Reactive Polymer-Functionalized Aligned Multiwalled Carbon

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 10328−10340

pubs.acs.org/IECR

Reactive Polymer-Functionalized Aligned Multiwalled Carbon Nanotube Bundles-Induced Porous Poly(ethylene terephthalate) Fibers Li Yuan,* Zehao Wang, Song Chen, Aijuan Gu, Guozheng Liang, and Guoqiang Chen* Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P.R. China Downloaded via BUFFALO STATE on July 20, 2019 at 13:22:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Porous poly(ethylene terephthalate) (PET) fibers were fabricated by the melt-spinning method. Here, reactive polymer-functionalized aligned multiwalled carbon nanotube bundles (RP@ACNTB) were introduced into a spinning system to create nanopores in PET fibers via the diffusion of gaseous products evolved from the decomposition of polymers. PET fibers with RP@ACNTB had outstanding mechanical property due to the CNT alignment and the good compatibility between PET and RP@ACNTB. As compared to PET fibers, PET fibers with 0.3−0.8 wt % RP@ACNTB displayed significantly increased tensile strength and improved thermal stability. Especially, the formation of a porous structure could markedly increase the water absorption of PET/RP@ACNTB fibers from 0.9% to 2.9−5.7%. When the same amount of fillers was applied, PET fibers with RP@ACNTB showed better water absorption and lower mechanical property than PET fibers with pristine ACNTB mainly because of the presence of pores in the PET matrix.

1. INTRODUCTION Poly(ethylene terephthalate) (PET) is a class of semicrystalline and semiaromatic thermoplastic polyesters synthesized from terephthalic acid (HOOC−C6H4−COOH) and ethylene glycol (HO−C2H4−OH). PET has been widely used in the textile industry, packaging, high strength fibers, filtration membrane, automobile parts, biomedical field, etc. because of its characteristics including high strength, transparency, good thermal property, and insolubility in most solvents, good spinnability, flexibility, and dimensional stability,1−3 but the application of PET is limited owing to its poor hydrophilicity, mechanical strength, thermal stability, and inflammable property.4 Many efforts have been made to improve PET’s performance or develop new functions. For example, to enhance the hydrophilicity of PET fibers, hydrophilic groups such as −COOH, −OH, and −NH2 have been introduced to the surfaces of PET fibers using enzymatic modification, electron beam grafting, laser treatment, annealing, and acidification,4−8 but these methods may destroy the molecular structure of PET, resulting in the decrease of physical property. To improve the thermal stability of PET fibers, high heat-resistant fillers such as organomodified clay, carbon nano tube (CNT), fumed silica, calcium carbonate, calcium silicate, and magnesium hydroxide have been added,9−14 but investigation shows that in most cases, a poor interfacial interaction between PET matrix and filler appears, limiting the effective improvement in thermal properties of PET fibers. In addition, fillers modified with PET fibers may have © 2019 American Chemical Society

poorer hydrophilicity than the neat polymer matrix owing to the barrier properties of fillers. So, it is very significant to improve the heat resistance as well as the hydrophilicity of PET fibers without destroying PET molecular structure. Previous studies have indicated that the pore structure can greatly expand the features of fibers including warm retention, water absorption, soft hand, filtering capability, storage and delivery capacity, etc.15,16 A very popular method to generate pores in fibers is to use a highly volatile solvent or binary solvent systems during the spinning process. The commonly used solvent systems include water, N,N′-dimethylformamide (DMF), acetic acid, methanol, tetrahydrofuran (THF), dichloromethane (DCM)/dimethylformamide, DCM/acetone, dimethylacetamide (DMAc)/acetone, DMF/chloroform, methylene chloride/ethanol, DMF/water, THF/DMF, DCM/ butanol, and DCM/hexane.17−27 Furthermore, when highly volatile solvents are used, the applied gas pressure can act as a driving force to expel the polymer solution, forming pores on fiber surface, and with this method mass production of polymeric fibers with pore structure can be facilitated.28 To blend salts with spinning polymer systems is another commonly used method for porous fibers, and salts such as sodium chloride, Received: Revised: Accepted: Published: 10328

January 10, 2019 May 21, 2019 May 28, 2019 May 28, 2019 DOI: 10.1021/acs.iecr.9b00166 Ind. Eng. Chem. Res. 2019, 58, 10328−10340

Article

Industrial & Engineering Chemistry Research Scheme 1. Schematic of the Synthesis Process of RP@ACNTB

absorption, and tensile strength of the resulting PET fibers were also evaluated.

lithium chloride, magnesium chloride, zinc chloride, and aluminum chloride, triethylbenzylammonium chloride, gallium trichloride, calcium carbonate, and sodium bicarbonate can be used.16,29,30 The spun fibers are immersed in salt-selective solvents (e.g., ethyl alcohol, tert-butyl alcohol, acetone, sodium hydroxide solutions, water, and hydrochloric acid) for the removal of salts to obtain porous fibers. Additionally, the bicomponent fibers with phase separation structure can be selectively removed with one of the components in water or solvent (e.g., ethyl alcohol, tert-butyl alcohol, acetone, and sodium hydroxide solutions) to create the porous structure.31 Polymer fibers containing a cryogenic liquid such as liquid nitrogen or colloidal templating can have a porous structure.32,33 Although the porous structure renders the fiber new promising functions, it can bring fibers with new problems such as lower mechanical property and thermal stability. Therefore, it is a great challenge to produce porous fibers with high mechanical property and thermal stability. As we know, the oriented CNT in the polymer matrix can give rise to significant improvements in thermal and mechanical properties in the alignment direction as compared to those with CNTs of random orientation.34,35 Then the highly aligned CNT arrays or bundles may offer an exciting opportunity to improve the integrated property of polymers owing to their aligned CNTs. There are considerable interests in fabricating polymer composites with aligned CNT bundles or arrays. For instance, the chemically modified aligned CNT bundles or arrays have been incorporated into polymers for enhancing the strength, thermal stability, and other properties of polymers.36−41 In the present paper, to improve the hydrophilicity and the mechanical property of PET fibers without sacrificing other excellent properties, the reactive polymers functionalized aligned multiwalled carbon nanotube bundles (RP@ACNTB) were synthesized and used to fabricate porous PET fibers by melt spinning. Because of no addition of solvent and no application of high pressure, the present method for porous fiber is an energy-saving and environmentally friendly technology. Moreover, all raw materials are commercial and inexpensive, and porous fibers can be manufactured at mass production prices by melt spinning machines, which has obvious advantages over the electrospinning technique. In this work, the morphology and structure of PET fibers with RP@ACNTB were systematically investigated, and the thermal property, flame retardancy, water

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial poly(ethylene terephthalate) (PET) chips used in this study were kindly provided by Yingxiang Chemical Fibers Company Ltd. (China). Urea, formaldehyde, and triethanolamine (TEA) were bought from Tianjin Chemical Reagents Factory (China). 2-Ethyl-4methylimidazole (2E4MZ) was obtained from Zhejiang Linhai Kaile Chemical Factory (China). Aligned carbon nanotube bundles (ACNTB) with lengths of 30−100 μm were purchased from Chengdu Organic Chemicals Co. Ltd. (China). Cyanate ester [bisphenol A dicyanate (2,2′-bis(4-cyanatophenyl)isopropylidene, CE] resins were provided from Yang Zhou Techia Chemical Co. Ltd. (China). Surfactant sodium dodecyl sulfate (SDS) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Diglycidyl ether of bisphenol A epoxy resin (EP, epoxy equivalent: 185−190 g/equiv) was purchased from Wuxi Resin Plant (China). 2.2. Functionalization of ACNTB. The steps for functionalized ACNTB were as follows. (1) A mixture of 20 g of urea and 10 g of formaldehyde was added to a 250 mL three-neck round-bottomed flask equipped with a reflux condenser and a mechanical stirrer at 25 °C. After urea was completely dissolved in formaldehyde, the pH value of the solution was adjusted with TEA to 8, the temperature was raised to around 60 °C, keeping the temperature for 1 h, and then the ureaformaldehyde (UF) prepolymer solution was obtained. (2) 100 mL of SDS solution with a concentration of 1 wt % and 10 g of ACNTB were added to a UF prepolymer solution, and the pH value of the solution was slowly adjusted with 10 wt % H2SO4 solution to 2 under stirring, raising the temperature to 60 °C. After 3 h, the reaction was ended, the suspension was washed with deionized water and acetone several times, filtered, and dried at room temperature for 24 h, and poly(urea-formaldehyde) coated ACNTB (PUF@ACNTB) powders were obtained. The yield of PUF@ACNTB is about 90%, which was the ratio of the mass of products to the total mass of urea, formaldehyde, and ACNTB. The schematic of the 10329

DOI: 10.1021/acs.iecr.9b00166 Ind. Eng. Chem. Res. 2019, 58, 10328−10340

Article

Industrial & Engineering Chemistry Research

The morphology was analyzed by using a light microscope (LM, Keyence, VHX-1000) and a scanning electric microscope (SEM, HITACHI, S-4700). The tensile strength of the fiber was measured according to the testing method for tensile properties of man-made staple fibers (GB/T14337-2008) using an electronic single fiber strength meter (YG001N, China) at an extension rate of 10 mm/min. 10−15 samples for each formulation were tested. The flame retardancy was evaluated with a microscale combustion calorimeter (MCC, FTT0001, UK). The sample was heated to a specified temperature at a heating rate of 1 °C/s in a mixed flow atmosphere (nitrogen 80% and oxygen 20%). The temperature range was 75−750 °C. Three samples for each formulation were tested. Limiting oxygen index (LOI) of fiber was evaluated according to the test method of flame retardancy of the polyester fiber (FZ/T 50017-2011). A twisted yarn (0.30 ± 0.03g) with a twisting number of 60 was prepared for the test. Dynamic mechanical analysis (DMA) was performed on a TA Instruments Q800 in tensile mode between 30−110 °C using a heating rate of 3 °C/min at 1 Hz. The glass transition temperature (Tg) was determined from the peak temperature in the tan δ-temperature plot. Thermogravimetric analysis (TGA) was performed on a TA Instrument Discover TGA from 30−800 °C at a heating rate of 10 °C/min using nitrogen flowing at 50 mL/min. Differential scanning calorimetry (DSC) experiments were performed (Q200, TA Instruments) between 30 and 350 °C at a heating rate of 10 °C/min in a nitrogen atmosphere. The degree of crystallinity (Xc) values for PET fibers with fillers were calculated based on the DSC data according to eq 110

synthesis process of PUF@ACNTB is shown in Scheme 1. (3) 30 g of PUF@ACNTB powders was dispersed in 800 mL of ethanol solution, then 10 g of CE, 5 g of EP, and 0.25 g of 2E4MZ were added, and subsequently the temperature of the solution was heated to 70 °C. After 6−7 h, the reaction was ended, CE and EP polymers were coated on the surface of PUF@ACNTB, and the resulting particle was designated as RP@ACNTB. Owing to the low reaction temperature for CE and EP resins, CE and EP polymers were still reactive for the residual reactive cyanate ester (−OCN) and epoxy groups (Scheme 1), and then ACNTB was functionalized with reactive polymers. To control the residual reactive −OCN and epoxy groups, the formation of RP@ACNTB particles and the reaction solution were monitored using FTIR at intervals. The changes of the absorption peak intensities of −OCN between 2370 and 2170 cm−1 and epoxy groups at 910 cm−1 could be analyzed by internal standard method of FTIR. Before each test, an equal amount of homogeneous solution was taken, and then the solid phase was separated from the solution. The solid phase and the components in the residual solvent after drying were analyzed using FTIR, respectively. The residual reactive −OCN or epoxy groups could be estimated from the absorption peak area ratio of −OCN or epoxy groups of RP@ACNTB particles at any given time to −OCN or epoxy groups of the initial reaction solution. The RP@ ACNTB particles were washed several times with acetone and dried before use. Figure S1 shows the possible reactions during the synthesis process of RP@ ACNTB.42−44 Figure S2 shows the representative FTIR spectra of RP@ACNTB particles and the components in the residual solvent at different reaction times. In this work, the relative contents of −OCN and epoxy groups for RP@ACNTB are 10% and 3%, respectively.

Xc =

ΔHm f ΔHm0

× 100 (1)

where ΔH0m was the heat of fusion of 100% crystalline PET 0 (ΔHm = 140 J/g), and f was the weight fraction of the PET polymer.45 The specific surface area (SSA) was analyzed by the Brunauer−Emmett−Teller (BET) method (ASAP 2020M) using nitrogen (N2) adsorption at −196 °C, and the pore size distribution was estimated by the Barrett−Joyner−Halenda (BJH) method. Pore volumes were estimated from the uptake of N2 at a relative pressure of 0.99 (P/P0). The porosity can be estimated from the apparent volume (V0) and absolute dense volume (V) of the sample according to eq 2. Here, the V0 of the sample was measured in distilled water at room temperature using a balance (FA1104J, 0.0001 g, China) with a density kit. The V could be obtained using the same method for measuring V0 after the pores in the same sample were eliminated by compression using a hot press at 20 MPa and 230 °C for 1−2 min.

2.3. Fabrication of Fiber via the Melt Spinning Method. The homogeneous mixture of dried PET chips and RP@ACNTB powder was put in feeding throats of an FDY spinning instrument, and fibers could be produced in an extruder via the melt spinning method. The temperatures of the screw extruder were set as 265, 275, 283, and 282 °C, respectively, and the drawing temperature of the fiber was 50−80 °C. The weight ratios of RP@ACNTB to PET were 0.3, 0.5, and 0.8 wt %, respectively, and correspondingly, the resulting PET fibers were designated as PET/0.3RP@ACNTB, PET/0.5RP@ACNTB, and PET/0.8RP@ACNTB, respectively. For comparison, PET fibers with 0.8 wt % pristine ACNTB (designated as PET/0.8ACNTB) were produced using the same procedure. 2.4. Characterization. The structure of the sample was characterized using an X-ray diffractometer (XRD, X’Pert-Pro MPD). The range of the test was 2θ = 5−80°, and the step size was 0.026°. Attenuated total reflection Fourier transform infrared (ATRFTIR) spectra were obtained using a Bruker Vertex 70 spectrometer (Germany) at room temperature. The spectra were collected at 4 cm−1 resolution over the range of 4000−400 cm−1 in absorbance mode. Before scanning each sample, the background spectrum was taken with an empty ATR crystal and was subtracted from the sample spectrum using the software.

P=

V0 − V × 100% V0

(2)

Water absorption of the fiber was evaluated by immersing the fiber in deionized water as follows. The dried fibers were weighed (m0) and immersed in deionized water for 24 h, then dehydrated, and dried in a centrifuge at 1000 r/min for 5 min. After having written down the weight of the fiber (m), the water absorption rate (R) of the fibers was calculated according to eq 3. 10330


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Figure 1. FTIR spectra (a) and XRD patterns (b) of ACNTB, PUF, PUF@ACNTB, and RP@ACNTB.

Figure 2. DSC (a) and TGA (b) curves of ACNTB, PUF, PUF@ACNTB, and RP@ACNTB.

R=

m − m0 × 100% m0

of C−OH at 1029 cm−1 can be observed in the FTIR curve of PUF.49 As compared to PUF and ACNTB, PUF@ACNTB shows the absorption peaks at 3360, 1637, and 1546 cm−1, implying that PUF polymers deposit on ACNTB. The FTIR spectrum of RP@ACNTB not only displays the absorption peaks of PUF at 3360, 1637, and 1546 cm−1 but also shows the stretching vibration absorption peaks of −OCN at 2170−2370 cm−1 and epoxy groups at 910 cm−1,50 respectively, indicating the introduction of CE/EP polymers to PUF@ACNTB and the potential reactivity of RP@ACNTB owing to the existence of −OCN and the epoxy group. Because the characteristic absorption peaks of CE and EP polymers, which occur at 1510−1600 cm−1 (CC stretching vibration in phenyl ring) and 1564−1637 cm−1 (CN stretching vibration in triazine ring),42 can be overlapped by the characteristic absorption peaks of −CO−NH− in PUF, it is difficult to distinguish them from the FTIR spectrum of RP@ACNTB. In addition, the new absorption peaks appear at above 3750 cm−1 in the FTIR spectrum of RP@ACNTB and should be attributed to the hydrogen-bonded OH-stretching vibrations.51 To further confirm the structure of RP@ACNTB, XRD experiments on ACNTB, PUF, PUF@ACNTB, and RP@ ACNTB were performed (Figure 1b). The XRD pattern of PUF displays a diffraction peak at 2θ = 22.3°, 24.6°, and 31.2° caused by the existence of a crystallite structure.52 For ACNTB, the XRD pattern shows diffraction peaks at 2θ = 26.0° and 43.2° corresponding to the interlayer spacing of the nanotube and the

(3)

Dynamic contact angle (CA) was analyzed using a dynamic contact angle meter and tensiometer (DCAT 21, Dataphysics Instruments, Germany) according to the Wilhelmy method. Fiber was dipped into the deionized water at a speed of 3 mm/ min and an immersion depth of 5 mm. Two specimens for each formulation were tested at 23 ± 2 °C. The advancing angle (θadv) and receding angle (θrec) can be evaluated based on the wetting curve during the advancing and receding cycle using SCAT software. The average CA (θav) was calculated according to eq 4.46 cos θav = 0.5 cos θadv + 0.5 cos θrec

(4)

