Synergistic Toughening of Poly (lactic acid)-Cellulose Nanocrystal

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Synergistic Toughening of Poly (lactic acid)-Cellulose Nanocrystal Composites through Cooperative Effect of Cavitation and Crazing Deformation Mechanisms Joseph Kinyanjui Muiruri, Songlin Liu, Jayven Chee Chuan Yeo, Junqiang Justin Koh, Junhua Kong, Warintorn Thitsartarn, Wern Sze Teo, and Chaobin He ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00201 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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ACS Applied Polymer Materials

Synergistic Toughening of Poly (lactic acid)-Cellulose Nanocrystal Composites through Cooperative Effect of Cavitation and Crazing Deformation Mechanisms

Joseph K. Muiruri †, ‡, Songlin Liu *, ‡, Jayven Chee Chuan Yeo †, ‡, Junqiang Justin Koh †, §, Junhua Kong ‡, Warintorn Thitsartarn ‡, Wern Sze Teo § and Chaobin He *, †, ‡

†Department

of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576

‡Institute

of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634

§ Singapore

Institute of Manufacturing Technology, Agency for Science, Technology and

Research (A*STAR), 2 Fusionopolis Way, Innovis #08-04, Singapore 138634

*Correspondence to [email protected] (SL Liu); or [email protected] (C. He)

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ABSTRACT In this work, we present poly (lactic acid) (PLLA)-cellulose nanocrystal (CNC) composites with simultaneous improvements in tensile toughness and impact strength using dual CNC-based fillers. The dual CNC-based fillers were designed to toughen PLLA by control of (a) the morphology, and (b) the interfacial interactions in the PLLA composites. The first filler denoted as CNCaq-rD comprised of rigid CNC, and a rubber layer (rD) which exhibited immiscibility with the PLLA matrix, albeit with weak interactions. The second filler denoted as CNCaq-rDPDLA comprised of rigid CNC, a middle rubber layer (rD), and an outer layer of PDLA blocks that could form strong interactions with PLLA matrix via stereocomplexation. The dual fillers superbly improved both tensile toughness and impact strength of PLLA through synergistic effects. Incorporating 10% CNCaq-rD and 10% CNCaq-rD-PDLA into the PLLA matrix, the strain at break (a measure of tensile toughness) and impact strength increased by 100-fold and 3fold, respectively. Scanning electron microscopy (SEM) and small angle x-ray scattering (SAXS) analysis revealed dual deformation mechanisms: (i) cavitation between CNCaq-rD and PLLA matrix, and (ii) matrix crazing induced by CNCaq-rD-PDLA. Our study suggests that these dual mechanisms result in dramatic property enhancements of the PLLA under tensile and impact conditions. The resultant PLLA-based composites with improved thermomechanical properties could have potential applications in packaging and in the medical fields.

Keywords: Immiscibility, Interfacial interactions, Stereocomplexation, Fillers, Rheological behavior, Small angle X-ray scattering, Microstructure.


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1. INTRODUCTION Because of the societal grand challenges, namely, resource constraints and carbon constraints from overexploitation of global ecosystem, sustainability has become more real in recent decades.1 Consequently, a paradigm shift has emerged from the traditional petroleum-based polymers to more ecofriendly and sustainable polymers.2-3 Such polymers include poly (lactide) (PLA)4, polysaccharides5-6, poly(hydroxyalkanoates) (e.g. PHB)7 among others. PLA, a semicrystalline aliphatic polyester derived mostly from corn, has become one of the most studied biodegradable polymers for numerous applications.8 The phenomenal success of PLA arises from its unique properties such as excellent biocompatibility, biodegradability, high strength and rigidity as well as processability. However, applications of PLA in certain areas are restricted due to its low impact strength (~ 2 kJm-2) and limited elongation at break (˂10%).9 Several techniques have been applied to improve the fracture toughness of PLA, such as plasticization10-11, copolymerization, blending with flexible polymers.9Although these techniques toughen PLA, each of them suffers from some demerits; for instance, copolymerization often induces loss in mechanical properties and is relatively expensive. Zeng et al.12 fabricated poly (ester-urethane) (PEU), and reported that PEU10 with similar weight fractions of PLA and poly (butylene succinate) (PBS) exhibited high elongation of 458% and low strength of 22 MPa. On the other hand, plasticization demands the use of high content of plasticizers to enhance toughness, which then leads to reduction in strength and stiffness. In a recent study, Notta-Cuvier and coworkers10, developed PLA plasticized blends with tributyl citrate (TBC). With the addition of 10% TBC, the blend was highly ductile, but with significant drop of about 30% in stiffness and strength. Other studies on plasticized PLA blends with other plasticizers reported similar



results.13-15 In this respect, it may be relatively easy to improve the tensile toughness (strain at break) but is much more demanding to improve the impact strength of PLA.2 The simultaneous improvement of tensile toughness and impact strength of PLA requires rational material designs. Interestingly, most of the designs for toughened PLA composites with significant improvements in the overall toughness incorporate petroleum-derived toughening agents.16-21 In the context of sustainability, these toughening agents are not ideal for PLA toughening due to their reduced biodegradability. In our previous work 5, we have demonstrated the production of tough and biodegradable PLA composites through the addition of modified spray-dried nanocellulose fillers. However, the impact strength of these composites was not significantly improved. To address this challenge, the design of rubber toughening agents should follow a criterion defined by NatureWorks LLC, which include: small rubber domains (usually 0.1-1.0 µm) in the matrix, sufficient interfacial interactions, glass transition temperature (Tg) of the rubber (at least 20 °C) lower than the service temperature, rubber should not be miscible with the matrix and possess good thermal processability.22 Based on the above-mentioned criterion, we purposefully designed dual biodegradable nanocellulose (CNC)-based fillers, aiming at controlling (i) the morphology and (ii) the interfacial interactions. As such, one of the fillers (CNCaq-rD) comprises of CNC as a core, and the copolymer from ε-caprolactone (ε-CL) and D-lactide (D-LA) as the outer rubber layer (rD). This rubber part is aimed at inducing immiscibility with PLLA matrix. On the other hand, the second filler (CNCaq-rD-PDLA) comprises of CNC as a core, middle rubber (rD) layer and PDLA blocks as outer layer to induce strong interactions with PLLA through


