Surface characterization of the hydroxy-terminated poly(ε-caprolactone)/poly(dimethylsiloxane) triblock copolymers by electron spectroscopy for chemical analysis and contact angle measurements

Abstract

The surface composition and free energy properties of two grades of amphiphilic and semicrystalline triblock copolymers consisting of a poly(dimethylsiloxane) (PDMS) midblock (Mw ≃ 2300) coupled to poly(∈-caprolactone) (PCL) end blocks having differing molecular weights (Mw ≃ 2000, sample P3, and Mw ≃ 3000, sample P2) and homopolymer PCL (Mw ∼ 40 000) were investigated by Fourier transform infrared, spectroscopy, electron spectroscopy for chemical analysis (ESCA), and contact angle measurements using critical surface tension, one-liquid and two-liquid methods. ESCA showed that the molar concentration of PDMS increased from 36.5% in the bulk up to 70.2% in the surface for sample P2 and from 46.3% in the bulk up to 79.2% in the surface for sample P3 in high vacuum. This indicates that the lower surface energy PDMS microdomains were segregated in the surface region to minimize the surface energy of the copolymer. The longer the PCL block, the higher the phase separation. One-liquid contact angle results were evaluated by using van Oss, Good, and Chaudhury's Lifshitz-van der Waals and Lewis acid-base (AB) methodology, and it was determined that the basicity surface tension coefficients (γs-) of the copolymers decreased with the increase of the PDMS content at the surface, a result in agreement with the ESCA results but not proportional to them, indicating that the surfaces of the copolymers are highly mobile and molecular rearrangement takes place upon contacting with a polar testing liquid drop. The strong AB interaction between the basic carboxyl groups of PCL segments with the Lewis acidic groups of the polar liquids restructured the surface molecular composition at the contact area by increasing PCL and decreasing PDMS concentration in polar environments. The two-liquid contact angle method was also applied, and it was determined that γs- decreased inverse proportionally with the increase of PDMS segments. Also, it was realized that the molecular restructuring did not take place in the two-liquid method.