polymers
Article
Tailoring Copolymer Properties by Gradual Changes
in the Distribution of the Chains Composition Using
Semicontinuous Emulsion Polymerization
Carlos Federico Jasso-Gastinel 1, *, Alvaro H. Arnez-Prado 1 , Francisco José Aranda-García 1 ,
Luis Octavio Sahagún-Aguilar 1 , Fernando A. López-Dellamary Toral 2 ,
María Elena Hernández-Hernández 1 and Luis Javier González-Ortiz 2
1
2
*
Chemical Engineering Department, Universidad de Guadalajara, 44100 Guadalajara, Mexico;
[email protected] (A.H.A.-P.); [email protected] (F.J.A.-G.);
[email protected] (L.O.S.-A.); [email protected] (M.E.H.-H.)
Chemistry Department, Universidad de Guadalajara, 44100 Guadalajara, Mexico;
[email protected] (F.A.L.-D.T.); [email protected] (L.J.G.-O.)
Correspondence: [email protected]
Academic Editor: Graeme Moad
Received: 21 December 2016; Accepted: 13 February 2017; Published: 22 February 2017
Abstract: To design the properties of a copolymer using free radical polymerization, a semicontinuous
process can be applied to vary the instantaneous copolymer composition along the conversion
searching for a specific composition spectrum of copolymer chains, which can be termed as weight
composition distribution (WCD) of copolymer chains. Here, the styrene-n-butyl acrylate (S/BA)
system was polymerized by means of a semicontinuous emulsion process, varying the composition
of the comonomer feed to obtain forced composition copolymers (FCCs). Five different feeding
profiles were used, searching for a scheme to obtain chains rich in S (looking for considerable
modulus), and chains rich in BA (looking for large deformation) as a technique to achieve synergy in
copolymer properties; the mechanostatic and dynamic characterization discloses the correspondence
between WCD and the bulk properties. 1 H-nuclear magnetic resonance (1 H-NMR) analysis enabled
the determination of the cumulative copolymer composition characterization, required to estimate
the WCD. The static test (stress-strain) and dynamic mechanical analysis (DMA) were performed
following normed procedures. This is the first report that shows very diverse mechanostatic
performances of copolymers obtained using the same chemical system and global comonomer
composition, forming a comprehensive failure envelope, even though the tests were carried out at
the same temperature and cross head speed. The principles for synergic performance can be applied
to controlled radical copolymerization, designing the composition variation in individual molecules
along the conversion.
Keywords: copolymer; free radical; mechanical properties; semicontinuous polymerization;
composition distribution; gradual composition changes; synergic performance
1. Introduction
Since the twentieth century, different blending methods, types of reactions, and polymerization
processes have been used to prepare two component polymers, attempting to combine or optimize
properties, expand applications, or simply optimize the cost/properties relationship [1–3]. It has been
demonstrated that the contribution of each polymeric component is improved if phase separation
occurs at the microscopic level, which implies that chemical blending offers better component
interaction than physical blending [4]. Additionally, the mechanical superiority of polymer blends
Polymers 2017, 9, 72; doi:10.3390/polym9020072
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Polymers 2017, 9, 72
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containing a gradient in composition over materials with uniform composition in the polymer
bulk (e.g., gradient interpenetrating polymer networks (gIPNs) versus regular IPNs obtained by
a three step method using bulk polymerization) has been shown [5–7]. Such superiority has been
attributed to the synergic contribution of polymeric region(s) rich in component A as well as of
region(s) rich in component B, in the spatial structure of the bulk polymer, which leads to the
optimization of properties [6]. Moreover, using the suspension process, forming gradients by
comers/monomer diffusion in polymer beads followed by an in situ polymerization, better control
in transport properties [8,9], or ionic exchange optimization [10,11] has been achieved in polymer
blends. Nevertheless, for materials obtained by conventional or traditional free radical polymerization
(FRP), if sequential homopolymerizations of A and B components are used in emulsion, a two-phase
core-shell type polymer is usually obtained, where phase separation generates a weak interaction
between the components, inhibiting the optimization of properties. However, by appropriate
comonomer mixing taking advantage of their common difference in reactivities [12], gradients in
composition have been formed since a long time ago to produce for example a gradient in properties
like refraction-index in copolymers [13]. Basically, gradients in polymers may be formed in one or
more constituents of a polymeric material, carrying out variations in space, structure, composition,
or synthesis method [14–17]. With that perspective, taking into consideration that covalent bonding
offers the strongest component interaction and that gradients in composition help for the synergic
contribution of the attributes of each component, the attainment of convenient gradual changes in
the composition spectrum of copolymer chains (weight composition distribution of chains; WCD)
could improve the mechanical properties. One way to search for these characteristics using free radical
copolymerization (FRC) is to prepare copolymers by means of a semicontinuous process varying the
feed comonomer composition along the reaction. That is, in principle forcing the reaction, copolymer
chains rich in A, as well as chains rich in B can be formed, regardless of the relative reactivities
(for not extremely distant values). Additionally, with a gradual variation in feeding composition,
the formation of chains with intermediate compositions of monomers A and B can also be attained;
such chains may serve as compatibilizers to reduce the tendency to separate the polymer bulk into
two phases. With that aim, using the styrene-2-ethylhexyl acrylate system with a seeded multistage
semicontinuous emulsion process, promising results were reported years ago using linear feeding
profiles for the comonomers with variations in global composition [18]; besides, seed type or particle
size may also be varied [19]. If the cumulative composition of the copolymer chains is followed along
the reaction and used to estimate the WCD of the copolymer bulk, the copolymer properties can be
correlated to that composition spectrum. With this aim, the styrene/n-butyl acrylate (S/BA) system
with styrene seed was polymerized in emulsion, considering several feeding profiles for the 70/30
w/w composition. For the copolymers obtained, significant changes in the toughening effect caused by
the BA contribution were observed, while their Young’s modulus was sustained to a certain extent at
room temperature. Those changes in mechanical behavior corresponded to variations in the copolymer
composition spectrum [20]. Based on those results, to demonstrate the full correspondence between
the composition spectrum of the copolymer chains and the mechanical properties (dynamic and static)
of a copolymer system, in this study several feeding profiles using the 50/50 w/w S/BA as chemical
system and a semicontinuous emulsion process were considered and the correspondent WCDs are
estimated. The S/BA monomer pair is a system of interest for the combination of rigidity-elasticity
properties with distant glass transition temperatures (–55 ◦ C for BA and 100 ◦ C for S correspond to the
homopolymers), and shows moderate phase segregation [21]. Their relative reactivities are 0.19 for BA
and 0.72 for S [22].
