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Comparison of the physical properties of different polyimide nanocomposite films containing organoclays varying in alkyl chain lengths

  • Seon Ju Lee , Moon Young Choi , Lee Ku Kwac , Hong Gun Kim and Jin-Hae Chang EMAIL logo
Published/Copyright: September 29, 2023
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REVIEWS ON ADVANCED MATERIALS SCIENCE
From the journal REVIEWS ON ADVANCED MATERIALS SCIENCE Volume 62 Issue 1

Abstract

Poly(amic acid) (PAA), a precursor of polyimide (PI), is synthesized by reacting dianhydride 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylic anhydride with diamine 3,3′-dihydroxybenzidine in N,N′-dimethylacetamide. Organoclays with different alkyl chain lengths were dispersed in PAA, and the weight percentages (wt%) of the organoclays varied. The PI hybrid films were prepared over multiple steps under heat treatment conditions. Bentonite (BTN) was used as the pristine clay, and octylamine (C8) and hexadecylamine (C16) were used to chemically modify the surface of BTN to obtain the desired organoclay samples. Organoclays C8-BTN and C16-BTN were dispersed in a PI matrix, and the organoclay content varied in the range of 1–9 wt%. The thermal, morphological, and optical properties of the PI hybrid films were investigated based on the organoclay content. Although the thermal stability of the PI hybrid film improved when a small amount of organoclay was added, it decreased when the nano-filler content exceeded a certain critical content. Specifically, in the hybrid containing C8-BTN, the critical content is 5 wt%, while in the hybrid with C16-BTN, the critical content is 7 wt%. In addition, the morphology of the clay dispersed in the matrix at the critical content showed the best dispersed phase. The physical properties (thermal characteristics, dispersibility, and optical transparency) of the PI hybrid film containing C16-BTN were better than those of the hybrid film containing C8-BTN. However, the thermal expansion of the C8-BTN hybrid was lower than that of the C16-BTN film at the same content.

Keywords: polyimide; nanocomposite film; organoclay

1 Introduction

Polyimide (PI), a super-engineering plastic, is a highly heat-resistant polymer that exhibits excellent mechanical properties and chemical resistance. PI is characterized by a low coefficient of thermal expansion (CTE) and a low dielectric constant [1,2]. It is easy to synthesize, can be used to make thin films, and does not require a crosslinking group for curing [3]. The unique insulation properties of the materials make them suitable candidates for the fabrication of electronic materials, such as flexible printed circuit boards. PI can also be used for the development of chip-on-film and tape-automated bonding technologies [4,5].

Aromatic PI is a polymer with low crystallinity or an amorphous structure. It presents a transparent and rigid chain structure. Aromatic PIs used in electronic materials exhibit excellent thermal stability, and these PIs are used to fabricate electronic structural materials and in the aerospace industry. Therefore, this material can be potentially used to fabricate flexible substrates and materials that can be used in high-temperature processes [6]. Because aromatic PIs are mainly developed for high heat resistance and strength, there are various imides, such as ether (–O–)-, amide (–CONH–)-, and sulfone (–SO2–), depending on the structure introduced [7,8,9]. Depending on the chemical structure, PIs can be used to fabricate a range of materials, such as films to molded parts on mechanical and electronic devices.

Extensive research has been carried out to improve the optical transparency of PIs by redesigning the monomer structure. One of the primary methods used to improve the optical transparency of PIs involves designing asymmetric and noncoplanar structures. To this end, highly electronegative fluorine atoms [10,11], asymmetric or rigid but noncoplanar segments [12,13], and alicyclic rings have been introduced into PIs [14,15]. These structural modifications (1) inhibit reactions between molecular chains, (2) prevent chain stacking, and (3) inhibit the formation process of charge transfer complexes (CTCs), resulting in colorless and transparent PI [7,8,16,17]. PIs composed of alicyclic monomers exhibit a higher degree of transparency and better solubility than aromatic PIs, as they are characterized by low polarity, low molecular density, and low CTC between chains. Although alicyclic PIs are characterized by high degrees of transparency, the temperatures at which they thermally decompose are lower than the temperatures at which commonly used PIs decompose. It is necessary to overcome this limitation by designing PIs of appropriate structures. The optical transparency and solubility of the compounds can be determined by introducing an appropriate amount of alicyclic units into the PI backbone to control the degree of interaction between the polymer chains [18,19].

PIs containing alicyclic monomers have attracted considerable attention as they can be potentially used to fabricate organic electroluminescent panels, flexible solar cell substrates, liquid crystal display substrates, low-dielectric materials, and light guides [20,21,22]. Commercial PIs containing alicyclic monomers are generally synthesized from a variety of alicyclic dianhydrides and aromatic diamines to achieve the desired properties. For example, alicyclic PIs developed using 1,2,3,4-cyclobutane tetracarboxylic dianhydride and 2,3,5-tricarboxycyclopentane acetic acid dianhydride have been commercialized as alignment layers for the fabrication of thin-film liquid-crystal displays [23]. Other colorless PI films synthesized using 1,2,4,5-cyclohexane tetracarboxylic dianhydride (CHDA) have also been successfully used to fabricate materials used in the fields of fiber-optic communication and solar energy applications [24]. The CTC effect decreased significantly, and the film transmittance attributable to the presence of a six-membered ring structure improved when CHDA was used. The T g value recorded for CHDA-based CPI was 350°C, and this could be attributed to chain stiffness.

It is well known that organoclay nano-fillers synthesized by chemically reacting clay with an organic modifier can be uniformly dispersed in a polymer matrix as nanometer (nm)-sized particles. Polymer nanocomposites obtained by uniformly dispersing nano-sized clay in a matrix polymer present good thermal stability, mechanical properties, and dimensional resistance that are better than those of composite materials obtained by blending composite materials. The heat deflection temperature of these materials is higher than the temperature of other composite materials fabricated from blended composite materials [25,26]. The common clay types include montmorillonite (MMT), bentonite (BTN), saponite, hectorite, and mica. BTN consists mostly of MMT (∼80%), whereas the rest consists of minerals such as mica, calcite, chlorite, illite, and quartz. BTN exhibits unique adsorption and catalytic properties because of its excellent dispersion ability. These characteristics make them suitable for use as fillers in ceramics, packaging materials, cosmetics, paints, and pharmaceuticals, and these act as effective nano-fillers for polymer matrices [27,28].

Matsumoto [22,29] have widely studied alicyclic polyimides. 4-(2,5-Dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylic anhydride (DTDA), known as tetralin-DA, has been used as a representative dianhydride monomer in the synthesis of soluble PI [18,30,31,32]. The asymmetric tetralin structure in DTDA increased the reactivity of the monomer, and this helped develop PIs with excellent physical properties. DTDA can be mass-produced at a low cost using maleic anhydride and styrene as starting materials under an oxygen-containing gas as a catalyst [33].

Poly(amic acid) PAA was synthesized using dianhydride DTDA and diamine 3,3′-dihydroxybenzidine (BZ-OH) as monomers. Organoclays of various contents were dispersed in the synthesized PAA, and then, PI hybrid films were prepared following a step-by-step heat treatment method. DTDA was used as the monomer to achieve excellent reactivity and increase the transparency of the synthesized PI films. In addition, BZ-OH was also used to increase the dispersibility of the clay in the PI matrix. PI containing –OH group in the main chain forms a hydrogen bond with hydrophilic BTN to increase the dispersibility of clay.

BTN was chemically modified with octylamine (C8) and hexadecylamine (C16) to synthesize two types of organoclays, C8-BTN and C16-BTN, respectively, to study the dependence of the physical properties of the materials on the alkyl chain lengths. In addition, the filler content of the polymer matrix was set in the range of 1–9 wt% to conduct a systematic study of the properties of the PI hybrids taking into account the organoclay content of the materials.

The alicyclic anhydride monomer DTDA was used to increase the colorless transparency of PI, and the amine monomer BZ-OH containing a –OH group was used to increase the dispersion and solubility of the hydrophilic clay in the PAA state. Due to this effect, it was possible to prepare a PI hybrid film in which a higher content of organic clay was dispersed. We fabricated a PI hybrid film following a solution intercalation method, and the influence of the type and amount of organoclay used on the thermal properties, morphology, and optical transparency of the hybrid film was investigated. The physical properties of the two types of organoclays differing in the length of the alkyl groups were also compared.

2 Experimental

2.1 Materials

DTDA and BZ-OH monomers were purchased from SEJIN-CI (Seoul, Korea). N,N′-Dimethylacetamide (DMAc) was obtained from Juncei (Tokyo, Japan) and used after completely removing moisture using molecular sieves (4 Å). C8, C16, and BTN were purchased from Sigma-Aldrich (Yongin, Korea). The cation exchange capacity of pristine BTN was 82.3 meq/100 g [25,27].

2.2 Synthesis of organoclay

C8-BTN and C16-BTN were synthesized by chemically modifying BTN with C8 and C16, respectively. Similar synthetic methods were followed to synthesize C8-BTN and C16-BTN. Hence, the process of preparation of C8-BTN was presented as an example. C8 (1.23 g; 9.51 × 10−3 mol) was added to a mixed solvent consisting of 100 mL of distilled water and 1.92 mL of concentrated HCl. The solution was stirred at 80°C for 1 h under an atmosphere of N2. Distilled water (100 mL) and 4.00 g of BTN were taken in a beaker, and the constituents were mixed at 80°C for 1 h. The resulting solution was added to the C8 solution and reacted at 80°C for 2 h. The white precipitate was filtered and washed with 100 mL of distilled water. The process was continued for 1 h at room temperature, and the separation process was repeated 2–3 times. The precipitate was washed with a mixture of ethanol and distilled water (ethanol:distilled water, 1:1, v/v) at room temperature for 0.5 h and filtered. The obtained solid product was dried under vacuum for 24 h at 25℃.

