Flow-Induced Dendritic β-Form Isotactic Polypropylene Crystals in

Article pubs.acs.org/Macromolecules

Flow-Induced Dendritic β‑Form Isotactic Polypropylene Crystals in Thin Films Bin Zhang,† Binghua Wang,† JiaJia Chen,† Changyu Shen,† Renate Reiter,‡ Jingbo Chen,*,† and Günter Reiter*,‡ †

School of Materials Science & Engineering, Zhengzhou University, Zhengzhou 450002, People’s Republic of China Institute of Physics, University of Freiburg, 79104 Freiburg, Germany

‡

S Supporting Information *

ABSTRACT: Flow-induced dendritic β-form isotactic polypropylene (iPP) lamellae in thin films have been investigated using optical microscopy (OM) and atomic force microscopy (AFM). Shear flow in a thin film was induced by scratching supercooled iPP melt with a sharp scalpel at a constant rate. After subsequent isothermal crystallization, an extremely high nucleation density of edge-on α-iPP crystals oriented perpendicular to the flow direction was observed. Increasing shear flow temperature from 130 to 190 °C led to a decrease in both number density of flow-induced edge-on α-iPP lamellae and subsequent β-iPP crystals. Interestingly, two distinct paths for the change in orientation of edge-on crystals could be identified. The most probable route was the transition from edge-on to leaf-shaped α-iPP flat-on crystals. The transition from edge-on α-iPP lamellae to β-iPP flat-on crystals with dendritic shape occurred less frequently. We suggest a stepwise process for flow-induced growth of β-iPP crystals associated with the initial nucleation of edge-on α-iPP crystals via aligned polymer chains induced by flow and a subsequent transition to the growth of β-iPP flat-on crystals.

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crystals of α-iPP can be acquired, including axialites, hedrites, dendrites, spherulites, cylindrites, and faceted single crystals. The β-form of iPP (β-iPP) is a metastable crystalline phase and can only be induced under special conditions, such as by adding an active β-iPP nucleation agent,28 using a crystallization temperature gradient, or by applying flow on a supercooled melt.29−31 In comparison with α-iPP, β-iPP demonstrates different performance characteristics, e.g., a lower melting temperature, a lower fusion heat, and remarkably improved elongation at break and impact strength. Thus, during the past five decades many research groups have focused their interests on the β-iPP.32−38 According to an early observation of Padden and Keith,39 depending on the crystallization temperature, two forms of spherulitic morphology of β-iPPthe radial (βIII) and the ringed (βIV) spherulites, both with strong negative birefringenceare involved when iPP crystallizes from the quiescent melt. Furthermore, it has been revealed that singlecrystal-like hedrites formed via lamellar branching and splaying in β-iPP form act as the precursors of spherulitic growth. Varga et al.40,41 have reported that hedrites can reach considerable sizes (∼100 μm), presumably due to a coordinated cooperative growth of a stack of flat-on lamellae developing around a screw dislocation. However, the nature of the formation of β-iPP nuclei still remains unclear, especially in the case of flow.

INTRODUCTION Polymer crystallization is a complex phenomenon where besides thermodynamics, a combination of concepts from nonequilibrium and nonlinear physics is required to explain how a material consisting random coiled polymeric chains can transform into a nearly perfect ordered state.1 Usually, in the course of crystallization of polymers, various ordered structures and morphologies as aggregates of folded-chain lamellar crystals (thermodynamically metastable)2−8 with defined geometrical arrangements rather than extended-chain crystals (in or near equilibrium).9 It is now well established that free energy barrier of crystal nucleation can be drastically reduced by the application of a flow field to an entangled polymer melt or polymer solution. The so-called “shish-kebab” structure9−18 is the result of such flow-induced crystallization, in which foldedchain lamellar crystals (kebabs) grow perpendicular to highly stretched and oriented fibrillar-like chains (shish).19 Recently, to investigate the molecular mechanism for the formation of the shish, many experiments and molecular dynamics simulation of polymer chains have been carried out to explore the early stages of flow-induced nucleation.10,20−25 As one of the most widely used polymer and due to its interesting polymorphic behavior, isotactic polypropylene (iPP) represents a suitable candidate for studying shear-induced polymer crystallization.14,26,27 iPP can crystallize in three major crystal forms, known as the monoclinic α-form, the hexagonal β-form, and the orthorhombic γ-form. Normally, when iPP crystallizes from a melt, several morphologies of folded-chain © XXXX American Chemical Society

