Local molecular dynamics of poly(ethylene glycol) dimethacrylate synthesized in the presence of branched poly(methyl methacrylate): A photochromic probe study

Article abstract of DOI:10.1007/s11172-009-0152-x

The local molecular dynamics and morphology of poly(ethylene glycol) dimethacrylate networks synthesized in the presence of 0-40% branched poly(methyl methacrylate) were studied using a photochromic probe (6-nitrospiropyran). The change in the free volume of the polymer in the vicinity of the photochromic probe was monitored by the rate of dark bleaching of merocyanine (colored form of 6-nitrospiropyran). The local free volume and the mobility of the polymer chain fragments decrease as the conversion of C=C bonds and the content of branched poly(methyl methacrylate) in the polymer increase. ; 2009 Springer Science+Business Media, Inc.

Full text of DOI:10.1007/s11172-009-0152-x

Russian Chemical Bulletin, International Edition, Vol. 58, No. 6, pp. 1166—1171, June, 2009
1166
Local molecular dynamics of poly(ethylene glycol) dimethacrylate
synthesized in the presence of branched poly(methyl methacrylate):
a photochromic probe study
S. V. Kurmaz, V. V. Ozhiganov, and M. P. Berezin
Institute of Problems of Chemical Physics, Russian Academy of Sciences,
1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (496) 515 5420. Eꢀmail: skurmaz@icp.ac.ru
The local molecular dynamics and morphology of poly(ethylene glycol) dimethacrylate
networks synthesized in the presence of 0—40% branched poly(methyl methacrylate) were
studied using a photochromic probe (6ꢀnitrospiropyran). The change in the free volume of the
polymer in the vicinity of the photochromic probe was monitored by the rate of dark bleaching
of merocyanine (colored form of 6ꢀnitrospiropyran). The local free volume and the mobility
of the polymer chain fragments decrease as the conversion of C=C bonds and the content of
branched poly(methyl methacrylate) in the polymer increase.
Key words: polydimethacrylate networks, branched poly(methyl methacrylate), photoꢀ
chromic probe, 6ꢀnitrospiropyran, merocyanine, kinetics, absorption spectroscopy.
Crossꢀlinking radical polymerization of multifuncꢀ
due to the fact that branched poly(methyl methacrylates)
represent multicomponent mixtures of macromolecules
of different composition and architecture, namely, linear,
branched, and hyperbranched with different content of
C=C bonds.5 During polymerization, bulky macromolꢀ
ecules of the branched polymers bearing reactive C=C
bonds are incorporated into the polymer chain and thus
increase the free volume of the polymer. At the same
time, those macromolecules of the branched polymers
that are not involved in polymerization act as polymeric
fillers and can exert an opposite effect on the properties
related to the molecular mobility. They can fill the free
volume of the polymer, distributed in the form of microꢀ
cavities, and restrict the intensity of motions of the side
groups, small backbone fragments, and segments comprisꢀ
ing a few backbone atoms. In this case, macromolecules
of the branched polymers distributed in the polymer netꢀ
work matrix can serve as sites with the increased molecuꢀ
lar mobility, as indicated by DSC data6 on the glass
transition temperatures (Tg). Depending on the polymer
structure and on the content of the branching agent and
chainꢀtransfer agent, the Tg values of branched polyꢀ
(methyl methacrylate) are 20—40 °C lower than those of
linear poly(methyl methacrylate).
tional monomers containing two and more C=C bonds
leads to simultaneous in situ formation of branched macꢀ
romolecules and microgel particles, i.e., rigid crossꢀlinked
structures built of small cycles,1 which define the topoꢀ
logical structure and morphology of the polymer network.
The polymerization of multifunctional monomers in the
presence of high concentrations of branched polymers
containing reactive C=C bonds produces a polymer netꢀ
work whose topological structure is mainly built of
branched macromolecules with many terminal chains
in the peripheral layer.2
Branched poly(methyl methacrylates) synthesized
according to a known method3 can be cured to give polyꢀ
mer networks4 and used for the synthesis of polymer netꢀ
works with unusual topology. Polymer networks formed
in the presence of branched poly(methyl methacrylates)
have not only an unusual topology, but also possess new
properties controlled by the molecular mobility. This is
Methods based on freezing of the mobility of molecuꢀ
lar probes became very popular for studying the local
molecular dynamics of polymer networks. For these purꢀ
poses, substances capable of undergoing isomerization in
the reaction system are often used.7 If the activation
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1137—1142, June, 2009.
1066ꢀ5285/09/5806ꢀ1166 © 2009 Springer Science+Business Media, Inc.

Local molecular dynamics of polydimethacrylate
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 6, June, 2009
1167
Scheme 1
volume is rather large, the isomerization rate can serve as
a measure of the free volume in the reaction system and
be an indirect characteristic of conversion.1 For instance,
the use of azobenzene and stilbene as photochromic probes
made it possible to monitor the changes in conversion
during polymerization, to suggest the distribution of the
free volume in the system, and to identify the microꢀ
heterogeneous structure of the polymer chain.7
The aim of the present work is to study the local moꢀ
lecular dynamics and morphology of ethylene glycol
dimethacrylate polymers synthesized in the presence of
various branched poly(methyl methacrylate) additives
using a photochromic probe.
6ꢀNitrospiropyran (1), which was preliminarily introꢀ
duced into the initial monomer—polymer mixture, served
as a photochromic probe. As was shown by the studies
of the polymerization kinetics of methyl methacrylate
(monofunctional analog of ethylene glycol dimethacrylate,
EGDM),* compound 1 is a weak inhibitor due to the
NO2 group and reduces the maximum polymerization rate.
The inhibition proceeds through reduction of the nitro
group to the NO group,8 so that the polymer molecules
contain no compound 1. Molecules 1 are distributed in
the polymer network due to structural physical transforꢀ
mations of the reaction medium, namely, the glass transiꢀ
tion and microsyneresis accompanying the threeꢀdimenꢀ
sional radical polymerization.1 The molecular mobility of
the reactants, including molecules 1, is “frozen” as a
result of the glass transition that develops at deep polyꢀ
merization stages. Owing to microsyneresis, the lowꢀ
molecularꢀweight components of the reaction system are
separated from highly crossꢀlinked regions to domains
with a lower degree of crossꢀlinking or to the liquid phase.
Microsyneresis of the liquid components causes a local
redistribution of the solids dissolved in the initial monoꢀ
mer. Owing to these processes, molecules 1 are found
in regions with different degrees of crossꢀlinking and
molecular mobility.
The photochromic probe method is based on the fact
that UV irradiation of compound 1 leads to cleavage of
the C—O covalent bond and subsequent transformation
of 1 to its colored form, merocyanine (1´) (Scheme 1).
Compound 1´ is an amphoteric ion capable of undergoꢀ
ing reversible reduction to the initial structure on heating
or irradiation with visible light.9
According to the estimates,10 the van der Waals volꢀ
ume of molecule 1 is ~300 Å3, which makes it possible to
evaluate the intensity of molecular motions in microꢀ
domains, whose volumes are not smaller than the intrinꢀ
sic size of the molecular probe. It should be noted that
this method has a limitation related to a decrease in the
optical transparency of the polymer in the region of
absorption of the colored form 1´ at certain concentraꢀ
tions of branched poly(methyl methacrylate).
Experimental
Poly(ethylene glycol dimethacrylate) (PEGDM, 2) films
with variable content of branched poly(methyl methacrylate)
(3) were produced by radical polymerization in the bulk at
T = 60 °C. The initiator was AIBN (0.1%). The film thickness
was about 0.1 mm.
Branched polymer 3 represents a copolymer of methyl
methacrylate with triethylene glycol dimethacrylate (TEGDM)
with the composition 100 : 5 (mol.%), which was synthesized by
threeꢀdimensional radical copolymerization in toluene, conꢀ
trolled by 1ꢀdecanethiol (5 mol.%) as the chainꢀtransfer agent.4
The main physicochemical characteristics of the branched
polymer 3 as follows: Mn = 5.2•103, Mw/Mn = 2.5, [C=C] =
= 1.2 mol kg–1, and Tg = 70 °C. The branched architecture of
polymer 3 was confirmed by viscosimetry.4 Unlike linear
poly(methyl methacrylate) 4, polymer 3 is characterized by the
weak dependence of the reduced viscosity on the polymer
concentration in solution, which indicates an increase in the
compactness (molecular packing density) of the macromolecules
and their branched architecture.11 The content of polymer 3 in
the monomer—polymer mixture varied from 0 to 40%.
Polymers 2 obtained in the presence of 0, 5, 10, and 40%
polymer 3, which are optically transparent in the region of
absorption of colored form 1´, were chosen for studies by the
photochromic probe method. Compound 1 (Aldrich), whose
content in the reaction mixture was ~0.1—0.2%, was not specially
purified. A DRTꢀ400 lamp equipped with a UFSꢀ5 filter was
used for the irradiation of the polymers containing compound 1.
The irradiation time was 10 s. The kinetics of transformation of
the colored form 1´ to the initial structure 1 was studied by
absorption spectroscopy (Specord M40 spectrophotometer) in
the dark. The change in the absorbance (D) of the colored form
1´ was detected as a function of time at 20 °C. The degree of
conversion (P) of 1´ was calculated by the formula
P = 1 – Dt/D0,
where D0 and Dt are the initial and current absorbances of form
1´. The apparent rate constants kapp and the halfꢀconversion
time t1/2 corresponding to 50% conversion of molecules 1´ were
chosen as quantitative characteristics of the bleaching kinetics
of the colored form 1´.
* The data were obtained by E. O. Perepelitsina.

1168
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 6, June, 2009
Kurmaz et al.
Table 1. The values of t1/2 and apparent rate constants k and k2
for bleaching of form 1´ in polymer networks 2 obtaine1d in the
presence of additives of compound 3 and in linear polymer 4
P/rel. units
2
3
1
4
0.75
Polymer C3 (%)a CC=C (%)b t1/2/min
k1
k2
5
/min–1
0.50
0.25
2
0
5
10
40
—
52.7
60.7
57.0
60.0
95.0
3.1
4.0
9.0
31.9
19.3
0.2
—
0.17 0.11
0.09 0.05
0.03 0.012
0.049 0.015
4
a Additive of 3.
b Degree of conversion.
25
50
75
t/min
Fig. 1. Kinetic curves of bleaching of form 1´ in linear polymer 4
(1) and in polymers 2 obtained in the absence (2) and in the
presence of 5 (3), 10 (4), and 40% (5) of additive 3.
The degrees of conversion of C=C bonds (C (%)) in the
polymers studied were determined by IR spectroscopy. The IR
spectra of polymers (Nujol mulls) were recorded on a Specord
75ꢀIR spectrophotometer. The internal standard was the ν(C=O)
stretching band at 1725 cm–1. The ν(C=C) stretching band at
~1638 cm–1 was used as a “fingerprint.” The degree of conversion
of C=C bonds was calculated by the formula
restricted segmental mobility of the polymer chains with
retention of the motion of the side groups or small backꢀ
bone fragments. As a result, the bleaching of 1´ in polyꢀ
mers 2 and 4 occurs much more slowly than in liquids12
and even in polymers in the rubbery or viscousꢀflow states.13
The data in Table 1 show that the bleaching of 1´ in the
polymer network 2 containing no additive of compound 3
proceeds faster than in linear polymer 4. The t1/2 value
decreases by ~6 times. Different kinetics of the reaction
1´ → 1 in the linear polymer and in the polymer network
are due to not only the differences in the C=Cbond conꢀ
version (see Table 1), but also to structural peculiarities of
the polymers. Polymer networks are characterized by high
degree of structural heterogeneity and the presence of
microdomains with different degrees of crossꢀlinking and
molecular mobility.1 The loosening effect of chemical
crossꢀlinks causes the molecular packing density to deꢀ
crease and the free volume of the polymer network to
increase; as a consequence, the rate of bleaching of the
colored form 1´ increases.
The addition of macromolecules 3 to polymer 2 reꢀ
sults in a decrease in the bleaching rate of form 1´ (Fig. 1,
curves 3—5); correspondingly, the t1/2 value increases.
For example, at a low content of compound 3 in polymer
2 (5%), the t1/2 value increases by ~1.3 times. Further
increase in the content of compound 3 in polymer 2
results in a greater decrease in the bleaching rate of form
1´. At the content of 3 in polymer 2 equal to 40%, the
t1/2 value increases by an order of magnitude relative to
that of polymer 2 containing no compound 3 (see Fig. 1,
curves 2 and 5). This suggests a decrease in the local free
volume in the reaction zone of bleaching of form 1´.
Since the degrees of conversion of C=C bonds in the
polymers with additives of compound 3 are close, i.e.,
the degree of chemical crossꢀlinking is approximately the
same, one can assume that the decrease in the local free
C = (1 – [(D1638/D1725 polym/(D1638/D1725)mix]) 100,
)
where D1638 is the absorbance of the ν(C=C) stretching band in
the polymers studied and of the EGDM—polymer 3 mixture,
and D1725 is the absorbance of the ν(C=O) stretching band of
the polymers and of an EGDM—polymer 3 mixture. According
to the IR spectroscopic data, the degree of conversion of the
C=C bonds in polymers 2 obtained in the presence of compound
3 is somewhat higher than in the initial polymer (Table 1). This
suggests a higher molecular mobility of the components in
the monomer—polymer 3 reaction system during radical polyꢀ
merization.
Results and Discussion
The kinetic curves of bleaching of the colored form 1´
(λmax ≈ 573 nm) in polymers 2 synthesized in the presence
of compound 3 (0—40%) and characterized by the ultiꢀ
mate conversion of the C=C bonds are shown in Fig. 1.
The kinetic curve of bleaching of form 1´ in a linear
poly(methyl methacrylate) film cast from a solution of
the polymer containing compound 1 is also presented
for comparison. It was shown by special studies that the
photochemical activity of compound 1 is independent of
the method of its incorporation into the polymer, namely,
it is almost the same in both polymer 4 obtained by radical
polymerization of a methyl methacrylate (MMA)—comꢀ
pound 1 (0.2%) reaction mixture and poly(methyl methꢀ
acrylate) film cast from a solution of the polymer conꢀ
taining compound 1.
The kinetics of bleaching of the colored form 1´ was
measured at 20 °C. At this temperature, polymers 2 and 4
are in the glassy state characterized by shortꢀrange order
in the arrangement of the macromolecules and strongly

Local molecular dynamics of polydimethacrylate
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 6, June, 2009
1169
volume and mobility of constituents of the surrounding
macromolecules is due to the additives of macromolecules
3, which are chemically inert in the radical polymerizaꢀ
tion of EGDM. They fill the free volume of the polymer
and restrict the intensity of motions of the side groups and
small backbone fragments.
sion contain linear regions, and the slopes of the depenꢀ
dences lnD0/Dt = f(t) in these regions were used to deterꢀ
mine the kapp values (see Table 1). Like for linear polymer
4, the deviation from the exponential law can be due to
the dispersion of the size distribution of the microcavities
in which photochromic molecules are localized and to
the heterogeneity of the supramolecular structure of the
polymer, which determines the differences in the converꢀ
sion rates of the colored form 1´.
Valuable information on the local molecular dynamꢀ
ics of the polymer network was obtained in studies of
cyclic conversions of compound 1 in polymers 2 syntheꢀ
sized in the absence and in the presence of 40% of polyꢀ
mer 3. In both cases, the t1/2 value depends strongly on
the number of irradiation cycles. It halved after the
second irradiation cycle in the initial polymer 2. In polyꢀ
mer 2 containing additive 3 (40%) t1/2 is ~18 min after the
third irradiation cycle. This implies that the free volume
in the microdomains, where the molecules of compound
1 are localized, changes substantially during photochroꢀ
mic transformations. According to published data,15 mulꢀ
tiple photochromic transformations of spiropyrans induce
an increase in the volume of small (submicronic) cavities
in the polymer matrix. This is a possible reason for accelꢀ
eration of the conversion of the colored form 1´ in the
polymers studied.
Figure 2 shows the kinetic curves of the reaction 1´ →
1 in the polymers 2 obtained in the presence of additive 3
(0—40%) in the coordinates of the firstꢀorder equation.
It is seen that the bleaching kinetics of form 1´ is satisfacꢀ
torily described by the exponential dependence in the
whole interval of conversions and characterized by
kapp = 0.2 min–1 only in the initial polymer 2. This means
that the major part of molecules 1 is localized either in
microdomains characterized by low degree of conversion
of the C=C bonds and enhanced molecular mobility of
macromolecular fragments or in the liquid monomer.
During threeꢀdimensional polymerization of EGDM,
molecules 1 can be redistributed between the liquid and
solid phases due to microsyneresis and can isolate to the
liquid phase. It is the reactions of coloring and bleaching
of compound 1 in liquids that usually obey a firstꢀorder
pattern, whereas in glassy matrices the reaction mechaꢀ
nism becomes more complicated.14 It should be noted13
that the exponential dependence in the whole interval of
conversions of form 1´ is also observed in linear and
branched polyacrylates in the rubbery state, which is charꢀ
acterized by high mobility of all constituents of the macꢀ
romolecule including atoms, groups, units, and segments.
The deviation from the exponential law is observed for
linear polymer 4 and polymers 2 obtained in the presence
of compound 3 (see Fig. 2). The kinetic curves of bleachꢀ
ing of form 1´ in polymers 2 with additives of compound
3 corresponding to the initial and deep stages of converꢀ
The local molecular dynamics of polymers 2 with difꢀ
ferent degrees of conversion of the C=C bonds in the
presence of additive 3 was studied using the photochroꢀ
mic probe. The polymerization of the EGDM—polymer
3 (40%) reaction mixture was carried out at 60 °C in thin
layers and stopped at a particular time interval, and then
the bleaching of form 1´ was monitored as described above.
The conversion of the C=C bond in the polymer obtained
between potassium bromide glasses was monitored by IR
spectroscopy.
lnD0/Dt
The kinetic curves of bleaching of form 1´ in polymers
2 with different degrees of conversion of C=C bonds in
the presence of additive 3 (40%) and their anamorphoses
in the coordinates of the firstꢀorder equation are shown in
Figs 3 and 4, respectively. The main parameters of the
reaction of bleaching of form 1´ in the polymers studied
are listed in Table 2. It follows that the bleaching kinetics
of the colored form 1´ depends strongly on the converꢀ
sion of C=C bonds in polymer 2. At low degrees of conꢀ
version (C = 3%) a loosely crossꢀlinked polymer with low
molecular packing density containing many “pendant”
C=C bonds, unreacted monomer, and unreacted macroꢀ
molecules of polymer 3 is formed. As a result, transformaꢀ
tion of form 1´ to the starting compound 1 is fast and
described by an exponential dependence in the whole
interval of conversions (see Fig. 4). The corresponding
kapp value for the bleaching of form 1´ is given in Table 2.
It should be noted that form 1´ in the starting monomer is
bleached very rapidly and highꢀperformance methods, e.g.,
2
2
1
4
3
5
1
25
50
75
t/min
Fig. 2. Anamorphoses of the kinetic curves of bleaching of form
1´ in linear polymer 4 (1) and in polymers 2 obtained in the
absence (2) and in the presence of 5 (3), 10 (4), and 40% (5) of
additive 3, plotted in the coordinates of the firstꢀorder equation
(T = 20 °C).

1170
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 6, June, 2009
Kurmaz et al.
P/rel. units
Table 2. Dependence of the kinetic parameters of bleaching of
form 1´ (T = 20 °C) on the degree of conversion of C=C bonds
in polymers 2 with an additive of compound 3 (40%) obtained at
60 °C and [AIBN] = 0.1 wt.%
1
2
0.75
0.50
0.25
CC=C (%)
t1/2/min
k1
k2
4
3
min–1
5
3.0
0.66
2.5
15.9
29.6
33.7
36.6
1.0
0.27
0.07a
0.03a
0.03a
0.04a
—
—
0.031b
0.017b
0.015b
0.013b
25.2
54.7
58.0
62.9
63.2
Note. The apparent rate constants for bleaching of form 1´, k1 and
k2, were determined in the linear portions of the lnD0/Dt = f(t)
dependence corresponding to the initial (0—20%)a and deep
(> 40%)b degrees of conversion of form 1´.
20
40
t/min
Fig. 3. Kinetic curves of bleaching of form 1´ in polymers 2
obtained in the presence of additive 3 (40%) with the degree
of conversion of C=C bonds: 3 (1), 25 (2), 55 (3), 58 (4), and
63% (5) plotted in the coordinates of the firstꢀorder equation
(T = 20 °C).
network is formed. Based on the bleaching rate of form
1´ (see Fig. 3, curve 3), this results in a considerable
decrease in the free volume. The bleaching rate of form 1´
in the polymer decreases substantially, and the t1/2 value
increases to ~16 min. The kinetic curve of bleaching of
form 1´ can be approximated by superposition of at least
two exponential curves corresponding to the monomoꢀ
lecular processes with different rate constants. It is most
likely that molecules of compound 1 are localized in the
microdomains with different densities of the chemical
network, free volumes, and molecular mobility.
Due to the glass transition of the reaction mixture and
“freezing” of the molecular mobility of its components,
including the “pendant” C=C bonds, polymerization
ceases and a highly crossꢀlinked polymer network with a
limiting conversion of ~60% is formed. Crossꢀlinking
“brings together” the macromolecules, resulting in
further decrease in the free volume and the mobility of
the side groups and backbone fragments. As a result, the
bleaching rate of form 1´ is almost halved and the t1/2
value increases to ~30—37 min (see Table 2). The kinetic
curves of bleaching of form 1´ can be approximated by a
superposition of several exponential curves.
flash photolysis, are needed to determine the quantitative
parameters.
Figure 3 (curve 2) shows that the bleaching rate of
form 1´ in the polymer at ~25% conversion decreases
compared to the polymer studied earlier, probably, due to
an increase in the density of the chemical network and
a decrease in its molecular mobility. The kinetics of the
1´ → 1 reaction is satisfactorily described by an exponenꢀ
tial dependence, which suggests that the major part of
molecules of compound 1 is localized in the microdomains
with the loose network or in the liquid phase.
When the polymerization rate decreases in the autoꢀ
retardation stage, a polymer with a C=C bond conversion
of ~55% and, hence, a higher density of the chemical
lnD0/Dt
1
2
2
Annealing of the samples of polymer 2 with the 40%
additive of compound 3 with the ~60% C=C bond conꢀ
version at T = 60 °C for ~120 h showed that the bleaching
rate of form 1´ increases and the t1/2 value decreases to
27 min. This suggests an increase in the free volume of the
polymer formed due to mobile holes as a result of internal
rotation of different fragments of polymer chains during
the heat treatment.16
1
3
4
5
Thus, the introduction of additives of compound 3
into polymer network 2 and the increase in their content
cause the bleaching rate of form 1´ to decrease. Macroꢀ
molecules of the branched polymer 3, which are not inꢀ
volved in polymerization, most likely, fill the free volume
20
40
t/min
Fig. 4. Anamorphoses of the kinetic curves of bleaching of form
1´ in polymer 2, obtained in the presence of additive 3, with the
degree of conversion of C=C bonds equal to 3 (1), 25 (2), 55 (3),
58 (4), and 63% (5).

Local molecular dynamics of polydimethacrylate
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 6, June, 2009
1171
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of the polymer and restrict the mobility of the side groups
and small backbone fragments. It was found that the halfꢀ
conversion time increases and the 1´ → 1 reaction is
retarded with an increase in the degree of conversion of
C=C bonds in the polymers studied due to the formation
of a highly crossꢀlinked polymer. The number of irradiaꢀ
tion cycles and annealing of polymers were shown to
affect the bleaching kinetics of form 1´. The free volume
and molecular mobility of polymer chains in the vicinity
of probes is controlled by the degree of conversion of
C=C bonds, the content of the additive of compound 3,
and the specific features of the microstructure of polymer
networks 2 formed in the presence of polymer 3. The
photochromic probe method is an efficient tool for studyꢀ
ing the local dynamics and microstructure of polymer
networks obtained in the presence of additives of comꢀ
pound 3.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 06ꢀ03ꢀ33018)
and the Division of Chemistry and Materials Science of
the Russian Academy of Sciences (Program No. 8).
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Received March 5, 2008;
in revised form January 21, 2009