Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE10 May 2022Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends Zhiyuan Wu, Chunyu Zhang, Youliang Zhu, Zhongyuan Lu, Heng Liu, Bin Xu, Xuequan Zhang and Wenjing Tian Zhiyuan Wu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Chunyu Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Rubber-Plastics, Ministry of Education, Shandong Provincial Key Laboratory of Rubber-Plastics, School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042 Google Scholar More articles by this author , Youliang Zhu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Zhongyuan Lu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Heng Liu Key Laboratory of Rubber-Plastics, Ministry of Education, Shandong Provincial Key Laboratory of Rubber-Plastics, School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042 Google Scholar More articles by this author , Bin Xu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Xuequan Zhang Key Laboratory of Rubber-Plastics, Ministry of Education, Shandong Provincial Key Laboratory of Rubber-Plastics, School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042 Google Scholar More articles by this author and Wenjing Tian *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202101726 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Identification and visualization of phase structures inside polymer blends are of critical importance in the understanding of their intrinsic structure and dynamics. However, the direct optical observation of the individual component phase in a dense bulk material poses a significant challenge. Herein, three-dimensional fluorescence imaging of phase separation and real-time visualization of phase transformation in immiscible polymer blends of polypropylene and polystyrene is realized through multiphoton laser scanning microscopy. Owing to the specific fluorescence behavior of the cyanostyrene derivative 2-(4-bromophenyl)-3-(4-(4-(diphenylamino)styryl)phenyl)fumaronitrile, the high-contrast imaging of the macrophase of the component polymer in two and three dimensions with a maximum depth of 140 μm and a high signal-to-noise ratio of 300 can be achieved. Detailed spectroscopic and structural studies reveal that the distinctive fluorescence features of each phase domain should originate from the formation of a completely different aggregate between probes and component polymer. Furthermore, visualizations of the internal morphology deformation and macrophase transformation were realized by employing a stretched dumbbell sample under constant tension. Download figure Download PowerPoint Introduction Polymer blends can provide excellent performance that can be achieved by combining various polymers with distinctive properties into a single material in many ways.1,2 These multi-component systems typically lead to a wide range of phase behaviors that directly influence the associated physical properties and play a crucial role in the final application.3 Thus, identifying the spatial structure of their individual phases is critical to understanding the relationships among the formation, structure, dynamics, and functionality of blend materials. Most polymer blends are immiscible, that is, each component phase usually separates into distinct, macroscopic domains during the mixing process, termed as macrophase-separated structures, which greatly affects the blend properties.4 For example, a co-continuous structure can lead to maximum contributions of stiffness, hardness, and impact properties of each component simultaneously, which can then be used in a variety of applications, such as electrically conductive blends and tissue scaffolds.5 A significant complication in blending is that the resulting structures are three-dimensional (3D) and have an interpenetrating anisotropic structure. Furthermore, the macrophase-separated domains and structures are usually formed and observed on the micrometer scale,6–9 which makes it difficult to clearly identify the intrinsic structures of phase behavior, especially for the structures inside the polymer matrix. Some analytical techniques,5,10,11 such as transmission electron microscopy,12 scanning electron microscopy (SEM),13 and atomic force microscopy,14 are used to characterize the surface topography, size, and distribution of the dispersed phase in polymer blends. Although these methods offer excellent spatial resolution, they are usually confined to the material surfaces or cross-sections that require an invasive sample preparation process.15,16 The lack of the third dimension leads to a misinterpretation of the true phase structure.17–19 More complicated morphologies will usually result in a less convincing interpretation of the surface images. Moreover, they do not directly provide intuitive insight into the morphologies and are unable to provide a representation of these internal complex structures.20 Therefore, the development of microscopes that are capable of 3D images for internal complex structures is anticipated. Recent advances in fluorescence-based techniques have emerged as a powerful tool to characterize morphologies and facilitate a 3D visualization of the exact morphology.21–30 Lopez-Barron and Macosko31 characterized the interface between the two phases of an immiscible polymer blend made of fluorescently-labeled polystyrene (PS) and styrene-ran-acrylonitrile copolymer by laser scanning confocal microscopy. Tang et al.32 observed the macrodispersion of montmorillonite fillers labeled with tetraphenylethene in a polymer matrix. However, visualization and precise localization of individual phase domains in a densely packed polymer mixture, especially for the internal phase structures, are still rare. Several factors limit the performance of current fluorescence microscopy to image macrophase structures of polymer blends, such as difficult or even impossible identification of different polymer species, an inability to distinguish the localization of phase-separated domains inside the sample, or unsatisfactory fluorescence imaging efficiency and contrast.33–38 These restrictions mainly involve the emission behavior of fluorescent probes in the polymer mixture, which ultimately influences the final imaging resolution, signal-to-noise ratio, and the visualization of an individual polymer within the bulk blends. Therefore, a direct 3D imaging technique for polymer blends without any invasive processes remains a significant challenge but would provide valuable information about both their intrinsic structures and dynamics. We have addressed the above challenge and developed a specific strategy of manipulating dye aggregates to identify polymer domains with or without phenyl moieties, where different aggregation pathways lead to two distinct emission properties of a specific label-free fluorescent probe, 2-(4-bromophenyl)-3-(4-(4-(diphenylamino)styryl)phenyl)fumaronitrile (TB). By exploiting TB in a way to selectively light up the target polymer, we have demonstrated ultrafast and non-invasive imaging of immiscible polymer blends composed of polypropylene (PP) and PS using a multiphoton fluorescence imaging technique (multiphoton laser scanning microscopy, MLSM). The distinct fluorescence of polymer domains with or without phenyl moieties allows us to visualize 3D structures of individual phases inside the polymer mixtures. This excellent depth resolution can clearly observe a more accurate ellipsoid radius and the transformation from a co-continuous phase to a sea-island phase under different annealing rates. Furthermore, we can also track the phase distribution in a stretchable dumbbell sample of PS/PP under constant tension in real time. The “invisible” information relative to the internal morphology deformation in the polymer specimens is transformed to visible fluorescent signals, which provides an essential insight into the relationship between the microstructure and mechanical properties of the polymer blends. Experimental Methods All-atom molecular dynamics simulation of the TB-doped PS films and TB-doped PP films An all-atom molecular dynamics (MD) simulation system consists of four TB molecules and 20 PS or PP chains with a length of 50 repeating units for each. Simulations were carried out using the Forcite module of the Materials Studio (Accelrys Inc., San Diego, CA, United States) with COMPASSII force field. The composite structure was equilibrated for 1 ns to fully mix the TB molecule with PS or PP chains by the simulation under the NPT ensemble at room temperature and 1 bar pressure. General preparation procedure of TB-doped polymer films Polymer and TB stock solutions were prepared by dissolving 1 g of polymer sample in 100 mL toluene and dissolving 0.001 g of TB in 2 mL toluene, respectively. Polymer blend solution with a PP mass fraction of 80% were prepared by mixing 0.4 mL of PP solution and 0.1 mL of PS solution. Afterward, 0.1 mL TB solution was mixed with 0.5 mL of as-prepared polymer solution under ultrasonication for about 2 h, generating a homogeneous solution with a polymer concentration of 10 mg/mL and 1.0 wt % content of TB. Uniform thin films of TB-doped polymers were fabricated by droplet coating the mixed solutions of TB and polymer blends onto quartz plates. The samples was kept at 180 °C for 10 min to volatilize toluene, and then the temperature was reduced to room temperature at rates of 1, 10, and 50 °C/min, respectively. General preparation procedure of TB-doped PS/PP bulk First, we mixed the TB (1.0 wt %) with PS and PP in a WLG10G twin-screw extruder (Shanghai Xinshuo Precision Machinery Co., Ltd., Shanghai, China) at 180 °C extruder temperature and 60 r/min screw speed. Tensile test specimens with dimensions of 75 × 10 × 2 mm made of TB/PS/PP were produced by injection molding, conducted by a WZS10D injection molding machine (Shanghai Xinshuo Precision Machinery Co., Ltd., Shanghai, China) at 180 °C injection temperature, 10 MPa packing pressure, and 5 s packing time. Results and Discussion Developing a specific fluorescence probe for polymer blends Our strategy to visualize and analyze the structures of immiscible polymer blends depends on the aggregation of the fluorescent probe, which differs from conventional methods that often need to label the polymer with a fluorescent probe through a complicated synthetic procedure. We synthesized a specific fluorescent probe TB ( Supporting Information Figures S1–S3) that can boost its emission when doping in polymers containing a benzene ring and can totally quench its emission in other polymers. Consequently, an appropriate binary polymer blend, containing benzene-based PS and its immiscible pair PP, is expected to serve as a potential imaging target by simply doping with the fluorescent probe TB (Figure 1a). The experimental samples can be prepared by using simple solution-processed methods, such as spin coating or droplet coating, in which 1 wt % TB is doped into the polymer. Figure 1 | Investigating photophysical properties of TB. (a) Luminous mechanism of the TB (1.0 wt %) doped in PS and PP and molecular structures of the polymers (PS and PP) and fluorescent probe (TB). (b) UV–vis absorption and (c) PL emission spectra (excitation wavelength: 365 nm) of the TB in THF solution, TB films, 1.0 wt % TB-doped PS films and 1.0 wt % TB-doped PP films. (d) Free energy of mixing TB with PS, PP using melting point depression measurements. Download figure Download PowerPoint The emission behaviors of TB in both PS and PP matrices were initially investigated by steady-state spectroscopy. As expected, the TB-doped PS films exhibited bright red fluorescence, whereas the TB-doped PP films and TB films exhibited almost no emission. The PL spectrum of TB-doped PS films showed a strong emission band peak at 614 nm, where the photoluminescence quantum yield (ΦF) was as high as 34%. In contrast, TB-doped PP films or pure TB films exhibited almost no emission and their PL spectra were a straight line along the bottom (Figure 1c). Furthermore, we found that TB films show an obviously red-shifted absorption band compared with that of TB in tetrahydrofuran (THF) solution, indicating that the existence of strong intermolecular interactions leads to aggregate formation in TB films, as shown in Figure 1b. Importantly, when doping PS and PP with TB, the absorption band of the TB-doped PS films showed a red-shift compared with the TB-doped PP films. These observations suggested that TB probably exhibits different intermolecular interactions with a PS or PP polymer chain, because the optical properties of the dye molecules strongly depend on the aggregate structure or packing arrangement. To gain further insight into the interactions between TB and polymer in a matrix, the compatibility of TB with the polymer was evaluated by the Flory–Huggins interaction parameter (χ), whose value can be derived from the melting point of TB in the presence of a polymer (TMmix). The melting point of the TB-doped PS or TB-doped PP mixture with different mass fractions was performed by differential scanning calorimetry. As shown in Supporting Information Figure S4, TB showed varying degrees of melting point depression in the PS and PP mixture. Thus, as shown in Figure 1d, we can obtain the values of the interaction parameter of TB with different polymers through Flory–Huggins lattice theory.39,40 In the mixture of TB and PS, the interaction parameter is −0.11, which indicates that they are miscible. The negative interaction parameter reflects a great thermodynamic driving force for mixing TB with PS, which suggests TB has good dispersion in the PS matrix and tends to form an isolated state. In contrast, the mixture of TB and PP results in a largely altered interaction parameter of 0.65. The highly positive value of the interaction parameter indicates that TB and PP are immiscible, and the large difference between the interaction parameters suggests that TB undergoes a different assembly process when mixed with PS and PP. It is worth noting that TB molecules possess a large dipole moment because of the asymmetric geometric and strong intramolecular charge transfer state, which usually leads to the formation of compact packing and strong intermolecular interactions. Indeed, strong π–π interactions and a closed packed structure are observed for the TB crystal ( Supporting Information Figure S5), which also exhibits no emission similar to its films ( Supporting Information Figure S6a). These findings indicate that TB would present a different aggregate structure in a PS and PP polymer matrix, resulting in distinct emission behavior. Additionally, to explore the fluorescence properties of TB-doped polymers with or without phenyl moieties, a variety of polymers with and without phenyl moieties were chosen, such as PS, poly-a-methyl styrene (PAMS), styrene butadiene styrene block copolymer (SBS), polyethersulfone (PES), PP, polyvinyl pyrrolidone (PVP), polyetherpolyol (PMPO/POP), and polycaprolactone (PCL). As shown in Supporting Information Figure S6, TB exhibited strong emission in PS, PAMS, SBS, and PES, but showed weak emission in PP, PVP, PMPO, and PCL. This suggests that TB can serve as a unique probe for the visualization of phase separation of biphasic blend polymers with and without phenyl moieties. We further investigated the excited-state dynamics of TB in a PS and PP polymer matrix. Time-resolved fluorescence spectra of TB shown in Supporting Information Figure S6b reveal a significantly increased fluorescence lifetime (τFL) of TB-doped PS films (5.01 ns) compared with that of TB-doped PP films (1.38 ns), as well as TB films (2.65 ns). Furthermore, a long fluorescence lifetime was also observed in TB-doped rigid polymers, as listed Supporting Information Table S1. Because the PL quantum yield ΦF equals the product of τFL and the radiative deactivation rate (kr), kr and the non-radiative deactivation rate (knr) can be approximately estimated ( Supporting Information Table S1). Compared with TB-doped PP films, there is one order of magnitude increase of kr in TB-doped PS films, while knr decreases to one-sixth. This shows that the fluorescence transition is favorable in TB-doped PS films but inhibited in TB-doped PP films. It is worth noting that TB has no emission in diluted THF solution but exhibits boosted emission at 77 K ( Supporting Information Figure S7). The frozen solution provides a rigid environment to fix the isolated TB molecules and restrict the intramolecular motions, which results in enhanced emissions compared with that in solution at room temperature. This is in good agreement with the strong emission observed when TB is doped into the PS matrix, where PS is analogous to a solid solution. However, TB films also show a low kr similar to the TB-doped PP film, which suggests that TB probably forms aggregates to cause emission quenching. Excited state dynamics of TB/PS and TB/PP polymer mixtures To explore the origin of the distinctive fluorescence behavior of TB in various aggregate states, we carried out ultrafast transient absorption (TA) spectroscopy to study the internal excited-state dynamics. Femtosecond-resolved TA spectra of TB are shown in Figures 2a, 2b, 2d, and 2e. After excitation, the characteristic excited-state absorption (ESA) bands at 500–750 nm were observed in THF solution within a few ps. Fast intramolecular non-radiative processes, including structural evolution accompanied by vibration and rotation, can dissipate the excited-state energy of TB on the sub-ps and ps scales. The significant changes in ESA within 2 ps is likely due to the configurational evolution after photoexcitation ( Supporting Information Figure S8). In sharp contrast, the characteristic stimulated emission (SE) bands at 580–610 nm overlapping with the ESA bands at 610–780 nm were observed in TB-doped PS films. Notably, a constant decrease at the SE band was observed, which is in good agreement with its PL spectrum at 625 nm (Figure 2a) and should result from the radiative S1–S0 decay. This spectral evidence indicates that a large number of excitons are deactivated through the radiative pathway, which is consistent with a high radiative deactivation rate. The all-atom MD simulations predict the dispersed state of TB in different polymer matrixes. As indicated in Figure 2c, TB was mono-dispersed in the PS matrix and tended to an isolated state owing to the good compatibility with PS. Consequently, the radiative transition of excited TB can be predominant when TB is mono-dispersed in a rigid PS matrix and intramolecular vibration and rotation are suppressed. Figure 2 | Excited state dynamics of TB in different states. TA spectra of (a) TB-doped PS films, (b) TB-doped PP films, TB in (c) THF and (d) TB films. Excitation: 400 nm, probe light: 450–780 nm. Snapshot of the all-atom MD simulations of (e) TB/PS films and (f) TB/PP films. Download figure Download PowerPoint Although the intramolecular vibration and rotation are greatly suppressed in the solid state, excimers are likely to form in TB films upon excitation, owing to the molecular stacking and intrinsic D–A structure of TB. A “dark” excited state can be formed within 1 ps, as evidenced by the gradually increasing ESA, which then undergoes a non-radiative pathway ( Supporting Information Figure S9). More importantly, an obvious change of the ESA in TB-doped PP films was also observed within 25 ps ( Supporting Information Figure S10), which lagged compared with that of TB in solution. This may originate from a slow configurational evolution after photoexcitation, where the intramolecular motions are not fully restricted by the PP matrix. Similar to TB films, the gradually increased and blue-shifted ESA indicated that the radiative transition of excited TB was inhibited, because a molecular aggregate was formed following the initial structural evolution. These findings imply that the “dark” excited state should originate from the formation of a unique aggregate of TB in the PP matrix, which agrees with the closed stacking of TB molecules from MD simulations (Figure 2f). 3D visualization of the phase separation of PS/PP films by MLSM Owing to the unique photophysical properties of TB doped in different polymer matrixes, we explored the potential application in directly imaging the morphologies of polymer blends by fluorescence microscopy. For this purpose, PS and PP without any fluorescent probe labels were chosen as binary immiscible polymer pairs, and TB served as the fluorescence probe with a low doping content of 1%. The blend films underwent a simple mixing process with different annealing treatments to obtain various phase-separated structures. As shown in Supporting Information Figure S11, a surface image of a phase-separated structure between the PS and PP phase domains was clearly observed by fluorescence microscopy, where the PS domains showed strong emission and the PP domains exhibited no fluorescence. This indicates that TB exhibits distinct fluorescence properties in different phase domains when doping in a PP/PS blend mixture. The results imply that the assembly processes of TB with PS and PP in the blend mixture are independent of each other, and the emission behavior is identical to that from doping with the single polymer. As shown in Supporting Information Figure S12, TB/PS films with different TB ratios had strong fluorescence emission while the fluorescence of TB/PP films was always weak. It means that TB can keep the same fluorescent characteristics when different mass fractions of TB are doped in polymers. Considering