For the reactions, starting with a polystyrene (PS) seed, predetermined feeding stages are used
with linear, parabolic, or linear V type (e.g., linear descendant-ascendant) profiles to change the
instantaneous S/BA comonomer composition and orientate, since at the beginning of the reaction the
composition changes in the new copolymer chains. With the feeding schemes used, the formation of
chains rich in S units (looking for a high modulus) and chains rich in BA units (for high deformation
Polymers 2017, 9, 72
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capacity) is intended. In the end, this approach may be used as a method to tailor the properties
by means of useful gradual changes in the WCD of the copolymer chains. To be used as references
for mechanical properties, an equivalent statistical copolymer (SC) and a core-shell type polymer in
two stages (T-S) were also prepared and tested.
2. Experimental Section Materials
Styrene and n-butyl acrylate monomers (both from Sigma Aldrich, St, Louis, MO, USA,
purity >99%) were disinhibited with ionic exchange resins; methyl ester hydroquinone resin was
used for BA, and 4-tert-butylcathechol for S. As surfactant, sodium dodecylsulfate (SDS; Aldrich,
St, Louis, MO, USA, purity >98%) was used as acquired. Potassium persulfate was used as initiator
(KPS; Aldrich, purity >98%). Sodium bicarbonate (Arm and Hammer, Ewing, NJ, USA) was used to
Polymers
2017, 9, 72 the reaction. Distilled water was used for every polymerization
3 of 11
buffer pH changes
during
reaction.
In all polymerizations,
nitrogen
gas
was
used
to
purge
the
reaction
system.
Twenty
wt
%
of solid
mechanical properties, an equivalent statistical copolymer (SC) and a core-shell type polymer in two
(T-S) were also prepared
and tested.at the end of every reaction. The reactions were carried
polymer (PS, T-Sstages
or copolymers)
was obtained
out at 70 ◦ C in a2. Experimental
4 L glass reactor,
stirring the system at 400 rpm; reaction times >12 h were used
Section Materials
to maximize monomer(s)
conversion
(followed
by from
gravimetry).
SC MO,
synthesis
was carried out
Styrene and n-butyl acrylate
monomers (both
Sigma Aldrich,The
St, Louis,
USA, purity
>99%)
were
disinhibited
with
ionic
exchange
resins;
methyl
ester
hydroquinone
resin
was
used
by adding quickly to the reactor the required amount of monomers (S/BA, 50/50forw/w) at room
BA, and 4-tert-butylcathechol for S. As surfactant, sodium dodecylsulfate (SDS; Aldrich, St, Louis,
temperature while
stirring. Before the addition of the required amount of initiator, the whole system
MO, USA, purity >98%) was used as acquired. Potassium persulfate was used as initiator (KPS;
purity
>98%).reaction.
Sodium bicarbonate
andpolymeric
Hammer, Ewing,
NJ, USA) was
used
to buffer
was heated up toAldrich,
start the
batch
For the(Arm
other
materials,
a PS
seed
was obtained first
pH changes during the reaction. Distilled water was used for every polymerization reaction. In all
by batch emulsion polymerization. The T-S polymer was obtained by adding to the reactor the required
polymerizations, nitrogen gas was used to purge the reaction system. Twenty wt % of solid polymer
amount of PS seed
the second
stage at
the
of reactions
BA monomer
(BA/PS,
(PS,and
T-S orfor
copolymers)
was obtained
therequired
end of everyamount
reaction. The
were carried
out at 50/50 w/w)
70 °C
a 4 Laglass
reactor,
stirring
the system
at 400
rpm;
reaction
times >12
h were
used up
to and a similar
was quickly added
toinsuch
reactor
while
stirring;
then,
the
whole
system
was
heated
maximize monomer(s) conversion (followed by gravimetry). The SC synthesis was carried out by
procedure to theadding
one quickly
followed
the the
SCrequired
synthesis
was
carried out.
forced
to thefor
reactor
amount
of monomers
(S/BA, The
50/50 five
w/w) at
room composition
temperature
stirring. by
Before
the addition
of the required
amount of initiator,
the whole system
copolymers (FCCs)
werewhile
obtained
means
of seeded
semicontinuous
emulsion
processes; for the
was heated up to start the batch reaction. For the other polymeric materials, a PS seed was obtained
FCCs the global first
styrene
content was also 50 wt %. At the start of each copolymerization reaction, the
by batch emulsion polymerization. The T-S polymer was obtained by adding to the reactor the
required
amount
of PSlatex
seed and
for the second
required
amount
of BA monomer
(BA/PS,water required
reactor was charged
with
a seed
containing
50stage
g ofthePS
and the
amount
of distilled
50/50 w/w) was quickly added to such a reactor while stirring; then, the whole system was heated up
to complete 1600
g
of
total
load.
Then,
10
sequential
“comonomer
feeding
stages”
of 12 min each
and a similar procedure to the one followed for the SC synthesis was carried out. The five forced
one were implemented
for(FCCs)
the Vwere
type
feeding
profile
where
20 stages of
6 min were used).
composition(except
copolymers
obtained
by means
of seeded
semicontinuous
emulsion
the FCCs
the global styrene
content
was also 5045wtg %.
At the for
start the
of each
At the beginningprocesses;
of eachforstage,
an aqueous
solution
containing
(except
reaction of the V
copolymerization reaction, the reactor was charged with a seed latex containing 50 g of PS and the
type feeding profile,
where
22.5
g
of
water
were
used)
of
distilled
water,
and
the
required
amounts
amount of distilled water required to complete 1600 g of total load. Then, 10 sequential “comonomer
feeding
stages”
of
12
min
each
one
were
implemented
(except
for
the
V
type
feeding
profile
where
of KPS, SDS, and NaHCO3 were added as a batch to the reaction system. In every stage, the amount
20 stages of 6 min were used). At the beginning of each stage, an aqueous solution containing 45 g
added of each salt
wasforthe
corresponding
to 2%
of where
the total
ofwere
theused)
comonomers
(except
the one
reaction
of the V type feeding
profile,
22.5 gmass
of water
of distilled to be added
water,
and
the
required
amounts
of
KPS,
SDS,
and
NaHCO
3
were
added
as
a
batch
to
the
reaction to the reactor
in that stage. The feeding flow was modified at the start of each stage, and pumped
system. In every stage, the amount added of each salt was the one corresponding to 2% of the total
(Masterflex L/Smass
7528-10)
at constant flow rate during the stage. Five feeding profile combinations
of the comonomers to be added in that stage. The feeding flow was modified at the start of each
for the S and BAstage,
comonomers
implemented
(M1 –M
), by means
linear
(L1 Five
or L2 ), parabolic
and pumped towere
the reactor
(Masterflex L/S 7528-10)
at5constant
flow rate of
during
the stage.
feeding
profile
combinations
for
the
S
and
BA
comonomers
were
implemented
(M
1–M5), by means of
(Pa), linear ascendant-linear descendant (A), and linear descendant-linear ascendant (D) trajectories.
linear (L1 or L2), parabolic (Pa), linear ascendant-linear descendant (A), and linear descendant-linear
Such individual ascendant
flow profiles
are shown
in Figure
1a–e with
theinfollowing
schemes:
L2 S/L1 BA (M1 );
(D) trajectories.
Such individual
flow profiles
are shown
Figure 1a–e with
the following
L2S/L1BA(M
(M1););PaS/L
1BA (M
2); L1(M
S/PaBA
(M
3); L1S/L1BA(M
(M4););DS/ABA
(M
5); in all
cases
the
PaS/L1 BA (M2 );schemes:
L1 S/PaBA
L
S/L
BA
);
DS/ABA
in
all
cases
the
feeding
time was
3
1
1
4
5
feeding time was predetermined as 2 h.
predetermined as 2 h.
6
5
Comonomer Flow (g/min)
Comonomer Flow (g/min)
6
L2S
L1BA
4
3
2
1
0
5
PaS
L1BA
4
3
2
1
0
0
20
40
60
80
100
120
0
20
40
60
Time (min)
Time (min)
(a)
(b)
Figure 1. Cont.
80
100
120
Polymers 2017, 9, 72
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Polymers 2017, 9, 72
4 of 11
6
8
L 1S
PaBA
Comonomer Flow (g/min)
Comonomer Flow (g/min)
10
6
4
2
0
0
20
40
60
80
100
L1S
L1BA
5
4
3
2
1
0
120
0
20
40
Time (min)
60
80
100
120
Time (min)
(c)
(d)
Comonomer Flow (g/min)
6
DS
ABA
5
4
3
2
1
0
0
20
40
60
80
100
120
Time (min)
(e)
Figure 1. Comonomers flow as a function of reaction time, for S/BA FCCs synthesis: (a) M1
(L2S/L1BA);
(b) Mas
2 (PaS/L
1BA); (c) M
3 (L
1S/PaBA); (d)
M4 (Lfor
1S/L1S/BA
BA); andFCCs
(e) M5 (DS/ABA).
Figure 1. Comonomers
flow
a function
of
reaction
time,
synthesis: (a) M1 (L2 S/L1 BA);
(b) M2 (PaS/L1 BA); (c) M3 (L1 S/PaBA); (d) M4 (L1 S/L1 BA); and (e) M5 (DS/ABA).
For the characterization of each FCC material, seven samples were extracted at predetermined
reaction times, to be measured by 1H-NMR (Varian Gemini 2000, Agilent Technologies, Palo Alto,
CA, USA) their respective global styrene percentage (
values) and, by gravimetry their
For the characterization
of each FCC material,
seven samples were extracted at predetermined
respective global conversion values (
values). For the calculations to build up the WCD, the
reaction times, toconversion
be measured
by 1 H-NMR
(Varian
Gemini 2000,
Agilent
interval (from
0% up to the
final experimental
conversion
value) Technologies,
was discretized in Palo Alto, CA,
subintervals of 0.1% in width. It was considered that all the chains produced inside of each
USA) their respective global styrene percentage (SPexp values) and, by gravimetry their respective
conversion subinterval i (characterized by its respective
value) have the same
value, which
global conversion
the calculations
to build
upabove
the WCD,
the conversion
expa values).
was values
estimated(X
with
third order For
polynomial
obtained by fitting
the seven
mentioned
experimental
values;
for experimental
this, the least squares
method was used.
The total
mass
of monomer j in
(S or
interval (from 0%
up to the
final
conversion
value)
was
discretized
subintervals of
BA) polymerized up to subinterval i (
) can be calculated with the following equations:
0.1% in width. It was=considered
that= all the
chains
produced
inside of each conversion subinterval
100 −
,
or,
being the total amount of monomer (S plus
i (characterizedBA)
byincluded
its respective
Xi (450
value)
have
theofsame
SPji polymerized
value, which
was estimated
with a
in the reaction
g). Thus,
the mass
monomer
in the subinterval
i
∆
can beobtained
estimated asby
follows:
∆
=
− above
the mass
percentage of
. Therefore,
third order polynomial
fitting
the
seven
mentioned
experimental
values; for
S in the chains formed in the subinterval i %
can be obtained with the following equation:
this, the least squares
method
was .used.
The total mass of monomer j (S or BA) polymerized up
% = ∆
∆
+∆
The respective masses of polymer produced with % values within
j
composition
intervalswith
(e.g., 0%–5%
of S, 5%–10% equations:
of S and so on) were
to subinterval ithe
(mpre-established
)
can
be
calculated
the following
mSupsuitably
to i = Xi MT SPi or,
up to i
summed up and, with such values, the histograms showing the WCD profiles for the copolymer
mBA
− CPi ),inM
the total
amount
of monomer
(Swere
plus
BA) included in the
(100contained
T being
each
FCC material
at the end
of the reaction
(Figure 2a,b)
constructed
up to i = Xi MTchains
j
[19,20].
reaction (450 g). Thus,
the mass of monomer j polymerized in the subinterval i (∆mi ) can be estimated
All latexes were dried by evaporation at room temperature; the collected high molecular weight
j
j
j
as the ones reported before [18–20]), were used to prepare samples by compression
as follows: ∆mi polymers
= (mup(such
to i +1 − mup to i ). Therefore, the mass percentage of S in the chains formed in
molding (Schwabenthan polystat 200T) to carry out all mechanical tests. Stress-strain measurements
the subinterval i (%Si ) can be obtained with the following equation: %Si = ∆mSi / ∆mSi + ∆mBA
.
i
The respective masses of polymer produced with %Si values within the pre-established composition
intervals (e.g., 0%–5% of S, 5%–10% of S and so on) were suitably summed up and, with such values,
the histograms showing the WCD profiles for the copolymer chains contained in each FCC material at
the end of the reaction (Figure 2a,b) were constructed [19,20].
All latexes were dried by evaporation at room temperature; the collected high molecular weight
polymers (such as the ones reported before [18–20]), were used to prepare samples by compression
molding (Schwabenthan polystat 200T) to carry out all mechanical tests. Stress-strain measurements
were performed at 25 ◦ C following the procedure of the American Society for Testing Materials (ASTM
D638, samples Type IV, crosshead speed: 0.0083 cm/s; Universal testing machine SFM10).
A dynamic mechanical analysis (DMA) was carried out to measure storage and loss moduli as a
function of temperature (ASTM D-5023, three point bending clamp, 1 Hz, heating rate: 1 ◦ C/min TA
Instruments Q800, Waters Corporation, New Castle, DE, USA).
0.30
Weight fraction of copolymer chains
Weight fraction of copolymer chains
were performed at 25 °C following the procedure of the American Society for Testing Materials
(ASTM D638, samples Type IV, crosshead speed: 0.0083 cm/s; Universal testing machine SFM10).
A dynamic mechanical analysis (DMA) was carried out to measure storage and loss moduli as a
function of temperature (ASTM D-5023, three point bending clamp, 1 Hz, heating rate: 1 °C/min TA
Instruments Q800, Waters Corporation, New Castle, DE, USA).
Polymers 2017, 9, 72
5 of 11
M1 (L2S/L1BA)
M2 (PaS/L1BA)
0.25
0.20
0.15
0.10
0.05
0.00
0
20
40
60
80
Styrene weight percentage in copolymer chains
(a)
100
0.30
M3 (L1S/PaBA)
M4 (L1S/L1BA)
M5 (DS/ABA)
0.25
0.20
0.15
0.10
0.05
0.00
0
20
40
60
80
100
Styrene weight percentage in copolymer chains
(b)
Figure 2. Weight fraction of copolymer chains of FCCs as a function of styrene weight percentage in
Figure 2. Weight fraction of copolymer chains of FCCs as a function of styrene weight percentage
the copolymer chains, estimated for: (a) M1, and M2, materials and (b) M3, M4, and M5 materials. For
in the copolymer chains, estimated for: (a) M1 , and M2 , materials and (b) M3 , M4, and M5 materials.
codes, see Figure 1.
For codes, see Figure 1.
3. Results and Discussion
3. Results and Discussion
The chosen feeding profiles (Figure 1a–e) allow the diversification of composition histograms
Therepresent
chosen feeding
profiles
1a–e)
allow the
of composition
histograms
that
that
the WCDs
of (Figure
the FCCs
presented
in diversification
Figure 2a,b. The
feed trajectories
allow the
represent
the
WCDs
of
the
FCCs
presented
in
Figure
2a,b.
The
feed
trajectories
allow
the
formation
formation of chains that are rich in styrene or butyl acrylate at different moments depending on theof
chains
that
are rich
or butyl
acrylatethe
at continuous
different moments
depending
the flow profile
flow
profile
and in
thestyrene
monomer
reactivities;
change in
monomeron
composition
allowsand
a
the biased
monomer
reactivities;
the continuous
change in
monomer composition
allows
a biased
change
change
in the instantaneous
copolymer
composition.
The composition
profiles
shown
in
Figure
2a,b definecopolymer
the mechanical
properties
that
are presented
in Figures
3–5.
Even though
the
in the
instantaneous
composition.
The
composition
profiles
shown
in Figure
2a,b define
copolymer
composition
determines
the
mechanical
performance
that
may
vary
from
highly
elastic
the mechanical properties that are presented in Figures 3–5. Even though the copolymer composition
(almost pure
poly(butyl acrylate)
(PBA),that
to highly
rigidfrom
behavior
(almost
pure
PS), the
deformation
determines
the mechanical
performance
may vary
highly
elastic
(almost
pure
poly(butyl
capacity
and
moduli
are
affected
by
both
components
depending
on
the
chains
buildup
along
the
acrylate) (PBA), to highly rigid behavior (almost pure PS), the deformation capacity and
moduli
composition
spectrum.
The
correlation
between
the
WCD
of
a
copolymer
and
its
correspondent
are affected by both components depending on the chains buildup along the composition spectrum.
canWCD
be disclosed,
quantifying
2a,b the weight
fraction
accumulation
Themechanical
correlationproperties
between the
of a copolymer
and in
itsFigure
correspondent
mechanical
properties
can be
or
wt
%
of
the
copolymer
chains
within
specific
composition
ranges
(e.g.,
0–60
wt
%,
which
can
be
disclosed, quantifying in Figure 2a,b the weight fraction accumulation or wt % of the copolymer chains
designated as styrene composition interval or simply SI 0–60).
within specific composition ranges (e.g., 0–60 wt %, which can be designated as styrene composition
For the general FCC behavior, even though the chains that are close to an extreme of the ordinate
interval or simply SI 0–60).
axis will have significant influence on the property related to the pure homopolymer (i.e., at 0 wt % the
For the general FCC behavior, even though the chains that are close to an extreme of the ordinate
behavior stands for PBA and at 100 wt % to PS), the synergic performance also requires the
axis will have significant influence on the property related to the pure homopolymer (i.e., at 0 wt % the
considerable presence of chains that contain both components (as a fundamental difference to T-S
behavior
standsInfor
PBA
and at
100 wtat
% to
thehistograms
synergic performance
requires
the considerable
materials).
that
sense,
looking
thePS),
FCC
as a wholealso
in Figure
2a,b,
the highest
presence
of
chains
that
contain
both
components
(as
a
fundamental
difference
to
T-S
materials).
In4 that
mechanical moduli can be expected for M1 and M2, but higher deformations can be expected for M
or
sense,
looking
at
the
FCC
histograms
as
a
whole
in
Figure
2a,b,
the
highest
mechanical
moduli
can be
M5.
expectedFor
forthe
M1FCCs
and M
deformations
canseen
be expected
forof
Mstress-strain
2 , but higher
4 or M5 .
presented
in Figure
3, it can be
that the set
curves at 25 °C
For
the
FCCs
presented
in
Figure
3,
it
can
be
seen
that
the
set
of
stress-strain
curvesenvelope
at 25 ◦ C
encompass an extensive range of mechanical performance, forming in fact a wide failure
encompass
range
of mechanical
performance,
formingspeed)
in fact[23],
a wide
failure
envelope
(althoughanit extensive
was obtained
at the
same temperature
and cross-head
which
only depends
(although
was obtained
at the same
temperature
and cross-head
which only
depends
on
on the itWCD
of such materials,
because
they possess
the samespeed)
global[23],
composition.
The
Young’s
(E) and
deformation
ranksthe
forsame
the global
FCCs can
be seen in
Table
1 and
their
the modulus
WCD of such
materials,
becausecapacity
they possess
composition.
The
Young’s
modulus
correlations
with capacity
the WCDranks
can be
by be
observing
Table12.and
In their
principle,
the higher
(E) and
deformation
forestablished
the FCCs can
seen in Table
correlations
withthe
the
modulus,
lower theby
deformation;
is, 2.
basically
the FCCs
position
values
inthe
E
WCD
can be the
established
observing that
Table
In principle,
theoccupy
higheropposite
the modulus,
the
lower
and deformation
However,
looking
at the
curves position
of Figurevalues
3, it caninbeEinferred
that the design
deformation;
that is,ranks.
basically
the FCCs
occupy
opposite
and deformation
ranks.
However, looking at the curves of Figure 3, it can be inferred that the design of the WCDs may be used
to obtain a good combination of E and deformation, searching for considerable rigidity and toughness.
Polymers 2017, 9, 72
6 of 11
of the WCDs may be used to obtain a good combination of E and deformation, searching for
6 of 11
and toughness.
Polymers
2017, 9, 72rigidity
considerable
8
M1
M2
M3
M4
M5
T-S
SC
Strees
6
4
2
2600%
0
0
10
20
30
40
50
60
Strain (%)
Figure
3. 3.
Stress-strain
behavior
of of
FCCs
(M(M
two
polymer
(T-S)
and
statistical
copolymer
Figure
Stress-strain
behavior
FCCs
1–M
twostage
stage
polymer
(T-S)
and
statistical
copolymer
1 –M
5 ),5),
◦ C (crosshead speed: 0.5 cm/min). For FCCs codes, see Figure 1.
(SC)
at
25
(SC) at 25 °C (crosshead speed: 0.5 cm/min). For FCCs codes, see Figure 1.
Table
1. Young’s
modulus
andand
ultimate
properties
of FCCs
(M1 –M
), as
as T-S
SC materials
at
5),well
as well
as and
T-S and
SC materials
Table
1. Young’s
modulus
ultimate
properties
of FCCs
(M15–M
25at◦ C25and
crosshead
speed
of 0.5ofcm/min.
For For
codes,
see Figures
1 and
2. 2.
°C and
crosshead
speed
0.5 cm/min.
codes,
see Figures
1 and
Property
Property
Young
(MPa)
YoungModulus
Modulus (MPa)
Ultimate
(MPa)
Ultimate Stress
Stress (MPa)
Ultimate Strain
Strain (%)
Ultimate
(%)
Toughness (MPa)
Toughness (MPa)
1
MM
M2M2
M3 M3
M4 M4
M5 M5
T-S T-S
SC SC
1
440
±
20
200
±
30
120
±
30
90
±
20
0.35
±
0.10
30
±
30
440 ± 20
200 ± 30
120 ± 30
90 ± 20
0.35 ± 0.10
30 ± 30
20 ± 420 ± 4
± 0.4 2.2 ±
2.20.3± 0.3 2.8 ±
2.80.4± 0.40.120.12
± 0.070.9 ±0.9
1.0 ± 0.1
7.47.4
±±
0.80.8 3.03.0
± 0.4
± 0.07
0.1 ± 0.1
1.0 ± 0.1
± 300± 300
13.9 ±
1.9 ± 1.9
35.5 ±35.5
1.1 ± 1.1
2.22.2
±±
0.20.2 15.6
± 2.3
15.6
± 2.3 6.1 ±
6.11.8± 1.8 54 ±5415± 1526002600
13.9
11.0 ± 0.6 48.5 ± 0.9 11.7 ± 1.0
150 ± 2
630 ± 5
11.7 ± 0.7 31.5 ± 2.4
11.0 ± 0.6 48.5 ± 0.9 11.7 ± 1.0 150 ± 2
630 ± 5
11.7 ± 0.7 31.5 ± 2.4
Table
2. 2.
Weight
fraction
of of
copolymer
chains
of of
FCC
samples
whose
composition
is is
within
a specific
Table
Weight
fraction
copolymer
chains
FCC
samples
whose
composition
within
a specific
interval.
For
codes,
see
Figure
1.
interval. For codes, see Figure 1.
M55
M1 M2 M2 M3M3
MM
44
0.2090.2070.207 0.112
0.112 0.134
0.134
0.001
0.001
0.0620.0630.063 0.177
0.177 0.035
0.035
0.005
0.005
0.017
0.0630.0780.078 0.086
0.086 0.040
0.040
0.017
0.081
0.075
0.050
0.153
0.064
0.081 0.075
0.050
0.153
0.091
0.064
0.073
0.533
0.0710.0940.091 0.070
0.064 0.349
0.073
0.533
0.103
0.0700.0940.094 0.065
0.070 0.137
0.349
0.103
0.044
0.033
0.0790.0890.094 0.072
0.065 0.056
0.137
0.044
0.000
0.0900.0810.089 0.081
0.072 0.020
0.056
0.033
0.122
0.197
0.105
0.110
0.1010.2700.081 0.289
0.081 0.169
0.020
0.000
0.006
0.1910.3480.122 0.375
0.197 0.209
0.105
0.110
0.023
0.812
0.2710.6140.270 0.585
0.289 0.682
0.169
0.006
0.789
0.3340.2670.348 0.209
0.375 0.473
0.209
0.023
0.186
0.134
0.422
0.636
0.539
0.614 0.585
0.682
0.812
0.183
0.137
0.193
0.077
0.2050.3860.267 0.415
0.209 0.318
0.473
0.789
0.188
0.1410.2920.186 0.350
0.134 0.182
0.422
0.636
0.143
0.110
0.1690.2030.183 0.278
0.137 0.125
0.193
0.077
1.000
0.4611.0000.386 1.000
0.415 1.000
0.318
0.188
70–100
0.382
0.292 0.350
0.182
0.143
80–100 show the highest
0.292and0.203
0.125
0.110 whose values
In Figure 3, M1 and M5 clearly
lowest0.278
modulus
respectively
0–100
1.000
1.000
1.000
1.000
1.000
can be seen in Table 1. In Table 2, it can be noticed that they present significant differences in favor of M
Styrene
% inChains
Cop. ChainsM1
Styrene
wt %wt
in Cop.
0–10 0–10
10–2010–20
20–3020–30
30–40
30–40
40–50
50–6040–50
60–7050–60
70–8060–70
80–9070–80
90–100
80–90
0–20
90–100
0–30
0–60 0–20
30–60 0–30
40–60
0–60
60–80
30–60
60–100
70–10040–60
80–10060–80
0–10060–100
0.209
0.062
0.063
0.064
0.071
0.070
0.079
0.090
0.101
0.191
0.271
0.334
0.539
0.205
0.141
0.169
0.461
0.382
0.292
1.000
1
for the wt % of copolymer chains that are rich in S (SI 70 or 80–100), and even since SI 60–100, because
In Figure 3, M1 and M5 clearly show the highest and lowest modulus respectively whose values
the wt % content of copolymer chains for that range are 46.1 for M1 versus 18.8 for M5 . In contrast,
can be seen in Table 1. In Table 2, it can be noticed that they present significant differences in favor of
for the composition ranges below 60 wt % S, the big differences are in favor of M5 especially for the
SI 40–50 region (53.3 wt % for M5 versus 7.1 wt % of chains for M1 ), or the accumulated for SI 0–60
M1 for the wt % of copolymer chains that are rich in S (SI 70 or 80–100), and even since SI 60–100,
because the wt % content of copolymer chains for that range are 46.1 for M1 versus 18.8 for M5. In
contrast, for the composition ranges below 60 wt % S, the big differences are in favor of M5 especially
for the2017,
SI 40–50
SI
Polymers
9, 72 region (53.3 wt % for M5 versus 7.1 wt % of chains for M1), or the accumulated 7for
of 11
0–60 (53.9 for M1 versus 81.2 for M5), denoting the high capacity of M5 for deformation. Such extreme
stress-strain behaviors may be used as a basis to look at the general trends for the FCCs.
(53.9 for M1 versus 81.2 for M5 ), denoting the high capacity of M5 for deformation. Such extreme
Comparing the differences for the M2–M4 materials regarding Young modulus, in Figure 3 the
stress-strain behaviors may be used as a basis to look at the general trends for the FCCs.
curves almost overlap at the origin and that proximity is reflected in Table 1 considering the
Comparing the differences for the M –M4 materials regarding Young modulus, in Figure 3 the
standard deviation of the experimental 2values.
Considering the general mechanical behavior the
curves almost overlap at the origin and that proximity is reflected in Table 1 considering the standard
contribution of the chains accumulated in SI 60–100 for the modulus and SI 0–60 for the deformation
deviation of the experimental values. Considering the general mechanical behavior the contribution of
capacity, those mechanical properties for M1–M5 materials are plotted in Figure 4a,b, to show their
the chains accumulated in SI 60–100 for the modulus and SI 0–60 for the deformation capacity, those
trends with weight fraction of copolymer chains. In addition, since M4 and M5 show high weight
mechanical properties for M1 –M5 materials are plotted in Figure 4a,b, to show their trends with weight
fraction values in SI 40–60, their deformation values are also plotted for that interval (Figure 4b). In
fraction of copolymer chains. In addition, since M4 and M5 show high weight fraction values in SI
Figure 4a, the positive slope shows how the modulus increases as the weight fraction increases in the
40–60, their deformation values are also plotted for that interval (Figure 4b). In Figure 4a, the positive
SI 60–100 interval. It can also be seen that the modulus difference is not very significant for M2, M3 and
slope shows how the modulus increases as the weight fraction increases in the SI 60–100 interval. It can
M4; however, looking at the ultimate strain in Figure 4b and Table 1, M2 and M3 show values close to
also be seen that the modulus difference is not very significant for M2, M3 and M4 ; however, looking at
each other, while M4 is considerable bigger than both of them (M4 shows 3.4 and 8.8 times the modulus
the ultimate strain in Figure 4b and Table 1, M2 and M3 show values close to each other, while M4 is
value of M2 and M3 respectively). This behavior clearly shows the synergic performance of M4, which
considerable
bigger
than both in
of Figure
them (M
shows 3.4
and
8.8 times the modulus value of M2 and M3
in fact can also
be perceived
3 4looking
at M
2–M4 curves, and comparing the toughness in
respectively).
behavior
shows
of M4 , which
fact canslope
also be
Table 1, whereThis
M4 shows
3.1clearly
and 12.8
timesthe
thesynergic
value ofperformance
M2 and M3 respectively.
Theinpositive
for
perceived
in
Figure
3
looking
at
M
–M
curves,
and
comparing
the
toughness
in
Table
1,
where
2 Figure
4
4
the deformation trend of the FCCs in
4b, that can be observed for both composition intervalsM
(SI
shows
3.1
and
12.8
times
the
value
of
M
and
M
respectively.
The
positive
slope
for
the
deformation
2
3
0–60 and SI 40–60), denotes the importance of the presence of copolymer chains with intermediate
trend
of the FCCs in Figure 4b, that can be observed for both composition intervals (SI 0–60 and SI
compositions.
40–60),
denotesatthe
of the presence
of 3,
copolymer
chains with
intermediate
compositions.
Looking
theimportance
reference materials
in Figure
the T-S polymer
shows
a low modulus
compared
Looking
the similar
reference
materials incapacity
Figure 3,tothe
polymer shows a low modulus compared
to four
FCCsatand
deformation
MT-S
2, while the SC shows almost three times its
to
four FCCs capacity,
and similar
deformation
to M2 , while
almost
times its
deformation
although
it has acapacity
lower modulus
than the
the SC
T-Sshows
material.
Suchthree
performances
deformation
capacity, although
it has acopolymers
lower modulus
the T-Sproposed
material.here
Such
performances
validate the convenience
to synthesize
by thethan
procedure
and
confirm that
validate
the
convenience
to
synthesize
copolymers
by
the
procedure
proposed
here
and
confirm
that
in the T-S polymeric materials, the properties combination of the components is not promoted
due
to
in
the
T-S
polymeric
materials,
the
properties
combination
of
the
components
is
not
promoted
due
to
phase separation of the homopolymers within the polymer bulk.
phase separation of the homopolymers within the polymer bulk.
3300
M1
M2
M3
M4
M5
400
300
3000
SI 60-100
SI 0-60
SI 40-60
2700
Ultimate Strain (%)
Young Modulus (MPa)
500
200
100
M1
M2
M3
M4
M5
2400
2100
100
80
60
40
20
0
0
0.0
0.2
0.4
0.6
0.8
Weight fraction of copolymer chains
(a)
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Weight fraction of copolymers chains
(b)
Figure 4. Young’s modulus and ultimate strain as a function of styrene weight fraction within FCCs
Figure 4. Young’s modulus and ultimate strain as a function of styrene weight fraction within FCCs at
at specific
copolymer
composition
intervals.
Young
modulus
SI 60–100;
Ultimate
strain
specific
copolymer
composition
intervals.
(a) (a)
Young
modulus
for for
SI 60–100;
(b) (b)
Ultimate
strain
for for
SI
SI
0–60
and
SI
40–60.
For
FCCs
codes,
see
Figure
1.
0–60 and SI 40–60. For FCCs codes, see Figure 1.
In Figure 5a, for the storage modulus (E’) of the two-component polymers it can be seen that the
In Figure 5a, for the storage modulus (E’) of the two-component polymers it can be seen that the
high value in SI 0–60 of the M5 copolymer promotes the E’ decay at low temperature, while the high
high value in SI 0–60 of the M5 copolymer promotes the E’ decay at low temperature, while the high
values in SI 60–100 and SI 80–100 of M1 impart the highest temperature resistance of the FCCs. The
values in SI 60–100 and SI 80–100 of M1 impart the highest temperature resistance of the FCCs. The M2
M2 material follows closely the M1 trajectory up to 40 °C with an E’ decrease at slightly lower
material follows closely the M1 trajectory up to 40 ◦ C with an E’ decrease at slightly lower temperature
temperature due to its lower value for the SI 60–100, but mainly to its lower value in SI 80–100. The
due to its lower value for the SI 60–100, but mainly to its lower value in SI 80–100. The E’ decay that
M1 and M2 materials show at PBA Tg essentially correspond to the high value in SI 0–20, that both
show in Figure 2a and Table 2. For the SI 0–30, the M3 copolymer that shows the highest value in
Polymers 2017, 9, 72
8 of 11
E’ decay that M1 and M2 materials show at PBA Tg essentially correspond to the high value in SI 0–20,
8 of 11
that both show in Figure 2a and Table 2. For the SI 0–30, the M3 copolymer that shows the highest
value in Table 2 presents a steeper E’ decay that starts slightly above PBA Tg, but its high value in SI
Table
2 presents
steeper resistance
E’ decay that
starts
aboveE’PBA
its80
high
80–100
allows aathermal
before
theslightly
catastrophic
finalTdecay
°C.value in SI 80–100
g , but at
allows a thermal resistance before the catastrophic E’ final decay at 80 ◦ C.
Polymers 2017, 9, 72
1000
M1
M2
M3
M4
M5
T-S
SC
1000
Loss modulus (MPa)
Storage modulus (MPa)
10000
100
M1
M2
M3
M4
M5
T-S
SC
100
10
10
1
-60
-40
-20
0
20
40
60
Temperature (°C)
(a)
80
100
120
-60
-40
-20
0
20
40
60
80
100
120
Temperature (°C)
(b)
Figure5.5.(a)
(a)Storage
Storagemodulus;
modulus;and
and(b)
(b)loss
lossmodulus
modulusasasa afunction
function
temperature
for:
FCCs
1–M5),
Figure
ofof
temperature
for:
FCCs
(M(M
1 –M5 ),
T-S
and
SC
materials
(frequency:
1
Hz).
For
codes,
see
Figures
1
and
2.
T-S and SC materials (frequency: 1 Hz). For codes, see Figures 1 and 2.
The M4 copolymer does not show a phase separation tendency in E’, starting a smooth decay at
The M copolymer does not show a phase separation tendency in E’, starting a smooth decay at
0◦ °C and, 4around 40◦ °C it presents a crossover point with the M3 curve, denoting a slightly higher
0 C and, around 40 C it presents a crossover point with the M3 curve, denoting a slightly higher
temperature resistance for the latter material, when both approach the terminal zone for E’. The SC
temperature resistance for the latter material, when both approach the terminal zone for E’. The SC
shows very poor temperature resistance for the E’ value, while the T-S shows a marked decay at PBA
shows very poor temperature resistance for the E’ value, while the T-S shows a marked decay at PBA
Tg (more than one order of magnitude) followed by a plateau that starts at −20 °C that presents a
Tg (more than one order of magnitude) followed by a plateau that starts at −20 ◦ C that presents a
final decay close to 100◦ °C. Its low E’ performance as in the static test can be attributed to the lack of
final decay close to 100 C. Its low E’ performance as in the static test can be attributed to the lack of
interaction between components.
interaction between components.
For loss modulus (E”) in Figure 5b, the M5 FCC shows a copolymer type behavior with a peak at
For loss modulus (E”) in Figure 5b, the M5 FCC shows a copolymer type behavior with a peak at
approximately 10 °C with an almost vertical decay afterwards, denoting an elastomeric behavior at
approximately 10 ◦ C with an almost vertical decay afterwards, denoting an elastomeric behavior at
room temperature. M1 and M2 copolymers follow very similar trajectories (as in E’ behavior) in the
room temperature. M1 and M2 copolymers follow very similar trajectories (as in E’ behavior) in the
whole test temperature range. The peak around −40◦ °C is assigned to a glass transition response, due
whole test temperature range. The peak around −40 C is assigned to a glass transition response, due
to the chains accumulated in the SI 0–20 (Figure 2a); then, E” maintains a certain plateau value, until
toitthe
chains
in thewith
SI 0–20
(Figure
2a); then,
certain
plateau
value,
until
falls
at 80◦ accumulated
°C, in accordance
E’ decay
(Figure
5a).E”
M3maintains
material, ashows
a wide
peak
at −25
°C,
◦ C,
itbecause
falls at 80
C,
in
accordance
with
E’
decay
(Figure
5a).
M
material,
shows
a
wide
peak
at
−
25
3 0–20; then, it shows a smooth decay as
there are a considerable amount of chains in the SI
it
because
there
are
a
considerable
amount
of
chains
in
the
SI
0–20;
then,
it
shows
a
smooth
decay
as
it
did in the E’ curve (Figure 5a). At around 40
°C, it starts a plateau like behavior due to the influence
did
in the
curveuntil
(Figure
5a). At
around
40 ◦ C, it starts a80plateau
like behavior due to the influence of
of the
SI E’
60–100,
its final
decay
at approximately
°C.
◦ C.
the SI The
60–100,
until
its
final
decay
at
approximately
80
M4 material shows a very wide peak from −40◦ to 40 °C (the widest of all polymeric
The
M
material shows
a very wide
peak from −
40 to 40
widest
of of
all compositions
polymeric materials)
materials)4 indicating
the presence
of copolymer
chains
withC a(the
wide
range
around
indicating
the
presence
of
copolymer
chains
with
a
wide
range
of
compositions
aroundby
thethe
highest
the highest peak value (approximately at −11 °C); that trajectory is highly influenced
chains
◦ C); that trajectory is highly influenced by the chains composition
peak
value
(approximately
at
−
11
composition profile in the SI 0–70 range. The slow decrease of E” that starts at ca. 0 °C continuous
profile
SI 0–70a range.
The formation
slow decrease
of E” that
starts
ca. 0 ◦ is
C continuous
until it presents
until in
it the
presents
shoulder
tendency
at 60
°C,atwhich
a signal influenced
by the
◦
a accumulation
shoulder formation
tendency
at
60
C,
which
is
a
signal
influenced
by
the
accumulation
chainsat
of chains in the SI 70–100 range (chains rich in S); the final modulus decayofoccurs
inlower
the SItemperature
70–100 range
(chains
rich
in S); the final modulus decay occurs at lower temperature than
than
M1–M
3 materials due to its lower SI 70–100 value. The high E” value from
◦ C,
M−55
materials
due
to
its
lower
SI
value.dissipation
The high E”
value from
55 ◦ C the
to almost
1 –M
3
°C to almost 40 °C, indicates a 70–100
high energy
capacity
(area−under
curve) 40
within
indicates
a high energy
capacity
(area under
thepresents
curve) within
that temperature
range.
that temperature
range.dissipation
For the reference
materials,
the SC
a copolymer
type behavior
with
For
the
reference
materials,
the
SC
presents
a
copolymer
type
behavior
with
a
lower
but
slightly
wider
a lower but slightly wider peak than the one presented by M5 (accounting for the dissipation capacity
peak
than
one
presented
M5 (accounting
for the dissipation
capacity
since −the
60 ◦random
C), that deviates
since
−60the
°C),
that
deviatesby
slightly
at 20 °C towards
its final decay,
denoting
structure
◦ C towards its final decay, denoting the random structure with very low temperature
slightly
at
20
with very low temperature resistance. The T-S material shows a peak at −45 °C showing a
◦ C showing a temperature peak value close to PBA
resistance.
Thepeak
T-S material
shows
a peak
at −45
temperature
value close
to PBA
Tg with
a significant
E” decay (due to is high PBA content) that
Tis
a significant
E” decay
(due
high
content) that
is followed
by a plateau
fromthe
0 toglass
ca.
g with
followed
by a plateau
from
0 to
to isca.
100PBA
°C (showing
there
a small peak),
denoting
◦
100
C (showing
there a values
small peak),
denotingthe
theones
glass
temperature values that resemble
transition
temperature
that resemble
oftransition
the pure homopolymers.
the ones of the pure homopolymers.
Polymers 2017, 9, 72
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In summary, the methodology proposed here to design the properties of a FCC by variations
in the WCD of the chains and its correlation with the mechanical properties has been demonstrated,
showing also an unusual failure envelope involving diverse mechanostatic performances of copolymers
obtained using the same chemical system and global comonomer composition, tested at the same
temperature and cross head speed. Additionally, as stated in this work, the correlation between WCD
profiles and the mechanical properties above described using FRP reactions (establishing the structure
characteristics required for synergic behavior) has an equivalent with controlled radical polymerization
(CRP). Recently, for the S/BA system using also a semicontinuous emulsion process with a multistage
feeding technique [18–20]), Guo et al. tested two types of gradient copolymers obtained by CRP that
exhibited yielding and elastic deformation [24], whose copolymer chains composition varied from
one component to the other, according to their former report [25]. Although they did not present
a discussion about their behavior related to the specific structures of those gradient copolymers,
the explanation for that behavior is in accordance with the synergic contribution of the components
proven here (as it was also demonstrated in chemical blends); the presence of long segments of each one
of the components along with transition chain segments of intermediate compositions of monomers A
and B in between each molecule, should lead to the optimization of polymer properties. The conformed
polymer bulk would be similar to the copolymers above described using FRC. In another report, similar
results on damping performance were found for materials obtained by FRC [20] and CRP [21] for the
S/BA system. The equivalence in properties has to do with the richness in S and BA units. For the
FRC case, such richness occurs in different chains, and for the CRP case it occurs within one molecule.
Such component richness in high MW chains is important for mechanical properties in both cases,
because the contribution of each component is enhanced in that way; it is well known that MW is
an important parameter for mechanical properties [26,27]. Additionally, the polydispersity index
should also be considered if looking for uniform properties in the product [28], or for easy processing,
particularly for high MW polymers [29]. To improve the synthesis processes, the use of programmed
feeding can help to control comonomer addition in FRC [30] or CRP [31,32].
4. Conclusions
The WCD-bulk mechanical properties relationship for FCCs obtained by FRP has been
demonstrated. Additionally, it has been shown that the WCD can be easily modified by variations
in the comonomer feeding profile if a semicontinuous emulsion polymerization is used (taking into
consideration the specific monomer reactivities). The wide failure envelope obtained confirms that the
methodology presented here can be used to tailor copolymer properties using a reaction system that
can easily be extended to industrial scale for a wide variety of applications.
Acknowledgments:
financial support.
The authors wish to thank CONACYT and Universidad de Guadalajara for the
Author Contributions: Carlos Federico Jasso-Gastinel: Conceived the experiments, codesigned the reactions, analyzed
data, and wrote the paper. Alvaro H. Arnez-Prado; Francisco José Aranda-García; Luis Octavio Sahagún-Aguilar:
Performed the experiments and part of the characterization. Luis Javier González-Ortiz: Developed the program to
estimate the WCDs, codesigned the reactions and discussed writing details with Carlos Federico Jasso-Gastinel.
Fernando A. López-Dellamary Toral; María Elena Hernández-Hernández: Contributed with some analytical
experiments and their interpretation.
Conflicts of Interest: The authors declare no conflict of interest.
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