Received: May 26, 2016 Revised: June 27, 2016

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DOI: 10.1021/acs.macromol.6b01123 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules To disclose the key mechanisms of flow-induced crystallization in iPP melts, different approaches have been adopted, based e.g. on fiber-pulling, forming homogeneous fiber/matrix composites, using a parallel plate rheometer, die extrusion, and so on. Generally, weak flow induces “bundles” of densely packed polymers which provide a crystal lattice perfectly matched for epitaxial growth of folded-chain lamellae and introducing a morphology in cylindrical symmetry. In previous works, certain observations have been made concerning the crystallization behavior of β-iPP: (1) Flow-induced iPP α-row nuclei can initiate growth of both, β-iPP as well as monoclinic α-iPP.42−45 (2) β-iPP crystals have been found to be composed of fan-like arrangement of lamellae aligned normal to the flow direction. (3) The kinetic requirement for the transition from α-iPP to β-iPP growth (nucleation of β-iPP) takes place in the temperature range from 100 to 141 °C. In this temperature range, the β-form has faster growth rate than the α-form.46 (4) It was unveiled that the density of β-iPP nuclei depends evidently on the shear rate. Even at very low shear rate (1 s−1), β-iPP nuclei can be created by shear flow. Hsiao47,48 and An28 suggested that the relative amount of the β-form increased with the shear rate and reached a plateau maximum value at 57 s−1 (in situ) and 20 s−1 (ex situ), respectively. In our previous studies, we proposed an influence of the initial structure of the melt on the formation of flow-induced β-iPP cylindrites under shear flow.42,49−51 The nucleation mechanism of β-iPP represents the key to understanding the formation of flow-induced β-iPP. Although structural features and proprieties of β-iPP have been investigated for more than 50 years, so far very little work has focused on characterizing the origin and formation of flowinduced β-iPP nuclei. Bundles of iPP chains oriented by shear flow in thin films therefore provide an opportunity to investigate the morphology of nucleation site for β-iPP lamellae on the nanometer scale. On the basis of the observations presented in the present paper, we propose a nucleation mechanism for flow induced β-iPP in thin film. Instead of direct nucleation on shear-induced oriented molecular bundles, β-iPP lamellae are most likely induced by dangling chains and cilia on the fold surface of α-iPP edge-on crystals.

adopted for our experiments is schematized in Figure 1 and consisted of the following steps: (1) Heating the sample from room temperature

Figure 1. Schematic diagram showing the experimental protocol of flow-induced β-iPP crystallization in thin films. to 190 °C at a rate of 10 °C/min. (2) Holding the temperature at 190 °C for 5 min to erase the thermomechanical history. (3) Cooling at 10 °C/min down to Ts. (4) Holding the temperature at Ts for 5 min. (5) Using the scalpel to scratch the thin films at Ts and then cooling the sample to the crystallization temperature (Tc). (6) After a subsequent quench to room temperature, samples were examined by AFM. For morphological observation, an Olympus BX-61 optical microscope (Olympus, Tokyo, Japan) equipped with a Linkam THMS 600 hot stage (Linkam Scientific Instruments, Tadworth, UK), in reflection dark field mode, was used. The color code of all optical microscopy images was inverted for better contrast than original images (see Figure S1). Characterization of details of the morphology of flow-induced β-iPP was performed by atomic force microscopy in tapping mode (TM-AFM, JPK Instruments, Germany).

3. RESULTS AND DISCUSSION During crystallization at Tc = 138 °C, we examined the surfaces of the sheared iPP films by OM at different crystallization times (tc). In Figure 2a, one can observe the typical flow-induced oriented crystalline lamellae patterns after isothermal crystallization for ∼7 min. At the border of the scratch, the reduction of the free energy barrier for the formation of nuclei led to an extremely high areal density of iPP lamellae. Intriguingly, at the chosen Tc, no crystalline lamellae appeared outside the sheared region. In Figure 2c, we could easily distinguish two kinds of morphologies of the crystals, characterized by their “leaf” and “hexagonal” shapes. In previous morphological studies of many investigators,53 using either optical microscopy, atomic force microscopy, or transmission electron microscopy, leaf-shaped crystalline lamellae patterns were identified as basic structural features of α-iPP. As shown in Figures 2a, 2b, and 2c, the fanshaped hexagonal crystals indicated by black arrows grew obviously faster than the α-crystals and even caused a stop of the growth of neighboring α-crystals. It is unambiguously confirmed by selective melting at 155 °C that the fan-shaped hexagonal crystal structures shown in Figure 3 were β-form iPP crystals.54 Details of the morphology of such hexagonal β-iPP crystals and the nature of their nucleation will be discussed later. Under our experimental conditions, the diffusion-limited aggregation process in thin films resulted in the formation of dendritic crystals (seen in Figure 2c). On top of the faceted dendritic β-form crystals, a number of “secondary” lamellae

2. EXPERIMENTAL SECTION 2.1. Materials and Samples Preparation. Isotactic polypropylene (iPP, trade name F401, Mw = 315 000, Mn = 83 000, MWD = 3.79), with a nominal melting temperature of 166 °C, used in this work was produced by Yangzi Petroleum and Chemical Corp. (Nanjing, China). iPP was dissolved in p-xylene at concentrations of 0.4−1.0 wt % by heating the solution for 30 min to 130 °C, which is well above the nominal dissolution temperature of ca. 100 °C.52 Thin films were prepared by spin coating the hot solution onto a hot silicon substrate (N100 type) surface with a KW-4A spin-coater (Institute of Microelectronics, Chinese Academy of Sciences, China). The spin speed and time were 3000 rpm and 30 s, respectively. A homemade hot stage has been added for heating the substrate before spinning. Before depositing the solution, the substrate was kept at 70 °C for 3 min. The resulting samples were then cooled rapidly from 70 °C to room temperature. The resulting thin films were scratched with a sharp scalpel at the shear flow temperature with the constant velocity of 5 cm/s (ts ∼ 0.2 s). The shearing was done at a sufficient load so that the whole film was removed from the line produced by moving the scalpel along one chosen direction. In our experiment, the width of the resulting line of removed material was about 10 μm. Prior to shearing, iPP thin films were subjected to a thermal treatment to erase possible effects of thermal or mechanical history. The thermomechanical protocol B

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Figure 2. Optical micrographs showing the growth of iPP crystals at the crystallization temperature Tc = 138 °C in a ca. 26 nm thick film after shearing at Ts = 138 °C. The images were taken after the crystallization time tc = (a) t0 ∼ 7 min, (b) t0 + 15 min, and (c) t0 + 45 min. Length of lamellae of (d) α-form and (e) β-form plotted against crystallization time (tc) for various Tc. (f) Growth rates (Gα,Gβ) and ratio (Gα/Gβ) of the two iPP forms as a function of Tc.

(Gβ) of β-form crystals also decreased significantly with increasing Tc (126−142 °C) from 22.2 to 0.55 μm min−1, respectively. Intriguingly, the ratio of Gα/Gβ increased with Tc (see Figure 2f). Measurements of the growth kinetics (see Figure 2f and Figure S2) also showed, at higher Tc (e.g., 142 °C), Gβ < Gα. Such behavior has been observed previously for bulk iPP.57 This phenomenon may be attributed to the higher equilibrium melting temperature of α-form iPP crystals. The temperature Ts at which shearing was performed was crucial for the resultant flow-induced crystalline morphology since the concentration of shear-induced nucleation precursors is affected by molecular relaxation processes occurring at Ts. Figure 4 depicts the flow-induced morphology of iPP crystals in thin films which were subjected to shear at various Ts. Obviously, the crystal morphology changed with increasing Ts. When Ts was set at 130 °C, about 36 °C below its nominal melting temperature (Tm ∼ 166 °C), we obtained at the scratch line a high nucleation density of fan-shaped β-form crystals separated by α-form crystals which developed into a sawtooth pattern. At Ts = 166 °C, as shown in Figure 4b, at the scratch

Figure 3. Optical micrographs of β-form iPP crystals induced by shearing at Ts = 138 °C in (a) 200 nm and (c) 26 nm thick film under the conditions Tc = 138 °C. (a, c) Polymorphic structures (α and β forms) after isothermal crystallization. (b, d) Texture after selective melting of the β-form crystals at 155 °C.

appeared along the junction lines where branches merged. Similar observations have been revealed for i-PS.55 More information on “secondary” lamellae can be found in Figure S3. Figures 2d and 2e exhibit the time dependence of length (defined by the position of the farthest tip with respect to the edge of the scratch-induced line) of α-form and β-form iPP crystals, determined by in situ optical microscopy. Supercooling (crystallization temperature, Tc) plays a crucial part in determining the growth rate of a polymer crystal.56 Growth rates of α-form and β-form iPP determined from the data in Figures 2d and 2e were plotted against Tc, as depicted in Figure 2f. The average growth rate (Gα) of α-form lamellae decreased with increasing Tc from 16.4 μm min−1 (126 °C) to 0.58 μm min−1 (142 °C). In parallel, the average lamellar growth rate

Figure 4. Optical micrographs of thin iPP films which were individually sheared at Ts = (a) 130, (b) 166, (c) 180, and (d) 190 °C and all isothermally crystallized at Tc = 134 °C for 20 min. C

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Figure 5. A set of AFM height (a−c) and the corresponding phase (d−f) images showing details of the morphology of the flow-induced crystallization of iPP (coexistence of α-iPP edge-on crystals and β-iPP/α-iPP flat-on crystals) induced in a ca. 26 nm thick film by shearing at Ts = 130 °C, followed by crystallization at Tc = 130 °C for about 2 min.

line predominantly α-form crystals were formed. Only one fanshaped β-form iPP crystal could be observed. When Ts was further increased to 180 °C (see Figure 4c), the nucleation density of crystals along the scratch line was found to be distinctly lower than that in Figures 4a and 4b. Individual αform crystals were clearly recognizable. This implies that less and less shear-induced nuclei survived as Ts was increased. Continuing to increase Ts, flow-induced oriented polymer chains obviously were able to relax practically completely and almost no crystals were induced (see Figure 4d). This observation suggested that molecular relaxation affected the formation of β-form iPP crystals more than that of α-form crystals. In other words, oriented chains play a major role in the formation of β-form iPP crystals, which will be presented in the following subsections. Because of their small size, nucleation sites of β-form iPP crystals cannot be observed with an optical reflection microscope. However, morphology in such length scale can easily be detected by atomic force microscopy. As shown in Figure 5, morphology and orientation of lamellae in the scratch area were found to be very similar to flow-induced oriented crystalline lamellae patterns, usually generated by stretching/ shearing an entangled polymer bulk melt or solution. In Figure 5b, it can be seen that all of the iPP crystals with leaves (αform)52 and dendritic (β-form) patterns grew perpendicular to the flow direction. The high-magnification micrographs (see Figures 5a and 5c) confirmed that leaf-like and dendritic patterns represented flat-on lamellae. However, at the very border of the scratched line, only edge-on lamellar crystals aligned normal to the flow direction were observed. Most of the initial edge-on crystals turned into the flat-on form when their length reached ∼200 nm. It should be pointed out that the transition from edge-on to flat-on crystals occurred abruptly, but it did not happen for all edge-on lamellae at the same time. Consequently, while some edge-on crystals stopped already after a few 10 nm, others continued to grow over distances up to ca. 800 nm.

As can be seen from Figure 5f, a few of the edge-on lamellar crystals were not fully straight but bended as they grew forward and thus inevitably collided with the growth of neighboring lamellae due to their slightly different growth directions. Actually, in most cases, the main reason for the stop in growth of the initial edge-on crystals was their collision with other edge-on or flat-on crystals. As flat-on crystals possessed a larger interface with the surrounding melt of polymer thin film, these crystals had a higher probability to capture polymer chains and integrate them, leading to faster growth.58,59 Our results indicate that only edge-on lamellae can be nucleated at the flow border. These lamellae were oriented of perpendicular to the flow direction. At the boundary of the scratch, the nucleation density for edge-on lamellae was so high that no further crystals could grow in between. The distance between neighboring edge-on crystals was only ca. 12 nm. This value is comparable to the thickness of lamellar crystals obtained in solution, thin films and in bulk samples.8,51 The nucleation site of a flat-on β-iPP crystal can be seen from Figures 5d and 6. A kinetically controlled mechanism and a mechanism of lamellar twisting have been proposed to explain a transition from edge-on to flat-on lamellar crystals observed in polymer thin films.58 However, in our study, we have no indications of screw dislocations or lamellar twisting close to edge-on lamellae, indicating that flat-on lamellae developed by growth initiated via edge-on lamellae. Additionally, selective washing experiment indicated that β-iPP crystals were easily dissolved in p-xylene (Figure S4). This difference in dissolution behavior is similar to the difference in melting behavior,54 suggesting that selective washing reflects differences in thermal stability of the crystals. After a washing procedure at a relatively high temperature, β-iPP crystals were removed, and only the αiPP edge-on crystals remained to which the β-iPP crystals were initially attached (see the white arrow in Figure S4b). A similar transition from an edge-on to a flat-on orientation of lamellae has been reported for edge-on crystals obtained by scratching (rubbing) polymer thin films.60−62 Here, our D

