CN111041584B

CN111041584B - Composite seed crystal for controlling polyvinylidene fluoride crystallization, preparation method and material

Info

Publication number: CN111041584B
Application number: CN201911255146.XA
Authority: CN
Inventors: 李先锋; 韩春梅; 王宁
Original assignee: Suzhou Wuchuangzhi New Material Technology Co ltd
Current assignee: Suzhou Wuchuangzhi New Material Technology Co ltd
Priority date: 2019-08-23
Filing date: 2019-12-10
Publication date: 2022-08-30
Anticipated expiration: 2039-12-10
Also published as: CN111041584A

Abstract

The invention discloses a composite seed crystal for controlling polyvinylidene fluoride crystallization, a preparation method and a material, and belongs to the technical field of polymer crystallization. The composite seed crystal comprises insoluble solid particles and a soluble surface modifier; the surface modifier is a polar organic molecule, one end of a molecular chain of the polar organic molecule is chemically grafted to the surface of the solid particle, and the other end of the molecular chain is a free end. The preparation method utilizes the composite seed crystal to prepare the polyvinylidene fluoride material by a double-diffusion solution phase separation method or a thermally induced phase separation method. According to the invention, by adding chemical substances which generate specific interaction with PVDF molecular chains, the steric hindrance effect during processing and molding in a solution or molten state is ensured, and the control of PVDF molecular conformation during molding is realized, so that the folding and aggregation modes of the molecular chains during crystallization are controlled. The steric hindrance of the seed crystal and the protection of the specific crystal face can be utilized to obtain the directionally aggregated high-energy crystal face, and the oriented assembly of the PVDF crystal face is realized.

Description

Composite seed crystal for controlling polyvinylidene fluoride crystallization, preparation method and material

Technical Field

The invention relates to the technical field of polymer crystallization, in particular to a composite seed crystal for controlling polyvinylidene fluoride crystallization, a preparation method and a material.

Background

Polyvinylidene fluoride (PVDF) is a hydrophobic fluorocarbon thermoplastic material, a white powdery semi-crystalline polymer, with a crystallinity of 60-80%, a general fluorine content of 59%, and a relative density of1.75-1.78, water absorption of 0.04%, glass transition temperature of about-39 deg.c, brittle temperature of-62 deg.c, crystalline smelting point of about 170 deg.c, thermal decomposition temperature over 316 deg.c, long-term use temperature range of-50-150 deg.c, high toughness and tensile strength of 500Kg/cm ² The composite material has excellent mechanical performance, impact resistance, wear resistance, weather resistance and chemical stability. Has the characteristics of low surface tension and high dielectric constant, the performance of the film is basically unchanged when the film is irradiated by an ultraviolet lamp with the wavelength of 20-400 nm for one year, and the film does not become brittle when the film is placed outdoors for twenty years. It is not corroded by acid, alkali, strong oxidant and halogen at room temperature, and is stable to organic solvents such as aliphatic hydrocarbon, aromatic hydrocarbon, alcohol and aldehyde.

Due to the characteristics of excellent chemical stability and mechanical strength and easy processing, the material can be widely used in the fields of chemical industry, environmental protection, food processing and the like as a separation membrane material. Since PVDF resin has superior weatherability, it can be used outdoors for a long time without maintenance, and fluorocarbon coatings prepared from it have been widely used in airports, power stations, high-rise buildings, highways, etc., and are also one of the best materials for valves, pumps, piping fittings, pipes, heat exchangers, and storage tanks of petrochemical plant fluid treatment systems as a whole or liners. A separator, a gel, a porous film, etc., made of PVDF resin, are important materials in the assembly of lithium secondary batteries, and are also important fields of application of PVDF. Particularly, as a crystalline polymer, the existence (alpha, beta, gamma, delta and epsilon) of various crystal forms makes the crystalline polymer become a good piezoelectric and ferroelectric material, has wide application prospect in the fields of industrial automation, explosion impact measurement, instruments and meters, medical appliances and the like, and is concerned by more and more researchers.

Crystallization is an efficient self-assembly process that enables the directed assembly of disordered molecules into a variety of ordered forms. For small molecules, a mesogenic theory is gradually accepted and utilized by researchers, as opposed to the traditional crystallization process (nucleation and growth mechanism). Mesomorphic theory states that: first, a plurality of monomers (molecules or ions) are aggregated to form nanoparticles, and of course, the interior of the particles may not completely reach the ordered state of crystals, and then ordered assembly is performed to form crystals. The monomers that are assembled are not individual molecules or ions, but rather preliminary aggregates thereof. The kinetics of which derive from the high surface energy of the nanoparticles. One-dimensional orientation aggregation (oriented agglomeration-OA) also often occurs if the grains have an asymmetric structure or different crystal plane energies. Recently, the process of directional assembly of nanoparticles has been clearly observed by high resolution transmission electron microscopy (Liao, H. -G et al. real-time imaging of Pt3Fe nano growth in solution. science 2012,336,1011; Li, D.; Nielsen, M.H. et al. direction-specific interactions control growth by oriented approach. science 2012,336,1014). In addition, stepwise ordered transformations of the internal structure of the nanoparticles were also demonstrated (Li, L.et al. non-crystaline-to-crystaline transformations in Pt nanoparticies.J.Am. chem. Soc.2013,135, 13062). At present, the directional assembling method of the nano particles also becomes a powerful technology for synthesizing the one-dimensional ordered nano structure material.

Unlike small molecule crystallization, for polymers with highly entangled molecular segments, the crystallization process is more complicated and has been one of the challenging basic problems in the field of polymer physics. Polymer crystallization tends to form metastable platelets with folded chains rather than directly generating the most stable extended chain crystals. Keller et al obtained lamellar single crystals of polymers by slow crystallization from dilute solutions (Keller, A.A note on single crystals in polymers: evidence for a folded chain crystallization. oil. Mag.1957,2,1171), recognizing that polymer platelets are of particular importance in polymer crystal morphology. The special morphological structure of the polymer crystal different from that of the small molecule crystal makes the research on the polymer crystal to consider not only the crystallization thermodynamics but also the kinetic process of the polymer crystal particularly important.

As to The theory of polymer crystallization, in addition to The traditional nucleation growth theory proposed earlier by Lauritzen and Hoffman (Hoffman J.D. et al The rate of crystallization of linear polymers with a chain folding. Treatise on Solid State Chemistry,1976,497-614), The theory of Mesomorphic phases (Mesomorphic) has also evolved gradually recently. In particular, Strobl has made intensive studies on polymer crystals, and it is proposed that the crystals are formed by the development of crystal nuclei as mesogens, and then gradually combined to fully develop into platelets (Strobl, G. from the crystalline sea mesomorphic and crystalline lamellae to lamellar crystals: A major route crystallized in polymer crystallization, Eur. Phys. J. E2000, 3, 165-); olmsted believes that the mesomorphic phase is formed by a spinodal mechanism via dendritic growth. Although not forming a generally accepted view, the view of mesomorphism with different degrees of ordered structure before crystal formation is gradually recognized, wherein the bundle structure model is a more representative one. The model indicates that macromolecular bundle crystal nuclei continuously exist in the solution at a certain temperature, are continuously destroyed and formed along with thermal fluctuation, and can form a stable solution with dynamic equilibrium.

For crystalline polymer materials such as PVDF, due to its highly entangled molecular chains and its complex and strong interaction, it is very difficult to control its crystallization (including controlling the degree of crystallization, the type of crystallization, and the micro-crystalline morphology) under the existing limited processing conditions, and even impossible to realize oriented assembly.

Disclosure of Invention

Problem (A)

In summary, it is an urgent need for those skilled in the art to provide a relatively effective technical means for controlling the crystallization of PVDF and further controlling the microstructure of the crystallized product.

(II) technical scheme

The invention aims to provide a composite seed crystal for controlling polyvinylidene fluoride crystallization, a preparation method and a material, so as to solve the technical problems.

In a first aspect of the embodiments of the present invention, a composite seed crystal for controlling crystallization of polyvinylidene fluoride is provided;

comprises insoluble solid particles and soluble surface modifier;

the surface modifier is a polar organic molecule, one end of a molecular chain of the polar organic molecule is chemically grafted to the surface of the solid particle, and the other end of the molecular chain is a free end.

Optionally, the composite seed crystal further comprises a soluble small molecule substance;

the soluble small molecule substance comprises organic small molecules and/or inorganic small molecule metal ions;

wherein the small organic molecule comprises C _5～9 Fatty acid ethylene glycol ester, C _7～9 One or more of fatty acid diethylene glycol ester, diethylene glycol butyl ether, carbitol ester, propylene carbonate and gamma-butyrolactone;

the small molecule metal ion comprises Mg ²⁺ 、Li ⁺ 、Na ⁺ 、K ⁺ One or more of (a).

Optionally, the solid particles comprise one or more of the following insoluble inorganic substances: sheet graphene, montmorillonite, transition metal sulfide, and transition metal carbide;

and/or, the solid particles further comprise crosslinked cured organic resin particles.

Optionally, the surface modification comprises one or more of the following soluble homopolymers or copolymers:

polyethylene glycol, methoxypolyethylene glycol, polyurethane, ethylene glycol-polyurethane-ethylene glycol copolymer, styrene and ethylene glycol copolymer, poly-N-methylpyrrolidone, a copolymer of N-methylpyrrolidone and N-methyl methacrylate, long-chain fatty amine or long-chain fatty acid;

the molecular weight of the surface modifier is 200-1.0 × 10 ⁵ In the meantime.

Optionally, the composite seed crystal is dispersed in a solvent or a plasticizer, and is maintained in a slurry state, and the solid particle content is 5-50 wt%.

In a second aspect of the embodiments of the present invention, there is provided a method for controlling polyvinylidene fluoride crystallization, including the steps of:

polyvinylidene fluoride products with different structures and performances are prepared by using the composite seed crystal through a double-diffusion solution phase separation method or a thermal phase separation method.

Optionally, polyvinylidene fluoride products with different structures and properties are prepared by a double diffusion solution phase separation method or a thermal phase separation method, and the method comprises the following steps:

putting 29-36 wt% of polyvinylidene fluoride, 20-26 wt% of composite seed crystal, 24-31 wt% of dibutyl phthalate and 14-20 wt% of dioctyl phthalate into a material dissolving tank, heating to 215-225 ℃, stirring and dissolving to form a transparent solution, performing vacuum defoaming for 250 minutes, applying pressure by nitrogen, extruding by a metering pump and a spinning nozzle, directly immersing into a water bath at 20-50 ℃, cooling and shaping to form the hollow fiber porous membrane. Preferably, after cooling and setting to form the hollow fiber porous membrane, the hollow fiber porous membrane is formed by soaking in alcohol to remove the plasticizer or solvent and other impurities.

Optionally, the method further comprises the following steps after cooling and shaping to form the hollow fiber porous membrane:

performing acid-base soaking on the hollow fiber porous membrane;

and/or, stretching and heat setting the hollow fiber porous membrane.

putting 22 wt% of polyvinylidene fluoride powder, 20 wt% of composite seed crystal, 30-58 wt% of dimethylacetamide and 0-30 wt% of dioctyl phthalate into a dissolving tank together, heating at 80-100 ℃, and stirring for dissolving 230-; keeping the membrane casting solution at a temperature for defoaming for 450-min, then supplying pressure by nitrogen, extruding by a metering pump and a spinning nozzle, directly immersing into a water bath at 20 ℃ for gelling and shaping to form a hollow fiber initial membrane, and extracting a solvent by water and/or alcohol to form a hollow fiber porous membrane;

and/or dissolving 8 wt% of polyvinylidene fluoride powder and 5 wt% of composite seed crystal in a dimethylacetamide solvent, then placing the mixture in a heating jacket at 78-82 ℃ for heating and stirring for 170-190 minutes, thereby obtaining a uniform and clear casting solution; then keeping the temperature of the casting solution, standing and defoaming for 350-370 minutes for later use; coating the obtained membrane casting solution on non-woven fabrics in an electrothermal blowing drying oven at 78-82 ℃, and soaking in a constant-temperature water bath at 18-22 ℃ under sealing for 18-22 minutes to crystallize the gel to form a gel membrane; placing the obtained gel film in a methanol extraction agent for washing, and drying the washed gel film to obtain a super-hydrophobic structure with a micro-nano-shaped surface;

and/or increasing the polyvinylidene fluoride content to 10 wt%, changing the composite seed crystal, dissolving the composite seed crystal in a dimethylacetamide solvent, and then placing the mixture in a heating jacket at 78-82 ℃ for heating and stirring for 170-190 minutes, thereby obtaining a uniform and clear casting solution; then keeping the temperature of the casting solution, standing and defoaming for 350-370 minutes for later use; coating the obtained membrane casting solution on non-woven fabrics in an electrothermal blowing drying oven at 78-82 ℃, and soaking in a constant-temperature water bath at 18-22 ℃ under sealing for 18-22 minutes to crystallize the gel to form a gel membrane; and (3) placing the obtained gel film in a methanol extraction agent for washing, and drying the washed gel film to obtain the beta-crystal-form polyvinylidene fluoride film, wherein the microstructure presents a nano-fiber crystal network structure.

In a third aspect of the embodiment of the present invention, a polyvinylidene fluoride material is further provided, which is prepared by the above preparation method.

(III) technical effects

Compared with the prior art, the invention can achieve the following technical effects:

one or more chemical reagents in the composite seed crystal provided by the embodiment of the invention have a special affinity effect with PVDF, so that part of high-energy crystal faces can be protected, the crystal faces can be greatly reduced and cannot be further developed or aggregated, and the development of the crystal is along a one-dimensional direction, for example, flexible macromolecules (namely polar organic molecules) on the surface of solid particles can partially participate in the crystallization of PVDF, so that the further aggregation of nanofiber crystals can be controlled to form a micron-sized ball cluster structure, and the porous form and the mechanical property of a final product are influenced; the special affinity action of the composite seed crystal and the PVDF incomplete fluorinated molecular chain is utilized to ensure the steric hindrance effect during the processing and forming of solution or molten state, and the control of the conformation of the PVDF molecule during the forming is realized, so that the folding and aggregation modes of the molecular chain during the crystallization, namely the crystallization structure and the microscopic morphology, are controlled. Thus, the quantity of nucleation of PVDF can be controlled, the high-energy crystal face directionally aggregated can be obtained by using the steric hindrance of the seed crystal and the protection of the specific crystal face, and the product forming process is combined, so that the oriented assembly (oriented aggregation) of the PVDF crystal face is obtained, and the PVDF high-molecular material product with ordered or disordered nanofiber crystal aggregation and specific structure and performance is formed.

Further, the soluble small molecule substances of the composite seed crystal in the embodiment of the invention are distributed around the PVDF macromolecules according to polarity characteristics in a solution state, and assist in stabilizing the pre-crystallization conformation of the PVDF.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.

FIGS. 1a and 1b are sectional structural views of a PVDF-product hollow fiber membrane in an embodiment of the present invention;

FIG. 2 is an enlarged partial cross-sectional structural view of a PVDF film in an example of the present invention;

FIGS. 3 a-3 e are structural diagrams of the nano-surface of a PVDF hydrophobic product in the examples of the present invention;

FIG. 4 is a diagram of a network of nanofiber crystals of a PVDF article in an embodiment of the invention.

Detailed Description

The technical solutions in the embodiments of the present invention are clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention. The present invention will be described in more detail with reference to the following embodiments in order to make the technical aspects of the present invention more apparent and understandable.

Seeds are additives that can nucleate during the crystallization process to accelerate or promote the growth of an enantiomer crystal of the same crystal form or configuration. The seed crystals are used to provide sites for crystal growth so that crystal nuclei can form from a homogeneous solution in which only one phase exists across an energy barrier, and the added seed crystals accelerate the growth rate of the target crystal form and contribute to the target product. In industrial products, in order to obtain a crystal product with large and uniform particle size, primary nucleation should be avoided as much as possible, secondary nucleation should be controlled, and it is usually necessary to add a proper amount of seed crystal as a core for crystal growth, and the preparation of seed crystal is therefore an important link for preparing crystal.

The embodiment of the invention discloses a multifunctional composite seed crystal for effectively controlling the crystallization degree, the crystallization type (crystal form) and the microscopic crystallization form of polyvinylidene fluoride (PVDF). The special affinity function of the multifunctional composite seed crystal and the PVDF incomplete fluorinated molecular chain is utilized to ensure the steric hindrance effect during the processing and forming in a solution or molten state, and the control of the conformation of the PVDF molecule during the forming is realized, so that the folding and aggregation modes of the molecular chain during the crystallization, namely the crystal structure and the microscopic form, are controlled. At the moment, the quantity of nucleation of the PVDF can be controlled, the directionally aggregated high-energy crystal face can be obtained by utilizing the steric hindrance of the seed crystal and the protection of the specific crystal face, and the oriented assembly (oriented assembly) of the PVDF crystal face is obtained by combining the molding process of the product, so that the ordered or disordered PVDF polymer material product with the specific structure and the specific performance and aggregated nanofiber crystal is formed. The microstructure morphology of the product can be controlled by the primary nucleation and the secondary or multiple nucleation in the forming process according to the characteristics of the composite seed crystal.

In the embodiment of the invention, the composite seed crystal is also characterized in that the seed crystal is compounded by a plurality of chemical materials, wherein the compounding is not only carried out physically, but also carried out by some chemical reaction methods. For example, as an embodiment, the composition includes not only soluble small molecule substances, rigid small molecule suspensions, but also flexible macromolecular organic materials. Wherein the soluble micromolecular substance can be divided into soluble inorganic salt and soluble organic micromolecular; the rigid micromolecule suspended substance is insoluble solid particles; the flexible macromolecular organic material is a polar macromolecular material chemically grafted on the surface of the solid particle.

In the embodiment of the invention, part of the seed crystals in the composite seed crystals are in a uniform liquid state at the temperature of more than 50 ℃, and become uniformly dispersed nano solid particles when the temperature is reduced to form the seed crystals. In the liquid state, PVDF molecules can be properly attracted and gathered, the polar conformation of a PVDF molecular chain can be stabilized, the steric hindrance effect of suspended solid particles and the protection of different crystal faces can be jointly utilized, the crystal development is limited in a one-dimensional direction, and therefore the nanofiber crystal is formed through assembly.

The composite seed crystal of the embodiment of the invention can be properly adjusted according to the characteristics and the process of the processed product, but the final composition not only comprises the soluble micromolecular substance, the rigid micromolecular suspension, but also comprises the polar organic molecular material. In the present embodiment, the insoluble solid particles are not completely chemically insoluble, but have soluble polar organic molecular chains on the surface, and when the solid particles are dispersed in a solution, they are chemically dissolved, that is, dispersed at a molecular level. Unlike the completely dissolved state, one end of the molecular chain of the soluble polar organic molecule is grafted onto the suspended solid particles. Therefore, the agglomeration of solid particles is protected, the solid particles are stably dispersed in the PVDF solution or melt, and the solid particles can be better combined with PVDF molecules to jointly complete the assembly of PVDF nano-fiber crystals.

