Wall Slip Effect on Shear-Induced Crystallization Behavior of Isotactic

Article pubs.acs.org/IECR

Wall Slip Effect on Shear-Induced Crystallization Behavior of Isotactic Polypropylene Containing β‑Nucleating Agent Baojing Luo,† Hongfei Li,*,‡ Yao Zhang,† Feifei Xue,† Peipei Guan,† Jing Zhao,† Chengbo Zhou,† Wenyang Zhang,† Jingqing Li,† Hong Huo,§ Dean Shi,∥ Donghong Yu,⊥ and Shichun Jiang*,† †

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P. R. China State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China § Institute of Polymer Chemistry and Physics, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China ∥ Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, China ⊥ Department of Biotechnology, Chemistry, and Environmental Engineering, Aalborg University, DK-9000, Aalborg, Denmark ‡

ABSTRACT: Shearing is unavoidable during the polymer process, and isotactic polypropylene (iPP) is one of the most used commercial polymers. iPP mixed with β-nucleating agent TMB-5 was isothermally crystallized at 135 °C from melts under various shear conditions and investigated via synchrotron radiation small-angle and wide-angle X-ray scattering (SAXS and WAXS) techniques with CCD detector. The shear-induced crystalline structure and the orientation of the samples were studied according to the obtained WAXS and SAXS patterns. The results showed that both the crystallinity of α-iPP and β-iPP are in direct proportion to the orientation degree rather than shear rate especially at high shear rate, which proves that wall slip should not be neglected when taking shear effect or rheological behavior into consideration. attributed the plateau content of β-phase to the maximum proportion of oriented polymer chains determined by the critical orientation molecular weight (M*). On the contrary, Huo et al.20 found a decreased content of β-iPP when shear rate exceeded 20 s−1, which was considered to be caused by the spatial restriction from the spread growth of α-crystals in iPP melt. Adding a β-nucleating agent is an efficient approach for iPP to form β-crystals,21−23 thus it can be assumed that under shear flow the β-nucleated iPP melt could form a higher fraction of βcrystals. However, the fact is far beyond expectation. Huo et al.20 performed a systematic study on the crystallization of sheared iPP with β-nucleating agent and revealed that shear suppresses the efficiency of the β-nucleating agent. Hsiao et al.22 investigated the competitive growth of α- and β-crystal in β-nucleated iPP, and found a counteraction of shear to the formation of β-iPP. In the present work, the influence of shear on the crystallization of β-nucleated iPP and pure iPP was investigated. We found the same decreased formation of βcrystals in β-nucleated iPP with shearing, which demonstrates the antagonism between shear and the β-nucleating agent. According to the Varga-Karger-Kocsis model13 the formation of the β-phase in sheared iPP depends on the formation of α-row nuclei which have dual nucleating ability. Thus, it is considered that the depressed formation of the β-crystal in β-nucleated iPP with initially increased shear rate is caused by the overgrowth of

1. INTRODUCTION In the processing of crystalline polymers such as injection molding, extrusion, film blowing, and spinning, molten polymer chains are subjected to intense flow fields before crystallization. 1 The flow fields have a strong impact on the conformation, orientation, arrangement, and distribution of polymer chains in the melt, thus affect the crystallization kinetics as well as the crystal structure and morphology of crystalline polymers which determines the final product properties.1−7 It is essential and attractive to optimize and improve the performance of crystalline polymer products via the research of shear-induced polymer crystallization. Isotactic polypropylene (iPP) has been extensively investigated in polymer research.8−11 Various crystalline modifications (α, β, and γ) appear in the formation of crystal structure depending on the crystallization conditions which exactly affect the morphologies of highly order structures.12 The performance of iPP materials mainly depends on the contents of α-, and βcrystals. Since β-iPP was known to possess excellent mechanical properties such as impact strength, toughness, ductility, and tensile strength, how to modify the content of β-crystals in iPP products has been a remarkable subject for decades. It was found that shear or elongation can induce the β-form of crystalline iPP,13−16 even at a very low shear rate.17 Up to date, most of the related studies focused on the relationship between shear rates and β-form of iPP.1 Hsiao et al.18,19 investigated the shear-induced crystallization structure development of iPP and found that the amount of β-iPP increased with shear rate at a constant strain and reached a plateau at a shear rate of 57 s−1. They associated the content of β-crystals with the surface area of oriented α-cylindrites/α-row nuclei, and © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13513

