Hydrolytic Stability of Crosslinked, Highly Alkaline Diallyldimethylammonium Hydroxide Hydrogels

Abstract

The aim of this study was to evaluate the persistence of alkaline hydrogels based on a common (N,N'-methylenebisacrylamide, BIS) and three recently published tetraallyl crosslinkers. Such hydrogels have been shown to be suitable materials for the rehabilitation of cementitious materials. Of the four crosslinkers under investigation, N,N,N',N'-tetraallylpiperazinium dibromide decomposed quickly in 1 m KOH solution and was not considered further. BIS showed the first signs of a decomposition after several days, while tetraallylammonium bromide and N,N,N',N'-tetraallyltrimethylene dipiperidine dibromide remained unaffected. In contrast to BIS, which suffers from low solubility in water, the two tetraallyl crosslinkers show unlimited miscibility with diallyldimethylammonium hydroxide solutions. For the study, gels with up to 50 wt % crosslinker were prepared. Of these, gels containing tetraallylammonium bromide always show the highest degrees of swelling, with a peak value of 397 g/g at a content of 2 wt %. Under accelerated ageing at 60 °C for 28 d, gels crosslinked with BIS ultimately turned liquid, while the storage modulus and the degree of swelling of the two tetraallyl-crosslinked gels remained unchanged. This indicates that alkaline gels can be suitable for long application periods, which are common for rehabilitation measures in the construction industry.

Keywords: copolymer; crosslinker; durability; hydrogel; hydrolysis; rheology; swelling.

Figure 1

Top: general scheme for the crosslinking polymerization of DADMAOH; bottom: structures of the cross-linkers 1a–c and BIS used in this study.

Figure 2

¹H-NMR spectra of the pure crosslinkers in 1 m KOH solution in D₂O over the course of 7 days: (a) tetraallyl ammonium bromide, (b) N,N,N′,N′-tetraallyl piperazinium dibromide, (c) N,N,N′,N′-tetraallyl trimethylene dipiperidine dibromide, (d) BIS. The numbers on the right axis indicate the time code: (1) reference spectrum without KOH, (2) 1 m KOH in D₂O on the same day, (3) after 24 h, (4) after 48 h, (5) after 7 days.

Figure 3

Potential beginning of the alkaline degradation of TAPB (1b) by Hofman-type elimination.

Figure 4

Swelling behaviour of poly(DADMAOH) gels with different crosslinkers as a function of the crosslinker concentration in bidistilled water. Reproducibility is approx. ±10–14%. Due to the logarithmic scale, the error bars are smaller than the symbols and are therefore omitted.

Figure 5

Storage modulus (a) and equilibrium degree of swelling (b) of gels crosslinked with 10 mol% TAAB (triangles) and TAMPB (squares), as a function of the molar fraction of methacrylamide. The storage modulus was determined at an amplitude of 1%, a frequency of 1 Hz, and a contact pressure of 5 N.

Figure 6

Storage modulus of a poly(DADMAOH) hydrogel with 2 mol% TAMPB and 8 mol% methacrylamid as a function of the applied normal force. The values were recorded at an amplitude of 1% and a frequency of 1 Hz 4 weeks after starting the polymerization.

Figure 7

Change of the storage moduli of poly(DADMAOH-co-MAA)-hydrogels crosslinked with 2 mol% TAAB (triangles) or TAMPB (squares) and 4 mol% BIS (circles) at 60 °C.

Figure 8

Equilibrium degree of swelling of poly(DADMAOH-co-MAA) hydrogels crosslinked with 2 mol% TAAB/2 mol% TAMPB/4 mol% BIS at different times after storage of the polymerized gels at 60 °C. All gels were lyophilized before the swelling tests.