Enthalpy Relaxation of Photopolymerized Thiol−Ene Networks: Structural Effects

Abstract

Physical aging behavior of photopolymerized thiol-ene networks was investigated by measuring the extent of enthalpy relaxation in terms of network density and molecular structure. The homogeneous network structure of the thiol-enes, having narrow glass transition temperature ranges, showed characteristic temperature and time dependency relationships for enthalpy relaxation. All thiol-ene films annealed at different temperatures (T a ) for 1 h according to the isochronal method showed maximum enthalpy relaxation peaks at approximately T g -10°C by DSC. The extent of enthalpy relaxation as a function of annealing time (t a ) was obtained by the isothermal aging method. Correlations between the extent of enthalpy relaxation and the heat capacity difference at T g were made and related to thiol-ene chemical group rigidity and network linking density. Pendulum hardness values for a selected thiol-ene film showed a clear change in hardness upon aging, indicating sub-T g mechanical relaxation, consistent with the related enthalpy relaxation process.

End Group Reactions of RAFT-Prepared (Co)Polymers

“…The reaction under base or nucleophilic catalysis is also accurately termed a conjugate addition or thiol-Michael reaction and is limited to activated substrates. [132] Of these general reactions, the radical thiol-ene process has been the most extensively used in the polymer and materials fields and most notably as a highly efficient method for preparing near-perfect networks and films, as exemplified by the work of Hoyle and coworkers [133][134][135][136][137][138][139][140][141] and Bowman et al [142][143][144][145][146][147][148] Recently, the thiol-ene reaction has attracted researchers in other areas of synthesis owing, in part, to the recognition of its click [149,150] characteristics. Desirable features associated with the thiol-ene reaction include: (1) hydrothiolation can proceed under a variety of conditions, including via a radical pathway and via catalytic pathways mediated by nucleophiles, bases and acids; (2) a wide range of enes serve as compatible substrates; (3) virtually any thiol can be employed, including highly functional species; (4) these reactions can be extremely rapid and are generally tolerant of the presence of oxygen and moisture.…”

Heating scans of stretched polystyrene films with stable baselines: Preparation method of samples for thermal analysis

“…The structural relaxation study is concerned with the slow evolution of thermodynamic properties toward the equilibrium, where the effect of physical ageing is frequently observed using differential scanning calorimetry (DSC) technique with the measurement of enthalpy variation as an actual thermodynamic quantity. Polymers with confined or constrained situations have recently attracted considerable attention regarding the physical ageing; including, layered glassy films [1,2], composites with nanoparticle [3,4], and polymer networks with different degrees of cross-linking [5].…”

Enthalpy relaxation of photopolymerized multilayered thiol‐ene films

“…[6][7][8][9][10][11][12][13] Uniform and highly dense network structure results in narrow glass transition temperature (FWHM < 20 C) and very high damping factor (tan d < 1.6) due to probably narrow distribution of relaxation times. 15,16 The overall relaxation is significantly affected by chemical structural parameters such as network uniformity, crosslink density, rigidity, bulky side groups, and hydrogen bonding. 15,16 The overall relaxation is significantly affected by chemical structural parameters such as network uniformity, crosslink density, rigidity, bulky side groups, and hydrogen bonding.…”