Structure of multi-temperature sensitive core-shell microgels
The internal and all over structure of multi-temperature sensitive core-shell microgels was studied in this work. The particles consist of chemically cross linked networks of two temperature-sensitive polymers, poly-N-isopropylacrylamide (PNIPAM) and poly-N-isopropropylmethacrylamide (PNIPMAM), which exhibit lower critical solution temperatures (LCST) at 34 and 44 °C in aqueous solution. The particles were prepared in a two-step seed-and-feed polymerization to obtain a core-shell morphology, which allows a spatial separation of the two polymers with sensitivity to different quantities of the environmental stimulus temperature. The influence of shell thickness, i.e., the shell/core mass composition, and the cross linker content of the shell on the overall structure of PNIPAM core-PNIMAM shell microgels were investigated by means of dynamic light scattering (DLS). It was found that the particle size decreased in two distinct transitions upon heating at temperatures according to the core and shell component, respectively. The core transition at 34 °C became less pronounced for higher cross linked and thicker shells. Small-angle neutron scattering (SANS) was used in order to explore the internal structure. A new core-shell form factor model was developed and employed to fit the obtained data. Real space information on the particle structure was obtained from an analytical expression of the form factor calculating radial density profiles. The structure of the core-shell microgels was evaluated at temperatures above, between and below the LCSTs of the two polymers. At high temperatures the radial density distribution is well described by a two-box profile with narrow interface at the particle surface and at the core-shell interface where both networks interpenetrate. Decreasing the temperature below the shell LCST revealed a highly swollen shell, a broadened core-shell interface and increased dimensions of the core. The core expansion was explained by lateral stretching forces exerted to the core from the swollen shell. Thicker shells developed stronger forces and thus led to more expanded cores. At a temperature below the core LCST both core and shell were highly swollen, however, the swelling of the core was restricted. The cross linked shell polymer close to the core-shell interface was stretched to a maximum extend and prevented further swelling of the core. An inverse PNIPMAM core-PNIPAM shell microgel was investigated for comparison. Already DLS indicated a mutual influence of core and swelling as the particle sizes observed for the core-shell particle were smaller than for the parent core at temperatures between the LCSTs. The form factor model demanded for modifications to allow an asymmetric shape of the interface between core and shell. The analysis of data taken at 39°C validated the core compression by the collapsed shell and further revealed an inhomogeneous swelling of the core due to loosely cross linked chains in the periphery which did not interpenetrate with the shell network. The calorimetric properties of the core-shell microgel series with varied shell/core mass ratios were examined by means of differential scanning calorimetry (DSC) and compared the structural information obtained from SANS. Two thermal transitions were found a temperatures correlated with the LCSTs of the components. A shift of the core transition towards higher temperatures was observed with increased shell masses and could be explained by a "chemo-mechanical" model. The forces which are exerted from the swollen shell to the core partially compensate the thermodynamic shrinking force of the core and thus the transition temperature is increased. In case of a very high shell mass a third thermal transition was observed between the LCST. The very thick shell led to a significant expansion of the core and the third transition corresponds to reorganized hydrogen bonds which are formed by the stretching of polymer chains close to the core-shell interface. The transition heats of the PNIPMAM shells were much smaller than compared to a pure PNIPMAM microgel. The comparison with SANS results revealed a restricted swelling in the confined geometry of a shell and could explain the lower heats.