Synthesis and swelling behavior of polyurethane networks based on hyperbranched polymer
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A series of six polyurethane (PU) networks was synthesized from Boltorn (R) hydroxy-functional hyperbranched polyester (HBP) of the second pseudo generation as a cross-linking agent, alpha,omega-dihydroxy-(ethylene oxide-poly(dimethylsiloxane)-ethylene oxide) (EO-PDMS-EO) and 4,4-methylenediphenyl diisocyanate, by two-step polymerization in solution. Each sample of the prepared PUs had different EO-PDMS-EO content. The chemical structure of the synthesized networks was analyzed by FTIR spectroscopy. The influence of the EO-PDMS-EO content and type of the solvent on the swelling behavior of the PUs in 2-propanol and toluene was investigated. During the swelling measurements, a certain amount of sal fractions was extracted from the PUs by solvents. According to the H-1-NMR results, sal fractions were mainly composed of the soluble hyperbranched PU, formed during the polymerization by partial modification of the end hydroxyl groups of HBP with NCO-terminated prepolymer synthesized in the ...first step of the reaction. Chains of the so reacted prepolymer can then fold back and form cyclic products by reaction of the free -NCO group from the NCO-terminated prepolymer with free -OH group of HBP. As the EO-PDMS-EO content increases the amount of the sol fractions and swelling degree also increased, indicating that networks with lower EO-PDMS-EO content are more cross-linked and have higher solvent resistance.