Chapter 9: Functional smart textiles using stimuli-sensitive polymers – Functional Textiles for Improved Performance, Protection and Health

9

Functional smart textiles using stimuli-sensitive polymers

A.K. Agrawal and M. Jassal,     Indian Institute of Technology, India

Abstract:

Immense interest has been shown in polymeric systems exhibiting stimuli-sensitive properties due to their wide range of applications. This chapter discusses the general features of stimulisensitive polymers. It also introduces the basic chemistry of synthesis of temperature-and pH-responsive polymers. Then, the chapter reviews the different techniques suitable for producing smart textiles using stimuliresponsive polymers. The response of these polymers can be enhanced by integrating to textiles or by conversion to stable, strong thin shapes.

Key words

temperature-responsive textile

transition temperature

poly(N-tert-butylacrylamide-ran-acrylamide)

coated yarn

shape changing fibers

pH-responsive fibers

water-vapor transmission rate

9.1 Introduction

Fibers are flexible, mechanically strong one-dimensional structures and can be converted into a variety of two-and three-dimensional flexible structures such as ligaments (artificial muscles), woven fabrics, non-woven webs and membranes. A smart fiber that changes shape in response to various stimuli from the environment would not only enable development of the above applications in a desirable manner, but also open up a wide window of new applications in areas such as intelligent textiles (both apparel and technical applications), robotics and aerospace. In this chapter, we aim to highlight recent results of stimuli-responsive polymers for converting them to strong responsive structures suitable for making smart textiles.

9.2 Stimuli-sensitive polymers

Stimuli-sensitive polymers (SSP), also known as ‘stimuli-responsive polymers’ or ‘smart-polymers’ or ‘intelligent-polymers’, are a class of polymers that show a reversible transformation from one state to another as a response to various stimuli from the environment. Other than the widely studied temperature stimulus, the other chemical and physical stimuli which bring about a reversible transition in such polymers include electric field, solvent-composition, light, pressure, sound, stress and magnetic field, and chemical and bio-chemical stimuli (i.e. pH and ions).13

Responsive polymers change their individual chain dimensions/size, secondary structure, solubility or the degree of intermolecular association in solution. Generally, the physical or chemical event that causes these responses is limited to formation or destruction of secondary forces (hydrogen bonding, hydrophobic effects, electrostatic interactions, etc.), simple reactions (e.g. acid–base reactions) of moieties pendant to the polymer backbone, and/or osmotic pressure differentials that result from such phenomena (Fig. 9.1). These polymers provide a big opportunity for creating intelligent materials that needs to be exploited.

9.1 Mechanism of action of SSP.

9.2.1 Temperature-responsive polymers

Immense interest has been shown in polymeric systems exhibiting stimulisensitive properties, ever since the observation of lower critical solution temperature (LCST) behavior in poly(N-isopropylacrylamide) (PNIPAm) solution.4, 5 The aqueous solutions of the temperature-sensitive polymers change their phase reversibly from a soluble state to an insoluble state across the LCST. A few other polymers for which solution properties similar to PNIPAm have been reported are: poly(N, N′-dimethylacrylamide), poly(N-n-butylacrylamide), poly(N-acryloylpiperidine), poly(methoxy methyl acrylic acid), poly(propionylethyleneimine), poly(vinyl methyl ether), poly(N, N′-dimethylaminoethyl methacrylate) (DEAEM).1, 5, 6

The SSP have both hydrophilic and hydrophobic groups in their structure. For example, PNIPAm has a hydrophobic backbone and a pendant group which has a hydrophilic amide moiety and a hydrophobic isopropyl moiety (Fig. 9.2). Depending upon which among the hydrophilic or hydrophobic interactions dominate, the polymer exists as an extended chain or a collapsed chain. PNIPAm has an LCST in the range of 31–34 °C where it undergoes transitions in the physical properties, such as mechanical, optical and thermal properties. These changes in temperature-sensitive polymers/gels are used to characterize the transition temperature of polymers/gels by dynamic mechanical measurement, infrared (FTIR), UV–vis-spectrophotometry (UV–vis), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), atomic force microscopy (AFM) and X-ray scattering.

9.2 Chemical structure of PNIPAm.

These polymers when synthesized in a hydrogel form, exist in a swollen state at temperatures below the transition temperature and in collapsed state at temperatures above the transition. The huge difference in waterholding capacity, which transforms reversibly according to the transition temperatures, has potential to be exploited for different applications. These polymers find various applications1 in the field of controlled drug-delivery,7, 8 molecular-separation,9 enzyme-activity control,10 artificial muscles,1113 etc.

Different applications require the transformation to occur at a desired temperature, in order to achieve the switching at that particular temperature. The design of the hydrophilic-hydrophobic balance therefore becomes necessary. The LCST of a polymer can be designed/tuned to a desired value by

 using additives14, 15

 designing monomer structure16, 17

 copolymerization.1821

Save et al.22 have reported synthesis of series of new temperature-sensitive copolymers based on of N-tert-butylacrylamide (NTBA) and acrylamide (AM) (Fig. 9.3). Random linear and cross-linked copolymers of NTBA and AM were prepared by solution polymerization method, using regulated dosing of comonomer AM having a higher reactivity ratio (rAM = 1.5) than NTBA (rNTBA = 0.5). Linear copolymers with varying feed ratios of NTBA and AM (80:20 to 20:80 mol%) were synthesized and characterized. For the synthesis of copolymer hydrogels, N, N′-methylenebisacrylamide (MBA) was used along with monomers. The incorporation of a higher percentage of the hydrophilic comonomer AM, in the structure resulted in the shifting of the transition temperature towards a higher value. The transition temperatures of the copolymers synthesized with feed compositions of 80:20, 70:30, 60:40, 50:50, 40:60, 30:70 and 20:80 mol% were found to be 2, 10, 19, 27, 37, 45 and 58 °C, respectively. DSC studies confirmed the formation of random copolymers.

9.3 Copolymer structure.

Another interesting case of a thermo-responsive polymer is the hyper-branched polyether structure synthesized by Jia et al.23 The reaction proceeded via proton transfer polymerization of a diepoxide and a triol. The LCST could be varied from 19 to 40.3 °C depending on the composition of the triol.

9.2.2 pH-responsive polymers

The pH-sensitive polymers24 undergo a change from a hydrophobic state to a hydrophilic state by the change in pH. These polymers consist of ionizable pendant groups (acidic such as carboxylic and sulfonic acids or basic groups like amine) that can accept and donate protons in response to the environmental change in pH. As the environmental pH changes, the degree of ionization in pendant groups undergoes dramatic change at a specific pH called pKa. This rapid change in the net charge of pendant groups causes an alternation of the hydrodynamic volume of the polymer chains. This results in a transition from collapsed hydrophobic state to soluble hydrophilic state of the polymer.24 The pH-sensitive hydrogels can be weakly acidic (anionic) or weakly basic (cationic) depending on the nature of the ionizable moieties on their polymer backbones. Many systems based on acidic carboxylic acid group comonomers or basic amino group containing comonomers have been reported.2528 The hydrogel swelling and de-swelling properties depend on several factors including their hydrophobic-hydrophilic nature, crosslink density(elasticity), charge density and pKa.29

In poly(acrylic acid), the carboxyl group gets ionized at alkaline pH and becomes hydrophilic. In this state, the ionized groups mutually repel due to their negative charge and force the polymer chains to unfold and dissolve in the medium. While at a low pH, the carboxylic groups lose their charge and the polymer collapses as it becomes hydrophobic (Fig. 9.4). Since these polymers are in solubilized form when hydrophilic, they are generally produced as a cross-linked structure for many applications. In cross-linked gel form, the polymer network swells when hydrophilic and de-swells when subjected to a pH at which it is hydrophobic. The cationic polymers like N, N’ diethyl amino ethyl methacrylate exhibit a similar behavior, but collapse at high pH and swell at lower pH. Since the swelling of polyelectrolyte hydrogels is mainly due to the electrostatic repulsion among charges present on the polymer chain, the extent of swelling is influenced by any conditions that reduce electrostatic repulsion such as pH, ionic strength and type of counter ions. The swelling and pH responsiveness of polyelectrolyte hydrogels can be adjusted by using neutral comonomers, such as 2-hydroxyethyl methacrylate, methylmethacrylate and maleic anhydride. Different comonomers provide different hydrophobicity to the polymer chain, leading to different pH-sensitive behavior.

9.4 Mechanism of action of pH-responsive polymer.

The synthesis of polymeric hydrogels is typically accomplished by one of two well-established schemes:

1. Polymerization of hydrophilic monomers.

2. Modification or functionalization of existing polymers.

Some pH-responsive polymer systems are given in Table 9.1.

