Stress-optical Behavior Research Articles

Mechano‐opticalbehavior in poly (ethylene terephthalate)/poly (ether imide) blends

AbstractMechano‐optical behavior and related structural evolution during uniaxial stretching of melt miscible poly (ethylene terephthalate) (PET)/poly (ether imide) (PEI) blends were studied near their glass transition temperature using an instrumented machine that measures true stress, true strain and spectral birefringence simultaneously. Stretching from amorphous state, two distinct stress‐optical regimes were observed at temperatures between Tgand Tcc(cold crystallization). Near Tg, a typical photoelastic behavior persists until a critical temperature above which temperature independent initial stress optical behavior is observed. At those temperatures above Tg, where glassy behavior is observed, decreasing stretching rate was also found to eliminate this glassy photo elastic regime leading to the observation of a linear initial stress optical behavior that becomes temperature independent as expected from linear stress optical rule. Increasing PEI concentration in the blends suppresses crystallizability and increases temperature at which initial elastic region disappears giving way to pure liquid behavior where linear stress optical behavior is observed. This is attributed to the increase and broadening of the glass transition temperature with the addition of noncrystallizable PEI. In PET/PEI blends, the stress‐optical coefficient (SOC), determined in a linear stress optical regime, was found to increase linearly with the increase in PEI concentration.

Thermally-inverted strain-optical and stress-optical behavior of poly(4-methyl-1-pentene) in rubbery state

Thermally-inverted strain-optical and stress-optical behavior of poly(4-methyl-1-pentene) in rubbery state

Stress Optical Behavior and Structure Development in Melt Spun PEEK/PEI Blends

Abstract Stress optical behavior as influenced by blend composition has been investigated by on–line stress monitoring during melt spinning of a series of melt miscible PEEK/PEI blends. At low stresses/draw downratios, the fibers are found to be amorphous. In this range, the linear stress optical rule is obeyed and stress optical constant was found to be composition dependent. Beyond a critical drawdown, the PEEK chains first form very highly oriented ordered domains that exhibit significant translational disorder along the fiber axis. In the processing range observed, this structure persists even at high spinline deformation/stresses. While the stress optical behavior remains linear for those compositions where no stress crystallization was detected, negative deviation from initial linearity was observed when these ordered PEEK regions began to be observed indicating that birefringence saturation is approached while stresses are increased.

Evolution of structural mechanisms in thermoplastic polyimide (BTDA-DAH) from amorphous precursors as revealed by real-time uniaxial mechano-optical behavior

The real-time mechano-optical behavior of Polyimide 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) 1,6-diaminohexane (DAH) [PI(BTDA-DAH)] films during uniaxial deformation was investigated in the glassy and rubbery states as a function of temperature and stretch rate to understand the stretching characteristics of the polymer and elucidate the structural evolution. The study combined real-time spectral birefringence with true stress and true strain measurements and the data were further augmented by offline DSC and WAXD measurements. During deformation near glass transition temperature, initial photoelastic regime exhibiting very small slope is followed by regimes II and III. As the deformation temperature increases, the initial photoelastic regime disappears and gives way to temperature-independent regime I following classic stress-optical behavior. Regime I is followed by either a negative deviation from SOR into regime IIIa at lower deformation temperatures, or a positive deviation from SOR into regime II at higher deformation temperatures which eventually transitions into regime IIIc at large strains. The polymer was essentially amorphous in regime I. As the deformation increased, strain-induced crystallization occurred in regime II where crystallinity as high as 24% was achieved. The strain-induced crystals enhanced network connectivity and increased the rate of polymer chain orientation before transitioning to regime IIIc. Under conditions where regime II was not observed, a linear mechano-optical response in regime I extended to higher deformations before transitioning into regime IIIa where the highly oriented chains exhibited a “nematic-like” order with translational disorder along the chain directions. Based on the above observations a schematic for the structural evolution of PI(BTDA-DAH) during uniaxial deformation was proposed.

Mechano-optical behavior of loosely crosslinked double-network hydrogels: Modeling and real-time birefringence measurement during uniaxial extension

Real-time uniaxial strain-induced birefringence in loosely crosslinked double-network (DN) hydrogels synthesized from acrylamide (AAm) and N,N-dimethyl(acrylamide) (DMA) was measured. Tensile tests were performed at different extension rates from quasi-static conditions to very rapid tests followed by a holding for relaxation. Both DN hydrogels exhibited negative birefringence whose absolute value increased with extension and remained constant during relaxation. DN hydrogel synthesized from AAm displayed a linear stress optical (birefringence – true stress) behavior, however, a nonlinear trend was observed for the DN gel synthesized from DMA. A simple photoelastic model was developed based on the mechanical behavior of the gels using Fung elastic potential and Brewster's stress-optical law, and the model was compared with the experimental data.

Role of Nanoparticles and Relaxation on Strain-Induced Crystallization Behavior in Uniaxially Stretched Polyethylene Naphthalate Films: A Mechano-Optical Study

The effects of nanoparticle concentration and processing conditions on the relaxation behavior of PEN nanocomposites are investigated using mechano optical techniques where birefringence, true stress and true strain are measured while subjecting the polymer films to uniaxial deformation followed by relaxation. For this purpose, two different stretching temperatures were employed to stretch the films containing 0.5 and 2 wt % nanoparticles: one above and the other below the Tll (liquid–liquid transition). Increasing the temperature as well as the addition of nanoparticles suppresses the spontaneous deformation processes leading to sharp increase in true strain. An instantaneous stress drop is observed during relaxation below the Tll corresponding to the initial glassy component observed in the stress-optical behavior during stretching. This stress is attributed to the presence of segmental rigid correlations that were not broken during the stretching stage. This behavior was found to be absent above the li...

A fast real time measurement system to track in and out of plane optical retardation/ birefringence, true stress, and true strain during biaxial stretching of polymer films

An instrumented and highly integrated biaxial stretching system was designed and constructed to obtain true stress, true strain, and optical behavior of polymeric films during biaxial stretching. With programmable drive motors, any form of temporally varying biaxial deformation profiles, including linear, exponential, logarithmic as well as cyclic, can be applied to a square-shaped films. This machine allows the investigation of mechano-optical behavior of films under profiles captured in industrial processes. To overcome the edge effects, the samples are painted with a dot pattern that is imaged using a high speed video capture system. This system accurately determines the locations of the each dot matrix in subsequent images acquired and calculates the true strains in both directions. The in-plane optical retardation is determined using spectral birefringence method that uses polarized white light and optical spectrometer in the optical train. This is carried out automatically at less than 10 nm in retardation resolution with the light beam passing through the symmetry center of the sample. Out of plane retardation is measured with an identical optical train tilted 45° to the plane of the film with its light beam going through the same spot on the sample as 0° beam. The true stress and birefringences are calculated with the determined instantaneous thickness of the film. With this system, the stress optical behavior of PET's is determined up to very large deformation levels at moderate to high deformation rates. Beyond the initial linear stress optical behavior, these films exhibit sudden positive deviation from linearity and this start of nonlinearity was directly associated with the stress induced crystallization.

Dynamic measurement of stress optical behavior of three amorphous polymers

In the present study, rheo-optical and mechanical properties of three amorphous polymers, i.e., PS (polystyrene), PC(polycarbonate) and COC(cyclo olefin copolymer), widely used for optical products have been investigated. Accurate measurement of stress optical coefficients and elastic modulus data across the glass transition region are essential for predicting optical anisotropy in many injection molded optical products like pickup lenses and waveguide in LCD module since the final products have both flow and thermal history from the melt to glass. To obtain stress optical behavior in wide range of frequency and temperature including rubbery, glassy and glass transition regime, frequency sweep tests with extensional bar and shear sandwich tools were undertaken. As a result, glassy and melt extreme values of stress optical coefficient of PS and PC were evaluated as well as master plots in wide frequency region. The sign change of stress optical coefficient was shown clearly for PS as the frequency increased. On the other hand, the sign of stress optical coefficient over the whole frequency region is always positive for PC. For COC’s of different composition, even though the glass transition temperature can vary, the stress optical coefficient of COC’s with different composition stays almost constant at two extremes.

Stress - Optical behavior of Poly(m-xylylenediamine adipamide) (Nylon MXD6): Influence of molecular weight

The effect of molecular weight and rubbery state uniaxial stretching conditions on the mechano-optical behavior of Poly(m-xylylenediamine adipamide) (Nylon MXD6) films was investigated using a real time spectral birefringence stretching machine. The stress optical behavior exhibits a multi stage behavior that depends on process conditions as well as molecular weight. At low stretching temperatures and high rates the stress optical behavior was found to start with an initial glassy photo elastic behavior. Decreasing stretching rate or increasing processing temperature was found to eliminate this glassy photoelastic regime leading to the observation of a linear initial stress optical behavior past a temperature of 95 °C (Tll). The stress optical constant (SOC) was about the same for both M.W. materials stretched at temperatures past Tll, at 2.7 GPa−1 for HPA6 and 2.61 GPa−1 for LPA6. Following this initial regime, the behavior is controlled by the competition between orientation and relaxation during deformation. If the chain orientation relaxation is not suppressed by increasing the stretching rate and/or the molecular weight or by decreasing temperature, the material strain crystallizes.

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.

Stress ‐ Optical Behaviour of Polymers

AbstractSummary: The collaborative research between Stepto and colleagues at Manchester University and Ward and colleagues at Leeds University is discussed in terms of its general significance for understanding the development of molecular orientation which occurs during the stretching of polymers. Cross‐linked polyethylene and polyethylene terephthalate are chosen as the key examples, with particular reference to their stress optical behaviour. Infra‐red spectroscopy is shown to be a valuable tool to validate the methodology developed by Stepto and his colleagues for modelling the behaviour of rubber‐like networks.

A molecular dynamics study of the stress–optical behavior of a linear short-chain polyethylene melt under shear

In this study, we present details of the stress–optical behavior of a linear polyethylene melt under shear using a realistic potential model. We demonstrate the existence of the critical shear stress, above which the stress–optical rule (SOR) begins to be invalid. The critical shear stress of the SOR of this melt turns out to be 5.5 MPa, which is fairly higher than 3.2 MPa at which shear thinning starts, indicating that the SOR is valid up to a point well beyond the incipient point of shear thinning. Furthermore, contrary to conventional wisdom, the breakdown of the SOR turns out not to be correlated with the saturation of chain extension and orientation: It occurs at shear rates well before maximum chain extension is obtained. In addition to the stress and birefringence tensors, we also compare two important coarse-grained second-rank tensors, the conformation and orientation tensors. The birefringence, conformation, and orientation tensors display nonlinear relationships to each other at high values of the shear stress, and the deviation from linearity begins at approximately the critical shear stress for breakdown of the SOR.

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.

Experimental studies and molecular modelling of the stress–optical and stress–strain behaviour of poly(ethylene terephthalate). Part III: Measurement and quantitative modelling of birefringence–strain, stress–strain and stress–optical properties

Experimental measurements and Monte-Carlo (MC) modelling of the birefringence–strain, stress–strain and stress–optical behaviour of poly(ethylene terephthalate) (PET) are used, together with the analysis of orientation–strain and conformation–strain behaviour reported in Paper I, to give a detailed, quantitative interpretation and characterisation of its deformation-related properties. The difference between the stress–strain and stress–optical behaviour of PET that had been reported previously is confirmed. Except for the stress, the measured values of all the properties studied are in agreement with those calculated using the MC modelling, which suggests that not all of the junctions or the chains in the entangled PET network are elastically active. The results given by Kuhn and Grün theory are compared with those given by the MC modelling. The expected shortcomings of Kuhn and Grün theory are found. However, distinct from the behaviour reported previously for polyethene, the theory can be used to evaluate, semi-empirically, the stress–optical coefficient of PET.

Influence of Nanosilica Particles on Hysteresis and Strain Induced Crystallization of Natural Rubber as Investigated by a Real Time True Stress-True Strain Birefringence System

Abstract In this paper, the influence of nanosilica particles on hysteresis and stress induced crystallization of natural rubber compounds is described. For this purpose a newly developed uniaxial stretching system that monitors birefringence, true stress and true strain was used in real time while stretching and retraction is applied to the nanocomposite sheets. This allowed us to link the continuous structural measurements - as represented by birefringence - and the true mechanical properties during the actual stretching and retraction stages as influenced by the presence of nanosilica particles. The birefringence measurement system was also shown to be quite robust in detecting very low levels of retardation prevalent in rubbers. The stress optical behavior of unfilled NR compounds was compared with those filled with 5 phr and 10 phr nanosilica particles. This study allowed the determination of a critical birefringence value above which strain - induced crystallization occurs in the NR compounds. The results indicate that the addition of nanosilica reduces the stress optical constant of natural rubber compounds. In addition, the critical birefringence, above which the strain - induced crystallization occurs, decreases in the presence of 5 phr nanosilica thus indicating the enhancement of the crystallization process at this loading. However, the nanosilica loading beyond 5phr was not found to be a further benefit to strain - induced crystallization mostly because of the introduction of large surface area nanosilica agglomerates that act as a physical barrier between the chains that also most likely suppresses the crosslinking reaction during the preparation.

Real Time Development of Structure in Partially Molten State Stretching of Polypropylene as Detected by Spectral Birefringence Technique: Effect of Isotacticity

The uniaxial deformation behavior of three polypropylene films with isotacticity indices varying from 43 to 95 was investigated in the partially molten state using real time, true stress, true strain, and birefringence measurement system. These polymers generally exhibit three regime stress‐birefringence behaviors during stretching. The unmelted crystalline regions act as physical junctions. If the films contain crystallinity that is too low (less than ca. 10%) the Regime I disappears. Under these conditions, the films were found to exhibit large linearity in strain and stress optical behavior. If, however, too low a deformation rate is employed, these relationships become non‐linear. The increase of isotacticity increases the magnitude of the birefringence at all deformation levels primarily as a result of the increase in crystallinity that helps in establishing a long‐range physical network. This increase in the long‐range connectivity was found to promote not only the orientation in crystalline domains but also in the amorphous domains. The increased long‐range connectivity in high isotacticity films was also found to result in destruction of local initial structures that generate a large fraction of the oriented amorphous chain regions. This translates into larger stress decrease and birefringence increase changes during the holding stage where the oriented amorphous chains undergo complex relaxation/oriented crystallization processes.

Large Deformation Mechano-Optical and Dynamical Phase Behavior in Uniaxially Stretched Poly(ethylene naphthalate)

Uniaxial deformation behavior of PEN films in the rubbery state is investigated as a function of processing temperature, draw rate, and molecular weight (Mw). Real-time measurement of true stress−true strain and birefringence of the films during the process of stretching is obtained by using a novel instrumented uniaxial stretching machine that couples the spectral birefringence system with the real-time strain detection system. This enables quantitative measurement of true stress−true strain and birefringence, enabling quick and nonstop measurement of stress−optical behavior (SOB) of PEN. A three-regime SOB was obtained for PEN films: (i) regime Istress optical rule applies with a stress optical constant (SOC) of 27.5 GPa-1; following this regime the behavior become nonlinear in (ii) regime II and birefringence rises steeply; finally in (iii) regime III, birefringence reaches a plateau while stress increases. All regimes are affected by the temperature, rate, and Mw. Decreasing temperature and increasing stretching rate and Mw extend the range of regime I over which the material remains amorphous. Regime II and III slopes were found to be dependent on rate and Mw. Measurement of strain rate indicates that affine deformation holds until the end of regime I, rising sharply in regime II, as a result of spontaneous deformation. The strain rate rapidly decreases to zero when regime III starts. The peak in strain rate coincides with the fast increase of birefringence during regime II. According to WAXD and DSC studies, the material remains amorphous in the regime I. Right after the transition to the regime II, the amorphous chains are extremely oriented but possess low translational order, which prevents them from crystallizing. Further stretching the structure evolves very rapidly to a crystalline structure, with approximately 40% crystallinity that is maintained throughout the regime III at the end of which finite chain extensibility prevents further changes in the structure. A model representing the structural mechanisms followed from the amorphous to the crystalline state is suggested. Both rate and temperature have a significant effect on the structural development of PEN: at intermediate to high rates and low temperatures a semicrystalline structure is obtained whereas at very high temperatures and low rates the structure remains completely amorphous despite the large deformation that has been applied. On the basis of the these data, a dynamic phase diagram was constructed.

Role of molten fraction on the structural evolution in stretching and cooling of crosslinked low-density polyethylene: Real-time mechano-optical measurements

We investigated the uniaxial deformation behavior of crosslinked low-density polyethylene in partially and substantially molten states using a real-time true stress–strain birefringence system. The stress–birefringence behavior exhibits a multiregime behavior during stretching and holding process. The details of this regime behavior are primarily governed by the degree of unmelted crystallinity as it has a dominant role in the long-range structural connectivity. When the long-range physical connectivity is present, a three-regime nonlinear stress–optical behavior was observed. When the long-range connectivity is substantially eliminated at higher temperatures, the regime I behavior disappears. Structural studies including cooling process reveal that the lower the proportion of molten material during stretching, the higher the concentration of fibrillar structure and the shorter are the lengths of the kebabs that exhibit twisted lamellae after solidification. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1825–1841, 2005

Experimental characterisation and further molecular modelling of the conformational and elastic behaviour of poly(ethylene terephthalate) network chains

The Monte-Carlo (MC) method developed to model the elastomeric stress–strain behaviour of polyethylene (PE), poly(dimethyl siloxane) (PDMS) and poly(ethylene terephthalate) (PET) networks and the stress-optical behaviour of PE networks is now developed to investigate further the stress–strain behaviour of PET networks. Accurate infrared (IR) spectrometry measurements have been used to determine the populations of gauche conformers in the glycol residues of PET chains in melts. The proportion of gauche states was found to be 76%, consistent with the rotational energy difference of −4.16 kJ mol−1 between trans and gauche states used previously. The greater conformational flexibility of the PET chain compared with the PE chain leads to lower network moduli and smaller deviations from Gaussian and affine network behaviour. Previous results are briefly reviewed and new comparisons of the elastic behaviour of PET and PE chains are made using normalised plots. Subsequent publications will apply the present results to interpreting the measured stress–strain and the stress-optical properties of entangled PET melts.

Experimental characterisation and further molecular modelling of the conformational and elastic behaviour of poly(ethylene terephthalate) network chains

The Monte-Carlo (MC) method developed to model the elastomeric stress–strain behaviour of polyethylene (PE), poly(dimethyl siloxane) (PDMS) and poly(ethylene terephthalate) (PET) networks and the stress-optical behaviour of PE networks is now developed to investigate further the stress–strain behaviour of PET networks. Accurate infrared (IR) spectrometry measurements have been used to determine the populations of gauche conformers in the glycol residues of PET chains in melts. The proportion of gauche states was found to be 76%, consistent with the rotational energy difference of −4.16 kJ mol −1 between trans and gauche states used previously. The greater conformational flexibility of the PET chain compared with the PE chain leads to lower network moduli and smaller deviations from Gaussian and affine network behaviour. Previous results are briefly reviewed and new comparisons of the elastic behaviour of PET and PE chains are made using normalised plots. Subsequent publications will apply the present results to interpreting the measured stress–strain and the stress-optical properties of entangled PET melts.