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Available online at ww w .sciencedirect.com

www.elsevier.com/locate/jmbbm

Research Paper

Effect of high-pressure polymerization on mechanical properties of PMMA denture base resin

Natsuko Murakamia, Noriyuki Wakabayashia,n, Rie Matsushimab, Akio Kishidab, Yoshimasa Igarashia

aRemovable Partial Denture Prosthodontics, Department of Masticatory Function Rehabilitation, Division of Oral Health Sciences, GraduateSchool, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, JapanbDepartment of Material-based Medical Engineering, Division of Biofunctional Restoration, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, Japan

a r t i c l e i n f o

Article history:Received 6 December 2012Received in revised form26 December 2012Accepted 30 December 2012Available online 8 January 2013

Keywords:High pressure Polymerization ResinDenture PMMA Toughness

a b s t r a c t

The aim of this study was to assess the effect of high-pressure polymerization on mechanical properties of denture base resin. A heat-curing denture base resin and an experimental PMMA were polymerized under 500 MPa of pressure by means of an isostatic pressurization machine at 70 1C for 24 h to make rectangular specimens whose dimensions were 30 mm 2 mm 2 mm. Each specimen was deflected on a three-point flexural test until either fracture occurred or the sample was loaded up to 8 mm in deflection. The molecular weight of the PMMA without filler was analyzed using the high-speed liquid chromatography system. Increased ductility without fracture was shown in the specimens subjected to high pressure, while most of the control specimens (ambient pressure) fractured. The mean toughness of the PMMA specimens polymerized under the high pressure was significantly higher than the same material polymerized under ambient pressure (po0.01). The high pressure groups of the denture resin and the PMMA revealed a significantly lower mean 0.2% yield stress, flexural strength, and elastic modulus than control groups (po0.01). There were certain amounts of higher molecular weight polymers in the high pressure specimens than were present in the controls. The increased toughness shown in the PMMA polymerized under the high pressure was presumably attributed to the higher molecular weight produced by the pressure. The result suggests a potential application of the high-pressure polymerization to the development of PMMA-baseddenture resin with improved fracture resistance.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The fracture resistance of acrylic denture base, whether for conventional or implant-supported dentures, is one of the

important factors in terms of safety, longevity, and patient satisfaction with removable prosthodontics. The most com- mon material used to fabricate the denture base is an acrylicresin made from a mixture of methyl-methacrylate (MMA)

nCorresponding author. Tel.: þ81 3 5803 4935; fax: þ81 3 5803 4946.E-mail addresses: n . mu r akami.r p ro@tmd.ac.jp ( N . Murakami), wak a ba y ashi.rpro@tmd.ac.jp ( N . W akab a yashi),

matsushima.mbme @ tmd.ac.jp (R. Matsushim a ), kishida.mbm e @tmd.ac . jp (A. Kishi d a), igarashi.rp r o@tmd.ac.jp ( Y . Igarashi).

1751-6161/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. h t tp://dx.doi. org/10.1016/j.jmbbm.2012.12.011

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and poly (methyl methacrylate) (PMMA). This material pos- sesses favorable working characteristics, polishability, stabi- lity in the oral environment, and excellent aesthetic appearance (Narva et al., 2005). However, the literature has reported that the frequency of acrylic fracture is estimated to be between 57% and 64% of all failures of removable dentures (Darbar et al., 1994; Vallittu et al., 1993). The physical proper- ties exhibited by this material are sensitive to the degree of conversion, processing technique, and conditions presented by the oral environment. Stress-susceptible sites often seen in removable partial dentures may also have a significant influence on the fracture initiation in the acrylic denture base (Bural et al., 2010; Carlsson et al., 1976; Shimizu et al., 2004). It is therefore recognized that the material does not meet the mechanical requirements for prostheses that are subjected to high stress under masticatory function (Koroglu et al., 2009; Memon et al., 2001; Narva et al., 2005).

