Influence of Polyol/Crosslinker Blend Composition on Phase Separation and Thermo-Mechanical Properties of Polyurethane Thin Films

Abstract

Polyurethanes (PUs) from Polyethylene glycol (PEG) and polycaprolactone diol (PCL) and a crosslinker, Pentaerythritol (PE), were synthetized with isophorone diisocyanate (IPDI). In this study, we investigated the effect of polyol and crosslinker composition on phase separation and thermo-mechanical properties. The properties were studied through dynamic mechanical analysis, X-ray scattering, atomic force microscopy (AFM), and thermogravimetric analysis (TGA). The results showed changes in PUs properties, microphase structure, and separation due to the composition of polyol/crosslinker blend. So, the largest concentration of PE produced multimodal loss factor patterns, indicating segment segregation while PUs with a PEG/PCL = 1 displayed a monomodal loss factor pattern, indicating a homogeneously distributed microphase separation. Additionally, the increase of the PEG concentration enhanced the damping capacity. On the other hand, agglomeration and thread-like structures of hard segments (HS) were observed through AFM. Finally, the thermal behavior of PUs was affected by chemical composition. Lower concentration of PE reduced the crosslinking; hence, the temperature with the maximum degradation rate.Polymers 2020, 12, 666 2 of 13 separation can lead to property changes because SS are responsible for the softness, flexibility, and rubbery behavior, while HS are related to stiffness and mechanical behavior. The elastomeric behavior of PUs results from embedding HS in SS, where the HS acts as crosslinker due to hydrogen bonding [5]. Hence, the segregation of HS and SS mediate PU properties. Aforesaid segregation can be studied by several microscopic, spectroscopic and crystallographic techniques like Atomic force microscopy (AFM), infrared spectroscopy (IR), both small (SAXS) and wide (WAXS) angle X-ray scattering.Previous research has described this segregation as a function of the composition and chemical structure of monomers, and its effect on PU properties. e.g., Klinedinst, D. et al. [4] studied the effects of varying the SS molecular weight and overall HS content thermoplastic segmented PUs. They used AFM and dynamic mechanical analysis (DMA) to identify a thread-like microphase separated structure in chain-extended systems. A higher polyol molecular weight increased the partial crystallization of SS at lower temperatures than room temperature, and larger SS/HS incompatibility, which induced greater microphase separation and a larger storage modulus plateau magnitude. The larger HS mass also increased the temperature at which HS melting occurred, broadening the storage modulus plateau.Some works have focused on the study of the chain extender or crosslinker structure. Like, Kim, H-N., et al.[1] who compared PU microphases with three chain extenders: isosorbide (ISB), isomannide (IMN), and 1,4-butanediol (BD). According to the SAXS results, the scattering widths of IMN and ISB-based PUs were larger than those of BD-based PUs, indicating that the HS domain sizes in the IMN and ISB-based PUs were smaller. It als...

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