2.3 Synthesis of PI hybrid film

All the PI hybrid films (differing in the organoclay content) were fabricated following the same method. Hence, the film fabricated using 5 wt% C8-BTN was considered as a representative example. About 3.00 g (1.00 × 10−2 mol) was taken in a 100 mL three-necked flask under an atmosphere of nitrogen. The constituent was dissolved in 20 mL of DMAc, and the mixture was stirred at 25℃ for 0.5 h. BZ-OH (2.16 g; 1.00 × 10−2 mol)) was dissolved in 15 mL of DMAc, and the mixture was stirred at 25℃ for 0.5 h under an atmosphere of nitrogen. PAA was synthesized by mixing the aforementioned two solutions and reacting the components at 25℃ for 16 h.

C8-BTN (0.26 g) was dispersed in 20 mL of DMAc over a period of 3 h. This solution was added to the PAA solution under conditions of vigorous stirring. The mixing process was conducted at 25°C for 1 h, and the mixture was heated to 50°C over 2 h under vacuum for solvent removal. Thereafter, the reaction temperature was increased to 80°C for 1 h. The obtained PAA hybrid solution was poured onto a glass plate and subjected to conditions of stepwise heat treatment to obtain the PI hybrid films. The detailed heat treatment conditions are presented in Table 1. The overall chemical structure of the synthesized PI hybrid is shown in Scheme 1. After heat treatment, the hybrid films were cooled to room temperature and removed from the glass plate. The film was synthesized under the same conditions, and the thickness of the obtained hybrid film was maintained in the range of 32–40 μm, regardless of the concentration of the organoclay, to improve the reliability of the results.

Table 1

Heat treatment conditions for PI hybrid films

Samples Temperature (°C)/time (h)/pressure (Torr)
PAA 25/1/760 → 50/2/1 → 80/1/1
PI hybrid 110/0.5/1 → 140/0.5/1 → 170/0.5/1 → 195/0.8/1 → 220/0.8/1 → 235/2/1
Scheme 1 Synthetic route for the fabrication of the PI hybrid films.
Scheme 1

Synthetic route for the fabrication of the PI hybrid films.

2.4 Characterization

Fourier transform infrared (FT-IR) spectroscopy (PerkinElmer, L1600300, London, UK) technique was used to identify the functional groups present in the films and confirm the synthesis of the PI films. Nuclear magnetic resonance (NMR) spectra of the PI films were recorded using a Bruker 400 MHz Advance II NMR spectrometer (Bruker, Berlin, Germany). The Larmor frequency (ω o/2π) for 13C magic-angle spinning (MAS) NMR was 100.61 MHz. The MAS speeds for 13C were measured at 12 kHz to minimize the number of spinning sidebands, and the NMR peaks were calibrated with respect to the signals corresponding to tetramethylsilane (TMS).

The morphology of the clay dispersed in the PI matrix was observed using a wide-angle X-ray diffractometer (XRD) (Rigaku, SWXD/X-MAX/2000PC, Tokyo, Japan) equipped with a Cu-Kα (λ = 1.54 Å) target. Measurement coverage was performed at 2ϑ = 2°–15° at a scan rate of 2°·min−1. The state of the clay dispersed in the hybrid film was confirmed using the transmission electron microscopy (TEM, JEOL, JEM 2100, Tokyo, Japan) technique. Samples (thickness: 90 nm) were prepared using a microtome after curing the epoxy resin at 70°C for 24 h in vacuum. The acceleration voltage was set at 120 kV.

The thermal properties of the hybrid films were measured at 20°C·min−1 under N2 conditions using a differential scanning calorimeter (DSC, 2-00915, Delaware, USA) and a thermogravimetric analyzer (TGA, SDT 0650-0439, Delaware, USA). The CTE values were measured using a thermomechanical analyzer (TMA, SS6100, Tokyo, Japan). The CTE value was obtained by analyzing the results obtained under conditions of secondary heating, and the heating rate was maintained at 5°C·min−1 under an expansion force of 0.1 N.

The cut-off wavelength (λ o) was determined, the light transmittance in the visible region was studied using an ultraviolet-visible (UV-vis) spectrophotometer (Shimadzu, UV-3600, Tokyo, Japan), and the yellow index (YI) of the hybrid film was measured using a spectrophotometer (Konica Minolta, CM-3600d, Tokyo, Japan).

3 Results and discussion

3.1 FT-IR and 13C-NMR

Analysis of the results obtained using the FT-IR technique revealed the successful synthesis of PI from heat-treated PAA. The peaks corresponding to the PI unit are shown in Figure 1. The presence of the hydroxyl group of the diamine BZ-OH moiety was revealed by the peak at 3,250 cm−1. The peaks corresponding to asymmetric and symmetric stretching of C═O appeared at 1,775 and 1,695 cm−1, respectively. The peak at 1,392 cm−1 indicated the formation of the final imide, and this peak was the characteristic peak representing C–N–C stretching in imides [34].

Figure 1 FT-IR spectral profile recorded for PI.
Figure 1

FT-IR spectral profile recorded for PI.

The chemical structure of the synthesized PI was confirmed using the solid-state 13C MAS NMR spectroscopy technique. The 13C NMR spectra were recorded at room temperature, and the profiles are presented in Figure 2. Analysis of the NMR spectrum reveals that the 13C peaks corresponding to the alicyclic moieties (a–c) appeared at 21.28, 38.09, and 44.00 ppm. Peaks (d–h) corresponding to the benzene structure were observed at 118.98, 128.83, and 141.99 ppm. Peaks corresponding to the carbons of the phenolic and imide structures (i and j, respectively) were observed at 152.83 and 177.82 ppm, respectively, and the spinning sidebands of phenol were marked with asterisks (Figure 2) [35]. The 13C NMR chemical shifts shown in Figure 2 were consistent with the chemical structure of the synthesized PI, and the NMR and FT-IR results revealed the successful synthesis of the PIs.

Figure 2 13C-NMR spectra recorded for PI.
Figure 2

13C-NMR spectra recorded for PI.

3.2 XRD

Figure 3 presents the XRD patterns recorded for pristine clay BTN, organoclays C8-BTN and C16-BTN (synthesized following an ion exchange reaction), and PI hybrids varying in organoclay contents. The characteristic peak of pristine BTN appeared at d = 12.26 Å (2ϑ = 7.20°), and the formation of the C8-BTN substituted with C8 was confirmed by the presence of the characteristic peak at d = 12.61 Å (2ϑ = 7.00°). The characteristic peak of the C16-BTN system substituted with a long alkyl group (C16) appeared at d = 17.03 Å (2θ = 5.18°). The d value of the organoclay was greater than that of pristine clay. Organic groups such as C8 and C16 were introduced on the clay surface through an ion exchange reaction. The reaction proceeded in the presence of an alkyl group, and this not only increased the compatibility of the molecule with the polymer but also facilitated the insertion of the polymer chain by widening the interlayer distance associated with the clay samples. The length of the alkyl chain in C16-BTN is longer than that of the chain in C8-BTN [36,37,38]. The former is characterized by a wider interlayer distance than C8-BTN, and hence, it is expected to penetrate more easily into the system and provide better dispersion compared to C8-BTN.

Figure 3 XRD patterns recorded for PI and PI hybrid films containing different organoclays. (a) C8-BTN and (b) C16-BTN.
Figure 3

XRD patterns recorded for PI and PI hybrid films containing different organoclays. (a) C8-BTN and (b) C16-BTN.

The XRD patterns of PI hybrids dispersed with various C8-BTN contents (range: 0–9 wt%) were recorded (Figure 3a), and the analysis of the patterns revealed that a peak of low intensity appeared at d = 13.08 Å (2ϑ = 6.75°) when 3 wt% of C8-BTN was dispersed in the PI hybrids. The intensity of this diffraction peak increased by a small amount when the C8-BTN content increased to 9 wt%. It was observed that the extent of agglomeration increased with an increase in the clay content. In addition, a new crystal structure was formed when the clay content in the PI matrix increased, and it was confirmed that the layered clay did not completely exfoliate on the PI in this hybrid material [39,40]. The presence of characteristic peaks was not observed in the patterns recorded for the C16-BTN hybrid, even when 7 wt% of organoclay was dispersed in the PI systems. A peak of significantly low intensity appeared at d = 12.76 (2ϑ = 6.92°) when 9 wt% of clay was dispersed in the system. The results revealed that up to 7 wt% of C16-BTN was well dispersed in the PI matrix, and exfoliation could be observed under these conditions. The results confirmed that the alkyl chain in C16-BTN was longer than the alkyl chains in C8-BTN. The former was characterized by a wider interlayer spacing than the latter, which facilitated the dispersion of the units in the polymer chain.

