Science and technology of perfluorocyclobutyl aryl ether polymers

This article highlights the preparation of perfluorocyclobutyl (PFCB) aryl ether polymers for a multitude of commercial technologies that are of academic and commercial global interest. In this account, the synthesis of various aryl trifluorovinyl ether (TFVE) monomers tailored for specific applications is discussed. The preparation of PFCB aryl ether polymers and their properties is then presented. Topics of PFCB aryl ether polymers and their applications include photonics, polymer light emitting diodes (PLEDs), proton exchange membranes (PEMs) for fuel cells, atomic oxygen (AO) resistant coatings, and hybrid composites. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5705–5721, 2007     

Photofabrication of surface relief gratings from azobenzene containing perfluorocyclobutane aryl ether polymer

An azobenzene based perfluorocyclobutane (PFCB) aryl ether polymer, poly[4‐(phenylazo)triphenylamine‐1,2‐hexafluorocyclobutyl ether], was synthesized by using a palladium catalyzed amination reaction and the 2π + 2π cyclodimerization of 4‐[(trifluorovinyl)oxy]bromobenzene. This polymer was designed and synthesized to permit azo chromophores to be incorporated, thus introducing photoinduced dynamic behavior into the PFCB containing polymer structure. The polymer exhibited a T_g of about 122 °C, and the maximum absorption of the azo chromophores was 407 nm in the film state. A thin film was prepared through a spin coating process, and the rapid growth of diffraction efficiency was then achieved by irradiation with an interference beam. The efficient formation of holographic surface relief gratings was also verified from AFM images, which show regularly defined and aligned grating structures. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3525–3532, 2005     

Synthesis and comparison of CF3 versus CH3 substituted perfluorocyclobutyl (PFCB) networks for optical applications

A novel aryl trifluorovinyl ether monomer, 1,1,1‐tris(4‐trifluorovinyloxyphenyl)‐2,2,2‐trifluoroethane ( 5 ), was prepared via a multistep reaction sequence adapted from previously reported procedures. Monomer 5 polymerizes by free‐radical mediated thermal cyclodimerization to produce a crosslinked perfluorocyclobutyl (PFCB) polymer. Substituting CH₃ for CF₃ did not affect the polymerization kinetics as measured by differential scanning calorimetry. Surprisingly, the refractive index of poly5 (1.4931 at 1550 nm) is slightly higher than that measured for poly6 (1.4876 at 1550 nm) despite the significant increase in fluorine content. Compared to the CH₃‐containing monomer 6 , fluorinated analogue 5 exhibits increased thermal and thermal oxidative stability and thus we expected lower optical loss for long‐term high performance applications. Copolymerization with existing aryl trifluorovinyl ether monomers should allow access to new PFCB network copolymers with a tailored performance. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5292–5300, 2004     

Synthesis and electronic factors in thermal cyclodimerization of functionalized aromatic trifluorovinyl ethers

A series of 19 p-substituted aromatic trifluorovinyl ether compounds were prepared from versatile intermediate p-Br-C(6)H(4)-O-CF=CF(2) and underwent thermal radical mediated cyclodimerization to new difunctional compounds containing the 1,2-disubstituted perfluorocyclobutyl (PFCB) linkage. The synthetic scope demonstrates the functional group transformation tolerance of the fluorovinyl ether, and the dimers are useful as monomers for traditional step-growth polymerization methods. (19)F NMR spectra confirmed that p-substitution affects the trifluorovinyl ether group chemical shifts. The first kinetic studies and substituent effects on thermal cyclodimerization were performed, and the results indicated that electron-withdrawing groups slow the rate of cyclodimerization. The data were further analyzed using the Hammett equation, and reaction constants (rho) of -0.46 at 120 degrees C and -0.59 at 130 degrees C were calculated. This study presents the first liner free energy relationship reported for the cyclodimerization of aromatic trifluorovinyl ethers to PFCB compounds.     

