We report on synthesis and characterization of doubly temperature sensitive core−shell microgels. These core−shell microgels are composed of a core of chemically cross-linked poly(N-isopropylacrylamide) (PNIPAM) and a shell of cross-linked poly(N-isopropylmethacrylamide) (PNIPMAM). PNIPAM exhibits in water a lower critical solution temperature (LCST) of ca. 34 °C, and the LCST of PNIPMAM is ca. 45 °C. Solution properties were investigated by means of dynamic light scattering (DLS), optical transmission, differential scanning calorimetry (DSC), and small-angle neutron scattering (SANS). Core−shell microgels of this composition display a temperature-dependent two-step shrinking behavior. The influences of the content of the cross-linking agent N,N‘-methylenbis(acrylamide) (BIS) and of the thickness of the PNIPMAM shell on the thermosensitive response of the PNIPAM/PNIPMAM core−shell microgels were investigated. Core−shell microgels with a thick shell do not show a size transition at the PNIPAM LCST anymore. The volume transition is adjustable by varying the cross-link density of the shell. The swelling behavior of the core−shell microgels is compared to that of pure PNIPAM and PNIPMAM. Additionally, an inverse system consisting of a PNIPMAM core and a PNIPAM shell was prepared and investigated by DLS. Here the collapsed shell at intermediate temperatures strongly restricts the core swelling so that the overall size of the core−shell microgel is smaller as compared to the pure core.

A doubly temperature-sensitive core-shell microgel composed of two temperature-sensitive polymers with different lower critical solution temperatures (LCSTs) in the core and shell has been studied by small-angle neutron scattering (SANS). The application of a novel universal form factor model in the analysis of the SANS data reveals that the radial density profile at temperatures above the LCSTs of both polymers can be well described by a two-box profile with narrow interfaces. At temperatures between the LCSTs, the radial density profile reveals that the core in the core-shell microgel has larger dimensions than the naked core. Thus the swollen shell pulls the core apart. At temperatures below both LCSTs, however, the shell restricts the core swelling, and the core is found to be smaller than in its native state. This clearly demonstrates the mutual influence of core and shell swelling.

Cores and effect: Using different temperature‐sensitive polymers a core–shell microgel is prepared, which at intermediate temperatures has a shell with a higher segment density Φ than the core (see radial density profiles: core (red), shell (blue)). More detailed information is obtained from small‐angle neutron scattering. A new form‐factor model describes the experimental scattering curves at different temperatures.

Swelling properties of doubly temperature sensitive core-shell microgels consisting of two thermosensitive polymers with lower critical solution temperatures (LCTS) at, respectively, 34 degrees C in the core and 44 degrees C in the shell have been investigated by small-angle neutron scattering (SANS). A core-shell form factor has been employed to evaluate the structure, and the real space particle structure is expressed by radial density profiles. By this means, the influences of both shell/core mass composition and shell cross-linker content on the internal structure have been revealed at temperatures above, between, and below the LCSTs. Higher shell/core mass ratios lead to an increased expansion of the core at temperatures between the LCSTs, whereas a variation of cross-linker in the shell mainly effects the dimensions of the shell. The influence on the core structure was interpreted as resulting from an elastic force developed from the swollen shell. At temperatures below the core LCST, the core cannot swell to its native size (i.e., in the absence of a shell), because the maximum expanded shell network prohibits further swelling. Thus, depending on temperature, the shell either expands or compresses the core.

The thermal transition of a thermoresponsive microgel of poly‐N‐isopropylacrylamide (PNIPAM; transition temperature=34 °C) is shifted to higher temperatures when it is embedded in a shell of temperature‐sensitive poly‐N‐isopropylmethacrylamide (PNIPMAM; transition temperature=44 °C). The magnitude of the shift depends on the shell/core mass ratio. A thick shell induces a third transition arising from strong mechanical forces exerted on the core.