Effect of Flowing Preformed Spherulites on Shear-Induced Melt

Feb 20, 2018 - The effects of flowing preformed spherulites on shear-induced melt crystallization behaviors of isotactic polypropylene (iPP) had been ...
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Effect of Flowing Preformed Spherulites on Shear-Induced Melt Crystallization Behaviors of Isotactic Polypropylene Junyang Wang, Xuehui Wang, Qiaojiao Wang, Cui Xu, and Zhigang Wang* CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: The effects of flowing preformed spherulites on shear-induced melt crystallization behaviors of isotactic polypropylene (iPP) had been investigated by using a Linkam shearing cell coupled with an optical microscope, atomic force microscope, and rheometer. By quenching iPP melt to 130 °C for a short time, preformed spherulites were intentionally obtained. While iPP films were heated up to temperatures below the nominal melting point of iPP (164 °C), various shear conditions were applied, and then the crystalline morphologies during crystallization at the isothermal crystallization temperature of 135 °C were traced by alternately using phase contrast optical microscopy and polarized optical microscopy modes. Two distinct highly orientated thread- and cylinder-like crystalline morphologies were induced by shearing with preformed spherulites present, which were dependent on the applied shear rate and temperature. The distinct crystalline morphologies were attributed to relative movements between shear flowing melt and preformed spherulites. Atomic force microscopy was used to reveal the crystal aggregates of the two typical morphologies, indicating that the former was composed of highly packed orientated crystal nuclei, while the latter was composed of loosed packed random crystal nuclei, for which the thin crystal lamellae grew perpendicularly to crystal nuclei, leading to large widths for the cylinders as seen by an optical microscope. Rheological measurements indicated that preformed spherulites were isolated in the iPP films, which could obviously retard the stress relaxation of iPP films at the lower shear temperature.

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INTRODUCTION Semicrystalline polymers are inevitably subjected to shear or elongation flows under various processing operations such as extrusion, injection molding, and blow molding. The shear flow accelerates the crystallization kinetics and induces the crystalline morphological transition from spherulites to shish kebab as well, which would dramatically enhance the physical properties of the final products.1−4 The exploration of the formation mechanism of shish kebab has been intriguing due to scientific and industrial values. Nowadays, two main models, the coil−stretch transition5 and stretch network model,4,6−10 dominate the community of shear-induced polymer crystallization, which account for the crystalline morphological development and crystallization kinetics studies. Nevertheless, these models are still under debate and deserve further investigation, especially for unveiling the nature of flow-induced precursors.11−13 As is well-known, transcrystallization could happen by heterogeneous nucleation at the interface.14−17 An essential prerequisite for transcrystallization is the presence of high density active nuclei. In general, a substrate with high surface energy tends to foster transcrystalline growth.16 In the shear flow case shear inhomogeneity has been claimed to dramatically affect polymer crystallization.18−21 Lin et al. proposed that shear rate near the melt−substrate interface was higher than © XXXX American Chemical Society

that in bulk when shear plates with high surface energy were used.22 Nevertheless, this situation is unavoidable especially in industrial processing, as indicated by the formation of highly orientated skin layer morphology during injection molding, where the hot melt next to the cold die wall undergoes fast cooling and heterogeneous nucleation simultaneously occurs during shear flow.23−25 Recently, Shen et al. discovered the comet-like shish kebab at the melt−substrate interface during weak flow especially when quartz plates were not specifically cleaned, in which shish grew as tails on preformed point-like nuclei.26,27 Shen et al. then advocated a wall slip mechanism.26,27 When point-like nuclei or spherulites, being regarded as filling particles with substantial interaction with melt, are produced during shear or prior to shear, both the crystalline morphologies and crystallization kinetics can be altered.28−32 Coccorullo and Pantani et al.28,29 studied the effect of continuous shear flow on nucleation and spherulitic growth rates of iPP, and they observed an increase of nucleation density, in which continuous nucleation and growth of spherulites in micrometer scale was actually the synergy of Received: December 18, 2017 Revised: February 8, 2018

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DOI: 10.1021/acs.macromol.7b02686 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules preformed spherulites and continuous weak shear flow with a shear rate lower than 0.3 s−1. In analogy with the particle-filled systems, preformed nuclei or spherulites can induce stress amplification and influence macromolecular chain orientation.30−32 The formation of spherulites at the melt−substrate interface is inevitable due to heterogeneous nucleation effect, thus leading to the complexity of stress environment and shearinduced crystallization, which remains challenging and has not been explored systematically in the literature. In the present work, preformed spherulites in iPP were produced before the supercooled iPP melt was subjected to any shear treatments. Both phase contrast and polarized optical microscopes were adopted for in situ observation of the crystalline morphologies during isothermal crystallization after cessation of shearing for the supercooled iPP melt with preformed spherulites, providing real time and real space analyses on the formation mechanisms of two distinct threadand cylinder-like crystalline morphologies. AFM was applied to reveal the nature of these highly orientated crystalline morphologies at the crystal lamellar scale. To examine the isolation of preformed spherulites in iPP films and effect of preformed spherulites on the chain relaxation, the rheological properties were measured, including the frequency sweeps and stress relaxation at different shear temperatures. The conclusions derived from the results enable us to propose a framework that well captures the morphological characteristics and provides potential guidance for industrial processing on semicrystalline polymers.

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Figure 1. Schematic indication of thermal and shear protocol as applied to iPP films for shear-induced crystallization study. erase thermal history, followed by a temperature quench at a cooling rate of 30 °C/min to Th = 130 °C, and then held for a holding time, th (about 1 min), to produce spherulites with relatively high density and small sizes on the micrometer scale. The sizes of preformed spherulites were controlled to be about 20 μm with a variation range of less than 5 μm. Step II. Shearing was applied to undercooled melt at chosen temperature, Ts. The iPP film was heated to Ts below the nominal melting point of iPP (164 °C) at a heating rate of 30 °C/min, held at Ts for 30 s, and then subjected to shearing. The Ts values of 155, 160, and 163 °C were chosen in this study. Various shear rates, γ̇, combined with different shear times, ts, were applied. Further growth of preformed spherulites was restrained at Ts because of low undercooling. Holding the melt for 30 s was also applied for reference tests such as quiescent crystallization and shearing with no preformed spherulites, which was essential to guarantee the same thermal history. Note that it was observed that shearing at lower Ts’s resulted in fracture of undercooled melt in the rheometer possibly due to fragility of the sample at lower temperatures. Step III. Isothermal crystallization at temperature, Tc = 135 °C. After a shearing step, the film was quenched at 30 °C/min to Tc = 135 °C, with the crystalline morphologies traced. The zero time as mentioned in the later sections was assigned to the instant, at which shearing was just stopped. The measurements were conducted in duplicate, demonstrating repeatability of the results. Orientated Crystalline Morphologies Observed by Atomic Force Microscopy (AFM). AFM observation on the orientated crystalline morphologies of the shear-induced crystallized iPP films was performed using a MFP-3D Stand Alone (MFP-3D-SA) atomic force microscope (Oxford Instruments Asylum Research, Inc.). Tapping/AC air topography operating mode was applied using OMCL-AC160TS-W2 (Olympus, Japan) cantilevers with a typical resonant frequency of 300 kHz and a spring constant of 42 nN/nm. The scan rate was 1 Hz. The sample line was 256, and the drive amplitude was 100 mV. The shear-induced crystallized iPP films were prepared by following the protocol shown in Figure 1 in the Linkam optical shearing system. After isothermal crystallization at Tc = 135 °C for 30 min, the power of shear cell was turn off to allow the films to be cooled to room temperature. Then the iPP films were taken off from the shear cell. Because AFM observation required a flat film surface, the iPP films were hot-pressed between two pieces of silicon wafer at 160 °C for 5 min and then were quenched to room temperature. During this melting process the crystallized portion formed during isothermal crystallization was molten and the highly orientated crystalline portion was expected to survive, which would be the main object for AFM observation. After removing one of the two pieces of silicon wafer, the iPP films stuck to another piece were etched for 4 h at room temperature, using 1 wt % potassium permanganate solution containing 2 parts of sulfuric acid and 1 part of phosphoric acid. The etched iPP films were washed sequentially with hydrogen peroxide, distilled water, and acetone and then were dried in a vacuum oven.

EXPERIMENTAL SECTION

Material. A commercial available isotactic polypropylene (iPP) (Aldrich) was used, which had weight-average molar mass, Mw = 392 kg/mol, polydispersity index, Mw/Mn = 5.8, and melt flow rate, MFR = 4 g/10 min (230 °C, 21.6 N). Sample Preparation. The granular iPP was first compression molded into a film with thickness of 1 mm. A small cubic sample was cut from the molded film before loading in a Linkam optical shearing system (CSS450) for the shear-induced crystallization study. Phase Contrast and Polarized Optical Microscope. A polarized optical microscope (POM, Olympus BX51, Japan) equipped with a Linkam optical shearing system (CSS450), providing the shear flow and temperature control, was primarily used in the present study for observation of crystalline morphologies under various shear conditions. The two quartz plates of the shear cell were carefully cleaned before each test. The gap was set at 50 μm for clarity of the optical microscope observation. The shear flow field was produced by rotation of the bottom plate while keeping the top one stationary. During heating and cooling process, the temperature and time profiles were recorded to ascertain the corresponding real-time temperature for each collected micrograph. Phase contrast optical microscopy (PCOM) and polarized optical microscopy (POM) modes were switched alternately at a time interval of 5 s to capture the crystalline morphological characteristics. The microscope was focused at the middle plane of the sample unless specified. POM was also applied to take the micrographs for the crystallized iPP samples during subsequent heating process after isothermal crystallization process at Tc = 135 °C. Thermal and Shear Protocol for Isothermal Crystallization. Each protocol of shear-induced crystallization of iPP films basically contained three sequential steps, i.e., formation of preformed spherulites at holding temperature, Th = 130 °C, shearing at chosen temperatures, Ts, and isothermal crystallization after cessation of shear at Tc = 135 °C. The three sequential steps are schematically illustrated in Figure 1 and are described respectively as follows: Step I. Preformed spherulites were produced at holding temperature, Th = 130 °C. The iPP film was held at Tm = 200 °C for 5 min to B

