Near-Zero Thermal Expansion and High Heat-Resistance Polyimide Films Based on a Symmetric and Rigid Pyrazine Structure

Abstract

Regarding the applications of polyimide (PI) films in emerging flexible devices and precision apparatus, their low coefficient of thermal expansion (CTE), especially near-zero expansion, is an urgent requirement for weakening the internal stress between the organic/ inorganic layers. Other properties of PI films including the high heatresistance and superior toughness are also demanded in actual scenarios. Herein, a set of PI films was developed by copolymerization of 2,5-bis(4aminophenyl)pyrazine (PRZ) into a pyromellitic dianhydride (PMDA)/ 4,4′-oxydianiline (ODA) system. The monocrystal analysis of the PRZ monomer reveals its rigid, symmetric, and nearly coplanar steric structure. Thanks to the synergy and balance of flexible ODA and rigid PRZ units in PI chains, the near-zero expansion, high heat-resistance, and optimal mechanical properties of the PI films are simultaneously achieved. Particularly, the PI film with ODA/PRZ = 4.5/5.5 presents a near-zero CTE of −0.41 ppm/K, high glass transition temperature (T g ) of around 417 °C, superior fracture strength of 191 MPa, and initial modulus of 3.83 GPa. Interestingly, via adjusting the ODA/PRZ ratios, the precise control of CTE values can be realized, enabling the thermal expansion matching of PI films on broad types of device substrates, e.g., Cu (15−17 ppm/K) and Si (3−5 ppm/K).

Highly Rigid and Functional Polyimides Based on Noncoplanar Benzimidazolone Diamines and Their Photolithography Application

“…The dimensional stability of PIs was evaluated by TMA (Figure c). The CTE of all prepared PI films ranged from 52 to 63 ppm/K, which is comparable to traditional aromatic PIs. , Previous studies have shown that the CTE is inversely proportional to the packing density of polymer chains . In general, the higher the structural stiffness/linearity is and the stronger the intermolecular interactions are, the lower the CTE is.…”

Development of Fluorinated Colorless Polyimides of Restricted Dihedral Rotation toward Flexible Substrates with Thermal Robustness

“…The dimensional stability of CPI substrates is strictly required, i.e., CTE ≤ 15 ppm/K, which aims to match with inorganic layers (SiN x and SiO x in TFTs, Cu in flexible copper clad laminate (FCCL), etc. ). − However, valuable CPIs face a fundamental conflict between high transparency and low CTE, rooted in the intrinsic difference in tightly or loosely packed chains. As shown in Figure d and Table , 6FDA-based CPI films exhibit apparent thermal expansion behaviors (CTE ≥ 60 ppm/K).…”

Comprehensive Performance of Polyimide Reinforced by Multiple Hydrogen Bonds for Flexible Electronics Application

“…The aggregation state of polymers is mainly related to the orientation of molecular chains and intermolecular chain interactions . Inter-HBs in PIs can not only greatly increase intermolecular interaction, bringing molecular chains closer together and enhancing crystallization, but also effectively improve the mutual orientation of molecular chains through their directional effect, resulting in more ordered packing. ,, In order to detect the aggregated state in the PI films, WAXD analysis was carried out at room temperature. It can be seen in Figure a that the 2θ peak gradually changed from a wide diffuse peak to a sharp peak with increasing DAPyBA content.…”

“…The thermal expansion behavior of polymers is mainly related to the rigidity and linearity of molecular chains and the interactions between the chains. Therefore, increasing the rigidity and linearity of molecular chains, as well as enhancing intermolecular interactions, can effectively improve the thermal dimensional stability of PI films. ,,− Here, TMA was used to test the CTEs of the PI films over the temperature range of 50–300 °C to evaluate their dimensional stability, and the results are shown in Figure and Table . Compared with PI-0 with a CTE of 31.9 ppm K –1 , when only 10% of ODA was replaced by DAPyBA, the CTE decreased by 26% to 23.7 ppm K –1 .…”

“…Numerous studies have proved that the CTE of polymer films mainly depends on their chemical structure, molecular chain packing, and molecular chain orientation . Linear molecular structures are beneficial for reducing the CTEs of polymers due to their strong self-orientation and tight molecular packing. − By comparing three PIs containing benzoxazole-imide, benzimidazole-imide, and pyrimidine-imide, respectively, Luo et al found that an in-plane orientation contributed more to dimensional stability and mechanical strength than the degree of crystallization, and they obtained a poly­(pyrimidine-imide) film with a CTE of less than 5.0 ppm K –1 and an elastic modulus greater than 5.0 GPa . For colorless and transparent PIs, the reduction of CTE is more difficult due to the use of large side groups or alicyclic structures.…”