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Functional Materials Letters Vol. 3, No. 4 (2010) 1–4 © World Scientific Publishing Company DOI: 10.1142/S1793604710001342
ENGINEERING ORIENTATION IN BLOCK COPOLYMERS FOR APPLICATION TO PROSTHETIC HEART VALVES JOANNA STASIAK∗ , GEOFF D. MOGGRIDGE Department of Chemical Engineering and Biotechnology University of Cambridge, Pembroke Street Cambridge, CB2 3RA, UK ∗
[email protected] ADRIANO ZAFFORA, ANNA PANDOLFI and MARIA L. COSTANTINO Department of Structural Engineering, Politecnico di Milano Milan 20133, Italy Received 1 July 2010; Revised 6 September 2010
This study demonstrates how the mechanical performance of polymeric material can be enhanced by morphology and phase orientation of block copolymers to achieve desired anisotropic mechanical properties. The material used was a new Kraton block copolymer consisting of styrene-isoprene-butadiene-styrene blocks having cylindrical morphology. We report a method of achieving long range uniaxial as well as biaxial orientation of block copolymer. Each microstructural organization results in a specific mechanical performance, which depends on the direction of the applied deformation. The method of tailoring mechanical properties by engineering microstructure may be successfully utilized to applications requiring anisotropic mechanical response, such as prosthetic heart valves. Keywords: Block copolymers; microstructure; uniaxial and biaxial orientation; mechanical properties.
In recent years polymeric materials have been widely studied as potential prosthetic heart valve materials, designed to overcome the clinical problems associated with both mechanical and bio-prosthetic valves.1 Polymeric heart valves can be moulded into a complete trileaflet without the need of suturing individual leaflets. The geometric similarity between the polymeric trileaflet heart valve and the natural valve provides a central, less disturbed, blood flow. Polymeric materials have also shown great potential in overcoming problems of material fatigue and maintaining functional characteristics. Ideally, they should provide long-term stable functioning in order to be suitable alternatives for tissue or mechanical leaflets. Using polydiene based block copolymer as a model material in numerical modeling studies, we have recently demonstrated that stress distribution for pressure loading is not uniform within a whole leaflet.2,3 Under the diastolic load, all three leaflets must assure appropriate coaptation. This causes a significant stress increase near the junction between
free-edge and commissural regions, while the leaflet centre is a region characterized by a moderate stress. Numerical simulations indicate that the performance of the polymeric heart valve leaflet may be enhanced by anisotropic mechanical properties, able to withstand the high stress in the border region. This may be achieved by choosing an appropriate material or design. The design aspect has recently been investigated by Burriesci et al.4 They suggest that energy and the produced stress arising from the deformation between the open and closed states can be reduced by designing for circumferential curvature in both the open and closed configurations. Here we focus on the microstructural aspect of enhancing the mechanical performance of a polymeric material by tailoring orientation of block copolymer microdomains to match specifications of trileaflet heart valve design. An additional advantage of this approach is that the microphase formation that block copolymers exhibit has been recognized for its potential ability to positively influence the biological response. The domains that form these materials can be of the size of tens of nanometers, based on styrene end-blocks
∗ Corresponding Author.
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with an elastic block of either isoprene, isobutylene, butadiene or ethylene-butylene. These domains are of the same order of magnitude as cell receptors and large proteins such as fibrinogen. Microphase separation has been confirmed to affect blood-material interactions, demonstrated by platelet adhesion.5 This study presents a method of making long range ordered block copolymer with tailored mechanical behaviour utilising thermo-mechanical processing of the polymer, which affords a bridge between the morphology and the mechanical properties. Kraton Polymers has recently developed a new family of styrenic block copolymers that feature a hybrid elastomer composition that combines a controlled distribution of isoprene and butadiene monomer units into the midblock styrene-isoprene-butadiene-styrene (SIBS). The new block copolymer has unique attributes such as a lower mid-block Tg than SIS and improved thermo-stability compared to SIS and SBS.6 We used a commercial block copolymer from Kraton D1171 PT, which is a clear, linear SIBS block copolymer with a polystyrene content of 19%. The molecular weight of the block was 180 kg/mol as determined by gel permeation chromatography (GPC). Styrene fraction has been verified by 1 H NMR using a Bruker 400 MHz NMR and CDCl3 solvent. The microstructure of Kraton D1171 PT exhibits cylindrical morphology of styrene rich cylinders in an isoprenebutadiene soft matrix. As determined by SAXS the d-spacing of the domains is 28 nm. SAXS patterns were recorded using a Bruker analytical X-ray system. CuKα lab source with λ = 1.54 Å was generated by a Siemens ceramic tube operated at 45 kV and 45 mA. The beam was collimated by a three pinhole collimator, and the specimen was placed 70 mm from the HiStar 2-D multiwire detector.
(a)
The oriented samples were prepared by compression moulding at 150◦C. Prior to channel-die compression, polymer pellets were heated to 150◦ C and squeezed to a dense cuboid block measuring 15 mm × 15 mm × 22 mm. This cuboid was then subjected to thermal compression in a Teflon coated channel die (Fig. 1(a)), or between two parallel plates (Fig. 1(b)) in a heated press at 150◦ C. The channel die moulded the polymer into rectangular strips 100 mm long, 15 mm wide and 0.7 mm thick. The geometry resulting from bidirectional compression was a circle of about 70 mm diameter and the same thickness, 0.7 mm. The compression ratio, defined as the height of the dense cuboid block divided by the thickness of the processed film, was approximately 20. Isotropic films were prepared by slowly evaporating a toluene solution of the polymer (ca. 75 wt % toluene). SAXS patterns for uniaxial orientation of Kraton D1171 PT are shown in Fig. 2. The cylinder-forming morphology forms a very well-ordered, single domain structure following extensional flow in the channel die, with the hexagonal unit cell well oriented with respect to the faces of the bulk sample (Fig. 2(a)). SAXS patterns for the X-ray beam normal to the flow direction, presented in Fig. 2(b), show two equatorial dots resulting from the orientation of styrene domains along the flow field. The microstructure organisation was mapped at various distances from the sample centre and well-ordered microstructure was confirmed within the whole sample. The compression between two parallel plates constrained the polymer to bidirectional flow (Fig. 1(b)). The circle-shaped sample was X-rayed along 3 different directions: vertical, horizontal and transverse (Fig. 3) with the radiation beam normal to the direction of flow. The X-ray patterns shown in Fig. 3 indicate that orientation is radially directed from the sample center, which is the only place where isotropic microstructure was retained.
(b)
Fig. 1. Schematic of sample treatment in (a) channel die and (b) between two parallel plates. Arrows indicate direction of applied force and also flow direction of the polymer during compression.
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Engineering Orientation in Block Copolymers for Application to Prosthetic Heart Valves 3
Fig. 4. Orientation function for uniaxially and radially oriented (vertical mapping) Kraton D1171 PT, as a function of distance from sample centre.
Fig. 2. SAXS patterns of unidirectionally oriented Kraton D1171 PT taken with X-ray beam (a) parallel to flow direction (b) normal to flow direction; and different distances from the sample centre.
Fig. 3. SAXS patterns of radially oriented Kraton D1171 PT taken at different distances and directions from the sample center.
Quantitatively the degree of orientation can be expressed by the second order orientation function P2 . The orientation function has been calculated as described in our previous study7 from azimuthal integration of SAXS patterns for vertical mapping.
Maximum value P2 = 1 is achieved for a perfectly uniaxially oriented structure, while a completely isotropic sample is characterized by P2 = 0. For both uniaxial and radial orientation the development of orientation occurred near sample center, where the polymeric material was loaded. Figure 4 shows that the order parameter for a uniaxially oriented sample varied from P2 = 0.68 at the loading central point to P2 = 0.83, 5 mm from the centre and remained stable at further distances. Radially oriented samples showed high orientation values only while mapped 15 mm and 25 mm from the center. The development of the order in all directions starts from the sample center, indicating that the isotropic region is located there. The maximum orientation value for the radially ordered specimen was P2 = 0.85. Mechanical properties of block copolymers have been investigated by means of uniaxial traction experiments. In tension measurements the force was applied by using a Texture Analyser TA-TX2 from Stable Micro System, at 1 mm/s stretching and relaxation rate, up to 100% of initial elongation. For deformation measurements dumbbell-shaped tensile specimens were cut from uniaxially and radially oriented block copolymers as shown in Fig. 5. The initial length and width of samples after clamping was 30 mm and 6 mm respectively with the exception of samples N, which were only 8 mm long. Tensile true stress (Cauchy stress) — true strain (logarithmic strain) curves for four ordered samples as well as for isotropic film are shown in Fig. 6. On loading the block copolymer parallel to the orientation of the cylinders (sample P), the stress was immediately transferred to the glassy cylinders while the perpendicular deformation (sample N) enabled the soft phase to receive the applied stress earlier. As a result, entirely different stiffness moduli were obtained on loading the
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(a)
(b)
Fig. 5. Schematic of polystyrene organization for (a) uniaxial and (b) radial orientation. Samples chosen for stretching had parallel (P), normal (N), quasiparallel (QP) and quasi-normal (QN) orientation with respect to stretching direction.
properties that are anisotropic in nature and stress-strain relationship differ substantially from the stress- strain curves of isotropic polymeric materials. Design of a valve using a polypropylene fibre reinforced composite material8 showed that specific alignment of the fibres gives significant reduction of maximum principal stress, and a more homogenous stress distribution was obtained. However due to laborious handcraft work this method of leaflet fabrication cannot become a commercial process. In this paper, we propose the application of block copolymers having cylindrical microstructure as a new material for prosthetic heart valves. The advantage of using these materials is that the hard cylinders may be aligned to reinforce the heart valve leaflet. Such a polymeric leaflet may be manufactured by compression moulding of the melt. It is essential to possess a knowledge of microstructural patterns and mechanical strength of these materials compressed in various ways. Outputs of the research provide further insight into the previously reported properties of these materials9 and may support a design of a mould for manufacturing polymeric heart valve leaflet, where cylinders oriented in specific directions mimic the collagen in natural tissues.
Acknowledgment The authors thank the EPSRC for financial support for this work under Grant EP/F007663/1. The support of the Italian MIUR through the grant 2007YZ3B24 is also acknowledged gratefully.
References
Fig. 6. True stress-strain curves for isotropic and oriented Kraton D1171 PT. Oriented samples presented parallel (P), quasi-parallel (QP), normal (N) and quasi-normal (QN) cylinders orientation with respect to stretching direction respectively.
block copolymers parallel (P) and normal (N) to the orientation direction. Samples taken from radially oriented polymer present orientation quasi-parallel (for samples QP) and quasi-normal (for samples QN) to the stretching direction. The observed effect during stretching of QP samples was softening compared to P, while stretching of QN samples revealed hardening compared to N. Finally as expected, a specimen of isotropic microstructure had stress-strain behavior between those oriented parallel and quasi-parallel to stretching direction and those oriented normal and quasi-normal. In the natural valve leaflets collagen fibres optimally align themselves for fatigue strength. This results in material
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