May 24, 2014 July 19, 2014 August 8, 2014 August 8, 2014 dx.doi.org/10.1021/ie502062k | Ind. Eng. Chem. Res. 2014, 53, 13513−13521

Industrial & Engineering Chemistry Research

Article

Figure 1. Selected WAXS patterns of crystallized pure iPP (upper row) and iPP/TMB-5 (bottom row) crystallized at 135 °C under different shear rates: 0 s−1 (a, f), 5 s−1 (b, g), 20 s−1 (c, h), 40 s−1 (d, i), and 60 s−1 (e, j). The arrow bar indicates the flow direction (FD).

α-phase due to the exceeded formation of α-row nuclei. However, considering the crystallization temperature (Tc) and shear temperature (Ts) were identically 135 °C, at which the kinetic requirement (Gβ > Gα) and the shearing temperature requirement (Ts < Tβα ≈ 141 °C) for α-row nuclei induced βmodification were satisfied,13 the β-formation in iPP bulk was supposed to be promoted by the enhanced formation of α-row nuclei as in pure iPP under the same range of shear rate. On the other hand, the temperature-dependent selective nucleating agent TMB-5 only promotes the β-nucleation at 135 °C.4 Furthermore, different from the previous results,20,22 when the applied shear rate was high, the content of β-crystals increased in β-nucleated iPP while the content decreased in pure iPP. Therefore, we considered that the wall slip reduces the shear flow imposed on the polymer melt. It is well-known that the classical Newtonian/small-molecule fluids stick to the solid wall during flow.24 Contrarily, wall slip usually occurs for molten polymers at the melt/wall interface under a certain wall shear stress τc.25−27 Wall slip has dramatic effects on the structure and properties of final industrial products of polymeric materials on account of its influence on the flow conditions during processing.28,29 Many studies have been undetaken to investigate crystallization behaviors in sheared iPP with or without β-nucleating agent; however, the essence of the β-nucleating mechanism still remains a longstanding argument. Herein, regardless of the original mechanism for β-iPP formation, influences of shear on crystallization of pure iPP and β-nucleated iPP were investigated with considering the effect of wall slip. From our research, it is found that wall slip is a non-negligible factor for the polymer structures especially under high shear rate.

2.2. Sample Preparation. iPP granules were melt-mixed with 0.1 wt % TMB-5 nucleating agents (assigned as iPP/TMB5) at 190 °C for 10 min via the 60 mL chamber of XSS-300 torque rheometer with a rotation speed of 30 rpm. The pure iPP was treated with the same process. A Linkam CSS-450 thermal stage (Linkam Scientific Instruments, Ltd., U.K.) equipped with the standard quartz optical windows on the top and bottom steel blocks was used to control the temperature and shear conditions. The samples with a thickness of 1 mm were prepared as reported.20,33 2.3. Measurement and Characterization. X-ray scattering measurements were performed at Beamline 1W2A with λ = 0.154 nm of Beijing Synchrotron Radiation Facility (Beijing, China). A Mar165-CCD was employed for collection of 2DWAXS and 2D-SAXS images. The sample-to-detector distance was 1593 mm for pure iPP and 1650 mm for iPP/TMB-5 in SAXS measurements, and was 103 mm in all WAXS measurements. Blank scattering patterns obtained separately were used for the background correction and normalization of the data.34 The aperture positions of all crystallized samples were marked to be investigated. The relative amounts of α-crystal and β-crystal (Kα and Kβ) were evaluated according to the method of Turner Jones et al.35 which was further modified by Hsiao et al.:15,18 Kβ =

Aβ(300) Aα(110) + Aα(040) + Aα(130) + Aβ(300)

Kα = 1‐Kβ

(1) (2)

where Aβ(300), Aα(110), Aα(040), and Aα(130) are the areas of β(300), α(110), α(040), and α(130) reflection peaks, respectively. Prior to calculation, a peak-fitting procedure was implemented to the integrated WAXS profiles.15,18,36 Lamellae orientation was calculated from the Azimuthal profiles of SAXS patterns using the Hermans’ formula, which is defined as follows:37

2. EXPERIMENT SECTION 2.1. Materials. iPP (T1701) was provided by Yanshan Petrochemical Corp. Inc. The Mw and Mn are 3.0 × 105 g/mol and 7.4 × 104 g/mol, respectively. The melting temperature of the iPP materials is approximately 165 °C measured by DSC (TA Q-2000) with a heating rate of 10 °C/min. The βnucleating agent is an aryl amides derivative (trade name TMB5,4,30−32 supplied by Chemical Institute of Shanxi, China). The raw TMB-5 nucleating agent would not melt during the melting process of iPP/TMB-5 mixture (up to 200 °C) observed by polarized optical microscope (POM).

fh =

3⟨cos2 φ⟩−1 2

(3)

where ⟨cos φ⟩ is the orientation parameter defined as 2

2

⟨cos φ⟩ =

∫0

π /2

I(φ) sin φ cos2 φ dφ

∫0 13514

π /2

I(φ) sin φ dφ

(4)

dx.doi.org/10.1021/ie502062k | Ind. Eng. Chem. Res. 2014, 53, 13513−13521


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Figure 2. WAXS profiles for (a) pure iPP and (b) iPP/TMB-5 under various shear rates obtained from corresponding WAXS patterns.

Figure 3. Shear-rate dependent crystallinity of α- and β-crystal of the investigated samples.

parent−daughter lamellae structure of α-iPP. The equator arcing diffractions originate from the mother lamellae with their normals (c-axis) along the FD. The meridian arcing diffractions are caused by the epitaxy growth of daughter lamellae on the lateral ac-faces of mother lamellae, with their a*-axis (originated by a-axis of the daughter lamella) along the FD.38,39 The quiescently crystallized iPP/TMB-5 presents a βdominating diffraction pattern (Figure 1f). With increasing shear rate, the α-diffractions turn to overwhelm the βdiffractions (Figure 1g−i), indicating the suppression of shear on the formation of β-crystals in β-nucleated iPP. On the other hand, the arcing diffractions of iPP/TMB-5 are sharper than that of pure iPP at the same shear rate. Obviously, this is caused by the addition of TMB-5 particles. Varga et al. observed a polymorphic structure around the nucleating agent surface with an α trans-crystalline layer besieged by the β-phase.40 They found that the chain-folded lamellae of the β-phase formed on the surface stand on their edge with the molecular chains lying nearly in the plane of the sample. Regardless of the controversy on β-nucleating mechanism, it is obvious that β-nucleating agents can enhance the orientation of the neighboring polymer chains and the subsequent epitaxy lamellae. In other words, the nucleating agents can provide a “structure template” which can determine the growth direction (orientation) of the epitaxy crystals, resulting in more oriented structures during crystallization. This may partially explain the remarkable lattice orientation of iPP/TMB-5 in Figure 1. When shear rate attains 60 s−1, the WAXS pattern of pure iPP pronounces isotropic diffractions with the absence of βdiffractions (Figure 1e), and the WAXS pattern of iPP/TMB-5 exhibits decreased α-diffractions with redominating β-diffractions (Figure 1j). The results indicate a decreased shear flow effect at high shear rates. To investigate the phenomena, it is

where φ is the angle between a unit of the crystal of interest (e.g., c-axis) and a reference direction (shear flow direction in this work), and I(φ) is the scattering intensity at φ. Herein, when f h is 1, all lamellar normals are perfectly parallel to the flow/shear direction, and when f h is 0, the lamellae are randomly distributed.

3. RESULTS AND DISCUSSION 3.1. WAXS Patterns. Figure 1 shows ex situ WAXS patterns for pure iPP and iPP/TMB-5 crystallized at 135 °C under different shear conditions. All patterns are in the same color scale. The flow direction (FD) is vertical. The designation of azimuthal angle (φ), equator, and meridian are marked in Figure 1f. Each WAXS pattern of pure iPP contains reflections from α-phase. With increasing shear rate (0−40 s−1), the typical β(300) diffraction emerges and enhances with an obvious transition of α-diffractions from isotropic form to arc-like form, indicating the formation of oriented crystals. It was reported22 that the oriented α-crystals formed immediately after shear, subsequently with the inception of oriented β-crystals which finally turned into isotropic form by the spontaneous growth of β-crystals without any preferential orientation.18,22 However, it is difficult to conclude whether the β-crystals are oriented here, because of the overlap between β(300) and α(040) diffractions. Chain orientation within the crystalline regions can be analyzed through the azimuthal intensity distribution. As known, the hk0 reflections of α-iPP represent lattice planes parallel to the cchain axis (coincides with the FD in our experiments). Considering the reciprocity relation between the real space and the reciprocal space, the maximum scatterings of α(040) around the equator indicate the presence of oriented crystalline along the FD. As for the four-arcing pattern of the α(110) diffraction, it is supposed to be caused by the characteristic 13515



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Figure 4. Selected SAXS patterns of crystallized pure iPP (upper row) and iPP/TMB-5 (bottom row). The conditions of a−j are the same as in Figure 2

slip is unavoidable at the polymer−solid interface for viscoelastic polymer fluids.26,44,45 There are two wall slip regions in the shear flow of molten polymers with a transition from weak to strong above a critical shear stress σs.44−47 Considering the relationship between shear stress and shear rate for non-Newtonian fluids σs = Kγ̇nc , a critical shear rate γ̇c = (σs/K)1/n was proposed above which a non-negligible strong wall slip occurs. Here, K is a constant, and n (less than 1 for pseudoplastic fluid) is a specific exponent for its corresponding polymer melt. Since wall slip occurrence at high shear rate significantly influences the actual flow field, simply analyzing the results from the perspective of shear rate seems unreliable. To investigate the real shear effect, SAXS measurements were conducted. 3.3. Lamella Orientation with Shear. The orientation in a sheared polymer originates from the competition of chain orientation during shear and relaxation after shear, which would be further reduced by the spatial growth of crystals. Therefore, orientation degree can be taken to evaluate the true shear effect preserved in polymer melts, which was an absent perspective in former studies20 due to the absence of two-dimensional X-ray diffraction measurements. The true shear effect with consideration of the wall slip influences could be obtained via SAXS measurements. Ex situ SAXS patterns are shown in Figure 4. The definition of azimuthal angle φ, equator, and meridian are marked in Figure 4f. The meridian scattering maxima are attributed to the periodic lamellae (kebabs) being perpendicular to the FD. Note that the SAXS images show no evident sign of equatorial scatterings in Figure 4, which indicate that we would not observe the formation of extended chain crystals (the shish structures). The absence of equator streaks may be caused by the final structure of the prepared samples. In this case, the shish structures may be too farther apart to be observed which was pointed out by Hsiao et al.,48 or the size of the shishes may be too small and/or the volume fraction of the shish structure may be too low to induce a detectable SAXS signal as explained by Somani et al.33 The SAXS pattern of quiescently crystallized iPP (Figure 4a) consists of a diffuse scattering, indicating the absence of detectable oriented structures. When applied with shearing (Figure 4b−d), the meridian scattering maxima appear implying the presence of oriented scatters (lamellae) that are perpendicular to the FD. When shear rate reaches 60 s−1 (Figure 4e), the SAXS pattern returns to a diffuse scattering, indicating the decreased formation of oriented structures.

