Pressure-induced polymerization of acrylic acid: a Raman spectroscopic study.

We report here the first in-situ Raman spectroscopic study of pressure-induced structural and polymeric transformations of acrylic acid. Two crystalline phases (I and II) were observed upon compression to approximately 0.3 and approximately 2.7 GPa. Phase I can be characterized with a single s-cis molecular conformation with possibly a similar structure to the low-temperature phase, while phase II suggests significantly enhanced molecular interactions toward polymerization and structural disordering. Beyond approximately 8 GPa, spectroscopic features indicate the onset of polymerization. The high-pressure polymeric phase together with the pressure-quenched materials was examined and compared with two commercial acrylic acid polymers using Raman spectroscopy. The characteristics of polymeric acrylic acid and their transformation mechanisms as well as the implications of hydrogen-bonding interactions are discussed.