摘要:

AbstractIt is shown that the entanglement junction may be modeled as a binary hooking contact of Kuhn nodes between two chains. The entanglement behavior is thus determined by chain tortuosity and given byNv= (1/β)C ∞α, whereNvis the number of real or virtual skeletal bonds in an entanglement strand,C∞is the characteristic ratio, α = 2 is the number of hooks involved at an entanglement junction, and β = 1/3 is the fraction of binary hooking configurations out of all possible configurations at a binary nodal contact. In other words, we haveNv= 3C ∞2, which is verified experimentally for 44 polymers, covering a wide variety of skeletal, pendant, and stereoisomeric (tacticity) structures. SinceC∞may be estimated by group additivity, the present equation may be used to predict the entanglement behavior from che

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270401

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe effect of the length of side chains on the optical configuration parameter Δaof poly(n‐alkyl acrylate) networks was theoretically studied using the valence optical scheme (VOS). The theoretical calculations suggest that Δashould be negative, and its absolute value should increase as the side‐chain length increases. Moreover, a positive temperature dependence is also predicted. By comparing theoretical and experimental results, it was found that the valence optical scheme gives a good qualitative interpretation of the side‐chain effects on the birefringence of the stretched networks. However, the method fails to give a quantitative description of Δa, presumably because strong intermolecular interactions enhance the polarizability along the chains. The fact that the difference between theoretical and experimental values of Δadecreases with increasing side‐chain length suggests that flexible long chains act as a diluent reducing intermolecular interactions that

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270402

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe α‐relaxation process of seven commercial diglycidyl ether of bisphenol‐A (DGEBA) epoxy resins of varying molecular weights, without hardener, was characterized by measuring the frequency and temperature dependence of the complex permittivity, (ϵast;). The temperature dependences of the frequency of maximum dipole loss ƒmaxand the ionic conductivity σ are non‐Arrhenius and can be described by the Williams‐Landel‐Ferry (WLF) equation. The frequency dependence of the molecular relaxation process is described by the Kohlrausch‐Williams‐Watts (KWW) relaxation function. The WLFC1parameter and the KWW β parameter, describing the temperature and frequency dependences, respectively, vary systematically with the molecular weight of the resin. These results are discussed in the context of recent theories of th

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270403

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractA new gel permeation chromatography indirect calibration method is proposed for a polymer for which samples of known molecular weight and narrow molecular weight distribution are not available. This method is based on conversion of an experimentally obtained calibration curve for another polymer (polymer standard) through the universal calibration concept but using a new intrinsic viscosity‐molecular weight relation. This indirect calibration curve is proposed especially for the low‐molecular‐weight region, where the method based on the Mark‐Houwink‐Sakurada equation gives erroneou

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270404

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe present investigation deals with the tensile mechanical properties of the melt‐blended polyethylene/polystyrene pair as controlled by poly(hydrogenated butadiene‐b‐styrene) copolymers. It is clearly demonstrated that moderate amounts of these copolymers (2–10%) significantly increase both the ultimate strength and elongation at break of blends of polystyrene with various types of polyethylene (low‐density, high‐density, linear low‐density, and hydrogenated polybutadiene) and synergistic effects may result. The mechanical performance is strikingly dependent on the molecular characteristics of the copolymers. Over a broad range of molecular weights (60,000–270,000), diblocks are more effective than graft, triblock, or star‐shaped copolymers. It is also demonstrated that using polymeric emulsifiers under usual processing conditions in the melt state is powerful technique for preparing valuab

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270405

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractAn allophanate‐cured polyurethane (PU) has been prepared. The relationship between trifunctional network structure and mechanical properties is discussed. Mooney‐Rivlin constantsC1andC2are determined from stress‐strain isotherms.C2is approximately constant. The average value of structure factorA3in the limit at large deformation is 0.432, in fair agreement with the theoretical prediction of 1/3. The structure factorA′3of all PU samples becomes roughly unity in the limit at small deformation. The experimental results are interpreted by the recent Flory theory (a fluctuation‐constraint model of junctions). The κ parameter decreases with increasing crosslink density; the ξ parameter is small and approaches zero. Increasing crosslink density tends to cause phantom deformation for extended PU networks. However, the absolute value of the reduced stress in the limit at large deformation (or for the equivalent phantom network) [f ph*] is believed to be overestimated for PU networks with high crosslink density. The origin of this behavior is discus

