Improved water barrier properties of polylactic acid films with an amorphous hydrogenated carbon (a-C:H) coating

For the first time a 120 nm thick layer of amorphous hydrogenated carbon (a-C:H) deposited on top of 50 mm thick polylactic acid (PLA) films with radio-frequency plasma-enhanced chemical vapor deposition has been used as barrier to impede water diffusion through free-standing PLA films. Surface structures were examined by AFM and the water uptake was measured with Fourier transform infrared attenuated total reflection (FTIR-ATR) spectroscopy, a powerful technique for studying the diffusion of water through polymeric membranes. We have compared the water uptake of uncoated and a-C: H coated PLA films by placing the initially dry films in contact with deionized water for 24 h and monitoring the time-dependent absorbance changes in the FTIR spectra of the OH stretching bands of water in the wavenumber region of 2960-3750 cm(-1). Mathematical modeling revealed that the water uptake was best described with a model consisting of three diffusion coefficients ascribed to monomeric and dimeric, clustered and bulk water having values between 1.1 x 10(-8) and 2.6 x 10(-10) cm(2)s(-1). We show that the thin a-C: H layer effectively decreased the water uptake of PLA with 55% after a contact time of 24 h and even more at shorter times. (C) 2017 Elsevier Ltd. All rights reserved.