Strong, tough and mechanically self-recoverable poly(vinyl alcohol)/alginate dual-physical double-network hydrogels with large cross-link density contrast

Abstract

Strong and tough poly(vinyl alcohol) (PVA)/alginate hydrogen-bonded-ionic dual-physical double-network (DN) hydrogels have been successfully prepared by a facile route of a freeze-thaw (25-25-25 °C) cycle followed by concentrated (1.0 mol L^-1 of) aqueous-Ca²⁺ immersion of PVA/Na alginate (SA) mixed aqueous solutions. It was found that, at mole ratios of the PVA- to SA repeat units of 20/1 to 80/1, the DN gels likely evolved a semi-interpenetrating polymer network (IPN) morphology of rigid alginate networks dispersed in while interlocking with ductile PVA network to accomplish DN synergy that gave their high strength and toughness, where the high alginate rigidity originated probably from its dense cross-link induced syneresis and dispersion along crosslink-defective voids to result in little internal stress concentration. Tentatively mechanistically, as the 20/1-80/1 DN gels were stretched steadily, their mechanical response was gradually differentiated into distinct synergistic states: the sparsely hydrogen-bonded PVA served as a ductile matrix to bear small fractions of the established stresses at its large elongations; whereas the densely ionically (i.e. Ca²⁺) cross-linked alginate functioned as a rigid skeleton to sustain the remaining larger stresses upon its smaller local strains. Promisingly, this ductile-rigid matrix-skeleton synergistic mechanism of semi-IPN morphology may be universally extended to all A/B DN hydrogels of large A-B rigidity (or cross-link density) contrast, whether the cross-link nature of network(s) A or B is covalent, ionic, hydrogen bonded or van der Waals interacted. The strong and tough DN gels also displayed satisfactory self-recovery of viscoelastic behaviour, in that their Young's modulus and dissipated energy in the uniaxial tensile mode and dynamic storage and loss moduli in the oscillatory shear mode all recovered significantly from non-linear viscoelastic regimes despite different degrees of failure to revert to (quasi)linear viscoelasticity.

Scheme 1. (a) Illustration of a sequential formation of poly(vinyl alcohol) (PVA)/alginate hydrogen-bonded-ionic dual-physical double-network (DN) hydrogels: (1) first, PVA/sodium-alginate (SA) aqueous solutions are subjected to a freeze–thaw (RT–−25 °C–RT) cycle of being frozen at −25 °C for 20 h and then thawed back at room temperature (RT) (∼25 °C) for at least 4 h to construct a hydrogen-bonded network of the major PVA; (2) second, the pregels are immersed into a large amount of fresh CaCl₂ aqueous solution of 1.0 mol L⁻¹ at RT for at least 8 h to establish an ionically cross-linked network of the minor SA through development of Ca-dicarboxylate chelate triplets. (b) Representation of the cross-link structures of the PVA/alginate DN hydrogels formed in (a): (1) the hydrogen-bonded structure of the first PVA network, showing a coexistence of mediate coordinations between the PVA hydroxyls that are bridged by a series of hydrogen-bonded water molecules and immediate interactions essentially present in the formed PVA crystallites; (2) the ionically cross-linked structure of the second alginate network.

Fig. 1. Stress, σ vs. stretch, ε curves, obtained by uniaxial tensile testing at room temperature (RT) (∼25 °C) at a crosshead speed of 50 mm min⁻¹, of poly(vinyl alcohol) (PVA)/alginate sequential, hydrogen-bonded-ionic dual-physical double-network (DN) hydrogels having mole ratios of the PVA- to sodium alginate (SA) repeat units of 20/1, 30/1, 40/1, 50/1, 60/1, 70/1 and 80/1, compared with those of their corresponding PVA hydrogen-bonded and alginate ionic single-network (SN) hydrogels. Both the DN-gels PVA networks and the PVA SN gel are formed by hydrogen bonding of an ∼13–20 wt% of PVA aqueous solution upon its freeze–thaw (RT–−25 °C–RT) cycle of being frozen at −25 °C for 20 h and then thawed back at RT for at least 4 h, followed by its immersion into a large amount of Ca²⁺ aqueous solution of 1.0 mol L⁻¹ at RT for at least 8 h, thus showing similar compositions (i.e. water contents) and morphologies (i.e. crosslink structures and -densities) to each other. Both the DN-gels alginate networks and the alginate SN gel are constructed through ionic cross-linking of an ∼1–3 wt% of SA aqueous solution with the large amount of Ca²⁺ immersion solution of 1.0 mol L⁻¹ at RT for at least 8 h, hence displaying analogous compositions and morphologies to each other.