Although the XRD technique can be used to measure the interlayer distance between clay layers accurately, this technique cannot be used to unambiguously determine the spatial distribution of the clay layers in the PI matrix. It is difficult to accurately study the dispersion of clays by solely analyzing the XRD peak intensities [41,42]. An electron microscope must be used to understand the dispersion behavior of clay units in the PI matrix, as the process of formation of nano-scale hybrids cannot be effectively described based on XRD results.

3.3 TEM

An electron microscope can be used to directly determine the degree of intercalation, exfoliation, or agglomeration of clay layers and complement the XRD results. Results obtained using the TEM technique validated the results obtained using the XRD technique. The interlayer distance between the clay units and the dispersion behavior of clay could also be studied using this technique. The TEM technique was used to study the dispersion behavior of clay in detail, and the direct observation of the interlayer structure provided a quantitative understanding of the dispersion properties of clay.

Figure 4 presents the results obtained when the PI system containing 3–7 wt% of C8-BTN was studied. The degree of magnification increased from left to right. The black line represents a 1 nm-thick clay layer with a plate-like structure, and the space between the black lines represents the PI that exists between the clay layers. Aggregates of dimensions <10 nm were formed when 3 wt% of organoclay was evenly dispersed (Figure 4a) in the PI matrix. When the organoclay content increased to 5 wt%, a higher amount of clay was found to be present in the system (Figure 4b). Under these conditions, excellent distribution of less than 20 nm thickness was maintained. However, when the C8-BTN content was further increased to 7 wt%, some clays were well dispersed in nano-size, but most of the clays were agglomerated with a large amount of approximately 100 nm (Figure 4c). The results indicated that the critical content for even dispersion of C8-BTN in the PI matrix in the nano-scale was 5 wt%, and agglomerates were formed when the content of the clay particles was higher than 5 wt%. Analysis of the XRD profiles and the presence of peaks of low intensity revealed that exfoliation was expected to occur when the content of organoclay in the sample was 7 wt% (Figure 3a). Analysis of the TEM images also revealed that some of the clay particles remained un-exfoliated, and this could be attributed to the formation of agglomerates. Several researchers have reported contradictory XRD and TEM results [41,43]. As stated previously, the TEM images provide a quantitative understanding of the nano-structure of the samples based on the orientation or dispersion of clay in the composite, and the results can be directly visualized. However, the XRD technique can be used to study diffracted forms that present the exfoliated states of the composites that have lost their periodic organic structure. This can be attributed to the fact that the XRD technique does not provide clear information about the structure of the materials.

Figure 4 TEM images of PI hybrid films containing different amounts of C8-BTN (images recorded under different magnification conditions).
Figure 4

TEM images of PI hybrid films containing different amounts of C8-BTN (images recorded under different magnification conditions).

Figure 5 presents the TEM images of the PI hybrid films containing varying amounts of C16-BTN. It was observed that organoclay samples were evenly and well dispersed in the samples when the organoclay content was 5 wt% (Figure 5a). In addition, even when the content of the C16-BTN increased to 7 wt%, the clay was observed with excellent dispersion of less than 10 nm (Figure 5b). It was also observed that nano-sized particles were formed when the amount of organoclay was 9 wt% and aggregates approximately 20 nm in size were formed under these conditions. The extent of agglomeration increased with an increase in the amount of clay dispersed, and the critical content of C16-BTN was determined to be 7 wt%.

Figure 5 TEM images of PI hybrid films containing varying amounts of C16-BTN (images recorded under different magnification conditions).
Figure 5

TEM images of PI hybrid films containing varying amounts of C16-BTN (images recorded under different magnification conditions).

The degree of dispersion realized in the C16-BTN hybrid was higher than that achieved in the C8-BTN hybrid. The critical clay content in the C8-BTN hybrid was 5 wt%, whereas the critical clay content in the C16-BTN hybrid was 7 wt%. This result could be explained by the fact that the interlayer distance in C16-BTN was higher than that in C8-BTN. Hence, it is easier to insert polymer chains and achieve a better extent of dispersion in the case of C16-BTN compared to the case of C8-BTN. This was also validated by the interlayer distance, determined using the XRD technique, between the organoclay layers (Figure 3). This characteristic can help improve the thermal and optical properties of the materials, and this has been described in the following sections.

3.4 Thermal property

The melting transition temperature (T m) of PIs cannot be recorded using the DSC technique as PIs are transparent amorphous polymers. Therefore, the thermal properties of PIs can be explained based on the glass transition temperature (T g) of the materials. T g is affected by the structure of the monomers constituting the polymer, type of substituent, free volume attributable to chain movement, secondary bonds such as hydrogen bonds, curing reactions, and additives dispersed in the composite material [44,45,46]. In addition, T g is significantly influenced by the stiffness and flexibility of the polymer chains and affected by the extent of chain interactions and changes in free volume.