High-pressure polymerization is one of the processing technologies that have been developed for the synthesis of high-molecular-weight polymers with well-defined structures. The effect of pressure has been reported for reversible addition fragmentation (chain) transfer polymerization (RAFT), result- ing in higher polymerization rates and in polymers with higher molecular weights and lower polydispersity (Rzayev and Penelle, 2004). High pressures up to 600 MPa have been reported to enable relatively fast synthesis of well-defined molecular weight polymethacrylates, even at room tempera- ture (Kwiatkowski et al., 2008). It has also been reported that polymerization under pressures up to 500 MPa at 60 1C facil- itate atom transfer radical polymerization (ATRP) of MMA, resulting in higher polymerization rates and in polymers with higher molecular weights and lower polydispersity (Arita et al.,2008; Rzayev and Penelle, 2002). A recent report indicated that polymerization under a high pressure of 250 MPa at a high temperature of 180 1C increased the flexural strength and hardness of commercially available dental resin composites (Nguyen et al., 2012). In that study, a significant increase of the fracture toughness (KIC) with the high-pressure/high tempera- ture condition was shown in two dental composite materials out of four. However, the effect of the high pressure polymer- ization on the mechanical properties of the PMMA denture base resin is not known.

In this study, we utilized a high-pressure reaction system incorporating a pressure-resistant chamber to polymerize the

PMMA specimens under pressures up to 980 MPa. The pur- pose of this study was to test the hypothesis that applying high pressure during polymerization increases the fracture resistance of the PMMA-based denture base resin.

2. Materials and methods

The materials used in this study, with their batch numbers, manufacturers, and powder/liquid compositions, are listed in Table 1.

2.1. Packing of samples

The specimens were prepared using a Teflons plate (PTFE Sheet, F-8035-04, Flon Industry, Tokyo, Japan) with eight rectangular mold cavities (30 mm 3 mm 3 mm for each). A commercially available PMMA-based heat-curing denture base resin was mixed at a room temperature of 25 1C in accordance with the manufacturers’ instructions at the powder/liquid ratio of 100 mg/43 mL. When the mixture reached the consistency of dough, it was poured into the mold and covered with a polyvinyl chloride plate sealed with a polyethylene film. Each mold plate was then packed and sealed in a polyethylene (PET) container (Seisannipponsha Ltd., Tokyo, Japan), followed by secondary packing with another PET container that was filled with water.

We prepared two experimental PMMA materials, one with filler material [denoted as PMMA/F(þ)] and the other without filler [denoted as PMMA/F( )] (Table 1). In the preliminary molecular weight measurements, the Acron and the PMMA/ F(þ) did not dissolve in the gel permeation chromatographic (GPC) solvent, probably due to the inclusion of the cross- linking polymers, thus it was not possible to measure the molecular weight of these materials accurately. Therefore, the PMMA/F( ) was prepared specifically to ensure the for- mation of high-molecular-weight polymers. The PMMA/F( ) was prepared by a mixture of MMA and benzoyl peroxide (BPO) as the polymerization initiator, with N,N-dimethyl-p- toluidine (DMPT) as the polymerization promoter. The pack- ing of each PMMA specimen was conducted using the same method as for the Acron specimens.

Table 1 – Materials used in the study.

Sample code Composition Batch no. Manufacture

Acron Powdera Acron pink 1012201 GC, Tokyo, JapanLiquid Acron 1203121

PMMA/F(þ) Powder: PMMA (Daiyanal BR88) rcclg1864-11 Mitsubishi Rayon., Tokyo, JapanLiquidb MMA DCE2809 Wako Pure Chemical Industries, Osaka, Japan

BPO 10160333 Alfa Aesar, MA

PMMA/F( ) Liquidc MMA DCE2809 Wako Pure Chemical Industries, Osaka, JapanBPO 10160333 Alfa Aesar, MADMPT MR0FI Tokyo Chemical Industry, Tokyo, Japan

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a Powder contains BPO according to the manufacturer’s note.b Liquid contains with BPO with the molar ratio of 180:1 for MMA:BPO.c Liquid contains BPO and DMPT with the molar ratio of 180:1:4 for MMA:BPO:DMPT.

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2.2. Polymerization

Polymerization was conducted under high pressure, with ambient pressure as the control. The high-pressure polymer- ization was performed by using an isostatic pressurization machine (Dr. Chef, Kobe Steel, Ltd., Hyogo, Japan). The sample containers were immersed in transmission fluid in the sample chamber of the machine. For the Acron and the PMMA/F(þ) samples, the pressure was increased at a rate of33 MPa/min at 70 1C until it reached 500 MPa. After the pressure was maintained for 24 h, the chamber was depres- surized at a rate of 33 MPa/min until it reached ambient pressure. The same temperature and polymerization time were employed for the control specimens. For the PMMA/F( ) samples, one of the high pressure levels of 500 MPa, 800 MPa, or 980 MPa was used, with the temperature maintained at25 1C during the polymerization process of 60 min. Control specimens were polymerized under ambient pressure in a heat-retention bath. Each specimen taken out from the mold was trimmed and finished using abrasive papers (250 grit to600 grit). The average final dimensions of all specimens measured using a digital caliper was 3073 mm 270.1 mm 270.1 mm in length, width, and thickness, respectively. Eight specimens for the high pressure group and 16 specimens for the control group were prepared for the Acron and the PMMA/ F(þ) and were stored in distilled water at 37 1C for 24 h before the flexural test.

2.3. Flexural test

Each specimen from the Acron and the PMMA/F(þ) groups was subjected to a three-point flexural test on a hydraulic servo testing machine with axial loading units (Instron 1123, Canton, MI, USA). The specimens were centered on the test rig so that the loading wedge, set to travel at a crosshead speed of 1 mm/ min, engaged the center of the upper surface of each speci- men, with a distance between the supports of 20 mm. The specimens were tested at room temperature under ambient pressure. Each specimen was deflected with a vertical cross- head speed of 1 mm/s until fracture occurred, or it was loaded until the loading rod reached a pre-set maximum vertical driving distance of 8 mm. The axial load and displacement were recorded by 5 kN load cells with transducers mounted on linear actuators. The transverse strength (S, N/mm2) was calculated from the following equation:

3FlS ¼ 2bh2

where F (N) is the applied load at the highest point of the load–deflection curve, l is the span length, b is the width of the test specimen, and h (mm) is the thickness of the test speci- men. The 0.2% yield stress of each specimen was also computed.

The modulus of elasticity (E, N/mm2) was computed fromfollowing equation:

F1 l3

E ¼ 4bh3 d

where d (mm) is the deflection corresponding to load F1 (N) at a point in the straight line portion taken from the

load–deflection curve. The point was automatically detected by the software of the testing machine.

Toughness is the amount of elastic and plastic deforma- tion energy required to fracture the acrylic resin specimens. It was measured as the total area under the load–deflection curve of the tested specimens. The values for toughness (T, N mm) were calculated from following equation:

T

EðbÞ¼

lbh

where E(b) is breaking energy calculated from the area under the load–deflection curve.

The mean values for each test group were calculated and the non-paired t-test was used to test the significant difference between the high pressure and the ambient polymerization. All statistical analyses were performed using SPSS (15.01, SPSS Inc., Chicago, IL, USA) on a significance level of 1%.

2.4. Microscopic observations

The fractured and non-fractured specimens after the load test were gold sputtered (250 A˚ thickness) and examined by a scanning electron microscope (S-3400NK, Hitachi, Tokyo, Japan) at a 5 kV accelerating voltage. The SEM photomicro- graphs were taken with 30 to 50 magnifications for visualobservations.

To observe the intermediate layer between the PMMA beads and polymer matrix, specimens were further polished using abrasive paper (800 grit to 2000 grit) and finally with0.1 mm alumina particles. To expose the PMMA beads by slightly dissolving the surface of resin, tetrahydrofuran (THF) was applied as a solvent for 30 s. The observation using the light microscope images (AX70 PROVIS, Olympus, Tokyo, Japan) was visually served to evaluate the interfacial characterization in the surface (Kawaguchi et al., 2011a, 2011b).

2.5. Gel permeation chromatography (GPC)

Polymerized samples from the PMMA/F( ) group were taken out for GPC measurement to determine their molecular weight and distribution. The analysis was carried out at40 1C on a high-speed liquid chromatography system (Shodex GPC-101) equipped with a guard column (Shodex GPC LF-G), two 30-cm mixed columns (Shodex GPC LF-804, exclusion limit of 2 106), and a differential refractometer (Shodex RI-71S). N,N-dimethyl formamide (DMF) containing 10 mM LiBr was used as an eluent at a flow rate of 0.8 mL/min. The GPC system was basically calibrated by PMMA standards {Shodex, STANDARD M-75, Mp ¼ [(2.89 103)–(9.65 105)]}.

3. Results

The specimens were safely polymerized by high pressure up to 980 MPa, and the external appearance of the high pressure specimens was not distinguishable from that of the control specimens. All the high pressure specimens were loaded up to a preset maximum displacement of 8 mm without fracture, except for two of the Acron and one of the PMMA/F(þ) samples, all of which fractured into two pieces (Figs. 1 and 2). The

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Fig. 1 – Stress–strain relationship of the Acron denture base resin. All high pressure specimens (n ¼ 8) were loaded up to a pre-set maximum deflection of 8 mm without fracture, except for two specimens that fractured. All the control specimens (n ¼ 16) exhibited fractures, except for two that were loaded up to the displacement limit.

Fig. 2 – Stress–strain relationship of the PMMA. All high pressure specimens (n ¼ 8) were loaded up to a pre-set maximum deflection of 8 mm without fracture, except for one specimen that fractured. All the control specimens(n ¼ 16) exhibited fractures.

stress–strain curve of the high pressure specimens showed elastic behavior in the early stage, a peak load, and subsequent plastic behavior. All the ambient control specimens exhibited a typical stress–strain curve until reaching a sharp decrease in stress due to fracture, except for two of Acron samples that were loaded up to the displacement limit. There was consider- able deviation in the maximum deflection of the fractured specimens in the control group.

The high pressure group of each material exhibited sig- nificantly lower mean values for the 0.2% yield stress

(po0.01), flexural strength (po0.01), and elastic modulus (po0.01) in comparison with the control group of the corre- sponding material (Table 2). For each material, no statistically

significant difference between the two pressure conditions was found in the strain at the maximum stress. No statisti- cally significant difference in the toughness between the different pressures was found in the Acron, whereas the high pressure group revealed significantly higher toughness (po0.01) than the control in the PMMA.

The SEM fracture surface microphotographs indicated that the side surface of all the non-fractured high pressure speci- mens revealed distributions of crazing on the tensile bottom surface (Fig. 3). Cleavage-like ratchet marks were evident on the bottom surface, and they created a number of curved crack lines that were originally the straight polishing marks. It was clearly indicated in the Acron and the PMMA/F(þ) that the numbers of the cracks were more for the non-fractured high pressure specimens (Fig. 3, left) than those for a few non-fractured control specimens (Fig. 3, right). The cracks generally propagated deeper along the polishing marks at the corner of the bottom plane. The surface cracks and the curved polishing marks were not observed on the side and the bottom surfaces of the fracture specimens (Fig. 3, center). The border between the PMMA beads and the polymer matrix were visible in all the high pressure specimens of the Acron and the PMMA/F(þ) (Fig. 4, upper left). The interpenetrating polymer network (IPN) layer was also confirmed in the high pressure groups after the THF treatment (Fig. 4, lower left). While, the border of the PMMA particles was slightly visible in the SEM of the fractured control specimens (Fig. 4, upper right) but not detected for the control groups (Fig. 4, lower right).

Fig. 5 shows the differential molecular weight distribution of the GPC analysis of representative specimens for the PMMA/F( ) that were polymerized under different levels of high pressure as well as under ambient conditions. The control specimen revealed a monophasic wide distribution, whereas all the high pressure specimens provided a diphasic distribution, in which the number-average molecular weight (Mn) of the larger molecular weight peak was higher than that of the control. The differential weight fraction demon- strated in the larger weight peak increased as the polymer- ization pressure increased. On the other hand, the peaks for all the high pressure groups were located above the 106

molecular weight, meaning that they were likely close to the exclusion limit of the method. This indicated that these high pressure specimens potentially possessed a higher molecular weight fraction than those detected with the method used in this study.

4. Discussion

The hypothesis that the fracture resistance is higher with high pressure than it is under ambient pressure was partially supported by the results, which showed significantly higher average toughness of the PMMA in the high pressure samples than in the ambient pressure samples. The increased tough- ness was consistent with the results reported in a previous study for dental composites that were polymerized under the high pressure of 250 MPa (Nguyen et al., 2012). The results of the weight measurement of the PMMA/F( ) indicated that there were certain amounts of higher molecular weight

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Table 2 – The mean and the standard deviations of the mechanical properties for the Acron denture base resin and the experimental PMMA with filler in the three-point flexural load test.

Acron

Control High pressure

PMMA/F(þ)

Control High pressure

Mean 83.9 656.3 85.2 66.3 0.2% yield stress (MPa)

(S.D.) 1.8 1.3 2.6 2.3Mean 124.0 95.3 128.0 97.4

Flexural strength (MPa)(S.D.) 17.3 9.9 4.8 2.5Mean 6.26 5.81 6.44 6.14

Strain at the maximum stress (%)(S.D.) 1.78 1.41 1.1 0.29Mean 3.36 2.75 3.32 2.79

Elastic modulus (GPa)(S.D.) 0.05 0.03 0.10 0.10Mean 0.94 1.56 0.72 1.76

Toughness (N mm)(S.D.) 0.81 0.33 0.18 0.55

n Asterisk indicates a significant difference between the high pressure and control within the same material groups (po.01)

Fig. 3 – The SEM images of representative Acron specimens after the loading test. The images on the left: a non-fractured high pressure specimen; center: a fractured control (ambient pressure) specimen; right: a non-fractured control specimen. The images on the upper row show the side surface ( 30), while the corresponding specimens on the low are viewed from the tensile bottom surface ( 50). White arrows indicate the crazing marks on the tensile bottom surface. S: side surface, T:tensile bottom surface, C: compressive upper surface.

polymers in the high pressure polymerization groups than were present in the control group. In addition, the fraction of the high-molecular-weight polymers increased as the pres- sure level increased. It was therefore highly likely that the increased toughness of the high pressure specimens was attributed to the increased average molecular weight of the polymer matrix. The increased molecular weight was con- sistent with previous studies reporting that the polymeriza- tion under pressure from 300 MPa (Kojima et al., 2002) to500 MPa (Arita et al., 2008) increased the average molecular weight and the polymerization rates of MMA. In a free radical polymerization, it was explained that high pressure remark- ably increased the polymerization rate, with an enhanced propagation rate constant and a reduced

termination rate constant (Arita et al., 2008; Kwiatkowski et al., 2008). Another potential reason for the increased toughness was the reduc- tion of the internal voids and defects (Brosh et al., 2002; Chadwick et al., 1989), because the high pressure environment

could reduce the distribution of the voids that were potentially created during the polymerization process (Carmai and Dunne, 2003; Onishi and Yoshikawa, 2002). It was reported in a study using dental composites that the high pressure/high temperature polymerization resulted in a reduction in the number and size of defects (Nguyen et al., 2012) that led to the improvement of the fracture toughness. The high-molecular- weight fraction could increase processing difficulties because of its enormous contribution to the melt viscosity (Ahmed et al., 2009; Sperling, 2001). For these reasons, the low end of the distribution might be created and act as a plasticizer, softening the material. Certainly it does not contribute as much to the tensile strength (Ahmed et al., 2009; Sperling,2001).

The high pressure of 500 MPa during the polymerization process decreased the elastic modulus, the yield, and the maximum flexural strength of the PMMA-based heat curing resin. Emphasis should be placed on the visible interface

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Fig. 4 – Typical magnified images of the fractured surfaces of the Acron specimens. The SEM images (upper) and the light microscopic images following the THF treatment (lower), are shown for typical high pressure (left) and control specimens (right). Black arrows indicate the interface between a PMMA particle and the polymer matrix. White triangles indicate the IPN layer (lower left).

Fig. 5 – GPC traces of PMMA polymerized under ambient(control) and high pressure (500 MPa, 800 MPa, and980 MPa). The x-axis in the logarithmic scale represents the molecular weight and the y-axis represents the differential weight fraction calculated from the cumulative weight percentage. Molecular weights were measured relative to PMMA standards [(Mp ¼ (2.89 103)–(9.65 105)].

between the PMMA particles and the polymer matrix in the high pressure specimens, which was not seen in the control specimens. In the heat-polymerized resin under the ambient pressure, the PMMA particles were likely to swell by absorb- ing the monomer, resulting in the homogenized appearance in the micrographs (Fig.4, right). However in the high pressure polymerization, the interface became visible in the SEM, presumably because the high pressure increased the inter- facial stress between the materials of dissimilar elastic modulus and coefficient of compressibility (Sasuga and Takehisa, 1978). Since the

Acron and the PMMA/F(þ) of this study were heat-cured after the dough stage samples were placed in the chamber, the effect of the compression by the

high pressure could play a significant role in creating the interfacial stress on the border between the particles and the matrix. Such negative effects of the high pressure polymer- ization may partially explain the crack initiation on the tensile bottom surface of the loaded specimens. The distribu- tions of the crazing potentially caused the decrease of some mechanical properties and the strengths, although they were still within the range of the requirement specified by the ISO standard for the denture base polymers and copolymers (ISO20795-1:2008).

In the high pressure specimens, the crazes propagated perpendicular to the applied tension but were arrested by the matrix polymer without causing bulk fracture. Although such crazing generally appears at highly stressed regions asso- ciated with flaws, stress concentrations, and molecular inho- mogeneities (Ishikawa and Takahashi, 1991), the process of craze growth prior to cracking absorbs fracture energy and effectively increases the fracture toughness of a polymer (Atkins et al., 1975; Green and Pratt, 1974). Therefore, the formation of crazing on the tensile surfaces was indicative of the increased ductility of the PMMA specimens.

During polymerization, the monomers dissolve and diffuse to the surface of the PMMA beads. By treating the polymer structure with a suitable solvent such as THF used in this study, the IPN layer can be observed surrounding PMMA beads in the microscope (Vallittu and Ruyter, 1997; Vallittu et al.,1997). In the high pressure specimens of this study, the distinctive topographical features of the PMMA particles became visible and the existence of the IPN layer was also detected. While the thickness of the IPN layer was dependent of temperature and the contact wetting time of the liquid monomers, it has been indicated that the increased thickness of the layer gained the creep resistance (Oysaed and Ruyter,1989) and other mechanical properties of the autopolymeriz- ing denture base resins (Kawaguchi et al., 2011a). However, the influence of the IPN layer on the mechanical properties of the heat-curing denture base resins remains to be elucidated.

The high pressure of 500 MPa was employed for the flexural test specimens, although the highest limit of the system used was 980 MPa. Previous literature indicated that the application of high pressure on a monomer mixture for polymerization decreases the intermolecular distances and reduces the free volume (Murli and Song, 2010). Due to reduced mobile capability under an extremely high pressure (above 1 GPa), the monomers are likely to transform into solids and form monomer crystals, thus reducing the poly- merization degree (Nguyen et al., 2012; Schettino et al., 2008). In fact, we found that some pilot specimens prepared under the high pressure of 980 MPa showed lower strengths than those made with the pressure of 500 MPa. It can be specu- lated that the higher pressure more than 500 MPa further reduced the interfacial integrity between the PMMA beads and the polymer matrix. Further studies are needed to optimize the pressure level in combination with the tem- perature and processing time for production of the PMMA denture base resin that can stably exhibit higher mechanical properties than those shown in this study. The results of this study may increase the understanding of the effect of the high pressure polymerization on the mechanical properties of the PMMA-based heat-curing denture base resin and lead

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to further developments of this polymerization method. Future investigations are also encouraged to assess the poten- tial application of the high pressure polymerization in the production of the CAD/CAM blocks for the dental prostheses and of the toughened PMMA particles for the conventional heat-activated or chemically activated denture base resins.

5. Conclusions

Polymerization under a high pressure of 500 MPa increased the toughness of the experimental PMMA with filler, while decreasing the yield and flexural strengths and the elastic modulus. The increased toughness obtained by the high- pressure polymerization in the commercial denture base resin and the experimental PMMA resin suggests potential development of biomedical PMMA materials with improved fracture resistance.

Acknowledgement

This study was supported by a Grant-in-Aid (#24592902 toN.W.) from the Japan Society for Promotion of Science/MEXT.

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