Unexpected Strength and Toughness Reinforcement of the Injection

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Unexpected strength and toughness reinforcement of the injectionmolded isotactic polypropylene parts with oriented #-crystals Yanhui Chen, Xu Bo, Song Yang, Haoqing Yang, Tom Lawson, Zhiqiang Wu, Qiuyu Zhang, and Zhongming Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03704 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

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Industrial & Engineering Chemistry Research

Unexpected

Strength

and

Toughness

Reinforcement

of

the

Injection-molded Isotactic Polypropylene Parts with Oriented β-crystals

Yanhui Chen,†,* Xu Bo,† Song Yang,† Haoqing Yang,† Tom Lawson,‡ Zhiqiang Wu,† Qiuyu Zhang,† and Zhongming Li#

†

Department of Applied Chemistry, School of Science, Northwestern Polytechnical University,

Xi’an 710072, China ‡

ARC Center of Excellence for Nanoscale BioPhotonics, Macquarie University, Sydney, NSW

2109, Australia #

College of Polymer Science and Engineering and State Key Laboratory of Polymer Materials

Engineering, Sichuan University, Chengdu 610065, China

Abstract: In this work, three profiles of injection temperatures were adopted to tune the morphology of β-nucleating agent and further affect the resulting β-crystal morphology of isotactic polypropylene (iPP) during an injection molding process. When the middle injection temperature profile was adopted (the maximum temperature was 220 °C), abundant oriented β-crystals with c-axis perpendicular to the injection direction were presented throughout the whole injection-molded part and their existence and distribution were confirmed by synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). These oriented 1

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β-crystals give rise to simultaneously enhanced tensile and impact strength of β-nucleated iPP part. In a novel outcome, this work transformed the usually mechanically weak and soft component (i.e., the β-crystals) of iPP into one that added to its self-reinforcement and overall strength. This finding provides a new idea that a soft structure can be utilized to reinforce itself. Keywords: isotactic polypropylene, β-crystal, injection temperature, orientation, WAXD, SAXS

1. INTRODUCTION Among the crystal phases of isotactic polypropylene (iPP) (i.e., α, β, γ, etc.),1-3 the β-phase has attracted intensive research interest because of its unique crystallographic characteristics,4-5 and positive influence on the toughness of iPP products.6-7 As a thermodynamically metastable phase, the β-phase can be produced by adding β-heterogeneous nucleating agents,8-10 by applying temperature gradients,11-12 or by applying shear flow fields.13-14 The addition of β-nucleating agents demonstrates many advantages to the fabrication of iPP products, such as a high β-nucleation efficiency, a low loading, little negative impact on the iPP fabrication process, and a high performance/cost ratio.15 To tune the toughness of iPP, the content of β-phase is usually considered first under investigation. Initially, it was supposed that a larger amount of β-phase could be achieved by increasing the amount of β-nucleating agent. Instead, even when an excessive amount was added, the content of β-phase cannot keep increasing, but become saturated. A higher β-phase content normally imparts a higher impact strength 2


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or toughness to the iPP part. As a matter of fact, the observed improvement in toughness dependent on β-phase content is limited and marginal in scope. Logically, the maximum β-phase content itself can never be more than 100%, and in the practical processing is much less due to the complicated external environment or insufficient selectivity of β-nucleating agents.16 The morphology of β-phase also shows a great potential to toughen iPP, which have become the focus of much recent research effort. Thanks to the epitaxial growth of β-crystal on the surface of β-nucleating agent crystals, the morphology of β-phase can be efficiently tuned by that of β-nucleating agent which was dependent on its loading,16-19 and the processing temperature.16, 20 Li, et al. found that well-developed β-spherulites formed at the critical content of β-nucleating agent (at 0.05 wt%) exhibited better ductility when compared to the bundle-like β-morphology at the supercritical content (0.2 wt%), due to the more integrated crystalline structure of the former.19 Luo, et al. observed a parabolic dependency of toughness on the content of β-nucleating agent. The highest toughness was achieved when adding 0.5 wt% β-nucleating agents at which concentration “flower”-like β-crystal agglomerates were formed.17 Zhang, et al. found that the leaf-like transcrystals at 0.1 wt% β-nucleating agents demonstrated the best impact strength.18 Besides the loading of β-nucleating agent, the processing temperatures adopted to produce iPP parts can also affect the final morphology of β-crystal due to the changeable self-organized morphology of β-nucleating agent.16, 20. With an increase of the final molten temperature, Luo, et al. sequentially obtained β-spherulites, β-transcrystalline entities and ‘‘flower’’-like 3



