According to the relaxation times from NMR-studies, it is concluded that there is more mobile structures of PNIPAM on PS core particles compared to the heterogenous microgel samples (a looser and/or more heterogeneous network structure).
Phase transitions and structural characteristics of microgels were further studied with 1H-NMR spectroscopy including the measurements of the signal intensities as well as the spin-lattice (T1) and spin-spin (T2) relaxation times for the protons of PNIPAM with changing temperature.
Howverer, the behavior of the system just at ξ = 1.5, suggests that the configuration of the particle plays a key roleas microgels reconfigure their intra-structure at each temperature, evidenced by DLS and SLS measurementsin dilute PNiPAM-PEG suspensions.
This contribution introduces the synthesis of poly(N-isopropylacrylamide) (pNIPAm)-based core/shell mi-crogels and gives an overview of investigations involving the physical properties of such systems. Core/shell mi-crogels are created via two-stage ”seed-and-feed” precipitation polymerization. Core particles serve as preexisting nuclei onto which a hydrogel-forming polymer of a different composition (the shell) is added. The physical properties of the resultant particles are then analyzed with photon correlation spectroscopy (PCS) and fluorescence resonance energy transfer (FRET). Investigations herein show how the overall particle behavior differs between core and core/shell systems. Addition of a shell impacts the swelling behavior of the core component and varies with shell thickness. Particle behavior can be explained by considering the preparation method, where a radial crosslinker density gradient is created, resulting in a distribution of polymer network densities in the core and shell components.
The results of core/shell synthesis can easily be realized by comparing the size differences between the original core and resultant core/shell particles via PCS. Fig. 2 shows the results of polymerizing a pNIPAm shell onto a pNIPAm core. Although this particle is not interesting from a functional standpoint, the data illustrate that core/shell particles are larger in size and exhibit the same volume phase transition (VPT) behavior as the original core. PCS analysis also reveals that the size distribution between the core and core/shell particles is the same, between 15% and 20%. Furthermore, there is no indication of a new particle population created from the second polymer synthesis step. This information supports the contention that the oligoradical capture rate of the core is greater than the propensity of growing polymer chains to form new particles during shell polymerization.
The most interesting core/shell microgels are those in which the hydrogel polymer in the core and shell components differ in composition. Visualization of the core/shell structure is possible when a pNIPAm core is used for subsequent polymerization of a pNIPAm-co-acrylic acid (pNIPAm-co-AAc) shell. The results of this core/shell polymer combination can be seen in the transmission electron micrograph (TEM) image shown in Fig. 3; selective staining of the AAc groups with uranyl acetate allows for easy visualization of the core/shell structure. Only one population of particles is shown in the figure, again illustrating that no new particles are formed in solution during the second step of polymerization. This image also suggests that the interface between the two materials is fairly sharp and not highly interpenetrated. It should be noted that the particle size measured by this technique is somewhat unreliable as the particles may have a tendency to flatten and spread on the TEM grid during the sample preparation. This can also lead to a larger apparent polydispersity; individual particles are not expected to interact with the substrate in a homogeneous fashion. It is for these reasons that particle sizing is performed via the less perturbing method of PCS.
However, we do not observe aggregation, indicating there must also bea repulsive contribution to the particle-particle interactions, which in our case results from the presenceof charge at the surface of the particles from the ionic initiator used in the microgel synthesis [11,17].
Fig. 3 Transmission electron micrograph of pNIPAm (core)/ pNIPAm-co-AAc (shell) hydrogel particles. The AAc component of the particles was preferentially stained with uranyl acetate to visualize the core/shell structure of the particles.
One aspect of microgel synthesis that is an essential consideration of core/shell particle behavior is the creation of a radially distributed crosslinker density gradient during particle formation. It has been shown that during particle growth, the crosslinker is statistically incorporated faster than other constituent monomers, leading to the creation of a radial distribution of crosslinks.[1,2,37,43,44] An illustrative model of the resultant particle morphology is shown in Fig. 5. Because the crosslinker is incorporated fastest, polymer chains toward the particle interior possess a greater number of crosslinking points per chain than those toward the particle exterior. However, this same population of loosely crosslinked chains will be hindered from reaching their fully extended state in the presence of added polymer (e.g., shell). The average number of chains that will be perturbed will differ according to the initial cross-linker concentration, with lower mol% crosslinked particles having a higher average number of loosely cross-linked chains near the periphery than highly crosslinked particles that do not have a gradient morphology.[45,46]
The thermodynamic and morphological impact of a hydrogel shell on a hydrogel core particle can be studied with a chosen experimental design relying on a pH- and temperature-responsive core particle, upon which a temperature-responsive, pH-innocent material is added. In this fashion, the shell-induced modulation of the pH response and thermoresponsivity of the core particle can be studied. The well-investigated pH/thermoresponsive hydrogel, pNIPAm-co-AAc, was chosen as the core material and a pH-innocent pNIPAm shell was added. The core/shell particles used for illustration are simply the inverse of those discussed above. In this previous case, the thermodynamics of particle deswelling was investigated, whereas the mechanical properties of this core/shell system are now being presented.
As shown in Fig. 6, a size decrease is observed at all temperatures and at pH 6.5 for these core particles following shell addition, suggesting that the pH-induced swelling of the core is now restricted by the presence of the shell. Indeed, below 31 °C, the volume of the core/shell particle is only 40% of the parent core particle volume at pH 6.5. In addition to mediating the solvation of the core, the added pNIPAm shell also controls the overall particle deswelling behavior above the polymer LCST by forcing the charged microgel core to undergo a volume phase transition. The added shell compresses the charged core at the LCST of neutral pNIPAm, despite the resistance of the parent core particle to deswelling when the AAc moieties are deprotonated.
Fig. 5 The synthesis of pNIPAm microgels via precipitation polymerization creates a radial crosslinker density gradient. Polymer chains near the particle periphery have fewer cross-linker points between chains than those located toward the interior. The various subchain lengths between crosslinking points affect particle deswelling as described in the text.