Azo Content Research Articles

Methacrylic azopolymers for holographic storage: A comparison among different polymer types

The photoinduced anisotropy and volume holographic storage in a series of polymers with different architectures and azo contents of 7% and 20% in weight have been investigated. Measurements of the birefringence (Δ n) induced with linearly polarised 488 nm light show that for polymers with an azo content of 20%, the highest and more stable Δ n values are achieved in copolymers with a di-block architecture. The Δ n induced in polymers with 7 wt% azo content is not stable. Volume holographic polarisation gratings have been stored in the polymers. Only in the case of the di-block copolymer with an azo content of 20% in weight, the stored gratings are stable. A diffraction efficiency higher than 90% and an angular selectivity of 4° have been achieved in gratings with a period of 2 μm. It can be concluded that the di-block copolymer with 20 wt% azo content is the most suitable, among the studied copolymers, as volume holographic storage material.

Synthesis, characterization and photoinduction of optical anisotropy in liquid crystalline diblock azo‐copolymers

AbstractDiblock copolymers with polymethyl methacrylate and side chain liquid crystalline (LC) azopolymethacrylate blocks were synthesized by atom transfer radical polymerization (ATRP). The azobenzene content in these copolymers ranges from 52 to 7 wt %. For an azo content down to 20% they exhibit a LC behavior similar to that of the azo homopolymers. Thin films of these copolymers were characterized by transmission electron microscopy (TEM). A lamellar nanostructure was observed for azo content down to 20 wt %, while no structure is observed for the copolymer with a 7% azo content. The optical anisotropy induced in these films by illumination with linearly polarized 488 nm light was studied and the results compared with those of the azo homopolymer and of a random copolymer with a similar composition. The formation of azo aggregates inside the azo blocks is strongly reduced in going from the homopolymer to the copolymers. Photoinduced azo orientation perpendicular to the 488 nm light polarization was found in all the polymers. The orientational order parameter is very similar in the homopolymer and in the block copolymers with an azo content down to 20 wt %, while it is much lower in the random copolymer and in the 7 wt %. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1899–1910, 2007

Conformation of Azobenzene‐Modified Poly(α‐L‐Glutamate) (AZOPLGA) in Thin Films: Solid State NMR Studies#

The conformations of azobenzene‐modified poly(α‐L‐glutamate)s (AZOPLGA) with a different degree of functionalization were examined by solid state 13C NMR. The polymer main chain conformations in AZOPLGA powders (precipitated from reaction system) changes from α‐helix to β‐sheet when the degree of functionalization increases from 12% to 56%. In addition, the solvent used for fabricating films plays an important role in organizing AZOPLGA backbones into characteristic conformation. For AZOPLGA56 (AZOPLGA with 56% of functionalization) cast films, the polymer backbones can assume conformations ranging from order state (β‐sheet) to random coil by changing the solvent for fabrication. In contrast, the effect of solvent on the conformation of AZOPLGA23 (AZOPLGA with 23% of functionalization) is not so significant. When compared with AZOPLGA23 powder (precipitated from reaction system), the helical conformation increases for AZOPLGA23 film cast from TFA. However, the fractions of α‐helix and β‐sheet conformation in AZOPLGA23 films (cast from DMF or pyridine) are nearly identical to that of AZOPLGA23 power. Moreover, even though the polymer backbones are random coil in AZOPLGA56 films when cast from TFA, some locally ordered domain can be observed. Lastly, the effect of the azo content appears to play a dominant role over the effect of solvents in directing the conformation of these polymers. #Dedicated to the Memory of Professor Sukant K. Tripathy.

Photoinduced Birefringence and Surface Relief Gratings in Polyurethane Elastomers with Azobenzene Chromophore in the Hard Segment

A series of polyurethane elastomers containing aromatic azobenzene chromophore in the hard segment was synthesized by a one-step method. AFM surface study showed that particles on the order of 100 nm to several micrometers appeared after annealing the polymer films. Birefringence was induced in these polymer elastomers. The stability of the photoinduced birefringence could be improved significantly by annealing the polymer films. This also induced microphase separation. The effects of azo contents and molecular weight of soft segments were investigated. Dynamic study shows that photoinducing rate decreased while relaxation rate increased after annealing. Surface relief gratings could be induced only on the pure polyurethane and the elastomer containing 75% bis(azo)−diol. Only a reversible volume hologram was observed for the other elastomers.

