bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular weight distribution in random crosslinking of polymer chains
Authors: Tobita Hidetaka
Published: April 2003

The molecular weight distribution (MWD) of crosslinked polymer molecules formed during polymeric network formation is the sum of the fractional MWDs containing 0, 1, 2, 3, … crosslinkages. The MWD for polymer molecules containing 𝓀 crosslinkages is investigated for the random crosslinking of polymer chains whose initial MWD is given by the Schulz‐Zimm distribution. For a very narrow initial MWD, each fractional MWD with 𝓀 = 0, 1, 2, … is independent and a multimodal distribution is obtained for the whole distribution. When the initial MWD is uniform, the average crosslinking density within the polymer fraction whose degree of polymerization is r, ρr is simply given by ρr = ρgel,c – 2/r irrespective of the extent of crosslinking reaction where ρgel,c is the crosslinking density within gel fraction at the gel point. On the other hand, the MWDs with 𝓀 crosslinkages overlap each other with different 𝓀 values significantly for the broader initial distributions, and ρr increases with the progress of crosslinking reactions. The value of ρr increases with increasing r but levels off asymptotically at large r. The average crosslinking density of polymer molecules containing 𝓀 crosslinkages ρ𝓀 is an increasing function of k but soon reaches a plateau; sooner for the broader initial MWDs. For 𝓀 ≥ 1, ρ𝓀 is always larger than the average crosslinking density of the whole reaction system ρ in the pregelation period, i.e., in terms of the crosslinking density, the difference between polymer molecules with and without crosslinkage is most significant. In general, the average crosslinking density ρ, which is convenient to use in describing the nature of the whole reaction system, cannot be considered as a characteristic degree of crosslinking for polymer molecules containing at least one crosslinkage. Consideration of the bivariate distribution of r and k reveals important aspects of the polymeric network formation that have been obscured in the conventional theories in which the averages including linear polymers are solely considered. © 1995 John Wiley & Sons, Inc.
Computer simulation of crystallization kinetics and morphology in fiber‐reinforced thermoplastic composites. III. Thermal nucleation
Authors: Mehl Nathan A. , Rebenfeld Ludwig
Published: April 2003

A computer simulation has been used to predict crystallization kinetics and crystalline morphology in composite materials based on thermally nucleated crystallizable matrices. As demonstrated for athermally nucleated composites, the presence of reinforcing fibers increases the complexity of the system. Fibers are shown to have a dual effect on the spherulitic crystallization process. The influence that fibers have depends on the interplay between the enhancing effects that fibers have on nucleation and the depressing effects that fibers have on spherulitic growth. Fibers that do not provide additional nuclei to the system depress the rate of crystallization relative to an unreinforced polymer, while fibers that add nuclei to the system increase the rate of crystallization. The transcrystalline morphologies that develop in thermally nucleated fiber‐reinforced polymers are controlled primarily by the relative numbers of bulk and fiber nuclei. The extent of transcrystalline regions can be suppressed either by increasing the rate of bulk nucleation, or by decreasing the rate of fiber nucleation. Finally, the qualitative appearance of the morphology in the transcrystalline region was found to be indicative of the mode of fiber nucleation. © 1995 John Wiley & Sons, Inc.
Influence of morphology on the fracture toughness of isotactic polypropylene
Authors: Ouederni Mabrouk , Phillips Paul J.
Published: April 2003

An adaptation of the fracture toughness test method, the J‐integral technique, is described within the general framework of polymer fracture behavior. It is shown that there is a strong interaction between different morphological parameters in the way they affect the fracture behavior of isotactic polypropylene (iPP). The fracture toughness decreases with increasing crystallinity at a fixed spherulite size. The fracture toughness also decreases slightly with increasing spherulite size at a constant crystallinity, but this may not be a pure spherulite size effect. The use of a nucleating agent results in a very fine spherulitic structure but facilitates crack growth and reduces the material toughness beyond the crack initiation stage. This suggests that the material behavior is dictated by the increase in crystallization temperature caused by the presence of the nucleating agent and not by the change in spherulite size. © 1995 John Wiley & Sons, Inc.
Dilation kinetics of glassy, aromatic polyimides induced by carbon dioxide sorption
Authors: Wessling M. , Huisman I. , Boomgaard Th. v. d. , Smolders C. A.
Published: April 2003

Over the past years, the equilibrium sorption of gases in polymers has been intensively studied. Mostly, glassy polymers were investigated because of their excellent selective mass transport properties. This work does not focus on the equilibrium sorption but on the kinetics to reach the equilibrium. We developed a new experimental method measuring the sorption‐induced dilation kinetics of a polymer film. Carbon dioxide and glassy, aromatic polyimides were chosen as model systems. Low‐pressure experiments demonstrate that the measured dilation kinetics represent the sorption kinetics. A significant delay between the sorption and dilation kinetics is based on the fact that dilation kinetics occurs simultaneously with the concentration increase in the center of the polymer film. High‐pressure experiments reveal significant differences in dilation kinetics compared to low‐pressure experiments. Generally, three regimes can be distinguished in the dilation kinetics: a first, fast volume increase followed by two much slower regimes of volume increase. The magnitude of fast and slow dilation kinetics strongly depends on the swelling history of the polymer sample. The results of the experiments are analyzed in the light of a model relating the fast dilation kinetics to a reversible “Fickian” dilation and the slower dilation kinetics to an irreversible, relaxational dilation. © 1995 John Wiley & Sons, Inc.
Structure, conformation, and motions of polytetrahydrofuran (PTHF) in the hexagonal form of the PTHF‐urea inclusion compound
Authors: Vasanthan Nadarajah , Shin I. Daniel , Tonelli Alan E.
Published: April 2003

Combination of differential scanning calorimetry, x‐ray diffraction, Fourier transform infrared, and C‐13 nuclear magnetic resonance observations made on the crystalline inclusion compound (IC) formed between polytetrahydrofuran (PTHF) and urea (U), together with their comparison to identical observations performed on bulk semicrystalline samples of PTHF, have permitted an analysis of the conformations, motions, and environments available to PTHF chains in both solid‐state phases. The isolated PTHF chains occupying the narrow channels of the PTHF‐U‐IC are highly extended, though small rotational deviations averaging 24° from the nearly all trans, planar zig zag conformation of bulk crystalline PTHF chains produce some significant differences in their behaviors. PTHF chains in PTHF‐U‐IC possess much greater mobility than bulk crystalline PTHF chains as evidenced by C‐13 spin lattice relaxation times, T1, 50 times shorter (1.5 s) than observed for bulk crystalline PTHF chains (75 s). FTIR observations are consistent with very little specific interaction between guest PTHF chains and host urea matrix molecules and result in similar spectra for bulk and IC PTHF, except for the presence of the CH2 rocking vibration band at 745 cm−1 observed for bulk PTHF. The absence of this band in the IC PTHF can be understood by considering the symmetry of the all trans, planar zig zag conformation of bulk crystalline PTHF chains, which prevents the CH2 rocking mode from coupling with skeletal stretching and bending modes as occurs in the nonplanar, helical PTHF chains in PTHF‐U‐IC. © 1995 John Wiley & Sons, Inc.
Viscoelasticity and birefringence of polyisobutylene
Authors: Okamoto Hirotaka , Inoue Tadashi , Osaki Kunihiro
Published: April 2003

The complex Young's modulus, E*(ω), and the complex strain‐optical coefficient, O*(ω), were measured for polyisobutylene (PIB) over a wide temperature range near and above its glass transition temperature. The master curves could be constructed well for each function with the method of reduced variables. The shift factor, aT, for E*(ω) is the same as that for O*(ω). The ratio of the imaginary parts of O*(ω) and E*(ω), O″(ω)/E″(ω), takes an extremum, which has never been observed for other polymers. The relation between O*(ω) and E*(ω) cannot be described by a modified stress‐optical rule (MSOR) which has been found valid for various polymers. The basic concept of the MSOR. i.e., the chain orientation and the orientation of flat monomer units in the stretch direction, is not sufficient to describe the behavior of PIB and another origin of stress, presumably due to the fluctuation of local stress, should be included. This term does not contribute to the birefringence. The main maximum of tan δ is ascribed to the relaxation of the chain orientation in contrast with many other polymers, such as polyisoprene and polycarbonate, for which the maximum of tan δ is ascribed to the rotational relaxation of monomer units. © 1995 John Wiley & Sons, Inc.
Metal particle adsorption and diffusion in a model polymer/metal composite system
Authors: Shull Kenneth R. , Kellock Andrew J.
Published: April 2003

We have developed a model polymer/metal composite system based on the adsorption of colloidal gold particles from a dilute aqueous suspension to the surface of poly(2‐vinylpyr‐idine) (PVP). Particle coverages and tracer diffusion coefficients for the particles within a PVP matrix phase were measured by Rutherford backscattering spectrometry. The adsorption process is quantitatively described by a diffusion‐limited mechanism where gold particles irreversibly adsorb to the surface of the polymer film. Model dispersions produced in this way are excellent model systems for studying the fundamental properties of metal particle dispersions, since the particle size and the areal density of particles on the surface are well‐controlled. Diffusion coefficients for the gold particles within PVP were also measured. The diffusion of the gold particles was found to be coupled to the bulk viscosity of the polymer, even though the size of the gold particles was only slightly larger than the mesh size of the entanglement network for PVP. © 1995 John Wiley & Sons, Inc.
Molecular weight characterization of soluble high performance polyimides. 2. Validity of universal SEC calibration and absolute molecular weight calculation
Authors: Konáš Martin , Moy Thomas M. , Rogers Martin E. , Shultz Allan R. , Ward Thomas C. , Mcgrath James E.
Published: April 2003

Six different soluble high‐performance aromatic polyimides, each prepared by solution imidization to three controlled average molecular weights, were analyzed by size exclusion chromatography (SEC) using on‐line parallel coupled refractometric and viscometric detectors. N‐methylpyrrolidone (NMP) with 0.06 M LiBr and NMP stirred over P2O5 were used as mobile phase for four of the polyimides; NMP with 0.06 M LiBr and NMP stirred over P2O5 were used as mobile phases for four of the polyimides; NMP with 0.06 M LiBr tetrahydrofuran (THF) and chloroform served as mobile phases for the other two polyimides. For all the samples the stationary phase in the SEC columns was cross‐linked polystyrene beads. Molecular weight averages of the polyimides were calculated using universal SEC calibration with polystyrene standards in each solvent. The agreement of the calculated molecular weight averages in the different solvents confirms that the universal SEC calibrations are valid for these semiflexible polymers. There was good agreement with weightaverage molecular weights obtained by low‐angle laser light scattering (LALLS) performed in pure NMP. Intrinsic viscosity and molecular weight data for a series of nine samples of one polyimide covering a Mw = 20,000–70,000 g mol–1 interval were treated to obtain Mark‐Houwink‐Sakurada constants. Unperturbed chain dimensions of this polyimide were obtained by application of the Stockmayer‐Fixman extrapolation procedure to these data. ©1995 John Wiley & Sons, Inc.
A nuclear magnetic resonance study of dynamics in toluene‐polyisobutylene solutions: 1. Penetrant diffusion and Fujita theory
Authors: Bandis Athinodoros , Inglefield Paul T. , Jones Alan A. , Wen Wen‐Yang
Published: April 2003