Table 9.1

Commonly used polymer systems for pH-sensitive polymers

9.2.3 Dual-sensitive (temperature and pH sensitive) polymers/hydrogels

These polymers respond to more than one environmental stimulus and are expected to provide more sophisticated responsiveness and greater potential for novel applications. Two main approaches have been used to prepare multi stimuli-responsive polymers. One approach involves the copolymerization of different responsive components.46 Many studies have been carried out, where the copolymers have been made either for having response to two stimuli, e.g. both temperature and pH, or for adjusting the transition temperature of the resulting copolymer by the design of the hydrophilic–hydrophobic balance. Though the polymers have response to two different stimuli, e.g. temperature and pH, the presence of the other comonomer also affects the transition value of each stimulus.

In the second approach, the multi-stimuli-sensitive hydrogels are obtained from interpenetrating polymer networks (IPN) of two polymers with independent stimuli responsiveness. This method has been used to make IPNs composed of temperature-responsive poly (N-isopropyl acrylamide), or PNIPAAM and a pH-responsive hydrogel composed of either poly (acrylic acid)47 By chain interpenetration, one may attain combination of properties from these two polymer networks. Since there is no chemical bonding between the two components networks, each network may retain its own property while the proportion of each network may also lead to much higher mechanical strength with respect to the homopolymer network.

Chitosan-polyacrylic acid IPNs have also been reported48 to show response to both alkaline as well as acidic pH. Dual response has also been discovered in acrylonitrile (AN) copolymers with acrylic acid (AA).40, 4951

9.3 Drawbacks and limitations of current SSP/hydrogels

The SSP used in the hydrogel form have poor mechanical properties and hence are used as thick gels when converted to structures. An important factor for application is not only the maximum amount of water absorbed or released but also the rate of swelling and shrinking.

The major drawback of SSP when polymerized in the gel form is their slow response at transition. A disk-shaped gel of PNIPAm–co-poly(ethylene oxide) of 0.7 cm diameter in dry state requires 75 h to attain equilibrium.52 In another study, pH and temperature-sensitive hydrogels (thickness 1–1.5 mm) based on N-acryloyl-N′-methylpiperazine and methyl methacrylate ions have been reported to show a reversible response to pH with a long response time of 150 min.53 The response time which is diffusion controlled is given by:

where, τ is the time for diffusion to achieve equilibrium, r represents the smallest gel dimension in the swollen state, and Dcoop is the cooperative diffusion coefficient of the network in the swelling solvent (Dcoop for PNIPAAm/ water is 3 × 10− 7 cm2 s− 1).

Due to slow response and poor magnitude of response, the potential applications of these copolymer hydrogel structures have not been fully exploited.

9.3.1 Approaches to improve the response

Several other approaches for making the structure of the polymers which result in a faster response time have been discussed in a review,3 but these studies also report loss in mechanical strength of the hydrogels. Some of the strategies that have been proposed in order to increase the response rate include

(i) forming a heterogeneous network structure of the hydrogel through a phase separation method

(ii) graft-copolymerization, where the free ends of the grafts act to accelerate the dehydration rate

(iii) use of pore-forming agent during the polymerization reaction.

All these modifications lead to improvement in the response time typically ranging by one order of magnitude. Similarly, the swelling ratios of the hydrogels increase from 150% to 3000% based on the thickness and cross-linking density of the hydrogels. However, to date improvements in response time have been limited, because these polymers have not been subjected to processing into mechanically strong thin shapes.

The simplest way to improve the response time is to make hydrogels stronger, thinner and smaller.

SSP are expected to overcome the limitations of slow response, limited transition, weak mechanical properties and find additional numerous novel applications if:

(a) new series of SSP could be synthesized with a structure that can render them processable,

(b) these new polymers could be processed into thin-films or fibers with thickness in the range of few microns,

(c) alternatively, these polymers could be integrated as coatings to nonactive substrates such as mechanically strong yarns and fabrics and

(d) the resultant SSP fiber and SSP-substrate composite could retain the thermo-sensitive property.

9.4 Smart functional textiles

As discussed above, the major drawbacks of the current SSP gel structures are their weak mechanical properties and poor transitional response. Processing of these materials into thin structurally strong shapes or their integration to textile materials (such as mechanically strong yarns and fabrics) is likely to solve the current drawbacks of hydrogels and also develop responsive textile materials for smart textile.

This has been achieved by converting the linear copolymer of poly(N-tert-butylacrylamide-ran-acrylamide) (27:73 mole %) containing polycarboxylic acid cross-linker and catalyst into fibers (30–50 μm), and coatings onto cellulosic yarns and fabrics.3, 22, 54, 55 The processed forms were dried and cured at 150–200 °C for 5–25 min. The presence of reactive side groups in the monomer has been utilized to introduce cross-links with the help of polyfunctional cross-linkers. This approach of incorporation of responsive polymers in the form of thin film/coating on reactive flexible substrate provides an active polymer layer on top of substrates. Such a structure also results in mechanically strong SSP composites with a large surface area.

9.4.1 Integration of stimuli-responsive polymers to textiles

The cross-linking appears to result due to condensation between the amide side-groups of the copolymer and carboxylic acid groups of the cross-linker. The carboxylic acid groups of polyfunctional cross-linker also react with hydroxyl groups of the cellulose substrate resulting in chemical bonding of copolymer onto the substrate. This approach of incorporation of responsive polymers in the form of thin film/coating on reactive flexible substrate would provide an active polymer layer on top of substrates. Such a structure would give mechanically strong SSP composites with a large surface area.

Coated yarn structures

The yarn samples were dip-coated with the above coating solution, dried at 120 °C for 5 min and cured at 180 °C for 6 min. The integrity of the copolymer coating to yarn was found to be excellent.

The transitional behavior of the coated yarn was studied with respect to temperature of the surrounding water bath. The yarn shows volumetric swelling of 4500% at 6 °C and collapse to a swelling percentage of 800 at 80 °C. A clear transition was observed between a temperature range of 15-35 °C with a mean transition temperature of about 25 °C. When the coated yarn was subjected to de-swelling, the de-swelling curve with temperature followed the swelling curve with a small hysteresis. The transition was found to be reversible to repeated swelling and de-swelling cycles. During swelling, the equilibrium was achieved within 5 min as shown in Fig. 9.5, while during de-swelling the equilibrium was attained in less than 10 s. The coated yarns showed much faster rates of transition compared to the hydrogels. This may be attributed to the higher surface area to volume ratio of the copolymer-yarn composite. However, the transition observed in coated yarn was broader compared to pure copolymer gel and also shifted to a lower temperature. This broad transition may be a result of localized heterogeneity in the cross-link density of the coated yarn structure. And the shift in transition temperature in coated yarns is due to the decrease in hydrophilicity of the copolymer caused by decrease in the number of amide groups as some of the amide groups are used in the cross-linking reaction.

9.5 Rate of transition of the coated yarn immersed in a water bath. Swelling rate was studied at 6 °C and de-swelling initiated after 6 min of the experiment time by changing the temperature to 80 °C.

Crespy has described the chemistry of and synthesis of temperature-responsive polymers (TSPs) and their application on textiles.56 Alternative methods for integration of SSP to textile substrate by radiation grafting57 or photo irradiated grafting (at λ < 300 nm)58 are also reported. In all these cases, the transitions of the integrated PNIPA remain similar to the pure PNIPA; however, it is shown to become significantly broad over a wide temperature range.

Coated fabric: responsive breathable fabric

We thought that it would also be interesting to investigate whether fabrics made from the above coated yarns would show response in air atmosphere as opposed to their tests in water baths, where ample supply of water is available for the transition. A breathable fabric55 was prepared by integrating the TSP onto a cotton fabric with 23% add-on. The coating on the fabric showed a swelling ratio of around 800% and a response time of 20 min to equilibrium swelling. The water-vapor transmission rate (WVTR) values of the TSP integrated breathable fabric were measured as a percentage of control uncoated substrate. The transmission percentage at 20% relative humidity for TSP fabrics (Fig. 9.6) were found to change across the transition temperature (15–45 °C) from 58% to 94% compared to a comparative non-responsive breathable fabric (made using poly(acrylamide)-coated fabric), which changed only from 70% to 94%. The difference in percentage transmission, due to a change in the environment temperature, shows the responsive (smart) behavior of the TSP fabrics. Similar results were obtained for other relative humidity conditions.

9.6 Water-vapor transmission for coated fabrics shown as a percentage of control (uncoated cotton fabric) at 20% relative humidity. (a) Responsive breathable sample made from TSP-coated cotton fabric and (b) non-responsive breathable sample made from polyacrylamide-coated cotton fabric.

