Polymer film physics

I. Pyroelectricity
Figure: Pyroelectric device. (a), Schematic of a sandwich-structured device and the electric poling process. The dipole domains are aligned along the direction of the electric field when a high voltage is applied. (b), An example of a flexible sensor array prepared by fully printing. (c), A sequence of thermal images captured by the infrared camera. A homogeneous temperature field is observed across the large area device (roughly 2 cm x 2 cm). Scale bar: 10 mm.

Polymer-based pyroelectric thin films are crucial functional materials at the core of flexible and lightweight electronic devices, such as wearable monitoring sensors, energy harvesters, and infrared detectors. Exposed to a temperature change, the polymer film generates electricity by changing the dipole moment in the polarized material. Nevertheless, the pyroelectric features of polymer films, such as poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), vanish when the surrounding or detecting temperature exceeds the ferroelectric-to-paraelectric transition temperature, and thus limits the pyroelectric performance in a low-temperature range. 

Herein, a novel strategy is proposed to mitigate this issue, in which a new class of P(VDF-TrFE) copolymer with less molar ratio of TrFE is employed. Experimentally, we reduced the defects through thermal annealing in a vacuum, which remarkably broadens the working temperature range (roughly up to 140 °C) where the polymer films still retain high pyroelectric properties (pyroelectric coefficient 50 μC/(m2K)). 

Our study provides an alternative choice for exploiting pyroelectric polymer films with enhanced pyroelecticity in applications, such as thermal/infrared detectors, that require a wide applicable temperature range.

Collaborators: Guillaume Fleury (Univ. Bordeaux); Hamid Kellay (Univ. Bordeaux); Georges Hadziioannou (Univ. Bordeaux)

Paper: Pyroelectricity and temperature stability in flexible polymer thin films (under review)

II. Wrinkling instability
Figure: Solvent drop interaction with a polymer thin film. (a) An experimental setup for introducing solvent drops and further wrinkling instability observation using an optical microscope. The number of wrinkles near the front of the drop spreading is found linearly increasing with the size of the drop. (b) The prediction model agrees well experimental data, in which we are able to estimate the adhesion energy between the film and the substrate.

Polymer based conductive and transparent thin films are an important class of functional materials at the heart of flexible organic electronic devices. Stacking printed thin films layer by layer is a low cost and frequently used technique to build structured devices with the desired electronic properties and functionalities. The mechanical stability and adhesion between the adjacent layers is key in the performance and lifetime of such devices. Thin flexible films are, however, prone to a number of instabilities: cracks, tears, or delamination and wrinkling. This is detrimental to their use especially in the case of multilayer devices. 

Here, it is shown that a simple water or solvent drop deposited on such films gives rise to a buckling instability and the formation of several folds due to the tendency of these films to swell in contact with the solvent. We found that the number of wrinkles at the front of drop spreading increases linearly with the drop size. Depending on the properties of the films such as thickness and Young’s modulus, the films could either exhibit wrinkling or stable exposed to the solvent drop. By using an energy balance (bending, adhesion, and interfacial energies due to wetting), we proposed a model to describe the wrinkling instability (see the Figure).  

CollaboratorsJean‐Michel Rampnoux (Univ. Bordeaux); Abdelhamid Maali (CNRS, Bordeaux); Eric Cloutet (CNRS, Bordeaux)

                           Georges Hadziioannou (Univ. Bordeaux); Hamid Kellay (Univ. Bordeaux)

PaperDelamination and Wrinkling of Flexible Conductive Polymer Thin Films. Advanced Functional Materials, 2021, 31(21): 2009039.