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of the higher degree of supercooling (the equilibrium melting temperature of α-form crystals were found to be higher than that of β-form crystals) and correspondingly smaller free energy barrier for nucleation, the probability of the α-to-α transition is expected to be higher.63 Corroborating this expectation, a transition from α-form edge-on to β-form flat-on crystals is not observed often.

4. CONCLUSION We have shown that shear flow has a strong influence on the selection of crystal polymorphs of iPP thin film in a desired area of the film. We found that shearing a supercooled melt of iPP induced a preferential orientation of polymer chains along the flow direction, which could act as nucleation site for epitaxial growth of edge-on crystals. Furthermore, a transition from edge-on to flat-on lamellae was observed during crystallization. Interestingly, the transition from α-iPP edge-on to α-iPP flat-on lamellae was frequently observed, while the transition from αiPP edge-on to dendritic β-iPP flat-on lamellae occurred only occasionally, probably due to a comparatively larger free energy barrier for nucleation. In conclusion, we note that flow-induced crystallization of β-iPP in thin film proceeds in two steps: First, α-form edge-on crystals are grown from preferentially oriented and aligned polymer chains induced by flow. Subsequently, the fold surface of the α-form crystals act as a nucleation site for βiPP flat-on crystals, representing a mechanism for a transition from edge-on to flat-on crystals and thus switching the growth of edge-on lamellae in the growth of flat-on lamellae.

Figure 6. A set of AFM height (a, b) and corresponding phase (c, d) images showing details of the flow-induced morphology at nucleation site of a flat-on β-iPP crystal. The sample was prepared by shearing at Ts = 130 °C in a ca. 26 nm thick film, followed by crystallization at Tc = 130 °C for 2 min.

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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.6b01123. Figures S1−S4 (PDF)

observations indicate that nucleation of β-form iPP flat-on lamellar crystals in thin films was induced by α-form edge-on lamellar crystals. A schematic illustration of the pathway of βform crystals originating from α-form edge-on crystal is shown in Figure 7. First, shish crystals, i.e., a bundle of oriented chains, parallel to the flow direction were induced by shear flow. Second, αform edge-on lamellar crystals grew epitaxially on the bundle of oriented chains. At the fold surface of these edge-on crystals, loose loops or cilia of reduced chain mobility are concentrated which finally transformed into nuclei of flat-on crystals on the surface of the parent edge-on crystals. As indicated in Figure 7, depending on kinetic effects two routes may be possible: a transition from α-iPP edge-on crystal to α-iPP flat-on crystal and from α-iPP edge-on crystal to β-iPP flat-on crystal. Because

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.C.). *E-mail [email protected] (G.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Science Foundation of China (No. 11172272 and 11372284), Outstanding Young

Figure 7. Schematic illustration of flow-induced β-iPP flat-on crystals in thin film. (a) Shish crystals were induced by flow. (b) Edge-on lamellar crystals could epitaxially grow from these shish crystals. (c) During the growth process, a transition from edge-on to flat-on α-iPP or β-iPP crystals occurs. E

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Talent Research Fund of Zhengzhou University (1521320004), China Postdoctoral Science Foundation (2016M592302), and Startup Research Fund of Zhengzhou University (1512320001).

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DOI: 10.1021/acs.macromol.6b01123 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01123 Macromolecules XXXX, XXX, XXX−XXX