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270406

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractChemical stress relaxation (CSR) of allophanate‐cured polyurethane (PU) was measured under two kinds of degradation conditions: (1) degradation in butylamine (BuNH2)/dimethyl sulfoxide solution; (2) degradation at high temperature in nitrogen. The allophanate groups of the crosslink sites in PU networks are selectively cleaved by BuNH2, and CSR is not influenced by the crosslink density within experimental error. This suggests the validity of Tobolosky's method for distinguishing between random scission along network chains and the cleavage of crosslinks from the dependence of CSR on crosslink densities. In addition, CSR of PU samples degraded at high temperature in nitrogen was also found to be independent of the crosslink density. It follows that thermal degradation of PU networks occurs by the cleavage of crosslink sites in the same manner. CSR is compared with the relative degree of thermal degradation of PU sample, determined by‐the amine‐decomposition method and the swelling method. Consequently, the total (or residual) crosslink density of degraded PU networks corresponds essentially to the intermitten

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270407

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe rheological behavior of the thermotropic liquid crystal copolyesters composed ofp‐hydroxybenzoic acid and 2‐hydroxy‐6‐naphthoic acid (indicated as HBA/HNA) is investigated for three different compositions: 75:25, 58:42, 30:70 (in mol%). The inherent viscosities are 9.2, 5.1, and 7.8 dl/g, respectively. Yield stress is observed for all three samples. This indicates the existence of crystallites in the melt which may be related to the shear thinning viscosity at low shear stress. Melt fracture, and a die swell ratio which increases with shear rate, are also observed at higher temperatures and at low shear stress for the two copolyesters having compositions 75:25 and 30:70. We have also estimated for 30HBA/70HNA at 335°C the entrance pressure loss, Δpent, and ΔPent/δw, where δwis the shear stress at the capillary wall. The large value of ΔPent/δwsuggest that HBA/HNA is a highly elastic polymeric material. It is found that both ΔPentand ΔPent/δwincrease with shear rate. However, the high elasticity does not account for the disappearance of melt fracture and contraction of die swell at high shear stress. This abnormal phenomenon is ascribed to the formation of a network of crystallites caused by blocky regions

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270408

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractAn apparatus has been constructed that permits the measurement of time‐dependent changes in pressure near the point of vitrification. The same instrument is used for measuring steady‐state PVT properties, which are necessary for a proper analysis of the dynamic measurements. The former experiments are referred to as pressure‐jump volume‐relaxation (PJVR) measurements and serve as a direct probe of the structural relaxation process that occurs in all glasses. Experiments have been performed on polystyrene from 110 to 150°C and up to 2 kbar using pressure steps of 500 bars. The qualitative observations are analogous to those obtained at atmospheric pressure by rapid changes in temperature, namely (1) nonlinearity, (2) asymmetry, and (3) memory effects associated with complicated temperature or pressure histories. Each of these effects is accounted for semiquantitatively by a phenomenological order‐parameter model that has been extended to include the effect of pressure. Deviations between theory and experiment increase as temperature and pressure increase, this being manifest mostly in a predicted recovery curve (expansion isobar) that recovers the equilibrium volume more quickly than the experimental data; the contraction isobars are in most cases predicted within experimental error. The adjustable parameters of the model are found to vary somewhat with pressure and temperature, apparently due to variations in δ and Δκ. The activation volume suggests that 10–20 monomer segments are involved in the recovery process, assuming that the activation volume itself represents only a fraction of the dynamic unit (as observed in mol

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270409

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe effect of plastic deformation on the chain dimensions of polymers in the semicrystalline state was investigated using linear hydrogenated polybutadiene (HPB), a model ethylene/butene‐1 copolymer having about 40% crystallinity at room temperature. Dilute blends of deuterium‐labeled chains with various molecular weights (20,000 ≤M≤ 214,000) in the same unlabeled matrix (M= 95,000) were uniaxially stretched at 25°C to extension ratios of α = 2.4 and 4.4. Radius of gyration normal to the stretch directionR⊥was measured for the labeled chains by small‐angle neutron scattering. The molecular extension ratio inferred from these data αm=R g2/R ⊥2was significantly smaller than α for short chains (M100,000. This variation in αm/α closely parallels the molecular weight dependence of mechanical strength and ductility i

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270410

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY