Understanding polymer deformation
A time-resolved monitoring technique has been used by UK researchers to measure the mechanical deformation of polymers. The findings could eventually lead to more durable artificial heart valves.
‘Many soft materials, such as elastomeric polymers and biopolymer fibrils, undergo reversible phase or alignment transitions in response to pressure, shear and temperature’, explains Dr Geoff Moggridge from the University of Cambridge. 'Such microscopic structural changes often have significant effects on the mechanical properties of the product. Understanding the dynamic response of soft materials to deformations of pressure, temperature and shear is therefore important for the control of manufacturing processes.’
The team used X-ray diffraction techniques, commonly used in data acquisition to reduce noise, to obtain information on the alignment and rheological response of soft materials.
‘This is the first time that data “binning” of X-ray patterns have been performed for cyclically sheared or stretched systems, in order to gather data over a relevant timescale for soft solids,’ says Moggridge. ‘Orientated samples with cylindrical morphology were stretched up to 200% both parallel and perpendicular to the cylinder axis.’
Stretching was paused for six seconds during a 60 second cyclical period to record deformation patterns during elongation and relaxation for better visualisation. By adding up the data captured within each of the six-second segments, the team has discovered what was happening at that point of the sinusoidal wave.
The results show that after a stretching/relaxation cycle, the length of sample increased, but, after external stress removal, the specimens show an increase in strength, depending on deformation direction.
‘Mechanical properties, such as toughness, modulus and strength depend on chemistry, microstructure, and local conformation of polymer chains,’ explains Moggridge. ‘What is particularly interesting is that orientated sample strength depends on deformation directions relative to the orientation axis, meaning that local stress, resulting from the contraction of rubbery matrix, can be relaxed through appropriate displacement of the cylinders.
‘When the materials arestretched, the mechanical properties change because the cylinders all align in one direction,’ Moggridge says. ‘What we have now started to do is try to control the mechanical properties of the material by controlling the alignment of the cylinders.’
By adjusting the alignment, not just in one direction, but in several, the team hopes to be able to reinforce soft materials where stress will concentrate.
Stuart Patrick, a polymers specialist, believes the work adds to a greater understanding of this area of polymer research.
‘The cylindrical phase is the most studied morphology of thermoplastic elastomers under strain, with far fewer investigations of styrenic copolymers of different morphologies’, he says. ‘It will be interesting to see how this technique can
be utilised to improve or modify mechanical properties in other end use applications.’
Moggridge is now working with the University of Reading to develop artificial heart valves that could potentially make use of the research.Materials World Magazine, 01 May 2010
‘Many soft materials, such as elastomeric polymers and biopolymer fibrils, undergo reversible phase or alignment transitions in response to pressure, shear and temperature’, explains Dr Geoff Moggridge from the University of Cambridge. 'Such microscopic structural changes often have significant effects on the mechanical properties of the product. Understanding the dynamic response of soft materials to deformations of pressure, temperature and shear is therefore important for the control of manufacturing processes.’
The team used X-ray diffraction techniques, commonly used in data acquisition to reduce noise, to obtain information on the alignment and rheological response of soft materials.
‘This is the first time that data “binning” of X-ray patterns have been performed for cyclically sheared or stretched systems, in order to gather data over a relevant timescale for soft solids,’ says Moggridge. ‘Orientated samples with cylindrical morphology were stretched up to 200% both parallel and perpendicular to the cylinder axis.’
Stretching was paused for six seconds during a 60 second cyclical period to record deformation patterns during elongation and relaxation for better visualisation. By adding up the data captured within each of the six-second segments, the team has discovered what was happening at that point of the sinusoidal wave.
The results show that after a stretching/relaxation cycle, the length of sample increased, but, after external stress removal, the specimens show an increase in strength, depending on deformation direction.
‘Mechanical properties, such as toughness, modulus and strength depend on chemistry, microstructure, and local conformation of polymer chains,’ explains Moggridge. ‘What is particularly interesting is that orientated sample strength depends on deformation directions relative to the orientation axis, meaning that local stress, resulting from the contraction of rubbery matrix, can be relaxed through appropriate displacement of the cylinders.
‘When the materials arestretched, the mechanical properties change because the cylinders all align in one direction,’ Moggridge says. ‘What we have now started to do is try to control the mechanical properties of the material by controlling the alignment of the cylinders.’
By adjusting the alignment, not just in one direction, but in several, the team hopes to be able to reinforce soft materials where stress will concentrate.
Stuart Patrick, a polymers specialist, believes the work adds to a greater understanding of this area of polymer research.
‘The cylindrical phase is the most studied morphology of thermoplastic elastomers under strain, with far fewer investigations of styrenic copolymers of different morphologies’, he says. ‘It will be interesting to see how this technique can
be utilised to improve or modify mechanical properties in other end use applications.’
Moggridge is now working with the University of Reading to develop artificial heart valves that could potentially make use of the research.Materials World Magazine, 01 May 2010
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