Tough polyion-complex hydrogels from soft to stiff controlled by monomer structure

• Published in: Polymer (Guilford) 2017-05, Vol.116, p.487-497
• Main Authors: Luo, Feng, Sun, Tao Lin, Nakajima, Tasuku, Kurokawa, Takayuki, Li, Xufeng, Guo, Honglei, Huang, Yiwan, Zhang, Huijie, Gong, Jian Ping
• Format: Article
• Language: English
• Subjects: Ammonium; Biomaterials; Biomedical materials; Bonding strength; Chemical bonds; Crosslinking; Dynamic relaxation spectrum; Elasticity; Elongation; Hydrogels; Kinetics; Mechanical properties; Microstructure; Monomer structure; Monomers; Phase separation; Phase transitions; Polycations; Polyelectrolytes; Polyion-complex; Rheological properties; Stiffness; Surgical implants; Temperature; Time-temperature superposition; Tough hydrogel
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/j.polymer.2017.02.042

Tough hydrogels with adjustable stiffness are expected for adapting application as various biomaterials. Oppositely charged polyelectrolytes form tough and self-healing physical polyion-complex (PIC) hydrogels via formation of inter-chain ionic bonds with a wide distribution in bond strength. The strong bonds serve as permanent crosslinking to impart elasticity and the weak bonds as reversible sacrificial bonds to dissipate energy and to self-heal. In this work, we fabricate four PIC hydrogels using four positively charged trimethyl-ammonium monomers with slightly different chemical moieties and a same negatively charged polymer. The obtained PIC hydrogels all show high toughness but large difference in stiffness, extensibility, and self-recovery kinetics. With slight difference in the monomer structure of the polycations, the modulus of the hydrogels varies over two orders in magnitude, from 0.36 to 56 MPa, and the difference in elongation at break is up to five times. The presence of acryloyl moiety and methyl moiety increase the stiffness of the hydrogels. In the temperature range studied, all the four PIC hydrogels exhibit the rheological simple behaviours, following the time-temperature superposition principle. The four samples show quite different dynamic relaxation spectra over wide frequency range, revealing large difference in the strength distribution of dynamic ionic bonds. SEM observation reveals quite different phase separation structure for the four samples, in which the polymer chain stiffness should play an important role. This understanding of structure-properties of the PIC hydrogels will merit the designing of various supramolecular tough hydrogels and therefore broaden the scope of hydrogels for the applications as biomaterials. [Display omitted] •Four polycations with slight difference in their monomer structure were used to form PIC hydrogels with a same polyanion.•The four PIC hydrogels obtained are all mechanically tough but they show large differences in stiffness.•The breaking of the ionic bonds accounts for the toughness by energy dissipation mechanism.•The stiffness difference is originated from different strength distribution of dynamic ionic bonds in the PIC hydrogels.•Characters of the moiety in monomers dominate the microstructure and mechanical properties.