John R. Dorgan

Single-Step Method for the Isolation and Surface Functionalization of Cellulosic Nanowhiskers

Surface modification of cellulosic nanowhiskers (CNW) is of great interest, especially to facilitate their use as polymer reinforcements. Generally, alteration of the surface chemistry is performed using multiple reaction steps. In contrast, this study demonstrates that the needed hydrolysis of amorphous cellulose chains can be performed simultaneously with the esterification of accessible hydroxyl groups to produce surface functionalized CNW in a single step. The reaction is carried out in an acid mixture composed of hydrochloric and an organic acid (acetic and butyric are both demonstrated). Resulting CNW are of similar dimensions compared to those obtained by hydrochloric acid hydrolysis alone; sizes are verified by multiangle laser-light scattering and transmission electron microscopy. However, narrower diameter polydispersity indices indicate that surface groups aid the individualization of the nanowhiskers (Px = 2.5 and 2.1 for acetic and butyric acid, Px = 3.0 for hydrochloric acid). More than half of the hydroxyl groups located on the CNW surface are substituted under the employed reaction conditions as determined by quantitative Fourier-transform infrared-spectroscopy. The resulting surface modified CNW are dispersible in ethyl acetate and toluene indicating increased hydrophobicity and thus are presumably more compatible with hydrophobic polymers when used as a reinforcing phase.

Thermal and Rheological Properties of Commercial-Grade Poly(Lactic Acid)s

Poly(lactic acid) is the subject of considerable commercial development by a variety of organizations around the world. In this work, the thermal and rheological properties of two commercial-grade poly(lactic acid)s (PLAs) are investigated. A comparison of the commercial samples to a series of well-defined linear and star architecture PLAs provides considerable insight into their flow properties. Such insights are valuable in deciding processing strategies for these newly emerging, commercially significant, biodegradable plastics. Both a branched and linear grade of PLA are investigated. The crystallization kinetics of the branched polymer are inferred to be faster than the linear analog. Longer relaxation times in the terminal region for the branched material compared to the linear material manifests itself as a higher zero shear rate viscosity. However, the branched material shear thins more strongly, resulting in a lower value of viscosity at high shear rates. Comparison of the linear viscoelastic spectra of the branched material with the spectra for star PLAs suggests that the branched architecture is characterized by a span molecular weight of approximately 63,000 g/mol. The present study conclusively demonstrates that a wide spectrum of flow properties are available through simple architectural modification of PLA, thus allowing the utilization of this important degradable thermoplastic in a variety of processing operations.

Polylactides: properties and prospects of an environmentally benign plastic from renewable resources

Pressing environmental and economic concerns dictate the need to develop new synthetic macromolecules based on renewable resources. The vast majority of existing materials are based on non-renewable fossil resources that will eventually be extinguished. Manufacturing synthetic polymers and disposal by incineration produces CO 2 and contributes to global warming. For these reasons, poly(lactic acid) (PLA) polymers are of increasing commercial interest because they are derived from a renewable resources, sequester significant quantities of carbon dioxide relative to petrochemical based materials, conserve energy, and easily degrade. The mechanical properties of PLA are compared to other commodity plastics and it is shown that PLA closely resembles polystyrene. The effects of blending linear and branched chain architectures are discussed and it is shown that this provides a convenient method for controlling the elasticity and viscosity of the composite material without affecting mechanical or permeation properties. The melt rheology of high L content linear PLAs shows two unique features; they may be drawn to large Hencky strains without breaking and they exhibit considerable strain hardening. As a result, PLA is easily processed into fiber form. Due to the excellent combination of mechanical, rheological, and environmental properties, the prospects for widespread commercialization of PLA are excellent.

Melt rheology of variable L-content poly(lactic acid)

Polylactides (PLAs) have been known for several decades and have recently gained considerable commercial significance. This development makes it highly desirable to have the rheological properties of these materials well characterized and reduced to useable mathematical models. However, comprehensive rheological characterization of PLAs is not yet available from the literature. In this study, rheological and thermal measurements were made on a comprehensive and well-characterized set of homopolymers and copolymers spanning wide ranges of molecular mass and stereoisomer proportions (L content). For all compositions within the weight average molecular weight range of 105–106(g∕mol) and a reference temperature of 180 °C, the zero shear viscosity is described by the relationship log(η0)=−14.26+3.4log(Mw), the plateau modulus is 1.0±0.2MPa, and average WLF parameters are c1=3.241∕K and c2=164.9K; the later correspond to a Vogel temperature of 288.25 K. The values of the glass transition temperatures at infinit...

Melt rheology of poly(lactic acid): Entanglement and chain architecture effects

Poly(lactic acids) (PLAs) are a family of polyesters available via fermentation from renewable resources and are the subject of considerable recent commercial attention. In this study, the melt rheological properties of a family of poly(lactic acid) stars are investigated and compared to the properties of the linear material. For polymers made from a 98:2 ratio of the L to D enantiomeric monomers it is found that the entanglement molecular weight is in the range of 9000 g per mole (Me≈8700 g/mol) while the molecular weight for branch entanglement is inferred to be approximately 3500 g per mole (Mb≈34 600 g/mol). In addition, the zero shear viscosity of the linear material increases with the 4.6 power of molecular weight. These results may suggest that PLA is a semistiff polymer in accordance with other recent findings. The increase in zero shear viscosity for the branched materials is measured and quantified in terms of appropriate enhancement factors. Relaxation spectra show that the transition zone for ...

Water transport in polylactic acid (PLA), PLA/ polycaprolactone copolymers, and PLA/polyethylene glycol blends

Polylactic acid (PLA) is a hydrolytically degradable aliphatic polyester, and water vapor permeability may have a significant influence on the rate of degradation. A method is devised to use bags prepared from PLA films and filled with molecular sieves to determine the water vapor permeability in the polymer, its copolymers with caprolactone, and blends with polyethylene glycol. The “solution-diffusion” model is used to determine the permeability parameters. These include the solubility coefficient,S, a measure of the equilibrium water concentration available for hydrolysis and the diffusion coefficient,D, which characterizes the rate of water vapor diffusion into the film under specific conditions. Values ofS andD at 50‡C and 90% relative humidity ranged from 400 × 10-6 to 1000 × 10-6 cm3 (STP)/(cm3 Pa) and 0.20 × 10-6 to 1.0 × 10-6 cm2/s, respectively. TheS andD coefficients were also measured at 20 and 40‡C and compared to those of other polymers. The degree of crystallinity was found to have little influence on the measured permeability parameters. The heat of sorption, δHS, and the activation energy of diffusion, ED, were used to show that the permeability process is best described by the “water cluster” model for hydrophobic polymers. Finally, the diffusion coefficient is used to compare the rate of water diffusion to the rate of water consumption by ester hydrolysis. Results indicate that hydrolytic degradation of PLA is reaction-controlled.

Physical Properties and Fiber Morphology of Poly(lactic acid) Obtained from Continuous Two-Step Melt Spinning

Fibers of poly(lactic acid) (PLA) produced by two-step melt-spinning are studied. The PLA resin used contains a 98:02 ratio of l:d stereochemical centers. A range of processing conditions is explored. The cold-draw ratio is varied from 1 to 8 under conditions of constant heating. In addition, three draw ratios are studied at three different heating rates. The thermal, mechanical, and morphological properties of the resultant fibers are determined. Properties can be widely manipulated through a combination of draw ratio and draw temperature. A maximum tensile strength and modulus of 0.38 GPa and 3.2 GPa, respectively, are obtainable. Using atomic force microscopy, the fiber morphology is found to be highly fibrillar; microfibril diameters are roughly 40 nm in diameter. Very high draw ratios cause the fiber to turn from shiny and translucent to dull and white; this transition is attributed to surface crazing. Significant molecular weight loss is observed upon processing (weight-average molecular weights drops between 27% and 43%).

Heavy oils: Their shear story

Heavy oils are important unconventional hydrocarbon resources with huge reserves and are usually exploited through thermal recovery processes. These thermal recovery processes can be monitored using seismic techniques. Shear-wave properties,inparticular,areexpectedtobemostsensitivetothechanges in the heavy-oil reservoir because heavy oils change from being solid-like at low temperatures to fluid-like at higher temperatures. To understand their behavior, we measure the complex shearmodulusandthusalsotheattenuationofaheavy-oil-saturated rock and the oil extracted from it within the seismic frequency band in the laboratory. The modulus and quality factor Qoftheheavy-oil-saturatedrockshowamoderatedependence on frequency, but are strongly influenced by temperature. The shear-wave velocity dispersion in these rocks is significant at steam-flooding temperatures as the oil inside the reservoir loses viscosity.Atroomtemperatures,theextractedheavyoilsupports a shear wave, but with increasing temperature, its shear modulus decreases rapidly, which translates to a rapid drop in the shear modulus of the heavy-oil-saturated rock as well.At these low to intermediate temperatures 30°C‐100°C, an attenuation peak corresponding to the viscous relaxation of the heavy oil is encountered also resulting in significant shear-wave velocity dispersion, well described by the Cole-Cole model. Thus, shearwaveattenuationinheavy-oilrockscanbesignificantlylargeand iscausedbyboththemeltingandviscousrelaxationoftheheavy oil. At yet higher temperatures, the lighter components of the heavy oil are lost, making the oil stiffer and less attenuative.The dramaticchangesinshearvelocitiesandattenuationinheavyoils should be clearly visible in multicomponent seismic data, and suggestthatthesemeasurementscanbequalitativelyandquantitatively used in seismic monitoring of thermal recovery processes.

Supra-molecular ecobionanocomposites based on polylactide and cellulosic nanowhiskers: synthesis and properties.

Successful filler dispersion and establishment of good interfacial contact with the surrounding matrix are essential for optimized reinforcement in polymeric nanocomposites. In particular, in renewable-based composites this can be challenging, where hydrophilic attractions between nanofillers facilitate aggregation. Here an innovative approach to prepare cellulosic nanowhisker (CNW) reinforced polylactide (PLA) is presented. The lactide ring-opening polymerization is initiated from CNW surface hydroxyl groups after partial acetylation to control the grafting density. Grafting of PLA chains is verified by Fourier transform infrared spectroscopy. The resulting nanocomposites display exceptional properties; a heat distortion temperature of 120 °C is achieved at 10 wt % CNW loading and can be further enhanced to reach 150 °C at 15 wt % CNW. The formation of a percolating network is verified by comparison of modulus data with an established theoretical model. Additionally, nucleation by CNWs reduces the crystallization half-time to 15 s compared with 90 s for PLA. Melt-pressed films retain transparency indicating good filler dispersion.

Phosphite stabilization effects on two-step melt-spun fibers of polylactide

The effects of molecular weight stabilization on mechanical properties of polylactide (PLA) fibers are investigated. The textilegrade PLA contains a 98:02 ratio of L:D stereocenters and fibers are produced by the two step method, involving a primary quench and cold drawing. Molecular weight loss, which is approximately 30% for unstabilized PLA, can be eliminated by the addition of small amounts of tris(nonylphenyl) phosphite prior to processing. The thermal and mechanical properties of fibers produced with two different concentrations of TNPP are compared to those of unstabilized PLA. Faster crystallization rates are obtainable with addition of the stabilizer, but final crystallinities are unaffected. Mechanical properties of the TNPP-enriched fibers are greatly improved over their unstabilized counterparts. Tensile strengths can be improved by 10–30% at a given draw ratio, while modulus may be improved by 10–25%. Excessive amounts of TNPP or insufficient mixingresult in inhomog eneities that are deleterious to mechanical properties. Based on the available information, chain extension is believed to be the most likely mechanism for the molecular weight stabilization. # 2002 Elsevier Science Ltd. All rights reserved.