Free Standing Dry and Stable Nanoporous Polymer Films Made through Mechanical Deformation

We offer a new straight forward approach to create nanoporous polymer membranes with well defined average pore diameters between 10 and 50 nm account for more than 70 % of the total pore volume.

The process is based on (fast) mechanical deformation of highly entangled polymer films at high temperatures and a subsequent quench below the glass transition temperature T_g. The methodology does not need any chemical processing, supporting substrate, or self assembly  process and is solely based on polymer inherent entanglement effects. The resulting membranes are stable at ambient conditions and display remarkable elastic properties.

Well controlled porous membranes can easily be made from various commodity polymers like polystyrene, polycarbonates, polyetherketones and -sulfones, polyamides, polyesters, polylactic acids, perfluoralkoxyalkanes, polyvinylchloride, polymethacrylamides, poly(meth)acrylates, polyphenyleneethers, -sulfides and -sulfones, polysulfones, styrene-acrylonitriles, polyarylates and cellulose acetates for example. Guidelines to optimize  the process parameters based on the used polymer and the wanted product specification is available.

For first laboratory proof of concept experiments monodisperse atactic polystyrene of M_n = 1×10⁶ Da obtained from Sigma Aldrich was used. Transparent PS films of about 100 μm thickness were prepared by pressing 250 mg of PS under 20 kN load and temperature 433 K (well above Tg = 380 K) for an hour.

To deform the film with a biaxial stress, the PS film was immobilized by the edges over circular aperture of 1.75 cm of diameter and deformed by applying a load using the negative pressure generated by a controlled vacuum of 50 mbar at a temperature of 473 K for different times of duration. Depending on the duration of vacuum pressure applied, different degrees of stretching were achieved in PS films as given in Table 1. To preserve the morphology of the stretched films, they were quenched by dipping them into liquid nitrogen. Computer simulation results reveal that such a quench is not needed and that  cooling down below Tg is sufficient. After that films were kept at room temperature without any special conservation precautions.

Because it was found that the transversal and longitudinal sound velocities C_T and C_L of the porous polymer films produced with the offered method are higher than the sound velocity of the bulk material, Brillouin Light Spectroscopy (BLS) can be used as a non-contact, non-invasive and zero-strain method for the identification of the nanoporous polymer material claimed by our patent application.

Kurt Kremer et al: "Free Standing Dry and Stable Nanoporous Polymer Films Made through Mechanical Deformation", Adv. Sci. 2023, 10, 2207472

EP Priority patent application filed in April 2023.