Molecular-Level Tuning toward Aggregation ... - ACS Publications

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Molecular-Level Tuning toward Aggregation Dynamics of SelfHealing Materials Feng-Min Nie,† Jing Cui,† Yu-Feng Zhou,‡ Li Pan,*,† Zhe Ma,*,† and Yue-Sheng Li†,§ †

Downloaded via IDAHO STATE UNIV on July 17, 2019 at 18:22:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Tianjin Key Lab Composite & Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300350, P. R. China ‡ School of Materials Science & Engineering, Zhengzhou University, Zhengzhou 450002, P. R. China § Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China S Supporting Information *

ABSTRACT: Polymerized ionic liquids (PILs) represent one class of promising candidates for self-healing materials because of the potential diffusion of ion pairs from one aggregation to another. Therefore, dynamics of ionic aggregation plays the crucial role in the self-healing process. However, a win−win situation to both self-healing efficiency and mechanical strength is a major challenge for PILs and even all intrinsic healable materials. To resolve this challenge, a series of novel imidazolium-based norbornene PILs with finetuned side-chain microstructures were synthesized in the present work. The inserted imidazolium groups divide side chains into two parts: spacer and tail. By tuning the length of these two parts independently, self-healing efficiency of PILs could be significantly improved without sacrificing the mechanical strength. The increase in spacer segments by eight methylenes decreased the glass transition temperature by 70 °C and turned the PIL from a strong material into a highly stretchable material. It was very useful to find that tuning the lengths of spacer and tail conversely could achieve comparable mechanical strength. More importantly, our results revealed that long tail segments with 5, 7, or 9 methylenes formed an additional tail region between ionic aggregations, which remarkably reduced the average aggregation distance and consequently accelerates healing kinetics. The understanding on structure-healing interplay provides a convenient and efficient molecular design approach for optimizing mechanical strength and healing efficiency simultaneously.

1. INTRODUCTION Self-healing materials are attracting rapidly increasing attention in recent years because of their excellent physical properties such as crack resistance, long-term durability, and low repairing cost.1−5 So far, there are mainly two approaches to endow materials with self-healing capability: one is to introduce active agents pre-embedded within microcapsules or hollow fibers and the other one utilizes reversible covalent bonds or supramolecular bonds.6−9 Materials based on the first approach rely on the leakage of active agents and accordingly they lack repeatable healing at the same damaged part due to consumption of the limited active agents.10 Hence, to achieve a persistent self-healing property, intrinsic healable materials based on reversible bonds are more promising. Compared with reversible covalent bonds such as Diels−Alder reactions11 and disulfide bonds,12 supramolecular interactions such as ionic interactions,13,14 hydrogen bonds,15 and metal−ligand interactions16,17 are relatively weak, so they are easier to associate and dissociate for achieving a reversible healing function. In recent decades, scientific interests in accelerating the selfhealing process while maintaining the mechanical properties have tremendously increased because of their enhanced reliability.18 Among the available supramolecular interactions, ionic interactions are easily regulated and show high stability © XXXX American Chemical Society

under ambient conditions. Therefore, ionic interactions are quite suitable for application in self-healing materials.19,20 According to the strong ionic interactions, the ion pairs in polymer matrixes easily form aggregations in which an equilibrium between association and dissociation existed. This equilibrium is established following the “ion-hopping” mechanism, which shows that the ion pair initially located within one aggregation hops or diffuses to another.21 In this way, those cracks generating to cause failure of materials could be repaired by the rearrangement of aggregations and the diffusion of polymer chains. Therefore, both mechanical property and healing capability are largely dependent on the dynamics of ionic aggregations.22,23 To quantify the dynamic nature of aggregations, the supramolecular bond lifetime (τ) is introduced, which is a measure of the average time that an ionic group spends within a particular aggregation of the ionic interaction system.19,24 Short lifetime indicates that the polymer is soft and easy to flow, whereas long lifetime means that the material is strong and dynamically stable.23 Polymerized ionic liquids (PILs) are a special type of the functional polymers that carry the ionic groups in every Received: April 28, 2019 Revised: June 22, 2019

A

DOI: 10.1021/acs.macromol.9b00871 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. (a) Synthesis Route of PILs; (b) Schematic Representation of Obtained PILs with Different Spacer and Tail Lengths

repeated unit.25,26 In this way, PILs combine the advantage properties of ILs and polymers to exhibit high ion conductivity, good chemical stability, and superior processability,25 so they are widely applied in solid electrolytes,27 sensors,28 gas separation/sorption,29 and so on.30 Benefited from their substantial reversible ionic interactions, PILs also represent the promising candidate for intrinsic self-healing polymers.31 Recently, it was found that in PILs, ion mobility and nanoscale structural arrangement are largely dependent on the microstructure of side chains.32,33 This inspires us that structural design of the side chain with ionic functional groups can be utilized to tune the self-healing property. To understand the relationship between the side-chain structure, self-healing, and mechanical properties, we designed a series of PILs by finetuning their side chains. The imidazolium groups in the side chain were used to separate side chains into the spacer and the tail. It deserves emphasis that these two parts can be tuned independently. As presented in Scheme 1, new PILs with different combinations of spacer segments (Sm) located between the imidazolium ring and the polymer backbone and of tail segments (Tn) pendant to the imidazolium ring were synthesized. For convenience, polymers are named by the number of carbon atoms in spacer segments and tail segments, which are indicated as m and n, respectively, that is, P(SmTn). Based on these new PILs with delicately designed side chains, we attempt to unveil the structure-to-healing interplay and optimize healing efficiency without sacrificing mechanical strength.

exhibited excellent functional group tolerance and high catalytic activity, was chosen to synthesize polymers.34 Polymerization was carried out in the N,N-dimethylformamide (DMF) and dichloromethane component solvent, which was stirred for 12 h at room temperature to ensure the full conversion. The 1H NMR spectra demonstrate that all monomers have been completely converted into PILs (Figures S1−S3). The number-average molecular weight (Mn) and polydispersity index (Mw/Mn) of the obtained PILs were measured by gel permeation chromatography (GPC). Because of the strong interactions between PILs and column fillers, DMF with 50 mM of LiBr was used as the eluent for GPC measurement.35−37 As observed, molecular weights (Mn) of the obtained PILs are larger than 105 and their polydispersity indexes (Mw/Mn) are smaller than 1.4 (Table S1). 2.2. Interplay between the Side-Chain Structure and Mechanical Property. Chain mobility plays a vital role to achieve excellent mechanical strength and self-healing function simultaneously. Too flexible chains cannot resist the external deformation strength, whereas too rigid chains are hard to move to rebuild the reversible bonds which are responsible for self-healing. As previously revealed, large imidazolium groups formed ionic aggregations in PILs, which behaved as physical cross-linkers to restrict the molecular movement.38−40 First of all, we explored the effect of side-chain length on molecular dynamics in the segmental scale by analyzing the glass transition temperature (Tg). As shown in Figure 1a, when the spacer length (m) increases from 3 to 5, Tg exhibits a sharp drop by 40 °C for PILs with all tail lengths (n = 1−9). As the spacer length is increased to m = 7 and 11, Tg further decreases to 20 and 10 °C, respectively. Evidently, Tg of the PILs has a strong dependence on the length of the spacer segments. Such pronounced decrease in Tg is caused by the reduced bondage effect on main chain originated from ionic aggregations. With short spacer segments (i.e., m = 3), ionic aggregations are very close to the main chains and thus the backbone movements are restricted. As the spacers inserted between main chains and imidazolium groups are increased (i.e., m = 11), the more flexible spacers allow main chains to move more freely, as reflected by the reduced Tg.

2. RESULTS AND DISCUSSION 2.1. Synthesis of PILs with Various Spacer and Tail Segments. To systematically study the relationships between molecular structures, mechanical property, and healing performance, imidazolium-based norbornene derivatives with various spacer segment lengths (m = 3, 5, 7, and 11) and tail segment lengths (n = 1, 5, 7, and 9) were prepared, as illustrated in Scheme 1. A series of PILs with different sidechain lengths were synthesized via ring-opening metathesis polymerization (ROMP). The Grubbs third-generation catalyst (G3) RuCl 2 (3-bromopyridine) 2 (H 2 IMes)(CHPh), (H2IMes = N,N-desityl-4,5-dihydroidazol-2-ylidene), which B


Article

Macromolecules

P(S11Tn) series polymers exhibit too high or too low Tg, respectively, they are not suitable for mechanical tests at room temperature. 2.3. Correlation Distance and Intensity of Ionic Aggregations. 2.3.1. Correlation Distance of Ionic Aggregations. After achieving the control of mechanical property, next step is to improve self-healing capability. The self-healing capability is determined by the reversibility of ionic aggregations.31 The above results of Figure 1a have already shown that the spacer segments dominate chain mobility. Therefore, P(SmT1) series polymers with the identical short methyl tail (indicated by the purple vertical line in Scheme 1b) were analyzed to reveal the influence of spacer length on the correlation distance of ionic aggregations. Figure 2a illustrates

Figure 1. (a) Glass transition temperatures (Tg) of PILs with different spacer lengths m and tail lengths n. (b) Typical stress−strain curves of P(SmTn).

On the other hand, increasing tail length allows the segments to move more freely. In addition, long tail segments may hinder the packing of ionic aggregations and, as a consequence, the molecular segments are easier to move. As observed, the length of tail segments also exhibits a slight influence on the Tg value, although the Tg is mainly dominated by spacer segments. As shown in Figure 1a, the Tg decreases with increase in the tail length (n) from 1 to 5 and then rises again with increasing n from 5 to 9. Increase in Tg with increase in the tail length may be associated with the possible interdigitation of the long tails, which could impose constrains of the main-chain motions. A similar observation has also been found in polymethacrylate system.41 These results suggest that spacer segments dominate the segmental mobility. This interplay can be utilized to control chain mobility for realizing strong mechanical strength and self-healing property. By tuning the side-chain microstructures, mechanical performance could be systematically tailored from highly stretchable to strong materials. Figure 1b displays the representative stress−strain curves of PILs, and relevant data have been summarized in Table S2. As the length of spacer segments reduces from m = 7 to m = 5 (for P(S7T1) and P(S5T1), respectively), mechanical strength can be improved markedly (Young modulus from 17 to 124 MPa). Reduction in the spacer length enhances the bondage effect of ionic groups to polymer backbone, impeding chain slippage and material failure, which has also been evidenced in the increase of Tg. All healing polymers studied in this work are strong and tough, and their Young’s modulus can be tuned within a broad range from 3 to 124 MPa. In addition, strain at break can also be enhanced from 780 to 2100%, as the tail length increases from n = 1 of P(S7T1) to n = 7 of P(S7T7). Interestingly, it is found that tuning the length of spacer and tail conversely can remain the comparable mechanical strength. As shown in Figure 1b, when the spacer length is decreased from m = 7 to m = 5 and, simultaneously, increase the tail length from n = 1 to n = 5 or 7, the polymers possess similar breaking stress [P(S7T1) vs P(S5T5) and P(S5T7)]. However, because the P(S3Tn) and

Figure 2. (a) Schematic representation of three characteristic lengths for P(SmT1) (m = 7), which can be measured by X-ray scattering. (b) 1D X-ray scattering curves of P(SmT1) (m = 3, 5, 7, 11). (c) Spacer length m dependence of correlation distances (dI, dII, and dIII). The black dash line represents theoretical length of the fully extended chains between ionic aggregations.

the ideal molecular model of PILs proposed by Salas-de la Cruz et al., where aggregation morphology is characterized by three correlation lengths (dI, dII, and dIII) and can be measured by the X-ray scattering method.32 Figures 2b and S4 show that all PILs studied in this work display those three characteristic peaks in all X-ray scattering curves. According to the aforementioned model, the lowest q peak at qI = 1−4 nm−1 is denoted as ionic aggregation distance, and the intermediate q peak at qII = 8−10 nm−1 is referred as the anion distance, C


Article

Macromolecules while the highest q peak at qIII = 14−16 nm−1 is assigned to the pendant-to-pendant distance.42,43 It can be seen that with increasing spacer length (m), qI shifts to lower q values, which indicates an extension in the correlation distance between the aggregations (dI). Figure 2c represents variations of all correlation lengths as a function of m, which are obtained by d = 2π/q. For P(SmT1) series polymers, the spacer length dependence of dI is linear and a positive slope is around 0.13 nm per −CH2− repeating unit. Ionic groups are placed at the end of side chains in P(SmT1), so the ionic aggregation distance (dI) is equivalent to lateral dimension of the nonpolar domain, called as “spacer domain” in this work (Figure 2a). This means that after increasing m from 3 to 11, the longer spacer segments efficiently push ionic groups away from the backbone and expand the intermediate spacer domains, similar to the observation in styrene-based ionomers.44 Figure 2b also shows that the qI peak becomes more pronounced with increasing lengths of spacer segments, which indicates a higher degree of the arrangement of ions. For short spacer segments, main chains can restrict the movement of imidazolium groups and consequently hinder the arrangement of ions. The increase of spacer length reduces the main chains restriction effect on imidazolium groups. In this case, imidazolium groups can be easily packed into the ordered domains with no need of the substantial adjustment of main chains. To figure out the physical states of side chains, we calculated the theoretical distance between ionic aggregations, supposing the spacer between backbones and imidazolium groups is fully extended (calculation method was given in Scheme S1). The theoretical values are represented by dash line in Figure 2c. For m = 3 and 5, the coincidence between theoretical and experimental dI values indicates that the spacer segments are near fully extended in P(S3T1) and P(S5T1). However, experimental dI values are much lower than those of extended spacer for m = 7 and 11. The enlarged deviation of dI has two possible reasons. One is that the side chains are fully extended but interdigitate with their neighboring side chains. The other one is that side chains are not fully extended. Actually, it has been demonstrated in poly(n-alkyl-vinylimidazolium bistrifluoromethylsulfonylide) P(CnVIm-TFSI) homopolymer that the neighboring imidazolium groups do not interdigitate in the case of short tail segments due to the strong coulombic repulsion between ionic domains, ruling out the first possibility.33 Thus, the observed negative deviation of dI between experimental and theoretical values indicates that for m = 7 and 11, the spacer segments are not fully extended, that is, squeezed. Differently, correlation lengths of dII and dIII remain unchanged with increasing spacer length. The anion-to-anion distance, dII, keeps constant at 0.715 nm and the characteristic spacing between pendants, dIII, is always around 0.517 nm. This indicates that a long spacer segment extends the lateral distances between main chains but does not adjust the vertical distances between side chains, though flexible spacer segments allow imidazolium groups to move more freely. Note that the long tail segments may form new tail domains, which could disrupt the ionic aggregations and introduce one more correlation distance dt, as illustrated in Figure 3a. With increase in the length of tail segments (n), the distance of ionic aggregations (dI) reduces first and then rises, as displayed by Figure 3b. Such a non-monotonic tail dependence phenomenon was observed for the first time in all PILs designed in this

Figure 3. (a) Schematic representation of two nonpolar domains in P(SmTn) (m = 7, n = 7). (b) Correlation distance of ionic aggregation (dI) plotted vs the tail segment length (n); (c) dI derived from XRD peak positions as a function of tail segment length (n) in comparison with theoretical lengths of fully extended segments in P(S7Tn) and P(S11Tn).

work. This observation is distinctly different from the case of P(CnVIm-TFSI) that dI only grows linearly with the expansion of tail segment.32,33,42,45,46 The difference lies in the preexisting spacer domains of our PILs. As shown in Figure 3a, there are two ionic correlation lengths at the same time, ds and dt, which correspond to the lateral dimensions of spacer domain and tail domain, respectively. The apparent value of dI determined from X-ray diffraction measurement actually is the d +d average value over these two domains, that is, dΙ = s 2 t . As the tail length increases from 1 to 5, new tail regions are formed and their small dt values with respect to the spacer regions ds notably reduce the average value of dI (Figure 3b). Figure 3b shows that for P(S3Tn) and P(S5Tn) series polymers, the minimum dI is found at n = 5. The increase of dI with further elongating tail (n > 5) is simply ascribed to the expansion of tail regions. Differently, for P(S7Tn) and P(S11Tm) series polymers, the minimum value of dI is observed at n = 7. The reason of why dI sustainably decreases with increasing tail length (from n = 5 to 7) is interpreted in this part. On one hand, the steric effect of tail segments becomes more significant with the augment of tail. On the other hand, long tail segments tend to spread out the ionic domains from adjacent chains, decreasing their coulombic repulsion compared with short methyl tail. These two effects have opposite influences in spacer domain. The expansion of tail regions tends to decrease spacer domains, whereas the decrease of coulombic repulsion between neighboring ionic domains allows the recovery of squeezed spacer segments, or to say, increasing spacer domains. When coulombic repulsion D


Article

Macromolecules dominates the variation of spacer domains, spacer domains increase with the elongation of tail segments, and therefore the spacer segments are very close to the fully extended chains. It is also demonstrated by the results of Figure 3c that the experimental dI of n = 5 is more close to the corresponding theoretical value than n = 1. However, when the tail length increases beyond n = 5, the steric effect of tail segments increases and becomes the dominant factor in determining the size of spacer domains, leading to a reduction of spacer domains. As also proved in Figure 3c, an enlarged deviation between experimental and theoretical dI for n = 7 can be observed. As the tail segment increases from n = 5 to n = 7, the reduction amplitude in the spacer domain is too large to be compensated by the increase of tail segments, and as a consequence the apparent averaged distance dI shows a continuous decrease in P(S7Tn) and P(S11Tn). As the length of tail was increased, the tail chains may interdigitate or entangle with their neighboring side chains. The vertical distance between two side chains (dIII) determined by X-ray scattering is around 0.517 nm, while the lateral tail domain dimension dt is dependent on the tail length. When the size of extended tail chain dn is larger than

Figure 4. Ionic aggregation energy calculated by DFT. (a) ΔEquad/ 2ΔEpair of P(SmT1) as a function of spacer length m; (b) ΔEquad/ 2ΔEpair of P(SmTn) as a function of dI.

dIII 2 + d t 2

, tail chains from neighboring molecules are possible to contact. However, it should be also noted that the longest alkyl tail designed in this work is only Mw = 127 Da, which is much smaller than PE critical entanglement molecular weight Me = 828 Da,47 so the practical number of entanglements seems quite low, even though they may be generated. 2.3.2. Intensity of Ionic Aggregations. Because of strong ionic interactions, ion pairs form aggregations and endow PILs with self-healing capability. The intensity of ionic aggregations is a result of the competition between cation−anion interactions and steric hindrance of side chains.31,48 The intensity of cation−anion interactions could be estimated from NMR spectra.31,49 In 1H NMR spectra, chemical shifts of imidazolium ring are comparable for PILs with different side chains, as shown in Figure S2. This observation indicates that the cation−anion interactions in various side chains are very similar, except for slightly weaker interactions of the methyl tail. In this case, the steric hindrance, which is dependent on side chain, dominates the intensity of ionic aggregations. To quantify aggregation intensity, formation energies of ion pairs (ΔEpair) and quadrupole associations (ΔEquad) of imidazolium salt-based NBE with CF3(CF2)3SO3 were calculated using density functional theory (DFT) (Table S3).50,51 There is an equilibrium between quadrupoles and two ion pairs in PILs, so the ΔEquad/2ΔEpair ratio is useful to compare the tendency to aggregate, where the large ratio indicates high tendency for ion pairs to associate, that is, intense aggregations.52 P(SmT1) series polymers with shortest methyl tails are selected as the model system to discuss the influence of spacer length on the intensity of ionic aggregations. As shown in Figure 4a, the ΔEquad/2ΔEpair ratio rises significantly with increase in the spacer length m from 3 to 7 and grows slightly with the extending spacer further to m = 11. The spacer segment connects the imidazolium group with the backbone and the elongation of spacer segments improves the mobility of ionic groups, which favors aggregation of ions. Furthermore, once the designed tail is larger than the methyl group, both the tail and spacer regions should be taken into account and their total steric hindrance can be represented by dI, which means the averaged correlation distance between 2

ionic aggregations. As shown in Figure 4b, the calculated intensity of ionic aggregations decreases with the increase of dI. It is interesting to observe that the length of spacer segments dominates the variation of aggregation intensity, and the dependency follows some kind of linear function. 2.4. Dynamics of Ionic Aggregation and Self-Healing Kinetics. In PILs, ionic aggregation is based on the dynamic equilibrium of ionic pairs between association and dissociation, that is, dynamic aggregations. It has been reported that the reversible association−dissociation of ionic aggregations is conducted by the “ion-hopping” mechanism.21 The ions hop or diffuse from one aggregation to another and this activity stimulates the movement of those ion-attached segments to rebridge (i.e., healing) the microcracks. The dynamics of ionic aggregations can be quantified by the relaxation time of aggregation (τs), which indicates the average residence time of an ionic group within a particular aggregation.21 Figure 5a shows the experimental determination of average residence time from the reciprocal of crossover frequency (ω) between storage modulus (G′) and loss modulus (G″).53,54 When τs is long, ionic aggregations are more stable from the dynamic point of view. Correspondingly, the polymer behaves more like an elastic solid with relatively higher storage modulus but slow self-healing process. When τs is very short, ion pairs are active to hop from one aggregation to another, which explicitly facilitates the healing behavior. On the other hand, material mechanical strength prefers rigid chains with relatively low mobility. Therefore, the relaxation time of aggregations is a key factor in determining healing efficiency with balance of sufficient mechanical strength. Actually, aforementioned τs is the time required for ion pairs to diffuse from one aggregation to another. This diffusion process includes two steps: (1) ion pairs break away from aggregation bondage, and (2) ion pairs hop a certain distance to reach another aggregation. The τs is the total time to implement both steps, that is, τs = τ1 + τ2, where τ1 and τ2 are the time required for above steps, respectively. For the first E


Article

Macromolecules

time τ2 = d/v depends on the competition between the ionhopping distance and the hopping speed. From Figure 5b, it is seen that P(S3T1) actually has a longer total time τs than P(S5T1) and P(S7T1). It is implied that the small hopping speed v leads to a large relaxation time τ2, which even exceeds the reduced relaxation time τ1. Thus, the slow segmental mobility of P(S3T1) is responsible for its slow aggregation dynamics. In addition, Figure 5b demonstrates that P(S5T1) and P(S7T1) have comparable total relaxation time τs. In this case, the increment of second relaxation time τ2 just compensates the shortening of the first relaxation time τ1, resulting in a similar aggregation dynamics between P(S5T1) and P(S7T1). Next, we fixed the spacer length at m = 5 in P(S5Tn) to explore the influence of tail on τs. As shown in Figure 4b, once the spacer length is fixed, aggregation energy ΔEquad/2ΔEpair and the corresponding relaxation τ1 have a negative correlation with distance dI. Meanwhile, it is observed from Figure 1a that Tg only changes slightly with increasing tail length. It is very likely that the correlation distance dominates the hopping time, where the relaxation time τ2 exhibits the positive dependence on dI. Figure 3b displays that among P(S5Tn) series, P(S5T1) has the largest dI of 2.392 nm and P(S5T5) has the shortest dI of 1.730 nm. Correspondingly, P(S5T1) has the shortest τ1 and longest τ2, whereas P(S5T5) has the longest τ1 and shortest τ2. Interestingly, Figure 5c demonstrates that P(S5T1) and P(S5T5) have comparable τs, which is larger than P(S5T7). Now, we obtained a series of novel PILs with adjustable aggregation dynamics and mechanical properties. The selfhealing performance of PILs is assessed using mechanical tests. The healing efficiency was defined as εt/ε0 × 100% to evaluate the healing degree, where εt is the breaking strain of the cut specimens after healing time t and ε0 is the breaking strain of the uncut original specimens. It has been proposed that selfhealing efficiency is related to the dynamics of ionic aggregations, which could be reflected by τs.54 In this work, those PILs that are able to self-heal at room temperature, including P(S5T5), P(S5T7), P(S7T1), and P(S7T7), were compared to disclose the correlation between dynamics of ionic aggregations and self-healing property. These PILs could self-heal to regain the same breakage strain with original noncut sample, but the self-healing kinetics varies largely with the designed microstructures of side chains, as shown in Figure 6. For P(S7T1) and P(S5T5), the time required to completely recover the initial breakage strain, that is, achieving 100% selfhealing, is 40 and 36 h, respectively. Unexpectedly, it takes just 1 and 24 h for P(S7T7) and P(S5T7) to achieve complete selfhealing, respectively. For polymers with similar Tg, distinct τs endows with different healing efficiency. Short relaxation time of P(S7T7) (546 s) and P(S5T7) (3000 s) implies high healing efficiency, with respect to that of P(S5T5) (4 × 104 s) and P(S7T1) (7 × 104 s) (Figure S5). Moreover, P(S5T7) has higher breaking stress and faster healing process than P(S7T1), as observed in Figure 6b,d. It is clear that the accelerated dynamics of ionic aggregations is favorable to the reparation process. Importantly, simultaneous increase in tail segments and decrease in spacer segments is proved to realize the improvement in healing efficiency without sacrificing the mechanical strength. On the other hand, the entanglements of main chain may be varied with the side-chain structure, which influences the healing process. We estimated the numbers of main-chain entanglement for these four polymers (calculation

Figure 5. (a) Storage modulus (G′) and loss modulus (G″) as a function of frequency of P(S5C5) at 100 and 80 °C. Relaxation time of ionic aggregations (τs) as a function of temperature (T) for (b) P(SmT1) with m = 3, 5, 7; (c) P(S5Tn) with n = 1, 5, 7.

releasing step of ion pair, aggregation energy directly determines the tendency of breakage and the essential time τ1. In the second hopping step, τ2 can be assessed simply by the formula τ2 = d/v, where d is the hopping distance and v is the average speed of ion pairs. In this work, the correlation aggregation distance (dI) is equal to the average length of ionhopping (d), while the inherent segmental mobility reflected by values of Tg is closely associated to the speed of ionhopping (v). Therefore, aggregation energy, segmental mobility, and the distance between aggregations together show an impact on the relaxation time. Then, we attempt to establish the relationship of the side chain structure with ionic aggregation dynamics in order to understand the intrinsic self-healing performance. The effects of spacer length and tail length on relaxation time will be discussed separately. The P(SmT1) polymers with the same methyl tail (n = 1) are used to discuss the effect of spacer on τs (Figure 5b). Figure 4a shows that in P(SmT1), as the spacer length increases, the aggregation energy increases and leads to the increase of relaxation time τ1. Moreover, Figure 2c presents that the ion-hopping distance (d) increases with increasing spacer length, where P(S3T1) has the shortest movement distance. However, P(S3T1) has a much higher Tg of 79.7 °C than 38.4 °C of P(S5T1) or 21.2 °C of P(S7T1), see Figure 1a. This observation indicates that the hopping speed (v) increases largely with increasing spacer length m. In this case, movement F


Article

Macromolecules

strong to highly stretchable. Differently, the increase of tail length from n = 1 to 9 could improve the PIL ductility. The correlation distance of aggregations dI enlarges with the increase of spacer length, whereas with the increase of tail length, it reduces first and rises again. The intensity of ionic aggregations enhances with the increase of spacer segments due to the larger mobility of ions. When the spacer segment length is fixed, aggregation intensity increases with the decrease of dI. The segmental mobility of PILs, distance of ionic aggregations, and energy of ionic aggregations together determine dynamics of ionic aggregations, which is quantified by the relaxation time. High segmental mobility, short correlation distance, and low aggregation intensity reduce the relaxation time, which favors the rearrangement of aggregations and consequently enhances healing process. The revealed structure-to-healing interplay opens a new approach for the design of high-performance self-healing materials for many important applications such as batteries and sensors.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00871. Experimental details and additional data including 1H NMR spectra, X-ray scattering curves, relaxation time, and master curves; theoretical fully extended distance between ionic aggregations; molecular and thermal characteristics of PILs; mechanical property of P(SmTn); interaction energies; and tabulated values for density, cation and anion volumes, volume fraction of polymer chains in the bulk, plateau modulus, entanglement molar mass, and the number of entanglement of studied PILs (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.P.). *E-mail: [email protected] (Z.M.). Figure 6. Stress−strain curves of original and healed (a) P(S7T7), (b) P(S5T7), (c) P(S5T5), and (d) P(S7T1).

ORCID

Li Pan: 0000-0002-9463-6856 Zhe Ma: 0000-0003-2458-4197 Yue-Sheng Li: 0000-0003-4637-4254

details were given in Table S4 and Figure S6).13,55−62 It was found that the numbers of main-chain entanglements are very small and similar. Thus, for the significant improvement in selfhealing efficiency among these four healing polymers, entanglement of main chains seems not to be the dominant factor.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China (nos. 21690071 and 21574097). This research is also supported by Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (grant no. 2018-K05). We thank Prof. Yi Luo and Dr. Jia-Jie Sun from Dalian University of Technology for their kind help in the calculation of ionic aggregation energy. Z.M. thanks Prof. Houyu Zhang from Jilin University very much for the calculations of cation and anion volumes.

3. CONCLUSIONS In this work, we designed and synthesized a series of novel imidazolium-based PILs via ROMP, where the imidazolium group divides the side chains into spacer and tail segments. The spacer and tail segments of PILs were varied independently in the length ranges of m = 3−11 and n = 1− 9, respectively. It was found that increasing the spacer length from m = 3 to 11 could reduce the glass transition temperature from 80 to 10 °C and vary mechanical performance from G


Article

Macromolecules

■

(21) Vanhoorne, P.; Register, R. A. Low-Shear Melt Rheology of Partially-Neutralized Ethylene−Methacrylic Acid Ionomers. Macromolecules 1996, 29, 598−604. (22) Bose, R. K.; Hohlbein, N.; Garcia, S. J.; Schmidt, A. M.; van der Zwaag, S. Relationship Between the Network Dynamics, Supramolecular Relaxation Time and Healing Kinetics of Cobalt Poly(butyl acrylate) Ionomers. Polymer 2015, 69, 228−232. (23) Aida, T.; Meijer, E. W.; Stupp, S. I. Functional Supramolecular Polymers. Science 2012, 335, 813−817. (24) Hackelbusch, S.; Rossow, T.; van Assenbergh, P.; Seiffert, S. Chain Dynamics in Supramolecular Polymer Networks. Macromolecules 2013, 46, 6273−6286. (25) Qian, W.; Texter, J.; Yan, F. Frontiers in Poly(ionic liquid)s: Syntheses and Applications. Chem. Soc. Rev. 2017, 46, 1124−1159. (26) Mecerreyes, D. Polymeric Ionic Liquids: Broadening the Properties and Applications of Polyelectrolytes. Prog. Polym. Sci. 2011, 36, 1629−1648. (27) Zhang, P.; Li, M.; Yang, B.; Fang, Y.; Jiang, X.; Veith, G. M.; Sun, X.-G.; Dai, S. Polymerized Ionic Networks with High Charge Density: Quasi-Solid Electrolytes in Lithium-Metal Batteries. Adv. Mater. 2015, 27, 8088−8094. (28) Zhao, Q.; Dunlop, J. W. C.; Qiu, X.; Huang, F.; Zhang, Z.; Heyda, J.; Dzubiella, J.; Antonietti, M.; Yuan, J. An Instant MultiResponsive Porous Polymer Actuator Driven by Solvent Molecule Sorption. Nat. Commun. 2014, 5, 4293. (29) Morozova, S. M.; Shaplov, A. S.; Lozinskaya, E. I.; Mecerreyes, D.; Sardon, H.; Zulfiqar, S.; Suárez-García, F.; Vygodskii, Y. S. Ionic Polyurethanes as a New Family of Poly(ionic liquid)s for Efficient CO2 Capture. Macromolecules 2017, 50, 2814−2824. (30) Yuan, J.; Mecerreyes, D.; Antonietti, M. Poly(ionic liquid)s: An Update. Prog. Polym. Sci. 2013, 38, 1009−1036. (31) Cui, J.; Nie, F.-M.; Yang, J.-X.; Pan, L.; Ma, Z.; Li, Y.-S. Novel Imidazolium-based Poly(ionic liquid)s with Different Counterions for Self-healing. J. Mater. Chem. A 2017, 5, 25220−25229. (32) Salas-de la Cruz, D.; Green, M. D.; Ye, Y.; Elabd, Y. A.; Long, T. E.; Winey, K. I. Correlating Backbone-to-backbone Distance to Ionic Conductivity in Amorphous Polymerized Ionic Liquids. J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 338−346. (33) Delhorbe, V.; Bresser, D.; Mendil-Jakani, H.; Rannou, P.; Bernard, L.; Gutel, T.; Lyonnard, S.; Picard, L. Unveiling the Ion Conduction Mechanism in Imidazolium-Based Poly(ionic liquids): A Comprehensive Investigation of the Structure-to-Transport Interplay. Macromolecules 2017, 50, 4309−4321. (34) Sutthasupa, S.; Shiotsuki, M.; Sanda, F. Recent Advances in Ring-opening Metathesis Polymerization, and Application to Synthesis of Functional Materials. Polym. J. 2010, 42, 905−915. (35) Yan, Y.; Deaton, T. M.; Zhang, J.; He, H.; Hayat, J.; Pageni, P.; Matyjaszewski, K.; Tang, C. Syntheses of Monosubstituted Rhodocenium Derivatives, Monomers, and Polymers. Macromolecules 2015, 48, 1644−1650. (36) Hadadpour, M.; Gwyther, J.; Manners, I.; Ragogna, P. J. Multifunctional Block Copolymer: Where Polymetallic and Polyelectrolyte Blocks Meet. Chem. Mater. 2015, 27, 3430−3440. (37) He, H.; Zhong, M.; Adzima, B.; Luebke, D.; Nulwala, H.; Matyjaszewski, K. A Simple and Universal Gel Permeation Chromatography Technique for Precise Molecular Weight Characterization of Well-Defined Poly(ionic liquid)s. J. Am. Chem. Soc. 2013, 135, 4227−4230. (38) Chen, L.; Hu, K.; Sun, S.-T.; Jiang, H.; Huang, D.; Zhang, K.Y.; Pan, L.; Li, Y.-S. Toughening Poly(lactic acid) with Imidazoliumbased Elastomeric Ionomers. Chin. J. Polym. Sci. 2018, 36, 1342− 1352. (39) An, C.; Lou, Y.; Li, Y.; Wang, B.; Pan, L.; Ma, Z.; Li, Y. S. Unusual II−I Phase Transition Behavior of Polybutene-1 Ionomers in the Presence of Long-Chain Branch and Ionic Functional Groups. Macromolecules 2019, 52, 4634−4645. (40) Meng, Q.; Wang, B.; Pan, L.; Li, Y.; Ma, Z. Synthesis and Properties of Isotactic Polypropylene Ionomers Containing Ammo-

REFERENCES

(1) Cordier, P.; Tournilhac, F.; Soulié-Ziakovic, C.; Leibler, L. Selfhealing and Thermoreversible Rubber from Supramolecular Assembly. Nature 2008, 451, 977−980. (2) Bao, C.; Jiang, Y.-J.; Zhang, H.; Lu, X.; Sun, J. RoomTemperature Self-Healing and Recyclable Tough Polymer Composites Using Nitrogen-Coordinated Boroxines. Adv. Funct. Mater. 2018, 28, 1800560. (3) Patrick, J. F.; Robb, M. J.; Sottos, N. R.; Moore, J. S.; White, S. R. Polymers with autonomous life-cycle control. Nature 2016, 540, 363−370. (4) Burattini, S.; Greenland, B. W.; Chappell, D.; Colquhoun, H. M.; Hayes, W. Healable Polymeric Materials: A Tutorial Review. Chem. Soc. Rev. 2010, 39, 1973−1985. (5) Ghosh, S. K. Self-healing Materials: Fundamentals, Design Strategies, and Applications; Wiley-VCH: Weinhe, 2009. (6) White, S. R.; Sottos, N. R.; Geubelle, P. H.; Moore, J. S.; Kessler, M. R.; Sriram, S. R.; Brown, E. N.; Viswanathan, S. Autonomic Healing of Polymer Composites. Nature 2001, 409, 794−797. (7) Diesendruck, C. E.; Sottos, N. R.; Moore, J. S.; White, S. R. Biomimetic Self-Healing. Angew. Chem., Int. Ed. 2015, 54, 10428− 10447. (8) Zhang, Z. P.; Rong, M. Z.; Zhang, M. Q. Mechanically Robust, Self-Healable, and Highly Stretchable “Living” Crosslinked Polyurethane Based on a Reversible C-C Bond. Adv. Funct. Mater. 2018, 28, 1706050. (9) Yang, Y.; Urban, M. W. Self-healing Polymeric Materials. Chem. Soc. Rev. 2013, 42, 7446−7467. (10) Yanagisawa, Y.; Nan, Y.; Okuro, K.; Aida, T. Mechanically Robust, Readily Repairable Polymers via Tailored Noncovalent Crosslinking. Science 2018, 359, 72−76. (11) Wu, S.; Li, J.; Zhang, G.; Yao, Y.; Li, G.; Sun, R.; Wong, C. Ultrafast Self-Healing Nanocomposites via Infrared Laser and Their Application in Flexible Electronics. ACS Appl. Mater. Interfaces 2017, 9, 3040−3049. (12) Wang, Z.; Tian, H.; He, Q.; Cai, S. Reprogrammable, Reprocessible, and Self-Healable Liquid Crystal Elastomer with Exchangeable Disulfide Bonds. ACS Appl. Mater. Interfaces 2017, 9, 33119−33128. (13) Guo, P.; Zhang, H.; Liu, X.; Sun, J. Counteranion-Mediated Intrinsic Healing of Poly(ionic liquid) Copolymers. ACS Appl. Mater. Interfaces 2018, 10, 2105−2113. (14) Cuthbert, T. J.; Jadischke, J. J.; de Bruyn, J. R.; Ragogna, P. J.; Gillies, E. R. Self-Healing Polyphosphonium Ionic Networks. Macromolecules 2017, 50, 5253−5260. (15) Yang, J.-X.; Long, Y.-Y.; Pan, L.; Men, Y.-F.; Li, Y.-S. Spontaneously Healable Thermoplastic Elastomers Achieved through One-Pot Living Ring-Opening Metathesis Copolymerization of WellDesigned Bulky Monomers. ACS Appl. Mater. Interfaces 2016, 8, 12445−12455. (16) Rao, Y.-L.; Chortos, A.; Pfattner, R.; Lissel, F.; Chiu, Y.-C.; Feig, V.; Xu, J.; Kurosawa, T.; Gu, X.; Wang, C.; He, M.; Chung, J. W.; Bao, Z. Stretchable Self-Healing Polymeric Dielectrics CrossLinked Through Metal−Ligand Coordination. J. Am. Chem. Soc. 2016, 138, 6020−6027. (17) Mozhdehi, D.; Ayala, S.; Cromwell, O. R.; Guan, Z. SelfHealing Multiphase Polymers via Dynamic Metal−Ligand Interactions. J. Am. Chem. Soc. 2014, 136, 16128−16131. (18) Liu, J.; Tan, C. S. Y.; Yu, Z.; Li, N.; Abell, C.; Scherman, O. A. Tough Supramolecular Polymer Networks with Extreme Stretchability and Fast Room-Temperature Self-Healing. Adv. Mater. 2017, 29, 1605325. (19) Cui, J.; Ma, Z.; Pan, L.; An, C.-H.; Liu, J.; Zhou, Y.-F.; Li, Y.-S. Self-healable Gradient Copolymers. Mater. Chem. Front. 2019, 3, 464−471. (20) Hager, M. D.; van der Zwaag, S.; Schubert, U. S. Self-healing Materials; Springer: Switzerland, 2015. H


Article

Macromolecules nium Ions. Acta Polym. Sin. 2017, 11, 1762−1772, DOI: 10.11777/ j.issn1000-3304.2017.17028. (41) Tejero, R.; Arbe, A.; Fernández-García, M.; López, D. Nanostructuration by Self-Assembly in N-Alkyl Thiazolium and Triazolium Side-Chain Polymethacrylates. Macromolecules 2015, 48, 7180−7193. (42) Liu, H.; Paddison, S. J. Direct Comparison of Atomistic Molecular Dynamics Simulations and X-ray Scattering of Polymerized Ionic Liquids. ACS Macro Lett. 2016, 5, 537−543. (43) Araque, J. C.; Hettige, J. J.; Margulis, C. J. Modern Room Temperature Ionic Liquids, a Simple Guide to Understanding Their Structure and How It May Relate to Dynamics. J. Phys. Chem. B 2015, 119, 12727−12740. (44) Moore, R. B.; Bittencourt, D.; Gauthier, M.; Williams, C. E.; Eisenberg, A. Small-Angle X-ray Scattering Investigations of Ionomers with Variable-length Side Chains. Macromolecules 1991, 24, 1376− 1382. (45) Allen, M. H.; Wang, S.; Hemp, S. T.; Chen, Y.; Madsen, L. A.; Winey, K. I.; Long, T. E. Hydroxyalkyl-Containing Imidazolium Homopolymers: Correlation of Structure with Conductivity. Macromolecules 2013, 46, 3037−3045. (46) Liu, H.; Paddison, S. J. Alkyl Chain Length Dependence of Backbone-to-Backbone Distance in Polymerized Ionic Liquids: An Atomistic Simulation Perspective on Scattering. Macromolecules 2017, 50, 2889−2895. (47) Dealy, J. M.; Larson, R. G. Structure and Rheology of Molten Polymers; Hanser Gardner Pubs: Cincinnati, 2006. (48) Suckow, M.; Mordvinkin, A.; Roy, M.; Singha, N. K.; Heinrich, G.; Voit, B.; Saalwächter, K.; Böhme, F. Tuning the Properties and Self-Healing Behavior of Ionically Modified Poly(isobutylene-coisoprene) Rubber. Macromolecules 2018, 51, 468−479. (49) Palomar, J.; Ferro, V. R.; Gilarranz, M. A.; Rodriguez, J. J. Computational Approach to Nuclear Magnetic Resonance in 1-Alkyl3-methylimidazolium Ionic Liquids. J. Phys. Chem. B 2007, 111, 168− 180. (50) Choi, U. H.; Ye, Y.; Salas de la Cruz, D.; Liu, W.; Winey, K. I.; Elabd, Y. A.; Runt, J.; Colby, R. H. Dielectric and Viscoelastic Responses of Imidazolium-Based Ionomers with Different Counterions and Side Chain Lengths. Macromolecules 2014, 47, 777−790. (51) Chan, B.; Radom, L. Assessment of Theoretical Procedures for Hydrogen-atom Abstraction by Chlorine, and Related Reactions. Theor. Chem. Acc. 2011, 130, 251−260. (52) Choi, U. H.; Lee, M.; Wang, S.; Liu, W.; Winey, K. I.; Gibson, H. W.; Colby, R. H. Ionic Conduction and Dielectric Response of Poly(imidazolium acrylate) Ionomers. Macromolecules 2012, 45, 3974−3985. (53) Feldman, K. E.; Kade, M. J.; Meijer, E. W.; Hawker, C. J.; Kramer, E. J. Model Transient Networks from Strongly HydrogenBonded Polymers. Macromolecules 2009, 42, 9072−9081. (54) Bose, R. K.; Hohlbein, N.; Garcia, S. J.; Schmidt, A. M.; van der Zwaag, S. Connecting Supramolecular Bond Lifetime and Network Mobility for Scratch Healing in Poly(butyl acrylate) Ionomers Containing Sodium, Zinc and Cobalt. Phys. Chem. Chem. Phys. 2015, 17, 1697. (55) Pedretti, A.; Villa, L.; Vistoli, G. VEGA-An Open Platform to Develop Chemo-bio-informatics Applications, Using Plug-in Architecture and Script Programming. J. Comput.-Aided Mol. Des. 2004, 18, 167−173. (56) Pedretti, A.; Villa, L.; Vistoli, G. VEGA: A Versatile Program to Convert, Handle and Visualize Molecular Structure on WindowsBased PCs. J. Mol. Graphics Modell. 2002, 21, 47−49. (57) Nakamura, K.; Fukao, K.; Inoue, T. Dielectric Relaxation and Viscoelastic Behavior of Polymerized Ionic Liquids with Various Counteranions. Macromolecules 2012, 45, 3850−3858. (58) Nakamura, K.; Saiwaki, T.; Fukao, K.; Inoue, T. Viscoelastic Behavior of the Polymerized Ionic Liquid Poly(1-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonylimide)). Macromolecules 2011, 44, 7719−7726.

(59) Lin, Y.; Shangguan, Y.; Zuo, M.; Harkin-Jones, E.; Zheng, Q. Effects of Molecular Entanglement on Molecular Dynamics and Phase-separation Kinetics of Poly(methyl methacrylate)/Poly(styrene-co-maleic anhydride) Blends. Polymer 2012, 53, 1418−1427. (60) Wu, S. Entanglement between Dissimilar Chains in Compatible Polymer Blends: Poly(methyl methacrylate) and Poly(vinylidene fluoride). J. Polym. Sci., Polym. Phys. 1987, 25, 557−566. (61) Rubinstein, M.; Colby, R. H. Polymer Physics; Oxford University Press: London, 2003. (62) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics, 3rd ed.; Wiley: New York, 1980.

I


Molecular-Level Tuning toward Aggregation ... - ACS Publications

Recommend Documents