Andreea Balaceanu

Monitoring the internal structure of poly(N-vinylcaprolactam) microgels with variable cross-link concentration.

The combination of a set of complementary techniques allows us to construct an unprecedented and comprehensive picture of the internal structure, temperature dependent swelling behavior, and the dependence of these properties on the cross-linker concentration of microgel particles based on N-vinylcaprolactam (VCL). The microgels were synthesized by precipitation polymerization using different amounts of cross-linking agent. Characterization was performed by small-angle neutron scattering (SANS) using two complementary neutron instruments to cover a uniquely broad Q-range with one probe. Additionally we used dynamic light scattering (DLS), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). Previously obtained nuclear magnetic resonance spectroscopy (NMR) results on the same PVCL particles are utilized to round the picture off. Our study shows that both the particle radius and the cross-link density and therefore also the stiffness of the microgels rises with increasing cross-linker content. Hence, more cross-linker reduces the swelling capability distinctly. These findings are supported by SANS and AFM measurements. Independent DLS experiments also found the increase in particle size but suggest an unchanged cross-link density. The reason for the apparent contradiction is the indirect extraction of the parameters via a model in the evaluation of DLS measurements. The more direct approach in AFM by evaluating the cross section profiles of observed microgel particles gives evidence of significantly softer and more deformable particles at lower cross-linker concentrations and therefore verifies the change in cross-link density. DSC data indicate a minor but unexpected shift of the volume phase transition temperature (VPTT) to higher temperatures and exposes a more heterogeneous internal structure of the microgels with increasing cross-link density. Moreover, a change in the total energy transfer during the VPT gives evidence that the strength of hydrogen bonds is significantly affected by the cross-link density. A strong and reproducible deviation of the material density of the cross-linked microgel polymer chains toward a higher value compared to the respective linear chains has yet to be explained.

Stimuli-responsive poly(N-vinylcaprolactam-co-2-methoxyethyl acrylate) core–shell microgels: facile synthesis, modulation of surface properties and controlled internalisation into cells†

Herein we report the synthesis of biocompatible stimuli-responsive core-shell microgels consisting of a poly(N-vinylcaprolactam) (PVCL) core and a poly(2-methoxyethyl acrylate) (PMEA) corona via one-step surfactant-free precipitation copolymerization. The copolymerization process was investigated by reaction calorimetry, microgel growth was monitored by in situ dynamic light scattering and the chemical structure of core-shell microgels was characterized by Raman spectroscopy. It was possible to incorporate up to 32 mol% MEA into the PVCL/MEA microgels without loss of colloidal stability and broadening of the size distribution. The core-shell morphology of microgels was confirmed by transverse magnetization relaxation 1H-NMR, dynamic light scattering (DLS), atomic force microscopy (AFM) and viscosimetry. By means of the NMR data, calorimetry and viscosity measurements it could be shown that MEA is mainly located in the microgel shell. This leads to hindered temperature-induced swelling and collapsing of the PVCL-core, as demonstrated by DLS measurements, due to the fact that the PMEA-shell exhibits a very low LCST around 5 °C. These results could also be confirmed by AFM: an increasing MEA-content leads to the formation of dense and compact core-shell microgels and results in a loss of their softness and deformability. Due to the presence of the PMEA-shell these microgels can be endocytosed much faster by HeLa cells maintaining their viability and can be suitable candidates for the design of drug carriers or imaging/diagnostic systems.

Thermoresponsive Core-Shell Microgels. Synthesis and Characterisation

Abstract Thermoresponsive microgels based on poly(N-vinylcaprolactam), poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide) with core-shell morphology were synthesized by two-step seed precipitation polymerisation. The microgel shell thickness was controlled by increasing the monomer amount added to the core seed during the second polymerisation step. 1H nuclear magnetic resonance and Raman spectroscopy were used to determine the real molar ratio between the components of core and shell. The volume phase temperature transition of microgels was investigated with dynamic light scattering and differential scanning calorimetry. The influence that the intrinsic critical solution temperature of the core and shell polymers has on the volume phase transition temperature of the microgels is discussed.