Microfluidic elaboration of polymer microfibers from miscible phases: Effect of operating and material parameters on fiber diameter

https://doi.org/10.1016/j.jtice.2022.104215Get rights and content

Highlights

  • Microfluidic capillary-based process for the continuous-flow production of microfibers from two miscible phases.

  • Thoroughly study of the effects of phase materials, phase flow rates, inner capillary diameter and monomer volume fraction on microfiber diameter (25 to 85 µm).

  • Determination of an empirical correlation which perfectly predicts microfiber diameter.

Abstract

Background

fiber diameter is one of the most important morphological parameters which drives the applications of microfibers. This creates a need for the development of processes capable of producing a large variety of microfibers with a given diameter. To this regards, microfluidic spinning has recently emerged as an outstanding and simple technique for the production of micro- and nanofibers with controllable size and morphology.

Methods

herein, microfibers were produced from (macro)monomers or prepolymers (core phase) by in situ photoirradiation using a capillary-based microfluidic device and a miscible sheath phase of various viscosity. The effects of the flow rate of both phases as well as the viscosity of the sheath fluid, the capillary dimensions and the monomer volume fraction in the core phase were thoroughly studied.

Significant findings

by calculating the capillary number ratio from the ratios of sheath to core flow rate and viscosity, an empirical relationship which perfectly predicts the microfiber diameter as a function of monomer volume fraction, the capillary number ratio and capillary inner diameter but independent of its outer diameter is extracted. This result paves the way to the continuous-flow production of microfibers with well-controlled morphological characteristics.

Introduction

Microfibers have attracted a lot of attention due to (i) their large surface area to volume ratio, (ii) their diverse properties arising from the great variety of materials they are made of, and (iii) their ability to assemble into 3D complex structures and foldability [1]. These benefits enable polymer microfibers to have an excellent potential in many applications such as biomedicine [2], [3], [4], [5], [6], fiber optics [7], sensors [8,9], and water treatment [10,11]. Different approaches have been employed to produce micro- and nanofibers such as melt spinning [12], wet spinning [13], draw spinning [14], macromolecular assembly [15], and electrospinning [16], techniques relying on the physical mechanism of solidification to produce fibers, i.e. the starting raw material is a polymer solution. As such they suffer from limitations regarding (i) the nature of the material employed and (ii) the morphologies of fibers that can be achieved [17]. Hence, it is difficult to produce fibers with diverse morphologies and with a broad range of materials. Recently, microfluidic spinning has shown a great potential for the production of microfibers with diverse compositions, morphologies, and surface functionalities. This technique consists in stretching a stream of monomer or polymer solution (core phase) by an immiscible or miscible solution (continuous or sheath phase) inside a microfluidic device. Due to the small size of the device microchannel (ca. 100 µm), laminar flow is commonly achieved affording reproducibility and stability to the flow; two highly desirable features required for the production of fibers with given diameters and morphologies. By manipulating the flow inertia (i.e. individual solution flow rates), solution viscosity, interfacial tension between the core and sheath phases and taking possibly advantage of gravity forces, fibers with diverse morphologies such as grooved, flat, core-shell, hollow, and Janus can be easily produced. The choice of material becomes broader due to the panel of solidification methods, i.e. photopolymerization, ionic and chemical crosslinking, solvent exchange, and solvent evaporation. Fiber surface functionalization is also possible by encapsulation method or in situ chemistry [1].

Surface to volume ratio is one of the foremost fiber parameters that drive their applications. Since the latter is inversely proportional to the fiber diameter [18], many properties such as mechanical [19,20], cell adhesion and proliferation [20], biomimicking extracellular matrix [21], optical extinction capacity [22], and filtration performance [23] depend on the fiber diameter. Although, fiber diameter is a crucial parameter, the literature scarcely and partially reports on all the possible parameters responsible for controlling the fiber diameter in microfluidic spinning. Most reports address only the effect of sheath/continuous fluid and core/disperse fluids flow rates independently [24,25] or in the form of sheath to core flow rate ratio (Qs/Qc) [26], [27], [28], [29]. Liu et al. observed the effect of the capillary diameter on the fiber diameter using the thermoinitiated polymerization induced phase separation technique [30]. But there is no literature in which researchers interpreted these results into some mathematical form to predict the fiber diameter.

Herein, we developed an empirical relationship which can predict the fiber diameter in relation with others operating and materials parameters for the case where the monomer and its polymer are miscible with the continuous phase. The investigation of operating parameters was not only limited to flow rates and capillary diameter, but the effect of viscosity of sheath fluid and monomer volume fraction in the core phase were also investigated. Two monomers, tri(propylene glycol) diacrylate (TPGDA), poly(ethylene glycol) diacrylate (PEGDA), and one prepolymer, UV-curable adhesive NOA 89, were used as reference materials for the synthesis of fibers using in situ photoirradiation in a capillary-based microfluidic device. All results were used to extract an empirical correlation which predicted the final microfiber diameter upon variation of the capillary number ratio, monomer volume fraction, and internal capillary diameter.

Section snippets

Results and discussion

The fibers were produced using a capillary-based microfluidic device (Fig. 1) involving two phases: (1) the core fluid (Φc) becoming the fiber upon photopolymerization and (2) the sheath fluid (Φs) consisting in poly(ethylene glycol) (PEG), whose flow rates are Qc and Qs respectively. PEG was used due to its miscibility with core fluid, commercial availability, high tunable viscosity with its molecular weight which prevents fast diffusion of Φc into Φs, and reasonable shearing force to produce

Conclusion

Polymer microfibers of two different monomers and one photocurable adhesive with diameters as low as 23 µm were produced by in situ photopolymerization using a capillary-based coaxial microfluidic device and a miscible PEG sheath fluid. The impact of different operating and material parameters such as volume fraction of monomer in core phase, flow rate ratio of sheath to core fluid, viscosity of the sheath fluid and dimensions of the capillary on the resulting fiber diameter were studied. The

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Authors thank W. Drenckhan for discussions and A. Collard for all technical aspects. Authors are grateful to P. Allgayer for mechanical engineering support. WR would like to acknowledge the Higher Education Commission of Pakistan (HEC) for his Ph.D. fellowship.

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