Modeling the interactions between light and crystallizing polymer during fast cooling

In this work, an experimental set-up able to quench thin polymer films whilst recording the sample thermal history as well as the overall and depolarized light intensities of a laser beam emerging from the sample is described. The interactions between the light beam and the crystallizing material have been modeled accounting for absorption and scattering phenomena. The proposed model was found to be able to reproduce the experimentally observed behavior of light intensities and it was validated by comparison with conventional DSC analysis. On the basis of this model, a method to obtain crystallinity evolution is proposed and applied to some fast cooling runs. The method was applied to quenching runs of an iPP carried out under cooling conditions more than one order of magnitude faster than those allowed for the DSC technique. The crystallization temperature dependence upon cooling rate was obtained from these runs in a cooling rate range never before explored. PACS 81.05.Lg; 81.70.Fy     

Optimal design for extending the lifetime of thin film luminescent solar concentrators

This study is concerned with the enhancement of the molecular, weathering stability of two models of thin film luminescent solar concentrators (LSCs). Two model systems of thin film LSCs had been proposed; the first model consists of a transparent PMMA/SiO₂ nanohybrid layer coated on a coumarin doped PMMA substrate. The second model designed as the ordinary configuration in which coumarin dye is dissolved in PMMA/SiO₂ nanohybrid layer then coated on a transparent PMMA substrate. The effect of nanosilica concentration on the prepared models was studied by TEM, SEM, DSC, FT-IR, UV–vis absorption and indoor photodegradation test. The obtained results suggested the first model for a durable design of thin film LSC applications specially in hot regions.     

Identification of polymer matrix yield stress in the wood cell wall based on micropillar compression and micromechanical modelling

Based on a combination of micropillar compression experiments and modelling of the secondary cell wall (cw) using continuum micromechanics, the shear yield stress of the polymer matrix is identified for both normal and compression wood of Norway spruce. It is shown that the model is able to capture the differences in mechanical properties between the two tissues based on the knowledge of composition of the samples, microfibril angle, as well as phase properties on the nanometer scale. By testing an isolated piece of the cell wall with a homogeneous and uniaxial stress field on the micrometer scale and using the micromechanical model to determine average stress fields on the nanometer scale, it is possible to identify the shear yield stress of the polymer matrix in wood, which was found to be in the range of 14.9–17.5 MPa for normal and compression wood. It was shown that this corresponds to a stress in the lignin phase of approx. 17 MPa. This combined study thus demonstrates a new approach for validating multiscale models predicting yield properties with uniaxial experiments at the microscale and measuring phase properties of inhomogeneous materials by a combination of modelling and experimental approaches.     

The timescale of spinodal dewetting at a polymer/polymer interface

We investigate the dynamics of spinodal dewetting in liquid-liquid polymer systems. Dewetting of poly(methyl-methacrylate) (PMMA) thin films on polystyrene (PS) “substrates” is followed in situ using neutron reflectivity. By following the development of roughness at the PS/PMMA interface and the PMMA surface we extract characteristic growth times for the dewetting process. These characteristic growth times are measured as a function of the molecular weight of the two polymers. By also carrying out experiments in the regime where the dynamics are independent of the PS molecular weight, we are able to use dewetting to probe the scaling of the PMMA thin film viscosity with temperature and molecular weight. We find that this scaling reflects bulk behaviour. However, absolute values are low compared to bulk viscosities, which we suggest may be due in part to slippage at the polymer/polymer interface. Received 25 June 2001 and Received in final form 5 December 2001     

Many-neighbour interaction and non-locality in traffic models

The optimal-velocity model, as proposed by Bando et al. [1], shows unrealistic values of the acceleration for various optimal-velocity functions [2,3]. We discuss different approaches of how to correct this problem. Multiple look-ahead (many-neighbour interaction) models are the most promising candidates in reducing accelerations and decelerations to realistic values. We focus on two such models and, in particular, their linear stability and how these affect the vehicle dynamics and wave solutions. As found earlier [4], multiple look-ahead models reproduce many real flow features, and our results further support the necessity of this ansatz. However, the problem of non-locality arises when they are transformed into the corresponding continuum model. We discuss three methods of how to interpret many-neighbour interaction in macroscopic models.Received: 27 March 2004, Published online: 12 July 2004PACS: 45.70.Vn Granular models of complex systems; traffic flow - 89.90. + n Other topics in areas of applied and interdisciplinary physics - 47.50. + d Non-Newtonian fluid flows     