Table 2 summarizes the thermal characteristics of the PI hybrid films based on the contents of the two types of organoclays. The T g values recorded before and after the dispersion of organoclays were compared, and the results revealed numerous changes. It was observed that when the C8-BTN content in the PI matrix was increased from 0 to 5 wt%, the T g increased from 235 to 268°C. This was the maximum increase (33℃) that was recorded when the studies were conducted at the critical content of organoclay. This could be attributed to the fact that the organoclay content increased when secondary bonds, such as hydrogen bonds, were formed between PI and clay. An increase in the clay content resulted in an increase in the degree of obstruction to the segmental movement of the polymer chain inserted into the clay systems. However, when the C8-BTN content in the PI matrix was increased from 5 to 9 wt%, the T g of the hybrid rapidly decreased by 35°C (to 233°C). The decrease in T g could be potentially attributed to the agglomeration of clay that occurs in the presence of an excess amount of organoclay in the polymer matrix. Several researchers have reported similar results [47,48].

Table 2

Thermal properties of PI hybrid films

Organoclay in PI (wt%) C8-BTN C16-BTN
T g (°C) T D ia (°C) wtR 600b (%) CTEc (ppm·°C−1) T g (°C) T D i (°C) wtR 600 (%) CTE (ppm·°C−1)
0 (pure PI) 235 253 52 38.1 235 253 52 38.1
1 240 264 52 35.3 242 281 53 38.2
3 262 286 52 32.3 252 294 58 37.5
5 268 296 57 31.2 269 305 60 37.1
7 256 258 57 33.4 285 313 60 37.2
9 233 254 60 35.0 263 233 59 37.6

aInitial decomposition temperature at 2% weight loss. bWeight residue at 600℃. cCoefficient of thermal expansion for 2nd heating (50–200℃).

The maximum T g was recorded under conditions of critical organoclay content in the case of the C16-BTN hybrid. It was observed that the T g value of the PI hybrid decreased with an increase in the clay content. For example, the maximum T g (285°C) was recorded when the organoclay content was 7 wt%, and the value decreased to 263°C when the C16-BTN content was 9 wt%.

A significant increase in T g was observed for the case of both the hybrid samples in the presence of a small amount of clay. This can be potentially attributed to the excellent dispersion achieved in the presence of intermolecular hydrogen bonds. The strong hydrophilicity of the diamine moiety BZ-OH, constituting PI, promoted the formation of the bonds between these units and the –OH group in clay BTN. It was observed that the overall T g value of the C16-BTN hybrid was higher than that recorded for the C8-BTN hybrid, and the critical organoclay content recorded for C16-BTN was also higher than that recorded for C8-BTN. This can be attributed to the fact that C16, which exhibits better thermal stability and consists of longer alkyl groups compared to C8 (Figure S1 and Table S1), opens the interlayer spaces between the clay layers more easily than C8, making it easier to insert the polymer chain into the hybrid. Figure 6 presents the DSC thermograms of the PI hybrids containing different types and varying amounts of organoclays.

Figure 6 DSC thermograms recorded for PI and PI hybrid films containing different organoclays. (a) C8-BTN and (b) C16-BTN.
Figure 6

DSC thermograms recorded for PI and PI hybrid films containing different organoclays. (a) C8-BTN and (b) C16-BTN.

When inorganic materials are mixed with organic materials, the thermal stabilities of hybrid materials increase. This can be attributed to the high thermal stability of inorganic materials. Table 2 presents the TGA results obtained by studying PI hybrids. It was observed that the results were influenced by the type and content of the two organoclays used. In the case of the C8-BTN hybrid, the initial decomposition temperature (T D i), after a 2% reduction, was recorded to be 253°C for pure PI. It was observed that when the content of C8-BTN was increased from 1 to 5 wt%, the T D i of the hybrid increased from 264 to 296°C. The increasing thermal stability achieved by mixing small amounts of clay into the systems has been previously reported by many researchers [44,47]. The increase in the stability can be attributed to the increase in the extent of bonding between the clay particles and the matrix polymer and the thermal stability of the clay samples. The increase in stability can also be attributed to the fact that evenly dispersed clay inhibits heat transfer and suppresses substances that are easily decomposed and volatilized [49,50]. It was observed that the T D i value decreased significantly to 254°C when 9 wt% clay was dispersed into the matrix. It was observed that the clay particles formed agglomerates when the clay content exceeded the critical content. This resulted in a deterioration in the insulating effect exerted by the clay layer on the polymer matrix. The deterioration in the insulating effect can be attributed to the low extent of dispersion of the clay layer in the matrix [48].

For both the C16-BTN and C8-BTN hybrids, the maximum T D i was recorded when the critical organoclay content was used in the matrix (Table 2). It was observed that the value decreased when the content was increased further. For example, when the C16-BTN content was 7 wt%, the T D i of the PI hybrid increased to 313°C, which was 60°C higher than the value recorded for pure PI. However, when the C16-BTN content was at 9 wt%, the T D i value decreased to 233°C.

The residual amount obtained after heating at 600°C (wtR 600) gradually increased from 52 to 60 % as the organoclay content was increased from 0 to 9 wt%. The increase was proportional to the clay content, and the content increased regardless of the type of organoclay used. The observations can be explained by the inherently high heat resistance of the clay samples that can be used to form tar (Table 2). Figure 7 presents the TGA thermograms recorded for various PI hybrids. It was observed that the nature of the thermograms was influenced by the type and content of organoclays.

Figure 7 TGA thermograms recorded for PI and PI hybrid films containing different organoclays. (a) C8-BTN and (b) C16-BTN.
Figure 7

TGA thermograms recorded for PI and PI hybrid films containing different organoclays. (a) C8-BTN and (b) C16-BTN.

When a polymer is heated, it relaxes in a direction perpendicular to the main chain. However, in the hybrids with strong and rigid plate-like clay layers, the polymer chains inserted between the clay layers did not readily expand upon heating. Therefore, heat transfer can be effectively blocked under these conditions. It was observed that small amounts of thermally stable clay could hinder the process of lateral thermal expansion of the polymer chains that proceeds under the influence of heat [51,52,53].

Table 2 summarizes the CTE values recorded for the two hybrid film series subjected to conditions of secondary heating. The CTE value recorded for the pure PI film was 38.1 ppm·°C−1, and the CTE value recorded for the C8-BTN hybrid was lower than that recorded for the pure film, irrespective of the filler content ranges. The minimum CTE was recorded for the C8-BTN hybrid when the critical clay content was used. Following this, the values increased with an increase in the filler content. For example, the CTE value was 35.3 ppm·°C−1 when the C8-BTN content was 1 wt%. The minimum value (31.2 ppm·°C−1) was recorded when the organoclay content was 5 wt%. However, when the C8-BTN content increased to 9 wt%, the value increased to 35.0 ppm·°C−1. CTE values in the range of 37.1–38.2 ppm·°C−1 were recorded for C16-BTN regardless of the filler content, and the suppression of the thermal expansion process was not observed under these conditions. This can be attributed to the fact that the long, flexible, and stretchable alkyl group (C16), substituted into the clay systems, offsets the effect of heat resistance. Figure 8 presents the TMA profiles recorded for various PI hybrids containing different types and contents of organoclays.

Figure 8 TMA thermograms recorded for PI and PI hybrid films containing different organoclays. (a) C8-BTN and (b) C16-BTN.
Figure 8

TMA thermograms recorded for PI and PI hybrid films containing different organoclays. (a) C8-BTN and (b) C16-BTN.

3.5 Optical transparency

The cut-off wavelength representing the initial transmission (λ o), the transmittance at a wavelength of 500 nm (500 nmtrans), and the YI corresponding to the PI hybrid films were studied to understand the optical properties of the materials.

YI can be obtained using the following formula (ASTM E313-96, DIN 6167):

Y I = 100 × ( a X − b Z ) / Y ,

where a and b values are red and yellow, respectively, and X, Y, and Z values correspond to tristimulus values [54]. However, the value of YI can be easily obtained using a spectrophotometer.

The UV-vis profiles recorded for the films are shown in Figure 9, and the results are summarized in Table 3. For the case of the PI hybrid film containing C8-BTN, the λ o value increased steadily from 331 to 345 nm when the content of C8-BTN in the hybrid increased from 0 to 9 wt%. When the filler content was increased from 0 to 9 wt%, the 500 nmtrans values corresponding to the hybrid films decreased rapidly from 83 to 31%, and the YI linearly increased from 4 to 25 under these conditions. The optical transmittance of the hybrid film fabricated using C16-BTN was not significantly different from that recorded for the C8-BTN hybrid film. As the content of C16-BTN increased from 0 to 9 wt%, the λ o value gradually increased from 331 to 341 nm. However, the 500 nmtrans values decreased from 83 to 45%, and the YI value increased from 4 to 22. The optical properties of the C16-BTN hybrid film (λ o, 500 nmtrans, and YI) were better than those of the C8-BTN hybrid film as the dispersion state of clay was better in the C16-BTN hybrid than in the C8-BTN hybrid. The dispersion state of clay was studied using the TEM technique (Figures 4 and 5).

Figure 9 UV-vis transmittance profiles of PI and PI hybrid films containing different organoclays. (a) C8-BTN and (b) C16-BTN.
Figure 9

UV-vis transmittance profiles of PI and PI hybrid films containing different organoclays. (a) C8-BTN and (b) C16-BTN.

Table 3

Optical properties of PI hybrid films

Organoclay in PI (wt%) C8-BTN C16-BTN
Thicknessa (μm) λ 0 b (nm) 500 nmtrans (%) YIc Thickness (μm) λ 0 (nm) 500 nmtrans (%) YI
0 (pure PI) 37 331 83 4 37 331 83 4
1 33 335 73 11 36 334 77 10
3 32 338 54 17 36 336 70 15
5 32 340 48 19 38 338 64 17
7 34 342 35 24 40 339 52 20
9 40 345 31 25 40 341 45 22

  1. aFilm thickness. bCut-off wavelength. cYellow index.

Images of the film obtained based on the logo are presented in Figures 10 and 11, and the images help confirm the optical transparency of the developed PI hybrid film. Pure PI appeared colorless and transparent. However, as the C8-BTN content increased, the intensity of the color of the hybrid film gradually increased, making it difficult to visualize logos or characters through the films. Similar results were observed for both series. The changes in color occurring with an increase in the clay content could be visualized through the films, as the pure PI films were not perfect colorless or transparent. The C16-BTN hybrid film appeared brighter than the C8-BTN hybrid film when visualized with the naked eye, even when the same organoclay content was present in both. This was confirmed by the YI results. It was observed that the transparency decreased significantly as the organoclay content increased. However, the YI values recorded for the two series of fillers were significantly lower than the values recorded for the commercially available Kapton® film (YI = 96) [10,55].

Figure 10 Photographs of PI and PI hybrid films containing varying amounts of organoclay. (a) 0 (pure PI), (b) 1, (c) 3, (d) 5, (e) 7, and (f) 9 wt% C8-BTN.
Figure 10

Photographs of PI and PI hybrid films containing varying amounts of organoclay. (a) 0 (pure PI), (b) 1, (c) 3, (d) 5, (e) 7, and (f) 9 wt% C8-BTN.

Figure 11 Photographs of PI and PI hybrid films containing varying amounts of organoclay. (a) 0 (pure PI), (b) 1, (c) 3, (d) 5, (e) 7, and (f) 9 wt% C16-BTN.
Figure 11

Photographs of PI and PI hybrid films containing varying amounts of organoclay. (a) 0 (pure PI), (b) 1, (c) 3, (d) 5, (e) 7, and (f) 9 wt% C16-BTN.

4 Conclusions

The effects of two types of organoclays on the physical properties of PI hybrid films were investigated. DTDA presenting an alicyclic moiety and BZ-OH, containing a hydroxyl group, were used as the monomers to synthesize the PI systems. C8-BTN and C16-BTN, where BTN is appended with octylamine (C6) and hexadecylamine (C16), respectively, were used as organoclays for hybridization.

The T g, T D i, and optical transparency of the C16-BTN hybrid were higher than those of C8-BTN. This can be explained by the fact that the distance between the clay layers in the sample containing C16 is wider than the distance between the layers in the sample containing C8, and this can be attributed to the length of the alkyl chains. The longer the chain, the better the extent of insertion of the polymer into the clay layer, and the better the effective dispersion. It was observed that the CTE of the C16-BTN hybrid containing a long alkyl group that could be readily expanded under thermal conditions was higher than the CTE recorded for the C8-BTN hybrid substituted with a short alkyl group.

PI is a high-performance material that can be used under extreme conditions to fabricate various materials, as PI exhibits excellent heat, thermodynamic, and chemical resistance. PIs with excellent physicochemical properties can be synthesized by varying the monomers and optimizing the reaction conditions. Hybrid materials in which fillers are uniformly dispersed at the nanoscale in the PI matrix have been developed using fillers designed to impart excellent dispersibility and excellent interfacial adhesion properties. These materials can be used to develop new super-engineered PI materials with excellent physical properties that cannot be developed following conventional manufacturing processes.

Acknowledgments

The authors acknowledge the support by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03012069), and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2022R1A2C1009863).

  1. Funding information: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03012069). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2022R1A2C1009863).

  2. Author contributions: J.-H. Chang designed the project and wrote the manuscript. L.K. Kwac and H.G. Kim reviewed and analyzed data. S.J. Lee and M.Y. Choi and prepared the samples and participated in the data analysis. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Conflict of interest: The authors state no conflict of interest.

  4. Data availability statement: All data generated or analyzed during this study are included in this published article.

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Received: 2023-06-10
Revised: 2023-07-07
Accepted: 2023-09-04
Published Online: 2023-09-29

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  94. Thermosetting polymer composites: Manufacturing and properties study
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https://doi.org/10.1515/rams-2023-0120
Keywords for this article
polyimide; nanocomposite film; organoclay
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BY 4.0

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