Stereocomplex-type polylactide with remarkably enhanced melt-processability and electrical performance via incorporating multifunctional carbon black

Journal Pre-proof Stereocomplex-type polylactide with remarkably enhanced melt-processability and electrical performance via incorporating multifunctional carbon black Zhenwei Liu, Fangwei Ling, Xingyuan Diao, Meirui Fu, Hongwei Bai, Qin Zhang, Qiang Fu PII:

S0032-3861(19)31141-3

DOI:

https://doi.org/10.1016/j.polymer.2019.122136

Reference:

JPOL 122136

To appear in:

Polymer

Received Date: 22 November 2019 Revised Date:

24 December 2019

Accepted Date: 28 December 2019

Please cite this article as: Liu Z, Ling F, Diao X, Fu M, Bai H, Zhang Q, Fu Q, Stereocomplex-type polylactide with remarkably enhanced melt-processability and electrical performance via incorporating multifunctional carbon black, Polymer (2020), doi: https://doi.org/10.1016/j.polymer.2019.122136. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

CRediT author statement Zhenwei Liu: Conceptualization, Methodology, Writing - Original Draft Fangwei Ling: Validation, Data Curation Xingyuan Diao: Investigation, Data Curation, sources Meirui Fu: Investigation, Software, Visualization Hongwei Bai: Supervision, Funding acquisition, Writing - Review & Editing Qin Zhang: Software, Visualization Qiang Fu: Supervision, Funding acquisition, Writing - Review & Editing

Graphical Abstract (TOC)

Stereocomplex-type polylactide with remarkably enhanced melt-processability and electrical performance via incorporating multifunctional carbon black Zhenwei Liu, Fangwei Ling, Xingyuan Diao, Meirui Fu, Hongwei Bai*, Qin Zhang, Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China

Stereocomplex-type polylactide with remarkably enhanced melt-processability and electrical performance via incorporating multifunctional carbon black Zhenwei Liu, Fangwei Ling, Xingyuan Diao, Meirui Fu, Hongwei Bai*, Qin Zhang, Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China

*Corresponding author

Tel./Fax: +86 28 8546 1795. E-mail: [email protected], [email protected] (H.W. Bai); [email protected] (Q. Fu).

Abstract: As a popular “green” engineering plastic, stereocomplex-type polylactide (SC-PLA) exhibits great application potential in various fields owing to its outstanding physicochemical performance and durability. However, the applications of SC-PLA still face formidable challenges mostly associated with its inferior melt-processability (i.e., the weak melt memory effect to motivate exclusive SC crystallization and extremely low melt viscosity) and the lack of necessary functional features (e.g., electrical conductivity) in some cases. Herein, we devise a facile and

robust strategy to overcome these obstacles by incorporating carbon black (CB) into equimolar poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) blend. It is interesting to find that the CB particles can adsorb many PLLA/PDLA chain segments on their surface and such strongly adsorbed PLA segments could interact with PLA chains outside the surface to form physical junctions capable of stabilizing the PLLA/PDLA chain assemblies in the melt, finally inducing the exclusive SC formation during subsequent crystallization. Meanwhile, the CB particles can substantially enhance the melt viscosity of the blend (from 3.9 Pa·s to 844.1 Pa·s when measuring at 250 °C and 50 Hz). Because of the greatly improved melt-processability, the PLLA/PDLA/CB composites have been successfully processed into highly crystalline products with exclusive SC crystallites and excellent thermomechanical performance by injection molding. Additionally, the CB particles can endow the composite products with fascinating electrical conductivity (19.0 S/m) and electromagnetic interference shielding effectiveness (26.6 dB). This work could open up a promising avenue towards high-performance and multifunctional PLA engineering Bioplastic. Key words: polylactide, stereocomplex, carbon black, melt processability, electrical performance

1. Introduction In the past decades, bio-based and biodegradable thermoplastic polyesters have sparked massive attention owing to the growing concerns on “white pollution” and petroleum crisis related to traditional synthetic polymers[1]. As one of the most popular eco-friendly polymers with high mechanical strength, versatile processability, and extraordinary transparency, polylactide (PLA) displays great application potential in numerous fields[2-8]. Nowadays, with the substantial drop in production cost, PLA has been utilized in diverse commodity applications such as packing and other short-time uses. Nevertheless, it still faces formidable challenges in practical engineering applications mostly because of its inadequate resistances to high temperature and hydrolysis[9, 10]. Fortunately, stereocomplex (SC) crystallization between poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) chains in a side-by-side fashion poses a promising solution to address these challenges[11-21]. Compared to PLLA or PDLA homocrystallites (HCs), the exclusive formation of SC crystallites with much denser chain packing can dramatically improve the thermomechanical performance of PLA including heat/hydrolysis resistances[22, 23]. In particular, the Vicat softening temperature of high-melting-temperature (ca. 220-230 °C) SC-type PLA (SC-PLA) is as high as 201 °C, much superior to that (ca. 143 °C) of the PLLA or PDLA[22]. These superb performance enable SC-PLA to use in high-temperature and durable applications as an eco-friendly engineering plastic. Unfortunately, the current applications of the fascinating SC-PLA have been remarkably limited by its poor melt-processability, namely the weak melt memory effect to motivate exclusive

SC crystallization[24-26] (both HCs and SC crystallites are simultaneously and competitively formed in the melt crystallization of high-molecular-weight (HMW) PLLA/PDLA

blends,

which

inevitably

deteriorates

the

thermomechanical

performance of final products) and the extremely low melt viscosity (the rigid chain structure of PLA does not allow the formation of effective chain entanglement in melt state[27] which is unbeneficial for any stable melt-processing process including blow molding)[28]. Furthermore, the lack of necessary functional features (e.g., electrical conductivity) makes SC-PLA hard to use in some practical applications, such as electronic industries[29, 30]. Therefore, it is of critical significance to develop multifunctional

SC-PLA

engineering

plastic

with

substantially

improved

melt-processability so as to fully exploit its application potential. To date, several strategies have been employed to improve the melt memory effect of HMW SC-PLA[31-35]. Synthesis of PLAs with special molecular structures (e.g., stereoblock and star-shaped PLAs) is believed to be exceedingly effective in attaining exclusive SC crystallization[36-38]. For instance, Biela et al.[39] found that star-shaped HMW PLLA/PDLA blends can be exclusively crystallized into SC crystallites when the arm number of the star-shaped PLAs reaches 13 due to the strong hardlock-type interactions between the enantiomoric arms. However, only linear PLLA and PDLA have been industrially produced and the synthesis of such special PLAs suffers from complicated preparation procedures. In contrast, physical blending with some miscible polymers or plasticizers represents a simple and efficient route to promote the SC formation in commercial linear PLLA/PDLA blends[40-42].

Recently, Pan et al.[43] reported that SC crystallites can be exclusively formed in HMW PLLA/PDLA (50/50) blend when certain amounts of miscible poly(vinyl acetate) (PVAc) are introduced. Nevertheless, high-content (e.g., 75 wt%) miscible polymers are required to realize exclusive SC formation, which could lead to the loss both in product performance and sustainability. Meanwhile, low-MW (1000-2000 g/mol) poly(ethylene glycol) (PEG) with lower content (10 wt%) can be used as plasticizer to trigger the exclusive SC crystallization of PLLA/PDLA blends by increasing segmental mobility of PLA chains[40] but the migration of the plasticizers could accelerate the product aging. Regarding to the enhancement in the melt viscosity of SC-PLA, several modifiers (e.g., chain extenders and high-viscosity polymers) have been successfully used[28, 44]. For example, the melt flow index of PLLA/PDLA blends can be decreased by half with incorporating small amounts (ca. 2 wt%) of chain extender (styrene-acrylic multifunctional-epoxide oligomeric agent)[44]. In recent years, the incorporation of functional fillers (e.g., carbon nanotube and graphene) has promoted the explosive development of polymer-based composites with new functionalities, such as electrical conductivity, electromagnetic performance, and thermal conductivity[45-49]. However, the functional modification of SC-PLA has been scarcely reported yet. More importantly, to the best of our knowledge, it is still a pending question to simultaneously improve the melt memory effect, melt viscosity, and functionality of SC-PLA. Carbon black (CB) is one of the most widely-used fillers in polymer industry for the

fabrication

of

mechanically

robust

and

multifunctional

polymer

composites[50-52].

Besides

some

useful

functionalities

including

electrical

conductivity and ultraviolet resistance, CB particles possess unique self-networking ability in polymer matrix and strong adsorbability for molecular chains on their surface, which are favorable for the mechanical reinforcement and even functionality improvement[52, 53]. In this work, we attempt to substantially improve the melt processability and electrical performance of SC-PLA by incorporating the CB as multifunctional filler into equimolar PLLA/PDLA blend. The CB particles were incorporated into the blend by melt mixing at a low temperature of 180 °C, where abundant cluster of PLLA/PDLA chain assemblies and resultant SC crystallites can be readily generated under the drive of shear forces[54]. It was expected that, on one hand, the incorporated CB particles could adsorb many PLLA/PDLA chain segments onto their surface and these strongly adsorbed PLA segments could collaborate with PLA chains between the CB particles to form physical junctions capable of stabilizing the precursory PLLA/PDLA chain assemblies in the melt, thus triggering exclusive SC crystallization upon cooling. On the other hand, the CB particles could also contribute to the significant enhancement in the melt viscosity of SC-PLA. Furthermore, the CB particles could impart SC-PLA with excellent electrical performance with the formation of network-like structure. As expected, we have demonstrated that both the melt processability and electrical performance of SC-PLA can be substantially enhanced with the incorporation of CB. The role of the CB self-networking ability and adsorbability in tailoring the performance of SC-PLA has been discussed and highlighted. We believe this strategy provides a new avenue to

fabricate high-performance and multifunctional SC-PLA engineering plastic using traditional melt-processing technologies.

2. Experimental 2.1 Materials PLLA with a weight average molecular mass ( M w ) of 1.72 × 105 g/mol, a D-lactide unit content of 1.4% and a polydispersity index (PDI) of 1.7 was obtained from Nature Works Co. Ltd., USA. PDLA with an M w of 1.85 × 105 g/mol, an L-lactide unit content of 0.5% and a PDI of 1.6 was provided by Zhejiang Hisun Biomaterial Co., Ltd., China. Two kinds of CB (trade name Printex XE2B and Printex 35) with similar particle sizes but different aggregate structures were purchased from Degussa Co. Ltd., Germany. The detailed parameters of the two CBs are listed in Table 1. For convenience, the CBs of Printex XE2B and Printex 35 are named as PX and P35, respectively.

Table 1. The parameters of the two kinds of CBs. Sample

Diameter

DBP adsorption number

Specific surface area

Self-networking

2

Adsorbability

name

(nm)

(mL/100 g)

(m /g)

ability

PX

~30

420

818

Strong

Strong

P35

~31

42

55

Weak

Weak

2.2 Sample preparation PLLA/PDLA/CB composites were prepared by melt mixing PLLA, PDLA and CB

using an internal mixer (Haake Rheomix, Germany) at 180 °C and 60 rpm for 5 min. The weight ratio between PLLA and PDLA was kept at 1:1, while the maximum contents of the PX and P35 in the composites were 10 wt% and 30 wt%, respectively. The obtained composites were denoted as PLLA/PDLA/xCB, where x refers to the weight fraction of CB in the composites. In order to investigate the mechanical properties of the composites, standard specimens were fabricated by injection molding using a Haake MiniJet

(Germany) at a cylinder temperature of 250 °C and

a mold temperature of 140 °C. The composite melts were isothermally crystallized in the hot mold for 5 min. All materials were vacuum dried at 60 °C for 12 h before processing. 2.3 Characterizations and measurements The adsorbability of CB particles for PLA chains was estimated by thermogravimetric analysis (TGA), which was performed using a TA Instruments Q500 analyzer (USA) at a heating rate of 10 °C from 30 to 600 °C under the protection of dry nitrogen atmosphere. CB samples used for the TGA estimation were separated from PLLA/PDLA/CB composites by extensively washing with excessive hexafluoroisopropanol and centrifugation at 10000 rpm, where the free PLA chains that were not strongly adsorbed on the CB surface can be completely removed. The washing-centrifugation procedure was repeated for at least 5 times to completely remove the free chains. The obtained CB specimens were vacuum dried at 80 °C for 24 h to remove any residual solvent. The interaction between CB particles and PLA chains was characterized using an

Andor SR-500i Raman microscope with a 532 nm laser source. The dispersion of CB in PLLA/PDLA/CB composites was observed using an FEI Inspect F field emission scanning electron microscope (FE-SEM, USA) at an accelerating voltage of 5 kV. The SEM specimens were prepared by cryofracturing the injection molded bars in liquid nitrogen. The cryo-fractured surface was sprayed with a thin layer of gold before the observation. The morphology of the pristine CB particles and the separated CB particles as well as the microstructures of PLLA/PDLA/CB composites were detected using an FEI Tecnai G2 F20 transmission electron microscopy (TEM, USA) at an accelerating voltage of 200 kV. The specimens of CB particles were prepared using the same procedure as that for the TGA estimations, while the specimens of PLLA/PDLA/CB composites in the form of thin slices were prepared by the ultra-cryo-microtomy of the composite samples on a Leica UCT ultramicrotome (Germany). The thermal analysis was carried out using a pyris-1 differential scanning calorimetry (DSC) instrument (USA) under the protection of dry nitrogen atmosphere. To examine the nonisothermal crystallization behavior of PLLA/PDLA/CB composites, the specimens were completely melted at 250 °C for 5 min to eliminate thermal history and then cooled down to 30 °C at a scanning rate of 5 °C/min, followed by reheating to 250 °C at a scanning rate of 10 °C/min. To investigate the isothermal crystallization behavior of PLLA/PDLA/CB composites, the specimens were cooled down to 180 °C at a rate of 150 °C/min after melting at 250 °C for 5 min, and then held at this temperature until the crystallization was finished. To

evaluate the matrix crystallinity of the injection molded composites, the specimens were heated from 30 °C to 250 °C at a heating rate of 10 °C/min. The crystallinities of HCs ( X c,HC ) and SC crystallites ( X c,SC ) are obtained by the following expressions:

X c,HC =

∆Η m,HC w f ∆Η mo , HC

(1)

X c,SC =

∆Η m,SC w f ∆Η mo , SC

(2)

where VH m, HC and VH m, SC represent the melting enthalpies of HCs and SC o o crystallites, respectively; ∆Η m , HC and ∆Η m , SC (selected as 93 J/g[14] and 142

J/g[14]) are the melting enthalpies of perfect HCs and SC crystallites, respectively; w f is the weight percent of PLLA/PDLA component in the composites. The fraction

of SC crystallites ( fSC ) is calculated by the following expression:

f SC =

X c,SC X c,HC + X c,SC

(3)

The crystal structure of PLLA/PDLA/CB composites was characterized using a PANalytical X'Pert pro MPD diffractometer (Holland) with a CuKa radiation (40 kV, 40 mA). The wide-angle X-ray diffraction (WAXD) patterns were recorded from 5° to 40° at a scanning rate of 3 °/min. The dynamic mechanical analysis (DMA) of PLLA/PDLA/CB composites was performed using a TA Q800 instrument (USA) in a single-cantilever mode. The rectangular specimens (50 mm × 10 mm × 4 mm, length × width × thickness) were heated from 0 °C to 220 °C at a scanning rate of 3 °C/min. The oscillating amplitude and frequency were set as 10 µm and 1 Hz, respectively. The tensile properties of PLLA/PDLA/CB composites were measured using a

SANS Universal tensile machine (China) with a crosshead speed of 5 mm/min. The measurement was carried out at room temperature (23 °C), and at least 6 specimens were measured for each sample. The rheological behaviors of PLLA/PDLA/CB composites were characterized using a Hakke Mars rheometer equipped with two parallel plates (25 mm in diameter) in a frequency sweep mode. The frequency was varied from 0.01 to 100 Hz, and the strain was set as 1%. The characterization was performed at 250 °C under the protection of dry nitrogen atmosphere. The electrical conductivity of PLLA/PDLA/CB composites was measured using a Keithley 6487 picoammeter (China) at a constant voltage of 1 V. The rectangular specimens (20 mm × 4 mm × 0.5 mm, length × width × thickness) were prepared by compression molding at 250 °C and 10 MPa for 5 min. The two ends of the specimens were coated with a thin layer of conductive silver to ensure good contact with the electrodes. The

electromagnetic

interference

(EMI)

shielding

effectiveness

of

PLLA/PDLA/CB composites was evaluated in the X-band frequency range of 8.2 GHz to 12.4 GHz by an Agilent N5230 vector network analyzer. The disk specimens (10 mm in diameter and 2 mm in thickness) used for the evaluation were prepared by compression molding.

3. Results and discussion 3.1 Adsorption of PLLA and PDLA chains on the CB surface

In CB-filled polymer composites, CB particles usually exhibit strong adsorbability for various polymer chains[52]. In order to verify the adsorption of PLLA and PDLA chains on the CB surface during melt mixing of PLLA/PDLA/CB composites, the CB particles separated from the PLLA/PDLA/10CB composites by washing with excessive hexafluoroisopropanol (good solvent for both HCs and SC crystallites) were characterized with TEM and the results are presented in Figure S1. Interestingly, the surface of pristine PX and P35 particles is relatively smooth, while the separated PX and P35 particles exhibit a blurry surface, which suggests that some PLA chains could be adsorbed on the CB surface. Moreover, the blurry area of the separated PX particles seems larger than that of the separated P35 particles probably due to the higher amounts of the adsorbed PLA chains.

Figure 1. TGA curves of PLLA/PDLA binary blend, pristine CBs, and CBs separated from PLLA/PDLA/10CB composites.

