Physical aging enhanced mesomorphic structure in melt-quenched poly(L-lactic acid)

Area of Science:

• Polymer Science
• Materials Science
• Spectroscopy

Background:

• Poly(L-lactic acid) (PLLA) is a biodegradable polymer with applications in various fields.
• Understanding the structural evolution during physical aging is crucial for predicting material properties.
• Isothermal physical aging below the glass transition temperature (Tg) influences polymer structure and dynamics.

Purpose of the Study:

• To investigate the structural evolutions and kinetics of melt-quenched PLLA during isothermal physical aging.
• To identify the nature of the local ordered structure formed during physical aging.
• To elucidate the relationship between physical aging and cold crystallization in PLLA.

Main Methods:

• Time-resolved infrared (IR) spectroscopy was employed to monitor structural changes during aging.
• Temperature-dependent IR spectroscopy was used to study the thermal behavior of the aged structures.
• Melt-quenched PLLA and poly(D,L-lactide) (PDLLA) samples were utilized.

Main Results:

• A local ordered structure, characterized by a band at 918 cm(-1), developed with aging time in PLLA.
• This structure was identified as a mesophase, similar to that formed during uniaxially drawn PLLA.
• A small amount of mesophase was present in the initial melt-quenched PLLA but absent in PDLLA.
• The aged local ordered structure was partially molten, not totally molten, upon heating.

Conclusions:

• The local ordered structure formed during physical aging of PLLA is a mesophase.
• The presence of mesophase in initial PLLA and its partial melting behavior explain enhanced cold crystallization.
• Physical aging kinetics and structural evolution are key factors in PLLA's post-processing behavior.