Thickening of crystals during rapid heating. I. Linear polyethylene and nylon-6

The change of long spacing in polyethylene and nylon-6 during heating at 18°C/min. was measured with an apparatus devised for rapid measurement of the small-angle x-ray diffraction. The apparatus made it possible to obtain a diffraction profile in 1.8 sec. Long spacings increased notably at high temperatures near the melting point, as has been observed during low-rate heating. However, the increase in long spacing during heating was very small in a methoxymethylated nylon-6 sample. This suggests that the increase in the long spacing at high temperature is due to the thickening of crystals. The long spacing in polyethylene samples in the vicinity of the melting point is apparently independent of thermal history.     

Thickening process of polyethylene single crystals at an early stage of annealing

By the use of a position sensitive proportional counter, changes in small and wide angle X-ray scattering during annealing of polyethylene single crystal mats were measured from the start in successive spans of very short measuring time. At high temperatures, the long period relating to stacking of lamellae rapidly increased at an early stage, passed through a plateau, and thereafter again increased gradually. With a decrease in annealing temperature, the amount of its first rapid increase was reduced and the plateau changed into an ascending slope. At much lower annealing temperatures, the long period increased following the logt law after an induction time. The integral breadth of a peak corresponding to the long period first increased rapidly, simultaneously with the rapid increase in the long period, and thereafter decreased. Wide angle X-ray measurement showed that the integrated intensity of 110 reflection first decreased and then increased during annealing at high temperatures. This fall and rise process was more marked, when the annealing temperature is higher and the initial thickness of lamellae is smaller. From these observations, it was inferred that in the thickening process, stacking order of lamellae at first decreased because of rapid reorganization due to partial melting or melt-recrystallization and subsequently increased through increasing evenness of lamellar thickness.     

Thickening process of polyethylene single crystals at an early stage of annealing

The thickening process of polyethylene single crystals was simulated with computer by the Monte Carlo method. According to the experimental results in the previous report, the time dependence of the long period changed greatly with annealing temperature; at lower temperatures the long period increased gradually, while at high temperatures the long period rapidly increased at a very early stage of annealing and then increased gradually after passing through a plateau. Through computer simulation, it was shown that such a great change in the time dependence of the long period with annealing temperature can be explained by combining two mechanisms: (A) sliding diffusion of molecular segments along the chain axis and (B) local melt-recrystallization (namely, local melting followed by recystallization).     

Thickening of Crosslinked Ethyl Methacrylate-Acrylic Acid Latexes

Latexes based on ethyl methacrylate (EMA), acrylic acid (AA), crosslinked with 1,6-hexanediol propoxylate diacrylate (HPDA), were synthesized via emulsion polymerization, yielding dispersions with different acidic group contents and crosslinking densities. The latexes were thickened using various NaOH/AA molar ratios and the resultant dispersions were characterized by pH-metry, zeta potentiometry, and turbidimetry. The process of thickening by carboxyl neutralization was shown to be dependent on the neutralization effect on particle–particle interactions and particle solubilization. Turbidimetry, pH-metry, and zeta potential measurements showed that neutralization resulted in partial latex solubilization and viscometry indicated that, for a given latex, an optimum thickening was achieved for and specific NaOH/AA molar ratio, showing the existence of a relationship between thickening, particle–particle interactions, latex solubilization, and polyelectrolyte effect.     

Extended-chain crystals. II. Crystallization of polyethylene under elevated pressure

A report on crystallization of polyethylene at elevated pressures to an extended-chain morphology is presented. The crystals have been characterized by electron microscopy and density determination. Pressure, supercooling (temperature), and crystallization time have been varied to find the best conditions for production of perfect crystals. At 10–30°C supercooling completely crystallized polyethylene was obtained between 4.5 and 7 kb crystallization pressure in 1–8 hr. Analysis of fracture surfaces of samples crystallized for different lengths of time shows an increase in size and number of crystal lamellae and an improvement of extended chain crystals in the early stages of crystallization. A further improvement of the less well crystallized material between the lamellae occurs after 15 min of crystallization time.     

