Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Extensional Stress-Induced Orientation and Crystallization can Regulate the Balance of Toughness and Stiffness of Polylactide Films: Interplay of Oriented Amorphous Chains and Crystallites Xin-Rui Gao,† Yue Li,† Hua-Dong Huang,† Jia-Zhuang Xu,† Ling Xu,† Xu Ji,‡ Gan-Ji Zhong,*,† and Zhong-Ming Li*,† College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering and ‡College of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China
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†
S Supporting Information *
ABSTRACT: Polylactide (PLA) films with an excellent balance of toughness and stiffness were realized by extensional stress efficiently. For the relatively low extensional stress, gauche− gauche conformers that originated from the oriented amorphous chains lead to super-toughening behavior. Among higher extensional stress, strain-induced orientation and crystallization act as the driving force of reinforcement. This mechanism is evidenced by the pronounced enhancement in the elongation at break from 16.9 up to 294.9% accompanying the variation yield strength from 45.3 up to 135.5 MPa. The highest elongation at break results from the early stretching stages, whereas the highest yield strength is obtained from a high draw ratio. More impressively, PLA films show temperature-invariant super-ductility and reinforcement at low temperatures (0 and −20 °C). This work provides a preferable and scalable method to fabricate competitive PLA materials, expanding the practical application of sustainable polymers served at a wide temperature range or in harsh environments. interconnection at the interface or in the network.9,10,12 The accordant principle of the above-mentioned strategies is influencing the formation of crystallites. In most instances, the role of the amorphous phase has not gained as much attention as it deserves. Just a few recent works addressed the concern about improving macroscopic properties (transport properties14 and water barrier properties15) by tailoring the rigid amorphous fraction.14−16 In particular, the influence of amorphous architecture on the mechanical properties of polymer glass has been confirmed. In glassy polymers, including polycarbonate,17−19 polystyrene,19−21 and poly(methyl methacrylate),19,21−23 the original two-component model was established by Haward and Thackray.24 It divided stress into the primary bonds (covalent bonding in the polymer network) and secondary bonds (intermolecular forces). Based on the understanding of this model, pioneering works have explained the temporary mechanical rejuvenation25,26 and answered why the melt stretched polystyrenes are flexible.27 Despite the different approaches adopted in the two phenomena mentioned above, loss of strain localization is convinced to be the unified root of flexibility. The compression in the rolling procedure weakens the secondary bonds, so the toughening effect in mechanical
1. INTRODUCTION As the mainstay of bio-based thermoplastic, polylactide (PLA) enjoys good reputation ascribed to the biodegradability and relatively high strength.1,2 However, the grievous clash between strength and toughness becomes the predicament that restricts its scope of utilization.3 Many attempts have been dedicated to achieve balanced mechanical performance. The two major methods are copolymerization4 and blending.5,6 Copolymerization requires a relatively sophisticated process, although the significant enhancement of toughness could be achieved.4 Blending is competent for manufacturing while the poor interfacial interaction may lead to the limited enhancement of performance.5,6 The obstacle for both is that the improved ductility is always accompanied by an enormous sacrifice of strength.4,5,7 Hence, there is still a lack of a pathway that can be conveniently used to simultaneously obtain remarkable strength and ductility without incorporation of any other component, especially at an industrial scale. As a semi-crystalline polymer, both crystalline and amorphous phases govern the macroscopic properties of PLA. Preparing special crystal structures (such as shishkebab8,9) by a special processing device (such as layermultiplying extrusion10 and oscillation shear injection molding11) is capable of ameliorating the performance, including heat resistance,11 gas barrier properties,12 and mechanical properties.8,13 It has been proven that the well-defined crystal structure could offer sufficient reinforcement and tenacious © XXXX American Chemical Society
Received: May 6, 2019 Revised: June 8, 2019
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2.2. Sample Preparation. To avoid degradation caused by hydrolysis, PLA pellets were dried at 80 °C for 12 h in a vacuum oven before processing. PLA films with diverse extensional stress were formulated by casting-thermal stretching. Figure S1 shows the digital photo of equipment. Extrusion was carried out on a single-screw extruder, temperatures were set to 175, 180, 190, and 175 °C from hopper to die. As delineated in Figure 1, the as-extruded film was
rejuvenation would disappear in 30 min after the progressive aging.25−27 The melt stretching induces the prestretched network and reduction of segmental friction, enhance the contribution of stress from the primary bonds, and the flexibility could be preserved for 6 months.27 Analogously, as an endeavor of explaining the brittle to ductile behavior in a glassy state, recent research studies presumed polymer glass as a hybrid embracing the network formed by van der Waals forces and the underlying load-bearing chain network arising from uncrossability. Upon cooling down below the glass transition temperature (Tg), all segments are vitrified and chain network freezes, thus most polymer glasses become brittle. At high temperatures, polymer glasses switch from brittle to ductile because of the enhanced molecular mobility. Once the temperature (T) is below brittle-to-ductile transition temperature (Tbd), the weak network of van der Waals forces is dominant, resulting in the brittle fracture. At T > Tbd, after the breakdown of van der Waals forces, the chain network acts as the load-bearing part and determines when macroscopic yielding takes place.17,19−21,28 As for PLA, tensile properties are strongly related to chain packing in the amorphous phase. It is reported that PLA cooled at slightly higher than Tg (60 and 80 °C) showed elevated toughness without obvious crystals. This abnormal behavior is due to the increased scale of high energy gauche−gauche (gg) conformers, which exhibit conformational change under a low-stress level and reduces the critical stress for shear yielding.29 This abnormal phenomenon also demonstrates that processing conditions could regulate the amorphous structure, and therefore tailor the mechanical performance. As a typical semi-crystalline polymer with a low crystallization rate and rigid chains,30 there is ample space for tailoring the amorphous structure and the crystal structure simultaneously in PLA. In the present case, we utilize various extensional stress on PLA films for the two following objectives. (a) To unravel and tailor the interplay between the molecular orientation in the amorphous phase and straininduced crystallization by imposing intense extensional stress. (b) To develop a scalable strategy, aiming to dramatically enhance stiffness and toughness without additives, and making it feasible in large scale and close to the actual manufacturing circumstance. Herein, a scalable and versatile “casting-thermal stretching” routine is adopted to manipulate the amorphous chain network and the crystalline phase simultaneously. Various intensities of extensional stress are applied on PLA chains, inducing the hierarchical orientation both in amorphous and crystalline phases. The molecular orientation is investigated by Fourier transform infrared (FTIR) spectroscopy. Temperature-modulated differential scanning calorimetry (TMDSC) is used for distinguishing and tracing different types of amorphous and crystalline phases, while the appearance of oriented crystallites is confirmed by wide-angle X-ray diffraction (WAXD). The morphology during deformation is revealed using small-angle X-ray scattering (SAXS) and a polarized optical microscope (POM).
Figure 1. Schematic diagram of casting-thermal stretching procedure. stretched between the preheat roll and drawing roll with the speed of V1 and V2, respectively. The chill roll rotated at V2, acting as the cooling and wind-up part. Different draw ratios (DRs) were achieved by manipulating V2, while V1 was constant. To quantify the intensity of extensional flow, the DR was calculated by the ratio of the crosssectional areas of the die and the sample. The temperatures of the preheat roll and drawing roll were controlled at 80 °C, which is slightly higher than Tg. The chill roll was unheated. We manufactured samples with different DRs, including 7.5, 9.8, 11.2, 13.3, 16.0, 19.0, and 21.7. Meanwhile, as-casted films without thermal stretching were prepared as control samples. To give a vivid description, we select the DR 16.0 sample as a representative, taking digital photos (see Figure S2). 2.3. Characterization. 2.3.1. Fourier-Transform Infrared (FTIR) Spectroscopy. FTIR (Nicolet 6700, Thermal Scientific) was intended for characterizing the conformational change caused by extensional stress. All spectra were collected in the reflection mode with 16 scans and a resolution of 4 cm−1. The polarized FTIR spectroscopy was implemented by FTIR equipped with a Polaroid. We did a multiangle scan from 0 to 180° per 5° through altering the polarizers′ angle, analyzing the absorption spectra of 1000−900 cm−1 wavenumber. The dichroic ratio (D) and orientation function (f) were deduced using relations31 D = A /A⊥
(1)
f = (D − 1)/(D + 2)
(2)
where A∥ and A⊥ are the parallel and perpendicular absorbance, respectively. For PLA, absorption at 921 cm−1 is bound to the crystalline phase while absorption at 956 cm−1 is due to the contribution of amorphous phases.32 The orientations of the amorphous phase (fa) and the crystalline phase (fc) are determined by 956 and 921 cm−1 bands, respectively. 2.3.2. Two-Dimensional Wide-Angle X-ray Diffraction (2DWAXD). Two-dimensional-WAXD was conducted at the beamline BL15U1 (wavelength λ = 0.124 nm) of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) with an X-ray CCD (Model SX165, Rayonix LLC.) as the detector, and the sample-todetector distance was set at 159.8 mm. 1D-WAXD intensity profiles as a function of q were obtained by integration in the azimuthal angular range of a whole circle (0−360°) from the sample patterns employing
2. EXPERIMENTAL SECTION 2.1. Materials. PLA (4032D) is a commercial product from NatureWorks with ∼2% D-LA. The number-average molecular weight and weight-average molecular weight were 1.06 × 105 and 2.23 × 105 g mol−1, respectively. B
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Figure 2. PLA films with diverse DRs. (a) Reflection mode FTIR spectroscopy in the wavenumber range of 1300−1240 cm−1. (b) The intensity changes at 1267 cm−1 as a function of DRs.
Figure 3. P-FTIR spectroscopy of the PLA films with various DRs, (a) as-casted, (b) DR 7.5, (c) DR 9.8, (d) DR 11.2, (e) DR 13.3, (f) DR 16.0, (g) DR 19.0, and (h) DR 21.7. The orientation factors of 956 cm−1 (fa) and 921 cm−1 (fc) are labeled on the front of each picture. The purple and yellow arrows mark the peak at 921 and 956 cm−1, respectively. XPolar software, while background scattering was subtracted in advance. The calculation methods of the average size of the crystal domain (Lhkl) and the orientation parameter (Herman’s orientation factor, f H) are shown in the Supporting Information. 2.3.3. Two-Dimensional Small-Angle X-ray Scattering (2DSAXS). Two-dimensional-SAXS was performed at the beamline BL16B1 of SSRF, with a sample-to-detector distance of 2365 mm.
The SAXS images were collected with an X-ray CCD detector (Mar165, a resolution of 2048 × 2048 pixels). Samples were prestretched to certain strains and then distinguished into three characteristic regions (undeformed zone, transitional zone, and necking zone), following the procedure shown in the Supporting Information. C
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contrast to the obvious absorbance at 956 cm−1. The lack of periodic variation indicates a relatively weak orientation at low DRs, but the elevated fa declares that the orientation of amorphous chains could be induced at the low extensional stress, whereas the change of fc is unapparent at samples with DR of 7.5. Once DR exceeds 9.8, the peak at 921 cm−1 becomes more and more prominent. Both of these two peaks exhibit distinct cyclical fluctuation from 0 to 180°. In the meantime, fa and fc increase simultaneously. It demonstrates that oriented amorphous and crystalline structures appear when the extensional stress is strong enough. For DR 21.7 samples, fa and fc manifest the slight reduction. Connecting to the evolution of gg conformers, it is reasonable to depict the structural change with the following statement: conformational change occurs at the first stage of stretching when the exclusively oriented amorphous phase arises. As DR elevates, because of the strain-induced crystallization, both oriented crystalline and amorphous phases are dominant, implying that the extensional stress largely motivates orientation and crystallization. Combining with the anomaly in Figure 2, crystallization suppresses the occurrence of the gg conformer and produces an obvious drop of the absorbance at 1267 cm−1, which is consistent with the previous research.35 3.2. Structure of Oriented Crystallites in Stretched PLA Films. WAXD is utilized to obtain information about the oriented crystalline structure. Figure 4 compares the 2D-
2.3.4. Thermal Behavior Testing. A TA Q2000 differential scanning calorimeter (DSC) instrument was used for standard DSC and temperature-modulated DSC (TMDSC) analysis. The experiments were performed with about 5 mg samples heated from 30 to 200 °C under a nitrogen atmosphere. The heating rates were 5 and 3 °C min−1 for DSC and TMDSC, respectively. In particular, the oscillation period was set to 60 s and the temperature modulation amplitude was ± 0.5 °C in TMDSC testing.33 The calculation procedures of crystallinity (Xc), the mobile amorphous fraction (XMAF), and the rigid amorphous fraction (XRAF) are shown in the Supporting Information. 2.3.5. Mechanical Property Testing. Tensile properties were measured at −20, 0, 25 °C (room temperature) with the crosshead speed of 5 mm min−1 using the Instron Instrument Model 5576 equipped with precision temp-enclosure. Each sample was cut into 6 mm (length) × 1 mm (width) rectangle, a minimum of 5 replicates were tested under the same conditions, and the average values were calculated with standard deviations. To test the low-temperature mechanical performance, samples were precooled at the corresponding temperature for more than 30 min to accommodate the actual low-temperature environment and maintain the heat balance before testing at 0 and −20 °C. 2.3.6. Polarized Optical Microscope (POM). As-casted, DR 9.8, DR 13.3, DR 16.0, and DR 21.7 samples with the corresponding preset strains were tested. The workflow of sample treatment is the same as that in the section of 2D-SAXS. The morphology in the undeformed zone, transitional zone, and necking zone was observed using POM (BX51, Olympus Co., Tokyo, Japan).
3. RESULTS AND DISCUSSION 3.1. Molecular Conformation and Orientation of Chain Segments in Stretched PLA Film. The band of 1267 cm−1 in FTIR is assigned to the C−O−C backbone stretching,34 which is greatly sensitive to high energy gg conformers of the PLA molecular chain.35 Thus, the proportion of gg conformers in stretched films can be evaluated by the intensity of 1267 cm−1. It is reported that when heating over the glass transition region, the energyfavorable gauche−trans (gt) conformers rearrange into the less energy-favorable gg counterparts because of the enhancement in free volume and chain mobility. In contrast, due to the formation of ordered domains, gg conformers readjust to lowenergy gt counterparts during physical aging.35 Yamaguchi et al.29 found that cooling at the temperatures slightly higher than Tg or prolonging cooling time at slightly lower than Tg could stimulate the closer chain packing and elevate the content of gg conformers. In this work, the effect of extensional stress on the conformational change is confirmed, as shown in Figure 2. It is believed that gg conformers originate from the stretching of the molecular chain. By promoting DRs, the proportion of gg conformers is supposed to grow continually, but an atypical declination of absorbance turns up after DR rises to 9.8. Another abnormal part is that this declination seems to experience a slowdown at DR 19.0 samples. Molecular orientation plays a pivotal role in the mechanical performance of semi-crystalline PLA films, which is systematically characterized by P-FTIR (Figure 3). The band at 921 cm−1 is related to the coupling of the C−C backbone stretching with the CH3 rocking mode,36 which is also sensitive to the crystalline phase, while 956 cm−1 originated from CH3 rocking is usually ascribed to the amorphous phase.32 The fa and fc calculated according to the absorbance change at 956 and 921 cm−1 are chosen to identify and compare the orientation of chain segments. In the as-casted sample and sample with DR of 7.5, the band at 921 cm−1 is absent, in
Figure 4. Two-dimensional-WAXD patterns of PLA films with various DRs. The corresponding DR is labeled on the upper left corner of each picture. The stretching direction is horizontal, as the double-headed arrow shows.
WAXD patterns of the as-casted and stretched PLA films. Onedimensional curves (1D-WAXD) are integrated circularly from the corresponding WAXD patterns as shown in Figure 5a. Two characteristic diffraction peaks assigned to PLA crystalline phase exist in samples with DRs higher than the critical DR of 7.5. 9 It suggests that extensional stress could evoke crystallization. In contrast, only amorphous halo diffraction exists in as-casted and DR 7.5 samples, reflecting the very low crystallinity under the weak extensional stress. With the increasing DR, the diffraction of the crystalline phase displays obvious reinforcement around the meridian, elucidating the formation of highly oriented crystals, which is consistent with the verdict in Figure 3. When DR reaches 16, the spot-like pattern becomes dispersed and diffraction peaks are visibly broader, suggesting that excessively strong stress may cause the fragment. To prove this hypothesis as well as quantify the evolution of crystal size, the mean size of the crystal domain (Lhkl) is shown in Figure 5b. Samples with higher DRs possess smaller crystal D
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Figure 5. (a) One-dimensional-WAXD diffraction profiles. (b) Average size of the crystal domain (Lhkl) of samples with different DRs.
sizes, indicating that the extensional field induces local ruptures in the crystalline region. Nucleation might appear at the same time, but which needs further confirmation. Specifically, compared to the 9.08 nm of DR 19.0 sample, the Lhkl of DR 21.7 sample is slightly larger. This indicates that the rearranged lattice with a larger scale might generate. Tracing back to the decreasing absorbance at 1267 cm−1 from DR 19.0 to DR 21.7, the slightly larger crystal domain adds growing evidence that recrystallization occurs under the extremely intense extensional stress. Based on the previous result, fragmentation and recrystallization are likely to emerge in samples with high DRs.37,38 To quantify orientation in the crystalline phase, the azimuthal profiles (see Figure S4) were extracted and orientation parameters (f H) of (200/110) reflection were calculated (see Figure 6). The trend of f H with improving DR
stress becomes strong, the orientation degree (f H, fa, and fc) keeps growing, corroborating the formation of the highly oriented structure. On the other hand, the local rupture of crystallites induced by extensional stress lowers the crystal size. (d) As for DR 21.7 samples, the recrystallization with insufficient orientation results in an abrupt diminution of f H and fc, leading to the reduction of the intensity at 1267 cm−1. 3.3. Heterogeneity of the Amorphous Phase in Stretched PLA Films. DSC could reflect the structural characteristics in the stretched PLA film, especially the amorphous information that cannot be identified by X-ray diffraction. The significant change of cold crystallization is closely relevant to the crucial transformation of the amorphous phase. It is known that both orientation and crystallization give rise to an earlier onset of cold crystallization. The cold crystallization range is approximately 90−110 °C in as-casted samples, whereas the cold crystallization peak distinctly moves to the low-temperature region at the DR 7.5 sample, showing the clear evidence of the ordered amorphous phase developed under relatively low extensional stress, which is also confirmed in P-FTIR as the oriented amorphous phase. Another notable case is the appearance of a weak endothermic peak before glass transition. Some previous works considered the weak endothermic peak before glass transition as the melting of mesophase,39−41 simultaneous devitrification and enthalpic relaxation of the RAF,33 and physical aging.35 The essence is the relaxation of the intermediate phase with certain molecular ordering. To explicitly reveal the ordered structure, we define this endothermic peak as the chain relaxation peak in a general concept, which is quantified and monitored by TMDSC. The curves of nonreversible heat flow are shown in Figure 8a. The corresponding temperatures of chain relaxation (T1) and cold crystallization (T2) are shown in Figure 8b. By elevating DR, T1 shifts to a higher temperature area whereas T2 demonstrates the reverse trend. The well-aligned region induced by extensional stress retards the amorphous mobility, so chain relaxation could only occur at a higher temperature range, showing the enhanced T1. As for cold crystallization, T2 drops dramatically from the as-casted films (100.60 °C) to DR 9.8 samples (75.17 °C) because the stretched chains with ordered constitution could rearrange into lattice easily and readily at low temperatures. Similar explanations have been proposed in the mesophase.39−42 The increasing population of the ordered amorphous phase also results in the greatly weakened enthalpy of cold crystallization in highly crystallized samples (from DR 11.2 to DR 13.3).33 Further increasing DR, samples with high DRs (from DR 16.0 to DR 21.7) exhibit growing enthalpy because of the stress-induced fragmentation
Figure 6. Orientation parameter (f H) of samples as a function of DRs.
is the same as the trend of fc shown in Figure 3. Taking stock of the information aforementioned, the structural evolution could be divided into different sections according to the intensity of extensional stress. (a) Upon the relatively weak extensional stress, samples with low DRs are almost amorphous. A large content of gg conformers as well as the oriented amorphous phase could be triggered at this stage, giving rise to the absorbance at 1267 cm−1. In contrast, the change of f H is negligible. (b) At the medium DR region, strain-induced orientation and crystallization are obvious. Abundant oriented crystallites are generated, showing an increasing f H and large Lhkl (17.83−30.14 nm). Meanwhile, the production of gg conformers is restrained due to the ordered structure formed under the moderate extensional stress. (c) As the extensional E
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plotted in Figure 9b. The detailed data are collected in Table S1. As such, the XMAF values are estimated to be 95.1 and 58.8% for as-casted and DR 21.7 samples, respectively, indicating that a large part of the mobile amorphous phase is consumed and immobilized because of crystallization. Correspondingly, Xc increases from 0.4 to 33.1%, being remarkably consistent with the observation in FTIR and WAXD. The elevated Xc also proves that the declination of Lhkl is partly attributed to the nucleation effect of extensional stress. Based on the minor increase of 10.82% form DR 9.8 samples to DR 21.7 samples, it illustrates that the nucleation effect is relatively weak while the local ruptures induced by the mechanical force are dominant. XRAF shows visible fluctuation at high DRs and nearly remains unchanged in small and medium DRs. It is deduced that the stress-induced fragmentation and recrystallization in high DRs could induce the conversion between RAF and MAF, which is not enough to create an impact on Xc. 3.4. Strength and Ductility of Stretched PLA Films. The roles of the gg conformer, oriented amorphous chains, and oriented crystallites in the mechanical properties (including strength and ductility) are evaluated by uniaxial tensile testing at room temperature, 0 and −20 °C. The yield strength and the elongation at break tested at room temperature are listed in Figure 10. As expected, compared to the typical brittle behavior of as-casted samples, all of the stretched films have a strikingly simultaneous enhancement in strength and ductility. The yield strength raises from 45.3 MPa for as-casted PLA films to 69.5, 72.6, 82.7, 95.9, 107.4, 115.2, and 135.5 MPa for the stretched samples with growing DRs (increase of 53.5, 60.2, 82.6, 111.6, 137.0, 154.4, and 199.2%), respectively. It should be emphasized that the extensional stress also results in remarkable ductility compared to as-casted samples, enhancing by 17.4, 11.7, 7.8, 5.7, 4.6, 3.2, and 2.1 times (from 16.9% of elongation at break to 294.9,199.3, 132.1, 96.2, 77.8, 55.0, and 34.9%), respectively. Compared with recent works based on the PLA toughened blends, our work obtains a great balance of strength and ductility (see Figure S4). Therefore, it can be concluded that the balanced and splendid mechanical performance is attained in the stretched films by tailoring the interplay among hierarchical orientation and crystallization. Extensional stress could effectively lead to the formation of oriented amorphous phase and oriented crystallites with increasing Xc, so the yield strength is significantly elevated as DR growing. With regard to the outstanding ductility, the conformational change and crystallization under strong extensional stress are the two essential factors. The high
and recrystallization, which is in great agreement with the findings in WAXD. The damage of the ordered structure impairs its inhibitory effect on gg conformers, which is the reason why the intensities at 1267 cm−1 of DR 16.0 and DR 19.0 samples are nearly the same (see Figure 2). In this region, T2 still holds around 75 °C, hinting that the relaxed amorphous chains have nearly the same degree of ordered structure thereby crystallizing at a similar temperature. It is reasonable to conclude that the fragmentation of the ordered structure starts at DR = 16.0 and becomes obvious at DR 19.0, the recrystallization turns apparent at DR = 21.7. Therefore, the enthalpy at the high DR region increases first and then decreases. The reversible heat flow curves are shown in Figure 9a. The less amplitude and the wider glass transition region in stretched samples are caused by wide relaxation time distribution of the anisotropic structure,15 i.e., the oriented structure. Besides, an exothermic peak around 75 °C arises in highly stretched samples, which is considered to be the mesophase transformation in previous work.39 In our opinion, it may be relevant to the stress-induced fragmentation and recrystallization occurred in samples with high DRs, the evolution of enthalpy agrees well with the observations in Figures 7 and 8. As the onset of fragmentation, the
Figure 7. DSC curves of PLA films with various DRs.
endothermic enthalpy in DR 16.0 samples is relatively low, whereas the largest enthalpy occurs at DR 19.0 samples due to the obvious fragmentation of the oriented structure. Improving DR to 21.7, the presence of recrystallization reduces the enthalpy. Xc, XMAF, and XRAF of samples with different DRs are
Figure 8. (a) TMDSC nonreversible heat flow curves for samples with various DRs. (b) Peak temperatures (T1, T2) of chain relaxation and cold crystallization for samples with various DRs. F
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Figure 9. (a) Reversible heat flow curves for samples with various DRs (b) Xc, XMAF, and XRAF versus DR (detailed data are collected in Table S1).
Figure 10. Tensile properties of samples with different DRs at room temperature.
content of gg conformers could produce conformational change under a low-stress level, which is a benefit to the toughness of PLA.29 Moreover, crystallization facilitates the transformation from the gg conformer into the gt conformer.35 It is reasonable to conclude that the extensional stress not only triggers crystallization, but also affects the chain packing in the amorphous phase, and then getting the promising balance between strength and toughness. Samples with low DRs present superior ductile behavior due to the large proportion of gg conformers and low crystallinity. Upon further drawing, strain-induced orientation and crystallization are getting dominant, restraining the content of gg conformers. As a consequence, the enhancement of ductility is slightly damaged. When the extensional stress is strong enough, the yield strength is sequentially enhanced to the highest value (135.5 MPa) and the flexibility is retained. In this case, stress-induced fragmentation and recrystallization appear at the molecular level and whitening occurs in a large picture. Beyond those mentioned before, the diversity of deformation behaviors is also notable. The pioneering study pointed out that the development of the deformation zone depends on the balance of strain softening and strain hardening.25 For this work, as a result of orientation, strain hardening is becoming more and more significant by improving DRs (see Figure 11).25 This indicates that the stretched chain could augment the contribution of stress from the polymer network,27 involving the promotion of ultimate strength. In addition, it is known that strain hardening is produced by the disorder− order transition during drawing,40 and strongly sensitive to extensional stress regarding both the onset and the slope of this phenomenon. By promoting DR, the onset of strain hardening continuously moves to lower strain, whereas the slope increases. The reason is that the originally oriented amorphous chain in the stretched sample could transform into an ordered structure more easily, so highly oriented samples show strain
Figure 11. Typical stress−strain curves of samples with different DRs at room temperature.
hardening at a lower onset value. The steeper slope is ascribed to higher cohesiveness,40 suggesting that samples with higher DRs could generate a more ordered structure which presents strong molecular interactions during drawing. As a sustainable packing material, temperature-invariant mechanical performance is crucial, especially for the packaging of frozen foods or drugs. However, very few research studies have addressed the ductility in the cold environment, which is far below Tg. In Figure 12, DR 9.8, DR 13.3, and DR 19.0 samples are selected as representatives, exhibiting strong and ductile behavior even at low temperatures. The variation tendency is approximately analogical at 25, 0, and −20 °C as a function of DRs. Ductility increases first and then decreases, together with the improved strength. As for the same sample at different temperatures, along with the reducing temperature, all of the samples present higher yield strength and slightly weakened ductility due to the loss of chain mobility and a more rigid structure. Stretched PLA films G
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Figure 12. Comparison of tensile properties at room temperature, 0 and −20 °C for samples with different DRs.
display a wide temperature range of reinforcement and toughening, not only providing a new method to enhance mechanical properties but also enlightening the production of low-temperature-resistant PLA materials. 3.5. Evolution of Morphology during Deformation in Stretched PLA Films. Materials are susceptible to go through premature failure due to the unstable strain localization (i.e., cracks and microcracks) when exposed to mechanical loading. Therefore, the inspection of deformation behavior and effective retard of cracks are vital for ensuring the mechanical reliability. The morphology of the as-treated samples in different regions are investigated by POM and SAXS (see Figure 13). For as-casted samples, the transitional zone and whitening zone are covered by numerous cracks perpendicular to the stretching direction while the undeformed zone is flat without any crack. These cracks propagate and become denser when close to the whitening part, acting as the omen of brittle fracture. As for the SAXS scattering of as-casted samples, the streak scattering along the equator is displayed in the transitional zone, elucidating that prolonged voids formed perpendicular to the stretching direction, which is also a harbinger of brittle fracture. The relatively weak vertical signals in the whitening region may come from the strain-induced ordered structure. Interestingly, shallow crazes develop through the transitional zone, then shift to 45 degrees direction in flexible DR 9.8 samples. Combined with the streaks in the meridian direction, the ductile behavior of DR 9.8 samples could be verified by the voids oriented along the stretching direction. The increasingly intense streaks originate from the elevating void or crazing concentration from the undrawn zone to the necking zone.41,43 When the crack tip meets the elongated void, it deflects along the direction of lowest fracture toughness leading to fibrillation.27 It is clearly evidenced by the 45 degrees tilt of crazes in the necking region, as shown in Figure 13b3, which is beneficial to prevent the propagation of strain localization and absorb more energy. However, with regard to DR 13.3 samples, only the shear band could be shaped, suggesting that the existence of oriented
Figure 13. POM observations in the undeformed zone (subscript 1), transitional zone (subscript 2), and necking zone (subscript 3) of (a) as-casted, (b) DR 9.8, (c) DR 13.3, and (d) DR 19.0 samples. The corresponding SAXS scattering patterns are displayed in the lower right corner. The stretching direction is horizontal, as the doubleheaded arrow shows.
crystallites hampers deformability. The similar transition from necking to shear banding is observed after aging.44 Because less energy is dissipated, the ductility is confined in DR 13.0 samples. Furthermore, no obvious strain localization emerges in the DR 19.0 samples, corresponding to the very weak strain softening captured in Figure 11.45 It demonstrates that the highly oriented network could stabilize the deformation region and increase the entanglement density, which impairs the strain softening and reinforce the strain hardening. Accordingly, no diffraction signal appears in the SAXS pattern of DR 13.3 and DR 19.0 samples, indicating that homogenous H
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Figure 14. Schematic representation of structural transmutation in samples with various DRs.
compared to the as-casted sample. With regard to the medium and high stress, the coupling effect of the flexible amorphous chain network and robust oriented crystallites is exerted. The strength is reinforced continually, leading to the impressive strength of 135.5 MPa, which displays a prominent increase of 199.2% in contrast to the as-casted samples. It is worth noting that these preeminent properties are mostly maintained even at −20 °C. Therefore, the mechanical performance could be controlled precisely by controlling the hierarchically oriented structure, wherein the function of the amorphous phase is emphasized and quantified. This work offers an economical and efficient approach to formulate superb PLA products in a large scale that could be extensively applied for green packing materials used in the cold environment (for example, the packing bags in express delivery), holding great potential to create a new paradigm for PLA modification. This route is likely to be transferable to other polymers, realizing an industrially interesting method of manufacturing high-performance materials.
deformation is ascribed to the strong oriented structure with high crystallinity. 3.6. Mechanism for Concurrent Reinforcement and Toughening. By imposing extensional stress, oriented amorphous network, and strain-induced crystallization are optimized and balanced, providing an ideal routine to fabricate superior and competitive PLA films with desirable strength and superior ductility. Figure 14 illustrates the structural evolution with the assistance of intense extensional flow. As a whole, we divide the samples into three stages according to different extensional stress, comprising the super-toughening stage, toughening and reinforcement stage, and super-reinforcement stage. Subjected to a relatively weak stress, the conformational change caused by the orientation in the amorphous network emerges. Abundant gg conformers are generated to display the supertoughening effect. As a result of the good mobility of the amorphous network and lack of crystallite, voids easily orient parallel to the drawing direction, preventing the propagation of cracks, the ductility is largely improved. Upon stretching to medium DR, the molecular chains among the entangled points are further tightened, the oriented crystallite arises and acts as the crosslink point to bear the load. As a consequence, the shear band develops after the yielding point and the yield strength is substantially promoted together with the limited improvement of toughness. At a high DR region, a rather strong structure is shaped by the highly oriented crystallites, mobile amorphous chains are strictly restricted owing to this rigid architecture, the samples attain the unexceptionable strength and acceptable toughness.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00932. Digital photo of casting-thermal stretching equipment; digital photo of DR 16.0 samples; the calculation method for the average size of the crystal domain (Lhkl); the calculation method for the orientation parameter (f H); sample treatment in two-dimensional small-angle X-ray scattering (2D-SAXS) and a polarized optical microscope (POM); calculation methods of crystallinity (Xc), mobile amorphous fraction (XMAF), and rigid amorphous fraction (XRAF); the azimuthal profiles of wide-angle X-ray diffraction (WAXD); comparison of elongation at break versus yield strength between conventional toughened PLA blends and our work; the calculation procedure of crystallinity (Xc), mobile amorphous fraction (XMAF), and rigid amorphous fraction (XRAF) (PDF)
4. CONCLUSIONS Through exploiting strong extensional stress, a wide variety of PLA films with simultaneously enhanced strength and toughness have been prepared by tailoring the hierarchical orientation and crystallization. The interplay between oriented amorphous chains and crystallites is investigated according to various stresses. The super-toughening behavior is achieved in the samples with relatively low extensional stress. Due to the formation of gg conformers and the great mobility of amorphous chain, samples exhibit an outstanding improvement of 1638.8 and 53.5% in breaking elongation and yield strength I
