Biqiong Chen

Poly(ε-caprolactone)/graphene oxide biocomposites: mechanical properties and bioactivity

Biomedical applications of graphene have recently attracted intensive attention, with graphene-based nanomaterials being reported as promising candidates in, for example, drug delivery, biosensing and bioimaging. In this paper, mechanical properties and bioactivity of nanofibrous and porous membranes electrospun from graphene oxide (GO) nanoplatelets reinforced poly(ε-caprolactone) (PCL) were investigated. The results showed that the presence of 0.3 wt% GO increased the tensile strength, modulus and energy at break of the PCL membrane by 95%, 66% and 416%, respectively, while improving its bioactivity during biomineralization and maintaining the high porosity of over 94%. The mechanical enhancements were ascribed to the change in the fiber morphology and the reinforcing effect of GO on PCL nanofibers, whereas the improvements on the bioactivity stemmed from the anionic functional groups present on the GO surface that nucleated the formation of biominerals. Systematic studies on the PCL/GO nanocomposite films with varying GO concentrations revealed that the reinforcing effect of GO on PCL was due to the strong interfacial interactions between the two phases characterized by Fourier transform infrared spectroscopy, the good dispersion of GO in the matrix and the intrinsic properties of GO nanoplatelets. The strong and bioactive PCL/GO nanofibrous membranes with a high porosity have great potential for biomedical applications.

Characterisation and drug release performance of biodegradable chitosan-graphene oxide nanocomposites.

Biodegradable chitosan-graphene oxide (GO) nanocomposites possess improved mechanical properties and drug delivery performance over chitosan and could prove to be a viable, controlled and pH-sensitive transdermal drug delivery system. Chitosan nanocomposites containing varying GO contents and drug loading ratios were investigated. The nanocomposite with 2 wt % GO provided the optimal combination of mechanical properties and drug-loading capacity. It offered a faster and a more substantial release of drug than chitosan as well as a slower biodegradation rate, owing to the abundant oxygenated functional groups, hydrophilicity and large specific surface area of GO sheets. The drug delivery profiles of the nanocomposite were dependent on the drug loading ratio, with 0.84:1 being the best ratio of drug to GO for a quick and high release of the loaded drug. The nanocomposite also demonstrated pH sensitivity of drug release, releasing 48% less drug in an acidic condition than in a neutral environment.

Polymer–clay nanocomposites: an overview with emphasis on interaction mechanisms

Abstract There has been a surge of interest in polymer–clay nanocomposites over the past decade. This review surveys these new materials with emphasis on the debate surrounding interaction mechanisms and the behaviour of intercalated polymer in clay galleries. Swelling properties, high cation exchange capacities, high aspect ratio and large surface area give smectite clays the new role of high performance filler for thermoplastic or thermosetting polymers for the creation of intercalated or exfoliated nanocomposites. These nanocomposites can be prepared by three routes: in situ polymerisation, solution methods, or melt processing. Modification of either clay or polymer can change the type of polymer–clay composite. X-ray diffraction and transmission electron microscopy are often employed as the main characterisation techniques to establish the state of the clay. A very low volume fraction of clay significantly improves the mechanical and barrier properties of the pristine polymer, which makes these nanocomposites very promising materials.

Impact strength of polymer-clay nanocomposites

The first reports of polymer-clay nanocomposites sprang out of industrial research from the motor industry in 1987. Since then, academic research has flourished and far outstrips industrial implementation of these new materials; approximately 700 papers were published in 2007. Much of this work, although it emphasises mechanical properties, neglects the characterisation parameter that industry regards as critical; impact strength. The expression ‘toughness’ has become ambiguous and inferences from the area under the tensile stress-strain curve (conducted at low strain rates) can directly contradict impact testing results. The mechanical, barrier and thermal stability properties of polymers are usually enhanced by incorporation of nano-clays, often to a remarkable extent and these improvements are well-documented. It seems to be important now to take a closer look at progress in retaining toughness when a polymer is converted to a nanocomposite and what matters is sensitivity to defects and energy absorption under high strain rate conditions. This is a lacuna that might be holding back the deployment of nanocomposites. This Review shows how difficult it is to identify the factors contributing to retention of toughness. Most of the polyamide studies report slight reductions in impact strength or very modest increases, explicable in terms of micro-cavitation as observed in the electron microscope. There is a growing view that clay reinforcement is only effective in retaining toughness above Tg. In other polymers, it is difficult to predict how nano-reinforcements will affect impact strength because of the confounding effects of crystallinty, spherulite size, preferred orientation or processing variables.

Novel thermoplastic starch-clay nanocomposite foams.

Novel thermoplastic starch (TPS)-clay nanocomposite foams were prepared by melt-processing. The use of urea as plasticizer avoids the cracking of TPS during storage and enhances the dispersion of ammonium-treated clay in TPS. X-ray diffraction shows an increase in the basal plane spacings of both natural and treated clays, suggesting formation of nanocomposites. Scanning electron microscopy shows spontaneously formed regular foam structures with 84% porosity in TPS-ammonium-treated clay. This does not form in TPS or TPS-natural clay nanocomposites. This result implies that the regular foam formation is due to the ammonium surfactant of clay, which produces ammonia gas acting as an internal blowing agent. Thermogravimetric analysis confirms this deduction.

