Influence of Chain Topology (Cyclic versus Linear)
on the Nucleation and Isothermal Crystallization of Poly(l‑lactide) and Poly(d‑lactide)
Posted on 2018-02-20 - 19:52
Ring
closure click chemistry methods have been used to produce
cyclic c-PLLA and c-PDLA of a number-average
molecular weight close to 10 kg/mol. The effects of stereochemistry
of the polymer chains and their topology on their structure, nucleation,
and crystallization were studied in detail employing wide-angle X-ray
scattering (WAXS), small-angle X-ray scattering (SAXS), polarized
light optical microscopy (PLOM), and standard and advanced differential
scanning calorimetry (DSC). The crystal structures of linear and cyclic
PLAs are identical to each other, and no differences in superstructural
morphology could be detected. Cyclic PLA chains are able to nucleate
much faster and to produce a higher number of nuclei in comparison
to linear analogues, either upon cooling from the melt or upon heating
from the glassy state. In the samples prepared in this work, a small
fraction of linear or higher molecular weight cycles were detected
(according to SEC analyses). The presence of such “impurities”
retards spherulitic growth rates of c-PLAs, making
them nearly the same as those of l-PLAs. On the other
hand, the overall crystallization rate determined by DSC was much
larger for c-PLAs, as a consequence of the enhanced
nucleation that occurs in cyclic chains. The equilibrium melting temperatures
of cyclic chains were determined and found to be 5 °C higher
in comparison with values for l-PLAs. This result
is a consequence of the lower entropy of cyclic chains in the melt.
Self-nucleation studies demonstrated that c-PLAs
have a shorter crystalline memory than linear analogues, as a result
of their lower entanglement density. Successive self-nucleation and
annealing (SSA) experiments reveal the remarkable ability of cyclic
molecules to thicken, even to the point of crystallization with extended
collapsed ring conformations. In general terms, stereochemistry had
less influence on the results obtained in comparison with the dominating
effect of chain topology.
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Zaldua, Nerea; Liénard, Romain; Josse, Thomas; Zubitur, Manuela; Mugica, Agurtzane; Iturrospe, Amaia; et al. (2018). Influence of Chain Topology (Cyclic versus Linear)
on the Nucleation and Isothermal Crystallization of Poly(l‑lactide) and Poly(d‑lactide). ACS Publications. Collection. https://doi.org/10.1021/acs.macromol.7b02638
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AUTHORS (10)
NZ
Nerea Zaldua
RL
Romain Liénard
TJ
Thomas Josse
MZ
Manuela Zubitur
AM
Agurtzane Mugica
AI
Amaia Iturrospe
AA
Arantxa Arbe
JW
Julien De Winter
OC
Olivier Coulembier
AM
Alejandro J. Müller