title: Influence of curing modes on crosslink density in polymer structures.
abstract: OBJECTIVE
  This study investigates the influence of curing modes on the crosslinking density of dental composites.
  
  
  METHODS
  A light-cure unit (BISCO VIP) that allowed for independent command over time and intensity was selected. Four different light-curing modes with constant light energy density were investigated (control (C), pulse delay (PD), soft-start (SS) and pulse cure (PC)). The degree of crosslinking was assessed directly by measuring the glass transition temperature of 1 mm thick composite (Z100, 3M-ESPE) specimens using differential scanning calorimetry (DSC 2920). Polymer softening in ethanol was used as an indirect method for assessing the degree of crosslinking. After light-curing, specimens were stored in air at 37 degrees C for 24 h and subjected to hardness testing using a digital microhardness tester (n = 6, load=500 g; dwell time=15 s). The specimens were then placed in 75% ethanol-water solution at 37 degrees C for 24 h and post-conditioning hardness was determined. Mean hardness (KHN)/hardness deterioration (DeltaKHN) was computed and data was subjected to analysis using one-way ANOVA/Scheffe's test.
  
  
  RESULTS
  Ranking of degree of crosslinking density by DSC was as follows: C>PC>SS>PD. For the indirect method of determining crosslinking density, DeltaKHN ranged from 10.8 to 12.9 and ranking was PC>SS>C>PD.
  
  
  CONCLUSIONS
  Specimens polymerized with PD were significantly more susceptible to softening in ethanol than specimens cured with PC. Results of this study suggest that polymerization with PD resulted in a lower crosslink density and gave rise to polymers with an increased susceptibility to softening in ethanol.
authors.affiliation: Department of Restorative Dentistry, Faculty of Dentistry, National University of Singapore, 5 Lower Kent Ridge Road, Singapore, Singapore 119074. g0306023@nus.edu.sg
authors.forename: M S
authors.initials: MS
authors.lastname: Soh
authors.forename: Adrian U J
authors.initials: AU
authors.lastname: Yap
mesh_terms: D000704:Analysis of Variance; D002152:Calorimetry, Differential Scanning; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003188:Composite Resins / Q000737:chemistry*; D000431:Ethanol / Q000737:chemistry; D006244:Hardness; D006801:Humans; D008027:Light; D008029:Lighting / Q000295:instrumentation; D044367:Phase Transition; D011108:Polymers / Q000737:chemistry; D012822:Silicon Dioxide; D012997:Solvents / Q000737:chemistry; D013499:Surface Properties; D015040:Zirconium
chemical_list: D003188:Composite Resins; D011108:Polymers; D012997:Solvents; C083568:Z100 composite resin; D000431:Ethanol; D012822:Silicon Dioxide; D015040:Zirconium