JP2024179552A - Surface-modified substrate and manufacturing method thereof - Google Patents

Abstract

To provide a surface reformed substrate having a surface reformed film which is a low-friction concentrated polymer brush having excellent durability such as friction resistance.SOLUTION: A surface reformed substrate comprises: a substrate; and a surface reformed film which is provided for a surface of the substrate, and formed of a polymer having a constitutional unit (i) derived from methyl methacrylate and a constitutional unit (ii) derived from fluorinated alkyl methacrylate, where one terminal of the polymer is bonded to the surface of the substrate via a connection structure, fluorinated alkyl methacrylate is 2,2,2-trifluoroethyl methacrylate or the like, the ratio of the constitutional unit (ii) to the total of the constitutional unit (i) and the constitutional unit (ii) is 5 to 60 mol%, the number-average molecular weight of the polymer is 100,000 to 3,000,000, the molecular weight distribution is 1.1 to 2.0, the thickness of the surface reformed film is 0.1 to 5 μm, and the occupancy ratio σ* of the polymer to the surface of the substrate is 0.1 or more.SELECTED DRAWING: None

Description

The present invention relates to a surface-modified substrate and a method for producing the same.

A method for modifying a substrate is known in which a polymer having groups at its terminals that can adsorb or react with the substrate is applied to the substrate to form a physically or chemically bonded polymer layer on the substrate surface. Another method is known in which a polymerizable group added to the substrate surface is used as a starting point to polymerize a monomer to form a grafted polymer layer from the substrate surface.

In recent years, research has been conducted on so-called "concentrated polymer brushes" that are densely grafted onto a substrate using the technology of living radical polymerization developed in the 1990s. In these concentrated polymer brushes, polymer chains are grafted onto the substrate at high density with intervals of 1 to 4 nm. It is known that such concentrated polymer brushes can modify the surface of a substrate, imparting properties such as low friction, suppression of protein adsorption, size exclusion properties, hydrophilicity, and water repellency (Patent Documents 1 and 2, Non-Patent Documents 1 to 3).

JP 2008-133434 A JP 2010-261001 A

Adv. Polym. Sci. , 2006, 197, 1-45 J. Am. Chem. Soc. , 2005, 127, 15843-15847 Polym. Chem. ,2012,3,148-153

However, in recent years, there has been a demand for the ability to easily modify the surfaces of various substrates with surface modification films that have lower friction. It has also been pointed out that, depending on the conditions of use, the mechanical energy that constitutes the surface modification film can cut the polymer molecular chains, making it easier for performance such as low friction to decrease. For this reason, there has been a demand for surface modification of substrates with a surface modification film that has superior durability.

The present invention has been made in consideration of the problems associated with the conventional technology, and its object is to provide a surface-modified substrate having a surface-modified film that is a dense polymer brush with low friction and excellent durability, such as abrasion resistance. Another object of the present invention is to provide a method for producing the above-mentioned surface-modified substrate.

That is, according to the present invention, there is provided a surface-modified substrate as shown below.
[1] A surface-modifying film comprising a substrate and a polymer having a structural unit (i) derived from methyl methacrylate and a structural unit (ii) derived from a fluoroalkyl methacrylate provided on a surface of the substrate, wherein one end of the polymer is bonded to the surface of the substrate via a linking structure, and the fluoroalkyl methacrylate is selected from the group consisting of 2,2,2-trifluoroethyl methacrylate, 2,2,3,3,3-pentafluoropropyl methacrylate, 1,1,1,3,3,3-hexafluoroisopropyl methacrylate, 2,2,3,4,4,4-hexafluorobutyl methacrylate, and 2,2,3,3,4,4,4-heptafluorobutyl The surface-modified substrate is at least one selected from the group consisting of methacrylate, 3,3,4,4,5,5,6,6,6-nonafluorohexyl methacrylate, and 3,3,4,4,5,5,6,6,6-nonafluorohexyl methacrylate, wherein the total proportion of the structural units (i) and (ii) in the polymer is 95 mass% or more, the proportion of the structural units (ii) in the total of the structural units (i) and (ii) is 5 to 60 mol%, the number average molecular weight of the polymer is 100,000 to 3,000,000, the molecular weight distribution (weight average molecular weight/number average molecular weight) is 1.1 to 2.0, the thickness of the surface-modified film is 0.1 to 5 μm, and the occupancy rate σ* of the polymer on the surface of the substrate is 0.1 or more.
[2] The surface-modified substrate according to [1], wherein the surface-modified film is swollen containing a first ionic liquid.
[3] The surface-modified substrate according to [1] or [2], wherein the linking structure is represented by the following general formula (A):

（前記一般式（Ａ）中、Ｒ_１は、水素原子、アルキル基、アシル基、又はアリール基を示し、Ｒ_２は、アルキル基又はアリール基を示し、「Ｐｏｌｙｍｅｒ」は前記ポリマーを示し、Ｙは、Ｏ又はＮＨを示し、「＊」は前記基材側の結合位置を示す）

(In the general formula (A), _R1 represents a hydrogen atom, an alkyl group, an acyl group, or an aryl group, _R2 represents an alkyl group or an aryl group, "Polymer" represents the polymer, Y represents O or NH, and "*" represents a bonding position on the substrate side.)

The present invention also provides a method for producing a surface-modified substrate, as described below.
[4] A method for producing a surface-modified substrate according to any one of [1] to [3] above, comprising the steps of: preparing an initiator group-imparting substrate having a polymerization initiating group represented by the following general formula (1) bonded to the surface of the substrate; and in the presence of the initiator group-imparting substrate and a polymerization solvent containing a second ionic liquid, surface-initiated living radical polymerization of a monomer containing the methyl methacrylate and the fluoroalkyl methacrylate under a pressure condition of 10 to 500 MPa to form the polymer having one end bonded to the surface of the substrate via the linking structure, and providing the surface-modified film formed of the polymer on the surface of the substrate.

（前記一般式（１）中、Ｒ_１は、水素原子、アルキル基、アシル基、又はアリール基を示し、Ｒ_２は、アルキル基又はアリール基を示し、Ｘは、塩素原子、臭素原子、又はヨウ素原子を示し、Ｙは、Ｏ又はＮＨを示す）

(In the general formula (1), _R1 represents a hydrogen atom, an alkyl group, an acyl group, or an aryl group; _R2 represents an alkyl group or an aryl group; X represents a chlorine atom, a bromine atom, or an iodine atom; and Y represents O or NH.)

[5] The method for producing a surface-modified substrate according to [4], wherein the second ionic liquid is at least one selected from the group consisting of N-(2-methoxyethyl)-N-methylpyrrolidinium bis(trifluoromethanesulfonylimide) and N,N-diethyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonylimide).

According to the present invention, it is possible to provide a surface-modified substrate having a surface-modified film that is a dense polymer brush with low friction and excellent durability such as abrasion resistance. In addition, according to the present invention, it is possible to provide a method for producing the above-mentioned surface-modified substrate. The surface-modified substrate of the present invention has a sufficient film thickness and has a surface-modified film that is a dense polymer brush with low friction and excellent durability, and is therefore useful as a material for constituting sliding members, sliding parts, sealing materials, etc. that require low friction and durability.

<Surface-modified substrate>
Hereinafter, the embodiment of the present invention will be described, but the present invention is not limited to the following embodiment. One embodiment of the surface-modified substrate of the present invention comprises a substrate and a surface-modified film formed of a polymer having a structural unit (i) derived from methyl methacrylate and a structural unit (ii) derived from fluoroalkyl methacrylate, which is provided on the surface of the substrate, and one end of the polymer is bonded to the surface of the substrate via a linking structure. In the polymer forming the surface-modified film, the total ratio of the structural unit (i) and the structural unit (ii) is 95 mass% or more, and the ratio of the structural unit (ii) to the total of the structural unit (i) and the structural unit (ii) is 5 to 60 mol%. In addition, the number average molecular weight of the polymer forming the surface-modified film is 100,000 to 3,000,000, and the molecular weight distribution (weight average molecular weight / number average molecular weight) is 1.1 to 2.0. And, the thickness of the surface-modified film is 0.1 to 5 μm, and the occupancy rate σ* of the polymer on the surface of the substrate is 0.1 or more. Hereinafter, the details of the surface-modified substrate of the present invention will be described.

(Substrate)
The type of substrate is not particularly limited, and any of natural products, artificial materials, inorganic materials, and organic materials can be used (excluding substrates containing a polymer having a vinyl alcohol unit). The shape of the substrate is also not particularly limited, and substrates of various shapes such as lumps, powders, sheets, films, pellets, and plates can be used. The surface-modified substrate of this embodiment is produced, for example, by polymerizing a monomer while a substrate to which a polymerization initiator group has been added is immersed in a polymerization solvent. For this reason, it is preferable to use a substrate that can withstand the polymerization solution without dissolving in the polymerization solution. Specific examples of substrates include films, fibers, and sheets formed of materials such as metals, metal oxides, metal nitrides, metal carbides, ceramics, wood, silicon compounds, thermoplastic resins, thermosetting resins, cellulose, and glass.

The substrate may be subjected to a surface treatment. Examples of the surface treatment include inorganic treatments such as silane coupling treatment, silica coating treatment, and silica alumina treatment; and cleaning and activation treatments such as plasma treatment, ultraviolet irradiation treatment, ozone oxidation treatment, radiation treatment, X-ray treatment, electron beam treatment, and laser treatment.

(Surface modified film)
One end of the polymer forming the surface-modified film is bonded to the surface of the substrate via a linking structure. That is, the surface-modified substrate of this embodiment has a surface-modified film that is a dense polymer brush formed by densely generating brush-like polymers on the surface of the substrate.

The polymer forming the surface-modified film (polymer layer) which is a concentrated polymer brush has a structural unit (i) derived from methyl methacrylate and a structural unit (ii) derived from fluoroalkyl methacrylate. It is preferable that the polymer is substantially formed only from the structural unit (i) derived from methyl methacrylate and the structural unit (ii) derived from fluoroalkyl methacrylate. By using a polymer having the structural unit (ii) derived from fluoroalkyl methacrylate, the heat resistance of the surface-modified film is expected to be improved. In addition, the high bond energy between fluorine and carbon is thought to lead to improved load-bearing and friction-resistant durability of the surface-modified film. Furthermore, the low surface free energy of fluorine reduces the adhesive force with the mating surface of the sliding part, and it is expected to reduce the starting torque of the sliding part and improve low friction.

The fluorinated alkyl methacrylate is at least one selected from the group consisting of 2,2,2-trifluoroethyl methacrylate, 2,2,3,3,3-pentafluoropropyl methacrylate, 1,1,1,3,3,3-hexafluoroisopropyl methacrylate, 2,2,3,4,4,4-hexafluorobutyl methacrylate, 2,2,3,3,4,4,4-heptafluorobutyl methacrylate, and 3,3,4,4,5,5,6,6,6-nonafluorohexyl methacrylate. If the fluorinated alkyl group of the fluorinated alkyl methacrylate is too long, the surface modified film formed may be soft. In recent years, there has been a trend toward increased regulation of perfluoroalkyl compounds due to their accumulation and difficulty in decomposing, but the above-mentioned fluorinated alkyl methacrylates are suitable not only from the viewpoint of performance but also from the viewpoint of the environment.

If the polymer forming the surface-modified film is composed only of structural units (ii) derived from fluoroalkyl methacrylate, the polymer will be less likely to dissolve and swell in commonly used organic solvents or ionic liquids. For this reason, the polymer has structural units (i) derived from methyl methacrylate. If the monomer copolymerized with fluoroalkyl methacrylate is a methacrylate other than methyl methacrylate, it may be difficult to synthesize a polymer with sufficient molecular weight due to steric hindrance, etc., and the glass transition temperature of the resulting polymer may decrease, making it more likely to have reduced durability.

In the polymer, the proportion of structural unit (ii) in the total of structural unit (i) and structural unit (ii) is 5 to 60 mol%, and preferably 10 to 50 mol%. If the proportion of structural unit (ii) is less than 5 mol%, the effect of the fluorine atoms is not fully exerted. On the other hand, if the proportion of structural unit (ii) is more than 60 mol%, the polymer becomes difficult to dissolve and swell in commonly used organic solvents and ionic liquids.

In the polymer forming the surface-modified film, the total proportion of the structural unit (i) and the structural unit (ii) is 95% by mass or more, preferably 98% by mass or more, and more preferably 100% by mass. The polymer may further have other structural units other than the structural unit (i) and the structural unit (ii) as long as the other structural units are less than 5% by mass. Examples of monomers constituting the other structural units include methacrylates other than methyl methacrylate (other methacrylates). Other methacrylates include aliphatic alkyl, alicyclic alkyl, and aromatic methacrylates such as ethyl, propyl, butyl, isobutyl, t-butyl, hexyl, octyl, 2-ethylhexyl, decyl, isodecyl, dodecyl, stearyl, behenyl, cyclohexyl, t-butylcyclohexyl, trimethylcyclohexyl, tricyclodecyl, isobornyl, adamantyl, phenyl, and benzyl; methacrylates which are esters of alcohols having oxygen, nitrogen, fluorine, silicon, and the like, such as tetrahydrofurfuryl, dimethylaminoethyl, diethylaminoethyl, t-butylaminoethyl, trifluoromethyl, perfluorooctyl, and trimethylsilyl; and the like. Among these, it is preferable to use a methacrylate having a functional group, since it is possible to impart functionality and durability by crosslinking the polymer or reacting it with other compounds. Examples of methacrylates having functional groups include hydroxyalkyl methacrylate, 3-chloro-2-hydroxypropyl methacrylate, glycidyl methacrylate, epoxycyclohexylmethyl methacrylate, methacryloyloxyethyl isocyanate, and blocked methacryloyloxyethyl isocyanate.

One end of the polymer that forms the surface-modified film is bonded to the surface of the substrate via a linking structure. An example of the linking structure is the structure represented by the following general formula (A).

（前記一般式（Ａ）中、Ｒ_１は、水素原子、アルキル基、アシル基、又はアリール基を示し、Ｒ_２は、アルキル基又はアリール基を示し、「Ｐｏｌｙｍｅｒ」は前記ポリマーを示し、Ｙは、Ｏ又はＮＨを示し、「＊」は前記基材側の結合位置を示す）

(In the general formula (A), R ₁ represents a hydrogen atom, an alkyl group, an acyl group, or an aryl group, R ₂ represents an alkyl group or an aryl group, "Polymer" represents the polymer, Y represents O or NH, and "*" represents a bonding position on the substrate side.)

The number average molecular weight (Mn) of the polymer is 100,000 to 3,000,000, preferably 500,000 to 2,000,000. If the Mn of the polymer is less than 100,000, the thickness of the surface modification film is insufficient. On the other hand, it is difficult to polymerize a polymer with an Mn of more than 3,000,000, and many termination reactions occur as side reactions, which may result in an excessively large molecular weight distribution (PDI). Note that the Mn and Mw of the polymer in this specification are values calculated as polymethyl methacrylate measured by gel permeation chromatography (GC).

The molecular weight distribution of the polymer (PDI = weight average molecular weight (Mw) / number average molecular weight (Mn)) is 1.1 to 2.0, preferably 1.15 to 1.8, and more preferably 1.2 to 1.6. PDI is a physical property that serves as an index of the variation in the molecular weight of a polymer. If the PDI exceeds 2.0, the variation in molecular weight is large, so that molecular chains with large molecular weight (long) tend to protrude from the surface of the surface-modified film formed, which may impair the characteristics of the dense polymer brush.

The molecular weight of the polymer can be measured by GPC using a surface-modified film obtained by treating the surface-modified substrate with an alkali or the like and cleaving the bond with the substrate as a sample. When synthesizing a polymer by living radical polymerization, polymerization proceeds uniformly from the polymerization initiator group. Therefore, if a monofunctional polymerization initiator compound such as ethyl isobutyric acid bromide is added to the polymer polymerization system, the molecular weight of the polymer extended from the monofunctional polymerization initiator compound can be estimated to be the same as the molecular weight of the polymer extended from the surface of the substrate.

The thickness of the surface modification film (dry film thickness) is 0.1 to 5 μm, preferably 0.2 to 3 μm, and more preferably 0.4 to 1.5 μm. If the thickness of the surface modification film is less than 0.1 μm, it is too thin, and when used as a sliding member, the surface roughness of the mating surface may be rougher than the thickness of the surface modification film, and the surface modification film may be easily scraped off. On the other hand, if the thickness of the surface modification film is more than 5 μm, the polymerization time of the polymer may become excessively long and the molecular weight distribution of the polymer may become excessively broad.

The variation in thickness of the surface-modified film is preferably within ±20%, and more preferably within ±10%, based on the average thickness of the surface-modified film. The average thickness of the surface-modified film is the average value of the thicknesses of the surface-modified film measured at any 10 points. If the variation in thickness of the surface-modified film exceeds ±20%, the surface smoothness of the surface-modified film may decrease slightly, and the brush effect may become insufficient.

The occupancy rate of the polymer on the surface of the substrate (surface occupancy rate σ*) is 0.1 or more, preferably 0.1 to 1.0, and more preferably 0.15 to 0.8. By setting the surface occupancy rate σ* of the polymer within the above range, the characteristics of a dense polymer brush can be exhibited. In other words, by forming a surface modified film with a polymer that is a dense polymer brush, an effective brush effect is exhibited, making it possible to use it in a variety of applications. Furthermore, by reducing frictional energy loss and preventing wear due to the extremely low friction, the substrate can be given effects such as a long life, antibacterial and antiviral properties due to size exclusion properties, contamination resistance, and protein adhesion prevention, as well as high lubricity and high slipperiness due to liquid retention.

The surface occupancy σ* can be calculated according to the following formula (Y) using the "graft density σ (pieces/nm ² )" calculated by the following formula (X).
σ=dLNAMn...(X)
d: density of polymer L: thickness of surface modification film (polymer layer) NA: Avogadro's number Mn: number average molecular weight of polymer σ*=a ² σ...(Y)
^a2 : Monomer cross-sectional area

The thickness of the surface modification film can be measured by a conventional method using, for example, an ellipsometer, an electron microscope, or an atomic force microscope. The density of the polymer can be measured by values described in conventional literature or by a method described in JIS K 7112:1999, etc.

It is preferable that the surface-modified film contains a solvent and swells in order to achieve a more effective brush effect. As a solvent for swelling the surface-modified film, it is preferable to use a liquid medium that has an affinity for dissolving the polymer. By swelling the polymer layer with such a liquid medium, the thickness of the surface-modified film can be increased by up to 1.1 to 4 times.

Examples of solvents (liquid media) that swell the surface-modified film include water-based lubricants and lubricating oils. Examples of water-based lubricants include water, glycerin, polyethylene glycol, and polypropylene glycol. Examples of lubricating oils include hydrocarbon-based lubricating oils, ester-based lubricating oils, silicone oils, and ionic liquids. By swelling the surface-modified film with these liquid media, the brush effect, which includes extremely low friction, size exclusion properties, and high liquid retention, can be effectively exerted. This makes it possible to provide various articles that are endowed with properties such as energy saving, long life, and high performance.

In particular, the surface-modified film is preferably swollen with an ionic liquid (first ionic liquid). The ionic liquid is nonvolatile and nonflammable. Therefore, by using the ionic liquid as a liquid medium for swelling the surface-modified film, the film is less likely to volatilize or ignite even when the surface of the surface-modified film is rubbed with a member or the like, and safety and long-term durability can be maintained. The ionic liquid is a compound having an organic salt structure and is liquid at around room temperature. Examples of the cations constituting the ionic liquid include phosphonium cations, ammonium cations, imidazolium cations, and pyridinium cations. Examples of the anions constituting the ionic liquid include chlorine anions, bromine anions, iodine anions, sulfonate anions, sulfonimide anions, tetrafluoroboron anions, and hexafluorophosphate anions. From the viewpoints of polymer solubility, low friction, high polarity, and polymerization progress, the first ionic liquid that swells the surface-modified film is preferably at least one selected from the group consisting of N-(2-methoxyethyl)-N-methylpyrrolidinium bis(trifluoromethanesulfonylimide) and N,N-diethyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonylimide).

The surface-modifying film may be provided on the entire surface of the substrate, or on a portion of the surface of the substrate. The surface of the substrate on which the surface-modifying film is not provided may be subjected to other surface treatments, coatings, etc. For example, by providing a surface-modifying film on one side of a film-like substrate and providing an adhesive layer or a pressure-sensitive adhesive layer on the other side, a surface-modified substrate that can be attached to various articles, etc. can be produced.

The surface-modified substrate of this embodiment has a surface-modified film that is a dense polymer brush that has sufficient thickness, is not easily detached from the substrate, and has excellent durability such as abrasion resistance. Therefore, the surface-modified substrate of this embodiment is suitable for use as sliding parts for, for example, machines and devices, robots, home appliances, vehicles, aircraft, and space-related items. Furthermore, by swelling the surface-modified film with a solvent (liquid medium), it is possible to provide various articles that are endowed with properties such as energy saving, long life, and high performance.

<Method for producing surface-modified substrate>
The surface-modified substrate of this embodiment can be produced, for example, by the method shown below. That is, one embodiment of the method for producing a surface-modified substrate of the present invention is a method for producing the above-mentioned surface-modified substrate, which includes a step of preparing an initiator-imparted substrate having a polymerization initiation group represented by the following general formula (1) bonded to the surface of the substrate, and a step of surface-initiated living radical polymerization of a monomer containing methyl methacrylate and a fluoroalkyl methacrylate under a pressure condition of 10 to 500 MPa in the presence of a polymerization solvent containing an initiator-imparted substrate and a second ionic liquid to form a polymer having one end bonded to the surface of the substrate via a linking structure, and a step of providing a surface-modified film formed of the polymer on the surface of the substrate.

（前記一般式（１）中、Ｒ_１は、水素原子、アルキル基、アシル基、又はアリール基を示し、Ｒ_２は、アルキル基又はアリール基を示し、Ｘは、塩素原子、臭素原子、又はヨウ素原子を示し、Ｙは、Ｏ又はＮＨを示す）

(In the general formula (1), _R1 represents a hydrogen atom, an alkyl group, an acyl group, or an aryl group; _R2 represents an alkyl group or an aryl group; X represents a chlorine atom, a bromine atom, or an iodine atom; and Y represents O or NH.)

Conventionally, a method for modifying the surface of a substrate by grafting a vinyl polymer from the substrate surface has been known. Examples of methods for grafting a polymer from the substrate surface include a so-called surface-initiated polymerization method in which a polymer is polymerized from the substrate surface. Examples of surface-initiated polymerization methods include a method in which polymerization is initiated from radicals generated by irradiating the substrate surface with radiation or the like; a method in which polymerization is initiated from a compound having an azo group or a peroxide group introduced to the substrate surface; and the like. In contrast, in the method for producing a surface-modified substrate of this embodiment, a substrate (substrate with initiator group) having a specific polymerization initiator group that generates radicals bonded to its surface is first prepared. Next, a polymer is grafted by surface-initiated living radical polymerization from the polymerization initiator group of the prepared substrate with initiator group, and a surface-modified film is provided on the substrate surface.

The polymerization initiating group has a structure represented by the following general formula (1). In general formula (1), the group represented by X is eliminated as a radical, and a carbon radical is generated. Then, a monomer is inserted into the generated carbon radical, and polymerization proceeds. This results in the formation of a polymer with the polymerization initiating group at one end.

（前記一般式（１）中、Ｒ_１は、水素原子、アルキル基、アシル基、又はアリール基を示し、Ｒ_２は、アルキル基又はアリール基を示し、Ｘは、塩素原子、臭素原子、又はヨウ素原子を示し、Ｙは、Ｏ又はＮＨを示す）

(In the general formula (1), _R1 represents a hydrogen atom, an alkyl group, an acyl group, or an aryl group; _R2 represents an alkyl group or an aryl group; X represents a chlorine atom, a bromine atom, or an iodine atom; and Y represents O or NH.)

In the general formula (1), the carbon atom to which X is bonded is a tertiary carbon atom or a quaternary carbon atom. At least one of _R1 and _R2 is preferably a group other than an alkyl group. That is, when at least one of _R1 and _R2 is an electron-donating group other than an alkyl group, X is easily eliminated. In particular, it is more preferable for the carbon atom to which X is bonded to be a quaternary carbon atom in order to generate a radical.

As a method for preparing an initiator group-imparted substrate to which a specific polymerization initiator group is bonded, that is, a method for introducing (bonding) a specific polymerization initiator group to the surface of a substrate, for example, a method for imparting a compound having a polymerization initiator group to a substrate by coating, vapor deposition, transfer, electrodeposition, lamination, monolayer film formation, or the like can be mentioned. Among them, a method for imparting a compound having a group that reacts with the substrate surface and a polymerization initiator group, or a polymer having a polymerization initiator group to a substrate is preferable. By such a method, an initiator group-imparted substrate to which a specific polymerization initiator group is bonded can be prepared. Therefore, according to the manufacturing method of the surface-initiating substrate of this embodiment, a surface-modified film (polymer layer) that is a dense polymer brush can be provided on the surface of a substrate that does not substantially have reactive groups such as hydroxyl groups or amino groups on its surface.

As the polymer having a polymerization initiating group, a polymer having a three-dimensional crosslinked structure or a polymer further having a group that reacts with the substrate surface is preferred. By applying a compound having a group that reacts with the substrate surface and a polymerization initiating group or a polymer having a polymerization initiating group to the substrate, it is possible to introduce a tightly bound polymerization initiating group to the substrate surface.

The group (reactive group) that reacts with the substrate surface may be appropriately selected according to the type of substrate. For example, when a silicon substrate, a metal, a metal oxide, or an inorganic compound having a hydroxyl group is used as the substrate, an alkoxysilyl group and an epoxy group are preferred as reactive groups. When an organic compound having a hydroxyl group such as cellulose is used as the substrate, an acid halide group, an acid anhydride group, an isocyanate group, and the like are preferred as reactive groups. As a compound having an alkoxysilyl group (silane coupling group) and a polymerization initiator group, it is preferred to use a compound represented by the following formula (2).

In addition, the initiator group having the polymerization initiator group bonded thereto can also be prepared by applying a polymer (polymer-type treatment agent) obtained by polymerizing a monomer component containing a monomer represented by the following formula (3) and a monomer having an alkoxysilyl group such as (meth)acryloyloxypropyltrimethylsilyl or (meth)acryloyloxypropyltriethoxysilyl to the surface of a substrate, and coating the surface of the substrate with the applied polymer.

In the presence of an initiator group-imparting substrate and a polymerization solvent, a monomer containing methyl methacrylate and a fluoroalkyl methacrylate is subjected to surface-initiated living radical polymerization under pressure conditions of 10 to 500 MPa. This results in the formation of a polymer with one end bonded to the surface of the substrate via a specific linking structure, and a surface-modified film formed from this polymer can be provided on the surface of the substrate.

For example, atom transfer radical polymerization (ATRP) is used as the surface-initiated living radical polymerization. In the ATRP method, the halide radical extracted from the polymerization initiator group is stabilized by changing the valence of the metal ion to form a halide salt. Then, a monomer is added to the organic radical, which is the residue from which the halide is removed. However, since the organic radical is unstable, the halogen is extracted as a radical from the metal halide to stabilize it and prevent termination reactions such as coupling. Then, the metal ion from which the halogen has been removed returns to its original valence. By repeating such oxidation-reduction reactions, the monomer is polymerized and grows with the organic halide as the polymerization initiator group, and a polymer is formed. Since the polymerization reaction proceeds in a state of low radical concentration, the polymer grows uniformly and tends to have a uniform molecular weight. By applying this ATRP method to surface-initiated living radical polymerization, a dense brush-like polymer can be formed.

In atom transfer radical polymerization, metal complexes are used. Examples of metal complexes include complexes of copper chloride or copper bromide with polyamines such as dinonylbipyridine, tridimethylaminoethylamine, and pentamethyldiethylenetriamine; dichlorotris(triphenylphosphine)ruthenium; etc.

Atom transfer radical polymerization is carried out in the presence of a polymerization solvent. As the polymerization solvent, a polymerization solvent containing an ionic liquid (second ionic liquid) is used. By carrying out polymerization in the presence of a highly polar ionic liquid, the polymerization rate increases, and a polymer with a high molecular weight can be obtained. As the ionic liquid (second ionic liquid), the same ionic liquid as the first ionic liquid described above that can be used when swelling the surface-modified film can be used. That is, the second ionic liquid is preferably at least one selected from the group consisting of N-(2-methoxyethyl)-N-methylpyrrolidinium bis(trifluoromethanesulfonylimide) and N,N-diethyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonylimide). The first ionic liquid and the second ionic liquid may be the same or different.

The polymerization solvent may further contain a solvent other than the ionic liquid (another organic solvent). Examples of the other organic solvent include hydrocarbon-based solvents, ester-based solvents, glycol-based solvents, ether-based solvents, amide-based solvents, alcohol-based solvents, sulfoxide-based solvents, and urea-based solvents.

In the manufacturing method of this embodiment, the monomer is polymerized under a pressure condition of 10 to 500 MPa, preferably under a pressure condition of 100 to 400 MPa. By polymerizing under such high pressure conditions, it is possible to prevent a termination reaction and improve the elongation reaction rate of the polymer. This produces a high molecular weight polymer, and a surface-modified film with a high thickness (up to 5 μm) can be formed with a single polymer brush chain.

As the polymerization vessel, it is preferable to use a vessel that can be sealed and can withstand high pressure. In addition, since pressure needs to be transmitted to the inside of the vessel, it is preferable to use a vessel that has a part that deforms under pressure, such as a soft part or an expandable part made of plastic. Specifically, various vessels such as polyethylene bottles, PET bottles, retort pouches, and blister containers can be used. In addition, a vessel made of a material that is resistant to deformation at the temperature during polymerization and has heat resistance is preferable. Furthermore, a vessel made of a material that is resistant to corrosion by the polymerization solvent and has properties such as chemical resistance and solvent resistance is preferable. Examples of materials that constitute the polymerization vessel include polyolefin resins, fluorine resins, polyester resins, polyamide resins, and engineering plastics. In addition, it is preferable to prevent gas from entering the polymerization vessel as much as possible during polymerization. For example, it is preferable to charge the polymerization solution to more than 90% of the volume of the polymerization vessel.

As described above, by polymerizing the polymer in a state where a monofunctional polymerization initiator compound having a polymerization initiator group is blended into the polymerization system, the molecular weight of the polymer extended from the monofunctional polymerization initiator compound can be estimated to be the same as the molecular weight of the polymer extended from the surface of the substrate. The amount of the monofunctional polymerization initiator compound blended into the polymerization system is preferably 0.01 mass% or less, more preferably 0.001 mass% or less, based on the entire polymerization system. If the amount of the monofunctional polymerization initiator compound is too large, the polymerization system becomes highly viscous, and it may be difficult to remove the resulting surface-modified substrate. In addition, it may take a long time to wash and remove the free polymer attached to the surface of the resulting surface-modified substrate.

The present invention will be described in detail below based on examples, but the present invention is not limited to these examples. Note that "parts" and "%" in the examples and comparative examples are based on mass unless otherwise specified.

<Production of Surface-Modified Substrate>
Example 1
A silicon substrate of 3 cm x 3 cm size was prepared and ultrasonically cleaned using methyl ethyl ketone (MEK), tetrahydrofuran (THF), and 2-propanol (IPA) in that order. Next, the substrate was irradiated with UV ozone using a UV ozone cleaner (manufactured by Japan Laser & Electronics Lab). Thereafter, the substrate was immersed for 18 hours in a container containing 89 parts of ethanol, 10 parts of 28% aqueous ammonia solution, and 1 part of 2-bromo-2-methylpropionyloxypropyltrimethoxysilane (BPM). The substrate was removed from the container and ultrasonically cleaned using ethanol, and then dried with a blower to obtain an initiator-imparted substrate-1.

Under a nitrogen atmosphere, 0.0001 parts of ethyl 2-bromo-2-methylpropionate (EBiB), 0.0096 parts of copper (II) bromide, 0.0558 parts of copper (I) bromide, 0.3884 parts of 4,4'-dinonyl-2,2'-bipyridyl (dNbpy), 10.81 parts of methyl methacrylate (MMA), 2.02 parts of 2,2,2-trifluoroethyl methacrylate (TFEMA), 11.96 parts of anisole, and 1.33 parts of N-(2-methoxyethyl)-N-methylpyrrolidium bis(trifluoromethanesulfonylimide) (hereinafter, MEMP-TFSI) were placed in a vial and stirred to prepare a polymerization solution.

The polymerization solution was filled into an aluminum laminate bag containing the initiator group-imparting substrate-1, and the bag was heat-sealed while removing the gas. The aluminum laminate bag was placed in a high-pressure device (product name "PV-400", manufactured by Shin Corporation) containing water as a pressure medium. Polymerization was carried out for 4 hours under conditions of 60°C and 400 MPa.

In the reaction system, a highly viscous solution containing a polymer that had been polymerized from the free polymerization initiator EBiB was produced. A portion of the solution was sampled, and the number average molecular weight (Mn) of the polymer was measured in terms of polymethyl methacrylate by gel permeation chromatography (GPC) using N,N-dimethylformamide (DMF) as the developing solvent. As a result, the Mn of the polymer was 2.44 million, and the molecular weight distribution (PDI = weight average molecular weight (Mw) / number average molecular weight (Mn)) was 1.45. In living radical polymerization, polymerization is uniformly initiated from the polymerization initiator group. It is known that polymers are simultaneously produced from the polymerization initiator group bonded to the substrate and from the free polymerization initiator EBiB, and the molecular weights of the two polymers produced are the same. In the following, the molecular weight of the polymer produced from the polymerization initiator group on the substrate surface was measured in the same manner, assuming that the molecular weight of the polymer produced from the polymerization initiator group on the substrate surface was the same as the molecular weight of the polymer produced from the free polymerization initiator (EBiB).

The contents removed from the aluminum laminated bag were thoroughly washed with THF to obtain surface-modified substrate 1. Ten randomly selected locations on the formed surface-modified film were used to measure the thickness (dry film thickness) of the surface-modified film using a spectroscopic film thickness meter, and the thickness was 940 nm. The surface occupancy rate (σ*) calculated using the method described above was 0.13.

The obtained surface-modified substrate 1 was immersed in MEMP-TFSI and heated at 80°C for 10 minutes using a hot plate to swell the surface-modified film. After shaking off excess MEMP-TFSI using a spin coater, 10 random locations on the swollen surface-modified film were selected, and the thickness of the surface-modified film (swollen film thickness) measured using a spectroscopic film thickness meter was 2,610 nm. The degree of swelling (swollen film thickness/dry film thickness) was 3.22.

(Examples 2 to 9)
Surface-modified substrates 2 to 9 were obtained in the same manner as in Example 1 described above, except that the conditions and the like shown in Tables 1-1 and 1-2 were used. The physical properties and the like of the obtained Surface-modified Substrates 2 to 9 are shown in Tables 1-1 and 1-2.
The abbreviations in Tables 1-1 to 1-3 are as follows.
DEME-TFSI: N,N-diethyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonylimide)
HFBMA: 2,2,3,3,4,4,4-heptafluorobutyl methacrylate NFHMA: 3,3,4,4,5,5,6,6,6-nonafluorohexyl methacrylate

(Comparative Example 1)
Except for using only MMA as a monomer without using TFEMA, which is a fluoroalkyl methacrylate, a surface-modified substrate 1H was obtained in the same manner as in Example 1. The physical properties of the obtained surface-modified substrate 1H are shown in Table 1-3.