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agglomerates of the β-crystallites. These β-“flower”-like agglomerate crystallites demonstrated the largest strain at break (the best ductility) when mechanically tested.20 Unfortunately, the toughness of iPP molded-parts is usually improved at the expense of the strength and this is also true in parts made from iPP with β-nucleating agent, due to the loose packing manner of β-crystal. Therefore, the hard to achieve, but being an important goal, simultaneous strength and toughness reinforcement is the aim in the fabrication of iPP parts. As far as the reinforcement is concerned, oriented crystals (like shish-kebabs) are well-known self-reinforced structures.21 Likewise, oriented β-crystals are thus expected to improve the strength of β-phase-rich iPP. However, oriented β-crystals cannot be easily obtained in the molded-parts due to their natural thermodynamic and mechanical instability. β-crystals are very easy to transform into the α-phase or the mesomorph phase under external conditions.22-24 Fabrication trials, such as pulling fibers in an iPP melt,25 mixing iPP fibers in a molten or partially molten state into an isotropic iPP melt,26 or annealing an oriented iPP melt with the aid of a temperature gradient,12, 27 only gained a limited amount of β-phase with a preferred orientation. Other pilot studies that applied shear flow to β-nucleated iPP samples also failed.28-29 However, later attempts with a certain sequential application of shear flow allowed the formation of β-crystal orientation.30-32 These attempts utilized an external flow field in the practical processing to orient needle-like β-nucleating agent crystals from which regular β-transcrystals were then generated. Distinguished from the normal c-axis orientation of shish-kebabs33 or hybrid 4


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shish-kebabs,34 the orientation of β-crystal is reported to be anomalous with its c-axis perpendicular to the flow direction.32,

35

This intriguing feature accounts for the

unique mechanical anisotropy that occurs in the injection-molded or the extruded β-nucleated iPP parts.30-31 For example, the ductile behavior was observed to occur in the transverse direction to the machine direction, whereas the sample was brittle in line with the machine direction. Currently, studies performed on the formation and performance of oriented β-crystals have only just started. Whether the soft β-crystal can reinforce iPP by means of its orientation is still unknown. In this work, three profiles of injection temperatures were employed in the injection molding processing to fabricate three different β-crystal morphologies of iPP in the presence of a β-nucleating agent. Advanced X-ray measurements (synchrotron wide-angle X-ray diffraction and small-angle X-ray scattering) with a high spatial resolution were used to characterize the crystal structures and orientation within each injection-molded β-nucleated iPP part. It was found that oriented β-crystals were distributed throughout the entire iPP part when the middle injection temperature profile was used. Moreover, these iPP parts demonstrated enhanced strength and toughness compared to the pure iPP part. The formation mechanism of oriented β-crystals and its contribution to the mechanical performance are discussed in detail in this work. Our work explores the possibility of utilizing the soft component of the part as a reinforcing component to the polymer matrix.

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2. EXPERIMENTAL SECTION 2.1. Materials iPP was purchased from Dushanzi Petroleum Chemical Co., Xinjiang, China. Its molecular weight was Mw = 39.9 × 104 g·mol-1, and its polydispersity was Mw/Mn = 4.6. Its melt flow rate was 3 g·(10 min)-1 (230 °C, 21.6 N). Aryl amide compound (TMB-5) was used as the β-nucleating agent; its chemical structure is similar to an aromatic amine β-nucleating agent, i.e., N,N’-dicyclohexyl-2,6- naphthalenedicarboxamide.16 This compound was kindly provided by Fine Chemical Institute of Shanxi, Taiyuan, China. 2.2. Sample Preparation Pure iPP granules and iPP granules with 0.2 wt % β-nucleating agent were separately melt-mixed via a twin-screw extruder. The screw speed was 82 rpm, and the processing temperature window was within the range of 170 to 180 °C from hopper to die. The extruded pellets were then injected into dumb-bell parts. Three barrel temperatures profiles (LT, MT and HT) were adopted in the fabrication of the β-nucleated iPP parts during the injection molding processing, as listed in Table 1. For comparison, the pure iPP sample was injection-molded at the low injection temperature. The other the injection molding parameters were as follows. Mold temperature: 40 °C, injection pressure: 40 MPa, holding pressure: 50 MPa, injection-packing time: 10 s, cooling time: 20 s, and screw rotation speed: 90 rpm.

