EFFECTS OF A METHACRYLIC SILANE ON SOME PHYSICOCHEMICAL PROPERTIES OF RESIN-BASED BIOMIMETIC COMPOSITE

Introduction

In the presence of water, organotrialkoxysilanes can undergo a series of hydrolysis-condensation reactions leading to three-dimensional silsesquioxane (SSO) structures that have potential as both resin matrix components and as molecular-sized silica fillers (1). It has been shown that SSO formation from silane can occur in dental monomers (2). In addition to the self-condensation reaction, organosilanes may form high molecular mass silyl derivatives with hydroxylated and carboxylated dental monomers. Functional silanes, in addition to serving as components in resin blends, also can be considered as molecular-sized fillers.

In this study, methacryloxypropyltrimethoxysilane (MPTMS) was combined with non-hydroxylated dental monomers, e.g., ethoxylated bisphenol A dimethacrylate (EBPADMA) and an aliphatic diurethane dimethacrylate (UDMA), photoactivated, and the resulting resins were utilized in formulating the composites with amorphous calcium phosphate, ACP (3, 4). We hypothesized that EBPADMA and UDMA would serve as polymerizable solvents for the in situ formation of methacrylic SSOs via hydrolysis-condensation reaction of MPTMS (4–7). If predominantly fully-condensed SSOs were formed, physicochemical and mechanical performance of the composites should improve compared to the non-SSO containing formulations. A series of physicochemical tests including degree of vinyl conversion (DC), polymerization shrinkage (PS), biaxial flexure strength (BFS), water sorption (WS) and ion release were performed on ACP composite specimens formulated with UDMA or EBPADMA alone (as controls) and their MPTMS (SSO)-containing hybrids.

Experimental

Formulation of the resins

The experimental SSO resins (S-resins) were formulated from commercially available EBPADMA and UDMA monomers with MPTMS and photo-activated by the inclusion of camphorquinone (CQ; mass fraction of 0.2 %) and ethyl-4-N, N-dimethylamino benzoate (EDMAB; mass fraction of 0.8 %). The monomers and the components of photo-initiator systems (with the corresponding acronyms that are used throughout this manuscript) and the composition of the resins (% mass fraction) are provided in Tables 1 and 2. Resins were kept at 23 °C for 28 d to allow sufficient time for the condensation reaction to take place before being utilized in composite preparation (2). The hydrolytic condensation reactions of MPTMS resulting in either incompletely- or fully-condensed SSOs are illustrated in Fig. 1.

Table 1.

Monomers and components of the photoinitiator system.

Monomer	Acronym
Ethoxylated bisphenol A dimethacrylate	EBPADMA
1,6-bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4- trimethyl hexane	UDMA
Methacryloxypropyltrimethoxysilane	MPTMS
Camphorquinone	CQ
Ethyl-4-N, N-dimethylamino benzoate	EDMAB

Table 2.

Composition (mass fraction, %) of the resins.

Resin acronym	EBPADMA	UDMA	MPTMS
E	99.0	-	-
ES*	73.0	-	26.0
U	-	99.0	-
US*	-	75.6	23.4

^*S indicates anticipated SSO formation.

Fig. 1.

Schematic presentation of pathways to incompletely and fully-condensed SSOs.

ACP synthesis and characterization

ACP was synthesized as detailed earlier (3, 4). It precipitated instantaneously in a closed system at 23 °C upon rapidly mixing equal volumes of a 800 mmol/L Ca(NO₃)₂ solution and a 536 mmol/L Na₂HPO₄ solution that contained a molar fraction of 2 % Na₄P₂O₇ as a stabilizing component for ACP. The suspension was filtered, the solid phase washed subsequently with ice-cold ammoniated water and acetone, and then lyophilized. To avoid exposure to humidity, ACP was kept under vacuum (2.7 kPa) over desiccant until utilized in composite formulations.

The amorphous state of ACP was verified by powder X-ray diffraction (XRD; Rigaku DMAX 2000 X-ray diffractometer, Rigaku/USA Inc., Danvers, MA, USA) and Fourier-transform infrared (FTIR; Nicolet Magna-IR FTIR 550 spectrophotometer, Nicolet Instrumentations Inc., Madison, WI, USA) spectroscopy. Particle size distribution of Zr-ACP was determined by gravitational and centrifugal sedimentation analysis (SA-CP3 analyzer, Shimadzu Scientific Instruments, Inc., Columbia, MD, USA).