necessary to further analyze the shearing induced crystalline structures of polymers. 3.2. Analysis of iPP Crystalline Structures. The integrated WAXS profiles obtained from corresponding WAXS patterns are illustrated in Figure 2. The characteristic reflections of α-crystals, that is, α(110), α(040), and α(130), are at 2θ = 14.1°, 16.9° and 18.5°, respectively, and the diffraction of β(300) at 2θ = 16.1° is taken to characterize the existence of β-crystals.41,42 The relative amounts of α-phase (Kα) and β-phase (Kβ) were calculated according to eqs 1 and 2. A standard peak-fitting method18 was used to deconvolute the peaks in WAXS profiles for the calculation of K values. The shear-rate dependent Kα and Kβ are exhibited in Figure 3. For pure iPP (Figure 3a), an initially increased Kβ and decreased Kα with the increasing shear rate is shown, which is consistent with the reported results.13,15,18−20,22,39 This can be explained as follows: the increasing shear can enhance the formation of α-row nuclei which exactly triggers the growth of β-crystals, considering the faster growth rate of β-iPP at 135 °C compared with α-iPP,43 the relative amount of β-crystals is finally enhanced. It is natural to speculate a further increase of Kβ at higher shear rate, however, a declined trace occurs above 10 s−1. Similar results were reported20 and were attributed to the spatial restriction from the spread growth of α-phase caused by the exceeded formation of α-row nuclei. We could not fully agree with this explanation because a reduced shear effect at high shear rate was observed in our experiment. Thus, the decreased formation of β-iPP at high shear rate involves rather complicated reasons. As for iPP/TMB-5, the variations of Kβ and Kα with shear rate are also nonmonotonic (see Figure 3b). Kβ decreases and Kα increases initially with increasing shear rate, indicating an antagonism between shear and the β-nucleating agent toward the formation of β-iPP, consistent with the reported results.20,22 Then the Kβ (Kα) values undergo a slight variation at 20 s−1 and 40 s−1 subsequently with an abrupt increase (decrease) at 60 s−1, which indicates that the antagonism toward the formation of β-iPP is reduced. Although the essence of the antagonism still remains a puzzle, and the dashed curves in Figure 3 just represent a rough trend of the results, it is still justified to conclude that the true shear field imposed on polymer melt is much weaker than the nominal/applied one at high enough shear rates. Considering the abnormal WAXS results at high shear rate, we speculate a wall slip which can decrease the shear flow occurs above a certain shear rate. Unlike Newtonian fluids, wall 13516



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Figure 5. Azimuthal profiles of (a) pure iPP and (b) iPP/TMB-5 according to corresponding SAXS patterns.

positive/effective shear region where the wall slip effects are weak and negligible (illustrated in Figure 7a). However, a decreased trace of orientation degree above 10 s−1 is seen, which is direct evidence of the decreased formation of oriented structures and a further indication of the reduced shear effect imposed on polymer melt. Since temperature is the predominant factor for the relaxation of the polymer chains/ segments and taking into account the constant shear/ crystallization temperature in our experiments, it can be considered that the shear-induced oriented chains/segments would undergo a similar relaxation process after the cessation of shear that was even applied with different shear rate. Thus, the reduced orientation is not caused by chain relaxation, but the essentially decreased shear flow caused by the strong wall slip under high shear rate. The occurrence of a relative velocity between polymeric fluids and the solid boundary in shear/elongation flow field is named wall slip.25−27,52−56 Both coil−stretch transition (illustrated in Figure 7,b1−b2) and desorption (illustrated in Figure 7,c1−c2) can produce massive interfacial slip between a highly entangled melt and a solid surface. However, which process preferentially occurs depends on the surface energy, that is, chain desorption is usually the dominant process on weakly adsorbing surfaces, while a coil−stretch transition is more likely to occur on high energy surfaces.45,46 Most inorganic solid surfaces possess high surface energy which allow sufficient polymer melt adsorption.45 Since we used bare silicon slices as the shear windows, it can be considered that the wall slip in our experiment is induced by the interfacial coil− stretch transition. It has been proven that wall slip occurrence is only possible at one surface.50 Massive researches only considered the polymer/solid slip on the stationary wall without taking into account the rotatory/moving wall. In our experiment, when the applied shear rate is within 10−40 s−1 (assigned as SR2), the crystallized samples have a circular scratch on their top surface. Since the stationary/top widow is smaller than the rotary/ bottom one in our experiment, it is easy to confirm the occurrence of strong wall slip on the top surface above a critical shear rate γ̇c (see Figure 71). However, when sheared above 40 s−1 (assigned as SR3), the samples are quite smooth on both surfaces, indicative of the absence of a strong wall slip on the top surface. In this case, we speculate the strong wall slip occurs on the rotatory/bottom surface above a higher critical shear rate γ̇c* (see Figure 7b2). It is noteworthy that although a large wall slip length (b) or wall slip velocity (vs) is equally attained in both situations, the resulting velocity gradient in the polymer melt is totally different.

