Window of Pressure and Flow To Produce β ... - ACS Publications

Jun 9, 2017 - ABSTRACT: Using a pressuring and shearing device (PSD), we explored the simultaneous effects of pressure and flow on β-crystal formatio...
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Window of Pressure and Flow To Produce β‑Crystals in Isotactic Polypropylene Mixed with β‑Nucleating Agent Shu-Gui Yang,† Yan-Hui Chen,‡ Bo-Wen Deng,† Jun Lei,*,† Liangbin Li,§ and Zhong-Ming Li*,† †

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ‡ Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an 710129, China § National Synchrotron Radiation Lab and College of Nuclear Science and Technology, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: Using a pressuring and shearing device (PSD), we explored the simultaneous effects of pressure and flow on β-crystal formation. Interestingly, pressure plays a versatile role in β-crystal formation in isotactic polypropylene (iPP) mixed with β-nucleating agent (β-NA) when a flow field exists simultaneously. At a low pressure (5 MPa), a mass of β-crystals can be obtained over the entire range of applied shear rates (0.0−24.0 s−1). As the pressure increases (50−100 MPa), a weak shear flow (3.2 s−1) can profoundly suppress the β-crystal formation. When elevating the pressure to 150 MPa, β-crystals cannot be generated. The pressure and flow window to produce β-crystals in iPP mixed with β-NA were successfully summarized for the first time. The diversified β-crystal formation behaviors in iPP mixed with β-NA under the simultaneous effects of pressure and shear flow were well elucidated by combining classical nucleation theory and the growth of different crystalline phases. Our current work lays a solid foundation to tailor β-crystals in iPP mixed with β-NA in practical processing and thus to optimize the mechanical properties of iPP products.

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NA-induced β-nuclei, eventually resulting in a decrease in βcrystal quantity.16,17 Aside from the shear flow, another important processing factor, i.e., pressure, also affects iPP modifications. It has been reported that the pressure is favorable for the nucleation and growth of γ-crystal, and the γ-crystal proportion was shown to increase from zero at normal pressure to almost 100% under a high pressure of 200 MPa.18−20 Thus, the pressure may work against β-crystal formation in iPP mixed with β-NA because the accelerated nucleation and growth rates of γ-crystal result in fewer spaces for the remainder of the melt to crystallize into β-crystal. Indeed, research results have verified that the growth of both αand β-crystal was significantly suppressed in iPP mixed with βNA when pressure increased to 150 MPa.21,22 Effects of a sole external field (flow or pressure) on the structural features and properties of iPP mixed with β-NA have been well documented as per the above results. However, very little work has focused to date on the mutual effects of pressure and flow on the crystallization kinetics and structure development in iPP mixed with β-NA. Pressure and flow coexist in most practical processing conditions, and thus, the simultaneous influence of flow and pressure on β-crystal formation is interesting and has

INTRODUCTION The β-crystals of isotactic polypropylene (iPP) have received substantial attention in scientific research and industrial applications since their discovery1 because they show excellent toughness (including enhanced impact strength and improved elongation at break).2−5 To date, the most effective and convenient method to prepare high concentration β-crystal has been to add β-nucleating agent (β-NA), which is widely implemented in industrial production.6−8 However, in practical processing, the conditions (e.g., the pressure and flow window) for generating β-crystal in iPP mixed with β-NA remain unclear because both the complicated and inevitable flow and pressure fields in practical processing have profound influences on the βcrystal formation. Leugering et al.9 found that flow was initially capable of inducing β-crystal formation. Subsequently, the flow-induced βcrystal mechanism was carefully elucidated by Varga using thermo-optical observations10−13 and Hsiao et al. using in situ synchrotron X-ray measurements.14,15 The flow-induced βcrystal is believed to originate from oriented α-row nuclei, whose surfaces possess β-crystal nucleation ability. Naturally, the combination of shear flow and β-NA is supposed to be capable of synergistically inducing more β-crystals. Unfortunately, the coexistence of shear flow and β-NA is, in fact, unfavorable for β-crystal formation. The underlying mechanism is competitive growth between flow-induced α-nuclei and β© XXXX American Chemical Society

Received: January 7, 2017 Revised: June 1, 2017

A

DOI: 10.1021/acs.macromol.7b00041 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules aroused our interest to determine the pressure and flow window for generating β-crystal, especially in iPP mixed with βNA. In our previous work on the formation and microstructure of γ-crystal in pure iPP under the coexistence of flow and pressure, we observed the special dependence of γ-crystal on the shear and pressure.23,24 In the current work, we systematically studied the crystalline structure of iPP mixed with β-NA crystallized under the coexistence of pressure and flow. For the first time, we obtained the window of pressure and flow to produce βcrystals in iPP mixed with β-NA. Moreover, the versatile role of pressure in β-crystal formation under flow was revealed by combining classical nucleation theory and the growth of different crystalline phases. Our work elucidates the β-crystal formation in iPP mixed with β-NA when pressure and flow are coexistent and lays a solid foundation for manipulating the polymorphic composition of iPP products in commercial processing.

