Highly fluorinated polyimide blends – Insights into physico-chemical characterization

Polymer 55 (2014) 4488e4497

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Highly fluorinated polyimide blends e Insights into physico-chemical characterization Mariana-Dana Damaceanu a, *, Catalin-Paul Constantin a, Maria Bruma a, Nataliya M. Belomoina b a b

“Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A, Iasi 700487, Romania Nesmeyanov Institute of Organoelement Compounds, ul. Vavilova 28, Moscow 119991, Russia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 April 2014 Received in revised form 20 June 2014 Accepted 23 June 2014 Available online 18 July 2014

A thorough study on the physico-chemical properties of novel blends of fluorinated aromatic polyimides prepared on the basis of the similarity of dianhydride monomer units was performed. High-temperature solution polycondensation reactions resulted in high molecular weights polyimides further used as blend components. The blend films and coatings were rootedly characterized by various methods, including FTIR, DSC, thermal stability, UVeVis and fluorescence spectroscopy, with emphasis on the miscibility behaviour. They exhibited high thermal stability and unique glass transition temperature, a critical evidence of the blends homogeneity. The presence of charge transfer interactions between neighbour polyimide chains was comprehended by UVeVis and fluorescence spectroscopy, and discussed in correlation with chemical structures. The hydrophobic blend films showed a porous structure or an interlamellar or interfibrillar segregated morphology, good mechanical and optical properties with high transparency. Furthermore, new insights into their physico-chemical behaviour induced by the trifluoromethyl group or other structural features are discussed. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Fluorinated polyimides Miscible blends Charge transfer complex

1. Introduction The growth of modern technology has posed a constantly increasing need for materials that can perform well under harsh conditions, such as elevated temperatures. Aromatic polyimides possess a great potential for this purpose, having excellent thermal, mechanical, dielectric and optical properties, along with good chemical resistance and high dimensional stability [1,2]. Polyimides are currently applied in many areas; as matrices for advanced composite materials, as thin films in electronic applications for interlayer dielectrics, passivation layers, substrates for flexible printed circuit boards and stress-relief layers in high-density electronic packaging, as structural adhesives and sealants, as high temperature insulators for aircraft wire coatings, as membranes for gas separation etc [3e8]. However, fully aromatic polyimides have rigid chains and strong interchain interactions which result in poor solubility and non-melting characteristics, thus low processability for such applications. These features are a result of the highly symmetrical and highly polar groups. Strong interactions originate

* Corresponding author. Tel.: þ40 232 217454; fax: þ40 232 211299. E-mail address: [email protected] (M.-D. Damaceanu). http://dx.doi.org/10.1016/j.polymer.2014.06.089 0032-3861/© 2014 Elsevier Ltd. All rights reserved.

from intra- and interchain charge transfer complex (CTC) formation and electronic polarization supported by the strong electron acceptor characteristics of imides and the electron donor characteristics of amine segments [9,10]. Therefore, some significant synthetic efforts in the area of high-temperature resistant polymers have been focused on several approaches in order to improve their solubility and processability without much sacrifice in thermal characteristics and other desired properties [11e13]. Polymer blending can be considered as a preferred route of modification in the light of its simplicity, reproducibility, processability and low development cost. Moreover, the blending of polymers may synergistically combine the advantages of different components. Materials selection works on the basis that the main advantages of one component will compensate for deficiencies of the other and vice versa. Thus, various polymer blends, derived either from pure polymers or from chemically modified ones, have been studied [14e16]. But the major disadvantage of polymeric blends is that they tend to phase separate. Blend miscibility is induced either by the similarity of the monomer units or by specific interactions between different segments, and most miscible polymers exhibit a miscibility window; that is, a temperature and composition range within which a single phase is formed. It has been reported that various polyimides are miscible with

M.-D. Damaceanu et al. / Polymer 55 (2014) 4488e4497

O

CF3

N

O

O

N

CF3

CF3

CF3

O O

CF3

N

O

N

N

N

CF3

CF3

P3 N O

CF3

N O

O

CF3 O

O

n

CF3

CF3

P4 N

n

O

O

O

O

O

N

CF3

n

O

CF3

CF3

O

O

O

O

CF3

P2

CF3

N

O

O

n

P1 O

O

N

O

O

O

O

CF3

N

N

CF3

O

N

4489

n

O

CF3 O

O CF3

N

O

O

O

n

P6

P5

Scheme 1. Chemical structures of fluorinated polyimides used in this study.

polybenzimidazoles [17], polyether-sulfones [18,19], polyetherether-ketones [20], sulfonated polyether-ether-ketones [21], poly(methyl-methacrylate) [22] and other polyimides [23]. The mechanism of miscibility was shown to be related in some cases to H-bonding between the chains of the two components [24], and for other blends the mechanism was addressed to charge-transfer complexes (CTC) [25]. The incorporation of fluorine into polyimide structures has been intensively explored with the hope of fine-tuning several properties of particular interest. Fluorine incorporation has been found to generally lower the dielectric constant [26] and moisture absorption [27], to bring about desirable changes to optical properties [28] and, also, to increase the thermal stability in some cases [29]. While there have been occasional forays into more complex fluorinated structures with valuable results, most work has concentrated on the incorporation of the hexafluoroisopropylidene moiety (6F) into dianhydride and diamine components or of trifluoromethyl groups on benzene rings [30e33]. Thus, their good solubility in common solvents enabled their use in polymer blending solutions. Blending fluorine-containing polyimides offers an attractive opportunity for the development of novel materials exhibiting useful combinations of the two polymers' respective properties [34e36]. However, the physical properties of the newly developed polymer blends are strongly dependent upon phase miscibility. In a recent study, we have reported on the synthesis and characterization of highly fluorinated polyimides with ortho-kink structure and their potential applications, especially as transparent coatings [33]. These polymers provided only brittle free-standing films probably owing to their very stiff polymer chains (insufficient entanglements) [32]. Therefore, to benefit of their potential properties for practical applications, such as membrane for hot gases separation or transparent flexible substrates in optoelectronic devices, it is necessary to combine them with polyimides having high film-forming ability provided that the desired properties of the new materials are not significantly affected. The main objective of this study is to explore the advantages of employing the simple while effective and efficient blending technique in tailoring the properties of new fluorinated polyimide materials. Thus, this work describes the synthesis and characterization of new fluorinated polyimide blends. The miscibility of the two components was studied by various techniques and the morphology of the obtained films was studied by AFM and polarized light microscopy. The optical properties involved in the

miscible blends were quantified according to the total light transmittance by using UVeVis spectroscopy, while the fluorescence spectroscopy was involved to evidence the CTC formation. The thermal stability, mechanical and wetting properties of these polyimide blend films were also investigated. 2. Experimental 2.1. Polymers Ortho-kink aromatic polyimides P1eP4 containing trifluoromethyl-substituted benzene rings and hexafluoroisopropylidene moieties were synthesized as described elswhere [33]. Their chemical structures are shown in Scheme 1. Filmforming polyimides P5 and P6 (Scheme 1) which were used for mixing with P1eP4 in the blends, have been prepared as described in Supporting Information section. 2.2. Preparation of polymer blend films The polymer blends were prepared in film form by casting mixed polyimide solutions onto glass plates. Thus, different volumes of polyimides in NMP having 10% concentration were mixed with stirring for 48 h at 100  C, followed by ultrasonation in a water bath for 8 h. Mixed solution samples were periodically taken and cast into small film pieces in order to check the miscibility by means of DSC. These series of tests established that 8 h of ultrasonation are necessary to achieve a complete mixing of the two components. The as-obtained solution mixtures with different proportion of each component (Table 1) were filtered and cast onto glass plates, followed by gradual heating from room temperature

Table 1 Composition of fluorinated polyimide blends. Polymer blend

(Wt %) P1

B1 B2 B3 B4 B5 B6

P2

P3

P4

P5

50

50 50

50 25

P6

75 75

25 25 25

75 75

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up to 180  C, and kept at 180  C for 4 h to remove the residual solvent. The resulting films were stripped off the plates by immersion in boiling water, followed by drying in oven at 110  C. These films had the thickness in the range of 10e30 mm and were used afterwards for various measurements. 2.3. Preparation of polymer blend coatings Very thin polymer coatings having the thicknesses of about 0.1 mm were deposited by drop-casting technique on glass or quartz substrates by using a solution obtained from 5 mg of polymer blend film in 1 ml NMP (0.5% concentration). These films, as-deposited, were gradually heated from room temperature up to 180  C and kept at 180  C for 1 h to remove the residual solvent. 2.4. Preparation of polymer films and coatings For comparison, films of neat polyimides P5 and P6 were prepared by casting a filtered solution of 10% concentration of the polymer in NMP onto glass plates, followed by gradual heating from room temperature up to 180  C, and kept at 180  C for 4 h. Transparent coatings resulted having strong adhesion to the glass support. The resulting films were stripped off the plates by immersion in water followed by drying in oven at 100  C. These films had the thickness of approx. 30 mm and were used for comparison. Very thin coatings having the thicknesses of about 0.1 mm were deposited by drop-casting technique on glass or quartz substrates by using diluted polymer solutions in NMP (0.5% concentration). These films, as-deposited, were gradually heated from room temperature up to 180  C and kept at 180  C for 1 h to remove the residual solvent. 2.5. Measurements The infrared (FTIR) spectra were recorded on FT-IR Bruker Vertex 70 Spectrophotometer in reflexion mode, by using freestanding films. Average-molecular weights were measured by means of gel permeation chromatography (GPC) using a Waters GPC apparatus, provided with Refraction and UV Photodiode array detectors and Shodex column. Measurements were carried out with polymer solutions having 2% concentration, and by using DMF/0.1 mol NaNO3 as solvent and eluent, with a rate of 0.6 mL/min. Polystyrene standards of known molecular weight in solution of DMF/0.1 mol NaNO3 were used for calibration. The thermal stability of the polymer blends was investigated by thermogravimetric analysis (TGA) using an STA 449F1 Jupiter derivatograph (Netzsch), operating at a heating rate of 10  C/min, in nitrogen atmosphere, from room temperature to 700  C. The 5% weight loss on the TG curve was considered to be the beginning of decomposition or the initial decomposition temperature (IDT). The temperature of maximum rate of decomposition which is the maximum signal in differential thermogravimetry (DTG) curves was also recorded. The glass transition temperature (Tg) of the polymer blends was determined by using a Pyris Diamond DSC Perkin Elmer calorimeter. Small film samples of each polymer were crimped in aluminium pans and run in nitrogen with a heat-cool-heat profile from room temperature to 350  C at 10  C/min. The mid-point temperature of the change in slope of the DSC signal of the second heating cycle was used to determine the glass transition temperature values of the polymers. The optical transmission and photoluminescence spectra were registered with a Specord M42 apparatus and Perkin Elmer LS 55

apparatus, respectively, by using as-prepared free standing films or thin polyimide blends deposited on quartz plates. The quality and morphology of the polymer blends was investigated by atomic force microscopy (AFM) using a Scanning Probe Microscopy Solver PRO-M, NT-MDT equipment made in Russia, in semi-contact mode, semi-contact topography technique. Polarized light microscopy was performed on blend films using an Olympus BH-2 microscope under cross polarizers with a THMS 600/HSF9I hot stage. The tensile properties were recorded on a Shimadzu AGS-J deformation apparatus at ambient temperature at a rate of deformation of 1 mm/min with a load cell capable of measuring forces up to 1 kN and a sample film of 25 mm  5 mm. For each data point, three samples were tested and the average value was used. Wetting properties were studied by measuring the static contact angles of water and ethylene glycol sessile droplets deposited onto the film surface. Deionised water and ethylene glycol microdrops with an average of 1e2 mL were deposited on the samples surface with a microliter syringe. The images of the drops were recorded by a video camera. The experiments were carried out by using a CAM 101 Optical Video Contact Angle System from KSV Instruments LTD, Finland, in open-air atmosphere and room temperature. Contact angles were calculated as average values over a large number of measurements (typically 10). The measurements were repeated three times on different parts of the film samples. Temperature and moisture were constant during the experiment (23  C and 68% respectively). Ethylene glycol and deionised water were used as solvents for calculation of the surface free energy of the polymer films. 3. Results and discussion 3.1. Miscibility and thermal behaviour of fluorinated polyimide blends Polyimides P1eP4 were firstly mixed with P5 or P6, based on the similarity of the anhydride monomer units, in 1:1 mass ratio. The blends of P3 and P4 with P6 (B1 and B2, respectively) were miscible and revealed single glass transition temperature (Tg), whereas the blends P1:P5 1:1 and P2:P5 1:1 were immiscible regardless of the variation in the preparation method. These immiscible blends were translucent, and their DSC thermograms showed two glass transition temperatures (Tg), indicating immiscibility at the molecular level. Because of the immiscibility of these blends, only B1 and B2 blends were used for further studies, and new blends, P1:P5 (B3), P2:P5 (B4), P3:P6 (B5) and P4:P6 (B6), in 1:3 mass ratio, were prepared. Fig. 1 shows the DSC thermograms of

Fig. 1. DSC thermograms of the polyimide blend P1:P5 1:3 (B3), neat polymer film P5 and of immiscible polymers P1:P5 1:1.

M.-D. Damaceanu et al. / Polymer 55 (2014) 4488e4497 Table 2 Thermal properties of fluorinated polyimide blend films B1eB6. Material

Tg ( C)

IDT ( C)

T10% ( C)

Tmax ( C)

W700 (%)

B1 B2 B3

214 194 236

59 56 56

250 220 199 297 229

515 526 435 511a 527 534 537 544 545

554 553 548

B4 B5 B6 P5 P6

467 494 360 430a 488 499 508 523 527

551 550 553 553 553

57 61 62 64 61

Tg ¼ glass transition temperature. IDT ¼ temperature of 5% weight loss on the TG curve. T10% ¼ temperature of 10% weight loss on the TG curve. Tmax ¼ temperature of maximum rate of decomposition. W700 ¼ char yield at 700  C. a Without accounting the residual solvent.

the polyimide blend P1:P5 1:3 (B3), neat polymer film P5 and of immiscible polymers P1:P5 1:1. The Tg values of polyimide blends B1eB6 are presented in Table 2; they are between those of the neat polymers, except for B3 whose Tg is lower than that of the neat polymers. It appears that a small percent (approximately 4.8%) of the residual N-methylpyrrolidone (NMP) solvent is present in this blend film (as was observed in TG analysis and will be discuss later), being retained in the virtue of H-bonding by polar groups of the polymer structural units. Thus, NMP molecules have a plasticizing effect upon the blend film, reducing its Tg value. Only one Tg value was found for each blend, indicating that the polyimide blends B1eB6 were homogeneous. During B1, B2, B5 and B6 film preparation, crystallization of P3 and P4 components from the amorphous state took place, as was indicated by polarized light microscopy (PLM) images and will be discussed in Section 3.2. It was previously shown that in the case of amorphous pellets and film of aromatic polyimides, crystallinity can be developed by cold crystallization or can be induced by solvent exposure from the rubbery amorphous state [37]. The orientation introduced during film processing results in an alignment of amorphous segments along the molecular axis in the plane of the film. Since the glass transition temperature of crystallizable polymers P3 and P4 are of 211  C and 142  C, respectively, and the blend film preparation was performed at temperature no higher than 180  C, it is reasonable to assume that solvent-induced crystallization occurred in all blend films, whereas in the case of B2 and B6 (based on P4) the development of the cold crystallization phenomenon can be also taken into consideration. This assertion is

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supported by a higher crystallinity observed for B2 and B6 blend films compared with B1 and B5 films. Thus, the DSC measurements were carried out on films of B1, B2, B5 and B6 frozen in crystallized state and only Tg was seen upon two heating scans up to 350  C with no other thermal transition associated with melting or recrystallization. Fig. 2 shows the thermogravimetric (TG) curves of the neat polyimides P5 and P6 and of their blends with P1, P2, P3 or P4 in different proportion, in form of films. As shown in Table 2, these polyimide blends have good thermal stability, as expected in case of fluorinated aromatic polyimides. The initial decomposition temperatures (IDT) of these blends were about 430e508  C, the temperature of 10% gravimetric loss (T10%), that is an important criterion for evaluation of thermal stability, ranged from 511 to 537  C, while the temperature corresponding to the maximum rate of decomposition was above 548  C, indicating a high thermal stability. For each sample, the degradation process is not complete, the char yields of those polymer blends at 700  C being higher than 56%. The thermogravimetric data shows a clear increase of thermal stability of the blends based on P6 with increasing the content of the most thermostable polymer (P6). The highest increasing rate was registered for the blends based on P3 which is less stable than P4 [33], while the crystallinity of the films does not appears as the main factor affecting the thermal stability. A perceptible percent (approximately 4.8%) of the residual NMP solvent is present only in B3 films, as evaluated from TG and DTG curves, although the formation of complexes between polar groups and NMP were observed in all blends in less than 1%. Moreover, an additional step of decomposition is assumed to take place at the piperazine moiety, a less thermally stable cycloaliphatic unit, similar to P1 component of B3 [33]. All the polyimide blend films show intermediate weight loss behaviour compared to the pure polyimides from their composition. The non-superposition of decomposition temperatures of neat and blended polyimides demonstrates ones more that the investigated blends are homogeneous. However, these new materials showed an important thermal stability increase, higher than 60  C, compared with that of neat polyimides P1eP4, demonstrating that blending is an useful tool for achieving high performance materials. The FTIR spectra of the polyimide blends B1eB6 provided evidence that mixing of the two components in those blends takes place at the molecular level, as listed in Table S2 and discussed in Supporting Information. It should be noted that the C]O imide stretching and bending bands of B1eB6 polyimide blends registered important frequency shifts compared to the neat polymer films P5 or P6. These frequency shifts were reported to be caused by intermolecular interactions between neighbouring polymer

Fig. 2. TG curves of the neat polyimides P5 (left) and P6 (right) and of their blends with P1eP4.

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Fig. 3. FTIR spectra of polymer blend B4 (P2:P5 1:3), neat polymer film P5 and of immiscible polymers P2:P5 1:1 in the range 1850e650 cm1.

chains, e.g. hydrogen bonding or due to the CTC formation, in which C]O bonds of imide rings are involved [24,38]. Further discussion on this topic is carried out in Section 3.4. Fig. 3 shows the FTIR spectra of polymer blend B4 (P2:P5 1:3), neat polymer film P5 and of immiscible polymers P2:P5 1:1 in the range 1850e650 cm1.

3.2. Morphology The morphology of fluorinated polyimide blend films B1 and B4 investigated by AFM technique are shown in Fig. 4 as representative examples. The compositional mapping of these polymer blends was investigated by phase contrast imaging that shows a contrast variation less than 10 , demonstrating no phase separation due to the good miscibility of two polyimide components at the molecular levels. On the other hand, the morphology of the B1eB6 blend films is strongly influenced by the nature of the components and their ability to form films of different morphology, when investigated by AFM or SEM. P1 and P2 are amorphous polymers and form porous polyimide films [33], P3 and P4 are crystallizable polymers and their films exhibited a morphology with vertically segregated structures, whereas P5 and P6 are amorphous polymers that form neat and very smooth films. Therefore, in the case of B1, B2, B5 and B6 we are discussing about a miscible blend of an amorphous, noncrystallizable polymer (P6) with a crystallizable polymer (P3 or P4); therefore, these blends are expected to exhibit a complex microstructure that is dependent upon blend composition. The binary blends B3 and B4 of the amorphous polymers (P1 or P2 with P5) still preserved the porous morphology observed for the neat polyimide films, in which the pores are sparsely (B3) or well (B4) dispersed in the amorphous mass of the polymer blends (Fig. 4). These spherical entities are non-uniform in size distribution and displayed an average diameter ranging from 0.1 up to 1 mm for B3 and from 50 to 100 nm for B4.

The presence of an amorphous polyimide in B1, B2 and B6 blends does not prevent the crystallization of P3 and P4 polymer components. When crystallization occurs, the amorphous polyimide P6 is rejected from the P3 or P4 crystalline domains, resulting in different complex morphologies. The three morphological features which are possible for miscible blends of crystallizable and noncrystallizable polymers are described as interlamellar segregation, interfibrillar segregation, and interspherulitic segregation [39]. From the three possible morphologies, the microscopy results showed for B2 and B6 containing the most crystallizable polymer, P4 (Fig. 5), an interlamellar segregation that occurs at both 50% and 25% P4 concentration. On the other hand, the interfibrillar morphology dominates for B1 blend having the crystallizable polyimide P3 in a concentration of 50%, whereas sparse crystallization was observed for B5 with 25% of P3, the high content (75%) of amorphous polyimide P6 practically preventing the crystallization of polyimide P3 (Fig. 5). The microscopy results were similar for both thin and thick films of B1, B2, B5 and B6, and no thickness-dependent crystallinity was observed. 3.3. Mechanical properties of polyimide blend films The free-standing polyimide blend films having the thickness in the range of 10e30 mm were flexible, tough, and maintained their integrity after repeated bending. The tensile properties of these materials are collected in Table 3. Elastic modulus, tensile strength and elongation to break have been determined as averages of three drawing experiments. All polymer blends showed similar type of behaviour with respect to the elastic deformation range at small strains. The values of tensile strength were in the range of 25e85 MPa, elastic modulus in the range of 0.92e2.18 GPa and tensile strain in the range of 2.19e5.75%. Some representative tensile tests of these polyimide blends in comparison with that of neat polyimide films are shown in Fig. 6.

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Fig. 4. AFM images of polyimide blend films B1 and B4 (a: 3D image, b: histogram, c: section).

By comparing the tensile properties of polyimide blends B1 and B5 with those of the neat polyimide film P6, we can assert that tensile stress and elastic modulus of polyimide blends increased with increasing of P3 content, being accomplished by a significant decrease of the tensile strain. In the case of polymer blends B2 and B6, the increase of the P4 content resulted in a higher elastic modulus, similar tensile stress and lower tensile strain. Thus, the introduction of crystallinity into our polymer blends resulting in more rigid films imparts between the improved solvent resistance

and increased modulus, the materials remaining stiff and tough, and the decrease of the elasticity. However, all the tensile data presented in Table 3 demonstrated that by blending polymers P5 or P6 with P1, P2 or P3, P4, respectively, films with satisfactory mechanical properties (toughness and modulus) were obtained. Accordingly, these new polyimide materials obtained by blending can be further used in applications in which the combination of the advantages of the two different materials' properties is required.

Fig. 5. Optical polarized light micrographs (40) of polyimide blends B1, B2, B5 and B6.

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Table 3 Tensile properties of films made from fluorinated polyimide blends and neat polymers. Polymer B1 B2 B3 B4 B5 B6 P5 P6

(P3:P6 (P4:P6 (P1:P5 (P2:P5 (P3:P6 (P4:P6

1:1) 1:1 1:3) 1:3) 1:3) 1:3)

Tensile stress (MPa)

Tensile strain (%)

Modulus (GPa)

85.75 25.12 32.43 44.73 52.03 27.6 63.82 69.38

3.42 1.67 3.42 2.19 5.75 3.9 9.64 12.09

2.08 1.96 0.92 2.18 1.02 1.17 0.97 1.25

3.4. Charge transfer complex (CTC) formation It is well proved that both intra- and interchain (CT) interactions do exist in wholly aromatic polyimides and they are key phenomena in controlling mechanical, thermal, photophysical and photooptical properties of these polymers. The aromatic polyimides exhibit broad and formless fluorescent emission spectra in the low energy range due to the intermolecular CTC, and therefore the CT fluorescence was applied as a powerful tool in studying the local ordering in polyimide films as well as the miscibility of polyimide blends [40]. The main characteristics of the existence of the chargetransfer fluorescence in polyimide films are the spectral red-shift and fluorescence yield decrease as the charge-transfer ability increases [9]. In order to survey the CTC ability formation in fluorinated polyimide blends B1eB6, the excitation and emission spectra were studied on thin blend films (coatings) deposited on quartz plates having the thicknesses of approximately 1 mm. All the blend films showed similar excitation and emission spectra, therefore the existence of CTC in the prepared blends are discussed in detail only for B2, as representative example. Fig. 7 shows the one-dimensional excitation and emission spectra of the fluorinated polyimide blend film B2 and of neat polyimide films P4 and P6. Those films (the neat polymers and the blend) showed apparent structureless fluorescence emissions in the visible region with multiple bands and excitation spectra with peaks in the region from 200 nm to 300 nm and above 300 nm. The large band centred at 337 nm in the excitation spectrum of P4, and at 397 nm, in the excitation spectra of P6 and B2 confirms the formation of the intermolecular charge-transfer complex in the

Fig. 6. Tensile tests of polyimide blends B1 and B5 and of neat polyimide film P6.

Fig. 7. Excitation and emission spectra of polymer blend film B2 and of neat polymer films P4 and P6.

neat and blended polyimides (CTC excitation band). The strong blue shift (aprox 60 nm) of the excitation band of P4 compared with that of P6 and B2 could be rationalized in terms of their structures. Firstly, the bulky CF3 substituents on the ortho-connected N-phenyl ring cause considerable distortion between the two molecular planes owing to steric hindrance and a decrease in conjugation. Consequently, structural relaxation of the CTC are allowed during the excited lifetime in P4, in comparison with P6 were the structural relaxation of the CTC seems to be more restricted and redshifted, although a very low intensity band at 337 nm could also be observed for P6. In the polyimide blend B2, both CTC bands are present at 337 and 397 nm, the first being weakened. This means that the population of the CTC coming from the CT interaction between the similar polymer P4 chains is hindered to some extent, thus proving the P4 and P6 miscibility at the molecular level. The lowest wavelength bands in the excitation spectra, at 232e239 nm, 252e258 nm and 281 nm were observed for both neat and blended polyimides and they were assigned to pep* benzenoid transitions. The existence of very weak intramolecular CT interaction should also be considered to exist in a lower extension tacking into account that overlapping of the bands based on local electronic transition and conjugations is possible. In order to determine more precisely the existence of CTC in polyimide blends B1eB6, the samples were excited at different wavelengths, located without or within of the CTC excitation band. Fig. 7 shows the fluorescence spectra of the blend film B2 and of corresponding neat polymer films P4 and P6 excited at the wavelength immediately before CTC band to occur, respectively 300 nm for P4 and 360 nm for B2 and P6. All polyimide materials showed an apparent strong and sharp fluorescent emission in the blue spectral region with one or two maxima of fluorescence at 420e423 nm and 439e443 nm, and one or more shoulders or weak bands at higher wavelengths. This main fluorescence emission originates from their locally excited (LE) states, in which the electrons excited at the dianhydride moieties were relaxed to a similar location in the ground state. The neat polymers P4 and P6 showed a shoulder at higher wavelength, 483e484 nm, whereas a clear peak appeared for the polyimide blend B2 film at this wavelength. This fluorescence originates most probably from excited intramolecular or intermolecular CT states generated by the energy transfer from the LE states [10]. The highest intensity of this emission in the case of blended polyimides compared with the neat polymers may account for an increased population of CTC in the blend film. Nevertheless, B2 film showed an additional weak fluorescence band at

M.-D. Damaceanu et al. / Polymer 55 (2014) 4488e4497

higher wavelength, centred at 528 nm, most probably due to the CT interactions between the two different polyimides in the blend. In order to elucidate the nature of CT interactions in the polyimide blend film B2, several wavelengths within the CTC excitation bands were involved in order to perform the emission spectra (Fig. 8). By excitation at 397 nm, corresponding to the maximum of CTC excitation band, the emission spectrum of B2 consists in two weak peaks located at 485 and 531 nm and a large weak tail centred at 575 nm. This emission generated directly from the excited CT state is characterized by electron transfer from the electron-donating diamine moiety to the electron-accepting dianhydride moiety. Usually, it is not easy to separate the intramolecular and intermolecular CT fluorescence, the first being much weaker than the second one. It was proposed that the intramolecular CT fluorescence is generated by coplanarization between the benzimide and the N-phenyl molecular planes via intermolecular conformational rearrangement, whereas intermolecular CTC formation is promoted by denser molecular aggregation through dense chain packing, and therefore may cause the spectral red shift of absorption and fluorescence bands by reduction of the donoreacceptor distance [9,41]. Accordingly, we may attribute the weak peak from 531 nm to intramolecular CT interactions in the blended polyimides (film B1), being proved that the intramolecular CT can take place at the excited state even in the significantly distorted conformations [42]. This is supported by no spectral change in shape and position of the fluorescence CT bands with increasing of the excitation wavelength (Fig. 8). Instead, the large but very weak band centred at the highest wavelength should be reasonable assumed to the intermolecular CTC between the two components of the blend. Being known that long-wavelength excitation originates in the intermolecular CTC, we performed the CT fluorescence spectra of B1 blend by excitation at 420 nm and 470 nm. It is clearly observed a red shift of the intermolecular CT band from 575 nm (when excitation was made with 397 nm) at 597 nm and 631 nm, when the B1 film was excited at 420 and 470 nm, respectively. Moreover this red-shift was accomplished by an increase of the intermolecular CT band intensity proving the increase of intermolecular CTC population with increase of the excitation wavelength, thus dense chain packing between the two polyimide components of the blend. These results lead us to conclude that both intramolecular and intermolecular CT interactions in solid state contribute to the good miscibility of P4 and P6 to form the B2 blend that is sensitive to CTC fluorescence intensity, shape and position.

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Fig. 9. Optical transmission spectra of the polyimide blend films with a thickness of z1 mm (the inset shows the photographs of free-standing blend films).

3.5. Optical transparency Optical transparent polyimide films are of special importance for opto-electronic devices such as optical half-waveplates for planar lightwave circuits, optical waveguides for communication interconnects or transparent flexible substrates in displays. Since the introduction of fluorine groups into the polyimide main chains is known to reduce the colour and increase the transparency of the films made therefrom, we have made a study on the optical transmission of the free-standing films and very thin coatings prepared from fluorinated polyimide blend films B1eB6. The B1eB6 free-standing films (about 10e30 mm thickness) are transparent and light coloured, except for B1 that is deep reddish yellow (Fig. 9). Table 4 summarizes the cut-off wavelength values (l0) estimated by the point where the optical transmission curve intersects a bisected line drawn through the intersection of the extrapolations of the two slopes. Their cut-off wavelength (l0) was in the range of 361e488 nm, and the transparency at 500 nm (T500) ranged between 5% and 70%. As expected, the transparency of the polyimide blend freestanding films was lower than that of the neat polymer films, due to their much more ability to form intermolecular charge transfer complex (CTC) than the neat aromatic polyimides. On the other hand, it was shown that the colour of the polyimide films, consequently their optical transparency, is affected not only by CT

Table 4 Optical properties of neat polyimides P1eP4 and their blends B1eB6. Polymer material

Fig. 8. Emission spectra of blend film B2 obtained at various wavelengths within the CTC excitation band.

B1 B2 B3 B4 B5 B6 P1 P2 P3 P4 P5 P6

(P3:P6 (P4:P6 (P1:P5 (P2:P5 (P3:P6 (P4:P6

1:1) 1:1) 1:3) 1:3) 1:3) 1:3)

Thick films (z10 e30 mm)

Thin coatings (z1 mm)

T500 (%)

l0 (nm)

T500 (%)

l0 (nm)

5 55 70 31 46 66 e e e e 75 78

488 371 361 412 389 371 e e e e 367 387

84 78 89 87 75 84 92 91 83 81 91 90

316 348 311 296 353 323 284 293 342 308 303 313

T500 e optical transparency at 500 nm; l0 e cut-off wavelength.

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interactions which are dictated by the polymer backbone structures, but is also very sensitive to processing conditions like monomer purity, kind of solvents, film thickness and cure conditions (temperature and atmosphere) [9]. Moreover, the use of NMP as solvent for polyimide synthesis and film preparation results in more intensely coloured films due to the traces of oxidized residual solvents. Since the variation in the thicknesses of B1eB6 blend free-standing films is between 1 and 20 mm, we have performed a study of the optical transparency of the polyimide blend coatings and of corresponding neat polymer coatings on quartz plates, having similar thicknesses (z1 mm), which were prepared under identical conditions. All the polyimide blend coatings revealed low l0 and high optical transparency, with a percentage of transmittance higher than 75% at 500 nm. As shown in Table 4, the optical transmission spectra of polyimide films P1, P2, P5 and P6 show high transparency in the visible region, over 90%. The other two polyimide films P3 and P4 displayed slightly lower optical transparency, of 83% and, respectively, 81%, demonstrating their higher ability for CTC formation in solid state. Therefore, the blends B1, B2, B5 and B6 derived from P3 and P4 exhibited slightly lower transparency, in the range of 75e84%, as a result of intermolecular CT interactions between similar or different polyimide chains in the blends (Table 4), compared to B3 and B4 derived from P1 and P2 whose transparency values were measured as 89% and 87%, respectively. Fig. 9 shows the optical transmission spectra of the polyimide blend films with a thickness of ≈1 mm. Comparing the optical data of the polyimide blends and of corresponding neat polymers we cannot assert a specific trend in variation of the optical transparency, colour and l0 of polyimide blends B1eB6 tacking into account multiple factors affecting these properties, and especially different possibilities and complexity of CTC formation. However, the investigated blends revealed still low cut-off wavelengths and high optical transparency, with a percentage of transmittance between 75 and 89% at 500 nm, comparable with that of reported fluorinated polyimides [43], as to allow their use in optical applications. 3.6. Wetting properties of fluorinated polyimide blends Static contact angles of water and ethylene glycol to the polymer blend film surfaces were tested to get information with regard the hydrophobic properties of binary polymer systems B1eB6. The water contact angle values were measured in the range of 83.03e96.95 , whereas ethylene glycol contact angle values ranged between 61.89 and 72.07 (Table 5). As shown in Table 5, the polymer blends do not display significant difference between the values of the contact angle, except for B3. However, it could be observed that B3 and B4 blends showed the extreme values, the lowest one being of 83.03 and the highest one of 96.95 . The other blend films exhibited similar values of water contact angle, between 90.35 and 93.70 , with no obvious difference, regardless the variation in composition and chemical structure of the blend components. The small differences that exist are quite difficult to attribute only to the structural feature of each

Table 5 Wetting and roughness properties of polyimide blend films B1eB6. Blend

B1

B2

B3

B4

B5

B6

qw ( ) qEG ( )

93.70 65.52 36.55

90.97 64.61 93.3

83.03 61.89 10.98

96.95 72.07 22.15

91.81 66.21 1.17

90.35 69.19 40.43

RMS (nm)

qw e water contact angle; qEG e ethylene glycol contact angle; RMS e root mean square roughness.

blend component. Being known that the wettability behaviour of a surface depends on the surface topography (geometry and roughness) as well [44] the roughness factor should be considered in discussing these properties (Table 5). It was shown that the introduction of alicyclic piperazine units has an important impact on the decrease of hydrophobic character of those polyimides containing such units [33]. Therefore, it was expected that those blends containing the piperazine units in the diamine segment, respectively B1, B3 and B5, to exhibit lower water contact angle values as compared to their analogues B2, B4 and B6. This is the case only for B3 that has a lower contact angle value compared with B4, the difference of the film roughness being too small to produce an important hydrophobic effect. Instead, B1 blend containing 50% of P3 has a higher value compared with B5 which contains only 25% of P3. This could be ascribed to the surface topography differences of the two blend films. B1 film shows a much more rough surface (RMS ¼ 36.55 nm) compared to B5 (RMS ¼ 1.17 nm), due to a higher degree of packing in solid state and this aspect is directly reflected in the contact angle value. In the case of B2 and B6, which do not contain piperazine units, the hydrophobic character is practically the same, the difference in the roughness parameter being to low to produce a significant change in wettability properties of these blends. B2 and B6 films showed the highest RMS, being of 93.3 nm and, respectively, 40.43 nm, and do not contain piperazine units, therefore, it was expected to have higher values of contact angle compared with B1 and B5. Instead, they have lower water contact angle values because of a higher content of oxygen atoms per structural units of the P4 blend component compared with P3 blend component, which interact with water or ethylene glycol molecules forming H-bonds. In conclusion, we can assert that the competition between the surface chemical composition and the topography of the films surface leads to those different values of contact angles, without following a general rule. However, the chemical composition appears to be predominant. 4. Conclusions In this work, the synthesis and physico-chemical characterization of new fluorinated aromatic polyimide blends are reported. Various characterization techniques have been involved to demonstrate the achievement of homogeneous mixtures at the molecular level in these polyimide blends. The DSC thermograms revealed a single glass transition temperature reflecting a high miscibility between the two polymer components. The frequency shift of the imide characteristics bands in the blend films indicated a high level of interaction between polymer chains, mainly due to charge transfer complex (CTC) formation. Both intramolecular and intermolecular CT interactions in solid state contributed to the good miscibility of these fluorinated polyimide blends, these interactions being sensitive to CTC fluorescence intensity, shape and position. Various morphologies were found for these new blend films, being in strong correlation with those of neat polymer films. Two of the blend films were amorphous with spherical entities dispersed on the surfaces, whereas the others displayed a morphology composed of vertically segregated structures, with interlamellar or interfibrillar segregation. All polyimide blends showed high thermal stability, their decomposition starting above 467  C. The blend films showed satisfactory mechanical properties, with tensile strength values in the range of 25e85 MPa, elastic modulus in the range of 0.92e2.18 GPa and tensile strain in the range of 2.19e5.75%. The values of the water contact angle to polyimide blend film surfaces were in the range of 83.03e96.95 , demonstrating a hydrophobic character induced by the presence of a high content of fluorine atoms in the polymer component structures. Each blend film was transparent in the visual light range

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and revealed low cut-off wavelengths and high optical transparency, with a percentage of transmittance between 75 and 89% at 500 nm. Thus, the blending proved to be a useful tool in achieving new highly fluorinated polyimide materials based on non-film forming polymers with improved thermal and mechanical behaviour and satisfactory optical properties compared with raw materials. Nevertheless, the useful properties of non-film forming polymers as components of the envisaged polyimide blends can now be exploited in applications such as gas-separation membranes, interlayer dielectrics or transparent flexible substrates. Acknowledgements The funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 264115 e STREAM is acknowledged with great pleasure. Paper dedicated to the 65th anniversary of “Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, Iasi, Romania. Appendix A. Supplementary data Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.polymer.2014.06.089. References [1] Hergenrother PM. High Perform Polym 2003;15:34e5. [2] Liawa DJ, Wang KL, Huanga YC, Leec KR, Lai JY, Ha CS. Prog Polym Sci 2012;37: 907e74. [3] Feger C, Franke H. In: Ghosh MK, Mittal KL, editors. Polyimides: fundamentals and applications. New York: Marcel Dekker; 1996. pp. 759e814. [4] Maier G. Prog Polym Sci 2001;26:3e65. [5] Bruma M, Sava I, Hamciuc E, Hamciuc C, Damaceanu MD. Rom J Inf Sci Tech 2006;9:277e84. [6] Kraftschik B, Koros WJ, Johnson JR, Karvan O. J Membr Sci 2013;428:608e61. [7] Youn HC, Park HS, Song JS, Cho SH, Suh SJ. Microelectron Eng 2013;103: 156e9. [8] Yang H, Deng L, Guo W, Zhang F, Duan J, Tang H, et al. Opt Laser Technol 2013;54:413e8. [9] Hasegawa M, Horie K. Prog Polym Sci 2001;26:259e335.

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