Main results

Within this project are obtaind and patterned novel azo-PI, with checked structures, high glass transition temperatures and thermostability, ability to be processed in the form of films with high flexibility and resistivity, photochromic behavior, adequate morphological features, good adhesion with the tested metals and good dielectric properties, resistance to certain solvents, improved piezoelectric properties induced by the anisotropy of the morphology, succesfuly tested as alternative substrates for flexible/stretchable electronic devices and sensors applications.

Stage I

In the first stage of the project, 5 series of azo-polyimides were synthesized (starting from the control polyimides precursors mixed with two types of azochromophores, in molar ratios of 1:0.25, 1:0.5, 1:0.75 and 1:1) with the supramolecular structure verified by means of spectroscopy in Fourier Transform Infrared Spectroscopy, by highlighting both the chemical structure and intermolecular hydrogen bonds.

The systems were obtained in the form of films, testing the flexibility, torsional strength and resistance to certain solvents used in the process of cleaning to which flexible substrates are subjected before the fabrication of various electronic devices over them.

Thermal characterization was performed by thermogravimetric analysis and differential scanning calorimetry. It was found that all investigated polymers showed a high thermal stability. The glass transition temperatures decreased slightly with the increase in the amount of azochromophore.

The wetting properties were evaluated by measuring the contact angle. With the increase in the amount of azo monomer, a significant increase in the dispersive component of the surface tension was observed.

The photochromic behavior was investigated using ultraviolet-visible spectroscopy (UV-Vis) and it was established that the surfaces can be structured using the third harmonic of a pulsed Nd:YAG laser (with a wavelength of 355 nm).

Dielectric properties were measured by dielectric spectroscopy technique. The dielectric characteristics increased with the increase in the amount of azo component. The investigation of the piezoelectric properties was performed before polarization, the piezoelectric constant d33 being between 7.9 and 8.6 pC/N and a significant increase is expected after polarization.

The supramolecular systems presented different morphology (assessed by atomic force microscopy), varying depending on the components introduced into the system, very low roughness and high isotropy. The local mechanical properties were determined by AFM spectroscopy. The adhesion force varied depending on the nature of the diamine used in the synthesis and the nature of the azochromophore, being higher for the systems with an azochromophore containing -CN. For both types of azochromophore, as the molar ratio increases, the adhesion force also increases.

Stage II

If in the first stage of the project 5 series of azo-polyimides were synthesized with a supramolecular structure verified by highlighting both the chemical structure and the intermolecular hydrogen bonds, in the second stage of the project, the selected systems obtained under the form of films with high flexibility and torsional strength were subjected to simple texturing processes such as phase mask UV laser irradiation (PMUVL), exposure to diffuse coplanar surface barrier discharge plasma (DCSBDP), microstructuring and functionalization by direct UV laser texturing in different gas atmospheres, but also combined, such as PMUVL/unidirectional prestressing (DS), DCSBDP/DS and PMUVL/DCSBDP. 

Simple texturing procedures led to the formation of micrometric, periodic structures, inducing morphological anisotropy in the case of the PMUVL (due to the photochromic behavior) and DCSBDP methods, but also randomly arranged structures on the surface, inducing morphological isotropy, in the case of direct UV laser texturing in different gas atmospheres.

Using the combined texturing processes, dual complex hierarchical patterns on the micrometric and nanometric scale were fabricated on the surface of the supramolecular structures with a high molar ratio between the control polyimide precursor and the azochromophore, their existence being proven by the complete analysis of the AFM data. Thus, topographic images with phase and amplitude contrast, maps obtained by the furrows method, which analyze the uniformity of the formed modulations, representations of the direction of the texture (based on the Fourier transform), representations of the isotropy of the texture (based on the autocorrelation function) and Abbott curves from which the 3D parameters characterizing the relief texture were calculated were used. 

These presented methodologies have the advantage of being cheap and fast, allowing the anisotropy/isotropy of the morphology to be obtained on an appreciable surface. Thus, new materials with high morphological and piezoelectric properties were obtained for applications in state-of-the-art technologies, highlighting their ability to perform their functions in a tribological contact, as anisotropic/isotropic substrates for target applications. 

The initial and textured flexible and extensible substrates exhibited good adhesion with metals and silver nanoparticle-based inks. The stability of sputter-deposited metal layers using a target magnetron assembly as well as inkjet-printed strain gauge or piezoresistive sensors (for process development in printed electronics and smart 3D printing) on initial and textured flexible and stretchable substrates was verified by SEM and AFM, no cracks or unevenness being observed on their surface following repeated bending.

Stage III

In stage III of the project the new flexible azo-polyimides films were tested to reveal the photo-switching behavior and the photo-induced surface changes for photo-actuator and sensor applications, the piezoelectric effect for piezo-actuator applications and the circuits printing for applications in flexible polymer memory devices.

For photoactuation measurements, the samples were kept under the XeCl excimer laser beam (laser wavelength 308 nm, duration pulse 20 ns), at different laser fluences, observing the change in the actuation angle. The best results were obtained on thin samples of 10 μm. A pronounced photoactivation behavior was observed for the P1:AzoCN(1:1) sample. The sample responded to the light stimulus from the first seconds after irradiation, showing the maximum bending of 24 degrees in 7 minutes. Compared to the initial sample, the morphology of the sample after irradiation changes significantly, creating a three-dimensional network aspect, but the photoactuation test does not induce chemical changes.

Piezoelectric materials have the property of generating an electrical charge when subjected to mechanical stress. To evaluate the performance of the studied materials and highlight the direct piezoelectric effect of the films before and after texturing, a d33 piezometer test system was chosen. The values of the piezoelectric constant d33 took high values (around 12 – 13 pC/N) when the static force is small compared to the data presented in the literature for polyimides with -CN groups. As the applied force increases, d33 decreases. It is also found that texturing by PMUVL improves the piezoelectric properties of the material, the piezoelectric constant being superior to that before texturing. The explanation could be that, during irradiation, the process of trans-cis isomerization of the azo segments in the exposed regions favors the orientation of the dipoles, inducing a more organized and compact structure, so that texturing by PMUVL produced a significant surface anisotropy, which induces an increase in the piezoelectric constant. In addition, dielectric properties have also been observed to improve upon surface texturing for the same reasons. A significant increase in dielectric constant is observed immediately after the texturing step, which then decreases slightly over several days.

To make a polymer memory device, the standard geometry was fabricated, consisting of planar electrodes (lower electrode - bit line, upper electrode - word line) forming a sandwich structure with a polyimide memory layer. The memory device was organized into memory cells, each storing one binary bit. For the tests, the supramolecular system P1:AzoCN(1:1) was used with the most relevant properties and the best ink from the point of view of adhesion to the polymer DM-SIJ-3200, taking into account the values of the contact angle, the spreading work and adhesion work. The electrode sizes and desired pattern were fixed, ink release nozzles and the most suitable pattern for printing were tested to achieve optimal results. Thus, 5 electrodes were printed on one side of the flexible film, and on the opposite side a single electrode, arranged perpendicular to the 5. After printing the electrodes, the polymer substrates were inserted into the tempering oven. After tempering, it is observed that the electrodes retain their integrity even after repeated bending of the substrate, without undergoing changes. Subsequently, the lower electrode must be grounded during the electrical measurement. It is necessary to modify the conductivity of the azo-polyimide layer by applying a voltage across the memory cell, allowing one bit of data to be stored, following the analysis of current response statistics and threshold voltage extracted from the flexible memory cell in plan and under bending conditions, to to highlight their efficiency in the flexed state.