Morphological Changes of Isotactic Polypropylene Crystals Grown in

Article pubs.acs.org/Macromolecules

Morphological Changes of Isotactic Polypropylene Crystals Grown in Thin Films Bin Zhang,† Jiajia Chen,† Baochen Liu,† Binghua Wang,† 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 and Freiburg Materials Research Center, University of Freiburg, 79104 Freiburg, Germany

‡

S Supporting Information *

ABSTRACT: Morphological variations of lamellae of isotactic polypropylene (iPP) grown in thin films have been examined experimentally by optical microscopy (OM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). A flower-shaped morphology of iPP crystals, composed of several petal-like lamellae radiating from a nucleus, was typically found. At crystallization temperatures (Tc) below 135 °C, initially petal-like lamellae with a flat α-iPP backbone and many regular branches were formed, which were able to induce epitaxial nucleation of γ-iPP, resulting in features similar to a dendrite growing in the plane of the slow growth direction (i.e., b-axis of α-iPP). With increasing Tc, these dendritic structures disappeared gradually, and the lamellae exhibited a faceted lathlike shape for Tc > 150 °C. Interestingly, periodic lateral splitting (the crystal splayed into a pair of branches) at the fast growth plane was observed at a critical width (Wmax) which increased with Tc. In particular, the measured temperature dependence of the products of Wmax2G (G represents the growth rate along the a*-axis) was found to be constant. We discuss the role of the diffusion field at the growth front and epitaxial crystallization with respect to morphological changes of iPP lamellae in thin films.

1. INTRODUCTION

The evolution of those morphologies is typically related to a gradient field at crystal growth font formed in thin films.29−32 According to the morphology diagram proposed by Brener,33 changes in crystal morphology may occur by varying the crystallization temperature (supercooling). For instance, Taguchi et al.34 studied the growth shape of crystals of isotactic polystyrene (iPS) in thin films as a function of crystallization temperature and revealed changes in morphology from faceted to dendritc crystals. Monte Carlo simulations experiment by Hu et al.35 yielded similar results. Furthermore, in our previous work, we reported that the characteristic width of finger-like branched structures is closely related to crystallization temperature.36 Motivated by industrial but also academic interests, polypropylene (PP), a widely used semicrystalline polymer, has been investigated for a long time. In the past few decades, single crystals of syndiotactic polypropylene (sPP) grown from a thin film, with two pairs of (010) and (100) sectors, which have boundaries along the diagonals of the rectangular shape crystals, have been studied in detail.37−40 Investigations of the morphology of iPP lamellar crystals grown in thin films can be traced back to the1960s.41 Toda and co-workers42 obtained isolated iPP lamellae from the melt and suggested that the

The phenomenon of crystallization of long polymer chains has always been one of the most important and striking aspects in condensed matter physics.1 During crystallization, disordered polymers orient sequences of their chains by adsorption on the surface of primary nuclei, leading preferentially to the formation of metastable folded-chain lamellae.2−4 In an isotropic bulk polymer melt, the resulting spherulites are composed of lamellar crystals in its inner part, continuously branched and often epitaxially crystallized.5−7 However, when polymers are confined thin films, three-dimensional isotropic spherulitic growth is not possible, favoring a preferential orientation of the lamellae.8−11 Probing morphology of polymer crystal in thin films may help to identify fundamental steps in the process of lamellar growth, leading to a better understanding on polymer crystallization. Because of spatial confinement and specific interaction at interfaces, characteristic features of the crystallization process (e.g., crystallization kinetics and the resulting crystal morphology) in thin films are often rather different from what is found in bulk.12−15 In particular, in the study of thin films much attention has been paid to diffusion-controlled crystal morphologies.16−18 Typically, when polymers crystallize in a thin film, crystals with various structures, including faceted, dentritic, seaweed, and finger-like patterns, are observed.19−28 © XXXX American Chemical Society

Received: June 28, 2017

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Macromolecules crystals were sectored and exhibited a difference in lamellar thickness of the (010) and (100) sectors. Recently, various techniques such as transmission electron microscopy (TEM),43 optical microscopy (OM),42 and atomic force microscopy (AFM)44 have been used to investigate iPP lamellae grown under different conditions. Despite some achievements of those preceding studies on the crystallization behavior and structure of iPP, features of the morphology of iPP lamellae crystals, in particular at micrometer and submicrometer length scales, have not been investigated in detail, mainly because of the difficulty of obtaining isolated lamellae without overgrowth resulting from e.g. epitaxial crystallization and microcrystallites covering lamellar surface.45−48 The main reason for such overgrowth is the existence of an amorphous layer on the surface of crystalline lamellae, from which microcrystallites are formed during the cooling process following isothermal crystallization. These microcrystallites prevent the observation of the detailed morphology of the lamellar crystals. In the present AFM study, we attempt to explore the detailed morphology of iPP lamellae in thin films. In particular, we explore the mechanism causing a morphological instability at the growth front leading to the formation of branches, which we reveal through a careful washing procedure. Because of the highly anisotropic shape (lath-type shape with a large aspect ratio) of iPP lamellae, two different morphological instabilities were observed, related to dendritic growth in (100) sectors, that is, along the slow b-axis and periodic splitting in fast growth direction (that is, the a*-axis43), respectively. The observed evolution of crystal morphology as a function of crystallization temperature suggested that both the diffusion field around the growing crystal and structural features play crucial roles in determining changes in the morphology of iPP lamellae, involving a transition from a dendritic to a faceted morphology.

Figure 1. Temperature protocol applied for the crystallization experiment. lamellae during cooling or quenching. Therefore, we have washed the crystals by putting the specimen into a hot solvent, i.e., p-xylene at 94 °C for 10 s. After retracting the specimen from the solvent, the residual solvent was allowed to evaporate for at least 24 h at 50 °C in a vacuum oven. In Figure S1, we present a comparison of the morphology of the initial sample and sample after being washed. 2.5. Observation Techniques. Observation of the detailed crystal morphology was carried out by using an atomic force microscope (TM-AFM, JPK Instruments, Germany). The height and phase images of iPP lamellar crystals were acquired in the tapping mode at ambient temperature. A JEOL JEM-2100 transmission electron microscopy (TEM) operating at 200 kV was used to study the crystals at room temperature. The corresponding iPP crystals used for electron diffraction experiment were obtained as follows: an amorphous carbon film (about 10 nm thick) was first evaporated on freshly exposed mica. Samples for electron diffraction were spin-coated and crystallized like AFM samples. The thin film was floated off onto a water surface, transferred to a copper grid, and then shadowed with a few nanometers of amorphous carbon for electron diffraction observations.

3. RESULTS AND DISCUSSION Typical overall morphologies of iPP crystals as a function of crystallization temperature and time are shown in Figure 2. One can observe that iPP crystals grown in thin films are greatly different from that found in bulk where typically spherulites are obtained. Here, iPP flower-shaped aggregated crystals growing out from a central nucleus consist of several petal-shaped lamellar crystals lying flat on the silicon wafer substrate. Using AFM at rather high magnification, it can be observed that the central nuclei did not contain impurities but consisted of crosshatched strands of edge-on crystals (see Figures S2 and S3). Interestingly, the growth of iPP lamellae at temperatures from 130 to 140 °C yielded a variety of patterns. When Tc was set to 130 °C, the shape of the lamellae was not stable. As a function of growth time, the lamellae obviously curved, yielding a form resembling a petal. The underlying morphological instability can be clearly identified, even in the image t0 + 2.5 min. At 140 °C, the beginning of a morphological instability can be observed only after a rather long growth time (e.g., t0 + 53 min). Continuing to increase Tc, lamellae were largely straight along the longitudinal growth direction, even when the crystals had a relatively large size (see e.g., image at t0 + 160 min). However, for all cases some features like the thicker parallel ridges (darker in Figure 2), which divided lamellae into different sectors, could be observed.49 In order to study the morphology and structure of iPP lamellae in more detail, AFM was used to record topographic and phase information. Figure 3 shows typical AFM height images of iPP crystals obtained at crystallization temperatures ranging from 130 to 150 °C. At low temperatures, we found edge-on crystals, indicated by a red dotted ellipse in Figure 3. For increasing

2. EXPERIMENTAL SECTION 2.1. Sample. We used isotactic polypropylene (iPP) (Mw = 1.9 × 105 g/mol, Mn = 5.6 × 104 g/mol, Đ = 3.5) with a nominal melting temperature (Tm) of 165 °C (determined by DSC at a heating rate of 10 °C/min), produced by China Petroleum & Chemical Corp. Jinan Company (Jinan, China). 2.2. Preparation of Thin Films. The polymer was dissolved in pxylene at a concentration of 0.25 wt % at 135 °C, which is well above the nominal dissolution temperature of 100 °C. Then, the hot solution was dropped onto a N100 type hot silicon wafer (UV treated for 30 min before being placed on the spin-coater; the silicon wafer should have temperature above 80 °C during spin-coating). The Si wafer was spun at a rate of 7000 rpm with a KW-4A spin-coater (Institute of Microelectronics, Chinese Academy of Sciences, China) for 30 s. Film thickness of the resulting iPP film, as determined with a homemade ellipsometer, was 12.3 ± 0.5 nm. 2.3. Crystallization Temperature Protocol. An Olympus BX-61 optical microscope (Olympus, Tokyo, Japan) equipped with a Linkam THMS 600 hot stage (Linkam Scientific Instruments, Tadworth, UK) was used for crystallization and the observation of morphological changes. The experimental temperature protocol is shown in Figure 1. The thin film was heated to the molten state at a rate of 10 °C/min under nitrogen, up to 175 °C, and kept at this temperature for 2 min. Then, the specimen was crystallized isothermally on the hot stage at a preset temperature in the supercooled state (120−160 °C) for variable times. After isothermal crystallization, the specimen was immediately quenched to room temperature. 2.4. Washing Procedure. In order to facilitate a clear observation of the surface morphologies of lamellae, it was necessary to remove amorphous portions and microcrystallites, formed on the surface of B

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Figure 2. Optical micrographs showing the growth of iPP crystals at different crystallization temperatures (Tc) in a ca. 12.3 nm thick film. The images were taken after crystallization at (a) Tc = 130 °C for t0, t0 + 2.5 min, t0 + 5 min, t0 + 7.5 min; (b) Tc = 140 °C for t0, t0 + 26 min, t0 + 53 min, t0 + 80 min; and (c) Tc = 145 °C for t0, t0 + 52 min, t0 + 106 min, t0 + 160 min.

crystals was only 2−7 μm. On the basis of the shape of these lamellae, we can conclude that growth rates along the two crystal directions differed significantly. Electron diffraction showed that the a*-axis was along the direction of the long size of the crystals.43 Accordingly, the b-axis was in the orthogonal axis of these α-form crystals of iPP.41 Interestingly, similar to syndiotactic polypropylene (sPP),37 sectors and diagonal axis were observed for the iPP lamellae found in the thin films studied here. For example, these features can be seen clearly in Figure 3c, indicated by black and red arrows, respectively. Because of a thickness difference between neighboring sectors, they can be distinguished easily by AFM. Sectors indicated by red arrows are thicker iPP lamellar crystals. Furthermore, patterns and aspect ratio of iPP crystals of their size along the a*-axis and the b-axis growth direction depended strongly on Tc. For the sample crystallized at 130 °C for 10 min, Figure 3a shows a curved lamellar crystal with dendritic pattern at the lateral edge. As Tc increased, the dendritic pattern gradually disappeared and transformed eventually at 150 °C into lamellar crystals with a lath shape, indicating the unit cell of the underlying iPP α-form (Figure 3d). The AFM images of Figure 3 demonstrate for increasing Tc the variation on the lateral edges of iPP lamellar crystal from dendrites (Figure 3a), to a serration pattern (Figures 3b and 3c), and finally to flat structures (Figure 3d). In Figure 4, the lateral outgrowths on the basal lamellae at different Tc reveal more morphological details. At low Tc (below 135 °C), when lamella grew forward along the direction of the b-axis, a typical dendrite morphology appeared. In the past years, many researchers have reported dendritc patterns in thin films, in both experiment and simulation.52,53 When increasing Tc to 145 °C (see Figure 4d), iPP lamellae exhibited a lath-like, almost rectangular shape without any visible side branches. We emphasize that crystals only grew very slowly at relatively high crystallization temperatures (e.g., 140−145 °C), allowing the perfecting of the crystals. Structurally, growth of the side branches along (110) and (−110) planes was decreasingly relevant, creating effectively a serrated growth

Figure 3. AFM height images showing typical patterns of iPP crystals grown from a 12.3 nm thick film by varying the crystallization temperature: (a) 130 °C for 10 min, (b) 135 °C for 28 min, (c) 145 °C for 280 min, and (d) 150 °C for 494 min.

crystallization temperatures, edge-on lamellae became smaller and shorter and eventually almost invisible at Tc = 150 °C. Flaton lamellar crystals were emerging from edge-on crystals as can be seen in Figure S2. Thus, we conclude that edge-on crystals were nucleated preferentially at the early stage of crystallization. Nevertheless, in 12.3 nm thick films, flat-on lamellae crystals were favored in the later stages of the growth process.50 According to the three-layer model,51 as Tc increased, the faster growth rate of flat-on lamellae resulted in the termination of growth of edge-on lamellae. In Figure 3, the distance from the center to the growth front of lamellar crystals was about 15−25 μm. The width of these C

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Figure 5. (a) Morphology and (b) electron diffraction pattern of the iPP crystals grown from a thin molten film at 135 °C.

Figure 4. AFM height images of iPP lamellar crystals grown in a 12.3 nm thick film at Tc = (a) 120, (b) 130, (c) 135, and (d) 145 °C.

front at a very small scale. Thus, initiation of epitaxial γ-iPP crystallization on the (100) growth front of α-iPP crystals was limited, resulting in relatively large sizes of smooth lateral faces of the lath-shaped crystals. In our experiments, the films had a thickness slightly thinner than the lamellar thickness of the resulting crystals. Close to the growth interface of the crystal, a depletion zone develops, introducing a concentration gradient for the diffusion field assuring the supply of amorphous polymers. Simultaneously with the slow growth rate, the fold surface of a lamellar crystal and the lateral growth front became more strongly populated by amorphous (molten) polymers. Polymers diffusing across the fold surface of a lamellar crystal may reach gaps on the serrated edge and fill up the serrated growth front. Induced by this filling process, lamellae may be nucleated epitaxially on the lateral surface of a dendritic side-branch of the α-iPP lamellae. The orientation of these lamellae becomes visible due to subsequent crystallization processes yielding a characteristic crack-like pattern (see Figures S4 and S5). Furthermore, selected area electron diffraction patterns, obtained from petal-like lamellae crystallized at 135 °C, represented in Figure 5b, show that these lamellae consisted of mixed α/γ crystals with a crystal structure that is similar to that observed for needle like γ-phase crystals.54 Nonparallelism of chain packing has been reported by Bruckner and Meille55−57 as one of the most important features of crystalline structures of γ-iPP crystals in an orthorhombic unit cell. Electron microscopy studies indicated that the γ-phase crystallizes on the lateral ac-faces of α-iPP.48 Based on the analysis of crystal polymorphism, an in-depth understanding of the morphology on a molecular level has indeed been reached through the elaborate work of Lotz et al.58−60 As shown in Figure 6, composite lamellar crystals consisted of a flat-on αiPP crystal that served as a substrate for γ-iPP overgrowth. The characteristic period of the overgrowth crystals was measured to be about 18 nm.

Figure 6. AFM phase images of an iPP crystal grown at 135 °C. The arrows in (b) and (c) show the branching sites along the a*-axis of the lamellae.

To visualize the crystal growth mechanism along the a*-axis of the lamellae, we had a closer look at an individual lamellar crystal, grown at 135 °C in a 12.3 nm thick film, as shown in Figure 3. Strikingly, we found that the forward growth of the lamellae was accompanied by a consecutive tip-splitting in the direction perpendicular to the growth face, as indicated by the arrows in Figure 6b. Interestingly, for a given Tc, the critical width, Wmax (indicated in Figure 6b), before the lamellar crystal splayed into a pair of branches was almost constant. One may ask: Why does the lamellar crystal become unstable and split into two branches along the a*-axis when its width reaches Wmax? In Figure 7, we examined lamellae grown at different Tc. One can clearly see that Wmax and branch frequency strongly depended on Tc; that is, Wmax increased with increasing Tc while the branch frequency decreased with Tc. In Figure 9b, we present the results of Wmax for a systematic series of experiments. For crystals grown from a thin film, as a direct evidence of the gradient field assuring mass transport, a depletion zone develops close to the crystalline growth interface, introducing a concentration gradient for the diffusion field assuring the supply of amorphous polymers. In Figure 8 for a nonwashed sample, a depletion zone can be seen clearly in between the crystal front and the amorphous film. The corresponding diffusion length is an important parameter in pattern formation in the course of polymer crystallization and relates to the length scales of these crystal patterns. Usually, the depletion zone represents the region D

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The splitting mechanism can be interpreted in the following way: Periodic lateral splitting observed in the AFM pattern showed a long crack (a series of button holes) and epitaxially growing lamellae on the crack edge. The sites of lateral splitting can be explained in terms of structural features. During the growth process, deposition of a stem in epitaxial registry results in chains having their main axis nearly perpendicular to the initial crystal. If a stem is deposited on the fast growing (010) lateral face, no growth is possible in the b-axis direction. This epitaxially attached stem acts as a poisoning site. Further lateral growth is hindered at this position. As the crystal continues to grow, the crystal must split at this position, initiating the formation of a long crack. This structural interpretation accounts for the fact that this phenomenon is only observed in iPP. Now, we attempt to clarify the origin of curved lamellar crystals at lower Tc (see Figure 2), where we observed smaller values of Wmax and a higher frequency of branching, inducing a curved shape of the lamellae. For Tc = 145 °C, the length of the crystal was not long enough to allow for the onset of branching. Thus, the crystals have a rectangular shape as shown in Figure 4d. The branching mechanism of lamellar crystals has been discussed previously.62−66 The possibility of an instability of the growth front related to the diffusion field is widely discussed since the original proposal by Padden and Keith.67 In our experiments, the depletion zone yielding a self-induced gradient field resulted from the mass transport ahead of the growth front, as discussed above. According to the Mullins−Sekerka52 (MS) instability, the minimum wavelength λ of the branching instability is equal to the geometric mean of the diffusion length l and the capillary length d0: λ = 2π ld0 . As described by the MS theory, due to composition or temperature gradients, a steadily moving planar front will eventually become unstable and form branches. For thin films, the diffusion length also can be replaced by the width of the depletion zone ahead of crystal front. In our previous paper, we have demonstrated that the Mullins−Sekerka wavelength λ, i.e., the length scale of the morphological instability, can be determined by the distance

Figure 7. AFM phase images of iPP lamellar crystals grown in 12.3 nm thick films at (a) 130, (b) 135, (c) 140, and (d) 145 °C. Branching sites are indicated by white arrows.

having a depressed thickness with a width related to the diffusion length.29,61 As shown in Figure 8, we have identified a nonconstant width of the “depletion zone” existing in front of the edges of the crystal branches along the fast growth plane. The width of the “depletion zone” ahead of the slowly growing plane was measured to be about 225 nm. Interestingly, as shown in Figure 8c, the width of depletion zone along the a*axis of the branching lamellae was not constant and varied between 270 and 780 nm. The lath-shaped crystal was predominately branching on the fast growth plane where the “leading branches” were always closest to the reservoir of the polymer melt and thus had a higher probability to capture and attach molecules. As a consequence, if branching is allowed, the growth front may become uneven.

Figure 8. (a) AFM height image of the growth front of iPP crystals (without washing) grown from 12.3 nm thick films at 135 °C. (b) Height profile of the cross section along the b-axis. (c) Height profile of the depletion zone at different positions along the a*-axis of an iPP lamellar crystal. E

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with Tc. We tentatively believe that the periodic lateral splitting in the a*-axis of iPP crystals is governed by two aspects: First, a stem in epitaxial registry may serve as branching site for periodic lateral splitting. Then, lamellar branching along both growth directions is affected by an unstable growth front caused by an anisotropic gradient field as described by the Mullins− Sekerka instability.

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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.7b01381. Figures S1−S5 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Figure 9. Temperature dependence of (a) lamellar thickness dc, (b) characteristic width Wmax of branches, (c) the average growth rate G along the a*-axis (the inset shows the thickness dependence of the G), and (d) Wmax2G obtained for 12.3 nm thick films at 135 °C.

ORCID

Bin Zhang: 0000-0002-8293-1321 Renate Reiter: 0000-0003-2294-1445 Günter Reiter: 0000-0003-4578-8316

between side branches.36 We found at high Tc that Wmax2G, where G is the growth rate of crystal, was independent of Tc and G. Thus, after having measured Wmax2 and G (shown in Figure 9), at high Tc, we found that Wmax2G was approximately constant. However, at high degree of supercooling (e.g., below 130 °C), the crystallization rate was too fast and the growth front was too narrow to allow for a precise determination of λ. The decrease of the growth rate (G) with film thickness is illustrated in Figure 9c. At lower crystallization temperatures (e.g., 135 °C), the growth rate decreased with film thickness due to the longer diffusion time needed to transport molecules to the crystal growth front. For crystallization temperatures higher than 145 °C, G varied little for thicknesses between 7 and 37 nm. Especially for 7 nm thick films, lamellar crystals grew approximately at the same rate at 145 or 150 °C. The average radius of gyration of the iPP chains used in this study is around 15 nm,68 which is larger than the thickness of some of the films. The growth rate as a function of film thickness might become constant for film thicknesses less than or close to the radius of gyration. Diffusion of macromolecules under such confinement may be difficult. Finally, for thicker films the growth rate depends on both crystallization temperature and film thickness.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Wenbing Hu and Prof. Yongfeng Men for inspiring and very helpful discussions. We acknowledge the help given to us by Prof. Huihui Li and Le Ma for the TEM measurements. The authors are grateful to the National Science Foundation of China (No. 11372284), Outstanding Young Talent Research Fund of Zhengzhou University (1521320004), China Postdoctoral Science Foundation (2016M592302), and Startup Research Fund of Zhengzhou University (1512320001).

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4. CONCLUSION Applying a careful washing procedure, variations in morphology of isotactic polypropylene lamellae, especially the periodic branching along the fast growth direction (a*-axis), could be observed for the first time. Depending on the growth rate of the individual sectors, the morphology of isotactic polypropylene crystals grown in thin films yielded two different patterns: lathshaped lamellae and dendritic patterns with periodic branches. Lath-shaped lamellae with smooth and straight growth fronts were observed at high Tc, e.g., above 150 °C, while the formation of dendrites at the lateral faces was observed at lower Tc. Interestingly, growth in the a*-axis of the lamellae was accompanied by the formation of branches at a characteristic width when they splayed into a pair of branches. The characteristic wavelength of this periodic splitting increased F

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DOI: 10.1021/acs.macromol.7b01381 Macromolecules XXXX, XXX, XXX−XXX