Triple-detector GPC characterization and processing behavior of long-chain-branched polyethylene prepared by solution polymerization with constrained geometry catalyst

Fourteen long-chain branched (LCB) polyethylene (PE) samples were prepared by a constrained geometry catalyst. The PE samples had average branching frequencies of 0.06-0.98 branches per polymer chain, as determined by the nuclear magnetic resonance spectroscopy (¹³C NMR). These samples, as well as five linear PEs were characterized using a gel permeation chromatography (GPC) coupled with online three-angle laser light scattering (LS), differential refractive index (DRI), and viscosity (CV) detectors. The root mean-square radius of gyration intrinsic viscosity ([η]), and molecular mass (M) of the PEs were measured for each elution fraction. Based on the comparison of the long-chain branching (LCB) PEs with their linear counterparts and the Zimm-Stockmayer equation, the distributions of long-chain branch frequency (LCBF) and density (LCBD) as function of molecular mass were estimated. It was found that although the LCBF increased with the increase of molecular mass, the LCBD showed a maximum value in the medium molecular mass range for most of the PE samples. The average LCBD data from the GPC analysis were in good agreement with the ¹³C NMR measurements. The rheological properties and processing behavior of these samples were also assessed. While the long chain branching showed significant effects on the modulus and viscosity, it did not improve the processing. Compared to linear PE, polymer melt flow instabilities such as sharkskin, stick-slip and gross melt fracture developed in extrusion of LCB PEs occurred at lower wall shear stresses and apparent shear rates.     

The effect of random branching on the balance between flow and mechanical properties of polyamide-6

The rheological and mechanical properties of a series of linear and randomly branched polyamide 6 samples, with varying molar mass and varying degree of moderate branching, have been investigated.As expected, it was found that random long-chain branching has a pronounced effect on the rheological behaviour of the materials in both shear and extensional flow. The zero shear viscosity increases with branching while the flow curve becomes more shear thinning. Randomly branched materials have an enhanced melt strength in elongational flow. Although branched, the materials show perfect melt stability in their rheology.The mechanical properties show minor differences between the linear and the moderately branched samples and are mainly dependent on the weight average molar mass. A small increase in modulus, yield stress and failure stress and a decrease in the strain at break are found, which is probably due to increased molecular orientation in the plane of injection moulding. The IZOD impact strength is similar to what is normally found for linear polyamide-6, and independent of branching. Also the fracture toughness K_IC is not affected by the incorporation of random branching. However, it is clearly dependent on the weight average molar mass.By using random branched polyamide-6, molecular weights and related mechanical properties can be obtained that are out of reach for linear polyamide-6.     

Critcal stress and recoverable shear for polymer melt fracture

The phenomenon of extrudate distortion, which is called melt fracture, was studied for polystyrene samples of narrow and broad molecular weight distribution, and commerical samples of polypropylene and linear and branched polyethylene. It was experimentally found that the shear stress at the onset of melt fracture (τ_cr) is of the order of 10⁶ dynes/cm² and independent of the distribution of molecular weights. As the weight average molecular weight increases the shear stress τ_cr decreases. For polystyrene extruded at τ_cr the recoverable shear strain, which is defined to be half the ration (first normal stress difference/shear stress), was found proportional to the factor M _zM _z+1/M _w² which represents the distrubution of molecular weights. The proportionality is expected to hold for other polymer systems. The polymer behavior at the onset of melt fracture was explained in terms of Graessley's entanglement theory and his correlation between true and Rouse shear compliance.     

Controlled degradation and crosslinking of polypropylene induced by gamma radiation and acetylene

Isotactic polypropylene (iPP) undergoes crosslinking and extensive main chain scissions when submitted to irradiation. The simultaneous irradiation of PP and acetylene is able to control chain scission and produce grafting. The grafted PP further reacts with PP radicals resulting in branching and crosslinking. In this work, commercial polypropylenes (iPP) of different molecular weights were irradiated with a ⁶⁰Co source at dose of 12.5 kGy in the presence of acetylene in order to promote the crosslinking. The mechanical and rheological tests showed a significant increase in melt strength and drawability of the modified samples obtained from resins with high melt flow index. The characterization of the molecular modifications induced by gamma irradiation of isotactic polypropylenes under acetylene atmosphere proved the existence of branching, crosslinking and chain scission in a qualitative way. The G′ and G″ indicated the presence of LCB in all samples. Therefore, PP irradiation under acetylene was proved to be an effective approach to achieve high melt strength polypropylene (HMSPP).     

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.     

Effects of the ethylene–propylene–diene monomer microstructural parameters and interfacial compatibilizer upon the EPDM/montmorillonite nanocomposites microstructure: Rheology/permeability correlation

Attempts have been made to investigate the effects of ethylene–propylene–diene monomer (EPDM) rubber structural parameters on the developed microstructure, mechanical properties, rheology, and oxygen gas permeability of EPDM/organically modified montmorillonite (O‐MMT) nanocomposite samples prepared via melt mixing. Maleic anhydride grafted EPDM (EPDM‐g‐MAH) has been employed as an interfacial compatibilizer. The influence of the EPDM melt viscosity and chain linearity on the extent of exfoliation of the clay nanolayers has been evaluated through the calculation of the nanolayer aspect ratio (length/thickness) with the Halpin–Tsai model. The results are consistent with the X‐ray diffraction patterns of the samples. The flocculation of the clay nanolayers has been found to be more probable when O‐MMT is mixed with highly branched, low‐molecular‐weight EPDM. More exfoliation occurs when EPDM rubber with a high molecular weight but low branching is used. This has been confirmed by more nonlinear melt rheology behavior and broadening of the retardation time spectra. Maleated EPDM has been shown to be effective in enhancing the molecular intercalation of the clay nanolayers and the prevention of flocculation in both low‐molecular‐weight and high‐molecular‐weight EPDM matrices. Dynamic melt rheology measurements have revealed nonterminal behavior within the low‐frequency range by interfacially compatibilized molten samples with an EPDM‐g‐MAH/clay ratio of 3, regardless of the matrix molecular weight and chain linearity. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Crystallization characteristics of weakly branched poly(ethylene terephthalate)

A series of branched poly(ethylene terephthalate) (BPET) samples were prepared from melt polycondensation by incorporation of various amount (0.4-1.2 mol%) of glycerol as a branching agent. These polymers were characterized by means of H¹ NMR, intrinsic viscosity. The general crystalline and melting behavior was investigated via DSC. It was found that the crystalline temperature T_cc from the melt shifted to high temperature and the T_hc from the glass got low for BPETs while the melting temperatures of BPETs kept almost unchanged. The kinetics of isothermal crystallization was studied by means of DSC and POM. It was found that the present branching accelerated the entire process of crystallization of BPETs, although prolonged the induced time. In addition, branching reduced nucleation sites; hence the number of nucleates for BPET got smaller. Therefore, more perfect geometric growth of crystallization and greater radius of spherulites could develop in BPET due to less truncation of spherulites.     

Effect of Method of Preparation on Molecular Packing of TMPC/PS Blends

Dielectric and differential scanning calorimetry measurements of polystyrene/tetramethyl polycarbonate (PS/TMPC) blends have been carried out for two series of samples prepared by a melt mixing method, using a Brabender, and a solvent casting method using methylene chloride. The dielectric measurements of the samples were carried out over wide ranges of temperature (50–220°C) and frequency (10^-2–10⁵Hz). The composition ratios of the samples measured were 12·5, 25, 50, 75 and 87·5wt% TMPC for solvent casting and 10, 25, 40, 50 and 60wt% TMPC for melt mixing. It has been found that melt mixing under the conditions used produces compatible blends for all the composition ratios measured without causing any pronounced degradation. The glass transition temperatures and dielectric relaxation behaviour of the blends prepared by the different methods showed differences in molecular packing and dynamics. The results obtained could be attributed to a variation in the size of structural units responsible for compatibility, which depended on the method of preparation. © of SCI.     

Long chain branching in low density polyethylene: 2. Rheological behaviour of the polymers

Investigations have been carried out by different methods on the rheological properties, both shear and tensile, of some unfractionated samples of low density polyethylene, the molecular characteristics and long chain branching content being known. From the comparison with analogous linear polyethylenes it clearly appears that correlations exist between the long chain branching content and the main viscous and elastic parameters of the melt. The results obtained illustrate the possibility of reaching a preliminary evaluation of the LCB degree in the integral polyethylene samples by simple rheological measurements.     

Relaxation Behavior of Polyethylene Oriented by Various Techniques

Abstract

Stress relaxation of high-density polyethylene extrudates and those crystallized from highly deformed melt (PE-1) have been investigated in a wide range of temperatures (?50 to +120°C) and draw ratios from 5.5 up to 12.2 at the different constant tensile strains from 1 up to 20%. The experimental data obtained have been summarized by the time-temperature superposition principle. Relative intensity of stress relaxation (the stress drop in 10³ s divided by the initial stress) has been observed to increase together with the growth of draw ratio despite the enhancement of the short-term properties. The radiation cross-linking of the PE-1 samples may only decrease the stress relaxation intensity by 30%. The relaxation properties of a number of oriented polyethylene samples produced by various techniques were compared. It has been established that all the investigated materials are characterized by similar values and high relative drops in stress, whereas the short-term properties are essentially different. It points out to the relaxation processes being intensive both in the oriented and unoriented PE.     

Untersuchungen zur Langkettenverzweigung in Polyäthylenen

Polyethylene samples of various densities and melt flow indices resulting from different polymerization processes have been investigated with respect to long chain branching (LKV). For that purpose several polymer fractions have been characterized by measurement of weight average molecular weights M_w and intrinsic viscosities [η], the latter ranging from 0,2 to 3,2 with high pressure samples and from 0,2 to 10 with low pressure material. The intrinsic viscosity difference of branched (high pressure) polyethylene compared to linear (low pressure) polyethylene is used as a measure of LKV. With high pressure polyethylene LKV increases with decreasing density. This dependence is strongest within the medium molecular weight range. Samples with varying LKV but constant density can be obtained by appropriate change of polymerization conditions. No LKV has been observed with low pressure polyethylene. This means a marked difference compared to high pressure material of equal density. Branching with low pressure polymers can therefore be ascribed to the short chain type only, which in particular results from copolymerization. Several mathematical approaches have been checked whether or not they can yield suitable information about n, the number of long chain branches per molecule. The best fit with our experimental data is obtained using the expression [η]v/[η]₁ = g^1,3 (n = f(g)) and assuming, that the average concentration of long chain branch points does not depend on molecular weight for fractions of the same sample (n/M = const.). If LKV ist taken into consideration, logarithmic normal molecular weight distributions are obtained for many high pressure polyethylenes (similar to low pressure material). Data are reported in support of the view, that performance characteristics are dependent on LKV. There is some evidence, that melt flow properties of polyethylene are improved with increasing LKV.     

Determination of the molecular characteristics of commercial polyethylenes with different architectures and the relation with the melt flow index

The molecular weight distribution curves of several commercial polyethylene samples were evaluated by high‐temperature gel permeation chromatography with two detectors (a refractive‐index detector and a viscometer) to determine the molecular sizes and architectures (branching). The polymer samples included high‐ and low‐density polyethylenes with different molecular weight distributions (wide, medium, unimodal, and bimodal) from nine producers. The results were tested against the melt flow index and zero‐shear melt viscosity to find correlations. The data for high‐density polyethylene correlated well with the molecular weight, whereas the data for low‐density polyethylene did not correlate. However, when the weight‐average molecular weight was corrected by the branching parameter and a factor form, all the polyethylene samples fit a single equation. These results indicate that the melt flow index is dependent not only on the molecular weight but also on the molecular shape, including branching. The relation accounted for samples of different resin producers, molecular weights (65,000–638,000), and polydispersities (2.9–20). The use of the branching parameter for the correction of the molecular weight allowed the correlation of these parameters despite differences in the technologies, molecular weights, and molecular architectures. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1572–1578, 2007     

Correlations of the first normal stress difference with shear stress and of the storage modulus with loss modulus for homopolymers

The effects of temperature, molecular weight and its distribution, side chain branching, and the structure of polymers on the elastic behavior of bulk homopolymers were investigated, by using logarithmic plots of first normal stress difference (N₁) against shear stress (σ₁₂) and logarithmic plots of storage modulus (G′) against loss modulus (G″). For the investigation, we have used data from the literature as well as our recent experimental results, covering a very wide range of temperature and shear stress or loss modulus. It has been found that such plots are very weakly sensitive to (or virtually independent of) temperature and to the molecular weight of high molecular weight polymers, but strongly dependent upon the molecular weight distribution and the degree of side chain branching. A theoretical interpretation of the observed correlations is presented, using molecular theories.     

Influence of long-chain branching on linear viscoelastic flow properties and dielectric relaxation of polycarbonates

The dynamic viscoelastic property, creep and creep recovery behavior, and dielectric relaxation of long-chain branched Bisphenol A polycarbonates were measured in parallel plate rheometer and dielectric analyzer. The linear polycarbonate (PC-L) as reference and three branched polycarbonates (PC-Bs) have similar molecular weights and molecular weight distributions, while the PC-Bs have different branching degrees, below 0.7 branch points/chain and above twice of M_c. The long-chain branched polycarbonates exhibit higher zero-shear viscosities, more significant shear shinning, higher flow activation energies, and much longer relaxation times. It was also found that long-chain branches increase the elasticity of melt characterized by the steady-state recoverable compliance and the storage modulus. The ‘dissident’ rheological behavior of long-chain branching exhibiting mainly in addition polymers such as polyolefin, is confirmed in condensation polymers. These behaviors resulted from additional molecular entanglements of long-chain branches can be understood qualitatively in terms of the tube model for topological constraints. The dielectric α-relaxation of linear polycarbonate and branched polycarbonates has been fitted with Vogel-Fulcher-Tammann-Hesse (VFTH) equation and the shape of relaxation time curves is also analyzed. The long-chain branched polycarbonates present longer relaxation times, but divergent α-relaxation temperatures, because the latter is dominated by the free volume.     

Gross melt fracture of polyethylene. II: Effects of molecular structure

The effect of molecular structure (MW, MWD and LCB) on the critical tensile stress (σ_c) for the onset of gross melt fracture (OGMF), proposed in Part I (1) as a material‐dependent criterion for fracture, was determined for a group of polyethylenes varying in structure. These included linear low and high‐density polyethylenes and several materials produced using metallocene and constrained geometry catalysts. It was found that the critical stress is independent of M_W, for constant polydispersity but increases with increasing long chain branching and polydispersity. The addition of boron nitride particles had no effect on the σ_c up to a level of 0.5% by weight.     

Polystyrenes of known structure: Part 3. Polymers with long-chain branching

The melt- and solution viscosity behaviour of some polystyrenes with long-chain branching is described. These polymers, which were prepared by reacting chloromethylated polystyrene with potassium polystyryl, are distinguishable from the comb-shaped polystyrenes previously described in having longer branches and lower branching frequencies. (The number-average molecular weight of the branches is greater than 4.6 × 10⁴ and in some cases greater than that of the backbone.) Their low shear melt viscosities and intrinsic viscosities in tetrahydrofuran and cyclohexane are above those of the backbone polymer and increase as the branch length increases for a given branching frequency, the rate of increase being greater the lower the branching frequency. In each solvent the intrinsic viscosities of the branched polymers are below those of linear polymers of comparable molecular weights. The melt viscosities of the majority of the branched polymers are also below those of linear polymers of the same molecular weights, but for a few, those with the longest branches in the series with the lowest branching frequency, the opposite is true.     

The particle flow of polystyrene during capillary extrusion

The melt flow of emulsion polymerized polystyrene has been investigated in accordance with the particle flow concepts developed by Berens and Folt. Particles were found to be present in the extrudate up to 210°C and resins with larger particles were found to have lower viscosities. The molecular weight appears to have no significant effect on the melt viscosity above a certain molecular weight. The energy of activation for viscous flow at 190°C and at shear stress of 5 × 10⁵ dynes/cm² was found to be 29–33 kcal/mol depending on type of resin.     

Influence of molecular structure on the rheological and processing behavior of polyethylene resins

The rheological and processing behavior (melt fracture performance) of linear lowdensity polyethylenes (LLDPEs) is studied as a function of both the weight average molecular weight (M_w) and its distribution (MWD). A number of LLDPE resins having different molecular characteristics were tested, with essentially one characteristic (M_w or MWD) changing at a time. The first series of resins consisted of nine samples having a wide range of polydispersities (3.3–12.7) and nearly constant M_w and short chain branching. The second series had six resins with varying M_w (51,000–110,000) but fixed MWD (about 4). The influence of M_w and MWD on the viscosity profiles, linear viscoelastic moduli as expressed by means of a discrete spectrum of relaxation times, extrudate swell, and melt fracture behavior for these resins is reported. Correlations between the molecular characteristics of the resins and their rheological and processing behavior are also reported. It is found that for a given molecular weight, the optimum melt fracture performance is obtained at a specific polydispersity value, and it is characterized by a minimum relaxation time for the resin defined in terms of recoverable shear.     

Shape memory and stress relaxation behaviour of oriented mono-dispersed polystyrene

The shape memory and stress relaxation behaviour of oriented samples of mono-dispersed polystyrene have been studied. It was found that the transient stress dip of Fotheringham and Cherry could successfully predict the recovery providing that the initial deformation which produces the molecular orientation is undertaken at a sufficiently fast strain rate. This means that if recovery is to be predicted, relaxation of the molecular orientation must not occur during the drawing process. This is attributed to the low fraction of the viscosity term of the total stress for polystyrene. This makes it advantageous to maximise the total stored stress, allowing the smaller component to be more accurately determined. It is proposed that the critical strain rate for maximising the stored stress is related to the smallest relaxation time in the melt, the entanglement time, τ_e.     

Rheological control in foaming polymeric materials: I. Amorphous polymers

The influence of rheological properties, especially melt strength, on foam structures, such as cell size, cell density and cell size distribution, of amorphous polymer was investigated. The rheology of polystyrene (PS) was controlled by molecular modification with free radical reaction, and PS with long chain branching (LCB) level ranging from 0.15 to 1.6 branching point per 10⁴ carbon atom was gotten. The shear and elongational rheology were found to be dependent on the LCB structure, and the strain hardening behavior of modified samples in transient elongational viscosity confirmed the existence of long branched chain. The effects of chain structure and foaming conditions such as temperature and pressure were studied by the analysis on the foam structures obtained by supercritical CO₂. The experimental results revealed that increasing LCB level would decrease cell size, make cell size distribution narrower and slightly increase cell density. The effects of chain topology on the foam structures were also investigated by numerical simulation, where Pom-Pom model was used to describe the effect of backbone length and arm length. The dependence of cell size on the arm length was consistently observed in experiments and simulation. It suggested that the arm length had greater influence on the cell radius than the backbone length. Therefore, the relationship among foam structures, rheological properties and molecular structures can be established from both experiments and simulation, which can be used as a guidance to control the foam structure by designing and controlling the molecular structures and the corresponding rheological properties.