Lamellar and spherulitic crystallization of poly(s-2-hydroxybutanoic acid) and its stereocomplexes

Abstract

Crystallization kinetics of poly (S-2-hydroxybutanoic acid) [P(S2HB)] and P(S2HB)/P (R2HB) stereocomplexes were investigated by in situ atomic force microscopy, polarized optical microscopy, and differential scanning calorimetry. For the homopolymer, an equilibrium melting temperature Tm° of 130.3 °C is obtained according to the Hoffman-Weeks method. In the case of thin films of 50 nm in thickness, a large number of edge-on lamellae form rapidly on the edge of the scratch line induced by the AFM tip. The edge-on lamellae change their orientation to flat-on lamellae at crystallization temperatures equal or higher than 75 °C. The lamellar and radial growth rates exhibit a bell-shaped temperature dependence with a maximum around 400 ± 50 nm/min at a crystallization temperature of 77 ± 2 °C, in contrast to the stereocomplexes whose growth rates peak at 19.6 μm/min. Analysis of the growth rates using the Lauritzen-Hoffman model shows that crystallization of homopolymers and stereocomplexes occurs in Regime II, with an end surface free energy σe of 6.2 × 10−2 J/m2 in the first case and 7.6–8.5 × 10−2 J/m2 in the second, and work of chain folding q between 18 and 24 kJ/mol for the homopolymer and 26–30 kJ/mol for the stereocomplexes, which is in the range of moderately stiff chains without bulky side-groups.