Polymer nanocomposites-polymer-based materials that incorporate filler elements possessing at least one dimension in the nanometer range-are increasingly being developed for commercial applications ranging from building infrastructure to food packaging to biomedical devices and implants. Despite a wide range of intended applications, it is also important to understand the potential for exposure to these nanofillers, which could be released during routine use or abuse of these materials, so it can be determined whether they pose a risk to human health or the environment. This article is the first in a series of two that review the state of the science regarding the release of engineered nanomaterials (ENMs) from polymer nanocomposites. Two ENM release paradigms are considered in this series: the release of ENMs via passive diffusion, desorption, and dissolution into external liquid media and release of ENMs assisted by matrix degradation. The present article focuses primarily on the first paradigm and includes (1) an overview of basic interactions between polymers and liquid environments and a brief summary of diffusion physics as they apply to polymeric materials; (2) a summary of both experimental and theoretical methods to assess contaminant release (including ENMs) from polymers by diffusion, dissolution, and desorption; and (3) a thorough, critical review of the associated body of peer-reviewed literature on ENM release by these mechanisms. A short outlook section on knowledge gaps and future research needs is also provided.

The assembly of dissolved technical lignins in aqueous and organic medium has been studied at the solid-liquid interface. Adsorption of alkali lignin onto gold coated crystals treated with a cationic polymer was determined using a quartz crystal microbalance with dissipation monitoring. Complete coverage of the cationic surface with alkali lignin occurred at low solution concentration, revealing a high affinity coefficient under both alkali and neutral conditions. With additional adsorption studies from organosolv lignin in organic solvent and spectroscopic analysis of mixtures of cationic polymer and alkali lignin, a noncovalent interaction between lignin's aromatic rings and the cation of the quaternary ammonium group was shown to exist. The work underscores how polyphenolic biopolymers can strongly interact with cations through noncovalent interactions to control molecular architecture.

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Concomitant with the development of polymer nanocomposite (PNC) technologies across numerous industries is an expanding awareness of the uncertainty with which engineered nanoparticles embedded within these materials may be released into the external environment, particularly liquid media. Recently there has been an interest in evaluating potential exposure to nanoscale fillers from PNCs, but existing studies often rely upon uncharacterized, poor quality, or proprietary materials, creating a barrier to making general mechanistic conclusions about release phenomena. In this study we employed semiconductor nanoparticles (quantum dots, QDs) as model nanofillers to quantify potential release into liquid media under specific environmental conditions. QDs of two sizes were incorporated into low-density polyethylene by melt compounding and the mixtures were extruded as free-standing fluorescent films. These films were subjected to tests under conditions intended to accelerate potential release of embedded particles or dissolved residuals into liquid environments. Using inductively-coupled plasma mass spectrometry and laser scanning confocal microscopy, it was found that the acidity of the external medium, exposure time, and small differences in particle size (on the order of a few nm) all play pivotal roles in release kinetics. Particle dissolution was found to play a major if not dominant role in the release process. This paper also presents the first evidence that internally embedded nanoparticles contribute to the mass transfer, an observation made possible via the use of a model system that was deliberately designed to probe the complex relationships between nanoparticle-enabled plastics and the environment.

Callus cells were initiated on cambial strips obtained from 4 to 8 y old Douglas-fir (Pseudostuga menziesii) trees, cultured on solidified Murashige and Skoog (MS) medium supplemented with 2,4-dichlorophenoxyaceticacid (2,4-D) and benzylaminopurine (BA). The cultures could be maintained by sub-culturing on fresh medium every four weeks. When the callus cells were subsequently transferred to liquid MS medium supplemented with different phytohormones, suspension cultures could be initiated and maintained by periodic sub-culture. Approximately 65% of the callus cells cultured on liquid MS medium supplemented with 2,4-D, when maintained for 6-7 weeks without sub-culture, differentiated to tracheary element (TE) like cells. The formation of TE like cells was confirmed histochemically by staining with phloroglucinol-HCl. Secondary thickening of the cell walls were confirmed by polarized light microscopy, which showed strong birefringence of the cell wall due the presence of crystalline cellulose. The presence of lignin was determined by pyrolysis-GC-MS and FTIR spectroscopy. The lignin content in differentiated cell wall samples was quantified at 21% by the lignothioglycolic acid assay. Analysis of monosaccharide composition of cell wall samples after acid hydrolysis showed that the percentage of glucose, xylose and mannose had increased in the differentiated cell walls. These increases correspond to the formation of cellulose, glucomannan and xylan, primarily associated with secondary cell walls.

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