EP2643079A1

EP2643079A1 - Pvdf membranes having a superhydrophobic surface

Info

Publication number: EP2643079A1
Application number: EP11802497.5A
Authority: EP
Inventors: André DERATANI; Damien Quemener; Denis Bouyer; Céline POCHAT-BOHATIER; Chia-Ling Li; Da-Ming Wang; Juin-Yih Lai
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite Montpellier 2 Sciences et Techniques; Arkema France SA
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Montpellier 2 Sciences et Techniques; Arkema France SA
Priority date: 2010-11-22
Filing date: 2011-11-22
Publication date: 2013-10-02
Also published as: US20130306560A1; SG191730A1; FR2967591A1; JP5792823B2; CN103347597B; CN103347597A; JP2014504946A; WO2012069760A1; FR2967591B1; KR101796637B1; KR20140037018A

Abstract

The present invention relates to the field of hydrophobic solid surfaces, and more particularly to polyvinylidene fluoride (PVDF) membranes having a superhydrophobic surface. The invention also relates to the process for preparing these membranes and also to the industrial applications thereof. The PVDF membranes according to the invention comprise a superhydrophobic surface comprising a structure that is porous on the nanometre scale and interconnected crystalline nodules of micrometre size.

Description

PVDF MEMBRANES WITH SUPERHYDROPHOBIC SURFACE

The present invention relates generally to the field of hydrophobic solid surfaces, and more particularly to polyvinylidene fluoride (PVDF) membranes with superhydrophobic surface. The invention also relates to the process for preparing these membranes as well as their industrial applications.

By "superhydrophobic" is meant the characteristic of a surface on which a drop of water forms with said surface a contact angle greater than or equal to 150 °. Superhydrophobia is a known physical property that responds to Cassie's law. By definition, the contact angle is a dihedral angle formed by two interfaces contiguous to their apparent intersection. In this case, the surface is described as "non-wetting" with respect to water. This property is commonly referred to as the Lotus effect. Superhydrophobic surfaces have a high roughness. Indeed, it is the nanometric roughness of a surface that confers the property of superhydrophobia, as shown in the publication by Lafuma A. and Quéré D. (2003): "Superhydrophobie States", Nature Materials, 2 (457-460 ).

Polymeric membranes are generally produced by a phase inversion process. Entry of a non-solvent into a polymer solution causes separation between a polymer-rich phase constituting the continuous matrix of the material and a discontinuous polymer-poor phase at the origin of the pores.

It is known to manufacture highly hydrophobic surfaces using various methods such as sol-gel techniques, plasma treatments, casting processes, vapor-induced phase inversion or precipitation processes from solutions.

In the vapor-induced phase inversion (VIPS) method, a wet-atmosphere evaporation step precedes immersion in the coagulation bath. In this method, moist air plays a crucial role in the formation of a highly hydrophobic hierarchical structure. This type of structure traps the air and prevents close contact of the water with the surface.

Such structures have been obtained by N. Zhao et al., Macromol. Rapid Commun., 2005, 26, 1075-1080, using the aforementioned VIPS method. These authors demonstrate that it is possible to form polycarbonate films, a semi- crystalline, superhydrophobic surface by drying in a humid atmosphere. The morphology obtained shows the formation of nodules with a flower-like structure ("flower like") on the surface.

However, this technology does not make it possible to manufacture mechanically stable superhydrophobic PVDF membranes.

Highly hydrophobic PVDF membranes have already been described.

T. H. Young et al., Polymer: 40 (1999) 5315-5333 obtained the following two morphologies from PVDF solutions:

by precipitation from a PVDF / DMF solution in water, the rapid entry of the non-solvent causes the mixture to be found very rapidly in the liquid-liquid demixing range; in this case the morphology is that of a conventional asymmetric membrane made of a dense surface skin supported on a spongy structure with more or less macrovoids; by precipitation from a PVDF / DMF solution in octanol, the slow entry of the non-solvent causes the mixture to remain a sufficiently long time in the solid-liquid demixing zone (crystallization domain), which gives a morphology in dense non-interconnected nodules.

