  According to the recycling model, neutron stars in low-mass X-ray binaries
were spun up to millisecond pulsars (MSPs), which indicates that all MSPs in
the Galactic plane ought to be harbored in binaries. However, about $20\%$
Galactic field MSPs are found to be solitary. To interpret this problem, we
assume that the accreting neutron star in binaries may collapse and become a
strange star when it reaches some critical mass limit. Mass loss and a weak
kick induced by asymmetric collapse during the phase transition (PT) from
neutron star to strange star can result in isolated MSPs. In this work, we use
a population-synthesis code to examine the PT model. The simulated results show
that a kick velocity of $\sim60~{\rm km~s}^{-1}$ can produce $\sim6\times10^3$
isolated MSPs and birth rate of $\sim6.6\times10^{-7} {\rm ~yr}^{-1}$ in the
Galaxy, which is approximately in agreement with predictions from observations.
For the purpose of comparisons with future observation, we also give the mass
distributions of radio and X-ray binary MSPs, along with the delay time
distribution.
