Rheological Study of the Miscibility of LLDPE/LDPE Blends and the Influence of Tmix

Blends of LLDPE and LDPE combine the favorable mechanical properties of the LLDPE with the ease of processing of LDPE. Yet, the melt rheological consequences of blending LLDPE and LDPE have received far less attention than have linear/linear or linear/ branched blends (1). Previous reports showed that blends of LLDPE/LDPE were either immiscible or partially miscible (2–5). Considering the components of these blends separately, melts of “pure” LLDPE are likely to phase separate as detected or suggested by different investigators (6–8). On the other hand, melts of LDPE possess a substantial amount of ordered phase as revealed by NMR studies of Bremner and Rudin (9). The existence of liquid-state molecular order and high-temperature transitions (208° and 227°C) in linear and lightly branched polyethylenes and their absence in highly branched LLDPEs was discussed in detail elsewhere (10–14). The existence of these high-temperature transitions prompted the extension of the miscibility study to cover the influence of the mixing temperature (Tmix) in the vicinity of those transitions. We are not aware of any previous study that discussed the influence of Tmix on the miscibility of LLDPE/LDPE blends. However, Hill’s group (15) presented experimental work with PE blends that indicated that a melt of 20% Linear PE and 80% LDPE is a single phase at 220°C and is biphasic at 170°C. Furthermore, DSC thermal analysis of a 50/50(w) blend of UHMWPE and normal MWPE showed one melting peak for blends mixed at 220°C or above (up to 280°C), suggesting miscibility, while two peaks (suggesting immiscibility) and melt segregation were obtained for blends mixed at 190°C (16). For immiscible blend systems, the state of dispersion and specifically the shape of the dispersed phase (i.e. droplets) greatly influence the rheological responses of immiscible polymer blends. As a basic rule, phase separation causes G and G to increase as a result of “emulsion morphology” present in phase separated systems. The emulsion morphology with consequent surface tension ( ) gives rise to a non-zero storage modulus (G ) even in a mixture of two Newtonian liquids. Scholz et al. (17) presented a constitutive equation for dilute emulsions of spherical, monodisperse Rheological Study of the Miscibility of LLDPE/LDPE Blends and the Influence of Tmix