A recent article published in RSC Applied Polymers proposed re-processible electronic substrates based on photopolymerizable polyimides to mitigate electronic waste (e-waste). These high-performance, photo-patternable substrates are degradable due to their ester linkages.
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Background
The accumulation of e-waste globally is escalating due to rapid technological advancements. The continued development of flexible electronics, which are promising in fields such as robotics, wearables, healthcare, and packaging, exacerbates this issue.
New polymeric electronic substrates (e-substrates) are being developed with inherent re-processability, allowing complete degradation or reuse of electronic components at the end of their life cycle. This innovation aims to minimize waste and reduce the costs associated with electronics.
E-substrates for consumer devices require a high dielectric constant, low dissipation factor, and the ability to withstand large-scale manufacturing conditions. Polyimides (PIs) such as Kapton® are popular due to their outstanding mechanical properties.
However, the re-processability of PIs is limited by their stable imide bonds and the need for high synthesis temperatures (approximately 250 °C). This study proposed using thiol-ene click chemistry to photopolymerize PIs with degradable ester networks.
Methods
Various dianhydride feedstock chemicals were explored to synthesize diallyl bisimide monomers with low melting points, resulting in eight monomers. Among these, three were unable to form stable liquids, leaving five monomers (diallyl ester, diallyl ether, diallyl cyclohexane, diallyl diether, and diallyl hexafluoro) for further experimentation.
Trimethylolpropane tris(3-mercaptopropionate) (TMPMP) was selected as the thiol monomer to synthesize poly(imide ester) thiol-ene networks (PI-ester, PI-ether, PI-cyclohexane, PI-diether, and PI-hexafluoro) from each diallyl bisimide monomer. The resins were polymerized using ultraviolet (UV) light (405 nm) at 80 °C and cast into different shapes using silicone molds for further analysis.
The glass transition temperature (Tg) of the PIs was determined through dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). Thermogravimetric analysis (TGA) was conducted to examine their thermal behavior, while Fourier-transform infrared spectroscopy (FTIR) was used to study their polymeric network.
Tensile strength was measured after 48 hours of curing using dog-bone-shaped samples, while disk-shaped samples were used to assess thermal conductivity with a thermal constant analyzer. A thermo-mechanical analysis was also performed to evaluate thermal expansion under a cool-heat-cool-cool cycle (-70 °C → 300 °C → -70 °C → 300 °C → 30 °C).
Vector network analysis was conducted to estimate the dielectric constant and dielectric loss of the prepared PIs, and their electrical conductivity was measured using a digital multimeter.
Results and Discussion
Monitoring the thiol peak in real-time FTIR analysis confirmed the rapid polymerization of the diallyl bisimide monomers, achieving 63 % thiol conversion within 10 seconds under 5 mW/cm2 UV light. The resulting polymer films were free-standing, glassy, tough, and transparent, with a thickness of approximately 200 μm.
The different diallyl imide monomers resulted in Tg values ranging from 66 to 92 °C, which are lower than those of commercial PIs but sufficient for wearables, IoT, and single-use applications. The stiff aromatic PIs (PI-hexafluoro and PI-diether) exhibited the highest Tg values, while the aromatic PIs with flexible linkages (PI-ester and PI-ether) had the lowest.
The synthesized PIs exhibited moderate extensibility, with elongation-at-break values around 6 %, except for PI-ether, which had a value of 47 % due to its flexible ether linkage that supports additional backbone mobility. Overall, all five thiol-ene PIs were mechanically robust.
The thermal conductivities of the synthesized polymers were comparable to Kapton® modified with specific additives. Additionally, their dielectric loss values ranged from 0.0145 to 0.0169, within the typical range for commercial PIs (0.0037-0.020).
Depolymerization of the proposed polymeric e-substrates was analyzed using a transesterification reaction with methanol, catalyzed by potassium carbonate. This process took over a week due to the limited diffusion of methanol into the glassy polymer network. However, complete depolymerization was observed within 24 hours when dichloromethane was added to the methanol solution.
Electrical components were successfully recovered from the depolymerized substrates and tested for reuse on a fresh patterned substrate, exhibiting good functionality. Furthermore, the PI substrates were compatible with multilayered processing for high-density electronic circuits.
Conclusion
Overall, the researchers successfully developed re-processible network PIs through thiol-ene photopolymerization of diallyl bisimide monomers. These PIs demonstrated promising mechanical strength, thermal stability, thermal conductivity, and dielectric behavior, making them potential alternatives to aromatic PIs such as Kapton®.
The proposed framework presents a viable solution to the growing e-waste problem, as depolymerization through transesterification reactions enables the selective removal of the synthesized PIs and the recovery of functional electrical components. Additionally, these photopolymers integrate seamlessly with existing commercial workflows for dense, multilayered circuitry.
However, the prepared materials require further improvement in areas such as thermal conductivity, thermal expansion coefficient, and dielectric constant to replace current flexible e-substrates fully. The researchers suggest using composites or oligomeric polyimides as precursors to enhance the properties of PIs for commercial applications.
Journal Reference
Reese, CJ., et al. (2024). Photopatternable, degradable, and performant polyimide network substrates for e-waste mitigation. RSC Applied Polymers. DOI: 10.1039/d4lp00182f, https://pubs.rsc.org/en/content/articlelanding/2024/lp/d4lp00182f
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