The detection of phase separation and identification of miscibility in biopolymer blends is an important aspect for the improvement of their physical properties. In this article, the phase separation in blends of poly(3-hydroxybutyrate) (PHB) with poly(L-lactic acid) (PLA) and poly(epsilon-caprolactone) (PCL), respectively, has been studied as a function of the blend composition by FT-IR imaging spectroscopy. For both polymer blend systems, a miscibility gap has been found around the 50:50% (w/w) composition of the two components. Furthermore, the separating phases have been identified as blends of the two polymer components and their compositions could be determined from calibrations based on the spectra of the blends in the compositional range of miscibility. The data derived from FT-IR spectroscopic imaging were corroborated by additional DSC analyses and mechanical stress-strain measurements of polymer blend films, which exhibited a characteristic fracture behavior as a function of PHB composition.

Introduction.Biodegradable polymers 1 are a group of polymers which can be produced from renewable resources by bacteria and have the fundamental advantage of being biodegradable. In this communication uniaxially elongated films of different blends of poly(3-hydroxybutyrate) and poly(lactic acid) were analyzed by FT-IR spectroscopic imaging. Poly(3-hydroxybutyrate) (PHB) 2 is the most common member of the poly(hydroxyalkonates) (PHAs), which has the disadvantage of being stiff and brittle because of its high crystallinity. In contrast, the poly(lactic acid) (PLA) 3 used here contains 10% of mesolactic acid, and as a consequence it has only a crystallinity of ∼32% (see below) and is flexible.Blends of PHB and PLA have been analyzed with different techniques by several research groups. [4][5][6][7] Recently, FT-IR imaging results on the phase behavior of PHB/PLA blends with different compositions have been published. 8 The present communication reports FT-IR imaging data obtained with polarized radiation on different PHB/PLA blends which had been previously investigated by rheo-optical FT-IR spectroscopy and subjected to uniaxial mechanical elongation.FT-IR spectroscopic imaging 9,10 offers the possibility to combine spectral and spatial information, thereby enabling a chemical visualization of samples. Thus, sample areas of 3.9 × 3.9 mm 2 or 260 × 260 µm 2 can be analyzed by FT-IR transmission spectroscopy without or in combination with a microscope, respectively, with a lateral resolution up to 10-15 µm. First reports on the use of polarized radiation for the production of FT-IR images have been reported by Wilhelm et al. 11 and Koenig et al. 12 Rheo-optical FT-IR measurements combine a stress-strain test with in-situ polarization measurements to detect the structural information on a molecular level simultaneously to the mechanical treatment. For the detailed experimental and theoretical principles of rheo-optical FT-IR/FT-NIR spectroscopy, the reader is referred to the relevant literature. [13][14][15] In previous investigations of the mechanical elongation of PLA-rich (g60 wt % PLA) PHB/PLA blend films (35 °C, 10% strain per minute) interesting orientation phenomena of the PHB and PLA chains have been detected. Thus, in these blends the PLA chains orient in the direction of elongation whereas the PHB chains orient perpendicular to the drawing direction. This has been attributed to a continuum mechanical alignment of the long axes of the lamellar PHB domains which are embedded in the PLA matrix (see below). 16 PHB/PLA blend films with PHB > PLA compositions, on the other hand, exhibit similar mechanical properties to PHB homopolymer and could only be oriented by cold drawing in ice-water after quenching from the melt. 17 In these films the polymer chains of both PHB and PLA exhibited a preferential chain alignment parallel to the drawing direction.

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