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
- 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
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.
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.
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.