D. J. Groves

Predicting low density polyethylene melt rheology in elongational and shear flows with “pom-pom” constitutive equations

A recent constitutive equation derived from molecular considerations on a model architecture containing two branch points a “pom-pom” captures the qualitative rheological behavior of low density polyethylene (LDPE) in shear and extension for the first time [, J. Rheol. 42, 82 (1998)]. We use a hypothetical melt of pom-poms with different numbers of arms to model the behavior of LDPE. The linear relaxation spectra for various LDPE samples are mapped to the backbone relaxation times of the pom-pom modes. Data from start-up flow in uniaxial extension fixes the nonlinear parameters of each mode giving predictions for shear and planar extension with no free parameters. This process was carried out for data in the literature and for our own measurements. We find that multimode versions of the pom-pom equation, with physically reasonable distributions of branching, are able to account quantitatively for LDPE rheology over four decades in the deformation rate in three different geometries of flows. The method sug...

Constriction flows of monodisperse linear entangled polymers: Multiscale modeling and flow visualization

We explore both the rheology and complex flow behavior of monodisperse polymer melts. Adequate quantities of monodisperse polymer were synthesized in order that both the materials rheology and microprocessing behavior could be established. In parallel, we employ a molecular theory for the polymer rheology that is suitable for comparison with experimental rheometric data and numerical simulation for microprocessing flows. The model is capable of matching both shearand extensional data with minimal parameter fitting. Experimental data for the processing behavior of monodisperse polymers are presented for the first time as flow birefringence and pressure difference data obtained using a Multipass Rheometer with an 11:1 constriction entry and exit flow. Matching of experimental processing data was obtained using the constitutive equation with the Lagrangian numerical solver, FLOWSOLVE. The results show the direct coupling between molecular constitutive response and macroscopic processing behavior, and differentiate flow effects that arise separately from orientation and stretch.

Predicting the rheology of linear with branched polyethylene blends

A series of melt blended commercial linear and branched polyethylenes are used to explore the generality of blending laws. The measured relaxation modulus G(t), and zero shear viscosity η0 for each blend and blend fraction, have been compared with prediction for miscible blends, particularly using equations derived by Tsenoglou (1987). Plus or minus deviation between theory and measurement is dependent on the relative molecular weights of the blend components. We have found empirically that a generalised form of the blending law for G(t) and for η0, with a floating index C, provides an improved prediction of the blend fraction data. In particular the function defining C is non-symmetrical, from which we infer the significance of branching as well as molecular weight. The optimum value of the index differs for each of our blends, in the range 1.25 to 4, the variability being accounted for by the different degrees to which branched and linear polymers relax co-operatively in the melt. Blends of two near linear polymers do not fit the floating index prediction and conform more closely, though not precisely, to the original Tsenoglou rule.

Gel fractions and chain reactions in irradiated polyethylenes

Abstract Atomistic computation, Flory–Stockmayer theory and rheological measurements have been applied to the analysis of gel-fraction data from two linear low density poly(ethylene)s (LLDPEs) irradiated and annealed either in vacuo or in the presence of acetylene. Good agreement between the three methods, as the extent of crosslinking at any given gel-fraction, confirm the presence of free radical mediated chain reactions, even during in vacuo irradiations. It is proposed that chain reactions, rather than polydispersity and structure, explain most of the deviation from ideal Charlesby–Pinner behaviour of irradiated polymers.

An investigation into the relationship between 'gel-effective' and total numbers of crosslinks in irradiated LLDPE

Rheological data for crosslinked LLDPE samples have been related to previous studies of similar materials. It has been found possible to attribute a range of time constants (λ0in) and characteristic moduli (G0i) to the ‘gel-network’ strength factor (S) of all these materials. Comparability established, the number of crosslinks estimated rheologically are compared with those computed by molecular modelling for equivalent gel fractions. Below about 0·4 gel fraction, all crosslinks are found to be effective in forming the gel. Beyond about 0·85 gel fraction, further increase in crosslinks produces only small changes in moduli; this is observed at around five crosslinks per pre-irradiated molecule. A simple power relationship is found between the rheological and ‘gel-effective’ numbers of crosslinks. A previously postulated relationship between the gel fraction and numbers of ‘gel-effective’ crosslinks appears to be universal for LLDPEs. ‘Gel-networks’ arise at low gel fractions. Rheological data suggest that there is a dose-related progression of ‘gel-network’, from initial viscoelastic polymer, through a medium containing a mobile distribution of ‘gel-networks’ of increasing size, to a temporary ‘gel-network’, still able to relax, and finally a saturated permanent network. ©1997 SCI