Štefan Chmela

Spectral characteristics and photochemical stability of fluorescence probes based on 1,8-naphthaleneimide in solution and in polymer matrix

Abstract Fluorescence probes of type N -(1′- R -2′,2′,6′,6′-tetramethyl-4′-piperidinyl)-1,8-naphthaleneimide exhibit spectral properties which are influenced by environment and substitution ( R ) on sterically hindered nitrogen of piperidine moiety. Absorption spectra of parent amine and its N substituted derivatives show the longest wavelength band around 340 nm (log ɛ ∼ 4.0) which is well resolved in cyclohexane. In polar methanol it is only slightly blue shifted. Substitution on sterically hindered nitrogen does not influence the absorption spectra in near UV region. Fluorescence spectra of the probes have maximum around 385 nm in methanol and comparable intensity as anthracene. In nonpolar cyclohexane the fluorescence intensity is low (more than 100×). In nonpolar isotactic polypropylene matrix the red shifted excimer like emission in the range 440–470 nm was observed. The N -oxy and N -hydroxy derivatives under the same conditions do not yield the excimer emission. In other polymer matrices as polystyrene, poly(methyl methacrylate) and polyvinylchloride similar emission as in methanol was observed, the emission in latter matrix being the most intense. The fluorescence lifetime in polar solvent and matrices is less then 1 ns and deviates slightly from monoexponential. In nonpolar iPP, the fluorescence lives longer around 5 ns and its decay is more complex. Intermolecular quenching of the probes by 1-oxy-2,2,6,6-tetramethyl-4-hydroxypiperidine in methanol occurs at the rate which is above the diffusion controlled limit what might indicate the involvement of the polar medium in the process. Intramolecular quenching, expressed as the ratio I NH / I NO for parent and oxidized amine respectively, strongly depends on the medium as well. In polar methanol this ratio is about 30 while in polymer matrices 2–4 only. Photolysis of probes doped in polymer matrices with radiation above 300 nm revealed that the parent amine is about 3× more stable as 1,8-naphthoic anhydride or N -octadecyl-1,8-naphthaleneimide.

Stabilising effect of monomers, homopolymers and copolymers of vinyl derivatives of sterically hindered amines on the photo-oxidation of polypropylene

Abstract Vinyl derivatives of sterically hindered amines, N-(2,2,6,6-tetramethyl-4-piperidyl)methacrylamide, 2,2,6,6-tetramethyl-4-piperidyl acrylate and methacrylate and 1,2,2,6,6-pentamethyl-4-piperidyl acrylate and methacrylate were prepared. Monomers, homopolymers and copolymers with dodecyl methacrylate (DMA) and octadecyl acrylate (ODA) were tested as stabilisers of the photo-oxidation of polypropylene film. The copolymers exhibited higher efficiency and lower extractability. The photostabilising efficiency increased in the order: homopolymer

Spectral Properties of Probes Containing Benzothioxanthene Chromophore Linked with Hindered Amine in Solution and in Polymer Matrices

Absorption and emission spectroscopy as well as laser flash photolysis was employed in order to characterize the spectral properties of novel probes based on benzothioxantheneimide chromophore covalently linked with different types of sterically hindered amines. These were chosen as 2-(2,2,6,6-tetramethyl-4-piperidyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXINH), the equivalent stable nitroxyl radical, i.e. 2-(1-oxo-2,2,6,6-tetramethyl-4-piperidyl)thioxantheno[2,1,9dej]isoquinoline 1,3-dione (BTXINO) and the alkoxy derivative 2-(1-(1′-phenylethoxy)-2,2,6,6-tetramethyl-4-piperidyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXINOR). Spectral properties, in solutions and in various polymer matrices such as polystyrene, polymethyl methacrylate, polyvinyl chloride and polypropylene, were compared with the compound 2-(1-dodecyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXID) taken in the present study as a reference compound. By means of the fluorescence decay and in the contrary to three other probes, BTXINO probe clearly showed a biexponential decay while the three other probes led to monoexponential decay. Two different singlet excited states with lifetimes of about 0.4 and 5 ns were proposed. They correspond to two dispositions of the nitroxyl radical chain above and along the fluorescent moiety of the molecule. Such behaviour depends on the surrounding media. Moreover, an efficient intramolecular quenching of the fluorescence emission was only observed with the short lived singlet excited state. The ratio BTXID/BTXINO was found equal to about 4 and 9 in solutions and polymer matrices respectively. Laser flash photolysis indicated that the novel probes as well as the model compound yielded transient absorption with maximum at 530 nm, corresponding to the triplet states. The intermolecular quenching of such species by molecular oxygen and by free N-oxyl, such as 1-oxy-2,2,6,6-teramethylpiperidine (TEMPO) and 1-oxy-2,2,6,6-teramethyl-4-hydroxypiperidine (TEMPOL), and the intramolecular quenching was not efficient.

Preparation, photochemical stability and photostabilizing efficiency of adducts of 1,8-naphthaleneimide and hindered amine stabilizers in polymer matrices

Adducts of 1,8-naphthaleneimide and hindered amine stabilizer (HAS) such as 1-R-substituted-2,2,6,6-tetramethyl-4-aminopiperidine were prepared where R is –H, –O., –OH, –COCH3 and –OCOCH3. The photochemical stability of adducts was determined at photolysis in isotactic polypropylene, polystyrene and polyvinylchloride. Their photostabilizing efficiency was determined at photooxidation of isotactic polypropylene. The photochemical stability of the adducts is low in all matrices. The N-oxy derivative was the most stable in all matrices. The photolysis of all derivatives is the slowest in polystyrene. Intramolecularly combined chromophore/HAS are less effective stabilizers than in 1:1 mixture of separated components. The 1,8-naphthaleneimide chromophore decreases the stabilization efficiency of hindered amine structural units as compared to its absence.

Photo‐Oxidation of sPP/Organoclay Nanocomposites

Photo‐oxidation of syndiotactic polypropylene–sPP/organoclay nanocomposites was performed. Nanocomposites were prepared in situ by melt compounding of sPP, compatibilizer (iPP grafted with maleic anhydride–iPP‐g‐MAN) and organoclay filler ME C18 (modified with octadecyl ammonium chains in intergaleries of layered silicate, of which silicate layers (about 1 nm thin) were exfoliated). The influence of ME C18 nanoparticles alone (in content region 1 to 15 wt%) and together with compatibilizer iPP‐g‐MAN on the photostability of the sPP nanocomposite was studied. It was found that the silicate ME C18 nanoparticles alone catalyze the photooxidation and shorten the induction period of photo‐oxidation to one fourth (at the content of 5 wt% of ME C18) in comparison with unfilled sPP) and the presence of compatibilizer supports the photo‐oxidation of sPP nanocomposite. The ME C18 nanoparticles decrease the efficiency of UV stabilizers. The rate of photo‐oxidation of sPP/clay nanocomposite after the induction period is significantly higher than unfilled sPP. The mechanism of photo‐oxidation is discussed.

Spectral characteristics of multifunctional probes based on pyrene in solution and in polymer matrices

Abstract The spectral properties of multifunctional probes derived from 4-(1-pyrene)butyric acid and 2,2,6,6-tetramethyl-4-hydroxypiperidine in solution and in polymer matrices were investigated. In the absorption spectra, the well-resolved absorption of pyrene dominates and is little influenced by the medium. The absorption of the hydrochloride derived from the amine in non-polar matrices is rather diffuse. All the fluorescence probes with different functional groups at the sterically hindered centre yield well-resolved emission spectra, which are the same in solution and in polymer matrices at low concentration. The main features of the emission spectra are not influenced by the polarity of the medium. A red-shifted excimer emission is observed for the hydrochloride at concentrations in solution (10 −3 mol dm −3 ) and in solid matrices (0.1 mol kg −1 ) at which the parent probe yields the monomer emission only, indicating the formation of ground state aggregates. Steady state and lifetime measurements indicate that the emission of the N -oxyl-type probe is quenched by about 50% irrespective of the medium. This new radiationless channel in the N -oxyl-type probe is comparable with the rate of the radiation channel.

HALS-phosphite combinations as light and heat stabilizers for polypropylene

Abstract Organic phosphites combined with Hindered Amine Light Stabilizers (HALS) were used as stabilizers in the photo- and thermoxidation of polypropylene film. The efficiency of the mixture of HALS and phosphite was compared with the efficiency of the combination of HALS and phosphite in one molecule. A strong synergistic effect was observed for the molecule with HALS and phosphite moieties. Generally, the mixture of HALS and phosphite exhibited synergistic, antagonistic or additive effects. The efficiency of the mixture depends upon the chemical structure of the phosphite and HALS structural units as well as on the ratio of the components.

Combined oligomeric light and heat stabilizers

Abstract A series of terpolymers containing structural units of hindered amine stabilizer (HAS), antioxidant of diphenylamine type and octadecyl alkyl chain has been prepared by radical oligomerization initiated by AIBN. Monomers bearing HAS represented acrylate, methacrylate or acrylamide of 2,2,6,6-tetramethyl and 1,2,2,6,6-penta-methylpiperidine respectively. Second monomer in terpolymers was 4-amino-diphenylamine acrylamide or methacrylamide and the third one n-octadecyl acrylate. Molecular weights of oligomers lay in the region 4000–6500. The molar ratio of monomers in oligomers was approximately the same as in the mixtures of monomers before oligomerization (1:1:1). Oligomeric stabilizers were tested as photostabilizers in polyoctenamer and polypropylene (PP) and as thermostabilizers in PP at 130°C. Stabilizing efficiency for all oligomeric stabilizers was roughly the same in photooxidation of PP. In the case of thermooxidation, the oligomer TMPA/DPA/ODA showed several times better efficiency than other oligomers.

Photochemical stability and photostabilizing efficiency of anthracene/hindered amine stabilizers in polymer matrices

Abstract Adducts of chromophore (anthracene substituted in position 9) and hindered amine stabilizers (HAS) such as 2,2,6,6-tetramethylpiperidine-4-yl 3-(9-anthracene)propanoate, 1-oxo-2,2,6,6-tetrametylpiperidine-4-yl 3-(9-anthracene)propanoate, [2,2,6,6-tetramethyl-piperidine-4-yl 3-(9-anthracene)propanoate]ium chloride as well as model compounds 3-(9-anthracene) propanoic acid and methyl 3-(9-anthracene)propanoate were investigated. The photochemical stability of the adducts was determined in photo-decomposition in polystyrene, poly(methyl methacrylate), poly(vinyl chloride), isotactic polypropylene (iPP) and low density polyethylene (LDPE). Their photo-stabilizing efficiency was determined in iPP and in LDPE. The rate of decomposition of adducts on irradiation λ> 310 nm was very fast in all matrices. Intramolecularly combined chromophore/HAS are more effective stabilizers than 1:1 mixtures of the separated components. The decomposition of anthracene produces free radicals containing HAS. These radicals are able to graft to the polymer chain of iPP and LDPE.

Photo‐oxidation and Stabilization of sPP and iPP/Boehmite‐Disperal Nanocomposites

Photo‐oxidation studies on polypropylene (PP)/organoclay nanocomposites were performed. Nanocomposites of isotactic (iPP) and syndiotactic (sPP) polypropylene were prepared by melt compounding. The nanofiller was Boehmite Disperal OS2–alumina hydrates (Al(OH)O) modified by C10–C13 alkylbenzene sulfonic acid. The nanofiller content was 1, 5, and 10 wt%. There is a clear pro‐degradation effect of filler for both types of polypropylene used. The extent of this effect depends on the amount of filler and type of polypropylene used. In the case of sPP samples, the pro‐degradation effect is proportional to the amount of filler in the whole concentration range of filler content used. In the case of iPP, there is a pro‐degradation plateau at 5 wt% content of filler and higher concentration of filler (10 wt%) does not increase the rate and the course of photo‐oxidation. Two long term stabilizers of HAS family were tested ‐ commercial oligomeric stabilizer Chimassorb 944 (CHIM) and synthesized combined HAS/phenol (TMP). Stabilizing efficiency depends on the filler content. CHIM is able to stabilize just the nanocomposites with the lowest content (1 wt%) of filler. There is no stabilizing effect of this HAS in the case of the higher amount of nanofiller (5 and 10 wt%) in both types of polypropylene. By contrast, the combined HAS/phenol‐TMP revealed some stabilizing efficiency over the whole range of filler content. The possible reasons for this difference are discussed. Interactions of filler with some HAS stabilizers were studied in cyclohexane as a model liquid for polypropylene by UV‐spectroscopy. Interaction resulted in the fixing of additive on filler. Much stronger interaction has been obtained for oligomeric CHIM in comparison with low molecular HAS.