Encapsulation and Thermally Triggered Release from Polymer Vesicles

University of Sheffield and Polymer Center academic Professor Steven Armes et al. have demonstrated the encapsulation of silica nanoparticles and bovine serum albumin within synthetic polymeric vesicles, and the consequent triggered release of the payload.
The encapsulation and subsequent release of active ingredients on the micro or nanoscale has drawn considerable academic interest over the recent years. Such processes find application in a wide range of industrial formulations, from medicine to laundry science and agrochemicals.

Similarly, the ability of certain polymers to self-assemble in solution to form hollow spheres, known as vesicles or polymersomes, provides us with a convenient vessel within which to encapsulate desired active ingredients. These vesicles contain an inner lumen encased by a spherical polymeric bilayer, somewhat similar to the way eukaryotic cells are encased by a lipid bilayer.

In the recent work reported by Professor Armes, the particular block copolymer used to produce these vesicles was poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate), which abbreviated to PGMA-PHPMA. The use of a technique known as polymerisation-induced self-assembly (PISA) enabled the convenient synthesis of these PGMA-PHPMA vesicles at relatively high concentrations and short reaction times.
Poly(2-hydroxypropyl methacrylate) has an unusual property: in water it becomes more soluble at lower temperatures. Furthermore, by cooling a solution of PGMA-PHPMA vesicles down to around 0 °C, the PHPMA block becomes increasingly solvated which causes the vesicles to break apart. The ability to disintegrate the encapsulating vesicles by cooling provides a convenient thermal trigger to release any payload that may be loaded within them. By cooling the silica loaded vesicles in ice for 30 minutes, it was demonstrated that the vesicles disintegrated and the silica nanoparticles were released back into solution.
The encapsulation and release process was characterised by a number of analytical techniques, including transmission electron microscopy (TEM), disc-centrifuge photosedimentometry (DCP) and small-angle x-ray scattering (SAXS). In addition, this encapsulation process was not limited to inorganic silica nanoparticles; biological material could also be encapsulated. Bovine serum albumin (BSA), a model globular protein, was successfully encapsulated in the PGMA-PHPMA vesicles and then released upon cooling.

For the original publication please see C. J Mable, S. P. Armes et al. J. Am. Chem. Soc. 2015, 137, 16098 or visit http://pubs.acs.org/doi/abs/10.1021/jacs.5b10415

Article by Matt Rymaruk; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.

Polymer Centre Highlight: A Greener Route to Green Energy

Polymer centre academics; Dr Alan Dunbar, Dr Ahmed Iraqi, Dr Alastair Buckley and Prof David Lidzey have reported the manufacture of organic photovoltaic devices from non-halogenated binary solvent blends. This work was carried out in collaboration with Dr Andrew Pearson from the Optoelectronics Group at Cambridge University.

Organic photovoltaic devices (OPVs) have seen recent improvements in the power conversion, now being able to achieve 9 % efficiency. The performance achievable is approaching the benchmark which would allow OPVs to become commercially viable. OPVs use a conjugated polymer and fullerene derivative to act as a semi-conducting layer. However, currently OPVs require the use of halogenated solvents to dissolve and then deposit this essential semi-conducting layer. In order to produce environmentally acceptable OPVs, a suitable non-halogenated solvent must be found for the semi-conducting layer though the majority do not readily dissolve in such solvents.

Hansen solubility parameters were used to predict solvent systems that would dissolve the organic semi-conductor. This was achieved by using a system with similar solubilising properties to that of the halogenated solvent. The solvent system used was carbon disulphide (CS2) and acetone. Both are used commercially and have a lower toxicity than the halogenated solvents previously used. The solvent blend had a solubility limit of 20 mg ml-1 compared to 10 mg ml-1 of the halogenated solvent. This increase is attributed to blending allowing for a closer match to the solubility parameters of the organic semi-conductor.

OPVs produced using the solvent blend achieved power conversion values higher than those obtained from using a halogenated solvent, for both conjugated polymers used.

Original publication: Organic photovoltaic devices with enhanced efficiency processed from non-halogenated binary solvent blends, Griffin, J.; Pearson, A. J.; Scarratt, N. W.; Wang, T.; Dunbar, A. D. F.; Yi, H.; Iraqi, A.; Buckley, A. R.; Lidzey, D. G., Org. Electron. 2015, 21, 216-222.

Article by Luke Fox; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.

Polymer Centre Highlight – Inkjet printing of self-healing polymers in aerospace applications

Polymer Centre academics Dr Simon Hayes and Dr Patrick Smith have been investigating an application for thermally-cured polymers to heal defects in composite materials in aircrafts, by applying this technology through inkjet printing.

Composite materials, defined as materials made from two or more distinctive components which have different physical and chemical properties, are used in a variety of industries including the aerospace sector, due to their superior properties including high strength, durability and stiffness. However, an emerging problem with composites is the delamination, or separation, of internal layers. This leads to concealed damage within the material- a difficult problem to detect and treat. The use of self-healing polymers has been investigated for in situ repair of composite materials in airplanes.

Composite specimens for testing were produced from two different monomers, which were synthesised and inkjet printed in layers as 1% and 5% w/v solutions onto a PTFE base. Heat was applied under vacuum, initiating the thermal polymerisation of monomers via a series of Diels-Alder reactions to form polymers. It is this thermal polymerisation that is being used to promote self-healing between damaged layers.

The critical energy release and the interlaminar shear strength were assessed for each specimen. It was found that 5% samples showed superior interlaminar properties over control and 1% samples.

A thermal cycle analysis was conducted on the composite specimens. The results indicated that after heat treating, the 5% samples showed less of a reduction in interlaminar properties than the control and 1% samples. This suggests that some rebonding took place in the laminate layers due to polymerisation. Overall printing of the monomers followed by the application of heat to start the polymerisation, therefore the self-healing process, has been shown to have a beneficial effect on the composite material.

Original article: Inkjet Printing of Self-healing Polymers for Enhanced Composite Interlaminar Properties, Elliot Fleet, Yi Zhang, Simon Hayes and Patrick Smith, Journal of Materials Chemistry, 2015 3, pp 2283-2293

Article by Kathryn Murray; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.

Polymer Centre Research Highlight- Superhydrophobic Ski Bases

Prof Peter Styring at the University of Sheffield has developed a technology which could be used to improve the speed of alpine skis, by embossing the base of the skis to reduce water adhesion. Reducing the water adhesion (or increasing hydrophobicity) has been shown to reduce the friction experienced by the ski and hence increase ski speed across snow.

The technology takes inspiration from hydrophobic surfaces in nature, such as those found on Lotus leaves. These surfaces use microscale structures such as dimples or pillars to reduce surface wetting.

Ski bases are made from ultrahigh molecular weight polyethylene (UHMWPE). Introducing microscale structures to this material presents a processing issue, since casting is not an option as the polymer degrades before its melting point. This problem has been overcome by applying heat and pressure (whilst remaining below the degradation temperature) to a steel mesh placed upon the ski base, in an embossing process (below). This pressing introduces the desired patterning to the UHMWPE.

An increased contact angle (indicating reduced surface wetting) is seen following embossing (below), indicating increased hydrophobicity and therefore increased speed of ski.

The original publication was Superhydrophobic Ski Bases for Reduced Water Adhesion. Nurul A. Nordin and Peter Styring, Procedia Eng., 2014, 72, pp 605–610.

Article by Stephen Knox; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.