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Low Modulus Dry Silicone-Gel Materials by Photoinduced Thiol−Ene
Chemistry
Otto van den Berg,† Le-Thu T. Nguyen,†,‡ Roberto F. A. Teixeira,† Fabienne Goethals,† Ceren Ö zdilek,§
Stephane Berghmans,§ and Filip E. Du Prez†,*
†
Department of Organic Chemistry, Polymer Chemistry Research Group, Ghent University, Krijgslaan 281, S4-bis, B-9000 Ghent,
Belgium
‡
Faculty of Materials Technology, Ho Chi Minh City University of Technology, Vietnam National University, 268 Ly Thuong Kiet,
10 District, Ho Chi Minh City, Vietnam
§
TYCO Electronics Raychem BVBA, Diestsesteenweg 692, 3010 Kessel-Lo (Leuven), Belgium
ABSTRACT: The curing behavior of telechelic vinylfunctionalized poly(dimethylsiloxane) (PDMS) with poly(dimethylsiloxane-co-propylmercaptomethylsiloxane) using
photoinitiated radical thiol−ene polyaddition was studied, by
means of rheology, mechanical analysis of the cured
elastomeric products, and high resolution magic angle spinning
NMR (HR-MAS). Postpolymerization modification of a
hydroxy-functionalized PDMS-derivative (OH-PDMS) yielded
a telechelic thiol-functionalized PDMS-derivative, which was
subsequently used as chain extender for the preparation of dry
silicone-gel materials with elastic moduli between 30 and 500
kPa. The rate of thiol−ene polyaddition of the chain extender
proved to be similar to that of the cross-linking process using multifunctional PDMS-based thiols. HR-MAS analysis of the
loosely cross-linked thiol−ene PDMS networks and their fluoride-solubilized counterparts proved a highly efficient cross-linking
with an optimal cure at 1:1 thiol to ene stoichiometric ratios. Using mechanical analysis, it was shown that the low molecular
weight thiol-functionalized chain extender was efficiently incorporated in the PDMS polymer network.
■
INTRODUCTION
Organogels are noncrystalline, nonglassy, solid materials
composed of a liquid organic phase entrapped in a threedimensionally cross-linked network.1 Such materials are used in
a large number of different applications including cosmetic,
medical and pharmaceutical (e.g., burn-wound care, padding
parts for medical devices), household (e.g., gel candles), sports
(e.g., gel-padded bicycle seating, shoe insoles), and electronics.
However, in some applications the physical properties of
organogels are required without the presence of a low
molecular weight liquid phase, for example in situations
where migration of organic liquids to the environment is a
problem, such as in biomedical devices or electronics. In such
cases, elastomer networks with low cross-link density can act as
suitable replacements. Synthesis of these materials can be
achieved by using either high molecular weight polymers as the
basis for the “gel” or by using lower molecular weight polymers
in combination with a chain extender. This class of materials is
identical to a classical cross-linked rubber, albeit with a very low
cross-link density.
Organogel-like materials containing no, or very little
entrapped organic phase, are often referred to as “dry-gels”.
Well-known representatives of this class of “dry-gel” materials
are certain silicone-based (poly(dimethylsiloxane), PDMS)
platinum-cured elastomers, where a fully cross-linked network
© 2014 American Chemical Society
is achieved by combining a well-defined cross-linker and a
bifunctional chain-extender with a telechelic vinyl-functionalized PDMS, resulting in a low cross-link density network. A
disadvantage of such chemistry is the use of an expensive
platinum catalyst, which in addition is very prone to poisoning
by traces of different kinds of chemicals, including amines,
metal salts and sulfur compounds (e.g., vulcanized rubber).
Thus, elimination of the platinum catalyst would circumvent
these issues.
One type of chemistry that is a potential candidate for
efficient (cross) linking of PDMS chains, without the use of a
platinum catalyst, is thiol−ene chemistry. A large number of
publications on thiol−ene chemistry involve modification and
functionalization of existing (polymer) structures and the
preparation of new molecular architectures. Some excellent
reviews dealing with this topic were published.2 In patent
literature, thiol−ene polymerization of multifunctional thiols
with multifunctional enes and ynes is described for the
preparation of materials with different types of functionality.
For example, Bowman et al. reported the polymerization of
several multifunctional unsaturated urethanes, allylethers,
Received: December 16, 2013
Revised: January 26, 2014
Published: February 7, 2014
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Scheme 1. Preparation and Optimization of Thiol−Ene Cured PDMS Low Cross-Link Density Networks, Indicating the
Investigated Combinations and the Involved Characterization Techniques
and nonoptimized cross-linked materials. In order to optimize
the formulation of a thiol−ene photocured PDMS elastomer
with low cross-link density, without the added complications of
a three-component system, the photo cross-linking process of
telechelic vinyl-functional PDMS with side-chain thiol-functionalized PDMS is analyzed using rheology of the curing process,
mechanical analysis and high resolution magic-angle spinning
NMR spectroscopy (HR-MAS) of the cured elastomeric
products, besides liquid state NMR of soluble components
and fluoride-solubilized cross-linked-PDMS (A + B in Scheme
1).17
The chain extension reaction of telechelic vinyl-functional
PDMS with telechelic thiol-functional PDMS (B + C in
Scheme 1) is studied separately from the cross-linking
formulations. Finally an optimized cross-linking formulation is
combined with a telechelic thiol-functional PDMS chainextender, to yield the desired soft, platinum-free and low
cross-link density silicone elastomers (A + B + C in Scheme 1).
acrylates and methacrylates with multifunctional thiols, yielding
cross-linked end products exhibiting shape memory properties,3
which are claimed for medical applications. Other examples of
applications of thiol−ene based polymeric structures include
the preparation of low gas permeability membranes,4 sealants,5
stamps for lithography,6 degradable polymeric structures for
biomedical7 and dental applications,8 liquid crystalline
compositions for optical applications,9 and polymer electrolytes
for, e.g., batteries.10
Silicone-based thiols have been described for different
applications,11 including fast-cure optical fiber coatings12 and
functionalized microfluidic devices13 and for the modification of
surfaces.14 An excellent study of photochemical thiol−ene
polyaddition kinetics of different silicone based telechelic ene
materials (vinyl, allyl, norbornyl, and many others) of moderate
molecular weights (Mn ∼ 6000 g mol−1) with a commercially
available polythiol has been published by Müller et al.15 With
swelling studies, online Raman and differential photocalorimetry, they showed that telechelic vinyl substituted PDMS
combines a high reactivity with a high final conversion in thiol−
ene PDMS networks, in contrast to, e.g., norbornyl-substituted
PDMS that showed a high reactivity but a rather poor final
conversion. This observation is especially interesting as it is
demonstrated by our group that thiol−ene reactions between
thiol-functional polymers and ene-functional polymers are
usually inefficient, yielding only partially reacted materials.16
In this paper, the viability of the reaction between
commercially available thiol-functionalized poly(dimethylsiloxane) (PDMS), (A, Scheme 1) telechelic vinylfunctionalized PDMS (B), and a dithiol PDMS chain-extender
(C) is explored to efficiently form gel-like rubbery materials.
A prerequisite for the synthesis of a soft gel-like rubber is a
low cross-link density. However, the exact functionality of the
reacting polymers is usually unknown. In addition the
stoichiometry is not a simple function of molar ratio of
functional groups, since steric factors (accessibility, molecular
weight distribution) play an important role as well in the final
conversion of functional groups. Introducing a chain extender
further complicates matters, since it could have a different
reactivity compared to the cross-linker, leading to unpredictable
■
RESULTS AND DISCUSSION
Characterization of cross-linked PDMS on a molecular level is
quite difficult due to the insolubility of the cross-linked
network. Raman spectroscopy and photo DSC have been used
to follow the cross-linking process of such systems.15 However,
if the concentration of functional groups decreases even further,
in our case at least 1 order of magnitude lower than any earlier
publication, quantification and characterization of chemical
functionality in cross-linked PDMS networks using Raman
spectroscopy and photo DSC becomes even more difficult.
Therefore, we opted for a combination of HR-MAS NMR
spectroscopy on swollen PDMS networks, with mechanical
characterization and liquid NMR on the soluble fraction of the
cross-linked materials, in order to gain insight into the curing
chemistry and kinetics of these materials.
To assess the influence of the thiol to ene ratio and the
molecular weight of the vinyl-functionalized PDMS (VPDMS)
(Table 1) on the network properties, a series of photocured
samples was prepared using a fixed concentration of photoinitiator (2,2-dimethoxy-2-phenylacetophenone, DMPA) of 3
mg/g of formulation.
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Table 1. Absolute Molecular Weight and Dispersity of the
PDMS-Based Polymers, Obtained by Triple Detection GPC
polymer
0.97 Pa s
VPDMS
(1)
4.8 Pa s
VPDMS
(2)
9.7 Pa s
VPDMS
(3)
PDMS
multi
thiol (4)
Mn (g mol‑1)
indicated by
supplier
2.8 × 10
4
thiol content
(mol %)
(from
NMR)
Mn (g mol‑1)
NMR
Mn (g mol‑1)
GPC
Đ
1.79 × 10
1.97 × 10
4
1.76
−
4
4.95 × 104
3.3 × 104
4.50 × 104
1.57
−
6.27 × 104
4.13 × 104
4.53 × 104
1.76
−
6.8 × 103
2.76
3.9
6 × 103 to
8 × 103
−
First, the samples were subjected to a tensile test in order to
determine the elastic modulus of the cross-linked networks,
followed by a Soxhlet extraction with pentane to determine on
the one hand the sol-fraction of the networks and on the other
hand the chemical composition of this sol-fraction using liquid
state NMR.
Figure 1A shows the moduli for three different series of
networks prepared from telechelic vinyl PDMS of different
molecular weight (1−3). While the moduli depend strongly on
the thiol to ene ratios, for all samples a maximum in the Emodulus is found close to the ratio of 1, which confirms that an
optimum in the network formation is reached at this ratio and
that, contrary to hydrosilylation cured silicone materials, no
excess of thiol-functionalized cross-linker is required for
obtaining an optimal cure process. The amount of extractable
material (pentane soluble fraction) per unit of cured material
follows a similar trend: all samples show a minimum in
extractable material at a thiol to ene ratio of approximately 1
(Figure 1B). For all molecular weights, a similar minimum solfraction of around 10 wt % was found.
An NMR-analysis of the pentane-soluble fractions of the
networks prepared from the high molecular weight telechelic
vinyl PDMS (3) was performed. Two different sets of protons
are of significance in the characterization of the extracted
material: the unsaturated vinyl protons (5.6−6.15 ppm) and
the α thiol/thioether methylene moieties (2.5 ppm). In Figure
2, the number of protons (all normalized to the methylsiloxane
signal at 0 ppm) of each set are plotted as a function of the thiol
to ene ratio.
The vinyl protons decrease in intensity with an increasing
thiol to ene ratio. At a ratio of just above the point of
equimolarity, the number has become equal to zero. The CH2S
(thiol and/or thioether) signal on the other hand first decreases
to zero at a ratio just below the point of equivalence and
increases steeply at higher ratios. The disappearance of the vinyl
signal just above the point of equivalence is indicative of the
efficient nature of the thiol−ene curing. Indeed, even at low
thiol and ene concentrations of ∼0.05 mol L−1, this high
molecular-weight thiol−ene curing elastomer is able to fully
cure at equimolarity of thiol and ene components. The
presence of α thiol/thioether methylene units at low thiol to
ene ratios can be explained by the initial formation of soluble,
branched structures containing thioether moieties and an excess
of double-bonds that are indeed observed as well. When the
amount of thiol cross-linker is increased even further, the
extractables, close to the point of equivalence, do not contain
Figure 1. (A) Elastic moduli of the networks formed from telechelic
vinyl PDMS of different molecular weight (1−3) and thiol
functionalized PDMS (4) as a function of the molar thiol to ene
ratio (NMR) used to prepare the networks. The error bars represent
the 90% confidence interval. (B) Soluble fraction of the networks (in
pentane) as a function of the molar thiol to ene ratio.
any α thiol/thioether methylene units anymore, indicating that
the extractable fraction consists at that point mainly of
nonfunctionalized PDMS, e.g., cyclic oligomers that are already
present in the used grade of telechelic vinyl PDMS.
The NMR analysis of extractables from cured thiol−ene
PDMS networks only gives information on the components
that do not take part in the network formation. NMR analysis
of the network itself is much more demanding and requires the
use of HR-MAS NMR techniques or the application of a
chemical degradation step, prior to liquid-state NMR analysis.
The first option was performed using a 700 MHz NMR
spectrometer fitted with a solid-state probe rotating at the
magic angle.
A typical HR-MAS NMR spectrum obtained for a fully cured
sample of thiol−ene elastomer is shown in Figure 3.
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bonds in the cross-linked thiol−PDMS allows the mild and
selective cleavage of network segments using tetrabutylammonium fluoride,18 resulting in smooth dissolution of the crosslinked material and the formation of tetrabutylammonium
siloxanolate and silylfluoride containing polymer fragments.
Ammonium chloride was added after dissolution of the network
in order to neutralize the siloxanolate to silanol moieties
(Scheme 2).
Scheme 2. Reaction of PDMS-Containing Materials with
Tetrabutylammonium Fluoride
Figure 2. Number of α thiomethylene protons (CH2S; thiol and/or
thioether corresponding to signals α and α′ in Figure 3, squares) and
vinyl protons (triangles) per 5000 SiCH3 protons (the average number
of methyl protons per vinyl-PDMS as calculated from the tripledetection GPC-data of 3), recorded for extractables from thiol−ene
cured PDMS networks prepared from vinyl PDMS 7, as a function of
the molar thiol to ene ratio used to prepare the networks.
From the NMR spectra of the degraded networks, the double
bond conversion was calculated in an identical way as was done
for the HR-MAS measurements, i.e., directly from the double
bond or the α signal, using the γ signal as an internal standard.
The NMR data of the solubilized thiol−ene PDMS networks
revealed that the integral of the dimethylsiloxane moiety,
normalized to the γ signal, is not a function dependent on the
amount of PDMS added to form the network. The reason for
this observation proved to be the loss of difluoro
dimethylsilane, which is a gas at room temperature. No loss
of either vinyl dimethyl silyl moieties or other functional groups
was observed upon exposure to fluoride ions.
In addition to the calculated double bond conversions
obtained from HR-MAS and fluoride dissolution, the
theoretical maximum conversion was calculated from the feed
ratio of thiol and ene, as represented in Figure 4. All double
bond conversion results, obtained from HR-MAS and fluoride
dissolution, coincide, within the margin of error, with the
theoretical maximum conversion-line. This proves on one hand
that the thiol−ene conversion in the formed networks is close
to the predicted theoretical maximum and on the other hand
that fluoride dissolution of thiol−ene PDMS networks is a
viable method for the chemical analysis of these cross-linked
materials. The errors in the double-bond conversions calculated
from the α/γ signal ratios gave the largest experimental errors
(∼15% for both fluoride dissolution and HR-MAS), which is
ascribed to the less accurate integration of the α signal as a
result of a partial overlapping with a small signal of disulfide
methylene protons, and to the fact that the calculation based on
the α-to-γ signal was done with an assumption of the absence of
disulfide formation.
Chain extension of telechelic VPDMS (2) with a low
molecular weight telechelic dithiol PDMS was explored in
order to assess the feasibility of using thiol−ene chemistry for
Figure 3. 700 MHz HR-MAS NMR spectrum of a thiol−ene PDMS
network prepared from 2 and 4 with initially equimolar amounts of
thiol and ene moieties, swollen in deuterated chloroform.
Both α methylene−silyl signals (labeled as γ and β′) are
present and are fully resolved from other signals that are related
to the α thiomethylene signals (thiol and thioether, labeled as α
and α′ in Figure 3), the β thiomethylene (labeled as β, overlap
with the water signal) and SH-signal (overlap with a signal from
a minor unknown impurity). From the HR-MAS data, the
double bond conversion was calculated using both the double
bond signal at 5.6−6.15 ppm directly or from the α
thiomethylene signal, both normalized to the γ signal since
this signal is indeed independent of the thiol−ene conversion
and is not overlapping with any other signal (see the
Experimental Section for calculation equations).
A second method for the analysis of cross-linked thiol−ene
PDMS networks, which could be applied in a more routine
way, was developed. In fact, the presence of ample siloxane
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In order to assess the speed and efficiency of thiol−ene
coupling of thiol and ene functionalized PDMS (B+C in
Scheme 1), the prepared dithiol (8) was combined with vinylfunctionalized PDMS (2, Table 1) in different ratios with the
addition of 3 mg/g of photo initiator (DMPA). During
irradiation of the degassed samples with 365 nm UV-light (∼12
mW/cm2), the viscosity was measured as a function of
irradiation time, using a Brookfield viscosimeter (Figure 5).
The viscosity of the final product obtained after photopolymerization highly depends on the ratio of thiol to ene
functionalized PDMS, as expected for an efficient linking
reaction (Figure 5A). At a 1:1 molar ratio, the viscosity of the
final product was above the limit (2.2 × 106 cPs) of what can be
measured. However, at this 1:1 ratio the maximum measurable
viscosity translates, via a formula linking melt-viscosity of
PDMS with its number-average molecular weight, into an
approximate minimum reached molecular weight Mn of 190
000,19 which means that on average at least 5 vinylfunctionalized PDMS units of Mn 33 000 (2) were linked to
a single chain by the dithiol chain-extender (8).
A decrease in the reaction induction time with an increase in
dithiol content is observed up to equimolar ratio (Figure 5B).
Dissolved oxygen, which acts as an inhibitor by converting
carbon-centered radicals into peroxy radicals, is most likely
consumed at a faster rate with increased thiol concentration due
to radical transfer from the peroxy radical to the thiol, which
reactivates the “dead” peroxy radical, thus increasing the overall
concentration of radicals and thereby the consumption of
oxygen.20 However, beyond the point of thiol to ene
equimolarity, the decrease in induction time levels off,
suggesting that the inhibition time, in this case, is not only
dependent on the thiol concentration but also on the thiol to
ene ratio. This, at first sight peculiar observation, indicates the
formation of a thiol−ene complex prior to the actual thiol−ene
radical addition process, as was already suggested by Cramer et
al.21 In such case, oxygen can be scavenged preferentially by an
activated charge-transfer thiol−ene complex, leading to a
steeper thiol concentration dependence of the inhibition time
up to the point of equimolarity, after which addition of extra
thiol moieties will not lead to the formation of extra charge
transfer complexes and thus to a less pronounced further
decrease in inhibition time.
The chain extension reaction of vinyl-functionalized PDMS
with a thiol-functionalized PDMS can be regarded as a
simplified version of the cross-linking reaction of vinylfunctionalized PDMS with multithiol-functionalized PDMS, as
described by Müller et al.,15 with the difference that the
Figure 4. Double bond conversion as a function of the thiol to ene
ratio for thiol−ene networks prepared from telechelic vinyl PDMS 2
and thiol-functionalized PDMS cross-linker 4, using 700 MHz HRMAS and fluoride dissolution.
the preparation of PDMS-networks with very low and controlled
cross-link densities. The telechelic dithiol PDMS was therefore
synthesized starting from Tegomer H−Si 2311 (5, Scheme 3),
a telechelic hydroxyl-functionalized PDMS, of which the
hydroxyl functions were first converted into mesylate groups
(6) followed by a nucleophilic substitution with potassium
thioacetate in DMF. The telechelic PDMS dithiol (8) was
obtained by treating the thioacetate (7) with a small excess of
dry propylamine to give, after work-up, a slightly yellow and
almost odorless oil with an overall yield of about 95%.
In order to precisely assess the polymerization and/or crosslinking of thiol- and ene- functionalized PDMS with regard to
stoichiometry, conversion and side-reactions such as disulfide
formation, the starting materials were thoroughly characterized
using NMR analysis and triple-detection GPC. 1H NMR
analysis of 8 showed a number-average molecular weight Mn of
3100, which is slightly higher than the parent hydroxyfunctionalized material 5 (Mn 2900). This difference can be
fully explained by a slight and almost unavoidable formation of
disulfide in the telechelic dithiol when exposed to atmospheric
oxygen (indicated by a triplet at 2.60 ppm).
Scheme 3. Synthesis of a Telechelic Thiol-Functionalized PDMS Chain Extendera
a
Key: (i) CH3SO2Cl, NEt3, THF, 0 °C; (ii) KSAc, DMF, 20 °C; (iii) n-PrNH2, 0 °C; (i−iii) 95% overall yield.
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The average molecular weight of the silicon polymer is Mn =
m/(Nt0 + Nt0 − y). Hence, y = Nt0 + Nt0 − (m/Mn). The
double bond conversion can then be expressed as
y
x=
=
2Nv0
(N
t0
+ Nt 0 −
(2Nv0)
m
Mn
)=MN
n v0
+ M nNt 0 − m
2M nNv0
in which Nv0 and Nt0 represent respectively the number of
moles of vinyl- and thiol-functionalized telechelic PDMS and in
which m represents the total mass of the silicone polymer.
The number molecular weight Mn was calculated using the
following formula:19
log η = 1 + 0.0123M n 0.5
The rate of polymerization Rp can then be expressed as15
R p = −[M ]0
dx
= k′(x)[M]α [S−H]γ I0β
dt
in which x is the conversion of the double bonds, k′(x) is a
conversion-dependent quantity, [M]0 is the initial double-bond
concentration in the silicones, [S−H] is the thiol concentration
of the system and I0 is the intensity of the incident light. The
maximum rate of polymerization Rpm (normalized by M0) was
determined from the maximum slope of a plot of x versus time.
From Figure 6 it is clear that Rpm increases steeply up to the
point of thiol to ene equimolarity, followed by a decrease in
reaction rate.
Figure 5. (A) Final viscosity after prolonged irradiation at an
approximate light intensity (365 nm) of 12 mW/cm2 and a
concentration of DMPA of 11.7 μmol g−1. (B) Induction time as a
function of the different molar ratios of telechelic dithiol PDMS (8) to
telechelic divinyl (ene) PDMS (2).
molecular weight of the vinyl-functionalized PDMS used in our
experiments is approximately a factor of 5 higher and that the
functionality of the thiol component is only 2. As the use of
calorimetric methods or Raman spectroscopy are no longer
feasible for determining the double bond conversion as a result
of the dilution of functional groups, viscosimetry was
considered to follow conversions because of this method’s
sensitivity to changes in molecular weight associated with high
molecular weight components. Consequently, the viscosity
during the chain extension reaction was measured in steady
shear as a function of irradiation time. Conversion (x) was
calculated as a function of time using the relationship between
number-average molecular weight and conversion. Initially we
have 2Nv0 moles of vinyl groups and (2Nt0) moles of thiol
groups. If y is the number of moles of the vinyl groups that
reacted, the remained number of moles of vinyl groups is 2Nv0
− y and the remaining number of moles of thiol groups is 2Nt0
− y. Thus, the number of moles of both vinyl and thiol end
groups of the silicon polymer is 2Nt0 + 2Nt0 − 2y while the
number of moles of silicon polymer chains is Nt0 + Nt0 − y.
Figure 6. Maximum reaction rate (100(dx/dt)) of thiol-functionalized
telechelic PDMS (Mn = 3100 g/mol) with vinyl-functionalized PDMS
(33000 g/mol) with a concentration of DMPA of 11.7 μmol/g and an
approximate light intensity (365 nm) of 12 mW/cm2.
This result is similar, both qualitative and quantitative, to
earlier observations by Müller et al. for the cross-linking of
relatively low molecular weight telechelic vinyl terminated
PDMS (Mn ∼ 6000 g/mol) with multifunctional PDMS-based
thiols. The increase in the maximum reaction rate up to the
point of equimolarity can be explained by an increase in the
propagation kinetics, which is caused by an increase of the
efficiency of the radical transfer to thiol with increasing thiol
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complete reaction of the radical-initiated thiol addition to
vinyl−silicone, the curing reaction is also efficient for curing of
high molecular weight systems. A bifunctional thiol chainextender was efficiently synthesized from a commercially
available bis(hydroxyfunctional) low molecular weight silicone
in three virtually quantitative steps, and used to link high
molecular weight PDMS chains to form even higher molecular
weight vinyl-functionalized PDMS. Viscosimetry was used to
follow the kinetics of the chain-extension process. It was found
that the maximum reaction rate nicely follows the description
of Müller et al. in which two regimes can be discerned for the
thiol−ene cross-linking reaction of telechelic vinyl-functionalized PDMS with a multifunctional thiol.
Chain extension involves the reaction of two telechelic
PDMS derivatives, leading to an increase in molecular weight.
Although this system is different compared to the cross-linking
system described by Müller et al., it was shown to follow the
same rules as the ones that apply for the cross-linking system.
This means that the synthesized chain extender can be freely
used to replace part of the multithiol cross-linker, leading to the
formation of low cross-link density networks with elastic
moduli of down to 0.1 MPa. Moreover, the use of this chain
extender allows the formulation of easy processable, low
viscosity photocuring materials that, after cure, yield elastomers
with tunable mechanical properties.
HR-MAS NMR-analysis of the cured thiol−ene PDMS
networks demonstrated that the reaction between thiol and ene
in this particular system is extremely efficient, even in the high
dilution conditions used for the production of low cross-link
density elastomers. The double bond conversion of the thiol−
ene networks were all, within the margin of error, identical to
the maximum possible conversion calculated from the thiol to
ene feed-ratio. In addition to HR-MAS NMR analysis, the
networks were subjected to fluoride dissolution, followed by
regular solution-state NMR analysis. The results show that this
degradation, using fluoride ions, selectively degrades the PDMS
main-chain without affecting the integrity of the original crosslinking points. Quantitative analysis of the double-bond
conversion of the fluoride-degraded cross-linked material gave
identical results to those obtained by HR-MAS analysis,
showing the validity of this analysis approach for cross-linked
PDMS networks.
In principle, functionality of the networks can be easily tuned
by either adding a slight excess of thiol or ene, resulting in the
presence of vinyl, thiol or even disulfide functionality in the
final network, allowing for facile (surface) modification.
concentration. However, after the point of equimolarity, the
termination by thiyl−thiyl radical recombination becomes more
dominant, leading to a drop in Rpm. This drop in Rpm is steep,
indicating a sudden change in reaction kinetics, once the point
of equivalence is passed.
From the rheological data, obtained for the curing of linear
thiol and ene-functionalized PDMS, it became on the one hand
clear that photoinitiated thiol−ene chemistry can be applied to
efficiently link different PDMS molecules and on the other
hand that the kinetics of this linking process are similar to the
one of the cross-linking of vinyl-functionalized PDMS with
multi thiol-functional PDMS. Combination of thiol functionalized chain-extenders with multifunctional cross-linkers should
therefore result in the formation of a cross-linked network with
long interlinked PDMS-segments between cross-links, provided
that the stoichiometric ratio of thiol to ene is correct. Hence,
the replacement of a thiol-cross-linker by a linear telechelic
thiol functional chain-extender should give a linear decrease in
elastic modulus as a function of the molar fraction of chainextender added to the formulation, if this assumption is valid.
Figure 7 indeed confirms this assumption, proving that the
Figure 7. The elastic modulus of thiol−ene networks prepared from
telechelic vinyl PDMS 2 and multi thiol-functionalized PDMS 4 with
different amounts of chain-extender (CE-thiol) (8) at constant overall
thiol concentration.
■
EXPERIMENTAL SECTION
Instrumentation. Nuclear magnetic resonance spectra were
recorded on a Bruker Avance 300, a Bruker DRX 500 or a Bruker
AvanceII 700 spectrometer at room temperature. Tensile testing was
performed on a Tinus-Olsen H10KT tensile tester equipped with a
100 N load cell, using cylindrical specimen with an effective gage
length of 25 mm, and a diameter of 4.5 mm. The tensile tests were run
at a speed of 10 mm/min. Test specimens were prepared by filling 1
mL polypropylene syringes with photocurable formulation and
photocuring them at an approximate light intensity (365 nm) of 12
mW/cm2 in a Metalight Classic irradiation chamber for 5 min,
resulting in reproducible cylindrical specimens. Triple detection GPC
was performed on a PL GPC50plus (pump + five detectors). The five
detectors are a low angle light scatter detector at 15°, a right angle light
scatter detector at 90°, a viscometer, a refractive index detector, and a
UV Knauer Wellchrom Spectro-Photometer K-2501. The GPC system
was further fitted with two Plgel 5 μm MIXED-D columns and an PL
kinetics of thiol−ene addition of both the cross-linker and the
chain-extender are similar and that soft gel-like low cross-linkdensity elastomers can predictably be synthesized using a
telechelic thiol-functional chain extender in combination with a
multi thiol-functional PDMS cross-linker.
■
CONCLUSIONS
Thiol−ene curing of high molecular weight PDMS was proved
to be an efficient way for creating soft gel-like cross-linked
elastomers. Since the method is not depending on the use of a
heavy metal catalyst, the reaction is not as easily inhibited by a
whole range of materials such as traces of amines and tin
catalysts, as usually occurring for, e.g., platinum catalyzed
hydrosilylation based curing systems. Moreover, due to the
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vacuo, to yield a clear slightly yellow viscous liquid (100 g, 98%) 1H
NMR (300 MHz, CDCl3, ppm): 2.44 (q, CH2SH), 1.53 (m,
CH2CH2SH), 1.15−1.39 (m, γ-, δ- and ε-CH2), 0.46 (broad t,
CH2Si), 0.00 (s, (CH3)2Si(OR)2), −0.03 (s, (CH3)2Si(CH2)(OR))
Calculations of Double Bond Conversion (Figure 6) from the
Proton-NMR Integrals of the Vinyl and α Thiomethylene Signals
and the Molar Thiol-to-Ene Feed Ratio. With r being the molar thiolto-ene feed ratio, Iα the integral of the α thiol and thioether methylene
signals (corresponding to 2 protons), Iγ the integral of the γ peak
(corresponding to 2 protons), IDB the integral of the vinyl signal
(corresponding to 3 protons), and assuming insignificant disulfide
formation, it follows that:
1 ≤ Iα/Iγ ≤ 2, with Iα/Iγ = 1 when no thiol−ene reaction occurs and
Iα/Iγ = 2 when all thiol groups undergo thiol−ene reactions. Thus, the
double bond conversion (DB %) calculated from the α signal
normalized to the γ signal is
AS RT auto sampler. Viscosity was measured using a Brookfield DVII
viscosimeter fitted with a LV4 spindle and a PCS10/k8047 data logger.
HR-MAS NMR Analysis. NMR samples were prepared as follows:
dry material was cut in small pieces and put in a 4 mm rotor (80 μL).
Next, solvent (CDCl3) was added to allow the material to swell. This
removes most of the dipolar line broadening typically associated with
the solid state, while residual line broadening caused by susceptibility
differences can be handled by spinning at the magic angle. The sample
was homogenized by stirring within the rotor. All 1H NMR spectra
were recorded on a Bruker Avance II 700 spectrometer (700.13 MHz)
using a hr-MAS probe equipped with a 1H, 13C, 119Sn and gradient
channel. Samples were spun at a rate of 6 kHz. To characterize the
gels, 1D 1H spectra were recorded. All spectra were measured with an
acquisition time of 1.136 s in which 32768 fid points were obtained,
leading to a spectral width of 20.6 ppm. For qualitative analysis, 8
transients were summed up with a recycle delay of 2 s. For
quantification, 32 scans were used with 30 s recycling delay to
guarantee full relaxation of the signal.
Materials. Vinyl-terminated poly(dimethylsiloxane) and poly(dimethylsiloxane-co-methylmercaptopropylsiloxane) were obtained
from ABCR, hydroxyhexyl-terminated poly(dimethylsiloxane)
(Tegomer H-Si 2311) was obtained as sample from Evonik.
