Hydration and Sorption Properties of Raw and Milled Flax Fibers.

The physicochemical and hydration properties of mechanically modified flax fibers (FFs) were investigated herein. Raw flax fibers (FF-R) were ball-milled and sieved through mesh with various aperture sizes (420, 210, and 125 μm) to achieve modified samples, denoted as FF-420, FF-210, and FF-125, respectively. The physicochemical and hydration properties of FF-R with variable particle sizes were characterized using several complementary techniques: microscopy (SEM), spectroscopy (FT-IR, XRD, and XPS), thermoanalytical methods (DSC and TGA), adsorption isotherms using gas/dye probes, and solvent swelling studies in liquid H2O. The hydration of FF biomass is governed by the micropore structure and availability of active surface sites, as revealed by the adsorption isotherm results and the TGA/DSC profiles of the hydrated samples. Gravimetric water swelling, water retention values, and vapor adsorption results provide further support that particle size reduction of FF-R upon milling parallels the changes in surface chemical and physicochemical properties relevant to adsorption/hydration in the modified FF materials. This study outlines a facile strategy for the valorization and tuning of the physicochemical properties of agricultural FF biomass via mechanical treatment for diverse applications in biomedicine, energy recovery, food, and biosorbents for environmental remediation.

Introduction

The hydration properties of biomaterials occupy a key role in their physical, chemical, and microbiological attributes.[1] In particular, the role of water activity and moisture adsorption/desorption processes form the basis of a wide range of applications in food processing, materials science, and advanced polymer technology.[2] Thus, a better understanding of solute–water interactions is required for the rational design of advanced materials, such as desiccants, humectants, and adsorbents, for energy harvesting, food preservation, and environmental remediation. Natural fiber-based composites are gaining greater interest in the fields of energy, food, and polymer technology because of their environmental sustainability, eco-friendly nature, and favorable mechanical strength and performance.[2,3] The continued interest in the use of natural flax fiber (FF) in biocomposites over traditional reinforcing materials (e.g., glass fiber, talc, and mica) relates to its low cost, low density, high mechanical strength, and biodegradability.[4] However, the hydrophilic nature and lack of good interfacial adhesion of natural flax limit its use in composites because of degradation of its mechanical properties upon swelling in water. The hydrophile–lipophile balance (HLB) of natural flax can be tuned by substituting the surface hydroxyl (−OH) groups via such reactions as acetylation, benzoylation, and peroxide treatment.[5,6] Consequently, the water affinity of natural fibers is reduced, whereas their compatibility with apolar polymer matrices is enhanced.[7] The chemical treatment of flax affords changes in the surface properties of the material, owing to changes in the relative biomass composition. Changes in surface properties are usually accompanied by changes in the surface morphology and textural properties.[8] More recently, research on natural flax has focused on its water uptake properties.[7,9] By contrast, the uptake properties of natural FFs with water vapor have been more actively studied because of their unique hydration properties and relevant applications to the textile industry.[10−12] Recently, advanced applications of fibers have emerged that utilize hydration phenomena, such as those related to the biomedical field (e.g., wound dressing),[13,14] biosorbents for environmental remediation (oil, dyes, and heavy metals),[15,16] energy harvesting in heat, ventilation, and air-conditioning (HVAC) systems and food-based products.[2] In particular, recent reports have detailed comprehensive reviews on the removal of metals, dyes, and other organic contaminants using biomaterials and their modified forms as versatile sorbents.[17,18]

