Molecular Weight Distribution Data Research Articles

Analysis of flow behavior of polyethylenes and polypropylenes with graessley's theory

AbstractGraessley's theory of non‐Newtonian behavior of linear polymers is tested with data on polyethylenes and polypropylenes having a wide range of molecular weights and molecular weight distributions, and for a polyethylene fraction. Theoretical flow curves derived by using molecular weight distribution data from column fractionations are in good agreement with experimental curves obtained from a cone‐and‐plate viscometer and an extrusion‐type rheometer, when the distribution is assumed to cut off at a maximum molecular weight. The experimentally obtained relaxation time for entanglement is found to be proportional to the Rouse relaxation time, though the former is about one decade smaller than the latter.

Molecular weight distributions in radiation‐induced polymerization. VII. γ‐ray‐induced polymerization of “wet” styrene in the solid state

AbstractIn the present paper kinetic and molecular weight distribution results are reported for the γ‐ray‐initiated polymerization of styrene in the solid state. “In‐source” polymerization over the temperature range −35°C to −55°C and post‐polymerization at −35°C have been investigated for “wet” styrene samples (water concentration ≈ 10−3 mole/l.). An interesting feature of the solid‐state polymerization of styrene is the bimodal nature of the molecular weight distribution. On a qualitative basis the results resemble those obtained previously for the polymerization of rigorously dried (“dry”) styrene. However, there are noticeable differences on a quantitative basis resulting from the considerable difference in the water content between wet and dry samples. On the basis of these studies, the kinetic and molecular weight distribution data have been interpreted as being indicative of polymerization occurring simultaneously via free‐radical and cationic mechanisms.

Molecular weight distributions in radiation‐induced polymerization. V. A simultaneous kinetic and molecular weight distribution analysis of the liquid‐state polymerization of styrene

AbstractThe γ‐ray initiated polymerization of styrene in the liquid state was investigated over the temperature range 0 to −29°C at constant dose rate. The kinetics and molecular weight distributions were studied for samples prepared by standard techniques and samples subjected to exhaustive drying to remove residual water. In the former case, the rates of reaction were comparable to those for purely free radical polymerization, however, the resulting molecular weight distributions were distinctly bimodal, indicating an additional contribution from the cationic mechanism. On the other hand, the rates of polymerization for rigorously dried samples were 2 to 3 orders of magnitude greater than accepted free‐radical values, and the molecular weight distributions were unimodal in nature. The experimental results were compared with theoretical kinetic data and molecular weight distribution data generated from a kinetic scheme taking into consideration polymerization via free‐radical, cationic, and radical‐cationic species, resulting in the evaluation of a number of quantities of interest. Substitution of determined values for the rate constants and G values results in good agreement between theoretically generated and experimentally determined kinetic data and molecular weight distribution data over the range of experimental conditions studied.

Experimental investigation of the concept of molecular migration within sheared polystyrene

AbstractSeveral important aspects of the flow in polymer melts through capillaries remain unexplored. This paper examines experimentally one such effect associated with the radial shear‐stress gradient in capillaries. During capillary melt flow of a polymer with a wide molecular weight distribution, migration of the large molecules away from the region of highest shear stress, i.e., at the capillary wall, has been predicted but only modestly investigated. This effect has the potential to produce a molecular weight spectrum over the cross section of extruded polymer. Studies of distribution in shear were conducted on a well‐characterized wide‐distribution polystyrene (M̄w = 234,000). An Instron Rheometer equipped with a long capillary (length/diameter ratio of 66.7) was used to perform the extrusion at temperatures of 160–250°C. A solvent coring procedure was used to dissolve away concentric layers of polymer from the extrudate for molecular weight analyses. The method has been shown to cut clean sections without selective extraction. Values of M̄w, M̄n and M̄w/M̄n were calculated from complete molecular weight distribution data obtained by calibrated gel permeation chromatography. For a wide range of shear rates and temperatures, no evidence for molecular fractionation was observed. Shear degradation of this polymer was found to be small. However, at high shear rates at 250°C, evidence indicating extensive shear‐induced thermal degradation was found. No evidence for oxidative degradation at the extrudate surface was found at either low or high shear rates at this temperature.

Correlation between steady state flow curve and molecular weight distribution for polyethylene melts

AbstractThis article uses Graessley's theory of viscosity to predict the flow curve for several high‐density and low‐density polyethylene melts using the molecular weight distribution data obtained from the gel permeation chromatograph. The agreement with the experimental flow curve obtained from the Weissenberg rheogoniometer and the Instron rheometer was not quantitative for many high‐density polyethylenes studied here. For the low‐density polyethylenes, it was shown that the agreement between the theory and the experiment was good even though the molecular weight distribution data were not corrected for long‐chain branching. For these samples, the experimental relaxation time τ0 obtained by superposition of the data with the theoretical master curve was of the order of the Rouse relaxation time τR. The systematic increase in the ratio τR/τ0 was ascribed to the increase in the molecular weight or to the increased number of long‐chain branches.

Modification of a Gel Permeation Chromatograph for Automatic Sample Injection and On-Line Computer Data Recording

Abstract Gel permeation chromatograms are usually subjected to mathematical analyses of varying degrees of complexity to obtain molecular weight distribution and analytical data. Hand reduction of the recorder output data is difficult and time consuming even on an occasional basis. This paper describes in detail the modifications necessary to adapt the Waters Gel Permeation Chromatograph Model 200 for on-line data acquisition by a computer equipped with an analog to digital converter (IBM 1800). Since the operation is done in the background mode, time sharing with several other users or instruments is possible. Modifications to the auto injection unit are also described. This system has proven to remove all of the usual hand data reduction steps and to be extremely flexible for nonroutine analyses.

Gel‐permeation chromatography and cellulose. I. Effect of degree of nitration of cellulose on molecular weight distribution data

AbstractConsideration is given to the effect on gel‐permeation chromatographic (GPC) data of the extent of substitution in nitrated cellulose. GPC parameters for samples containing 13.55–13.81% nitrogen (14.14% corresponds to complete substitution, DS = 3) were hardly affected by this variation in substitution. Variations that were observed are considered to arise within the samples themselves. Experiments with low molecular weight organic iodides, nitrates, and hydroxyl compounds indicate longer chain lengths than actual; this is attributed to extensive solvation of the substituent groups. The very long chain lengths obtained for cellulose nitrate by the present GPC procedure may arise from such an affect.

Molecular weight distributions in radiation‐induced polymerization. III. γ‐Ray‐induced polymerization of styrene at low temperatures

AbstractThe kinetics of the γ‐radiation‐induced polymerization of styrene was studied at radiation intensities of 8 × 104, 2.4 × 105, 3.1 × 105, and 8.3 × 105 rad/hr over a temperature range of −10°C to 30°C. The water content of the irradiated samples varied from 1.0 × 10−3 to 7.5 × 10−3 mole/l. The power dependence of the rate of polymerization on the dose rate at −10°C varied from 0.53 to 0.71 as the water content of the sample varied from 7.5 × 10−3 to 1.0 × 10−3 mole/l. A value of 3.1 kcal/mole was determined for the overall activation energy. Molecular weight distribution studies by gel‐permeation chromatography indicated the presence of two distinct peaks. The contribution of each peak was dependent on specific experimental parameters. Kinetic data and molecular weight distribution data indicate the coexistence of two propagating species. Analysis of the data strongly suggests that a free‐radical mechanism and a cationic mechanism are involved.

Flow Behavior of Low Molecular Weight Polybutadiene, Carboxyl-Polybutadiene, and Butadiene-Acrylonitrile Copolymers

The flow behavior of some low molecular weight polymers was studied as a function of shear rate, shear stress, and temperature. The polymers included polybutadienes, random carboxyl and carboxyl-terminated polybutadienes, carboxyl-terminated butadiene-acrylonitrile, and acrylonitrile-butadiene copolymers. Measurements were made in the temperature range 3.8–73°C. A cone and plate viscometer and pressure capillary viscometer were used to cover the shear rate range 1−105 sec−1. Measurements were carried out with various capillary radius to length ratios. M̄w and M̄n values were calculated from molecular weight distribution data obtained by gel permeation chromatography. Limiting viscosity numbers were also determined. In the range of shear rates studied, all polymers showed a limiting viscosity at low rates of shear and a region of shear rate thinning. The log viscosity-log shear rate flow curves for each sample at the various temperatures were superimposable by linear shifts. Energies of activation calculated according to the method of Fox and Loshaek were found to have values characteristic of the molecular structure. Normal stresses developed in capillary flow as measured by entrance effects showed some dependence on structure. Flow results were compared with the Bueche-Harding experimental standard curve and the Bueche theoretical curve. The presence of polar groups in the polymer chain increased the energy of activation and the relative magnitude of the viscosity. The log viscosity-log shear rate flow curves for all the samples at any constant temperature were superimposable by linear shifts in two distinct classes according to differences in molecular weight distribution.

Numerical analysis and kinetic interpretation of molecular weight distribution data

AbstractA novel numerical method is described for calculating both the distribution moments and the molecular weight distribution curve from fractional on data. The method is based upon Gauss‐Laguerre integration techniques and enables the user to calculate six distribution moments by finite algebraic operations on the cumulative distribution curve. Results obtained by this method on a sample of thermally polymerized polystyrene are described and compared with the theoretical predictions of polymerization kinetics. The data are found to be equally consistent with an initiation mechanism which is either second‐ or third‐order in monomer concentration. The calculated activation energy for second‐order initiation compares favorably with values already reported in the literature, but insufficient data are available to make the same comparison for third‐order initiation.

Gel permeation chromatography. I. A new method for molecular weight distribution of high polymers

AbstractPolystyrene gels crosslinked in the presence of diluents have been made in fine‐mesh bead form suitable for packing into chromatographic columns. A series of narrow molecular weight range polymer fractions was eluted through such columns with aromatic and chlorinated solvents. Effluent concentrations were detected and recorded by a continuous differential refractometer. The fractions were shown to be efficiently separated. Columns capable of separating adjacent polymeric samples of high molecular weight were prepared from gels crosslinked in the presence of large amounts of diluents having little or no solvent action on polystyrene. Smaller proportions of diluents and those with more solvent action yielded columns with lower molecular weight permeability limits. Such studies provided a unique quantitative view of the topology of the gels. They also demonstrated that rapid repetitive molecular weight distribution data can be obtained in this way on polymers for which solvents compatible with the gels are available.