finding, showing that the smallest and most stable PEC particles were found for the
PAMAC with charge density closest to that of PDADMAC, while the PAMAC with
low charge density had an opposite effect.
However, seemingly in contrast to the latter findings are recent studies of
Claesson and coworkers [54] based on a cationic copolymer of methacryloxyethyl
trimethylammonium (METAC) with varying portions of the nonionic poly(ethylene oxide) ether methacrylate (PEO45MEMA) and anionic PSS. They showed that
with decreasing charge density it was possible to obtain soluble PEC particles with
decreasing hydrodynamic radii to around 20 nm and increasing stability, even for
1:1 stoichiometry. This finding was explained by steric stabilization via the hydrophilic PEO-decorated shell, which prevented secondary aggregation.
3.1.2 Molecular Weight
In a classical paper by Dautzenberg [26] on the standard PDADMAC(250 kg/mol)/
PSS system, no systematic change in the size and structural density with change in
M W (8–1,000 kg/mol) of PSS was found. This finding was interpreted as a large
kinetic effect in the complexation process that suppressed the influence of molecular weight, but conflicts with the findings of several other authors.
The size of spherical PLL/DNA complexes deposited and dried on mica substrates
was studied by Wolfert and Seymour [39] by SFM using PLL M w ranging from 4 to
225 kg/mol. From the height and radius of the flattened particles, the volumes were
determined and showed an increase with increasing PLL M w . Figure 9 shows a plot of
the PLL/DNA particle dimension versus PLL M w . SFM images for DNA complexed
with PLL of M w 4,000 or 225,000 g/mol are given in the insets of Fig. 9.
Recently, Hu and coworkers [55] reported on the influence of CHT M W on the
particle size of carboxymethylpachyman (CMP)/CHT nanoparticles. Increasing
CHT M W from 12,000 to 46,000 g/mol resulted in an increase of PEC particle
size from 135 to 279 nm. As an explanation, they emphasized that longer chains of
positively charged CHT molecules can complex with a larger number of negatively
charged CMP molecules.
Figure 10 shows our own results on the influence of molecular weight of PEI and
PAC on the size of PEC-0.6 and PEC-1.5 particles mixed at pH ¼ 7/7 (i.e., PEC
solution at pH 7 and PAC solution at pH 7) [48]. In the case of PEC-0.6 particles,
only the M w of PEI was varied and for PEC-1.5 particles only the M w of PAC was
varied, because we assume that the excess PEL dominates the shell region and
therefore is more effective for particle size changes. Only for the pH combination of
pH ¼ 7/7 and only in the case of PEI was there a significant particle size enlargement of D H ¼ 120 nm to D H ¼ 380 nm upon increasing the M W of PEI from
1,300 to 750,000 g/mol (Fig. 10) for PEC-0.6 particles. For pH ¼ 10/4 PEC-0.6
particles showed no significant dependence on the M w of PEI, for which we have no
straightforward explanation. Presumably, in the more compact state of PEI at
pH ¼ 10/4 the formed PEI/PAC particles are not so sensitive to M w variation.
Furthermore, neither for pH ¼ 10/4 nor for pH ¼ 7/7 was there a significant
enlargement of particle size with increasing M w of PAC. Even for pH ¼ 10/4
Sizing, Shaping and Pharmaceutical Applications of Polyelectrolyte Complex. . .
209
PAMAC with charge density closest to that of PDADMAC, while the PAMAC with
low charge density had an opposite effect.
However, seemingly in contrast to the latter findings are recent studies of
Claesson and coworkers [54] based on a cationic copolymer of methacryloxyethyl
trimethylammonium (METAC) with varying portions of the nonionic poly(ethylene oxide) ether methacrylate (PEO45MEMA) and anionic PSS. They showed that
with decreasing charge density it was possible to obtain soluble PEC particles with
decreasing hydrodynamic radii to around 20 nm and increasing stability, even for
1:1 stoichiometry. This finding was explained by steric stabilization via the hydrophilic PEO-decorated shell, which prevented secondary aggregation.
3.1.2 Molecular Weight
In a classical paper by Dautzenberg [26] on the standard PDADMAC(250 kg/mol)/
PSS system, no systematic change in the size and structural density with change in
M W (8–1,000 kg/mol) of PSS was found. This finding was interpreted as a large
kinetic effect in the complexation process that suppressed the influence of molecular weight, but conflicts with the findings of several other authors.
The size of spherical PLL/DNA complexes deposited and dried on mica substrates
was studied by Wolfert and Seymour [39] by SFM using PLL M w ranging from 4 to
225 kg/mol. From the height and radius of the flattened particles, the volumes were
determined and showed an increase with increasing PLL M w . Figure 9 shows a plot of
the PLL/DNA particle dimension versus PLL M w . SFM images for DNA complexed
with PLL of M w 4,000 or 225,000 g/mol are given in the insets of Fig. 9.
Recently, Hu and coworkers [55] reported on the influence of CHT M W on the
particle size of carboxymethylpachyman (CMP)/CHT nanoparticles. Increasing
CHT M W from 12,000 to 46,000 g/mol resulted in an increase of PEC particle
size from 135 to 279 nm. As an explanation, they emphasized that longer chains of
positively charged CHT molecules can complex with a larger number of negatively
charged CMP molecules.
Figure 10 shows our own results on the influence of molecular weight of PEI and
PAC on the size of PEC-0.6 and PEC-1.5 particles mixed at pH ¼ 7/7 (i.e., PEC
solution at pH 7 and PAC solution at pH 7) [48]. In the case of PEC-0.6 particles,
only the M w of PEI was varied and for PEC-1.5 particles only the M w of PAC was
varied, because we assume that the excess PEL dominates the shell region and
therefore is more effective for particle size changes. Only for the pH combination of
pH ¼ 7/7 and only in the case of PEI was there a significant particle size enlargement of D H ¼ 120 nm to D H ¼ 380 nm upon increasing the M W of PEI from
1,300 to 750,000 g/mol (Fig. 10) for PEC-0.6 particles. For pH ¼ 10/4 PEC-0.6
particles showed no significant dependence on the M w of PEI, for which we have no
straightforward explanation. Presumably, in the more compact state of PEI at
pH ¼ 10/4 the formed PEI/PAC particles are not so sensitive to M w variation.
Furthermore, neither for pH ¼ 10/4 nor for pH ¼ 7/7 was there a significant
enlargement of particle size with increasing M w of PAC. Even for pH ¼ 10/4
Sizing, Shaping and Pharmaceutical Applications of Polyelectrolyte Complex. . .
209