Polystyrene Microgel Research Articles

New insights into re-entrant melting of microgel particles by polymer-induced aggregation experiments.

While microgels are in general described as soft particles, polystyrene (PS) microgels can be synthesized in a way that cross-link density has only a minor influence on their physical properties. Even though the particles swell in a good solvent, the imparted slight softness still allows a mapping on hard sphere behaviour for a large range of cross-link densities [Schneider et al., Soft Matter, 2017, 13, 445]. Nevertheless, the hard sphere analogy breaks down as soon as polymer chains are added to these systems. Quantitative differences between PS microgels and true hard sphere systems appear and the differences between stronger and weaker cross-linked PS microgels can be observed. To gain deeper insights into the origin of these deviations from true hard sphere behaviour, this work is addressed to a systematic study of the colloid-polymer interactions in PS microgel-polymer mixtures. We investigated the aggregation behaviour (namely aggregation concentration and cluster structure) as a function of colloid size, cross-link density and colloid-polymer size ratio in very dilute colloidal suspensions. Our results show that the interplay of cross-link density and polymer size is a key parameter for the strength of the colloid-polymer interactions. Furthermore, the centre-to-centre distance of the colloidal particles in the formed clusters decreases if the cross-link density is decreased, allowing for a higher packing density. This may also explain the unusually high fluid packing fractions observed in the re-entry region of the phase diagram of PS microgel-free PS polymer mixtures.

The role of initiator on the dispersibility of polystyrene microgels in non-aqueous solvents

Non-aqueous microgel particles are commonly synthesised in water, dried, and then redispersed in non-aqueous solvents. An important factor to consider when synthesising such particles is the initiator, which can determine how well the particles disperse in solvents. Polystyrene microgel particles were made with three different initiators. When a neutral, oil soluble initiator (azobisisobutyronitrile) was used the particles dispersed in toluene as well as cyclohexane and decalin. In contrast, anionic, water-soluble initiators (potassium persulfate or azobis(4-cyanovaleric acid)) created particles that only redispersed in toluene and not the other two solvents. Of the three considered, toluene is the best solvent for polystyrene and also has the highest polarizability, making it most effective at redispersing particles with polar or ionisable functional groups. Zeta potential and conductivity measurements, however, did not detect a direct relationship between particle charging and redispersibility. Oil soluble initiators result in “inside out” polymerisation where the initiator groups are buried inside the growing particle, whereas water-soluble initiators result in “outside in” polymerisation, with the polar initiator groups residing on the particle surface. By tailoring the ratio between water and oil soluble initiators, it may be possible to synthesise microgel particles with uniform or designed charge profiles from the core to the surface.

On tuning microgel character and softness of cross-linked polystyrene particles.

Polystyrene (PS) microgel colloids have often been used successfully to model hard sphere behaviour even though the term "gel" invokes inevitably the notion of a more or less soft, deformable object. Here we systematically study the effect of reducing the cross-link density from 1 : 10 (1 cross-link per 10 monomers) to 1 : 100 on particle interactions and "softness". We report on the synthesis and purification of 1 : 10, 1 : 25, 1 : 50, 1 : 75 and 1 : 100 cross-linked PS particles and their characterization in terms of single particle properties, as well as the behaviour of concentrated dispersions. We are able to tune particle softness in the range between soft PNiPAM-microgels and hard PMMA particles while still allowing the mapping of the microgels onto hard sphere behavior with respect to phase diagram and static structure factors. This is mainly due to a rather homogeneous radial distribution of cross-links in contrast to PNiPAM microgels where the cross-link density decreases radially. We find that up to a cross-link density of 1 : 50 particle form factors are perfectly described by a homogeneous sphere model whereas 1 : 75 and 1 : 100 cross-linked spheres are slightly better described as fuzzy spheres. However the fuzziness is rather small compared to typical PNiPAM microgels so that a hard sphere mapping still holds even for these low cross-link densities. Finally, by varying the reaction conditions - changing from batch to semibatch emulsion polymerization and varying the feed rate or by adjusting the monomer to initiator ratio we can tune the fuzziness or significantly alter the inner structure to a more open, star-like architecture.

Reducing hole transporter use and increasing perovskite solar cell stability with dual-role polystyrene microgel particles.

Perovskite solar cells (PSCs) are a disruptive technology that continues to attract considerable attention due to their remarkable and sustained power conversion efficiency increase. Improving PSC stability and reducing expensive hole transport material (HTM) usage are two aspects that are gaining increased attention. In a new approach, we investigate the ability of insulating polystyrene microgel particles (MGs) to increase PSC stability and replace the majority of the HTM phase. MGs are sub-micrometre crosslinked polymer particles that swell in a good solvent. The MGs were prepared using a scalable emulsion polymerisation method. Mixed HTM/MG dispersions were subsequently spin-coated onto PSCs and formed composite HTM-MG layers. The HTMs employed were poly(triaryl amine) (PTAA), poly(3-hexylthiophene) (P3HT) and Spiro-MeOTAD (Spiro). The MGs formed mechanically robust composite HTMs with PTAA and P3HT. In contrast, Spiro-MG composites contained micro-cracks due the inability of the relatively small Spiro molecules to interdigitate. The efficiencies for the PSCs containing PTAA-MG and P3HT-MG decreased by only ∼20% compared to control PSCs despite PTAA and P3HT being the minority phases. They occupied only ∼35 vol% of the composite HTMs. An unexpected finding from the study was that the MGs dispersed well within the PTAA matrix. This morphology aided strong quenching of the CH3NH3PbI3-xClx fluorescence. In addition, the open circuit voltages for the PSCs prepared using P3HT-MG increased by ∼170 mV compared to control PSCs. To demonstrate their versatility the MGs were also used to encapsulate P3HT-based PSCs. Solar cell stability data for the latter as well as those for PSCs containing composite HTM-MG were both far superior compared to data measured for a control PSC. Since MGs can reduce conjugated polymer use and increase stability they have good potential as dual-role PSC additives.

Photoactive composite films prepared from mixtures of polystyrene microgel dispersions and poly(3-hexylthiophene) solutions.

Whilst polystyrene microgels belong to the oldest family of microgel particles, their behaviours when deposited onto substrates or prepared as composites have received little attention. Because polystyrene microgels are solvent-swellable, and inherently colloidally stable, they are well suited to form composites with conjugated polymers. Here, we investigate the morphology and light absorption properties of spin coated composite films prepared from mixed dispersions of polystyrene microgels and poly(3-hexylthiophene) (P3HT) for the first time. We compare the morphologies of the composite films to spin coated microgel films. The films were studied using optical microscopy, SEM, AFM, wide-angle X-ray diffraction and UV-visible spectroscopy. The films contained flattened microgel particles with an aspect ratio of ∼10. Microgel islands containing hexagonally close packed particles were evident for both the pure microgel and microgel/P3HT composite films. The latter were electrically conducting. The composite film morphology was dependent on the microgel and P3HT concentration used for film preparation and a morphology phase diagram was constructed. The P3HT phase acted as an electrically conducting cement and increased the robustness of the films to solvent washing. The composite films were photoactive due to the P3HT component. The absorbance for the films was tuneable and increased linearly with both microgel and P3HT concentration. The results of the study should apply to other organic swellable microgel/conjugated polymer combinations and may lead to new colloidal composites for future optoelectronic applications.

Crystallization kinetics of colloidal binary mixtures with depletion attraction.

In this work the crystallization kinetics of colloidal binary mixtures with attractive interaction potential (Asakura-Oosawa) has been addressed. Parameters such as fraction of crystals, linear crystal dimension and crystal packing have been quantified in order to understand how the crystal formation is driven in terms of the depth of the attractive potential and the composition of the binary mixture (described by the number ratio). It was found that inside the eutectic triangle, crystallization is mainly governed by nucleation and the crystal packing is close to the close-packing of hard spheres. Moving out from the eutectic triangle towards small component results in the crystallization of small spheres. Enrichment of the eutectic mixture with large component results in the crystallization of both large and small spheres, however, the kinetics are completely different from those of the eutectic composition. Crosslinked polystyrene microgels with nearly hard sphere interactions were used as model systems. Attraction was introduced by addition of linear polystyrene. The time evolution of crystallization has been followed by static light scattering.