Biopolymer hydration is ubiquitous to many chemical processes and stabilization of biological structures.[19−21] Theoretical and experimental techniques have been employed to investigate the hydration properties of biopolymers.[20,22] For example, single-crystal X-ray diffraction techniques and high-resolution NMR spectroscopy have provided unique insight into biopolymer hydration phenomena.[20,22] More recently, new insight into surface-mediated hydration processes was obtained using complementary material characterization studies: spectroscopy, thermal analyses, solvent uptake, and adsorption isotherm methods. Recent studies[23,24] on the hydration of starch- and cellulose-based materials in mixed solvent systems reveal that the relative biopolymer–solvent affinity depends on various physicochemical properties: (1) HLB of the biopolymer, (2) relative polarity of the biopolymer–solvent system, (3) textural [surface area (SA) and pore structure] properties, and (4) solvent properties such as relative polarity and molar volume. The ability of natural FFs to adsorb water relates to the presence of abundant polar functional groups (−OH and COOH) that characterize surface adsorption sites. The cell wall of a plant fiber is considered to be a natural composite that consists of crystalline microfibrils embedded in an amorphous lignin–hemicellulose–pectin matrix.[9,25] The variable composition of cellulose, hemicellulose, pectin, and lignin imparts a variable HLB character to the composite structure of FFs because of the variable amorphous nature and abundance of the functional groups.[9] Independent studies report the composition of raw flax biomass as ∼70% cellulose, 18% hemicellulose, 2% pectins, 2% lignins, and ∼1.7% oil/waxes. The respective components of flax contribute differentially to the water uptake properties because of the variable level and accessibility of surface functional groups.[26] Also, the overall composition of the FFs likely varies upon chemical and/or mechanical treatment because of changes in surface accessibility. Thus, it can be concluded that chemical and/or mechanical treatment of natural FFs afford structurally modified materials with tunable physicochemical properties for tailored applications, in accordance with numerous literature reports.[25−28] Despite extensive research on the relationship between water sorption and the role of particle size in the surface and textural properties of ball milled/ground flax, there are limited studies that report on the hydration properties of these modified materials.

Herein, various complementary techniques were used to study the physicochemical and hydration properties of natural flax in its pristine and mechanically modified forms: spectroscopy [Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), and scanning electron microscopy (SEM)], thermoanalytical (differential scanning calorimetry, DSC; thermogravimetric analysis, TGA), and sorption methods (g/l adsorption isotherms and solvent swelling). Modified FFs were prepared via ball milling, where the ball-milled raw FF (FF-R) was sieved using various mesh sizes with variable apertures (425, 210, and 125 μm) to prepare FF-420, -210, and -125 materials, respectively. The objectives of this study are (1) to characterize the physicochemical properties of natural raw flax with variable particle sizes modified via ball milling and sieving, (2) to investigate the water (g/l) uptake properties of the raw and modified materials, and (3) to gain insight into the structure–adsorption relationship for FF biomass and its hydration properties. Herein, we demonstrate that the hydration and water-swelling/retention properties of the pristine (FF-R) and modified (FF-420, -210, and -125) FFs vary markedly, in agreement with the variable textural (surface and pore structure) properties of the materials. The results of this study are likely to contribute to valorization of flax using a facile milling approach to modify the biomass structure, along with tailoring of the biomass properties. The results of this study are likely to have practical utility because of the ubiquitous importance of hydration phenomena and emerging fiber-based technologies in medicine, energy, food, textiles,[30,31] and environmental remediation.[32]

Experimental Section

Materials and Chemicals

The FF-R material was procured from Biolin Research Inc. (Saskatoon, SK, Canada). Spectroscopic-grade potassium bromide (KBr) and wire mesh sieves [USA standard sieves; no. 40 (420 μm), no. 70 (210 μm), and no. 120 (125 μm)] were purchased from Sigma-Aldrich (ON, Canada).

Modification of the Raw Flax

Mechanically modified FF materials were processed by ball milling and rotary grinding of FF-R. Ball milling was achieved by mixing the raw flax material with ZrO2 balls (6–10 mm in diameter) in a stainless steel milling jar with a weight ratio of 1:10. The rotation speed of the disk and milling jar was set to 450 rpm, where the milling jar was rotated alternately in the forward and reverse directions at intervals of 2 min. The ball-milled FF samples were further ground in a rotary coffee grinder and sieved using variable mesh sizes (no. 40, 70, and 120) to achieve FF samples denoted as FF-420, FF-210, and FF-125, respectively. The raw flax sample (FF-R) and the mechanically modified samples (FF-420, -210, and -125) were characterized using complementary techniques, as described below.