9.4.2 Conversion of SSP to fibers

Stimuli-sensitive fibers are likely to show fast transition and better utilization of functional sites available inside the structure. Stimuli-sensitive fibers would also have an advantage over grafted SSP membranes in that they would not require support of inactive (passive) substrate, and therefore, would be able to show much higher functionality per unit weight of the material used. However, the technology of fiber spinning is complex and confined to only a few known non-responsive polymers. Therefore, it would be highly desirable to find methods to produce fibers from SSP. Linear SSP can be processed into fibers and then stabilized by cross-linking for producing SSP fibers that are stable during use.

Temperature-responsive fibers

A high molecular weight poly(N-tert-butylacrylamide-ran-acrylamide::27:73) was converted to insoluble strong fibers with fineness of 30–50 μm by solution spinning, drawing and subsequent cross-linking (using the approach mentioned above for coated yarns and fabric). The mechanism of cross-linking using NMR and FTIR techniques has been separately investigated and published by our group.

The cured fiber was placed at 6 °C for 15 min and then transferred to 80 °C for 15 min. The SSP fibers were found to change shape (with respect to both diameter and length). Figure 9.7 shows an optical micrograph of the diametric change of the fiber immersed in water. The rate of transition of the fiber is shown in Fig. 9.8(a) and the transition temperature is shown in the Fig. 9.8(b).

9.7 The optical microscopic view at × 100 of SSP fiber produced from poly(NTBA:Am::27:73) copolymer with 37.9 mol% cross-linker concentration. (a) As-spun and cured fiber and (b) fiber at equilibrium swelling in water at 6 °C.

9.8 Rate of transition and transition temperature of SSP fibers. () Fiber with 37.9 mol% cross-linker concentration, () fiber with 88.4 mol% cross-linker concentration and () copolymer gel of the same composition shown for comparison.

The transition was sharp and occurred reversibly in a very narrow range of temperature (3–4 degrees). Also, the transition temperatures of the cross-linked fibers were found to shift towards the lower temperature from 37 °C (in linear copolymer) to 22–25 °C. As shown in Fig. 9.9, these engineered fibers display sharp temperature sensitivity, extremely high reversible change in dimensions (1000% in diameter and ~ 70% in length), and extremely fast response time (< 20 s for expansion and < 2 s for contraction).

9.9 Change in shape of different SSP fibers when subjected to swelling/de-swelling cycles between 6 °C and 80 °C, respectively. Trace A, fiber with 37.9 mol% cross-linker concentration; trace B, fiber with 88.4 mol% cross-linker concentration; trace C, fiber with 63.2 mol% cross-linker concentration with draw ratio of 6; and trace D, fiber with 63.2 mol% cross-linker concentration with draw ratio of 3.

pH-responsive fibers

(a) Hydrogel fibers by chemical cross-linking

PVA/PAA blend fibers have been thermally cross-linked to form hydrogel fibers.59, 60 The hydrogel fibers exhibit pH-sensitive behavior and show hysteresis loop in the pH range from 2.5 to 12.5. The transition pH value was reported to shift to lower value with increasing content of PAA. The cross-linking of fiber caused a drop in the swelling ratio (i.e. magnitude of response decreased) and a shift in the transition pH to a higher pH value. The oscillatory swelling/contracting behavior of the hydrogel fiber exhibited a well reversible pH-responsive property. But these fibers have very poor mechanical properties because of the cross-linked structure.

(b) Hydrogel fibers by modification of an existing precursor fiber

It is known that AN and AA copolymers are used as pH-sensitive materials. An interesting approach has been reported to modify the existing poly (acrylonitrile) (PAN) fiber to produce the pH-sensitive fiber.23, 61 The modification involves two steps:

(i) Thermal stabilization of fiber at 210 °C to impart cross-links that provide the strength and structural integrity.

(ii) Saponification of nitrile groups to responsive carboxylic acid groups in the stabilized fibers.

Figure 9.10 shows the structure of stabilized and saponified acrylic fiber (pH-responsive fiber).

9.10 Chemical structure of modified PAN fiber.

The modified PAN fibers were found to change shape with pH of environmental solution. The structure, properties and response behavior of modified fibers are determined by stabilization and saponification conditions. The modified PAN fiber (stabilized for 5 h and hydrolyzed) exposed to pH 10 attained equilibrium in 180 min with the percentage increase in length of about 55%.

