Langmuir and Langmuir−Blodgett Films of Poly(ethylene\noxide)-<i>b</i>-Poly(ε-caprolactone) Star-Shaped Block Copolymers

Abstract

Self-assembly of poly(ethylene oxide)-<i>block</i>-poly(ε-caprolactone) five-arm stars (PEO-<i>b</i>-PCL) was studied at the\nair/water (A/W) interface. The block copolymers consist of a hydrophilic PEO core with hydrophobic PCL chains\nat the star periphery. All the polymers have the same number of ethylene oxide repeat units (9 per arm), and the number\nof ε-caprolactone repeat units ranges from 0 to 18 per arm. The Langmuir monolayers were analyzed by surface\npressure/mean molecular area isotherms, compression−expansion hysteresis experiments, and isobaric relaxation\nmeasurements, and the Langmuir−Blodgett (LB) films' morphologies were investigated by atomic force microscopy\n(AFM). PCL homopolymers crystallize directly at the A/W interface in a narrow surface pressure range (11−15\nmN/m). In the same pressure region, the star-shaped block copolymers undergo a phase transition corresponding to\nthe collapse and the crystallization of the PCL chains as shown by the presence of a pseudoplateau in the isotherms.\nThe LB films were prepared by transferring the Langmuir monolayers onto mica substrates at various surface pressures.\nAFM imaging confirmed the formation of PCL crystals in the LB monolayers of the PCL homopolymers and of the\ncopolymers, but also showed that the PCL segments can undergo additional crystallization after monolayer transfer\nduring water evaporation. The PCL crystal morphologies were also strongly influenced by the surface pressure and\nby the PEO segments.