DOI: 10.1021/acs.macromol.7b02686 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. Selected PCOM (left panel) and POM (right panel) micrographs for iPP films during isothermal crystallization at Tc = 135 °C after holding the melt at Ts = 155 °C for ts= 30 s (a, b) and after a shearing at shear rate γ̇ = 10 s−1 for ts = 5 s at Ts = 155 °C without preformed spherulites (c, d). The scale bar represents 50 μm and is applied to other micrographs as well. The arrow on the right indicates the shear direction, and this note is applicable to all other figures. Differential Scanning Calorimetry (DSC). A differential scanning calorimetry (DSC TA Q2000, TA Instruments) was used to measure the DSC heat flow curves of iPP films taken off from the Linkam optical shearing system after experiencing different shear crystallization treatments. A heating rate of 10 °C/min was applied. Rheological Behaviors by Rotational Rheometry. A rotational rheometer (AR2000ex, TA Instruments) was used to measure the rheological properties for neat iPP and iPP films with preformed spherulites. The iPP samples were compression molded at 200 °C into circular discs of 8 mm in diameter and 0.3 mm in thickness by using a homemade vacuum laminator. For the comparison with optical microscope, the disc thickness must be as thin as possible. Herein, the thickness of 0.3 mm had to be chosen because the samples for rheological test must have sufficient thickness to provide enough torque values. The strain sweep tests from 0.1 to 1000% were performed at an angular frequency of 1.0 rad/s to determine the linear viscoelastic regime of the iPP samples. For the rheological property measurements, dynamic frequency sweeps in the linear viscoelastic regime were performed at Ts’s with a frequency range from 500 to 0.01 rad/s. Prior to the dynamic frequency sweeps at Ts’s, the disc-shaped neat iPP films were kept at 200 °C for 5 min to remove previous thermal histories. The disc-shaped iPP films with preformed spherulites were prepared in a rheometer by following the protocol in Figure 1, and then the dynamic frequency sweeps at Ts’s were performed. Note that the cooling rate in the rheometer was 8 °C/min, which was relatively lower than that in the Linkam optical shearing system. However, no much difference in the number of preformed spherulites was seen at 130 °C when a cooling rate of 8 °C/min was applied in the Linkam optical shearing system. More importantly, neat iPP and iPP films with preformed spherulites were subjected to a 30% strain in the linear viscoelastic regime (see Figure S1 in the Supporting Information) to examine their stress relaxation behaviors.

can be taken as the reference. The selected micrographs taken using phase contrast optical microscopy (PCOM) and polarized optical microscopy (POM) modes for iPP films during crystallization at Tc = 135 °C under quiescent or shearing at Ts = 155 °C without preformed spherulites are shown in Figure 2. It shows that without preformed spherilites the nucleation density can be increased more by shearing than the quiescent condition. The homogeneity of spherulitic sizes shown in Figure 2 confirms that holding at Ts for 30 s produces no extra nuclei. Shear intensity at the shear rate γ̇ = 10 s−1 for ts = 5 s at Ts = 155 °C is insufficient to induce any orientated crystalline morphology because for shear-induced crystallization of polymers the formation of orientated crystalline morphology requires a critical shear intensity.33−35 However, if preformed spherulites were produced in the iPP films prior to shearing, the morphology of subsequent isothermal crystallization became greatly different. The selected PCOM and POM micrographs for iPP films with preformed spherulites during crystallization at Tc = 135 °C after shearing at Ts = 155 °C are shown in Figure 3. Compared to Figure 2, it is apparent that the crystallization kinetics is accelerated by shearing with preformed spherulites in iPP films since nucleation and crystal growth occur within a short period of 120 s. Note the time of 120 s was the earliest time when the thread-like crystals appeared at the measurement conditions. Prior to this time, the thread-like crystals could not be observed by POM. The initial formation of the thread-like crystals might be a challenging topic for the early crystallization of semicrystalline polymers with preformed spherulites under shear, which is subjected to a future study. More interesting is that the crystalline morphology in Figure 3 is highly orientated, which is greatly distinguished from that shown in Figure 2. In detail, thread-like crystals extend from the preformed spherulites with varying lengths along the shear direction, as pointed out by red arrows in Figure 3A. These thread-like crystals show decreasing widths starting from preformed

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RESULTS AND DISCUSSION Quiescent and Shear-Induced Crystallization at Ts = 155 °C without Preformed Spherulites. In this work, the quiescent and shear-induced crystallization for iPP films without preformed spherulites at the chosen crystallization temperature, Tc = 135 °C, was first examined, and the results C

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Figure 3. Selected PCOM (A) and POM (B) micrographs for iPP films with preformed spherulites during isothermal crystallization at Tc = 135 °C after shearing at shear rate γ̇ = 10 s−1 for ts = 5 s at Ts = 155 °C. The scale bar represents 200 μm and is applied to other micrographs.

shear rate increased to γ̇ = 20 s−1 as shown in Figure 4B(b), not only extra point-like nuclei but also thread-like crystals grow from preformed spherulites, indicating that the shear rates, γ̇ = 10 s−1 or above, are sufficient for the morphological transition from spherulites to orientated thread-like crystals when preformed spherulites are present. Furthermore, γ̇ = 20 s−1 tends to induce longer thread-like crystals compared with that of γ̇ = 10 s−1 (Figure 3). Further analysis on the micrographs in Figure 4 tells that the integrity of thread-like crystalline is disrupted as the length increases; i.e., the long thread-like crystals growing out from preformed spherulites are broken up into several short thread-like crystals or even row-aligned nuclei at some “tail” regions. The disruption of morphological integrity infers to inhomogeneity of the shear field extending from preformed spherulites in shear direction, with the actual shear flow more intense near preformed spherulites than the locations away from them. Shear Temperature Dependence for Orientated Crystalline Morphologies. Shear temperature, Ts, plays an essential role in the shear-induced crystallization for polymers. To validate the effect of Ts on the morphological transition, other two Ts’s (160 and 163 °C) were chosen, which were slightly lower than the nominal melting point of iPP (about 164 °C). Figure 5 shows the selected PCOM and POM micrographs for iPP films with preformed spherulites during isothermal crystallization at Tc = 135 °C after shearing at shear

spherulites to their ending tails. Some thread-like crystals show discontinuous threads, like fracture or breaking along shearing. POM micrographs shown in Figure 3B indicate that the birefringence of thread-like crystals extending from preformed spherulites is weaker than that of preformed spherulites. At prolonged crystallization time the thread-like crystals evolve into cylinder-like crystalline morphology with widths comparable to those of preformed spherulites, possibly because of the space-restricted lamellar crystal growths perpendicular to the thread-like core crystals. Note that the three regimes depending on the applied shear rate with a constant shear time have been proposed according to the crystalline morphological transitions,36−39 and for the present iPP system, preformed spherulites obviously shift the shear effect directly from an intermediate regime to orientated regime, as will be discussed more in the later section. Effect of Shear Rate on Orientated Crystalline Morphology at Ts = 155 °C. To well understand the effect of shear rate on the crystalline morphologies of iPP films with preformed spherulites,33−35 two other shear rates, γ̇ = 5 s−1 and γ̇ = 20 s−1, with shear time ts = 5 s, were applied. The selected PCOM and POM micrographs for iPP films at different shear rates γ̇ = 5 s−1 and γ̇ = 20 s−1 for ts = 5 s at Ts = 155 °C are shown in Figure 4. For low shear rate γ̇ = 5 s−1, only extra point-like nuclei with much smaller sizes than preformed spherulites appear, which finally develop into spherulites. As D

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Figure 4. Selected PCOM micrographs (A) and POM micrographs (B) for iPP films with preformed spherulites during isothermal crystallization at Tc = 135 °C after shearing at shear rate γ̇ = 5 s−1 (a) and γ̇ = 20 s−1 (b) for ts = 5 s at Ts = 155 °C. The scale bar represents 200 μm and is applied to other micrographs.

rate γ̇ = 10 s−1 for ts = 5 s at Ts = 160 °C and Ts = 163 °C, respectively. It can be clearly seen from the micrographs taken at the early crystallization stage (the left column in Figure 5) that preformed spherulites gradually become indistinct until disappear as Ts increases from 160 to 163 °C, indicating melting of preformed spherulites.40−44 Different melting states of preformed spherulites under shear have obvious effects on the crystalline morphology. At Ts = 160 and 163 °C, orientated cylinder-like crystalline morphology forms. Apparently the cylinder-like crystalline morphology shows some difference from that formed at Ts = 155 °C, especially at the early crystallization stage. At Ts = 163 °C, the cylinder-like crystals extend with comparable widths, more close to a spindle-shaped pattern. At Ts = 160 °C, the orientated cylinder-like crystals are similar to that formed at Ts = 155 °C, but with much wider tail regions. It is interesting that the relatively higher birefringence appears on the head regions of the cylinder-like crystals in the POM micrographs (Figure 5B), indicating that preformed spherulites still function although they become invisible due to melting in PCOM. The applied shear rate exhibits an obvious effect on the formation of orientated cylinder-like crystalline morphologies.