The self‐diffusion coefficients of toluene in polyisobutylene (PIB) solutions were determined using the pulsed field gradient nuclear magnetic resonance technique. The volume fraction of toluene in the polymer was varied from 0.045 up to 0.712 and the temperature was varied from 225 K up to 368 K. The concentration dependence of the data was interpreted using the Fujita free volume theory and the temperature dependence was interpreted with the WLF equation. These models describe separately the concentration and temperature dependencies of the toluene self‐diffusion coefficients very well and the resulting free volume parameters are in good agreement with the ones extracted from the analysis of viscosity data on the same system. ©1995 John Wiley & Sons, Inc.
Multiplet formation in calcium partially neutralized poly(acrylamidocaproic acid)
Authors: Kim Hee‐tak , Park Jung‐ki , Lee Kew‐ho
Published: April 2003

The multiplet formation in calcium partially neutralized poly(acrylamidocaproic acid) was studied by using differential scanning calorimetry and Fourier transform infrared spectroscopy. At 20 mol % of neutralization of acid groups, the ionic groups started to aggregate to form multiplets. The hydrogen bonding of amide group was destroyed during multiplet formation due to the reduction of the motional freedom of the side chain. The extension of the short side chain started to be loosened at the neutralization of 44 mol %. In this state, the multiplets could be formed without severe side chain stretching due to the proximity of the ionic groups. It is expected that the more stable multiplet is formed with increasing the degree of neutralization. © 1995 John Wiley & Sons, Inc.
Coagulation of poly(p‐phenylene benzobisoxazole) in polyphosphoric acid using aqueous and nonaqueous solutions
Authors: Gillie J. K. , Newsham M. D. , Sen A. , Bubeck R. A.
Published: April 2003

Diffusion of aqueous and nonaqueous coagulants into lyotropic/nematic solutions of poly(phenylene benzobisoxazole) in polyphosphoric acid using fluorescence microscopy is reported. The fluorescence microscopy technique offers advantages in that the experiment is independent of the coagulant and the experiments can be performed in a front surface mode so that optically thin samples are not required. The diffusion process is modeled as Case II diffusion as described by Crank and Park. The apparent diffusion coefficients are found to range from ∼ 2 × 10−6cm2/s to ∼ 2 × 10−7cm2/s for aqueous and nonaqueous coagulants. The apparent diffusion coefficients can be varied by several orders of magnitude (e.g., down to ∼ 3 × 10−10cm2/s) by varying the concentration of polyphosphoric acid in the coagulant. © 1995 John Wiley & Sons, Inc.
Thermodynamic interaction parameters in polymer solutions and gels
Authors: Horkay Ferenc , Hecht Anne‐Marie , Geissler Erik
Published: April 2003

Polymer‐solvent interaction parameters are reported for poly(vinyl acetate)‐acetone, poly(vinyl acetate)‐toluene, and poly(dimethyl siloxane)‐toluene systems using different techniques. Results obtained by osmotic deswelling are compared with those from quasi‐elastic light scattering and small‐angle neutron scattering (SANS). In gels, the latter techniques involve separation of the time‐dependent from the static component of the scattered radiation+ Separation is achieved in quasi‐elastic light scattering through the heterodyning properties of the light, and in SANS by subtracting an appropriate static structure factor. The interaction parameters obtained by different separation procedures are consistent with measurements using the osmotic method. © 1995 John Wiley & Sons, Inc.
Dynamic mechanical properties of ionomer/ionomeric microbead composites
Authors: Cousin P. , Smith P.
Published: April 2003

Composites of sulfonated polystyrene (PS‐SSA) (0‐8 mol % sulfonation) mixed with submicronic styrene‐4 vinylpyridine (PS‐4VP) (31 mol % 4VP content) microspheres were investigated by differential scanning calorimetry, Fourier transform infrared spectroscopy, and dynamic mechanical spectroscopy at 1 Hz in the glass transition region. The resulting proton transfer reaction from the SSA to the 4VP units was confirmed by IR spectroscopy, and led to a significant increase in the post‐Tg Young's modulus as well as a lengthening of the rubbery plateau. Surprisingly, the addition of sulfonated polystyrene microspheres to styrene‐vinyl pyridine copolymers had no such effects, possibly because of steric factors. © 1995 John Wiley & Sons, Inc.
High density polyethylene/hydrogenated oligo(cyclopentadiene) blends: Tensile stress‐strain behavior
Authors: Cimmino S. , Di Pace E. , Martuscelli E. , Silvestre C. , Mendes L. C. , Bonfanti G.
Published: April 2003

