Key words CVD polymerization - biointerfaces - area-selective deposition - multifunctional interfaces
- surface engineering
1 Introduction
Polymer coatings have impacted nearly every part of modern life due to their unique
physicochemical qualities, including low density, restricted permeability, and resistance
to degradation, which are largely the result of a diverse polymer chemistry.[1 ],[2 ] Thin-film coatings of functional polymers, typically no thicker than 800 nm, have
been developed to take advantage of polymersʼ extensive organic functions without
compromising desirable bulk qualities (such as mechanical strength) of the underlying
substrate.[1 ],[3 ] Functional polymer coatings have been widely employed to define the surface properties
of a variety of devices, thereby transforming research fields from energy and sustainability
to medical devices. Despite the fact that solution-based techniques, such as layer-by-layer
assembly,[4 ] inkjet printing,[5 ] and spin-dip coating,[6 ] continue to be important methods for forming polymer coatings in biological applications,
vapor-deposited coatings have emerged as a promising alternative.
Chemical vapor deposition (CVD) is a chemical process involving the reaction of volatile
precursors in the gas phase to produce a solid product that deposits on surfaces.[7 ] This method is commonly employed in the semiconductor industry to manufacture dense
inorganic films.[8 ] Related to polymer thin films, precursors can adsorb to the surface first followed
by a substrate-supported polymerization.[9 ],[10 ] CVD processes tend to operate under low-pressure conditions because of the need
to vaporize liquid (or solid) precursors and to avoid interactions of the precursors
in the gas phase prior to deposition onto the substrate.[9 ] CVD polymer coatings have been successfully adopted in various applications during
the last few decades, necessitating a systematic review. The purpose of this paper
is to present a conceptual overview of recent advances in the control of surface properties
through the fabrication of diverse poly-p -xylylene (PPX) structures employing CVD polymerization.
2 CVD Polymerization as a Sustainable Coating Technology
2 CVD Polymerization as a Sustainable Coating Technology
Numerous conventional chemical procedures include substantial quantities of poisonous
and volatile organic solvents.[11 ] One of the primary goals of green chemistry is the replacement of such a harmful
reaction solvent.[12 ] With increasing worldwide environmental consciousness, the design of solvent-free
(“green”) procedures has attracted researchersʼ attention. Many processes using solid-state
or solvent-free conditions have been prioritized due to reduced pollution and expense.[13 ] These reactions make procedural work much simpler and more efficient; hence, they
have gained vital importance and popularity in a short period of time.[13 ],[14 ] Following the mindset of sustainability, future polymer coatings necessitate a strong
focus on greener approaches. The concept of using more sustainable solvents, such
as water, polyethylene glycol, molten organic salts, and supercritical CO2 , to replace conventional organic solvents is thus gaining support.[15 ] Alternatively, a wide variety of reactions have been advanced to occur in the solid
form entirely without the need for a solvent. CVD polymerization as an entirely solvent-free
procedure fits into this classification and thus broadens the applications for polymeric
surface modification.[16 ] Removing the requirement for solvents has also important performance implications,
because it eliminates the possibility of bridging, meniscus and swelling effects.
Moreover, curing or thermal treatments are unnecessary and leaching of contaminants
is generally not a concern typically resulting in superior biocompatibility.[16 ],[17 ] In addition to its advantages for highly controllable film production, the fabrication
of solvent-free polymeric layers using the CVD process may have a significant technological
impact that appears to favorably align with current green and sustainable trends in
surface engineering.
3 CVD Instrumentation
CVD systems typically have four parts ([Figure 1a ]): a gas inlet, where vaporized precursors (like monomers) are delivered into the
deposition chamber; sublimation and pyrolysis zones, where the precursor is converted
into reactive monomers; and a deposition chamber, where the precursors are chemically
activated to enable subsequent polymerization reactions on or near the substrate surface.[2 ],[18 ] The primary classification of CVD methods is based on either their reaction mechanisms
or their energy sources [for example, plasma-enhanced CVD (PECVD)].[19 ] Functional polymer thin films can be created using CVD procedures via chain-growth
or step-growth polymerization mechanisms. The chain-growth mechanism is used in methods
such as initiated CVD (iCVD)[7 ],[20 ] and PECVD,[21 ] whereas the step-growth mechanism allows for the reaction to occur between any two
nearby molecules with the right moieties. The latter is frequently observed in PPX
polymers[22 ] and oxidative CVD (oCVD).[23 ]
Figure 1 a) A side-view schematic of a two-source CVD system. CVD reactor typically consists
of 4 main parts: gas inlet, sublimation zone, pyrolysis zone and deposition chamber.
b) Polymerization process of [2.2]paracyclophane to PPX under the conditions given
in a CVD system.
The processes described above, including iCVD, “parylene-type” CVD polymerization,
PECVD, and oCVD, are among the methods that are utilized the most frequently in the
production of functional polymer thin films for use in biological applications. In
interface engineering for biological applications, CVD polymerization of [2.2]paracyclophanes
(PCPs) is one of the most prevalent techniques.
4 Poly-p -xylylene Coatings: Background of Polymerization Process and Functionalized Films
4 Poly-p -xylylene Coatings: Background of Polymerization Process and Functionalized Films
In the ground-breaking research carried out by Gorham, PCP vapor was subjected to
thermal cracking ([Figure 1b ]).[24 ],[25 ] In 1947, Szwarc first described the production of PPX using vacuum polymerization.[26 ] Building upon these earlier efforts, the Gorham approach for CVD polymerization
of PCP broadened its practical applicability.[25 ] The PCP precursor is converted into reactive monomers using sublimation and vacuum
pyrolysis at temperatures exceeding 600 °C ([Figure 1b ]).[27 ] William Gorham demonstrated that PCP can be quantitatively transformed into monomers
at 600 °C.
Soon after the discovery of PPX, scientists demonstrated that functionalized PPX films
can further enhance coating performance.[28 ] The chemical structures and trade names of most common derivatives are illustrated
in [Figure 2 ]. PPX-C is the most widely used PPX derivative because of its strong dielectric and
moisture barrier properties.[24 ] The demand for functional polymer coatings is significant, particularly in biomaterials
research. The surface functionality of PPX can be utilized to derivatize the coating
after deposition, resulting in the formation of intriguing biological interfaces.[28 ] By altering surface properties based on polymerization precursors, this could open
up new paths for biomaterials engineering. To improve implant biocompatibility, reactive
PPXs could be functionalized with biomolecules, such as proteins or biopolymers.[28 ],[29 ]
Figure 2 Chemical structure of the most common commercial PPX or parylenes (trend name). Adapted
from Ref. [24 ] published under a creative commonslicense (CC BY).
For this approach, chemical groups that can serve as chemical anchors and to which
biomolecules can form covalent bonds are necessary. Employing functionalized PPX,
for instance, has enabled the surface modification of metallic stents with a range
of reactive coatings, such as poly(p -xylene-2,3-dicarboxylic acid anhydride), which was essential for the covalent immobilization
of r-hirudin.[30 ] In a separate investigation, scientists discovered that poly(hydroxymethyl-p -xylylene-co-p -xylylene)-coated stents exhibit excellent mechanical stability, outstanding biocompatibility,
and low in vitro and in vivo cytotoxicity.[31 ] The precursor is functionalized prior to polymerization, resulting in reactive CVD
coatings. It is challenging to produce precursors that can be sublimed, are thermally
stable at pyrolysis temperatures, and capable of forming p -quinodimethane (i.e., PX) as the reactive monomers of CVD polymerization.[24 ] Since the establishment of the Gorham process, many projects have used functionalized
PCP with success.[32 ]
As precursor, difunctionalized PCP and monofunctionalized PCP are the two most common
ways to obtain PPX derivatives.[24 ] By adding bulky groups to the benzyl ring, it was possible to produce solvent-soluble
polymers.[33 ] Due to their solubility, these polymers may be studied using traditional techniques
such as gel permeation chromatography and nuclear magnetic resonance (NMR) spectroscopy.
Introduction of n-alkyl chains at the benzylic ring also resulted in polymers where
glass transition temperature, decomposition temperature, and Youngʼs modulus decreased
with increasing alkyl chain length. In addition to solubility, functionalization can
affect a large number of other characteristics. Siloxane substituents, for instance,
produced an amorphous, colorless polymer with a glass transition temperature of −10 °C
and a 5% decomposition temperature of 442 °C.[33 ] This polymer possessed superior mechanical properties compared to unsubstituted
PPX, with a Youngʼs modulus of 0.02 GPa and an average elongation at break of 470%.
Additionally, the siloxane groups altered the wetting behavior. As previously demonstrated
for the siloxane-modified PPX, the elongation at break (maximum value of 380%) was
also enhanced compared to commercial PPX. Using a monosubstituted PCP, several functionalized
PPX surfaces were prepared.[24 ] CVD polymerization of these precursors led to the formation of a copolymer composed
of PX and the substituted PX building blocks. Due to the dissimilar vapor pressures
of the quinodimethanes, it is difficult to produce copolymers with equal amounts of
each monomer moiety.[34 ] Lahann and colleagues employed monosubstituted PCP as the starting material for
a variety of functionalized coatings.[28 ],[35 ] They used their coatings primarily to immobilize biomolecules such as proteins and
cells. Additionally, functionalized PPX was employed to immobilize proteins using
site-specific and nonspecific coupling strategies. Maleimide- and N -hydroxy-succinimide-substituted PPX copolymer was created as a result.[36 ] The simultaneous pyrolysis of two distinct precursors in distinct sublimation and
pyrolysis zones permitted the formation of copolymers with three building units.[37 ]
5 Main Applications of Poly-p -xylylenes
5 Main Applications of Poly-p -xylylenes
The application fields are dominated by two categories: electronics and medicine.[38 ] In both domains, polymers perform coating and protection functions. PPX is a typical
protective coating used in the sectors of electronics, aerospace, and medicine.[39 ],[40 ] Up to a given thickness, the resulting film is chemically resistant, thin, conformal,
and free of pinholes. The coatings confer a multitude of properties simultaneously,
including enhanced lubricity, electrical insulation, chemical and moisture isolation,
and mechanical protection. Dust and soil are repelled by the low adhesion to the surface.
The surface of PPX coatings can be altered using a specialized plasma coating. Thus,
an interlayer is placed to provide excellent adhesion to both the substrate and the
subsequent primer. Following this procedure, the hydrophobic PPX polymer can once
again be painted using solvent-borne or waterborne spray primers.[39 ]
PPX can be applied to change biological surface properties. Diverse chemical anchor
groups produce a coating platform that is flexible.[28 ],[41 ] The most prevalent use of PPXs in the medical industry is as protective coatings
for a vast array of devices.[28 ] Surface engineering for microfluidic devices is made possible by the CVD polymerizationʼs
ability to synthesize a variety of functional groups.[42 ] Insulin has been immobilized on surfaces with CVD coatings to facilitate in vitro
cell adhesion and proliferation. As functionalization anchors, amino groups that dangle
from copolymers with amino-p -xylylene and PX moieties can act as amino groups. These amino groups are capable
of immobilizing thrombin inhibitors such as r-hirudin. The functionalization is useful
for devices that come into contact with native blood.[43 ] Inhibitor of thrombin r-hirudin has been introduced to the pharmaceutical release
system.[44 ] Frequently, the capacity of biosensing and bio-MEMS to accomplish their functions
depends on their surface attributes (such as wetting characteristics, binding to biomolecules,
etc.).[45 ] Therefore, interface engineering is crucial. Consequently, biosensing apparatus
and microfluidic devices usually require functional polymer coatings on the device
surface for their construction and/or performance enhancement.[2 ],[28 ] As a result of its conformability, CVD processes are widely employed as coating
techniques. During manufacture, surface micro- and nanostructures are routinely added
to biosensors and bio-MEMS devices to boost their surface-to-volume ratio, enabling
sensitive detection and precise manipulation at the biointerface.[46 ] Coatings made of PPX have been utilized to increase the sensitivity of fluorescence
imaging as well as to create sensor arrays that are employed in label-free molecular
identification.[37 ] CVD coatings are therefore new substrates for studying biomolecule–surface interactions
when quantitative and spatial information is sought.[28 ],[37 ] In addition, a wide variety of biomolecules have been immobilized using functionalized
PPX containing aldehydes,[47 ] amines,[48 ] anhydrides,[49 ] or active esters.[41 ],[50 ] In biosensing, fluorescence imaging is frequently used to visualize cells or other
living things in situ with high spatial resolution. PPX coatings have been used to
overcome the signal-to-noise ratio issues of certain biological systems.[51 ] Additionally, the varied functional moieties of CVD polymer thin films enabled facile
biofunctionalization for precise interaction with target molecules. These imaging
substrates with CVD alterations revealed ultrasensitive in situ detection of target
biomolecules.[2 ]
6 Area-Selective CVD Polymerization
6 Area-Selective CVD Polymerization
In order to generate arrays of nanoscale features at high densities, numerous surface
preparation and treatment techniques involving multiple steps are developed. Microcontact
printing (µCP) is one of the most widely used patterning techniques, employing patterned
elastomers to imprint reactive chemicals on polymer-modified substrates.[52 ] Applying a photomask to the vapor deposition of polymer films on perfect substrates
is an additional frequent patterning technique. These two methods are based on photo-
or soft-lithographic processes.[52 ],[53 ] These techniques are restricted to traditional two-dimensional surfaces due to the
limitations of photomasks and elastomeric stamps.[54 ]
Combining nanolithography,[56 ] e-beam lithography,[57 ] and two-photon laser with vapor-deposited polymer films enables the creation of
micro- or nanostructures on surfaces with complex geometries. This is due to the fact
that vapor-deposited polymer films offer conformal coverage, adjustable ultra-thin
(20 nm) thickness, pinhole-free construction, and programmable topology and chemistry.[58 ] In addition to being used in the patterning of three-dimensional structures, they
are also widely employed in electronics as a tunable organic dielectric layer and
in biology for localized surface modification.[59 ] Nevertheless, these methods are costly, time-consuming, chemically demanding on
the environment, wasteful, and toxic. Thus, area selective deposition (ASD) is a more
suitable technique for achieving this objective.[60 ],[61 ] In general, ASD results in conformal deposition of polymer layers on desired ʼfavorableʼ
substrates/regions and no or minimal deposition in intended ʼunfavorableʼ regions.[61 ]
Since the beginning of the 20th century, ASD has been used in CVD polymerization.
Gladfelter determined the selectivity of area-selective CVD using [Equation 1 ].[62 ]
Instead of measuring the quantity of nuclei on the surface, Gladfelter compared the
preference of deposition during initial deposition using the easily measured nucleus
coverage. The overall selectivity, S , can be determined by comparing the difference between the nuclei coverage on surface
1, θ
1 , and surface 2, θ
2 . Vaeth and Jensen discovered that iron and its salt had significant implications
for preventing poly(p -phenylene vinylene) formation on the substrate surface during CVD by limiting the
nucleation and propagation steps.[63 ]
The polymerization inhibition effect was extended to transition metals, transition
metal salts, and organo-transition metal complexes.[64 ] As the most prevalent PPX, PPX-N and PPX-C also exhibit ASD. Suh et al. produced
nanoscale structures by combining CP and selectively deposited CVD polymerization.[55 ] Their methods ([Figure 3a ]) include creating PDMS stamps with photolithographic patterns, coating inhibitor
layers with titanium and iron films solely on flat top and bottom surfaces using e-beam
evaporation, and applying PPX via CVD polymerization. PPX deposited only on the walls
of PDMS stamps due to ironʼs inhibiting effect. Further scanning electron microscopy
(SEM; [Figure 3b ]) measurements were used to identify the structures with a high aspect ratio. Using
µCP, the technical utility of the modified stamp was further evaluated; [Figure 3c,d ] demonstrates successful pattern transfer.
Figure 3 a) A schematic diagram of the experimental procedure. b) A SEM image of a PDMS stamp
with a selectively grown PPX film along the sidewall. Note the sharp difference in
colors between PDMS and PPX. The lateral thickness of PPX layer is 100 nm, which gives
an increased aspect ratio of 1.9. The bar scale indicates 500 nm. c) and d) SEM images
of Au lines obtained by µCP onto a gold substrate followed by wet etching. Adapted
with permission from Ref. [55 ]. Copyright 2003 American Institute of Physics.
As shown in [Figure 4 ],[65 ] Chen et al. examined the specific inhibition of CVD polymerization by a variety
of metals using a systematic approach. They applied selective inhibition to reactive
polymer coatings such as functionalized PPX. No area-selective property was identified
on transition metals for oxygen- or nitrogen-containing substituted PPX. This suggested
that the metal and the heteroatoms interact preferentially. In addition, not all metals
exhibited selectivity for the same polymer. Certain substrates, such as iron, copper,
silver, platinum, and their salts, are more likely to inhibit CVD polymerization.[65 ] Using density functional theory calculations, Vitos[66 ] and his colleagues confirmed that substances with high melting temperatures typically
have high bond strengths; for example, the surface energy of tungsten is anticipated
to be more than that of its neighbor molybdenum and substantially greater than those
of noble metals. In addition, high-index facets often have a greater surface energy
than low-index facets due to a higher density of low-coordinated surface atoms; for
instance, the surface energy of Rh{830} is more than that of Rh{100}.[67 ] In fcc, bcc, and hcp materials, the {111}, {110}, and {0001} planes have the highest
atomic density per unit area, respectively.[68 ] According to the classic broken bond model, the minimizing of broken bonds in these
surfaces leads to the lowest surface energy values.[68 ] Chen et al. reported that the growth of poly[4-vinyl-p -xylylene-co-p -xylylene] was selectively inhibited on titanium and confirmed its reactivity for
spatially controlled cross-metathesis reactions.[65 ] With its functional groups, this selectively deposited reactive coating may offer
an exceedingly straightforward technique for designing micro- and nano-structured
biointerfaces.
Figure 4 (a) Area selectivity on nine different metal surfaces (Au, Ag, Ni, Cu, Ir, Pt, W,
Ta, and Ti) for poly-p -xylylenes deposited via CVD polymerization. (b) Typical spectra for poly(dichloro-p -xylylene) (3) on Au, Ag, Ni, Cu, Ir, Pt, W, Ta, and Ti. The spectra are dominated
by characteristic C–Cl stretches at 1030 – 1100 cm−1 , which were present on Au, Ni, Pt, W, and Ta surfaces, but not on Ag, Cu, Ir, or
Ti surfaces. (c) Schematic illustration shows the CVD polymerization process of PCP
that yields nonreactive (1 – 4) as well as reactive (5 – 14) poly-p -xylylenes. Adapted with permission from Ref. [65 ]. Copyright 2008 John Wiley and Sons.
Physisorption of the monomer on the surface and subsequent chemisorption are two steps
in the multistep process that is described as the growth mechanism in CVD. The chemisorption
step is analogous to a propagation reaction between the monomer and a radical chain
end, and each chemisorption produces a new chemisorption site.[69 ] The plausible explanation is that the initiation reaction happened when two or more
monomer molecules combined to form a biradical oligomer following the adsorption of
monomers on the substrate. The propagation process began when a monomer molecule reached
one of the free-radical polymer chain ends.[70 ] Fortin and Lu developed a kinetic model in which the rate-limiting phase of polymer
synthesis was precursor adsorption on the surface.[69 ]
In the Fortin model, the sticking coefficient, which is determined by the energetics
of the monomer–substrate interaction, is defined as the probability of a precursor
species adsorbing or reacting each time it strikes the surface, and is normally controlled
by the substrate temperature. It is essential to remember that the sticking coefficient
only applies to chemisorption processes. Each initial monomer that chemisorbs in the
absence of sufficient coverage will result in a new radical chain termination (similar
to nucleation in crystal formation). In [Figure 5a ], the experimental data indicate clearly that the sticking coefficient of PX reactive
species is proportional to temperature; consequently, the deposition rate is likewise
proportional to the deposition temperature.
Figure 5 a) Sticking coefficient of PX reactive species as a function of temperature. b) Deposition
rate as a function of temperature at pressure = 4.0 mTorr. c) Deposition rate as a
function of pressure at temperature = 22 °C. Adapted with permission from Ref. [69 ]. Copyright 2002 American Chemical Society.
As depicted in [Figure 5b ], the deposition rate of PPX generally increases as substrate temperature decreases.
indicating that the adsorption is the rate-limiting step. Working pressure inside
the CVD system is another crucial factor that influences the deposition rate, shown
in [Figure 5c ], as it influences the concentration of gas-phase precursor species. In this case,
the deposition rate increases as the operating pressure rises. Nowadays, it is of
technological importance to scale down electronic devices, especially semiconductors
and microchips, to get faster performance with smaller sizes, which fuels a need for
complex three-dimensional structures and thin-film materials. ASD, as one of the technologically
relevant bottom-up technologies, allows for synthesis of nanomaterials with well-defined
shapes, sizes, and chemical compositions that are formed through the growth and self-assembly
of atoms and molecules as their building blocks.[71 ] Unlike atomic layer deposition, which requires removal of residual deposits to avoid
the deposition on a non-target area during ASD, CVD polymerization provides a new
ASD strategy.[61 ] Moreover, recent kinetic studies suggest the deposition conditions during CVD polymerizations,
including temperature and pressure, are the main effect of deposition rate. Although
the precise underlying mechanism of area-selectivity in CVD polymerization is yet
to be understood, it is critical to investigate the relationship between area-selective
CVD polymerization and deposition conditions.
7 Fabrication and Applications of Topological Structures
7 Fabrication and Applications of Topological Structures
For many years, approaches for the fabrication of topological structures have been
of interest as a progression beyond surface film coating. Numerous attempts have been
made to leverage the unique properties that certain topologies, dimensions, and chemistries
can provide, such as chirality, targeted drug delivery, and supramolecular assembly;
however, the majority of techniques have limited access to offer tailoring the shape
of a material with multiple functionalities. In this context, CVD polymerization offers
distinct benefits for the fabrication of PPX structures with the appropriate forms
and characteristics. In particular, adding a template for polymerization allows its
shape and physicochemical properties to be transferred to the resultant structures.
Chen and colleagues employed ice particles as the template for CVD polymerization
to produce porous PPX architectures in their research.[72 ] They sprayed water mist onto a superhydrophobic surface (modified silicon or glass)
to generate water droplets with a high contact angle (static, advancing, and receding
angles of 152.0° ± 0.9°, 154.1° ± 0.6°, and 150.2° ± 0.8°, respectively). The specimen
was then vitrified by immersion in liquid nitrogen and placed on the CVD reaction
chamber. Under the conditions of 100 mTorr and 4 °C, vaporized monomers of the PPX
were deposited onto the sublimating ice particles. This diffusion-limited deposition/sublimation
technique produced porous PPX particles with sizes constrained by the ice particle
templates. By initiating sublimation at a specified time, the sizes of ice particles
and consequent PPX particles may be regulated. They then adopted an ice-templated
CVD system for applications in multifunctional, multicompartmental, and cell-laden
scaffolds.[73 ] They also designed multicompartmental scaffolds by spatially assembling two ice
templates with different geometries and microenvironments.
In a recent work by Tsotsalas and co-workers, crystal-controlled CVD polymerization
using metal–organic frameworks (MOFs) as the template was used to create porous microparticles
([Figure 6 ]).[74 ] They used PPX and PPX-C for the deposition and HKUST-1 as a sacrificing MOF crystal.
After CVD deposition, PPX@MOF composites were formed and then placed in a basic solution
in order to remove the template and create monolithic PPX particles. To evaluate the
depth of PPX precursors into the MOF templates, the PPX particles were analyzed via
focused ion beam SEM (FIB-SEM). Cross-sectional SEM images of PPX particles revealed
a solid core, whereas PPX-C particles had a hollow interior ([Figure 6b ]). The authors further utilized PPX and PPX-C porous particles as reversible ethanol
gas adsorbent materials showing substantial uptake of the gas by ~ 320 ng/µg and ~ 40 ng/µg,
respectively. The difference in uptake rate was attributed to the porous morphology
of PPX particles which was able to host more ethanol vapors to the particles compared
to the PPX-C that behaved similarly to the reference sensor. Compared to other precedent
methods, such as free-radical polymerization,[75 ] electropolymerization,[76 ] and click reaction,[77 ] MOF-templated CVD allows for fabrication of nanoscale porous gas-hosting particles
without yielding solvents, catalysts, and byproducts.
Figure 6 Top: Schematic illustration of the polymerization of PPX via cyclophane-based CVD
polymerization using MOF crystals as a template. EDTA stands for ethylenediaminetetraacetic
acid. Bottom: Crystal shape and cross-section of the PPX particles PPX and PPX-Cl
before and after the FIB cuts. Reproduced with permission from Ref. [74 ]. Copyright 2022 American Chemical Society.
To further exploit the merits of templated micro- and nanoscale materials, it is important
to modulate their chemical and structural features diversely. Topological architectures,
in many cases, leverage their large surface-to-volume ratio or porous properties for
specific applications. In this context, fibrillar structures are another class of
materials that have attracted large interests for a variety of applications. Greiner
and co-workers reported PPX microfibers templated by electrospun poly(L-lactide) (PLA)
fiber.[78 ] The PPX-PLA core-shell fibers were annealed at 250 °C under vacuum to degrade the
PLA templates and obtain PPX hollow tubes. They also explored electrospun poly(vinyl
alcohol)/bovine serum albumin (BSA) nanofibers for the application of release time
control of BSA via coating nanometer-thick (40 – 300 nm) PPX coating on the composite
fibers.[79 ] Demirel and co-workers have shown a template-free synthesis of submicron wires using
oblique angle deposition.[80 ] They have outlined a template-free synthesis of submicron wires using oblique angle
deposition.[80 ]
They have reported a template-free synthesis of submicron wires using oblique angle
deposition.[80 ] There are two challenges to overcome to make fibrous structures without using template
materials: 1) CVD tends to result in dense polymer films, which is not a desirable
feature with respect to engineering fibers that are inherently porous, and 2) the
flux of monomer deposition is non-directional. To resolve these issues, they customized
CVD by adding a nozzle for the directional deposition and assembled two rotating motors
with two axes that are vertical and planar to the substrate. They discovered that
slanting the substrates away from the deposition axis offers porosity and leads to
formation of sculptured thin films of PPX. From the oblique angle deposition, as represented
in [Figure 7A ], a columnar morphology (aspect ratio of ~1 : 1000) that is tilted away from the
surface normal was obtained on a substrate.
Figure 7 Cross-sectional SEM micrographs of columnar (A) columnar, (B) helical, (C) chevron
and (D) planar PPX films. Planar PPX films do not possess nanostructured morphology.
Scale bars for all micrographs are 20 µm. Reproduced with permission from Ref. [80 ]. Copyright 2008 Elsevier.
They were able to further modulate the structures by rotating the stage planarly to
fabricate helical and chevron morphologies ([Figure 7B,C ]). Templated CVD polymerization has demonstrated its potentials to make tailored
structures of fibers owing to their excellent coverage on the solid-state materials
and modular process parameters (e.g., pyrolysis temperature, stage temperature, and
deposition rate of the PCP precursors). While solid templates enable straightforward
deposition on the surfaces, using fluid templates offers more unique architectures
due to their dynamic interplay with guest materials. Aizenberg and co-workers coprecipitated
BaCl2 and Na2 SiO3 under constant flow of CO2 to spontaneously form cylinder, petal, coral, or helical structures depending on
the concentration of carbonate nucleation density and overall pH of the solution.[81 ] While chemical interaction plays key roles in templating, turbulent flow-induced
molding was also demonstrated to engineer polymer into spherical, fibrous, or dendritic
polymer particles.[82 ]
Anisotropic liquids, specifically liquid crystalline (LC) phases, are also an interesting
option for templating material owing to their unique long-range order of alignment
and elastic properties.[83 ] Kato and co-workers have used smectic LCs to make a hydrogen bond-assisted nanofiber
network.[84 ] The alignment of nanofibers was in accordance with the local LC orientation and
changed by slight tuning of the temperature. Akagi and co-workers templated polyacetylene
nanofibers with a variety of LCs with catalysis into chiral nanofibers.[85 ] Integrating the advantages of CVD with LCs, LC-templated CVD polymerization resulted
in the synthesis of end-attached nanofiber arrays supported by a solid surface.[86 ] A film of LC supported by a glass substrate was transferred to the CVD reaction
chamber and pyrolyzed monomers diffused into the LC template to form nanofiber arrays
([Figure 8D ]). After removing LC by rinsing with organic solvents, they noted that the alignment
of nanofibers was in accordance with the molecular ordering of the LC template. For
instance, nematic LC film with homeotropic (vertical) anchoring to the substrate formed
straight nanofibers, while adopting cholesteric LC film-induced helical nanofibers.
Optical characterization with crossed and transmission electron microscopy diffraction
pattern revealed nematic-like ordering of polymer chains within the nanofibers ([Figure 8F–I ]). Lahann and team further modulated the morphology of nanofibers into enantiomerically
pure nanohelices using a precursor with chiral center.[87 ] Compared to conventional methods to fabricate chiral nanofibers (e.g., electrospinning[88 ] or molecular self-assembly[89 ]), LC-templated CVD decorated the surface with an ordered array of nanohelices in
the achiral LC ([Figure 9 ]). They reported that the molecular chirality of chiral PCP was transferred to cause
helicity in nanofibers and spiraling of nanofibers in a higher order of scale. Depending
on the ratio of chiral ([Figure 9B,C ]) and achiral precursors ([Figure 9D ]), their copolymerization using CVD enabled the control over chirality of nanohelices.
After dispersing nanohelices in methanol by bath sonication, circular dichroism (CD)
spectroscopy revealed mirrored signals at 242 nm with characteristic Cotton effects
indicating that molecular chirality being transferred to the nanohelices. In contrast,
achiral nanofibers did not exhibit any conspicuous signals of CD spectrum. To further
understand the role of the molecular chirality of PCP to its surrounding LC template,
they replaced nematic LCs with cholesteric LCs to investigate competing chirality
effects between the precursors and templates. When the chirality of the precursor
and the LC template was identical, CVD polymerization resulted in supercoiled nanohelices
while they obtained straight nanohelices with precursor and LC with countering chirality.
This discovery has a couple of important implications: 1) understanding how the chirality
is transferred across the multiple scales in artificial and natural systems and 2)
a new platform to fabricate nanohelices with control over the alignment and scales.
Figure 8 Templated synthesis of nanofiber arrays via CVD into anisotropic media. (A) CVD of
1a – h yields polymers 2a – h. (B, C) Representative chemical structures of cyanobiphenyl-based
(5CB and E7) and halogenated (TL205) LCs. (D) Fabrication of polymer nanofibers via
CVD into a LC phase aligned perpendicular to the substrate. (E) SEM images of nanofibers
polymerized from 1a (10 mg) in 5CB. After the nanofiber synthesis, the LC template
was removed. (F) Optical micrograph (crossed polars) of a nanofiber. Orientations
of the analyzer (A) and polarizer (P) are shown in the double-arrow cross. (G, H)
Micrographs (crossed polars) of the nanofiber with a quarter wave plate with its slow
axis (γ, green double arrow) (G) perpendicular or (H) parallel to the fiber axis;
lower order interference colors (yellow) indicate a decrease in retardance. (I) Analysis
of interference colors of the nanofiber in (G) and (H) indicates orientation of the
polymer chains aligned along the fiber axis. Adapted with permission from Ref. [86 ]. Copyright 2018 The American Association for the Advancement of Science.
Figure 9 Templated synthesis of polymer nanohelices via CVD polymerization into a nematic
LC film. (A) Schematic representation of nanohelices 2S and 2R templated into the
nematic E7 phase. Inset: Chemical representation of CVD polymerization of chiral and
achiral precursors. (B – D) SEM images of nanohelices 2S and 2R and achiral nanofibers
2A prepared by CVD polymerization of 1S (B), 1R (C), and 1A (D), respectively (the
LC is homeotropically anchored on a surface before polymerization and was removed
prior to SEM). (E) High-resolution C1s XPS spectra of 2S and 2R confirming identical
chemical composition for nanohelices with opposite handedness. (F) CD spectra of nanohelices
2S (blue) and 2R (green) and achiral nanofibers 2A (black). Adapted with permission
from Ref. [87 ]. Copyright 2022 John Wiley and Sons.
Advancing the fundamental understanding on nanofiber-templates, it is critical to
explore how nanofibers share elastic strain with LC template under an electric field.[90 ] One of the interesting properties of LC is dielectric property, which causes changes
in molecular orientation when an electric field is applied. From morphological modulation
by inherent LC phases in their previous report, they demonstrated the synthesis of
bent nanofiber arrays that exhibit programmed response under electric application,
thereby stretching in z -direction by the elastic strain from the LC template ([Figure 10 ]). In this report, they used 65 mol% 4-(trans -4-pentylcyclohexyl) benzonitrile (PCH5) and 35 mol% 4-(trans -4-propylcyclohexyl) benzonitrile (PCH3) to induce bent anchoring at the LC–air interface
and homeotropic anchoring at the indium tin oxide (ITO) glass substrate.[91 ]
Figure 10 Changes in the optical appearance of an NFF-embedding LC under a) 0 V, b) 2.5 V,
c) 10 V, and d) 0 V at 1 kHz (sine waveform, thickness ≈ 100 µm). Proposed mechanisms
of side view (bottom) and top view (top) of nanofiber reorganization corresponding
to each micrograph are described on the right-hand side. (e, f) A schematic illustration
of the deformation of a single nanofiber on various planes. (g) The optical appearance
of an NFF-embedding LC (10 V) with crossed polarizers and a wave plate (λ = 530 nm).
The direction of the polarizer, the analyzer and the waveplate is indicated with the
arrows. (h) Change in the average retardance of LC (a – c) as a function of voltage.
The error bars are 1 S. D. of retardance measured across the sample (a – c, n = 92 points each). Adapted with permission from Ref. [90 ]. Copyright 2022 John Wiley and Sons.
Optical appearance under crossed polars showed dark brushes before the CVD polymerization,
which comes from the extinct signals from the nanofibers that are parallel to either
the analyzer or the polarizer orientation. Then PCP-HM was polymerized using CVD to
synthesize bent nanofibers within the PCH mixture. Instead of removing the LC template,
they placed another ITO substrate on top to prepare a sandwich cell. They applied
an alternating current (AC) electric field (1 kHz) across the sandwich cell while
observing under the cross-polarized microscope to characterize the changes in tilt
angle of the LC–nanofiber composite. As shown in [Figure 10a–d ], optical retardance gradually decreased from 178 ± 22 nm to 5 ± 1 nm ([Figure 10h ]), which indicates that mechanically coupled PCH/nanofibers were stretched by the
elastic torque of the surrounding LC template during the electric field application.
After removal of the AC, optical retardance increased again reversibly. To provide
direct evidence of nanofiber actuation, they infused polymerizable LC (RM 257) and
initiator to the PCH/nanofiber composite specimens. For comparison, they crosslinked
the composite without an electric field and during the presence of an electric field.
Then, both specimens were vitrified in liquid nitrogen to observe a cross-sectional
view under cryogenic SEM. After coating the surface with Au/Pd in a vacuum environment,
SEM images revealed that the tilt angle of nanofibers from the surface normal notably
decreased under the presence of an electric field. Finally, a model was developed
that predicts the equilibrium tilt angle of nanofibers under certain voltage of AC
electric field. Their model predicted that the tilt angle changes from the initial
angle of 60° to 27° upon 50 V of AC and this result was in good consistency with our
observation under cryo-SEM. This report demonstrates a new type of system with shape-encoded
actuation of end-attached nanofibers that resemble the behavior of a coral reef when
capturing the preys into the core. Furthermore, mechanical modeling discussed in this
report for the first time provides a quantitative assessment tool to evaluate the
effect of elastic strain of LC matrix on the shape of end-attached nanofibers.
8 Conclusions and Outlook
8 Conclusions and Outlook
This report provides a summary of significant research on CVD polymerization, which
enables conformal deposition of polymer layers on a desired substrate. Functionalized
PPXs can serve as reactive coatings and will facilitate the design of micro- and nano-structured
biointerfaces. In addition, an overview of CVD of PCPs that can be performed on surfaces
coated with thin films of LC to produce organized assemblies of end-attached polymer
nanofibers is presented. The LC-templated CVD process is compatible with cholesteric
LCs or blue-phase LC templates, allowing for the synthesis of a variety of nanofiber
shapes. By virtue of the elastic energy stored in stretched nanofibers, these various
nanofiber shapes offer the potential to program distinct electromechanical and electrooptical
responses in LC–nanofiber composites. These results are encouraging for the development
of advanced light valves and optical tweezers based on vortex light beams. The process
resulted in the fabrication of soft nanocomposite materials encoding complex properties,
such as responses to external fields that are promising for the design of new classes
of electrooptical devices or soft actuators. The CVD polymerization of chiral precursors
into LCs also results in superhierarchical arrangements of enantiomorphically pure
nanofibers. Other LC phases exhibit distinct molecular orderings, and crystalline
MOF networks with variable pores, crystal sizes, and shapes can serve as confined
templates for the synthesis of three-dimensional polymer nanostructures.
The vapor deposition of PPX is a unique technique with numerous benefits that produces
a polymer coating with exceptional properties. In 2022, the global PPX coating market
was split by end-user industry (military and aerospace, electronics, medical, automotive),
type (PPX-N, PPX-C, PPX-D), and geography (Asia-Pacific, North America, Europe, South
America, and Middle-East and Africa).[92 ] The PPX coating market is anticipated to increase at a 5.8% compound annual growth
rate from 882.0 US$ in 2022 to 1532.75 US$ in 2030.[92 ] These projections show a significant increase in demand for PPX polymers in the
future, highlighting the importance of this material across numerous industries. The
conformal, functional, and responsive nature of CVD polymers makes them highly desirable
in surface-modification applications. It was these materialsʼ electronic and military
uses that initially sparked scientific and industry attention. Having finally settled
into a routine, the PPX polymers found new uses at the close of the 20th century,
the most prevalent of which being MEMS (micro-electro-mechanical systems) technology.
Due to MEMS, organic semiconductor, and biomedical applications, PPX has recently
undergone a rebirth and appears to have extremely promising future prospects. However,
there are disadvantages, such as the difficulties in synthesis of novel precursors,
their high costs, and the sublimation of cyclophanes, which prevented a high throughput.
Thus, research was conducted into employing different molecules as a beginning material.
These chemicals must be volatile, thermodynamically stable, and capable of forming
the required quinodimethane moiety. To obtain quantifiable PPX yields, quinodimethane
moieties must be quantitatively generated from the starting material. All of these
limits are unquestionably impediments to the growth of PPX, but their elimination
will further establish the procedure as a real cross-sectional technology. Future
studies may focus on CVD deposition kinetics under nano-confinement, ultrahigh-aspect-ratio
structures, and/or innovative templates. All of these factors play an ever-increasing
significance in the creation of cutting-edge biomaterials. Additionally, the thickness
of a polymer coating critically influences its mechanical properties. This mostly
unexplored aspect has the potential to provide an additional control mechanism for
fine-tuning local interactions.
Funding Information
This work has been in part supported by the National Science Foundation through Grant
1 916 654 (J. Lahann). The Helmholtz Foundation is acknowledged for financial contributions.
