2 research outputs found
Formation of Well-Defined Vesicles by Styrene Addition to a Nonionic Surfactant and Their Polymerization Leading to Viscous Hybrid Systems
Self-assembled
structures in aqueous solutions can be fixed by polymerization after
adding hydrophobic monomers and can thereby be used as templates which
allow to substantially alter the properties of these systems. In this
work, we started from a self-assembled micellar system consisting
of the nonionic surfactants tetradecyldimethylamine oxid and Pluronic
L35 to which styrene was added as a polymerizable monomer. Interestingly,
it was observed that styrene induces a transition from micelles to
well-defined vesicles in a similar manner as a typical cosurfactant.
The structural transition of the aggregates upon styrene addition
as well as the structures formed after initiating a polymerization
reaction were investigated by means of turbidity, dynamic and static
light scattering, small-angle neutron scattering, and rheology measurements.
Especially the scattering results confirmed the interesting effect
of styrene on the mesoscopic structure and showed a structural evolution
from rod-like micelles for low styrene concentrations to vesicles
at intermediate styrene amounts, and then finally the formation of
microemulsion droplets for high styrene content. Their polymerization
of the vesicles again leads to a shape change to wormlike, polymerized
aggregates, whose presence then results in rather viscous systems.
In contrast, the microemulsions with higher styrene content then are
templated and retain their size after polymerization, thereby leading
to nanolattices
Monitoring the Internal Structure of Poly(<i>N</i>‑vinylcaprolactam) Microgels with Variable Cross-Link Concentration
The combination of a set of complementary
techniques allows us
to construct an unprecedented and comprehensive picture of the internal
structure, temperature dependent swelling behavior, and the dependence
of these properties on the cross-linker concentration of microgel
particles based on <i>N</i>-vinylcaprolactam (VCL). The
microgels were synthesized by precipitation polymerization using different
amounts of cross-linking agent. Characterization was performed by
small-angle neutron scattering (SANS) using two complementary neutron
instruments to cover a uniquely broad Q-range with one probe. Additionally
we used dynamic light scattering (DLS), atomic force microscopy (AFM),
and differential scanning calorimetry (DSC). Previously obtained nuclear
magnetic resonance spectroscopy (NMR) results on the same PVCL particles
are utilized to round the picture off. Our study shows that both the
particle radius and the cross-link density and therefore also the
stiffness of the microgels rises with increasing cross-linker content.
Hence, more cross-linker reduces the swelling capability distinctly.
These findings are supported by SANS and AFM measurements. Independent
DLS experiments also found the increase in particle size but suggest
an unchanged cross-link density. The reason for the apparent contradiction
is the indirect extraction of the parameters via a model in the evaluation
of DLS measurements. The more direct approach in AFM by evaluating
the cross section profiles of observed microgel particles gives evidence
of significantly softer and more deformable particles at lower cross-linker
concentrations and therefore verifies the change in cross-link density.
DSC data indicate a minor but unexpected shift of the volume phase
transition temperature (VPTT) to higher temperatures and exposes a
more heterogeneous internal structure of the microgels with increasing
cross-link density. Moreover, a change in the total energy transfer
during the VPT gives evidence that the strength of hydrogen bonds
is significantly affected by the cross-link density. A strong and
reproducible deviation of the material density of the cross-linked
microgel polymer chains toward a higher value compared to the respective
linear chains has yet to be explained