Vesicle-to-micelle transition induced by grafted diblock copolymers

We study by small-angle neutron scattering the vesicle-to-micelle transition induced by anchoring diblock copolymers on the surfactant membranes. Vesicles are made using the ternary system SDS (sodium dodecyl sulfate)/octanol/PS-POE (polystyrene-polyoxyethylene), chosen as a model of more complex biological systems. The anchored polymer chains destabilize the membranes and lead to the formation of mixed polymer-surfactant micelles. We show that there is no influence of the polymer mass on the membranes destabilization. We compare this experimental result with recent theoretical predictions concerning the polymer-grafted membranes. We have a good agreement with predictions involving membrane curvature effects, and also with predictions focusing on more local deformations of membranes created by the grafted chains. Received 12 June 2001 and Received in final form 18 March 2002     

Incorporating DNA in a lamellar phase: A Flory model

The European Physical Journal E - Following an experimental work demonstrating that DNA pieces can be incorporated in large quantity in a neutral lamellar phase, we propose a theoretical model...     

模型化研究高分子凝胶体积相变中的共非溶性作用

基于Flory-Huggins理论,我们建立理论模型,研究在共非溶剂(CNS)中,高分子凝胶(PG)体积相变中的共非溶性作用.理论模型考虑PG中CNS的桥接作用、各种分子的混溶效应.研究发现,当CNS与高分子单体间的吸引强度较小时,CNS与高分子单体间桥接作用的减弱,会导致PG体积相变.当CNS与高分子单体间的吸引作用强度较大时,随着CNS与高分子单体间桥接作用的减弱,PG的体积分数呈现两次台阶式的转变,表明PG体系出现两次体积相变.这是由于桥接作用的减弱,虽然会有部分CNS分子被排挤出PG,但是并未完全消除CNS与高分子单体间的桥接吸引作用.所得理论结果符合实验观测,由此表明了共非溶性作用会在很大程度上调控PG的相变行为.     

Controlled construction of nanostructures in graphene

We report on the laser-assisted fabrications of nanostructures in graphene membranes supported on polymer films. By using a laser beam to deposit heat locally, irradiated polymer instantaneously melts and vaporizes. During laser drilling of the polymer, the single-layer graphene membrane adheres to the polymer surface and consequently forms tens of nanometer deep wells. Due to the short time scale of laser irradiation, heat diffusion in the polymer is negligible, and the excitation energy is highly confined in the polymer. As a result, graphene nanowells of hundreds of nanometers in diameter can be pat- terned with high fidelity. With the increasing of nanowell density, we observe the spontaneous formation of nanowrinkles connecting pairs of nanowells in the graphene membranes. Importantly, Raman spectra confirm that no defects are intro- duced in graphene membranes by laser irradiation under our experimental conditions. Our results highlight the possibility to construct nanostructures and to design novel devices based on graphene.     

DNA单分子弹性理论

随着单分子操纵技术的发明与发展，人们已经可以对单个生物大分子施以力或力矩，并测量它们的物理性质，DNA单分子的力学实验表明，在分子尺度上理解生物大分子的生化过程，力与能量是同等重要的结构与功能参数。一个梯子模型被用来描述双链DNA的外力拉伸曲线，在这个模型中，DNA是由许多碱基对(梯子的横杆，横杆之间存在吸引势)连接两条聚核苷酸虫链(梯子的两侧)形成的高分子，利用路径积分法得出的理论曲线与实验曲线吻合得很好，对于单链DNA，用分立的杂化高分子链统计理论的母函数方法来计算其弹性行为，得出与实验相符合的外力引起的解链相变结果。此外，对于抑瘤蛋白p53识别序列DNA微环弹性进行分析，发现其弹性模量只是通常随机序列的三分之一。     

Supramolecular polymorphism of DNA in non-cationic L<Subscript>α</Subscript> lipid phases

The structure of a complex between hydrated DNA and a non-cationic lipid is studied, including its phase diagram. The complex is spontaneously formed by adding DNA fragments (ca. 150 base pairs in length) to non-cationic lipids and water. The self-assembly process often leads to highly ordered structures. The structures were studied by combining X-ray scattering, fluorescence and polarized microscopy, as well as freeze-fracture experiments with transmission electron microscopy. We observe a significant increase of the smectic order as DNA is incorporated into the water layers of the lamellar host phase, and stabilization of single phase domains for large amounts of DNA. The effect of confinement on DNA ordering is investigated by varying the water content, following three dilution lines. A rich polymorphism is found, ranging from weakly correlated DNA-DNA in-plane organizations to highly ordered structures, where transmembrane correlations lead to the formation of columnar rectangular and columnar hexagonal superlattices of nucleotides embedded between lipid lamellae. From these observations, we suggest that addition of DNA to the lamellar phase significantly restricts membrane fluctuations above a certain concentration and helps the formation of the lipoplex. The alteration of membrane steric interactions, together with the appearance of interfacial interactions between membranes and DNA molecules may be a relevant mechanism for the emergence of highly ordered structures in the concentrated regime.     

Reflective-type configuration for nearly phase-matching cerenkov second-harmonic generation in a nonlinear-optical polymer waveguide

A new technique for achieving efficient Cerenkov-type second-harmonic generation (SHG) in a nonlinear-optical (NLO) polymer waveguide is presented. The configuration, which can prevent the losses of light caused by relatively long-distance propagation and the multiple reflections that appear in the conventional Cerenkov technique, exhibits ease of fabrication and compactness. We experimentally observed a conversion efficiency of 1.6% W(-1) cm(-1), which to our knowledge is the highest value reported for Cerenkov SHG in polymer, by tuning both the thickness and the refractive index of the polymer film close to phase matching between a guided fundamental wave and a guided harmonic wave. The experimental results agreed well with the theoretical prediction.     

Polymer-mediated interactions of fluid membranes in a lyotropic lamellar phase: a small angle X-ray and neutron scattering study

Small angle X-ray and neutron scattering data on an effective three-component lamellar phase composed of water, a non adsorbing water-soluble polymer (polyvynilpyrolidone), fluid membranes, made from a mixture of a cationic surfactant (cetylpiridiumchloride) and a cosurfactant (hexanol), are presented for various membrane as well as polymer concentrations. The data are fitted with a recently proposed model which takes into account the geometry and the fluctuations of these periodic structures. This allows a quantitative study of the polymer contribution to the smectic compression modulus of the lamellar phase. Four different regimes of polymer confinement are expected. The associated variations in are compared to a recent theoretical model, which predicts the polymer-mediated contribution to the smectic compression modulus. Received 20 January 1998     

Polymers and scaling

We review the statistical mechanics of polymer solutions with reference to the theories of scaling, current in the theory of phase transitions. Topics include the theoretical background, the relation of the polymer problem to magnetic systems, numerical calculations, Monte-Carlo work, self-consistent field theories, recent field-theory work and experimental work.     

Preparation of PVDF-HFP Microporous Membranes via the Thermally Induced Phase Separation Process

The thermally induced phase separation (TIPS) process was employed to prepare poly (vinylidene fluoride-co-hexafluoropropylene; PVDF-HFP) microporous membranes using sulfolane as the diluent. The phase diagram of the PVDF-HFP/sulfolane system was drawn and analyzed. The effects of polymer content in casting solution and cooling rate on the cross-sectional morphology, crystallinity, crystal structure, and porous structure of the resulting membranes were investigated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and a mercury porosimeter, respectively. The mechanical properties of the membranes were evaluated by tensile tests. It was found that a solid-liquid phase separation occurred in the PVDF-HFP/sulfolane system. Spherulites and “net-shaped” structures coexisted in the obtained membranes. Polymer content and cooling rates had some influences on the crystallinity, porous structure, and mechanical properties of the membranes, but no influence on the polymer crystal structure of the membranes.     

Shear-induced phase transitions in ternary polymer blends

We present a study of flow-induced phase transitions in microemulsion phases of ternary polymer blends. The results match qualitatively with the recent experimental observations on such systems but differ from the behavior expected and observed in the analogous system of surfactants. We rationalize this contrast from a molecular viewpoint suggesting that the interplay between polymer chain conformations and their flow deformations can lead to novel flow effects upon the phase, structural, and rheological behavior of multicomponent polymer systems.     

Fabrication of piezoelectric components for a tunable and efficient device for DNA delivery into mammalian cells

We fabricated three piezoelectric components (PZT) that can produce ultrasonic waves with various generated power in order to improve the delivery of DNA molecule and polymer/DNA complexes into cells. Two cationic polymers (PEI and PDMAEMA) were interacted with DNA to form nano-scaled DNA/polymer complexes with/without the help of PZT devices. The application of PZT devices under optimal conditions helped to avoid cytotoxicity and greatly increased the transfection (DNA delivery) efficiency of these complexes in mammalian cells. The cytotoxicity and transfection efficiency were found to be correlated with the PZT-generated power, waveforms and duration of ultrasonic treatment. There was no observable cytotoxicity in our experimental models and, a maximum transfection efficiency 700% greater than that of polymer/DNA complexes without applying ultrasound was achieved. The transfection efficiency of plain polymer/DNA complexes (without PZT treatment) corresponded to a 630-fold increase in comparison to the naked DNA. The waveforms of generated ultrasound greatly influenced the transfection efficiency, while cytotoxicity was not significantly affected. This means that, for optimal DNA delivery, duration of the peak voltage (V_max/Div) also plays a role. In addition, the generated waves from PZT do not cause dissociation of polymer/DNA complexes or a change in the particle sizes of these complexes. In conclusion, these results suggest that the operation of PZT devices can be a tunable/safe way to greatly improve DNA delivery for gene therapy.     

Why is the DNA denaturation transition first order?

We study a model for the denaturation transition of DNA in which the molecules are considered as being composed of a sequence of alternating bound segments and denaturated loops. We take into account the excluded-volume interactions between denaturated loops and the rest of the chain by exploiting recent results on scaling properties of polymer networks of arbitrary topology. The phase transition is found to be first order in d = 2 dimensions and above, in agreement with experiments and at variance with previous theoretical results, in which only excluded-volume interactions within denaturated loops were taken into account. Our results agree with recent numerical simulations.     

(Chicken feathers keratin)/polyurethane membranes

Actually, chicken feathers are considered as waste from the poultry industry; however, 90% of feather structure is constituted by a protein called keratin. In this research, the properties of feather keratin and polyurethane are combined in order to synthesize hybrid synthetic–natural membranes. Both polymers are linked by urethane bonds which are similar to peptide bonds found in proteins. Keratin is incorporated onto the polyurethane matrix by dissolving protein in a salt solution (urea and 2-mercaptoethanol) at different concentrations: 11, 13, 15, 17, 19, and 21% (w/w). In order to know the effect of urea on membranes, keratin is incorporated to polyurethane in two ways; as keratin salt solution and after dialyzing. Both membrane types were characterized by Scanning Electron Microscopy (SEM) to observe their morphologic changes. Fourier Transformed Infrared Spectroscopy (FT-IR), Termogravimetric Analysis (TGA), and Differential Scanning Calorimetry (DSC) were used to study membrane structures. Results show that keratin is grafted in polyurethane and, therefore, there is an influence of amino acids through the amino and carboxylic groups (NH and COOH) into the synthetic polymer structure. According with characterization results, the obtained membranes are functional materials that can be useful in diverse applications, among them the separation process can be emphasized.     

Large scale ordered topographical and chemical nano-features from anodic alumina templates

We have used ordered anodic alumina membranes as masks to create large scale ordered arrays of either holes or chemical islands on silica. Regularly spaced holes were obtained by direct etching of silica/silicon or glass substrates through the membranes used as etching masks. To create an array of chemically functional islands, the membrane is first glued on the substrate using a soft polymer and subsequently the polymer is etched gently though the mask. Finally organo-silane molecules are deposited through the alumina/polymer hybrid mask and the mask is removed chemically leaving nanoislands on the substrate. We anticipate that this technique will be useful in future biological and biomedical applications.     

Geometry-dependent electronic properties of highly fluorescent conjugated molecules

We present a combined experimental/theoretical study of the electronic properties of conjugated para-phenylene type molecules under high pressure up to 80 kbar. Pressure is used as a tool to vary the molecular geometry and intermolecular interaction. The influence of the latter two on singlet and triplet excitons as well as polarons is monitored via optical spectroscopy. We have performed band structure calculations for the planar poly(para-phenylene) and calculated the dielectric function. By varying the intermolecular distances and the length of the polymer repeat unit the observed pressure effects can be explained.     

Optical nonlinearity and structural phase-transition observation of organic dye-doped polymer silica hybrid material

The optical nonlinearity of organic dye-doped poly(methyl methacrylate) (PMMA)-silica-gel hybrid material was investigated by second-harmonic-generation measurement. We found that incorporation of in situ polymerized solgel precursors into the organic dye-doped PMMA significantly improved the nonlinear optical stability of the system. However, improvement of thermal stability occurred only when a sufficient amount of silica gel was incorporated. A structural phase transition from pure polymer to a hybrid system was found near a 10-mol.% silica-gel concentration. The optimum polymer/tetraethoxysilane molar ratio is 2:1 to 1:1.     

Biaxial nematic phase in a thermotropic liquid-crystalline side-chain polymer

We report on variable-angle deuterium NMR measurements of nematic liquid-crystalline side-chain polymers, where we have found proof of the existence of a biaxial nematic phase in a system with a side-on attachment of the mesogenic group to the polymer backbone. This provides efficient coupling to the polymer backbone and thus stabilizes the as yet elusive biaxial phase. The experimental approach is validated on a uniaxial end-on nematic polymer, and the reliability of the results is investigated in detail using two-dimensional correlation spectra, providing information on director distribution effects.     

Molecular Dynamics simulation of a polymer chain translocating through a nanoscopic pore

The detection of linear polymers translocating through a nanoscopic pore is a promising idea for the development of new DNA analysis techniques. However, the physics of constrained macromolecules and the fluid that surrounds them at the nanoscopic scale is still not well understood. In fact, many theoretical models of polymer translocation neglect both excluded-volume and hydrodynamic effects. We use Molecular Dynamics simulations with explicit solvent to study the impact of hydrodynamic interactions on the translocation time of a polymer. The translocation time τ that we examine is the unbiased (no charge on the chain and no driving force) escape time of a polymer that is initially placed halfway through a pore perforated in a monolayer wall. In particular, we look at the effect of increasing the pore radius when only a small number of fluid particles can be located in the pore as the polymer undergoes translocation, and we compare our results to the theoretical predictions of Chuang et al. (Phys. Rev. E 65, 011802 (2001)). We observe that the scaling of the translocation time varies from τ ∼ N ^11/5 to τ ∼ N ^9/5 as the pore size increases (N is the number of monomers that goes up to 31 monomers). However, the scaling of the polymer relaxation time remains consistent with the 9/5 power law for all pore radii.