Investigation of the linear flow regime of commercial polymers by numerical conversion of MVM creep measurements

A new experimental and numerical method has been developed to characterize the terminal flow behavior of polydisperse, commercial grade polymer melts over a wide dynamic range of time/frequency scales. Experimentally, an MVM rheometer specifically designed for long time scale ( t ∼ 10 4 s) creep measurements is used to measure the creep compliance of three commercial polymers: two high density polyethylenes and one polystyrene. The long time scale MVM creep data are complemented in the short time scale regime by creep data from an industrial plate-plate rheometer. The time-dependent creep data is combined and converted to a discrete retardation spectra using a nonlinear regularization algorithm to address the ill-posed nature of the interconversion. The retardation spectrum is analytically converted to dynamic moduli and compared with independently measured dynamic moduli. In the overlapping frequency region, calculations and measurements show excellent agreement and the combined data span a much larger dynamic range than either independent data set. The calculated and measured dynamic moduli data are combined and a retardation spectrum with a vastly expanded dynamic range is generated. Combining long time scale MVM creep compliance data and dynamic moduli data exploits the intrinsic sensitivities of controlled strain and controlled stress rheological experiments and is a powerful means to greatly expand the experimentally accessible dynamic range of time/frequency. This approach is particularly useful for commercial polymers with broad molecular weight distributions and commensurately large distributions of relaxation times.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47213/1/397_2004_Article_BF00367362.pd

Myosin VI in PC12 cells plays important roles in cell migration and proliferation but not in catecholamine secretion

Myosin VI (MVI) is the only known myosin walking towards minus end of actin filaments and is believed to play distinct role(s) than other myosins. We addressed a role of this unique motor in secretory PC12 cells, derived from rat adrenal medulla pheochromocytoma using cell lines with reduced MVI synthesis (produced by means of siRNA). Decrease of MVI expression caused severe changes in cell size and morphology, and profound defects in actin cytoskeleton organization and Golgi structure. Also, significant inhibition of cell migration as well as cell proliferation was observed. Flow cytometric analysis revealed that MVI-deficient cells were arrested in G0/G1 phase of the cell cycle but did not undergo increased senescence as compared with control cells. Also, neither polyploidy nor aneuploidy were detected. Surprisingly, no significant effect on noradrenaline secretion was observed. These data indicate that in PC12 cells MVI is involved in cell migration and proliferation but is not crucial for stimulation-dependent catecholamine release