Table 1. Three profiles of barrel temperatures were adopted to produce pure iPP and 6


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β-nucleated iPP parts during the injection molding processing. Sample Name

Barrel Temperatures from Nozzle to Feeding Zone

Pure iPP

185 °C

190 °C

190 °C

180 °C

165 °C

LT (Low Temperature)

185 °C

190 °C

190 °C

180 °C

165 °C

MT (Middle Temperature)

215 °C

220 °C

220 °C

200 °C

165 °C

HT (High Temperature)

235 °C

240 °C

240 °C

200 °C

165 °C

2.3. X-ray Measurements To analyze β-crystal distribution in the injection-molded β-nucleated iPP parts, the samples were characterized layer by layer along the width direction by means of synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS), as shown in Figure 1. A block with 6.0 mm width and 4.0 mm thickness was cut first from the dumbbell bar and then the extra material was removed in the thickness direction except for reserving a 1.0 mm thick central piece, while the width was still kept at a 6.0 mm thickness. The X-ray beam, set perpendicular to the part, was moved along the bar from the edge to center. A picture was taken at every 0.5 mm interval, and stopped at the position of 3.0 mm from the edge since the remaining part was symmetrical. The distance from the edge to a 0.5 mm depth of the part was considered as the skin layer; from about 0.5mm to 1.5mm of the part as the middle layer (or intermediate layer); and from about 1.5mm to 3 mm as the core layer. Synchrotron WAXD and SAXS measurements were carried out at the Advanced Polymers Beam line (λ=0.1371 nm) in the National Synchrotron Light Source (NSLS), at the Brookhaven National Laboratory (BNL) in the United States. A MAR CCD 7



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X-ray detector (MARUSA) was employed for the detection of 2D-WAXD and 2D-SAXS images, and had a resolution of 1024 × 1024 pixels (pixel size = 158.44 mm). The sample-to-detector distance was 113 mm for WAXD (calibrated by an aluminum oxide (Al2O3) standard) and 1783 mm for SAXS (calibrated by a silver behenate (AgC22H43O2) standard), respectively.

Figure 1. A schematic diagram of the testing positions of the injection-molded parts for the WAXD/SAXS measurements. MD, the injection molding direction (i.e. the flow direction); TD, the transverse direction; ND, the direction normal to the MD-TD plane.

1D-SAXS profiles were obtained from the circular average of the 2D-SAXS images, where the scattering intensity was plotted as a function of the modulus of the reciprocal space vector, s (|s| = 2 sin θ/λ, with λ representing the wavelength of the incident beam). In the 1D-SAXS profile, the long period (LB) defines the statistical average of the distance between two lamellar crystals, as calculated by LB = 1/sm. 1D-WAXD profiles were obtained by integrating the intensity of 2D-WAXD 8


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images. Based on 1D-WAXD profiles, the crystallinity (Xc) was calculated according to Equation 1.

Xc =

∑ Acryst (1)

∑ Acryst + ∑ Aamorp

where Acryst and Aamorp represent the areas of diffraction peaks emitted from the crystals and the diffused scattering from the amorphous phase, respectively. The relative content of β-crystal (Kβ) was evaluated by the modified method by Hsiao et al.,13 as shown in Equation 2.

Kβ =

Aβ (110) Aβ (110) + Aα (110) + Aα (040) + Aα (130)

(2)

where Aβ(110) represents the area of the diffraction peak of β(110) of β-crystal; Aα(110), Aα(040), and Aα(130) correspond the signature diffraction peaks of α-crystal, namely, α(110), α(040) and α(130). The crystallinity of β-crystal (Xβ) is calculated on the basis of Equation 3.

Xβ = Kβ × Xc

(3)

The relative orientation parameter (Orel) was used to determine the degree of orientation of β-crystal.36 Orel =

180° − FWHM ∈ (0;1) 180°

(4)

where FWHM represents the full-width at half maximum of the azimuth scan of the characteristic of lattice plane of β-crystal (i.e., β(110)). If multiple diffraction peaks were observed in the azimuth scan curves, the minimum FWHM was used. 2.4. Scanning Electronic Microscope To observe β-crystal morphology, the co-fractured cross-sections of the 9



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injection-molded iPP parts were etched by a mixed acid solution.37 The fractured surface (mainly around the core layer) were gold-coated and observed using a VEGA3-JMQ SEM instrument (Tescan, Czech Republic). 2.5. Mechanical Property Tests The tensile tests were performed at 23 °C and at a crosshead speed of 50 mm/min according to the standard ASTM D-638. The notched impact tests were carried out at 23 °C according to the standard ASTM D256-05, using a 1.2 mm deep V-notch. Over five samples were tested for each measurement.

