23 research outputs found
Block Copolymer Response to Photothermal Stress Fields
Block copolymer materials can be
aligned using shear forces; in
order to fully exploit this phenomenon for controlling nanoscale order,
the coupling between applied forces and molecular properties must
be elucidated. We use a photothermal method to generate extreme and
controllable thermal and stress fields in thin films of cylinder-forming
block copolymers. By studying morphological ordering as a function
of time, shear rate, polymer material, molecular weight, and film
thickness, we elucidate the critical parameters with respect to efficient
ordering. We find that ordering efficiency depends weakly on the block
copolymer interaction parameter and strongly on the difference in
mechanical response of the two phases. Morphologies can be aligned
only when the inverse shear rate is smaller than the material’s
relaxation time. Overall, photothermal shear alignment provides an
efficient means of ordering and aligning nanoscale morphologies over
macroscopic areas, using a surprisingly short (subsecond) shear pulse
Millisecond Ordering of Block Copolymer Films <i>via</i> Photothermal Gradients
For the promise of self-assembly to be realized, processing techniques must be developed that simultaneously enable control of the nanoscale morphology, rapid assembly, and, ideally, the ability to pattern the nanostructure. Here, we demonstrate how photothermal gradients can be used to control the ordering of block copolymer thin films. Highly localized laser heating leads to intense thermal gradients, which induce a thermophoretic force on morphological defects. This increases the ordering kinetics by at least 3 orders of magnitude compared to conventional oven annealing. By simultaneously exploiting the thermal gradients to induce shear fields, we demonstrate uniaxial alignment of a block copolymer film in less than a second. Finally, we provide examples of how control of the incident light field can be used to generate prescribed configurations of block copolymer nanoscale patterns
Rapid Ordering in “Wet Brush” Block Copolymer/Homopolymer Ternary Blends
The ubiquitous presence of thermodynamically
unfavored but kinetically
trapped topological defects in nanopatterns formed <i>via</i> self-assembly of block copolymer thin films may prevent their use
for many envisioned applications. Here, we demonstrate that lamellae
patterns formed by symmetric polystyrene-<i>block</i>-poly(methyl
methacrylate) diblock copolymers self-assemble and order extremely
rapidly when the diblock copolymers are blended with low molecular
weight homopolymers of the constituent blocks. Being in the “wet
brush” regime, the homopolymers uniformly distribute within
their respective self-assembled microdomains, preventing increases
in domain widths. An order-of-magnitude increase in topological grain
size in blends over the neat (unblended) diblock copolymer is achieved
within minutes of thermal annealing as a result of the significantly
higher power law exponent for ordering kinetics in the blends. Moreover,
the blends are demonstrated to be capable of rapid and robust domain
alignment within micrometer-scale trenches, in contrast to the corresponding
neat diblock copolymer. These results can be attributed to the lowering
of energy barriers associated with domain boundaries by bringing the
system closer to an order–disorder transition through low molecular
weight homopolymer blending
Latent Alignment in Pathway-Dependent Ordering of Block Copolymer Thin Films
Block copolymers spontaneously form
well-defined nanoscale morphologies during thermal annealing. Yet,
the structures one obtains can be influenced by nonequilibrium effects,
including processing history or pathway-dependent assembly. Here,
we explore various pathways for ordering of block copolymer thin films,
using oven-annealing, as well as newly disclosed methods for rapid
photothermal annealing and photothermal shearing. We report the discovery
of an efficient pathway for ordering self-assembled films: ultrarapid
shearing of as-cast films induces “latent alignment”
in the disordered morphology. Subsequent thermal processing can then
develop this directly into a uniaxially aligned morphology with low
defect density. This deeper understanding of pathway-dependence may
have broad implications in self-assembly
Self-Assembled Phases of Block Copolymer Blend Thin Films
The patterns formed by self-assembled thin films of blended cylindrical and lamellar polystyrene-<i>b</i>-poly(methyl methacrylate) block copolymers can be either a spatially uniform, single type of nanostructure or separate, coexisting regions of cylinders and lamellae, depending on fractional composition and molecular weight ratio of the blend constituents. In blends of block copolymers with different molecular weights, the morphology of the smaller molecular weight component more strongly dictates the resulting pattern. Although molecular scale chain mixing distorts microdomain characteristic length scales from those of the pure components, even coexisting morphologies exhibit the same domain spacing. We quantitatively account for the phase behavior of thin-film blends of cylinders and lamellae using a physical, thermodynamic model balancing the energy of chain distortions with the entropy of mixing
Cooperative Ordering and Kinetics of Cellulose Nanocrystal Alignment in a Magnetic Field
Cellulose
nanocrystals (CNCs) are emerging nanomaterials that form
chiral nematic liquid crystals above a critical concentration (<i>C</i>*) and additionally orient within electromagnetic fields.
The control over CNC alignment is significant for materials processing
and end use; to date, magnetic alignment has been demonstrated using
only strong fields over extended or arbitrary time scales. This work
investigates the effects of comparatively weak magnetic fields (0–1.2
T) and CNC concentration (1.65–8.25 wt %) on the kinetics and
degree of CNC ordering using small-angle X-ray scattering. Interparticle
spacing, correlation length, and orientation order parameters (η
and <i>S</i>) increased with time and field strength following
a sigmoidal profile. In a 1.2 T magnetic field for CNC suspensions
above <i>C</i>*, partial alignment occurred in under 2 min
followed by slower cooperative ordering to achieve nearly perfect
alignment in under 200 min (<i>S</i> = −0.499 where <i>S</i> = −0.5 indicates perfect antialignment). At 0.56
T, nearly perfect alignment was also achieved, yet the ordering was
36% slower. Outside of a magnetic field, the order parameter plateaued
at 52% alignment (<i>S</i> = −0.26) after 5 h, showcasing
the drastic effects of relatively weak magnetic fields on CNC alignment.
For suspensions below <i>C</i>*, no magnetic alignment was
detected
Dynamic Thermal Field-Induced Gradient Soft-Shear for Highly Oriented Block Copolymer Thin Films
As demand for smaller, more powerful, and energy-efficient devices continues, conventional patterning technologies are pushing up against fundamental limits. Block copolymers (BCPs) are considered prime candidates for a potential solution <i>via</i> directed self-assembly of nanostructures. We introduce here a facile directed self-assembly method to rapidly fabricate unidirectionally aligned BCP nanopatterns at large scale, on rigid or flexible template-free substrates <i>via</i> a thermally induced dynamic gradient soft-shear field. A localized differential thermal expansion at the interface between a BCP film and a confining polydimethylsiloxane (PDMS) layer due to a dynamic thermal field imposes the gradient soft-shear field. PDMS undergoes directional expansion (along the annealing direction) in the heating zone and contracts back in the cooling zone, thus setting up a single cycle of oscillatory shear (maximum lateral shear stress ∼12 × 10<sup>4</sup> Pa) in the system. We successfully apply this process to create unidirectional alignment of BCP thin films over a wide range of thicknesses (nm to μm) and processing speeds (μm/s to mm/s) using both a flat and patterned PDMS layer. Grazing incidence small-angle X-ray scattering measurements show absolutely no sign of isotropic population and reveal ≥99% aligned orientational order with an angular spread Δθ<sub>fwhm</sub> ≤ 5° (full width at half-maximum). This method may pave the way to practical industrial use of hierarchically patterned BCP nanostructures
Linear Mesostructures in DNA–Nanorod Self-Assembly
The assembly of molecules and nanoscale objects into one-dimensional (1D) structures, such as fibers, tubules, and ribbons, typically results from anisotropic interactions of the constituents. Conversely, we found that a 1D structure can emerge <i>via</i> a very different mechanism, viz, the spontaneous symmetry breaking of underlying interparticle interactions during structure formation. For systems containing DNA-decorated nanoscale rods, this mechanism, driven by flexible DNA chains, results in the formation of 1D ladderlike mesoscale ribbons with a side-by-side rod arrangement. Detailed structural studies using electron microscopy and <i>in situ</i> small-angle X-ray scattering (SAXS), as well as analysis of assembly kinetics, reveal the role of collective DNA interactions in the formation of the linear structures. Moreover, the reversibility of DNA binding facilitates the development of hierarchical assemblies with time. We also observed similar linear structures of alternating rods and spheres, which implies that the discovered mechanism is generic for nanoscale objects interacting <i>via</i> flexible multiple linkers
Injectable Anisotropic Nanocomposite Hydrogels Direct in Situ Growth and Alignment of Myotubes
While
injectable in situ cross-linking hydrogels have attracted
increasing attention as minimally invasive tissue scaffolds and controlled
delivery systems, their inherently disorganized and isotropic network
structure limits their utility in engineering oriented biological
tissues. Traditional methods to prepare anisotropic hydrogels are
not easily translatable to injectable systems given the need for external
equipment to direct anisotropic gel fabrication and/or the required
use of temperatures or solvents incompatible with biological systems.
Herein, we report a new class of injectable nanocomposite hydrogels
based on hydrazone cross-linked poly(oligoethylene glycol methacrylate)
and magnetically aligned cellulose nanocrystals (CNCs) capable of
encapsulating skeletal muscle myoblasts and promoting their differentiation
into highly oriented myotubes in situ. CNC alignment occurs on the
same time scale as network gelation and remains fixed after the removal
of the magnetic field, enabling concurrent CNC orientation and hydrogel
injection. The aligned hydrogels show mechanical and swelling profiles
that can be rationally modulated by the degree of CNC alignment and
can direct myotube alignment both in two- and three-dimensions following
coinjection of the myoblasts with the gel precursor components. As
such, these hydrogels represent a critical advancement in anisotropic
biomimetic scaffolds that can be generated noninvasively <i>in
vivo</i> following simple injection
Nanostructured Surfaces Frustrate Polymer Semiconductor Molecular Orientation
Nanostructured grating surfaces with groove widths less than 200 nm impose boundary conditions that frustrate the natural molecular orientational ordering within thin films of blended polymer semiconductor poly(3-hexlythiophene) and phenyl-C<sub>61</sub>-butyric acid methyl ester, as revealed by grazing incidence X-ray scattering measurements. Polymer interactions with the grating sidewall strongly inhibit the polymer lamellar alignment parallel to the substrate typically found in planar films, in favor of alignment perpendicular to this orientation, resulting in a preferred equilibrium molecular configuration difficult to achieve by other means. Grating surfaces reduce the relative population of the parallel orientation from 30% to less than 5% in a 400 nm thick film. Analysis of in-plane X-ray scattering with respect to grating orientation shows polymer backbones highly oriented to within 10 degrees of parallel to the groove direction