4 research outputs found
Dangling-End Double Networks: Tapping Hidden Toughness in Highly Swollen Thermoplastic Elastomer Hydrogels
This report introduces a unique method
of significantly improving
toughness in highly swollen block copolymer-based thermoplastic elastomer
(TPE) hydrogels by converting an intrinsically large population of
dangling chain ends into a mechanically active second network. In
one form, the TPE hydrogels developed by our group are based on swelling
of a vitrified melt-blend of two amphiphilic block copolymer species,
sphere-forming polystyrene-polyÂ(ethylene oxide) (SO-H) diblock and
triblock (SOS) copolymers. Here, the PEO midblock in the SOS triblock
copolymer serves to tether adjacent PS spherical aggregates, producing
hydrogel networks that are incredibly elastic and mechanically robust,
preserving their shape even at the very high intrinsic swelling ratios
produced at low SOS concentrations (e.g, 37 g of H<sub>2</sub>O/(g
of polymer) at 3.3 mol % SOS). In this report, we advance the utility
of this framework by exploiting the hundreds of dangling PEO chain
ends per spherical aggregate to form a second, mechanically active
network. The approach is based on a stepwise installation of two tethering
SOS triblock copolymer populations. The first is present directly
during <i>melt-state</i> self-assembly of the original diblock/triblock
copolymer blend and inherently determines the equilibrium swelling
ratio of resulting hydrogel. The second population is then introduced <i>postswelling</i>, by simply coupling the dangling SO diblock
copolymer chain ends under conditions largely free of the mechanical
stress osmotically imposed on the primary network. Notably, this action
simply shifts the ratio of diblock and triblock copolymer without
compromising the thermoplasticity of the network. Here, we use the
facile water-based coupling of PEO-terminal azide and alkyne groups
to demonstrate the scale of toughness enhancements possible through
conversion of dangling ends into a second network. The dangling-end
double networks produced exhibit remarkable improvements in tensile
properties (tensile modulus, toughness, strain at break, and stress
at break), including a 58-fold increase in mean toughness (to 361
kJ/m<sup>3</sup>) and a 19-fold increase in mean stress to break (to
169 kPa) in highly swollen samples containing up to 95% (g/g) water.
Importantly, these improvements could be realized without altering
water content, shape, small-strain dynamic shear, and unconfined compressive
properties of the original TPE hydrogels
The Role of Architecture in the Melt-State Self-Assembly of (Polystyrene)<sub>star</sub>-<i>b</i>‑(Polyisoprene)<sub>linear</sub>-<i>b</i>‑(Polystyrene)<sub>star</sub> Pom-Pom Triblock Copolymers
Using
a unique one-pot convergent anionic polymerization strategy,
18 (polystyrene)<sub>star</sub>-<i>b</i>-(polyisoprene)<sub>linear</sub>-<i>b</i>-(polystyrene)<sub>star</sub> (S<sub><i>n</i></sub>IS<sub><i>n</i></sub>) pom-pom
triblock copolymers were synthesized varying a range of architectural
parameters including PS arm molecular weight (<i>M</i><sub>n,star</sub>), the number of arms contained in the star (<i>n</i>), and the PI midblock molecular weight (<i>M</i><sub>n,PI</sub>). A selected series of five of these 18, in which <i>M</i><sub>n,star</sub> was held approximately constant between
14.3 and 16.5 kDa, but with the numbers of arms in the star and PI
midblock molecular weight varied, were selected for detailed characterization
using rheology, AFM, and SAXS. The five selected all shared PS as
the minority component, with star volume fractions (<i>f</i><sub>PS</sub>) varying between 0.11 and 0.22. All samples showed
clear phase separation, with three of the five adopting a highly ordered
hexagonal packing of cylinders (HPC) confirmed through SAXS and AFM.
The remaining two systems were limited to liquid-like packing of cylindrical
domains (LLP). Longer midblock molecular weights and increased numbers
of arms in the star both showed a propensity to hinder formation of
a highly ordered hexagonal lattice. Increasing the number of arms
in the star also favored transitions to a disordered phase at lower
temperatures when overall S<sub><i>n</i></sub>IS<sub><i>n</i></sub> molecular weight was held constant. The behavioral
trends identified suggest interfacial packing frustration plays a
prominent role in determining the ability of the system to develop
highly ordered periodic structures. The chain crowding produced by
the PS star architecture intrinsically favors interfacial curvature
toward the majority PI component, contrary to that intrinsically favored
by the block composition alone. In the two systems in which the frustration
was architecturally most severe (largest <i>n</i> of 7.1,
highest <i>M</i><sub>n,PI</sub> of 191 kDa), evolution of
a hexagonal lattice could not be induced, even after significant thermal
annealing. The pom-pom architecture itself also appears to have a
significant impact on entanglement relaxation dynamics, with development
of HPC morphologies only possible at elevated temperatures
Morphological Phase Behavior of Poly(RTIL)-Containing Diblock Copolymer Melts
The development of nanostructured polymeric systems containing
directionally continuous polyÂ(ionic liquid) (polyÂ(IL)) domains has
considerable implications toward a range of transport-dependent, energy-based
technology applications. The controlled, synthetic integration of
polyÂ(IL)Âs into block copolymer (BCP) architectures provides a promising
means to this end, based on their inherent ability to self-assemble
into a range of defined, periodic morphologies. In this work, we report
the melt-state phase behavior of an imidazolium-containing alkyl–ionic
BCP system, derived from the sequential ring-opening metathesis polymerization
(ROMP) of imidazolium- and alkyl-substituted norbornene monomer derivatives.
A series of 16 BCP samples were synthesized, varying both the relative
volume fraction of the polyÂ(norbornene dodecyl ester) block (<i>f</i><sub>DOD</sub> = 0.42–0.96) and the overall molecular
weights of the block copolymers (<i>M</i><sub>n</sub> values
from 5000–20 100 g mol<sup>–1</sup>). Through
a combination of small-angle X-ray scattering (SAXS) and dynamic rheology,
we were able to delineate clear compositional phase boundaries for
each of the classic BCP phases, including lamellae (Lam), hexagonally
packed cylinders (Hex), and spheres on a body-centered-cubic lattice
(S<sub>BCC</sub>). Additionally, a liquid-like packing (LLP) of spheres
was found for samples located in the extreme asymmetric region of
the phase diagram, and a persistent coexistence of Lam and Hex domains
was found in lieu of the bicontinuous cubic gyroid phase for samples
located at the intersection of Hex and Lam regions. Thermal disordering
was opposed even in very low molecular weight samples, detected only
when the composition was highly asymmetric (<i>f</i><sub>DOD</sub> = 0.96). Annealing experiments on samples exhibiting Lam
and Hex coexistence revealed the presence of extremely slow transition
kinetics, ultimately selective for one or the other but not the more
complex gyroid phase. In fact, no evidence of the bicontinuous network
was detected over a 2 month annealing period. The ramifications of
these results for transport-dependent applications targeting the use
of highly segregated polyÂ(IL)-containing BCP systems are carefully
considered
Network Formation in an Orthogonally Self-Assembling System
Many supramolecular motifs self-assemble into nanorods,
forming
the basis of the mechanical properties of supramolecular polymers.
When integrated as end-caps in a bifunctional telechelic polymer,
the motifs can phase segregate into the same or into another nanorod.
In the latter case, a functional cross-link is formed by the bridging
chain that strengthens the polymer network. This study introduces
a supramolecular polymeric system that consists of two different nanorod
forming supramolecular motifs. When end-capped to monofunctional polymers,
these supramolecular motifs self-assemble in an orthogonal fashion
in two separate types of noncross-linked nanorods, resulting in a
viscous liquid lacking macroscopic properties. The addition of 15
mol % of an α,ω-telechelic polymer containing both supramolecular
motifs, each on one end, transforms this viscous sticky liquid to
a solid material with elastomeric properties due to network formation
between the two types of nanorods