Amandaa K. Brewer, PhD, Tosoh Bioscience LLC
For polymeric materials the molar mass and molar mass distribution play a vital role in the determination of mechanical, bulk,
and solution properties. These properties govern polymer processing and the end-use performance of a given material (1,2).
Unlike small molecules, which have discrete molar mass distributions, synthetic polymers are typically composed of hundreds
to thousands of chains of different molar mass that result in a distinctive molar mass distribution. The shape and breadth
of a polymer's molar mass distribution will depend on the mechanism, kinetics and condition of the polymerization, and will
dictate the end-use properties of the polymer. Polymer properties such as hardness, tear strength, impact resistances, wear,
brittleness, toughness, tackiness, etc., are important in determining the successes or failure of a given material.
One polymeric material of particular interest to the automotive industry is an alloyed grade thermoplastic: polycarbonate
acrylonitrile-butadiene-styrene (PC/ABS). High-impact and heat-resistant grades of PC/ABS alloys are used in the instrument
panels, armrests, interior trim panels, seatbelt retainers, glove compartment doors, and lift gates of automobiles, while
plating grades of PC/ABS are used in wheel covers, grilles, headlight bezels, mirror housings, and decorative trim (3). The
numerous uses of PC/ABS in the automotive industry makes the characterization of the PC/ABS essential for the determination
of whether an end-use material will be successful or not. The physical and chemical properties of PC/ABS resins are traditionally
analyzed by techniques such as, infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermogravometric
analysis (TGA), gel permeation chromatography (GPC), and melt flow (4). Each of these techniques provides various details
about the PC/ABS material being used to form the given automotive part, e.g. FT-IR, DSC and TGA all provide information about
crosslinking, while GPC (depending on the detection methods implemented) can provide information regarding molar mass, molar
mass distribution, and polymeric size.
The difference between a successful and unsuccessful polymer based material can be determined by observing the molar mass
and molar mass distribution of the polymer(s) encompassing the end-use material. Here we have implemented the use of the EcoSEC®
GPC System encompassing dual flow refractive index (RI) and UV detectors to perform failure analysis on two PC/ABS automobile
parts. The use of GPC for the failure analysis allowed for determination of the molar mass averages, molar mass distributions,
and a comparison of successful and unsuccessful PC/ABS automobile parts.
PC/ABS samples were prepared by dissolving shaved off portions of the sample in tetrahydrofuran (THF) (Fisher Chemical) for
a final sample concentration of 1.0 g/L. Samples were shaken manually, allowed to sit overnight, and filtered using a 0.45
μm PTFE syringe filter (Acrodisc) before analysis was performed. Sample analysis was performed on a system consisting of an
EcoSEC GPC System (HLC-8320) (Tosoh Bioscience LLC, King of Prussia, PA) equipped with RI and UV detectors. The UV absorbance
was monitored at a wavelength of 254 nm. Separation of filtered 10 μL injections occurred over a column bank consisting of
two 4.6 mm i.d. × 15 cm, 4 μm particle size TSKgel® SuperMultiporeHZ-M columns (exclusion limit 1,000,000 g/mol) proceeded
by the appropriate guard column (Tosoh Bioscience LLC). The mobile phase was THF at a flow rate of 0.35 mL/min. Detector,
pump oven, and column oven were maintained at 35 °C.
For all chromatographic determinations, results are averages of three injections from two separate sample dissolutions. Data
was processed with the EcoSEC GPC Workstation software, version 1.08. The molar mass and molar mass distribution of the PC/ABS
samples was determined based on a polystyrene relative calibration curve created from a PStQuick MP-M polystyrene mix standard
(Tosoh Bioscience LLC) ranging in molar mass from 530 to 780,000 g/mol under the same experimental conditions as sample analysis.
Calibration curve data was fitted with a linear function and error values were less than 1%.