Organoclay reinforced polyimide hybrid films based on hexadecyl modified montmorillonite and synthetic mica

Abstract

Poly(amic acid) (PAA) was synthesized via the polycondensation of 4,4′-oxydiphthalic anhydride and 3,3′-dihydroxybenzidine, followed by the incorporation of organoclays and subsequent multistep thermal imidization to fabricate polyimide (PI) hybrid films. Montmorillonite (MMT) and synthetic mica (Mica) were employed as clay fillers and chemically modified with hexadecylamine (C16) to obtain C16-MMT and C16-Mica, respectively. The organoclays were incorporated into the PI matrix at loadings ranging from 0.5 to 3.0 wt%, and the effects of clay type and content on thermomechanical properties, dispersion behavior, and optical transparency were systematically investigated. The incorporation of a small amount of organoclay significantly enhanced the thermomechanical performance of the PI hybrid films; however, exceeding a critical filler content resulted in property deterioration due to filler agglomeration. Among the hybrids, PI films containing C16-Mica exhibited superior thermomechanical properties compared to those containing C16-MMT, which is attributed to the higher aspect ratio and rigid layered structure of Mica. In addition, the C16-Mica hybrids showed slightly higher optical transparency, although the difference was marginal. Transmission electron microscopy analysis confirmed uniform nanoscale dispersion at low filler contents, while partial agglomeration was observed beyond the critical loading, correlating well with the observed decline in hybrid film properties.

Introduction

Recent research on polyimides (PIs) has focused on the development of nanocomposite materials by incorporating nanofillers such as graphene, clay, and carbon nanotubes to enhance their thermal and mechanical properties^1–3. These efforts aim to maintain excellent thermal stability, radiation resistance, and electrical insulation while achieving functional PIs suitable for aerospace and electronic applications.

Nanocomposites consist of ultrafine inorganic particles with sizes on the order of nanometers that are homogeneously dispersed in a polymer matrix⁴. Nanocomposites possess properties that are superior to those of conventional composites because of their superior interfacial adhesion. Several methods^5–7 have been used to obtain nanocomposites of polymers with organoclays, specifically solution intercalation, melt intercalation, and in-situ interlayer intercalation. Among them, the solution intercalation method is a widely used technique for fabricating polymer nanocomposites, offering effective dispersion of nanofillers such as clay and graphene within the polymer matrix. In this method, polymers dissolved in a solvent interact readily with layered fillers, facilitating intercalation or exfoliation and resulting in improved dispersion and enhanced material properties^5,6. The process operates at relatively low temperatures and allows easy control of composition and filler content. However, the use of organic solvents poses environmental and health concerns, and residual solvent removal can be problematic. Additionally, poor compatibility between polymer and filler may cause agglomeration, negatively affecting composite performance. Thus, careful material selection and process optimization are essential⁸.

Clays have sandwich-type structures that typically consist of one octahedral Al sheet and tetrahedral Si sheets, the so-called philo-silicate. There are many types of philo-silicates: kaolinite, montmorillonite (MMT), hectrite, saponite, synthetic mica (Mica), etc^9–11. In this study, MMT and Mica were employed as clay fillers in the synthesis of clay/polymer nanocomposites. MMT and Mica have platelet-like structures composed of stacked silicate sheets and share the same thickness of approximately 1 nm; however, Mica exhibits a significantly larger lateral dimension (≈1,230 nm) than MMT (≈218 nm), resulting in a higher length-to-width ratio (aspect ratio)^10,12. This difference in aspect ratio has a substantial influence on the properties of the dispersed hybrid films, depending on the type and loading of the clay discussed in the following sections. MMT has a high swelling capacity, which is essential for efficient intercalation of the polymer, and is composed of stacked silicate sheets that provide good thermal, tensile, and molecular barrier properties, as do its hybrid materials with polymers. Mica is a different kind of clay that has been widely used as a reinforcing filler in polymeric matrices because of its excellent mechanical, electrical and thermal properties. Pristine Mica-type layered silicates usually contain hydrated Na⁺ or K⁺. Micas also have the highest layer charge density among the clays¹². Although naturally occurring Micas are non-swelling, synthetic Micas which swell in water can be prepared by hydrothermal, solid-state, and topotactic exchange methods^13–15.

