Effects of monomer structure and imidization degree on mechanical properties and viscoelastic behavior of thermoplastic polyimide films

Preparation and Properties of Low-Dielectric Polyimide Films Containing Tert-Butyl

The design of high-performance polyimide (PI) films and understanding the relationship of the structure-dielectric property are of great significance in the field of the microelectronics industry, but are challenging. Herein, we describe the first work to construct a series of novel tert-butyl PI films (denoted as PI-1, PI-2, PI-3, and PI-4) based on a low-temperature polymerization strategy, which employed tetracarboxylic dianhydride (pyromellitic anhydride, 3,3',4,4'-biphenyl tetracarboxylic anhydride, 4,4'-diphenyl ether dianhydride, and 3,3',4,4'-benzophenone tetracarboxylic anhydride) and 4,4'-diamino-3,5-ditert butyl biphenyl ether as monomers. The results indicate that introducing tert-butyl branches in the main chain of PIs can enhance the free volume of the molecular chain and reduce the interaction between molecular chains of PI, resulting in a low dielectric constant. Particularly, the optimized PI-4 exhibits an excellent comprehensive performance with a high (5) wt% loss temperature (454 °C), tensile strength (117.40 MPa), and maximum hydrophobic angle (80.16°), and a low dielectric constant (2.90), which outperforms most of the results reported to date.

Monomer Dependence of Colorless and Transparent Polyimide Films: Thermomechanical Properties, Optical Transparency, and Solubility

Six poly(amic acid)s (PAAs) were synthesized by reacting bis(3-aminophenyl) sulfone with various dianhydride monomers such as pyromellitic dianhydride, 4,4'-biphthalic anhydride, dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride, 4,4'-oxidiphthalic anhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, and 4,4'-(hexafluoroisopropylidene) diphthalic anhydride. These PAAs were then converted to polyimide (PI) films by thermal imidization at various temperatures. To obtain colorless and transparent PI (CPI), the dianhydride monomer used in this study had an overall bent structure, a structure containing a strong electron-withdrawing -CF3 substituent or an alicyclic ring. In addition, some monomers contained ether or ketone functional groups in their bent structures. The thermomechanical properties, optical transparency, and solubility of CPI films with six different dianhydride monomer structures were investigated, and the correlation between the monomer structure and CPI film properties was clarified. Overall, CPI with an aromatic main chain structure or a linear structure had excellent thermal and mechanical properties. In contrast, CPI with a bent structure containing functional groups or substituents in the main chain exhibited excellent optical transparency and solubility.

Challenge for Trade-Off Relationship between the Mechanical Property and Healing Efficiency of Self-Healable Polyimide

Polyimide is actively applied in various industrial fields because of its strong mechanical properties, owing to the interactions between the polymer chains. Fully aromatic imide structures exhibit high glass-transition temperatures due to the strong interactions between their chains, which hinder chain mobility. Therefore, preparing a material that exhibits self-healing at a low temperature of ≤100 °C and good mechanical properties is challenging. Thus, we prepared imides with four-component semiaromatic structures by adjusting the contents of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride and 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) to yield four-component self-healable colorless polyimides (f-SH-CPIs) with novel structures, flexibilities, good mechanical properties, and low healing temperatures. The flexibilities and distances between the polymer chains, as the basis of the trade-off relationship between the mechanical properties and healing efficiency, were controlled. These materials may be used as substrates in wearable devices and multilayer insulation that may protect from space dust, cosmic rays, and satellite fragments.

Fabrication and mechanical, electrical properties study of polyimide films curing at low-temperature condition assisted by microwave

Polyimide (PI) films with excellent mechanical and electrical properties were produced at a low temperature assisted by microwave. Depending on the reciprocating movement of dipole molecules excited by microwave, even though under a low temperature, the rotation and conformational changes of functional groups could also be promoted. Thus, reaction between N-H bond and -OH in carboxyl which could realize dehydration and cyclization of polyamic acid proceeded normally. Results revealed that the combination between ambient temperature only at 70°C and microwave power at 2000 W or higher could also produce PI films with imidization degree of 100%. For PI-2000W, compared with traditional PI film (PI-300°C) that generated only at temperature of 300°C, even though on the basis of no any change in molecular structure and addition of filler, the tensile strength, elastic modulus, crystallinity, and apparent density increased by about 21%, 51%, 37%, and 5%, respectively. For electrical properties, conductivity enhanced by one to two orders of magnitude comparing with PI-300°C. Results indicated that while keeping the original molecular structure of PI films unchanged, this work provided a new and effective method to assist the enhancement in mechanical and electrical properties of PI films.

