FTIR spectroscopy for analysis of crystallinity of poly(3-hydroxybutyrate-co-4 -hydroxybutyrate) polymers and its utilization in evaluation of aging, orientation and composition

Abstract

ATR FTIR (attenuated total reflectance Fourier transform infrared) spectroscopy was applied for direct evaluation of the crystallinity of poly(3- hydroxybutyrate -co- 4- hydroxybutyrate) (P(3HB-co-4HB) polymers. At low 4HB content, this is believed to be a much more accurate method compared to the crystallinity calculated by differential scanning calorimetry (DSC). The FTIR method is also simpler and easier to interpret compared to x-ray diffraction or scattering. As the crystallinity of semi-crystalline P(3HB-co-4HB) polymers reduces with increased 4HB content, it could be used in composition analysis of copolymers. ATR FTIR spectroscopy also provides good insight into the morphological changes of the P(3HB-co-4HB) molecules due to orientation (e.g. fiber drawing, filaments or film orientation, post draw, etc). As the percent fraction of the mobile amorphous phase is shown to reduce with aging in P(3HB) homopolymer and in its semi-crystalline copolymers with 4HB, easy monitoring of aging could also be offered based on this technique. The biodegradation specifics, e.g. morphological changes caused by the biodegradation in soil and compost, would be difficult to measure by transmission FTIR due to high absorbance of films, but quite possible with surface ATR FTIR. Among other factors, e.g. glass transition temperature and crystal size, the biodegradation rates in P(3HB) copolymers have been reported to decrease with an increase in crystallinity, e.g. in [1,2]. For the purposes of providing materials with controlled biodegradation rates, multi layer coextruded films are industrially offered and ATR FTIR could be very useful in monitoring the morphological changes of these films over time. We present the calculation of the crystallinity indicator of P(3HB-co-4HB) polymers as a ratio of crystalline phase specific absorption at 1227 cm−1 to amorphous phase specific absorption at 1184 cm−1, which is shown to correlate well with crystallinity calculated from DSC measurements. The developed method could be used in water based PHA latexes as water does not absorb at the selected wavelengths.