Solution Blow Spinning (SBS) is a process in which solid nanofibers are produced from a polymeric fluid stream delivered through a milimeter-scale inner nozzle ...
Key words: Nanofibers, solution blow spinning, poly-L-lactic acid
Krzysztof FALISZEWSKI1, Michał WOJASIŃSKI1, Tomasz CIACH1 1 Warsaw University of Technology, Faculty of Chemical and Process Engineering
INFLUENCE OF PROCESS PARAMETERS ON THE SIZE AND MORFOLOGY OF POLY-L-LACTIDE ACIDNANOFIBERS OBTAINED BY SOLUTION BLOW SPINNING Solution Blow Spinning (SBS) is a process in which solid nanofibers are produced from a polymeric fluid stream delivered through a milimeter-scale inner nozzle and elongate by pressured gas supplied to outer nozzle in coaxial nozzles system. Nanofibers have been prepared by SBS of 5 wt% poly-L-lactide acid (Mw > 200kDa) solution in 3/1 (v/v) chloroform/acetone mixture. An analyze of an influence of two process parameters – polymer solution feed rate and gas pressure on the size and morphology of obtained nanofibers was investigated.Changing the gas pressure has a significant impact on the size of the formed nanofibers while the feed rate is primarily responsible for the stability of the process. Effect on the morphology of nanofibers of both parameters is negligible. However, the appropriately adjusting of them allows reduction of the amount of contaminants in nonwoven.
1. INTRODUCTION Nanofibers made of biodegradable materials may have many applications in medicine and biotechnology. Examples of such applications are tissue scaffolds, bone fixations devices, implants covers, wound dressings. Poly-L-lactideacid (PLLA) is one of more frequently used biodegradable polymers. Most commonly used in biomedicine for the production of dental implants, resorbable sutures and bone scaffolding [1]. ‘A growing interest of nanomaterials requires the development of new, more efficient methods for their preparation.A solution blow spinning technique was developed using elements of both electrospinning and melt blowing technologies as an alternative method for making non-woven webs of micro- and nanofibers with diameters comparable with those made by the electrospinning process with the advantage of having a nanofibers production rate (measured by the polymer injection rate) several times higher’ [2]. The studies were performed to analyze the influence of process parameters, such as polymer solution feed rate and gas pressure, on the size and morphology of poly-Llactideacid (PLLA)nanofibers obtained by Solution Blow Spinning (SBS) method. Obtaining nanofibers of known and preconceived properties allows designing the product like tissue scaffold or implant coating with the required characteristics.
2. EXPERIMENTAL METHODS To obtain poly-L-lactideacid (PLLA) nanofibers Solution Blow Spinning (SBS) method was used. SBS is a method that allows to receivenanofibers, developed on the basis of modifications of Electrospinning (ES) and Melt Blowing (MB) methods.Nanofibers are produced by blow-molding of the polymer solution from coaxial nozzles system using compressed gas. Using as a driving force of the process pressurized gas supplied to the outer nozzle is taken from the MB method. Application of polymer solution instead of molten forms is inspired by the ES method. Main SBS advantage over ES and MB are obtaining nanofibers with a size similar to those obtained by ES, but withseveral times higher production efficiency of the process and possibility of using a low-temperature gas if the solvent volatility is high enough. 2.1. PREPARATION OF SOLUTION.
The polymer used in experiments waspoly-L-lactideacid (PLLA) with molecular weight MW > 200 kDa (Biomer L9000). The following two solvents were used: chloroform and acetone (both Carlo Erba Reagents, Italy). Solvents were of analytical purity. The solvent was a 3:1 (in volume) mixture of chloroform and acetone. The polymersolution concentration was 5 wt%. Polymer solution was stirred over night before spinning. 2.2. EXPERIMENTAL SETUP AND PROCEDURE.
