An infrared linear dichroism study of carboxylic acids oriented in stretched polyethylene

Polarized IR spectra of four carboxylic acids have been recorded using the stretched polymer method. Benzoic, propanoic and propynoic acids form cyclic dimers in polyethylene, while formic acid forms an α type polymer. The transition moment directions of 23 vibrations of benzoic acid dimer are given and the carboxylic ring in-plane vibrations of the studied acids are discussed in the light of the obtained results.     

Linear dichroism and fluorescence polarization of diphenyl polyenes in stretched polyethylene films

A study of the linear diehroism and fluorescence polarization of diphenylpolyenes (C₆H₅—(CH=CH)_n—C₆H₅) with n = 1,2,3,4,6,8 in stretched polyethylene films shows that the polyenes orient in the anisotropic matrix with their transition dipole (emission and absorption, π → π^*) aligned with the direction of stretch. The maximum dichroic ratio is observed for 1,6-diphenyl-1,3,5-hexatriene (DPH), with n = 3, whereas the values are substantially lower for n < 3 or n > 3. The high orientation of DPH and poor alignment of the higher polyenes may be accounted for in terms of the growing flexibility of the polyene chains with increasing chain length. The results justify the use of DPH as a convenient fluorescence probe for biological and synthetic membranes.     

Optical properties of stretched polymer dispersed liquid crystal films

The basic mechanisms determining the formation of optical anisotropy in stretched, thin polymer dispersed liquid crystal (PDLC) films with micron sized nematic droplets have been studied experimentally and the results analysed in terms of a proposed theoretical model. The experiments were performed on PDLC films with the bipolar nematic director configuration in the droplets, where the film transmittance, microscopic structure, and birefringence of the polymer matrix were studied. It is shown that the orientational ordering of bipolar nematic droplets, introducing the main contribution to the ability of stretched PDLC film to polarize the transmitted light, is strongly dependent upon initial droplet shape and the elastic properties of the polymer matrix. The 'anomalous' nematic director orientation is also observed in a portion of elongated droplets where the axes of bipolar configurations do not coincide with the major axes of the droplet cavities due to the presence of inclusions at the cavity walls. The effect of alternation of droplet size and shape upon stretching and the influence of optical anisotropy of the polymer matrix on film transmittance are analysed. On the basis of the results obtained, simple criteria for optimization of main PDLC polarizer performance are formulated.     

Infrared linear dichroism of the T1 state of acridine in stretched polyethylene

We report the first determination of the symmetry of an excited-state vibration observed with direct infrared absorption. We assign the excited triplet-state vibration of acridine at 751 cm^?1 to b₁ symmetry.     

A linear dichroism study of pyrromethene-, pyrromethenone- and bilatriene-abc-derivatives

The linear dichroism of several pyrromethene, pyrromethenone and bilatriene-abc derivatives embedded in a polyethylene film and oriented by stretching is investigated. On dissolving these pigments in the polymer matrix no changes in their conformations are observed, as compared to solutions in apolar solvents. Polarisation of the absorption bands of the pigments is deduced from the polarisation spectra. To enhance the solubility and orientation in the polymer, a pyrromethenone and a bilatriene-abc bearing a C₁₈-alkyl chain were synthesised for the first time.

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Zum Lineardichroismus von Pyrromethen-, Pyrromethenon- und Bilatrien-abc-Derivaten

Zusammenfassung Der Lineardichroismus einiger Pyrromethen-, Pyrromethenon- und Bilatrienabc-Derivate, die in eine Polyethylenfolie eingebettet vorliegen, wird untersucht. Bei der Lösung dieser Pigmente in der polymeren Matrix wird keine Veränderung der Konformation im Vergleich mit Lösungen in apolaren Lösungsmitteln beobachtet. Aus den Polarisationsspektren wird die Polarisation der Absorptionsbanden dieser Chromophore abgeleitet. Ein Pyrromethenon und ein Bilatrien-abc mit C₁₈-Alkylseitenkette wurden zur Verbesserung der Löslichkeit und Orientierung in der Polymerfolie erstmals dargestellt.

     

TEM-cross section investigations of plasma polymer silver composite films

Plasma polymer silver composite films were investigated by means of cross section transmission electron microscopy (XTEM). The silver is encapsulated in the form of small particles in a nearly homogeneous plasma polymer matrix. The shape and the size of the particles vary with the polymerization power density. At lower polymerization power density the silver particles appear almost spherical and a three-dimensional particle distribution can be found in the polymer matrix. However the shape of particles at higher power density is more elliptic and the particle distribution is two-dimensional. The different kinds of encapsulation can be interpreted as being due to the different densities and porosities of the plasma polymer matrix.     

Analysis of the visible-near-ultraviolet spectrum of 9-aminoacridine using dichroic spectra in stretched polymer films

We have achieved a deconvolution of the absorption spectrum of 9-aminoacridine dispersed in polyvinyl alcohol films into 11 separate harmonic vibronic progressions of which 5 are polarised along the long axis and 6 along the short axis of the dye. The correlation between the experimental electronic transitions in this range and the calculated (π^*, π) transitions is very satisfactory.     

Light transmission, linear dichroism and birefringence of nematic/polymer dispersions

A theoretical study of light transmission, linear dichroism and birefringence of partially ordered dispersions of bipolar nematic droplets in a polymer matrix is presented. The treatment rests on the single scattering approach to an ensemble of uncorrelated and noninteracting anisotropic particles. Theoretical evaluations of the extinction cross sections and phase functions are performed in the anomalous diffraction approximation. Four basic model systems are analysed: PDLC and NCAP films in an external electric field, and PDLC and NCAP films under uniaxial mechanical deformation. The calculated dependences of the generalized parameters characterizing the light transmission, dichroism and birefringence on the angle of incidence of the probing light, droplet size and shape, refractive indices of the LC and polymer, and parameters of the external field are presented graphically and discussed in detail.     

Heterogeneous drying of colloidal polymer films: dependence on added salt

Using magnetic resonance profiling coupled with dynamic light scattering, we have investigated the mechanisms leading to the formation of a partly coalesced surface layer, or "open skin", during film formation from waterborne polymer dispersions. We present the first use of the skewness of the distribution of free water as a model-free indicator of the spatial nonuniformity of drying. The skewness reaches a maximum at the same time at which a strong, static component, presumably originating from a skin at the film/air interface, appears in the light scattering data. Addition of salt to the dispersion increases both the skewness of the distribution of free water and the propensity for skin formation. Surprisingly, the drying is influenced not only by the concentration and valency of the ions in the salt but also by the particular ion. At intermediate particle densities, added salt strongly lowers the cooperative diffusion coefficient, Dcoop. When the particles reach close packing, Dcoop sharply increases. If the particles readily coalesce, the effects of the increased diffusivity will be counteracted, thereby inducing the formation of a skin. A modified Peclet number, Pe, using Dcoop, is proposed, so that the presence of salt is explicitly considered. This modified Pe is able to predict the nonuniformity in drying that leads to skin formation.     

Synthesis of metal/polymer colloidal composites by the tailored deposition of silver onto porous polymer microspheres

A new colloidal silver system is presented in which a fine colloidal silver is in situ deposited onto functionalized porous poly(ethylene glycol dimethacrylate) [poly(EGDMA)] microspheres. The effectiveness of the silver deposition has been investigated through an examination of the surface characteristics of poly(EGDMA) microspheres. The result reported in this study demonstrate that the control of the surface area and surface functionality (in this study, a hydroxyl group) of poly(EGDMA) microspheres is an important factor that practically determines the degree of deposition of colloidal silver. X‐ray analysis has shown that silver nanoparticles are dispersed evenly on inner and outer surfaces and have a face center cubic phase. Preservation testing has shown that silver‐containing poly(EGDMA) microspheres have powerful antibacterial properties and, therefore, have significant potential as new preservatives. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2551–2557, 2004     

Optical properties of plasma polymer films with encapsulated silver particles

Plasma polymer thin films with encapsulated small metal particles were prepared by simultaneous plasma polymerization and metal evaporation. Based on transmission electron microscope (TEM) micrographs, particle size and shape were analysed on films with continuously varying filling factor. Thermal annealing causes dramatic changes of the particle size and shape. The optical (UV, VIS, NIR) properties of the films were determined by the UV-absorption of the plasma polymer and by the plasma resonance absorption of the metal particles. The changes in the transmission spectra during thermal annealing were simulated with different effective medium theories. The calculated transmission spectra were fitted to the experimental spectra.     

A small-angle neutron scattering study of adsorbed polymer structure in concentrated colloidal dispersions

Poly(ethylene oxide) (PEO) adsorption on colloidal silica particles was studied by small-angle neutron scattering under the core-contrast-matching condition. The volume fraction profile of the adsorbed layer was derived by modeling the average layer scattering term. It was found that, with increasing colloid concentration, the adsorbed PEO layers collapse due to the repulsions between adsorbed layers on neighboring particles. At the same time, the correlation length in the adsorbed layer obtained by fitting the layer fluctuation scattering term was found to decrease, indicating that denser polymer layers are formed. These two observations are self-consistent.     

Relaxation behaviour of stretched polymer melts

The relaxation behaviour of stretched polymer melts is described with the aid of the Lodge-model, in which a finite deformation time is incorporated. The theoretical predictions, based on this model, are in fair agreement with experimental results, at least for relatively small deformations. A simple shift factor is introduced to fit the theoretical and experimental curves in the case of larger deformations.     

Composite spring model in stretched polymer knots

The deformation of a knotted polymer under a stretching force is studied by modeling the deformed knot as a composite spring system. Our results predict that the elastic modulus of a knotted polymer is larger when compared to an equal‐length linear chain. More complex knots are in general stiffer. The increase in stiffness of knots relative to the linear chain is also derived. Monte Carlo simulations are also performed to investigate the streching of polymer knots. Chain lengths up to N = 82 and prime knots 0₁ , 3₁ , 4₁ , 5₁ , 6₁ and 8₁ are considered. Segregation of the crossings into a small tight region of the knot structure at strong forces is observed. The increase in stiffness predicted by the composite spring model agree well with the simulation data. Our simulation data show that the scaling laws proposed by de Gennes and Pincus for a single linear chain under traction force still hold for the knotted type polymers.     

Interactions between colloidal particles in polymer solutions: A density functional theory study

We present a density functional theory study of colloidal interactions in a concentrated polymer solution. The colloids are modeled as hard spheres and polymers are modeled as freely jointed tangent hard sphere chains. Our theoretical results for the polymer-mediated mean force between two dilute colloids are compared with recent simulation data for this model. Theory is shown to be in good agreement with simulation. We compute the colloid-colloid potential of mean force and the second virial coefficient, and analyze the behavior of these quantities as a function of the polymer solution density, the polymer chain length, and the colloid/polymer bead size ratio.     

Orientation effects in biaxially stretched linear polyethylene

The literature on polymer morphology contains many studies of structural and orientation changes occurring upon uniaxial stretching of films and fibers, but it has only an occasional reference to such studies on biaxially stretched film. This paper presents data on a structural change due to simultaneous biaxial stretching up to 6 × 6 stretch ratio of quenched linear polyethylene slightly below the melting temperature. At low stretch ratios the b axis of the orthorhombic unit cell orients predominantly in the biaxial plane of stretching or film plane. At higher than 4 × 4 stretch ratio, a second crystal orientation appears which is a (110) orientation in the film plane. Differential scanning calorimetry scans show two melting peaks occurring concurrently with diffraction effects of two crystal orientations. The evidence for two populations of crystals differing in orientation are discussed in the light of current concepts of folded-chain lamellae and their fragmentation with elongation.     

Fabrication of superhydrophobic polymer films with hierarchical silver microbowl array structures

Flexible superhydrophobic polyvinyl alcohol (PVA) films with silver bowl-like array structure are fabricated based on the thermal evaporation with sphere monolayer as templates and the modification of 1H, 1H, 2H, 2H-perfluorodecanethiol on silver surface. The silver microbowl arrays were composed of silver nanoparticles with an average diameter size of ca. 10 nm. The polymer films exhibit excellent stability and remarkable superhydrophobicity with a high water contact angle (CA) of about 163° and a low sliding angle (SA) of less than 3°.     

Azulene: polarized absorption in stretched polymers and magnetic circular dichroism

Linear dichroic and magnetic circular dichroic spectroscopy is used to characterize the excited states of azulene. Comparison with crystal spectra underlines the necessity of using two-parameter equations for evaluations of stretched sheet spectra despite some previous claims to the contrary. The MCD spectrum reveals strong vibronic interactions in the second transition.The results are interpreted on the basis of Pariser—Parr—Pople (PPP) type calculations including some with extensive configuration interaction (CI). Calculated signs of the first three B terms spectrum agree with experimental signs irrespective of the details of the PPP model, and even the numerical values are in an order-of-magnitude agreement. Only the calculation with extensive CI gives agreement for signs of higher B terms. It suggest the possible presence of two excited states, but the present experimental evidence for these is insufficient. One of these new states would be best described as a predominantly doubly excited state.The origin of the B terms is traced to contributions due to magnetic mixing of individual zero-order excited states and the results permit determination of the absolute sense of magnetic dipole transition moments between excited states with respect to the sense of the vector product of the electronic dipole transition moments from the ground state to these excited states. A comparison is performed with B terms calculated using finite perturbation technique and gauge-invariant orbitals.     

Formation of ordered mesoporous films from in situ structure inversion of azo polymer colloidal arrays

This work shows that mesoporous polymeric films with spherical and elliptical pores can be obtained by in situ structure inversion of the azo polymer colloid arrays through selective interaction with solvent. The epoxy-based azo polymer contained both the pseudo-stilbene-type azo chromophores and the hydrophilic carboxyl groups. The colloidal spheres of the azo polymer were prepared by gradual hydrophobic aggregation of the polymeric chains in THF-H2O media, induced by a steady increase in the water content. Ordered 2D arrays of the hexagonally close-packed colloidal spheres were obtained by the vertical deposition method. After the solvent (THF) annealing, the ordered 2D arrays were directly transformed to mesoporous films through the sphere-pore inversion. Under the same condition, the 2D arrays composed of the ellipsoidal colloids, which were obtained by the irradiation of a polarized Ar+ laser beam on the colloidal sphere arrays, could be transformed to films with ordered elliptical pores. To our knowledge, this is the first example to demonstrate that mesoporous structures can be directly formed from the colloidal arrays of a homopolymer through structure inversion. This observation can shed new light on the nature of self-assembly processes and provide a feasible approach to fabricate mesoporous structures without the infiltration-removal step. By exploring the photoresponsive properties of the materials, mesoporous film with special pore structure and properties can be expected.