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Direct observation of a stereocomplex crystallite network in the asymmetric polylactide enantiomeric blends
Accepted Manuscript Direct observation of a stereocomplex crystallite network in the asymmetric polylactide enantiomeric blends Jie Wang, Ruihua Lv, Bin Wang, Bing Na, Hesheng Liu PII:
S0032-3861(18)30299-4
DOI:
10.1016/j.polymer.2018.04.012
Reference:
JPOL 20492
To appear in:
Polymer
Received Date: 13 December 2017 Revised Date:
27 February 2018
Accepted Date: 2 April 2018
Please cite this article as: Wang J, Lv R, Wang B, Na B, Liu H, Direct observation of a stereocomplex crystallite network in the asymmetric polylactide enantiomeric blends, Polymer (2018), doi: 10.1016/ j.polymer.2018.04.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Direct Observation of a Stereocomplex Crystallite Network in the Asymmetric
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Polylactide Enantiomeric Blends
Jie Wang, Ruihua Lv, Bin Wang, Bing Na*, Hesheng Liu
Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices, School of
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Chemistry, Biology and Materials Science, East China University of Technology, Nanchang, 330013,
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People’s Republic of China
Abstract: In the past a stereocomplex crystallite network was inferred in the asymmetric poly (L-lactide) (PLLA)/poly (D-lactide) (PDLA) blends. This study directly witnessed a honeycomb stereocomplex
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crystallite network in the blends with high-molecular-weight PDLA as a minor component via selective dissolution. High-molecular-weight PDLA spanned and connected stereocomplex crystallites to produce the honeycomb network that survived during selective dissolution. The stereocomplex crystallite
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network significantly accelerated melt crystallization of PLLA matrix owing to remarkable nucleating
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effects. Finally, the origin of the stereocomplex crystallite network was illustrated. Keywords: stereocomplex crystallite network; nucleating effects; asymmetric PLLA/PDLA blends
*
Correspondence author. E-mail address: [email protected], [email protected]
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1. Introduction Polylactide is a kind of semi-crystalline polymers derived from natural resources. It has two
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enantiomers, i.e. poly (L-lactide) (PLLA) and poly (D-lactide) (PDLA).1 PLLA, similar to PDLA, has low crystallization ability and usually behaves amorphous during practical processing. Much effort has been made to enhance PLLA crystallization, for instance by adding nucleation agents.2 Stereocomplex 4
are highly efficient to accelerate PLLA
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crystallites, co-crystallization from PLLA and PDLA,3,
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crystallization. Stereocomplex crystallites, having a higher melting point by about 50 ℃ than PLLA homocrystals, can survive in the PLLA melt and act as nucleation sites to induce rapid PLLA crystallization upon subsequent cooling.5-8 Practically, PDLA as a minor component is mixed with PLLA to produce asymmetric blends. Different from other nucleation agents, stereocomplex crystallites are
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simultaneously generated in the PLLA matrix during mixing.
Accelerated PLLA crystallization is affected by the morphology of resultant stereocomplex crystallites in the asymmetric blends. The decrease in the size of stereocomplex crystallites via
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re-crystallization contributes to high nucleation density and thus enhanced PLLA crystallization.9 On the other hand, stereocomplex crystallites tend to form a network via cross-linking effects in the asymmetric
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blends above a critical amount of PDLA, e.g. 2 wt%.10 It is manifested by the abrupt increasing of viscosity and prolonged relaxation behavior in the melt.10, 11 The stereocomplex crystallite network can retard growth of PLLA homocrystals to some extent, albeit of enhanced nucleation density. In addition, it was argued that stereocomplex crystallite network could induce pronounced local stress, responsible for acceleration in the PLLA nucleation.12 To date, the presence of stereocomplex crystallite network in the asymmetric blends was mainly
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deduced from rheological results.10-12 An exception is that some bright lines were observed in the melt by optical microscopy and were ascribed to stereocomplex crystallite network.13 However, a clear
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structure of stereocomplex crystallite network is still absent. In this study, a honeycomb stereocomplex crystallite network in the asymmetric blends with high-molecular-weight PDLA as a minor component was for the first time disclosed by electron
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microscopy with aid of selective dissolution. High-molecular-weight PDLA chains spanned and
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connected stereocomplex crystallites to produce the honeycomb network. Correspondingly, crystallization in PLLA matrix was remarkably promoted by the stereocomplex crystallite network. 2. Experimental Section 2.1. Materials and Sample preparation
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The PLLA and PDLA were supplied by Changchun Sinobiomaterials Co., Ltd, China. The viscosity-average molecular weight of PLLA was 140 kg/mol. Two kinds of PDLA with viscosity-average molecular weight of 49 and 528 kg/mol, respectively, were used. The low- and
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high-molecular-weight PDLAs were designated as l-PDLA and h-PDLA, respectively. Solution mixing
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in chloroform with a concentration of 0.1 g/ml at room temperature was adopted to prepare PLLA/PDLA blends. After complete dissolution the homogenous solutions were slowly poured into a large amount of ethanol for precipitation. The PLLA/PDLA blends were dried at 40 ℃ under vacuum for a week. The content of PDLA in the blends was 1, 2, 5 wt%, respectively. For comparison neat PLLA was also subjected to the above procedures. Samples for measurements were fabricated by hot pressing at 200 ℃ for 3 min, followed by quenching into liquid nitrogen. 2.2. Characterizations
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XRD. X-ray diffraction measurements were conducted by a Bruker D8 ADVANCE X-ray diffractometer at room temperature. The wavelength of the X-ray was 0.154 nm; and the reported XRD profiles were
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subtracted from background diffractions. SEM. Morphological observations were explored by a Nova NanoSEM 450 scanning electron microscope (SEM). Samples were prepared by selective dissolution in dichloromethane to remove
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excess PLLA in the blends at room temperature. The floccules undissolved in dichloromethane were
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collected by a stainless steel mesh for SEM measurements after drying.
Rheology. Melt properties were evaluated by a TA discovery hybrid rheometer (DHR-2) with a 25 mm parallel-plate geometry at 200 ℃ under a flow nitrogen atmosphere. Dynamic oscillatory shear measurements were carried out in the linear viscoelastic regime that was determined by a prior strain
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sweep.
DSC. Differential scanning calorimetry measurements were preformed by a TA Q2000 DSC instrument under a flow nitrogen atmosphere. To record non-isothermal crystallization traces samples were cooled
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at a rate of 5 ℃/min after being held at 200 ℃ for 3 min. For isothermal crystallization samples were
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rapidly cooled to a desired crystallization temperature from 200 ℃. OM. Morphology change during heating was checked by an optical microscope equipped with a hotstage. Samples, same to those for SEM measurements, were stepwise heated to a desired temperature. After being isothermally held for 3 min images were captured. 3. Results and Discussion As shown by XRD profiles in Figure 1, stereocomplex crystallites are presented in both PLLA/h-PDLA and PLLA/l-PDLA blends. The characteristic diffractions at 2θ of 12, 20.8 and 24o are
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ascribed to the (110), (300)/(030), and (220) planes of stereocomplex crystallites, respectively.12, 14, 15 Meanwhile, there is absence of diffractions from homocrystals in the blends, same to that observed for
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neat PLLA. It suggests that at 200 ℃ excess PLLA in the blends remains molten state but stereocomplex crystallites prevail. Note that stereocomplex crystallites and homocrystals have a melting point of about 222 and 174 ℃, respectively (Figure S1, Supporting Information). On the other hand, formation of
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stereocomplex crystallites seems to be irrelevant with molecular weight of PDLA. There is little
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difference in the amount of stereocomplex crystallites between PLLA/h-PDLA and PLLA/l-PDLA blends at the same PDLA content (see crystallinity labeled in Figure 1).
Intensity
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Figure 1 XRD profiles of the blends with indicated PDLA content quenched from 200 ℃: (a) PLLA/h-PDLA, (b) PLLA/l-PDLA. The crystallinity (Xc) of stereocomplex crystallites was labeled for each sample.
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Stereocomplex crystallites are physically cross-linked with PLLA matrix due to co-crystallization between PLLA and PDLA chains.10-12 The cross-linking effects could result in the formation of a
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stereocomplex crystallite network in the PLLA matrix. Selective dissolution in dichloromethane was adopted to verify structural formation of stereocomplex crystallites in both blends. It is based on that stereocomplex crystallites can survive in dichloromethane but homocrystals are completely dissolved.7, Figures 2a-b presents optical images of PLLA/h-PDLA and PLLA/l-PDLA blends in dichloromethane,
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respectively. Some floccules are observed in the transparent solutions for PLLA/h-PDLA blends, irrespective of PDLA content. In contrast, turbid solutions are exhibited by PLLA/l-PDLA blends, as a result of severe light scattering from dispersed stereocomplex crystallites in dichloromethane.10 It means that an integrated stereocomplex crystallite network should exist in the PLLA/h-PDLA blends.
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PLLA/l-PDLA blends.
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Otherwise, turbid solutions could be obtained for PLLA/h-PDLA blends, similar to those of
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PLLA/I-PDLA
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Figure 2 Optical photographs of the blends with indicated PDLA content in dichloromethane: (a) PLLA/h-PDLA, (b) PLLA/l-PDLA. The concentration of the blends in dichloromethane was 30 mg/ml. (c) schematic diagrams of structural formation of stereocomplex crystallites in both blends.
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High-molecular-weight PDLA chains should span and connect several stereocomplex crystallites to produce the integrated network. The anchoring of high-molecular-weight PDLA chains in the
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undissolved stereocomplex crystallites is thus responsible for the network integrity in dichloromethane. In contrast, stereocomplex crystallites in the PLLA/l-PDLA blends can not be directly bridged by low-molecular-weight PDLA chains. PLLA chains in the matrix should combine the stereocomplex
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crystallites, as suggested in a previous study.10 The dissolution of PLLA chains in the matrix results in
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the collapse of stereocomplex crystallites in dichloromethane. Figure 2c gives schematic diagrams of structural formation of stereocomplex crystallites in PLLA/h-PDLA and PLLA/l-PDLA blends, respectively.
The microscopic morphology of floccules is demonstrated by SEM micrographs in Figure 3. A
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honeycomb network consisting of highly interconnected threads is observed for PLLA/h-PDLA blends with different PDLA content. It suggests that stereocomplex crystallites generated in the PLLA/h-PDLA blends are indeed bridged by high-molecular-weight PDLA. The excess PLLA should be trapped in the
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meshes of the network, which was removed by dichloromethane during selective dissolution. The
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network becomes dense with increasing of PDLA content in the blends, as a result of enhanced amount of stereocomplex crystallites.
On the other hand, the honeycomb network is thermally stable below melting point of the stereocomplex crystallites (~222 ℃). As shown by optical micrographs in Figure 4, the network almost persists at 220 ℃ but melts completely at 230 ℃. It provides a direct evidence for complete survival of the stereocomplex crystallite network while being held at 200 ℃, which is unprecedented in the previous studies. Therein, stereocomplex crystallites rather than the network were confirmed in the
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asymmetric PLLA/PDLA blends at elevated temperatures.16, 17
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Figure 3 SEM micrographs revealing a stereocomplex crystallite network at (a-c) low and (a’-c’) high magnifications, respectively, in the PLLA/h-PDLA blends after selective dissolution by dichloromethane:
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(a, a’) 1 wt%, (b, b’) 2 wt%, (c, c’) 5 wt%. Scale bars: 200 µm (left column), 50 µm (right column).
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Figure 4 Optical micrographs revealing thermal stability of the stereocomplex crystallite network in the PLLA/h-PDLA blends with 2 wt% PDLA after selective dissolution by dichloromethane: (a) 30 ℃, (b) 200 ℃, (c) 220 ℃, (d) 230 ℃. Scale bar: 50 µm (for all images).
Formation of stereocompelx network significantly reinforces the melt of PLLA/h-PDLA blends. As
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shown in Figure 5, remarkable melt elasticity is exhibited by PLLA/h-PDLA blends with 5 wt% PDLA. It is demonstrated by that storage modulus (G’) is higher than loss modulus (G’’) in the low frequency region.18, 19 Meanwhile, the stereocomplex crystallite network can afford high stress. Therefore, high
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storage modulus is exhibited by PLLA/h-PDLA blends. At a frequency of 0.1 Hz, storage modulus is
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about 3400 Pa for PLLA/h-PDLA blends, which is almost 4 times higher than that of PLLA/l-PDLA blends. Correspondingly, high complex viscosity is achieved in the PLLA/h-PDLA blends due to remarkable reinforcement effects from the stereocomplex crystallite network. In addition, it is expected that the stereocomplex crystallite network could be slipped under stress, responsible for shear thinning in the PLLA/h-PDLA blends. The other possibility is disentanglement of excess PLLA chains that are intertwined with the threads of the stereocomplex crystallite network.
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Figure 5 Comparison of (a) storage and loss modulus and (b) complex viscosity between PLLA/h-PDLA and PLLA/l-PDLA blends with 5 wt% PDLA at 200 ℃. As demonstrated by DSC cooling trances in Figure 6, significant enhancement of PLLA
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crystallization occurs in the PLLA/h-PDLA blends. The crystallization exotherm in the blends is shifted
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to high temperatures, as compared to that of neat PLLA. The crystallization temperature is enhanced by about 40 ℃ in the PLLA/h-PDLA blends with 5 wt% PDLA. On the other hand, at the same PDLA content PLLA crystallization occurs earlier in the PLLA/h-PDLA blends than that in the PLLA/l-PDLA blends. For instance, at 2 wt% PDLA content crystallization temperature is about 142 and 120 ℃ for PLLA/h-PDLA and PLLA/l-PDLA blends, respectively. Rapid PLLA crystallization in the PLLA/h-PDLA blends is further demonstrated by isothermal crystallization at various temperatures. The blends with 2 wt% PDLA were selected as examples; and the corresponding results are given in Figure 7.
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The peak time of exotherms is much shorter for PLLA/h-PDLA blends at each crystallization temperature. For example, it is about 2 and 35 min for PLLA/h-PDLA and PLLA/l-PDLA blends,
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Figure 6 DSC cooling traces of (a) PLLA/h-PDLA and (b) PLLA/l-PDLA blends with indicated PDLA content from 200 ℃, (c) PLLA crystallization temperature versus PDLA content in both blends.
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Figure 7 DSC heat flow traces of (a) PLLA/h-PDLA and (b) PLLA/l-PDLA blends with 2 wt% PDLA content while isothermal crystallization at indicated temperature, (c) peak time versus crystallization temperature in both blends.
Different PLLA crystallization habits should be correlated with morphology of stereocomplex crystals in the blends. Abundant thin threads within the stereocomplex crystallite network in the PLLA/h-PDLA blends could trigger high nucleation density towards PLLA crystallization around them. It is similar to transcrystallization induced by stereocomplex fibers embedded in the PLLA melt.20 High
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nucleation density is thus responsible for remarkable PLLA crystallization in the PLLA/h-PDLA blends. On the other hand, the stereocomplex crystallite network is less likely to be synchronously shrunk with
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PLLA melt due to its rigidity while temperature jumps down. Therefore, significant interfacial stress could be encountered by PLLA melt trapped in the stereocomplex crystallite network prior to crystallization.12 The interfacial stress could promote nucleation to some extent in the PLLA/h-PDLA
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blends. The above two aspects regarding nucleation effects toward PLLA matrix are insignificant in the
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PLLA/l-PDLA blends without the stereocomplex crystallite network. Thus, accelerated crystallization becomes less remarkable in the PLLA/l-PDLA blends.
The stereocomplex crystallite network in the PLLA/h-PDLA blends was already induced during precipitation process, accompanied by abundant homocrystals (Figure S2, Supporting Information).
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Annealing at 200 ℃ results in the melting of homocrystals but the stereocomplex crystallite network is retained. To solidify this argument, the precipitates of PLLA/h-PDLA blends were heated up to 240 ℃ to eliminate possible stereocomplex crystals, followed by quenching into liquid nitrogen. The
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down at 5 ℃/min.
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amorphous blends were annealed at 200 ℃ for 3 min to produce stereocomplex crystals, and then cooled
Figure 8 shows the corresponding DSC results of PLLA/h-PDLA blends subjected to the above thermal procedures. The stereocomplex crystals, generated from amorphous blends at 200 ℃, also have nucleating effects on melt crystallization of PLLA matrix. The crystallization temperature is enhanced by about 30 ℃ in the PLLA/ h-PDLA blends with 5 wt% PDLA. On the other hand, selective dissolution demonstrates that there are few floccules in dichloromethane (the insets in Fig. 8b). However, the Tyndall scattering indicates that the generated stereocomplex crystals are in the form of very fine
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particles. It means that stereocomplex crystals, produced only by annealing, are isolated rather than interconnected in the PLLA matrix. It is different from the stereocomplex crystallite network shown in
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Figs. 2-3. High melt viscosity should be responsible for the retarded formation of the stereocomplex crystallite network. Correspondingly, nucleating effects from very fine stereocompelx crystals are
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weaker than that of the stereocompelx crystallite network (Fig. 6).
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Figure 8 (a) DSC cooling traces and (b) crystallization temperature of PLLA/h-PDLA blends with respect to PDLA content. In (b) optical photographs of the blends in dichloromethane, irradiated by a laser, are included as insets. Sample preparation is described in the text. 4. Conclusion Stereocomplex crystallites are highly interconnected to form a honeycomb network in the asymmetric blends with high-molecular-weight PDLA as a minor component. It is absent in the blends
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with low-molecular-weigh PDLA where stereocomplex crystallites are isolated in the PLLA matrix. The honeycomb network is much pronounced to accelerate crystallization of PLLA matrix due to enhanced
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nucleation effects; and the crystallization temperature is enhanced by about 40 ℃ with 5 wt% PDLA. Finally, a comparative study indicates that formation of the stereocomplex crystallite network is induced by precipitation process. This study gives further insights into structural formation of the stereocomplex
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crystallite network and its remarkable effect on PLLA crystallization in the asymmetric PLLA/PDLA
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blends.
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (No.
20133ACB21006).
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References
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21664001, 21364001) and the Major Program of Natural Science Foundation of Jiangxi, China (No.
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ACCEPTED MANUSCRIPT A stereocomplex crystallite network was directly witnessed in the PLLA/PDLA blends The network was favored in the blends with high-molecular-weight PDLA
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The network enhanced crystallization temperature of PLLA matrix by about
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S0032-3861(18)30299-4
DOI:
10.1016/j.polymer.2018.04.012
Reference:
JPOL 20492
To appear in:
Polymer
Received Date: 13 December 2017 Revised Date:
27 February 2018
Accepted Date: 2 April 2018
Please cite this article as: Wang J, Lv R, Wang B, Na B, Liu H, Direct observation of a stereocomplex crystallite network in the asymmetric polylactide enantiomeric blends, Polymer (2018), doi: 10.1016/ j.polymer.2018.04.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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140
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Direct Observation of a Stereocomplex Crystallite Network in the Asymmetric
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Polylactide Enantiomeric Blends
Jie Wang, Ruihua Lv, Bin Wang, Bing Na*, Hesheng Liu
Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices, School of
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Chemistry, Biology and Materials Science, East China University of Technology, Nanchang, 330013,
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People’s Republic of China
Abstract: In the past a stereocomplex crystallite network was inferred in the asymmetric poly (L-lactide) (PLLA)/poly (D-lactide) (PDLA) blends. This study directly witnessed a honeycomb stereocomplex
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crystallite network in the blends with high-molecular-weight PDLA as a minor component via selective dissolution. High-molecular-weight PDLA spanned and connected stereocomplex crystallites to produce the honeycomb network that survived during selective dissolution. The stereocomplex crystallite
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network significantly accelerated melt crystallization of PLLA matrix owing to remarkable nucleating
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effects. Finally, the origin of the stereocomplex crystallite network was illustrated. Keywords: stereocomplex crystallite network; nucleating effects; asymmetric PLLA/PDLA blends
*
Correspondence author. E-mail address: [email protected], [email protected]
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1. Introduction Polylactide is a kind of semi-crystalline polymers derived from natural resources. It has two
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enantiomers, i.e. poly (L-lactide) (PLLA) and poly (D-lactide) (PDLA).1 PLLA, similar to PDLA, has low crystallization ability and usually behaves amorphous during practical processing. Much effort has been made to enhance PLLA crystallization, for instance by adding nucleation agents.2 Stereocomplex 4
are highly efficient to accelerate PLLA
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crystallites, co-crystallization from PLLA and PDLA,3,
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crystallization. Stereocomplex crystallites, having a higher melting point by about 50 ℃ than PLLA homocrystals, can survive in the PLLA melt and act as nucleation sites to induce rapid PLLA crystallization upon subsequent cooling.5-8 Practically, PDLA as a minor component is mixed with PLLA to produce asymmetric blends. Different from other nucleation agents, stereocomplex crystallites are
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simultaneously generated in the PLLA matrix during mixing.
Accelerated PLLA crystallization is affected by the morphology of resultant stereocomplex crystallites in the asymmetric blends. The decrease in the size of stereocomplex crystallites via
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re-crystallization contributes to high nucleation density and thus enhanced PLLA crystallization.9 On the other hand, stereocomplex crystallites tend to form a network via cross-linking effects in the asymmetric
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blends above a critical amount of PDLA, e.g. 2 wt%.10 It is manifested by the abrupt increasing of viscosity and prolonged relaxation behavior in the melt.10, 11 The stereocomplex crystallite network can retard growth of PLLA homocrystals to some extent, albeit of enhanced nucleation density. In addition, it was argued that stereocomplex crystallite network could induce pronounced local stress, responsible for acceleration in the PLLA nucleation.12 To date, the presence of stereocomplex crystallite network in the asymmetric blends was mainly
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deduced from rheological results.10-12 An exception is that some bright lines were observed in the melt by optical microscopy and were ascribed to stereocomplex crystallite network.13 However, a clear
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structure of stereocomplex crystallite network is still absent. In this study, a honeycomb stereocomplex crystallite network in the asymmetric blends with high-molecular-weight PDLA as a minor component was for the first time disclosed by electron
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microscopy with aid of selective dissolution. High-molecular-weight PDLA chains spanned and
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connected stereocomplex crystallites to produce the honeycomb network. Correspondingly, crystallization in PLLA matrix was remarkably promoted by the stereocomplex crystallite network. 2. Experimental Section 2.1. Materials and Sample preparation
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The PLLA and PDLA were supplied by Changchun Sinobiomaterials Co., Ltd, China. The viscosity-average molecular weight of PLLA was 140 kg/mol. Two kinds of PDLA with viscosity-average molecular weight of 49 and 528 kg/mol, respectively, were used. The low- and
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high-molecular-weight PDLAs were designated as l-PDLA and h-PDLA, respectively. Solution mixing
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in chloroform with a concentration of 0.1 g/ml at room temperature was adopted to prepare PLLA/PDLA blends. After complete dissolution the homogenous solutions were slowly poured into a large amount of ethanol for precipitation. The PLLA/PDLA blends were dried at 40 ℃ under vacuum for a week. The content of PDLA in the blends was 1, 2, 5 wt%, respectively. For comparison neat PLLA was also subjected to the above procedures. Samples for measurements were fabricated by hot pressing at 200 ℃ for 3 min, followed by quenching into liquid nitrogen. 2.2. Characterizations
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XRD. X-ray diffraction measurements were conducted by a Bruker D8 ADVANCE X-ray diffractometer at room temperature. The wavelength of the X-ray was 0.154 nm; and the reported XRD profiles were
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subtracted from background diffractions. SEM. Morphological observations were explored by a Nova NanoSEM 450 scanning electron microscope (SEM). Samples were prepared by selective dissolution in dichloromethane to remove
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excess PLLA in the blends at room temperature. The floccules undissolved in dichloromethane were
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collected by a stainless steel mesh for SEM measurements after drying.
Rheology. Melt properties were evaluated by a TA discovery hybrid rheometer (DHR-2) with a 25 mm parallel-plate geometry at 200 ℃ under a flow nitrogen atmosphere. Dynamic oscillatory shear measurements were carried out in the linear viscoelastic regime that was determined by a prior strain
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sweep.
DSC. Differential scanning calorimetry measurements were preformed by a TA Q2000 DSC instrument under a flow nitrogen atmosphere. To record non-isothermal crystallization traces samples were cooled
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at a rate of 5 ℃/min after being held at 200 ℃ for 3 min. For isothermal crystallization samples were
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rapidly cooled to a desired crystallization temperature from 200 ℃. OM. Morphology change during heating was checked by an optical microscope equipped with a hotstage. Samples, same to those for SEM measurements, were stepwise heated to a desired temperature. After being isothermally held for 3 min images were captured. 3. Results and Discussion As shown by XRD profiles in Figure 1, stereocomplex crystallites are presented in both PLLA/h-PDLA and PLLA/l-PDLA blends. The characteristic diffractions at 2θ of 12, 20.8 and 24o are
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ascribed to the (110), (300)/(030), and (220) planes of stereocomplex crystallites, respectively.12, 14, 15 Meanwhile, there is absence of diffractions from homocrystals in the blends, same to that observed for
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neat PLLA. It suggests that at 200 ℃ excess PLLA in the blends remains molten state but stereocomplex crystallites prevail. Note that stereocomplex crystallites and homocrystals have a melting point of about 222 and 174 ℃, respectively (Figure S1, Supporting Information). On the other hand, formation of
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stereocomplex crystallites seems to be irrelevant with molecular weight of PDLA. There is little
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difference in the amount of stereocomplex crystallites between PLLA/h-PDLA and PLLA/l-PDLA blends at the same PDLA content (see crystallinity labeled in Figure 1).
Intensity
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Figure 1 XRD profiles of the blends with indicated PDLA content quenched from 200 ℃: (a) PLLA/h-PDLA, (b) PLLA/l-PDLA. The crystallinity (Xc) of stereocomplex crystallites was labeled for each sample.
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Stereocomplex crystallites are physically cross-linked with PLLA matrix due to co-crystallization between PLLA and PDLA chains.10-12 The cross-linking effects could result in the formation of a
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stereocomplex crystallite network in the PLLA matrix. Selective dissolution in dichloromethane was adopted to verify structural formation of stereocomplex crystallites in both blends. It is based on that stereocomplex crystallites can survive in dichloromethane but homocrystals are completely dissolved.7, Figures 2a-b presents optical images of PLLA/h-PDLA and PLLA/l-PDLA blends in dichloromethane,
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10
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respectively. Some floccules are observed in the transparent solutions for PLLA/h-PDLA blends, irrespective of PDLA content. In contrast, turbid solutions are exhibited by PLLA/l-PDLA blends, as a result of severe light scattering from dispersed stereocomplex crystallites in dichloromethane.10 It means that an integrated stereocomplex crystallite network should exist in the PLLA/h-PDLA blends.
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0 wt% 1 wt% 2 wt% 5 wt%
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PLLA/l-PDLA blends.
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Otherwise, turbid solutions could be obtained for PLLA/h-PDLA blends, similar to those of
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PLLA/h-PDLA
PLLA
PLLA/I-PDLA
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Figure 2 Optical photographs of the blends with indicated PDLA content in dichloromethane: (a) PLLA/h-PDLA, (b) PLLA/l-PDLA. The concentration of the blends in dichloromethane was 30 mg/ml. (c) schematic diagrams of structural formation of stereocomplex crystallites in both blends.
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High-molecular-weight PDLA chains should span and connect several stereocomplex crystallites to produce the integrated network. The anchoring of high-molecular-weight PDLA chains in the
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undissolved stereocomplex crystallites is thus responsible for the network integrity in dichloromethane. In contrast, stereocomplex crystallites in the PLLA/l-PDLA blends can not be directly bridged by low-molecular-weight PDLA chains. PLLA chains in the matrix should combine the stereocomplex
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crystallites, as suggested in a previous study.10 The dissolution of PLLA chains in the matrix results in
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the collapse of stereocomplex crystallites in dichloromethane. Figure 2c gives schematic diagrams of structural formation of stereocomplex crystallites in PLLA/h-PDLA and PLLA/l-PDLA blends, respectively.
The microscopic morphology of floccules is demonstrated by SEM micrographs in Figure 3. A
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honeycomb network consisting of highly interconnected threads is observed for PLLA/h-PDLA blends with different PDLA content. It suggests that stereocomplex crystallites generated in the PLLA/h-PDLA blends are indeed bridged by high-molecular-weight PDLA. The excess PLLA should be trapped in the
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meshes of the network, which was removed by dichloromethane during selective dissolution. The
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network becomes dense with increasing of PDLA content in the blends, as a result of enhanced amount of stereocomplex crystallites.
On the other hand, the honeycomb network is thermally stable below melting point of the stereocomplex crystallites (~222 ℃). As shown by optical micrographs in Figure 4, the network almost persists at 220 ℃ but melts completely at 230 ℃. It provides a direct evidence for complete survival of the stereocomplex crystallite network while being held at 200 ℃, which is unprecedented in the previous studies. Therein, stereocomplex crystallites rather than the network were confirmed in the
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a’
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b’
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a
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asymmetric PLLA/PDLA blends at elevated temperatures.16, 17
c’
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c
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Figure 3 SEM micrographs revealing a stereocomplex crystallite network at (a-c) low and (a’-c’) high magnifications, respectively, in the PLLA/h-PDLA blends after selective dissolution by dichloromethane:
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(a, a’) 1 wt%, (b, b’) 2 wt%, (c, c’) 5 wt%. Scale bars: 200 µm (left column), 50 µm (right column).
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Figure 4 Optical micrographs revealing thermal stability of the stereocomplex crystallite network in the PLLA/h-PDLA blends with 2 wt% PDLA after selective dissolution by dichloromethane: (a) 30 ℃, (b) 200 ℃, (c) 220 ℃, (d) 230 ℃. Scale bar: 50 µm (for all images).
Formation of stereocompelx network significantly reinforces the melt of PLLA/h-PDLA blends. As
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shown in Figure 5, remarkable melt elasticity is exhibited by PLLA/h-PDLA blends with 5 wt% PDLA. It is demonstrated by that storage modulus (G’) is higher than loss modulus (G’’) in the low frequency region.18, 19 Meanwhile, the stereocomplex crystallite network can afford high stress. Therefore, high
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storage modulus is exhibited by PLLA/h-PDLA blends. At a frequency of 0.1 Hz, storage modulus is
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about 3400 Pa for PLLA/h-PDLA blends, which is almost 4 times higher than that of PLLA/l-PDLA blends. Correspondingly, high complex viscosity is achieved in the PLLA/h-PDLA blends due to remarkable reinforcement effects from the stereocomplex crystallite network. In addition, it is expected that the stereocomplex crystallite network could be slipped under stress, responsible for shear thinning in the PLLA/h-PDLA blends. The other possibility is disentanglement of excess PLLA chains that are intertwined with the threads of the stereocomplex crystallite network.
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5
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G' (PLLA/h-PDLA) G'' (PLLA/h-PDLA) G' (PLLA/l-PDLA) G'' (PLLA/l-PDLA)
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Complex viscosity (Pa s)
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Modulus (Pa)
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Figure 5 Comparison of (a) storage and loss modulus and (b) complex viscosity between PLLA/h-PDLA and PLLA/l-PDLA blends with 5 wt% PDLA at 200 ℃. As demonstrated by DSC cooling trances in Figure 6, significant enhancement of PLLA
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crystallization occurs in the PLLA/h-PDLA blends. The crystallization exotherm in the blends is shifted
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to high temperatures, as compared to that of neat PLLA. The crystallization temperature is enhanced by about 40 ℃ in the PLLA/h-PDLA blends with 5 wt% PDLA. On the other hand, at the same PDLA content PLLA crystallization occurs earlier in the PLLA/h-PDLA blends than that in the PLLA/l-PDLA blends. For instance, at 2 wt% PDLA content crystallization temperature is about 142 and 120 ℃ for PLLA/h-PDLA and PLLA/l-PDLA blends, respectively. Rapid PLLA crystallization in the PLLA/h-PDLA blends is further demonstrated by isothermal crystallization at various temperatures. The blends with 2 wt% PDLA were selected as examples; and the corresponding results are given in Figure 7.
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The peak time of exotherms is much shorter for PLLA/h-PDLA blends at each crystallization temperature. For example, it is about 2 and 35 min for PLLA/h-PDLA and PLLA/l-PDLA blends,
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Tc ( C)
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respectively, while isothermal crystallization at 150 ℃.
Figure 6 DSC cooling traces of (a) PLLA/h-PDLA and (b) PLLA/l-PDLA blends with indicated PDLA content from 200 ℃, (c) PLLA crystallization temperature versus PDLA content in both blends.
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Figure 7 DSC heat flow traces of (a) PLLA/h-PDLA and (b) PLLA/l-PDLA blends with 2 wt% PDLA content while isothermal crystallization at indicated temperature, (c) peak time versus crystallization temperature in both blends.
Different PLLA crystallization habits should be correlated with morphology of stereocomplex crystals in the blends. Abundant thin threads within the stereocomplex crystallite network in the PLLA/h-PDLA blends could trigger high nucleation density towards PLLA crystallization around them. It is similar to transcrystallization induced by stereocomplex fibers embedded in the PLLA melt.20 High
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nucleation density is thus responsible for remarkable PLLA crystallization in the PLLA/h-PDLA blends. On the other hand, the stereocomplex crystallite network is less likely to be synchronously shrunk with
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PLLA melt due to its rigidity while temperature jumps down. Therefore, significant interfacial stress could be encountered by PLLA melt trapped in the stereocomplex crystallite network prior to crystallization.12 The interfacial stress could promote nucleation to some extent in the PLLA/h-PDLA
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blends. The above two aspects regarding nucleation effects toward PLLA matrix are insignificant in the
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PLLA/l-PDLA blends without the stereocomplex crystallite network. Thus, accelerated crystallization becomes less remarkable in the PLLA/l-PDLA blends.
The stereocomplex crystallite network in the PLLA/h-PDLA blends was already induced during precipitation process, accompanied by abundant homocrystals (Figure S2, Supporting Information).
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Annealing at 200 ℃ results in the melting of homocrystals but the stereocomplex crystallite network is retained. To solidify this argument, the precipitates of PLLA/h-PDLA blends were heated up to 240 ℃ to eliminate possible stereocomplex crystals, followed by quenching into liquid nitrogen. The
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down at 5 ℃/min.
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amorphous blends were annealed at 200 ℃ for 3 min to produce stereocomplex crystals, and then cooled
Figure 8 shows the corresponding DSC results of PLLA/h-PDLA blends subjected to the above thermal procedures. The stereocomplex crystals, generated from amorphous blends at 200 ℃, also have nucleating effects on melt crystallization of PLLA matrix. The crystallization temperature is enhanced by about 30 ℃ in the PLLA/ h-PDLA blends with 5 wt% PDLA. On the other hand, selective dissolution demonstrates that there are few floccules in dichloromethane (the insets in Fig. 8b). However, the Tyndall scattering indicates that the generated stereocomplex crystals are in the form of very fine
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particles. It means that stereocomplex crystals, produced only by annealing, are isolated rather than interconnected in the PLLA matrix. It is different from the stereocomplex crystallite network shown in
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Figs. 2-3. High melt viscosity should be responsible for the retarded formation of the stereocomplex crystallite network. Correspondingly, nucleating effects from very fine stereocompelx crystals are
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weaker than that of the stereocompelx crystallite network (Fig. 6).
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Endo
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Figure 8 (a) DSC cooling traces and (b) crystallization temperature of PLLA/h-PDLA blends with respect to PDLA content. In (b) optical photographs of the blends in dichloromethane, irradiated by a laser, are included as insets. Sample preparation is described in the text. 4. Conclusion Stereocomplex crystallites are highly interconnected to form a honeycomb network in the asymmetric blends with high-molecular-weight PDLA as a minor component. It is absent in the blends
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with low-molecular-weigh PDLA where stereocomplex crystallites are isolated in the PLLA matrix. The honeycomb network is much pronounced to accelerate crystallization of PLLA matrix due to enhanced
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nucleation effects; and the crystallization temperature is enhanced by about 40 ℃ with 5 wt% PDLA. Finally, a comparative study indicates that formation of the stereocomplex crystallite network is induced by precipitation process. This study gives further insights into structural formation of the stereocomplex
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crystallite network and its remarkable effect on PLLA crystallization in the asymmetric PLLA/PDLA
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blends.
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (No.
20133ACB21006).
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References
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21664001, 21364001) and the Major Program of Natural Science Foundation of Jiangxi, China (No.
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ACCEPTED MANUSCRIPT A stereocomplex crystallite network was directly witnessed in the PLLA/PDLA blends The network was favored in the blends with high-molecular-weight PDLA
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The network enhanced crystallization temperature of PLLA matrix by about
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