title: Novel dental restorative materials having low polymerization shrinkage stress via stress relaxation by addition-fragmentation chain transfer.
abstract: OBJECTIVE
  To produce a reduced stress dental restorative material while simultaneously maintaining excellent mechanical properties, we have incorporated an allyl sulfide functional group into norbornene-methacrylate comonomer resins. We hypothesize that the addition-fragmentation chain transfer (AFCT) enabled by the presence of the allyl sulfide relieves stress in these methacrylate-based systems while retaining excellent mechanical properties owing to the high glass transition temperature of norbornene-containing resins.
  
  
  METHODS
  An allyl sulfide-containing dinorbornene was stoichiometrically formulated with a ring-containing allyl sulfide-possessing methacrylate. To evaluate the stress relaxation effect as a function of the allyl sulfide concentration, a propyl sulfide-based dinorbornene, not capable of addition-fragmentation, was also formulated with the methacrylate monomer. Shrinkage stress, the glass transition temperature and the elastic modulus were all measured. The composite flexural strength and modulus were also measured. ANOVA (CI 95%) was conducted to determine differences between the means.
  
  
  RESULTS
  Increasing the allyl sulfide content in the resin dramatically reduces the final stress in the norbornene-methacrylate systems. Both norbornene-methacrylate resins demonstrated almost zero stress (more than 96% stress reduction) compared with the conventional BisGMA/TEGDMA 70/30wt% control. Mechanical properties of the allyl sulfide-based dental composites were improved to the point of being statistically indistinguishable from the control BisGMA-TEGDMA by changing the molar ratio between the methacrylate and norbornene functionalities.
  
  
  CONCLUSIONS
  The allyl sulfide-containing norbornene-methacrylate networks possessed super-ambient T(g), and demonstrated significantly lower shrinkage stress when compared with the control (BisGMA/TEGDMA 70-30wt%). Although additional development remains, these low stress materials exhibit excellent mechanical properties which are appropriate for use as dental restorative materials.
authors.affiliation: Department of Chemical and Biological Engineering, Engineering Center, ECCH 111, University of Colorado, Boulder, CO 80309-0424, USA.
authors.forename: Hee Young
authors.initials: HY
authors.lastname: Park
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Kloxin
authors.forename: Ahmed S
authors.initials: AS
authors.lastname: Abuelyaman
authors.forename: Joe D
authors.initials: JD
authors.lastname: Oxman
authors.forename: Christopher N
authors.initials: CN
authors.lastname: Bowman
mesh_terms: D000498:Allyl Compounds / Q000737:chemistry*; D001643:Bridged Bicyclo Compounds; D003188:Composite Resins / Q000737:chemistry*; D018772:Dental Marginal Adaptation; D003799:Dental Stress Analysis; D055119:Elastic Modulus; D006536:Heptanes; D055115:Light-Curing of Dental Adhesives; D008422:Materials Testing; D008689:Methacrylates / Q000737:chemistry*; D015394:Molecular Structure; D018583:Pliability; D058105:Polymerization; D013314:Stress, Mechanical; D013440:Sulfides / Q000737:chemistry*; D044366:Transition Temperature
chemical_list: C504926:(2.2.1)bicyclohept-2-ene; D000498:Allyl Compounds; D001643:Bridged Bicyclo Compounds; D003188:Composite Resins; D006536:Heptanes; D008689:Methacrylates; D013440:Sulfides; C038491:allyl sulfide