were simultaneously collected. The spectra shown in Figure 9b (for high frequency region, see Figure S12) were taken from the corresponding positions marked along the red line (direction of radial growth) in Figure 9a, which covers the melt, growth front, and spherulitic entity. Similar to the spectra in Figure 2, the bands at 922 and 918 cm−1 are related to the crystalline and mesomorphic phase, respectively, whereas the bands located between 918 and 922 cm−1 are ascribed to the intermediate structure between G

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density and anisotropy. Because of the high mobility, this layer instantly thickens, up to a critical value (step 1), and then reorganizes into intermixed ordered structure (step 2, preordering) with Xordering up to a critical value (i.e., ca. 15%, see Figure 9c) at the growth front, subsequently leading to the solidification of preceding preordering into the observed entity (crystal or spherulite, see Figures 9a and 10) (step 3, metastable crystal) with increased Xordering but diminished mobility. Note that this freshly formed crystal is metastable and undergoes further gradually slow structural reorganization and densification, ultimately resulting in an equilibrium crystal structure (step 4) with the specific crystal thickness, packing density (lattice spacing), and ordering degree (Xordering = ca. 70%, see Figure 9c), solely depending on the crystallization temperature. It is worth emphasizing that the above multistage process is spatially and temporally continuous (see Figure S13). Microscopically, the structure and related structural characteristics at each region along the radial direction are similar to those at adjacent ones. Similarly, with time going on, for each nanoscopic location during the real growth, the structure and its related structural characteristics in each step closely resemble those in the preceding ones. As such, this multistage process we proposed for the crystallization scenario is exactly consistent with the well-known Ostwald’s step rule, which argues that the first phase that forms (e.g., nucleates) is not the most energetically stable, but the one whose free energy is closest to the original disordered (e.g., liquid/melt) state.85 For example, from melt to mesomorphic phase, preordering, fresh metastable crystal, and then to final equilibrium crystal, the Gibbs free energy should be sequentially decreased. Therefore, it is reasonably assumed that the nucleation is also an event of multistage process similar to that occurred at any nanoscopic location along the growth direction. In a size regime of several tens of nanometers, the metastable mesomorphic phase is prone to be transformed to preordering with relatively high Xordering up to ca. 15% as quickly as it is produced, resulting in the first crystallite, which can be referred to as the crystal nucleus. Meanwhile, the crystallite/nucleus is in contact with surrounding intermediate preordering. Similar to the scenario as shown in Figure 10 (or Figure S13), afterward, the crystallite/nucleus undergoes the reorganization and densification, whereas the preordering and melt repeat the respective preceding processes; this is somewhat analogous to the primary and secondary nucleation of classical theory.64 However, owing to the low supercooling for normal crystallization in PLLA at 130 °C, the active mesomorphic phase and thus its contribution to the nucleation are remarkably limited, leading to an extremely low nucleation density (Figures 4a and 9a) and the formation of lamellae/sheets of crystals (Figure 5a) that grow

Figure 9. Optical microscope image (a) and the corresponding contour (d) of intensity distributions of 1267 cm−1 band for PLLA spherulite that crystallized at 130 °C (quickly cooled from 210 °C) for 30 min; FTIR spectra (b) and the corresponding calculated fraction (c) of structural ordering at the positions marked along the red line in panel a.

contour map (Figure 9d) of the intensity distribution of the 1267 cm−1 band corresponding to the OM image (Figure 9a). The corresponding X ordering was further calculated to quantitatively evaluate the spatial distributions of the crystallite formation and conformational ordering against the position marked in Figure 9a. As shown in Figure 9c, the Xordering slightly increases before (positions 26−24) the observation of spherulite and then (positions 23−21) increases quickly until approaching a plateau, showing a similar trend as was observed in Figure 2b. The trend is also qualitatively consistent with the previous results observed on iPP by other techniques61,62,82 and the earlier arguments and simulations.83,84 This result consequently leads us to put forward a mechanistic pathway of normal crystallization (Figure 9) as a multistage process in which PLLA chains are crystallized via transient and metastable intermediate phases including mesomorphic phase, as demonstrated in Figure 10, which is slightly different from Strobl’s model.54,56 Along the radial direction, the crystal growth starts with the rearrangement of random chain sequences from the isotropic melt to form a thin mesomorphic layer, which has structural ordering characteristics close to the melt (isotropic), i.e., high conformational defects and thus low ordering degree (Xordering), low packing

Figure 10. Schematic (not to scale) representation of multistage model proposed for the formation of the PLLA crystals (spherulites). During the crystal growth, the mesomorphic phases are transformed to preordering as quickly as they are formed at the growing front and then to fresh crystals and final equilibrium crystals. H

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Macromolecules from the front metastable species acting as growth units starting from a central nucleus (Figure 10). The situation in CO2-treated PLLA is markedly different. Owing to the high supercooling and moderately increased mobility in low temperature/pressure CO2, nodular mesophase formation from the glassy state was significantly enhanced with dramatically increased nucleation density (∼1015 nuclei/mm3) (Figure S5). Moreover, because of the RAF and low mobility, the transformation of mesophase nodules into ordered structure and subsequent lateral growth were kinetically impeded, thereby freezing the evolution state in steps 1 and 2; however, with increasing tt, step 3 was prone to occur to some extent, especially for longer tt during the mesophase formation. To verify this point, we annealed the treated samples (tt = 0.5−360 min) at a lower temperature of Ta = 78 °C (around the Tg of RAF) for a very short period of ta = 5 s, mimicking the devitrification of RAF, thereby revealing the metastability and Xordering of the mesophase formed in different tt. Otherwise, a high temperature would lead to crystallization even in a very short ta in particular for longer tt (see Figure 2). As shown in Figure 11, with increasing tt, a new band appears at

Figure 12. (a) Fraction of structural ordering for CO2-treated PLLA before and after annealing at Ta = 78 °C for ta = 5 s; the data were calculated from the FTIR spectra shown in Figure 11. Dashed lines are shown to guide the eye. (b) DSC curves for CO2-treated (2 MPa and Tt = 0 °C for tt = 360 min) PLLA samples before and after annealing at Ta = 78 °C for ta = 5 s.

Xordering of all samples diminished but are still larger than that of the melt-quenched one. On the other hand, the AFM results (Figure S5) show the morphological appearance before and after annealing in addition to FTIR data. Except for tt = 0.5−2 min, the morphology remained substantially unchanged for tt = 5−360 min upon annealing. Moreover, DSC results (Figure 12b) show that the endothermic peak attributed to the RAF of mesophase disappears after annealing. These observations collectively suggest that the CO2enhanced mesophase underwent a disordering instead of melting process, namely the devitrification of RAF, which endowed the annealed samples with restored original mobile amorphous fraction, as evidenced by IR data (Figure 11) and the heat capacity increment at Tg in the DSC curve (Figure 12b). Meanwhile, the Xordering slightly increases with tt for tt = 0.5−10 min and increases apparently for tt = 45−360 min, verifying the enhanced Xordering. Therefore, despite the annealing and thus disordering, it is concluded that some structural ordering was preserved particularly in mesophase sample with large tt = 45−360 min, confirmed by the unchanged cold temperature peaks (Figure 12b). Similarly, compared to Ta = 78 °C, once annealing at much higher temperatures, e.g., Ta = 130 °C, the metastable mesophase definitely undergoes a much faster disordering and subsequent ordering (structural reorganization and densification) process to be nuclei. It is clear that the larger the tt of initial treatment, the lesser is the time for structural

Figure 11. FTIR spectra in the wavenumber range of 1400−850 cm−1 for CO2-treated PLLA (mesophase) before and after annealing at Ta = 78 °C for ta = 5 s. The CO2 treatments were conducted at 2 MPa and Tt = 0 °C for different tt as indicated. The FTIR spectrum for meltquenched sample (black dotted line) is also shown for comparison.

1212 cm−1 with enhanced absorbance, whereas the intensity of the C−O−C (C−H bending-coupled) stretching band at 1267 cm−1 (characteristic of amorphous phase) decreases, indicating the conformational ordering on expense of random coils. Meanwhile, the absorbance of the 918 cm−1 band was significantly increased, indicating the enhanced mesophase formation, which agrees well with previous work.38 However, upon annealing at Ta = 78 °C for 5 s, an opposite trend of the changes in these characteristic IR peaks is observed, especially in samples of tt = 0.5−10 min, indicating a reverse process of the structural ordering, i.e., disordering. Figure 12a shows the fraction of conformational ordering (Xordering) for the CO2treated samples before and after annealing. Upon annealing, the I

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Macromolecules reorganization to form a critical Xordering value of ca. 15%. For example, upon annealing, the Xordering for samples of long tt = 45−360 min (Figure 12a) are close to ca. 15%, indicating their negligible time (∼1 s) required to be nuclei. In contrast, as for shorter tt (e.g., tt = 0.5−2 min), upon annealing, the disordered samples containing less residual ordering degree need relatively longer time (∼2−3 min) to undergo a process of progressive structural reorganization to reach a critical Xordering of ca. 15% for yielding crystal nucleation. Moreover, on the basis of the growth rate of ca. 4.3 μm/min (i.e., ca. 70 nm/s) at 130 °C (data not shown) and the very small distance (ca. 10−30 nm) between the centers of nodules (Figure S5), which serve as nuclei, the growth (solidification of preordering) process can be completed in less than 1 s. As such, according to our multistage model, at a given ta longer than 1 s, the crystallization rate is strongly dependent on the state (i.e., ordering degree) of potential nuclei. Accordingly, while maintaining a constant ta of ta = 30 s (Figure 1) and ta = 2 s (Figure 2), Xordering increase with increasing tt, resulting in nearly completed crystallization for samples of tt = 5 min and tt = 45 min, respectively. Alternatively, for a given tt = 30 s, Xcrystal increases with increasing ta, e.g., Xordering = 7% for ta = 2 s (Figure 2), Xcrystal = 43% for ta = 30 s (Figure 1), and Xcrystal = 56% for ta = 1 min (Figure 3). On the other hand, owing to the exceptionally high nucleation density of the CO2-induced mesophase (Figure S5), it is highly expected that the lamellae (sheets of crystals) cannot form in crystallized PLLA via nodular mesophase, which is therefore markedly distinct from the situation in direct crystallization and is generally consistent with other polymers crystallized via mesomorphic nodules.46,47 In contrast, the disordered nanoscopic nodules acting as nuclei may undergo a coalescence process54 by further ordering and crystal growth, leading to the rodlike nanostructure morphology (Figures 5b− f). Consequently, the previously CO2-induced nodular mesophase with extremely high nucleation density not only acted as active nuclei that significantly enhanced the crystallization (Figures 1−3), but more importantly, they also served as nanoscale building blocks that constitute the nanorods in crystallized samples (Figures 5b−f). Furthermore, the results in Figure 9 and the proposed model (Figure 10) nicely account for the final Xcrystal of ca. 55−60% due to the limited tt (e.g., 30 s) and/or ta (e.g., 2 s) (Figures 2 and 3), which are insufficient to complete the further densification/perfection (step 4) of the newly formed crystalline aggregates before obtaining equilibrium crystal with Xcrystal = ca. 70% (Figure 9c). Moreover, compared with long ta, the limited ta (e.g., 2 s) lead to incomplete packing although they are long enough (ta > 1 s) to finish the crystal growth (e.g., see Figure 3 and POM result in Figure S14) at Ta = 130 °C (a typical temperature for forming α form), resulting in loose packing density and thus weak characteristic peaks of α form PLLA (see FTIR results in Figure S15), which are qualitatively similar to those of equilibrium crystals formed at Ta = 105−115 °C (Figure 7). Along the same line, our proposed multistage pathway also provides a general understanding of the polymorphic behavior of PLLA. That is, the formation of the crystalline polymorphs (α and α′ forms) mainly relies on the packing state of the structural ordering (reorganization and densification) (step 4 in Figure 10) during the crystallization, which mainly depends on the crystallization temperatures. High temperatures are favorable for the structural perfection and densification to form ordered α form with dense packing, whereas low

temperatures and thus limited chain mobility retard the reorganization and densification, creating less-ordered α′ form with loose packing. As such, the moderate temperatures (105− 115 °C) would lead to a moderate packing instead of mixture of α′ and α forms, indicating that the normally crystallized α′ form should be the result of kinetic metastability rather than polymorph selection. Therefore, polymorphic behavior is essentially independent of the initial mesophase with different tt (Figures 6−8). Additionally, our proposed model reasonably accounts for the blending effect (signifying slow kinetics and thus long time for perfect packing) of PDLLA that enables α crystal to form in PLLA at reduced temperatures75 and the partial transformation (implying further slight packing) of α′ form into α crystal during the cold crystallization of PLLA.76

4. CONCLUSIONS Using spectroscopic and microscopic techniques, the effect of the CO2-induced mesophase on the isothermal crystallization at atmospheric pressure was investigated to reveal the role of metastable mesophase in directing the crystallization and resulting structure and morphology of PLLA. It was found that the crystallization of PLLA underwent a multistep process, in which the crystal formation was preceded by the formation of various metastable intermediate phases, including mesomorphic phase, preordering, and even metastable crystal. That is, the mesomorphic phase continuously evolved and reorganized into preordering with relatively higher ordering degree up to a critical value until the onset of crystallization. When grown in contact with the front intermediate preordering and mesomorphic phase, the metastable crystal continually underwent gradually slow reorganization and densification to form the equilibrium crystal at the temperature (e.g., 130 °C). The multistep process also applies to the nucleation event. For the direct crystallization at low supercooling, the contribution of mesomorphic phase to the nucleation was negligible, leading to extremely low nucleation density and thus formation of spherulite with the lamellae, which were continuously solidified from the front metastable species acting as growth units. On the other hand, upon annealing, the CO2induced mesophase instantaneously underwent the devitrification of RAF and subsequent reorganization toward the preordering to be crystal nuclei. Following that, the growth of the nanorods that constituted the nonspherulitic morphology advanced through the multistep pathways of crystallization and coalescence process of the nanograins of mesophase; this was accompanied by the significantly enhanced crystallization rate based on the exceptionally high nucleation density as opposed to the case of direct crystallization. For the samples of tt = 5 min and tt = 45 min, the crystallization were nearly completed in ta = 30 s and ta = 2 s, respectively. Moreover, compared with the situation of direct crystallization, the polymorphic behavior was not affected much by the CO2induced mesophase served as potential nuclei, while it intrinsically relied on the temperatures and thus the mobility for packing. Consequently, crystallized PLLA having identical crystalline polymorphs of largely different superstructures and nanostrutured morphologies was obtained. We believe that these results provide new insights into the understanding and regulation of the crystallization of polymers. Owing to the fast kinetics, it was challenging to trace the formation process of the rodlike nanostructure. Also, the nanorods were observed even in tt = 30 s sample crystallized at 130 °C, implying a relatively J

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high melting temperature of mesophase, requiring further investigation by flash calorimetry.

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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.6b01442. Figures S1−S15 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] or [email protected]; Fax +86 574 8791 2309; Tel +86 574 8669 4770 (Q.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21404012) and China Postdoctoral Science Foundation (Grant No. 2015M581967). Q. L. gratefully acknowledges Corbion Purac (The Netherlands) for providing PLLA for this research.

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DOI: 10.1021/acs.macromol.6b01442 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01442 Macromolecules XXXX, XXX, XXX−XXX