Biaxial reinforcements for polybutene-1 medical-tubes achieved via flow-design controlled morphological development of incorporated polystyrene: In-situ microfibrillation, alignment manipulation and performance optimization
Introduction
Polymeric materials have a wide range of desirable attributes that lend themselves to increased use in medical applications. Offering disposability, amenability to gamma and e-beam sterilization, which effectively reduce the occurrence of infection, polyolefin single-use devices, e.g. tubes, pipelines, catheters, are recognized as the most recommended medical products [1], [2].
It is vital to note that serious damage and infection could be caused to the patient due to the failure of medical piping systems, especially when they are implanted as catheters, artificial tracheas or drug-infusing tubes [3]. To lower the risk of device-failure caused medical accident, the mechanical resistance of medical tubes can't be overemphasized given the inherently mediocre mechanical properties of many polyolefin-based devices especially under the condition of high strains, loads and elevated temperatures [2], [3], [4]. Moreover, the commercialized manufacturing of medical tubes (i.e. melt extrusion) brings about deleterious features to the devices in terms of weld lines and axial orientation, which lead to even flawed end-use mechanical performances of the plumbing devices particularly in the hoop direction [4], [5]. When the tubes were under hydrostatic pressure, infusing patients with drugs in intensive care, cracks often generate at the welded joints and propagate along the axial direction leading to the complete failure of the tubes [3], [4], [6], [7].
Studies from academics and industry, by means of introducing fibers (glass, carbon, Kevlar etc.), or nano-sized fillers (carbon nano-tubes, graphene etc.) for the purpose of reinforcing polymeric devices, have been extensively carried out for decades [8], [9]. Nevertheless, due to the inherent deficiencies of these fillers, namely, poor dispersion, weak interfacial load transfer, random alignment etc., the “reinforcements direct-filling” techniques often fail to live up to expectations [10], [11]. In recent years, the emergence of a new group of polymer micro- or nano-composites, distinguished by perfect distribution of micro- or nano-fibrillar structure via the concept “converting” instead of “adding”, i.e. converting the bulk polymer into micro- or nano-composites instead of adding the fillers into the matrix, have been arousing widespread research interests [5], [11], [12], [13], [14]. Nevertheless, this technique, via which the microstructure (in this case, microfibers) were gained by the “blending” “drawing” “isotropization” three–step process, is of low-efficiency for continuous tubing extrusion. Moreover, since the in-situ formed microfibers tend to align along the extrusion direction (flow direction) [11], [12], [13], [14], [15], this microfibrillation technology still confronts the difficulty of balancing the mechanical properties of the tubes in both axial and hoop directions, especially in current case, the mechanical performance in hoop direction (perpendicular to flow) is of particular importance.
For the purpose of bi-axially reinforcing polybutene-1(PB-1), a semi-crystalline polyolefin which finds potential application in medical-purpose plumbing systems with excellent resistance to chemicals, solvents and creep even at elevated temperatures [16], [17], we reported a novel microfibrillar candidate: polystyrene (PS) in combination with a self-designed flow controlled extrusion device. Helical convergent flow was adopted to induce the in-situ formation of helically-aligned PS microfibers and continuously mass-produce PB-1 tubes with superior mechanical properties [18]. In this study, firstly, rheological manifestations of the chosen polymer systems were characterized and analyzed to indicate the morphological evolution mechanism and requirements, to guide the subsequent processing. Then we systematically revealed that by controlling the flow field parameters (convergent flow ratio, hoop velocity, flow pattern), the formation, alignment direction and helical configuration of the microfibers can be simultaneously manipulated to achieve excellent biaxial reinforcement for PB-1 tubes. As a result of the microfibers' excellent dispersion and alignment, good interfacial load transfer and optimized hierarchical configuration, the mechanical performances of the PS/PB-1 composite tubes in both axial and hoop direction were significantly improved as compared to conventional neat PB-1 tubes.
Section snippets
Materials
The raw materials used in this study were polybutene-1(PB-1) and polystyrene (PS). The PS (666H), the microfibrillar candidate, was a general purpose polystyrene supplied in pellets by Styron with a melt flow index (MFI) of 8.0 g/10 min (200 °C/5 kg). The PB-1 matrix was P5250, provided by Mitsui Chemicals Co. Ltd used for tubing extrusion, and its MFI was 0.4 g/10 min (190 °C/2.16 kg).
Sample preparation
The dried granules of PB-1 and PS in a weight ratio of 90/10, 80/20 and 70/30, were mixed and melt blended in a
Results and discussion
Rheological predetermination: It is well-established that the lower viscosity ratio between the dispersed phase and the matrix facilitates the microfibrillation [21]. Then the viscosities of two polymers are major factors in determining the feasibility of microfibrillation. According to the Cox-Merz rule, the apparent shear viscosity () at a given shear rate is equal to the complex viscosity () at the same frequency [22], as verified in SI-Fig. 3. As can be seen in Fig. 1a, the complex
Conclusion
In this study, an advanced polymer processing technique for the continuous large-scale manufacturing of biaxially-reinforced medical-purpose tubes was reported. This paper systematically demonstrated that, by applying flow fields with different patterns and parameters to the off-die melt, the formation, alignment angle and hierarchical configuration of the PS microfibers was manipulated. In this study, the optimal conditions and parameters of this technique were determined to achieve maximized
Acknowledgment
This work is financiered by the National Natural Science Foundation of China (51127003 and 51121001). Synchrotron 2D SAXS and WAXD were carried out in Shanghai Synchrotron Radiation Facility (SSRF), Shanghai, China.
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