This work reviews the literature published over the last ten years on polymer mechanical properties as a function of molecular weight and molecular weight distribution, Thermal properties, stress-strain properties, impact, fracture, fatigue, creep, stress relaxation and cracking and crazing are examined for a wide variety of homopolymers and a limited number of copolymers. In general, mechanical properties increase as the molecular weight increases. However, above some limiting molecular weight the mechanical property is usually unaffected. Although much work has been done to describe the effects of molecular weight on mechanical properties, little quantitative correlation exists. The available equations to predict such properties as cracking and crazing, Tg, Tm and tensile strength from molecular characteristics are discussed in detail. However, a more quantitative description incorporating a wider range of mechanical properties would be more useful. This would facilitate use of the vast amount of information available and enable it to be applied more readily to new polymer systems.

Influence of molecular weight and molecular weight distribution on mechanical properties of polymers

The photodynamic, actue toxicities of 20 polycyclic aromatic hydrocarbons (PAHs) to Daphnia magna were predicted by photophysical and physiochemical parameters. The photophysical parameters considered were lowest singlet energy, lowest triplet energy, singlet-triplet splitting energy and phosphorescence lifetime. The physiochemical parameters were first- and second-order connectivity indices and log P values. D. magna were exposed to aqueous solutions of each PAH such that equimolar concentrations in D. magna were achieved. The organisms and the PAHs were then exposed to 120 μW/cm2 UV-A and 25 μW/cm2 UV-B light. Mortality times (min) were recorded and the results reported as median lethal time (LT50). Potency factors (π) were calculated and used to rank the PAHs in terms of relative photodynamic toxicity. Some statistically relevant correlations between individual physical parameters and toxicity were observed. Linear, multiple regression models were poor predictors of photoinduced PAH toxicity. A curve-linear model was developed to predict photoinduced toxicity from triplet energy. Goodness-of-fit chi-square tests were performed and demonstrated that triplet energy was an effective predictor both of observed LT50 and of LT50 values adjusted to a constant PAH concentration. Toxicity data were also analyzed using discriminant functional analysis. A stepwise, canonical correlation parameter selection method separated the PAHs into three toxic categories by using triplet energy and phosphorescence lifetime as variables. This model, which classifies PAHs as very toxic, moderately toxic or nontoxic, was 100% accurate when the model developed from one set of PAHs was tested with a different set.

Predictive models for photoinduced acute toxicity of polycyclic aromatic hydrocarbons to Daphnia magna, strauss (cladocera, crustacea)

Modelling the linear viscoelastic rheological properties of bituminous binders

Molecular dynamics simulation was used to calculate rotational relaxation time, diffusion coefficient, and zero-shear viscosity for a pure aromatic compound (naphthalene) and for aromatic and aliphatic components in model asphalt systems over a temperature range of 298–443 K. The model asphalt systems were chosen previously to represent real asphalt. Green–Kubo and Einstein methods were used to estimate viscosity at high temperature (443.15 K). Rotational relaxation times were calculated by nonlinear regression of orientation correlation functions to a modified Kohlrausch–Williams–Watts function. The Vogel–Fulcher–Tammann equation was used to analyze the temperature dependences of relaxation time, viscosity, and diffusion coefficient. The temperature dependences of viscosity and relaxation time were related using the Debye–Stokes–Einstein equation, enabling viscosity at low temperatures of two model asphalt systems to be estimated from high temperature (443.15 K) viscosity and temperature-dependent relaxa...

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Relaxation time, diffusion, and viscosity analysis of model asphalt systems using molecular simulation

High-boiling-point petroleum fractions (350°C+ ) are known to be enriched in the most polar compounds such as resins and asphaltenes. Asphaltenes contain molecules of variable aromaticity with different contents of heteroatoms, metals and functional groups. An asphaltene fraction is a complex mixture of polydispersed molecules in terms of size and chemical composition. Such structures cannot be represented by only one model molecule. Several models have been proposed in the literature to describe these structures (continental and archipelago types). Due to this high polydispersity, specific properties such as aggregation are observed. Detailed characterization of such highly complex products is difficult and the choice of analytical techniques that can be used is quite limited. Average structural information has been obtained, but it cannot be representative of all chemical and structural variety that such matrices may contain. The aggregation state of asphaltene macromolecules strongly depends on the experimental conditions (concentration, temperature and solvent). Significant variations in measurement may be induced if the operating conditions used are not similar. The techniques being based on different theories and being run in different experimental conditions are consequently not equivalent. The asphaltenic structures are not measured in the same aggregation state. Heavy product characterizations can be divided into two types: the chemical and colloidal characterizations. The first one gives information on both chemical composition and main functional groups as well as the structural state of metals and heteroatoms in macromolecules. Numerous techniques are used such as high-performance liquid chromatography (HPLC), pyrolysis coupled with gas chromatography and mass spectrometry (PyGC-MS), 13 C nuclear magnetic resonance (NMR), and spectroscopic techniques (IR, XPS, EXAFS and XANES). Colloidal characterization consists of the investigation of the dispersion of heavy fractions (asphaltenes and resins) as macromolecules in a good solvent or in their natural medium (maltenes). The main parameters that can define the colloidal structure are the molecular weight and the mass or size polydispersity of aggregates. Various techniques may be employed, such as mass spectrometry, scattering techniques (small-angle X-ray and neutron scattering), pulsed-field gradient nuclear magnetic resonance (PFG-1 H NMR), as well as different separation methods (ultracentrifugation, solvent separation and size-exclusion chromatography). As a preliminary step, fractionations have to be carried out in order to simplify these matrices, too complex to be directly characterized.

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Physicochemical Characterization of Petroleum Fractions: the State of the Art

The influence of molecular weight (900 to 1.8 × 106) on the glass transition temperature of low polydispersity polystyrene (anionically prepared) has been studied by differential scanning calorimet...

Effect of Molecular Weight on Glass Transition by Differential Scanning Calorimetry

Effect of molecular weight and composition on the glass transition temperatures of asphalts

The equilibrium anionic polymerization of α-methylstyrene in p-dioxane, with potassium as initiator, has been investigated at 5, 15, 25, and 40°C by using high-vacuum techniques. The comparison of these results with those obtained previously for the equilibrium polymerization of α-methylstyrene in tetrahydrofuran revealed that, although the values of ΔG1c, the free-energy change upon the polymerization of 1 mole of liquid monomer to 1 bases-mole of liquid amorphous polymer of infinite chain length, are the same for both systems, there is a distinct effect of the solvent. This effect is reflected in the value of monomer equilibrium concentration and its variation with polymer concentration and is explained in terms of a solvent–monomer and solvent–polymer interaction parameter.

Equilibrium anionic polymerization of α‐methylstyrene in p‐dioxane

The thermal decomposition and the glass transition temperature Tg of poly(phenyl methacrylate) (PPhMA) and poly(cyclohexyl methacrylate) (PcHU) were studied with a differential scanning calorimeter (DSC). The undecomposed and decomposed polymers were analyzed by gel permeation chromatography (GPC) for molecular weight distributions and by DSC for changes in the thermal properties, e.g., Tg. For all values of weight-loss α, the thermal stability of the polymers follows the order: Poly-(methyl methacrylate) (PMMA) = PcHMA > poly(ethyl methacrylate) (PEMA) > PPhMA > poly(n-butyl methacrylate) (PnBuMA) > poly(isobutyl methacrylate) (PiBuMA). In the depolymerization reactions that occur during the isothermal decomposition of PPhMA, there is no specific preference for longer or shorter chains although a minor fraction of the volatilized fraction with an [Mbar]w 10−5 of 2.5 and an [Mbar] n |MX 10.−5 of 1.5 does undergo chain recombination yielding high molecular weight products with an Mw × 10−6 of 1.35...

Thermal decomposition and glass transition temperature of poly(phenyl methacrylate) and poly(cyclohexyl methacrylate).

Kinetic studies on the decomposition of poly-α-methylstyrene samples with molecular weights ranging from 3.7 × 103 to to 2.0 × 105 have been carried out with the differential thermo-gravimetric technique. Changes in molecular weight distributions with decomposition, at different temperatures, have been studied by gel-permeation chromatography. A depolymeriza-tion mechanism was shown to be responsible for the decomposition phenomenon. The order of reaction for this depolymerization reaction was found to be one. The values of the activation energy for samples of different molecular weights showed no specific trends; however, it would appear that polymers with relatively higher molecular weights have lower activation energies of decomposition. The proportions of the three steric forms, viz., isotactic, heterotactic, and syndiotactic, in the polymer before and after thermal treatments did not change, suggesting that stereoregularity has no apparent effect on the decomposition of poly-α-methylstyrene.

S. L. Malhotra

Papers

Effect of Molecular Weight on Glass Transition by Differential Scanning Calorimetry

Effect of molecular weight and composition on the glass transition temperatures of asphalts

Equilibrium anionic polymerization of α‐methylstyrene in p‐dioxane

Thermal decomposition and glass transition temperature of poly(phenyl methacrylate) and poly(cyclohexyl methacrylate).

Thermal Decomposition of Poly-α-methylstyrene