Thiol-ene functionalized siloxanes for use as elastomeric dental impression materials

Abstract

Objectives

Thiol- and allyl-functionalized siloxane oligomers are synthesized and evaluated for use as a radical-mediated, rapid set elastomeric dental impression material. Thiol-ene siloxane formulations are crosslinked using a redox-initiated polymerization scheme, and the mechanical properties of the thiol-ene network are manipulated through the incorporation of varying degrees of plasticizer and kaolin filler. Formulations with medium and light body consistencies are further evaluated for their ability to accurately replicate features on both the gross and microscopic levels. We hypothesize that thiol-ene functionalized siloxane systems will exhibit faster setting times and greater detail reproduction than commercially available polyvinylsiloxane (PVS) materials of comparable consistencies.

Methods

Thiol-ene functionalized siloxane mixtures formulated with varying levels of redox initiators, plasticizer, and kaolin filler are made and evaluated for their polymerization speed (FTIR), consistency (ISO4823.9.2), and surface energy (goniometer). Feature replication is evaluated quantitatively by SEM. The T g , storage modulus, and creep behavior are determined by DMA.

Results

Increasing redox initiation rate increases the polymerization rate but at high levels also limits working time. Combining 0.86 wt% oxidizing agent with up to 5 wt% plasticizer gave a working time of 3 min and a setting time of 2 min. The selected medium and light body thiol-ene formulations also achieved greater qualitative detail reproduction than the commercial material and reproduced micrometer patterns with 98% accuracy.

Significance

Improving detail reproduction and setting speed is a primary focus of dental impression material design and synthesis. Radical-mediated polymerizations, particularly thiol-ene reactions, are recognized for their speed, reduced shrinkage, and ‘click’ nature.

Introduction

Synthetic elastomeric impression materials are widely used in clinical dentistry to obtain negative replicas of hard and soft intraoral tissues from which positive gypsum casts can be prepared. Since the casts serve as templates for the fabrication of dentures, crowns, and various orthodontic appliances, precise and transferable detail reproduction is demanded of the impression. To achieve a high degree of detail reproducibility, the impression material must possess sufficient hydrophilicity to coat moist oral surfaces and adequate fluidity to surround small features . Furthermore, the impression material should be biocompatible, have reasonable working and setting times, resist permanent deformation upon removal from the mouth, and maintain dimensional stability after setting for multiple casts to be poured .

Toward these objectives, four classes of synthetic elastomeric dental impression materials are currently available: polysulfide, condensation silicone, addition silicone, and polyether. Although most impression materials on the market today are recognized for providing acceptable detail reproduction , they remain limited by their long setting times and susceptibility to dimensional instabilities . Average setting times range from 6 min (addition silicon) to 13 min (polysulfide), with each second within the mouth serving as a source of motion-induced distortion by the patient . Moreover, each class of impression material has the potential to diminish in accuracy over time. Polysulfide and condensation silicones release liquid by-products as they set, while the addition silicones (or polyvinyl siloxanes, PVS) release gas by-products. Consequently, polysulfide and condensation silicone impressions may shrink upon evaporation of water or ethanol, respectively . Meanwhile, casts poured from PVS impressions may contain pits or voids if adequate time is not provided between full set and positive rendition . Lastly, the hydrophilic nature of the polyether class of materials makes the impressions subject to swelling through absorption of moisture from the surrounding environment .

Clearly, the setting mechanisms employed by the current classes of materials do not offer an optimal route for a quick-setting, dimensionally stable material. Free radical-mediated polymerizations, on the other hand, are known to occur rapidly while remaining amenable to various modes of initiation including light, heat, and chemical processes . Of particular interest for an impression material application is the radical-mediated thiol-ene polymerization. The thiol-ene ‘click’ reaction is well documented in the literature as proceeding at rapid rates while remaining uninhibited by oxygen, releasing no by-products, and requiring no solvents to attain quantitative conversions . Moreover, the thiol-ene reaction mechanism proceeds through a series of alternating propagation and chain transfer events prior to termination that makes the reaction a step growth rather than a chain growth process ( Fig. 1 ) . As a result, networks formed via thiol-ene reactions exhibit delayed gelation and are quite homogenous. The delay in gelation is of particular importance for an impression material application since the preservation of the liquid state will allow the working time to be extended without compromising the reaction rate. Furthermore, the limited bond capacity of sulfur leads to less volumetric shrinkage in the thiol-ene polymerization than would be seen in a comparable vinyl-based system, such as that seen in the PVS class of materials .

Fig. 1
Schematic of the thiol-ene reaction mechanism. The thiol-ene polymerization proceeds through a cyclic step growth mechanism consisting of alternating propagation/chain transfer steps following initiation and prior to termination. The reaction mechanism assumes ideal conditions in which the alternating steps proceed at the same overall rate and homopolymerization of the ene is minimized.

While these advantages of thiol-ene reactions have led to a significant increase in its implementation and the general reference to it as one of the most prominent of the click reactions, there are some drawbacks to the thiol-ene reaction under some circumstances as well, all of which were circumvented here by a careful selection of monomers and conditions. Odor is often cited as a significant potential issue with any thiol-containing resin; however, here, the use of higher molecular weight, purified monomers eliminates the low molecular weight impurities and compounds that cause the odor. Careful selection of the ene is also necessary to eliminate the homopolymerization reaction as has been done here. Further, others have noted that the thiol-ene reaction is not well-suited for polymer–polymer conjugation and other reactions that involve dilute concentrations of functional groups, particularly when large concentrations of photoinitiator are used . Under these dilute functional group conditions, side reactions such as chain transfer to oxygen and radical–radical termination that are unimportant in bulk polymerizations such as those used here become relatively much more important.

Thiol-ene chemistry could potentially be incorporated into a wide range of monomer species; however, impression materials must be elastomeric at room temperature with adequate strength to resist tearing when removed from the mouth or significantly compressing under the weight of casting agents. Consequently, siloxanes are a fitting material selection for implementation with a thiol-ene-based setting/polymerization reaction given their noted flexibility, mechanical integrity, and biocompatibility . Siloxanes are also highly amenable to functionalization, and multifunctional polymers can be readily synthesized through the condensation of pendant Cl, OH, or OR groups by a variety of catalytic species . Hence, the primary objective of this study is to evaluate the use of thiol- and ene-functionalized siloxanes as a viable alternative to current impression materials. Specifically, a single thiol-ene functionalized siloxane formulation was synthesized, its polymerization kinetics monitored, and its network properties compared with a leading brand of PVS impression material. We hypothesize that the use of thiol-ene chemistry will produce a material with improved setting time and detail reproduction without statistically significant alteration in mechanical properties relative to a PVS impression material of similar consistency.

Materials and methods

Materials

3-Aminopropyl(methyl) diethoxysilane (SiNH 2 , 95%), 3-mercaptopropyl(methyl) dimethoxysilane (SiSH, 96%), diphenyl dimethoxysilane (SiDP, 98%), di- n -octyl dimethoxysilane (SiDO), and trimethylmethoxysilane (SiMe) were purchased from Gelest, Inc. (Morrisville, PA) and used without further purification. Allylchloroformate (97%), triethylamine (≥99%), N,N -bis(2-hydroxyethyl)- p -toluidine (DHEPT, ≥97%), benzoyl peroxide (BPO, ≥97%), monomethyl ether hydroquinone (MEHQ, ≥98%), and kaolin were purchased from Sigma–Aldrich (St. Louis, MO) and used as received. Aquasil Ultra Smart Wetting Impression Material Monophase (lot no. 091202) and LV (lot no. 091119, Dentsply, Tulsa, OK) was provided by Septodont.

3-(Aminopropylmethyl)diethoxy silane was combined with allylchloroformate according to a method described in the literature to produce an allyl-functionalized silane monomer (82%) with hydrogen bonding capabilities . Thiol- and allyl-functionalized siloxane oligomers were then synthesized via the acid-catalyzed condensation of alkoxysilane monomers, and their structural characteristics were reported previously . The condensation process yielded a thiol-functionalized oligomer (99%, SiSH DP, Fig. 2 a) with 7:3 SH:DP and an allyl-functionalized oligomer (99%, SiNHC C DP DO, Fig. 2 b) with 5:4:1 C C:DP:DO.

Fig. 2
Silane oligomers used in this study: (a) SiSH DP (SH:DP 7:3), (b) SiNHC C DP DO (C C:DP:DO 5:4:1).

Methods

Polymerization conversion studies were performed on formulated thiol-ene mixtures in the near IR (Nicolet Magna-IR 750 series II FTIR spectrometer) using glass slides separated by a 300 μm spacer as the sample holders. Real-time kinetics of samples containing no kaolin filler were collected at a resolution of 4 cm −1 and at a rate of 5 scans every 2 s at both room temperature (23 °C) and oral temperature (35 °C). The final conversions of thiol and allyl functionalities were calculated as one minus the ratio of final to initial peak areas centered at 2570 cm −1 (SiSH DP, S H stretch) and 4490 cm −1 (SiNHC C DP DO, C C stretch), respectively. All measurements were completed in triplicate (i.e., n = 3). All samples were stored at ambient conditions (temperature and humidity) and not otherwise pretreated prior to their use in subsequent experiments.

The consistency of fully formulated siloxane samples with adequate working time was measured in accordance with ISO 4823, Section 9.2 dental standards. Briefly, 0.5 mL of unset siloxane material was injected between two glass slides (7″ × 5″) and compressed for 5 s with 14.7 N of force. Following the designated 15 min polymerization period, the major and minor diameters were measured, and the average of the two lengths was recorded ( n = 3 for each sample).

Surface energy was quantified by measuring the static contact angle (DROPimage Advanced, v.2.0.10) of DI water atop a film of each crosslinked network with a goniometer (Ramé-Hart Instruments, Model 500 Advanced). Samples were prepared by injecting uncured formulations between two glass slides separated by plastic spacers (300 μm thick). Complete conversion of each experimental siloxane sample was confirmed by FTIR; the commercial PVS samples were allowed to set for the manufacturer-instructed timeframe prior to testing.

Dynamic mechanical analysis (DMA) was performed in triplicate on crosslinked thiol-ene siloxane networks and on set PVS light and medium body samples (300 μm thickness) in tension using a TA Instruments Q800 scanning at 1 °C/min from −40 to 40 °C at a frequency of 1 Hz and a strain of 0.1%. The glass transition temperature ( T g ) was defined as the temperature corresponding to the maximum in the tan δ curve. Creep recovery and stress relaxation of the medium and light bodied thiol-ene functionalized siloxanes and commercial PVS materials were also measured in tension by DMA. Creep recovery was performed by extending samples under a constant load of 0.1 MPa for 10 min followed by 20 min recovery. Stress relaxation was conducted with an initial strain of 15% with 10 min recovery. Both test methods were run in triplicate on 600 μm thick samples at 35 °C.

Feature replication was evaluated qualitatively on a centimeter size scale and quantitatively by scanning electron microscopy (SEM, JEOL JSM 7401F) on a nanometer size scale. Centimeter size scale features were constructed of rectangular solids, four-sided pyramids, and elliptical half domes in three sizes: (blocks, length × width × height) 3 cm × 3 cm × 6 cm, 4 cm × 3 cm × 8 cm, 5 cm × 3 cm × 10 cm; (pyramids, length × width × height) 3 cm × 3 cm × 6 cm, 4 cm × 4 cm × 8 cm, 5 cm × 5 cm × 10 cm; (domes, diameter major × diameter minor ) 3 cm × 6 cm, 4 cm × 8 cm, 5 cm × 10 cm. The rectangular solid features were spaced 75 μm from the pyramidal features, which were separated by 50 μm from the half dome features. Replication of patterns with nanometer periodicities on silica wafers was quantified by SEM image analysis. ImageJ public domain software (NIH) was used to enhance contrast and smooth images, as well as to provide initial estimates of dimensions. MATLAB software (The MathWorks, Inc.) was then used to calculate the Fast Fourier Transform of the image, which allowed the periodicity of the sample to be measured. Feature replication was quantified as the absolute percent difference between sample and substrate periodicities.

Statistical analysis

The Student’s t -test ( n = 3, α = 0.05) was used to compare the difference of means between PVS and thiol-ene siloxane samples at two consistency levels (light body and medium body) for the material properties under investigation. In particular, statistical analysis was performed on storage modulus, creep recovery, stress relaxation, and contact angle data sets.

Materials and methods

Materials

3-Aminopropyl(methyl) diethoxysilane (SiNH 2 , 95%), 3-mercaptopropyl(methyl) dimethoxysilane (SiSH, 96%), diphenyl dimethoxysilane (SiDP, 98%), di- n -octyl dimethoxysilane (SiDO), and trimethylmethoxysilane (SiMe) were purchased from Gelest, Inc. (Morrisville, PA) and used without further purification. Allylchloroformate (97%), triethylamine (≥99%), N,N -bis(2-hydroxyethyl)- p -toluidine (DHEPT, ≥97%), benzoyl peroxide (BPO, ≥97%), monomethyl ether hydroquinone (MEHQ, ≥98%), and kaolin were purchased from Sigma–Aldrich (St. Louis, MO) and used as received. Aquasil Ultra Smart Wetting Impression Material Monophase (lot no. 091202) and LV (lot no. 091119, Dentsply, Tulsa, OK) was provided by Septodont.

3-(Aminopropylmethyl)diethoxy silane was combined with allylchloroformate according to a method described in the literature to produce an allyl-functionalized silane monomer (82%) with hydrogen bonding capabilities . Thiol- and allyl-functionalized siloxane oligomers were then synthesized via the acid-catalyzed condensation of alkoxysilane monomers, and their structural characteristics were reported previously . The condensation process yielded a thiol-functionalized oligomer (99%, SiSH DP, Fig. 2 a) with 7:3 SH:DP and an allyl-functionalized oligomer (99%, SiNHC C DP DO, Fig. 2 b) with 5:4:1 C C:DP:DO.

Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Thiol-ene functionalized siloxanes for use as elastomeric dental impression materials
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