Preparation of composite specimens

Composite pastes were made by mixing the appropriate resin (mass fraction 60 %) with ACP filler (mass fraction 40 %) using hand spatulation. The homogenized pastes were kept under a moderate vacuum (2.7 kPa) overnight to eliminate the air entrained during mixing. The pastes were molded into disks (14.9 mm to 15.3 mm in diameter and 1.31 mm to 1.53 mm in thickness) by filling the circular openings of flat Teflon molds, covering each side of the mold with a Mylar film plus a glass slide, and then clamping the assembly together with spring clips. The disks were photo-polymerized by irradiating sequentially each face of the mold assembly for 60 s with visible light (Triad 2000; Dentsply International, York, PA, USA).

Degree of vinyl conversion (DC)

DC attained in the unfilled resins (copolymers) and the corresponding ACP composites was determined by FTIR spectroscopy. Spectra were acquired before photo-cure and 24 h after cure. DC was calculated from the decrease in the integrated peak area of the 1637 cm⁻¹ absorption band between the polymer (cured specimen) and monomer (uncured specimen). Absorption bands at 1538 cm⁻¹ and 775 cm⁻¹ were used as the internal reference for EBPADMA and UDMA formulations, respectively.

Polymerization shrinkage (PS)

PS of composites was measured by a computer-controlled mercury dilatometer (8) (PRC- ADAF), Gaithersburg, MD, USA). Composite pastes ((100 ± 10) mg) were irradiated twice: initially for 60 s followed by a second irradiation for 30 s after 60 min. Data were collected for a total of 90 min. PS was calculated based on the known mass of the sample and its density. The latter was determined by means of the Archimedean displacement principle using an attachment to a microbalance (Sartorius YDK01 Density Determination Kit, Sartorius AG, Goettingen, Germany).

Biaxial flexure strength (BFS)

BFS testing was employed to compare the mechanical strength of dry (after 24 h storage in the air at 23 °C) and wet (after 3 weeks of immersion in 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid (HEPES)-buffered saline solution (0.13 mol/L NaCl; pH = 7.4) at 23 °C; 100 mL saline solution/specimen) composite specimens. Piston-on-three-ball loading cell and a computer-controlled Universal Testing Machine (Instron 5500R, Instron Corp., Canton, MA, US) operated by Testworks 4 software were utilized. BFS values were calculated according ASTM F394-78 (9).

Water sorption (WS)

WS of composite specimens was determined as follows. Specimens were initially dried over anhydrous CaSO₄ until a constant mass was achieved (± 0.1 mg), and then were exposed to an air atmosphere of 75 % relative humidity (RH) at 23 °C by keeping them suspended over saturated aqueous sNaCl slurry in closed systems. Gravimetric changes of dry-padded specimens were recorded at predetermined time intervals. WS of individual specimens was calculated as WS = [(W_t − W₀)/W₀] × 100, where W_t represents sample mass at any given time and W₀ is the initial mass of dry sample.

Mineral ion release

Ion release from composite disk specimens was examined at 23 °C, in continuously stirred, saline solutions (initial volume of saline per composite specimen: 100 mL). Aliquots were taken at predetermined time intervals and the kinetic changes in the Ca and PO₄ concentrations were determined by utilizing spectro-photometric analytical methods (3).

Statistical data analysis

One standard deviation (SD) is identified in this paper for comparative purposes as the estimated standard uncertainty of the measurements. These values should not be compared with data obtained in other laboratories under different conditions. Experimental data were analyzed by analysis of variance (ANOVA; α = 0.05). Significant differences between the groups were determined by all pair-wise multiple comparisons (Tukey-test).

Results and Discussion

The amorphous character of the Zr-ACP filler was confirmed by XRD (two diffuse broad bands in the 2θ = (4 to 60)° region) and from the FTIR spectrum (two wide phosphate absorbance bands at (1200 to 900) cm⁻¹ and (630 to 550) cm⁻¹). Filler particle sizes ranged from submicron up to approximately 100 μm; calculated median diameter, d_m, of the powder was (5.9 ± 0.7) μm.

The results of the physicochemical evaluation of E, ES, U and US composites are summarized in Table 3.

Table 3.

Physicochemical assessment of the composites. Indicated are mean values with one standard deviation in parenthesis. Number of specimens: n=3 (PS and ion release), n=5 (BFS and WS) and n=6 (DC).

Property	E	ES	U	US
DC (%)	29.4 (9.3)	40.0 (8.7)	44.3 (9.0)	54.2 (8.5)
PS (vol %	4.5 (0.9)	6.5 (1.7)	3.5 (0.3)	7.0 (2.0)
BFS dry (MPa) wet	55.9 (10.0) 49.2 (5.9)	44.2 (4.0) 33.9 (7.1)	61.1 (13.9) 41.3 (8.9)	53.2 (3.9) 42.2 (8.0)
WSmax (%)	2.0 (0.5)	3.2 (0.5)	3.9 (0.2)	4.3 (0.7)
Ca (mM/L) PO4 (mM/L)	1.4 (0.2) 1.2 (0.3)	0.6 (0.1) 0.5 (0.1)	1.2 (0.2) 1.3 (0.2)	0.7 (0.1) 0.8 (0.1)

DC values attained at 24 h post-cure decreased in the following order: US > U > ES > E (one way ANOVA; p ≤ 0.001). All pair wise multiple comparisons, however, indicated that the differences between the U and US and between the E and ES formulations were not statistically significant (due to the high SDs seen in all groups). Similarly, apparently higher PS values in ES and US composites compared to E and U specimens were not statistically significant (Tukey test; reason: high SDs in SSO groups and small sample size). Although the differences in the mean BFS values among the treatment groups appeared statistically significant for both dry and wet specimens (one-way ANOVA; p= 0.049 and 0.048, respectively), all pair-wise multiple comparisons showed no significant difference between the means (p > 0.05). ES, U and US composites weakened significantly after aqueous exposure (23 %, 33 % and 21 %, respectively) compared to dry specimens. The plateau WS levels (WS_max) were reached after 45 d in all systems. WS_max in E composites was significantly lower than the WS_max in US, U and ES composites (p≤0.034). Both Ca and PO₄ solution concentration rapidly increased during the first 24 to 48 h of immersion and a significant reduction in the rate of release after 200 h. The ion release was generally lower ion from the composites formulated with MPTMS.

In situ silanization, a simple technique that involves adding the silane agent as a co-monomer to the usual dental monomer systems, has been identified as an attractive alternative to the presilanization methods for surface activation of glass fillers used in dental composites (10, 11). Indirect evidence obtained via mechanical testing shows that in situ silanization affects coupling of the glass filler and polymer matrix phases (1). In addition, the exchange reactions between the organosilane and hydroxylated monomer or water absorption by polar monomers (EBPADMA or UDMA) can lead to the formation of oligomeric SSO products. It is generally expected that blends of SSOs and silyl derivatives in dental resins could ameliorate effects of shrinkage and stress that develops upon polymerization and aid in keeping the composites mechanically stable upon aqueous immersion. In the case of ES resin formation of silyl ether can be ruled out since there are no sources of labile hydrogen in EBPADMA monomer. As shown by FTIR analysis (1), the presence of modest amounts of water in the relatively hydrophobic EBPADMA may, however, be sufficient to convert MPTMS to SSOs via un-catalyzed hydrolysis-condensation reaction of methyl silyl ether groups. A similar mechanism apparently controls the interactions in US mixtures despite the fact that UDMA is more polar than EBPADMA and has two potentially, somewhat labile hydrogens in the urethane group.

The experimental data on DC and PS did not confirm the hypothesis that inclusion of SSOs in dental resins could ameliorate effects of shrinkage that develops upon polymerization. The apparent trend of higher DC in ES and US composites probably contributed tothe unfavorable increase in PS of these composites. The unexpectedly large data scattering in both DC and PS measurements was most likely caused by the random, uneven dispersion of large ACP agglomerates throughout the composite specimens (12).

The BFS data suggest that the potential formation of SSO had no effect on the ACP filler or the filler/matrix interface. It seems that, as already shown in our earlier work with different resin formulations (3, 12, 13), the inability of ACP filler to closely interact with the resin was not altered by introduction of MPTMS into the resin, and this shortcoming had a major role in controlling the mechanical behavior of the composites.

The WS of ACP composites is a result of the water absorbed by both the resin and the ACP filler. The composites formulated with more hydrophilic UDMA absorbed more water compared to composites formulated with less hydrophilic EBPADMA. Addition of MPTMS to the resin only marginally increased the WS of US composites but has significantly increased the WS of ES composites. No accelerated ion release was seen in ES composites that could be correlated with increased WS values. However, the remineralizing potential of ACP composites has not been severely impeded by in situ silanization.

Conclusions

The addition of MPTMS and its SSO products to EBPADMA and UDMA resins did not adversely affect the critical remineralizing potential of these composites and had little effect on other properties. A series of better controlled hydrolysis/condensation experiments and identification of the condensation products may be necessary to better understand the effects of in situ silanization reactions on ACP composites.