Polymer electrolyte membranes from ﬂuorinated polyisoprene-block-sulfonated
polystyrene: Structural evolution with hydration and heating
Akinbode I. Isaacs Sodeye
, Tianzi Huang
, Samuel P. Gido
, Jimmy W. Mays
Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003, USA
Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA
Chemical Sciences Division, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Received 28 February 2011
Received in revised form
2 May 2011
Accepted 7 May 2011
Available online 14 May 2011
Small-angle neutron scattering (SANS) and ultra-small-angle X-ray scattering (USAXS) have been used to
study the structural changes in ﬂuorinated polyisoprene/sulfonated polystyrene (FISS) diblock copoly-
mers as they evolved from the dry state to the water swollen state. A dilation of the nanometer-scale
hydrophilic domains has been observed as hydration increased, with greater dilation occurring in the
more highly sulfonated samples or upon hydration at higher temperatures. Furthermore, a decrease in
the order in these phase separated structures is observed upon swelling. The glass transition tempera-
tures of the ﬂuorinated blocks have been observed to decrease upon hydration of these materials, and at
the highest hydration levels, differential scanning calorimetry (DSC) has shown the presence of tightly
bound water. A precipitous drop in the mechanical integrity of the 50% sulfonated materials is also
observed upon exceeding the glass transition temperature (T
), as measured by dynamic mechanical
Ó 2011 Elsevier Ltd. All rights reserved.
Ionomers have increasing utility in various areas of research and
industry such as batteries, fuel cells, electrolysis cells, ion exchange
membranes, sensors, electrochemical capacitors, modiﬁed elec-
trodes, and even golf balls . Ionomers typically are comprised of
a hydrophilic acid-bearing phase embedded in a hydrophobic
phase. The hydrophilic phases are known to form due to aggrega-
tion of the acid groups into multiplets or larger clusters [2e4].
NaﬁonÔ, which is the most used ionomer material in proton
exchange membrane (PEM) applications, is composed of a hydro-
phobic ﬂuoropolymer backbone and hydrophilic ﬂuorosulfonic
acid-bearing side chains . Other PEM materials range from ﬂu-
oropolymer to aromatic to hydrocarbon backboned materials,
bearing pendant acid groups in one conﬁguration or another .
The clusters formed by the acid groups at the end of the side
chains are essential in facilitating ionic conductivity by absorbing
water which dissociates the protons, forming hydronium ions
which may hop from one acid site to the next during transport 
or, alternatively, protons may migrate directly through the aqueous
phase or hop across hydrogen bonds of adjacent molecules. Thus,
the quantity, shape, size, and connectivity of these ionic aggregates
dictate the observed transport properties of such materials.
When block copolymers of suchionomers are made, typically in
the diblock, triblock, or graft copolymer architectures , an extra
level of morphological complexity is introduced, which yields
a hierarchical structure. As in normal block copolymers, nano-
meter-scale phase separation occurs between the blocks, creating
separate hydrophobic and hydrophilic domains with morphologies
that can be similar to those observed for neutral block copolymers.
The acid groups in the hydrophilic domains further form clusters at
a smaller length scale [9,10].
Swelling or dilation of ionomers upon increasing water or
methanol content and heating to service temperature is also an
important factor affecting conductivity and mechanical integrity.
Block copolymer ionomers with one block being a ﬂuoropolymer
have been shown to exhibit enhanced network formation and
mechanical integrity, especially when hydrated . Backbone
stiffness and the nature of the counterions associated with the acid
sites are known to affect the degree of water uptake, and hence,
swelling [6,12]. The structure of NaﬁonÔ has been shown to
undergo evolution and phase inversion in order to conserve speciﬁc
surface as water content increases .
Previously, we reported the synthesis of well-deﬁned block
copolymer ionomers comprised of a ﬂuorinated hydrophobic block
Corresponding author. Tel.: þ1 413 577 1216; fax: þ1 413 545 0082.
Corresponding author. Department of Chemistry, University of Tennessee,
Knoxville, TN 37996, USA. Tel.: þ1 865 974 0747; fax: þ1 865 974 9304.
E-mail addresses: firstname.lastname@example.org (S.P. Gido), email@example.com,
firstname.lastname@example.org (J.W. Mays).
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Polymer 52 (2011) 3201e3208