Robert A. Bubeck

Structure–property relationships in metallocene polyethylenes

The development of new polyolefins based on metallocene technologies represent a considerable advance in the performance of polyethylenes available for a wide range of applications. The ability to obtain a homogeneous short chain branching distribution and control molecular weight is remarkable. The added ability to introduce long chain branching (LCB) into what would otherwise be linear polyethylenes has opened up a broad range of processing possibilities. Through a review of this technology, it becomes apparent that significant elements of the fundamentals underlying the materials science of these new polyolefins are in need of clarification. The morphology of metallocene polyethylenes, particularly in the low-crystallinity range, has yet to be conclusively resolved. The occurrence of multiple melting endotherms for ethylene/α-olefin copolymers with, ostensibly, a homogeneous branching distribution is in need of explanation. LCB incorporated into the linear copolymers enhances shear thinning and melt elasticity. Yet to be fully determined, however, are the types of long chain branch architectures that most effectively promote these desirable rheological attributes, and to what degree these types are present in metallocene polyolefins.

Modes of deformation in rubber-modified thermoplastics during tensile impact

Real-time small-angle X-ray scattering (RTSAXS) studies were performed on a series of rubber-modified thermoplastics. Scattering patterns were measured at successive time intervals as short as 1.8 ms and were analysed to determine the plastic strain due to crazing. Simultaneous measurements of the absorption of the primary beam by the sample allowed the total plastic strain to be computed. The plastic strain due to other deformation mechanisms, e.g. particle cavitation and macroscopic shear deformation was determined by the difference. Samples of commercial thicknesses can be studied at high rates of deformation without the inherent limitations of microscopy and its requirement of thin samples (i.e., plane strain constraint is maintained on sample morphology).Contrary to the conclusions drawn from many previous dilatation-based studies, it has been demonstrated that the strain due to non-crazing mechanisms, such as rubber particle cavitation, and deformation of the glassy ligaments between rubber particles, occurs before that due to crazing mechanisms. Crazing accounts for at most only half of the total plastic strain in HIPS (high impact polystyrene) and ABS (rubber-modified styrene-acrylonitrile copolymer) materials. The proportion of strain attributable to crazing can be much less than half the total in thermoplastic systems with considerable shear yield during plastic deformation.The predominant deformation mechanism in polycarbonate-ABS blends is shear in the PC (polycarbonate) with associated rubber gel particle cavitation in the ABS. This cavitation means that there appears to be a direct relationship between gel particle rubber content in the ABS and toughness of the blend. The mechanism is the same whether the tensile stress is in the direction parallel or perpendicular to the injection-moulded orientation, with simply less total strain being reached before fracture in the weaker perpendicular direction. Crazing, although the precursor to final fracture, occurs after the predominant mechanism and contributes only a few per cent to the total plastic deformation.

The influence of branch length on the deformation and microstructure of polyethylene

Abstract The length and number of side chain branches have a profound influence on the microstructure and physical properties of polyethylene (PE). For a series of linear PE copolymers: environmental stress cracking resistance (ESCR), melting points, creep resistance and modulus, and equilibrium spherulite size were all found to increase with increasing branch length (methyl to hexyl) at a given density and molecular weight. It is proposed that (at a fixed molecular weight) branch length and branch concentration determine spherulite size and, consequently, spherulitic boundary areas, in which the dry crazing/voiding occurs during the incubation period of environmental stress cracking (ESC). At a fixed density, decreased spherulite size contributes to greater spherulite boundary slip and increased creep at low (less than 2 MPa) stresses.

Kinetics of environmental stress cracking in high density polyethylene

Abstract The observation of environmental stress crack (ESC) growth in high density polyethylene (HDPE) in a 10% lgepal CO-630 solution is reported using double-edge- notched specimens, which allow a fracture mechanics approach. Below the initial stress intensity factor K1 value of 0.4 MPa m1/2, the cracking process consisted of both an incubation time for cracking, td′ and a crack growth stage. The incubation time is stress dependent (decreasing with increasing stress), while the crack growth exhibits a root time (t 1 2 ) dependence and is relatively stress independent. The incubation time is the time necessary to generate a dry void craze structure sufficient to allow the PE to absorb the aggressive liquid. As a consequence of the liquid transport in the craze structure, the crack growth is believed to be controlled by the velocity of the liquid entering the void/fibril structure where capillary pressure is the driving force. The incubation times were determined to be more significant than the actual average crack growth rates for the PE samples tested. Injection moulding orientation increases the average crack growth rate without significantly changing the incubation time.

Macromolecular orientation in hot stretched and injection moulded polystyrene

Abstract Small-angle neutron scattering (SANS) and optical birefringence measurements were used to investigate a wide range of macromolecular orientation in hot stretched and injection moulded polystyrene samples. These two techniques show similar trends even though each is sensitive to different forms of macromolecular orientation. The hot stretched samples are found to deform nonaffinely in relation to the external drawing (for draw ratios higher than 3). The cold condition injection moulded samples showed more orientation than the hot condition injection moulded samples, which were not birefringent even though they did have a residual orientation as seen by SANS. SANS is evidently sensitive to factors controlling orientation at extremes for which birefringence is not.

Effect of deformation ratio on fibril deformation in fatigue of polystyrene

Small-angle X-ray scattering has been used to measure the deformation of craze fibrils during mechanical fatigue of polystyrene. The maximum deformation of the sample in the fatigue cycle was kept constant while the minimum deformation was varied. When the minimum deformation was 50% or more of the maximum, the load on the craze fibrils remained tensile. When the minimum deformation was reduced below this, the load on the fibrils became compressive and they buckled. The main effect of minimum sample deformation on fatigue life occurred in the regime where the fibrils remained straight. In this regime a decrease in minimum sample deformation caused a considerable decrease in fatigue life. At low minimum sample deformations the effects of minimum deformation on fatigue life were not large. These effects probably stem from the fibril strains involved in the deformation processes.

Monofilament drawing device for in situ x‐ray scattering studies of orientation development in polymeric fibers

The physical properties of polymer fibers are directly related to the development of orientation in the draw zone under extensional conditions. Although the final properties and microstructure of fibers can be evaluated after processing is completed, it is desirable to understand the development of orientation during drawing independently from the effects superimposed by subsequent unit operations. In situ x‐ray scattering measurements of the extrudate in the draw zone provide an ideal means of accomplishing this goal. Depending on the polymeric system to be studied, rotating anode or synchrotron‐based x‐ray sources may be required, making portability of the fiber drawing device highly desirable. This paper describes an easily transportable in situ x‐ray filament drawing device, and discusses typical data obtained using the device to produce filaments from a liquid crystalline nematic solution of poly(cis‐benzoxazole) in polyphosphoric acid.

Surfaces of Semi-Fluorinated Block Copolymers Studied Using Nexafs

The molecular orientation within a surface liquid crystalline layer made up of semi-fluorinated side-groups [-CO-(CH{sub 2}){sub x{minus}1}-(CF{sub 2}){sub y}F] (SF groups) attached to the isoprene block of a styrene-isoprene diblock copolymer was determined by analyzing the partial electron yield C-edge NEXAFS signal. The results show that in contrast to the bulk, where the SF groups lie parallel to the diblock copolymer lamellae and thus parallel to the surface, the surface SF groups make an average angle with the surface normal of between 29 and 46{degree} depending on x and y.

Molecular Origins of Deformation Behavior and Physical Aging in Polycarbonate Copolymers

The molecular origins of yield and deformation behavior in polycarbonate copolymers were studied with a combination of mechanical property, dynamic mechanical spectrometry (DMS), solid state variable temperature NMR (VTNMR), and Fourier transform infrared (FTIR) measurements. Changes in yield associated with physical aging were included in the study. The VTNMR measurements were performed using deuterium-tagged PC samples in the glassy state. As a general rule, increased resistance to embrittlement due to physical aging coincides with increasing relative linearity of the copolymer molecular structure. The concepts of “short range order” in polycarbonate glasses proposed by Haward et al.1 and by Schaefer et al.2 are discussed in the light of the results presented here and were found to be partially applicable.

An apparatus for in situ x-ray scattering measurements during polymer injection molding

We report a novel instrument for synchrotron-based in situ x-ray scattering measurements during injection molding processing. It allows direct, real-time monitoring of molecular-scale structural evolution in polymer materials undergoing a complex processing operation. The instrument is based on a laboratory-scale injection molding machine, and employs customized mold tools designed to allow x-ray access during mold filling and subsequent solidification, while providing sufficient robustness to withstand high injection pressures. The use of high energy, high flux synchrotron radiation, and a fast detector allows sufficiently rapid data acquisition to resolve time-dependent orientation dynamics in this transient process. Simultaneous monitoring of temperature and pressure signals allows transient scattering data to be referenced to various stages of the injection molding cycle. Representative data on a commercial liquid crystalline polymer, Vectra(R) B950, are presented to demonstrate the features of this apparatus; however, it may find application in a wide range of polymeric materials such as nanocomposites, semicrystalline polymers and fiber-reinforced thermoplastics.