  In the present work, crystallization in melts and poor-solvent solutions of
semiflexible polymers with different concentration was studied by means of
dissipative particle dynamics simulation technique. We use a coarse-grained
polymer model trying to catch general principles of crystallization in such
systems on large time and length scales. We observe the crystallization process
starting from an initial randomly prepared system with different polymer volume
fractions in a poor solvent. Because the solvent is very poor, the macrophase
polymer-solvent separation takes place very fast and is accompanied by partial
polymer crystallization. We have found that the overall crystalline fraction at
the end of crystallization process decreases upon increasing the polymer volume
fraction in the initial randomly prepared system, while the steady-state
crystallization speed is almost the same at polymer volume fractions larger
than 50\%. At the same time, the average crystallite size differs considerably
and has a maximum value in the systems with 90\% polymer volume fraction. We
assume that this polymer concentration is an optimal value in a sense of a
balance between the amount of polymer material available for increasing
crystallite size and chain entanglements preventing crystallites growth and
merging.