DOI: 10.1021/acs.macromol.9b00932 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
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crystalline superstructure: achieving high-performance polylactide. ACS Appl. Mater. Interfaces 2017, 9, 25818−25829. (11) Zhang, Z.-C.; Sang, Z.-H.; Huang, Y.-F.; Ru, J.-F.; Zhong, G.-J.; Ji, X.; Wang, R.; Li, Z.-M. Enhanced heat deflection resistance via shear flow-induced stereocomplex crystallization of polylactide systems. ACS Sustainable Chem. Eng. 2017, 5, 1692−1703. (12) Zhou, S.-Y.; Huang, H.-D.; Ji, X.; Yan, D.-X.; Zhong, G.-J.; Hsiao, B. S.; Li, Z.-M. Super-robust polylactide barrier films by building densely oriented lamellae incorporated with ductile in situ nanofibrils of poly(butylene adipate-co-terephthalate). ACS Appl. Mater. Interfaces 2016, 8, 8096−8109. (13) Xie, X.-L.; Li, Y.; Xu, J.-Z.; Yan, Z.; Zhong, G.-J.; Li, Z.-M. Largely enhanced mechanical performance of poly(butylene succinate) multiple system via shear stress-induced orientation of the hierarchical structure. J. Mater. Chem. A 2018, 6, 13373−13385. (14) Sangroniz, A.; Chaos, A.; Iriarte, M.; del Rio, J.; Sarasua, J.-R.; Etxeberria, A. Influence of the rigid amorphous fraction and crystallinity on polylactide transport properties. Macromolecules 2018, 51, 3923−3931. (15) Delpouve, N.; Stoclet, G.; Saiter, A.; Dargent, E.; Marais, S. Water barrier properties in biaxially drawn poly(lactic acid) films. J. Phys. Chem. B 2012, 116, 4615−4625. (16) Nassar, S. F.; Domenek, S.; Guinault, A.; Stoclet, G.; Delpouve, N.; Sollogoub, C. Structural and Dynamic Heterogeneity in the Amorphous Phase of Poly(l,l-lactide) Confined at the Nanoscale by the Coextrusion Process. Macromolecules 2017, 51, 128−136. (17) Cheng, S.; Johnson, L.; Wang, S.-Q. Crazing and strain localization of polycarbonate glass in creep. Polymer 2013, 54, 3363− 3369. (18) van Melick, H. G. H.; Govaert, L. E.; Meijer, H. E. H. Localisation phenomena in glassy polymers: influence of thermal and mechanical history. Polymer 2003, 44, 3579−3591. (19) Zartman, G. D.; Cheng, S.; Li, X.; Lin, F.; Becker, M. L.; Wang, S.-Q. How melt-stretching affects mechanical behavior of polymer glasses. Macromolecules 2012, 45, 6719−6732. (20) Liu, J. N.; Lin, P. P.; Cheng, S. W.; Wang, W. Y.; Mays, J. W.; Wang, S. Q. Polystyrene glasses under compression: ductile and brittle responses. ACS Macro Lett. 2015, 4, 1072−1076. (21) Li, X. X.; Wang, S. Q. Mapping brittle and ductile behaviors of polymeric glasses under large extension. ACS Macro Lett. 2015, 4, 1110−1113. (22) Hebert, K.; Ediger, M. D. Reversing strain deformation probes mechanisms for enhanced segmental mobility of polymer glasses. Macromolecules 2017, 50, 1016−1026. (23) Lee, H. N.; Ediger, M. D. Mechanical rejuvenation in poly(methyl methacrylate) glasses? molecular mobility after deformation. Macromolecules 2010, 43, 5863−5873. (24) Haward, R. N.; Thackray, G. The use of a mathematical model to describe isothermal stress-strain curves in glassy thermoplastics. Proc. R. Soc. London, Ser. A 1968, 302, 453−472. (25) Govaert, L. E.; van Melick, H. G. H.; Meijer, H. E. H. Temporary toughening of polystyrene through mechanical preconditioning. Polymer 2001, 42, 1271−1274. (26) van Melick, H. G. H.; Govaert, L. E.; Raas, B.; Nauta, W. J.; Meijer, H. E. H. Kinetics of ageing and re-embrittlement of mechanically rejuvenated polystyrene. Polymer 2003, 44, 1171−1179. (27) Huang, Q.; Madsen, J.; Yu, L.; Borger, A.; Johannsen, S. R.; Mortensen, K.; Hassager, O. Highly anisotropic glassy polystyrenes are flexible. ACS Macro Lett. 2018, 7, 1126−1130. (28) Liu, Z.; Li, X.; Zheng, Y.; Wang, S.-Q.; Tsige, M. Chain network: key to the ductile behavior of polymer glasses. Macromolecules 2018, 51, 1666−1673. (29) Huang, T.; Miura, M.; Nobukawa, S.; Yamaguchi, M. Chain packing and its anomalous effect on mechanical toughness for poly(lactic acid). Biomacromolecules 2015, 16, 1660−1666. (30) Xu, P. W.; Lv, P.; Wu, B. G.; Ma, P. M.; Dong, W. F.; Chen, M. Q.; Du, M. L.; Ming, W. H. Smart design of rapid crystallizing and nonleaching antibacterial poly(lactide) nanocomposites by sustainable
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Phone: +86-28-8540-0211. Fax: +86-28-8540-6866 (G.-J.Z.). *E-mail:
[email protected] (Z.-M.L.). ORCID
Jia-Zhuang Xu: 0000-0001-9888-7014 Ling Xu: 0000-0002-1157-0741 Gan-Ji Zhong: 0000-0002-8540-7293 Zhong-Ming Li: 0000-0001-7203-1453 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Key R&D Program of China (2018YFB0704200), the National Natural Science Foundation of China (Grant Nos 51673135, 51822305, 21776183, and 21776186), the Youth Foundation of Science & Technology Department of Sichuan Province (Grant No. 2017JQ0017). Thanks to the Beamline BL16B and BL15U staff at the Shanghai Synchrotron Radiation Facility (SSRF) for supporting the X-ray measurements.