Poly(glycerol sebacate urethane)–Cellulose Nanocomposites with Water-Active Shape-Memory Effects

Biodegradable and biocompatible materials with shape-memory effects (SMEs) are attractive for use as minimally invasive medical devices. Nanocomposites with SMEs were prepared from biodegradable poly(glycerol sebacate urethane) (PGSU) and renewable cellulose nanocrystals (CNCs). The effects of CNC content on the structure, water absorption, and mechanical properties of the PGSU were studied. The water-responsive mechanically adaptive properties and shape-memory performance of PGSU-CNC nanocomposites were observed, which are dependent on the content of CNCs. The PGSU-CNC nanocomposite containing 23.2 vol % CNCs exhibited the best SMEs among the nanocomposites investigated, with the stable shape fixing and shape recovery ratios being 98 and 99%, respectively, attributable to the formation of a hydrophilic, yet strong, CNC network in the elastomeric matrix. In vitro degradation profiles of the nanocomposites were assessed with and without the presence of an enzyme.

Intercalation and in situ polymerization of poly(alkylene oxide) derivatives within M+-montmorillonite (M = Li, Na, K)

We have synthesized a range of montmorillonite-based clay–polymer nanocomposites by intercalation of a variety of functionalized molecules having poly(ethylene oxide) and poly(propylene oxide) backbones from aqueous solution using a facile batch process. We focus on montmorillonite clays charge-balanced by cation exchange with Li+ and K+, but otherwise unmodified. Analysis by X-ray diffraction and thermal methods showed that intercalation occurred in all cases and that the composites displayed a range of interlayer spacings and organic content, from monolayer arrangements to pseudo-trilayer arrangements. Intercalated K+-montmorillonites had a propensity to exfoliate, in marked contrast to their resistance to swelling by water. Large-scale molecular dynamics simulations of selected composites were used to elucidate possible interlayer arrangements of the composites. Materials property studies showed that these clay–polymer composites had significantly increased Youngs moduli compared to the unfilled polymer.

Biomimetic poly(glycerol sebacate)/poly(l-lactic acid) blend scaffolds for adipose tissue engineering

Large three-dimensional poly(glycerol sebacate) (PGS)/poly(l-lactic acid) (PLLA) scaffolds with similar bulk mechanical properties to native low and high stress adapted adipose tissue were fabricated via a freeze-drying and a subsequent curing process. PGS/PLLA scaffolds containing 73vol.% PGS were prepared using two different organic solvents, resulting in highly interconnected open-pore structures with porosities and pore sizes in the range of 91-92% and 109-141μm, respectively. Scanning electron microscopic analysis indicated that the scaffolds featured different microstructure characteristics, depending on the organic solvent in use. The PGS/PLLA scaffolds had a tensile Youngs modulus of 0.030MPa, tensile strength of 0.007MPa, elongation at the maximum stress of 25% and full shape recovery capability upon release of the compressive load. In vitro degradation tests presented mass losses of 11-16% and 54-55% without and with the presence of lipase enzyme in 31days, respectively. In vitro cell tests exhibited clear evidence that the PGS/PLLA scaffolds prepared with 1,4-dioxane as the solvent are suitable for culture of adipose derived stem cells. Compared to pristine PLLA scaffolds prepared with the same procedure, these scaffolds provided favourable porous microstructures, good hydrophilic characteristics, and appropriate mechanical properties for soft tissue applications, as well as enhanced scaffold cell penetration and tissue in-growth characteristics. This work demonstrates that the PGS/PLLA scaffolds have potential for applications in adipose tissue engineering.

Mechanical behavior of transparent nanofibrillar cellulose-chitosan nanocomposite films in dry and wet conditions

Transparent, biocompatible and biodegradable chitosan (CS) nanocomposite films reinforced with nanofibrillar cellulose (NFC) were prepared by solution casting. The effects of NFC content on the mechanical properties in dry and wet conditions were investigated. The incorporation of NFC significantly enhanced the mechanical properties, especially in wet conditions. The ultimate tensile strength and Young׳s modulus of chitosan were improved by 12 times and 30 times, respectively, for the nanocomposite containing 32wt% of NFC in wet conditions. The mechanism of the remarkable reinforcements was studied by analyzing the swelling behavior of NFC-CS nanocomposites. The mechanical properties of wet NFC-CS nanocomposite films matched well with those of human skin, which demonstrate potential for uses as artificial skin and wound dressings.

Relative modulus–relative density relationships in low density polymer–clay nanocomposite foams

Polymer–clay nanocomposite (PCN) foams represent an important class of new materials in structural engineering, biomedical fields and packaging. This paper reports the relative modulus–relative density relationship, a crucial correlation in cellular solids, for low-density PCN foams. Polyurethane (PU)–natural clay nanocomposite foams with a porosity of 97% were used for studies of such relationship. The foam structures were characterised by Scanning Electron Microscopy and X-ray Micro-Computed Tomography and the modulus was obtained from compressive testing. It was found the relative modulus–relative density relationship of low-density PCN foams with porosities higher than 95% closely followed the normalised Gibson–Ashby models for open cells and closed cells, and in the case of PU–clay nanocomposite foams the geometric constant of foam C1 was determined to be approximately 0.45–0.88 in the well-established model for conventional open-cell foams, namely Ef/Es = C1(ρf/ρs)2 where E and ρ refer to modulus and density and subscripts f and s stand for foam and solid. The effects of clay, clay content and mixing sequence on the cell structure, physical and mechanical properties of the polymer foam were also discussed.