Green process for green materials: viable low-temperature lipase-catalysed synthesis of renewable telechelics in supercritical CO2

We present a novel near ambient temperature approach to telechelic renewable polyesters by exploiting the unique properties of supercritical CO2 (scCO2). Bio-based commercially available monomers have been polymerised and functional telechelic materials with targeted molecular weight were prepared by end-capping the chains with molecules containing reactive moieties in a one-pot reaction. The use of scCO2 as a reaction medium facilitates the effective use of Candida Antarctica Lipase B (CaLB) as a catalyst at a temperature as low as 35 °C, hence avoiding side reactions, maintaining the end-capper functionality and preserving the enzyme activity. The functionalised polymer products have been characterised by 1H-NMR, MALDI-TOF, GPC and DSC in order to carefully assess their structural and thermal properties. We demonstrate that telechelic materials can be produced enzymatically at mild temperatures, in a solvent-free system and using renewably sourced monomers without pre-modification, by exploiting the unique properties of scCO2. The macromolecules we prepare are ideal green precursors that can be further reacted to prepare useful bio-derived films and coatings

Mild synthesis of poly(HEMA)-networks as well-defined nanoparticles in supercritical carbon dioxide

Free-radical dispersion polymerisation of 2-hydroxyethyl methacrylate was carried out in supercritical carbon dioxide (scCO2) in the presence of stabilisers based on polyethylene oxide (PEO) and poly(heptadecafluorodecyl acrylate) (PFDA). Different architectures of copolymers (random, palm-tree and diblock) were tested for their surface tension, cloud point and as a stabilising agent. The diblock architecture was found to be the best candidate resulting in poly(HEMA) spherical particles with a size of 316 nm. Furthermore, the effect of the CO2-phobic block (PEO) in the diblock architecture was investigated by using three different chain lengths (1000, 2000, 5000 g mol−1). By optimizing the stabiliser composition and structure, mild reaction conditions have been identified allowing us to obtain well-defined spherical cross-linked poly(HEMA) particles with a mean diameter of unprecedented low size (216 nm) at a temperature as low as 35 °C

Sustainable terpene triblock copolymers with tuneable properties for pressure sensitive adhesive applications

A series of triblock copolymers in a hard-soft-hard block configuration with varying hard block α-pinene methacrylate content and molecular weight and butyl acrylate soft segment have been synthesised and investigated for viability in pressure sensitive adhesive (PSA) applications. The morphologies vary from pockets of hard phase distributed within a continuous soft matrix, through to lamellar with co-continuous phases, and finally continuous hard phase with pockets of soft phase dispersed. Uniaxial tensile properties, probe adhesion performance and cyclic adhesive behaviour are presented for seven compositions including four short chain and three long chain copolymers, alongside a commercial benchmark PSA. Structure-property relationships for the novel elastomers are evaluated, establishing that short chain materials with 20-25 wt% αPMA offer similar tensile and adhesion performance to the commercial elastomer. Raising the hard phase concentration has been observed to provide a considerable increase in ultimate tensile strength, stiffness and peak tack force, but at the expense of significant reductions in ultimate tensile strain, adhesive bond displacement and vibrational dissipation. The results suggest that the performance of these sustainable materials can be tuned to produce viable PSAs with a range of useful properties

Synthesis and control of crosslinked poly(acrylic acid) based viscosity modifiers using dense phase carbon dioxide as a solvent

We investigate the clean synthesis of a cross-linked poly(acrylic acid) viscosity modifier using supercritical CO2 to replace more hazardous volatile organic solvents that are typically used for this process. The polymers were analysed by aqueous swell ratio studies to demonstrate the effect of process conditions such as pressure and temperature on the cross-link density of the materials. The reactions were optimised to yield high swelling polymers of up to 134 g water/g polymer. Dynamic mechanical analysis (DMA) was also applied to investigate the mechanical properties of the synthesised cross-linked poly(acrylic acid) materials and these data were directly correlated with swell ratio. In addition, rheological studies demonstrate that the hydrated gels are comparable with commercially available equivalents

How does dense phase CO2 influence the phase behaviour of block copolymers synthesised by dispersion polymerisation?