3. RESULTS AND DISCUSSION 3.1 Distribution of β-crystal in the Injection-molded iPP Parts To analyze β-crystal distribution in the injection-molded iPP parts, synchrotron WAXD was first applied to detect the iPP parts from the surface to the center. Three 2D-WAXD patterns targeting three positions (i.e., 0.5 mm, 1.5 mm and 3.0 mm) were chosen to represent the skin, intermediate and core layer of each iPP sample, respectively (Figure 1). Five intense characteristic lattice planes of α-crystals, i.e., α(110), α(040), α(130), α(111), and α(-131) from the inner to the outer layer, are observed in pure iPP sample (Figure. 2a), and this is accompanied by a shallow diffraction of β(110) appearing between α(110) and α(040). Moreover, broad equatorial arc-like diffraction of α(040) and β(110) are observed in the skin and intermediate layer of pure iPP sample, indicating a slight orientation of α- and β-crystal. Flow field, which is inevitable during an injection molding process, is well reported to efficiently induce the formation of β-crystal in iPP.38 The literature 10


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consensus proposes that shear flow field is able to orient iPP molecular chains to generate the partially ordered molecular bundles on the surface of which β-crystal epitaxially and successively grow.25,

39-40

If oriented α-crystals are formed in the

injection molding, β-crystals will be endowed with the same preferred orientation. The diffraction intensity of α-crystal in the β-nucleated iPP parts gets weaker and weaker as the injection temperature increases, and the orientation of α-crystal is unobservable in the β-nucleated iPP parts. By contrast, β(110) diffraction was relatively apparent, as shown in Figure 2b-d. Here, isotropic β(110) diffraction is observed from the skin to the core layer of the LT sample (i.e., β-nucleated iPP parts were processed at a low injection temperature). An intriguing phenomenon is found in the MT sample. Six well-defined spots of the β(110) lattice plane, of which two spots are presented at the equator and four around ±30° off the meridian, can be seen in the WAXD patterns from the skin to the core layer, illustrating that a neat orientation of β-crystals exists throughout the whole β-nucleated iPP part.18, 41-43 In the HT sample, the oriented diffraction of β(110) is gradually converted into isotropic diffraction as approaching the core layer. To reveal the content distribution of β-crystal in the injection-molded parts, 1D-WAXD curves of a series of representative positions are displayed in Figure 3. Shallow β(110) diffraction peaks are observed in every layer of pure iPP sample, where the strongest intensity appears at 1.0 mm, due to the strong shear flow and proper super-cooling degree at the intermediate layer.28 All the surface layers of β-nucleated iPP parts were formed immediately when the hot melt contacted the cold 11



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mold, where a high super-cooling degree and strong shear flow was experienced. Therefore, the formation of β-crystal at the surface layer is greatly depressed.15 As the core layer is approached, the shear flow gradually attenuates and accordingly the nucleation ability of β-nucleating agent becomes revived. As a result, β-crystal of all the β-nucleated iPP parts progressively dominates from the skin to the core layer. Furthermore, as the injection temperature increases, the diffraction peaks of β-crystal become higher and higher, indicative of the increase of β-crystal. The specific β-crystal content integrated from the 1D-WAXD curves was given in Figure 4. The highest content of β-crystals in pure iPP sample is 0.06 and appears at 1.00 mm. The content of β-crystals in all the β-nucleated iPP parts is lowest at the surface (at around 0.02), and increases dramatically at the intermediate layer (i.e., at about 0.5 mm to 1.5 mm) and reaches the plateau at the core layer. The maximum content of β-crystal in the LT, MT and HT samples were 0.32, 0.44 and 0.54, respectively.

12


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Figure 2. Selected 2D-WAXD patterns of skin, intermediate and core layer of the injection-molded pure iPP part (a) and iPP parts with 0.2 wt% β-nucleating agent prepared at a low (b), middle (c) and high (d) injection temperature. The distance to the edge of samples for the skin, intermediate and core layer is about 0.5 mm, 1.5 mm and 3.0 mm, respectively. The injection molding direction is vertical.

(a)

(b)

13



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(c)

(d)

Figure 3. 1D-WAXD curves obtained from circularly integrated intensities of 2D-WAXD patterns from the skin to the core layer of the injection-molded pure iPP part (a) and iPP parts with 0.2 wt% β-nucleating agent prepared at a low (b), middle (c) and high (d) injection temperature.

Figure 4. The distribution of β-crystal content from the skin to the core layer of the injection-molded pure iPP part (a) and iPP parts with 0.2 wt.% β-nucleating agent prepared at a low (b), middle (c) and high (d) injection temperature.

Analogous to β-crystal content distribution, the orientation of β-crystal also demonstrates a certain distribution along the thickness (or width) of the injection-molded parts, and can be clearly seen in the azimuthal scan curves of β(110) 14


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diffraction peaks in Figure 5. Moreover, the relative orientation parameter of β-crystal (Orel) at different layers of the injection-molded parts is displayed in Figure 6. Wide and shallow diffraction peaks of β-crystal are observed at the equator in every layer of pure iPP sample (Figure 5a), indicating that the β-crystal has a preferable orientation along the injection molding direction although the β-crystal content is very low. Owing to the high super-cooling and strong shear flow, β-crystal at the surface layer of all the β-nucleated iPP parts also show c-axis orientation. For the rest layers of the LT sample, broad diffraction peaks are located at around the meridian, suggestive of a weak β-crystal orientation perpendicular to the injection molding direction (Figure 5b). Obvious β-crystal orientation perpendicular to the injection molding direction was observed in the MT sample, as displayed by two equatorial diffraction peaks and four around ±30° off the meridian (Figure 5c). β-crystal orientation decays at the core layer (2.00 mm) in the HT sample, as shown by the almost flat azimuth scan curve at the core layer (Figure 5d). From the detailed Orel in Figure 6, it can be observed that the MT sample demonstrates a steady high degree of β-crystal orientation, at about 0.92, from the surface to the core layer. As approaching the core layer, a drop of Orel is clearly seen in the other two β-nucleated samples. The highest drop happens in the HT sample, from 0.91 (at 2.0 mm) to 0.56 (at 2.5 mm), and is consistent with the azimuthal scan curves in Figure 5d. However, the lowest Orel is observed in the LT sample instead of pure iPP sample. Compared with the increased β-crystal content from the skin to the core layer, the Orel of β-crystal demonstrates an opposite attenuation. 15



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(a)

(b)

(c)

(d)

Figure 5. Azimuthal scan curves of β(110) lattice plane from the skin to the core layer of the injection-molded pure iPP part (a) and iPP parts with 0.2 wt% β-nucleating agent prepared at a low (b), middle (c) and high (d) injection temperature.

16


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Figure 6. The relative orientation parameter (Orel) of β-crystal from the skin to the core layer of the injection-molded pure iPP part (a) and iPP parts with 0.2 wt% β-nucleating agent prepared at a low (b), middle (c) and high (d)injection temperature. 3.2 Lamellar Structures of the Injection-molded iPP Parts Lamellar structures in the injection-molded iPP parts were further characterized using SAXS measurement, as shown in Figure 7. Strong meridian scattering gets wider and wider from the skin to the core layer of pure iPP sample (Figure 7a), indicating a gradual attenuation of lamellar orientation, and this agrees with the WAXD measurement in Figure 2a. This meridian scattering mainly originates from the oriented α-crystal lamellae, since oriented α-crystal lamellae are parallel to the injection molding direction in pure iPP sample.44 Besides the strong meridian scattering, the equatorial scattering in the LT sample is relatively enhanced compared to that in pure iPP sample and the scattering maxima is observed to shift to a smaller s (Figure 7b). As revealed in the WAXD measurement (Figures 2 and 5), β-crystal in the β-nucleated iPP parts were found to be oriented perpendicular to the injection molding direction. This oriented β-crystal lamellae may contribute to the observed equatorial scattering. The converged equatorial scattering is apparent in the MT sample except that at the skin layer (Figure 7c), corresponding to the refined orientation of β-crystal lamellae. The scattering gets diffuse from the skin to the core layer in the HT sample (Figure 7d), suggesting that gradual attenuation of the lamellar orientation. All of these phenomena are consistent with the WAXD finding, although SAXS and WAXD focus on sizes at the micro- and the nano-scales, respectively. 17



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Figure 7. Selected 2D-SAXS patterns of the skin, intermediate and core layer of the injection-molded pure iPP part (a) and iPP parts with 0.2 wt % β-nucleating agent prepared at a low (b), middle (c) and high (d) injection temperature. The distance to the edge of samples for skin, intermediate and core layer is about 0.5 mm, 1.5 mm and 3.0 mm, respectively. The injection molding direction is vertical. 18


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Meridian

Equator

19



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Figure 8. 1D-SAXS intensity profiles (Is2) (after a Lorentz correction) were measured and graphed along the meridian and equator as a function of reciprocal space vector (s) from the skin to the core layer of the injection-molded pure iPP part (a) and iPP parts with 0.2 wt % β-nucleating agent prepared at a low (b), middle (c) and high (d) injection temperature.

Given that scattering maxima are shown at the equator and meridian of 2D-SAXS patterns in Figure 7, 1D-SAXS intensity profiles were separately analyzed along the equator and meridian, as shown in Figure 8. Furthermore, changes of the long periods at the equator and meridian from the skin to the core layer of all the samples are given in Figures 9 and 10, respectively. Long periods of the equator and meridian of all the sample are basically the same. For pure iPP sample and LT sample, they keep steady from the skin to the core layer, around 14.0 nm for the former and around 16 nm for the latter. However, long periods taken from the equator and meridian of the MT and HT samples show an increased trend from the skin to the core layer. The plateau values are about 19.0 nm for the MT sample and at about 19.2 nm for the HT sample. In the whole, as the injection temperature increases, long periods at the equator and meridian both increase. This may be attributed to the increased 20


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content of β-crystals. It is well known that the α-crystal has a monoclinic unit cell with a = 0.665 nm, b = 2.096 nm, and c = 0.650 nm, possessing a compact structure called cross-hatched parent and daughter lamellae, where the daughter lamellae nucleates and grows from the previously formed primary lamellae (i.e., the parent lamellae).45 The β-crystal has a trigonal unit cell with a=b=1.103 nm and c=0.649 nm.46 The growth of β-crystal proceeds mainly by branching around screw dislocations, where new lamellae diverge from its initial lamellar arrangement.47 Normally, the β-lamellae at a loose stacking mode normally demonstrate a larger long period, compared to α-lamellae.48 In pure iPP sample, α-crystal lamellae are dominant, while β-crystal lamellae dominate in the other three β-nucleated iPP samples and the content of β-crystal increases with the increase of the injection temperature (Figure 4). Therefore, the highest β-crystal content gives rise to the largest long periods observed in the HT sample (Figure 4d). As aforementioned, the equatorial and meridian scattering maxima are mainly considered to originate from oriented β- and α-crystal lamellae, respectively. Normally, the equatorial and meridian long periods (i.e., the long periods of oriented β- and α-crystal lamellae) are usually inconsistent, due to their different stacking modes. It was also verified in our previous work that the oriented β-crystal lamellae, crystallized under quiescent conditions, exhibited a 6.5nm higher long period than the oriented α-crystal lamellae.48 However, in this work the equatorial long periods of the β-nucleated iPP parts did not display an obvious discrepancy from the counterpart. Different from the quiescent crystallization, in the practical processing the formation 21



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of oriented α- and β-crystal suffers from a complex external environment and thus goes through a fast crystallization rate and an enhanced secondary crystallization. As a result, the growth of crowded crystal lamellae eventually eliminates the discrepancy of lamellar stacking.

Figure 9. Long period at the meridian from the skin to the core layer of the injection-molded pure iPP part (a) and iPP parts with 0.2 wt% β-nucleating agent prepared at a low (b), middle (c) and high (d) injection temperature.

Figure 10. Long period at the equator from the skin to the core layer of the injection-molded pure iPP part (a) and iPP parts with 0.2 wt% β-nucleating agent prepared at a low (b), middle (c) and high (d)injection temperature.

22


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3.3 The Morphology of β-crystal in the Injection-molded iPP Parts The resultant β-crystal morphologies after suffering from the inevitable external fields in the injection molding processing are shown in Figure 11. Smaller coarse β-spherulites are observed in the LT sample; β-transcrytals are oriented along the injection direction in the MT sample, where the needle-like β-nucleating agent crystals are etched; β-flower-like aggregates demonstrate sufficient connections within crystallites in the HT sample. Therefore, it turns out that variable β-crystal morphologies are formed when adopting different injection molding barrel temperatures.

(a)

(b)

23



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(c)

(d)

Figure 11. SEM photographs of fractured cross-section (mainly the core layer) of the injection-molded pure iPP part (a) and iPP parts with 0.2 wt % β-nucleating agent prepared at a low (b), middle (c) and high (d) injection temperature. The injection molding direction is vertical.

3.4 Mechanical Property of the Injection-molded iPP Parts The mechanical properties (i.e., tensile strength, impact strength and elongation at break) of the injection-molded pure iPP part and β-nucleated iPP parts are displayed in Figure 12. It is surprising to see that the MT sample demonstrates the highest tensile strength (43.2MPa), about 4 MPa higher than pure iPP sample, while the lowest tensile strength appears in the LT sample. This is a unique phenomenon that the β-nucleated iPP part with abundant soft β-crystals has a higher tensile strength than the pure iPP part with stiff α-crystals. We consider that the key should be that β-crystals are oriented. In addition, an opposite tendency of elongation at break is observed. 24


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Interestingly, the impact strength of the β-nucleated iPP parts increases with the injection temperature, although the same amount of β-nucleating agent was added to each part. The impact strength of the HT sample is almost three times than that seen in pure iPP sample. As observed in Figure 4, β-crystal content also increased with the increase of the injection temperature. We considered that the improved impact strength was attributed to the higher β-crystal content of the β-nucleated iPP parts.

Figure 12. Tensile strength, impact strength and elongation at break of the injection-molded pure iPP part (a) and iPP parts with 0.2 wt% β-nucleating agent prepared at a low (b), middle (c) and high (d) injection temperature.

3.5 The Influence of β-crystal Content and Morphology on the Mechanical Properties Distinguished from the ideal flow fields that samples undergo in the 25



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laboratory,49-50 the external fields experienced in the practical processing (e.g. extrusion, injection molding and blow molding) are more complicated, and are usually coupled with a heterogeneous shear flow field, an elongation flow field and a temperature gradient field. In our current work, the complicated external fields existing in the injection molding were mainly utilized to tune the content and the morphology of β-crystal, eventually endowing the iPP parts with enhanced mechanical properties. A skin-core structure given by the flow fields was demonstrated in all the injection-molded iPP parts. For the β-nucleated iPP parts, the skin layer had the lowest β-crystal content (Figure 4), but the highest orientation degree (Figure 6), while the core layer was found to be totally opposite. This phenomenon was closely related to the crystallization process of polymer melt during the injection molding processing. When the polymer melt was immediately injected into the mold, the melt was oriented. Once the oriented melt contacted the cold mold wall, the crystallization rate was markedly accelerated due to its drastic super-cooling affect. Then, the oriented melt was frozen to form oriented α-crystals with a high degree of orientation and overwhelmed the growth of β-crystals, due to the competitive growth of α- and β-nuclei.15 At the packing stage of the injection molding, the crystallization environment of the rest part was not as severe as that experienced at the skin layer. The crystallization was almost conducted under a quasi-static condition. The oriented melt was gradually relaxed, and the crystallization rate slowed down, leading to an attenuation in α-crystal orientation. The growth of β-crystals thus become revived, 26


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overpassing that of α-crystals. When approaching the core layer, β-crystal content gradually increased, until it reached a plateau (Figure 4). As a matter of fact, it can be concluded that the competitive growth of α- and β-crystals, to different extent, happened at every layer of just one injection-molded part. Heterogeneous injection temperatures during the injection molding processing had a significant impact on the morphology and content distribution of β-crystal in the β-nucleated iPP parts. With the increase in the injection temperature, β-crystal content in the core layer increased from 0.32 for the LT sample to 0.54 for the HT sample. As shown in Figure S1, β-nucleating agent crystals transformed from dot-like, to needle-like, to flower-like aggregates as the temperature increased, predicting the reduction of β-crystal due to the gradual tight connection within β-nucleating agent crystallites. However, the result was opposite. At this time, another factor, i.e., the high injection temperature may promote the mobility of molecular chains, which were accelerated to fast diffuse to the surface of β-nucleating agent crystals to form β-crystals. It appeared that the positive effect of the high injection temperature on the β-crystal growth far surpassed the negative effect of the reduction of β-heterogeneous nucleation sites. Certainly, the injection temperature should not be too high, as this will cause the degradation of polymer molecular chains. In addition, the inevitable complicated external flow fields altered the arrangement of the β-nucleating agent crystals, and further affected the resultant β-crystal morphology. Taking the MT sample for example, the initial β-nucleating agent crystals were in the form of needle-like state (Figure S1), which were oriented 27



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by the flow fields along the flow direction (Figure 11c). Owing to the increased viscosity and the bulky size of needle-like β-nucleating agent crystals, once the needle-like β-nucleating agent crystals were oriented along the injection direction, they were hard to relax back to a random state. Subsequently, the molecular chains were absorbed to the surface of oriented needle-like β-nucleating agent crystals where the oriented β-crystals epitaixally grew. Apparent β-crystal orientation was observed in every layer of the MT sample except at the surface layer (Figs. 2c, 6c and 7c), as evidenced by two equatorial diffraction peaks and four around ±30° off the meridian (Figure 5c) and the unique cross scattering pattern (Figure 7c). For the high orientation degree of β-crystals at the skin layer of the HT sample (Figs. 2d, 6d and 7d), two possible reasons were proposed. When the melt was injected into the mold, the originally formed needle-like β-nucleating agent crystals were oriented along the flow direction and fixed to induce oriented β-crystals before they were further self-assembled into flower-like β-nucleating agent crystals. Another possibility was that the formed flower-like β-nucleating agent crystals were squeezed into bundle-like aggregation under the effect of strong flow field, and then arranged along the flow direction. At the core layer, the oriented melt was basically relaxed back to the random state and the crystallization rate slowed down, so that β-nucleating agent crystals gained enough time to form or to recover to their flower-like state, eventually inducing isotropic β-flower-like crystals. Therefore, it is easy to understand the attenuation of the orientation degree of β-crystals in the core layer of the HT sample. For the LT sample, the precipitated dot-like β-nucleating agent crystals were isotropic 28


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and cannot be oriented. Therefore, homogenous distribution of β-crystals without apparent orientation was seen throughout the LT sample (Figs. 2b, 5b, 6b and 7b). Generally, the tensile strength of the β-nucleated iPP parts is lower than that of pure iPP part and lowers further with the increase of β-crystal content.20 The tendency of elongation at break is usually the opposite. In this work, the presence of oriented β-crystal morphology manifests a significant effect on the tensile property of the injection-molded parts. With the presence of abundant oriented β-crystals (in which the c-axis of β-crystal lamellae perpendicular to the injection molding direction or the tensile direction) throughout the whole part, the MT sample exhibited the highest tensile strength. This intriguing result demonstrated that the injection-molded parts were self-reinforced by the intrinsically soft β-crystals. When the tensile stress was applied to the isotropic spherulites or lamellae along the tensile direction, the stress normally transmitted from the amorphous region in the matrix to the restrained amorphous region within the lamellae, then to the lamellae itself, leading to chain slips, lamellar slips or the complete separation of lamellae

51-52

. However, when the

tensile stress was applied to the lamellae perpendicular to the tensile direction, unzipping of lamellae happened first, followed by the lamellar dislocation, which induced the formation of microvoids, further developed into deformation bands, and eventually into fibrils.23-24, 53 This distinguished deformation response of iPP parts with oriented β-crystals to the tensile stress accounted for its enhanced tensile strength.

29



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4. CONCLUSION In this work, the inevitable flow fields in the injection molding processing firstly oriented needle-like β-nucleating agent crystals, thus inducing the formation of abundant oriented β-crystals (i.e., oriented β-transcrytals) throughout the whole injection-molded iPP parts. The β-nucleated iPP part with oriented β-crystals not only demonstrated higher impact strength, but also surprisingly exhibited enhanced tensile strength, compared to the pure iPP part. A concurrent improvement to the tensile strength and toughness of iPP part using β-crystal is an unusual, but a welcome outcome.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Polarized Light Microscopy (PLM) photographs of isothermal crystallization of iPP sample with 0.2 wt% β-nucleating agent at 134 °C after annealing for 5 min at different temperatures. (Figure S1)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID 30


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Yanhui Chen: 0000-0001-6511-1738 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support for this work from the National Natural Science Foundation of China (Nos. 51473135, 51503170, 51573147, and 21676217), the Opening State Key Laboratory of Polymer Materials Engineering (Sichuan University) (Grant No. sklpme2015-4-22) and the Fundamental Research Funds for the Central Universities (No. 3102016BJY01).

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Unexpected Strength and Toughness Reinforcement of the Injection

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