F. B.
Madsen
,
A. E.
Daugaard
,
C.
Fleury
,
S.
Hvilsted
and
A. L.
Skov
*
Danish Polymer Centre, Department of Chemical and Biochemical Engineering, Technical University of Denmark, DTU, Søltofts Plads, Bldg. 227, 2800 Kgs. Lyngby, Denmark. E-mail: al@kt.dtu.dk
First published on 6th January 2014
The existence of short-chain domains in heterogeneous bimodal PDMS networks has been confirmed visually, for the first time, through confocal fluorescence microscopy. The networks were prepared using a controlled reaction scheme where short PDMS chains were reacted below the gelation point into hyperbranched structures using a fluorescent silicone compatible cross-linker. The formation of the hyperbranched structures was confirmed by FTIR, 1H-NMR and size exclusion chromatography (SEC). The short-chain hyperbranched structures were thereafter mixed with long-chain hyperbranched structures to form bimodal networks with short-chain domains within a long-chain network. The average sizes of the short-chain domains were found to vary from 2.1 to 5.7 μm depending on the short-chain content. The visualised network structure could be correlated thereafter to the elastic properties, which were determined by rheology. All heterogeneous bimodal networks displayed significantly lower moduli than mono-modal PDMS elastomers prepared from the long polymer chains. Low-loss moduli as well as low-sol fractions indicate that low-elastic moduli can be obtained without compromising the network's structure.
The mechanical properties of PDMS elastomers, as an alternative, can be improved by creating bimodal networks which are prepared by applying advanced mixing schemes. Bimodal networks consist of very short and relatively long chemically identical polymer chains cross-linked into a network, and their mechanical properties are superior to traditional elastomers.5,6 Several studies have investigated the unexpected high tear resistance and ultimate strength of bimodal networks.7–9 The elastic modulus of bimodal networks increases significantly at high elongations, so the ultimate strength of the networks is very high. Improvements in ultimate strength stem from the limited extensibility of the short chains, which gives the networks toughness.10,11 Furthermore, the relatively long chains retard the rupture process and provide extensibility to the network.9 Therefore, bimodal networks exhibit both a substantial ultimate stress and ultimate strain.6 Bimodal networks can be created by the random distribution of short polymer chains within the long chains,6,12–14 or as heavily cross-linked short-chain domains joined to the long-chain network.15–17 Bimodal networks with homogenous (random) distributions of short and long chains are prepared in one-step procedures where the chains are mixed and cross-linked simultaneously to form random networks. In contrast, heterogeneous bimodal networks are prepared in two-step procedures. The short-chain domains are prepared in a reaction that happens prior to the cross-linking, where the short chains are reacted below the gelation threshold into hyperbranched structures which are subsequently mixed and cross-linked together with the long chains. Consequently, short-chain domains are created within the long-chain network. Furthermore, the long chains can also be reacted with cross-linker molecules below the gelation threshold – prior to the network formation – in order to obtain even higher control over the network and the distribution of long- and short-chain domains.17
We have previously demonstrated that the advantage of forming heterogeneous bimodal networks with a controlled distribution of short-chains is that the elastic modulus becomes lower compared to both homogenous bimodal networks with similar short chain concentrations and mono-modal PDMS networks.17 In so doing, softer and more stretchable PDMS elastomers are obtained. The films are at the same time easily handled as thin films, which for the homologues mono-modal PDMS network is difficult without the addition of silica particles. Furthermore, heterogeneous bimodal networks have been shown to exhibit very low viscous losses in combination with low elastic moduli.17 So they are therefore excellent alternatives to both mono-modal PDMS elastomers, which require fillers to have sufficiently high tear strength to make them manageable as thin films, and to homogenous bimodal networks, which have higher elastic moduli due to the random distribution of short chains.
We herein present a novel method for analysing the size of short-chain domains in heterogeneous bimodal PDMS networks. A recently developed silicone compatible cross-linker allowing for orthogonal chemistry18 is used to tag the short-chain hyperbranched structures at their cross-linking points with fluorescent molecules. The size of the short-chain domains will furthermore be correlated to the final mechanical properties of the networks. The design scheme offers an unprecedented way of tuning the properties of heterogeneous bimodal networks and thereby paves the way for the implementation of such networks in advanced applications where strong, unfilled and largely extensible networks are needed. The properties of the resulting networks will fulfil the demands of many commercial PDMS elastomer applications, including soft lithography applications where higher fracture energy is needed,19 optical applications where strong elastomers without optically distorting fillers are desired3,4 and dielectric elastomers where soft networks with high extensibilities and low viscous losses are essential.20–25
Fig. 1 Schematic illustration of the two-step reaction procedure employed to form heterogeneous bimodal networks. |
In a similar manner, the long-chain hyperbranched structures were prepared by using a commercial cross-linker (28000 g mol−1, bearing, on average, 16 vinyl groups per chain) and a higher molecular weight PDMS (28000 g mol−1). The hyperbranched structures, based on the short and long chains, were mixed and cross-linked by using the commercial cross-linker in excess. The fluorescent and reinforcing short-chain hyperbranched structures thereby became covalently bound to the long-chain hyperbranched structures, ensuring sufficient separation of the short-chain structures to maximise the reinforcing effect.
The heterogeneous bimodal networks were prepared with varying contents of short-chain hyperbranched structures, in order to illustrate the effect of the ratio between long- and short-chain hyperbranched structures. The concentration of short-chain structures varied from 0 to 30 wt%, corresponding to 0 to 92 mol% (of the total number of hydride functional polymers). Films based entirely on the short-chain polymer could not be prepared, as the polymer was of a too low molecular weight to form a mechanically stable film.
The prepared heterogeneous bimodal networks were characterised by confocal microscopy, which uses single-point illumination and rejects out-of-focus light. Thus, images with better vertical optical resolution than traditional fluorescence microscopy are obtained. The acquired images had reduced background haze and represent a thin cross-section of the sample, as shown in Fig. 2 (original fluorescence confocal microscopy images of all samples can be found as ESI†).
The fluorescent domains are evenly distributed within all networks. Furthermore, it can be seen from Fig. 2 that the fluorescent domains vary in size and abundance, depending on the ratio between the short- and long-chain hyperbranched structures in the network. The mean diameters of the fluorescent domains were determined as an average of 100 domains, the results for which are summarised in Table 1. As seen in Table 1, the sizes of the fluorescent domains vary only moderately for the concentrations of short-chain hyperbranched structures, from 3 wt% to 20 wt%, whereas substantially larger domains are observed at 30 wt%. Furthermore, it is clear from Fig. 2 that the domains are very well distributed, and the number of domains is high for both samples. When the concentration of the short-chain hyperbranched structures is increased to 30 wt%, a substantial increase in the domain's size is evident (Fig. 2), due to the short-chain hyperbranched structures becoming so tightly packed that even upon ideal distribution they will interconnect covalently and turn into larger agglomerates. This is also substantiated by the large increase in the standard deviation of the measured average short-chain domain diameter.
