DEFORMATION OF SOFT COLLOIDAL CRYSTALLINE STRUCTURE — THEORETICAL CONSIDERATIONS AND EXPERIMENTAL EVIDENCES BY SYNCHROTRON SMALL-ANGLE X-RAY SCATTERING ON TENSILE STRETCHED POLYMERIC LATEX FILM

 Films obtained via drying a polymeric latex dispersion are normally colloidal crystalline where latex particles are packed into a face centered cubic (fcc) structure. Different from conventional atomic crystallites or hard sphere colloidal crystallites, the crystalline structure of these films is normally deformable due to the low glass transition temperature of the latex particles. Upon tensile deformation, depending on the drawing direction with respect to the normal of specific crystallographic plane, one observes different crystalline structural changes. Three typical situations where crystallographic c-axis, body diagonal or face diagonal of the fcc structure of the colloidal crystallites being parallel to the stretching direction were investigated. Tilting angle and d-spacing of several crystallographic planes as a function of draw ratio at each situation were derived. Experimental evidences for such relationships were also given by considering in-situ synchrotron small angle X-ray scattering data of a typical latex film during stretching. It turns out that the experimental results are fully in accordance with the mathematical calculations.     

DEFORMATION OF SOFT COLLOIDAL CRYSTALLINE STRUCTURE-THEORETICAL CONSIDERATIONS AND EXPERIMENTAL EVIDENCES BY SYNCHROTRON SMALL-ANGLE X-RAY SCATTERING ON TENSILE STRETCHED POLYMERIC LATEX FILM

Films obtained via drying a polymeric latex dispersion are normally colloidal crystalline where latex particles are packed into a face centered cubic (fcc) structure.Different from conventional atomic crystallites or hard sphere colloidal crystallites,the crystalline structure of these films is normally deformable due to the low glass transition temperature of the latex particles.Upon tensile deformation,depending on the drawing direction with respect to the normal of specific crystallographic plane,one ...     

Monitoring the transformation of colloidal crystals by styrene vapor using atomic force microscopy

The stages of transformation of a colloidal crystalline film of latex spheres to a new periodic structure were imaged by atomic force microscopy. Colloidal crystalline films were prepared with 320 nm diameter poly(styrene-co-2-hydroxyethyl methacrylate) (PSt/HEMA) spheres. The hexagonally ordered surfaces of the colloidal crystalline films were transformed with styrene vapor at room temperature to a new morphology having holes in the surface and the same periodicity as the original films. The surfaces of colloidal crystals and the transformed films have a raspberry-like texture superposed on the 320 nm hexagonal periodicity. Both height images and phase images reveal that the latex spheres shrink and the transformation proceeds by an order-disorder-order sequence. The final structure is an interconnected colloidal array with smaller polystyrene particles dispersed in a continuous PSt/HEMA matrix.     

Structural reorganization of a polymeric latex film during dry sintering at elevated temperatures

Structural rearrangement in a latex powder during dry sintering at temperatures higher than the minimum film formation temperature was investigated by means of synchrotron small-angle X-ray scattering. Two major effects were identified: (1) Deformation of latex particles leads to a closure of voids between them and an extensive perfectioning of the face centered cubic colloidal crystalline ordering. Such an improvement of the colloidal crystalline structure involves preferential crystal growth along certain crystallographic directions as was evidenced by the measured unmatched relative diffraction intensity distribution of the crystallographic (111) and (220) planes. (2) Interdiffusion of polymeric chains between adjacent particles promotes a nanometer sized aggregation of nonpolymeric materials previously located in the interstices between particles. Size and size distribution of the aggregates at different dry sintering conditions were evaluated by using a model considering spheres dispersed in the system.     

