FR2620712A1 - Hydrophilic block copolymers of vinylidene fluoride and of N-alkylacrylamides and process for their production - Google Patents

Abstract

Polymer chemistry. The invention relates to hydrophilic block copolymers of vinylidene fluoride, of an amide containing terminal unsaturation, such as N,N-dimethylacrylamide and (optionally) of an alkyl methacrylate such as methyl methacrylate, together with the process for their production. Application to the production of ultrafiltration membranes.

Description

 The present invention relates as a whole to hydrophilic vinylidene fluoride copolymers and it relates more particularly to the production of sequential hydrophilic copolymers of vinylidene fluoride with N-alkylacrylamides or 1vinyl-2-pyrrolidinone and, optionally, a alkyl methacrylate. The polymers are useful in the formation of filter membranes, coatings, films and fibers.

 Fluorocarbon polymers such as polyvinylidene fluoride are resistant to chemical and biological attack. However, they are hydrophobic, which limits their possibilities of use in the formation of filter membranes. A hydrophilic membrane must allow the rapid passage of the liquid during the filtration of aqueous solutions. Hydrophobic polymers are also incompatible with many polymer additives and many other polymers.

 The modification of these polymers in order to produce a hydrophilic membrane is disclosed in US Pat. No. 4,340,482. The surface of a preformed polymer is treated with a base to create active sites used for the grafting of hydrophilic reagents such as amino acids. For example, a mixture of glycine, sodium hydroxide and water is used to graft glycine in the form of the glycinate ion to the surface of a microporous membrane preformed from polyvinylidene fluoride.

 The present invention provides a vinylidene fluoride polymer, and it is the polymer itself, rather than just its surface, that is made hydrophilic.

The polymer retains its excellent physical and chemical resistance properties.

 In accordance with the present invention, there is provided a hydrophilic sequential copolymer comprising approximately 70 to 99% by weight of a preformed vinylidene fluoride polymer which has reacted with approximately 1 to 30% by weight of at least one unsaturated amide terminal chosen from the group comprising N- (lower alkyl) acrylamides, N, N-di (lower alkyl) -acrylamides and l-vinyl-2-pyrrolidinone and O has approximately 29% by weight of a methacrylate lower alkyl.

 The invention also provides a process for the production of a sequential hydrophilic copolymer of vinylidene fluoride and an amide with terminal unsaturation, which consists in polymerizing vinylidene fluoride in aqueous emulsion in the presence of a free radical initiator to form a vinylidene fluoride polymer latex and then to add a free radical initiator supplement and approximately 1 to 30% of monomer to said latex, based on the total weight of the copolymer, said monomer comprising at least one amide to non - terminal saturation chosen from the group comprising N- (lower alkyl) -acrylamides, N, N-di- <lower alkyl) -acrylamides and l-vinyl-2-pyrrolidinone and O at approximately 29% by weight, on based on the total weight of the copolymer, of a lower alkyl methacrylate, and in polymerizing the emulsified monomer in the presence of said latex to form a copolymer latex of said vinylidene fluoride and said monomer.

 The copolymers of the invention are based on vinylidene fluoride polymers which are both structurally stable and chemically inert and which are therefore useful for the formation of thin microporous filter membranes. However, vinylidene fluoride homopolymers have low water permeability. The reaction of these polymers with certain hydrophilic amides with terminal unsaturation gives copolymers which are hydrophilic. Ultrafiltration membranes formed from these copolymers have much greater water permeability. Copolymers are also more compatible both with other polymers and with additives for polymers with high surface energies, such as inorganic oxides. The reaction of mixtures of the amide and a lower alkyl methacrylate with vinylidene fluoride polymer gives terpolymers which, although less hydrophilic than amide copolymers, surprisingly have better water permeability and better retention of dissolved bodies.

 The copolymers of the invention are sequential copolymers because a preformed vinylidene fluoride polymer latex is reacted with a hydrophilic amide amide with terminal unsaturation in the presence of free radical initiators, so that in the resulting hydrophilic copolymer, the amide (or amide and acrylate) polymerized is in chemical bond with the vinylidene fluoride polymer.

 The expression "vinylidene fluoride polymer" is used in the present specification for the sake of brevity to include in its definition homopolymers, and copolymers with other fluorocarbon monomers, normally solid and of high molecular weight.

These copolymers include those which contain at least 50 mol% of vinylidene fluoride copolymerized with at least one comonomer chosen from the group comprising tetrafluoroethylene, trifluorethylene, chlorotrifluoroethylene, hexafluoropropene, vinyl fluoride and pentafluoroprene. acts for example of copolymers composed of at least about 70 and up to 99 mol% of vinylidene fluoride and, correspondingly from 1 to 30% of tetrafluoroethylene, as disclosed in British patent N 827 308; and from about 70 to 99% of vinylidene fluoride and from 1 to 30% of hexafluoropropene (see for example the patent of the United States of America Ne 4,178,399), and from about 70 to 99 mol% of vinylidene fluoride and 1 to 30 mole% of trifluoroethylene. Terpolymers of vinylidene fluoride, hexafluoropropene and tetrafluoroethylene, as described in US Pat. No. 2,968,649 and vinylidene fluoride terpolymers, Trifluoroethylene and Tetrafluoroethylene are also representative examples of the class of vinylidene fluoride copolymers which can be used as preformed polymers in the preparation of the sequential copolymers which are discussed in this specification.

 Processes which are suitable for the preparation of these vinylidene fluoride polymers are known in the prior art, as described, for example, in US Pat. Nos. 4,360,652 and 4,569,978. In these processes, approximately 0.01 to 0.5% by weight, based on the total amount of monomer to be used in the process, of a fluorinated surfactant, about 0.03 to 0.30% by weight, on the base of the monomer, of a wax or of a saturated oil with a long chain (to avoid adhesion of the polymer to the surface of the reactor) and deionized water are charged into a reactor which is equipped with an agitator and a heat control device. The charge is freed from oxygen by heating (100 ° C.) and stirring (50 to 72 rpm), purged at rest, brought to the desired reaction temperature (60 × 95 ×, preferably 75 ° C.). from about 0.1 to 8% by weight, based on the total amount of monomers used in the process, of a chain transfer agent such as 2-propanol or CC13F is added and the gauge pressure in the reactor is brought to a value of 1.4 to 7 MPa (preferably 4.55-5.95 MPa) by the addition of monomer (vinylidene fluoride plus all comonomers).

The polymerization is triggered by the addition of a free radical initiator such as dilsopropyl peroxydicarbonate which is emulsified in water with the addition of a surfactant. Thereafter, the monomer, initiator and, optionally, additional chain transfer agent are added portionwise or continuously to the reactor during the polymerization cycle until the desired amount of monomer has been polymerized by forming a vinylidene fluoride polymer latex.

Hydrophilic monomeric amides with terminal nonsaturation which are advantageous for use in the present invention include N-vinylpyrrolidinone and N-alkylacrylamides in which one or both of the hydrogen atoms carried by the nitrogen atom are replaced by a lower alkyl group (C1 to C4). Examples of these acrylamides are N-methylacrylamide,
N-ethylacrylamide, N-propylacrylamide, N-butylacrylamide, N, N-dimethylacrylamide and N, N-diethylacrylamide. It would be expected that acrylamide would also promote hydrophilicity, but it does not react effectively with the vinylidene fluoride polymer.

 The sequential copolymers of the invention may also contain a lower alkyl methacrylate (C1 to C4) such as methyl methacrylate. The acrylates give membranes which have better retention of dissolved bodies and better water permeability although the polymers containing an acrylate are less hydrophilic.

 It takes about 1% of amide to create an important hydrophilic character. Sequential copolymers containing up to about 30% by weight of amides can be prepared by the process of the invention. If the copolymer is to be used to form filter membranes, less amounts of amide should be used because amounts exceeding about 20% have the effect that membranes prepared from such copolymers become impermeable. Consequently, compositions which are suitable for filter membranes must contain approximately 1 to 20% by weight of amide, the remainder being formed from vinylidene fluoride polymer and, optionally, alkyl methacrylate in amounts up to to about 29% by weight. Polymers containing an alkyl methacrylate which are preferred for membranes contain about 70 to 90% by weight of vinylidene fluoride polymer units, about 5 to 15% by weight of amide polymer and about 5 to 15% by weight of alkyl methacrylate polymer units.

 All the above percentages are based on the total weight of the sequential copolymer.

 The amide (or the amide and the acrylate) is added after the formation of the vinylidene fluoride polymer latex. It is desirable, but not essential, to add the terminal unsaturated amide and, where appropriate, the acrylate to the vinylidene fluoride polymer latex in the reactor at the end of the polymerization and, preferably, immediately the vinylidene fluoride has been fully loaded into the reactor and before the pressure in the reactor has dropped. Not only is this more practical, but the degree of reaction between the vinylidene fluoride polymer and the added monomer is maximized. When the amide is used alone, it is also advantageous to quickly add the entire charge of amide to the reactor to obtain the maximum "copolymerization yield" (that is to say the percentage of comonomer units chemically bonded to the vinylidene fluoride polymer) in order to minimize the amount of amide homopolymer. The optimal rate of addition of the monomeric amide varies depending on the scale of the polymerization process involved and this rate can be determined empirically from one case to another.

Thus, in the following examples, in which a 7.57 l reactor is used, a monomer loading speed of 101 grams / minute gave a copolymer yield of 98%, compared to yields of 83% and 87 % only for respective speeds of 48 and 36 grams / minute. However, when the monomer charge also contains an alkyl methacrylate, rapid addition does not improve the copolymerization yield. Other factors which affect the copolymerization yield are the nature and amount of the chain transfer agent and the ratios of the reactants.

 The monomeric amide can be added in the form of an aqueous solution and the alkyl acrylate in the form of an aqueous emulsion. If both amide and acrylate are used, they are added as an aqueous emulsion. The free radical initiator is added both during the introduction of the monomer and the polymerization. The same pressures, the same temperatures and the same initiators can be used as in the polymerization of vinylidene fluoride. The total reaction times for the two polymerizations are generally in the range of 1 to 8 hours and preferably 2 to 5 hours. The solids content of the latex generally ranges from 10 to 25% by weight.

 Other details of the invention emerge from the following examples, given without limitation, in which all the parts are by weight, unless otherwise specified.

Example 1
VF2 / DMA sequential copolymer (85/15)
An initial charge consisting of deionized water (4400 ml), an aqueous solution at 1% by weight of ammonium perfluoroctanoate (surfactant, 100 ml) and a paraffinic wax (4 g) was charged in a 7.7 l horizontal reactor fitted with an L-shaped stirrer with 4 stainless steel blades, electrically driven. The initial charge was deoxygenated by heating to 110 ° C. with stirring (72 rpm) using a mixture of steam and water in the reactor jacket, and then purging without stirring. The stirrer was restarted at 72 rpm and the reactor was cooled to 75 ° C.

 A 5% by weight aqueous solution of 2propanol (107 g) was pumped into the reactor from a stainless steel bottle under nitrogen pressure of 140 kPa.

 Vinylidene fluoride (VF2) under gauge pressure of 5.6 MPa was loaded into the reactor from a hot bottle placed on a balance equipped with a digital weight reading system, until the pressure in the reactor has reached 4.55 MPa (about 450 grams of vF2).

 A cooled, initiator aqueous emulsion, prepared using a non-aeration agitator, and containing diisopropyl peroxydicarbonate (PPI, 2% by weight) and a fluorinated surfactant (0.15 ammonium perfluoroctanoate % by weight), was introduced by pumping into the reactor at a flow rate of approximately 8 ml / min. The polymerization started after an 18 minute induction period as shown by a pressure drop. During the polymerization, the initiator emulsion was charged at a speed necessary to maintain the reaction (about 2 ml / min) and VF2 was continuously charged to the reactor to maintain the gauge pressure at 4, 55 MPa until a total of approximately 1022 grams, including the initial charge, has been introduced. The time required was 1 hour and 11 minutes. The supply of VF2 was then stopped.

 A 40% by weight aqueous solution of N, Ndimethylacrylamide (DMA) 4450 g) was then rapidly loaded into the reactor over a period of 5 minutes at a flow rate of 36 g / min from a stainless steel bottle, using a nitrogen pressure of 5.25 MPa.

The IPP initiator emulsion (2% by weight) was introduced by pumping into the reactor at a flow rate of 2 ml / min during the DMA supply and then for 30 minutes. Deionized water (35 ml) was pumped into the DMA line to discharge the DMA remaining in the reactor. The gauge pressure rose to 5.53 MPa. The final VF2JDMA ratio was 85/15 (weight ratio) or 90/10 (molar ratio). The total of the charged VF2 and DMA monomers reached 1202 g. The total amount of Ipp emulsion used was 387 ml (7.7 g of PPI).

Stirring in the reactor was continued at 75 C for two hours after the end of the introduction of
DMA. The gauge pressure dropped to 5.32 MPa.

The agitator was then stopped and the residual gas was removed in a separator charged with wet ice and in a tower charged with carbon.

 The latex obtained as product was filtered through cheesecloth and its weight was 4982 g, with a solid content of 14.2% and a pH of 3.9. The dry coagulum weighed 41 g and the adhesions were 2 g. The total polymer yield was 749 g (63.2%). The average rate of polymerization was 36 grams / liter / hour.

 The latex could not be coagulated by freezing, and it was therefore dried in a container filled with "Teflon" polymer in an oven at 90 ° C. overnight.

Part of the resulting copolymer was compression molded into a transparent flexible film at 180 ° C. A portion of the film was weighed and extracted with boiling deionized water to remove any polymerized DMA. The aqueous extract was evaporated and the residue (poly-DMA) was dried and weighed. The percentage of extractables has been calculated. The% yield of DMA copolymer was then calculated as follows
(% of DMA in monomers -% of ex
Yield = film tractables x 100
% of DMA in the monomers
The water absorption of the film was measured by weighing before and after immersion in water for 24 hours.

 An ultrafiltration membrane was produced by a process analogous to that which has been described in the United States of America patent N 4,384,047. A solution of the above copolymer (20% by weight), of glycerol (5 % by weight) and triethyl phosphate (75% by weight) was prepared by stirring with a magnetic stirrer and heating to 108C. The transparent viscous solution, after standing to remove the bubbles, was poured at 108 ec onto a transparent glass plate of 30.48 x 30.48 cm, using a Gardner knife set at 1.016 mm. The poured solution was allowed to stand in a place equipped with a hood for 5 minutes and then immersed in a 15 x 15 city water bath for 4 days. A coherent upper layer separated from the membrane. remaining and it was cut into a 90 mm diameter circle using a graduated steel die. The membrane was tested using an Amicon TCF-10 thin channel ultrafiltration device. The results of the test are shown in Table 1.

Example 2
VF2 / DMA sequential copolymer (90/10)
The synthesis was analogous to that described in Example 1, except: 1) the molar ratio
VF2 / DMA which was 90/10 by weight and 93/7 by moles 2) no chain transfer agent was used; 3) a period of 32 minutes was allowed for the reaction to be completed between the end of step 1 and the start of step 2.

 As regards the identity and the proportions of the ingredients, 913 g of VF2, 522 g of aqueous emulsion i 19.5% by weight of DMA and 293 ml of emulsion at 2% by weight of PPI were used. The pressure range was 4.69 to 0.7 MPa. The reactor was purged after step 1 (polymerization of VF2) and a layer of nitrogen under pressure of 0.7 MPa was created during step 2) (addition of DMA and reaction). The total reaction time was 2 hours and 45 minutes. The filtered latex weighed 5870 g with a solid content of 12.8%. The total polymer yield, including 9 g of coagulum and 3 g of adhesions, was 762 g (75%). The average rate of polymerization was 53 g / l / h. The results of the tests are shown in Table 1.

Example 3
VF2 / DMA sequential copolymer (88/12)
The synthesis was analogous to that described in Example 1, except: 1) the ratio of the VF2 / DMA monomers which was 88/12 by weight and 92/8 by moles, 2) the chain transfer used consisted of "ISOTRON ~ -11" (CFC13).

 VF2 (1062 g), 50% DMA solution (aqueous, 286 g) and a 2% by weight emulsion of IPP (492 ml) were used. The pressure range was 5.42 to 3.15 MPa. The total reaction time was 2 hours and 20 minutes. The filtered latex weighed 5621 g with a solids content of 18.9%. The total polymer yield was 1136 g (94%). The average polymerization rate was 96 g / l / h. The results of the tests are shown in Table 1.

Example 4
80/20 VF2 / DMA sequential copolymer
The synthesis was analogous to that which is described in Example 1 except the ratio of the monomers, VF2 / DMA, which was 80/20 by weight and 96/14 by moles.

 As for the identity and the proportions of the ingredients, an aqueous solution at 5% by weight of 2-propanol (115 g), VF2 (957 g), an aqueous solution at 40% by weight of DMA was used. (597 g) and an emulsion at 2% by weight of IPP (365 ml). The gauge pressure range was 4.55-5.42 MPa. The total reaction time was 3 hours and 50 minutes. The filtered latex weighed 530 g with a solid content of 13.9%. The total polymer yield, including 33 g of coagulum and 3 g of adhesions, was 774 g (64.7%). The average rate of polymerization was 36 g / l / h. The results of the tests are shown in Table 1.

Hydrophilic VF2 / DMA copolymers
Example 1 2 3 4
Monomers VF2 / IMA VF2 / DMA VF2 / DMA VF2 / DMA
weight ratio 85/15 90/10 88/12 80/20
molar ratio 90/10 93/7 92/8 86/14 Charge speed of the como- 36 101 48 80 nomere (g / min)
Charge time of the comono- 5 2 3 3 mother (min) concentration of the chain transfer agent (g / kg of monomer IPA, 4.5 None CCl3F, 76 IPA, 4.8
Pressure range (MPa) 5.53-4.55 4.69-0.7 * 5.42-3.15 5.42-4.55 Total reaction time
(h) 3.80 2.75 2.33 3.83
Encopolymer yield,% 62 75 94 65
Melting point of the copolymer (C) 162 164 165 157
Heat of fusion (J / g) 39 54 44 26
Water absorption,% 7.6 2.05 4.4 12.4
Water extractable bodies,% 1.95 0.18 2.1 1.8
DAMA copolymerization yield,% 87 98 83 91
Viscosity in the molten state
(Rp) - - 20.0
Flow rate in the molten state (g / 10 min)
Temp. C 180 230 - 180
21.6 kg 3.7 2 - 5.6%, N - - 1.76
IMA copolymerized,% - - 12.4
TABLE 1 (continued)
VF2 / IMA hydrophylic copolymers
Example 1 2 3 4
H20 flow through the membrane (140kPa)
initial (ml / ai? / iin) 0.875 0.19 0.17 0.33
final (ml / ofymin) 0.56 0.079 0.09 0.26
0.1% Blue solution
Dextran (140 kPa)
flow (ml / cm / min) 0.15 0.067 0.08 0.22
retention% 95>95>95> 95 * End of reaction in step 1
Example 5
VF2 / DMA / NNA sequential terpolymer (85/5/10)
your synthesis was analogous to that of Example 1, except that three monomers were used. VF2 / DMA / MMA in a weight ratio of 85/5/10 or a molar ratio of 90/7/3. The chain transfer agent consisted of ISOTRON-ll (CFC13). VF2 (1026 g), ISOTRON-ll (86 g), an aqueous emulsion (400 g) containing 60 g of DMA, 120 g of MMA washed with a base and 0.4 g of "Surf" were used. lon SilîS "as fluorinated surfactant and an emulsion at 2% by weight of IPP (676 ml). The pressure range was from 6.26 to 4.27 MPa. The total reaction time was 2 hours and 13 minutes. The filtered latex weighed 5531 g with a solid content of 15.5% and a pH of 3.0. The total polymer yield was 911 g (76%). The average polymerization rate was 76 g / l / h. The results of the tests are shown in Table 2.

Example 6
VF2 / DMA / MMA sequential terpolymer (80/10/10)
The synthesis was analogous to that of Example 5, except that the proportion VF2 / DMA / MMA of the monomers was 80/10/10 by weight or 86/7/7 in moles.

 VF2 (958 g), ISOTRON-11 (86 g), an aqueous emulsion containing 120 g of DMA, 120 g of MNA washed with a base and 0.4 g of fluorinated surfactant were used. "Surflon SllSN and an emulsion at 2% by weight of IPP (385 ml). The pressure range was from 5.81 to 4.51 MPa.

The total reaction time was 2 hours and 25 minutes. The filtered latex weighed 5449 g with a solids content of 15.2% and a pH equal to 3. The total polymer yield was 901 g (75%). The average polymerization rate was 71 g / l / h. The results of the tests are shown in Table 2.

Example 7
VF2 / DMA / MMA sequential terpolymer (70/10/20)
The synthesis was analogous to that of Example 1, except that liron used three monomers: VF2 / DMA / NMA in a weight ratio of 70/10/20 or a mole ratio of 79/7/14. No chain transfer agent was used. VF2 (699 g) was first polymerized and a reaction time of 30 minutes was observed before the addition of the aqueous amido-acrylic emulsion (600 g) containing 33.3% by weight of MMA, 16.7% DMA and 0.15% fluorinated surfactant "Surflon SlllS"). The total volume of IPP initiator emulsion used was 459 ml (9.2 g IPP). The gauge pressure was in the range of 4.65-2.38 MPa. your total reaction time was 4 hours and 21 minutes. The filtered latex weighed 5988 g with a solids content of 12.5% by weight and a pH of 4.0. The total polymer yield was 776 g (77.6%), including 26 g of coagulum and 3 g of adhesions. The average polymerization rate was 35 g / l / h. The results of the tests are shown in Table 2.

TABLE 2
VF2 / DMA / MMA hydrophylic terpolymers Example 5 6 7 Control
Monomers VF2 / DMA / MMA VF2 / DMA / MMA VF2 / DMA / MMA VF2
weight ratio 85/5/10 80/10/10 70/10/20 100%
molar ratio 90/3/7 86/7/7 79/7/14
Feed speed in between
(g / min) 90 60 2.5
Feeding time in
(min) 2 4 120
Concentration of chain transfer agent (g / kg) relative to total monomers CCl3F, 71 CC13F, 72 None
Pressure range
(MPa) 6.27-4.27 5.81-4.51 4.65 # 2.38 *
Total duration of
reaction (h) 2.2 2.42 4.35
Terpo- Yield
lymer,% 76 75 78
Fu temperature
sion, C 164 161 153 160 Heat of fusion,
(J / g) 41 32 21 51
Water absorption,% 1.5 4.6 2.8 <0.1
Water extractable bodies (%) 0.9 0.9 0.15 <0.1
DMA co-polymerization yield,% 82 91 98.5
Viscosity in the state
fade (kp) 18.6 21.4 58.7200C -
TABLE 2 (continued)
VF2 / IMA / MMA hydrophylic terpolymers
Example 5 6 7 Witness
Flow velocity in state f # of 180C
(g / 10 min) - - 0.5 1.2
21.6kg
N,% 0.92 1.90 1.9
DMA copolymerized,% 6.5 13.4 13.4
H2O flow through
a membrane (140 kPa)
initial (ml / ai? / min) 1.3 1.8 0.25 0.083
final (ml / cm / min) 0.50 0.73 0.125 0.058
0.1% solution of
Dextran Blue (140 kPa)
flow (ml / year2 / min) 0.43 0.34 0.125 0.058
retention,% 95 95> 95 95 * End of reaction in step 1.

Example 8
VF2 / 1-vinyl-2-pyrrolidinone sequential copolymer
(w) (85/15)
The synthesis was analogous to that of Example 1, except for the composition of the monomers (VF2 / VP in a weight ratio of 85/15 or a molar ratio of 91/9).

VF2 (1026 g), a 5% by weight aqueous solution of 2-propanol (107 g), a 40% by weight aqueous solution of VP (445 g) and a 2% by weight emulsion were used. PPI (514 ml). The pressure was varied from 5.84 to 4.41 MPa. The total reaction time was 3 hours and 53 minutes. The filtered latex weighed 3727 g with a solid content of 8.3% and a pH of 3.2. The total polymer yield was 660 g (55%). your average polymerization speed was 31 g / l / h. The test results are shown below
Melting temperature of
copolymer (* C) 166.170 (double peak)
Heat of fusion (J / g) 54
Water absorption (%) 0.15
Hot water extractable bodies (%) 0.19
Copolymerization yield of
PV,% 98.7
Flow rate in the molten state,
g / 10 min, 180C, 21.6 kg 73
Water flow through a membrane
(140 kPa)
initial (ml / cm '/ min) 0.28
final 0.17
0.1% solution of Bleu Dextran
(140 kPa)
flow (ml / cm2 / min) 0.15
Retention,% 80
The polymers of the invention offer a flow of water greater than that which is offered by the control homopolymer of vinylidene fluoride (KYNAR 461 resin), the properties of which are indicated in Table 2. They give filtration membranes whose water permeability is enhanced while retaining the macromolecules. Because the hydrophilic amides are chemically bound throughout the polymer, the wettability of the entire polymer rather than just its surface is promoted. This not only improves its filtration properties, but also improves its compatibility with other polymers and polymer additives which are mixed with it.

Claims (14)

 1. Hydrophilic sequential copolymer, characterized in that it comprises approximately 70 to 99% by weight of a preformed vinylidene fluoride polymer with which reacted approximately 1 to 30% by weight of at least one amide with selected terminal nonsaturation in the group comprising N (lower alkyl) -acrylamides, N, N-di (lower alkyl) -acrylamides and 1-vinyl-2-pyrrolidinone and 0 to about 29% by weight of lower alkyl methacrylate.  2. Polymer according to claim 1, characterized in that it contains approximately 1 to 20% by weight of polymeric amide units and approximately 80 to 99% by weight of polymeric vinylidene fluoride units.  3. Copolymer according to claim 1, characterized in that it contains approximately 70 to 90% by weight of polymeric units of vinylidene fluoride, approximately 5 to 15% by weight of polymeric units of amide and approximately S to 15% in weight of polymer units of lower alkyl methacrylate.  4. Copolymer according to one of claims 2 and 3, characterized in that it is in the form of a filtering membrane.  5. Copolymer according to claim 1, characterized in that it contains approximately 80 to 90% by weight of vinylidene fluoride homopolymer units and approximately 10 to 20% by weight of polymer units of N, Ndimethylacrylamide.  6. Copolymer according to claim 3, characterized in that the vinylidene fluoride polymer is a homopolymer of vinylidene fluoride.  7. Method for producing a hydrophilic sequential copolymer of vinylidene fluoride and an amide with terminal unsaturation, characterized in that it consists in polymerizing vinylidene fluoride in aqueous emulsion in the presence of a radical initiator free to form a vinylidene fluoride polymer latex and then i add to said latex an additional free radical initiator and approximately 1 to 30% by weight of monomer, based on the total weight of copolymer, said monomer comprising at least one amide with terminal unsaturation chosen from the group comprising N (lower alkyl) -acrylamides, N, N-di (lower alkyl) -acrylamides and N-vinylpyrrolidinone and in polymerizing said monomer in the presence of said latex so as to form a latex of vinylidene fluoride copolymer and said monomer .  8. Method according to claim 7, characterized in that the monomer contains up to about 29% by weight, based on the total weight of the copolymer, of a lower alkyl methacrylate so that said vinylidene fluoride is copolymerized both with said amide and lower alkyl methacrylate.  9. Method according to claim 7, characterized in that a portion of about 1 to 20% by weight of amide is added, based on the weight of polymer.  10. Method according to claim 7, characterized in that about 5 to 15% by weight of said amide and about 5 to 15% by weight of said methacrylate are added.  11. Method according to claim 7, characterized in that it involves the presence of a chain transfer agent and a fluorinated surfactant.  12. The method of claim 7, characterized in that it is carried out at a temperature of about 60 to 95iC and a gauge pressure of about 1.4 to 7 MPa.  13. Process for the production of a hydrophilic sequential copolymer of vinylidene fluoride and of an amide i terminal non-saturation, characterized in that it consists in rapidly adding approximately 1 to 20%, based on the total weight of the copolymer of at least one terminal unsaturated amide chosen from the group comprising a N (lower alkyl) -acryiamide, N, N-di (lower alkyl) -acrylamide and N-vinylpyrrolidinone and a free radical initiator in a preformed vinylidene fluoride homopolymer latex and i polymerize said amide in the presence of said latex so as to form a copolymer of said vinylidene fluoride and said amide.  14. Method according to claim 12, characterized in that the amide is N, N-dimethylacrylamide.