A rheological study of long branching in polyethylene by blending

Abstract

SynopsisSteady shear viscosities, dynamic viscosities and moduli, and the corresponding activation energies for flow were examined for a branched polyethylene, a linear polyethylene, and three of their blends a t 150' and 190°C. The polyethylenes were chosen to have closely matched molecular weights and distributions. An R-17 Weissenberg rheogoniometer and an Instron capillary rheometer were used. At lower stress, the branched polymer had a higher viscosity than the linear one, possibly because of the contribution of long branches to entanglements. At high stress, this contribution is reduced and the inherently smaller coil dimensions likely become responsible for the lower viscosity of the branched polymer. The activation energy for the branched polymer is high and decreases with stress, in contrast to the low and almost-constant value for the linear polymer. The effects here of pressure on compression are considered. The entanglements of long branches may also decrease with increasing temperature. With decreasing stress, the activation energy for branched polymer tends to become constant, corresponding to an absence of pressure effects and an equilibrium entanglement of long branches for a given temperature range. The linear relationship between activation energy and blend composition problably means that any compressional effects, like free volume, are additive and that long-branch entanglements rearrange with added linear molecules. The linearity may be the result, in part, of a broad distribution for the lengths of long branches.

Effect of long chain branching on linear-viscoelastic melt properties of polyolefins

“…However, despite extensive work on the effects of LCB on the viscoelastic properties of these types of material in the melt in the 1960s, there is no unified picture of their dependence on molecular variables. The following general properties were established for a moderate to high degree of LCB (>> 1 LCB/10 4 carbon atoms): a) lower Newtonian or zero-shear viscosity η o and a higher critical shear rate o γ& for the onset of shear thinning behaviour than linear polymers of the same weightaverage molecular weight, M w [3][4][5][6][7][8][9][10][11][12][13][14]; b) less intense pseudoplastic behaviour [11,[15][16][17]; c) increased activation energy of flow, E a [18][19][20][21][22][23][24][25][26][27][28][29]; and d) enhanced melt elasticity expressed in terms of first normal stress difference N 1 , steady-state compliance J e o and extrudate swell d j /D [4,5,12,13,16,17,[30][31][32].…”

“…[7] When the LCB content is very low (less than 1 LCB /10 4 C), its effect on the radius of gyration goes unnoticed and values of g ≈ 1 are obtained [42]. The effect of such a low amount of LCB has been explored in LDPE and in modified linear polymers of ethylene/α-olefins and propylene (PP) by irradiation, electron-beam treatment, peroxide reaction, and thermal/mechanical degradation [7,10,17,[44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59]. In principle, lowpressure processes (chromium-based and Ziegler-Natta type catalysts, developed in the 1950s and 1970s, respectively) yield linear species.…”

Development of Branched Polycarbonate by an Ultrasound-Assisted Melt Mixing Process with Multifunctional Agents

“…Hence, shear-thinning can occur because entanglements of the branched polymers are more easily detached than the entanglements of the linear chains at the higher shear rates [8]. By adding 5phr of MFA, the level of complex viscosities was higher than those of the others and shear thinning was more evident.…”

Radical Chain Polymerization

“…The long branches, formed by the ''normal'' chain transfer to polymer reaction (Eq. 3-127), affect the melt flow (viscosity) properties of the polymer and thus greatly influence its processing characteristics [Jacovic et al, 1979;Starck and Lindberg, 1979]. The short branches, which outnumber the long branches by a factor of 20-50, have a very significant effect on the polymer crystallinity.…”