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Figure 1. Schematic diagram of the temperature, pressure, and shear flow (linearly distributed shear rates range from 0.0 to 24.0 s−1) employed as a function of time.

during the isothermal period, and thus, the crystallization during the cooling process was leaved out in the current work; see the Supporting Information), the specimens were cooled to room temperature (the cooling rates can be maintained at approximately 20 °C/min from 210 to 100 °C and approximately 10 °C/min from 100 to 25 °C by the circulated water cooling method). The specimen was disk-shaped with a radius r = 10 mm and a thickness d = 0.5 mm. The shear rates applied to the iPP melt increased linearly with the radius and ranged from 0.0 to 24.0 s−1 (γ̇ = ωr/d, where ω is the angular velocity of the rotator disk, r is the radial position of the disk, and d is the thickness of the sample). 2D-WAXD/SAXS Measurements. To identify the crystalline structure in the radial direction, a fan-shaped area in the specimen was cut out (see Figure S3). The X-ray beam moves along the radial direction from the center with its incident direction perpendicular to the surface of the specimen. Two-dimensional wide-angle X-ray diffraction and small-angle X-ray scattering (2D-WAXD/SAXS) measurements were conducted at the beamlines BL15U1 and BL16B1, respectively, of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China). For the 2D-WAXD measurements, a monochromatic X-ray beam with a wavelength of 0.124 nm was focused on an area of 3 × 2.7 μm2 (length × width), and the sampleto-detector distance was 172.5 mm. As to the 2D-SAXS measurements, the monochromatic X-ray beam operated at a wavelength of 0.124 nm with a beam size of 300 × 500 μm2 (length × width), and the sample-to-detector distance was maintained at 1908 mm. The 2DWAXD/SAXS images were collected with an X-ray charge-coupled device (CCD) detector (Model SX165, resolution of 2048 × 2048 pixels, Rayonix Co. Ltd., USA). Each 2D-WAXD/SAXS pattern was background corrected and normalized using a standard procedure. The scattering patterns after calibration were circularly averaged at a constant q, resulting in onedimensional WAXD (1D-WAXD) intensity curves. For the 1D-WAXD profiles, the relative content of the β-crystal (Kβ) can be calculated using the methodology developed by Obadal, when the system consists of α-, β-, and γ-crystals:21,22

EXPERIMENTAL SECTION

Materials. The iPP used in this experiment was a commercial grade iPP with a melt flow rate (MFR) of 3.0 g/10 min (230 °C, 21.6 N), a weight-average molecular mass Mw of 3.99 × 105 g/mol, and a molecular weight distribution index Mw/Mn of 4.6. The iPP was purchased from Dushanzi Petroleum Chemical Co., Dushanzi, China. The β-NA arylamide derivative (TMB-5) with a chemical structure similar to N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide was kindly donated by the Fine Chemicals Department of the Shanxi Provincial Institute of Chemical Industry, China. Sample Preparation. Using a HAAKE POLYLAB OS torque rheometer, β-NA was melt mixed with iPP at 190 °C for 10 min at a rotation speed of 30 rpm. Then, the obtained iPP mixed with β-NA with 0.2 wt % β-NA was cut into pellets. The combined effects of pressure and shear flow on the iPP mixed with β-NA were investigated using a custom-designed pressuring and shearing device (PSD, see Figure S1) that can simultaneously provide pressure and flow. This device is described in the Supporting Information. To explore the window of pressure and flow to produce β-crystals, the crystallization temperatures were carefully selected. The β-crystal grows faster than the α-crystal in the temperature range 100−140 °C;7,25−27 hence, the crystallization temperature was set at 134 °C when a pressure of 5 MPa was applied. Subsequently, the experimental crystallization temperatures increased with the pressure to maintain a constant undercooling (ΔT = T0m(P) − Tc(P), where T0m(P) and Tc(P) are the equilibrium melting temperature and the crystallization temperature of iPP at pressure, P) because the equilibrium melting temperature increased with the increase in pressure. The relation between the equilibrium melting temperature of iPP and pressure can be expressed as the equation28

Tm0(P) = Tm0 + ζ(P − P0)

(1)

where T0m is the equilibrium melting temperature of iPP at atmosphere pressure, P0 is the atmospheric pressure, and ζ is the pressure dependence of the melting temperature (the most frequently used values of 186.1 °C for T0m and 0.3 °C/MPa for ζ were applied).28,29 Hence, to maintain a undercooling of ∼51 °C, the crystallization temperatures were set to 134, 149, 164, and 179 °C as the pressures were 5, 50, 100, and 150 MPa, respectively. The experimental protocol is shown in Figure 1. iPP mixed with β-NA were first heated and annealed at 210 °C for 15 min to erase the memory of previous thermal and mechanical histories and then cooled to a desired temperature (Tc = 134, 149, 164, and 179 °C). Then, programmed pressures (5, 50, 100, and 150 MPa, corresponding to the crystallization temperatures from low to high, respectively) and shear flow with a fixed duration of 10 s for every specimen were applied. After maintaining the temperature at Tc for 30 min (this time was sufficient to ensure that iPP mixed with β-NA mainly crystallized

Kβ =

Aβ (110)

Aα (110) + Aβ (110) + Aα (040) + Aα (130) + A γ (117) (2) where Aβ(110) is the area of the β(110) diffraction peak of β-crystal (the characteristic diffraction peak of β-crystal was designated as (300) for a hexagonal unit cell in the past,30 and now it turned out to be the (110) plane in a trigonal unit cell31,32); Aγ(117) is the area of the γ(117) diffraction peak for γ-crystal; and Aα(110), Aα(040), and Aα(130) are the areas of the α(110), α(040), and α(130) diffraction peaks, respectively, of α-crystal. The crystallinity index of β-crystal (Xβ) in this case is given by the equation Xβ = Xc × Kβ B

(3) DOI: 10.1021/acs.macromol.7b00041 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. 2D-WAXD patterns of iPP mixed with β-NA crystallized at various shear rates (0.0−24.0 s−1) and different pressures (5, 50, 100, and 150 MPa). The shear flow is in the vertical direction. where the crystallinity (Xc) is evaluated by the ratio of the area under the resolved Gaussian crystalline peaks to the total area under the unresolved diffraction curve.33 The shared crystallinity of both α- and γ-crystal (Xα+γ) in the threephase crystalline system can be defined by the equation Xα + γ = Xc − Xβ

macroscopically isotropic lamellar structure. As a pressure of 5 or 50 MPa is applied, the appearance of strong characteristic diffraction ring of β(110) manifests the formation of β-crystal due to the high nucleation efficiency of β-NA. However, when the pressure increases to 150 MPa, instead of the β(110) diffraction ring, the signature γ(117) diffraction ring emerges. This result means that the application of 150 MPa tremendously restrains the β-crystal formation. The most likely scenario is that the elevated pressure (150 MPa) facilitates the nucleation and growth of γ-crystal, and thereby the crystallization process is overwhelmingly dominated by the γcrystal. When a shear flow is imposed, polymer chains are stretched to induce oriented crystals. We first observed the lamellae orientation before turning to the β-crystal in the iPP mixed with β-NA that experienced simultaneous effects of pressure and shear flow. As observed in Figure 2, in the relatively high shear rate regions (i.e., γ̇ = 24.0 s−1), the diffraction arcs α(110), α(040), and α(130) were located at the equator, regardless of pressure, which is indicative of the generation of paralleloriented α-crystal (or a c-axis orientation), where the c-axis of the unit cell is parallel to the flow direction. The off-axis arcs of the α(110) and α(130) planes, with the α(110) arc being close to the meridian, come from the daughter lamellae of the αcrystal.32,36 For γ-crystal, the γ(008) arc resides at the equator. In accordance with the work of Hsiao et al. and De Rosa et al., it is evident that γ-crystals are oriented in “parallel chain axis orientation”, where one-half of the chain axes are parallel to the flow direction and the other half are aligned at an angle of ∼81° with respect to the flow direction.32,37,38 However, for β-crystal, only a rather weak orientation is observed, possibly because the β-NA was not completely melted at the temperature of 210 °C; thus, the β-NA self-organization did not occur due to the high melt viscosity.4 In this case, β-NA existed as dot nuclei, which ultimately resulted in randomly dispersed β-crystals. After determining the orientation of the different crystalline phases, we turned our attention to the β-crystal formation. As shown in Figure 2, the 2D-WAXD patterns reveal that the diffraction ring β(110) evolution with increasing shear rate shows a clear distinction between different pressures. Strong diffraction rings of β(110) can be observed over the entire shear rate range when P = 5 MPa, a very low pressure level. As the pressure increases to 50−100 MPa, they fade away quickly with increasing shear rate and then disappear entirely at a relatively low shear rate (e.g., 8.0 s−1). When the pressure further rises to 150 MPa, the β(110) diffraction ring disappears at all shear rates. These diversified behaviors certainly resulted from the

(4)

Most diffraction peaks of α- and γ-crystal overlap, but α(130) and γ(117) are the unique reflections of α- and γ-crystal, respectively, and were used to determine their fraction, as follows:34 Kγ =

A γ (117) A γ (117) + Aα (130)

(5)

Therefore, the crystallinity values of α-crystal (Xα) and γ-crystal (Xγ) are given by the following equations: X γ = Xα + γ × K γ

(6)

Xα = Xα + γ × (1 − K γ )

(7)

The SAXS data were azimuthally integrated to obtain intensity patterns, I(φ), for calculating the degrees of orientation of lamellae, i.e., Hermans’ orientation function ( f H), which is defined as follows:35 fH =

3⟨cos2(ϕ)⟩ − 1 2

(8)

where 2

⟨cos (ϕ)⟩ =

∫0

π /2

I(ϕ) cos2 ϕ sin ϕ dϕ

∫0

π /2

I(ϕ) sin ϕ dϕ

(9)

is the average angle between the lamellar normal and a reference direction (e.g., the flow direction). Differential Scanning Calorimetry (DSC) Measurements. DSC measurements were performed to characterize the melting temperatures using a differential scanning calorimeter (TA DSC Q2000) at a heating rate of 10 °C/min purged in a nitrogen atmosphere (50 mL/min). The temperature scale was calibrated using indium as a standard to ensure the reliability of the acquired data. The data were collected from 40 to 190 °C, and the sample weight was approximately 5 mg.

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RESULTS The iPP mixed with β-NA was crystallized under a given pressure (5, 50, 100, and 150 MPa) and step shear (ranging from 0.0 to 24.0 s−1). To examine the β-crystal in the crystallized iPP mixed with β-NA, 2D-WAXD measurements were performed. A series of representative 2D-WAXD patterns are shown in Figure 2. Under a static condition (γ̇ = 0.0 s−1), a series of Debye−Scherrer rings appear, representative of a C

DOI: 10.1021/acs.macromol.7b00041 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. 1D-WAXD profiles as a function of the scattering vector (q) of iPP mixed with β-NA, obtained from circularly integrated intensities of 2DWAXD patterns in Figure 2.

Figure 4. Crystallinities of iPP mixed with β-NA crystallized at various shear rates (0.0−24.0 s−1) and different pressures (5, 50, 100, and 150 MPa). The crystallinities of α-, β-, and γ-crystals are duplicated with color gradation, where the color change from blue to red indicates the increase in crystallinity.

γ-crystals are presented in Figure 4. At a low pressure of 5 MPa, the increase in shear rate caused only a slight decrease in the intensity of the β(110) diffraction peak (Figure 3a). The crystallinity of β-crystal remains at 0.44, occupying a relative content of 68.6% even when the shear rate increases to 24.0 s−1. The high concentration of β-crystal is mainly attributed to the high nucleation effectiveness of β-NA and the favorable crystallization temperature for β-crystal growth (134 °C).

application of pressure, whose the underlying mechanism will be discussed in detail in the Discussion section. The above results factually portrayed a map of β-crystal formation as a function of both pressure and flow in iPP mixed with β-NA. To obtain the window of pressure and flow for generating β-crystal, we calculated the crystallinities of the three crystalline phases after deconvoluting the peaks in the 1DWAXD profiles (Figure 3). The crystallinities of the α-, β-, and D

