Solubility behavior of amphiphilic block and random copolymers ...

516 LAMBERMONT-THIJS ET AL.
INTRODUCTION
The solution properties of amphiphilic copolymers,
such as solubility and aggregation, are of
major importance for their use in, for example,
personal care, medical, or pharmaceutical applications.
In addition, such applications require biocompatible
polymers and nontoxic solvents like
water–ethanol mixtures. Surprisingly, little is
known about solution properties of amphiphilic
copolymers in water–ethanol mixtures despite
that it is well-known that such mixtures exhibit
interesting abnormal properties due to the presence
of hydration shells around the ethanol
molecules. 1–3 Thepresenceofsuchshellsresultin
solubility maxima for drug molecules in water–
ethanol mixtures. 4–6 To gain more detailed knowledge
and understanding on the effects of binary
solvent mixtures on solution properties of polymers,
systematic investigations are required in
which the composition of both the polymer and
the solvent mixture should be varied. Poly(2-oxazolines)
facilitate the research on solution properties
because of the ease and versatility to vary the
side groups, and thus, the polymer properties. By
simply changing the length of the alkyl side
group, the nature of the polymers can be varied
from hydrophilic with methyl and ethyl substituents
to hydrophobic polymers with longer alkyl or
aromatic side groups. 7,8 In literature, a large
number of studies discuss structure-property relationships
for poly(2-oxazolines), in some cases also
using systematic variations in monomer composition.
9,10 Such copolymer series mainly focused up
to now on the combination of hydrophilic and
hydrophobic monomers because amphiphilic
copolymers exhibit interesting thermal, surface,
and solution properties. For example, the lower
critical solution temperature (LCST) in water,
which is based on the hydrophilic–hydrophobic
balance of various copolymers, has been studied
in detail. 11,12 It is known that the LCST can be
controlled by incorporating specific compositions
of hydrophilic and hydrophobic 2-oxazoline monomer
units within the main chain, 13–16 as in the
case of other thermosensitive polymers. 17 In addition,
such amphiphilic poly(2-oxazoline)s have
gained interest for use in, for example, aqueous
self-assembly, micellar catalysis, drug delivery,
and hydrogels. 18–30
In a previous solubility screening, a series of
gradient 2-methyl-2-oxazoline or 2-ethyl-2-oxazoline
in combination with 2-phenyl-2-oxazoline
copolymers was investigated revealing that the
solution properties of these amphiphilic gradient
copolymers could be tuned in a wide range by
only changing the composition of the water–ethanol
mixtures. 31 Here,wereportsystematicinvestigations
on the solution properties of 2-ethyl-2-
oxazoline (EtOx) and 2-nonyl-2-oxazoline (NonOx)
containing random and block copolymers. The solution
properties of these copolymers were investigated
in water–ethanol mixtures ranging from
pure water to pure ethanol (steps of 20 wt %).
This systematic screening for random and block
copolymers with similar compositions allows a
detailed investigation of the effect of solvent composition
as well as the polymer structure, which
has not been reported before. Different solution
properties were investigated such as LCST, selfassembly,
and the formation of dispersions. Furthermore,
the self-assembled structures were
analyzed by DLS to correlate the hydrodynamic
radius of the formed aggregate with the effect of
the binary water ethanol solvent mixture.
EXPERIMENTAL
The synthesis and characterization of the EtOx-
NonOx random and block copolymers was already
reported elsewhere. 32
Instrumentation
The solubility screening was performed by heating
the polymer (5.0 0.2 mg) in a solvent mixture
of ethanol (Biosolve) and deionized water (1.0
mL). The investigated temperature range was
20–75 C with heating and cooling steps of 1 C
min 1 . During these controlled heating and cooling
cycles (two cycles per sample), the transmission
through the solutions was monitored in a
Crystal16 from Avantium Technologies. 31,33 All
vials were visually inspected after the heating
program to facilitate the interpretation of the
observed transmission profiles. A more detailed
solubility screening was performed for selected
samples in a broader temperature range from
20 Cto100 C. The presented upper critical solution
temperature (UCST) temperatures correspond
to the dissolution temperatures at 50%
transmittance from the second heating run.
Detailed LCST measurements were performed in
a wider temperature range from 25 Cto105 C
with heating and cooling steps of 1 Cmin 1 .All
presented LCST temperatures represent the dissolution
temperatures at 50% transmittance in
Journal of Polymer Science: Part A: Polymer Chemistry
DOI 10.1002/pola