Polarized light‐induced photoisomerization in glassy poly(methyl methacrylate) and local relaxation processes of the polymer matrix

Photoisomerization kinetics of trans 4‐methoxystilbene induced by irradiation with linearly polarized light was investigated in the glassy state of poly(methyl methacrylate)(PMMA). The reaction was strongly selected by linearly polarized light as revealed by a large dichroic absorption generated upon irradiation. The optical anisotropy increases with increasing irradiation time and passes through a maximum before approaching an equilibrium value, which strongly depends on the experimental temperatures. From the decay of the total absorbance of the trans‐isomer observed upon irradiation with 313 nm UV light, it was found that the rates of the photoisomerization trans → cis were not significantly changed within the temperature range of the experiments. On the other hand, the reorientational relaxation times of the stilbene, which were directly measured by the annealing‐after‐irradiation techniques, depend strongly on temperature. These relaxation data were compared to the previous results obtained with dopants of the size larger than the stilbene and were discussed in conjunction with the local relaxation processes of the PMMA matrix. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 682–690, 2000     

Comparative study of the mechanical relaxation behavior of polymethacrylates containing different bulky side groups

The syntheses of poly(1,3‐dioxan‐5‐yl methacrylate), poly(cis‐2‐phenyl‐1,3‐dioxan‐5‐yl methacrylate), poly(trans‐2‐phenyl‐1,3‐dioxan‐5‐yl methacrylate), poly(cis‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate), and poly(trans‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) are reported. The mechanical relaxation spectrum of the simplest polymer, poly(1,3‐dioxan‐5‐yl methacrylate), exhibits a prominent β relaxation centered at ?98 °C, at 1 Hz, followed in increasing order of temperature by an ostensible glass–rubber relaxation process. In addition to the β relaxation, the loss curves of poly(trans‐2‐phenyl‐1,3‐dioxan‐5‐yl methacrylate) and poly(trans‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) display in the glassy state a high activation energy relaxation, named the β* process, that seems to be a precursor of the glass–rubber relaxation of these polymers. The mechanical spectra of poly(trans‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) and poly(cis‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) exhibit a low activation energy process in the low‐temperature side of the spectra, which is absent in the other polymers. The molecular origin of the mechanical activity of these polymers in the glassy state is discussed in qualitative terms. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1154–1162, 2002     

Investigation of polyimides containing naphthalene units. IV. Mechanism of naphthalisoimides formation and their isomerization to naphthalimides

During high-temperature condensation of six-membered naphthalene anhydrides with aromatic primary amines, at first, naphthalisoimides were formed, having cis or trans structures in dependence on absence or presence of acidic catalysts. Only the trans naphthalisoimides isomerized to the naphthalimides in the presence of basic catalysts during heating or UV irradiation. The mechanism of naphthalisoimides formation and their rearrangement to naphthalimides has been proposed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3523–3529, 1999     

Relaxation–structure relationship in bulk and plasticized polyamide 11

A series of blends of high molecular weight polyamide 11 (PA11) and three different polar molecules [i.e., N-butyl benzene sulfonamide (BBSA), δ-valerolactam, and ω-laurolactam] have been studied by differential scanning calorimetry (DSC), Fourier transform infrared microscopy (FTIR), differential mechanical thermal analysis (DMTA), and wide- and small-angle X-ray scattering (WAXS and SAXS) experiments. FTIR analysis shows that the concentration of free NH groups in bulk or plasticized PA11 is lower than 15% up to 260°C and less than 1% at room temperature. DMTA data show a β relaxation for dry PA11 but not for dry PA6 and PA12. On the other hand, when ω-laurolactam, which allows trans conformation of amide groups, is added to PA11 the intensity of the α relaxation increases and a strong antiplasticizing effect is observed. This effect is associated with a decrease of the PA11 free volume by increasing the chains packing. On the contrary, when δ-valerolactam, which allows cis amide groups conformations, and BBSA are blended with PA11, the α relaxation temperature and β peak intensity decrease as a function of added molecules concentration. This is associated with a plasticizing effect. It is suggested to attribute the bulk β relaxation to segmental motions involving H-bonded CONH in a cis conformation in amorphous domains of the PA11. In turn, the α relaxation is related to segmental motions involving CONH groups in a trans conformation. Therefore, antiplasticizing and plasticizing effects depend upon the ability of the additive molecule to change the initial conformational structure of polyamide (i.e., the ratio cis over trans H-bonding of the amide group conformations). Overall, a point of interest to note seems to be the difference between the trans and cis conformations of the amide groups in term of bonding with each other in a PA11 chain and additives such as BBSA. In addition, the limited influence of BBSA on the crystalline microstructure of PA11 is explained by the fact that 85% of the PA11 amorphous phase is intraspherulitic and that a great part of the plasticizer is located in these domains. © 1996 John Wiley & Sons, Inc.     

Cis-trans isomerization and 13C-NMR chemical shift of polyphenylacetylene

Before and after cis-trans isomerization, the observed ¹³C-NMR chemical shifts of poly(phenylacetylene) (PPA) in the solid state were investigated on the basis of ¹³C-NMR chemical shift calculations within AM1 for the cis-transoidal and deflected trans-transoidal forms. Two ¹³C-resonance peaks in the observed CP/MAS ¹³C-spectrum were assigned theoretically by the ¹³C chemical shifts of the main and side chains. After thermal isomerization, the ¹³C peak of the main chain for PPA shifted upfield by 3.5 ppm, in contrast to the downfield shift of the ¹³C peak for polyacetylene. This upfield shift of trans-PPA largely was attributed to the increases of the excitation energy from the ground state to the lowest φ_π–π* state in the paramagnetic terms of ¹³C chemical shift on the main chain carbons with the increase in deflected angle τ of 0 to 80°. The ±80° deflected conformation of the trans-transoidal chain due to the cis-trans isomerization was confirmed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1657–1664, 1999     

Molecular weight and configurational stability of poly[(fluorophenyl)acetylene]s prepared with metathesis and insertion catalysts

Series of high‐cis and cis/trans poly[(fluorophenyl)acetylene]s (PFPhA) have been prepared by polymerization of (2‐fluorophenyl)acetylene, (3‐fluorophenyl)acetylene, and (4‐fluorophenyl)acetylene with catalysts: [Rh(1,5‐cyclooctadiene) OCH₃]₂ (high‐cis PFPhAs) and tungsten(VI) oxychloride/tetraphenyltin (cis/trans PFPhAs). The molecular weight and configurational stability under various conditions at room temperature were studied for both PFPhAs series by means of size exclusion chromatography, ¹H‐NMR, and UV‐vis techniques. All samples exhibited slow degradation when exposed to the atmosphere in the solid state; the rate of degradation was independent on the F‐position on the Ph ring. The rate of degradation increased up to three orders of magnitude in the tetrahydrofuran solution where it was higher for high‐cis polymers compared with their cis/trans counterparts. The degradation of high‐cis PFPhAs was accompanied by significant cis‐to‐trans isomerization in aerated tetrahydrofuran solution. Rate of degradation and isomerization exhibited the same dependence on the F‐position on the Ph ring. The hypothesis was postulated that the degradation of high‐cis PFPhAs in solution was accelerated by cis‐to‐trans isomerization due to which the content of unpaired electrons on the main chains is enhanced. In both high‐cis and cis/trans series of polymers the ortho‐substituted isomers exhibited an enhanced stability compared with meta‐ and para‐substituted isomers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4296–4309, 2010     

Thermally stable,photoactive polymerizable diazenes. Applications in polymer technology

New and useful nonpolymerizable and polymerizable diazene free radical initiators have been designed, based on molecular orbital and empirical linear free energy calculations. The design of the polymerizable diazenes has been based on four requirements: independent reactivities of azo and vinyl moieties, high thermal stability of the trans-diazene isomers, efficient convertability of the trans isomer to the cis isomer, and high thermal reactivity (to produce free radicals) of the cis-diazene isomers. The cis-diazene isomers are obtained from the trans isomers by irradiation with visible light. A new free radical initiator has been shown to be useful in preparing graft copolymers and in crosslinking polymer chains by visible light activation. Poly(styrene-co-2-(meta-styrylazo)-2-methoxypropane) (2) was prepared by the copolymerization of styrene monomer and 2-(meta-styrylazo)-2-methoxypropane (1), either thermally or via a free radical initiator. Copolymer 2 then was reacted with methyl methacrylate monomer in the presence of visible light, to produce poly (styrene-g-methyl methacrylate) (6). Poly(styrene-co-2-(meta-styrylazo)-2-methylpropane) (17), prepared by the copolymerization of styrene monomer and 2-(meta-styrylazo)-2-methylpropane (15), was successfully crosslinked at elevated temperatures. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3203–3213, 1999     

Origin of the color of π‐conjugated columnar polymers: Poly(p‐n‐octylthiophenyl)acetylene prepared using a [Rh(norbornadiene)Cl]2 catalyst

Highly stereospecific polymerization of a novel sulfur containing aromatic acetylenes, that is, (p‐n‐octylthiophenyl)acetylene (pOctSPA), was successfully performed using the Rh complex, [Rh(norbornadiene)Cl]₂‐TEA, catalyst in the presence of various solvents under mild conditions. The resulting polymers were characterized in detail by ¹H NMR, ESR, laser Raman, diffuse reflective UV‐Vis (DRUV‐Vis), and wide angle X‐ray diffraction methods. The data showed that the resulting polymers bear cis‐transoid form, which can induce the cis‐to‐trans isomerization when the cis polymers are subjected to pressure at room temperature under vacuum, breaking rotationally the cis C?C bonds in the main‐chain giving two kinds of π‐radicals, the so‐called cis radical and trans radical as the origin of a polymer magnet like a novel spin glass material. Further, the resulting cis poly(acetylene)s were found to have a helical main‐chain, which is packed in pseudohexagonal crystal called π‐conjugated columnar or nano π‐conjugated columnar as a novel color controllable material. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2836–2850, 2005     

Siloxane polymers containing azo moieties synthesized by click chemistry for photo responsive and liquid crystalline applications

Three new types of siloxane‐based photoactive liquid crystalline polymers containing azo side groups were synthesized through the click chemistry route. The polymers having molecular weight range of 14,000–34,000 g mol^?1 were soluble in most of the polar solvents like chloroform, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, and dichloromethane. The photoresponsive trans–cis photoisomerization under UV radiation and cis–trans relaxation process in dark for the polymers were studied. The isomerization rate constants were found to be 0.01–0.04 sec^?1 and 1.16*10^?4–4.67*10^?4 sec^?1, respectively. It has been noted that the polymers showed high intensity absorption for n‐π* in chloroform. Both trans and cis forms of azide monomers having azo moiety exhibited molar extinction coefficient ( ? _max) in the range of 22,000–33,000 L mol^?1 cm^?1. The thermotropic behavior of the polymers was studied by polarizing optical microscope (POM) and differential scanning calorimetry (DSC) experiments. Polymer P1 showed liquid crystalline textures of nematic droplets, whereas P2 showed smectic focal conic texture and nematic droplets. Polymer P1 was also studied for photomechanical bending on exposure to UV radiation. The polymers showed initial degradation temperature in the range of 210–275°C. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Solid‐state NMR studies of cis‐1,4‐polyisoprene crosslinked with dicumyl peroxide in the presence of triallyl cyanurate

The network of dicumyl peroxide (DCP)/triallyl cyanurate (TAC) crosslinked cis‐1,4‐polyisoprene was studied by solid‐state NMR techniques such as direct‐polarization (DP), cross‐polarization (CP), and proton T₂ experiments. Line broadening and cis/trans isomerization of mobile carbons were observed in the DP experiments. The information on rigid carbons of network structures was observed with the CP technique. Motional heterogeneity was examined by proton T₂ relaxation experiments. Decreases in long T₂ (T_2L) values from the mobile non‐network structures and short T₂ (T_2S) values from the rigid network structures were observed with an increase in peroxide or coagent concentration. The percentage of T_2S in T₂ relaxation, which is related to network density, was observed to increase with peroxide and coagent addition. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1417–1423, 2000     

Optimization of the isocratic non-aqueous reverse phase (NARP) HPLC separation of trans/cis-α-and β-carotenes

Non-aqueous reverse phase chromatography was used to optimize the separation of all-trans-α- and β-carotenes and their cis isomers extracted from dried carrots. The technique can easily be applied to routine assays. Optimization was carried out by modifying the concentrations of methylene chloride and acetonitrile, and by studying the influence of temperature on the capacity factors and selectivity. The effect of the type of C 18 stationary phase bonding (mono- and polyfunctional) on separation was also examined. The need for polymeric bonding on the stationary phase is stressed for the realization of the trans/cis isomer separation.     

Unusually facil cis to trans isomerization of a polypropiolate bearing a long alkyl chain polymerized using a [Rh(norbornadiene)Cl]2 catalyst: An ESR study

Alkylpropiolate having n-hexadecyl moiety in the ester group was stereospecifically polymerized with a [Rh(norbornadiene)Cl]₂ complex catalyst in alcohol to give rise to cis-transoid polyacetylenes in high yields. The unusually facile cis to trans isomerization for the polymer was found to be induced when the polymer was warmed to even less than ca. 57°C. Under these conditions ESR spectra bearing hyperfine structures due to the radicals generated by the rotational scission in the cis–π bond were observed. This isomerization induced a large g value shift in the ESR spectra together with an increase of the radical concentration during the isomerization, indicating formation of planar conjugated trans sequences that stabilize mobile unpaired electrons as solitons. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2457–2461, 1998     

A visible light responsive azobenzene‐functionalized polymer: Synthesis,self‐assembly,and photoresponsive properties

A novel visible light responsive random copolymer consisting of hydrophobic azobenzene‐containing acrylate units and hydrophilic acrylic acid units has been prepared. The azobenzene molecule bearing methoxy groups at all four ortho positions is readily synthesized by one‐step conversion of diazotization. The as‐prepared polymer can self‐assemble into nanoparticles in water due to its amphiphilic nature. The tetra‐o‐methoxy‐substituted azobenzene‐functionalized polymer can exhibit the trans‐to‐cis photoswitching under the irradiation with green light of 520 nm and the cis‐to‐trans photoswitching under the irradiation with blue light of 420 nm in both solution and aggregate state. The morphologies of the self‐assembled nanoparticles are revealed by TEM and DLS. The controlled release of loaded molecules from the nanoparticles can be realized by adjusting pH value since the copolymer possesses pH responsive acrylic acid groups. The fluorescence of loaded Nile Red in the nanoparticles can be tuned upon the visible light irradiation. The reversible photoswitching of the azobenzene‐functionalized polymer under visible light may endow the polymer with wide applications without using ultraviolet light at all. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2768–2775     

Deformation induced evolution of mobility in PMMA

The stress relaxation response in the glassy state just below T_g was measured for poly(methylmethacrylate) following application of constant strain rate uniaxial tensile deformation at various locations on the stress–strain curve, including the yield and post‐yield region. The macroscopic mobility was determined from analysis of the relaxation response. Up to a factor of 3 decrease in relaxation time was observed with the fastest relaxation occurring in the post‐yield softening region. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

NMR signal assignment of cis/trans‐conformational segments in MEH‐CN‐PPV

Poly[2‐(2′‐ethylhexyloxy)‐5‐methoxy‐1,4‐phenylene‐(1‐cyanovinylene)] MEH‐CN‐PPV and its all‐trans model compound 1,4‐bis(α‐cyanostyryl)‐2‐(2‐ethylhexyloxy)‐5‐methyloxybenzene were synthesized via Knoevenagel condensation. All‐cis isomer and cis–trans isomer of 1,4‐bis(α‐cyanostyryl)‐2‐(2‐ethylhexyloxy)‐5‐methyloxybenzene were prepared by the photoisomerization reaction. Comparison of the ¹H NMR spectra between MEH‐CN‐PPV and three model compounds proved the occurrence of cis‐vinylene in the backbone of MEH‐CN‐PPV. According to the ratio between the cis‐vinylene signal and trans‐vinylene signal, the content of the cis‐vinylene could be estimated to be 15% in MEH‐CN‐PPV. This large cis‐vinylene content came from the rapid photochemical isomerization of cyanovinylene and was likely relative to the poor electroluminescence property of MEH‐CN‐PPV. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1105–1113, 2008     

Filler–polymer interactions in both silica and carbon black‐filled styrene–butadiene rubber compounds

