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Nanocomposites Based on High Impact Polystyrene/Silver Nanoparticles: Effect of Silver Nanoparticles Concentration
on the Reaction Evolution, Morphology and Impact
Strength.
Journal: Polymer Engineering & Science
Manuscript ID: PES-10-0253.R1
Wiley - Manuscript type: Research Article
Date Submitted by the Author:
n/a
Complete List of Authors: Morales, Graciela; Centro de Investigación en Química Aplicada, Polymer Synthesis Soriano, Florentino; Centro de Investigación en Química Aplicada, Plastic Proccesing and Technology
Keywords: high performance polymers, nanocomposites, radical polymerization, nanoparticles
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Nanocomposites Based on High Impact Polystyrene/Silver
Nanoparticles: Effect of Silver Nanoparticles Concentration on the
Reaction Evolution, Morphology and Impact Strength.
F. Soriano-Corral, G. Morales*
Centro de Investigación en Química Aplicada (CIQA), Blvd. Enrique Reyna No.
140, 25253, Saltillo Coahuila, México.
*Presented at the III International Congress of Metallurgy and Materials, Monclova,
Coahuila, México, 2009
∗ To whom correspondence should be addressed
E-mail: [email protected]
Phone: 52- 844-4389830
Fax: 52-844-4389839
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ABSTRACT
Nanocomposites based on HIPS/silver nanoparticles were synthesized via in situ
bulk-suspension polymerization adding a colloidal suspension of silver nanoparticles
(AgNP’s) in styrene from the beginning of the reaction. The concentrations of AgNP’s
in the final nanocomposites were 0, 0.025, 0.10 and 1.0 wt-%. The rate of
polymerization and free radicals concentration were found to decrease with increasing
AgNP’s concentration.
For nanocomposites with 0.025 and 0.10 wt-% of AgNP’s, the phenomenon of phase
inversion (PI) during the mass polymerization occurred within the same range as that
for the blank HIPS. Further, the impact strength of these nanocomposites did not present
any changes as compared to the blank HIPS. However, there was no sign of the PI
phenomenon in the case of 1.0 wt-% of AgNP’s, due to a decrease in the amount of free
and graft polystyrene (PS) onto the rubber chain as the free radicals concentration
diminishes with an increase in AgNP’s. In this case the impact strength doubles the
values of the blank HIPS due to the presence of a interpenetrated polymer network of
crosslinked grafted rubber and PS instead of the formation of a defined morphology.
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INTRODUCTION
The incorporation of different types of nanoparticles into a polymeric matrix has
shown to be a very effective way to generate marked improvements on the physical
and/or mechanical properties of the new polymeric nanocomposite, as compared to the
original pure polymer. These nanoparticles, which by definition should have at least one
dimension in the order of 1-100 nm, may be either organic, such as carbon nanotubes or
inorganic, such as many metallic oxides [1], metallic salts or pure metals [2].
With respect to the incorporation of metallic nanoparticles into different
polymeric matrices, and particularly AgNP´s, there are many reports covering different
methods to prepare the nanocomposites. One of these methods consists in the synthesis
of the polymer in a first step followed by the incorporation of silver ions Ag+, which are
reduced to Ag0 by a reducing agent [3-5], by heating or by γ irradiation. Another
simpler and more widely used method is the incorporation of a dispersion of AgNP´s
into the polymer matrix via melt mixing [6].
Finally, some reports in the literature deal with the in-situ polymerization of
vinyl monomers in the presence of AgNP´s [7-9]. This procedure has shown to be more
effective than melt mixing, since it allows a better distribution and dispersion of the
AgNP´s within the polymer matrix. In this sense, Yeum et al. reported the in situ
suspension polymerization of vinyl acetate [7] and methyl methacrylate [8] in the
presence of AgNP´s, where a decrease in the polymerization rate, due to the presence of
the silver nanoparticles was observed, as well as the formation of AgNP´s agglomerates
in the final nanocomposite. Nonetheless, the in situ polymerization of heterogeneous
systems in the presence of mineral or metallic nanoparticles has been scarcely studied.