The transition rate for the swelling cycle in 2 N NaOH solution was extremely fast and was found to be complete in about 4 s. Compared to swelling, the de-swelling was still faster and the entire change occurred spontaneously (< 1 s). The fast transition obtained in the pH-responsive fiber is due to the large surface to volume ratio provided by the fine fiber structure compared to hydrogels.

The fibers could be repeatedly subjected to cycles of elongation and contraction (Fig. 9.11) indicating the stability of structure. And the transition was sharp, stable and occurred reversibly.

9.11 Cyclability behavior of modified acrylic fiber.

This process appears to be simple and involves the modification of an existing precursor fiber but it has some drawbacks:

(i) pH-sensitive fibers are black in color due to chemical conjugation of nitrile groups to form six membered aromatic rings, which makes them inappropriate for smart textile applications.

(ii) Production rate of these fibers is considerably slow due to slow oxidation step.

(iii) The parameters available for controlling the amount of cyclization and cross-linking in the fiber are limited.

(iv) Production of fibers of varying chemical and physical architecture for tunable response is difficult.

(c) Hydrogel fibers with physical cross-links

The above limitations of chemically cross-linked responsive fibers can be overcome and these can find additional numerous novel applications if:

(i) Stimuli-sensitive copolymers could be synthesized with a controlled architecture (enriched segments of strength imparting monomer and enriched segments of responsive monomer).

(ii) Interestingly, these new polymers could be processed into thin oriented fibers with physical cross-links (instead of chemical cross-links). Rather the physical structure of the fiber has been tuned to give both responsiveness and structural stability.

pH-sensitive copolymers of controlled architecture based on AA and AN have been synthesized by specially designed free radical polymerization. AN was chosen to impart fiber-forming properties to the copolymer while AA was selected to provide pH response. The approach was to control the distribution of both AN and AA (AA) moieties in the polymer chains in such a way that small block segments of each are possible. On wet spinning of the copolymers into fine fibers and their subsequent annealing, it was found that the segments of AN moieties from different chains could come together (phase separate), crystallize and form various tie-points (AN domains) to provide strength to the formed structure, whereas the block segments of AA could form responsive domains which provided pH response through ionization under suitable environment. The mechanical properties as well as pH response of the fibers could be significantly enhanced by modifying the chemical architecture of the copolymers and the morphology of the resultant fibers, while keeping their physical dimensions fixed. The proposed structure of the copolymers is also shown in Fig. 9.12. Spinning of the copolymers was studied in detail with respect to their (a) thermodynamic behavior during spinning and (b) effect of spinning and post-spinning operations on their pH response and mechanical properties.62

9.12 Proposed structure of the copolymers and fiber. (a) Chemical structure AA30R, (b) schematic of fibers AA30R, (c) chemical structure of AA30B and (d) schematic of fibers consisting of domain morphology.

The fibers from block copolymer (AA50B) showed significantly higher strength, extensibility and retractive forces compared to fibers from random type copolymers (AA50R) (Table 9.2). These properties could be further improved by increasing the AN content to up to 90 mol% (AA40B-AA10B). Swelling behavior was also superior for fibers from block type copolymers. The extent and rate of swelling was higher while the hysteresis was lower for fibers from block type structures. However, the extent and rate of swelling decreased with increasing AN content. As a result of these changes, the fibers from AA30B (30 mol% AA) were able to show significantly better mechanical properties and retractive forces compared to fibers from AA50R while still maintaining a similar swelling ratio.

Table 9.2

Equilibrium swelling of different pH-responsive fibers without any load and retractive force during de-swelling

AA50B: represents 50 mol% of monomer AA in blocky architecture and R represents random architecture.

The finer fibers from AA50B (38 μm) showed a stable, reversible transition with significantly higher equilibrium volumetric swelling of 22200% at a pH of 10 compared to their thicker version of 120 μm, which showed volumetric swelling of only 3300-3600%. Also, the finer fibers from AA50B (38 μm) showed significantly higher strength (1.6 times), and higher retractive stress (1.2 times) during the transition compared to the fibers with higher diameter (120 μm). The rate of transition was also found to be higher for the finer fibers and their equilibrium de-swelling time was also significantly lower at 20 s. These changes in properties of the two fibers is attributed to the formation of better domain structure due to higher drawing imparted to the coagulating fiber. The fibers heat-set at 100 °C were not able to withstand repeated cycling and were disintegrated in a few cycles, while the fiber heat-set at 150 °C could withstand more than 50 cycles without any deterioration in their mechanical properties. However, the increased heat-setting temperature was found to have a negative effect on the equilibrium swelling as well as the response rate. The FTIR spectroscopy of the heat-set samples suggested that though samples had undergone some chemical changes in relation to disappearance of OH groups of COOH, there were no significant indications of formation of cyclized structure at these temperatures.

The effect of block length of individual monomer moieties on response and mechanical properties of the pH-sensitive fibers has also been reported.51 Copolymers of AN and AA with AA (feed ratio of 30 mol%) but varying average block length of AA were converted to fine fibers by solution spinning in DMF-water system, drawn in coagulation bath, and annealed at 120 °C for 2 h to develop domain morphology. The domains formed were of nano dimensions (5–10 nm) possibly because of small segment lengths of AA and AN in the copolymers and were of different sizes as the segment lengths changed from one copolymer to another as evident from changing thermal shrinkage and X-ray diffraction patterns. The fibers with segregated nano-domain morphology (AA30B-AA30BII) were found to have significantly higher swelling percentage, faster response and higher stability to repeated cycling compared to the fiber obtained from random copolymer (AA30R) (Fig. 9.13). Also, the fibers from block copolymers (AA30B-AA30BII) showed better tensile properties during swelling and higher retracting forces during de-swelling.

9.13 Cycles of swelling and de-swelling of the pH-sensitive fibers (AA30R, AA30B, AA30BI and AA30BII).

The fibers made from copolymers with higher AA block length showed significantly higher swelling ratio (an increase from 775% in AA30R to 8800% AA30BII) and a much faster response (0.25% per min to 12% per min) having a similar AA content. This was explained based on formation of possibly larger nano domains of AA moieties that allowed better and faster opening of the AA regions during swelling.

To understand the effect of bulky -CH3 group of methacrylic acid (MA) on the properties of the pH-responsive fibers,63 poly(acrylonitrile-co-methacrylic acid) was synthesized.

The fibers from MA30B showed higher volumetric swelling, faster response and lower mechanical properties compared to AA40B (with ~40 mol% AA) possibly because of the bulky –CH3 group in MA, which opens the responsive domains of the fibers (Fig. 9.14).

9.14 Cycles of swelling and de-swelling of the pH-sensitive fibers (AA40B and MA30B).

Clearly the replacement of AA with MA improved the responsive properties significantly; however, mechanical properties were compromised. Therefore, only a small amount of MA comonomer was incorporated along with AA in the AA30B type block copolymer.

The copolymer of AN and AA containing about 27 mol% AA and about 7 mol% of MA (AAMA30B) was synthesized using the method of regulated dosing. The properties of fibers from AAMA30B were compared with those from polymers having block type architecture with AA (AA30B) and from polymers with random architecture with AA (AA30R), where all copolymers had similar composition of acidic monomer (~30 mol%). The fibers developed from AAMA30B showed swelling values much higher than that of AA30B, which in turn showed higher values than that of AA30R. The fibers from AAMA30B showed swelling of 3103-3305%, which was more than 100% increase over the values of AA30B and gave a response rate of ~ 12%/min versus ~ 1.2%/min in AA30B, which was ten times faster. However, the mechanical properties were found to be still a bit lower in AAMA30B compared to AA30B. The retractive stress was 0.23 MPa (a decrease of 50%), tenacity in swollen condition was 11.02 MPa (a decrease of ~ 45%), and in de-swollen condition was 63.51 MPa (a decrease of ~ 15%). The reduction in mechanical properties suggested that the introduction of bulky MA moieties in the polymer chains had hampered the formation of proper AN domains, which were responsible for creating the stress bearing backbone of the fiber.

These fibers also exhibited sensitivity to temperature.40 When subjected to change in temperature at an alkaline aqueous medium, the fibers showed temperature-dependent swelling behavior. The swelling first increased with increasing temperature and then suddenly decreased to swelling values close to the original state. For example, fibers containing 30 mol% of AA (AA30B) showed a clear and sharp transition temperature between 49 °C and 50 °C, where the swelling suddenly decreased from 6440% to 4200%. The behavior was reversible and stable to repeated cycling. The transition temperature at pH 10 could be modulated from 40 °C to 62 °C by varying the composition of the copolymer from 10 mol% AA to 50 mol% AA. While the transition temperature for AA30 B could be changed from 33 °C to 61 °C as the pH of the medium was increased from 7 to 12. Below pH 7, the thermal transition could not be observed probably due to the poor ionizability of the carboxylic groups.

The transition appears to be a result of three simultaneous effects:(a) first increased ionization of carboxylic groups due to increasing pH; (b) increased ionization of carboxylic groups due to increasing temperature, and (c) LCST transition of copolymer while going from lower to higher temperature regime. The dual response to pH and temperature stimulus in acrylic fibers is an important discovery and is expected to support the development of new application in actuators, smart textiles and related areas.

Electro actuating fibers

Many of the polyelectrolytes/pH-responsive polymers (like Chitosan) have been reported to show actuation to changes in the applied voltage to some extent. The electrical actuation amongst these polyelectrolyte hydrogels is limited due to the insulating nature of the polymeric hydrogels. Attempts, such as blending with a conducting polymer, have been made to improve its sensitivity towards electric voltage,6466 but satisfactory results have not yet been obtained. Another problem that arises is the stability of these materials towards the cyclic changes in the pH. Thus, to improve its stability, blending with some other polymers or copolymerization is carried out which, in turn, adversely affects the response behavior.

When the electric voltage is applied, chitosan fiber is observed to show some response, but the magnitude and the speed of response is less. Therefore to induce the electrical conductivity within the chitosan matrix blending with a conducting filler material has been reported. Suitably functionalized single wall carbon nano-tubes (SWCNT) were dispersed in chitosan and wet spun in an alkaline coagulation system to produce SWCNT/ chitosan composite fibers.67 The incorporation of SWCNT loading on the mechanical properties of the composite fiber was investigated and the tensile stress–strain curves are shown in Fig. 9.15. The tenacity of neat chitosan fiber was recorded as 96 MPa. The incorporation of functionalized carbon nano-tubes resulted in continuous and significant improvement in tenacity even at higher SWCNT concentrations.

9.15 Effect of SWCNT concentration on the tenacity of SWCNT/chitosan composite fiber.

Carbon nano-tube reinforced chitosan fibers were subjected to electric field for a known time interval at constant potential difference of 10 V in a 0.5% NaCl solution. The specimens were connected to cathode and anode alternately after every 5 min.

The pure chitosan fibers remain unaffected even on application of electric voltage or rather it can be assumed to be a too slow response and of very low magnitude. The SWCNT/chitosan composite fibers responded immediately to the change in applied voltage. Figure 9.16 illustrates the behavior of chitosan fibers with the cyclic switching of the potential difference.

9.16 Response of SWCNT/chitosan composite fibers to the changes in applied voltage (10 V in 0.5% NaCl solution).

Figure 9.17(a) and (b) shows the response behavior towards the applied voltage. With the increase in the carbon nano-tube concentration some strain is observed when the fiber is connected at the cathode, but up to the carbon nano-tube concentration of 0.05% the fiber cannot survive beyond three cycles of switching electric field. As shown in Fig. 9.17(c) and (d), on incorporation of non-functionalized SWCNT, the composite fibers are not able to survive beyond three cycles even at higher concentration of the SWCNT. In case of the functionalized SWCNT, the stability of the fibers can be observed until ten cycles of changing electric field. This can be attributed to the interaction between the amine groups of the chitosan and the carboxyl groups present on the functionalized carbon nano-tubes. At higher concentration of nano-tubes (0.2% of SWCNT), the strain observed during the changing electric field is lower than that observed for 0.1% concentration. This may be due to the increase in the rigidity of the composite fibers at the higher carbon nano-tube concentration.

9.17 Effect of functionalized SWCNT on the magnitude and strain rate of the composite fibers.

It has been observed that the strain rate with respect to the applied voltage and the magnitude of strain are a function of the carbon nano-tube concentration. The presence of carbon nano-tubes introduces a conductive path for free ions in the chitosan hydrogel. Thus with the increase in the SWCNT concentration, the strain rate increases. However, the presence of carbon nano-tubes makes the fiber structure more rigid and therefore the magnitude of strain decreases.

9.5 Conclusions

The study suggested that the control of the chemical architecture and physical morphology of the pH -responsive structure may be the key to producing responsive fibers with both higher mechanical properties and response that are suitable for their applications in artificial muscles, actuators and smart textiles.

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