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Enthalpy relaxation of the glass of poly (l-lactic acid) of different d-isomer content and its effect on mechanical properties

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Abstract

The relaxation of the glass of poly (l-lactic acid) (PLLA) and its random copolymers with 2 and 4 mol% d-isomer co-units has been analyzed by conventional differential scanning calorimetry. It was found that the presence of up to 4% d-isomer co-units in the PLLA chain has no measurable effect on the glass relaxation kinetics in fully amorphous samples. At ambient temperature, after rapid cooling of the melt at a rate of 50 K/min, to avoid crystallization, the glass relaxation begins after about 10 min and is not finished within 5000 min. The relaxation of the glass is accompanied by a major change of mechanical properties as it was revealed by time-resolved measurements of the micro-indentation hardness and Young’s modulus. During annealing of amorphous PLLA at ambient temperature, the microhardness and Young’s modulus increase by more than 40 and 10%, respectively, within a time period of about one month, and with an equilibrium value not yet achieved even after such a long annealing time. As in case of the enthalpy-relaxation kinetics, both, absolute values and the time evolution of the analyzed mechanical properties are independent on the presence of d-isomer co-units in the PLLA chain within the investigated concentration range.

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Acknowledgements

Financial support by the Deutsche Forschungsgemeinschaft (DFG) (Grant AN 212/20) is greatly acknowledged.

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Authors and Affiliations

  1. Center of Engineering Sciences, Martin Luther University Halle-Wittenberg, 06099, Halle/Saale, Germany

    H. M. Naeem Iqbal, Chanita Sungkapreecha & René Androsch

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  1. H. M. Naeem Iqbal
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  2. Chanita Sungkapreecha
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  3. René Androsch
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Correspondence to René Androsch.

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Naeem Iqbal, H.M., Sungkapreecha, C. & Androsch, R. Enthalpy relaxation of the glass of poly (l-lactic acid) of different d-isomer content and its effect on mechanical properties. Polym. Bull. 74, 2565–2573 (2017). https://doi.org/10.1007/s00289-016-1854-5

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  • DOI: https://doi.org/10.1007/s00289-016-1854-5

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