Article pubs.acs.org/Macromolecules
Temperature-Dependent Recrystallization Morphologies of CarbonCoated Isotactic Polypropylene Highly Oriented Thin Films Le Ma,† Zhenzhen Zhou,† Jie Zhang,† Xiaoli Sun,† Huihui Li,† Jianming Zhang,‡ and Shouke Yan*,†,‡ †
State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡ Key Laboratory of Rubber-Plastics, Qingdao University of Science & Technology, Qingdao 266042, China S Supporting Information *
ABSTRACT: Temperature-dependent recrystallization morphologies of carbon-coated iPP oriented thin films were studied by electron microscopy combined with electron diffraction. It was found that the vacuum evaporated carbon layer always results in oriented melt-recrystallization of the preoriented iPP thin films. The morphology of the recrystallized carbon-coated iPP thin films is, however, dependent on both melting and crystallization temperatures. At low melting temperatures, e.g., lower than 200 °C, parallel-aligned edge-on lamellae with the same molecular chain orientation as the prepared samples were always generated regardless of crystallization temperature. When melting at 300 °C and crystallization at low temperature, e.g., 120 °C, both edge-on and flat-on lamellae were observed. At elevated crystallization temperature, e.g., 150 °C, the flat-on iPP crystals show more or less a regular lath shape. At the same time, quadrites caused by wide-angle lamellar branching originating from homoepitaxy of α-iPP were seen. It was further found that all of the three kinds of iPP crystals have a fixed mutual orientation. While a portion of the lamellae in the quadrites possess the same molecular chain orientation as the parallel-aligned edge-on lamellae, the flat-on iPP crystals arranged with their a-axes along the molecular chain direction of the parallel-aligned edge-on lamellae. This suggests that both the quadrites and flat-on crystals of iPP are initiated by the parallel-aligned edge-on lamellae. These results indicate unambiguously the strong fixing effect of vacuumevaporated carbon layer on the iPP oriented thin film. The fixing effect is, however, inhomogeneous. Raman spectroscopy study indicates the existence of chemical bonding between the carbon layer and the iPP film. It is those molecular chains fixed to the carbon layer by chemical bonds that produce always oriented edge-on lamellae. The molecular chains fixed weakly to the carbon layer can be released from the carbon layer at high temperature and generate flat-on lamellar crystals by crystallization at high temperatures.
1. INTRODUCTION Thin and ultrathin polymer films have received a great deal of attention, both scientifically and technologically.1−10 They play an important role in a variety of applications, such as electronics, liquid crystal alignment, adhesion, and so on.11,12 For crystalline polymers, the crystalline morphology, especially the special arrangement of the crystals in thin or ultrathin films, significantly influences the physical and mechanical properties. Therefore, the study of thin film crystallization of polymers can not only help our understanding of the nature of polymer crystallization but also provide us a way to optimize the properties of polymer thin films by controlling their crystalline morphology and structure. It was well-known that the polymeric crystals in thin films can be arranged with molecular chains in the film plane (edgeon lamellar orientation) or normal to the film plane (flat-on lamellar orientation).13−15 The factors influencing the orientation of lamellae in polymer thin films are the film thickness,13,16−22 crystallization temperature,19,22−25 and interaction between the polymer and its supporting substrate.23,26−28 Schönherr et al.13 have observed the crystal© XXXX American Chemical Society
lization of poly(ethylene oxides) with different thicknesses on Si substrates. They found that the films were mainly composed of flat-on lamellar crystals when the films are thinner than 300 nm, while edge-on lamellae were dominant in films thicker than 1000 nm. A similar thickness dependence was also observed for other polymers, such as linear low-density polyethylene.22 On the other hand, crystallization temperature is an important factor for governing the crystallization process and the resultant morphology of polymers in all cases, i.e., bulk and thin film crystallization. For thin film crystallization, it was confirmed that easy homogeneous nucleation at the film surface at low temperatures favors the formation of edge-on lamellae, while the quicker heterogeneous nucleation at the polymer/substrate interface results in the growth of flat-on lamellae at high temperature.19,23 Moreover, the interfacial interaction between polymer and substrate is another vital factor which influences not only the structure and morphology but also the physical Received: February 13, 2017 Revised: April 17, 2017
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DOI: 10.1021/acs.macromol.7b00299 Macromolecules XXXX, XXX, XXX−XXX
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at a 2 cm−1 resolution. The orientation function of the iPP thin films was calculated by equation f = (R − 1)/(R + 1), where R is the dichroic ratio defined as the ratio between parallel and perpendicular polarized intensities, i.e., A∥/A⊥. The bands 998 and 2920 cm−1 were chosen to estimate the orientation of iPP crystals ( fc) and the average orientation ( fav) of the iPP thin films, respectively.58−62 For Raman spectroscopic measurements, a Renishaw In Via Raman microscope was used. The excitation lines are 633 nm with a power of 10 mW.
