polymers Article
Structure of Composite Based on Polyheteroarylene Matrix and ZrO2 Nanostars Investigated by Quantitative Nanomechanical Mapping Maria P. Sokolova 1,2, *, Michael A. Smirnov 1,3 , Alexander N. Bugrov 3,4 , Pavel Geydt 2 , Elena N. Popova 3 , Erkki Lahderanta 2 , Valentin M. Svetlichnyi 3 and Alexander M. Toikka 1 1
2 3
4
*
Department of Chemical Thermodynamics & Kinetics, Saint Petersburg State University, Universitetsky pr. 26, Peterhof, Saint Petersburg 198504, Russia;
[email protected] (M.A.S.);
[email protected] (A.M.T.) Laboratory of Physics, Lappeenranta University of Technology, Skinnarilankatu 34, 53850 Lappeenranta, Finland;
[email protected] (P.G.);
[email protected] (E.L.) Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoy pr. 31, Saint Petersburg 199004, Russia;
[email protected] (A.N.B.);
[email protected] (E.N.P.);
[email protected] (V.M.S.) Department of Physical Chemistry, Saint Petersburg Electrotechnical University “LETI”, ul. Professora Popova 5, St. Petersburg 197376, Russian Correspondence:
[email protected]; Tel.: +7-812-328-68-76
Academic Editors: Francesco Paolo La Mantia and Maria Chiara Mistretta Received: 23 May 2017; Accepted: 2 July 2017; Published: 6 July 2017
Abstract: It is known that structure of the interface between inorganic nanoparticles and polymers significantly influences properties of a polymer–inorganic composite. At the same time, amount of experimental researches on the structure and properties of material near the inorganic-polymer interface is low. In this work, we report for the first time the investigation of nanomechanical properties and maps of adhesion of material near the inorganic-polymer interface for the polyheteroarylene nanocomposites based on semi-crystalline poly[4,40 -bis (4”-aminophenoxy)diphenyl]imide 1,3-bis (30 ,4-dicarboxyphenoxy) benzene, modified by ZrO2 nanostars. Experiments were conducted using quantitative nanomechanical mapping (QNM) mode of atomic force microscopy (AFM) at the surface areas where holes were formed after falling out of inorganic particles. It was found that adhesion of AFM cantilever to the polymer surface is higher inside the hole than outside. This can be attributed to the presence of polar groups near ZrO2 nanoparticle. QNM measurements revealed that polymer matrix has increased rigidity in the vicinity of the nanoparticles. Influence of ZrO2 nanoparticles on the structure and thermal properties of semi-crystalline polyheteroarylene matrix was studied with wide-angle X-ray scattering, scanning electron microscopy, and differential scanning calorimetry. Keywords: polyheteroarylene; zirconia; nanocomposite; polymer structure; quantitative nanomechanical mapping
1. Introduction In recent years increasing interest has been focused on the elaboration of polymer–inorganic composites (mixed matrix membranes) as functional hybrid materials, which combine the best properties of both phases: mechanical properties and processability of polymers on the one hand, and electrical, optical, catalytic or transport properties of inorganic materials on the other hand. Polyheteroarylenes, in particular aromatic polyimides and their composites, are promising materials for a wide range of applications due to their superior chemical stability, good mechanical properties, Polymers 2017, 9, 268; doi:10.3390/polym9070268
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and excellent thermal stability [1]. Introduction of inorganic particles into the polyimides can regulate the physicochemical properties of polymers and attain desirable properties in the material. Recently, it was demonstrated that such composites can be promising as membrane materials for separation of gas and liquid mixtures [2–6], films for optoelectronic fabrications [7,8], and high performance dielectric materials [9]. Different types of inorganic nanofillers are reported as regulators of polyimide properties. It was shown that titania nanoparticles are able to regulate transparency of polyimide films prepared from polymer containing benzimidazole side groups [10]. It is worth noting that successful dispersion of filler in the polymer requires additional dispersants or specific groups attached to the polymer. The authors of [11] describe the preparation of barium titanate/polyimide nanocomposite films using two types of dispersants 2-phosphonobutane-1,2,4-tricarboxylic acid and acrylic-acrylate-amide copolymers. It was shown that surface modification of nanoparticles improves their dispersibility in a polymer matrix and enhances dielectric properties of the films. In the work [12], the end –Si(OH)3 groups were attached to the polymer to increase its compatibility with inorganic particles. Additionally, it was shown that inorganic nanoparticles can regulate structure of polyimide membranes, which influence the selectivity of membranes in separation applications [13–15]. For example, it was shown that CO2 /N2 and CO2 /CH4 selectivity increase with the addition of ZnO nanoparticles to the polyimide bearing pendent naphthyl groups. It is generally accepted that introduction of inorganic nanoparticles into organic membrane leads to: decrease of chain mobility near polymer-particle interface [16], change of the free volume of film [4,5], change of the degree of crystallinity, the size of crystallites of polymer and distribution of crystalline phase inside the polymer volume [17,18]. Appearance of new selective diffusion pathways along polymer-inorganic interface affects the selectivity of a membrane. Orientation of polymeric molecules near the surface of carbon nanotubes is important for enhancing mechanical properties of polyimides [19]. It was shown recently [20] by molecular dynamics simulations of polyheteroarylene matrix that polyimide macromolecules form subsurface layers near the surface of single-walled carbon nanotubes. In the case of polyimide prepared from 1,3-bis (30 ,4-dicarboxyphenoxy) benzene and 4,40 -bis (4”-aminophenoxy) diphenyl (R-BAPB), the elongation of chains near inorganic particles was observed, while in the case of polymer synthesized with the same dianhydride and 4,40 -bis (4”-aminophenoxy) diphenylsulfone (R-BAPS) chains become compact. Thus, local characteristics and behaviors of polymer in proximity with the surface of inorganic particle are of great practical and theoretical interest. Atomic force microscopy (AFM) is a versatile method, which can be used for simultaneous mapping of the surface morphology with electrical, mechanical, and adhesive properties of a material with high spatial resolution and with an ability to operate within different ambient conditions [21,22]. For studying of nanomechanical properties PeakForce Quantitative Nanomechanical Mapping (PF-QNMTM ) technique, which allows the investigation of soft polymeric materials with a spatial resolution of ~50 nm is often applied [23]. PF-QNM has been successfully used for studies of nano-mechanical properties of polymeric materials under different impacts. For example, amyloid fibers at ageing [24], structural evolution, and mechanical properties of a deformed isoprene rubber [25] and the influence of silica on the nanomechanical properties of chitin–silica hybrid film [26,27]. In [28], it was pointed out that PF-QNM is the delicate and simple method to understand the nanoscale structure and to investigate interfacial interactions involved in the compatibilization process in polymer nanocomposites. At the same time, PF-QNM technique has never been used for characterization of nanomechanical properties of polyimide-inorganic composites. In the present work, we report on our investigation of R-BAPB polyimide/ZrO2 nanostars composite using PF-QNM, wide-angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The aim was to understand the difference between the properties of polymer matrix near organic/inorganic interface and in the bulk of the composite, which is necessary for further understanding of separation properties of mixed matrix membranes.
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2. Materials andMethods Methods 2. Materials and 2. Materials and Methods
2.1.2.1. Materials Materials 2.1. Materials Zirconium(IV) (IV) oxychloride oxychloride octahydrate (98.5%, Neva-Reactive, Saint Saint Petersburg, Russia, CAS: Zirconium octahydrate (98.5%, Neva-Reactive, Petersburg, Russia, Zirconium (IV) oxychloride octahydrate (98.5%, Neva-Reactive, Saint Petersburg, Russia, CAS: 7699-43-6); sodiumsodium acetate trihydrate (99.5%, Neva-Reactive, Saint Petersburg, Russia, CAS: 6131-90-4); CAS: 7699-43-6); acetate trihydrate (99.5%, Neva-Reactive, Saint Petersburg, Russia, 7699-43-6); sodium acetate trihydrate (99.5%, Neva-Reactive, Saint Petersburg, Russia, CAS: 6131-90-4); N-methyl-2-pyrrolidone (97%, Sigma-Aldrich, St. Louis, MO, USA, CAS: 120-94-5); 4,4′-bis (4″CAS: 6131-90-4); N-methyl-2-pyrrolidone (97%, Sigma-Aldrich, Louis, MO, USA, CAS: 120-94-5); N-methyl-2-pyrrolidone (97%, Sigma-Aldrich, St. Louis, MO, St. USA, CAS: 120-94-5); 4,4′-bis (4″0 0 aminophenoxy) biphenyl (97%, TCI, CAS: 13080-85-8); 1,3-bis (3′,4-dicarboxyphenoxy) benzene 4,4aminophenoxy) -bis (4”-aminophenoxy) biphenyl CAS: 13080-85-8); 1,3-bis (3 ,4-dicarboxyphenoxy) biphenyl (97%, TCI,(97%, CAS:TCI, 13080-85-8); 1,3-bis (3′,4-dicarboxyphenoxy) benzene (OOO (OOO «Tech. «Tech. Chim. Prom.», Yaroslavl, Russia) were used as received without purification. benzene Chim. Prom.», Yaroslavl, Russia) were used as receivedpurification. without purification. (OOO «Tech. Chim. Prom.», Yaroslavl, Russia) were used as received without 2.2. Synthesis of ZrO 2 Nanostars 2.2.2.2. Synthesis of of ZrO 2 2Nanostars Synthesis ZrO Nanostars Star-shaped ZrO 2 nanoparticles were synthesized in hydrothermal conditions from ·8H2 O, Star-shaped ZrOZrO were synthesized in hydrothermal conditions from ZrOCl2from 2 nanoparticles Star-shaped 2 nanoparticles were synthesized in hydrothermal conditions ZrOCl 2·8H2O, by the method described previously in [29]. Zirconium oxychloride (0.805 g) and by ZrOCl the method described previously in [29]. Zirconium oxychloride (0.805 g) and sodium acetate (0.103 2·8H2O, by the method described previously in [29]. Zirconium oxychloride (0.805 g) and g), sodium acetate (0.103 g), i.e., with mole ratio 1:2, were dissolved in 15 mL of distilled water under i.e.,sodium with mole ratio 1:2, were dissolved in 15 mL1:2, of were distilled water under stirring for 1 h.water The obtained acetate (0.103 g), i.e., with mole ratio dissolved in 15 mL of distilled under stirring for 1 h. The obtained solution was transferred to a teflon cell and was hold in autoclave at solution to a teflon cell and was holdtoina autoclave at temperature 240 ◦ C at and stirringwas for 1transferred h. The obtained solution was transferred teflon cell and was hold in of autoclave temperature of 240 °С and pressure of 150 atm for 4 h. The detailed structure of zirconia nanostars temperature 240for °С4and pressure of 150 atm forof4zirconia h. The detailed structure of zirconia nanostars pressure of 150 of atm h. The detailed structure nanostars prepared by this method was prepared by this method was described earlier in [13]. preparedearlier by thisinmethod described [13]. was described earlier in [13]. 2.3. Preparation of Composites 2.3.2.3. Preparation of ofComposites Preparation Composites Two monomers were used for preparation of R-BAPB polyimide: (1,3-bis (3′,4Two monomers used for preparation of R-BAPB(1,3-bis polyimide: (1,3-bis (3′,4Two monomers were were used for preparation of R-BAPB polyimide: (30 ,4-dicarboxyphenoxy) dicarboxyphenoxy) benzene and 4,4′-bis (4″-aminophenoxy) biphenyl). The chemical structure of the dicarboxyphenoxy) benzene and 4,4′-bis (4″-aminophenoxy) biphenyl). The chemical structure ofunit the of benzene and 4,40 -bis (4”-aminophenoxy) biphenyl). The chemical structure of the repeating repeating unit of R-BAPB polyimide is given in Figure 1. ZrO2 nanostars with concentration 5 wt % repeating unit of is R-BAPB polyimide given in Figurewith 1. ZrO 2 nanostars with 5 wt % R-BAPB polyimide given in Figure 1.isZrO concentration 5 wtconcentration % based on the weight 2 nanostars based on the weight of a polymer were dispersed in N-methyl-2-pyrrolidone. After that, the diamine based on the weight of a polymer were dispersed in N-methyl-2-pyrrolidone. After that, the diamine of and a polymer were dispersed in N-methyl-2-pyrrolidone. After that, the diamineinand in a dianhydride in a molar ratio of 0.97:1.03 were consistently dissolved thedianhydride dispersion of and ratio dianhydride in awere molar ratio of 0.97:1.03 were consistently dissolved in the dispersion of of molar of 0.97:1.03 consistently dissolved in the dispersion of nanoparticles. Formation nanoparticles. Formation of polyamic acid was conducted under argon flow during 6 h with nanoparticles. Formation ofunder polyamic conducted argon flow during 6 h with polyamic acid was conducted argonacid flowwas during 6 h withunder continuous Composites were continuous stirring. Composites were prepared by casting the solution stirring. onto glass plates with continuous stirring. Composites were prepared by casting the solution onto glass plates with◦ prepared by casting the of solution with subsequent removing of solvent (12 h at1 h 80at C) subsequent removing solventonto (12 hglass at 80plates °С) and imidization by stepwise thermal treatment: subsequent removing of solvent (12 h at 80 °С) and imidization stepwise treatment: 1 h at◦ ◦ C, 1 hby ◦ C, 1thermal and imidization by °С, stepwise thermal treatment: 100 °С, h atFinally, 200 ◦ C,the 1 hobtained at 250 C, 100 °С, 1 h at 150 1 h at 200 °С, 1 h at 250 °С, 11 h at 280 and at 0.5150 h at 300 °С. 100 °С, 1◦h at 150 °С, 1 h at 200 °С, h at 250 °С, 1 h at 280 °С, and 0.5 h at 300 °С. Finally, the obtained ◦ C.1Finally, 1 hfilms at 280 C, removed and 0.5 h from at 300glass thefurther obtained films were removed glass plates for were plates for investigation. Prepared from polymer–inorganic films were removed from glass plates for further investigation. Prepared polymer–inorganic further investigation. Prepared polymer–inorganic is denoted the text asfilms R-BAPB-ZrO composite is denoted in the text as R-BAPB-ZrO2composite . For comparison, the in polyimides without . composite is denoted in the text as R-BAPB-ZrO2. For comparison, the polyimides films without 2 filler the waspolyimides also prepared and it is denoted as R-BAPB. of all and filmsitwas 20 µm. as Forinorganic comparison, films without inorganic filler wasThickness also prepared is denoted inorganic filler was also prepared and it is denoted as R-BAPB. Thickness of all films was 20 µm. Optical images of prepared films are shown in Figure 2. R-BAPB. of all films wasare 20 shown µm. Optical images OpticalThickness images of prepared films in Figure 2. of prepared films are shown in Figure 2.
Figure 1. Thechemical chemical structure of of the polyimide R-BAPB repeating unit. Figure polyimideR-BAPB R-BAPBrepeating repeatingunit. unit. Figure1.1.The The chemicalstructure structure of the the polyimide
Figure 2. Optical images of pristine R-BAPB (left) and composite membrane R-BAPB-ZrO2 (right). Figure 2. Optical images of pristine R-BAPB (left) and composite membrane R-BAPB-ZrO2 (right). Figure 2. Optical images of pristine R-BAPB (left) and composite membrane R-BAPB-ZrO2 (right).
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2.4. Characterization Methods 2.4.1. Microscopic Investigation Scanning electron microscopy (SEM) micrographs of the films’ surfaces were obtained with a Zeiss Merlin SEM (Carl Zeiss, Oberkochen, Germany). For investigation of cross-sections, the films were frozen in liquid nitrogen and fractured perpendicularly to their surface. Coating with carbon layer was used for preparation of the samples for SEM. Scanning probe microscope multimode 8 (Bruker, Santa Barbara, CA, USA) operating in PeakForce TUNATM mode was used for atomic force microscopy (AFM) experiments. Scanning was done in PeakForce QNM mode with feedback adjusted automatically by ScanAsyst program protocol. Major PeakForce parameters were: amplitude 100 nm and frequency 2 kHz. ScanAsyst-Air probe (Bruker, Santa Barbara, CA, USA) with a tip radius of 5 nm and spring constant 0.47 N·m−1 was used for accurate topography measurements with setpoint force 2 nN. Then, a considerably stiffer probe Tap525a (Bruker, Santa Barbara, CA, USA) with tip radius ~10 nm, spring constant ~120 N m−1 , and resonance frequency of 447 kHz was utilized to perform the QNM measurements under a force of ~50 nN that allowed to deform the sample in depth of approximately 1 nm. 2.4.2. Wide-Angle X-ray Diffraction Study Structure of initial polyimide and composite film was studied by the wide-angle X-ray diffraction (WAXD) with using a D8 DISCOVER diffractometer (Bruker, Rheinstetten, Germany). Scattering angles varied from 5◦ to 40◦ with 0.05◦ step using Cu-Kα radiation. 2.4.3. Analysis of Thermal Properties Differential scanning calorimetry (DSC) was conducted using a DSC 204 F1 (Netzsch, Selb, Germany) differential scanning calorimeter to obtain the glass transition temperature (Tg ) of samples. The analysis was conducted under inert atmosphere with samples of approximately 4–5 mg at a scan rate of 10 ◦ C min−1 from 20 to 350 ◦ C. Thermobalance TG 209 F1 Libra (Netzsch, Selb, Germany) was used for thermogravimetric analysis (TGA), which was performed under inert atmosphere with samples having a weight of approximately 2–4 mg at a scan rate of 10 ◦ C·min−1 . 2.4.4. FTIR Spectroscopy Investigation In this work, the IR Fourier spectrometer Vertex 70 (Bruker, Ettlingen, Germany) and the ATR reflector (Pike Technologies, WI, USA) were used. Zn-Se crystals in the form of prisms with an incidence angle of the radiation on the object θ = 45◦ were used as ATR elements. 3. Results and Discussion 3.1. WAXD Data Structure of R-BAPB film and composite (R-BAPB-ZrO2 ) were characterized by WAXD. The diffraction patterns are presented in Figure 3. According to pattern 1 (Figure 3), the initial polyimide represented semi-crystalline structure and positions of the peaks are resemble the spectra reported previously [30]. The sample exhibit two strong reflections at 18.9◦ and 22◦ , which correspond to the interplanar distances of d = 4.7 and 4.1 Å, respectively, and weak reflections at 2Θ = 8.1◦ , 10.3◦ , 13.3◦ , 15.7◦ , 19.8◦ , 20.7◦ , 26.6◦ , 28.0◦ , and 28.9◦ (d = 10.9, 8.6, 6.6, 5.7, 4.5, 4.3, 3.4, 3.2, and 3.1 Å, respectively).
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pattern. The presented data confirm the chemical structure of initial polyimide and qualitative 5 of 12 chemical composition of its composite with zirconia.
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Figure Thewide-angle wide-angleX-ray X-ray diffraction (WAXD) patterns of pristine polyimide (R-BAPB) Figure 3. The diffraction (WAXD) patterns of pristine polyimide (R-BAPB) (1), (1), its composite zirconia (R-BAPB-ZrO and reference for monoclinic 2) (2) and patternpattern for monoclinic zirconiazirconia (JCPDS its composite withwith zirconia (R-BAPB-ZrO 2 ) (2)reference (JCPDS 37-1484). 37-1484).
3.2. Investigation of Morphology withof Electron Microscopy From the diffraction pattern composite film (Figure 3, pattern 2) it is seen that the main reflections are located at the same positions as the initial polymer. However, their SEM results of cross-sections and surfaces ofpeaks initialofR-BAPB and its composite with intensity ZrO2 are is slightly reduced in comparison with the pure polymer matrix. In addition, strong reflections with presented in Figure 4. A lot of fracture lines are clearly visible on the image of cross-section of initial ◦ , 28.1◦ , 31.5◦ , 34.2◦ , and 35.2◦ (d = 3.7, 3.2, 2.8, 2.6, and 2.5 Å) appear, which corresponds 2Θ = 24.1 polyimide (Figure 4a). This is connected to the semi-crystalline nature of the polymer. Addition of zirconia to the presence of ZrO nanostars with monoclinic Positions of the peaks(see for ZrO 2 from nanoparticles leads to 2the significant changes in thesingony. morphology of cross-section Figure 4b). the JCPDS card No. 37-1484 are shown in Figure 3 for comparison with an experimental diffraction Roughness of cross-section significantly increases, which can relate to the presence of nanoparticles pattern. The presented confirm thefrom chemical structure initial polyimide qualitative chemical in the polymer matrix.data It can be seen Figure 4c thatofthe upper surfaceand of pure R-BAPB film is composition of its composite with zirconia. composed from uniformly distributed crystalline flake-like domains with sizes about 400 nm. Introduction of zirconia nanoparticles decreases the sizes of crystalline domains to 100–200 nm 3.2. Investigation of Morphology with Electron Microscopy (Figure 4d). This is connected to possible ability of zirconia to act as crystallization center for the SEM which resultsleads of cross-sections surfaces amount of initialofR-BAPB andwith its composite with ZrO2 are polymer, to formation and of increased crystallites reduced size. presented Figure 4. A lot fracture lines are clearly visible the image of ZrO cross-section of initial Thus,inSEM images of ofcomposite membranes based ononR-BAPB and 2 nanostars show polyimide (Figure 4a). of This connected to the semi-crystalline nature of theparticles polymer. of excellent homogeneity theisprepared films, where agglomerated inorganic areAddition not visible. zirconia leadsdue to the changes the2 morphology (see Figure 4b). It can benanoparticles proposed that, to significant possible ability of in ZrO particles to of actcross-section as crystallization centers, Roughness of cross-section significantly increases, which relate to from the presence of nanoparticles growing polymer crystallites around ZrO 2 particles push can them apart each other. It is possible in thethe polymer matrix. It cansurface be seen from Figure that the oxygen upper surface of carbonyl pure R-BAPB film that interaction between -OH groups of4c ZrO 2 and atoms of groups of is composed from uniformly distributed crystalline domainsofwith sizeschains aboutand 400their nm. imide cycles of polymer via hydrogen bounding leads flake-like to the orientation polymer Introduction of in zirconia nanoparticles decreases the ofbulk crystalline domains This to 100–200 nm denser packing the vicinity of the nanoparticle thansizes in the of the polymer. interaction (Figure 4d). This is connected to possible ability of zirconia to act as crystallization center for will be discussed in the Section 3.3. As a result, the uniform distribution of isolated nanoparticlesthe is polymer, achieved.which leads to formation of increased amount of crystallites with reduced size. Presented images confirm that the preparation method based on introduction of nanoparticles before the synthesis of polyamic acid leads to strongly uniform distribution of filler in the composite. Figure 4e,f show morphology of bottom side of the composite film with different magnifications. It can be seen that surface of the film is smooth and uniformly distributed nanoparticle are clearly
or upper surface, which can be explained by sedimentation of bigger particles during solvent evaporation from composite (R-BAPB-ZrO2). At the same time, it must be noticed that the SEM method is unable to provide a definite answer to the question if visible stars are inorganic particles or only their imprint on the surface of polymer. However, appearance of such objects makes it possible to investigate the structure and properties of polymeric matrix near inorganic nanoparticles. Polymers 2017, 9, 268 6 of 12 This was performed with AFM measurements of the bottom side of the composite membrane.
Figure microscopy (SEM) images of pristine polyimide film film R-BAPB (a,c) and Figure 4. 4. Scanning Scanningelectron electron microscopy (SEM) images of pristine polyimide R-BAPB (a,c) composite film with zirconia (b,d,e,f). Cross-section morphology (a,b),(a,b), images of upper side (c,d) and and composite film with zirconia (b,d,e,f). Cross-section morphology images of upper side (c,d) and bottom of films. bottom side side (e,f) (e,f) of films.
3.3. FT-IR Thus,Spectroscopy SEM images of composite membranes based on R-BAPB and ZrO2 nanostars show excellent homogeneity of the prepared films, where agglomerated inorganic particles are not visible. Itwith can The chemical structure of pristine polyimide R-BAPB and composite film was investigated be proposed that, due to possible ability of ZrO to act as crystallization 2 particles FT-IR spectroscopy (Figure 5). Comparison of FT-IR spectra of zirconia nanostarscenters, (Figure growing 5a) and polymer crystallites around ZrO particles push them apart from each other. It is possible that the 2 composite film (Figure 5c) demonstrates that the incorporation of nanostars to the polyimide leads to −1 interaction surface –OH groupsofofthe ZrO of carbonyl to groups of imide cycles 2 and the decreasebetween in the integrated intensity band atoxygen 3700 cmatoms corresponding free -OH groups on of polymer via hydrogen leads the orientation of polymer packing the filler surface and thebounding increase of thetointensity of the band in thechains regionand of their 3300 denser cm−1, which is in the vicinity of the nanoparticle than in the bulk of the polymer. This interaction will be discussed in attributed to the associated hydroxyl groups (Figure 5a,c). These results can be attributed to the the Section a result, uniformbonds distribution of isolated nanoparticles is achieved. changing of3.3. the As network of the hydrogen in a composite in comparison with the initial polymer. Presented images confirm that the preparation method based on introduction of nanoparticles FT-IR spectra of initial polyimide and composite film with ZrO2 (Figure 5b,c) demonstrate typical −1 −1 (C=O before the polyamic acid leads to strongly uniformstretching), distributionatof 1715–1720 filler in thecm composite. bands for synthesis the imideofcycle: at 1775 cm (C=O asymmetric Figurestretching) 4e,f show morphology of−1bottom side of the composite with different symmetric and at 735 cm (C=O banding), with the C–Nfilm stretching peak atmagnifications. 1370 cm−1. It can be seen that surface of the film is smooth and uniformly distributed nanoparticle are clearly visible. Along with the small nanostars with diameters about 50 nm, the bigger ones with diameters 500 nm are seen. They appear only on the bottom side of the composite film, not in the cross-section or upper surface, which can be explained by sedimentation of bigger particles during solvent evaporation from composite (R-BAPB-ZrO2 ). At the same time, it must be noticed that the SEM method is unable to provide a definite answer to the question if visible stars are inorganic particles or only their imprint on the surface of polymer. However, appearance of such objects makes it possible to investigate the structure and properties of polymeric matrix near inorganic nanoparticles. This was performed with AFM measurements of the bottom side of the composite membrane.
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3.3. FT-IR Spectroscopy
1715
The chemical structure of pristine polyimide R-BAPB and composite film was investigated with FT-IR spectroscopy (Figure 5). Comparison of FT-IR spectra of zirconia nanostars (Figure 5a) and composite film (Figure 5c) demonstrates that the incorporation of nanostars to the polyimide leads to the decrease in the integrated intensity of the band at 3700 cm−1 corresponding to free –OH groups on the filler surface and the increase of the intensity of the band in the region of 3300 cm−1 , which is attributed to the associated hydroxyl groups (Figure 5a,c). These results can be attributed to the changing of the network of hydrogen bonds in a composite in comparison with the initial polymer. FT-IR spectra of initial polyimide and composite film with ZrO2 (Figure 5b,c) demonstrate typical bands for the imide cycle: at 1775 cm−1 (C=O asymmetric stretching), Polymers 2017, 9, 268 −1 12 at 1715–1720 cm (C=O symmetric stretching) and at 735 cm−1 (C=O banding), with the7 ofC–N stretching peak at 1370 cm−1 .
735
1775
λ, cm-1
3700
3300
1370
3 2 1
4000
3500
3000
1500
1000
500
λ, cm-1 Figure 5. FT-IR spectra of zirconia nanostars (1), pristine polyimide (2) and composite film with ZrO2 (3). Figure 5. FT-IR spectra of zirconia nanostars (1), pristine polyimide (2) and composite film with ZrO2 (3).
3.4. AFM Results 3.4. AFM Results Comparison between typical images of surface topography of pure R-BAPB film and its composite Comparison typicalinimages surface topography pure R-BAPB and its membrane with between ZrO2 is given Figure of 6a,c, respectively. The of pristine polymer film demonstrates composite membrane with with ZrO2 considerable is given in difference Figure 6a,c, respectively. The pristineparts polymer significantly rough surface in height for the neighboring of the demonstrates rough surface6b,d). with considerable in height the neighboring surface (see significantly profiles curves in Figure Amplitude ofdifference topography profilefor curves is about 350 parts (see profilesand curves in Figure 6b,d). Amplitude of topography profile curves can is and of 30 the nm surface for initial polymer composite, respectively. Higher amplitude for pure polyimide about 350 and to 30the nmformation for initialofpolymer and composite, amplitudeformations for pure be attributed bigger crystallites, whichrespectively. pack in largeHigher supramolecular polyimide can be attributed to the formation of bigger crystallites, which pack in large during the preparation of the film from the pristine polymer. The root mean squared roughness supramolecular formations during the preparation of the film from the pristine polymer. The (Rq ), which was averaged from the values obtained for different parts of surface with sizes root 5 µm mean squared from theand values obtained for respectively. different partsTheir of × 5 µm wereroughness 14 ± 2 and(R8q),±which 0.2 nmwas foraveraged initial polyimide composite film, surface with sizes 5 µm × 5 µm were 14 ± 2 and 8 ± 0.2 nm for initial polyimide and composite film, difference is emphasized by comparing surface topography profiles which are presented in Figure 6b,d. respectively. Their is emphasized by comparing surface profiles are We suggested that difference lower roughness and amplitude of profile curve fortopography composite relates to which the possible presented in Figure 6b,d. We suggested that lower roughness and amplitude of profile curve for ability of ZrO2 nanoparticles to act as crystallization centers for polymer. This leads to an increased composite relates to the possible ability of ZrO 2 nanoparticles to act as crystallization centers for amount of crystallites, but with reduced sizes. This is in agreement with our SEM results (Figure 4a,b). polymer. This leads to an increased amount of crystallites, but withparticles reduced sizes. is in AFM pictures also demonstrate uniformity of distribution of inorganic inside theThis composite. agreement with our SEM results (Figure 4a,b). AFM pictures also demonstrate uniformity of The reduced roughness due to changes in crystalline structure of composite leads to an increasing distribution of inorganic particles inside the composite. The reduced roughness due to changes in glossiness in polymer film with incorporation of inorganic nanoparticles (Figure 2). crystalline structure of composite leads to an increasing glossiness in polymer film with incorporation of inorganic nanoparticles (Figure 2).
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Figure 6. Surface topography of upper side of R-BAPB film (a), composite film with zirconia (c), and Figure 6. Surface of upper side of R-BAPB film (a), composite film with zirconia (c), and profile curvestopography (b,d). profile curves (b,d).
The abovementioned AFM images were collected from the upper surface of the film, which was The abovementioned AFM images were collected from the upper surface of the film, which was in contact with air during preparation of sample. Further investigation of the bottom side of the in contact with air during preparation of sample. Further investigation of the bottom side of the film film provided additional information about changes of polymer structure near the organic/inorganic provided additional information about changes of polymer structure near the organic/inorganic interface. The simultaneously captured surface topography, mechanical stiffness, and adhesiveness interface. The simultaneously captured surface topography, mechanical stiffness, and adhesiveness of the same area of the bottom surface of composite film are presented in Figure 7a,c,e, respectively. of the same area of the bottom surface of composite film are presented in Figure 7a,c,e, respectively. These results were also obtained in the PeakForce QNM mode. As it is seen from the topography map, These results were also obtained in the PeakForce QNM mode. As it is seen from the topography during removal of the film from substrate, some star-shaped ZrO2 particles, which were deposited map, during removal of the film from substrate, some star-shaped ZrO2 particles, which were on the bottom surface of the film, fall out of polymer matrix. The imprint of the star marked with deposited on the bottom surface of the film, fall out of polymer matrix. The imprint of the star marked blue arrow in Figure 7a was chosen for measurements of profiles and discussion of results due to the with blue arrow in Figure 7a was chosen for measurements of profiles and discussion of results due flatness of the surrounding surface. A smooth and flat surface allows to minimize errors, which can to the flatness of the surrounding surface. A smooth and flat surface allows to minimize errors, which arise from different contact area between probe and sample in various points of the rough surface. can arise from different contact area between probe and sample in various points of the rough surface. Figure 7b demonstrates that the depth of the hole remaining in the polymer after removing of the Figure 7b demonstrates that the depth of the hole remaining in the polymer after removing of the ZrO2 nanoparticles is ~4 nm. Therefore, it can be concluded that ZrO2 nanostars are composed of flat ZrO2 nanoparticles is ~4 nm. Therefore, it can be concluded that ZrO2 nanostars are composed of flat crystallites with a thickness of ~4 nm and a width of ~450 nm. crystallites with a thickness of ~4 nm and a width of ~450 nm.
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Figure 7. Surface topography of bottom side of composite (a), map of elastic modulus (c), and map of Figure 7. Surface topography of bottom side of composite (a), map of elastic modulus (c), and map of adhesion (e); (b,d,f) show profiles for these corresponding channels taken at the selected region of the adhesion (e); (b,d,f) show profiles for these corresponding channels taken at the selected region of the surface with an imprint from zirconia nanoparticle (shown with white line segment). surface with an imprint from zirconia nanoparticle (shown with white line segment).
Figure on the the same same region region of of surface surfaceas as Figure7c,d 7c,dshow showthe the map map of of elastic elastic modulus modulus and its profile on presented increased stiffness stiffnessnear nearthe the presentedininFigure Figure7a,b. 7a,b.ItItisisclearly clearly seen seen that that the the sample sample demonstrates demonstrates increased position packing of of polymeric polymericchains chainsnear near positionofofinorganic inorganicparticle. particle. This This gives gives evidence evidence for more dense packing thenanoparticle nanoparticlethan thanin inother otherparts parts of of the the sample. sample. The selected nanoparticle the nanoparticle influence influenceon onthe theelastic elastic modulusininthe thelateral lateraldirection directionisisaadistance distance of of approximately approximately 500 nm. The modulus The same same qualitative qualitativeresults results canbe beseen seenfor forother otherregions regionsof of the the surface. This data is can is in inthe theagreement agreementwith withother othertheoretical theoreticalresults, results, whichdemonstrates demonstratesthe theability ability of of R-BAPB R-BAPB chains chains to elongate near the surface which surface of of inorganic inorganic particle particle (carbonnanotube) nanotube)[20], [20],which whichwas wasconsidered considered as as a pre-crystallization stage. (carbon stage. Experimental Experimentalresults resultsfor for composites of polyimides also demonstrate an increase of Young’s modulus of polymeric films with composites of polyimides also demonstrate modulus of polymeric films with introduction of inorganic nanoparticles [13,31]. At the same time, only nanomechanical
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introduction of inorganic nanoparticles [13,31]. At the same time, only nanomechanical measurements measurements with AFM can provide direct experimental evidence for the increasing of local with AFM can provide direct experimental evidence for the increasing of local stiffness of polymer stiffness of polymer matrix near organic/inorganic interface. matrix near organic/inorganic interface. Additional insight into the local structure of composites can be obtained from the map of Additional insight into the local structure of composites can be obtained from the map of adhesion adhesion (Figure 7e). It is seen that the bottom part of the imprint is significantly more adhesive than (Figure 7e). It is seen that the bottom part of the imprint is significantly more adhesive than the the surrounding polymeric surface. The profile in the Figure 7f demonstrates that the border of the surrounding polymeric surface. The profile in the Figure 7f demonstrates that the border of the adhesive region coincides perfectly with the walls of the imprint. It can be suggested that the adhesive adhesive region coincides perfectly with the walls of the imprint. It can be suggested that the adhesive properties of this area are connected with high local concentration of polar chemical groups on the properties of this area are connected with high local concentration of polar chemical groups on the bottom surface of the filler imprint. bottom surface of the filler imprint. Prepared samples were also investigated with DSC (Figure 8). It was found that glass-transition Prepared samples were also investigated with DSC (Figure 8). It was found that glass-transition temperature (Tg) was 203–205◦ °C for both samples. Peaks corresponding to melting of crystalline temperature (Tg ) was 203–205 C for both samples. Peaks corresponding to melting of crystalline phase phase are clearly visible in the range 315–319 °C, which confirms the semi-crystalline nature of are clearly visible in the range 315–319 ◦ C, which confirms the semi-crystalline nature of prepared prepared films as it was observed by WAXD. The thermal stability of the prepared samples was films as it was observed by WAXD. The thermal stability of the prepared samples was evaluated by evaluated by TGA. The TGA curves indicate that solvent has been successfully eliminated from TGA. The TGA curves indicate that solvent has been successfully eliminated from polyimide film and polyimide film and also from composite film with ZrO2 because there is no weight loss below 100 °C. also from composite film with ZrO2 because there is no weight loss below 100 ◦ C. Both TGA curves Both TGA curves show one region of weight loss, which correspond to the decomposition of the show one region of weight loss, which correspond to the decomposition of the polymer backbone. polymer backbone. The step of weight was 500 °C for pristine polyimide and 480 °C for composite The step of weight was 500 ◦ C for pristine polyimide and 480 ◦ C for composite film with ZrO2 , which film with ZrO2, which agrees with data published in for R-BAPB [30]. This demonstrates a high agrees with data published in for R-BAPB [30]. This demonstrates a high thermal stability of the thermal stability of the prepared samples. prepared samples.
Figure film: first scan (1),(1), second scanscan (2) and of composite film with Figure 8. 8. DSC DSC curves curvesofofinitial initialpolyimide polyimide film: first scan second (2) and of composite film ZrO nanostars: first scan (3), second scan (4). with2 ZrO2 nanostars: first scan (3), second scan (4).
4. Conclusions 4. Conclusions 0 -bis (4”-aminophenoxy) Nanomechanical properties of Nanomechanical properties of composite composite of of semi-crystalline semi-crystalline poly poly [4,4 [4,4′-bis (4″-aminophenoxy) 0 diphenyl]imide 1,3-bis (3 ,4-dicarboxyphenoxy) benzene (R-BAPB) with zirconia diphenyl]imide 1,3-bis (3′,4-dicarboxyphenoxy) benzene (R-BAPB) with zirconia nanostars nanostars were were studied using the the quantitative quantitative nanomechanical nanomechanical mapping mapping (QNM) (QNM) mode mode of of atomic atomic force force microscopy. microscopy. studied using Experimental evidence of Experimental evidence of increased increased rigidity rigidity of of polymer polymer near near the the organic/inorganic organic/inorganic interface interface was was obtained for the first time. The distance on which nanoparticle gives influence on the mechanical obtained for the first time. The distance on which nanoparticle gives influence on the mechanical properties is in by properties of of polymeric polymeric matrix matrix is in the the range range of of hundreds hundreds of of nanometers, nanometers, which which was was observed observed by nanomechanical mapping. Comparison of adhesive properties between surface in the imprint nanomechanical mapping. Comparison of adhesive properties between surface in the imprint of of nanoparticle and upper upper surface surface of of the nanoparticle and the membrane membrane material material revealed revealed the the existence existence of of adhesive adhesive polar polar groups groups in in the the surface, surface, which which was was formed formed at at the the contact contact with with zirconia. zirconia. Investigation Investigation of of surfaces surfaces and and cross-sections of pure pristine polyimide (R-BAPB) and composite (R-BAPB-ZrO ) films shows that cross-sections of pure pristine polyimide (R-BAPB) and composite (R-BAPB-ZrO2) films shows that addition of zirconia before the acid can addition of zirconia nanoparticles nanoparticles before the formation formation polyamic polyamic acid can achieve achieve aa highly highly uniform uniform distribution of inorganic Introduction of distribution of inorganic filler filler inside inside the the polymer polymer matrix. matrix. Introduction of zirconia zirconia nanoparticles nanoparticles significantly decreases the This can can relate relate to to the the ability ability of significantly decreases the roughness roughness of of film’s film’s surface. surface. This of zirconia zirconia nanoparticles to act act as centers during during the the thermal thermal treatment treatment of of aa film. As aa result, nanoparticles to as crystallization crystallization centers film. As result, the the
number of crystalline domains in the composite increases, while their size decreases. Therefore, an ordering effect of inorganic nanoparticles toward the polymer matrix was observed with advanced
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number of crystalline domains in the composite increases, while their size decreases. Therefore, an ordering effect of inorganic nanoparticles toward the polymer matrix was observed with advanced AFM technique. These results are of interest for using polyheteroarilene films as membrane materials in separation technology. Acknowledgments: This work was supported by the Russian Science Foundation (RSF), grant 16-13-10164: Maria Sokolova, Michael Smirnov, and Alexander Toikka acknowledge the RSF for support in the synthesis of membranes, studies of their structural characteristics and analysis of results. Alexander Bugrov acknowledges St. Petersburg State University for the postdoctoral fellowship (grant 12.50.23.2014). Valentin Svetlichnyi and Elena Popova acknowledge RFBR (17-03-00733 a). The experimental work was facilitated by the equipment of the Resource Center of X-ray Diffraction Studies, Nanotechnology Interdisciplinary Resource Center, and of Thermogravimetric and Calorimetric Resource Centre at St. Petersburg State University. Authors are grateful to Elena N. Vlasova from Institute of Macromolecular Compounds RAS for the study of samples obtained in this article by the FT-IR spectroscopy. Author Contributions: Alexander M. Toikka, Michael A. Smirnov, and Maria P. Sokolova planned and determined the overall structure of the study, perform and analyze structural and thermal properties of the composite materials and wrote the paper; Alexander N. Bugrov synthesized the polyheteroarylenes, ZrO2 nanostars, and prepared the composites; Pavel Geydt performed atomic force microscopy; Elena N. Popova performed DSC measurements, Erkki Lahderanta and Valentin M. Svetlichnyi also participated in the conception and design of experiments. All authors contributed to the preparation of the text of the paper. Conflicts of Interest: The authors declare no conflict of interest.
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