A water-soluble thermally-crosslinkable polymeric material, comprising: (a) one or more first polymer chain of an azetidinium-containing polymer having azetidinium groups; (b) one or more second polymer chains of a poly(meth)acrylamide-based copolymer comprising: (i) first repeating units of at least one (meth)acrylamide monomer selected from the group consisting of (meth)acrylamide, N,N-dimethyl (meth)acrylamide, N-2-hydroxyethyl (meth)acrylamide, N-3-hydroxypropyl (meth)acrylamide, N-2-hydroxypropyl (meth)acrylamide, N-4-hydroxybutyl (meth)acrylamide, N-5-hydroxypentyl (meth)acrylamide, N-6-hydroxyhexyl (meth)acrylamide, N-(2,3-dihydroxypropyl) (meth)acrylamide, N,N -bis(2-hydroxyethyl) (meth)acrylamide, N-tris(hydroxymethyl)methyl (meth)acrylamide, N,N-dimethylaminopropyl (meth)acrylamide, and combinations thereof, and (ii) second repeating units each of which is an acrylic repeating unit of any one of formula to in which R o is hydrogen or methyl, X o and X 1 independent of each other are -O- or-NR n - in which R n is hydrogen or a C 1 -C 4 alkyl group, L 1 is a C 1 -C 4 alkylene divalent radical, L 2 and L 4 independent of each other are a C 2 -C 4 alkylene divalent radical having 0 or 1 hydroxyl group, L 3 is -CH 2 -CHOH-CH 2 -, t1 is an integer of 2 to 25, t2 is an integer of 2 to 4, and t3 is 0 or 1; and (c) azetidinium groups each of which is embedded in or covalently attached to one of said one or more first polymer chains; wherein each second polymer chain is covalently attached to one of said one or more first polymer chains via at least one linkage formed between one azetidinium group of the azetidinium-containing polymer and one reactive pendant chain of the poly(meth)acrylamide-based copolymer; wherein the poly(meth)acrylamide-based copolymer comprises at least 50% (preferably at least 55%, more preferably at least 60%, even more preferably at least 70%, most preferably at least 80%) by mole of all (meth)acrylamide repeating units relative to all repeating units of the poly(meth)acrylamide-based copolymer. (a) one or more first polymer chain of an azetidinium-containing polymer having azetidinium groups; (b) one or more second polymer chains of a poly(meth)acrylamide-based copolymer comprising: (i) first repeating units of at least one (meth)acrylamide monomer selected from the group consisting of (meth)acrylamide, N,N-dimethyl (meth)acrylamide, N-2-hydroxyethyl (meth)acrylamide, N-3-hydroxypropyl (meth)acrylamide, N-2-hydroxypropyl (meth)acrylamide, N-4-hydroxybutyl (meth)acrylamide, N-5-hydroxypentyl (meth)acrylamide, N-6-hydroxyhexyl (meth)acrylamide, N-(2,3-dihydroxypropyl) (meth)acrylamide, N,N -bis(2-hydroxyethyl) (meth)acrylamide, N-tris(hydroxymethyl)methyl (meth)acrylamide, N,N-dimethylaminopropyl (meth)acrylamide, and combinations thereof, and (ii) second repeating units each of which is an acrylic repeating unit of any one of formula to in which R o is hydrogen or methyl, X o and X 1 independent of each other are -O- or-NR n - in which R n is hydrogen or a C 1 -C 4 alkyl group, L 1 is a C 1 -C 4 alkylene divalent radical, L 2 and L 4 independent of each other are a C 2 -C 4 alkylene divalent radical having 0 or 1 hydroxyl group, L 3 is -CH 2 -CHOH-CH 2 -, t1 is an integer of 2 to 25, t2 is an integer of 2 to 4, and t3 is 0 or 1; and (i) first repeating units of at least one (meth)acrylamide monomer selected from the group consisting of (meth)acrylamide, N,N-dimethyl (meth)acrylamide, N-2-hydroxyethyl (meth)acrylamide, N-3-hydroxypropyl (meth)acrylamide, N-2-hydroxypropyl (meth)acrylamide, N-4-hydroxybutyl (meth)acrylamide, N-5-hydroxypentyl (meth)acrylamide, N-6-hydroxyhexyl (meth)acrylamide, N-(2,3-dihydroxypropyl) (meth)acrylamide, N,N -bis(2-hydroxyethyl) (meth)acrylamide, N-tris(hydroxymethyl)methyl (meth)acrylamide, N,N-dimethylaminopropyl (meth)acrylamide, and combinations thereof, and (ii) second repeating units each of which is an acrylic repeating unit of any one of formula to in which R o is hydrogen or methyl, X o and X 1 independent of each other are -O- or-NR n - in which R n is hydrogen or a C 1 -C 4 alkyl group, L 1 is a C 1 -C 4 alkylene divalent radical, L 2 and L 4 independent of each other are a C 2 -C 4 alkylene divalent radical having 0 or 1 hydroxyl group, L 3 is -CH 2 -CHOH-CH 2 -, t1 is an integer of 2 to 25, t2 is an integer of 2 to 4, and t3 is 0 or 1; and (c) azetidinium groups each of which is embedded in or covalently attached to one of said one or more first polymer chains; wherein each second polymer chain is covalently attached to one of said one or more first polymer chains via at least one linkage formed between one azetidinium group of the azetidinium-containing polymer and one reactive pendant chain of the poly(meth)acrylamide-based copolymer; wherein the poly(meth)acrylamide-based copolymer comprises at least 50% (preferably at least 55%, more preferably at least 60%, even more preferably at least 70%, most preferably at least 80%) by mole of all (meth)acrylamide repeating units relative to all repeating units of the poly(meth)acrylamide-based copolymer.