PVDF Blends

Article Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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Mechanism of Reactive Compatibilization of PLLA/PVDF Blends Investigated by Scanning Transmission Electron Microscopy with Energy-Dispersive X‑ray Spectrometry and Electron Energy Loss Spectroscopy Wenyong Dong,†,‡ Hideki Hakukawa,† Naohiro Yamahira,† Yongjin Li,‡ and Shin Horiuchi*,† †

ACS Appl. Polym. Mater. Downloaded from pubs.acs.org by UNIV PARIS-SUD on 04/04/19. For personal use only.

Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-0031, Japan ‡ College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, P. R. China S Supporting Information *

ABSTRACT: The localization of reactive compatibilizers at the interfaces in polymer blends of PLLA (poly(L-lactic acid)) and PVDF (poly(vinylidene fluoride)) was investigated by scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectrometry (EDX) and electron energy loss spectroscopy (EELS). Polyhedral oligomeric silsesquioxane (POSS) functionalized with epoxide groups and poly(methyl methacrylate) (PMMA) chains was applied as compatibilizer in the immiscible PLLA/PVDF (50/ 50 wt %) blend. The blends were successfully compatibilized by adding the functionalized POSS through the chemical reaction of epoxide group with PLLA and the dissolution of PMMA into PVDF phase. The localization behaviors of the POSS compatibilizers at the PLLA/PVDF interfaces, which were influenced by melt-blend conditions, were characterized by EDX elemental analysis. We also investigated the local chemical structures in the interfacial regions by energy-loss near edged fine structure in EELS. We proposed the mechanism of the reactive compatibilization of immiscible polymer blends by the STEM-EDX/EELS analysis of the PLLA/PVDF interfaces compatibilized with the POSS derivatives. KEYWORDS: polymer blend, compatibilization, interface, scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectrometry (EDX), electron energy loss spectroscopy (EELS), energy-loss near edged fine structure (ELNES)

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INTRODUCTION Development of novel polymeric materials by simple dry blending of more than two polymers with different properties has invoked much interest for its economy and efficiency advantage.1−3 However, simple multicomponent systems usually show poor properties due to the low mixing entropy and high interfacial tension. It has been recognized that reactive compatibilization is an effective route to achieve high mechanical properties of polymer blends. Small amount of compound is added to multicomponent polymer systems as a compatibilizer which is expected to chemically or physically interact with the polymer components and then to improve the interfacial strength and decrease the size of the dispersion phase.4,5 Thus, a series of molecular structures like block, graft, and comb polymers have been designed and applied as compatibilizers in immiscible polymer blends. The parameters of their molecular structures like block length in block polymers and number and length of side chains in graft and comb polymers are optimized to manipulate the compatibilizers to be enriched at the matrix/domain boundary.6−15 © XXXX American Chemical Society

However, it is difficult to identify the location of the compatibilizers in the polymer blends owing to its small amount, which is usually less than a few weight percent. Scanning transmission electron microscopy (STEM) has been recognized as a powerful tool for investigating local material structures, allowing us to perform high spatial resolution and high contrast imaging with angle-selective annular detectors for elastic scattering. Moreover, it allows us to perform the spectroscopic analysis in local areas in a specimen by energy-dispersive X-ray spectrometry (EDX) and electron energy loss spectroscopy (EELS) using small focused electron beam.16 Those two analytical techniques rely on the interaction between electron beam and specimen electrons.17 The incident beam excites an electron in an inner shell of the specimen, ejecting it while creating an electron hole where the electron was. An electron in higher-energy shell then fills the Received: January 22, 2019 Accepted: March 25, 2019 Published: March 25, 2019 A

DOI: 10.1021/acsapm.9b00061 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

from Hybrid Plastic (United States) with the product name of EP0409. The reactive POSS(epoxy)x-g-PMMAy, where x and y are the number of epoxide groups and grafted PMMA chains attached to POSS core, respectively, were obtained by ring-opening reaction of the epoxide groups in POSS(epoxy)8 with carboxyl-terminated PMMA (PMMA-COOH) as reported in detail in our previous paper.29 POSS(epoxy)8 (4 g, 0.003 mol), PMMA-COOH (45.6 g, 0.012 mol), and N,N′-dimethyl lauryl amine (0.077g, 3.6 × 10−4 mol) were dissolved into 250 mL of xylene and then heated at 140 °C for 20 h. Then, the reaction solution was first diluted by acetone and then precipitated into cold diethyl ether. The precipitate was dried in a vacuum oven at 40 °C for 24 h, and 48 g of POSS(epoxy)5-g-PMMA3 was obtained. The x and y were determined by Fourier-transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1H NMR). PMMA-COOH (Mn = 3800 g/mol, Mw/Mn = 2.74) was synthesized by telomerization of MMA with 4,4′-azobis(4-cyanovaleric acid) (ACVA) as initiator and thioglycolic acid (TGA) as chain transfer agent.30 POSS, PMMA, polyethylene terephthalate (PET), poly(1, 4-dimethyl-phenylene ether) (PPO), poly(ethyl methacrylate) (PEMA), poly(amide-66) (PA66), and polycarbonate (PC) were purchased from Sigma-Aldrich and used as received. Melt Mixing Process. PLLA, PVDF, and the POSS derivatives were dried in the vacuum oven overnight at 80 °C before use. A Haake batch mixer (Polylab QC, Thermo Fisher Scientific, Inc.) was used to mix the blends at 190 °C for 10 min with a rotation speed of 50 rpm. The PLLA/PVDF ratio was fixed at 50/50 by weight, and the POSS compatibilizer was added at 3 wt % based on the total weight amount of PLLA and PVDF. In this work, binary PLLA/PVDF (50/ 50) blend, ternary blends of PLLA/PVDF with POSS(epoxy)8, and PLLA/PVDF with POSS(epoxy)5-g-PMMA3 were prepared. All of the ingredients were fed into the chamber of the mixer at once (one-step mixing). The PLLA/PVDF/POSS(epoxy)5-g-PMMA3 ternary blend was also prepared by two-step mixing where PLLA and POSS(epoxy)5-g-PMMA3 were mixed for 5 min, and then PVDF was added into the premixed blend for another 5 min. At the end of mixing, the obtained blends in molten state were removed from the chamber and cooled to room temperature in atmosphere. Then, a certain amount of sample was compression-molded into film with a size of 100 × 100 × 0.5 mm by hot pressing at 10 MPa and 190 °C for about 5 min, followed cold pressing for about 5 min to allow the temperature of the film to decrease to room temperature. STEM Characterization. STEM was performed with a TECNAI Osiris instrument (FEI Company, Hillsboro, OR, United States) equipped with four windowless silicon drift EDX detectors (FEI Super X) and an EELS spectrometer (Enfinium SE model 976, Gatan Inc., CA, United States). All works were performed at an accelerating voltage of 200 kV, and the measurements were carried out at 110 K to reduce the radiation damage of specimens using a cryo-specimen holder (MTC-002, Mel Build, Corp., Fukuoka, Japan). The EDX detectors were placed symmetrically around the optical axis near the specimen area, which were able to enhance the EDX detection sensitivity significantly and enabled us to detect small amounts of lowZ elements in the polymer materials rapidly.18,20,31−33 EDX elemental mapping was performed using an electron probe of 0.83 nm diameter and 0.6 nA current with the dwell time of 30 μs/pixel. 300 × 300 pixel maps were obtained by integration of the signals for 20 min scanning. The maps and the extracted EDS spectra were quantified by CliffLorimer method.34 The removal of continuum background and deconvolution of the EDS spectra were carried out using ESPRI software (Bruker AS, Germany). The dispersity of the blend samples was quantitatively evaluated by digital image analysis of STEM images using ImageJ 1.51 software. STEM-EELS was performed in spectrum-imaging (SI) mode in STEM. Spectra were acquired in a spatially serial manner: Energy loss spectra were recorded for each position of a small probe scanned over two-dimensional (2D) regions in a specimen. Then, we obtained a three-dimensional (3D) data cube regarding local chemical information consisting 2D spatial distribution (x and y) and energy losses levels (E) of the probe.35 The spectra were acquired using a Dual EELS system, which allowed the recording of both the low loss

hole, and the difference in energy between the two states is released in the form of an X-ray. As the energies of the X-ray are characteristic of the atomic structures of the emitting element, EDX allows the elemental composition of specimen to be measured.18−21 The incident electrons, at the same time, lose the energy due to the excitation of the specimen electrons during the travel in a specimen, which produces inelastically scattered electrons. EELS is intensity profile of inelastically scattered electrons as a function of loss of energy.22 Especially, an inner-shell ionization edge (core loss) in EELS contains information on chemical structures concerned with covalent bonds and valence state of atoms, which is called energy-loss near edged fine structure (ELNES).23 In STEM, high energy electron probe of which size is less than 1 nm is focused onto a specimen, and thus, the analytical area can be reduced as small as the probe diameter. But, at the same time, the damage of a specimen is a serious issue for polymer samples, which limits the high-resolution chemical analysis by EELS. Therefore, most of the works associated with polymer structures by EELS have been done with energyfiltering transmission electron microscopy (EFTEM).24−27 EFTEM allows us to perform EELS in TEM mode where electron beam is irradiated on wide area of a specimen. The energy density of electron probe in TEM mode is much lower than that in STEM mode. It reduces radiation damage of a specimen, but the spatial resolution of EELS analysis is limited. We in this work attempt the STEM-EDX/EELS analysis for investigating the localization of a compatibilizer at the interface in melt mixing process of immiscible polymer blend. We have reported that POSS functionalized with both epoxide groups and with PMMA long chains works as an excellent compatibilizer for immiscible PLLA/PVDF blends. This POSS compatibilizer was designed under the concept that the epoxide groups were able to react with the carboxyl end groups of PLLA and the grafted PMMA chains were thermodynamically miscible with PVDF phase.28 The wellcompatibilized blends of PLLA/PVDF (50/50 wt %) exhibited remarkable enhancement in the elongation at break as high as 375% without a decrease of either the tensile modulus or strength, which were approximately 100 times higher than those in the uncompatibilized PLLA/PVDF binary blend.29 The optimized properties were obtained by two-step mixing, where the POSS compatibilizer was premixed with PLLA, followed by mixing with PVDF, and the obtained blend exhibited much finer dispersed phase and longer elongation at break compared to the one-step mixing where all components were blended simultaneously. This indicates that the location of the compatibilizer is significantly affected by the mixing process. The PLLA/PVDF/POSS−compatibilizer multicomponent polymer system is a convenient system for STEM analysis because each component contains key elements for locating their local distributions, i.e., fluorine for PVDF, oxygen for PLLA, silicon for POSS, and also oxygens with different chemical bonds in PLLA and the POSS derivatives. Throughout this work, we propose the compatibilization mechanism of reactive polymer blends in melt mixing process.

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EXPERIMENTAL SECTION

Materials. PLLA was provided by Natureworks (United States) with the product name 3001D, and PVDF was provided by Kureha Chemistry (Japan) with the product name of KF850. Epoxyfunctionalized POSS (POSS(epoxy)8), where the epoxide groups attached at the eight corners of the silsesquioxane cage, was obtained B


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Figure 1. STEM-BF images of PLLA/PVDF binary blend (a), PLLA/PVDF/POSS(epoxy)8 blend (b), one-step PLLA/PVDF/POSS(epoxy)5-gPMMA3 ternary blend (c), and two-step PLLA/PVDF/POSS(epoxy)5-g-PMMA3 ternary blend (d). and core loss spectra simultaneously at the same position before moving on to the next.36−38 To deal with the large differences in signal intensity in the two regions, the low loss region was recorded with a short dwell time in a few microseconds, and the core loss regions were recorded with a longer dwell time with several seconds without the change of beam condition. The energy loss positions of the core loss spectra could accurately be corrected by correcting the drift of the zero-loss peaks (ZLPs), and the effects of plural scattering in the core loss region could be removed by Fourier logarithmic deconvolution using the shapes of the low loss spectra. The blend films were first embedded in a light curable resin, LCRD800 (Toagosei Corp., Japan), and then were microtomed by a Reichert-Jung Ultracut E microtome with a diamond knife to obtain sections with a thickness of about 50 nm. The sections were collected on copper meshes covered with lacy carbon.

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RESULTS AND DISCUSSION STEM-EDX Analysis for Imaging the POSS Compatiblizers’ Distribution in the PLLA/PVDF Blends. Figure 1 shows phase-separated morphologies of the PLLA/PVDF (50/ 50) blends in low magnification STEM bright-field (STEMBF) images. The PLLA/PVDF binary blend (Figure 1a) and the ternary blend of PLLA/PVDF/POSS(epoxy)8 (Figure 1b) show poor dispersion where irregular large domains are distributed, while the PLLA/PVDF blends with POSS(epoxy)5-g-PMMA3 prepared by one step (Figure 1c) and two steps (Figure 1d) show remarkable improvement of the dispersion. Quantitative digital image analysis revealed that the average diameters and the standard deviations of the dispersed domains in one-step and two-step blends were 3.0 ± 0.13 and 2.2 ± 0.05 μm, respectively, as shown in Figure S1. The average diameters of those two blends indicate no significant differences, but the two-step blend apparently shows uniform and fine dispersion of the domains, as is represented by the standard deviations. In all the blends, we can observe the PLLA/PVDF phase separation clearly without heavy metal staining where the dark domains are dispersed in the bright matrix. Figure 2 displays the EDX spectra extracted from 300 × 300 nm regions in the domain and the continuous matrix of the binary blend (Figure 1a), which allows us to easily assign the dark dispersed domains to PVDF phase due to the presence of the fluorine (F)-Kα peak at 0.68 keV. The EDX spectra indicate that the signal count obtained from the PLLA phase is much lower than that from the PVDF phase. The intensity of the carbon (C)-Kα peak at 0.27 keV in the PLLA spectrum is only 23% of that in the PVDF spectrum. Moreover, even though the PLLA contains large number of oxygen (O) atoms (22% in atomic ratio), the peak corresponding to the O-Kα is quite small, which is close to the background signals of the PVDF spectra. This strongly indicates that PLLA is degraded by electron beam much faster than PDVF, where the chemical bonds of PLLA are cleaved by either backbone main-chain scission or

Figure 2. EDS spectra extracted from the dark domain (PVDF phase) and the bright matrix (PLLA phase) in the PLLA/PVDF binary blend (Figure 1a). The inset shows the magnified spectra, including the oxygen Kα peak obtained from PLLA region.

hydrogen abstraction, and low-molar compounds are thus produced and evaporated away from the specimen. This loss of mass in PLLA results in good contrast in the STEM-BF images. To investigate the distribution of the POSS compatibilizers in the blends, STEM-EDX analysis was performed. Figure 3 displays STEM-HAADF images, where the bright phase is corresponded to the PVDF phase, and the elemental maps of F, Si, and O in the PLLA/PVDF interfacial regions in the three POSS-containing blends are presented. The blend with POSS(epoxy)8, where all the corners of POSS cage are functionalized only with epoxide groups, indicates that a Si-rich aggregate is localized at the PLLA/PVDF boundary (Figures 3a−d). The distributions of the POSS-compatibilizer are concentrated at the PLLA/PVDF boundaries when some of the epoxide groups are replaced with PMMA chains in POSS(epoxy)8, as shown in Figures 3e−l. In the one-step blend, the compatibilizer tends to be distributed along the PLLA/PVDF boundary region, but it seems that the small aggregates are unevenly distributed along the interfacial region. On the other hand, the two-step blend shows more uniform distribution of the enriched compatibilizer along the PLLA/ PVDF boundary (Figure 3k). It is noticed that the POSS compatibilizer is also distributed in the PVDF domains in the one-step blend (Figure 3g), while it seems to be distributed evenly both in the PVDF and the PLLA phases in the two-step blend (Figure 3k). The thicknesses of the interfacial layers of the compatibilizer are estimated by the line profiles perpendicular to the interface in the Si maps, as shown in Figure S3. The thickness of the one-step blend is about 30 nm, C


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Figure 3. STEM-HAADF images and STEM-EDX elemental maps of (a−d) PLLA/PVDF/POSS(epoxy)8, (e−h) one-step PLLA/PVDF/ POSS(epoxy)5-g-PMMA3, and (i−l) two-step PLLA/PVDF/POSS(epoxy)5-g-PMMA3 ternary blends. Panels a, e, and i are STEM-HAADF images, and green, red, and blue maps represent the elemental distributions of F, Si, and O, respectively. Regions assigned by serial numbers in panels a, e, and i are for the acquisition of EDX spectra.

Figure 4. Si-Kα peaks appeared in the quantified EDX spectra extracted from the regions indicated in the STEM-HAADF images of Figure 3. (a) PLLA/PVDF/POSS(epoxy)8, (b) one-step blend of PLLA/PVDF/POSS(epoxy)5-g-PMMA3, and (c) two-step blend of PLLA/PVDF/ POSS(epoxy)5-g-PMMA3.

indicated that small amounts of POSS(epoxy)8 is distributed in the two polymers. The one-step and two-step blends with the PMMA-grafted POSS compatibilizer show the different compatibilizer distributions as indicated from the Si-Kα peak intensities shown in Figures 4b and c. The Si-Kα peaks detected from the three regions in the one-step blend, where region 1 is interfacial region, region 2 is the PVDF phase, and region 3 is the PLLA phase, exhibit different intensities of peaks. That is, the interfacial region gives the highest intensity, the PVDF the second, and the PLLA the lowest intensity. In the two-step blend, on the other hand, the intensity of the SiKα peak obtained from the interfacial region (region 1 in Figure 3i) is the highest, while the other two regions give almost equal lower intensities. The EDX elemental mapping and spectroscopy indicate that the PMMA-grafted POSS compatibilizer can be localized effectively at the interfacial regions in the one-step and two-step blends, but the blend sequence influences the distribution of the compatibilizer significantly.

while it is about 100 nm in the two-step blend. The elemental maps are the projection of the cross sections of spherical domains. Thus, the estimated interfacial thicknesses are larger than the actual values. But, the elemental line profiles indicate the difference in the interfacial structures of the POSS aggregates formed at the PDVF/PLLA boundary in one-step and two-step blends. The oxygen maps shown here represent mainly the distribution of the POSS compatibilizers, not that of PLLA. Owing to the degradation of PLLA, the contribution from PLLA to the O elemental maps is small as compared to that from POSS. Si-Kα peaks that appeared in the quantified EDX spectra extracted from the selected regions indicated in Figure 3 are presented in Figure 4, and the overall EDX spectra are shown in Figure S2. Figure 4a shows the spectra obtained from the four regions in Figure 3a in the blend of PLLA/PVDF/ POSS(epoxy)8. As displayed in the elemental distribution image (Figure 3c), the Si-rich aggregate in the interfacial region (region 2) gives significantly high Si-Kα peak. It is also D


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ACS Applied Polymer Materials STEM-EELS Analysis for Locating the POSS Compatibilizers in the PLLA/PVDF Blends. As described above, the localization of the POSS compatibilizers at the PLLA/ PVDF boundary region can be visualized and quantitatively analyzed by STEM-EDX Si mapping and spectrometry. For performing detail chemical analysis of the phase-separated structures, STEM-EELS was performed in SI mode. In the EDX analysis, the radiation damage of PLLA made it difficult to detect oxygen with high signal intensity. First of all, an attempt to find the optimum condition to acquire the oxygen ionization K-edge (O K-edge) of PLLA with high signal-tonoise (S/N) ratio was made. Figure S4 shows as-obtained EELS O K-edges of PLLA with different probe conditions. The spectra were acquired in SI mode from 500 × 500 nm area with 50 nm point-to-point distance and with the acquisition time for one spectrum of 3 s. Therefore, the exhibited spectra are the average of 100 spectra. It shows that the reduction of the beam size to 0.74 nm with 0.293 nA beam current gives the highest jumping ratio of the O K-edge, which is the ratio of the intensities at the edge (I1) and at the top of the peak (I2). Further reduction of the probe size to 0.66 nm results in a noisy spectrum with no core loss signals. Figures 5a and b are

Figure 6. Oxygen ELNES features of (a) PLLA, (b) PMMA, (c) PET, and (d) PPO with different electron probe conditions. Probe size, current, and acquisition time are shown in (a). From lower to upper spectrum, beam size and current are increased. Intensities of the spectra are normalized with the peak at 533 eV.

the O K-edges of (a) PLLA, (b) PMMA, (c) PET, and (d) PPO acquired with different electron probe conditions. Those are the average of the spectra acquired from 500 × 500 nm regions with 50 nm point-to-point distance and with 1 s acquisition time. It was revealed that the second peak at 537 eV is significantly sensitive to the beam condition in PLLA, PMMA, and PET. The peak becomes gradually weaker and is finally eliminated with increase of the beam size and current. Especially, the second peak intensity of PLLA decreases much faster than the others, representing poor stability of PLLA against the electron beam. The second peak in PET remains at the high level even after irradiation with the large probe. The O K-edges of PC, PA66, and PEMA are also shown in Figure S5. Those polymers also show the same trend in terms of the changes in the second peak with the probe condition. It is indicated that PEMA exhibits fast degradation in a similar manner of PLLA, while PC and PA66 show relatively high stability similar to that of PET. The investigated polymers except for PPO contain ester bond, while PPO contains only the ether bond. PPO, as shown in Figure 6d, shows a tiny peak at 537 eV even with the smallest probe, suggesting that the beam-sensitive peak at 537 eV is mainly contributed from the oxygen in the carbonyl group in the ester bond. Chain scissions of the polymers by electron beam occur easily in the carbonyl group in the ester bond, which may produce gaseous products like carbon oxide evaporated away from the specimen.

Figure 5. STEM-EELS O K-edge spectra obtained from (a) PLLA phase in the PLLA/PVDF binary blend, (b) PLLA phase in the twostep PLLA/PVDF/POSS(epoxy)5-g-PMMA3 ternary blend, and (c) POSS. Background signals were removed by fitting the low energy loss part below the edges.

O K-edges obtained from the PLLA phase in the PLLA/PVDF binary blend and in the two-step PLLA/PVDF/POSS(epoxy)5-g-PMMA3 ternary blend, respectively. The O Kedge of the binary blend exhibits two peaks at 533 and 543 eV, while that of the ternary blend possesses an additional peak at 537 eV. Figure 5c is the O K-edge obtained from POSS powder sample, showing the shift of the edge to higher energy loss position and different edge shape features from those obtained in the two blend samples, where a sharp peak at 540 eV is followed by a broad peak at 560 eV. The O K-edges obtained from the two blend samples (Figures 5a and b) are contributed by the ester bonds of PLLA, but they give different ELNES features from each other. To consider the reason for such differences, we investigated the influence of the beam irradiation on the ELNES of the O Kedges of various oxygen containing polymers. Figure 6 presents E


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g-PMMA3 binary blend. This blend is obtained after the initial mixing step in the two-step PLLA/PVDF/POSS(epoxy)5-gPMMA3 blend. As reported in the previous paper,29 the epoxide groups in POSS(epoxy)5-g-PMMA3 react with PLLA during the melt-mixing process, and POSS(epoxy)5-g-PMMA3 forms small domains in the PLLA matrix with the size of about 100 nm. The domains form core−shell structure with the POSS shell surrounding the PMMA cores. Those two spectra show clearly different features associated with the intensities of the two peaks at 533 and 537 eV. The PLLA phase gives no beam-sensitive peak at 537 eV due to the absence of the POSS, while the PMMA domain gives strong peak at 537 eV, suggesting the inclusion of the POSS in the PMMA domain. The peak at 537 eV obtained from the PMMA domain is much stronger than that obtained in the pure PMMA shown in Figure 6b. This means that the presence of POSS in the PMMA domain significantly improves the stability against the electron probe. Thus, we can distinguish PLLA and PMMA phases by ELNES of O K-edges. Effect of the Mixing Process on the Localization of the Compatibilizer in the PLLA/PVDF Blends. We successfully acquired the O K-edges of the beam-sensitive PLLA with sufficient intensities by reducing the probe size to the minimum at which the edge can be detected. To understand the effect of the mixing processes on the compatibilization of the PLLA/PVDF blend with POSS(epoxy)5-g-PMMA3, the distribution of the compatibilizer was investigated by characterizing the variation of the O and the F core loss edges in EELS spectra across the PLLA/PVDF interfaces. Figure 8 shows the variation of the EELS spectra in the energy loss region, including the O K- and F K-edges, when the investigated regions are shifted from the PLLA phase (region 1) to the PVDF phase (region 4). EELS spectra were acquired over the 700 × 700 nm regions including the PLLA/ PVDF interfaces by the SI mode with the probe size of 0.74 nm and the acquisition time of 1 s for the one-step (Figure 8a) and two-step (Figure 8b) blends. In both blends, the PLLA phase (region 1) gives only the O K-edge, and the F K-edge at 685 eV appears when the region is shifted toward the PVDF phase. The interfacial regions (regions 2 and 3) give both O and F K-edges in the spectra, indicating that the POSS compatibilizer, PLLA, and PVDF coexist in the regions. The region which locates deeply inside the PVDF phase (region 4) in the one-step blend gives O K-edge with relatively high intensity, while that in the two-step blend gives extremely low intensity edge. The background signals, which are produced mainly due to the contribution of the plural scattering, are calculated by fitting the lower energy part below the O K-edge according to the power-law function23−27 and then by extrapolating them to the entire part of the spectra, as shown in the red lines. The corresponding backgroundsubtracted spectra are shown together with the as-obtained spectra. One can recognize that the beam-sensitive peak at 537 eV remains in the O K-edges obtained in the PLLA phase (regions 1−3) in the two-step blend, which does not exist in the one-step blend. This means that the POSS is not distributed in the PLLA phase in the one-step blend. O and F elemental distribution images were created using the corresponding core loss edges. For those two elements, the intensities of the background-subtracted spectra in the energy windows of 540−550 and 690−700 eV were mapped for representing O and F distributions, respectively. Figure 9 shows the maps of those two elements of (a) one-step and (b)

Considering those results, it is interpreted that the different ELNES features obtained in the PLLA phases shown in Figures 5a and b are caused by the difference in the degradation behavior by the electron probe. In the ternary blend of PLLA/ PVDF/POSS(epoxy)5-g-PMMA3, small amount of POSS is dissolved in the PLLA phase as indicated in Figures 3 and 4, which can retard the cleavage of the ester bonds, and thus, the beam-sensitive peak remains in the spectrum. The effect of the POSS on degradation behavior of PLLA can also be detected in the EDX results shown in Figure 4. The ratio of the C-Kα peaks of PLLA against PVDF in the two-step blend is 29%, while it is 23% in the binary blend. The ELNES features in the O K-edges can be obtained in the localized areas of the blend samples as shown in Figure 7.

Figure 7. O K-edges ELNES features obtained from (a) PLLA/ PVDF/POSS(epoxy)8 and (b) PLLA/POSS(epoxy)5-g-PMMA3 blends. The areas displayed in the green rectangles were investigated, and the spectra were extracted from the red and blue regions by SI mode with 35 nm point-to-point distance. The acquisition times for one spectrum are (a) 1 s and (b) 2 s.

Figure 7a shows the O K-edges obtained from the PLLA region and the POSS(epoxy)8 aggregate at the PLLA/PVDF boundary in the blend of PLLA/PVDF/POSS(epoxy)8. EELS spectra were acquired by SI mode in the 500 × 500 nm area, as indicated in Figure 7a, with point-to-point distance of 35 nm, and then the spectra from the two regions, one of which is the PLLA phase and the other of which is the POSS aggregate, were extracted. Those two spectra show different ELNES features. That is, the spectra extracted from the PLLA phase show the beam-sensitive peak at 537 eV, while those from the aggregate show the characteristic broad peak at 560 eV, indicating the presence of POSS. The results indicate that small amount of the POSS compatibilizer is dissolved in the PLLA. The spectrum obtained from the aggregate (red spectrum) suggests that the POSS aggregate includes PLLA due to the reaction between PLLA and epoxide groups. Figure 7b shows the spectra obtained from the PLLA/POSS(epoxy)5F


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Figure 8. STEM-EELS analyses of the interfacial regions of PLLA/PVDF in (a) one-step and (b) two-step blends. 700 × 700 nm regions, indicated in the images, were investigated by SI mode with 35 × 35 nm point-to-point distance and with 0.74 nm probe. The spectra extracted from the regions 1−4 are shown below the corresponding images. In each spectrum, the background-subtracted spectra (green) are shown calculated by the subtraction of background (red) from the as-obtained spectra (blue).

(region 4 in Figure 8a) is not enough to discuss the ELNES features. But, the EDX result (Figure 3g) suggests that the O K-edge obtained from the PVDF domain originated from PMMA and POSS in the compatibilizer. This means that POSS(epoxy)5-g-PMMA3 compatibilizer prefers to be distributed into the PVDF phase when all the components are fed into the mixing chamber at the same time. In the two-step

two-step blends together with the intensity profiles along the lines indicated in the maps. The elemental line profiles clearly indicate the difference in the concentration of oxygen in the PDVF phase. The oxygen concentration was kept at high level in the PVDF phase in the one-step blend, while it was almost zero in the PVDF phase in the two-step blend. Unfortunately, the resolution of the spectra obtained from the PVDF domain G


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Figure 9. F and O elemental maps and intensity profiles along the horizontal lines across the interfaces of (a) one-step and (b) two-step blends created by STEM-EELS. The background contributions below the O and F K-edges were calculated using the energy loss range from 480 to 530 and 600 to 680 eV, respectively, by the power-law function.

blend, at the stage of the first step where POSS(epoxy)5-gPMMA3 and PLLA are mixed preliminary, the compatibilizer reacts with PLLA and forms small domains in the PLLA phase.29 At the second stage when PVDF is fed into the premixed blend, some of the PLLA-grafted compatibilizers move toward the PVDF phase but are trapped at the interface due to the graft of PLLA onto the compatibilizer. Mechanism of Reactive Compatibilization in the PLLA/PVDF/POSS(epoxy)5-g-PMMA3 Blends. The Si elemental maps shown in Figure 3 indicate the selective localizations of the POSS(epoxy)5-g-PMMA3 compatibilizer at the interfaces of PLLA/PVDF in the two blends that were prepared by the different mixing sequences. Moreover, Figure S3 shows the differences in the state of the localization of the compatibilizer in these two blends. As mentioned before, the interfacial layer formed by the compatibilizer in the two-step blend seems to be thicker and more uniform than that of the one-step blend. Furthermore, the Si and F elemental concentration profiles shown in Figure S3 suggest the difference in the localization of the compatibilizer between the two blends. That is, the compatibilizer resides in the PVDF phase in the one-step blend, while it almost exists in the PLLA

side in the interfacial region. The oxygen analysis by STEMEDX is limited due to the degradation of PLLA and PMMA by STEM probe because sufficient signal gains in EDX spectrum require the scanning for long integration time with relatively high-current large probe. STEM-EELS analysis with lowcurrent small probe allowed us to detect the oxygen of PLLA and/or PMMA, as shown in Figure 8, which indicates that the compatibilizer is distributed in the PVDF phase in the one-step blend. Considering those results, the mechanisms of the reactive compatibilization in the two blends with different blending strategies are depicted in Figure 10. In the one-step blend (Figure 10a), the compatibilizer prefers to be distributed in the PVDF phase due to the thermodynamic miscibility of PVDF and PMMA. During the melt blending, there is little chance for the compatibilizers existing near the PLLA/PVDF boundary to react with PLLA, which is able to enrich the compatibilizers at the interface. So, most of the compatibilizers are distributed in the PVDF phase and thus do not contribute to the improvement of the PVDF phase dispersion in PLLA matrix and of the interfacial strength in this system. Some of the reacted compatibilizers could be pulled off from the interface H


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process affects the localized structure of the compatibilizer significantly, which suggests that the localization of the reactive compatibilizer is determined by the competition between the dissolution of the PMMA chains into the PVDF and the chemical reaction of the epoxide groups with PLLA during the melt blending. In our case, the former occurs faster than the latter. The results suggest the high miscibility between PMMA and PVDF. The PMMA and PVDF system is known as one of the rare systems that is miscible in all the compositions and temperatures. This system was investigated by Wu et al., in which the interfacial layer formed via interdiffusion in a layered sample was about a few micrometers thick at 190 °C for 161 min.28 This interfacial thickness is extremely thick as compared with thicknesses of other miscible pairs such as PMMA/ poly(styrene-ran-acrylonitrile)25 and poly(vinyl chloride)/ poly(caprolactone).40 Those two pairs show interdiffusion with a few hundreds of nanometers in similar conditions. Therefore, PMMA/PVDF combination has strong thermodynamic affinity even among other miscible polymer pairs. We are now investigating the interdiffusion behavior between PMMA and PVDF by the same techniques introduced in this work. It is important for compatibilizers to be localized effectively at the domain/matrix interfaces, and for achieving such interfacial structures, the balance between miscibility and reactivity of compatibilizers with the polymers to be blended is well-considered.

Figure 10. Illustration representing the mechanism of the reactive compatibilization of PLLA/PVDF blends with POSS(epoxy)3-gPMMA5: (a) one-step blend and (b) two-step blend.

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and occluded in the PVDF phase due to the strong affinity of PMMA and PVDF. The two-step blend (Figure 10b), on the other hand, where PLLA and the compatibilizer are preliminarily mixed, has a large amount of the compatibilizers remaining in the PLLA phase. In the second step, in which the premixed PLLA/compatibilizer is further mixed with PVDF, the compatibilizer migrates toward the PVDF phases due to the thermodynamic affinity of the PMMA chains attached to POSS with the PVDF phase. However, the reaction of PLLA with the compatibilizer prohibits the dissolution of the compatibilizer into the PVDF phase thoroughly. Therefore, the compatibilizer can form a thick and uniform layer surrounding the PVDF domains. Some compatibilizers which do not react with PLLA during the first step could be distributed in PVDF phase in the second step. The interfacial layer formed with the reactive compatibilizer contributes to the inhibition of coalescence of the PVDF domains during the melt mixing, resulting in the improvement of the dispersion and interfacial strength.39

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.9b00061.

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Statistical image analyses, overall EDX spectra, STEMEDX composite maps with line profiles, EELS spectra of O K-edges with different probe conditions, and O Kedge ELNES features of polymers (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yongjin Li: 0000-0001-6666-1336 Shin Horiuchi: 0000-0001-8256-0498

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Notes

The authors declare no competing financial interest.

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CONCLUSION We performed STEM-EDX/EELS analysis for locating the reactive compatibilizers of POSS functionalized with epoxide groups and PMMA chains in PLLA/PVDF (50/50 wt %) blends. STEM with high-energy electron probe has caused difficulty in the analysis of polymer samples. Optimization of the probe condition and use of highly sensitive detection systems for EDX and EELS enabled us to perform detailed elemental and chemical analyses of the polymer blend samples by STEM. It is the first time to show the possibility of the chemical analysis of oxygen containing polymers by ELNES features of O K-edges in EELS. The POSS functionalized only with epoxide is distributed along the PLLA/PVDF interfacial region as inhomogeneous aggregates. The replacement of some epoxide groups with PMMA chains improves the dispersity and mechanical properties of the blend through localization of the reactive compatibilizer at the interface. The melt mixing

ACKNOWLEDGMENTS This work was funded by the New Energy and Industrial Technology Development Organization of Japan (NEDO) Grant (P16010).

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PVDF Blends

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