Influence of the polymer size on depletion attraction—induced gel and glass transitions of microgel colloids

In previous work of our group on the re-entrant melting of colloidal glasses we observed an unexpectedly huge re-entry region for a binary system of 1:50 crosslinked polystyrene (PS) microgel spheres when a depletion attraction range ξ=Rg,polymer/Rcolloid=0.08 was induced by non-adsorbing (free) PS chains. To investigate whether this fluid region can even be increased by reducing polymer size and therefore the attraction range as predicted by mode coupling theory (MCT), we studied an almost identical system. It consists of a binary 1:50 PS microgel mixture (Rh,L=105.8nm, Rh,S=82.9nm; Γ=Rh,S/Rh,L=0.85) with a reduced attraction range of ξ=0.059 by use of an appropriate free PS polymer (Mw=79,600g/mol, Rg,polymer=9.18nm). Here the glass transition is shifted from ϕg=0.577 to higher values, allowing for fluid states up to volume fractions of ϕ=0.715. This is significantly higher than the maximum fluid state of ϕ=0.69 reached for ξ=0.08. Tentatively correcting for osmotic deswelling these volume fractions reduce to 0.673 and 0.66, respectively. This is in qualitative agreement with MCT, but in quantitative disagreement because the theory predicts a shift of only a few percent. Such fluid states beyond the maximum amorphous packing of hard spheres (ϕrcp∼0.64) do not occur for PMMA colloids and 1:10 crosslinked PS microgel spheres that can both be considered as hard spheres. Thus, we tentatively attribute the observation of fluid states at such high volume fractions to a combination of the slight softness of the 1:50 microgels, leading to osmotic deswelling and particle deformability. In addition, polydispersity and polymer non-ideality effects should be taken into account.

Structural and dynamical evolution of colloid-polymer mixtures on crossing glass and gel transition as seen by optical microrheology and mechanical bulk rheology

We present a study of the evolution of structural properties and dynamical behaviour of colloidal complex systems, as seen by two techniques working on two different length scales. Our system consists of a binary mixture at high volume fraction of internally crosslinked polystyrene microgels dispersed in an organic solvent. In this colloidal binary mixture the total polydispersity of the system has been artificially increased in order to “tentatively” suppress crystallization. We have studied concentrated repulsive and attractive systems. In the first case, the colloidal particles have a ‘nearly’ hard sphere interaction. In latter case, we have introduced an attractive interaction via Asakura-Oosawa potential using free linear polymer added to this system. We characterize the mechanical properties applying dynamic light scattering based optical microrheology at the microscopic level, and mechanical bulk rheology at the macroscopic level. We compare the results obtained by both techniques finding, under certain conditions, discrepancies attributed to existing heterogeneities. Analyzing the observed discrepancies, we propose a qualitative model of structural and dynamical evolution of the studied systems.

Polymer-enforced crystallization of a eutectic binary hard sphere mixture

We prepared a buoyancy matched binary mixture of polydisperse polystyrene microgel spheres of size ratio Γ = 0.785 at a volume fraction of Φ = 0.567 just below the kinetic glass transition. In line with theoretical expectations, a eutectic phase behaviour was observed, but only a minor fraction of the samples crystallized at all. By adding a short non-adsorbing polymer we enforce inter-species fractionation into coexisting pure component crystals, which in turn also shows signs of intra-species fractionation. We show that in formerly inaccessible regions of the phase diagram binary hard sphere physics are made observable using attractive hard spheres.

Photocleavable coumarin crosslinkers based polystyrene microgels: phototriggered swelling and release

Two types of photocleavable crosslinkers (CLA and CLB) based on 7-amino coumarin moieties have been synthesized. The ingenious design for the crosslinkers with different vinyl functionalities was aimed to investigate the influence of the crosslinker’s structure on the photodegradation and photoswelling behavior of the constructed microgels. Analysis of DLS, UV spectra and fluorescence spectra verified the original intention and the generated microgels revealed structure-dependent photo-regulated swelling and degradation behavior. The successive loading and release of the hydrophobic Nile Red (NR) dye of the microgel upon visible light irradiation further showed its ability to be used as an excellent nanocarrier for hydrophobic compounds.

