Microgel Heterogeneous Morphology Reflected in Temperature-Induced Volume Transition and¹H High-Resolution Transverse Relaxation NMR. The Case of Poly(N-vinylcaprolactam) Microgel

Abstract

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...

Heterogeneous Morphology of Random Copolymer Microgels as Reflected in Temperature‐Induced Volume Transition and1H High‐Resolution Transverse Relaxation NMR

“…The Flory-Rehner state equation for homopolymer microgel in the homogeneous approximation is, as reported before, [ 28 ] correlation spectroscopy (PCS). A Malvern Zetasizer Nano SZ system equipped with a 4 mW helium-neon laser ( λ = 633 nm) was employed, with the back scattering angle set at 173 ° .…”

“…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. [ 28 ] In this work we are extending the Flory swelling/ deswelling volume transition theory applied to copolymer microgels in order to account for the bimodal internal structural heterogeneities in combination with 1 H transverse relaxation NMR measurements. This approach allows characterizing quantitatively the core-corona morphology of PVCL-PNIPAAm and PVCL-PNIPMAAm random copolymer microgels.…”

Stimuli-Sensitive Microgels from Native Elastin: An Easy Approach for a Drug Release System

“…These sizes are in good agreement with the hydrodynamic radii determined by light scattering (see Table 1). In many cases, the electron micrographs demonstrate the presence of protrusions, indicating a gradient in density from the core towards the outside, as established for microgels prepared by precipitation polymerization [4]. In order to verify that the microgel samples are in general biocompatible we used a cell proliferation assay.…”

“…Typically, they have a gradient in the distribution of their crosslinks, such that the corona contains more dangling ends and the crosslinks are more densely distributed in the core. This gradient not only results in a gradient of the swelling but also in a gradient of the solubility/solubilization, mostly due to endgroup effects [4]. Thus, if the solvent quality is decreased, the hydrogel particles tend to collapse first in the core and maintain a solubilizing corona.…”

Aggregation behaviour of biohybrid microgels from elastin-like recombinamers

“…Proton transverse magnetization decays of microgels are sensitive to the core/corona morphology [46][47][48] or to the presence of hydrophobic and crosslinking domains like, as seen in the case of native a-elastin microgels. 10 In the last case the amino acid side chains in the hydrophobic domains have a larger amplitude and frequency of the segmental dynamics and hence a long transverse magnetization decay.…”