Plastic deformation kinetics for glassy polymers and blends

Measurements of the plastic deformation kinetics for several glassy (PS, PC, PI-polyimide, PET, epoxy-amine network), semi crystalline polymers (PBT, PET) and blends (ABS, PC:ABS, PC: PBT) were performed for the unidirectional compression loading conditions by using constant temperature deformation calorimetry. The experiments have permitted us to follow the changes of the mechanical work (A), the heat of deformation (Q) and differences between these quantities, i.e., internal energy (U) stored in samples during their loading and unloading. Experiments have shown that the large portion (45–85%) of the mechanical work of deformation (A) is converted to heat (Q). The rest ofA is converted to internal energy (U) stored in deformed samples. U is quite high as compared with metals [1,2]. After complete unloading of plastically deformed samples, i.e., samples carrying irreversible atT _def plastic deformation ( _irr ), some amount (U) of stored energy disappeared. The amount of (U and (U) are different for different polymers. All data are analyzed in the framework of the model proposed in [3,4]. The experiments support the deformation model where the plasticity of glassy polymers is the process of nucleation and development of so-called PDs-plastic local shear defects of nonconformational and nondilatational nature.Dedicated to Prof. Dr. W. Pechhold on the occasion of his 60th birthday     

Effect of anisotropic deformation on the differential scanning calorimetry response of polymer glasses

Large anisotropic deformation affects the physical state of a polymer glass, where the changes in the state of material are revealed by performing a differential scanning calorimetry (DSC) experiment. Previously, the deformation was applied to polymers well below their glass transition temperatures, and it was found that uniaxial compressive loading–unloading resulted in a broad exothermic peak on the DSC trace. Here we report on the effect on the subsequent DSC response of a deformation experiment performed in uniaxial extension on a ductile 50:50 co-polymer poly(BMA-co-MMA) (PBMA/MMA). The deformation of up to 80% strain was applied at T_g − 30°C and T_g − 40°C, that is, closer to T_g than in the previous work. Unlike in the well below T_g deformation case, the DSC trace contains an endothermic peak followed by an exothermic peak. The magnitude of the endothermic peak as well as the asymptotic glassy heat capacity increase with the amount of mechanical work performed during the deformation cycle.     

Review of polymer oxidation and its relationship with materials performance and lifetime prediction

All polymers are intrinsically susceptible to oxidation, which is the underlying process for thermally driven materials degradation and of concern in various applications. There are many approaches for predicting oxidative polymer degradation. Aging studies usually are meant to accelerate oxidation chemistry for predictive purposes. Kinetic models attempt to describe reaction mechanisms and derive rate constants, whereas rapid qualification tests should provide confidence for extended performance during application, and similarly TGA tests are meant to provide rapid guidance for thermal degradation features. What are the underlying commonalities or diverging trends and complications when we approach thermo-oxidative aging of polymers in such different ways? This review presents a brief status report on the important aspects of polymer oxidation and focuses on the complexity of thermally accelerated polymer aging phenomena. Thermal aging and lifetime prediction, the importance of DLO, property correlations, kinetic models, TGA approaches, and a framework for predictive aging models are briefly discussed. An overall perspective is provided showing the challenges associated with our understanding of polymer oxidation as it relates to lifetime prediction requirements.     

Photoelasticity of textured heterogeneous polymer materials

The photoelastic properties of textured polymer composites are considered for the high deformation limit. Photoelasticity equations for incompressible polymers are derived in the correlation function theory approximation. It is shown that the birefringence may be zero for critical stretching across the texture axis. This effect can be used to measure the dispersion of shear moduli of polymer composites. © 1993 John Wiley & Sons, Inc.     

In-line monitoring of droplets deformation and recovering and polymer degradation during extrusion

An extruder can be operated as a torque rheometer by setting an external control of the processing variables and adding an in-line optical detector and an on-off mechanical valve at the extruder die exit. Various operational modes can be used including constant, ramp and sinusoidal changes of the die-head pressure. With the valve closed, a fixed amount of polymer is added and the extruder put into operation, controlling the screw rotation speed via software, having a proportional/integral/derivative controller. Polymer degradation can be followed recording changes in barrel pressure and torque. After processing, the valve is opened and the molten polymer discharged under a controlled die-head pressure, manipulating again the screw rotation speed. The polymer mixture morphology can be scanned during the discharge of the melt flow by the in-line turbidimeter, showing the deformation/recovery of the second phase droplets.     

Characterisation and effect of polymer network deformation in reverse-mode polymer-stabilised cholesteric texture

In reverse-mode polymer-stabilised cholesteric texture (PSCT), the dynamic response is derived from local director reorientation governed by dielectric coupling effect/self-assembly and polymer network deformation. A double exponential rise/decay model is proposed to investigate the underlying physical mechanisms. Through simulation of the transient rise and decay processes, the polymer network deformation in PSCT can be quantitatively evaluated. Less deformation and faster restoration speed of the polymer network can suppress hysteresis. These results provide useful guidelines for future PSCT fabrication and performance optimisation.     

Degradation issues of polymer materials used in railway field

Polymer materials used in railway field are degraded by environmental factors such as thermal, oxidative, photolytic, hydrolytic, and mechanical. The expected service life of the polymer materials used in railway field is approximately 20 years that is relatively long period for the polymer material; therefore, respective degradation factors should be well considered. Some of the degradation conditions indicate similar mechanism. The oxidative reaction was seen in every degradation conditions under air atmosphere. The hydrolytic reaction was mainly observed in chemical and biological degradations. The degradation behavior of the polymer material was analyzed by various methods. FTIR, thermal analysis (TG, DSC), and molecular weight determination were mainly applied for its purpose. However, the degradation mechanism of practical products made of polymer material was insufficiently studied and the exchange criteria of the products depended on the visual inspection without the suitable degradation analysis not only in the railway field but also in other commercial and industrial fields. In addition, most of the methods to analyze degradation of polymer material are performed on the standard specimen forms. For the installed product, some kinds of damage are generated through the sample collection process; therefore, the damaged products have to be exchanged for brand new ones or repaired totally to be used for more period. Moreover, it is hard to suspend the railway service for the degradation analysis of polymer products. From these backgrounds, the prospect of degradation analysis related to polymer materials used in railway field was proposed.     

Regulation and control of polymer network deformation in reverse-mode polymer-stabilised cholesteric texture

By simulating the dynamic response processes of reverse-mode polymer-stabilised cholesteric texture (PSCT) via double exponential rise/decay model, the deformation of polymer networks were quantitatively characterised. The polymer network deformation is shown to be controlled by regulating monomer concentration and the applied electric field. Increased monomer concentration facilitates the production of polymer fibrils of a higher polymer molecular weight while suppressing distortion of the polymer network. Decreased polymer network deformation assists in the decrease of the undesired hysteresis phenomenon while also demonstrating a shorter field-off response time of PSCT; however, the trade-off is increased operation voltage and decreased contrast ratio. The observed results provide useful guidelines for future PSCT material and device optimisations.     

Thermodynamics of the deformation of segmented polyurethanes with various percentages of hard block I. The initial deformation

The thermodynamic analysis of the uniaxial stretching of the polybutadiene polyurethanes of various compositions and mechanical history was carried out using deformation calorimetry. The initial small strain deformation was found to result from the volume elasticity of the hard phase. Samples containing up to 50% of the hard block behave like typical thermoelastoplastics, which are characterized by the plastic-to-rubber transition and large reversible deformations. More rigid samples behave similar to typical solid crystalline polymers aboveT _g.It is great pleasure for us to dedicate this paper to Prof. Dr. H.-G. Kilian on the occasion of his 60th birthday.     

Thermal analysis techniques for characterization of polymer materials

Routine DSC and TGA techniques, used to characterise polymer thermal stability, have been further used for assessment of comparative thermal stability of various polymer materials and for prediction of material lifetimes. The following materials were investigated: (1) commercial and experimental polymer materials - results for poly(vinyl chloride) (PVC) and bisphenol A polycarbonate (PC) are presented; (2) a polydimethylsiloxane-polytetrafluoroethylene (SIL-PTFE) coating system; and (3) commercially available linear low density polyethylene (PE-LLD), unmodified and modified chemically and physically. The plot of reciprocal temperature of initial decomposition 1/T_di vs log heating rate β has been recommended for assessment of comparative thermal stability. The lifetime of polymer materials was calculated from the plots of log time-to-failure, log t_f, vs reciprocal temperature 1/T, where t_f values were obtained using T_di from TGA measurements or directly from the oxidation induction time (OIT) data as criteria for initial deterioration of polymer thermal stability. The following sequences of increasing thermal stability were found for investigated materials:

(1)

PVC ? PC;

(2)

SIL < SIL-PTFE 20% < SIL-PTFE 50% ? PTFE;

(3)

(B) PE-LLD, grafted < (A) PE-LLD, unmodified < (C) PE-LLD, filled.

The lifetime of polymer materials predicted from the plots of log t_f vs 1/T are in reasonable agreement with experimental data and users' observations, e.g. approximately 1 year for PC and unmodified PE-LLD both at 373 K (100 °C) and for PVC at temperature of outdoor conditions about 298 K (25 °C).     

Polymer–polymer interactions via analog calorimetry. 3. Interaction between styrene and acrylonitrile repeat units

Analog calorimetry is used to study the interaction between styrene and acrylonitrile repeat units. Electrostatic charge calculations were used as a guide to divide the polymer repeat units and analogs into groups. A mean-field binary interaction model was used to evaluate group interaction energies. The enthalpic interaction energy for the styrene-acrylonitrile pair from this study is 7.63 ± 0.12 cal/cm³ which is consistent with values obtained from phase behavior studies of poly(styrene-co-acrylonitrile) blends. The cyano group, C(TRIPLE BOND)N, of the acrylonitrile repeat unit has a permanent dipole. The results of this study suggest that the orientation of this dipole with respect to the backbone of the acrylonitrile unit strongly affects its interaction with styrene repeat unit. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 831–839, 1997     

Polymer–polymer interactions via analog calorimetry. 2. Blends of polystyrene with bisphenol-A polycarbonate and tetramethyl bisphenol-A polycarbonate

Analog calorimetry is used as a tool to study the interaction of polystyrene, PS, with bisphenol-A polycarbonate, PC, and with tetramethyl bisphenol-A polycarbonate, TMPC. Electrostatic charge calculations were used as a guide to divide polymer repeat units and analogs into groups. A mean-field binary interaction model was used to evaluate group interaction energies. The enthalpic interaction energy obtained from this study for the PS-PC pair is 0.41 ± 0.13 cal/cm³ while that for the PS-TMPC pair is 0.19 ± 0.34 cal/cm³. The result for PS-PC blends is in good agreement with values obtained from studies using the critical molecular weight approach and the phase behavior of copolymer blends. The value for PS-TMPC does not correctly predict the phase behavior of this blend; however, its standard deviation (on both an absolute and relative basis) is large and the range of possible interaction energies includes the negative values obtained from neutron scattering. The results of this study indicate that the presence of methyl groups on the aromatic ring of TMPC repeat unit is the main factor favoring the miscibility of PS-TMPC blends. © 1997 John Wiley & Sons, Inc.     

Fluorescence polarization studies of multicomponent polymer materials — anthracene labelled non-aqueous dispersions

Fluorescence polarization measurements have been carried out in hydrocarbon solvents on non-aqueous colloidal polymer dispersions labelled with anthracene [A] groups. The polymer particles are composed (dry volume) of 9% polyisobutylene and 91% poly(methyl methacrylate) [PMMA]. The A groups were introduced during particle synthesis using 9-anthrylmethyl methacrylate as a comonomer with MMA. The extent of polarization decreases with increasing A content of the PMMA phase and increasing temperature increases the steady state fluorescence polarization. These results can be explained in terms of immobile A groups which are sufficiently close that some energy transfer [A^*+AA+A^*] contributes to fluorescence depolarization.     

Relationship between heat of immersion and surface Gibbs energy of fluorite and cassiterite

Calorimetric measurements were made of the heat of immersion in water of cassiterite that was either untreated or treated with 60% HNO₃. The heats of immersion of cassiterite and fluorite were also calculated theoretically from the surface Gibbs energy components, and compared with the heat of immersion measured for cassiterite and that taken from the literature for fluorite. The results of the measurements and calculation revealed that the heat of immersion depends on the degree of hydration of the surface of cassiterite and fluorite. It was also found that it is possible to predict the heats of immersion in water of cassiterite and fluorite from the Lifshitz-van der Waals and acid-base components of the surface Gibbs energy.     

Essential work of fracture,crack tip opening displacement,and J-integral relationship for a ductile polymer film

A unique set of double-edge notched tension specimens of a Polyethylene Terephthalate Glycol-modified film was tested in mode I, plane stress. The load was registered on a universal testing machine. The displacements, ligament lengths, and video frames were recorded by a Digital Image Correlation system. With these registered data, the essential work of fracture, J-integral, and crack tip opening displacement (CTOD) fracture concepts have been applied. The onset of crack initiation was through a complete yielded ligament. The analysis showed that the intrinsic specific work of fracture, w_e, is the specific energy just up to crack initiation, which is an initiation value. w_e has both a coincident value and the same conceptual meaning as J_o, the J-integral at the onset of crack initiation. The relationship between J_o and CTOD is also determined. The influence on the notch quality when the specimens were sharpened by two different procedures, femtosecond laser ablation and razor blade sliding, was analysed in detail.     

Influence of deformation and chemical structure on elementary free volumes in glassy polymers

In the scope of the mechanism of positronium formation in polymers before trapping by elementary free volumes (EFV), we demonstrate an approach to quantitative analyses of experimental results. Correspondingly, a relation between the EFV concentration and annihilation characteristics (lifetimes and intensities) is derived. We use PATFIT and CONTIN analyses of positron annihilation lifetime spectra and obtain information on some heterogeneity of the structure of glassy polymers and on the mechanism of the plastic flow of glassy material.     

Solution calorimetry to monitor swelling and dissolution of polymers and polymer blends

The observed rate of drug release from a polymeric drug delivery system is governed by a combination of diffusion, swelling and erosion. It is thus not a simple task to determine the effects of the polymer on the observed drug release rate, because the swelling characteristics of the polymer are inferred from the drug release profile. Here we propose to use solution calorimetry to monitor swelling. Powdered polymer samples (HPMC E4M, K4M, K15M and NaCMC, both alone and in a blend) were dispersed into water or buffer (pH 2.2 and 6.8 McIlvaine citrate buffers) in a calorimeter and the heat associated with the swelling phenomena (hydration, swelling, gelation and dissolution) was recorded. Plots of normalised cumulative heat (i.e. q_t/Q, where q_t is the heat released up to time t and Q the total amount of heat released) versus time were analysed by the power law model, in which a fitting parameter, n, imparts information on the mechanism of swelling.

For all systems the values of n were greater than 1, which indicated that dissolution occurred immediately following hydration of the polymer. However, while not suitable for determining reaction mechanism, the values of n for each polymer were significantly different and, moreover, were observed to vary both as a function of particle size and dissolution medium pH. Thus, the values of n may serve as comparative parameters. Properties of the polymer blends were observed to be different from those of either constituent and correlated with the behaviour seen for polymer tablets during dissolution experiments. The data imply that solution calorimetry could be used to construct quantitative structure–activity relationships (QSARs) and hence to optimise selection of polymer blends for specific applications.     

聚合物光伏材料与器件研究进展

高效率的聚合物太阳电池依赖于光吸收活性层材料对太阳光能量的充分利用．电极界面材料将光吸收活性层产生的空穴和电子分别快速高效地抽取到阳极和阴极,并通过进一步改进光伏器件的结构提升能量转换效率和稳定性．本课题组在光吸收活性层中新型聚合物给体材料、新型电极界面材料、利用水／醇性电极界面材料制作新型倒装器件结构的太阳电池方面取得重要进展,推动了太阳电池在能量转换效率和稳定性方面的突破．     

The fracture testing of ductile polymer films: Effect of the specimen notching

The fracture of a ductile polymer film, a heterophase ethylene-propylene block copolymer, has been studied, combining a range of characterisation methods in an attempt to provide a better understanding of the intricate details that play an important role in the repeatability and reproducibility of the essential work of fracture test. The experimental factors that have a strong influence on the resulting parameters are clearly explained, with particular attention to the effect of the quality of the notches, the non-collinearity of the two edge notches in double edge notched tension specimens, and the lack of alignment of the specimen with the load axis once it is mounted on the load train. Furthermore, the influence of these experimental factors on the registered stress-displacement curves is also studied, and a criterion and the method for separating non-valid specimens are established.