Biaxial stretching and structure of various LLDPE resins

The purpose of this study is to investigate the biaxial stretchability, the structure developed, molecular orientation and shrinkage of linear low‐density octene copolymers (LLDPEs) biaxially stretched using a laboratory biaxial stretcher. Seven resins having different molecular characteristics were used in this study. The effect of stretching temperature and rate on stretchability is assessed. Biaxial orientation factors for the crystalline axes as well as that of the amorphous phase were determined using FTIR spectroscopy and shrinkage of oriented films was measured. The results indicate that the high molecular weight tail (Mz) and comonomer content play important roles in orientation of the resins. Higher Mz, MWD and resin content eluting above 90°C (by TREF technique) tends to increase orientation, and finally, some correlation between orientation and Mz, MWD and resin content eluting above 90°C and between shrinkage and amorphous orientation were observed.     

Study on phase separation of PET/PEN blends by dynamic rheology

Blends of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) were processed into biaxially drawn films, and samples taken from the bi‐oriented films were then investigated by dynamic rheology experiments in the melt state. Storage modulus G′ and loss modulus G″ were determined in the frequency range of 10^?2–10² rad/s at temperatures between 260 and 300°C. Although the time–temperature superposition (TTS) principle was found to hold in the high frequency regime, a breakdown of TTS was observed at low frequencies, and the terminal behavior of the storage modulus G′ of the blends departs drastically from the terminal behavior observed for the blend components. This is caused by interfacial surface tension effects. The results indicate that despite the effect of transesterification reactions, the PET/PEN blend systems investigated consist of a microseparate phase of PEN platelets in a matrix of PET. This morphology is produced when the blends are processed into biaxially oriented PET/PEN films, and droplets of PEN are deformed into a lamellar structure consisting of parallel and extended, separate layers. The large interfacial surface area of the bi‐oriented PET/PEN blends leads to remarkably strong interfacial tension effects in dynamic rheology measurements. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Oxygen permeability of biaxially oriented polypropylene films

The effect of thermal history on the oxygen permeability of biaxially oriented polypropylene (BOPP) films was investigated. Compression‐molded sheets prepared with different cooling rates were biaxially oriented at several temperatures in the range between the onset of melting and the peak melting temperature and at a strain rate similar to that encountered in a commercial film process. The stress response during stretching was found to depend on the residual crystallinity in the same way regardless of the thermal history of the compression‐molded sheet. Biaxial orientation reduced the oxygen permeability measured at 23°C; however, the reduction did not correlate with the amount of orientation as measured by birefringence or with the fraction of amorphous phase as determined by density. Rather, the decrease in permeability was attributed to reduced mobility of amorphous tie molecules. A single one‐to‐one correlation between the oxygen permeability and the intensity of the dynamic mechanical β‐relaxation was demonstrated for all the films used in the study. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Primary structure and physical properties of poly(ethylene terephthalate)/poly(ethylene naphthalate) resin blends

The morphology and properties of blends of poly(ethylene naphthalate) (PEN) and poly(ethylene terephthalate) (PET) that were injection molded under various conditions were studied. Under injection molding conditions that make it possible to secure transparency, blends did not show clear crystallinity at blending ratios of more than 20 mol% in spite of the fact that crystallinity can be observed in the range of PEN content up to 30 mol%. Because both transparency and crystallinity could be secured with a PEN 12 mol% blend, this material was used in injection molding experiments with various injection molding cycles. Whitening occurred with a cycle of 20 sec, and transparency was obtained at 30 sec or more. This was attributed to the fact that transesterification between PET and PEN exceeded 5 mol% and phase solubility (compatibility) between the PET and PEN increased when the injection molding time was 30 sec or longer. However, when the transesterification content exceeded 8 mol%, molecularly oriented crystallization did not occur, even under stretching, and consequently, it was not possible to increase the strength of the material by stretching. PET/PEN blend resins are more easily crystallized by stretch heat‐setting than are PET/PEN copolymer resins. It was understood that this is because residual PET, which has not undergone transesterification, contributes to crystallization. However, because transesterification reduces crystallinity, the heat‐set density of blends did not increase as significantly as that of pure PET, even in high temperature heat‐setting. Gas permeability showed the same tendency as density. Namely, pure PET showed a substantial decrease in oxygen transmission after high temperature heat‐setting, but the decrease in gas permeability in the blend material was small at heat‐set temperatures of 140°C and higher.     

Correlation of thermally stimulated current and blow molding condition in poly(ethylene terephthalate) bottles

Thermally stimulated current (TSC) was used to study molecular relaxations in polyethylene terephthalate (PET) bottles. Unstretched PET film, which was used as a model for the bottle preform, exhibited two peaks at 77 and 90°C that correspond to the α and ρ relaxation processes, respectively. The bottles exhibited only the ρ relaxation, which is located within the temperature range for blow molding PET bottles. The α peak is associated with the main glass transition temperature (T_g) and the ρ peak may be associated with a second T_g. The second T_g is attributed to a “constrained state,” which shows dipolar behavior. Heat‐shrinkage behavior was examined at 90°C. The maximum TSC (I_m) of the ρ peak decreased with increasing heat set temperature, and with decreasing shrinkage. Bottles blown at 113°C showed a lower I_m and shrinkage than those blown at 103°C for equivalent heat set temperatures. The higher blowing temperature allowed a higher stretch speed that produced higher crystallinity bottles with self‐heat generation during rapid deformation. A relationship between the shrinkage mechanism and the dipole relaxation was proposed.     

Gas transport properties of poly(ethylene‐2,6‐naphtalene dicarboxylate) films: Influence of crystallinity and orientation

H₂ and CO₂ transport properties were investigated on semicrystalline poly(ethylene‐2,6‐dicarboxylate naphtalene) (PEN) films and biaxially stretched films and analyzed in terms of solubility and diffusion. The decrease of the permeability observed on the thermally crystallized samples has been described by Maxwell's law. No change of the sorption properties of the amorphous phase has been noticed as a function of the thermal treatment and the evolution of the diffusion coefficient has been related to a tortuosity effect. The low permeability measured on the biaxially stretched film is related to both a change of the free volume sizes distribution and a tortuosity effect. The interesting barrier properties of the biaxially stretched film are kept even after annealing the film at 250°C. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1849–1857, 2003     

Morphology–property relationship in oriented PET films: Microstructural reorganization during heat treatment

Morphology–property relationships for simultaneously biaxially stretched films and heatset with fixed dimensions in the temperature range of 100–240°C have been studied. The observed transition in various properties at 180°C can be explained on the basis of microstructural changes caused by competition among several processes, such as crystallization, solid-state thickening, melting, and molecular relaxation as well as by melting and recrystallization. The resulting structures and, thereby, the properties are different in temperature Regime-II (T_g to T_max) and Regime-III (T_max to T_m). In Regime-II, the high rate of crystallization compared to the rate of molecular relaxation develops a constrained amorphous phase, whereas the predominant melting and recrystallization process in Regime-III generates the relaxed amorphous phase. The structural reorganization during heat treatment is almost the same for uniaxially oriented film, fibers, and biaxially oriented films prepared under similar processing conditions. © 1994 John Wiley & Sons, Inc.     

Texture and morphology of biaxially stretched poly(ethylene naphthalene‐2,6‐dicarboxylate)

The orientation of poly(ethylene naphthalene‐2,6‐dicarboxylate) (PEN) films with different morphologies were studied by wide‐angle X‐ray diffraction. Different structures were obtained by thermally treating biaxially stretched PEN samples. Virgin and thermally treated (1 h at 240, 250, and 260°C) samples of PEN bioriented films were characterized by DSC to determine the glass‐transition temperature and the crystallinity ratio. To define the orientation of crystallites in the 25 μm thick bioriented samples, pole figures were recorded for various PEN samples, as a function of their position in the transverse drawing direction. The significant result is that there is a dominant crystal population, whose c‐axis direction varies from +45° at one sample edge to ?45° at the other edge, the orientation at the center being parallel to the transverse direction. There is also a secondary population, which can be seen only near the center. DSC studies also showed that by increasing the annealing temperature the crystallinity ratio was increased and pole figures showed that the texture was modified, probably because of disorientation mainly from an annealing temperature of 260°C. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2224–2232, 2003     

Enhancement of the dimensional stability of poly(ethylene‐2,6‐naphthalene dicarboxylate) filament by multistep zone annealing spinning

Both good tensile properties and good resistance to thermal shrinkage are prerequisites for tire cord applications. For these purposes, poly(ethylene‐2,6‐naphthalene dicarboxylate) (PEN) filaments were prepared by multistep zone annealing (MSZA) spinning with a specially devised system. The melting temperature of the PEN filaments so obtained was slightly increased with an increasing total draw ratio. All the filaments exhibited a sharp melting peak around 270°C, but glass‐transition behavior was barely visible via differential scanning calorimetry. Rheovibron experiments showed α relaxation in the vicinity of 175°C. Increasing the draw ratio above 4 did not increase the birefringence value much, but it did lead to increases in the tensile properties. The PEN filaments consisted exclusively of α‐form crystals. The PEN filaments showed excellent resistance to thermal shrinkage, which was less than 1% even with heating to 140°C. In the MSZA spinning process, increasing the degree of hot drawing proved more effective than increasing the degree of cold drawing for obtaining PEN filaments with better dimensional stability at elevated temperatures. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 916–922, 2002     

A comparative study of structure–property relationships in highly oriented thermoplastic and thermotropic polyesters with different chemical structures

Structure and properties of commercially available fully oriented thermoplastic and thermotropic polyester fibers have been investigated using optical birefringence, infrared spectroscopy, wide‐angle X‐ray diffraction and tensile testing methods. The effect of the replacement of p‐phenylene ring in poly(ethylene terephthalate) (PET) with stiffer and bulkier naphthalene ring in Poly(ethylene 2,6‐naphthalate) (PEN) structure to result in an enhanced birefringence and tensile modulus values is shown. There exists a similar case with the replacement of linear flexible ethylene units in PET and PEN fibers with fully aromatic rigid rings in thermotropic polyesters. Infrared spectroscopy is used in the determination of crystallinity values through the estimation of trans conformer contents in the crystalline phase. The analysis of results obtained from infrared spectroscopy data of highly oriented PET and PEN fibers suggests that trans conformers in the crystalline phase are more highly oriented than gauche conformers in the amorphous phase. Analysis of X‐ray diffraction traces and infrared spectra shows the presence of polymorphic structure consisting of α‐ and β‐phase structures in the fully oriented PEN fiber. The results suggest that the trans conformers in the β‐phase is more highly oriented than the α‐phase. X‐ray analysis of Vectran® MK fiber suggests a lateral organization arising from high temperature modification of poly(p‐oxybenzoate) structure, whereas the structure of Vectran® HS fiber contains regions adopting lateral chain packing similar to the room temperature modification of poly(p‐oxybenzoate). Both fibers are shown by X‐ray diffraction and infrared analyses to consist of predominantly oriented noncrystalline (63–64%) structure together with smaller proportion of oriented crystalline (22–24%) and unoriented noncrystalline (12–15%) structures. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 142–160, 2006     

Hot compaction of polyethylene naphthalate

In this article, we describe the production of single polymer composites from polyethylene naphthalate (PEN) multifilaments by using the hot compaction process. In this process, developed at Leeds University, highly oriented tapes or fibers are processed at a critical temperature such that a small fraction of the surface of each oriented element is melted, which on cooling recrystallizes to form the matrix of the composite. This process is, therefore, a way to produce novel high‐volume fraction polymer/polymer composites where the two phases are chemically the same material. A variety of experimental techniques, including mechanical tests and differential scanning calorimetry, were used to examine the mechanical properties and morphology of the compacted PEN sheets. Bidirectional (0/90) samples were made at a range of compaction temperatures chosen to span the melting range of the PEN multifilaments (268–276°C). Measurement of the mechanical properties of these samples, specifically the in‐plane modulus and strength, allowed the optimum compaction temperature to be ascertained (～ 271°C), and hence, the optimum mechanical properties. The optimum compacted PEN sheets were found to have an initial modulus close to 10 GPa and a strength of just over 200 MPa. The glass transition temperature of the optimum compacted sheets was measured to be 150°C, nearly 40°C higher than compacted poly(ethylene terephthalate) (PET) sheets. In previous work on polypropylene and PET hot compacted materials, it proved instructive to envisage these materials as a composite where the original oriented multifilaments are regarded as the reinforcing phase, and the melted and recrystallized material are regarded as the matrix phase. Dynamic mechanical bending tests (DMTA) were used here to confirm this for PEN. DMTA tests were carried out on the original fibers and on a sample of completely melted material to determine the fiber and matrix properties, respectively. The composite properties were then predicted by using a simple rule of mixtures and this was found to be in excellent agreement with the magnitude and measured temperature dependence of the hot compacted PEN material. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 796–802, 2004     

Structure and physical properties of biaxially stretched polyethylene terephthalate sheets under different heat‐set and stretch conditions

The thermal shrinkage of heat‐set polyethylene terephthalate (PET) sheets at 85°C after stretching at low speed and low temperature was similar to that for sheets stretched at high speed and high temperature if the crystallinities of these sheets were the same. However, differences between other physical properties, such as yield strength, were observed, which may be due to differences in the amorphous phase. The sheets stretched at the higher temperature and higher speeds may have different structures in the molecular segments in the amorphous phase, more relaxed than in the sheets prepared at the lower speed and temperature.     

Reactively formed ENPT copolymers as compatibilizers in ternary blends of poly(ethylene naphthalate)/poly(pentamethylene terephthalate)/poly(ether imide)

The compatibility of ternary blends of poly(ethylene naphthalate)/poly(pentamethylene terephthalate)/poly(ether imide) (PEN/PPT/PEI) was studied by examining the transesterification of PEN and PPT. ENPT copolymers were formed in situ as compatibilizers between PPT and PEI components in ternary blends. Differential scanning calorimetric (DSC) results for ternary blends showed the immiscibility of PEN/PPT/PEI, but ternary blends of all compositions were phase‐homogeneous after heat treatment at 300°C for more than 60 min. Annealing samples at 300°C yielded amorphous blends with a clear, single glass transition temperature (T_g), as the final state. Additionally, ENPT copolymer improved the compatibility of ENPT/PPT/PEI blends, yielding a homogeneous phase in the ENPT‐rich compositions. The morphology of the ENPT/PPT/PEI blends was altered from heterogeneous to homogeneous by controlling the concentration of PPT in the ENPT copolymers as well as the concentration of the ENPT copolymers. Moreover, a homogeneous phase with a clear T_g was observed when the concentration of PPT in the ENPT copolymer fell to 70 wt% in the ENPT/PEI = 50/50 blends. Experimental results indicate how the concentration of PPT in the ENPT copolymer affects miscibility in the ENPT/PEI blends. POLYM. ENG. SCI. 46:337–343, 2006. © 2006 Society of Plastics Engineers     

Study of nucleation activity and application of a nucleating agent to copolypropylene

The influences of nucleating agent content on crystallization peak temperature, crystallization curve shape, crystallization onset temperature, and crystallinity of a copolypropylene were studied in detail by differential scanning calorimetry. The results showed that crystallization onset temperature and crystallization peak temperature increased 17°C–22°C and 15°C–19°C, respectively, with increasing nucleating agent content. The behavior of the nucleating agent showed excellent efficiency. But the shape of the crystallization curve broadened with increasing nucleating agent content, and the crystallinity came to an extremum when the nucleating agent content was 0.2%. These results generally differed from those for crystallization of i‐PP by a nucleating agent. Modification of copolypropylene was studied by use of a nucleating agent, and the characteristics of the effects of the nucleating agent on copolypropylene were mastered. The results showed that the hardness of a copolypropylene improved observably. So the abrasion resistance of biaxially oriented polypropylene (BOPP) film for cigarette packaging was improved by adopting the modified copolypropylene as the skin‐layer heat‐seal material of BOPP film. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101:3915–3919, 2006     

Morphology and texture development of uniaxially stretched poly(ethylene naphthalene‐2,6‐dicarboxylate)

The texture development of PEN films with different semicrystalline morphologies have been studied by X‐ray diffraction. These different structures have been obtained by uniaxially stretching PEN amorphous films at 100 and 160°C (below and above T_g) at different drawing ratios. Samples have also been characterized by DSC to determine the crystallinity ratios, the crystallization, and melting temperatures. To define the orientation of crystallites in the oriented samples, pole figures have been constructed, as a function of temperature and drawing ratio (DR) in the range 1.5–4. In the range from DR = 2 to 4 the orientation is clearly uniplanar‐axial. At T_draw = 100°C the crystallinity shown by DSC analysis is higher than the sample stretched at 160°C. The orientation is also higher when samples are stretched at 100°C. The naphthalene rings mainly stay in the plane of the film with a lower fraction perpendicular to the plane of the film. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 395–401, 2007     

Influence of heat‐setting temperature on the properties of a stretched polypropylene microporous membrane

The influence of heat‐setting temperature on the stress–strain curves, differential scanning calorimetry (DSC) curves and properties of a stretched polypropylene microporous membrane was studied. It was found that with an increase of heat‐setting temperature, a plastic plateau region in the stress–strain curves and a melting endotherm plateau in the DSC curves became apparent. The permeability and porosity firstly increased with the temperature to 145 °C and then decreased. The pore structure arrangement showed similar changing trend. On the one hand, higher heat‐setting temperature could decrease the shrinkage of the microporous membrane. On the other hand, the crystallization of some chains around separated lamellae during heat‐setting was unfavorable to the properties of the microporous membrane. The occurrence of crystallization during heat‐setting explained the origin of the plastic plateau. For the fabrication of microporous membranes based on the melt‐stretching mechanism with good dimensional stability, heat‐setting is necessary, but too high a heat‐setting temperature can destroy the permeability. © 2013 Society of Chemical Industry     

Mechanical Behavior of Poly(Ethylene 2,6-Naphthalene-Dicarboxylate) (PEN) Fibres near the Glass-Rubber Transition Temperature

Abstract

The mechanical properties of as-spun poly(ethylene 2,6-naphthalene-dicarboxylate) (PEN) fibres were studied in order to characterize this relatively new material near its glass-rubber transition.

Tensile tests were carried out on amorphous (low-speed spun) PEN filaments. The temperature range of 90°C up to 160°C was covered using increments of 10°C. A transition from necking and cold drawing to rubber-like behavior was observed in the stress-strain relationship. Dynamic mechanical experiments were carried out on PEN yarns spun at speeds from 500 to 4000 m min^?1. Both temperature and frequency were varied. The maxima in loss modulus depend on spinning speed. Tensile behavior and dynamic mechanical behavior of PEN fibres demonstrate that the glass-rubber transition temperature of PEN is approximately 125°C.     

Thermal properties of poly(vinyl cyclohexane)

Isotactic poly(vinyl cyclohexane) (PVCH) was studied by thermal analysis. The deduced equilibrium melting point, T, is 405°C (678 K). The heat of fusion, Δ H was found to be 50.82 J/g (5.60 kJ/mol) and Δ C_p at T_g, is 0.273 J/(gK) [30.1 J/(molK)]. The glass transition temperature, T_g, of the amorphous PVCH is 80°C (353 K). In semicrystalline samples, T_g increases up to 165°C (438 K) for crystallinities > 40%. Beside crystalline and flexible amorphous, a rigid amorphous phase is postulated in the semicrystalline polymer.     

Creep and shrinkage behavior of improved ultrathin polymeric films

Long‐term creep‐deformation and shrinkage characteristics of improved ultrathin polymeric films for magnetic tapes are presented. These films include poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), and aromatic polyamide (ARAMID). PET film is currently the standard substrate used for magnetic tapes, and thinner tensilized‐type PET, PEN, and ARAMID have recently been used as alternate substrates with improved material properties. The thickness of the films ranges from 6.2 to 4.8 μm. More dimensional stability is required for advanced magnetic tapes, and the study of creep and shrinkage behavior is important for estimating the dimensional stability. Creep measurements were performed on all available substrates at 25, 40, and 55°C for 100 h. Based on these data, master curves were generated using time–temperature superposition to predict dimensional stability after several years. The amount of creep deformation is considerably smaller for ARAMID and tensilized‐type PET than for PEN, although Standard PET shows the largest amount of creep. In addition, creep measurements under high humidity also show similar trends. Shrinkage measurements at 55°C for 100 h show that the shrinkage of ARAMID is lower than that of PET and PEN. The relationship between the polymeric structure and dimensional stability are also discussed. Based on the creep and shrinkage behavior, ARAMID and tensilized‐type PET seem to be suitable for advanced magnetic tapes. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1477–1498, 2002; DOI 10.1002/app.10012     

Morphology and barrier mechanism of biaxially oriented poly(ethylene terephthalate)/poly(ethylene 2,6‐naphthalate) blends

To improve the barrier properties of poly(ethylene terephthalate) (PET), PET/poly(ethylene 2,6‐naphthalate) (PEN) blends with different concentrations of PEN were prepared and were then processed into biaxially oriented PET/PEN films. The air permeability of bioriented films of pure PET, pure PEN, and PET/PEN blends were tested by the differential pressure method. The morphology of the blends was studied by scanning electron microscopy (SEM) observation of the impact fracture surfaces of extruded PET/PEN samples, and the morphology of the films was also investigated by SEM. The results of the study indicated that PEN could effectively improve the barrier properties of PET, and the barrier properties of the PET/PEN blends improved with increasing PEN concentration. When the PEN concentration was equal to or less than 30%, as in this study, the PET/PEN blends were phase‐separated; that is, PET formed the continuous phase, whereas PEN formed a dispersed phase of particles, and the interface was firmly integrated because of transesterification. After the PET/PEN blends were bioriented, the PET matrix contained a PEN microstructure consisting of parallel and extended, separate layers. This multilayer microstructure was characterized by microcontinuity, which resulted in improved barrier properties because air permeation was delayed as the air had to detour around the PEN layer structure. At a constant PEN concentration, the more extended the PEN layers were, the better the barrier properties were of the PET/PEN blends. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1309–1316, 2006