Structural evaluation of branched polyethylene by combined use of GPC and gradient‐elution fractionation

Abstract

A method is described and a computer program outlined whereby gel‐permeation chromatographic analysis of fractions from gradient‐elution fractionation of branched polyethylenes provides a complete molecular weight evaluation of each fraction and the parent resin. The procedure involves the use of the universal calibration concept of Benoit et al. in a way that eliminates the ambiguities present in attempts to apply it directly to whole polymers. The resultant molecular weight data for fractions, when related to their solution viscosity and low‐shear melt viscosity and to their infrared analysis, provides a total structural evaluation of a branched polyethylene resin, including molecular weight molecular weight distribution and the distribution of both long‐and short‐chain branching. The potential of this method for providing a comprehensive structural evaluation of branched polyethylene is illustrated by examples of its application in the analysis of some commercial resins.

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

“…Note that η o is extremely sensitive to branch content and that the dependence is non-monotonic. This result qualitatively resembles that obtained by Bersted [30,47,50]. In both cases, as LCB density increases, η o is predicted to increase to a maximum at around 1 -3 …”

“…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].…”

Rheological Features and Flow‐Induced Crystallization of Branched Poly[ethylene‐co‐(1,4‐cyclohexanedimethylene terephthalate)] Copolyesters

“…As a matter of fact, enhanced melt elasticity was reported years ago for long chain branched polyethylenes, compared to linear samples. [45][46][47][48][49] More recently, the same effect has been recalled for isotactic and syndiotactic poly(propylene). [32,[49][50] According to these results, we assumed that long chain branching caused by the incorporation of PER to polymerization process produced the observed melt elasticity increase.…”

On‐line viscometry combined with gel permeation chromatography. II. Application to branched polymers

“…The intrinsic viscosity of branched polymers is smaller than that for linear polymers of the same molecular weight M.+ This ratio is related to the ratio of square radii of gyration There is still considerable uncertainty about the form of the relationship G = G ( g ) (5) and how it is affected by polymer-solvent interactions and the nature of the branching.14J5 In this paper we will utilize only theoretical expressions for g , calculated from random flight ~tatistics.~ Theoretical values of g depend on the type and location of the branches as well as B , the number of branch points per molecule: l 6 g = g (structure, B )…”