title: Structural and dynamical studies of acid-mediated conversion in amorphous-calcium-phosphate based dental composites.
abstract: OBJECTIVE
  To investigate the complex structural and dynamical conversion process of the amorphous-calcium-phosphate (ACP)-to-apatite transition in ACP based dental composite materials.
  
  
  METHODS
  Composite disks were prepared using zirconia hybridized ACP fillers (0.4 mass fraction) and photo-activated Bis-GMA/TEGDMA resin (0.6 mass fraction). We performed an investigation of the solution-mediated ACP-to-apatite conversion mechanism in controlled acidic aqueous environment with in situ ultra-small angle X-ray scattering based coherent X-ray photon correlation spectroscopy and ex situ X-ray diffraction, as well as other complementary techniques.
  
  
  RESULTS
  We established that the ACP-to-apatite conversion in ACP composites is a two-step process, owing to the sensitivity to local structural changes provided by coherent X-rays. Initially, ACP undergoes a local microstructural rearrangement without losing its amorphous character. We established the catalytic role of the acid and found the time scale of this rearrangement strongly depends on the pH of the solution, which agrees with previous findings about ACP without the polymer matrix being present. In the second step, ACP is converted to an apatitic form with the crystallinity of the formed crystallites being poor. Separately, we also confirmed that in the regular Zr-modified ACP the rate of ACP conversion to hydroxyapatite is slowed significantly compared to unmodified ACP, which is beneficial for targeted slow release of functional calcium and phosphate ions from dental composite materials.
  
  
  CONCLUSIONS
  For the first time, we were able to follow the complete solution-mediated transition process from ACP to apatite in this class of dental composites in a controlled aqueous environment. A two-step process, suggested previously, was conclusively identified.
authors.affiliation: Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA. Electronic address: fan.zhang@nist.gov.
authors.forename: Fan
authors.initials: F
authors.lastname: Zhang
authors.affiliation: Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Allen
authors.affiliation: Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA.
authors.forename: Lyle E
authors.initials: LE
authors.lastname: Levine
authors.affiliation: Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA.
authors.forename: Mark D
authors.initials: MD
authors.lastname: Vaudin
authors.affiliation: Volpe Research Center, American Dental Association Foundation, Gaithersburg, MD 20899, USA.
authors.forename: Drago
authors.initials: D
authors.lastname: Skrtic
authors.affiliation: Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA.
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Antonucci
authors.affiliation: Volpe Research Center, American Dental Association Foundation, Gaithersburg, MD 20899, USA.
authors.forename: Kathleen M
authors.initials: KM
authors.lastname: Hoffman
authors.affiliation: Volpe Research Center, American Dental Association Foundation, Gaithersburg, MD 20899, USA.
authors.forename: Anthony A
authors.initials: AA
authors.lastname: Giuseppetti
authors.affiliation: X-ray Science Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA.
authors.forename: Jan
authors.initials: J
authors.lastname: Ilavsky
mesh_terms: D000143:Acids / Q000737:chemistry*; D002130:Calcium Phosphates / Q000737:chemistry*; D003188:Composite Resins; D015394:Molecular Structure; D033362:Powder Diffraction
chemical_list: D000143:Acids; D002130:Calcium Phosphates; D003188:Composite Resins; C020243:calcium phosphate
keywords: ACP-based composites; Acid-medicated conversion; Amorphous calcium phosphate; Amorphous conversion; Dental composites; Dental material; Structure
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