Abstract
The effects of long chain branching (LCB) degree on the shear-induced isothermal crystallization kinetics of a series of LCB polylactides (LCB PLAs) have been investigated by using rotational rheometer, polarized optical microscopy (POM) and scanning electron microscopy (SEM). Dynamic viscoelastic properties obtained by small-amplitude oscillatory shear (SAOS) tests indicate that LCB PLAs show more broadened relaxation time spectra with increasing LCB degree. Upon a pre-shear at the shear rate of 1 s−1 LCB PLAs show much faster crystallization kinetics than linear PLA and the crystallization kinetics is enhanced with increasing LCB degree. By modeling the system as a suspension the quantitative evaluation of nucleation density can be derived from rheological experiments. The nucleation density is greatly enhanced with increasing LCB degree and a saturation in shear time is observed. Crystalline morphologies for LCB PLAs observed by POM and SEM demonstrate the enhancement of nucleation density with increasing LCB degree and a transformation from spherulitic to orientated crystalline morphologies. The observation can be ascribed to longer relaxation time of the longest macromolecular chains and broadened, complex relaxation behaviors due to the introduction of LCB into PLA, which is essential in stabilizing the orientated crystal nuclei after pre-shear.
Highlights
Studied the quiescent crystallization behavior of linear PLA and long chain branched PLA (LCB PLA) with different branching degrees and found that LCB PLAs crystallized much faster than linear PLA, for which the branched chains play as a role of nucleating sites[18]
The much more pronounced nonterminal behaviors for LCB PLAs with higher branching degrees suggest that the relaxation time can be greatly increased by improving the branching degree of LCB PLAs, which is consistent with the results for LCB PLAs prepared with other methods[16,18,30]
A series of long chain branched polylactides (LCB PLAs) with different long chain branching degrees was prepared by γ irradiation after melt blending of linear PLA with trimethylolpropane triacrylate (TMPTA) of different amounts
Summary
Rheological properties of linear PLA and LCB PLAs with different branching degrees. The melt rheological properties can be greatly influenced by the changes of topological molecular structures, such as the LCB structures[16,17,25,28,29]. This work concentrates on the shear-induced crystallization of linear PLA and LCB PLAs with different branching degrees with relatively weak shear intensity, the enhancement of crystallization kinetics and transformation of crystalline morphologies concluded mainly from rheological results and optical micrographs may provide potential guidance for industrial processing for PLA, especially LCB PLAs to achieve products with excellent performances
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