Measurement of size‐dependent glass transition temperature in electrospun polymer fibers using AFM nanomechanical testing

The glass transition temperature (T_g) of individual electrospun polymer polyvinyl alcohol fibers of varying diameter was measured using atomic force microscopy (AFM) based nanomechanical thermal analysis. Indentation and bending of individual electrospun fibers using AFM allowed the calculation of the elastic modulus of the polyvinyl alcohol (PVA) fibers across a range of different temperatures. The elastic modulus of electrospun PVA fibers was observed to decrease significantly when passing through T_g, which allowed accurate determination of T_g. The T_g of electrospun PVA fibers was shown to decrease for smaller fiber diameters especially for fiber diameters below 250 nm. This size‐dependent glass transition behavior of electrospun PVA fibers is indicated as being due to polymer chain confinement. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Is the alignment of nematics on a polymer slab always along the rubbing direction? A molecular dynamics study

ABSTRACT

We have studied the alignment and molecular organisation within a thin film of the popular nematic 5CB sandwiched between two flat polymer slabs, examining the effect of polymer chemical nature and morphology with atomistic molecular dynamics simulations. We have chosen two common polymers: polystyrene (PS) and polymethylmethacrylate (PMMA), either with their chains in random coil conformation (disordered) or with chains unidirectionally stretched (ordered). We found that, independently on the arrangement of the chains, both surfaces align planarly the liquid crystal, in accord with experimental observation. Moreover, while 5CB molecules align along the chains stretching direction of the PMMA ordered surface, on the combed PS surface they arrange on average perpendicularly to the stretching direction. This behaviour is attributed to the chemically specific interactions between the respective aromatic moieties of PS and 5CB.     

A study on the polymer structures and electro-optical properties of epoxy-mercaptan-based polymer dispersed liquid crystal films

ABSTRACT

In this paper, polymer dispersed liquid crystal (PDLC) films based on epoxy-mercaptan system were prepared by thermal-initiated polymerization. The effects of the liquid crystal (LC) content, the proportion and the functionality of epoxy monomers on the polymer structures and electro-optical properties of the as-made PDLC films were investigated systematically. It was found that the morphologies of the polymer matrix can be altered from polymer meshes to polymer balls by increasing the LC content as well as the functionality of epoxy monomers. Accordingly, the electro-optical properties could be regulated by the morphologies of polymer networks. Especially, the as-made PDLC films with homogeneous porous structures exhibited the optimal electro-optical properties. Consequently, this work offers a meaningful approach to control the microstructures and optimize the electro-optical properties of PDLC films, which indeed can form a wonderful footstone for the wide application of PDLC.     

Hydrogen bonding interactions between adsorbed polymer molecules and crystal surface of acetaminophen

The objective of this work was to investigate whether or not the hydrogen bonding interaction between polymer and crystal surface can be detected by the etching pattern changes in the presence of polymers. The (010) face of acetaminophen single crystal was used as a model solid surface. The etching patterns on the (010) face of acetaminophen crystal by water are in the directions of a- and c-axes, which are the same as the directions of the dominant attachment energies on the (010) face. In the presence of polymer, the hydrogen bonding interactions between adsorbed polymer and crystal surface can affect surface diffusion of acetaminophen molecules and change the etching patterns in the direction of a-axis, i.e., the direction of one hydrogen bond chain. Studies with 2-hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxypropyl methylcellulose (HPMC) and poly(vinyl alcohol) (PVA) showed that polymers, which can form hydrogen bonds with acetaminophen crystal surface, can change etching patterns in the direction of a-axis. Study with Dextran suggested that if a polymer cannot form hydrogen bonds with crystal surface due to steric repulsion, it will not change the etching pattern in the direction of a-axis. Studies with poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) further confirmed that only if a polymer can form hydrogen bonds with acetaminophen on crystal surface, the etching patterns in the direction of a-axis will be affected. The study results suggest that in the presence of polymers, the etching pattern change in the direction of hydrogen bond chain, the a-axis of acetaminophen crystals, can be used to indicate the existence of the hydrogen bonding interactions between adsorbed polymers and acetaminophen crystal surface.     

Characterization of the surface charge property and porosity of track-etched polymer membranes

As an important property of porous membranes, the surface charge property determines many ionic behaviors of nanopores, such as ionic conductance and selectivity. Based on the dependence of electric double layers on bulk concentrations, ionic conductance through nanopores at high and low concentrations is governed by the bulk conductance and surface charge density, respectively. Here, through the investigation of ionic conductance inside track-etched single polyethylene terephthalate (PET) nanopores under various concentrations, the surface charge density of PET membranes is extracted as ∼−0.021 C/m² at pH 10 over measurements with 40 PET nanopores. Simulations show that surface roughness can cause underestimation in surface charge density due to the inhibited electroosmotic flow. Then, the averaged pore size and porosity of track-etched multipore PET membranes are characterized by the developed ionic conductance method. Through coupled theoretical predictions in ionic conductance under high and low concentrations, the averaged pore size and porosity of porous membranes can be obtained simultaneously. Our method provides a simple and precise way to characterize the pore size and porosity of multipore membranes, especially for those with sub-100 nm pores and low porosities.     

Modification of electro-optical properties of polymer dispersed liquid crystal films by iniferter polymerization

In this letter, iniferter polymerization was employed to prepare polymer dispersed liquid crystal (PDLC) films. Polystyrene (PS) was prepared as a macro-iniferter (MI). With the addition of MI in PDLC films, poly(methyl acrylate)-b-polystyrene was prepared in situ and used as polymer matrix in photopolymerization induced phase separation (PIPS). A reduction in driving voltages and an improvement in the ON state transmittance were observed for the sample prepared with a small amount of MI; while a poor electro-optical performance was obtained for that without any MI. Moreover, molecular weight and refractive index of the polymer matrix could be easily adjusted by the concentration of MI, and the matrix seems to be a prospective material for the PDLC devices.     

Photochemical formation of nonlinear optical and photorefractive image in polymer layer

The paper is devoted to the development of the light-sensitive layers in which the image with nonlinear optical (NLO), photoelectric and photorefractive (PR) properties arises as a result of the photochemical reaction. Layers were obtained by using poly(hydroxyaminoether) (PHAE) containing electron-donor aromatic amino-groups in polymer chains doped with tetrabromomethane as an acceptor. The amino-groups and tetrabromomethane molecules form donor–acceptor complexes the photoexcitation of which induces electron transfer from the donor to the acceptor leading to the synthesis of a colored polycation product which possesses charge carrier photo-generating and NLO properties. Besides, these layers are electron-transporting. This allowed to obtain light-sensitive layers for recording NLO and PR images and pictures.     

The effect of the resultant microphase-separated structures of polymer matrices on the electro-optical properties of polymer dispersed liquid crystal films by Iniferter polymerization

Iniferter polymerization was employed to prepare polymer dispersed liquid crystal (PDLC) films and an additional photoinitiator was introduced to induce the phase separation of polymer matrices themselves on the process of preparing the PDLC. The effect of the polymerization kinetics and the resultant microphase-separated structures of polymer matrices on the electro-optical properties of PDLC films were studied. It was found that the bigger length scale of phase separation of polymer matrices induced strong light-scattering resulting in low ON-state transmittance. And faster polymerization kinetic induced higher threshold and saturation voltages.     

Interpreting the conductive atomic force microscopy measured inhomogeneous nanoscale surface electrical properties of Al‐doped ZnO films

In this work, conductive atomic force microscopy is used to study the inhomogeneous surface electrical conductivity of Al‐doped ZnO thin films at a nanoscale dimension. To this end, Al‐doped ZnO films were deposited onto the soda lime glass substrates at substrate temperature (T_s) varying from 303 to 673 K in radio frequency magnetron sputtering. The obtained local surface electrical conductivity values are found to be influenced by their bulk electrical resistivity, surface topography and tip geometry. Further, the average (local) surface conductivity from the film surface is found to increase with increasing T_s from 303 to 623 K, beyond which they decrease until 673 K. Copyright © 2016 John Wiley & Sons, Ltd.     

Competitive adsorption of fibrinogen and albumin onto polymer microspheres having hydrophilic/hydrophobic heterogeneous surface structures

The competitive adsorption of bovine fibrinogen (BFb) and bovine serum albumin onto polymer microspheres from the mixture solution was examined under various protein-to-microsphere ratios using various homopolymer microspheres and poly(2-hydroxyethyl methacrylate)/polystyrene composite microspheres having heterogeneous surface structures consisting of both hydrophilic and hydrophobic parts. They were produced by emulsifier-free (seeded) emulsion polymerizations. The selective adsorption of BFb was not observed for the homopolymer microspheres, but observed for the composite polymer microspheres having optimum compositions.Part CXXXVIII on the series Studies on Suspension and Emulsion     

Antibacterial nanocomposite supporting cell growth and spheroid formation by chemical surface treatment of polymer foil

Octenidine dihydrochloride (OCT) has a wide spectrum of antibacterial, antifungal and virucidal activity. OCT is also newly used in tissue engineering. The aim of this work was to create a new nanocomposite consisting of OCT-grafted polymer with (i) antibacterial effect and/or (ii) surface for better cell adhesion and proliferation. The polymer foils were chemically activated with Piranha solution and subsequently grafted with OCT. Changes in surface properties before and after modifications were detected by electrokinetic analysis, goniometry, atomic force microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The final nanocomposite polymer/OCT exhibits antibacterial activity against Staphylococcus epidermidis (S. epidermidis). The new nanocomposite material has also been shown to support the growth of B14 cell culture on the substrate and to form cell multilayers, which could lead to the formation of spheroids. This behaviour strongly depends on the concentration of OCT grafted onto the polymer surfaces. This new nanocomposite could be used in medicine, for bioapplications, environmental protection.     

Quantifying the elasticity and viscosity of geometrically confined polymer films via thermal wrinkling

We apply thermal wrinkling, which is a surface instability that occurs during thermal annealing of polymer films geometrically confined by a rigid substrate and a flexible superstrate, to study the elasticity and viscosity of chemically crosslinked polymer systems. Specifically, we study the thermal wrinkling of aluminum‐capped polyhydroxystyrene films with different extent of chemical crosslinking and find that that the rate of change of the wrinkling wavelength with annealing time and temperature has unique relationships with the elasticity and viscosity of the polymer network. With the aid of analytical expressions that relate the time‐ and temperature‐dependent evolution of the wrinkle wavelength to the elasticity and viscosity, we are able to quantify the elastic modulus and shear viscosity of geometrically confined polymer thin films as a function of the degree of crosslinking. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Facile measurement of surface heat loss from polymer thin films via fluorescence thermometry

Quantitative determination of heat loss and transport within complex systems having inhomogeneous temperatures and several different components is important for applications ranging from electronics to solar cells. An approach and material system to study heat transport within and heat loss from polymer thin films is presented. In a thin film configuration with a cylindrical heating source, the theoretical solution for temperature as a function of radial position can be determined from fundamental principles. Use of embedded fluorescent molecules as temperature probes and manipulation of the relative location of heating and thermometry light sources allows experimental measurements of temperature versus position within the plane of the film. For a large range of practical cases, the exact theoretical solution can be well‐approximated by a single term, which enables a fit to experimental data, and subsequent determination of either the heat loss coefficient at the film's surface or the material's effective thermal conductivity. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 643–651     

成膜溶剂与氟化嵌段共聚物膜的表面富集行为

利用接触角、XPS、SFG、AFM等技术研究了环己酮、甲苯和三氟甲苯为成膜溶剂所得聚甲基丙烯酸甲酯-b-聚（甲基丙烯酸-2-全氟辛基乙酯）（PMMA—b—PFMA）嵌段共聚物膜的表面结构与性能．发现浇铸成膜时成膜溶剂对聚合物氟化组分向表面富集程度的影响相对较小,而旋涂成膜时溶剂的影响很大．不管以何种形式成膜,三氟甲苯溶剂最有利于氟化组分向表面富集,甲苯次之,环己酮最差．这一现象与溶剂的挥发速度无关．聚合物在溶液中的聚集结构、气／液界面结构是造成成膜方式对聚合物表面结构与性能产生巨大影响的主要原因．当聚合物在溶液中形成以PFMA为核、PMMA为冠的胶束结构时,在溶液固化过程中氟化组分向表面富集需要较长的时间,这时由于成膜方法直接影响溶液的固化速度,造成其对氟化组分向表面富集的程度影响很大．当聚合物在溶液中以单分子或松散聚集体存在,在溶液固化过程中氟化组分向表面富集的速度很快,这时成膜方法对氟化组分向表面富集的程度影响很小．以上结果无论对理论研究还是应用研究都具有重要意义．     

Application of inverse gas chromatography in the surface characterization of diethanol amine modified polystyrene based polymer

In the present work, diethanol amine modified polystyrene based polymer (PVBC-Diethanol amine) was synthesized and characterized, then surface properties of the polymer were examined by inverse gas chromatography method at infinite dilution. The retention diagrams obtained based on the interaction of polar and nonpolar probes with the polymer were drawn over a temperature range from 30 to 55 °C. Through the diagrams, the dispersive component of the surface free energy, g _S ^D of the polymer surface, and the specific enthalpy of adsorption, DH _A ^S , of probes on the polymer were also calculated. Lewis acid, K _A , and Lewis base, K _D , parameters of PVBC-Diethanol amine surface were determined. The values of K _A and K _D indicated that PVBC-Diethanol amine surface exhibited a basic behavior.     

A novel technique to measure the surface electrical potential of polymer films by using dielectric spectroscopy

A novel method that allows the determination of the electrical potential of a polymer surface has been applied for polypropylene film treated by N₂ + H₂ cold plasma. The plasma treatment results in formation on the film surface of NH₂ groups, which then gets transformed to NH₃⁺ in contact with an electrolyte and leads to the formation of an electrical double layer. The method consists of theoretical calculation of electrical model potential, using the measurement of the electrical capacitance of the film in contact with an electrolyte by dielectric spectroscopy. Comparison with the results obtained by theoretical model shows similar dependencies of the electrical potential as a function of amino‐groups density and electrolyte concentration, but systematic differences of absolute values are observed. Copyright © 2005 John Wiley & Sons, Ltd.     

Strong host-guest interaction enables facile and controllable surface modification of cucurbit[6]uril-based polymer nanocapsules for in vivo cancer targeting

Cucurbit[6]uril-based polymer nanocapsules (CB[6]PNs), covalently conjugated with a near-infrared dye (Cy 7), was exploited as a stable platform (Cy7-CB[6]PNs) for in vivo cancer targeting. The strong host-guest interaction between CB[6] and spermidine (spmd) enabled us to modify the surface of the nanocapsules with spmd-conjugated cancer targeting functional tags in a facile, controllable and non-covalent manner. Thanks to the strong host-guest interaction retained even in the bloodstream of a mouse, enhanced accumulation of Cy7-CB[6]PNs on cancer tissues of mice was successfully detected via NIR imaging when the nanocapsules were treated with more number of the cancer targeting functional tags. This result showed the potential of the CB[6]-based strong host-guest interaction as a controllable and efficient tool for surface modification of in vivo cancer targeting nanomaterials.     

Structural aspects of deformation of amorphous polymers

Experimental and theoretical data on the inelastic deformation of amorphous glassy polymers were analyzed. The decisive role of direct structural methods in determination of the deformation mechanism of glassy polymers was established. A new mechanism of deformation and thermally stimulated recovery of strained glassy polymers was considered on the basis of structural data analysis.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 1–6, January, 2005.     

Effect of surface modification on the properties of plasma-polymerized hexamethyldisiloxane thin films

Plasma-polymerized hexamethyldisiloxane (pp-HMDSO) thin films have been deposited in a radiofrequency (RF) remote plasma-enhanced chemical vapor deposition (PECVD) system, on different types of substrates: silicon wafers, glass, quartz crystals, and chemiresistor structure. The as-grown thin films have been post treated in two types of reactive plasmas produced in SF₆ and O₂ gases. The effect of this surface modification on different properties of the as-grown pp-HMDSO thin film (chemical structure, elemental composition, surface morphology, film density and thickness, optical bandgap, and electrical resistivity) has been investigated. It is found that SF₆ plasma and O₂ plasma surface modifications of the as-grown pp-HMDSO thin film induce property changes different from each other. SF₆ plasma converted the as-grown pp-HMDSO film to a more porous material and caused a narrowing of its optical band gap of about 33%, while O₂ plasma induced a lowering of film electrical resistivity of about two orders of magnitude.     

The stiffening of ultrathin polymer films in the rubbery regime: The relative contributions of membrane stress and surface tension

We have previously reported on a new nanobubble inflation technique we have developed to measure the absolute creep compliance of polymer films at thicknesses down to ∼10 nm (1–5). One of the more surprising results to come out of these studies was the observation that as the film thickness decreased the rubbery plateau compliance was seen to decrease dramatically. At the thinnest film thickness (∼10 nm) the compliance decreased (or conversely the stiffness increased) by some 3 orders of magnitude from the bulk value. It is known that, at the nanometer scale, surface tension effects can become significant. We examine here the contribution of surface tension to the observed stiffening and show that, while the contribution is important, it does not account for the degree of stiffening observed. This suggests that molecular or confinement induced stiffening is the major factor. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2441–2448, 2009