Rp

Ja

Sb

(c

a

A

R

R

9

A

K

C

P

M

P

FT

0d

d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 529–534

Available online at www.sciencedirect.com

jo u rn al hom epa ge : www.int l .e lsev ierhea l th .com/ journa ls /dema

esin composite blocks via high-pressure high-temperatureolymerization

ean-Francois Nguyena, Véronique Migonneyb, N. Dorin Rusec,∗, Michaël Sadouna

Unité de Recherches Biomatériaux Innovants et Interfaces (URB2I-EA4462), Faculté de chirurgie dentaire, Université Paris Descartes,orbonne Paris Cité, Paris, FranceCNRS UMR- Université Paris 13. Laboratoire de Chimie, Structure et Propriétés de Biomatériaux et d’agents Thérapeutiques

CSPBAT).UFR SMBH, Bobigny, France, Institut Galilée, Villetaneuse, FranceFaculty of Dentistry, University of British Columbia, Vancouver, Canada

r t i c l e i n f o

rticle history:

eceived 29 July 2011

eceived in revised form

November 2011

ccepted 1 December 2011

eywords:

omposites

ressure polymerization

echanical properties

hysical properties

a b s t r a c t

Objectives. The aim of this study was to thermo-polymerize under high pressure four com-

mercially available dental resin composites to obtain and characterize composite blocks

suitable for CAD/CAM procedures.

Methods. Gradia (GC, Japan), Vita VM LC (Vita Zahnfabrik, Germany), Grandio (VOCO,

Germany), and EsthetX (Dentsply, Germany), were selected for this study. Paradigm (3 M

ESPE, USA), a CAD/CAM composite block, was included for comparison. Composite blocks

were obtained through polymerization at high-temperature high-pressure (HT/HP). Samples

for mechanical/physical characterizations were cut from Paradigm and HT/HP composite

blocks while control samples were obtained by photo-polymerizing (PP) the materials in

molds. Flexural strength (�f), fracture toughness (KIC), hardness, and density (�) were deter-

mined and compared by pairwise t-tests ( = 0.05). Fractured surfaces were characterized

under a scanning electron microscope.

Results. The results have shown that HT/HP polymerization resulted in a significant (p < 0.05)

increase in �f, hardness, and � for all composites investigated. Even if KIC of all materials

was increased by HT/HP polymerization, significant increases were detected only for Gra-

dia and EsthetX. The Weibull modulus of HT/HP polymerized composites was higher than

that of PP counterparts. HT/HP materials had higher �f, Weibull modulus, and KIC com-

pared to Paradigm. The most significant SEM observation of fractured KIC specimens from

all the materials tested was the presence of fewer and smaller voids in HT/HP polymerized

composites.

Significance. The results of this study suggest that HT/HP polymerization could be used

to obtain dental resin composite blocks with superior mechanical properties, suitable for

CAD/CAM processing.

emy

© 2011 Acad

∗ Corresponding author at: Faculty of Dentistry, University of British Caculté de chirurgie dentaire, Université Paris Descartes, Sorbonne Pariel.: +33 1 58 07 68 01; fax: +33 1 49 65 99 89.

E-mail address: dorin@dentistry.ubc.ca (N.D. Ruse).109-5641/$ – see front matter © 2011 Academy of Dental Materials. Puoi:10.1016/j.dental.2011.12.003

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

olumbia, 2199 Wesbrook Mall, Vancouver, BC V6T 1Z3 Canada,s Cité, 1 rue Maurice Arnoux, 92120 Montrouge, France.

blished by Elsevier Ltd. All rights reserved.

l s 2

530 d e n t a l m a t e r i a

1. Introduction

Due to esthetic considerations and to the growing concernrelated to metal ion release from alternative restorative mate-rials (amalgam), it is expected that “future developmentswill focus on highly esthetic organic–inorganic systems withgood bonding to tooth structure, long-term stability and wearresistance, high biocompatibility, and simplicity of clinicalhandling” [1]. Resin composites, along with adhesive dentistry– the associated procedure of attaching them to hard toothtissues, represent probably the most promising developmentin restorative dentistry. Over the last ten years, significantprogress has been achieved in improving the properties ofdental resin composites to enable their usage in posterior,load bearing restorations [2,3]. Furthermore, the use of resincomposites has expanded from a direct restorative material tolaboratory processed composite blocks to be used in CAD/CAMsystems for the fabrication of indirect restorations, such asinlays, onlays, crowns, and even bridges [4]. The foreseenadvantages of a CAD/CAM processed composite block over adirect composite restoration are multiple and comprise a bet-ter polymerized material, less porosity, more homogeneity,the avoidance of the in vivo polymerization shrinkage, andthe avoidance of operator-related variables, to mention only afew. Moreover, using resin composite blocks instead of ceramicblocks for CAD/CAM procedures could significantly reduce thetime of fabrication and have less wear on the cutting equip-ment [5]. Resin composite restorations could be maintainedand repaired easier than ceramic ones. In comparison, how-ever, with ceramic blocks intended for the same application,resin composite blocks have inferior mechanical propertiesand, therefore, major concerns exist with regards to their longterm in vivo performance.

The aim of this study was to thermo-polymerize underhigh pressure four commercially available resin compositesto obtain and then characterize resin composite blocks suit-able for use in CAD/CAM procedures. The null hypothesistested was that physical and mechanical properties of com-posite blocks obtained via thermo-polymerization under highpressure are no different from those of photo-polymerizedcounterparts.

2. Materials and methods

Four commercially available dental resin composites, Gradia(GC, Japan), Vita VM LC (Vita Zahnfabrik, Germany), Grandio(VOCO, Germany), and EsthetX (Dentsply, Germany), wereselected for this study. Paradigm (3 M ESPE, USA), a com-mercially available resin composite block for CAD/CAM, wasalso included for comparison. Table 1 summarizes the detailsregarding the materials used, while samples preparation andcharacterization are described in the following sections.

Gradia and VITA VM LC are light-curable indirect com-posites and are indicated for inlays, onlays, veneers, and for

full and partial crowns, whereas Grandio and EsthetX arelight-curable direct composites. In order to replicate manu-facturers’ recommended laboratory curing procedures for theindirect composites, the initial photo-polymerization of all

8 ( 2 0 1 2 ) 529–534

composites was followed by a 30 min post-cure in a curingchamber (PLC 4000, Schütz Dental, Germany). This proce-dure, furthered referred to as “photo-polymerization” (PP),would increase the degree of conversion and would there-fore maximize the mechanical/physical properties of allmaterials.

2.1. Thermo-polymerization under high pressure

A differential scanning calorimeter (DSC 823, Mettler Toledo,Greifensee, Switzerland) was used to determine the thermo-polymerization temperature of each material at atmosphericpressure (0.1 MPa). Since the determined polymerization tem-peratures were in the (160–180) ◦C range, it was decided toperform all the polymerization reactions at 180 ◦C. There-after, approximately 100 g of each material was placed insidea flexible silicone tube (25 mm internal diameter), which wasthen introduced into an autoclave (custom-built for this study)with pressure and temperature control (LabVIEW version 8.2,National Instruments, USA). A thermocouple was placed in theproximity of the sample to enable accurate monitoring and,via feed-back, control of the temperature. In the first stage, thepressure within the autoclave was increased to 250 MPa at arate of 0.1 MPa/s at ambient temperature. In the second stage,the temperature was increased to 180 ◦C, at a rate of 2 ◦C/min.The sample was then maintained at 250 MPa and 180 ◦C for60 min before being cooled off and the pressure released. Fourcylindrical composite blocks (25 mm × ∼200 mm), one fromeach composite, were made.

2.2. Flexural strength

One part of each high-temperature high-pressure (HT/HP)polymerized composite block was cut, with an Isomet saw(Buehler) under water irrigation, into 25 to 30 rectangular bars[(2 mm × 2 mm × 20 mm)]. Paradigm blocks were cut similarlyto obtain 25 samples.

Control specimens (n = 30) were obtained by photo-polymerizing resin composites packed into (2 × 2 × 20) mmstainless steel molds. The samples were irradiated three times(once on the middle of the sample and one time on eachextremity of the sample) for 40 s with a LED curing unit(Radii, SDI, Victoria, Australia) operating at a power densityof 897 mW/cm2 (measured with a Curing radiometer, DentsplyCaulk, Milford, USA). After de-molding, the samples were post-cured for 30 min in a curing chamber.

Each sample was polished on 4000 grit silicon carbide (SiC)paper on a water-irrigated grinding wheel and its dimensionswere measured with a digital caliper (Mitutoyo Co., Kawasaki,Japan) before being tested.

Flexural strength (�f) was determined by loading the sam-ples in a three point bending device (with a 16 mm spanbetween the supports) at a cross-head speed of 1 mm/min,using a computer controlled (NexyGen®, Lloyd, UK) Lloyd LRX(Lloyd, UK) universal testing machine.

Flexural strength was calculated using the formula:

�f = 3FL

2hc2

d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 529–534 531

Table 1 – Details of materials used.a

Composite (manufacturer) Matrixb Filler Filler content Group Polymerizationparameters

Gradia (GC) UDMA, EDMA Silica &Pre-polymerizedfiller

Weight:75% GCL Light polymerization

GCP 250 MPa + 180 ◦C for1 h

Grandio (VOCO) BisGMA, TEGDMA,Urethane-BisGMAadduct

Barium-alumino-borosilicate; fumedsilica

Weight: 87%Volume:71.4%

VOL Light polymerization

VOP 250 MPa + 180 ◦C for1 h

EsthetX (Dentsply) BisGMA, TEGDMA,bis-EMA adduct

Barium-fluoro-alumino-borosilicate; fumedsilica

Weight:77%Volume: 60%

DEL Light polymerization

DEP 250 MPa + 180 ◦C for1 h

VitaVM LC (VITA Zahnfrabrik) UDMA, TEGDMA,BisGMA, 2-dimethylaminoethylmethacrylate

Silica filler Weight: (45-48) % VIL Light polymerization

VIP 250 MPa + 180 ◦C for1 h

Paradigm (3 M ESPE) BisGMA, TEGDMA Silane-treatedceramic

Weight: (80-90) % P As receivedCAD/CAM blocks

a The composition of the organic matrix and filler content was obtained from manufacturers’ data.negly

ws

2

Ocegtt

bci(osp

lwsatuicTf

b BisGMA is bisphenol A glycol-dimethacrylate; TEGDMA is triethyleethylenemethacrylate; UDMA is urethanedimethacrylate.

here F is the load at fracture, L the specimen span, h thepecimen width, and c the specimen height.

.3. Fracture toughness

ne part of each HT/HP polymerized composite block wasut, with an Isomet saw (Buehler) under water irrigation, intoight rectangular bars [(8 × 8 × 15) mm], which were then wetround on 800 grit SiC to obtain (6 × 6 × 6 × 12) mm equilateralriangular prisms. Eight Paradigm samples were obtained inhe same manner.

Eight control specimens for each material were obtainedy photo-polymerizing (PP) resin composites, packed andured in increments, into (6 × 6 × 6 × 12) mm teflon molds. Thencrements were irradiated for 40 s with a LED curing unitRadii, SDI, Victoria, Australia) operating at a power densityf 897 mW/cm2 (measured with a Curing radiometer, Denst-ply Caulk, Milford, USA). After de-molding, the samples wereost-cured for 30 min in a curing chamber.

Fracture toughness (KIC) was determined using the notch-ess triangular prism (NTP) specimen KIC test. [6] The prismsere secured into one half of the specimen holder and a

harp scalpel was used create a small (<0.1 mm-deep) defectlong the loading edge before securing the second half ofhe specimen holder. The specimens were loaded in tension,sing a computer controlled (Bluehill, Instron) universal test-

ng machine (Instron model 4301, Instron Canada Inc.), at arosshead speed of 0.01 mm/min until crack arrest or fracture.he maximum load recorded before crack arrest or complete

ailure (Pmax) was used to calculate KIC in MPa m1/2 using the

col dimethacrylate; EDMA is ethyleneglycoldimethacrylate; EMA is

following equation, proposed by Barker [7] and adopted byASTM standard E1304:

KIC = Y∗minPmax

DW1/2

where Y∗min is the minimum dimensionless stress inten-

sity coefficient (28 for NTP samples [6]) D the specimenholder diameter (12 mm), and W the specimen holder length(10.4 mm).

2.4. Hardness

Fractured flexural strength specimens were used for micro-hardness determinations. Experimental HT/HP, Paradigm, andcontrol PP samples, were surface coated with a thin (∼10 nm)gold layer, in a sputter-coater (SC500, Bio-Rad, UK), in orderto improve reading. Surface microhardness was measured bymeans of a Vickers indenter (MH3, Metkon, Bursa, Turkey),under a 10 N loading and a 20 s dwell time. Thirty determi-nations on five specimens were made for each material.

2.5. Density

Fractured KIC specimens were used for density determina-tions. The density of HT/HP, Paradigm, and control PP samples,was determined based on Archimedes’s principle using aXS205 (Mettler Toledo, Greifensee, Switzerland) balance. Theywere weight in air and in deionized water and the density was

then calculated using the following formula:

� = A(�w − �a)A − B

+ �a

l s 2 8 ( 2 0 1 2 ) 529–534

Res

ult

s

(Mea

n

±

SD

) of

ph

ysic

al/m

ech

anic

al

char

acte

riza

tion

s

and

stat

isti

cal a

nal

ysis

.*

pro

per

ty

GC

L

GC

P

Perc

ent

incr

ease

VO

L

VO

P

Perc

ent

incr

ease

DEL

DEP

Perc

ent

incr

ease

VIL

VIP

Perc

ent

incr

ease

P

)

84.9

7

±

12.6

1

160.

73

±

15.9

9

89.1

6*12

1.20

±

12.2

9

192.

36

±

9.46

58.7

1*11

1.92

±

17.3

171.

84

±

21.4

7

53.5

4*84

.9

±

14.7

2

140.

85

±

11.9

2

65.9

0*13

1.85

±

36.3

8Pa

·m1/

2)

0.85

±

0.08

1.58

±

0.18

85.8

8*1.

03

±

0.11

1.17

±

0.21

13.5

9 1.

26

±

0.21

1.52

±

0.10

20.6

3*0.

97

±

0.15

0.99

±

0.17

2.06

0.78

±

0.21

(in

HV

N)

33.0

1

±

5.11

53.7

8

±

2.26

62.9

2*85

.66

±

5.65

110.

02

±

4.31

28.4

3*38

.95

±

3.48

79.6

4

±

2.52

104.

46*

20.0

1

±

1.98

40.7

3

±

3.16

103.

54*

114.

80

±

4.34

in

g/cm

3)

1.62

50

±

0.00

23

1.63

37

±

0.00

18

0.53

*2.

1795

±

0.00

17

2.18

68

±

0.00

12

0.33

*2.

0754

±

0.00

38

2.10

08

±

0.00

68

1.22

*1.

5044

±

0.00

12

1.50

87

±

0.00

10

0.29

*2.

1261

±

0.00

42od

ulu

s7.

04

10.3

4

11.4

2

22.3

9

5.62

8.87

4.66

13.8

8

5.61

s

sign

ifica

ntl

y

dif

fere

nt

(p

<

0.05

) res

ult

s

base

d

on

pai

rwis

e

t-te

st

com

par

ison

s.ra

dia

ligh

t

cure

d; G

CP

–

Gra

dia

HP/

HT

pol

ymer

izat

ion

; VO

L

–

Gra

nd

io

ligh

t cu

red

; VO

P

–

Gra

nd

io

HP/

HT

pol

ymer

izat

ion

; DEL

–

Esth

etX

ligh

t

cure

d; D

EP

–

Esth

etX

HP/

HT

pol

ymer

izat

ion

; VIL

M

LC

ligh

t

cure

d; V

IP

–V

ita

VM

LC

HP/

HT

pol

ymer

izat

ion

; P

–Pa

rad

igm

.

532 d e n t a l m a t e r i a

where � is density of the sample, A the mass of the samplein air, B the mass of the sample in deionized water, �w thedensity of deionized water determined from the measurementof water temperature, and �a the density of air (0.0012 g/cm3).Thirty determinations on five specimens were made for eachmaterial.

2.6. Fractured surface morphology

Representative fractured NTP specimens of HT/HP polymer-ized, Paradigm, and control PP composites were sputter-coated (SC500, Bio-Rad, UK) with gold. The surface mor-phology was then characterized under a scanning electronmicroscope (SEM) (JSM-6400, JEOL Ltd., Tokyo, Japan) at lowand high magnification.

2.7. Statistical analysis

Since the main purpose of this study was to assess the effect ofHT/HP polymerization on the physical/mechanical propertiesand not to compare materials among themselves, the resultsof mechanical/physical tests for each material were analyzedby t-tests, performed at a 0.05 level of significance. The resultsobtained for Paradigm were used as a reference only and werenot included in the statistical analysis.

Weibull statistics parameters were calculated for the flex-ural strength data. The description of the Weibull distributionis given by

Pf = 1 − e−(�/�0)m

where Pf is the fracture probability, defined by the relation

Pf = �

N + 1

where K is the rank in strength from least to greatest, Ndenotes the total number of specimens in the sample, m isthe shape parameter (Weibull modulus), and �0 is the scaleparameter or characteristic strength �63,21% [8,9]. The Weibullmodulus, characteristic strength, and the strength at a proba-bility of failure of 5% were obtained using the Weibull statisticsoption in Excel® (Microsoft, USA).

3. Results

The results of the physical/mechanical characterizations aresummarized in Table 2, along with the results of the statisticalanalysis. Fig. 1 presents Weibull plots of the flexural strengthresults where, to facilitate observation of the effect of HT/HPpolymerization, open symbols were used for PP composites,corresponding filled symbols for their HT/HP counterparts,and a distinct symbol for Paradigm. The results have shownthat HT/HP polymerization resulted in a significant (p < 0.05)increase in flexural strength, hardness, and density for allcomposites investigated (Table 2) in comparison with their PPcounterparts. Even if the fracture toughness of all materialswas increased by HT/HP polymerization, significant (p < 0.05)

increases were detected only for Gradia and EsthetX. TheWeibull modulus of the HT/HP polymerized composites wasalso higher than that of PP counterparts. The results for �f, itsassociated Weibull modulus, and KIC obtained for the HT/HP

Tabl

e

2

–

Gro

up

a

�f

(in

MPa

KIC

(in

MH

ard

nes

sD

ensi

ty

(W

eibu

ll

m∗

Iden

tifi

ea

GC

L

–

G–

Vit

a

V

d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 529–534 533

Fig. 1 – Weibull plots of flexural strength results; open symbols were used for photo-polymerized (PP) materials andfilled-symbols were used for high-temperature high-pressure (HT/HP) polymerized materials.

Fig. 2 – SEM micrographs of specimens fractured during fracture toughness (KIC) testing: (a) Grandio photo-polymerized (PP);(b) Grandio polymerized under high-temperature high-pressure (HT/HP).

ma

ttphmm

4

HaArttp

aterials were higher than those of the commercially avail-ble CAD/CAM material Paradigm.

The most significant observation during the SEM charac-erizations of fractured KIC specimens from all the materialsested was the presence of fewer and smaller voids in HT/HPolymerized composites. As an example, Fig. 2a and b showsigh (1500×) magnification micrographs of PP and HT/HP poly-erized Grandio samples, respectively. No attempt had beenade to quantify the differences identified.

. Discussion

igh pressure polymerization is an appealing approach in thettempt to improve the properties of polymers/composites.s early as 1930, Conant and Tongberg [10] reported their

esult on the application of high pressure [(0.9–1.2) GPa] inhe polymerization of isoprene. A high pressure appara-us used to study a series of organic reactions and severalolymerization reactions under pressure was described by

Fawcett and Gibson. [11]. Later on, high density polyethyleneand poly(methyl methacrylate) (PMMA) [12,13], among severalother polymers, were obtained under high pressure. Study-ing the polymerization of tetraethylene glycol dimethacrylate(TEGDMA) under pressure (1 GPa) and with no catalyst, Kamin-ski et al. [14] reported a degree of crosslinking of 12% after96 h, similar to that obtained in the industrial production ofPMMA.

The primary effects of pressure on a monomer mixtureare to decrease intermolecular distances and to reduce thefree volume [15]. Furthermore, as the pressure increases, themobility of the monomers is reduced and limited to relativelysmall translations, which suggests that the polymerizationkinetics are slower as the pressure increases. It is likely thatas the pressure increases above the GPa range, monomerslose their mobility necessary for polymerization and most

likely transform into solids, forming monomer crystals [16].Low degrees of conversion would result in polymers withpoor mechanical properties while pressures in the GPa rangewould make the process unsuitable for large scale industrial

l s 2

r

534 d e n t a l m a t e r i a

production. It is for this reason that in the present study wehave decided to adopt a polymerization protocol that com-bines high temperature (180 ◦C) and relatively high pressure(250 MPa), conditions that could be used in large scale indus-trial applications.

The results obtained in this study (Table 2) have shownsignificant increase in the density of all the materials inves-tigated. Considering that density change in the compositescan result only due to a change in the density of the matrixand that the volume percentage of the matrix is between30 and 40, an overall increase in density of (0.29–1.22) % isquite substantial. The increase in density of the matrix occursdue to a reduction in the number and size of defects underthe isotropic compaction of the mixture, as identified in theSEM characterization of fractured specimens (Fig. 2a and b).A reduction in the number and size of defects should leadto an improvement of mechanical properties [17,18], a factclearly identified in the results of this study. The flexuralstrength of the HT/HP composites was significantly (p < 0.05)increased [by (53–89)%] over that of their PP counterparts andso was hardness [by (28–104)%] and fracture toughness [by(2–85)%] (Table 2). Previous reports have indicated that highermolecular weight polymers are obtained under high pressurepolymerization as propagation rate is enhanced and termi-nation rate is reduced [19]. Consequently, another possiblecontributor to the improved mechanical properties obtained inthis study may be the fact that a higher degree of crosslinkinghad been achieved under the selected polymerization proto-col.

It could be further hypothesized that HT/HP polymerizationdiminishes polymerization shrinkage by bringing monomerscloser together. Moreover, high pressure could also lead toa more homogeneous in-mass polymerization compared toPP, where the reaction is initiated differentially through themass of the composite as a function of the distance from thelight source. These plausible aspects, in turn, could result ina decrease in the internal stresses that are normally associ-ated with polymerization reactions [20]. Supporting the abovehypotheses is the identified higher reliability of the HT/HPpolymerized materials compared to their PP counterparts, asrevealed by Weibull moduli comparisons.

5. Conclusions

The results of this study suggest that HT/HP polymeriza-tion can afford dental resin composite blocks with superiormechanical properties, suitable for CAD/CAM processing.

Acknowledgement

Bulk materials and Paradigm blocks were kindly donated bythe manufacturers.

8 ( 2 0 1 2 ) 529–534

e f e r e n c e s

[1] Craig RG. Advances in biomaterials from 1957 to 1997. J OralRehabil 1999;26:841–6.

[2] Willems G, Lambrechts P, Braem M, Vanherle G. Compositeresins in the 21st centur. Quintessence Int 1993;24:641–58.

[3] Ferracane JL. Resin composite—state of the art. Dent Mater2011;27:29–38.

[4] Miyazaki T, Hotta Y, Kunii J, Kuriyama S, Tamaki Y. A reviewof dental CAD/CAM: current status and future perspectivesfrom 20 years of experience. Dent Mater J 2009;28:44–56.

[5] Giordano R. Materials for chairside CAD/CAM-producedrestorations. J Am Dent Assoc 2006;137:14S–21S.

[6] Ruse ND, Troczynski T, MacEntee MI, Feduik D. Novelfracture toughness test using a notchless triangular prism(NTP) specimen. J Biomed Mater Res 1996;31:457–63.

[7] Barker LM. A simplified method for measuring plane strainfracture toughness. Eng Fracture Mech 1977;9:361–9.

[8] Weibull W. A statistical distribution function of wideapplicability. J Appl Mech 1951;18:293–7.

[9] Bona AD, Anusavice KJ, DeHoff PH. Weibull analysis andflexural strength of hot-pressed core and veneered ceramicstructures. Dent Mater 2003;19:662–9.

[10] Conant JB, Tongberg CO. Polymerization reactions underhigh pressure. I. Some experiments with isoprene andbutyraldehyde. J Am Chem Soc 1930;52:1659–69.

[11] Fawcett EW, Gibson RO. The influence of pressure on anumber of organic reactions, in the liquid phase. J Chem Soc1934:386–95.

[12] Arita T, Kayama Y, Ohno K, Tsujii Y, Fukuda T. High-pressureatom transfer radical polymerization of methylmethacrylate for well-defined ultrahigh molecular-weightpolymers. Polymer 2008;49:2426–9.

[13] Kojima Y, Matsuoka T, Takahashi H. Structure of poly(methyl methacrylate) synthesized under high pressure. JMater Sci Lett 2002;21:473–5.

[14] Kaminski K, Paluch M, Wrzalik R, Ziolo J, Bogoslovov R,Roland CM. Pressure-induced polymerization oftetraethylene glycol dimethacrylate. J Polym Sci Part APolym Chem 2008;46:3795–801.

[15] Schettino V, Bini R, Ceppatelli M, Citroni M. Activation andcontrol of chemical reactions at very high pressure. Phys Scr2008;78:1–5.

[16] Murli C, Song Y. Pressure-induced polymerization of acrylicacid: a Raman spectroscopic study. J Phys Chem B2010;114:9744–50.

[17] McCabe JF, Ogden AR. The relationship between porosity,compressive fatigue limit and wear in composite resinrestorative materials. Dent Mater 1987;3:9–12.

[18] Brosh T, Ferstand N, Cardash H, Baharav H. Effect ofpolymerization under pressure on indirect tensilemechanical properties of light-polymerized composites. JProsthet Dent 2002;88:381–7.

[19] Kwiatkowski P, Jurczak J, Pietrasik J, Jakubowski W, MuellerL, Matyjaszewski K. High molecular weightpolymethacrylates by AGET ATRP under high pressure.

Macromolecules 2008;41:1067–9.

[20] Ferracane JL. Developing a more complete understanding ofstresses produced in dental composites duringpolymerization. Dent Mater 2005;21:36–42.