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stereocomplexation. Such a combination of CNC-based fillers in which each filler has a defined role in PLLA toughening has not been reported so far. By varying the total contents and the ratios of the dual fillers in PLLA, the strain at break (a measure of tensile toughness) and the impact strength were simultaneously enhanced. These significant changes in mechanical performances could be correlated with change in deformation mechanisms. In particular, three major deformation mechanisms have been identified in semicrystalline polymers under stretching conditions.9 These mechanisms include cavitation, crazing and shear yielding. Cavitation refers to the formation of voids during deformation due to excessive stress, whereas crazing is the formation of microvoids with fibrils that span from one face to the other and are thus able to support significant stresses before failure.5, 23 On the other hand, shear yielding involves chain slippage after the material yields during deformation. In this work, the concurrent improvements of the tensile toughness and the impact strength correlates with the dualism of cavitation and crazing deformation mechanisms exhibited by the composites through SEM and SAXS analysis. We therefore propose a model in which the CNCaq-rD filler acts as the cavitation component and the CNCaq-rD-PDLA filler induces crazing of the PLLA matrix. It is worth noting that except for rubber modified polystyrene 24, cavitation and crazing dualism in PLLA composites has not been reported so far. Overall, the thermomechanical properties, morphologies and deformation mechanisms of these PLLA composites are discussed in depth. More importantly, the resultant tough PLA-cellulose based composites are fully biodegradable and would be ideal for food packaging, medical applications and so on, where both the high ductility and impact strength are prerequisites.



2. EXPERIMENTAL SECTION 2.1 Materials PLLA 3051D (Mn = 160 kDa, D = 1.7) was obtained from Natureworks. Nanocellulose (CNCaq) aqueous solutions were supplied by University of Maine. D-lactide monomer was purchased from Corbion Purac. ε-caprolactone (ε-CL) monomer, tin (II)-ethylhexanoate (Sn(Oct)2) catalyst and anhydrous toluene were purchased from Sigma Aldrich. All the other chemicals were used as received. 2.2 Synthesis of CNCaq-rD and CNCaq-rD-PDLA Fillers 17g of CNC aqueous solution (11.8 wt %) equivalent to 2g of CNC was solvent exchanged from water to acetone in several successive centrifugation and re-dispersion steps. The resultant acetone-based suspension was air-dried under ambient conditions to obtain a powder, denoted as CNCaq. Using 2g CNCaq as an initiator ， copolymerization of ε-caprolactone (ε-CL) and Dlactide to obtain CNCaq-rD and CNCaq-rD-PDLA was done by ring opening polymerization (ROP), which was described in our previous study.5 The yield of CNCaq-rD-PDLA was 94 wt%. However, only one-step was involved in the case of CNCaq-rD because no PDLA blocks were grafted as an outer layer. The resultant products were washed with excess methanol, and then dried overnight under vacuum. The synthesis pathway of the CNCaq-rD and CNCaq-rD-PDLA by ROP is as shown in Figure 1


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Figure 1. (A) Synthesis pathway for CNCaq-rD and CNCaq-rD-PDLA fillers

2.3 Fabrication of PLLA / CNCaq-rD / CNCaq-rD-PDLA Composites PLLA / CNCaq-rD / CNCaq-rD-PDLA composites were fabricated by solution castinginjection molding technique.5 Predetermined quantities of PLLA, CNCaq-rD, CNCaq-rD-PDLA as listed in Table 1 were solution cast, pulverized and injection molded into standard tensile and impact test specimens by injection molding (Haake MiniJet, Thermo Fisher Scientific) operated at injection temperature (200 °C), mold temperature (65 °C), injection pressure/time (900 bar/15s) and post pressure/time (300 bar/15s). Neat PLLA and PLLA composites for impact tests were prepared in the same way using impact specimens’ mold. For brevity, the PLLA composites are coded using numerals, for instance 80/10/10 to represent 80% PLLA/ 10% CNCaq-rD / 10% CNCaq-rD-PDLA. Neat PLLA samples were prepared in the same way. 7 ACS Paragon Plus Environment


Table 1. The compositions and the notations of the PLLA composites Sample notation

PLLA (wt %)

Composition CNCaq-rD CNCaq-rD-PDLA (wt %) (wt %)

Neat PLLA

100

0

0

95/2.5/2.5

95

2.5

2.5

90/5/5

90

5

5

80/20/0

80

20

0

80/15/5

80

15

5

80/10/10

80

10

10

80/5/15

80

5

15

80/0/20

80

0

20

2.4 Characterizations Differential scanning calorimetry (DSC) measurements were performed using a (DSC) (TA Instruments Q100) under nitrogen atmosphere. The samples were equilibrated at -80 °C, then heated from -80 °C to 220 °C, cooled to -80 °C and re-heated to 220 °C using a ramping rate of 10 °C/min. Rheological properties of the PLLA and selected PLLA composites were determined using Anton Paar modular compact rheometer (MCR 302) at 190 °C. Parallel plates of 25 mm diameter and a gap of 1 mm were employed for the measurements. To ensure that the strain used was within the linear viscoelastic region (LVER), we performed frequency sweep from 0.1 to 100 rad/s in the region of 1% strain. X-ray diffraction (XRD) diffractograms of PLLA and PLLA composites were obtained by Bruker GADDS diffractometer (D8 Discover µMR). The operating voltage was 50kV for the area detector and a current of 800 µA, with Cu Kα radiation (λ = 1.5418 Å) at room temperature. The XRD spectra were obtained as a function of 2θ in the range of 5-40°.


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Tensile tests were performed at a crosshead speed of 1.0 mm/min using an Instron 5569 universal tensile machine, according to ASTM D638, Type V standard. Average values of five samples were recorded. Notched Izod impact tests were determined using pendulum impact tester (Zwick HIT25P, Germany), according to ASTM D256 with typical specimen dimensions of 63.5 mm x 12.7 mm x 3.2 mm. All the tests were performed at ambient temperature. Five specimens of each sample were tested and their average values were recorded. The average absorbed impact energy (in joules) per unit area was calculated as the impact strength for each sample. Dynamic mechanical analyzer (DMA Q800, TA instruments, USA) was used to probe the viscoelastic properties of PLLA and PLLA-based composites. Tests were performed on injection-molded samples of dimension 17.5 x 10 x 1 mm3 at a frequency of 1 Hz and amplitude of 20μm in a temperature range of −80 °C to 130 °C at a heating rate of 3 °C/min. The storage modulus and tan δ were recorded as a function of temperature. Small angle X-ray scattering (SAXS) was performed on a Xenocs Xeuss 2.0 SAXS instrument at 50 KV, 0.6 mA and Cu Kα radiation (1.5418 Å) was used on selected PLLA composites to probe the microstructure evolution. The 1D SAXS scattering intensities were normalized by sample thickness. Morphologies of impact specimens were examined using field emission scanning electron microscopy (FESEM) (JEOL JSM 6700F). A thin layer of gold was sputter coated on the samples prior to the morphology observations. In order to study the dispersion of the fillers, Philips CM 300 high-resolution transmission electron microscope (HRTEM) at an acceleration voltage of 300 kV was employed. Ultra-thin specimens (ca. 100 nm) were cut using a cryo-ultra



microtome (Leica EM FCS) at the temperature of -30 °C or -50 °C depending on the ductility of the specimen, and then attached onto copper grids for the TEM observation. 3

RESULTS AND DISCUSSION

3.1 Thermal Behavior and Structure of CNCaq-rD and CNCaq-rD-PDLA Fillers Figure 2 shows the thermal behavior of CNCaq-rD, CNCaq-rD-PDLA, and CNCaq-rD: CNCaqrD-PDLA (1:1), probed using DSC. In Figure 2a, CNCaq-rD and CNCaq-rD-PDLA exhibit a glass transition temperature (Tg) of about −38 °C and −29 °C, respectively. Apparently, CNCaqrD is largely amorphous as only a Tg is observed, whereas, CNCaq-rD-PDLA is semicrystalline with a cold crystallization peak (Tcc) and a melting peak (Tm) at 75 °C and 145 °C, respectively. The crystallizable part of CNCaq-rD-PDLA is due to the presence of the PDLA end block. In order to discriminate the overlapping peaks and accurately locate the apparent maximum, the first derivatives are shown in Figure 2b.


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Figure 2. (A) DSC traces for CNCaq-rD, CNCaq-rD-PDLA and CNCaq-rD: CNCaq-rD-PDLA (1:1) from the first heating scan and (B) its first derivatives By mixing CNCaq-rD and CNCaq-rD-PDLA in a 1:1 ratio, the Tg region broadens at the high temperature side (Figure 2b). This broadening indicates the presence of composition 11 ACS Paragon Plus Environment


heterogeneity and broad distribution of the relaxation time connected to the transition mechanism.25 Moreover, the width (intensity) of this broadened Tg region is suppressed as compared with that of individual fillers, which arises from the segmental immobilization at the interface of the two fillers due to strong interaction. The results agree with the noticeable increase in Tcc about 18 °C and Tm about 5 °C of the CNCaq-rD: CNCaq-rD-PDLA (1:1) mixture when compared with CNCaq-rD-PDLA. The shift of Tcc to high temperature could be resulted from the hindered chain mobility required for cold crystallization of PDLA. Therefore, the delayed cold crystallization and melting of the PDLA crystals occur at a relatively high temperature. Although, there is interaction between CNCaq-rD and CNCaq-rD-PDLA mixture (1:1) as depicted in Figure 2b, it is worth noting that the interphase dynamics may change in the presence of PLLA matrix in a ternary composite. This is primarily because PDLA has stronger affinity to the PLLA matrix due to stereocomplex formation. Therefore, the properties of the resultant PLLA ternary composites will depend on both the filler-filler and matrix-filler interactions. Overall, these thermal analysis results indicate that the dual fillers are ideal for application as impact modifiers according to NatureWorks criterion mentioned in the introduction section. The structure of each component in a composite material has strong influence on the overall physical and mechanical properties of the composite. Figure 3 shows X-ray diffraction (XRD) and atomic force microscopy (AFM) employed to understand the structural changes and the surface morphology of the cellulose nanocrystals (CNCaq), CNCaq-rD, CNCaq-rD-PDLA and a combination of CNCaq-rD: CNCaq-rD-PDLA (1:1). As seen in Figure 3a, CNCaq exhibits diffraction peaks at (101), (101), (021), (002), and (004) lattice planes, which are located at 2θ of 15.1, 16.5, 21, 22.5, and 34.8°, respectively. These peaks show that the CNCaq used in this study


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is in the alpha (α) form, and the morphology of this alpha (α) cellulose is shown using arrows in Figure 2b. After grafting the copolymer (ε-CL and D-LA) onto the CNCaq surface to obtain CNCaq-rD, the CNCaq diffraction peaks designated to α-cellulose are still observable, though in very low intensities. Additionally, the presence of PCL crystal plane indices at (111/211) confirms the successful grafting of the copolymer from the CNCaq surface. It also clearly indicates that the copolymer (ε-CL and D-LA) is quasi-random in nature, with both PDLA and PCL blocks in the structure although no melting peak was observed in DSC (Fig. 2(A)), probably due to the low sensitivity of DSC. The average block lengths of DLA and CL have been calculated to be ~2.51 (Figure S2), which lies in between the amorphous and crystalline morphology as reported in literature in which copolymers from LLA and CL showed random sequence when the block length of LLA or CL was 2.0 and below, and blocky sequence when the block length of LLA or CL was 3.0 and above, which is dependent on the initiators used and the monomer ratios.26



Figure 3. (A) XRD diffraction patterns of CNCaq, CNCaq-rD, CNCaq-rD-PDLA and CNCaqrD/CNCaq-rD-PDLA (1:1), and (B) 1 x 1μm2 AFM image and the corresponding 3D AFM images of CNCaq. Subsequent grafting of the PDLA end blocks onto CNCaq-rD, reveal that the form and structural integrity of the CNCaq has been maintained after the grafting procedure. More importantly, after PDLA grafting, stronger peaks appear at 2θ = 16.5° and 19° in CNCaq-rDPDLA, corresponding to PDLA lattice planes of (200/110) and (203), respectively. The 1:1 filler mixture shows much stronger diffraction peaks at 2θ = 16.5° and 19° for the PDLA blocks in CNCaq-rD-PDLA, whereas PCL peaks at crystal plane indices at (111/211) become weaker. These structure changes suggest that there is a noticeable interaction between CNCaq-rD and CNCaq-rD-PDLA. Ultimately, these interactions could affect the thermal behavior of the resultant PLLA composites.


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3.2 Thermal behavior and Structure of PLLA/ CNCaq-rD/CNCaq-rD-PDLA Composites Figure 4 shows the typical DSC thermograms of the PLLA and the PLLA composites. The incorporation of the dual fillers in equal quantities (wt %) into the PLLA matrix shifts the Tg and Tcc to lower temperature compared to neat PLLA, by between 1-4 °C and 0-6 °C, respectively. This should be resulted from the rubber phase in the fillers.

Figure 4. DSC curves for (A) neat PLLA, (B) 95/2.5/2.5, (C) 90/5/5, (D) 80/10/10, (E) 80/20/0 and (F) 80/0/20 from the first heating scan. Moreover, the 80/20/0 composite exhibits a lower Tg of about 55 °C and also the lowest Tcc (ca. 94 °C) as compared with neat PLLA and the other composites. This behavior can be attributed to the high content of CNCaq-rD in the PLLA matrix. On the other hand, the 80/5/15 and the 80/15/5 show comparable Tg and Tcc, but the former has a higher melting enthalpy. It is worth noting that both neat PLLA and the 80/20/0 composite depict double melting peak, with 15 ACS Paragon Plus Environment


the lower melting peak as a small shoulder. The double melting behavior has been widely reported in literature27-29, and is attributed to a transition of imperfect PLLA crystals (α’) to more perfect PLLA crystals (α).28 Except for neat PLLA, all other composites show a stereocomplex-melting peak (Table 2), albeit a weak one for the 80/20/0, which agrees with the quasi-random character of the CNCaqrD in Figure 3A. On the other hand, the 80/0/20 composite depicts a higher Tcc (ca. 120 °C) peak and the highest enthalpy associated with stereocomplex crystallites. This behavior can be attributed to the high content of -PDLA block from CNCaq-rD-PDLA in the composite, thus facilitating stereocomplexation between -PDLA end chains in the filler and the PLLA matrix. Besides the thermal behavior of PLLA and PLLA composites, the rheological behavior of the polymer melt is critical in polymer processing. In order to better understand the rheological behavior, we conducted rheological study on PLLA and selected PLLA composites. The results of the rheological studies which provide some insight on the frequency dependency of storage modulus (G’), loss modulus (G”) and complex viscosity (η*) are provided in Figure S1(a, b and c), respectively. Figure 5 shows the WAXD diffraction patterns obtained for neat PLLA and the PLLA composites. Neat PLLA exhibits a largely amorphous character with a very weak α-crystal peak at 2θ = 16.7°, corresponding to (200/110) planes.30 Except for the neat PLLA, all the PLLA composites show clear diffraction peaks at 2θ = 16.7, 19.1 and at 2θ = 12, 21 and 24°, corresponding to homo-crystals (α) and stereocomplex crystals (SC), respectively 5.


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Figure 5. WAXD profiles for the neat PLLA and the PLLA composites These structural changes in composites with CNCaq-rD-PDLA filler suggest that the CNCaqrD-PDLA provides a surface that promotes PLLA crystal formation by reducing the free energy for nucleation, and the presence of -PDLA end-blocks in CNCaq-rD-PDLA enables stereocomplex formation. However, the 80/20/0 show weak diffraction peaks at 2θ = 12, 21 and 24°, which confirms that the rubber segment in CNCaq-rD is quasi-random. This is supported by the facts of: 1) the existence of very short -PDLA blocks in the rubber segment that form imperfect stereocomplex crystals with PLLA matrix; and 2) the presence of PCL diffraction peaks in Figure 3A. However, the melting of these “blocks” was not detectable in DSC, probably due to the short block lengths and extremely small thermal process associated with the melting of the short blocks.26, 31-32 The quasi-random structure suggests that the CNCaq-rD filler could have very weak interaction with the matrix, which is a necessary condition for toughening PLLA.20



The presence of SC peaks at 2θ = 12, 21 and 24° implies that the composites should demonstrate improved thermal properties.4,

33-34

This is because stereocomplexation, a

stereoselective association of enantiomeric PLLA and PDLA, forms a dense network of weak (1.3 kJ/mol) CH3…O=C hydrogen bonds.34-35 These intimate structural conformations would result in decreased mobility of the polymer chains, and hence the improved thermal stability of the stereocomplex composites.36-37 3.3 Mechanical Properties of PLLA and PLLA/CNCaq-rD/CNCaq-rD-PDLA Composites 3.3.1 Tensile behavior The mechanical properties of PLLA are vital in engineering applications. In this context, mechanical properties of neat PLLA and PLLA composites were studied using uniaxial tensile and notched Izod impact tests. Figure 6a illustrates the typical stress-strain curves of neat PLLA and the PLLA composites, and Table 2 summarizes the thermo-mechanical property results.

Figure 6. Mechanical properties of neat PLLA and PLLA composites: (a) stress-strain curves and (b) notched Izod impact strength. From Figure 6a, it is seen that the stress-strain diagram exhibit two regions: (i) the brittle failure region with a strain at break ˂ 15 % and (ii) the ductile failure region with a strain at 18 ACS Paragon Plus Environment

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break > 100%. In the brittle failure region, the neat PLLA, 80/20/0, and 80/0/20 exhibit little or no plastic deformation before failure. This brittle failure is characterized by low strain at break, but with high Young’s modulus and yield strength. On the other hand, all the ternary PLLA composites exhibit pervasive plastic deformation in the ductile failure region. Notably, PLLA composites with equal contents of the dual fillers show extensive ductility of about 40, 60 and 100-fold strain at break for 95/2.5/2.5, 90/5/5 and 80/10/10 composites, respectively, compared with the neat PLLA. Interestingly, the extreme tensile ductility of these PLLA composites does not have substantial effect on the maximum stress of the composite. For instance, 90/2.5/2.5 composite have a tensile strength of about 50 MPa, compared with 58 MPa for the neat PLLA (Table 2). The distinct mechanical behavior of the PLLA ternary composites from the neat PLLA and the binary composites (80/20/0 and 80/0/20) suggests that there exists a joint effect of the fillers. Such synergetic effect could result in changes in deformation mechanisms in the composites, leading to notable enhancements in mechanical properties, especially the ductility. Previous research2, 9, 21, has indicated that it is relatively easier to improve the tensile toughness than the impact strength of PLLA, due its low crack initiation and propagation energy. Therefore, the simultaneous improvements of tensile toughness and impact strength of PLLA is an important achievement in enlarging the scope of engineering applications of PLLA. In light of this, we tested the effect of the filler content in the PLLA composites under impact conditions. 3.3.2 Impact behavior The impact strength refers to the ability of a material to absorb fracture energy under sudden loading. Figure 6b shows the notched Izod impact strength values of the PLLA and the PLLA composites. Neat PLLA show very low impact strength of about 2.3 kJm-2, corresponding to its



brittle nature. Interestingly, all the PLLA composites show improved impact strength as compared with the neat PLLA, except for the 80/0/20 composite. The impact strength of the 95/2.5/2.5 and 80/10/10 composites is improved by about 20% and 230%, respectively, compared with neat PLLA. The 80/10/10 composite displays both the highest impact strength (~7.9 kJm-2) and strain at break (~798 %), which suggest high cooperative effects of the dual fillers under uniaxial (tension) and triaxial (impact) testing conditions. Contrarily, the 80/20/0 shows an increase in impact strength of about 75%, whereas the 80/0/20 composite show a decrease of about 20%, compared with neat PLLA. These results imply that the 80/20/0 composite absorbs and dissipates about 2-fold impact energy, compared with the 80/0/20 composite. This clearly indicates a change in deformation mechanism for the PLLA composites, which will be elaborated in detail in the deformation mechanism section. This change in deformation mechanism could be best exemplified for the PLLA composites containing dual fillers, due to the changes in interphase dynamics such as the filler-filler interactions elucidated in Figure 2B and the expected matrix-filler interactions.


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Table 2. Thermal and Mechanical Properties of Neat PLLA and PLLA composites Tg

Tcc

Tmhc

Tmsc

ΔHm

χhc

(°C)a

(°C)b

(°C)

(°C)

(J/g)c

(%)d

σ (MPa)e

E (GPa)f

ε (%)g

(MJm-3)h

(kJm-2)i

Neat PLLA

59

106

6.5

7

58 ± 1.7

3.9 ± 0.3

8.0 ± 3.6

5.1 ± 0.1

2.3 ± 0.1

95 / 2.5 / 2.5

58

106

149

193

14.1

16.6

49 ± 5.1

3.2 ± 0.6

310 ± 0.3

87.3 ± 0.3

2.8 ± 0.2

90 / 5 / 5

56

104

149

194

19.5

23.3

39 ± 1.8

2.6 ± 0.1

500 ± 6.4

134.2 ± 0.1

2.9 ± 0.1

80 / 20 / 0

55

94

148

181

4.4

5.9

55 ± 3.7

3.3 ± 0.4

7.5 ± 1.0

3.5 ± 0.2

4.2 ± 0.5

80 / 15 / 5

54

109

149

184

3.3

4.4

41 ± 0.2

2.9 ± 0.4

207 ± 0.6

50 ± 0.1

3.7 ± 0.3

80 / 10 / 10

55

100

150

195

17.9

24.1

32 ± 0.5

2.7 ± 0.1

798 ± 5.1

204.7 ± 0.9

7.9 ± 1.1

80 / 5 / 15

58

110

150

193

2.8

3.8

37 ± 0.6

2.2 ± 0.4

238 ± 5.8

51 ± 0.7

3.3 ± 0.2

80 / 0 / 20

59

120

152

196

2.5

3.4

56 ± 1.5

3.6 ± 0.1

2.5 ± 0.8

1.0 ± 0.1

2.2 ± 0.2

Samples

149/151 n.d

Tensile Properties

Toughness

Impact Strength

n.d - not detected in DSC. Tmhc and Tmsc peak temperature of the melting endotherms of homocrystals and stereocomplex crystals, respectively, detected using DSC. a Tg - Glass transition temperature detected using DSC. b Tcc - Cold crystallization temperature recorded using DSC. c ΔHm – net enthalpy of PLLA homocrystals obtained from subtracting melting cold crystallization enthalpy from the melting enthalpy determined by DSC first runs. d Crystallinity percent of PLLA homocrystals calculated according to the following equation: χ % = 100 × ΔHm ⁄ (W × ΔHm°), where ΔHm – net enthalpy of PLLA homocrystals, W is the weight fraction of PLLA taken up in the samples and ΔHm° is 93 J/g for 100% crystalline PLLA. e Tensile stress at yield. f Elastic modulus. g Strain at break. h Tensile toughness determined by integrating the stress-strain curves. i Izod impact strength determined using ASTM D256.



3.4 Deformation Mechanisms in PLLA and PLLA/CNCaq-rD/CNCaq-rD-PDLA

Composites 3.4.1 Morphology of the neat PLLA and PLLA composites From previous studies

2, 5, 20, 23-24, 30, 38-39,

deformation mechanisms such as crazing, shear

yielding, cavitation, debonding etc., are correlated with both the tensile ductility and the impact strength of semicrystalline polymers. To understand the deformation mechanisms of neat PLLA and selected PLLA composites with high filler content, we studied the impact-fractured surfaces near the notch area using SEM as shown in Figure 7. The scheme of the observed fracture surface is shown in Figure 7g. Clearly, neat PLLA shows relatively smooth fracture surface without appreciable plastic deformation features, implying a typical brittle failure with low impact strength (ca. 2.3 kJm-2). In contrast, the fracture surface of the 80/20/0 composite displays phase separation morphology with CNCaq-rD dispersed in PLLA matrix, in accordance to the purposefully designed immiscible CNCaq-rD. This immiscibility of the CNCaq-rD in PLLA matrix was confirmed using DMA analysis at the low temperature region (Figure S3). The tan delta curve for the 80/20/0 composite shows a clear relaxation peak at about -35 °C. The presence of this relaxation suggests that interaction between the CNCaq-rD filler and the matrix is very weak despite of the possible weak stereocomplexation.20 This immiscibility behavior and the resultant morphology result in impact strength enhancement, which correlates with the impact energy (4.2 kJm-2) obtained for this composite shown in Table 2. Additionally, this immiscibility behavior suggests that the level of interactions between PLLA matrix and the PDLA blocks in the quasi-random copolymer (CNCaq-rD) is very weak. On the contrary, the SEM images obtained from the 80/0/20 composite exhibit relatively coarse surface that is indicative of craze22 ACS Paragon Plus Environment

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crack transition. This morphology suggests that the high content of perfect stereocomplex crystallites in the matrix overcomes the energy barrier for their deformation and thereby initiating crazes from which cracks grow. Eventually, these cracks lead to premature breakdown that is in agreement with its low strain at break shown in Table 2.

Figure 7. SEM micrographs for the impact fracture surface of (A) Neat PLLA, (B) 80/20/0, (C) 80/15/5, (D) 80/10/10, (E) 80/5/15, and (F) 80/0/20, (G) Schematic diagram of the fracture surface observed using SEM. Scale bar = 1μm



The PLLA ternary composites (Figure 7c, d and e), corresponding to 80/15/5, 80/10/10 and 80/5/15, respectively, shows much rougher fracture surfaces, an indication of ductile behavior that is in agreement with tensile toughness values and impact results in Table 2. Exceptionally, the 80/10/10 composite exhibits very rough fracture surface that is characteristic of energydissipative toughening mechanisms with high impact strength (~7.9 kJm-2). Clearly, the fracture surface displays high deformation zones with microvoids and microfibrils emanating from simultaneous cavitation and crazing mechanisms. As anticipated, cavitation and crazing mechanisms occur in tandem in the PLLA matrix in ternary composites because of: 1) the immiscibility and the presence of weak interactions of PLLA matrix with the CNCaq-rD and 2) the stronger stereocomplex interactions with the CNCaq-rD-PDLA. However, comparing the tensile and impact strength values of the neat PLLA and the selected PLLA composites in Table 2 confirms the differences in toughening mechanisms for the composites. For instance, the 80/20/0 composite show low tensile toughness but high impact strength due to cavitation; whereas the 80/0/20 composite depicts both low tensile toughness and impact strength due to craze-crack transition. On the other hand, the 80/10/10 composite shows high toughness under both impact and tensile conditions, which suggests the existence of synergy from the dual fillers in PLLA matrix during deformation. Notably, cavitation and crazing are the two competitive deformation mechanisms involved in the toughening of the 80/10/10 composites, resulting in high impact and tensile toughness values. To clearly observe the dispersion of CNCaq-rD and CNCaq-rD-PDLA within the PLLA matrix, we used TEM micrographs of the 80/10/10 composite (Figure S4). CNCaq-rD and CNCaq-rD-PDLA fillers are identified as the dark regions due to the electron density differences from the PLLA matrix. Generally, CNC-rD and CNC-rD-PDLA fillers in the 24 ACS Paragon Plus Environment

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sizes ranging between 200 to 300 nm are well dispersed within PLLA matrix. The result agrees with the nominal sizes of CNCaq shown in Figure 2b. However, it is not possible to distinguish between CNCaq-rD and CNCaq-rD-PDLA due to lack of clear contrast. To get deeper insight into the microstructure-deformation relationship in the PLLA composites, we performed a post-deformation analysis on selected deformed tensile samples using small angle X-ray scattering (SAXS). 3.4.2 Microstructure deformation mechanism through SAXS analysis SAXS is a versatile technique to characterize microstructural evolution and molecular motion dynamics in semicrystalline polymers. The deformation structures usually probed using SAXS include: cavitation, crystal transition, chain orientation, lamellar structure and so on, which result in electron density differences in the polymers. Generally, undeformed samples exhibit isotropic rings reminiscent of random orientation of the fillers, but with a slight scattering if there is pre-existing heterogeneities. Upon stretching (deformed), the isotropic scattering in 2D SAXS pattern converts to an ellipsoidal pattern with intense streaks appearing along certain directions. More importantly, SAXS pattern for craze development is distinguishable in the form of a cross-like signature consisting of two intense streaks perpendicular and parallel to the tensile direction. Overall, the 2D scattering intensity has its major axis along the equatorial direction and the minor axis along the meridional direction. Normally, these intensities are obtainable from 2D SAXS patterns processed using software, such as ImageJ, and reduced to 1D volume plot of intensities I (q) vs. q., as shown in Figure S5. Figure 8 shows the 2D-SAXS patterns and their corresponding volume plots (graphs of intensity I (q) vs. q) of PLLA and selected PLLA composites, where 𝑞 = 4𝜋sin 𝜃/𝜆 and θ is half of the scattering angle. 1D SAXS spectra for the neat PLLA reveal low scattering 25 ACS Paragon Plus Environment


intensities in both meridional and equatorial directions, due to low electron density fluctuations. However, for the 80/20/0 composite, considerable scattering intensity exists due to the phase separated CNCaq-rD particles within the PLLA matrix, as also seen in SEM images (Figure 7b). This composite exhibits scattering streaks in both meridional and equatorial directions, although peaks in the equatorial direction appear to be more intensive. This suggests that the 80/20/0 composite cavitates prior to the initiation of crazing and the crazes formed are unstable, leading to brittle failure under tensile conditions. Under impact conditions, the 80/20/0 composite absorbs considerable energy probably due to the high content of CNCaq-rD with low cavitation resistance and the negative pressure created under triaxial stress. In contrast, the 80/0/20 composite exhibits an order of magnitude more intensive than the 80/20/0 composite, in both equatorial and meridional directions. The most plausible explanation is that the stronger network of stereocomplex crystallites initiates internal cavitation of rubber instead of debonding cavitation, thus delaying the matrix yielding and developing catastrophic cracks before failure. This agrees with Liu et al40, who reported that in terms of impact strength of blends, debonding cavitation followed by matrix shear yielding is a much more important toughening mechanism than internal cavitation of rubber particles.


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Figure 8. 2D SAXS images of (a) undeformed and, (a’) deformed PLLA and PLLA composites, and their corresponding equatorial and meridional intensity profiles of the deformed samples. Stretching direction is vertical.



Page 28 of 36

Interestingly, the 80/15/5, 80/10/10 and 80/5/15 composites exhibit high scattering intensities in both meridional and equatorial directions as compared with the neat PLLA and the binary composites (80/20/0 and 80/0/20). This suggests the existence of high electron density contrasts in both directions, which could be ascribed to the close proximity of cavities and fibrils resulting from the cavitation and crazing processes. A striking feature is the comparable equatorial and meridional intensities of the 80/10/10 composite and the resultant diamond-shaped 2D SAXS pattern. This diamond shape indicates that morphology is strongly and equally oriented in both equatorial and meridional directions. This also suggests the coexistence of multiple deformation mechanisms in the PLLA matrix resulting in massive plastic deformation. These balanced deformation mechanisms result in superb toughening of the 80/10/10 composite in tensile toughness and impact strength by 40-fold and 3-fold, respectively, compared with the neat PLLA (Table 2). 3.4.3 Proposed model for toughness enhancements Brittle polymers can only absorb energy to a certain load limit, and then crack growth occurs. However, there is a general consensus that in order to toughen a brittle polymer matrix, another material with a modulus lower than the matrix should be incorporated.2 Addition of the low modulus material into the matrix therefore delays

the fracture

process.9 In light of this, it would be interesting to explore the effect of adding the dual fillers with lower modulus but each of them has a specific role in toughening PLLA. Figure 9a shows a possible schematic model by which the dual fillers simultaneously toughen PLLA under impact and tensile conditions.