Scheme 2. Representation of a tentative ductile-rigid matrix-skeleton mechanism of the synergistic mechanical enhancement in poly(vinyl alcohol) (PVA)/sodium alginate (SA) sequential, hydrogen bonded-ionic dual-physical double-network (DN) hydrogels: (a) the DN gel comprises a major PVA network sparsely hydrogen bonded and a minor SA network densely ionically cross-linked with Ca²⁺ to form an interpenetrating polymer network (IPN), which is mechanically dominated by the alginate network that, despite its high intrinsic rigidity, has as poor mechanical properties as its parallel alginate single-network (SN) gel due to the significant presence of internal crosslink-defective voids (i.e. stress concentration) upon a cross-link induced syneresis that poses its much lower effective rigidity; (b) with an increase in the PVA/SA ratio above a critical value, the DN gel turns into a semi-IPN of small, dispersed alginate networks locally interlocking with global PVA network, which enables synergy of the PVA-matrix's ductility and the alginate-skeleton's high effective rigidity from its little internal stress concentration upon crosslink-induced syneresis and dispersion along the crosslink-defective voids, and hence gives greater mechanical properties than either of the parallel PVA- and alginate SN gels; (c) with increasing the PVA/SA ratio further beyond another higher critical value, the DN gel evolves into an alginate-“filled” PVA where the smaller, dispersed alginate networks, although more rigid owing to the presence of fewer internal crosslink-defective voids, do not locally interlock and thus may have basically van der Waals interactions with the global PVA network, leading to a predominance of the weak PVA network that has comparably poor mechanical properties to its parallel PVA SN gel. Specifically, the PVA-alginate mechanical synergism in (b) during a tensile or tearing process is illustrated sequentially from (1) to (4): (1) the interlocking PVA and alginate networks deform synchronously and equally upon initial stretches of the DN gel; (2) the DN gel is progressively differentiated upon further stretches into a ductile PVA matrix and a rigid alginate skeleton of distinct mechanical responses (stretchable vs. stressable, respectively), until a large critical stretch of the former when the stress of the latter is large enough to begin to rupture the Ca dicarboxylate cross-links; (3) the sacrificial ionic bonds of Ca dicarboxylate are all broken upon a further small stretch from the onset of their rupture, leaving the hydrogen bond cross-links of the well-extended PVA network nearly intact; (4) the evolved large stress turns to be suffered essentially by the global, weak PVA network, resulting in a catastrophic failure of the DN gel upon instantaneous breakage of the intermolecular hydrogen bonds possibly accompanied by a small degree of backbone chain scission of the PVA.

Fig. 2. Tensile hysteresis loops, at room temperature (∼25 °C), of a poly(vinyl alcohol) (PVA)/alginate sequential, hydrogen-bonded-ionic dual-physical double-network (DN) hydrogel of maximum mechanical properties (tensile strength, elongation at break and toughness) having a mole ratio of the PVA- to Na alginate repeat units of 60/1, in which the Ca-dicarboxylate ionic cross-links of the minor, dispersed alginate networks are dramatically denser than the hydrogen bond cross-links of the major, global PVA network. To obtain the hysteresis loops, a fresh specimen of the DN gel, kept gripped on a universal testing machine as it is over the whole process at an initial separation of 25 mm, is subjected to sequential tensile-unloading (to zero stretch) cycles of predetermined constant-incremental stretches at a crosshead speed of 50 mm min⁻¹ without intermission between the neighbouring cycles: (a) at its small stretches, one fresh specimen is run from an initial stretch of 0.07 mm mm⁻¹ until 0.55 mm mm⁻¹ at an increment of 0.08 mm mm⁻¹; while (b) at its larger stretches, another fresh, parallel specimen is run from an initial stretch of 0.70 mm mm⁻¹ until its fracture at an increment of 0.80 mm mm⁻¹. As it essentially overlaps completely with the bottom of the immediately subsequent tensile curve, any of the unloading curves is omitted from either of Graphs a and b.

