Low Draw Ratios Research Articles

Influence of Low-Molecular-Weight Esters on Melt Spinning and Structure of Poly(lactic acid) Fibers.

Poly(lactic acid) has great potential in sectors where degradability is an important advantage due to its polymer nature. The medical, pharmaceutical, and packaging industries have shown interest in using PLA. To overcome the limitations of stiffness and brittleness in the polymer, researchers have conducted numerous modifications to develop fibers with improved properties. One such modification involves using plasticizing modifiers that can provide additional and desired properties. The scientific reports indicate that low-molecular-weight esters (LME) (triethyl citrate and bis (2-ethylhexyl) adipate) affect the plasticization of PLA. However, the research is limited to flat structures, such as films, casts, and extruded shapes. A study was conducted to investigate the impact of esters on the process of forming, the properties, and the morphology of fibers formed through the melt-spinning method. It was found that the modified PLA required different spinning and drawing conditions compared to the unmodified polymer. DSC, FTIR, WAXD, and GPC/SEC analyses were performed for the modified fibers. Mechanical tests and morphology evaluations using SEM microscopy were also conducted. The applied plasticizers lowered the temperature of the spinning process by 40 °C, and allowed us to obtain a higher degree of crystallinity and a better tenacity at a lower draw ratio. GPC/SEC analysis confirmed that the polymer-plasticizer interaction is physical because the booth plasticizer peaks were separated in the chromatographic columns. The use of LME in fibers significantly reduces the temperature of the spinning process, which reduces production costs. Additives significantly change the production process and the structure of the fiber depending on their rate, which may affect the properties, e.g., the rate of degradation. We can master the degree of crystallinity through the variable amount of LME. The degree of crystallization of the polymers has a significant influence on polymer application.

Formation of highly oriented crystalline structure for polyoxymethylene fiber enables superior intrinsic thermal conductivity

Polyoxymethylene (POM) as an engineering plastic with high performances, has been widely applied, while endowed with superior intrinsic thermal conductivity for POM fibers was conducive to broaden their application in thermal management field, which were usually prepared through melt spinning-hot drawing/heat setting process. In this work, the tension annealing process was further incorporated into fiber production to develop highly orientated crystalline structure of fibers. All fibers have a uniform diameter with axial textures on surface, and the tensile strength/modulus increased remarkably by drawing and annealing process. In melting spinning process, the POM chains pre-oriented under stress, resulting in a slight increase of thermal conductivity. In hot drawing/heat setting process, through stress-induced crystallization of POM fiber, the shish-kebab crystalline structure formed at a low draw ratio (DR), while at high DR, a homogeneous and compact oriented microfibrillar crystalline structure formed. As a result, a significant improvement of crystallinity (Xc: 84.42 %) and orientation degree (fc, fa: 0.94) in crystalline/amorphous region of POM chains contributed to the superior axial thermal conductivity, which increased from 0.286 Wm−1K−1 of bulk POM to 2.131 Wm−1K−1. In annealing process, although Xc increased further to 93.37 % under high temperature, forming a nearly perfect crystal, while the f of annealed fiber remained under tension force but without further increase, the axial thermal conductivity showed no obvious improvement, revealing that orientation of POM chains had a more significant promotion on improving thermal conductivity of POM fibers than crystallization.

Blown Composite Films of Low-Density/Linear-Low-Density Polyethylene and Silica Aerogel for Transparent Heat Retention Films and Influence of Silica Aerogel on Biaxial Properties.

