The effect of molecular weight blending on PVC melt rheology

The effect of molecular weight blending on melt flow characteristics has been studied with a 50/50 mixture of suspension PVC resins with the respective M _w of 56,300 and 123,000. The dynamic shear measurements were made with the Rheometrics Visco-Elastic Tester at angular frequencies of 0.1 to 40 radians/s. In the temperature range of 160 to 215° C, all samples showed three distinct flow regions marked by three different values of the activation energy. The high molecular weight fraction introduced a relatively strong influence on the melt flow characteristics of the blend due to the effect of its relatively high crystalline content. These samples also failed to show a Newtonian flow behavior at 190°C at an extremely low shear rate corresponding to 10^?4 radians/s., possibly reflecting the effect of the remnant crystallinity of the material.     

Structure and properties of polyethylene produced by γ-radiation polymerization in flow system

The radiation-induced polymerization of ethylene was carried out by use of a benchscale plant with a flow-type reactor of 1 liter capacity under the following conditions: pressure, 200–400 kg/cm²; temperature, 30–90°C; irradiation intensity, 3.8 × 10⁵ rad/hr; and ethylene flow rate, 300–3000 nl/hr. The molecular weight of polymer formed was shown to decrease with increasing reaction temperature and to increase with increasing pressure. When the ethylene flow rate increases, the molecular weight decreases in the polymerization at 30–60°C, but it does not change in the polymerization at 75–90°C. Methyl group content, which is a measure of short-chain branching of the polymer, increases with increasing reaction temperature, i.e., ca. 1 CH₃/1000 CH₂ at 30°C and ca. 9 CH₃/1000 CH₂ at 90°C. Methyl content is independent of the ethylene flow rate. The changes in the melt index of polymer with reaction conditions corresponds to the change of the molecular weight. The density, crystallinity, and melting point of polymer decrease with the reaction temperature as the short-chain branching increases, and they are almost independent of ethylene flow rate and pressure.     

Free radical branching of polylactide by reactive extrusion

The reactive extrusion of polylactide (PLA) with a free radical initiator resulting in a branched polymer was accomplished. Reaction conditions were in the range of 160°C to 200°C with an initiator concentration between 0.0 and 0.5%. Triple detector size exclusion chromatography, melt flow index, thermal gravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis were used to characterize the polylactide polymers. PLA without initiator showed extensive degradation as was evidenced by a decrease in both molecular weight and melt viscosity. The optimum range for branching resulting in a high molecular weight and low melt flow index polylactide was found to be around 170°C to 180°C and 0.1 to 0.25% initiator.     

Rheological properties of poly(trimethylene terephthalate) in capillary flow

The shear viscosity, extensional viscosity, and die swell of the PTT melt were investigated using a capillary rheometer. The results showed that the PTT melt was a typical pseudoplastic fluid exhibiting shear thinning and extensional thinning phenomena in capillary flow. There existed no melt fracture phenomenon in the PTT melt through a capillary die even though the shear rate was 20,000 s^?1. Increasing the shear rate would decrease the flow activation energy and decline the sensitivity of the shear viscosity to the melt temperature. The molecular weight had a significant influence on the flow curve. The flow behavior of the PTT melt approached that of Newtonian fluid even though the weight‐molecular weight was below 43,000 s^?1 at 260°C. The extensional viscosity decreased with the increase of the extensional stress, which became more obvious with increasing the molecular weight. The sensitiveness of the extensional viscosity to the melt temperature decreased promptly along with increasing the extensional strain rate. The die swell ratio and end effect would increase along with increasing the shear rate and with decreasing the temperature, which represented that the increase of the shear rate and the decrease of temperature would increase the extruding elasticity of the PTT melt in the capillary die. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 705–709, 2005     

Characterization of poly(3,3-bisethoxymethyl oxetane) and poly(3,3-bisazidomethyl oxetane) and their block copolymers

The homopolymers, poly(3,3-bisethoxymethyl oxetane) (polyBEMO), poly(3,3-bisazidomethyl oxetane) (polyBAMO), and triblock copolymers based on these homopolymers and a statistical copolymer center block composed of BAMO and 3-azidomethyl-3-methyl oxetane AMMO were synthesized and characterized by differential scanning calorimetry, modulus-temperature, optical microscopy, membrane osmometry, and solution and melt viscosity. The values of K and a for the Mark-Houwink equation were found to be 7.29 × 10^?3 mL/g and 0.80, respectively, for polyBEMO at 25°C using number-average molecular weights. Glass transition temperatures were in the range ?25 to ?40°C and melting temperatures were between 65 and 90°C for all polymers. The melting temperature was found to increase as expected with molecular weight. Melt viscosities of triblock copolymers with polyBAMO end blocks were at least an order of magnitude lower than those with polyBEMO end blocks and clear optically, suggesting that the polyBAMO-based triblock copolymers formed one phase in the melt, while the polyBEMO-based triblock materials (milk white) phase separated. The addition of filler raised the melt viscosity to a level between that predicted by the Guth-Smallwood and the Mooney equations.     

Synthesis of narrow distribution polytetrahydrofuran

Bulk polymerizations of tetrahydrofuran (THF) have been studied kinetically at reaction temperatures in the range ?10 to +80°C using p-chlorophenyldiazonium hexafluorophosphate initiator. Initiation has been studied to enable selection of a ‘clean’ initiation condition (95°C for 4 min). Factors causing broadening of the molecular weight distribution are discussed, the main causes of such broadening being chain transfer reactions and concurrent initiation with propagation. These could be minimized by using a low reaction temperature (?10°C). Molecular weight distributions were measured by gel permeation chromatography. Propagation rate constants were determined and found to increase with increasing temperature according to an Arrhenius expression giving an activation energy of 51 kJ/mol. The method will produce monodisperse samples of THF polymer over a wide molecular weight range from 5 × 10³ to 10⁶.     

Some rheological properties of molten polytetrafluoroethylene

Measurements of melt viscosity on samples of polytetrafluoroethylene of different molecular weight were carried out at 360°C by means of tensile creep tests in the linear viscoelasticity range. The apparent activation energy for viscous flow in the range between 330° and 380°C was estimated to be 20 kcal/mole. A value of about 7,500 was also determined for the average molecular weight between entanglement points (M_e), from the equilibrium compliance (D_e). Melt viscosity data were compared with zero strength time (ZST) values and a linear correlation was found on a bilogarithmic scale. The dependence of ZST on the applied stress and temperature was also studied and the results are discussed on the basis of Bueche's theory on the creep at rupture above the glass transition temperature.     

Stereocomplexation and morphology of enantiomeric poly(lactic acid)s with moderate‐molecular‐weight

The thermal behavior and spherulitic morphologies of poly(L ‐lactic acid) (PLLA)/poly(D ‐lactic acid) (PDLA) 1/1 blend with weight‐molecular‐weight of 10⁵ order, together with those of pure PLLA and PDLA, were investigated using differential scanning calorimetry and polarized optical microscopy. It was found that in the blend, stereocomplex crystallites could be formed exclusively or coexisted with homocrystallites depending on thermal history. Banded to nonbanded spherulitic morphological transition occurred for melt‐crystallized PLLA and PDLA, while the blend presented exclusively nonbanded spherulitic morphologies in the temperature range investigated. The spherulite growth of the blend occurred within a wider temperature range (≤180°C) compared with that of homopolymers (≤150°C), while the spherulite growth rates were comparable for both the blend and homopolymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Sorption of benzene by poly(ethylene oxide)

The activity of benzene in poly(ethylene oxide), PEO, has been determined over the concentration range 0.3 to ～35 wt% benzene using the piezoelectric sorption method. The temperature range was 64° to 97°C and the molecular weights of polymer samples were 1.0 × 10⁵, 6.0 × 10⁵, and 5.0 × 10₆ g/g mol. The Flory-Huggins interaction parameters, χ, determined in this work agree within experimental error with χ values determined by gas chromatography and by vapour pressure measurement. The values of χ extrapolated to zero solvent concentration, χ^∞, have a minimum and negative value in the vicinity of 85°C for the sample of molecular weight 1.0 × 10⁵, 84°C for the sample of molecular weight 6.0 × 10⁵, and 77°C for the sample of molecular weight 5.0 × 10⁶. In the vicinity of 65°C, χ^∞ is negative for each molecular weight of polymer and increases to positive values with an increase in temperature. For all samples studied, the χ^∞ parameters reach constant values (0.22 to 0.30) at temperatures higher than 90°C.     

Rheological properties of molten poly(ethylene terephthalate)

The complete steady-state flow properties of molten poly(ethylene terephthalate) for shear stresses ≦4.14 × 10⁶ dynes/cm² were determined. A single, complete master curve had been constructed in earlier work by Gregory and Watson; the curve interrelates the shear stress, shear rate, temperature, and molecular weight (inherent viscosity) by using a temperature superposition scheme from the literature and a similar molecular weight superposition scheme. Equations in agreement with theory and with other published experimental data were derived from the master curve. Results presented here make possible the direct calculation of the melt viscosity of poly(ethylene terephthalate) at shear stresses ≦4.14 × 10⁶ dynes/cm². The effects of a unit temperature change and/or a unit change in inherent viscosity (I. V.) on the melt viscosity were determined. For poly(ethylene terephthalate) with a 0.6 I. V., a 0.0025 change in I. V. accounts for about the same change in melt viscosity as a 1°C change in temperature.     

Thermal analysis of poly(phenylene sulfide) polymers. I: Thermal characterization of PPS polymers of different molecular weights

Thermal properties of Fortron®

1 ®Registered trademark of Hoechst Celanese Corporation.

poly(phenylene sulfide) (PPS) polymers of different molecular weights were studied by DSC. Crystallization studies revealed that the ability of these polymers to crystallize decreases with increasing molecular weight. The Avrami equation poorly describes the isothermal crystallization of PPS. Lamellar crystallization was observed for the lowest molecular weight sample. For the other, higher molecular weight polymers the Avrami exponent is always between 2 and 3, suggesting development of distorted spherulites with heterogeneous nucleation. The temperature dependence of the solid and melt heat capacities have been determined. The solid specific heat capacity did not exhibit a molecular weight dependence. The heat capacity increase at the glass transition, T_g, has been calculated to be 28.1 J°C^?1 mole^?1. The equilibrium melting point of PPS has been estimated to be 348.5°C using the Hoffman–Weeks method. The T_g of PPS increases with molecular weight. The T_g of the highest molecular weight evaluated is 92.5°C. A DMA relaxation peak corresponding to the onset of the phenylene ring rotation occurs at ?92°C. Only the highest molecular weight could be quenched to a completely amorphous state.     

High-pressure polymerization of ethylene using methyl isobutyl ketone peroxide as initiator

An investigation was carried out into the suitability of methyl isobutyl ketone peroxide (MIKP) as initiator for the high pressure polymerization of ethylene. For this purpose, polymerization tests were carried out in a stirred autoclave at a pressure of 1000 to 2000 bar and a temperature of 195 to 310°C. The initiator concentration of the feed was varied between 6.5 and 42 mol ppm while the residence time was kept at a constant 30 s. Apart from the rate of polymerization, the conversion and the initiator consumption were also determined. The characteristic properties of polyethylene (PE) were determined by measuring the melt flow index, the density and, in some cases, the molecular weight distribution. Conversion levels of 5 to 27% were achieved with rates of polymerization between 1 and 5.5 kg PE l^?1 h^?1. The initiator consumption at 1700 bar was in the region of 1 to 2 g I kg^?1 PE^?1 over a temperature range of 220 to 315°C. In view of these results, MIKP can be considered as suitable to initiate ethylene high pressure polymerization at 220 to 310°C, particularly in the tubular reactor. The density of the polyethylene thus prepared is ranging between 0.910 and 0.927 g ml^?1. Without the addition of modifiers or cross-linking agents, the melt flow index varies considerably. The polydispersity of the polymers prepared at a pressure of 1700 to 2000 bar was between 6 and 8 and therefore within the range to be expected for stirred autoclaves. Reducing the pressure to 1000 bar resulted in surprisingly low polydispersity values of 2.4 to 3.7.     

Influence of processing parameters on the degradation of poly(L‐lactide) during extrusion

The influence of processing conditions during melt extrusion on the degradation of poly(L ‐lactide) (PLLA) has been investigated. PLLA polymer was processed by melt extrusion in a double screw extruder at 210 and 240°C. For each extrusion temperature, two screw rotation speeds, 20 and 120 rpm, were used. To investigate the influence of moisture on the thermal degradation during processing, the PLLA granules were dried at 100°C for 5 h and then either extruded directly or conditioned at 65% RH, 20°C for 24 h prior to extrusion. The results show that a decrease in molecular weight measured as number‐average (M_n) molecular weight occurs for all combinations of process parameters used. At processing temperature of 210°C, the change in molecular weight for the dry granules was shown to be dependent on the residence time (i.e., screw rotation speed) in the melt. By changing the screw rotation speed from 120 to 20 rpm at 210°C, M_n decreased from 33,600 to 30,200 g/mol. When the processing temperature was increased to 240°C, the dry granules showed an M_n of 25,600 and 13,600 g/mol when extruded at 120 and 20 rpm, respectively. M_n for the conditioned specimens extruded at 210°C was 18,400 g/mol when processed at 120 rpm and 12,300 g/mol at 20 rpm. When processed at 240°C, 20 rpm, M_n is independent of whether the granules were dry or moist prior to extrusion. It is probably due to the fact that the degradation at 240°C is so extensive that the presence of moisture in the polymer does not contribute further to the degradation process. The stress and strain at break decreased due to degradation and were dependent on the molecular weight of the samples. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2128–2135, 2001     

Effects of frequency,molecular weight and thermal oxidation on the dynamic mechanical response of poly (ethylene oxide)

The technique of dynamic mechanical thermal analysis (DMTA), operated in the dual cantilever mode, was used to characterize the effects of frequency, crystallinity, molecular weight (MW) and the extent of thermal oxidation on the dynamic mechanical response of poly(ethylene oxide) (PEO). The glass transition temperature (T_g) of PEO (MW = 9 × 10⁵ Dalton) was found to be ?44°C. For PEO (MW = 1 × 10⁵ Dalton) the T_g is ?39°C and this value increases by 2–9°C for every decade increase in the measuring frequency. Two minor, second-order transitions of PEO are also discernible at ?33 and 32°C. An inverse dependence of T_g on molecular weight was found in the molecular weight range studied and this is contrary to the Fox-Flory theory. It was also found that a partially crystalline sample is obtained despite very rapid quenching of PEO from the melt into liquid nitrogen. Thermal oxidation of PEO before processing leads to an increase in the amplitude of the loss tangent peak. This reflects the effect of oxidation products in restricting polymer crystallization and the subsequent increase in the amorphous fraction of the polymer. The position of the T_g peak in PEO remains reasonably fixed with progressive ageing and this was attributed to crosslinking having occurred in addition to chain scission during thermal oxidation.     

Polymerization of methyl methacrylate in water in presence of the metal oxides TiO2 and Cu2O

Rates of sodium bisulfite-initiated polymerization of methyl methacrylate in water were determined in absence and in presence of the metal oxides TiO₂ and Cu₂O at 30°, 40°, 50°, and 60°C. Cuprous oxide and titanium dioxide enhanced the rate of polymerization and reduced the molecular weight as compared with the figures obtained in absence of oxide, the effect of the former being more pronounced than the latter. With TiO₂, the rate was increased from 2.3 to 3.2 × 10^?5, while with Cu₂O, it was increased to 8.6 × 10^?5 mole/l./sec, both at concentrations of 9 g/l. water. The apparent energy of activation for the polymerization of methyl methacrylate between 40°C and 50°C was found to be 15.6 kcal/mole in absence of the metal oxides, and 7.6 kcal/mole and 2.8 kcal/mole in presence of titanium dioxide and cuprous oxide, respectively. The number-average molecular weight was found to decrease slightly with the addition of TiO₂ but to decrease greatly when Cu₂O was added.     

Curing reactions in plastic prepolymers and propellants. I. Polystyrene

Curing reactions of the viscous PS prepolymer and PS/AP propellant slurry have been studied. The molecular weight of the binder (separated from the propellant) and the prepolymer was found to increase to a maximum value, remain constant for some time, and then fall off between 50–125°C. The molecular weight of the binder was found to be less than corresponding prepolymer between 100–150°C but at lower temperatures (50–75°C) the reverse was found to be true. The increase in the molecular weight during curing at lower temperatures has been explained on the basis of Trommsdorff effect which gets support from the estimated activation energy (9 kcal mole^?1) for the curing process. Curing was recognized as chain extension where the rate of polymerization becomes diffusion controlled below 75° C.     

Influence of heating rate on the pyrolysis of oil shale

The decomposition of oil shale from the Kvarntorp deposit in Sweden has been investigated using isothermal and non-isothermal methods. It was found that the heating rate during the initial non-isothermal period in the isothermal experiments had a considerable effect on the rate of devolatilization. At heating rates in the order of 10⁵°C/s the total weight loss exceeded the weight loss predicted from the volatile matter according to a proximate analysis.The effect of particle and sample size on the rate of decomposition (weight loss) was studied and found to have a significant influence at heating rates of the order of 10⁵°C/s while no effect was detected at heating rates around 200 °C/s (isothermal) or below 50°C/min (non-isothermal). The decomposition at low heating rates was completed at temperatures below 600°C and about 50 percent of the devolatilization could be described by first order kinetics with an apparent activation energy of 130 KJ/mole.At heating rates of 200°C/s (isothermal) the decomposition could also be described by simple first order kinetics but with an activation energy of 67 KJ/mole, thus indicating a different rate-determining mechanism.     

Shear and thermal history effects in polypropylene melts

Fiber grade polypropylenes with melt flow indices of 3 and 12 were studied in the aspolymerized (powder) state and after pelletization. Pelletizing operations caused very little change in the molecular weight distributions of these polymers. The lower melt flow index material exhibited much greater apparent viscosity and melt elasticity in the powder than in the pellet from during screw extrusion at 190°C. These results are consistent with the existence of a higher entanglement density in the powder version. Instron rheometer data showed no difference between the two polymer forms because of the possibility for entanglement in the rheometer reservoir during rheological experiments. The effects of sample history noted with the 3 melt flow polymer were less pronounced with the lower molecular weight 12 melt flow material. The differences in flow curves of powder and pelletized forms of the latter polymer were negligible at 175 and 190°C. Differences in die swell were more noticeable, however. The effects observed are attributable to reversible shear-induced decreases in entanglement density. Similar phenomena have been reported for other polymers. The results reported here have implications in quality control procedures for thermoplastics and in the production of polymers with desired property balances.     

Morphology and melt crystallization of poly(L‐lactide) obtained by ring opening polymerization of L‐lactide with zinc catalyst

The morphology and melt crystallization of zinc catalyzed poly(L ‐lactide) (PLLA) were investigated by using differential scanning calorimetry (DSC), polarized optical microscopy, and scanning electron microscopy. Isothermal melt crystallization performed at 95–135°C showed that the morphology depends on the degree of supercooling, as illustrated by crystallite perfection and lamellar thickening behaviors. Double melting peak was observed on DSC thermograms and attributed to the melt‐recrystallization mechanism, small and imperfect crystals becoming gradually more stable ones. Circumferential and hexagonal cracks were detected in PLLA spherulites, which were formed during melt‐crystallization at 135°C and quenching in liquid nitrogen. Rhythmic growth and thermal shrinkage are suggested to be the two main factors accounting for the formation of periodic cracks. Spherulite growth rates of PLLA were evaluated by using combined isothermal and nonisothermal procedures, and were analyzed by the secondary nucleation theory. The maximum growth rate reached 9.1 μm/min at 130°C. The temperature range investigated (120–155°C) belongs to the Regime II of crystallization. The value of U* was found to be 1890 cal/mol, instead of 1500 cal/mol commonly used in literature, and K_g and σ were estimated to be 3.03 × 10⁵ K² and 1.537 × 10^?4 J/m², respectively. As a result, no distinct difference between PLLA catalyzed by zinc metal and those prepared with stannous octoate catalyst exists in this work. POLYM. ENG. SCI., 46:1583–1589, 2006. © 2006 Society of Plastics Engineers.     

Eigenschaftskennfunktionen thermisch hergestellter polystyrole unterschiedlichen molekulargewichts und unterschiedlicher molekulargewichtsverteilung

Mechanical properties and melt flow behaviour have been measured on thermal polystyrenes in order to study the effect of different molecular weight and different molecular weight distribution. The mechanical properties (impact strength, tensile strength and flexural strength) were found to depend upon a critical molecular weight M_w = 1.5 × 10⁵, where the polystyrenes reached ultimate strength. An influence of molecular weight distribution only can be observed when the average molecular weight is near the critical molecular weight limit. The broader the distribution, the poorer are mechanical properties. Injection molded specimens with different molecular weight distribution but equal average molecular weight showed different molecular orientation and different anisotropic mechanical properties. On the basis of equal orientation the narrow distributed specimens showed higher mechanical strength in direction of orientation. The broader the distribution, the higher the elastic properties of melt. An optimum of physical properties and ease of fabrication will be obtained with narrow molecular weight distributed polystyrenes with an average molecular weight slightly above the critical molecular weight limit.