A Universal Study on the Effect Thermal Imidization Has on the Physico-Chemical, Mechanical, Thermal and Electrical Properties of Polyimide for Integrated Electronics Applications

Polyimides (PI) are a class of dielectric polymer used in a wide range of electronics and electrical engineering applications from low-voltage microelectronics to high voltage isolation. They are well appreciated because of their excellent thermal, electrical, and mechanical properties, each of which need to be optimized uniquely depending on the end application. For example, for high-voltage applications, the final polymer breakdown field and dielectric properties must be optimized, both of which are dependent on the curing process and the final physico-chemical properties of PI. The majority of studies to date have focused on a limited set of properties of the polymer and have analyzed the effect of curing from a physicochemical-, mechanical- or electrical-centric viewpoint. This paper seeks to overcome this, unifying all of these characterizations in the same study to accurately describe the universal effect of the cure temperature on the properties of PI and at an industrial processing scale. This paper reports the widest-ranging study of its kind on the effect that cure temperature has on the physico-chemical, mechanical, thermal and electrical properties of polyimide, specifically poly (pyromellitic dianhydride-co-4, 4′-oxydianiline) (PMDA/ODA). The optimization of the cure temperature is accurately studied not only regarding the degree of imidization (DOI), but also considering the entire physical properties. Particularly, the analysis elucidates the key role of the charge–transfer complex (CTC) on these properties. The results show that while the thermal and mechanical properties improve with both DOI and CTC formation, the electrical properties, particularly at high field conditions, show an antagonistic behavior enhancing with increasing DOI while degrading at higher temperatures as the CTC formation increases. The electrical characterization at low field presents an enhancement of the final PI properties likely due to the DOI. On the contrary, at high electric field, the conductivity results show an improvement at an intermediate temperature emphasizing an ideal compromise between a high DOI and PI chain packing when the thermal imidization process is performed over this equilibrium. This balance enables maximum performance to be obtained for the PI film with optimized electrical properties and, overall, optimal thermal and mechanical properties are achieved.

Effects of diamine isomers on the properties of colorless and transparent copoly(amide imide)s

To fully understand the structure–property relationship of aromatic copoly(amide-imide)s (Co-PAIs) and determine which factors lead to chain rigidity, we prepared two series of Co-PAIs. They were synthesized from two types of amine monomers containing m- and p-isomers and different ratios of 4,4′-(hexafluoroiso-propylidene)diphthalic anhydride (6FDA) and 4,4′-biphthalic anhydride (BPA). m-Substituted and p-substituted N,N′-[2,2′-bis(trifluoromethyl)-4,4′-biphenylene]bis(aminobenzamide) (MPAB) diamine isomers were synthesized from 3- and 4-nitrobenzoyl chloride and 2,2′-bis(trifluoromethyl) benzidine (TFB), respectively. The Co-PAI films were synthesized from poly(amic acid) (PAA), via solution-casting, followed by thermal imidizations. The thermal- and mechanical-properties and optical transparency of the Co-PAI films with different BPA monomer contents were investigated. We also investigated the effects of the different MPAB isomers on the Co-PAI structures. Compared with the m-substituted MPAB Co-PAI films, the p-substituted MPAB Co-PAI films have superior thermo-mechanical properties at the same monomer content. However, the optical transparencies of the m-MPAB Co-PAIs are slightly better than those of the p-MPAB Co-PAIs.

The thermal- and mechanical-properties and optical transparency of the copoly(amide imide) films with different BPA monomer contents were investigated.

Structural Evolution of Macromolecular Chain During Pre-imidization Process and Its Effects on Polyimide Film Properties

Pre-imidization has been found to have a determining role on the final properties of polyimide (PI) films. In this work, a series of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)/2,2'-bis(trifluoromethyl)benzidine (TFMB) PI models with specified pre-imidization degree (pre-ID) were constructed and analyzed on the basis of molecular dynamic (MD) simulation to reveal the real-time evolution of structure and properties that occurred during the pre-imidization process. The MD results indicated that the T_g of the models increased obviously with increasing pre-ID, which corresponded to the increase of rigid PI chain segments that restricted the mobility of molecular chains. In addition, the increase of fractional free volume and mean square end-to-end distance indicated looser chain packing and more extended chain conformation during the pre-imidization process. As a further verification, a series of corresponding PI films were experimentally prepared via a controlled partially pre-imidization process. Mechanical properties of the prepared PI films were tested to be significantly enhanced, and the coefficient of thermal expansion decreased from 61.5 to 47.6 ppm/°C with pre-ID increasing from 0% to 100%, which could be attributed to the orderly molecular chain arrangement formed during the chemical pre-imidization process, as disclosed by MD simulation. This work paves the way for the observation of the real-time structure and property evolutions of PI materials, especially during the pre-imidization process.

Intrinsically heat-sealable polyimide films with atomic oxygen resistance: Synthesis and characterization

Intrinsically heat-sealable polyimides with atomic oxygen (AO) resistance (ARPIs) were synthesized from 2,3,3′,4′-oxydiphthalic anhydride (aODPA), 2,5-bis[(4-aminophenoxy)phenyl]diphenylphosphine oxide (BADPO), and para-phenylenediamine (PDA). The effects of the molecular structure and diamine ratio were investigated on the properties of the ARPI, including mechanical property, thermal property, heat sealability, and AO resistance. Heat sealability and AO resistance were realized for the ARPI film by combining the asymmetry of the aODPA moiety and the passivated layer forming characteristic of diphenylphosphine phosphine oxide group. Meanwhile, the deficiency of low mechanical strength and thermal resistance, commonly existing in a completely BADPO-derived polyimide system, was remedied effectively by the higher reactivity and rigidity of PDA.

Synthesis, characterization, and properties of thermoplastic polyimides derived from 4,4’-(hexafluoroisopropylidene)diphthalic anhydride in diethylene glycol dimethyl ether

By regulating the order of monomer addition, four kinds of copolymer polyimides (PIs) were prepared using diethylene glycol dimethyl ether (DEGDE) and N, N-dimethylacetamide (DMAc) as the solvents. The molecular weights of polyamide acids (PAAs) ranged from 3.8 × 105 to 9.2 × 105. All of the films displayed high glass transition temperatures ( Tgs) ranging from 313°C to 346°C. The polymer films show excellent thermal stabilities with 5% weight loss at temperatures of 505–524°C and char yields at 800°C were as high as 55% under nitrogen. The peel strengths of flexible copper (Cu) clads were in the range from 0.337 N cm−1 to 0.598 N cm−1. Compared to the molecular weight and peel strength of fresh PAA, those of PAAs prepared using DMAc significantly decreased after storage for 3 months at 0°C. However, when DEGDE was used as the solvent, the molecular weights of the PAAs and the thermal properties of the PIs were maintained after long storage time.

Characterization and thermal degradation kinetics of thermoplastic polyimide based on BAPP

Two kinds of thermoplastic polyimides (PIs) were synthesized via a two-step method with 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), 4,4′-oxydianiline (ODA) diamine, and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), and their thermal degradation kinetics was studied by thermogravimetric analysis at different heating rates under nitrogen. Derivative thermogravimetric analysis curves indicated a simple, single-stage degradation process in PI BTDA-BAPP and a two-stage degradation process in PI BTDA-ODA-BAPP. The activation energies ( Eas) of the thermal degradation reaction were determined by the Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose methods without a knowledge of the kinetic reaction mechanism. By comparing the values of Ea and weight loss temperatures, it was demonstrated that the thermal stability of PI BTDA-ODA-BAPP was superior to that of PI BTDA-BAPP.

Polyimide thin film composite (TFC) membranes via interfacial polymerization on hydrolyzed polyacrylonitrile support for solvent resistant nanofiltration

High performance solvent resistant nanofiltration membranes are fabricated via interfacial polymerization between m-phenylenediamine and 1,2,4,5-benzenetetra acylchloride on hydrolyzed polyacrylonitrile supports followed by chemical imidization.

Synthesis and characterization of low-CTE polyimide films containing trifluoromethyl groups with water-repellant characteristics

A novel fluorinated ester-bridged aromatic diamine, bis(2-trifluoromethyl-4-aminophenyl)terephthalate (CF3-BPTP) was synthesized, which was employed to prepare a series of fluorinated ester-bridged polyimide (PFEI) films with controlled ester-bridged segments and fluorine contents in the polymer backbone. The PFEI films were prepared by copolymerization of biphenyl tetracarboxylic dianhydride as aromatic dianhydride monomer and the aromatic diamine monomer mixture consisting of p-phenylenediamine and different amounts of CF3-BPTP. Experimental results indicated that the films’ water uptakes (Wus) reduced with increasing of the CF3 groups loadings in the ester-bridged polyimide backbones while keeping the films with low enough coefficient of thermal expansion (CTE). By controlling CF3 group loadings, polyimide films with desirable combination of thermal, mechanical, and dielectric properties for application in high density and thinner flexible printed circuits (FPCs) have been obtained. Thus, polyimide films with CTE of ≤20 × 10−6 1/K (ppm/K) at 50–200°C, glass transition temperature of ≥310°C, Young’s modulus of ≥6.0 GPa, Wu of as low as 0.7 wt%, dielectric constant ( ε) of 3.3 have been obtained. The two-layer flexible copper clad laminate prepared by coating the polyimide precursor resin poly(amid acid) solution on the surface of copper foil followed by thermal imidization at elevated temperature did not show apparent curling due to its closed CTE value.

Preparation and properties of thermally conductive polyimide/boron nitride composites

Hexagonal boron nitride micro particles functionalized by γ-MPS, were used to fabricate PI/BN composites. The thermal conductivity of the composites with 40 wt% m-BN content was increased to 0.748 W m⁻¹ K⁻¹, 4.5 times higher than that of the pure PI.

Aligned carbon nanotube film enables thermally induced state transformations in layered polymeric materials

The energy losses and geometric constraints associated with conventional curing techniques of polymeric systems motivate the study of a highly scalable out-of-oven curing method using a nanostructured resistive heater comprised of aligned carbon nanotubes (A-CNT). The experimental results indicate that, when compared to conventional oven based techniques, the use of an "out-of-oven" A-CNT integrated heater leads to orders of magnitude reductions in the energy required to process polymeric layered structures such as composites. Integration of this technology into structural systems enables the in situ curing of large-scale polymeric systems at high efficiencies, while adding sensing and control capabilities.

The effect of liquid stabilization on the structures and the conductive properties of polyimide-based graphite fibers

This stabilization method could effectively avoid a sudden jump in temperature and local heat storage during the carbonization, as well as help improve the stability of PI fibers.