摘要:

Amorphous polymers are viscoelastic materials. When they are subjected to dynamical loads, their behavior can be modeled by transform functions of stress and strain in the complex plane. In our work, a model based on the fractional calculus concept is proposed in order to predict the viscoelastic behavior of polymethylmethylacrylate (PMMA) over a wide temperature range between (Tg-190°C) and (Tg+15°C). The extended fractional solid model is shown to be capable of describing experimentally observed dynamic viscoelastic behavior over a wide temperature range, including multiple relaxations, using a limited number of free parameters. Structural recovery of PMMA was studied by dynamic mechanical spectrometry, and its effect on the different parameters is also discussed. Furthermore, from the fractional differential and fractional integral formulations. most of the relevant viscoelastic functions that quantify the degree of molecular mobility of amorphous polymers, likeE(t),E*(τ). andH(t), can be derived analytically.

ISSN:0022-2348    

DOI:10.1080/00222349908248154

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

A complex aging regime occurs in the course of thermal aging of elastomers. Depending on the type and the content of the rubber filler materials, temperature, chemical environment (normally air), and time, a different aging process can be observed also by nuclear magnetic resonance (NMR) [1–6]. The methods used are the common spin-echo1H-NMR, including variable echo times and parameter-selective NMR-1H-imaging (material properties imaging). The decay of the echo-magnetization is discussed on the basis of a single-chain model with a distribution of dipolar interactions. This model is based on the influence of a very fast, but anisotropic, local motion, as well as larger and slower motions, which are able to diminish the residual dipolar interaction. Carbon-black-filled natural rubber, as well as silica and carbon-black-filled E-SBR (emulsion-polymerized styrene butadiene rubber) and S-SBR (solution-polymerized SBR) are the systems under investigation, with the results showing some characteristic features of the course of aging observable by NMR.

ISSN:0022-2348    

DOI:10.1080/00222349908248155

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Proton spin relaxation above the glass transition temperature and differential scanning calorimetry (DSC) have been used to investigate dynamical aspects of aging by a time-dependent change of the chemical composition in cis-1,4-polybutadiene samples. The polymer chains in bulk polybutadiene were cross-linked by exposure to ionizing radiation. The transition of the1H-NMR (nuclear magnetic resonance) signal from a liquidlike spin system response to a solidlike one is caused by cross-linking and network formation. This effect depends on the radiation dose. The description of the relaxation data is based on a stochastic model of translational diffusion of structural defects between fluctuating and fixed junctions of polymer chains. The assigned composite stochastic process has non-Markovian character. It is shown that the radiation yields changes in the constrained detect diffusion processes modeling the chain dynamic. Correlation time and memory time for the local and coherent segmental motion strongly depend on radiation dose. Based on this stochastic model, an understanding of aging processes on the molecular level is possible.

ISSN:0022-2348    

DOI:10.1080/00222349908248156

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The relaxation and crystalline properties of ethylene vinyl acetate (EVA) co-polymers inserted in steel/polymer/steel assemblies were studied. To investigate the properties of the interfacial region, polymers of different thickness inserted in the assemblies were analyzed. The studied EVA copolymers are semicrystalline polymers. The relaxation properties of the amorphous phase were investigated by dynamic mechanical measurements performed on the steel/polymer/steel assemblies, and the crystalline properties were studied by differential scanning calorimetry (DSC). The results indicate that, for low polymer thicknesses, the mobility of the amorphous phase is significantly reduced. Significant changes in the crystalline organization also were observed when the polymer thickness decreased, with the presence of more numerous disorganized crystals for thin EVA layers. These crystals can act as physical ties that reduce the mobility of the neighboring amorphous chains. These results indicate the formation of an interphase layer of reduced mobility.

ISSN:0022-2348    

DOI:10.1080/00222349908248157

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Amplitude and frequency dependence of reversible melting of polycaprolactone (PCL) and an ethylene octene copolymer (EOM) were studied using temperature-modulated differential scanning calorimetry (TMDSC) (2*10−1Hz <f< 0.05 Hz) and shear spectroscopy (dynamic mechanical analysis, DMA) (5*10−4Hz <f< 100 Hz). It was found that the excess heat capacity of PCL is constant for temperature amplitudes in the range 5 mK <AT< 2 K. The excess heat capacity decreases with frequency of temperature perturbation and tends to zero at about 0.1 Hz and 100 Hz for PCL and EOM, respectively. The constant excess heat capacity and the frequency dependence support the idea that reversible melting is related to a relaxation process on the surface of the polymer crystals. The occurrence of such a relaxation process was shown by shear modulus measurements in the same frequency and temperature region. The relaxation process is, in the melting region, much slower than main relaxation (glass transition). At low temperatures, a crossover can be seen, indicating the independence of both processes because of spatial separation. The main relaxation is related to the melt, while the other is related to the crystal surface.

ISSN:0022-2348    

DOI:10.1080/00222349908248158

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The segment relaxation in two series of binary, finely dispersed poly(butylene terephthalate)/polycarbonate (PBT/PC) and polyamide-6/acrylonitrile-buta-diene-styrene (PA6/ABS) blends was studied by the method of thermally stimulated depolarization currents (TSDC) both in normal mode (global TSDC spectra) and in thermal-sampling mode (TSDC-TS). The resulting temperature dependencies and distribution functions of segment relaxation activation energy Easrand the influence of annealing on the relaxation behavior of the mixed phases are discussed, considering the phase morphology. Common to all blends under study are lowerEasrp(the most probable value ofEasr), narrowerEasrdistribution functions, and broader temperature ranges of the glass transitions in both phases of the blend compared to those of the initial components. The relationships are in good agreement with the hypothesis on the spontaneous fractionation of polymers in blends and on the breakdown of the cooperative segment mobility regions caused by the interactions between the molecular chains of different polymers. In finely dispersed small particles of the PBT-rich phase (particle diameter ≥ 0.5 μm), a degeneration of the cooperative segment (a) relaxation in a noncooperative segment (β) relaxation caused by the solution of PC molecules in PBT was detected.

ISSN:0022-2348    

DOI:10.1080/00222349908248159

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Thermoplastic elastomers (TPEs) are conventionally made of block copolymers or partly cross-linked polymer blends. Alternatively, TPEs can be prepared from structured latices, too. Hard-soft latex particles with a thermoplastic core and an elastomeric shell yield highly extendable elastic films, the strength of which depends sensitively on the relative core size and the particle architecture. Core-shell particles were prepared, by two-step emulsion polymerization, with the thermoplastic polystyrene (PS) in the core and the elastomer polyethylacrylate (PEA) in the shell. PEA particles were synthesized first. The PS cores were then incorporated in them in the second step. This method permits the design of monocore. as well as multicore, particles. These PS-PEA particles were not cross-linked in the core or in the shell. They can be classified as microblends. Compression-molded films of them exhibited, therefore, a coarsened microphase morphology that was, however, still much finer than that of simple melt-mixed blends PS/PEA. The film morphologies of monocore and multicore particles were different as far as the former yielded spherical PS domains, while the latter yielded extended PS clusters. This was strongly reflected by the stress-strain behavior: Films from multicore particles responded in a viscoelastic, rubbery manner, while films from monocore particles behaved like viscous liquids.

ISSN:0022-2348    

DOI:10.1080/00222349908248160

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The application of pentabromobenzyl acrylate monomer (PBB-MA) extrusion polymerized in ethylene-propylene-diene monomer (EPDM) terpolymer allows the production of flame-retardant polypropylene (PP) with very high impact and elongation at break and good resistance to thermal aging.

ISSN:0022-2348    

DOI:10.1080/00222349908248161

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The diffusion of acetophenone in peroxide cross-linked low-density polyethylene (LDPE) medium-voltage cable was investigated with Fourier transform infrared (FTIR) microscopy. The influence of storage under normal climate and for up to 2 years on the acetophenone profile are discussed. The diffusion rate and the coefficient of diffusion of acetophenone in cross-linked LDPE were determined. It could be shown that the coefficient of diffusion is independent of the morphology of the medium-voltage cables investigated.

ISSN:0022-2348    

DOI:10.1080/00222349908248162

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

ISSN:0022-2348    

DOI:10.1080/00222349908248113

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor