FTIR studies of polyimides: thermal curing

Temperature-Controlled Chain Dynamics in Polyimide Doped with CoCl2 Probed Using Dynamic Mechanical Analysis

Cobalt(II) chloride (CoCl2) being in the vicinity of polyimide chains entails modifications in terms of the molecular dynamics, which are mainly governed by the possible presence of amic acid residual groups, by the transition-metal-type characteristics of cobalt and by the CoCl2 content. Polyimide was synthesized using poly(amic acid) according to the reaction of 2,2'-bis(3,4-dicarboxylphenyl)hexafluoropropane dianhydride (6FDA) with 3,3'-dimethyl-4,4'-diaminodiphenylmethane (MMDA) in N,N-dimethylacetamide. CoCl2 was added before the thermal imidization of the poly(amic acid). An experimental approach was designed to establish the interaction between the polyimide and CoCl2 and whether the interaction depends on the quantity of the salt. Evidence for the existence of residual amic acid groups was obtained using second derivative Fourier Transform Infrared Spectroscopy (FTIR) and with the help of 2D correlation spectroscopy (2D-COS). Moreover, FTIR, along with X-ray photoelectron spectroscopy (XPS), revealed the interaction between the polymer and CoCl2, primarily in the form of Co(II)-N coordinated bonds. Nevertheless, the coordination of cobalt with suitable atoms from the amic acid groups is not precluded. The results of dynamic mechanical analysis (DMA) featured a specific relaxation assigned to the presence of CoCl2 in the polymeric film and demonstrated that its (non)reinforcing effect depends on its content in the polyimide.

Redox-active polyimides for energy conversion and storage: from synthesis to application

As the demand for next-generation electronics is increasing, organic and polymer-based semiconductors are in the spotlight as suitable materials owing to their tailorable structures along with flexible properties. Especially, polyimide (PI) has been widely utilised in electronics because of its outstanding mechanical and thermal properties and chemical resistance originating from its crystallinity, conjugated structure and π-π interactions. PI has recently been receiving more attention in the energy storage and conversion fields due to its unique redox activity and charge transfer complex structure. In this review, we focus on the design of PI structures with improved electrochemical and photocatalytic activities for use as redox-active materials in photo- and electrocatalysts, batteries and supercapacitors. We anticipate that this review will offer insight into the utilisation of redox-active PI-based polymeric materials for the development of future electronics.

PI-based composites with high dielectric constant and low loss by filling with self-derived carbon

In this work, we prepared polyimide (PI) composite films by directly filling the matrix with self-derived carbon particles. Without any surface modification layer, these specially made particles were compatible with their parent matrix quite well. For the composite film with 25 wt% filler content, the dielectric constant was 72.5 at 1 MHz at room temperature, and the dielectric loss was only 0.069. The conductivity of the corresponding composite was measured to be below 10−6 Sm−1, and the breakdown strength was 165 MVm−1. In addition, the tensile strength of the composite film was measured to be 73 MPa. These results indicate that carbonized PI can be used as an excellent filler to prepare PI-based composite films with high dielectric constant.

Progress in Aromatic Polyimide Films for Electronic Applications

Aromatic polyimides have excellent thermal stability, mechanical strength and toughness, high electric insulating properties, low dielectric constants and dissipation factors, and high radiation and wear resistance, among other properties, and can be processed into a variety of materials, including films, fibers, carbon fiber composites, engineering plastics, foams, porous membranes, coatings, etc. Aromatic polyimide materials have found widespread use in a variety of high-tech domains, including electric insulating, microelectronics and optoelectronics, aerospace and aviation industries, and so on, due to their superior combination characteristics and variable processability. In recent years, there have been many publications on aromatic polyimide materials, including several books available to readers. In this review, the representative progress in aromatic polyimide films for electronic applications, especially in our laboratory, will be described.

Structure and dynamics of small polyimide oligomers with silicon as a function of aging

The effect of UV curing and shearing on the structure and behavior of a polyimide (PI) binder as it disperses silicon particles in a battery electrode slurry was investigated. PI dispersant effectiveness increases with UV curing time, which controls the overall binder molecular weight. The shear force during electrode casting causes higher molecular weight PI to agglomerate, resulting in battery anodes with poorly dispersed Si particles that do not cycle well. It is hypothesized that when PI binder is added above a critical amount, it conformally coats the silicon particles and greatly impedes Li ion transport. There is an "interzonal region" for binder loading where it disperses silicon well and provides a coverage that facilitates Li transport through the anode material and into the silicon particles. These results have implications in ensuring reproducible electrode manufacturing and increasing cell performance by optimizing the PI structure and coordination with the silicon precursor.

A Patternable and In Situ Formed Polymeric Zinc Blanket for a Reversible Zinc Anode in a Skin-Mountable Microbattery

Owing to their high safety and reversibility, aqueous microbatteries using zinc anodes and an acid electrolyte have emerged as promising candidates for wearable electronics. However, a critical limitation that prevents implementing zinc chemistry at the microscale lies in its spontaneous corrosion in an acidic electrolyte that causes a capacity loss of 40% after a ten-hour rest. Widespread anti-corrosion techniques, such as polymer coating, often retard the kinetics of zinc plating/stripping and lack spatial control at the microscale. Here, a polyimide coating that resolves this dilemma is reported. The coating prevents corrosion and hence reduces the capacity loss of a standby microbattery to 10%. The coordination of carbonyl oxygen in the polyimide with zinc ions builds up over cycling, creating a zinc blanket that minimizes the concentration gradient through the electrode/electrolyte interface and thus allows for fast kinetics and low plating/stripping overpotential. The polyimide's patternable feature energizes microbatteries in both aqueous and hydrogel electrolytes, delivering a supercapacitor-level rate performance and 400 stable cycles in the hydrogel electrolyte. Moreover, the microbattery is able to be attached to human skin and offers strong resistance to deformations, splashing, and external shock. The skin-mountable microbattery demonstrates an excellent combination of anti-corrosion, reversibility, and durability in wearables.

Three-Dimensional Monolithic Organic Battery Electrodes

Design of freestanding electrodes incorporated with redox-active organic materials has been limited by the poor intrinsic electrical conductivity and lack of methodology driving the feasible integration of conductive substrate and the organic molecules. Single-walled carbon nanotube (SWCNT) aerogels, which possess continuous network structure and high surface area, offer a three-dimensional electrically conducting scaffold. Here, we fabricate monolithic organic electrodes by coating a nanometer-scale imide-based network (IBN) that possesses abundant redox-active sites on the 3D SWCNT scaffold. The substantially integrated 3D monolithic organic electrodes sustain high electrical conductance through a 3D electronic pathway in their compressed form (∼21 μm). A thin and controllable layer (<8 nm) of IBN organic materials has a strong adhesion onto the ultra-lightweight and conductive substrate and facilitates multielectron redox reactions to deliver a specific capacity of up to 1550 mA h g^-1 (corresponding to the areal capacity of ∼2.8 mA h cm^-2). The redox-active IBN in synergy with the 3D SWCNT scaffold can enable superior electrochemical performances compared to the previously reported organic-based electrode architectures and inorganic-based electrodes.

Robust Micron-Sized Silicon Secondary Particles Anchored by Polyimide as High-Capacity, High-Stability Li-Ion Battery Anode

Silicon is an attractive high-capacity anode material for lithium-ion battery. With the help of nanostructures, cycling performance of silicon anode has improved significantly in the past couple of years. However, three major shortcomings associated with nanostructures still need to be addressed, namely, their high surface area, low tap density, and poor scalability. Herein, we present a facile and practical method to produce micron-sized Si secondary particle cluster (SiSPC) with a high tap density and a low surface area from bulk Si by high-energy ball-milling. By coupling SiSPC with a mechanically robust polyimide binder, more than 95% of the initial capacity is retained after 500 cycles at 3500 mA g^-1 (1C rate). Reversibility of electrode thickness change is confirmed by in situ dilatometry. In addition, the polyimide binder suppresses the surface reaction of the particles with electrolyte, resulting in a high Coulombic efficiency of 99.7%. Excellent cycling performance is obtained even for thick electrodes with an areal capacity of 3.57 mAh cm^-2, similar to those in commercial lithium-ion batteries. The presented Si electrode system has a high volumetric capacity of 598 mAh cm^-3, which is higher than that of the commercial graphite anode materials.

Structural evolution from poly(amic acid) to polyimide fibers during thermal imidization process

In this study, poly(amic acid) (PAA) precursor fiber was prepared via a two-step wet spinning method and subsequently heat-treated to obtain the polyimide fiber by thermal imidization. The structural evolution of the PAA precursor during the imidization was traced using various measurements. The imidization degree of the PAA fibers treated at different temperatures was calculated by Fourier transform infrared analysis, and the cyclization reaction occurred accompanied by the decomplexation of the H-bonded N, N′-dimethylacetamide (DMAc). The thermogravimetry test illustrated that the residual solvent evaporation took place prior to imidization. Moreover, in situ wide-angle X-ray diffraction and small-angle X-ray scattering measurements were employed to investigate the development of the aggregation structure and microvoids in PAA fibers. The results indicated that molecular chains were thermally extended during the thermal imidization, resulting in the increase of Hermans’ orientation factor and the increased strain. The thermal imidization process also led to the orientation and elongation of microvoids along the molecular chains.

Polyimide film with low thermal expansion and high transparency by self-enhancement of polyimide/SiC nanofibers net

A facile approach to synthesize a polyimide (PI) film with enhanced dimensional stability, a high mechanical property and optical transparency is presented by embedding the partial imidized PI/SiC nanofiber-net in a poly(amic acid) (PAA) solution, followed by removing the solvent and imidization of the PAA. The nanofiber-network self-filled PI film demonstrates a much lower thermal expansion coefficient (CTE), an excellent mechanical property and high transparency retention in comparison to the film fabricated by solution cast. When the SiC content is 6 wt% in PI/SiC nanofibers, the CTE values for the PI film containing 25 wt% PI/SiC nanofibers are 2.80 times lower than the solution cast PI/SiC film. The tensile strength and modulus for the PI/SiC fiber filled film are also improved by 159% and 91% respectively in comparison to the solution cast SiC/PI film. In addition, the PI/SiC nanofiber-network filled PI film exhibits a high optic transparency. The significant improvement in aforementioned properties is contributed to by the long and continuous nanonetwork which acts as a frame to maintain the stable dimension and endow the film with high mechanical properties. Moreover, the nanosized SiC particles were constricted within the nano-fiber to avoid light scattering, so the high transparency of the film was retained.

Polyimide films with low thermal expansion and high transparency were fabricated by homogeneity enhancement of nanofibers net. The nanofibers net that composed of polyimide and SiC nanoparticles is obtained by electrospinning technic.

Synthesis of Polyimides in Molecular-Scale Confinement for Low-Density Hybrid Nanocomposites

In this work, we exploit a confinement-induced molecular synthesis and a resulting bridging mechanism to create confined polyimide thermoset nanocomposites that couple molecular confinement-enhanced toughening with an unprecedented combination of high-temperature properties at low density. We describe a synthesis strategy that involves the infiltration of individual polymer chains through a nanoscale porous network while simultaneous imidization reactions increase the molecular backbone stiffness. In the extreme limit where the confinement length scale is much smaller than the polymer's molecular size, confinement-induced molecular mechanisms give rise to exceptional mechanical properties. We find that polyimide oligomers can undergo cross-linking reactions even in such molecular-scale confinement, increasing the molecular weight of the organic phase and toughening the nanocomposite through a confinement-induced energy dissipation mechanism. This work demonstrates that the confinement-induced molecular bridging mechanism can be extended to thermoset polymers with multifunctional properties, such as excellent thermo-oxidative stability and high service temperatures (>350 °C).

Effect of pre-imidization on the structures and properties of polyimide fibers

The pre-imidization process was adopted in preparing high-performance polyimide fibers, resulting in the effectively improved mechanical properties of the resultant polyimide fibers.

Structures and properties of polyimide fibers containing fluorine groups

The fluorine groups were successfully incorporated into the polymer backbone, and the structure–property relationship of the resulting polyimide fibers were systematically investigated.

Investigation on cyclization process of co-polyimides containing 2-(4-aminophenyl)-5-aminobenzimidazole units

As one type of high-performance polymers, polyimides have been extensively used in various fields. The cyclization reaction plays a significant role in the change of the polymeric chemical structure and physical properties. In this work, a series of random co-poly(amic acid)s (PAAs) were synthesized by introducing an aromatic heterocyclic diamine monomer (2-(4-aminophenyl)-5-aminobenzimidazole (BIA)) into the homopolyimide backbones of pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA). Fourier transform infrared (FTIR) spectroscopy was employed to study the cyclization kinetics in the temperature range of 180–300°C. The results indicate that the addition of BIA decreases the cyclization rates, which is mainly attributed to the rigid-rod structure of the BIA. The mechanism of thermal imidization of the solid state co-PAA was also studied by FTIR using two-dimensional correlation spectroscopy method. The signs of the cross peaks in asynchronous spectra suggested that the imide-related modes changed prior to the amide mode, which indicates that cyclization occurred before the amide proton was abstracted. The result illustrates the truth that the introduction of BIA units into PMDA-ODA segments obviously increases the barrier for the meta-segment of PAA into corresponding polyimide, which is the essential reason for the decreasing cyclization rate.

Two-dimensional fourier transform infrared (FT-IR) correlation spectroscopy study of the imidization reaction from polyamic acid to polyimide

The mechanism of the thermal imidization of solid-state polyamic acid was studied using Fourier transform infrared (FT-IR) spectroscopy using the two-dimensional correlation spectroscopy method. It is assumed that two isomers exist in a polyamic acid segment: one is called the para-segment, which favors imidization reaction, the other is the meta-segment, which is not in favor for imidization unless the temperature is high enough. The results show that the imidization process differs for the two states of polyamic acid segments. The para-segment is more sensitive to the heat environment for the formation of imide ring, and it will take several intermediate steps to complete the ring closure at the aid of the solvent. As for the meta-segment, the ring will be closed before the imide formation due to the powerful energy provided in the high-temperature environment, and the ever-increasing chain rigidity and the loss of solvent during the heating process make this path the only option to continue the imidization process.

A facile approach to fabricate porous nanocomposite gels based on partially hydrolyzed polyacrylamide and cellulose nanocrystals for adsorbing methylene blue at low concentrations

Porous nanocomposite gels were fabricated by a facile method consisting of the electrospinning and subsequent heat treatment based on partially hydrolyzed polyacrylamide (HPAM) of ultra-high molecular weight, with cellulose nanocrystals (CNCs) as crosslinker. The effects of three electrospinning parameters (i.e., solution concentration, composition of solvent mixture, and CNC loading level) on morphology and diameter of electrospun fibers were systematically investigated. The swelling properties of porous gels and their application in the removal of methylene blue dye (as a compound representative of contaminants) were evaluated. Electrospun fiber morphologies without beads, branches, and ribbons were achieved by optimizing the electrospinning solutions. The thermal crosslinking between HPAM and CNCs was realized through esterification, rendering the product nanocomposite membranes insoluble in water. Electrospun fibers of approximately 220 nm in diameter comprised the 3D porous nanocomposite gels, with porosity greater than 50%. The porous nanocomposite gels displayed a rapid swelling rate and an efficient adsorption capacity in removing methylene blue at low concentrations from aqueous solutions.