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crystals in the iPP mixed with β-NA. It is evident that a high concentration of β-crystal can be obtained under a low pressure (∼5 MPa) in iPP mixed with β-NA, where the flow only slightly reduces the β-crystal crystallinity. However, under a higher pressure (50−100 MPa) during iPP mixed with β-NA processing, a static condition (without shear flow) is more favorable for producing a high fraction of β-crystals due to the remarkable suppression effect of shear. The above results also elucidate the amazing phenomenon reported in our previous work that the β-crystal disappeared entirely in the intermediate layer of injection-molded bar of iPP mixed with β-NA.33 The reason that the shear flow completely prevented the β-crystal formation was perplexing at that time, but now it seems reasonably apparent that the pressure existing in the mold cavity played an unfavorable role in the β-crystal formation, i.e., the remarkable suppression of β-crystal formation when flow exists simultaneously. According to the distinct shear rate dependence of the β-crystal, the pressure range can be divided into three regions. At a low pressure (∼5 MPa), β-crystals are dominant in a polymorphic composition over the whole shear rate range. At a higher pressure (50−100 MPa), a high concentration of β-crystals can be obtained only under a nearly static condition, and α-crystals become dominant when a strong shear flow applies. When the pressure exceeds 100 MPa, the formation of β-crystal seems impossible at any shear rate, and the γ-crystal become dominant because of the favorable thermodynamic conditions for γ-crystal crystallization. These results allow the elucidation of the polymorphic composition under varied pressures and shear strengths. More importantly, we will be able to manipulate the polymorphic composition of iPP products and thus enhance their properties in real processing. Melting behavior is strongly related to the crystalline structure. For a comparison, the DSC heating curves are presented in Figure 5. At a low pressure (5 MPa), iPP mixed with β-NA exhibits double endothermic peaks at the temperatures of 155.7 and 165.3 °C over the entire shear rate range, where the endothermic peak at the lower melting temperature (155.7 °C) is unequivocally ascribed to the fusion of β-crystals because of their higher degree of disorder2,39 and the second endothermic peak (165.3 °C) can be assigned to the fusion of α-crystals. Considering the WAXD results that β-crystals are dominant in a polymorphic composition over the entire shear rate range under 5 MPa, it is likely that the α-crystal endothermic peak is due to β to α recrystallization during the heating process because of the thermal instability of βcrystals.40,41 When the pressure increases to 50 MPa, the first endothermic peak shifts to a higher temperature (157.6 °C) under static conditions due to the γ-crystal formation (the melting temperature of γ-crystals is slightly higher than that of the β-crystals, while lower than that of the α-crystals). In particular, as the pressure increases to 100 MPa, iPP mixed with β-NA shows triple endothermic peaks at the temperatures of 150.7, 161.2, and 170.2 °C under static conditions, mostly likely corresponding to the fusion of β-, γ-, and α-crystals, respectively. Furthermore, the application of shear flow gives rise to a broad endothermic peak located from 160 to 170 °C at both 50 and 100 MPa, implying the extinction of β-crystals, which is in good agreement with the WAXD results. When the pressure is 150 MPa, iPP mixed with β-NA again exhibits double endothermic peaks, at the temperatures of 163.3 and 170.5 °C, indicative of the fusion of γ- and α-crystals.

Meanwhile, a low pressure (5 MPa) would not significantly promote the thermodynamic conditions for γ-crystal crystallization, and therefore no γ-crystal formed, which also favored the formation of β-crystal. In contrast, when P = 50 MPa, a very low shear rate (3.2 s−1) leads to a remarkable intensity drop in the β(110) diffraction peak (Figure 3b), and the β-crystal crystallinity decreases from 0.52 (under a static condition) to 0.05. Continuing to increase the shear rate (8.0 s−1), the βcrystals are totally suppressed. Clearly, the β-crystal formation depends not only on the shear flow intensity but also on the imposed pressure level because the pressure determines the thermodynamic state and affects the nucleation and growth of different crystalline phases. When the pressure further increases to 150 MPa, the thermodynamic state is favorable for γ-crystal crystallization. Both the nucleation and growth rates of γ-crystal were increased by the elevated pressure; thus, γ-crystal dominated the crystallization process of iPP mixed with βNA. As a result, β-crystals were so completely suppressed that there was no trace of β-crystals at any shear rate (Figure 3d). Additionally, note that the application of shear flow no longer induces β-crystal formation when the imposed pressure is above 5 MPa (see the 2D-WAXD in Figure S5, the crystalline structure of iPP crystallized over an identical shear rate range (0.0−24 s−1) and identical pressures (5, 50, 100, and 150 MPa) as those of the iPP mixed with β-NA samples). Thus, β-crystals can only be induced by β-NA, which is one of the reasons for their disappearance under higher pressures (>5 MPa). The above results indicate an intriguing phenomenon wherein the suppression of β-crystal formation by shear flow is negligible at a low pressure (e.g., 5 MPa) but becomes dominant at a higher pressure (e.g., 50 MPa). To determine the role of pressure in the iPP mixed with β-NA crystallization when shear flow exists simultaneously, we performed a precise examination of the relative content of β-crystal (Kβ) and compared the Kβ values of samples obtained under different pressures at the same shear rate (∼15.0 s−1) and undercooling (∼51 °C). As illustrated in Table 1, when the shear flow is Table 1. β-Crystal Percentage for IPP Mixed with β-NA Crystallized under Different Pressures 5, 50, 100, and 150 MPa, under Static or Shearing Conditions (Shear Rate 15.0 s−1)a pressure (MPa) Kβ-Q (%) Kβ-S (%)