Highly Fluorinated and Crosslinkable Dendritic Polymer for Photonic Applications

Summary: A novel crosslinkable dendritic polymer has been synthesized by the thermal polymerization of peripheral aryl trifluorovinyl ether moieties of a highly fluorinated dendrimer. The resulting perfluorocyclobutane(PFCB)‐containing dendritic polymer exhibited excellent processability, low optical loss (0.36 dB · cm⁻¹ at 1 310 nm with 1% dye doping), high thermal stability, and good solvent resistance for waveguide‐based photonic applications.

Structure of the crosslinkable fluorinated dendrimer synthesized.     

Perfluorocyclobutyl-linked hexa-peri-hexabenzocoronene networks

Hexa-peri-hexabenzocoronene (HBC) derivatives possess many interesting properties ranging from liquid crystallinity, to hole transport, to variable emission behavior, yet are plagued by insolubility and general accessibility of polymerizable substituents. The first synthesis of trifluorovinyl ether-substituted hexabenzocoronene and its polymerization to perfluorocyclobutyl (PFCB) polymers and copolymers is shown. Unlike hydrocarbon ethers, fluoro vinyl ethers are stable under HBC oxidation conditions. Discrete HBC units in PFCB polymers provide access to potentially processable HBC optical materials.     

A novel semi-fluorinated graft copolymer containing perfluorocyclobutyl aryl ether-based backbone

The synthesis of a series of novel semi-fluorinated graft copolymers bearing perfluorocyclobutyl (PFCB) aryl ether-based backbone and polystyrene side chains is described. This work initially focused on the synthesis of a trifluorovinyl ether (TFVE) monomer containing a bromine atom, which could be employed as an initiating site for atom transfer radical polymerization (ATRP). Thermal cyclopolymerization of this TFVE monomer provided a macromolecular initiator followed by subsequent initiating ATRP of styrene to afford the desired PFCB aryl ether-based graft copolymers.     

Synthesis and characterization of a biphenyl perfluorocyclobutyl (BP‐PFCB) polyethylene glycol (PEG) blend compatibilizer

Polyethylene glycol (PEG) end‐capped trifluorovinyl ether (TFVE) telechelomer was synthesized in one step via esterification of 4‐(trifluorovinyloxy) benzoic acid. The new telechelomer was characterized by attenuated total reflectance Fourier transform infrared (ATR‐FTIR), elemental analysis, and by ¹⁹F and ¹H nuclear magnetic resonance (NMR) spectroscopy. The telechelomer and 4,4′‐bis(4‐trifluorovinyloxy)biphenyl (BPVE) were thermally copolymerized via step‐growth [2 + 2] cycloaddition at 160°C. The polymerization afforded PEG enchained biphenyl perfluorocyclobutyl (BP‐PFCB) copolymers that are solution processable and film forming. These copolymers were characterized by ATR‐FTIR, ¹⁹F NMR, and ¹H NMR. Gel permeation chromatography (GPC) gave number‐average molecular weight (M_n) ranging 11,000 to 12,000. Compatibilization of PEG and a commercial polymer BP‐PFCB was achieved utilizing the new PEG BP‐PFCB copolymer, 3‐co2‐4 . It was found that 5 wt% of 3 ‐ co2 ‐ 4 was ideal to reduce interfacial tension by scanning electron microscope (SEM). In addition, phase homogeneity was studied by differential scanning calorimetry (DSC). Copyright © 2016 John Wiley & Sons, Ltd.     