High Impact Polystyrene (HIPS), is a heterogeneous system constituted by a
continuous polystyrene phase and a polystyrene-grafted polybutadiene elastomer
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disperse phase in the form of discrete particles. These elastomeric particles are slightly
crosslinked and in many cases they present polystyrene homopolymer occlusions [10,
11]. These morphology structures are the key for obtaining a good balance between the
two opposite characteristic properties of these types of materials, such as rigidity and
toughness.
One of the methods widely used for the synthesis of HIPS is the in situ
polymerization of styrene (St) in the presence of a rubber where the following events
take place: a) at very low St conversion, close to 2%, a phase separation occurs
generating a continuous phase of St/rubber and a discontinuous phase of St/PS. In the
St/rubber phase, the production of PS homopolymer and the graft copolymer (PS
grafted onto the rubber) take place while in the St/PS phase only the PS homopolymer
production occurs. b) As the amount of PS increases and the volume fraction of both
phases are roughly similar, the reaction mixture presents a transition period with co-
continuous structures followed by the phase inversion, where the solution St/PS
becomes the continuous phase containing discrete rubber droplets with a practically
well defined complex morphological structure. c) When the monomer conversion
reaches about 100%, the final material is composed by a dispersion of crosslinked
rubber particles within a matrix of PS so that the elastomeric phase gains integrity,
which prevails during further thermal processing.
It is the purpose of this work to study the effect of the addition of AgNP´s during
the in situ polymerization of HIPS upon the evolution of monomer conversion, the
morphology development and the physical and mechanical properties, especially the
impact strength, of the HIPS/Silver nanocomposites obtained.
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EXPERIMENTAL
Materials
Styrene (industrial grade, from Poliformas de México) was used as received. The
rubber used was a Styrene/butadiene (St/Bd) graded block copolymer (BC) [i.e.:
(butadiene)-(butadiene→styrene)-(styrene)] prepared by anionic polymerization, and
was supplied by Dynasol Elastómeros S.A de C.V (Tampico, México) and with a St:Bd
composition of 30:70 wt-%. The AgNP´s, of 7-28 nm in size, were provided by
Servicios Industriales Peñoles S.A. de C.V. (Torreón, México), as a colloidal
suspension in styrene.
The initiators used were benzoyl peroxide (BPO), for the bulk polymerization
stage (Promotores y Catalizadores Orgánicos de México) and 3, 3-di-(terbutylperoxi)
ethylbutyrate (Lupersol 233 M75), for the suspension polymerization stage (Atofina
Peróxidos de México). The solvents used during the characterization of the obtained
materials, were methyl-ethyl ketone (MEK, industrial grade), toluene and methanol
(Baker); N,N-dimethyl formamide (DMF, analytical grade), tetrahydrofuran (THF) and
osmium tetra-oxide (Sigma-Aldrich). Polyvinyl alcohol, sodium chloride, and nonyl
phenol for the suspension stage, were acquired from Sigma-Aldrich and used as
received
Synthesis of HIPS and HIPS/Silver nanocomposites
Both, HIPS and HIPS/Silver nanocomposites, were synthesized by the bulk-
suspension process in a one gallon capacity stainless steel reactor, with an anchor-
turbine stirrer and at room temperature: i) a fixed 8 wt-% of graded block copolymer
with respect to monomer was dissolved into styrene at a stirring rate of 20 rpm until the
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total dissolution of the copolymer; ii) 0.1 wt-% of BPO and 0.1 wt-% of Lupersol 233
M75 were added to the reaction mixture, the stirring rate and temperature were
increased to 40 rpm and 90ºC, respectively, and the stage of bulk pre-polymerization
took place until the monomer conversion (x) was approximately 0.35-0.40; iii) during
the last stage the suspension medium was added, and the polymerization was carried out
up to total monomer conversion. The suspension medium is constituted by water (2 L),
polyvinyl alcohol (1.7 g), sodium chloride (1.8 g), and nonyl phenol (0.66 g). During
this stage the temperature was increased to 125 °C for a period of 2 hours and
afterwards to 150 °C for another 2 hours until total monomer conversion.