5

50

100

150

86.9 84.0

77.7 0.9

49.5 0.4

0 0

a

They are denoted as Kβ-Q and Kβ-S, respectively, under static and shear.

applied at 5 MPa, Kβ changes from 86.9% (static condition) to 84.0%, representing a slight decrease of approximately 2.9%. However, this situation is entirely changed at a higher pressure (50−100 MPa). For instance, when P = 50 MPa, the flow results in a notable reduction in the Kβ (to ∼0.9% from 77.7%, a change of 76.8%). This reduction is much more remarkable than the total reduction caused by the sole effects of pressure (∼13.7%) or shear flow (∼21.2%), demonstrating that the βcrystal suppression resulting from the shear flow and higher pressure (50−100 MPa) is not a simple summation but rather a synergistic effect. The contour map of the β-crystal crystallinity (Figure 4) clearly depicts the window of pressure and flow to fabricate βE

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Figure 5. DSC heating curves of iPP mixed with β-NA crystallized at various shear rates (0.0−24.0 s−1) and different pressures (5, 50, 100, and 150 MPa). Five representative shear rates 0.0, 6.0, 16.0, 20.0, and 24.0 s−1 were selected for studying the melting behaviors.

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DISCUSSION In the above section, we systematically examined the crystalline structures of iPP mixed with β-NA under the coexistence of pressure and flow. For the first time, we obtained the window of pressure and flow to produce β-crystals in iPP mixed with βNA. Moreover, we observed an intriguing phenomenon in which the pressure played multiple roles in the iPP mixed with β-NA crystallization under a flow field, which is vital to the successful manipulation of β-crystals in practical processing. In this section, we will discuss the origin of the diversified crystallization behaviors of β-crystals in iPP mixed with β-NA under pressure and flow. From a conventional perspective, polymer crystallization from melt starts with stable nuclei formation. The nucleation and subsequent crystal growth determine the resultant crystalline structure. In the case of iPP mixed with β-NA in the current work, nucleation is mainly controlled by the crystallization temperature, pressure, flow, and β-NA. β-NA can selectively induce the formation of β-crystal because the crystal planes of β-NA are inclined to induce epitaxial growth of βcrystal based on the dimensional lattice matching theory proposed by Lotz et al.27,42,43 Heterogeneous nuclei (β-nuclei) originating from the β-NA are only slightly influenced by the pressure and flow because of the unchanged nucleation surface (the β-NA exists as dot nuclei in our case). Thus, the nucleation density of the β-nuclei induced by the β-NA is consistent over various shear rates and pressures. Here, we focus on the nucleation possibilities of the three crystalline phases under the coexistence of pressure and flow. In Turnbull−Fisher’s classical nucleation theory, the nucleation rate is dominated by the free energy of the nucleation barrier (ΔG*) and the free energy of activation (ΔGη).44 Using a folded-chain lamellar nucleus

model, the nucleation barriers for different crystalline phases under a static condition can be expressed as follows:45 ΔGi*,q =

32σi 2σi ,eTi0,m

2

(ΔHiΔTiρi ,c )2

(10)

where σi and σi,e are the specific surface free energies of the lateral and end surfaces, ΔHi and ρi,c are the melting enthalpy and density of the crystal, respectively, and T0i,m and ΔTi are the equilibrium melting temperature and undercooling, respectively. The subscript i represents the specific crystalline phase of α-, β-, or γ-crystals. The effect of pressure on ΔGi,q * is largely due to its influence on T0i,m, where the pressure dependence of T0i,m can be estimated by the Clapeyron equation, which is defined as follows:46 dTi0,m dP

=

Ti0,m ⎛ ρi ,c − ρa ⎞ ⎜ ⎟ ΔHi ⎜⎝ ρi ,c ρa ⎟⎠

(11)

where ρa is the density of the amorphous phase. Combining eqs 10 and 11, ΔGi,q * for the three crystalline phases under a given pressure can be calculated; the results are shown in Figure 6 (a summary of the relevant parameters used in the comparison is given in the Supporting Information). The variation tendencies of ΔGi,q * with the increase in pressure are different for the three crystalline phases. The nucleation barriers of α- and β-crystals increase with pressure, while the nucleation barrier of γ-crystals slightly decreases with pressure. At a low pressure (∼5 MPa), ΔGα,q * < ΔGβ,q * < ΔGγ,q * . Thus, from a thermodynamics perspective, the nucleation rate of α-crystals is faster than that of β- and γ-crystals. However, because of the addition of βNA, the secondary nucleation introduced by β-NA has a substantial effect on the nucleation of the β-crystal. Abundant F

