Abstract
An intelligent textile with optimal environmental responses was developed from electrospun nanowebs with controllable pore structures via the spinning conditions. The use of shape memory polyurethane allowed the material’s shape to be retained and recovered through heating. The thickness of the electrospun nanoweb was varied so as to manufacture the membrane with various pore diameters. The samples were evaluated for shape memory performance, air and water vapor permeability.
All samples showed shape recoveries of at least 99% and shape retentions of at least 94%. Increasing the thickness, increased the shape retention but slightly reduced the shape recovery. Pore size decreased with increasing thickness, thus decreasing air and water vapor permeability.
Stretched nanowebs showed the greatest differences in water vapor permeability at 10°C with 90% RH and at 15°C with 90% RH compared with original nanowebs.
Keywords
Introduction
Shape memory materials can be temporarily deformed and recover their original form through exposure to conditions such as heat, light, electromagnetic field, or pH.1–5 Shape memory polymers show more limited transformation ranges than shape memory alloys but have advantages of low density, mass, and price, while being easily manufactured and dyed. Therefore, they have received much research attention.6–9
Polyurethane is a block copolymer that consists of hard and soft segments. It can be produced with a variety of physical properties by varying its structure, the molecular weight of the segments, and the ratio of hard to soft segments. Shape memory polyurethane (SMPU) can show shape retention and shape recovery by adjusting the temperature to around its transition temperature (Ttrans). Ranta et al. reported that the Ttrans of SMPU can be the glass transition temperature or melting point of the soft segments.7,8,10,11
Chung et al.1,2 reported that polyurethane showed better shape memory characteristics when manufactured as a nanoweb rather than a film or foam. Nanoweb also has advantages applicable for use in clothing, such as efficient water vapor and air permeability.12–14 Electrospinning can control fiber diameters and can control the material’s thickness through the degree of fiber stacking, which allows adjustment of pore size and the nanoweb membrane properties.12–16
This work reports the effects of the thickness – and hence the pore structure – of SMPU nanowebs on their environmental responses.
Various thickness nanowebs were electrospun under optimal conditions and their shape memory characteristics, thermal properties, pores, air and water vapor permeabilities were measured to aid the development of intelligent textiles with optimal responses to environmental changes.
Experimental
Materials
Pellet-type MM3520 SMPU was used (Diaplex, Japan; Mn = 24473; Mw = 116483). SMPU was prepared from diphenylmethane-4,4’-diisocyanate (MDI), adipic acid, ethylene glycol, ethylene oxide, polypropylene oxide, 1,4-butanediol, and bisphenol A. Polyether-type soft segments were composed of polypropylene glycol and polyethylene glycol; the hard segments were aromatic type (MDI). 11 The solvent used for electrospinning the nanoweb contained N, N-dimethylformamide (DMF, HCON(CH3)2 = 73.10, DAE JUNG) and tetrahydrofuran (THF, C4H8O = 72.11, DAE JUNG).
Sample preparation
To make the nanowebs, we used an electrospinning machine (eS-robot® Electrospinning/spray system, Nano NC, Korea). Electrospinning conditions were optimized by testing concentrations of 4–15 wt%, feed rates of 0.5–2.5, and tip-to-collector distances of 8.5–16.5 cm. 20, 40, and 60 µm thick nanowebs were then manufactured under the optimal conditions.
Appearance and pore characteristics
Nanoweb uniformity and fiber diameters were characterized by field emission scanning electron microscopy (FE-SEM, S-4800, HITACHI Japan). Pore sizes and distributions were measured using a porometer (CFP-1500AEL, PMI, USA) before stretching, after 50% elongation, and after samples returned to their original shapes.
Physical properties
40 mm × 10 mm sample tensile characteristics were assessed at room temperature using a UTM (WL 2100, Withlab Co. Ltd., Korea) with jaws set at 20 mm apart and a crosshead speed of 10 mm/min.
Thermal properties
Differential scanning calorimetry (DSC) (200 F3 Maia(R), NETZSCH, Germany) assessed Ttrans of the samples over the range −30°C to 200°C with heating at 10°C/min. The thermal characteristics of the MM3520 pellets were examined based on the results obtained from the second scan.
Thermomechanical properties
Shape memory behavior was assessed using 100 mm × 10 mm samples as follows:
1
Maximum transformation (εm) was applied at Ttrans+25°C and maintained for 5 min. The sample was cooled to Ttrans-25°C. The extension was maintained at this temperature for 5 min, fixing the stretched form. At Ttrans − 25°C, a load was applied for 10 min and removed. The sample did not return to its original shape as the temperature was below the Ttrans; it returned to the retained shape. Without any applied load, the temperature was raised to Ttrans + 25°C and maintained for 10 min, allowing the sample to recover its original shape because the temperature was above the Ttrans.
Transformation values of the retained and recovered shapes can be used in equations (1) and (2) respectively to calculate the degrees of fixation and recovery.
1
Air and water vapor transmission
Air permeabilities were measured using an air permeability tester (FX3300, Testest, Switzerland) based on the ASTM D 737 Frazier method. Water vapor permeability was measured based on a testing method employing calcium chloride as described in ASTM E 96.
Results and discussion
Nanoweb electrospinning
Electrospinning was optimized at 0.5 mL/h/14 kV, 0.5 mL/h/16 kV, 1.5 mL/h/16 kV, and 1.5 mL/h/18 kV for concentrations of 12, 13, 14, and 15 wt%, respectively, in agreement with previous work, 1 which reported that optimal voltage increased with increasing spinning solution concentration. Optimal discharge rate also increased with increasing spinning solution concentration.
