General description of optical dichroic orientation factors for relating optical anisotropy of bulk polymer to orientation of structural units

The relationship between the optical anisotropy of high polymeric materials in bulk and the orientation of structural units within the materials was described in general by using several types of mean values of the orientation distribution function of three Eulerian angles, i.e., the orientation factors, under some assumptions about the symmetry of the function being applicable for the most of the industrial products. A newly defined biaxial orientation factor, F_θηⁱ = 〈sin² θ_j cos 2η_j〉, where θ_j and η_j are the polar and azimuthal angles of the jth axis within the structural unit with respect to the bulk axes, may relate the biaxial orientation of the structural units to the dichroic orientation factors, which are measurable optical anisotropic indices of the bulk materials. Some applications of the results to the birefringence and infrared and dye dichroism are also discussed.     

Investigation of molecular orientation distributions by polarized raman scattering and polarized fluorescence

It is assumed that the intensity of the vibrational Raman scattering from each of the structural units (molecular segments or crystallites) in an oriented polymer solid is determined by a symmetric second-rank tensor. The distribution of orientations of the principle axes of the tensors is expanded in a series of generalized spherical harmonics, Z_lmn(θ)e^?imψe^?in?, and it is shown that relations among the coefficients in this expansion can be obtained from suitable intensity measurements using polarized radiation. If the orientation of the tensor axes within the structural unit is known (in the general case, for several Raman lines), then the coefficients M_lmn of a similar expansion for axes fixed in the units can be obtained for l, m, and n even and l ≤ 4. The limitation to even m follows from the assumption that the solid has at least orthotropic statistical symmetry but the limitations on n and l arise from the nature of the Raman effect. Some simple special cases are considered and some orientation-independent intensity sums are derived. It is shown that, with the simplifying assumption usually made, the theory of the polarized fluorescence method for determining molecular orientations is a special case of the theory for the Raman method.     

Methods of description of orientation in polymers

The nonrandom orientational distribution of structural units, such as crystallites and chain segments, prevailing in an anisotropic bulk polymer sample can be represented fully by an orientation distribution function. Measurements of fluorescence polarization and wide-line NMR are, in principle, capable of yielding information on the moments of the distribution function up to the fourth order. This work presents the method of analysis required to determine these moments. For this purpose, the distribution function is expanded in a series of generalized spherical harmonies. The method is an extension of a similar technique previously proposed for analysis of x-ray diffraction data for determination of a complete crystallite orientation distribution function.     

General description of orientation factors in terms of expansion of orientation distribution function in a series of spherical harmonics

A mathematical representation of orientation distribution of structural units within the bulk polymer is given in terms of an expansion of the distribution function in a series of spherical harmonics. Each coefficient of the expanded series is discussed in general relation to the orientation factors, average degrees of orientation distribution, defined by several different authors independently. Several optical techniques to evaluate the orientation factors, the second and fourth moments of orientation distribution of crystalline and noncrystalline structural units from optical dichroic quantities, are discussed. Some graphical representations of the state of orientation are proposed, and the estimation of orientation distribution from the orientation factors of different orders is discussed quantitatively.     

Optical anisotropy of polyisobutylene: Strain birefringence

Strain birefringence measurements on crosslinked polyisobutylene (butyl rubber) confirm earlier work of Stein and Tobolsky on the linear polymer indicating the optical anisotropy to be much greater than should have been expected from the structural symmetry of the polyisobutylene (PIB) chain. The configuration–optical anisotropy parameter Δa for PIB at 25°C is 4.1(±0.1) × 10^?24 cm³, or about half the value for crosslinked polymethylene, both polymers being undiluted and amorphous. Swelling with cyclohexane, CCl₄, and CBrCl₃ lowers Δa to values of 3.8, 3.4, and 2.8 × 10^?24 cm³, respectively. Contributions from intermolecular correlations in the bulk polymer and from form anisotropy in the diluted systems are small, if not negligible. Temperature coefficients measured isometrically yield d In Δa/dT ≈ 0.2 × 10^?3 deg^?1. Both Δa and its temperature coefficient are much greater than calculated from rotational isomeric state theory assuming additivity of bond polarizabilities. The disparity (more than tenfold for Δa) cannot be relieved by any rational adjustment of the structural parameters. It is suggested that the severe crowding of groups in the PIB chain may affect the anisotropies of group polarizabilities.     

Noninvasive optical characterization of hydrogels

This paper presents results based on concentration quenching of fluorescence polarization (cqfp), which demonstrate the feasibility of using cqfp to measure changes in hydrogel-membrane hydration. This is accomplished by binding fluorescent molecules to the hydrogel matrix, and showing that the fluorescence polarization is a monotonic function of fluorophore concentration. Films based on crosslinked 2-hydroxyethyl methacrylate containing lissamine side groups (10⁻⁴-10⁻⁷ mol/l) were mounted in a special cell which provided an aqueous environment, and in which polarized fluorescence could be measured. An argon-ion laser provided polarized excitation at 488nm, and the polarized fluorescent emission was detected. The correlation between the fluorescence polarization and the bound dye concentration was found to correspond to the theoretically expected behavior.     

RESOLUTION OF THE FLUORESCENCE EXCITATION SPECTRUM OF INDOLE INTO THE 1La AND 1Lb EXCITATION BANDS*

Abstract— The fluorescence excitation spectrum and the excitation polarization spectrum of indole in propylene glycol were measured at — 58°C, after selecting by optical filters the emission originating from the ¹L_a electronic level. From the analysis of these spectra, the excitation spectrum was resolved into the ¹L_a and ¹L_a excitation bands. A similar resolution of the excitation spectrum of tryptophan is given. This method can also be applied to the resolution of the emission spectrum in cases of dual emission.     

Crystal orientation in a semicrystalline polymer in relation to deformation of polymer spherulites. IV. Crystal orientation mechanism of nylon-6 under uniaxial stretching

A model relating crystal orientation to the deformation of nylon-6 spherulites under uniaxial stretching is discussed in terms of the orientation distribution functions of reciprocal lattice vectors of crystal planes, such as the (002) and (200) planes. The distribution functions calculated from the model are compared with those obtained from x-ray diffraction experiments. It is found that the crystal a axis and, consequently, the direction of hydrogen bonds within the crystal (α modification) orient parallel to the lamellar axis in the undeformed state, and that the crystal orientation behavior of nylon-6 is much different from that of low-density polyethylene, being characterized by much smaller values of the reorientation parameters of crystallites within orienting lamellae. Moreover, small-angle light scattering for H_v and V_v polarization is also calculated on the basis of the spherulite deformation model by taking the nylon-6 crystal as having orthogonal–biaxial symmetry in optical anisotropy. It is concluded that the H_v scattering can be realized in terms of the proposed model for spherulite deformation by taking into account a considerable contribution of hydrogen bonds to the molecular polarizability, so as to make the polarizability along the crystal a axis larger than that along the b axis. In other words, this conclusion suggests positively birefringent spherulites in the nylon-6 samples studied.     

Influence of the rotational mobility of probe molecules on their orientation factors

The influence of a small rotational mobility on the measured orientation factors f₂ and f₄ (connected with the second and fourth moments of the orientation distribution function), of uniaxial probe molecules in uniaxial polymer systems is calculated with special regard to the fluorescence polarization method. A mean angle of rotation of about 10° within the time scale of the experiment, i.e., the fluorescence lifetime, affects f₄ considerably, while it has practically no influence on f₂. On the other hand, the knowledge of the true f₄ of the sample and some assumptions on the mobility—orientation correlation make it possible to evaluate the mean rotational angle θ from the measured f₄. Moreover, the measured f₂ value can be precisely corrected. The effect of the assumptions on the reliability of f₂ and θ is analyzed. Experimental results on two partially mobile probe molecules in drawn polyethylene are discussed.     

Studies of micro-brownian motion of a polymer chain by the fluorescence polarization method. I. Fluorescent conjugates of polyacrylamide

Polyacrylamide having a fluorescent residue at the chain end was prepared by polymerization of acrylamide in the presence of a fluorescent dye. The segmental motion of the chain end in dilute solution was studied by the fluorescence polarization method on the fluorescent polyacrylamide conjugates thus obtained. The linear relation between 1/p and T/η₀ held for every sample studied in aqueous media, where p is the degree of polarization of the fluorescence, T is the absolute temperature, and η₀ is the viscosity of the medium. The mean relaxation time 〈ρ〉 of the conjugate was evaluated from these data as a function of the molecular weight of the conjugate. The value of 〈ρ〉 increased slightly with molecular weight, varying from 3.3 × 10^?9 to 7 × 10^?9 sec. The absolute values of 〈ρ〉 and its molecular weight dependence suggest that 〈ρ〉 represents the mean rotational relaxation time for the cooperative motion of about ten monomeric units at the chain end. The effect of the mean extension of polymer chain on the segmental motion was found to be negligible.     

Development of high-resolution optical scanning fluorescence microscopy

Instrumentation of a high-resolution optical scanning fluorescence microscope is presented. This instrument provides a fluorescent image of the structural surface of various kinds of thin membrane samples with high resolution on the molecular scale. For this instrument, a new excitation optical tip and a three-dimensional scanning unit have been designed. The optical tip consists of a sharpened optical fibre covered with evaporated aluminium in vacuo, on which the top has a pinhole. The optical tip is mounted in the centre of a crosslinked piezoelectric scanning head. On thez-axis, a piezoelectric positioner is equipped with a differential micrometer mechanism. All operations are controlled with a micro-computer system.The scanning area in this instrument is 4000*4000 nm₂ for measurements of biological specimens. Several optical scanning microscopic images as well as the STM image are presented.     

Synthesis and Molecular Properties of Acceptor‐Substituted Squaraine Dyes

A broad series of more than 20 acceptor‐substituted squaraines was synthesized that feature different acceptor functionalities at the central squaraine four‐membered ring. The influence of these acceptor units on the reactivity of semisquaraine precursors and stability of the respective squaraines were explored. Thereby the dicyanovinyl group was found to be the most versatile acceptor group that enabled various modifications at the donor moiety of the squaraine scaffold, leading to an extended series of dicyanovinyl‐functionalized squaraines. The variation of donor units afforded a set of NIR fluorophores that cover a wavelength region from the visible at about 650 nm far into the NIR up to 920 nm with fluorescence quantum yields between 0.93 and 0.11 and outstanding optical brightness. This excellent optical property is related to a rigid molecular scaffold that is fixed in an all‐cis configuration by the additional dicyanovinyl acceptor unit. The change of the molecular symmetry from C_2h to C_2v upon functionalization of the squaraine core with dicyanovinyl acceptor group has been confirmed in solution by electro‐optical absorption (EOA) spectroscopy, revealing permanent ground‐state dipole moments μ_g in the range between 4.3 and 6.4 D. These dipole moments direct an antiparallel packing of the molecules in the solid state according to single‐crystal X‐ray analyses achieved for four dicyanovinyl‐functionalized squaraines. The structural properties, the EOA results, as well as the band shapes of the optical spectra indicate that these polymethine dyes are cyanine‐type chromophores. It is worth noting that the orientation of the dipole moment vectors is orthogonal to the orientation of the transition dipole moment vectors, which is an uncommon but characteristic feature of this rather novel class of polymethine dyes. With regard to applications of these dyes in organic solar cells, their redox properties were also studied by cyclic voltammetry.     

Synthesis and properties of through‐space conjugated polymers based on cyano‐substituted poly(p‐arylenevinylene)s

New through‐space cyano‐substituted poly(p‐arylenevinylene)s containing a [2.2]paracyclophane unit were synthesized by the Knoevenagel reaction. Polymers 5 and 7 have cyano groups at α‐positions and β‐positions from the dialkoxyphenylene unit, respectively. Their optical and electrochemical behaviors were investigated in detail in comparison with their model compounds. Polymers 5 and 7 exhibited through‐space conjugation via the cyclophane units. Polymer 5 showed greenish blue emission (λ_max = 477 nm) in diluted solution with fluorescence quantum efficiency (?_F) of only 0.007, whereas polymer 7 emitted in the bluish green region (λ_max = 510 nm) with ?_F of 0.32. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5979–5988, 2009     

Synthesis and optical properties of fluorene‐based polymers incorporating organosilicon unit

The synthesis and optical properties of polymers bearing the repeating unit of terfluorene and various organosilicon groups were investigated. Polymers with high molecular weight and good solubility could be obtained by Suzuki coupling polymerization from silylene‐containing fluorene‐based dibromo monomers and 9,9‐dihexylfluorene‐2,7‐bis(trimethyleneborate). From UV spectra of polymers bearing acyclic silylene bridge, the organosilicon units not only interrupted a π‐conjugation but also contributed to an electronic communication between connected fluorenes. The emission maximum wavelengths (ca. 400 nm) blue‐shifted when compared with that of polyfluorene (418 nm) and the fluorescence quantum yields were considerably high (>0.82) in the CHCl₃ solution. On the other hand, rather broad emission was observed at 480 nm and the fluorescence quantum yield was quite low (0.004) in the solution‐state PL spectrum of tetraphenylsilole‐containing polymer. The polymer emitted visible green light in the spin‐coated film. The fluorescence peak intensity at 486 nm gradually decreased when the film was illuminated with the UV light of 359 nm in air. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4786–4794, 2007     

Synthesis of π‐conjugated polymer containing both thiophene and phosphole sulfide in the main chain by simultaneous post‐element transformation of organotitanium polymer

The synthesis and unique optoelectronic features of a π‐conjugated polymer containing both thiophene and 1‐phenylphosphole sulfide units (multiple heteroles) in the main chain by the post‐element transformation of a regioregular organometallic polymer possessing titanacyclopentadiene‐2,5‐diyl unit are described. The π‐conjugated polymer containing multiple heteroles was obtained in 73% yield by the simultaneous reaction of the organotitanium polymer with sulfur monochloride and dichlorophenylphosphine (0.6 equiv each), whose number‐average molecular weight (M_n) and the molecular‐weight distribution (M_w/M_n) were estimated to be 11,000 and 3.4, respectively, by the size exclusion chromatography (SEC). The π‐conjugated polymer thus obtained was found to have the high HOMO and the low LUMO energy levels due to the electron‐rich thiophene and electron‐deficient phosphole sulfide units, respectively, as supported by its cyclic voltammetry (CV) analysis. Compared to a mixture of a polymer containing sole thiophene‐unit and that containing sole phosphole sulfide units, the π‐conjugated polymer‐containing multiple heteroles proved to exhibit interesting optical properties. For example, a specific emission peak was observed at 608 nm in the photoluminescence spectrum, which was not observed in the case of the thiophene‐containing polymer, the phosphole‐containing polymer, and their mixture. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2519–2525     

Fluorophores as Optical Sensors for Local Forces

The main aim of this study is to investigate correlations between the impact of an external mechanical force on the molecular framework of fluorophores and the resultant changes in their fluorescence properties. Taking into account previous theoretical studies, we designed a suitable custom‐tailored oligoparaphenylenevinylene derivative (OPV5) with a twisted molecular backbone. Thin foils made of PVC doped with 100 nM OPV were prepared. By applying uniaxial force, the foils were stretched and three major optical effects were observed simultaneously. First, the fluorescence anisotropy increased, which indicates a reorientation of the fluorophores within the matrix. Second, the fluorescence lifetime decreased by approximately 2.5 % (25 ps). Finally, we observed an increase in the emission energy of about 0.2 % (corresponding to a blue‐shift of 1.2 nm). In addition, analogous measurements with Rhodamine 123 as an inert reference dye showed only minor effects, which can be attributed to matrix effects due to refractive index changes. To relate the observed spectroscopic changes to the underlying changes in molecular properties, quantum‐chemical calculations were also performed. Semiempirical methods had to be used because of the size of the OPV5 chromophore. Two conformers of OPV5 (C₂ and C_i symmetry) were considered and both gave very similar results. Both the observed blue‐shift of fluorescence and the reduced lifetime of OPV5 under tensile stress are consistent with the results of the semiempirical calculations. Our study proves the feasibility of fluorescence‐based local force probes for polymers under tension. Improved optical sensors of this type should in principle be able to monitor local mechanical stress in transparent samples down to the single‐molecule level, which harbors promising applications in polymer science and nanotechnology.     

Tunable fluorescence conjugated copolymers consisting of tetraphenylethylene and fluorene units: From aggregation‐induced emission enhancement to dual‐channel fluorescence response

A series of new conjugated polymers PTPE_xF_y, which consist of tetraphenylethylene (TPE) units and fluorene (F) units, have been designed and synthesized by Suzuki cross‐coupling polymerization. The polymers PTPE_xF_y exhibited aggregation‐induced emission enhancement and dual‐channel fluorescence response (DCFR) when they were aggregated in solution, and these properties are related with their TPE‐to‐F ratio in the polymer backbone. For PTPE and PTPE_0.5F_0.5 , the fluorescence emission was enhanced by aggregation when water was added into their THF solutions. For the copolymers PTPE_0.3F_0.7 , PTPE_0.2F_0.8 , and PTPE_0.1F_0.9 , the DCFRs were observed when they were aggregated by adding water into their solution, which can be attributed to the different responses of fluorene segments and TPE segments to aggregation. The fluorene segments have an aggregation‐caused quenching characteristic, whereas the TPE segments have an aggregation‐induced emission characteristic. According to the fluorescence lifetime and quantum yield data of the polymer solutions, we have discovered that the polymer's natural life time increases as its TPE content increases. In the solid film, PTPE_0.3F_0.7 and PTPE_0.2F_0.8 showed better quantum yield than other polymers, due to the combination of the excellent fluorescent property of the fluorene groups and the nonplanar conformation of the TPE groups. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Polymerization of allyl esters of unsaturated acids. I. Cyclopolymerization of methyl allyl maleate

Radical polymerization of methyl allyl maleate is kinetically discussed in terms of cyclopolymerization using 2,2′-azobisisobutyronitrile (AIBN) as an initiator and benzene as a solvent at 60°C. The ratios of the rate constants of the unimolecular cyclization reaction to those of the bimolecular propagation reaction of the uncyclized allyl and vinyl radicals, K_A and K_V, are estimated to be 9.7 and 1.35 mole/liter by fitting the kinetic equations obtained here to the dependence of the degree of cyclization on monomer concentration, respectively; the large difference between K_A and K_V is also discussed in detail. On the basis of these results the formation mode and the sequence distribution of the structural units of the polymer produced are discussed in detail; thus, for the polymer obtained in the bulk polymerization, about 90% of the cyclic structures can be formed via the intramolecular attack of uncyclized allyl radical on maleic double bond and the probability of succession of the cyclic structural units in diad sequence is exemplified as 0.27.     

Controlled synthesis and fluorescent properties of poly(9-(4-vinylbenzyl)-9H-carbazole) via nitroxide-mediated living free-radical polymerization

The functional polymer containing carbazole unit, [(poly(9-(4-vinylbenzyl)-9H-carbazole) (PVBK)], was successfully prepared via nitroxide-mediated living free-radical polymerization of 9-(4-vinylbenzyl)-9H-carbazole (VBK). The controlled features of the polymerization were confirmed by the linear increase in the molecular weight with the monomer conversion while keeping the relative narrow molecular weight distribution (M_w/M_n ? 1.51), and successful chain extension with styrene. The resulting polymer absorbed light in the 305-355 nm range and exhibited fluorescent emission at 350 nm. The fluorescent intensity of the polymer was lower than that of monomer and was affected by the properties of the different solvents, which decreased in the following order: DMF > THF > CHCl₃ at the same concentration of carbazole units. The fluorescence intensity of the polymer was apparently influenced by chromophore concentration, and the maximum value was obtained when the carbazole unit concentration was around 8 × 10⁻⁵ mol/L. Moreover, it was shown that the strong fluorescence of PVBK can be quenched by methyl acrylate (MA).     

An Atomistic Modeling and Quantum Mechanical Approach to the Hydrolytic Degradation of Aliphatic Polyesters

This paper reports computational simulations at two different scales employed to investigate the hydrolytic degradation of two homopolyesters: polyglycolide, PGA and poly(L-lactide), PLLA. Atomistic bulk models were used to investigate the dry and various hydrated states of the two systems. In addition, the first moments of contact between the polymers and water were studied employing atomistic interface models. A higher affinity of water to polyglycolide in comparison with poly(L-lactide) was observed, while diffusion of water was found to be lower in the first polymer. Quantum chemical calculations for the first step of the water-assisted hydrolysis revealed a higher resistance to hydrolytical scission of the L-lactyl units in comparison to glycolyl units.