Compared with that of pure iPP, the SAXS patterns of iPP/ TMB-5 (Figure 4f-j) consist of much stronger meridian scatterings, even under quiescent condition (Figure 4f). On the other hand, the scattering patterns of iPP/TMB-5 scarcely change with increasing shear rate (Figure 4g−i) until 60 s−1 (Figure 4j) when an obvious decrease of meridian scatterings occurs. Since the scattering intensity of the SAXS pattern originates from the contribution of all the scatters in the polymer samples,48 the azimuthal distribution of scattering intensity in Figure 5 provides an intuitional way to analyze the distribution/orientation of the overall scatters (lamellae) in the crystallized samples. The scattering peaks around 90° and 270° in Figure 5 represent the meridian maximum scatterings. The intensity and azimuthal width of these scattering peaks can be used to understand the structure parameters and the orientation of the scatters in crystallized polymer; that is, the smaller is the azimuthal width, the higher is lamellar orientation.48 The lamellar orientation degree f h was evaluated by eqs 3 and 437 according to the corresponding azimuthal profile and is shown in Figure 6. Noteworthy is that the defined reference directions

Figure 6. Shear-rate dependent orientation degree of pure iPP and iPP/TMB-5.

in eq 3 are lamella normal and the FD, thus, the higher f h, means there is a greater fraction of chain-folded lamellae perpendicular to the FD. For pure iPP, the oriented structures are originally induced by shear flow according to the common physical intuition that flow leads to local orientation of polymer chains/segments. Thus, the orientation degree is supposed to increase with the increasing shear rate shown as the initial trace (within 0−10 s−1, assigned as SR1) in Figure 6. The positive correlation implies a 13517



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Figure 7. Schematic diagrams of wall/polymer interfacial slip under different shear conditions (based on the previous work of Wang,45,49 Hatzikiriakos,46 Spikes and Granick,50 Mhetar and Archer.51).

The oriented polymer chains/segments are essentially induced by the inner entanglement in a velocity gradient flow, and the relaxation of polymer chains/segments is defined at a constant temperature; therefore, the final orientation degree is in proportion to the actual intensity of shear flow imposed on polymer melt and vice versa. As aforementioned, the strong wall slip occurs on the stationary/top surface within SR2, at which the orientation degree decreases slightly as shown in Figure 6. This because the entanglement between the bottom-adsorbed chains and the adjacent inner free chains is still sufficient to drive the whole polymer melt moving with the rotatory/bottom plate. However, the “inverse dragging” by the stationary/top plate trends to disappear due to the coil−stretch transition of the top-adsorbed chains. In this case, the velocity gradient in the polymer melt is supposed to decrease obviously;

however, the increased shear rate imposed on the rotatory/ bottom plate partially counteract the weakening effect from wall slip, reducing the degree of the declined velocity gradient in the polymer melt. Alternately, when the strong wall slip occurs on the rotatory/bottom plate within SR3, a large decrease of the orientation degree is seen (Figure 6), which can be explained as follows: when applied with a high enough shear rate, the bottom-adsorbed polymer chains undergo a large coil−stretch transition, resulting in a disentanglement between the bottomadsorbed polymer chains and the neighboring chains in the melt. Considering the low content of TMB-5 (0.1 wt %), the viscosity of iPP/TMB-5 melt is comparable with that of pure iPP, which means the wall slip occurrence in iPP/TMB-5 is similar to that in pure iPP. This can explain the consistent 13518



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Figure 8. Orientation dependent Kβ and Kα of (a) pure iPP and (b) iPP/TMB-5.

either unconspicuous in pure iPP or inhibited in iPP/TMB-5, which indicates that only a special extent of oriented polymer chains can promote the formation of β-iPP. However, the underlying reasons still remain a puzzle.

