Polymers, Polymer Networks and interpenetrated polymer networks

Almost every materials developed correspond to the combination of compounds that are generally thermodynamically incompatible. Thus, the architecture of interpenetrated networks of polymers is the only way to develop a mixture of two polymers that is stable over time. It corresponds to the combination of crosslinked polymers, at least one of which is synthesized in the presence of the other. These various pathways of synthesis are schematized below:

One of the objectives is to combine within a material the different properties of the associated partners while blurring the weaknesses of each. IPNs are stable dimensionally over time and may have improved properties of resistance to chemical and physical aging.

Some associations made:

In order to understand and control the phases separation mechanisms that play a crucial role on the properties of the final material, the laboratory also realize and studies such structures from micro-emulsion but also from 2D systems formed at the interface air – water or developed as thin film. Under these conditions, the interface phenomenon become more important and therefore easier to characterize. We have thus demonstrated that synthesis at the air/water interface leads to the formation of 2D semi-interpenetrated networks without phases separation, with partners as different as polydimethylsiloxane (PDMS) and cellulose aceto-butyrate. (A.El Haitami and all. 2015, 2014 )

Interpenetrating Networks of Polymer

The architecture of interpenetrated networks of polymers (IPNs) corresponds to the combination of cross-linked polymers, at least one of which is synthesized in the presence of the other. The different ways of synthesizing IPNs are summarized below:

The IPNs represent the only possible mode of association of two cross-linked polymers, that is to say the only way of elaborating a stable «mixture» in the time of two polymers. If one of the polymers is not cross-linked, then we are talking about a semi-interpenetrated network of polymers.
One of the objectives of the construction of these architectures is to combine within a material the different properties of the associated partners while blurring the weaknesses of each. IPNs are dimensionally stable over time and may have improved properties of resistance to chemical and physical ageing.

Parameters for the synthesis of IPNs conditioning the final properties of the material

We have shown that the synthesis parameters significantly affect the morphology of the final materials.

For example, polyisobutene (PIB) with an average molar mass (between 10,000 and 100,000 g/mol) can be immobilized in a polymethacrylate (semi-IPN) network in a solvent-free synthesis. The images below show that the PCHMA-rich phase (dark domains) is in the form of spherical domains from 0.1 to 0.3 µm in diameter, regardless of the initiator of the radical polymerization used. On the other hand, PIB-rich domains (clear domains) are smaller when network formation is initiated by peroxide dicyclohexyl peroxydicarbonate ((PCDH) t½ = 10:00 to 40°C - low temperature primer) rather than benzoyl peroxide (POB- t½ = 10:00 to 80°C).

MET image of semi-IPNs PIB/Polymethacrylate (50/50) whose polymerization is initiated by PCDH (left) or POB (right). The polymethacrylate network is marked with RuO4 and therefore appears dark, the PIB corresponds to the clear areas.

This difference in morphology was confirmed by dynamic thermomechanical analyses (DMTA). Thus, when the synthesis temperature is higher (POB), the viscosity of the reaction medium decreases, which favours the separation of phases between polymers during synthesis.

The control of the synthesis parameters is even more important during the OPN architecture synthesis. For example, the synthesis of POE/NBR IPNs was performed sequentially or sequentially in-situ. When the POE network is formed in the presence of already cross-linked NBRs (sequenced pathway), phase separation is not detected by MET (figure right below). However, when the POE network is formed first (in-situ), it takes the form of microgels whose size increases with the proportion of POE in the material. The resulting materials then present a macro-dispersion of phases, leading to a morphology of POE nodules dispersed in a continuous phase of elastomer (left figure below). As the mass proportion of POW increases, the POW domains percolate, totally or partially, leading to a continuous conductive phase.

MET images of IPN POE/NBR according to a sequential in-situ synthesis (left) or sequenced (right). The NBR is specifically marked with OsO4.

2D hybrid structures

Model 2D structures are formed by adsorption of metal ions under a stabilized fats acid monolayer at the air-water interface. These structures attempt to mimic those of certain biomaterials such as mother-of-pearl. Metallic ion interaction – organic polar head appears subtle: some divalent cations form a 2D network of ionic complexes commensurable to the organic network (substructure) while others do not organize. In order to analyse the relevant physical processes causing the growth of a given mineral structure, the adsorption kinetics of different cations were studied according to different physico-chemical parameters (ion concentrations, pH, counter-ions). Characterizations by thermodynamic measurements, Brewster microscopy and X-ray rasante and high resolution (ESRF) diffractions have shown that the 2D hybrid structures formed by the Mn2+ and Mg2+ ions exhibit remarkable rigidity and ensure an organization of the organic part not observed on pure water. The study of the adsorption kinetics of Cu2+ ions revealed the role of pH on the phase transitions leading to the final state, which is itself independent of pH.

2D organic structures

The methods of making photosensitive surfaces play a significant role in the photochemical processes then involved. Thus the properties of films, of a cellulosic derivative bearing cinnamate groups (CABg) obtained by Langmuir-Blodgett deposit and spin-coating have been compared with those of the corresponding materials synthesized in the form of networks. Photo-irradiation in the UV of these systems causes the trans cis isomerisation of the cinnamate photosensitive groups but may also be accompanied by their dimerisation, the importance of which depends on the method of organization of the surface. The organization of the CABg in the form of ultrathin films (Langmuir-Blodgett) or their cross-linking within materials prevents the reaction of dimerisation.On the contrary, this irreversible reaction is predominant in spin-coating films.

The UV-related surface properties of an azobenzene acrylate and fluorinated acrylate (poly(Azo-co-AcRf6)) copolymer were also investigated. In this case, the isomerisation of the azobenzene groups is only favoured when the film of copolymers obtained by spin-coating.

The combined study of these different methods of making photosensitive surfaces has therefore made it possible to determine, for each system, the best way of implementing photomodulatable wettability surfaces.