Effect of atmospheric-pressure plasma on adhesion characteristics of polyimide film

Localized Surface Hydrophilicity Tailoring of Polyimide Film for Flexible Electronics Manufacturing Using an Atmospheric Pressure Ar/H2O Microplasma Jet

The poor hydrophilicity of polyimide (PI) films limits their applications in flexible electronics, such as in wearable and implantable bio-MEMS devices. In this paper, an atmospheric pressure Ar/H2O microplasma jet (μAPPJ) with a nozzle diameter of 100 μm was utilized to site-selectively tune the surface hydrophilicity of a PI film. The electrical and optical characteristics of the μAPPJ were firstly investigated, and the results showed that multi-spikes occurred during the plasma discharge and that diverse reactive species, such as O atoms and OH radicals, were generated in the plasma plume. The physical and chemical properties of pristine and microplasma-modified PI surfaces were characterized by the water contact angle (WCA), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The wettability of the PI surface was significantly enhanced after microplasma modification, and the WCA could be adjusted by varying the applied voltage, water vapor content, plasma treatment time and storage time. The AFM images indicated that the surface roughness increased after the plasma treatment, which partially contributed to an improvement in the surface hydrophilicity. The XPS results showed a reduction in the C content and an increase in the O content, and abundant hydrophilic polar oxygen-containing functional groups were also grafted onto the PI film surface. Finally, the interaction mechanism between the PI molecular chains and the microplasma is discussed. The breaking of C-N and C-O bonds and the grafting of OH radicals were the key pathways to dominate the reaction process.

Understanding Electrical Failure of Polyimide-Based Flexible Neural Implants: The Role of Thin Film Adhesion

The lack of long-term stability of polymeric neural interfaces remains one of the most important and less tackled issues in this research field. To address this issue, we fabricated two test structures based on interdigitated electrodes (IDEs) encapsulated with polyimide (PI). One of the test samples was pretreated with barrel oxygen plasma prior to spin coating of the second PI layer. The second test structure was pretreated using a reactive ion etching (RIE) process. The test samples were immersed in an electrolyte solution at elevated temperatures to mimic the conditions inside the human brain. The samples were then electrically and mechanically stressed to accelerate their degradation. Real-time monitoring of the electrical insulation stability was used to compare the impact of the pretreatment on the long-term stability. Barrel-plasma-activated test samples showed a mean lifetime of 1.5 days, whereas RIE pretreatment increased the mean lifetime to 24 days. Therefore, RIE-pretreated test samples exhibited 16 times longer mean stability compared to purely chemically activated test samples. Furthermore, the electrical measurements were correlated with mechanical adhesion tests. Chemically activated test samples showed significant delamination, whereas RIE pretreatment enhanced the adhesion, and no delamination could be observed. The correlation of these investigations suggests that the adhesion between different layers is higher following RIE pretreatment compared to pretreatment with chemical barrel plasma. In conclusion, the adhesion between the two PI foils seems to play a key role in the long-term stability of such devices.

Fabrication and Characterization of Non-Equilibrium Plasma-Treated PVDF Nanofiber Membrane-Based Sensors

The effect of a self-pulsing non-equilibrium plasma discharge on piezoelectric PVDF nanofiber membrane was investigated. The plasma discharge was generated in air with a DC power source, with a discharge current of 0.012 mA, a nominal interelectrode separation of 1 mm, and discharge voltage of ~970 V. In a continuous fabrication process, the electrospinning method was used to generate thin nanofiber membrane with a flow rate of 0.7-1 mL h^-1 and 25-27 kV voltage to obtain the nanofiber with high sensitivity and a higher degree of alignment and uniformity over a larger area. Plasma treatment was applied on both single layer and multi-layer (three layers) nanomembranes. In addition, simultaneously, the nanofiber membranes were heat-treated at a glass transition temperature (80-120 °C) and then underwent plasma treatment. Fourier-transform infrared (FTIR) spectroscopy showed that the area under the curve at 840 and 1272 cm^-1 (β phase) increased due to the application of plasma and differential scanning calorimeter (DSC) indicated an increase in the degree of crystallinity. Finally, PVDF sensors were fabricated from the nanofibers and their piezoelectric properties were characterized. The results suggested that compared to the pristine samples the piezoelectric properties in the plasma and plasma-heat-treated sensors were enhanced by 70% and 85% respectively.

An Assessment of Surface Treatments for Adhesion of Polyimide Thin Films

Polyimide films are currently of great interest for the development of flexible electronics and sensors. In order to ensure a proper integration with other materials and PI itself, some sort of surface modification is required. In this work, microwave oxygen plasma, reactive ion etching oxygen plasma, combination of KOH and HCl solutions, and polyethylenimine solution were used as surface treatments of PI films. Treatments were compared to find the best method to promote the adhesion between two polyimide films. The first selection of the treatment conditions for each method was based on changes in the contact angle with deionized water. Afterward, further qualitative (scratch test) and a quantitative adhesion assessment (peel test) were performed. Both scratch test and peel strength indicated that oxygen plasma treatment using reactive ion etching equipment is the most promising approach for promoting the adhesion between polyimide films.

Preparation of Polymer-Immobilized Polyimide Films Using Hot Pressing and Titania Coatings

Recent studies have revealed that polymer molecules at film surfaces exhibit unique physical properties compared to those in bulk. On the other hand, such a topic has not been extensively focused for the cases of rigid polymers such as polyimide (PI). This study investigated whether hot pressing could induce the immobilization of other polymers, poly(4-vinylphenol) (PVP), on PI film surfaces. Results supported the immobilization of PVP on the PI film surfaces, and the increase of hot-press temperature resulted in the increase of the immobilization amount of PVP. The mechanism of immobilization is discussed considering the effects of hot pressing on the interactions between PVP and PI at the interfaces of their films. Sol-gel titania coatings were further conducted to the obtained PVP-immobilized PI films. The effect of PVP immobilization on formability and the adhesion of titania layers on the film surfaces were evaluated. These results demonstrate that hot pressing of other polymers is a useful approach for the surface modification of PI films, particularly introducing certain functional groups, and indicate that the polymer immobilization mechanism might be correlated with the surface physical properties of PI films.

Surface modification of a PES membrane by corona air plasma-assisted grafting of HB-PEG for separation of oil-in-water emulsions

The main goal of this study is to modify a polyethersulfone (PES) membrane by grafting with hyperbranched polyethylene glycol (HB-PEG) using corona air plasma to intensify the anti-fouling properties of the prepared membrane. The separation efficiency and fouling tendency of the modified membranes were evaluated for the treatment of a synthetic oily wastewater. A mechanism was proposed for the HB-PEG grafting on the surface of the corona treated PES membranes and all steps of the grafting were described in detail. The effects of corona treatment operating conditions on the morphology, surface properties, separation performance and anti-fouling efficiency of the modified PES membranes were investigated. Also, the HB-PEG grafted PES membranes were characterized by FTIR, AFM and contact angle analysis. Finally, the HB-PEG grafting on the surface of the PES membranes altered the surface hydrophilicity and led to the improvement of the anti-fouling property and oil–water permeation flux of all modified membranes without any remarkable changes in oil rejection.

The main goal of this study is to modify a polyethersulfone (PES) membrane by grafting with hyperbranched polyethylene glycol (HB-PEG) using corona air plasma to intensify the anti-fouling properties of the prepared membrane.

Surface Plasma Characterization of Polyimide Films for Flexible Electronics

Flexible electronics have drawn much attention due to vast application possibilities. Polyimide was the substrate of choice as a flexible substrate owing to its properties such as good mechanical strength, high temperature resistance, good dimensional stability, and low dielectric constant. The adhesion between metal and polymer substrate plays a crucial role for reliability of these applications and low adhesion was the cause for most failures. In this study, plasma surface treatments were applied on polyimide surface by inductively coupled plasma (ICP) treatment system. The results of contact angle measurements and atomic force microscopy (AFM) show a large increase in surface roughness with increasing treatment time. Complete wetting was found for both argon and oxygen plasma treatment. Analysis of chemical composition by FTIR reveals an increase in carbon-oxygen functional groups and the concentration of oxygen on the surfaces.

The changes in electrical and interfacial properties of polyimide exposed to dielectric barrier discharge in SF₆ medium

The formation mechanism of space charges in polyimide (PI) which was exposed to dielectric barrier discharge (DBD) in SF6 medium and the effects of the space charges on interfacial and electrical properties of PI were investigated. The variation of normalized surface charge density on PI sample was calculated and illustrated for different DBD exposure times. The surface potential was measured to determine the effect of the space charges on the sample. Then, the contact angle values were measured to obtain the relation between the surface energy and the surface charge density. The expressions for the total charge and the concentration of trapped electrons were derived by using Poisson and continuity equations at stationary state. The space charges were determined experimentally by using thermally stimulated depolarization current (TSDC) method. Also, SEM image and FTIR spectrum of virgin and treated samples were presented to observe the structural variations. It was seen that the approach for the formation mechanism of the space charges agreed with the experimental data. However, it was concluded particularly for the short-time DBD treatments that the space charges accumulated in the sample should be considered besides the effects of surface functionalization in the determination of the surface energy.

Synthesis and characterization of organo-soluble polyimides containing phthalazinone and bicarbazole moieties in the main chain

Organo-soluble aromatic polyimides with good comprehensive properties are favorable for insulating layer material applications in flexible copper-clad laminates. A novel aromatic diamine, bis( N-4-amine-phenyl)-3,3′-bicarbazole (BCZDA) was firstly successfully synthesized. The structure of BCZDA was definitely confirmed via Fourier transform infrared and 1H-NMR spectroscopy and elemental analysis. Then a series of soluble polyimides were prepared from BCZDA and the diamine containing phthalazinone moiety as coreactant by one-pot solution copolycondensation with two kinds of anhydrides, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and 2-bis[4-(3,4- dicarboxyphenoxy) phenyl] propane dianhydride in sulfolane. Experimental results indicated that all the copolyimides having the glass transition temperature in range of 295 to 304 °C exhibited good solubility in N-methyl-2-pyrrolidinone (NMP), m-cresol, and N, N-dimethyl acetamide (DMAc). Their onset temperatures at 5% weight loss were all higher than 505 °C in nitrogen. Their flexible films cast in NMP exhibited good adhesive property (2.32–3.42 N mm−1) to copper foil and had good mechanical properties with the tensile strength of 69.5–79.3 MPa, elongation to break of 16–23% , and tensile modulus of 1.2–1.5 GPa. The copolyimides possessed dielectric constants of 3.08–3.26 (100 kHz) and low water absorption in the range of 0.92–1.06%, indicating that these obtained copolyimides are good candidates for the fields of microelectronics.

Synthesis and Properties of Soluble Copolyimides Containing Phthalazinone Moieties for Flexible Copper-clad Laminates

Soluble aromatic polyimides with good comprehensive properties are potential materials for microelectronics. A series of soluble copolyimides containing phthalazinone moieties were prepared from 1,2-dihydro-2-(4-aminophenyl)-4-[4-(4-(aminophenoxyl)phenyl)] (2H) phthalazin-1-one, 4,4'-diaminodiphenyl ether, as comonomers, and appropriate aromatic dianhydrides by one-pot solution polymerization in sulfolane. The corresponding copolyimide films had a good balance in thermal, adhesive and dielectric properties. The resulting copolyimides showed good solubility in polar aprotic solvents such as N-methyl-2-pyrrolidinone (NMP) and N, N-dimethyl acetamide and high thermal stability with glass transition temperatures in the range of 252—266 ° C and decomposition temperature at 5% weight loss being above 505 °C in nitrogen. The copolyimide solutions in NMP were processed into flexible films which exhibited good adhesive property (1.66—2.28 N mm-1) and showed a predominantly amorphous nature measured by wide-angle X-ray diffraction. These free-standing films had dielectric constants of 2.3—3.1 (100 kHz) and low water absorption in the range of 0.76—0.93%. These outstanding properties make the obtained copolyimides good candidates for polyimide-based flexible copper-clad laminates.

Remote atmospheric-pressure plasma activation of the surfaces of polyethylene terephthalate and polyethylene naphthalate

The surfaces of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) were treated with an atmospheric-pressure oxygen and helium plasma. Changes in the energy, adhesion, and chemical composition of the surfaces were determined by contact angle measurements, mechanical pull tests, and X-ray photoelectron spectroscopy (XPS). Surface-energy calculations revealed that after plasma treatment the polarity of PET and PEN increased 6 and 10 times, respectively. In addition, adhesive bond strengths were enhanced by up to 7 times. For PET and PEN, XPS revealed an 18-29% decrease in the area of the C 1s peak at 285 eV, which is attributable to the aromatic carbon atoms. The C 1s peak area due to ester carbon atoms increased by 11 and 24% for PET and PEN, respectively, while the C 1s peak area resulting from C-O species increased by about 5% for both polymers. These results indicate that oxygen atoms generated in the plasma rapidly oxidize the aromatic rings on the polymer chains. The Langmuir adsorption rate constants for oxidizing the polymer surfaces were 15.6 and 4.6 s(-1) for PET and PEN, respectively.