Figure 6 shows the selected POM micrographs for iPP films with preformed spherulites, which isothermally crystallized at Tc = 135 °C after shearing at shear rates γ̇ = 5 s−1 and γ̇ = 20 s−1 for ts = 5 s at Ts = 160 and 163 °C, respectively. It can be found from Figure 6A that at Ts = 160 °C the iPP films with shearing at shear rate γ̇ = 5 s−1 for ts = 5 s do not show obvious orientated crystals, while shearing at shear rate γ̇ = 20 s−1 for ts = 5 s leads to orientated cylinder-like morphology, with the lengths of the crystals much longer than that obtained by shearing at shear rate γ̇ = 10 s−1 for ts = 5 s as shown in Figure 5B. Some thread-like crystals are obviously seen in Figure 6A(b), similar to that shown in Figures 3A and 4A. At Ts = 163 °C, shearing at γ̇ = 5 s−1 or γ̇ = 20 s−1 for ts = 5 s produces quite similar orientated cylinder-like crystals, except that shearing at γ̇ = 20 s−1 for ts = 5 s produces much longer cylinder-like crystals. Unveiling Formation of Orientated Cylinder-like Crystalline Morphologies. It is indisputable that the formations of two types of cylinder-like crystalline morphologies are correlated to the combined effect of shearing and preformed spherulites in the iPP films. Shear temperature dependence verifies that an increase in temperature leads to the crystalline morphological transition. Figure 7 shows enlarged E

DOI: 10.1021/acs.macromol.7b02686 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. Selected PCOM (A) and POM (B) micrographs for iPP films with preformed spherulites during isothermal crystallization at Tc = 135 °C after shearing at shear rate γ̇ = 10 s−1 for ts = 5 s at (a) Ts = 160 °C and (b) Ts = 163 °C. The scale bar represents 200 μm and is applied to other micrographs.

PCOM and POM micrographs taken for iPP films with preformed spherulites during isothermal crystallization at Tc = 135 °C after shearing at γ̇ = 10 s−1 for ts = 5 s at different Ts’s. At Ts = 155 °C the thread-like pattern extending from preformed spherulites shown in Figures 7a and 7d illustrates a smooth surface, and the width of the thread-like crystalline pattern near preformed spherulites is much larger than that far from them, suggesting a shear inhomogeneity starting from preformed spherulites along shear direction. At Ts = 160 °C, the orientated pattern extending from preformed spherulite is no longer thread-like, but cylinder-like, and the width of the cylinder is relatively uniform and comparable to that of preformed spherulites. A close observation tells that the rough surface of the cylinder-like pattern shown in Figures 7b and 7e is composed of a large amount of point-like nuclei with certain birefringence. At Ts = 163 °C, the orientated cylinder-like pattern shown in Figures 7c and 7f does not contain preformed spherulite at the head; however, it seems to have point-like nuclei just like those formed at Ts = 160 °C. It might be appropriate to consider that the case at Ts = 163 °C is much closer to that at Ts = 160 °C than that at Ts = 155 °C. However, due to the closest to Tm, the nominal melting point of iPP, the

preformed spherulites at Ts = 163 °C become invisible, although the residual traces of preformed spherulites might still function for the formation of the cylinder-like crystalline morphology at this high shear temperature. More interesting, an obvious difference in the heading direction exists for the orientated patterns with preformed spherulites; for the one at Ts = 155 °C, the preformed spherulite is heading to shear direction as pointed by red arrow, leaving the tail behind; while at Ts = 160 °C, the preformed spherulite seems to be heading to the opposite direction of shearing. This finding is subjected to more detailed discussion in the next section. In the present study, the preformed spherulites are actually confined between the parallel quartz plates. Some of these spherulites might originally grow from the heterogeneous nuclei at the surface of quartz plates.14−16 Therefore, in the shearing process, there exist two different situations according to the relative motion between preformed spherulites and the quartz plate. Figure 8 shows the selected PCOM micrographs taken for iPP films before shear, after shear, and following crystallization at Tc = 135 °C for 150 s after shearing at γ̇ = 10 s−1 for ts = 5 s at Ts = 155 °C and Ts = 160 °C, respectively. It can be seen from Figure 8A that at Ts = 155 °C the relatively F

DOI: 10.1021/acs.macromol.7b02686 Macromolecules XXXX, XXX, XXX−XXX

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Figure 6. Selected POM micrographs for iPP films with preformed spherulites during isothermal crystallization at Tc = 135 °C after shearing at (a) shear rate γ̇ = 5 s−1 for ts = 5 s, and (b) γ̇ = 20 s−1 for ts = 5 s at (A) Ts = 160 °C, and (B) Ts = 163 °C. The scale bar represents 200 μm and is applied to other micrographs.