Block copolymers synthesised in supercritical CO2 dispersion undergo in situ self-assembly which can result in a range of nanostructured microparticles. However, our previous study revealed that copolymers with different block combinations possessed different microphase separated morphologies at identical block volume fractions. In this paper, we follow up those initial observations. By examining the phase behaviour of a selection of structurally diverse block copolymers, we explore the structural factors which influence the conflicting self-assembly behaviours. The composition dependence of the morphology is found to be strongly related to the CO2-philicity of the second block relative to poly(methyl methacrylate) (PMMA). Whilst PMMA-b-poly(benzyl methacrylate) (PBzMA) and PMMA-b-poly(N,N-dimethylaminoethylmethacrylate) (PDMAEMA) phase behaviour follows traditional diblock copolymer phase diagrams, PMMA-b-poly(styrene) (PS) and PMMA-b-poly(4-vinyl pyridine) (P4VP), which comprise blocks with the greatest contrast in CO2-philicity, self-assemble into unexpected morphologies at several different block volume fractions. The morphology of these copolymers in the microparticulate form was found to revert to the predicted equilibrium morphology when the microparticles were re-cast as films and thermally annealed. These findings provide strong evidence that CO2 acts as a block-selective solvent during synthesis. The CO2-selectivity was exploited to fabricate various kinetically trapped non-lamellar morphologies in symmetrical PMMA-b-PS copolymers by tuning the ratio of polymer:CO2. Our data demonstrate that CO2 can be exploited as a facile process modification to control the self-assembly of block copolymers within particles

Surface enhanced Raman scattering using metal modified microstructured optical fibre substrates

We report the fabrication of metallic metamaterials using microstructured optical fibres as templates. The resulting fibres serve as excellent substrates for surface enhanced Raman spectroscopy and represent an exciting platform for in-fibre plasmonic devices

High-pressure rheological analysis of CO2-induced melting point depression and viscosity reduction of poly(ε-caprolactone)

High-pressure rheology has been used to assess the effects of supercritical carbon dioxide (scCO2) on the melting point (Tm) and viscosity of poly (ε-caprolactone) (PCL) over a range of temperatures and pressures up to 300 bar over a wide range of shear rates. Plots of the storage and loss moduli against temperature show a significant shift of Tm to lower temperatures in the presence of CO2, indicating that the polymer crystals melt at temperatures much lower than the ambient pressure Tm. Furthermore, a significant decrease in the viscosity of two PCL grades with different molecular weight (Mn ~ 10 kDa and 80 kDa) was also detected upon increasing the CO2 pressure to 300 bar. Experimental viscosity data were fitted to the Carreau model to quantify the extent of the plasticising effects on the zero-shear viscosity and relaxation time under different conditions. Similar analyses were conducted under high-pressure nitrogen, to compare the effects obtained in the presence of a non-plasticising gas

Surface-enhanced Raman spectroscopy using silver impregnated polycarbonate substrates

Novel substrates based on noble metal nanoparticles are currently the subject of extensive research in fields such as biological sensing, medicine, spectroscopy and nano-photonics due to the large electromagnetic fields generated in the vicinity of the metal surface via a surface plasmon resonance. The dependence of the resonance wavelength on the size, shape, local dielectric environment and interparticle spacing, enables engineering of the metal nanoparticle substrates to target specific requirements [1]. Here we report the fabrication of silver impregnated polycarbonate composites as substrates for surface enhanced Raman spectroscopy (SERS) where the plasmonic properties can be controlled via the reaction parameters. Although silver has superior plasmonic properties to other metals, it is often over looked for SERS substrates as it is highly prone to oxidation. The embedding of silver nanoparticles into polymer substrates offers substantial environmental protection, allowing for the construction of temporally stable plasmonic devices that can exploit the mechanical flexibility of the polymers and prevent particle agglomeration. These nanoparticle composites offer a number of advantages as SERS substrates as they are cheap, easily processed, and are bio-compatible