Gradient Structure of Crystalline Morphology in Injection-Molded

Article pubs.acs.org/IECR

Gradient Structure of Crystalline Morphology in Injection-Molded Polylactide Parts Tuned by Oscillation Shear Flow and Its Influence on Thermomechanical Performance Zi-Hong Sang, Xu-Long Xie, Sheng-Yang Zhou, Yue Li, Zheng Yan, Ling Xu, Gan-Ji Zhong,* and Zhong-Ming Li* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China S Supporting Information *

ABSTRACT: The inhomogeneity of the cooling rate and shear rate during polymer processing such as injection molding usually leads to the harvest of a layered structure, which is often difficult to tune. By introducing oscillation shear flow during the packing stage of an injection molding cycle, special injection-molded polylactide (PLA) parts with different thicknesses and crystallinity of skin layers were fabricated via controlling shear durations and oscillation frequencies, as revealed by microbeam wide-angle X-ray diffraction results. For the part of the 2000 μm thick layer with 50% crystallinity, the heat distortion temperature and Vicat softening temperature reach 96.6 and 159.3 °C, respectively. Moreover, the Young’s modulus rises remarkably with the increase of thickness and crystallinity. Significantly, the maximal shear rate appeared 850 μm from the surface, about 1068 s−1 (up to 103 s−1), which plays a crucial role for the formation of oriented crystalline morphology.

1. INTRODUCTION Polylactide (PLA), an aliphatic thermoplastic polymer derived from renewable resources having extraordinary strength and modulus, impressive biocompatibility, and favorable transparency, holds tremendous potential to replace traditional petroleum-based and nonbiodegradable polymers.1−5 Large scale availability of PLA at a reasonable price has opened up more common application areas such as one-dimensional (1D) fibers, two-dimensional (2D) films, and three-dimensional (3D) parts in recent years.6−8 In addition to the recent popular 3D printing technology, on considering that PLA cartridges are the most commercialized and sold.9 Because of high precision, high efficiency, and low cost, injection molding is the first choice for batch manufacturing three-dimensional PLA parts with complex shapes and varying size such as tableware, cosmetics, and outdoor products.10−12 Nevertheless, owing to © 2017 American Chemical Society

slow crystallization dynamics and semirigid chain backbone, injection molded PLA parts are normally amorphous as a result of rather high cooling rate and relatively low mold temperature, thus inevitably giving rise to low heat resistance, often expressed by the heat deflection temperature (HDT). Largescale applications of PLA are significantly hindered especially in areas that ask for high levels of mechanical strength and heat resistance.13 Naturally, crystallization mediation mainly including kinetics and the crystallinity provides the most practical and simplest approach for enhancing the thermomechanical performance of Received: Revised: Accepted: Published: 6295

March 5, 2017 April 29, 2017 May 9, 2017 May 9, 2017 DOI: 10.1021/acs.iecr.7b00930 Ind. Eng. Chem. Res. 2017, 56, 6295−6306

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Industrial & Engineering Chemistry Research PLA.7,14−16 For the purpose of increasing the crystallinity of PLA parts, a variety of tactics have been implemented. In general, increasing the optical purity of the monomer contributes to the increase in the crystallization rate due to the regular molecular chain structure.17,18 Also, a chemical method for stereocomplexation of PLA attracts much attention from researchers. Some focus on the direct synthesis of multiblock PLA of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA),19−21 while others employ nanostereocomplexation such as graphene oxide (GO) nanosheet-grafted PDLA,22 carbon nanotube (CNT) grafted PLLA−PDLA,23 nanoclaygrafted PLLA,24 cellulose nanocrystal grafted PLLA,25 etc. Despite the chemical method having its own advantages, it seems that an effective strategy to directly manipulate melt processing or optimize parts in the postprocessing is highly desirable allowing for economy and versatility, and some chemical methods may take much time to scale up based on our knowledge. A few effective strategies during (or after) melt processing have been proposed to improve the crystallinity of PLA. One is adding nucleating agent into the polymer. Some nucleating agents are evidenced to be very effective, possessing extremely large specific surface area as one of the vital factors to ensure their superior nucleation ability.26 In melt processing of PLA, the most effective nucleating agents mainly include talc and stereocomplex. By adding 1 wt% talc, a more than 20-fold reduction of crystallization half-time was achieved.27 Stereocomplex showed a superior nucleating efficiency to that of talc, but the overall crystallinity of PLA was reduced.28 Some of our previous work also shows that the presence of nucleating agents exhibited an active effect on crystallization kinetics by providing nucleating sites for initializing the crystallization process.29,30 Nevertheless, in fact, due to high cooling rates and high production efficiency in the melt processing, the important role of these effective nucleating agents has not given full play. The PLA parts obtained in conventional injection molding are still amorphous. As such, many studies attempt to increase the mold temperature for longer cooling time in injection molding,31 which undoubtedly increases power consumption and cost. On the other hand, thermal annealing of PLA parts is an effective and easy method to modify the crystallinity, crystal forms, and crystal size.32 The dramatically increased HDT of PLLA/ EGMA (poly(ethylene-glycidyl methacrylate)) binary system after thermal annealing is founded in Oyama’s work;33 a similar result is also manifested in thePLLA/TPU (thermoplastic polyurethane)/PDLA ternary system.34 And yet, the deformation of injection-molded PLA parts because of gravity effect and uneven shrinkage during the thermal annealing process is still an inevitable problem. Besides, its low efficiency and high cost are likely factored into the decision. Another important finding uncovers that shear flow can promote the formation of oriented morphology and enhance the nucleation and crystallinity of PLA.35,36 Following this principle, we utilized a modified injection molding technologyoscillation shear injection molding (OSIM)to impose a strong shear on the melt during actual melt processing,.32,37−39 Specifically, OSIM processing can provide intense oscillation shear flow on melt in the mold cavity during solidification by the reciprocating motion of two pistons at the same frequency. We verified that the crystallinity of PLA parts by OSIM is remarkably improved;39,40 also, we also have successfully prepared PLA parts with shish-kebab superstructure,40 in which the shish-kebab superstructure largely