variation traces with increasing shear rate in Figure 6 between pure iPP and iPP/TMB-5. Furthermore, Figure 6 exhibits a higher orientation degree of iPP/TMB-5 than that of pure iPP at the same shear rate, which is caused by three reasons: first, the needle-like TMB-5 particles are solid throughout the experiment and trend to align along the FD, causing a local shear by different velocities at the interface between molten iPP and the particles, which would induce a further orientation of polymer chains; second, the epitaxy growth of lamellae on the surfaces of the nucleating agent particles is determined; third, the nucleating agent could lower the free energy barrier for the formation of shish or secondary nucleation57 which exactly promotes the formation of perpendicular lamellae. Despite the widely accepted fact that oriented structure cannot be formed without the action of flow even with the presence of nucleating agent,57 the quiescently crystallized iPP/TMB-5 in our experiment still possesses a nonzero orientation degree (about 0.57). It might be ascribed to the preserved orientation during gap-changing: polymer melt between the two parallel windows underwent a shearing when the gap changed from 2500 to 1000 μm. Although an annealing at 200 °C for 5 min was subsequently applied, the arrangement of TMB-5 particles tending to the FD and the local shear around them were hardly eliminated due to the absence of the “relaxation” process for the particles. Furthermore, Figure 6 shows that the orientation degree of iPP/TMB-5 is less sensitive to shear rate compared with that of pure iPP, which means the TMB-5 particles play an important role in stabilizing the oriented structures. According to the above analyses, it is supposed that the β-nucleating agents predominate the final orientation degree of the βnucleated iPP. The orientation degree indicates the reserved orientation with considering the wall slip factor, making it an intuitional physical parameter to judge the actual shear effect imposed on polymer melt. Thus, it is more reasonable and reliable to investigate the dependence of crystallization behavior on the orientation degree rather than the shear rate. Figure 8 shows the variation of Kβ and Kα with orientation degree. The positive correlation in Figure 8a agrees well with the widely approved viewpoint that the β-crystals of iPP emerge more likely in the region composed of highly oriented structures.58 Figure 8b exhibits an initially high Kβ before a large decrease above the orientation degree of about 0.63. Although the underlying mechanism is still unknown, the decreased Kβ demonstrates that a positive/efficient shear flow can significantly inhibit the β-inducing ability of TMB-5. From Figure 8, specific ranges of orientation degree in favor of the formation of β-iPP can be found, that is, 0.55−0.60 for pure iPP and 0.57−0.63 for iPP/ TMB-5. Beyond such orientation, the formation of β-iPP is

4. CONCLUSION In this work the influence of shear on the formation of βcrystals both in pure iPP and β-nucleated iPP was studied. The lamellar orientation degree was characterized by SAXS measurements, and the relative amounts of β-crystals and αcrystals (Kβ and Kα) were obtained according to WAXS data. It is found that the shear flow influences the formation of oriented structures in pure iPP. With effective shear flow, the formation of oriented structures in pure iPP is promoted, which is in favor of the formation of β-crystals. On the contrary, the formation of β-crystals in iPP/TMB-5 is inhibited by the effective shear flow. A strong wall slip is proven to occur by the decreased orientation degree above a critical shear rate γ̇c (about 10 s−1 herein), with which the formation of β-crystals in β-nucleated iPP is surprisingly promoted. Thus, the strong wall slip can be used to attain high levels of β-crystals in β-nucleated iPP during processing. There exists a second critical shear rate γ̇c* (about 40 s−1 herein) above which a transition of wall slip location from the stationary surface to the rotatory/moving surface occurs leading to a further decrease of the shear flow in polymer melt. According to the dependence of Kβ (Kα) on orientation degrees, there is a special range of chain orientation within which the formation of β-crystals is largely promoted both in pure iPP and β-nucleated iPP.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: hfl[email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (20974077, 21374077, 21274149 and 51173130) and the Special Funds for National Basic Research Program of China (2010CB631102).

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REFERENCES

(1) Liu, Q.; Sun, X.; Li, H.; Yan, S. Orientation-induced crystallization of isotactic polypropylene. Polymer 2013, 54, 4404− 4421.

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Wall Slip Effect on Shear-Induced Crystallization Behavior of Isotactic

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