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology of RP@ACNTB. Figure 1 shows the FTIR spectra and XRD patterns of ACNTB, PUF, PUF@ACNTB, and RP@ACNTB. No significant absorption peaks occur in the FTIR curve of ACNTB except for the stretching vibration absorption peak of −OH in sorbed water and water vapor at about 3700 cm−1 (Figure 1a).47,48 The absorption peaks of the stretching vibrations of −OH, −NH, and −NH2 at around 3360 cm−1, the stretching vibrations of CO at 1637 cm−1 and C−N at 1546 cm−1 in −NH−CO− NH− and −CO−NH−, the bending vibrations of −CH3 at 1378 cm−1 and N−H at 1242 cm−1, and the stretching vibration 10331


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ACNTB, and RP@ACNTB is 697, 58, 189, and 226 °C, respectively. RP@ACNTB shows higher Tdi and residual carbon yield (Yc) than PUF@ACNTB owing to the better thermal properties of aromatic ring-containing CE-EP/PUF polymers than PUF (Table 1 and Figure S3). The decomposition temperatures (Tdmax1) at the maximum weight loss rate for PUF@ACNTB and RP@ACNTB are 269 and 270 °C, respectively, which are close or equal to the Tdmax1 of PUF. Owing to the decomposition of CE and EP polymers, a new weak peak occurs at 425 °C (Tdmax2) in a derivative thermogravimetric curve of RP@ACNTB as compared to that of PUF@ACNTB. Based on the Yc values of ACNTB, PUF, PUF@ACNTB, RP@ACNTB, and CE-EP/PUF polymers at 800 °C (Table 1 and Figure S3), the calculated relative content of ACNTB (MACNTB) in RP@ACNTB is about 33%. To further confirm the structure of PUF@ACNTB, the SEM images of ACNTB, PUF@ACNTB, and RP@ACNTB are provided as shown in Figure 3. The pristine ACNTB shows many tiny interspaces between each CNT. For PUF@ACNTB, it can be observed that PUF polymers fill the interspaces and coat the surface of CNTs. After the introduction of CE and EP polymers, more polymers lie on the surface of ACNTB as shown in Figure 3c′, mainly owing to the deposition of polymers formed by the reactions of CE and EP with 2E4MZ (Figure S1). The morphology of RP@ACNTB is different from that of polymers-functionalized nonaligned multiwalled carbon nanotubes (RP@MWCNT) (Figure S5) owing to the partial alignment of the CNTs in ACNTB. 3.2. Structure and Morphology of PET/RP@ACNTB Fiber. Figure 4 shows the LM and SEM images of PET fibers with ACNTB and different contents of RP@ACNTB. The color of PET/RP@ACNTB fiber gradually changes from white to gray and then to black with increasing RP@ACNTB content. When the same contents of ACNTB and RP@ACNTB are applied, PET/RP@ACNTB fibers show a lighter color than PET/ ACNTB fiber because black ACNTB is coated by a layer of polymers. All PET fibers have a diameter of about 50 μm and similar surface morphologies, but the cross sections of PET fibers with ACNTB and RP@ACNTB are quite different. The introductions of ACNTB and RP@ACNTB can increase the

reflection of the carbon atoms, respectively.53 In the case of PUF@ACNTB, the diffraction peaks at 2θ = 22.3°, 26.0°, 31.2°, and 43.2° can be observed in an XRD pattern. The XRD pattern of RP@ACNTB is similar to that of PUF@ACNTB mainly because of the physical deposition of amorphous CE and EP polymers on PUF@ACNTB. Figure 2a shows the DSC curves of ACNTB, PUF, PUF@ ACNTB, and RP@ACNTB. For all samples, the endothermic peaks below 150 °C in the DSC curves result from the volatilization of small molecules such as water or formaldehyde. In the case of PUF, the endothermic peaks above 250 °C are mainly caused by the thermal decomposition of PUF. The endothermic peaks above 250 °C for PUF@ACNTB should be attributed to the thermal decomposition of PUF by comparing the TGA results of PUF and ACNTB. RP@ACNTB shows a significant endothermic peak at about 273 °C that is higher than the lowest decomposition temperature (about 250 °C) of PUF@ACNTB; the main reason is the fact that CE-EP/PUF polymers have better thermal properties than PUF (Figure S3). In addition, owing to the possible reactions of residual −OCN/ epoxy groups with −OH and −NH− (Figure S4) during the heating process, a very weak and broad exothermic peak between 160 and 190 °C can be observed in the DSC curve of RP@ACNTB, which can indicate the reactivity of RP@ ACNTB. Figure 2b shows the TGA curves of ACNTB, PUF, PUF@ ACNTB, and RP@ACNTB, and Table 1 summarizes the Table 1. Characteristic Temperatures of ACNTB, PUF, PUF@ACNTB, and RP@ACNTB sample

Tdi (°C)

Tdmax1 (°C)

ACNTB PUF PUF@ACNTB RP@ ACNTB

697 58 189 226

269 269 270

Tdmax2 (°C)

Yc (%)

425

92.2 13.2 40.8 43.8

characteristic temperatures of ACNTB, PUF, PUF@ACNTB, and RP@ACNTB. The initial thermal decomposition temperature (Tdi) at 5 wt % weight loss of ACNTB, PUF, PUF@

Figure 3. SEM images of ACNTB (a, a′), PUF@ACNTB (b, b′), and RP@ACNTB (c, c′). 10332


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Figure 4. LM and SEM images of surface and cross section of PET fibers with ACNTB and RP@ACNTB: PET (a, a′), PET/0.8ACNTB (b, b′), PET/ 0.3RP@ACNTB (c, c′), PET/0.5RP@ACNTB (d, d′), PET/0.8RP@ACNTB (e, e′), and pore size distribution curves and porosity (f).

Scheme 2. Schematic Representation of the Formation of Pore Structures within the PET Fiber during the Melt Spinning Process

fracture surface roughness of PET fiber, and PET/RP@ACNTB fibers show an apparent pore structure as compared to PET fiber with or without ACNTB. Those pores are radially distributed around ACNTB, which is completely different from the solvent induced pores that are on both the outer and inner surfaces of fibers.17−20 The formation of the pore structure within PET/ RP@ACNTB is mainly attributed to the decomposition of PUF polymers in RP@ACNTB. As we know, PUF can decompose above 250 °C (Figure 2), and the spinning temperature range is 265−282 °C; then during the spinning process of fiber, PUF in RP@ACNTB can decompose and produce gases, which can diffuse from the inside to the outside of the polymer matrix and

leave the traces within PET fibers, forming the pores around ACNTB. Scheme 2 illustrates the formation mechanism of pore structure within PET fiber. The residual reactive −OCN and epoxy groups may react with −NH in PUF during the fabricating PET fiber, and the formed products have high thermal stability (Figures S3 and S4), which should depress the decomposition of PUF and stop the formation of pores; but the phenomena can be neglected since it happens at the interface between CE/epoxy polymers and PUF. During the fabricating process of PET fiber, the pore structure is only induced by the decomposition of PUF. During the fabrication of PET fiber, the CE and EP polymers on the ACNTB are like a wall that can protect the alignment of 10333


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PET fiber (Figure S4).56,57 Although the changes of the stretching vibration absorption peaks of −OH and −COOH and possible groups such as the oxazolidinone ring at 1752 cm−1 and the triazine ring at 1564 and 1367 cm−1 in the FTIR spectrum of RP@ACNTB are difficult to detect because those peaks have weak intensity and can be overlapped by the characteristic absorption peaks of PUF, the reactions between the compounds containing −OH or −COOH in PET fiber and −OCN/epoxy groups in RP@ACNTB can be indicated by a new weak exothermic peak at 316 °C in the DSC curve of PET powders with 20 wt % RP@ACNTB, when compared to PET and RP@ACNTB (Figure S7). 3.3. Thermal Behavior of PET/RP@ACNTB Fiber. Figure 6 shows the DSC curves of PET fibers with ACNTB and

the carbon nanotube bundles. As we know, CE and EP polymers have high thermal stability, and they are thermally stable during the fabrication of the fibers as indicated by Figure S3, thus they can maintain the structure of ACNTB. Also, the possible reactions between −OCN/epoxy groups and −OH/−NH groups can strengthen the protection of the structure of ACNTB (Figure S4). Nanometer pore structures can be observed from Figures 4c′−e′. The porosities of PET/0.3RP@ ACNTB, PET/0.5RP@ACNTB, and PET/0.8RP@ACNTB are 9%, 16%, and 27%, respectively. Increasing the content of RP@ACNTB may create more pores within PET fibers. The SSA values of PET/0.3RP@ACNTB, PET/0.5RP@ACNTB, and PET/0.8RP@ACNTB are 6.3, 21.5, and 56.7 m2/g, respectively. The pore sizes of PET/RP@ACNTB fibers range from 1.9 nm to about 750 nm as shown in Figure 4f, and the average pore size increases with the content of RP@ACNTB. Interestingly, the pore size is close to that of electrospun porous fibers from volatile solvent mixtures.18,19,23 The PET fibers with 0.8% ACNTB and 0.8% RP@ACNTB have similar surface morphology to PET/0.8WMCNT and PET/0.8RP@ MWCNT (Figure S6), respectively. Owing to the higher degree of CNT orientation in ACNTB than in WMCNT, more aligned CNTs in ACNTB are closely packed together, and then the distance between aligned CNTs in PET/0.8ACNTB is shorter than that in PET/0.8WMCNT as indicated by the cross section of PET fibers (Figure 4b′ and Figure S6a′). Figure 5 shows the FTIR spectra of PET fibers with ACNTB and RP@ACNTB. PET fibers with and without ACNTB display

Figure 6. DSC curves of PET fibers with ACNTB and different contents of RP@ACNTB.

different contents of RP@ACNTB. All DSC curves display an exothermic peak at 119−127 °C attributed to cold crystallization temperature (Tcc) and the endothermic peaks at 256 and 258 °C due to the melting of PET. The additions of ACNTB and RP@ACNTB basically do not affect the melting temperature of PET fiber possibly owing to the very low content of filler; similar phenomena occur in PET powder systems with ACNTB and RP@ACNTB (Figure S8). However, they can decrease Tcc values of PET fibers because ACNTB and RP@ACNTB can act as prealigned nucleation sites,58,59 and then the crystallization rate of PET is accelerated by the addition of the filler, which also can be proved by the higher crystallization temperature of PET powders with ACNTB and RP@ACNTB during cooling (Figure S8). PET/0.8RP@ACNTB fiber shows lower Tcc than PET/0.8ACNTB when the same amount of filler is applied, which is attributed to the good compatibility between RP@ ACNTB and PET as indicated by the weak exothermic peak at 316 °C in the DSC curve of the PET powder with 20 wt % RP@ ACNTB (Figure S7). The finding indicates that the initial crystalline structure easily develops for PET/[email protected] The calculated Xc values of PET fibers with ACNTB and RP@ ACNTB from DSC heating scans are listed in Table 2. ACNTB and RP@ACNTB can increase the Xc value of PET. PET/ 0.8RP@ACNTB fiber has a higher Xc value than PET/ 0.8ACNTB fiber, which proves that RP@ACNTB can effectively increase the crystalline structure of PET fiber. Although the addition of RP@ACNTB can improve the Xc of PET, the pore structure around ACNTB can weaken the interaction between PET matrix and ACNTB in fibers, which

Figure 5. FTIR spectra of PET fibers with ACNTB and different contents of RP@ACNTB.

the stretching vibration absorption peaks of −OH in adsorbed bound water, residual diethylene glycol, or carboxylic end groups at 3400−3700 cm−1, CO at 1721 cm−1, C−O−C at 1248 and 1090 cm−1, C−O in primary alcohol at 1018 cm−1, benzene ring at 1500 and 1450 cm−1, and C−H at 2972 cm−1.7,8,52−56 For PET/RP@ACNTB fibers, the FTIR spectrum not only shows all characteristic absorption peak of PET but also displays the stretching vibration absorption peaks of −NH2− and −NH− at 3260−3350 cm−1 and −CO−NH− at 1637 cm−1, which indicate that function groups such as −NH−, −NH2−, and −CO−NH− have been introduced to PET fiber. It must be mentioned that PET contains a small number of compounds containing −OH or −COOH, which may react with −OCN/ epoxy groups in RP@ACNTB during the spinning process of 10334


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436 °C. The main weight loss stage of PET between 350 and 600 °C is ascribed to the structural decomposition of the polymers. The addition of ACNTB can slightly increase the Tdi and Tdmax of PET fibers to 397 and 439 °C, respectively. The introduction of RP@ACNTB can also improve the Tdi and Tdmax of PET fibers, which are 7−21 °C and 2−18 °C higher than those of PET fiber, respectively. However, PET/RP@ACNTB fibers do not show sustainable enhancement in Tdi and Tdmax with increasing RP@ACNTB content, and this phenomenon is attributed to the increased pores in PET/RP@ACNTB. The increased pores can reduce the thermal stability for the easy heat diffusion within fibers. In this present work, PET fiber with 0.5 wt % RP@ACNTB has the optimum thermal stability. As compared to PET/0.8ACNTB fiber, PET/0.8RP@ACNTB fiber shows higher Tdi and Tdmax although the same contents of fillers are used. Figure 8 shows the FTIR spectra of the gas products evolved from the decomposition of fibers between 50 and 800 °C. PET can decompose by a random scission of the ester links involving a six centered cyclic transition state to give a vinyl ester and carboxylic acid. Subsequently these primary products of pyrolysis can undergo secondary processes to give a wide variety of products such as CO, CO2, acetaldehyde, aromatic acids, and their vinyl esters.61 The FTIR spectra of the gas products display significant characteristic peaks of the OHstretching vibration in water and ethanol at 3570−3900 cm−1, C−H stretching vibration in vinyl esters at 3000 and 2735 cm−1, CO stretching and deformation vibrations at 2326 and 667 cm−1 in CO2, and CO stretching vibration in vinyl esters and acetaldehyde at 1754 cm−1.61 The additions of ACNTB and RP@ACNTB can decrease the intensity of the absorbance peak of vinyl esters and acetaldehyde at about 1754 cm−1 as indicated by the FTIR spectra of the gas

Table 2. Xc Values of PET Fibers with ACNTB and RP@ ACNTB sample

ΔHm (J/g)

Xc (%)

PET PET/0.8ACNTB PET/0.3RP@ACNTB PET/0.5RP@ACNTB PET/0.8RP@ACNTB

18.3 21.0 27.7 20.9 26.2

13.1 15.1 19.8 15.0 18.9

decreases Xc of PET, and then increasing RP@ACNTB content may not continuously increase Xc. Therefore, PET/0.8RP@ ACNTB has lower Xc than PET/0.3RP@ACNTB. TGA curves of PET fibers with ACNTB and RP@ACNTB are shown in Figure 7. PET fiber has Tdi of 390 °C and Tdmax of

Figure 7. TGA curves of PET fibers with ACNTB and different contents of RP@ACNTB.

Figure 8. FTIR spectra of gaseous products evolved from the decomposed PET fibers with ACNTB and RP@ACNTB during the heating process: PET (a), PET/0.8ACNTB (b), [email protected]@ACNTB (c), [email protected]@ACNTB (d), and [email protected]@ACNTB (e). 10335


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Figure 9. DMA curves of PET fibers with ACNTB and different contents of RP@ACNTB: (a) storage modulus and (b) tan δ.

products evolved from PET fibers with ACNTB and RP@ ACNTB, which demonstrates that the addition of ACNTB and RP@ACNTB may retard the initial decomposition process of PET. Additionally, for PET fiber with higher RP@ACNTB content, the temperature at the maximum intensities of absorption peaks of gas products appears at about 455 °C, which is higher than that (about 436 °C) of gas products evolved from PET fiber with and without ACNTB. The intensity of the absorbance peak of CO2 at 2326 cm−1 for PET fiber with RP@ ACNTB changes irregularly with the content of RP@ACNTB, which is mainly attributed to the combined effects of the increased pores and RP@ACNTB content. In a word, PET fibers with higher content of RP@ACNTB can have better thermal stability than PET fibers; the reason can be the good compatibility between PET and RP@ACNTB and the high thermal stability of RP@ACNTB. Additionally, the content of small molecules such as residual alcohols and carboxylic acid in PET fibers can reduce owing to the reaction between −OH in PET and −OCN/epoxy groups in RP@ ACNTB, which can improve the thermal properties of PET fibers. The addition of ACNTB and RP@ACNTB has significant impact on the storage modulus of PET fiber (Figure 9). Since CNT has high stiffness, the addition of ACNTB can improve the storage modulus of PET fibers. However, RP@ACNTB can basically maintain or even reduce the storage modulus of PET fibers. As we know, the porous structure appears within polymers in favor of the motion of polymer molecular chains, which can result in the decrease of the storage modulus of polymers. Therefore, PET/RP@ACNTB fibers with pore structure do not exhibit significantly increased storage modulus. The addition of ACNTB can slightly improve the glass transition temperature (Tg) of PET fiber because of the inhibition role of ACNTB on the motion of the polymer chain of PET and the improved Xc value of PET. For the same reason, PET/RP@ ACNTB fibers have Tg close to PET despite the formation of the pore structure within PET. From Figure S9, it can be observed that PET/0.8ACNTB and PET/0.8RP@ACNTB have similar thermal properties and Tg values to PET/MWCNT and PET/ 0.8RP@MWCNT, respectively. It seems that the orientation state of CNTs has no significant influence on the thermal property of PET fibers. 3.4. Mechanical Property of PET/RP@ACNTB Fibers. Figure 10 shows the tensile strength and elongation at a break of PET fibers with ACNTB and RP@ACNTB. The addition of

Figure 10. Tensile strength and elongation at break of PET fibers with ACNTB and RP@ACNTB.

ACNTB and RP@ACNTB increases the tensile strength of PET fibers from 3.3 cN·dtex−1 to 4.2−8.3 cN·dtex−1 and reduces the elongation at break of PET fibers from 260% to 146−210%. As the content of RP@ACNTB increases, the tensile strength of PET fibers increases first and then gradually declines. The improved mechanical properties of PET/RP@ACNTB fibers can be attributed to the facts as follows. First, ACNTB has high mechanical strength, and the pullout and fracture of the aligned CNTs consume more energy as shown in Figures 4b′−4e′, leading to improvement of the mechanical property. Second, PET and RP@ACNTB have a good compatibility that can effectively transfer the stress, improving the tensile strength of PET.62 Third, due to the nucleating agent effects of RP@ ACNTB (Table 2), PET molecular chain orientation around RP@ACNTB can increase the tensile strength of PET fibers. Although RP@ACNTB can improve the strength of PET fiber, increasing RP@ACNTB content can create more pore structures that decrease the mechanical strength of PET fiber mainly owing to the stress concentration phenomenon. Therefore, PET fibers with higher RP@ACNTB content have reduced tensile strength and the elongation at break. Here, 0.3 wt % RP@ACNTB can realize the highest tensile strength that is 152% higher than that of PET fibers. Owing to the presence of pore structure, PET/0.8RP@ACNTB has lower tensile strength and elongation at break than PET/0.8ACNTB fibers although the same amounts of fillers are used. PET/0.8ACNTB and PET/ 10336


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−CO−NH− in RP@ACNTB have been introduced to PET fiber, leading to the increase of water absorption. Second, the pores can make it easier for water to diffuse in fibers,7 thus increasing the water absorption of the fibers. For the same reason, PET/0.8RP@ACNTB fiber shows higher water absorption than PET/0.8ACNTB fiber. Compared with PET/ 0.8MWCNT and PET/0.8PR@MWCNT (Figure S11), PET/ 0.8ACNTB and PET/0.8RP@ACNTB have higher water absorption for the lower interface areas of the ACNTB/PET matrix. In this work, the higher water absorption of PET fibers with RP@ACNTB can indicate better hydrophilic properties and good dyeability. As shown by Figure S12, as compared with the dyed PET fiber, the dyed PET fibers with RP@ACNTB experience lower weight loss after washing owing to the loosely adsorbed dye loss, and they also possess higher color strength. After five washing cycles, the washing fastness of the dyed PET fibers with RP@ACNTB still can reach grade 5. Pictures show that the fluorescent disperse dyes dyed fibers with RP@ACNTB can glow under the irradiation of UV light (Figure S13). The change of surface wetting behavior of fibers is evaluated by the average dynamic contact angle (θav), which can be obtained according to the wetting curves during the advancing and receding cycle for PET fibers with ACNTB and different contents of RP@ACNTB (Figure S14). Although ACNTB is hydrophobic and the hydrophilic −NH−, C−O−C, and −CO− NH− are present in RP@ACNTB, the addition of ACNTB and RP@ACNTB has no significant influence on the θav values (64.9 ± 24°) of PET fibers. The reason is the fact that ACNTB and RP@ACNTB are coated with the PET matrix (Figures 4b−4e), and then PET fibers with ACNTB and RP@ACNTB have surface-wetting behavior similar to PET fiber. Due to the above reason, PET/0.8MWCNT and PET/0.8PR@MWCNT have similar θav values with PET/0.8ACNTB and PET/0.8PR@ ACNTB, respectively (Figure S11). 3.6. Flame Retardancy. Figure 12 shows the MCC curves and LOI values of PET fibers with ACNTB and RP@ACNTB. The values of microcalorimetric correlation data including heat release capability (HRC), peak heat release rate (PHRR), total heat release (THR), the temperature at PHRR (TPHRR), and the yield of char residual (Y′c) are listed in Table 3. 0.8 wt % ACNTB can lead to an 11.2%, 11.0%, and 4.2% decrease in HRC, PHRR, and THR of PET fibers, respectively, and a 47.7% increase in Y′c of PET fibers. The addition of RP@ACNTB also can decrease the HRC, PHRR, and THR of PET fibers that are

0.8RP@ACNTB can show much better tensile strength than PET/0.8MWCNT and PET/0.8RP@MWCNT owing to the aligned CNTs (Figure S10). It also can be found that PET/ 0.8MWCNT shows lower tensile strength than PET fiber; the main reason is the fact that CNTs are disorderly distributed in the PET matrix, the orientations of some CNTs may be perpendicular to the axial direction of the fiber, and the weak interfaces formed between those CNTs and PET can significantly reduce the tensile strength of PET fiber. In addition, the possible formation of aggregated MWCNT in PET fiber can decrease the tensile strength of PET fiber. 3.5. Water Absorption of PET/RP@ACNTB Fibers. Figure 11 shows the water absorption and the average dynamic

Figure 11. θav and water absorption and of PET fibers with ACNTB and RP@ACNTB.

contact angle (θav) of PET fibers with ACNTB and different contents of RP@ACNTB. The additions of ACNTB and RP@ ACNTB can change the water absorption of PET fibers. PET/ ACNTB fiber has higher water absorption than PET fiber, which is attributed to the weak interface interaction between PET and CNT that makes water easily permeate through the interface. RP@ACNTB can significantly increase the water absorption of PET fiber. The water absorption of PET fibers with 0.3, 0.5, and 0.8 wt % RP@ACNTB is 2.9−5.7%, which is much higher than that of PET fiber (about 0.9%). The improved water absorption of PET/RP@ACNTB fibers can be attributed to the facts as follows. First, hydrophilic groups such as −NH, C−O−C, and

Figure 12. MCC curves (a) and LOI values (b) of PET fibers with ACNTB and RP@ACNTB. 10337


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Industrial & Engineering Chemistry Research Table 3. MCC Data of PET Fibers with ACNTB and RP@ACNTB sample

HRC (J/g·K)

PHRR (W·g−1)

THR (KJ/g)

TPHRR (°C)

Y′c (%)

PET PET/0.8ACNTB PET/0.3RP@ACNTB PET/0.5RP @ACNTB PET/0.8RP@ACNTB RP@ACNTB ACNTB

446 396 420 400 401 115 11

400.9 356.7 385.6 361.7 365.7 103.7 4.249

19 18.2 17.9 17.7 18.4 10 2

432.8 434.7 431.2 431.7 431.6 268.5 88.92

13.27 19.6 13.8 13.5 19 48.8 92.8

Figure 13. Digital images of PET fibers with ACNTB and RP@ACNTB after combustion: PET (a), PET/0.8ACNTB (b), [email protected]@ACNTB (c), [email protected]@ACNTB (d), and [email protected]@ACNTB (e).

