Thermoreversible Behavior of Associating Polymer Solutions:  Thermothinning versus Thermothickening

Abstract

Self-assembling properties of poly(sodium acrylate) grafted with dodecyl [C12], PAAgC12, or poly(N-isopropylacrylamide) [PNIPA] side chains, PAAgPNIPA, were studied in unentangled semidilute aqueous solution. While PAAgC12 self-associates through hydrophobic interactions, the gelation of PAAgPNIPA is triggered by heating in response to the lower critical solution temperature of PNIPA (LCST ∼ 32 °C). The local structure of the physical networks was investigated by small-angle neutron scattering, and the scattering patterns were described using a polydisperse sphere model taking into account hard-sphere interactions. This model allow us to draw a realistic picture of physical gels with quantitative information concerning the size of hydrophobic cores, the volume fraction of stickers in the aggregates, the fraction of stickers which take part in the aggregation process, the range of repulsive interactions, and the structural modifications induced by temperature. The description of the network is in good agreement with complementary data obtained from DSC and 13C NMR. In the present work, a special emphasis has been given to the important relationship existing between the viscoelastic properties of associating polymer solutions and the binding energy of stickers leaving temporarily the micellar junction. Depending on the endothermic or exothermic nature of the disengagement process of the sticker (heat of demicellization), the relaxation time of the network and the viscoelastic properties will either decrease or increase with the temperature. The consequence is that aqueous solutions of PAAgC12 and PAAgPNIPA exhibit opposite rheological properties with the temperature, namely thermothinning and thermothickening. By mixing these two copolymers, we show that intermediate properties can be obtained but in that case a microphase-separated network is obtained as a result of copolymer segregation.

Physical Gelation of Amphiphilic Poly(N‐isopropylacrylamide): Influence of the Hydrophobic Groups

“…This association is strong at low temperature as the enthalpy associated to the extraction of a dodecyl group from a hydrophobic domain is positive. [31] The hydrophobic microdomains constitute the crosslinking points of the reversible network ( Figure 9b). As was mentioned in the previous work of hydrophobically modified PNIPAM [56] , a non-negligible proportion of the NIPAM units neighboring the hydrophobic microdomains should be dehydrated.…”

“…On the other hand, rheological properties of hydrophobically modified water-soluble polymers such as polyacrylamide [26][27][28] , poly(dimethylacrylamide) [29] , and poly(sodium acrylate) [29][30][31] have attracted an increasing interest in the last decade. Beyond a threshold concentration, the viscosity increases rapidly with the concentration, due to the formation of interchain aggregates between the hydrophobic groups; these aggregates act as crosslinks leading to reversible physical network.…”

“…Regarding the temperature effect, these systems are usually thermo-thinning. Recently new systems based on a hydrophilic backbone, poly(sodium acrylate), grafted with PNIPAM [31][32][33] have been developed. Such copolymers showed thermo-thickening behavior, which depends both on structural (side chain lengths) and physicochemical parameters (concentration, shear rate).…”

Rheology of associative polymer solutions

“…Due to the polyelectrolyte nature of the backbone, polymers with higher contents of hydrophobic groups remain soluble. Hourdet et al [112] have studied PAA grafted with dodecyl chains and found a strong increase of the viscosity with increasing concentration. The temperature dependence of the terminal relaxation time could be characterized by an activation energy of 150 kJ/mol, which differs from that of HEUR containing the same alkyl groups (Ea = 42 kJ/mol).…”

Thermothickening Behavior of Graft Copolymers Containing Poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) Side Chains in Aqueous Solution

“…Thermothickening polymers have attracted much attention as a result of their special behaviors and several aqueous systems which display such thermoassociative properties have been developed in the last four decades. All these aqueous systems can be classified into three different groups: aqueous solutions of cellulosic derivatives (6-9), block copolymers (10)(11)(12)(13)(14) and graft copolymers (15)(16)(17)(18)(19)(20). Although all the systems exhibit a thermothickening behavior, only the graft copolymers, which were developed by Hourdet at the beginning of the 1990s, really provide a systematic way to explore the key relations existing between the main structure of the copolymer and its flow properties with the temperature.…”