Changes In Molecular Weight Distribution Research Articles

Physico-chemical behaviour of β irradiated plastic materials currently used as packagings and medical products

The combined chromatographic technics and thermal analysis constitute an informative methodology for studying the modifications which could occur following a radiotreatment of plastic material at different doses (25 to 100 kGy). Several plastic materials used as packagings (PVC, PE, PS) were investigated. SEC method coupled with UV and DDL detections was applied to document any changes in molecular weight distribution. Reticulation and scission were the main observed degradation phenomena. These structural modifications were supported by TGA data, while the DSC provided information on modifications in crystallinity. In addition, RP-HPLC was carried out for the evaluation of the radiochemical behaviour of the additives and monomers. Firstly we demonstrated the degradation of high molecular weight phenolic antioxidants in BHT within the PEVA. Secondly, the modifications of amino 6 caproic acid and ε caprolactam, present in polyamid 6, depend on the irradiation doses.

Change of molecular weight distribution of statistical poly(styrene‐co‐acrylonitriles) with compositio of copolymer

AbstractStatistical poly(styrene‐co‐acrylonitrile) (SAN) copolymes with varying mole fractions of comonomer units have been synthesized in a batch reactor. The intrinsic viscosity and gel permeation chromatography (GPC) patterns of model copolymer samples were the basic experimental results. Chromatographic patterns of SAN copolymer samples were converted into molecular weight distributions and afterwards fitted to the theoretical distribution functions. The correlation between the composition of copolymers and the best fitted model distribution (Schulz's two parameters distribution in this case) was established. The treatment of data shows that parameter α is inversely proportional to the content of acrylonitrile units in the sample and indicates the molecular weight interval. Parameter b shows the invariability of the molecular weight distribution shape.

Time-of-Flight Secondary Ion Mass Spectrometric Measurements of Molecular Weight Distributions for Functionally Terminated Oligomers and Transferred Langmuir-Blodgett-Kuhn Monolayers

We report the first direct measurements of the molecular weight distribution in transferred condensed monolayers of amphiphilic oligomers determined by time-of-flight secondary ion mass spectrometry (TOFSIMS). Two α,ω-functional oligomers of poly(dimethylsiloxane) are investigated, comprising pentylamine and propylcarboxy end groups. Measurements are performed directly on condensed monolayer films transferred to silver-coated substrates using the Langmuir-Blodgett-Kuhn (LBK) technique. These results are compared to the original distributions measured on submonolayer thin films of the original oligomers prepared by spin coating onto silver-coated substrates and to molecular weight determinations provided by end group titration and size exclusion chromatography analyses. Different families of ions are found for the two different thin film preparation methods. A number of tentative assignments are proposed for these masses, based upon consideration of the effects of the LBK film deposition process and the influence of interactions between the end groups and the substrate. The molecular weight distributions for LBK films of both oligomers is found to be narrower and shifted to higher molecular weights than are those for the corresponding spin-coated films. The changes in molecular weight distribution observed are attributed to dissolution of lower molecular weight species into the aqueous subphase during the LBK film deposition process.

Structural degradation of polyurethane-based elastomeric modules

Degradative characteristics were studied using specimens of three polyurethane-based elastomeric modules that were treated in solutions of varying acidity, oxygen content and temperature. After periods of 10 and 100 days, molecular weight distribution changes, mechanical property changes and elemental chemistry changes were studied using gel permeation chromatography (GPC), stress-relaxation testing and X-ray photoelectron spectroscopy (XPS), respectively. As determined through analyses of the molecular, mechanical, and elemental changes after treatment, the degradative mechanism was influenced by whether the polyurethane was polyester-based or polyether-based, and the degree of structural degradation was influenced by the type and duration of conditioning treatment. Degradation of the polyester-based materials after treatment was dominated by a chain scission mechanism; conversely, the degradation of the polyether-based material was dominated by a crosslinking mechanism. Among the conditioning treatments, the combined effects of an increase in acidity, oxygen content and temperature most influenced the degree of degradation with time, with the increase in temperature having the greatest effect of any of the single variables investigated.

Lifetime prediction of polymers

AbstractNatural and artificially accelerated weathering trials were carried out on low‐density polyethylene samples used for agricultural and disposable purposes. Modifications in polymer characteristics were studied using Fourier transform infrared (FTIR) spectroscopy, different scanning calorimetry (DSC), gel permeation chromatography (GPC), scanning electron microscopy (SEM), and mechanical properties testing. The natural exposure trials were conducted at five geographical locations representing varying ambient temperature and humidity conditions and receiving different doses of solar radiation. The artificially accelerated exposure trials were carried out in an Atlas weatherometer for 5,000 hours, whereas the natural exposure was for 24 months. Rates of photo‐oxidative and thermal degradation have been determined by measuring the formation of non‐volatile carbonyl oxidation products which absorb in the infrared region with a maximum absorbance level at 1710cm−1. Thermal characteristics were noticed to vary in terms of the shape and size of melting peak for both the natural and artificial weathering. This implies a change in crystallinity, which has indicated an increasing trend with exposure time. The crystalline melting temperature (Tm) remains almost steady for both exposure tests. The drop in mechanical properties and the modification in polymer chain length were also monitored. The GPC analysis indicated the change in molecular weight distribution (MWD), which shows the formation of lower molecular‐weight species. Surface modifications were revealed in terms of abrasion effects apparent from SEM micrographs. A correlation between natural and artificial weathering was considered for lifetime prediction in a short exposure time. It was found that the confidence level of predicting lifetime on the basis of artificially accelerated exposure trials is dependent on many parameters, which include time, material, equipment, etc. © 1995 John Wiley & Sons, Inc.

Chain transfer to solvent in two‐state anionic polymerization, 2. Slow exchange between propagating species

AbstractThe effect of slow exchange between growing chains on the average degree of polymerization (DP) and the molecular weight distribution (MWD) of polymers formed in a two‐state polymerization accompanied by chain transfer to solvent is considered. For the case when centres of only one type are capable of chain transfer it is shown that the exchange can be regarded as fast or slow, depending on the ratio of characteristic times of exchange (τex) and chain transfer (τtr) for these centres. The conditions for fast exchange, when the MWD of the polymer becomes independent of the rate of exchange, are derived. Under otherwise the same conditions, polymer of the narrowest MWD is formed if the exchange is fast, the number‐average DP being independent of the exchange rate. With increasing ratio τex/τtr, the weight‐average DP of the polymer increases and the MWD changes from unimodal to bimodal. For the case when “non‐transferring” centres are dormant, it was also found that at slow exchange the number‐average DP is greater and the MWD is less broadened than at fast exchange.

Butadiene polymerisation using ternary neodymium-based catalyst systems

Ternary catalyst systems for the polymerisation of 1,3-butadiene to high cis content were studied. The systems Nd(carboxylate)3/tert-butyl X /diisobutyl aluminium hydride (carboxylate=naphthenate, versatate; X=Cl, Br, I) were studied with respect to the halide:Nd ratio and halide type on catalyst activity and polymer characteristics. A lowering of the halide:Nd ratio results in lower conversions to polymer and a change in polymer molecular weight distribution. Catalyst stability is affected by halide type; instability, or tendency to precipitate, following the order I>Br>Cl. Less active catalysts (e.g. based on tert-butyl iodide) give low conversions and broad polymer MWD. Cis content remains at 98% and is unaffected by a lowering of halide:Nd ratio or a change in halide type.

Computer modelling of the degradation of linear polymers

Mechanisms of degradation of linear polymers are considered with the help of a computer program, which uses a pseudo-random number generator to carry out a Monte Carlo-like simulation of the processes. Randomly chosen bonds are degraded either at random positions along a molecule or by a number of specific systematic processes, such as breaking the molecule in half or slicing off a fixed number or percentage of units. The resulting molecular weight distribution (MWD) changes are compared to experimental measurements reported in the literature and generated in the authors' own laboratory. Totally random processes do not explain observed movements of the MWD to lower molecular weight or the formation of new peaks. Both processes require specific systematic scission of the polymer chain, such that, for instance, the length of one of the degradation products falls within the range of the original MWD. New peaks require the splitting off of fragments of a systematic length and/or recombination of fragments with each other or with other molecules in the distribution. Peak movement requires the preferential scission of the longest molecules. Random processes tend to follow standard first-order kinetic plots of reciprocal DP vs time, provided all the products of degradation are included in the DP calculation. Systematic scission processes tend to introduce curvature.

Simulation model for the modification of polymers via crosslinking and degradation

The Monte Carlo sampling technique, in which polymers are sampled randomly from an infinite number of polymer molecules in the reaction system, is used to make computer simulations for the molecular-weight distribution ( MWD) change during crosslinking and degradation processes. In this method, since one can investigate each polymer molecule directly, very detailed structural information can be easily obtained. This present method is not restricted by the condition of long primary chains with a low degree of crosslinking and degradation, and it is also straightforward to account for the effect of the residence time distribution. When the initial MWD is extremely narrow, say P w/ P n< 1.1, a multimodal distribution may be obtained by crosslinking reactions; however, in general, the crosslinking and/or degradation reactions tend to produce a unimodal MWD as long as the size and structural dependence of crosslinking and scission reactions is not important. When the expected crosslinking density is not the same for all primary polymer molecules, namely there exists a crosslinking density distribution among the primary polymer molecules, the average crosslinking density at the gel point is smaller than the value of 1/ P wp that is expected from the Flory-Stockmayer theory, where P wp is the weight-average chain length of the primary polymer molecules.

Acid Hydrolysis of K‐Carrageenan in a Batch Reactor: Stochastic Simulation of Change of Molecular Weight Distribution with Time

AbstractThe degradation of K‐carrageenan by acid hydrolysis in a LiCl/HCl (pH 2) buffer solution has been studied at various temperatures in a batch reactor. The reduction in the weight‐average molecular weight and the variation in the molecular weight distribution (MWD) have been followed over time by an SEC‐MALLS‐RI chromatography system. While a deterministic rate equation can be fit to the average molecular weight‐time data, this is not the case for the MWD‐time data. Since depolymerization by hydrolysis usually proceeds via nonspecific (random) bond cleavage, a stochastic approach can be appropriate for the resultant MWD‐time data. In the present work, the master equation has been derived from the stochastic population balance for this batch hydrolysis system characterized by a set of discrete states, each representing a particular molecular weight range. The governing equations for the means, variances, and covariances of the random variables, each representing the number of carrageenan molecules in an individual state, have been extracted from the master equation. The equations for the means have been applied to the MWD‐reaction time data, and the unknown reaction rate parameters have been estimated. These parameters can be used to generate the MWD at any hydrolysis reaction time from any initial MWD. The applicability, limitations, advantages, and sources of error of the derived model have been discussed.

Mechanism of shear modification of low density polyethylene

AbstractIn this paper we address the aspects associated with shear modification of low density polyethylene by extrusion. By successive passes of the polymer through an extruder it can be shown that both the melt viscosity and melt elasticity are reduced by shear modification. These reversible variations are not accompanied by significant changes in molecular weight distribution. The major effects of extrusion are to decrease the amount of elastically effecitve material. Study of proton relaxation times at 150°C shows that the melt comprises regions of relatively ordered, entangled, and unentangled polymer. Shearing reduces the proportion of more ordered material. The elastically effective anchors that are reduced by shearing are regions with restricted segmental mobility rather than conventional entanglements or intermolecular knots. Successive extrusions produce a relatively small decrease in the number of such constraints to molecular motion. Repeated shearing has a diminishing effect on the proportion of more ordered material in the polyethlene melt. This parallels observations of the effects of repeated shear modification on the properties of polyethylene films and other extrudates.

Inhibition of degradation in styrene-based thermoplastic elastomers by hydrogen-donating hydrocarbons

The effects of hydrogen-donating hydrocarbon additives derived from petroleum (HHAPs) on styrene-based thermoplastic elastomers, styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS), during mixing at 180°C in air were investigated and the following results were obtained: 1. (1) Changes in torque, temperature, gel fraction, mechanical properties and molecular weight distribution were investigated for SBS (crosslinking is the main cause of degradation), and mechanical properties and molecular weight distribution were studied for SIS (scissions are the main cause of degradation). 2. (2) The obvious effects against these types of degradation can be attributed to the high radical scavenging abilities of the additives. Thus, the inhibiting effects correspond precisely to the reaction rates with DPPH (2,2-diphenyl-1-picrylhydrazyl). HHAPs are new conceptual inhibitors because of their pure hydrocarbon constituents without functional groups containing heteroatoms, and they are interesting enough to be investigated further.

The reactive modification of polyethylene. I: The effect of low initiator concentrations on molecular properties

AbstractCommercial samples of high density, linear low density, and low density polyethylene were modified by injection of low concentrations of free radical initiator during extrusion. Molecular properties monitored included molecular weight distribution, degree of unsaturation, and branching. When the polyethylene teed to this reactive extrusion process had similar values of Mw, but varying polydispersity, degree of branching and degree of unsaturation, the magnitude of the change in molecular weight distribution was found to be in the following order: HDPE 1 &gt; LDPE2 &gt; LLDPE. In general, terminal vinyls enhanced molecular weight increase, and branching promoted degradation. However, for a second high density polyethylene sample with Mw = 154,000 (rather than the previous sample's Mw of 85,600), the change in molecular weight distribution was small and located at the lower molecular weight end. This work provided data for the kinetic model development detailed in Part II.

The reactive modification of polyethylene. II: Mathematical modeling

AbstractIn Part I of this work, experimental data showed that the effect of low concentrations of free radical initiator injected into polyethylene during extrusion depended upon the degree of unsaturation and branching in the feed as well as the molecular weight. This paper shows attempts to quantitatively explain these reactive extrusion results through development of two kinetic models based upon the rate equations for reactions considered dominant. The first model developed incorporated unsaturation via consideration of simultaneous crosslinking‐endlinking reactions. It contains two variable parameters: the overall initiator efficiency and a ratio of two rate constants reflecting the reactivity of the unsaturated bonds. The model was able to fit the changes in molecular weight distribution of both the low density and linear low density polyethylene but not the high density polyethylene samples. In addition to fitting the molecular weight distribution, this model also provided reasonable values of initiator efficiencies for crosslinking, endlinking, and chain extension reactions, as well as the number of terminal vinyls of the products. The second model is a special case of the first: it neglects the presence of unsaturation in the feed. This second model is actually the usual “crosslinking” model widely known from a derivation based upon statistical arguments. It was not able to fit most molecular weight distributions obtained. However, the model was shown to be useful for accounting for observed molecular weight distribution for a high density polyethylene sample of low initial unsaturation. Also, it was able to explain the amount of gel formed as a function of initiator concentration.

Classification and protein distribution in a series of intracapsular cataracts.

The crystallin profiles of the cortices and nuclei of intracapsular cataractous lenses were studied by high pressure liquid chromatography (HPLC), polyacrylamide gel electrophoresis (PAGE), and dot blotting. The complete personal and medical history of 381 patients and the Cooperative Cataract Research Group (CCRG) classification of each were obtained. Few statistically significant associations between patient personal history and cataract types were found. Protein profiles of selected cataracts which had specifically located opacities (i.e., nuclear only, cortical only, etc.) were studied in detail. Sodium dodecyl sulphate-PAGE revealed few differences in lens-soluble proteins between cataractous and normal cortices or nuclei. By HPLC, the proteins of cataractous cortices and their nuclei differed very little from age-matched controls. The cortical proteins of nuclear cataracts appeared normal. However, two major alterations of proteins were observed in the nuclei of dense nuclear cataracts. Increased high molecular weight protein and increased components with molecular weights < 20,000 Da were found in cataractous nuclei as compared with normal age-matched control lens nuclei. Dot blot (immunological) analyses identified the crystallins of normal lenses that eluted from the HPLC column more efficiently than those of cataractous lenses. Cortical protein HPLC samples had the most specificity. Nuclear protein HPLC samples of older normal and cataractous lenses had little if any alpha crystallin specificity in the void volume peak. A relation between the presence of opacities and changes in molecular weight distribution of crystallins was found in the opaque nuclei but not in the opaque cortices.(ABSTRACT TRUNCATED AT 250 WORDS)

Radiation degradation of poly(methyl methacrylate) in the soft x‐ray region

AbstractThe molecular weight distribution change has been measured for the photoresist poly(methyl methacrylate) [PMMA] after in‐vacuo exposure to monochromatic soft x‐rays from the Canadian Synchrotron Radiation Facility [CSRF]. The experimental changes in the mo‐lecular weight distribations derived from gel permeation chromatography [GPC], were compared to a simple Monte Carlo simulation model that assumes random main chain scission. Using this model a scission radiation chemical yield of G(S) = 1.28± 0.10 at room temperature was found to give the best fit at a photon energy of 621 eV. This value is similar to values reported previously in the literature using electron beam and γ‐ray sources, but significantly larger than those reported for fast neutrons, α‐particles, or energetically charged particles. It was found that in this soft x‐ray energy regime, that degradation of PMMA involves primarily a random scission process of the main chain. The results of a least‐squares fit of this soft x‐ray G(S) data and all available literature values from other radiation sources, to the linear energy transfer [LET] dE/dx are discussed. © 1993 John Wiley &amp; Sons, Inc.

Degradation of Degradable Starch-Polyethylene Plastics in a Compost Environment

The degradation performance of 11 types of commercially produced degradable starch-polyethylene plastic compost bags was evaluated in municipal yard waste compost sites at Iowa State University (Ames) and in Carroll, Dubuque, and Grinnell, Iowa. Masterbatches for plastic production were provided by Archer Daniels Midland Co. (Decatur, Ill.), St. Lawrence Starch Co. Ltd. (Mississauga, Ontario, Canada), and Fully Compounded Plastics (Decatur, Ill.). Bags differed in starch content (5 to 9%) and prooxidant additives (transition metals and a type of unsaturated vegetable oil). Chemical and photodegradation properties of each material were evaluated. Materials from St. Lawrence Starch Co. Ltd. and Fully Compounded Plastics photodegraded faster than did materials from Archer Daniels Midland Co., whereas all materials containing transition metals demonstrated rapid thermal oxidative degradation in 70 degrees C-oven (dry) and high-temperature, high-humidity (steam chamber) treatments. Each compost site was seeded with test strips (200 to 800 of each type) taped together, which were recovered periodically over an 8- to 12-month period. At each sampling date, the compost row temperature was measured (65 to 95 degrees C), the location of the recovered test strip was recorded (interior or exterior), and at least four strips were recovered for evaluation. Degradation was followed by measuring the change in polyethylene molecular weight distribution via high-temperature gel permeation chromatography. Our initial 8-month study indicated that materials recovered from the interior of the compost row demonstrated very little degradation, whereas materials recovered from the exterior degraded well. In the second-year study, however, degradation was observed in several plastic materials recovered from the interior of the compost row by month 5 at the Carroll site and almost every material by month 12 at the Grinnell site. The plastic bags collected from each community followed a similar degradation pattern. To our knowledge, this is the first scientific study demonstrating significant polyethylene degradation by these materials in a compost environment.

The effect of high-energy electron radiation on poly(arylene ether)s

Thin films of four experimental poly(arylene ether)s of similar chemical structure were exposed to 1 MeV electrons while under high vacuum. The films received total exposures of 5 x 107 and 1 x 109 rads at a dose rate of 5 x 107 rads h-1 and a pressure of 2 x 10-7 torr. Films exposed to 5 x 107 rads showed dramatic changes in molecular weight distribution. After exposures of 1 x 109 rads the films were only partially soluble in chloroform and exhibited no detectable changes in the glass transition temperatures. Thin-film tensile properties were also altered by the exposure to electron radiation. The effect of the exposures as determined by various analyses is discussed.

Polydisperse Dextran as a Diffusing Test Solute to Study the Membrane Permeability of Alginate Polylysine Microcapsules

Applications of alginate polylysine (APL) microcapsules in cell culture engineering and hybrid artificial organs require strict control of membrane permeability. For example, hybrid artificial organs must permit the diffusion of smaller molecules including peptides and proteins and the exclusion of leucocytes and immunoglobulins (MW > 150,000). Single molecular weight solutes such as proteins have been used to study the molecular weight cut-off of the membrane. A new approach that uses a heterogenous mixture of dextrans of different molecular weights: -MW 10,000-500,000 as a test solute in diffusion experiments is described. Intra/extra capsular changes in concentration and molecular weight distribution of the heterogenous dextran mixture are monitored and determined by high performance gel chromatography. MW can then be related to diffusional Stokes radius, a more useful parameter when comparing permeability of cell membrane to different test solutes.

A Novel Method for Detection of Endo-Xyloglucan Transferase

A new approach has been developed for quantification of the activity of endo-xylogluc an transferase, a novel enzyme that mediates the transfer of a segment of one xyloglucan molecule to another xyloglucan molecule. Purified xyloglucans with defined molecular-weight distributions and their fluorescent derivatives (pyridylamino xyloglucans) were used as substrates for the enzymatic reaction. The transferase activity was quantified by monitoring changes in molecularweight distributions of substrates by an alkali compatible gel permeation chromatographic system, equipped with a pulsed amperometric detector and a fluorescence detector. This new method was applied to the rapid detection and characterization of a novel transferase derived from plant tissues.