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Applications of Polymer, Composite, and Coating Materials

MESOPOROUS AND NANOCOMPOSITE FIBROUS MATERIALS BASED ON POLY(ETHYLENE TEREPHTHALATE) FIBERS WITH HIGH CRAZE DENSITY VIA ENVIRONMENTAL CRAZING: PREPARATION, STRUCTURE, AND APPLIED PROPERTIES Olga V. Arzhakova, Alla A. Dolgova, and Aleksandr L. Volynskii ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Materials & Interfaces

MESOPOROUS

AND

NANOCOMPOSITE

FIBROUS

MATERIALS

BASED

ON

POLY(ETHYLENE TEREPHTHALATE) FIBERS WITH HIGH CRAZE DENSITY VIA ENVIRONMENTAL

CRAZING:

PREPARATION,

STRUCTURE,

AND

APPLIED

PROPERTIES O.V. Arzhakova, A.A. Dolgova, and A.L. Volynskii Faculty of Chemistry, Lomonosov Moscow State University, Leninskie Gory, Moscow, 119991 Russia ABSTRACT Preparation of mesoporous and nanocomposite fibrous polymer materials based on commercial poly(ethylene terephthalate) (PET) fibers with high density of crazes (HCD fibers) via environmental crazing (EC) is described. Multiple crazes in pristine PET fibers were initiated by the pre-crazing procedure, and the density of the initiated crazes in the starting HCD fibers is equal to ~200 crazes per mm. The scenario of environmental crazing of the HCD PET fibers was studied by on-line microscopic observations. The mechanism of environmental crazing of the HCD fibers is found to be different from the classical well-known scheme: new crazes are initiated over a broad interval of tensile strains up to 250%, splitting of thin craze walls takes place, and the collapse of the fibrillar-porous structure of crazes is prevented. The HCD fibers preserve their porosity even upon the complete removal of the physically active liquid environment from the volume of crazes. As a result, the overall porosity of the HCD fibers can achieve ~60 vol. % and pore dimensions are estimated to be below ~6 nm. Applied properties of the mesoporous HCD fibers (gas storage potential, sorption, insulating properties) are studied. The bottom-up synthesis of silver nanoparticles in the mesoporous HCD fibers via reduction of silver ions is described, and the resultant silver-containing nanocomposite fibers are characterized by a uniform distribution of silver nanoparticles with an average size of 3 nm. The silver content in the HCD fibers is 6 times higher than that in the pristine PET fibers with the same tensile strain. The silver-loaded fibers show high bactericide activity against Gram positive (Staphylococcus aureus) and Gram negative bacteria (Escherichia coli) and antifungal activity against Candida guilliermondii. The proposed EC approach allows preparation of sustainable mesoporous polymeric fibers and related functional nanocomposite materials with valuable functional properties for diverse applications. Keywords: mesoporous polymer fibers, environmental crazing, crazes, nanocomposite materials, silver nanoparticles, antibacterial and antifungal properties INTRODUCTION Porous polymeric materials present an immense interest because of their high potential for diverse

applications

as

sorbents,

insulating

and

breathing

light-weight

materials,

filtration/separation membrane materials, fuel cell membranes, hemodialysis, membrane distillation, etc.1-5Porous organic polymers can be used as effective separation and gas storage materials, host matrices for the encapsulation of target agents for controlled drug delivery and release of drugs, supports for immobilization of catalysts and sensors, as precursors of ACS Paragon Plus Environment

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nanostructured carbon materials, matrices for biomolecular immobilization and cell scaffolds, packaging materials, etc.4-9 In general, depending on pore dimensions, all porous materials are classified into three major groups: macroporous materials with the pore dimensions above 50 nm, mesoporous materials with pore dimensions of 2–50 nm, and microporous materials with the pore dimensions below 2 nm.4 In common practice, porous polymer fibers are prepared by the traditional methods of thermally induced phase separation (TIPS), air casting of polymer solutions, etching, immersion precipitation, electrospinning, and stretching.5, 8-15 Usually, the resultant pore dimensions of commercial membranes range from 100 nm to 10 microns. At the present time, preparation of mesoporous polymer materials, especially fibrous materials, presents a challenging task. Among synthetic fibers, of special value is poly(ethylene terephthalate) (PET) with its maximum market share in the worldwide production of fibers. Environmental crazing of polymers offers an alternative approach for the preparation of mesoporous polymeric materials, including mesoporous fibers. In general, crazing features the fundamental mode of plastic deformation of solid polymers (films, tapes, fibers, tubes), which proceeds via a spontaneous stress-induced development of porosity, which is provided by initiation and growth of discrete structural plastic zones referred to as crazes.16-22 The structure of each single craze is presented by thin nanoscale fibrils (10-15 nm in diameter) bridging the opposite craze walls, which are separated by pores with dimensions below 15 nm.20,

23-24

Initiation and growth of crazes in solid polymers upon loading can provided by tensile drawing in air (so called dry crazing)16, 19, 24-27 or in the presence of physically active liquid environments (PALEs) (solvent crazing or environmental crazing).20, 27 In contrast to dry crazing, which is commonly treated as an undesirable phenomenon triggering an early macroscopic fracture of polymers,19 environmental crazing (EC) of polymers can proceed without their failure up to high tensile strains up at the stage of strain hardening,20 thus leading to the development of a profuse porosity in polymers. Specific features of EC are primarily associated with the action of PALEs on solid polymers (the Rehbinder phenomenon).20,

27-29

Environmental crazing can be also

proposed as a universal route for the preparation of mesoporous materials with high porosity and good mechanical characteristics, which can serve as scaffolds and supports for the development of diverse multifunctional nanocomposite polymeric materials containing target additives incorporated within mesoporous crazes. In brief, the general scenario of environmental crazing can be presented as follows.15, 23 Upon loading below the yield point, surface defects and structural imperfections existing in any real polymer serve as craze initiation sites (the stage of craze initiation).23, 30-33 The development ACS Paragon Plus Environment

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of a fibrillar-porous structure in each individual craze proceeds via fibrillation according to the Rayleigh-Taylor meniscus instability mechanism.18-20,

23

With increasing tensile strain ε, the

initiated crazes start to grow via a progressive craze tip advance in the direction perpendicular to the direction of the applied stress.20, 23, 26, 30 Once both stages of craze initiation and craze tip advance are completed (within the initial strain interval of ~10% for PET), the initiated crazes start to widen along the direction of tensile drawing (the stage of craze widening). This stage is associated with a large-scale transformation of the initial bulk polymer into the oriented fibrillar state within crazes with their specific mesoporous structure and overall porosity increases over a wide interval of tensile strains (from 10% to 250% and higher for PET). In an ideal case when all deformation is assumed to proceed via crazing, volume porosity W of the polymer increases according to the linear law as 𝑊 =

𝜀 𝜀+1

× 100%, where ε is the tensile strain.

However, usually, the experimental W-ε curve is appreciably lower than the theoretical curve and starts to deviate from the theoretical expectations even at low tensile strains.20, 22 The reasons behind this decline (porosity W gradually decreases with ε down to zero) are concerned with the collapse of the fibrillar-porous structure of crazes. In other words, environmental crazing and collapse are two independent processes, which act in opposite directions: upon EC, porosity increases, while, due to the collapse, porosity shuts down.22 In most cases, the inevitable collapse outweighs and fully discredits the potential advantages of environmental crazing as a universal approach for the preparation of mesoporous polymer materials with high porosity. The driving force of this undesirable collapse phenomenon is provided by the unique structural organization of each individual craze and general macroscopic properties of the crazed polymers are controlled by the collective behavior of all crazes in the system. In general case, the inner craze structure is presented by numerous load-bearing fibrils (10-15 nm in diameter) oriented along the stretching direction (the length of fibrils can achieve even several millimeters) with their ends anchored to the craze walls (interfaces), thus bridging them,20, 23-24 the craze fibrils are separated by interconnected voids (pores) of the same dimensions.23, 34-36 It is also important to mention an unusual state of thin (nanoscale) fibrils. In thin nanoscale polymeric films or surface layers, glass transition temperature appears to be markedly reduced as compared with that of the bulk polymer and, hence, the polymer in fibrils exists in a more softened state with enhanced flexibility.37-40 Hence, craze fibrils can be envisaged as specific asymmetric flexible colloidal particles, and each craze can be treated as a certain closed colloidal system with high specific surface and excessive surface energy. This system is ACS Paragon Plus Environment


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characterized by high thermodynamic instability and, as any unstable colloidal system, it tends to reduce its well-developed surface and high surface energy via coalescence or, more likely, coagulation of the colloidal particles,40 which are craze fibrils. At the stage of craze widening, as the tensile strain ε increases, the length of craze fibrils increases along the direction of the applied stress, and their mobility increases (by analogy with a slack flexible string). When the flexible craze fibrils become long enough, they are able to perform elementary coagulation acts via interaction by their side surfaces due to action of contact forces (van-der-Waals forces).40 Coagulation can proceed via contacts of neighboring fibrils through thin layers of a liquid medium.40 Therefore, coagulation of craze fibrils provides a marked decrease in the overall free surface of the fibrillar-porous structure of the crazed polymers and high level of interfacial surface energy. In the case of fibers, their deformation via the EC mechanism is characterized by an early and intensive collapse at comparatively low tensile strains and this process gradually progresses until all crazes with their porous structure are degenerated into dense micronecks, thus leading to the development of a unique surface ridged relief with alternating slim and fat regions corresponding to the healed crazes (micronecks) and bulk polymer, respectively.22 This process is markedly boosted when the PALE is removed from the porous structure of crazes.22 However, in literature, this ridged structure with micronecks is mistakenly assumed to be originally developed in the PET fibers upon EC rather than being the consequence of the collapse.41-46 This misleading assumption is likely to be related to the fact that all experiments were performed only for dry samples after the removal of the PALE when the collapse is complete. Hence, the EC process can be potentially used for the preparation of highly porous polymeric fibers if the collapse could be prevented. This challenge can be accomplished, for example, via reducing the mobility of flexible craze fibrils by increasing the number of crazes. The objective of this work is concerned with the preparation of sustainable mesoporous fibrous materials based on the commercial PET fibers via preliminary initiation of multiple crazes and environmental crazing, detailed study of the EC mechanism of environmental crazing of the fibers with high density of crazes (HCD fibers), detailed description of the structure and applied properties of the prepared fibrous EC materials as well as the feasibility of the preparation of hybrid nanocomposite materials based on the HCD PET fibers. EXPERIMENTAL SECTION Materials

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In this work, we used the commercial unoriented fibers of glassy amorphous PET (Khimvolokno, Russian Federation) (glass transition temperature is 75°С) as a yarn. Each yarn contains 25-30 filaments with a diameter of 35-38 µm. As a physically active liquid environment (PALE) promoting environmental crazing of PET, n-butanol (Aldrich) was used. At room temperature, the degree of swelling of PET in this PALE is negligible. Small-angle X-ray scattering The HCD fibers and pristine fibers with a tensile strain of 100% were examined by the SAXS method at the DIKSI beamline of the Kurchatov Centre of Synchrotron Radiation (Moscow, Russia). The synchrotron radiation wavelength was 0.16 nm, and the vacuum chamber length was 2.4 m, allowing SAXS measurements in the scattering vector range of 0.07−1.1 nm−1. Preliminary initiation of crazes To generate multiple crazes in the initial PET fibers, the PET fibers (slightly wetted by the PALE) were stretched in air under the continuous mode by a tensile strain of 3-5% (below the yield point) using a 5-point bending unit on the standard DACA Instruments equipment. In all experiments, we used the same batch of the HCD polymer fibers prepared under the continuous mode. Mechanical tests The stress-strain behavior of the PET fibers was studied on a HOUNSFIELD H1KS tensile machine (Great Britain) at room temperature. Crosshead speed was 5 mm/min. The tests were performed in air and in PALEs for, at least, 5-7 samples, and the results were averaged. The experimental error did not exceed 3-5%. On-line microscopic observations In this work, the EC mechanism of the pristine and HCD PET fibers was studied in on-line microscopic observations according to the procedure described elsewhere22 using an Opton polarization microscope (Carl Zeiss, Germany) equipped with a Canon A450 digital camera. The strain rate was 5 mm/min. The optical images were analyzed using the FemtoScan software (Advanced Technologies Center, Russian Federation). Porosity Porosity W of the PET fibers was estimated from changes in the geometric dimensions (length and diameter) of the samples upon tensile drawing as: (1) ACS Paragon Plus Environment


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where V0 is the initial volume of the sample, ΔV is the difference between the volume of the deformed sample and the initial sample. The measurements were performed for, at least, 3-5 samples. The experimental error was 3%. Dye-contrasting tests The structure of the HCD fibers was characterized by dye-contrasting tests using the solutions of Rhodamine 6G (1.7 nm) in the PALE (n-butanol) (concentration was 10-4 g/L). The HCD fibers were either stretched in this dye-containing solution or allowed to stay at room temperature. Then, the stained fibers were carefully washed with running water for 30 min and dried for 30 min under the compressed air. Scanning electron microscopy (SEM) Structure of the PET fibers samples was studied using an EVO 40 XPV scanning electron microscope (Zeiss). Prior to the SEM observations, the fractured samples were decorated with a conducting gold layer with a thickness of 50–70 nm using a Giko IB-3 setup. Atomic force microscopy (AFM) The AFM studies were performed in the contact mode using a Solver PRO-M atomic force microscope (Russia). Low-temperature nitrogen adsorption Adsorption measurements were performed with an ASAP 2020 automatic analyzer (Micromeritics). Before the measurements, all samples were evacuated for 4–5 h. Pore size was estimated according to the Barrett-Joyner-Halenda (BJH) method using the Micromeritics software. Preparation of silver-containing HCD fibers The HCD PET fibers were subjected to stretching by 200% in the PALE solution containing dissolved AgNO3 (isopropanol (IPA)-water solution was used as the PALE; the IPA/water ratio was 3:1; concentration of silver nitrate was 5 wt %;). Then, the PALE solution was removed by evaporatio until the constant weihgt was attained. The content of silver nitrate in the HCD PET fibers was estimated gravimetrically. The experimental error was 2%. Then, the fibers were irradiated by the UV light (exposure 15 min). Transmission electron microscopy (TEM) For TEM observations, the silver-containing PET fibers were immersed into a cementing glue solution and allowed to stay for 24 h. Then, ultrathin sections were cut from the samples with a diamond knife (ultramicrotome, Reichert Jung) at room temperature and placed onto copper


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grids coated with formvar. The samples were examined on a Leo_912, AB Omega transmission electron microscope (Carl Zeiss). The TEM images were analyzed using a FemtoScan software. Fluorescence microscopy Fluorescence microscopy images of PET fibers were collected using an Olympus IX81 fluorescence microscope with UV excitation and a Hamamatsu digital CCD camera C7780. Antimicrobial tests To assess the antimicrobial activity47 of the HCD fibers with silver nanoparticles, we used the strains of Staphylococcus aureus (Gram-positive bacterium), Escherichia coli (E.Coli, Gram negative bacterium), and Candida guilliermondii (a species of yeast of the genus Pichia). The Petri dishes were inoculated with a standartized inoculum of test microorganisms. The silvercontaining HCD fibers were placed into the Petri dishes. The strains were grown in the BSA/agar at 37°C (Staphylococcus aureus and Escherichia coli) and at 28°C (Candida guilliermondii) and allowed to stay with the silver-containing samples for 24 h. The size of the inhibition growth zone was measured from the corresponding images using the FemtoScan software. RESULTS AND DISCUSSION Preparation of PET fibers with high craze density Polymer fibers with high density of crazes (HCD fibers) were prepared according to the following procedure: multiple crazes in a pristine polymer sample were initiated by bending and slight stretching in air at room temperature by low tensile strains (below yield tooth). In this work, the initial PET fibers were deformed by 3-5% in air under the continuous mode using a custom-made five-point bending module. Note that, due to this craze initiating procedure (precrazing), transparent fibers change their visual appearance and become milky white; their diameter remains unchanged. Figure 1 presents the optical micrograph and the SEM image of the PET fiber after the craze initiation (pre-crazing) procedure. To visualize the induced structural changes in the sample, the pre-treated PET fibers were dye-contrasted by the dye-containing IPA solution (Rhodamine 6G) and dried in air. As a result, the pre-crazed sample becomes slightly colored (purpulish), and this fact suggests that the pre-treated fibers contain crazes with open pores, where dye molecules are accommodated. As follows from Figs. 1a and 1b, the pre-treated PET fibers contain multiple thin crazes. The number of crazes N per unit length (craze density) is estimated to be more than 200-300 crazes per millimeter. Let us mention that craze density of the PET fibers stretched in the PALE ACS Paragon Plus Environment


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by 3-5% is well below 10-15 crazes per mm.22 Hence, one can conclude that the pre-crazing procedure provides the initiation of multiple crazes and preparation of fibers with high craze density (HCD fibers).

Figure. 1. Optical micrograph (a) and SEM image (b) of the PET fibers with high craze density (optical images for dye-contrasted samples) and stress-strain curves (c) of pristine PET fibers (1, 2) and HCD fibers (3, 4) upon stretching in air (1, 3) and in PALE (2, 4). Figure 1c shows the stress-strain diagrams of the pristine and HCD PET fibers upon stretching in air (curves 1 and 3) and in the PALE (curves 2 and 4). As compared with pristine PET fibers (Fig. 1c, curves 1 and 2), the corresponding stress-strain diagrams of the HCD fibers upon their stretching in air and in the PALE (Fig. 1c, curves 3 and 4) are seen to be lower as expected, upon stretching in the PALE, deformation proceeds at the lower stress level that that in air due to the adsorptional action of the PALE. Environmental crazing of HCD fibers in on-line microscopic observations Structural evolution of the HCD fibers upon their tensile drawing in the PALE (n-butanol) was studied by direct on-line microscopic observations according to the procedure described in our earlier publication.22 Figures 2a and 2b show the on-line micrographs of the HCD fibers at the early stages of deformation (10-30%).


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Figure 2. Micrographs of the HCD PET fibers upon their stretching in the PALE (nbutanol) by tensile strains of (a) 10 and (b) 30% (on-line observations) and (c) schematic representation of pristine fibers and HCD fibers upon classical crazing at the fixed tensile strain and the corresponding SAXS pattern of the crazed pristine and HCD PET fibers. At tensile strains below 30%, deformation of the HDC PET fibers proceeds via the development of porosity according to the mechanism of classical environmental crazing (Figs. 2a and 2b). As compared with the conventional PET fibers,22 the number of the nucleated crazes in the HCD fibers is seen to be much higher. At a given strain, the overall elongation of conventional fibers and HCD fibers upon EC is the same but provided by the different number of crazes. In this case, the contribution from each individual craze into the overall porosity can be estimated as W/N, where W is the overall porosity, and N is the number of crazes in the sample. Evidently, in the case of the HCD fibers, the porosity W is delocalized over multiple crazes, and the contribution from each craze to W is lower. Figure 2c shows the schematic representation of the structure of pristine fibers and HCD fibers and the classical SAXS pattern (the insert) upon environmental classical crazing. This "butterfly" pattern shows two mutually perpendicular X-ray reflections, which correspond to scattering from craze fibrils and craze walls and reflects the interior structure of crazes composed of thin oriented fibrils (along the direction of tensile drawing) that bridge the opposite craze ACS Paragon Plus Environment


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walls. Let us mention that the SAXS patterns for pristine and pre-crazed HCD fibers appear to be nearly the same, and this observation seems to be reasonable as, in both cases, the fibers contain similar structural elements (craze fibrils and craze walls). In the case of the HCD fibers, the opening of each craze is seen to be much lower (at a given tensile strain ε) and, hence, the length of craze fibrils bridging the opposite craze interfaces is shorter. Hence, in the case of the HCD fibers, the flexibility of craze fibrils is suppressed as their length is short and they are unable to interact. Hence, one can expect that, for the HCD fibers, the phenomenon of collapse can be partially diminished or even prevented, thus offering the route for the preparation of highly porous fibers. Let us prove this expectation by the microscopic images of the HCD PET fibers at higher tensile strains up to 200% (Fig. 3A). As the tensile strain is further increased, deformation of the HCD PET fibers proceeds without any visible changes in the diameter of the fiber in the crazed regions. Hence, volume porosity of the HCD fibers with ε = 200% is above 60%.


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Figure 3. Optical micrographs (A) of the HCD PET fibers and (B) pristine PET fibers upon environmental crazing (in on-line observations). For comparison, it seems interesting to consider the structural evolution of pristine fibers and HCD fibers (namely, two individual crazes shown by blue and red arrows) within the same interval of tensile strains (Fig. 3B).22 In the case of the pristine PET fibers, no new crazes are seen to be initiated at higher strains above the yield strain; at tensile strains ranging from 100% to 200% (Fig. 3B), the initiated crazes are involved only in craze widening, and the width of crazes markedly increases (from several microns to 30-40 microns). The diameter of the fibers ACS Paragon Plus Environment


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within the crazed regions is seen to gradually thin down at the central part due the collapse of the porous structure of crazes via coagulation of nanoscale craze fibrils.22 Comparing Figs. 3A and 3B, one can conclude that, at the same tensile strains, the HCD fibers preserve their diameter, porosity increases, and no collapse is seen to occur. More detailed information on the structural evolution of the HCD PET fibers upon crazing can be gained from the snapshots at higher magnifications. Figure 4 shows the corresponding micrographs of the HCD PET fibers.

Figure 4. Optical micrographs of the HCD PET fibers upon deformation in the PALE by 25% (a), 75% (b), 150% (c), and 250% (d) (in on-line observations) and optical micrograph of the HCD PET fiber after deformation in the dye-containing PALE (n-butanol) by 50% (enlarged image). The micrographs of the HCD fibers at higher magnifications clearly show that, as the tensile strain increases from 25% to 75%, the craze walls become thinner (Figs. 4a and 4b). Evidently, in the HCD fibers with multiple crazes, the regions of unoriented bulk polymer between crazes are short (not more than 1-2 µm). As the tensile strain increases (Figs. 4c and 4d), the regions of the bulk polymer between crazes become even shorter due to the gradual ACS Paragon Plus Environment

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consumption of this material and its transformation into oriented state within the craze volume. The thinned craze walls are unable to resist the applied pressure and tend to split down, thus giving rise to the initiation of new crazes. Comparing the scenario of environmental crazing of pristine fibers (Fig. 3B) and HCD fibers (Figs. 3A and 4), one can conclude that, at tensile strains above 150%, deformation of the conventional PET fibers with low craze density proceeds via widening of existing crazes, which were nucleated at the early stages of tensile drawing (at the yield point of 5-7%), and the collapse of wide crazes via coagulation of craze fibrils shuts down the fibrillar-porous structure of crazes and leads to their complete monolithization and degeneration into monolithic micronecks.22 Mechanism of environmental crazing of HCD fibers In general, the mechanism of environmental crazing of the HCD fibers appears to be different from that of the pristine PET fibers. As compared with the pristine PET fibers, environmental crazing of the HCD fibers proceeds via a progressive development of nanoscale porosity without any collapse even at high tensile strains. This observation implies that the initiation of multiple crazes in the HCD fibers allows preparation of polymeric fibers with high porosity due to the fact that, in the case of the HCD fibers, all strain and porosity appears to be distributed over numerous crazes. As a result, the width of each craze is small and, hence, the length of craze fibrils bridging the opposite craze walls is also short. Therefore, short craze fibrils possess no freedom to interact with neighboring fibrils, and the collapse of the porous structure via coagulation of craze fibrils is prevented. The striking feature of the environmental crazing of the HCD fibers is concerned with the progressive initiation of new crazes at strains well above the yield point while, for pristine PET fibers, crazes are initiated only at strains near the yield point, and this process is finished after the yield tooth. All further deformation proceeds via the growth of the once initiated crazes. The reason behind this behavior is related to the fact that, in the HCD fibers with numerous crazes, craze walls appear to be thin and weak (becoming even thinner and weaker with increasing tensile strain) and they are unable to resist the applied stress and readily split down, thus leading to the formation of new and new crazes. Figure 4 shows the optical image of the dye-contrasted HCD fibers when the HCD fibers were stretched in the PALE containing the dissolved dye (Rhodamine 6G). After the deformation in the dye-containing PALE the fiber becomes brightly colored, thus indicating the penetration of the dye into the porous structure of crazes. In this micrograph, bulk polymer (so-called craze walls) is seen as white regions, whereas the porous structure of crazes is brightly colored. This



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micrograph also clearly visualizes the process of craze wall splitting and initiation of new crazes at the early stages of deformation (enlarged fragments in the lens). Hence, this new scenario of environmental crazing of the HCD PET fibers allows preparation of mesoporous polymeric fibers with high porosity as well as preparation of nanocomposite fibers containing diverse target additives. Removal of the PALE from the mesoporous HCD fibers Let us now discuss whether the highly porous HCD fibers preserve their porosity when the PALE is removed from the volume of crazes. Usually, the removal of the PALE via evaporation from crazes always triggers an intensive collapse and "kills" their porous structure, thus leading to their degeneration into monolithic micronecks.22 Figure 5 shows the SEM and AFM images of the HCD fibers and, for comparison, the SEM image of the pristine PET fibers after the complete removal of the PALE by evaporation. To our knowledge, the fibers with crazes were for the first time studied by the AFM method.


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Figure 5. SEM (a, b) and AFM images (c) of the HCD PET fibers and pristine fibers (d) with ε = 100% after the complete removal of the PALE. As follows from Fig. 5, the HCD fibers with ε = 100% fully preserve their thickness and their visual appearance is seen to be different from that of the pristine PET fibers at ε = 100% (Fig. 5c) when the fibers have characteristic fat-slim ridged surface relief (Fig. 5c) and thick regions (bulk polymer) alternate with thin regions (crazes degenerated into monolithic micronecks). All HCD fibers are seen to be stuffed with multiple crazes. For the HCD PET with ε = 100%, the density of crazes is seen to be equal to ~1000 per mm. The AFM image of the HCD fiber clearly shows the formation of a new craze in the sample via craze wall splitting. However, according to the classical scenario, all crazes are initiated within the narrow interval of tensile strains around the yield point (below 7-10%), whereas further deformation proceeds via their growth and opening but without any initiation of new crazes. In the case of the HCD fibers, the density of crazes progressively increases with increasing the tensile strain: the number of crazes ACS Paragon Plus Environment


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increases from 200-300 (the initial pre-crazed sample with ε = 5%) to ~1000 (the HCD sample with ε = 100%) per 1 mm of the initial sample or, in other words, in addition to 200 crazes per 1 mm, ~1700 new crazes per the same length are nucleated!

Figure 6. SEM images of the HCD PET fibers with ε = 200% after the complete removal of the PALE. When the tensile strain further increases to ε = 200%, the craze density increases to 15002000 crazes per mm (Fig. 6). According to our earlier reasoning, this fact implies that the number of crazes increases from 200-300 (the initial precrazed sample with ε = 5%) to 2000 (the HCD sample with ε = 200%) per 1 mm of the initial sample. Hence, in other words, in addition to 200 crazes per 1 mm, ~5000-6000 new crazes within the same length are nucleated! In whole, the structure of the HCD fibers at higher strains resembles the structure of the Dutch Stroopwaffels as a stack of thin round waffels. The analogy seems to be even more justified because the waffels are glued by the caramel filling like the space between thin craze walls is filled with the fibrillar matter. This marked rise in the number of crazes is a unique feature of environmental crazing of the HCD fibers, which is provided by the progressive splitting of thin craze walls and progressive initiation of new crazes. Pore dimensions of the HCD fibers were estimated by the method of low-temperature nitrogen sorption. According to the BJH method, the average pore size of the HCD fibers is ~6 ACS Paragon Plus Environment

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nm, and this estimation well agrees with the earlier data on the pore size of the crazed PET films. Hence, the EC approach provides an efficient tool for the preparation of mesoporous polymeric fibers with a high level of volume (~60%). THE HCD FIBERS AS APPLIED MATERIALS Sorption properties of the HCD fibers To answer the question whether the HCD fibers can serve as efficient sorbents, the dried PET fibers were allowed to stay in the dye-containing solution (for 15 min, Rhodamine 6G). Note that the removal of the PALE from the pristine PET fibers leads to the complete collapse of the porous structure, and no coloration is observed. Within this period of time, all crazed HCD fibers with different tensile strains become brightly colored (brightly purple). This coloration is provided by the effective dye sorption and the fibers retain their color even after the prolong repeated washing in a flowing hot water (for 2 h and longer). The structure of the dye-containing HCD fibers was studied by the methods of optical and fluorescence microscopy (Figs. 7 and 8). Figure 7 shows the micrographs of the colored PET fibers with different tensile strains. The dye-contrasting tests for the dry HCD fibers allow a clear visualization of structural evolution of the HCD PET fibers upon environmental crazing (Fig. 7). In whole, noteworthy is that the visual appearance of the samples after sorption is similar to that of the samples after stretching in the dye-containing solutions. In this case, sorption-assisted coloring of the HCD fibers even at high tensile strains (ε = 320%) seems to be unexpected. Hence, the HCD PET fibers are proved to preserve their open porosity over a broad range of tensile strains.



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Figure 7. Light micrographs of the HCD fibers with different tensile strains (from 100% to 320%) after their exposure to dye-containing IPA solution (Rhodamine 6G). Figure 8 illustrates the specific scenario of environmental crazing of the HCD PET fibers: the absence of any collapse of the porous craze structure and preservation of an open porosity at high tensile strains, splitting of craze walls, initiation of new crazes over a broad interval of tensile strains, fragmentation and disintegration of thin craze walls into debris which are seen as small islands and veins within the crazed structure. This structure can be compared to the structure of marble meat with intramuscular fatty marbling. It seems likely that, in the case of the HCD fibers, these debris of craze walls (marbling elements) can serve as reinforcing elements.


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Figure 8. Fluorescence micrographs of dye-contrasted HCD PET fibers with (a) ε = 100%, (b) 50%, and (d) 270% (blue regions - craze walls, white regions - crazes) and (c) optical image of the HCD PET fiber at 300%. Micrographs illustrating certain structural details of the HCD PET fibers: (b) wall splitting and (c, d) wall fragmentation into chips. Hence, the HCD fibers can serve as efficient sorbents and can provide the efficient removal of dyes (or some other colorants or contaminants) from their solutions. Environmental crazing of the HCD fibers can be considered as a route for the preparation of mesoporous and light-weight PET fibers with high porosity. Noteworthy is that this process can be implemented in a continuous technological process using the standard equipment for orientational drawing. HCD fibers as gas storage and insulating materials Highly porous HCD PET fibers can be applied as specific nanocomposite gas storage materials composed of a polymer and a gas. In the case of the porous materials with mesoscale pore dimensions, the gas appears to be confined within nanoscale voids. Hence, thermal conductivity of the gaseous phase is expected to be depressed due to the well-known Knudsen effect.10,

48

Then, the effective thermal conductivity 𝜆′𝑔 of the gas in the gas-filled porous

materials reads as:

𝜆′𝑔 =

𝜆′𝑔0 𝑙𝑔

(2)

(1 + 𝛽(𝐷 ) 𝑝



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where 𝜆′𝑔0 is the thermal conductivity of free air (0.026 W/m K at room temperature), β is the coefficient describing the energy transfer between gas molecules and solid material (~2 for air), 𝑙𝑔 is the mean free path of gas molecules (lg = 70 nm at room temperature), and Dp is the mean pore diameter (6 nm for the crazed PET). Then, 𝜆′𝑔 of air in the HCD PET fibers is equal to 0.00107 W/m K. As a result, thermal conductivity of the gaseous phase within the HCD fibers is ~25 times lower than that of free air. Hence, the HCD fibers can serve as effective insulating materials and can be effectively used, for example, for heat retention in smart and comfort textiles.10 Hybrid nanocomposite HCD fibers with silver nanoparticles To validate the advantages of the environmental crazing of the HCD fibers for the preparation of functional nanocomposite materials, the functional additive was incorporated into the mesoporous PET fibers upon tensile drawing in the PALE or by the subsequent impregnation of the crazed mesoporous structure. The HCD PET fibers were subjected to the tensile drawing in the PALE solution containing dissolved silver nitrate to a tensile strain of 200%. After evaporation of the PALE, the mesoporous structure of the HCD PET fibers was loaded with AgNO3. Further in situ reduction of silver ions to silver was performed using the UV irradiation, which is safe for biological purposes as it does not involve the potentially contaminants or cytotoxic reducing agents. The reaction within mesopores proceeds as: 2AgNO3 = 2Ag↓ + 2NO2↑ + O2↑. After the UV irradiation, the HCD PET fibers are seen to be colored: from slightly blondish to brownish, thus validating the occurrence of the reduction reaction and formation of silver. The content of silver in the HCD PET fibers was estimated to be ~30 wt.%. Note that, in the case of the pristine fibers with low density of crazes, the content of silver (weight gain) is only 4-5 wt.% (at the fixed tensile strain). In other words, the concentration of silver nanoparticles in the HCD fibers appears to be nearly 6-7 times higher. To prove the formation of silver nanoparticles within the HCD PET fibers, the fibers were examined by the TEM method.


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Figure 9. TEM images of the thin section of the silver-loaded HCD PET fibers (ε = 200%) (a, c), X-ray diffraction pattern (b), and size distribution of silver nanoparticles in the HCD PET fibers (ε = 200%) (d). The X-ray diffraction data can provide the supporting evidence that, upon the UV irradiation of the AgNO3-loaded HCD PET fibers, silver ions are reduced to silver (Fig. 9b). The XRD pattern reveals the formation of the face centered cubic crystalline phase of silver nanoparticles. The diffraction peaks at 2θ = 38.32, 44.53, 64.73, and 77.66° correspond to the (111), (200), (220) and (311) reflection planes, respectively. This pattern fully agrees with the standard data reference for silver (ICCD card number: 00-003-0921). The corresponding TEM images of ultrathin sections of the silver-containing HCD PET fibers are shown in Fig. 9c. Silver nanoparticles (Ag NPs) are found to be uniformly distributed within the HCD PET fibers without ACS Paragon Plus Environment


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any agglomeration. Their dimensions are well below 10 nm. Figure 9d shows the histogram as the size distribution of Ag NPs in the HCD PET fibers: this distribution is unimodal with a maximum at 3 nm. The histogram has the tails corresponding to the presence of small-sized Ag NPs (3 nm). The absence of the dots in the XRD pattern is likely to be associated with small dimensions of the silver nanoparticles. Therefore, the HCD PET fibers can serve as efficient host matrices for the impregnation and immobilization of various cargo and preparation of the functional nanocomposite fibers with the uniform distribution of nanoparticles. In the case of the silver-containing HCD fibers, these materials can be used as broadspectrum alternatives for routine antibiotic therapy and can inhibit the growth of microbes as silver ions are efficient against diverse drug resistant bacteria and can suppress the growth of diverse bacterial strains and fungus by binding Ag/Ag+ with the biomolecules present in the microbial cells.49-51 Nowadays, Ag NPs are approved for biomedical applications and widely used in diverse commercial products as wound dressings, cosmetic and medical creams, etc. Silver is characterized by low toxicity and presents the minimal risk to human health. The antimicrobial action of silver nanoparticles is provided by their ability to alter the cellular permeability and produce reactive oxygen species (ROS). The HCD fibers containing Ag NPs were tested as antibacterial materials against Grampositive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria as well as antifungal materials against Candida guilliermondii (Fig. 10).


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Figure 10. Microbiological tests on antimicrobial and antifungal activity of the silverloaded HCD fibers: (A) Staphylococcus aureus, (B) Escherichia coli, (C) Candida guilliermondii. As follows from Fig. 10, the HCD fibers with Ag NPs are seen to have high antimicrobial performance against Gram positive (Staphylococcus aureus) (Fig. 10A) and even Gram negative bacteria (Escherichia coli) (Fig. 10B). The latter fact is of special importance as the Gram negatives are quickly developing resistance against antibiotics and pose major challenges. Noteworthy is that, even for Gram-negative bacteria (Escherichia coli), the inhibition zone around an isolated silver-loaded fiber seen as the transparent region around the fiber is equal to ~2 mm. This assessment proves the high antimicrobial performance of the silver-loaded HCD fibers because the thickness of the fiber is only 30 µm (Fig. 10B, b, c, d). Of special interest is that the silver-loaded HCD fibers also demonstrate the antifungal activity and can inhibit the growth of Candida guilliermondii, which is known to be an "opportunistic" fungal pathogen52. ACS Paragon Plus Environment


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Even though the inhibition zone is moderate, the area under the HCD fibers remains transparent thus indicating that the growth of yeasts is suppressed (Fig. 10C). In this work, the Ag-loaded HCD fibers were shown to preserve their antimicrobal activity for, at least, three months but, in principle, no deterioration in the long-term performance is expected. This bottom-up approach can be also applied for the incorporation of other metal ions and their in situ reduction, including noble metals (palladium, gold, platinum, etc.), thus allowing preparation of a broad scope of nanocomposite materials for different applications (catalytic systems, sensors, drug delivery systems, electroconducting materials, etc.). CONCLUSIONS Mesoporous fibrous materials with high porosity (~60 vol. %) and with a pore diameter below 10 nm based on commercial PET fibers were prepared by the method of environmental crazing. The prerequisite condition for the preparation of the highly porous mesoporous PET fibers is concerned with the initiation of multiple crazes in the traditional commercial PET fibers (craze density is equal to ~200 crazes per mm) via their preliminary treatment by bending and benign stretching by low strains well below the yield point. Direct on-line microscopic studies reveal that the EC mechanism of the HCD PET fibers appears to be different from the classical wellknown and expected scenario. New features are concerned with the fact that new crazes are initiated over a broad interval of tensile strains up to ~250% when thin craze walls are unable to resist the built-up stress level and split down, thus giving rise to the initiation of new crazes. The mobility of short craze fibrils within narrow crazes appears to be suppressed: they are unable to interact with each other and reduce the highly developed surface via coagulation. As a result, the undesirable collapse of the highly developed fibrillar-porous structure of crazes is suppressed and their degeneration into monolithic micronecks is fully prevented. This approach offers the route for the design of mesoporous PET fibers with high volume porosity (up to 60 vol. %) and small pore dimensions (< 10 nm). Applied characteristics of the mesoporous HCD fibers are studied, and the benefits of the environmental crazing for the preparation of nanocomposite PET fibers are highlighted. The bottom-up synthesis of silver nanoparticles within the mesopores of the HCD fibers allows the preparation of hybrid nanocomposite fibers containing silver nanoparticles with the average size of 3 nm and provides their uniform distribution throughout the polymer matrix without any tendency for agglomeration. The silver content in the hybrid silver-containing HCD fibers ACS Paragon Plus Environment

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appears to be 6 times higher than that in the pristine PET fibers with the same tensile strain. The resultant silver-loaded PET fibers show high antimicrobal and antifungal activity against diverse microorganisms. The practical value of the proposed approach is also concerned with the fact that, among all polymer fibers, the global market share of PET fibers in worldwide production in 2017 has achieved 49%.53 However, this approach can be extended to other polymers. Noteworthy is that this process can be performed on the existing technological equipment for orientational drawing of fibers with minor modifications. The proposed mesoporous PET fibers can be used in practice as breathable materials, insulating heat-retention materials, high-performance clothing materials, comfort textiles, sorbents, etc. Introduction of diverse functional additives to the mesoporous PET fibers offers a universal strategy for the preparation of functionalized and nanocomposite fibers which can be used as flame retardant fibers, antimicrobial materials for medical applications, antistatic fibers, conductive materials and textiles, fibers with antiultraviolet protection, infrared textiles, superhydrophobic and hydrophilic materials, insect repellent materials, protective clothing, smart textiles, intelligent consumer apparel products, etc.

Corresponding author: O.V. Arzhakova; e-mail address: [email protected] Author Contributions # The authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the Russian Foundation for Basic Research (project no. 18-29-17016). This work was performed using the equipment provided by the Lomonosov Moscow State University Program of Development. We would like to thank Dr. S. Abramchuk for his helpful assistance in the TEM observations, Dr. T.A. Cherdyntseva for microbiological tests, and I.I. Nikishin for the AFM image of the HCD fiber.



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