Molecular dynamics simulation studies of the miscibility and thermal properties of PMMA/PS polymer blend

This investigation is an attempt to improve our understanding of the thermal properties of PMMA (Polymethyl methacrylate) by using PS(Polystyrene); the miscibility of PMMA/PS polymer blend is studied. Our work aims to study the impact of the percentage of PMMA/PS polymer blend on the simulated values of the glass transition temperature (Tg) using the dilatometric method. Compass was chosen as the force field (second category force field). The results reveal a single value of the glass transition temperature Tg that is found for all the curves of the PMMA/PS blend system (molar ratio: (50:50, 60:40, 54:46 and 80:20)); this could be a good criterion for predicting the miscibility. Additionally, the solubility parameters of PMMA and PS are calculated and used to obtain the Flory–Huggins parameter, and the morphology of our polymer blend is simulated using the dissipative particle dynamics method (DPD). Our results exhibit an increase in the Tg of PMMA whenever PS is added; hence, we can confirm the miscibility of the PMMA/PS polymer system.     

采用密度泛函理论与分子动力学对聚甲基丙烯酸甲酯双折射性的理论计算

双折射性是各种光学材料的重要性能之一,具有高双折射率的光学材料在诸多研究及工业领域的应用越来越广泛.然而,作为常用的光学薄膜及光波导材料之一的聚合物材料的双折射性通常却很弱,只能通过实验对其双折射率进行大致的表征,缺乏对其双折射率的系统性理论计算,从而限制了提高聚合物双折射性的研究.本文建立了从聚合物的单体分子结构到多分子链的系统性的双折射率理论计算方法,并借助此方法研究了导致聚合物弱双折射性的限制因素.以聚甲基丙烯酸甲酯(PMMA)为研究对象,运用密度泛函理论研究了其本征双折射率,这里的本征双折射率是指分子链完全取向时其单体单元的双折射率.计算结果表明其本征双折射率高达0.0738左右,并且通过计算给出了PMMA单体单元的平均双折射率色散曲线.采用分子动力学方法研究了该聚合物(包含20个分子链)的材料双折射率.理论计算结果表明,尽管该聚合物具有较大的本征双折射率,但是由于聚合物中分子链取向度极低,聚合物材料最终表现出来的双折射率只有0.00052.本文建立的研究方法及研究结果为研究增强聚合物材料双折射性提供了理论依据.     

Numerical simulation of flat-tip micro-indentation of glassy polymers: Influence of loading speed and thermodynamic state

Flat-tip micro-indentation tests were performed on quenched and annealed polymer glasses at various loading speeds. The results were analyzed using an elasto-viscoplastic constitutive model that captures the intrinsic deformation characteristics of a polymer glass: a strain-rate dependent yield stress, strain softening and strain hardening. The advantage of this model is that changes in yield stress due to physical aging are captured in a single parameter. The two materials studied (polycarbonate (PC) and poly(methyl methacrylate) (PMMA)) were both selected for the specific rate-dependence of the yield stress that they display at room temperature. Within the range of strain rates experimentally covered, the yield stress of PC increases linearly with the logarithm of strain rate, whereas, for PMMA, a characteristic change in slope can be observed at higher strain rates. We demonstrate that, given the proper definition of the viscosity function, the flat-tip indentation response at different indentation speeds can be described accurately for both materials. Moreover, it is shown that the model captures the mechanical response on the microscopic scale (indentation) as well as on the macroscopic scale with the same parameter set. This offers promising possibilities of extracting mechanical properties of polymer glasses directly from indentation experiments.     