(Comparative Example 2)
Under a nitrogen atmosphere, 0.0039 parts of EBiB, 0.0134 parts of copper (II) bromide, 0.0861 parts of copper (I) bromide, 0.5934 parts of dNbpy, 10.81 parts of MMA, 2.02 parts of TFEMA, 9.96 parts of anisole, and 1.11 parts of MEMP-TFSI were put into a vial and stirred to prepare a polymerization solution. Thereafter, the surface-modified substrate 2H was obtained in the same manner as in Example 1 described above, except that the polymerization was carried out for 12 hours under conditions of 60 ° C. and 0.1 MPa. The physical properties of the obtained surface-modified substrate 2H are shown in Table 1-3.

(Comparative Example 3)
Initiator group-imparting substrate-1 was placed in a blower dryer and heated at 200°C for 2 hours to produce initiator group-imparting substrate-2. This initiator group-imparting substrate-2 was prepared by heating initiator group-imparting substrate-1 to decompose some of the polymerization initiating groups and reduce the density of the polymerization initiating groups. Then, a surface-modified substrate 3H was obtained in the same manner as in Example 1, except that the initiator group-imparting substrate-2 produced was used. The physical properties of the obtained surface-modified substrate 3H are shown in Table 1-3.

<Evaluation>
A friction sliding tester (trade name "TRB3", manufactured by Anton Paar) was used to carry out a friction sliding test on the surface modified film of the manufactured surface modified substrate, and the friction coefficient was measured and calculated. The results are shown in Table 2. Specifically, a friction test was carried out by rubbing the surface of the surface modified film at a linear velocity of 0.5 cm/s under room temperature (25°C) conditions, using MEMP-TFSI as the lubricating solvent and a 10 mm diameter SUJ2 ball as the mating material. The load was changed in the range of 1 to 30 N.

The surface-modified substrate of the present invention has a surface-modified film with excellent durability such as abrasion resistance and low friction, and is therefore useful as a component that constitutes the sliding parts of various machines, devices, systems, etc.

Claims (5)

A substrate;
A surface-modified film formed of a polymer having a structural unit (i) derived from methyl methacrylate and a structural unit (ii) derived from a fluoroalkyl methacrylate, the surface-modified film being provided on the surface of the substrate;
one end of the polymer is bound to the surface of the base material via a linking structure;
the fluorinated alkyl methacrylate is at least one selected from the group consisting of 2,2,2-trifluoroethyl methacrylate, 2,2,3,3,3-pentafluoropropyl methacrylate, 1,1,1,3,3,3-hexafluoroisopropyl methacrylate, 2,2,3,4,4,4-hexafluorobutyl methacrylate, 2,2,3,3,4,4,4-heptafluorobutyl methacrylate, and 3,3,4,4,5,5,6,6,6-nonafluorohexyl methacrylate;
In the polymer, the total proportion of the structural unit (i) and the structural unit (ii) is 95 mass% or more,
The proportion of the structural unit (ii) in the total of the structural unit (i) and the structural unit (ii) is 5 to 60 mol %,
The number average molecular weight of the polymer is 100,000 to 3,000,000 and the molecular weight distribution (weight average molecular weight/number average molecular weight) is 1.1 to 2.0;
The thickness of the surface modified film is 0.1 to 5 μm;
A surface-modified substrate, in which the occupancy rate σ* of the polymer on the surface of the substrate is 0.1 or more. The surface-modified substrate according to claim 1, wherein the surface-modified film contains a first ionic liquid and is swollen. The surface-modified substrate according to claim 1 , wherein the linking structure is represented by the following general formula (A):

(In the general formula (A), R ₁ represents a hydrogen atom, an alkyl group, an acyl group, or an aryl group, R ₂ represents an alkyl group or an aryl group, "Polymer" represents the polymer, Y represents O or NH, and "*" represents a bonding position on the substrate side.) A method for producing a surface-modified substrate according to any one of claims 1 to 3,
A step of preparing an initiator group-imparting substrate having a polymerization initiating group represented by the following general formula (1) bonded to the surface of the substrate;
a step of performing surface-initiated living radical polymerization of a monomer including the methyl methacrylate and the fluoroalkyl methacrylate under a pressure condition of 10 to 500 MPa in the presence of the initiator group-imparting substrate and a polymerization solvent including a second ionic liquid, thereby forming the polymer having one end bonded to the surface of the substrate via the linking structure, and providing the surface-modified film formed of the polymer on the surface of the substrate;
A method for producing a surface-modified substrate having the above structure.

(In the general formula (1), _R1 represents a hydrogen atom, an alkyl group, an acyl group, or an aryl group; _R2 represents an alkyl group or an aryl group; X represents a chlorine atom, a bromine atom, or an iodine atom; and Y represents O or NH.) The method for producing a surface-modified substrate according to claim 4, wherein the second ionic liquid is at least one selected from the group consisting of N-(2-methoxyethyl)-N-methylpyrrolidinium bis(trifluoromethanesulfonylimide) and N,N-diethyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonylimide).