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     

Infectious Behavior in Photo-oxidation of Polymers

When a polymer is used together with others,its aging process will be affected by the adjacent polymers.This infectious behavior between polymers makes the aging process more complex than that of an individual material.In this study,infectious behavior in photooxidation of polymers was investigated.Polypropylenes(PPs),an unstabilized PP and a commercial PP,were chosen as the infection sources.Six typical polymers,high density polyethylene(HDPE),low density polyethylene(LDPE),polystyrene(PS),polycarbonate(PC),poly(ethylene terephthalate)(PET),and polyamide 6(PA6),were used as the targets.The degree of oxidation of the targets was evaluated by attenuated total reflection-Fourier transform infrared spectroscopy(ATR-FTIR).An accelerating effect of two infection sources on the photo-oxidation of the target polymers was observed.Potential infectious agents from the infection sources were detected by pyrolysis-gas chromatography-mass spectrometry(Py-GC-MS)and gas chromatography(GC).The acceleration effect of two main infectious agents,i.e.acetone and acetic acid,on the photo-oxidation of the commercial PP was verified.The infectious effect of the infection source on the target polymer was considered to be a comprehensive result of the effects of a variety of infectious agents.     

New approaches to mapping through-thickness variations in the degradation in poly (lactide-co-glycolide)

Biodegradable poly (lactide-co-glycolide) (PLGA) copolymers have been used for many years for biomedical applications such as soluble sutures, orthopaedic implants and more recently as potential tissue scaffold materials. The rate at which the copolymers degrade can be manipulated from a period of days to months by changing the lactide/glycolic acid ratio. Degradation of PLGA copolymers occurs by hydrolysis of the ester bonds in the polymer backbone. The hydrolysis reaction is autocatalytic and is accelerated by the build up of degradation products in the bulk of the material. As a consequence, material degradation is expected to be non-uniform through the specimen thickness with the material at the centre degrading at a faster rate than at the surface. Despite many studies of PLGA degradation, information on this local variance is sparse as the techniques used to track the process are usually bulk measures. In this study, two new approaches for monitoring degradation have been developed that enable local measurements of degradation to be made throughout the specimen over an extended period of time. Chemical and mechanical variations in the structure of the polymer have been mapped using attenuated total reflectance infrared spectroscopy (ATR-FTIR) and nanoindentation. These have produced comparable results and show that the degradation rate at the centre of the specimens is almost an order of magnitude higher than at the surface.     

有机小分子对聚丙烯光氧老化的传染作用

以聚丙烯(PP)为研究对象, 选择代表典型小分子降解产物结构的酸、 酯、 醛、 酮和醇类共18种模型小分子, 研究有机小分子对PP光氧老化的传染作用. 研究发现, 所有的小分子都能不同程度地加速PP的光氧老化. 其中, 酸类、 醛类和酮类小分子的加速作用较强, 酯类和醇类小分子的加速作用较弱. 进一步研究了丙酮和乙酸对PP光氧老化的作用机理. 结果表明, 丙酮易光解产生甲基自由基, 通过引发PP氧化的方式加速其老化进程; 乙酸不具备引发能力, 通过催化氢过氧化物分解的方式促进PP中氧化产物的产生和积累.     

Reactive processing of polymers: Effect of bifunctional and tri-functional comonomers on melt grafting of glycidyl methacrylate onto polypropylene

Two reactive comonomers, divinyl benzene (DVB) and trimethylolpropane triacrylate (TRIS), were evaluated for their role in effecting the melt free radical grafting reaction of the monomer glycidyl methacrylate (GMA) onto polypropylene (PP). The characteristics of the GMA-grafting systems in the presence and absence of DVB or TRIS were examined and compared in terms of the yield of the grafting reaction and the extent of the main side reactions, namely homopolymerisation of GMA (poly-GMA) and polymer degradation, using different chemical compositions of the reactive systems and processing conditions. In the absence of the comonomers, i.e. in a conventional system, high initiator concentrations of peroxides were typically required to achieve the highest possible GMA grafting levels which were found to be generally low. Concomitantly, both poly-GMA and degradation of the polymer by chain scission takes place with increasing initiator amounts. On the other hand, the presence of a small amount of the comonomers, DVB or Tris, in the GMA-grafting system, was shown to bring about a significant increase in the grafting level paralleled by a large reduction in poly-GMA and PP degradation. In the presence of these highly reactive comonomers, the optimum grafting system requires a much lower concentration of the peroxide initiator and, consequently, would lead to the much lower degree of polymer degradation observed in these systems. The differences in the effects of the presence of DVB and that of TRIS in the grafting systems on the rate of the GMA-grafting and homopolymerisation reactions, and the extent of PP degradation (through melt flow changes), were compared and contrasted with a conventional GMA-grafting system.     

Characterization of chemical heterogeneity in polymer systems using hydrolysis and tapping‐mode atomic force microscopy

Characterization of polymer coatings microstructure is critical to the fundamental understanding of the corrosion of coated metals. An approach for mapping the chemical heterogeneity of a polymer system using chemical modification and tapping‐mode atomic force microscopy (TMAFM) is demonstrated. This approach is based on the selective degradation of one of the phases in a multiphase polymer blend system and the ability of TMAFM to provide nanoscale lateral information about the different phases in the polymer system. Films made of a 70:30 polyethyl acrylate/polystyrene (PEA/PS) blend were exposed to a hydrolytic acidic environment and analyzed using TMAFM. Pits were observed to form in the PEA/PS blend films, and this degradation behavior was similar to that of the PEA material. Using these results, the domains in the 70:30 blend were identified as the PS‐rich regions and the matrix as the PEA‐rich region. This conclusion was confirmed by Fourier transform infrared‐attenuated total reflection analyses that revealed the hydrolysis of the PEA material. TMAFM phase imaging was also used to follow pit growth of the blend as a function of exposure time. The usefulness of the chemical modification/AFM imaging approach in understanding the degradation process of a coating film is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci B Part B: Polym Phys 39: 1460–1470, 2001     

Study of the early deactivation in pyrolysis of polymers in the presence of catalysts

In this work, the early degradation step of the pyrolysis of some polymers in the presence of certain catalysts has been studied using thermogravimetric analysis (TGA). Three commercial polymers (PE, PP and EVA) and three catalysts were studied (ZSM-5, MCM-41a, and MCM-41b), and the MCM-41a catalyst has been selected for the analysis of the earlier steps of the pyrolysis process carried out in the presence of catalysts. Several cycles of heating–cooling were performed using a thermobalance, in order to analyze the influence of the first stages of decomposition on the activity of the more accessible active sites involved. In this way, the behaviour of the polymer–catalyst mixtures (20% (w/w) of catalyst) was studied and compared with that observed in the corresponding thermal degradation as well as in the pyrolysis in the presence of catalysts, in a single heating cycle.The results obtained clearly show the existence of an early degradation step. For a polymer–catalyst system with low steric hindrances such as PE-MCM-41, this early degradation step causes a noticeable decrease of the catalyst activity for the main decomposition step (i.e., cracking of the chain). The decrease of the catalytic activity is lower for a polymer–catalyst system with higher steric restrictions, as occurs in the EVA-MCM-41 degradation process. However, in this case, the catalyst activity in the first decomposition step (i.e., the loss of the acetoxi groups) is noticeable decreased after one pyrolysis run, thus reflecting that the active sites involved are mainly the most accessible ones.     

Viscoelastic characterization of phenolic resin–carbon fiber composite degradation process

Viscoelastic characteristics of cured phenolic resin–carbon fiber composite materials were investigated through glass transition and degradation reaction processes in the high‐temperature region up to 400°C. A typical glass transition of the crosslinked thermoset polymer was followed by irreversible degradation reactions, which were exhibited by the increasing storage modulus and loss modulus peak. A degradation master curve was constructed by using the vertical and horizontal shift factors, both of which complied well with the Arrhenius equation in light of the kinetic expression of degradation rate constants. Using an analogy to the Havriliak–Negami equation in dielectric relaxation phenomena, a viscoelastic modeling methodology was developed to characterize the frequency‐ and temperature‐dependent complex moduli of the degrading thermoset polymer composite systems. The temperature‐dependent relaxation time of the degrading composites was determined in a continuous fashion and showed a minimum relaxation time between the glass transition and degradation reaction regions. The capability of the developed modeling methodology was demonstrated by describing the complex behavior of the viscoelastic complex moduli of reacting phenolic resin composite systems. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 907–918, 1999     

Carbon dioxide evolution and carbonyl group development during photodegradation of polyethylene and polypropylene

In situ infrared (FTIR) spectrometry has demonstrated that more CO₂ is photogenerated from polypropylene (PP) than from polyethylene (PE) films. Potential applications of the method include investigation of polymer degradation mechanism and ranking of polymer photo-stabilities in as little as 3 h.This study focuses on clarifying the mechanism of this rapid CO₂ formation from PE and PP, and complementary insight was obtained from changes in the IR transmission spectra of films irradiated by UVA for hundreds of hours. A 30 min induction time observed for CO₂ photogeneration from PP, but not PE, was reflected, on a much longer time scale, in the induction time for carbonyl development in PP, but not PE. This suggests that, in PP, the CO₂ induction time is a consequence of the slow development of carbonyl groups, a hypothesis that is supported by the elimination of the PP induction time when, prior to the CO₂ measurements, films are pre-exposed to UVA, to generate carbonyl groups. In addition, more CO₂ is evolved from both PE and PP films if they are pre-exposed.     

Development of a methodology to predict material properties from environmental exposure. I. Kinetic treatment

This study focuses on bisphenol A polycarbonate and tracks changes in the properties of this material as a function of the degree of degradation, τ, induced by a hostile environment. Among many possible environmental effects, we have limited this investigation to those associated with elevated temperature, although the methodology is general. This τ parameter is a product of the kinetic rate constant, k, found from isothermal kinetics, and the time of degradation, t. The elucidation of τ has been linked to the measurement of the molecular weight distribution, which, in turn, can be related to various properties to yield valuable predictive relationships. Only the thermal history of the polymer and its initial properties are required for the model. This technique is not limited to a specific polymer or even to thermal degradation. As long as the kinetics of the process can be mathematically modeled, this approach should apply to a host of other situations, providing property prediction simply from knowledge of the material history. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 794–801, 2002     

The effect of adding virgin material or extra stabilizer on the recyclability of polypropylene as studied by multi-cell imaging chemiluminescence and microcalorimetry

The effect of adding virgin material or new stabilizers on the recyclability of polypropylene (PP) was determined. Stabilized PP was subjected to oven ageing at 130 °C for 100, 250 or 500 h before and after upgrading with virgin material (0, 20, 50, 80 or 100%) or additional stabilizer during reprocessing. The effect of upgrading recycled PP with extra stabilizer or virgin material was determined by measuring the elongation at break, yellowness change, oxidation induction time (OIT) or total luminous intensity (TLI) by chemiluminescence (CL) techniques. Selected specimens were analysed by microcalorimetry (MC).It has been shown that upgrading recycled PP with virgin material was not effective. Adding 80% virgin material did not increase the lifetime more than adding 20% virgin material. This indicates that a small amount of recycled material can induce degradation by contamination. Adding extra stabilizers instead of virgin material was much more effective. Of all techniques used the OIT as determined with CL showed a clear difference in residual stability of differently treated materials and seems to be the most valuable analytical method to determine the recyclability of PP. A comparison between CL and MC shows that MC is sensitive enough for measuring the solid-state oxidation of stabilized PP. Slightly lower OITs were detected by MC probably associated with the static atmosphere used during the measurements. The TLI method is not suitable to determine the degradation state of to be recycled material.     

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.     

Effect of residual contaminants and of different types of extrusion processes on the rheological properties of the post-consumer polypropylene

Rheological measurements were conducted to verify the influence of different mechanical recycling processes and the presence of contaminants on the degradation of post-consumer polypropylene. Firstly, polypropylene (PP) was contaminated to simulate a post-consumer material, following the protocol recommended by the FDA. PP was subsequently recovered (washed and dried) and the samples were submitted to different extrusion processes. The rheological data demonstrated that the different types of processing applied and the presence of contaminants altered the molecular structure of the samples. The contaminants acted as agents that accelerated the polymer degradation. The contaminated samples submitted to higher shear rates exhibited greater decrease in their molar mass and a slight narrowing in the molar mass distribution. Also, it was observed that the most degraded samples showed decrease in their molar mass, in the viscosity and in the level of their molecular entanglements. These samples also exhibited a more Newtonian behavior and their molar mass distribution showed a slight narrowing. By calculating the ratio of the molar mass it was possible to quantify the degree of degradation of PP samples, confirming the results obtained.     

Thermal oxidation of ester-modified isotactic polypropylene

The relationship between the thermal oxidation of isotactic PP samples modified by esters miscible and immiscible with the polymer and the structure of these samples has been studied. An analysis of kinetic features of oxygen uptake, buildup of oxidation products, and changes in the mechanical properties and structural parameters of PP in the course of oxidation has shown that the effect of a modifier on the kinetics of thermal oxidation of the polymer depends on compatibility of an additive and a polymer matrix. The addition of ester that is partially miscible with PP accelerates oxidation. In the case of an immiscible ester, the effect is quite the reverse. This phenomenon is rationalized by the fact that the phase state of the system determines changes in the initial structure of the polymer matrix and, hence, manifestation of structural effects in the reaction kinetics and the participation of additives in chain reactions of cooxidation with PP.     

Two-dimensional lattice Monte Carlo simulation of the compatibility of A/B/functionalized A ternary polymer mixtures coupled with chemical reaction

Compatibility of A/B and functionalized A ternary polymer mixtures was studied by Monte Carlo simulation in a two-dimensional lattice. Polymer A was a nonreactive polymer, whereas polymer B was a reactive polymer and immiscible with polymer A. Functionalized polymer A could react with the end group of polymer B, leading to the formation of block copolymers. Simulation results showed the phase domain sizes dropped considerably with increasing functionalized polymer A content, indicating that the compatibility between polymer A and B could be markedly improved with the introduction of functionalized polymer A. Moreover, it was shown that the resulting block copolymers tended to distribute at the phase interface between polymer A and B, and the block copolymer conformation depended on the structures of polymer B and functionalized polymer A. In case 1, i.e., both polymer B and functionalized polymer A were with single end group, it could be found that the block A and block B of resulting A–B copolymer inserted into polymer A and polymer B phase domains, respectively. In case 2, i.e., functionalized polymer A was with single end group and polymer B was with double end groups, it was found that the resulting A–B–A triblock copolymer tended to connect two neighbor separated polymer A phase domains. However, in case 3, namely functionalized polymer A was with double end groups and polymer B was with single end group, it was found that the resulting B–A–B triblock copolymer was likely to form a folding conformation. These lead to the different compatibilizing effects for different polymer structures. Comparing with case 1 and case 2, functionalized polymer A with double end groups (case 3) had less effective to compatibilize the A/B polymer blends. For the purpose of comparison, same simulations were carried out in a three-dimensional lattice. The results showed the compatibility behavior of the mixtures was similar to those in the two-dimensional lattice with the addition of functionalized polymer A. However, the conformation of the resulting block copolymers was different from that in the two-dimensional lattice.     

Heterogeneous oxidative degradation in irradiated polymers

In the presence of oxygen, polymeric materials may undergo diffusion-limited heterogeneous degradation with significant oxidation occurring only near the surfaces. It is important in studying polymer degradation (and in designing accelerated aging experiments) to be able to determine under what conditions heterogeneous degradation occurs, and to estimate the extent of significant oxidative penetration under those conditions. We describe here the use of several techniques for identification of oxidative degradation gradients. A rapid determination of oxidation depth is accomplished by optical examination of metallographically polished cross-sectioned samples; oxidized and nonoxidized regions are distinguished by differences in surface reflectivity. A more detailed determination of the shapes of degradation gradients is accomplished by performing a series of sensitive determinations of relative hardness changes across the surface of cross-sectioned, polished samples. Typical oxidation depths for the commercial polymers examined are on the order of fractions of millimeters over a dose-rate range of 10⁴–10⁶ rad/h. Significant variations among different materials are found, as would be expected given differences in oxygen consumption and permeation rates. A detailed example is given of the tensile property behavior of a Viton material over a range of dose rates where the degradation is seen to change from strongly heterogeneous at high dose rates to homogeneous as the dose rate is lowered. Degradation differences in this material are very pronounced: at high dose rates the polymer undergoes primarily crosslinking to give a hard brittle material, whereas under lower dose rates, where oxygen permeation is complete, the polymer undergoes predominantly scission to yield a soft, easily stretchable material.     

Membrane extraction with sorbent interface-gas chromatography as an effective and fast means for continuous monitoring of thermal degradation products of polyacrylonitrile

A novel sample preparation technique, membrane extraction with a sorbent interface (MESI) has been optimized and used for continuous monitoring thermal degradation products in polyacrylonitrile (PAN) polymer headspace at different temperatures, followed by gas chromatography-mass spectrometry (GC-MS). MESI with a flat sheet poly(dimethyl siloxane)-polycarbonate (PDMS-PC) membrane and Tenax trap was used. The system is very simple, fast and reliable and allowed us to extract, enrich and continuously monitor major volatile compounds released from the polymer at different temperatures. The volatile and semi-volatile gaseous degradation products were identified. Sensitivity of the method depends on the length of time for trapping.     

Exploring rheological properties of aqueous polyaniline‐PVP dispersion

Stable aqueous dispersion of polyaniline (PAn) stabilized by a hydrophilic polymer poly(vinyl pyrrolidone) (PVP) exhibits interesting rheological properties different from its components. Shear thinning observed for both PVP and PAn–PVP colloid (PP) indicates partially entangled nature of the later. Linear viscoelastic response of PVP solution exhibit strong frequency dependence of elastic (G′) and viscous (G″) modulus over the whole frequency range (0.1–100 ras/s) where G′ never exceeds G″ indicating the applicability of the Rouse‐Zimm model to this system. On the other hand, there is a crossover of G′ and G″ in the rheological profile of PP dispersion so that a single relaxation time model can be applicable. Therefore, PVP presents an entangled polymeric system and supposed to have a spectrum of relaxation times, whereas PP resembles to a physically crosslinked system with a single relaxation time. Increasing the extent of hydrogen bonding within the system (by raising the fraction of PAn or by leaving the solution undisturbed for long) relaxation time also becomes longer. The large difference in values of steady and complex shear viscosity (η and η*) within LVE regime reflects that original Cox‐Merz rule is obviously inapplicable to these systems. But at larger strain amplitude, η and η* are satisfactorily coincident that indicates a broader applicability of the modified Cox‐Merz rule. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2443–2455, 2008     

Thermal oxidative degradation kinetics of flame-retarded polypropylene with intumescent flame-retardant master batches in situ prepared in twin-screw extruder

The thermal oxidative degradation kinetics of pure PP and the flame-retarded (FR) PP materials with intumescent flame-retardant (IFR) master batches in situ prepared in twin-screw extruder were investigated using Kissinger method, Flynn-Wall-Ozawa method and Coats-Redfern method. The results showed that the activation energy order of PP and FR PP samples with different blowing agent/char former ratios obtained by Kissinger method agrees well with that obtained by Coats-Redfern one, which well illustrates the relationship between the composition of IFRs and their flame-retardancy, i.e. FR material with richer carbonization agent has higher activation energy for thermal oxidative degradation, hence leading to a better flame-retardancy. For Flynn-Wall-Ozawa method, due to its adoption of Doyle approximation, the obtained activation energy and its order of samples are very different from those of both Kissinger and Coats-Redfern methods. Criado method was finally used to determine the degradation reaction mechanism of various samples.