Craig J. Carriere

Estimation of Interfacial Tension Using Shape Evolution of Short Fibers

Interfacial tension plays an important role in the manufacture of thermoplastic matrix composites and in the development and stabilization of blend morphologies. The purpose of this work was to develop a rapid method to determine the interfacial tension. The work entailed the microscopic tracking of shape changes of polystyrene fibers imbedded in poly(methyl methacrylate). The measured interfacial tension was found to agree with a value extrapolated from literature data to ±20%. This method reduces the analysis time over traditional techniques, does not require complicated equipment, and provides reasonable precision.

The effects of short‐chain branching and comonomer type on the interfacial tension of polypropylene‐polyolefin elastomer blends

The imbedded fiber retraction method was used to assess the effect of increasing octene content and comonomer type on the compatibility of polypropylene-polyolefin elastomer (PP-POE) blends via direct measure of the interfacial tension. The interfacial tension was found to decrease monotonically with increasing octene content from a starting value of 1.5 ± 0.16 dyn cm at an initial octene level of 9% down to 0.56 ± 0.07 dyn cm at an octene content of 24%. These effects can be interpreted in terms of the effective decrease in the molecular weight between chain ends for the branched POE materials. The experimental data were found to be described well by a modification of the empirical relationship used to describe the effect of molecular weight on the interfacial tension for linear materials. The power-law parameter was found to be numerically equivalent for that obtained for the molecular weight dependence of linear materials. The measured interfacial tension was also found to be dependent on the type of comonomer used in the PP-POE systems. The interfacial tension ranged from 1.07 ± 0.09 dyn cm for a PP-POE system made using ethylene-propylene down to 0.56 ± 0.07 dyn cm for a PP-POE made using ethylene-octene (24% octene).

EVALUATION OF THE INTERFACIAL TENSION BETWEEN HIGH MOLECULAR WEIGHT POLYCARBONATE AND PMMA RESINS WITH THE IMBEDDED FIBER RETRACTION TECHNIQUE

The interfacial tension between molten, high molecular weight samples of polycarbonate and poly(methyl methacrylate) was measured using the imbedded fiber retraction (IFR) technique. A value of 1.44 ± 0.16 dyn/cm was measured at 240 °C. This value was obtained on commercial resins with zero‐shear bulk viscosities and at a temperature typically encountered in the processing of blends and/or composites. Refinements to the IFR method were implemented which reduced the experimentation time. Consequently, measurements of the interfacial tension can now be routinely obtained in several hours for materials with zero‐shear bulk viscosities on the order of 104 Pa s.

Correlation between glass transition temperature and chain structure for randomly crosslinked high polymers

The empirical form for the dependence, Tg(n) ≅ Tg(∞)·(1 + α/n), of the glass transition temperature Tg on the average number n of repeat units between crosslinks, is generalized for randomly crosslinked high polymers. The new form, Tg(n) ≅ Tg(∞) · [1 + c/(n·Nrot)], is based on a correlation study of data for 77 samples of 10 different sets of resins. The fitting parameter α is resolved into composition-dependent Nrot and composition-independent c terms. Nrot summarizes the average number of rotational degrees of freedom per repeat unit, and is estimated in a straightforward manner from the structure and mol fraction of each repeat unit. The value of c is found from data analysis to be 5 ± 2. The results of this work are consistent with expectations based on the entropy theory of glasses, and provide improved understanding and predictive ability for the properties of crosslinked polymers.

Dilute‐solution dynamic viscoelastic properties of xanthan polysaccharide

The frequency dependences of the storage and loss shear moduli, G’ and G‘, of dilute solutions of highly purified xanthan polysaccharide were measured at 20.0 °C using the Birnboim–Schrag multiple‐lumped resonator. The frequency range was 150–8000 Hz and the concentration range was 0.1–0.4 g/l. Three solvents were used, one of which contained 75% by weight glycerol to increase the solution viscosity. Measurements of oscillatory flow birefringence were also made in one solvent over the frequency range from 1–630 Hz and agreed well with the viscoelastic data in the range of overlap. The intrinsic viscosity in both water and 25%/75% water/glycerol (with 0.085 mol/l sodium chloride) was determined as 5200 ml/g. The frequency dependences of G’ and G‘, extrapolated to infinite dilution, could be fitted to a hybrid model for semiflexible rods which was modified to take into account a moderate degree of molecular weight distribution (Mw/Mn=1.4). From the experimental data the persistence length of the native xant...

Compatibility of high polymers probed by interfacial tension

Imbedded-fiber retraction (IFR) has been applied to study the compatibility of high polymers. IFR measures the interfacial tension between two immiscible high-viscosity thermoplastic resins in their molten states. Ten nonreactive blend pairs were studied. One blend component was a poly(styrene-co-acrylonitrile-co-fumaronitrile) terpolymer resin (S/AN/FN). The other component was one of a set of ten S/AN resins with an AN level between 0 and 51%. These high-molecular-weight resins were particularly challenging for IFR since they were nearly isorefractive, had high melt viscosities (103–105 Pa s), and could chemically age when molten. Interfacial tensions γ12 ranged from 0.00 to 5.5 dyn/cm at 200 °C. Miscible bends had γ12 = 0 and a single Tg.Immiscible blends had γ12 > 0 and two Tgs. Compatibility was quantitatively assessed from the monotonic rise in γ12 as compatibility decreases. The results demonstrate that IFR can rank the compatibility of high polymers. It is expected that IFR can also rank the compatibility of polymers with similar Tgs,and rank the ability of additives to enhance blend compatibility.

Failure and deformation studies of syndiotactic polystyrene

Syndiotactic polystyrene (sPS) is a chemically resistant, high-heat, semicrystalline polymer which is currently under development by The Dow Chemical Co. The research reported herein was undertaken to determine the critical fracture strength, i.e., the critical stress intensity factor, K 1C , and the fracture energy, G 1C , of sPS. The studies were aimed at developing a basic understanding of the failure mechanism and toughness of sPS. This work included investigations of the effect of molecular weight, as well as flow-induced anisotropy. Scanning electron microscopy (SEM) was used to aid in the determination of the failure mechanism. During failure testing, it was observed that sPS fails with a slow, controlled crack growth and ruptures with an almost nondetectable amount of yielding, as based on a tensile dilatometry investigation and a plane strain, biaxial yield experiment. The proposed failure mechanism, based on the scanning electron micrographs, is one of constrained crazing, followed by void coalescence with the spherulite nucleators acting as stress concentrators in the system. The damage appears to be greatly confined, with little initial cold-drawing of the spherulites. Addition of a nucleator reduces the K 1C values somewhat, as added nucleation sites proliferate the sites for stress concentration across the sample.

Evaluation and modeling of the high‐temperature short‐term creep performance of selected glass‐filled semicrystalline and liquid crystalline polymers

The creep performance of a material is one of the main criteria currently used to assess the long-term performance of thermoplastic composites. In a wide number of applications, including electrical connectors and various automotive applications, dimensional stability at elevated temperatures (i.e., short-term creep performance) is an essential design requirement. In many of these applications the material can be exposed to elevated temperatures and high applied stresses as part of the fabrication process or in the end-use application. The elevated temperature creep behavior of 30%, and 40% glass-filled syndiotactic polystyrene, 30% glass-filled poly(butylene terephthalate), 30% glass-filled poly(phenylene sulfide), and a 30% glass-filled liquid crystalline polymer has been evaluated. The creep behavior for each of the materials has been modeled using the formulation proposed by Findley, Kholsa, and Petersen (FKP). The FKP model was found to provide a good description of the creep performance of these glass-filled materials. The parameters in the FKP model were evaluated for each of the materials as a function of temperature and applied stress. Comparison of the predicted short-term creep behavior using the FKP model with experimental data has shown that the predicted creep behavior is within ±10% of the measured value over the temperature and applied stress ranges examined.

Fracture behavior of magnetic media films

The fracture behavior was determined for three polymeric films which are used for magnetic media and other applications. Both fracture initiation and propagation were characterized by a specialized video-based real-time film fracture technique. The toughness of each of these films was evaluated from the fracture mechanism and material properties of the films. The results showed that a materials resistance to crack propagation is not proportional to its resistance to crack initiation; and the stress associated with the crack initiation is usually smaller than the maximum stress.

Method for the evaluation of interfacial tension in high viscosity systems

The ability to develop a strong interface in a composite between the polymer matrix and fiber reinforcement depends on the wetting of coated fibers by the surrounding material. In order to characterize this phenomenon, one is required to measure the interfacial tension between the coating and the matrix at elevated temperatures which are characteristic of composite manufacturing. The method presented here allows the evaluation the interfacial tension between high viscosity materials. It is based on the analysis of the transitional shape evolution of a short fiber of one material imbedded in a matrix of a second. The uniqueness of the method is related to its ability to provide a value for the interfacial tension for high viscosity materials. Additional benefits of the method include a relatively short period of required experimentation and a lucid data acquisition procedure.The paper addresses both experimental and theoretical aspects of the method.