Measurements of relaxation time of a polyethylene melt

Abstract

SynopsisA device for measuring the elasticity of polymer melts: has been designed by one of us (B. Maxwell). The device was used to obtain the relaxation modulns in shear of a linear polyethylene melt. From these data a discrete relaxation spectrum was derived. The range of the obtained spect,rum was confirmed to correspond to the terminal zone of the "entanglement plateau" of the spectrum. The limiting dynamic viscosity (as frequency approaches zero) was obtained by integrating the relaxation modulus with respect to time. The viscosity and its activation energy were found to agree closely with the flow viscosity and the flow activation energy, respectively, involved in capillary flow.Even though it is well known that polymer melts are generally viscoelastic, in their flow studies they are often treated as if they are purely viscous liquids. This oversimplification is particularly serious in some materials where the elastic response to an external force is of the same magnitude as the viscous response. In processing polymeric materials, for example, there is a strong indication that the elastic energy plays an important role in melt the extent to which strains are frozen into the final product ifi also related to the elastic contribution and affects the mechanical properties of the solid.While there is a very large amount of data available on the flow viscosity of polyethylene melt^,^'-^ only a few investigations have been carried out on the viscoelastic properties within the linear range. ' We have constructed a device suitable for making measurements of stress relaxation on a polymer melt at elevated temperatures. In stress relaxation a strain is applied and held constant while the decaying stress is measured as a function of time. Hence, the possibility of errors arising from friction, as in a creep experiment, is eliminated. If the strain is kept small enough, the mathematics developed for linear viscoelasticity is applicable, thus simplifying the analysis and practical application of the data.The device for performing stress relaxation experinients in shear is shown in Figure 1. The sample is premolded in the form of two hollow cylinders. I t is then loaded in the apparatus and melted under nitrogen atmosphere. The shear strain is applied by rotating the inverted cup.

Investigation of Temperature-Dependent Relaxation Processes in Polyethylene Films by Covalently Attached Anthryl Probe

“…I F (T)/I F (0) ratios vs temperature are plotted in Figure 6, along with the assignments of observed transition temperatures to polymer relaxation processes from the literature (ranges between bars). [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] The intensity data have also been plotted in Figure 7 according to an Arrhenius-type treatment (eq 1) (vide infra) to aid in the location of onset transition temperatures and to calculate the associated activation energies, E Α , reported in Table 2. The assignments of the onset temperatures in the Arrhenius plots (Figure 7) are somewhat subjective.…”

“…Arrows have been drawn at positions of slope changes detected in Arrhenius-type plots of the data (Figure 7) and associated with temperatures for the onset or cessation of the relaxation processes assigned from literature values. [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] 1 -I F (T) Slope changes for the LLDPE/1 film appear around 160 K (onset of γ-relaxations), 250 K (onset of β-relaxations), 330 K (onset of R-relaxations), and around 370 K (onset of melting). Further evidence for the presence of an R-relaxation in LLDPE is found in its DSC thermogram that shows a small peak around 315 K (Figures 3 and 12 of Supporting Information).…”

Dynamic mechanical properties of polymer melts: The dependence of viscoelastic properties on molecular weight distribution

“…Both samples were prepared by anionic polymerization. Subsequent rigorous characterization showed that they have heterogeneity indices M , / M , 4 1.06.…”

Calendering thermoplastic materials

“…; this is much smaller than the relaxation time of most polymer melts at a processing temperature. 22 If eq. (40) is substituted into the equation of motion without inertia terms for the modified second-order viscoelastic materials, then we have…”