b S Supporting Information ' INTRODUCTIONMaterials of micro/nanosize dimensions often show unique characteristics which are not expected from the bulk. For this reason, studies on such micro/nanomaterials are attracting a lot of attention and are widely extending into diverse fields of science and technology. In this way, when focusing into the polymer area, microgels are currently becoming the focus of considerable scientific studies.Microgel particles are cross-linked polymeric particles with dimensions in the colloidal range. A promising feature deriving from their microsize is their rather fast capability to swell compared to the macroscopic gels. This is one of the reasons why these materials have generated an increasing interest in the past decades, particularly stimulus-responsive microgels. These microgels are able to alter their volume and properties in response to external stimuli, such as pH, temperature, pressure, and ionic strength. This fact proved them to be attractive candidates for many potential applications including drug delivery, biosensing, or separation techniques. 1,2 Special interest is focused on the hydrogels based on polymers, which have a lower critical solution temperature (LCST) near the temperature of the human body. The temperature-responsive nature of these polymers leads to a variety of biological applications. Hydrogels made from these polymers have been considered as drug delivery devices, materials for tissue engineering, and materials for preventing surgical adhesion. Poly(Nisopropylacrilamide) (PNIPAAm) is the most widely studied hydrogel and has a LCST of 34°C (ref 3 and references therein). Our systems are based on poly(N-vinylcaprolactam) (PVCL), which also has the LCST in the physiological range of around 32°C. It is biocompatible and materials based on this polymer can be potentially used in biomedical applications.The attempts to determine the structure of aqueous microgel particles have been reported in the literature. Wu and Pelton 4 reported that during the formation of poly(NIPAM) particles by precipitation polymerization the cross-linking agent (bis-(acrylamide)) was consumed faster compared to NIPAM and therefore preferentially incorporated into microgels. Because of the fact that poly(NIPAM) or poly(VCL) microgel particles are prepared at above the LCST of the linear polymer, it is believed that the core regions of the final particles contain a relatively higher amount of the monomers consumed in the early ABSTRACT: The Flory temperature-induced volume transition theory for homopolymer microgels was generalized for the case of bimodal heterogeneous morphology. The most probable morphological parameters were selected from the microscopic and thermodynamic constraints imposed by 1 H transverse relaxation NMR and Flory equation of state in the approximation of a homogeneous morphology. Proton transverse magnetization relaxation NMR proved directly the existence of a bimodal heterogeneous morphology of the PVCL microgel particle. The volume polymer fractions in the...

We report synthesis of amphoteric microgels by copolymerization of N-vinylcaprolactam (VCL), itaconic acid dimethyl ester (IADME), and vinylimidazole (VIm) in the precipitation−polymerization process. After hydrolysis of ester groups of IADME, component microgels contain acidic and basic groups in their structure. Proton high-resolution transverse magnetization relaxation under magic angle sample spinning (MAS) was used to measure the dynamic heterogeneity corroborated with the chemical structure of a multicomponent amphoteric microgel. NMR results indicate that itaconic acid groups (originated from hydrolyzed IADME component) are localized mostly in the microgel core. The core−shell morphology of poly(N-vinylcaprolactam)-based microgels was suggested with carboxylic acid groups in the core and imidazole groups in the shell. The variation of the IADME and VIm content in microgel structure allows varying microgel charge and swelling degree in basic and acidic pH, respectively. Obtained amphoteric microgels exhibit narrow size distribution and superior colloidal stability.

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.