Recombination Of Macroradicals Research Articles

Development of a spatially structured polymeric matrix under UV irradiation of polylactide-based composites filled with aluminosilicate microspheres

The relevance of the study is conditioned by the fact that increased consumption of synthetic polymers leads to an increase in environmental pollution due to the long decomposition time of plastic waste. As a result, it is necessary to develop polymer composites based on a biodegradable polymer matrix, and to improve the performance properties of finished plastic products, it is necessary to purposefully select cheap and affordable inorganic fillers. Thus, the purpose of this study is to investigate the regularities in the generation of a spatially structured polymer matrix under UV irradiation of polylactide-based composites filled with aluminosilicate microspheres (ASM). The leading approach to the given problem is to melt polymer composites of various compositions and to determine the physical, mechanical, and thermophysical characteristics of the prototypes, including the supermolecular structure of the polymer matrix under the influence of ultraviolet irradiation. The study suggests that the filling of polylactide with ASM particles leads to an increase in the elastic modulus, a decrease in the strength at static rupture and resistance to dynamic destructive effects, as well as heat resistance. Small aluminosilicate microspheres, when added to polylactide, perform the function of nucleation and, even with a small content, increase the crystallinity degree by 3.7 percentage points. In the range of ASM content from 1 pph to 10 pph, the absolute value of the crystallinity degree practically does not depend on the filler concentration in the polymer composite. UV (ultraviolet) irradiation in the presence of air oxygen initiates the thermooxidative destruction of polylactide and leads to the establishment of a spatially structured polymer phase using the electrostatic intermolecular interaction of additionally formed oxygen-containing functional groups in macrochains, as well as partial intermolecular crosslinking during recombination of macroradicals. The establishment of spatial structures in the polymer matrix under UV irradiation determines an increase in the resistance of experimental samples to thermal effects. It is manifested in an increase in the bending temperature under load by 7-10 percentage points, a decrease in the crystallinity degree by 1.2-2.6 percentage points, a decrease in the fluidity of the meltage and also an increase in the glass transition and melting temperature. The materials of the study are of practical value for the development of biodegradable composites based on polylactide filled with inorganic components.

Stability of Polymer–Monomer Particles of Synthetic Latexes

The presence of “living” macroradicals in the volume of a polymer–monomer particle may be one of the factors responsible for the loss of the aggregative stability of latexes. If the adsorption protection of a polymer–monomer particle is insufficient, high content of “living” macroradicals leads to gelation in the latex in the course of storage. Correlation between the latex life time, hydration of nonionic surfactant molecules in the adsorption layer of a polymer–monomer particle, and extent of the action of the macroradicals was determined. Naphthalenesulfonic dispersing agents enhance the stability of latex systems in the step of polymerization and distillation of the monomers owing to a decrease in the critical micelle concentration of the emulsifier, to extension of the micellar period of the polymerization, to an increase in the degree of saturation of polymer–monomer particles, and to an increase in the probability of macroradical recombination in the volume of a polymer–monomer particle. With an increase in the degree of polycondensation of naphthalene-containing dispersing agents, their surface activity increases, whereas the ability to support the aggregative stability of latexes decreases.

Impact of photooxidative degradation on the oxygen permeability of poly(ethyleneterephthalate)

This article reports a study of the influence of PET photooxidation on the oxygen barrier properties of this polymeric material. It is shown that the photochemically induced oxidation of PET provokes a decrease of the oxygen permeability coefficient, which affects the properties of the material. This effect is shown to result from crosslinking by recombination of macroradicals formed by the photooxidation of PET. The global effect that is observed is thickness-dependent. As a consequence of the attenuation of the UV light that penetrates the sample, photochemical oxidation of PET only occurs in the first 35 microns from the exposed surface, meaning that for samples with a thickness larger than that of the oxidised layer, the permeability decreases at the surface, whereas that of the core of the sample is not modified. This is corroborated by measurements of the variations of permeability coefficients for films with different thicknesses.

