STRUCTURE AND DYNAMICS OF STRUCTURE FORMATION IN MODEL TRIARM STAR BLOCK-COPOLYMERS OF POLYSTYRENE, POLY(ETHYLENE OXIDE), AND POLY(EPSILON-CAPROLACTONE)
G. Floudas et al., STRUCTURE AND DYNAMICS OF STRUCTURE FORMATION IN MODEL TRIARM STAR BLOCK-COPOLYMERS OF POLYSTYRENE, POLY(ETHYLENE OXIDE), AND POLY(EPSILON-CAPROLACTONE), Macromolecules, 31(21), 1998, pp. 7279-7290
The structure and dynamics of structure formation have been studied in
model triarm star block copolymers composed of two crystallizable blo
cks (poly(ethylene oxide) (PEO) and poly(epsilon-caprolactone) (PCL))
and one amorphous block (polystyrene(PS)). Crystallization starts from
the homogeneous phase. For the structure investigation, X-ray scatter
ing, optical microscopy, and atomic force microscopy have been employe
d, whereas for the kinetics, we have used differential scanning calori
metry, optical microscopy, and rheology. In the stars, there is a comp
etition for crystallization between the two crystallizable blocks whic
h have similar mobilities and melting temperatures but crystallize in
different unit cells (monoclinic vs orthorhombic). When the crystalliz
able block length ratio is 3 or higher, only the longer block will cry
stallize. For comparable lengths both blocks can crystallize-however,
not within the same molecule-but the crystallinity, long period, and c
rystalline lamellar thickness are reduced with respect to the pure PEO
and PCL. The latent heats, obtained in the isothermal crystallization
calorimetric experiments, are analyzed in terms of the Avrami theory.
Although similar Avrami exponents were found for all stars (n = 2, re
flecting a disklike growth from heterogeneous nuclei), the crystalliza
tion times were different depending on the nature of the crystallizabl
e blocks. Optical microscopy revealed the formation of different super
structures (spherulites/axialites) depending on the type of crystalliz
able block (PEO/PCL). The growth rates of these superstructures were o
btained and analyzed in terms of a kinetic nucleation theory, and the
fold surface free energies were extracted. Notwithstanding the larger
specific surface of bulk PCL as compared to that of PEG, the fold surf
ace free energies in the stars were similar to that in PEG, indicating
a pure PEO crystal and mixing of the amorphous blocks with the PCL cr
ystal. This is supported from the results of the atomic force microsco
py measurements on thin films, which have indicated the formation of p
erforated PCL crystals.