The Role of Melt Memory and Template Effect in Complete

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The Role of Melt Memory and Template Effect in Complete Stereocomplex Crystallization and Phase Morphology of Polylactides Yan-Fei Huang, Zheng-Chi Zhang, Yue Li, Jia-Zhuang Xu, Ling Xu, Zheng Yan, Gan-Ji Zhong, and Zhongming Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01562 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Crystal Growth & Design

The Role of Melt Memory and Template Effect in Complete Stereocomplex Crystallization and Phase Morphology of Polylactides

Yan-Fei Huang, Zheng-Chi Zhang, Yue Li, Jia-Zhuang Xu, Ling Xu, Zheng Yan, Gan-Ji Zhong,* and Zhong-Ming Li

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, People’s Republic of China

* Corresponding author. Tel.: +86-28-8540-0211; Fax: +86-28-8540-5402. E-mail: [email protected] (G. J. Z.)

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Stereocomplex crystals (SC) formation induced by thermal treatment is the most promising approach to achieve the heat resistance of polylactides (PLA). However, a hierarchical morphology of SCs, formed during differently thermal environment, is rarely reported although it is crucial for us to understand their enhanced effect on heat resistance. This work exams morphology of 1:1 stoichiometric blend of poly(L-lactide)/ poly(D-lactide) (PLLA/PDLA) after thermal treatment. In order to explore the formation mechanism of the morphology, in situ wide-angle X-ray diffraction (WAXD) was conducted during the entire thermal treatment. Interestingly, a phase separation structure resulted from the melt memory effect of homocrystallites is observed, for the first time, when treated the amorphous sample at a relatively low temperature (190 oC). Increasing the treating temperature to a medium one (210 oC), two template effects, i. e. the template of unmelted SCs during the isothermal process, and new templates formed due to the supercooling during the cooling process, produce different crystallization kinetics, promoting the formation of sunflower-like spherulites. It is worth noting that under this thermal environment, complete SC could be achieved simply, which has never been realized from the melt of a high molecular weight PLLA/PDLA blend. The template effect of unmelted SC also endows the sample thermally treated at high temperature (230 oC) with incredible high crystalline temperature, giving rise to squeezed spherulites. This work sheds light on the mechanism of the SCs formation, providing effective guidance for hierarchical morphology control of SC for highly heat deflection resistance, and bringing about a 2


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facile approach to achieve exclusive SC crystallites in melt processed products.

INTRODUCTION As one of the most innovative biopolymers that can degrade into nontoxic compounds in landfills, polylactide (PLA) exhibits many advantages, such as excellent biodegradability, biocompatibility, and sustainability.1-6 It is extremely attractive to use PLA materials practically and manufacture them commercially as a good alternate for petroleum-based polymers such as isotactic polypropylene and polyethylene.7-9 However, the application of PLA is limited due to its inferior heat resistance,10-11 which largely lies in its low glass transition temperature (Tg, 60 °C) and low crystallinity. The formation of stereocomplex crystals (SCs), through the stereocomplexation between enantiomeric poly(D-lactide) (PDLA) and poly(L-lactide) (PLLA),12-14 is perceived as one of the most promising strategies to prepare highly heat-resistant PLA products. In a pioneer study, Hideko, et al. reported a PLA alloy with high SC crystallinity presented a higher thermal stability than neat PLA, retaining high storage modulus up to 210 oC.15 Unfortunately, not all of the PLLA/PDLA racemic blends can achieve exclusive stereocomplex crystallization, especially for high molecular weight (Mw) PLLA/PDLA blends during melt processing,

where

homocrystallites

(HC)

crystallization

prevails

over

SC

crystallization.16-21 This is quite disappointing since high Mw is a prerequisite for superior comprehensive performances of PLA. Numerous endeavors have been geared towards preparing well-stereocomplexed 3



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samples from high Mw PLLA/PDLA blends, including synthesis,22-26 blending,27 using nucleation agent,28 repeat casting,29 supercritical fluid technology,30 thermal treatment,20, 31-32, and so on. Among all these methods, thermal treatment never fails to draw people’s attention due to its effective promotion in the SC formation, easy-to-operate, cost-saving and environmental benignancy. Till now, a few theoretical studies were carried out associated with the effect of thermal treatment on the formation of SC in PLLA/PDLA blends.20,

