Linear Rheology of Supramolecular Polymers Center-Functionalized

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Linear Rheology of Supramolecular Polymers Center-Functionalized with Strong Stickers X. Callies,*,⊥,† C. Véchambre,‡ C. Fonteneau,§,∥ S. Pensec,§,∥ J.-M. Chenal,‡ L. Chazeau,‡ L. Bouteiller,§,∥ G. Ducouret,⊥,† and C. Creton*,⊥,† Sciences et Ingénierie de la Matière Molle, CNRS UMR 7615, École Supérieure de Physique et de Chimie Industrielles de la Ville de Paris (ESPCI), ParisTech, PSL Research University, 10 rue Vauquelin, F-75231 Paris cedex 05, France † SIMM, UPMC Univ Paris 06, Sorbonne-Universités, 10 rue Vauquelin, F-75231 Paris cedex 05, France ‡ Laboratoire MATEIS, CNRS, INSA Lyon, 7 avenue Jean Capelle, Villeurbanne, 69100, France § Chimie des Polymères, UPMC Univ Paris 06, UMR 8232, IPCM, Sorbonne Université, F-75005 Paris, France ∥ Chimie des Polymères, UMR 8232, IPCM, CNRS, F-75005 Paris, France ⊥

S Supporting Information *

ABSTRACT: We investigate the linear viscoelastic properties of a series of low polydispersity poly(n-butyl) acrylate chains center-functionalized with a triurea interacting moiety, able to self-associate by six hydrogen bonds. Depending on the molecular weight (Mn) of the side chains, the polymers can self-associate and form supramolecular structures in the melt state. For Mn < 40 kg/mol, the polymers form large bundles of parallel rods which relax stress as large colloidal objects. For Mn ≥ 40 kg/mol, the self-assembly of stickers form smaller and randomly oriented rods and the viscoelastic properties are mainly governed by the side chains relaxations, as observed for weak stickers. However, in both regimes, the side chains relax at short time scales, and then, the relaxation of fibrillar aggregates follow (by scission or diffusion process) at long time scales. The weak correlation between both relaxation modes makes the tuning of the linear viscoelastic properties relatively easy by varying independently the length of the side chains and the associating strength of the sticker. The good control of the molecular dynamics via the chemical structure makes the supramolecular center-functionalized polymers extremely interesting for materials applications where dissipative rheological behaviors are targeted over large frequencies ranges, such as vibration damping or adhesive applications.

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INTRODUCTION Supramolecular chemistry, defined as the reversible association of molecules through weak interactions, is at the center of molecular associations in living organisms and has been recently harnessed to form new materials exploiting the ability of molecules to self-assemble.3−7 Most of these self-assembly processes are studied in solution but if the material is a fluid itself, they can be studied in the absence of solvent. This is typically the case for flexible polymers above their glass transition, where chains enjoy the long-range molecular mobility necessary for self-assembly, but can store significant elastic energy, a characteristic of solids, to result in interesting soft materials. One particular type of new soft material which has emerged is a reversible network of polymers, obtained by functionalizing flexible polymer chains with strongly interacting groups.8−11 Seiffert and Sprakel,8 have recently reviewed the dynamics of these supramolecular structures and pointed out the complex interplay between the bond energy of individual stickers, the presence of large-scale supramolecular structures and the resulting rheological properties of the material. It is © XXXX American Chemical Society

therefore essential to systematically characterize the rheology of well-defined model systems where the molecular weight of the polymer, the position of the sticker groups on the chain and the sticker concentration are systematically varied. Most past studies have focused on systems containing more than one sticker group per chain and which therefore tend to form reversible elastic networks.10−14 However, for applications where dissipative properties may be more important than elasticity, self-associating groups can be also used to control the molecular dynamics and introduce additional relaxation processes. Along those lines, our group has focused in the past on self-assembly and adhesive properties of 3 kg/mol polyisobutylene (PIB) chains bearing a single sticker per chain located in its middle. In solution these chains self-associate into cylindrical structures15 while in the melt they form highly dynamic structures which show some long-range order below Received: July 17, 2015 Revised: September 11, 2015

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DOI: 10.1021/acs.macromol.5b01583 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules 50 °C and have very interesting and unique dissipative properties which makes them potentially useful as soft adhesives.16 Motivated by this first proof of concept study, we carried out a more comprehensive investigation on the rheological and adhesive properties of polymer chains (above their glass transition temperature), center-functionalized with an associating group. Two types of associating groups were tested: the bisurea xylene using quadruple H-bonds associations (the corresponding polymers were named PnBAX) and the triurea toluene associating more strongly by sextuple H-bonds (PnBA3U). For both sticker types the molecular weight of the polymer was varied from 5 to 100 kg/mol exploring systematically the effect of increasing the molecular weight of the flexible polymer and decreasing the concentration of the stickers. PnBAX does not form self-assembled structures with long-range order as the PIB center functionalized with bis-urea toluene does and yet its rheological properties are clearly affected by the presence of the sticker group as shown in a first paper of this series.2 On the other hand, PnBA3U moieties show both long-range supramolecular self-organization and a gel to fluid transition, which markedly depend on temperature and PnBA molecular weight. The results of such a parallel and systematic investigation are the focus of two papers, a companion paper focusing on thermodynamics and supramolecular self-assembly1 and the current paper focusing on the rheological properties. The molecular structure of the model system is schematically shown on Figure 1. A series of center-functionalized PnBA with

Table 1. Structural Parameters of PnBA3U Systems

PnBA3U systems

reference PnBA

DP

Φs (%)

Tg (°C)

5200

1.24

36

6.8

−47.2

8000 12000 18000 40000 57000 85000 107000

1.2 1.24 1.23 1.32 1.3 1.34 1.4

58 89 140 310 440 660 990

4.4 3.0 1.9 0.89 0.62 0.42 0

−48.2 −46.2 −46.4 −47.7 −47.1 −49.7 −54.8

Mn (g/mol)

PnBA3U5 PnBA3U8 PnBA3U12 PnBA3U18 PnBA3U40 PnBA3U60 PnBA3U85 PnBA107

supramolecular polymers were synthesized by a functional initiator approach. First self-assembling compounds bearing three urea groups and two carbon−halogen moieties were prepared. In a second step these compounds were used as initiator for the ATRP polymerization of n-butyl acrylate. Therefore, several poly n-butyl acrylates of known molecular weight and center-functionalized with a triurea sticker were obtained (see Figure 1). The molecular characteristics of the supramolecular polymers studied in this paper are reported in Table 1. The number-average molar mass Mn and dispersity (Đ = Mw/Mn with the weight-average molar mass Mw) were determined by size exclusion chromatography (SEC) in tetrahydrofuran, with a refractive index detector and a polystyrene calibration curve for samples PnBA3U5, PnBA3U8, PnBA3U12, and PnBA3U18 or with a triple detection setup for the other samples (see Figure S1 for representative SEC curves). The average degree of polymerization DP and the sticker density Φs in the polymer matrix are then estimated: DP = (Mn− Ms)/ Mbu and Φs = Ms/Mn with the molar mass Ms of the sticker and the molar mass Mbu of the butyl acrylate monomers. In order to rule out the presence of any residual solvent in PnBA3U after synthesis, samples are analyzed by 1H NMR (Bruker Avance 200) (see Figure S2). Rheology. Oscillatory shear tests were carried out on a stresscontrolled rheometer Anton Paar Physica MCR 501 in the linear regime over a wide temperature range (−30 to 50 °C). Before each experiment, samples were annealed for 1 day at 70 °C and 200mbar in the presence of silica dryer. The sample was set up between the two parallel plates of the rheometer (diameter 8 or 25 mm) at room temperature. As PnBA3Us flow under stress, the final gap was reached by compressing the sample with the upper plate. The sample was annealed at 50 °C for 2 h and then, frequency sweeps (0.02 to 20 Hz) of G′(ω) and G″(ω) were carried out in 5 °C increments down to −30 °C in order to construct master curves. After the overall process, a frequency sweep was repeated at 25 °C to check if the supramolecular structure did not change during the experiments. For all samples, the same frequency dependence at 25 °C was observed before and after the experimental investigation (see Figure S3 in Supporting Information).

Figure 1. Schematic chemical structure of the PnBA3U supramolecular polymer synthesized by Fonteneau17 by ATRP. The polar sticker is linked to two linear poly n-butyl acrylates (R). The degree of polymerization changes from one sample to another.

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RESULTS Rheological Investigations at 25 °C. Figure 2 shows the rheological behavior of different molecular weights of PnBA3U in the linear domain at 25 °C. Two different behaviors are clearly observed. For the molecular weights between 5 kg/mol and 20 kg/mol, the rheological response is that of a soft elastic solid. The level of the low frequency elastic plateau decreases when the molecular weight increases. For Mn ≥ 40 kg/mol, our supramolecular polymers are viscoelastic liquids: G″ remains higher than G′ on all the experimental range of angular frequencies. As discussed later, the Mn dependence of the rheological behavior must be analyzed in light of the multiscale structure investigated in the companion paper.1 The structure of all

a narrow molecular weight distribution was synthesized (see Table 1) with a range of molecular weights ranging from below to above the average molecular weight between entanglements of pure PnBA (Me ∼ 20−30 kg/mol).18 The linear rheological properties of this series of model polymers were characterized over a wide range of temperatures (−30 to 50 °C) and master curves have been constructed when possible, by using the time−temperature superposition principle. The respective role of the sticker and of the molecular weight of the side chain in controlling the rheological properties will then be discussed.

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Đ

samples

EXPERIMENTAL PART

Materials. The synthesis procedure of the supramolecular polymers was previously described by Fonteneau et al.17 Briefly, B


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Figure 2. Linear rheology curves at 25 °C for different molecular weight PnBA3U. The storage modulus G′ is represented by filled markers, the loss modulus G″ by unfilled markers.

materials were investigated at 25 °C by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). For short PnBA chains (Mn ≤ 18 kg/mol), SAXS and AFM data show a clear correlation peak around a 5−10 nm indicating that the molecules form cylindrical rods or filaments oriented parallel to each other, organized in bundles over extended regions while for Mn ≥ 40 kg/mol the cylindrical structures exist but are shorter in length and are not bundled.1 Rheology Master Curves. Master curves were constructed at the reference temperature of 7 °C for all materials by using the time−temperature superposition (TTS) principle. For all molecular weights investigated in this study, the superposition of the viscoelastic moduli works well for T ≤ 30 °C (see Figures 3 and 4),while at higher temperatures, the superposition fails slightly for Mn ≥ 40 kg/mol (see Figures S4 and S5). The horizontal shift factors a(T) were first determined by superposing the experimental tan δ (ω, T) curves while the vertical shift factors b(T) were subsequently determined by superposing G″(a(T)ω, T). These shift factors do not depend

Figure 4. G′ and G″ master curves for PnBA3U40 and PnBA3U60 at Tref = 7 °C. The color of the experimental points indicates the temperature at which frequency sweeps were carried out. Additional vertical shifts were applied for PnBA3U60 (×0.2).

Figure 3. G′ and G″ master curves for PnBA3U5, PnBA3U12, and PnBA3U18 at Tref = 7 °C. The color of the experimental points indicates the temperature at which frequency sweeps were carried out. Additional vertical shifts were applied for PnBA3U5 (×8) and PnBA3U12 (×5). The dashed line follows a power law ω0.5.

on the molecular weight and thus, on the density of stickers in the PnBA matrix (see Figure S7). The factors a(T) of PnBA3U are similar to the factors measured for poly(butyl acrylate) homopolymers and thus follow the classic WLF relation18 (see Figures S6 and S7) while b(T) are close to 1 for all samples. The G′(ω) and G″(ω) master curves give a clear snapshot of the viscoelastic properties over a wide frequency range. For all materials, (at low and high Mn), different regimes can be identified on the G′ and G″ plots. The master curve for our two lowest Mn (PnBA3U5 and PnBA3U12) can be divided in a solid regime (G′ ≫ G″) at low ω, a transition regime ((G′∼ G″ ∼ ω0.5) at intermediate ω and a dissipative regime (G′ < G″) at high ω (see Figure 5). As the Mn of the PnBA increases, the master curve can now be more clearly divided into four regimes (see PnBA3U18 in Figure 3 or 5). In between the previously described regimes, an elastic “plateau” (G′ > G″) is observed between the low frequency gel and the high frequency dissipative part. As shown in Figure 4, the master curves for Mn ≥ 40 kg/mol look more like master curves of lightly entangled polymer chains with a viscous flow at low ω, an elastic plateau at intermediate ω and a dissipative part at high C


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Figure 6. tan δ master curves for PnBA3U5 (red), PnB3U12 (yellow), PnBA3U18 (green), and PnBA3U40 (blue) for Tref = 7 °C. The arrows indicates the frequency ωd for the different samples and the frequency ωe when it exists.

molecular weights PnBA3U (Mn ≥ 40 kg/mol) is characterized by a terminal zone (G″ > G′). This gel/fluid transition is directly related to the formation of cylindrical structures which self- organize in bundles as shown by Véchambre et al.1 The formation of bundles (detected by SAXS) disappears at the same temperature as the material properties shift from solid to liquid; suggesting strongly that it is not the associations between stickers (which exist for all molecular weights) which control the sol−gel transition but the long-range order between rods. This order−disorder transition bears similarities with the order−disorder transitions of nematic liquid crystal phases (for orientational order) and to the thermodynamics of selfassembly of hairy rods polymers.20 At higher temperatures yet, the equilibrium size of the rods may decrease as a result of the competition between the strength of the H-bond and the conformational entropy penalty of the side chains forming a supramolecular bottle brush.1,21,22 In addition to the elastic behavior at long time scales, master curves reveal that these supramolecular gels are extremely dissipative over a large frequency and modulus range (4 decades in frequency and 2 decades in modulus for PnBA3U12 and PnBA3U20). The master curves show that the structure of the bundles remains unchanged as a function of T. The dynamics of scission/recombination of stickers is too slow to contribute to the viscoelasticity of the supramolecular material. This interpretation is consistent with the order/disorder transition seen by X-ray scattering at much higher temperatures for these systems1 (TODT ∼ 170 °C for PnBA3U5). The similarity between the shift factors measured for the PnBA3U series and the PnBA107 (without stickers) suggests that the dissipative behavior at short time scales is due to the relaxation of the PnBA side chains and that the supramolecular rods simply structure the material in a way that extends the frequency range of this dissipative behavior. At high frequency (ω > ωd), the dissipative behavior of PnBA3U is independent of the density of stickers or of the length of the side chains, since ωd ∼ 5 × 103s−1 is quasiindependent of Mn. Interestingly, a similar value for ωd ∼ 3 × 103s−1 is observed for the reference polymer PnBA107 (see Figure S6). By analogy with unentangled or entangled polymers23 this dissipative relaxation mechanism should be attributed to the α-relaxation of the PnBA matrix. This

Figure 5. Critical frequency for the description of the master curves (Tref = 7 °C) (left) and their identification on the master curve of PnBA3U18.

ω. The variation of the shape of the master curves with material structure can be summarized by noting the value of specific frequencies where G′ and G″ cross (see Figure 5): • the frequency ωg at the high frequency end of the gel regime • the frequency ωe at the low frequency end of the elastic “plateau” • the frequency ωd at the low frequency end of the dissipative regime. As reported in Figure 5, ωd is measured for all PnBA3U systems but ωe can be only estimated for the longest supramolecular chains and ωg for the shortest supramolecular polymers. While ωd is independent of the molecular weight, both ωg and ωe decrease when Mn increases. For adhesive applications and also for vibration damping, the dissipative properties of the intermediate molecular weights PnBA3U18 and PnBA3U40 are particularly interesting. The value of tan δ is nearly unity over four decades of frequency suggesting very interesting properties for vibration damping19 and as soft adhesives16 (see Figure 6).

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DISCUSSION From Soft Dissipative Gels to Non-Newtonian Fluids. At low frequency, the decrease of ωg with increasing Mn reveals the progressive transition from a solid gel to a viscoelastic fluid (i.e., the density of stickers decreases) and confirms the trend observed at room temperature in Figure 2. For Mn ≤ 20 kg/ mol, the solid character is clearly enhanced by an elastic low frequency plateau (G′ > G″) while the liquid behavior of higher D


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stickers are diluted in the polymer matrix and the molecular dynamics is mainly controlled by the dynamics of entanglements. Regardless of the chemical nature of the sticker, the value of |η*| tends toward that unfunctionalized PnBA when Mn increases. When the bis-urea sticker is replaced by a triurea one, the variation of |η*| with Mn is less pronounced and the critical molecular weight Mc is shifted from 20 to 40 kg/mol. The selfassembly of the triurea sticker seems to promote the formation of large domains of parallel rods and thus high viscoelastic moduli, as previously suggested by the stronger level of interaction between stickers detected by infrared in solution.17 To investigate further the physics of center-functionalized polymers, we will now compare master curves of both systems at a fixed molecular weight, below and above Mc. As expected, bis-urea and triurea center-functionalized polymers both behave as lightly entangled polymers for Mn ≥ Mc. This is illustrated with the comparison between PnBAX40 and PnBA3U40 in Figure 8. For both materials, the same order

interpretation is consistent with the glass transitions observed by DSC (see Table 1): Tg ∼ −49 °C was observed for all PnBA3Us. At intermediate frequency, the decrease of ωe with increasing Mn reveals the growth of the entanglement plateau as Mn increases from 18 to 57 kg/mol. However, it is worth noting that the molecular weight of PnBA3U18 is much lower than the molecular weight of lightly entangled poly(butyl acrylate) chains18 (Mn ∼ 2Me ∼ 50−60 kg/mol) suggesting that the supramolecular structure influences the relaxation of the side chains toward a more elastic behavior. For Mn ≥ 40 kg/mol, a viscous response (i.e., G″ > G′) is observed at low frequency ω < ωe but the damping factor tan δ still tends toward 1 when a(T)ω decreases (see Figure S5). Strength of Supramolecular Interactions. The construction of master curves highlights the major role played by the PnBA matrix in the dissipative behavior at short time scales. Like Yan and co-workers in their study of end-functionalized poly(isobutene),11 we can now examine the influence of the strength of interaction between stickers on the rheological properties of center-functionalized polymers. To answer this question, the triurea center-functionalized PnBAs will be compared to linear PnBA chains center-functionalized by a weaker sticker, a bis-urea xylene sticker.2 The linear rheology of this model system (PnBAX) was presented and discussed in our previous study.2 The synthesis procedure is identical and all details can be found in the previous article. In order to offer a first insight into the differences between PnBA3U and PnBAX, it is interesting to plot the magnitude of the complex viscosity |η*(ω)| = √(G′2 + G″2)/ω versus the molecular weight Mn for both systems and for unfunctionalized poly(butyl acrylate)18 at room temperature (T = 25 °C) and at ω = 1 rad/s (see Figure 7). Although no supramolecular objects

Figure 8. G′ and G″ master curve for bis-urea and triurea centerfunctionalized polymers at Mn = 40 kg/mol (left) and 18−20 kg/mol (right) at Tref = 7 °C. The master curve of PnBAX20 and PnBAX40 is taken from our previous study.2 A vertical shift factor (=1/2) was applied on that of PnBAX40. The color of the experimental points indicates the temperature at which frequency sweeps were carried out.

of magnitude is observed for the terminal relaxation time τ, determined at the crossing point between G′ and G″: τ = 0.04 s for PnBAX40 and τ = 0.07 s for PnBA3U40. The plateau modulus G0, estimated at the minimum of tan δ, is also similar for both stickers: G0 = 2 × 105 Pa for PnBAX40 and G0 = 4 × 105 Pa for PnBA3U40. The absence of a Newtonian regime for PnBA3U40 could reveal the presence of small aggregates, which are obviously absent in PnBAX40. For Mn ∼ 20 kg/mol, the effect of strengthening the supramolecular interactions is clearly observed at long time scales (see Figure 8). At high frequency, both systems are highly dissipative and are characterized by a short elastic plateau (G0 ∼ 6 × 105 Pa for PnBA3U20 and G0 ∼ 4 × 105 Pa for PnBAX20). At low frequency, PnBA3U is characterized by an elastic plateau while PnBAX behaves as a viscoelastic liquid. Such differences between both systems are consistent with SAXS and AFM observations1 showing that the supramolecular cylinders bundle together for the PnBA3U while they do not for the PnBAX. The critical sticker concentration Φc (or the critical molecular weight Mc) to form bundles (and form a gel at low ω) depends on the strength of the sticker. may be higher

Figure 7. Variation of magnitude of the complex viscosity |η*| (at ω = 1 rad/s, 25 °C) with Mn for triurea and bis-urea functionalized poly(butyl acrylate) and unfunctionalized PnBA. The dashed lines are used to guide eyes. The data of PnBAX and PnBA are taken from.2

or long-range order could be detected in PnBAX by AFM and SAXS,2 the variation of the rheological behavior with the chemical structure is similar for both types of sticker. Below a critical molecular weight (Mc) estimated at the minimum of |η*|(Mn), the linear rheology depends mainly on the selfassembly of stickers in the polymer matrix and the magnitude of the complex viscosity is much higher than that of unfunctionalized poly(butyl acrylate). However, for Mn ≥ Mc, E


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Macromolecules than 4% for the PnBAX (Mc ≤ 20 kg/mol) while it is between 1 and 2% for PnBA3U (20 ≤ Mc ≤ 40 kg/mol). At Mn = 5 kg/mol, both systems are in the stickers’ dominated regime. At high frequency, the dissipative response of both materials depends mainly on the Rouse-like relaxation of the side chains and thus, is quasi-independent of the strength of stickers. At intermediate ω, an elastic plateau revealing the supramolecular aggregates is observed for both materials. However, unlike PnBA3U5, the superposition of the viscoelastic data fails clearly for PnBAX5 and a liquid response is observed at low frequency. This reveals a change of supramolecular structure with T and possibly, the evidence of an order/disorder transition over the investigated T range. The comparison between PnBAX5 and PnBA3U5 in Figure 9 shows

The two rheological studies and the multiscale structural investigation1 reveals a series of complex new materials, the rheology of which cannot be fully and quantitatively compared with existing models, especially at low Mn. The dissipative response observed at high frequency may be analyzed in light of the recently reported relaxation of flexible arms of comb-shaped or bottle-brush polymers.26,27 However, at low frequency, the elastic behavior depends on the molecular organization over extended regions (up to 100 nm to 1 μm), as previously observed in nanostructured block copolymers28 and thus, is difficult to predict quantitatively. As the side-chain molecular weight increases, the equilibrium length of the hairy supramolecular rod decreases1 and the structures will tend toward stars and/or comb-shaped polymers, the architecture of which depends on temperature and could be more quantitatively predicted by thermodynamics models including chain stretching and H-bond interaction energy.22

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CONCLUSION

The linear viscoelastic properties of a series of linear triurea center-functionalized PnBA was extensively characterized over a large Mn range from 5 to 85 kg/mol. Unlike in our previous study on PnBAX,2 the non covalent interactions between the polar stickers are strong enough for the rod-like aggregates to bundle in a hexagonal structure which remains quasi unchanged over the investigated temperature range. This good stability of the structure allows us to construct master curves for all our samples and thus, identify the contribution of the stickers to the self-assembled structure and that of the side chains’ relaxation to the viscoelastic properties of center-functionalized polymers. For the frequency dependence of G′ and G″ at a fixed and relatively low temperature (T < 30 °C), a general shape emerges from our experimental master curves, as illustrated with the experimental data of PnBA3U18 and PnBAX20 in Figure 10. At high frequency, where a dissipative behavior is observed, viscoelastic moduli do not depend on the density of stickers and seem to be closely linked to the monomer friction

Figure 9. G′ and G″ master curve for PnBA3U5 (triangle) and PnBAX5 (circle) at Tref = 7 °C. The master curve of PnBAX5 is taken from our previous study.2 The color of the experimental points indicates the temperature at which frequency sweeps were carried out. For both systems, the vertical and horizontal shift factors are similar to that of unfunctionalized PnBA.

that the strength of stickers may indirectly modify the linear viscoelastic properties via the stability of the supramolecular structure, i.e. the temperature Tc of the gel/fluid transition as discussed more extensively in our companion paper.1 The stronger the interaction between stickers and the more stable the self-assembly of stickers is, resulting in a higher Tc. Finally, it is worth pointing out that the interpretation of the differences in rheology through the strength of supramolecular interactions is only possible if we assume the existence of the same type of aggregates in PnBAX and PnBA3U. This is reasonable because of the central location of the sticker on the linear poly(butyl acrylate) chain; the steric hindrance between the side chains favors a unique type of aggregates, comb-shaped filaments.24 In the cases of supramolecular copolymers14 or telechelic polymers,25 a high diversity of supramolecular objects is possible and the relaxation mechanisms will vary from one system to another. For these structures, the correlation between supramolecular interactions and rheology in the melt state is thus complex, while the effect of strength of stickers in centerfunctionalized polymers is more directly characterized via the critical parameters Tc and Φc.

Figure 10. Schematic of the frequency and temperature dependence of the viscoelastic moduli for linear polymers functionalized by strong stickers in the middle. The colors indicate the molecular origin of the different zones identified in the plots. F


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coefficient of the PnBA matrix, i.e. its glass transition. At intermediate frequency, a viscoelastic behavior (G′ ∼ G″) over several decades characterizes the relaxation of the side chains between the supramolecular aggregates and depends mainly on the length of the side chains. If the side chain is long enough, an elastic plateau is observed due to topological constraints between the PnBA arms, as observed in Figure 10. At low frequency, the rheological behavior depends mainly on the formation or not of bundles and on the interplay between sticker strength and molecular weight of the side chain. For strong stickers and below a critical molecular weight of the side chains, the supramolecular rods form bundles where rods are oriented parallel to each other into large domains.20 These large colloidal objects relax very slowly and are responsible for the elastic plateau observed at low a(T)ω, as schematized in Figure 10. For weaker stickers this oriented structure is not observed in the melt at room temperature and the material flows at low frequency. At higher temperatures, the oriented rods structure eventually disappears1 as the strength of the H-bonds in the sticker decreases and the entropic penalty of forming a brush of flexible chains increases. One expects also the size of the rod structures to decrease in length with increasing T22 but we did not observe it directly. The identification of the contribution of the supramolecular structure and that of stickers underlines the fact that the viscoelastic properties of center-functionalized may be easily adjusted by varying independently the nature and the length of the side chains, as well as the nature of the sticker. This is a strong advantage compared to telechelic polymers or supramolecular copolymers where both mechanisms are linked to each other. The good control of the molecular dynamics via the chemical structure makes the supramolecular center-functionalized polymers extremely interesting for materials applications where highly dissipative rheological behaviors are desired over large frequency ranges, such as vibration damping or adhesive16 applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01583. Size exclusion chromatograms, NMR spectra, frequency sweeps, tan δ master curves, G′ and G″ master curves, and shift factor plots (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(X.C.) E-mail: [email protected]. *(C.C.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the ANR SUPRADHESION program (Project ANR-10-BLAN-0801). We also thank D. Vlassopoulos and F. Snijkers for helpful discussions.

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REFERENCES

(1) Véchambre, C.; Callies, X.; Fonteneau, C.; Ducouret, G.; Pensec, S.; Bouteiller, L.; Creton, C.; Chenal, J. M.; Chazeau, L. SupraG


Linear Rheology of Supramolecular Polymers Center-Functionalized

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