I am writing a PhD dissertation that contains two main themes/topics. These topics are fairly different from each other, but they do have (some!) common threads.
Within each theme, I have two rather related sub-themes. Each sub-theme is substantial enough to form a chapter.
How should I structure the dissertation. I was thinking about having two parts, as follows:
Ch.1 - Introduction (discussing general structure & motivation) Part I - Theme 1 - Ch.2 - theme 1 - topic 1 - Ch.3 - theme 1 - topic 2 Part II - Theme 2 - Ch.4 - theme 2 - topic 1 - Ch.5 - theme 2 - topic 2 Ch.6 - Conclusion Appendices and References
The above structure seems a little odd to me, though. Are the purpose of parts to partition the paper completely? That is, should each chapter belong to a part? In the above structure, Ch.1 and Ch. 6 are not in any part.
In addition, would including an introduction and conclusion for each part be appropriate?
My recommendation is to consider "parts" as an auxiliary notion. The thesis should make sense to the reader even if he ignores the division into parts.
That's fairly doable with your proposed solution, it just needs clever organization of introductory and preliminary texts.
Actually, I don't think you can do much better if your 4 chapters are too long to become 2 chapters.
The only alternative I can think of is the following:
Ch.1 - Introduction (discussing general structure & motivation) Part I - Theme 1 Ch.2 - theme 1 - deeper introduction & preliminaries Ch.3 - theme 1 - topic 1 Ch.4 - theme 1 - topic 2 Part II - Theme 2 Ch.5 - theme 2 - deeper introduction & preliminaries Ch.6 - theme 2 - topic 1 Ch.7 - theme 2 - topic 2 Ch.8 - Conclusion Appendices and References
Also, as a last remark, if you realize there's nothing to put into the conclusions, I'm quite sure you can create the thesis without them, but better ask your supervisor.
Actually, have you discussed this with your supervisor?