Fractionation of poly(ethylene-co-vinyl acetate) in supercritical propylene: Towards a molecular understanding of a complex macromolecule

Abstract

A commercial low-density polyethylene copolymer, poly(ethylene-co-vinyl acetate) (EVA), synthesized via the high-pressure free-radical polymerization process, was fractionated with supercritical propylene by isothermal increasing pressure profiling and critical, isobaric, temperature rising elution fractionation (CITREF TM ). Extensive characterization of the fractions by nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC) in combination with low-angle laser light scattering (LALLS), and differential scanning calorimetry (DSC) was used to map not only the molecular-weight and chemical composition distributions of the parent copolymer, but also its short-chain branch (SCB) and long-chain branch (LCB) distributions. Fractionation by increasing pressure profiling confirmed the broad molecularweight distribution and the narrow acetate-branch distribution expected for this random copolymer but revealed the presence of a small amount ( Ç 2 wt %) of low molecularweight amorphous species containing a high level of alkyl SCBs (80 branches/1000 C). The LCB density estimated from the Zimm-Stockmayer relationship using the GPC data monotonically increases with increasing molecular weight above 60,000 g/ mol, in agreement with the kinetics of free-radical polymerization. CITREF TM was found to fractionate this copolymer by crystallinity, which is influenced by both the alkyl SCBs and the acetate branches. Up to 18% difference in total branch density (õ5% in crystallinity) between EVA molecules was identified using CITREF

Spectroscopic Measurement of Solute and Cosolvent Partitioning between Supercritical CO2 and Polymers

“…Supercritical fluids have been used for a number of polymer processes including polymer fractionation, [1][2][3][4] extraction of molecules from polymer matrices, [5][6][7][8] and impregnation of polymers with additives. [9][10][11][12] An advantage of a supercritical fluid is that its solvent strength can be adjusted with pressure, which enables tuning of the selectivity and degree of extraction and the degree of loading during an impregnation process.…”

Branched polyethylenes fractionated in supercritical propane

“…There has been growing interest in Super Critical Fluids (SCF) to fractionate polymers with respect to molar mass, 11 architectures (branching degree), and chemical composition. 12 The same physico-chemical principle active in liquid solvents (decreasing solubility of the polymer with increasing molar mass) exists with supercritical fluid solvents, but an additional separation factor derives from their pressure-dependent dissolving power: At some low pressure (specific for each polymer) only low molar mass chains (of a homologous series) will be dissolved. By increasing the extraction pressure in a sequential stepwise manner narrow fractions of increasing molar masses are obtained.…”

Observations on HDPE Characterization with a Microcalorimeter as a Complementary Tool to TREF and CRYSTAF

“…Experimental conditions for TREF (Soares and Hamielec, 1999), CRYSTAF (Monrabal, 1996) and CITREF (Folie et al, 1997) are described elsewhere. Microcalorimetric measurements were obtained using two different cooling rates of l.8°C/h (as normally used in TREF) and 6.0°C/h (as normally used in CRYSTAF), followed by the same heating rate of 12.0°C/h.…”

“…An important advantage of this technique is that large polymer fractions can be obtained without generating hazardous liquid organic solvent waste (Britto et al, 1999b;Folie et al, 1997). Several versions of differential scanning calorimetry (DSC) proposed recently for analyzing polyolefins appear to be complementary to TREF and CRYST AF (Parikh et al, 1993;Starck, 1996).…”