4. CONCLUSIONS The reactive RP@ACNTB particles were successfully prepared and applied to PET to fabricate porous fibers by melt blending spinning. During the fabrication process of PET/RP@ACNTB fiber, RP@ACNTB could partly decompose and produce gaseous products, which diffused in the spinning system and left pores in PET fibers. Because of the pullout and fracture behavior of the alignment of CNTs in RP@ACNTB in PET fibers and the good compatibility between the PET matrix and RP@ACNTB, the addition of RP@ACNTB could significantly improve the tensile strength of PET fiber. When the content of RP@ACNTB was 0.3 wt %, PET/RP@ACNTB fiber had the highest tensile strength that was 152% higher than PET fiber. The appropriate addition of RP@ACNTB could slightly increase the thermal property of PET fiber mainly for the excellent thermal property of RP@ACNTB. The RP@ACNTB could consume heat and increase the viscosity of a melting droplet of PET and the residue char yields, which protected the PET matrix and enhanced the flame retardancy of PET fiber. It was easier to obtain high water absorption for PET/RP@ ACNTB fiber mainly for the presence of the porous structure. High water absorption of 2.9−5.7% for PET fiber was obtained by adding 0.3−0.8 wt % RP@ACNTB. However, the introduction of RP@ACNTB had no significant influence on the surface wetting behavior of PET fiber. In general, high performance PET/RP@ACNTB fibers with pore structures were successfully fabricated directly by melt spinning without involving the use of solvent, which was obviously different from the traditional solvent etching methods for producing porous fibers by a two-step process. Additionally, the resulting PET fibers had the potential to develop functions with good dyeability and warm retention with the presence of pore structure.

5.8−10.1%, 3.8−8.8%, and 5.8−3.2% lower than those of PET fibers, respectively. Whereas the Y′c of PET fiber with RP@ ACNTB increases by 9.3−43.2%, as compared to PET fiber. However, ACNTB and RP@ACNTB have no significant influence on the temperature at PHRR (TPHRR) of PET fiber. The addition of ACNTB and RP@ACNTB can increase the LOI value of PET fiber from 19% to 24% and 22−26%, respectively. Evidently, ACNTB and RP@ACNTB can improve the flame retardancy of PET fibers. The enhancement of flame retardancy for PET fibers with ACNTB and RP@ACNTB can be explained as follows. First, ACNTB and RP@ACNTB have high thermal properties and can take away a part of the heat to slow the combustion process. Second, ACNTB and RP@ACNTB can increase the viscosity of the melting droplet and reduce the melting droplet phenomenon of PET, and they also can be exposed during the combustion process and cover the combustion surface of PET, which can be indicated by the larger region of char residue left on the surface of PET fibers with ACNTB and RP@ACNTB after combustion (Figure 13); then they can act as thermal insulation barriers to protect PET, resulting in the retardation of the thermal decomposition of PET fiber. Third, the presence of ACNTB and RP@ACNTB promotes the char formation of fiber as implied by the higher Y′c in Table 3, thus reducing the heat transfer between the heat source and the polymer surface and limiting the diffusion of oxygen into the PET matrix, consequently enhancing the flame retardancy of PET fibers.62 The flame retardancy of PET/RP@ACNTB fibers gradually increases with RP@ACNTB content. For all PET/RP@ ACNTB fibers, PET/0.8RP@ACNTB fiber shows relatively lower HRC, PHRR, THR, and the highest LOI values. However, PET/0.8RP@ACNTB fiber does not exhibit better flame retardancy than PET/0.8ACNTB mainly due to the presence of pores in PET/RP@ACNTB fibers. Owing to the lower interface areas between ACNTB and PET matrix, PET/ 0.8ACNTB and PET/RP@ACNTB display lower flame retardancy than PET/0.8MWCNT and PET/0.8RP@ MWCNT (Figure S15 and Table S2).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b00166. Additional possible reactions during synthesis process of RP@ACNTB; FTIR spectra of RP@ACNTB particles 10338


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and components in residual solvent at different reaction times; TGA and DTG curves of CE-EP/PUF; possible reactions of residual cyanate ester/epoxy groups with −OH and −NH− groups; SEM images of MWCNT and RP@MWCNT; LM and SEM images of surface and cross section of PET fibers; DSC curves of PET fibers, RP@ ACNTB, and PET powders; heating and cooling DSC curves of PET powders; TGA and DMA curves of PET fibers; tensile strength and elongation at break of PET fibers; θav and water absorption and of PET fibers; weight loss and color strength of dyed fiber after washing; washing fastness for dyed fibers; pictures of fibers dyed with red fluorescent disperse dyes under irradiation of UV light; wetting curves during advancing and receding cycle for PET fibers; MCC data of PET fibers (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Phone: +86 512 65880967. Fax: +86 512 65880089. E-mail: [email protected] (L.Y.). *E-mail: [email protected] (G.C.). ORCID

Li Yuan: 0000-0003-2059-5235 Aijuan Gu: 0000-0002-2235-1018 Guozheng Liang: 0000-0001-9690-7931 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the support of the National Natural Science Foundation of China (No. 51273135) and of the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials.

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REFERENCES

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Reactive Polymer-Functionalized Aligned Multiwalled Carbon

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