appears in the skin layer, while almost amorphous PLA exists in the core layer. The perfect superstructure contributes to significant improvement of the tensile strength and impact toughness and, meanwhile, a slight enhancement of the thermomechanical performance. However, it is known that the presence of a gradient of temperature and shear rate along the thickness direction of injection molded parts usually leads to the harvest of layered structure during conventional injection molding for semicrystalline polymers. Few investigations have shown the evolution of a special layered crystal structure of injection-molded PLA; thus, the existence of a quantitative correlation between the thickness of the skin layer and the thermomechanical performance of an injection-molded PLA part is not yet uncovered. One of the major reasons is the absence of an efficient way to control shear layer thickness within a wide range. Herein, combining our previous advances in application of OSIM technology and crystal structure characterization, we attempt to qualitatively reveal the relationship between internal crystal structure (thickness and averaged crystallinity of skin layer) and the thermomechanical performance of injectionmolded PLA parts. We carefully selected a PLA with high optical purity (∼99.8%) for the sake of the control of crystallinity and gradient crystal structure within a wide range. First, we design a series of injection-molded PLA parts with different skin layers by tailoring the shear duration and oscillation frequency via OSIM technology. To reveal different structures of the skin layer, the inner structures of the OSIM samples were thoroughly characterized by scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), etc. Then, we also test the thermomechanical performance of the parts and get valid evidence to demonstrate the qualitative relationship between different skin layers and the thermomechanical performance of injection molded PLA parts for the first time.

2. EXPERIMENTAL SECTION 2.1. Materials. The raw PLA pellets (Grade L130), a semicrystalline grade PLA with 99.8% optical purity of L-lactic acid, was kindly supplied by Corbion Co. Ltd. (Amsterdam, The Netherlands). Its weight-averaged molecular weight and polydispersity index are 1.73 × 105 g/mol and 1.94, respectively, according to gel permeation chromatography (GPC) measurements. Sodium hydroxide (NaOH AR grade) and methanol (CH3OH AR grade) were purchased from Chengdu Kelong Chemical Reagent Factory (China). 2.2. Molding of Injection-Molded PLA Parts. The raw PLA pellets after being dried at 80 °C overnight in a vacuum oven were injection molded into bars through OSIM processing. Detailed information on the OSIM machine is available in our previous work (Figure S1). The barrel temperature profiles were set at 170, 180, 190, 180, and 175 °C from hopper to nozzle, respectively. Through early exploration, we observed the gradient crystal structure of PLA along the thickness direction of injection-molded parts which can be obtained especially in a suitable mold temperature (∼80 °C); thereby the 80 °C mold temperature is fixed in this research. For the sake of clarity, samples will be denoted as PLA-X with X being the shear duration (s), and the oscillation frequency is 0.5 Hz. Specifically, PLA-X@hf stands for imposing a high shear oscillation frequency (2 Hz) different from PLA-X with 0.5 Hz. Static PLA (denoted as PLA-S) was injection molded into bars under the same conditions for the 6296

