Assessing the Suitability of a Green Solvent for GPC and TREF Analyses of Polyethylene

May 01, 2014

The study presents results of gel permeation chromatography (GPC) and analytical temperature rising elution fractionation (TREF) of polyethylene in butylal (dibutoxymethane, a halogen-free solvent). A comparison is made with the results obtained using 1,2,4-trichlorobenzene showing that changing the solvent to butylal does not require any modification of the existing GPC instruments and has additional advantages such as lower toxicity, increased detector signal for polystyrene standards, and broadening domain of crystallization temperatures for TREF.

The molecular structure of polyethylene (PE) is usually analyzed by two techniques: gel permeation chromatography (GPC) and temperature rising elution fractionation (TREF).

GPC separates the macromolecules according to their hydrodynamic volume and allows for a measurement of the molecular weight distribution (MWD). TREF separates semicrystalline polymers as a function of their melting temperature in solution and allows for the evaluation of short chain branching distribution (SCBD).

In both cases, the samples are analyzed in dilute solutions obtained in chlorinated solvents as dichlorobenzene or trichlorobenzene. Usually, the polymer is separated in fractions using a set of columns and the concentrations of the eluted fractions are measured with differential refractive index (dRI) detection. The resulting chromatograms are converted in MWD or SCBD by calibrating the method with standards.

The solvent 1,2,4-trichlorobenzene (TCB) is frequently used for these analyses, mainly because of the following properties:

  • Its boiling point is 214 °C, allowing the melting and dissolution of all types of polyethylenes.
  • The differential index of refraction (dn/dc) of polyethylene solutions is -0.1 mL/g, which gives good signals in dRI detection for injected concentrations as low as 1 mg/mL.
  • Polystyrene is soluble in TCB, which offers the possibility to use the polystyrene–divinylbenzene gel columns and calibrate the GPC systems with narrow standards.

However, TCB also has some disadvantages:

  • Its freezing point is 17 °C, close to room temperature, so precautions have to be taken to avoid TCB crystallization, which can damage the GPC column gel and GPC detectors.
  • Polystyrene solutions have a dn/dc of 0.05 mL/g, which renders the GPC calibration difficult, especially for low molecular weights of standards obtained by butyl lithium induced anionic polymerization, in which the residual butyl part of initiator with a high negative dn/dc further diminishes the peak intensity.
  • It has a relatively high toxicity, causes skin irritation, and is labeled as very toxic to aquatic life with long-lasting effects.

Because of environmental concerns there is a continuous trend toward the replacement of chlorinated solvents with solvents that have a lower toxicity, mostly in the cleaning industry (1). In a recent application patent (2), acetals were described as a valuable replacement for perchlorethylene or trichloroethylene for dry cleaning of textile, leather, or fur goods. In addition to dissolving a large spectrum of compounds, acetals produced by reactions between alcohols and aldehydes can be prepared from renewable or potentially renewable alcohols. With regard to their toxicity, only a few studies are available, but acetals are expected to show very good health and environment profiles and are also biodegradable.

In this study, we show the potential of using butylal in GPC and TREF analyses of polyethylenes. We chose this acetal because of its high boiling point (180 °C), low volatility, and ability to dissolve polystyrene. The evaluation was performed by comparing the results obtained with TCB and butylal on the same samples with the same GPC and analytical TREF systems and methods.

Experimental

Solvents

We used 1,2,4-trichlorobenzene (TCB, Spectropure dry, Biosolve Chimie) and dibutoxymethane (butylal, ultra-pure grade, Lambiotte & Cie, CAS 2568-90-3) for GPC and analytical TREF analyses.

Samples

Polyethylene standard (SRM 1475a, molecular weight [MW] = 52,000 g/mol, polydispersity = 2.9, National Institute of Standards and Technology [NIST]) and a narrow distribution polystyrene standard (MW = 135,000 g/mol, polydispersity = 1.1, Agilent Technologies) were analyzed by GPC. Four commercial metallocene polyethylenes with narrow short-chain branching distributions and the following densities were analyzed by analytical TREF: 0.923, 0.934, 0.947, and 0.955 g/cm3.