Abstract
The spatial elastic modulus distribution of microgel networks in presence and absence of bifunctional crosslinkers is studied by AFM. Thermoresponsive poly(N-isopopylacrylamide) (PNIPAM) and poly(2-(2-methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol)methacrylate) (P(MEO2MA-co-OEGMA)) microgels are synthesized via precipitation polymerization above their lower critical solution temperature (LCST). High-resolution elastic modulus profiles are acquired using AFM force-indentation mapping of surface-deposited microgels at 25 °C. For both microgel systems, the use of a bifunctional crosslinker leads to a strong elastic modulus gradient with stiff microgel cores and soft networks toward the edge. In absence of a dedicated crosslinker (self-crosslinking), PNIPAM microgels show a homogeneous elastic modulus distribution, whereas self-crosslinked P(MEO2MA-co-OEGMA) microgels still show decreasing elastic moduli from the centre to the edge of the microgels. However, POEGMA microgels without comonomer showed no elastic modulus gradient suggesting that different incorporation rates of MEO2MA and OEGMA result in a radial variation of the polymer segment density. In addition, when varying the molecular weight of OEGMA the overall elastic modulus was affected, possibly due to molecular weight-dependent phase behavior and different reactivity. This shows that quite different microgel architectures can be obtained by the simple "one-pot" precipitation reaction of microgels which may open to new avenues toward advanced applications.
Highlights
Previous work showed that Atomic force microscopy (AFM) nanoindentation measurements on adsorbed BIS-crosslinked PNIPAM microgels reveal radial elastic modulus gradients, where higher elastic moduli were found in the centre of the microgels.[9,22,40]
Using neutron scattering, Wellert and co-workers found high density inhomogeneities in EGDMA-crosslinked P(MEO2MA-co-OEGMA) microgels which they interpreted as MEO2MA rich-domains forming during the reaction.[21]. These results suggest that the observed radial elastic modulus gradients for P(MEO2MA-co-OEGMA500)EGDMA microgels are due to the increased reactivity of the bivalent crosslinker and due to different monomer diffusion rates and phase behaviour of the oligomeric methacrylates forming during the reaction
We aimed to investigate the spatial elastic modulus distribution of soft colloidal thermoresponsive microgels adsorbed at a solid surface as a function of the monomer composition
Summary
Micro and nanoparticles composed of stimuli responsive polymers paved the way toward several promising applications, such as triggered drug delivery systems,[1,2,3,4,5] materials with advanced optical properties,[6,7] or bioactive coatings that are capable of responding to environmental parameters.[8,9,10,11,12,13] A very prominent type of responsive microparticles are thermosensitive microgels composed of polymers with a lower critical solution temperature (LCST).[14]. In comparison to macroscopic polymer gels, microgels show a range of interesting properties, such as rapid and strong volume changes upon temperature variation, narrow size distribution, and straightforward processing toward coatings.[15,16,17,18] Their synthesis via free radical polymerization is comparatively simple. Since the crosslinkers are bivalent they tend to be incorporated at higher rates into the growing microgels during precipitation polymerization, leading to higher crosslinking in the microgel centre.[20,21] The resulting microgels show a crosslinking density gradient, i.e. a stiff, highly crosslinked core with a soft and very fuzzy outer perimeter due to low crosslinking.[22,23,24] For some applications, e.g. in drug release or tissue engineering, different network structures and higher overall deformability are required To address these needs, hollow microgels and ultra-soft microgels have been developed.[25,26] It has been shown that such microgel structures can be obtained by omitting bifunctional crosslinkers but instead using chain transfer
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