Segmented poly(urethane-urea)s have been synthesized with mixed soft segments of ultra-low monol content poly(propylene glycol) (PPG) and tri(propylene glycol) (TPG) which allows the fabrication of quality elastomers without crosslinking. The narrow molecular weight distribution of the ultra-low monol content PPG polyols allows for the probing of the influence of the low molecular components of the molecular weight distribution through the inclusion of low molecular homologs of PPG such as TPG. Structure-property relationships for these materials were investigated as average soft segment molecular weight was varied by blending 8000 g/mol PPG with TPG to achieve molecular weights of 2500, 2000, and 1500 g/mol. Morphological features such as microphase separation, interdomain spacing and interphase thickness were quantified and revealed with SAXS. AFM was utilized to verify the microphase separation characteristics inferred by SAXS. The thermal and mechanical behavior was assessed through applications of DMA, DSC, and conventional mechanical tests. It was found that as the average soft segment molecular weight was decreased through the addition of TPG, the interdomain spacing distinctly increased contrary to the trend seen for decreasing soft segment molecular weight in PPG based systems without TPG. Additionally, the inclusion of TPG in the poly(urethane-urea) formulations resulted in the formation of larger hard domains as evidenced by AFM. These results and supporting evidence from DMA, DSC, birefringence, and mechanical testing led to the conclusion that TPG apparently acts more as a chain extender as well as, or in contrast to, a soft segment.