Mao Peng et al J. Appl. Polym. Sci .: 98 (2005) 1358-1363 prepared PVDF membranes from a 20% by weight solution of PVDF in DMAc using three methods:

a first method by precipitation in a coagulation bath consisting of water (conventional phase separation already described in the work of TH Young with DMF as solvent), giving asymmetric membranes with a smooth filtering surface layer having a water contact angle of 85.2 ± 3.2 °; a second method by adding DMAc in the coagulation bath, which gives symmetrical membranes whose surface has a water contact angle of about 140 ± 5 ° when the proportion of DMAc is between 65 and 75; % (see the data in Table 1 of this document). The membranes obtained by this process are highly swollen, mechanically unstable and their surface is not homogeneous (as shown on page 1362, right column, ^1st paragraph). Moreover, this method has the disadvantage of consuming a lot of solvent;

a third method by precipitation with VIPS in moist air, which gives symmetrical membranes consisting of crystalline nodules of 4 microns resulting from the agglomeration of dense spheres of a few hundred nm in size and whose surface has a contact angle to water generally between 144 to 149 ° with an example at 150.6 ± 0.4 °.

C. Y. Kuo et al, Desalination 233 (2008) 40-47, studied the precipitation of a solution of PVDF / NMP in light alcohols such as methanol, ethanol, n-propanol and n-butanol. It has been shown that precipitation with a single alcohol bath leads to highly hydrophobic membranes with a water contact angle of 144 ° (for methanol) up to 148 ° C for water. n-propanol. The morphology obtained is bi-continuous. The use of precipitation with a double bath, first in alcohol (2s) and then in water gives membranes bi-continuous morphology but with a lower contact angle (136 ° for the n- propanol).

Q. Li et al, Polym. Adv. Technol. DOI: 10.1002 / pat. 1549 (2009) describe three other routes for preparing highly hydrophobic PVDF membranes (water contact angle of up to 136.6 °):

- From a solution of PVDF in a PET / DMAc mixture, a 60 min evaporation step is applied in a relative humidity of 60% followed by precipitation in water. A morphology of leaf lettuce leaf type is obtained, having a weak interconnection; by precipitation in ethanol, the same morphology is obtained in the mass of the membrane but a rough and dense surface layer;

- Precipitation in a double bath (the first compound of a greater or lesser proportion of solvent, followed by a second water bath) increases the surface porosity without losing the mechanical strength. However the morphology remains that of the "leaf lettuce" with a contact angle to the water of maximum 136,6 °. The object of the present invention is to prepare superhydrophobic PVDF membranes. These membranes are porous and have a hierarchical surface morphology. The porosity of the membrane, associated with a double level of organization, at the micrometric scale and at the nanoscale, is capable of trapping air and makes it possible to generate superhydrophobic surface properties also known under the name lotus effect. It is an approach inspired by structures found in nature (biomimicry) on lotus leaves and legs of water spiders (Hydrometra Stagnorum). Using the VIPS process described above, it was not possible to prepare mechanically stable PVDF membranes suitable for industrial applications. Indeed, in this case, the crystalline nodules are not interconnected. It is therefore desirable to prepare PVDF membranes having a hierarchical structure of crystalline nodules whose surface has a porous structure at the nanometer scale (100 to 600 nm) and whose nodules are interconnected (a structure also called "nanostructured morphology" ).

For this purpose, and according to a first aspect, the subject of the invention is a PVDF membrane comprising a superhydrophobic surface comprising a porous structure at the nanometric scale and interconnected micron-sized crystalline nodules. Typically, said superhydrophobic surface has a water contact angle greater than or equal to 150 °. The contact angle is measured by deposition of a drop of water of 8 under ambient conditions of temperature (21 ± 3 ° C) and pressure. The indicated value is an average of at least 4 independent measurements.

According to a second aspect, the invention relates to a process for preparing the superhydrophobic PVDF membrane according to the invention, comprising a precipitation operation from a dual alcohol-water bath system.

The invention and the advantages it provides will be better understood in the light of the detailed description which follows and the appended figures in which:

Figure 1 illustrates the membranes prepared in Example 1;

FIG. 2 illustrates the membranes prepared in example 2;

Figure 3 illustrates the membranes prepared in Example 3;

Figure 4 illustrates the membranes prepared in Example 4;

Figure 5 illustrates the membranes prepared in Example 5;

FIG. 6 is the image obtained by scanning electron microscopy (SEM) of a superhydrophobic membrane according to the invention obtained by precipitation of PVDF in a double iso-propanol-water bath;

FIG. 7 illustrates the structure of several membranes observed with SEM, prepared by the VIPS method and by precipitation of PVDF in a double bath: methanol-water; ethanol-water; n-propanol-water; iso-propanol-water; 1-butanol-water; 1-octanol-water and 1-decanol-water, respectively.

The hydrophobic membranes of PVDF are used on a large scale thanks to their multiple qualities: hydrophobicity, thermal resistance, chemical resistance, resistance to UV radiation, etc. PVDF is a semicrystalline polymer containing a crystalline phase and an amorphous phase. The crystalline phase gives good thermal stability, while the amorphous phase confers flexibility to the membranes made in this polymer. It is desirable to have PVDF membranes, some of whose properties have been further improved. A route developed in recent years aims to increase the hydrophobicity properties of PVDF membranes, while retaining good mechanical properties, which would make them even more suitable for certain industrial applications, such as membrane distillation, filtration, Lithium batteries, etc.

The techniques previously used to prepare high hydrophobic PVDF membranes are based on phase separation induced for example by electrospinning, steam or coagulation. The latter method consists in separating the phases by adding a non-solvent to a PVDF solution. The known processes described above make it possible to produce highly hydrophobic PVDF membranes, which do not reach however the qualification of superhydrophobicity, defined as being a superhydrophobic surface having a contact angle to water greater than or equal to 150 ° C.

The present invention therefore proposes to provide superhydrophobic PVDF membranes, as well as a method of manufacturing these membranes.

The PVDF membranes according to the invention comprise a superhydrophobic surface comprising a hierarchical structure with two levels of organization, namely inter-nodule porosity at the micrometric scale and intranodular porosity at the nanoscale, and nodules. interconnected crystalline Said superhydrophobic surface has a water contact angle greater than or equal to 150 ° C. Scanning electron microscopy images show that said nodules have a size of between 5 and 12 microns, preferably between 6 and 8 microns. These nodules have an inter-nodular porosity of less than 5 microns, whereas the intra-nodular pores have a submicron size (of a few hundred nanometers), giving a sponge-like morphology. The images also show that the nodules are connected to each other, which gives mechanical strength to the whole. Moreover, the PVDF membranes according to the invention have a pore volume greater than 70%, preferably greater than 75% and advantageously equal to or greater than 80%.

The structure of the PVDF membranes according to the invention is of the interconnected type.

This type of structure is obtained when the phase separation takes place by spinodal decomposition, unlike the phase separation by nucleation and growth which leads to a dispersed phase in the form of spherical nodules. The notion of "phase" can be defined as a portion of "uniform" material that has stable properties and reproducible. In other words, the properties of a phase are exclusively a function of the thermodynamic variables and are independent of time.

The superhydrophobic PVDF membrane according to the invention is characterized by the presence of a hierarchical structure:

- micrometric (crystalline nodules), and

- nanometric (the porous morphology of sponge-shaped nodules),

which is at the origin of the superhydrophobic property. This type of structure traps the air and prevents a close contact of water with the surface resulting in very high contact angles.

Advantageously, the membrane has a resistance to a pressure of up to at least 5 bar, testifying to its good mechanical strength. The reinforced membrane (especially textile) is subjected to water under pressure and it is verified that it remains intact.