Optical anisotropy and non-linear optical properties of azobenzene methacrylic polymers

Photoinduced optical anisotropy has been studied in several methacrylic copolymers, either amorphous or liquid crystalline, with cyano azobenzene chromophores in the side chain. Illumination with linearly polarised 488 nm light gives rise to high and stable values of birefringence and dichroism, mainly in liquid crystalline films. Both dichroism and birefringence decrease with the azo content in the copolymer series while some increase can be achieved by the incorporation of biphenyl molecules to the copolymers. Photoinduced anisotropy disappears above the glass transition temperature in amorphous polymers, whereas it increases in liquid crystalline polymers (LCP) due to a thermotropic self-organisation. The non-linear optical (NLO) properties of the films have been studied by second harmonic generation (SHG) measurements. The intensity of the harmonic signal from in situ corona poled polymeric films has been measured. The effect of 488 nm light irradiation on the azo chromophores orientation and consequently on the NLO response of the films was investigated at different temperatures. The intensity and thermal stability of the second harmonic signals obtained after the two different (thermal and photoassisted) corona poling processes have been compared.

Optical anisotropy and non-linear optical properties of azobenzene methacrylic polymers

Photoinduced optical anisotropy has been studied in several methacrylic copolymers, either amorphous or liquid crystalline, with cyano azobenzene chromophores in the side chain. Illumination with linearly polarised 488 nm light gives rise to high and stable values of birefringence and dichroism, mainly in liquid crystalline films. Both dichroism and birefringence decrease with the azo content in the copolymer series while some increase can be achieved by the incorporation of biphenyl molecules to the copolymers. Photoinduced anisotropy disappears above the glass transition temperature in amorphous polymers, whereas it increases in liquid crystalline polymers (LCP) due to a thermotropic self-organisation. The non-linear optical (NLO) properties of the films have been studied by second harmonic generation (SHG) measurements. The intensity of the harmonic signal from in situ corona poled polymeric films has been measured. The effect of 488 nm light irradiation on the azo chromophores orientation and consequently on the NLO response of the films was investigated at different temperatures. The intensity and thermal stability of the second harmonic signals obtained after the two different (thermal and photoassisted) corona poling processes have been compared.

Synthesis, characterization, and photochromic properties of PMMA functionalized with 4,4′‐diacryloyloxyazobenzene

AbstractAttempts to introduce the azo chromophore into poly(methyl methacrylate) (PMMA) molecular chains were made in search for a new approach to obtain functionalized‐PMMA (f‐PMMA) with high glass transition temperature and an adequate azo content. A novel functionalized monomer, 4,4′‐diacryloyloxyazobenzene (DAOAB), was synthesized and used as grafting agent in the polymerization of methyl methacrylate. 1H‐NMR spectrum, FTIR spectrum, elemental analysis, and MDSC measurement confirmed that the proposed structure of DAOAB was synthesized successfully. Furthermore, the thermal properties and photochromic behavior of f‐PMMA samples were also analyzed by DSC, TGA, and UV–visible spectroscopy. The results have shown that f‐PMMA exhibited a better photochromic behavior and considerably improved thermal properties than pure PMMA. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1061–1068, 2002

Main Chain-Containing Azo-Tetraphenyldiaminobiphenyl Photorefractive Polymers

The design of polymeric materials with optimized photoconductivity and nonlinear optical properties is of major interest for emerging optical applications using photorefractive properties. This paper reports the synthesis and preliminary optical characterization of a series of novel, fully functionalized, photorefractive polymers. They were synthesized by a post azo coupling reaction of a polyether carbonate polymer based on the charge transport N,N,N‘,N‘-tetraphenyl-diaminobiphenyl group with 4-nitrobenzene-diazonium tetrafluoroborate by phase transfer catalysis. The amount of the inserted 4-nitro-phenyl-azo group was between 0.1 and 1.7 per structural unit. These polymers absorb in the UV−vis range, because of the presence of donor−acceptor substituted azobenzene chromophores. Light induced change of the refractive index was studied by birefringence measurements (λprobe = 632.8 nm) as a function of azo content in thin films with thickness ranging from 200 to 500 nm. Surface relief grating inscription wa...

Improvement of LPS-based command surfaces: effect of inserting a flexible disiloxane segment into the azo side chain on photo-driven response

A new synthetic route for azo grafted ladder-like polysilsesquioxanes (LPS) used for fabricating command surfaces has been developed. In comparison with previous work, the grafting ratio of azobenzene on the LPS backbone could be controlled more accurately and the low molar mass impurity, which might influence the concerted action of the azobenzene units as command moieties on the surrounding LC molecules, could be readily removed by repeated precipitation. The results of our measurements indicate that inserting a flexible disiloxane segment into the spacer of the side group can improve the photo-driven response. The response steepness of a LC cell is 20 times that of our previous result. However, the response speed also depends on the spacer length and the content of azo side groups. Using azo grafted LPS with a shorter spacer as the material for the command surface, the response speed is faster than with a longer spacer. Optimum response can be achieved at an azo content of 10 mol %.