properties of semicrystalline polymers.29−33 Especially, the existence of special interactions can result in the formation of specific crystal structure and/or preferred orientation of a polymer. For example, favored crystallographic interaction between polymer and substrate will lead to epitaxial crystallization of polymers. This has been successfully used to regulate the chain orientation and crystal structure of polymorphic polymers as well as the spatial arrangement of planar backbone molecular chains.34−48 In general cases, it was found by Monte Carlo simulations that thin films on a slippery substrate show mainly edge-on lamellae but exhibit flat-on lamellae while on a sticky substrate.27 Based on different interaction strength between polymer and substrate, the structure and morphology of thin polymer films can also be well controlled.26 To enhance the interaction between polymer with the supporting substrate, hydrogen bonding has been frequently utilized. As an example, by placing polyesters on poly(vinylphenol), the formation of hydrogen bonds influences both the crystallization and the melting behavior of the polyesters significantly.30−32,49 In our previous studies, we found that a vacuum-evaporated carbon layer exhibits a very strong fixing effect on surface polymer chains, and this has been successfully utilized to maintain the chain orientation of the preoriented polymer films during melt-recrystallization.50−56 The interaction between the polymers and the coated carbon layers is even stronger than hydrogen bonding. Recently, we have studied the meltrecrystallization of carbon-coated isotactic polypropylene (iPP) oriented thin films. It was found that the iPP film always recrystallizes in a way of highly oriented edge-on lamellae with α-form crystals when melting and crystallization temperatures are lower than 200 and 130 °C, respectively. As a succeeding work, we followed the temperature-dependent recrystallization of the carbon-coated iPP oriented thin films at elevated melting and crystallization temperatures with the aim of revealing the importance of interfacial interaction on the crystallization of polymer thin films and the mechanism of the fixing effect of vacuum-evaporated carbon films.
3. RESULTS The melt-drawn iPP thin films exhibit highly oriented edge-on lamellar structure with molecular chains arranged predominately in the drawing direction. The BF micrograph and corresponding electron diffraction pattern of the melt-drawn iPP films have been reported in several previous publications.63 For a better comparison of the readers, a representative BF image and an electron diffraction pattern of an as-drawn iPP thin film are shown in the Supporting Information as Figure S1. It was confirmed in a previous work63 that when melting the carbon-coated iPP oriented films at temperatures lower than 200 °C and recrystallization at temperatures below 130 °C, highly oriented edge-on lamellae of α-iPP are always observed. It is further confirmed that the highly oriented edge-on lamellar morphology is actually independent of crystallization temperature. As an example, Figure 1 shows the TEM BF micrographs
2. EXPERIMENTAL SECTION The iPP used in this study is obtained from Aladdin. The melt flow index of it is 0.50 g/10 min. Its highly oriented thin films were prepared according to a melt-draw technique introduced by Peterman and Gohil.57 According to this technique, a small amount of a 0.5 wt % iPP solution in xylene was dispersed uniformly on a preheated glass plate at ca. 140 °C, where the xylene solvent was allowed to evaporate. After evaporation of the solvent, the thin molten iPP layer of ca. 1 μm in thickness was then picked up by a motor-driven cylinder with a drawing speed of about 20 cm/s and highly oriented ultrathin iPP films of ca. 50 nm were collected. The thus-prepared iPP thin films were then vacuum carbon deposited on one side and then heat-treated under different conditions (as indicated in the text) for electron microscopy study. Vacuum carbon deposition was conducted with a JEOL JEE-420 vacuum evaporator under 3 × 10−4 Pa, and the electric current was controlled between 20 and 30 A. For transmission electron microscopy (TEM) observation, a JEOL JEM-2100 TEM operated at 200 kV was used in this study. Phase contrast bright-field (BF) electron micrographs were obtained by defocus of the objective lens. In order to minimize radiation damage by the electron beam, focusing was carried out on an area; then the specimen film was translated to an adjacent undamaged area for recording the images immediately. For FTIR analysis, a Spectrum 100 FT-FTIR spectrometer (PerkinElmer) was used. FTIR spectra in the wavenumber ranges of 3000−2850 and 1010−985 cm−1 were obtained by averaging 16 scans
Figure 1. BF images and corresponding electron diffraction patterns of carbon-coated melt-drawn iPP thin films heat-treated at 200 °C for 5 min and then isothermally crystallized at 140 °C (a, b) and 150 °C (c, d) for 6 h. The arrows in the BF images represent the drawing directions of the oriented iPP thin films.
and corresponding electron diffraction patterns of the carboncoated iPP oriented films molten at 200 °C for 5 min and then recrystallized at 140 and 150 °C, respectively. They both display highly oriented edge-on lamellae of α-iPP. This is different from the results of Monte Carlo simulations and indicates the strong fixing effect of carbon layer on the surface iPP molecular chains which prohibits the upright orientation of iPP chains that would produce flat-on lamellae. B
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Figure 2. Polarized FTIR spectra of an oriented iPP thin film in the regions of 3000−2850 and 1010−985 cm−1 before and after meltrecrystallization. 0° and 90° indicate the electron vector parallel and perpendicular to the drawing direction of the iPP thin film during preparation, respectively.
To characterize the orientation quantitatively, polarized IR study was carried out on the oriented iPP thin films before and after melt recrystallization. Figure 2 shows the polarized FTIR spectra of the oriented iPP thin films in the regions of 3000− 2850 and 1010−985 cm−1 before and after melt-recrystallization. It is clear that the FTIR spectra with the electron vector parallel and perpendicular to the drawing direction of the iPP thin film are evidently different. The orientation functions of iPP thin film in crystalline phase before and after meltrecrystallization determined from 998 cm−1 are 0.45 and 0.41, respectively. The slight decrease of the orientation function of the iPP crystals may be associated with a reduced molecular chain orientation. This is actually not the case. It is caused by the occurrence of wide-angle lamellar branching, which produces a portion iPP crystalline lamellae aligned 100° apart from the drawing direction of the film as confirmed by the electron diffraction (please see Figure S2). The average orientation function of iPP thin film estimated from the 2920 cm−1 decrease from 0.39 for the original thin film to 0.16 after melt-recrystallization. This originates from the fact that the amorphous phase of the original thin film exhibits also certain extent chain orientation, as reported for the melt-drawn polyethylene thin films.64 These more or less oriented molecular chain segments get relaxed during melting and recrystallization. As a result, the average orientation function of iPP thin film drops significantly. To check the strength of fixing effect, the carbon-coated iPP thin films were heated to 300 °C for different times and then crystallized at various temperatures. Figure 3 shows the BF image and the electron diffraction pattern of a carbon-coated iPP oriented thin film after melting at 300 °C for 5 min and subsequently crystallized isothermally at 120 °C for 2 h. From the BF image (see Figure 3a) we can still see the highly oriented edge-on lamellae of iPP. However, the electron diffraction pattern shown in Figure 3b consists actually of three sets of α-iPP diffractions. As sketched in Figures 3c, two sets of
Figure 3. A BF electron micrograph (a) and its corresponding electron diffraction pattern (b) of a carbon-coated melt-drawn iPP thin film, which has been heat-treated at 300 °C for 5 min and then isothermally crystallized at 120 °C for 2 h. Parts (c) and (d) are the sketches of the diffraction patterns related to the cross-hatched edge-on and flat-on iPP crystals, respectively. The subscripts “e” and “f” in part b indicate the electron diffractions contributed by the edge-on and flat-on α-iPP crystals, respectively. The arrow in the BF image indicates the drawing direction of the oriented iPP thin film.
them are contributed by the cross-hatched edge-on lamellae due to the unique wide-angle lamellar branching of α-iPP. The other one is composed of (hk0) reflections resulting from the flat-on lamellar crystals of α-iPP (Figure 3d). For a better C
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diffraction pattern of a vacuum carbon-evaporated melt-drawn iPP thin film which has been heat-treated at 300 °C for 5 min and subsequently recrystallized at 145 °C for 20 h. Now, flat-on crystalline ribbons can be more clearly observed with their long axes aligned in the molecular chain direction of the edge-on iPP crystals (Figure 5a). Moreover, the BF image shows highly oriented edge-on lamellae with less pronounced wide-angle branching. However, quadrite structures, an indication of wideangle lamellar branching, can be identified in some areas. The appearance of strong diffraction spots of flat-on crystals and weakened diffraction spots of edge-on lamellar crystals on the electron diffraction pattern (Figure 5b) is in accordance with the BF observation. When the sample was crystallized at 150 °C, the edge-on, flat-on, and quadrite structures are also clearly displayed as seen from Figure 6a. Figure 6b reveals that the quadrite structure
understanding, the electron diffraction patterns corresponding to the highly oriented edge-on lamellae with and without wideangle lamellar branching as well as the flat-on crystals are presented in the Supporting Information as Figure S2. This suggests that a portion of iPP molecular chains have released from the vacuum evaporated carbon film and arranged themselves upright in flat-on form crystals. It is further confirmed that the population of released molecular chains of iPP increases with melting time at 300 °C. Figure 4 shows the BF electron micrograph and corresponding
Figure 4. A BF electron micrograph (a) and corresponding electron diffraction pattern (b) of a vacuum carbon-evaporated melt-drawn iPP thin film which has been heat-treated at 300 °C for 10 min and then isothermally crystallized at 120 °C for 2 h. The arrow in the picture indicates the drawing direction of the iPP film during preparation.
electron diffraction pattern of a carbon-coated melt-drawn iPP thin film which has been heat-treated at 300 °C for 10 min and then cooled to 120 °C for isothermal crystallization. In the BF image (Figure 4a), there are some places that do not clearly show edge-on lamellar structure. These parts may correspond to the flat-on crystal regions. The electron diffraction confirms an increase of flat-on crystals as judged by the intensity increase of the (200)f reflection spots (Figure 4b). The above experiments have illustrated the influence of melting time at 300 °C on the recrystallization behavior of the carbon-coated iPP oriented thin film. Actually, the crystallization temperature also has a pronounced effect on the crystalline morphology of iPP.19,23−25 Taking this into account, the influence of recrystallization temperature on the morphology of carbon-coated iPP oriented thin films was studied. Figure 5 shows the BF electron micrograph and corresponding
Figure 6. BF electron micrographs (a−c) and an electron diffraction pattern (d) of the vacuum carbon-evaporated melt-drawn iPP thin films, which have been heat-treated at 300 °C for 5 min and then isothermally crystallized at 150 °C for 20 h. The arrows in the pictures indicate the drawing direction of the film during preparation.
consists of a double population of lamellae seen edge-on and 80° or 100° apart from each other, which is frequently obtained for α-iPP on either solution or bulk crystallization.48,65 It can be recognized from Figure 6b that a portion of edge-on lamellae in the quadrite region has the same orientation while the other part of the lamellae are inclined at 80° with respect of the original parallel aligned lamellae. This means that the quadrites are initiated with the same mechanism as the parallel oriented lamellae. On the other hand, it seems that there are some edgeon lamellae grown from the flat-on ones (see Figure 6a,c). The corresponding electron diffraction also confirms the coexistence of three kinds of lamellar structures (see Figure 6d).
4. DISCUSSION According to the above-obtained experimental results, several aspects should be addressed here. First of all, it was confirmed that the vacuum-evaporated carbon film exhibits strong fixing effect on the polymer chains.53 The fixing effect prevents the
Figure 5. BF electron micrograph (a) and corresponding electron diffraction patterns (b) of a vacuum carbon-evaporated melt-drawn iPP thin film, which has been heat-treated at 300 °C for 5 min and then isothermally crystallized at 145 °C for 20 h. The arrow in the picture indicates the drawing direction of the film during preparation. D
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originally included in the oriented crystal lattice will result in the oriented recrystallization with unchanged chain orientation and generate a parallel aligned edge-on lamellar structure. In the regions where the iPP chains are loosely anchored to the carbon layer, the molecular chains can be released from the carbon layer and relaxed completely at high temperature. The crystallization of these relaxed iPP molecular chains is temperature dependent. It was well documented that iPP forms mainly edge-on lamellae at low temperature while flat-on lamellar structure is dominant at high temperature. There is a temperature window in which the edge-on and flat-on lamellae coexist. For example, the melt-recrystallization of iPP oriented thin films simply supported by preformed carbon film at 120 °C produces only flat-on crystals (see Figure S6). If the crystallization temperature is over 145 °C, a single-crystal-like lath structure can be produced.66,67 In the present case, due to the strong fixing effect of the vacuum evaporated carbon layer to the oriented iPP thin film, only some flat-on lamellar crystals have been produced after melt-recrystallization by melting at high temperatures. This happens at the places where molecular chains are released from the carbon layer. For the quadrites, as can be noted in Figure 7b marked by the red circles, there are some areas where most of the iPP was removed from the carbon layer while some tiny crystalline dots remained. These crystalline dots are tightly fixed by the carbon layer and cannot relax during melting. They will be unambiguously the nucleation sits during recrystallization after melting. It was well documented that unique wide-angle lamellar branching of α-iPP can produce the quadrites.68,69 The quadrites have the ac planes parallel to the film plane with a single-crystal-like structure. This has been confirmed by epitaxial crystallization of polyethylene (PE) on the melt-recrystallized carbon-coated iPP thin films. The PE deposited on the quadrite give rise upon recrystallization to a highly ordered lamellar structure with the PE lamellae oriented parallel to the bisector of the obtuse angle made by the branches of the quadrite (see Figure S5a). On the other hand, the PE crystallized on the parallel aligned iPP lamellae produces a cross-hatched structure with PE lamellae being ±50° apart from the iPP parallel lamellae (see Figure S5b). This indicates that the iPP quadrite was initiated by a single nucleus and has a unique crystallographic a- and c-axes orientation in the film plane. On the other hand, the paralleloriented iPP lamellae exhibit a uniaxial fiber orientation. Therefore, biaxial orientation of the PE on parallel aligned iPP lamellae was observed. Third, the orientation relationship between different crystals should be discussed. It was found that the diffraction patterns of all observed crystals, i.e., the parallel aligned edge-on lamellae, the flat-on lamellae, and the quadtites, exhibit a unique fixed orientation. The molecular chains of the parallel aligned edgeon lamellae oriented always in the drawing direction of the film during preparation. This originates the fixing effect of the vacuum-evaporated carbon on the surface molecular chains of the oriented iPP film. For quadrites, the fixed orientation can also be understood when the lamellae created by fixed iPP molecular chains are considered as the mother lamellae to induce the daughter lamellae through wide-angle branching. For the flat-on crystals, the upright chain orientation suggests that the carbon layer loses the fixing effect on the surface molecular chains. It should be pointed out, however, that the flat-on crystals grown on a carbon supporting film exhibit a random orientation in different areas (see Figure S6). In present case, the flat-on crystals produced in carbon-coated iPP
relaxation of molecular chain stems in the surface monolayer and induces the oriented recrystallization of the films. A recent Raman spectroscopy study shows that after carbon coating of the oriented polyethylene thin films some new C−C chemical bonds were created between the carbon layer and the polymer film.56 Raman spectroscopy study on the carbon-coated iPP oriented thin films indicates also the formation of some new C−C chemical bonds (see Figure S3). This also suggests that the strong fixing effect is related to chemical bonding. However, the present results indicate that the fixing effect can be destroyed to some extent at 300 °C. This is hard to be understand if chemical bonding is considered to be the sole reason, since the temperature of the onset of thermal decomposition of iPP is higher than 336 °C (see Figure S4). Therefore, there may exist different fixing mechanisms of the coated-carbon layer on polymer thin films. There are some iPP molecular chains connected to the carbon layer by chemical binding, which cannot be destroyed by heating to 300 °C. These molecular chains maintain the oriented recrystallization of highly oriented edge-on lamellae. On the other hand, some iPP molecular chains may be just anchored to the vacuumevaporated carbon layer by a physical bonding, such as diffusion of carbon into the amorphous interlamellar regions of the iPP, forming pillars as anchors to moor the entangled macromolecular ties.50 To confirm this hypothesis, dissolution test of the carbon-coated iPP thin films in xylene at 100 °C has been performed. As shown in Figure 7, when the carbon-coated iPP
Figure 7. BF electron micrographs of carbon-coated melt-drawn iPP thin films, which have been partially dissolved in xylene at 100 °C for 2 h (a) and 4 h (b). The arrows show the molecular chain directions of the as-prepared samples.
thin films were put into xylene at 100 °C for 2 h (Figure 7a), only iPP in some small domains was removed from the carbon layer. Furthermore, the iPP remaining on the carbon layer shows always the same chain orientation as the original sample (see the electron diffraction shown in Figure S7). This demonstrates the very strong adhesion between vacuumevaporated carbon layer and the iPP thin film. After partial dissolving in xylene at 100 °C for 4 h (Figure 7b), the domains where iPP were detached from carbon layer get larger. Nevertheless, the attempt to release all of the iPP from the carbon layer has not succeeded in xylene at 100 °C. This suggests the existence of iPP chains being connected to the carbon layer due to a physical interaction. Second, according to the different fixing effects of the carbon layer on the iPP molecular chains in different regions, the observed morphologies can be addressed here. The iPP molecular chains tightly connected to the carbon layer via chemical bonding cannot relax during melting. Therefore, those E
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Macromolecules thin film exhibit always an oriented alignment with a-axis parallel to the drawing direction during film preparation (see all of the electron diffraction patterns superimposed of edge-on and flat-on crystals presented in this paper). The fixed mutual orientation of them with respect to the edge-on lamellae demonstrates that either the coated carbon layer or the fixed edge-on lamellae have a remarkable influence on the recrystallization of the flat-on iPP crystals. We consider first the carbon layer effect. One may consider that the vacuumevaporated carbon layer records the fine surface topology of the preoriented iPP thin film, which in turn induces the graphoepitaxy of iPP after melting-recrystallization. In this way, edge-on lamellae are expected. This is actually not the case, since flat-on iPP crystals are observed. Another possibility is that the carbon layer connects with the iPP chains in the amorphous regios, so that the iPP chains can simply obtain an upright orientation. If this is the case, considering the fiber orientation of the melt-drawn film, a fixed mutual orientation of the a-axes of flat-on crystals parallel to the original c-axis direction is also hard to be understood. Taking all those into account, a reasonable explanation may rest on the influence of the fixed edge-on lamellae on the later-growing flat-on crystals. Considering that the mother and daughter lamellae of the quadrites have common b-axes, the recrystallized iPP edge-on lamellae should have (010), i.e., the ac planes, parallel to the film surface. A close inspection of Figures 5a and 6c finds that many parallel-aligned edge-on lamellae grow into the flat-on ones. This may indicate that the flat-on crystals are initiated at the growth front of the edge-on lamellae, namely on the (100) lattice plane. This differs from wide-angle lamellar branching, which is initiated on the lateral (010) faces of the lamellae with a two-dimensional lattice matching between the c- and a-axes and vice versa.64 In this case, a one-dimensional lattice matching between the a-axis of the flat-on crystal and the c-axis of the edge-on crystal can be found. Therefore, we speculate the fixed mutual orientation between the edge-on and flat-on crystals to be the result of homoepitaxy based on a one-dimensional matching rather than the two-dimensional lattice matching fulfilled in lamellar branching. Further studies on the exact mechanism of this unique mutual orientation are still underway.
show more or less a regular lath shape. At the same time, quadrites caused by lamellar branching originating from homoepitaxy of α-iPP were seen. The formation of these different morphologies demonstrates that the fixing effect of the vacuum-evaporated carbon layer on the oriented iPP thin film is not homogeneously over the whole film. There exist strongly fixed regions where oriented edge-on lamellae were always obtained regardless of melting and crystallization temperatures. In the areas where the iPP molecular chains are weakly bonded to the carbon layer, detaching of the iPP molecules from the carbon layer and relaxation of them lead to the formation of flat-on iPP crystals. Moreover, if there are some nuclei with edge-on orientation in these areas, quadrites are then the observed morphology. It was further found that all of the three kinds of iPP crystals have a fixed mutual orientation. While a portion of the iPP lamellae in the quadrites possess the same molecular chain orientation as the parallel aligned edge-on lamellae, the flat-on iPP crystals arranged with their a-axes along the molecular chain direction of the parallel-aligned edge-on lamellae. This suggests that both the quadrites and the flat-on crystals of iPP are initiated by the parallel aligned edge-on lamellae fixed by the carbon layer.
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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.7b00299. A representative BF image and its corresponding electron diffraction pattern of a melt-drawn iPP oriented thin film; electron diffraction patterns of uniaxially oriented edgeon iPP lamellar crystals, cross-hatched edge-on lamellar iPP crystals, and flat-on iPP crystals; Raman spectra of oriented iPP thin films with and without carbon coating as well as the vacuum-evaporated amorphous carbon film; thermogravimetric analysis (TGA) of iPP; BF electron micrographs of epitaxial crystallization of polyethylene (PE) on the melt-recrystallized carboncoated iPP thin films; a BF electron micrograph and the electron diffraction patterns of the an iPP oriented thin film supported by a carbon film, which has been heattreated at 180 °C for 5 min and then crystallized isothermally at 120 °C; electron diffraction pattern of carbon-coated melt-drawn iPP thin films, which have been dissolved in xylene at 100 °C for 4 h (PDF)
5. CONCLUSIONS Temperature-dependent recrystallization morphologies of carbon-coated iPP highly oriented thin films were studied by electron microscopy combined with electron diffraction. It was found that the vacuum-evaporated carbon layer results always in an oriented melt-recrystallization of the preoriented iPP thin films, even though the result is three unique oriented morphologies, i.e., oriented edge-on lamellae, oriented flat-on lamellae, and oriented quadrites. The formation of different morphologies is dependent on both melting and crystallization temperature. At low melting temperatures, e.g., lower than 200 °C, parallel aligned lamellae edge-on orientation was always generated, regardless of the crystallization temperature. These edge-on lamellae exhibit the same molecular chain orientation as the prepared samples. When melting the carbon-coated iPP thin film at temperature of 300 °C, the recrystallizing morphology depends strongly on both the melting time and crystallization temperature. At low crystallization temperature, e.g., lower than 120 °C, a morphology composed of both edgeon and flat-on lamellae was observed. The content of the flat-on crystals increases with melting time at 300 °C. At high crystallization temperatures, e.g., 150 °C, the flat-on iPP crystals
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]; Tel 0086-10-64455928; Fax 008610-64455928 (S.Y.). ORCID
Huihui Li: 0000-0001-5745-4079 Jianming Zhang: 0000-0002-0252-4516 Shouke Yan: 0000-0003-1627-341X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Prof. Dr. J. M. Schultz (University of Delaware) for his help in revision of the English. The financial F
DOI: 10.1021/acs.macromol.7b00299 Macromolecules XXXX, XXX, XXX−XXX
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support of the National Natural Science Foundations of China (No. 21434002, 51521062 and 51573015) and the program of Introducing Talents of Discipline to Universities (B08003) is gratefully acknowledged.
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