A NOVEL APPROACH TO RHEOLOGICAL CHARACTERIZATION FOR THE GELATION IN POLYMER CRYSTALLIZATION

The isothermal crystallizations of three kinds of commercial isotactic polypropylene have been studied by DSC and rheometric experiments,in a range of temperatures where the rate of crystallization is moderate.As the crystallization proceeds,volume contraction induces tensile force upon the parallel plates.The tensile force relaxed quickly in liquid states, but after a critical amount of the relative crystallinity,it starts to accumulate in the static test,that is,with the motionless parallel plates.A new method to determine the liquid-solid transition by the static tensile force is proposed and compared with two dynamic methods of detecting liquid-solid transition,that is,the power-law modulus theory and the yield modulus model.The tensile force method predicted considerably earlier transition than the dynamic methods,and the corresponding DSC data indicate relative crystallinity of larger than 0.2 at the transition times.The limitation of dynamic methods and other possible errors have been analyzed.While the dynamic methods are suitable for slow crystallization,the tensile force method is more appropriate for the crystallization of moderate rates.Moreover,it has the advantage of almost not disturbing the crystallizing materials before the transition.

SOLUTION BEHAVIOR OF THE FALLING SPHERE PROBLEM IN VISCOELASTIC FLOWS

The hp-finite element method and pseudo arc-length method are combined to track solution behavior of the steady flow of a sphere falling in a tube filled with viscoelastic fluids. The computation is proved to be convergent and stable by a posteriori error analyses; the solutions smooth and with extremely steep stress gradient are obtained by using appropriate high-order interpolation distributions. The commonly used drag coefficient is proved not a reliable indicator for the approximation error. The solution curves of the upper convected Maxwell (UCM) and the Oldroyd-B fluids clearly indicate that there exist limiting points, parameterized by the Deborah number, where the magnitude of the solutions and the axial normal stress gradients behind the rear stagnation rise up rapidly, maybe go to infinity. The limiting points can be simply removed by employing the modified Chilcott and Rallison (MCR) fluid, which replaces the infinitely extendable, linear Gaussian chain with the finitely extendable nonlinear elastic spring (the FENE chain). Therefore the limiting behavior is caused by the physical model; it is not a numerical artifact.

Effect of uniaxial pressure on conduction behavior of carbon black filled poly(methyl vinyl siloxane) composites

The piezoresistive behavior of carbon black (CB) filled poly(methyl vinyl siloxane) (PMVS) vulcanites under uniaxial compression was studied.At filler weight frac- tions φ slightly above the percolation threshold φc,resistance first increased with pressure and then turned to decrease at a critical compressive stress,exhibiting a positive pressure co- efficient of resistance (PPCR) and a negative pressure coeffi- cient of resistance (NPCR) effects ,respectively.The NPCR effect became much more pronounced at φ>>φc,while com- pressive cycles facilitated the occurrence of the weak PPCR effect during compression.The PPCR-NPCR transition was a process related to true stress.It is believed that the changes of microstructure in the percolating network,i.e. the breakdown and the reformation of infinite conducting clusters,under pressure would be responsible for the uniaxial piezoresis- tance and the plastic deformation of the filled vulcanites.