With respect to the silver nanocomposites, the colloidal suspension of silver in
styrene was added from the beginning of the reaction together with the block copolymer
and initiators in the proper amount so as to obtain HIPS/silver nanocomposites with 0,
0.025, 0.1 and 1.0 wt-% silver content in the final product. The nanocomposites thus
obtained were identified as HIPS1 (blank HIPS), HIPS1-0.025, HIPS1-0.1 and HIPS1-1
for the different AgNP’s concentrations used.
Characterization
Monomer Conversion was determined by dissolving a sample of 5 g of HIPS
and/or HIPS/AgNP’s nanocomposite in 25 mL of toluene, followed by precipitation in
250 mL of methanol. In order to obtain solely x to PS, the 8 wt-% of BC added into the
reaction was subtracted from the initial weight value of the sample considered. Thus,
conversion is indicated as the ratio of the amount in g of the precipitated sample, to the
amount in g of the initial sample.
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Grafting degree (GD) and grafting density (Nt*) were determined by dissolving
0.5 g of sample of HIPS and/or HIPS/AgNP’s nanocomposite in 25 ml of a MEK/DMF
mixture (1:1), which is a selective solvent for the PS homopolymer. Thereafter, the non-
soluble BC and BC-g-PS were removed from the PS homopolymer via ultra-
centrifugation at 20,000 rpm and -20 °C. The amounts of insoluble material and of PS
homopolymer were determined gravimetrically. Then, through Eqs. 1-3, GD and Nt*
were calculated [12]. The PS homopolymer was then precipitated from methanol, and
then re-dissolved in THF (HPLC grade) in order to evaluate the average molecular
weight via size exclusion chromatography (SEC).
freetotalgrafted PSPSPS −= (1)
where PSgrafted, PStotal and PSfree represent the amount of grafted PS, the total amount of
grafted and non-grafted PS, and the amount of free PS, in g, respectively. GD was
calculated from:
100*initial
grafted
BC
PSGD = (2)
where PSgrafted is the amount in g of grafted PS and BCinitial is the initial amount in g of
BC in the reaction, which is maintained constant. Nt* was obtained as follows:
=
freePS
BC
initial
grafted
Mn
Mn
BC
PSNt ** (3)
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where PSgrafted is the amount in g of grafted PS, BCinitial is the initial amount in g of BC
in the reaction, BCMn is the number average molecular weight of the BC used, and
freePSMn is the number average molecular weight of the free PS homopolymer.
Morphology of the samples was examined by transmission electron microscopy
(TEM) using a JEOL JSM-7401-FE-SEM in transmission mode. The examined samples
were first cut under cryogenic conditions with a Leica Ultracut microtome with a
diamond knife to a thickness of ca. 70 nm and tinted with osmium tetraoxide. The
average particle diameter (Dp) and the volume fraction of the disperse phase (Φ) were
determined using the Image Analyzer Software “Image Pro 3.0”. In each case,
measurements were made on at least 300 particles at 20,000 X magnification. Impact
strength was determined according to ASTM D-256.
Mathematical Model
The mathematical model used [13] considers two phases in thermodynamic
"instantaneous" balance (i.e., it assumes that the transfer of species between the phases
is much more rapid than the polymerization).
The homogeneous stage before the phase separation is modeled as a particular
case of the heterogeneous model, where there only exists a rubber rich phase. The
kinetic equations considered in the mathematical model are described in Table 1.
In these equations I , St, )(PS s and P represent the initiator, styrene, PSfree
molecules, with s repetitive units, and graft copolymer molecules, respectively. ⋅I , ⋅S ,
⋅0P , y ⋅P represent the primary radicals of the initiator, polystyril radicals and primary
and non-primary radicals of the graft copolymer, respectively and K3, 2, 1, , =n, ms The
residual PB is considered as a particular case of the graft copolymer with no grafts.
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The corresponding material balance for the involved species, based on the
kinetics shown in Table 1, are the following:
Initiator
∑=
−=2,1
d [I]d
}[I]d{
j
jjVkt
V
(4)
1I2 ]I[I][ K= (5)
where the sub-indexes 1 and 2 represent the PS and BC rich phase, respectively and KI
is the initiator partition coefficient.
Monomer
Assuming the hypothesis of long chain, where the monomer is only consumed
during the propagation reaction, the material balance for the monomer is:
∑=
−=2,1
p,d
}d{[St]
j
jjVRt
V
(6)
2,1[St])][P][S( ..pp, =+= jkR jjjj (7)
1St2 ]St[]St[ K= (8)
where jRp, is the polymerization rate in the phase j (j=1, 2) in mol/(m3 s), and KSt is the
monomer partition coefficient.
The differential ordinary equations were solved through the discretization of the
equations with the method of finite differences. After resolving equations (4) to (8), it is
possible to determine the monomer conversion (monomer concentration can be known
from Eq. (6)).
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Afterward, an algorithm that changes the initiator's initial concentration (the rest of
the recipe was used as it was measured experimentally) was employed, to minimize the
difference between the experimental conversion, and the conversion predicted by the
model [14]. Certainly, this initiator's concentration is fictitious, and does not correspond
with the concentration used experimentally. The above-mentioned difference is directly
related to the presence of the AgNP’s
RESULTS AND DISCUSSION
Effect of the incorporation of silver nanoparticles on the HIPS polymerization rate.
Figure 1a shows the variation of x with reaction time during the mass
polymerization stage, for the blank HIPS and for the HIPS/Silver nanocomposites. A
considerable decrease in the x values can be observed as the AgNP’s concentration
increases from 0 to 1 wt-%, and at the same time the average number molecular weight
increases (Fig. 1b). This behavior can be attributed either to a possible physical
interaction between the silver nanoparticles and the free radicals present in the reaction
medium in a similar manner as reported by Yeum et al. [7-9], or to the reduction of Ag+
(adsorbed onto the AgNP´s surface) in the presence of free radicals, as mentioned by
Yanagihara et al. [3, 4] and Kong et al. [5].
In order to corroborate the possible interaction between free radicals and
AgNP’s, the concentration of free radicals was calculated in each case and the plot with
respect to x values is shown in Figure 2. The radical concentration was calculated
through Eq. 9 which was obtained by re-arraigning the expression of monomer
consumption during the propagation step [15], and taking into account the experimental
values of x and reaction time.
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[ ]dt
dx
xkR
p )1(
1*
−= (9)
where: dt
dx is the slope of the curve of conversion as a function of time and pk is
the propagation rate constant T
pk/35577 e100.1 −×= L/mol-s [16].
From Figure 2 it can be observed that the free radicals concentration tends to
decrease with x, this decrease becomes more noticiable after ca. 20 % conversion except
for HIPS1-0.025, where the free radicals concentration remains relatively constant. In
addition, an evident decrease in the free radicals concentration is observed as the
concentration of AgNP’s increases, where the maximum drop for HIPS1-1 is 74%
(6*10-8
mol/L) with respect to the [R*] for the blank HIPS, meanwhile for HIPS1-0.025
and HIPS1-0.1 the decrease in [R*] is in the order of 33 and 22%, respectively.
On the other hand, through the use of the mathematical model described in the
experimental section [13], and using experimental data, the theoretical initiator
concentration that had to be used to obtain the results presented in Figure 1a was
determined (Fig. 3). These results indicated a decrease of 20, 40 and 86 wt-% in the
concentration of BPO when HIPS/Silver nanocomposites with 0.025, 0.10 and 1.0 wt-%
silver were synthesized, respectively. These results are indicative of a decrease in the
free radicals concentration from the initial stage of the reaction, which can be directly
associated with an interaction between the free radicals generated upon BPO
decomposition and AgNP’s. Another possible cause for the decrease in the free radicals
concentration might be attributed to the existence of an interaction between the
surfactant used to disperse the silver nanoparticles in St and the free radicals. With
respect to this surfactant effect, Figure 4 shows the experimental results obtained when
HIPS was synthesized in the presence of a similar surfactant concentration as that used
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for the dispersion of 1 wt-% of silver nanoparticles. As it can be observed, there is a
significative effect on conversion, and more detailed studies are currently carried out in
order to provide a better understanding of this phenomenon.
Effect of the incorporation of silver nanoparticles on the grafting degree (GD) and
the phase inversion (PI) phenomenon.
After assessing the effect of silver nanoparticles on the x values, the effect of
AgNP´s on the grafting degree and on the phase inversion was evaluated. The phase
inversion can be estimated from the plot of grafting degree as a function of time and/or
conversion (Fig. 5) where a sudden decrease in the GD, after reaching a maximum
value, indicates that phase inversion occurred. This behavior can be interpreted as
follows: a) the first stage of the curve relates to a reaction mixture presenting a rubber
rich continuous phase and involves an increase of the amount of graft copolymer during
the reaction, b) it reaches a maximum followed by a decrease in the GD; during this
period a co-continuous phase is present in the reaction mixture and the decrease of the
grafting degree can be attributed to the removal of the PS and graft copolymer occluded
inside some micelles, which at this point can be easily removed, and c) at the end of this
period, the phase inversion occurs and the GD values start to increase due to new
grafting reactions taking place at the occlusions of the rubber particles and the reaction
mixture present a rich continuous PS phase.
Taking into account this situation, from Figures 5a and 5b it can be observed that
the GD of the HIPS/silver nanocomposites is always lower than the value achieved by
the blank HIPS. These results agree with those presented in Figure 2, in the sense that
the presence of the AgNP’s diminishes the free radicals concentration [R*]. Since
freePSMn which can be considered the same as the Mn of the PS grafts [17], remains
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approximately constant for the HIPS/silver nanocomposites, the increment in the
AgNP´s concentration (from 0.025 to 0.1 wt-%) lead to a decrease in the GD as a
consequence of a reduction in the number of grafts per rubber backbone (Nt*) at the PI
phenomenon, as it can be observed from the values reported in Table 2.
Nevertheless, it is apparent that for 0.025 and 0.1 wt-% silver concentration, the
PI during the bulk polymerization stage of the HIPS/silver nanocomposites occurs
within the same range as that reported for the blank HIPS (between 27-34 % monomer
conversion and between 135 -180 min of reaction time). However, there was no sign of
the PI phenomenon for the 1.0 wt-% silver nanocomposite, even for reaction times as
long as 400 min (Fig. 5b); which could be attributed to the very low amount of PS
molecules produced, due to the low concentration of free radicals in the presence of the
higher concentrations of the silver nanoparticles.
The occurrence of the phase inversion phenomenon was validated through the
evolution of the developed morphologies observed by electron microscopy at different
predetermined periods of reaction time (different conversion values). Figures 6a, 6b,
and 6c for 0, 0.025 and 0.10 wt-% silver concentration, show the presence of extended
structures constituted by PS stabilized by the graft copolymer BC-g-PS formed in situ at
x = 0.19, 0.26 and 0.23, respectively. The rubber phase shows the presence of micelles
structures, which according to Sardelis [18], are due to the interfacial interaction
between the BC used as the precursor rubber and the PS generated during the reaction,
which can be solubilized into the PS domains of the block copolymer as a consequence
of a very similar number average molecular weight of the PS in the BC (BC
PSMn ) and
the freePSMn (for all nanocomposites
freePSMn = 80-100 kg/mol and BC
PSMn = 80
kg/mol).
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As monomer conversion reaches 0.28, 0.33 and 0.28 for HIPS with 0, 0.025 and
0.1 wt-% of silver nanoparticles, respectively, the micelles give place to lamellar
structures, which are indicative of a co-continuous phase. Finally, at x = 0.34, 0.36 and
0.35 for the systems mentioned above, core-shell morphologies are observed, which
indicate that the phase inversion has occurred.
On the other hand, in Figure 6d, for a silver concentration of 1.0 wt-%, only the
first two stages are observed and no formation of the core-shell structure occurs, at least
at the evaluated periods of time, corroborating the results presented in Figure 5, in the
sense that there is no phase inversion at this high silver concentration due to the low
amount of PS formed (x = 0.26 as compared to 0.34, 0.36 and 0.35 for HIPS1, HIPS-
0.025 and HIPS-0.10).
Effect of the incorporation of silver nanoparticles on the impact strength as a
consequence of the morphology developed.
Figure 7 shows the micrographs of the blank HIPS (Fig. 7a) and the HIPS/silver
nanocomposites with 0.025 and 0.10 wt-% silver concentration (Figs. 7b and 7c),
respectively. All the micrographs correspond to the final product, where core-shell
morphologies can be observed; as well as that of the nanocomposite with 1.0 wt-%
silver concentration (Fig. 7d), where an undefined morphology is present, which is
precisely related to the fact that the bulk polymerization stage did not reach the
necessary conversion value for the phase inversion to occur and for the core-shell
morphology to be established.
With respect to the parameters that define the morphology and influence the
impact properties, the average particle diameter (Dp) and the volume fraction of the
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disperse phase (Φ) shown in Table 3, present a slight increase with the silver
nanoparticles concentration. As the grafting degree at the phase inversion point
decreases with the silver nanoparticles concentration (Fig. 5), the interfacial tension
increases, giving rise to a larger particle size (157 and 140 nm, as compared to 134 nm
for the blank HIPS). Nevertheless, the results for impact strength are relatively similar
for the blank HIPS and the nanocomposites with 0.025 and 0.10 wt-% silver, whereas, it
doubled for the nanocomposite with 1.0 wt-% silver. This behavior is due to the fact
that the amount of PS homopolymer formed in the 1.0 wt-% silver nanocompósito is
low (26% conversion) and the system, made up of mainly rubber phase when phase
inversion would occur (which in this case does not take place), tends to form an
interpenetrated polymer network of crosslinked rubber grafted with PS, in agreement
with reports by Amos [19] and Keskkula [20] in the absence of agitation.
CONCLUSIONS
It was found that silver nanoparticles strongly interfered with the bulk
polymerization reaction of HIPS, killing the free radicals and decreasing the rate of
polymerization and conversion values.
For 0.025 and 0.10 wt-% silver concentration, the phase inversion during the
bulk polymerization stage of the HIPS/silver nanocomposites occurred within the same
range as that for the blank HIPS. However, there was no sign of phase inversion for the
1.0 wt-% silver nanocomposite, even for reaction times as long as 400 min. The above-
mentioned effect shifted the phase inversion point to longer reaction times.
The impact strength of those nanocomposites with 0.025 and 0.10 wt-% silver
that is, the systems that presented the phase inversion phenomenon, remained similar as
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that of the blank HIPS, but it almost doubled for the nanocomposite with 1.0 wt-%
silver which showed no phase inversion.
ACKNOWLEDGEMENT
The authors would like to thank Pablo Acuña, Mario Palacios, Guadalupe
Mendez and María Luisa López for their technical support and Dr. René D. Peralta
Rodríguez for reviewing the manuscript.
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Evolution of a) monomer conversion vs reaction time and b) free PS Mn as a function of monomer
conversion for blank HIPS (HIPS1) and HIPS/silver nanocomposites (error bars 5%).
43x20mm (600 x 600 DPI)
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Figure 2. Evolution of free radicals concentration with monomer conversion for blank HIPS (HIPS1) and HIPS/Silver nanocomposites.
64x58mm (600 x 600 DPI)
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Figure 3. Calculation of the initial initiator concentration through adjusting the experimental conversion values vs reaction time data by using a mathematical model, for a) blank HIPS (HIPS1)
and for b), c) and d) HIPS/silver nanocomposites with 0.025, 0.10 and 1.0 wt-% of silver, respectively.
171x148mm (600 x 600 DPI)
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Figure 4. Plot of monomer conversion as a function of reaction time for blank HIPS(HIPS1), HIPS with 1 wt-% silver (HIPS1-1) and HIPS with 0.2 wt-% surfactant alone (HIPS1-0.2D2) used to
disperse silver nanoparticles (error bars 5 %). 59x50mm (600 x 600 DPI)
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Figure 5. Plot of GD as a function of a) monomer conversion and b) reaction time, for blank HIPS (HIPS1) and HIPS/Silver nanocomposites.
75x38mm (600 x 600 DPI)
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Figure 6. Developed morphology structures before (left column), during (middle column) and after (right column) the phase inversion point during the synthesis of HIPS/Silver nanocomposites; a)
HIPS1, b) HIPS-0.025, c) HIPS-0.10 and d) HIPS-1.0 (scale bar 1µm). 150x189mm (300 x 300 DPI)
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Figure 7. Micrographs of the morphology structure for (a) blank HIPS (HIPS1) and (b, c, d) HIPS/Silver nanocomposites with 0.025, 0.10 and 1.0 wt-% silver, respectively (scale bar 1 µm).
150x158mm (300 x 300 DPI)
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Table 1. Kinetic equations considerd in the mathematical model according to [13].
Chemical Initiation
⋅⋅
⋅⋅
⋅
→+
→+
→
0
1
PPI
SStI
2II
i2
i1
d
k
k
k
Thermal Initiation ⋅
→ 1S2St3 i0k
Propagation
⋅⋅
−
⋅⋅
⋅⋅
−
→+
→+
→+
n
k
n
k
s
k
s
PStP
PStP
SStS
p
p0
p
1
10
1
Transfer to Monomer
⋅⋅
⋅⋅
⋅⋅
+→+
+→+
+→+
1'
0
1
1S
SPStP
SPStP
S)(PStS
fm
fm
fm
k
kn
ks s
Transfer to the rubber
⋅⋅
⋅⋅
+→+
+→+
0
0S
PPPP
P)(PPS
fg
fg
k
n
k
n n
Termination by coupling
PSP
)(PSS
tc
tc
S
→+
→+
⋅⋅
⋅⋅
−
knm
knns s
PSP tc''
0 →+⋅⋅ kn
PPP tc''
0 →+⋅⋅ kn
PPP tc'
→+⋅⋅ kmn
PPP tc'
00 →+⋅⋅ k
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Table 2. Grafting Density (Nt*) and Number Average Molecular Weight ( Mn ) at the
phase inversion point for the synthesized blank HIPS and HIPS/silver nanocomposites.
HIPS1 HIPS-0.025 HIPS-0.10 HIPS-1.0
Silver content (wt-%) 0.00 0.025 0.10 1.00
Grafting density (Nt*) 1.20 0.89 0.80 NA
Mn PS free at PI (kg/mol) 81 101 90 NA
Mn PS free at PI: Average Number Molecular Weight of the PS matrix at the phase inversion point.
NA: Not applicable (the phase inversion was not reached).
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Table 3. Grafting degree (GD), average particle diameter (Dp), volume fraction of the
disperse phase (Φ) and impact strength (IS) of the synthesized HIPS and HIPS/silver
nanocomposites.
Materials Parameter
HIPS1 HIPS-0.025 HIPS-0.10 HIPS-1.0
GD (%) 133 98 78 ND
Dp(nm) 134 157 140 ND
Φ 0.23 0.24 0.22 ND
IS (J/m) 37 34 33 66
ND: Not determined due to the absence of an established morphology
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FIGURE CAPTION
Figure 1. Evolution of a) monomer conversion vs reaction time and b) free PS Mn as a
function of monomer conversion for blank HIPS (HIPS1) and HIPS/silver
nanocomposites (error bars 5%).
Figure 2. Evolution of free radicals concentration with monomer conversion for blank
HIPS (HIPS1) and HIPS/silver nanocomposites.
Figure 3. Calculation of the initial initiator concentration through adjusting the
experimental conversion values vs reaction time data by using a mathematical model,
for a) blank HIPS (HIPS1) and for b), c) and d) HIPS/silver nanocomposites with 0.025,
0.10 and 1.0 wt-% of silver, respectively.
Figure 4. Plot of monomer conversion as a function of reaction time for blank
HIPS(HIPS1), HIPS with 1 wt-% silver (HIPS1-1) and HIPS with 0.2 wt-% surfactant
alone (HIPS1-0.2D2) used to disperse silver nanoparticles (error bars 5 %).
Figure 5. Plot of GD as a function of a) monomer conversion and b); reaction time, for
blank HIPS (HIPS1) and HIPS/silver nanocomposites.
Figure 6. Developed morphology structures before (left column), during (middle
column) and after (right column) the phase inversion point during the synthesis of
HIPS/silver nanocomposites; a) HIPS1, b) HIPS-0.025, c) HIPS-0.10 and d) HIPS-1.0.
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Figure 7. Micrographs of the morphology structure for (a) blank HIPS (HIPS1) and (b,
c, d) HIPS/silver nanocomposites with 0.025, 0.10 and 1.0 wt-% silver, respectively.
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Corrections to the article entitled: “Nanocomposites Based on High Impact
Polystyrene/Silver Nanoparticles: Effect of Silver Nanoparticles Concentration on
the Reaction Evolution, Morphology and Impact Strength” by F. Soriano-Corral, G.
Morales*
We read carefully the article and took into account the editorial changes required as well
as the comments from the reviewers which are indicated in the text as follows:
Answers to the reviewer 1:
1) and 3) We included in the experimental section more discussion about the
mathematical model described by Luciani and the corresponding reference (reference
13) written in English. In this case the complete mathematical model was not included
as it is not our intention to describe in this paper the mathematical model and its
development but we included the material balance for the initiator and the monomer in
order to determine the conversion behavior and then the initial BPO concentration
during the HIPS/Silver nanoparticles composite synthesis. The model herein was used
only as a tool in order to describe the behaviors observed in the presence on AgNP’s.
It was included as well as the reference (reference 15) that was used in order to obtain
equation 4. With respect to this last equation we describe herein all the mathematical
arrangements in order to reach equation 4 but we consider that it is not necessary to
include all of them in the article text.
From the experimental data the [M*] was obtained as a function of X, where the rate of
polymerization is given by Eq. 1 (Odian, 1991):
*]][[][
MMKdt
Mdp=−
Eq. 1
Where [M] represents the monomer conversion, [M*] the concentration of free radicals and
kp the polymerization rate constant. It is known from the experimental data that:
0
0
][
][][
M
MMX
−=
Eq. 2
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where, X corresponds to monomer conversion and [M]0 represents the concentration of
initial monomer. By re-arrangement of Eq.2,
][][][ 00 MMXM −= Eq. 3
Applying d/dt on Eq. 3:
dt
Md
dt
dXM
][][
0−=
Eq. 4
Substitution of Eq. 1 into Eq. 4 yields:
*]][[][ 0 MMKdt
dXM p=
Eq. 5
By rearrangement of Eq.5, [M*] can be calculated as (Equation 4 in the article):
dt
dX
XKM
p )1(
1*][
−
=
Eq. 6
Where the dX/dt was experimentally obtained from the derived polynomial function that
describes the evolution of X as a function of time, with a correlation equal to 0.99, where
the T
pk/35577 e100.1 −
×= is given in L/mol-s. In this case we included in the text the
corresponding reference (Reference 16)
2) On page 8, the observation made by the reviewer was wrong, the average molecular
weight increases with increasing silver nanoparticles concentration, there was a mistake
not in the text but in the corresponding Figure and it was properly changed (see Figure
1).
3) It is correct, the maximum decrease is 74% for HIPS1-1 with respect to blank HIPS.
The correction was made in the text.
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4) In the case of HIPS with 1% AgNP’s, the values of impact strength are due
principally to the fact that in this case, the phase inversion was not reached at the
evaluated periods of time so that as the sample contains less of the PS brittle portion and
mostly PB at this point (x=0.27), the morphological final structure (core-shell) is not
formed and the rubber phase becomes crosslinked. It must be noted that in all the
HIPS/Silver nanoparticles composites synthesized (final product), the amount of rubber
is the same (8 wt-%).
5) The manuscript was now corrected by an English speaking people, so we hope there
are no more mistakes through out the text
Answers to the reviewer 2:
1) We tried to make the abstract more concise but it couldn´t be shorten so much.
2) The words “silver” and “High Impact Polystyrene” weren´t added to the keywords
list, because it didn´t exist in the manuscript center glossary.
3) More references were included about the in situ polymerization of vinyl monomers
(references 7-9) but it must be pointed out that the in situ polymerization of
heterogeneous systems in the presence of mineral or metallic nanoparticles has been
scarcely studied as it is mentioned in the introduction section.
4) We revised the introduction paragraph
5) The core shell morphologies are very well defined but we agree with the reviewer
that the “interprenetrated network” is not well defined (instead it is in accord with the
explanation given for reviewer 1, answer 4) so we changed the expression to “semi-
interpenetrated polymer network of crosslinked rubber grafted with PS” in order to be
more clear about this behavior that was previously described by Amos (1974) and
Keskkula (1979), both of them included in the text and in the references.
After considering all the corrections mention below it is our hope that the article
can be published without any further corrections. Thank you in advance.
G. Morales, F. Soriano-Corral
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