128. Shear-induced nucleation and morphological evolution for bimodal long chain branched polylactide
作者：Huagao Fang, Yaqiong Zhang, Jing Bai and Zhigang Wang*
關鍵字：聚乳酸，長鏈支化，剪切，結晶
論文來源：期刊
具體來源：Macromolecules, 2013, 46, 6555-6565
發表時間：2013年
ABSTRACT: The effects of long chain branching on the nucleation density
enhancements and morphological evolution for polylactide (PLA) materials during
shear-induced isothermal crystallization process were thoroughly investigated
by using rotational rheometer and polarized optical microscopy (POM). Shear-induced
nucleation density enhancements for the long chain branched PLA (LCB PLA) were
studied on the basis of the determination of the critical shear rate, for which
the stretch of the longest chains of the linear component is expected. The
results of shear-induced isothermal crystallization kinetics show that the
crystallization process under shear is greatly enhanced compared to the quiescent
conditions and the crystallization kinetics is accelerated with the increases
in shear rate and/or shear time. LCB PLA crystallizes much faster than linear
PLA under the same shear condition. A saturation effect of shear time on
crystallization kinetics is observed for both linear PLA and LCB PLA. In-situ POM observations demonstrate that LCB PLA not only possesses
higher nucleation density under the identical shear time and a constant lower
value of spherulitic growth rate compared with that of linear PLA but also
forms the shish-kebab structure after sheared for sufficient time. The
quantitative evaluation of the shear-induced nucleation densities from
rheological measurements is based on the space filling model by using the
Avrami equation and the obtained nucleation density values are well consistent
with that estimated from POM observations. A saturation of nucleation density
under shear can be reached for both linear PLA and LCB PLA. The saturated
nucleation density values are higher than that under the quiescent condition by
a factor of over 3 orders of magnitude and the saturated nucleation density
value for LCB PLA is more than that for linear PLA by a factor of one order of
magnitude under the same shear condition. The enhancement of nucleation ability
and the morphological evolution from the spherulitic to shish-kebab structures
induced by shear flow can be ascribed to the broadened and complex relaxation
behaviors of LCB PLA.