Photoinduced Alignment of Polymer Liquid Crystals Containing Azobenzene Moieties in the Side Chain. 5. Effect of the Azo Contents on Alignment Behavior and Enhanced Response

A series of side-chain polymer liquid crystals (PLCs) of 6-[4-(4-ethylphenyl)diazenylphenyloxy]hexyl methacrylate and 4‘-[6-(methacryloyloxy)hexyloxy]-4-cyanobiphenyl were synthesized and characterized. The effects of contents of the azo units and the wavelength of irradiation light on photoinduced alignment behavior were investigated. On irradiation of linearly polarized light (LPL) at 366 nm directly, alignment change was induced only in the PLC with 6 mol % azo units (P6), while phase transition took place in the other PLCs with higher contents of the azo units. On irradiation of LPL at 436 nm directly, alignment was induced with a fair efficiency in P15 and P35, low efficient alignment was observed in P6 and P100, and no alignment change was induced in P55 and P75, which exhibited a homeotropic state. A high alignment efficiency was achieved in all PLCs by using another method: irradiation with 366 nm unpolarized light at first and then with 436 nm LPL. The alignment change induced by this method was...

Azo polymers for reversible optical storage: 13. Photoorientation of rigid side groups containing two azo bonds

Copolymers of methyl methacrylate with a methacrylate containing a rigid group with two azo bonds (3RM) were prepared and their photoinduced birefringence levels and rates studied. Birefringence levels of 0.11 for the copolymer with 11.6 mol% azo structural units and 0.13 for the copolymer with 30.0 mol% azo structural units were found; this is higher than the birefringence inducible in a typical azo homopolymer containing a chromophore with only one azo group, poly{4′-[(2-(acryloyloxy)ethyl)ethylamino]-4-nitroazobenzene} [poly(DRIA)]. The birefringence per azo structural unit for a copolymer containing 11.6 mol% 3RM is about five times that for a DRIA copolymer with similar azo content, because of the intrinsic structural properties of 3RM (high length/diameter ratio). Dichroism in both u.v. and visible regions of the spectrum contribute to the overall photoinduced birefringence. The rate of inducing birefringence in the 3RM copolymers is lower than in poly(DRIA) and the birefringence stability (91–96% of the induced birefringence is maintained after the writing laser is off) is much better than that for poly(DRIA) (about 80%). The good stability and slow birefringence growth rate are due to the lesser mobility of the larger side group.

Water Soluble and Photoactive Copolymers Containing Amidic Aryldiazosulfonate Groups

Photoactive polymers could be synthesized by copolymerization of methyl methacrylate and sodium-(4-methacryloylaminophenyl)-diazosulfonate which became water soluble at high azo content. Determination of the copolymerization parameters revealed an almost ideal copolymerization behavior of the two monomers. The photoactivity of the azo chromophore could be further increased by introduction of two methoxy substituents into the aromatic unit which renders these polymers more suitable for application as photo resins.

Azo Polymers for Reversible Optical Storage. 10. Cooperative Motion of Polar Side Groups in Amorphous Polymers

4-Nitrophenyl 4-[[2-[(2-methyl-1-oxo-2-propenyl)oxy]ethyl]oxy]benzoate (BEM) was synthesized and copolymerized with 4‘-[[[2-[(2-methyl-1-oxo-2-propenyl)oxy]ethyl]ethyl]amino]-4-nitroazobenzene (DR1M) in dioxane at 60 °C. BEM and DR1M copolymerize in a random fashion with reactivity ratios rDR1M = 0.82 and rBEM = 1.03. The copolymers are amorphous. Interactions between the azo and the BEM side groups produce a shift of absorption maxima in the electronic spectra. Birefringence can be photoinduced and photoerased on films of all copolymers. The photoinduced birefringence per azo chromophore is very high in copolymers with low azo contents, about 4 times that of poly(DR1M). A cooperative motion of BEM groups with the azo groups is postulated as the reason for the additional birefringence. To our knowledge this is the first example of cooperative motion in amorphous polymers. The growth and relaxation of the birefringence are fitted by biexponential equations and “writing” rate constants are calculated for two processes, one is assigned to the fast motion of side groups and the other to the slow motion of the main chain. The fast “writing” rate increases with the DR1M content while the slow “writing” rate is almost constant. From the relaxation of the birefringence, about 85% of it is maintained in copolymers with high BEM contents. The maximum “writing” efficiency obtained for poly(DR1M-co-BEM) is 5 × 10-6 cm3/J (at about 50 mol % DR1M) which is higher than that of poly(DR1M).

