Hi ThePhDer,
It's slightly difficult to answer your question, as you don't say what stage you're at or field you're in. But every PhD is unique. Some people's work is sequential, so one chapter leads naturally into another. My chapters were pretty much defined methodologically (I'm a science PhD), and that worked. I seem to recall that at the end of my second year, I was expected to produce a thesis plan outlining the chapters and what would go in them. My supervisors seemed quite happy with it and my finished product pretty much followed that plan. I should add that there is no dictate that says your chapters have to be immediately connected, but it doesn't sound like that's your problem. On the contrary. There's also no dictate that states how many chapters you need. Theoretically, you could produce a single 150 page research chapter. But that would still have to be structured to avoid p*****g your examiners off. And if you can structure it, you can divide it into chapters.
If you're really stuck, you should speak to your supervisor and pick their brains. It is his/her job, after all!