The poly-L-actideacid (PLLA) nanofibers was spun from its solution by using experimental setup, which is depicted in Fig. 1a. The blown nanofibers were collected on a rotating drum set at a distance of 20 cm from the nozzle. Rotation speed of collector was set at 50 rpm. Construction of coaxial nozzles system is shown in Fig. 1b.
Fig. 1 a) experimental setup b) self-designed coaxial nozzles system
Gas pressure was generated by a compressor (HYDROVANE, UK) and was supplied to the outer nozzle. The gas pressure was varied between 500 and 1500kPa. The polymer solution was delivered by infusion pump (AP12 ASCOR, Poland) to control the polymer solution feed rate between 10 and 50 ml/h. 2.3. MICROSCOPY.
For observations of the blown nanofibers scanning electron microscopy (SEM) was used. In particular, SEM images were prepared using SEM FEI PHENOMTM, USA microscope. Prior to observation with SEM, all samples were sputter-coated with Ag layer witha thickness of 15 nmin gold sputter (K550X Emitech, UK). 3. RESULTS AND DISSCUSION 3.1. SOLUTION BLOWING OF PLLA NANOFIBERS.
‘In solution blowing, after the solution streams are pressed out of the nozzle, they are stretched to the extreme by the high-velocity gas flow to the collector in accompany by solvent evaporation and nanofibers formation. The process is similar to electrospinning but with different nanofibers formation driving forces. For solution blowing, high-velocity gas flow deforms the solution streams, evaporates the solvent, and solidifies them into nanofibers; while electric force per-forms in electrospinning’ [3]. The goal of research was to determine the effect of process parameters on the size and morphology of nanofibers obtained by SBS. As two process variables having the greatest impact on the process feed rate of the polymer solution in the inner nozzle space and air pressure supplied to the outer nozzle were selected. In order to conduct experiments for determining the effect of processing parameters on manufactured material the DEO tool from STATISTICA software was used. Box-Behnken plan was applied. Experimental plan based on the software is presented in table 1. Results shown in the table 1 are derived from the average of three series of measurements made under given conditions. Feed rate [ml/h]
10
30
50
10
30
50
10
30
50
Pressure [kPa]
500
500
500
1000
1000
1000
1500
1500
1500
Diameter [nm]
223.6
222.3
217.8
204.7
205.3
201.6
237.5
231.1
226.9
Standard deviation[nm]
0.042
0.047
0.042
0.035
0.036
0.043
0.049
0.042
0.044
Tab. 1 The mean diameter of nanofibers obtained under given conditions.
3.2. Feed rate and gas pressure effect.
To study the effect of process parameters three series of measurements at the nine different setup settings were performed. To measure the diameter of the nanofibersSEM images were analyzed by Photoshop CS5 commercial software. Example of diameter distribution in the single sample is shown in Fig. 2 as a histogram. For all samples, diameter of fibers distribution is narrow, as shown in the Fig 2, which is highly preferable.The mean results of the measurements with the standard deviation are presented in table 1. Percentage of nanofibers[%]
30 25 20 15 10 5 0 149 170 192 213 235 256 278 299 321 342 nanofibers diameter [nm] Fig. 2 Histogram showing the distribution of diameter in single sample.
Based on data from Table 1, by using the STATISTICA software,best fitted surface (Fig. 3) and Pareto (Fig. 4) diagrams were generated. Both diagrams are based on linear (L) and quadratic (Q) models of fitting the data. On this basis it can be concluded that the effect of polymer feed rate on the size of obtained nanofibers is insignificant – with confidence interval of 95% for (L) and (Q) models.However, a significant increase in feed rate leads to a small reduction in nanofibers diameter.
Fig. 3 3D best fitted surface diagram.
Fig. 4 Pareto chart.
