X < 0.5 up to around 0.5 g/cm
3 at X ¼ 1 was found, which was interpreted to
mean that the formation of new particles rather than further growth dominates the
density/mixing ratio profile. Furthermore, PDADMAC concentration had no significant influence on r, which stayed constant for c PDADMAC of 10
À5 to 10
À3 M,
and neither did ionic strength show any influence on r. The results implied that
individual PEC particles may aggregate, but in principle their internal structure
remains unaltered. However, based on the P(DADMAC-co-acrylamide)/PSS system [36] with decreasing DADMAC content, and thus charge density, PEC
particles were found to decrease in r due to increasing charge mismatch. Therefore, charge density was claimed to be an important parameter for the swelling
degree of PEC particles. Of course, in PEC systems of oppositely charged
copolymers of PNIPAAM dramatic temperature-dependent increases from r % 0.1
g/cm
3 at 25
C to r % 0.7 g/cm
3 at 50
C can be found. Therefore, such thermotropic PEC systems are most interesting for applications such as triggerable drug
delivery.
Related to this, Wagberg and colleagues [78] studied subtle molecular effects on
the internal structure of PECs. These authors found that replacing PAC by poly
(methacrylic acid) (PMAA) in PECs containing PAH revealed a significant increase
in the water content and particle size.
3.3.2 Internal Structure of PEC Aggregates
In the previous section, considerations on the internal structure of PEC systems
were raised on the singular particle level. This section discusses aggregated
systems. Secondary PEC aggregation, where primary PEC particles aggregate to
larger clusters [16, 22] in some analogy to Ostwald ripening, is of relevance. Such
aggregate particles may adopt “raspberry” structures, where large spherical
particles form the envelope of many smaller particles. However, only few experimental proofs (such as microscopy) for such structures have been reported up to
now. This could be explained by the soft and water-rich nature of such raspberrylike PEC particles, where the soft primary particles might show no sufficient
distinction due to fusing (PEC dispersions might be better described as emulsions).
Nevertheless, raspberry structures were obtained in some selected examples, where
hydrophobic PELs were used for complexation (unpublished results).
Recent contributions on the internal structure of macroscopic PEC systems
circle around the term “saloplasticity” raised by Schlenoff and coworkers
[79–81]. Saloplasticity denotes and makes use of the phenomenon that upon
addition of salt the PEC material becomes more fluid-like and hence formable.
This property is seen as analogous to thermoplasticity, which denotes changes in
polymer material properties with increasing temperature. Earlier results on PECrelated PEL multilayer (PEM) saloplasticity (denoted therein as “salt softening”)
were reported by Fery and coworkers [82], who studied the effect of salt
concentration on the mechanical elasticity and compressibility of hollow PEM
capsules.
224
M. M€ uller
3 at X ¼ 1 was found, which was interpreted to
mean that the formation of new particles rather than further growth dominates the
density/mixing ratio profile. Furthermore, PDADMAC concentration had no significant influence on r, which stayed constant for c PDADMAC of 10
À5 to 10
À3 M,
and neither did ionic strength show any influence on r. The results implied that
individual PEC particles may aggregate, but in principle their internal structure
remains unaltered. However, based on the P(DADMAC-co-acrylamide)/PSS system [36] with decreasing DADMAC content, and thus charge density, PEC
particles were found to decrease in r due to increasing charge mismatch. Therefore, charge density was claimed to be an important parameter for the swelling
degree of PEC particles. Of course, in PEC systems of oppositely charged
copolymers of PNIPAAM dramatic temperature-dependent increases from r % 0.1
g/cm
3 at 25
C to r % 0.7 g/cm
3 at 50
C can be found. Therefore, such thermotropic PEC systems are most interesting for applications such as triggerable drug
delivery.
Related to this, Wagberg and colleagues [78] studied subtle molecular effects on
the internal structure of PECs. These authors found that replacing PAC by poly
(methacrylic acid) (PMAA) in PECs containing PAH revealed a significant increase
in the water content and particle size.
3.3.2 Internal Structure of PEC Aggregates
In the previous section, considerations on the internal structure of PEC systems
were raised on the singular particle level. This section discusses aggregated
systems. Secondary PEC aggregation, where primary PEC particles aggregate to
larger clusters [16, 22] in some analogy to Ostwald ripening, is of relevance. Such
aggregate particles may adopt “raspberry” structures, where large spherical
particles form the envelope of many smaller particles. However, only few experimental proofs (such as microscopy) for such structures have been reported up to
now. This could be explained by the soft and water-rich nature of such raspberrylike PEC particles, where the soft primary particles might show no sufficient
distinction due to fusing (PEC dispersions might be better described as emulsions).
Nevertheless, raspberry structures were obtained in some selected examples, where
hydrophobic PELs were used for complexation (unpublished results).
Recent contributions on the internal structure of macroscopic PEC systems
circle around the term “saloplasticity” raised by Schlenoff and coworkers
[79–81]. Saloplasticity denotes and makes use of the phenomenon that upon
addition of salt the PEC material becomes more fluid-like and hence formable.
This property is seen as analogous to thermoplasticity, which denotes changes in
polymer material properties with increasing temperature. Earlier results on PECrelated PEL multilayer (PEM) saloplasticity (denoted therein as “salt softening”)
were reported by Fery and coworkers [82], who studied the effect of salt
concentration on the mechanical elasticity and compressibility of hollow PEM
capsules.
224
M. M€ uller