End-linked poly[2-(dimethylamino)ethyl methacrylate]-poly(methacrylic acid) polyampholyte conetworks: Synthesis by sequential RAFT polymerization and swelling and SANS characterization

Article abstract of DOI:10.1021/ma200668v

Four well-defined end-linked triblock polyampholyte conetworks composed of positively ionizable 2-(dimethylamino)ethyl methacrylate (DMAEMA) repeating units and negatively ionizable methacrylic acid (MAA) repeating units were synthesized using one-pot, sequential reversible addition-fragmentation chain transfer (RAFT) polymerization, by employing 1,4-bis[2-(thiobenzoylthio)prop-2- yl]benzene (1,4-BTBTPB) as the chain transfer agent, and ethylene glycol dimethacrylate (EGDMA) as the cross-linker. From the four end-linked conetworks, three were based on ABA triblock polyampholytes with polyDMAEMA midblocks with a constant degree of polymerization (DP) and polyMAA end-blocks of different DPs. The fourth end-linked polyampholytic conetwork was based on an equimolar BAB triblock polyampholyte with a polyMAA midblock. Furthermore, two polyampholyte networks were also prepared: one based on an end-linked equimolar statistical polyampholyte, and one with a randomly cross-linked, rather than an end-linked, architecture. Finally, the two homopolymer networks based on the DMAEMA and the MAA monomers were also synthesized. The MAA units were introduced in the (co)networks via the polymerization of 2-tetrahydropyranyl methacrylate (THPMA) followed by its acid hydrolysis after (co)network formation. The linear precursors to the (co)networks were found to have molecular weights and compositions close to the expected values, whereas the extractables from the (co)networks were determined to be lower than 30%. In water, the degrees of swelling (DS) of all the polyampholyte (co)networks presented a characteristic minimum at intermediate pH values, around the (co)network isoelectric point (pI), while they increased at acidic and basic pHs. The pI values of the ampholytic (co)networks were estimated as the midpoints of the regions of reduced swelling and ranged between 5.3 and 6.8, decreasing with the increase of the MAA content in the (co)networks. Finally, small-angle neutron scattering (SANS) studies of the polyampholyte (co)networks swollen in D₂O provided SANS profiles without any peaks but broad shoulders whose location was consistent with the spacing of the cross-linking cores.

Full text of DOI:10.1021/ma200668v

ARTICLE
pubs.acs.org/Macromolecules
End-Linked Poly[2-(dimethylamino)ethyl Methacrylate]ꢀ
Poly(methacrylic acid) Polyampholyte Conetworks: Synthesis
by Sequential RAFT Polymerization and Swelling and
SANS Characterization
Kyriaki S. Pafiti,† Zelina Philippou,† Elena Loizou,† Lionel Porcar,‡ and Costas S. Patrickios*,†
†Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus
‡Institute Max von LaueꢀPaul Langevin, BP 156, F-38042, Grenoble, Cedex 9, France
ABSTRACT: Four well-defined end-linked triblock polyampholyte
conetworks composed of positively ionizable 2-(dimethylamino)ethyl
methacrylate (DMAEMA) repeating units and negatively ionizable
methacrylic acid (MAA) repeating units were synthesized using one-pot,
sequential reversible additionꢀfragmentation chain transfer (RAFT)
polymerization, by employing 1,4-bis[2-(thiobenzoylthio)prop-2-yl]-
benzene (1,4-BTBTPB) as the chain transfer agent, and ethylene glycol
dimethacrylate (EGDMA) as the cross-linker. From the four end-linked
conetworks, three were based on ABA triblock polyampholytes with
polyDMAEMA midblocks with a constant degree of polymerization
(DP) and polyMAA end-blocks of different DPs. The fourth end-linked
polyampholytic conetwork was based on an equimolar BAB triblock
polyampholyte with a polyMAA midblock. Furthermore, two polyam-
pholyte networks were also prepared: one based on an end-linked equimolar statistical polyampholyte, and one with a randomly
cross-linked, rather than an end-linked, architecture. Finally, the two homopolymer networks based on the DMAEMA and the MAA
monomers were also synthesized. The MAA units were introduced in the (co)networks via the polymerization of 2-tetrahydropyr-
anyl methacrylate (THPMA) followed by its acid hydrolysis after (co)network formation. The linear precursors to the (co)networks
were found to have molecular weights and compositions close to the expected values, whereas the extractables from the
(co)networks were determined to be lower than 30%. In water, the degrees of swelling (DS) of all the polyampholyte (co)networks
presented a characteristic minimum at intermediate pH values, around the (co)network isoelectric point (pI), while they increased
at acidic and basic pHs. The pI values of the ampholytic (co)networks were estimated as the midpoints of the regions of reduced
swelling and ranged between 5.3 and 6.8, decreasing with the increase of the MAA content in the (co)networks. Finally, small-angle
neutron scattering (SANS) studies of the polyampholyte (co)networks swollen in D2O provided SANS profiles without any peaks
but broad shoulders whose location was consistent with the spacing of the cross-linking cores.
’ INTRODUCTION
better-defined polyampholyte networks13 would enable the deriva-
tion of accurate structureꢀproperty relationships and may broaden
their applications horizon.
Polyampholyte1ꢀ5 networks6ꢀ12 are cross-linked polymers
bearing both acidic and basic units which are usually randomly
distributed. When these units are weakly acidic or weakly basic,
the polyampholyte networks display a great pH-responsiveness,
with large aqueous degrees of swelling and high chain extensions
at extreme pH values, and minimum swelling and chain contrac-
tion at an intermediate pH range around the isoelectric point, pI,
the pH of zero net change. These materials have found applica-
tions in medicine,6ꢀ8 most notably for drug release9 and DNA
adsorption,10 and in metal-ion adsorption and removal.11
Despite the large number of studies on polyampholyte
networks,6ꢀ12 the structure of most of these materials is poorly
defined, with the chains between the network junctions (so-called
elastic chains) displaying broad molecular weight distributions,
a result of the simultaneous free radical cross-linking copolym-
erization of the monomers and the cross-linker. Availability of
Control over the length and composition of the network
polyampholyte chains can most conveniently be achieved by the
use of “living” polymerization techniques, such as anionic poly-
merization.14 To date, the only reports on the preparation of well-
defined polyampholyte (co)networks made use of sequential group
transfer polymerization (GTP),15 a controlled, “quasiliving”16
anionic polymerization method best-suited for methacrylates, for
the one-pot synthesis of end-linked ABA triblock polyampholytes,17
and end-linked star polyampholytes.18 As the acidic and basic
units were placed in separate segments, these well-defined
Received: March 23, 2011
Revised:
May 15, 2011
Published: June 07, 2011
r
2011 American Chemical Society
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Figure 1. Chemical structures and names of the main reagents used for the (co)network synthesis.
materials are called polyampholyte “conetworks” rather than just
“networks”.
from ethanol. The polymerization solvent 1,4-dioxane was dried over
CaH2 and vacuum distilled prior to use.
