Polymer Scission Research Articles

Drag reduction by linear flexible polymers and its degradation in turbulent flow: A phenomenological explanation from chemical thermodynamics and kinetics

Based on chemical thermodynamics and kinetics analysis, this work provides a phenomenological explanation of drag reduction and its degradation by linear flexible polymers. We propose that drag reduction happens due to the unstable thermodynamic environment created by the elongation of free polymers and aggregates, and degradation of drag reduction happens due to the unstable kinetic environment with the chain scission of the free polymer and aggregate. Experimental results from microscopic and macroscopic scales further validate the proposed theory. Fourier series is employed to explain the molecular weight distribution that happens in the drag reduction.

CoFe2O4 Nano-particles for Radical Oxidative Degradation of High Molecular Weight Polybutadiene

Polymer waste production increased dramatically in the last decades, and has reached to 380 million tons (Mt.) in 2015. Due to their long-term stability, these materials impose a serious environmental challenge. Currently, recycling of polymer waste focuses on re-use of actual products, mechanical processing, chemical recycling, and bio-degradation into environmentally friendly materials. In our previous work, we proposed a new approach for radical-initiated oxidative degradation of polymers using cobalt ferrite (CoFe2O4) nanoparticles. In our current work, we focus on the use of CoFe2O4 nanoparticles as catalysts for radical degradation of high molecular weight polybutadiene. Cobalt ferrite nanoparticles were embedded into the polybutadiene polymeric matrix, with the aim of studying degradation in polymeric products that can be manufactured with catalytic nanoparticles. The polymer degradation process was characterized using gel permeation and size exclusion chromatography measurements, thermogravimetric analysis, FTIR, NMR, and mass spectroscopy. Based on the results from these diverse measurements, we propose a mechanism for the degradation process. Overall, our results show that the radical processes within the polybutadiene polymer lead to two parallel processes: polymer crosslinking and polymer scission. Moreover, we show that the ratio between crosslinking and degradation can be controlled by the reaction duration and catalyst concentration.

Cp2ZrHCl induced catalytic chain scission of diene-based polymers under mild conditions: Influence of chemical environment around C=C bonds

Controlled chain scissions of diene-based polymers, including cis-1,4-polybutadiene (cPB), cis-1,4-polyisoprene (cPIP), trans-1,4-polyisoprene (tPIP), 1,4-polychloroprene (PCP) and polycyclooctene (PCOE), were carried out under mild conditions via the hydrozirconation between C=C bonds and bis(cyclopentadienyl) zirconium hydrochloride (Cp2ZrHCl). The molecular weights (Mn) of the chain-scission products mainly depended on the loadings of Cp2ZrHCl and reaction time regardless of reaction media and temperature; however, cis/trans configuration ratio of C=C bonds in chain-scission products was influenced by reaction media, temperature and time. By means of NMR spectroscopy, chain-scission mechanism was confirmed to be β-alkyl elimination reaction. Deuterium (2H) tracing experiments showed that the hydrogenation of C=C bonds was a detrimental competing reaction to β-alkyl elimination. Furthermore, the hydrogenation was strongly influenced by steric and electronic effects around C=C bonds. However, chemical environments around C=C bonds did not affect chain-end functionalization of products, as chain-end alkylzirconium complexes generated from β-alkyl elimination were facilely quenched by electrophiles to obtain chain-end functionalized products. More importantly, under the assistance of hydrogen (H2), highly-efficient catalytic chain scission was achieved. Even more, cross-linked rubbers (such as cross-linked cPIP) could be also cleaved catalytically into soluble oligomers.

Physicochemical Modeling for Pressurized Hot Water Extraction of Spruce Wood

The study of reaction kinetics and process modeling plays a vital role in the development and optimization of industrial processes. In this study, a physicochemical model is presented for pressurized hot water extraction of spruce wood meal to facilitate process development and optimization. The model takes into account several relevant phenomena during the extraction process, including acid–base reaction equilibrium, irreversible reaction kinetics, ion exchange, and mass transfer. The system is modeled by assuming two liquid phases, i.e., liquid external to wood fibers and fiber-bound liquid to enable inclusion of ion exchange. The nonideal behavior of the system (due to presence of ions) is considered by utilizing activities instead of concentrations. The scission of hemicellulose polymers into oligomers and monomers is modeled by a population balance approach of discretized categories. Model parameters are optimized by fitting the model output to experimental data reported in the literature. The presen...

Physical modeling of chemical membrane degradation in polymer electrolyte membrane fuel cells: Influence of pressure, relative humidity and cell voltage

Chemical membrane degradation causes deterioration of critical membrane properties such as gas separation which finally causes failure of polymer electrolyte membrane fuel cells (PEMFCs). In order to identify the underlying physical processes, a physics-based model of chemical membrane degradation is implemented into the novel numerical framework NEOPARD-X [1]. The existing 2D PEMFC model is extended to incorporate the mechanisms of hydrogen peroxide formation and reduction, a redox cycle of iron contaminants in the ionomer phase, radical formation due to Fenton's chemistry and radical attack on the polymer structure. Unzipping of the polymer backbone and scission of the side chains are considered as degradation mechanism. The degradation model is validated against experimental data obtained in accelerated stress tests (ASTs). From theoretical considerations, the influence of chemical membrane degradation on the cell performance is revealed. The influence of pressure, relative humidity and cell voltage on the chemical degradation is rationalized. The operating conditions strongly influence the kinetics and spacial distribution of the membrane degradation. Degradation is found to be most pronounced at elevated pressure, high relative humidity and high cell voltage close to the interface of anode catalyst layer and PEM.

The photoregulation of a mechanochemical polymer scission

Control over mechanochemical polymer scission by another external stimulus may offer an avenue to further advance the fields of polymer chemistry, mechanochemistry, and materials science. Herein, we demonstrate that light can regulate the mechanochemical behavior of a diarylethene-conjugated Diels–Alder adduct (DAE/DA) that reversibly isomerizes from a weaker open form to a stronger closed form under photoirradiation. Pulsed ultrasonication experiments, spectroscopic analyses, and density functional theory calculations support the successful photoregulation of the reactivity of this DAE/DA mechanophore, which is incorporated at the mid-chain of a polymer, and indicate that higher force and energy are required to cleave the closed form of the DAE/DA mechanophore relative to the open form. The present photoregulation concept provides an attractive approach toward the generation of new mechanofunctional polymers.

Photochemical Kinetic Modeling of Degradation of Aqueous Polyvinyl Alcohol in a UV/H2O2 Photoreactor

This study presents a photochemical kinetics model to describe the degradation of water-soluble PVA (Polyvinyl Alcohol) polymer in a UV/H2O2 batch reactor. Under the effect of UV light, the photolysis of hydrogen peroxide into hydroxyl radicals can generate a series of polymer scission reactions. For a better understanding and analysis of the UV/H2O2 process in the cracking of the PVA macromolecules, a chemical reaction mechanism of the degradation process and a relevant photochemical kinetics model are developed to describe the disintegration of the polymer chains. Taking into account the probabilistic fragmentation of the polymer, the statistical moment approach is used to model the molar population balance of live and dead polymer chains. The model predicts the PVA molecular weight reduction, the acidity of the solution, and hydrogen peroxide residual. In addition to previously published data collected in this laboratory, a new set of experiments were conducted using a 500 mg/L PVA aqueous for different hydrogen peroxide/PVA ratios for model validation. Measurements of average molecular weights of the polymer, hydrogen peroxide concentrations and pH of the PVA solution were determinant factors in constructing a reliable photochemical model of the UV/H2O2 process. Experimental data showed a decrease in the PVA molecular weight and a buildup of the solution acidity. The experimental data also served to determine the kinetics rate constants of the PVA photochemical degradation and validate the model whose predictions are in good agreement with data. The model can provide a comprehensive understanding of the impact of the design and operational variables.

Unraveling the Correlations between Conformation, Lubrication, and Chemical Stability of Bottlebrush Polymers at Interfaces.

In the present study, we monitored the conformation and chemical stability of a hydrophilic bottlebrush (BB) polymer in pure water and buffered saline solutions. We correlated these parameters to lubricating and wear protecting properties. Using the surface forces apparatus (SFA), we show that the BB polymer partially adsorbs on mica surfaces and extends half its contour length toward the aqueous media. This conformation gives rise to a strong repulsive interaction force when surfaces bearing BB polymer chains are pressed against each other. Analysis of these repulsive forces demonstrated that the adsorbed polymer chains could be described as end-attached elastic rods. After 2 months of aging at temperatures ranging from 4 to 37 °C, partial scission of the BB polymer's lateral chains was observed by gel permeation chromatography with a half-life time of the polymer of at least two years. The thickness of the BB polymer layer assessed by SFA appeared to quickly decrease with aging time and temperature, which was mainly caused by the adsorption to the substrate of the released lateral chains. The gradual loss of the BB polymer lateral chains did not significantly impact the tribological properties of the BB polymer solution nor its wear protection capacity. The friction coefficient between mica surfaces immersed in the BB polymer solution was μ = 0.031 ± 0.002, was independent of the aging conditions, and remained constant up to an applied pressure P = 0.2 to 0.25 MPa. Altogether, this study demonstrates that, besides the gradual loss of lateral chains, the BB polymer is still able to perform adequately as a lubricant and wear protecting agent over a time period suitable for in vivo administration.

Towards improved predictions for the enzymatic chain-end scission of natural polymers by population balances: The need for a non-classical rate kernel

Enzymatic chain-end depolymerization is commonly employed for the transformation of biomass into important products. To date, investigation on the validity of the rate kernel which is critical to model success, has been conveniently avoided. Through a case study with extensive confrontation with experimental data, we uncover this critical relationship by inspecting every minute detail in the mechanistic modelling procedure. Using a newly proposed shape-evolving function for the rate kernel, model calibration reveals that the commonly employed constant rate kernel is inappropriate for modelling the scission step, and that the apparent rate kernel of hydrolysis resembles an endothermic activation energy barrier function. Facilitated by the adoption of this non-classical rate kernel, good predictions are attained by the model at different hydrolysis conditions with a global parameter set. Being the first to predict distributed data, the methodology here serves as a guide for future studies on the enzymatic disruption of polymeric biomass, i.e. for guiding substrate and enzyme structure modifications.

Low temperature gamma sterilization of a bioresorbable polymer, PLGA

Medical devices destined for insertion into the body must be sterilised before implantation to prevent infection or other complications. Emerging biomaterials, for example bioresorbable polymers, can experience changes in their properties due to standard industrial sterilization processes. Gamma irradiation is one of the most reliable, large scale sterilization methods, however it can induce chain scission, cross-linking or oxidation reactions in polymers. sterilization at low temperature or in an inert atmosphere has been reported to reduce the negative effects of gamma irradiation. The aim of this study was to investigate the impact of low temperature sterilization (at −80°C) when compared to sterilization at ambient temperature (25°C) both in inert atmospheric conditions of nitrogen gas, on poly(lactide co-glycolide) (PLGA).PLGA was irradiated at −80 and 25°C at 40kGy in a nitrogen atmosphere. Samples were characterised using differential scanning calorimetry (DSC), tensile test, Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (1H NMR) spectroscopy and gel permeation chromatography (GPC).The results showed that the molecular weight was significantly reduced as was the glass transition temperature, an indication of chain scission. FTIR showed small changes in chemical structure in the methyl and carbonyl groups after irradiation. Glass transition temperature was significantly different between irradiation at −80°C and irradiation at 25°C, however this was a difference of only 1°C. Ultimately, the results indicate that the sterilization temperature used does not affect PLGA when carried out in a nitrogen atmosphere.

Performance of UV weathered HDPE composites containing hull fiber from DDGS and corn grain

The availability of distiller’s dried grain with solubles (DDGS), the main co-product of corn ethanol production, exceeds its demand as feedstuff. A physical separation process named elusieve can separate the hull fiber from DDGS or corn flour to obtain DDGS with enhanced protein and oil content, or corn flour with higher starch content. This study was performed to investigate the potential of using the hull fiber separated with elusieve process on DDGS or corn flour as fillers in high density polyethylene (HDPE) composites at the commercial scale. Additionally, the degradation effects of UV accelerated weathering on this material were investigated. Commercial scale composite samples were made with six combinations of oak fiber, corn hull fiber and DDGS fiber, with the fiber loading maintained at 50%, and then the samples were subjected to UV accelerated weathering for 2000h. The unexposed DDGS samples showed better resistance to moisture absorption (less than 5%) than the corn filler samples (18%). The specific gravity for all the filler composites was more than one for both unexposed and UV weathered composites. The DDGS fiber also showed higher flexural properties in oak25/DDGS25 than other filler composites. For all samples, the accelerated weathering resulted in a lightening during the first 1000h of exposure, followed by a darkening at 2000h of UV exposure. Overall, UV weathering resulted in a chain scission of the HDPE polymer increasing the crystallinity of the polymer in the weathered filler composites.

High temperature polymer degradation: Rapid IR flow-through method for volatile quantification

Accelerated aging of polymers at elevated temperatures often involves the generation of volatiles. These can be formed as the products of oxidative degradation reactions or intrinsic pyrolytic decomposition as part of polymer scission reactions. A simple analytical method for the quantification of water, CO2, and CO as fundamental signatures of degradation kinetics is required. This study describes an analytical framework and develops a rapid mid-IR based gas analysis methodology to quantify volatiles that are contained in small ampoules after aging exposures. The approach requires identification of unique spectral signatures, systematic calibration with known concentrations of volatiles, and a rapid acquisition FTIR spectrometer for time resolved successive spectra. The volatiles are flushed out from the ampoule with dry N2 carrier gas and are then quantified through spectral and time integration. This method is sufficiently sensitive to determine absolute yields of ∼50 μg water or CO2, which relates to probing mass losses of less than 0.01% for a 1 g sample, i.e. the early stages in the degradation process. Such quantitative gas analysis is not easily achieved with other approaches. This approach opens up the possibility of quantitative monitoring of volatile evolution as an avenue to explore polymer degradation kinetics and its dependence on time and temperature.

Alternative Approach to Synthesize Methylated Chitosan Using Deep Eutectic Solvents, Biocatalyst and “Green” Methylating Agents

Conventional synthesis of N-methylated chitosan involves the use of organic solvents in alkaline conditions, using methyl iodide as the methylating agent. However, the method does not result in selective N-methylation and is known to cause heavy polymer scission. In this investigation, we have reported alternative “green” approaches for methylated chitosan synthesis. Two types of deep eutectic solvents (DESs), viz., DES(Urea) and DES(Gly), either alone or in combination with other solvents, were screened as media for facilitating the methylation of chitosan. Our results indicated that DESs mediated selective N-methylation in the absence of NaOH, with no polymer scission, when compared with the reported methods. 1H NMR spectra confirmed selective N-methylation in the case of products obtained using DES(Urea), while that obtained using DES(Gly) demonstrated some O-methylation. Another green method investigated the unexplored property of biocatalyst lipase for methylating chitosan in the presence of “green” ...

POLYMER DEGRADATION IN TURBULENT DRAG REDUCING FLOWS IN PIPES

The drag reduction by addition of high molecular additives has been investigated by a number of researchers since it was reported by Toms more than 60 years ago. One of the most significant limitations in drag reduction is the polymer degradation, which is caused by the turbulent structures. Researches have demonstrated that many parameters affect the polymer efficient, as: molecular weight, Reynolds number, concentration and temperature. In the present work we investigate this degradation phenomenon in a pipe flow apparatus device, for aqueous solutions of three different polymers: Polyethylene Oxide (PEO), Polyacrylamide (PAM) and Xanthan Gum (XG).The first two are known as flexible molecules while the last one is considered rigid. The dependence of polymer scission on molecular weight, concentration and Reynolds number is analyzed. We report how the drag reduction decreases when the flow pass repeatedly through the pipe and how the pressure loss measured in the apparatus increases, despite to the fact that the experiment was conducted at a fixed inlet pressure. It is worth noting that the mechanism of loss of efficiency for the XG solutions seems to be completely different from that observed for PEO and PAM, the flexible materials.

Pyrolysis characteristics of glass fiber-reinforced plastic (GFRP) under isothermal conditions

The pyrolysis characteristics of GFRP (glass fiber reinforced plastic) were determined under isothermal conditions at 500–1000°C using a thermo-balance reactor (TBR) and wire mesh reactor (WMR). The kinetics parameters for the GFRP were derived using the activation energy, of 28.17kJmol−1, of frequency factor, 2.12s−1, and reaction order, of 0.48–0.8. The structural variations in the GFRP char were determined using BET, SEM, and FT-IR techniques. Organic compounds and polymers were decomposed with increasing temperature. The experimental results showed that increasing the temperature and reaction time favored polymer scission at temperature of 600–1000°C with residence time of 2–10s.

Mechanochemical Reactions Reporting and Repairing Bond Scission in Polymers.

The past 10 years have seen a resurgence of interest in the field of polymer mechanochemistry. Whilst the destructive effects of mechanical force on polymer chains have been known for decades, it was only recently that researchers tapped into these forces to realize more useful chemical transformations. The current review discusses the strategic incorporation of weak covalent bonds in polymers to create materials with stress-sensing and damage-repairing properties. Firstly, the development of mechanochromism and mechanoluminescence as stress reporters is considered. The second half focuses on the net formation of covalent bonds as a response to mechanical force, via mechanocatalysis and mechanically unmasked chemical reactivity, and concludes with perspectives for the field.

Comparison of Mechanochemical Chain Scission Rates for Linear versus Three-Arm Star Polymers in Strong Acoustic Fields.

The effect of star versus linear polymer architecture on the rates of mechanochemically induced bond scission has been explored. We determined rate constants for chain scission of parent linear and star polymers, from which daughter fragments were cleanly resolved. These studies confirm a mechanistic interpretation of star polymer chain scission that is governed by the spanning rather than total molecular weight. We further demonstrate the preserved rate of site-selective mechanophore activation across two different polymer structures. Specifically, we observed consistent activation rate constants from three-arm star and linear polymer analogues, despite the Mn of the star polymer being 1.5 times greater than that of the linear system.

Drag reduction induced by flexible and rigid molecules in a turbulent flow into a rotating cylindrical double gap device: Comparison between Poly (ethylene oxide), Polyacrylamide, and Xanthan Gum

Polymer-induced drag reducing flow has been investigated for over 60years. One reason for this is that the drag reducers in flow systems have been successfully applied and represent a great potential benefit to many industrial processes. However, the phenomenon is not completely understood and many aspects of the problem remain unclear. Some important issues are related to the development of turbulent structures and to the breaking of the polymer molecules. These two phenomena impose a transient behavior on the polymer efficiency and the drag reduction, DR, can be clearly divided into three periods of time. Over time, at the very beginning of the test, DR assumes a minimum value (sometimes negative) before reaching its maximum efficiency. When degradation becomes important, DR starts to decrease until it achieves its asymptotic value, a time in which the polymer scission stops and the molecular weight distribution reaches a steady state. In the present paper, we study the drag reduction development from the very beginning of a turbulent flow into a rotating cylindrical double gap device. DR is induced by three different polymers: Poly (ethylene oxide) (PEO), Polyacrylamide (PAM) and Xanthan Gum (XG). The first two are known as flexible molecules while the last one is considered rigid. The goal here is to compare the effect of the different polymers on DR over time, paying particular attention to the difference between the rigid and the flexible molecules. The tests are conducted for a range of Reynolds numbers, concentrations and temperatures, from the very start to the time when the drag reduction achieves its final level of efficiency. The time to achieve the maximum efficiency is an increasing function of concentration and decreases with Reynolds and temperature in PEO solutions. Such time seems to be very short for the other polymers, less than 3s. It is worth noting that no loss of DR was observed for high concentrations of PAM, which suggests that PAM is more resistant than PEO. It is also shown that DR induced by XG is qualitatively different from that of the other agents. XG’s solution is highly influenced by a pre-shearing, which suggests the existence of polymer aggregates. In addition, it seems that degradation do not occurs for solutions of XG. The observed loss of efficiency in high concentrations is, possibly, caused by de-aggregation during the test.

The effect of styrene-methyl methacrylate monomeric arrangement on the ultrasonic degradation of copolymers

Ultrasonic degradation is a convenient means of studying the degradation of macromolecules in transient elongational flows. Cavitation results in non-random, near-midchain scission of linear polymers, which reach a limiting molar mass Mlim beyond which degradation is no longer possible. Also, the flow fields produced as a result of ultrasonic bubble collapse are kinetically and fluid-mechanically equivalent to those produced during more experimentally intractable manifestations of transient elongational flow. Here, we describe results from the ultrasonic degradation of dilute solutions of block, random, and alternating copolymers of styrene and methyl methacrylate, along with the degradation of their respective homopolymers. Results from the degradation experiments were monitored using size-exclusion chromatography with on-line triple detection including multi-angle static light scattering, differential viscometry, and differential refractometry. The influence of monomeric arrangement on solution structure was found to have concomitant influence on copolymer degradation, both mechanistically and kinetically. Noteworthy was the correspondence between persistence length and both Mlim and degradation rate k when comparing the alternating to both the block and random copolymers, in qualitative agreement with the previously-postulated modified path theory of polymer degradation in transient elongational flows. Most differences in Mlim and k could be explained by a combination of effects due to molar mass, monomer ratio, monomer arrangement, and resultant chain stiffness in solution, except for differences in Mlim between the random and block copolymers studied, which proved too subtle to measure.

Shear degradation resistance of star poly(ethyleneimine) - polyacrylamides during elongational flow

AbstractAn experimental study of the flow-induced scission behaviour of four star hydrolyzed polyacrylamides (HPMA) with different arms during planar elongational flow in a cross-slot flow cell is described. The results showed that the shear stability of linear HPAM in distilled water was not essentially different from star HPAM. Polymer scission was not observed in either system in a shear rate range from 20,000 to 100,000s-1, which can be attributed to the strong polyelectrolyte behaviour of HPAM in distilled water. However, at the same shear rate, the star HPAMs exhibited superior shear stability in comparison to the linear HPAMs in aqueous solutions containing NaCl (CNaCl=0.2-1.0%wt) and, in particular, the initial reduction rate of relative viscosity (R) decreased with the degree of branching of the HPAMs. In addition, it was found that the R of five HPAMs in NaCl aqueous solutions exhibited an exponential dependence on shear rate, in which the coefficient C1 can be used to quantitatively evaluate shear stability. In star HPAM NaCl aqueous solutions, the increase of R with shear rate is very likely due to the decrease of the hydrodynamic radius (Rh) of these HPAMs, while the increase of R with NaCl concentrations can be attributed to the relatively low viscosity of these polymers at high NaCl concentrations.