Strain-induced crystallization of poly(ethylene terephthalate)

The fabrication of poly(ethylene terephthalate), PET, into fibers, films, and containers usually involves molecular orientation caused by molecular strain, which may lead to stress- or strain-induced crystallization (SIC). The SIC of PET was studied by the methods of birefringence, density, thermal analysis, light scattering, and wide-angle X-ray. The development of crystallinity is discussed in relation to the rate of crystallization, the residual degree of orientation, and stress relaxation. The experimental procedure involves stretching samples at temperatures above the glass transition temperature, Tg, to a given extension ratio and at a specific strain rate of an Instron machine. At the end of stretching, the sample is annealed in the stretched state and at the stretching temperature for various periods of time, after which the sample is quickly quenched to room temperature for subsequent measurements. During stretching, the stress strain and the stress relaxation curves are recorded. The results indicate that the SIC of annealed, stretched PET can proceed in three different paths depending on the residual degree of orientation. At a low degree of residual orientation, as indicated by the birefringence value, annealing of stretched PET leads only to molecular relaxation, resulting in a decrease of birefringence. At intermediate orientation levels, annealing causes an initial decrease in birefringence followed by a gradual increase and finally a leveling off of birefringence after a fairly long period of time. At higher orientation levels, annealing causes a rapid increase in birefringence before leveling off. The interpretation of the above results is made using the measurements of light scattering, differential scanning calorimetry, and wide-angle X-ray. The rate of the SIC of PET is also discussed in terms of specific data analysis.     

Control the strain-induced crystallization of polyethylene terephthalate by temporally varying deformation rates: A mechano-optical study

The mechano-optical behavior of PET is, for the first time, investigated under temporally varying rates to influence the basic mechanisms of structural organization leading to strain-induced crystallization. For this purpose, four rate profiles, Linear, Sigmoidal, Logarithmic and Exponential, were chosen and films were stretched in Uniaxial Constrained Width mode using newly developed biaxial stretching machine. This machine allows real time direct measure of true stresses, strains, in-plane and out-of-plane birefringences during the deformation. Substantial differences in the mechano-optical behavior and resulting structural mechanism were observed in all of the chosen rate profiles. Linear profile, taken as a standard, yields three stress-optical regimes (SOR) during deformation. At early stages of deformation the birefringence remains linear with stress and material remains amorphous. This is designated as Regime I representing classical stress-optical behavior observed in large number of non-crystallizable polymers. In Regime II, a fast increase of birefringence accompanies formation of crystalline structure with establishment of long-range connected network. In the final Regime III birefringence levels off as the chains approach their finite extensibilities.All three regimes observed in Linear profile are also observed in Logarithmic and Exponential cases. However, Sigmoidal deformation shows only the first two regimes even though the film was stretched to the same total engineering strain as applied to all profiles. Logarithmic profile was found to induce early strain crystallization leading to early development of strain hardening. Exponential profile on the other hand retards the formation of “potentially constraining” long-range physical networks. This allowed the development of higher birefringence and crystallinity levels using this mode. A logarithmic birefringence-work relationship with two distinct stages was found to apply to all temporally varying profiles.     

Dynamic phase diagram derived from large deformation non-linear mechano-optical behavior of polyethylene naphthalate nanocomposites

The effect of nanoparticles on the mechano-optical behavior of PEN was investigated using a real time true stress-true strain birefringence measuring system [Toki S, Valladares D, Sen TZ, Cakmak M. ANTEC Proc 2001:1830; Koike Y, Cakmak M. Polymer 2003;44:4249]. The large deformation stress-optical behavior revealed that there are at most three distinct regimes that may be augmented by an initial additional glassy component at lower temperatures and/or high rates. The presence of nanoparticles did not change the birefringence development of PEN before strain induced crystallization. However, after strain induced crystallization occurred in PEN, the birefringence increase was found to be diminished due primarily to the decrease in the amount of crystallinity and amorphous chain orientation with the addition of nanoparticles. The final structure and deformation behavior of the nanocomposites have been mapped out in a dynamic phase diagram as a function of stretching temperature, rate and nanoparticle content. This dynamic phase diagram shows that the material undergoes three critical structural transitions: (i) nematic-crystalline transition wherein the material stretched below a certain temperature does not undergo orientation-induced crystallization but develops a highly ordered nematic state. (ii) A kinematic transition wherein the material transforms from a ‘structured liquid’ to a ‘true liquid’ state exhibited by the disappearance of the initial glassy component as the material becomes devoid of the inherent structure due to segmental correlations. (iii) While unfilled PEN remains amorphous at high temperatures due to high relaxation rates combined with decreased thermal crystallizability, the presence of nanoplatelets was found to facilitate strain induced crystallization even at such high temperatures primarily due to the decrease in orientation relaxation in their presence.     

Mechano optical behavior of polyethylene terephthalate films during simultaneous biaxial stretching: Real time measurements with an instrumented system

We present an experimental study of real time true stress–strain–birefringence measurements to elucidate the sequence of structural mechanisms that occur during simultaneous biaxial stretching of PET films from amorphous precursors in rubbery state. Stress–birefringence relationship, wide angle X-ray diffraction, Raman spectroscopy and DSC thermal analysis were used to identify the stages of the mechano-optical behavior of the films during stretching, and to identify their structural origins. The measurements revealed four regimes for the relationship between the stress and birefringence. In the first regime the stress has a linear relationship with birefringence where the linear stress optical rule holds and the stress optical constant for PET is 5.8 GPa⁻¹ (5800 Brewster). In the second regime, the relationship is also nearly linear with a steeper positive slope, and in the third regime the relationship is nonlinear. At very high stretching rates, a fourth stage could be seen, where the stress increases while the birefringence reaches a plateau. This stage is reached when the polymer chains attain their finite extensibilities. This stage was not observed if low rates of stretching employed, where high relaxation movements dominate the orientation effects. The deviation from the initial linear stress optical rule coincides with the onset of the stress-induced crystallization as revealed by the off line measurements. This transition was found to be rate dependent and increased rate delays this transition to higher stresses.     

Kinetics of structural evolution during crystallization of preoriented PET as followed by dual wavelength photometric birefringence technique

A two-color laser photometric measurement system was used to follow birefringence changes in annealing of prestretched PET films. This technique was shown to be capable of detecting large retardation (and thus birefringence) changes beyond one wavelength of the light. The use of green and red lasers in the photometric system allows the detection of the reversals in birefringence during the course of annealing. Unless critical orienialion/erystallinity levels are developed by stretching the films, heat setting the films at temperatures close to glass transition temper ature results in complete elimination of preferential orientation. At Intermediate stretch ratios, crystalline regions form and Ihe acl as anchor points establishing a network for the orienied amorphous chains. These Dims exhibit partial relaxation followed by rapid increase in birefringence due to the accelerating effect of orientation on crystallization. At high stretch ratios, where substantially oriented and strain crystallized structures are obtained, the initial relaxation stage disappears and birefringence continue to increase throughout the heat setting process even at temperatures very close to glass transition temperature. In these films, however, the total change of birefringence decreases as more of the chains are oriented and crystallized in the stretching stage, leaving a smaller fraction of polymer chains to rearrange during Ihe annealing stage. The kinetics of the structural change exhibit a complex behavior and the largest rates of structural changes were observed in films exhibiting intermidiate birefringence levels.     

Real time development of structure in partially molten state stretching of PP as detected by spectral birefringence technique

Our main focus in this study is to investigate the deformation behavior of PP in temperature range where the PP is partially molten using a newly developed uniaxial stretching system. This system allows the real time study of the structural reorganization processes as reflected in birefringence coupled with true stress and true strain at temperature and deformation rates used in industrial film process. This instrument revealed that birefringence-stress relationship is linear beyond an initial yielding point until the onset of strain hardening beyond which negative deviation from this linearity is observed. At lower strains the films were found to continue to change significantly during holding stage leading to increase in birefringence. These changes observed in holding stage decreases with decrease of stretching speed and temperature and with increase of total strain. Increased strain rates result in destruction of crystallites that involves block rotations of these regions leading to observation of lower birefringence at faster rates. The latter process generates large amount of stretched amorphous chains that gradually convert to crystalline state during holding.     

Mechanical/optical behaviors of imogolite hydrogels depending on their compositions and oriented structures

A robust acrylamide (AAm) hydrogel reinforced by imogolite (IG), a perfect rigid nanotubular clay mineral, exhibited distinct tensile stress–strain characteristics and strain‐induced birefringence in accordance with the compositions of the gels. The gel showed a reversible anisotropic/isotropic structural transition in response to stretching/releasing before the breakdown strain. The strain‐induced birefringence of the IG‐reinforced gels could be fixed by the in situ interpenetrating polymerization of other AAm monomers that were impregnated into the gels in the stretched states. This resulted in gels with nonvolatile anisotropic birefringence, and therefore, the fixed anisotropic IG ordering showed specific stress–strain characteristics depending on the orientation of IG. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41691.     

Kinetics of rapid structural changes during heat setting of preoriented PEEK/PEI blend films as followed by spectral birefringence technique

The influence of composition and preorientation on the development of structural hierarchy during heat setting of PEEK/PEI films was investigated using on-line birefringence, and off-line wide-angle and small-angle X-ray scattering, infrared dichroism and thermal analysis techniques. When the PEEK/PEI blends are drawn to deformation levels below the onset of strain hardening, the subsequent heat setting at high temperature starts with a large relaxation process followed by a fast crystallization and a long-term slow structural rearrangement stages. When the films are predrawn beyond a critical structural level (crystallinity and orientation), the initial relaxation stage disappears. This signifies that beyond a critical structural order a long-range physical network, where the nodes consist of crystallized domains and chain—self and cross-entanglements—are formed. This physical network allows the entropy driven shrinkage stresses to be maintained that results in the development of oriented crystalline phase. The addition of non-crystallizable PEI chains was found to retard the formation of this ‘network structure’ resulting in lower orientation levels.     

The suppression of strain induced crystallization in PET through sub micron TiO2 particle incorporation

The effects of TiO₂ particles on the crystallization and uniaxial stress-strain behavior of poly(ethyleneterephthalate) (PET) films from amorphous precursors were investigated. The addition of small fraction of sub micron sized TiO₂ particles were found to suppress the mechanical relaxation processes associated with high temperature side of the β relaxation while enhancing the low temperature relaxation. When crystallized from unoriented precursors, TiO₂ particles act as a nucleation agent and enhance the thermally induced crystallization of the PET chains. However, when stretched from the amorphous precursors in rubbery temperature range (T_g<T_p<T_cc), the TiO₂ concentration levels as low as 0.35 wt% was found to reduce the overall stress and retard strain hardening and accompanying stress induced crystallization. As a result, under the same stretching conditions, the films containing TiO₂ were found to possess lower crystallinity and orientation levels. This was attributed to suppression of stress induced crystallization as these particles act to disrupt the formation of crystalline lattice by their physical presence. This may be as a result of the reduction of chain entanglements in the presence of these sub micron sized TiO₂ particles in the structure of the polymers that retard the formation of physical network whose nodes are made up of entanglements and small crystalline domains. The development of this long range ‘connected’ network is primarily responsible for the rapid upturn in the stresses at the onset of strain hardening observed in stress strain curves.This method represent a unique way to apply ‘anti-nucleating agent’ effect to control the development of stress induced crystallization that will help control the film and fiber forming processes.     

Crystal structure of uniaxially stretched bio-based polyamide 510 films

The crystal structure changes of PA510 films during uniaxially stretching at 80°C, 110°C, 140°C and 170°C had been investigated as a function of stretching ratio and stretching rate. The stress–strain relationship curves showed that the stress of the PA510 films gradually increased when the stretching ratio increased. The wide-angle X-ray diffraction results verified that only one distinct equator reflection of stretched films was clearly identified at 80°C, 110°C and 140°C, namely γ(100) at 2θ = 20.6°. However, when the stretching temperature reached 170°C, the γ(004), γ(006) and γ(008) crystal form appeared in the meridional direction at λ = 12. Combined with differential scanning calorimeter analysis, it was found that the Xc increased from 7% to 40% as a result of the strain induced crystallization phenomenon and the stretching promoted the appearance of γ crystal form. In addition, the increase in the crystallinity and the molecular chain orientation increased the strength of the PA510 films in the tensile direction. And it also found that the microcracks occurred in the stretched films at high stretching ratio (λ = 12).     

Preparation and characterization of biaxially oriented films from polybutylene terephthalate based thermoplastic elastomer block copolymers

Film casting and biaxial stretching of a series of polyester thermoplastic elastomers (TPEs) were studied. Biaxial orientation in the stretched films was characterized by wide‐angle X‐ray diffraction and birefringence measurements. Biaxial orientation factors were determined. The X‐ray diffraction and birefringence clearly indicated the development of planar biaxial orientation in the stretched films with biaxial stretching. The phenyl groups in the stretched PBT and TPE films gradually became more parallel to the film surfaces with increasing biaxial orientation. The lower the PBT content in the stretched TPE films, the lower the planar biaxial orientation achieved. The β form of crystalline PBT was found only in the stretched PBT films, but not in the TPE films.     

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     

Study of the structure transformation of wool fibers with Raman spectroscopy

Raman spectroscopy and tensile tests were used to investigate the structure transformation of stretched wool fibers with stretching ratios up to 110%. The typical bands analyzed in this article included the amide I region, the amide III region, the C? C skeletal vibration region, and the S? S and C? S bond vibration region. To investigate the variations of the crystallinity and orientation of the wool fibers, the density and birefringence of the fibers were also measured. The results showed that the secondary structure of the wool fibers was transformed from an α‐helical structure to a β‐pleated‐sheet structure during the early stage of stretching. When the fiber was stretched more than 80%, the mechanism of stretching mainly relied on the slippage of the peptides. Meanwhile, the pretreatment of the wool fibers with sodium bisulfite and the setting processing resulted in the reduction of the concentration of the S? S bonds. The results for the density and birefringence showed that the degree of crystallinity of the wool fibers decreased, whereas the degree of orientation increased during the stretching. The tensile behavior of the stretched wool also supported the α→β microstructure transformation. The diameter results showed that the extent of slenderization was about 25.3% when the stretching ratio was 80%. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1–7, 2007     

Poly(ethylene terephthalate)/poly(ethylene glycol‐co‐1,3/1,4‐cyclohexanedimethanol terephthalate)/clay nanocomposites: Effects of biaxial stretching

In this study, we fabricated poly(ethylene terephthalate) (PET)/clay, PET/poly(ethylene glycol‐co‐1,3/1,4‐cyclohexanedimethanol terephthalate) (PETG), and PET/PETG/clay nanocomposite plates and biaxially stretched them into films by using a biaxial film stretching machine. The tensile properties, cold crystallization behavior, optical properties, and gas and water vapor barrier properties of the resulting films were estimated. The biaxial stretching process improved the dispersion of clay platelets in both the PETG and PET/PETG matrices, increased the aspect ratio of the platelets, and made the platelets more oriented. Thus, the tensile, optical, and gas‐barrier properties of the composite films were greatly enhanced. Moreover, strain‐induced crystallization occurred in the PET/PETG blend and in the amorphous PETG matrix. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42207.     

Structure and properties of an oriented fluorosulfonated/PTFE copolymer

This polymer is a precursor of Nafion, an ionomer developed for ion‐selective membranes. In this work various stretching procedures were carried out on the precursor, in order to explore the possibilities of enhanced performance. First, the crystallization behavior and properties of the unoriented precursor were investigated. It was found that the precursor copolymer contains crystallinity, which melts over a very wide temperature range. The amount of crystallinity is modified both by thermal history and applied shear. Thermal analysis can be used to determine processing/annealing temperatures for samples with unknown thermal history. It was found that the precursor possessed a maximum elongation at break at 70°C. Stretching was carried out at 70°C using a laboratory‐based tentering process, to produce both uniaxial and biaxial films. Films were characterized to monitor their shrinkage behavior, and orientation was measured by birefringence. Their tensile properties were measured with respect to stretching direction. Using tentering, which is carried out at relatively low temperatures, large amounts of orientation were produced, resulting in a significant increase in tensile strength, and decrease in elongation at break.     

Enhancement of stain‐induced crystallization in polylactide via thermal preannealing

Stretching of amorphous polylactide above glass transition temperature can lead to strain‐induced crystallization, which is mainly determined by stretching rate and temperature. This study demonstrates that thermal annealing prior to stretching is alternative to enhance strain‐induced crystallization of a given polylactide at the identical stretching conditions. The local order generated during thermal annealing acts as physical crosslinks to effectively alleviate molecular relaxation and thus facilitates the advent of strain‐induced crystallization. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39993.     

Mechanisms of structural organizational processes as revealed by real time mechano optical behavior of PET film during sequential biaxial stretching

The main focus of this research is to identify the structural mechanisms that are responsible for changes observed in various stages of coupled mechano-optical (stress and strain optical) relationships as they are influenced by the deformation mode, level and rate. This involves in situ real time studies of the true-stress-strain-birefringence of sequential biaxially stretched PET films using a highly instrumented biaxial stretching machine and ex situ structural studies. These ex-situ studies include wide angle X-ray diffraction, Raman spectroscopy and DSC thermal analysis to examine the orientation, conformation and crystallization behaviors as well as formation of long range connectivity through physical network. The relationship between the stress and birefringence exhibited three Regimes. Regime I, linear relationship with a stress optical constant of 5.8 GPa⁻¹. Regime II, non-linear relationship with the establishment of chain tautness and nematic like two-dimensional order. Regime III, where the chains reach their finite extensibility.     

Stress-induced crystallization of branched polyethylene terephthalate films

Polyethylene terephthalate (PET) prepolymer containing 0.0 to 0.3 mole percent pentaerythritol was polymerized to an inherent viscosity of 0.63-0.70 dl/gm by the fluidized bed technique. Rheological studies and gel permeation chromatography (GPC) examination showed the samples to be branched in character. Amorphous films were stretched at 82 and 93°C at elongation rates of 54, 161 and 267 percent-s^?1 using the T. M. Long machine. The extent of stress-induced crystallization was established by a density determination. The branched samples behaved very similarly to the linear PET material in crystallization and birefringence studies. Neither the percent crystallinity nor the birefringence appears to be a strong function of strain rate over the range 54-267 percent-s^?1. A cursory examination of the crystallization kinetics of the oriented samples suggests that extent of branching in our samples does not markedly reduce the crystallization rate at annealing temperatures of 180 and 220°C.     

Comparing elastomeric behavior of block and random ethylene–octene copolymers

This work compared the elastomeric properties of two low‐crystallinity ethylene–octene copolymers. One was a block copolymer with lamellar crystals and the other was a random copolymer with fringed micellar crystals. The comparison of the stress–strain behavior at 23°C revealed that the initial elastic modulus and the yield stress depended only on the crystallinity of the copolymer. When the temperature was raised above 23°C, melting of the fringed micellar crystals of the random copolymer caused a rapid decrease in the modulus. Some decrease in the modulus of the block copolymer over the same temperature range was attributed to the crystalline α‐relaxation. Both polymers exhibited strain‐hardening, ultimate fracture at high strains, and high recovery after fracture. However, in the block copolymer, the onset of strain‐hardening and the ultimate fracture occurred at higher strains. The block copolymer also showed higher recovery from high strains. The initial stretching resulted in a permanent change in the stress–strain curve. It was suggested that following the onset of crystal slippage at the yield, the crystals underwent permanent structural changes through the course of the strain‐hardening region. The transformation of the fringed micellar crystals occurred at lower strains than the transformation of the lamellar crystals. The extent of the structural transformation was described by the crosslink density and the strain‐hardening coefficient extracted from elasticity theory. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Evolution of phase behavior and orientation in uniaxially deformed polylactic acid films

The development of crystalline structure and orientation during uniaxial stretching of cast amorphous linear and branched lactic acid films were investigated in the rubbery temperature ranges that spans between glass transition temperature and cold crystallization temperature. This material exhibited almost ideal stress‐strain behavior in the temperature range 65–80°C. Because of its strain crystallizability, films with uniform thickness can be obtained at high deformation levels as a result of self‐leveling. Branching was found to retard this self‐leveling through its slightly detrimental effect on the strain hardening. Upon stretching the material undergoes rapid orientation in the amorphous state and beyond a critical level very sharp and highly oriented β crystalline form chains with ?3/1 helix. If the temperature is at or below Tg, with additional stretching, the films were found to revert to a highly oriented amorphous state through the destruction of the crystalline domains. At higher temperatures, further stretching results in continuation of improvement in crystalline order.