In order to quantitatively evaluate the adsorbed amount of PLA chains, TGA measurements were also carried out and the results are exhibited in Figure 1. Noticeably, for pristine PX and P35, no obvious weight loss can be observed when the

temperature rises to 600 °C. In contrast, the separated CB particles show a significant weight loss in the temperature range of 300-400 °C due to the decomposition of the adsorbed PLLA and PDLA chains, which makes it possible to estimate the amount of the strongly adsorbed PLA chains on the CB surface using the weight loss recorded at 600 °C. The adsorbed amount in the separated PX particles is estimated to be ca. 13.9 wt%, much higher than that (ca. 3.6 wt%) in the separated P35 particles, signifying that the PX particles have a much stronger adsorbability for PLA chains. Additionally, it should be noted that the weight percent of the strongly adsorbed PLLA/PDLA chains in the PLLA/PDLA/CB composites is relatively small (typically, ca. 1.5 wt% for the PLLA/PDLA/10PX composite and ca. 0.4 wt% for the PLLA/PDLA/10P35 composite).

Figure 2. Raman spectra of pristine CBs and CBs separated from PLLA/PDLA/10CB composites.

The Raman spectra of the pristine CB particles and the separated CB particles were recorded to confirm the interactions between CB and the adsorbed PLA chains, and the results are shown in Figure 2. The peak at around 1580 cm-1 originates from the

vibration of an ideal graphitic lattice, and is generally referred as G band[55]. The peak at around 1340 cm−1 is arisen from the defects of the graphitic structures, and is usually named as D band[55]. For the separated CB particles, there exhibits a slight up shift for both G and D bands as compared to the pristine CB particles. This shift could be resulted from the interaction between PLA chains and CB particles. It has been reported that there are many oxygen-containing polar groups on the CB surface, such as carboxylic, phenolic, and lactonic groups[56, 57]. On one hand, these groups could collaborate with the ester groups of PLA by dipolar or/and hydrogen bonding interactions [57]. On the other hand, these groups could react with the end groups of PLA chains during melt processing [56]. Both the physical and chemical interactions could contribute to the adsorptions of PLA chains on the CB surface. Moreover, the separated PX particles show higher spectra shifts than those of the separated P35, indicating the stronger interactions between PX particles and PLA chains. In addition, as compared to the P35 particles, the larger specific surface area of PX particles may be another reason for the superior adsorbability of PLA chains[58].

3.2 Formation of CB network structure in PLLA/PDLA blend matrix The morphological structures of PLLA/PDLA/CB composites were observed with SEM and TEM. Figure 3 displays some representative SEM micrographs. With the incorporation of 2 wt% PX into PLLA/PDLA blend characterized by a smooth fractured surface (Figure 3a), small agglomerates of nearspherical CB particles are noticed to homogeneously disperse in the blend matrix and no network-like structure

of CB particles can be detected (Figure 3b). When the PX content reaches to 5 wt%, the CB particle agglomerates tend to self-organize into unique network-like structure and the CB network becomes much denser with further increasing PX content to 10 wt% (Figure 3c and 3d). However, for the PLLA/PDLA/P35 composites, the CB particles are found to aggregate into big agglomerates and the dispersion of the agglomerates in the blend matrix is relatively poor (Figure 3e-g). Thus, the critical content of P35 particles for the formation of CB network is as high as 10 wt%. It suggests that the self-networking ability of PX particles is much stronger than that of the P35 particles, which is further supported by the TEM micrographs shown in Figure 4. Evidently, distinctly different from PX particles, the serious aggregation makes the P35 particles hard to self-organize into particle network at low content.

Figure 3. SEM micrographs of the cryo-fractured surfaces for (a) PLLA/PDLA, (b) PLLA/PDLA/2PX,

(c)

PLLA/PDLA/5PX,

(d)

PLLA/PDLA/10PX,

(e)

PLLA/PDLA/5P35, (f) PLLA/PDLA/10P35, and (g) PLLA/PDLA/30P35 composites.

Figure 4. TEM micrographs for (a) PLLA/PDLA/5PX, (b) PLLA/PDLA/10PX, (c) PLLA/PDLA/10P35, and (d) PLLA/PDLA/30P35 composites.

Figure 5. DSC thermograms of PLLA/PDLA/PX (a and a') and PLLA/PDLA/P35 composites (b and b') recorded during (a, b) the cooling scans after complete melting at 250 °C for 5 min and (a', b') the subsequent heating scans. The cooling and the heating rates were 5 °C/min and 10 °C/min, respectively.

Figure 6. Relative fraction of SC crystallites ( fSC ) formed in PLLA/PDLA/CB composites as a function of CB content.

3.3 Improved melt-processability of PLLA/PDLA/CB composites The melt memory effect of PLLA/PDLA/CB composites with different CB particles has been comparatively investigated with DSC and the results are presented in Figure 5. For PLLA/PDLA binary blend, two crystallization peaks associated with the formation of both HCs and SC crystallites can be observed during cooling the blend melt from 250 °C to 30 °C due to the weak melt memory effect (Figure 5a). Moreover, the formation of HCs is predominant over that of SC crystallites, as evidenced by the much larger characteristic melting peaks of HCs at 160-180 °C (the one at ca. 165 °C for PLLA with a low optical purity and the another at ca. 170 °C for PDLA with a higher optical purity) in the second heating curveas compared to that of SC crystallites at 200-220 °C (Figure 5 a'), and the fraction of SC crystallites ( fSC ) is as low as 14.3% (Figure 6). However, with the incorporation of 2-3 wt% PX, the

formation of SC crystallites becomes predominant, indicating that the PX particles can improve the melt memory effect of the PLLA/PDLA blend. Impressively, the SC crystallites are exclusively formed in the PLLA/PDLA/PX composite with 10 wt% PX. In contrast, for PLLA/PDLA/P35 composites, although the incorporation of P35 gives rise to a great increase in the fSC (Figure 5b and 5b'), exclusive SC formation can be obtained only when the P35 content reaches to 30 wt% (Figure 6). To examine the effect of CB particles on the crystallization kinetics of PLLA/PDLA blend, the isothermal crystallization behavior of PLLA/PDLA/PX composites was investigated by DSC at 180 °C. As shown in Figure 7, only a slight decrease in the crystallization time of PLLA/PDLA blend matrix can be observed with the incorporation of PX particles, indicating that CB cannot act as effective nucleating agent for the SC crystallization of PLLA/PDLA blend.

Figure 7. DSC thermograms of PLLA/PDLA/PX composites recorded during isothermal crystallization at 180 °C.

Figure 8. WAXD patterns of (a) PLLA/PDLA/PX and (b) PLLA/PDLA/P35 composites prepared by melt-crystallization at a cooling rate of 5 °C/min after complete melting at 250 °C for 5 min.

The WAXD characterization was also carried out to further confirm the substantially improved melt memory effect of the CB filled PLLA/PDLA blend. In the WAXD patterns shown in Figure 8, the characteristic peaks at 12.1°, 20.9°, and 24.0° are attributed to the (110), (300/030), and (220) crystal planes of SC crystallites, while those at 16.9° and 19.° are ascribed to the (110/200) and (203) crystal planes of HCs. As expected, the characteristic peaks of both HCs and SC crystallites can be clearly found on the WAXD patterns of PLLA/PDLA binary blend, whereas a dramatically decreased intensity in the characteristic peaks of HCs is observed for PLLA/PDLA/CB composites. Notably, the critical content of PX for the exclusive SC formation is much lower than that of P35, which is consistent with DSC results.

Figure 9. DSC second heating curves of PLLA/PDLA/5PX composites prepared by different melt-mixing procedures. The pre-mixing time at 220 °C is given in the profile.

It has been reported that the crystallization behavior of equimolar PLLA/PDLA blends is very sensitive to the melt structure[59]. For the exclusive formation of high-content SC crystallites in the blend, the generation of the heterogeneous melt containing abundant PLLA/PDLA chain assemblies (i.e., precursors for SC crystallization) is a prerequisite. In order to reveal the vital role of CB particles in substantially improving the melt memory effect of PLLA/PDLA blend, the two-step melt-mixing procedure was also utilized to prepare the PLLA/PDLA/5PX composite, namely the premixing of PLLA, PDLA and PX at a high temperature of 220 °C (where only the CB particles can adsorb PLA chains but the SC crystallites cannot form) for different time periods and subsequent melt mixing at 180 °C for 5 min. Figure 9 gives the DSC second heating curves of the obtained composites. It is

interesting to find that increasing the high-temperature melt-mixing time gives rise to an apparent enhancement in the adsorbed amount of PLLA/PDLA chains on the PX surface, and thereby facilitates the formation of more SC crystallites. More importantly, we observe that the melt memory effect of the PLLA/PDLA/5PX composite becomes poor (the fSC decreases from 92.1 to 72.8) when the PLLA and PDLA chains are separately adsorbed on the PX surface (the PLLA/5PX and PDLA/5PX master batches were prepared by melt mixing at 220 ° C) before melt-mixing at 180 °C, although the total adsorbed amount of PLA chains is almost the same (approximately 17.5 wt% as shown in Figure S2). These results distinctly indicate that the adsorption of PLLA/PDLA chain assemblies rather than the PLLA or PDLA chain clusters is favorable to the substantial improvement in the melt memory effect. One possible molecular mechanism for the chain adsorption induced improvement in the melt memory effect is illustrated in Scheme 1. For the PLLA/PDLA binary blend, the PLLA/PDLA chain assemblies could be totally destroyed after melting of SC crystallites pre-generated in the melt blending[59, 60], thus causing the predominant formation of HCs upon subsequent cooling (Scheme 1a). However, for PLLA/PDLA/PX composites, the PLLA/PDLA segments strongly adsorbed on the CB surface could readily interact with PLA chains outside the surface to form physical network junctions capable of preventing the complete decoupling of enantiomeric PLA chain segments from their precursory assemblies, finally inducing exclusive SC crystallization (Scheme 1b). Although the adsorption amount is relatively small (e.g., only 1.5 wt% for the PLLA/PDLA/10PX composite with

outstanding melt memory effect), the role of the adsorbed PLA chains in stabilizing the PLLA/PDLA assemblies could be extraordinary. In fact, the significant effect of the similar thin adsorbed layer on the performance has been widely reported in the CB-filled rubber composites[61, 62]. Most importantly, if the PLLA and PDLA chains are separately adsorbed on the surface of different CB particles, it is difficult to form effective physical junctions capable of stabilizing the PLLA/PDLA precursory assemblies and thus manly HCs are formed in the PLLA or PDLA rich regions (Scheme 1c).

Scheme 1. Schematic illustration showing the molecular mechnism of CB-promoted SC crystallization of PLLA/PDLA blend.

Besides the positive effect of CB particles on the melt memory effect, the CB induced remarkable enhancement in the melt viscosity of PLLA/PDLA blend was also noticed during melt mixing. The rheological behaviors of PLLA/PDLA/CB composites were measured with a dynamic rheometer at 250 °C. As presented in Figure 10a, PLLA/PDLA binary blend exhibits a Newton platform in the low

frequency region and a surprisingly low melt viscosity of 3.9 Pa·s at 50 Hz (which is close to that in practical internal mixing). With the incorporation of CB, the Newtonian platform gradually disappears and the melt viscosity increases significantly (Figure 10a and 10b). In particular, the melt viscosity of PLLA/PDLA/10PX and PLLA/PDLA/30P35 composites at 50 Hz are as high as 580.1 Pa·s and 844.1 Pa·s, respectively, approximately two orders of magnitude higher than that of the PLLA/PDLA blend (3.9 Pa·s). The significantly enhanced melt viscosity could be ascribed to the filler effect and cross-linking effect of CB particles on the PLLA/PDLA matrix in melt state.

Figure 10. Frequency dependence of complex viscosity (η) for (a) PLLA/PDLA/PX and (b) PLLA/PDLA/P35 composites.

Based on the above results, it is clear that the use of CB can substantially improve the melt processability of PLLA/PDLA blend because of the strong adsorbability of CB particles for PLA chains in polymer matrix, which makes it possible to fabricate highly crystalline PLLA/PDLA/CB composite products with exclusive SC crystallites by simple melt-processing technologies, such as injection molding.

3.4

Superb

thermomechanical

performance

of

injection-molded

PLLA/PDLA/CB composite products Due to the substantial improvement in the melt-processability, PLLA/PDLA/CB composites have been successfully processed into highly crystalline products by injection molding. Figure 11 displays the DSC heating curves of the injection-molded composite products. As shown in Figure 11a, both HCs and SC crystallites are formed in PLLA/PDLA blend product because of the poor melt memory effect. However, with the enhancement in the melt memory effect, the content of SC crystallites formed in the composite products increases remarkably (Figure 11a). Importantly, the SC crystallites are exclusively formed in the PLLA/PDLA/10PX and PLLA/PDLA/30P35 composite products without any trace of HCs.

Figure 11. DSC heating curves of injection molded (a) PLLA/PDLA/PX and (b) PLLA/PDLA/P35 composites with various amounts of CB.

DMA was performed to evaluate the heat resistance of the PLLA/PDLA/CB composite products, and some representative DMA curves are shown in Figure 12.

For PLLA/PDLA blend product, the storage modulus drops significantly when the temperature surpasses the melting temperature (ca. 160-180 °C) of HCs due to the existence of large amounts of HCs. However, for PLLA/PDLA/CB composite products with exclusive SC crystallites, only slight decrease in the storage modulus can be observed before the melting of SC crystallites at a much higher temperature (ca. 215-220 °C). Especially, the storage modulus of PLLA/PDLA/10PX at 200 °C is as high as 153.3 MPa, much superior to that of the PLLA/PDLA blend product (0.4 MPa). This distinctly indicates the greatly enhanced heat resistance of PLLA/PDLA/CB composite products.

Figure

12.

Temperature-dependent

storage

modulus

for

injection

molded

PLLA/PDLA blend and PLLA/PDLA/CB composites.

Figure 13 gives the tensile properties of PLLA/PDLA/CB composite products. It is interesting to find that, with increasing CB content up to 10 wt%, the Young’s modulus of both PLLA/PDLA/PX and PLLA/PDLA/P35 composite products

enhances greatly without obviously compromising the tensile strength. The evidently decreased tensile strength of PLLA/PDLA/P35 composite products with high-content P35 (i.e., 20-30 wt%) may be resulted from the serious aggregation of CB particles (Figure 5c). Furthermore, it should be mentioned that the relatively low mechanical strength of the composites (<45 MPa) may be associated with the possible thermal degradation involved in the injection molding at a high temperature of 250 °C.

Figure 13. (a) Tensile strength and (b) Young's modulus of injection molded PLLA/PDLA/CB composites as a function of CB content.

3.5 Favorable electrical performance of injection-molded PLLA/PDLA/CB composite products The electrical conductivity of PLLA/PDLA/CB composite products was measured and the results are shown in Figure 14. It is found that the PLLA/PDLA/CB composite products show typical electrical percolation behavior. The electrical conductivity is extremely low for the PLLA/PDLA blend product, but enhances rapidly when the content of the incorporated CB particles reaches to a certain content. Similar electrical behavior has been reported in the PLLA/carbon-based fillers

composites[63]. Moreover, the electrical conductivity of PLLA/PDLA/PX composite products is much higher than that of PLLA/PDLA/P35 composite products with the same CB content. For example, the electrical conductivity of PLLA/PDLA/10PX is 19.0 S/m, two orders of magnitude higher than that of PLLA/PDLA/10P35 (0.15 S/m). The results indicate that the PX particles are more efficient in improving the electrical conductivity of PLLA/PDLA/CB composite products, which can be attributed to the more perfect CB particle network formed in the blend matrix.

Figure 14. Electrical conductivity evolution of PLLA/PDLA/CB composites as a function of CB content.

Figure 15. Electromagnetic interference shielding effectiveness (EMI SE) values of PLLA/PDLA/CB composites measured in the frequency range of 8.2-12.4 GHz.

As well known, the high conductivity is favorable for developing materials with high electromagnetic interference shielding effectiveness (EMI SE). Figure 15 shows the EMI SE values of PLLA/PDLA/CB composite products in the frequency of X-band from 8.2 to 12.4 GHz. It can be seen that the PLLA/PDLA blend product shows a particularly low EMI SE due to the absence of CB conductive networks. The EMI SE of PLLA/PDLA/CB composite products enhance greatly with the increase of CB content. Moreover, the EMI SE of PLLA/PDLA/PX composite products is much superior to the PLLA/PDLA/P35 composite products, which is consistent with the variations of electrical conductivity. The EMI SE of the PLLA/PDLA/10PX and PLLA/PDLA/30P35 composite products is about 26.6 dB and 25.1 dB, respectively, which can meet the commercial application requirements (about 20 dB)[64]. Considering that the higher mechanical strength relative to the PLLA/PDLA/30P35 composite product, the PLLA/PDLA/10PX composite offers more potential in practical applications.

4. Conclusions In summary, we have devised a straightforward approach to develop high-performance and multifunctional SC-PLA engineering plastic by substantially improving the melt-processability and functionalities of PLLA/PDLA blends with the aid of CB. The CB particles were incorporated into the blends by simple melt mixing, where some PLLA/PDLA chains could be strongly adsorbed onto the surface of CB

particles and collaborate with the matrix chains to form physical junctions capable of stabilizing the PLLA/PDLA chain assemblies in the melt, finally triggering the exclusive SC formation in the melt crystallization of PLLA/PDLA/CB composites. The incorporation of CB particles can also substantially enhance the melt viscosity of the PLLA/PDLA blend. The strong adsorbability of CB particles for PLA chains in the blend matrix could contribute to the significant improvement in the melt-processability. Owing to the substantially improved melt-processability, highly crystalline PLLA/PDLA/CB composite products with exclusive SC crystallites were obtained by injection molding. The composite products exhibit superb heat resistance and good tensile properties. Furthermore, the CB particles can endow the composites with impressive electrical conductivity and electromagnetic interference shielding effectiveness due to the formation of CB particle network. These results suggest that our strategy could open up a new and facile pathway toward high value-added PLA engineering Bioplastic.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51873129 and 51721091).

Appendix A. Supplementary data

Supplementary

data

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http://www.sciencedirect.com.

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this

article

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be

found

at

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Highlights

1. CB particles are used to improve the melt-processability and electrical performance of SC-PLA. 2. The CB particles exhibit strong adsorbability for PLLA/PDLA chain segments. 3. The adsorbed PLA segments contribute to the greatly enhanced melt memory effect and melt viscosity. 4. The PLLA/PDLA/CB composites can be injection molded into products with exclusive SC crystallites. 5. The obtained products possess superb thermomechanical and electrical properties.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: