Chemistry Reference
In-Depth Information
The multi-layer when immersed in an acidic solution with pH
1.8,
experiences a pH-induced spinodal-type phase-separation, due to
protonation of the -COOH group. A brief rinse in water (pH
∼
5.5)
and subsequent drying result in a porous and swollen (increased
thickness) film exhibiting a graded-RI, which coated with
∼
/4
optical thickness on both sides of a PS substrate, demonstrated the
reflection loss as low as 0.01% at
l
= 650 nm.
Broadband AR covering from VIS to NIR wavelengths (400−
2000 nm) was obtained by using three sequential spin-coatings of
PS-
l
-PMMA copolymers with different volume fractions of PMMA
block on a glass [100]. Although this multi-layer film can achieve
∼
b
1% reflectance in the entire VIS-NIR range; the interfaces among
air/each layer film/ substrate still exist and the methods involve
several steps. Li et al. [101] fabricated a porous film with GRIN
structure by spin-coating the (PS-
-PMMA)/PMMA blend onto an
octadecyltrichlorosilane (OTS)-modified glass substrate, achieving
a gradient distribution of PMMA domains in the vertical direction
of the entire microphase-separated film. After the removal of PMMA
domains, a PS porous structure with an excellent gradient porosity
ratio in the vertical direction of the film is formed, with the RI
increasing gradually from the top to the bottom of the film. Glass
substrates coated with such porous film exhibited both broadband
and omnidirectional AR properties, with an excellent transmittance
of >97% for VIS-NIR range. Here, the volume fraction of the PMMA
block (
b
N
) and the weight
percent of the homopolymer played a key role in the formation of
the gradient-porosity index inner structure and its stability [102].
The best broadband AR performance was achieved from the 45 mg/
mL solution of PS-
f
), the total degree of polymerisation (
c
PMMA
25%)
blended with 30 wt% PMMA: a mean transmittance of 98.4% at full
spectrum, and >98.0% in the VIS-NIR range between 600-2000 nm
[102].
In a recent progress [99], broadband quasi-omnidirectional
AR property was achieved from polyhedral oligomeric
silsesquioxane-based (POSS)-based nanostructures. Thermal free
radical copolymerisation (methacryl POSS cage mixture) during
nanoimprint lithography produced a uniform array of 'moth-eye'
nanostructures on both sides of a glass substrate (Fig. 2.19d) with
yields
b
-PMMA (
M
= 68000-33500,
f
=
∼
n
PMMA
∼
90% to 100%. The transmittance of the resulting glass
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