water. This structured water leads to entropically driven polymer–polymer
interactions via the hydrophobic effect [31]. Under the conditions where
pNIPAm has a random coil structure, the solvent–polymer interactions are
stronger than the polymer–polymer interactions. At higher temperatures, the
hydrogen bonds with the water molecules break and there is an entropically
favored release of bound and structured water, leading to the formation of a
globular polymer conformation. In this case, the polymer–polymer hydro-
phobic interactions become stronger than the polymer–solvent interactions,
and the polymer phase separates. The temperature at which this phase
separation occu rs is called the lower critical solution temperature (LCST). It is
this behavior that makes pNIPAm a very attractive candidate for the
fabrication of stimuli-responsive hydrogels. It is worthwhile noting, however,
that one must consider more than simply hydrophilic and hydrophobic side
chain contributions to polymer solvation when describing LCST behavior. For
example, the polymer formed from N-isopropylmethacrylamide (NIPMAm)
[33–38], which differs from NIPAm by only a single methyl group, has a higher
LCST in water, which suggests that it is more hydrophilic despite a greater
organic content. Apparently, this ‘‘increased hydrophilicity’’ does not arise
from an increase in polymer polarity, but instead comes from a decrease in
chain flexibility. This changes the entropic contribution to the free energy of
mixing, and thus increases the LCST.
4.1.4 Microgels and Nanogels
Colloidally stable particles made from hydrogels, also referred to as micro- or
nanogels, have similar properties as their macrogel counterparts; that is, a
pNIPAm microgel, like the bulk gel, will also undergo a volume phase
transition temperature (VPTT) near the LCST of the parent polymer [13, 39].
In addition to these properties, microgels have other characteristics of colloidal
dispersions such as zeta potentials [13, 40, 41] and can also form ordered
phases when prepared as a highly monodispersed sol [42–45].
Some very important studies have focused on the differences between
macro- and microgels with respect to their phase behavior [13, 39, 43, 46–58].
These are too numerous to describe in detail here, so we offer a single example
of the complexity of these materials. Wu et al. have shown that the VPTT of
the microgels is slightly higher than the LCST of pNIPAm (Fig. 4.2), and also
that the transition region is less sharp than that of bulk gels [46]. The reason for
this continuous transition is due to a greater heterogeneity in the subchain
lengths of the microgels as compared to traditionally prepared macrogels.
When the microgels are subjected to T > VPTT, the regions of the particle with
longer subchain lengths collapse at a lower temperature than the regions with
shorter subchains. Thus, one can think of the observed phase transition for a
microgel as being the summation of the phase transitions of the different
subnetworks in the particle. We have also observed this behavior in core/shell
64 BIOMEDICAL NANOSTRUCTURES