Production of porous materials by the expansion of polymer gels

Abstract

A method produces porous materials by expansion of polymer gels. The porous materials can be a micro- or nano-porous polymer materials.

Claims

1. A micro- or nano-porous polymer material, wherein the polymer material is of a density in a range from 10 to 300 kg/m.sup.3, wherein when counting at least 300 pores of the polymer material, an average size of the at least 300 pores is in a range from 0.05 to 0.5 μm, wherein the micro- or nano-porous polymer material is not soluble in its own monomer, wherein the polymer material is selected from the group consisting of a polystyrene consisting of styrene as the monomer and divinyl benzene (DVB) as a crosslinker and a polymethyl methacrylate (PMMA) consisting of methylmethacrylate as the monomer and N, N′ methylene bisacrylamide (MBAA) as a crosslinker.

2. The micro- or nano-porous polymer material according to claim 1, wherein the polymer material and/or the starting material are partially crosslinked.

3. The micro- or nano-porous polymer material according to claim 1, wherein arithmetic mean of thickness of webs between pores of the polymer material is in a range from 5 to 50 nm.

4. The micro- or nano-porous polymer material according to claim 3, wherein the arithmetic mean of thickness of webs is in a range from 10 to 35 nm.

5. The micro- or nano-porous polymer material according to claim 1, wherein the micro- or nano-porous polymer material is partially closed-cell.

6. The micro- or nano-porous polymer material according to claim 1, wherein the polymer material has a density in a range from 30 to 200 kg/m.sup.3.

7. The micro- or nano-porous polymer material according to claim 1, obtained by a method comprising: swelling a polymer starting material using a plasticizer at a predetermined temperature to make the polymer starting material visco-elastic deformable, wherein the polymer starting material is at least partially crosslinked and the crosslinker content in the polymer starting material is in a range from 0.01 to 10 mol %, wherein the predetermined temperature is in a range from 0 to 100° C.; subsequently, contacting the swollen polymer starting material with a foaming agent under a first pressure; and subsequently, depressurizing from the first pressure to a second pressure such that the swollen polymer starting material further expands and solidifies to obtain a micro- or nano-porous material.

8. A molded body made of a micro- or nano-porous polymer material according to claim 1.

9. The molded body according to claim 8, wherein the moulded molded body is sealed.

10. The moulded molded body according to claim 8, wherein the molded body has a thermal conduction in a range from 1 to 30 mW/(m.Math.K).

11. The molded body according to claim 10, wherein the molded body has a thermal conduction in a range from 10 to 26 mW/(m.Math.K).

12. The micro- or nano-porous polymer material according to claim 1, wherein a content of the crosslinker is 0.1 to 1 mol %.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1: Scanning electron micrographs of purchased acrylic glass foamed by the NF-GAFFEL method. The mass ratio of acrylic glass to acetone was 1:3. The sample was subjected to a CO.sub.2 atmosphere at p=250 bar, T=55° C., t=15 min and subsequently expanded (t.sub.exp≈1 s). The foam density is 0.35±0.05 g/cm.sup.3.

(2) FIG. 2: Scanning electron micrographs of PMMA foamed by the NF-GAFFEL method. The PMMA gel was saturated with acetone. The samples were subjected to a CO.sub.2 atmosphere at p=250 bar, T=55° C., t=15 min and subsequently expanded (t.sub.exp≈1 s). The crosslinker concentration (MBAA) was ν=0.2 mol % in the left foam and ν=0.7 mol % in the right foam. The foam densities have values of 0.35±0.05 g/cm.sup.3 (left) and 0.40±0.05 g/cm.sup.3 (right).

(3) FIG. 3: Scanning electron micrographs of PMMA foamed by the NF-GAFFEL method. The PMMA gel was saturated with acetone. The samples were subjected to a CO.sub.2 atmosphere at p=250 bar, t=15 min and subsequently expanded (t.sub.exp≈1 s). The crosslinker concentration (MBAA) was ν=0.7 mol % in all foams. The left sample was foamed at a temperature of T=35° C., the middle sample at T=55° C. and the right sample at T=75° C. The foam densities have values of 0.40±0.05 g/cm.sup.3 (left), 0.40±0.05 g/cm.sup.3 (middle) and 0.40±0.05 g/cm.sup.3 (right).

(4) FIG. 4: Scanning electron micrographs of PMMA foamed by the NF-GAFFEL method. The PMMA gel was saturated with acetone. The samples were subjected to a CO.sub.2 atmosphere at p=250 bar, T=55° C. and subsequently expanded (t.sub.exp≈1 s). The crosslinker concentration (MBAA) was ν=0.7 mol % in all foams. The left sample was foamed after t=5 min, the middle sample after t=15 min and the right sample after t=60 min. The foam densities have values of 0.45±0.05 g/cm.sup.3 (left), 0.40±0.05 g/cm.sup.3 (middle) and 0.40±0.05 g/cm.sup.3 (right).

(5) FIG. 5: Scanning electron micrographs of PMMA foamed by the NF-GAFFEL method. The PMMA gel was saturated with acetone. The samples were subjected to a CO.sub.2 atmosphere at p=250 bar, T=55° C., t=15 min and subsequently expanded (t.sub.exp≈1 s). The crosslinker concentration (MBAA) was ν=0.7 mol % in the left foam and ν=1.2 mol % in the right foam. In both samples p=0.5 mol % of modifier (EHTG) were present. The foam densities have values of 0.10±0.05 g/cm.sup.3 (left) and 0.25±0.05 g/cm.sup.3 (right).

(6) FIG. 6: Scanning electron micrographs of PS foamed by the NF-GAFFEL method. The mass ratio of PS to acetone was 1:1, resp. The samples were subjected to a CO.sub.2 atmosphere at p=250 bar, T=55° C., t=15 min and subsequently expanded (t.sub.exp≈1 s). The crosslinker concentration was ν=0.5 mol % in the left foam and ν=1.0 mol % in the right foam. The foam densities have values of 0.10±0.05 g/cm.sup.3 (left) and 0.25±0.05 g/cm.sup.3 (right).

(7) FIG. 7: Scanning electron micrographs of PS foamed by the NF-GAFFEL method. The mass ratio of PS to acetone was 1:1, resp. The samples were subjected to a CO.sub.2 atmosphere at p=250 bar, t=15 min and subsequently expanded (t.sub.exp≈1 s). The crosslinker concentration (DVB) was ν=1.0 mol % in all foams. The left sample was foamed at a temperature of 35° C., the middle sample at 65° C. and the right sample at 75° C. The foam densities have values of 0.25±0.05 g/cm.sup.3 (left), 0.25±0.05 g/cm.sup.3 (middle) and 0.20±0.05 g/cm.sup.3 (right).

(8) FIG. 8: Scanning electron micrographs of PS foamed by the NF-GAFFEL method. The mass ratio of PS to acetone was 1:1, resp. The samples were subjected to a CO.sub.2 atmosphere at T=65° C., t=15 min and subsequently expanded (t.sub.exp≈1 s). The crosslinker concentration (DVB) was ν=1.0 mol % in both foams. The left sample was subjected to a CO.sub.2 saturation pressure of p=250 bar and the right sample to a CO.sub.2 saturation pressure of p=150 bar. The foam densities have values of 0.25±0.05 g/cm.sup.3 (left) and 0.25±0.05 g/cm.sup.3 (right).

(9) FIG. 9: Scanning electron micrographs of PS foamed by the NF-GAFFEL method. The mass ratio of PS to acetone was 1:1. The sample was subjected to a CO.sub.2 atmosphere at p=250 bar, T=60° C., t=15 min and subsequently expanded (t.sub.exp≈20 s). The crosslinker concentration (DVB) was ν=2.0 mol %. The foam density is 0.15±0.05 g/cm.sup.3.

(10) FIG. 10: Scanning electron micrographs of PVC foamed by the NF-GAFFEL method. The sample saturated with acetone was subjected to a CO.sub.2 atmosphere at p=250 bar, T=70° C., t=10 min and subsequently expanded (t.sub.exp≈1 s). The foam density is 0.20±0.05 g/cm.sup.3.

(11) FIG. 11: Scanning electron micrographs of PE foamed by the NF-GAFFEL method. The sample saturated with cyclohexane was subjected to a CO.sub.2 atmosphere at p=250 bar, T=70° C., t=15 min and subsequently expanded (t.sub.exp≈1 s). The foam density is 0.35±0.05 g/cm.sup.3.

(12) FIG. 12: Scanning electron micrograph of the nanostructure of the polystyrene material given in example 6.

(13) FIG. 13: Scanning electron micrograph of a nanoporous polymer material used for the determination of pore diameters with the Datinf Measure computer program.

DETAILED DESCRIPTION OF THE INVENTION

(14) In the inventive method (NF-GAFFEL method) according to aspect (1) of the invention, the polymer starting material consists of one or several polymers, preferably thermoplastic polymers, alternatively preferably crosslinked polymers, very particularly preferably of a copolymer of the group of polymethyl methacrylate (PMMA/crosslinker), polystyrene (PS/crosslinker), polyvinyl chloride (PVC/crosslinker), polylactide (PL/crosslinker), polyethylene (PE/crosslinker), polypropylene (PP/crosslinker), polycarbonate (PC/crosslinker) and cellophane/crosslinker. The plasticiser is preferably selected from the group of ketones (such as acetone) and other polar aprotic solvents and short chain alkanes such as butane, pentane, hexane and cyclohexane, for example; however, acetone is particularly preferred. Here, the mass ratio of the polymer starting material to the plasticiser is preferably in a range from 10:0.5 to 1:3, particularly preferably in a range from 10:2 to 1:1. Preferably, the swelling time is in a range from 0.1 s to 100 h, particularly preferably in a range from 1 s to 1 h.

(15) The starting material preferably contains from 0.01 to 10 mol % of plasticiser. The crosslinker is preferably divinylbenzene (DVB), ethylene glycol dimethacrylate (EGDMA) or methylene bisacrylamide (MBAA).

(16) The preset temperature in step (a) and/or (b), independently of one another, is preferably in a range from 0 to 100° C. and is particularly preferably in step (a) in a range from 15 to 30° C. and independently thereof in step (b) in a range from 30 to 70° C.

(17) It is further preferred that the swelling in step (a) results in a dense packing of polymer lattices preferably having a mean particle size in the micrometre and nanometre ranges. It is also preferred that step (a) is carried out in an extruder.

(18) In another preferred embodiment of the method according to the invention, the foaming agent in step (b) is selected from CO.sub.2 and other foaming agents which are completely miscible with the plasticiser under high pressure, in particular short chain alkanes such as methane, ethane and propane. The gel is preferably contacted with the foaming agent under a pressure of 10 to 300 bar, particularly preferably from 50 to 200 bar.

(19) In another preferred embodiment the pressure lowering in step (c) takes place within a time ranging from 0.1 s to 60 s in which the polymer material cools and solidifies.

(20) The obtained micro- or nanoporous polymer material preferably has a density from 0.5% to 50% of the density of the polymer starting material and/or a mean pore size from 0.01 to 10 μm.

(21) The NF-GAFFEL method according to the invention could already successfully be carried out in a batch process for copolymers such as PMMA/crosslinker, PS/crosslinker and PVC/crosslinker. The preparation of a coherent polymer gel is important for a successful production of nanoporous polymer materials.

(22) Preferably, the method according to the invention is carried out as a batch method and not as a continuous method.

(23) Preferably, the parameters of pressure and temperature are selected to be above the binary miscibility gap of the plasticiser and particularly preferably thermostatting is carried out for a sufficient time (depending on the sample thickness or the sample volume) so that after expansion foams with pore diameters in the lower micrometre or nanometre range will always be obtained.

(24) Especially desired properties such as, for example, low densities, small pores, open-/closed-pored foam structures, etc. may be obtained by varying the following parameters which can be classified into three production processes:

(25) 1. Production of the polymers: In the production of the polymers, the following parameters are particularly preferably important for the final product: Crosslinker, modifier and copolymer and the concentrations thereof The type of radical starter and the quantity used Temperature and duration of the polymerisation reaction Homogenisation during polymerisation After treatment of the polymer (purification, extrusion, annealing, etc.)

(26) 2. Production of the polymer gels: In the production of the polymer gels, the following parameters are particularly preferably important for the final product: Plasticisers and the ratio between the solid polymer and the plasticiser The duration of exposure of the plasticiser in the polymer gel The saturated polymer gel or defined plasticiser concentrations

(27) 3. Production of the polymer foams: here, the following parameters are particularly preferably important: The type of foaming agent Pressure, temperature, time Expansion speed

(28) Preferably, the following parameters are required for the exact composition of the polymer gels.

(29) The mass fraction of the radical starter used refers to the total mass of the sample.

(30) $\sigma =\frac{m_{\mathrm{starter}}}{\underset{i}{\overset{j}{.Math.}}{m}_i}$

(31) The mass fraction of the plasticiser in a plasticiser/polymer mixture is defined as follows:
λ=m.sub.gelling agent/(m.sub.gelling agent+m.sub.polymer).

(32) For an exact composition of the polymer, the concentrations of all additives such as, for example, the crosslinker and the modifier are given on a molar basis as follows:

(33) The molar ratio of a crosslinker in a crosslinker/monomer mixture is given by
υ=n.sub.crosslinker/(n.sub.crosslinker+n.sub.momomer).Math.100.

(34) The molar ratio of additional additives such as, for example, a modifier in an additive/monomer crosslinker mixture is given by:

(35) $\rho =\frac{n_{\mathrm{additive}}}{\underset{i}{\overset{j}{.Math.}}{n}_i}.Math.100.$

(36) The polymer gel is preferably obtained in time-economic steps depending on the processing form. Thus, for thin films of the order of 1 μm preferably a direct contacting with the plasticiser is expedient. Preferably, bulk materials require either polymer latex powders which are compacted and then soaked with the plasticiser or polymer granulates which are obtained by various methods such as milling or freeze drying. Preferred are polymer starting materials with one dimension, length, width or height, on the order of micrometres to obtain a fast swelling.

(37) After the provision of the required polymer gels the desired foam can be produced by the method (NF-GAFFEL) according to the invention. Both a batch process and a continuous method are suitable for a large-scale implementation of the foaming process. A batch process is particularly preferred for steps b) and/or c).

(38) 1. The production of the foams in a batch process can be carried out in accordance with examples 1 to 5. Preferably, this requires matching the scale of the pressure-proof equipment to the desired foam quantity and maintaining the above-mentioned parameters such as pressure, temperature and residence time of the polymer gel in the foaming agent atmosphere, for example. Possible pressure-proof equipment is preferably autoclaves which resist the required process parameters and are already used in industrial processes. Hence, the polymer gels can preferably be contacted with the compressed foaming agent in a closed container and expanded to a solid nanofoam after a sufficient thermostatting.

(39) 2. The production of the foams in a continuous process could be realised in an extruder, for example. Since the polymer gels are easily formable already at room temperature, they can be continuously processed in an extruder without supplying heat energy and foamed by adding the foaming agent by the process of the invention (NF-GAFFEL). In this process, energy is introduced into the system preferably in the form of shearing which provides for a steady increase of the polymer gel surface. Here, the formation of the homogeneous mixture of foaming agent and plasticiser in the polymer gel is preferably accelerated and therefore reduces the time of contact between foaming agent and polymer gel. Preferably, large-scale extruders allow adjusting the required parameters of pressure and temperature easily provided that the extruder is sufficiently gasproof. The time of contact between the foaming agent and the polymer gel can preferably be varied by the extrusion canal length and the screw speed. Preferably, the polymer gel filled with foaming agent can be expanded at the extruder end to form a solid nanofoam by opening a valve. Preferably, a pressure gradient resulting in a slow expansion (t.sub.exp≈20 s) is advantageous here (see Example 2.4).

(40) The invention will be further elucidated by the following, non-restrictive examples.

EXAMPLES

Example 1: Polymethyl Methacrylate (PMMA) Nanofoams

Example 1.1: Production of a PMMA Nanofoam by the NF-GAFFEL Method

(41) A PMMA gel was prepared by adding acetone to a sample of a conventional acrylic glass and subsequently foamed with CO.sub.2. The expansion time t.sub.exp from the initial pressure p=250 bar to the normal pressure of 1 bar was approx. 1 second. The mass ratio of PMMA to acetone in the polymer gel was 1:3 (λ.sub.acetone=0.75). The gel was contacted with CO.sub.2 in a high-pressure cell at p=250 bar, T=55° C. and t=15 min and subsequently expanded. FIG. 1 shows the foam structure at two different magnifications.

(42) The gel/foam composition was: σ=unknown, λ.sub.acetone=0.75, n=unknown, ρ=unknown.

Example 1.2: Production of a PMMA Nanofoam by the NF-GAFFEL Method

(43) A PMMA consisting of the monomer methyl methacrylate (MMA) and the crosslinker N,N′-methylene bisacrylamide (MBAA) was polymerised using the radical starter azobisisobutyronitrile (AIBN) at T=95° C. and a period of 2 h. Example 1.2 illustrates the different impacts of two different crosslinker concentrations on the obtained foam structure. A polymer saturated with acetone was prepared and contacted with CO.sub.2 and foamed as in Example 1 at p=250 bar, T=55° C. and t=15 min. This example demonstrates the successful application of the NF-GAFFEL method using a PMMA sample with a precisely known composition.

(44) The left-hand portion of FIG. 2 shows the effect on the resulting foam structure at a crosslinker content of ν=0.2 mol % and the right-hand portion of FIG. 2 shows a distinctly smaller foam structure due to the effect of ν=0.7 of crosslinker.

(45) The gel/foam composition was: σ.sub.AIBN=0.004, λ.sub.acetone=saturated, ν.sub.MBAA=0.20 mol % (left)/0.70 mol % (right).

Example 1.3: Production of a PMMA Nanofoam by the NF-GAFFEL Method

(46) A PMMA consisting of the monomer methyl methacrylate (MMA) and the crosslinker N,N′-methylene bisacrylamide (MBAA) was polymerised using the radical starter azobisisobutyronitrile (AIBN) at T=95° C. and a period of 2 h as in Example 1.2. Example 1.3 illustrates the effect of the foaming temperature on the foam structure. For this purpose the PMMA gel was produced as in Example 2 and contacted with CO.sub.2 and foamed at p=250 bar, t=15 min. The CO.sub.2 contacting temperature and thus also the expansion temperature was varied. The left-hand portion of FIG. 3 shows the resulting foam structure at T=35° C., in the middle portion the temperature was T=55° C. and in the right-hand portion the temperature was T=75° C.

(47) The gel/foam composition was: σ.sub.AIBN=0.004, λ.sub.acetone=saturated, ν.sub.MBAA 0.70 mol %.

Example 1.4: Production of a PMMA Nanofoam by the NF-GAFFEL Method

(48) A PMMA consisting of the monomer methyl methacrylate (MMA) and the crosslinker N,N′-methylene bisacrylamide (MBAA) was polymerised using the radical starter azobisisobutyronitrile (AIBN) at T=95° C. and a period of 2 h as in Examples 2 and 3.

(49) Example 4 illustrates the effect of the residence time of the gels in the CO.sub.2 atmosphere on the foam structure. For this purpose the PMMA gel was produced as in Example 2 and foamed at p=250 bar. The crosslinker concentration was ν=0.7 mol % and the foaming temperature was T=55° C. in all experiments.

(50) FIG. 4 shows the results of three foaming experiments varying in the time the gel was subjected to the CO.sub.2 atmosphere. The left foam was obtained after a saturation time of t=5 min, the middle foam after t=15 min and the right after t=60 min.

(51) The gel/foam composition was: σ.sub.AIBN=0.004, λ.sub.acetone=saturated, ν.sub.MBAA 0.70 mol %.

Example 1.5: Production of a PMMA Nanofoam by the NF-GAFFEL Method

(52) A PMMA consisting of the monomer methyl methacrylate (MMA), the crosslinker N,N′-methylene bisacrylamide (MBAA) and the modifier 2-ethylhexylthioglycolate (EHTG) was polymerised using the radical starter azobisisobutyronitrile (AIBN) at T=95° C. and a period of 2 h. Example 5 illustrates the effect of the modifier on the foam structure of two polymers at different crosslinker concentrations. For this purpose the polymers were saturated with acetone and contacted with CO.sub.2 and foamed as in the above examples at p=250 bar, T=55° C. and t=15 min. The modifier concentration of p=0.50 mol % was the same in both foams.

(53) The left-hand portion of FIG. 5 shows the effect on the resulting foam structure at a crosslinker content of ν=0.7 mol % and the right-hand portion of FIG. 5 shows the effect at ν=1.20 mol % of crosslinker. The modifier (EHTG) has an enormous influence on the properties of the foam pores. Monodisperse closed-cell foams having pore sizes in the lower micrometre range (left) and foams with regions of different pore diameters (right) can be produced easily. This allows to produce foams with regions having different properties.

(54) The gel/foam composition was: σ.sub.AIBN=0.004, λ.sub.acetone=saturated, ν.sub.MBAA=0.70 mol % (left)/1.20 mol % (right), ρ.sub.EHTG=0.50 mol %.

Example 2: Polystyrene (PS) Nanofoams

Example 2.1: Production of PS Nanofoams With Different Pore Sizes by the NF-GAFFEL Method

(55) A PS consisting of the monomer styrene and the crosslinker divinylbenzene (DVB) was polymerised using the radical starter azobisisobutyronitrile (AIBN) at T=90° C. and a period of 2 h.

(56) A PS gel was prepared from PS and the same weight of acetone (λ.sub.acetone=0.50) and contacted with CO.sub.2 in a high-pressure cell at ρ=250 bar, T=55° C. and t=15 min and subsequently expanded. FIG. 6 shows the foam structure of two samples containing different crosslinker concentrations. The left sample has a crosslinker concentration of ν=0.5 mol % and the right sample has a crosslinker concentration of ν=1 mol %.

(57) The gel/foam composition was: σ.sub.AIBN=0.004, λ.sub.acetone=saturated, ν.sub.DVB=0.5 mol % (left)/1.00 mol % (right).

Example 2.2: Production of PS Nanofoams by the NF-GAFFEL Method

(58) PS gels were produced as in Example 2.1 and contacted with CO.sub.2 at p=250 bar, T=55° C. and t=15 min and subsequently expanded. The crosslinker concentration (DVB) was ν=1 mol %. Three foaming experiments were conducted at different temperatures. The foam in the left-hand portion of FIG. 7 was obtained at a temperature of T=35° C., the foam in the middle portion at T=65° C. and foam in the right-hand portion at T=75° C.

(59) The gel/foam composition was: σ.sub.AIBN=0.004, λ.sub.acetone=0.50, ν.sub.DVB 1.00 mol %.

Example 2.3: Production of PS Nanofoams by the NF-GAFFEL Method

(60) PS gels were produced as in Example 2.1 and subsequently contacted with different CO.sub.2 pressures and foamed at T=65° C. after t=15 min. The crosslinker concentration (DVB) was ν=1 mol %. In FIG. 8 the left foam resulted from a CO.sub.2 pressure of p=250 bar, whereas the right foam was obtained at a CO.sub.2 pressure of p=150 bar.

(61) In both experiments the parameters are chosen such that they are above the binary miscibility gap between CO.sub.2 and acetone.

(62) The gel/foam composition was: σ.sub.AIBN=0.004, λ.sub.acetone=0.50, ν.sub.DVB 1.00 mol %.

Example 2.4: Production of PS Nanofoams by the NF-GAFFEL Method

(63) PS gels were produced as in Example 2.1 and contacted with CO.sub.2 at p=250 bar, T=60° C. and t=15 min and subsequently expanded. The crosslinker concentration (DVB) was ν=2 mol %. The decisive difference to all previous foaming experiments was the duration of the expansion process from 250 to 1 bar. FIG. 9 shows the foam obtained with a foaming time of t.sub.exp≈20 s.

(64) The gel/foam composition was: σ.sub.AIBN=0.004, λ.sub.acetone=0.50, ν.sub.DVB 2.00 mol %.

Example 3: Polyvinyl Chloride (PVC) Nanofoams; Production of a PVC Nanofoam by the NF-GAFFEL Method

(65) A saturated PVC gel was prepared by adding acetone to a sample of a conventional PVC polymer and subsequently foamed with CO.sub.2. The gel was contacted with CO.sub.2 in a high-pressure cell at p=250 bar, T=70° C. and t=10 min and subsequently expanded. FIG. 10 shows the foam structure at two different magnifications.

(66) The gel/foam composition was: σ=unknown, λ.sub.acetone=saturated, ν=unknown, ρ=unknown.

Example 4: Polyethylene (PE) Nanofoams; Production of a PE Nanofoam by the NF-GAFFEL Method

(67) A saturated PE gel was prepared by adding cyclohexane to a sample of a conventional PE polymer at 60° C. and subsequently foamed with CO.sub.2. The gel was contacted with CO.sub.2 in a high-pressure cell at p=250 bar, T=70° C. and t=15 min and subsequently expanded. FIG. 11 shows the foam structure at two different magnifications.

(68) The gel/foam composition was: σ=unknown, λ.sub.cyclohexane=saturated, ν=unknown, ρ=unknown.

Example 5: Preparation of 10 g of Nanoporous Polystyrene Particles

(69) Ten grams of crosslinked polystyrene particles (mean diameter 1 mm, 1 mol % of DVB as crosslinker) is contacted with 20 g of acetone in a sealed vessel for 180 minutes at room temperature and under normal pressure. Due to the different refractive indexes of the polymer and the polymer gel, swelling could be followed visually to determine when the polystyrene particles were completely converted into the polymer gel. The polystyrene gel particles were dimensionally stable and had a spherical shape but were—contrary to the starting polymer—deformable by slight mechanical impact. Then, the swollen polystyrene particles were subjected to a CO.sub.2 atmosphere at 200 bar and 70° C. for 90 minutes. A pressure relief resulted in nanoporous polystyrene particles with a mean diameter of 2 mm and a density <0.30 g/cm.sup.3 and a mean nanopore diameter <500 nm (see FIG. 12).

Example 6: Determination of the Structure of the Nanoporous Materials

(70) In order to image the structure of the produced nanoporous materials, first a fresh fracture edge was created. Subsequently, the sample was fixed on the sample plate with the fracture edge facing upward. In order to dissipate the charge generated during measurement, conductive silver lacquer was used for fixation. Prior to imaging, the sample was coated with gold in order to avoid local charging effects. For this purpose the K950X coating system with the K350 sputter attachment from EMITECH was used. In all cases gold sputtering was performed under an argon pressure of approx. 10.sup.−2 mbar, always applying a current of 30 mA for 30 seconds. The layer thickness of the gold layer coated in this way was approximately between 5 and 15 nm. The electron photomicrographs were taken with a device of the SUPRA 40 VP type from ZEISS. Acceleration voltages up to 30 kV and a maximum resolution of 1.3 nm are possible with this device. Micrographs were recorded with the InLens detector at an acceleration voltage of 5 kV. In order to determine the mean pore diameter and the mean web thickness of the nano- and microporous foams, a micrograph taken with the described scanning electron microscope was chosen and at least 300 pores or webs were measured with the Datinf Measure computer program to ensure a sufficiently good statistics. Since each scanning electron micrograph contains only a limited number of pores and webs, several scanning electron micrographs are used for the determination of the mean pore and web diameters. Here it should be ensured that the magnification is chosen such that the error in the length determination is kept as small as possible. An example is shown in the Annex in FIG. 13.

(71) The features of the invention disclosed in the present description, in the drawings as well as in the claims both individually and in any combination may be essential to the realization of the various embodiments of the invention.