Polymer brush

1. A polymer brush comprising a substrate and a polymer layer,

wherein the polymer layer comprises polymer chains each of which has one end fixed on the substrate and other end free-ended,

wherein the polymer chains are formed by fixing a brush initiator having a polymerization initiating group on a surface of the substrate and polymerizing a monomer,

wherein a state of phase-separation between a polymer dense part and a polymer thin part reversibly changes in the polymer layer, and

wherein the maximum height difference between the polymer dense part and the polymer thin part is not less than 1 nm.

2. The polymer brush according to claim 1 , wherein the polymer layer thickness is not less than 1 nm and not more than 50 nm provided that the polymer layer is in a non-phase-separated state, and wherein the number average molecular weight of the polymer chain is not less than 2,000 and not more than 1,000,000.

3. The polymer brush according to claim 1 , wherein the phase-separated structure is reversibly controlled with the exterior environment change.

4. The polymer brush according to claim 3 , wherein the exterior environment is any one selected from the group consisting of temperature, pressure, and ion concentration.

5. The polymer brush according to claim 1 , wherein a polymer dense part and a polymer thin part is formed in the polymer layer.

6. The polymer brush according to claim 5 , wherein a plurality of the polymer dense parts and a plurality of the polymer thin parts appear periodically and repeatedly in the planar direction in the polymer layer.

7. The polymer brush according to claim 5 , wherein a structure formed by a phase-separation between the polymer dense part and the polymer thin part is any one selected from the group consisting of sea-island structure, cylinder structure, interlinked structure, and lamellar structure.

8. A process for controlling a surface state comprising

contacting the polymer brush according to claim 1 with a solvent, and

changing exterior environment.

9. The process for controlling a surface state according to claim 8 , wherein the exterior environment change is provided by any one selected from the group consisting of solvent temperature, pressure and ion concentration.

10. A process for producing a phase-separated type polymer brush, said process comprising

fixing a brush initiator having a polymerization initiating group on a surface of a substrate,

polymerizing a monomer to form a polymer brush (A) comprising the substrate and a polymer layer comprising at least one polymer chain each of which has one end fixed on the substrate with fixed-end density of σ (nm −2 ) and other end free-ended,

contacting the polymer brush (A) with a solvent (B) at a temperature where a solution of the polymer chain with the polymer concentration determined by the following formula (1) is in a phase-separated state in a phase diagram provided that the polymer chain is in a free state without bonding to the substrate,

Polymer concentration (volume %)=σ (nm −2 )×number average molecular weight of the polymer chain (g/mol)×10 21 ) ÷(root mean square end-to-end distance of free polymer chain (nm)×Avogadro number, Na (mol −1 )×polymer chain density (g/cm 3 )  (1),

and

replacing the solvent to a gas below the glass transition temperature of the polymer while keeping the temperature within the phase-separated state in the phase diagram.

11. The process for producing a polymer brush according to claim 10 , wherein the fixed end density of the polymer chain, σ is not less than 0.001 (nm −2 ) and not more than 0.1 (nm −2 ).

12. The process for producing a polymer brush according to claim 10 , wherein the solvent (B) is a solvent with a binodal start temperature of not less than 1° C. and not more than 100° C. provided that the solvent (B) is mixed with the polymer chain in a free state without bonding to the substrate at a polymer concentration determined by the previous formula (1).

13. The process for producing a polymer brush according to claim 10 , wherein the combination of the polymer and the solvent (B) is

(1) polymer: aromatic hydrocarbon polymer, and solvent (B): hydrocarbon solvent, or

(2) polymer: (meth)acryl polymer, and solvent (B): water-based solvent.

14. A process for preparing phase-separated structure comprising

heating the polymer brush according to claim 1 over glass transition temperature of the polymer.

15. A surface modified member having the polymer brush according to claim 1 formed on the surface of the member.