Synthesis and-characterization-of-cross-linked-polymeric-nanoparticles-and-their-composites-for-reinforcement-of-photocurable-dental-resin 2013-reacti

1. c
Synthesis and characterization of cross-linked polymeric nanoparticles
and their composites for reinforcement of photocurable dental resin
Melinda Szaloki a
, Jozsef Gall b
, Katalin Bukovinszki a
, Janos Borbely c
, Csaba Hegedus a,⇑
a
Department of Prosthetic Dentistry, University of Debrecen Medical and Health Science Center, Faculty of Dentistry, Nagyerdei korut 98, H 4032 Debrecen, Hungary
b
Department of Finance and Controlling, University of Debrecen, Faculty of Economics and Business Administration, H 4028 Debrecen, Hungary
c
Institute of Water and Environmental Management, University of Debrecen Centre for Agricultural Sciences and Engineering, Faculty of Agriculture, H 4032 Debrecen, Hungary
a r t i c l e i n f o
Article history:
Received 22 May 2012
Received in revised form 20 October 2012
Accepted 21 November 2012
Available online 29 November 2012
Keywords:
Photopolymerization
Reinforced resin
Mechanical properties
Cross-linked prepolymers
a b s t r a c t
Reactive polymeric nanoparticles were formed for reinforcement of photocurable dental resin. Cross-
linked polymeric nanoparticles were synthesized by emulsion polymerization of mono- (methyl methac-
rylate; MMA) and trifunctional (trimethylol propane trimethacrylate; TMPTMA) monomers. The nano-
particles were dispersed in bisphenol A glycol dimethacrylate (Bis-GMA) based dental resin matrix in
the range of 5–25 wt% to form photocurable nanocomposites.
The effect of reactive polymeric particles on the mechanical properties of photocurable dental resin
was investigated. Polymerization shrinkage, polymerization shrinkage stress, viscosity, diametral tensile
strength, compressive, and flexural strength of the nanocomposites have been studied.
It was observed that the cross-linked nanoparticles significantly influenced the mechanical properties
of the reinforced dental resin nanocomposites.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Cross-linked network structure can be formed by polymeriza-
tion of dimethacrylates, which network is reinforced by inorganic
filler for several applications as dental restorative materials. The
main ingredients of resin based dental composites are resin matrix,
inorganic filler, photoinitiator and coupling agents. The most
important mechanical properties of dental composites are hard-
ness, elastic modulus, flexural strength, tensile strength, shrinkage
stress, and shrinkage, which can be greatly influenced by their
components. The dental composite as brittle materials exhibit a
very low plastic deformation, cannot be subjected to traditional
tensile strength test. A compression test for tension also referred
to as, an indirect tensile test or diametral tensile test is used [1–
3]. Diametral tensile strength is also an important property when
characterizing dental composites since many materials for intra-
oral use have measurements of tensile strengths that are markedly
lower than their corresponding compressive values. Low values of
tensile strength may contribute to early intra-oral failure of mate-
rials. However amount of resin is lower than the filler by weight%
[4] in a dental composite material, the composition, structure, and
reactivity of resin matrix play important role in determining of the
properties [5,6]. The inorganic filler not only directly determine the
mechanical properties of restoratives but also allow reducing
amount of monomer thus negative effects of presence of resin re-
duce as polymerization shrinkage, polymerization shrinkage stress,
and residual monomer. Developments in the filler system may lead
to improved mechanical and aesthetic properties compared with
earlier composite materials [7,8]. In general, the main ingredients
of the resin matrix are bisphenol A glycol dimethacrylate (Bis-
GMA), triethylene glycol dimethacrylate (TEGDMA) and urethane
dimethacrylate (UEDMA), which form polymer network around
the inorganic filler particles by photopolymerization process.
The strength, shrinkage behavior, handling, rheological proper-
ties and hardness of restorative dental composites can be con-
trolled by many factors such as amount, shape, type and particle
size of filler materials [9–11]. Different types and the size distribu-
tion of inorganic fillers are used by the manufacturers thus, their
comparison is complex.
Intensive research work has been done in order to reduce the
negative effects of dental composites [12,13]. Polymerization
shrinkage, polymerization stress, marginal gap forming, postoper-
ative pain, secondary caries, and residual monomers are the most
important negative effects. High shrinkage and/or high contraction
stress may lead to failure between the resin composites and the
tooth structure surfaces. An undesirable contraction stress is
caused due to polymerization shrinkage of dental resins. The
shrinkage stress is actually a tensile stress on the cavity wall
[14,15]. The shrinkage stress values depend on many factors as
1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2012.11.013
⇑ Corresponding author. Tel.: +36 52255308; fax: +36 52255208.
E-mail address: hegedus.csaba.prof@dental.unideb.hu (C. Hegedus).
Reactive & Functional Polymers 73 (2013) 465–473
Contents lists available at SciVerse ScienceDirect
Reactive & Functional Polymers
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2. elastic modulus of tooth and dental filling materials, amount and
type of inorganic filler, polymerization conversion and cavity de-
sign factor (C factor) [16]. The C factor is a quotient which ex-
presses the ratio of bounded surface to unbounded surface. The
higher value of C factor means higher polymerization stress. At first
class cavities the value of C factor is 5, at third class the C factor is 2
[17]. Numerous methods have been applied to measure the poly-
merization shrinkage stress of dental composite [18–23]. Conse-
quently the values of shrinkage stress exhibit a wide range. In a
number of studies the contraction stress was determined in a di-
rect manner [14,16,24,25]. In the most common test a resin com-
posite sample was placed between opposing adhesive surface
which were connected to the frame and load-cell of tensilometer.
The shrinkage is unfavorably influenced by the TEGDMA/Bis-
GMA ratio, forasmuch as TEGDMA-rich matrix has shown a higher
shrinkage [26]. Types and ratio of matrix monomers had effect on
the network structure [27–29], on the shrinkage stress [30] and on
the viscoelastic properties [6,31].
Beyond the changing of monomers, the polymerization shrink-
age and stress are controlled by other modifications, such as add-
ing prepolymers, application of inorganic fillers with different
level, shape and size [8,18], synthesis of new monomers [32,33]
and their matrices [34]. The prepolymers are known as micro-sized
milled, cured dental composite fillers [35]. The application of nano-
sized prepolymers as nanogels-modified monomers [36] was a no-
vel attempt to reduce the polymerization shrinkage of dental com-
posite. Besides the applying of spherical nanoparticles, the
nanofibers could be used to improve the flexural properties of
Bis-GMA/TEGDMA dental composites by preparation of nylon 6/
fibrillar silicate nanocomposite [37].
Mechanical factors of dental resin composites were evaluated
and it was found that the prepolymerized clusters improved the
shrinkage/contraction stress properties [38]. It was expected that
the nano-sized silica reduced the negative effect of dental resin
composite as polymerization shrinkage, polymerization stress,
marginal gap forming, low polymerization conversion, postopera-
tive pain, and secondary caries. The nanotechnology can be up-
swing to the composite chemistry [39,40]. Universal, microfilled,
microhybrid dental resin composites were compared to nano-filled
composites published by several researchers [41,42]. The structure
integrity and mechanical features of dental composites were af-
fected by the agglomerates of nanoparticles. The agglomerates of
inorganic particles can worsen on the advantages of nano-fillers
due to their porous structure.
In our previous work reactive polymeric nanoparticles were
synthesized and the effect of cross-linker monomer was investi-
gated on the size, reactivity and swelling feature of formed nano-
particles [43–45]. In this study cross-linked polymeric
nanoparticles were synthesized for reinforcement of dental resin
matrix. Mechanical properties of reinforced dental resin were
investigated in order to confirm the efficiency of nanoparticles.
2. Experimental
2.1. Materials
Two types of nano-sized copolymers were synthesized in differ-
ent mole ratio by emulsion copolymerization. The monofunctional
monomer was methyl methacrylate (MMA) (Sigma–Aldrich Co., St.
Louis, MO, USA). The trifunctional monomer was trimethylol pro-
pane trimethacrylate (TMPTMA) (Sigma–Aldrich Co., St. Louis,
MO, USA). Mole ratios of MMA/TMPTMA monomers were 3/7
and 7/3 in the feed. The monomers were emulsified in solution
of sodium dodecyl sulphate (SDS) (Sigma–Aldrich Co., St. Louis,
MO, USA). The thermoinitiator was potassium persulfate (Sigma–
Aldrich Co., St. Louis, MO, USA).
The photocurable resin matrix was a mixture of bisphenol A
glycol dimethacrylate (Bis-GMA) (Sigma–Aldrich Co., St. Louis,
MO, USA) and triethylene glycol dimethacrylate (TEGDMA) (Sig-
ma–Aldrich Co., St. Louis, MO, USA) monomers in 50/50 weight ra-
tio, containing 5 wt% ethoxylated bisphenol A glycol
dimethacrylate (Bis-EMA) (Sigma–Aldrich Co., St. Louis, MO, USA)
and 0.2 wt% camphorquinone (CQ) (Sigma–Aldrich Co., St. Louis,
MO, USA) as photoinitiator. In the most of dental composites, in
which negative effects often occur, similar composition can be
found such as in our experimental resin matrix. The materials were
used as received without further purification.
2.2. Preparation of cross-linked nanoparticles
Reactive polymeric nanoparticles were synthesized by emulsion
polymerization of mono- (MMA) and trifunctional (TMPTMA)
monomer. In details the reaction conditions were published in
our previous work [44]. Briefly, the monomers were emulsified in
distilled water by adding SDS surfactant, than sonicated for
10 min. The emulsions were performed under inert atmosphere
(N2) during 120 min reaction time using potassium peroxide initi-
ator at 60 °C. After the polymerization reaction the cross-linked
nanoparticles were precipitated from aqueous latex by adding a
threefold excess of methyl alcohol and then polymer dispersions
were centrifuged for 15 min at 14,100 rpm.
2.3. Preparation of reinforced photocurable resin
Two series of reactive polymeric nanoparticles (RPNPs) were
prepared, type A: MMA/TMPTMA = 3/7 and type B: MMA/
TMPTMA = 7/3, and dispersed in Bis-GMA based resin by sonica-
tion. The sonication process helped the ingress of dental resin
monomers into the cross-linking structured polymeric nanoparti-
cles. Two different types of nanocomposites (NCA and NCB) were
made by mixing reference resin matrix and different amount of
RPNPs in the range of 5–25 w/w%. NCA marked reinforced resin
contains type A nanoparticles in 5% (R1), 10% (R2), 15% (R3), 20%
(R4), 25% (R5) and reference resin. NCB marked nanocomposite is
made by mixing of 5% (R6), 10% (R7), 15% (R8), 20% (R9), and
25% (R10) of type B nanoparticles and reference resin. The applied
ratios of nanocomposites and notations are showed in Table 1.
2.4. Nuclear magnetic resonance spectroscopy measurements
Reactivity and structure of RPNPs were detected by Nuclear
Magnetic Resonance Spectroscopy (1
H NMR, Bruker Avance II
500, USA) at 500 MHz operating frequency. The samples were dis-
solved in 98 atom% chloroform-D (Sigma–Aldrich Co., St. Louis,
MO, USA). The chemical shifts were represented in parts per mil-
lion (ppm) based on the signal for tetramethylsilane (TMS) as a
reference.
Table 1
Notation of nanocomposites.
Amount of
RPNPs (w/w%)
in resin matrix
(%)
NCA nanocomposite
containing type A
nanoparticles: MMA/
TMPTMA = 3/7
NCB nanocomposite
containing type B
nanoparticles: MMA/
TMPTMA = 7/3
5 R1 R6
10 R2 R7
15 R3 R8
20 R4 R9
25 R5 R10
466 M. Szaloki et al. / Reactive & Functional Polymers 73 (2013) 465–473

3. 2.5. Transmission and scanning electron microscopy measurements
The polymer samples were dissolved in toluene with a concen-
tration of 1 mg/ml. The sample solution was dropped to a carbon-
coated copper grid then the sample was dried at room temperature
for one night. The size, morphology and size-distribution of nano-
particles were studied in dried form by Transmission Electron
Microscopy (TEM, JEOL2000 FX-II TEM, Japan). Fracture surface of
reinforced resin specimen was recorded by Scanning Electron
Microscopy (SEM, Hitachi 3000N, Japan).
2.6. Dynamic light scattering
The hydrodynamic diameters of polymeric nanoparticles were
assessed by using a Zetasizer Nano ZS instrument (Malvern Instru-
ments Ltd., Worcestershire, UK), at an operating wavelength of
633 nm. The size distribution and the Z-average size of the nano-
particles were measured at 25 °C with an angle detection of 173°
in optically homogeneous quartz cuvettes. The samples were pre-
pared from the reaction mixture and dissolved in toluene at a con-
centration of 1 mg/ml. Each sample was measured three times and
average serial data were calculated.
2.7. Rheology
The viscosity values of reference and reinforced resins were
measured by a rotation rheometer (AR 550, TA Instruments, New
Castle, USA) equipped with cone–plate accessory [46]. This con-
sisted of a rotating cone and a stationary plate with the sample
filled in the gap between them (cone angle 1°590
3800
, cone diameter
60 mm, 2° standard steel cone, gap 60 lm). The device contains an
air bearing, which required the provision of a minimum of 2.5 bars
pressure. The apparatus is equipped with a temperature unit (Pel-
tier plate) that gives a good temperature control (20 ± 0.05 °C) over
an extended time. To measure each sample – three times average –
about 8 ml of mixture was placed on the surface of the plate. The
data were analyzed by Rheology Advantage Data Analysis software
(TA Instruments Ltd., version 4.1.2).
2.8. Photopolymerization shrinkage measurements
The polymerization shrinkage was determined by measuring
the density of unfilled resin and nanocomposites according to
the Archimedes principle with the commercial density determi-
nation kit of an analytical balance (Adam Equipment PW 254,
UK). The tests were done in distilled water at controlled temper-
ature (20 ± 4 °C) and a relative humidity of 50% (DIN 13907). The
resin and modified resins were cured by a photopolymerization
in a chamber unit (Dentacolor XS, Heraeus Kulzer GmbH, Ger-
many) at 435 nm wavelength for 90 s. For each measurement fif-
teen specimens (n = 15) were prepared. The weighing of sample
was very fast (about 10 s) in distilled water so the sorption and
desorption processes of resin and composites were negligible in
water [47]. The uncured resin and reinforced resin were opti-
mally wetted by distilled water during the immersing the
sample.
2.9. Photopolymerization shrinkage stress measurements
During the photopolymerization process a tensile force affect to
the cavity wall. This tensile stress, as photopolymerization shrink-
age stress, was measured with a mechanical testing device (IN-
STRON 5544, USA). Two glass rods (8 mm in diameter  25 mm
in height) were place opposite each other in a vertical position in
the testing machine. Before every measurement the surface of
glass rods were gently sandblasted with 50 lm sized Al2O3 (clini-
cal-sandblaster; Henry Schein Inc., USA). The remaining alumi-
num-oxide was removed by compressed air and after that the
glass surface was rinsed with acetone. The sandblasting process
ensured an increased surface roughness and removed residual
cured sample from area. The samples were photopolymerized with
a dental halogen light-curing unite (Translux EC; Heraeus Kulzer
GmbH, Germany) at 435 nm wavelength for 200 s. For each mea-
surements fourteen specimens (n = 14) were prepared.
The distance between the upper and lower rods was standard-
ized in 0.8 mm. This gap was filled with sample resin. The C factor
expressed the ratio of bounded surface to unbounded surface. This
configuration resulted in C-value of 5 (C = r/h, where r is the radius
of disk sample, h is the height of sample, i.e. 0.8 mm). The shrink-
age stress measuring started with switching on the polymerization
lamp. The shrinkage stress was measuring in real time while the
distance between two glass rods was kept constant 0.8 mm, while
maximum tensile stress was measured by load cell.
2.10. Flexural strength
Flexural strength of unfilled resin and composites was investi-
gated with a mechanical testing device (INSTRON 5544, USA).
The three point flexural strength tests were implemented on pris-
matic specimens. The parameters of specimens were
2 mm  2 mm and its length is 20 mm. The samples were photopo-
lymerized for 90 s. In average, fourteen specimens (n = 14) were
stored at room temperature 24 h before testing. The distance of
spams was 18 mm. The cross head speed was 1 mm/s. The flexural
stress (FS) and flexural modulus (FM) data were calculated by MSZ
EN ISO 178.
2.11. Compressive strength
Compressive strength of reference and reinforced resins were
investigated with a mechanical analyzer (INSTRON 4302, USA).
The compressive tests were performed on cylindrical specimens
described above with the full scale load range of 10 kN and with
1 mm/min cross head speed. The cylindrical samples had a diame-
ter of 5 mm and specimen length of 5 mm for the each specimen.
The samples were photopolymerized for 90 s. In average, 14 spec-
imens (n = 14) were prepared and stored 24 h at room temperature
before testing. The compressive stress (CS) data were calculated by
MSZ EN ISO 604.
2.12. Diametral tensile strength
The diametral tensile stress (DTS) of reference and reinforced
resin was measured with mechanical testing device (INSTRON
5544, USA) equipped with 2 kN load cell at cross-head speed of
1 mm/min. The specimens were 3 mm in height and 6 mm in
diameter. The uncured resins were placed into a Teflon mold and
the samples were covered by a thin cured in a chamber unit (Den-
tacolor XS, Heraeus Kulzer GmbH, Germany) at 435 nm wave-
length for 90 s. Number of observations were between 18 and 24
in all specimens (n = 18–24).
The parameters of specimens (height (h); diameter (d)) and
maximum compression load (F) at specimens fracture in diametral
position were recorded. DTS was calculated as DTS = 2F/(phd)
where p constant is 3.14 [1,44,48–50].
2.13. Statistical analysis
The (non-parametric) Mann–Whitney (MW) test was applied to
compare the different groups of data. This test is also referred as a
natural non-parametric alternative of the Student-t test for the
comparison of means. In our case, it is suggested to prefer MW test
M. Szaloki et al. / Reactive & Functional Polymers 73 (2013) 465–473 467

4. to t test due to the relatively small sample sizes and the possible
lack of normality of the distributions. Note that one can calculate
both the exact significance and asymptotic significance for this
test. Since in all cases the same result was concluded, based on
both, but only the later one is shown in the tables. The statistical
analyses were performed by using SPSS version 17.0.
3. Results and discussion
3.1. Characterization of polymeric nanoparticles
3.1.1. Formation of RPNPs
Polymeric nanoparticles were formed by non-linear free radical
polymerization of mono- (MMA) and trifunctional (TMPTMA) ac-
rylic monomers. For reducing negative effects of resin based com-
posites, the reactive polymeric nanoparticles (RPNPs) were added
to dental resin matrix. For MMA–TMPTMA copolymer the yield
of polymerization was 83.86% at mole ratio of 3:7, and 98.67% at
mole ratio of 7:3 monomer composition. The structure of these
polymeric nanoparticles was a cross-linked network containing
reactive pendant vinyl groups due to the trifunctional methacry-
late monomer.
3.1.2. NMR results
The structure of obtained cross-linked polymeric nanosystems
was characterized by 1
H NMR spectroscopy. The assignments and
chemical shifts of the 1
H signals of nanosystems showed that the
RPNPs contained reactive pendant vinyl groups (Fig. 1). The inten-
sity of vinyl signals of reactive nanoparticles depended on the ratio
of trifunctional TMPTMA monomers. At higher ratio of trifunction-
al monomer (Fig. 1 part A), as cross-linker, the plurality of residual,
pendant vinyl groups of nanoparticles is more considerable.
The 1
H NMR spectra of polymers were assigned by indentified
protons in a diad of the copolymer. The proton NMR chemical
shifts (ppm) were summarized in Fig. 1. Corresponding structural
units were indicated on a schematic structure of a diad of the
copolymer (Fig. 1).
3.1.3. Particle size of cross-linked polymeric nanoparticles
The preparation of RPNPs was performed in emulsion by using
surfactant. The advantage of this technique is that the size and size
distribution of prepared polymeric particles were regulated by the
size of micelles [5,27,51,52].
TEM micrographs (Fig. 2) supported that nano-sized and spher-
ical particles were formed by cross-linking polymerization. It was
Fig. 1. Schematic structure of a diad of the copolymer, 1
H NMR spectra and peak assignation of an MMA–TMPTMA copolymer. (a) MMA/TMPTMA = 3/7 and (b) MMA/
TMPTMA = 7/3. The incorporation of cross-linking monomers into the polymer cross-linked network were indicated by the NMR signals of reactive vinyl groups. Intensity of
signals depended on the mole ratio of cross-linker monomer amount.
Fig. 2. TEM micrographs and size-distribution of RPNPs. (a) MMA/TMPTMA = 3/7 and (b) MMA/TMPTMA = 7/3.
468 M. Szaloki et al. / Reactive & Functional Polymers 73 (2013) 465–473

5. observed that RPNPs had a cluster forming ability, mainly at higher
concentration of TMPTMA (Fig. 2 part A) [44]. According to TEM
pictures, the particle size of nanoparticles varied in the range of
67–650 nm for MMA/TMPTMA = 3/7 copolymers and the size dis-
tribution is broader. For MMA/TMPTMA = 7/3 copolymer the parti-
cle size was in the range of 53 and 630 nm.
For hydrodynamic size measurement of swollen RPNPs, they
were dispersed in toluene. The organic solvents like toluene dif-
fused into the polymer network structure thus the nanoparticles
became swollen state, which was confirmed by DLS measure-
ments. According to DLS, the mean diameter of nanoparticles by
intensity was 208 ± 28.92 nm at MMA/TMPTMA = 3/7 mol ratio,
and 595 ± 89.9 nm at MMA/TMPTMA = 7/3 copolymer.
Based on the results, it can be established that the ratio of tri-
functional monomer is one of the main influencing factors for
determination of the cross-linking density, reactivity, as well as
size and size distribution, and swelling behavior of RPNPs. At high
mole ratio of trifunctional monomer as cross-linker, more cross-
linked structure of particles were formed, therefore the swelling
ability of these systems is moderated by the intramolecular cova-
lent bonding.
3.2. Characterization of reinforced dental resin
The reinforced dental resins as nanocomposites were formed by
mixing increasing amount of polymeric nanoparticles and dental
resin. The different features of nanoparticles resulted different
physical and mechanical properties in the nanocomposite mix-
tures. Small size, porous structure and big surface area of nanopar-
ticles can be resulted incomplete wetting of polymer surface by
dental resin. The incomplete monomer penetration into the poly-
mer pore and among particles can lead to formation of internal
and interfacial voids. The background of this process can be ex-
plained by the agglomeration of nanoparticles. This effect may ap-
pear on the deteriorating mechanical properties.
3.2.1. Rheological results
The fluidity is an important factor at the characterization of a
composite, which influences the movement of monomers in the
matrix, gel point of polymerization, values of shrinkage and shrink-
age stress. For this reason, viscosity value of resin and modified re-
sin was analyzed depending on the amount of RPNPs (Fig. 3).
Rheological results showed that viscosity values of nanocom-
posites were arisen exponential by weight% of RPNPs. The cross-
linked polymeric nanoparticles were in swelling form in the
mixture of organic monomers opposing to inorganic filler of resin
based dental filling materials. The monomers of resin matrix
penetrated into the cross-linked structure of RPNPs network. How-
ever, the moving of swelled nanoparticles was limited inside the
resin in a function of amount and cross-linking density of the
RPNPs.
The rheological data of NCA and NCB were fairly different from
reference resin matrix; therefore we investigated the effect of the
cross-linking density of RPNPs in nanocomposites (namely NCA
versus NCB). The RPNPs were swollen differently in the mixtures
of Bis-GMA, TEGDMA, Bis-EMA and camphorquinone, depending
on the mole ratio of methyl methacrylate and trimethylol propane
trimethacrylate in prepolymerized polymers. It was observed that
the viscosity value varied considerably (except at 5 w/w% organic
filler content). At lower cross-linker concentration (MMA/
TMPTMA = 7/3) the formed nanoparticles had lower cross-linking
density, thus the monomers of organic matrix were able to diffuse
easily inside of filler particles. The viscosity values were elevated
on a bigger rate at lower cross-linker density (MMA/
TMPTMA = 7/3) because of looser, more permeable network struc-
ture. Based on the shape of the viscosity - shear rate curves it could
be established the reinforced nanocomposites behaved as Newto-
nian fluids and showed coherence with the nanoparticles load.
However at the highest nanoparticles content, namely 25 w/w%,
hysteresis loops were appeared on the viscosity (Pas)-shear rate
(1/s) curves, indicating the structural viscosity of reinforced resin
matrix. These viscoelasticity features are known among the dental
composites. According to the viscosity data, it was declared that
nanoparticles had a considerable effect on the fluidity properties
inside the nanocomposites, which depended on the amount and
cross-linking density of nanoparticles.
3.2.2. Volumetric shrinkage results
The polymerization or volumetric shrinkage (PS) and its effects
on lack of durable application of resin based dental filling materials
are a well known problem in the research area of dentistry. In our
study the goal was to prepare such reactive polymeric nanoparti-
cles, by which application in dental resin matrix can be reduced
the negative effects of dental restoratives. The formed chemical
binding between the reactive groups of resin monomers and pre-
polymers influenced the mechanical properties of cured nanocom-
posite as volumetric shrinkage, flexural and compressive strength.
The efficacy of the MMA–TMPTMA copolymers on the polymer-
ization shrinkage was investigated. The shrinkage analyses were
performed by the buoyancy method. According to the volumetric
shrinkage measuring results, it was established that the shrinkage
reduced in the presence of RPNPs compared to the reference resin
(Fig. 4 at 0%).
Mann–Whitney statistical analysis showed that shrinkage
values of modified dental resin (except R6) were significantly
Fig. 3. Mean viscosity values of unfilled reference resin and reinforced resin
depending on reactive polymeric nanoparticles content (w/w%). NCA nanocompos-
ite means MMA/TMPTMA = 3/7 copolymer in dental resin matrix; NCB nanocom-
posite means MMA/TMPTMA = 7/3 copolymer in dental resin matrix.
Fig. 4. Mean volumetric shrinkage data of unfilled reference resin and nanocom-
posites depending on the nanoparticles content (w/w%). NCA nanocomposite means
MMA/TMPTMA = 3/7 copolymer in dental resin matrix; NCB nanocomposite means
MMA/TMPTMA = 7/3 copolymer in dental resin matrix.
M. Szaloki et al. / Reactive & Functional Polymers 73 (2013) 465–473 469

6. different (P-value < 5%) from the reference unfilled resin (see
Table 2). This effect verified that the prepolymerized spherical
nanoparticles had a considerable effect to contraction reduction
in the resin matrix. At the comparison of two type of nanocompos-
ites NCA and NCB (NCA versus NCB) it was acquired that there was
not a significant difference (P-value exceeding 10%) in the effect of
the copolymers (types A and B) except at 25 w/w% (Asymp. Sign. 2-
tailed is 0.056 namely 5.6%). The asymptotic significance was in the
range of 17% and 26% (data not shown). In this study we suggest to
apply strictly interpreted 10% significance limit (taking also into
account the relatively small samples sizes).
Consequence was that the photopolymerization shrinkage
reducing was effected by the presence of RPNPs, but the PS values
were not significantly influenced by the current monomer compo-
sition of nanoparticles.
3.2.3. Polymerization shrinkage stress results
The polymerization of dental monomers leads to a bulk contrac-
tion and exerts a tensile force to the cavity wall. Shrinkage and
shrinkage stress are disadvantage of applicability of the filling
material. The effect of polymeric nanoparticles was studied on
shrinkage stress of nanocomposite. The results of PSS tests showed
that the shrinkage stress values of nanocomposite decreased with
increasing of nanoparticles content (see Fig. 5).
At the comparison of the two type nanocomposite it was found
that the shrinkage stress reducing was higher at type NCA nano-
composite than NCB one. The nanoparticles act as a stress relaxing
particles, which are able to absorb the stress during the photopoly-
merization shrinkage [35]. In our nanocomposite the RPNPs are
able to absorb shrinkage stress during the curing process. The
relaxing effect is influenced by the cross-linking density of RPNPs.
In type NCB nanocomposite the prepolymers has lower cross-link-
ing density thus the monomers of dental resin get easily into the
polymer network structure than at higher cross-linker density
(type NCB).
The background of the decreasing shrinkage stress may be also
that extent of PCS are not able to reach the cohesion energy which
impact between particles, voids will not form on the filler-resin
interface [53]. The incomplete penetration into the RPNPs resulted
empty voids inside the prepolymers. Surface area of empty voids
reduced, thus energetically favorable by reducing the total surface
free energy. The shrinkage of the void means an increased resin
volume thus the total volume equals those of the resin and the
void. The polymerization of trapped resin among the aggregated
RPNPs operates as constrained shrinkage so trapped resin move
away from the surface during the polymerization ignoring the
whole resin shrinks so decrease the surface areas and the PCS.
According to this theory intermolecular intramolecular voids have
meaningful effect to reducing the polymerization shrinkage stress.
The measured data are correlated with data of similar methods
in the literature despite the fact that there is no uniform method of
measuring [23].
We have compared the shrinkage stress of each nanocomposite
with the reference resin. The Mann–Whitney statistical tests
(Table 3) showed that shrinkage tensile stress values of modified
dental resin were significantly different (P-value < 10%) from those
of the reference unfilled resin for the cases R4, R5 and for R7–R10,
furthermore, for R8–R10 we have P-value < 1%. Note also that the
significance vanishes as one goes from R1 to R5 or from R6 to R10.
3.2.4. Flexural strength results
The efficacy of reactive polymeric nanoparticles was observed
on flexural strength as flexural stress (FS) and flexural modulus
(FM). Taking into account the mechanical forces in the mouth,
the flexural properties of dental composite are important factors
beside many other features. The three-point bending tests were
used to measure the flexural stress and modulus of reference resin
and modified resin (Fig. 6).
Statistical analysis of FS showed that the nanocomposites sig-
nificantly differed from the reference resin. Based on the FM statis-
tical analysis it was found that the most of data were significantly
different from the reference resin (P-value < 5%) except R5 and
R10, where the asymptotic significance was 17.2% and 17.8%,
respectively (Table 4). The significant differences can be explained
by the presence of the reactive prepolymers inside the resin
matrix.
According to the flexural stress and modulus data it was ob-
served that the flexural values of modified dental resins were high-
er than unfilled dental resin. The average flexural stress and
Fig. 5. Mean polymerization shrinkage stress (PSS) data of unfilled reference resin
and nanocomposites depending on the nanoparticles content (w/w%). NCA nano-
composite means MMA/TMPTMA = 3/7 copolymer in dental resin matrix; NCB
nanocomposite means MMA/TMPTMA = 7/3 copolymer in dental resin matrix.
Table 2
Descriptive statistics and Mann–Whitney U and Z test statistics with P-values
(Asymp. Sig.) of volumetric shrinkage, in each case the nanocomposite is compared to
the reference resin (PS R0).
Mean Std.
deviation
Mann–
Whitney U
Z Asymp. Sig.
(2-tailed)
PS R0 6.972 0.466
PS R1 6.486 0.470 31.00 À2.109 0.035
PS R2 5.951 0.543 10.00 À3.563 0.000
PS R3 5.353 0.842 4.00 À3.911 0.000
PS R4 5.345 0.400 2.00 À3.907 0.000
PS R5 4.915 0.481 0.00 À4.031 0.000
PS R6 6.743 0.569 67.00 À0.598 0.550
PS R7 5.666 0.541 5.00 À3.853 0.000
PS R8 5.733 0.326 0.00 À4.031 0.000
PS R9 5.401 0.178 0.00 À4.031 0.000
PS R10 4.336 0.770 0.00 À4.243 0.000
Table 3
Descriptive statistics and Mann–Whitney U and Z test statistics with P-values
(Asymp. Sig.) of polymerization shrinkage stress (PSS, in MPa), in each case the
nanocomposite is compared to the reference resin (PSS R0).
Mean Std.
deviation
Mann–
Whitney U
Z Asymp. Sig. (2-
tailed)
PSS R0 0.6401 0.0612
PSS R1 0.6422 0.0605 97.000 À0.046 0.963
PSS R2 0.6251 0.0585 89.00 À0.414 0.679
PSS R3 0.6122 0.0427 69.00 À1.332 0.183
PSS R4 0.5982 0.0487 56.00 À1.930 0.054
PSS R5 0.5748 0.0663 42.00 À2.378 0.017
PSS R6 0.6229 0.0501 85.00 À0.597 0.550
PSS R7 0.6016 0.0384 56.00 À1.930 0.054
PSS R8 0.5614 0.0364 23.00 À3.446 0.001
PSS R9 0.5407 0.0412 18.00 À3.676 0.000
PSS R10 0.5235 0.0411 10.00 À4.043 0.000
470 M. Szaloki et al. / Reactive & Functional Polymers 73 (2013) 465–473

7. modulus showed a maximum at NCB type nanocomposite 15 w/
w% nanoparticles content. This can be explained by flow properties
of resin matrix and nanocomposites. As it was shown that the vis-
cosity data of nanocomposites increased by nanoparticles content.
The viscosity value increased in smaller rate up to 15 w/w% RPNPs
content, similar to flexural strength data. Above 15% the viscosity
of nanocomposites elevated in bigger rate, but the flexural factors
decreased in same rate. At 15% composition, the rheological prop-
erties of nanocomposites were similar to unfilled resin. However,
the FS and FM data showed a maximum at 15%, which clearly ex-
plained by the effect of nanoparticles. Above 15% the movements
of RPNPs in resin matrix were hindered by the swelling phenom-
ena and clustering. In fact, 25% of polymeric nanoparticles caused
structural viscosity.
At the comparison of NCA and NCB nanocomposites it was ob-
served that the FS values of nanocomposites significantly differed
from each other, except at 20 and 25 w/w% nanoparticles content
(data not shown). The change of flexural factors was greater at
NCB reinforced resin, in which the cross-linking density is lower
because of the lower amount of trifunctional monomer. This phe-
nomenon can be explained by the different swelling ratio of pre-
polymers depending on the cross-linking density. More
monomers of dental resin diffused into the more permeable
cross-linking network.
The nanoparticles have embedded into the polymer matrix by
swelling ability which is confirmed on the following SEM picture
(Fig. 7). The picture was taken from a broken specimen of flexural
investigating. The sample was 10 w/w% of MMA:TMPTMA = 7:3
nanoparticles, namely R7, in cured dental resin matrix.
3.2.5. Compressive strength results
The compressive strength method helps investigation of the
efficacy of nanoparticles in bulk and its effect on the compressive
stress (CS) of cured nanocomposites. The mean CS values are
shown in Fig. 8 depending on the nanoparticles content (w/w%).
Compressive stress of NCA and NCB nanocomposites showed a
maximum at 10 w/w% nanoparticles content. At higher nanoparti-
cles content (namely 20 and 25 wt%) the mean stress value de-
creased. This reducing can be explained by small number of
Table 4
Descriptive statistics and Mann–Whitney U and Z test statistics with P-values
(Asymp. Sig.) of flexural strength (flexural stress at max [MPa] and flexural modulus
[MPa]), in each test the nanocomposite is compared to the reference resin (FS R0).
Mean Std.
deviation
Mann–
Whitney U
Z Asymp. Sig.
(2-tailed)
FS R0 54.814 5.820
FS R1 79.264 5.919 0.00 À4.491 0.000
FS R2 84.788 10.527 0.00 À4.815 0.000
FS R3 86.888 8.239 0.00 À4.743 0.000
FS R4 76.733 18.044 19.00 À3.616 0.000
FS R5 67.961 9.561 22.00 À3.318 0.001
FS R6 99.635 7.726 0.00 À4.392 0.000
FS R7 99.574 12.114 0.00 À4.666 0.000
FS R8 114.110 11.223 0.00 À4.943 0.000
FS R9 80.863 11.654 1.00 À4.539 0.000
FS R10 68.108 9.827 26.00 À3.588 0.000
FM R0 1390.267 177.940
FM R1 1702.548 194.107 20.00 À3.570 0.000
FM R2 1936.363 119.003 0.00 À4.815 0.000
FM R3 1763.462 103.295 4.00 À4.585 0.000
FM R4 1498.615 328.204 54.00 À2.004 0.045
FM R5 1489.557 192.463 62.00 À1.366 0.172
FM R6 1818.121 110.423 3.00 À4.245 0.000
FM R7 1914.310 136.755 0.00 À4.666 0.000
FM R8 2198.973 252.550 3.00 À4.839 0.000
FM R9 1593.036 198.3903 43.00 À2.706 0.007
FM R10 1586.682 330.033 80.00 À1.348 0.178
Fig. 7. SEM micrograph of surface fracture of reinforced resin composites (R7
nanocomposite, 10 w/w% of MMA:TMPTMA = 7:3 copolymer in cured dental resin).
Fig. 6. Mean flexural stress and modulus data of unfilled reference resin and
nanocomposites depending on the nanoparticles content (w/w%). NCA nanocom-
posite means MMA/TMPTMA = 3/7 copolymer in dental resin matrix; NCB nano-
composite means MMA/TMPTMA = 7/3 copolymer in dental resin matrix.
Fig. 8. Mean compressive stress data of unfilled reference resin and nanocompos-
ites depending on the nanoparticles content (w/w%). NCA nanocomposite means
MMA/TMPTMA = 3/7 copolymer in dental resin matrix; NCB nanocomposite means
MMA/TMPTMA = 7/3 copolymer in dental resin matrix.
M. Szaloki et al. / Reactive & Functional Polymers 73 (2013) 465–473 471

8. sample units, inhomogeneous distribution of nanoparticles inside
the cured resin matrix network (aggregation) and the movements
of swelled nanoparicles. The rheological results also indicated that
addition of prepolymers to resin matrix increased the viscosity of
dental composites (Fig. 3) depending on cross-linking density
and amount of nanoparticles. Moreover, structural viscosity also
occurred at higher nanoparticles content (R5 and R10). The swelled
nanoparticles represented a higher volume and additionally the
nanoparticles aggregated. The movements of aggregated, swollen
nanoparticles were sterical constrained in dental resin. The chem-
ical structure, viscosity of resin matrix, and moving of cross-linked
polymeric nanoparticles have an effect to gelation of polymeriza-
tion process, conversion, polymerization kinetic and mechanical
properties [5,27,52] during the photopolymerization process. The
penetration of dental resin can be incomplete among the aggre-
gated RPNPs. These internal voids in bulk material (empty voids)
generate an internal stress during the compression load thus the
compression strengths of these composite can decrease.
In addition the air bubbles in the bulk and the non-compliant
distribution of nanoparticles entailed difficulties during the mea-
suring. Nevertheless nowadays the individuality of nanoparticles
in a matrix signifies a big challenge for the researchers.
Comparison of nanocomposite and reference resin was per-
formed using the Mann–Whitney test. The results of the statistical
tests are shown in Table 5.
The mean compressive stress values of NCA nanocomposites
(CS R1 – CS R5) did not show significant differences in the compar-
ison of NCA and reference resin (CS R0). The P-values did not ex-
ceed the 10% significance limit, except 5% and 15% nanoparticles
content (Table 5). Major changing in CS values was observed at
NCB nanocomposite, similar to that seen in the flexural stress re-
sults. At low nanoparticles content (namely CS R6 and CS R7) the
compressive stress data of NCB nanocomposites were significantly
higher than the unfilled reference resin. The P-values did not ex-
ceed the strictly interpreted 10% significance limit that we suggest
to use here. The most of P-values was less than 5% except CS R8 (P-
value = 32.5%). At this composition the prepolymers have loose
cross-linking network, which is more permeable for the monomers
of dental resin (Fig. 3). After the photopolymerization process the
less cross-linked nanoparticles have reinforced the dental resin
what occurred in the CS data elevation, thus the effect of nanopar-
ticles better expressed.
3.2.6. Diametral tensile strength results
The diametral tensile strength measures the cohesive strength
of the material. The cohesive properties of the material depend
on the load necessary to fracture, independently of the deforma-
tion values. As expected the diametral tensile strength of reference
resin represented the highest value showing homogeneity and
flexibility of unfilled resin matrix. The reinforced resins (NCA and
NCB) showed lower DTS value due to their inhomogeneity (see
Fig. 9).
In the comparison of DTS of reference and reinforced resin it
was found that all reinforced resin differed significantly from the
reference (Table 6). The significance had already appeared at
5 w/w% nanoparticles content (R1 and R6) while this significance
such a low concentration displayed at higher nanoparticles content
in other mechanical testing method as PSS and CS. These findings
indicated that tensile strength test is much more sensitive in pre-
dicting differences between resin composites [54]. Tendency is not
really observed between the neighbors (5 w/w% versus 10 w/w%)
however all nanocomposite (NCA and NCB) differ significantly
from the reference resin. The DTS values of NCB nanocomposites
were higher than NCA. In NCB nanocomposite the cross-linking
density of RPNPs was lower, the particles were more permeable
for dental resin, thus the tensile strengths were improved com-
pared to NCA composites. At higher amount of nanoparticles (from
10 to 25 w/w%) the significant difference remained between the
reference and filled resin composite. Due to the high number of
samples the small differences detected better.
In the comparison of the diametral tensile stress values of each
nanocomposite with the reference resin the Mann–Whitney
statistical tests showed that all of the modified dental resins were
significantly different (P-value < 10%) from the reference unfilled
resin. Furthermore, in most of the cases (R1–R3, R5, R8 and R10)
we obtained an asymptotic P-value even less than 1% showing that
the two resins are fairly different.
DTS is an acceptable test for nano-filled composite materials be-
cause it is sufficiently brittle [1–3]. In this study DTS method was
used for analyzing the efficacy of RPNPs in dental resin in respect
of cohesive strength. According to applicability of DTS the plastic
Table 5
Descriptive statistics and Mann–Whitney U and Z test statistics with P-values
(Asymp. Sig.) of compressive strength (compressive stress at max [MPa]), in each case
the test compares reference resin (CS R0) and the nanocomposite one.
Mean Std.
deviation
Mann–
Whitney U
Z Asymp. Sig. (2-
tailed)
CS R0 167.594 26.992
CS R1 167.868 18.949 54.00 À0.070 0.944
CS R2 189.648 13.829 33.00 À1.806 0.071
CS R3 169.882 32.180 60.00 À0.033 0.974
CS R4 146.290 20.254 33.00 À1.806 0.071
CS R5 125.421 14.222 8.00 À3.153 0.002
CS R6 193.571 23.062 28.00 À2.134 0.033
CS R7 223.722 36.103 16.00 À3.077 0.002
CS R8 175.627 22.851 50.00 À0.985 0.325
CS R9 144.634 18.656 33.00 À2.031 0.042
CS R10 120.029 11.846 7.00 À3.631 0.000
Fig. 9. Mean diametral tensile stress (DTS) data of unfilled reference resin and
nanocomposites depending on the nanoparticles content (w/w%). NCA nanocom-
posite means MMA/TMPTMA = 3/7 copolymer in dental resin matrix; NCB nano-
composite means MMA/TMPTMA = 7/3 copolymer in dental resin matrix.
Table 6
Descriptive statistics and Mann–Whitney U and Z test statistics with P-values
(Asymp. Sig.) of diametral tensile stress (MPa), in each case the test compares
reference resin (DTS R0) and the nanocomposite one.
Mean Std.
deviation
Mann–
Whitney U
Z Asymp. Sig. (2-
tailed)
DTS R0 25.736 9.513
DTS R1 17.824 7.133 77.00 À2.856 0.004
DTS R2 16.045 5.182 52.00 À3.001 0.003
DTS R3 17.741 4.101 96.00 À2.916 0.004
DTS R4 18.451 2.833 114.00 À2.443 0.015
DTS R5 15.247 3.730 71.00 À3.573 0.000
DTS R6 19.902 4.245 136.00 À1.865 0.062
DTS R7 19.287 5.190 131.00 À2.160 0.031
DTS R8 18.523 4.333 111.00 À2.669 0.008
DTS R9 20.806 3.906 151.00 À1.652 0.099
DTS R10 16.604 2.833 78.00 À3.507 0.000
472 M. Szaloki et al. / Reactive & Functional Polymers 73 (2013) 465–473

9. deformation of materials was negligible. However, some plastic
deformation occurred in the contact regions of the composite
mainly at low filler content [55] and at low cross-linking density
(namely NCB).
In consequence of viscosity, shrinkage, shrinkage stress, flex-
ural, diametral tensile, and compressive strength measurements
it can be concluded that the RPNPs as cross-linked prepolymers
had considerable effect on these mechanical factors of reinforced
photocurable dental resin. According to the result of this study, it
was found that 10 wt% of reactive polymeric nanoparticles content,
regardless of its composition, influenced favorably the mechanical
properties of their nanocomposites. Rheology properties of 10 w/
w% nanocomposites were similar to the reference resin matrix,
but the difference in mechanical properties (FS, FM, CS, PS, PSS,
and DTS) was remarkable due to the presence of prepolymerized
nanoparticles in the nanocomposites. At the comparison of types
A and B nanoparticles it can be established that the prepolymers
with low cross-linking density and smaller size distribution have
favorable effect on shrinkage, shrinkage stress, flexural-, diametral
tensile-, and compressive strengths at this type of prepolymers.
The nanoparticle-parameters can be changed to reach the optional
factor, thus the properties of their nanocomposite can be influ-
enced correctly.
4. Conclusion
In this work, cross-linked polymeric nanoparticles as prepoly-
mers were synthesized and characterized for reinforcement of
photocurable dental resin. The nanocomposites were formed by
mixing dental resin and different range of prepolymers. It was ob-
served that reactive polymeric nanoparticles (RPNPs) swell by the
penetration of dental resin monomers. The prepolymers have a sig-
nificant effect on the investigated mechanical parameters. It was
found that the polymerization shrinkage could be reduced by add-
ing prepolymerized cross-linked nanoparticles. The viscosity of
nanocomposites was increased exponentially by the amount of
the nanoparticles. The less cross-linked prepolymers showed high-
er viscosity values because of more permeable network structure
for dental resin. This is the reason that the measured flexural-,
and compression strength improved in the comparison of unfilled
and reinforced dental resin. The reinforced resin more strongly re-
sisted the flexural and compression load than the reference un-
filled resin however this resistance depend on the type of
prepolymer. The flexural- and compressive strengths of nanocom-
posites showed a maximum at 10 wt% of nanoparticles; 10% nano-
particles content influenced favorably the mechanical properties of
their nanocomposites. The advantage of presented prepolymers,
namely the porous structure, appeared in the shrinkage stress
reducing. The prepolymers acted as flexible, relaxing particels
which relieved the internal stress during the photopolymerization.
The shrinkage stress expanded to filler–matrix interface which
compensate the volumetric losses and help to reduce the effect
of polymerization contraction stress. This is the explanation that
porous nanoparticles reduce the contraction stress. The decreased
diametral tensile strength data of nanocomposite showed that the
nanoparticles increased the inhomogeneity of the system, reduced
the cohesive strength of dental resin. Nevertheless indicated that
tensile strength test is much more sensitive in predicting differ-
ences between resin and composites. The advantage of presented
prepolymers, namely the porous structure, appeared in the
mechanical properties improving. The optimization of nanoparti-
cle-parameters (reaction circumstances and applied monomers)
and variation of RPNPs ratio in dental resin can result an attractive
reinforced photo-curable resin for dental application.
Acknowledgement
This work was supported by the grant of TÁMOP 4.2.1/B-09/1/
KONV-2010-0007 project ‘‘Research University Project’’.
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