Structure-Thermomechanical Property Correlations of Highly Branched Siloxane-Urethane Networks Someshwarnath Pandey, Sangram K. Rath, and Asit. B. Samui* Naval Materials Research Laboratory, Shil-Badlapur Road, Addl. Ambernath, Ambernath 421506, Maharastra, India * S Supporting Information ABSTRACT: A series of B 3 core-terminated highly branched siloxane-urethane polymers was synthesized through the A 2 +B 3 route. Isophorone diisocyanate (IPDI)-terminated polydimethylsiloxane (PDMS) was used as the A 2 unit and triethanol amine as the B 3 core. Size exclusion chromatography (SEC) studies revealed decreasing number-average molecular weights for the branched polymers and increasing tendency toward lower molecular weight species formation with increased proportion of B 3 core in the branched polymers. The degree of branching and fraction of dendritic units, evaluated from 1 H NMR, increased monotonically with increasing B 3 core in the branched polymers. Cross-linked networks of the highly branched polymers were prepared by reaction of the terminal hydroxyl groups with tetraethoxysilane (TEOS) at room temperature. The sol fractions obtained for the networks from solvent extraction studies were consistent with the non-network-forming low molecular weight fractions obtained from the deconvoluted SEC traces. The solubility parameter, Flory-Huggins interaction parameter, and cross- link density of the networks were evaluated from swelling studies. FTIR spectroscopy was used to evaluate the degree of hydrogen bonding of the branched networks. The thermomechanical properties of the networks were evaluated by stress-strain measurements and dynamic mechanical analysis, and the results were correlated with the structural parameters, such as degree of branching, extent of hydrogen bonding, and cross-link density. ■ INTRODUCTION Hyperbranched polymers represent a class of highly branched soluble macromolecules, which has attracted a lot of attention during the past decade. This has resulted in development of several new and simpler synthetic approaches for their preparation. 1-7 Branched polymers containing urethane or urea groups within the backbone are well established as precursors for various polyurethane (PU) resins, foams, and coatings and thus have high industrial importance. 8-13 In general, isocyanate chemistry is thoroughly studied for polymers due to the high versatility and potential to tailor material characteristics by varying the structure of the monomers, monomer combinations, as well as morphology and branch point density control. Hydrogen bonding plays a critical role in this class of materials, and these noncovalent secondary interactions often significantly influence the material properties. 14,15 Spindler and Fré chet first reported the preparation of high molecular weight hyperbranched polyurethanes using AB 2 -type monomers containing a hydroxyl (A) and two blocked isocyanate groups (B 2 ). 16 Kumar et al. also used an AB 2 -type monomer and reported the preparation of fully aromatic hyperbranched polyurethane from 3,5-dihydroxybenzoylazides using Curtius-type rearrangement reactions. 17 Since the AB x approach necessitates rather innovative chemistry approaches due to the high reactivity of the NCO group, researchers resorted to the A 2 +B y approach for hyperbranched polyureas and polyurethanes to simplify the process and facilitate work with more commonly available monomers. Thus, hyper- branched polyurethane polyols and polyisocyanates could be synthesized based on conventional raw materials, exploiting the differences in reactivity in an A 2 + CB 2 or AA′ + CB 2 (or AA′ + B′B 2 ) approach. 18-20 On the basis of the present work, we discuss certain literature reports on the A 2 +B 3 approach to prepare hyperbranched polyurethanes. Sheth et al. reported the preparation of segmented hyperbranched polyurethanes by an oligomeric A 2 +B 3 approach. 21 They used diisocyanate-capped polyethylene oxide or polypropylene oxide as the A 2 units, while oligomeric triamines were used as the B 3 core. The resulting products with degree of branching in the range 30- 50% showed microphase-separated morphologies and mechan- ical properties close to their linear analogues. Unal et al. demonstrated the preparation of segmented, hyperbranched polyurethaneureas using the same approach, 22 where A 2 was an isocyanate end-capped polyether glycol, such as poly- (tetramethylene oxide) glycol (PTMO), and B 3 was an aliphatic triamine. They reported the importance of slow addition of A 2 units onto the B 3 core to control the gelation, structural regularity, and minimization of cyclic species in preparation of branched polyurethanes. Oguz et al. reported the structure development in hyperbranched polymers prepared by the oligomeric method through experimental studies and Monte Carlo simulations. 23 They observed a strong influence of solution concentration on the gel point and the extent of cyclization on the polymers formed. Although there are several such reports on synthesis and characterization of hyper- branched or highly branched polyurethanes and polyureas, studies on further use of end-functionalized polymers to prepare cross-linked networks are limited. Further, in all these Received: April 2, 2011 Revised: February 1, 2012 Accepted: February 1, 2012 Published: February 1, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 3531 dx.doi.org/10.1021/ie200629w | Ind. Eng. Chem. Res. 2012, 51, 3531-3540