The influence of hydrogenated olio(cyclopentadiene) (HOCP) on tensile mechanical properties of its blends with high density polyethylene (HDPE) has been studied at 20, 80, and 100°C. The nominal stress‐strain curves performed at 20°C indicate an increase of the modulus and a conspicuous decrease of the ultimate properties increasing the HOCP content in the blend. Blends with HOCP content > 20%‐wt are found to be very fragile at 20°C. These behaviors have been related to the presence of the HOCP‐rich phase that has the glass transition at about 55°C. At 20°C the HOCP oligomers act as a hardener for the HDPE. The nominal stress‐strain curves performed at 80 and 100°C show a strong decrease of the modulus (compared to the value at 20°C), plastic deformation for all the blends tested, and rupture of HDPE and blend samples at the beginning of the cold‐drawing. Moreover, the modulus and the ultimate parameters are found to decrease with the composition. The behaviors at 80 and 100°C have been related to several factors: the higher mobility of HDPE molecules at these temperatures compared to that at 20°C; the HOCP‐rich phase that is at temperatures higher than its glass transition, and so it acts as a plasticizer for the HDPE; and the decreases of crystallinity and number of entanglements when the HOCP is added to the HDPE component. ©1995 John Wiley & Sons, Inc.
Dielectric relaxation spectroscopy in poly(hydroxyethyl acrylates)/water hydrogels
Authors: Kyritsis A. , Pissis P. , Grammatikakis J.
Published: April 2003

The electrical and dielectric properties of poly(hydroxyethyl acrylate), PHEA, hydrogels were studied by means of dielectric relaxation spectroscopy in wide ranges of frequencies (5–2 × 109 Hz), temperatures (173–363 K) and water contents (0.065–0.46, g of water per gram of dry material). The secondary dipolar mechanisms (γ and βsw) and the dc conductivity mechanism were studied in detail by analyzing the dielectric susceptibility data within the complex permittivity formalism, the modulus formalism, and the complex impedance formalism. For both mechanisms molecular mobility was found to increase with increasing temperature or increasing water content (T‐f‐h superposition principle). The energy parameters and the shape parameters of the response were determined for both mechanisms at several water contents and temperatures. The temperature dependence of dc conductivity was found to change from Vogel‐Tamman‐Fulcher (VTF) type to Arrhenius type at water contents of ca. 0.30. At water contents lower than about 0.30 the hydrogels are homogeneous whereas at higher water contents a separate water phase appears. In terms of the strong/fragile classification scheme our results suggest that the PHEA hydrogels are fragile systems. ©1995 John Wiley & Sons, Inc.
Kinetic gelation predictions of species aggregation in tetrafunctional monomer polymerizations
Authors: Anseth Kristi S. , Bowman Christopher N.
Published: April 2003

During the polymerization of tetrafunctional monomers, a heterogeneous distribution of species exists throughout the reaction mixture and dramatically complicates the network structural evolution. This work attempts to quantify this heterogeneity using kinetic gelation simulation predictions of a defining parameter called the heterogeneity index. The heterogeneity index directly measures how heterogeneous the polymerization is with respect to interactions of like species or pairs of species. Examples of the species are polymer segments, monomeric double bonds, pendant double bonds, initiator molecules, radicals, and free volume. By implementing this index, it is clear that the heterogeneity in the polymer network dramatically changes as a function of conversion and polymerization conditions. In particular, microgel formation and monomer pooling were characterized and quantified with the heterogeneity index. In addition to characterizing the structural heterogeneity, the influence of the heterogeneity on the initiator efficiency and trapping of free radicals was also studied and qualitatively compared to experimentally observed behavior. ©1995 John Wiley & Sons, Inc.
Ramp rate effect on polyimide film properties and morphology
Authors: Jou Jwo‐Huei , Lin Chen‐Piao , Sheu Wei‐Hwa
Published: April 2003

The stress in films of semirigid and rigid polyimides (PIs) on silicon (Si) substrate has been measured in situ during curing using a newly modified bending beam apparatus. By using the apparatus, the onset of residual stress in the initially solvent‐rich films has also been investigated. The stress characteristics of the PI films are strongly ramp rate dependent. Different ramp rates result in markedly different stress patterns, thermo‐mechanical properties, and film morphologies. The residual stress at room temperature after curing is scattered around 23–31 MPa for the films of pyromellitic dianhydride‐4,4′‐oxydianiline (PMDA‐ODA). For pyromellitic dianhydride‐p‐phenylenediamine (PMDA‐PDA), it systematically increases from −6 to 28 MPa for ramp rate increasing from 0.5 to 10°C/min. The residual stress is very low in the slowly cured PMDA‐PDA films. However, these films have gone through a very high‐stress transient state during curing. The maximum transient stress is comparatively higher in the films of PMDA‐PDA than in PMDA‐ODA. Slowly cured PMDA‐PDA films exhibit high structural ordering, high in‐plane anisotropy, and low coefficient of thermal expansion. ©1995 John Wiley & Sons, Inc.
Self‐diffusion in melts of statistical copolymers: The effect of changes in microstructural composition
Authors: Losch Andreas , Salomonovic Radan , Steiner Ullrich , Fetters Lewis J. , Klein Jacob
Published: April 2003

