Chinese Journal of Chemical Physics  2016, Vol. 29 Issue (6): 725-728

The article information

Hui-li Liu, Guang-ming Liu, Guang-zhao Zhang
刘慧丽, 刘光明, 张广照
Modulation of Self-healing of Polyion Complex Hydrogel by Ion-specific Effects
基于离子特异性效应调控聚电解质络合物水凝胶自修复性能
Chinese Journal of Chemical Physics, 2016, 29(6): 725-728
化学物理学报, 2016, 29(6): 725-728
http://dx.doi.org/10.1063/1674-0068/29/cjcp1605109

Article history

Received on: May 13, 2016
Accepted on: May 24, 2016
Modulation of Self-healing of Polyion Complex Hydrogel by Ion-specific Effects
Hui-li Liua, Guang-ming Liua, Guang-zhao Zhangb     
Dated: Received on May 13, 2016; Accepted on May 24, 2016
a. Department of Chemical Physics, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China;
b. Faculty of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China
*Author to whom correspondence should be addressed. Guang-ming Liu, E-mail:gml@ustc.edu.cn
Abstract: We have prepared polyion complex (PIC) hydrogel consisting of poly (3-(methacryloylami no) propyl-trimethylamonium chloride) and poly (sodium p-styrenesulfonate) polyelectrolytes via a two-step polymerization procedure and have investigated specific ion effects on the selfhealing of the PIC hydrogel. Our study demonstrates that the mechanical properties of the PIC hydrogel are strongly dependent on the type of the ions doped in the hydrogel. The ion-specific effects can be used to modulate the self-healing efficiency of the PIC hydrogel. As the doped anions change from kosmotrops to chaotropes, the self-healing efficiency of the PIC hydrogel increases. A more chaotropic anion has a stronger ability to break the ionic bonds formed within the hydrogel, leading to a higher efficiency during the healing.
Key words: Polyelectrolyte hydrogel    Self-healing    Ion-specific effects    Mechanical property    Counterions    
Ⅰ. INTRODUCTION

Self-healing materials have been investigated extensively during the last decade due to their important applications to extend the lifetime of materials and lower the production cost [1-5]. Polymeric hydrogels, e.g., polyelectrolyte hydrogels, have been regarded as good biomaterials [6, 7], with promising applications in many fields such as extra-cellular matrix [8-10] and artificial biorgans [11-13]. If the polymeric hydrogels possess good self-healing properties, then the applications of the hydrogels will be significantly widened [5, 14-16].

On the other hand, the mechanical properties also play an important role in the applications of the hydrogels [17-19]. To improve the mechanical properties of polymeric hydrogels including stiffness, strength, and toughness, the novel polyion complex (PIC) hydrogels were developed recently [20-22]. Although the self-healing of the PIC hydrogels can be achieved by reforming the non-covalent ionic bonds with the aid of external salts [20, 22], the self-healing efficiency of the PIC hydrogels is not high even at a high salt concentration. For example, the self-healing efficiency of the PIC hydrogel is~66% in terms of the work extension assisted by a 3 mol/L NaCl solution [20].

In this work, we have developed a method to modulate the self-healing of the PIC hydrogels by ion-specific effects. As different types of ions have distinct ability to break the ionic bonds formed between the oppositely charged polyelectrolytes [23], the self-healing efficiency is expected to be tuned by the external ions. Poly (3-(methacryloylamino) propyl-trimethylamonim chloride) (PMPTC) and poly (sodium p-styrenesulfonate) (PNaSS) were employed to prepare the PIC hydrogel. We find that the self-healing efficiency of the PIC hydrogel increases as the doped anions change from kosmotropes to chaotropes.

Ⅱ. EXPERIMENTS

3-(Methacryloylamino) propyl-trimethylammonium chloride (MPTC) was purchased from Sigma Aldrich (50wt% aqueous solution), which was passed through a basic alumina column to remove polymerization inhibitor prior to use. Sodium p-styrenesulfonate (NaSS) was purchased from Aladdin. NaSS was recrystallized from the mixture of ethanol and water. 4, 4'-azobis (4-cyanovaleric acid) (ACVA) was employed as a radical initiator. 2-Oxoglutaric acid bought from Wako Pure Chemical Industries, Ltd. was used as a photoinitiator. All the salts (AR grade) were purchased from Aladdin.

The PIC hydrogel was synthesized according to the procedure reported in Ref.[20]. First, PMPTC was synthesized by free radical polymerization from an aqueous solution containing 1.0 mol/L MPTC and 0.15mol% ACVA (refer to the MPTC monomer). Afterwards, PMPTC was lyophilized and made into powder. The powder was mixed with NaSS monomer with a mole ratio of NaSS to MPTC of 0.49:0.51. 0.15mol% 2-oxoglutaric acid (refer to the NaSS monomer) was added as the photoinitiator. The mixture was stirred to form a homogeneous solution with a concentration of 1.5 mol/L ([MPTC]+[NaSS]) in a 0.5 mol/L NaCl solution at 45 ℃. Then, the solution was injected into a cell composed of a pair of silica glass plates with 1.5 mm spacing to achieve the polymerization by UV light for at least 12 h. After the polymerization, the as-prepared sample was put into Milli-Q water to remove small ions from the PIC hydrogel.

