Low Modulus Dry Silicone-Gel Materials by ... - ACS Publications

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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.

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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 1292

dx.doi.org/10.1021/ma402564a | Macromolecules 2014, 47, 1292−1300

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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

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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. 1293


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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. 1294


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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 1295


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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. 1296


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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 1297


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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.

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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.

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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 1298


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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

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (F.E.D.P.) fi[email protected]. Notes

The authors declare no competing financial interest.

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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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REFERENCES

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