Stabile, Thermoresponsive Colloidal Clusters: An Unusual Morphology of Polymer Dispersions

Abstract

A new class of colloidal polymeric particles consisting of polystyrene spheres grown in a poly(N-isopropyl acrylamide) precursor scaffold is synthesized via redox-initiated heterophase polymerization. The morphology and thermoresponsiveness of these assemblies is proven by electron microscopy investigations and temperature-dependent measurements of the change of both the speed of sound travelling through the dispersion and the hydrodynamic particle size. Electron microscopy (EM) micrographs (transmission and scanning EM as well cryo-scanning EM) prove the existence of colloidal clusters when the freeze-dried copolymer is redispersed in pure water. The clusters have a size of several micrometers, contain about 800 polystyrene particles with diameter below 100 nm, and show a highly reproducible thermoresponsive behavior with a lower critical solution temperature corresponding to that of pure poly(N-isopropyl acrylamide).

Borderline Particles: Approaching the Limit between Colloidal Stability and Instability during Heterophase Polymerization

“…Particularly, the electron microscopy images of Figures A (reductant is β‐CD) and 4D (reductant is glycerol) illustrate that both the overall size of the clusters and the phase‐separated PS particles in them differ in dependence on the reductant. A quantitative estimation shows that the specific MSP cluster obtained with β‐CD has a diameter of about 3.3 μm and contains about 800 PS particles with diameters between 20 and 200 nm . On the contrary, the cluster obtained with glycerol (Figure D) has a size of about 9.2 μm and is composed of about 100 PS particles with an average size of about 600 nm (see Figure SI‐4, Supporting Information).…”

“…For the production of MSP with β‐CD as reductant, the following detailed recipe was employed at polymerization temperature of 60 °C which might be considered as kind of standard, also see ref …”

“…For the production of MSP with β-CD as reductant, the following detailed recipe was employed at polymerization temperature of 60 °C which might be considered as kind of standard, also see ref. [ 14 ] : 92 g of water (5.11 mol), 0.08 g of β-CD (7.05 × 10 −5 mol), 2.5 g of NIPAM (22.1 × 10 −3 mol), 0.45 g of CAN (8.208 × 10 −4 mol), 5 mL HNO 3 1 M (5 × 10 −3 mol), and 1.5 g styrene (1.44 × 10 −2 mol).…”

“…The formation of stable block copolymer particles with MSP morphology has been straightforwardly explained by the peculiarities of the β-CD molecular structure. [ 14 ] On the one hand this molecule contains seven methylol groups, which can theoretically lead to the formation of maximum seven radicals at the C-6 position of the β-CD molecule serving as kind of core or branching point. On the other hand, it was assumed that the β-CD ring might be threaded onto the PNIPAM chain or the side groups might be dipped into the cavity of the β-CD.…”

“…Recently, we found a surprising morphology of colloidal polymer particles for which we coined the name multiple suspension particles (MSP) in analogy to the well-known multiple emulsions. [ 14 ] Each MSP has a size of up to several micrometers and contains about 800 polystyrene (PS) particles with diameter below 100 nm which are covalently attached to and evenly dispersed in a PNIPAM matrix. This peculiar particle morphology was obtained if instead of a hydrophilic polymer, β-cyclodextrin (β-CD) was used as reductant to start the polymerization.…”