The self-assembly mechanism has been studied in poly(γ-benzyl-L-glutamate)-poly(ethylene glycol)-poly(γ-benzyl-L-glutamate) (PBLG-PEG-PBLG) triblock copolymer melts using X-ray scattering, polarizing optical microscopy, differential scanning calorimetry, and FTIR spectroscopy. Intrinsic competing interactions (crystallization, hydrogen bonding, liquid crystallinity, microphase separation) give rise to different levels of organization. Depending on the peptide volume fraction f, two cases can be discussed: for low peptide volume fractions, microphase separation results in PBLG and PEG phases rich in all secondary structures (α-helices, β-sheets, and chain-folded PEG) notwithstanding the large undercooling necessary to induce PEG crystallization. For f > 0.4, interfacial mixing results in the destruction of the less coherent peptide secondary structures (β-sheet). Interfacial mixing may prove to be a key factor in controlling the appearance of β-sheets in low molecular weight peptides.