Fabrication and Characterisation of Amine‐Rich Organic Thin Films: Focus on Stability

Abstract

Thin films, rich in primary amines (CNH2), were deposited from nitrogen (N2) or ammonia (NH3) and ethylene (C2H4) with different gas mixture ratios, R, using three different methods: atmospheric‐pressure‐ or low‐pressure plasma polymerisation (PP), and vacuum‐ultraviolet photo‐polymerisation. They are designated H‐plasma‐polymerised ethylene (PPE):N, L‐PPE:N and ultraviolet‐polyethylene (UV‐PE):N, respectively. Of interest in cell‐culture and tissue engineering, all three coating‐types were examined with regard to stability in air and solubility in water, compared with other deposits in the literature that were obtained from single precursors such as allylamine (AA) or n‐heptylamine (HA), PP‐AA and PP‐HA, respectively. The three types of deposits, prepared using comparable R values, were characterised by X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy and atomic force microscopy and found to vary significantly among themselves in regard to their [N]‐ and [NH2] concentrations, and their chemical stabilities during long‐term exposures to air or aqueous solvents. UV‐PE:N and L‐PPE:N films were found to compare very favourably with their best PP‐AA and PP‐HA counterparts; we conclude that the additional important fabrication parameter (the gas mixture ratio, R) is a major asset for preparing stable NH2‐rich organic coatings with optimal properties.

Considering the degradation effects of amino-functional plasma polymer coatings for biomedical application

“…Plasma polymers containing primary amino groups are known to show aging effects, i.e. oxidation reactions yielding the formation of amides (among other functional groups) [11,13,14]. Immersion in water results in weight loss and removal of nitrogen groups from the residual deposit, e.g., by hydrolysis of imine groups.…”

“…The oligomeric parts of the films are released due to degradation accompanied by pore formation in the remaining films [12]. This degradation occurs already within a few hours of storage in water and leaves the cross-linked portion of the plasma polymer on the substrate [12,14,26].…”

“…Degradation effects such as oxidation and leaching of low molecular weight fragments that are known to occur for amino-functional plasma polymers, however, require special handling [9,[11][12][13][14][15][16]. Prior to cell attachment a-C:H:N-coated samples are therefore stored in air for several weeks, thermally treated (in vacuum) and/or washed with solvents, while the media is frequently exchanged during cell growth [17][18][19].…”

Photo-initiated chemical vapor deposition as a scalable particle functionalization technology (a practical review)

“…For example, the functional coating will be subject to the same problems. Therefore, work conducted by Wertheimer et al on coating aging (hydrophobic recovery) [49] as well as stability of film by spectrometry [20] is quite useful. The critical issues of PECVD demonstrated by Bunshah [35] coupled with the mass transport considerations proposed by Goyal et al [50] and the critical review from Liston et al [51] can be used to assist in reactor design.…”

“…Nonetheless, due to its high energy transfer to the reactive species, PECVD is the most powerful CVD technique. PECVD has been proven to functionalize nanoparticles very e ciently [11,20], but tends to make reaction happen too quickly, resulting in unstructured coatings with low crosslinking. A variant of this technique consists in pulsing the plasma in order to allow the functional coating to restructure itself between pulses.…”