dark spherulites pointed by yellow arrows remain stationary during shear, which are thought to be stuck to the top stationary plate, while the other relatively bright spherulites pointed by blue arrows are moving during shearing. Following crystallization at Tc = 135 °C for 150 s after shearing, the thread-like crystals can be seen with preformed spherulites heading along shear direction and tails extending in the opposite direction. In such a case, the moving spherulites along the shear direction are responsible for the formation of the orientated thread-like crystalline pattern. Thus, we conclude that these moving spherulites, which are stuck to the bottom rotational plate, keep moving with the same rate as the bottom moving plate. It can be further seen from Figure 8B that at Ts = 160 °C no stationary spherulites can be observed, suggesting that due to partial melting of the preformed spherulites, the spherulites were not stuck to any of the plates of the shear stage. Therefore, a relative motion between the growing spherulites and the surrounding melt can be imagined. It is believed that the surrounding melt moves faster along the

bottom rotational plate and the growing spherulites were floating in the moving melt. Because the slow spherulites float and the fast melt moves with the bottom plate, the orientated cylinder-like crystalline patterns show the opposite tail direction as compared with those in Figure 8A. Figure 9 provides an illustration about the two cases depicted in Figure 8. At low Ts, preformed spherulite can move simultaneously with the bottom rotational plate due to strong adhesion of spherulite and plate (Figure 9A), from the initial location x0 to xt. Such strong adhesion possibly comes from the initial nucleating site on the bottom rotational plate, where the preformed spherulite does not melt at low Ts. However, at high Ts, partial melting of preformed spherulite reduces adhesion between spherulite and the bottom rotational plate; on the one hand, the spherulite becomes fragmented; on the other hand, the spherulite behaves to move slowly forward with the flowing melt like a heavy impurity, exhibiting a reduced moving distance. Because of obvious interaction between molecular chains in the matrix and preformed spherulites, chain G

DOI: 10.1021/acs.macromol.7b02686 Macromolecules XXXX, XXX, XXX−XXX

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Figure 7. Enlarged PCOM (A) and POM (B) micrographs taken for iPP films with preformed spherulites during isothermal crystallization at Tc = 135 °C after shearing at γ̇ = 10 s−1 for ts = 5 s at different Ts’s. The scale bar represents 50 μm and is applied to other micrographs.

Figure 8. Selected PCOM micrographs for iPP films taken before shear, after shear, and crystallization at Tc = 135 °C for 150 s after shearing at γ̇ = 10 s−1 for ts = 5 s at (A) Ts = 155 °C and (B) Ts = 160 °C. The microscope was focused at the bottom rotational plate. The representative stationary and moving spherulites are pointed out by yellow and blue arrows, respectively. The scale bar represents 200 μm and is applied to other micrographs.

sequently, at low Ts high orientation of molecular chains and nucleation density can be induced behind a preformed

orientation or even stretching can be induced preferentially behind the fast moving preformed spherulites.34,45−47 ConH

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Figure 9. Schematic illustration of moving of preformed spherulites during shear at (A) low Ts and (B) high Ts and subsequent nucleation and growth at Tc = 135 °C between the two parallel plates of shear cell and the corresponding characteristic cylinder-like crystalline morphologies. x0 and xt represent the initial location of preformed spherulites before shear and a distance the bottom plate has moved after shear, respectively.

POM (Figures 7 and 8) can be clearly exposed to AFM; otherwise, they might be inaccessible because they could be buried among many later-formed tiny spherulites. The AFM images shown in the first row of Figure 10 represent the features clearly shown in Figures 7 and 8. In Figure 10, the images in (a) and (b) of the first row immediately represent the clear thread-like pattern, which is orientated along the shear direction, apparently corresponding to that shown in Figure 7A(a), while the images in (c) and (d) of the first row represent only visible trace of the cylinder-like pattern with a large width, which is parallel to the shear direction, apparently corresponding to that shown in Figure 7A(b). The average width of the cylinder-like pattern is 6.60 ± 0.53 μm. The purple arrows in the first row of (d) point at one width of the pattern. Although the trace of cylinder-like pattern in (c) and (d) of the first row is not obvious, the perpendicularly growing crystal lamellae to the cylinder-like pattern make the trace clearly visible by AFM. At enlarged magnifications, the thread-like or cylinder-like crystalline structures can be disclosed, which is not able to see under PCOM or POM. It can be found from the images in the second and third rows of Figure 10, the thread-like crystalline pattern in (a) and (b) is composed of the high densely packed crystals, while the cylinder-like crystalline pattern in (c) and (d) is composed of loosely packed crystals, among which the perpendicularly growing crystal lamellae are distributed. The average width of the thread-like pattern is 0.49 ± 0.07 μm. The purple arrows in the second row of (b) point at one width of the pattern. It can be further seen from the images in the last row of Figure 10 that the densely packed crystals have relatively uniform sizes and are highly orientated along the shear direction, as pointed out by the purple arrows in the last row of (b). The orientated crystals have an average length of 210 ± 23 nm. Because of the high density of the orientated crystals in the thread-like pattern, there is no space for the crystal lamellae to grow among them. The images of (c) and (d) in the last row demonstrate that for the loosely packed crystals of the cylinderlike pattern the crystals have no uniform sizes and are randomly

