Shiwang Cheng

Big Effect of Small Nanoparticles: A Shift in Paradigm for Polymer Nanocomposites

Polymer nanocomposites (PNCs) are important materials that are widely used in many current technologies and potentially have broader applications in the future due to their excellent property tunability, light weight, and low cost. However, expanding the limits in property enhancement remains a fundamental scientific challenge. Here, we demonstrate that well-dispersed, small (diameter ∼1.8 nm) nanoparticles with attractive interactions lead to unexpectedly large and qualitatively different changes in PNC structural dynamics in comparison to conventional nanocomposites based on particles of diameters ∼10-50 nm. At the same time, the zero-shear viscosity at high temperatures remains comparable to that of the neat polymer, thereby retaining good processability and resolving a major challenge in PNC applications. Our results suggest that the nanoparticle mobility and relatively short lifetimes of nanoparticle-polymer associations open qualitatively different horizons in the tunability of macroscopic properties in nanocomposites with a high potential for the development of advanced functional materials.

Controlling Interfacial Dynamics: Covalent Bonding versus Physical Adsorption in Polymer Nanocomposites

It is generally believed that the strength of the polymer-nanoparticle interaction controls the modification of near-interface segmental mobility in polymer nanocomposites (PNCs). However, little is known about the effect of covalent bonding on the segmental dynamics and glass transition of matrix-free polymer-grafted nanoparticles (PGNs), especially when compared to PNCs. In this article, we directly compare the static and dynamic properties of poly(2-vinylpyridine)/silica-based nanocomposites with polymer chains either physically adsorbed (PNCs) or covalently bonded (PGNs) to identical silica nanoparticles (RNP = 12.5 nm) for three different molecular weight (MW) systems. Interestingly, when the MW of the matrix is as low as 6 kg/mol (RNP/Rg = 5.4) or as high as 140 kg/mol (RNP/Rg= 1.13), both small-angle X-ray scattering and broadband dielectric spectroscopy show similar static and dynamic properties for PNCs and PGNs. However, for the intermediate MW of 18 kg/mol (RNP/Rg = 3.16), the difference between physical adsorption and covalent bonding can be clearly identified in the static and dynamic properties of the interfacial layer. We ascribe the differences in the interfacial properties of PNCs and PGNs to changes in chain stretching, as quantified by self-consistent field theory calculations. These results demonstrate that the dynamic suppression at the interface is affected by the chain stretching; that is, it depends on the anisotropy of the segmental conformations, more so than the strength of the interaction, which suggests that the interfacial dynamics can be effectively tuned by the degree of stretching-a parameter accessible from the MW or grafting density.

High-performance aromatic polyimide fibres

A new polyimide has been synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-dimethyl-4,4′-diaminobiphenyl (DMB). A high-strength, high-modulus, high-temperature fibre has been developed from this polyimide via a dry-jet wet spinning method. The tensile strength of BPDA-DMB fibres is 3.3 GPa and the tensile modulus is around 130 GPa. The compressive strength of the fibres has been investigated through a tensile recoil test (TRT), while the fibre morphology after compression has been studied via polarized light microscopy (PLM) and scanning electron microscopy (SEM). From the TRT measurements, we have observed that the compressive strength of this fibre is 665 (±5) MPa, which is higher than those of other aromatic polymer fibres. The effect of fibre diameter on the compressive strength of BPDA-DMB fibres is not substantial. The critical compressive strain for this fibre at which the kink bands start appearing under the observation of PLM is at 0.51–0.54%. Subglass relaxation processes have been observed and the measure of an apparent relaxation strength may serve as one of the factors which significantly affect the compressive strength of the fibres. Tensile tests of pre-compressed fibres reveal a continuous loss in tensile strength (up to 30%) with increasing the compressive strain (up to 2.6%). PLM and SEM observations show that during the compression BPDA-DMB fibres form regularly-spaced kink bands at ±60 ° (±2 °) with respect to the fibre axis. The kink band density initially increases with the compressive strain, and reaches a maximum at around 1.1%. Further increase of the compressive strain decreases this density due to the merge of the neighbouring bands. The size of kink bands also correspondingly increases within this compressive strain region. The morphological observation via SEM implies the existence of a skin-core structure and microfibrillar texture which are common features in polymer fibres.

Unexpected molecular weight effect in polymer nanocomposites

The properties of the interfacial layer between the polymer matrix and nanoparticles largely determine the macroscopic properties of polymer nanocomposites (PNCs). Although the static thickness of the interfacial layer was found to increase with the molecular weight (MW), the influence of MW on segmental relaxation and the glass transition in this layer remains to be explored. In this Letter, we show an unexpected MW dependence of the interfacial properties in PNC with attractive polymer-nanoparticle interactions: the thickness of the interfacial layer with hindered segmental relaxation decreases as MW increases, in sharp contrast to theoretical predictions. Further analyses reveal a reduction in mass density of the interfacial layer with increasing MW, which can elucidate these unexpected dynamic effects. Our observations call for a significant revision of the current understandings of PNCs and suggest interesting ways to tailor their properties.