The multifunctional composite seed crystal provided by the embodiment of the invention is characterized in that flexible macromolecules on the surface of suspended solid particles in the seed crystal can partially participate in the crystallization of PVDF (polyvinylidene fluoride), so that the further aggregation of nano-fiber crystals can be controlled to form a micron-sized ball cluster-shaped structure, and the porous form and the mechanical property of a final product are influenced.

In addition, the soluble micromolecular substances in the composite seed crystal provided by the embodiment of the invention are distributed around the PVDF macromolecules according to the polarity characteristics in a solution state, so as to assist in stabilizing the pre-crystallization conformation of the PVDF.

According to the embodiment of the invention, the insoluble inorganic substance in the composite seed crystal comprises one or more of two-dimensional materials such as sheet graphene, montmorillonite, Transition Metal Sulfides (TMDs), Transition Metal Carbides (TMCs) and the like, and can also comprise crosslinked and cured organic resin particles.

In the present embodiment, the solid particles must be chemically modified, i.e. chemically grafted as described above, which mainly comprises two layers: the first is that the modifier can not be dissolved in the solution or dispersed in the melt independently, and the second is that one end of the modified polar organic molecule (flexible polymer in most cases) can still move freely, thus ensuring the free end of the molecular chain of the polar organic molecule to be in a free dispersion state in the solution, and aiming at ensuring the dispersion stability of the solid particles in the solution or the melt and the proper fusion property with PVDF.

Preferably, in the embodiment of the present invention, the surface modifier includes one or more of soluble homopolymers or copolymers of polyethylene glycol, methoxypolyethylene glycol, polyurethane, ethylene glycol-polyurethane-ethylene glycol copolymer, styrene-ethylene glycol copolymer (PS-b-PEG), poly N-methyl pyrrolidone (PVP), copolymer of N-methyl pyrrolidone and N-methyl methacrylate (PVP-b-PMMA-b-PVP), long-chain fatty amine or long-chain fatty acid, and the molecular weight of the surface modifier is 200-1.0 × 10 ⁵ In between.

After the solid particles are modified, the composite seed crystal of the embodiment of the invention is dispersed in a common solvent such as Dimethylacetamide (DMAc) or a common plasticizer such as Dibutyl phthalate (DBP) and is kept in a slurry state. The solid particle content is 5-50 wt%.

In the embodiment of the invention, soluble organic small molecules and inorganic small molecule metal ions are added into the slurry according to requirements. The organic small molecules include: c _5～9 Fatty acid ethylene glycol ester, C _7～9 Fatty acid diethylene glycol ester, diethylene glycol butyl ether, carbitol ester, propylene carbonate and gamma-butyrolactone; the metal ions comprising Mg ²⁺ 、Li ⁺ 、Na ⁺ 、K ⁺ And the like.

Preferably, the composite seed crystal is added in an amount of 1 to 25 wt% in the PVDF solution or melt.

Multifunctional composite crystal using embodiments of the present inventionThe main reason for obtaining the nano-fiber crystal is that one or more chemical agents in the composite seed crystal have special affinity with PVDF, polar organic molecules used as surface modifier, and soluble small molecular substances such as C _5～9 Fatty acid ethylene glycol ester, C _7～9 The fatty acid diethylene glycol ester, diethylene glycol monobutyl ether, carbitol ester, propylene carbonate, gamma-butyrolactone and the like have special affinity with PVDF, can protect part of high-energy crystal faces, greatly reduce the crystal faces, and cannot be further developed or aggregated, and the development of the crystal is along a one-dimensional direction.

Two specific examples of the composite seed crystal of the present invention are listed below.

Example 1

Polystyrene-b-polyethylene glycol (PS-b-PEG) grafted nano silicon dioxide particles (with the particle size of 20-300 nanometers and 20wt percent), light nano calcium carbonate (with the particle size of 20-300 nanometers and 5wt percent), diethylene glycol monobutyl ether (5wt percent) and C _5～9 Fatty acid ethylene glycol ester (20 wt%), dimethylacetamide (DMAc) (50 wt%) were mixed, and stirred mechanically with a stirrer at 3000 rpm to form a slurry-like composite seed crystal.

Example 2

Polyurethane grafted nanometer silicon dioxide particles (with the particle diameter of 20-300 nanometers and 10wt percent), gamma-butyrolactone (5wt percent) and C _7～9 Fatty acid diethylene glycol ester (20 wt%), lithium chloride (5 wt%), and DMAc (60 wt%) were mixed, and stirred mechanically with a stirrer at 3000 rpm to form a slurry-like composite seed crystal.

The composite seed crystal provided by the embodiment of the invention can be used for preparing PVDF products with different structures and performances by adopting various common product processing technologies. The common product processing technology comprises the common double-diffusion solution phase separation method and the thermally induced phase separation method. The PVDF porous membrane prepared by the method has uniform structure, better water flux and higher porosity, and can be used for the aspects of filtration and separation, tap water purification, sewage treatment and recycling and the like.

Specific examples of PVDF articles prepared using the composite seed crystals provided by the present invention are listed below.

Example 3

Putting 29 wt% of polyvinylidene fluoride, 20 wt% of composite seed crystal, 31 wt% of dibutyl phthalate and 20 wt% of dioctyl phthalate into a dissolving tank together, heating to 220 ℃, stirring and dissolving to form a transparent solution, defoaming in vacuum for 4 hours, applying pressure by nitrogen, extruding by a metering pump and a spinning nozzle, directly immersing into a water bath at 20 ℃, cooling and shaping to form a hollow fiber initial membrane, and extracting a diluent to form the hollow fiber porous membrane.

The obtained hollow fiber porous membrane had an outer diameter of 2.2mm and an inner diameter of 1.2mm in a uniform porous state, as shown in FIG. 1, and a spherical crystal aggregate structure was avoided, and it was tested that the water flux of the hollow fiber porous membrane was 1250L.m ^-2 h ^-1 The breaking force was 4.2 newtons, and the void ratio was 66.3%.

Comparative example 3.1:

the content of the polyvinylidene fluoride in example 3 is increased to 36 wt%, the composite seed crystal is increased to 26 wt%, other proportions are correspondingly reduced, for example, 24 wt% of dibutyl phthalate and 14 wt% of dioctyl phthalate, other preparation steps are the same as those in example 1, the prepared hollow fiber porous membrane has the outer diameter of 2.3mm and the inner diameter of 1.2mm, the microstructure of the membrane is uniform, and the water flux is tested to be 850L.m ^-2 h ^-1 The breaking force was 7.3N, and the porosity was 60.3%.

Comparative example 3.2:

the hollow fiber membrane in comparative example 3.1 is soaked in acid and alkali, and the specific method of acid and alkali soaking is as follows: firstly, soaking in 2mol/L hydrochloric acid for 2 hours, washing with a large amount of water, then soaking in 2mol/L sodium hydroxide solution for 8 hours, and taking out, wherein the water flux of the obtained hollow fiber porous membrane is improved by 20%, the breaking force is equivalent, and the void ratio is improved to 71%.

Comparative example 3.3:

after the hollow fiber membrane in the comparative example 3.1 is stretched and heat-set, the water flux of the membrane is improved by 60-196%, the breaking force is equivalent, and the void ratio is improved to 66.3-70.5%. As an implementation mode, the stretching and the heat setting are 50-120% of the stretching of a double-roll stretcher, and the hot oil bath treatment is carried out for 30s at 140 ℃.

Comparative example 3.4:

according to the film-making conditions of comparative example 3.1, the temperature of the cooling water bath is changed to 50 ℃, the water flux of the film is equivalent, the breaking force is reduced to 4.8 newtons, and the void ratio is equivalent.

Example 4

Putting 22 wt% of polyvinylidene fluoride powder, 20 wt% of composite seed crystal and 58 wt% of dimethylacetamide into a dissolving tank together, and then heating and stirring at 80 ℃ for dissolving for 4 hours to obtain a uniform casting solution; and then keeping the film casting solution at a constant temperature, standing and defoaming for 8h, then supplying pressure by nitrogen, extruding by a metering pump and a spinning nozzle, directly immersing into a water bath at 20 ℃ for gelling and shaping to form a hollow fiber initial film, and extracting a solvent by a large amount of water to form the hollow fiber porous film.

The hollow fiber porous membrane has an outer diameter of 1.6mm and an inner diameter of 0.9mm, has a uniform membrane structure, a cross section of a three-dimensional network structure, and has a breaking strength of 4.6 Newton and a water flux of 610L.m, as shown in FIG. 2 ^-2 h ^-1 The porosity was 71.3%.

Comparative example 4.1:

according to the film-forming method of example 4, half of the 60 wt% dimethylacetamide solvent was replaced by dioctyl phthalate, i.e., 30 wt% of each of the two solvents. Then heating at 100 ℃ and stirring for dissolving for 4 hours to obtain uniform casting solution; and then keeping the film casting solution at a constant temperature, standing and defoaming for 8h, then supplying pressure with nitrogen, extruding through a metering pump and a spinning nozzle, directly immersing into water bath gel at 20 ℃ and shaping to form a hollow fiber initial film, and extracting a solvent with a large amount of water and alcohol to form a hollow fiber porous film with the outer diameter of 1.8mm and the inner diameter of 1.0 mm. The membrane structure is uniform, the breaking strength is 6.6 newtons, and the water flux is 1630L.m ^-2 h ^-1 The void ratio was 67.3%.

Example 5

The preparation method of the embodiment 5 of the invention mainly comprises three steps of preparing the membrane casting solution, preparing the gel solution and post-treating:

preparing a casting film liquid: dissolving 8 wt% of polyvinylidene fluoride powder and 5 wt% of composite seed crystal in a dimethylacetamide solvent, and then placing the mixture in a heating jacket at 80 ℃ to heat and stir for 3 hours, so as to obtain a uniform and clear casting solution; then keeping the temperature of the casting solution, standing and defoaming for 6 hours for later use;

preparing a gel film: coating the obtained casting solution on a non-woven fabric in an electrothermal blowing drying oven at 80 ℃, and soaking the non-woven fabric in a constant-temperature water bath at 20 ℃ under sealing for 20min to crystallize the gel to form a gel film;

performing post-treatment: and washing the obtained gel film in a methanol extracting agent for three times, wherein the first washing time is 30min, the second washing time is 2h, and the third washing time is 24 h. And (3) drying the washed gel film to obtain the super-hydrophobic structure with the micro-nano-shaped surface, wherein the contact angle between the super-hydrophobic structure and water is 161 degrees as shown in figure 3, and the water drops on the surface of the obtained gel film are spherical.

Example 6

Increasing polyvinylidene fluoride to 10 wt% in example 5, changing the composite seed to a composite seed containing lithium salt, specifically, lithium chloride in this example, the same procedure was followed to obtain a β crystal type PVDF film having a network structure of nano-fiber crystals in the microstructure, as shown in fig. 4.

In the prior art, the size effect of the ordered nanofiber is very obvious, and the ordered nanofiber shows many novel characteristics in the aspects of light, electricity, heat, magnetism and the like, while the material taking the ordered polymer nanofiber as a main body shows outstanding hydrophobic, porous, separation, electric conduction, piezoelectricity and mechanical properties, so that great interest is increasingly brought to people in preparing one-dimensional nanostructures and researching the properties and functions of the one-dimensional nanostructures by applying theoretical and experimental means. The preparation method of the ordered nano structure mainly comprises the following steps: electric field induction, magnetic field induction, solvent volatilization induction, liquid crystal assisted orientation, nanoimprint lithography, direct stretch orientation, and the like (Su, b.et. the art of alignment one-dimensional (1D) nanostructures. chem.soc.rev.2012,41,7832), however, the above methods either require special instruments and equipment or special reagent raw materials, are difficult to prepare on a large scale, and are expensive. For the assembly of the ordered micro-nano structure of the macromolecule, the assembly is mostly completed through the specific interaction (hydrogen bond, pi-pi bond, van der waals or electrostatic interaction) generated by the macromolecule (such as protein, DNA, phospholipid, block copolymer, amphiphilic polymer and the like) with a specific structure. The other method is to process the formed product into an ordered structure by a top-down method, namely, a specific physical technical means.

Different from the prior art, the invention provides the multifunctional composite seed crystal which can effectively control the crystallization degree, the crystallization type (crystal form) and the microscopic crystallization form of polyvinylidene fluoride (PVDF). The special affinity function of the multifunctional composite seed crystal and the PVDF incomplete fluorinated molecular chain is utilized to ensure the steric hindrance effect during the processing and forming in a solution or molten state, and the control of the conformation of the PVDF molecule during the forming is realized, so that the folding and aggregation modes of the molecular chain during the crystallization, namely the crystal structure and the microscopic form, are controlled. At the moment, the quantity of nucleation of PVDF can be controlled, the oriented aggregation high-energy crystal face can be obtained by using the steric hindrance of the seed crystal and the protection of the specific crystal face, and the forming process of the product is combined, so that oriented assembly (oriented attachment) of the PVDF crystal face is obtained, and the PVDF high polymer material product which is ordered or disordered and has nanofiber crystal aggregation and a specific structure and performance is formed, the preparation cost is low, and the large-scale implementation is easy.

The principles and embodiments of the present invention have been described herein using specific examples, which are provided only to help understand the method and the core concept of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the above, the present disclosure should not be construed as limiting the invention.