Linear and branched fluoroazo‐benzene chromophores with increased compatibility in semifluorinated polymers

A family of fluorinated azobenzene‐based push‐pull chromophores with one, two, and three trifluorovinyl ether (TFV) groups in linear and branched architecture was synthesized and utilized as active materials in the low optical loss electro‐optic (EO) composites. The fluorinated azobenzene chromophores exhibited increased solubility (30–50 wt %) in semifluorinated polymer host, such as perfluorocyclobutane (PFCB) aromatic ether resin after crosslinking, compared with the commercially available nonfluorinated azobenzene chromophore Disperse Red 1 (1–2 wt %). The impact of this approach on the optical properties on the polymer blends is assessed through optical propagation loss measurements and EO characterization. The resulting fluorinated EO composites showed excellent optical clarity, low birefringence, and low optical loss less than 0.5 dB/cm, while giving EO coefficients of about 3–7 pm/V at 1550 nm. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3166–3177, 2007     

The Photolysis of Polyvinylpyrrolidone in Aqueous Solutions in the Presence of Oxygen and Cupric Ions

Abstract

The oxidative photolysis of polyvinylpyrrolidone with light of λ = 2537 Å has been studied over a range of oxygen pressures and polymer concentrations. The results show that chain scission and cross linking take place simultaneously. In the range where cross linking is a negligible component, a mechanism based on chain scission due to components which lead to chain scission without intervention of oxygen, and a component which leads to chain scission via hydroperoxide side groups, has been proposed. This mechanism accounts satisfactorily for all observed features of the reaction. The degree of degradation at any one time decreases with oxygen pressure. Cupric ions, with or without oxygen present, have very little influence on the degradation process. However, the UV spectra of PVP in the presence of cupric ions are different from those without them.     

Kinetics of thermal degradation of poly(-caprolactone)

The thermal degradation/modification dynamics of poly(-caprolactone) (PCL) was investigated in a thermogravimetric analyzer under non-isothermal and isothermal conditions. The time evolution of the molecular weight distribution during degradation was studied using gel permeation chromatography. Experimental molecular weight evolution and weight loss profile were modeled using continuous distribution kinetics. The degradation exhibited distinctly different behavior under non-isothermal and isothermal heating. Under non-isothermal heating, the mass of the polymer remained constant at initial stages with rapid degradation at longer times. The Friedman and Chang methods of analysis showed a 3-fold change (from 18 to 55–62 kcal mol⁻¹) in the activation energy from low temperatures to high temperatures during degradation. This suggested the governing mechanism changes during degradation and was explained using two parallel mechanisms (random chain scission and specific chain end scission) without invoking the sequential reaction mechanisms. Under isothermal heating, the polymer degraded by pure unzipping of specific products from the chain end.     

Hydrolytic degradation of poly(rac‐lactide) and poly[(rac‐lactide)‐co‐glycolide] at the air–water interface

The understanding of the simultaneous transport and chain‐scission phenomena involved in the hydrolysis of bulk‐degrading polymers requires the experimental separation of chain cleavage and water diffusion. The hydrolytic chain cleavage of poly(rac‐lactide) rac‐(PLA) and poly[(rac‐lactide)‐co‐glycolide] (PLGA) is analysed on the basis of monolayer degradation experiments combined with an improved data reduction procedure. Different, partly contradictory models of the hydrolytic degradation and erosion mechanism of PLA and PLGA, namely random chain scission and chain‐end scission, are discussed in the literature. The instantaneous linear area reduction observed for the polymer Langmuir films indicates a chain‐end scission mechanism. As monolayers of end‐capped and non‐end‐capped polymers degrade with exactly the same rate, the observed differences in the degradation kinetics of bulk samples do clearly result from differences in the water penetration into these polymers. A pronounced ‘auto‐inhibition’ effect is observed for the polymers degraded at initially high pH of the aqueous subphase in the absence of buffers. Copyright © 2007 John Wiley & Sons, Ltd.     

Enzymatic chain scission kinetics of poly(epsilon-caprolactone) monolayers

The hydrolytic and enzymatic degradation behavior of poly(epsilon-caprolactone) (PCL) is investigated using the Langmuir monolayer technique, and an improved data acquisition and data reduction procedure is presented. Hydrolytic and enzymatic monolayer degradation experiments of PCL with various molecular weights by Pseudomonas cepacia lipase have been carried out to analyze the influence of subphase pH, subphase temperature, enzyme concentration, and the packing density of polymer chains on the degradation kinetics. The enzymatic monolayer degradation results in an exponential increase in the number of dissolved degradation fragments with increasing degradation time, which confirms random chain scission to be the dominant scission mechanism. The increase in the enzymatic scission rate constant with decreasing initial average molecular weight of the polymers is assigned to the influence of the area density of polar terminal groups on the substrate-enzyme complex formation.     

