  Given a rank two trianguline family of $(\varphi,\Gamma)$-modules having a
noncrystalline semistable member, we compute the Fontaine--Mazur
$\mathcal{L}$-invariant of that member in terms of the logarithmic derivative,
with respect to the Sen weight, of the value at p of the trianguline parameter.
This generalizes prior work, in the case of Galois representations, due to
Greenberg--Stevens and Colmez.