Azo Polymers for Reversible Optical Storage. 5. Orientation and Dipolar Interactions of Azobenzene Side Groups in Copolymers and Blends Containing Methyl Methacrylate Structural Units

Abstract : Two amorphous copolymer series and one blend series based on Disperse Red I and methyl methacrylate are compared using absorption spectra and optically induced birefringence on spin-coated films. Observed blue shifts in the absorption spectra with increasing azo concentration suggest that, locally, azo dipoles are paired antiparallel both intramolecularly and intermolecularly. Induced birefringence studies indicate that the intramolecular dipolar interaction affects the orientability of the azo dipoles. The intermolecular dipolar interaction does not appear to have a large effect on orientability. The dependence of induced birefringence on azo content in the sample is different in the blend series (linear) and in the two copolymer series (nonlinear). A triad sequence distribution model is used to explain this difference. Observed writing efficiencies as a function of composition are also analyzed using a sequence distribution model. Dynamics of the inducing and birefringence relaxation processes are compared using biexponential fitting equations. jg p.2

Preparation and characterization of polyurethane‐type macroazo initiators for block copolymerization

AbstractThe use of dipropylene glycol in the polyaddition of 2,2′‐azobis(2‐cyanopropanol) (ACP) with hexamethylene diisocyanate results in soluble polyurethane‐type macroazo initiators (PUMAI) in high yields. By adjusting the amount of ACP used, the number‐average molecular weight of the polyurethane segments was adjusted in the range from 2000 to 6000 at an azo content in the PUMAI's of 0,18 to 0,5 mmol/g‐polymer (determined by UV). 13C NMR spectroscopy of the PUMAI's showed that no significant decomposition of azo groups occurred during polyaddition. At 80°C the PUMAI's initiated effectively the radical solution polymerization of vinyl monomers. As supposed from extraction results and the polymerization mechanism, block copolymers are formed. The whole process seems to be adequate for industrial applications.

Azobenzene‐Containing polypeptides: Photoregulation of conformation in solution

AbstractPhotochromic polypeptides, with 16 to 56% azobenzene groups in the side chains, have been prepared by reaction of poly(L‐glutamic acid) with p‐aminozaobenzene, both in the presence of dicyclohexyl carbodiimide/N‐hydroxybenzotriazole and of pivaloyl chloride. Analogous modification reactions carried out on poly(L‐aspartic acid) were unsuccessful owing to the formation of N‐succinimide rings. In trimethylphosphate, all the azopolypeptides exhibit the α‐helix CD pattern. Irradiation produces the trans‐to‐cis isomerization of the azo side chains, but does not induce any variations of the backbone conformation. In water, the CD spectra indicate the presence of appreciable amounts of α helix in 16 and 21% mol azo‐containing poly‐(Lglutamates), while a β structure is present in a 36% mol azopolypeptide. Light produces conformational changes of the polypeptide conformation which are completely reversed in the dark. The extent and kind of photobehavior depend on the azo content and the pH value at which irradiation is carried out. The light‐induced effects are discussed on the basis of the pH‐induced order‐disorder conformational transitions. In fact, the pK values and the transition curves of the dark‐adapted samples were found to be different from those of the irradiated ones.

Azobenzene-containing poly( l-glutamates). Photochromism and conformation in solution

Photochromic polypeptides having various contents of azobenzene chromophores attached to th side chains have been prepared by condensing poly( l-glutamic acid) with p-amino-azobenzene. The photochemical and thermal cis-trans isomerizations of the azo chromophores have been investigated by absorption and circular dichroism spectroscopy, and the photochromic behaviour has been related to the conformation in solution. The azopolypeptides exhibit the α-helix c.d. pattern, in trimethylphosphate. The α-helix content markedly depends on the azo content, but it is not affected by the cis or trans geometric forms of the azo side chains. Strong solvent effects by H 2O or trifluoroethanol on the extrinsic azo c.d. bands, suggest the existence, in trimethylphosphate solution, of a super-ordered secondary structure involving a regular arrangement of the side chains, on th periphery of the helical peptide backbone.