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polymer melt by a factor of TΔSf. The corresponding nucleation barrier under a shear can be expressed as ΔG*i,f = ΔGi,q * − TΔSf, where ΔSf is the entropic reduction, which is related to the chain deformation.48 The ΔSf increases with the shear rate. Nevertheless, the variation tendency of ΔG*i,f with an increasing shear rate is undoubtedly the same as that of ΔG*i,q despite their different quantitative values. At a low pressure (∼5 MPa), two types of nuclei, i.e., flow-induced nuclei for α-crystal growth24 and β-NA-induced nuclei for β-crystal growth, can coexist simultaneously in the iPP melt. Meanwhile, it is wellknown that the density of flow-induced nuclei is significantly greater than that when using nucleating agents especially when an intense flow is imposed on the polymer melt.17,49 Therefore, as shown in the careful research performed by Chen et al.,17 competitive growth of α- and β-crystals occurs under a shear flow, leading to a decrease in β-crystals compared to that under quiescent crystallization conditions. Additionally, the application of a low pressure (5 MPa) results in a mild augmentation of the equilibrium melting temperature. In other words, the undercooling of the melt is slightly enhanced, which causes a more substantial increase in the growth rate of the β-crystals than that of the α-crystals.47 Therefore, the β-crystal crystallinity at a low pressure shows a gentle decrease with shear rate compared with that under atmospheric pressure conditions (under the same crystallization temperature). As discussed above, flow-induced nuclei for α-crystal growth are responsible for the decrease in the β-crystal quantity at a low pressure (e.g., 5 MPa). Thus, a relationship should exist between the flow intensity and the fraction of the β-crystal. To explore this relationship, we examined the sample of iPP mixed with β-NA crystallized at 5 MPa and a shear flow. The results are shown in Figure 7. The insets (SAXS patterns) display a

Figure 6. Calculated free energies of the nucleation barriers (ΔGi*) for three crystal forms (α-, β-, and γ-crystals) plotted versus pressure.

β-crystals can be obtained because β-NA provides a large quantity of β-nucleation sites for β-crystal growth. Additionally, the β-crystal grow faster than the α- and γ-crystal because the temperature of iPP mixed with β-NA crystallized at 5 MPa is 134 °C, which is within the temperature range of 100−140 °C;7,25−27 hence, β-crystals are the dominant crystalline phase under static conditions. When the crystallization pressure increases (50−100 MPa), the nucleation barriers of the three phases have the relationship ΔG*α,q < ΔG*γ,q < ΔG*β,q; therefore, nuclei of γ-crystal are likely to generate. For a molecular conformation perspective, αcrystals, whose (010) crystallographic planes are composed of a highly symmetrical, lozenge-shaped array of methyl groups, can associate with the contact face of the β(110) plane, resulting in a growth transition to β-crystals.27 The γ-crystal has a peculiar crystalline structure with nonparallel stem axes within the unit cell, and they are incapable of producing β-crystal epitaxial growth. Moreover, the maximum growth rates of the α- and γcrystals are found to increase with increasing pressure, leading to adverse conditions for β-crystal growth.47 Under this circumstance, although the pressure (50−100 MPa) is insufficient to completely suppress the β-crystal formation for the strong heterogeneous nucleation of β-NA, it leads to a decrease in β-crystals. Hence, all three crystalline phases can be found in the iPP mixed with β-NA. As the pressure further increases to 150 MPa, the nucleation barriers of the three phases change (ΔGα,q * < ΔGγ,q * < ΔGβ,q * ). The nucleation possibility of γ-crystal increases and numerous nuclei of γ-crystal are likely to form. Besides, the γ-crystal tends to be more stable at a higher pressure according to Phillips et al.20 Hence, an elevated pressure creates a thermodynamically challenging situation for β-crystal nucleation. Meanwhile, the γcrystal growth rate is significantly accelerated at elevated pressures,47 resulting in unfavorable crystallization conditions for β-crystal growth. Therefore, even though the addition of βNA offers a large quantity of β-nucleation sites for β-crystal growth, there are few possibilities for the nucleation or for the growth of the β-crystals under an elevated pressure (150 MPa), and the γ-crystals become dominant despite the loading of βNA. When a shear flow is imposed, the situation is more complex. Flow-induced chain orientation and stretch can lower the conformational entropy and increase the free energy of a

Figure 7. Relative content of the β-crystal (Kβ) and Hermans’ orientation parameter ( f H) for iPP mixed with β-NA crystallized at 5 MPa accompanying with shear flow (0.0−24.0 s−1). The insets are SAXS patterns taken at four designated shear rates (3.4, 10.3, 17.2, and 24.0 s−1). White arrows in the insets represent the shear flow direction.

