Quantitative determination of short‐chain branching content and distribution in commercial polyethylenes by thermally fractionated differential scanning calorimetry

Abstract

A method for rapid quantitative analysis of the content and distribution of short chain branching (SCB) for α‐olefin/ethylene copolymers based on thermally fractionated DSC is presented. Eight commercial polyethylenes, four made with conventional Ziegler‐Natta catalysts and four made with metallocene catalysts, were analyzed by differential scanning calorimetry (DSC), after having been thermally segregated by successive nucleation annealing (SNA). The polyethylenes were also analyzed by temperature rising elution fractionation (TREF) and carbon‐13 nuclear magnetic resonance (13C‐NMR). The SNA‐DSC procedure segregates polyethylenes according to methylene sequence lengths (MSL). The relationship between DSC melting temperature and SCB content was obtained by calibration with linear hydrocarbons; TREF results were not used in the SNA‐DSC calibration. Deconvolution of the SNA‐DSC endotherms yielded estimates of the average SCB contents and SCB distributions. The SCB contents obtained from the SNA‐DSC for linear low density polyethylenes agreed very well with the SCB contents obtained by 13C‐NMR and TREF, and the SCB distributions measured by SNA‐DSC were very similar to those obtained by TREF. The SCB contents obtained by SNA‐DSC for ultra‐low density polyethylenes, made with metallocene catalysts, were about 20% lower than the values obtained by 13C‐NMR; the values obtained by TREF were even lower.

Optimisation and Application of Polyolefin Branch Quantification by Melt‐State 13C NMR Spectroscopy

“…[9][10][11][12] Such branches form structural defects during crystallisation and thus strongly affect crystallisation rates, ultimate crystallinity, melting point (T melt ), glass transition (T g ) and other bulk mechanical properties. [13] Similarly, long-chain branches (LCB) are formed by macromonomer incorporation during polymerisation. With these branches typically being longer than the entanglement molecular weight their presence strongly affects the processability of the bulk polymer.…”

“…branching is known to influence the zero-shear viscosity even at concentrations of 2 LCB per 100 000 CH 2 , as characterised by 13 C NMR. [14,15] Commonly, differential scanningcalorimetry (DSC), [13,16] triple-detection gel permeation chromatography (GPC) [17][18][19] and rheology [14,[20][21][22][23][24][25][26][27] are used for branch determination.…”

“…Although these techniques may be very sensitive to the presence of branching, they only provide qualitative insight into the degree of branching. Alternatively, 13 C NMR spectroscopy is one of the few methods capable of providing quantitative values of the branch content, albeit averaged over all molecules present. [14,15] The typical implementation of NMR undertakes analysis of the sample in the solution-state: however, when applied to polyethylene a number of problems arise.…”

Thermal and mechanical analysis of metallocene‐catalyzed ethene–α‐olefin copolymers: The influence of the length and number of the crystallizing side chains

“…5 Segregation studies by stepwise annealing with differential scanning calorimetry (DSC) have been applied to different types of Z-N- [6][7][8][9][10] and metallocene-catalyzed ethene copolymers. [8][9][10][11][12] The results show that differences in the ethylene sequence distributions can be seen when the polymers are annealed in steps at successively lower temperatures. The multiple endothermic peaks observed in the melting curves after this treatment correspond to a fraction of segregated molecules that crystallized at a certain temperature.…”

Effect of Branch Length on 13C NMR Relaxation Properties in Molten Poly[ethylene‐co‐(α‐olefin)] Model Systems

“…One of the parameters that determines the macroscopic properties of polyethylene is the length of the branches along the backbone, with short chain branching (SCB) and long chain branching (LCB) influencing the solid [1,2] and melt-state [3,4] properties and processability of the polymer. While 13 C NMR allows accurate quantification of the number of branch points, it is of limited use for the determination of branch length.…”