摘要:

AbstractThis study clarifies and quantifies factors which increase the ductility of a low‐molecular‐weight propylene homopolymer having an intrinsic viscosity of 0.89 dl/g. The tensile behavior of homopolymer/ethylene‐propylene rubber (EPR) blends was studied from the viewpoint of the associated molecular structure of EPR and its compatibility with the homopolymer. When EPR is “dissolved” in a homopolymer, the glass transition temperature (Tg) of the amorphous phase of a homopolymer was found to shift to a lower temperature, with homopolymer/EPR compatibility being subsequently evaluated using this shift, i.e., Δtg. Results show two conditions are required to improve the ductility of the low‐molecular weight propylene homopolymer: ΔTgmust be ≥ 3°C and ≥ 30 wt% EPR must be blended wi

ISSN:0032-3888    

DOI:10.1002/pen.760351602

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractIn the present study of resin transfer molding, an effective gap located between the side wall and the fiber mats is proposed. The concept of the effective gap leads to a permeable boundary condition on the interface between the fiber mats and the side wall. The porosity is no longer assumed to be a constant in the whole domain. Results show that the effective gap concept can explain the phenomena of edge effects reasonably. In our model, the filling process is simulated by the body‐fitted finite element method and the limitation of the time increment for the quasi‐steady state process is discussed in det

ISSN:0032-3888    

DOI:10.1002/pen.760351603

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractThe creep behavior of polymers depends on the physical age of the material at the time of stress application. Creep curves shift to longer times for more highly aged material and, in poly(vinyl chloride), (PVC), this can be modeled by an empirical equation in which the magnitude of an effective mean retardation time parameter is dependent upon, and increases with, the age of the polymer. Creep compliances for PVC also depend on the magnitude of the applied stress when this exceeds about 4 MPa. This nonlinear behavior is caused, at short creep times, by a reduction in the value for the retardation time parameter on application of the creep stress. Specimens appear therefore to be initially de‐aged by elevated stresses. Subsequently, this parameter increases with creep time implying that physical aging has been reactivated, but the rate of increase also depends on the stress level. These influences of elevated stresses can be described by an extension of the creep model, and parametric expressions have been derived which relate creep compliance values to time, stress, and the age of the polymer. It is shown how the parameters can be determined from a short series of creep experiments and thus how creep deformations can be calculated over wide ranges of time, stress, and ag

ISSN:0032-3888    

DOI:10.1002/pen.760351604

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractSeveral important polymerizations [e.g., poly(methyl methacrylate) (PMMA), polystyrene] exhibit gel and glass effects during polymerization. These are associated with the decrease of the diffusivities of the macroradicals and monomer molecules with increasing viscosities of the reaction mass. A new model has been developed to account for the gel and glass effects, both under batch and semibatch reactor operations. The free volume theory of Vrentas and Duda has been used to account for the changes in the diffusion coefficients. The model parameters are tuned using experimental data on the isothermal bulk and solution polymerizations of MMA in batch reactors. Conversion histories and molecular weights have been obtained for cases where solvent, initiator or monomer are added to (or removed from) the reaction mass after some reaction has taken place. In addition, studies involving step changes in temperature hav been carried out. It is found that the conversionvs.time behavior after such step changes depend upon a complex interplay of three factors: Polymer concentration, molecular weight, and the temperature at the point these changes are effected.

ISSN:0032-3888    

DOI:10.1002/pen.760351605

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractThe computer aided design approach used in current applications of semicrystalline polyoxymethylene (POM) requires high strain‐rate mechanical data. The primary aim of this work has been to measure the room temperature modulus and tensile strength of injection molded samples of POM of different molecular weights at cross‐head speeds of between 10−5ms−1. We observe no major transition in bulk mechanical behavior in this range of test speeds, the Young's modulusE, in particular, showing little strain rate dependence. This is rationalized on the basis of tensile tests over a range of temperatures, these indicating room temperature to correspond to the plateau in theE(T) curves (Tgfor these materials is taken to be −70°C, and the DSC melting onset occurs at ∼ 170°C).The tensile strength increases as ∼log(dϵ/dt) and the behavior is found to be highly nonlinear, strains to fail of the order of 1 being observed even at the highest strain rates, depending on the molecular weight. It is believed that the yield stress of th crystalline regions determines the tensile strength aboveTg, the higher degree of crystallinity associated with lower molecular weights resulting in a slightly higher tensile strength. Nevertheless, failure is qualitatively brittle, with no necking and relatively little permanent deformation. This behavior is discussed in terms of morphological investigations of the fractured samples by optical and scanning electron microscopy (SEM).In attempting to relate ultimate failure to the molecular/crystalline structure of the samples, measurements of the critical stress intensity for crack initiation in mode I opening,KIC, as a function of crystallization temperatureTchave been carried out using compact tension specimens machined from injection molded and compression molded plaques.KICincreases with molecular weight and decreases withTcat low test speeds (in spite of an increase in crystallinity withTc). This is accounted for in terms of a crack shielding model for crack initiation and of molecular rearrangements occurring during crystallization which lead to a decrease in the effective entanglement density withTc. The implications of this model are then compared withKICresults over a range of cross‐head speed

ISSN:0032-3888    

DOI:10.1002/pen.760351606

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractIn this study we used continuous oxidizing equipment to produce carbon fiber. The results show when the stretch of the fiber during continuous oxidation is increased, the orientation and modulus of resultant carbon fiber is unexpectedly decreased. This effect is much different from other published results. In this case, the stretching occurs after the cyclization of the molecular chains in the amorphous region. The stretching leads to the scission of some cyclized molecular chains in the amorphous region and the shrinkage of the crystalline region.

ISSN:0032-3888    

DOI:10.1002/pen.760351607

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractThe network formation process for bisphenol A‐based cyanate ester resin was examined using size exclusion chromatography (SEC) and differential scanning calorimetry (DSC). A kinetic scheme based on the SEC data was used to predict the species distribution in the reaction medium in the pregel region. After gelation, the molecular species undergo retroversion as the high molecular species are predominantly attached to the growing network structur

ISSN:0032-3888    

DOI:10.1002/pen.760351608

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractPolyetherimides (PEI) based on dianhydrides of 3,3′, 4,4′‐dipheniloxytetracarboxylic acid and 1,3‐bis‐(3,4‐di‐carboxyphenoxy)benzene that contain fragments of such thermoplastic polymers as poly(ether sulfone) or poly(phenylene oxide) have been synthesized. Thermoplastic properties of synthesized PEI were characterized by flow temperatureTfand melt viscosity ηm. The properties obtained are outstanding for the production of PEI as fusible film binders, and composites can be produced without toxic solvents. Carbon fiber reinforced plastics (CFRP) based on PEI films were analyzed using an Acoustic Emission

ISSN:0032-3888    

DOI:10.1002/pen.760351609

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760351601

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY