Performances of Chitosan Grafted onto Surface of Polyacrylonitrile

Aug 20, 2013 - Generally, the acrylic fibers have a low chemical reactivity. Two types of treatments were used in order to increase the chemical react...
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Performances of Chitosan Grafted onto Surface of Polyacrylonitrile Functionalized through Amination Reactions Vasilica Popescu*,† and Emil Ioan Muresan‡ †

“Gheorghe Asachi” Technical University, Faculty of Textiles, Leather Engineering and Industrial Management, 29 Blvd. Mangeron, TEX 1 Building, Iasi 700050, Romania ‡ “Gheorghe Asachi” Technical University, Faculty of Chemical Engineering and Environment Management, 73 Blvd. Mangeron, Iasi 700050, Romania S Supporting Information *

ABSTRACT: Generally, the acrylic fibers have a low chemical reactivity. Two types of treatments were used in order to increase the chemical reactivity: a pretreatment for functionalization and a treatment for polymer grafting. Each stage permitted the generation of new functional groups whose reactivity was much higher than that of the groups existing in the untreated PAN. The pretreatments were carried out with dihydroxyethyl amine (DHEA) and hydroxylamine (HA) in the presence or absence of NaOH. The functionalized samples were treated with chitosan (CS) in order to produce the grafting. The modifications resulting from the treatments were highlighted by Fourier transform infrared (FTIR) and scanning electron microscopy (SEM) analyses, the density of the new functional groups, grafting degree, energy dispersive X-ray (EDAX), yellowness index, color measurements/CIE LAB graphs. The performances referred to hygroscopicity, possibility of dyeing with acid dyestuffs (nonspecific to acrylic fibers), and excellent color fastness properties. A comparative study was performed between the results obtained through functionalization and grafting.

1. INTRODUCTION Many acrylic fibers have a low chemical reactivity. In time, there have been numerous preoccupations concerning the creation of new functional groups onto PAN through various chemical treatments. For instance, the acrylic fibers were chemically modified with amines such as dimethylaminopropylamine,1 ethylenediamine,2 hydrazine,3 urea, hydroxylamine,4−6 primary or secondary acyclic aliphatic amines, diethylamine, and diethylenetriamine.7 In order to increase the accessibility to water or aqueous solutions (including dyestuff solutions, at dyeing), treatments with NaOH were carried out.8,9 As the result of these treatments, the area of application of these functionalized fibers was extended, including ion exchange membranes1,2 and absorbents for metal ions from wastewaters.10,11 All these treatments were based on the technique of surface modification via chemical modification or wet chemical12,13 at temperatures of 70 °C at most, in the presence or absence of catalysts. A manner to attach a polymer on a support is the direct grafting of the polymer on that support if this has a high reactivity. If the support has no reactive groups, the grafting only occurs after activation on a previously modified surface. In this work, the activation (functionalization) of the PAN support surface was performed in five ways, by treating it with five chemical agents: DHEA, DHEA + NaOH, HA, HA + NaOH, and NaOH. These treatments result in both chemical modifications and alteration of physical-mechanical properties of the support. The conditions in which the functionalizations were performed were 2.5−5% concentration of chemical agents, 100−120 °C, durations of 30− 120 min, in presence/absence of catalysts. A new type of reactive groups (amidines, amides, amidoximes, hydroxamic acid, and carboxylic acid) was obtained at the end of each functionalization stage. These new reactive groups represented the reactive © 2013 American Chemical Society

sites where the next grafting stage occurred. The grafting consisted of coupling a natural polymer onto the functionalized supports by means of the etheric chemical bonds (except for the functionalization with NaOH, when the polymer coupling occurred by means of etheric and esteric bonds). The chitosan was preferred among the natural polymers, since it has two functional groups and shows interesting properties, such as biocompatibility, biodegradability, as well as bactericide capacities.14,15 After the grafting stage, the PAN fibers acquired several OH and NH2 groups (coming from CS). The results of the functionalization and grafting stages respectively were confirmed by qualitative analyses (Fourier transform infrared (FTIR), scanning electron microscopy (SEM)) and quantitative methods (elemental analysis, determination of functionalized groups density, grafting degrees, yellowness index, and color measurements/CIE LAB graphs). The performances of the functionalized/grafted PAN were referred to hygroscopicity, tensile strength, capacity of dyeing with anionic dyestuffs, as well as the excellent washing and rubbing resistances. The effects acquired after functionalization with amines and grafting with CS respectively were presented by comparison all along the work.

2. EXPERIMENTAL SECTION 2.1. Materials. The utilized chemicals were HA (pKb = 8.02) and CS (pKb = 7.8) from Fluka (dissolved with 2% acetic acid solution) and DHEA (pKb = 5.5), NaOH (pKb = 0.2), and oxalic acid (the acid medium for dyeing) from Merck company. Received: Revised: Accepted: Published: 13252

May 10, 2013 August 10, 2013 August 20, 2013 August 20, 2013 dx.doi.org/10.1021/ie401494a | Ind. Eng. Chem. Res. 2013, 52, 13252−13263

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Two polymers were used: PAN fiber as textile support and CS as grafting agent. PAN was supplied by a Romanian company (Savinesti SA), and CS from Fluka Company. After functionalization/grafting, the dyeings were realized with two acid dyestuffs (C.I. Acid Red 88 (AR 88) and C.I. Acid Violet 48 (AV48)) obtained from Alibaba and Bezema companies. The chemical structure of dyestuffs is indicated in Supporting Information Table S1; the physical-chemical characterization of the polymers utilized in this study is also presented in the Supporting Information, S1. 2.2. Chemical Modifications of PAN. 2.2.1. Functionalization through Amination. Functionalization is the initial stage for PAN modification in order to acquire highly reactive groups, that is, to create reactive sites. The chemical modification of PAN fiber was made during pretreatments performed in separated baths, using the reagents: 2.5% NaOH, 2.5−5% amines (DHEA or HA) ± 2.5% NaOH. Pretreatments with NaOH and HA ± NaOH have been performed on Mathies Policolor machine at 100 °C, for 30−120 min, using a liquor ratio of 1:150. Pretreatments with DHEA ± NaOH were carried out in more severe conditions: 120 °C, 2 atm pressure, and MgCl2 as catalyst, 30−120 min time period, and 1:150 liquor ratio. These pretreatments determine modifications of the chemical structure of PAN fiber, at acetate and acrylonitrile levels, modifications that were evaluated by measuring the density of the new groups (amidines, amidoximes, amides, and hydroxamic or carboxylic-types acids), calculated according to eq 1.16

Da =

Wa M

Wf

× 106

bromide−iodide, having 25 reflexions and the investigation angle of 45°. This accessory device was attached to the Spectrophotometer FTIR IRAffinity-1 Shimadzu (Japan), the spectra were recorded with 250 scans in the 4000−600 cm−1 range. After recording, the absorption spectra have been electronically analyzed with spectral subtraction method using Panorama 3.2 software from LabCognition (details in Supporting Information S2). 2.3.2. SEM Analysis. A QUANTA 200 3D DUAL BEAM electron microscope was utilized for SEM analysis. This apparatus is a combination of two (SEM and focused ion beam (FIB)) systems by whose means three-dimensional images can be obtained by sending an electron beam on the pretreated samples, with a magnification of 100 000×. Moreover, by using energy dispersive X-ray (EDAX), the elemental analyses were performed for surface characteristics identification and high resolution chemical analysis. 2.3.3. Elemental Analysis. The elemental analyses were carried out with Quanta 200 3D Dual Beam electron microscope using energy dispersive X-rays (EDAX) to identify the percentage of each chemical element from chemical modifications throughout functionalization and grafting stages. 2.3.4. Yellowness Index, YI. Yellowness index (YI) is associated with the light reflection of samples modified through chemical exposure and processing. YI was determined by using the Spectroflash SF300/Datacolor Spectrophotometer and the ASTM Method E313-73. The formula utilized for YI is indicated by eq 3. ⎛ Z⎞ YI = ⎜1 − 0.847 ⎟ × 100 ⎝ Y⎠

(1)

where Da is the density of functionalized groups (amidines, amidoximes, amides, and hydroxamic or carboxylic type acid), [mmol/kg]; Wa is the mass of reagent (amine/base or amine + NaOH, respectively) used in the pretreatment, [g]; M is the molecular weight of reagent, [g]; Wf = PAN fiber final weight after pretreatment, [g]. After the functionalization stage, the samples were intensely washed with deionized warm water (40 °C) and then with cold water and dried at room temperature. 2.2.2. CS Grafting. The reactive groups obtained during the functionalization play the part of reactive sites for polymer grafting. CS grafting onto the already functionalized supports was performed in the presence of the oxalic acid (as catalyst) at a temperature of 100 °C. CS concentration varied from 2.5 to 4%. The coupling of CS was realized by means of etheric/esteric covalent bonds. The treatments were carried out on Mathies Polycolor machine. The surface grafting degree was calculated according to the eq 2: DG =

WaG − WaF × 100 WaF

(3)

where YI is the Yellowness Index of PAN sample, and Y, Z are the CIE tristimulus values obtained using D65/10° or C/2° as illuminant/observer. 2.3.5. Physical-mechanical Properties: Hygroscopicity and Tensile Strength. The determination of the hygroscopicity was carried out according to BSEN ISO 12571-2000. The formula for calculating the hygroscopicity is given by eq 4. H=

M100 − M 0 × 100 M0

(4)

where H = hygroscopicity index, [%]; M100 = average mass of five identical samples contained in an environment with 100% relative humidity, for 24 h, [g]; M0 = medium mass, absolutely constant of five identical samples, held in an environment with relative humidity of 0%, for a period of 24 h, [g]. The tensile strength of fibers was determined on a FAFEGRAF (Germany) dynamometer respecting the ISO 13934-2:1999 standard. 2.3.6. Color Strength, K/S. Usually, the acrylic fibers can only be dyed with cationic dyestuffs. After functionalization and grafting, samples were dyed with two acid dyestuffs, AR 88 and AV48, respectively. The dyeing processes were realized on Mathies Polycolor machine, according to the following dyeing formula: liquor ratio M = 1:75, addition of oxalic acid (pH = 4) for protonation (samples were kept 10 min, at 20 °C in this acid medium), dyeing with 1 g/L acid dyestuff, temperature raise to T = 98 °C, and dyeing at this temperature for 1 h. After dyeing, the samples were washed with 1 g/L Lavotan DSU, M = 1:75, t = 30 min, T = 50 °C. They were hot and cold rinsed and dried in air. Finally, the color strength, K/S, was measured on a Datacolor Spectrophotometer (Spectroflash SF300).

(2)

where DG = grafting degree [%]; WaG and WaF = weights of PAN supports after grafting and after functionalization, respectively, [g]. After grafting, the samples were intensely washed with deionized warm water (40 °C) and then with cold water and dried at room temperature. 2.3. Methods of Analysis. 2.3.1. FTIR Analyses. FTIR analyses were performed to identify chemical transformations produced by pretreatments with the five reagents used at functionalization stage and CS grafting, respectively. FTIR analyses were carried out on a Multiple Internal Reflectance Accessory (SPECAC, SUA) with ATR KRS-5 crystal of thallium 13253

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*

Notations 1 and 2(below each arrow) refer to the type of chemical reaction that occurs at AV respectively AN levels from PAN.

Figure 1. Chemical transformations during functionalization and grafting stages. A is PAN functionalized with NaOH; A.CS is CS grafted onto A; B is PAN functionalized with DHEA; B.CS is CS grafted onto B; C is PAN functionalized with DHEA + NaOH; C.CS is CS grafted onto C; D is PAN functionalized with HA; D.CS is CS grafted onto D; E is PAN functionalized with HA + NaOH; E.CS is CS grafted onto E.* 2.3.7. Fastness Properties. The dyed samples (functionalized and/or grafted, respectively) were tested according to ISO standard AO5-CO6/1999 (using the gray scale) to determine the resistances to washing. Color fastness to rubbing was determinate according to ISO 105-X16/2001.

range between 0.5 and 8%; therefore, in Figure 1 are valid the relations 5: x > xi > xi′

and

y > yi > yi′

where

i ∈ [15]

(5)

Thus, being a strong base (pKb = 0.2), NaOH converted the ester group from AV into an −OH group through a saponification reaction. At the level of the CN group, a hydrolysis occurred up to the carboxylic group (Figures 1 and 2) via an amide group.17,18 In this way, NaOH hydrolyzed the CN group (partly or completely), functionalizing it. At the end of pretreatment with NaOH, the A functionalized fiber was obtained. DHEA (pKb = 5.5) and HA (pKb = 8.02) amines converted the ester group from AV in an −OH group through N-acylation

3. RESULTS AND DISCUSSION 3.1. Modifications through Functionalization/Grafting. 3.1.1. Chemical Mechanisms. By treating PAN with NaOH and with amine ± NaOH systems, respectively, only a part of the CN groups from AN, and acetate groups from AV, was converted according to the scheme from Figure 1. The conversion degrees 13254

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with CS. In the presence of the oxalic acid as catalyst and at T = 100 °C, CS was grafted on each functionalized support by means of etheric covalent bonds. The OH groups newly acquired from PAN through functionalization took part in the reaction, and the OH groups attached to the carbon atom C (6) from CS. In the case of the fiber functionalized with NaOH, besides the etheric bridge, an esteric bridge was formed between PAN and CS (between the COOH groups from the functionalized PAN and part of the primary OH groups from CS). After the CS grafting stage, the following products were obtained: A.CS; B.CS; C.CS; D.CS; and E.CS (Figure 1). 3.1.2. Density of Functionalized Groups. The modifications of acetate and nitrile groups from PAN were established by means of the density of functionalized groups, Da index values, calculated according to eq 1 (Figure 2). In Figure 2, one can notice that Da depended less on the basicity of the utilized amine and more on the molecular mass/volume. A 30 min interval was enough for HA to get Da comparable to those obtained with NaOH. Even if HA is a weak base, with small molecular volume, it was easy for it to penetrate inside the acrylic fiber and to determine the N-acylation reaction at the level of acetate group and amination at the level of CN group. In the case of functionalization with NaOH + amine mixture, the NaOH aggressiveness (due to the basicity value) cumulated with the bigger or smaller movement capacity of the mixture (depending on the mixture molecular volume) determined the diminution of Da with its increasing molecular mass. 3.1.3. Grafting Degrees. The functionalized samples A−E were subjected to grafting reaction with CS. Grafting was based on an etherification reaction produced between two asymmetric alcohols, in the presence of an organic acid (oxalic acid, pH = 4), at a temperature of 100 °C, and different periods of time: 60 and 120 min. The values for grafting degrees are presented in Table 1.

Figure 2. Density of functionalized groups after 30 min functionalization.

reaction. The N-acylation was based on nucleophile addition of DHEA and HA, respectively, to the acetate group from PAN.19 An unstable tetrahedral intermediary product was formed, which easily came off and released the weak base (in this case PAN− OH, i.e. the functionalized acrylic fiber) and acylated amine. Through a severe amination recipe (temperature of 120 °C, pressure of 2 atm, and MgCl2 as catalyst), DHEA converted the CN group into imine group or rather into a disubstituted amidine group20 (with two −CH2−CH2−OH radicals). A condensation reaction occurred between the two terminal OH groups from these radicals, with the formation of a morpholine group. At the result, at the end of pretreatment with DHEA, the functionalized fiber B was obtained. HA acted on the CN group converting it into an imine group.21 Yet, this quickly passed into its tautomeric form (Figure 1), generating a hydroxamic compound, namely an amidoxime.22 At the end of the pretreatment, the functionalized fiber D was obtained. At the level of ester groups from AV, the mixtures DHEA + NaOH and HA + NaOH respectively determined saponification reactions,18 due to the action of the base with the smallest pKb, that is, the strongest base (NaOH). The ester groups were converted into −OH groups, and the sodium acetate resulted as a secondary reaction product. At the level of CN group from PAN, NaOH from DHEA + NaOH and HA + NaOH mixtures respectively determined the CN conversion into a carboxyl group (via amide group, yet without N having any substitute), creating the possibility of addition of nucleophile agents (DHEA and HA, respectively).19 These amines added to the hybridized carbon atom sp2 (from the fresh functional group −COOH) and, after the stage of water and ammonia evacuation, resulted a N,N- disubstituted (in the case of DHEA) or N- monosubstituted (in the case of HA) amide group. Practically, from the standpoint of the chemical mechanism, these N-di/monosubstituted amides were obtained through a nucleophile substitution via tetrahedral intermediary. At the end of N-acylation reaction, at the level of amide group resulted from DHEA, the reaction medium determines the occurrence of a condensation reaction, that is, water elimination between the two component OH groups, and the formation of a morpholine ring (Figure 1). At the end of the reactions performed with the DHEA + NaOH and HA + NaOH mixtures, the functionalized C and E fibers resulted. In Figure 1, one can notice that PAN was functionalized through each pretreatment applied. In this way, PAN acquired at least two reactive groups able to participate in surface grafting

Table 1. Values of Grafting Degrees, [%] grafting degrees after 60 min

a

grafting degrees after 120 min

supporta

2.5% CS

4% CS

2.5% CS

4% CS

A B C D E

0.6915 0.7358 0.8250 1.0523 0.9013

0.7128 0.8245 0.9131 1.3725 0.9978

0.8878 0.7698 0.8718 1.3511 1.0151

0.9525 0.9799 1.0188 1.5250 1.2341

A, B, C, D, E are functionalized samples (legend as in Figure 1).

From Table 1, one can see that amidoxime groups (from D) and the groups of hydroxamic acid type (from E) are the most reactive ones; they lead to the highest grafting degrees. The lowest reactivity belongs to acrylic sample A, which manifests its character of weak acid and leads to the lowest grafting degrees. The A compound has COOH groups, which appeared from the conversion of CN groups, as well as OH groups from the conversion of acetate groups from AV. In this case, grafting of CS on this sample can be done through etheric bond (at OH group), or through esteric bond (at COOH group). The achievement of both types of covalent bonds is not excluded. 3.2. Changes Confirmed by Qualitative/Quantitative Analyses. 3.2.1. Analysis in IR Range. In order to highlight the chemical modifications produced by the functionalization/ grafting treatments, the spectral subtraction method was used, consisting of the subtraction of a reference spectrum from another spectrum (of analyzed sample). A description of this method is presented in detail in Supporting Information S2.1. 13255

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Figure 3. FTIR spectra: (a) overlapping of spectra for untreated PAN and A sample (PAN f unctionalized with NaOH); (b) result of spectra subtraction operation.

Figure 4. FTIR spectra: (a) spectra overlapping for untreated PAN and sample B (B is PAN f unctionalized with DHEA); (b) result of the spectral subtraction.

For the subtraction operations, the Panorama 3.2 software from LabCognition was used. The spectrum of untreated PAN was used as a reference when functionalized samples were analyzed. PAN fiber is a ternary copolymer of 85% AN + 10% AV + 5% αMS, and its bands within the IR range are presented in Supporting Information Table S3. The ester groups were converted during functionalization into secondary alcohols

(through saponification and N-acylation reactions, respectively); that is why on the spectra of functionalized samples (from A to E), the heights of the principal peaks assigned to the ester group COOR were decreased (1732 cm−1 assigned23,24 to CO stretching (s) and 1233 cm−1 for COC stretching vibration(s)) (Figures 3−7). Other peaks corresponding to AV have dropped also: 1626 cm−1 (CO asymmetric stretching (m)), 13256

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Figure 5. FTIR spectra: (a) spectra overlapping for untreated PAN and sample C (PAN f unctionalized with DHEA + NaOH); (b) the difference spectrum.

1445 cm−1 (overlapping aymmetric and asymmetric deformations for CH2 (m) and CH3 (m)), 1366 cm−1 (symmetric deformation for CH3 (m)), 1069 cm−1(CO stretching (m)). The presence of secondary alcohols is confirmed25 by the peaks from 3385 cm−1(OH stretching (s)), 1069 cm−1 (C−OH stretching (s)), and near 1400 cm−1 for O−H in-plane deformation (m)). These can be noticed better after spectral subtraction (Figures 3.b−7.b) by means of the bands above the zero line (positive values on the difference spectrum), which indicate that all the functionalized samples were enriched in such groups. Other modifications appeared in the length of the C−C chain: the peaks corresponding to C−H stretching vibrations (2930−2857 cm −1 (s), 1445 cm −1 (m), and 1366 cm −1 (Supporting Information Table S3)) decreased in all functionalization reactions16 as result of saponification/N-acylation reactions caused by NaOH ± amine. This is proved by the appearance of bands below the zero line (in these ranges), on the difference spectrum (Figures 3.b−7.b). Changes also occurred at the level of the CN groups16,22−27 after the functionalization stage. The peaks assigned to CN vibrations were smaller after all functionalizations (in Figures 3.b−7.b appear bands below the zero line, negative values at 2239 cm−1 (CN stretching)). These were caused by the conversion of CN groups in new functional groups (such as COOH, amidine, amide, amidoxime, and hydroxamic acid groups). In case of sample A (Figure 3), the presence of carboxylic acid groups9 (via amide groups) is shown at 1732 cm−1 (CO symmetric stretching (s)), 3600−3400 cm−1 (OH stretching (s), broad band), 945 cm−1 (OH out-of plane deformation (m)), 1445 and 1366 cm−1 (C−O stretching (m) and O−H deformation). At 1732 cm−1, one can notice a decrease of the peak resulted from the conversion of the ester group in secondary OH groups

(saponification reaction) on the one side, and an increase due to the vibrations of CO from carboxylic acid groups appeared after the conversion of the CN groups, on the other side. It seems that the saponification reaction created a more pronounced effect (bands bellow the zero line in Figure 3.b). Not all ester groups were converted to carboxylic groups; some of them have changed only up to the stage of primary amide19,23 groups: near 3385 and 3270 cm−1 (overlapping symmetric and asymmetric stretchings for NH2 (s)), 1626 cm−1 (CO stretching (s)), 1550 cm−1 (N−H out of plane (m)). In the case of sample B the presence of amidine groups is revealed in Figure 4 at 1628 cm−1 (CN stretching (m-s)). This peak increases even if it overlaps on a peak that suffered a considerable decrease (1626 cm−1 assigned to CO asymmetric stretching (Supporting Information Table S3), for ester group from AV that was converted in secondary alcohol groups). In the spectra of B and C samples (Figures 4 and 5), there are also present vibrations characteristic to morpholine: 945 cm−1 (ring deformation for morpholine group (m)), 2930−2830 cm−1 (CH2 asymmetric and symmetric stretchings (s)), 1465− 1444 cm−1 (C−H bend, CH2/CH3 (m)), 1390−1350 cm−1 (C−H bend/CH3 (w)), 1305−1275 cm−1 (C−H skeletal (w)), 1240−1175 cm−1 (C−N stretching (m)), 1200−1150 cm−1 (C−O stretching (m-s) at 1167 cm−1), 1045−1000 cm−1 (C−O stretching (m)), and 850 −550 cm−1 (ring skeletal (s)). Peaks at 2930 cm−1 and 2857 cm−1 increased significantly in this case; this can be explained by a more pronounced conversion of CN group (caused by the reaction of amination, i.e., addition of DHEA that has many CH2 groups). The presence of tertiary amide groups is confirmed in spectrum C (Figure 5) at 1650 cm−1 (CO stretching). The band 13257

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Figure 6. FTIR spectra: (a) spectra overlapping for untreated PAN and sample D (PAN f unctionalized with HA); (b) result of the spectra subtraction operation.

Figure 7. FTIR spectra: (a) spectra overlapping for untreated PAN and sample E (PAN f unctionalized with HA + NaOH); (b) result of the spectra subtraction operation.

and OH stretchings (s)) and near 937 cm−1 (NO stretching (w)) (positive values in Figure 6.b). In spectrum E, the hydroxamic acid groups appear at 1670− 1600 cm−1 (combination of CN and CO stretching (m-s)

above the zero line (positive values) appears for this peak in Figure 5.b. In case of spectrum D, the amidoxime groups16,22 appear at 1645 cm−1 (CN stretching (m)), 3650−3150 cm−1 (NH 13258

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Figure 8. FTIR spectra: (a) spectra overlapping for A and A.CS; (b) result of spectral subtraction between A.CS and A (same signif ication as in Figure 1).

Figure 9. FTIR spectra: (a) spectra overlapping for B and B.CS; (b) result of spectral subtraction between B.CS and B (same signif ication as in Figure 1).

vibrations), 1534 cm−1(NH deformation and CN stretching (m)) and near 937 cm−1 (NO stretching (w)) (confirmed by the positive values in Figure 7.b). After the grafting operation, the presence of a new etheric bond (that confirms the grafting) between the functionalized samples and CS was highlighted by the spectral subtraction (Figures 8 and 9 and Supporting Information Figures S4−S6). The etheric bonds are confirmed by modifications that appeared

in the height of peaks around 1200 and 1024 cm−1 (assigned to C−O−C asymmetric (s) and weak symmetric stretches). The forming of etheric bonds lead to disappearance of bands assigned to secondary alcohols, a fact evident only in 3600−3400 cm−1. In the case of A.CS, both esteric and etheric bonds were formed. The esteric bonds are confirmed by the increase of the height of peaks from 1732 cm−1 assigned to CO stretching (s) and 1231 cm−1 (s) for C−O−C stretching vibration (positive values 13259

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Figure 10. SEM images for PAN after functionalization/grafting.

stretching (m-w)), 1060 cm−1 (overlapping C−O stretching of C(3)−OH (s) with CN stretching (w)), near 1024 cm−1 (C−O stretching of C(6)−OH (s)), 893 cm−1 (deformation of pyranozic ring from CS (w) and NH2 out-of-plane deformation (broad, s), 706 cm−1 (NH wagging broad), and 3600−3200 cm−1 (overlapping asymmetric and symmetric stretchings for O−H and NH2 (broad, very weak)). After grafting with CS, another increase of the peak from 1732 cm−1 occurred due to CO stretching vibration resulted from CS dissolution in acetic acid (CS-NH3+ −OOC−CH3 is formed by dissolution). Still another proof of grafting consists in the modification of the C−C chains length (2930−2857 cm−1). The increases of these peaks are

in Figure 8). Another proof of grafting is the CS presence on the final samples (prior to IR analysis, the samples were intensely washed in order to remove the superficially bonded CS). The presence of CS is proved by the following bands: at 1649 cm−1 (s) (for CO stretching amide I), at 1530 cm−1 (broad, m-s) (NH2 in-plane deformation), 1427 cm−1 (w) (CH2 bending from C6), 1372 cm−1 (CH bending and sym. CH3 deformation (m-w)), band at 1302 cm−1 (OH overlapping from C(3) (deformation in plane) (w) with CH2 (from C(6) deformation (m-w)), 1238 cm−1 (combination N−C−O stretching (amide IV) and OH bending (m)), 1151 cm−1 (overlapping asymmetric oxygen bridge (C−O−C) stretching with CN 13260

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The extension of the functionalization durations from 30 to 120 min resulted in the increase of hygroscopicity in all the studied cases. Taking into account the nature of the utilized functionalization agents, one can make the following observations: the HA and DHEA amines (alone or mixed with NaOH) resulted in higher values of hygroscopicity as compared to those obtained only with NaOH. Of all reagents, HA resulted in the largest hygroscopicity. Perhaps HA, having a smaller molecular volume, penetrated more easily into the PAN fiber, whose crystalline zones it disturbed, and determined a change of the crystalline/amorphous ratio. Holes and pores of different size appeared, which significantly influenced the hygroscopicity. According to the above reasoning, the order of hygroscopicity decrease is

highlighted by the bands above the zero line (positive values both in Figures 8 and 9 and in Supporting Information Figures S4−S6), after the spectral subtractions. 3.2.2. SEM Analysis. The pretreatments determine modifications of chemical structure, as well as of physical structure. From chemical point of view, modifications appeared at acetate and nitrile groups levels and these changes are very evident in Figure 10. From physical point of view, disturbances appeared in crystallinity, the widening of amorphous areas, and the appearance of pores,16 which will increase the accessibility of PAN fiber over other chemical agents. In cases of functionalization of PAN with amine ± NaOH/grafting with 4% CS, physical modifications were highlighted by SEM analysis, and the supplementary details were given in Supporting Information, S3. 3.2.3. Nitrogen (N) Content. The N content was determined using EDAX analysis. Only the pretreatments performed with DHEA and HA resulted in the increase of the N content in the PAN fiber. This can be explained by the transition from a group with a single N atom (the CN group from the untreated PAN) to groups with two N atoms each (amidine (NCNH) and amidoxime (HONHCNH) groups, respectively.) The N content not only depended on the amine nature and the duration of the pretreatment performed for functionalization but also on amine concentration, and it increased with the increase of this (Table 2 and Supporting Information S4).

HA > HA + NaOH > DHEA + NaOH > DHEA > NaOH > witness sample

In the case of NaOH, it is known that part of NaOH quantity acted for the conversion of CN group and another part acted for saponification of the ester group from AV. In this case, the diffusion of this base inside the acrylic copolymer was more reduced because the saponification reaction (eq 6) resulted in the acetic acid as intermediate product, which diminished the reactivity of other NaOH neighboring molecules (still uninvolved in saponification) through an unwanted neutralization reaction.19

Table 2. N Content Acquired after Functionalization and Grafting, [%] CS grafting duration functionalization duration functionalization recipe witness sample 2.5% NaOH 2.5% DHEA 5% DHEA 2.5% DHEA + 2.5% NaOH 5% DHEA + 2.5% NaOH 2.5% HA 5% HA 2.5% HA + 2.5% NaOH 5% HA + 2.5% NaOH

60 min

120 min

60 min

120 min

2.5% CS

4% CS

2.5% CS

4% CS

14.30 10.03 15.32 15.96 14.30

14.30 11.13 15.47 16.13 14.30

14.30 15.24 16.71 16.93 16.62

14.30 16.30 21.79 21.95 20.00

14.30 17.04 20.90 21.05 18.03

14.30 18.40 21.93 22.42 20.23

14.30

14.30

16.76

21.34

19.97

21.09

16.21 17.30 14.30

17.82 18.59 14.30

17.36

18.20

19.99

14.95

16.94

18.99 20.34 19.54

14.30

14.30

16.55

17.23

17.88

18.99

Equation 6 is based on the elimination of CH3COO− and can justify the mass loss appeared after any saponification reaction. Thus, the functionalization performed with NaOH resulted in 0.89% mass loss. The other reagents determined smaller mass loss (between 0.34% and 0.77%), as antagonistic actions occurred in the treatment mediums: elimination (at the level of ester groups) and addition (at the level of CN groups). Only the HA amine resulted in mass increase (0.32%). After grafting with CS, all the samples recorded mass increases. The mass variation after the performed treatments was also reflected in the values of the supports tensile strength (Supporting Information Table S9). The tensile strength slightly decreased as compared to the standard at all the functionalized samples, except for those treated with HA (increased with 0.87%). After grafting with CS, the tensile strengths became bigger than after functionalization and closer to the value corresponding to the standard sample. The samples C, D, and E grafted with CS have tensile strengths higher than the standard (with 0.73%, 5.97%, and 3.22%, respectively). As the result of the acquired performances, the functionalized/ grafted PAN fibers can have various utilizations. The functionalized fibers can be used as ion exchangers, filters, mortar liners (in constructions), products that retain water vapors or even water, or as absorbing membranes for the metals from

19.89

CS grafting onto every functionalized samples A−E led to an increase percentage of N. The increase was greater as long as grafting duration and chitosan concentration were higher. 3.2.4. Yellowness Index and Color Measurements. Yellowness index (YI) indicates the degree of departure toward yellow of an object color from colorless or from a preferred white. YI increased after each functionalization (more in HA case) but decreased significantly after grafting. The YI and spectrophotometric data were analyzed in details in Supporting Information, S5. 3.3. Performances of Treated PAN. 3.3.1. Hygroscopicity and Tensile Strength. All the pretreatments performed for functionalization resulted in the increase of the capacity of water and implicitly of water vapors absorption, as compared to the standard sample, as the result of the modification of PAN physical structure (Table 3). 13261

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Table 3. Values of Hygroscopicity after Functionalization/Grafting, [%]a CS grafting functionalization

a

30 min

120 min

support

30 min

60 min

120 min

2.5% CS

4% CS

2.5% CS

4% CS

witness sample A B C D E

1.15 ± 0.10 5.42 ± 0.20 6.23 ± 0.14 6.72 ± 0.17 12.03 ± 0.21 6.93 ± 0.13

1.15 ± 0.10 5.56 ± 0.02 6.91 ± 0.11 8.69 ± 0.13 16.29 ± 0.24 6.36 ± 0.12

1.15 ± 0.10 6.44 ± 0.03 7.07 ± 0.06 7.78 ± 0.20 21.46 ± 0.30 7.37 ± 0.15

1.15 ± 0.10 4.44 ± 0.10 6.15 ± 0.16 6.54 ± 0.13 7.43 ± 0.15 5.51 ± 0.06

1.15 ± 0.10 4.91 ± 0.08 5.93 ± 0.14 6.35 ± 0.10 6.40 ± 0.08 5.35 ± 0.13

1.15 ± 0.10 3.81 ± 0.07 3.96 ± 0.10 3.35 ± 0.13 3.76 ± 0.07 4.83 ± 0.12

1.15 ± 0.10 3.47 ± 0.08 3.77 ± 0.12 3.14 ± 0.09 3.21 ± 0.06 4.04 ± 0.10

A−E are functionalized samples (same legend as in Figure 1). The values indicated with ± are the standard deviations.

Table 4. K/S and Fastness to Washing (FW) Values for Dyed Samples dyeing after functionalization 1 g/L AR 88

a

dyeing after CS grafting

1 g/L AV 48

1 g/L AR 88

1 g/L AV 48

functionalization recipe

K/Sa

FWb

K/S

FW

K/S

FW

K/S

FW

2.5% NaOH 2.5% DHEA 2.5% DHEA + 2.5% NaOH 2.5% HA 2.5% HA + 2.5% NaOH

0.400 0.483 0.614 5.530 1.795

5/5/5 5/5/5 5/5/5 4−5/5/4−5 4−5/5/4−5

0.170 0.190 0.230 0.290 0.260

4−5/4−5/4−5 4−5/4−5/4−5 4−5/4−5/4−5 4/4−5/4 4/4−5/4

0.980 0.770 0.830 6.390 1.938

5/5/5 5/5/5 5/5/5 5/5/5 5/5/5

0.680 0.435 0.524 0.534 0.505

5/5/5 5/5/5 5/5/5 5/5/5 5/5/5

K/S is color strength after dyeing with 1 g/L dyestuff. bThe three digits are change in color/staining on acrylic/staining on cotton.

wastewaters. Due to the bactericide character of CS14,15 and its very good breaking/cutting resistance, the fibers grafted with CS can be especially used to manufacture textile equipments from surgical fields. 3.3.2. Tinctorial Answers to Functionalization and Surface Grafting. The tinctorial method was used to prove that PAN modifications after functionalization/grafting are those which influenced the capacity of dyeing with anionic dyestuffs (untreated PAN can only be dyed with cationic dyestuffs, not with anionic dyestuffs).The protonation stage from the beginning of dyeing process was responsible for the cationization of the samples functionalized or grafted with CS. The cationization created the premises for dyeing with acid dyestuffs; ionic bounds appeared between the fiber cations and the anions from acid dyestuffs; it was noticed that the values of color strength (K/S) for the functionalized and dyed samples depended on both the basicity of functional groups and their density (frequency). After dyeing the grafted samples, higher values of K/S were obtained than those of the functionalized/dyed samples for both AR 88 dyestuff and AV48 (Table 4). This fact proved that the amino groups from chitosan are protonized much easier. 3.3.2.1. Fastness Properties. The durability of the performed treatments is generally tested through the method of repeated washings. In this work, we resorted to the method of washing after dyeing to prove that the functionalization/grafting/dyeing effects are resistant in time. The excellent resistances of dyeings after washing (Table 4) and rubbing (Supporting Information Table S.10) prove the strength of the bonds between chemically modified PAN and the used acid dyestuffs and confirm the acquirement of a deep dyeing. That is why the CS grafted fibers can be used to make blanket and sheets for hospital beds, medical suits, or other articles of clothing used by the medical personnel in the surgeries. They could also be blended with wool fibers in order to create yarns and/or garment products that could have high resistance, hygroscopicity, and an excellent dyeing capacity (high K/S values, color fastness, and could be dyed easily in the same dye bath).

4. CONCLUSIONS Functionalization and surface grafting are two ways to increase the chemical reactivity of PAN. The effects that appeared through functionalization depended on the basicity and size of reagents molecules, as well as on the reagents concentration and treatment duration. CS grafting’ effects were stable over time, but they depended on the reactivity of functional groups, on CS concentration, and on process duration. The effects of functionalizations/graftings were confirmed by the good values of color strengths after dyeing processes (the nonfunctionalized/ ungrafted PAN cannot be dyed with anionic dyestuffs). After grafting, all the samples were dyed more intensely than the functionalized samples, as a proof of the amine groups presence (from CS) on PAN surface. Functionalizations with amines and surface graftings with CS resulted in changes in polymer (in physical and chemical structure), which determined the increasing utilization of PAN. Thus, PAN acquired higher performances: high reactivity, hygroscopicity/wettability, antimicrobial effects (due to NH2 groups from CS), and an excellent dyeability with a nonspecific dyestuff class. Moreover, all the effects acquired through functionalization/grafting had excellent durability.

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ASSOCIATED CONTENT

* Supporting Information S

S1: Characterization of Dyestuffs and Polymers (PAN and CS). S2: FTIR Analysis. S3: SEM Analysis. S4: Nitrogen (N) Content. S5: Yellowness Index and Color Measurements. S6: Tensile strength. S7: Fastness properties. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +40 (0)726371108. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors, equally. 13262

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors are grateful to Mr. Phys. G. E. Grigoriu from the Institute of Macromolecular Chemistry “P. Poni”, Iasi, for the realization and interpretation of FTIR spectra. Also, many thanks to Dr. Prof. C. Munteanu from “Gheorghe Asachi” Technical University for performing the SEM and EDAX analyses.

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