Surface and Textural Characterization of the FF Materials

Scanning Electron Microscopy

SEM images were obtained using FEG-SEM SU6600 instruments at an accelerating voltage of 15 kV, using low (30×) and high (1.5k×) magnification.

X-ray Photoelectron Spectroscopy

XPS measurements were carried out using a Kratos (Manchester, UK) AXIS Supra system. All survey scan spectra were collected in the 5–1200 binding energy range in 1 eV steps with a pass energy of 160 eV. High-resolution spectra were also conducted using 0.05 eV steps with a pass energy of 20 eV. An accelerating voltage of 15 eV and an emission current of 15 mA were used for data collection.

X-ray Diffraction

XRD patterns of the FF-R and modified forms were recorded with a PANalytical Empyrean powder X-ray diffractometer. Monochromatic Co Kα1 radiation was used while the applied voltage and current were set to 40 kV and 45 mA, respectively. The FF films were mounted in a horizontal configuration after evaporation of methanol. The XRD patterns were measured in the continuous mode over a 2θ range (7–50°) with a scan rate of 3.2° min–1.

FT-IR Spectroscopy

The FT-IR spectra of the FF samples were recorded using a BIO-RAD FTS-40 spectrophotometer operating in the diffuse reflectance (DRIFT) mode. The samples (ca. 10 mg) were mixed with spectroscopic-grade KBr (ca. 90 mg) using a spatula with minimum grinding to maintain the particle size and integrity of the samples. A total of 132 scans were acquired for each sample for a spectral region of 400–4000 cm–1 with a resolution of 4 cm–1.

Nitrogen Adsorption Isotherms

The SA and pore structure properties of the raw and modified samples were measured using a Micromeritics ASAP 2020 (Norcross, GA) with an accuracy of ±5% and calibrated using alumina (Micromeritics) with a known pore volume (PV) and SA. Each sample (1 g) was degassed at an evacuation rate of 5 mmHg s–1 at 100 °C for 48 h. The micropore SA was estimated using a t-plot (de Boer method), whereas the Barret–Joyner–Halenda (BJH) method was employed to estimate the PV and pore diameter (PD) from the adsorption isotherm using the Kelvin equation.[33]

Hydration and Sorption Properties

Dye Adsorption Kinetics

The sorption uptake kinetics of the FF-R and mechanically modified samples (FF-420, -210, and -125) were measured using a one-pot kinetic method,[34] where the uptake of a chromophore dye probe (methylene blue; MB) was measured as a function of time. Briefly, the FF adsorbents (ca. 120 mg) were added into dialysis bags that were previously equilibrated in water. The fiber sample was enclosed by clipping both ends of the dialysis bag in a manner analogous to a tea sachet. The adsorbents were immersed in a beaker containing ca. 150 mL of 10 μM MB aqueous solution, equipped with a Teflon stirrer bar, with mixing at ∼150 rpm. Aliquots of solution (∼2.6 mL) were taken at variable time intervals during continuous stirring, where the equilibrium dye concentration was measured by UV–vis spectrophotometry (Varian CARY 100). The temporal uptake (Qt) was plotted as a function of time (h) according to a method detailed elsewhere.[34]

Thermogravimetric Analysis and Differential scanning calorimetry

TGA (Q50 TA Instruments) of the flax samples was performed with a heating rate of 5 °C min–1 from 30 to 500 °C, under a N2 purge gas. Solid samples were analyzed in the dry and wet states in aluminum pans, where the samples weights were fixed at 20 ± 0.2 mg. The sample dosage in Millipore water was 10% (w/w). The integration of the thermal events in the TGA profile was measured using the TA Q50 software.

Differential scanning calorimetry (TA, Q20 TA Analyzer) of the raw and modified flax materials was performed between 30 and 150 °C at a scan rate set to 10 °C/min. Solid samples (5 ± 0.5 mg) were analyzed using hermetically sealed aluminum pans, and nitrogen gas was used to regulate the sample temperature and sample compartment purging.

Water Swelling and Water Retention Value

The various FF materials were evaluated for their uptake of water at equilibrium in order to estimate the degree of solvent swelling in liquid water. Dry FF samples (ca. 20 mg) were equilibrated in 7 mL of Millipore water for 48 h. The degree of swelling (Sw) was calculated using eq Ws refers to the wet sample and Wd refers to the dry sample after oven drying to a constant weight (±0.1 g) at 60 °C.

The water retention value (WRV) was evaluated by equilibrating 40 mg of each sample in de-ionized water for 1 h, followed by centrifugation (Precision Scientific Co.) at 4000 rpm for 1 h to separate the solid from the liquid water. The hydrated solid sample was weighed (w1), followed by drying in a conventional oven at 105 °C. Finally, the sample was placed in a desiccator to cool for 12 h and weighed again (w2). Each measurement was carried out in triplicate and the WRVs (%WRV) were estimated using eq , where W1 (g) and W2 (g) are the weights of the wet and dry samples, respectively.

Water Vapor Analysis

The water vapor adsorption isotherms were obtained using the Intelligent Gravimetric Analyzer system (IGA-002, Hiden Isochema, UK). Sample weights of ca. 30 mg were placed in a stainless steel container that were housed in a vessel equipped with a microbalance, a thermostat, and ultrahigh vacuum capability. Prior to the start of the isotherm measurements, samples were degassed at 70 °C and 10–8 mbar for 6 h, where the desired temperature inside the vessel was controlled accurately using an external water bath. The vapor adsorption isotherms were obtained at 25 °C for a range of pressures from 0 to 30 mbar with 5 mbar increments.

Results and Discussion

The milling of FF-R led to the preparation of mechanically modified biomass according to the particle size and SA. On the basis of previous reports,[4,5,25,35] it can be inferred that changes in the textural properties are accompanied by variations in the surface chemical properties. These effects are posited to have a direct impact on the hydration phenomena of the various FF samples. To gain insight into the role of mechanical treatment in the physicochemical and hydration properties of FF-R samples, complementary techniques were used to characterize the structure and physicochemical properties of the raw and treated FFs, as outlined in the sections below.

Textural and Surface Properties

The surface morphology of the raw and chemically modified FFs was characterized using SEM. In Figure , the SEM micrographs of the FF-R reveal long strands with fibrous features, along with an incremental reduction in the mean particle size upon mechanical treatment, as noted for FF-420, -210, and -125 samples. A previous report[25] for biomass materials indicate that a reduction in particle size occurs upon grinding, along with concomitant changes in the physical properties and chemical composition. These changes are accompanied by a reduced degree of polymerization, reduced crystallinity, and greater accessibility of surface functional groups.[25] The SEM images provide support of the disintegration of the waxy or organophilic layers that are present in the raw fiber sample, as evidenced in Figure .

Figure 1

SEM micrographs of FF-R and the modified materials: FF-420, FF-210, and FF-125, obtained at low (30×) and high (1.5k×) magnification.

Nitrogen adsorption isotherms provide characterization of the surface and textural properties of the flax biomass. In particular, the Brunauer–Emmett–Teller (BET) model affords estimates of the textural properties: SA, pore width (PW), and pore volume (PV) parameters that can provide insight into the structure–function relationship of such biomass. Figure shows the N2 adsorption isotherms for FF-R and the mechanically treated materials (FF-420 and -210) and reveal that the modified and unmodified materials have limited porosity and SA. The isotherm results for the FF-125 sample are not reported herein because of instability during sample degassing for such fine particle sizes. However, the main nitrogen uptake event in the adsorption profile relates to the adsorption at the powder grain interface, according to the pronounced uptake at relative pressures (P/P0) near unity. The corresponding SA and pore structure parameters of the raw and milled fiber materials are relatively low, as listed in Table . In general, all the flax materials show very small SA (∼1 m2/g) that is characteristic of nonporous fibril materials,[23,24] with negligible pore volumes ranging ∼0.35 to 1.3 × 10–4 cm3/g (cf. Table ). Although the PV is measurably smaller, it is marginally higher for the modified materials, where values are ca. 3- to 4-fold greater. The specific SA of the modified flax decreases with increasing pore structure, which may relate to the fiber geometry of the ground samples.[36] The increased hysteresis loop in the desorption profiles of the modified materials in Figure relates to the effect of capillary condensation. The results are consistent with the greater pore structure accessibility, in agreement with the PV data.[37]

Figure 2

N2 adsorption isotherms of raw and milled flax plotted as the amount of N2 adsorbed as a function of relative pressure at 295 K.

Table 1

Surface and Textural Parameters of Various Flax Materials Estimated From N2 Adsorption Isotherm Results at 77 Ka

parameter	FF-R	FF-420	FF-210	FF-125
SA (cm2/g)	1.29	0.583	0.230	ND
PW (nm)	0.903	0.945	2.30	ND
PV (10–5·cm3/g)	3.50	11.8	13.2	ND
water swelling (%)	672	735	1660	2010
vapor uptake (g/g)	4.26	492	5.26	5.32
KBET (L/mol)	5.34	9.89	10.2	10.5

ND (not detected): due to sample degassing instability for such fine particle sizes.

The crystallinity of the FF materials was estimated from the PXRD results shown in Figure . The XRD profiles reveal that the crystallinity of the FF-R material (fwhm ≈ 2.0) decreases upon modification (fwhm > 2.4) and follows the trend FF-R < FF-420 < FF-210 < FF-125, according to changes in linewidth and intensity of the cellulose I reflections at θ ≈ 15 and 22°.[25] These trends are understood in terms of the changes in the macromolecular structure of the biopolymers upon rupture of intermolecular bonds during grinding that leads to enriched content of the noncrystalline fraction of cellulose, in accordance with greater fibrillation upon milling of the biomass.[38] A greater proportion of amorphous cellulose has been reported for biomass through mercerization of natural fibers via alkali treatment because of disruption of the hydrogen bonding network of the biopolymer.[27,28]

Figure 3

X-ray diffraction spectra of the raw and mechanically modified FF samples.

The surface composition of the raw and ground FFs provides an account of the structural modification upon mechanical treatment of the samples. FT-IR and XPS are sensitive spectral techniques that can be used to characterize the surface chemistry of biopolymer and inorganic composites.[39,40] Previous reports[41,42] reveal that specific components of cellulosic fibers were studied using DRIFT and attenuated total reflection (ATR) methods, where the relative intensities of key spectral signatures were used to estimate the proportion of the respective fiber components. In Figure a, the various IR bands were assigned according to established reports:[35,41,43] ∼2900 cm–1 (C–H stretching), ∼1700 cm–1 (C=O carboxylic acid or ester stretching band for hemicellulose), and ∼1615 cm–1 (pectins). In an ATR characterization study of FFs by Garside and Wyeth,[42] the IR band ∼1600 cm–1 was assigned to adsorbed water and the signature ∼1430 cm–1 corresponds to the C=C in-plane aromatic vibrations of lignins.[41,42] In general, a greater spectral intensity of the IR bands at ca. 1735 and 1615 cm–1 for the mechanically modified FF samples provides support that a greater proportion of hemicellulose and pectin fraction reside at the treated biomass surface. It is inferred that reduction of particle sizes for the FF materials occurs upon milling, where the exposure of fresh biomass surface sites is accompanied by alteration of physicochemical properties relevant to sorption processes (e.g., surface charge, chemical functionality, and porosity). The surface accessible functional groups in the modified materials are likely altered upon milling, along with a reduction in the molecular weight because of changes in the biomass particle size and fiber morphology. The flax biomass is posited to become more amorphous with greater structural defects upon mechanical treatment, in agreement with the SEM and PXRD results. Consequently, the greater abundance of the surface functional groups (e.g., −C=O and −OH) is evidenced by the FT-IR results. In addition, XPS was used to study the surface composition of the FFs according to variable levels of mechanical milling.

Figure 4

(a) FT-IR and (b) XPS spectra of the raw and mechanically modified FF samples. The expanded bonding energy region for N 1s ca. 400 eV is shown as an inset.

The survey profiles (cf. Figure b) of the XPS data for the raw (FF-R) and the mechanically modified fibers (FF-420, -210, and -125) show unique spectral signatures at bond energies ∼285 (C 1s; 75–85%), 532 (O 1s; 16–24%), and 400 eV (N 1s; 1–2%). It is noteworthy that the N 1s region was not observed for the FF-R sample, as shown in Figure b, and a summary of spectral signatures for the convoluted XPS bands is shown in Table . In general, the C 1s spectra of the fiber materials were deconvoluted into three bonding states: C1 ≈ 285 eV (C–C/C–H), C2 ≈ 286 eV (C–OH/C–O), and C3 ≈ 288 eV (C=O), where the latter was of lesser abundance (cf. Table S1 in the Supporting Information). Similarly, the O 1s spectra were deconvoluted as two bonding states: O1 at ∼531 eV and O2 ∼532 eV. It is noteworthy that the bond energies corresponding to the N 1s region are associated with proteins (amides) that are observed solely for the modified fibers. It is noteworthy that the amide (R–NH–C=O) spectral band can be deconvoluted into various contributions because of N 1s (∼400 eV), O 1s (∼531 eV), and C 1s (∼288 and 286 eV) regions,[44,45] in agreement with the results shown in Table S1 in the Supporting Information. The foregoing suggests that mechanically modified fibers have greater abundance of functional groups that become exposed upon milling because of the removal of the waxy surface layer. Furthermore, some of the O 1s bond energies are associated with absorbed water onto the fiber surface sites,[44] in agreement with the FT-IR results presented above.

Table 2

XPS Composition and Bond Energy Data for the Raw and Modified FF Materials

	FF-R			FF-125			FF-210			FF-420
	bond E	FW	%	bond E	FW	%	bond E	FW	%	bond E	FW	%
O 1s	532.40	3.25	16.65	532.60	3.33	22.83	532.45	3.12	23.73	532.50	22.62	22.62
C 1s	285.40	3.10	83.35	284.60	3.48	76.15	285.45	3.55	75.04	285.50	75.64	75.64
N 1s	negligible	negligible	negligible	399.60	3.09	1.02	400.45	2.92	1.24	399.50	1.74	1.74

Fiber Hydration Properties

The physicochemical properties of biomaterials influence their hydration state and water adsorption properties from the vapor and in liquid-phase media. The results described above reveal that mechanical milling results in structural changes: reduced crystallinity and the relative abundance/accessibility of the surface functional groups and pore domains. Hydration properties of flax encompass biomass-solvent (water) affinity via surface interactions (e.g., hydrogen bonding, van der Waals, and ion–dipole or dipole–dipole interactions), according to the surface chemical and textural properties. Thus, the dye adsorption properties of FFs under kinetic conditions provide an estimate of the surface accessible functional groups. In Figure , the kinetics of methylene blue (MB) dye uptake among the various FF samples adopt the following trend: FF-125 ≥ FF-210 > FF-420 > FF-R. The faster dye uptake kinetics relate to samples with greater surface area due to the greater abundance of surface functional groups and the enhanced microporous nature of the material, as supported by SEM, XPS, and FT-IR results. In contrast, the raw material (FF-R) is characterized by a biomass surface with limited functional groups and porosity. Similar kinetic profiles for the FF-R and FF-420 samples at t ≤ 1 h indicate that the structures of these samples are similar, in agreement with their similar pore structure and surface properties estimated from the N2 uptake isotherms, as shown in Table . The increased kinetics for the FF-420 sample at t > 1 h relative to the FF-R sample indicate an improved accessibility of the micropore structure of the milled samples, where diffusion of the dye is greater and adsorption within the pore domains prevails at longer time intervals. The relative hydration state of the flax biomass can be inferred from the kinetic dye uptake results shown in Figure because of the hydrophilic nature of MB and its affinity to the polar heteroatom sites of the fiber surface.[46] A study of FF materials immersed in liquid water (and with water vapor) under equilibrium conditions is presented to afford a complementary understanding of the role of sorption in the hydration properties (vide infra) of this biomass.

