the work function will be change, but I dont know in which way. The work fuction its the energy to take an electron from the Fermi level to the vacuum level in the surface. It will be depend with change on surface. Maybe the difference it will be for the MWNT without Polyaniline and with Polyaniline.. If the work function diminish its possibly related with better electron transfer.
I would assume the work function will already change a lot depending on a) how the PAni was produced (maybe even not reproducible from PAni batch to batch) b) same for MWNT c) how the nanocomposite would be made. This is because any surface modification will change the work function.
Therefore, I suggest to first test the reproducibility of work function for the pure components by measuring several batches.
Work function will depend how you design your nanocomposite with PANI. Do MWCNT have some sort of van der waal's interaction or how both are attached with each other will affect the work function. Do i-v measurements and see how experimentally the change is taking place. Wessling is right that you have to carry number of experiments to validate your data.