Effect of solvent type and content on monomer conversion of a model resin system as a thin film.

OBJECTIVE: The purpose of this study was to examine the effect of solvent concentration on the degree of conversion of a model photo-activated resin formulation when placed as a thin film in ambient air. METHODS: A photo-activated co-monomer mixture (ethoxylated bis-GMA/TEGDMA) (Bisco Inc.) was diluted into six concentrations (from 1.0 to 13.0M) with either acetone or ethanol. A controlled volume of diluted, uncured resin was placed on the horizontal surface of an attenuated reflectance unit and the infrared (IR) spectrum obtained. A light emitting diode light-curing source (Bluephase, Ivoclar/Vivadent) then immediately irradiated the specimen for 10s (n=5). Five minutes after exposure, IR spectra of the cured material were obtained, and monomer conversions were calculated using standard methods that monitored changes in aliphatic-to-aromatic CC absorbance ratios in the uncured and cured states. RESULTS: In the acetone/model resin system, maximum conversion occurred with 2.5-5.0M solvent. In the ethanol/model resin system, conversion peaked at 2.5M solvent. Above 5.0M solvent, conversion values declined rapidly for both solvents. A 13.0-M solution resulted in near 0% conversion for both solvents. At 2.5 and 5.0M acetone, conversions exceeded those of equivalent concentrations of the ethanol-based system. CONCLUSIONS: For both an acetone- and ethanol-solvated model resin system, conversion did not immediately decrease with addition of solvent, but instead increased over that of the model resin alone. At higher solvent content (greater than 2.5M ethanol and 5.0M acetone), conversion rapidly declined, with ethanol causing less conversion at equal-molar solvent concentrations.

Student accuracy and evaluation of a computer-based audience response system.

We have incorporated an audience response system into our curriculum to increase student interaction in the teaching process. Classroom Performance System (CPS) is a computer-based audience response system that allows students to answer questions posed to the entire class by entering responses on a keypad. The responses are tallied and displayed on the classroom screen for all students to see. The purpose of our study was to determine student accuracy using the system with three different methods of administering questions. A secondary purpose was to assess students' perceptions about using the system. Our hypothesis for the study was that there should be no difference in volunteer accuracy or questionnaire responses to the three methods of gathering responses. Sixty-two dental students volunteered to participate. Using three methods (projected on a screen, verbal, and written), volunteers were given "responses" to enter into the system using CPS wireless remote answering devices. In the projected and verbal formats, the teacher managed the assessment by controlling the pace of input. In the written format, students were given responses on paper to input into the system at their own pace. At the end of the sessions, volunteers completed an anonymous questionnaire regarding their experiences with the system. The accuracy of responses was similar in the teacher-managed assessments (projected and verbal format). There was a statistical difference in the accuracy of responses in the student-managed assessment (p=<0.000001). Questionnaire responses also showed that students preferred teacher-managed assessments. The hypothesis was disproved. The overall response to this audience response system and its methods of gathering information was very positive.

Effect of temperature on unpolymerized composite resin film thickness.

STATEMENT OF PROBLEM: Room temperature (RT) composite resins are viscous, and this condition may affect the ability of the material to flow and adapt to preparation walls. PURPOSE: The purpose of this study was to compare the film thickness of a variety of commercial composite resins heated prior to light polymerization. The film thickness of these heated materials was also compared to those of flowable products at RT. MATERIAL AND METHODS: Five flowable and 7 conventional composite resins were used: 2 microfills, a nanofill hybrid, 2 submicron hybrids, a trimodal hybrid, and a packable. Composite resin (0.05 mL) was pressed between 2 Mylar-covered glass plates (15 kg) for 180 seconds, light polymerized, and the thickness measured using a micrometer. When comparing conventional composite resin heated to 54 degrees C or 60 degrees C, the RT value was control, while the RT flowable values were control when compared to heated composite resin. (n=5/group). Data were analyzed using 1- and 2-way analysis of variance and the Tukey-Kramer post hoc test (alpha=.05). Correlation of filler content to flow potential was evaluated. Scanning electron microscopic analysis of polymerized specimens was characterized by filler size and shape. RESULTS: Room temperature conventional composite resin values varied in film thickness. Not all preheated products reduced in thickness. Thickness of the nanofill did not reduce, and one of the submicron hybrids reduced the greatest. No correlation existed between composite resin classification, filler content or shape, and film thickness. No difference in thickness existed between composite resins preheated to 54 degrees C and 60 degrees C. Room temperature and preheated conventional composite resin provided film thickness greater than that of flowable materials. CONCLUSIONS: Preheating conventional composite resin yields lower film thickness for some products, but flow cannot be attributed to composite resin classification, filler content, or shape. Preheated composite resin thickness was greater than that of all flowables.