Dielectric relaxation analysis of hybrid acrylic–polyurethane gels

In this paper we present a dielectric relaxation analysis of three acrylic–polyurethane gel samples in connection with their synthesis conditions. The dielectric analysis was carried out using broadband dielectric spectroscopy (BDS). Additionally, differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) measurements were performed. The thermodynamic characterization outcomes evidence very similar characteristics of the glass transition phenomena among the investigated samples without any signature of melting or crystallization. X-ray diffraction patterns evidence distinctive features for the three gel samples in two different regions, i.e., at high and low values of the scattering vector q. BDS measurements, identify three distinct dielectric relaxation processes above Tg. Particularly, differences in the dielectric parameters of the slowest relaxation process have been rationalized in connection with the synthesis method used. Our results show that the analysis of these acrylic–polyurethane gels by BDS – in combination with SAXS – provides access to characteristics of the network structure that would ultimately influence the properties of these materials.The authors gratefully acknowledge the financial support of the Basque Government project codes: IT-654-13 and nanoIKER2011-2013 (Etortek program) and also the Spanish Ministerio de Economía y competitividad and EU, project codes: MAT2012-31088 and MAT2015-63704-P.Peer reviewe

MoS₂ Nanoplatelet Fillers for Enhancement of the Properties of Waterborne Pressure-Sensitive Adhesives

Nanocomposite pressure-sensitive adhesives (PSAs) composed of polyurethane (PU)/(meth)­acrylates reinforced with MoS₂ nanoplatelets were prepared by blending aqueous dispersions. MoS₂ crystals were exfoliated by sonication in water in the presence of poly­(vinylpyrrolidone) (PVP, molecular weight of 10 000 g mol^–1) to prepare an aqueous dispersion. Waterborne colloidal polymer particles (latex) were synthesized by miniemulsion photopolymerization in a continuous tubular reactor. The adhesive and mechanical properties from the resulting nanocomposite films were determined as the MoS₂ fraction was increased. A superior balance of viscoelastic properties was achieved with 0.25 wt % loading of the MoS₂ nanoplatelets, leading to a tack adhesion energy that was three times greater than that for the original PSA

High-Performance Adhesives Resulting from Spontaneous Formation of Nanogels within Miniemulsion Particles

Molecular structure plays a crucial role in determining the final properties of pressure-sensitive adhesives. Here, we demonstrate that the molecular structure of polyurethane/(meth)­acrylic hybrids synthesized by miniemulsion photopolymerization changes during storage of the dispersion at room temperature because of the spontaneous formation of nanogels by the assembly of polymer chains within the polymer particles. Analysis of the nanogel structure by asymmetric-flow field-flow fractionation allows identification of the molecular structure that provides the unusual combination of high tack adhesion and excellent shear resistance at high temperature [maximum value of the shear-adhesion failure temperature (SAFT) test, >210 °C]