DOI: 10.1021/acs.iecr.7b00930 Ind. Eng. Chem. Res. 2017, 56, 6295−6306

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Industrial & Engineering Chemistry Research sake of comparison. In order to clearly describe the experimental process, we specifically offer the OSIM operation of the digital pictures (Figure S2) and schematic illustration for the two pistons motion (Figure S3.). During the packing stage of an injection molding cycle, the two pistons move reciprocally at the same frequency. Before solidification of the gate, for example, PLA-6 represents the reciprocal movement of two pistons only for one time, as shown in Figure S3 (n = 1, n is defined as the number of repeated movements of pistons). The two pistons move continuously from the top to the bottom (about 7 cm) for 3 s, and then from the bottom to the top for 3 s, so the shear duration is for 6 s in this process. In this case, the shear duration is defined as 6 s. The oscillation frequency between the two pistons’ movement is 0.5 or 2 Hz, which is mainly manipulated by changing the interval of the reciprocating pistons motion. That is, a long interval for 2 s corresponds to a high oscillation frequency for 0.5 Hz, while a short interval for 0.5 s corresponds to a high oscillation frequency for 2 Hz. By analogy, PLA-18 (n = 3, 2 s), PLA-30 (n = 5, 2 s) and PLA-90 (n = 15, 2 s) respectively stand for the shear duration of 18, 30, and 90 s with same oscillation frequency (0.5 Hz), while PLA-120@hf (n = 20, 0.5 s) stands for the shear duration of 120 s with a higher shear oscillation frequency (2 Hz) than that of other samples (0.5 Hz). Finally, worth noting was that we quantitatively calculated the shear rate of samples during solidification along the thickness direction, finding that the shear rate is about 353 s−1 (up to 102 s−1) on the surface of the samples and then rapidly increases with an increase of the thickness of solidified layer until the gate is frozen. The maximum shear rate reaches 1068 s−1 (up to 103 s−1) when the thickness of the solidified layer increases to 850 μm. After the gate is solidified, the shear rate plunges to zero in the core region. The detailed schematic illustrations are shown in Figures S4 and S5. 2.3. Microbeam Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD). Two-dimensional WAXD measurements were performed to determine molecular orientation, crystal structure, and their distribution along the thickness of injection-molded PLA bars at the beamline BL15U1 of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China). The monochromated X-ray beam (a wavelength of 0.124 nm) was focused to an area of 3 × 2.7 μm2 (length × width), with the sample-to-detector distance of 165.5 mm, while the 2D-WAXD images were collected with an X-ray CCD detector (Model SX165, Rayonix Co. Ltd., USA). The sample with a dimension of 8 × 4 × 1 mm3 (length × width × thickness) was carefully machined from the standard impact bar. The incident direction of the X-ray beam was normal to the MD−ND plane and the sample was scanned with the X-ray beam from the skin to the core of the sample down from the MD−ND surface with a step of 50 μm as shown in Figure 1. Moreover, the 1D-WAXD intensity profiles for each q were obtained by integration in the azimuthal angular range of a whole circle (0−360°) from the WAXD patterns employing the Polar package after subtracting background scattering. Then we used the deconvoluting-peak technique to calculate the degree of crystallinity using the method in our previous work.32,41,42 By a similar data analysis, azimuthal scans (0−360°) of 2D-WAXD were made for the (110) planes of PLA in order to obtain the degree of orientation along the flow direction. Hermans’ orientation functions were calculated according to Picken’s method.43−45

Figure 1. Schematic of sampling method for X-ray diffraction and scattering measurements. MD, molding direction; TD, transverse direction; ND, direction normal to the MD−TD plane.

2.4. Scanning Electronic Microscopy (SEM). To identify crystalline morphology in injection-molded PLA bars, we selectively etched the amorphous PLA samples with a solution mixture compounded of water−methanol (1:1 by volume) and 0.05 mol/L sodium hydroxide for 24 h at 25 °C. Prior to SEM observations, all treated surfaces were cleaned by distilled water and ultrasonication for 30 min. A field-emission SEM (Nova Nano, FEI, USA) operating at an accelerating voltage of 5 kV and working distance of 5 mm was utilized to observe the crystalline morphology of the etched samples, which were sputter-coated with gold before SEM imaging. 2.5. Two-Dimensional Small-Angle X-ray Scattering (2D-SAXS). To further examine the crystalline morphology in injection-molded PLA parts, sample preparation was the same as that for WAXD shown in Figure 1. Then 2D-SAXS measurements were performed at the beamline BL16B1 of SSRF, Shanghai, China (the monochromated X-ray beam with a wavelength of 0.124 nm), with a sample-to-detector distance of 2140 mm. The SAXS images were collected with an X-ray CCD detector (Mar165, a resolution of 2048 × 2048 pixels). The intensities I(q) (q = 4π sin θ/λ) were obtained through integration in the azimuthal angular range of the whole circle of the diffraction pattern, where 2θ is the scattering angle and λ represents the X-ray wavelength (0.124 nm). 2.6. Thermal Deflection Performance Test. Two tests were used for the thermal deflection performance: (1) heat distortion temperature (HDT); (2) Vicat softening temperature (VST). 1. The HDT was estimated by a HDT tester HDV2 (ATLAS, USA) at edgewise mode under a load of 0.45 MPa at a heating rate of 120 °C/h. HDT is the temperature at which the specimen deflection increases to 0.32 mm during the heating process. The dimensions of rectangular specimens for the HDT measurement are 80 × 10 × 4 mm3 (length × width × thickness) (ISO 75). 2. The VST test was carried out with injection-molded PLA bars according to ISO 306. The heating rate during tests was 120 °C/h, and the area of the indenter was 1 mm2 applying a load of 1 MPa. A minimum of three bars for each sample were 6297


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obvious gradient structure especially along the thickness direction appears under oscillation shear flow, as shown in Figure 2b−f. The region showing crystal diffraction signals markedly enlarges with the increase of shear duration. Figure 3

tested at the same conditions, and the averaged values were presented with standard deviations. 2.7. Tensile Property Test. According to ASTM standard D638, the tensile properties were measured at room temperature on an Instron universal test instrument (Model 5576, Instron Instruments, USA) with a crosshead speed of 20 mm/ min and a gauge length of 25 mm. A minimum of six bars for each sample were tested at the same conditions, and the averaged values were reported with standard deviations. 2.8. Dynamic Mechanical Analysis (DMA). To obtain thermomechanical properties and distinct insights into the thermal behavior of injection-molded PLA part, a cubic specimen for DMA measurement with a size of 60 × 10 × 4 mm3 (length × width × thickness) was obtained. The dynamic mechanical properties were measured on a DMA Q800 (TA Instruments, USA) in a multifrequency strain mode with a dual cantilever clamp (ASTM standard D4065). The apparatus was operated with a frequency of 1 Hz, over the temperature range 20−120 °C at a heating rate of 3 °C/min. A minimum of three bars for each sample were tested at the same conditions.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Molecular Orientation of Injection-Molded PLA Parts with Layered Structure. Microbeam WAXD is first used to identify whether the expected layered structure is achieved in different regions of injection-molded PLA parts, as shown in Figure 2. For PLA-S sample (Figure 2a), the crystal diffraction signal is overall weak at the mold temperature of about 80 °C because of its low crystallization rate at this temperature. In sharp contrast, an

Figure 3. WAXD intensity profiles in injection-molded PLA part with different shear durations and oscillation frequencies: (a) PLA-S, (b) PLA-6, (c) PLA-18, (d) PLA-30, (e) PLA-90, and (f) PLA-120@hf, corresponding to the 2D-WAXD patterns. Notably, the inserted numbers 50, 400, 800, 1200, 1600, and 2000 respectively stand for the distance from the surface of the part (μm).

shows one-dimensional WAXD intensity profiles which clearly display the main reflection peaks at q = 11.73 and 13.10 nm−1, corresponding to (110)/(200) and (203) for all samples indicative of characteristics of α-crystals.46 We intuitively see that the diffraction intensity significantly increases for all sheared samples in the skin layer with the increase of shear duration (Figure 3b−f). Moreover, the diffraction becomes relatively stronger along the thickness direction compared with other samples for high oscillation frequency (Figure 3f). It should also be noted here that there is no change about crystal form (a more ordered crystal structure form (α-form)) of PLA along the thickness direction under different shear durations or oscillation frequencies, mainly because α′-form crystal with less compact chain packing mode and chain conformation usually crystallizes under relatively low temperature (no higher than 120 °C) due to slow crystallization kinetics of PLA,36 while shear induced crystallization occurs at higher temperature as observed in our previous work.30 As a consequence, only typical α-crystals are generated in all samples. Moreover, the molecular orientation appears at the position of 400−800 μm from the skin at a low shear duration (Figure 2c, PLA-6; or Figure 2d, PLA-18).32,41 Further evidence will be given in section 3.2. Interestingly, as the shear duration further increases (Figure 2e, PLA-90; or Figure 2f, PLA-120@hf) the molecular orientation is inconspicuous. To the best of our knowledge, an effective method to get injection-molded PLA samples with gradient structure of the skin layer has not been well-established yet;

Figure 2. Typically selected WAXD patterns along the thickness direction of injection-molded PLA part with different shear durations and oscillation frequencies: (a) PLA-S, (b) PLA-6, (c) PLA-18, (d) PLA-30, (e) PLA-90, and (f) PLA-120@hf. Notably, “@hf” shows that high shear oscillation frequency (2 Hz) is imposed in injection molding. The inserted numbers 50 μm, 400 μm, 800 μm, 1200 μm, 1600 μm, and 2000 μm stand for the distance from the surface of the part. 6298


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In addition, a sharp step change in the HDT is noticed when the crystallinity is 25%, indicating a threshold for crystallinity content.50 If we consider the position where the crystallinity can exceed 25% belongs to the skin layer, then the relationship between thickness, averaged crystallinity of the skin layer, and different shear durations or oscillation frequencies is summarized in Figure 5 from the above WAXD result. For a

however, in this work, we have successfully gained different skin layers in injection-molded PLA parts by flexibly manipulating the shear duration and oscillation frequency during injection molding for the first time. The influence of shear duration and oscillation frequency on the crystallinity and its distribution of injection-molded PLA parts is illustrated in Figure 4. It is generally accepted that an

Figure 4. Crystallinity and its distribution along the thickness direction of injection-molded PLA part with different shear durations and oscillation frequencies.

