Effects of molecular weight and chain ends on glass transition of polystyrene

The main glass transition temperatures, T_g of narrow distribution anionic polystyrene samples have been measured by differential thermal analysis (d.t.a.). The d.t.a. data were verified by differential scanning calorimetry. Effects of heating rates were eliminated by extrapolating observed T_g values to a standard heating mode of 1°C/min In a T_g vs. log (heating rate) relation. The variations of T_g peak height and temperature with heating rate and polymer molecular weight (M) agree with conclusions from other reports of d.t.a. work. The T_g values corrected for heating rate effects agree well with figures reported in studies with d.t.a. and other techniques for locating vitrification temperatures.The volumes attributable to end groups differ significantly between anionic and thermally initiated polystyrenes at temperatures above T_g. A comparison of T_g values of both polystyrene types provides a direct test of the iso-free volume model of the vitrification process. A common T_g vs. M relation was found for anionic and thermally initiated polystyrenes, regardless of chain end nature. This comparison is not consistent with the concept that vitrification is governed by free volume defined as the difference between specific volume of the polymer and a reference volume. Our results are compatible with several other mechanisms which have been suggested for the glass transition.     

Effects of molecular weight distribution and branching on rheological properties of polyolefin melts

The objective of this work was to determine the relationships among molecular and melt parameters of polyolefins. The polyolefins studied are polypropylene, poly-1-butene, poly-1-hexene, poly-1-dodecene, these have regularly spaced short-chain branches. Conclusions from previous work, as well as some new data, on polyethylene are given. As the molecular weight increases, the critical shear rate decreases but the melt viscosity and non-Newtonian ratio increase. As the molecular weight distribution broadens, the critical shear rate decreases, whereas the normal forces and the non-Newtonian ratio increase. Increasing the number of short-chain branches increases the energy of activation and the melt viscosity but decreases the non-Newtonian ratio. As the length of the short-chain branches increases, the non-Newtonian ratio increases, but the melt viscosity, critical shear rate, and energy of activation decrease. Increasing the number of long-chain branches decreases the non-Newtonian ratio, but the normal forces and the melt viscosity increase. Such information allows the polymer chemist to design a polyolefin molecule having the critical melt properties required for a given production technique.     

Prediction of dynamic and transient rheological properties of polystyrene and high-density polyethylene melts from the molecular weight distribution

A model relating the steady-shear melt viscosity and elasticity to the molecular weight distribution in HDPE and polystyrene melts has been extended to predict the dynamic viscosity, modulus, and loss modulus. Limitations in the model as applied to the dynamic properties are discussed. The model is also applied to the transient response of stress growth during steady shearing. This application is considered useful because it may help describe nonsteady-state flow of polymer melts in short dies and cyclic operations as employed in commercial molding equipment.     

Electrochemically controlled molecular weight of polystyrene production

Summary It is well known that physical properties such as mechanical and thermal characteristics of a polymer are highly dependent on its molecular weight and molecular weight distribution (MWD). This feature encourages our interest for finding suitable conditions that allow us to control the molecular weight of the formed polymer, by controlling the experimental parameters. We are reporting some results about the electropolymerization of styrene in tetrahydrofuran with tetrabutylammonium tetrafluoroborate and sodium tetraphenylboron as supporting electrolytes. From the electrolyses using tetrabutylammonium tetrafluoroborate two molecular weight distribution curves, showing weight average molecular weight values ( ) of 3.6×10⁵–1.7×10⁶ for the first and 5×10³ for the second one were observed. When using sodium tetraphenylboron a unique distribution curve was displayed suggesting that the weight average molecular weight of the polystyrene, increases by increasing the current density; the observed MWD being rather low.     

Molecular weight and molecular weight distribution from dynamic measurements of polymer melts

A method for determining the molecular weight distribution (MWD) of a polymer melt has been developed using the dynamic elastic modulus (G'), plateau modulus (G), and zero shear complex viscosity (η). The cumulative MWD was found to be proportional to a plot of (G'/G)^0.5 vs. measurement frequency (ω). Frequency (ω) was found to be inversely proportional to (MW)^3.4, as expected. Results were scaled to absolute values using the empirical relationship η ∝ (M?_w)^3.4, where M?_w is the weight-average MW. M?_w, M?_n (number-average MW) and M?_w/M?_n calculated from melt measurements were found to agree with size exclusion chromatography usually well within 10 percent for broad and bimodal distribution samples. M?_w/M?_n tended to be approximately 20 percent higher for narrow distribution samples (M?_w/M?_n < 1.2) because we did not account for a finite distribution of relaxation times from a collection of monodisperse polymer chains. We also did not account for the plasticizing effect of short chains mixed with long ones which caused peak positions to be closer together for Theological vs, size exclusive chromatography (SEC) determinations of MW for bimodal distribution blends.     

Molecular motions in low molecular weight melts of polystyrene studied by n.m.r. relaxation

The proton spin-lattice relaxation times of polystyrene were determined selectively for the phenyl and chain protons at 88 MHz in the temperature range of the α-process using a usual FT-inversion-recovery experimental procedure. The temperature and molecular weight dependence of the relaxation times are discussed qualitatively from a phenomenological point of view. The phenyl ring and main chain are coupled and have cooperative character.     

Effects of mineral oil on the rheological behavior of polystyrene melts

The effects of mineral oil on the small strain rheological functions of polystyrene are shown to be simply related to the oil content and the rheological behavior for the neat material. Reduced variables, which are analogous to the effects of temperature, are found to successfully reduce the data for all levels of oil. The use and form of these reduced variables are shown to be predicted by the model of Bersted, which relates the rheological functions directly to the molecular weight distribution. The effects of the oil are argued to suggest that the mineral oil acts as a plasticizer for polystyrene.     

Low molecular weight polyacrylic acid with nitrilodi(methylene-phosphonic acid) chain ends for scale inhibition

Poly(acrylic acid) with molecular weight of 5000 was produced by using 2-aminoethanethiol hydrochloride as chain transfer agent. Amine chain end groups of the resulting polyacrylic acid were transformed into nitrilodi(methylenephosphonic acid) by reacting formaldehyde-phosphorous acid in the presence of hydrogen chloride. ¹H NMR, ³¹P NMR and microanalysis were used for structural analysis.The modified polyacrylic acid had much better calcium carbonate scale inhibition effect than commercial poly(acrylic acid).     

Effects of molecular weight and comonomer content on the optical clarity of crystallized syndiotactic polystyrene sheets

Clear and high-crystallinity sheets can be prepared consistently from syndiotactic polystyrene (sPS) simply by annealing amorphous sPS sheets at a temperature ranging from 150 to 180°C. The combination of its high clarity and high crystallinity makes sPS sheet suitable for many food/medical packaging applications. A series of sPS polymers with various molecular weight and para-methylstyrene (PMS) comonomer content were used in this study for gaining a mechanistic understanding of the cause for this unusual coexistence of high crystallinity and high clarity. Results show that high molecular weight is preferred for high sheet clarity. The effect of the comonomer content, however, is more complicated; instead of a monotone response to % PMS, the sheet clarity was found to peak at 4% PMS. A mechanism based on the regime growth theory of crystallization was used to interpret successfully the above behavior. Briefly, increasing molecular weight reduces the crystal growth rate and thus shifts the growth pattern to regime III, resulting in more clear sheet. Increasing % PMS reduces both the nucleation and crystal growth rates and thus shifts the growth pattern to either regime I (lower clarity) or III (higher clarity) as controlled by the relative magnitude of the two rate reductions. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2455–2461, 1999     

Influence of molecular weight on fatigue behavior of polyethylene and polystyrene

Fatigue tests in reversed tension-compression have been carried out on samples of polyethylene and polystyrene of widely varying molecular weights, extending up to 2, 000, 000. All tests on polystyrene specimens were made at 1600 rpm. For polyethylene, tests speeds had to be reduced to 100 rpm in order to avoid serious temperature effects. For both materials, increasing molecular weight leads to improved resistance to alternating loading. For polystyrene, this improvement in ultimate properties even continued well beyond molecular weight values where T_g, becomes effectively independent of molecular weight. For polyethylene, samples of high molecular weight did not fail even after 10⁷ cycles of alternating loading at a stress level of 3000 psi.     

Effects of molecular weight distribution and long-chain branching on the viscoelastic properties of high- and low-density polyethylene melts

Using the Han slit/capillary rheometer, rheological measurements were taken of several commercially available low- and high-density polyethylene melts, namely, three low-density polyethylene samples of Chemplex Corp. (CX 1005, CX 1016, and CX 3020), three low-density polyethylene samples of U.S. Industrial Chemicals Co. (NA 205, NA 244, and NA 279), two high-density polyethylene samples of Union Carbide Corp. (DMDJ 5140 and DMDJ 4306), and two high-density polyethylene samples of Mitsui Petrochemical Industries, LTD. Molecular characterization of these samples was carried out by the resin suppliers. The rheological measurements included (1) entrance pressure drop, (2) exit pressure, (3) pressure gradient, (4) die swell ratio. These then permitted us to determine the shear viscosity and normal stress differences. The rheological measurements were interpreted to identify the effects of long-chain branching and molecular weight distribution on the rheological properties of polyethylenes in the light of the existing molecular viscoelasticity theories. It was found that fluid elasticity is greater for polymers having a broader molecular weight distribution and that, for polymers having more long-chain branching, viscosities are lower while elasticities are higher.     

阴离子溶聚丁戊橡胶分子量的研究

以正丁基锂为引发剂，分别以二氧六环（DOX）和二乙二醇二甲醚（2G）为极性调节剂研究了异戊二烯和丁二烯阴离子共聚物分子量随反应时间的变化规律。实验发现，聚合物的分子量与活性种的活性有关，高温严重破坏活性种的活性，调节剂用量是杀死活性种活性的另一因素。聚合前期，分子量随时间的增加而增大；温度升高后，高温破坏了活性种的活性，活性链解缔合导致的粘度下降，所以聚合后期分子量降低。2G对活性链的破坏受温度的影响要小于DOX受温度的影响。     

Interfacial tension in polymer melts. Part II: Effects of temperature and molecular weight on interfacial tension

Interfacial tension is one of the most important parameters that govern the morphology of polymer blends and the quality of adhesion between polymers. However, few data are available on interfacial tension due to experimental difficulties. A pendant drop apparatus was used for the determination of the interfacial tension for the polymer pair polypropylene/polystyrene (PP/PS). The effects of temperature and molecular weight were evaluated. The range of temperatures used was from 178° to 250°C, and the range of molecular weights used was from 1590 to 400,000. The interfacial tension decreased linearly with increasing temperature. With only one exception, higher molecular weight systems showed weaker dependence of interfacial tension on temperature than lower molecular weight systems. Also, polydisperse systems showed a stronger dependency on temperature than the monodisperse systems. The value of the interfacial tension, which increases with molecular weight, appears to level off at molecular weights above the entanglement chain length. For the polymer pair PP/PS, the dependency of the interfacial tension on the number average molecular weight appears to follow the well-known semi-empirical (?2/3) power rule over most of the range of molecular weights. Comparable correlations were obtained with values of the power between ?1/2 and ?1.0.     

Variation of refractive index of polystyrene with molecular weight: Effect on the determination of molecular weight distributions

Sensitive refractive index measurements on solutions have indicated that the narrow molecular weight polystyrene samples prepared by the Mellon Institute method exhibit a small systematic molecular weight–refractive index dependence up to 300,000 and higher. This upper limit is higher than had previously been suggested. Sensible consistency is generally assumed above a few thousand molecular weight. The effect on this molecular weight–refractive index dependence on gel permeation chromatography and light-scattering molecular weight measurements on polystyrene standards is illustrated.     

The effect of molecular weight on the fatigue behavior of polystyrene

Significant improvements in the fatigue life of polystyrene can be realized by increase in molecular weight. In this investigation, samples of polystyrene of varying molecular weight were subject to alternating cycles of axial tension and compression at a test speed of 1600 rpm. One set of fatigue tests was made on samples machined from commercial whole polymer rod. A second set of samples was prepared from a PS standard having an average molecular weight of 160,000 and a narrow molecular weight distribution. A third set of samples had a high average molecular weight of 860,000. For a whole polymer, the S–log N curves tended to have the same general shape as for metals, and the endurance limit appeared to be about 1400 psi. The test results show that the average fatigue life, at any given stress amplitude, is significantly increased by increase in molecular weight. For example, the average life of the high molecular weight standard at any given stress level was found to be more than tenfold that of the low molecular weight standard.     

The atactic polystyrene molecular weight effect on the thermal properties and crystal structure of syndiotactic polystyrene/atactic polystyrene blends

This work examined how the molecular weight of atactic polystyrene (aPS) affects the thermal properties and crystal structure of syndiotactic polystyrene (sPS)/aPS blends using differential scanning calorimetry, polarized light microscopy and wide angle X-ray diffraction (WAXD) technique. For comparative purposes, the structure and properties of the parent sPS was also investigated. The experimental results indicated that these blends showed single glass transition temperatures (T_gs), implying the miscibility of these blends in the amorphous state regardless of the aPS molecular weight. The non-isothermal and isothermal melt crystallization of sPS were hindered with the incorporation of aPSs. Moreover, aPS with a lower molecular weight caused a further decrease in the crystallization rate of sPS. Complex melting behavior was observed for parent sPS and its blends as well. The melting temperatures of these blends were lower than those of the parent sPS, and they decreased as the molecular weight of aPS decreased. Compared with the results of the WAXD study, the observed complex melting behavior resulted from the mixed polymorphs (i.e. the α and β forms) along with the melting-recrystallization-remelting of the β form crystals during the heating scans. The degree of melting-recrystallization-remelting phenomenon for each specimen was dependent primarily on how fast the sPS crystals were formed instead of the incorporation of aPSs. Furthermore, the existence of aPS in the blends, especially the lower molecular weight aPS, apparently reduced the possibility of forming the less stable α form in the sPS crystals.     

Rheological behavior of low molecular weight polystyrene composites containing monodisperse crosslinked polystyrene beads

Steady shear viscosities and dynamic moduli of polymer composites, consisting of crosslinked polystyrene beads and low molecular weight polystyrene matrix, were measured in a cone-and-plate rheometer at different temperatures. Viscosities and dynamic moduli were found to be very sensitive to filler loading and measurement temperature. Steady shear viscosities of 30% and 40% loaded low molecular polystyrene composites showed a power-law behavior over the entire range of shear rates. Storage and loss moduli were initially linear with frequency on double logarithmic plots, with limiting slopes of 0.3 and 0.1. At high concentration of filler particles, they showed a flat plateau at low frequencies, indicating that these systems exhibit a yield behavior. A 20% PS composite loaded with beads of high crosslink densities resulted in poor dispersion of beads as a result of poor dispersion of particles. PS beads 1.16 μm in diameter showed a higher viscosity. It is due to the apparent increase in loading resulting from broken particles. At low measurement temperature, filler effects were suppressed by high viscosity matrix and showed a similar rheological behavior to high molecular weight by PS matrix. We suggest that rheological behavior reflects the state of dispersion of beads in the matrix.