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Melt Strength and Rheological Properties of Biodegradable Poly(Lactic Aacid) Modified via Alkyl Radical-Based Reactive Extrusion Processes

Dean, Katherine M., Petinakis, Eustathios, Meure, Sam, Yu, Long, Chryss, Andrew
Journal of polymers and the environment 2012 v.20 no.3 pp. 741-747
biodegradability, branching, crystal structure, crystallization, extrusion, foams, gel chromatography, hydrogen, loss modulus, melting, molecular weight, polylactic acid, rheology, temperature, thermal analysis, viscoelasticity
Poly(lactic acid) (PLA) has been modified using twin-screw reactive extrusion to improve its melt properties and crystallinity. In this work lauroyl peroxide was used as an alkyl free radical source, abstracting hydrogen atoms from the PLA backbone leading to branching and chain extension reactions. Once the linear viscoelastic region was determined for these polymers, changes in dynamic rheology (dynamic viscosity real and loss modulus) were measured. Gel permeation chromatography showed that the molecular weight and polydispersity increased to a maximum with the addition of 1.00 and 0.50 wt% peroxide, respectively. Low temperature β transitions in dynamical mechanical thermal traces gave further evidence that branching had also occurred. Göttfert Rheotens measurements showed a three fold increase in melt strength due to both increased chain length and branching. Thermal analysis showed the level of crystallisation had decreased also possibly due to branching. Reductions in crystallinity and improved melt strength are known to be critical for film and foam formation.