Short-chain concentration | Domain diametera (D) | C N b × 10−9 | G′@1 Hz | G′′@1 Hz | |
---|---|---|---|---|---|
[Wt%] | [Mol%] | [μm] | [g−1] | [kPa] | [kPa] |
a The mean diameters were determined with the imaging processing program ImageJ. b The number average concentration of short-chain domains is calculated as: CN = ((6 × wt%)/(πD3))(1/ρPDMS), where ρPDMS is the density of PDMS. | |||||
0 | 0 | — | — | 93.4 | 1.19 |
3 | 45 | 2.9 ± 0.9 | 2.4 | 11.8 | 0.32 |
10 | 75 | 2.8 ± 0.8 | 9.0 | 21.5 | 0.20 |
20 | 87 | 2.1 ± 0.5 | 43 | 78.1 | 0.98 |
30 | 92 | 5.7 ± 1.6 | 3.2 | 59.5 | 0.70 |
The influence on the distribution and size of the observed domains on the elastic and mechanical properties of the bimodal networks was determined by employing small amplitude rheology. The resulting shear storage moduli (G′) and loss moduli (G′′) are summarised in Table 1. The mono-modal PDMS reference network, with 0% of short chains, has a storage modulus of 93.4 kPa. Upon the addition of the short-chain hyperbranched structures, the modulus decreases significantly to 11.8 kPa. From fundamental theories on rubber elasticity the elastic moduli are expected to scale linearly with the inverse average molecular weight of the polymer chains in the network.28 As such, the addition of very short polymer chains to the system would be expected to decrease the average molecular weight of the system and thus increase the elastic modulus significantly. This decrease in the elastic modulus in heterogeneous bimodal networks has been shown previously but not explained with supporting data.16,17 Based on the unprecedented correlation between the network composition, size and distribution of the fluorescent domains, and the elastic properties of heterogeneous bimodal networks, we believe that the decrease in the elastic modulus can be explained by a multi-domain theory. When short-chain hyperbranched structures are added, they naturally contain a large concentration of unreacted hydride groups on the surface. These groups react with cross-linker molecules, which upon initial reaction are hindered from diffusion by reptation.29 Consequently, there is a large concentration of cross-linker molecules on the surface of the hyperbranched structures. Small cross-linker molecules, such as those used in our previous study,17 are packed tightly around the surface, whereas the long-chain cross-linker molecules used in this study tend to fold around the short-chain hyperbranched structures. In both cases, this creates areas around the cross-linker molecules where the effective cross-linking density is lower than in the bulk long-chain network, as a large number of the cross-linking sites are present around the short-chain domains, as illustrated in Fig. 3. In this study, with a high molecular weight cross-linker, folding back to the short-chain domains also leads to the formation of loops, which do not contribute to elasticity to the same extent as the true cross-linking sites.30 Domains with surrounding cross-linker molecules therefore create a local softening effect in the PDMS networks. The heterogeneous bimodal elastomers will consequently be softer and have a lower elastic modulus than the homologous mono-modal networks despite higher average cross-linking density. The reduction in the modulus, from mono-modal to a small amount of short-chain domains, is confirmed experimentally. The elastic modulus is then increased with increasing short-chain concentration. At high concentrations of short chains the elastic modulus drops again. This can be explained by a drop in the number average concentration of short-chain domains CN = ((6 × wt%)/(πD3))(1/ρPDMS), with the additional increase of short chains (see Table 1). From the calculations it can be seen that for the bimodal networks, the elastic modulus increases with increasing CN, although, not in a trivial way.
Interfaces with a lower cross-linking density are still perfectly cross-linked due to the high number of functional groups on each hyperbranched structure, and as such they cannot be regarded as network imperfections, since their mechanical properties are not destroyed.
Very soft silicone elastomer behaviour can also be observed for imperfect networks, which occurs as a result of unreacted groups. This lack of reactivity can be introduced either from stoichiometrically imbalanced reactions27,31 or from incomplete catalyst inhibition reactions.32 It can also arise naturally from steric hindrance or heterogeneity, thus leading to local cross-linker and polymer excesses, respectively. Therefore it is important to ensure that heterogeneous bimodal networks do not contain a large fraction of solubles, so-called sol fractions. The sol fractions of heterogeneous bimodal PDMS networks were determined by conducting swelling experiments with toluene, the results for which can be found in Table S1 in the ESI.† The sol fractions were found to lie in the range of 5–10% and were apparently independent of short chain concentration. The addition of the short chains as domains therefore does not create additional imperfections in the infinite networks, which means that there is complete compatibility between the short-chain domains and the long-chain matrix.
Swelling experiments were performed in order to determine the relationship between the volume degree of equilibrium swelling (Qv) and the mechanical properties. The results can be found in the ESI.†Qv is found to decrease with increasing short-chain concentration, due to the strong increase in cross-link density. Furthermore, as expected, there is not a linear relationship between the elastic moduli (3G′) and the equilibrium swelling degree (see Fig. S3 in the ESI†), which is observed for traditional mono-modal PDMS elastomers. Heterogeneous bimodal networks therefore behave quite different to mono-modal PDMS networks, since the modulus decreases following the addition of short-chain domains, i.e. with increasing cross-link density. This means that swelling capacity is determined from the continuous long-chain part of the network, whereas elasticity is governed by a combination of short- and long-chains properties.
In general, it is difficult to obtain PDMS elastomers with low elastic moduli without deteriorating other network properties. Soft PDMS elastomers therefore usually suffer from high viscous losses. The frequency-dependent viscous loss can be determined from the shear loss modulus G′′. All the prepared bimodal elastomers are seen to have lower viscous loss than the mono-modal PDMS network. Furthermore, the heterogeneous bimodal networks are well cross-linked, as the storage moduli (G′) is 10 to 100 times larger than the loss moduli (G′′) for the investigated frequency range, with differences being smallest at high frequencies. A plot of the storage and loss moduli as functions of frequency can be found as ESI.† Viscous loss at the low strain low frequency limit is measured as a few percentage points only. This is comparable to our previously reported results for heterogeneous bimodal networks.17 Viscous loss in a polymer network is due to dangling chains and sol molecules, i.e. incomplete network formation.27 It is therefore evident from the low viscous losses of the heterogeneous bimodal networks that the obtained low moduli are not a result of incomplete reactions but rather of network artefacts, as shown in Fig. 3. The loops do not contribute significantly to the elastic modulus, but at the same time they do not contribute a loss, since they cannot relax. Heterogeneous bimodal networks are therefore an effective and simple method of creating soft elastomers with very low viscous losses.
The thermal transition behaviour of the heterogeneous bimodal networks was determined using differential scanning calorimetry (DSC). The resulting thermograms can be found as ESI,† while the crystallisation and melting temperatures are summarised in Table S1 in the ESI,† too. A large decrease in the area under the crystallization peaks can be seen in the thermograms, along with an increasing concentration of short-chains. This means that there is a smaller degree of crystallinity in the material in line with increasing short-chain content. Consequently, the PDMS elastomers become inherently amorphous at large concentrations of short chains, as almost no crystalline behaviour is observed. This is most likely due to an increased cross-linking density as the short chain domains are highly dense structures in elastomers. This high cross-link density means that the polymer chains are no longer able to arrange into the positions necessary for crystalline regions to form. Melting temperatures are also observed to decrease with higher concentrations of short chains because of the dilution of the crystalline segments. Furthermore, areas under the melting peaks are reduced at higher concentrations, which mean that the energy required for melting is reduced at larger concentrations of short chains.
Moreover, the effect of short-chain domains on the thermal stability of the PDMS networks was investigated by thermal gravimetric analysis (TGA). The determined degradation temperatures for the bimodal PDMS networks with different short-chain concentrations are summarised in Fig. S5 and Table S1 in the ESI.† The first degradation step occurs due to the cleavage of Si–CH3 bonds. This degradation temperature increases significantly with increasing amounts of short chains (from ∼430 °C to ∼530 °C), which demonstrates that heterogeneous bimodal networks also have a beneficial influence on the thermal stability of PDMS elastomers.
Footnote |
† Electronic supplementary information (ESI) available: 1H-NMR spectra and SEC traces of the short-chain hyperbranched structures before and after the reaction, tabulated data from DSC, TGA and swelling experiments, plot of the elastic moduli (3G′) as a function of the degree of equilibrium swelling (Qv), DSC thermograms, TGA curves, fluorescent confocal microscopy images of all samples and plots of the storage and loss moduli as functions of frequency. See DOI: 10.1039/c3ra47522k |
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