31-34

For instance, Sarasua, et al.,

noticed that SC crystallization was accompanied or induced by hydrogen bonds formation during thermal treatment.33 Fujita, et al. demonstrated that the SC formed by the mechanism that molecular chains mutually diffused between the adjacent crystals and rearranged side by side.32 Zhang, et al. clarified two key points to form SC during thermal treatment, that is, using the 1:1 stoichiometric blend of high Mw PLLA and middle Mw PDLA, and choosing the annealing temperature between the melting points of α crystal and SC-crystal.31 Xiong, et al. reported that the initial crystals played an important role in influencing the formation of subsequently SC and HC during thermal treatment.34 In practical studies, thermal treatment has been employed to improve SC content in PLA products such as spun fibers,35-38 electrospun sc-PLA

membranes,39

hot-pressed

films,40

biaxial-stretched

films,41

and

injection-molded parts.42 For example, Takasaki, et al. elucidated that the annealing gave a high-speed melt spinning fiber mainly consisting of highly oriented SC crystal. In our recent exploration, the injection-molded PLLA/PDLA parts were thermally treated to facilitate the SC formation.42 It was found that large amounts of SC were 4


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effectively formed due to the template effect, i.e. the effect of initial crystallinity. Superb heat resistance and Vicat softening temperature of ~200 oC were endowed for the thermally treated sample. However, a relatively large-scale morphology including crystalline superstructure and phase morphology of SCs induced by the thermal treatment is rarely explored, although it is crucial for us to control hierarchical structure of SCs for highly heat deflection resistance. What is more, complete stereo-complexation of high Mw PLA has not been found. In the present work, the crystalline morphology of injection-molded high Mw PLA after thermal treatment is observed by scanning electron microscope (SEM) with the aid of selective etching technology. An interesting phase separation structure of SCs and HCs is detected, for the first time, when using a low thermal treating temperature. After thermal treating at medium and high temperature, SCs exhibit sunflower-like spherulites and squeezed spherulites, respectively. In situ wide-angle X-ray diffraction (WAXD) was performed to trace the crystalline behavior during the entire thermal treatment process to help to understand the formation mechanism of SC. Interestingly, complete stereo-complexation of PLA was achieved under suitable thermal environment. The role of melt memory effect and template effect in SCs formation is discussed.

EXPERIMENTAL SECTION Materials. PLLA with around 0.2% of D-LA (trademark L130 with Mw = 1.7 × 105 g/mol, Mn = 8.9 × 104 g/mol, and PDI = 1.95 according to GPC tests) were kindly 5



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supplied by Corbion (Purac Biochem B.V., Amsterdam, the Netherlands). And PDLA (with Mw = 1.1× 105 g/mol, Mn = 6.5 × 104 g/mol, and PDI = 1.62 according to GPC tests) was kindly supplied by Sinobiom Co. Ltd. (Changchun, China). Sample preparation. PLLA and PDLA pellets were dried at 80 °C under vacuum overnight to avoid degradation due to hydrolysis before extrusion. The PLLA/PDLA blended pellets (PLLA/PDLA = 1:1, wt/wt) were obtained through a twin-screw extruder for the subsequent injection molding. Temperatures of the barrel were 190-230 °C from the hopper to die and the screw speed was set at 240 rpm. Extruded pellets were dried at 80 °C under vacuum overnight again and then, were injection molded into dumbbell bars with a cross-sectional area of 4 × 6 mm2 and 100 mm in length by employing a commercial injection-molding machine. The temperature profile for injection molding was 210, 240, 250, 250, and 230 °C from the hopper to nozzle, respectively. The mold temperature and packing pressure were kept constant at 40 °C and 60 MPa, respectively. Scanning

electronic microscopy

(SEM).

To

observe

the crystalline

morphologies of treated samples, a small block was cut from the center part (1−2 mm away from the sample surface, and 40−50 mm away from the sample ends) of the injection-molded component, then the block was dried at 80 °C in a vacuum oven overnight. The dried blocks were placed in preheated silicone oil at different temperatures (190, 210 and 230 °C) for 10 min. The treated blocks were soaked in liquid nitrogen for 30 min and cryogenically fractured. The smooth fracture surfaces were etched by a water-methanol (1:1 by volume) mixture solution containing 0.05 6


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mol/L of sodium hydroxide for 48 h at an ambient temperature, subsequently, the etched surfaces were cleaned by distilled water with the help of ultrasonication. Then the etched surfaces were sputtered with thin golden layers and observed by a field-emission scanning electronic microscopy (SEM, Inspect F50, FEI, USA) operating at 3 kV. Characterization of SC Crystallization during Thermal Treatment. A thin slide with a dimension of 6 × 6 × 0.5 mm3 was cut from the center (1−2 mm away from the sample surface, and 40−50 mm away from the sample ends) of injection-molded PLLA/PDLA bars for in situ WAXD experiments. These experiments were carried out at BL16B, Shanghai Synchrotron Radiation Facility (SSRF) in order to investigate the crystallization behavior during thermal treatment. A MAR CCD detector (MAR-USA) with resolution of 1024 × 1024 pixels (pixel size = 80 µm) was used to acquire WAXD patterns. The wavelength of incident X-ray was 0.124 nm and the distance from the sample to CCD was 145 mm. A Linkam THMS-600 stage whose windows were replaced by Capton film capable of X-ray tests was used to precisely control thermal history. The thermal procedures were set as follows: (i) heating from room temperature to treatment temperature (190, 210 and 230 °C) at a rate of 30 °C/min; (ii) keeping at treatment temperature for 10 min; (iii) cooling to 60 °C at a rate of -10 °C/min. During the whole thermal treatment process, WAXD patterns were collected at 15 s/frame with a clear time of 2 s. Analysis of WAXD data. The profiles of one dimensional (1D)-WAXD were 7



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gained by circularly integrated intensities of two dimensional (2D)-WAXD patterns. And the total crystallinities (Xc-total) was calculated by formula (1) with the help of deconvoluting-peak method. = ∑

∑

(1)

∑

where Acryst and Aamorph are the fitted areas of all crystalline and amorphous phases, respectively. Similarly, the crystallinities of SC (Xc-sc) were obtained by formula (2), =

/

(2)

∑ ∑

where A110, A300/030, and A220 are the areas of (110), (300)/(030), and (220) reflection peak of PLA stereocomplex crystallites, respectively.43-44 Accordingly, the crystallinities of HC (Xc-hc) was given as: = −

(3)

The degree of stereo-complexation (Fsc) is calculated as: Fsc =

Xc −sc Xc −sc + Xc − hc

(4)

RESULTS Crystalline morphology of SCs after thermal treatment. High-resolution SEM observations are performed to visually exam the crystalline morphology of SCs after thermal treatment. For the sample treated at low temperature (190 oC), some tiny voids (Figure 1a1) are seen in the cryo-fracture surfaces of the sample after etching using alkali solution. Further enlarging the area without holes, an uneven surface is easy to be found (inset of Figure 1a1). This potholed surface is reminiscent of the 8


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phase separation structure where one of the immiscible phases showing a relatively unstable state can be removed by alkali solution. Considering that the thermal treatment is an efficient way to induce SCs formation, we speculate both SCs and HCs were generated in the sample. It is reported that SCs with densely crystalline structure resulted from strong stereoselective interactions between PLLA and PDLA chains possess better hydrolysis resistance than that of HCs.45-47 Thus, we assume that HCs are easier to detach from the surface during the etching process, which, together with the detached amorphous phase, are two reasons for the generation of the porous and sunken surface as shown in Figure 1a1. To the best of our knowledge, this is the first time to observe a phase-separation-like structure in enantiotropic PLLA/PDLA blends after thermal treatment. The mechanism behind such structure formation will be discussed in detail in the Discussion section. In order to exam the crystalline morphology, SEM image with increased magnification is given in Figure 1a2. Unexpectedly, only imperfect spherulites with small sizes and lamellar structures (see the arrows in Figure 1a2) can be observed. Different from the phase-separation-like morphology observed in the sample thermally annealed at low temperature, only smooth surfaces are noticed in the sample thermally treated at medium temperature (210 °C) as shown in Figure 1b1. The absence of the cavity is probably related to the undetectable or absence of the HCs with relatively low hydrolysis resistance. That is to say, the medium temperature treatment is more beneficial for the SCs formation than the low temperature. It is of interest to find the presence of spherulites in Figure 1b1, which stay apart from each 9



other and show a uniform diameter of 10-20 µm. More interestingly, as shown in the inset of Figure 1b1 with higher magnification, a sunflower-like superstructure is detected, where the classic spherulites act as flower disc, while the lamellae aligned along the axial direction (Figure 1b2) serve as the petal. Such different lamellar morphologies between the “flow disc” and the “petal” may have some relevance to the different assembly conditions of the lamellae, and we will discuss it in detail in the Discussion section.

Figure 1. Hierarchical morphology of samples thermally treated at (a) low, (b) medium and (c) high temperature with different magnifications. 10


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The crystalline morphology of the sample thermally treated at high temperature (230 oC) is also depicted in Figure 1. The cavities and holes appear again as shown in Figure 1c1, indicating the formation of a detectable level of HCs. More importantly, a large number of spherulites are observed throughout the whole sample. Different from the classical spherulites, as well as sunflower-like one as mentioned above, spherulites displayed in Figure 1c1 are polygonal and squeezed with each other, displaying an irregular shape and straight interfaces (Figure 1c2). Moreover, spherulites formed at high-temperature treatment shows diameter over 20 µm, which is bigger than that (10-20 µm, Figure 1b1) of the spherulites formed after medium temperature treatment. The increased crystal size is often associated with the promoted crystals growth due to the enhanced chain mobility at high temperature.

Crystallization process of PLLA/PDLA blends during thermal treatment. To deeply understand the temperature-dependent morphology, in situ WAXD was employed to detect the crystallization process and kinetics during the thermal treatment. Figure 2 displays the evolution of Xc-sc, Xc-hc, and Xc-total, calculated by integration of the 1D-WAXD profiles (see Figures S1), during thermal treatment at low temperature. When the temperature is 50 °C, Xc-hc, Xc-sc, and Xc-total are 0, which indicates that the injection-molded sample is amorphous before the thermal treating. Cold crystallization of both HC and SC is noticed simultaneously when the temperature reaches 87 °C. After that, HC and SC present distinctly different cold crystallization behaviors. It is seen that Xc-hc increases intensely from 0 to 0.35 and 11



reaches a plateau at about 120 °C. On the contrary, Xc-sc rises slowly and only approaches to 0.04 at the plateau. In other words, the kinetics of SC’s is much slower than that of HC during cold crystallization. This makes sense since the formation of SC needs the participation of both PLLA and PDLA chains,31 which set higher demands for chain mobility than HC formation where only one type of chains is required. After cold crystallization, both Xc-hc and Xc-sc stay constant before the temperature reaches 150 °C. It is clear that Xc-hc starts to decrease as the temperature reaches 150 °C, indicating some HCs begin to melt at this temperature. However, no obvious change of Xc-sc is noticed until 160 °C. That is to say, the increase of Xc-sc lags behind the decrease of Xc-hc. This is because the released PLLA and PDLA chains cannot stack side by side to form SC immediately, which agrees with our previous work.42 When the temperature approaches to 183 °C, all HCs melt, displaying Xc-hc of 0 and Xc-sc of ca. 0.17. In the subsequent isothermal process, Xc-hc is always 0, while Xc-sc keeps increasing from 0.17 to 0.25 with the increased holding time. In the cooling process,

Xc-sc first shows a moderate increase from 0.26 to 0.31 at the temperature zone from 190 to 153 °C, and then exhibits a constant value of ca. 0.32 when temperature further decreased to 60 °C. HCs start to crystallize at 138 °C, which is about 20 °C higher than that in neat PLLA melt.48-49 This is because the SCs can act as a nucleating agent facilitating the crystallization of HCs. When further decreasing the temperature, a gradually increased Xc-hc from 0 to 0.11 was observed. This confirmed our speculation in “crystalline morphology” section that both SCs and HCs were induced during 12


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thermal treatment at low temperature. On this occasion, it is quite reasonable to claim a phase separation structure of SCs and HCs as shown in Figures 1a1.

Figure 2. Xc-sc, Xc-hc, and Xc-total as a function of time during thermal treatment at low temperature.

Figure 3 shows the time-dependent crystallinities (calculated by integration of the 1D-WAXD profiles as shown in Figures S2) during thermal treatment at medium temperature. The heating process is almost the same as that of thermal treatment at low temperature. Some feature temperature was labeled on the picture, and will not be described here for brevity. In the isothermal process, Xc-sc dramatically increases from 0.16 to 0.33 with an increment of 0.17, which is 112.5% larger than that of the sample treated at low temperature. This implies that medium temperature is more efficient in inducing SC formation than low temperature. In the following cooling process, Xc-sc experiences a vigorous growth when the temperature decreases from 210 °C to 155 °C, and finally reaches a high value of 0.50. This is much higher than the Xc-sc (0.32, Figure 2) achieved in low-temperature thermal treatment, confirming again the 13



medium temperature has an advantage in facilitating SCs over the low temperature. It is essential to add that undetectable level of HC is formed when the cooling process ends, which is the reason for the absence of a cavity in the etched sample shown in Figure 1b1.

Figure 3. Xc-sc, Xc-hc, and Xc-total as a function of time during thermal treatment at medium temperature.

When using a high thermal treatment temperature, the evolution of Xc-sc, Xc-hc, and Xc-total (calculated by integration of the 1D-WAXD profiles as shown in Figures S3) follows a similar tendency as using low and medium temperatures before the temperature reaches 211 °C (Figure 4). Once exceeding this temperature, SCs start to melt and the Xc-sc dramatically decreases to 0.02, in clear contrast to the rising trend in the heating process when treated at the low and medium temperature (Figures 2 and 3). In the isothermal process, Xc-hc is 0, and Xc-sc (or Xc-total) keeps constant of 0.02. This phenomenon means a dynamic equilibrium of melting and recrystallization of SCs was achieved. Unexpectedly, Xc-sc exhibits a rapid increase as soon as the temperature begins to drop. A substantial improvement of Xc-sc from 0.02 to 0.38 is observed when 14


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the temperature drops from 230 to 182 °C. The physical origin of the surprisingly high crystallization temperature will be described in the Discussion section. When the temperature decreases to 143 °C, HCs start to crystallize and Xc-hc increasingly boosts from 0 to 0.11 at the temperature zone from 143 °C to 108 °C. As mentioned before, the relatively high crystallization temperature of HCs also results from SC’s nucleation effect. When the cooling procedure ends, the final crystallinity of HC, SC and the total is 0.11, 0.39 and 0.50, respectively. The existence of small amounts of HCs can be removed during the etching, contributing to the cavity as shown in Figure 1c1.

Figure 4. Xc-sc, Xc-hc, and Xc-total as a function of time during thermal treatment at high temperature.

Table 1. Fraction of SC crystallites by WAXD for injection molded PLLA/PDLA parts under different thermal environment Low temperature

Medium temperature

High temperature

(190 oC)

(210 oC)

(230 oC)

15



Fsc

0.74

1.00

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0.78

It is worth adding that Fsc values are controllable by the thermal environment (Table 1). It reaches 1.00 for the sample treated at medium temperatures, indicating complete SC crystallites can be achieved in the injection-molded parts. This is inspiring since our method is clean and inexpensive, which could be readily introduced into current industrial-scale production lines for manufacturing of high-performance PLA components. After exploring the evolution of the crystals during the thermal treatment at various temperatures, we also studied the crystallization kinetics to have a better understanding of the SCs formation. Since the crystallization behavior of HC and SC is almost the same for all the samples during the heating process, we will not discuss this procedure here. Figure 5a illustrates the Xc-sc before and after the isothermal process at different thermal treating temperatures. When isothermally treated at low temperature, Xc-sc increases slightly from 0.15 to 0.25. In comparison with that, Xc-sc dramatically enhances from 0.16 to 0.33 after isothermal treating at medium temperature, with an increment of 106.3%. In the isothermal process at high temperature, Xc-sc keeps constant of 0.02. These results indicate that the temperature shows a significant impact on the crystallization kinetics of SCs during the isothermal process. Figure 5b represents the evolutions of Xc-sc during the following cooling process. It is necessary to note that Xc-sc does not change linearly with the temperature, but for comparison, a simple linear fit is employed here to reflect the growth rate of 16


