Research On Some Basic Rheological Problems Of Solid-liquid Interface

Several rheological methods are used to determine the characteristic times at a fixedtmperature for a commercial polydimethylsiloxane (PDMS) and their consistence and relationto the linear relaxation spectrum are examined. The experimental damping functions of stepdeformation for the PDMS, a polymethylvinylsiloxane(PMVS) and a commercial highdensity polyethylene (HDPE) are compared with predictions of the Doi-Edwards theory andmodel of Marrucci et al. The effect of wall slip on the damping function data is discussed.The appearance of stress peaks due to the material instability as the strain increases above acritical value is detailed. By applying the stress decomposition method proposed by Cho et al.to large amplitude oscillatory shear(LAOS) for the PDMS sample, the relationship betweenthe generalized elastic modulus G′_N(ω,γ)and the relaxation modulus G(γ, t) is investigated.It is found that in the linear and initial nonlinear regimes, as the angular frequency increases,G′_N(ω,γ) approaches to G(γ, t) on the time scale t=1/ω.Steady shear experiments were done for PDMS on aluminium parallel plates of differentsurface roughness with three gaps in which the wall slip was characterized by slip velocityand slip extrapolation length. It was found that the wall slip was suppressed by the roughnessin a certain range which agreed approximately with that determined by Javierand Lynden..Steady and dynamic shear experiments were done for PDMS and a linear low densitypolyethylene(LLDPE) on parallel plates with the same surface roughness and differentchemical composition. It was found that the surface energy has an important effect on wallslip for PDMS. In the experimental range, no obvious wall slip was observed for LLDPEmelt. The relationship between the surface energy and wall slip was investigated bymeasuring the contact angle of the PDMS melt on the fixture's surface. The result shows thatstrong surface energy weakens the level of wall slip.Wall slip of the melts of PDMS, PMVS, HDPE and polypropylene(iPP) were studiedexperimentally by using a rotational rheometer with parallel plate fixtures and checking thegap dependence of the stress/strain data. In the steady shearing flow it was discovered that,contrast to HDPE, there seems no critical shear stress for the onset of slip of PMVS. In thedynamic oscillatory shear flow, as the strain amplitude increasing, the stress data obtainedunder different gaps first overlapped well in the linear and the early part of nonlinear regions,then, at a certain strain amplitude, started to diverge, indicating that wall slip or strain stratification occurred. Employing the elastic/viscous stress-decomposition techniqueproposed by Cho et al.'s, we have examined possible candidates among various parametersthat could determine the onset of wall slip. The dimensionless parameterτ′_max|G^*|, i.e., theratio of the maximum elastic stress in a cycle to the linear complex modulus, was found tovary in the range from 0.26 to 0.49 for the four kinds of polymer melts when the wall slip orstrain stratification occurred. In that range, the onset value decreases weakly as the angularfrequency increases and is almost independent of the temperature in considerable large ranges.Thus elastic stress is the key factor causing the wall slip or strain stratification of polymermelts, which supports the conclusion of Vinogradov et al.'s earlier investigation.Wall slip in LAOS for PDMS was further studied experimentally with one gap of theparallel plates fixtures and various angular frequencies and strain amplitudes. As the strainamplitude increased, no→obvious→decreasing even obsolescent wave form distortion insequence were observed, which showed that stick→weak slip→stick/slip→strong/full slipeven unsteady flow occurred between the melt and the wall. At the same time the nonlinearviscoelasticity in LAOS was measured by using the dimensionless storage moduliG′_N(ω,γ₀) proposed by Cho et al.'s. As angular frequency increases, both nonlinearviscoelasticity and slip increases. A dynamic spectrum concerning two key parameters of thestrain amplitude and angular frequency was shown to predict the flow behavior in LAOS. Thedynamic spectrum reflects the early stage of the LAOS, and time is another crucial factorwhich affects the wall slip behavior.Isothermal crystallization behavior of HDPE was studied experimentally by using arotary rheometer with aluminum parallel plates fixtures with different surface roughness andparallel plates fixtures with the same surface roughness and different chemical composition.It was found that the crystallization rate depends on the surface roughness of the aluminumfixtures. As the surface roughness increases, the crystallization rate first increases thendecreases which means there is a range of surface roughness where the crystallization ratecan reach the maximum value. Surface roughness effectively enhances the interfacial area,thus decreases the heat resistance; but possible trapped air pockets in the valley of the roughsurface produce a considerable interfacial heat resistance, both affect the crystallization rateof HDPE. The isothermal crystallization rate decreases in sequence as the HDPE specimencontacted with aluminum, brass and stainless steel plates whose surface roughness are on thesame level. The sample crystallizes more quickly on the fixtures with higher heat conductioncoefficient, thus with lower heat resistance. It was found that the crystallization rate of HDPE was insensitive to the surface energy of the fixture whose surface was pasted with analuminum foil.