Increase Of Long Spacing Research Articles

Direct investigations of deformation and yield induced structure transitions in polyamide 6 below glass transition temperature with WAXS and SAXS

Deformation and yield induced structure transitions of polyamide 6 (PA6) were detected with the combination of the wide- and small-angle X-ray scattering (WAXS and SAXS) at 30 °C below glass transition temperature (Tg) of PA6. During deformation, γ-α phase transition was found at elastic stage. The concentrated stress in crystals at elastic stage provided adequate energy for the direct γ-α phase transition under Tg. The force to promote the γ-phase into α phase directly is insufficient at the yield stage and a transient phase as a compromise was formed. The transient phase was confirmed by DSC measurements and assisted the γ-α phase transition indirectly. The γ-phase slips into incomplete fragments at yield point, and the parts along tensile direction are responsible for the formation of transient phase. The γ-fragments after yield is oriented by shear force following α-phase. The yield of PA6 was intimately related with the γ-transient phase transition and the slippage of γ-fragments. In addition, the increase of long spacing at elastic deformation along tensile direction was mainly contributed by the amorphous region. After yield, the phase transition and crystalline slippage affected the lamellar evolution. A simplified schematic diagram was provided to illustrate the structure evolution of PA6.

Decorated MCM-41/polyethylene hybrids: Crystalline details and viscoelastic behavior

Polyethylene-based nanocomposites have been prepared by in-situ polymerization of ethylene with mesostructured MCM-41. Different approaches are used to improve interfacial adhesion between components, high density polyethylene (HDPE) matrix and MCM-41. These consist either of the incorporation of UA in the polymer chains by copolymerization in a very small content or of the modification of MCM-41 surface with the system based on undecenoic acid, UA/triisobutylaluminum, TIBA, before polymerization. The materials attained exhibit minimal changes in the crystallinity resulting from first melting process although a slight increase of long spacing at room temperature is seen as MCM-41 is incorporated in the hybrids. Moreover, a confinement effect is noticed and, thus, a delay in the crystallization process of the macrochains within pores and channels is detected. These novel self-reinforced nanocomposites show higher stiffness than other similar hybrids with poorer interfacial adhesion. Accordingly, they exhibit an enhancement in their mechanical performance without changing the final processing temperature since Tm is the same for all of the specimens.

Deformation behaviour of poly(ether ester) thermoplastic elastomers as revealed by small-angle X-ray scattering

The scattering behaviour of thermoplastic elastomers based on poly(ether ester) (PEE) under stress is studied. Bristles of PEE consisting of poly(butylene terephthalate) as hard segments and polyethylene glycols ( M w = 1000) as soft segments in a ratio of 50 50 wt% are drawn five times and then annealed with fixed ends. Small angle X-ray scattering (SAXS) measurements are carried out with single bristles subject to stress and with deformations up to 200%. The increase of long spacing L is directly proportional to strain ϵ (up to ϵ = 100%) supporting an affine deformation mechanism. At ϵ = 75 and 100% two long spacings L 1 and L 2 are observed, L 2 being close to the initial L 0 1 and remaining constant with further increase in ϵ up to ϵ = 200%. The L values measured after removal of the stress (up to ϵ = 30%) revert almost to those obtained without stress; further increase in the deformation of the samples leads to a decrease in L values. The effect of stress on the scattering intensity is similar. A model is proposed to explain the scattering behaviour under stress. The affine increase of L with ϵ is related to the reversible deformation of the amorphous regions. Further increase in ϵ leads to the elimination of the interfibrillar contacts since tie molecules are pulled out. As a result relaxation of some microfibrils occurs and a second long spacing with a value close to that of the initial one is observed. A microscopic picture of the deformation mechanism and the influence of tie molecules on L are discussed.

Real time small‐angle x‐ray scattering during annealing of polymer single crystals

AbstractExperiments at the Cornell high energy synchrotron source (CHESS) have shown that it is possible to obtain 2‐D small‐angle x‐ray scattering patterns from single crystal mats of high density polyethylene with a time resolution of 0.3 s. However, it took up to 5 s to heat the 0.1‐mm‐thick specimen to the annealing temperature. A logarithmic increase of long spacing was quickly established, after an induction time of <2 s at the higher annealing temperatures. At lower temperatures the original reflection remains, weakening, while a continuous scatter to smaller angles builds up. At 10–20 s annealing time a new maximum becomes clear, and then the logarithmic increase of long spacing begins. The intensity of this reflection is intially low and increases with annealing time. On cooling it decreases again. It seems that here we can directly observe two mechanisms of lamellar thickening, melting then recrystallization at short times and diffusional thickening at long times.

Change in fine structure of nylon 6 gut yarn in twisting, annealing, and untwisting processes. I. Observation by WAXD, SAXD, and EM

To obtain information on the change in fine structure of nylon 6 taking place during practical false-twisting processes, the manner of change in the three elemental processes, i.e., twisting, annealing, and untwisting, was studied. For simplicity, nylon 6 gut yarn was used instead of multifilament yarn. Wide- and small-angle x-ray diffraction (WAXD and SAXD) together with electron microscopy (EM) were used here. The degree of molecular orientation in the crystalline region of the twisted yarn gradually decreases with increase of the twist number (TN) in the region of TN ≫ 100. The long spacing, determined by SAXD, of the twisted yarn increases with increases in TN. The increase in long spacing cannot be interpreted only by macroscopic strain or elongation of the yarn in the twisting process. This difference seems to arise from the contribution of the decrease of lateral size of lamellae to the average long spacing; therefore the increase in long spacing should be attributed to the elongation of the amorphous region, deduced from the crystallinity measured by WAXD and the long spacing diffraction. The angle between the streak line on the surface of twisted yarn observed by EM and the fiber axis agrees well with the twist angle of the yarn. The crystal lamellae are stacked normal to the streak line at the initial stage, i.e., at a low value of TN, but they begin to deviate from the normal direction with increase in TN, accompanied by their partial destruction. Based on SAXD and density measurements, the internal strain of yarn annealed after twisting is fully relaxed. When the yarn is untwisted after twisting and annealing, the crystal orientation recovers gradually to that of the untreated yarn. The chain axis within the lamellae in the center region of the yarn becomes nearly parallel to the fiber axis, but the chain axis in the outer region does not.

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

AbstractThe 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.

Redrawing of oriented polyethylene film

AbstractDrawn and subsequently annealed polyethylene film was restretched along the original draw axis at various temperatures. The internal deformation was analyzed in terms of the structural parameters of a simplified model. The elementary deformations are the rotation of crystals around the b axis and shear at the crystal interface. The rigidity of the crystal plays an important role during extension; and as a result, disorientation of chains in the crystal occurs at high strain. At the same time, crystals deform in such a way that the crystalline chains tilt about the b axis along the (h00) plane. This deformation of the crystal is affected by temperature. The increase in long spacing with extension can be interpreted roughly by the changes in structural parameters. The strain in amorphous region in also discussed in relation to these parameters.

Liquid‐induced reversible long‐spacing changes in polyethylene single crystals and their implications for the fold‐surface problem

AbstractThe long spacing of single‐crystal mats of polyethylene was found to increase reversibly on addition of liquids which normally act as swelling agents. The magnitude of the increase follows the sequence of the solvent power which these liquid exhibit at higher temperatures. The increase in long spacing depends to a minor extent on crystallization conditions, such as concentration and crystallization temperature, but increases very markedly with the molecular weight of the polymer. Heat treatment reduces the amount of swelling, even in the range of low treatment temperatures which do not produce a foldlength increase, implying a reorganization of the fold surface. It is inferred that there is a layer along the fold surface which expands by swelling which in turn implies a disordered amorphous component. Nevertheless, such disordered material need not be an intrinsic consequence of chain folding, as its amount can be reduced to insignificant proportions, e.g., by taking molecules which are short but still fold. Thus both, the presence of amorphous disorder and the nonunique nature of such a disorder along the surface is demonstrated. Further implications for the fold‐surface problem are discussed.

Properties and structures of films drawn from polyethylene single‐crystal mats

AbstractThe properties and structures of polyethylene films drawn from mats of single crystals were investigated in comparison with films drawn from bulk polymer. The most important result obtained is that the structural reorganization stimulated by mechanical stress and annealing occurs with much greater difficulty in the mat‐drawn film. The chief mechanical characteristics of the film are a very low extensibility and a very high modulus. Structural characteristics, such as the double‐orientation of the crystalline region and the morphology of the crystals, do suffer no substantial changes during annealing at high temperatures. This stability of the structure can be related to the characteristic fine structure of the mat‐drawn film in which there are a much larger number of tie chains, connecting neighboring crystals, than in the bulk‐drawn specimen. Relaxation of the amorphous region and a notable increase in long spacing take place in the mat‐drawn sample as in ordinary bulk‐drawn samples. However, the morphological changes of the crystals accompanied by the folding back of chains seem to take place with great difficulty during annealing even in the vicinity of the melting point.