WO2006089529A1 - Invention concerning dimensioning of meso-and nanostructures - Google Patents

Abstract

The invention concerns the field of dimensioning or making meso- and nanostructures having a dimension ranging between 1 nm and 1000 µm using a cutting edge and a suitable cooling process.

Description

Invention relating to the dimensioning of meso- and nanostructures

The invention relates to the field of sizing of meso- and nanostructures. In the following, this means the comminution or shortening of structures with typical dimensions (diameter, cross sections, etc.) of less than 1000 μm.

State of the art:

Numerous methods for the production of meso and nano structures are known in the prior art. These include e.g. the extrusion processes.

Electrospinning certainly constitutes one of the currently most important methods in science and technology for the production of meso- and nanostructures, in particular polymers, i. Essentially, in electrospinning, a polymer melt or polymer solution is exposed to a high electric field on a cannula or tip serving as an electrode. By electrostatic charging of the polymer melt or polymer solution, a flow of material directed at the counter electrode is formed, which solidifies on the way to the counter electrode. Depending on the electrode geometries, so-called nonwovens or ensembles of ordered fibers are obtained. While with polymer melts so far only fibers with diameters significantly greater than 1 micron are obtained, one can produce from polymer solutions fibers with diameters down to 5 nm. Of particular technical interest, e.g. for filtration applications, fibers with diameters smaller than 1 μm.

In electrospinning and in many other known processes for the production of meso or nanostructures, such as the extrusion process arise almost exclusively continuous fibers or other structures, which in their dimension - either directly during manufacture or afterwards must be scaled down in order to put the structures to a meaningful use.

Disadvantages in the prior art:

To our knowledge, in electrospinning, both ends of one and the same fiber have been observed so far. We have made a number of modifications to the electrospinning process for the preparation of short fibers unsuccessfully. Attempts have also been made to produce short pieces of fiber by simply cutting the fibers with sharp blades, but this did not work. Again and again, it has been observed that the fibers are cut but welded together at the fiber ends or otherwise deformed (see Figures 1A and B). FIGS. A and B show the results of cutting tests on structures in the range of 0.3 to 3 μm, the structures consisting of polyamide 6 (FIG. 1A) and polylactide (FIG. 1B, PLA). The cutting experiments were carried out with a scalpel at room temperature.

Similar or similar results were observed with structures ranging from a few nanometers to several 100 μm.

Despite these difficulties, it would be technologically very interesting to be able to produce short electrospun or otherwise made fibers or other small meso nanostructures, as this opens up access to a number of potential applications. It must always be kept in mind that electrospinning is the only technically realized process to date that allows the continuous production of fibers with diameters below 1 μm of almost any polymer. For example, chamfers less than 1000 μm in length are of interest for inhalation applications or as short cut fibers in filtration applications. However, short electrospun fibers can also be used for the production of short TUFT tubes (see DE 100 234 56 A1) or for the preparation of novel membranes. Other new areas of application could be in galenics or cosmetics. Also as composite fibers, z. B. to reinforce materials (eg., Polymers) or to achieve special effects, eg. As optical effects, or variation of the electrostatic behavior such fibers or structures could be used. Especially in the field of modification of the electrical conductivity of such fibers or structures can play a special role. It is well known that the so-called percolation threshold (at about 40% content) can be significantly undermined for mixtures of conductive and nonconductive materials if anisotropic materials are used. So z. B. electrical conductivity of insulating materials by addition of less than 1% carbon nanofibers achieved. The technological advances that would be possible with short electrospun fibers is obvious. It should also be noted that of course not only short polymer fibers of homopolymers, copolymers, blends and composites are of interest, but also other electrospun fibers which are converted via a polymer precursor by a post-treatment to another material, e.g. As electrospun polyacrylonitrile fibers in carbon fibers, metal fibers, ceramic fibers, etc ..

task

The object is to provide a method with which it is possible to dimension meso or nanostructures consisting of one or more materials, preferably including at least one polymer so that a deformation of the structures and / or connection of the structures with other structures at the points of intervention for sizing - usually for shortening or reduction - as completely as possible avoided.

This could then be used e.g. Electrospun cut fibers of defined length after or during the manufacturing process are produced. A filming of the cut fiber ends should not occur, otherwise the properties of isolated cut fibers is lost.

Problem solving and technical progress

It has surprisingly been found that with sharp blades or other sharp-edged objects with initial or periodic or permanent cooling of the structures and / or the cutting edges to low temperatures, eg. B. under liquid nitrogen, electrospun fibers can be cut without the cut ends significantly deform or / and stick together. So succeeded, cut fibers with lengths between 80 and several make a hundred micrometers in length. A significant increase in slicing efficiency was achieved by using parallel blade arrays (see Fig. 2). The distance between the blades determines the length of the fibers after the cut.

Fibers of particularly uniform length have been coated with already, i. see, inter alia, Hou et al; DE 0010053263A1, 08.05.2002; Dersch et al., J. Polym., Part: Polym., Chem., Ed., 41, 545 (2003) In principle, however, unoriented fibers can also be cut as described.

In further experiments it could be found that, as already briefly described above, it is also sufficient to cool the structures before the cut to such an extent that no deformation occurs due to the cut.

An alternative to this was the cooling of the cutting device, i. Of course, the structures and / or the cut edge could of course also be cooled during the cut in order to avoid the deformation of the structures.

With regard to the most studied polymeric structures, it has been found that it is sufficient to bring the cutting edges before the cutting process to a temperature which is substantially in the range of the glass transition temperature of the polymer, which (in the case of a mixture of several polymers) the largest weight fraction having the polymeric structure.

embodiments

Preparation and cutting of oriented electrospun fibers: Polyamide 6 fibers, polylactide fibers, and poly (methyl methacrylate) fibers were first prepared by electrospinning the polymer solutions onto paper. Sufficient parallel orientation was achieved by using a fast rotating metal roller as the counter electrode. Another method for producing a parallel orientation of the fibers is to wind the fibers on a substantially rectangular structure, which by the Electrospinning used at least one electrode in the region of the exit opening of the thread or the later fiber, is polarizable. That is, the structure then acts as a counter electrode and the two longer sides of the substantially rectangular structures provide an orientation of the fibers. Preferably, a rectangular and rotating metal frame is used, which can be filled in hollow or with a non-polarizable medium.

The paper thus coated with polymer fibers (over the metal roll) was then cooled in a metal bowl in a cold bath with liquid nitrogen to - 196 ° C and cut with a likewise cooled metal blade (see Fig. 2 C, D, E). The figures show from left to right with a scalpel under liquid nitrogen cut fiber mats of polyamide 6 (C), polylactide (D), poly (methyl methacrylate) (E, PMMA). In all structures, no significant deformations or even sticking of the fiber ends were found.

Chopped fibers of defined length: When cutting with a scalpel initially no chopped fibers of uniform length were produced. Only cutting with blade blocks, consisting of several mutually parallel blades of disposable razors, led to cut fibers of defined length (see Fig. 3 F, G). Fig. 3 shows 80 μm short cut fibers of PLA (F) and PMMA (G) by cutting with a blade block.

A majority of the cut fibers remained after cutting between the blades, (see Fig. 4, 5) which offers an advantageous possibility to remove the cut fibers from the cold bath separated from the substrate (substrate) paper.

Fig. 4, H, J shows about 50 (H) to about 80 (J) microns short cut fibers of PLA between the blades in the blade block.

Fig. 5, K, L shows about 50 (K) to about 80 (L) microns short cut PMMA fibers between the blades in the blade block.

When cutting, it was also important not to move the blade against the fibers, so not with a draft cut, but to cut with a roll or pressure cut. The minimum length of the cut fibers was determined only technically by the width of the blade body, longer fibers were through the use of spacers between the blades (see Fig. 6). 6, M, N show a blade block with and without spacers (M) and thus produced 600 μm long cut fibers made of PLA (N)

- Suspending the cut fibers: When the blade block was immersed in ethanol, the cut fibers easily detached from the blades and succeeded in suspending, resulting in mutually isolated cut fibers. This could be clearly demonstrated by dripping the suspension onto a paper filter (see Fig. 7). 7 shows PMMA cut fibers (P ₁ Q) suspended and deposited on a paper filter for this purpose.

Thus, a method was also found which allows the orderly release of the wonderfully oriented between the edges of the cutting device structures.

In other materials and structures similarly good results could be achieved with respect to the substantially lacking deformation of the cut ends, the fixation between the edges of the cutting device and with respect to triggering the ordered cut structures from the cutting device by liquids such as ethanol.

As further structures, e.g. Nano and meso tubes or multi-layer nano and meso ribbons or cables, or nanostructured surface (e.g., apertured) nano and meso tubes.

In further experiments, a method was developed in order to further develop the method according to the invention for dimensioning, applied to the case of meso / nanofibers, so that meso / nanostructured fibers of defined length can be continuously produced therewith.

For this purpose, an electrospinning apparatus with a metal tip acting as a nozzle to which a first high potential was applied (and to which a chamber under pressure with the polymer mixture to be spun connected) equipped with a rectangular metal frame as a counter electrode.

In the metal frame, a substantially unpolarisierbares material was attached, which then attached to a cutting edge provided with cutting device as a mating surface. That The fibers oriented between the edges of the metal frame, substantially parallel to the longer metal edges, were then reduced by pressing the cutting edges against the surface of the unpolarizable material.

In this case, the cutting device was matched in its movement to the movement of the rotating metal frame, that the cut was always carried out when a surface of the unpolarisierbaren material was on the, the nozzle side facing away. The cut was carried out while performing a relative to the cut edges rolling movement.

The self-rotating cutting device had to two surfaces

Cutting edges. The cut fibers were then exposed under the action of

Ethanol or another at the temperature of the cooled cut edges still liquid washed out.

The washout was then performed on the surface, which was not in cutting interaction with the metal frame.

The washed out, cut structures could then on a

Liquid surface collected and then arranged relatively on the edge of the

Liquid surface are collected.

As an alternative to the cutting device, of course, a cutting device can be used, which consists of a plurality of cutting edges, which lie on a

Drum which is then in rotating, cutting interaction with said metal frame or a metal roller. Figure legends

Fig. 1: cut with a scalpel at room temperature electrospun

Polyamide 6 (A) or polylactide (B, PLA) fibers

Fig. 2: cut with a scalpel under liquid nitrogen fiber mat

Polyamide 6 (C), polylactide (D) ₁ poly (methyl methacrylate), (E, PMMA)

Fig. 3: 80 micron short cut fibers of PLA (F) and PMMA (G) by cutting with a blade block

4: Overview (H) and detail (J) of 80 μm short cut fibers made of PLA between the blades in the blade block

Fig. 5: Overview (K) and detail (L) of 80 micron short cut fibers of PMMA between the blades in the blade block

Fig. 6: blade block with and without spacer (M) and 600 microns long

Cutting fibers made of PLA (N)

7: suspended PMMA cut fibers (P ₁ Q) deposited on paper filters

Claims

claims 1) A method for sizing meso or nanostructures of a dimension of 1 nanometer to 1000 microns using at least one cutting edge, characterized in that the structures, or the at least one cutting edge, directly or indirectly before or during the execution of the cut once, periodically or permanently cooled. 2) Method according to claim 1, characterized in that the dimensioning is carried out by a vertical or rolling movement of the at least one cutting edge relative to the structure to be dimensioned. 3) Method according to claim 1 or 2, characterized in that the cooling is carried out so that the structures to be dimensioned after dimensioning substantially no deformation, in particular no deformation, which for connection, e.g. in the form of a bond or fusion with adjacent structures leads.