The disclosure provides a method for producing low molecular weight polytetrafluoroethylene containing less C6-C14 perfluorocarboxylic acids or salts thereof. The method for producing low molecular weight polytetrafluoroethylene includes: (1) irradiating high molecular weight polytetrafluoroethylene to provide low molecular weight polytetrafluoroethylene having a melt viscosity of 1.0×10.sup.2 to 7.0×10.sup.5 Pa.Math.s at 380° C.; and (2) irradiating the low molecular weight polytetrafluoroethylene to a dose that does not decompose the low molecular weight polytetrafluoroethylene.

The disclosure provides a method for producing low molecular weight polytetrafluoroethylene containing less C6-C14 perfluorocarboxylic acids or salts thereof. The method for producing low molecular weight polytetrafluoroethylene includes: (1) irradiating high molecular weight polytetrafluoroethylene to provide low molecular weight polytetrafluoroethylene having a melt viscosity of 1.0×10.sup.2 to 7.0×10.sup.5 Pa.Math.s at 380° C.; and (2) irradiating the low molecular weight polytetrafluoroethylene to a dose that does not decompose the low molecular weight polytetrafluoroethylene.

Provided is a composition including a polyvinylidene fluoride (A); and a vinylidene fluoride polymer (B) excluding the polyvinylidene fluoride (A), wherein the polyvinylidene fluoride (A) comprises vinylidene fluoride unit and a pentenoic acid unit represented by formula (1): CH.sub.2═CH—(CH).sub.2—COOY wherein Y represents at least one selected from the group consisting of an inorganic cation and an organic cation, a content of vinylidene fluoride unit of the polyvinylidene fluoride (A) is 95.0 to 99.99 mol % based on all monomer units of the polyvinylidene fluoride (A), and a content of the pentenoic acid unit of the polyvinylidene fluoride (A) is 0.01 to 5.0 mol % based on all monomer units of the polyvinylidene fluoride (A).

Provided is a composition including a polyvinylidene fluoride (A); and a vinylidene fluoride polymer (B) excluding the polyvinylidene fluoride (A), wherein the polyvinylidene fluoride (A) comprises vinylidene fluoride unit and a pentenoic acid unit represented by formula (1): CH.sub.2═CH—(CH).sub.2—COOY wherein Y represents at least one selected from the group consisting of an inorganic cation and an organic cation, a content of vinylidene fluoride unit of the polyvinylidene fluoride (A) is 95.0 to 99.99 mol % based on all monomer units of the polyvinylidene fluoride (A), and a content of the pentenoic acid unit of the polyvinylidene fluoride (A) is 0.01 to 5.0 mol % based on all monomer units of the polyvinylidene fluoride (A).

A fluorine-containing polymer has a fluorine content of 50% by mass or more, a degree of unsaturation of 0.1 mEq/g or more, and a glass transition temperature of −20° C. or higher, and the relationship between the maximum value E*1 of the complex elastic modulus in the range of from −50° C. to (glass transition temperature−10° C.) and the minimum value E*2 of the complex elastic modulus in the range of from (glass transition temperature+10° C.) to 250° C. satisfies the following inequality: E*2/E*1≤0.01.

A fluorine-containing polymer has a fluorine content of 50% by mass or more, a degree of unsaturation of 0.1 mEq/g or more, and a glass transition temperature of −20° C. or higher, and the relationship between the maximum value E*1 of the complex elastic modulus in the range of from −50° C. to (glass transition temperature−10° C.) and the minimum value E*2 of the complex elastic modulus in the range of from (glass transition temperature+10° C.) to 250° C. satisfies the following inequality: E*2/E*1≤0.01.

Provided is a fuel filler pipe including: an inner layer that contains a fluorine-containing copolymer having a carbonyl group-containing group and having a melting point of 250° C. or lower; an intermediate layer that contains a non-fluorine copolymer having a unit based on ethylene and a unit based on vinyl alcohol; and an outer layer that contains a polyolefin, wherein the inner layer, the intermediate layer, and the outer layer are directly laminated in that order, and the outer layer is a layer that contains a polyolefin having a unit based on an acid anhydride or is a layer in which a layer that contains a polyolefin having a unit based on an acid anhydride and a layer that contains a polyolefin having no unit based on an acid anhydride are directly laminated.

A method for producing a modified polytetrafluoroethylene, the method including: polymerizing tetrafluoroethylene in an aqueous medium in which a polymer having units based on a fluorine-free monomer is present. The fluorine-free monomer is a monomer represented by formula (1): CH.sub.2═CR.sup.1-L-R.sup.2, where R.sup.1 is a hydrogen atom or an alkyl group, L is a single bond, —CO—O—*, —O—CO—* or —O—, * is a bonding position to R.sup.2, and R.sup.2 is an alkyl group. To the total amount of tetrafluoroethylene supplied to the polymerization, a proportion of the polymer is from 0.001 to 0.050 mass %.

A method for producing a modified polytetrafluoroethylene, the method including: polymerizing tetrafluoroethylene in an aqueous medium in which a polymer having units based on a fluorine-free monomer is present. The fluorine-free monomer is a monomer represented by formula (1): CH.sub.2═CR.sup.1-L-R.sup.2, where R.sup.1 is a hydrogen atom or an alkyl group, L is a single bond, —CO—O—*, —O—CO—* or —O—, * is a bonding position to R.sup.2, and R.sup.2 is an alkyl group. To the total amount of tetrafluoroethylene supplied to the polymerization, a proportion of the polymer is from 0.001 to 0.050 mass %.

A method for producing a modified polytetrafluoroethylene, the method including: polymerizing tetrafluoroethylene in an aqueous medium in which a polymer having units based on a fluorine-free monomer is present. The fluorine-free monomer is a monomer represented by formula (1): CH.sub.2═CR.sup.1-L-R.sup.2, where R.sup.1 is a hydrogen atom or an alkyl group, L is a single bond, —CO—O—*, —O—CO—* or —O—, * is a bonding position to R.sup.2, and R.sup.2 is an alkyl group. To the total amount of tetrafluoroethylene supplied to the polymerization, a proportion of the polymer is from 0.001 to 0.050 mass %.