Mixed-crystal infrared studies of chain-folding in polyethylene single crystals: Low-temperature and molecular weight studies

Infrared spectra of 40 PEH/1 PED mixed crystals have been obtained at low temperature as a function of the molecular weight of the PED. The band contour in the CD₂ bending region was resolved into crystalline and noncrystalline components by a least-squares fitting of Gaussian-plus-Lorentzian bands. The splittings of the crystalline component were analyzed in terms of the theory for finite chains of coupled oscillators, with the help of normal coordinate calculations for certain structures. Together with some results on n-paraffin mixed crystals, our studies show that (i) the spectroscopic observations are associated with interactions of crystalline stems of a single molecule; (ii) the observed crystalline splittings are incompatible with an essentially “switchboard” model of chain folding; (iii) adjacent reentry folding predominates, with folding along single (110) and (200) planes occurring; and (iv) for high molecular weights of PED double (110) fold planes prevail. These results are seen to be consistent with the morphologies known to be associated with the conditions of crystallization.     

Vibrational analysis of chain folding in polyethylene crystals

The vibration frequencies of a polyethylene crystal lattice have been computed for various structures in which a fully deuterated chain is mixed with a normal chain. Intermolecular potential functions previously developed by the authors were used in the calculation. It is shown that such structures make it possible to distinguish experimentally between chain folding in the (110) plane and folding parallel to the (100) plane. Calculations have also been done for a regular polymer of (CH₂CD₂). Such a structure would permit the determination of the parity of the number of methylene groups in the fold.     

Mixed crystal infrared study of chain folding in crystalline polyethylene

Infrared spectroscopic studies have been made of mixed crystals of linear polyethylene and perdeuteropolyethylene. On the basis of normal vibration analyses by Tasumi and Krimm it had been shown that the study of crystal splittings of internal chain modes in such mixed crystals could provide information on the geometry of chain folding. The present results, which include a study of n-paraffin (C₃₆) mixed crystals, confirm these predictions. They show that (110) folding predominates in dilute solution grown crystals, and that this is transformed to (200) folding in melt-crystallized polymer. Folding with adjacent re-entry is favored, a random re-entry model being clearly eliminated.     

Fourier-transform infrared study of polyethylene single crystals in suspension

The infrared spectra of polyethylene single crystals suspended in liquid are reported and compared to dried mats of the same material. In order to observe the polyethylene bands, liquids with partial windows in complementary regions of the spectra were chosen. Any remaining bands of the suspending media were removed by substraction of the spectrum of the pure liquid. The crystalline bands of suspended polyethylene are much sharper than those of the dried material, indicating a decrease in lateral crystalline order upon drying. Drying also results in a change in the type and distribution of conformations associated with amorphous and fold structures, as determined by the observed frequencies of the methylene wagging mode.     

Coordinated pair chain folding model for solution-grown polyethylene single crystal

A new regular chain folding model, namely the “coordinated pair chain folding model” (CPCFM) is proposed to revive the consideration that a polymer chain should fold regularly rather than randomly in solution-grown polyethylene single crystals.     

Chain folding in single crystals of polytetrafluoroethylene

Potential-energy calculations are performed in order to obtain satisfactory models for folding in single crystals of polytetrafluoroethylene crystallized in the phase stable below 19°C. The folds are assumed to be tight with adjacent reentry. Folds with a variable number of bonds are analyzed. Their conformation is deduced by a method which obtains closure between two semifolds into which each fold is decomposed. The allowed models are subsequently investigated by using an energy minimization program. Some possible models are proposed. The calculations show that the folds must be described by a number of rotational isomeric states higher than is usually assumed.     

Chain folding in single crystals of tetrafluoroethylene copolymers

Molecular-mechanics calculations are performed on model folds proposed in the literature for single crystals of polytetrafluoroethylene crystallized in the phase stable below 19°C, in order to evaluate how they are modified when a fluorine atom is substituted by a bulky group, as occurs in fluorinated copolymers of tetrafluoroethylene. Only intramolecular effects are taken into account (isolated-chain model). An exhaustive analysis has been carried out of tetrafluoroethylene-hexafluoropropylene copolymer. Moreover, the lower energy folds found for such a copolymer have been examined for copolymers with chlorotrifluoroethylene and perfluoroalkylvinylethers, as comonomers. Our calculations show that all the considered comonomers can be arranged in the model folds proposed for the homopolymer, according to the imposed geometric constraints.     

Effect of temperature on frying oils: infrared spectroscopic studies

Frying oils were studied by Fourier-transform infrared (FT-IR) spectroscopy, in the range 4,000–200 cm^?1, at different temperatures, in the liquid and solid states. The infrared spectrum at 15 °C was similar to that at 200 °C. The band at 730 cm^?1 which was assigned to the rocking mode of (–CH₂) disappeared at higher temperature because of the rotational isomerism which occurred in the oil structure. The activation energy (E _a) of the disappearing (–CH₂) band, calculated by use of the chemical dynamic method using the Arrhenius equation, is 8.45 kJ mol^?1. The enthalpy difference (ΔH) between the two rotational isomer bands of the conformational structures of the oil at 730 and 1,790 cm^?1, at different high temperatures, was also calculated, by use of the Van’t Hoff equation; the value obtained was ?10.85 kJ mol^?1.     

Observations of slip traces in polyethylene single crystals

Single crystals of polyethylene are investigated by transmission electron microscopy. A pattern of lines is observed in the crystals. The properties of these lines suggest that the lines represent slip traces (slip steps) generated by the movement of dislocations in the polyethylene crystals.     

Nanoscale calorimetry of isolated polyethylene single crystals

We used thin‐film differential scanning calorimetry to investigate the melting of isolated polyethylene single crystals with lamellar thicknesses of 12 ± 1 nm. We observed the melting of as few as 25 crystals. Over a wide number of crystals (25–2000 crystals), the heat of fusion was 40% larger than the bulk value. The melting temperature of the isolated single crystals was 123 ± 2 °C, 9 °C lower than that of the bulk material. We also measured the heat of fusion of quenched crystals (±15%) over a wide range of heating rates (20,000–100,000 K/s). Annealing the quenched crystals resulted in shifts in the endotherm peak by as much as 15 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1237–1245, 2001     

A fourier transform infrared spectroscopic investigation of polyethylene single crystals. I. Methylene wagging mode

The application of the spectral subtraction routine to the determination of the morphology of polyethylene single crystals is discussed in detail. An analysis of the CH₂ methylene wagging mode is presented. A band at 1346 cm^?1 is observed to be unique to polyethylene single crystals. The assignment of this band to a regular, tight fold structure is discussed.     

Fast growth rates of polyethylene single crystals grown at high temperatures and their relevance to crystallization theories

The rates of growth of polyethylene single crystals grown from dilute solution in hexadecane and tetradecanol have been measured over the temperature range T_c = 98–120°C by following the change in turbidity during crystallization of a suspension of crystals of known shape and final size. The rates decrease similarly with T_c in each solvent, but for a given supercooling crystals grow much faster in tetradecanol where the corresponding crystallization temperature is higher. Similarly, the rates are much higher in hexadecane than those previously reported from xylene at equivalent supercoolings but lower T_c. Changes in the corresponding crystal morphologies as T_c is raised are quantified in terms of the axial ratio and the degree of curvature of the nominally {100} faces, both of which increase with T_c. The results can be interpreted as showing a transition from regime I to regime II growth in both solvents, which agrees both qualitatively and quantitatively with the predictions of the nucleation-based kinetic theories. Such a transition has never before been reported for solution crystallization. Using this analysis, reasonable values are obtained for the crystal side-surface energy σ of 7.4–7.5 erg cm^?2 and for the regime I substrate length L of 0.14 μm. No correlation is found between crystal morphology and growth rate and there are no discontinuous changes in morphology at the proposed transition points. The occurrence of curved crystal edges raises the fundamental issue of how to reconcile noncrystallographic growth surfaces with nucleation-controlled growth. A new approach to polymer crystal growth based on equilibrium surface roughening, which does not require nucleation, is therefore very pertinent in this respect and this is discussed.     

Growth of polyethylene single crystals from the melt: Morphology

Three-dimensional shape of polyethylene single crystals grown from the melt has been studied. Two distinct types of lateral habit have been obtained: lenticular shape (type A) and truncated lozenge (type B) in the range of regime I and II. Electron microscopy has revealed chair-like shape of type B crystal and reconfirmed the planar shape of type A crystal. In the type B crystal, spiral growth has occurred frequently in the {110} sectors and the sense of the handedness of spiral terraces has been maintained. It has been, suggested that the frequens occurrence of spiral growth is responsible for a morphological change (axialite-spherulite) accompanying the regime I–II transition. The origin of the chair-like crystals has been discussed and, a possible mechanism has been suggested for the formation of spiral terraces; the mechanism is based on a distortion caused by the three-dimensional shape of chair-like crystals. It has been found that the chair-like crystals are curved in the opposite way to S-shaped lamellae observed by Bassett and Hodge in banded spherulites. In fact, the present work has led to the recognition of further classes of crystal with curving cross-sections and of distinctions between them. In final analysis, a unifying thread has been identified between lateral habits, growth kinetics and three-dimensional shape of lamellae, in turn, leading to some rationalization of multilayer developments including twisting in banded spherulites, the latter based on existing suggestions in the literature.