which were stable in a much broader pH range than PEC particles containing the
anionic homopolymer. These PEC particles were claimed to feature a neutral core
of complexed PELs decorated by a PEO shell, which can be denoted as block
copolymer micelles.
N-Isopropylacrylamide Comonomers
It is well known that poly(N-isopropylacrylamide) (PNIPAAM) systems undergo
thermotropic phase transitions, resulting in polymer segment density increases at
temperatures higher than the lower critical solution temperature (LCST) [60]. This
phase transition is associated with a loss of external hydrogen bonds of the amide
units to water and a gain of internal hydrogen bonds between amide units expelling
water from the aggregated phase. Copolymers of NIPAAM with charged monomer
units can result in a loss of this thermotropic behavior. Dautzenberg and coworkers
checked whether the complexation of a cationic copolymer of NIPAAM [poly
(methacryloyl-oxyethyldimethyl benzylammonium chloride)] with an anionic
copolymer of NIPAAM [poly(2-acrylamido-2-methylpropanesulfonate)] can
regain the known thermotropic properties of the homo PNIPAAM [61]. Indeed,
they could show significant decreases of the particle size of a PEC-0.6 at a NaCl
concentration of 0.01 M from R H ¼ 200–240 nm at temperatures below LCST to
around R H ¼ 100–140 nm at temperatures above the LCST. Furthermore, this
process was reversible and accompanied by a respective increase and decrease in
structural density of the PEC system. Such a thermoswitchable change in structural
density may have interesting consequences for applications like the controlled
uptake and release of drugs or enzymes.
Kleinen and Richtering [62] complexed microgels of weakly crosslinked
P(NIPAAM-co-methacrylic acid) P(NIPAAM-co-MAA) with PDADMAC and the
resulting submicron particles showed thermosensitive behavior (see Fig. 13). A significant drop in the hydrodynamic radius of PEC from around R H ¼ 300–350 nm
(X ¼ 1:0.2–1:1.25) at 20
C to around R H ¼ 120–170 nm at 45
C took place. The
lower the PDADMAC content in the PEC, the higher was the particle size in the
swollen state and the higher was the size difference between swollen and compact state.
Nolan, Serpe, and coworkers showed that thermoresponsive PEL complex films
containing poly(N-isopropylacrylamide-co-acrylic acid) and PAH fabricated by the
layer-by-layer (LbL) technique could load and release doxorubicin [63] or insulin
[64] under temperature control. No such behavior was shown to our knowledge for
PEC particle systems of equivalent oppositely charged PNIPAAM-derived PEL
components.
Amphoteric Terpolymers of Oppositely Charged and Neutral Blocks
Recently, water-soluble micellar PEC particles formed by the self-complexation of
polyampholytic amphiphilic polybutadiene-block-quaternized poly(2-vinylpyridine)block-poly(methacrylic acid) (PB-b-P2VPQ-b-PMAA) triblock terpolymers were
214
M. M€ uller
anionic homopolymer. These PEC particles were claimed to feature a neutral core
of complexed PELs decorated by a PEO shell, which can be denoted as block
copolymer micelles.
N-Isopropylacrylamide Comonomers
It is well known that poly(N-isopropylacrylamide) (PNIPAAM) systems undergo
thermotropic phase transitions, resulting in polymer segment density increases at
temperatures higher than the lower critical solution temperature (LCST) [60]. This
phase transition is associated with a loss of external hydrogen bonds of the amide
units to water and a gain of internal hydrogen bonds between amide units expelling
water from the aggregated phase. Copolymers of NIPAAM with charged monomer
units can result in a loss of this thermotropic behavior. Dautzenberg and coworkers
checked whether the complexation of a cationic copolymer of NIPAAM [poly
(methacryloyl-oxyethyldimethyl benzylammonium chloride)] with an anionic
copolymer of NIPAAM [poly(2-acrylamido-2-methylpropanesulfonate)] can
regain the known thermotropic properties of the homo PNIPAAM [61]. Indeed,
they could show significant decreases of the particle size of a PEC-0.6 at a NaCl
concentration of 0.01 M from R H ¼ 200–240 nm at temperatures below LCST to
around R H ¼ 100–140 nm at temperatures above the LCST. Furthermore, this
process was reversible and accompanied by a respective increase and decrease in
structural density of the PEC system. Such a thermoswitchable change in structural
density may have interesting consequences for applications like the controlled
uptake and release of drugs or enzymes.
Kleinen and Richtering [62] complexed microgels of weakly crosslinked
P(NIPAAM-co-methacrylic acid) P(NIPAAM-co-MAA) with PDADMAC and the
resulting submicron particles showed thermosensitive behavior (see Fig. 13). A significant drop in the hydrodynamic radius of PEC from around R H ¼ 300–350 nm
(X ¼ 1:0.2–1:1.25) at 20
C to around R H ¼ 120–170 nm at 45
C took place. The
lower the PDADMAC content in the PEC, the higher was the particle size in the
swollen state and the higher was the size difference between swollen and compact state.
Nolan, Serpe, and coworkers showed that thermoresponsive PEL complex films
containing poly(N-isopropylacrylamide-co-acrylic acid) and PAH fabricated by the
layer-by-layer (LbL) technique could load and release doxorubicin [63] or insulin
[64] under temperature control. No such behavior was shown to our knowledge for
PEC particle systems of equivalent oppositely charged PNIPAAM-derived PEL
components.
Amphoteric Terpolymers of Oppositely Charged and Neutral Blocks
Recently, water-soluble micellar PEC particles formed by the self-complexation of
polyampholytic amphiphilic polybutadiene-block-quaternized poly(2-vinylpyridine)block-poly(methacrylic acid) (PB-b-P2VPQ-b-PMAA) triblock terpolymers were
214
M. M€ uller