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www.rsc.org/softmatter Volume 7 | Number 8 | 21 April 2011 | Pages ...

www.rsc.org/softmatter Volume 7 | Number 8 | 21 April 2011 | Pages 3653–4072

ISSN 1744-683X

PAPER

Genzer et al.

Photochromic materials with tunable

color and mechanical flexibility

1744-683X(2011)7:8;1-H


PAPER

www.rsc.org/softmatter | Soft Matter

Photochromic materials with tunable color and mechanical flexibility†

Hyun-Kwan Yang, A. Evren € Ozçam, Kirill Efimenko and Jan Genzer*

Received 3rd September 2010, Accepted 28th October 2010

DOI: 10.1039/c0sm00928h

Florescence switches based on photochromic compounds have been fabricated previously and

identified as potential candidates for information technology. Recently, optically responsive materials

with tunable color have been prepared by dissolving photochromic compounds, such as spiropyran

(SP), into solutions of various pH or embedding them into sol–gel matrices with adjusted chemical

compositions. Here we report on fabricating flexible rubbers with tunable color by embedding SP

molecules inside silicone elastomer networks (SENs) based on poly(vinylmethylsiloxane) (PVMS). SPcontaining

PVMS networks have been further modified either physically by exposing them to

ultraviolet/ozone treatment or chemically by attaching functional thiols to the vinyl bonds in PVMS via

UV-activated thiol–ene addition. The color hue of the SP–PVMS SENs after exposing to UV light

depends on either the UVO dose or the chemical end-functionality of the thiol modifier, respectively.

We also present simple methodologies enabling patterning regions in SP-doped SENs with various

shapes and colors.

Introduction

Photochromic compounds (PCs) represent a family of chemicals

whose color changes depending on the wavelength of light to

which they are exposed. 1 Spiropyran (SP) and its chemical

derivatives have been amongst the most-widely studied PCs for

more than half a century. 2 The color variation of SP is associated

with transformation from a closed form to an open form caused

by electromagnetic radiation, or application of mechanical

force. 3 While under visible light or at elevated temperatures SP

adopts the closed form, when exposed to UV light SP switches to

either a yellow protonated merocyanine (MEH) or a red merocyanine

(ME) open form. The MEH (strong absorption at 400

to 500 nm) and ME (strong absorption at 500 to 700 nm) forms

typically coexist; the color of the open form is dictated by the

relative population of the MEH- and ME-mers. In addition,

the transition of SP molecules from the non-polar closed form to

the polar (zwitterionic) open form results in increased wettability

in SP-based coatings. 4 Previous research has established

conclusively that the concentration of both forms depends

strongly on the nature of the surrounding matrix. To that end,

under acidic conditions, the open conformation of SP is

predominantly MEH (the reaction between proton and O in SP:

H + +O / –OH) while in other environments SP adopts the ME

open conformation (due to the hydrogen bonding with OH). The

stability of the open form depends on the environmental conditions

(i.e., wavelength of light or temperature) and the nature of

the matrix. While in polar environments the open form of SP is

relatively stable, in less polar or non-polar matrices, SP prefers

typically the more stable closed form. Hence, the rate of the back

reaction (open / closed) in a nonpolar solvent is higher than

that in polar solvents. 5–7

Department of Chemical & Biomolecular Engineering, North Carolina

State University, Raleigh, NC, 27965-7905, USA. E-mail: Jan_Genzer@

ncsu.edu; Tel: +1-919-515-2069

† Electronic supplementary information (ESI) available: Fig. S1, S2, and

S3. See DOI: 10.1039/c0sm00928h

Switching between the closed and open forms of SP has been

investigated traditionally in solution. Several groups have

demonstrated recently that SP can be incorporated successfully

into a solid matrix. Specifically, sol–gel methods have been

developed that enabled formation of solid-state SP-based materials.

The color type of the open form was adjusted by tailoring

the chemical composition of the sol–gel material. 8–10 While in

those studies SP was embedded physically into the matrix

materials, some researchers demonstrated that SP-based

compounds can also be bound chemically to soft polymeric

matrices. 11–14 The latter approach was then exploited in mechanochemical

studies where mechanical stresses acting onto the

material led to changes in color of the composite, which was

associated with the switch of PC compounds from their closed

form to the open form.

This study builds upon previous research pertaining to

embedding SP-based compounds into solid matrices. In contrast

to previous work, however, we use flexible silicone elastomer

network (SEN) as a matrix material. As will be demonstrated

later in the paper, the SEN platform offers a few key advantages

over the aforementioned sol–gel method. First, the intrinsic

flexibility of SENs enables fabrication of flexible light-responsive

films. Second chemically tailored SENs offer the ability to

change gradually the matrix material color (determined by the

relative population of the MEH and ME forms) ranging from

purple to yellow. Third, by chemical patterning, various portions

of the SP-based SENs can be tailored to possess various colors

when exposed to UV light. Finally, functional SENs can be

converted into mechanically harder materials. Combining the

flexibility of the SENs with their ability to be converted into

semi-rigid solids further facilitates preparing films with various

shapes and forms.

Results and discussion

Fig. 1 depicts schematically the formation of SP-based SENs.

Detailed description of the preparation procedures is given in the

3766 | Soft Matter, 2011, 7, 3766–3774 This journal is ª The Royal Society of Chemistry 2011


Experimental section. Briefly, SP–PVMS SEN composites are

prepared either: (1) by mixing PVMS/SP/cross-linker/catalyst and

curing the network or (2) by first preparing PVMS SENs and

impregnating them with SP. The loading of SP inside SP–PVMS

networks generated by the impregnation method is generally

smaller than that formed by mixing. While the concentration of

spiropyran in SP-doped PVMS film can be determined by the

Beer–Lambert law, we do not attempt to do so here. The actual

concentration of SP in the impregnated sample may vary from

sample to sample. Specifically, depending on the processing

conditions the SP concentration in the SP-doped PVMS SENs

ranges from 40 to 80% of the concentration present in the

specimen prepared by the mixing technique. Because the SP

concentration does not alter significantly the actual color of the

SP-doped PVMS network (see Fig. S1 in the ESI†), the conclusions

presented herein are all generally valid. As will be demonstrated

later in the paper, the chemical composition of the PVMS

SEN can be tailored chemically by implementing thiol–ene addition

reaction of short thiols with various chemical compositions.

Upon UV irradiation, the color of the SP-based PVMS network

can thus be tuned to range from purple to yellow, depending on

the type of thiol molecule used in the thiol–ene addition reaction.

At ambient conditions the rate of transformation from the

closed form to the open form depends on the UV dose that is

Fig. 1 (A) Poly(vinylmethylsiloxane) (PVMS) network is formed by

cross-linking PVMS with poly(vinylmethoxysiloxane) (PVMES) in the

presence of Sn octoate as a catalyst (the cartoon is not drawn to scale).

(B) Spioropyran (SP)-containing PVMS networks are modified by

soaking the PVMS networks with low molecular weight thiol–ene reaction

with alkanethiols and exposing to UV light of 254 nm for various

time intervals (see text for details). SP is introduced into the PVMS

network by either (1) mixing with the individual network-forming

components (upper right) or (2) by swelling the PVMS network with

tetrahydrofuran and exposing to SP solution (lower right). The bottom

portion of the figure depicts the closed and open forms of SP.

proportional to the UV irradiation time. In SP–PVMS SENs

prepared by the mixing method the fraction of the open form SP

increases with increasing UV light exposure time, as demonstrated

by the images in Fig. 2. Increasing the UV dosage leads to

the increase of the absorption peak at 550 nm, as documented

by the corresponding UV/Vis spectra in Fig. 2A. The reverse

transformation from the open form to the closed form can be

triggered by either exposing the specimen to visible light or by

increasing temperature. As can be seen from the photographs

taken under ambient conditions in visible light, the time it takes

to switch from the open form to the closed form depends strongly

on the initial UV dose. Samples kept under UV light for a short

time (1 min) exhibit the return to the closed form within the

first 24 h, while the longer exposure to UV irradiation retards the

relaxation ability of PVMS-embedded SPs to the closed form.

One way to decrease the transition time from the open to the

closed form is to increase the temperature of the sample, as

indicated in the top row of Fig. 2.

The time it takes for the specimen to return from the colored

state (after UV irradiation) to the colorless form (after exposing

to visible light or after increasing temperature) will also depend

on the population of the MEH and ME open forms and their

relaxation rates to the closed form of SP. The populations and

relaxation rates of both open forms of SP will, in turn, depend on

the chemical nature of the matrix material in which they are

embedded. To illustrate the above points, SP-doped PVMS

networks were first subjected to ultraviolet/ozone (UVO) for 3

min and subsequently exposed to visible light. In Fig. 3 the

corresponding UV/Vis spectra and optical images of a few

selected samples are shown. As apparent, the color of the samples

changes slowly from dark brown to yellow over 48 h. Importantly,

the UV/Vis spectra reveal that the SP molecules

embedded in PVMS exposed to UVO adopt simultaneously and

freely the ME and MEH forms when irradiated with UV light.

The relative population of the two open forms of SP and their

relaxation rates must be considered in the context of matrix

chemical changes that occur inside the PVMS network under the

UVO treatment. Previous work in our group has revealed that

the exposure of silicone networks to UVO treatment causes

a series of events that affect significantly the chemical and

physical nature of matrix material. 15 Oxygen radicals formed

during the UVO process react with the elastomer material and

can result in irreversible transformation of chemical and

mechanical properties of PVMS. Even at short UVO exposure

times the originally hydrophobic PVMS matrix gets converted

progressively into a hydrophilic material. This, in turn, determines

the relative populations of the ME and MEH forms of SP

and also their relaxation rates. As demonstrated in Fig. 4,

increasing the UVO treatment leads to the generation of a large

fraction of hydroxyl and carboxyl groups as indicated by

increased population of the MEH form (strong absorption

between 400 and 500 nm). This finding is supported by IR spectra

shown in Fig. 4, which indicate clearly an increase in the

concentration of –OH groups (3200–3600 cm 1 ) and a corresponding

increase in the concentration of –COOH (1720 cm 1 ).

While we cannot comment on the detailed molecular level

interaction between SP and the newly generated functional

groups inside the UVO-treated PVMS SEN, we can make a few

conclusions about the relative population of the MEH and ME

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Fig. 2 Optical images of spiropyran-doped PVMS networks after irradiation with UV light (l ¼ 254 nm) for various times at 25 C (bottom-left) and 3

min UV exposure and aging at various temperatures (top-right). Corresponding UV/Vis spectra after exposing to UV light for various times at 25 C (A)

and subsequent exposure to visible light at 25 C for 1 day (B) are also shown. The various lines on the UV/Vis spectra indicate UV exposure times.

forms depending on the matrix material. Increasing UVO

treatment time results initially in a fast increase of the ME open

form and a subsequent increase in the MEH form. Comparing

this observation with the rate of formation of the –OH and

–COOH functionalities, we arrive to the conclusion that the ME

form interacts strongly with the –OH groups and the MEH form

prefers the –COOH environment. We also note that the UV/Vis

spectra in Fig. 3 reveal that while the ME open form (500–700

nm) returns to the closed form relatively fast, the MEH form

(400–500 nm) is fairly stable for SP-doped PVMS networks after

UVO irradiation. Consequently, the fact that the relaxation

times from the open to the closed form of SP-doped PVMS

networks exposed to UV light are shorter than those in SP-doped

PVMS networks exposed to UVO for the same amount of time

can be explained by the different relaxation rates of the MEH

and ME forms to the closed form due to the interactions with the

matrix material. To test these hypotheses, we fabricated SPcontaining

SENs that were modified with either –COOH or –OH

functionalities (the method leading to the formation of such

networks based on thiol–ene reaction of functionalized thiols is

described later in the paper). After exposing the specimens to UV

light for a brief period of time, we monitored the visual

appearance and UV/Vis spectra of the specimens as a function of

time. The data, shown in Fig. S2 of the ESI†, confirm that the

MEH form interacts strongly with the –COOH based matrix

while the ME form prefers the –OH based matrix. Our data also

confirm that the relaxation kinetics of the MEH open form is

much slower than that of the ME form.

After the initial exposure of SP-doped PVMS networks to

UVO and relaxation from the open to the closed form, we have

re-exposed the samples briefly to another UVO cycle. The relative

populations of the open form of SP after the UVO reexposure

were different than those observed during the initial

UVO treatment. Moreover, the changes depended critically on

the initial UVO treatment time. Specifically, the population of

the open forms of SP after the second UVO exposure is higher

than that observed after the initial short UVO treatment only in

cases when the initial treatment was relatively short (2.5 min).

Samples that were exposed initially to the UVO for longer time

periods exhibited a lower population of the open form of SP after

the second UVO exposure (12.5 and 42.5 min). Interestingly,

specimens irradiated with UVO for very long periods of time

3768 | Soft Matter, 2011, 7, 3766–3774 This journal is ª The Royal Society of Chemistry 2011


Fig. 3 UV/Vis spectra and optical images of spiropyran-doped PVMS

networks after exposure to UVO treatment for 3 minutes and subsequent

exposure to visible light.

excitation and relaxation rates between the closed and open

forms of SP is the altered hardness of the matrix material due to

UVO modification which may hinder the ability of SP to alternate

between the closed and open forms. We will address this

point later in the paper. We note that the UVO exposure does not

lead to any changes in the structure of SP, as documented by

NMR taken before and after UVO exposure of SP (see Fig. S3 in

the ESI†).

The capability to vary the extent of PVMS hydrophilization

via UVO treatment was, in turn, explored to pattern various

regions of the matrix material by exposing the SP-doped PVMS

specimens to the UVO treatment across blocking masks of

various shapes. Capitalizing on the ability to tune the degree of

modification of the substrate (and thus the nature of the open

form of SP) by tailoring the UVO dosage delivered into the

specimen, the samples were exposed to the UVO across three

different masks thus creating six regions on the samples that

received various doses ranging from 0 to 42.5 min (cf. Fig. 5 in

the middle). After the UVO treatment, the samples were exposed

to UV light at variable temperatures. As seen before, the sample

color resulted from the competition between the UV light

(promoting the open form) and higher temperature (promoting

the closed form). While increasing the temperature led to the

transition from the open to the closed form, at lower temperatures,

the UV irradiation stimulated the excitation of SP molecules

into their open form. The UV/heat cycle was fully

Fig. 4 (Left) UV/Vis spectra collected from SP-doped PVMS networks

exposed to UVO for 2.5 (top row, red), 12.5 (middle row, green), and 42.5

(bottom row, blue) minutes immediately after the UVO treatment (solid

lines), after exposure to visible light for 1 week (dotted lines), and reexposure

to UV light for 1 minute (dashed line). The solid grey thin line

denotes the UV/Vis spectrum of parent SP-doped PVMS network.

(Right) IR spectra of SP-doped PVMS networks after exposure to UVO

for various times.

(42.5 min) and were subsequently exposed to visible light do not

exhibit dramatic changes after the second re-exposure to the

UVO light (cf. dotted and dashed blue lines). These experiments

confirm our earlier observations, namely, that the transition

from the open to the closed form depends on the chemical nature

of the matrix, which governs the lifetime of the individual open

forms of SP. An additional factor contributing to the altered

Fig. 5 Spiropyran-doped PVMS sample was exposed to UV/ozone

treatment across three masks for various times (top panel). The figure

depicts optical images of the sample after exposing to UV light for

3 minutes at various temperatures.

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eversible. Exposure of SP to UV or UVO (for the dosages

studied here) did not alter the chemical nature of the SP molecule,

as confirmed by NMR before and after the respective

irradiation (see ESI†).

The UVO treatment of silicone elastomer networks affects not

only the chemical nature of the matrix but also its physical

properties. Specifically, we have established previously that UVO

treatment results in hardening of portions of the network

exposed to prolonged UVO treatment. 16 In order to establish the

effect of UVO treatment on the surface mechanical properties of

SP-doped SENs we have carried a series of mechanical tests by

nanoindentation. In Fig. 6 we plot the reduced modulus (E r )of

bare PVMS (blue triangles), SP–PVMS prepared by the mixing

method (red squares) and that of SP–PVMS generated by the

impregnation method (green circles). The penetration depth of

the tip was 1 mm. At short UVO times, the E r for PVMS stays

approximately constant but increases considerably for prolonged

UVO exposure. E r for SP–PVMS prepared by impregnation/

swelling is lower than that of bare PVMS and raises only

marginally with increasing the UVO time (30% increase over 40

min of UVO exposure time). E r for SP–PVMS formed by the

mixing technique is the lowest of all moduli and it remains

constant for all UVO times studied. The decrease in the modulus

of SP–PVMS composites compared to PVMS is caused by the

presence of SP molecules embedded inside the PVMS matrix;

higher loadings of SP lead to lower E r values. Increasing UVO

time leads to the formation of reactive functionalities (i.e.,

carboxyls, hydroxyls, aldehydes, ketones, radicals) inside PVMS,

which contribute to subsequent densification of the network due

to the formation of additional cross-link points within the PVMS

network. 15 The process comprises various physical routes that

are too complex to deconvolute completely. It is known that

UVO converts molecular oxygen (O 2 ) into ozone (O 3 ) and very

reactive atomic oxygen [O] molecules. The conversion of O 2 into

O 3 and [O] above the sample produces large amounts of reactive

species that start to modify the PVMS specimen close to the

Fig. 6 Reduced modulus of PVMS/SP and PVMS networks (E r )as

a function of UVO treatment time (E r is defined in the Experimental

section). Results for PVMS/SP are shown for samples prepared by the

mixing method (squares) and the swelling method (circles). The tip

penetration depth is 1.0 mm.

surface. Activation of O 2 molecules trapped inside the PVMS

network is not very likely as PVMS is not transparent for

wavelengths below 240 nm and becomes even less transparent

with increasing UVO treatment time. 15 Interestingly, we have

noticed recently that the conversion from elastomers to solid-like

materials can be induced by UV alone. 17 This would suggest that

the conversion has to start at the surface and with time the

modification progresses deeper into the sample because of the

penetration of the reactive species. While we cannot provide

direct molecular-level description of phenomena contributing to

the conversion of flexible PVMS to a glass-like material, we note

that the process is quite rapid (10–20 min) and it has to originate

at the very surface of the PVMS film. Doping PVMS coatings

with SP causes two effects. First, the presence of SP in the PVMS

SEN prevents the formation of additional cross-link points

within SEN because of steric hindrance; interestingly SP does not

lower the glass transition of PVMS. Second, SP absorbs UV light

(leading to the transformation of the closed form to the open

form) and hence minimizes additional UV-induced modification

of PVMS. Considering that the concentration of SP inside the

PVMS SEN matrix prepared by the impregnation method is

lower than that in specimens formed by the mixing technique one

would expect that the E r of the former samples would be smaller

than that of the latter ones. This is exactly what we detect in our

experiments.

One of the benefits of using PVMS SEN as a matrix material is

its ability to be modified chemically. To this end, the PVMS

networks were reacted with functional thiols of various endfunctionalities.

The chemical nature of the thiol molecules was

expected to influence the type of the open form of SP. The thiolmodified

PVMS coating was then dipped into THF solution of

SP for 1 minute, washed with MeOH, and exposed to UV irradiation

for 1 minute, the appearance of the film changed from

transparent to colored. The color of the UV-exposed films

depended on the type of thiol molecule used. Fig. 7 summarizes

the data. Parent SP–PVMS samples (cf. Fig. 7A) converted into

a purple color (cf. Fig. 7B) when exposed to UV light, indicating

strong abundance of the MEH open form. Specimens modified

with 1-propanethiol were hydrophobic; they resulted in light

yellow color after exposing the sample to UV light (cf. Fig. 7C).

We also attached two hydrophilic moieties to the PVMS

network. SP-doped PVMS networks modified with 3-mercaptopropionic

acid (MPA) and 2-mercaptoethanol (BME) turned

yellow (cf. Fig. 7D) and bright purple (cf. Fig. 7E), respectively,

when exposed to UV irradiation. In the MPA matrix, SP adopts

primarily the MEH open form after exposure to UV light which

is due to the presence of acidic proton in the thiol. In the BME

matrix, most spiropyrans adopt the ME open form when irradiated

with UV light due to hydrogen bonding between O in SP

and –OH in BME. In addition, a matrix comprising a mixture of

MPA and BME resulted in an orange color (cf. Fig. 7F); here

these two strong absorption peaks indicate the coexistence of the

MEH and ME open forms.

The ability to change the color of the SP-doped PVMS films by

varying the chemical nature of the matrix can, in turn, be

employed in generating specimens comprising color gradients.

The preparation of a specimen whose color changes gradually

from red to yellow is described in Fig. 8. First, a PVMS SEN

sheet was prepared (cf. Fig. 8A) as described in the Experimental

3770 | Soft Matter, 2011, 7, 3766–3774 This journal is ª The Royal Society of Chemistry 2011


Fig. 7 UV/Vis spectra and optical images of spiropyran-doped PVMS networks before (A) and after (B–F) irradiation with UV light (l ¼ 254 nm). The

color (and UV/Vis spectrum) of the spiropyran-doped PVMS network after UV irradiation depends on the chemical environment in which the spiropyran

molecules are placed; they are either (B) dark purple for parent PVMS, (C) light yellow for PVMS modified with methyl-terminated thiol, (D)

dark yellow for PVMS modified with carboxy-terminated thiol, (E) light purple for PVMS modified with hydroxy-terminated thiol, and (F) orange for

PVMS modified with a mixture of carboxy- and hydroxyl-terminated thiols. All samples were irradiated for 1 minute.

section. Subsequently, a sheet of poly(dimethylsiloxane) (PDMS)

network (thickness z 0.5 mm) with a rectangular opening in the

center was placed on top of the substrate (cf. Fig. 8B) and the

resultant well was filled with MPA (cf. Fig. 8C). The well was

subsequently covered with a mask bearing a transparency

gradient (cf. Fig. 8D) through which it was exposed to UV light

(l ¼ 254 nm) for 30 min. During the UV light exposure thiol–ene

addition reaction occurred between MPA molecules and vinyl

groups present in PVMS. The concentration of MPA grafts in

the PVMS matrix was expected to increase with increasing the

Fig. 8 Preparation of substrates with gradients in color. Spiropyran-doped PVMS network (A) is covered with a PDMS-based well (B) that is filled with

3-mercaptopropionic acid (MPA) (C). The system is enclosed with a mask bearing a transparency gradient (D) and exposed to UV light (l ¼ 254 nm).

The substrate contains a gradient in MPA (E). After exposing to UV light for 30 minutes, the substrate exhibits gradual variation in color (F).

This journal is ª The Royal Society of Chemistry 2011 Soft Matter, 2011, 7, 3766–3774 | 3771


dosage of UV light and should thus vary gradually along the

sample (cf. Fig. 8E). We assumed that no significant diffusion

occurred in the MPA solution and hence thiol-radical formed in

the regions that received a higher dosage of UV light reacted

quickly with neighboring vinyls. After removing the samples

from the assembly and washing with MeOH, the specimens were

exposed to THF solution of SP for 1 minute, washed with

MeOH, and exposed to UV light (l ¼ 254 nm) for 1 minute. As

indicated by the optical image shown in Fig. 8F, the sample color

changed gradually from yellow (right) to red (left). The yellow

color is associated with the MEH form due to the high concentration

of MPA. Moving along the sample from the right to the

left, the concentration of the MPA groups decreased while that

of free vinyl increased resulting in mixing of purple and yellow

colors that yields the resultant red appearance (mixture of ME

and MEH open forms of SP). This method can, obviously, be

extended to other thiols and even more complex gradient

arrangements.

Our final installment illustrating the generality of our

approach involves demonstration of chemical patterning of

selected portions of the substrate with various shapes having

different colors. This method is more general than that described

in Fig. 3 because the color of UV-irradiated sample depends

exclusively on the type of thiol used. The specimens were

prepared by an analog to the method described earlier in Fig. 8.

Specifically, PVMS SEN film was formed and was covered with

a PDMS sheet bearing holes of various shapes. The thus formed

wells were filled with selected thiols (note that each well can be

filled with a different thiol), and the substrate was covered with

quartz slide. Subsequent exposure of the sample through the

quartz slide to UV light (l ¼ 254 nm) for 30 min resulted in thiol–

ene grafting of the thiols to the PVMS network. The specimens

were then removed from the assembly and exposed to THF

solution of SP for 1 minute, washed with MeOH, and exposed to

UV light (l ¼ 254 nm) for 1 minute in order to induce the

transition of SP from its closed form to the open form. The

spatial arrangement of the colors inside the sample depends on

the shape of the PDMS mask and the type of thiol used. We

formed a ‘‘Big Dipper’’ shape by attaching chemically 1-propanethiol

in the form of 7 stars to the PVMS matrix. While at

ambient conditions the sample was completely transparent (cf.

Fig. 9A), after exposure to UV light for 1 min, the sample

featured 7 yellow stars resting in purple matrix (cf. Fig. 9B).

Fig. 9C–E demonstrate that the resulting samples are completely

flexible, both under ambient conditions and after exposure to UV

light. They can be folded and stretched without losing their color.

The color is completely reversible. Leaving the UV-exposed

samples in visible light for 1 h resulted in de-coloring of the

specimens (not shown). In agreement with our earlier observation,

the color of the patterned areas can be varied readily by

attaching chemically thiol molecules to the PVMS matrix (cf.

Fig. 9F–I).

Conclusions

In summary, we developed a novel method leading to the

formation of flexible substrates that change color upon UV illumination.

We used flexible silicone elastomers as matrices for

photochromic spiropyran molecules and demonstrated that the

sample color can be adjusted by chemically altering the matrix

environment without compromising the sample flexibility. We

have also demonstrated that the color inside the specimen can be

adjusted to either possess distinct chemical patterns (multiple of

each sample) or can change gradually as a function of position on

the sample. While we have detected a small increase in modulus in

SP–PVMS composites upon exposure to UVO treatment, the

materials remained relatively soft. It remains to be seen whether

the materials harden when UVO times in excess of 42 min are used.

While this new technique offers new opportunities for creating

novel functional materials, it is not perfect. One main drawback

is that the SP molecules are not linked chemically to the PVMS

matrix. While this arrangement enables facile transition between

the closed and open forms of SP, some SP molecules can be

released from the network when exposed to organic solvents that

swell PVMS. We have not studied this phenomenon in detail but

we noted that in SP–PVMS matrices under exposure to aqueous

solution no release of SP has been detected. In the future, it may

be worth developing chemical routes that would enable chemical

attachment of SP to the matrix and study the effect of the

confinement on the molecular rearrangement of the

photochromes.

Experimental

Hydroxy-terminated poly(vinylmethylsiloxane) (PVMS) was

synthesized using the methodology described elsewhere. 18 The

molecular weight of PVMS used in this study was 35 kDa, as

determined by size exclusion chromatography (SEC). Complete

description of the SEC characterization of PVMS can be found

in ref. 19. The SENs were formed by cross-linking PVMS with

poly(vinylmethoxysiloxane) (PVMES, Gelest Inc. PA, USA) by

means of a Sn-based catalyst. Chemical modification of PVMS

SENs was accomplished by implementing thiol–ene addition of

short functionalized thiols (i.e., 3-mercaptopropionic acid, 2-

mercaptoethanol, 1-propanethiol) with different polarities.

Thiol–ene reactions were initiated by means of a pen-Ray 5.5

watt low pressure mercury arc lamp (Ultra-Violet Product,

Upland, CA) with primary output at 254 nm. No photoinitiator

was used. After the attachment of chemical compounds, the film

was washed by using the ultrasonic cleaner with methanol for 1

hour. The presence of the thiol in PVMS was confirmed via FT-

IR by the decrease of the C]C peaks at 960, 1407, and 1587

cm 1 . The C]O stretching vibration of a carboxyl group in

MPA appeared at 1720 cm 1 . The stretch of the –OH peak in

BME was located between 3200 and 3600 cm 1 . The –CH 2 – and

–CH 3 stretches in 1-propanethiol appeared at 1454 cm 1 and

1376 cm 1 .

SP-doped PVMS films were prepared by two methods: (1)

mixing method and (2) swelling method. In the mixing technique,

SP was mixed with PVMS and the PVMES cross-linker using

ethyl acetate as a solvent and the mixture was allowed to form

a gel. In the swelling method, SP was impregnated by dipping the

transparent flexible PVMS network into a THF solution of SP.

While there is a small difference in the loading of SP inside the

PVMS SEN (see Fig. S1 in the ESI†), we use those two methods

interchangeably because we are concerned primarily with qualitative

description of the behavior. More quantitative studies are

currently underway.

3772 | Soft Matter, 2011, 7, 3766–3774 This journal is ª The Royal Society of Chemistry 2011


Fig. 9 Patterning of spiropyran-doped PVMS substrates with various shapes. The procedure is the same as described in Fig. 6, except the PDMS well is

covered with quartz substrate. The ‘‘big dipper’’ shape is formed by filling the PDMS well pattern with 3-mercaptopropionic acid (MPA) (A). After

exposing the substrate to UV light (l ¼ 254 nm) (B), the parent spiropyran-doped PVMS network turns dark purple, while the regions containing MPA

are yellow. The substrates are flexible both before (C) and after UV exposure (D and E). Filling the PDMS well with thiol of various chemical functionalities

results in different colors of the thiol-exposed regions: (F) –COOH based, (G) –OH based, (H) –COOH and –OH based, and (I) –CH 3 based.

We used a low pressure Hg pen-Ray 5.5 Watt UV lamp (Ultra-

Violet Product, Upland, CA) with a primary output at 254 nm.

We measured the areal power density of 1.16 mW cm 2 for the

UV source–sample distance used in the irradiation experiments.

Ultraviolet/ozone (UVO) treatment of the PVMS surface was

carried out in a commercial UVO chamber (Jelight Company,

Inc., Model 42). The power of the UV lamp is 8 mWcm 2 .

Ultraviolet-visible (UV/Vis) spectroscopic studies were conducted

with the Jasco V-550 UV-vis spectrometer. The spectra

were collected over a frequency range from 4000 to 450 cm 1 with

a resolution of 2 cm 1 . Fourier transform infrared spectroscopy

in the attenuated total reflection mode (FTIR-ATR) was used to

characterize chemical changes using the Nicolet 6700 spectrometer.

Ge-crystal detector was used for collecting the data by 64

scans under extra dry airflow. Optical images were taken with

digital camera equipped with a 18–55 mm lens. Annealing was

performed using a temperature-adjustable hot plate (Corning).

Testing of the mechanical properties of SP-doped PVMS

networks was performed by using the Hysitron Triboindenter,

which was equipped with quasi-static and dynamic mode of

operation, force and displacement controlled (feedback) and an

integrated AFM head. Indentations were performed at room

temperature in the acoustic enclosure of the Triboindenter. A

46 mm conical diamond tip was used to perform the indentations

This journal is ª The Royal Society of Chemistry 2011 Soft Matter, 2011, 7, 3766–3774 | 3773


and the calibration of the diamond tip was performed on standard

fused quartz. Force–displacements curves of indents were

analyzed by using the Oliver–Pharr method by using the software

TriboScan supplied by Hysitron. Only the results of displacement

controlled quasi-static indentations were reported. The

reduced modulus (E r ) was calculated using eqn (1) and (2):

E r ¼ S p ffiffiffi p

p

2 ffiffiffiffi

(1)

A

1

E r

¼

1 n

2

E

þ

sample

1 n

2

E

indenter

where S is the stiffness of the unloading curve, A is the projected

contact area, and n is Poisson’s ratio (take to be 0.5). For

a standard diamond indenter probe, E indenter is 1140 GPa and

n indenter is 0.07.

Acknowledgements

This work was supported by the Office of Naval Research (Grant

No. N00014-07-1-0258). Partial support from the DTRA (Grant

No. W911NF-07-1-0090) is also acknowledged.

(2)

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