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.
Flat sheets have been used successfully for the immobilization of biomolecules
(biochips). However, in some cases the density of functional groups cannot be
sufficiently high on flat substrates. Porous materials can have a much higher number
of functional groups per unit area.
Activation inside the pores
In a first step the pore surface has to be activated, i.e. functional groups are
formed which then can be used for chemical reactions. An oxygen plasma activates the
pore surface in a very efficient and clean way. The surface oxygen concentration is
stable against washing with water and other solvents.
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XPS oxygen distribution over the cross-section of porous PE (5 mm x 5 mm) |
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: optimised treatment and washed
with water.
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Porous PE as received (back), plasma oxidized (front) |
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)
Functionalization with NH2
With our surface chemistry toolbox we ca prepare
materials with a range of functional groups for the coupling of functional molecule at
the surface.
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Determination of basic functional groups |
With diaminoethane (R = -(CH2)2-) the last reaction can be used
to demonstrate the number that can be obtained with various porous material. The
concentration of amino groups was determined with Tropaeolin which is acidic and binds
to the basic amino groups. After washing with water the dye is removed by a sodium
hydroxide solution. The dye concentration (proportional to the amine concentration) is
measured with UV/Vis spectrometry.
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Tropaeolin stained cross-section of 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 treatement 60 s; right: pore size 7 µm, plasma treatement 1200 s. |
Determination of the amino concentration with Tropaeolin gives the numbers in the table.
| membran | 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 |
Instead of diaminoethane we can couple poly(ethylene imine). After Staining with tropaeolin
we are able to inspect the cross-section of a piece of the porous material. We found that
the plasma penetrates deeply into materials with large pores while small pores prevent the
deep penetration even at long treatment times.