Main Chain Scission Research Articles

Free radical macrophotoinitiators: an overview on recent advances

This paper provides an overview of the recent advances on polymeric photoinitiators for UV curing. During the last decade, significant developments have been achieved in the synthesis of macrophotoinitiators, due to the advantages derived of their macromolecular nature, in comparison with their corresponding low molecular weight analogues. In particular, a variety of macromolecules containing the two main types of free radical photoinitiators: hydrogen-abstracting (thioxanthone, benzil, anthraquinone, camphorquinone) and photofragmenting chromophores (benzoin ether, acylphosphine oxides) are described. For hydrogen-abstracting photoinitiator, the photoinitiation activity have been examined in terms of volume and nature of substituent in the polymeric coil, and their influence to prevent the recombination of radicals favouring their reaction with the monomer. Also, copolymers bearing chromophore and amine groups with potential synergistic effects of activity are reported. It has been found that the approach of the tertiary amine to the chromophore to produce the corresponding exciplexes is dependent on both the monomeric or polymeric nature of chromophore and the tertiary amine. Type II polymeric photoinitiators, such as benzoin ether derivatives having the benzoin methyl ether moieties connected to the main chain through the benzyl aromatic are reported. And a fragmentation mechanism involving the formation of an stable quinoid structure and aliphatic acyl radical is proposed for the above copolymers, which would justified their lower initiating efficiency than the corresponding low molecular weight model. In addition, polymers bearing phosphine oxide moieties are described. The efficiency in the polymerisation of all photoinitiators was found to be similar and irrespective of the presence of flexible spacer in their structure. However, it was found that the flexible oligomethylene spacer enhanced the compatibility of the new polymeric photoinitiators in acrylic adhesive formulations. Finally, polysilanes as photoinitiators are reported. Under UV irradiation, polysilanes undergo main-chain scission leading to free silyl radicals capable of reacting with olefinic monomers. The silyl radicals generated by photolysis can be oxidised by appropriate onium salts to yield cationic initiating species (photoinitiated radical promoted cationic polymerisation). The photoinitiation efficiency of polysilanes having different aliphatic and aromatic side groups has been investigated and compared with commercial low molecular weight photoinitiators as benzoin.

Catalytic decomposition of perfluoropolyether lubricants

With the ongoing decrease in magnetic spacing, the chemical durability of the lubricant film on magnetic recording media becomes a major factor in the reliability of the head-disk interface. There are several reports in the literature that have attributed deterioration of lubricant film to catalytic decomposition, and a reaction mechanism has been proposed by Kasai involving chain scission of the lubricant main chain promoted by Lewis acids. In this work, an alternative mechanism is proposed, wherein perfluoropolyether (PFPE) lubricants are catalytically decomposed via a free radical degradation loop. The catalytic decomposition of Fomblin Zdol in the presence of the Lewis acid Al/sub 2/O/sub 3/ was studied using thermogravimetric analysis (TGA). We show that various types of antioxidants can retard PFPE catalytic decomposition to different degrees, which we attribute to their different roles in breaking the free radical degradation loop. These data provide useful guidance in the development of new lubricants that are resistant to free radical initiated decomposition.

Oxidative degradation of cis- and trans-1,4-polyisoprenes and vulcanized natural rubber with enzyme-mediator systems.

Oxidative degradation of cis- and trans-1,4-polyisoprenes by two types of enzyme-mediator systems, lipoxygenase/linoleic acid and horseradish peroxidase/1-hydroxybenzotriazole, was investigated at 37 degrees C in aqueous media and analyzed by gel permeation chromatography. Lipoxygenase and horseradish peroxidase activate their substrates, linoleic acid and 1-hydroxybenzotriazole, respectively, for scission of main chains of both 1,4-polyisoprenes. Molecular weights of 1,4-polyisoprenes decreased during the treatment under both enzyme-mediator systems, and the depolymerization was completely inhibited by the addition of butylated hydroxytoluene. When the enzyme or the mediator from a reaction system was omitted, depolymerization did not progress, indicating that the scission of polymer chain is induced by the radicals generated only in the presence of both enzyme and mediator. Fenton reagent with linoleic acid was also effective against the degradation of both 1,4-polyisoprenes. Vulcanized natural rubber latex gloves were treated under these three methods, and surface degradation with hole formation was observed with a scanning electron micrograph.

