摘要:

AbstractThe molecular structure, crystallization, solid‐state morphology, thermal properties, and phase behavior of two copolymers consisting of a poly(dimethylsiloxane) (PDMS) mid‐block coupled to polycaprolactone (PCL) end‐blocks were investigated. Both copolymers (which differ only in the molecular lengths of the PCL end‐blocks) were found to be lamellar systems, whose core consists of PCL chains having the same crystal structure as PCL homopolymer, and whose amorphous interlayers contain the PDMS blocks and the PCL noncrystalline segments. From x‐ray and electron‐microscopy results, it is expected that the PCL blocks may be folded once in the longer copolymer and not at all in the shorter. As a result of their differing PCL lengths, the former crystallizes as regular PCL spherulites (at a growth rate reduced with respect to PCL homopolymer), whereas the latter yields only defective, immature axialites of low overall crystallinity. Electron diffraction showed that these spherulites grow preferentially alongbcrystallographic axis and that the PCL crystalline stems are arranged perpendicularly to their lamellae. © 1993 John Wil

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310201

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractWe report some highly unusual phase behavior, of general implication for condensed matter, on the polymer poly(4‐methyl‐pentene‐1) (P4MP1) induced by changes in pressure and temperature, as observed in situ by x‐ray diffraction. Upon increasing pressure beyond a threhold, the polymer, crystalline under ambient conditions, loses its crystalline order isothermally, passing through a continuously varying sequence of mesomorphic states, the process being reversible. This behavior is observed in two widely separated temperature regions, suggesting, for the first time in a single component system, the possibility of reentrant liquid‐crystal and amorphous phases. At the upper temperature region (ca. 250°C) there is a consecutive increase and decrease of melting point with pressure. In the lower temperature region (room temperature) the pressure converts the crystal into an amorphous‐like glass obviating the need for going through the melt first, and this in a reversible manner. The latter pressure‐induced disordered phase converts into crystal on raising the temperature, and reverts to the glassy, disordered phase on lowering the temperature. Some aspects of this behavior have been found quite recently in water‐ice and silica but the process of “melting on cooling” has no precedent in any known system. Other unexpected findings include a new pressure‐induced modification of P4MP1 with a one‐way only entry with temperature, but full reversibility with pressure leading to a triple point in the PT phase diagram. The above highly uncommon results are putting several prevailing preconceptions to test which are being scrutinized. In the course of it some early expectations on general phase behavior, allowing among others for reentrant phases in one component systems, are being invoked as potentially appropriate for certain polymeric systems, if not for condensed matter in general. ©

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310202

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe ultimate properties of liquid crystalline polymers depend upon the achievement of high orientation, usually by means of flow fields. The properties are limited by disorientation which can occur before the product is solidified. Such cooperative orientation and disorientation phenomena also underlie the complex fluid rheology and product microstructure of these materials. The orientation and subsequent disorientation can be followed dynamically by optical and x‐ray techniques. Normally, monitoring of orientation is possible only by “fast” techniqes, such as birefringence, these are not applicable to opaque and strongly scattering liquid‐crystalline systems. To enable examination of the full dynamic response of concentrated nematic solutions of poly(1,4‐phenylene‐2,6 benzobisthiazole) (PBZT) the Daresbury Synchrotron Radiation Source was used. PBZT is among the most rigid macromolecules and serves as a good model for other materials of its class. The orientation process determined optically and from x‐rays is correlated with fluid rheology and availabel theoretical approaches. During relaxation from near perfect mesogen alignment three principal stages of the disorientation process were identified corresponding to solvent disorientation (first stage of stress relaxation), banding (slow stress relaxation and mesogen disorientation), and finally a very slow banding to polydomain transition. © 1993 John W

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310203

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractAn unusual conductivity enhancement occurs in PEO‐based ZnBr2/LiBr electrolytes of composition, [xZnBr2+ (1 − x)LiBr](PEO)16withx= 0.00, 0.05, 0.50, 0.75, 1.00. The conductivity of the mixed‐salt electrolytes is higher than that of either pure salt electrolyte. The highest conductivity, observed for x = 0.5, is two orders of magnitude higher than that of pure LiBr(PEO)16and one order higher than ZnBr2(PEO)16. In contrast, the conductivity of mixed Mg (CIO4)2/LiCIO4electrolytes, [xMg(CIO4)2+ (1 − x) LiCIO4](PEO)16wherex= 0.00, 0.20, 0.50, 0.80, 1.00, increases monotonically with the mole fraction of the higher conductivity component, LiCIO4(PEO)16. The conductivity and differential scanning calorimetry (DSC) results suggest that the conductivity enhancement in the ZnBr2/LiBr electrolytes results from a change in charge carrier type and concentration, whereas the conductivity change in the Mg(CIO4)2/LiCIO4electrolytes arises from a change in the microscopic viscosity of the electrolytes. © 1993 John Wiley&S

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310204

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractLangmuir‐Blodgett (LB) film formation has been investigated for polymers in which non‐linear optical (NLO) moieties are attached to the side chain of a polymethacrylate (PMA) backbone. Polymer monolayers were successfully transferred onto hydrophilic glass substrates using a moving‐wall type LB trough. The LB films were characterized by pressure‐area isotherms, polarized ultraviolet visible spectra, ellipsometry, and second harmonic generation measurements. The characterization shows that NLO moieties in the side chain are inclined at a shallow angle to the plane of the substrate. The second‐order nonlinear optical coefficient and molecular hyperpolarizability were also calculated. © 1993 John Wiley

