Molecular weight distribution of polyamides by GPC-viscometry. A comparison between high temperature and low temperature eluants

Synthetic polyamides (PA) owe many outstanding properties to their semicris talline morphology and to the intermolecular hydrogen bonding of the amide groups. Although the strength of the Van der Waals network confers an excep tional solvent resistance to this class of polymers, it may lead to serious difficulties in their solution characterization, such as the determination of molecular weight distribution (MWD). Over the years, a large number of dissolution schemes have been developed for the GPC characterization of PA. A recent IUPAC round-robin test on the GPC of aliphatic PAs nevertheless s howed large variations in the experimental data not only between laboratori es employing different methods, but also between laboratories adopting simi lar protocols. The purpose of the present investigation is to understand th e origins of these disparities by comparing GPC-Viscometry data obtained wi th some frequently used eluant systems: a) benzyl alcohol at high temperature; b) hexafluoro-isopropanol, pure or mixed with dichloromethane, at low tempe rature; c) common GPC solvents (THF, CH2Cl2,) after N-triflouracetylation. of the P A. Apart from pure HFIP, all the methods tested show consistent MW results whe n measured in the same laboratory. Viscometry data reveal, however, that th e expected linear viscosity law is obeyed only in benzyl alcohol. With the other eluants, significant departure from the Mark-Houwink relation denotes presence of non-size exclusion effects, rendering data evaluation imprecis e with the Universal Calibration. The problems are particularly acute for p olyamides with short methylene sequences which can only be dissolved in mix tures containing a high concentration of ME To improve accuracy, it is prop osed that GPC calibration for this class of polymer be performed with absol ute detection (on-line viscometer, light scattering) or with a broad MWD st andard of identical chemical composition to the analyzed sample.