Abstract
Variable-temperature Fourier transform infrared spectroscopy (FT-IR) and Raman
spectroscopic mapping measurements were applied to study the phase separation of a
poly(3-hydroxybutyrate) (PHB)-poly(l-lactic acid) (PLA) (50 : 50 wt.%) polymer-blend
film as a function of temperature between 25 and 175 °C. Because of the better band
separation compared with the fundamental absorptions, the first overtones of the
ν(C=O) bands of PHB and PLA were used to evaluate the temperature-dependent FT-IR
images as PLA-PHB and PHB-PLA band-ratio contour plots, respectively. From the
visualization of the band-ratio FT-IR images, it could be derived that even beyond
the melting point of PLA (145 °C), the lateral position and the geometry of the
PHB-rich and PLA-rich phases were retained up to 165 °C. Furthermore, the FT-IR
images derived during and after the melting of PHB (174 °C) provided an interesting
insight into the homogenization process of the polymer melt. By exploiting its
higher lateral resolution, valuable additional information became available from the
Raman mapping measurements. Based on the Raman data, the scenario of phase-separated
PHB-rich and PLA-rich domains of about 50 μm size, based on the FT-IR imaging
measurements, had to be revised. Instead, the originally interpreted PHB-rich and
PLA-rich domains are actually clusters of much smaller grains. Additionally, the
Raman images measured in the same temperature interval revealed that the clusters of
small PHB-rich grain structures aggregated as a function of temperature increase.
These investigations prove that FT-IR and Raman imaging in combination with
variable-temperature measurements can provide new (and so far unavailable) insights
into structural phenomena of phase-separated polymer blends.
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