Dynamic Mechanical Behavior of Ordered Off-Stoichiometric Polyurethane Systems at the Gel Point Threshold

Abstract

Dynamic mechanical and thermal behavior of ordered off-stoichiometric polyurethane (PU) systems, before and after the gel point, based on the mesogenic diol 6,6′;-[ethylenebis(1,4-phenyleneoxy)]dihexan-1-ol (D),2(4)-methyl-1,3-phenylene diisocyanate (DI), and poly(oxypropylene)triol (T) were studied. Polymer samples were prepared at various initial molar ratios of the reactive groups, r = [OH]_T/[NCO]_DI/[OH]_D, ranging from 1/10/9 to 5/10/9 (the ratio [NCO]_DI/[OH]_D = 10/9 was constant); the total mole ratio of hydroxy (OH) and isocyanate (NCO) groups, rOH = [OH]/[NCO] = ([OH]_D)/[NCO]_DI,changed from 1 to 1.4. Dynamic mechanical measurements during the curing reaction showed that the power law parameters that characterize the critical gel state (gel strength S and relaxation exponent n) are dependent on the initial composition (the ratio r _OH). The gel-point critical ratio of reactive groups r^c _OH, found during curing in the ordered state of the diol (at low curing temperature), has revealed that the critical gel (CG) structure is determined by a contribution of strong physical interactions as well as chemical junctions and does not correspond to pure chemical gelation (CG structure formed at low temperature exhibits flow at elevated temperatures in the isotropic state). This fact suggests that formation of the mesophase enhances the connectivity of the molecular structure at the gel point. Dynamic mechanical behavior of fully cured chemical networks (r _OH < r _OH ^c ) and un-cross-linked (r _OH > r _OH ^c ) samples (and a CG sample) has also been investigated. Decreasing the r_OH ratio (increasing concentration of chemical cross-links in the systems) inhibits conformational rearrangements required for ordering; at the same time, the intensity of the slow relaxation process in the rubbery region decreases.

Keywords:

• Dynamic modulus
• Frequency-temperature superposition
• Gel point
• Polyurethanes
• Power law behavior
• Sol-gel transition

ACKNOWLEDGMENT

Financial support of the grant agency of the Academy of Sciences of the Czech Republic (A4112901/1999), the grant agency of Charles University (grant GAUK 46/1998/B), and the Ministry of Education of the Czech Republic (project VZ 1132000) is gratefully acknowledged.