Inducing Stereocomplex Crystals by Template ... - ACS Publications

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Inducing Stereocomplex Crystals by Template Effect of Residual Stereocomplex Crystals during Thermal Annealing of InjectionMolded Polylactide Zheng-Chi Zhang, Xin-Rui Gao, Zhong-Jie Hu, Zheng Yan, Jia-Zhuang Xu, Ling Xu, Gan-Ji Zhong,* and Zhong-Ming Li* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China S Supporting Information *

ABSTRACT: In the present work, the injection-molded poly(L-lactide)/poly(D-lactide) parts were thermally treated at nearly below the melting point of stereocomplex crystals (SCs) (∼210 °C) to prepare polylactides (PLAs) with good heat resistance, and it is found that exclusive formation of SC is effectively achieved. It is worth mentioning that injectionmolded PLAs used in thermal treatment were first prepared by employing a high mold temperature (120 °C) to ensure proper crystallinity and SC content, which makes sure that no warpage of PLA was noticed during thermal treatment. Consequently, superbly heat resistant injection-molded parts with Vicat softening temperature of ∼200 °C were successfully achieved. Meanwhile, the template effect of residual SCs, as evidenced by in situ X-ray diffraction results, was proposed to explain the high efficiency of thermal treatment. Specifically, the residual SC in PLA melts with perfect surface and well-defined lattice parameters can act as template to guide formation of new SCs.

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the two key factors that determine the final SC content in PLLA/PDLA blends. According to the results reported by Sarasua and his co-workers, regardless of cooling rate and annealing conditions, PLLA/PDLA blends that are close to equimolar composition show the higher SC content, which indicates a preferential crystalline development of SC.15,16 What is more, Ikada et al. reported that SC crystallization is significantly suppressed in a solution mixture of PLLA and PDLA when Mw increases to 4000−40 000 g/mol.17 Similarly, Sarasua et al. also reported that predominant SC formations occur in blended films when the Mw of PLLA is below 100 000 g/mol, but when its Mw is higher, both HC and SC coexist.18 When it comes to melt blending and crystallization, the critical Mw of PLLA and PDLA to attain exclusive SC is only 6000 g/ mol in melt crystallization.8 Therefore, preparing PLA samples with high SC content from high Mw raw material is a bottleneck issue for obtaining PLA products with high heat resistance and balanced performance. Under these circumstances, several researchers are trying to develop an efficient method to prepare well stereocomplexed samples from high-molecular weight PLLA and PDLA through

INTRODUCTION Polylactides (PLAs, poly(D-lactide) and poly(L-lactide)), derived from renewable and naturally occurring corns, sugars, or beets, are biodegradable and compostable. Although their initial usages are for medical applications due to its biocompatibility and biodegradability, PLA has nowadays been used as commodity thermoplastic for large-scale production of injection-molded parts. However, in comparison with other commercial thermoplastics derived from petroleum, injection-molded PLA parts exhibit a relatively low heat resistance because of its slow crystallization rate and fast injection-molding cycle involving high cooling rate and relatively low mold temperature. Therefore, in the past 2 decades, both academia and industry are trying to enhance the heat resistance of PLA products.1,2 Since the pioneering works of Ikada et al. in 1990s, poly(Llactide) (PLLA) and its enantiomeric opposite, poly(D-lactide) (PDLA), have been known to form specific stereocomplex crystal (SC) upon mixing in solution or in bulk.3 SCs melting at a temperature of about 50 °C higher than the melting point of PLA homochiral crystals (HC) provide a promising solution to highly heat resistant PLA products.3,4 Many previous results have revealed the formation of SC in PLLA/PDLA blends affected by various factors, such as blend ratio,5−10 optical purity,9−11 thermal history,12−14 and molecular weight.5−7 Among these factors, blend ratio and molecular weight are © XXXX American Chemical Society

Received: June 5, 2016 Revised: August 31, 2016 Accepted: September 2, 2016

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DOI: 10.1021/acs.iecr.6b02169 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

crystallinity and residual SCs ensure enough self-support elements during thermal treatment, which is beneficial for avoiding warpage or deformation. Thus, injection-molded PLA parts with exclusive SC and without any warpage or unwanted deformation were successfully prepared through a short-term thermal treatment, leading to superb heat resistance. Meanwhile, with the aid of in situ wide-angle X-ray diffraction (WAXD), the template effect of residual SCs was proposed. Specifically, the residual SC with perfect surface and welldefined lattice parameters can act as template to guide formation of new SCs. With the presence of templates, new SCs form easily due to epitaxial crystallization and conformational entropy reduction provided by templates. Therefore, this work offers a commercial, practical, and simple approach to fabricate PLA products with complete SC structure, superior heat resistance, and promising dimension stability.

melting blend. Biela et al. found that the formation of the PLA SC in the melt was complete and perfectly reversible in starshaped enantiomeric PLAs and the in linear PLLA/PDLA blends with the presence of PLA grafted MWCNT.19−21 Nevertheless, complex processes are usually used to synthesize these polymers with special architectures, which limits their commercialization. Therefore, it is more desirable if exclusive formation of SCs can be achieved from commercially available, linear, high molecular weight PLLA/PDLA blends. With the aid of supercritical carbon dioxide, complete SC formation was obtained by Purnama and Kim.22 Complete SC samples were also successfully prepared by Bao et al. through a low temperature approach, i.e., blending at temperatures much lower than the melting point of SC.23 Unfortunately, when it comes to injection-molded SC parts, the widely used processing method with high efficiency, only limited researchers focus on this project. Although SC with high melting point is a promising method to promote the heat resistance of PLA products, injection-molded PLLA/PDLA blends usually remain in amorphous state or exhibit very limited SC content due to slow crystallization rate of PLA and fast injection-molding cycle.24−26 The Vicat softening temperature of injectionmolded PLLA/PDLA was only 74.7 °C as reported by Zhang et al.26 Similarly, an obvious drop in storage modulus obtained by dynamic mechanical analysis (DMA) was observed lower than 60 °C in Samuel’s work, indicating poor heat resistance.24 Thus, the crux of the matter to obtain injection-molded PLA samples with considerable heat resistance is to increase of crystallinity and SC content. However, to the best of our knowledge, an effective method to prepare injection-molded PLA samples with considerable SC content and good heat resistance has not been reported yet. As is well-known, thermal treatment or annealing is an efficient way to modify the crystal forms, crystallinities, and crystal dimensions. Fukushima et al. invented a novel synthetic technique called “solid state polycondensation”, where melt blended PLLA/PDLA mixtures were maintained at a constant temperature higher than the melting point of HC. As a result, complete SC sample with high molecular weight was approached.27,28 Recently, Lopez-Rodriguez et al. found that he PLLA/PDLA 50/50 blends are shown to be crystallizable exclusively as SC with a high degree of crystallization by performing recrystallization of the blends at 190 °C.29 Some theoretical research also observed further growth of SC in stretched,30 oriented31 PLLA/PDLA blends, the mixtures of PLLA and PDLA single crystals32 and casted equimolar mixture of PLLA and PDLA33 when they were heated or annealed between the melting point of HC and SC. From all these pioneering works, immature conclusion can be drawn that proper temperature procedure between the melting point of HC and SC that is preferable for the formation of SC and unsuitable for HC crystallization is an efficient approach to prepare SC dominated PLLA/PDLA blends. Although employing thermal treatment or annealing processing in injectionmolded parts is easy to approach, it is still challenging to avoid the deformation or warpage of injection-molded PLLA/PDLA parts due to uneven shrinkage and gravity effect during thermal treatment. In the current work, PLLA/PDLA blends were injectionmolded into samples at appropriate mold temperature to achieve samples with proper crystallinity and SC content. Then these injection-molded parts were thermally treated at optimal thermal treatment parameters. It is found that the proper initial

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EXPERIMENTAL SECTION Materials. PLLA (trademark L130 with Mw = 17.3 × 104 g/ mol, Mn = 8.9 × 104 g/mol, and PDI = 1.95 according to GPC tests) and PDLA (trademark D1010 with Mw = 8.5 × 104 g/ mol, Mn = 4.7 × 104 g/mol, and PDI = 1.82 according to GPC tests) were supplied by Corbion Co. Ltd. (Amsterdam, The Netherlands), in which around 0.2% of D-LA or L-LA is present for PLLA or PDLA resin. Preparation of PLLA/PDLA Injection-Molded Parts. To avoid degradation due to hydrolysis and the formation of voids during processing, PLLA and PDLA pellets were dried at 80 °C under vacuum overnight before extrusion. The PLLA/PDLA blends (PLLA/PDLA = 1:1, wt/wt) were melt mixed by employing a twin-screw extruder to produce pellets for subsequent injection molding. The screw speed of the twinscrew extruder was kept constant at 240 rpm, and the processing temperature profile was 190−230 °C from hopper to die. The blended pellets were injection molded into dumbbell bars with cross-sectional area of 4 × 6 mm2 and 100 mm in length by utilizing a commercial injection molding machine. The temperature profile for injection molding was 210, 240, 250, 250, and 230 °C from hopper to nozzle, respectively. The mold temperature and packing pressure were 120 °C and 60 MPa, respectively. Characterization of SC Crystallization during Thermal Treatment. DSC Measurements. DSC experiments were carried out by utilizing DSC Q2000 (TA Instruments, U.S.) to determine optimal thermal treatment temperature and duration. The endothermic and exothermic information on samples cut from the intermediate layer (1−2 mm away from sample surface) of injection-molded bars was collected during following procedure: (i) treating at different temperatures for 3 min or at 210 °C for different durations with a foregoing heating process (30 °C/min) and subsequent cooling process (−10 °C/min); (ii) heating at a rate of 10 °C/min to 260 °C to determine crystalline information (crystal forms and crystallinities). In Situ WAXD Measurements. Synchrotron two-dimensional wide-angle X-ray diffraction (2D-WAXD) experiments were carried out at BL16B (λ = 0.124 nm), Shanghai Synchrotron Radiation Facility (SSRF) in order to investigate the crystallization behavior during thermal treatment of samples cut from injection-molded PLLA/PDLA bars. A MAR CCD detector (MAR-USA) with a resolution of 1024 × 1024 pixels (pixel size = 80 μm) was used to acquire WAXD pattern. A modified Linkam THMS-600 stage was employed to precisely B


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Figure 1. DSC heating curves of (a) original sample before treatment and samples after thermal treatment at different temperatures (190, 210, 230, 250 °C) for 3 min and (b) different durations (3, 10, 30 min) at 210 °C.

Table 1. Crystalline Information of HC and SC in Samples after Thermal Treatment at Different Temperatures and for Different Durations

a

temp (°C)

time (min)

Tm‑hc (°C)

ΔHm‑hc (J/g)

Tcc‑sc (°C)

ΔHcc‑sc (J/g)

Tm‑sc (°C)

ΔHm‑sc (J/g)

Fsca

190 210 210 210 230 250 originala

3 3 10 30 3 3

172.6 171.6 171.4 172.1 171.1 172.3 173.1

18.6 13.5 6.1 9.9 15.8 31.2 41.5

203.4

3.8

198.2

4.5

225.4 224.3 224.2 225.2 234.6 223.3 225.4

42.8 57.1 67.6 62.1 61.7 35.3 37.6

0.58 0.74 0.88 0.81 0.72 0.43 0.34

Relative content of SC is calculated as Fsc = Xc‑sc/(Xc‑sc + Xc‑hc).

Taking the thickness of the injection-molded bars (4 mm), the annealing time was set as 15 min to achieve adequate thermal treatment in core layer of injection molded bars. Crystal Structure Measurement of Injection-Molded Parts. 2D-WAXD and small-angle X-ray scattering (SAXS) measurements were conducted at the BL15U (λ = 0.124 nm) and BL16B at Shanghai Synchrotron Radiation Facility (SSRF), with a Mar CCD (with a resolution of 1024 × 1024 pixels and pixel size = 80 μm) as a detector. The distances between sample and detector for WAXD and SAXS were 175 and 2240 mm, respectively. The samples for X-ray measurements were obtained from the injection-molded parts before and after thermal treatment as shown in Figure S1 in Supporting Information with a dimension of 6 × 6 × 1 mm3. The incident direction of Xray beam is from the surface to the core of injection-molded parts. Four positions, 0, 1000, 2000, and 3000 μm away from surface, were scanned, respectively. WAXD patterns were processed using aforementioned method to obtain 1D-WAXD profiles, Xc, Xc‑sc, Xc‑hc, and Fsc. Linear SAXS profiles were obtained from circular integration of intensities from 2D-SAXS images. The intensity was plotted as a function of the scattering vector, q, where q = 4π sin θ/λ with λ being the wavelength of the incident beam (0.124 nm) and 2θ being the scattering angle. The long period of crystalline lamellae (L) and lamellar thickness (Lc) were obtained from the electron density correlation function K(z) that can be derived from the inverse Fourier transformation of the experimentally intensity distribution I(q).36−39 Vicat Softening Temperature (VST) Tests. The VST tests were carried out with injection-molded samples before and after thermal treatment according to ISO-306. The heating rate during tests was 12 °C/6 min, and the area of indenter was 1 mm2 applying the stress of 1 MPa.

control thermal history. The detailed experimental procedures were set as follows: (i) heating at a rate of 30 °C/min from room temperature to 210 °C; (ii) holding the temperature at 210 °C for 10 min; (iii) cooling down to 60 °C at a rate of −10 °C/min. WAXD patterns were collected at 10 s/frame during the whole thermal treatment procedure. The profiles of 1D-WAXD were gained on the basis of circularly integrated intensities of 2D-WAXD patterns. Then, by use of deconvoluting-peak technique, the total crystallinity (Xc‑total) was calculated by the following equation: Xc‐total =

∑ Acryst ∑ Acryst + ∑ A amorph

(1)

where Acryst and Aamorp are the fitted areas of crystalline and amorphous phases, respectively. The crystallinity of SC (Xc‑sc) was given by Xc‐sc =

A110 + A300/030 + A 220 ∑ Acryst + ∑ A amorph

(2)

where A110, A300/030, and A220 are the areas of (110), (300)/ (030), and (220) reflection peak of PLA stereocomplex crystallites, respectively.14,34 Correspondingly, the crystallinity of HC (Xc‑hc) and the relative amount of SC (Fsc) could be calculated by using the formula as follows:35 Xc‐hc = Xc − Xc‐sc

(3)

Xc‐sc Xc

(4)

Fsc =

Thermal Treatment of the Injection-Molded PLLA/ PDLA Bars. The injection molded PLLA/PDLA bars dried in vacuum oven at 80 °C overnight were annealed at 210 °C in order to promote the formation of SC in PLLA/PDLA samples. C


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RESULTS Dependence of Thermal Treatment Conditions: Temperature and Duration. It is well-known that the thermal treatment temperature and duration are key to controlling the final crystalline state including crystallinity and crystal forms. In order to achieve the desirable results, we carefully determined the proper thermal treatment temperature and duration through a series of DSC tests. First, the crystallinity and crystal forms of original injection-molded PLLA/PDLA parts before thermal treatment (named as BT for brief) were detected. As shown in Figure 1a (original), the melting curve of BT sample exhibited two endothermal peaks located at 173 and 225 °C, respectively, which are ascribed to the melting process of HC and SC, respectively. In addition, an obvious exothermic peak is also observed located just before the melting of SC, indicating the cold crystallization of SC. The corresponding crystalline information listed in Table 1 also reveals that HCs and SCs are coexistent in original samples. The melting enthalpy of HC (ΔHm‑hc) is 41.5 J/g and that of SC (ΔHm‑sc) is 37.6 J/g, leading to a low relative SC content (Fsc), i.e., 0.34, as shown in Table 1. It is easy to conclude that BT samples are HC dominated and SC content is quite limited. Similarly, samples after thermal treatment (name as AT for brief) also exhibit the melting peak of both HC and SC, as shown in Figure 1a. No cold crystallization of HC is noticed in all heating curves, and only AT samples treated at 190 °C show the cold crystallization peak of SC. To clarify temperature dependence, the crystallization information on AT samples treated at different temperatures for 3 min is summarized in Table 1. When the thermal treatment temperature increases from 190 to 210 °C, ΔHm‑sc is enhanced from 42.8 to 57.1 J/g accompanied by the decrease of ΔHm‑hc leading to an obvious increase of Fsc. When the treatment temperature reaches 230 °C, no obvious enhancement in ΔHm‑sc and a slight decrease in Fsc are observed. In addition, SC’s melting temperature is obviously promoted due to the relatively high treatment (recrystallization) temperature. With further increase of the treatment temperature to 250 °C, an opposite tendency is observed; i.e., SC content decreases, and HC content increases. In conclusion, although SCs with high melting temperature are obtained after treatment at 230 °C, taking the degradation of PLA, energy costs during heating, and dimensional stability of AT samples into consideration, the thermal treatment of around 210 °C with adequate duration is highly desirable for practical application, which is close to the usual processing temperature for PLA (around 200 °C). To determine the optimal treatment duration, the DSC heating curves of AT samples for different durations at 210 °C are plotted in Figure 1b, and the corresponding crystalline information is listed in Table 1. With the increase of thermal treatment duration from 3 to 10 min, an obvious increase in ΔHm‑sc companied by a decrease in ΔHm‑hc is noticed, indicating that longer treatment durations is in favor of SC’s formation. After treatment at 210 °C for 10 min, the HC dominated BT samples are turned into SC dominated ones after a simple thermal treatment, which is of great significance to prepare superbly heat resistant PLA injection-molded samples. However, with further increase in treatment time to 30 min, the SC content remains almost constant with a slight reduction, which may result from the saturation of thermal treatment and the degradation of PLA. From these results, a clear conclusion can be drawn that 10 min is more than

sufficient to achieve considerable SC content higher than 80% of Fsc. From the DSC results shown before, SC content in AT samples can be obviously promoted through an efficient less time-consuming (only 10 min) thermal treatment process at 210 °C, where HCs are melted and SCs remain in unmelted or partial melt sate. Crystalline Structure of Injection-Molded Samples after Thermal Treatment. A thermal treatment process at 210 °C was applied to injection-molded SC samples based on the DSC results, and the crystal structure in BT and AT samples was characterized through WAXD and SAXS tests.

Figure 2. 2D-WAXD patterns at different positions of injectionmolded PLLA/PDLA samples: (a) AT and (b) BT.

Figures 2 and 3 show the 2D-WAXD patterns and 1DWAXD profiles at different positions, viz., 0, 1000, 2000, and 3000 μm away from surface, of AT and BT samples, respectively. The 2D-WAXD patterns of AT samples basically show three diffraction reflections associated with different lattice planes of SC, from inner to outer circles, corresponding to (110), (300)/(030), and (220) crystal planes, respectively.40 In addition to the diffraction reflections of SC, (110)/(200) and (203) lattice planes appear in BT sample, corresponding to α-crystal of HC, as shown in Figure 2b.41 Same results are observed in corresponding 1D-WAXD profiles; see Figure 3. Only three diffraction peaks of SC are observed in AT samples, indicating complete SC in treated samples, which has great potential to enhance thermal treatment of PLA injectionmolded parts. On the contrary, diffraction peaks of both SC and HC emerge in 1D-WAXD profiles of BT samples, indicating samples containing both SC and HC. To understand the crystallinity of different samples as a function of position and thermal treatment process, the estimated crystallinities of SC (Xc‑sc) and total crystals (Xc-total) are shown in Figure 4a. Due to the absence of HC in AT samples, only Xc‑total is presented, which is equal to Xc‑sc. As shown in Figure 4a, the crystallinity of SC is enhanced from the range of 0.14−0.18 to around 0.55 after thermal treatment, revealing exclusive SC. Meanwhile, the total crystallinities are also promoted from about 0.40 to over 0.50, which is also in favor of the enhancement of heat resistance. In addition, Xc‑total/sc is almost the same from surface to core in treated samples, i.e., homogeneous inner structure. This result demonstrates the efficiency of thermal treatment, although poor thermal conductivity is expected in the injection molded PLA samples. What is more, the total crystallinity of BT D


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Figure 3. 1D-WAXD profiles circularly integrated from 2D-WAXD patterns at different positions of injection-molded PLLA/PDLA samples: (a) AT and (b) BT.

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DISCUSSION From the results shown above, a solid conclusion can be drawn that injection-molded PLA samples with high SC content and resulting superb heat resistance can be successfully achieved through a simple thermal treatment process. Sarasua et al. noticed both SC crystallization and the red shift for CH3 and CαH stretching regions in FTIR spectra during isothermal crystallization of PLLA/PDLA films at 190 °C, which indicates that stereoselective crystallization is accompanied or induced by hydrogen bonds formation during annealing.33 Zhang et al. found the phenomenon in stretched PLLA/PDLA blends that some parts of molten chains migrate onto the surface of preexisting SC after HC is melted, resulting in the further growth of SC.30 Similar phenomenon was also noticed by Xiong et al. in highly oriented PLLA/PDLA blends, and they revealed the nucleating effect of the residual SC on the formation of new SC.31 Inspired by these results, a clear conclusion can be drawn that the initial (or preexisting) SC plays an important role in the formation of subsequent SC at high temperature. In comparison with reported results, our work reveals that high SC content can be successfully achieved in conventional injection-molded PDLA/PLLA blends. Taking the differences between our work and reported results and its industrial values into consideration, the formation of SC during our experiments and the mechanism of formation of SCs in isotropic PLLA/PDLA blends with the presence of preexisting SCs should be understood clearly via in situ characterization. In Situ Characterization of Crystallization during Thermal Treatment. To investigate the crystallization of SC during thermal treatment, in situ WAXD was employed to trace this process. The 1D-WAXD profiles obtained from circularly integrated intensities of 2D-WAXD patterns (data not shown here) during thermal treatment are plotted in Figure 6. The 1D-WAXD profile at 40 °C in Figure 6a shows five main diffraction peaks corresponding to the crystal planes (110)/ (200) and (203) of HC α crystal form41 located at 11.6 and 13.3 nm−1 and the crystal planes (110), (300)/(030), and (220) of SC crystals40 located at 8.2, 14.4, and 16.7 nm−1, respectively. It is easy to conclude that the BT samples are HC dominated as judged by the height of diffraction peaks, which is in agreement with DSC results. With the increase of temperature, the intensities of HC diffraction peaks decrease gradually indicating melting process of HC. Meanwhile, the intensities of SC diffraction peaks are gradually enhanced when the temperature is further promoted, which could be resulting from the recrystallization of SC. Following the end of heating process, the samples are held at 210 °C for 10 min; i.e., the optimal thermal treatment

samples is in the range of 0.35−0.40 with the SC content of around 0.15. The initial crystallinity and SC content in the injection-molded parts are a benefit for the dimensional stability of samples during the thermal treatment to avoid warpage or unwanted deformation. The 1D-SAXS profiles and corresponding 2D-SAXS patterns of AT and BT samples are shown in Figure 4b. Meanwhile, the long period and lamellar thickness obtained from K(z) profiles (shown in Supporting Information for brief) are illustrated in Figure 4c. In comparison with BT samples, narrower and sharper peak and clearer scattering pattern are observed in AT samples, indicating bigger electron density differences in these sample; i.e., relatively well-defined and impeccable crystals are induced through a simple thermal treatment process. Only a slight increase in long period is noticed between AT and BT samples. However, the relative lamellar thickness is dramatically enhanced from 6.3 to 13.1 nm after the thermal treatment process. These results are also responsible for the enhanced heat resistance. Heat Resistance and Dimensional Stability of Treated Samples. The Vicat softening temperatures (VSTs) of AT and BT samples are shown in Figure 5a to clarify the effect of SC on the heat resistance of PLA samples. With the presence of SC and relatively high crystallinities, the VST of BT samples is 164 °C. Furthermore, the VST of AT samples containing only SC is enhanced to 199 °C indicating the superior heat resistance with the aid of considerable SC content. In addition to the enhancement of heat resistance, it is also exciting to see the excellent dimensional stability of injectionmolded samples during thermal treatment with the aid of proper initial crystallinity and SC content even without any dimensional contraction. As exhibited in Figure 5b, no warpage or any unwanted deformation can be noticed. For comparison, obvious bending is noticed in treated amorphous samples prepared with low mold temperature (40 °C) as shown in Figure S3 in Supporting Information. Therefore, no extra effort is required to ensure the final dimensions of treated injectionmolded parts, which overcomes a common and bottleneck problem of thermal treatment. The methodology to obtain PLA parts with complete SC and outstanding heat resistance is applicable to almost all conventional processing like injectionmolding, compression, and extrusion. In conclusion, injection-molded PLLA/PDLA samples with exclusive SC and superb heat resistance can be successfully prepared through a simple thermal treatment process, which is significant to enlarge the application field of PLA materials and easy to be employed in practical processing. E


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Figure 5. (a) Vicat softening temperatures of AT and BT samples. (b) Digital photos of AT and BT samples.

To quantitatively clarify the melting process of HC and recrystallization process of SC during heating process, the evolution of crystallinities of HC (Xc‑hc), SC (Xc‑sc), total crystals (Xc‑total), and relative SC content (Fsc) is shown in Figure 7a. It is clear that Xc‑hc starts to decrease as the temperature reaches 135 °C, which results in the decrease of Xc‑total and an obvious improvement of Fsc. However, no obvious change of Xc‑sc is noticed until 165 °C. In other words, the increase of Xc‑sc lags behind the decrease of Xc‑hc denying the direct transformation from HC to newly formed SC and confirming the migration process of PLLA and PDLA chains onto the surface of preexisting SC.30 Finally, all HC melts along with the further increase of SC content and Fsc reaches 100%. It is also worthy to mention that although Xc‑sc increases monotonically as shown in Figure 7a, it does not mean that SCs do not melt during heating. There should be a competition between melting and recrystallization of SC. In our experiment, taking the relatively low temperature (210 °C) into consideration, the recrystallization should prevail over melting. Meantime, some SCs remain in partly melted or unmelted state and are left in PLA melts during or after heating process. These remaining SCs are named as residual SC in this work. These residual SCs with well-defined lattice parameter may act as templates for the formation of new SCs to accelerate kinetics of SC formation. The mechanism of this will be discussed later. In the following isothermal process, Xc‑hc (always 0), Xc‑total (always equal toXc‑sc), and Fsc (always 1) are not shown in Figure 7b for brevity due to the absence of HC. Two distinct regions can be noticed in the isothermal part of Figure 7b, i.e., a relative faster crystallization of SC before 2 min and a relatively slower growth process after 2 min, seemingly indicating that most surfaces of residual SC are occupied by the new SC after treatment for 2 min at 210 °C. After the isothermal process, a controlled cooling was applied and the SC content during this process is plotted in the cooling part of Figure 7b. At the temperature zone from 210 to 160 °C,

Figure 4. (a) Crystalline information on AT and BT samples: crystallinities of SC and total crystals. (b) 1D-SAXS curves of AT and BT samples and corresponding 2D-SAXS patterns inserted into the graph. (c) Long period and lamellar thickness obtained from K(z) profiles of AT and BT samples.

condition suggested by DSC results and the corresponding WAXD profiles is presented in Figure 6b. The intensities of SC diffraction peaks increase monotonically with time. During the following cooling process, similar results are observed as shown in Figure 6c. The intensities of SC diffraction peaks are gradually promoted and finally reach a plateau. In addition, even when the temperature reaches the crystallization temperature of HC, i.e., around 120 °C, no diffraction peaks of HC are observed. It is worthy to note that the absence of HC’s diffraction peaks does not mean the absence of HC but that the low HC content (could be traced by DSC as mentioned above) is out of the scale of WAXD technique. F


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Figure 7. (a) Temperature dependence of fraction of HC (Xc‑hc), SC (Xc‑sc), total crystals (Xc‑total) and relative SC content (Fsc) during heating process of thermal treatment. (b) Evolution of SC as a function of temperature and duration during the entire thermal treatment.

and SC network.42 And due to the same reason, even when the temperature reaches the crystallization temperature of HC, i.e., around 120 °C, it is hard for HC to form in the dense SC network with crystallinity over 0.50 because of extremely strong confinement. Mechanism of Crystallization of PLLA/PDLA with Coexistence of SC and HC during Thermal Treatment. On the basis of the above results, a schematic illustration of the structural evolution of PLLA/PDLA blends during thermal treatment processes is shown in Figure 8. The initial sample contains both HC and SC as illustrated in part a of Figure 8. Upon heating to 210 °C, the melting of HC liberates the PLA chains confined in HCs (shown in part b of Figure 8), providing abundant raw material to build new SC. After a migration process, PDLA and PLLA chains reach the surface of residual SCs as proved by the delay of SC formation after HC melts. PLA chains that reach the surface of residual SCs will assemble into new SCs with the aid of a template: the residual SCs (part c of Figure 8). Different from normal nuclei and nucleating sites provided by nucleating agent, particular attention must be paid to the two characters of residual SCs. First, the content of template is about 12% as indicated in Figure 7a, which is higher than the content of crystal nuclei during cooling (usually less than 5%) or additional nucleating agent (usually less than 10%). In this case, they can provide sufficient nucleating site to help the formation of new SCs. The well-defined surfaces of residual SCs acting as nucleating site can interact with PLLA or PDLA chains. In this case, the mobility and conformational entropy of PLLA or PDLA chains may be obviously reduced, leading to a remarkable decline in energy barrier of SC nucleation. Second, residual SCs have the same lattice parameters as normal SCs. Thus, lattice matching and epitaxial crystallization could be expected on the interface between residual SCs and PLLA/PDLA melts as reported in

Figure 6. (a) WAXD profiles of the samples at different temperatures during heating process of thermal treatment. (b) WAXD profiles of the samples at different durations during isothermal process of thermal treatment. (c) WAXD profiles of the samples at different temperatures during cooling process of thermal treatment.

an obvious increase of Xc‑sc from 0.35 to 0.55 can be observed. This phenomenon may be the result of newly formed template during cooling. Then, Xc‑sc reaches and is maintained at 0.55 after the temperature cools down to 155 °C because of the depressed mobility of PLA chains caused by lower temperature G


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Figure 8. Schematic illustration of the crystallization behavior of the PLLA/PDLA samples with coexistence of SC and HC during thermal treatment process.

°C), injection-molded PLA samples with exclusive SC and corresponding considerable heat resistance are successfully achieved (shown in part g of Figure 8). Meanwhile, the correctness of the proposal, i.e., template effect of residual SC during thermal treatment, is also proved by the SAXS results and corresponding lamella information after and before treatment, as shown in Figure 4c, where the slight increase in long period and dramatically enhanced lamellar thickness were noticed. During thermal treatment, HCs melt after 180 °C and SCs are reserved at 210 °C. The residual SCs act as template for the formation of new SC as discussed before. Then, the newly formed SCs grow from the surface of residual template along the c-axis direction until they are blocked by other newly formed or residual SCs. In other words, thermal treatment process only transforms HCs between original SC lamellae into newly formed SCs but does not change the original periodic structures too much, which is reflected by the negligible difference in long period between AT and BT samples. Meanwhile, due to the further growth from the surface of residual SCs, the lamellar thickness of SC should be obviously promoted as shown in Figure 4c. What is more, it is worthy to note that the template effect can also be expected when lower treatment temperature (190 °C) is employed. However, the mobility of PLA chains is not sufficient to migrate to the surface of residual SC templates. As a result, the efficiency of template effect is not appreciable. When high thermal treatment temperature (250 °C), higher than melting point of SC, is used, quite limited SC can be reserved to act as template. Therefore, only self-nucleation of SC is expected like normal melt crystallization process, finally resulting in relatively low SC content. In conclusion, the thermal treatment temperature has a profound influence on final SC content. In brief, injection-molded PLLA/PDLA parts containing both HC and SC can be successfully transformed into exclusive SC ones through a simple thermal treatment with the help of template effect as discussed before according to the evidence of in situ WAXD results. In our opinion, residual SCs play an important role in inducing the formation of new SCs, which ensure the efficiency of thermal treatment. However, more detailed research is still needed to clarify template effect, and results will be reported in the following works.

polyethylene melts.43,44 Because of the two reasons listed above, the residual SCs can act as template to guide the formation of new SC and the crystallization rate of new SC can be obviously promoted with the aid of the template effect. What is more, we have to claim that the template effect should also be effective for the crystallization of HC; i.e., crystallization kinetics of HC can also be accelerated by residual SCs.35−47 However, it cannot be observed at temperatures much higher than the melting point of HC. Then, in the following heating process and initial stage of isothermal process, more and more new SCs are induced on the surfaces of template as illustrated in part d of Figure 8. The template effect is so efficient that the saturation point is reached in only 2 min during isothermal process where all surfaces of residual SCs are occupied by new SCs, as illustrated in part e of Figure 8. In addition, at this moment, the newly formed SCs still remain in precursors, chain bundles, or crystal fragments due to high temperature without well-defined lattice parameters and surfaces that cannot act as template. All of the reasons listed above cause SC crystallization kinetics to decelerate after a short time during the isothermal process. In the cooling process, driven by large supercoiling, the SC precursors, chain bundles, or crystals fragments can transform into well-defined SCs with smooth surfaces that can act as new templates for the further formation of SCs, as illustrated in part f of Figure 8. Meanwhile, self-nucleated SCs induced by undercooling also emerge, which can also serve as new templates. Thus, the procedure that newly formed SCs act as new templates to induce further formation of SCs may happen repeatedly and SCs are induced just like the falling of dominoes, which is responsible for the accelerated crystallization kinetic of SC during cooling in comparison with the late stage of isothermal process. Finally, high SC content is achieved and dense SC network is built in treated samples. As reported by Wei et al., the confining effect of SC network on the crystallization of PLLA is observed with just 2 wt % of PDLA.42 In our experiments, with the formation of SC network with crystallinity over 0.50 and the decrease of temperature, the mobility of PLLA and PDLA chains is observably suppressed. Thus, the stagnation of SC formation and the absence of HC are noticed as evidenced by WAXD tests. As a result, through a simple thermal treatment process at optimal temperature (210 H


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CONCLUSIONS Injection-molded parts prepared from PLLA and PDLA blends with proper initial crystallinity and SC content were thermally treated to achieve injection-molded PLA parts with considerable SC content. The optimal thermal treatment temperature is 210 °C and the optimal thermal treatment time is 10 min according to our DSC results. After the thermal treatment process, exclusive formation of SC was effectively achieved in injection-molded PLA parts without any warpage or deformation with the help of proper initial crystallinity and SC content, which provides enough self-support elements during thermal treatment. Consequently, injection-molded PLA parts with superb heat resistance and Vicat softening temperature of 199 °C were successfully approached through a simple thermal treatment process. The template effect, in which new formed SCs are induced by the surface of residual SCs with high content, the same lattice parameters as normal SCs, and well-defined surfaces during thermal treatment, was proposed in terms of in situ WAXD results. With the aid of the template effect, the crystalline kinetics of SC was obviously promoted during thermal treatment process and exclusive stereocomplex structure was achieved in thermally treated injection-molded PLA part. A commercial, practical, and simple method to fabricate fully biodegradable PLA injection-molded parts with considerable SC content and superb heat resistance was achieved here, which is significant to enlarge the application field of PLA materials. It is also worthy to mention that the simple thermal treatment process is also available for the products obtained through extrusion, compression, fiber spinning, and any processing method after a minor adjustment in treatment condition.

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and BL15U at Shanghai Synchrotron Radiation Facility (SSRF) for supporting the X-ray measurements.

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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.iecr.6b02169. Diagrammatic drawing of X-ray tests, correlation function K(z) profiles obtained from SAXS results, and digital photos of treated samples through amorphous samples (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*G.-J.Z.: telephone, +86 28 85400211; e-mail, ganji.zhong@ scu.edu.cn. *Z.-M.L.: telephone, +86 28 85400211; e-mail, [email protected]. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grants 51533004, 51403139, 21576173, and 51473101), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 51421061), the Innovation Team Program of Department of Science & Technology of Sichuan Province (Grant 2014TD0002), and Purac Biochem B.V. We also are thankful for Beamlines BL16B I


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