Increase In Melt Viscosity Research Articles

Orientation of Aromatic Ion Exchange Diamines and the Effect on Melt Viscosity and Thermal Stability of PMR-15/ Silicate Nanocomposites

ABSTRACTNanocomposites of PMR-15 polyimide and a diamine modified silicate were prepared by addition of the silicate to the PMR-15 resin. The orientation of the ion exchange diamine within the silicate gallery was determined by x-ray diffraction and found to depend on the clay cation exchange capacity. The oligomer melt viscosity exhibited a dependence on the orientation of the diamine in the silicate interlayer, and in some cases, on the length of the diamine. A correlation was observed between the oligomer melt viscosity and the crosslinking enthalpy, where nanocomposites with an increased melt viscosity exhibited a decrease in enthalpy on crosslinking. After crosslinking, those nanocomposites with a high melt viscosity had poorer thermal oxidative stability compared to the less viscous systems. The melt viscosity was tailored by co-exchange of an aromatic diamine and an aliphatic amine into the silicate. Nanocomposites prepared with this silicate exhibited an increase in thermal oxidative stability compared to the neat resin.

Homogeneous distribution of 211 secondary-phase particles in single-grain melt-grown (Nd,Eu,Gd)Ba 2Cu 3O 7 bulk

The microstructural features of the (Nd,Eu,Gd)Ba 2Cu 3O 7 (NEG) bulks were studied by optical microscopy in polarized light. The quantitative microstructural data were obtained using the image processing system, Image-Pro Plus. Unlike YBa 2Cu 3O y melt-processed bulks, the NEG bulk superconductors prepared by oxygen-controlled-melt-growth process do not exhibit macroscopic 211 secondary-phase inhomogeneity due to 211 particle pushing. The lowering of interface energy and increase of melt viscosity are suggested to be responsible for the essential suppression of 211 pushing in NEG melt-grown bulks. This result opens a way to achieve a homogeneous distribution of the current densities in melt-grown bulk superconductors.

Effects of physical crosslinking on properties of plasticized high molecular weight PVC

A series of high molecular weight PVC (HMW-PVC) resin was synthesized by suspenion process at various temperatures and compounded to prepare plasticized HMW-PVC. Plasticized HMW-PVC is physically crosslinked by microcrystallites and chain entanglements. The molecular weight and degree of crystallinity of PVC resin increase with decrease of polymeri-zation temperature. More dense physical crosslinking networks are formed in plasticized PVC with the decrease of polymerization temperature of PVC resin. Plasticized HMW-PVC exhibits thermal reversible processability, but the gelation temperature and melt viscosity increase with increasing physical crosslinking density. Physical crosslinking also has great effects on the mechanical properties of plasticized HMW-PVC. The compression set and deformation decrease as molecular weight and crystallinity of PVC resin increase, but the softening temperature and tensile strength increase. The crosslinking network of plasticized HMW-PVC is more similar to the dissociable network of gels than to the permanent network of crosslinked rubber and is not stable under stretching at high stress.

The influence of low-melting-point alloy on the rheological properties of a polystyrene melt

The rheological behavior of polystyrene melts filled with different loadings of Sn-Pb alloy has been studied. The filled polystyrene melts showed a pseudoplastic behavior, and the viscosity varied dramatically at the melting point of the alloy. The temperature dependence of the viscosity has been found to follow two separate Arrhenius equations for high and low temperatures, respectively. The presence of alloy filler was found to increase melt viscosity below the melting point of the alloy, but decreased it on reaching the alloy melting point. The melt elasticity was found to decrease with increase of alloy concentration, and increase abruptly at the alloy melting point. However, viscous flow was the dominant mechanism of deformation over the entire range of temperature studied.

Shear rheology and thermal properties of linear and branched poly(ethylene terephthalate) blends

Linear poly(ethylene terephthalate) (LPET) was blended with branched poly(ethylene terephthalate) (BPET) by varying the branching comonomer and blend proportion of BPET. The crystallinity and melting temperatures of LPET in blends with BPET was measured using differential scanning calorimetry (DSC). Thermal analysis revealed that the crystallinity and crystallisation/melting temperatures of LPET–BPET blends were increased compared with LPET. The rheological measurements of the blends were conducted using a controlled stress rheometer, with dynamic oscillatory shear, as well as steady shear measurements. The acceleration of the LPET crystallisation was dependent on the blend proportion and on the branching comonomer. The rheological measurements indicated a 50% increase of melt viscosity of LPET when blended with 30mass% of BPET. This was attributed to increasing chain entanglements with the proportion of branching comonomer of the BPETs. Also, blends of LPET with BPET of different branching comonomer were investigated, and the rheology was found to be highly sensitive to composition. Dynamic rheological measurements of the blends were made, and compared with the data for LPET, to evaluate the effect of BPET on the viscoelasticity.

Mechanical, microscopical and fire retardant studies of ABS polymers

Antimony trioxide (Sb 2 O 3 ) is a common additive in ABS flame retardant formulations and the effects of adding it, with four commercial brominated materials, to ABS are reported here. The results focus upon mechanical, rheological, microscopical and flame retardant properties, and the effects of four Sb 2 O 3 materials with average particle sizes ranging from 0.5 to 11.8 μm. Compounds were produced using a twin-screw co-rotating machine and subsequently injection moulded into flexural and impact specimens, the latter being tested with instrumented falling weight equipment. The effects on rheological properties were studied using a capillary rheometer. Dynamic mechanical thermal analysis (DMTA) and LOI flame testing were also carried out. The brominated materials had more influence than Sb 2 O 3 on the ABS rheology, although a trend of increased melt viscosity with increasing Sb 2 O 3 particle size and loading was noted. The brominated materials generally increased the flexural modulus and reduced the deflection at peak force values, whereas the modulus and strength were relatively unaltered with Sb 2 O 3 at the loadings studied. The impact strength of the ABS was detrimentally affected by both additives as loading level and average particle size increased. The presence of 5 wt% sub-micron Sb 2 O 3 caused a 20% reduction in impact strength whilst the reduction due to the brominated materials depended upon the specific halogenated compound used. Scanning Electron Microscopy (SEM) showed good dispersion of Sb 2 O 3 regardless of particle size and Transmission Electron Microscopy (TEM) revealed that Sb 2 O 3 resided in the SAN phase of the polymer. The DMTA response of ABS was not altered by Sb 2 O 3 .

The Degradation Process Observed during Step Annealing of 73/27 HBA/HNA Copolyester

Stepwise annealing near the crystal-liquid crystal transition temperature was applied to a 73/27 4-hydroxybenzoic acid (HBA)/2-hydroxy-6-naphthoic acid (HNA) copolyester to increase the melting transition from 281 to 316°C. An unexpected degradation process was observed when further annealing was applied at 305 °C. An irreversible loss of crystallinity was confirmed by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The melt rheology of the step ordered 73/ 27 HBA/HNA copolyester was investigated using a dynamic rheometer. In contrast to the untreated and ordered sample, the degraded material exhibited an increase in melt viscosity (0.3 orders) after heating at 370 °C for 1 min, indicating some form of irreversible reaction. Furthermore, a plot of storage modulus versus loss modulus showed a completely different pattern for the degraded material. These observations suggest that a small amount of branching or cross-linking has occurred in the 73/27 HBA/HNA copolyester during step annealing at 305 °C. An attempt was made to simulate this process by adding 1 wt % of a cross-linkable oligomer to the 73/27 HBA/HNA copolyester. From the melt viscosity, it could be concluded that a small amount of branching (below 0.5 wt %) has occurred most likely in the crystalline regions of the ordered copolyester.

Effect of compatibilizer on the crystallization, rheological, and tensile properties of LDPE/EVOH blends

The effect of the compatibilizer on the crystallization, rheological, and tensile properties of low-density polyethylene (LDPE)/ethylene vinyl alcohol (EVOH) (70/30) blends was investigated. Maleic anhydride-grafted linear low-density polyethylene (LLD-g-MAH) was used as the compatibilizer in various concentrations (from 1 to 12 phr). The interesting effect of compatibilization on the crystallization kinetics of the blends was noted, and the correlation between the morphology and the rheological and tensile properties was also discussed. Morphological analysis showed that the blends exhibited a regular and finer dispersion of the EVOH phase when LLD-g-MAH was added. Nonisothermal crystallization exotherms of the compatibilized LDPE/EVOH blends showed the retarded crystallization of the dispersed EVOH phase, which probably resulted from the constraint effect of the grafted EVOH (EVOH-g-LLD) as well as the size reduction of the EVOH domains. The blends exhibited increased melt viscosity and storage modulus and also enhanced tensile properties with the addition of LLD-g-MAH, which seemed to be attributable to both dispersed particle-size reduction and improved interfacial adhesion. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1245–1256, 1998

A mechanism for the lateral transport of gas bubbles in silicic lava rising in a vertical conduit

It is well-known that the shear viscosity of silicate-rich melts is very sensitive to water content. Thus, as such, a material rise towards the earth's surface in an eruption, the pressure falls, water exsolves to form bubbles or voids, and the melt viscosity increases. This is responsible for the (by now) well-known large departures of the pressure gradient from hydrostatic. The exsolved gas is known to escape from the rising lava in to the surrounding rock and since gas content is believed to have an important role in the transitions between eruption regimes, it is of some interest to understand the mechanism of escape. A key difficulty is to explain how small gas bubbles could migrate laterally across the conduit, in view of their small size and the large liquid viscosity. It is shown here that the increase of melt viscosity with height produces a horizontal pressure gradient directed so that the pressure at the centre of the conduit is larger than at the walls. Thus the fluid in the centre is less viscous and more dense; this configuration is (very probably) dynamically unstable. The resulting instability will have the effect of mixing the lava, thus tending to transport gas bubbles from the centre to the (depleted) regions near the walls.

Plane strain fracture toughness of polyphenylene sulfide

AbstractThe plane strain fracture toughness of polyphenylene sulfide (PPS), tested with different molecular structures such as linear PPS, heat‐treated PPS, and branched PPS, was investigated over a wide range of melt viscosity and crystallinity and compared with other mechanical properties. The fracture toughness of linear PPS increased with increasing melt viscosity, and it reached the highest fracture toughness calculated from linear elastic fracture mechanics (LEFM) observed in this study. In this high melt viscosity region, the linear PPS specimen showed slow crack growth instead of brittle fracture. Although the tensile properties of heat‐treated PPS and branched PPS are the same as those of linear PPS, the fracture toughness of linear PPS is superior. The fracture toughness is affected by the crystallinity of the specimen, and the effect of crystallinity on fracture toughness is smaller than that of melt viscosity and molecular structure.

Effect of chemical modification on the flow behaviour of LDPE and its blends with linear LDPE

The flow behaviour of LDPE and its blends with linear LDPE (LLDPE) chemically modified with DCP was studied as a function of blend composition and shear rate at a constant temperature using a capillary rheometer. The effect of chemical modification on the viscous and elastic properties of LDPE and two blend systems LDPE/LLDPE (80:20, 60:40) is reported. Chemical modification which is in essence a free radical process induces changes that increase melt viscosity and melt elasticity of LDPE and the two blend systems. Modification also enhances compatibility of the molten polymers. Incorporation of a small quantity of linear LDPE having a relatively narrow molecular weight distribution to the conventional LDPE having a long chain branched structure and broader molecular weight distribution lessens the shear thinning behaviour of the modified blends of the two component polymers. Melt elasticity at a constant shear stress and temperature was also investigated using die swell measurements and relating them to recoverable shear strain, normal stress and shear modulus.

Crystallization of glassforming melts under hydrostatic pressure and shear stress: Part I Crystallization catalysis under hydrostatic pressure: possibilities and limitations

This is the first part of a thorough study of the kinetics of melt crystallization under applied static pressure, P, and under shear stress. The thermodynamic and kinetic consequences of increased external pressure on nucleation rate, non-steady-state time lag, rate of crystal growth and overall crystallization kinetics in undercooled melts are analysed. Two types of undercooled liquids (with either positive or negative volume dilatation upon crystallization) are considered. Particular attention is given to the effect of pressure on the specific interface energy, σ, at the crystal/melt phase boundary. Using an appropriate thermodynamic model it is shown that for one-component systems, (∂σ/∂p)<0 is to be expected as a rule. Thus an additional decrease of the thermodynamic barrier of nucleation in pressurized melts is to be expected. However, it is also shown that the increase of melt viscosity with pressure in most cases reduces the effect of this decrease. Thus increased pressure has a limited effect as a nucleation catalyst. The possibilities in this respect are analysed and conditions under which static pressure may lead to enhanced crystallization are outlined.

Thickening of Melt During Processing of PES: A Structural Investigation

Treatment under normal processing temperature has been carried out by kneading for different times and at different roller rotating speeds, and also by successive extrusion cycles. There is an increase in melt viscosity of PES samples after processing. A phenomenon of thickening of melt during processing of PES may occur due to prolonged shear, heat, and oxidation under normal processing temperature (315°–335°C). A systematic study was conducted to investigate the structure changes during processing of PES by a variety of techniques including: Fourier–transform infrared spectroscopy, 13C nuclear magnetic resonance, and x-ray photoelectron spectroscopy. The results indicate that chain scission reactions occur and that long-chain branches are formed due to shear and heating. A branching mechanism provoking increase in molecular weight and thickening of melt during processing of PES is presented based on experimental data.

Compatibilization of PP/PBT and PP/PA6 blends with a new oxazoline-functionalized polypropylene

Aim of this work was to study the effectiveness of a novel oxazoline-functionalized polypropylene as a compatibilizer for PP/PBT and PP/PA6 blends. This polypropylene-based compatibilizer mixes well with the polypropylene and is capable of reacting with the carboxylic and amine end groups of PBT and PA6. Significant improvements in blend toughness were achieved without reduction in strength and stiffness. These effects were related to stabilized morphology of finely dispersed minor phase well attached to the matrix. The enhanced interfacial interactions between the two phases, in particular at high PBT content were evidenced by increased melt viscosity.

Crystallization behaviour of poly( p-phenylene sulfide): Effects of molecular weight fractionation and endgroup counter-ion

The crystallization behaviour of poly( p-phenylene sulfide) (PPS) has been studied. Two PPS samples with 〈 M W〉 = 44 K and 〈 M W〉 = 64 K were fractionated by a process which selectively removes a portion of the low molecular weight species yielding fractionated PPS samples with 〈 M W〉 = 56 K and 〈 M W〉 = 104 K respectively. The fractionated samples were then treated with an ion exchange process to allow control over the nature of endgroup counter-ion, i.e. to introduce Na + and Zn 2+ ions. Using a Hoffman-Weeks analysis, the equilibrium melting temperature of these polymers was estimated to be 320°C irrespective of endgroup counter-ion or polymer molecular weight. The nucleation density was observed to increase as a function of molecular weight by small-angle light scattering (SALS). The spherulitic growth rates and nucleation densities were studied as a function of the chemical nature of endgroup counter-ion for PPS with 〈 M W〉 = 56 K that had been fractionated to remove low molecular weight species. Additionally, isothermal rates of bulk crystallization were analysed as a function of molecular weight of PPS and chemical nature of the endgroup counter-ion. It was found that as a function of endgroup counter-ion, crystal growth rates and overall crystallization rates decreased in the following order: H + > Zn 2+ > Na +. The order of decreasing crystal growth rates corresponded to a similar increase in melt viscosity as a function of endgroup counter-ion, suggesting that the reason for decreasing growth rates could originate from increasing secondary interchain interactions. Optical microscopy studies showed the nucleation density decreased in the order H + > Na +. The ion-exchange reactions were shown to be reversible by differential scanning calorimetry (d.s.c.) and optical microscopy studies. Crystallinity determinations by wide angle X-ray diffraction (WAXD) and measurements of the heats of melting illustrated that higher molecular weight PPS attained lower levels of crystallinity than PPS of lower molecular weight when crystallized under identical conditions.

New developments in the melt rheology of nylons. I: Effect of moisture and molecular weight

AbstractPeculiar observations on the melt rheology of ultra‐dry nylon resins, nylon 6 in particular, are reported. One aspect of this study deals with a sharp increase in zero shear melt viscosity (e.g. 2 to 5 times) as the nylon 6 resin moisture is taken from 0.10 down to 0.00%; the effect being reversible. Changes of such magnitude are unexpected considering that there are no detectable variations of the chemical/compositional/molecular weight type in the starting resin, when subjected to the imposed drying conditions. Another aspect of this study deals with a deviation of nylons (6, 6,6, and 12) from the Bueche (1952) relationship, well accepted for polymers to date. Under moderate drying conditions (e.g. 50°C/17 h/110 millitorr), the molecular weight exponent is found to be 3.8, which is within the range of 3.4 to 3.8 reported for nylon 6. However, under more severe drying conditions (e.g. 110°C/17 h/110 millitorr), the molecular weight exponents for nylon 6, nylon 66, and nylon 12 are 4.8, 5.4, and 4.6, respectively. We are proposing that a sharp increase in melt viscosity of ultra‐dry nylon 6 is partly due to an increase in the molecular weight of the melt (extrudate) which then, has a more pronounced impact on melt viscosity in view of the 4.8 exponent. Such unique results, in contrast to polyethylene (free radical polymer) and poly(ethylene terephthalate) (condensation polymer) are tentatively attributed to H‐bonding in nylon melts. Yet another aspect of this study deals with the rheology of supercooled molten polymers that can offer advantages for analytical characterization.

Influence of melt stability on the crystallization of bis(4‐aminophenoxy)benzene‐oxydiphthalic anhydride based polyimides

AbstractNovel high performance semicrystalline polyimides, based on controlled molecular weight phthalic anhydride (PA) endcapped 1,4‐bis(4‐aminophenoxy)benzene (TPEQ diamine) and oxydiphthalic dianhydride (ODPA), were synthesized. They exhibited excellent thermal stability in nitrogen and air atmospheres as determined by thermogravimetric analysis (TGA). The glass transition temperatures (Tg) for these polymers ranged from 225°C for the 10,000 Mn (10K) polymer, to 238°C for the 30,000 (30K) Mn material. The observed melting temperatures for all the polymers were ∼420°C. The crystallization behavior of these polymers showed a strong molecular weight dependence, as illustrated by the observation that the 10K and 12.5K polymers crystallized with relative ease, whereas the 15K, 20K, and 30K polymers showed little or no ability to undergo thermal recrystallization. The thermal stability of these polymers above Tm was investigated by studying the effect of time and temperature in the melt on the cold crystallization and melting of these polymers. Increased time and temperature in the melt resulted in lower crystallinity because of melt state degradation, such as crosslinking and branching, as evidenced by an increase in melt viscosity, which was more prominent for the higher molecular weight polymers.

Crystallinity in starch bioplastics

Thermoplastic starch (TPS) materials have been prepared by kneading, extrusion, compression moulding and injection moulding of several native starches with the addition of glycerol as a plasticizer. Two types of crystallinity can be distinguished in TPS directly after processing: (i) residual crystallinity: native A-, B- or C-type crystallinity caused by incomplete melting of starch during processing; (ii) processing-induced crystallinity: amylose V H−, V A− or E H−type crystallinity which is formed during thermomechanical processing The amount of residual crystallinity is related to processing conditions like processing temperature or applied shear stress. The composition of the mixture indirectly influences the amount of residual crystallinity. Lower amounts of glycerol cause a reduction in residual crystallinity. This effect is attributed to the increase in melt viscosity at decreasing plasticizer content, which causes an enhancement of shear stress on the melt. It has been concluded that composition and processing parameters are interrelated. Processing-induced crystallinity, also influenced by processing parameters, is caused by the fast recrystallization of amylose into single-helical structures. Increasing the screw speed during extrusion or increasing residence time during kneading causes an increase in single helical type crystallinity. The amount of crystallized amylose is proportional to the amount of amylose. The addition of complexing agents like calciumstearate or the presence of lysophospholipids cause the crystallization of amylose into these type of structures. In waxy starches, containing no amylose, obviously no V- or E-type crystallinity is formed.

Graft copolymers and ionomeric associations from mixtures of phenoxy with acid functionalized polyolefins—Part II: Compatibilizers for HDPE/PET blends

AbstractGraft copolymers and coionomeric mixtures produced by mixing a phenoxy polymer (polyhydroxyether of bisphenol A) respectively with ethylene–propylene copolymers, containing grafted maleic anhydride groups (EP‐g‐MA), and the sodium ionomers of the terpolymers of ethylene, an alkyl acrylate and a carboxylic acid, the ionomeric character of which being enhanced by the addition of either sodium benzoate (weak base) or sodium ethoxide (strong base), were used as compatibilizers for blends of PET and HDPE in different ratios and with varied molecular weight of the HDPE component. The preparation of the compatibilizer and the production of the blends were carried out on a twin‐screw extruder. Both types of compatibilizers acted as effective surfactants for the dispersion of the two polymers, at all weight ratios, and the blends exhibited a substantial increase in melt viscosity as a result of the addition of the compatibilizer. Only the coionomeric compatibilizers, however, produced substantial improvements in both tensile strength and elongation at break, irrespective of the ratio of the two polymers in the blends. This was attributed to the strong nucleating effects on the crystallization of the polymer components and to a strong interfacial adhesion between the two polymer phases. © 1994 John Wiley &amp; Sons, Inc.

Effect of Rubber Content in Acrylonitrile–Butadiene–Styrene and Additional Rubber on The Polymer Blends of Polycarbonate and Acrylonitrile–Butadiene–Styrene

Higher ABS rubber content in the polycarbonate (PC)/acrynitrile–butadiene–styrene (ABS) blend resulted in toughness improvement in terms of tensile, Izod impact and strain energy release rate at a cost of higher melt viscosity. The extra-added core-shell rubber particles with poly(methyl methacrylate) (PMMA) or acrylonitrile–styrene (SAN) outer shell resided mainly in the ABS phase and further improved the resulted blends toughness. The addition of styrene–maleic anhydride (SMA) copolymer in the PC/ABS blends caused severe coagulation of the rubber particles in the ABS phase and left a greater fraction of the SAN phase without presence of rubber. The toughness improvement by increasing rubber content must be compensated by the increase of melt viscosity and this fact has to be taken into consideration in selecting the starting materials of the PC/ABS blends.