Abstract
The isothermal crystallization kinetics of PLA stereocomplex (scPLA) and scPLA/graphene oxide nanocomposites was investigated. scPLA was prepared by symmetric blending the two isomers, PLLA and PDLA and the scPLA nanocomposite by polymerization of the respective lactides in presence of commercial graphene oxide nanoplatelets (GNPO+) followed of blending the two composites. The effect of the filler on the crystallization behavior and thermal properties of scPLA was investigated by using differential scanning calorimetry (DSC). Avrami equation was used to describe the isothermal crystallization kinetics and melting behavior. Addition of 0.5 wt% GNPO+ to PLA matrix increases the crystallization rate, lowering the half-time of crystallization t1/2 to around 5.5 min for scPLA nanocomposites, which is about 2.7 min below that found for scPLA when isothermally crystallized at 165oC. During the isothermal crystallization, homochiral and stereocomplex crystals are formed, depending on the crystallization temperature. As crystallization temperature reaches 160 and 165oC, the crystallization of homocrystals become prevented and only stereocomplex crystals are formed.
Highlights
Poly(lactic acid) (PLA) is a biopolymer widely used as an alternative to petroleum-derived polymers[1]
Molecular weight characterization of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) matrix of the PLA/graphene nanocomposite obtained by in situ polymerization of lactides in presence of graphene were done by Gel permeation chromatography (GPC)
It can be seen that both patterns show the same three peaks around 12, 21, and 24o which are related to the formation of PLA stereocomplexes crystallized in a triclinic unit cell.[21,23]
Summary
Poly(lactic acid) (PLA) is a biopolymer widely used as an alternative to petroleum-derived polymers[1] It has two isomeric forms, poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) that when blended either from solution or melt can result in stereocomplexation. PLA stereocomplex (scPLA) formed from these blends contains stereocomplex crystallites (sc-crystallites) which are quite different from the PLLA or PDLA homocrystallites[2,3,4,5]. The study aimed to see the nucleating effect on the formation of scPLA and enhancement of crystallization rate. The nucleating effect of carbon nanotubes (CNT) on PLLA/PDLA blends was recently reported[18,19]. The filler addition leads to a lower crystallization activation energy of scPLA, mainly due to the heterogeneous nucleating effect of the well-dispersed PDLA covalently bonded GO sheets. Graphene nanoplatelets HDPlas, GNPs grade 4, O2 functionalized (GNPO+) (lateral dimensions 1-2 μm, surface area above 750 m2/g) purchased from Cheap Tubes (Brattleboro, VT, USA) were used to prepare the nanocomposites
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