Random Chain Scission Process Research Articles

Direct Insertion Mass Spectrometric Analysis of Thermal Degradation of Poly(2‐alkyl‐2‐oxazoline)

AbstractDirect pyrolysis mass spectrometry is applied to investigate the thermal behavior of poly(2‐isopropyl‐2‐oxazoline) (PIPOX, a thermoresponsive polymer) and poly[2‐(3‐butenyl)‐2‐oxazoline] (PBOX, a “clickable” polymer). It is found that the thermal degradation of PIPOX is started by a loss of side chains. At slightly higher temperatures the degradation of the polymer backbone occurs by random chain scission processes. In the case of PBOX, vinyl polymerization of the side chains produces chains with variable thermal stabilities. The number of repeating units of the polymer has almost no effect on the thermal behavior of PIPOX, but significantly affects both thermal stability and degradation product distribution of PBOX.

Thermo-mechanical degradation and VOC emission of unstabilized and stabilized polypropylene copolymer during multiple Extrusions

The thermo-mechanical degradation during the multi-extrusion of unstabilized and stabilized polypropylene copolymer (cPP) was analyzed using the Chain Scission Distribution Function (CSDF) method. During the first extrusion of unstabilized cPP almost 60% of the initial polymeric chains were submitted to chain scission. The calculations using CSDF show a random chain scission process of chains with molecular weight below 100 kg.mol-1, and above that a preferential chain scission process. When stabilized the cPP molecular weight is kept constant, even after four extrusions, independently of the stabilizers concentration used. Its chain scission is greatly reduced, only being noteworthy at high values of molecular weight, presenting in this case a preferential chain scission process. During extrusion the unstabilized cPP evolves Volatile Organic Compounds (VOC) due to the volatilization of low molecular weight oxidized chain segments. VOC's emissions are greatly reduced during the melt processing of stabilized cPP, even after multiple extrusions.

Thermal degradation behavior of styrene‐butadiene‐styrene tri‐block copolymer/multiwalled carbon nanotubes composites

AbstractThe thermal degradation behavior of styrene‐butadiene‐styrene triblock copolymer (SBS) and SBS/multiwalled carbon nanotubes (MWCNTs) composites prepared by solution processing and melt mixing, respectively, was investigated using nonisothermal thermogravimetric analysis (TGA). The kinetic parameters of the activation energy (Ea) for degradation, preexponential factor A, and the reaction order (n) were evaluated by the Flynn‐Wall‐Ozawa, Kissinger, and Coats‐Redfern methods, respectively. Ea increased and n decreased after the incorporation of 3 wt % of MWCNTs into the SBS. The Ea of SBS/MWCNTs composite prepared by melt mixing was higher than that by solution processing, which was attributed to the good dispersion of MWCNTs in SBS and the interactions between MWCNTs and SBS. The gases evolved during thermal degradation at a nitrogen atmosphere were studied by Fourier transform infrared spectroscopy (FTIR) coupled with the TGA. Aliphatic and aromatic CH peaks appeared simultaneously in FTIR spectra, indicating the thermal degradation of SBS proceeds by a random chain scission process. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

The UV/H2O2 - photodegradation of poly(ethyleneglycol) and model compounds

The general mechanism for the photodegradation of polyethyleneglycol (PEG) by H2O2/UV was determined studying the photooxidation of small model molecules, like low molecular weight ethyleneglycols (tetra-, tri-, di-, and ethyleneglycol). After 30 min of irradiation the average molar mass (Mw) of the degradated PEG, analysed by GPC, fall to half of its initial value, with a concomitant increase in polydispersitivity and number of average chain scission (S), characterizing a random chain scission process yielding oligomers and smaller size ethyleneglycols. HPLC analysis of the photodegradation of the model ethyleneglycols proved that the oxidation mechanism involved consecutive reactions, where the larger ethyleneglycols gave rise, successively, to smaller ones. The photodegradation of ethyleneglycol lead to the formation of low molecular weight carboxylic acids, like glycolic, oxalic and formic acids.

A critical molar mass separating the ductile and brittle regimes as revealed by thermal oxidation in polypropylene

The thermal oxidation of unstabilized polypropylene films has been studied at 70, 90, 110 and 130 °C in air by IR, SEC, rheometry and tensile testing. Embrittlement is observed when the weight average molar mass reaches 210±10 kg mol −1. Polydispersity data are consistent with a homogeneous random chain scission process. This especially high sensitivity of fracture properties to chain scission is assumed to be specific of semi-crystalline polymers having their amorphous phase in rubbery state.

Thermal degradation of monodisperse polystyrene in bean oil

Thermal degradation of monodisperse polystyrene (PS) in bean oil was investigated in the temperature range of 305–350°C. The degradation products were identified by means of gel permeation chromatography (GPC) and Fourier transform infrared spectrometry (FTIR). A significant feature for the investigated system was that PS in the presence of bean oil thermally degraded into larger fragments of the polymer chain instead of monomer or oligomers which are usually generated in the direct thermal degradation. The average molecular weights of degradation products of PS decreased and the polydispersity indexes ( M w/ M n) increased but did not exceed 2 as the reaction time increased. The kinetics of the degradation reaction was also examined based on the GPC measurements. The results revealed that the degradation of PS in bean oil was a first order reaction and the energy of activation for chain scission was 215 kJ mol −1. The thermal degradation of PS in bean oil is believed to be a random chain scission process involving radical intermolecular transfer between bean oil and intermediate macroradicals of PS.

Study on thermal degradation of sol and gel of natural rubber

The thermal degradation of natural rubber (NR) in air at a constant heating rate was studied by using of the thermogravimetry (TG) and thermogravimetry and differential thermal analysis (TG–DTA) simultaneous techniques. It indicates that the temperature of thermal degradation of gel and sol of NR rises linearly along with the increment of the heating rate, whereas the heating rate has little effect on the degree of thermal degradation. Accompanying other side reactions, the thermal degradation of NR is not a simple random chain scission process, and it is an exothermic reaction. The dynamic variation of molecular structure of NR during the thermal degradation was studied using Fourier transform infrared spectroscopy (FTIR). It shows that the products of the thermal degradation of both sol and gel of NR are hydroperoxide, carbonyl, and hydroxyl compounds. The formation of gel makes the temperature of the thermal degradation of NR decrease and the rate of the thermal degradation increase; thus, the thermal stability of NR is reduced. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1339–1344, 2000

Chain scission distribution function for polypropylene degradation during multiple extrusions

The degradation of polypropylene subjected to multiple extrusions was evaluated by following the changes in the molecular weight distribution curves (MWD) obtained by size exclusion chromatography (SEC). The curves shift to the low molecular weight side and we calculate the chain scission distribution function (CSDF) as a function of the initial molecular weight. Assuming that the SEC curves of both, virgin and degraded PP follow Gaussian behaviour the calculated CSDF data would be a straight-line function of the original molecular weight (MW). A random chain scission process gives a constant and positive CSDF value independent of MW. If the process is no longer random the CSDF value increases as a result of the increase in the chain scission probability. Polypropylene subjected to multiple extrusion shows that the chain scission processes during thermo-mechanical degradation display a CSDF curve with a constant positive value in the low molecular weight region which rises in the higher MW region. This indicates that the probability of chain breaking is dependent on the degradation aggressiveness and the molecular weight of the chain. At low MW, it is independent of it, i.e. the chain scission is random, but at higher MW it becomes dependent, increasing with MW. The behaviour has been measured during multiple extrusions and using different screw configurations. The CSDF values are higher the greater the aggressiveness of the degradation, i.e. number of extrusions and screw configuration: kneading elements with 45° (KB45) degrade more than conveying elements (CON), increasing the overall level of chain scission.

Effect of abiotic factors on the degradation of poly(3-hydroxybutyrate- co-3-hydroxyvalerate) in simulated and natural composting environments

The effect of abiotic factors such as water and air on the degradation of poly(3-hydroxybutyrate- co-3-hydroxyvalerate) (PHBV) in a compost was investigated using simulated and natural environments. These include exposure in sterile water at 60°C, in air at 60°C and in a garden waste compost. In addition, the chain scission degradation mechanism of PHBV on immersion in sterile water at 60°C was studied in detail. The results show that during a period of 50 days, water and air have little or no effect on the degradation of PHBV in a garden waste compost. The degradation is due to microbial action only. According to the literature, the degradation of PHBV in sterile water follows a random chain scission process. We have found that degradation follows random chain scission kinetics only in the beginning of the immersion in sterile water at 60°C. After longer immersion times (200 days), the degradation process no longer obeyed first order kinetics and a decrease in mol% HV in the copolymer was observed.

Hydrolysis of poly(ethylene terephtalate). A steric exclusion chromatography study

The hydrolysis of PET ( M w 27 kg mol −1) in boiling water has been studied by steric exclusion chromatography. It appears that both the average molar mass and the polydispersity index decrease in a pseudohyperbolic way and tend towards asymptotic values, respectively M n ∞ →2 kg mol −1 and I ∞→1.50. This behaviour indicates that hydrolysis is homogeneous at dimension scales higher than Lc (lamellae thickness). Hydrolysis occurs in the amorphous phase and at chain folds at the crystal surface, whereas crystals are unaffected due to their impermeability to water. The chain length decreases to tend, thus, towards L c . Embrittlement is expected to occur when the entanglement network is destroyed in the amorphous phase. Since the corresponding critical chain length L e and the initial chain length L 0 are such as L e and L 0>> L c , it can be considered that in the conversion range of practical interest (e. g. before embrittlement). PET hydrolysis, despite its above quoted peculiarities, can be considered as a random chain scission process in kinetic studies.

Molecular weight distribution and mass changes during polyamide hydrolysis

Samples of polyamide 6, 11 and 12 have been hydrolysed at various temperatures ranging from 80 to 120°C in water baths of pH ranging from 0 to 4. The polymer degradation was monitored by steric exclusion chromatography (SEC), viscometry and gravimetry. SEC data indicate that the polydispersity of the polyamides tested does not increase during ageing. This can be considered as proof of the homogeneity of the reaction. Average molecular weight data from SEC or viscometry appear to be consistent with a random chain scission process, however gravimetry (at least in the case of relatively soft conditions, for instance pH ≥ 4) indicates a high monomer yield. These apparently contradictory results are tentatively explained by the equilibrated character of hydrolysis. Acid—amine pairs resulting from hydrolysis events would recombine easily when both belong to macromolecular fragments. When one fragment is the monomer or a low oligomer, it can escape easily from the cage and the corresponding recombination yield is decreased. The monomer extraction by water complicates the gravimetric behaviour, especially in the most severe hydrolysis conditions.

A viscometric method for the determination of induction period for polymer thermal oxidation

On the basis of viscometry technique a novel method was developed to determine the induction period for polymer thermal oxidation (τi). Natural rubber films from Manihot glaziovii degradated at 90°C were used as a model. Mv values were calculated by using the equation: [η] = 1.9 × 10−Mv0.745 in toluene at 30°C. A random chain scission process was applied. A plot of 1nN (N is the number of broken bonds, C=C in case of rubber) versus time of heating shows two lines. The first indicates no degradation (1nN=1nNo, to t≤τi) while the second represents the real kinetics of the process (1nN=1nNo-k (t-τi), to t≥τi). The intersection of these lines is the induction period of thermal oxidation. The effect of a protective agent, carnauba wax, was tested. The resolution of the method is comparable to that of chemiluminescence and is superior to that of IR and DSC.

Hydrolysis of poly(ethylene terephthalate): a kinetic study

The hydrolysis of poly(ethylene terephthalate) (PET) was studied at 100°C in distilled water, using molecular weight, crystallinity ratio, water equilibrium concentration and gravimetric (in the dry state) determinations. The crystallinity ratio and water equilibrium concentration increase noticeably during exposure. The classical second order kinetic model was applied to the amorphous phase, taking into account the increase in its hydrophilicity. This leads to a relatively good representation of the polymer behaviour, the second order rate constant being k = 1·83 × 10 −8 kg mol −1 s −1. The sample (dry) weight increases slowly at first for about 20 days of exposure and then decreases rapidly. A kinetic study of the weight balance was made assuming that only monomers (ethylene glycol and terephthalic acid) or the monoester are released. This leads to good predictions at the semi-quantitative level. It thus appears that neither autocatalysis nor preferential hydrolytic attack at chain ends are needed to explain the behaviour of PET during its hydrolysis. A simple second order, random chain scission process seems to be sufficient to describe this behaviour.

Kinetics of Chain Scission of Natural Rubber

Abstract The first-order and zero-order kinetic models of chain scission, based on random chain scission processes, are critically examined. It is likely that for many practical situations, the first-order chain scission kinetics can be represented by pseudozero-order kinetic types of equations. The kinetic results indicate that chain scission of NR occurs either by a pseudofirst-order or a pseudozero-order process. The pseudozero-order chain scission kinetics of NR are in consonance with the result that chain scission involves the bimolecular reaction of peroxy radicals in the termination step of the oxidation mechanism. However, this does not preclude unambiguously other chain scission reactions. The chain scission activation energy of NR is determined with the use of expressions derived for the oxidative degradation of NR as measured in terms of Wallace plasticities.

Radiation-induced degradation of polycarbonate: Electron spin resonance and molecular weight measurements

Elementary processes in the γ-irradiated degradation of bisphenol A polycarbonate (PC) have been studied by electron spin resonance (ESR) and molecular weight measurements. γ-Irradiation of PC films at −196°C gave an ESR spectrum attributable to trapped electrons, phenyl and phenoxy type radicals. An ionic species produced in PC was observed by optical absorption measurements. The viscosity average molecular weight, Mv , decreased with increasing irradiation dose both in vacuum and in air. A linear relationship between ( 1 Mn(t) − 1 ( Mn (0)) and irradiation dose was obtained, where Mn (t) is the number average molecular weight at irradiation time t. Random chain scission processes are suggested for γ-ray induced degradation of PC films. The ratio of the apparent rate constant of chain scission of PC in air to that in vacuum, k s(air)/k s(vac), was estimated to be 2·4. A mechanism for γ-ray induced degradation of PC is proposed on the basis of the experimental results.

Thermal degradation of polyurethane based on MDI and propoxylated trimethylol propane

AbstractA polyurethane based on diphenyl methane diisocyanate (MDI) and propoxylated trimethylol propane was thermally degraded by using the techniques of pyrolysis and thermogravimetric analysis (TGA) in an inert atmosphere. Identification of pyrolysis gaseous products at 600°C showed that the first degradation step consists of a reversal of the polycondensation process, i.e., dissociation into starting polyol and diisocyanate, followed by the polyol degradation and a probable diisocyanate polymerization. Kinetic parameters were determined using dynamic and isothermal TGA curves. It is shown that the degradation can be closely compared with a random chain scission process.

Evaluation of molecular weight averages resulting from random chain scission process for wide distributions as in polyolefins

AbstractThe effects of random degradation on the molecular weight averages for broad Schulz‐Zimm and Wesslau type distributions are evaluated by blending two most probable Flory type distributions so that the Mn:Mw:Mz ratios of the blend were equal to that of the distribution models for any particular Mw/Mn ratio. The results indicate a rather extreme sensitivity of broad Wesslau distributions to small degrees of degradation and suggest that this function is not a good description for polyolefins.

Thermal degradation of vinyl polymers in solution. I. Degradation of polystyrene in naphthalene solution

AbstractThe thermal degradation of polystyrene has been studied in naphthalene solutions in high vacuum. The rates of loss of weight have been determined over a range of temperatures from 350 to 380°C., and the energy of activation has been derived. Also the decrease in intrinsic viscosity with time and the loss of weight as function of polymer concentration have been investigated. The mechanism of degradation in naphthalene solution is similar to that of the thermal bulk degradation; it consists of an initial random chain scission process due to weak links, followed by a reverse polymerization process with chain end initiation.