Basic characteristics of uniaxial extension rheology: Comparing monodisperse and bidisperse polymer melts

We have carried out continuous and step uniaxial extension experiments on monodisperse and bidisperse styrene-butadiene random copolymers (SBR) to demonstrate that their nonlinear rheological behavior can be understood in terms of yielding through breakdown of the chain entanglement network and rubberlike rupture via non-Gaussian chain stretching leading to chain scission, respectively. In continuous extension, the sample with bidisperse molecular weight distribution showed greater resistance, due to double-networking, against the yielding-initiated failure. An introduction of 20% high molecular weight (106 g/mol) SBR to an SBR matrix (2.4 × 105 g/mol) could postpone the onset of nonuniform extension by as much as two Hencky strain units. In step extension, the bidisperse blends were also found to be more resistant to elastic breakup than the monodisperse matrix SBR. Rupture in both monodisperse and bidisperse SBR samples occurred when the finite chain extensibility was reached at sufficiently high rates....

Is shear banding a metastable property of well-entangled polymer solutions?

Using simultaneous rheometric and particle-tracking velocimetric measurements, we show that the long-time rheological states of well-entangled polymer solutions are not unique in simple shear. Shear banding emerges upon a sudden startup shear as well as during conventional large amplitude oscillatory shear at rates higher than the overall chain relaxation rate. However, shear homogeneity prevails when the final conditions of continuous shear and oscillatory shear are approached gradually from rates lower than the terminal relaxation rate. This suggests that the observed shear banding as nonlinear response to sudden large deformation is only metastable and not unique.

Morphology and crystal structure in single crystals of poly(p-phenylene terephthalamide) prepared by melt polymerization

Using the confined thin film melt polymerization technique lamellar single crystals, of two morphologies, have been grown for poly(p-phenylene terephthalamide). Electron diffraction (e.d.) patterns from these crystals and from fibre-like samples polymerized from sheared monomer, are in best agreement with a modified phase I cell. The lattice parameters (a = 7.88, b = 5.22, c = 12.9 A; α, β, γ = 90°) are similar to those in previously proposed cells, but the Pla1 space group rather than Pn or P21/n space groups more closely fits the e.d. data. In particular, 210, 120, 320 and 410 reflections are present with moderate intensity, which are forbidden for the latter structures and do not result from double diffraction.

Letter to the Editor: Sufficiently entangled polymers do show shear strain localization at high enough Weissenberg numbers

This Letter concludes that the recent data of Li et al. [J. Rheol. 57, 1411–1428 (2013)] are entirely consistent with the previous observations of the occurrence and absence of shear banding during startup shear and nonquiescent relaxation after large stepwise shear. In other words, based on the linear viscoelastic characteristics of these solutions depicted in Fig. 5(a) of Li et al., we find their results to follow from the previous analysis: One insufficiently entangled solution naturally exhibited homogeneous shear under the explored conditions. The two more entangled solutions did not exhibit shear banding and nonquiescent relaxation, because the samples appear to have significant polydispersity in the molecular weight distribution and because the applied shear rates were much lower than those needed to produce shear banding. Thus, the observations of Li et al. support rather than refute the existing knowledge concerning nonlinear rheological responses of entangled polymer solutions to startup and stepwise shear.

Mechanical reinformcement and thermal transition behaviors in nylon 6-b-polyimide-b-nylon 6 triblock copolymers☆

Abstract The mechanical properties and thermal transition behaviors of nylon 6-b-polyimide-b-nylon 6 triblock copolymers have been studied with varying block length and rigidity of the polyimide backbone and compared with those of pure nylon 6, in situ and physical solution blends. From differential scanning calorimetry experiments for liquid nitrogen quenched triblock copolymers, it is found that the crystallization temperature, which appears on successive heating, increases by 10–15°C, indicating relatively slow crystallization kinetics. Dynamic mechanical (DM) experiments show that the triblock copolymers exhibit a storage modulus which is 2 times higher than that of pure nylon 6 in the temperature range of −50°C to 150°C. As the rigidity of the polyimide backbone increases, the storage modulus of the copolymers decreases. In addition, the system with a polyimide block length of 12 000 g mol −1 shows the highest E ′ than any others with the block length shorter or longer than this value. Glass transition behaviors of these materials are also studied via α relaxations from DM observations. The relaxation peak temperature of tan δ increases from 60°C of pure nylon 6 to 82°C with 5% incorporation of the polyimide block component in the triblock copolymers. By incorporating a fully aromatic polyimide backbone into the triblock copolymer system, the tan δ peak of the α relaxation increases even further to 105°C with the same weight percentage of polyimide block incorporation.

Focus: Structure and dynamics of the interfacial layer in polymer nanocomposites with attractive interactions

In recent years it has become clear that the interfacial layer formed around nanoparticles in polymer nanocomposites (PNCs) is critical for controlling their macroscopic properties. The interfacial layer occupies a significant volume fraction of the polymer matrix in PNCs and creates strong intrinsic heterogeneity in their structure and dynamics. Here, we focus on analysis of the structure and dynamics of the interfacial region in model PNCs with well-dispersed, spherical nanoparticles with attractive interactions. First, we discuss several experimental techniques that provide structural and dynamic information on the interfacial region in PNCs. Then, we discuss the role of various microscopic parameters in controlling structure and dynamics of the interfacial layer. The analysis presented emphasizes the importance of the polymer-nanoparticle interactions for the slowing down dynamics in the interfacial region, while the thickness of the interfacial layer appears to be dependent on chain rigidity, and has been shown to increase with cooling upon approaching the glass transition. Aside from chain rigidity and polymer-nanoparticle interactions, the interfacial layer properties are also affected by the molecular weight of the polymer and the size of the nanoparticles. In the final part of this focus article, we emphasize the important challenges in the field of polymer nanocomposites and a potential analogy with the behavior observed in thin films.