spherulite, leading to formation of a thread-like crystalline pattern (Figure 8A). The growth of enormous nuclei inside thread is hindered, with mere lamellae growing perpendicular to the thread of nucleus aggregate at the late crystallization stage. However, at high Ts due to partial melting of preformed spherulite, the connection between molecular chains in the matrix and fragmented spherulite becomes weak. Furthermore, the slow moving rate of fragmented spherulite reduces shear intensity nearby (Figure 9B), leading to formation of the loosely packed nuclei. Subsequently, the confined crystals grow among these loosely packed nuclei, and crystal lamellae further grow around these confined crystals at the late crystallization stage, leading to formation of a cylinder-like crystalline pattern possessing large width (Figure 8B). AFM Observation on Orientated Cylinder-like Crystalline Morphologies. Although PCOM and POM have been used to capture the evolution of the crystalline morphologies during isothermal crystallization after shearing at different temperatures, the limited resolution of optical microscope restricts direct observation on the distinct crystalline morphologies at high resolution, such as at the crystal lamellar level. Therefore, we further used atomic force microscope (AFM) to take a close observation of the crystalline morphologies induced at different Ts’s with preformed spherulites existing. The shear-induced crystallized iPP films for AFM observation were prepared by following the protocol as shown in Figure 1, which were similar to those films shown in Figure 8. Figure 10 shows the AFM height and amplitude images with gradually enlarged magnifications for the shearinduced crystallized iPP films after shearing at γ̇ = 10 s−1 for ts = 5 s at Ts = 155 and 160 °C, respectively. Note that after shear the iPP films were left to crystallize at Tc = 135 °C for 30 min and cool to room temperature with the power off for the shear cell. As stated in the Experimental Section, the iPP films were hot pressed at 160 °C to melt some crystallized portions with low melting points. Therefore, the highly orientated thread- or cylinder-like crystalline morphologies seen under PCOM and I

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Figure 10. AFM height (a, c) and amplitude (b, d) images with gradually enlarged magnifications from top to bottom for iPP films with preformed spherulites isothermally crystallized at Tc = 135 °C for 30 min after shearing at γ̇ = 10 s−1 for ts = 5 s at (A) Ts = 155 °C and (B) Ts = 160 °C. The shear direction is indicated by the azure arrows. The scale bars in (d) are applied to each one of the row from (a) to (c).

DSC curves do provide the existence of β-form iPP crystals in the iPP samples with the shear conditions; however, the melting peak of β-form iPP crystals at 150 °C is very weak as compared with that of α-form iPP crystals at 164 °C, illustrating that the quantity of β-form iPP crystals is quite low in the samples. POM observation during the heating process can also pick out the first melting of β-form iPP crystals and the later melting of α-form iPP crystals if both of them coexist in the samples.53 Figures S3 and S4 show the selected POM micrographs taken at different temperatures during heating for the shear-induced crystallized iPP films with preformed spherulites (Ts = 155 and 160 °C, respectively). These POM micrographs basically did not show the trace of the melting of β-form iPP crystals, confirming the very tiny quantity of β-form iPP crystals in the samples if coexisting with the dominant αform iPP crystals. The above results infer that the final crystal form in the shear-induced crystallized iPP samples is α-form iPP crystals. Retarded Relaxation Behaviors Due to Preformed Spherulites. Although PCOM and POM have been used to capture the evolution of the crystalline morphologies of iPP films with preformed spherulites after shear at the micrometer scale and AFM has been used to take a close look at the crystalline morphologies at the lamellar level, the structural development due to the mechanism at the molecular level is

distributed with no clear local orientation. These random orientated crystals have an average length of 230 ± 26 nm. With large volume spaces among these random orientated crystals, thinner crystal lamellae grow perpendicular to fill up these liquid pockets, similar to the thinner crystal lamellar insertion mode as proposed for the secondary crystallization of polymers.48−50 We note here that Hsiao et al. reported the change in the shish length as a function of time after shear at γ̇ = 60 s−1 for ts = 5 s at Ts = 165 °C for iPP with weight-average molar mass, Mw = 368 kg/mol, and polydispersity index, Mw/ Mn = 4.0, by using small-angle X-ray scattering technique, and their final shish lengths for their iPP were about 200 nm. Our above AFM result about the orientated crystals having average lengths of 200 nm is apparently consistent with the shish length result as reported by Hsiao et al.51 Herein the final crystal forms in the crystallized iPP samples are necessary to concern. For this purpose, the DSC measurements were performed on the shear-induced crystallized iPP samples with preformed spherulites. The iPP samples with no shear were measured also as references. Because β-form iPP crystals have much lower melting point than α-form iPP crystals, DSC heating scan can disclose the coexistence of β-form iPP crystals.52 Figure S2 in the Supporting Information shows DSC heat flow curves for the iPP films taken off from the Linkam optical shearing system after experiencing different shear crystallization treatments. The J

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spherulites are introduced into the iPP films. However, this is not obvious at Ts = 160 °C and only slight increases in storage modulus can be seen. Therefore, the preformed spherulites in the iPP films produce different rheological responses at the low frequency region at the two Ts’s, corresponding to long time scale chain mobility difference. The slopes in Figure 11a also infer that the preformed spherulites in iPP films are relatively isolated, and these discrete spherulites have not built up connections to form a crystal network, which can be also visually inspected in Figure 8.56 Although the crystal network does not form through the discrete preformed spherulites in iPP films, the relaxation behaviors in these films behave differently. Figure 11b shows the changes of stress relaxation modulus, G(t), as functions of time at Ts’s for neat iPP and iPP films with preformed spherulites. The stress relaxation curves for neat iPP films at Ts = 155 and 160 °C are quite close, with the one at Ts = 155 °C slightly slower than another one at Ts = 160 °C. On contrast, the preformed spherulites if produced make the cases different. Preformed spherulites retard the stress relaxation of iPP films at Ts = 160 °C to a certain extent, while retardation to the stress relaxation becomes more obvious at Ts = 155 °C. Therefore, the result in Figure 11b illustrates that at Ts = 155 °C the discrete preformed spherulites produce more delayed chain relaxation in iPP films. It is considered that the retardation to stress relaxation at the two Ts’s should not be so much different as shown in Figure 11b if preformed spherulites in iPP films have no any adhesion to the bottom rotational plate. In the other words, the result in Figure 11b confirms the illustration schematically drawn in Figure 9, in which at low Ts some preformed spherulites move with the bottom rotational plate, and the shear-induced orientated chains take longer time to relax; whereas at high Ts the preformed spherulites might float in the iPP melts, and the shear-induced orientated chains can relax rapidly. In these two circumstances, the formation of two orientated crystalline morphologies is eventually observed as shown in Figures 7 and 8. Nevertheless, further investigations on the nucleation and crystal growth under various shear conditions at different Ts’s are still undergoing in our group.

certainly expectative. In recent work by Miyoshi and coworkers, the chain-folding (CF) structure of the iPP α phases and the chain-packing process were extensively studied by using 13 C−13C double quantum (DQ) NMR with selective isotopic labeling and spin-dynamics simulation, which was applied to verify various crystallization theoretical models at the molecular levels.54,55 While this unique NMR technique and the related analysis method are not available for us at the moment, we would like to examine the relaxation behaviors of the iPP films under shear due to existence of preformed spherulites, which might be helpful to understand the molecular mechanism for the formation of orientated crystalline morphologies. This kind of study can be operated in a rheometer. Figure 11 shows the changes of storage modulus with frequency for neat iPP and iPP films with preformed spherulites

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CONCLUSIONS The effect of flowing preformed spherulites on melt crystallization of iPP has been investigated by using optical microscopy, atomic force microscopy, and rheometry. The preformed spherulites were intentionally formed by crystallization at the low temperature of 130 °C, followed by preserving at higher shear temperatures, which were slightly lower than the nominal melting point of iPP. Under these circumstances, the crystalline morphologies could be obviously transformed from isotropic spherulites to other two unique highly orientated morphologies. When the iPP films were subjected to shearing at the low temperature, a thread-like crystalline morphology was observed extending from preformed spherulites along the opposite of shear direction, which could finally develop into cylinder-like crystalline morphology, while, when iPP underwent shearing at the higher temperature, a cylinder-like crystalline morphology with large cylinder widths was observed. The crystal aggregates of thread- and cylinderlike crystalline morphologies were further disclosed by using atomic force microscopy. The thread-like crystalline morphology formed because of highly packed orientated crystal nuclei, while the cylinder-like crystalline morphology formed because of loosely packed locally random crystal nuclei, among which

Figure 11. (a) Changes in storage modulus, G′, with frequency with the fixed strain of 1%, and (b) changes of stress relaxation modulus, G(t), as functions of time at Ts’s for neat iPP and iPP films with preformed spherulites. Neat iPP films were melted at Tm = 200 °C for 5 min prior to the frequency and time sweeps at Ts’s. The iPP films with preformed spherulites were prepared in rheometer by following the protocol in Figure 1.

at Ts = 155 and 160 °C, respectively. It can be seen that the storage modulus values at low frequencies at Ts = 155 °C are slightly higher than those at Ts = 160 °C for neat iPP films, which is understandable due to the temperature effect. It can be further seen that the storage modulus values at low frequencies at Ts = 155 °C become obviously higher when preformed K

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the thin crystal lamellae grew perpendicularly to them, leading to large widths of cylinders. The rheological measurements further demonstrated that the dispersion of preformed spherulites in iPP films were relatively isolated, while these preformed spherulites retarded the stress relaxation behaviors of iPP films at the low shear temperature, which brought about the different orientated crystalline morphologies as observed in this study. The iPP films containing preformed spherulites are subjected to further investigation, including the nucleation and crystal growth and the related mechanical performance.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02686.

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Figures S1−S4 (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel +86 0551-63607703; Fax +86 0551-63607703; e-mail [email protected] (Z.W.). ORCID

Zhigang Wang: 0000-0002-6090-3274 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z. G. Wang acknowledges the financial support from the National Science Foundation of China (Grant No. 51673183). The work was also financially supported by the opening project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (Grant No. Sklpme2017-4-06).

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