Chemically cross-linked poly(N-isopropylacrylamide) (PNIPAM) microgels and PNIPAM with different amounts of acrylic acid groups (PNIPAM-co-PAA) were synthesized and the temperature-induced aggregation behaviors of aqueous suspensions of these microgels were investigated mainly with the aid of dynamic light scattering (DLS) and turbidimetry. The DLS results show that the particles at all conditions shrink at temperatures up to approximately the lower critical solution temperature (LCST), but the relative contraction effect is larger for the microgels without acid groups or for microgels with added anionic surfactant (SDS). A significant depression of the cloud point is found in suspensions of PNIPAM with very low concentrations of SDS. The compression of the microgels cannot be traced from the turbidity results, but rather the values of the turbidity increase in this temperature interval. This phenomenon is discussed in the framework of a theoretical model. At temperatures above LCST, the size of the microgels without attached charged groups in a very dilute suspension is unaffected by temperature, while the charged particles (pH 7 and 11) continue to collapse with increasing temperature over the entire domain. In this temperature range, low-charged particles of higher concentration and particles containing acrylic acid groups at low pH (pH 2) aggregate, and macroscopic phase separation is approached at higher temperatures. This study demonstrates how the stabilization of microgels can be affected by factors such as polymer concentration, addition of ionic surfactant to particles without charged acid groups, amount of charged groups in the polymer, and pH.

a b s t r a c tConcretes with a high thermal energy storage capacity were fabricated by mixing microencapsulated phase change materials (MPCM) into Portland cement concrete (PCC) and geopolymer concrete (GPC). The effect of MPCM on thermal performance and compressive strength of PCC and GPC were investigated. It was found that the replacement of sand by MPCM resulted in lower thermal conductivity and higher thermal energy storage, while the specific heat capacity of concrete remained practically stable when the phase change material (PCM) was in the liquid or solid phase. Furthermore, the thermal conductivity of GPC as function of MPCM concentration was reduced at a higher rate than that of PCC. The power consumption needed to stabilize a simulated indoor temperature of 23°C was reduced after the addition of MPCM. GPC exhibited better energy saving properties than PCC at the same conditions. A significant loss in compressive strength was observed due to the addition of MPCM to concrete. However, the compressive strength still satisfies the mechanical European regulation (EN 206-1, compressive strength class C20/25) for concrete applications. Finally, MPCM-concrete provided a good thermal stability after subjecting the samples to 100 thermal cycles at high heating/cooling rates.

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The thermoresponsive nature of aqueous solutions of poly(N‐isopropylacrylamide) (PNIPAAM) star polymers containing 2, 3, 4, and 6 arms has been investigated by turbidity, dynamic light scattering, rheology, and rheo‐SALS. Simulations of the thermosensitive nature of the single star polymers have also been conducted. Some of the samples form aggregates even at temperatures significantly below the lower critical solution temperature (LCST) of PNIPAAM. Increasing concentration and number of arms promotes associations at low temperatures. When the temperature is raised, there is a competition between size increase due to enhanced aggregation and a size reduction caused by contraction. Monte Carlo simulations show that the single stars contract with increasing temperature, and that this contraction is more pronounced when the number of arms is increased. Some samples exhibit a minimum in the turbidity data after the initial increase at the cloud point. The combined rheology and rheo‐SALS data suggest that this is due to a fragmentation of the aggregates followed by re‐aggregation at even higher temperatures. Although the 6‐arm star polymer aggregates more than the other stars at low temperatures, the more compact structure renders it less prone to aggregation at temperatures above the cloud point.

The behavior of a range of different stimuli-responsive star poly­(N-isopropylacrylamide) (PNIPAAM) polymers in water was investigated using small-angle X-ray scattering (SAXS). The samples of 1, 2, and 5 wt % PNIPAAM stars with 2, 3, 4, and 6 arms were measured at seven temperatures ranging from 15 to 36 °C, which covers the known lower critical solution temperature (LCST) for linear PNIPAAM. The data were fitted with a Gaussian star form factor, and interactions were accounted for by a random-phase approximation (RPA) expression. A clear LCST was observed at ∼32 °C for the four polymers in agreement with turbidity data on the same polymers. The molecular weight was calculated from the forward scattering, which showed reasonable agreement with the values expected from the synthesis and characterization. From the fits the root-mean-square radius of gyration ⟨R g 2⟩1/2 and the second virial coefficient A 2 were obtained and compared with the literature.

Chemically cross-linked nanoparticles from dilute aqueous alkali solutions of hydroxyethylcellulose (HEC) in the presence of a cross-linker agent (divinyl sulfone, DVS) were prepared from a reaction mixture, which was exposed to different stirring speeds during the crosslinking process. At various stages during the cross-linking procedure, the reaction was terminated and the species were characterized by means of turbidimetry, asymmetric flow field-flow fractionation (AFFFF), dynamic light scattering (DLS), and rheo-small-angle light scattering (rheo-SALS) methods. During the cross-linking of a dilute polymer solution, the competition between intrapolymer and interpolymer is a prominent feature. The DLS results show that at early times in the cross-linking process, intrachain crosslinking with contraction of the complexes is promoted by high stirring speeds; at later times the growth of aggregates is inhibited by high stirring speeds. The results from the rheo-SALS measurements disclosed that at early times during the cross-linker reaction, the complexes are fragile against shear forces if the reaction mixture had been subjected to low stirring speeds. At a later state of cross-linking, more cross-links lead to better stability of the species, even for solutions that have been exposed to low stirring speeds during the cross-linking process. This study shows that the sizes of the particles can be tuned by exposing the solutions to different stirring speeds during the cross-linker reaction and to terminate the reaction at desired reaction times. The strategy discussed in this work for the preparation of particles of various sizes is of special interest in connection with enhanced oil recovery applications.

The interaction of the anionic surfactant sodium dodecyl sulfate (SDS) and the cationic surfactant hexadecyl trimethyl ammonium bromide with poly(N-isopropylacrylamide) (PNIPAAM) microgels with and without poly(acrylic acid) (PAA) was investigated by means of dynamic light scattering (DLS), zeta potential, and turbidimetry measurements. The DLS results show that the PNIPAAM microgels with PAA will contract when an anionic or cationic surfactant is added to the suspension, while the PNIPAAM microgels without PAA expand in the presence of an ionic surfactant. A collapse of the PNIPAAM microgels is observed when the temperature is increased. From the zeta potential measurements, it is observed that the charge density of PNIPAAM microgels in the presence of an ionic surfactant is significantly affected by temperature and the attachment of the negatively charged PAA groups. The turbidity measurements clearly indicate that the interaction between PNIPAAM and SDS is more pronounced than that of the cationic surfactant.

Interactions between a chemically cross-linked poly(N-isopropylacrylamide) (PNIPAAM) microgel and the anionic surfactant sodium dodecyl sulfate (SDS) have been investigated by dynamic light scattering (DLS) and turbidimetry in both a buffer system (pH = 7) with an ionic strength of 0.05 M and water. Both the turbidity and DLS measurements revealed significant polymer-surfactant interactions between PNIPAAM and SDS in the buffer system, whereas in the absence of buffer only the highest SDS concentration clearly deviates from what is observed in the absence of surfactant. The DLS results show that the particles shrink with increasing temperature at all conditions; a macroscopic phase separation is approached at higher temperatures for the microgels without or with low SDS concentration in the buffer system. Mixtures of microgels and surfactants may be promising for enhanced oil recovery applications. 1. Introduction Unstable oil price and increasing oil demand is a fact of life, even though a global peak of conventional oil production is expected to decay, energy insufficiency is one of the most important problems that faces humanity (Deffeyes, 2001; Reynold, 2002; Mohanty, 2003; Hirsch et al., 2005). Liquid fuel prices will probably increase as a consequence of the decreasing production of cheap, easy-to-access oil. Without a practical solution, the economic, environmental, social, and political costs will be unprecedented. With the opportunities of discovering new reservoirs decreasing (Hirsch et al., 2005; Thomas and Farouq-Ali, 1999), enhanced oil recovery (EOR) has become an important option to moderate shortage of oil production in reservoirs after they reach their peak production (Mohanty, 2003; Hirsch et al., 2005). However, only approximately 30 % of the total oil in place can be recovered naturally leaving approximately 70 % oil in the reservoir itself (Curbelo et al., 2007). During the last years, the synergism between chemically cross-linked hydrogels and surfactants in aqueous solution has attracted much interest from both fundamental and applied research of chemical flooding for EOR (Chhetri et al.,2009; Dong et al.,2009; Liang and Zhang, 2009; Moura de Luna et al., 2009; Al-Manasir et al., 2009; Zhao et al., 2009; Aalaie and Rahmatpour, 2008; Aalaie et al., 2008; Aalaie et al., 2008; Al-Manasir et al., 2009). In the oil field, polyacrylamides and polysaccharide hydrogels (Al-Manasir et al., 2009; Zhao et al., 2009; Aalaie and Rahmatpour, 2008; Aalaie et al., 2008; Aalaie et al., 2008; Al-Manasir et al., 2009), or various types of surfactant (Chhetri et al.,2009; Dong et al.,2009; Liang and Zhang, 2009; Moura de Luna et al., 2009; Al-Manasir et al., 2009) are widely used in chemical flooding. Previous studies showed that by using very dilute surfactant solutions in the chemical formula, the oil/water interfacial tension could be significantly reduced and heavy oil was more easily dispersed into the water phase (Dong et al.,2009; Ma, 2005; Liu et al., 2006). Poly(N-isopropylacrylamide) (PNIPAAM) is a widely used polymer, which is temperature sensitive in an aqueous environment, with a lower critical solution temperature (LCST) of approximately 32 °C(Schild, 1992). In the case of low molecular weights, the value of the LCST is dependent on both the molecular weight and the polymer concentration (Pamies et al., 2009). At temperatures below the LCST, PNIPAAM is highly solvated due to hydrogen bonding between water molecules and amide residues of the polymer chain. When the temperature is increased the hydrogen bonds are disrupted and multichain association and growth of huge aggregates occur. PNIPAAM microgels are relatively large, cross-linked PNIPAAM particles. In this paper we have examined the temperature dependent behavior of aqueous suspensions of PNIPAAM microgels. We have carried out dynamic light scattering (DLS) and turbidity experiments on these microgels in the presence of different amount of SDS at two different solvent conditions (in water and in a 0.05 M buffer at pH =7).