Thermal stability and degradation of chitosan modified by benzophenone

N-(biphenylmethylidenyl) chitosan polymer was prepared, characterized and thermal stability was compared with chitosan. Thermal degradation products of the modified polymer were identified by GC-MS technique. It seems that the mechanism of degradation of the prepared polymer is characterized by formation of low molecular weight radicals, followed by random scission mechanism along the backbond chain.     

Reaction of nitrogen dioxide with polystyrene films

The reaction of nitrogen dioxide with thin polystyrene films has been investigated at 35°C with different partial pressures of NO₂ (0.1, 2, 15, 30, and 60 cm Hg) and at several temperatures (25, 35, 45 and 55°C). The films were thin enough (ca. 20 μ) so that the reaction was independent of the diffusion of gas into the polymer. The experimental results can be represented by a chain mechanism. The whole degradation process is controlled by the diffusion of polymer radicals out of cages. This diffusion in turn, is affected by the decrease in viscosity or decrease in weight-average molecular weight as degradation proceeds. This leads to an acceleration of the degradation process. A straight-line relationship between the logarithm of the reciprocal weight-average molecular weight and the logarithm of a reaction–time function was found. The dependence on the rate was substantiated by degrading polymer fractions. The energy of activation for the process is small, in agreement with a diffusion process for chain scission. Nitro and nitrite groups are incorporated along the backbone of polystyrene during exposure. The number of these polar side groups appears to pass through a maximum with time, as is evidenced by aggregation of polymer molecules in benzene solution only during the middle stage of the degradation. The final stage of the process is slowed down by retarder being produced. This retarder can be removed by reprecipitation of exposed polymer films. Degradation in solution is similar to that of films. Isotactic polystyrene shows less irregularities in its degradation curve than the atactic polymer. This is, presumably, due to its more homogeneous morphology, large molecular weight, and broader molecular size distribution. The plot of the degree of degradation versus time for the isotactic polymer can be satisfactorily approximated by a straight line.     

Mechanical degradation of polypropylene solutions under large pressure drops

The degradation of polypropylene (PP), dissolved in n‐alkanes at high temperatures and pressures, during the solution discharge to ambient conditions was experimentally studied. Molecular weight distributions (MWD) of the solubilized PP were measured by gel permeation chromatography. The MWD curves of PP obtained after discharge of the polymer solution shift to the low molecular weight side of the distribution and the polydispersity is reduced. In this work, a systematic study on the discharge products was performed to elucidate the degradation mechanism and the effects of temperature and concentration on this phenomenon. Initially, pure polymers, PP and polystyrene (PS) were studied varying the solution temperature. In a second stage, the effect of polymer concentration on chain scission was assessed using experiments on physical blends of PP/PS. In all cases, thermal and oxidative degradation were previously analyzed. Mechanical degradation was found to be the main chain scission mechanism. A negative linear functionality of the chain scission was found in both temperature and polymer concentration. To analyze the relationship between polymer degradation and molecular weight, the chain scission distribution function was calculated. On this basis, a critical molecular weight for the beginning of chain scission was obtained. This value is a function of temperature but remains constant with concentration. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 455–465, 2007     

Thermal degradation of biodegradable edible films based on xanthan and starches from different sources

Films based on xanthan/wheat starch and xanthan/maize starch were prepared in different proportions and analysed in terms of thermal stability in relation to the degradation reaction in an inert atmosphere. The main gaseous decomposition products were identified by infrared spectroscopy. Xanthan was found to be less thermally stable than starch from different sources. The starch addition to xanthan induced an increase in the film thermal stabilities. The thermal degradation mechanism of xanthan starts with scission of the side chain groups with subsequent scission of the main chain. Starches from maize and wheat presented a constant mechanism of thermal degradation. The proportion of xanthan/starch (50/50) presented a synergistic behaviour which may be associated with changes in the mechanism of the thermal degradation process. The FTIR spectra of the gaseous products during thermal degradation of xanthan showed a broad absorption in the 2750–3300 cm⁻¹ region, probably indicating decomposition of β-d-mannopyranosyl groups, present in xanthan side chains. For pure starches, absorptions related to alcohol O–H stretching (3500–3750 cm⁻¹), CO stretching of CO₂, CO and carbonyl compounds (2361, 2177 and 1743 cm⁻¹, respectively) were observed. For xanthan/starch mixtures (50/50) the same compounds as those for the pure polymer degradation were evolved. However, a shift of the CO to higher wavenumber values indicates that new carbonyl compounds are being formed due to changes in the thermal degradation mechanism.     

Thermo-oxidative processes in biodegradable poly(butylene succinate)

Aliphatic polyesters have acquired significant interest as environmentally friendly thermoplastics for a wide range of applications, and understanding their degradation behaviour has relevance both for processing and end uses. We have investigated the thermal and thermo-oxidative degradation processes occurring in synthetic and commercial poly(butylene succinate) (PBSu). Thermal oxidation was performed in atmospheric air using extremely thin polymer films at 170 °C for up to 6 h. The oxidized compounds were analyzed by size exclusion chromatography (SEC), NMR spectroscopy, and Mass Spectrometry (MALDI-TOF MS). A measurable reduction of the molar mass of the polyesters was soon apparent, promoting the formation of PBSu oligomers with different end groups. MALDI mass spectrometry combined with the use of extremely thin polyester films provided a virtual magnifying glass to obtain exhaustive information on the structure of the oxidation products. An α-H abstraction mechanism has been unambiguously ascertained to be the primary step in PBSu oxidation. The oxidized polymer chains originating from the decomposition of the hydroperoxide intermediate by radical rearrangement reactions had not been revealed before. The latter products subsequently undergo chain scission processes, which can be accurately traced from the chemical species identified in our work. Thermal degradation experiments were also performed under nitrogen at 240-260 °C. The new species identified in the MALDI spectra support a decomposition pathway taking place through a β-hydrogen-transfer mechanism, followed by the production of succinic anhydride from succinic acid end molecules via a back-biting process.     

Photo- and radiation-induced degradation of vinyl polymers in solution. I. Oxidative degradation of poly(α-methylstyrene) initiated by photodecomposition of AIBN

The oxidative degradation of poly(α-methylstyrene) initiated by the photodecomposition of azobisisobutyronitrile was studied at 30°C in benzene solution. The progress of the reaction was followed by measuring the rate of chain scission of the polymer. It has been confirmed by GPC measurements that random scission of the polymer chain occurs in the present system. Chain scission did not occur in the absence of AIBN and oxygen. The rate was proportional to the initiation rate and independent of polymer concentration and oxygen pressure under these experimental conditions. Phenol was an effective inhibitor to this reaction. The mechanism of chain scission during oxidation is discussed, and a six-membered transition is proposed.     

Thermal stability of ferrocene derivatives and ferrocene-containing polyamides

Thermogravimetric data and the kinetic interpretation of the curves of mass loss for ferrocene, ferrocenecarboxylic and ferrocenedicarboxylic acids and a series of ferrocene-containing polyamides are presented. The results indicate that the degradation process occurred with more than one stage of thermal degradation. The apparent activation energy values and the FTIR spectra of the degradation products suggest that the degradation mechanism occurred by either scission of weak links or by random scission of the chain. Apparently, the N-vicinal methylene group was the primary site of attack of oxygen on the polymer chain. This revised version was published online in August 2006 with corrections to the Cover Date.