Phase transition behavior of PLLA ultrathin film studied by grazing angle reflection absorption infrared spectroscopy

With a home-made heating cell, the structural evolutions of PLLA ultrathin film during the heating and cooling process were investigated in real time by grazing angle IR reflection absorption spectroscopy (IRRAS). It is found that the phase transition behaviors, including the crystallization, melting and even the glass transition of PLLA ultrathin film, can be clearly revealed by detailed analysis on the temperature-dependent IR spectra collected with IRRAS technique. The quantitative analysis shows that temperature-dependent intensity changes calculated by vibrational modes associated with the CO and COC groups are suitable to reflect the subtle glass transition of PLLA ultrathin film. The qualitative spectral analysis on the CC backbone vibrational modes suggest that the 10/7 helical chain of PLLA formed during the heating process (i.e. cold crystallization) is preferably flat on the Au substrate, whereas it is perpendicular to substrate during the cooling process (i.e. melt crystallization). Our studies show that temperature dependent IRRAS technique is not only valid to study the glass transition but also the confined crystallization behavior of PLLA ultrathin film deposited on a substrate, which is hard to test by the conventional DSC technique. Besides, it can provide more structural information to understand the various phase transition behaviors of polymer ultrathin films on a molecular level.