摘要:

AbstractVarious approaches to the preparation and verification of single‐molecule single crystals are discussed for polyethylene and poly (oxyethylene). Analytic tools are electron microscopy, electron diffraction, and differential scanning calorimetry. The main difficulty in producing a single‐molecule single crystal is to keep crystals from joining during gro

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290201

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractSecond virial coefficientsA2and third virial coefficientsA3for benzene solutions of ten polystyrene fractions ranging in weight‐average molecular weightMwfrom 104to 2 × 107at 25°C were determined by light scattering. The third coefficient is represented approximately byA3= 8.0 × 10−6M 0.6wmol g−3cm6forMwabove 105. In this molecular weight region, the factor g defined byA3/A 22Mw/ This trend of g is consistent with predictions of early two‐parameter theories but not with those of renormalization group theories. In particular, quantitative agreement is observed between the present experiments (forMw≳ 2 × 105) and the mean‐field two‐parameter theory of S

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290202

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe phase behavior of narrow molecular weight distribution samples of polystyrene and polyisoprene in the presence of toluene was investigated by means of gel permeation chromatography. Equilibrium phase diagrams, tie lines, and critical points for a number of partially miscible polystyrene‐polyisoprene‐toluene systems were generated at 15 and 30°C and 1 atm pressure. The data were combined with previously reported results at 45°C. Using the experimentally determined phase compositions along with literature values of the polymer–solvent interaction parameters, the polymer–polymer interaction parameters were evaluated using the Flory–Huggins theory. The influence of temperature, polymer molecular weight, and polymer–solvant interaction parameters on the size, shape, and location of the equilibrium phase curve, the location of the critical point, and the polymer–polymer interaction parame

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290203

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractIn order to improve the selectivity and the stability and the stability for gas permeation of poly (1‐trimethylsilyl‐1‐propyne) (PTMSP) membrane, it was chemically modified by grafting polydimethylsiloxane (PDMS) chains. The graft copolymers were synthesized by four different methods via metallation of PTMSP withn‐butyllithium. PDMS content of the graft co‐polymers was controlled in the range of 4–92 mol %. Very tough, thin membranes could be prepared from these graft copolymers using a solvent casting method. Thermal property and gas permeability of the copolymer membranes thus obtained were evaluated. These membranes were relatively thermally stable, and the softening points were over about 150°C. Oxygen permeability coefficients Po2and selectivity Po2/PN2of PTMSP/PDMS graft copolymers depended on the PDMS content, the former was in the range of 1 X 10−8to 2 × 10−7cm3(STP)· cm/(cm2· s · cm. Hg) and the latter was 2.0–3.1. Minimum values of PO2and PN2occured at PDMS content of about 55 mol %. The introduction of more than 60 mol % of PDMS resulted in oxygen permeability coefficient which was maintained for more than one moth (PO2= 2 − 6 × 10−8cm3(STP)· cm/(cm2

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290204

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractFiber formation via the cellulose/ammonia/ammonium thiocyanate system by wet spinning has been investigated. This report presents a characterization of the structure and tensile properties of fibers spun under various coagulation conditions. Microscopic observations showed that the molecular size of coagulant was the dominant factor governing the crosssectional shape of the fibers. Density, birefringence, and crystallinity data indicated that a higher cellulose concentration and lower coagulation temperature favored development of a fiber with a denser and more oriented structure. Under optimum conditions, a welldefined fibrillar structure was obtained. Fiber tensile property measurements suggested the existence of a linear relationship between the fiber breaking tenacity and the product of the square of the Hermans' orientation factor and the infrared crystallinity index.

