The use of a fatty acid/β-Hydroxyester blend to enhance the surface hydrophilicity of crosslinked poly(ethylene glycol) coatings

Abstract

By reaction between stearic acid (SA) and a di-epoxidized oligomeric poly(ethylene glycol) under stoichiometric conditions, an amphiphilic β-hydroxyester (β-HE) bearing pendant alkyl chains was obtained with a typical yield of 93 wt%. The remaining 7 wt% consisted of SA. This blend, named β-HESA, was used as modifier of a reactive solvent based on poly(ethyleneglycol) dimethacrylate (PEGDMA). The system was activated with a visible-light photoinitiator, and crosslinked films were obtained after five minutes of irradiation. A drastic change in the contact angle from 43.2° to 14.6° was attained by adding only 10 wt% of β-HESA, indicating that this modifier had a profound effect on the surface hydrophilicity of the films. Morphological analysis showed that a phase separation process took place in these modified systems that led to the formation of β-HESA-rich crystalline domains dispersed in an amorphous phase mostly formed by cross-linked PEGDMA. An explanation for the marked increase of the surface hydrophilicity of the films was found both in the formation of a micro-structured surface texture and in the orientation of polar groups of SA towards the solid-air interface. This interpretation was based on the evidence that crystals were formed by fully extended n-alkyl chains of the β-HE which were interdigitated by chains of free SA. This configuration allowed accommodating free SA chains within the same crystal structure of the β-hydroxyester and exposing the polar carboxylic acid groups of SA towards the air/polymer interface. The results of this work show that the combination of a phase separation process with the proper design and arrangement of amphiphilic molecules can lead to a unique surface behavior with potential interest in technological applications.