We have used nuclear reaction analysis to measure diffusion coefficients D in couples consisting of hydrogenated polybutadienes of structure (C2H3(C2H5))x(C4H8)1−x and their partly deuterated counterparts. The 1,2‐ and 1,4‐olefinic isomers are randomly distributed along the chains and the mean vinyl fraction x varies between 0.38 and 0.94. We find that the effective monomeric mobility D0 [defined by D = D0(Ne/N2) for each copolymer, where N is the backbone length and Ne the entanglement spacing] decreases monotonically with increasing vinyl content x. Over the range of microstructures and temperatures T (−14−40°C) investigated we find log(D0/T) varies smoothly with (T − Tg), where Tg is the glass transition temperature of the respective melts. An analysis of our data in terms of a simple activated rate process model suggests that D0 is controlled by thermally activated hopping of segments whose effective volume is close to that of the respective statistical segment lengths of the copolymeric chains. ©1995 John Wiley & Sons, Inc.
Interfacial tension between low molecular weight polymers from the static analysis of cylindrical drops and their break‐up dynamics
Authors: Quirion François , Pageau Johanne
Published: April 2003

A spinning drop tensiometer was used to measure the interfacial tension between the coexisting phases of mixtures of low molecular weight polypropylene glycol, Mw = 1000, and polyethylene glycol, Mw = 300, 400, or 550. Two types of experiments gave concordant interfacial tensions. First, the static analysis of the equilibrium diameter of cylindrical drops according to Vonnegut and, second, Tomotika's dynamic analysis of Rayleigh instabilities caused by a sharp decrease of the angular speed. The end‐pinching process also lead to the break‐up of the drops and it was characterized by a rapid expansion of the hemispherical ends followed by a slower pinching step. The pinching rate increases as the final angular speed decreases while the growth rate of Rayleigh instabilities shows no definite trends in the range 118‐245 rad · s−1. It is shown that the end‐pinching of long cylindrical drops can be analyzed in terms of short fiber retraction to get the interfacial tension. © 1995 John Wiley & Sons, Inc.
Evaluation of the orientation coefficient for the c axis in poly(ethylene terephthalate) fibers
Authors: Auriemma Finizia , Guerra Gaetano , Parravicini Lidia , Petraccone Vittorio , Russo Giuseppe
Published: April 2003

The literature methods for the determination of the mean of the crystallite orientation distribution for the c axis, that is of the orientation coefficient fc, for poly(ethylene terephthalate) (PET), based on the azimuthal scan of the (105) reflection, are reviewed. These methods appear unsuitable for samples presenting the “tilted orientation”; that is, the molecular chain axis inclined by some degrees with respect to the fiber axis, as frequently occurs for PET fibers. A new method for the determination of fc for PET, also based on the azimuthal scan of the (105) reflection (which can be applied also to samples with “tilted orientation”), is proposed. This method implies as a first step the determination of the tilt angle, for which the complete fiber pattern is required. A possible simplifying assumption, which allows use of the sole azimuthal (105) profile and makes the method also applicable to poorly oriented samples (for which the determination of the tilt angle is not easy), is also discussed. © 1995 John Wiley & Sons, Inc.
The effects of annealing on fatigue crack propagation in polyethylene
Authors: Strebel J. J. , Moet A.
Published: April 2003

Fatigue crack propagation tests on annealed and quenched medium‐density polyethylene showed the annealed specimens to have much lower resistance to crack initiation and subsequent propagation. Although the same fracture mechanism, in which the brittle crack gradually becomes more ductile, prevailed in both cases, the voided and fibrillated crack tip root craze in the annealed material was much weaker that the nonfibrillated quenched root craze. Microstructural analyses indicate that the annealed material had separate crystallite populations, whereas the quenched material had a more homogeneous morphology. The highest melting fraction of the annealed material was composed of lamellae that were about 270 Å thick, and the quenched lamellae were estimated to be 160 Å thick. The reduced fatigue crack propagation resistance of the annealed material was suggested to be a result of a lower concentration of tie molecules and its reduced damping capability, compared to the quenched material. © 1995 John Wiley & Sons, Inc.
Structure and molecular conformation of tussah silk fibroin films: Effect of methanol
Authors: Tsukada Masuhiro , Freddi Giuliano , Monti Patrizia , Bertoluzza Alessandro , Kasai Nobutami
Published: April 2003

Structural changes of tussah (Antheraea pernyi) silk fibroin films treated with different water‐methanol solutions at 20°C were studied as a function of methanol concentration and immersion time. X‐ray diffraction measurements showed that the α‐helix structure, typical of untreated tussah films, did not change for short immersion times (2 min), regardless of methanol concentration. However, crystallization to β‐sheet structure was observed following immersion of tussah films for 30 min in methanol solutions ranging from 20 to 60% (v/v). IR spectra of tussah films untreated and methanol treated for 2 min exhibited strong absorption bands at 1265, 892, and 622 cm−1, typical of the α‐helix conformation. The intensity of the bands assigned to the β‐sheet conformation (1245, 965, and 698 cm−1) increased for the sample treated with 40% methanol for 30 min. Raman spectra of tussah films with α‐helix molecular conformation exhibited strong bands at 1657 (amide I), 1263 (amide III), 1106, 908, 530, and 376 cm−1. Following α → β conformational transition, amide I and III bands shifted to 1668, and to 1241, 1230 cm−1, respectively. The band at 1106 cm−1 disappeared and new bands appeared at 1095 and 1073 cm−1, whereas the intensity of the bands at 530 and 376 cm−1 decreased significantly. ©1995 John Wiley & Sons, Inc.
Characterization and promotion of dilatation bands in toughenable thermosetting resins
Authors: Sue H.‐J. , Bertram J. L. , Garcia‐Meitin E. I. , Puckett P. M.
Published: April 2003

Formation of dilatation bands has been found in a series of moderately crosslinked thermosets with molecular weights between crosslinks, Mc, ranging from 560 to 1000 g/mole. These dilatation bands, although they do not show distinct craze fibrils inside the band, do behave mechanically like the well‐known crazing phenomenon observed in polystyrene. Although the cause for the formation of dilatation bands is still largely unknown, the occurrence of the craze‐like fracture phenomenon (dilatation bands) is found to be strongly influenced by the stress state the matrix experiences, by the type of loading applied to the specimen, and by the type/size of toughener particles incorporated in the matrix. The tendency for dilatation band formation in engineering thermosets can be greatly enhanced by the use of craze‐prone toughener particles, such as nylon particles. The toughening effect via these newly observed dilatation bands is found to be quite impressive, and in some cases, can approach that via the shear‐banding process. The potential significance of the present finding for a better design of toughened thermosets for structural applications is discussed. ©1995 John Wiley & Sons, Inc.
Novel cellulose derivatives. III. Thermal analysis of mixed esters with butyric and hexanoic acid
Authors: Glasser Wolfgang G. , Samaranayake Gamini , Dumay Michelle , Davé Vipul
Published: April 2003

Cellulose derivatives with low degrees of substitution (i.e., DS < 1.5) often fail to reveal glass transition temperatures (Tg) by virtue of their tenacious adherence to moisture, thus preventing systematic analysis of substituent effects (size and DS) on Tg and Tm transitions. On the other hand, cellulose triesters have Tms that decline with acyl substituent size except when the substituent size becomes very large (i.e., > C6), and they have Tgs within 5–20°C of their Tms. This proximity is unusual for a semicrystalline material, and it interferes with the crystallization process that occurs between Tm and Tg. Triesters of cellulose with mixed acyl substituents (one smaller and one larger) allow not only unambiguous observation of Tgs and Tms but also an adjustable Δ(Tm − Tg) window that depends upon the size and the DS of the larger substituent. The materials studied including cellulose acetate butyrate triesters (DSbu 0.8–2.6), cellulose acetate hexanoate triesters (DShex 0–3.0), and cellulose acetate (DSac 2.44), revealed that only the mixed esters, in which the bulkier acyl group is in the range of DS 0.3–1.0, had a Δ(Tm − Tg) value in excess of 40°C. Although the Tm of cellulose acetate hexanoate declined by ca. 150°C per unit of DShex as DShex rose from 0 to 1, this was only ca. 25°C between DShex of 1 and 3. Frequently observed dual‐melt endotherms were attributed to two separate crystal morphologies. ©1995 John Wiley & Sons, Inc.
Thermal conductivity and thermal expansivity of thermotropic liquid crystalline polymers
Authors: Choy C. L. , Wong Y. W. , Lau K. W. E. , Yang Guangwu , Yee A. F.
Published: April 2003

The thermal conductivity and thermal expansivity of a thermotropic liquid crystalline copolyesteramide with draw ratio λ from 1.3 to 15 have been measured parallel and perpendicular to the draw direction from 120 to 430 K. The sharp rise in the axial thermal conductivity Kpar; and the drastic drop in the axial expansivity α∥ at low λ, and the saturation of these two quantities at λ > 4 arise from the corresponding increase in the degree of chain orientation revealed by wide‐angle x‐ray diffraction. In the transverse direction, the thermal conductivity and expansivity exhibit the opposite trends but the changes are relatively small. The draw ratio dependences of the thermal conductivity and expansivity agree reasonably with the predictions of the aggregate model. At high orientation, Kpar; of the copolyesteramide is slightly higher than that of polypropylene but one order of magnitude lower than that of polyethylene. In common with other highly oriented polymers such as the lyotropic liquid crystalline polymer, Kevlar 49, and flexible chain polymer, polyethylene, αpar; of the copolyesteramide is negative, with a room temperature value differing from those of Kevlar 49 and polyethylene by less than 50%. Both the axial and transverse expansivity show transitions at about 390 and 270 K, which are associated with large‐scale segmental motions of the chains and local motions of the naphthalene units, respectively. ©1995 John Wiley & Sons, Inc.
Solution viscosity behavior of complexes of poly(γ‐benzyl‐L‐glutamate) with lightly sulfonated polystyrene or its zinc salt
Authors: Shao L. , Weiss R. A. , Lundberg R. D.
Published: April 2003

The rheological behavior of solutions containing blends of poly(γ‐benzyl‐L‐glutamate) (PBLG) and either the free acid or the zinc salt of lightly sulfonated polystyrene (SPS) was studied as a function of blend composition, polymer concentration, degree of sulfonation of the SPS, and the polypeptide molecular weight. The zinc salt of SPS formed a transition metal complex with the amine‐end groups of the PBLG, and this resulted in an enhancement of the solution viscosity relative to a weighted average of the viscosities of the individual polymer solutions. The ZnSPS/PBLG solutions showed no anomalous time or shear dependencies. In contrast, solutions containing PBLG and the sulfonic acid derivative of SPS also had enhanced viscosities, but in addition, they exhibited time‐dependent viscosities (thixotropic behavior) and shear thickening (dilatant behavior). This was attributed to a nonequilibrium structure of the interpolymer complex due to a competition between acid‐amine and acid‐ester interactions. Although the acid‐amine interaction is enthalpically favored, when sufficient sulfonic acid groups were available, interactions between the sulfonic acid and the glutamate ester side groups of PBLG developed and this interaction promotes a helix‐to‐coil transition of the PBLG. ©1995 John Wiley & Sons, Inc.
Miscibility of poly(ε‐caprolactone), a semicrystalline polymer, with amorphous poly(styrene‐4‐hydroxystyrene) copolymers
Authors: Vaidya Milind M. , Levon Kalle , Pearce Eli M.
Published: April 2003

The miscibility of poly(ε caprolactone) (PCL) with poly(styrene‐co‐4‐hydroxystyrene) (PHS) copolymers was investigated as a function of comonomer composition experimentally and with calculations by two models; the binary interaction model and the association model. PCL was found to be completely miscible with PHS copolymers containing 5 or more mole percent of 4‐hydroxystyrene (HS) comonomer units for the entire range of blend compositions. Segmental interaction densities, Bijs, were determined by the analysis of the equilibrium melting point depression and by the application of the binary interaction model. By correlating the segmental interaction energy densities with the binary interaction model, thermodynamic miscibility is for comonomer composition over five mole percent of 4‐hydroxystyrene, which is in agreement with the experimental phase behavior. Application of the association model for specific interactions to blends also predicts the experimental miscibility boundary and phase behavior for all the PHS copolymers/PCL blends. © 1995 John Wiley & Sons, Inc.
Crowned polyacetylene. 2. cis form determination of poly(4′‐ethynylbenzo‐15‐crown‐5) by solid‐state NMR method
Authors: Kakuchi Toyoji , Matsunami Shigeyuki , Kamimura Hiroyuki , Ishii Fumiaki
Published: April 2003

The chain structure of cis‐poly(4′‐ethynylbenzo‐15‐crown‐5) (PEB15C5), prepared using the rhodium complex catalyst, was studied at −175°C using the solid‐state NMR technique. The second moment of PEB15C5 was 7.0 G2 for the main chain and 19.0 G2 for the side chain. These values agreed well with the respective theoretical ones for the cis form in the rigid state rather than the trans one. This agreement between experiment and theory suggested that the plane of the side chain (i.e., the benzo‐15‐crown‐5 group) is orthogonal to the main chain plane of the cis‐transoidal form. It was verified that the strong dipoledipole interaction between the side chains plays an important role as a clue to theoretically determine the cis‐transoidal form. NMR results in solid state confirmed theoretically that multipeaks in the 1H and 13C NMR spectra in CDCl3 solution are attributed to cis‐transoidal form in the chain. © 1995 John Wiley & Sons, Inc.
Phase‐matched second‐harmonic generation in poled polymer waveguide based on a main chain polymer
Authors: Tao X. T. , Watanabe T. , Zou D. C. , Shimoda S. , Usui H. , Sato H. , Miyata S.
Published: April 2003

The electric field‐induced dynamic phase‐matching of second harmonic generation (SHG) waveguide was demonstrated by using a main chain polyarylamine. The linear and nonlinear optical properties of this polymer are presented. The optimum phase‐matching thickness was controlled by applying an electric field to the polymer waveguide. The effective phase‐matching thickness variation induced by poling is about 0.025 μm that is six times larger than full width at the half‐maximum (FWHM) of phase‐matching thickness in conventional slab waveguide. The efficient phase‐matched SHG was observed from a taperless slab wave‐guide. The optical loss of poled polymer on glass substrate at 632.8 nm was 2.7 dB/cm. © 1995 John Wiley & Sons, Inc.
Hydrodynamic behavior of anionically prepared linear polyisoprenes and polystyrenes in carbon tetrachloride
Authors: Allorio Serena , Pispas Stergios , Siakali‐Kioulafa Ekaterini , Hadjichristidis Nikos
Published: April 2003

Intrinsic viscosity, [η], weight‐average molecular weight, Mw, relationships are reported for narrow molecular weight distribution linear polyisoprene and polystyrene samples in CCl4 at 25°C. Molecular weight values cover a range nearly two orders in magnitude, extending as low as 3 × 103. In the case of polystyrene there exists a molecular weight range (around Mw = 16,700) corresponding to a change in the Mark‐Houwink‐Sakurada (MHS) exponent from α = 0.71 to α = 0.54. Comparisons between the viscometric and hydrodynamic radii, from literature data, are made. For polyisoprene the MHS relationship is reported in CCl4, for the first time. For this case α = 0.713 for the whole range of molecular weights studied. Values for the second virial coefficient from low‐angle light‐scattering measurements support the conclusions drawn from viscometry that CCl4 is a good solvent for both polymers studied. The different behavior of the MHS exponent may be attributed to the difference in chain flexibility. © 1995 John Wiley & Sons, Inc.
Ostwald ripening in immiscible polyolefin blends
Authors: Mirabella Francis M. , Barley Jeffrey S.
Published: April 2003

The coarsening in the quiescent melt of the phase‐segregated particles of a polymer blend, composed of a narrow molecular weight fraction of an unbranched high‐density polyethylene (HDPE) and a highly branched (100 ethyl branches/1000 C atoms) hydrogenated polybutadiene (HPB) was studied. The system was effectively binary, due to the narrow molecular weight and composition distributions of each component. The system was composed of 90 wt % of the HDPE and 10 wt % of the HPB and it formed a two‐phase system in the melt at 177°C. The blend was precipitated from xylene solution in order to obtain an initially intimately mixed system. This was the third study in a series of studies of the coarsening of phase‐segregated particles in polymer blends. This study was unique in that the system studied was binary in this case while the previous systems were multicomponent. Since the present system was binary, exact thermodynamic calculations of the phase state of this system could be applied with a high level of confidence. The droplet phase particles, which were mainly composed of the HPB, were observed to coarsen on storage in the melt for times of from 5 s to 1 h. At the shortest storage time of 5 s the particles had an average radius of about 0.05 μm and coarsened to about 0.2 μm after 1 h storage in the melt state. Particle dimensions were measured by scanning electron microscopy of n‐heptane‐etched and gold‐coated sections. It was found that the volume of the particles increased linearly with time and that the rate constant of coarsening was Kexp = 1.23 × 10−18 cm3/s and this agreed fairly well with the rate constant calculated from Ostwald ripening theory of Kce = 0.86 × 10−18 cm3/s. In contrast the rate constant for direct particle diffusion and coalescence was Kc = 3.6 × 10−20 cm3/s. Since this was two orders of magnitude smaller than the rate constant for Ostwald ripening, it was concluded that, although the linear increase of volume with time was also consistent with the particle diffusion and coalescence mechanism, this was not a significant contributor to the coarsening mechanism. The major cause for the insignificance of the particle diffusion and coalescence mechanism was the high melt viscosity of the matrix polymers. The application of the Ostwald ripening theory to this system could be made with a high level of confidence because it was binary. It was found that the phase concentration of the droplet phase apparently underwent a rapid increase during the first 1‐2 min of storage in the melt, indicating that the system did not reach phase equilibrium (i.e., did not completely phase‐segregate) for about 1‐2 min. This further indicated that the long‐time coarsening regime was not entered until after this length of time. The particle size distributions remained approximately self‐similar over the period of coarsening, as predicted by Ostwald ripening theory. © 1995 John Wiley & Sons, Inc.
Self‐consistent field equations in the distribution function theory of polymeric liquids
Authors: Gan Hin Hark , Eu Byung Chan
Published: April 2003

In the distribution function approach to the conformational and thermodynamic properties of polymeric liquids site‐site (pair) distribution functions are essential components of the theory. These site‐site pair distribution functions are basically mean fields obeying integral equations. In our recent works, a set of self‐consistent field equations has been proposed for site‐site pair correlation functions which allow us to study conformational and thermodynamic properties of polymeric liquids. In this article, we present a short review of the theory and its applications to a number of aspects of polymeric liquids we have made until now. We also present a self‐consistent version of the polymer reference interaction site model where the integral equations for the intramolecular site‐site correlation functions are obtained from the Kirkwood hierarchy on the basis of the present theory. The present theory is shown to predict correctly the scaling properties associated with swollen and collapsed polymers in good and poor solvents, respectively. At finite densities, self‐consistent solutions of the intra‐ and intermolecular equations yield the structures and thermodynamics of polymer melts which are favorably compared with Monte Carlo simulation results. Self‐consistent theory results are found to be more accurate than the non‐self‐consistent approaches that use an ideal Gaussian chain conformation distribution function. © 1995 John Wiley & Sons, Inc.
Light‐scattering studies on phase separation in a binary blend with addition of diblock copolymers
Authors: Sung Lipiin , Han Charles C.
Published: April 2003

Time‐resolved light‐scattering measurements have been conducted to investigate the influence of a diblock copolymer additive on the phase boundaries and the kinetics of the phase separation of a polymer blend. The blend studied was a polystyrene‐d8/polybutadiene (PSD/PB) mixture with a diblock copolymer composed of the same homopolymers. It was observed that the critical temperature of the blend, which has an upper critical solution temperature (UCST), decreased with increasing copolymer content and the kinetics of the phase separation via a spinodal decomposition mechanism slowed down in the presence of the copolymer. The features of the spinodal peak position and intensity as a function of time with and without copolymer additive were analyzed for near and off‐critical compositions in various temperature jumps. The intermediate and late‐stage growth rates do not follow a universal scaling function with the addition of diblock copolymers. © 1995 John Wiley & Sons, Inc.
Study of the molecular mobility of collagen
Authors: Mezghani S. , Lamure A. , Lacabanne C.
Published: April 2003

The dielectric relaxations of a biopolymer, collagen, have been studied by means of the thermally stimulated current (TSC) technique. To investigate effects of water on TSC spectra, complementary measurements by differential scanning calorimetry (DSC) have been performed. In dehydrated collagen, three spectral bands were recorded in the temperature range of ‐180‐60°C, labelled α, β1, and γ in the order of decreasing temperature. The TSC spectrum of hydrated collagen exhibits a supplementary band labelled β2. By using the fractional polarization method, the γ and β1 modes have been attributed to localized movements of nonpolar (apolar) sequences and polar sequences, respectively. The α mode has been associated with delocalized movements of the collagen macromolecules. Finally the β2 mode has been attributed to reorientations of bound water molecules inside the collagen fibers. © 1995 John Wiley & Sons, Inc.