Comment Get a lawyer (Score 2) 848
Seriously. As soon as you try to negotiate or assert your position, the overwhelming chances are they will balk or claim your employment contract grants them rights to your work. It's much easier to talk to a lawyer now who knows the circumstances unique to your location and can advise you hat your chances are.
Don't forget, you can also 'donate' it to your job and position yourself as the good guy, which might help you out down the line, depending on your bosses and the company. Not to mention, it's also a nice thing to put on your resume that will look great at the next interview you go to.