Showing papers in "Journal of Polymer Science Part B in 1997"

Structure and thermal/mechanical properties of poly(l‐lactide)‐clay blend

Abstract: Organophilic montmorillonite was obtained by the reaction of montmorillonite (MON) and distearyldimethylammonium chloride (DSAC). The modified clay and poly(l-lactide), (PLLA), were solvent-cast blended using chloroform as cosolvent. The structure and properties of the PLLA-clay blends were investigated. Thermal measurements revealed that cold crystallization took place in the as-cast PLLA, and that the clay served as a nucleating agent. From small and wide-angle x-ray scattering measurements, it was found that silicate layers forming the clay could not be individually well dispersed in the PLLA-clay blends prepared by the solvent-cast method. In other words, the clay existed in the form of tactoids, which consist of several stacked silicate monolayers. However, these tactoids formed a remarkable geometrical structure in the blend films. That is, their surfaces lay almost parallel to the film surface, and were stacked with the insertion of PLLA crystalline lamellae in the thickness direction of the film. During the blend drawing process, fibrillation took place with the formation of plane-like voids developed on the plane parallel to the film surface. Furthermore, delamination of the silicate layers did not occur even under the application of a shearing force. Finally, Young's modulus of the blend increased with the addition of a small amount of the clay. © 1997 John Wiley & Sons, Inc.

Relaxations of confined chains in polymer nanocomposites: Glass transition properties of poly(ethylene oxide) intercalated in montmorillonite

Abstract: The relaxation behavior of poly(ethylene oxide) (PEO), intercalated in montmorillonite, a naturally occurring mica-type silicate, was studied by differential scanning calorimetry ( DSC and thermally stimulated dielectric depolarization ( or thermally stimulated current, TSC). The materials were synthesized by melt or solution-mediated intercalation. In both intercalates, the PEO chains were confined to ca. 0.8-nm galleries between the silicate layers. The solution intercalate contained a fraction of uninterca-lated PEO chains which exhibited a weak and depressed PEO melting endotherm in DSC. In contrast, the melt intercalate was starved such that almost all the PEO chains were effectively intercalated. For these melt intercalates, no thermal events were detected by DSC. TSC thermal sampling technique was used to examine the glass transition regions and to estimate the extent of cooperativity of chain motions. The motions of the intercalated PEO chains are inherently noncooperative relative to the cooperative T g motions in the amorphous portion of the bulk polymer. This is presumably due to the strong confining effect of the silicate layers on the relaxations of the intercalated polymer.

A SAXS study of the internal structure of dendritic polymer systems

Abstract: Small-angle x-ray scattering was used to characterize the single-particle scattering factors produced by poly(amidoamine) dendrimers, poly(propleneimine) dendrimers, and polyol hyperbranched polymers in dilute solutions with methanol as solvent. Fits from electron density modeling reveal similar overall densities of the dendrimers as a function of dendrimer generation. The seventh through tenth generation poly(amidoamine) dendrimers exhibit higher order scattering features that require nearly monodisperse, spherical particles with essentially uniform internal segment densities. Dilute hyperbranched polymer solutions exhibit scattering more indicative of the inherent irregularity of internal segment densities and overall sizes to be expected within these systems. Radii of gyration estimated from electron density modeling agree reasonably well with those estimated by standard Guinier methods used in previous studies.

Conformationally asymmetric block copolymers

Abstract: The standard parameters controlling AB diblock copolymer phase behavior are xN and fA, where x is an A-B segment interaction parameter, N is the overall degree of polymerization, and fA is the volume fraction of the A block. Recently, it has been recognized that the ratio of the A and B statistical segment lengths aA/aB also represents another important parameter. Here, we theoretically examine the effects of this latter parameter on the phase behavior using the standard Gaussian chain model. Calculations are performed using both self-consistent field theory (SCFT) and strong segregation theory (SST). The ratio aA/aB is shown to have strong effects on order- order phase boundaries. Furthermore, it significantly affects the relative stability of the complex phases. In particular, it enhances the metastability of the perforated lamel- lar phase and may actually cause it to become an equilibrium structure. We also illustrate that varying aA/aB produces large changes in the relative domain spacings at order-order phase boundaries, which could strongly affect the kinetics of these transitions. q 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 945-952, 1997

A new attempt to reconcile the statistical and phenomenological theories of rubber elasticity

Abstract: The statistical and phenomenological theories of rubber elasticity are reviewed briefly. Combining recent concepts proposed by Yeoh and Gent, a new theory is proposed. The proposed constitutive model for rubber vulcanizates invokes two mechanisms; one influences behavior at small strains while the other dominates behavior at large strains. Network flaws, such as entanglements, are suspected to be responsible for the first mechanism. Finite extensibility of network chains is identified as the cause of the second. Thus, macroscopic behavior is directly linked to molecular concepts. The proposed theory allows prediction of the stress–strain behavior of a family of four rubber vulcanizates in different modes of deformation (simple extension, compression, and simple shear) from regression analysis of tensile data alone from just one member. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1919–1931, 1997

Processing and physical property relationships in injection‐molded isotactic polypropylene. 1. Mechanical properties

Abstract: The aim of the research reported in these two articles was to explore the relationship between processing conditions and the physical properties of different grades of isotactic polypropylene injection moldings and propylene/ethylene copolymers. This first article describes the methods and processing conditions used for molding, together with mechanical test results. Both conventional and shear-controlled orientation injection molding (SCORIM) have been employed for the production of moldings. SCORIM is based on the application of specific macroscopic shears to a solidifying melt, which in turn, facilitates enhanced molecular alignment. SCORIM results in more pronounced molecular orientation than conventional injection molding, which is consistent with the substantial increase in Young's modulus of moldings produced by SCORIM. By controlling the processing parameters it is possible to control and enhance the stiffness without loss of tensile strength. An increase of up to four times in impact strength has been achieved with SCORIM as well as a substantial increase in Young's modulus. The conventional injection moldings containing pronounced molecular orientation exhibited impact resistance well below that for the SCORIM moldings. The mechanical tests carried out at 80°C showed that the high-temperature mechanical properties of all the materials, converted into moldings using SCORIM, exhibited substantial enhancement when compared with moldings of the same material converted by conventional injection molding. © 1997 John Wiley & Sons, Inc.

Water absorption and desorption in an epoxy resin with degradation

Abstract: Hygrothermal aging at elevated temperatures tends to induce degradation in epoxy resins. To predict the effects of this degradation, a knowledge of absorption and transport behavior of water is needed. In this work, a model material (DGEBA/DDA) has been employed to study the water absorption and absorption/desorption behavior during hygrothermal aging at 90°C, accompanied by degradation. The absorption results show an weight increase during the initial aging period followed by a decrease at later times. Absorption/desorption results show a similar phenomenon but with a net, overall weight loss after a certain period of aging. By assuming that water diffusion is approximately Fickian and that degradation of the resin is mainly caused by hydrolysis reactions, a model has been developed to describe the above-observed phenomena. Results show that the model is in good agreement with experimental data. Moreover, the model proposed can be used to estimate the average molecular weight of the intercrosslink chains after aging. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2659–2670, 1997

Adhesion of silicon oxide layers on poly(ethylene terephthalate). I: effect of substrate properties on coating's fragmentation process

Abstract: Reference LTC-ARTICLE-1997-008View record in Web of Science URL: http://www3.interscience.wiley.com/cgi-bin/jhome/36698 Record created on 2006-06-26, modified on 2016-08-08

Processing and Physical Property Relationships in Injection-Molded Isotactic Polypropylene .2. Morphology and Crystallinity

Abstract: A preceding article referred to the processing of various grades of isotactic polypropylene (iPP) and propylene/ethylene copolymers by conventional and shear-controlled orientation injection molding (SCORIM), and considered the mechanical properties of the molded polymers in relation to the processing conditions applied. This article is principally concerned with the morphology and phase relationships that apply in injection-molded iPP. The γ-phase of isotactic polypropylene is reported to occur in moldings with pronounced molecular orientation. The presence of γ-phase in injection moldings is indicative of enhanced Young's modulus, and tensile strength provided that the melt has not been subjected to excessive deformation during processing. The common view that isotactic polypropylene exhibits only α- and β-phases in commercially produced moldings is challenged. SCORIM moldings contain less β-phase than conventional moldings, and exhibit greater overall crystallinity than moldings produced by conventional methods. The difference in the relative proportions of α-, β-, and γ-phases is marked and depends on processing conditions and the molding method. Shear-controlled orientation injection molding (SCORIM) results in more pronounced molecular orientation than conventional injection molding, which is consistent with the substantial increase in Young's modulus of moldings produced by SCORIM. The improved mechanical properties of iPP moldings is attributed to shish-kebab morphology developed by the action of shear to the solidifying melt. A new model is proposed for the oriented region morphology in iPP injection moldings based on the results presented in this article. © 1997 John Wiley & Sons, Inc.

Measurement of crystalline index in nylons by DSC: Complexities and recommendations

Abstract: Differential scanning calorimetry (DSC) is one of the most widely used technique for measuring crystallinity in the polymer industry. The major source of error in the crystalline index (CIDSC) of low crystallinity polymeric articles, is the development of further crystallinity during the DSC scan. Although, this type of cold crystallization is obvious, and thus accounted for in polymers like polyethylene terephthalate, nylons are a difficult class of materials in that respect. The major contributing factors to the failure of DSC in measuring low levels of crystallinity in nylons are identified to be (1) silent crystallization between the glass (Tg) and melting (Tm) transitions, (2) extreme difficulties in packing a moisture-free nylon in the sample pan (the response due to traces of moisture being a broad endotherm competing with a broad exothermic crystallization), and (3) a sub-Tm exotherm, especially in low crystallinity nylons, due to relaxation of the processing-induced stresses. These factors, specific to nylons, mask the observation of cold crystallization and lead to substantially higher than real crystallinities. This manuscript deals with such complications and corrective actions using commercial nylon 6 films of CIDSC = 0−40%. X-ray diffraction measurements have been included to support the validity of our improved DSC methodology. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2219–2231, 1997

Self‐organization and polyolefin nucleation efficacy of 1,3:2,4‐di‐p‐methylbenzylidene sorbitol

Abstract: Recent studies have demonstrated that addition of a small quantity of dibenzylidene sorbitol (DBS) to a molten polymer may result in a physical gel if conditions permit the DBS molecules to self-organize into a three-dimensional network composed of highly connected nanofibrils. If the polymer crystallizes, DBS may also serve as a nucleating agent, promoting the formation of spherulites, especially in commercially important polyolefins such as polypropylene. We examine the thermal and mechanical properties, as well as the morphological characteristics, of an isotactic polypropylene copolymer with 3 wt % ethylene upon addition of less than 1 wt % of 1,3:2,4-di-p-methylbenzylidene sorbitol (MDBS). From dynamic rheological measurements, pronounced complex viscosity increases, attributed to MDBS nanofibril network formation, are observed at concentration-dependent temperatures above the melting point of the nucleated copolymer. Transmission electron micrographs of RuO4-stained sections confirm the existence of MDBS nanofibrils measuring on the order of 10 nm in diameter and, at higher concentrations, fibrillar bundles measuring up to about 200 nm across and several microns in length. The addition of MDBS at different concentrations is also found to promote increases in optical clarity, yield strength, tensile strength, and ultimate elongation of modified copolymer formulations. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2617–2628, 1997

Adhesion of silicon oxide layers on poly(ethylene terephthalate). II: Effect of coating thickness on adhesive and cohesive strengths

Abstract: Reference LTC-ARTICLE-1997-007View record in Web of Science URL: http://www3.interscience.wiley.com/cgi-bin/jhome/36698 Record created on 2006-06-26, modified on 2016-08-08

Use of living radical polymerizations to study the structural evolution and properties of highly crosslinked polymer networks

Abstract: Crosslinked polymer networks are used in a wide variety of applications. To use these materials effectively, a fundamental understanding of their structural evolution and the relationship between material properties and structure is essential. In this article, a novel technique employing “iniferters,” i.e., living radical polymerizations, to photopolymerize these networks is utilized to study the property and structural evolution of these highly desirable materials. Living radical polymerizations are used in this work since this technique avoids the problem of carbon radical trapping encountered while using conventional initiators. Dynamic mechanical measurements are performed on highly crosslinked methacrylate networks to glean information regarding their structural heterogeneity. By performing these measurements on homopolymerized samples at various stages of the reaction and on copolymerized samples of multifunctional methacrylates, the mechanical properties are characterized as a function of double bond conversion and comonomer composition. From such analyses, with respect to both temperature and frequency, quantitative conclusions regarding the structure of the networks are drawn. This effort is aimed at exploiting the living radical polymerizations initiated by p-xylylene bis(N,N-diethyl dithiocarbamate) (XDT), to study the mechanical property evolution and structural heterogeneity of crosslinked polymers which is nearly impossible otherwise. Polymers examined in this study include networks formed by homopolymerization of diethylene glycol dimethacrylate (DEGDMA) and polyethylene glycol 600 dimethacrylate (PEG600DMA) as well as copolymers of DEGDMA and PEG600DMA. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2297–2307, 1997

Temperature-induced phase transition of poly(N,N-dimethylaminoethyl methacrylate-co-acrylamide)

Abstract: Copolymers of N,N-dimethylaminoethyl methacrylate (DMAEMA) and acrylamide (AAm) were prepared to demonstrate a temperature-induced phase transition. Poly DMAEMA has a lower critical solution temperature (LCST) around 50°C in water. With copolymerization of DMAEMA with AAm, the LCST shifts to the lower temperature was observed, probably due to the formation of hydrogen bonds between amide and N,N-dimethylamino groups. FT-IR studies clearly show the formation of hydrogen bonds which protect N,N-dimethylamino groups from exposure to water and result in a hydrophobic contribution to the LCST. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 595–598, 1997

Structure?conductivity relationships of iodine-doped polyaniline

Abstract: Polyaniline, synthesized by using potassium dichromate as the oxidant, was doped with iodine in order to increase its electrical conductivity. The iodine-doped polyanilines attained a conductivity of 1.83 × 10−3 S/cm, which was about eight orders of magnitude greater than that of intrinsic polyaniline. The iodine-doped polyanilines did not absorb moisture readily when compared to the protonic-acid-doped polyanilines. Fourier transform infrared (FTIR) and x-ray photoelectron spectroscopy (XPS) results indicated that iodine-doping reactions occurred at the N-atoms in the quinoid structural units of the polyaniline molecular chains and consequently formed the charge transfer complexes. The iodine in the iodine-doped polyanilines existed mainly in the forms of I and I anions. As the doping level increased, the relative content of I anions increased. Thermogravimetric analysis (TGA) results showed that there was about 6 wt % of iodine strongly bonded to the polyanilines since they would not evolve even at the structural decomposition temperatures of the polymer backbones. Wide-angle x-ray diffraction spectroscopy (WAXD) results revealed that the intrinsic polyaniline was an amorphous polymer but the regularity of polyaniline chains increased after iodine-doping. The iodine-doped polyanilines also showed a decrease in thermal stability. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1993–2001, 1997

The first 85% kraft lignin-based thermoplastics

Abstract: Interest in the development of lignin-containing polymeric materials has been upheld more or less continuously for the past 20 years. Tendencies toward high moduli and poorly defined thermal transitions have been regarded as imposing inevitable limitations upon the use of lignin derivatives for such purposes. Incorporation of more than 25–40% (w/w) lignin had usually resulted in materials that were brittle and weak. For the first time, however, from homogeneous blends containing 85% (w/w) underivatized industrial kraft lignin with poly(vinyl acetate) and two plasticizers, a series of thermoplastics has been fabricated with promising mechanical properties. The tensile behavior of these new polymeric materials depends directly upon the degree of association between the intrinsic kraft lignin components. In extending to values about 25 MPa and 1.5 GPa, respectively, the tensile strengths and Young's moduli vary linearly with the effective Mw for the kraft lignin species, under conditions where the proportions of the individual molecular components, both associated and discrete, do not change. Moreover, melt-flow index measurements indicate that these polymeric materials are amenable to thermal processing by extrusion molding. Thus a significant step has been taken toward developing a new generation of thermoplastics that are lignin-based in a very fundamental way. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1899–1910, 1997

Physical aging of a polyetherimide: Volume recovery and its comparison to creep and enthalpy measurements

Abstract: Volume recovery measurements have been used to study the physical aging behavior of a polyetherimide. Isothermal aging temperatures near Tg were studied with aging times ranging up to several days. The volume decreases during physical aging and levels off at equilibrium. For comparison purposes, the data are normalized to yield the departure from equilibrium which varies from unity at very short aging times to zero when equilibrium is reached. As the aging temperature decreases, the normalized curves are shifted to longer times without a significant change in shape. Hence, the data can be reduced by aging time—temperature superposition. The temperature dependence of the shift factors used to reduce the volume recovery data and the times to reach equilibrium for the volume recovery follow the WLF equation and agree within experimental error with the values from enthalpy and creep measurements obtained in previous work. However, the approach to equilibrium for volume appears to differ from that of enthalpy, with volume recovery being faster than the enthalpy recovery at short times. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 929–936, 1997

Vapor sorption in emptied clathrate samples of syndiotactic polystyrene

Abstract: The analysis of chloroform vapor sorption at 35°C in semicrystalline syndiotactic polystyrene samples shows remarkably different sorption isotherms, depending on the crystalline form of the samples. In particular, “emptied” clathrate (“emptied” δ form) samples are characterized by higher equilibrium sorption levels and the differences are particularly relevant for low vapor activities. Moreover, sorption kinetics detected at a vapor activity equal to 0.5 show that in the case of “emptied” δ form samples the sorption rate is much higher than for the other semicrystalline samples. The larger sorption equilibrium uptakes and sorption rates of the “emptied” δ form samples are essentially due to their ability to absorb chloroform, already for low activities, by clathration in the crystalline phase. The measured equilibrium uptakes and sorption kinetics suggest that “emptied” δ form samples of syndiotactic polystyrene could be suitable for removing polluting chlorinated compounds from vapor and liquid streams. © 1997 John Wiley & Sons, Inc.

Monte Carlo simulations of polymer dynamics: Recent advances

Abstract: A brief review is given of applications of Monte Carlo simulations to study the dynamical properties of coarse-grained models of polymer melts, emphasizing the crossover from the Rouse model toward reptation, and the glass transition. The extent to which Monte Carlo algorithms can mimic the actual chain dynamics is critically examined, and the need for the use of coarse-grained rather than fully atomistic models for such simulations is explained. It is shown that various lattice and continuum models yield qualitatively similar results, and the behavior agrees with the findings of corresponding molecular dynamics simulations and experiments, where available. It is argued that these simulations significantly enhance our understanding of the theoretical concepts on the dynamics of dense macromolecular systems. © 1997 John Wiley & Sons, Inc.

Interfacial tension coefficient from the retraction of ellipsoidal drops

Abstract: Keywords: interfacial tension ; polymer blends ; ellipsoid retraction Reference LTC-ARTICLE-1997-011 URL: http://www3.interscience.wiley.com/cgi-bin/jhome/36698 Record created on 2006-06-26, modified on 2016-08-08

The relative role of coalescence and interfacial tension in controlling dispersed phase size reduction during the compatibilization of polyethylene terephthalate/polypropylene blends

Abstract: The breaking thread and the sessile drop methods have been used to evaluate the interfacial tension between a polypropylene ( PP ) and a polyethylene-terephthalate (PET). An excellent correlation was found between the two. The breaking thread technique was then used to evaluate the interfacial tension of these blends at various levels of a styrene-ethylene butylene-styrene grafted with maleic anhydride (SEBS-g-MA) compatibilizer. In order to evaluate the relative roles of coalescence and interfacial tension in controlling dispersed phase size reduction during compatibilization, the morphology of PP/PET 1/99 and 10/90 blends compatibilized by a SEBS-g-MA were studied and compared. The samples were prepared in a Brabender mixer. For the 10/90 blend, the addition of the compatibilizer leads to a typical emulsification curve, and a decrease in dispersed phase size of 3.4 times is observed. For the 1/99 blend, a 1.7 times reduction in particle size is observed. In the latter case, this decrease can only be attributed to the decrease of the interfacial tension. It is evident from these results that the drop in particle size for the 10/90 PP/PET blend after compatibilization is almost equally due to diminished coalescence and interfacial tension reduction. These results were corroborated with the interfacial tension data in the presence of the copolymer. A direct relationship between the drop in dispersed phase size for the 1/99 PP/PET blend and the interfacial tension reduction was found for this predominantly shear mixing device.

Morphological stability, interfacial tension, and dual-phase continuity in polystyrene-polyethylene blends

Abstract: The morphological stability of polystyrene high-density polyethylene (PS/PE) blend is investigated in the region of dual-phase continuity. The effect of the addition of a triblock SEBS copolymer to the blends on the stability of these morphologies, is examined. The results show that the morphology of the unmodified blends changes from co-continuous to droplet matrix for PS-rich blends whereas the morphology of a 50/50 blend maintains continuity but coarsened significantly upon annealing at 200°C. In the presence of the copolymer, these morphologies are much more stable. Selective solvent extraction of polystyrene in di-ethyl ether reveals that the level of PS continuity in the 50/50 blend is higher for the unmodified system than for the modified one. Upon annealing, the level of PS continuity significantly increases for the unmodified 50/50 PS/PE blend. The effect of the copolymer content in the blend on the interfacial tension between the two components is also investigated using the breaking thread method. The interfacial tension is found to be reduced from 5.6 to 1.1 mN/m by the addition of 20 parts of the copolymer to the blend. © 1997 John Wiley & Sons, Inc.

Structure and molecular conformation of Tussah silk fibroin films : Effect of heat treatment

Abstract: Structural changes of tussah (Antheraea pernyi) silk fibroin films induced by heat treatment were studied as a function of the treatment temperature in the range 200–250°C. The DSC curve of tussah films with α-helix molecular conformation displayed characteristic endo and exo peaks at 216 and 226°C, respectively. These peaks first weakened and then completely disappeared after heating at 230°C. Accordingly, the TMA thermal shrinkage at 206°C disappeared when the films were heated at 230°C. The onset of weight loss was monitored at 210°C by means of TG measurements. X-ray diffraction profiles gradually changed from α-helix to β-sheet crystalline structure as the treatment temperature increased from 200 to 250°C. On raising the heating temperature above 200°C, the intensity of IR and Raman bands characteristic of β-sheet conformation increased in the whole ranges of amide and skeletal modes. The sample treated at 200°C showed a spectral pattern intermediate between α-helix and β-sheet molecular conformation. The IR marker band for random coil structure, still detectable at 200°C, disappeared at higher treatment temperatures. Spectral changes attributable to the onset of thermal degradation appeared at 230°C. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 841–847, 1997

Rheology of polydimethylsiloxane swollen with supercritical carbon dioxide

Abstract: Viscosity curves were measured for polydimethyl siloxane (PDMS) melts swollen with dissolved carbon dioxide at 50 and 80°C for shear rates ranging from 40 to 2300 s−1, and for carbon dioxide contents ranging from 0 to 21 wt %. The measurements were performed with a capillary extrusion rheometer modified for sealed, high-pressure operation to prevent degassing of the melt during extrusion. The concentration-dependent viscosity curves for these systems are self-similar in shape, exhibiting low-shear rate Newtonian plateau regions followed by shear-thinning “power-law” regions. Considerable reduction of viscosity is observed as the carbon dioxide content is increased. Classical viscoelastic scaling methods, employing a composition-dependent shift factor to scale both viscosity and shear rate, were used to reduce the viscosity data to a master curve at each temperature. The dependence of the shift factors on polymer chain density and free volume were investigated by comparing the shift factors for PDMS-CO2 systems to those obtained by iso-free volume dilutions of high molecular weight PDMS. This comparison suggests that the free volume added to PDMS upon swelling with dissolved carbon dioxide is the predominant mechanism for viscosity reduction in those systems. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 523–534, 1997

The relationship between intramolecular hydrogen bonds and certain physical properties of regioselectively substituted cellulose derivatives

Abstract: This article tries to provide some direct evidence about the relationship between the intramolecular hydrogen bonds in cellulose and their corresponding effect on physical properties. The formation of intramolecular hydrogen bonds has been proved to contribute directly to certain physical properties of cellulose, such as its solubility in solvents having different polarities, the relative reactivities of the hydroxyls in a repeating unit and its crystallinity, using a 6-0-methylcellulose (6MC) film that was known' to have intramolecular hydrogen bonds. The excellent solubility of 6MC when compared with other cellulose derivatives indicated a lack of interchain hydrogen bonds. A comparison of the relative reactivities between the C-2 and C-3 position hydroxyls in 6MC also indicates that intramolecular hydrogen bonds once formed in 6MC films are possibly maintained even after dissolution in solvents. In addition, the poor crystallinity exhibited by 6MC supports the idea that crystallization in cellulosics may be dependent more upon preferencial interchain hydrogen bonding at the C-6 position hydroxyls than upon a uniform structure such as that found in 6MC, where every structural unit is completely and regioselectively substituted, distinguishing it from other synthetic polymers such as polyolefins and polyesters.

Conformation transition of silk fibroin induced by blending chitosan

Abstract: The conformation of silk fibroin in silk fibroin/chitosan (SF/CS) blend membrane was analyzed by infrared spectrum, X-ray diffractometry, and Raman spec- trum. The results demonstrated that the SF could show b-sheet conformation when the SF content in blend membranes was 10% (w/w) and 60-80% ( w/w), while the pure SF membrane showed random coil conformation. A mechanism of the conformation transition was suggested in that the SF chain could use the rigid CS chain as a mold plate to stretch itself to form a b-sheet structure according to the strong hydrogen bond between CS and SF. Therefore, a new concept, named ''Polymer-Induced Conformation Transition,'' was proposed. q 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2293-2296, 1997

Acrylic acid polymerization kinetics

Abstract: The kinetics of the isothermal polymerization of acrylic acid were determined utilizing 1H-NMR spectroscopy. The polymerization rate was observed to depend approximately on the power of monomer and the power of sodium persulfate concentration. This is consistent with a model in which the rate of initiation is itself dependent on the monomer concentration. The polymerization rate was also observed to have a strong dependence on percent neutralization, decreasing with increasing level of neutralization up to 75 to 100% neutralization, and then increasing again. The activation energy for the rate of polymerization was between 9 and 13 kcal/mol except for 100% neutralized acrylic acid, which had an activation energy of 18 kcal/mol. These data suggest that a transition in mechanism occurred at 100% neutralization. Increasing the ionic strength by the addition of sodium chloride also increased the rate. The dependence of the molecular weight on the above variables was also quantified for use in the model. It decreased with increasing conversion, decreasing ionic strength and increasing initiator. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2029–2047, 1997

Multicomponent latex IPN materials: 2. Damping and mechanical behavior

Abstract: The integrals of the linear loss shear modulus vs temperature (loss area, LA) and linear tan δ vs temperature (tan δ area, TA) were characterized for various core/shell latex particles with synthetic rubber, poly(butadiene-stat-styrene) [P (Bd/S), 90/10], and interpenetrating polymer networks (IPN) as the cores The IPN cores were composed of P(Bd/S) (Tg ≃ − 70°C) and an acrylate based copolymer (Tg around 10°C) for potential impact and damping improvement in thermoplastics Poly(styrene-stat-acrylonitrile) (SAN, 72/28) was the shell polymer for all these polymers Under the same loading, for both toughening and damping controls, among the IPN core/shell, blend of separate core/shell, and multilayered core/shell polymers, the IPN core/shell polymers were the best dampers However, the other core/shell polymers also showed higher LA values than P(Bd/S)/SAN core/shell polymer A comparison of LA values via a group contribution analysis method was made, the effect of particle morphology and phase continuity on damping being studied Inverted core/shell latex particles (glassy polymer SAN was synthesized first) showed much higher LA and TA values than normal core/shell ones (rubbery polymer was synthesized first) Models for maximum LA and TA behavior are proposed The damping property was essentially controlled by the phase miscibility and morphology of the core/shell latex particles The LA values for each peak in these multiphase materials provided some indication of the several fractional phase volumes © 1997 John Wiley & Sons, Inc J Polym Sci B: Polym Phys 35: 1501–1514, 1997

Morphology of anisotropic polymer‐dispersed liquid crystals and the effect of monomer functionality

Abstract: The morphology of transmission gratings with varying Bragg spacings formed using polymer-dispersed liquid crystals were examined using high-resolution scanning electron microscopy and image analysis techniques. The effect on the morphology of small changes in the overall functionality of the prepolymer syrup was observed. An increase in the amount of monofunctional compound resulted in small, nearly spherical domains (<100 nm in diameter) confined in well-defined lamellae for samples with a 0.49 μm Bragg spacing. A decrease in concentration (an effective increase in monomer functionality) at this Bragg spacing resulted in larger domains (100-200 nm) with much greater distribution of sizes and shapes. The local volume fractions of discrete liquid crystalline (LC) domains was considerably larger in the latter case. An increase in the Bragg spacing to 1.35 μm also resulted in well-defined lamellae of LC domains, although much more coalescence into irregularly shaped individual domains was observed. Surprisingly, the local volume fraction of LC increased in the larger Bragg spacing samples. The morphology results are discussed qualitatively in terms of liquid-gel demixing where the inherent crosslink density and elasticity of the polymeric host must be considered in phase separation processes on the nanoscale.

Structure and viscoelasticity of matched asymmetric diblock and triblock copolymers in the cylinder and sphere microstructures

Abstract: A polystyrene-polyisoprene ( PS-PI ) diblock copolymer (10,000-50,000 g/ mol) and a matched PS-PI-PS triblock (10,000-100,000-10,000 g/mol) were employed to study the effect of chain architecture on the rheological response of ordered block copolymer melts. Both samples adopt hexagonal microstructures with PS cylinders embedded in a PI matrix; on further heating, an order-order transition (OOT) into a cubic array of spheres takes place prior to the order-disorder transition. Each morphology was verified by SAXS and TEM. Interestingly, at the OOT the low-frequency elastic modulus of the diblock increased abruptly, whereas that of the triblock decreased. In contrast, the modulus of the cubic phase was roughly independent of chain architecture. Chain relaxation parallel and perpendicular to the cylinders was probed by measuring the elastic modulus of a macroscopically aligned sample in directions parallel (G' || ) and perpendicular (G'⊥) to the cylinder orientation. For both materials G' || < G' R < G'⊥ , where G' R is the elastic modulus of a randomly oriented sample. This result is attributed to the ability of the unentangled PS blocks to move along the direction of the cylinder axis, and thus relax the stress in the PI matrix in the parallel alignment. In each of the three cylindrical orientations the triblock had a larger modulus than the diblock, which is attributed to the presence of bridging PI blocks that connect distinct PS domains. About 20° below the OOT G' || showed a distinct change in its temperature dependence, which, coupled with SAXS measurements, is indicative of the onset of an undulation in the cylinder diameter that presages the pinching off of cylinders into spheres, as recently predicted by theory. The use of oriented samples also permitted SAXS confirmation of an approximate epitaxial relationship between the cylinder and the sphere unit cells, although a distinct change in the location of the structure factor maximum, q*, is noted at the OOT.