The effect of end groups on the thermal degradation of poly(methyl methacrylate)

The thermal degradation of poly(methyl methacrylate) was studied using a system of thiols as initiators. Depolymerization is the dominant reaction in the thermal degradation of such polymers, according to thermal analysis. The degradation mechanism involves only main-chain scission. Three methods of degradation were applied to elucidate the thermal degradation behaviour. The activation energy for the degradation of PMMA-thiols is 170 kJ/mol, higher than that of PMMA-AIBN (60 kJ/mol), as measured by the Flynn method. According to Ozawa's method, the activation energy is 210 kJ/mol higher than that of an AIBN-initiated polymer (148–151 kJ/mol). By Kissinger's measurement method, this system has a frequency factor (∼10 10) five orders of magnitude higher than the ordinary PMMA (∼10 5). The observed results are explained as a “blocking effect”.

Chemically Amplified Main Chain Scission: Chopping the Influence of Polymer Dimensions on Line Edge Roughness

The concept of Main Chain Scission of photoresist polymers (MCS) is used to reducing the influence of molecular dimensions on the quality of photoresist patterns. Independent from the wavelength or lithography technique, the concept will raise in relevance as critical dimensions approach molecular dimensions. After the introduction of the MCS concept on last years conference of photopolymer science and technology, initial results are given in this paper to verify this concept.[1] Starting with polymer synthesis, after optimization of resist formulation and process conditions, ebeam patterns with the MCS resist and a commercial ebeam resist were generated. The patterns were compared with respect to their roughness. It was found, that lines printed in the MCS resist show a significant lower LER than the same patterns in the ebeam resist.

Electron irradiation effects on partially fluorinated polymer films: Structure–property relationships

The effects of electron beam irradiation on two partially fluorinated polymer films i.e. poly(vinylidene fluoride) (PVDF) and poly(ethylene-tetrafluoroethylene) copolymer (ETFE) are studied at doses ranging from 100 to 1200 kGy in air at room temperature. Chemical structure, thermal and mechanical properties of irradiated films are investigated. FTIR show that both PVDF and ETFE films undergo similar changes in their chemical structures including the formation of carbonyl groups and double bonding. The changes in melting and crystallisation temperatures ( T m and T c) in both irradiated films are functions of irradiation dose and reflect the disorder in the chemical structure caused by the competition between crosslinking and chain scission. The heat of melting (Δ H m) and the degree of crystallinity ( X c) of PVDF films show no significant changes with the dose increase, whereas those of ETFE films are reduced rapidly after the first 100 kGy. The tensile strength of PVDF films is improved by irradiation compared to its rapid deterioration in ETFE films, which stemmed from the degradation prompted by the presence of radiation sensitive tetrafluoroethylene (TFE) comonomer units. The elongation at break of both films drops gradually with the dose increase indicating the formation of predominant crosslinked structures at high doses. However, the response of each polymer to crosslinking and main chain scission at various irradiation doses varies from PVDF to ETFE films.

The influence of UV-irradiation on poly(vinyl chloride) modified by iron and cobalt chlorides

The influence of ultraviolet radiation ( λ=254nm) on thin PVC films modified by addition of 1–5%(wt.) iron (III) chloride or cobalt(II) chloride is reported. The photoinduced degradation, oxidation and dehydrochlorination were studied using Gel Permeation Chromatography, viscometry, UV-Visible and FTIR absorption spectroscopy. The photocrosslinking yield was estimated by separation of insoluble gel. Sample morphology was observed by optical microscopy. It was found that both salts change the course of photochemical reactions in PVC. These changes depend on salt type and content as well as on time of UV-irradiation. FeCl 3 and CoCl 2 hinder main chain scission in PVC in the initial stage of irradiation (after 1 h) but accelerate this reaction during prolonged exposure (10–30 h). Photodehydrochlorination is retarded in doped PVC (with the exception of PVC+1%CoCl 2). PVC photocrosslinking is less efficient in the presence of additives after longer period of degradation. The influence of salts on photooxidation is different—Fe chloride slightly decelerates whilst Co chloride accelerates the formation of carbonyl groups in PVC. The differences of both salts on photooxidative degradation in PVC are caused by their varying chemical nature and by differences in sample morphology.

A MALDI, TGA, TG/MS, and DEA study of the irradiation effects on PMMA

Poly(methyl methacrylate) (PMMA) ( M w=6.4×10 3, PD=1.06) was irradiated under vacuum. The constant dose rate was 1.66×10 4 rad/min at doses between 10 and 100 Mrad using a cobalt-60 source. The samples were then analyzed by matrix assisted laser desorption/ionization (MALDI), hyphenated thermogravimetric/mass spectrometry (TG/MS), and dielectric analysis (DEA), all novel methods for the analysis of polymers damaged by radiation. Gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), and differential scanning calorimetry (DSC) were also used for analysis. This study evidenced main chain scission, the removal of ester side groups, and the production of monomer as a result of ionizing radiation.

EPR studies of high dose gamma-irradiated poly(methyl methacrylate)

Gamma-irradiated poly(methyl methacrylate) (PMMA) with high doses 320, 480, 640 and 800 kGy were studied by EPR spectroscopy at 25 °C. The EPR spectra can be accounted for by the presence of two radicals: Ra and Rb. Ra has a well-known nine-line spectrum, which is attributed to the main chain scission radical. Rb has a broad single-line spectrum, which is observed only under high dose irradiation condition. The line positions of the EPR spectrum of Ra with irradiated dose of 13.6 kGy, can be simulated with the following parameters: g=2.0036; a=23(3H), 7.3(1H) and 13.3(1H) G; peak–peak linewidth (Δ H pp): 7.3 G. However, serious lineshape discrepancy is obvious. On the other hand, an artificial spectrum, which is the appropriate sum of a five-line spectrum ( a=23 G; g=2.0036; Δ H pp=4.6 G) and a four-line spectrum ( a=23 G; g=2.0036; Δ H pp=8.2 G) can faithfully reproduce the experimental spectrum of Ra. The experimental EPR spectrum of PMMA under high dose γ-irradiation can be separated into the artificial spectrum of Ra and the spectrum of Rb. Quantitative analyses of the spin concentrations indicate that [Ra] decreases while [Rb] increases with increasing irradiation dose. When annealed at 40 °C, the concentrations of [Ra] and [Rb] are followed a second order annihilation process and their time constants are 251 and 230 min, respectively.

Radiochemical Yields for Cross-Links and Branches in Radiation-Modified Poly(tetrafluoroethylene)

For the enhancement of the compatibility of fluoropolymers with other materials radiation modification is applied, for which irradiation with high-energy electrons provides an efficient and flexible possibility. Poly(tetrafluoroethylene) (PTFE) is known to predominantly undergo chain scission under irradiation. To counteract this chain scission and the resulting deterioration of the mechanical properties, PTFE has been irradiated at temperatures above the melting temperature under inert atmospheres. Under these conditions, in addition to chain scission long chain branches and cross-links are formed. The resulting structures have been studied by high-resolution fluorine-19 solid-state NMR applying highspeed magic-angle sample spinning. The structural changes as a function of energy dose and irradiation condition have been investigated upon irradiation under nitrogen atmosphere at 385 °C and under vacuum at 365 and 385 °C. Structures of interest are terminal groups, CF 3 side groups, branches, and cross-links. From quantitative information derived from the 19 F NMR spectra, the radiochemical yields, G(x) values, for the formation of CF 3 side groups, branches, and cross-links have been calculated. G(x) values of side groups and branches decrease with radiation dose, while the radiochemical yield of cross-links is nearly constant. These results are interpreted that in the first step in addition to main-chain scission the CF 3 side groups and branches were formed and in the second step after forming a sufficient quantity of side groups and branches the cross-links are formed.