Bound rubber in a filled rubber compound is formed by physical adsorption and chemisorption between the rubber and the filler. Styrene–butadiene rubber (SBR) is composed of four components of styrene, cis‐1,4‐, trans‐1,4‐, and 1,2‐units. Filler–polymer interactions in both silica and carbon black‐filled SBR compounds were studied by analyzing microstructures of the bound rubbers with pyrolysis‐gas chromatography. Differences in the filler–polymer interactions of the styrene, cis‐1,4‐, trans‐1,4‐, and 1,2‐units were investigated. The filler–polymer interactions of the butadiene units were found to be stronger than that of the styrene unit. The interactions of the cis‐1,4‐ and trans‐1,4‐units were stronger with carbon black than with silica, whereas the 1,2‐unit interacted more strongly with silica than with carbon black. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 439–445, 2001     

The thermal reaction of 2-pentene around 500°C at low extent of reaction

The thermal reaction of 2-pentene (cis or trans) has been performed in a static system over the temperature range of 470°–535°C at low extent of reaction and for initial pressures of 20–100 torr. The main products of decomposition are methane and 1,3-butadiene. Other minor primary products have been monitored: trans-2-pentene, trans- and cis-2-butenes, ethane, 1,3-pentadienes, 3-methyl-1-butene, propylene, 1-butene, hydrogen, ethylene, and 1-pentene. The initial orders of formation, 0.8–1.1 for most of the products and 1.5–1.8 for 1-pentene, increase with temperature. The formation of the products and the influence of temperature on their orders can be essentially explained by a free radical chain mechanism. But cis–trans or trans–cis isomerization and hydrogen elimination from cis-2-pentene certainly involve both molecular and free radical processes. The formation of 1-pentene mainly occurs from the abstraction of the hydrogen atom of 2-pentene by resonance stabilized free radicals (C₅H₉.).     

Proton NMR investigation of the local dynamics of PEO in PEO/PMMA blends

Longitudinal relaxation of proton magnetisation was used to characterize the molecular motions of PEO chains in compatible PEO (hydrogenated)/PMMA (deuterated) blends. Both the temperature and the PEO concentration, Φ, were varied. A maximum in the spin–lattice relaxation rate was observed and its properties were analyzed as a function of Φ. For Φ ≤ 0.50, the maximum is observed below the glass transition temperature of the blend; this shows that PEO chains dispersed in a matrix of PMMA remain highly mobile on a local scale even below T_g(Φ). A frequency–temperature correspondence procedure, applied to the measurements performed at two Larmor frequencies, 32 and 60 MHz, leads to a characteristic correlation time for PEO molecular motions. Its temperature dependence obeys a WLF free volume relation above the glass transition of the blends. The PEO free volume fraction and its thermal expansion are strongly reduced by the presence of the PMMA chains. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1095–1105, 1997     

Synthesis and photoisomerization of mesogenic 4-alkoxystyrylpyridines

Introduction of long-chain alkoxy substituents into molecules of styrylpyridines as potential ligands for photosensitive metal complexes endows them with liquid crystalline properties over a wide temperature range. trans-cis Photoisomerization of 4-alkoxystyrylpyridines in solution was revealed by NMR and electronic absorption spectroscopy, and the kinetics of transformations of their molecular forms after irradiation were estimated. Photoinduced trans→cis isomerization of 4-alkoxystyrylpyridines is characterized by a higher rate, and intramolecular cyclization of their cis isomers after irradiation is slower, as compared to unsubstituted analog.     

Photoswitchable Adhesives Using Azobenzene‐Containing Materials

Photoinduced reversible solid‐to‐liquid transitions of azobenzene‐containing materials can control adhesion. Photoswitchable adhesives based on azobenzene‐containing small molecules and polymers are under intense investigation. The melting points or glass transition temperatures of such azobenzene‐containing materials in trans and cis forms are above and below room temperature, respectively. Photoswitching of these materials results in reversible trans‐cis isomerization and solid‐to‐liquid transitions. The solid trans azobenzene‐containing materials have strong adhesion and the liquid cis azobenzene‐containing materials have weaker adhesion. In this Minireview, we introduce adhesives based on azobenzene‐containing small molecules and polymers. The remaining challenges and perspectives in the field of photoswitchable adhesives using azobenzene‐containing materials are also discussed.