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290205

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractA theoretical model is proposed to describe quantitatively the steady‐state deformations of fibrils in crazes. In the model, the total force is represented as the sum of high elastic, plastic (actually, nonlinear viscous), and surface tension forces; the first two being modeled by using elongational rheology and the third represented in exact form. The model description is reduced to a boundary value problem for a second‐order ordinary differential equation describing the profile of fibrils. This problem results in an energy balance which, in turn, leads readily to a thermodynamic description of the equilibrium conditions for the deformations of homogeneous fibrils far from the interface (z → ∞). Two possible regimes of fibril deformation were found, depending on the periodicity of the fibrils in the craze. The first is Paredes and Fischer's regime of static deformations of fibrils under action of elastic forces and surface tension where the periodicity is determined by the condition for the static regime to exist. The second is Kramer's drawing regime where the fibril periodicity is found from the Fields and Achby maximization of craze tip advance speed. The choice between these two regimes is made to minimize the complete Gibbs' free energy including surface tension for the equilibrium state of fibrils at z → ∞. The model predicts that the static regime usually appears under moderate stresses and changes for the drawing regime when stresses increase. Hence, this model solves the contradictions between the static and drawing approaches proposed in literature. Examples of both the static and drawing regimes of fibril deformations are demonstrated and some comparisons with data are

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290206

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe dilute solution complexation equilibrium between linear macromolecules and smaller complementary oligomers is considered when: (1) the oligomers are free in solution; and (2) the oligomers are covalently attached at one end to the polymer. A general statistical mechanical framework is developed and is illustrated using a simple random walk model for polymer conformation. The statistical mechanical partition functions are formulated using a generating function technique, allowing thermodynamic averages in the complexed state to be calculated. Loops, trains, and tails of all possible length are allowed in the conformation of a complexed oligomer. Simulation results for the free oligomer case are compared with those obtained for oligomers covalently attached to the polymeric molecular. The model provides a theoretical explanation for the experimentally observed enhancement of complexation of oligomers grafted to the complementary polymers.

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290207

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractSorption and dilation isotherms for argon in poly(ethyl methacrylate) (PEMA) are reported for pressures up to 50 atm over the temperature range 5–85°C. At temperatures below the glass transition (Tg=61°C), sorption isotherms are well described by the dual‐mode sorption model; and isotherms aboveTgfollow Henry's law. However, isotherms for dilation due to sorption are linear in pressure at all temperatures over the range investigated. Partial molar volumes of Ar in PEMA are obtained from these isotherms. The volumes are approximately constant aboveTg(about 40 cm3/mol), whereas the volumes belowTgare smaller and dependent on both temperature and concentration (19–26 cm3/mol). By analyzing the experimental data according to the dual‐mode sorption and dilation model, the volume occupied by a dissolved Ar molecule and the mean size of microvoid in the glass are estimated to be 67 129 Å3, respectively. The cohesive energy density of the polymer is also estimated as 61 cal/cm3from the temperature dependence of the dual‐mod

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290208

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractRaman mechanical spectroscopy was used to examine interfacial effects on the stress distribution in model polydiacetylene fiber/epoxy composites. Epoxy release agents were coated on fiber surfaces to modify the interfacial adhesion properties. The modified fiber surfaces were then characterized by scanning electron microscopy and x‐ray photoelectron spectroscopy as well as optical microscopy. No difference in the maximum stress value or stress distribution was observed for the two types of fibers, coated or uncoated, used in composites. This suggests that adhesion properties at the composite interface do not affect tensile stress transfer efficiency nor, therefore, the composite tensile modulus along the fiber axis direction in uniaxial composites. Experimental data were also compared with theoretical calculations assuming perfect bonding between fiber and matrix, and idealized frictional force transfer mechanism at the fiber–matrix interf

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290209

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractMiscible 80/20 and 90/10 by weight blends of poly (n‐vinyl pyrrolidone) [PNVP] and phenoxy polymer were swollen with 73–85% water to produce rubbery hydrogels with shear moduli as high as 103dyne/cm2. Small‐angle x‐ray scattering, rheological, and calorimetric measurements showed that the gel consisted of tie chains, most probably of PNVP embedded in glassy phenoxy particles with radius of gyration 50–200 Å. A ternary phase diagram was calculated assuming athermal mixing between PNVP–H2O and PNVP and the known endothermic interaction between H2O–phenoxy. Phase separation into a phenoxy phase containing minimal H2O and PNVP and a water‐swollen PNVP phase was predicted in accordance with experimental results. Phase separation was suspected as forming first an interconnected phenoxy phase which was later dispersed by swelling forces transmitted through PNVP tie chains. The presence of localized internal stress in the swollen network was thought to be responsible for the irreversible decrease in the shear moduli observed at shear

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290210

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY