Experimental tests of the correlated chromophore domain model of self-healing in a dye-doped polymer

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by  Shiva K. Ramini

Experimental tests of the correlated chromophore domain model of self-healing in a
dye-doped polymer
∗
Shiva K. Ramini and Mark G. Kuzyk
Department of Physics and Astronomy,
Washington State University
Pullman, Washington 99164-2814
Sheng-Ting Hung
Department of Physics and Astronomy,
3
Washington State University
1
Pullman, Washington 99164-2814
0
and
2
Department of Chemistry

n Katholieke Universiteit Leuven
a Celestijnenlaan 200D
J B-3001 Leuven, Belgium

7 (Dated: December 11, 2013)

] Temperature dependent photodegradation and recovery studies of Dipserse Orange 11 (DO11)
h dyedissolved inpoly(methylmethacrylate)andpolystyrenepolymerhostsareusedasatest ofthe
p recently proposed correlated chromophore domain model.[1] This model posits that dye molecules
- form domains or aggregates. The nature of aggregation or how it mediates self healing is not yet
m
well understood. In this paper we present qualitative evidence that supports the hypothesis that
e the dye molecules undergo a change to a tautomer state with higher dipole moment and hydrogen
h bond with the amines and keto oxygensof the polymer. Groupings of such molecules in a polymer
c
chainformwhatwecalladomain,andinteractionsbetweenmoleculesinadomainmakethemmore
.
s robust to photodegradation and mediate self healing.
c
i
s PACSnumbers:
y
h
p I. INTRODUCTION a molecular level that we believe involves a cooperative
[ process of aggregates of molecules. The materials of in-

terest to our work have applications as optical materials
1 Structural damage and degradation of a polymer is
where photodegradation is a common cause of optical
v usually associated with cracking. Mitigating damage or
8 developingmethodstopromotehealinginpolymericma- and optoelectronic device failures, either as catastrophic
8 failure or a slow deterioration of the performance.[3–6]
terials after cracking is an active area of research moti-
0 vated by its practical utility. White and coworkers re- Though originating at the microscopic level, self heal-
1
ported on a structural polymeric material with the abil- ingindye-dopedpolymersalsoleadstomacroscopicheal-
.
1 ity to autonomically self-repair cracks.[2] Such polymers ing. For example, DesAutels et al have shownthat holes
0 incorporateamicroencapsulatedhealingagentthatisre- drilled into a dye-doped polymer with high-fluence laser
3 pulses heal, i.e. contract over time, in the presence of
leasedin the crackingprocess with polymerizationbeing
1
: triggeredbycontactofacatalystwiththe healingagent, the dye.[7] Without the dopant molecules, the holes en-
v large and develop ragged edges. Analogously, self heal-
thus bonding the crack faces. White observed as much
Xi as 75% recovery in toughness. ing is not observed when molecules are in solution in a
monomer solvent,[8] but only in solid solution with the
r Our work presented here is different in two regards.
a First, the sample is a dye-doped polymer rather than polymerasthehost.[9]Thus,thehealingprocessappears
to be a cooperative one between the solute dyes and the
a neat polymer and the degradation process is through
polymer.
optically-induced burning, so chemical changes are in-
The discovery of reversible photodegradation was a
duced rather than solely mechanical/structural damage
serendipitous one. Peng first observed recovery of fluo-
- though cracking can accompany burning. The dopant
rescence after photodegradationof a dye-doped polymer
molecules thus mediate the phenomena. The degree of
optical fiber,[10] and, Howell and Kuzyk discoveredthat
damageisobservedusingopticaltechniques,thesimplest
Disperse Orange11(DO11)dye dissolvedin poly (lethyl
of which is the detection of a color change. Secondly,
methacrylate) (PMMA) polymer undergoes full recov-
the healing process is a microscopic one, originating at
eryafterphotodegradationusingAmplifiedSpontaneous
Emission(ASE)tomonitortheprocess.[9,11]Embayeet
al showed that molecules photo degrade to a long-lived
∗Current address: Wyatt Technology Corporation, 6300 Hollister damagedstate,whichrequiresthepolymerhost.[12]The
Ave,SantaBarbara,CA93117-3253 processwas modeled using two higher-energystates, one
2
the damaged state, that both decay back to the undam-
TABLEI:Parameters determinedfor DO11in PMMA using
agedstate. These stateswereprobedwithlinearabsorp- 2
tion spectroscopy. Ramini et al further investigated the an average pumpintensity of Ip =0.202W/cm .
underlying mechanisms and showed that dye molecules α(min−1W−1cm2) β(10−4min−1) δµ(eV)
when embedded in polymer matrix appear to form do- Decay Rate Recovery Rate Free energy/molecule
mains, and cooperative effects within a domain lead to 7.09(±0.13) 3.22(±0.26) 0.29(±0.01)
both protection from photo-damage and recovery.[1]
There have been many proposed mechanisms for re-
covery, including photo reorientation (orientational hole fit the data wellfor DO11 dye doped in PMMA polymer
burning),photo-inducedisomerization,anddiffusion. All as a function of concentration, time, and intensity.[1]
of these have been shown not to be responsible.[12, 13] Integrating Equation 1 yields
Our present working hypothesis is that domains are in-
volvedandthatthedistributionofdomainsaregoverned (N αI/βN)n0
n(t)= − ,
by a condensation process.[1] Whilst the energetics of n +(N n αI/βN)exp[ (Nβ αI/N)t]
0 0
− − − −
the process are consistent with a tautomer of the DO11 (2)
moleculehydrogenbondingtothepolymerbackbone,[14] where n0 is the initial undamaged population at t=0.
the same aggregation model may be consistent with the When the pump intensity is turned off (I = 0), the
formation of a twisted charge transfer structure.[15] For population recovers according to
the purposes of this paper, the distinction between the
dn
two are unimportant. =βn(N n). (3)
In the present work, we provide additional tests dt −
of the correlated chromophore domain model using With n(t ) as the undamaged population right at the
0
temperature-dependent measurements. Past studies fo- time the pump laser is turned off, the solution to Equa-
cused on poly (methyl methacrylate) (PMMA) polymer tion 3 can be written as,
as the host. Here, we also test a random copolymer of
PMMA and polystyrene (PS) to investigate the effect of N
n= . (4)
changingthe polymer host. The new measurements pro- 1+ N 1 e(−Nβt)
vide further support for our model. hn(t0) − i
The above calculation is for a single domain. The ob-
II. CHROMOPHORE CORRELATION servedbehaviorofasample originatesinthe averagebe-
MODEL-BRIEF OVERVIEW havior over the domains. We assume that the domains
are distributed according to a condensation model. It is
In our earlier paper[1] we proposed a model of re- energetically favorable for a domain to form – the more
versiblephotodegradationofDisperseOrange11(DO11) molecules in a domain, the lower the energy. However,
dye doped in Poly(methyl methacrylate) (PMMA) that the temperature, kT breaks up the domains. At zero
has as its basis localized correlated regions of molecules, temperature, one would expect a single large domain,
which we call domains. These domains have not been andatinfinite temperature,anequalnumberofdomains
physically characterized; rather, indirect evidence sup- ofeachsize. Thus,sincetheaveragedomainsizeislarger
ports the hypothesis. Here we provide additional evi- atlowtemperature,therecoveryrateinthismodelwould
dence. increase as the temperature is lowered. This behavior is
To summarize the model, for a domain of size N, the opposite to a barrier model, in which elevated temper-
undamagedpopulationdecaysinproportiontotheinten- ature would accelerate healing. Thus, the temperature
sityandthepopulationofavailableundamagedmolecules dependence of photodegradation and recovery is a criti-
in the domain, n, while recovery is in proportion to the cal test of the theory.
number of damaged molecules, N n. Empirically, we Formally,thedomaindistributioniscalculatedbymin-
−
havedeterminedthatthebulkmaterialbehavesinaway imizingthe free energyofthe system. After considerable
suchthatthedecayrateisslowerforlargerdomains,and calculation, the distribution is found to be given by[1]
therecoveryrateisacceleratedinproportiontothenum-
N
ber of undamagedmolecules in a domain. This behavior 1 (1+2ρz) √1+4ρz
Ω(N)= − , (5)
is quantified by the phenomenological model of a single z (cid:20) 2ρz (cid:21)
domain given by
where z = exp δµ . Ω(N) is the number density of
dn αI kT
=βn(N n) n (1) domains of size(cid:16)N, (cid:17)ρ is the average number density of
dt − − N
molecules in the sample, δµ is the free energy advan-
where α is the rate per unit intensity at whichmolecules tage per molecule – which is the energy released when
decay,β istherecoveryrate,andI isthe intensityofthe a molecule is added to a domain, T is the temperature,
lightthatdamagesthesample. Thismodelwasshownto and k is the boltzmann constant.
3
UsingEquations2and5,the meannumberofundam-
aged molecules in a domain, n, is thus given by the en-
semble average,
∞
n(t;ρ,T,I,n )= n(t;N,I)Ω(N;ρ,T)
0
NX=1
∞
n(t)Ω(N)dN (6)
≈Z
1
Thesignalobservedfromopticalmeasurementsisrelated
to this mean population.
In our earlier paper, we used the
concentration/intenisity-dependent data to deter-
mine the material parameters. As a test of the model,
we used one set of parameters determined from popu-
lation decay data to predict the recovery dynamics at
several dye concentration and showed that experimental
FIG. 1: Custom sample holder and heating/cooling system
observations agree with the predictions. Table I sum-
used for temperaturedependent studies.
marizes the values of the three parameters in the model
which characterize a particular composite material, in
our case, DO11 dye in PMMA polymer.
copper heat exchanger is placed on top (hot side) of the
TE element to extract heat out of the system. A thick
film resistor is attached on the side to heat the sample
III. TEMPERATURE DEPENDENT
holder.
MEASUREMENTS
Twothrough-holesaredrilledintothealuminumblock,
onenormaltothesampletopassthepumplaserandthe
A. Experimental Setup
other one at a 300 angle to the normal for probing with
white light. The center axis of the two holes overlap
Weuseamplifiedspontaneousemission(ASE)asasen-
at the sample allowing the white light and pump laser
sitiveprobeofthepopulationofundamagedmoleculesby
to cross each other. An conical opening is made in the
taking advantage of the fact that undamaged molecules
side of the holder for emitted ASE to be detected. The
lasestronglywhiledamagedmoleculesdonot. Thesensi-
sample holder is placed on an insulating base made of
tivity of this measurement originates in the nonlinearity
‘Ultem’ plastic to avoidheat exchange with the environ-
of the process. Small changes in the population due to
ment. This plate is attached to an xyz-translation stage
damage yield large changes in the ASE intensity. The
(not shown). The whole setup is covered with a plexi-
experimental configuration is essentially a strong laser
glassenclosuretoreduceheatexchangewiththeenviron-
beam that both damages the sample and excites the un-
ment due to convection. Sapphire windows are installed
damagedmolecules,whichthenemitcoherentlight. The
in the enclosure walls where the beams enter and exit
ASE light, which is red, is emitted perpendicular to the
the enclosure. Desiccant is placed inside the enclosure
direction of the green pump beam. Thus, spectral and
to avoid water condensation at lower temperatures. A
spatialfilteringcanbeusedtosperatetheredsignalfrom
K-type thermocouple is inserted in the holder through
the green pump beam.
a drilled hole to measure the instantaneous tempera-
The sample chamber must provide a means for heat-
ture near the sample. Thermoelectric element, heating
ing and cooling the sample while passing the pump light
resistor, and thermocouple are connected to an Omega
and collecting the ASE light. There is also the option
CN7833 temperature controller which is interfaced with
of simultaneously measuring changes in absorbance by
a computer.
allowing a probe beam to overlap the burned area gen-
eratedby the pump. A schematic crosssectional view of
the chamber is shown in the Figure 1.
B. Results and Discussion
The sample holder is made of aluminum due to its
good thermal conductivity and is cooled using a ther-
1. Temperature Dependence
moelectric(TE) element. The TE element works on the
principle of the Peltier effect. When current is passed
through the terminals, it extracts heat from one side of A 10g/l DO11 dye mass to PMMA volume sample is
the element and deposits it out the other side. The TE used for the temperature-dependent experiments. This
elementisplacedontopofthe sampleholdersothatthe concentration is chosen because it produces good ASE
cooling side faces down and in contact with the sample signaland has a domaindistribution oflarge enoughav-
holder using thermal conductive paste. A water-cooled erage size to exhibit good self healing. The decay and
4
1.0
00..1101 Normalised ASE intensity000...468 136000000CCC pulation1.0 223890333KKK
1)0.09 0.2 d po0.9 331233KK
min- 0 2 4 Tim6e (minut8es) 10 12 14 age 333K
e (0.08 am0.8
ay rat0.07 d und
c e0.7
e z
D0.06 ali
m
or
0.05 N0.6
0.04
0.5
280 290 300 310 320 330 340 0 2 4 6 8 10 12 14 16
[h]
Temperature (K) Time (minutes)
FIG.2: Decay ratesestimated from single exponentialfitsto FIG. 4: Population decay upon irradiation at several tem-
the ASE decay data(inset) as a function of sample tempera- peratures along with the predictions from the Chromophore
ture. Correlation Model.

0.040 except a globalnormalizing constantat the beginning of
Equation 6 which takes into account the detector effi-
0.035 ciency and alignment changes from run to run.
ThetrendinFigures2and3canbeexplainedusingthe
0.030
n-1) chromophore correlation model as follows. As the tem-
ery rate (mi00..002205 pwtoeitrlhaotwtuherereintinegncrdoeefansacevyse,rftoahrgeemrmdoolaemlcmualioentsisotionz,efooarrmsenedtxorpomlpaayini,necsdolmeinapdetitnheges
ov literature[1]. If domains are caused by a condensation-
ec0.015
R likeprocess,the domainsizegrowsasthe temperatureis
decreasedanddissociatewhenthethetemperatureinin-
0.010
creased. In this picture, an increase in the temperature
0.005 leads to a decrease in the average domain size. With
smaller domains, there are fewer healthy molecules to
260 270 280 290 300 310 320 330 340
coax the damaged ones to heal. Since the decay rate is
Temperature (C) [h]
inversely proportional to the domain size, the net effect
is that as the temperature is increased,the decay rate of
FIG.3: Estimated recovery ratesfrom single exponentialfits
theundamagedpopulationincreases,consistentwithour
to therecovery data as a function of sample temperatures.
observationinFigure2. Therecoveryratealsodecreases
as the domain size decreases. This leads to a slower re-
coveryratealsoconsistentwithourobservations. Hence,
recovery data is obtained at several temperatures.
themodelcorrectlypredictsthetemperaturedependence
Ratherthanusing the fullmodel, we initiallyestimate
of self healing.
the decay and recovery rates using a single exponential
fit. Exponential functions yield a good fit to the data,
and provide a good estimate of the decay and recovery
rates. Figures2and3showthe decayandrecoveryrates 2. Nature of the Domains
so determined. Each data point at each temperature is
the weighted average of several runs. The data clearly While the chromophore correlation model correctly
shows that recovery is slower at high temperature and predictsthe behaviorofthe dye-polymersystem,the na-
is consistent with the domain model and eliminates the ture of aggregation and the role of the polymer is yet
barrier hypothesis. to be understood. Based on the fact that recovery of
Figure 4 shows population decay curves at six differ- ASE in DO11 is not observedin solution in liquid MMA
ent temperatures. The data is plotted along with the monomer, the solid polymer clearly plays an important
chromophore correlation model predictions (lines). The role. Kuzyketaldiscussedseveralpossiblesourcesofag-
predictions from the model are calculated using the pa- gregation and the role of the polymer.[14] The hypoth-
rameters in Table I. No adjustable parameters are used esis that interactions between a tautomer of the DO11
5
toberefinedinordertobettertestthehypothesisquanti-
tatively. Also,additionalexperimentsthatmeasureboth
decay and recovery;and, better estimate the various pa-
0.6 rameters are needed in order to strengthen our case.
-1 n)
mi
e ( 0.4
at
y r
a
ec 0.2 IV. CONCLUSIONS
D
We have shown that the correlated chromophore
0.0
domain model predicts all the observations of de-
0.25 0.50 0.75 1.00
PMMA proport ion in Copolymer cay/recovery of a dye-doped polymer as a function of
time, intensity, concentration, and temperature using
only three fixed parameters. Most importantly, the
-1 n) 0.6 temperature-dependent data – which shows accelerated
mi
e ( healingatdecreasedtemperatures,eliminatesthebarrier
at 0.4 hypothesis and is consistent with our domain picture of
y r the degradation and healing process.
a
c
De Also tested is the hypothesis that domain formation
0.2
originates in hydrogen-bonding of the DO11 molecules
with the PMMA polymer chain by evaluating copoly-
mers in which one of the components does not provide
0.0
bonding sites. Complementary evidence that supports
2 4 6 8 10 12
the domain picture is the observation that the decay
DO11 concentration in PMMA (g/l)
dynamics as a function of PMMA fraction is similar to
FIG. 5: (top) DO11 population decay rate as a function of the decay dynamics as a function of dye concentration.
PMMA content in PMMA/PS copolymer while dye concen- In both cases, the number of dye molecules in proxim-
tration is kept constant. (bottom) DO11 population decay ity to PMMA polymer chains is the same, so interac-
rate as a function of dye concentration in pure PMMA poly- tions between the two are responsible. Since hydrogen
mer.
bonding of the DO11 molecule with the polymer is the
mostpronounceddifferencebetweenPMMAandPS,and
the energyparameter δµ is consistentwith the hydrogen
molecule with the PMMA polymer chain is the origin bonding energy between a DO11 molecule and PMMA
of the interaction that forms a domain is based on the polymer, we propose that hydrogen bonding is the most
observation that the parameter δµ is about the same as obvious mechanism.
the hydrogenbinding energycalculatedforthe tautomer
interacting with PMMA.[14] It is also possible that it is Hydrogenbondingasthemechanismofdomainforma-
a TICT state, but its energetics are less well known or tiondoes notexplainthe healing mechanism. It is likely,
inaccurate to test this hypothesis.[15] however,that healing originates in and is acceleratedby
interactions that make healing energetically more favor-
The model predicts that the average domain size nat-
able. Clearly, a more quantitative analysis is required
urally decreases as the concentration is decreased. An-
with supplemental data to make possible a more defini-
other method for decreasing the domain size is to form
tive conclusion.
a copolymer in which the second component does not
hydrogen-bond with the DO11 tautomer. Polystyrene is The concept that a material would exhibit such com-
suchapolymer. Totestthishypothesis,wemeasuredthe plex behavior without intentional design by the exper-
populationdecayratesinsamplesofdyesoffixedconcen- imenter is an interesting one. Though self healing is a
tration(9g/L)in PMMA-PScopolymer. By varying the process with great practical utility, it is intriguing that
weight ratio of styrene to MMA, the domain size can be nature has been kind enough to provide an inherently
controlled. As a point of reference, we can compare this smart material system that appears to behave in a way
data with dye concentration-dependent data in PMMA. contrarytomostothers;itmediatesrecoveryinaworldin
Figure5showsthedata. ReducingthePMMAcontent whichirreversibledamageisthenorm. Furtheradvances
inthecopolymerbehavesinasimilarwaytoreducingthe inunderstanding the physics underlyingthis phenomena
dyeconcentrationinapurePMMAsample. Thisbehav- willsurelyenablenewapplicationsthatrequirematerials
ior is consistentwith our model and the domainpicture. to withstandhighlightintensities; and, mayleadto new
Themodelofdomainformationinacopolymerwillneed physics.
6
V. ACKNOWLEDGEMENTS
We thank the Air Force Office of Scientific Research
(FA9550-10-1-0286)for their generous support.
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