Firstly, the spiral arms are not material, they are not groups of stars that stay together and rotate around. If that were the case they would wind up tighly in the center and disappear. Spiral arms are like traffic slowdowns. Stars orbit the galaxy at a speed higher or lower than the arms do, depending on the distance from the center. At a certain radius, the "corrotation radius" they move at the same speed. Inside this radius the stars move faster, outside they move slower than the arms. Dust, gas and stars enter the arm, slow down, become compressed, make new stars, then exit and move on at a higher speed until they encounter the next arm. A star in one arm right now might be in a different arm 100 million years from now.
Secondly, the dynamics of this is highly a function of the density, which is much greater near the center of the galaxy. There is a field that studies this called "density wave theory". At a certain density, nearer the center, it is not possible to maintain the spiral shape and a bar results.
From reading up on it, that's about the best I can do.