Some relationships between molecular structure and flow in linear polyethylene

Abstract

synopsisThe flow curves of linear polyethylenes are characterized by a set of empirically defined parameters, including the apparent melt viscosity at zero shear stress, the shear streas for onset of non-Newtonian flow, and the s l o p of linear sections fitted to data in three shear stress ranges of the flow curve. Not all of these parameters have theoretical significance but all are easily meagured and therefore should be useful for broad, practical application. The dependence of the parameters on molecular weight, shape, and breadth of the molecular weight distribution has been demonstrated by using fractions and blends of fractions of linear polyethylenes. The zero shear viscosity and degree of non-Newtonian flow at low shear stress depend primarily on polymer molecular weight. The degree of non-Neenian flow a t higher shear stress can be related quantitatively to molecular weight distribution factors, while the stress for onset of non-Newtonian flow also correlates with molecular weight distribution. Formally, the derived correlations are specific to the apparatus used, but may in fact have much broader validity because of the minor importance in these polymer samples, of elastic effects such as capillary die entry effects.

Flow behavior of well characterized polyethylene melts

“…PSEUDOPLASTIC FLOW Schreiber (20,21) showed that plotting the value of ( v/r]o)1/2 versus log T (for the range v/l;lo > 0.65) and extrapolation to ?/.lo = 1 yields the critical shear stress, rcT, for onset of non-Newtonian flow. Narkis ( 2 2 ) successfully used this method for LDPE and HDPE, and this was the case also in the present work.…”

Onset of non‐Newtonian flow: A correlation with an average molecular weight

“…= 10log (MZtIMZ/iiW) or = 1 o ' O (iiw/iz+&)Although this functionality is not concrete at the present time, relation(2) gives an interesting prediction for the onset of non-Newtonian behavior of polymer systems. A monodisperse polymer with molecular weight of l o 5 starts to show non-Newtonian behavior at shear stress above lo5 dynes/cm.…”

“…2, while a normal distribution polymer with M, = lo5 at shear s t r e s s above 105/6 = 1.67 x lo4 dynes/cm2. The above tentative calculations may not be far from the experimental results presented in the literature(13,14), although precise values of variables are not available.The value of 7, (used in this work) is not determined with high accuracy and is only a practical measure of shear stress for onset of non-Newtonian flow, and further relation(2) is not concrete at the present time. Nevertheless, the experimental fact of a close correlation of T, with fiz+ ~~z /~w may be useful for theoretical consideration of non-Newtonian behavior.…”