Inner structure and dynamics of microgels with low and medium crosslinker content prepared via surfactant-free precipitation polymerization and continuous monomer feeding approach

The preparation of poly(N-isopropylacrylamide) microgels via classical precipitation polymerization (batch method) and a continuous monomer feeding approach (feeding method) leads to different internal crosslinker distributions, i.e., from core–shell-like to a more homogeneous one. The internal structure and dynamics of these microgels with low and medium crosslinker concentrations are studied with dynamic light scattering and small-angle neutron scattering in a wide q-range below and above the volume phase transition temperature. The influence of the preparation method, and crosslinker and initiator concentration on the internal structure of the microgels is investigated. In contrast to the classical conception where polymer microgels possess a core–shell structure with the averaged internal polymer density distribution within the core part, a detailed view of the internal inhomogeneities of the PNIPAM microgels and the presence of internal domains even above the volume phase transition temperature, when polymer microgels are in the deswollen state, are presented. The correlation between initiator concentration and the size of internal domains that appear inside the microgel with temperature increase is demonstrated. Moreover, the influence of internal inhomogeneities on the dynamics of the batch- and feeding-microgels studied with neutron spin-echo spectroscopy is reported.TU Berlin, Open-Access-Mittel - 201

Internal structure and dynamics of PNIPAM based microgels in bulk and adsorbed state at different internal crosslinker distributions

Stimuli-responsive microgels are a unique class of polymer structures which can undergo a fast response to an external trigger such as light, temperature or pH. It provides a wide potential application spectrum in optical devices, smart surface coatings, emulsion stabilisation etc. The most prominent example of these microgels are poly(N-isopropylacrylamide)(PNIPAM) microgels. During the last decades PNIPAM-based microgels were widely studied and serve nowadays as model systems for the investigation of the basic properties of microgels and the principles of supported transport of active substances, e.g. drug delivery. The responsive behaviour of the microgels is governed by the polymer network structure, i.e. the crosslinking of the polymer chains has a considerable influence on the physico-chemical and mechanical properties of the microgels. Especially the amount and the crosslinker distribution within a single microgel have a high relevance for the application as well as for the understanding of the microgel nature. This thesis focuses on a detailed analysis of microgels in solution and at interfaces with scattering experiments. Neutron and X-ray scattering provide a unique insight into the structure and dynamics of microgels, especially at the interface with grazing incidence small angle neutron scattering and neutron spin-echo spectroscopy (GISANS and GINSES). New insights into the inner structure and dynamics have been gained with improved experimental conditions and data analysis. This thesis is divided into the two main parts. In order to characterise the internal structure and dynamics of the PNIPAM microgels with respect to the crosslinker (N,N'-methylenebis(acrylamide), BIS) distribution, sophisticated analysis of the internal microgel (domain-like) structure and studies of its influence on the polymer dynamics in nanometer and nanosecond scales are presented in the first part of the thesis. In the context of smart polymer coatings, the influence of the confinement to the solid surface on the internal architecture and the thermoresponsiveness of the adsorbed PNIPAM microgels is investigated. It is shown that in contrast to the atomic force microscopy, the inhomogeneities of the polymer network of the adsorbed soft microgels can be achieved with surface sensitive neutron reflectometry and GISANS. The second part of the thesis aims at the peculiarities of the scattering experiments of thin polymer layers under grazing incidence conditions. Simulation of the scattering signal within the Distorted Wave Born Approximation is presented in order to improve the analysis of the GINSES data and to simplify the initial planing and performance of the grazing incidence experiments. The main impacts of this thesis are (i) an extended description of the complex internal structure, dynamics and the thermoresponsiveness of the PNIPAM microgel before and after adsorption onto a solid surface and (ii) demonstration the advantages of the investigation of the polymer systems at grazing incidence conditions in combination with numerical simulations

Mobility of bound water in PNIPAM microgels

Polymer–solvent interactions play a crucial role in the stimuli-responsive behaviour of polymer networks. They influence the swelling/deswelling behaviour as well as the dynamics of the polymer chains. Scattering experiments provide insight into the polymer–water interaction of poly(N-isopropylacrylamide) (PNIPAM) microgels cross-linked with N,N′-methylenebisacrylamide (BIS) in dried and humidified state. The water mobility is studied by means of neutron spin-echo spectroscopy and neutron backscattering spectroscopy. The residual water amount has been determined with Karl Fischer titration. For both degrees of humidification, the relaxation time of the water molecules is much larger than that of free water due to the strong interactions with the polymer network and is only weakly depending on temperature and length scale of observation. The possible influence of the water on methyl group rotations is discusse

Grazing Incidence Small-Angle Neutron Scattering: Background Determination and Optimization for Soft Matter Samples

Grazing incidence small-angle neutron scattering (GISANS) provides access to interfacial properties, e.g., in soft matter on polymers adsorbed at a solid substrate. Simulations in the frame of the distorted wave Born approximation using the BornAgain software allow to understand and quantify the scattering pattern above and below the sample horizon, in reflection and transmission, respectively. The small-angle scattering from the interfacial layer, visible around the transmitted beam, which might contribute also on the side of the reflected beam, can be understood in this way and be included into the analysis. Background reduction by optimized sample cell design is supported by simulations, paving the way for an optimized GISANS cell

Structure and dynamics of homogeneously and heterogeneously crosslinked PNIPAM microgels

Thermoresponsive poly(N-isopropylacrylamide) microgel particles have been largely studied in the past due to their interesting fundamental properties as well as their potential for application in drug delivery, sensor technology or biotechnology. In this contribution, microgel particles with homogeneous and heterogeneous crosslink distribution and different crosslinker concentrations will be studied with small angle neutron scattering (SANS) and neutron spin echo spectroscopy (NSE).The segmental dynamics of polymer chains in solution, as described by the Zimm model, is modified in microgels due to the presence of crosslinks and the crowded environment inside the particle, both inducing heterogeneities, for example “frozen” inhomogeneities or density fluctuations. Such structural heterogeneities have an impact on the segmental chain dynamics, which makes NSE, the highest resolution neutron spectroscopy technique, the experiment of choice for accessing variations and changes induced by changes in crosslink distribution and crosslink density.An outlook on interface effects on the internal structure and inhomogeneities of microgel particles will be given.*Support of the DFG (grant number WE5066/3-1 (S. Wellert) and HO 5488/2-1 (O. Holderer) is acknowledged

Understanding near-surface polymer dynamics by a combination of grazing-incidence neutron scattering and virtual experiments

Neutron spin-echo spectroscopy is a unique experimental method for the investigation of polymer dynamics. The combination of neutron spin-echo spectroscopy with grazing-incidence geometry (GINSES) opens the possibility to probe the dynamics of soft-matter materials in the vicinity of the solid substrate in the time range up to 100 ns. However, the usage of the GINSES technique has some peculiarities and, due to the novelty of the method and complexity of the scattering geometry, difficulties in further data analysis occur. The current work discusses how virtual experiments within the distorted-wave Born approximation using the BornAgain software can improve GINSES data treatment and aid the understanding of polymer dynamics in the vicinity of the solid surface. With two examples, poly(N-isopropyl acrylamide) brushes and poly(ethylene glycol) microgels on Si surfaces, the simulation as well as the application of the simulation to the GINSES data analysis are presented. The approach allowed a deeper insight to be gained of the background effect and scattering contribution of different layers

Looking inside Poly( N -isopropylacrylamide) Microgels: Nanomechanics and Dynamics at Solid–Liquid Interfaces

The continuous improvement of synthesis leads to a great variety in the internal architecture and functionality of colloidal hydrogels. A majority of envisioned applications use microgels as colloidal building blocks for layer formation at solid substrates. In this context, a fundamental understanding of the influence of these substrates on the internal structure and physical properties is essential. Especially, the nanomechanical properties of adsorbed poly(N-isopropylacrylamide) microgel particles are important for their application in cell cultivation, functional surface coatings, and others. Furthermore, these properties are closely related to the swelling behavior and the internal structure and dynamics of these microgels. However, the number of methods that are able to probe the viscoelasticity of adsorbed microgels over the entire vertical particle profile is limited. Grazing incidence neutron scattering techniques are suited to probe soft-matter samples with limited sample volumes at planar solid substrates and allow a resolution in the z-direction. We used neutron spin echo spectroscopy under grazing incidence to access fast thermal fluctuations (10–9 s) over the entire vertical particle profile. Atomic force microscopy nanoindentation was used to characterize the nanomechanical properties of adsorbed microgel particles prepared by batch and continuous monomer feeding methods. The resulting force maps revealed that batch microgels were “hard” and heterogeneous in their Young’s moduli, while the microgels from the continuous method were “soft” and homogeneous. Finally, neutron spin echo spectroscopy under grazing incidence revealed major differences in the vertical dynamic profile of both types of microgels