phiL[R_]:=25.68*Log[Sqrt[2/R+1]]

X[R_,phi_]:=R/2*(Exp[phi/25.68]-1)+3*(1+R/2)*(Exp[-phi/25.68]-1)

Manipulate[Plot[X[R,phi],{phi,0,phiL[R]},PlotRange->{-3,0}],{R,0.001,100}]