Orientation effects and cooling stresses in amorphous polymers

This paper deals with two kinds of internal stresses in amorphous polymers. First, the internal stresses connected with molecular orientation are discussed. A framework is presented that describes the interrelations of the various orientation effects (anisotropy, birefringence, etc.) and their dependence on the thermomechanical history of the material. The second part of the paper deals with the residual thermal stresses generated by rapid inhomogeneous cooling through the glass transition range. These so-called cooling stresses are well-known in glass technology. The theory of the quenching of flat glass plates is described, generalized to objects of arbitrary geometry, and shown to be applicable to polymers. The practical importance of cooling stresses is discussed.     

Residual birefringence of amorphous polymers for optical-disk substrates

Based on the Mueller matrix approach, we have successfully designed and constructed a highly sensitive instrument for measuring optical birefringence (LB) by using a polarization modulation method and phase-sensitive technique. Our instrument can measure the retardation of a sample up to 10^?2 radian. Using this instrument, we have measured the residual birefringence of thermosetting resins, CR-39 resin and epoxy resin, for opticaldisk substrates. CR-39 resin results are summarized as follows: (1) Annealing is very effective in reducing LB of samples; (2) samples prepared from raw material 2 (monomer purity, 96%, oligomer, 4%) have smaller LB than those from raw material 1 (monomer purity, 99.9%); and (3) annealed samples prepared from 2 satisfy the requirement on LB to be used as optical-disk substrates. For epoxy resin: (1) “Second curing” largely reduces LB of samples; (2) “slow cooling” is more effective in reducing LB of samples than “fast cooling;” (3) after the second curing, cutting does not induce any residual stress birefringence in samples; and (4) slow-cooled samples have good optical properties to be used as optical disk substrates.     

Residual stresses in polymers and their effect on mechanical behavior

Plates of bisphenol-A polycarbonate and poly(methyl methacrylate) have been quenched in ice water from temperatures slightly above their glass transition temperatures. Residual stresses are thus created, Measurement of these residual stresses has been accomplished by the “layer removal” method and the stress distributions through the thickness are presented. Compressive stresses, approximately 3000 psi, exist at the surface while tensile stresses-of at least 1000 psi exist in the interior. It is shown that these residual stresses can influence the notched Izod impact strengths for polycarbonates. The mechanism is thought to be suppression of craze initiation in advance of the notch due to the presence of residual compressive stresses for specimens notched prior to quenching. In the case of poly(methyl methacrylate), it is shown that compressive residual stresses at the surface can cause plastic yielding to occur in bending experiments resulting in permanent deformation and greater energy absorption.     

Residual stress,aging and fatigue fracture in injection-molded glassy polymers. II. Polycarbonate

Fatigue test have been conducted on notched polycarbonate bars molded using different conditions and having different post-molding conditioning. Fatigue crack propagation rates were not significantly modified by mechanical preconditioning prior to notching, but annealing in air caused an increase in crack growth rate. Annealing in vacuum did not cause the same effect, but specimens annealed in air or in vacuum showed a fracture surface morphology different from that obtained with unannealed bars. Changes in birefringence during cyclic loading and measurements of skin and core densities as a function of aging time were recorded, but a comprehensive interpretation of the fatigue behavior has not yet been achieved.     

Residual stresses in polymers I: The effect of thermal history

The distribution of residual stresses in quenched modified poly(phenylene oxide) (PPO) specimens was investigated. Quenching was carried out from temperature level above T_g to various temperatures below T_g. As expected, compressive stresses were measured at the surface layers while tensile stresses were in the inner layers. The ratio between the tensile and compressive stresses varied, depending on the thermal history. The level of residual surface stresses was found to depend on both the total temperature difference during cooling and the initial specimen temperature. At constant initial temperature the surface stresses are proportional to the total temperature difference, whereas, at constant final temperature the surface stresses are inversely proportional to the total temperature difference. An empirical correlation describing the surface stresses as a function of thermal history has been suggested. The differences between present theories and experimental data are discussed. emphasizing the apparent discrepancy regarding the influence of initial temperature above T_g on the level of residual stresses.     

Residual stresses in polymers. II. Their effect on mechanical behavior

The distribution of density and tensile properties in quenched modified poly(phenylene oxide) specimens was investigated. Quenching was carried out from temperature level above T_g to below T_g temperatures. Simultaneous to buildup of residual stresses, profiles of density and tensile properties were observed. The profiles were obtained using the layer removal technique, which was found not to affect the measured properties. Quenching of the material results in a steep density gradient in the surface layers. Correspondingly, the tensile modulus increases significantly from the surface to the inner layers and so are also the ultimate tensile properties. This behavior could be accounted for neither by the conventional packing volume approach nor by superposition of internal and external stresses. However, observations of the fracture surfaces are very supportive and indicate that the fracture initiation sites are influenced by the residual stresses. Hence, the mechanical behavior is strongly affected by both density and residual stresses profile. Density is the controlling factor in determining the elastic properties whereas residual stresses determine the ultimate strength and fracture mechanism.     

Residual stresses in polymers III: The influence of injection-molding process conditions

Following the evaluation of Residual Stresses (R.S.) in quenched specimens (Part I) and the resulting mechanical-physical properties (Part II), the, present study deals with the effect of injection-molding process conditions on R.S. and the respective properties of amorphous polymers. Melt temperature, mold temperature, injection rate, and injection pressure were the parameters studied. Experimental results indicated that the melt temperature caused two maxima in R.S. The second one reverses from compressive to tensile. In general, most changes occur in the surface regions, while R.S. decreases with increasing melt temperature, as is the case in zones far away from the gate. Furthermore, tensile modulus increased, in general, with rising melt temperature. In the case where the effect of mold temperature was studied, it was found that R.S. are compressive in the surface layers and tend to decrease upon increase in mold temperature and distance from the entrance region. Significant changes in R.S. were also detected in the interior layers. As the mold temperature approached Tg, low values of R.S. were measured, as was the case in quenched specimens. Injection rate affects surface R.S. to a large extent. With low flow rates, tensile stresses were developed in the exterior, reversing to compressive stresses at higher speeds. The reversal in sign depends on the location relative to the gate. Once compressive stresses were formed, further increase in rate caused a reduction in R.S. In addition, variations in tensile modulus, as high as 30 percent, were measured at high injection rates. As far as injection and holding pressures are concerned, experimental results showed that a maximum in R.S. was obtained, with increasing pressure, at the surface. Close to the gate entrance, a reverse from compressive to tensile R.S. was detected at high injection pressures. As in the other cases, injection pressure influenced mostly the exterior layers. Only in zones close to the entrance and at high pressures were high levels of R.S. measured in the core regions.     

Residual stresses in the quasi‐simultaneous laser transmission welding of amorphous thermoplastics

Laser transmission welding is frequently being used increasingly for joining complex, assembly‐oriented components, thanks to its small heat affected zone. As a result of the cooling processes, residual stresses can develop inside the components, potentially leading to stress cracking and premature component failure. One subgoal of process design should therefore be to reduce the level of residual stresses as far as possible when laser welding is employed. This work looks into experimental testing for residual stresses, as a factor for the weld parameters in quasi‐simultaneous laser transmission welding, with the objective of determining the optimum welding parameters. The ARAMIS optical measuring system for deformation analysis was used to this end, in conjunction with a modified hole‐drilling method. POLYM. ENG. SCI., 50:1520–1526, 2010. © 2010 Society of Plastics Engineers     

The influence of residual stresses and orientation on the properties of amorphous polymers

The effect of cold rolling on impact strength of poly(vinyl chloride) (PVC) acrylonitrile-butadiene-styrene (ABS), and an alloy of ABS and polycarbonate (Cycoloy) was studied. The results were presented in terms of Izod impact strength and present oil thickness reduction. The residual stresses molecule orientation and modification of mechanical properties due cold rolling were measured. It was shown that the mechanism of toughness enhancement in PVC, ABS, and Cycoloy different than the one observed in rolled polycarbonates. In the case of a highly localized failure initiation event (a single eraxe), which appears to be present in polycarbonate, the influence of residual stresses, is great, whereas in rubber modified polymers having multiple erasing over a large volume, orientation is more important than residual stress. Hot rolling of poly(methyl methaerylate) holds promise for a continuous process to achieve orientation and toughness enhancement.     

Birefringence of injection-molded glassy polymers during relaxation and recovery

Birefringence measurements have been made on six glassy polymers during stress–relaxation and recovery experiments at temperatures below T_g. Of the six polymers tested only one pair, poly(ethylene terephthalate) and a glassy polyamide (“Trogamid”), show a strong mutual resemblance in the behavior of birefringence under these conditions. The results are discussed with reference to molecular structure, and, although detailed interpretations are not offered, it is observed that the behavior is less complex for polymers which possess their most polarizable groups in the main chain or attached rigidly to it than for those polymers having polarizable side groups with relaxations which do not involve the main chain. All six polymers have been tested in injection-molded form, and the possible consequences of this are considered. The major differences observed to occur between the different materials do not appear to be related to processing, however, and some results obtained using specimens prepared in other ways are also presented to illustrate this.     

Residual stresses in high-temperature ceramic eutectics

This paper explores residual thermal stresses in directionally solidified ceramic eutectics, a class of materials that has much promise for high-temperature structural applications. Residual strain tensors of both phases in a eutectic composite are measured by single crystal X-ray diffraction techniques. In the analysis the material is treated as fully anisotropic and the strain tensors, subsequently converted to stress tensors, are measured. Results are presented for two oxide eutectics, NiO–cubic ZrO₂ and YAG–Al₂O₃, the former having a large thermal expansion mismatch between the two phases and the latter having similar thermal expansion properties. It is discovered that large residual stresses (of the order of 1 GPa) can be present at room temperature in as-processed eutectic materials unless the thermal expansion behaviors of the constituent materials are very similar. Ultimately these measurements not only elucidate the stress state but, when compared to theory, give information about the degree of interfacial constraint between the two phases.     

Calculation of residual stresses in amorphous wires

The residual internal stresses in a cylindrical wire produced in the rotating-water melt spinning process and a coated wire obtained by drawing from a melt have been calculated within the thermal viscoelasticity and structural relaxation theories. The coated wire consists of the core and the sheath with different thermal properties. The problem is considered with allowance made for the generation and the relaxation of stresses in the core and the sheath in the temperature range from initial (corresponding to the liquid state of a two-layer wire) to room temperature. The distributions of the residual stresses have been calculated for the free amorphous metallic wire and the amorphous wire with the sheath having a different elastic modulus and thermal expansion coefficient. The influence of preparation conditions and thermal properties of materials on the calculated parameters is analyzed.     

Calculation of residual stresses in amorphous wires

The residual internal stresses in a cylindrical wire produced in the rotating-water melt spinning process and a coated wire obtained by drawing from a melt have been calculated within the thermal viscoelasticity and structural relaxation theories. The coated wire consists of the core and the sheath with different thermal properties. The problem is considered with allowance made for the generation and the relaxation of stresses in the core and the sheath in the temperature range from initial (corresponding to the liquid state of a two-layer wire) to room temperature. The distributions of the residual stresses have been calculated for the free amorphous metallic wire and the amorphous wire with the sheath having a different elastic modulus and thermal expansion coefficient. The influence of preparation conditions and thermal properties of materials on the calculated parameters is analyzed.     

Residual thermal stresses in injection molded products

Nonisothermal flow of a polymer melt in a cold mold cavity introduces stresses that are partly frozen-in during solidification. Flow-induced stresses cause anisotropy of mechanical, thermal, and optical properties, while the residual thermal stresses induce warpage and stress-cracking. In this study, the influence of the holding stage on the residual thermal stress distribution is investigated. Calculations with a linear viscoelastic constitutive law are compared with experimental results obtained with the layer removal method for specimens of polystyrene (PS) and acrylonitrile butadiene-styrene (ABS). In contrast to slabs cooled at ambient pressures, which show the well-known tensile stresses in the core and compressive stresses at the surfaces, during the holding stage in injection molding, when extra molten polymer is added to the mold to compensate for the shrinkage, tensile stresses may develop at the surface, induced by the pressure during solidification.     

Neutron scattering and amorphous polymers

During the past ten years neutron scattering has become a much more widely used technique. The use of neutron scattering to study the conformation and dynamics of polymer chains in the bulk amorphous state and in solution is reviewed here. The basic theory of neutron scattering is introduced. The types of instruments which are currently used and the factors affecting neutron scattering experiments are discussed. The following sections are each concerned with a different type of scattering experiment and the information which has been obtained. At the beginning of each of these sections the theory relating to the particular topic under discussion is introduced. The three topics covered by this review are: conformation studies of polymers; dynamics of polymer chains and studies of side group motion in polymers.     

The structure of amorphous polymers

Over the past ten years the structure of amorphous polymers has been extensively investigated. Publications on this topic were concerned with various aspects of the amorphous structure and were often based on specific molecular models. This review article attempts to discuss the interrelations between the different structural properties which were studied. The topics covered are the short range positional order, the orientational order, the chain conformation, the supermolecular structure and the relation between molecular and macroscopical properties both of the fluid and the glassy state. The basic theory of the fluid state is introduced at the beginning as well as the experimental techniques currently used to characterize the amorphous structure. Experimental results on different amorphous polymers and oligomers are then discussed and the information is pointed out which can be derived from these data. Finally models on the structure of amorphous polymers are considered briefly.