Thermal decomposition characteristics of poly[diethyl-2-(methacryloyloxy)ethyl phosphate] using thermogravimetric analysis/mass spectrometry

The thermal decomposition of poly[diethyl 2-(methacryloyloxy)ethyl phosphate] (PDMP) by free-radical polymerization was studied by the combination of pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) with thermogravimetric analysis/mass spectrometry (TG/MS), in which the former identified directly the compositions of the evolved gases and the latter could further obtain real-time evolution curves of pyrolysates. A decomposition of PDMP was a two-stage process: The dominant mechanism of the first stage was initialized by hydrolysis at the phosphate ester linkage to evolve hydroxyethyl methacrylate (HEMA), triethylphosphate, and ethanol as major products; then, subsequently, the dominant mechanism of the second stage was a main-chain random scission to form acetaldehyde and water as major products along with carbonization of about 24% char yield of a solid residue at 600°C. Therefore, the reaction of phosphate groups into the backbone of polymeric materials could provide better flame retardancy. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 821–830, 2002

Study on increase in crystallinity in γ-irradiated poly(vinylidene fluoride)

Poly(vinylidene fluoride)(PVDF) was irradiated in vacuum. Low dose γ-irradiation induced the increase in crystallinity in PVDF is studied by means of X-ray diffraction and scattering, differential scanning calorimetry (DSC), gel data and mechanical properties analysis. It is suggested that the low dose irradiation results in intermolecular crosslinking and main-chain scission. The latter creates shorter polymer chain and reduces the number of entanglements per molecule which, in turn, increase molecular mobility. The increase in mobility permits the molecules to form crystallites in the amorphous regions.

Chemically Amplified Main Chain Scission: New Concept to Reduce Line Edge Roughness and Outgassing.

Chemically Amplified Main Chain Scission: New Concept to Reduce Line Edge Roughness and Outgassing.

Photosensitized degradation and crosslinking of linear aliphatic polyesters studied by GPC and ESR

Photosensitized reaction on aliphatic polyesters, 3g4 and M 23g4, prepared from succinic acid and propanediol derivatives were investigated by spectrophotometry, GPC, and electron spin resonance (ESR) methods. From change of GPC curves for 3g4 polyester doped with N, N, N′, N′-tetramethyl- p-phenylenediamine (TMPD) as a photosensitizer, it was confirmed that not only degradation but also crosslinking reaction occurred for 3g4, M 23g4, and poly(lactic acid), by UV irradiation. By detailed analysis of ESR spectra, it was confirmed that several radical species are produced through photosensitized reaction; TMPD radical cation, ester radial anion of polyester, main chain alkyl radical at the center of diol component, and acyl-type radical. Although there was no direct evidence for producing the main chain scission radical for 3g4 and M 23g4, it was strongly suggested that the photosensitized degradation and the crosslinking reactions were originated from the ester radical anion that was produced by electron capturing.

Photosensitive Polyetherimide (Ultem) Based on Reaction Development Patterning (RDP).

Photosensitive polyetherimide (PEI) consisting of non-photosensitive PEI (Ultem®) and photosensitive diazonaphthoquinone (DNQ) compounds gave positive-tone behavior by UV irradiation, followed by development in solution including ethanolamine. The SEM of the resultant images showed fine patterns with 10-15μm film thickness. GPC and 1H-NMR measurements that can give information on the structure of dissolved components in the irradiated region were carried out in order to clarify the mechanism of pattern formation. The imaging of photosensitive PEI is based on reaction development patterning (RDP), where the amine causes main-chain scission directly related to the pattern formation in development step to form low molecular weight amidized products.

Radiation-induced crosslinking of acetylene-impregnated polyesters

Enhanced crosslinking and reduction in chain scission are found in the amorphous regions of polycrystalline polyesters, when they are irradiated in the presence of acetylene. Increasing the chain length of the alkyl moiety is also found to decrease the extent of damaging main chain scission.

New material synthesis by radiation processing at high temperature—polymer modification with improved irradiation technology

Gamma-ray or electron beam irradiation at high temperature and at a small dose improved the Rockwell hardness and resistance to wear for polycarbonate and polysulfone. The effective temperature during irradiation was the glass transition temperature ( T g) of respective polymer, and the dose at maximum hardness was only 3–5 kGy. The effect for hardness was same between 60Co-gamma ray and electron beam and the main chain scission was predominant for both the polymers. The improvement of hardness and wear resistance was supposed to be dense molecular packing in matrix by rearrangement of molecules with synergistic effect of radiation and temperature.

NMR study of the gamma radiolysis of poly(dimethyl siloxane) under vacuum at 303 K

The structural changes which occur on the γ-radiolysis of poly(dimethyl siloxane) (PDMS) under vacuum at 303K have been investigated using 29Si and 13C NMR. New structural units consistent with main chain scission and crosslinking through both H-linking and Y-linking reactions have been identified. The results obtained at various absorbed doses have been used to calculate the G-values for scission and crosslinking. G-values for scission of G(S)=1.3±0.2, for H-linking of G(DCH2–R)=0.34±0.02 and for Y-linking of G(Y)=1.70±0.09 were obtained for radiolysis under vacuum at 303K. Thus crosslinking predominates over scission for radiolysis of PDMS under these conditions, and, by contrast with previous studies, Y-links have been shown to be the predominant form of crosslinks.

Radiation crosslinking and scission of poly(vinyl methyl ether) in aqueous solution

For a wide range of poly(vinyl methyl ether) (PMVE) concentrations (1–16 g dm −3), in anoxic conditions, polymer-derived radicals recombine in two major pathways: (i) crosslinking and (ii) disproportionation. Both these processes proceed inter- and intramolecularly. The radiation-chemical yields and kinetics of crosslinking have been studied by pulse radiolysis with light scattering intensity detection (LSI). In the absence of oxygen, G-values of intermolecular crosslinking were determined on the basis of LSI changes versus applied dose and compared with the results obtained previously for γ-irradiated samples. It has been found that the first half-life time of intermolecular crosslinking decreases with increasing dose per pulse. Addition of small amounts of macroradical scavenger (cysteamine hydrochloride) decreases, drastically, the increase of LSI signal. On increasing the PVME concentration, intermolecular crosslinking becomes more efficient. In the presence of oxygen, for diluted PVME solution (0.1 g dm −3), decrease of LSI signal consisting of the kinetic of a first-order reaction was observed. The rate constant of LSI decrease was found to be 1.1×10 3 s −1 and it was attributed to the main-chain scission.

Positive-Tone Imaging of Photo-Decomposable Polymer LB Films

Abstract We describe the lithographic properties of poly(N-isopentylmethacrylamide-co-t-butyl 4-vinylphenyl carbonate) [p(iPMA-tBVPC)] which has a structure that is subject to the main chain scission and to deprotection of t-butoxycarbonyloxy group by deep UV irradiation. The positive-tone patterns of the p(iPMA-tBVPC) LB film with 60 layers could be obtained by deep UV light irradiation, followed by development with alkaline aqueous solution. The resolution of the pattern was 0.75 μm, which is the resolution limit of the photomask employed. The photopattern formation with the copolymer [p(iPMA-tBVPC)] LB film was more sensitive than the homopolymer poly(N-isopentylmethacrylamide) [p(iPMA)] LB film. The etching resistance of the p(iPMA-tBVPC) LB film deposited for the pattern of the gold film is also investigated.