Tetrahydrofuran (THF, Aldrich, HPLC grade), anhydrous dimethylformamide (Aldrich, 99.8%), anhydrous toluene (Aldrich, 99.8%),
mesyl chloride (Acros Organics, 99.5%), propylamine (Aldrich, 98%),
potassium hydroxide (Aldrich, 90%), and thioacetic acid (Aldrich,
96%) were used as received. Triethylamine (Aldrich, 99%) was
distilled from calciumhydride prior to use.
Synthesis. Methylsulfonyl-Terminated Telechelic Poly(dimethylsiloxane) (6). Hydroxy-terminated poly(dimethylsiloxane)
(5, 100 g, 34.5 mmol) was dissolved in dry toluene (100 mL) and
evaporated under a nitrogen atmosphere to remove traces of water.
Then dry THF was added (200 mL) and the mixture was brought
under argon and cooled to 0 °C. Dry triethylamine (10.6 mL, 76
mmol) was added followed by a dropwise addition of mesyl chloride
(5.9 mL, 76 mmol) under vigorous stirring. The stirring was continued
for 30 min at 0 °C and another 4 h at room temperature. The reaction
mixture was then diluted with diethyl ether (200 mL), washed with
brine (2 × 200 mL), dried on anhydrous magnesium sulfate and
concentrated in vacuo, to yield a clear colorless viscous liquid (106.6 g,
100%) 1H NMR (300 MHz, CDCl3, ppm): 4.15 (d, CH2OMs), 3.41
(q, C H2CH2OMs), 2.92 (s, CH3SO2), 1.68 (m, γ-CH2), 1.2−1.3 (m,
δ- and ε-CH2), 0.46 (broad t, CH2Si), 0.00 (s, (CH3)2Si(OR)2), −0.03
(s, (CH3)2Si(CH2)(OR)).
Thioacetyl-Terminated Telechelic Poly(dimethylsiloxane) (7). To
a stirred solution of thioacetic acid (5.4 mL, 76 mmol) in
dimethylformamide (500 mL) was added potassium hydroxide (4.26
g, 76 mmol). The mixture was then cooled to 0 °C and
methylsulfonyl-terminated telechelic poly(dimethylsiloxane) (6,
105.4 g, 34.5 mmol) was added all at once. After an initial exothermic
reaction the mixture formed a stiff gel due to the formation of
potassium mesylate. After 1 h at 0 °C the mixture was kept at room
temperature for 8 h. Water (750 mL) was then added, which dissolves
the gel forming a two-layer system. Subsequently the mixture was
extracted with diethyl ether (3 × 200 mL). The combined organic
layers were washed with brine (2 × 300 mL), dried on anhydrous
magnesium sulfate and concentrated in vacuo, to yield a clear slightly
yellow viscous liquid (104.2 g, 99%) 1H NMR (300 MHz, CDCl3,
ppm): 2.79 (t, CH2SAc), 2.24 (s, SCOCH3), 1.49 (m, CH2CH2SAc),
1.16−1.36 (m, γ-, δ-, and ε-CH2), 0.45 (broad t, CH2Si), 0.00 (s,
(CH3)2Si(OR)2), −0.03 (s, (CH3)2Si(CH2)(OR)).
Thiol-Terminated Telechelic Poly(dimethylsiloxane) (8). A stirred
solution of 7 (104.2 g, 34.2 mmol) in tetrahydrofuran (100 mL) was
brought under argon and was cooled down to 0 °C. Subsequently, npropylamine (6.2 mL, 75 mmol) was added and the stirring was
continued for 1 h. Under a steady stream of argon, dilute hydrochloric
acid (50 mL, 10%) and brine (300 mL) were then added.
Subsequently the mixture was extracted with diethyl ether (3 × 200
mL). The combined organic layers were washed with brine (2 × 300
mL), dried on anhydrous magnesium sulfate and concentrated in
DB % = (Iα /Iγ − 1) × r × 100
On the other hand, the double bond conversion can also be
calculated from the vinyl signal normalized to the γ signal, following
DB % = (1 − 2rIDB/3Iγ ) × 100
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: (F.E.D.P.) fi[email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
O.v.d.B. is thankful to the FWO for the financial support. Prof.
José C. Martins and Dr. Krisztina Feher are acknowledged for
the help with the HR-MAS measurements. F.G. thanks the
Research Foundation-Flanders (FWO) for the funding of her
Ph.D. fellowship. F.E.D.P. acknowledges the Belgian program
on Interuniversity Attraction Poles initiated by the Belgian
State, the Prime Minister’s Office (P7/05), and the European
Science Foundation Precision Polymer Materials (P2M)
program for financial support.
■
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