Controlled radical polymerization (CRP) methods represent
a modern and powerful synthetic tool for the preparation of
well-defined polymers. Among them is reversible additionꢀ
fragmentation chain transfer (RAFT) polymerization,19 which
can be applied to a great range of monomers, including acrylates,
methacrylates, styrenics and acrylamides. This method has
extensively been used for the preparation of various types of
copolymers, including block, graft, and star copolymers. How-
ever, there are only a small number of reports on the application
of RAFT polymerization for the synthesis of well-defined conet-
works, all of which concern amphiphilic rather than ampholytic
systems.20,21 The aim in this investigation is to extend previous
work on well-defined polyampholyte conetworks synthesized by
GTP17,18 and prepare well-defined end-linked polyampholyte
conetworks using RAFT polymerization. This extension proved
to be nonstraightforward as certain challenges had to be addressed,
including the protection of the acidic monomer and the compat-
ibility of this protected monomer with the RAFT process. After
these issues were resolved, the synthesis of several well-defined
polyampholyte (co)networks was accomplished. These materials
covered a range of compositions and architectures, and were
thoroughly characterized in terms of their aqueous swelling
properties as a function of pH gravimetrically and also in terms
of their structure in water using small-angle neutron scattering
(SANS).
Synthesis of THPMA. The THPMA monomer was synthesized
based on a modification of the procedure reported by Hertler.22 In
particular, THPMA was prepared by the esterification reaction of MAA
with a 150% excess of 3,4-dihydro-2H-pyran at 65 °C over a 48 h period.
In the mixture, a small amount of the free radical inhibitor DPPH was added
to protect THPMA and MAA from undesired thermal polymerization.
Synthesis of the Chain Transfer Agent. The bifunctional chain
transfer agent (CTA) 1,4-bis[2-(thiobenzoylthio)prop-2-yl]benzene
(1,4-BTBTPB) was prepared by the reaction of 2 g of dithiobenzoic
acid (DTBA) (13.0 mmol) with 0.98 g of 1,4-diisopropenyl benzene
(6.20 mmol) in the presence of a catalytic amount (0.1 g) of p-toluene-
sulfonic acid (0.5 mmol) as described in the literature.23 The required
DTBA was prepared following the procedure given by Jayalakshmi
et al.24 Briefly, 1.1 g of Mg (45.8 mmol) and 127 mL of THF were
transferred into a three-necked round-bottom flask, and then, 4.8 mL of
bromobenzene (7.1 g, 45.5 mmol) was added dropwise into the reaction
mixture. The reaction was carried out at 40 °C until the Mg was
completely consumed and disappeared. Then, 2.7 mL of carbon disulfide
(3.5 g, 45.4 mmol) was added dropwise over 20 min and left to react for
1 h at ꢀ5 °C. The successful synthesis of the CTA was confirmed by 1H
and 13C NMR spectroscopies.
(Co)network Preparation. Figure 1 displays the chemical struc-
tures and names of the monomers, the cross-linker, and the bifunctional
CTA used for the synthesis of the polyampholyte (co)networks.
The synthesis of the (co)networks was accomplished by a one-pot,
sequential monomer, and cross-linker RAFT polymerization in Schlenk
tubes, without isolation of the intermediate polymer precursors. A 6 : 1 molar
ratio of EGDMA : CTA was used, following the conclusions of a previous
study21 in which the synthesis of DMAEMA homopolymer networks
was optimized. (Co)networks of different compositions and different
architectures were prepared. Composition variation was accomplished
through the variation of the polyTHPMA end-block lengths at a
constant polyDMAEMA middle block length, whereas architecture
variation was achieved by the variation the order of addition of the
reagents. End-linked conetwork synthesis required the sequential addi-
tion of monomers (in the appropriate order) and the cross-linker, while
for the synthesis of randomly cross-linked polyampholyte network all
the reagents were simultaneously terpolymerized. The THPMA units in
the (co)networks were converted to MAA units by acid hydrolysis after
(co)network formation and washing.
’ EXPERIMENTAL SECTION
Materials. The monomers 2-(dimethylamino)ethyl methacrylate
(DMAEMA, 99%) and methacrylic acid (MAA, 99%), the cross-linker
ethylene glycol dimethacrylate (EGDMA, 98%), basic alumina, calcium
hydride (CaH2, 90ꢀ95%), 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH,
95%), magnesium (98%), silica gel (60 Å, 70ꢀ230 mesh), n-hexane
(g96%), diethyl ether (g99.7%), and 1,4-dioxane (99%) were pur-
chased from Aldrich, Germany. The monomer 2-tetrahydropyranyl
methacrylate (THPMA) was synthesized in our laboratory as detailed
in a following section. Bromobenzene (g99.5%) was purchased from
Fluka, Germany. 1,4-Diisopropenyl benzene (97%) was purchased from
TCI Europe, Belgium. 2,20-Azobis(isobutyronitrile) (AIBN, 95%), carbon
disulfide (purity g99.5%), ethanol (99.9%) and deuterated chloroform
(CDCl3) were purchased from Merck, Germany. Tetrahydrofuran
(THF, 99.8%, both HPLC and reagent grade) was purchased from
Scharlau, Spain.
This paragraph details the procedure followed for the synthesis of one
of the conetworks based on an end-linked ABA triblock copolymer. In
particular, we describe the preparation of the equimolar ABA triblock
copolymer-based end-linked conetwork with the structure EGDMA3-
grad-THPMA25-grad-DMAEMA50-grad-THPMA25-grad-EGDMA3.
Note that the sequential rather than stepwise (which would have
involved polymer precipitation and isolation after each synthetic step)
polymerizations, employed to enhance the “livingness” of the system
(to secure the preservation of more active polymerization sites), resulted
in gradient rather than pure block copolymer structures. A 25 mL Schlenk
tube containing a magnetic stirring bar was loaded with 83.0 mg of the
Methods. Inhibitors and acidic impurities were removed from the
DMAEMA monomer and the EGDMA cross-linker by passing these
materials through basic alumina columns. This was followed by stirring
DMAEMA and EGDMA over CaH2, in the presence of the free radical
inhibitor DPPH, for 72 h to neutralize the last traces of moisture, and a
vacuum distillation. The AIBN radical initiator was recrystallized twice
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bifunctional CTA 1,4-BTBTPB (0.178 mmol), 18.3 mg of AIBN (0.1 mmol),
1.5 mL of freshly distilled DMAEMA (1.4 g, 8.9 mmol), and 1.5 mL of
freshly distilled 1,4-dioxane. The contents of the tube were subsequently
degassed by three freezeꢀvacuumꢀthaw cycles, were placed under an
inert nitrogen atmosphere, and were heated in an oil bath at 70 °C for
12 h. The produced DMAEMA homopolymer was sampled for gel
the DMAEMA monomer at 4.1 ppm divided by the area of the oxymethy-
lene protons of polyDMAEMA at 3.96 ppm, while, for THPMA,
monomer conversion was calculated from the ratio of the area of the
peak due the methinic proton of THPMA at 5.98 ppm divided by the
area of the methinic proton of polyTHPMA at 5.89 ppm.
Characterization of the (Co)networks. Determination of the
Sol Fraction (Extractables). Upon (co)network formation, a small piece
was cut from each (co)network, placed in a small volume of CDCl3
overnight, and the thus-extracted material was promptly characterized
using 1H NMR spectroscopy in terms of its composition, and, in
particular, the final monomer and cross-linker conversions based on
the signal from the unreacted olefinic protons. These extractables were
called “early” extractables.
1
permeation chromatography (GPC) and H NMR spectroscopy ana-
lyses (monomer conversion by 1H NMR spectroscopy = 87.5%; GPC
number-average molecular weight = Mn = 9150 g molꢀ1; theoretical
molecular weight (MW) = 8000 g molꢀ1; Mw/Mn = polydispersity index
(PDI) = 1.5; Mw is the weight-average molecular weight). Afterward,
1.5 mL of THPMA (1.5 g, 8.8 mmol) was added and was left to poly-
merize for 12 h. A sample of the synthesized ABA triblock copolymer
was also obtained for GPC and 1H NMR spectroscopy analyses
(THPMA conversion by 1H NMR spectroscopy = 83.4%; GPC
Mn = 16000 g molꢀ1; MW theory = 16500 g molꢀ1; PDI = 1.5;
The rest of each (co)network was placed in a large (1-L) glass-jar, and
left to equilibrate in THF for 2 weeks to remove all the extractables.
Subsequently, the extractables solution was separated from the THF-
swollen conetwork pieces by filtration, and was dried by evaporating off
the solvent using a rotary evaporator. This was followed by the complete
drying of the thus-predried extractables by placing them in a vacuum
oven at room temperature for 72 h. The sol fraction was calculated as
the ratio of the dried mass of the extracted polymer divided by the
theoretical mass of the gel which was estimated as the sum of the masses
of the monomers, the cross-linker and the CTA. Finally, the extractables
were characterized using GPC and 1H NMR spectroscopy analyses.
Measurement of the Degree of Swelling (DS) in THF. All (co)networks
were characterized before their hydrolysis in terms of their degrees of
swelling (DSs) in THF. The (co)networks were cut in small pieces and
were weighed to determine their THF-swollen mass. The pieces were dried
in a vacuum oven at room temperature for 72 h and their dry mass was,
subsequently, determined. The DSs were calculated as the ratio of the
average swollen (co)network mass (determined three times) divided by the
dry (co)network mass.
Hydrolysis of the THPMA Units. After the extraction of the sol fraction
and the measurement of the DS in THF, each THF-swollen (co)network
was transferred to a 1-L glass jar, which contained approximately 400 mL
of pure water plus a volume of a 10 M HCl solution. The number of
moles of HCl was three times the number of moles of THPMA plus
DMAEMA (DMAEMA is not hydrolyzed but captures HCl to get
ionized). The system was allowed to hydrolyze for 1 week, followed by
washing with distilled water for another 2 weeks to remove the excess of
HCl. The water was changed every day. Before each water change, the
pH of the supernatant solution was measured. The pH of the super-
natant solution of the (co)networks immediately after the hydrolysis was
approximately 1.5, but after the washings for 2 weeks, it rose to 3.5ꢀ4.0.
This hydrolysis step converted the THPMAꢀDMAEMA amphiphilic
(co)networks to MAAꢀDMAEMA ampholytic (co)networks.
1
THPMA/DMAEMA molar ratio in copolymer by H NMR spectros-
copy = 0.95), EGDMA cross-linker (0.2 mL, 0.2 g, 1.1 mmol) was added,
resulting in gel formation within 24 h. Before the addition of EGDMA,
1 mL of 1,4-dioxane was added into the polymerization mixture to
reduce the viscosity of the mixture.
For the preparation of the BAB triblock (with a THPMA midblock)
copolymer-based end-linked conetwork, the addition of the two como-
nomers was reversed, with THPMA polymerized first. However, during
its RAFT homopolymerization, THPMA was partially (45%) hydro-
lyzed to MAA units. This observation was consistent in all three
repetitions of this synthesis. This problem was overcome by the addition
of triethylamine to the polymerization solvent, which protected
THPMA from hydrolysis. The idea of adding a tertiary amine to protect
THPMA from hydrolysis was the result of the observation that THPMA
always smoothly polymerized, without hydrolysis, when this was added
as the second monomer, in the presence of the homopolymer of
DMAEMA, a tertiary amine itself. Thus, rather than using 1.5 mL of
1,4-dioxane as polymerization solvent, a mixture composed of 0.75 mL
of triethylamine and 0.75 mL of 1,4-dioxane was used instead. The same
procedure with triethylamine addition was also employed for the syn-
thesis of the THPMA homopolymer-based end-linked network.
Finally, the statistical copolymer end-linked polyampholyte network
was synthesized by the simultaneous copolymerization of the two
comonomers followed by the addition of cross-linker, while the randomly
cross-linked network based on statistical polyampholytes was prepared
by the simultaneous terpolymerization of the two comonomers and the
cross-linker, without addition of triethylamine.
Characterization of the Linear Precursors by GPC and 1H
NMR Spectroscopy. Samples of linear homopolymers and linear
copolymers extracted from the polymerization flask before cross-linking
were characterized in terms of their MW and composition using GPC
and 1H NMR spectroscopy. A Polymer Laboratories system, equipped
with an ERCꢀ7515A Polymer Laboratories refractive index (RI) detector
and a PL Mixed “D” column was employed for the GPC measurements,
using THF as the eluent which was delivered at 1 mL minꢀ1 by a Waters
515 isocratic pump. MW calibration was performed using poly(methyl
methacrylate) (PMMA) standards supplied by Polymer Laboratories.
The 1H NMR spectra of the polymer solutions in deuterated chloroform
(CDCl3) were recorded using a 300 MHz Avance Bruker NMR spectro-
meter equipped with an Ultrashield magnet, and calibration was
performed using the signal from residual protonated (CHCl3) solvent
(peak at 7.26 ppm). The copolymer composition was found from the
ratio of the relative areas of the peak due to the two oxymethylene
protons in polyDMAEMA at 3.96 ppm and that due to the methinic
proton of polyTHPMA at 5.89 ppm. Furthermore, the conversion of the
two monomers to polymer was also calculated from 1H NMR spectros-
copy as follows. For DMAEMA, monomer conversion was calculated
from the ratio of the area of the peak due to the oxymethylene protons of
Measurement of the DSs in Water. After their hydrolysis and washing
with water, all the (co)networks were characterized in terms of their
aqueous DSs within the pH range between 1.5 and 13 without added
salt. To this end, several pieces from each (co)network were first cut and
weighed to determine their swollen mass in water at a pH ∼3.5ꢀ4.0.
Then, these pieces were dried under vacuum for 72 h at room temperature
and were, again, weighed to determine their dry mass. Their aqueous
DSs were calculated as the ratio of the water-swollen mass divided by the
dry mass. Subsequently, the dried ampholytic (co)network pieces were
transferred in a volume of water where the appropriate volume of 0.5 M
NaOH (pH range from 3.5 to 13) or 0.5 M HCl (pH range from 1.5 to
3.5) standard solutions had been added to adjust the pH within the range
between 1.5 and 13. The number of moles of NaOH required for each
sample was calculated as the product of the desired degree of deprotona-
tion times the total number of moles of DMAEMA plus MAA units
contained in the sample. The latter was calculated from the (co)network
dry mass, and the polymerization stoichiometry. For these calculations,
a linear hydrogen ion titration curve was assumed, in which the degree of
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Figure 2. Schematic representation of the synthetic procedure followed for the preparation of the end-linked conetwork based on the gradient triblock
copolymer THPMA25-grad-DMAEMA50-grad-THPMA25. The DMAEMA units are shown in light blue, the THPMA units are painted purple and the
MAA units are shown in light pink. Black dumbbells represent the EGDMA cross-linker units.
protonation or deprotonation varied from 0 to 1 within the pH range
from 1.5 to 13. Although the assumption for linearity in the hydrogen ion
titration may sound too simplistic, experimental hydrogen ion titration
curves of linear block polyampholytes are almost linear,25 a result of the
strong electrostatic interactions in these highly charged systems. In any
case, this simplification did not adversely affect our data, because the
actual equilibrium pH value was always measured experimentally.
The above calculation simply aimed at securing samples with various
pH values, with the adjacent samples having degrees of ionization differing
roughly by the same value. To adjust the pH within the range between
1.5 and 3.5, one and two drops of the 0.5 M HCl standard solution were
added to two pieces of each (co)network. The samples were equilibrated
for 3 weeks and the swollen masses were measured. The DSs of all samples
were again calculated by dividing the average value of the swollen mass by
the dry mass for the different samples of each (co)network.
formation. tert-Butyl protection was avoided due to the harsher
reagents and more extreme conditions required for hydrolysis
(trifluoroacetic acid at room temperature or hydrochloric acid
under reflux conditions). Likewise, the very labile trimethylsilyl
group was also avoided as its detachment spontaneously occurs
even with atmospheric moisture. It is noteworthy, however, that
trimethylsilyl-protected (meth)acrylic acid has been successfully
incorporated in amphiphilic conetworks via conventional free
radial copolymerization for the introduction of carboxylic acid
groups (after its deprotection).26ꢀ31
Synthesis of the End-Linked (Co)networks. In the present
work, four end-linked triblock polyampholyte conetworks, one
end-linked statistical and one randomly cross-linked polyampho-
lyte networks, as well as two homopolymer end-linked networks,
were synthesized by sequential RAFT polymerization using
AIBN as the initiator, 1,4-BTBTPB as the chain transfer agent,
1,4-dioxane as the solvent, DMAEMA as the basic monomer,
THPMA as the protected acid (MAA) monomer, and EGDMA
as the cross-linker. The synthetic procedure for the preparation
of an ABA triblock copolymer-based end-linked conetwork by
sequential monomer and cross-linker additions, followed by the
acid hydrolysis of the THPMA units, is presented schematically
in Figure 2. In this figure, the DMAEMA units are shown in light
blue, the THPMA units are shown in purple, and the MAA units,
obtained upon the hydrolysis of the THPMA units, are shown
in light pink. The black dumbbells in the conetwork represent
the EGDMA cross-linker units. As is shown in the figure, the
synthesis of the linear DMAEMA homopolymer with both active
ends (due to the use of the bifunctional CTA) was accomplished
first, followed by the addition of the THPMA monomer which
resulted in chain growth and formation of the THPMA-grad-
DMAEMA-grad-THPMA (gradient) triblock copolymer, again
with two active ends. Finally, the addition of the cross-linker led
to the formation of a three-dimensional conetwork via the inter-
connection of the active copolymer chain ends. As already explained,
the synthesis of the conetworks involved sequential rather than
stepwise monomer and cross-linker additions to enhance the
Determination of the Isoelectric pH (pI) of the Polyampholyte
(Co)networks. The pI of the polyampholyte (co)networks was taken
as the pH where the (co)network presented a minimum DS. In some
cases, however, the (co)networks did not present a clear minimum in
their swelling profile but they formed a plateau. In these cases, the pI was
estimated as the middle point of this plateau region.
Small-Angle Neutron Scattering (SANS). SANS measurements were
performed on the 30 m NG7 instrument at the Center for Neutron
Research of the National Institute of Standards and Technology (NIST)
ꢀ
in Gaithersburg, Maryland. The incident wavelength λ was 6 Å. Three
sample-to-detector distances of 1, 4.5, and 13.5 m were employed,
covering a q-range [q = 4π/λ sin(θ/2)] from 0.0335 to 5.715 nmꢀ1. The
samples were loaded in quartz cells. The scattering patterns were
isotropic, and therefore, the measured counts were circularly averaged.
The averaged data were corrected for empty cell and background.
’ RESULTS AND DISCUSSION
Selection of Monomers. DMAEMA and MAA were selected
as the most common methacrylate weak base and weak acid
monomers, respectively. For the MAA protection, we elected the
tetrahydropyranyl group which can be readily removed under
mildly acidic conditions after the polymerizations and (co)network
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Table 1. Monomer and Cross-Linker Conversions, Molecular Weight, and Composition Characteristics of the Linear Precursors
to the (Co)networks Synthesized
conversion (mol %)
GPC results
% mol T
no.
polymer structurea
gelation
time (min) time (min) theor MWd
T
D
E
Mn
Mw/Mn theor 1H NMR
1
2
3
D100
E3-grad-D100-grad-E3
T50
---
85.7b
91.7c
---
---
95.6c
710
1440
1150
1440
668
16 000
---
14900b
---
12300b
1.5b
---
1.3b
---
1.5b
1.5b
0
---
---
180
---
100b
---
0b
46.1b
99.0b
99.0c
---
85.3b
89.2c 100c
---
9000
---
100
---
E3-grad-T50-grad-E3
D50
---
99.1c
---
e540
---
96.2b
99.3b
8000
12 500
---
8830b
13300b
---
9150b
16000b
---
9500b
14200b
---
9310b
16400b
---
0
T12.5-grad-D50-grad-T12.5
E3-grad-T12.5-grad-D50-grad-T12.5-grad- E3
D50
---
86.0c
690
33.0
1440
660
610
505
---
---
0
---
0b
47.9b
4
5
6
---
87.5b
---
8000
16 500
---
1.5b
1.5b
T25-grad-D50-grad-T25
E3-grad-T25-grad-D50-grad-T25-grad-E3
T50
83.4b
95.7c
98.0b
100b
99.0c
96.7b
99.9c
---
66.3b
98.5c 100c
---
88.6c
675
50.0
---
1440
705
---
---
100b
60.3b
---
9000
16 500
---
1.6b
1.9b
100
D25-grad-T50-grad-D25
E3-grad-D25-grad-T50-grad-D25-grad-E3
D50
---
750
50.0
---
1440
660
e540
625
---
---
0b
55.3b
---
94.5b
---
8000
21 000
---
1.4b
1.5b
0
T37.5-grad-D50-grad-T37.5
E3-grad-T37.5-grad-D50-grad-T37.5-grad- E3
D50-co-T50
81.8b 100b
88.5c 100c
92.6b
100c
100c
---
82.3c
690
60.0
---
1440
740
---
1.4b
---
---
---
7
8
87.2b
99.6c
100c
---
99.7c
100c
16 500
---
15300b
50.0
---
48.5b
---
E3-grad-(D50-co-T50)-grad-E3
1440
1440
540
75
---
D50-co-T50-co-E6
---
---
---
---
a D: 2-(dimethylamino)ethyl methacrylate. T: 2-tetrahydropyranyl methacrylate. E: ethylene glycol dimethacrylate. b From the analysis of the 1H NMR
spectra or the GPC traces of the linear precursors. c From the analysis of the 1H NMR spectra of the “early” extractables. d MWtheor = [M]/[CTA] ꢁ
(monomer conversion) ꢁ MWmonomer þ MWCTA
.
“livingness” of the polymerization (to preserve a greater number
of active polymerization sites) and enable facile conetwork forma-
tion. In initial experiments, the conetwork preparation was accom-
plished by stepwise additions of the monomers and the cross-linker.
After the formation of each polymer, the polymerization was
stopped and the polymer was recovered. In those experiments, a
6 : 1 EGDMA : CTA molar ratio was again used where it was
observed that this ratio was insufficient to effect conetwork formation
and, thus, a higher cross-linker loading was necessary for gelation.
Molecular Weights and Composition of the Linear Pre-
cursors and Monomer Conversion. Table 1 lists the theoretical
chemical structures of all the (co)networks and their linear
precursors, the MWs and compositions of the linear precursors,
and the monomer and cross-linker conversions as determined by
GPC and 1H NMR spectroscopy. Note that the final comonomer
and cross-linker conversions in the (co)networks were calculated
Figure 3. GPC traces of the homopolymers and the triblock copolymer
linear precursors to all the (co)networks. D: 2-(dimethylamino)ethyl
methacrylate. T: 2-tetrahydropyranyl methacrylate.
1
from the H NMR spectra of the “early” extractables. Figure 3
shows the GPC traces of the homopolymer and triblock copoly-
mer linear precursors to all (co)networks. The Mn and PDI values
for all linear homopolymer and copolymer precursors were
calculated from these GPC traces and are listed in Table 1.
The Mn values of the triblock copolymers were higher than those
of their parent homopolymers, as expected. Furthermore, all Mn
values, both of the homopolymers and the copolymers, were very
close, within 20%, to those expected from theory, with the
that of its homopolymer precursor. The spectrum of the triblock
copolymer presents the characteristic methinic proton peak f of
the tetrahydropyranyl ring at 5.89 ppm in the polyTHPMA
blocks, indicating the incorporation of THPMA units in the end-
blocks. Note that peak k at 5.98 ppm in the same spectrum is due
to the methinic proton of the THPMA monomer. Comparison
of the areas of peaks f and k allowed the calculation of the
conversion of THPMA monomer to polymer in the triblock.
Similarly, the conversion of the DMAEMA monomer to polymer
(in the DMAEMA homopolymer and in the triblock) was
calculated from the areas of the peaks of the oxymethylene
protons of polyDMAEMA and DMAEMA monomer at 3.96 and
4.1 ppm, respectively. The monomer conversions in the linear
homopolymer and copolymer precursors to the (co)networks
exception that of the largest triblock copolymer, THPMA37.5
-
grad-DMAEMA50-grad-THPMA37.5. The PDIs were relatively
high (g1.3) but comparable with those in other studies20b,21 for
polymers synthesized using the same CTA.
Figure 4 displays the 1H NMR spectrum of the ABA triblock
copolymer THPMA25-grad-DMAEMA50-grad-THPMA25 and
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Figure 4. 1H NMR spectra of (a) the DMAEMA50 homopolymer precursor and (b) the THPMA25-grad-DMAEMA50-grad-HPMA25 triblock copolymer.
are listed under the “Conversions” heading in Table 1, in the
corresponding row (of homopolymer or copolymer). The
monomer conversions in the homopolymers were always high,
ranging from 85.7 to 96.2% for DMAEMA, and 98.0 to 99.0% for
THPMA. The monomer conversions in the copolymers were
also fairly high, ranging from 66.3 to 100.0% for DMAEMA, and
81.8 to 100.0% for THPMA. The lowest value for the DMAEMA
conversion of 66.3% might be attributed to the polymerization
of this monomer after the polymerization of THPMA as first
block; even some partial hydrolysis of THPMA would delay the
polymerization of the second comonomer. Also note that in all
cases, the conversion of the monomer in the first block of a
copolymer was higher in the copolymer compared to the parent
homopolymer, indicating further progress of the polymerization,
as expected. Finally, the compositions of the linear precursors
were calculated from the normalized areas due to the peaks of the
characteristic protons of polyTHPMA (peak f) and polyDMAEMA
(peak c). The thus-calculated composition is also given in the last
column of Table 1 as the THPMA % mol, and, in all cases, this
was close to the theoretically expected composition.
was moderate (<30%), indicating sufficient incorporation of
linear precursors into the (co)network. Reduction of the extrac-
tables via the use of a greater charge of cross-linker was avoided
because of the risk of the formation of a second network totally
based on cross-linker.32 The lowest percentages of extractables
were presented by the network based on the end-linked statistical
polyampholyte and the randomly cross-linked polyampholyte
network. This can be attributed to the fewer polymerization steps
until cross-linking in these two networks, one and zero, respec-
tively, as compared to two steps in the case of the ABA and BAB
triblock copolymer end-linked conetworks. Note that the end-
linked statistical polyampholyte network and the randomly cross-
linked polyampholyte network also presented quantitative EGDMA
polymerization conversions (Table 1).
From the 1H NMR spectra of the extractables, it was observed
that these mainly consisted of linear chains (polyDMAEMA,
polyTHPMA or poly(DMAEMA-grad-THPMA)) which were
probably deactivated before the addition of the cross-linker. In
particular, the composition of the extractables was richer in the
polymer of the midblock, as this participated in the polymeriza-
tion from the beginning and was, therefore, subjected to deac-
tivation longer. The extractables also contained THPMA monomer
The final conversions of the DMAEMA and the THPMA
1
monomers in the (co)networks were calculated from the H
NMR spectra of the “early” extractables using the same peaks as
and EGDMA cross-linker, while only in one case (E3-grad-D100
grad-E3) did they contain DMAEMA monomer.
The Mn value of the main peak of the extractables, determined
by GPC, is also shown in Table 2. For (co)networks E3-grad-T50-
grad-E3, E3-grad-T12.5-grad-D50-grad-T12.5-grad-E3, E3-grad-T25-
-
those used for the calculations for the linear copolymers men-
1
tioned above. The H NMR spectra of the “early” extractables
were also used to estimate the conversion of the EGDMA cross-
linker in the (co)networks from the peak of the oxymethylene
protons of (nonpolymerized) EGDMA at 4.40 ppm. These three
conversions are also listed in Table 1 under the “Conversions”
heading for the rows corresponding to (co)networks. The
conversions of the three components in the (co)networks were
high, between 98.5 and 100.0% for DMAEMA, between 88.5 and
100.0% for THPMA, and between 82.3 and 100.0% for EGDMA.
The lowest EGDMA conversion of 82.3% corresponded to the
end-linked ABA triblock copolymer conetwork with the longest
chains (overall DP of 125), whereas somewhat higher EGDMA
conversions of ca. 87% were measured for the end-linked ABA
triblock copolymers with shorter chains (DPs of 75 and 100).
The EGDMA conversion in the remaining (co)networks were
95% or higher. Note that these differences in the cross-linker
conversion will be reflected in the DSs of the (co)networks,
which will be presented and discussed later in the manuscript.
Properties of the Sol Fraction. The extracted material (sol
fraction; final extractables rather than “early”extractables) was
characterized in terms of its percentage, Mn, and PDI values. The
results are listed in Table 2. The percentage of the extractables
grad-D50-grad-T25-grad-E3 and E3-grad-T37.5-grad-D50-grad-T37.5
-
grad-E3, the Mn values of the extractables were lower compared
to that determined by GPC for their corresponding linear
precursors (Table 1), probably due to the early deactivation of
the active centers of the linear precursors. In contrast, in the cases
of (co)networks E3-grad-D100-grad-E3 and E3-grad-D25-grad-
T50-grad-D25-grad-E3, the Mn values of their extractables were
higher than that determined by GPC for their linear precursors
(Table 1), suggesting possible interconnection of the free poly-
mer chains during the extraction procedure.
DSs in THF. The DSs in THF of all the (co)networks before
their hydrolysis were also measured and are listed in Table 3. The
DSs for the three ABA triblock copolymer-based end-linked
conetworks increased as the EGDMA cross-linker conversion
decreased (Table 1), leading to less compact materials. This DS
increase followed also the increase of the overall chain DP from
75 to 125 (THF is a nonselective solvent for DMAEMA and
THPMA), but we believe that the effect of cross-linker conver-
sion was more pronounced. From the three isomeric equimolar
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Table 2. Percentage, Compositions, and Molecular Weights of the Extractables from the (Co)networks
1H NMR results (mol %)
GPC results
no.
network structurea
E3-grad-D100-grad-E3
% w/w extract.
24.2
PDb
88.1
PTc
0
D
T
E
Mn
PDI
1
9.4
---
2.5
19 000
240
1.8
1.1
1.5
1.1
1.6
1.1
1.4
1.1
1.3
1.1
2.2
1.1
1.1
1.1
1.1
2
3
4
5
6
E3-grad-T50-grad-E3
22.3
29.8
24.3
25.1
25.0
0
97.2
29.9
25.0
60.8
43.4
---
0
2.8
5.9
9.2
0
9500
230
E3-grad-T12.5-grad-D50-grad-T12.5-grad-E3
E3-grad-T25-grad-D50-grad-T25-grad-E3
E3-grad-D25-grad-T50-grad-D25-grad-E3
E3-grad-T37.5-grad-D50-grad-T37.5-grad-E3
56.8
55.9
37.4
42.8
0
0
0
0
7.4
9.9
1.8
9.9
7400
220
8000
200
17 400
351
3.9
10 200
230
7
8
E3-grad-(D50-co-T50)-grad-E3
D50-co-T50-co-E6
4.4
0
0
0
0
0
0
0
0
210
11.1
55.8
44.2
9560
220
a D: 2-(dimethylamino)ethyl methacrylate. T: 2-tetrahydropyranyl methacrylate. E: ethylene glycol dimethacrylate. b PD: poly(2-(dimethylamino)ethyl
methacrylate). c PT: poly(2-tetrahydropyranyl methacrylate).
Table 3. Degrees of Swelling in THF, in Water at the Conditions of Hydrolysis Following Washing (pH ∼ 3.5) and at the
Isoelectric Points (pI) of all the (Co)networks, and pI Values
degree of swelling
DS in water after hydrolysis
no.
network structurea
E3-grad-D100-grad-E3
DS in THF before hydrolysis
at pH = 3.5ꢀ4.0
at pI
pI
1
2
3
4
5
6
7
8
21.0
---
24.0
19.6
39.6
47.7
22.7
54.9
23.9
22.0
---
---
---
5.5
E3-grad-A50-grad-E3
---
E3-grad-A12.5-grad-D50-grad-A12.5-grad-E3
E3-grad-A25-grad-D50-grad-A25-grad-E3
E3-grad-D25-grad-A50-grad-D25-grad-E3
E3-grad-A37.5-grad-D50-grad-A37.5-grad-E3
E3-grad-(D50-co-A50)-grad-E3
54.0
56.4
---
6.0
7.9
3.7
7.9
9.2
7.9
6.1
6.3
5.4
6.8
5.3
62.7
5.60
20.5
D50-co-A50-co-E6
a D: 2-(dimethylamino)ethyl methacrylate. A: methacrylic acid. E: ethylene glycol dimethacrylate.
(co)networks, the end-linked statistical polyampholyte network
and the randomly cross-linked polyampholyte network pre-
sented the lowest DSs in THF, which was consistent with the
complete polymerization of EGDMA in these two polyampho-
lyte networks and the minimum percentage of extractables.
DSs in Water at pH ∼ 3.5ꢀ4.0. The DSs in water, without
added salt (to maximize electrostatic interactions), of all the
(co)networks after their hydrolysis and washings (pH between
3.5 and 4.0) are also shown in Table 3. The trends for these DSs
followed the same ones for the DSs in THF discussed in the previous
paragraph. In particular, (co)networks where the EGDMA was
polymerized to a higher conversion (Table 1) presented lower
DSs. For example, polyampholyte conetwork 6 with the lowest
EGDMA conversion exhibited the highest DS also at pH ∼ 3.5.
pH-Dependence of the Aqueous Degrees of Swelling of
the Polyampholyte (Co)networks. The experimentally mea-
sured aqueous DSs of all the polyampholyte (co)networks as well
as those of the two homopolymer polyelectrolyte networks are
plotted against the pH of the supernatant solution in Figure 5,
while, Table 3 lists the pI values of the polyampholyte (co)networks.
The aqueous DSs of the DMAEMA homopolymer end-linked
network increased at pH < 7 due to the ionization of the DMAEMA
units in this pH range. Similarly, the aqueous DSs of the MAA
homopolymer end-linked network also increased upon the ioniza-
tion of the MAA units. However, with MAA being a weak acid
rather than a weak base, ionization occurred upon increasing the
pH, and the swelling increased at pH > 6. On the other hand, the
aqueous DSs of all the polyampholyte (co)networks presented a
characteristic minimum at intermediate pH values, while they
increased at acidic and basic pHs. This behavior is typical of
polyampholyte (co)networks, due to the existence of the iso-
electric point, pI, which is the pH of zero net charge.25,33 At and
around the pI, the net repulsive forces are zero, while the van der
Waals and hydrophobic attractive forces dominate, leading to a
reduced extension of the polyampholyte chains. The slight decrease
in the DSs at very high or very low pH values was probably due to
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Figure 5. Degree of swelling as a function of the pH of the supernatant solution for all the polyampholyte (co)networks.
the increase of the ionic strength, effected by the relatively high
concentration of NaOH or HCl under these conditions. Thus,
the behavior of the polyampholyte (co)networks at extreme
pH values was similar to that of simple polyelectrolyte networks,34 in
which the osmotic pressure established by the counterions to the
ionized units and the electrostatic repulsive forces between the
charged polymer chains both promoted network swelling.
The pIs of the polyampholyte (co)networks, estimated from
the position of the swelling minimum, ranged from 5.3 to 6.8.
The pIs of end-linked polyampholyte conetworks E3-grad-A25-
grad-D50-grad-A25-grad-E3 and E3-grad-A37.5-grad-D50-grad-A37.5
-
grad-E3 were 6.1 and 5.4, respectively, decreasing with the MAA
content, as expected. However, the most basic end-linked poly-
ampholyte conetwork E3-grad-A12.5-grad-D50-grad-A12.5-grad-E3
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Figure 6. Degrees of swelling of the polyampholyte (co)networks at low, high and isoelectric pH. (a) Effect of polyampholyte composition. (b) Effect of
polyampholyte architecture.
unexpectedly presented a relatively low pI value of around 5.5.
This discrepancy might arise from the approximate method of
estimation of the pI from the DS vs solution pH curve. For the
particular polyampholyte conetwork, this curve is highly asym-
metrical, with the branch at lower pH values being much steeper
than the one on the alkaline side, which may render the pI
estimation method less accurate. Note, however, that the max-
imum acidic DS of this polyampholyte conetwork was higher
than the maximum alkaline DS, correctly reflecting the excess
DMAEMA units relative to the MAA units. Also note that the
reverse was true for the most acidic end-linked polyampholyte
conetwork E3-grad-A37.5-grad-D50-grad-A37.5-grad-E3, while for
the equimolar polyampholyte conetwork the maximum acidic
and maximum alkaline DSs were approximately equal, again in
line with our expectations. In particular, at the pH extremes, the
swelling behavior of the polyampholyte conetworks was ex-
pected to be dominated by the corresponding fully ionized units.
Thus, at low pH values the high DSs were due to the ionization
of the DMAEMA units, while at high pH values the high DSs
were due to the ionization of the MAA units. The pIs of the three
equimolar end-linked polyampholyte (co)networks span a range
of values from 6.1 to 6.8. This variation is probably due to
the sensitivity of pI to composition around the equimolar
composition.25,33,35
Aqueous DSs at Low and High pH and at the pI.. The
aqueous DSs of the polyampholyte (co)networks at low pH (pH ∼3),
at high pH (pH ∼ 11) and at around the pI (pH ∼ 5ꢀ7) were
extracted from Figure 5 and are plotted in Figure 6. Figure 6a
presents the effect of polyampholyte composition, while Figure 6b
presents the effect of polyampholyte architecture.
Figure 7. SANS profiles of all the (co)networks in D2O at pH = 3.5. (a)
Effect of polyampholyte composition. (b) Effect of polyampholyte
architecture.
Figure 6a shows that the isoelectric aqueous DSs of the end-
linked ABA triblock polyampholyte-based conetworks were constant
and around 8, indicating that the polyampholyte conetworks
were in a less expanded state due to the absence of counterions
and repulsive forces. The low pH aqueous DSs were highest for
the DMAEMA-rich ABA triblock polyampholyte conetwork
which bore the greatest number of positively charged units
under these conditions (lowest number of MAA units). This
was despite the higher EGDMA conversion (Table 1) upon the
synthesis of this conetwork compared to that of its two homo-
logues with higher MAA contents, plotted in the same figure,
suggesting the dominance of the charge effect on swelling at this
instance. In contrast, the fact that the low pH aqueous DS of the
MAA-rich polyampholyte conetwork was not lower than but
equal to that of the equimolar polyampholyte conetwork may be
due to the lower EGDMA conversion upon the synthesis of
the former conetwork. The high pH DSs plotted in the same
figure were highest for the MAA-rich polyampholyte conetwork
which bore the greatest number of carboxylate anions at alkaline
conditions, although its greatest overall DP of the chains and the
lowest EGDMA conversion must have also contributed. In fact,
the high pH aqueous DSs of the three ABA triblock polyampholyte
conetworks increased monotonically with the number of MAA
units in the polyampholyte chain. Finally, it is noteworthy that the
high and the low pH aqueous DSs of the equimolar polyampho-
lyte conetwork were the same because of the same number of
charged units per chain at the two pH extremes, leading to the same
electrostatic repulsion and same driving force for swelling.
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Table 4. Experimental (from DSs) and Maximum Theoretical Distances between the EGDMA Cores and DSs in H2O (pH∼3.5)
of the Polyampholyte (Co)networks
core spacing (nm)
no.
polymer structurea
maximum theoreticalb
from DS
DS in H2O (pH ∼ 3.5)
1
2
3
4
5
6
E3-grad-A12.5-grad-D50-grad-A12.5-grad-E3
E3-grad-A25-grad-D50-grad-A25-grad-E3
E3-grad-D25-grad-A50-grad-D25-grad-E3
E3-grad-A37.5-grad-D50-grad-A37.5-grad-E3
E3-grad-(D50-co-A50)-grad-E3
19.0
25.4
25.4
31.8
25.4
---
23.2
25.4
19.0
26.2
12.5
---
39.6
47.7
22.7
54.9
6.00
22.0
D50-co-A50-co-E6
a D: 2-(dimethylamino)ethyl methacrylate. A: methacrylic acid. E: ethylene glycol dimethacrylate. b The spacing between the EGDMA cores was
calculated as the sum of the DPs of the DMAEMA plus the MAA units in each (co)network chain multiplied by 0.254 nm which is the contribution of one
monomer repeating unit to the contour length.
Figure 6b illustrates the effect of (co)network architecture on
the aqueous DSs of the four equimolar polyampholyte (co)networks.
At the pIs, all four isomeric polyampholyte (co)networks were
less expanded, exhibiting approximately the same DSs, around 8.
This is in agreement with previous studies17,18 which indicated
the independence of the DS at the pI on polyampholyte (co)network
architecture. The low and high pH aqueous DSs of the poly-
ampholyte (co)networks presented values that depended both
on EGDMA conversion upon (co)network synthesis and on
comonomer unit distribution along the chains. In particular, the
equimolar ABA triblock polyampolyte-based end-linked conet-
work presented the highest low and high pH DSs, which reflected
the lowest conversion of EGDMA (88.6%, less compact conetwork).
In contrast, the quantitative EGDMA conversion upon the
synthesis of the BAB triblock polyampholyte-based end-linked
conetwork exhibited the lowest high and low pH DSs (more
compact conetwork). Finally, the other two equimolar poly-
ampholyte networks, the statistical polyampholyte-based end-
linked network and the randomly cross-linked polyampholyte
network, had DSs closer to those of the BAB triblock poly-
ampholyte-based end-linked conetwork, as EGDMA conversion
in all three (co)networks was complete. The lower DSs of the
randomly cross-linked polyampholyte network compared to
those of its statistical polyampholyte-based end-linked counterpart
might be attributed to the existence of a distribution of longer
and shorter chains in the former network and the dominating
effect of the shorter chains.
Structure of Swollen (Co)networks. All the (co)networks
were also characterized in terms of their structure in D2O using
SANS. Figure 7 shows the SANS profiles of all the polympholyte
(co)networks in D2O at pH = 3.5. Figure 7a illustrates the effect
of polyampholyte composition, while Figure 7b presents the
effect of polyampholyte architecture. The scattering profiles from
all polyampholyte (co)networks displayed an upturn of the intensity
in the very low q-region, and a shoulder in the intermediate
q-region, without exhibiting any distinct peak. Such a peak has
previously been observed with end-linked amphiphilic conet-
works in D2O, prepared using a similar polymerization method,21
and also with other amphiphilic conetworks in the bulk.36 In the
case of the amphiphilic conetworks, the peak was attributed to
the micelle-like structures formed by the aqueous self-assembly
of the ABA triblock copolymer amphiphilic chains. In the present
polyampholyte (co)networks, the two hydrophilic comonomers,
DMAEMA and MAA, were not as effective at driving microphase
separation, and the peak was replaced by the shoulder. The
shoulder was located in the q-range between 0.1ꢀ0.2 nmꢀ1
,
corresponding to a correlation length of 2π/q = 30ꢀ60 nm, in
reasonable agreement with the EGDMA core spacing of about
25 nm calculated from the experimentally measured DSs37,38 in
water at pH = 3.5 and assuming 20 chains32 radiating out of each
core. This calculation was also based on the GPC Mn values, after
subtracting the molar mass loss associated with the removal of
the protecting groups from the THPMA units, and an average
polymer bulk density of 1.1 g cmꢀ3. Table 4 lists the values of the
thus-calculated spacings between the cores, which were close to
and slightly lower than the values of the maximum core spacings
calculated for fully stretched polyampholyte chains, and are also
listed in Table 4. The scattering observed in the very low q-region
(<0.06 nmꢀ1) probably arises from large-scale inhomogeneities
(>100 nm = 2π/q) in the system, most likely caused by spatial
variations in the concentration of the (co)network, such as existence
of condensed areas separated by less dense regions. Note that
these inhomogeneities are more than 1 order of magnitude larger
than the (random coil) polymer chain dimensions (∼5 nm).
Thus, the SANS profiles might relate to the coreꢀcore distances
within the polyampholyte (co)networks.
’ CONCLUSION
Four well-defined end-linked polyampholyte conetworks, one
end-linked statistical polyampholyte network and one randomly
cross-linked polyampholyte network, as well as two end-linked
homopolymer networks, were synthesized using RAFT poly-
merization. The (co)networks contained positively ionizable
DMAEMA units and negatively ionizable MAA units. The extrac-
tables from all the (co)networks were moderate, indicating successful
(co)network formation. The aqueous DSs of all polyampholyte
(co)networks presented a minimum around the isoelectric point
(pI) where the net charge was zero, while they increased at
alkaline and acidic pHs due to the ionization of the weakly acidic
and the weakly basic monomer repeating units, respectively.
Moreover, the low pH DSs were highest for the DMAEMA-rich
ABA-triblock polyampholyte end-linked conetwork, and the high
pH DSs were highest for the MAA-rich ABA triblock polyam-
pholyte end-linked conetwork, while the equimolar ABA triblock
polyampholyte end-linked conetwork presented more balanced
low and high pH DSs. The DSs at the pI of all the polyampholyte
(co)networks were around 8, and largely remained composition-
and architecture-independent. The SANS profiles of the
(co)networks did not show any distinct peaks but only shoulders,
5361
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Macromolecules
ARTICLE
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Corresponding Author
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’ ACKNOWLEDGMENT
We thank the Cyprus Research Promotion Foundation and
the EU Structural and Cohesion Funds for Cyprus for supporting
this work in the form of a PENEK2008 Doctoral Research Grant
(Project ENISX/0308/048) to K.S.P. We also thank our colleague
Dr. Paul D. Butler of the Center for Neutron Research at the
National Institute of Standards and Technology (NIST) for
helpful discussions on the interpretation of the SANS data.
Furthermore, we are grateful to the National Institute of Standards
and Technology and the U.S. Department of Commerce in
providing the neutron research facilities used in this work. Finally,
we acknowledge a generous donation by the A. G. Leventis
Foundation, which enabled the establishment of the NMR
infrastructure at the University of Cyprus.
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