Physical aging in poly(methyl methacrylate)poly(styrene-co-acrylonitrile) blends. Part II: Enthalpy relaxation

An investigation of the effect of physical aging on excess enthalpy of compatible polymer blends was carried out. Poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (SAN) were chosen for this study. Blends of different ratios of PMMA and SAN were physically aged at different times and temperatures below their glass transition (T_g) and then subjected to enthalpy relaxation measurement in a differential scanning calorimeter (DSC). An improved procedure was developed and, employed to analyze the data. The error associated with the calculation of the normalized deviation in enthalpy, known as the “Φ” function, was below 4%. The relaxation was observed to proceed faster at higher aging temperature. It was also found that at higher aging temperatures of T_g – 20 and T_g– 35°C, enthalpy relaxation in SAN-rich blends proceeds faster than in PMMA rich blends, while at the low aging temperature of T_g– 50°C the rate of relaxation becomes independent of the composition.     

Enthalpy relaxation in polymeric glasses

Constitutive equations are derived for enthalpy recovery in a glassy polymer after quench from above the glass transition temperature T_g to a temperature T in the sub‐T_g region. The model is based on the trapping concept, which treats a disodered medium as an ensemble of cooperatively rearranging regions (CRR) hopping in potential wells as they are thermally activated. Rearrangement occurs when a CRR reaches some liquid‐like energy level in a hop. The rate of hops is described by the theory of thermally activated processes, whereas the probability to change trap in a hop is determined by the difference between the current and equilibrium concentrations of cages. A nonlinear parabolic equation is developed for the distribution of traps. This equation ia used to describe entropy recivery in amorphous and semicrystalline polymers. Fair agreement is demonstrated between experimental data for poly(ether imide), poly(ethyl terephthalate) and polystyrene and results of numerical simulation. Some phenomenological relations are suggested to predict the effect of temperature, molar mass and degree of crystallinity on material parameters.     

Enthalpy relaxation in glassy polymers

Summary Constitutive equations are derived for enthalpy recovery in amorphous glassy polymers. The model is based on the concept of cooperative relaxation which treats a polymer as an ensemble of flow units rearranging at random times. The rate of rearrangement is determined by the Eyring theory of thermally activated processes. Fair agreement is demonstrated between results of numerical simulation and experimental data for polycarbonate, poly(ether imide), poly(methyl methacrylate), polystyrene and a glycerol glass. Received: 5 December 1999/Revised version: 28 February 2000/Accepted: 2 March 2000     

Enthalpy relaxation of filled copolymers

Excess enthalpy following annealing at various periods of time at T_g ?10°C was measured by differential scanning calorimetry of electrophotographic toners, copolymers of styrene and butyl methacrylate containing carbon black. An approximate equilibrium enthalpy relaxation after annealing the pure copolymers for one month was 3.30 kJ/kg for a copolymer with 66.5% styrene and 2.72 kJ/kg for a copolymer containing 49.8% styrene. The rate of enthalpy relaxation was reduced by increasing the styrene content of the pure copolymer. The incorporation of a carbon black with high surface area reduces the rate of enthalpy relaxation, increasing in effectiveness with butyl methacrylate concentration.     

Fracture behavior of rubber-modified thermoplastics after aging

Experiments show that the effects of outdoor aging on rubber-modified thermoplastics can be reproduced by laminating a layer of a glassy polymer onto the surface of unaged specimens. This technique is used to study the effects of fracture temperature, specimen geometry, and polymer composition on the impact strength of aged HIPS and ABS. Aging reduces the energy of crack initiation, so that the impact strength is determined by the crack-propagation energy, which is in turn governed by the nature and concentration of the rubber and by the fracture temperature.     

Molecular weight dependence in a relaxation phenomenon at glassy state: Physical aging of polycarbonate

At the glassy state of polymers, the reptation-type molecular motion should be frozen-in and the relaxation should be only by local motions, such as rotation, vibration, and torsion. Such local motions would not depend on molecular weight. This may be current understanding of the polymer glass. By contrast, we found that the rate of enthalpy relaxation in polycarbonate at 100 °C (50 °C below T_g) clearly depends on molecular weight. Deterioration of impact strength was also found to depend on molecular weight. The surprising results are presented in this letter.     

Physical aging in polystyrene: Comparison of the changes in creep behavior with the enthalpy relaxation

Physical aging in polystyrene was studied by annealing samples isothermally (at 22°, 57°, and 93°C) for various lengths of time and measuring the changes in the enthalpy and the flexural creep property. The enthalpy decrease reached a constant value after about 10 hours of aging at 93°C, indicating an apparent attainment of equilibrium, whereas the creep behavior continued to change with prolonged aging even after 10 hours. At room temperature it took about a year of aging to induce a measurable change in enthalpy, while the creep behavior showed changes clearly only after hours of aging. These and other items of evidence indicate that the effect of aging on the creep behavior cannot be interpreted solely in terms of the free volume concept. Thus the hope of being able to predict the mechanical properties of aged polymers from the measurement of specific volume or enthalpy alone is not likely to be realized.     

Enthalpy recovery and structural relaxation in layered glassy polymer films

Recent studies of physical aging in confined polymer glasses have revealed that aging behavior in confinement often differs from bulk behavior. This study used DSC to characterize physical aging and structural relaxation in bulk polysulfone (PSF) and co-extruded multilayered films of PSF and an olefin block copolymer (OBC) that have average PSF layer thicknesses of 640 nm, 260 nm, and 185 nm. The films were aged isothermally at 170 °C, and the recovered enthalpy upon reheating was measured over time. The films with 640 nm and 260 nm PSF layers had aging rates very similar to that of bulk PSF, while the film with 185 nm PSF layers had an aging rate slightly greater than the bulk value. The cooling rate dependence of the limiting fictive temperature (T_f′) in multilayered and bulk PSF samples was also characterized. Values of T_f′ were similar for all films at each cooling rate. The results of this work are in general agreement with our previous gas permeation aging study of multilayered PSF films aged at 35 °C, in which the effect of layer thickness on aging behavior was minimal. This stands in contrast to studies with thin, freestanding PSF films, which exhibit accelerated aging relative to bulk and have aging rates that depend strongly on film thickness.     

Enthalpy relaxation studies in isotactic polystyrene. Effects of crystallinity

Summary The enthalpy relaxation of the amorphus isotactic polystyrene is strongly affected from the crystalline phase induced by annealing at temperatures between Tg and Tm. All the parameters describing the relaxation process, H, T_max and T_ons depend also on the above T_g annealing conditions as the induced crystallinity alters the quantity and the quality (i.e.Tg, T_gons, T_g) of the remaining amorphous phase.     

Enthalpy relaxation in poly(ethylene terephthalate) and related polyesters

Enthalpic relaxation data are presented on poly(ethylene terephthalate), poly(ethylene naphthalate) and their copolymers. Analysis of these data allows the determination of the amount of energy absorbed at the glass transition, Q_t, and the location of the enthalpic recovery peak, T_max, as a function of the time of ageing of the samples. Ageing measurements were carried out for periods of up to 2016 h and at temperatures between 40 °C and 110 °C, depending upon the chemical composition of the system being investigated. The enthalpic relaxation rates are influenced by the chemical structure and reflect the effects of local order pinning the chains and influencing the rate of enthalpic recovery. © 2000 Society of Chemical Industry     

Enthalpy recovery and physical aging of polymer-diluent binary systems: A network epoxy and water

Physical aging of a network epoxy resin in both the dry and water plasticized state has been investigated using calorimetric methods. Enthalpic retardation (recovery) towards equilibrium below the glass transition temperature (TK) has been qualitatively and quantitatively evaluated over a wide temperature range by following the enthalpy recovered (Er) on re-heating through to and beyond the glass transition. Results indicate that water, acting as a plasticizer, accelerates aging at any given temperature. However, by using the respective Tg as a point of reference, a simple comparison of the kinetics of aging in the dry and plasticized state suggests that in the temperature range studied there are no significant differences. These observations, together with observed dilational and relaxational effects of water which appears to erase all accumulated aging effects in dry aged material, are commented upon and their importance in the' context of transport-phenomena and moisture sorption-desorption cycles are discussed.     

Enthalpy relaxation studies in polymethyl methacrylate networks with different crosslinking degrees

Structural relaxation of PMMA networks with distinct crosslink density has been studied by differential scanning calorimetry (DSC). The crosslinking agent used was ethylene glycol dimethacrylate (EGDMA). The experiments were carried out on heating after the samples have been subjected to distinct thermal histories, namely isothermal stages at different temperatures below the glass transition temperature for distinct times and cooling at different rates. These studies revealed a broadening of the glass transition with increasing crosslinking degree due to the constraints imposed by the crosslinks and suggested the presence of crosslink heterogeneity in the networks. A phenomenological model based on the configurational entropy concept was used to simulate the structural relaxation phenomenon and to evaluate the temperature dependence and distribution of the relaxation times of the conformational rearrangements for these networks. The agreement between the experimental results and the simulated thermograms was quite satisfactory.In addition, the kinetic fragility of the networks was evaluated from the results corresponding to the thermal treatments at distinct cooling rates. It was found an increase of the fragility index m with increasing crosslinking degree.     

Enthalpy relaxation of photopolymerized multilayered thiol‐ene films

Multilayered thiol‐ene network films with two and three different components were fabricated by spin coating and photopolymerization. The distinctive glass transition temperatures of each layer component were observed at corresponding glass transition regions of each bulk sample. Sub‐T_g aging of 10‐, 21‐, and 32‐layered thiol‐ene films was investigated in terms of enthalpy relaxation. Enthalpy relaxation of each layer component occurred independently and presented the characteristic time and temperature dependency. Overlapped unsymmetrical bell‐shaped enthalpy relaxation distribution having peak maximum at T_g‐10°C of each layer component was observed, resulting in broad distribution of enthalpy relaxation over wide temperature range. In addition, enthalpy relaxation of each layer component in the multilayered thiol‐ene films was significantly accelerated comparing to that of bulk thiol‐ene samples. Dynamic mechanical thermal properties of multilayered thiol‐ene films also showed two and three separated glass transition temperature. However, for 32‐layered thiol‐ene film consisting of three different layer components, glass transition and damping region are overlapped and the width is extended more than 100°C. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Physical aging in isotactic polypropylene

Physical aging studies were made using commercial and laboratory samples of isotactic polypropylene. Linear and nonlinear viscoelastic responses were measured after quenching the glass from above the glass transition temperatures to below the glass transition. Results show that aging is not eliminated by large mechanical deformations; rather, the time required for the glass to age into equilibrium is independent of the applied stress. Results obtained are purely kinetic and interpreted as the effects accompanying the process of glass formation in a semicrystalline polymer. © 1996 John Wiley & Sons, Inc.     

Physical properties of some injection molded thermoplastics of various glass contents

This paper discusses the influence of fiber-glass reinforcement content on the physical properties of four theroplastic injection molded materials. In addition, the effects of binder difference (i.e., thermoplastic vs. thermosetting), associated with the reinforcement, are described. Several properties that were expected to show response to glass content and binder differences have been examined. Among these are short- and long-term tensile strength, flexural properties, compressive resistance, impact strength, deflection temperature under load (DTL), and environmental stress cracking. This work has demonstrated that fiber-glass reinforcement of the particular thermoplastics described in this paper has led to substantial improvements in both the stiffness and strength characteristics of the base resins. The responses to binder chemistry difference was found to be slight in many instances for the resins, as reflected by the tests described herein. However, the long-term tensile, impact, DTL, and stress-cracking tests have shown considerable sensitivity to binder change in some cases, especially with respect to the polypropylene, SAN and polyamide plastics. The particular response to a given binder system, however, appears to vary both with the material under test and the property used to detect such response.     

Physical and chemical stress relaxation of elastomers

In this paper we have developed a method whereby physical and chemical relaxation processes can be distinguished, using stress relaxation experiments as a function of temperature. We assumed that there exists some temperature range above the glass transition temperature over which the chemical effects can be neglected for the time scale of the experiments. The data in this low temperature range were then used to determine the WLF constants and other physical relaxation parameters. The physical component of the stress relaxation could then be subtracted from high temperature experiments in order to extract chemical kinetic information. Based on certain reasonable assumptions, an equation was developed for the relaxation modulus of a chemically reacting system. This equation could be used to determine the time dependence of the crosslink density, or conversely could be used to predict the long term relaxation modulus from an assumed kinetic mechanism. These calculations were demonstrated for ethylene propylene and butyl elastomers.     

酚酞基聚芳醚砜和聚乙烯的共混研究

以聚乙烯接枝聚乙二醇(PE—g—PEO)为增容剂并用于酚酞基聚芳醚砜(PES—C)与聚乙烯(PE)共混体系。用力学性能测定等方法研究了增容剂用量、PEO加入量对PES—C与PE共混物拉伸强度的影响。结果表明.一定量的增容剂可以改善:PES—C与PE的相容性并提高PE的拉伸强度;同时.加入少量聚乙二醇(PEO).可以通过改善PES—C的加工熔融性,提高PES—C和PE的相容性,从而改善共混物的力学性能。     

Physical aging of nylon 66

The physical aging behavior of nylon 66 as a function of aging temperature, magnitude of applied stress, and aging time has been investigated. Creep tests were performed for samples quenched from a stabilization temperature of 160°C to aging temperatures between 41°C and 65°C, for aging times ranging from 0.5 to 64 h, and at stresses levels from 4 MPa to 24 MPa. Volume recovery was investigated for samples quenched from 160°C to aging temperatures of 41°C and 65°C. The creep compliance curves at different aging times were able to be superimposed. The double logarithmic shift rate was calculated and its dependence on aging temperature and applied stress was determined. Also, it was found that the volume recovery rate depends on the aging temperature. The time-temperature and stress-temperature superpositions were not possible for our data.