Figure 5

Methylene blue (MB) dye adsorption by FF biomass with variable mechanical milling under kinetic conditions at ambient temperature and pH.

TGA of Dry and Hydrated FFs

In spite of the structural complexity of bound water at biopolymer surfaces, thermoanalytical (DSC and TGA) methods and spectroscopic (NMR, FT-IR, and Raman) techniques provide useful information about the molecular level details of hydration phenomena for biopolymer-solvent systems.[23,24] In the case of DSC and TGA, unique thermal profiles of free and bound (weak vs strong) water provide complementary insight into biopolymer hydration. The DSC profiles of the raw and milled materials are shown in Figure a, where the DSC traces reveal broader dehydration transitions at higher temperature for the modified FF samples versus FF-R. The broader transitions for the milled samples suggest that the bound water exists in an ensemble of micro-environments that concurs with the amorphous nature of the biomass, in agreement with the PXRD results. Despite the reduced crystallinity of the ground samples, the appearance of the DSC profiles at higher temperatures is more evident for the FF-125 sample. These results provide support that the milled biomass samples have greater hydrophilicity upon mechanical treatment that affords greater accessibility of the surface functional groups, as supported by the FT-IR and XPS results. The biomass hydration properties are governed by several factors: (1) surface accessibility of functional groups, (2) greater SA and pore structure that favor water sorption, and (3) suitable functional groups that favor biomass–water interactions. The greater fwhm/peak areas of the modified samples in the DSC profiles indicate that the fiber materials have greater affinity for water because of enhanced textural and surface functional properties, consistent with the greater change in dehydration enthalpy (ΔHdeh) which concurs with the SEM, PXRD, and nitrogen adsorption results.

Figure 6

Thermal analyses of the raw and mechanically milled samples: (a) DSC thermograms of hydrated biomass in H2O (l) at equilibrium and (b) TGA spectra of hydrated samples at equilibrium and in the dry state.

Similar to the DSC results, the TGA profiles in Figure b indicate that greater weight loss occurs for the wet/dry milled materials (FF-R > FF-420 > FF-210 > FF-125) because of dehydration effects. The results indicate that the FF-125 sample has the greatest water binding affinity, in agreement with its greater hydrophilic character, as supported by the DSC and XPS results. In general, the thermal degradation of lignocellulosic natural fibers involves several thermal events:[47] release of bound water (50–150 °C), degradation/depolymerization of hemicellulose (250–370 °C), and degradation of α-cellulose (340–370 °C) and lignins (200–500 °C). The additional event at ∼300 °C for the milled samples in the dry state supports that the presence of greater fractions of hemicellulose/pectin occur at the surface of the treated biomass, in agreement with FT-IR results. It is noteworthy that the dry FF-R sample decomposes at a slightly higher temperature according to the TGA results because milling is inferred to degrade the biomass assembly. This trend occurs according to the lower decomposition temperatures of FF-420, -210, and -125 samples.[48] It is noteworthy that the diminishing trend in the weight losses occur because of the decomposition of the FFs ∼300–350 °C in the wet state, as observed in Figure b. The greater hydration of the mechanically modified biomass contributes to the disruption of intermolecular H bonding that results in a reduced decomposition temperature (cf. Figure b).[24]

Water Swelling and Retention Values

The thermogravimetric profiles described above were complemented by gravimetric swelling of the FF biomass in water in order to assess the ability of the flax biomass to retain sorbed water (Figure ). The greater swelling and WRVs in Figure (FF-125 > FF-210 > FF-420 > FF-R) reveal unique trends that parallel the greater hydrophilicity and hydration of the modified materials. These observations corroborate the DSC/TGA results, where parallel trends are noted for the greater surface/textural properties and amorphous nature of the modified FF samples. Also, this trend is supported by the XPS, SEM, gas/dye adsorption, and PXRD results. According to the DSC/TGA results, the FF-125 sample was characterized by tightly bound water in contrast to the weakly bound water for the FF-R sample. The thermal analysis results are further supported by the WRV results shown in Figure . The greater swelling of the mechanically modified flax materials indicates that the crystallinity of the biomass fiber structure affects the water uptake and hydration properties, in agreement with the greater amorphous (fibrillated) nature of the milled samples.[49] As indicated, the modification of FFs by milling attenuates the specific SA (cf. Table ).

