Thermal and Mechanical Properties of Cross-Linked Photopolymers Based on Multifunctional Thiol−Urethane Ene Monomers

Abstract

Urethane-based multiene monomers were synthesized and photopolymerized with a trifunctional thiol monomer to form highly cross-linked thiol−ene networks. Real-time FTIR was used to monitor the conversion of thiol and urethane ene monomers as a function of irradiation time. For stoichiometric thiol−urethane ene photopolymerizations, monomers reacted in a 1:1 molar functional group ratio, reaching ∼90% monomer conversion within several seconds. The effects of the ene chemical structure and concentration on thermal and mechanical properties were characterized by DMA, DSC, TGA, tensile, and energy absorption upon nondestructive impact. The temperature at tan δmax of the thiol−urethane ene networks was around 39 °C and decreased to lower temperatures with the addition of a reactive diluent diallyl ether ene monomer. Tensile and impact results were combined with fracture toughness measurements to elucidate the effect of the urethane and bisphenol A chemical structures. The energy absorption was dependent on the glass transition temperature of the thiol−urethane ene cross-linked networks. Improvement in fracture toughness and tensile properties was observed with the incorporation of 10 mol % of an ene with a bisphenol A structure. Scanning electron micrographs of fractured surfaces were used to interpret the nature of the brittle fracture. All of the properties of the thiol−urethane ene networks were compared to a conventional trithiol−triallyltriazine trione-based matrix.

Thio-urethane oligomers improve the properties of light-cured resin cements

“…The stress reduction potential of thiolfunctionalized materials and the toughening potential of thio-urethane networks have been well documented in the literature [13,14]. In this study, the kinetics of polymerization was used as a way to assess network formation and final degree of conversion for methacrylates modified with thio-urethane oligomers.…”

“…In general, the use of thiourethane- modified materials not only increased the degree of conversion in relation to the control, but was also able to reduce the rate of polymerization and delay vitrification for selected compositions. Chain-transfer reactions also likely contributed to the formation of more homogeneous networks, with decreased internal stresses [13], as will be discussed later. Assuming that the thiol concentration and the molecular weight of the oligomer were similar in all groups (which was verified in a pilot study using Ellman’s reagent titrations and gel permeation chromatography), it can be inferred that the effect on conversion and network formation was dependent on the thio-urethane structure, as well as on the secondary matrix to which they were added.…”

“…As an alternative to conventional thiol-enes or thiol-methacrylates, others have proposed the use of thio-urethane networks in applications where mechanical properties in general, but more specifically toughness and resistance to impact, are desirable [13,14]. Some studies have demonstrated the more homogeneous nature of thiourethane networks compared to the simple urethane counterparts, as well as the increased toughness values [13,14].…”

“…Some studies have demonstrated the more homogeneous nature of thiourethane networks compared to the simple urethane counterparts, as well as the increased toughness values [13,14]. In those studies, thiols are combined with isocyanates in situ , in a reaction catalyzed by a base.…”

Preparation of Novel Hydrolyzing Urethane Modified Thiol-Ene Networks

“…Urethane bonds form quickly through the reaction of isocyanate (NCO) and alcohol (OH) functional groups to produce relatively inexpensive and durable coatings, having strong mechanical properties, good abrasion resistance, and thermal stability [21]. Recent efforts of the Hoyle research group [22][23][24] demonstrate that the modification of thiol-ene networks with urethane segments may result in the improvement of selected polymer network properties specifically imparting impact adsorption, elastic properties and fracture toughness [22,24]. Polyisocyanates can be end-capped by reacting with ene containing alcohols, resulting in urethane bridged oligomers that may be efficiently polymerized with thiols.…”