FE-SEM images of the nanowebs were recorded at magnifications of 300×, 1000× and 5000× (Figure 1) and showed the fibers were of uniform diameter, which increased with increasing solution concentration. The lower diameter fibers produced from lower concentration solution showed the formation of beads because of imbalanced attraction between the needle to which the voltage was applied and the collector. Such an imbalance has been reported to be due to voltage, solution concentration, and type of solution.
15
Therefore, nanowebs were manufactured under optimal conditions of 12 wt% concentration, 0.5 mL/h flow rate, and 14 kV voltage, which produced fiber diameters of 0.177–0.926 µm, the smallest possible diameters without beading (Figure 1).
FE-SEM images of SMPU nanowebs according to various concentration solutions.
Tensile properties
In Figure 2, it is shown that the thicker the nanowebs, the higher the tensile stress. It was believed that increasing thickness increased the number of crossover points between the nanofibers, increasing the resisting force against the load and increasing the stress. When a load was applied to the nanowebs, it deformed by aligning the fibers in the stretching direction first and then the fibers slipped after overcoming the friction from the fiber entanglement, which is similar to the mechanism for nonwovens reported in the literature.
17
Increasing elongations of 157.15%, 200.35%, and 245.25% were recorded with increasing thickness because elongation increased for the same reason as did the force against the load.
Stress-strain curves of SMPU nanowebs according to various thicknesses.
Thermal properties
DSC curves of SMPU pellet and nanowebs show inflection points at 25.3 and 29.3°C, respectively (Figure 3), corresponding to the Tg of their soft segments. Chung et al.
1
reported that the orientation of the soft segments developed due to elongation during spinning so as to increase the melting heat and Ttrans. Therefore SMPU nanowebs showed slightly higher Ttrans than PU polymers due to elongation. Table 1 lists the SMPU nanoweb shape retentions at around the Ttrans. A wide temperature range of changes was shown for fixing and recovery around the Ttrans. Debdatta et al.
10
reported an advantage of the Ttrans being the glass transition temperature of the soft segments: energy can be stored elastically. A disadvantage is that control of Ttrans is not easy and the transition range is wide. The range of changes during differential scanning calorimetry was much wider than that of a material with a Ttrans of the melting point of the soft segments because the Ttrans of MM3520 was the glass transition temperature of the soft segments. The SMPU nanowebs showed wider ranging temperature changes than MM3520.
DSC curves of SMPU pellet and nanowebs. Shape retentions of SMPU nanowebs at various temperatures
Shape memory performance
All samples showed excellent shape retention and recovery (Figure 4), with shape retention increasing 95.27%, 96.02%, 97.01% and shape recovery decreasing slightly 99.94%, 99.86%, 99.60% as thickness increased from 20 to 40 to 60 µm. Shape retention increased with increasing thickness due to the increased number of crossover points between the fibers, which led to an increased force being more effectively transmitted throughout the fiber networks.
Shape retention and recovery values of SMPU nanowebs according to various thicknesses: (a) shape retention of SMPU nanowebs, (b) shape recovery of SMPU nanowebs.
Shape recovery of all samples was excellent because elongation of the fibers occurred during spinning and the orientations of the soft and hard segments were well established. 1
Pore size and distribution
Pore diameters decreased with increasing sample thickness and more small pores were formed (Figures 5 and 6). Increased fiber stacking with increasing thickness reduced the pore size. The thicker samples, with more crossover points between fibers, showed less variation of pore size under elongation. The stretched and fixed nanowebs increased by 1.5–2 times when compared with the original nanowebs. It was observed that the pore size of the recovered nanowebs increased slightly compared to the original nanowebs because the shapes of nanowebs were not completely recovered to the original shape.
Average pore diameter of SMPU nanowebs according to various thicknesses. Pore size distributions of original nanowebs according to various thicknesses.
Air and water vapor transmission
Air permeability consistently decreased with increasing thickness (Figure 7). It increased when samples were elongated by 50%; increases of 2.37, 0.78, and 0.65 cfm were respectively shown by the 20, 40, and 60 µm thick samples. A greater increase of air permeability was shown in the thinner samples, because of less fiber stacking.
Air permeability of SMPU nanowebs according to various thicknesses.
Water vapor transmissions (g/m2·24 h) at various temperatures
Conclusions
Electrospun SMPU nanowebs of various thicknesses were characterized and their performances were optimized. Their tensile strength and elongation increased with their increasing thickness.
Excellent shape recovery and shape retention were shown by all the nanowebs. Shape retention slightly increased with increasing thickness and there was little significant difference in shape recovery. Pore sizes, and thus air and water vapor permeabilities, decreased with increasing thickness.
The stretched and fixed nanowebs showed greater differences of water vapor permeability at 10°C with 90% RH; and 15°C with 90% RH, than the original nanowebs did, because stretched shapes of the samples could be retained by more than 70% at 10°C or 15°C.
The 40 µm SMPU nanoweb showed the best performance, maintaining high shape memory and the largest difference in water vapor permeabilities between when stretched and not stretched at 15°C with 90% RH.
Footnotes
Funding
This work was supported by the Korea Science and Engineering Foundation (KOSEF) (grant number R11-2005-065).