Determination of the rate constants of recombination of macroradicals and stable radicals via the competitive-inhibition method

The rate constants of recombination, kX, of propagating radicals with nitroxides in pseudoliving radical polymerization are determined via the competitive-inhibition method with the use of ESR spectroscopy. This method is applicable to determination of kX in the reactions of propagating radicals of styrene, acrylic acid, and methyl methacrylate with two stable radicals, the nitroxide diethylphosphono-2,2-dimethylpropyl nitroxide and the phenoxide galvinoxyl. The values of kX determined at 50°C increase in the following sequence: diethylphosphono-2,2-dimethylpropyl nitroxide-TEMPO-galvinoxyl. The selectivity of the low-activity propagating radicals of styrene in reactions with stable radicals is shown.

Radiation modified high impact polystyrene

The purpose of applying high energy (ionising) radiation with absorbed doses up to 1MGy was to achieve controllable changes in mechanical properties of high impact polystyrene (PS-HI) and, at the same time, to investigate the possibility of using reprocessed irradiated polymeric material. Dielectric relaxation of a radiation modified high impact polystyrene (PS-HI) has been investigated by the time dependence of charging and discharging current. The transient currents for the irradiated PS-HI were well approximated by the power function of the logarithm of time and related to the fractal dimension. It was also shown that yield strength and tensile strength increase while elongation at break decreases with increasing absorbed dose. The specimen prepared by a post-irradiation moulding gave higher melt flow rate than those of specimen formed before irradiation. These results indicate that after radiation the system of PS-HI is reprocessable. It is concluded that an oxygen environment at the beginning of irradiation leads to enhanced chain scission at the expense of crosslinks via peroxide formation and causes oxidative degradation of the main polymer chain of irradiated PS-HI at a low absorbed dose. However, at higher absorbed doses the quasi-inert environment has been established and crosslinking, due to recombination of macroradicals, is dominant.

The preparation and rheology characterization of long chain branching polypropylene

In order to study the rheological behavior of long chain branching (LCB) polypropylene (PP), linear polypropylene was modified by melt grafting reaction in the presence of 2,5-dimethyl-2,5( tert-butylperoxy) hexane peroxide and pentaerythritol triacrylate (PETA) in mixer. The transient torque curves and Fourier transformed infrared spectroscopy (FTIR) results indicated that macroradical recombination reactions took place and PETA had been grafted onto PP backbone. Various rheological plots including viscosity curve, storage modulus, loss angle, Han plot, Cole–Cole plot were used to distinguish LCB PP from linear PP. On the other hand, to quantify the LCB level in modified PPs, a new method was suggested on the basis of macromolecular dynamics models. The results showed that the level of LCB was in the range of 0.025–0.38/10 4 C . Moreover, the length of the branched chains and the content of the branched component increase with PETA concentration. Furthermore, the LCB efficiency of monomer can also be calculated, less than 20% of grafting monomers was used to form branch structure.

Laser flash photolysis of carboxymethylcellulose in an aqueous solution

AbstractLaser flash photolysis with excitation at 248 nm was used to study photochemically derived changes of carboxymethylcellulose (CMC) in aqueous solutions. Transient absorption spectra of solutions after photolysis revealed a broad band with a maximum of approximately 720 nm, which could be ascribed to the signal of the hydrated electron. The interaction of the hydrated electron with CMC was slow (<107 dm3 mol−1 s−1), but the OH radical, formed by the decomposition of H2O2, reacted with CMC at a high rate constant (9.5–11.0 × 108 dm3 mol−1 s−1). The rate constant of the reactions of CMC with hydroxyl radicals depended on the conformation of the macromolecules, which was determined by the pH of the solution. Transient absorption was recorded at a wavelength shorter than 370 nm for CMC solutions photolyzed in the presence of H2O2. As a result of OH attack, long‐lived radicals were formed on CMC. The recombination of macroradicals led to the formation of crosslinking bonds between side‐chain groups, and as a result of it an insoluble gel arose in low‐pH solutions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 505–518, 2005

Radiation-induced chemical processes in polystyrene scintillators

The regularities established for macroradical accumulation and intensity of radioluminescence under γ-irradiation of a polystyrene scintillator prove benzyl macroradicals to be efficient quenchers of the excited scintillator molecules. Dissolved oxygen was determined to have a constant of the quenching rate 100 times lower than that of macroradicals. Oxygen is an efficient antirad because of participating in oxidation reactions and subsequent recombination of macroradicals. The method was developed to obtain a polymeric scintillator with a polystyrene matrix containing a dispersed system of pores and channels. Radiation resistance of such a scintillator is 5–10 times higher than that of standard types.

Squalane as Model for Free Radical Reactivity of PP

Abstract The degradation, grafting and crosslinking of polypropylene have been studied with a free modelling system: the squalane. This study used conditions similar to reactive extrusion in terms of temperature and concentration of actives species. The products of reactions were characterized by mass spectrometry and IR spectroscopy. The reaction between peroxide and squalane confirms the importance of the half-life of the peroxide in limiting the degradation of the squalane chain. In presence of monomer, we observed simultaneously grafting and β-scission of squalane. Comparison between methacrylates and acrylates shows that acrylates are much more effective for grafting than methacrylates. With trifunctional monomers, the main reaction is the crosslinking of squalane (or recombination of macroradicals via the monomer). In this case, triacrylates were also more effective.

Photodegradation in Aqueous Solution of a Polyamidehydroxyurethane Type Polymer. Temperature Effect on the Process of Degradation

The present research shows some aspects of temperature effect on the process of photodegradation in aqueous solution of a polyamidehydroxyurethane type polymer. The process of photodegradation in aqueous solution of the studied polymer proceeds with chain scissions, what is promoted at elevated temperatures. At the temperatures 45°C and 65°C the values of the number average of chain scissions, S, and of the quantum yield of chain scission, φcs, are greater at the initial moments of the process than the correspondent values at 25°C and 35°C. At elevated temperatures there is an increase in the probability of the process of recombination of macroradicals, formed during chains scission. The values of S at 45°C and 65°C by the end of the process are smaller than those at 35°C and 25°C. It can also be noticed, that the values of φcs at 45 °C and 65°C continuously decrease after the initial moments of irradiation till the end of the process of degradation, while at 25°C the values of φcs increase practically during the whole process. At 35°C the behaviour of the system is more unusual.

Grafting and Degradation Reactions at the Synthesis of Interpenetrating Polymer Networks in Situ from Polyethylene and Butyl Methacrylate

Abstract The grafting and degradation reactions accompanying the synthesis of interpenetrating polymer networks (IPN) in situ consisting of polyethylene (PE) and poly(butyl methacrylate) (PBMA) were investigated under model conditions. After polymerization at 110°C it was found that the 2,5-dimethyl-2,5-di-(t-butylperoxy)hexane (Luperox-101) had a destructive effect on the PBMA network formed provided the temperature was raised to 160°C. Simultaneously in this reaction stage, further linkage of the PBMA chains with PE originates from a recombination of macroradicals from both polymers. In the thermally initiated polymerization of BMA (in the absence of peroxide), PE is grafted with BMA but no crosslinking of PE takes place. If a mixture of initiators (2,2′ - azo-bis-isobutyronitrile and Luperox-101) was used, a retardation of PE grafting with BMA was observed. A considerable degree of PE crosslinking and grafting of PBMA onto a PE network was achieved if both polymers were mixed with 3 mass % of Luperox-101.

Chemical aspects of the radiation‐induced conductivity in polymers

AbstractIt has been demonstrated that radiation‐induced conductivity (RIC) is strongly dependent on the chemical structure of the macromolecules, this dependence being influenced by the position of the active molecular group acting as a hopping center for charge carriers: main chain vs pendant case. Both RIC magnitude and its nature (free ion vs geminate) are affected. Annealing of the radiation‐induced conductivity (RIC) in four common polymers (PS, PET, LDPE and PTFE) continuously irradiated in vacuum to doses of about 5 × 104 Gy has been studied. While substantial recovery (up to 75%) has been observed in PS and PET, only steady degradation of RIC has been encountered in LDPE and especially in PTFE (annealing at elevated temperatures has been fulfilled only in air). Unlike other polymers tested, PS shows recovery that depends critically on the surrounding atmosphere (air or vacuum). To describe these results one relies heavily on the radiation‐induced oxidation as a chain radical process with degenerate branching (a case of LDPE and PTFE) and catalytic recombination of macroradicals (as in PS).

Styrene polymerization in silica gel porous grains

The effect of pore dimensions of silica gel grains on the peroxide initiated styrene polymerization was investigated. The observed changes in shape of conversion curves and rate of polymerization are interpreted as due to decrease of the rate of recombination of macroradicals separated by the walls of grains pores, accelerated decomposition of peroxide in cavities of submicrondiameters and ionic polymerization in the smallest pores of 10 nm diameter. The resulting course of polymerization shows influences of silica gel structure on all elementary polymerization steps, depending on size of pores.

Welding Mechanisms of Plastics: A Review

Abstract Strength of welded joints is a function of technological parameters of the production process. The type of function is dependent on the welding mechanism. Different mechanisms were found under various welding conditions. The processes included in the plastic welding mechanism are divided into two groups: 1) Processes which realize the joining of the parts. 2) Processes which create conditions for the first group to proceed. The first series of processes includes: a) diffusion of macroradicals, molecular segments or molecules of the polymer which can be either in a solid, melted or dissolved state. b) convective mass transfer. c) recombination of macroradicals across the contact surface. d) physical (surface) interaction. e) any combination of processes described above. The second group contains: a) formation of the real contact surface. b) formation of the macroradicals. c) destruction and removal of inert layers which prevent real contact of active material. Each process and the conditions of it...

Graft polymerization of methyl methacrylate onto chemically modified wools using LiBr–K2S2O8 redox system as initiator

AbstractGraft polymerizations of methyl methacrylate onto natural, reduced, S‐carboxymethylated, S‐aminoethylated, sulfated, acetylated, and oxidized wools were performed at 30°C using a LiBr–K2S2O8 redox system as initiator. The extent of grafting was found to be dependent on the thiol and amino contents of wool fibers; carboxylic, sulfated groups, and oxidation intermediates had no significative influence on graft yields. The polymethacrylic chains obtained after hydrolysis of the natural part of the copolymer and analyzed by infrared and NMR spectroscopies were characterized by an atactic structure quite comparable to that found for a corresponding homopolymer. There was no stereoregulating effect on methyl methacrylate polymerization due to the crystalline components of the wool structure. A quantitative determination of the number of amino acid residues linked to the end of polymethacrylic chains after hydrolysis indicated that chain termination occurs both by disproportionation and recombination of macroradicals for natural, sulfated, S‐carboxymethylated, and S‐aminoethylated wools; for reduced and oxidized wools, only a recombination by disproportionation was observed in accordance with the open structure of these wool fibers. The chain lengths of the isolated poly(methyl methacrylates) were also found to be consistent with the participation of thiol and amino groups in grafting. It seems, however, that amino groups do not themselves act as initiating sites.

A model of macroradical recombination in dilute solutions

The dependence of exchange rate constants and diffusion coefficients of stable macroradicals (modified polydimethylsiloxane ( P = 15, 29, 97 ) and polyethylenoxide ( P = 25, 47, 140 ) on the length of polymer chain in various solvents is investigated. The results are compared with the rate constants for diffusive bimolecular recombination of macroradicals of various lengths. The dependence of rate constants of exchange and combination for macroradicals of different lengths obey the same law. The rate of combination for macroradical with P = 15-10 5 is shown to depend mainly on the mobility of an active centre, limited by diffusion of the whole macromolecule. The diffusion rate constant of combination can be described by the Smoluchowski equation in which the radius of an active centre is used as the effective radius of interaction. A model of bimolecular termination of macroradicals, a model of “freely penetrating coils” (FPC), is proposed. It implies that k ter ∼ P −0.9 ± 0.1 (for “good” solvents). The reactivity of an active centre in recombination is independent of the length of a macroradical chain for P > 15. The exchange interaction in a radical pair broadens the ESR lines of radicals and recombination with equal effectiveness.

The rate constants of macroradical recombination as a function of chain length (review)

The rate constant of spin exchange k s and the diffusion coefficients of the stable radicals of polyoxyethylene derivatives (average degree of polymerization P ̄ = 25, 47 and 140 ) and of polydimethylsiloxane ( P ̄ = 15, 29 and 97 ), were investigated as functions of chain length in various solvents. The results were compared with the rate constants of diffusive mutual termination k t for macroradicals of various lengths. The dependence of k s and k t on chain length conform to a single rule. Macroradicals in the P ̄ = 15−10 5 range were found to have their termination reactions limited chiefly by the mobility of the reactive centres, which depends on the diffusion of the entire macromolecules. Coefficient D is described by the Smolukowski equation in which the effective radius of reaction is that of the reactive centre. A model for the mutual termination of the macroradicals, i.e. the “freely permeated coils” model, has k ∼ P ̄ 0·9±0·1 as a valid function (in the case of good solvents). It is concluded that the reactivity of the reactives centres at P ̄ > 15 depends on chain length in the recombination reactions. The electron-exchange reactions will result just as effectively in ESR line broadening with pairs of radicals as in the recombinations.

Mechanism and kinetics of polyethylene crosslinking by α,α′‐bis(tert‐butylperoxy)‐p‐diisopropylbenzene

AbstractThe study of the mechanism of polyethylene crosslinking is realized by a kinetic analysis of the α,α‐bis(tert‐butylperoxy)‐p‐diisopropylbenzene decomposition, as well as by the determination of its decomposition products and crosslink formation in the polymer. The experiments were carried out in a temperature range of 118°–148°C in both polyethylene and its low‐molecular model, n‐octane. From the results obtained it follows that the peroxide decomposition in both hydrocarbon media is kinetically a unimolecular reaction with an activation energy of 36 ± 2 kcal/mole and with an equivalent participation of both peroxidic groups, whereby a biradical formation is improbable. Macroradicals arise by a dehydrogenization reaction in which mainly primary oxyradicals of various types take part and methyl radicals are also formed by a transformation process of the former. Both types of radicals decay exclusively in a substitution reaction with polymer chains. The whole process is terminated by macroradical recombination so leading to crosslink formation in polyethylene.

Investigation of polymerization mechanisms by means of molecular weight distributions of reaction products

AbstractThe study of MWD functions gives insight into the mechanism of polymerization and polycondensation reactions. Especially all kinds of side reactions, by altering the already formed macromolecules, give characteristic changes in the MWD. Some special cases are discussed. For polycondensation products, two types of MWD are found. In the early stages of the reaction before the monomer has been consumed, broad distribution curves of the Flory shape are observed. In the later stages, after the polymer has been heated, quite narrow distributions of the Gaussian type are found. The theoretical explanation involves consideration of scission and macromolecular combination reactions, which result in random interchange of monomer units among macromolecules. For products of addition polymerization of dienes in the presence of a catalytic agent butyllithium, exceedingly narrow distribution functions are observed, the polymer being almost homogeneous. This is the result of simultaneous initiation and further growth of all the chains at the same rate during the entire reaction period without termination. For radical polymerization, sometimes simple unimodal, broad distribution curves are found and the termination constants (recombination and disproportionation rates) can be calculated from the curves. In special cases, however, the distribution functions are complicated and consist of many sharp maxima. It is suggested that secondary reactions of chain scission and macroradical recombination occur, which results in a sharpening of the MWD. Subsequent crosslinking of macromolecules from such sharpened distributions gives doubling, tripling, etc. of average molecular weights. This is the explanation for the polymodal sharpened distribution curves observed.