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310205

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractX‐ray diffraction patterns of linear and branched polyethylenes typically show two sharp reflections and an amorphous halo. The position of the halo depends on branch content and temperature. A single curve describes the position of the halo maximum (2θhalo) for a range of liquid hydrocarbons and polyethylenes in the 20–140°C range. At temperatures well below their melting point, branched polymers give 2θhalovalues which differ significantly from those observed for the liquid Linear polymers show a greater divergence, indicating that some of the material giving rise to the halo is much better packed than in the liquid.Parallel13C NMR spin‐lattice relaxation studies suggest that this relatively ordered material has a trans conformation but a low average T1cvalue. © 1993 John Wiley&

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310206

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe mechanical behavior of glassy polymers is time and temperature dependent as evidenced by their viscoelastic and viscoplastic response to loading. The behavior is also known to depend strongly on the prior history of the material, changing with time and temperature without chemical intervention. In this investigation, we examine the effects of this process of physical aging on the yield and postyield behavior and corresponding evolution in the structural state of glassy polymers. This has been achieved through a systematic program of uniaxial, isothermal, constant strain–rate tests on poly(methyl methacrylate) (PMMA) specimens of different thermal histories and by performing positron annihilation lifetime spectroscopy (PALS) measurements prior to and after mechanical deformation. PALS is an indicator of the free volume content, probing size and density of free volume sites and can be considered to be a measurement of structural state. The results of the mechanical tests show that aging acts to increase both the initial yield stress and the amount of strain softening which occurs subsequent to yield. Moreover, the amount of strain softening was found to be independent of strain rate indicating that softening is related to an evolution in structure as opposed to deformation kinetics. Furthermore, after sufficient inelastic straining, the initial thermal history is completely erased as evidenced by identical values of flow stress following strain softening, for both annealed and quenched polymer. Strong confirmation of the structural state or free volume related nature of the strain softening process is obtained by our companion PALS measurements. PALS detects anincreasein the size of free volume sites following inelastic deformation and finds the initially annealed and quenched specimens to posses the same post‐deformation distribution. The size of sites is found to evolve steadily with inelastic strain until it attains a steady‐state value. This evolution of free volume with strain follows the observed softening of the flow stress to a steady‐state value. These results provide experimental evidence that an increase in free volume with inelastic straining accompanies the strain softening phenomenon in glassy polymers and that strain softening is indeed a de‐aging process. Based on our experimental results a mechanistically based constitutive model has been formulated to describe the effects of thermal history on the yield and postyield deformation behavior of glassy polymers up to moderate strains. The model is found to successfully capture the effects of physical aging, strain softening, strain rate, and temperature on the inelastic behavior of glassy polymers when compared with experimental results. © 1993 John Wiley

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310207

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractFirst observations of a decrease in heat capacity onisothermalvitrification of a thermosetting polymer are reported. The method developed for this study allows simultaneous measurements of both the heat capacity of a material and the enthalpy released as chemical reactions occur under virtually isothermal conditions. Control experiments have been done on a nonstoichiometric composition which does not vitrify and for which total enthalpy released can be measured. During isothermal curing, the heat capacity of a thermoset first increases slightly, undergoes an abrupt decrease in a narrow range of time, and thereafter slowly decreases. The abrupt decrease represents the transition of the liquid to a glassy solid, which is irreversible and which shifts to shorter times as the curing temperature is increased. This transition occurs at different extents of cure for different temperatures. The thermodynamic consequence of kinetic slowing onirreversible conversionof a molecular liquid to a macromolecule under isothermal conditions is similar to that observed onreversible clustering, as a result of decrease in the thermal energy on supercooling a molecular liquid. © 1993 John Wiley&Sons, Inc

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310208

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractAbsorbance, excitation, and emission measurements have been performed with methyl benzoate and five model compounds, C6H5COO (CH2)xOOCC6H5,x= 2–6. Under appropriate conditions, three of the model compounds (those withx= 3, 4, 5) show evidence for the formation of intramolecular ground‐state dimers. The model compound withx= 5 can form two types of dimers which emit with different energies. The model compound withx= 3 forms one of these dimers, and the model compound with x = 4 prefers the other ground‐state dimer. Molecular modeling of the dimers suggests that the two conformations of the ground‐state dimers differ in the orientation of the two CO bonds. In the one dimer these two bonds are nearly parallel, but in the other they make an angle of about 120°. © 1993 John Wile

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310209

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe ratio of the intensities of excimer and monomer emission,ID/IM, has been measured for PyCOO(CH2)mOOCPy (m= 2–6, Py = 1‐pyrenyl) in solvents of different viscosity, η. These compounds are dimeric models for the corresponding polymers, selected to eliminate the influence of energy migration on the population of an excimer. The solvents are H(CH2)nOH,n= 1–7, and ethylene glycol. The values ofID/IMdecrease as η increases, and the rate of this decrease is a function of m. Therefore the qualitative dependence ofID/IMon m is a function of η. In the limit as η → ∞,ID/IMis an odd–even function ofmform= 2–5, and the values atm= 5 and 6 are nearly identical. A rotational‐isomeric state analysis of the conformations of the diesters can rationalize the appearance of the odd–even effect. This result is insensitive to assumptions about the extent of overlap of the two pyrene ring systems that is required for the production of emission in the excimer band

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310210

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY