Structural Evolution of Dimethacrylate Networks Studied by Dielectric Spectroscopy

Abstract

In this work, the structural heterogeneity of copolymers formed by photopolymerizations of diethylene glycol dimethacrylate (DEGDMA) and poly(ethylene glycol 600) dimethacrylate (PEG600DMA) has been characterized as a function of double bond conversion and cross-linking density (as measured by comonomer composition) by dielectric analysis (DEA). The heterogeneity has been characterized by a distribution parameter which represents the breadth of the polymer relaxation spectrum. It was found that the distribution parameter as measured by DEA matched well with those estimated previously by dynamic mechanical analysis. In addition to obtaining fundamental structural information from the α-transition (or glass transition) region, β-transitions (or secondary transitions) were studied in the poly(DEGDMA-co-PEG600DMA) networks studied. These secondary transitions are observed as a result of unreacted pendant as well as monomeric double bonds that are present in the samples. The heights of the damping factor peaks at the secondary transitions increase as the cross-linking density of the copolymer is increased (by increasing the DEGDMA composition). Further, the peak height decreases as the conversion increases in a given sample, strongly suggesting that the residual unsaturation in the copolymer network is responsible for the β-transition. Evidence is also presented that precludes the possibility that the initiator or its fragments cause the secondary transitions.

Synthesis and photopolymerization of acrylic acrylate copolymers

“…A second relaxation was present at low temperature on the tan ␦ curve. Such a behavior with two distinct transitions has been reported by Kannurpatti and Bowman for dimethacrylate networks 29 and by Barbeau et al for PUA networks.…”

Cure depth in photopolymerization: Experiments and theory

“…Furthermore, as the energy dosage increases, the optimal photoinitiator concentration decreases, in accordance with Eq. (22). The magnitude of the maximum cure depth at a given energy dosage is obtained from Eq.…”

“…[17][18][19][20] Since photon propagation through the medium is graded rather than discretized, gel is only formed up to the point at which the degree of cross-linking and polymerization is sufficient to form a solid gel network. 21,22 The curing depth thus corresponds to a thickness attainable via experimental measurement.…”

Photopolymerization as a promising method to sense biorecognition events

“… Delay in the attainment of equilibrium properties might be related to the kinetic evolution of the monomer blending 113, 116 .  Heat and mass gradients may appear in the curing step and affect the physical characteristics of the final material [134][135] creating a heterogeneous shrank product with stressed and fragile structure [136][137][138][139] . Such heterogeneities can cause different glass transition temperature varying up to 100ºC inside the very same sample [140][141] .…”