Rheology and Photo-Cross-Linking of Thiol−Ene Polymers

Abstract

A dynamic rheological technique, Fourier transform mechanical spectroscopy (FTMS), was used to monitor in real time the evolving rheological properties during UV cross-linking of two thiol−ene systems. These systems comprised a trifunctional thiol (trimethylolpropane tris(2-mercaptoacetate)) together with a trifunctional allyl monomer (triallyl isocyanurate) and a tetrafunctional thiol (pentaerythritol tetrakis(2-mercaptoacetate)) with the same allyl monomer. FTMS, in conjunction with specially designed quartz plates, provided an in situ method to elucidate the effects of temperature and monomer functionality on the photoinitiated polymerization of these systems. It was found that the tetrafunctional thiol system cross-linked at a faster rate than the trifunctional thiol system over the temperature range (25−50 °C) studied. Moreover, increasing the temperature increased the cross-linking rates for both systems. The Winter−Chambon criterion was applied to determine the gel point and the two parameters which characterize the material at its gel point, the gel stiffness, S, and the relaxation exponent, n. The gel stiffness was found to be greater for the trifunctional thiol system, which was consistent with the higher value of conversion calculated from the Flory−Stockmayer theory of gelation. Relaxation exponents of 0.80 and 0.81−0.82 were determined for the tri- and tetrafunctional thiol systems, respectively, indicating similar fractal structures at the gel point. These relaxation exponents were also invariant over the temperature ranges studied, suggesting that the cross-linking mechanisms remained unchanged with temperature. From the temperature dependence of the gel times, apparent activation energies of 6.6 and 14 kcal/mol were calculated for the tri- and tetrafunctional thiol systems, respectively.

Photopolymerized Injectable Water-soluble Maleilated Chitosan/ Poly(ethylene glycol) Diacrylate Hydrogels as Potential Tissue Engineering Scaffolds

“…In situ dynamic photorheology [18] was used to measure the elastic and viscous moduli during photopolymerization. A Haake Mars Rheometers (Thermo Fisher Scientific Inc.) equipped with a UV curing attachment and 20 mm parallel plate geometry was used to characterize the phtocrosslinking kinetics.…”

Imprint Molding of a Microfluidic Optical Cell on Thermoplastics with Reduced Surface Roughness for the Detection of Copper Ions

“…Figure 1(g) shows some components anticipated to be contained in the NOA 61 prepolymer, which include vinyl, thiol, and ester functionalities. 23 When the NOA 61 prepolymer was exposed under UV light for 2 h, chemical reactions took place, possibly resulting in the formation of vinyl polymers (-CH2-CH2-CH2-CH2-) 24,25 and chemical bonds between vinyl and thiol functionalities (-CH2-CH2-S-) [25][26][27][28][29] as intermediate components of partially cured NOA 61 polymer, as shown in Fig. 1(h).…”

Modeling the Kinetics of Enhanced Photo-Polymerization under a Collimated and a Reflecting Focused UV Laser

“…Experimental crossover time defines the time required to reach the gel point which is defined by the storage-loss moduli of the polymer [4,[18][19][20]. However, a theoretical crossover time (T*) has not been defined previously.…”