Claims

1. The composite crystal seed for controlling the crystallization of the polyvinylidene fluoride is characterized by comprising insoluble solid particles and a soluble surface modifier;

the solid particles include one or more of the following insoluble inorganic substances: sheet graphene, montmorillonite, transition metal sulfide, and transition metal carbide; and/or, the solid particles further comprise crosslinked cured organic resin particles;

the surface modification comprises one or more of the following soluble homopolymers or copolymers: polyethylene glycol, methoxypolyEthylene glycol, polyurethane, ethylene glycol-polyurethane-ethylene glycol copolymers, styrene and ethylene glycol copolymers, poly-N-methylpyrrolidone, copolymers of N-methylpyrrolidone and N-methyl methacrylate, long-chain fatty amines or long-chain fatty acids; the molecular weight of the surface modifier is 200-1.0 × 10 ⁵ To (c) to (d);

the surface modifier is a polar organic molecule, one end of a molecular chain of the polar organic molecule is chemically grafted to the surface of the solid particle, and the other end of the molecular chain is a free end; the free end is in a free dispersion state in the solution; the polar organic molecules are used for partially participating in the crystallization of PVDF;

the composite seed crystal is used for dispersing in a solvent or a plasticizer and is kept in a slurry state, and the content of the solid particles is 5-50 wt%;

the composite seed crystal also comprises a soluble small molecule substance; the soluble small molecule substance comprises organic small molecules and/or inorganic small molecule metal ions; the soluble small molecule substance is used for being distributed around the PVDF macromolecules according to polarity characteristics in a solution state so as to assist in stabilizing the pre-crystallization conformation of the PVDF.

2. The composite seed crystal for controlling crystallization of polyvinylidene fluoride according to claim 1,

the organic small molecule comprises C _5～9 Fatty acid ethylene glycol ester, C _7～9 One or more of fatty acid diethylene glycol ester, diethylene glycol butyl ether, carbitol ester, propylene carbonate and gamma-butyrolactone;

3. A preparation method for controlling polyvinylidene fluoride crystallization is characterized by comprising the following steps:

the composite seed crystal of claim 1 or 2 is used for preparing polyvinylidene fluoride products with different structures and properties by a double diffusion solution phase separation method or a thermal phase separation method, and the method specifically comprises the following steps:

putting 29-36 wt% of polyvinylidene fluoride, 20-26 wt% of the composite seed crystal, 24-31 wt% of dibutyl phthalate and 14-20 wt% of dioctyl phthalate into a dissolving tank, heating to 215-225 ℃, stirring and dissolving to form a transparent solution, performing vacuum defoaming for 250 minutes, supplying pressure with nitrogen, extruding through a metering pump and a spinning nozzle, directly immersing into a water bath at 20-50 ℃, cooling and shaping to form a hollow fiber porous membrane;

or putting 22 wt% of polyvinylidene fluoride powder, 20 wt% of the composite seed crystal, 30-58 wt% of dimethylacetamide and 0-30 wt% of dioctyl phthalate into a dissolving tank together, and then heating and stirring at 80-100 ℃ to dissolve 230-acetic acid for 250 minutes to obtain uniform membrane casting solution; keeping the membrane casting solution at a temperature for defoaming for 450-;

or dissolving 8 wt% of polyvinylidene fluoride powder and 5 wt% of the composite seed crystal in a dimethylacetamide solvent, then placing the mixture in a heating jacket at 78-82 ℃ for heating and stirring for 170-190 minutes, thereby obtaining a uniform and clear casting solution; then preserving the temperature of the casting solution, standing and defoaming for 350-370 minutes for later use; coating the obtained casting solution on non-woven fabrics in an electrothermal blowing dry box at 78-82 ℃, and soaking the non-woven fabrics in a constant-temperature water bath at 18-22 ℃ under sealing for 18-22 minutes to crystallize the gel to form a gel film; placing the obtained gel film in a methanol extraction agent for washing, and drying the washed gel film to obtain a super-hydrophobic structure with a micro-nano-shaped surface;

or increasing polyvinylidene fluoride to 10 wt%, changing the composite seed crystal into a composite seed crystal containing lithium chloride, dissolving the composite seed crystal in a dimethylacetamide solvent, and then placing the composite seed crystal in a heating jacket at 78-82 ℃ for heating and stirring for 170-190 minutes to obtain a uniform and clear casting solution; then preserving the temperature of the casting solution, standing and defoaming for 350-370 minutes for later use; coating the obtained casting solution on non-woven fabrics in an electrothermal blowing drying oven at 78-82 ℃, and soaking the non-woven fabrics in a constant-temperature water bath at 18-22 ℃ under sealing for 18-22 minutes to crystallize the gel to form a gel film; and (3) placing the obtained gel film in a methanol extraction agent for washing, and drying the washed gel film to obtain the beta-crystal polyvinylidene fluoride film, wherein the microstructure presents a nano fiber crystal network structure.

4. The preparation method of controlling the crystallization of polyvinylidene fluoride according to claim 3, wherein the cooling and shaping are carried out to form the hollow fiber porous membrane, and then the method further comprises the following steps:

performing acid-base soaking on the hollow fiber porous membrane;

and/or, stretching and heat setting the hollow fiber porous membrane.

5. A polyvinylidene fluoride material produced by the production method according to claim 3 or 4.