Facile fabrication of mechanochromic-responsive colloidal crystal films

This paper presents a simple approach to fabricate a reversible mechanochromic-responsive crystal film based on the room-temperature film-formation of monodisperse polymer latex by the aid of nanosilica particles. In this approach, when the "soft" colloidal polymer spheres were blended with colloidal silica particles and then cast on a substrate, followed by drying at room temperature for self-assembly, an elastic crystal film was directly obtained. This crystal film has not only reversible and repeatable mechanochromic-responsive property, but also tunable color and peak position covering almost entire visible spectral region, depending upon the sizes of polymer spheres and strains. This optical response is attributed to the variation of lattice spacing during deformation.     

Effect of solvent annealing on the tensile deformation mechanism of a colloidal crystalline polymeric latex film

The influence of solvent annealing on microscopic deformational behavior of a styrene/n-butyl acrylate copolymer latex film subjected to uniaxial tensile deformation was studied by small-angle X-ray scattering. It was demonstrated that the microscopic deformation mechanism of the latex films transformed from a nonaffine deformation behavior to an affine deformation behavior after solvent annealing. This was attributed to the interdiffusion of polymeric chains between adjacent swollen latex particles in the film. It turns out that solvent annealing is much more efficient than thermal annealing due to a much slow evaporation process after solvent annealing.     

Temperature and relative humidity dependency of film formation of polymeric latex dispersions

Thermogravimetric analysis and a synchrotron small-angle X-ray scattering technique were employed to characterize the structural evolution of a polymeric latex dispersion during the first three stages of film formation at different temperatures and relative humidities. Three intermediate stages were identified: (1) stage I*, (2) stage I**, and (3) stage II*. Stage I* is intermediate to the conventionally defined stages I and II, where latex particles began to crystallization. The change of drying temperature affects the location of the onset of ordering, whereas relative humidity does not. Stage I** is where the latex particles with their diffuse shell of counterions in the fcc structure are in contact with each other. The overlapping of these layers results in an acceleration of the lattice shrinkage due to a decrease of effective charges. Stage II* is where the latex particles, dried well above their T(g), are deformed and packed only partially during film formation due to incomplete evaporation of water in the latex film. This is because of a rapid deformation of the soft latex particles at the liquid/air interface so that a certain amount of water is unable to evaporate from the latex film effectively. For a latex dispersion dried at a temperature close to its minimum film formation temperature, the transition between stages II and III can be continuous because the latex particles deform at a much slower rate, providing sufficient surface area for water evaporation.     

Close-packed colloidal crystalline arrays composed of polystyrene latex coated with titania nanosheets

We have demonstrated that polystyrene latex coated with titania nanosheets can be fabricated into a close-packed colloidal crystalline array, and that these coated colloidal spheres can be used to control the peak position of optical stop bands through the coating. The titania-nanosheets-coated polystyrene latex was prepared by the layer-by-layer (LBL) assembly coating process, involving alternating lamination of cationic polyelectrolytes and anionic titania nanosheets on monodisperse polystyrene latex particles. The Bragg diffraction peak of the colloidal crystalline array shifted to longer wavelengths with the coating of titania nanosheets. This red shift was caused by an increase in refractive index upon coating, as revealed by angle-resolved reflection spectra measurements. The current work suggests new possibilities for the creation of advanced colloidal crystals having tunable optical properties from tailored colloidal spheres.     

Effects of self-assembly process of latex spheres on the final topology of macroporous silica

This paper surveys the topology of macroporous silica prepared using latex templates covering the submicrometric range (0.1-0.7 mum). The behavior of latex spheres in aqueous dispersion has been analyzed by dynamic light scattering (DLS) measurement indicating the most appropriate conditions to form well-defined cubic arrays. The optical behavior of latex spheres has been analyzed by transmittance and reflectance measurements in order to determine their diameter and filling factor when they were assembled in bidimensional arrays. Macroscopic templates have been obtained by a centrifugation process and their crystalline ordering has been confirmed by porosimetry and scanning electron microscopy. These self-assembled structures have been used to produce macroporous silica, whose final topology depends on the pore size distribution of the original template. It has been seen that latex spheres are ordered in a predominant fcc arrangement with slipping of tetragonal pores due to the action of attractive electrostatic interactions. The main effect is to change the spherical shape of voids in macroporous silica into a hexagonal configuration with possible applications to fabricate photonic devices with novel optical properties.     

Temperature-induced crystallization in concentrated suspensions of multiarm star polymers: a molecular dynamics study

In this work, we study temperature-induced crystallization in dense suspensions of multiarm star polymers. This is a continuation of a previous study, which identified and studied the emergence of "glassy" amorphous states, in accordance with experimental observations. We performed molecular dynamics simulations on two types of star polymers: 128-arm stars and 64-arm stars dissolved in n-decane in the temperature range of 20-60 degrees C. These supramolecules are modeled as "soft spheres" interacting via a theoretically developed potential of mean field. Both systems attain a crystalline structure with the characteristics of a face-centered-cubic (fcc) crystal beyond a certain temperature. Kinetics is sensitive on initial configuration. Interestingly, kinetic trapping in "temporary" energy wells leads to highly crystalline structures, yet less ordered than their genuine equilibrium fcc structure. This complication illustrates the difficulty in reaching the equilibrium state, which is crystalline at high temperatures. A structural analysis of the final conformations is presented. The effect of size dispersity and star functionality of soft spheres on microstructure is also examined. Both factors influence crystallization and their effect is quantified by our study.     

聚苯乙烯胶晶膜及三维有序大孔SiO2膜的制备及表征

采用垂直沉积法组装了三维聚苯乙烯胶晶膜,并用其为模板制备了三维有序大孔（3DOM）SiO2膜．SEM观察表明,制备的胶晶膜和3DOMSiO2膜具有fcc结构,有序性很好．考察乳液浓度对胶晶膜结构的影响表明,浓度越高,胶晶膜越厚,有序性也越高,膜在30层内都能很好的粘附在载玻片上．通过调整前驱物溶液的浓度和滴加方式,可得到表面为球形或孔状的3DOM SiO2膜．     

Fabrication of spherical colloidal crystals using electrospray

We demonstrated the use of electrohydrodynamic atomization to prepare uniform-sized emulsion droplets in which equal spheres of silica or polystyrene were dispersed. The size of the emulsion droplets was easily controlled by the electric field strength and the flow rate, independently of the diameter of the nozzles. During the evaporation of solvent in the droplets, spherical colloidal crystals were formed by self-assembly of the monodisperse colloidal spheres. The diameter of the spherical colloidal crystals was in the range of 10-40 microm. Depending on the stability of colloidal particles, the morphology of the self-assembled structure was varied. In particular, silica spheres in ethanol droplets were self-assembled into compactly packed silica colloidal crystals in spherical shapes, whereas polystyrene latex spheres in toluene droplets self-assembled into spherical colloidal crystal shells with hollow cores. The silica colloidal assemblies reflected diffraction colors according to the three-dimensionally ordered arrangement of silica spheres.     

Doped colloidal photonic crystal structure with refractive index chirping to the [111] crystallographic axis

A three-dimensionally ordered array of close-packed colloidal spheres, a photonic crystal structure in which the refractive index of the medium interstitial lattice in a colloidal crystal spatially changes in the [111] crystallographic axis, is demonstrated. The colloidal photonic crystal structure with refractive index chirping was produced by infiltration of a monomer and organic dopants with a high refractive index into a silica opal, followed by interfacial gel polymerization. The resulting photonic crystal structure has a gradually varying stop band at each different (111) plane in the face-centered cubic (fcc) crystal structure at a normal incidence. This novel structure exhibited optical characteristics that have band-gap broadening by the superposition of stop bands at each plane of the crystal with different dielectric functions. Moreover, the refractive index perturbation in the [111] fcc opal also showed a defect state within a pseudo-photonic band gap. This new type of photonic crystal structure should be useful for the band-gap engineering of photonic-band-gap materials.     

Crystal structure of thermosensitive gel spheres of poly(N-isopropylacrylamide) in the deionized aqueous suspension as studied by the static light-scattering measurements

Static light-scattering measurements of deionized suspensions of the thermosensitive gels of poly(N-isopropylacrylamide) with various degrees of cross-linking and sizes were made at 20 and 40 °C. Sharp scattering peaks are observed in the scattering curve, and they were attributed to the face-centered cubic (fcc) and/or body-centered cubic lattices (bcc) in the distribution of gel spheres. The fcc and bcc crystal structures formed in the stable and unstable conditions, respectively, i.e., the former formed more favorably at high sphere concentrations and/or low temperatures. The closest intersphere distances were much longer than the hydrodynamic diameters of the gel spheres especially at low sphere concentrations. These experimental results emphasize the important role of the extended electrical double layers in the crystallization of gel spheres, though the contribution of the double layers in gel systems is weak compared with that in the typical colloidal spheres.     

Photoinduced deformation of amphiphilic azo polymer colloidal spheres

Photoinduced shape deformation of colloidal spheres made of an amphiphilic azo polymer has been demonstrated in this work. The polymer contains the donor-and-acceptor-type azobenzene chromophores and can form uniform colloidal spheres by dropwise adding water into its THF solution. When the colloidal spheres obtained were exposed to the interfering p-polarized Ar+ laser beams (150 mW/cm2), the colloidal spheres changed to prolates (i.e., "rugby-balls"), "spindles", and finally "rods", depending on the irradiation times. The elongated direction of the spheres was observed to be the same as the polarization direction of the laser beam. The average major-to-minor ratio of the ellipsoids could be easily adjusted by controlling the irradiation time. The deformation effect observed in this work can offer a new way to prepare nonspherical colloids from colloidal spheres and will shed new light on the correlation between the photodriven shape deformation and photoinduced surface relief gratings for the same type of polymers.     

Poisson's ratio of the fcc hard sphere crystal at high densities

Elastic constants and the Poisson ratio of the fcc hard-sphere crystalline phases, free of defects and with vacancies, are determined by two Monte Carlo methods: (i) the analysis of the box fluctuations in the constant pressure ensemble with variable box shape (N-P-T) and (ii) by the free-energy differentiation with respect to deformation in the fixed box ensemble (N-V-T). Very good agreement is observed for the extrapolated to the infinitely large system limit results of both the methods. The coefficients of the leading singularities of the elastic constants near close packing are estimated; they are well described by the free volume approximation. Two mechanisms influencing the Poisson ratio are studied. (i) It is shown that at high densities particle motions decrease the Poisson ratio with respect to the static case which corresponds to zero temperature. Simulations performed for systems of soft spheres, interacting through n-inverse-power potentials, r-n, show that the elastic constants of the hard spheres can be obtained in the limit n-->infinity. When T-->0 the elastic constants of the soft spheres tend to those of the static model. (ii) It is also shown that vacancies decrease C11 and C44 and increase C12 and, hence, increase the Poisson ratio with respect to the defect-free state of the system.     

Azo polymer colloidal spheres containing different amounts of functional groups and their photoinduced deformation behavior

In this work, colloidal spheres composed of azo polymers with different chromophore loading densities were prepared, and their photoinduced deformation behavior was studied. The colloids were constructed by using a series of amphiphilic epoxy-based random copolymers containing 4-carboxylazobenzene functional groups with different degrees of functionalization (DFs). The colloidal spheres were fabricated through gradual hydrophobic aggregation of the polymeric chains in tetrahydrofuran-H2O dispersion media, which was induced by gradually adding water into the systems. The colloidal spheres were characterized by using transmission electron microscopy and dynamic light scattering. The photoinduced deformation behavior was studied by irradiating the colloidal spheres with a linearly polarized Ar+ laser beam. Results showed that the critical water content (CWC) for the colloid formation is related to the DF of the polymers, and CWC increases with the increase of DF. The hydrodynamic diameter of the colloidal spheres is also related to the DF of the polymers. When the DF of the polymers increases, the average size of the colloids gradually decreases. The hydrodynamic diameter of the colloidal spheres increases as the water dropping rate decreases. When the dropping rate is below 20 microL/s, the size of the colloidal spheres increases abruptly as the dropping rate further decreases. Upon the linearly polarized Ar+ laser beam irradiation, the colloids composed of polymers with different DFs can all be elongated along the polarization direction of the laser beam. As DF increases, the deformation degree characterized by the axial ratio (l/d) almost linearly increases. These observations can give some insight into the photoinduced deformation mechanism and can be used to construct colloids with different sizes and photoresponsive ability.     

Silver hierarchical bowl-like array: synthesis, superhydrophobicity, and optical properties

We present a facile synthetic route to a silver bowl-like array film with hierarchical structures on glass substrate using the colloidal monolayer as a template. In these special hierarchical structures, microstructures were provided by a colloidal template of polystyrene latex spheres and nanostructures resulting from the thermal decomposition of silver acetate. These structures were chemically modified with 1-hexadecanethiol, and a corresponding self-assembled monolayer (SAM) was formed on their surfaces. Due to the lotus leaf-like morphology with hierarchical micro/nanostructures, the film displayed an extraordinary superhydrophobicity after chemical modification. Water contact angle and sliding angle were 169 degrees and 3 degrees (the weight of water droplets: 3 mg), respectively. Additionally, its optical property has also been investigated. This structure could be used in microfluidic devices, optical devices, and biological science.     

双组分偶氮聚合物胶体球制备及其光致形变行为研究

研究了2种环氧树脂类含4-氨基-4′-硝基偶氮苯和4-氨基-4′-羧基偶氮苯生色团聚合物(BP-AZ-NT、BP-AZ-CA)双组分胶体球的制备和光致形变行为.通过在上述聚合物的四氢呋喃溶液中逐步加水诱导自组装的方法,得到了BP-AZ-CA/BP-AZ-NT双组分胶体球.在上述两种聚合物自组装形成胶体球过程中,较为疏水的BP-AZ-NT分子先发生聚集,而较亲水的BP-AZ-CA则在形成的胶体颗粒表面发生进一步聚集.在胶体球形成过程中,体系的临界水含量(CWC)主要由BP-AZ-NT发生聚集时的水含量决定,双组分胶体球的外层则含较多的BP-AZ-CA分子.比较单组分胶体球与双组分胶体球在线偏振Ar+激光(488nm,100mW/cm2)照射下的形变行为,进一步证实了通过上述方法可以制备BP-AZ-CA和BP-AZ-NT双组份的胶体球;胶体球形变时的初始拉伸速率由胶体球的外层聚合物分子的性质所决定。     

Layer-by-layer synthesis of multilayer core-shell latex and the film formation properties

Polymer latex particles were synthesized with multilayer core-shell structure via surface cross-linking emulsion polymerization. The latex core is coated with a five-layer shell. The polymerization was done in a semicontinuous fashion monitored by a dynamic laser scattering (DLS). The copolymer in each layer is designed with alternating high and low glass transition temperature (T(g)). Divinylbenzene (DVB) was added as the cross-linking agent in the synthesis of the "hard" layers to prevent the molecular diffusion from the adjacent "soft" layers. The layer-by-layer increment on the latex core is proved by the alternating changes on the film-formation capabilities of different latex emulsions at room temperature in correspondence with the variance in the T(g) of the outermost polymer layer. The detailed morphologies of the films formed by the latex with different number of layers were characterized by atom force microscopy (AFM). The deformation of the latex particles is largely depended on the nature of the polymer in the outermost layer of the latex particles. Further characterization carried out by multifrequency temperature-modulated differential scanning calorimetry (TOPEM-DSC) confirmed the layer-by-layer structure of the particles, although the molecular redistribution and the interlayer structures were observed. The work provides a routine toward the synthesis of multilayer polymer latexes.