The degradation of high molecular weight isotactic polypropylene (iPP) subjected to gamma rays irradiation up to 100 kGy in inert atmosphere was analyzed. The investigation relied upon complex viscosity, elastic modulus, gel fraction, morphology of the insoluble fraction and deconvoluted molecular weight distribution (MWD) curves. At low irradiation doses, already at 5 kGy, the MWD curve is strongly shifted to the low molecular weight side showing chain scission, which is confirmed using the calculated chain scission distribution function (CSDF). At high dose levels, the appearance of a shoulder in the high molecular weight side of the MWD curve indicates the formation of chain branching. The presence of a considerable insoluble fraction at these high dose levels indicates also the formation of cross-linking, which has different morphology then the insoluble fraction present in the original iPP. The rheological results show changes in the molecular structure of irradiated samples in agreement with the gel content data. The chromatographic and rheological data has shown that gamma irradiation of iPP produces chain scission, branching and cross-linking. 相似文献
Branched polymers like LDPE are known to possess a wide range of architectures. In this paper a modelling approach is developed, describing the relation between architectures, chemistry and reactor conditions with the general objective of improving characterisation and controlling visco‐elastic properties. More specifically, the particular scission kinetics of branched molecules as strongly contrasting with linear scission is described. A new method to synthesise branched architectures is developed as an alternative to full Monte Carlo (MC) sampling. It employs MC sampling for coupling of primary polymers only. Graph theory is used as an efficient storage method containing all topological information of individual molecules. The algorithm synthesises molecules for any given combination of chain length (n) and number of branches (N). The explicit and detailed knowledge of branched architectures allows finding the correct topological scission kinetics. Distributions of fragment lengths and numbers of branches on fragments after scission are obtained, showing a preference for short and long fragments. Approximate functions describing this have been implemented in another model, predicting molecular weight (MWD) and degree of branching (DBD) distributions using a Galerkin finite element method. Topological scission is seen to give MWD broadening and a higher branching density for long chains. Distributions of longest end‐to‐end distances could be computed for all architectural alternatives for given n, N. In conclusion, it is demonstrated that this method yields distributions of architectures consistent with MWD/DBD for radical polymerisation with long‐chain branching and random scission. 相似文献
Summary: Degradation of a polymer in a reactor by the degrading agent(s) follows a distinct pattern, primarily influenced by structural integrity and reactor environment. This distinct pattern is recorded in the changes in the evolved molecular weight distribution (MWD) or polymer chain length distribution (PCLD) curve characteristics from the initial intact state. Modern size exclusion chromatography (SEC) is the best laboratory‐based method that can clearly provide these plots in the form of chromatogram; however, detailed molecular information is not available. The nature of molecular destruction can be well‐characterised if the distinct MWD shift patterns can be simulated to fingerprint the different chain scission dynamics. This is investigated by our current research using the power of computer simulation techniques to gain insight into the polymer ageing processes. One such technique for studying simple decay processes is presented here, and the results are compared with experimental findings. The concept of a binary tree scission model is introduced to show chain rupture as a sequence of probabilistic events and as a non‐linear function of time. Two new mathematical algorithms, an iterative Monte Carlo structured probability scheme and a semi‐iterative algebraic exact statistical formulation method, are investigated to implement this model and simulate the evolution of resultant temporal MW distribution. The latter, an innovative approach to mathematical modelling, has the potential to generate a statistically perfect instant MWD decay curve. A statistical comparison of the product yield is presented from the data obtained using a wide variety of simulated scission regimes to determine the sources of variability.
Simulated MWD lateral shift for percent cut scission model showing deviation from the initial MWD (red) over degradation time zones Tj(0 ≥ j ≤ 9) with bimodal and curve broadening effect due to accumulation of varied percent cut range 5–30%. 相似文献
Abstract A new eluting solvent, cyclohexane, for steric exclusion liquid chromatography (SELC) at 70°C of i-polypropylene (PP) has been applied to the study of molecular weight distribution (MWD) of samples of PP under repeated extrusion and under oxidative degradation in the presence of incorporated organic peroxide. Various parameters characterizing the changes in MW and MWD of the samples are listed in TABLE 1. Mz changed pronouncedly with increasing extent of degradation in both degradation processes, while Mn changed very slightly indicating the average number of chain scission per chain had been very small, much less than one. Difference in the mechanism of the mechanical as compared to the oxidative degradation has shown up in the way the low MW end of the MWD curves changed during the progress of the degradation process. 相似文献
The oxidative and thermo-mechanical degradation of HDPE was studied during processing in an internal mixer under two conditions: totally and partially filled chambers, which provides lower and higher concentrations of oxygen, respectively. Two types of HDPEs, Phillips and Ziegler-Natta, having different levels of terminal vinyl unsaturations were analyzed. Materials were processed at 160, 200, and 240 °C. Standard rheograms using a partially filled chamber showed that the torque is much more unstable in comparison to a totally filled chamber which provides an environment depleted of oxygen. Carbonyl and transvinylene group concentrations increased, whereas vinyl group concentration decreased with temperature and oxygen availability. Average number of chain scission and branching (ns) was calculated from MWD curves and its plotting versus functional groups' concentration showed that chain scission or branching takes place depending upon oxygen content and vinyl groups' consumption. Chain scission and branching distribution function (CSBDF) values showed that longer chains undergo chain scission easier than shorter ones due to their higher probability of entanglements. This yields macroradicals that react with the vinyl terminal unsaturations of other chains producing chain branching. Shorter chains are more mobile, not suffering scission but instead are used for grafting the macroradicals, increasing the molecular weight. Increase in the oxygen concentration, temperature, and vinyl end groups' content facilitates the thermo-mechanical degradation reducing the amount of both, longer chains via chain scission and shorter chains via chain branching, narrowing the polydispersity. Phillips HDPE produces a higher level of chain branching than the Ziegler-Natta's type at the same processing condition. 相似文献
Ultrasonically induced degradation of polystyrene in tetrahydrofuran solutions has been studied. Four narrow molecular weight distribution (MWD) samples, a material with a wide MWD, and a bimodal MWD sample were degraded at various concentrations, temperatures, and ultrasonic intensities. The MWDs after a number of ultrasonation time periods were obtained from gel permeation chromatograms corrected for instrumental spreading. Degradation index DI, defined as the number of bonds broken per original number of molecules, has been used to describe the extent of degradation. The results of the experiments were used to determine the values of two parameters occurring in the degradation model developed previously. It was found that the course of the changes in MWDs predicted by the model are in good agreement with those observed experimentally when the following values of the model parameters are used. The probability of scission of a molecule with molecular weight M is proportional to the product of its number fraction and the 1.25 power of M. The probability density function of the location of scission along the chain is described by a Caussian curve centered at the midpoint of the chain with a standard deviation of 35% of the chain lengths and truncated 3I the chain ends. The agreement between observed and calculated MWDs was good for degrees of degradation as high as DI = 15. The course of the changes in MWDs was independent of the initial MWD and of the experimental conditions of concentration, temperature, and sonation intensity and, therefore, independent of the rate of degradation which varied greatly. 相似文献
The elution curves of size exclusion chromatography (SEC) for branched polymers formed through free‐radical polymerization that involves chain transfer to polymer were theoretically investigated by using a Monte Carlo method. We considered two types of measured molecular weight distribution (MWD), (1) the calibrated MWD relative to standard linear polymers, and (2) the MWD obtained by using a light scattering photometer (LS) in which the weight‐average molecular weight of polymers within the elution volume is determined directly. It was found that the calibrated MWD clearly underestimates the high molecular weight tail, and the measured distributions are narrower than the true MWD. On the other hand, the present simulation results showed that the LS method gives reasonable estimates of the true MWDs. The mean square radius of gyration of the polymer molecules having the same molecular weight was also investigated. The radii of gyration showed clear deviation from the Zimm‐Stockmayer equation[1] because of the non‐random nature of branched structure and the difference in the primary chain length distribution. 相似文献
The molecular weight distribution (MWD) curves for polymerization systems with chain transfer to polymer leading to reshuffling of polymer segments (and broadening of the MWD), but not changing chain functionalities, were simulated by the Monte Carlo method. The bimodality observed in some distributions was explained by different distribution functions of chains which did not undergo reshuffling and of those which underwent the chain transfer reaction. Using this observation, a numerical integration method for computing DP w/DP n (and the MWD curves) in the systems under consideration was devised. Plots relating DP w/DP n to monomer conversion and ktr/kp are presented and a method of determination of ktr/kp from the DP w/DP n data is proposed. 相似文献
Mathematical Modeling of non‐linear polymerization systems subject to gel formation is a challenging endeavor. At the gel point, the second and higher molecular weight moments diverge to infinity making it impossible to obtain the molecular weight distribution (MWD). The numerical fractionation (NF) technique utilizes a refinement of the method of moments to model non‐linear polymerization systems that form gel. Since the method of moments yields results in terms of average quantities, some information is lost when reconstructing the MWD using NF. As a consequence, a broad shoulder appears at the high chain length end of the MWD tail. This study demonstrates that the validity of the gamma distribution deteriorates for the broader branched polymer generations and evaluates the performance of various alternative model distributions. Proper selection of the model distribution enhances the NF‐reconstructed MWD. 相似文献
In the present study, the influence of electron-beam irradiation on plasticity-controlled and crack-growth-controlled failure in high-density polyethylene (HDPE) is investigated and the effect of both molecular weight distribution (MWD) and short chain branching (SCB) content are taken into account. Size exclusion chromatography (SEC) is used to study the evolution of the MWD of the sol fraction as a function of irradiation dose. Here, it is seen that chains shorter than the percolation threshold (5 kDa) are largely unaffected by electron beam radiation, while the fraction of longest chains (M > 300 kDa) is nearly entirely incorporated into the cross-linked network. Both yield stress and Young's modulus increased with irradiation dose, where the magnitude of the increase appears to be connected to the gel fraction. The (fatigue) crack growth kinetics of the grades changed relatively little with irradiation dose, which is unexpected. Furthermore, convergence of the crack growth kinetics parameter to a narrow range of values could be observed for the investigated grades at relatively high gel fractions. This would imply that the crack growth kinetics become increasingly independent of the MWD upon irradiation cross-linking, which could be attributed to a shift in the underlying crack growth mechanism from chain slip to chain scission. 相似文献
Summary: Heterogeneity of active centers (AC) of titanium-magnesium catalysts (TMC) and vanadium-magnesium catalyst (VMC) in ethylene-hexene-1 copolymerization has been studied on the base of data of polymer molecular weight distribution (MWD) deconvolution technique and copolymer fractionation onto narrow fractions. It was found that 3 and 4 Flory components (groups of active centers) are required to describe experimental MWD curves of copolymers produced over TMC with different Ti content. In the case of VMC MWD of homopolymer and copolymer are characterized by set of 5 Flory components (5 groups of AC). Different character of inter-relationship between MW and short chain branching (SCB) was found for ethylene-hexene-1 copolymers produced over different catalysts: heterogeneous type in the case of TMC and more uniform for copolymer prepared over VMC. The content of Ti affects on the slope of that profile in copolymers produced over TMC. The results indicated that TMC and VMC are different greatly on the heterogeneity of active centers to the formation of polymers with different molecular weights and to formation of copolymers with different inter-relationship between MW and short chain branching. TMC produces polymers with more narrow MWD but it contains highly heterogeneous centers regarding comonomer reactivity ratios. VMC produces polymers with broad and bimodal MWD but it contains more homogeneous centers regarding comonomer reactivity ratios. 相似文献
The molecular weight distribution (MWD) is possibly the most important characteristic of a polymer. Polymers derive many of their physical properties from their MWD. Therefore, since the origins of polymer science, the theory provides a link between the kinetic mechanism and the mathematical expression of the MWD, and there are analytical solutions for ideal cases. However, the MWD formed in real‐life polymerization processes is usually more complex; the solution of the mathematical models that describe them can be quite challenging and has been the focus of enormous research efforts. These models may consist of systems of very large dimension: thousands of differential equations, often stiff, which demand special numerical techniques for their solution. In this paper the numerical techniques that can be used to solve this challenging problem are reviewed and contrasted, including weighted residual methods, direct integration, numerical inversion of transformed equations, and lumping methods. Stochastic techniques are also surveyed. 相似文献
The hydrolytic and enzymatic degradation behavior of poly(epsilon-caprolactone) (PCL) is investigated using the Langmuir monolayer technique, and an improved data acquisition and data reduction procedure is presented. Hydrolytic and enzymatic monolayer degradation experiments of PCL with various molecular weights by Pseudomonas cepacia lipase have been carried out to analyze the influence of subphase pH, subphase temperature, enzyme concentration, and the packing density of polymer chains on the degradation kinetics. The enzymatic monolayer degradation results in an exponential increase in the number of dissolved degradation fragments with increasing degradation time, which confirms random chain scission to be the dominant scission mechanism. The increase in the enzymatic scission rate constant with decreasing initial average molecular weight of the polymers is assigned to the influence of the area density of polar terminal groups on the substrate-enzyme complex formation. 相似文献