Poly(ethylene glycol)(45)-b-poly(trimethylene carbonate)(n) PEG(45)-b-PTMCn diblock copolymers were synthesized with five different PTMC degrees of polymerization (n = 38, 96, 144, 170, 332) and their self-assembly properties in water were studied using a manual nanoprecipitation procedure. We confirmed that the copolymer's hydrophilic weight fraction (f(PEG)) is controlling nanoparticles morphology. We determined that the PEG(45)-b-PTMC96 with f(PEG) approximate to 17% is the optimal hydrophilic fraction for the stabilization of well-defined unilamellar vesicles with a membrane thickness of delta approximate to 14.6 nm. Maintaining this fraction constant and modulating the overall molar mass of the block copolymers allowed the establishment of a power law of delta proportional to (M-n) over bar (0.56) which provides a robust correlation between the molar mass of PTMC and vesicles' membrane thickness. Finally, we proved that controlling nanoprecipitation's conditions by microfluidics allowed fine-tuning and control of the nanoparticles size and polydispersity index while maintaining their shape with a perfect batch-to-batch reproducibility.