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RESEARCH REPORTS |
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;
* corresponding author, hmurata{at}hiroshima-u.ac.jp
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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= 53.8%; confirming the hypothesis) and molecular weight of polymer powders ( = 26.7%) had a greater influence on the gelation times of PEMA-based systems than did the molar volume of plasticizers ( = 9.0%) and concentration of polymers (i.e., powder/liquid ratio) ( = 4.5%). The results suggest that the gelation of tissue conditioners based on PEMA can be controlled over a wide range by varying the polymer molecular weight, and especially ethanol content.
KEY WORDS: tissue conditioners • sol-gel transition • dynamic viscoelastic properties • activation energy • orthogonal array
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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A wide range of gelation characteristics is found in commercial tissue conditioners, due to differences in composition and structure (Jones et al., 1986; Murata et al., 1998). The gelation of the materials has previously been characterized with the use of a reciprocating rheometer (Jones et al., 1986) and oscillating rheometer (Murata et al., 1993, 1998). These methods provide a comparative evaluation among the materials, but do not measure absolute values of elasticity and viscosity.
In general, the gelation of a physical gel is rheologically described by three stages: pre-gelation (sol), sol-gel transition, and post-gelation (gel). During the transition through these stages, materials undergo marked changes in rheological properties, and the monitoring of these changes offers a means of studying the influence of composition on gelation. This requires a sophisticated approach which enables well-defined rheological parameters to be determined. One method which is suitable for meeting this requirement is a dynamic mechanical test. Such a test measures the response of a material to a sinusoidal or other periodic stress (Nielsen and Landel, 1994). Previous studies evaluated the dynamic viscoelasticity of elastomers during setting by means of a controlled-stress rheometer (McCabe and Carrick, 1989; McCabe and Arikawa, 1998). However, little information is available on dynamic viscoelasticity of tissue conditioners during gelation.
The purpose of this investigation was to evaluate the influence of molecular weight of polymer powders, plasticizer type, ethanol content in liquids, and powder/liquid ratio on dynamic viscoelastic properties of concentrated polymer solutions based on PEMA, i.e., tissue conditioners, in sol-gel transition by the orthogonal array in addition to the one-way layout method. The relationship between activation energy of gelation and the above factors was also examined. Our hypothesis was that the gelation speed would be influenced most by the ethanol content of the materials.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Analysis of Dynamic Viscoelastic Properties
Dynamic viscoelastic properties of the test materials during gelation were determined at 37°C. A controlled-stress rheometer (CSL500, Carri-Med Ltd., Dorking, Surrey, England) was used with a parallel plate test configuration (diameter = 20 mm, gap = 1000µm) in oscillatory mode (Fig. 1
). Immediately after the powders and liquids were mixed for 15 sec at 23 ± 1°C, the resulting paste was placed on the plate of the rheometer. Torque was monitored every 30 sec at constant oscillating frequency (1 Hz) and angular displacement (3 mrad).
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) are defined as follows:
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where i2 = –1, and is the phase angle between stress and strain.
Determination of Gelation Times
The gelation time was defined as the time at which tan = 1 (G' = G'') was reached, that is, the gel point (Fig. 1
) (Winter and Chambon, 1986; Nielsen and Landel, 1994).
The various combinations of the 16 different materials were tested to determine the contribution ratio () of each factor to the gelation times by means of an orthogonal array L16(45) (Taguchi, 1991) (Appendix Table A1, www.dentalresearch.org). Second, 26 different materials were used for the determination of the effect of each factor by a one-way layout method.
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Determination of Activation Energies
The gelation times of 13 different materials were also determined at 4 temperatures: 25, 30, 35, and 40°C. An apparent activation energy of gelation (Ea) was determined according to the following kinetic equation (Oyanguren and Williams, 1993; Parker and Braden, 1996; Ponton et al., 2002):
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where tgel is the gelation time, C a pre-factor, R the gas constant, and T the absolute temperature.
Five tests were carried out for each material and temperature.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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, www.dentalresearch.org). The ethanol content ( = 53.8%) had more influence on gelation time than the molecular weight of polymer powders ( = 26.7%), which in turn had more influence than the type of plasticizer ( = 9.0%) and the P/L ratio ( = 4.5%).
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with time were noted with the higher-molecular-weight polymer powders, with the higher P/L ratio materials, and with liquids containing the larger percentages of ethanol (Fig. 2). A negative linear relationship was found between the log of the gelation time and both the log of the polymer molecular weight (r = –0.960 and –0.992, p < 0.0005) and the ethanol content (r = –0.995 and –0.979, p < 0.0005) (Fig. 3). There was also a negative linear relationship between the gelation time and the P/L ratio (r = –0.924 and –0.904, p < 0.0005). (The first r was for the materials containing butyl phthalyl butyl glycolate, and the second for dibutyl sebacate in the regression analyses for each factor.) With regard to plasticizer type, the greatest rate of increase of G' and G'' and decrease of tan with time were noted for liquids containing benzyl benzoate, and the smallest rate of change in these values was noted with dibutyl sebacate (Fig. 2). The shortest gelation time was with benzyl benzoate, and the longest was observed with dibutyl sebacate (Student-Newman-Keuls test, p < 0.05) (Fig. 3).
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; Appendix Table A3). Increasing the polymer molecular weight from 1.08 x 105 to 5.30 x 105, the P/L ratio from 0.8 to 1.4, and ethanol content from 2 to 20 wt% led to reductions in the activation energy of 32.2, 8.9, and 46.3 kJmol–1, respectively. Differences in plasticizer type led to a change in activation energy of 19.7 kJmol–1.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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At the gel point, G' and G'' are congruent, i.e., tan = 1, and this behavior is valid in the entire range 0 < frequency < 
. It has been reported that monitoring tan is an effective way of evaluating the viscoelasticity of impression materials during setting (McCabe and Arikawa, 1998). Therefore, in this study, the gelation time was defined as the time to reach tan = 1.
Viscoelasticity of the tissue conditioners during gelation can conveniently be discussed if the curves of tan are divided into 3 regions: region 1, where tan decreases with time rapidly; region 2, where tan = 1 (the gel point); and region 3, where tan continues to decrease. At the initial stage in region 1, the PEMA polymers dissolve in the plasticizer, and polymer chain entanglements are still scarce. All the solutions are essentially viscous Newtonian fluids. Entanglements then begin to occur, and the viscosity gradually increases with time until the solutions can no longer be described as viscous fluids. Before the gel point, although G' is always smaller than G'', G' increases much more rapidly than G'', and thus tan decreases rapidly. In region 2, an infinitely long molecule is produced, and the system reaches the gel point (liquid-solid transition), which occurs near where tan = 1. In region 3, pseudo cross-links consisting of chain entanglements are forming. The samples behave more elastically with increasing reaction time. G' increases rapidly and becomes larger than G'', and thus tan continues to decrease.
In the orthogonal array method, stress is laid on only factors that have a consistent effect, even when the conditions of other factors differ, because such factors are regarded as having great effect (Taguchi, 1991). Therefore, although the statistical results in this study were based on a limited number of compositions, the tendency of main effects, i.e., relationship between the levels of the factor and measured values, can be determined. It was found that the orthogonal array method was useful for evaluation of the relationship among many factors, such as composition and structure.
The compatibility of different polymers and solvents can be predicted by means of the solubility parameter (SP), which is the square root of the cohesive energy density or vaporization energy (
Ev) divided by the molar volume (V):
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In general, polymers and solvents should be mutually soluble when their solubility parameters are equal. It has been reported that a method which considers solvents as poorly, moderately, or strongly hydrogen-bonded could give relatively accurate predictions of the solubility of a solvent in a polymer (Bellenger et al., 1997).
Plasticizers are added to a polymer to lower the glass transition temperature and soften the rigid polymer. The solubility parameters of plasticizers, which are moderately hydrogen-bonded solvents, are within the range of those of PEMA and poly(methyl methacrylate) (PMMA) (Appendix Tables A3, A4, www.dentalresearch.org), resulting in mutual solubility. Gelation times were independent of the solubility parameters and molecular weight of plasticizers. Diffusion of the plasticizer into the polymer would be influenced more by the molar volume, which expresses the effect of molecular size, than by the solubility parameter, which is a measure of all of the intermolecular forces, consisting of dispersion forces, dipole-dipole interactions, and hydrogen bonding. It has also been reported that the miscibility of a given polymer-solvent pair is a decreasing function of the molar volume of the solvent (Bellenger et al., 1997). Therefore, higher-molar-volume plasticizers tended to lead to longer gelation times. However, the effect of plasticizer type was small compared with that of other factors.
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, www.dentalresearch.org), and it is not dissolved by ethanol. The higher-molecular-weight polymer powders produced the shorter gelation times. This can be explained by two contradictory factors. First, the plasticizer takes longer to diffuse into the higher-molecular-weight polymer powders, because the diffusion coefficient of polymer in the solvent is inversely proportional to (Mw)2. However, the higher-molecular-weight polymer powders would produce greater polymer chain entanglements (Han and Bae, 1998), because the distance between polymer chains is closer in polymer solutions with higher molecular weight and concentration (P/L ratio) (Hong and Chen, 1998). The gelation time, which was determined by the variation of viscoelasticity resulting from chain entanglements, would be influenced more by the entanglements than by the diffusion, resulting in shorter gelation times.
A higher P/L ratio also produced shorter gelation times. Although a longer time would be necessary for saturating the liquid with polymer powders at a higher P/L ratio, the higher concentration would also be associated with more entanglements. The influence of the entanglement would be greater, resulting in shorter gelation times.
The materials with the shorter gelation times were found to give lower values of activation energy. Gelation takes place through the activated state, when the system has greater energy than the activation energy. The greater entanglements and diffusion produced by varying composition and structure would lead more easily to the activated state. These changes in the activation energies were found to reflect the contribution of each factor to the gelation process.
The hypothesis that ethanol content is the most influential factor in determining the gelation times of the PEMA-based systems can be supported by the orthogonal array approach, by regression analyses, and by activation energies. Furthermore, it was also found that the polymer molecular weight has a great influence on the gelation process. An ideal tissue conditioner should have stable durability when applied in the mouth. However, it has been reported that materials based on PEMA exhibit a marked reduction in their flow properties with time, due to leaching out of ethanol (Jones et al., 1988; Murata et al., 1996, 1998). From the standpoint of gelation and manipulation after mixing, ethanol is considered to be an essential additive of these materials. Thus, if tissue conditioners which contain less or no alcohol are to be developed, PEMA with higher molecular weight should be used, or, alternatively, a new type of polymer should be developed. The findings of this study may help in the development of improved materials.
In conclusion, the strong polar bonding of ethanol and polymer molecular weight have a greater influence on the entanglement speed of concentrated polymer solutions based on PEMA than do the molar volume of plasticizers and concentration of polymers. The results suggest that the gelation of the PEMA-based system can be controlled over a wide range by varying the polymer molecular weight and especially ethanol content.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Received September 19, 2003; Last revision December 16, 2004; Accepted January 2, 2005
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REFERENCES |
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Braden M (1970). Tissue conditioners: I. Composition and structure. J Dent Res 49:145–148.
Han CK, Bae YH (1998). Inverse thermally-reversible gelation of aqueous N-isopropylacrylamide copolymer solutions. Polymer 39:2809–2814.[ISI]
Hong P-D, Chen J-H (1998). Network structure and chain mobility of freeze-dried polyvinyl chloride/dioxane gels. Polymer 39:5809–5817.
Jones DW, Sutow EJ, Graham BS, Milne EL, Johnston DE (1986). Influence of plasticizer on soft polymer gelation. J Dent Res 65:634–642.
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McCabe JF, Carrick TE (1989). Rheological properties of elastomers during setting. J Dent Res 68:1218–1222.
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Ponton A, Warlus S, Griesmar P (2002). Rheological study of the sol-gel transition in silica alkoxides. J Colloid Interface Sci 249:209–216.
Taguchi G (1991). System of experimental design. 4th ed. New York: Quality Resources. Dearborn, MI: American Supplier Institute Inc.
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| Journal of Dental Research ® | Critical Reviews (1990-2004) |
) and shear stress (
) are determined experimentally as follows: 
, 
R3) the shear stress factor; 
