  We present polarization profiles for 24 millisecond pulsars observed at 820
and 1500 MHz with the Green Bank Telescope by the NANOGrav pulsar timing array.
We use Mueller matrix solutions calculated from observations of PSRs B1929+10
and J1022+1001 to calibrate the data. We discuss the polarization profiles,
which can be used to constrain pulsar emission geometry, and also present the
discovery of very low intensity average profile components ("microcomponents")
in four pulsars. Using the rotation measures we measured for each pulsar, we
calculate the Galactic magnetic field parallel to the line of sight for
different lines of sight through the interstellar medium. We fit for linear and
sinusoidal trends in time in the rotation measure, dispersion measure, and
Galactic magnetic field. We detect rotation measure variations with a period of
one year in some pulsars but overall find that the variations in these
parameters are more consistent with a stochastic origin.
