Nanomechanical characterization of hemp fiber with atomic force microscopy

Abstract

Hemp and its composites are becoming increasingly popular for their environmental benefits and mechanical strength, making them critical in the development of sustainable materials. This study investigated the nanomechanical properties of industrial hemp fibers through the application of Atomic Force Microscopy (AFM). An AFM-based examination of individual fibers allows for the accurate assessment of nanoscale characteristics without interference from extraneous particles, providing a clear insight into individual fiber features. Using Sneddon’s analytical model for a paraboloid shape AFM tip geometry, the elastic moduli of the hemp fiber were calculated at different indentation depths. The maximum elastic modulus of $3.01 \pm 0.22$ GPa was observed at the lowest indentation depth. These findings demonstrated the mechanical nature of hemp fibers, showing that radial measures deviate significantly from longitudinal measurements commonly reported in the literature.

Keywords

hemp (Cannabis sativa L.)atomic force microscopy (AFM)elastic modulus nano indentation

Introduction

Fibers are thread-like structures that can be derived from natural or synthetic sources and used as reinforcement materials in composite products.¹ They have been widely used in various applications including their utilization in building materials,² particle and insulation boards, automotive parts,³ medicine,⁴ human food, animal feed,⁵ cement composites,⁶ and for other biopolymers.⁷ Despite their advantages such as improved strength and durability, synthetic fibers can cause problems for the environment since they are not biodegradable, require a lot of energy to produce, and leak microplastic particles over their lifetime. Their productions also have higher energy emissions and can have harmful effects on human health.^8–10 On the other hand, natural fibers can be sourced from plants or animals.¹¹ High degrees of processing flexibility, thermal stability, renewability, acoustic insulation, low specific weight that results in high specific strength, and stiffness are only a few of their characteristics.^12–14 Moreover, they are eco-friendly and biodegradable which makes them even more valuable in the present world.

Hemp is an example of natural fibers that fits into the categories of both bast and core fibers, noted for its environmental advantages.¹⁵ Naturally, hemp is the oldest and most environmentally beneficial material. While known for its psychoactive tetrahydrocannabinol (THC) content in marijuana and hashish, industrial hemp with less than 0.2% THC is not narcotic.¹⁶ Figure 1 shows an image of industrial hemp (Cannabis sativa L.). Advancements in modifying lignin in hemp fiber-based composites have greatly improved their capacity to be designed for specific structures, have several functions, and perform well mechanically.¹⁷ As a result, they are well-suited for use in construction and soundproofing. Furthermore, hemp fibers are increasingly recognized for their structural applications in sectors such as automotive,¹⁸ aerospace, and construction, where they are utilized in natural fiber-reinforced polymer composites and engineered composites for sustainable, lightweight, and mechanically strong alternatives to synthetic materials. Moreover, hemp’s environmental sustainability is improved by its capacity to absorb heavy metals, act as a carbon sink, and prevent the growth of weeds. Due to its versatility hemp fiber is getting popular as green and sustainable material in many industries.^19–21 Despite the advantages, it is often difficult for hemp fiber composites to match synthetic counterparts in mechanical strength. For instance, after being exposed to the weather, its tensile strength may drastically decrease. It is possible for the mechanical qualities of hemp ropes to noticeably deteriorate due to aging factors such as moisture and UV radiation.^22,23

Figure 1.

Bunch of industrial hemp fiber from europe.

The chemical composition of hemp mainly includes different cellulose, hemicellulose, pectin and lignin.^16,24 Due to the nature and versatility of hemp’s chemical composition, in situ lignin modification is necessary for structural applications of hemp fiber composites.¹⁷ There are a number of factors that affect the properties of hemp. These encompass macroscopic features like fineness, porosity, size, and form of the lumen in addition to microscopic ones like crystallinity, microfibril angle, and crystal alterations. Another significant consideration is also the historical background of the fibers, which includes their age, retting and separating circumstances, geographic origin, and rainfall during growth. Furthermore, tensile speed, gauge length, moisture content, temperature, and the different cross-sections of the fibers at different locations all have a major impact on the mechanical properties under measurement.²⁵ The tensile modulus of a bundle of hemp fiber is found to be 20-70 GPa by researchers and the fracture toughness to be 270-900 MPa.²⁶ Shahzad²⁷ investigated the tensile properties of a single hemp fiber and the tensile modulus was found to be $9.5 \pm 5.8$ GPa and the tensile strength to be $277 \pm 191$ MPa.

Due to its high strength, hemp is considered as a powerful addition in composite matrices by researchers. Hemp fiber-reinforced composites (HFRCs) are crafted by combining hemp fibers with a matrix material, which enhances the mechanical properties of the final composite through fiber reinforcement. The matrix material’s composition plays a crucial role in determining the mechanical properties of HFRCs. According to studies, performance can be greatly impacted by the kind of matrix that is employed. For instance, compared to hemp composites employing cellulose acetate or glass fiber-reinforced polymer matrices, those using a vinyl ester matrix exhibit better tensile and flexural strengths.²⁸ Studies reveal that hemp fibers are more elongated and have a higher ultimate strength than synthetic fibers, which improves the tensile and flexural properties of composite materials. These composites, which have been utilized for over a millennium, are witnessing a resurgence in interest due to their low cost, environmental benefits, and potential to substitute synthetic fibers in various applications.^29–32 Since hemp fiber shows so much promises, the macro-nanoscale characterization of a single hemp fiber is indispensable. The macro and nanoscale characterization helps eliminate the influence of foreign particles. One way to find out the nanoscale properties is Atomic Force Microscopy (AFM). AFM-based methods can both measure a broad range of mechanical properties in living and non-living organisms and apply precise mechanical forces to them.³³ Owing to its nanometer-scale probe tips, exceptional force sensitivity, and high translational precision, AFM has been an optimal instrument for conducting nanoindentation experiments on a diverse range of nanoscale structures in recent years.³⁴ AFM also provides sample surface information in nanometer scale. AFM-based nanoindentation provides a force versus sample deflection data that can be analyzed and used to measure the local elastic modulus of the samples.²⁶ Furthermore, hemp fiber is heterogenous and its mechanical properties are anisotropic. Accurate model and design efforts for hemp fiber structures require the elastic properties of these fibers in the radial direction. According to the best knowledge of the authors, there were no particular studies found in literature emphasizing on the AFM based nanomechanical properties of hemp fiber. In this article, AFM was used to characterize the elastic properties of industrial hemp fiber in its radial direction. Sneddon’s model for paraboloid sized tip was used to measure the elastic modulus of the hemp fiber.

Materials and experimental procedure

Materials

Industrial hemp from Europe (Country of origin: Germany) was used as the test material. The bleached hemp had a light golden color. Bleached hemp fibers exhibit reduced mechanical strength and elasticity compared to raw hemp due to the removal of hemicellulose and lignin during bleaching. Although bleaching enhances processing characteristics, it compromises fiber integrity, particularly in structural applications.³⁵ 10 individual hemp fibers were separated from the bunch and the diameter was measured at five locations for each fiber using a digital optical microscope (Keyence, model: VHX, Osaka, Japan) at 21°C and $30 % humidity$ .

Sample preparation

For AFM (D3100, Veeco) imaging and nanoindentation, several hemp fibers were cut at 3-6 mm lengths. To avoid impact of any residual stresses generated from cutting force, the scanning and indentation were conducted at the locations far away from the fiber ends. Reactive resin (LeapTech, model: Huntsman^TM Araldite GY 6010, Charleston, SC, USA) was utilized for keeping the hemp fiber rigidly attached to a glass slide. The resin and hardener (LeapTech, Charleston, SC) were taken in a 2:1 ratio in separate containers first. Both the containers were heated up to $45 °$ Celsius to increase their reactivity and then the chemicals were mixed together. The mixture was stirred rigorously manually and then a very thin layer was spread on the glass slide. After 1 h, when the mixture had jelly-like texture on the glass slide, the small fibers were gently placed on the resin film and another clean glass slide was used to press them. This was done to avoid epoxy contamination of the hemp fiber surface. After 12 h, the resin mixture became firm enough for the AFM imaging and nanoindentation. To confirm that the resin had no effect on the hemp top surface, SEM and 3D optical microscopy (Keyence, model: VHX, Osaka, Japan) were conducted on AFM indented hemp samples. For SEM, glass slides containing hemp fibers embedded in epoxy were attached to cylindrical aluminum mounts with conductive tape (3M XYZ-Axis Tape, Ted Pella, Redding, California, USA). The mounted slides then were coated with a conductive layer of gold (Cressington Sputter Coater 108Auto, Ted Pella, Redding, California, USA). Images were obtained with a JEOL JSM-6490LV SEM (JEOL USA, Inc., Peabody, Massachusetts, USA) operated at 15 kV. Both scans shown in Figure 2 indicate that there was no detectable resin contamination on the top surface of the fiber where indentations were performed. Moreover, the 3D scan of epoxy attached fiber showed that the epoxy surface was only in contact with the fiber at its lower surface. The gap between the top surface of fiber and epoxy was more than the radius of the fiber. This ensured that epoxy surface had no effect on the AFM readings made the top surface of the fiber.

Figure 2.

Scan of prepared sample surfaces for AFM (a) SEM scan (b) 3D optical microscopic scan.

Testing

Figure 3(a) shows the picture of AFM equipment and 3(b) depicts a schematic of the AFM used for this experiment. The AFM imaging was done using a tapping mode probe (PPP-NCHR-20, Nanosensors). The scanning size was $5 μ m \times 5 μ m$ . The scanning rate was $0.256 H z .$ After surface scanning and finding a flat region, a diamond probe (DNISP, Bruker) was utilized to perform nanoindentations. The AFM indentations were made away from the surface irregularities. To minimize piezo hysteresis the scan size was gradually lowered from $5 μ m \times 5 μ m$ to $1 n m \times 1 n m$ before nanoindentation. The indentation voltage of the piezo sensor was set at 1 V and 30 (6 indentation at 5 separate fibers each) indentations were done on the hemp fiber. A distance of $1.5 μ m$ was set between each indentation to avoid any effect from neighboring indentations. Then the retract curve from each indentation was used to find the elastic modulus of the fiber at all 30 positions and different indentation depths. The 3D profile of the diamond tip was measured by microscopically 3D scanning it by using Keyence optical microscope prior to the indentation test.

Figure 3.

(a) Veeco D3100 AFM (b) Schematic of Atomic Force Microscopy.

Theoretical background and analysis

Different methods have been established to set up mathematical models which can analyze the force-sample deflection data found from AFM nanoindentations. The Hertz model is widely used because it is simple and effective for analyzing soft biological samples.³⁶ The Sneddon model, which treats the sample as a linearly elastic half-space, is another method used in AFM experiments to assess mechanical properties.³⁷ The PLR model, which is based on finite element modeling, provides information on the mechanical properties of samples under various loading conditions in AFM indentation tests.³⁸ The DMT model takes surface tension effects into account and focuses on adhesive contact behavior, particularly for soft materials.^39,40 Furthermore, adhesive contact reactions in AFM studies are analyzed using the JKR model, which offers the theoretical foundations for precise analysis.⁴¹ Every model provides a different perspective on the mechanical characteristics of samples and is essential for comprehending the behavior of indentation in AFM studies.

Sneddon’s model was applied to extract the elastic modulus by assuming a paraboloid indenter in this case. It examines how a firm, symmetrical indenter presses into a flexible material (visually interpreted in Figure 4), using a mathematical formula that includes the elastic modulus.⁴² The Sneddon elastic model is also useful for investigating contacts other than basic spheres, as it can accommodate more complicated geometries.⁴³

Figure 4.

Schematics of a rigid punch pressing against an elastic material with modulus E in an axisymmetric manner.

Sneddon’s⁴⁴ analytical model uses integrals describing a contact between a plane surface of the sample and an axisymmetric tip.⁴⁵ The model led to following integrals⁴² :

δ = \int_{0}^{1} \frac{f^{'} (x)}{\sqrt{1 - x^{2}}} dx

(1)

F = 2 a (\frac{1 - v^{2}}{E}) \int_{0}^{1} \frac{x^{2} f^{'} (x)}{\sqrt{1 - x^{2}}} d x

(2)

where

v

is the sample Poisson’s ratio which in the case of hemp fiber is found to be 0.40,⁴⁶ and the fiber is considered as a linear elastic material,

a

is the contact radius. In these equations, the indenter shape is described by the function

f (x)

. The analytical solution of the integrals (1) and (2) can be found in literature for common geometries.

To find the indentation force and sample deflection, the spring constant $k_{c}$ and the sensitivity of the diamond probe $S$ was provided by the manufacturer. The retract curve data had calculated Z in nanometers in the x-axis and deflection V in volts in the y-axis. To convert this to force $F$ versus sample deflection $δ$ curve, the following equations are used:

δ = Z - V \times S

(3)

F = V \times S \times k_{c}

(4)

The force $F$ versus sample deflection $δ$ data was then analyzed to find the elastic modulus of the hemp fiber in radial direction. During the retraction process, the indenter initially remains attached due to adhesive forces such as van der Waals forces, electrostatic force, capillary force, and chemical bonds.^47,48 However, when it reaches a certain point of retraction, the indenter abruptly detaches as these adhesive forces become insufficient to handle the pulling force exerted by the piezo.⁴⁹In this article these forces were neglected during elastic modulus calculation by considering the initial contact point in the plot as a point which has the same amount of force as in the flat region.

The apex of the tip was considered as a paraboloid structure for this research. The relations for a paraboloid geometry can be interpreted into the following equations:

F = \frac{4 E \sqrt{R}}{3 (1 - v^{2})} δ^{\frac{3}{2}}

(5)

a = \sqrt{δ R}

(6)

where,

R

can found out using the radius of the indenter tip

R_{i}

and radius of the sample

R_{s}

\frac{1}{R} = \frac{1}{R_{i}} + \frac{1}{R_{s}}

(7)

Since the radius of the tip is very small compared to the radius of the sample

\frac{1}{R_{s}}

is considered to be zero. So, R is considered as the radius of the indenter. Here, E is the elastic modulus of the sample. For Bruker DNISP tip R had a nominal value of 40 nm. A simple image pixel calculation was also done to verify this value.

Rewriting equation (5) as the following:

F = C \times {(\sqrt{δ})}^{3}

(8)

Where

C = \frac{4 E \sqrt{R}}{3 (1 - v^{2})}

. Using this equation, a cubic curve fitting in MATLAB was done to find out the value of C. Then the elastic modulus was found using the following equation:

E = \frac{3 C (1 - v^{2})}{4 \sqrt{R}}

(9)

Using the above equation, the elastic modulus was calculated for different nanoindentation depths. The nanoindentation depth was considered after the contact between the sample and the AFM tip.

Result and discussion

Figure 5 shows the profile of the AFM nanoindenter tip. The tip had a three-sided pyramid structure and a paraboloid structure at the apex region. By doing image analysis by pixel calculation the apex of the tip was found to have a diameter of around $80 n m$ . The pixel width of the processed image was 8.5 nm at 10,000x image taken at 4k resolution. Thus, a relative error of ∼0.1 at the apex diameter calculation could be generated due to the image processing. So, the elastic modulus calculation with equation (9) can have a relative error of ∼5%.

Figure 5.

Optical Microscope image of the nanoindenter tip.

A representative optical image of a fiber is shown in Figure 6(a), the average diameter was measured as $28.01 \pm 2.84 μ m$ . Figure 6(b) shows the AFM height image of the surface of the hemp fiber in $5 μ m \times 5 μ m$ scale.³⁷ The image shows an almost a flat surface within this scaled region.

Figure 6.

(a) Determination of diameter of fiber by Optical Microscope; (b) AFM height image a of fiber surface.

The retract curves for all 30 indentations are shown in Figure 7. The negative deflection region in the plot is the region of contact between the tip and the fiber. The flat region along the horizontal axis in the plot is the region before the tip is in contact with the surface.

Figure 7.

AFM 30 nanoindentation retract curves interpreted into Force versus Sample Deflection.

The elastic modulus was calculated using the Sneddon’s model for linear elastic region for a plane surface. Elastic modulus E is shown for different indentation depths. Table 1 shows the summary of the elastic modulus at different indentation depth. The empirical formula for calculating

E

(GPa) of a single hemp fiber for a specific sample deflection

δ

(nm) was found to be:

E = 8.1862 δ^{- 0.413}

(10)

Table 1.

Summary of elastic modulus calculated in different indentation depth.

$I n d e n t a t i o n D e p t h δ (n m)$	Elastic Modulus E (GPa)	p Value
$11.17 \pm 0.65$	$3.01 \pm 0.22$ 7	0.0016
$24.13 \pm 0.95$	$2.20 \pm 0.112$	0.0029
$37.39 \pm 1.34$	$1.84 \pm 0.074$	0.0062
$51.08 \pm 1.78$	$1.61 \pm 0.055$	0.0168
$65.21 \pm 2.22$	$1.46 \pm 0.044$	0.0374
$101.01 \pm 3.38$	$1.22 \pm 0.030$	0.1105
$137.79 \pm 4.54$	$1.07 \pm 0.023$	0.1649
$175.24 \pm 5.68$	$0.97 \pm 0.020$	0.1882
$213.42 \pm 6.69$	$0.89 \pm 0.017$	0.1839

Figure 8 shows the elastic modulus versus indentation depth plot. The trend in the plot is matched with the plots for other materials in literature.^49,50 It can be seen that the elastic modulus gets lower as the indentation depth increases. The initial decrease of the elastic modulus with respect to the indentation depth or the slope is much higher than the latter portion. Also, it is seen that in lower indentation depths the elastic modulus varies more in 30 different indentations than in higher indentation depth. This variation could be due to the environmental effects on the surface in the peripheral region. Additionally, the declining trend of elastic modulus with increasing indentation could be a result of multiple effects. Firstly, the AFM tip’s cross sectional size increases and shape of the change from a paraboloid shape to three-sided pyramid shape as the indentation depth increases. In Figure 8 two distinct slopes (dashed lines) for changing elastic modulus could be identified with varying indentation depth. The tangent of the two sections of elastic modulus curves intersects at ∼ 40 nm which is also the radius of the paraboloid section of the tip. This supports that the transition from paraboloid to pyramid shape of indented profile could be a contributor to this transition of rates of elastic modulus change. Secondly the material composition of the hemp fiber is different at different indentation depths. Thirdly, when the indenter starts penetrating the lamellae and fibrils starts moving causing a relative movement among the fibrils which is different for indentation depth. Hemp fibers have a microfibrillar angle that varies between $2 ° and 6 °$ .⁵¹ This microfibrillar angle can also have an effect on the relative motion among the fibrils and thus having an effect on elastic modulus upon nano indentation. The average elastic moduli values for different indentation depths were statistically analyzed with single factor ANOVA (α = 0.05). The Least Significant Difference (LSD) of ANOVA model was 0.0584. By comparing the differences of average elastic moduli at different indentation depths, it was observed that all the calculated mean elastic moduli for nine mean indentation depth levels were statistically significantly different at 95% confidence level.

Figure 8.

Variation of elastic modulus with respect to indentation depth.

As hemp fiber is heterogenous and its properties can vary significantly with variation of location and selection of fiber, elastic moduli measured from different fiber were analyzed statistically. Statistical significance of difference of elastic moduli calculated from each fiber was analyzed with single factor ANOVA at 95% confidence level. The p-values of this statistical analysis at each indentation depth are shown in Table 1. A trend of increasing p-values with increasing indentation depth can be observed in this p-value distribution. The calculated mean elastic moduli statistically significantly different among different fibers at indentation depths up to 65.21 nm. In contrast, elastic moduli at mean indentations ≥101.01 nm, showed no statistically significant difference among measurements form different fibers. This implies that elastic moduli calculations are more consistent when the indentation is deeper than 101.01 nm.

The elastic moduli of hemp, found in this study for all indentation depths, are much lower than what is previously reported in the literature because they were all measured in longitudinal direction. The elastic modulus in the longitudinal direction was previously reported as $9.5 \pm 5.8$ GPa.²⁷ This kind of mismatches also observed for bamboo fiber⁵² and kenaf fiber.⁵³ Such phenomena can be explained by the fact that the single components of the fiber called fibrils are oriented towards the longitudinal direction which results in a higher modulus towards that direction. In radial direction when the indenter presses into the fiber the fibrils and lamella start to move away from each other whereas in longitudinal direction when tensile test is done the components of the fiber starts to move towards each other. So, in these two different loading situations the loading pattern on the binding components are very different. This can be one of the reasons why the modulus in radial direction is significantly less than in longitudinal direction. Knowing the radial elastic properties is essential for predicting critical instability conditions, bonding responses, failure modes, and the effective moduli of materials.⁵⁴ Additionally, Determining the elastic modulus in the radial direction is particularly vital, as it directly correlates with the bending stiffness and strength of fiber tubes, thereby providing a precise assessment of their mechanical performance and structural integrity.⁵⁵

Conclusion

This study effectively used AFM to describe the nanomechanical properties of industrial hemp fibers, providing important insights into their nanoscale structural behavior. Sneddon’s model for axisymmetric indenter and plane surface was used to predict the elastic modulus of the single hemp fibers. The study demonstrated the variation in elastic modulus of hemp fibers with respect to indentation depth using multiple AFM nanoindentations and the Sneddon’s analytical model for paraboloid geometries. The results highlight the unique mechanical properties of hemp fibers when examined in the radial direction, as opposed to the longitudinal metrics commonly described in the literature. This radial elastic modulus is a significant information for modeling and predicting the anisotropic behavior of hemp fiber structures. Elastic modulus was found to be in the range of $3.81 G p a - 0.89 G p a$ for the indentation depth of $11.17 n m - 213.42 n m .$ This directional variation in elastic characteristics is critical for the development of sophisticated composite materials, providing a route to optimizing fiber-reinforced products for specific structural requirements. This study also investigated the statistical variation of measured elastic moduli at different fibers and at different indentation depths. The study reported a minimum indentation depth threshold above which the elastic modulus measurements are statistically consistent.

Footnotes

Acknowledgments

This material is based upon work supported by the National Science Foundation under Grant No. 0619098. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Science Foundation; 0619098.

ORCID iDs

Sowmik Chowdhury

Xinnan Wang

Chad A Ulven

Data availability statement

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.*

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