2 research outputs found
Polymer-Grafted Lignin Surfactants Prepared via Reversible Addition–Fragmentation Chain-Transfer Polymerization
Kraft
lignin grafted with hydrophilic polymers has been prepared
using reversible addition–fragmentation chain-transfer (RAFT)
polymerization and investigated for use as a surfactant. In this preliminary
study, polyacrylamide and polyÂ(acrylic acid) were grafted from a lignin
RAFT macroinitiator at average initiator site densities estimated
to be 2 per particle and 17 per particle. The target degrees of polymerization
were 50 and 100, but analysis of cleaved polyacrylamide was consistent
with a higher average molecular weight, suggesting not all sites were
able to participate in the polymerization. All materials were readily
soluble in water, and dynamic light scattering data indicate polymer-grafted
lignin coexisted in isolated and aggregated forms in aqueous media.
The characteristic size was 15–20 nm at low concentrations,
and aggregation appeared to be a stronger function of degree of polymerization
than graft density. These species were surface active, reducing the
surface tension to as low as 60 dyn/cm at 1 mg/mL, and a greater decrease
was observed than for polymer-grafted silica nanoparticles, suggesting
that the lignin core was also surface active. While these lignin surfactants
were soluble in water, they were not soluble in hexanes. Thus, it
was unexpected that water-in-oil emulsions formed in all surfactant
compositions and solvent ratios tested, with average droplet sizes
of 10–20 μm. However, although polymer-grafted lignin
has structural features similar to nanoparticles used in Pickering
emulsions, its interfacial behavior was qualitatively different. While
at air–water interfaces, the hydrophilic grafts promote effective
reductions in surface tension, we hypothesize that the low grafting
density in these lignin surfactants favors partitioning into the hexanes
side of the oil–water interface because collapsed conformations
of the polymer grafts improve interfacial coverage and reduce water–hexanes
interactions. We propose that polymer-grafted lignin surfactants can
be considered as random patchy nanoparticles with mixed hydrophilic
and hydrophobic domains that result in unexpected interfacial behaviors.
Further studies are necessary to clarify the molecular basis of these
phenomena, but grafting of hydrophilic polymers from kraft lignin
via radical polymerization could expand the use of this important
biopolymer in a broad range of surfactant applications
Lignopolymer Superplasticizers for Low-CO<sub>2</sub> Cements
Grafting
hydrophilic polyacrylamide from a kraft lignin core results in a lignopolymer
that effectively plasticizes portland cement paste. Here, the potential
of this sustainably sourced lignopolymer is examined for improving
the workability of portland cement blended with two natural, finely
divided mineral materials (kaolin clay and clinoptilolite zeolite),
which can be potentially used in combination with portland cement
as a means to reduce the cement clinker content and the concomitant
embodied energy and greenhouse gas emissions in concrete. Both mineral
materials are known to affect cement hydration reactions but can significantly
reduce workability when blended with portland cement, presenting a
challenge for their practical large scale use., The
plastic behavior of mineral-cement combinations dosed with polyacrylamide-grafted
kraft lignin was compared to the behavior of those with a lignosulfonate
developed for plasticizing portland cement and a commercial polycarboxylate
ether (PCE). Compared to the other admixtures, the lignopolymer was
found to adsorb strongly to both kaolin and clinoptilolite and resulted
in the lowest yield stresses in pastes containing 25% mineral material
and 75% Type I/II portland cement at both high (5 mg/mL, 0.25 wt %)
and low (0.5 mg/mL, 0.025 wt %) admixture content, but no significant
retardation was observed in calorimetry measurements of hydration
kinetics. In contrast, the PCE had a low affinity for both mineral
admixtures at higher concentrations compared to that of both lignin
superplasticizers, and the resulting tests of slump spread and viscosity
were similar for these two minerals. Lignosulfonates had affinity
for the mineral phases similar to that of the other superplasticizers
but were ineffective at improving workability, pointing to the need
for steric interactions in plasticizing mineral materials used in
combination with cement. These results indicate that lignopolymers
could be an effective superplasticizer for a broad range of high surface
area minerals to be used in combination with portland cement