Size-dependent linear rheology of silica filled poly(2-vinylpyridine)

Intermediate Polymer Relaxation Explains the Anomalous Rheology of Nanocomposites with Ultrasmall Attractive POSS Nanoparticles

The rheological properties of entangled polymers loaded with very small, strongly attractive polyhedral oligomeric silsesquioxane (POSS) fillers differ from that of nanocomposites with larger fillers by (1) the shorter breadth of the entanglement plateau and (2) the relatively unchanged terminal viscosity with increasing POSS loading. Although such anomalous rheological properties can rewrite the property-processing map of materials (e.g., high glass transition temperature and low viscosity), their mechanism remains unclear. In this study, we report that polymer relaxations on intermediate time scales between α and entire-chain relaxation, so-called "slower processes", are responsible for this unusual rheological behavior of poly(2-vinylpyridine)/octa(aminophenyl)silsesquioxane (P2VP/OAPS) nanocomposites. To uncover the effects of entanglements on the nanocomposite dynamics, rheometry is used for variable matrix molecular weights. Results show a systematic change in the rheological response, which is independent of the molecular weight, and in turn, the presence of entanglements. This supports a physical interpretation that a slower process dominates the rheological response of the material at intermediate frequencies on length scales larger than the segment length or the OAPS diameter, while the underlying physical time scales associated with the entanglement relaxation remain unchanged. Such insights are anticipated to assist the future rational design of other highly attractive and ultrasmall nanoparticles that enable a fine-tuned rheological response of nanocomposites across multiple length scales.

Rationalizing the Composition Dependence of Glass Transition Temperatures in Amorphous Polymer/POSS Composites

We report that the fractions of "bonded" or "unbonded" monomers at a filler interface dictate the composition dependence of the glass transition temperatures (T_g) of polyhedral oligomeric silsesquioxane (POSS)-containing nanocomposites. T_g is arguably the single most important material property; however, predicting T_g in nanocomposites is often challenging because of confounding interfacial effects. To this end, we design a model nanocomposite to systematically study T_g of nanocomposites by leveraging the "all-interfacial" nature of ultrasmall POSS fillers loaded into random copolymers of styrene and 2-vinylpyridine (2VP). The amine-functionalized POSS forms hydrogen bonds only with 2VP, which behaves as a "bonded" monomer. The influence of copolymer composition and POSS loading on the T_g of this model composite is successfully explained by a Fox equation framework. This model also captures the T_g increase of other POSS-based polymer composites and potentially directs the future design of nanocomposite materials with tailored T_g.

Plasticized starch-based biocomposites containing modified rice straw fillers with thermoplastic, thermoset-like and thermoset chemical structures

In this work, rice straw (RS) as an abundant biomass was applied to prepare some renewable thermoplastic materials by using soda-pulping and benzylation processes. The obtained RS products including untreated RS, RS pulp, benzylated RS pulp and pulping liquor as well as benzylated RS were incorporated into the thermoplastic starch through a twin-screw extrusion process to obtain all green composites. The successful thermoplasticization reaction of RS products was confirmed by spectroscopy results and morphological observations. The interfacial adhesion between the plasticized starch matrix and the RS products is enhanced by the chemical modifications, which confirmed by investigating through the morphological observations and linear rheological responses. The partial phase miscibility of the plasticizer/starch mixtures is improved by adding the benzylated RS and RS pulp. The RS pulp having cellulosic microfibers enhances the Young modulus and tensile strength of the plasticized starch even more than untreated RS. However, their thermoset and thermoset-like structure leads to the brittle failure mode of the starch biocomposites, similar to the common natural fiber biocomposites. The thermoplasticization reaction changes the failure mode and significantly improves the toughness of the plasticized starch/RS product biocomposites owing to better phase miscibility.