These polymers are used in pharmaceutical formulations processed
by other technologies and their properties, as related to
these technologies, have been studied extensively. In the melt
extrusion process, rheology of the polymer melt is an important
factor affecting processing conditions and properties of pharmaceutical
product. Knowledge of the rheology is required for process
optimisation, troubleshooting, design of process equipment and
for computational fluid dynamics simulations.
Extruder torque and die head pressure has been studied as
indicators of melt viscosity [10,11]. Chokshi et al. [12] studied
shear rheology of drug–polymer melt blends and correlated it with
polymer–drug miscibility and extrudability. Lyons et al. [13] studied
the effect of supercritical fluids on polymer plasticization, extrusion
speeds, and temperature of polymer blend using parallel plate
rheometry.
Rotational shear rheology has been useful in establishing relationships
between material structure and its properties; however
these are generally limited to flows in the linear viscoelastic region,
which is characterised by small strain and low rate of material
deformation. The flowof polymer melt experienced inHMEis rapid
and largely nonlinear viscoelastic by nature. Extensional properties
become important during flow into the die cavity, wheremelt flow
converges and the polymer molecules undergo stretching, alignment
and alteration in chain entanglement.Resistance to flow in convergent regions such as the die entry region are indicated by
extensional viscosity and therefore have relevance to HME processing.