BULK AND SOLUTION PROPERTIES OF PEPTIDE-POLYMER CONJUGATES - Biologically-Responsive Hybrid Biomaterials - page 26

increase in hydrodynamic radius (determined by DLS) from 47.1 nm for the
crosslinked micelle to 55.1 nm for the DTT-reduced micelles.
In contrast to the PB- b -PLGA system in which morphological changes (from
micelles to vesicles) were induced by variation of the hydrophobic content or pH,
no such morphological changes were observed with the poly( L -lactide)- b -
poly(aspartic acid) system despite similar variation [93]. Throughout the
considered range of hydrophobic content (21 to 74 mol % aspartic acid) and pH
(4.4 to 8.6), scanning force microscopy and light scattering data continued to
reveal the presence of solely spherical aggregates with hydrodynamic radius
varying from 10 to 80 nm. Two possible explanations for this behavior have
been postulated. On one hand, a metastable state adopted by the semi-crystalline
PLLac core material could be locking each vesicle into a spherical conformation
[94]. On the other hand, the system may have attained a state of equilibrium, in
which an interplay of core/corona forces - hydrogen bonding of the PAsp
corona, hydrophobic forces in the PLLac core, and electrostatic repulsion
between the two - could produce similar aggregation behaviour [93].
Another block copolymer, polystyrene- b -poly( L -lysine) (PS 8, 10 - b -PLLys 9-72 ),
described by Klok et al . [95] exhibited original behavior at neutral pH and in
aqueous dilute solution. Despite variation of the PLLys block length, solely
cylindrical micelles were observed by dynamic/static light scattering (DLS/SLS),
small angle neutron scattering (SANS), and analytical ultracentrifugation (AUC).
Micelles were formed by the block copolymer polystyrene 388 - b -poly( L -
lysine) 138 . Circular dichroism (CD) spectroscopy reveals a conformational change
of the poly( L -lysine) portion from random coil to Ȳ -sheet as pH increases beyond
11. Data obtained by atomic force microscopy (AFM) corroborates this pH-
dependent conformational transition: material deposited by solvent evaporation
on graphite at pH 8 shows less wetting ability than the same material deposited
at pH 11 [96].
Poly(butadiene) m - b -poly( L -lysine) n (m-n = 107-200, 107-100, and 60-50)
was reported by Savin et al. [97] to form spherical or rod-like micelles
depending on the block composition at high pH. Specifically, TEM and DLS
data supported the appearance of more elongated rod-like shape in addition to
spherical micelles at pH 9 than at pH 2 in the PB 107 -PLLys 100 and PB 60 -PLLys 50
systems. Further, micelles shrank with decreasing pH from 82 ± 2 nm at pH 2
to 62 ± 2 nm at pH 9.2, as determined by DLS.
Recently, formation of micellar aggregates was reported for a material with
polytetrahydrofuran (PTHF) as a central hydrophobic block flanked by two
segments of poly( L -lysine) as outer hydrophilic blocks to form the ABA triblock
copolymer [poly( L -lysine)- b -polytetrahydrofuran- b -poly( L -lysine) or (PLLys 30 - b -