Poly (N-isopropylacrylamide) (poly-NIPAM) microgel particles are soft spheres with a core-shell structure having a dense core and a thin shell consisting of less cross-linked polymers and dangling polymer chains (here after referred to as hairs). These poly-NIPAM particles with core-shell structure and dangling polymer chains are referred to as CPNIPAM soft-spheres. Poly-NIPAM microgel soft-spheres with homogeneous core (HPNIPAM) are also synthesised by altering synthesis conditions to unravel the role of hairs on the phase behaviour, particle dynamics and the yielding behaviour of microgel suspensions under dense conditions (i.e., volume fractions ϕ ≥ 0.68). This article presents results obtained from dynamic light scattering (DLS), and rheology measurement on dense disordered (glassy) structures of poly-NIPAM microgel suspensions with volume fractions, ϕ < 0.68 and under over packed (ϕ > 0.68) conditions. We discuss two important results obtained on dense CPNIPAM microgel colloidal glasses, namely, (i) two-step yielding and (ii) sub-diffusive behaviour at short-time scales. The entanglement of hairs between neighbouring soft spheres under dense (also referred to as over packed) conditions is shown to be responsible for the above-mentioned results. To provide unambiguous evidence in favour of the existence of entanglements, we prepared HPNIPAM particles that have only homogeneous polymer core and no hairs. Under over packed conditions HPNIPAM suspensions showed single step yielding and absence of sub-diffusive behaviour at short times, respectively. These observations provide evidence in support of entanglements between hairs which are responsible for the unusual behaviour of dense microgel colloidal glasses of CPNIPAM particles. Molecular dynamics and oscillatory shear simulations have been carried out using multi-Hertzian (MH) pair-potential proposed by Bergman et al. (Nat. Commun. 2018, 9, 5039) for core-shell structured thermo-responsive poly-NIPAM microgel suspensions. We show that though MH pair-potential explains the phase behaviour of microgel suspensions qualitatively, it fails to explain the above-mentioned experimental observations. The need for improving MH pair-potential that accounts for the role of hairs and their entanglements under dense conditions to explain experimental observations reported over a volume fraction ranging from 0.80 to 0.89 is discussed. Graphical abstract

Dynamics and yielding in stimuli-responsive dense polymer microgel colloidal glasses

期刊论文
发表日期：

2026年2月25日

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Sivaram Vintha,
Saisavadas Meethale Veedu,
Butchi Venkata Rao Tata

摘要

Poly (N-isopropylacrylamide) (poly-NIPAM) microgel particles are soft spheres with a core-shell structure having a dense core and a thin shell consisting of less cross-linked polymers and dangling polymer chains (here after referred to as hairs). These poly-NIPAM particles with core-shell structure and dangling polymer chains are referred to as CPNIPAM soft-spheres. Poly-NIPAM microgel soft-spheres with homogeneous core (HPNIPAM) are also synthesised by altering synthesis conditions to unravel the role of hairs on the phase behaviour, particle dynamics and the yielding behaviour of microgel suspensions under dense conditions (i.e., volume fractions ϕ ≥ 0.68). This article presents results obtained from dynamic light scattering (DLS), and rheology measurement on dense disordered (glassy) structures of poly-NIPAM microgel suspensions with volume fractions, ϕ < 0.68 and under over packed (ϕ > 0.68) conditions. We discuss two important results obtained on dense CPNIPAM microgel colloidal glasses, namely, (i) two-step yielding and (ii) sub-diffusive behaviour at short-time scales. The entanglement of hairs between neighbouring soft spheres under dense (also referred to as over packed) conditions is shown to be responsible for the above-mentioned results. To provide unambiguous evidence in favour of the existence of entanglements, we prepared HPNIPAM particles that have only homogeneous polymer core and no hairs. Under over packed conditions HPNIPAM suspensions showed single step yielding and absence of sub-diffusive behaviour at short times, respectively. These observations provide evidence in support of entanglements between hairs which are responsible for the unusual behaviour of dense microgel colloidal glasses of CPNIPAM particles. Molecular dynamics and oscillatory shear simulations have been carried out using multi-Hertzian (MH) pair-potential proposed by Bergman et al. (Nat. Commun. 2018, 9, 5039) for core-shell structured thermo-responsive poly-NIPAM microgel suspensions. We show that though MH pair-potential explains the phase behaviour of microgel suspensions qualitatively, it fails to explain the above-mentioned experimental observations. The need for improving MH pair-potential that accounts for the role of hairs and their entanglements under dense conditions to explain experimental observations reported over a volume fraction ranging from 0.80 to 0.89 is discussed.

Graphical abstract