UV-Curable Antismudge Coatings - ACS Applied Materials

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UV-Curable Antismudge Coatings Chao Zheng, Guojun Liu, and Heng Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05732 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 14, 2017

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ACS Applied Materials & Interfaces

UV-Curable Antismudge Coatings Chao Zheng, Guojun Liu,* and Heng Hu Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6

E-mail: [email protected]

Keywords: Coatings, UV curable, antismudge, patterned wettability, graft copolymer, and self-assembly

Abstract.

Current antismudge coatings that bear nano-pools of a grafted liquid

ingredient for dewetting enablement (NP-GLIDE) are cured at high temperatures, which are undesirable for application on heat-sensitive substrates.

Reported herein

is the development of a NP-GLIDE coating that can be photo-cured at room temperature.

Of the various formulations that have been tested, robust coatings were

obtained from one recipe consisting of a photo-initiator, a trifunctional monomer that bears three double bonds, and a graft copolymer.

The last bears pendent double

bonds and a poly(dimethyl siloxane) (PDMS) side chain as the antismudge agent. Coatings were prepared by casting films from a solution containing these three components and then photolyzing the resultant films.

A systematic study revealed

that the liquid sliding property developed on the coating at a lower crosslinking density than that required for ink to contract.

Further, retaining the ability to

contract ink traces after many writing and erasing cycles was the most demanding of

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the three antismudge tests.

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For our optimized formulation, only 5 min of irradiation

was required to yield a transparent coating with superior antismudge properties. Moreover, irradiating selected regions and then removing, with a solvent, reagents in the non-irradiated regions can yield a surface with patterned wettability.

These

advantageous properties of the new photo-curable coating facilitate its applications.

Introduction Grafting a low-surface-tension liquid polymer such as poly(dimethyl siloxane) (PDMS)1-3 or a perfluorinated polyether (PFPE)4-5 onto one component of a polyurethane (PU) or an epoxy formulation and curing the resultant compound with the other components of a PU or an epoxy formulation yielded an NP-GLIDE PU or epoxy coating.

Here the term NP-GLIDE derives from the observation that many

test liquids have no problem to glide cleanly down such a coating at substrate tilt angles of less than 5° and that the coating contains nano-pools of a grafted liquid ingredient for dewetting enablement.

These coatings are anti-smudge because

marker inks and paints also readily contract on them and the contracted ink patches are easily removed.

Additionally, the coatings are transparent because the nanopools

of the anti-smudge agent on the surfaces and in the matrices of the coatings are much smaller than the wavelength of light and do not scatter light effectively.

These

advantageous features of the NP-GLIDE coatings facilitate their applications. However, the PU and epoxy formulations that we have worked with would cure thermally at high temperatures.

While thermal curing is convenient for many 2


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applications, it would be problematic if one cures in an oven an NP-GLIDE coating that has been applied onto a heat-sensitive substrate, such as the touchscreen on a cellphone.

Thus, the development of a UV-curable coating that can be rapidly cured

at room temperature would broaden the applications of NP-GLIDE coatings.

This

paper reports a novel UV-curable NP-GLIDE coating. While various reactions such as a ring-opening polymerization initiated by photo-generated cations may have been utilized to cure a NP-GLIDE coating,6-7 we subjected vinyl units to photo-initiated free radical polymerization for this purpose. In identifying a working system, we immediately thought of derivatizing P0-g-PDMS, where P0 denotes a polyol and P0-g-PDMS denotes P0 bearing a PDMS side chain, as a suitable candidate for this project (Scheme 1a) because of our previous use of P0-g-PDMS in the preparation of the NP-GLIDE PU coating.

We reasoned that

double bonds could be introduced to the P0 backbone by reacting the hydroxyl groups of P0 with 2-isocyanatoethyl methacrylate (Scheme 1b) to yield P0II-g-PDMS, where P0II denotes P0 after double bond introduction.

We further envisioned that a film

could be cast from a mixture consisting of a photoinitiator, P0II, and P0II-g-PDMS and that subsequent photolysis would yield a UV-cured NP-GLIDE coating. While a pristine coating from the above design could indeed contract ink, the ink contraction ability diminished on regions from which ink had been previously wiped off.

Robust

UV-cured NP-GLIDE coatings that could withstand at least 30 writing and ink removal cycles were obtained only after P0II was replaced with a liquid tri-functional monomer (TM) that was prepared from reacting 3 moles of 2-hydroxyethyl 3



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methacrylate (HEMA) with 1 mole of a hexamethylene diisocyanate trimer (HDIT, Scheme 1c).

Scheme 1. Chemical structures of a) P0-g-PDMS, b) 2-isocyanatoethyl methacrylate, c) HDIT, and d) PDMS-OH.

We note that fluorinated compounds have been previously incorporated into UV-curable formulations to enhance the chemical resistance of the resultant coatings.8-12

However, most prior studies failed to go beyond reporting the static

CAs of several liquids in the characterization of the wetting properties of the final products.

The contraction of ink and paint traces, which is more challenging to

achieve than liquid sliding but deemed an essential qualification for an anti-smudge coating, were seldom reported.

When PDMS was used,13-17 this polymer was bound

to the coating matrix by both ends, which did not facilitate the formation of an enriched PDMS brush layer.18

Thus, no anti-smudge properties were reported for

those systems despite the report of improved water repellency.14

Our NP-GLIDE

coatings have no problems with gliding different test liquids because the PDMS 4


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chains were grafted via only one end to a coating surface and helped convert a crosslinked solid coating surface into a slippery liquid-like one. Our NP-DLIDE coatings were inspired by prior reports of grafted liquid-like monolayers19-22 and of slippery liquid-infused porous surfaces (SLIPS).23-30

In the

former case, a monolayer of poly(dimethyl siloxane) (PDMS)19-21 or highly mobile organic molecules22 are grafted onto flat solid substrates to provide the de-wetting capabilities.

We think that NP-GLIDE coatings should be more wear-tolerant than

monolayer coatings because polymers do wear and the NP-GLIDE coatings are much thicker than monoayer coatings.

Our previous study has demonstrated that the

NP-GLIDE coatings maintained their antismudge properties even after their surfaces were much abraded.1-2, 4 We attributed this to the ability for the NP-GLIDE surfaces to self-regenerate: when the surface layer was worn away, nanopools that were initially embedded underneath were ruptured and chains of the freshly released liquid polymer replenished the worn surface.

In the case of a SLIPS, the pores of a

pre-made porous surface are filled with a low-surface-tension liquid.23-31

An

NP-GLIDE coating differs from a SLIPS because the liquid anti-smudge agent is covalently attached to the coating matrix and a grafted anti-smudge agent should be less prone to loss by evaporation or leaching.3

Results and Discussion P0II-g-PDMS Synthesis and Characterization. commercial polyol, P0, for this project.

For relevance, we used a

This polymer has been meticulously

characterized previously via various 2-dimensional 1H NMR techniques.4 5


The


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polymer is unduly complex because various monomers had been introduced by the manufacturer to provide different properties of the coating solution and the final coating.

However, the key components of P0 are the hydroxyl-containing monomers,

which have a molar fraction of ~ 40%.

Since the repeat unit number of the polymer

is ~ 20, an average of 8 hydroxyl groups exist in each P0 chain. To prepare P0II-g-PDMS, P0-g-PDMS was first synthesized.1

The protocol

involved reacting PDMS-OH, PDMS bearing a terminal hydroxyl group (Scheme 1d), with excess oxalyl chloride, to yield PDMS-COCl (PDMS bearing a terminal acid chloride functionality).

The excess oxalyl chloride was then removed via vacuum

distillation at 60 °C before PDMS-COCl was added into a P0 solution in anhydrous chloroform to eventually yield P0-g-PDMS.

While Figure S1a compares the 1H

NMR spectra of P0 and P0-g-PDMS, the SEC traces of the two polymers are compared in Figure S1b.

Our quantitative analysis of the NMR spectra concluded

that the P0-g-PDMS sample consisted of an average of 1.0 PDMS chain grafted onto each P0 chain and the PDMS weight fraction in the graft copolymer was 33 wt%. The second step involved reacting the residual hydroxyl groups of P0 with 2-isocyanatoethyl methacrylate at a molar ratio of 1.00/0.97.

While the 1H NMR

spectra of P0-g-PDMS and the produced P0II-g-PDMS are compared in Figure S1a, the FTIR spectra of the two are compared in Figure S1c.

The data show that all of

the hydroxyl groups of P0-g-PDMS were consumed during this process.

The

preparation of P0II-g-PDMS was also accompanied by an anticipated shift to the higher molecular weight side for the SEC peak of P0II-g-PDMS relative to that of 6


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P0-g-PDMS (Figure S1b). TM Synthesis.

The tri-functional monomer was synthesized by reacting HEMA

with HDIT at a molar ratio of 3.21 using dibutyltin dilaurate as the catalyst.32

The

consumption of isocyanate under these conditions has been confirmed by a FTIR study of samples collected from the reaction mixture at different times.

The FTIR

data in Figure S2 suggest that the isocyanate groups were consumed 2.0 h after the reactants were mixed. The TM used for our coating was prepared by reacting HDIT with HEMA for at least 2.0 h. DM-PDMS denotes a molecule

Synthesis of Mixtures of TM and DM-PDMS. that bears two double bonds and a PDMS arm.

To prepare mixtures that consisted of

DM-PDMS and TM, PDMS-OH and HDIT at hydroxyl-to-isocyanate molar ratios ranging between 1/99 and 1/3 were initially heated at 45 °C for 2.0 h with the catalyst dibutyltin dilaurate.

Sufficient HEMA was then added to react with the residual

isocyanate groups. Identification of a Robust NP-GLIDE Formulation.

Our prior studies suggest

that a NP-GLIDE coating could be prepared by grafting PDMS to a multifunctional reactive component, such as a polyol, and then curing the resultant compound with other components of a coating formulation.1-2,

4

Bearing this in mind, we

experimented with three formulations as illustrated in Scheme 2. While formulation 1 or F1 consisted of a DM-PDMS and TM mixture, formulation 2 (F2) consisted of P0II and P0II-g-PDMS.

Meanwhile, F3 consisted of TM and P0II-g-PDMS.

Lastly,

a reference coating F0 was prepared from TM alone together with the photoinitiator. 7



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To prepare a coating, a particular formulation was diluted with dimethyl carbonate (DMC)

and

N,N-dimethylformamide

(DMF)

2-hydroxy-2-methylpropiophenone was added.

before

the

photoinitiator

DMC and DMF which were

selectively poor for PDMS were used to facilitate the formation of micelles from DM-PDMS or P0II-g-PDMS.1, 4 Micelle formation helped hide PDMS in the cores and has been shown to improve the compatibility of the system during coating formation and to facilitate the uniform distribution of the PDMS nanopools throughout the coating matrix.1

The solvents were subsequently left to evaporate in

an oven under a gentle nitrogen flow overnight before the coatings were cured via UV irradiation for 5.0 min.

The complete consumption of the double bonds for

formulations F0, F2, and F3 after 300 s of irradiation is confirmed by comparing FTIR spectra of the samples before and after their photolysis with the data shown in Figures S3 and S4.

For comparison purposes, all coatings were 32 µm thick and

contained 5.0 wt% of the photoinitiator and 2.5 wt% of PDMS.

8


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Scheme 2.

Illustrations of the formulations tested.

The F1 formulation after casting on a glass plate and solvent evaporation was opaque and the coating remained so after photolysis, suggesting that macrophase separation occurred in this system.

Macrophase separation occurred probably

because the HEMA-bearing arms of the dimer were not sufficiently thick or large to isolate PDMS into individual nanopools but allowed the different PDMS nanodomains to overlap.

Because of sample opacity, the F1 coatings were not

further characterized. The F2 and F3 coatings appeared clear both before and after sample photocuring. The clarity of the cured coatings is seen from the high transmittance values for 500 nm light (Table 1).

The high transparency of the F2 coatings must be due to the

ability of the large P0II backbone to prevent the grafted PDMS side chains from undergoing macrophase separation and due to the compatibility between P0II and the P0II backbone of P0II-g-PDMS.

P0II-g-PDMS did not undergo macrophase

separation from the crosslinked TM in the F3 coatings probably partially because of the moderate compatibility between P0II and the crosslinked TM and partially because of the rapid crosslinking between TM and the P0II backbone under photolysis.

Table 1. Sample F0 F2 F3

Comparison of the water and hexadecane SAs and CAs on different coatings. T% at Hexadecane Water 500 nm CA (°) SA (°) CA (°) SA (°) 99 ± 1 Spreads Spreads 73 ± 1 > 85 98 ± 1 28 ± 1 2.4 ± 1 101 ± 1 66 ± 3 98 ± 1 28 ± 1 2.8 ± 1 102 ± 1 66 ± 4 9



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The clear F2 and F3 coatings were further characterized by measuring the static CAs of 5 µL water and hexadecane droplets as well as the sliding angles (SAs) of 5 µL hexadecane and 20 µL water droplets on these surfaces.

The CAs of hexadecane

and water were higher on the F2 and F3 coatings than on the F0 coating (Table 1). This increase should be due to the enrichment of the surfaces of the latter two coatings by the low-surface-tension PDMS chains.

As the surface tension ߛ஼ of the

coating surface decreased, test liquids spread less on the coating and their CA θ increased according to Young’s equation:

cos ߠ =

ఊ಴ ିఊಽ಴ ఊ

(1)

where γ is the tension of the test liquid, and ߛ௅஼ is the interfacial tension between the test liquid and the coating. While neither hexadecane nor water glided down the F0 coating as intact droplets, these liquids slid down the F2 and F3 coatings cleanly.

In the case of 5 µL

hexadecane droplets, the measured sliding angles (SAs) were lower than 3°. hexadecane sliding was facile on these coatings.

Thus,

Water droplets with a volume of 20

µL also slid on the substrate at a tilt angle of 66 ± 4°.

Therefore, PDMS

incorporation enhanced the liquid sliding properties of the base coatings. The F2 and F3 coatings were also subjected to an ink contraction test.

While a

red ink containing ethyl alcohol and ethylene glycol monobutyl ether as the solvent left a continuous mark on the F0 coating (Figure 1), the ink shrank on both the F2 and F3 coatings.

Additionally, the ink traces could be readily wiped off with a 10


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Kimwipes® tissue from the F2 and F3 coatings.

However, the traces could not be

removed under identical conditions from the F0 coating.

Thus, both F2 and F3

exhibit anti-smudge behavior.

Figure 1. Comparison of marker tests performed on the F0, F2, and F3 coatings: a) Ink trace left on a pristine coating. b) Status of the ink trace after cleaning with Kimpwipes® tissue. c) Ink trace left on the F2 and F3 coatings after writing on them the second time.

We repeated the ink contraction experiment on a previously marked region on the F2 and F3 coatings. diminished.

Surprisingly, the ink contracting ability on the F2 coating

On the other hand, the ink contracting ability remained on the F3

coating after 30 writing and erasing cycles.

Thus, only the F3 formulation afforded a

robust NP-GLIDE coating. The surprising difference between the F2 and F3 coatings suggests that the formulation of a robust NP-GLIDE coating is not as straightforward as we had originally envisioned.

The ink contracting performance noticeably diminished on

the F2 coating after writing erasure, probably because the exposed PDMS was mostly wiped off the surface together with the ink during the ink removal step.

11


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behavior suggested that the exposed PDMS chains were not tightly bound to the coating matrix in this case. Two factors could have contributed to the performance difference between the F2 and F3 coatings.

First, the maximal possible crosslinking density should be higher

in the F3 than in the F2 coatings. and TM, respectively.

The F2 and F3 coatings consisted mostly of P0II

A simple calculation revealed that the double bond contents in

P0II and TM were 1.6 and 3.6 mmol/g, individually.

Second, we suspect that the

mobility and the spacing of the double bonds in the TM are larger than those in P0II. Thus, the extent of intrachain or intramolecular polymerization was probably higher in the F2 than in the F3 coating.

Since the intramolecular polymerization did not

contribute much to the binding between different molecules such as between P0II and P0II-g-PDMS, the P0II-g-PDMS chains were more tightly bound with the coating matrix in the F3 than in the F2 coating. F3 Curing.

The wetting behavior of the F3 coating was studied as a function of

its radiation time to establish the conditions for robust NP-GLIDE coatings.

Figure

2 compares the FTIR spectra of a F3 sample that was irradiated for different periods of time.

While the marked peak at 1638 cm-1 in Figure 2 corresponds to the C=C

stretching of the HEMA double bonds,33 the peak at 1164 cm-1 arises from the stretching of the CO-O bond in the -OOC-C(CH3)=CH2 group.33

The intensities of

these two peaks decreased rapidly during the initial 30 s of irradiation.

After 60 s,

further decreases in the intensities of these peaks became barely noticeable.

12


Thus,

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the F3 coating was rapidly cured, which is typical for most UV curable formulations.34

Figure 2.

Comparison of the FTIR spectra of a F3 coating that was irradiated for different periods of time.

As irradiation progressed, the hardness of the coating increased.

Figure 3 shows

how the hardness measured using a series of pencils invoking the ASTM D3363 protocol varied with irradiation time for the F3 coating samples.

Below the

photolysis time of 10 s, the coating was softer than the softest pencil with the hardness designation of 9B. time of 30 s.

A measurable hardness of 8B was seen only by the photolysis

The hardness eventually reached 3H by the photolysis time of 300 s.

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Figure 3. Variation in the static CAs and SAs of a) water and b) hexadecane as a function of the F3 coating irradiation time. Also shown is the variation in the hardness of the coating as function of irradiation time.

As the coating hardness increased, the rate of water CA decrease due to surface reconstruction diminished.

Figure 4 plots how the water CA varied with their sitting

time on coating samples that had been irradiated for various times.

The water CAs

on the sample irradiated for 5 s rapidly decreased to approach 73 ± 1°, the static water CA on a crosslinked F0 coating.

This result suggests the ability for the more

hydrophobic PDMS that was initially in contact with water to recede into the coating matrix, thus exposing the lightly crosslinked TM component.

Surface reconstruction

was also observed on the coating that was irradiated for 10 s.

Meanwhile, no

significant surface reconstruction was observed on the coatings that were irradiated for 30 and 300 s.

Figure 4.

Water CA variations as a function of time on F3 coatings that were irradiated for 5, 10, 30, and 300 s, respectively.

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Figure 3a plots the variation in the static or equilibrium water CAs, measured 5 min after droplet dispensing, as a function of time that was used to irradiate F3 coating samples.

The CAs increased steeply with time between 5 and 30 s and

plateaued after 60 s.

These trends were consistent with those observed for the

double bond conversion data obtained via FTIR characterization, suggesting that double bond crosslinking was essential for impeding surface reconstruction. Figure 3a also plots the variation in the SAs of 30-µL water droplets as a function of F3 irradiation time. The SA variation trend contradicted the CA variation trend, i.e. the SA decreased as photolysis time increased and levelled off above the photolysis time of 120 s. The water CA variation trend also contrasted with that exhibited by the hexadecane CAs (Figure 3b). higher.

At shorter irradiation times, the hexadecane CAs were

They decreased with irradiation time initially and plateaued after 60 s.

The

higher CAs at shorter irradiation times might be due to the migration of the PDMS component to the surface under a hexadecane droplet, which should be more compatible with PDMS but helped increase hexadecane CAs. At irradiation times shorter than 30 s, hexadecane spread on the coatings.

It

started to glide down cleanly only on samples that were irradiated for 30 s and longer. The SA plateaued only after the irradiation time exceeded 120 s (Figure 3b). We also performed an ink contraction test on samples irradiated for different periods of time and the results are shown in Figure 5.

No ink contraction was

observed on samples that were irradiated for less than 30 s. 15


The ink contraction


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capability further improved only beyond the irradiation time of 30 s and excelled at the irradiation time of 300 s.

Figure 5.

Ink traces left on F3 coatings that were irradiated for 2, 5, 10, 30, 60, 120, and 300 s.

The data discussed above suggest that the CAs and SAs plateaued at shorter photolysis times than that required for the emergence of the ink shrinking behavior. Thus, ink shrinking is a more challenging test than the liquid CA and SA tests. While liquid CA and SA values have often been reported in the past,12-16 high CA values and clean liquid sliding do not automatically indicate that a coating possesses anti-smudge properties.

This study suggests that a high degree of coating

crosslinking is essential for achieving a robust and full-fledged anti-smudge coating. NP-GLIDE Coating.

F3 coatings that had been irradiated for 300 s were robust.

Figure 6a compares photographs taken of dyed hexadecane droplets that were immediately and 4 s after their dispense on an uncoated and a F3-coated glass plate. By 4 s, the droplet had cleanly slid down the F3-coated plate. droplet spread on the uncoated plate.

On the other hand, the

Other organic solvents including

16


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diiodomethane, decane, and dodecane, which had surface tensions of 50.8, 23.8, and 25.4 mN/m also cleanly glided down these coatings, exhibiting respective SAs of 12 ± 2°, 2 ± 1°, and 2 ± 1° for 5 µL droplets. We suspect these coatings should be similar to the PDMS-containing PU- or epoxy-based coatings that we have examined in the past and thus they should repel all liquids with surface tensions above ~ 23 mN/m.2-3

Figure 6. a) Photographs showing the clean sliding of hexadecane on a F3-coated glass slide in contrast with its spreading on an uncoated slide. b) Photographs showing the contraction of a red paint on a F3-coated glass slide but the consistent covering of an uncoated slide by this paint.

We note that the SA of 66 ± 4o for 20-µL droplets on F3 containing 2.5 wt% of PDMS was substantially higher than those on the prior PU or epoxy-based NP-GLIDE coatings.1-2

To gain insight into this, we analyzed this coating by XPS

and determined a Si atomic abundance of 6.2% on the coating surface (SI).

This

value was significantly lower than 11.7% reported for a NP-GLIDE PU coating that had a bulk PDMS content of 9.0 wt%1 or than 14.9% reported for a NP-GLIDE epoxy coating that had a bulk PDMS content of 7.4 wt%.2 lower PDMS bulk content of 2.5 wt%.

Indeed, the F3 coating had a

To examine the effect of increasing the bulk 17



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PDMS content, we prepared another F3 coating that contained 5.0 wt% of PDMS. The SA determined for 20 µL water droplets was still 67 ± 2°, which was the same as 66 ± 4o on the F3 coating containing 2.5 wt% of PDMS.

Thus, we suspect that the

surface PDMS content was already saturated by the bulk PDMS content of 2.5 wt% and did not further increase with bulk PDMS content beyond this value. We further speculate that the surface PDMS content difference between the current coating and the prior NP-GLIDE coatings probably arose from the different curing protocols used and the different coating compositions. The coating composition difference should affect the extent of PDMS surface stratification because we believe that a higher incompatibility between PDMS and the coating matrix should promote PDMS surface enrichment.

The polyol P0 and

isocyanate precursors of a NP-GLIDE formulation should be more polar than P0II and TM.

Thus, the higher incompatibility between PDMS and the PU components

probably rendered a higher propensity for PDMS to segregate to the surface in a PU coating. We further speculate that the much faster room-temperature UV curing was not as effective as the thermal curing in promoting the surface stratification of PDMS.

To

check if this hypothesis was true, we subjected the F3 formulation containing 2.5 wt% PDMS to thermal curing at 170 °C for 2 h.

On this thermally cured coating, the SA

of 20 µL water droplets was decreased to 41 ± 1°, suggesting an increased extent of PDMS surface chain segregation.

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A more effective method to promote water sliding is to incorporate some free PDMS into a NP-GLIDE coating and to prepare a silicone-infused UV-curable NP-GLIDE coating.3

The infused free silicone increases the surface coverage by

PDMS because it, unlike P0II-g-PDMS, does not contain a tethering P0II backbone that competes with PDMS for surface sites. Despite the water SA, the F3 coating readily contracted paint sprays.

Figure 6b

compares the behavior of a commercial red paint after its application onto a fully cured F3 coating and onto a glass glide.

After 15 s, the paint was seen to have

almost fully contracted on the F3-coated glass slide.

However, the uncoated plate

remained covered by the paint by this time and thereafter. The nanopool-containing structure of the F3 coating was confirmed via atomic force microscopy (AFM).

Figure 7 compares the phase images of a cross section

and an upper surface of a cured F3 coating containing 5.0 wt% of PDMS, where the cross-section was obtained by microtoming a cured sample.

Also shown in Figure

S5 are a topography and a phase image for a cross-sectional surface of a F0 coating. A comparison between Figure 7a and Figure S5b allows the immediate conclusion of the existence of nanopools in the F3 but not in the F0 coating matrix.

However, the

nanopool structure of Figure 7b on the surface of the coating was not as evident as that observed on a previous epoxy NP-GLIDE coating.2

Nonetheless, the surface

had a biphasic structure and one component had to be PDMS, because Si was seen on the surface by XPS and the surface demonstrated antismudge properties.

19



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Page 20 of 27

The small diameters of the PDMS nanopools dictated that the coatings did not scatter much light and were highly transparent. While Figure 8a plots how the transmittance of 500 nm light through 32 µm coatings varied as a function of PDMS content, Figure S6a shows how the average transmittance of F3 samples at 2.5 wt% PDMS varied as a function of coating thickness up to the coating thickness of 83 µm. The transmittance was high (above 96%) for all the samples examined.

The optical

clarity can also be appreciated from the insert shown in Figure 8a where coatings containing different PDMS weight fractions were placed onto written text.

One can

crisply discern the text beneath the coatings regardless of the PDMS content.

Figure 7.

AFM phase images of a) the cross-section and b) the upper surface of a F3 coating cured for 300 s.

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Figure 8. a) Variation in the transmittance at 500 nm for 32 µm cured F3 coatings as a function of the PDMS weight fraction in the coatings. b) Variation of static CAs of water and hexadecane on the cured F3 coatings as a function of the PDMS weight fraction in the coatings. We also measured the CAs of water and hexadecane on F3 coatings at a fixed thickness of 32 µm but different PDMS contents or at a fixed PDMS content of 2.5 wt% but different thickness and the results were plotted in Figure 8b and Figure S6b, respectively.

The CAs did not change significantly with coating thickness as

anticipated.

More interestingly, they did not change with PDMS bulk content when

it exceeded 0.50 wt%. In contrast, the contact angles of these two test liquids on a series of NP-GLIDE epoxy coatings did not plateau until the bulk PDMS content surpassed 4.0 wt%.2

Thus, PDMS readily self-enriched on the coating surface in the

current UV-curable system.

However, the equilibrium or saturated surface PDMS

content as revealed from the contact angle values and XPS data was not as high as in the thermally-cured epoxy system. Surfaces with Patterned Wettability.

An anticipated advantage of a

UV-curable NP-GLIDE formulation is the creation of surfaces with patterned wettability.

In demonstrating this, we cut the letters Q and U and as well as star and 21



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moon symbols into aluminum foils.

Page 22 of 27

These masks were then placed between a UV

lamp and the films and the samples were irradiated.

After 5 min of irradiation, the

samples were then rinsed with acetone to remove the non-irradiated regions.

Upon

drying, each plate was scribbled on multiple times with a marker so that the ink covered and surrounded the irradiated region.

This treatment yielded the results

shown in Figure 9.

Figure 9. Anti-smudge patterns generated on glass plates after their coatings were irradiated with light passing through the photomasks (shown as inserts) and the coating precursor in the non-irradiated regions were removed with acetone. The patterns were revealed by multiple marker stokes that eventually surrounded the non-irradiated regions but contracted in the crosslinked regions. Figure 9 clearly shows that anti-smudge patterns were created on the glass plates. The reagents in the irradiated regions underwent crosslinking and were not rinsed away by acetone.

In contrast with the surrounding regions that were not irradiated,

these crosslinked regions exhibited anti-smudge behavior.

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Conclusions Reacting a limiting amount of PDMS-OH and HEMA with HDIT yielded a mixture of DM-PDMS and TM.

Mostly TM was prepared by reacting HEMA with

HDIT at a molar ratio of 3.21/1.00.

Furthermore, P0II or P0II-g-PDMS was prepared

by reacting a polyol P0 with 2-cyanatoethyl methacrylate or with PDMS-COCl and 2-cyanatoethyl methacrylate, respectively.

Formulations consisting of DM-PDMS

and TM (F1), P0II-g-PDMS and P0II (F2), or P0II-g-PDMS and TM (F3) were then cast along with 2-hydroxy-2-methylpropiophenone to yield films on glass plates. These films were subsequently vitrified via photolysis.

The F1 coatings were cloudy

probably due to the inability of the small DM head groups to prevent different PDMS domains from overlapping and forming PDMS-rich macrodomains.

Meanwhile, the

F2 coatings were optically clear but readily lost their ink shrinking ability after a single writing and erasing cycle.

Only the F3 formulation yielded transparent robust

anti-smudge coatings that withstood more than 30 writing and erasing cycles, suggesting that the bonding between P0II-g-PDMS and the crosslinked TM matrix was stronger than that between P0II-g-PDMS and the crosslinked P0II matrix. A F3 formulation containing 2.5 wt% PDMS was photocrosslinked to different degrees and the evolution of its wetting performance was monitored as a function of irradiation time.

This systematic study revealed that the hexadecane and water CA

and SA values plateaued at irradiation times longer than 60 s.

However, full-fledged

ink contraction ability developed only by irradiation time of ~ 300 s. contraction test is more stringent than the CA and SA tests. 23


Thus, the ink


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The P0II-g-PDMS and TM coating possessed the transparency, internal structure, as well as ink and paint contraction capabilities that are desired of an NP-GLIDE coating.

More importantly, the coating could be irradiated under masks and then

developed (via removal of the uncrosslinked polymer) to yield crosslinked domains with patterned wettability.

We anticipate practical applications for the developed

UV-curable antismudge coating and believe in the guidance value of lessons learned from this study.

Supporting Information.

The Supporting Information is available free of

charge on the ACS Publications website at DOI: xxx Experimental section, polymer characterization results, coating hardness and adhesion test results, XPS data for a F3 coating, and AFM images of a F0 coating. Corresponding author email: [email protected]

Author Information. ORCID:

Guojun Liu:

Acknowledgements.

0000-0002-9643-5070

The Lorama Group Inc., NSERC, and OCE are

acknowledged for sponsoring this research.

Liu thanks the CRC program for a

Canada Research Chair position in Materials Science.

We also thank Dr. Kunzhi

Shen for taking videos of the wettability tests, Dr. Igor Kozin for useful discussion on FTIR analysis, Jane Wang for taking the AFM images, and Dr. Gabrielle Schatte for obtaining the XPS data.

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Graphic for Manuscript

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UV-Curable Antismudge Coatings - ACS Applied Materials

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