Blending Polyamide (PA) with another polymer (like Polyethylene PE) may be the easiest way to tune/enhance some compromise of properties (barrier properties to solvent, modulus, impact strength, etc). Most of polymer blends like PA/PE are immiscible and exhibit multi-phase morphology. The performances of the blends depend on the properties of each component, on composition and on the morphologies which are developed. Thus, the challenge is to control and stabilize a given morphology according to the desired set of properties. By decreasing interfacial tension and limiting coalescence, compatibilization allows stabilizing morphologies and thus insuring the reproducibility of the properties.
We have studied the relationships between (1) the formulation and process parameters and (2) the obtained morphologies and microstructures in reactively compatibilized PA6/HDPE/Maleic Anhydride-grafted-HDPE blends over a broad range of compositions, specifically with high amounts of compatibilizer.
Both the evolution of the dominant size as a function of the distance to co-continuity, and the distribution of sizes in a blend close to co-continuity, are modeled using percolation concepts. The mechanism underlying these concepts rely on the breakup-coalescence equilibrium, which shows that coalescence is not completely suppressed at large scale, even though the overall fraction of compatibilizer is large.
The morphologies exhibit two characteristic sizes: subdispersions of a few tens of nm coexist with larger (micrometric) scale morphologies. Large scale morphology regions have been identified in ternary compositions diagrams. The location of the co-continuity region agrees with theoretical models based on rheological properties. The observed large scale morphologies, however, are not compatible with the high fraction of copolymer formed in the system after reactive extrusion. This implies that most of the copolymers are under the form of nanometric subdispersions within large size domains.
Crystallinity also plays a key role on final properties. In the PE phase, as soon as a fraction of the phase is confined in nanometric domains, a distinct peak at lower temperature was observed, corresponding to confined crystallization. It was checked that this is coherent with the density of nucleating sites estimated from isothermal crystallization kinetics experiments. Fractionated crystallization at lower temperature was also observed for PA6 when it is confined in nanometric domains dispersed in a PE matrix.
In the recent PhD work of Chloé Épinat, we have studied the effect of the interfacial chemical reaction between PA66 and MA-g-HDPE in static conditions at a macroscopically flat interface. Interface destabilization and the nucleation and growth of ordered microphase separated copolymer domains, somehow similar to myelin figures observed in surfactants put in the presence of water, have been observed for the first time in this system. The morphologies which are formed depend on the architecture of the copolymer, namely essentially on the relative length of the blocks on each side of the interface. We have studied the conditions of stability of the plane interface in the case of non-symmetrical formed graft copolymer. This observation is very important, since it confirms that nanometric domains are certainly formed during reactive extrusion, in addition to purely rheological processes, like Taylor and/or Rayleigh instabilities leading also to micrometric domain formation.
The dependence of blend viscosity on the dispersed phase composition/morphology has also been studied. We have compared our measurements with modified Krieger-Dougherty model and demonstrated that the specificities of the rheological behavior can be explained by the creation of a percolating network thanks to nano-dispersions and potentially due to very small distances between dispersed droplets and the entanglement of copolymer brushes.