A CONTIN-Based Approach Towards the Data Analysis of Particle Sizing Experiments by Centrifugal Photosedimentometry

In order to analyse the output data of centrifugal photosedimentation experiments using CONTIN software, a mathematical model has been derived. This model is based on Fick's second law and describes the sedimentation behaviour of monomodal particles in a centrifugal field: it predicts that the extinction at the detection point decays exponentially until the particles are sedimented past the detection point, which gives a sharp drop of the curves to the baseline. In practice, however, a more fluently decaying profile was obtained. Simulations revealed that the polydispersity of the sample was responsible for this behaviour. When centrifugal photosedimentation data for effective monomodal and bimodal samples of fairly monodispersed PMMA lattices were analysed with CONTIN, using the model proposed, accurate particle size distributions were obtained, provided that the sample cell was completely filled. This was imperative in order to minimize the deviations of the practical application as compared with the theoretical model.     

Shear-induced concentration fluctuations and form factor changes in polymer solution in the good-solvent regime

Small-Angle Scattering from sheared semidilute polymer solution is reported in the good-solvent regime, at variance with former light and neutron measurements in the regime. First, concentration fluctuations are observed: the scattering increases noticeably along the flow at low q, but at variance with former results for the theta-solvent regime, no demixing is observed at higher shear. Here, instead, the effects follow a time-temperature superposition and saturate above a Weissenberg number around 5, like the stress which is known to present a plateau for these systems. Using the Zero Average Contrast technique, we have also measured the form factor, which displays the same saturation effect reaching a deformation ratio of the order of 2. These results agree with the Convective Constraint Release models (CCR) elaborated in order to predict the stress effects in the non-Newtonian regime (Marrucci-Ianniruberto) and their extension predicting also the scattering (Likhtman-Milner-McLeish).Received: 5 July 2004, Published online: 1 October 2004PACS: 61.12.Ex Neutron scattering (including small-angle scattering) - 47.15.-x Laminar flow - 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling     

Preparation of Poly(vinylidene fluoride)/Poly(methyl methacrylate) Blend Microporous Membranes via the Thermally Induced Phase Separation Process

The thermally induced phase separation (TIPS) process was employed to prepare poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blend microporous membranes. The effect of PMMA content on the dynamic crystallization temperature of the PVDF/PMMA/sulfolane system was analyzed. The effects of PMMA weight fraction and cooling rate on the cross-sectional morphology, crystallinity, crystal structure, thermal stability, and porous structure of the resulting membranes were investigated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and a mercury porosimeter, respectively. The mechanical properties of the membranes were evaluated by tensile tests. It was found that solid–liquid phase separation occurred in the PVDF/PMMA/sulfolane system. Scanning electron microscopy revealed that either increasing PMMA weight fraction or decreasing cooling rate will lead to a macroscopical phase separation between PVDF and PMMA. PMMA weight fraction and cooling rate had some influence on the crystallinity, porous structure, and mechanical properties, but no influence on the polymer crystal structure of the membranes. PMMA weight fraction influenced thermal stability of the final membranes but cooling rate did not.     

Chemically selective soft X‐ray patterning of polymers

The chemically selective modification of polymer mixtures by monochromated soft X‐rays has been explored using the high‐brightness fine‐focused 50 nm beam of a scanning transmission X‐ray microscope. Four different polymer systems were examined: a polymethylmethacrylate (PMMA) polyacrylonitrile (PAN) bilayer film; a PMMA‐blend‐PAN microphase‐separated film; a poly(MMA‐co‐AN) copolymer film; and a poly(ethyl cyanoacrylate) homopolymer film. A high level of chemically selective modification was achieved for the PMMA/PAN bilayer; in particular, irradiation at 288.45 eV selectively removed the carbonyl group from PMMA while irradiation at 286.80 eV selectively reduced the nitrile group of PAN, even when these irradiations were carried out at the same (x,y) position of the sample. In the last two homogenous polymer systems, similar amounts of damage to the nitrile and carbonyl groups occurred during irradiation at either 286.80 or 288.45 eV. This is attributed to damage transfer between the C[triple‐bond]N and C=O groups mediated by primary electrons, secondary electrons or radical/ionic processes, aided by their close spatial proximity. Although the overall thickness of the bilayer sample at 70 nm is smaller than the lateral line spreading of 100 nm, the interface between the layers appears to effectively block the transport of energy, and hence damage, between the two layers. The origins of the line spreading in homogeneous phases and possible origins of the damage blocking effect of the interface are discussed. To demonstrate chemically selective patterning, high‐resolution multi‐wavelength patterns were created in the PMMA/PAN bilayer system.     

Dynamics of photoinduced second order nonlinearity in dimethylamino-nitrostilbene polymer thin films

All-optical encoding of macroscopic second order susceptibility in a dimethylamino-nitrostilbene (DANS) polymer thin film is demonstrated. We show that the main mechanism responsible for the induced nonlinearity in DANS polymer is the angular hole burning effect due to trans–cis isomerization as distinct from azo-polymers, in which considerable reorientation of trans-molecules occurs. The DANS sample exhibits better dark stability in comparison with azo-systems such as disperse red 1 polymethylmethacrylate (DR1–PMMA). The results are explained by the absence of thermal cis–trans relaxation in DANS–PMMA.     

Three-dimensional reversible laser micromachining with subnanojoule femtosecond pulses based on two-photon photochromism

Three-dimensional reversible laser micromachining of polymer materials based on two-photon photochromism is demonstrated. Unamplified 60-fs, 0.5-nJ pulses of 790-nm Ti:sapphire laser radiation are used to induce a refractive-index change in a polymethyl methacrylate (PMMA) sample doped with spiropyran molecules through a two-photon absorption process. Waveguides are written in the bulk of spiropyran-doped PMMA samples by scanning these samples with respect to a tightly focused Ti:sapphire laser beam. Laser-induced fluorescence is used for on-line monitoring of the laser-micromachining process. The structures written in photochromic samples can be erased and reconfigured due to the reversibility of the photochromic effect. Received: 23 April 2003 / Published online: 6 June 2003 RID="*" ID="*"Corresponding author. Fax: +7-095/939-51-74, E-mail: zheltikov@top.phys.msu.su     

Glass transition of the two distinct single-chain particles of poly(N-isopropylacrylamide)

Two distinct single-chain particles of poly(N-isopropylacrylamide) (PNIPAM) in the state of loose coil and compact globule, have been prepared successfully below and above the lower critical solution temperature (LCST) in extreme dilute aqueous solution by the freeze-drying method, respectively. During the preparation of the compact globular single-chain sample, the surfactant of sodium n-dodecyl sulfate (SDS) was added into the system to prevent aggregation of globular single chains formed at a temperature above the LCST. After all the coil has been transformed into the compact globular particle, the SDS molecules were removed by dialysis. The glass transition temperature (T_g) of the two single-chain samples has been measured by differential scanning calorimetery (DSC) in comparison with that of bulk polymer. It was found that the T_g of the single-chain sample in compact-globule state was very near to that of the bulk polymer, whereas the T_g of the single-chain sample in loose-coil state was approximately 6 K lower than that of the bulk polymer. After treating the sample with repeated DSC cycles, the T_g of the single-chain sample in loose-coil state rose up successively near to that of the bulk polymer. These results have been explained in terms of the effect of entanglement on the mobility of the polymer segments in the two distinct single-chain samples.     

Conductivity and thermal studies of blend polymer electrolytes based on PVAc–PMMA

The polymer electrolytes comprising blend of poly(vinyl acetate) (PVAc) and poly(methylmethacrylate) (PMMA) as a host polymer and LiClO₄ as a dopant are prepared by solution casting technique. The amorphous nature of the polymer–salt complex has been confirmed by XRD analysis. The DSC thermograms show two T_g's for PVAc–PMMA blend. A decrease in T_g with the LiClO₄ content reveals the increase of segmental motion. Conductance spectra results are found to obey the Jonscher's power law and the maximum dc conductivity value is found to be 1.76 × 10^− 3 S cm^− 1 at 303 K for the blend polymer complex with 20 wt.% LiClO₄, which is suitable for the Li rechargeable batteries. The conductivity–temperature plots are found to follow an Arrhenius nature. The dc conductivity is found to increase with increase of salt concentration in the blend polymer complexes.     

Preparation and characterization of Tb and Tb(sal)3·nH2O doped PC:PMMA blend

Tb doped polycarbonate:poly(methyl methacrylate) (Tb-PC:PMMA) blend was prepared with varying proportions of PC and PMMA. Thermal and spectroscopic properties of the doped polymer have been investigated employing Fourier Transform Infrared (FTIR) absorption and differential scanning calorimetric (DSC) techniques. PC:PMMA blend (with 10 wt% PC and 90 wt% PMMA) shows better miscibility. Optical properties of the dopant Tb³⁺ ions have been investigated using UV-vis absorption and fluorescence excited by 355 nm radiation. It is seen that luminescence intensity of Tb³⁺ ion depends on PC:PMMA ratio and on Tb³⁺ ion concentration. Concentration quenching is seen for TbCl₃·6H₂O concentration larger than 4 wt%. Addition of salicylic acid to the polymer blend increases the luminescence from Tb³⁺ ions. Luminescence decay curve analysis affirms the non-radiative energy transfer from salicylic acid to Tb³⁺ ions, which is identified as the reason behind this enhancement.     

The influence of UV-irradiation and support type on surface properties of poly(methyl methacrylate) thin films

Thin poly(methyl methacrylate) (PMMA) films were prepared by a solution casting on different supports (glass and aluminium plates with different gloss). UV-irradiation (λ = 254 nm) was used for polymer modification. Surface properties of PMMA were studied by contact angle measurements, attenuated total reflection infrared spectroscopy and optical microscopy. It was found that support type has no influence on surface properties of virgin PMMA, however, the changes in these properties were observed during UV modification of polymer film. The most efficient photochemical reactions appeared in sample placed on the rough Al, whereas the smallest effect was observed in polymer on the glass.     

Model for UV induced formation of gold nanoparticles in solid polymeric matrices

UV irradiation of polymeric PMMA films containing HAuCl₄ followed by annealing at 60-80 °C forms gold nanoparticles directly within the bulk material. The kinetics of nanoparticle formation was traced by extinction spectra of nanocomposite film changes vs annealing time. We propose that UV irradiation causes HAuCl₄ dissociation and thus provides a polymeric matrix with atomic gold. The presence of an oversaturated solid solution of atomic gold in the polymeric matrix leads to Au nanoparticle formation during annealing. This process can be understood as a phase transition of the first order. In this paper we apply several common kinetic models of the phase transition for describing Au nanoparticle formation inside the solid polymer matrix. We compare predictions of these models with the experimental data and show that these models cannot describe the process. We propose that the stabilization effect of the matrix on the growing gold nanoparticles is important. The simplest model introducing some probability for the transition from growing nanoparticle to the non-growing, stabilized form is suggested. It is shown that this model satisfactorily describes the experimentally observed evolution of the extinction spectrum of Au nanoparticles forming in a polymer matrix.     

Heterogeneous nature of the dynamics and glass transition in thin polymer films

Recent experiments have demonstrated that the dynamics in liquids close to and below the glass transition temperature is strongly heterogeneous, on the scale of a few nanometers. We use here a model proposed recently for explaining these features, and show that the heterogeneous nature of the dynamics has important consequences when considering the dynamics of thin films. We show how the dominant relaxation time in a thin film is changed as compared to the bulk, as a function of the thickness, the interaction energy with the substrate, and the temperature. The corresponding time scales cover the so-called VFT (or WLF) regime and vary between 10^-8 s to 10⁴ s typically. In the absence of interaction, our model allows for interpreting suspended films experiments, in the case of small polymers for which the data do not depend on the polymer weight. The interaction leads to an increase of for an interaction per monomer of the order of the thermal energy T. This increase saturates at the value corresponding to strongly interacting films for adsorption energies slightly larger and still of order T. In particular, we predict that the shift can be non-monotonous as a function of the film thickness, in the case of intermediate interaction strength.Received: 1 July 2004, Published online: 26 October 2004PACS: 64.70.Pf Glass transitions - 61.41. + e Polymers, elastomers, and plastics - 68.15. + e Liquid thin films