Figure 7

Gravimetric solvent (water) swelling and WRVs of pristine and modified FFs.

However, solvent swelling in water is anticipated to result in a marked increase in the “apparent” SA because of hydration-induced structural changes in the biomass.[50] Adsorption studies of the various flax materials in the presence of water vapor provide further insights into the hydration properties. As such, hydration properties are of key importance to applications for adsorption-based energy wheels in HVAC systems.

The vapor adsorption isotherms and the related adsorption parameters are listed in Table , based on results presented in Figure . In general, the S-shaped isotherms shown in Figure correspond to the classification system described by the BET model that describe trends noted for cellulose-based hydrophilic materials.[7] The greater moisture capacity for the milled FFs are higher: FF-420 (Qm ≈ 4.92 g/g), FF-210 (Qm ≈ 5.26 g/g), and FF-125 (Qm ≈ 5.32 g/g), when compared to the pristine FF-R material (Qm ≈ 4.26 g/g). Moreover, the binding affinity (KBET) values for the milled samples exceed those of the pristine FF-R (cf. Table ) that further supports an increase in the textural properties, along with greater surface accessibility of the functional groups for the milled samples. It has been shown that modification of biomass by acetylation reduces the moisture uptake of the modified materials by up to 65%,[29] indicating the key role of accessible polar active sites for water uptake (Table ).

Table 3

Water Vapor Adsorption Isotherms and Adsorption Parameters for the Raw and Modified FF Samples at 298 K

FF-samples:	FF-R	FF-420	FF-210	FF-125
Qm (g/g)	4.26	4.92	5.26	5.32
KBET (L/mol)	5.34	9.89	10.2	10.5
C	1.15	1.17	1.17	1.16
R2	0.993	0.998	0.999	0.999
reduced (χ2)	0.392	0.104	0.0575	0.104
SA (m2/g)	153	177	183	191

Figure 8

Water vapor adsorption isotherms and the uptake parameters for the raw and modified FF samples.

Conclusions

This contribution reports on a study of the structural and physicochemical properties of FF-R and modified forms, according to variable levels of mechanical milling and grinding. The modified FFs were sieved through various mesh sizes (420, 210, and 125 μm) denoted as FF-420, FF-210, and FF-125, respectively. The structural features of the FF-R and mechanically modified forms (FF-420, -210, and -125) reveal differing surface chemistry and textural properties (SA and pore structure) according to several complementary techniques: microscopy (SEM), spectroscopy (FT-IR, XPS, and PXRD), and thermal analyses (DSC and TGA). The adsorption properties of the materials were assessed using solvent (water) swelling and adsorption isotherms using nitrogen gas, water vapor, and a hydrophilic dye probe in aqueous solution. The hydration properties of the FF biomass are governed by several factors: (1) the textural properties (SA and pore structure), (2) surface properties (e.g., surface charge), and (3) the availability of accessible surface sites, as revealed by the complementary methods reported herein. Gravimetric water swelling, WRV, and vapor adsorption results provide further support that size reduction of FFs upon milling is accompanied by changes in physicochemical properties such as surface charge, crystallinity, thermal stability, morphology, and accessibility of surface functional groups. Because natural FFs are composed of various biopolymer components (cellulose, hemicellulose, pectins, and lignins) with variable composition and C/OH ratios, changes in the chemical composition are anticipated upon mechanical grinding of the fibers because of changes in the particle size and fiber fibrillation effects. Consequently, alteration of the structure and physicochemical properties of the milled flax samples results in significant changes to their hydration properties, as evidenced by the enhanced water affinity for both the liquid and vapor. A key outcome of this study reveals that mechanical milling of flax biomass contributes to valorization of fibers to yield biomaterials with tunable physicochemical properties. In turn, promising applications of agricultural biomass waste are anticipated in energy, food, biomedicine, and adsorbents for environmental remediation.[51]