A linear polymerized photopolymer orienting a nematic liquid crystal within nonextensive formalism

The distribution function of photogenerated cross-links in thin films of poly(vinyl cinnamate) or poly(vinyl 4-methoxy-cinnamate) is investigated within nonextensive statistics, in terms of the irradiation time of these films with linearly polarized UV light. The scalar order parameter is obtained from the generalized distribution function, and is compared to the measured birefringence for some values of the entropic index q. The exposure time dependence of the nematogen scalar order parameter is plotted, and the effect of the nonextensive formalism on its exposure time dependence is presented.     

Temperature‐Induced Pattern Transition in Crystallizing Ultrathin Polymer Films

The crystallization patterns of ultrathin poly(ethylene oxide)/poly(methyl methacrylate)(PEO/PMMA) blend films crystallized at different undercooling were investigated by atomic force microscopy. Dendrite pattern formed as a result of crystalline anisotropy at low undercooling and evolved into seaweed pattern with increasing undercooling. Although the configuration of macromolecules is far different from that of simple small molecules, the crystallization pattern transition in ultrathin polymer films can be interpreted by the classical morphology diagram developed on the basis of metals and simple molecules, indicating that polymer chains can be considered as simple dynamical units in a quasi‐two‐dimensional confined state.     

Characterization of unidirectional photopolymerization in poly(vinyl cinnamate) by surface optical second-harmonic generation

Surface optical second-harmonic generation measurements were used to determine the change in the surface density and the surface orientational order parameter of the trans-cinnamoyl side groups of unidirectionally photopolymerized poly(vinyl cinnamate) (PVCN) films. The observed surface features were compared to the bulk properties of the films, which were probed by their UV absorption spectra and birefringence measurements. We found that the surface and the bulk photoreaction kinetics of the trans cinnamoyl side groups of the PVCN are very similar. The two photoreaction processes (trans-cis photoisomerization and photo-induced cross linking) appear to affect the linear and nonlinear optical properties of the polymer differently. This may explain the discrepancy between the birefringence and the azimuthal anchoring force of the unidirectionally photopolymerized PVCN.     

Stretched exponential kinetics for photoinduced birefringence in azo dye doped PVA films

We fabricated azo dye (methylorange) doped poly vinyl alcohol (MO/PVA) thin films and measured the photoinduced birefringence (PIB) kinetics for several pump beam intensities and for various MO concentrations by using the pump-probe technique. A novel approach to explain the transient behaviors of the photoinduced anisotropy is presented by employing an empirical stretched exponential time response in the course of the trans-cis-trans photoisomerization of azo molecules and is compared with the experimental data, showing excellent agreement. The stretched exponent is estimated to be β = 0.34 ± 0.04, revealing amorphous nature of the MO/PVA system.     

Photoinduced Anisotropy in Poly(Methyl Methacrylate)/Sudan Red III Thin Films

The dynamic behavior of photoinduced optical anisotropy of amorphous PMMA/Sudan Red III thin films has been investigated using the pump-probe method. The results showed that the anisotropy increased rapidly in an exponential fashion in the first seconds of pumping, so that a photostationary state was reached as a result of the photoisomerization process cycles. The anisotropy relaxation phase decreased exponentially in a slow manner, which means the anisotropy did not disappear instantaneously when the irradiation was switched off. Evidence of the existence of the Angular Hole Burning (AHB) and the Angular Redistribution (AR) effects in the Sudan Red III molecules in the PMMA polymeric host is presented. The relative photoinduced optical anisotropy was calculated. The results showed that, at room temperature, the thermal isotropic relaxation of the photoisomer was negligible compared to the anisotropic reverse photoisomerization that occurred as soon as the pump beam was cut-off.     

偶氮苯衍生物的全息光存储和双折射特性研究

对6-（4-（（4-（十八烷氧基-）苯基）二氮烯基）苯氧基）-1-己醇（E-1-ol）的光致变色、双折射和全息光存储特性进行了研究.以488 nm的氩离子（Ar+）激光为抽运光,632.8 nm 的氦氖（He-Ne）激光为探测光,研究了透射信号强度随时间的变化关系;在10 mW~160 mW的范围内,研究了透射信号强度的最大值随抽运光功率的变化;经计算,在各抽运光功率下,获得的光致双折射值（δn）达10－2量级,最大激光功率下,获得的δn为2.8×10－2.对透射信号强度随时间的变化曲线进行了理论拟合,得出曲线的上升和下降符合双指数增长和衰减,时间响应常量分别是：上升阶段：t1=4.8 s,t2=85.8 s;下降阶段：t1=1.9 s,t2=27.6 s.用等强度s偏振的两束Ar+激光为写入光, He-Ne光为读出光,研究了衍射信号强度随时间的变化及写入光功率改变时,衍射信号强度最大值与写入光功率的变化关系;对衍射信号强度随时间变化的动力学过程进行了理论拟合,得出光栅生长和衰减也有快和慢两个过程,光栅生长时间响应常量为：t1=3.9 s,t2=33.9 s,光栅衰减时间响应常量为 t1=1.8 s,t2=60.1 s.     

Photoinduced Structural Changes in Poly(4-Vinyl Pyridine): A Luminescence Study

In the present work we show a way of controlling photoluminescence (PL) properties through photoinduced quasi-crystal formation in a system based on poly(4-vinyl pyridine) (P4VPy). Under UV irradiation at 380 nm, concentrated solutions of P4VPy in pyridine turn into gel. This phase transition results in changes in the optical properties of this polymer. The position of the PL maximum can be changed continuously from 440 to 480 nm during irradiation. After several minutes of UV irradiation a new red-shifted PL at 492 nm appears upon excitation by light of a wavelength corresponding to that of the initial PL maximum, which is also red-shifted during irradiation. Solutions of P4VPy in pyrimidine show similar behavior, but those in pyridazine do not exhibit such behavior. We have found that the reason for the observed changes in the electronic properties is a photoinduced directional ordering of polymer molecules in a special quasi-crystal formation. The process originates from a structural change in the side chain of P4VPy, namely, protonation of the polymeric pyridine after solvation. During irradiation, the polymeric pyridinium ion interacts with neutral polymeric pyridine molecules. Interchain interaction through hydrogen bonds lead to an electronic property change. We observed that the process of photoinduced sol-gel transformation is reversible. Mechanical perturbation or heating can convert the gel back to a fluid solution. The red-shifted PL is not observed, and the initial PL is blue-shifted to 450 nm and stays there.     

Electro-optical properties and photopolarization anisotropy of methacrylic azopolymer films

We have synthesized polymethacrylates with azobenzene side moieties and “spacers” of different lengths. We have studied the spectra and kinetics of the electro-optical effect appearing in films of such azopolymers after they are exposed to linearly polarized light. The nature of the electro-optical effect is explained by the appearance of photoinduced optical anisotropy in the films, due to a change in the ratio of the concentrations of trans and cis isomers of the azobenzene moieties. The characteristic time required for establishment and relaxation of photoinduced optical anisotropy correlates with the times required for a change in the dielectric characteristics of the studied films when exposed to linearly polarized light. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 2, pp. 307–310, March–April, 2009.     

偶氮苯介晶基元取向方式对其取向稳定性影响

根据取向透射率变化研究了偶氮苯侧链液晶聚合物在不同光照功率条件下的取向,用锥光干涉法表征了侧链介晶基元的取向方向,并研究了升温对取向膜的稳定性影响.实验结果表明介晶基元的取向速度和取向度都随光照时间和光照功率增加而增加|超过一定阈值功率（20 mW/cm2）时,随着光照时间延长薄膜的透射率（取向度）反而降低.高功率光照使侧链介晶基元发生了面内和面外两种取向,面外取向使聚合物膜的透射率降低.面外倾斜取向的介晶基元不稳定,在取向未达到饱和时停止光照,倾斜取向的介晶基元容易发生解取向,而面内取向的介晶基元则能维持取向的状态.用不饱和取向的方法,在光照功率为 20 mW/cm2时,取向时间缩短为饱和取向的1/20,薄膜的取向度提高1.3倍.4 ms的一次曝光足以使聚合物膜产生可读出、稳定的面内取向,取向的结果能保持2年不发生变化.     

Postexposure evolution of a photoinduced grating in a polymer material with phenanthrenequinone

The specific features of the postexposure evolution of photoinduced (holographic) gratings in a model medium that consists of poly(methyl methacrylate) with distributed phenanthrenequinone and is characterized by a diffusion enhancement are investigated using the holographic relaxation technique. It is established that the evolution process occurs in four stages controlled by different mechanisms: (i) initial nonmonotonic changes governed by the diffusion of an intermediate radical photoreaction product and its transformation into a stable product due to attachment to the macromolecules, (ii) hologram enhancement through the diffusion of phenanthrenequinone molecules, (iii) rapid partial degradation associated with the spatially confined motion of polymer chain segments, and (iv) slow degradation as a result of macromolecular diffusion. In the course of polymer relaxation after a temperature jump, the processes associated with the mobility of molecules are retarded and the depth of rapid degradation decreases. The effective diffusion coefficient characterizing the destruction of reflection gratings is smaller than that for transmission gratings. This can be explained by the spatial inhomogeneity of the polymer.     

Conversion of light energy into the electric potential in Langmuir-Schaefer organic molecular films

The conversion of light energy into the electric potential due to photoinduced directed electron transfer was studied in Langmuir-Schaefer (LS) films completely consisting of phthalocyanine-fullerene dyad molecules. It was shown that photovoltaic signal amplitudes in LS films continued to increase for ～150 µs after turning off the optical excitation pulse. The signal amplitudes and the separate-charge state lifetime significantly exceeded the similar parameters for Langmuir-Blodgett films formed from dyad molecules mixed with neutral matrix molecules. The relative sensitivity of new structures to exciting radiation increased by more than three orders of magnitude.     

Characterization of blend polymer PVA-PVP complexed with ammonium thiocyanate

Thin films of blend polymer electrolytes comprising poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) complexed with ammonium thiocyanate (NH₄SCN) salt in different compositions have been prepared by solution casting technique using distilled water as solvent. The prepared films have been investigated by different experimental techniques. The complexation of these films has been studied by FTIR spectroscopy. The increase in amorphousness of the films with increase in NH₄SCN content has been confirmed by XRD analysis. The addition of ammonium thiocyanate salt to PVA-PVP polymer blend shows a shift in T_g of the blend. The effect of salt concentration and temperature on the ionic conductivity of the polymer blend films has been analyzed using AC impedance spectroscopy. The maximum conductivity of 6.85 × 10^?4 S cm^?1 at room temperature has been observed for the blend with 50 mol% PVA-50 mol% PVP complexed with 40 mol% NH₄SCN. The activation energy has been found to be minimum (0.24 eV) for this sample. Wagner’s polarization technique shows that the charge transport in these blend films is predominantly due to ions. Using the highest conductivity blend polymer electrolyte, a proton battery has been fabricated and its discharge characteristics have been studied.     

Effect of electric field on the optical absorption of amorphous molecular semiconductors doped with polymethine dyes

The effect of a dc electric field on the evolution of the electronic absorption spectra of poly(N-epoxypropylcarbazole) (PEPC), poly(styrene) (PS), and poly(vinyl butyral) (PVB) films doped with methine dyes is investigated. It is revealed that these spectra in the absorption range of the dye undergo transformations depending on the dipole moment and the symmetry of the π-electron structure of dye molecules. The inference is made that the electron density in the dye molecule is redistributed in an external electric field. This redistribution gives rise to the electric field-induced anisotropy in the spatial distribution function of photogenerated electron-hole pairs in amorphous molecular semiconductors.     

Nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic liquid-crystalline polymer

The nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic liquid-crystalline polymer were investigated. The single beam Z-scan measurement showed the polymer film possessed a value of nonlinear refractive index n₂ = −1.07 × 10⁻⁹ cm²/W under a picosecond 532 nm excitation. Photoinduced anisotropy in the polymer was studied through dichroism and photoinduced birefringence. A photoinduced birefringence value Δn ∼ 10⁻² was achieved in the polymer film. The mechanism for the nonlinear optical response and the physical process of photoinduced anisotropy in the polymer were discussed.     

Photo-induced anisotropy in ZnO/PVA nanocomposites prepared by modified electrochemical method in PMA matrix

Substantial photo-induced optical anisotropy was discovered in ZnO/PVA nanocomposites under the influence of external bicolor laser illumination. Zinc oxide nanoparticles were synthesized by electrolysis of a sodium chloride aqueous medium including poly-methacrylic acid (PMA) in a cell system having a soluble zinc anode. The structural analysis of the ZnO powder samples has been carried out by X-Ray Diffractometry (XRD) and Scanning Electron Microscopy (SEM). The polyvinyl alcohol (PVA) embedded ZnO films obtained from the powder samples possess larger grain sizes than those in powder form. The films were prepared from the same polymer matrix but elaborated with two different PVA contents which are respectively 15% and 30%. The photoinduced anisotropy was identified by using two bicolor Er: glass laser beams incident at different angles. Substantial influence of the technological processes on the embedded nanoparticle sizes and related birefringence was explored. The process of laser induced anisotropy shows an occurrence of birefringence saturation.     

Photoinduced dichroism and birefringence in films of Mordant Pure Yellow/poly(vinyl alcohol): simultaneous real-time investigations at two wavelengths

Photoinduced anisotropy in films of azo-dye Mordant Pure Yellow (MPY) dissolved in a polyvinyl alcohol matrix, is investigated. The photodichroism and photoinduced birefringence are measured in real time for two wavelengths, 488 nm (in the absorption band) and 633 nm (outside it), on excitation with an Ar⁺ laser beam. It is shown that strong dichroism is induced without changing the average optical density of the film, and the kinetic curves of the birefringence and the dichroism are substantially different. The conclusion is made that the photoprocesses of the dye molecules cause a reconstruction of the matrix.     

Mussel‐Inspired Polymer Grafting on CsPbBr3 Perovskite Quantum Dots Enhancing the Environmental Stability

Mussel‐inspired chemistry, motivated by the adhesive proteins secreted by mussels for attachment to wet surfaces, is an emerging technique for manipulating the surface properties of a variety of materials. Here, a facile mussel‐inspired poly(N‐vinyl pyrrolidone) (PVP) polymer encapsulation with trivial influence on inherent optical properties of perovskite quantum dots (QDs) is demonstrated to efficiently combat the low intrinsic thermal, chemical, and photostability of CsPbBr₃ QDs. The suitability of photopolymer‐processed PVP‐CsPbBr₃ QDs flexible films as color components in white light–emitting devices is substantiated.     

不同基体刚性偶氮化合物光致光学各向异性及其唯象模拟

在不同刚性的聚合物基体聚甲基丙烯酸甲酯（PMMA）、聚乙烯咔唑（PVK）和聚苯乙烯（PS）薄膜基体中掺入相同质量分数（均为0．003）的分散红13（DRl3）偶氮染料。以线偏振连续Ar^＋激光（514nm）作为控制光,连续He-Ne激光（632．8nm）作为信号光考察了具有相同厚度的三种样品在相同控制光功率（15mW）下的光致双折射效应。实验发现刚性较大的聚乙烯咔唑基体样品的双折射效应最小,刚性最小的聚苯乙烯基体样品的双折射效应最大。在偶氮生色团光致异构的四能级模型基础上,建立一新的唯象的模型。在模型中引入描述聚合物基体刚性的参量s,通过数值计算描述了不同基体刚性参量s对偶氮生色团光致异构取向的影响。计算结果表明,基体的刚性越大,样品的光致双折射效应越小,定性地解释了实验现象。     

Photoinduced Birefringence in Poly(Methyl Methacrylate) Polymeric Thin Films Doped with Di-Azo Sudan G

Photoinduced birefringence in poly(methyl methacrylate) (PMMA), polymeric thin films doped with di-azo Sudan G was investigated. A pump-probe method was used to study the dynamic behavior of the birefringence. The mechanism for the inducement of birefringence is discussed in terms of the photoisomerization of the di-azo Sudan G molecules. The results showed that the dynamic evolution of the photoinduced birefringence was such that the average birefringence increased, saturated, and then decreased with respect to the laser pump intensity. Also, an exponential-like relationship was observed between both the saturation and the isotropic states of the averaged transmitted probe beam (expressing the residual anisotropy) and the birefringence on one hand, and the intensity amplitudes of the pumping powers on the other hand.     

Photoinduced linear and/or circular birefringences from light propagation through amorphous or smectic azopolymer films

The self-induced rotation of the azimuth of elliptically polarized light passing through birefringent azopolymer thin films is investigated. The experiments were carried out on thin films of the amorphous p(DR1M-co-MMA) and p(DR1M) azopolymer samples and of the p(6MAN) derivative in its glassy and liquid-crystalline phases. In fact, using various controlled input light ellipticities, linear birefringence (LB) and/or circular birefringence (CB) measurements were performed separately and in conjunction with polarization analyses of the transmitted pump beam. According to a general theoretical analysis based on Jones’ matrix formalism, it is thus shown that the induced rotation angle through the films depends mainly on the ellipticity of the input light, on the generated LB level and, to a lesser extent, on the CB photoinduced in the liquid-crystalline phase. In the latter case, it is concluded that irradiation with circularly polarized light does induce a chiral arrangement in the polymer film, although the photosensitive chromophores do not contain any optically active group. Received: 22 April 2002 / Revised version: 28 May 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-5/5684-8402; E-mail: csouri@morgane.lsmc.u-bordeaux.fr