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Figure 9. (A) Schematic illustration showing dual toughening mechanisms in the 80/10/10 composite upon stretching, and (B) the resultant diamond-shaped 2D SAXS pattern. The stretching direction is vertical, and the figure not drawn to scale. Addition of high content (ca. 20%) of CNCaq-rD filler provides manifold sites of premature interfacial failure largely through cavitation. This suggests the PLLA / 20% CNCaq-rD composites could only afford high impact toughness but the tensile toughness is not high due to lack of mass craze. On the other hand, addition of high content of CNCaq-rD-PDLA filler is unfavorable because it leads to delay in matrix yielding, and eventually brittle failure occurs. Therefore, these binary PLLA composites may not show simultaneous improvements in tensile toughness and impact strength, which agrees well with the results presented in Table 1. One way to obtain synergetic effect is to incorporate both CNCaq-rD and CNCaq-rDPDLA fillers into the PLLA matrix. In this model, the CNCaq-rD filler acts as a cavitation 29 ACS Paragon Plus Environment


component that creates voids and absorbs fracture energy due to the negative dilatational effects. At the same time, the CNCaq-rD-PDLA filler acts as preferential craze nucleation sites. This is in agreement with Figure 9b, which shows the resultant diamond-shaped 2D SAXS pattern that corroborates the active co-existence of both cavitation and crazing components in the structural evolution of 80/10/10 composite. Because of the cooperative effect of the cavitation of the CNCaq-rD filler and massive crazing of the matrix, the cavities and large density of crazes results in simultaneous improvement in tensile toughness and impact strength. The cooperative effect of the dual fillers is also evident in the 80/15/5 and 80/5/15 composites that show almost equal tensile toughness (ca. 50 MJm-3). However, the 80/15/5 composite has superior impact strength by about 27% when compared with the 80/5/15 composite. This result suggests that cavitation which often results in enhanced impact strength in composites could be the dominant mechanism in the 80/15/5 composite.

4

CONCLUSION In summary, we have successfully prepared PLLA composites through solution cast-

injection molding method by utilizing synthesized CNCaq-rD and CNCaq-rD-PDLA as fillers in the PLLA matrix. Both tensile toughness and impact strength were improved tremendously when equal quantities of the dual fillers were incorporated into the PLLA matrix. In the case of 80/10/10 ternary composite, the strain at break and impact strength were increased by 100-fold and 3-fold, respectively, when compared with the neat PLLA. On the other hand, the 80/20/0 and the 80/0/20 binary composites showed low strain at break comparable to the neat PLLA. In light of this, there is clearly a synergistic effect of the dual fillers in the toughening of PLLA under tensile and impact conditions. The synergistic enhancements in mechanical properties were resulted from the filler-filler 30 ACS Paragon Plus Environment

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interactions, and filler-matrix interactions. SAXS and SEM analysis revealed the existence of dualism in deformation mechanisms in the composites. The CNCaq-rD filler facilitated cavitation under stress and created voids, thus absorbing a lot of fracture energy. The CNCaq-rD-PDLA filler acted as craze nucleation sites, thereby inducing crazing in the PLLA matrix. The cooperative effect of cavitation of the CNCaq-rD and matrix crazing induced by CNCaq-rD-PDLA led to pervasive plastic deformation, and this resulted in huge improvements in both tensile toughness and impact strength. These highly toughened PLLA-cellulose nanocrystal composites are very promising materials for packaging and biomedical (orthopedic) applications, where both the high ductility and impact strength are required. Supporting Information Dependency of (a) storage modulus (G’), (b) loss modulus (G”) and complex viscosity (η*) with frequency for PLLA and selected PLLA composites. Carbonyl region of

13C

NMR

spectrum of CNCaq-rD. Tan delta curves showing the low temperature relaxation for the neat PLLA and the selected PLLA composites. TEM image of the 80/10/10 composite. 2D SAXS contour plots showing coordinate axis and slicing in both equatorial and meridional directions used to obtain the intensities. Acknowledgments We sincerely thank Mr. Lim Poh Chong for X-ray diffraction analysis Conflict of interest The authors declare no competing interest.



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For Table of Contents Use Only Synergistic

Toughening

Nanocrystal

Composites

of

Poly

through

(lactic

acid)-Cellulose

Cooperative

Effect

of

Cavitation and Crazing Deformation Mechanisms

Joseph K. Muiruri, Songlin Liu, Jayven Chee Chuan Yeo, Junqiang Justin Koh, Junhua Kong, Warintorn Thitsartarn, Wern Sze Teo, and Chaobin He

Synopsis: Stress transfer through cooperative effect of cavitation and crazing deformation mechanisms in Poly (lactic acid)-Cellulose nanocrystal composite using dual fillers


Synergistic Toughening of Poly (lactic acid)-Cellulose Nanocrystal

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