Functionalized porous materials, membranes

There is a number of applications which demand to provide a functionalization of the inner surface of a porous material.

Sintered powders of polyethylene can be used for example as water proof pressure equilibration devices in equipment housings. Decreasing the surface energy can considerably improve the performance of such devices. For their application as filters, an altered surface chemistry can modify the permeation properties.

Membrane properties can be optimized or altered by funcionalizing the surface of the active layer. Hydrophobic coatings on pervaporation membranes increases the time before failure due to degradation.  

3D printed parts often are porous. Activation and coating can be applied to close the pores.

XPS oxygen distribution over the cross-section of porous PE (5 mm x 5 mm).

Porous PE as received (back), plasma oxidized (front).

Activation inside the pores  

An oxygen plasma activates the pore surface in a very efficient and clean way. Surface oxygen concentration (XPS) over a cross-section of sintered polyethylene cylinders with a diameter of 5 mm and a height  of 5 mm (size 5 mm by 5 mm). Left: normal treatment; middle: optimised treatment; right: optimized treatment and washed with water.

The surface free energy (SFE) of polyethylene is small and it results in a negative capillary effect, i.e. water does not penetrate into the pores unless there is a pressure that forces it. (see upper part of the picture). Due to the oxidation the SFE increases and, finally, the water penetrates into the pores easily. (lower part of the picture)

The ability of a low-pressure plasma to penetrate into pores was investigated using sintered PE powders (Surface oxidation inside of macroscopic porous polymeric materials). It largely depends on the size of the pores. For materials with a nominal pore size of 80 µm the activation with an oxygen plasma penetrates many mm while the penetration is limited to few mm in the case of pores sized of 7 µm.

Cross-section of tropaeolin stained porous PE which was treated in an oxygen plasma (RF, 20 W) and than functionalized with poly(ethylene imine). Left: pore size 120 µm, plasma treatment 60 s; right: pore size 7 µm, plasma treatment 1200 s.

Determination of basic functional groups.

Amino functionalization

Surfaces can be prepared with various functional groups by coupling molecules ranging from small ones to polymers.

Coupling diaminoethane or poly(ethylene imine) to an activated (oxidized) polymer are a facile way of creating an amino-functionalized surface. The distribution of the amino groups in the porous material can be visualized by staining with a dye as tropaeolin.

The concentration of amino groups was determined photometrically after the dye was removed by a sodium hydroxide solution.

Membrane functionalization

Amino-functionalized membranes were prepared as described above: 1) activation with an oxygen low-pressure plasma and 2) covalent coupling of poly(ethylene imine) from an aqueous solution.  

Various materials and various pores sizes were used. The determination of the amino concentration with tropaeolin gives the numbers in the table.

membranE

pore size

[NHx]

  μm nmol/cm2
PET 0.2 112
Polysulfon 0.2 1.7
PE 0.65 2
PE 1 1.8
PE 0.65 21
PA 0.2 140
Cellulose 0.2 1

 

These amino groups provide the opportunity for a further functionalization with bio-molecules such as proteins, enzymes for a wide variety of applications. All the treatments can be carried out in a continuous manner in roll-to-roll equipment.  

This is an example for many different ways of tailoring the surface properties of membranes. Plasma treatments can be done with many different process gases to obtain a simple activation, but also permanently hydrophilic or hydrophobic properties. Corona and flame treatments can by applied for functionalization at atmospheric pressure.