Using fluorinated ether composition and fluorinated ether composition

A composition including at least one first divalent unit represented by formula: Each Rf is independently selected from the group consisting of Rfa-(O)t-CHF-(CF2)n-; [Rfa-(O)t-C(L)H-CF2-O]m-W-; Rfb-O-(CF2)p-; F(CkF2k)-(O-CkF2k)P-O-CF2-; and CF3-O-(CF2)3-(OCF(CF3)-CF2)Z-O-L1-. Each Q is independently selected from the group consisting of a bond, -C(O)-N(R1)-, and -C(O)-O-. Each X is independently selected from the group consisting of alkylene and arylalkylene, wherein alkylene and arylalkylene are each optionally interrupted by at least one ether linkage and optionally terminated by -N(R1)-C(O)- or -O-C(O)-. R and R1 are each independently selected from the group consisting of hydrogen and alkyl having from 1 to 4 carbon atoms. Rfa represents a partially or fully fluorinated alkyl group having from 1 to 10 carbon atoms and optionally interrupted with at least one oxygen atom. Rfb is selected from the group consisting of CF3CFH- and F(CjF2j)-. Methods of reducing surface tension of a liquid, making foams, and treating a surface using the compositions are also disclosed.