The 1H NMR spectra were conducted on a Bruker AV400 NMR spectrometer. The PIC hydrogel was dried in an oven to remove the water from the sample. Then, the dried gel was dissolved in a 3 mol/L NaSCN D2O solution to perform the NMR measurement. The mole ratio of PMPTC to PNaSS was determined by elemental analysis on a VarioELIII elemental analyzer. The mechanical properties of the PIC hydrogel were measured on a tensile tester with a stretching speed of 100 mm/min at room temperature.

The self-healing of the PIC hydrogel was performed at room temperature. The sample was cut into two parts by a blade. The two newly formed surfaces were dipped into aqueous solutions containing different kinds of salts with a salt concentration of 3 mol/L for~2 min. Afterwards, the two newly formed surfaces were brought together for~1 min and then put into a sealed plastic bag for~12 h, followed by a dialysis of the sample in Milli-Q water for at least 4 days to remove the small ions from the healed zone to facilitate the reformation of ionic bonds.

Ⅲ. RESULTS AND DISCUSSION

As can be seen from Fig. 1(a), the prepared PIC hydrogel is white and compact after dialysis. Moreover, the PIC hydrogel can be easily processed into different shapes, e.g. (Ⅰ) rectangle shape, (Ⅱ) snowflake shape, and (Ⅲ) leaf shape, which is important for practical applications. The assignments of the H1 NMR peaks are given by the atom label in the inset molecules in Fig. 1(b) and (c). The absence of the peaks of vinyl group in the H1 NMR spectra in Fig. 1(b) and (c) indicates that the MPTC monomers almost convert into PMPTC completely and no monomers exist in the PIC hydrogel after the dialysis of the sample in water. The mole ratio of PMPTC to PNaSS is about 0.499:0.501 obtained from the elemental analysis. The mole ratio of cationic polyelectrolyte to anionic polyelectrolyte plays an important role in the mechanical properties of the PIC hydrogel. The PIC hydrogel is expected to have the best mechanical properties with the mole ratio of 1:1.

FIG. 1 (a) The prepared PIC hydrogel with different shapes: (Ⅰ) rectangle shape, (Ⅱ) snowflake shape, and (Ⅲ) leaf shape. (b) 1H NMR spectrum of PMPTC. (c) 1H NMR spectrum of the PIC hydrogel dissolved in the 3.0 mol/L NaSCN D2O solution.

Figure 2(a) shows the mechanical properties of the PIC hydrogel doped with different kinds of salts. A very high salt concentration would make the samples too soft to be handled. Here, all the samples were doped in the salt solutions with a salt concentration of 0.3 mol/L for~24 h. An obvious specific ion effect is observed in the stress-strain curves. As the anions change from Ac- to SCN-, the maximum stress of the hydrogel decreases form~1.1 MPa to~0.2 MPa along the order of Ac->Cl->3->Br->SCN-. Meanwhile, the strain at break increases from~5 mm/mm (~500%) to~10 mm/mm (~1000%) following the order of Ac- < Cl- < ClO3- < Br- < SCN-. The specific anion effect on the mechanical properties of the PIC hydrogel mainly originates from the ion-specific interactions between the doped anions and the positively charged -N+(CH3)3 groups associated with the PMPTC chains. It is known that the anionic hydration strength decreases following the order of Ac->Cl->3->Br->SCN- and the -N+(CH3)3 group is a weakly hydrated group [24, 25]. According to the Collins' concept of matching water affinities [26], the interactions between the -N+(CH3)3 group and the anions should increase following the order of Ac- < Cl- < ClO3- < Br- < SCN-. Therefore, the effectiveness of the anions to weaken the ionic bonds between the sulfonate and trimethylammonium groups should also increase as the anions change from Ac- to SCN- along the anion order. Because the mechanical properties of the PIC hydrogel are strongly dependent on the strength of the ionic bonds formed between the oppositely charged polyelectrolytes, the mechanical performance can be tuned by the anion-specific effects.

FIG. 2 (a) Mechanical performance of the PIC hydrogel doped with different kinds of salts in 0.3 mol/L salt solutions. (b) Toughness calculated from the stress-strain curves in Fig. 2(a).

The toughness values of the PIC hydrogel are shown in Fig. 2(b) obtained from the stress-strain curves. Obviously, the toughness of the hydrogel decreases as the anions change from Ac- to SCN-. The break strength and toughness decrease and the elongation at break increases as the anions change from Ac- to SCN- along the anion order, indicating that the mobility of the polyelectrolyte chains within the PIC hydrogel increases as the anions change from kosmotropes to chaotropes. This is important for designing self-healing PIC hydrogel as the chain mobility is a critical factor in determining the healing efficiency of hydrogels [27].

The self-healing ability of the PIC hydrogel is related to the interdiffusion of the polyelectrolyte chains across the cut surfaces from the two parts accompanied by the formation of the dynamic ionic bonds. As the breaking of the ionic bonds within the hydrogel is dependent on the type of doped anions, the chain mobility and the interdiffusion of polyelectrolyte chains should be tuned by specific ion effects. Thus, it is expected that the self-healing efficiency of the PIC hydrogel can be modulated by the ion-specific effects.

In Fig. 3(a), the virgin hydrogels (Ⅰ) were cut into two parts (Ⅱ) with a blade. The newly formed surfaces were dipped into 3.0 mol/L salt solutions for~2 min. Afterwards, the cut surfaces were brought together to contact for~1 min and then sealed in a plastic bag for~12 h, followed by a dialysis of the sample in Milli-Q water for at least 4 days. Finally, the healed gels were cut into dumbbell-shaped samples (Ⅲ) for tensile test (Ⅳ).

FIG. 3 (a) Self-healing of the PIC hydrogel assisted by 3.0 mol/L salt solutions. (b) Stress-strain curves of the virgin and healed samples assisted by 3.0 mol/L salt solutions for different types of anions. Inset: The stress-strain curve of the healed sample assisted by the NaAc solution. (c) Toughness and healing efficiency of the PIC hydrogel obtained from the stress-strain curves in Fig. 3(b).

In Fig. 3(b), it is evident that the fracture strain of the healed gels decreases following the order of SCN->Br->ClO3->Cl->Ac-. The stress-strain curve for Ac- is shown in the inset of Fig. 3(b) for clarity. The toughness of the healed hydrogels also decreases following the same series as that of the fracture strain (Fig. 3(c)). Therefore, the healing efficiency can be estimated based on the ratio of the toughness of the healed samples to the toughness of the virgin sample. Figure 3(c) shows the healing efficiency of the PIC hydrogels increases following the order of Ac- < Cl- < ClO3- < Br- < SCN-. That is, a more chaotropic anion can more effectively heal the sample. This is understandable because a more chaotropic anion has a stronger ability to break the ionic bonds formed within the hydrogel, leading to a higher mobility of the polyelectrolyte chains and a faster interdiffusion of polyelectrolyte chains across the cut surfaces, thereby giving rise to a higher efficiency during the healing. The mechanism of the modulation of the self-healing process of the PIC hydrogel by the ion-specific effects is schematically illustrated in Fig. 4.

FIG. 4 Schematic illustration of the self-healing mechanism of the PIC hydrogel modulated by ion-specific effects.
Ⅳ. CONCLUSION

In this work, we have investigated the modulation of the self-healing of the PIC hydrogel by ion-specific effects. The PIC hydrogel is composed of cationic PMPTC and anionic PNaSS. Due to the ion-specific interactions between the positively charged -N+(CH3)3 group associated with the PMPTC chains and the anions, the mobility and interdiffusion of polyelectrolyte chains can be tuned by specific ion effects. That is, the ion-specific effects can be used to modulate the self-healing of the PIC hydrogel. The self-healing efficiency of the PIC hydrogel increases as the anions change from kosmotropes to chaotropes because a more chaotropic anion has a stronger ability to break the ionic bonds, leading to a higher mobility of the polyelectrolyte chains and a faster interdiffusion of polyelectrolyte chains across the cut surfaces.

Ⅴ. ACKNOWLEDGMENTS

his work is supported by the National Natural Science Foundation of China (No.21374110, No.21574121, No.21622405, and No.21234003), the National Program on Key Basic Research Project (No.2012CB933802), the Youth Innovation Promotion Association CAS (No.2013290), and the Fundamental Research Funds for the Central Universities (No.WK2340000066).

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基于离子特异性效应调控聚电解质络合物水凝胶自修复性能
刘慧丽a, 刘光明a, 张广照b     
a. 中国科学技术大学化学物理系, 合肥 230026;
b. 华南理工大学材料科学与工程学院, 广州 510640
摘要: 通过两步聚合法制备了甲基丙烯酰胺丙基三甲基氯化铵(MPTC)和苯乙烯磺酸钠(NaSS)聚电解质络合物(PMPTC/PNaSS)水凝胶,研究了基于离子特异性效应调控聚电解质络合物水凝胶的自修复性能.通过不同种类离子对PMPTC/PNaSS水凝胶的特异性掺杂,可影响水凝胶中聚电解质链间静电相互作用以及链的运动能力,从而控制水凝胶的力学性能以及自修复性能.结果表明,当掺杂阴离子从结构构造型的强水化离子变为结构破坏型的弱水化离子,聚电解质络合物水凝胶的自修复效率会逐渐增加.主要是由于结构破坏型阴离子可以更加有效地打开水凝胶中的离子键,提高聚电解质链的运动能力和自修复效率.
关键词: 聚电解质水凝胶    自修复    离子特异性效应    力学性能    反离子