Conversion-dependent shrinkage in (meth)acrylates as a function of irradiance

Objectives: The objective of this study was to evaluate, at different irradiances, the conversion-dependent shrinkage behavior of (meth)acrylates as a function of monomer reactivity, resulting network stiffness and hydrogen bonding potential.

Methods: Triethylene glycol dimethacrylate (TEGDMA), polyethylene glycol dimethacrylate (and acrylate; PEGDMA/PEGDA, respectively), urethane dimethacrylate (UDMA), glycerol dimethacrylate (GlyDMA) and 1,14-tetradecanediol dimethacrylate (C14DMA) were mixed with 0.1 wt% 2,2-dimethoxy-2-phenylacetophenone (λ_max=365 nm). Shrinkage (linometer) and conversion (near-IR) were followed simultaneously during photopolymerization (3 or 30 mW/cm²). Conversion (DC) and shrinkage (VS) at the point of maximum rate of shrinkage development (Rv_max) were analyzed with 2-way ANOVA/Tukey’s test (alpha=5%).

Results: When comparing glassy x rubbery methacrylates of similar structure (TEGDMA x PEGDMA), Rv_max is similar at both irradiances, but TEGDMA presents much greater DC/VS@Rv_max as irradiance decreases than does the rubbery polymer, pointing to the much greater free volume entrapment at higher irradiances for glassy networks. For rubbery networks, free volume is independent of reaction rate, as is evidenced by the greatest increase in VS@Rv_max with the decrease in irradiance presented by PEGDA in comparison to PEGDMA. For glassy methacrylates, as the hydrogen bonding potential increases (C14DMA<TEGDMA<UDMA<GlyDMA), the increase in DC/VS@Rv_max for higher irradiances is greater, due to additional mobility restrictions imposed by stronger secondary intermolecular interactions.

Monomer	Rvmax (%)		DC@Rvmax (%)		VS@Rvmax (%)
	High I0	Low I0	High I0	Low I0	High I0	Low I0
TEGDMA	0.5±0.0	0.6±0.1	13±2	42±3	0.9±0.2	2.4±0.2
PEGDMA	0.3±0.0	0.3±0.0	22±3	26±2	0.8±0.1	1.2±0.1
PEGDA	2.1±0.1	1.1±0.2	80±5	81±4	2.1±0.2	3.3±0.2
GlyDMA	1.6±0.1	0.4±0.1	32±2	19±2	4.1±0.2	1.5±0.2
UDMA	2.7±0.1	0.7±0.2	13±1	30±3	0.9±0.1	0.9±0.2
C14DMA	0.1±0.0	0.2±0.0	14±1	35±3	0.3±0.0	2.2±0.3

Conclusions: The delay in shrinkage in relation to conversion has been demonstrated to be dependent on irradiance, monomer reactivity, secondary intermolecular interactions and stiffness of the network.