4 research outputs found
Branched Wormlike Micelles Formed by Self-Assembled Comblike Amphiphilic Copolyelectrolytes
The structure of the self-assemblies
formed by amphiphilic comblike
copolyelectrolytes dispersed in water has been investigated by scattering
techniques (light and neutron) and by transmission electronic microscopy.
The comblike polymers consisted of a polystyrene backbone grafted
with a fixed amount of pendant <i>N</i>,<i>N</i>-dimethyl quaternary ammonium alkyl groups of various lengths ranging
from C12 up to C18. In aqueous solution, the polymers self-assembled
into small spherical aggregates at low concentrations and into cylindrical
aggregates above a critical concentration with a diameter that increased
with the length of the alkyl side chains. The length of the cylindrical
aggregates increased with increasing concentration, and branching
occurred at higher concentration, which induced gelation above a critical
percolation concentration. Growth and branching were favored by increasing
the ionic strength of the solution. The dynamics slowed down with
decreasing temperature and increasing alkyl length, and the assemblies
of polymers with C16 and C18 pendant chains were kinetically frozen
at 20 °C
Dynamic Mechanical Properties of Networks of Wormlike Micelles Formed by Self-Assembled Comblike Amphiphilic Copolyelectrolytes
The
rheological properties of viscoelastic aqueous solutions of
wormlike micelles formed by the self-assembly of comblike copolyelectrolytes
have been investigated by flow and dynamic measurements. The comblike
polymers consisted of a polystyrene backbone grafted with a fixed
amount of pendant <i>N</i>,<i>N</i>-dimethyl quaternary
ammonium alkyl groups of various lengths ranging from C12 up to C18.
Upon increasing concentration, the increase in size of the wormlike
micelles and their branching results in the formation of a system
spanning network through a percolation process at a critical concentration
that decreases when salt is added or when the temperature is decreased.
In this manner transient gels are formed with a viscoelastic relaxation
time that does not depend on the polymer concentration or on the ionic
strength, but their elastic modulus increases with increasing polymer
or salt concentration. When the size of the alkyl groups is increased
from C12 to C16, the relaxation time increases very strongly, but
the temperature dependence remains characterized by the same activation
energy. For C18, the systems are frozen at least up to 80 °C
Structure and Dynamics of Dendronized Polymer Solutions: Gaussian Coil or Macromolecular Rod?
We
investigate the conformation of well-defined dendronized polymers
(denpols) based on poly(norborene) (PNB) and poly(<i>endo</i>-tricycle[4.2.2.0]deca-3,9-diene) (PTD) backbones employing static
and dynamic light scattering. Their synthesis by ring-opening metathesis
polymerization (ROMP) led to fully grafted and high molecular weight
denpols with narrow polydispersity. In dilute solutions, the persistence
lengths were estimated by static (radius of gyration) and dynamic
(translational diffusion) chain conformational properties of the denpols
and were compared to their homologue precursor PNB. The conformation
of denpols with a third generation side dendron conforms to a semiflexible
chain with a persistence length of about 6–8 nm, virtually
independent of the contour length. In the semidilute regime, the thermodynamics
and cooperative diffusion of denpols resemble the behavior of the
precursor solutions as described by the scaling theory of flexible
polymers above the crossover concentration. The assumption of extremely
high chain rigidity for this class of polymers is clearly not supported,
at least for the third generation dendron
Smoothing and Cicatrization of Isotactic Polypropylene/Fe<sub>3</sub>O<sub>4</sub> Nanocomposites via Magnetic Hyperthermia
We prepare industrially relevant magnetoresponsive thermoplastic
nanocomposites capable of being cicatrized and smoothed after additive
manufacturing through the application of an oscillatory magnetic field
(OMF). The materials are made of an isotactic polypropylene (iPP)
matrix filled with magnetite nanoparticles (NPs; 2–22 wt %,
75 nm in diameter) synthesized from steel waste, providing them with
a limited ecological impact on top of their ability to be repaired.
NPs are found to have no significant impact on the thermal properties
of iPP, which allows one to compare directly the magnetothermal effects
measured on the different nanocomposites. Beyond the primary temperature
increase generated by magnetic hysteresis loss, we show that the OMF
irradiation triggers a second heating mechanism from the iPP melting.
This phenomenon, which was assigned to NP magnetization and subsequent
rotation causing high-frequency mechanical friction, is investigated
here in a systematic way. Our results indicate that while the specific
power generated by NP friction is (expectedly) proportional to the
irradiation time, it is independent of the NP content as long as the
temperature is well above the polymer melting point. These observations
therefore suggest that the (local) filler–polymer interfacial
rheology dictates the amount of heat generated through friction. From
an application point of view, 7 wt % of the NPs is found to be enough
to induce iPP melting from the magnetothermal effect, which enables
the postprocessing of a hot-pressed and 3D-printed specimen through
“cicatrization” and “smoothing” experiments.
In the former case, rewelding a sample cut into two pieces is found
to provide a Young modulus and a yield point similar to those in native
hot-pressed samples (exhibiting, however, a lower strain at failure).
In the latter case and beyond the improved specimen appearance, smoothing
is found to double both the stress and strain at failure of large
3D-printed samples that present, nevertheless, significantly lower
properties than those of their hot-pressed counterparts