the significant difference of emission intensity in blends, we next sought to realize 3D visualization of the phase-separated structure inside blends by MLSM. It is worth noting that TB not only exhibits a unique emission behavior but also possesses highly nonlinear optical properties. Our previous study demonstrated that TB shows a large two-photon absorption cross-section in its aggregate state, which is beneficial for two-photon fluorescence (2PF) under two-photon excitation ( Supporting Information Figure S13). More importantly, the assembly process with PS enhances the radiative transition of TB, which results in a high photoluminescence quantum yield of a TB-doped PS mixture. These advantages of the unique features of TB are critical for the high imaging resolution and quality of TB-doped polymer blends. Figure 3 presents MLSM images of PS/PP binary polymer blends, where the blend film was prepared by simply mixing in toluene followed by droplet coating the mixed solutions of TB and polymers onto quartz plates. The samples were kept at 180 °C for 10 min to volatilize the toluene solvent, and then the temperature was cooled to room temperature at a rate of 1 °C/min. Femtosecond lasers with wavelengths of 800 nm were focused on the surface of the films through an objective lens (20×). The imaging time for each frame was approximately 4 s with 10-μm steps in the z direction. Interestingly, the image shows a distinguishable phase-separated structure of polymer blends, in which the red region represents the PS phase domain and the black region corresponds to the PP phase domain. The boundary between the two regions is well defined and was easy to identify one from the other. The strong red signal from the PS domain is attributed to the two-photon fluorescence of TB, but, in sharp contrast, no signal from the PP domain was detected owing to the aggregate-caused quenching. In addition to these two-dimensional images, we successfully obtained 3D reconstructed images via the z-scanning technique (Figures 3a and 3b). The images clearly show two distinct red and black regions in which the PS and PP domains are separated from each other, even in the z-axis direction. Eventually, the highest signal-noise ratio (SNR) of 220 and maximum imaging depth of 110 μm were achieved in the blends. Compared with fluorescence microscopy, there was a 100-times enhancement of the SNR by MLSM ( Supporting Information Figure S14), which helped to eliminate the interference of background fluorescence and achieve a high resolution. Owing to high-quality imaging of the blend, the isolated island structures with different radii of the PS domain were obtained from the scanning images of each section at every 10 μm. Notably, we were able to directly observe the true ellipsoidal shape of PS from the side view of the panoramic reconstructed fluorescence micrograph, where most of the PS ellipsoids with a maximum diameter of approximately 25 μm are arranged spatially separated from one another. Remarkably, we found that some of adjacent spherical PS domains have already merged into one observed in the depth of 40 µm, whereas they are completely separated in upper section of 10 and 20 µm, respectively (Figures 3c–3e). These observations are consistent with Ostwald ripening theory that states that components of the discontinuous phase can diffuse to form larger droplets through the continuous phase. In other words, the diffusion and merging process of the isolated PS phase probably occur through the continuous PP phase. Figure 3 | 3D visualization of the phase separation of TB/PS/PP films. (a) Top view of 3D reconstructed two-photo fluorescence imaging of TB/PS/PP films from a depth of 0–110 μm. (b) Side view of stacked imaging of TB/PS/PP films. The MSLM scans TB/PS/PP films every 10 μm in the range of 0–120 μm. PS region imaging at a depth of 10 μm (c), 20 μm (d), and 40 μm (e) Insert: partial enlargement of PS phase. Scale bar: 40 μm. Download figure Download PowerPoint Different from other methods,41–44 we demonstrate the successful imaging of binary immiscible polymer blends via non-covalent fluorescent probes by MLSM. We directly observed an integral and sharp 3D image of the complicated morphologies inside blends owing to excellent SNR and depth resolution, which is notoriously difficult to realize by conventional electronic and optical microscopy techniques. Visualization of the effect of annealing rate on the phase separation of PS/PP films by MLSM MLSM imaging allows us to directly evaluate the dynamic properties of phase separation in immiscible polymer blends. According to Oswald ripening theory, the phase separation of polymer blends is a thermodynamically-driven process, which suggests that the thermal annealing process will have a significant impact on the phase-separated structure. Figure 4 shows the images of TB-doped PS/PP blends with different annealing rates. The images were obtained through MLSM with 10-μm steps in the z direction ( Supporting Information Figures S15–S17). The well-defined co-continuous structures in blends were observed when the temperature dropped at a rate of 50 °C/min (Figure 4a and Supporting Information Video S1), where the PS and PP components showed 3D spatial interpenetrating and intertwining structures. With cooling rates down to 10 °C/min, we found that the discrete, large, nucleated islands or elongated islands