Fig. 3. Effects of the (predetermined) stretch of a tensile-unloading (to zero stretch) cycle on (a) the Young's modulus (E_c) of the cycle, i.e. slope at the onset point of the tensile curve of the cycle where the stress begins to become non-zero, and on (b) the degree of hysteresis (U_hys/W_t), i.e. ratio of the irreversible viscous work (U_hys) to the total viscoelastic work (W_t), until the cycle for a poly(vinyl alcohol) (PVA)/alginate sequential, hydrogen-bonded-ionic dual-physical double-network (DN) hydrogel of maximum mechanical properties (tensile strength, elongation at break and toughness) having a mole ratio of the PVA- to Na alginate repeat units of 60/1 and Ca-dicarboxylate ionic cross-links of the minor dispersed alginate networks dramatically denser than the hydrogen bond cross-links of the major global PVA network, which, standing gripped on a universal testing machine as it is across the entire process at an initial separation of 25 mm, is subjected to sequential tensile-unloading (to zero stretch) cycles of predetermined constant-incremental stretches at a crosshead speed of 50 mm min⁻¹ without intermission between the neighbouring cycles: at its small stretches, one fresh specimen is run from an initial stretch of 0.07 mm mm⁻¹ until 0.55 mm mm⁻¹ at an increment of 0.08 mm mm⁻¹; while, at its larger stretches, another fresh, parallel specimen is run from an initial stretch of 0.70 mm mm⁻¹ until its fracture at an increment of 0.80 mm mm⁻¹.

Fig. 4. (a) Evolution, with the aging time upon a tensile-unloading (to zero stretch) cycle, of the hysteresis loop re-formed from the cycle for a poly(vinyl alcohol) (PVA)/alginate sequential, hydrogen-bonded-ionic dual-physical double-network (DN) hydrogel of maximum mechanical properties (tensile strength, elongation at break and toughness) having a mole ratio of the PVA- to Na alginate repeat units of 60/1 and Ca-dicarboxylate ionic cross-links of the minor dispersed alginate networks dramatically denser than the hydrogen bond cross-links of the major global PVA network. To conduct the experiments, five fresh parallel specimens of the gel first undergo a tensile-unloading cycle of 1.0 mm mm⁻¹ stretch at a crosshead speed of 50 mm min⁻¹ at room temperature (RT) (∼25 °C), then stand in a large amount of CaCl₂ aqueous solution of 1.0 mol L⁻¹ (i.e. age at the sacrifice of neither their water content nor cross-link morphology) at RT for increasing times of 0, 2, 10, 25 and 60 min, respectively, and are finally subjected to the tensile-unloading cycle again. Since the five fresh parallel specimens exhibit similar hysteresis loop profiles to each other, only the hysteresis loop of the fresh specimen to be subsequently aged for 0 min is shown as the initial one (i.e. control) for analysis; and, exceptionally, the hysteresis loop of the gel on 0 min of aging is realised by the tensile-unloading cycle run in situ immediately after the control, with the specimen kept gripped as it is and actually not immersed into the CaCl₂ solution for aging. (b) Percent recoveries of the Young's (i.e. tensile elastic) modulus, E, and dissipated energy, U_hys, towards those of the control as functions of the aging time for the PVA/alginate DN hydrogel of the maximum mechanical properties, where the E of all the loops (including the control) was read as the slope at the onset (i.e. non-zero stress) point of their tensile curve and the U_hys estimated as their area from (a).

Fig. 5. Rheology, in the oscillatory shear mode under a 25 mm parallel-plate fixture at room temperature (∼25 °C) and a frequency of 1.0 Hz, of a poly(vinyl alcohol) (PVA)/alginate sequential, hydrogen-bonded-ionic dual-physical double-network (DN) hydrogel of maximum mechanical properties (tensile strength, elongation at break and toughness) having a mole ratio of the PVA- to Na alginate repeat units of 60/1, in which the Ca-dicarboxylate ionic cross-links of the minor, dispersed alginate networks are dramatically denser than the hydrogen bond cross-links of the major, global PVA network: (a) plots of strain-amplitude sweep (i.e. step strain-amplitude) from 0.1% to 1000% using a fresh specimen, where a critical strain-amplitude beyond which the viscoelastic behaviour of the gel transitions from linear to non-linear is identified as ∼300%, as reflected from the onset point of sharp changes in its constant (apparent) dynamic storage and loss moduli, G′ and G′′ respectively; (b) plots of step strain-amplitude alternate without intermission between 0.1% and 700% for four cycles using another fresh parallel specimen, where the 0.1% is safely enough within the linear viscoelastic region of the gel as revealed from (a) while the 700% larger enough than the critical strain-amplitude, ∼300%, obtained from (a) to fall well within its non-linear viscoelastic region, thus showing the fatigue behaviour of the gel's dynamic shear self-recovery from the non-linear towards linear viscoelastic regime.