Abstract
Due to the brittle nature of poly(lactic acid) many attempts have been made to flexibilize this polyester for applications such as thin films and foils. However, due to complex phase behavior, many drawbacks for plasticizer and blend components are described. To overcome miscibility, post crystallization and migration issues a principle of click-chemistry was employed to change the molecular characteristics from external to internal plasticization by fixation of a plastisizing unit with help of a stereocomplex crystallization. Hydroxyl terminated polycaprolactone oligomers were used as a macroinitiator for the ring opening polymerization of d-lactide, resulting in blockcopolymers with plasticizing unit polycaprolactone and compatibilizing poly(d-lactic acid)-blocks. The generated block copolymers were blended with a poly(l-lactic acid)-matrix and formed so called stereocomplex crystals. In comparison to unbound polycaprolactone the polycaprolactone blocks show a lower migration tendency regarding a solution test in toluene. Besides that, trapping the plasticizing units via stereocomplex also improves the efficiency of the plasticizer. In comparison to polymer blends with the same amount of non-bonded polycaprolactone oligomers of the same molecular weight, block copolymers with poly(d-lactic acid) and polycaprolactone can shift the glass transition temperature to lower values. This effect can be explained by the modulated crystallization of the polycaprolactone-blocks trapped into the matrix, so that a higher effective amount can interact with the poly(l-lactic acid)-matrix.
Highlights
The production of bio-based and biodegradable plastics is one important part of the bio-economy.The use of bio-based plastics saves fossil resources and, in most cases, helps reducing greenhouse gas emissions
By measuring the mechanical properties of PLLA/poly(d-lactic acid) (PDLA) cast films and polarization microscopy images, Tsuji and Ikada have developed a model that explains the changed mechanical properties of the stereocomplex poly(lactic acid) (PLA) compared to pure PLLA by the morphology of the stereocomplexes [7]
The researchers describe an increased crystallization rate of the blends with PLLA and the triblock copolymers compared to blends of PLLA and PDLA measured in isothermal differential scanning calorimetry (DSC)
Summary
The production of bio-based and biodegradable plastics is one important part of the bio-economy. By measuring the mechanical properties of PLLA/PDLA cast films and polarization microscopy images, Tsuji and Ikada have developed a model that explains the changed mechanical properties of the stereocomplex PLA compared to pure PLLA by the morphology of the stereocomplexes [7] Both spherulites of homo crystals and crystallites of stereocomplexes are connected by tie chains. The researchers describe an increased crystallization rate of the blends with PLLA and the triblock copolymers compared to blends of PLLA and PDLA measured in isothermal differential scanning calorimetry (DSC) The reason for this effect is the diluting effect of the miscible PEG midblock and increasing the chain mobility and rearrangements [12]. The improved elongation at break of the blend compared to the PLLA homopolymer can be explained by the compatible content of soft PEG and by a difference in morphology between stereocomplexes and homocrystals. As the miscibility of PCL and PLLA depends on the molecular weight of the components, defined block lengths of both, the PCL oligomers and the PDLA-blocks were synthesized
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