What if a transparent plastic film could repair its own scratches with heat? This study builds that idea into a polyimide, while keeping more than eighty-seven percent light transmittance and reaching ninety-one point eight percent healing efficiency.

Self-healing materials can detect damage or defects caused by environmental changes and repair them using external stimulations such as light, heat, and electricity. Optically transparent polyimides have potential applications in flexible displays, flexible circuit boards, flexible thin-film solar cells, and artificial skin because of their heat resistance, light transmission, and flexibility.

But their brittle nature means that frequent handling and movements can cause scratches and cracks during use, leading to failure of the entire device. Although several strategies have been developed for self-healing polyimides, solutions to these problems still face more challenges.

Self-healing materials are classified as extrinsic or intrinsic systems, depending on the mechanism of healing. Extrinsic systems release polymerizable or cross-linkable agents from capsules or vessels in the matrix, while intrinsic systems do not rely on repairing agents and instead use specific, reversible interactions within the matrix.

Disulfide bonds attract attention because they use abundant raw materials, mild reaction conditions, and respond strongly to a variety of stimuli. Functional monomers containing disulfide bonds can trigger molecular-chain reorganization through exchange reactions between disulfide bonds.

Aromatic polyimides have rigid molecular structures because of strong inter- and intra-molecular charge-transfer interactions, which hinder polymer-chain mobility. Flexible structures can improve chain mobility and self-healing, but long-chain aliphatic diamines produced elastomer-like materials.

A semi-aromatic polyimide reached a self-healing efficiency of ninety-eight percent, but its glass transition temperature was below one hundred degrees Celsius and its tensile strength was below sixty megapascals. Aromatic disulfides can also create a yellowish appearance and reduced transparency, so the trade-off between self-healing and mechanical, thermal, and optical properties still requires molecular design.

The designed self-healing polyimide was made by copolymerizing cystamine containing aliphatic disulfide bonds with BPADA and HFBAPP. Cystamine provides S-S bonds capable of restoring intra-chain bonds in damaged areas, while the flexible dianhydride linker in BPADA increases polymer-chain mobility.

Bulky trifluoromethyl groups in HFBAPP can inhibit charge-transfer interaction, increase intermolecular free volume, and reduce molecular-chain stacking, giving the films high transparency. Adjusting the monomer ratio produced a transparent SHPI film with both excellent self-healing capacity and mechanical properties.

SHPIs were synthesized by copolymerizing BPADA with HFBAPP and cystamine using a two-step polycondensation method. The molar ratio of BPADA to the total content of HFBAPP and cystamine was maintained at one to one, while the HFBAPP-to-cystamine ratios ranged from ten to zero to four to six.

BPADA and NMP were stirred under nitrogen until dissolution, followed by HFBAPP and then a mixture of cystamine and NMP at the corresponding molar ratios. The solution was stirred at room temperature for twenty-four hours to obtain polyamic acid, then treated with acetic anhydride and anhydrous pyridine at fifty degrees Celsius for ten hours to synthesize SHPIs by chemical imidization.

The SHPIs were precipitated in methanol and water, collected by filtration, washed three times with methanol, and dried overnight at seventy degrees Celsius under vacuum. The films were scratched with a zero point two millimeter DN-52 cutter blade under a constant load of one hundred grams, then healed in an oven under different conditions.

Scratched surfaces were monitored using a BX41 polarizing microscope, and an AI-7000M tensile testing machine evaluated the healing efficiencies. The specimens were cut into rectangular strips measuring fifty by ten by zero point zero eight millimeters, and each sample received three individual tensile tests.

Figure 2 presents UV–Vis transmittance spectra for SHPI films, showing a sharp transition near the ultraviolet-to-visible boundary and transmission approaching ninety percent across much of the visible range. The authors report that the eighty-micrometer films have cut-off wavelengths below four hundred nanometers, transmittance above eighty-seven percent at five hundred nanometers, and yellowness indices below ten.

The inset photograph visibly illustrates their optical transparency. The eighty-micrometer-thick SHPI films showed very high optical transparency, with cut-off wavelengths below four hundred nanometers, transmittance above eighty-seven percent at five hundred nanometers, and a yellowness index below ten.

The paper associates this transparency with decreased charge transfer and electronic conjugation as the content of aliphatic diamines increases. Highly electronegative trifluoromethyl groups can reduce charge transfer from electron-donating diamines to electron-withdrawing dianhydrides, prohibiting visible-light absorption by the polymers.

Figure three compares the thermal behavior of SHPI films with different cystamine contents using TGA, DTG, DSC, and DMA. The TGA and DTG panels show that cystamine-containing films decompose in a two-stage pattern, with an earlier stage associated with less stable S-S and S-C bonds.

The DSC and DMA curves show glass-transition behavior, while the reported DSC glass-transition temperatures decrease as cystamine content increases, helping explain the films’ greater low-temperature chain mobility and self-healing capability. Unlike SHPI-0, SHPI-0.1, SHPI-0.3, and SHPI-0.5 showed two stages of weight loss.

The first stage, around three hundred to four hundred degrees Celsius, involved S-S and S-C bonds in the cystamine and BPADA repeating unit, which are less stable at high temperatures than C-C or C-O bonds. The second decomposition stage was related to the repeating unit formed by condensation of HFBAPP and BPADA, similar to the ordinary SHPI-0 film.

Although cystamine lowered the five-percent mass-loss decomposition temperature, films containing fifty percent or less cystamine had decomposition temperatures of at least four hundred fifteen degrees Celsius. The SHPI films exhibited typical glass transition behavior in differential scanning calorimetry, and films containing cystamine had lower glass transition temperatures than SHPI-0 without cystamine.

The glass transition temperatures identified from dynamic mechanical analysis showed the same trend and ranged from one hundred eighty-four to two hundred eighteen degrees Celsius for the copolymer films. These changes indicate that cystamine, with its aliphatic chain structure, remarkably improved the flexibility of the polymer backbone.

Although cystamine reduced the glass transition temperature, it enhanced self-healing by introducing dynamic covalent disulfide bonds and flexible methylene groups. Figure four tracks scratched SHPI films under a polarizing microscope immediately after scratching and after heat treatment for six, twelve, and twenty-four hours.

The rows compare SHPI-zero, SHPI-zero point one, SHPI-zero point three, and SHPI-zero point five, while the one-millimeter scale bars indicate the image scale. This visual matters because it directly documents how the scratch appearance changes during thermal self-healing at the specified healing temperature.

The initial scratch depth was set to half the film thickness, and the scratches were produced as zigzag scratches under a constant load. After heat treatment at the healing temperature, defined as the differential-scanning-calorimetry glass transition temperature plus thirty degrees Celsius, for six, twelve, and twenty-four hours, the scratched films were observed with an optical microscope and hot stage.

After six hours, scratches on all SHPI films diminished, with SHPI-0.5 showing the fastest healing rate. After twenty-four hours, the scratch on SHPI-0.5 was completely healed, while scratches on SHPI-0.3, SHPI-0.1, and SHPI-0 remained visible.

The results suggest that increasing the disulfide-bond content substantially improved self-healing ability. Figure 5 combines surface imaging with chemical analysis of SHPI-zero-point-five films before and after healing. The SEM images show a pronounced zigzag scratch in panel a, while panel b shows the corresponding surface after heat treatment, and the EDS spectra identify carbon, oxygen, fluorine, and sulfur in both states.

Panel d further compares sulfur two-p XPS peaks before scratching and after healing, providing chemical evidence alongside the visible recovery of the film surface. The pristine SHPI-0.5 film was eighty point three micrometers thick, and the scratch depth was forty point seven micrometers; after heat treatment at one hundred ninety degrees Celsius for twenty-four hours, the scratches were almost completely healed.

The sulfur contents on scratched and healed surfaces were almost the same, indicating no significant weight loss during self-healing. The scratched and healed films showed no significant changes in high-resolution sulfur two-p X-ray photoelectron spectra, indicating no chemical changes during self-healing through disulfide exchange reactions.

The self-healing process did not affect the chemical bonds and structure of the films, enabling repeated healing in the same position. Figure 6 compares pristine and healed SHPI films through stress–strain curves and grouped measurements of tensile strength, strained strength, and healed strength.

These results matter because they connect the films’ mechanical performance with healing capacity as cystamine content changes. Adding cystamine decreased the tensile strengths of the SHPI films, consistently with changes in SHPI molecular weight. At fifty percent cystamine, SHPI-0.5 had a tensile strength of about ninety-nine megapascals, higher than most reported self-healing polyimide materials.

After heat treatment at the differential-scanning-calorimetry glass transition temperature plus thirty degrees Celsius for twenty-four hours, self-healing efficiency increased with cystamine content, and SHPI-0.5 reached ninety-one point eight percent. Self-healing requires elements capable of restoring intra-chain bonds or attraction and sufficient polymer-chain mobility.

Increasing cystamine supplied dynamic S-S bonds for exchange reactions in the scratched region, while decreased HFBAPP increased the amount of methylene groups and the flexibility and motility of the molecular chain. Figure seven tracks normalized stress relaxation under a constant two-percent strain, with panels a through d showing how each SHPI composition relaxes at several temperatures; the dashed line marks the one-over-e, or thirty-seven-percent, criterion used to define relaxation time.

Panel e applies an Arrhenius analysis to those times, while panel f summarizes the corresponding activation energies. These measurements matter because relaxation reflects migration, diffusion, and reversible disulfide-bond exchange, linking faster structural rearrangement and lower activation energy to self-healing behavior.

The relaxed tensile modulus was recorded over time at different temperatures under a constant strain of two percent. Relaxation time is defined as the time required for SHPI films to relax to one over e, or thirty-seven percent, of the initial modulus. Compared with the other SHPI films, SHPI-0.5 relaxed more rapidly at temperatures slightly above its dynamic-mechanical-analysis glass transition temperature of one hundred eighty-four degrees Celsius.

At one hundred ninety degrees Celsius, its relaxation time was about forty-five seconds and decreased as temperature increased, indicating faster disulfide-bond cleavage and network-topology transformation at higher temperatures. Activation energy is calculated by the Arrhenius equation to display the energy needed for fracture and generation of dynamic bonds in self-healing polymers.

Lower activation energies can facilitate topological rearrangements through rapid exchange reactions, leading to superior self-healing ability. The study prepared self-healing transparent polyimide films using cystamine as a functional monomer and adjusted the ratios of HFBAPP and cystamine through copolymerization.

Self-healing ability increased significantly with cystamine content, and SHPI-0.5 showed the best performance, with a repair efficiency as high as ninety-one point eight percent. SHPI-0.5 had a short relaxation time of forty-five seconds at one hundred ninety degrees Celsius, while its low relaxation activation energy of fifty-four point fifty-nine kilojoules per mole facilitated topological rearrangements through rapid exchange reactions.

Cystamine gives transparent polyimide films dynamic disulfide bonds that can reorganize under heat. The best formulation keeps about ninety-nine megapascals of tensile strength while healing up to ninety-one point eight percent of its damage.