Figure 5. Thickness and averaged crystallinity of skin layer in injection-molded PLA part with different shear durations and oscillation frequencies. The error bars stand for the fluctuations of the crystallinity within the skin layer.

intense flow can dramatically enhance the crystallinity compared to static PLA on account of the remarkably enhanced oriented chain segments along the flow direction (so-called precursor). We can clearly see that, in the position of about 50 μm from the surface, the crystallinity of sheared PLA reaches the maximum value of ∼45% with the increase of shear durations, which is about 7-fold as high as the static PLA one (∼6%). For neat PLA, it is still a huge challenge to realize the significant improvement of crystallinity for injection-molded parts without postprocessing such as thermal annealing.47,48 Moreover, because the PLA melt experiences stronger shear force and longer duration in the middle layer (about 400−1200 μm from the surface) than that in a position closer to the surface before solidification, the crystallinity of sheared PLA further increases, even above 50%. Generally, the crystallinity of PLA with low optical purity has proved to be low, no more than 30%.41 These results are interesting compared to polyolefins such as polyethylene and polypropylene which are frequently investigated in shear induced crystallization. For polyolefins, only the crystallization kinetics can be observably accelerated under shear flow, while the crystallinity is difficultly increased.41,49 Whereas, for the sheared PLA sample in our work, largely promoted crystallinity is achieved, which indicates that such PLA with high optical purity is more sensitive to shear flow. Besides, the increase of crystallinity can be reasonably attributed to the formation of highly oriented crystals proven in section 3.2. Toward the core (about 1200−2000 μm from the surface), the crystallinities of both static PLA and sheared ones decline for a low shear duration remarkably. Interestingly, though, for a higher shear duration or higher oscillation frequency, the averaged crystallinity maintains a relatively high level, about 40% for PLA-15 or 50% for PLA-120@hf in overall 3D parts along the thickness direction. Above all, the result of crystallinity and its distribution of injection-molded PLA parts definitely show that the different skin layers can be quantitatively manipulated under shear flow field during injection molding.

low shear duration of 6 s (PLA-6), the thickness and averaged crystallinity of the skin layer respectively increase to 400 μm and 31% from 0 μm and 9% of the static PLA. For a moderate shear duration of 18 or 30 s, the thickness and averaged crystallinity continue to increase to 1200 μm and 36%. When a high shear duration of 90 or 120 s is imposed on PLA melt, the thickness reaches to 2000 μm; that is, the whole injectionmolded PLA part is a crystal structure in this circumstance. But worth noting is that the averaged crystallinity continues to increase to 50% for a high oscillation frequency of 2 Hz (PLA120@hf), which will undoubtedly dramatically increase the thermomechanical properties of PLA parts in our following result. To further uncover the gradient of molecular orientation in injection-molded PLA parts, the orientation parameter ( f H) is shown in Figure 6. The f H of the static PLA (Figure 6a) holds almost the same low level of about 0.3 along the thickness direction because of the absence of highly oriented crystals. Also, the f H in the position close to or far from the surface (viz., 50, 1600, 2000 μm) for all sheared samples is as low as the static PLA showing formation of weakly oriented or isotropic crystals on account that short duration or long relaxation time is difficult to make PLA chains stretch and orient to some critical extent. However, the f H of samples with different shear durations varies along the thickness direction, showing a higher value in intermediate layer (400−1200 μm from the surface) compared to that in other positions. Notably, at a low shear duration (Figure 6, PLA-6, PLA-18), the f H has an almost identically high level of ∼0.87 in the layer about 400−800 μm from the surface, which confirms the formation of highly oriented crystals induced by the intense oscillation shear. Yet, the f H sharply decreases at a high shear duration (in the case of a high mold temperature about 80 °C), which may be due to the shear heating generated by long shear durations, finally 6299


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Figure 7. Typical SEM micrographs of injection-molded PLA parts with different shear durations and oscillation frequencies, corresponding to the position of 50 μm from surface (corresponding to the position “1” in Figure 4). Samples (a) PLA-S, (b) PLA-6, (c) PLA-18, (d) PLA-30, (e) PLA-90, and (f) PLA-120@hf. The scale bar is 5 μm.

the WAXD result. It is mainly because PLA with a semirigid chain backbone and low molecular weight45,46 is hard to form highly oriented crystals under a weak shear flow field; meanwhile, considering crystallization kinetics, the rapid cooling in the position close to the surface does not seem conducive to stretching of molecular chain. Therefore, pointlike nuclei tend to form and afterward abundant spherulites are achieved in these zones (Figure 7b−f) compared with static PLA (Figure 7a). Though, for polyolefins (e.g, polyethylene) with flexible long chain structures and molecular weight polydispersity, the long chains tend to entangle in the melt, which are easy to deform and orient under flow field to form “shish”, and free short chain segments in network fold to crystallize into “kebabs”.37 Moreover, we also observe these spherulites with a narrow diameter distribution ranging from 3 to 7 μm, which indicates shear flow induced crystallization via enhancing crystal nucleation of PLA, while the crystallization time is not enough for a big size spherulite considering a short molding cycle. In Figure 8, at the position about 400 μm from the surface, the static PLA has a few spherulites, but the oriented lamellae are observed in sheared samples, especially for a low shear duration (Figure 8b−d). We have a general idea of only some lamellae growing perpendicularly to the flow direction, which is similar to the shish-kebab superstructure, but there is no obvious shish structure. On one hand, the shear flow in this region is possibly not strong enough to form visible shishkebabs with large size because intense shear flow and sufficient duration are necessary to realize the transition of crystal structure. On the other hand, shish structure is difficult to be observed via SEM. In clear contrast, distinctive crystalline morphology is also observed at the position about 800 μm from the surface where the shear rate reaches almost the maximum of about 103 s−1 (the highest f H in along the thickness direction from WAXD