Effect of cross-link density on re-entrant melting of microgel colloids

We investigated concentrated suspensions of newly synthesized highly cross-linked (1:10) polystyrene (PS) microgel particles dispersed in a good, iso-refractive organic solvent that allowed for a detailed study using light scattering techniques. Inducing depletion attraction by addition of non-adsorbing (PS) polymer, we analyzed the re-entrant melting behavior of the particle system. We compared our results with that of previous studies on polystyrene microgel particles with a lower cross-linking ratio (1:50) and on PMMA suspensions. In contrast to the (softer) 1:50 microgels where an unexpectedly huge re-entry effect was found with fluid samples extending up to a volume fraction ϕ≈0.69, the re-entry region here extends only up to ϕ≈0.61 and the attractive glass transition line is found at significantly lower polymer concentrations.Instead we find a closer similarity of the re-entry behavior with that observed for hard sphere-like PMMA particles. Whereas the highest volume fractions up to which fluid states could be observed are comparable, the polymer concentration needed to induce re-entrant melting and the formation of attractive glass is shifted to higher polymer concentrations. While the latter effect may be attributed to the non-ideal behavior of the free polymer, the extension of the fluid region with respect to colloid volume fraction appears to depend sensitively on system details like particle softness or particle size distribution.

Optical experiments on a crystallizing hard-sphere–polymer mixture at coexistence

We report on the crystallization kinetics in an entropically attractive colloidal system using a combination of time resolved scattering methods and microscopy. Hard sphere particles are polystyrene microgels swollen in a good solvent (radius a=380 nm, starting volume fraction 0.534) with the short ranged attractions induced by the presence of short polymer chains (radius of gyration r g=3 nm, starting volume fraction 0.0224). After crystallization, stacking faulted face centered cubic crystals coexist with about 5% of melt remaining in the grain boundaries. From the Bragg scattering signal we infer the amount of crystalline material, the average crystallite size and the number density of crystals as a function of time. This allows to discriminate an early stage of conversion, followed by an extended coarsening stage. The small angle scattering (SALS) appears only long after completed conversion and exhibits Furukawa scaling for all times. Additional microscopic experiments reveal that the grain boundaries have a reduced Bragg scattering power but possess an increased refractive index. Fits of the Furukawa function indicate that the dimensionality of the scatterers decreases from 2.25 at short times to 1.65 at late times and the characteristic length scale is slightly larger than the average crystallite size. Together this suggests the SALS signal is due scattering from a foam like grain boundary network as a whole.

Synthesis of Photo-Cross-Linkable Microgel Colloids for Cluster Formation Studies

In many physical processes involving colloidal particles, transient structures (e.g., in transient particle gels, phase-separating suspensions) are created. Freezing these structures by chemically inter-cross-linking the particles could be an intriguing route to create network structures with special (e.g., mechanical) properties. Alternatively, photochemically fixing transient structures, thereby making them accessible to detailed analysis, could provide new insight in processes such as colloidal aggregation or crystal nucleation, which are of fundamental importance for soft matter physics as well as for applications. For this purpose we synthesized microgel particles, which carry photoreactive groups on the surface, and explored the possibility of creating chemical interparticle cross-links by UV illumination. Via seeded growth emulsion polymerization, we synthesized monodisperse polystyrene microgels with a radius of about 210 nm. In organic solvents, the particles swell to a hydrodynamic radius of 350 nm. These polystyrene microgels were functionalized by using a polymer analogous Friedel-Crafts benzoylation to introduce benzophenone groups onto the surface. The colloidal particles were characterized by static and dynamic light scattering, optical microscopy, and transmission electron microscopy. The interparticle cross-linking behavior of the modified microgels in concentrated dispersions was examined by dynamic light scattering and optical and scanning electron microscopy. To study the effect of short-range attractive forces between the particles, linear polystyrene chains acting as depletion agents were added. In time-dependent irradiation studies, we observe a coexistence of individual particles and clusters with a size range around 2 mum in radius already at shortest irradiation times upon addition of linear polymer. These (transient) clusters-introduced by depletion attraction-are permanently fixed by photo-cross-linking and, thus, on dilution of the system made amenable to analysis by light scattering.

