Can my part-time employer restrict me from also working for another employer PRN because they may wa

I work part time for my employer. I would like to work more hours, but they are unable to offer me additional hours at this time. I am a professional and prefer the type of work that I am currently doing. I would like to pursue some PRN opportunities with other employers in my field to supplement my current employment. My employer tells me that I cannot work in the type of setting that I am interested in because they may want to pursue relationships with these providers in the future. Basically I am being prevented from doing any extra work in my professional area of interest in any of the surrounding counties in Pennsylvania because my employer might want to seek these contracts in the future. Full time opportunities for my type of work are few and far in between in my geographic are, so to get more hours I really need more than one job.