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SCs. As marked on Figure 5b, the slope of the straight line obtained through the linear fit of Xc-sc is -0.0022, -0.0045, and -0.0117 after isothermal treatment at low, medium and high temperature, respectively. These results suggest that crystallization kinetics of SCs accelerates with the increase of the isothermal temperature during the cooling process. In conclusion, from Figures 2-5, the crystallization kinetics of SCs shows obvious dependence on thermal treatment environment. When the thermal treatment temperature is low, stagnant crystallization process of SCs is noticed during both isothermal and cooling process. For the sample thermal treated at the medium temperature, noticeable and mild increases of Xc-sc are observed in the isothermal and cooling process, respectively. When it comes to thermal treatment at relatively high temperature, Xc-sc is constant during the isothermal process but expedites vigorously during the cooling process.

Figure 5. (a) The comparison of Xc-sc before and after isothermal process during thermal treatment at different temperatures. (b) The dependence of Xc-sc on temperatures during the cooling process after thermally treated at different temperatures. 17



DISCUSSION In this work, we demonstrate that thermal treatment has an observable effect on

the crystalline morphology and crystalline kinetics of SCs in PDLA/PLLA blends. Some interesting structures such as phase separation structure, sunflower-like spherulites, and squeezed spherulites were found when thermal treating at low, medium and high temperature, respectively. Inspired by the unique morphology and structure of the thermally treated sample, we are interested to clarify the formation mechanism of these structures.

Phase separation of samples thermally treated at low temperature due to the memory effect. First of all, we concentrate on the formation of phase separation structure in samples thermally treated at low temperature. A schematic diagram is presented here to help to explain the mechanism of such structure formation, as shown in Figure 6. Before thermal treatment (Figure 6a), PLLA and PDLA chains are well dispersed due to their similar chemical and physical characters, and high temperatures during the extrusion and injection-molding. With the increase of temperature during thermal treatment, the cold crystallization happens, inducing a large number of HC (Xc-hc = 0.35, Figure 2) and a few amounts of SC (Xc-sc = 0.04, Figure 2). As for HCs (taking those formed by PLLA as an example), PLLA chains assemble into lamellae or spherulites of HC, and simultaneously PDLA chains are excluded out of HCs formed by PLLA. Thus, a PLLA-rich zone that centers on HCs of PLLA is induced after cold crystallization as indicated by blue regions illustrated in Figure 6b. In the 18


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same way, PDLA-rich zones can also be induced as shown in yellow regions of Figure 6b. In the boundary area near the PLLA-rich region and PDLA-rich region, some PLLA and PDLA encounters and cold crystallization of SCs happens (the green areas in Figure 6b).

Figure 6. Schematic diagram for the formation of phase separation structure in samples thermally treated at low temperature.

After the cold crystallization, HCs start to melt with the increase of temperature. However, it is difficult to achieve a homogeneous melt with uniformly dispersed PLLA and PDLA molecular chains. This is because polymers usually show melt memory effect, or more precisely, the slowing down of crystallization kinetics.50-54 In order to erase the melt memory, the sample should be heated to a high temperature for a short time or at lower temperatures (always above the melting temperature) for a longer time.52,55 In our case, the relatively low treatment temperature (190 °C) and short isothermal time (10 min), give no chance to eliminate the melt memory effect. As a result, clusters of PLLA and clusters of PDLA chains resulted from HCs are expected in melts. That is to say, there still exist PLLA-rich and PDLA-rich regions 19



(Figure 6c). On this occasion, HCs are more likely to form since in these regions it is difficult for PLLA and PDLA to meet with each other to form SCs. SCs can only be found at the interface of the PLLA-rich and PDLA-rich region, which act as fences (or network) preventing further interdiffusion of PLLA and PDLA chains, thus further stabilizing PLLA-rich and PDLA-rich zones. Finally, after the thermal treatment, HCs mainly abound in the independent phases, while SCs mainly locate in the interfaces, which results in the phase separation structure (Figure 1a1). It is worth to note that the crystallinity of HC after cold crystallization is 0.35 while the final crystallinity after the thermal treating is 0.11. This indicates that not all HCs formed during cold crystallization can behave memory effect. Some of them relax toward the fully equilibrated random state and transform into homogeneous melt when the temperature is higher than the melting point of HC.

Dependence of morphologies and crystalline kinetics on thermal treatment temperatures. From the SEM observations (Figure 1), and in situ WAXD results (Figures 2-5), we are confident to claim that the morphology and crystalline kinetics of SCs are obviously influenced by the thermal environment. This can be also proved by DSC results (Figure S4). In this part, we will give a detailed discussion of this phenomenon. As discussed above, before the thermal treatment, samples are homogeneous with well dispersed PLLA and PDLA chains, as schematically shown in Figure 7a1. During the heating procedure, cold crystallization of HCs prevails over that of SCs (Figures 2-4), which leads to samples after cold crystallization mainly contain HCs formed by PLLA or PDLA (Figure 7a2). This phenomenon is common to 20


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all the samples treated at different temperatures, so the heating process will not be discussed here again.

Figure 7. The morphology evolution of injection-molded PLLA/PDLA amorphous parts during thermal treatment under different temperatures.

When the isothermal temperature is low, as discussed above, the sample displays a heterogeneous melt due to the melt memory effect. Although the unmelted SCs with perfect surface and well-defined lattice parameters can serve as templates to induce the formation of new SCs (see detailed description of template effect in Ref.42), the heterogeneous melt where PLLA and PDLA rich zones are separated is definitely an adverse factor for the crystallization of SC that needs the participation of both PLLA and PDLA chains. In this case, crystalline rates of SCs are obviously constrained 21



during both isothermal and cooling process (Figures 2-5). Meanwhile, because of the phase-separated structure resulted from the heterogeneous melt, the chances of contact between PLLA and PDLA chains is low. Therefore, SCs with high content cannot be expected in samples thermally treated at the low temperature and the final crystallinity of SC is only 0.32 (Figure 2), which is the lowest compared to those treated at medium and high temperatures (Figures 3 and 4). In addition, confined by the sizes of the separated phases, it is difficult for PLLA and PDLA chains to assemble into well-defined crystals with big sizes. What’s worse, the relatively low isothermal temperature leads to a slow chain mobility and a high nucleation density that further restricting the mobility of PLA chains. This results in a suppressed crystalline rate of SCs (Figure 2), and formation of imperfect spherulites with small sizes (Figure 1a2). In a word, the crystallization kinetics and morphologies in samples thermally treated at low temperature mainly controlled by melt memory effect of HCs and resulted phase separation structure and the mechanism is illustrated in Figures 7b1 and 7c1. Increasing thermal treatment temperature to a medium one, crystalline morphology and kinetics of SCs become distinctly different. Firstly, because the thermal treatment temperature, i.e. 210 °C, is much higher than the melting and equilibrium melting temperature of HCs (170 – 180 °C),32 no clusters of PLLA and PDLA chains can survive and PLLA and PDLA chains tend to relax into random coils. Consequently, phase separation cannot be observed and homogeneous melts are obtained again. With the aid of the template effect, new SCs are efficiently induced on 22


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the surfaces of unmelted (residual) SCs (stage I in Figure 7b2), which leads to the obvious increase of Xc-sc as shown in Figures 3 and 5. With isothermal time elapsing, almost all surfaces of residual SCs are occupied by newly formed SCs, which is termed as template saturation (stage II in Figure 7b2) and results in the deceleration of SC’s crystallization during the isothermal process (Figure 3). Due to template saturation, the growth of spherulites may cease during the isothermal process. In the following cooling process, new templates can be formed due to the supercooling,42 and spherulites continue to grow with the decrease of temperature. However, thermal history of SCs formed during the isothermal and cooling process is distinctly different, which brings different morphologies between the inner and outer region of spherulites. Therefore, the resulted sunflower-like spherulites are observed in Figure 1b2. Meanwhile, self-nucleation can take place during isothermal and the following cooling process (stage III in Figure 7b2), which also makes a contribution to the crystallization of SCs. Finally, complete SC formation can be achieved in the sample thermal treating at medium temperature. In conclusion, the crystallization kinetics and morphologies in samples thermally treated at medium temperature mainly depended on the template effect of SCs. In future work, we will make more detailed studies about the thermal treatment environment, and identify the critical temperature that the template effect of SCs begins to affect the crystallization kinetics. When it comes to high isothermal temperature, only a small quantity of SCs with the crystallinity of 0.02 can survive in the homogeneous melt. Conceivably, SCs are mainly chain bundles or crystal frames due to the elevated temperature (Figure 7b3). 23



On one hand, the unmelted SCs can serve as templates to induce the formation of SCs. On the other hand, the new SCs keep melting due to the high temperature. That’s the reason for the dynamic equilibrium of melting and recrystallization observed in Figure 4. Thanks to the template effect, the recrystallization of SCs can take place at a quite high temperature (~230 oC, Figure 4) in the following cooling process. This is evidenced by the crystalline behavior of the sample thermally treated at 250 °C (see Figures S5 and S6), where the onset crystallization temperature of SC is 179 °C because no SC exists to act as a template in the beginning of the cooling process (Xc-sc = 0, Figure S6). Compared to the sample treated at low and medium temperatures, the high crystalline temperature endows the PLLA and PDLA chains with high mobility, which is a benefit for the growth of crystals. In addition, the low initial number of SCs that will not suppress the mobility of PLA chains is also in favor of the growth of crystals. As a result, the crystallization kinetics of SCs during cooling process after thermally treated at high temperature is the fastest among all thermal treatments at different temperatures (Figure 5). At the same time, due to the high chain mobility, collided spherulites with big sizes tend to form, contributing to the squeezed spherulites (Figure 1c and Figure 7c3). Taking low content of SC nucleation into consideration, exclusive SCs are difficult to be obtained in thermally treated samples and the formation of HCs is noticed in the cooling process as shown in Figure 4 and Figure 7c3. In a word, the crystalline morphologies and kinetics are mainly dominated by the chain mobility of PLA chains and growth process of SC at relatively high temperature. 24


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CONCLUSION The hierarchical morphology of SCs induced by the thermal treatment of amorphous PLA was investigated. For the first time, phase separation textures resulted from melt memory effect, and stagnant crystallization process of SCs are observed when thermally treated at low temperature. Increasing thermal treatment temperature to a medium one, a rapid growth of SC’s crystallinity is noticed during the isothermal process due to template effect of the unmelted SCs. After template saturation, a further increase of SC’s crystallinity generates due to the formation of new templates arising from the supercooling during the cooling process. The different thermal history of SCs formed during isothermal and cooling process contributes to the sunflower-like spherulites. Interestingly, exclusive SC crystallites can be achieved under this thermal environment. After a high-temperature treatment, the crystallinity of SCs undergoes a notable increase as soon as the temperature starts to drop. The incredible high crystalline temperature benefits from the template effect of unmelted SC. Thanks to the high-temperature crystallization and thus high chain mobility, crystal growth is promoted distinctly. As a result, squeezed spherulites are formed. The in-depth discussion about the relationship between morphologies and crystalline kinetics in this work is attractive in both academic and industrial, which will help us understand the mechanism behind the SCs formation, and provide guidance for fabrication of controlled structure, such as complete SC crystallites, in PLLA/PDLA components. 25



ASSOCIATED CONTET

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ***. 1D-WAXD files, DSC results, description of the crystallinity change during thermal treatment at 250 oC.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] ORCID Yan-Fei Huang: 0000-0001-9383-5063 Jia-Zhuang Xu: 0000-0001-9888-7014 Gan-Ji Zhong: 0000-0002-8540-7293 Zhong-Ming Li: 0000-0001-7203-1453 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors gratefully thank the financial support from the National Natural

Science Foundation of China (51533004, 51673135, 21576173 and 51573116), the 26


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Youth Foundation of Science & Technology Department of Sichuan Province (Grant No. 2017JQ0017). We also thank Beamlines BL16B in Shanghai Synchrotron Radiation Facility (SSRF) for supporting X-ray measurement.

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For Table of Contents Use Only

The Role of Melt Memory and Template Effect in Complete Stereocomplex Crystallization and Phase Morphology of Polylactides

Yan-Fei Huang, Zheng-Chi Zhang, Yue Li, Jia-Zhuang Xu, Ling Xu, Zheng Yan, Gan-Ji Zhong,* and Zhong-Ming Li

Phase separation structure, sunflower-like spherulites, and squeezed spherulites have been induced by memory effects and/or template effects after treating at various thermal environments in sterocomplexed polylactide.

35


The Role of Melt Memory and Template Effect in Complete

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