Abstract :
[en] Plasma polymerization is an attractive and powerful tool for synthesizing functional polymeric thin films with highly cross-linked structures. So far, in plasma many non-negligible side reactions promote the synthesis of polymers with a structure different from those obtained by conventional polymerization processes. Namely, the network architecture could be a consequence of the functional group's sacrifice on behalf of the cross-link’s formation. Remarkably, conducted at atmospheric pressure plasma polymerization is evidenced to promote the conventional polymerization pathway assuring good retention of functional groups from monomers in a vapor or liquid phase. Yet, sacrificed to the benefit of the functional groups - the cross-links, lack further contribution to the mechanical and chemical stability, limiting the long-term application. In other words, the current state of the atmospheric pressure plasma polymerization requires a balance between available functional groups and cross-links. Consequently, as an effective method for an adequate reinforcement of cross-linked structure, maintaining a high concentration of the functional groups arises the combination of two polymeric networks in Interpenetrating Polymer Networks (IPN).
However, the formation of IPN architecture in the plasma process has yet to be reported. In this context, this thesis aimed to examine the deposition of the thin films with the IPN architecture, utilizing an atmospheric pressure continuous sinusoidal plasma in the synthesis pathway. The primary objective was to evaluate the formation of the IPN system in-situ, in a sequential manner from telechelic oligomers. The convenience of the one-pot, in-situ, IPN formation involves foremost an immediate network's interlocking already during synthesis, thus providing forced compatibilization and reducing possible phase separation. To facilitate direct network formation, thin films were prepared from telechelic poly(ethylene glycol) (PEG) oligomers with different reactive end-groups to assure two different polymerization pathways. The latter factor was just a prerequisite to enable an IPN synthesis in a non-interfering manner, whereas a shared PEG backbone was intended as a compatibilization factor between the networks. In particular, methyl methacrylate (MA) and benzoxazine (Bz) were studied as reactive end-groups. These two distinct functional groups allowed the formation of the first network by atmospheric pressure plasma-induced polymerization of the telechelic MA-PEG, followed by the second network formation by thermal curing of the telechelic Bz-PEG thermoset.
The formation of the IPN systems in a non-interfering sequential manner was corroborated by Fourier-Transform Infrared Spectroscopy (FTIR) measurements and thermal analyses. Moreover, the negligible effect of the plasma exposure on the Bz chemical structure was assessed. The evaluation of the macroscopic and microscopic properties of the IPN systems were discussed, considering telechelic PEG oligomers with two different numbers of ethylene oxide units (n=1 or 8). Nano-scratch tests evidenced an apparent effect of the IPN formation and the Bz reinforcing role with up to a threefold increase of the mechanical load resistance compared with the MA-PEG thin film. Nano-viscoelastic analysis by Atomic Force Microscopy (AFM) confirmed the formation of the IPN structure and the absence of phase separation. In addition, loss tangent and Young modulus parameters indicated the formation of a stiffer IPN when the Bz-PEG with the lower number of ethylene oxide units was used. The well-known thermal properties of Bz thermosets were reflected in the IPN systems allowing the formation of PEG thin films with enhanced thermal stability, remarkably increased up to 100 °C.
A systematic study was carried out to deepen the understanding of the IPN systems prepared by this novel approach, particularly the structure-to-property relationship. Thermal studies allowed to associate the increment of the ratio between ethylene oxide units and reactive end-groups in the IPN system as a hindering effect on Bz network formation. Otherwise, the synergism between the formation of the two networks was revealed by the IPN systems prepared with the Bz-PEG with the lower number of ethylene oxide units, reaching thermal stability beyond that of their single constituents. Comparative studies conducted by nanomechanical AFM mapping between IPN thin films consisting of the same molar fraction of Bz enabled to determine the influence of the concentration of ethylene oxide units on the viscoelastic properties of the thin films.
With the ultimate goal of proposing an approach for the in-situ and simultaneous formation of IPNs by a plasma process, the plasma-induced ring-opening polymerization was considered an alternative to the thermal ring-opening polymerization mechanism. For this aim, model allyl substituted cyclic carbonates were investigated. Since the driving force behind ring-opening is the release of the cycle strain, the model molecules consisted of 6-membered and N-substituted 8-membered cyclic carbonates. While the allylic 6-membered cyclic carbonate was found to mainly polymerize by a free-radical mechanism through the double C=C bond, in the more reactive allylic N-substituted cyclic carbonate, a competition of two polymerization mechanisms occurred. Interestingly, ring-opening polymerization could be prioritized over the free-radical mechanism by actuating on plasma exposition parameters such as power and time.
