Domain Stability During Capillary Flow of Well Dispersed Two Phase Polymer Blends. Polystyrene/Polymethylmethacrylate Blends

Synopsis

Hypotheses for domain stability, i.e. the conditions where each domain of a dispersed phase remains as a coherent domain during capillary flow is tested with data for polystyrene -polymethylmethacrylate melts. It is found that stable domains exist vhen the ratio between the (Trouton) zero shear viscosity of the dis-crete phase to the continuous phase is larger than approximately one. Blends of two thermodynamically incompatible polymers, polystyrene (PS) and poly(methyl-methacrylate) (PMMA), were melt blended in a Brabender Plasticorder over the compositions: 100$ PS, 75$, 50$ 25# and 0% PS. Viscous and die swell behaviour of pure polymers and the blends at different temperature were obtained using an Instron capillary rheometer and the microstructure of the melt blends was studied by transmission electronmicroscopy.

The question of domain stability is “tested” by a) comparing the average volume of coherent discrete domains before and after extrusion through a capillary and b) by an indirect method where a model prediction for the viscosity for stable domain flow is compared with experimental data.

It has been found that viscosity-shear rate data of PS-PMMA blends at different temperatures constitute master curves at constant blend composition when plotted as (η/η₀ ) versus (γη₀ /pT). Furthermore all the master curves of the blends and homopolymers with different molecular weight distributions superposed into a single master curve when plotted as (η/η_o) versus (γ_oM_cH/ pRT) where M_c is twice the molecular weight between entanglements and H is the heterogeniety (M _w /M _n).

From the observed melt rheology results and the agreement of experimental results with model predictions supported by direct evidence of electron photomicrographs, it is concluded that the morphology of incompatible polymer blends depends on the composition ratio and Newtonian (or Trouton) viscosity ratio of the components. When the blend has the component with low Newtonian viscosity as dispersed phase, the dispersed droplets seem to be unstable and to break up into smaller droplets in capillary flow. On the other hand if the blend has the higher Newtonian viscosity component as a dispersed phase the dispersed droplets appear to form a stable morphology with continuous threads or a fibrillar pattern or elongated droplets in capillary flow.

In order to explain the experimental observations a hypothesis is formulated for the stability of coherent domains in the socalled relaxation region near the inlet region in the capillary for viscoelastic domains in viscoelastic media at high shear rates.

According to this hypothesis a necessary criterion for domain stability for a domain near the capillary axis (critically condition) is that the ratio between the relaxation time λ_D of the domains to that of the continuous phase λ_K is larger or equal to 1.

The relaxation time is given by $\lambda = {{a {\eta _0}{M_C}H\rho } \over {{C^2}RT}}$where η is the zero shear viscosity, M_C is twice the molecular weight between entanglements, H = M _W /M _n , p the polymer density, c is the polymer concentration, is the gas constant, T is the absolute temperature an a is a constant.