Studies on crystalline forms of nylon 6. II. Crystallization from the melt

The relative amounts of the α and the γ crystalline forms of nylon 6 obtained from the melt under different crystallization conditions have been studied by an x-ray diffraction procedure by comparison with a calibration curve obtained from the diffraction of standard samples. The weight fraction of the α form decreases with increasing crystallization temperature and that of the γ form increases. Growth of the α form is predominant in crystallization at 100°C and of the α form at 200°C. The amount of the α form tends to increase on annealing at 200°C for specimens crystallized at any temperature.     

Crystallization during polymerization of diazomethane. II. Studies of model compounds and chain extension of etched polyethylene single crystals by chemical reaction

In order to study the mechanism of crystallization during polymerization, a method to produce living oligomer crystal nuclei was developed. The living oligomers are diazoketones prepared from their carboxylic acids or acid chlorides. These diazoketones were proved to be reactive toward boron trifluoride etherate. The complex could be further chain-extended through a living polymerization mechanism with diazomethane. The optimal conditions for the reactions were worked out using model compounds. These reactions were then applied to chain extension of etched polyethylene single crystals. The reactions were confirmed by a combination of time-dependent differential thermal analysis, molecular weight determination, nuclear magnetic resonance and infrared analysis. The importance and potential application of such reactions for the study of nucleation and crystallization, and final properties of the resulting polymers are discussed.     

Crystallinity and the effect of ionizing radiation in polyethylene. II. Crosslinking in chain-folded single crystals

In order to ascertain whether the crystal core of irradiated polyethylene single crystals is free from crosslinks—as would follow from the results of Part I—the fold surface of irradiated single crystals was shaved off with ozone and the resulting product examined by GPC for molecular weight. It was found that on ozone degradation the entire material, which had been insolubilized by radiation-induced crosslinking, has become soluble hence became available for the GPC analysis. The chromatograms displayed the same peaked development with degradation as the unirradiated crystals leading eventually to single traverse dicarboxylic acids. This proves the absence of crosslinks within the crystal interior of the material as examined by GPC. The appearance of some additional low molecular weight material, is attributed to scission at the radiation-induced double bonds due to ozone which eliminates the possibility of the existence of crosslinks within the lattice such as might provide scission sites for ozone. The conclusion could therefore be reached that, to the extent assessable by our GPC test, there are no radiation-induced crosslinks within the crystal lattice, hence the crosslinks produced must be entirely confined to the fold surface region of the crystals.     

Real-time X-ray scattering study during heating of oriented injection-molded polyethylene

Highly oriented linear polyethylene was prepared by elongational flow injection molding. The changes in crystal orientation were investigated as a function of temperature by real-time wide-angle X-ray diffraction. Additionally, the influence of molecular weight upon the microstructure and the changes in orientation, during heating near the melting point, and after cooling have been examined. A shish-kebab structure is inferred for the high molecular weight samples (M_w≥10⁵) from SAXS observations, while for samples with M_w<10⁵ only an oriented lamellar structure is found. Consequently, a higher thermal stability is shown by the higher molecular weight samples. Furthermore, a recovery of crystal orientation on rapid cooling of the samples from the melt is only observed for samples with M_w≥10⁵. The results are discussed in terms of a preferential recrystallization of chain-folded lamellae, on cooling, onto the shish fibrils which survive at high temperature.     

Investigations of morphological changes during annealing of polyethylene single crystals

The morphological evolution of isolated individual single crystals deposited on solid substrates was investigated during annealing experiments using in situ and ex situ atomic force microscopy techniques. The crystal morphology changed during annealing at temperatures slightly above the original crystallization temperature of the crystals, far below their melting temperature. Evenly distributed cavities penetrated the crystals, and the number of cavities increased with a rising annealing temperature until the adjacent cavities coalesced. The thickness of the crystals increased during annealing at temperatures slightly above the crystallization temperature. Annealing experiments at fixed temperatures showed that the reorganization process (cavity formation and single‐crystal thickening) was fast. Depending on the annealing temperature, the final morphology was formed in seconds. This behavior suggests high chain mobility as well as a homogeneous solid‐state reorganization of the entire single crystal at low annealing temperatures. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 763–770, 2001     

Photocrosslinking of polyethylene. II. Properties of photocrosslinked polyethylene

Some properties of photocrosslinked high density polyethylene (2 mm thick sheets) have been studied. The homogeneity of the network is greatly improved by the application of triallylcyanurate (TAC) as crosslinking agent. The role of TAC in promoting the crosslinking is discussed. The crosslinked PE is found to be durable towards immersion in boiling water. Oxygen permeability increases while density and melting point decrease with increasing degree of crosslinking. The network chain density is obtained using conventional swelling method in boiling xylene, which together with data from melting point depression show that contribution of chain entanglement is insignificant to the gel formation of PE in this work.     

Investigation of phase morphology of incompatible polyethylene/nylon 6 blends containing EPDM-based functionalized compatibilizers

Graft copolymer and graft terpolymer were prepared by solution grafting of maleic anhydride (MAH) or acrylonitrile (AN) alone and mixture of MAH and AN on to ethylene–propylene–diene terpolymer (EPDM) using benzoyl peroxide (BPO) as an initiator. The resulting EPDM-g-MAH, EPDM-g-AN and EPDM-g-(MAH-co-AN) have been used to obtain a binary blend of Nylon 6/functionalized EPDM and a ternary blend of polyethylene/Nylon 6/functionalized EPDM by melt blending. The effects of the nature and the amount of the grafted species on the phase morphology, crystallization behavior and mechanical properties of the blends were characterized through scanning electron microscopy, optical microscopy, infrared spectroscopy and using a dynamic mechanical analyzer. From the morphological study, it can clearly be seen that the presence of the functionalized EPDMs in these blends resulted in an improvement of the dispersion degree in incompatible polyethylene/Nylon 6 blends.     

Effect of polymer surface morphology on adhesion and adhesive joint strength. II. FEP Teflon and nylon 6

Heterogeneous nucleation and crystallization of FEP Teflon and nylon 6 melts against high energy surfaces (i.e., gold) produce an interfacial region, in these polymers, of high mechanical strength. Dissolution of the metal substrate rather than removal by mechanical means results in a polymer surface which is amenable to conventional structural adhesive bonding. Nucleation and crystallization of the polymer melts in contact with phases of low surface energy (e.g., vapor) result in the generation of weak boundary layers.     

Nucleation and growth of new crystals within polyethylene films during surface-controlled recrystallization

Observations by transmission electron microscopy suggest that surface-controlled recrystallization of polyethylene occurs by the solid state nucleation and growth of new crystals with (110) or (11 0) planes parallel to the free surface. The nucleation site of these crystals is the free surface. The growth process involves the migration of [002] tilt boundaries suggesting that these boundaries have high mobilities. The migration process may occur by the lateral rearrangement of the macromolecules in the [002] tilt boundary.     

The electron microscope characterization of the fine structure of nylon 6: II. On the rearrangement of the structure during cold-drawing

The transmission electron microscope (TEM) visualization of the supermolecular structure of cold-drawn, oriented nylon 6 bulk material (bristles) by stained ultra-thin sections is reported. For evaluating the electron micrographs optical diffraction (OD) has been applied in comparison with small angle X-ray scattering (SAXS). The deformation of the spherulites was followed by polarization microscopy. In addition, investigations were carried out on commercial nylon 6 fibres. As the main result a transverse structure was revealed within the drawn samples at draw ratios between =4 and 4.5, consisting of mosaic crystals which show some lateral alignment. The structure is described by a modified layer lattice model. While the long period may increase slightly during drawing, the crystallite thickness remains almost constant. Fibres with =3.4 show a similarly oriented structure though the lateral alignment of the crystals is not so pronounced.     

Plastic deformation of polyethylene II. Change of mechanical properties during drawing

The crystallinity, elastic modulus, and tensile strength of samples of various draw ratios together with the true stress—strain curves of high-density polyethylene were determined to establish correlations with morphological changes occurring during deformation. Changes of crystallinity at draw ratios below 5, i.e., constancy during drawing of quenched film and a decrease during drawing of annealed film, are explained by the formation of microfibrils with crystallinity independent of the thermal history of the film. The microfibrils slide past each other at higher draw ratios, generating an increasing number of interfibrillar tie molecules, which is reflected in the increasing number of interfibrillar tie molecules, which is reflected in the increase of crystallinity, elastic modulus, and tensile strength. From the true stress—strain curves, the differential work density for the deformation of the volume element was calculated as a function of the draw ratio. It contains two components which reflect two different mechanisms of deformation. The first component, decreasing with increasing draw ratio, can be associated with the destruction of the original microspherulitic structure; the second one, increasing with increasing draw ratio, can be associated with the deformation of the new fiber structure, i.e., with the sliding motion of the microfibrils formed during the first deformation step.     

Extended-chain crystals. III. Size distribution of polyethylene crystals grown under elevated pressure

Extended-chain crystals of high molecular weight polymethylene, a polyethylene with a broad molecular weight distribution, and three fractions of polyethylene were grown from the melt under elevated pressure. Comparison of the crystal size distribution in the molecular chain direction (measured on fracture surfaces by electron microscopy) with the molecular weight distribution (measured by gel-permeation chromatography) gave the following results. Up to molecular weight 10,000 all samples showed eutectic separation into fully extended chain crystals of narrow molecular weight distribution. Above molecular weight 10,000 mixed crystals were formed. Under the chosen crystallization conditions larger chain extension was achieved with higher molecular weights. However, an increase in molecular weight by a factor of 1000 led only to a tenfold increase in chain extension. These facts are discussed in the light of a proposed mechanism of crystal growth.     

Structures for monodisperse oligoamides. A novel structure for unfolded three-amide nylon 6 and relationship with a three-amide nylon 6 6

Three-amide oligomers of nylon 6 and nylon 6 6 have been investigated using electron microscopy (imaging and diffraction), X-ray diffraction, and computational modeling. A new crystal structure has been discovered for the three-amide oligomer of nylon 6. This material crystallizes from chloroform/dodecane solutions into an unfolded crystal form that has progressively sheared hydrogen bonding in two directions between polar (unidirectional) chains. This structure is quite different from the usual room temperature α-phase structure of chain-folded nylon 6 crystals, in which alternatingly sheared hydrogen bonding occurs between chains of opposite polarity in only one direction. The occurrence of this new structure illustrates the extent to which progressively sheared hydrogen bonding is preferred over alternatingly sheared hydrogen bonding. Indeed, the progressive hydrogen bonding scheme occurs in the three-amide nylon 6 material even though it requires a disruption to the lowest potential energy all-trans conformation of the chain backbone, and requires all the chains in each hydrogen-bonded layer to be aligned in the same direction. We believe the presence of chain folding, which necessarily incorporates adjacent chains of opposite polarity into the crystal structure, prevents the formation of this new crystal structure in the nylon 6 polymer. In contrast, the three-amide nylon 6 6 crystal structure is analogous to the polymeric nylon 6 6 α-phase structure, found in both fibers and chain-folded crystals, and consists of progressive hydrogen-bonded sheets which stack with a progressive shear. In both structures, the molecules (≈ 3 nm in length) form smectic C-like layers with well-orchestrated stacking of 2.2 nm to form a three-dimensional crystal. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2849–2863, 1998