Crystallization kinetics of polydisperse hard-sphere-like microgel colloids: Ripening dominated crystal growth above melting

Highly cross-linked polystyrene microgel colloids dispersed in an index and density matching solvent provide a system with hard-sphere-like interactions, where gravity effects are effectively minimized. They are a suitable target for time-resolved observations of solidification in purely repulsive systems. We have investigated the crystallization kinetics at increasing undercooling using time resolved light scattering. Crystallization starts always with the formation of compressed, structurally heterogeneous precursor domains. In the coexistence region the precursors, after being converted into true crystallites, start growing fast by assimilating particles from the melt. The resulting polycrystalline material consists of high quality crystals and seems not to undergo long time-scale rearrangements. As the particle concentration grows, the higher undercooling and reduced particle mobility increasingly compromise the conversion-growth process. The growth of crystallites relies then on much slower ripeninglike processes, while refining of the crystal structure is detected up to the longest observed times.

Synthesis of polystyrene microgel with a hyperbranched polyglycerol scaffold as core: Effect of shell congestion

AbstractTwo hyperbranched polyglycerols (PG) (1a: Mn = 2000, 1b: Mn = 8000), both with 61% of the overall hydroxyl groups being transformed into trithiocarbonates (2a, 2b, with 16 and 65 OH groups being replaced by trithiocarbonates, respectively), can be used as a reversible addition‐fragmentation chain transfer (RAFT) agent. Polymerization of styrene mediated by 2a/2b was in a living manner, resulting in star‐like, amphiphilic, core‐shell structured polymers with 16 arms (3a) and 65 arms (3b), respectively. It was found that for system mediated by 2b, more serious side reaction (radical–radical coupling) occurred than that by 2a, most probably due to their difference in shell congestion. A shell‐crosslinked polymer could be prepared in the presence of divinylbenzene via a one‐pot route at solid content up to 20%, but analysis proved the crosslinking was incomplete. Furthermore, the 65‐arm‐star polymer (3b) more tended to undergo macroscopic rather than intramolecular crosslinking than the 16‐arm‐star polymer (3a) did. The phenomenon might again be attributed to their difference in shell congestion. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Polystyrene Microgel Amphiphilies with Saccharide to Achieve a Fullerene‐Coating on a Hydrophilic Surface

AbstractSynthesis of a polystyrene (PSt) microgel with glucose and maltohexaose, in which a good coating property was combined with an excellent hydrophilic property, is described. The glycoconjugated PSt microgel showed the ability to solubilize fullerene in aqueous solution, thus having a potential application to achieve the coating of a functional but incompatible compound, such as fullerene due to the lack of polarity, on the surface of various hydrophilic materials.

Polystyrene Microgel Amphiphiles with Maltohexaose. Synthesis, Characterization, and Potential Applications

4-Vinylbenzyl maltohexaoside peracetate (1) was copolymerized with divinylbenzene (DVB) using 1-phenyl-1-(2',2',6',6'-tetramethyl-1'-piperidinyloxy)ethane (2) in m-xylene. The copolymerizations were performed at 138 degrees C for 20 h using the mole fraction of 1 in the total feed of 1 and DVB (F1: [1]/[1]+[DVB]) varying from 0.11 to 0.38, affording polymeric products in yields ranging from 32 to 40%. The characterizations by linear PS-calibrated size exclusion chromatography (SEC), dynamic laser light scattering (DLS) measurements, and 1H NMR spectroscopy indicated that the product was assignable to the cross-linked poly(4-vinylbenzyl maltohexaoside peracetate) particle which is able to produce stable solutions, i.e., the PSt microgel with acetyl maltohexaose, 3. The specific rotations ([alpha]D23, c = 1.0 CHCl3) of 3 ranged from +43.3 degrees to +85.6 degrees. The average molar masses determined by the static laser light scattering (SLS) measurement of 3, M(w,SLS)'s, were from 64,700 to 118,000, which were calculated using the respective refractive index increments, dn/dc's, ranging from 0.03387 to 0.08340. The apparent numbers of the 1, 2, and DVB units in 3, N1, N2, and N(DVB), which were estimated from the respective [alpha]D23 values, M(w,SLS)'s, and real yields, ranged from 22 to 35, from 7 to 26, and from 146 to 506, respectively. The deacetylation of 3 was achieved by treatment with sodium methoxide in dry 1,4-dioxane to produce the PSt microgel with maltohexaose as the hydrophilic segment, 4, as a white solid. The solubility of 4 in various solvents was examined, indicating that a hydrophilic property was effectively introduced. Notably, 4 gave clear solutions in the mixed solvent of 1,4-dioxane and H2O. The ability to solubilize fullerite (mixture of fullerenes, C60/C70 = ca. 9/1) in aqueous solutions was examined according to the literature method. Approximately, 100 mg of 4 (1.7 micromol) solubilizes 1.3 mg of fullerite (1.7 micromol).

Synthesis of polystyrene microgel with glucose as hydrophilic segment via controlled free-radical polymerization

4-Vinylbenzyl glucoside peracetate (1) was copolymerized with divinylbenzene (DVB) using 1-phenyl-1-(2′,2′,6′,6′-tetramethyl-1′-piperidinyloxy)ethane (2) as an initiator in m-xylene at 138°C for 20h ([DVB]/[2]=28; [DVB]=0.62molL−1). The copolymerizations were performed using the mole fraction of 1 in the total feed of 1 and DVB (F1: [1]/[1]+[DVB]) ranging from 0.11 to 0.38 that produced the polystyrene (PSt) microgel with acetyl glucose, 3, in 46–53% yields. Dynamic laser light scattering (DLS) measurements showed that 3 was stably suspended in toluene as particles with average diameters (d's) ranging from 12 to 22nm. A static laser light scattering (SLS) measurement gave the average molar mass, Mw,SLS, of 3 that ranged from 9.69×104 to 6.96×105. The numbers of the 1, 2, and DVB units in 3 (N1, N2, and NDVB, respectively) were from 111 to 238, from 17 to 208, and from 350 to 4510, respectively. The deacetylation of 3 was achieved by treatment with sodium methoxide in dry 1,4-dioxane to produce the PSt microgel with glucose as the hydrophilic segment, 4. The solubilities of 4 in toluene, CHCl3, THF, 1,4-dioxane, pyridine, DMF, DMSO, and H2O, and the mixture of H2O and 1,4-dioxane were examined, indicating that a hydrophilic property had been effectively introduced into 4.

Distribution of CdSe Quantum Dots within Swollen Polystyrene Microgel Particles Using Confocal Microscopy

CdSe quantum dots (QDs) are semiconducting nanoparticles that fluoresce when stimulated by visible light. This property has been exploited in their use as tracer particles in biomedical applications. In this study, confocal microscopy has been used to determine the distribution of QDs within polystyrene microgel particles, dispersed in an organic solvent. It was found that the extent of microgel swelling affected the penetration of the QDs into the particles. Only when the microgel particles were swollen to their maximum extent were the QDs able to penetrate into the central core region of the particles.

Glass transition dynamics of hard sphere like microgel colloids with short-rangedattractions

We have studied by dynamic light scattering the glass transition dynamicsof a binary mixture of polystyrene microgel particles with a size ratio ofRsmall/Rlarge = 0.81 and a number ratio of , where a short-ranged depletion attraction () was induced by addition of linear polystyrene. This system shows a reentrantglass transition. We have determined the glass transition lines of this system inthe experimental control parameter space given by the colloid volume fractionφ and the polymerconcentration cp by employing the power law of mode coupling theory (MCT) for theα-relaxation times. We find a reentry region which is much larger thanpredicted by theory and reported for another colloidal system with depletionattractions. Analysing the fluid dynamics along the transition lines with theβ-scaling law ofMCT we extract the cp dependence of the exponent parameterλ and the non-ergodicityparameter fqc. The results are in qualitative agreement with the prediction ofMCT for increasing attraction strength. The observed ‘jump’ offqc isindicative of a change of the close by glass phase from a packing-driven to a bonding-driven glass. Theincrease of λ to 0.91 and its subsequent decrease may indicate the neighbourhood of anA4 singularity.