Microstructure and properties of poly(urethane-siloxane)s based on hyperbranched polyester of the fourth pseudo generation

Poly(urethane-siloxane) networks based on hydroxyethoxy propyl terminated poly(dimethylsiloxane) (PDMS) as the soft segment and 4,4'-methylenediphenyl diisocyanate (MDI) and two hyperbranched polyesters with different core as the hard segments were characterized by swelling experiments, thermal analyses (DSC and TG), thermomechanical analysis (DMTA), X-ray scattering studies, SEM and AFM analyses, water contact angle and water absorption measurements, as well as surface free energy determination. From these studies, structure-property relationships were elucidated. Hyperbranched polyesters based on 2,2-bis(hydroxymethyl)propionic acid and ethoxylated pentaerythritol or di-trimethylolpropane as core (BH-40 and HBP-4) were used as crosslinkers for the samples of different series. Both series are composed of samples having different PDMS (i.e., soft segment) content. The crosslinking density and extent of hydrogen bonding showed an influence on the polyurethane (PU) properties. It was found that higher crosslinking density and better thermal stability of PUs based on BH-40 compared to HBP-4 based PUs are due to the less dense structure of BH-40. DMTA experiments revealed that the networks exhibit two glass transition temperatures, of the soft and hard segments, and one secondary relaxation process. The crosslinking density and extent of the microphase separation increased and thermomechanical properties were improved with decreasing content of PDMS. With increasing PDMS content, the surface of the polyurethane networks became more hydrophobic, the surface free energy decreased and thermal stability was improved. The obtained results revealed that synthesized PUs have good thermal and thermomechanical properties, which can be tailored for the potential use in the coating technology by changing the type of hyperbranched polyester or PDMS content