Blown films based on low-density polyethylene (LDPE)/linear low-density polyethylene (LLDPE) and silica aerogel (SA; 0, 0.5, 1, and 1.5 wt.%) were obtained at the pilot scale. Good particle dispersion and distribution were achieved without thermo oxidative degradation. The effects of different SA contents (0.5–1.5 wt.%) were studied to prepare transparent-heat-retention LDPE/LLDPE films with improved material properties, while maintaining the optical performance. The optical characteristics of the composite films were analyzed using methods such as ultraviolet–visible spectroscopy and electron microscopy. Their mechanical characteristics were examined along the machine and transverse directions (MD and TD, respectively). The MD film performance was better, and the 0.5% composition exhibited the highest stress at break. The crystallization kinetics of the LDPE/LLDPE blends and their composites containing different SA loadings were investigated using differential scanning calorimetry, which revealed that the crystallinity of LDPE/LLDPE was increased by 0.5 wt.% of well-dispersed SA acting as a nucleating agent and decreased by agglomerated SA (1–1.5 wt.%). The LDPE/LLDPE/SA (0.5–1.5 wt.%) films exhibited improved infrared retention without compromising the visible light transmission, proving the potential of this method for producing next-generation heat retention films. Moreover, these films were biaxially drawn at 13.72 MPa, and the introduction of SA resulted in lower draw ratios in both the MD and TD. Most of the results were explained in terms of changes in the biaxial crystallization caused by the process or the influence of particles on the process after a systematic experimental investigation. The issues were strongly related to the development of blown nanocomposites films as materials for the packaging industry.

The influence of the multi-level structure under high drawing on the preparation of high strength Lyocell fiber

In order to research the multi-level structure of Lyocell fiber at different draw ratios and to reveal the limiting factors for preparing the high strength Lyocell fiber, the paper reports on the effect of draw ratio including low drawing (1–5), high drawing (6–11) and excessive drawing (12–20) on the multi-level structure and the mechanical properties of Lyocell fiber. The structure was determined by wide-angle X-ray diffraction, small-angle X-ray scattering and fibrillation test, and the result showed that, at low draw ratio stage, the breaking strength, yield strength and modulus of the fiber increased with the draw ratio owing to crystallinity as well as orientation increased while the micropore decreased, and there were almost no microfibrils on the fiber surface. At high draw ratio stage, the orientation of amorphous region increasing was the principal reason for the increase of fiber mechanical properties, and the micropores continued to decrease and a few short and thick microfibril was formed. At excessive draw ratio stage, the breaking strength remained constant mainly due to the basically unchanged crystallinity and orientation of the fibers, the yield strength and modulus decreased due to the slip of the highly crystallized and oriented elementary fibril. Meanwhile, the micropores still decreased and became more slender, the number of microfibrils increased and the microfibrils showed tenuous structure. It can be summarized that Lyocell fiber has the characteristics of multi-level structure, and the fundamental reason limiting the improvement of mechanical properties with draw ratio increasing is the slip of elementary fibril.

Constructing highly aligned crystalline structure to enhance sliding wear performance of bulk polyamide 6

High performance wear-resistant polyamide 6 (PA6) not only needs low friction and wear, but also requires high thermal conductivity and outstanding mechanical strength. Introducing lubricants into polymers has proved to be a feasible method, however, the contribution of intrinsic structure of PA6 to tribological properties is often ignored. Herein, a novel approach to fabricate high performance wear-resistant bulk PA6 via constructing a highly aligned crystalline structure was proposed. Firstly, an intense flow field applied by force assembly elements (FAEs) was introduced into extrusion processing to prepare PA6 precursor sheets with initially aligned molecular chains. Next, the precursor sheets were uniaxially drawn at 120 °C to achieve highly aligned crystalline structure at low draw ratio. The X-ray diffraction results showed the crystal orientation of the precursor sheets is 42% higher than that of samples prepared without FAEs. Furthermore, tensile stress induced γ-α phase transition and improved the crystallinity. As a result, the friction coefficient and wear rate of the bulk PA6 decreased 50.8% and 64.9%, respectively. Note that the maximum contact surface temperature reduced from 109.8 to 64.8 °C during high-speed friction because of the increasement of the thermal conductivity, effectively avoiding the aggravation of wear caused by surface softening. Moreover, the bulk PA6 also has excellent mechanical properties with a yield strength of 242 MPa and an elongation at break of 90%. Such remarkable enhancements in both mechanical and tribological properties are primarily attributed to the highly aligned crystalline structure with high α crystal form content. This work provides a way to design mechanically robust, wear-resistant polyamide applied in industry, transportation, etc.

Thermal stability enhancement of oriented polyethylene by formation of epitaxial shish-kebab crystalline structure

Oriented polyethylene (PE) products usually exhibit thermodynamic instability in oriented crystalline structure under high temperature during long term service. In this work, self-assembly rare-earth nucleating agent (RENA) was innovatively introduced to fabricate oriented PE and induced PE molecules to form stable epitaxial oriented crystalline structure. Compared with oriented neat PE, oriented PE-RENA showed much higher dimensional/strength retention during aging, which reached as high as 90.13%/86.46% after aging at 100 °C for 1000 h, exhibiting high dimensional/mechanical stability. At low draw ratio, PE-RENA exhibited hybrid oriented shish-kebab structure, for which oriented RENA assembly acted as shish structure and PE lamellae acted as kebab structure, while, after aging, new stable anisotropic shish-kebab crystalline structure formed and the crystallinity (Xc) changed slightly. At high draw ratio, highly oriented shish-kebab structure formed, most of which was retained after aging and the changes of Xc/orientation degree was insignificant, while the molecular mobility in crystalline phase/intermediate phase was weakened, exhibiting enhanced thermal stability in oriented crystalline structure.

Nanosilicates in Compatibilized Mixed Recycled Polyolefins: Rheological Behavior and Film Production in a Circular Approach.

Currently, plastic packaging represents a global challenge and has become a key point of attention for governments, media and consumers due to the visibility of the waste it generates. Despite their high resource efficiency, the perceived non-recyclability of polymeric films risks precluding them from being a relevant packaging solution in a circular economy approach. In this regard, the aim of this study was to implement a strategy to try closing the loop, via the mechanical recycling of post-consumer flexible packaging of small size (denoted as Fil-s) to obtain new films. In particular, two lots of Fil-s were used, which are PE/PP blends differing for the PP content and the presence of polar contaminants. The suitability for film blowing extrusion of these recycled materials, as such and after the addition of a compatibilizer and/or a lamellar nanosilicate, was evaluated. It was first evidenced that the difficulty of producing blown films with the pristine recycled materials, due to the frequent bubble breakages, occurring even at low draw ratios. Moreover, the shear and extensional rheological behavior of all Fil-s based systems was usefully correlated with their processability features, evidencing the key roles of the nanofiller to stabilize the bubble and of the compatibilizer to ensure a uniform film deformation, avoiding its premature breakage. Even if the adopted upgrading strategies allowed the production of blown films with both types of Fil-s, the different components of the recycled matrices were proven to significantly affect their processability and final film performances.

Photo-Actuation Stress of Ultra-Drawn, Chain-Extended Polyethylene

The influence of solid-state drawing on the photo-actuation stress of oriented semi-crystalline polymers is explored experimentally and theoretically. Photo-responsive and ultra-drawn linear polyethylenes (PEs) containing a commercial benzotriazole dye (BZT) are studied as a model material. In accordance with previous studies, it is found that the actuation stress as a function of draw ratio exhibits an optimum and that the stress first increases and then decreases again slightly. A semi-empirical model is presented that describes the photo-actuation stress as a function of the draw ratio, Young's modulus, thermal expansion coefficient, and temperature difference between the illuminated and non-illuminated state. The Young's modulus and thermal expansion coefficient versus draw ratio curves are described using models based on the pseudo-affine deformation scheme in combination with a two-phase structural model in which highly oriented elements are connected in series with a disordered phase. Rather surprisingly, it is found that the model predicts that the photo-actuation stress is virtually independent of the draw ratio and the degree of chain orientation and extension. An excellent agreement is observed between model predictions and experimental results if the actuation stress is corrected for temperature effects related to the conversion of light into heat. The structure-property relationships derived here provide guidelines for the design of light-driven artificial muscle systems based on ultra-drawn polymers. For instance, it is predicted that ultra-drawn polyamides and polyesters are potentially excellent candidates for producing photo-actuators provided that suitable additives are used and this despite their low maximum attainable draw ratio, Young's modulus and strength.

Tuning wettability and mechanical property of polylactide composite films with in-situ nanofibrils of poly(butylene adipate-co-terephthalate)

The poor hydrophobicity and toughness of polylactide (PLA) films make it difficult to meet the requirements of barrier, printing and moisture-proof material. How to improve the two drawbacks of PLA simultaneously is a daunting challenge at present. In this work, we introduce poly (butylene adipate-co-terephthalate) (PBAT) as a second flexible phase into PLA matrix and concomitantly impose intensive elongational flow by two-stage drawing. Under the elongational flow field, the PLA/PBAT film can form the oriented structure combining in-situ nanofibrils with oriented lamellae, exhibiting excellent stiffness-toughness balance. Then surface treatment via moderate alkaline etching makes the micro/nano structure expose on the surface, which can effectively improve the hydrophobicity of the surface. We further investigates the influence of blend composition, drawing ratio and etching time on the oriented structure, mechanical and wetting properties. It is found that the PLA/PBAT film with the weight ratio of 70/30, low draw ratio of 2.7 and etching time of 24 h has the optimal properties, which possesses Young's modulus of 2.53 GPa, yield strength of 51.9 MPa, elongation of break of 131.8% and water contact angle of 131.8°. It is noteworthy that the film can mainly keep original mechanical after surface treatment. This work could offer a facile and effective method to fabricate the green PLA based film with good mechanical properties and hydrophobicity in a large scale.

Switchable Isotropic/Anisotropic Wettability and Programmable Droplet Transportation on a Shape-Memory Honeycomb.

Programmable droplet transportation is required urgently but is still challenging. In this work, breath figure was employed to fabricate shape-memory poly(lactic acid) (PLLA) honeycombs in which tiny crystals and an amorphous network act as the shape-fixed phase and recovery phase, respectively. Upon uniaxial tension, circle pores from the breath figure were deformed to elliptical pores, producing contact angle differences and anisotropic wetting behaviors in two directions. Both pore geometry and anisotropic wettability can be tailored via the draw ratio. On the PLLA honeycomb surface with a lower draw ratio, the contact angle difference is too small to induce droplet transportation along the desired direction. In the case of a higher draw ratio, however, the movement of water droplets has been controlled absolutely along the tension direction. The transition between them can be achieved reversibly during uniaxial tension and recovery processes based on the shape-memory effect. The enhanced flow control, which can be attributed to the synergism between optimal hydrophobicity and enlarged anisotropic wetting behaviors, endows water droplets with the ability to turn a corner spontaneously on a V-shaped surface including two regions exhibiting different oriented directions.

Self‐assembled fibrillar polyethylene crystals with tunable properties

AbstractDue to its simple linear chain structure, crystal morphology of linear low‐density polyethylene (LLDPE) fibers can be controlled to fulfill the needs of diverse advanced applications. This study presents a simple two‐step method to produce LLDPE fibers with self‐assembled fibrillar crystals and highly oriented amorphous phase. Rather than conventional melt spinning, fibers were treated in a two‐step eco‐friendly bath without drawing after extruded fibers emerge from the spinneret. Treated fibers through the baths demonstrated lower crystallinity, but significantly higher degree of crystal orientation when compared to control fibers of traditional melt spinning. Morphological analysis revealed that a unique microstructure was formed after spinning through a two‐step eco‐friendly bath. As‐spun fibers demonstrated spherulitic morphology which can be transformed into a fibrillar structure followed by post‐drawing process. Cross sectional images of the treated LLDPE fibers produced at 400 m/min showed fibrillar PE crystals which can be more dominant upon post‐drawing. After two‐step bath treatment, produced fibers need low draw ratios to exhibit high performance. Our novel modification followed by hot drawing process can manipulate internal structure with performance of PE fibers to an outstanding level of 0.35 GPa strength and 3 GPa modulus at a production speed of 400 m/min.

Force field in coagulation bath at low temperature induced microfibril evolution within PAN nascent fiber and precursor fiber

AbstractThe coagulation process was vital for the microfibril evolution and mechanical properties of polyacrylonitrile (PAN) fibers. The PAN nascent fibers and precursor fibers were prepared by controlling drawing ratio of coagulation bath at low temperature during the dry‐jet wet spinning process. The microfibril morphological changes induced by force fields in coagulation bath were investigated by scanning electron microscopy and high‐resolution transmission electron microscopy. During the coagulation process, the spinning solution evolved into an interconnected network composed of random microfibrils and tie joints as a building block of the network. At low drawing ratio, the random interconnected network existed in nascent fibers. Increasing drawing ratio, the fiber filaments underwent shrinkage and the network was transformed into the transverse lamellae. Furthermore, the lamellar thickness also decreased. After the treatment of post‐spinning, similar transverse lamellae were formed in all of precursor fibers, and random microfibrillar network was stretched and oriented to develop into the aligned microfibrils in precursor fibers. The transverse fold‐chain crystal layers were densely stacked in the microfibrils. Increasing drawing ratio, the lamellae and microfibrils in precursor fibers were packed more densely and orderly. Consequently, the order and homogeneity of microfibrils in nascent fibers and precursor fibers were improved by increasing drawing ratio, which were benefit to increase fiber tensile strength and modulus.

Fast, Light-Responsive, Metal-Like Polymer Actuators Generating High Stresses at Low Strain

Summary Producing lightweight polymeric actuators able to generate high stresses typical of hard metals and/or ceramics remains challenging. The photo-mechanical responses of ultra-drawn ultra-high molecular weight polyethylene (UHMWPE) actuators containing azobenzene photo-switches with symmetrically attached polyethylene (PE) side chains are reported. Long PE side chains promote dispersion within the apolar UHMWPE matrix, and the ultra-drawn films are highly aligned. The ultra-drawn azobenzene-doped UHMWPE films have high Young's moduli (∼100 GPa) and are viscoelastic at room temperature at strains below 1%. The photo-mechanical response of the films is fast ( 6 × 104 Pa (kg m−3)−1) to UV or visible light at a low strain (∼0.06%). The actuator responds to rotating linearly polarized light, causing a photo-induced stress wave response. Such rapid, high-stress, low-strain, photo-mechanical responses are unique in soft polymer systems with physical values approaching hard metals/ceramics.

Вплив орієнтаційного витягування на властивості сумішей поліефірів ПЕТg/ПЕТ

Rational conditions for orientational drawing of PETg/PET polyester blend during processing into tape products are established. Tensile strength and elongation at break for all test specimens were determined according to ISO 527-2: 2012. Tensile modulus - ISO 527-1: 2019, specimen density - PN-EN ISO 1183-1, change of linear dimensions of specimens - ISO 16012: 2015. It was found that the introduction of PET into PETg leads to an increase in tensile strength of the blend, which is probably due to the process of orientational crystallization of PET. When the orientations are higher than 5.5 times in the PETg /PET mixtures, cavitation foaming occurs, which leads to a decrease in the blend density, its turbidity and a decrease in tensile strength. Blends with a high content of PETg have a significant thermal shrinkage, which should be taken into account when processing and applying these polymers. Physical and mechanical properties of PETg/PET polyesters blend with different ratio in the process of orientational stretching are established. The tensile strength increases with the increase drawing ratio for blends with high PET content due to its orientation crystallization. The phenomenon of cavitation foaming in polyester blend with draw ratio more than 5.5 times is found, which is accompanied by a decrease in the density of the samples and their tensile strength. Presence of PETg in PET matrix decrease tensile strength and elongation and significantly increase thermal shrinkage. Low elongation at break found for PETg/PET blend specimen without orientation which increases dramatically with low draw ratios and then decrease with higher drawing ratio. The technological modes of realization of cavitation foaming for the PETg/PET mixture at orientation drawing ratio above 5.5 due to the different phase structure of the components of the mixtures have been established. The introduction of PETg into PET leads to a significant increase in thermal shrinkage and reduces the physical and mechanical performance of oriented products.

Local process-dependent structural and mechanical properties of extrusion blow molded high-density polyethylene hollow parts

Although applied for several decades, production of hollow plastic parts by extrusion blow molding (EBM) is still over-dimensioned. To overcome this issue, a thorough investigation of the process-structure-property relationship is required. In this study, the local process-structure-property relationship for high-density polyethylene EBM containers is analyzed with differential scanning calorimetry and dynamic mechanic analysis microindentation. Local process-dependent crystallinity and complex modulus data at various processing conditions are supplemented with wide-angle X-ray diffraction and transmission electron microscopy (TEM). The crystallinities and the complex moduli clearly show lower values close to the mold side than at the inner side and the middle of the cross-section, which reflects the temperature gradient during processing. Additionally, the orientation of the polymer chain (c-axis) reveals a low level of biaxiality with a slight tendency towards transverse direction. The biaxiality increases for low mold temperature and high draw ratio. Finally, biaxiality is confirmed with TEM, which reveals no preferred lamellar orientation.

Mechanical properties of PTT fibers by sustainable horizontal isothermal bath process

The melt spinning process of poly (trimethylene terephthalate) was modified by the inclusion of horizontal isothermal bath (HIB) in the extrusion line. Several parameters including liquid depth, liquid temperature, and take-up speed were utilized in this study. The structural and mechanical properties of the fibers were characterized by DSC, X-ray, SEM, optical birefringence, and tensile testing. The results showed that the bath treatment increased considerably the fiber birefringence up to 0.066 by 66% and increased the crystallinity up to 47.94% by 100% compared to the control (no HIB) fibers. Drawing the HIB fibers with a low draw ratio (DR) of 1.11 increased the tensile strength up to 4.76 g/d and reduced the tensile strain down to 51.76%. Such strength is greater than the maximum value of 3.3 g/d reported before. The obtained results can widen application of PTT fibers in technical woven/nonwoven fiber products.

Influence of an Increased Fiber Filler Content on the Elongation Behavior of Filled Films in the Thermoforming Process

Thermoforming belongs to one of the most important processes in polymer processing, especially in the packaging industry. It enables the forming of thermoplastic components into shaped parts at high temperatures. Since the thermoforming of films takes place in the rubbery state, amorphous thermoplastics are mainly processed, which have a wide rubbery state. Radiation crosslinking can be used to widen the thermoforming window of semi-crystalline thermoplastics. A benefit of the crosslinking is the increased short-term temperature resistance. In general, there are only a few investigations concerning the thermoforming of filled thin films. Within this investigation, the influence of an increasing glass fiber content up to 15 vol.-% as well as the effect of radiation crosslinking on the elongation behavior and the wall thickness distribution was examined. It can be summarized that especially thermoforming with an increased filler content at high areal draw ratios represents a challenge. Whereas non-crosslinked glass fiber filled films are thermoformable only at low areal draw ratios, radiation crosslinked films can be also formed at higher areal draw ratios without difficulties. For high filler contents and high areal draw ratios, no forming is possible at high areal draw ratios, although the films have been crosslinked. The use of radiation crosslinking enables the process limit in thermoforming of thin filled films to be increased and thus the range of applications to be extended greatly.

Influence of melt spinning parameters on electrical conductivity of carbon fillers filled polyamide 12 composites

Nowadays, the market is flooded by smart textiles due to the major advances made in the development of flexible materials. Electrical conductivity is one of the most important properties, which can be assured by the introduction of metallic filaments in the yarns. However, such introductions of metallic filaments can cause a decrease of the characteristics of textile (flexibility, durability). Thus, a lot of studies focus on the development of electrical conductivity that does not decrease the characteristics of textile. An alternative to develop conductive polymer composites (CPC), is conductive fillers which can be integrated into the polymer. The current technic a bulk functionalization of the polymer during melt spinning allowing a thermoplastic yarn to become electrically conductive. In this study, the electrical conductivity of carbon black (CB) fillers filled multifilaments based on polyamide 12 (PA12) was optimized. The CPC were prepared in two steps: the first step was the incorporation of the fillers in PA12 by twin screw extrusion, and the second step was the melt spinning to obtain multifilament yarns. After extrusion, the blend has an electrical conductivity of 101 S/m. However, after the melt spinning of this blend, the electrical conductivity dropped sharply to 10-6 S/m. Several experimental parameters during the melt spinning can modify the structure filaments and impact on their electrical conductivity, which could explain this loss of electrical conductivity. Drawing out of rod during extrusion, as well as different melt spinning parameters, such as volumetric pump output and speed variation of the drawing rolls, were identified as key factors that not only influence the electrical conductivity but also the mechanical properties of the yarn. A low drawing ratio combined with a low pump output can increase the electrical conductivity. With the same value of volumetric pump output and the same supply roll speed, if the drawing roll speed increases from 225 rpm (rpm) to 450 rpm, the electrical conductivity decreases by a factor of 10, respectively 7.10-5 S/m and 7.10-6 S/m. Observations by transmission electron microscopy (TEM) of the two CBs inside PA12 give insight the dispersion of nanofillers, which effects on the CB physical network and its subsequent electrical conductivity. At the scale of the yarn, twisting can increase the contact path between the monofilaments and increase (from 10-6 S/m to 10-5 S/m) the global conductivity until a plateau for the multifilament. Thus, the key factors explaining this loss of electrical conductivity between the rod and the multifilament were identified. However, these parameters also effect the mechanical characteristic of the materials too.

Cavitation in Irradiated and Oriented High-Density Polyethylene Cast Films with Stacked Lamellar Morphology

ABSTRACTCavitation (void formation) in high-density polyethylene cast films, having a stacked lamellar morphology, subsequent to tensile stretching of both nonirradiated and irradiated films was investigated. With nonirradiated films, the cavitation was not observed in the nonannealed films with lower draw ratios, while a limited number of voids was detected in the films of higher draw ratios. Annealing caused an intensive cavitation, and uniform distribution of cavities was noticed in highly oriented films. It was shown that cross-linking exerted a suppressive influence on cavitation in the annealed and oriented films; and with increasing the irradiation dose, the cavitation was completely eliminated.

The evolution of crystalline structures during gel spinning of ultra-high molecular weight polyethylene fibers.

Most studies are focused on the final mechanical properties of the fiber and the processing window required to achieve high moduli and tensile strength. Several studies have alluded to the fact that the crystalline morphologies developed during gel spinning and post-drawing are very important in determining the final mechanical properties. However, it is surprising to know that no clear correlation exists between the crystalline structure and initial, evolving, and final mechanical properties. In an attempt to define structure-property relationships, we have developed novel tools to quantify the effect of processing on crystalline structure evolution. We examine through controlled gel-spinning and SAXS analysis the effect of flow kinematics on the development of crystalline structures. Direct correlations are made between polymer solution relaxation time, extension rates, crystallization time and gel-spun crystalline morphologies. We report direct evidence of flow induced crystallization, which approaches an asymptotic crystallization rate at high Weissenberg numbers. For Wi < 1, the crystalline structure is only slightly affected by equilibrium. For Wi > 1, the crystalline structure is highly anisotropic due to chain orientation/stretch during spinning. Fibers spun at different Weissenberg numbers are drawn to low draw ratios at constant temperature to measure the initial structure evolution. A qualitative SAXS analysis clearly shows similar evolution of different starting structures with the formation of more straight chain crystals upon drawing. However, there remain quantitative differences between the length of straight chain crystals and the size and distribution of lamellar domains depending on the starting structure.