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Viscoelasticity of Dental Tissue Conditioners during the Sol-gel Transition

¹ Department of Prosthetic Dentistry, Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima, 734-8553, Japan; and
² Dental Materials Science Unit, The Dental School, University of Newcastle upon Tyne, Framlington Place, Newcastle upon Tyne, NE2 4BW, UK;

View larger version (16K): [in a new window]   Figure 1. Measurement of dynamic viscoelastic properties during the sol-gel transition. (A) Schematic diagram of a controlled-stress rheometer in a parallel plate configuration. In the parallel plate system, the shear strain () and shear stress () are determined experimentally as follows: = F , = F T, where F (= R/d) is the shear strain factor; F (= 2/R3) the shear stress factor; the angular displacement; T the torsional force; R the radius of the plate; and d the shear gap. (B) Schematic representation of dynamic viscoelastic properties of gelation system as a function of reaction time. G' represents the elastic component of material behavior, whereas G'' represents the viscous component of material behavior. The gelation time was defined as the time to reach the gel point at which tan = 1 (G' = G'').

View larger version (30K): [in a new window]   Figure 2. Relationship between variations of G', G'', and tan of concentrated polymer solutions based on PEMA with time and each factor at 37°C. The material that had a median value of gelation time was selected from 5 materials measured. Data are given as median values (n = 5). (A) Influence of molecular weight (Mw) of polymer powders. PEMA polymer powders of various molecular weight were mixed with 10 wt% ethanol/90 wt% dibutyl sebacate at a P/L ratio of 1.2. (B) Influence of P/L ratio. PEMA polymer powders of molecular weight of 3.75 x 105 were mixed with 10 wt% ethanol/90 wt% dibutyl sebacate at various P/L ratios. (C) Influence of plasticizer type. PEMA polymer powders of molecular weight of 3.75 x 105 were mixed with various plasticizers/10 wt% ethanol at a P/L ratio of 1.2. (D) Influence of ethanol content in liquids. PEMA polymer powders of molecular weight of 3.75 x 105 were mixed with various rates of ethanol/dibutyl sebacate at a P/L ratio of 1.2. The materials containing no ethanol were also measured, so that the effect of ethanol could be demonstrated. A greater rate of changes in G', G'', and tan with time was noted with the higher-molecular-weight polymer powders, with the higher P/L ratio materials, with the lower molar-volume plasticizer, and with liquids containing the larger percentages of ethanol.

View larger version (16K): [in a new window]   Figure 3. Relationship between gelation times of concentrated polymer solutions based on PEMA and each factor at 37°C. The materials consisting of the PEMA polymer powder of molecular weight of 3.75 x 105 and the liquid containing 10 wt% ethanol/90 wt% plasticizer [butyl phthalyl butyl glycolate (aromatic ester), dibutyl sebacate (aliphatic ester)] (P/L = 1.2) were used as baseline, because they represented an average composition and structure of tissue conditioners. We determined the 26 materials by varying the levels of each factor of the standard materials (baseline). (A) Influence of molecular weight (Mw) of polymer powders. Four PEMA polymer powders, with Mw of 1.08 x 105, 2.39 x 105, 3.75 x 105, and 5.30 x 105, were used. There was a negative linear relationship between the log of the gelation time and the log of the polymer molecular weight (p < 0.0005). (B) Influence of P/L ratio. Four P/L ratios, 0.8, 1.0, 1.2, and 1.4, were used. There was a negative linear relationship between the gelation time and the P/L ratio (p < 0.0005). (C) Influence of plasticizer type. Four ester plasticizers—butyl phthalyl butylglycolate (BPBG), dibutyl phthalate (DBP), benzyl benzoate (BB), and dibutyl sebacate (DBS)—were used. All differences were significant (p < 0.05, ANOVA and Student-Newman-Keuls test). (D) Influence of ethanol content in liquids. Four ethanol contents, 2, 5, 10, and 20 wt%, were used. There was a negative linear relationship between the log of the gelation time and the ethanol content (p < 0.0005). Data are given as means and standard deviations (n = 5).