In this study, we used 4,4’-oxidiphthalic anhydride (ODPA) as a dianhydride and 3,3'-dihydroxy-benzidine (HBZ) as a diamine to synthesize a poly(amic acid) (PAA), and thermal imidization was performed with dispersed nanofillers to synthesize PI hybrid films. ODPA is a widely used dianhydride monomer in the synthesis of PIs, known for enhancing both solubility and optical transparency. Due to these advantageous properties, ODPA-based PIs exhibit high optical clarity, and excellent thermal and mechanical stability. Therefore, they are highly suitable for advanced applications such as flexible displays, insulating layers in semiconductor processes, optoelectronic materials, and solar cell substrates, where high-performance and multifunctional properties are required^16–18. Meanwhile, HBZ has a biphenyl structure in which two benzene rings are connected, and each ring has a hydroxyl group (–OH) substituted at the 3,3'-positions, forming a symmetric molecule. The –OH groups can form intermolecular hydrogen bonds, which enhance interactions between polymer chains. This structural feature can improve the mechanical and optical properties of polymers, making it suitable for the synthesis of high-performance PIs^19,20. To prevent CTC formation and increase the optical transparency, ODPA and HBZ were used to fabricate an overall curved PI structure.

To fabricate PI-based nanocomposites, hexadecylamine (C16) was reacted with MMT and Mica, respectively, to synthesize hexadecylamine-MMT (C16-MMT) and hexadecylamine-Mica (C16-Mica) nanofillers. By using an alkyl organic modifier of the same chain length, the effects of MMT and Mica on the PI hybrid films could be compared under identical filler loadings, despite the difference in clay type.

The purpose of this study is to evaluate the dispersion effect according to the two nanofillers used in the PI hybrid and the various contents. A method to manufacture a PI hybrid film through solution intercalation is described, and thermomechanical properties, optical transparency, and clay morphologies of the PI hybrid films are explored.

Experimental

Materials

Kunipia-F (Na⁺-MMT) was supplied by Kunimine Co. (Tokyo, Japan). The raw Na⁺-MMT was passed through a 325-mesh sieve to remove impurities, yielding a purified clay with a cation exchange capacity of 119 meq/100 g^10,12. Sodium-type fluorinated synthetic mica (Na⁺-Mica), in which the free –OH groups on the Mica surface were replaced by fluorine, was obtained from CO-OP Ltd. (Tokyo, Japan), with a cation exchange capacity in the range of 70–80 meq/100 g^10,12.

All reagents were purchased from TCI (Tokyo, Japan) and Aldrich Chemical Co. (Yongin, Korea). Commercial solvents were purified by distillation, while other common reagents were used as received without additional purification.

Synthesis of C16-MMT

As the synthesis methods for the organo-modified clays C16-MMT and C16-Mica are identical²¹, only the synthesis procedure of C16-MMT is described herein. C16 (11.47 g) was dissolved in 100 mL of H₂O containing 4.8 mL of HCl and stirred under a nitrogen atmosphere at 80 °C for 1 h. Separately, Na⁺-MMT (20 g) was dispersed in 400 mL of H₂O and maintained at 80 °C for 1 h. The two suspensions were then combined and stirred vigorously at 80 °C for an additional 2 h. The resulting white precipitate was collected by reduced-pressure filtration, washed with a 1:1 (v/v) mixture of distilled water and ethanol, and subsequently freeze-dried under vacuum at room temperature for 24 h. The process yielded approximately 93% of the desired white powder.

Synthesis of PI hybrid films

The purity, molar ratio, concentration, and solvent of the PAA precursor are critical parameters influencing the properties of the resulting PI films. To prepare the PAA, 4.03 g (1.3 × 10⁻² mol) of ODPA was dissolved in 40 mL of N,N'-dimethylacetamide (DMAc) under a nitrogen atmosphere and stirred at 25 °C for 30 min. A separate solution containing 2.81 g (1.3 × 10⁻² mol) of HBZ in 40 mL of DMAc was then added to the ODPA solution, and the mixture was stirred at room temperature for 16 h.

In most polymer nanocomposites, the clay loading is typically limited to below 5 wt%. This is because polymer–clay nanocomposites generally exhibit effective reinforcement and good dispersion at low filler concentrations, whereas higher filler loadings tend to induce filler aggregation, which can lead to a deterioration of optical and mechanical properties. Therefore, in this work, the nanofiller loading was controlled in the range of 0–3.0 wt% for the preparation of the hybrid films. As the same general procedure was applied for all PI hybrid films, the synthesis of a PI hybrid containing 1.0 wt% C16-MMT is presented here as a representative example. Specifically, 6 × 10⁻² g of C16-MMT was dispersed in 70 mL of DMAc via sonication at room temperature for 3 h, followed by mixing with the prepared PAA solution and sonication for an additional 1.5 h. The resulting PAA hybrid solution was cast onto a glass plate and subjected to vacuum drying at 50 °C for 2 h and at 80 °C for 1 h to remove residual solvent. Thermal imidization was subsequently carried out at various temperatures and durations to obtain the PI hybrid films. The detailed heat-treatment conditions are summarized in Table 1, and the overall chemical structure along with the reaction pathway for PI hybrid synthesis is illustrated in Scheme 1.

Table 1.

Heat treatment conditions of PI hybrid films.

Samples	Temperature (oC)/Time (Hr)/Pressure (Torr)
PAA	25/14/760 → 50/0.5/1 → 80/1/1
PI Hybrid	110/0.5/1 → 140/0.5/1 → 170/0.5/1 → 195/1/1 → 220/1/1 → 235/2/1

Scheme 1.

Synthetic rout for the fabrication of PI hybrid films.

Characterization

Fourier transform infrared (FT–IR) spectroscopy (Spectrum Two, PerkinElmer, Llantrisant, UK) was employed to verify the successful synthesis of the PI. Solid-state ¹³C cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopy (Bruker 400 DSX, Berlin, Germany) was performed at room temperature with a Larmor frequency of 100.61 MHz.

The interlayer spacing of the nanofillers within the PI matrix was determined using wide-angle X-ray diffraction (WAXD, Rigaku SWXD/X-MAX/2000PC, Tokyo, Japan) with Cu-Kα radiation (λ = 1.5406 Å) over a 2θ range of 3°–15° at a scan rate of 2°/min. Transmission electron microscopy (TEM, JEOL JEM-2100, Tokyo, Japan) was conducted to observe the dispersion state of clay in the PI matrix. For TEM, specimens were embedded in epoxy resin, cured at 70 °C for 24 h, and sectioned into 90 nm slices under vacuum using a glass-knife-equipped microtome. The TEM acceleration voltage was 120 kV.

Thermal properties were evaluated using differential scanning calorimetry (DSC, 2–00,915, Delaware, USA) and thermogravimetric analysis (TGA, SDT 0650–0439, Delaware, USA) at a heating rate of 20 °C/min. The coefficient of thermal expansion (CTE) was determined via thermomechanical analysis (TMA, SS6100, Tokyo, Japan) using film samples (5 × 30 mm²) heated at 5 °C/min under a 0.1 N load, with values obtained from the second heating cycle.

The tensile mechanical properties of the films were evaluated using a universal testing machine (Shimadzu, AG-50KNX, Tokyo, Japan) according to the ASTM D882 standard method for thin plastic films to ensure reproducibility. Specimens with dimensions of 5 mm × 50 mm were tested at a crosshead speed of 5 mm/min. Each measurement was repeated more than ten times, and data outside the acceptable error range were discarded. The reported values represent the average of the remaining measurements. Error bars shown in the figures correspond to the standard deviations obtained from repeated measurements.

Optical properties were measured using an ultraviolet–visible (UV–vis.) spectrophotometer (UV-3600, Shimadzu, Tokyo, Japan) in the range of 300–800 nm to determine the cut-off wavelength (λ₀) and transmittance at 500 nm (500 nm^trans). The yellowness index (YI) of PI hybrid films was determined using a spectrophotometer (CM-3600D, Konica Minolta, Tokyo, Japan).

Results and discussion

FT-IR and ¹³C NMR spectroscopy

The chemical structure of PI was confirmed using FT-IR and ¹³C MAS NMR spectroscopy, and the results are shown in Figs. 1 and 2, respectively. In Fig. 1, the FT-IR spectrum of PAA exhibits a very broad –OH absorption band in the range of 2800–3600 cm⁻¹, along with C = O stretching bands at 1710 cm⁻¹ (carboxylic acid) and 1637 cm⁻¹ (amide). In addition, a C = C stretching band is observed at 1397 cm⁻¹. In contrast, the FT-IR spectrum of PI obtained from PAA via thermal treatment shows characteristic C = O stretching bands at 1780 and 1698 cm⁻¹, as well as a distinct band at 1370 cm⁻¹ corresponding to the stretching of the C–N–C bond in the imide structure. The appearance of these imide-related bands confirms that the thermal treatment induced a reaction between the carboxylic acid and amide groups, leading to the formation of imide linkages²².

Fig. 1.

FT-IR spectra of PAA and PI.

Fig. 2.

¹³C-MAS NMR spectrum of PI.

Solid-state ¹³C NMR spectroscopy is employed to analyze the NMR signals of ¹³C nuclei, thereby elucidating the structures of solid-state organic compounds through their chemical shifts. It is also widely used to verify and complement FT-IR results, particularly in determining the chemical structures of polymers. In this study, the chemical shifts of the ¹³C nuclei were calibrated to 0 ppm at 25 °C using the tetramethylsilane (TMS) standard peak (set at 38.3 ppm).

In Fig. 2, the peaks corresponding to the carbons of the imide carbonyl and phenol groups were observed at 169.05 ppm (a) and 155.54 ppm (b), respectively. The peaks associated with phenyl carbons were observed at 144.48 ppm (c), 136.71 ppm (d), 129.42 ppm (e), and 120.60 ppm (f). Spinning sidebands were indicated by open circles (o) and asterisks (_*)²³. From the chemical shifts of all carbons, the expected chemical structure of the synthesized PI was confirmed.

XRD pattern

Figure 3 presents the XRD patterns of pristine clay, organoclays, and PI/organoclay hybrid films. Organically modified clays exhibit improved compatibility with polymers, enabling the polymer chains to be readily intercalated into the clay galleries. As expected, ion exchange between Na⁺-clays (Na⁺-MMT and Na⁺-Mica) and hexadecylamine (C16) resulted in an increase in the basal interlayer spacing compared with pristine clays, accompanied by a pronounced shift of the XRD peak toward lower 2θ values. For example, in the case of C16-MMT, the interlayer spacing increased from d = 11.96 Å (2θ = 7.34°) to d = 18.39 Å (2θ = 4.80°). In addition, a very small XRD peak corresponding to C16-MMT was observed at 2θ = 8.24° (d = 10.72 Å) in Fig. 3a, which is assigned to the (002) plane of the organoclay layers.

Fig. 3.

XRD patterns of PI and PI hybrid films with various filler contents. (a) C16-MMT and (b) C16-Mica.

When the PI hybrid films were formed, the peaks observed in the organoclay completely disappeared for C16-MMT loadings up to 1.5 wt% (Fig. 3a), indicating that the clay was fully dispersed within the polymer matrix, achieving a nanoscale exfoliated structure. However, when the filler content was increased to 2.0–3.0 wt%, a very small and broad peak appeared, suggesting aggregation of excess clay beyond the critical loading^24–26.

Similar to C16-MMT, C16-Mica showed comparable results. In pristine mica, peaks were observed at 2θ = 7.22° (d = 12.23 Å) and 2θ = 9.32° (d = 9.48 Å). In the organo-Mica, these peaks shifted to 2θ = 5.37° (d = 16.44 Å) and 8.85° (d = 9.98 Å), respectively, indicating increases in the d-spacing values. In addition, a very small and weak diffraction peak was also observed at 2θ = 13.78° (d = 6.39 Å). Generally, an increase in interlayer spacing facilitates the intercalation of polymer chains and promotes easier delamination of the clay, resulting in hybrids with improved clay dispersion¹². As shown in Fig. 3b, no peaks appeared in the XRD patterns of PI hybrid films containing up to 1.0 wt% C16-Mica. However, when the organoclay content was increased from 1.5 to 3.0 wt%, a peak emerged at 2θ = 9.25° (d = 9.55 Å). This suggests that the Mica layers of the organoclay were in an intercalated rather than exfoliated state and were not uniformly dispersed in the PI matrix. The XRD peak intensity increased markedly as the clay loading rose from 1.5 to 3.0 wt%, indicating that dispersion was more effective at lower clay contents, while higher loadings led to increased agglomeration of some clay within the PI matrix.

The dispersion state of fillers in polymer nanocomposites is generally evaluated by XRD and TEM. XRD provides a convenient means of determining the interlayer spacing arising from the periodic arrangement of clay layers in pristine clay and polymer nanocomposites, while TEM enables direct visualization for a qualitative assessment of the microstructure²⁷. These techniques are considered complementary in characterizing the morphology of polymer nanocomposites. Accordingly, the electron microscopy analyses presented in the following section serve as the primary evidence for nanocomposite formation^25,28,29.

TEM micrograph

A TEM allows a qualitative understanding of the internal structure through direct observation. More direct evidence of the formation of true nanocomposites is provided by our TEM micrographs of the ultramicrotomed sections. The TEM micrographs in Figs. 4 and 5 show the structures of the nanosheets for different magnifications. The dark lines are the intersections of the 1 nm thick sheet layers. Increasing the magnification in an area occupied by an aggregate reveals that the individual sheets of MMT clay are separated by a layer of polymer. Thus the morphology consists of a mix of intercalated and exfoliated sheets, i.e., there are regions where a regular stacking arrangement is maintained with a layer of polymer between the sheets, and also regions where completely delaminated sheets are dispersed individually³⁰.

Fig. 4.

TEM micrographs of (a) 1.5 wt% and (b) 2.0 wt% C16-MMT in PI hybrid films increasing the magnification levels.

Fig. 5.

TEM micrographs of (a) 1.5 wt% and (b) 2.0 wt% C16-Mica in PI hybrid films increasing the magnification levels.

Figure 4 presents the TEM analysis results of PI hybrid films containing 1.5 and 2.0 wt% C16-MMT. From left to right, the magnification increases, and the black lines represent plate-like clay layers with a thickness of approximately 1 nm, while the spaces between these lines correspond to the PI polymer chains intercalated between the clay layers. When 1.5 wt% of organoclay was uniformly dispersed (Fig. 4a), clay aggregates smaller than 10 nm were observed. Even when the organoclay content was increased to 2.0 wt% (Fig. 4b), an excellent dispersion state of less than 20 nm was maintained. These results indicate that C16-MMT was homogeneously dispersed at the nanoscale within the PI matrix.

XRD analysis suggested that exfoliation occurred when the organoclay content was 1.5 wt% (see Fig. 3a). At 2.0 wt% filler content, very small peaks were detected, which were attributed to the presence of aggregates; however, due to the minimal extent of agglomeration, no significant differences were discernible in the TEM images. As noted earlier, TEM images enable a quantitative assessment of the nanoscale structure of the composites based on the orientation and dispersion state of the clay layers, allowing direct visualization of the results. In contrast, the XRD technique analyzes the diffraction patterns of composites in the exfoliated state, where the periodic organic structure is lost, but it may not provide explicit information on the overall structure of the materials.

Figure 5 shows the TEM images of PI hybrid films containing 1.5 wt% and 2.0 wt% C16-Mica. When the C16-Mica content was 1.5 wt%, the Mica platelets exhibited a dispersed morphology with an approximate size of 30 nm within the PI matrix (Fig. 5a). However, when the content of organo-mica increased to 2.0 wt%, partial agglomeration of the Mica platelets was observed, forming aggregates approximately 40 nm in size (Fig. 5b). These aggregates correspond to the very small peaks observed in the XRD pattern of Fig. 3b.

The degree of dispersion of C16-MMT in the PI hybrids was superior to that of C16-Mica. As shown in Fig. 3, this result can be attributed to the larger interlayer spacing of C16-MMT (d = 18.39 Å) compared with that of C16-Mica (d = 16.44 Å). The larger d-spacing facilitates the intercalation of polymer chains between the clay layers, thereby enabling more efficient dispersion.

Ultimately, XRD and TEM analyses (Figs. 3, 4 and 5) confirmed that the clay layers were uniformly dispersed within the matrix polymer, indicating the formation of nanoscale hybrids despite the presence of some clay agglomerates. Therefore, clays such as MMT and Mica are exfoliated and uniformly dispersed within the PI matrix through solution intercalative polymerization. The excellent thermal, mechanical, and optical transparency properties exhibited by these nanocomposites can be attributed to the homogeneous dispersion of the particles, as discussed in the following section.

Thermal behaviors

Since PI is a transparent amorphous polymer, its thermal properties are primarily characterized by the glass transition temperature (T_g). The T_g is influenced by various factors^31–35, including the structure of the constituent monomers, the type of substituents, the free volume generated by chain mobility, secondary interactions such as intermolecular hydrogen bonding, curing reactions, and, in the case of hybrids, the presence of dispersed fillers. Moreover, T_g is strongly dependent on the molecular conformation, which is determined by the stiffness and flexibility of the polymer chains^34,35.

Table 2 summarizes the thermal properties of PI hybrid films prepared with different loadings of two types of organoclays. Various changes in T_g were observed upon organoclay incorporation. For instance, when the C16-MMT content in the PI matrix increased from 0 to 0.5 wt%, T_g rose from 229 °C to 245 °C. This behavior can be attributed to the formation of secondary interactions, such as hydrogen bonding between the –OH groups of HBZ-based PI and the hydrophilic organoclay, which restricts the segmental motion of polymer chains intercalated between clay layers, thereby reducing the free volume. However, as the C16-MMT content increased from 0.5 to 3.0 wt%, the T_g gradually decreased from 245 to 215 °C. This decrease in T_g at higher filler loadings is likely attributed to the partial aggregation of clay nanoparticles and the formation of interfacial free volume within the polymer matrix. At low filler contents, well-dispersed clay layers effectively restrict the mobility of polymer chains, thereby contributing to an increase in T_g. In contrast, excessive filler loading may disturb the regular packing of polymer chains, leading to increased local chain mobility and a slight reduction in T_g. Similar trends have been reported in our previous studies^15,36,37.

Table 2.

Thermal properties of PI hybrid films.

Organoclay in PI (wt%)	C16-MMT				C16-Mica
	Tg (oC)	TDia (oC)	wtR600b (%)	CTEc (ppm/oC)	Tg (oC)	TDi (oC)	wtR600 (%)	CTE (ppm/oC)
0 (pure PI)	229	258	70	36	229	258	70	36
0.5	245	279	70	32	235	273	68	35
1.0	241	276	72	34	237	278	70	34
1.5	228	270	71	36	240	278	70	30
2.0	217	264	72	36	250	284	72	28
3.0	215	261	71	38	238	266	71	32

^a Initial decomposition temperature at 2% weight loss. ^b Weight residue at 600 ^oC. ^c Coefficient of thermal expansion for 2^nd heating is 50 ~ 200 ^oC.

For the C16-Mica-based hybrids, T_g also exhibited a maximum at a certain critical filler content, followed by a decrease at higher loadings, similar to the behavior observed for C16-MMT. Specifically, as the filler content increased from 0 to 2.0 wt%, T_g increased from 229 to 250 °C, reaching its maximum value; however, further increasing the filler content to 3.0 wt% resulted in a decrease in T_g to 238 °C. Figure 6 presents the DSC thermograms of the PI hybrids containing various loadings of the two organoclays.

Fig. 6.

DSC thermograms of PI and PI hybrid films with various organoclay contents. (a) C16-MMT and (b) C16-Mica.

Incorporation of clay into the polymer matrix increases the initial decomposition temperature (T_Dⁱ) by functioning as both a thermal insulator and a mass-transport barrier, thereby impeding the release of volatile decomposition products^38,39. The enhanced thermal stability can also be ascribed to the inherent high thermal resistance of the clay and the interfacial interactions between the clay platelets and the polymer chains⁴⁰. In addition to other factors, the delayed polymer degradation observed with increasing clay nanoparticle loading can be explained by the fact that inorganic nanoparticle fillers, which are non-degradable materials with extremely high specific surface areas, reduce the mobility of polymer chains when dispersed within the matrix. When the nanoparticles are uniformly dispersed throughout the polymer matrix, they hinder the diffusion of free radicals, thereby enhancing the thermal stability of the hybrids^41,42.

Table 2 summarizes the T_Dⁱs as a function of organoclay content in the hybrids. The T_Dⁱ values exhibited a trend similar to that observed for T_g. For the C16-MMT hybrids, the T_Dⁱ of pure PI was 258 °C; however, increasing the filler content to 0.5 wt% raised the T_Dⁱ to 279 °C. Such an improvement in thermal stability with the addition of a small amount of clay has been reported by several researchers, and the underlying mechanisms have been discussed in detail earlier^43,44. When the clay content increased from 0.5 to 3.0 wt%, however, the T_Dⁱ gradually decreased to 261 °C. This decline is attributed to clay aggregation above the critical loading, which reduces the insulating effect of the clay layers within the polymer matrix due to their lower degree of dispersion. A similar trend was observed for the C16-Mica hybrids, where the maximum T_Dⁱ of 284 °C was obtained at a critical filler content of 2.0 wt%; further increasing the filler loading to 3.0 wt% resulted in a significantly decrease to 266 °C.

In addition, the residual weight at 600 °C (wt_R⁶⁰⁰) remained nearly constant at 68–72% for both hybrid series as the organoclay content increased from 0 to 3.0 wt%. This invariance indicates that, irrespective of clay type or loading, the thermally less stable organic moiety—specifically, the hexadexyl alkyl (C16-) chain—had already decomposed at elevated temperatures, and the measured residue corresponds primarily to the thermally stable inorganic component. The TGA thermograms of the organoclay and the PI hybrid films with varying organoclay loadings are presented in Fig. 7.

Fig. 7.

TGA thermograms of PI and PI hybrid films with various organoclay contents. (a) C16-MMT and (b) C16-Mica.

As shown in the TGA thermograms in Fig. 7, two stages of thermal decomposition were observed for both series of PI films upon heating. This phenomenon occurs during the transformation of PI containing –OH groups, originating from the HBZ monomer, into polybenzoxazole (PBO) through a thermal rearrangement (TR) process at elevated temperatures^45–47. For both series, the PI was completely converted to PBO via the TR process in the temperature range of approximately 350–450 °C. Scheme 2 illustrates the reaction process in which PI containing –OH groups is transformed into PBO through the TR process.

Scheme 2.

Thermal rearrangement of polybenzoxazole from PI by heat treatment.

In general, when a polymer is exposed to heat, its chains relax in a direction perpendicular to the main chain. However, in hybrids containing dispersed rigid, planar clay layers, the polymer chains confined between the clay layers do not readily expand upon heating, thereby effectively blocking heat transfer. In other words, thermally stable clay can suppress the lateral thermal expansion of polymer chains with only a small amount of filler^48,49.

Table 2 summarizes the CTE values of the two series of hybrid films measured under secondary heating conditions. The CTE of the pure PI film was 36 ppm/°C. For the C16-MMT hybrids, the CTE reached a minimum value of 32 ppm/°C at a filler content of 0.5 wt%, but increased again to 38 ppm/°C when the content was further increased. Similarly, in the case of the C16-Mica hybrids, the CTE reached a minimum value of 28 ppm/°C at a filler content of 2.0 wt%, but increased to 32 ppm/°C when the content was raised to 3.0 wt%. Ultimately, the minimum CTE value was observed at a critical clay content, beyond which the CTEs increased with further filler loading. Figure 8 shows the TMA thermograms of PI hybrids containing various types and loadings of organoclays.

Fig. 8.

TMA thermograms of PI and PI hybrid films with various filler contents. (a) C16-MMT and (b) C16-Mica.

As mentioned in the Introduction, because Mica (aspect ratio ≈ 1,230) has a much higher aspect ratio than MMT (aspect ratio ≈ 218), Mica-based hybrids exhibit overall superior thermal stability compared to MMT-based hybrids at the same filler content. This improvement is attributed to their rigid, non-swelling layered structure, higher dehydroxylation temperature, and enhanced thermal barrier effect against polymer chain motion and thermal degradation. Also, the critical filler content of Mica is generally higher than that of MMT because the rigid, non-swelling layered structure of Mica suppresses restacking and aggregation even at higher loadings.

Mechanical tensile property

Mechanical properties describe how a material responds to external forces or deformation, and in the case of polymer composites, these properties depend on the polymer’s structure, bonding state, crystallinity, as well as the type and dispersion of fillers⁵⁰. To evaluate these properties, a UTM is used to measure load and displacement, and the resulting stress–strain curve is analyzed to assess the material’s characteristics. In this study, to investigate the fundamental mechanical properties of the hybrid films as a function of organoclay content, the ultimate tensile strength, initial tensile modulus, and elongation at break (EB) were measured.

The ultimate tensile strength of the C16-MMT hybrid increases with the addition of clay up to a critical clay loading, and then decreases above that critical content. Table 3 summarizes the mechanical properties of the PI hybrid films with varying organoclay contents. For example, the ultimate tensile strength of 0.5 wt% PI hybrids is 122 MPa, which is about 65% higher than that of pure PI (74 MPa). This improvement arises because the organoclay layers are dispersed and intercalated in the PI matrix. This result is consistent with the general observation that the introduction of an organoclay into a matrix polymer increases its tensile strength. When the amount of organo-MMT in PI reaches 3.0 wt %, the ultimate tensile strength decreases to 87 MPa. The initial modulus also exhibited a similar trend with increasing filler content; that is, the C16-MMT-filled hybrid reached a maximum value of 1.43 GPa at a filler content of 0.5 wt%, whereas further increasing the filler content beyond the critical loading to 3.0 wt% resulted in a gradual decrease in the initial modulus to 1.07 GPa. This decrease in ultimate strength and initial modulus is mainly due to the agglomeration of clay particles above the critical clay loading, as we have previously explained.

Table 3.

Mechanical properties of PI hybrid films.

Organoclay in PI (wt%)	C16-MMT			C16-Mica
	Ult. Str.a (MPa)	Ini. Mod.b (GPa)	EBc (%)	Ult. Str (MPa)	Ini. Mod (GPa)	EB (%)
0 (pure PI)	74	1.20	4	74	1.20	4
0.5	122	1.43	6	77	1.33	4
1.0	120	1.31	6	84	1.32	6
1.5	105	1.16	6	106	1.47	5
2.0	103	1.08	5	136	1.78	6
3.0	87	1.07	6	92	1.24	4

^a Ultimate strength.^b Initial modulus.^c Elongation at break.

Similar results to those of C16-MMT were observed for the PI hybrid containing C16-Mica. The ultimate tensile strength and initial modulus increased to 136 MPa and 1.78 GPa, respectively, as the filler content increased to 2.0 wt%. However, when the filler content exceeded the critical level and reached 3.0 wt%, the strength and modulus decreased to 92 MPa and 1.24 GPa, respectively. These results are closely related to the critical filler content.

At the same filler content, Mica has a higher aspect ratio than MMT; therefore, Mica-based hybrids generally exhibit superior mechanical properties compared to MMT-based hybrids due to their rigid platelet structure, enhanced load transfer efficiency, and suppressed filler aggregation at higher loadings. Figure 9 shows the variations in tensile strength and initial modulus of the PI hybrid films as a function of filler content, with error bars representing the experimental deviations.

Fig. 9.

Mechanical properties of PI and PI hybrid films with various filler contents. (a) C16-MMT and (b) C16-Mica.

In contrast to the previously obtained results for ultimate tensile strength and initial modulus, the EB of all hybrid films was observed to be in the range of approximately 4–6%, regardless of the type and content of organoclaycharactre.

Optical transparency

To investigate the optical properties of the PI hybrid films, the λ_o, the 500 nm^trans, and the YI value were measured. The UV–vis. Analysis results for the hybrid films are shown in Fig. 10, and the data are summarized in Table 4. To ensure the reliability of the results, the thickness of all measured films was maintained within the range of 19–22 μm. For PI hybrid films containing either C16-MMT or C16-Mica, increasing the filler content from 0 to 3.0 wt% resulted in nearly constant λ₀ values regardless of the filler type, whereas slight differences were observed in the transmittance at 500 nm (500 nm^trans) and the YI values. For example, as the filler content increased from 0 to 3.0 wt%, λ_o remained constant at approximately 368–370 nm. The 500 nm^trans values decreased slightly from 62 to 54% for C16-MMT and from 62 to 53% for C16-Mica. Similarly, the YI values increased from 46 to 55 for C16-MMT and from 46 to 52 for C16-Mica.

Fig. 10.

UV–vis. transmittances of PI and PI hybrid films with various filler contents. (a) C16-MMT and (b) C16-Mica.

Table 4.

Optical properties of PI hybrid films.

Organoclay in PI (wt%)	C16-MMT				C16-Mica
	Thicknessa (μm)	λ0b (nm)	500nmtrans (%)	YIc	Thickness (μm)	λ0 (nm)	500nmtrans (%)	YIc
0 (pure PI)	21	370	62	46	21	370	62	46
0.5	22	370	59	48	22	370	61	46
1.0	22	370	58	50	20	370	59	47
1.5	19	370	57	52	21	370	56	49
2.0	22	369	56	53	19	370	55	50
3.0	19	370	54	55	19	368	53	52

^a Film thickness.^b Cut off wavelength.^c Yellow index.

Consequently, the optical transparency of the films decreased progressively as the organoclay content increased. This result can be attributed to enhanced light scattering arising from refractive index mismatch between the polymer matrix and clay platelets, as well as the formation of clay aggregates with sizes comparable to the wavelength of visible light at higher loadings^51,52.

To verify the transparency of the two PI hybrid film series containing different types of clay, images of the films placed over a logo background are presented in Figs. 11 and 12. Compared to pure PI, the color intensity of the hybrid films gradually increased with the addition of organoclay fillers, a trend that was already evident from the YI results for both series. Although the transparency of the films decreased gradually as the clay content increased, the logo could still be identified without significant difficulty.

Fig. 11.

Photographs of PI hybrid films with filler contents. (a) 0 (pure PI), (b) 0.5, (c) 1.0, (d) 1.5, (e) 2.0, and (f) 3.0 wt% C16-MMT.

Fig. 12.

Photographs of PI hybrid films with filler contents. (a) 0 (pure PI), (b) 0.5, (c) 1.0, (d) 1.5, (e) 2.0, and (f) 3.0 wt% C16-Mica.

Conclusions

This study investigated the effects of reinforcing a PI matrix with two types of organoclays. Nanoscale composites were prepared, and their thermomechanical property, clay dispersion, and optical transparency were compared at various organoclay contents.

A small amount of nanofiller was found to enhance the thermomechanical properties of the PI hybrid films, with C16-Mica exhibiting superior performance to C16-MMT due to its higher aspect ratio and rigidity. The critical content for maximum performance was 0.5 wt% for C16-MMT and 2.0 wt% for C16-Mica; however, exceeding these levels led to filler agglomeration and a subsequent deterioration in properties. XRD and TEM analyses confirmed that both fillers were well dispersed at the nanoscale, and optical analysis showed that C16-Mica maintained slightly higher transmittance than C16-MMT at the same loading, although the difference was minimal.

Overall, this study provides valuable insights into the structure–property relationships of organoclay-reinforced polyimide hybrid systems. The results offer a useful guideline for the molecular and nanostructural design of high-performance polyimide nanocomposites for advanced applications in flexible electronics, transparent display substrates, semiconductor packaging, and high-temperature insulating materials.

Author contributions

J.-H. Chang designed the project and wrote the manuscript and reviewed and data analyzed. A. R. Lim prepared the samples and participated in the data analysis. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Regional Innovation System & Education (RISE) program through the Jeonbuk RISE Center, funded by the Ministry of Education (MOE) and the Jeonbuk State, Republic of Korea (2025-RISE-13-JJU).

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.