Influence of topology of LCST-based graft copolymers on responsive assembling in aqueous media

Abstract

This work, based on structure/properties relationships of associating polymers, aims to investigate the role of topology in the self-assembling behavior of responsive graft copolymers. For that purpose, two graft copolymers with inverse topologies were prepared with similar amounts of water-soluble chains (poly(N,N-dimethylacrylamide) = PDMA) and LCST polymer chains (poly(N-isopropylacrylamide) = PNIPA). In pure water, and above 3 wt%, PNIPA-g-PDMA and PDMA-g-PNIPA exhibit very similar macroscopic properties with a sol/gel transition above 35 °C related to the microphase separation of PNIPA sequences. From complementary experiments, performed by DSC, 1 H NMR and small angle neutron scattering, we show that the phase transition of PNIPA is more abrupt when NIPA units are located within the backbone, compared to side-chains. Nevertheless, well above their transition temperature, the two copolymers display very similar bicontinuous structures where PNIPA sequences self aggregate into concentrated percolating domains (about 70 wt% at 60 °C) characterized by a frozen dynamics. On the other hand, when salt or surfactant molecules are added into unentangled semi-dilute aqueous solution, the PNIPA-g-PDMA sample does not percolate anymore above the transition temperature while PDMA-g-PNIPA still demonstrate thermothickening properties that are correlated to the ability of water-soluble PDMA chains to bridge PNIPA aggregates.

Steric stabilization of modified nanoclays triggered by temperature

“…Similar trends were observed for block copolymers which undergo the aggregation process upon heating [36]. Moreover, the volume increase is consistent with the positive enthalpy determined for transition of copolymers containing the PNIPA block [37]. A careful analysis of the thermodynamic data allows us to evidence some important points.…”

Thermoresponsive Toughening with Crack Bifurcation in Phase‐Separated Hydrogels under Isochoric Conditions

“…Structure is depicted as bicontinuous domains: the percolating rich‐PNIPAm domains that are topologically defined as the network containing the chemical crosslinks (•), embedded into a PDMA hydrated matrix. The PNIPAm volume fraction in collapsed domains is estimated to be around 0.70–0.75 as calculated according to the scattering invariant …”

“…Above approximately 50 °C, the position of the correlation peak gives the periodicity of the structure with an estimated characteristic length scale of d ≅ 885 Å. From the scattering invariant and assuming that almost all the PNIPAm chains are involved in the phase transition, the volume fraction of PNIPAm inside the collapsed domains is estimated to be around 0.70–0.75, which suggests that the dynamics in those domains could be slowed down …”

Thermoresponsive Toughening in LCST-Type Hydrogels: Comparison between Semi-Interpenetrated and Grafted Networks

“…After stirring overnight, almost full conversion was achieved, and polymers were purified by dialysis against pure water (membrane cut-off=50 kDa) during one week and recovered by freeze-drying. The syntheses of PDMA and PNIPA macromonomers were performed as previously described 22,23 using a telomerization process with cysteamine hydrochloride followed by modification of the amino end-group with acrylic acid in order to get the vinyl function. From previous works performed on similar systems, 24 it was shown that the functionalization of telomers with amino end-group is almost quantitative, at least greater than 90 %, as well as the subsequent vinyl modification of amino-end groups due to the large excess of activated…”