clear scattering lobe along the flow direction due to the packages or stacks of lamellae when the shear rate is approximately 3.4 s−1. Furthermore, the azimuthal width becomes narrower with shear rate because of the increase in lamellar orientation degree. Quantitative studies on the orientation of lamellar structure were performed using Hermans’ orientation function ( f H).35 It is obvious that the variation tendency of f H with shear rate is inverse to the relative G

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Macromolecules content of β-crystals. Based on the value of f H calculated at higher pressures, a color gradation map of f H is given in Figure S6. The color gradation map of f H remains consistent with the color gradation map of Xβ at a low pressure (∼5 MPa); large deviations arise at a higher pressure (≥50 MPa). These intriguing results demonstrate that the competitive growth mechanism between α- and β-crystals is insufficient at a higher pressure (≥50 MPa), thus requiring a new explanation. According to the results observed under a higher pressure (50−100 MPa) and at a weak shear flow, i.e., that the β-crystal concentration drops precipitously compared to that at a low pressure (∼5 MPa), we propose a plausible explanation as follows: As a higher pressure is applied, the nucleation barrier for γ-crystal decreases, so, except for flow-induced nuclei for αcrystal growth, pressure-induced nuclei for γ-crystal growth appear. In this state, there are three types of stable nuclei (α, β, and γ) at the beginning of isothermal crystallization. The βnuclei mainly originate from the β-NA, while the nucleation density of α-crystal greatly increases due to the application of shear flow. When the shear is relatively weak, the abundant flow-induced α-nuclei can not only enhance the final concentration of α-crystal but also provide favorable conditions for the γ-crystal crystallization because the (010) lateral face of the α-crystal is ideal surfaces for epitaxial crystallization of γcrystal.50−52 As a result, the concentrations of both α- and γcrystals increase under a weak flow compared to the static condition (in agreement with Figure 4). Correspondingly, the β-crystal concentration is low for the iPP mixed with β-NA when a medium pressure (50−100 MPa) and a weak shear flow (∼3.2 s−1) coexist. In addition, under such a medium pressure, if the shear flow is so strong that abundant oriented-nuclei for α-crystal growth can be induced and fewer spaces are left for the remaining melt to crystallize into other crystalline phases, both β- and γ-crystals begin to be suppressed (see Figure 4) and the α-crystal becomes dominant. When a flow field is imposed simultaneously with an elevated pressure (150 MPa), flow-induced nuclei for α-crystal growth remarkably increase in quantity; simultaneously, the γ-crystals demonstrate great nucleation and growth rates. Thus, there is no chance for βcrystal formation (see Figure 4), and the β-crystal disappears. In this case, the flow only changes the relative contents of α- and γ-crystals, as discussed in our previous work.24

NA can be well elucidated. The current results provide a potential method to tailor the concentration of the β-form, thereby optimizing the mechanical properties of iPP products in real processing.

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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.7b00041. Figures S1−S6 and Tables S1, S2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yan-Hui Chen: 0000-0001-6511-1738 Jun Lei: 0000-0001-6803-5216 Liangbin Li: 0000-0002-1887-9856 Zhong-Ming Li: 0000-0001-7203-1453 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 51533004, 51573119, 51473135, and 51421061), the Programme of Introducing Talents of Discipline to Universities (B13040), and the Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University, sklpme 2015-1-01 and sklpme2016-2-02). The authors are also grateful for the kind help and support of Beamline BL16B and BL15U in Shanghai Synchrotron Radiation Facility (SSRF) in SAXS and WAXD measurement and the results analysis.

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REFERENCES

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CONCLUSIONS The combined effects of shear flow (0.0−24.0 s−1) and pressure (5, 50, 100, and 150 MPa) on β-crystal crystallization in iPP mixed with β-NA were revealed using a PSD (exerting pressure and shear flow) and ex-situ WAXD/SAXS/DSC (detecting crystalline structure and melting behavior). The results show that the β-crystal formation in iPP mixed with β-NA strongly depends on the pressure and shear flow. A window of pressure and flow to generate β-crystals in iPP mixed with β-NA was revealed. A high fraction of β-crystals can be obtained under a low pressure (e.g., 5 MPa), where the β-crystal formation is slightly suppressed with an increase in the shear rate. As the pressure increases (50−100 MPa), the β-crystal crystallinity significantly decreases, even under a weak shear flow, because of the combined suppression effects of pressure and shear flow. With further increasing the pressure up to 150 MPa, β-crystals cannot be generated for thermodynamic reasons. Combining classical nucleation theory with the growth rates of the three crystalline phases, the simultaneous effects of pressure and shear flow on the β-crystal crystallization in iPP mixed with βH

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