The enthalpy of fusion and degree of crystallinity of polymers as measured by DSC

Crystalline polymers are not in thermal equilibrium and thermodynamic parameters such as enthalpy of fusion as determined by differential scanning calorimetry from the area under the melting endotherm over a wide temperature range have not been measured under equilibrium conditions. Accordingly measurements of the degree of crystallinity based on the enthalpy of fusion reflect experimental conditions, are incorrect in that they do not usually agree with those determined by other analytical procedures, such as density and WAX scattering, particularly when measured at ambient temperatures. While this has been repeatedly pointed out procedures used to determine the fractional crystallinity of polymers based on the enthalpy of fusion continue to be widely used.Using the First Law method, the enthalpy of fusion and fractional crystallinity have been measured as a function of temperature for metallocene polyethylene and polyethylene terephthalate during heating. The crystallinity measured on the sample prior to heating is in good agreement with that determined by density and wide angle X-ray diffraction.     

Characterization of thermoreversible polymer gels by the degree of crystallinity by surface area

A characterization for crystalline junction polymer gels has been extended by introducing a new parameter which implies a measure of the number of the polymer molecules passing through the crystallizing junctions. The parameter has been considered in a theoretical gel melting relation derived by Takahashi, Nakamura, and Kagawa and a crystallite size distribution has also been considered. The equation obtained was examined on ethylene–propylene copolymer gels by using the previous data of melting temperatures determined from DSC, crystallite sizes from x-ray and DSC measurements, and reaction probabilities from ¹³C-NMR measurements. Consequently, the parameter was found to be helpful as one additional measure. Also, it was confirmed that the average crystallite sizes estimated in several different ways were all in good agreement with the conventional data. © 1996 John Wiley & Sons, Inc.     

Orientation of crystalline polymers by non-classical means

Crystalline polymers can be oriented by epitaxy on various substrates, including low molecular weight, usually aromatic compounds and polymers oriented by epitaxy and by mechanical means (strain induced polyolefin films and friction-deposited PTFE layers). The resulting oriented films are thin, but display unusual morphologies with lamellae standing edge-on and, often, single crystal structure. They are adequate investigation materials for structure determination, show enhanced or maximized orientation-dependent properties, and may contribute to mechanical strengthening in composite polymer films.     

NMR observations of drawn polymers. V. Sorption into drawn and undrawn polyethylene

NMR measurements on undrawn polyethylene (PE) samples in contact with a solvent such as C₂Cl₄ indicate an increase in the mobility of the mobile chain segments as compared to dry samples. Highly drawn PE shows no such effect. This is because S_a, the sorption per unit mass of noncrystalline material present, decreases from 20.9 wt.-% (dry basis), found for undrawn quenched PE, to 0.63 wt.-% after drawing (S_a determined at 25°C. and 0.80 vapor activity). Drawing also reduces the segment mobility according to the NMR spectrum. It is shown that these effects are caused by considerable structural changes occurring in the noncrystalline regions of PE upon drawing. Annealing of drawn PE samples at successively higher temperatures leads to a gradual relaxation of the noncrystalline regions towards the state characteristic of undrawn PE. With increasing annealing temperature S_a as well as the mobility approach values found with undrawn PE.     

Colouration efficiency measurements in electrochromic polymers: The importance of charge density

A poly(3,4-ethylenedioxythiopene) (PEDOT)/polyaniline (PANI) electrochromic device (ECD) was fabricated for the purpose of examining colouration efficiency (CE) as a function of charge density ingress/egress. CE is typically measured in cm²/C with the aim being to produce as large an optical density change as possible with the least amount of charge being consumed. Results indicate that CE is not constant but is highly dependent on charge density insertion and the switching voltage. At a switching voltage of 1.9 V the maximum CE_max was 1186 cm²/C, recorded at 60% of the full optical switch where as the CE_95% was 302 cm²/C at 95% of the full optical switch. Furthermore, CE_max varied depending on the switching voltage from a high of 2212 cm²/C at a switching voltage of 1.2 V, 1528 cm²/C at 1.6 V and down to 1186 cm²/C at 1.9 V. The results highlight the fact that the current practice of quoting CE as a single-valued number may not reveal enough detail about the performance of ECDs and polymers.     

Determination of ethanol in beverages by flow injection, pervaporation and density measurements

A fast, clean and easy to automate flow injection-pervaporation method for the determination of ethanol in different beverages using density measurements is proposed. The method is based on separation of the ethanol from the sample using a pervaporation module; the analyte being collected in water as acceptor liquid. The density of this water-alcohol mixture is measured at the detector. After optimisation by either a univariate or multivariate approach as required, a linear range between 0 and 40% was established. Then, the assessment of the method versus a reference one was studied in terms of repeatability (0.12% v/v), reproducibility (0.32% v/v), detection limit (0.11% v/v) and traceability. The sample throughput was 15 samples h⁻¹. The method was in agreement with the reference methods used in the European Union.     

On the CO2 permeation of uniaxially drawn polymers

The CO₂ permeation coefficient and the difficient were measured using the permeation time-lag method for films of atactic polystyrene and high-density polyethylene, each as a function of uniaxial draw ratio. The reduction of permeability with draw ratio is observed for polystyrene and for polyethylene. In the latter it is associated with an increase in crystallinity. In both cases the premeability decreases and the solubility constant remains unchanged. The reduction of permeability is thus caused only by the reduction in diffusion of CO₂ in the drawn polymers. The mechainism is different for the two polymers, as is confirmed by measurements of birefringence, glass transition temperature, and crystallinity.     

The degree of crystallinity of monoclinic isotactic poly(propylene)

The degree of crystallinity of a set of monoclinic (alpha) isotactic poly(propylenes), prepared by a metallocene‐type catalyst, were determined at room temperature. Three different methods were used: density, enthalpy of fusion, and wide‐angle X‐ray scattering, and the results compared. The relation between the heat of fusion and the specific volume of these poly(propylenes) was found to be nonlinear, thus precluding any linear extrapolation to obtain the heat of fusion of the pure crystal (ΔH_u). The value of ΔH_u obtained from depression of the melting temperature by diluents is used. Based on the unit cell density of monoclinic crystals formed from a low defected fraction, the density obtained crystallinity levels were found to be between 0.l5–0.25 higher than those calculated from the heat of fusion. This relatively large difference holds for the isothermally crystallized and quenched isotactic poly(propylenes), and reflects the contribution of the interphase to the density determined crystallinity, which does not contribute to the heat of fusion. Paralleling results found in other systems, the crystallinity levels obtained from wide‐angle X‐ray scattering agree with those obtained from density, indicating a significant contribution of the partially ordered phase to the total diffraction. Emphasis is given on the need to account for the large differences in the crystallinities of poly(propylene) measured by different techniques when evaluating the dependence of properties on this quantity. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 323–334, 1999     

Heat conduction anisotropy of drawn high density polyethylene samples

In the paper the influence of the morphology of cold drawn samples on heat conduction anisotropy is discussed. It was shown that the important role is played by the existence of air gaps between fibrils of oriented polymer and the perpendicular microcracks appearing with the increase of the draw rate of the specimens.Rolled drawn samples have a significantly reduced air gap contents thus the heat conduction anisotropy measurements on these rolled samples provide real data on polymeric material properties.To Professor F. H. Müller on the occasion of his 75. birthday.     

Variation of crystallite orientation with degree of crystallinity: Isotactic polystyrene

The orientation of the crystallographic c axis (chain axis) was followed by x-ray diffraction during the crystallization of four samples of isotactic polystyrene differing in elongation ratio. The crystallite orientation can be expressed by 〈cos² χ_c〉, where χ_c is the angle between the c axis and the stretching direction. The degrees of crystallinity w were estimated from the diffraction data by using density for calibration. It was found that 〈cos² χ_c〉 decreases in a linear manner with crystallinity, the rate of decrease being very small when the elongation ratio α is 5, but becoming progressively larger as α is decreased toward unity. A qualitative measure suggests that amorphous orientation decreases during crystallization at a rate which is nearly independent of α. The variation of 〈cos² χ_c〉 with w is therefore governed by the orientation of the statistical chain segments prior to crystallization. If the elongation ratio is small, the supply of well oriented statistical segments is limited, and 〈cos² χ_c〉 will decrease at a rapid rate during crystallization. A treatment due to Krigbaum and Roe permits evaluation of the ratio, ν/N, where ν and N are the average numbers of statistical segments in the crystallization nucleus of critical size, and in a network chain, respectively. Our polystyrene samples were not crosslinked, so chain entanglements must serve as junction points. Values of ν could not be obtained, since N was unknown. However, the (ν/N) ratio for isotactic polystyrene decreases slowly with α, and the values agree reasonably well with those obtained in a previous study of oriented polychloroprene networks. After nearly complete crystallization (ω ca. 0.30), the long period spacing measured by low angle diffraction was approximately 135 Å, and varied only slightly with elongation ratio in the range α = 1 to 5. It therefore appears that chain folded lamellae are present in both drawn and undrawn samples of isotactic polystyrene.     

Estimation of Thoron incorporation by means of in-vivo measurements

Thoron (²²⁰Rn) is a gaseous decay product of²³²Th decay chain which becomes a potential source of internal contamination for individuals who handle significant amounts of ores and raw materials containing thorium. With the objective of establishing an occupational control of such workers, a methodology which permits, by means of in-vivo measuremens, the determination of the level of internally deposited²²⁰Rn daughters (²¹²Pb,²¹²Bi, and²⁰⁸TI), was developed at the Institute of Radiation Protection and Dosimetry/Brazilian Nuclear Energy Commission (IRD/CNEN) whole body counter. Previous calibration of an 8×4 NaI(Tl) detector was carried out using the lawrence Livermore National Laboratory (LLNL) thorax phantom with the lung cavity filled withi plastic bags containing²⁸Th standard solution. After sealing, the bags are stored until radioactive equilibrium is reached. Patients are measured for I h in two steps, at an interval of approximately 20 h, Positioning the detector over the thorax. A comparison of the activities determined at each measurement allows the distrimination of the contribution of the internally deposited shoret-and longlived radionuclides. This technique was recently applied to a group of workers at a gas mantle industry in Rio de Janeiro. Results of these measurements will be shown.