Figure 6. (a) Orientation parameter ( f H) of samples along the direction perpendicular to flow direction fitted from the intensity of a (110) reflection along the azimuthal angle from the inset figure (b) PLA-S, (c) PLA-6, (d) PLA-18, (e) PLA-30, (f) PLA-90, and (g) PLA120@hf.

accelerating the relaxation of highly oriented crystals, from closely packed lamellae to distorted spherulites (as proved in section 3.2). In fact, the orientation and chain relaxation of molecular chains exist simultaneously and competitively under intense shear flow. 3.2. Crystalline Morphology in Injection-Molded PLA Parts. To gain more evidence for the formation of highly oriented crystals, high-resolution SEM observation is performed to obtain the crystalline morphology of different positions (along the thickness direction, respectively corresponding to the position from “1” to “4” in Figure 4) with different shear durations in the etched 3D samples, as clearly shown in Figures 7−10. In the position of about 50 μm from the surface, as shown in Figure 7, sporadically isotropic spherulite is observed for static PLA, while a large amount of spherulites is expected under intense shear field; moreover, the size and number of spherulites increase a little with the increase of shear durations, which is in line with the crystallinity from 6300


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observed in other orientation-induced semicrystalline polymers such as polyethylene and polypropylene.8,51 In accordance with the result in Figure 5, the degree of orientation is high for low shear duration in this region. When a high oscillation frequency is applied on PLA melt such as PLA-120@hf (Figure 9f), distorted spherulites are observed. In the core (the position about 2000 μm from the surface; Figure 10), sporadic


result), as shown in Figure 9. In this region, spherulites are expected to be observed for the static PLA (Figure 9a); however, it is particularly gratifying that an assembly of closely spaced lamellae grows perpendicularly to the flow direction, especially for low shear duration samples such as PLA-6 or PLA-18 (Figure 9b,c). This architecture is quite similar to the representative feature of shish-kebab superstructure commonly



spherulites are observed in all PLA samples except for PLA90 and PLA-120@hf (that is, high shear duration or oscillation frequency), in which large size and uniform spherulites (diameter of ∼7 μm) turn up. This is ascribable to the fact of weak or no shear force in the core for low shear duration, showing a similar crystalline morphology to static PLA in the position of about 50 μm from the surface. But multiple shear duration or high oscillation frequency leads to shear heating, and finally highly oriented crystals tend to transform from closely packed lamellae to distorted spherulites, in accordance with the trend of orientation parameters from WAXD results. To further identify the existence of the highly oriented shishkebab superstructure in the shear layer, typical 2D-SAXS patterns at the position of 800 μm (the highest f H along the thickness direction from the WAXD result in Figure 6) from the surface are depicted in Figure 11. For static PLA sample, the SAXS patterns show no scattering reflection as depicted in Figure 11a, showing random lamellae and low crystallinity, corresponding to isotropic spherulite in Figure 9a. However, typical scattering patterns with two sharp triangular streaks in the equatorial direction and two bulb-shaped lobes in the meridional direction are observed in all sheared samples (Figure 11b−f), which are regarded as an evidence of shishkebab superstructure.41,52,53 However, we easily discover a representative scattering signal in the equatorial direction also observed in distorted spherulites. This unexpected signal of spherulites (PLA-120@hf) may be originated from the 6301


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Figure 11. Representative 2D-SAXS patterns and 1D-SAXS intensity profiles of injection-molded PLA parts with different shear durations and oscillation frequencies at the position of 800 μm from surface (corresponding to the position “3” in Figure 4). Samples (a) PLA-S, (b) PLA-6, (c) PLA-18, (d) PLA-30, (e) PLA-90, and (f) PLA-120@hf.

Figure 12. Correlation of heat resistance properties of injection-molded PLA parts and thickness and averaged crystallinity of skin layer: (a) heat deflection temperature at 0.45 MPa and (b) Vicat softening temperature. The inset tables are directly derived from the test results. Notably, “ X@Y” stands for thickness and averaged crystallinity of the sample corresponding to the WAXD result.

influence of cavitation under intense shear during processing. The 1D-SAXS intensity profiles through circular integration of intensities from corresponding 2D-SAXS patterns are inserted in Figure 11 in order to quantitatively investigate the lamellar structure in shish-kebab superstructure. The Bragg equation, L = 2π/q*, is used to calculate the long period between the adjacent lamellae; here L is the long period and q* represents the peak position in the scattering curves. The intensities of sheared PLA parts appear to have the same maximum around q = 0.3 nm−1 in the curves. Furthermore, the calculated result shows that the long period (L) of all sheared PLA parts is about 20.9 nm, suggesting a regular aligned lamellar structure along the flow direction. Moreover, it is noteworthy that the shear duration and oscillation frequency have little effect on the value of the long period.

