Macromolecular structural evolution of polyimide chains during large-ratio uniaxial fiber orientation process revealed by molecular dynamics simulation

The Effect of Mechanical Elongation on the Thermal Conductivity of Amorphous and Semicrystalline Thermoplastic Polyimides: Atomistic Simulations

Over the past few decades, the enhancement of polymer thermal conductivity has attracted considerable attention in the scientific community due to its potential for the development of new thermal interface materials (TIM) for both electronic and electrical devices. The mechanical elongation of polymers may be considered as an appropriate tool for the improvement of heat transport through polymers without the necessary addition of nanofillers. Polyimides (PIs) in particular have some of the best thermal, dielectric, and mechanical properties, as well as radiation and chemical resistance. They can therefore be used as polymer binders in TIM without compromising their dielectric properties. In the present study, the effects of uniaxial deformation on the thermal conductivity of thermoplastic PIs were examined for the first time using atomistic computer simulations. We believe that this approach will be important for the development of thermal interface materials based on thermoplastic PIs with improved thermal conductivity properties. Current research has focused on the analysis of three thermoplastic PIs: two semicrystalline, namely BPDA-P3 and R-BAPB; and one amorphous, ULTEM^TM. To evaluate the impact of uniaxial deformation on the thermal conductivity, samples of these PIs were deformed up to 200% at a temperature of 600 K, slightly above the melting temperatures of BPDA-P3 and R-BAPB. The thermal conductivity coefficients of these PIs increased in the glassy state and above the glass transition point. Notably, some improvement in the thermal conductivity of the amorphous polyimide ULTEM^TM was achieved. Our study demonstrates that the thermal conductivity coefficient is anisotropic in different directions with respect to the deformation axis and shows a significant increase in both semicrystalline and amorphous PIs in the direction parallel to the deformation. Both types of structural ordering (self-ordering of semicrystalline PI and mechanical elongation) led to the same significant increase in thermal conductivity coefficient.

New Strategy for Inducing Surface Anisotropy in Polyimide Films for Nematics Orientation in Display Applications

The operability of liquid crystal displays is strongly impacted by the orientation aspects of nematics, which in turn are affected by the alignment layer surface features. In this work, two polyimide (PI) structures are obtained based on a cycloaliphatic dianhydride and aromatic or aliphatic diamines with distinct flexibility. The attained PI films have high transmittance (T) for visible radiations, i.e., at 550 nm T > 80%. Here, a novel strategy for creating surface anisotropy in the samples that combines rubbing with a cloth and stretching via pressing is reported. Birefringence and atomic force microscopy (AFM) scans reveal that the generated orientation of the chains is affected by the chemical structure of the polymer and order of the steps involved in the surface treatment. Molecular modeling computations and wettability tests show that the PI structure and produced surface topography are competitive factors, which are impacting the intensity of the interactions with the nematic liquid crystals. The achieved results are of great relevance for designing of reliable display devices with improved uniform orientation of liquid crystals.

Revealing the High-Modulus Mechanism of Polyimide Films Prepared with 3,4'-ODA

To prepare PIs (polyimides) with desirable thermal and mechanical properties is highly demanded due to their widespread applications in flexible optoelectronic devices and printed circuit boards. Here, the PI films of BPDA/4,4′-ODA, BPDA/3,4′-ODA, PMDA/4,4′-ODA, PMDA/3,4′-ODA systems were prepared, and it was found that the PIs with 3,4′-ODA always exhibit a high modulus compared with the PIs with 4,4′-ODA. To disclose the mechanism of high-modulus PI films with 3,4′-ODA, amorphous PI models and uniaxial drawing PI models were established and calculated based on MD simulation. The PI structural deformations at different length scales, i.e., molecular chain cluster scale and repeat unit scale, under the same stress were detailed and analyzed, including the variation of chain conformation, bond length, bond angle, internal rotation energy, and torsion angle. The results indicate that PIs with 3,4-ODA have higher internal rotation energy and smaller deformation with the same stress, consistent with the high modulus.