The method according to the invention comprises the following steps:

a) dissolving a quantity of PVDF in a solvent at a temperature of at least 60 ° C, said solvent being used in pure form or added with water between 3 and 5% by weight relative to the weight of the solvent;

b) spreading the PVDF solution thus obtained on a solid support to form a film on the surface of said support;

c) immersing said film in a first bath containing an alcohol selected from among methanol, ethanol, n-propanol, iso-propanol and n-butanol, for a duration greater than or equal to 1 minute, preferably greater or equal to 5 minutes and then

d) immersing said support in a second water bath.

In a first step, the PVDF is dissolved in a solvent, chosen for example from the list: HMPA, DMAc, NMP, DMF, DMSO, TMP, TMU. The homogeneous solution obtained is deposited on a glass plate and then spread with a knife. The glass plate is then immersed in a first coagulant bath containing either a low molecular weight alcohol such as methanol, ethanol, n-propanol or isopropanol, or an alcohol of higher molecular weight such as n butanol, n-octanol or n-decanol. Said plate is then immersed in a second water bath, and then it is dried. Membranes comprising a superhydrophobic surface, comprising a nanoscale rough structure, and interconnected crystalline nodules were obtained when the alcohol was methanol, ethanol, n-propanol, iso-propanol or n-propanol. butanol. The nodules are interconnected and have a "sponge" morphology as shown in the appended FIG. 6, which illustrates the precipitation of the PVDF when the non-solvent is isopropanol.

The membranes obtained after a first bath in 1-octanol or 1-decanol have dense nodules. The denser the nodules, the less they can trap the air and the lower the hydrophobicity of the surface.

The formation of these morphologies is explained by a control of the composition paths in the ternary diagram which allows to play on a mixture of mechanisms S-L (crystallization) and L-L (precipitation).

Pore size, porosity and morphology of nodules from porous nodules to dense nodules in a bi-continuous structure through "sponge" nodules of all shapes can be obtained by varying the concentration of polymer, the temperature and the alcohol considered (Figure 7).

Competition between L-L phase separation and crystallization was analyzed during the separation process using FTIR (Fourier Transform Infrared Spectroscopy) microscopy. This method has shown that the surface of the PVDF membrane can vary from a bi-continuous morphology to a morphology of sponge-shaped nodules to arrive at dense nodules by acting on the coagulants with different solvent powers towards the PVDF. The use of low molecular weight alcohols, such as methanol and iso-propanol, leads to bi-continuous membranes and sponge-shaped nodules, respectively, whereas coagulation by means of Alcohols of higher molecular weight, such as n-octanol, lead to mixed structures containing dense nodules.

The use of FTIR microscopy made it possible to study the crystallization process during the coagulation reaction. When low molecular weight alcohols are used as nonsolvents, the LL (precipitation) mechanism dominates that of crystallization. Crystallization continues to occur sequentially, but only the rich polymer phase can form nodules. As the crystallization took place during the LL demixing, the membrane is formed of nodules with a very porous surface (sponge type nodules). When high molecular weight alcohols are used as a non-solvent, the LL separation curve has been shifted to the non-solvent. Crystallization prevailed on LL demixtion. Therefore, the polymer chain can form dense nodules when crystallization occurs before the LL separation phase.

The invention also relates to the application of the membranes described herein for the distillation of water, filtration and Li-ion batteries.

The invention will now be described with the aid of the following examples given for illustrative and not limiting. Example 1

A homogeneous solution of PVDF at 20% by weight is prepared by dissolving it in NMP or DMAc at 60 ° C. The solution obtained is deposited on a glass plate and then spread with a knife whose air gap is set at 250 μιη. The glass plate is then either subjected to moist air (VIPS process) to generate the phase separation (Comparative Example), or immersed in a first coagulant bath containing a low molecular weight alcohol such as methanol (Example 1b). ), Ethanol, n-propanol, isopropanol (Example 1c), 1-octanol (Comparative Example) and water (Comparative Example 1) for 10 min at 25 ° C. Said plate is then immersed in a second bath consisting of water (except in the case of VIPS where it is immersed in water or in ethanol), then it is dried at room temperature.

The membranes thus obtained were observed under a scanning electron microscope. Their resistance to a pressure of 5 Bar was also measured, when the membranes are reinforced, especially on textile. Finally, the angle of contact with water is measured by depositing a drop of water of 8 under ambient conditions of temperature (21 ± 3 ° C) and pressure. The indicated value is an average of at least 4 independent measurements. Table 1 summarizes the characteristics of the membranes formed. The images corresponding to these membrane samples, obtained by scanning electron microscopy, are shown in FIG.

Table 1

The presence of dense spheres is obtained when the crystallization outweighs the liquid-liquid phase separation (case of the VIPS process, eg la). Obtaining nodules with a porous structure occurs when liquid-liquid demixtion begins before crystallization (as in the case of coagulation in light alcohols, eg lb, le and ld). The use of alcohols heavier than butanol gives intermediate structures with dense nodules inserted into porous nodules (eg the). The bi-continuous structure, usually encountered in the case of commercial PVDF membranes, is obtained when coagulation is carried out with a single water bath (eg 1f).

These results show that the use of light alcohols as the first coagulation bath makes it possible to obtain superhydrophobic surface membranes whose structure of interconnected porous nodules guarantees a mechanical strength of 5 bar sufficient to allow filtration applications. The presence of dense nodules, even in small quantities, weakens the structure of the membrane.

Example 2 Influence of the Clotting Time in the First Bath

A homogeneous solution of PVDF at 20% by weight is prepared by dissolving it in NMP at 60 ° C. The solution obtained is deposited on a glass plate and then spread with a knife whose air gap is set at 250 μιη. The glass plate is then immersed in a first coagulant bath containing methanol for varying times at 25 ° C. The said plate is then immersed in a second bath made of water, then it is dried at room temperature. Table 2 shows the water contact angles of the membranes formed.

Table 2

These results show that the increase of the coagulation time in an alcohol bath makes it possible to slow down the liquid-liquid demixing mechanism and to change from a bi-continuous morphology to a morphology into connected porous nodules. This change in morphology is accompanied by an increase in the contact angle to water which becomes superhydrophobic in the case of methanol from a time of immersion of between 15 and 60 s (Ex. 2a - 2d ). The images corresponding to these membrane samples, obtained by scanning electron microscopy, are shown in FIG. 2. EXAMPLE 3 Influence of the Percentage of Water in the Pouring Solution

A homogeneous solution of PVDF at 20% by weight is prepared by dissolving it at 80 ° C in wet NMP with varying amounts of water (up to 6% by weight). The solution obtained is deposited on a glass plate and then spread with a knife whose air gap is set at 250 μιη. The glass plate is then immersed in a first coagulant bath containing a low molecular weight alcohol such as iso-propanol for 10 minutes at 25 ° C. The said plate is then immersed in a second bath consisting of water, and then it is dried at room temperature.

Table 3 shows the water contact angles of the membranes formed. The images corresponding to these membrane samples, obtained by scanning electron microscopy, are shown in FIG. 3. These results show that the addition of a few percent of water in the polymer solution makes it possible to adjust the contact angle. membranes prepared according to Example 3 without modifying the morphology obtained in porous nodules. Table 3 shows that superhydrophobic membranes are obtained for water addition values in the casting solution of between 3 and 5% (Ex. 3c, 3d and 3e).

Table 3

Example 4 Influence of the Dissolution Temperature

A homogeneous solution of PVDF at 20% by weight is prepared by dissolving it in NMP at temperatures between 32 and 110 ° C. The solution obtained is deposited on a glass plate and then spread with a knife whose air gap is set at 250 μιη. The glass plate is then immersed in a first coagulant bath containing a low molecular weight alcohol such as methanol, ethanol or iso-propanol for 10 min at 25 ° C. The said plate is then immersed in a second bath consisting of water, and then it is dried at room temperature. Table 4 shows the water contact angles of the membranes formed. The images corresponding to these membrane samples, obtained by scanning electron microscopy, are shown in FIG. 4.

Table 4 The results in Table 4 show that the dissolution temperature of the PVDF influences the morphology of the membrane obtained. Thus, bi-continuous morphologies are obtained below 50 ° C in either ethanol or iso-propanol. A temperature higher than this value is necessary to obtain the morphology in connected nodules with a porous structure essential for obtaining superhydrophobic membranes as seen in the preceding examples.

Example 5 Influence of the Polymer Concentration on Pore Size

A homogeneous solution of PVDF at different concentrations is prepared by dissolving it in NMP or in wet DMAc with 4% water at temperatures between 60 and 120 ° C. The solution obtained is deposited on a glass plate and then spread with a knife whose air gap is set at 250 μιη. The glass plate is then immersed in a first coagulant bath containing a low molecular weight alcohol such as iso-propanol for 10 minutes. Said plate is then immersed in a second bath consisting of water and then dried at room temperature.

Table 5 shows the water contact angles of the membranes prepared according to Example 5. The images corresponding to these membrane samples, obtained by scanning electron microscopy, are shown in FIG. 5.

Table 5

The results summarized in Table 5 show that superhydrophobic membranes having the morphology of connected porous nodules can be prepared by the method proposed in the present invention from different solvents, different compositions and different temperatures. These parameters allow to adjust the size of the largest inter-nodular pores (determined by the minimum water intrusion pressure in the membrane) in a range of 4 to 0.25 microns.

Abbreviations:

PVDF - polyvinylidene fluoride

DMF - dimethylformamide

NMP - N-methylpyrrolidone

PET - triethylphosphate

DMAc - N, N-dimethylacetamide

HMPA - hexamethylphosphoramide

DMSO - dimethylsulfoxide

TMP - trimethylphosphate

TMU - 1,1,3,3 - tetramethylurea

Claims

A PVDF membrane comprising a surface having a water contact angle greater than or equal to 150 °.

The membrane of claim 1 wherein said surface comprises interconnected crystalline nodules having a size of between 5 and 12 microns, preferably between 6 and 8 microns.

3. The membrane of claim 2 wherein said nodules have a porous structure with an intra-nodular pore size less than 1 μιη.

4. Membrane according to any one of claims 1 to 4 wherein the nodules have an inter-nodular pore size of less than 5 microns.

5. Membrane according to any one of claims 1 to 4 having a pore volume greater than 70%, preferably greater than 75% and advantageously equal to or greater than 80%.

6. Membrane according to any one of claims 1 to 6 characterized in that it has a resistance to a pressure of up to at least 5 Bar.

7. A method of manufacturing the PVDF membrane according to one of claims 1 to 6, said method comprising the following steps:

at. dissolving a quantity of PVDF in a solvent at a temperature of at least 60 ° C, said solvent being used in pure form or added with water between 3 and 5% by weight relative to the weight of the solvent;

b. spreading the PVDF solution thus obtained on a solid support to form a film on the surface of said support;

vs. immersing said film in a first bath containing an alcohol chosen from methanol, ethanol, n-propanol, isopropanol and n-butanol, for a duration greater than or equal to 1 minute, preferably greater than or equal to 5 minutes, then

d. immersing said support in a second water bath.

The process of claim 7, wherein said alcohol is isopropanol.

The process of claim 7, wherein said alcohol is methanol.

10. Method according to one of claims 7 to 9 wherein said solvent is selected from the list: HMPA, DMAc, NMP, DMF, DMSO, TMP, TMU.

11. Method according to one of claims 7 to 10 wherein the PVDF membrane is prepared by drying said support at room temperature.

12. Use of a membrane according to one of claims 1 to 6 or obtainable by the method according to one of claims 7 to 11 for the distillation of water.

13. Use of a membrane according to one of claims 1 to 6 or obtainable by the method according to one of claims 7 to 11 as a filtration membrane.

14. Use of a membrane according to one of claims 1 to 6 or obtainable by the method according to one of claims 7 to 11 in a lithium battery.