In summary, the SEM and SAXS results have consistently shown that the static PLA parts have few crystals along the thickness direction on account of the high cooling rate. What is more, highly oriented shish-kebab superstructure tends to develop in the middle layer with a stronger shear flow than that in the position close to or far from the surface (viz., 50, 1600, 2000 μm). More interestingly, according to the SEM results, the highly oriented nuclei are easier to keep for a lower shear duration (Figure 9b−d), while there is no obvious shish-kebab superstructure in the middle layer (Figure 9f) of injectionmolded PLA part at high shear duration and oscillation frequency (PLA-120@hf), presumably due to the shear heating generated and then inducing relaxation of oriented crystals. But an in situ technique is still required to monitor changes of 6302


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clearly shown in Figure 12, which plays an enormously important role in deciding the HDT of injection-molded PLA parts. Before the thickness reaches 1200 μm, the heat resistance shows an upward tendency with increase of the thickness of the skin layer, while above that the heat deflection resistance changes very little. Meanwhile, the improvement of crystallinity of parts contributes to improving the heat resistance,56 especially when thicknesses of the skin layer can reach the critical value for these 3D parts. Therefore, this work is valuable in understanding the quantitative relation between thermal resistance properties and the thickness and averaged crystallinity of the skin layer. The mechanical performance, particularly modulus and strength, is of vital importance to practical application. Figure 13a describes typical stress−strain curves of injection-molded PLA parts with thickness and averaged crystallinity of the skin layer. The sheared samples show a similar brittle fracture to that of the static PLA one with very low elongation at break and no obvious yielding. Note that, for the parts of PLA-6 and PLA-18, the elongation at break and tensile strength increase slightly compared with the static one, which may be due to a favorable orientation of the molecular chain, confirmed via SEM observation (shish) and WAXD results (Figure 2b,c), while lamellar orientation may make little difference. That is, the shish structure serves as a dulcet reinforcement in injectionmolded PLA parts. Compared with static PLA, moreover, the sheared samples reveal higher modulus. For clarity, Figure 13b summarizes the relationship between detailed tensile properties (Young’s modulus, tensile strength, elongation at break) with thickness and averaged crystallinity of the skin layer. The Young’s modulus gradually increases from approximately 1978 MPa for PLA-S (0 μm@9%) to 2517 MPa for PLA-120@hf (2000 μm@50%) mainly because of the increase of thickness and averaged crystallinity of the skin layer. Through further comparison, such as PLA-90 and PLA-120@hf with almost the same thickness of 2000 μm, while the modulus of PLA-120@hf (50% crystallinity) is higher than that of PLA-90 (50% crystallinity), showing crystallinity is also a critical impact factor for a material’s modulus. The tensile strength and elongation at break are intuitively shown in Figure 13b; PLA-6 (72.9 MPa, 7.5%) and PLA-18 (72.3 MPa, 6.3%) have higher values compared with other samples. In view of the results above, the mechanical properties are influenced by a combination of multiple factors such as the orientation of molecular chain, thickness, averaged crystallinity of skin layer, etc. Our result can reflect that Young’s modulus is mainly affected by the thickness and averaged crystallinity of the skin layer, while the improvement of tensile strength or toughness depends on whether shish structure (or molecular orientation) forms or not. DMA is performed to obtain further information on the thermomechanical performance of the injection-molded PLA parts with thickness and averaged crystallinity of the skin layer. The storage modulus (E′), loss modulus (E″), and damping parameter (tan δ) are described as a function of temperature in Figure 14. Clearly, the E′ of static PLA falls off sharply across the glass transition temperature of PLA (Tg of PLA, ∼60 °C), manifesting an inferior heat resistance. The subsequent increase of E′ at high temperatures (85−95 °C) is closely tied to the cold crystallization of the amorphous PLA occurring during the heating process.57 Interestingly, enormous enhancement of heat resistance can be observed in sheared PLA samples, where the decrement of E′ becomes much smaller than the static one. At

crystalline morphology in the process of injection molding and to confirm the above speculations. 3.3. Thermomechanical Properties of InjectionMolded PLA Parts with Thickness and Averaged Crystallinity of Skin Layer. Figures 12 and 13 testify to the

Figure 13. Mechanical properties of injection-molded PLA parts with thickness and averaged crystallinity of skin layer: (a) typical stress− strain curves; (b) tensile strength, Young’s modulus, and elongation at break.

extraordinary combination of favorable heat resistance and prominent mechanical performance of PLA parts. From Figure 12, the inset table reveals that unexpected increase of HDT (a) and VST (b) of the injection-molded PLA parts is evidently obtained compared to the static one manipulated by shear duration and oscillation frequency under a strong shear flow field. To see more clearly, parts a and b of Figure 12 respectively reflect the change rule of HDT and VST of samples with thickness and averaged crystallinity. Notably, when the skin layer is only ∼400 μm thick (i.e., PLA-6), the HDT and VST separately rise from 55.7 and 59.3 °C to 65.1 and 81.5 °C. Moreover, the heat resistance continues to increase with the thickness of the skin layer, but it changes slightly when the thickness reaches up to ∼1200 μm remaining stable about 77.5 °C (i.e., PLA-18, PLA-30, PLA-90). Specifically, the thickness of the skin layer of ∼1200 μm can just make the VST of parts approach the melting point (Tm ∼ 153 °C), as high as 2.5 times over a static part. That is, once the thickness reaches 1200 μm, the increase of thickness becomes less effective. More interestingly, the increase of averaged crystallinity from 40 to 50% makes the HDT significantly increase to 96.6 °C (i.e., PLA-120@hf), which is a uppermost level for an as-molded neat PLA system without postprocessing so far.27,54,55 It is worth noticing that improved heat deflection resistance is not in good proportion to the thickness of the skin layer, but there is a thickness threshold of about 1200 μm 6303


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Figure 14. Temperature dependence of (a) storage modulus (E′), (b) loss modulus (E″), and (c) damping parameter (tan δ) for injection-molded PLA part with thickness and averaged crystallinity of skin layer. The inset table summarizes the DMA results in detail.

about 83 °C, the E′ increases from 109.6 MPa for PLA-S (0 μm@9%) up to 251.2 MPa for PLA-6 (400 μm@31%) and further up to 489.8 MPa for PLA-120@hf (2000 μm@50%), which could be explained by the fact that cold crystallization is suppressed with the increase of thickness and averaged crystallinity of the skin layer, so that the part deformation becomes not so easy to happen under stress with increasing temperature. Additionally, almost no much impact of shear on PLA stiffness at room temperature can be obviously seen in the inset table in Figure 14, maintaining the superior E′ value of about 2150 MPa for all samples. The plots of E″ versus temperature presented in Figure 14b are useful for detecting molecular mobility transitions, and the peak of E″ is interpreted as Tg,58 which shows a slight increment of 1 °C in sheared samples compared to the static one. This is because the highly oriented shish-kebab superstructure or spherulites may confine the chain (segment) mobility in the amorphous region. As shown in Figure 14c, reduction in intensity and a broad peak of the tan δ curve for sheared samples are due to the same suppressed effect of perfect crystal structure on the mobility of PLA chains in the amorphous region. The tan δ values (in the inset table) are clear to observe the change of the Tg. Sheared samples emerge in a gradually increasing trend compared with static PLA. Thus, the DMA results indicate that the mechanical properties are closely linked to the thickness and averaged crystallinity of the skin layer once again.

during injection molding. Note that the designed injectionmolded PLA parts are not uniform along the thickness direction, consisting of different skin layers and amorphous layers. Then, we characterize their crystal structure and crystalline morphology with WAXD, SAXS, and SEM techniques, and test the thermomechanical performance. The results show that these injection-molded PLA parts exhibit significantly different thermomechanical performances strongly dependent on the crystalline morphology, thickness, and averaged crystallinity of the skin layer. Young’s modulus increases with the increase of thickness and averaged crystallinity of the skin layer, while the improvement of tensile strength depends on whether shish structure (or molecular orientation) forms or not. Moreover, when the thickness of the skin layer only increases to 1200 μm (i.e., PLA-18, PLA-30), the HDT increases to 77.5 °C from 55.7 °C for PLA-S. More interestingly, the increase of averaged crystallinity from 40 to 50% with a thickness of 2000 μm makes the HDT significantly increase to 96.6 °C (i.e., PLA-120@hf). That is, if the products require less demanding heat resistance, a thin skin layer of about 1200 μm is enough. But if applying in a high temperature environment (e.g., above 100 °C), the formation of thickness of 2000 μm with above 50% crystallinity is indispensable. Moreover, it is found that the maximal shear rate at a magnitude of 103 s−1 seems to be a critical condition for formation of oriented lamellae during injection molding for PLA. These results provide a bridge of process−structure− property that is pivotal for the injection-molded PLA with outstanding thermomechanical performance. We believe that the microscopic structure−macroscopic property correlation established herein can provide useful guidance for the design of the PLA materials with high performance.

4. CONCLUSION We provide qualitative evidence to demonstrate that different skin layers of an injection-molded PLA part have a significant impact on its thermomechanical performance for the first time. We first design a series of injection-molded PLA parts with different skin layers (crystalline morphology, thickness, and averaged crystallinity) by tailoring shear duration and oscillation frequency with a concept of “structuring process” 6304


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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.iecr.7b00930. Schematic illustration for injection mold with an oscillation shear supplier, digital pictures for injection mold with an oscillation shear supplier, schematic illustration for the two pistons motion during oscillation of OSIM technique, schematic illustration for the injection mold with an oscillation shear supplier and calculation methodology of shear rate distribution in OSIM sample, shear rate profile during solidification along the sample thickness direction (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.-J.Z.). *E-mail: [email protected] (Z.-M.L.). ORCID

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 thank the National Natural Science Foundation of China (51533004, 51673135, 51473101, and 21576173) and the Innovation Team Program and Youth Foundation of Science & Technology Department of Sichuan Province (Grants 2014TD0002 and 2017JQ0017) for financial support. We also thank the Shanghai Synchrotron Radiation Facility (SSRF), Beamlines BL16B and BL15U, for supporting X-ray measurements.

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REFERENCES

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