On the other hand, pressure of the gas supplied to the outer nozzle hascrucial influenceonthe process of nanofibers air spinning. It can be apparent for both (L and Q) models that changing the gas pressure about 500 kPais causing a change of the nanofibers diameter between 9 to 16 %. There is a minimum diameter of nanofibers, which can be obtained by SBS method, only by changing the gas pressure, which can be seen in surface diagram (Fig. 3). In the case of PLLA nanofibers this minimum is around 1000 kPa.Minimum occurs, probably due to changes in the character of the gas flow in the outer nozzle. 3.3. Nanofibers morphology.
Nanofibers morphology was evaluated on the basis of images taken by scanning electron microscope. As it is presented in the Fig. 5a, nanofibers obtained by the SBS in the conditions of the experiments are smooth and non-porous. Non-woven have irregular structure, nanofibers are arranged in different directions and are entangled with each other. Changes in feed rate and the gas pressure do not affect the morphology of the nanofibers (Fig. 5b/c).Increasing of these parameters may lead to increased contaminants of nonwoven, which means the higher presence of non-fibrous structures.
Fig. 5 SEM images of nanofiberstakenatpresetfeedrate (q) and gaspressure (p) a)q = 10 ml/h, p = 1500 kPa b)q = 10 ml/h, p = 500 kPa c)q = 30 ml/h, p = 1000 kPa.
4. CONCLUSIONS To produce nanofibers of defined size and morphology Solution Blow Spinning (SBS) method can be used. Process parameter with the greatest influence on the diameter of obtained nanofibers is gas pressure supplied to the outer nozzle. The impact of the gas pressure is significant, but is not a linear. In the initial gas pressure reduction obtained nanofibers diameter decreases until the minimum size is reached. Further decreasing the gas pressure results with slow increase ofnanofibers diameter. This may be caused by changes in the character of the gas flow in the outer nozzle from turbulent, through mixed flow as to laminar flow. This phenomenon will be the next step in the study of stability and efficiency of the SBS method. Influence of the
second process parameter – polymer solution feed rate, on the size of obtained nanofibers is negligible.Changes in the values of both examined parameters do not affect on the morphology of the obtained nanofibers. However, their appropriately adjusting is important due to the amount of contaminants that occur in the nonwoven. 5. REFERENCES [1] Gupta B., Revagade N., Hilborn J.,Poly(lactic acid) fiber: Anoverview, Progress in Polymer Scince, 2007, Prog. Polym. Sci. 32 (2007) 455–482 [2] Medeiros E. S., Glenn G. M., Klamczynski A. P., Orts W. J., Mattoso L. H. C., Solution Blow Spinning: A New Method to Produce Micro- and Nanofibers from Polymer Solutions, Wiley InterScience, 2009, DOI 10.1002/app.30275. [3] Zhuang Z., Yang X., Lei S., Cheng B., Guan K., Kang W., Solution blowing of submicron-scale cellulose fibers, Carbohydrate Polymers, 2012, G Model CARP6731.
6. SUMMARY Rozdmuch roztworu polimeru (ang. Solution Blow Spinning - SBS)jest procesem wytwarzania nanowłókien polimerowych. Polega on na wyciąganiu strumienia roztworu polimeru, podawanego do wewnętrznej części współosiowej dyszy, poprzez sprężony gaz wprowadzony do zewnętrznej przestrzeni dyszy w układzie współosiowo umieszczonych dysz. W celu zbadania wpływu dwóch parametrów procesowych, którymi są szybkość podawania roztworu polimeru i ciśnienie gazu, na rozmiar i morfologię otrzymywanych nanowłókien. Nanowłókna wytworzono z poliL-laktydu, korzystając z jego 5 wt% roztworu w mieszaninie chloroformu i acetonu (3/1 v/v).Na podstawie przeprowadzonych badań i analiz statystycznych można stwierdzić, że kluczowym parametrem wpływającym na zmianę średnicy nanowłókien jest ciśnienie gazu. Wpływ szybkości podawania polimeru jest mało znaczący. Oba parametry powodują niewielkie zmiany morfologii nanowłókien, ich wzajemne dopasowanie ma jednak wpływ na ilość zanieczyszczeń we włókninie.
