Solubility behavior of amphiphilic block and random copolymers ...

518 LAMBERMONT-THIJS ET AL.
DLS measurements confirmed the presence of isolated
pNonOx chains in solution rather than
aggregated ones (a single population with a
hydrodynamic radius of 3 nm was observed which
hasbeenattributedtosinglechains).Thispeculiar
solubility behavior of pNonOx in ethanol can
be rationalized as follows. To dissolve the polymer,
the solvent needs to penetrate into the pNonOx
crystals which can be facilitated by both the
increased Brownian motion of ethanol as well as
the partial melting of the crystals at higher temperatures.
Once the first solvation shells are
formed, the polymer dissolution would be accelerated
and, after full solvation, the polymer would
remain in solution. This observed effect is similar
to the formation of hydration shells in the dissolution
process of poly(ethylene glycol) as described
in literature. 37 All block copolymers containing 40
wt % or more NonOx required elevated temperatures
to become soluble in pure ethanol and
stayed in solution during the second heating and
cooling cycle.
The solubility phase diagram for the random
copolymers (Fig. 1, bottom) shows general trends
similar to the ones observed for the block copolymers,
that is, decreasing solubility with increasing
pNonOx content, increasing solubility with
increasing ethanol fraction, and dissolution of the
pNonOx copolymers in ethanol with elevated temperatures.
Nonetheless, some clear differences
between the random and block copolymers were
also found. In comparison to the block copolymers,
the random copolymers with a high pNonOx content
revealed less interactions with the water rich
solutions. Most of these samples gave clear solutions
with solid particles indicating that no part
of the copolymer was hydrated or dissolved. Moreover,
the solubility phase diagram shows that the
random copolymers displayed no translucent solutions
indicating that the random copolymers do
not form micellar aggregates. In contrast, two of
the random copolymers containing 60 and 70 mol
% NonOx reversibly dissolved in 80 wt % EtOH
upon heating, indicating an UCST. The dissolution
temperature found for the copolymer containing
60 mol % pNonOx is 26 C, whereas the polymer
containing 70 mol % pNonOx revealed a dissolution
temperature of 67 C due to the higher
amount of hydrophobic monomer indicating that
solvation of the pNonOx chains only takes place
at elevated temperatures. This difference between
the block and random copolymers can be understood
by the close proximity of the NonOx units in
the block copolymer resulting in an insoluble
block and, thus, micellization. In the random copolymer,
the insoluble NonOx units are uniformly
distributed over the chain resulting in a lower
total solubility. The random copolymer containing
10 mol % pNonOx exhibited a LCST in water and
in the binary solvent mixture containing 20 wt %
EtOH, whereas the block copolymer only revealed
a LCST transmission in water (LCST of 69.8 C).
A more detailed investigation of the LCST transition
focusing on the nature of the copolymer as
well as the solvent composition was performed
and will be discussed in detail below.
Solubilization Temperatures of Random
and Block Copolymers in Ethanol
The random copolymers containing 60 mol % or
more NonOx and the block copolymers with 40 wt
% NonOx or more dissolved in pure ethanol during
the first heating step. During subsequent cooling
and heating steps the copolymers remained in
solution. The solubilization temperatures for both
the random and block copolymers were extracted
from the transmission measurements (the dissolution
temperature was taken at 50% transmittance)
and are plotted in Figure 2. The random
copolymers reveal a lower crystallinity when
compared with the block copolymers as determined
by the thermal measurements. 32 The
decreased crystallinity in the random copolymers
can be rationalized by the presence of EtOx units
that disturb the packing in the pNonOx crystallites.
Therefore, ethanol can easily penetrate into
the crystals of the random copolymers and the
copolymers are dissolved at lower temperatures in
comparison to the block copolymers. This proposed
correlation between the crystallinity and dissolution
temperature is further evidenced by the close
resemblance of the dissolution temperature
against the composition plots and the melting temperature
against the composition plots. 32
Detailed LCST Investigations of Random
and Block Copolymers
A more detailed investigation of the LCST temperature
was performed for the random and block
copolymers containing 10 mol % NonOx. A series
of solvent mixtures ranging from 0 to 24% EtOH
(in steps of 3%) was prepared and the cloud points
(indicative for the LCST temperature) were determined
from the transmission plots at 50% transmittance
in the second heating run. The obtained
cloud point temperatures are plotted in Figure 3
for both the random and block copolymers,
Journal of Polymer Science: Part A: Polymer Chemistry
DOI 10.1002/pola