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REFERENCES
(1) Wu, B.; Zeng, Q.; Niu, D.; Yang, W.; Dong, W.; Chen, M.; Ma, P. Design of supertoughened and heat-resistant PLLA/elastomer blends by controlling the distribution of stereocomplex crystallites and the morphology. Macromolecules 2019, 52, 1092−1103. (2) Deng, S.; Bai, H.; Liu, Z.; Zhang, Q.; Fu, Q. Toward supertough and heat-resistant stereocomplex-type polylactide/elastomer blends with impressive melt stability via in situ formation of graft copolymer during one-pot reactive melt blending. Macromolecules 2019, 52, 1718−1730. (3) Spinella, S.; Cai, J.; Samuel, C.; Zhu, J.; McCallum, S. A.; Habibi, Y.; Raquez, J.-M.; Dubois, P.; Gross, R. A. Polylactide/poly(omegahydroxytetradecanoic acid) reactive blending: a green renewable approach to improving polylactide properties. Biomacromolecules 2015, 16, 1818−1826. (4) Zeng, X. Q.; Wu, B. S.; Wu, L. B.; Hu, J. J.; Bu, Z. Y.; Li, B. G. Poly(L-lactic acid)-block-poly(butylene succinate-co-butylene adipate) multiblock copolymers: from synthesis to thermo-mechanical properties. Ind. Eng. Chem. Res. 2014, 53, 3550−3558. (5) Jiang, L.; Wolcott, M. P.; Zhang, J. W. Study of biodegradable polyactide/poly(butylene adipate-co-terephthalate) blends. Biomacromolecules 2006, 7, 199−207. (6) Wang, Y. B.; Hillmyer, M. A. Polyethylene-poly(L-lactide) diblock copolymers: synthesis and compatibilization of poly(Llactide)/polyethylene blends. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2755−2766. (7) Gu, L.; Nessim, E. E.; Li, T.; Macosko, C. W. Toughening poly(lactic acid) with poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) triblock copolymers. Polymer 2018, 156, 261− 269. (8) Xu, H.; Zhong, G.-J.; Fu, Q.; Lei, J.; Jiang, W.; Hsiao, B. S.; Li, Z.-M. Formation of shish-kebabs in injection-molded poly(L-lactic acid) by application of an intense flow field. ACS Appl. Mater. Interfaces 2012, 4, 6773−6783. (9) Zhou, S.-Y.; Niu, B.; Xie, X.-L.; Ji, X.; Zhong, G.-J.; Hsiao, B. S.; Li, Z.-M. Interfacial shish-kebabs lengthened by coupling effect of in situ flexible nanofibrils and intense shear flow: achieving hierarchy to conquer the conflicts between strength and toughness of polylactide. ACS Appl. Mater. Interfaces 2017, 9, 10148−10159. (10) Li, C.; Jiang, T.; Wang, J.; Wu, H.; Guo, S.; Zhang, X.; Li, J.; Shen, J.; Chen, R.; Xiong, Y. In situ formation of microfibrillar J
DOI: 10.1021/acs.macromol.9b00932 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules aminolysis grafting and in situ interfacial stereocomplexation. ACS Sustainable Chem. Eng. 2018, 6, 13367−13377. (31) Tabatabaei, S.; Carreau, P.; Ajji, A. Microporous membranes obtained from polypropylene blend films by stretching. J. Membr. Sci. 2008, 325, 772−782. (32) Zhang, T.; Hu, J.; Duan, Y.; Pi, F.; Zhang, J. Physical aging enhanced mesomorphic structure in melt-quenched poly(L-lactic acid). J. Phys. Chem. B 2011, 115, 13835−13841. (33) Lan, Q.; Li, Y.; Chi, H. Highly enhanced mesophase formation in glassy poly(L-lactide) at low temperatures by low-pressure CO2 that provides moderately increased molecular mobility. Macromolecules 2016, 49, 2262−2271. (34) Kister, G.; Cassanas, G.; Vert, M. Effects of morphology, conformation and configuration on the IR and Raman spectra of various poly(lactic acid)s. Polymer 1998, 39, 267−273. (35) Pan, P.; Zhu, B.; Dong, T.; Yazawa, K.; Shimizu, T.; Tansho, M.; Inoue, Y. Conformational and microstructural characteristics of poly(L-lactide) during glass transition and physical aging. J. Chem. Phys. 2008, 129, No. 184902. (36) Zhang, J. M.; Duan, Y. X.; Sato, H.; Tsuji, H.; Noda, I.; Yan, S.; Ozaki, Y. Crystal modifications and thermal behavior of poly(L-lactic acid) revealed by infrared spectroscopy. Macromolecules 2005, 38, 8012−8021. (37) Jiang, Z.; Tang, Y.; Men, Y.; Enderle, H.-F.; Lilge, D.; Roth, S. V.; Gehrke, R.; Rieger, J. Structural evolution of tensile-deformed high-density polyethylene during annealing: scanning synchrotron small-angle X-ray scattering study. Macromolecules 2007, 40, 7263− 7269. (38) Men, Y. F.; Rieger, J.; Strobl, G. Role of the entangled amorphous network in tensile deformation of semicrystalline polymers. Phys. Rev. Lett 2003, 91, No. 095502. (39) Stoclet, G.; Seguela, R.; Lefebvre, J. M.; Rochas, C. New insights on the strain-induced mesophase of poly(D,L-lactide): in situ WAXS and DSC study of the thermo-mechanical stability. Macromolecules 2010, 43, 7228−7237. (40) Stoclet, G.; Seguela, R.; Lefebvre, J. M.; Elkoun, S.; Vanmansart, C. Strain-induced molecular ordering in polylactide upon uniaxial stretching. Macromolecules 2010, 43, 1488−1498. (41) Zhou, C.; Li, H.; Zhang, Y.; Xue, F.; Huang, S.; Wen, H.; Li, J.; de Claville Christiansen, J.; Yu, D.; Wu, Z.; Jiang, S. Deformation and structure evolution of glassy poly(lactic acid) below the glass transition temperature. CrystEngComm 2015, 17, 5651−5663. (42) Wang, Y.; Li, M.; Wang, K.; Shao, C.; Li, Q.; Shen, C. Unusual structural evolution of poly(lactic acid) upon annealing in the presence of an initially oriented mesophase. Soft Matter 2014, 10, 1512−1518. (43) Sun, Y.; Yang, L.; Lu, X.; He, C. Biodegradable and renewable poly(lactide)-lignin composites: synthesis, interface and toughening mechanism. J. Mater. Chem. A 2015, 3, 3699−3709. (44) Zhang, R.; Bai, P.; Dong, L.; Rui, X. Aging-dependent strain localization in amorphous glassy polymers: from necking to shear banding. Int. J. Solids Struct. 2018, 146, 203−213. (45) Govaert, L. E.; Timmermans, P. H. M.; Brekelmans, W. A. M. The influence of intrinsic strain softening on strain localization in polycarbonate: modeling and experimental validation. J. Eng. Mater. Technol. 2000, 122, 177−185.
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