1

Introduction

Polymeric materials have been used in dental applications for decades. Typical denture base resin is fabricated using polymethyl methacrylate (PMMA) resin. Such polymers are chosen based on availability, dimensional stability, handling characteristics, color, and compatibility with oral tissue . However the fractures of denture occasionally arise and then the repair of it needs. It is still far from ideal in fulfilling the mechanical requirements of denture made of PMMA . Numerous investigations have been directed toward reinforcing the mechanical properties . The resin should possess adequate strength to resistant a various force that occur in the oral environment.

Denture base polymers are usually supplied as a mixture of PMMA powder beads and methyl methacrylate (MMA) monomer liquid. Cross-linking agents are incorporated into the component of monomer liquid at concentration of a few percentages by volume. There is an intermediate layer between the PMMA beads and crosslinked polymer matrix, called semi-IPN layer . The monomers most often used as a crosslinking agent for MMA are ethyleneglycol dimethacrylate (EGDMA) which provides a short cross-linking chain between linear monomers. Dimethacrylate monomers polymerize to highly cross-linked, three-dimensional network . Cross-linking forms bridges between polymer chains and increases molecular weight. The extent of crosslinking plays an important role in the properties of polymer . Composition is influenced not only by remaining double bonds but also by the crosslink density of the resulting polymer structures . Cross-linkage provides a sufficient number of bridges between linear macromolecules to form a three-dimensional network that decreases water sorption, decreases solubility, and increase the strength and rigidity of the resin . The monomer systems with increased mechanical properties may change essentially the feature of denture base resin and expand their applications.

One of the new approaches for dental polymers to further improve the properties is the use of methacrylated dendritic monomer. Dendrimers contain symmetrically arranged branches emanating from a core molecule with a well-defined structure . Dendrimer has been synthesized and evaluated for dental applications . New dendrimers have been thought to improve the mechanical strength of the polymer , but by other kinds of dendrimers they may also lead to inferior mechanical strength . Due to their spherical form, the ability of dendrimer having a number of methacrylated groups with double bonds may give the opportunity to establish a good crosslink in the resin polymer.

The hypothesis in this study was that dendrimer-crosslinked resin would give better mechanical properties than EGDMA resin. The purpose of this study was to investigate the mechanical properties of denture base resin cross-linked with methacrylated dendrimer. In addition, formation of semi-IPN layer between PMMA beads and polymer matrix with various quantities of crosslinker was studied.

2

Materials and methods

Table 1 lists the materials in this study. The monomer liquid of resin was a mixture of methylmethacrylate (MMA) and crosslinker dendrimer (DD1, VTT Processes, Espoo, Finland) or crosslinker ethyleneglycol dimethacrylate (EGDMA) with five different volume percentages. The dendrimer used in this study is a large methacrylated polyester molecule with 12 methacrylate groups ( Fig. 1 ). The molecular weight of DD1 was 3617 g/mol. It was experimental, and not commercially available. Due to the high viscosity of the dendrimer, a 20 wt% amount of MMA was added to the DD1 liquid. The amount of crosslinking agent used was at 1.1, 2.3, 4.6, 6.9, 9.1 vol% relative to MMA. Powder were made by adding 2% benzoyl peroxide by weight as initiator. The molecular weight of PMMA powder was 350,000 g/mol.

The structural formulas of (a) dendrimer DD1 and (b) ethyleneglycol dimethacrylate (EGDMA).

The test specimens were fabricated from autopolymerizing resin with the powder/liquid ratio of 10 g/7 ml. The fluid was poured into a rectangular-shaped mold (3 mm × 10 mm × 65 mm) and waited for 3 min before polymerization. The specimens were polymerized in distilled water maintained at 55 °C under pressure of 0.4 MPa for 20 min. The cured specimens were removed from the mold, and cleaned under running water with up to No. 800 silicone-carbide paper (Stuers, UK). The number of specimens were eight each group. Test specimens were stored dry at room temperature before testing.

The flexural strength and flexural modulus of each group was measured using three-point bending test with a universal testing machine (model LRX; Lloyd Instruments Ltd., Fareham, England). The specimens were tested dry at room temperature. A crosshead speed of 5 mm/min was used, and the distance between the supports was 50 mm. Load was applied until fracture occurred. Results were recorded with PC software (Nexygen; Lloyd Instruments Ltd., Fareham, England). Flexural strength (FS) was calculated from the formula:

where F is the applied load (N) at the highest point of the load–deflection curve, l is the span length (50.0 mm), b is the width of the test specimen, and h is the thickness of the test specimen.

The flexural modulus (FM) of the test specimen was calculated from the formula:

where d is the deflection corresponding to load F at a point in the straight line portion of the trace.

For Surface microhardness test (MHN), further eight specimens each group were fabricated as above. The test specimens for hardness measured were further polished using No. 1200, 2400 and 4000 SiC paper, and then 0.1 μm alumina particles. Before testing, tetrahydrofuran (THF) was applied to the surface of resin for 30 s for visualizing the PMMA beads. Surface microhardness testing was performed on randomly selected portion of matrix area of polymer. A load of 245.3 mN for 10 s was employed to measure hardness in the field of matrix area. One randomly selected indentation was carried out each specimen. Surface microhardness number (MHN) was calculated from:

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