5 research outputs found
Simulations of Morphology Evolution in Polymer Blends during Light Self-Trapping
Simulations are presented for binary
phase morphologies prepared
via coupling the self-trapping properties of light with photopolymerization
induced phase separation in blends of reactive monomer and inert linear
chain polymer. The morphology forming process is simulated based on
a spatially varying photopolymerization rate, dictated by self-trapped
light, coupled with the Cahn–Hilliard equation that incorporates
the free energy of polymer mixing, degree of polymerization, and polymer
mobility. Binary phase morphologies form with a structure that spatially
correlates to the profile of the self-trapped beam. Attaining this
spatial correlation emerges through a balance between the competitive
processes entailed in photopolymerization-induced decreases in diffusion
mobility and the drive for the blend components to phase separate.
The simulations demonstrate the ability for a self-trapped optical
beam to direct binary phase morphology along its propagation path.
Such studies are important for controlling the structure of polymer
blends, whereby physical properties and critical physical and chemical
phenomena may be enhanced
Synthesis of Micropillar Arrays via Photopolymerization: An in Situ Study of Light-Induced Formation, Growth Kinetics, and the Influence of Oxygen Inhibition
We report a study
on the growth kinetics and resultant structures
of arrays of pillars in photo-cross-linkable films during irradiation
with a periodic array of microscale optical beams under ambient conditions.
The optical beams experience a self-focusing nonlinearity owing to
the photopolymerization-induced changes in refractive index, thereby
concentrating light and driving the concurrent, parallel growth of
microscale pillars along their path length. We demonstrate control
over the pillar spacing and pillar height with the irradiation intensity,
film thickness, and the size and spacing of the optical beams. The
growth of individual pillars in a periodic array arises from the combination
of intense irradiation in the beam regions and oxygen inhibition afforded
by the open, ambient conditions under which growth is carried out.
We propose a kinetic model for pillar growth that includes free-radical
generation and oxygen inhibition in thick films of photoinitiated
media in order to interpret the experimental results. The model effectively
correlates micropillar array structure to the oxygen inhibition effects.
This approach of growing micropillar arrays through photopolymerization
is straightforward and scalable and opens opportunities for the design
of textured surfaces for applications
Optical Autocatalysis Establishes Novel Spatial Dynamics in Phase Separation of Polymer Blends during Photocuring
We
report a fundamentally new nonlinear dynamic system that couples
optical autocatalytic behavior to phase evolution in photoreactive
binary polymer blends. Upon exposure to light, the blend undergoes
spontaneous patterning into a dense arrangement of microscale polymer
filaments. The filaments’ growth in turn induces local spinodal
decomposition of the blend along their length, thereby regulating
the spatially dynamics of phase separation. This leads to the spontaneous
organization of a large-scale binary phase morphology dictated by
the filament arrangement. This is a new mechanism for polymer blend
organization, which couples nonlinear optical dynamics to chemical
phase separation dynamics, and offers a new approach to light-directed
patterning and organization of polymer and hybrid blends
Tunable Nonlinear Optical Pattern Formation and Microstructure in Cross-Linking Acrylate Systems during Free-Radical Polymerization
We report cross-link-tunable, nonlinear
optical pattern formation
of transmitted light in a photopolymer undergoing free-radical polymerization.
Photopolymerization induces microscale filamentation of a uniform,
broad transmitted beam, which corresponds to a concurrent spatial
evolution in cross-linked morphology in the photopolymer. Because
the photopolymerization is permanent, the ensemble of filaments imprint
a microstructure comprising a cross-link gradient pattern. Tuning
the system’s capability to cross-link and branch changes the
magnitude of the refractive index change (Δ<i>n</i>), which both induces nonlinear conditions and also changes the strength
of the optical nonlinearity. Only a monomer with sufficient functionality
shows stable optical pattern formation, and its nonlinear regime exists
for a specific range of exposure intensities. A monomer of lower functionality
can be pushed into the nonlinear regime by formulating it with higher
functional monomers, whereby Δ<i>n</i> is increased
to provide a stronger response to light. In such formulations, the
strength of the nonlinearity, as evidenced by changes in light confinement
in the optical pattern, is tuned by varying this monomer’s
functionality or its relative weight fraction. The strong correlation
among polymerization-induced refractive index change, optical pattern
feature size, and cross-linked morphology demonstrates tunable optical
nonlinearity through variations in the inherent polymer structure