Are interfaces the key? Validation of interfacial interactions in bio-based polyurethane-wood composites by quantitative nanomechanical property mapping
- Open Access
- 01-01-2026
- Original
Published in:
Activate our intelligent search to find suitable subject content or patents.
Select sections of text to find matching patents with Artificial Intelligence. powered by
Select sections of text to find additional relevant content using AI-assisted search. powered by (Link opens in a new window)
Abstract
Introduction
Progressive degradation of the natural environment and climate change are the main problems of society. These processes are mainly caused by excessive pollution of water, soil and air by industrial production, transport and daily human activities. Further intensive exploitation of natural resources and an increase in pollutant emissions may lead to the depletion of fossil fuels and irreversible changes in the environment (Omer 2008; Mondal and Palit 2022). Currently, researchers and industry are developing new lower-emission production technologies and sustainable materials which will meet market expectations. One of the possible solutions to this problem may be the development of materials, which production will be based on the utilization of natural resources. Biomass is a promising alternative to petroleum-derived substances and is characterized by availability, renewability, biodegradability, and environmental compatibility (Isikgor and Becer 2015; Karmakar et al. 2020). To date, researchers have documented a wide range of renewable resources which are suitable for the manufacturing of functional materials with reduced carbon footprints and limited energy and water consumption (Yáñez-Pacios and Martín-Martínez 2018). One of the most promising natural resources is wood and its components (Akpan et al. 2021).
Wood and wooden resources have been used by humans for thousands of years, mainly as a construction material, source of fuel and raw material for toolmaking. According to the FAO reports, ~ 4.0 billion m3 of roundwood is produced annually all over the world (FAO 2023). An important issue that determines its use is the unique structure. Wood has a hierarchical porous structure, which depends on the type of wood, provides different features such as lightweight, exceptional mechanical properties and elasticity (Chen et al. 2020). The rigid cell walls of wood are composed of three biopolymers - cellulose, hemicellulose, and lignin. These substances do not simply occupy distinct spaces, but they also interact physically and chemically. For this reason, wood can be identified as a biomass composite material (Effah et al. 2015). The properties of wood are strongly correlated with the arrangement of cellulose fibrils within the lignin and hemicellulose matrix. These fibrils have crystalline and non-crystalline regions and consist of bundles of cellulose microfibrils. The microfibrils are composed of elementary fibrils with diameters around 3 nm. At the molecular level, cellulose is composed of D-glucose units, which are connected through covalent bonding, resulting in the formation of linear cellulose chains. Moreover, between chains, there are intra-chain and inter-chain hydrogen bonds, which contribute to the unique wood properties (Chen et al. 2020; Lou et al. 2023).
Advertisement
On a bigger scale, each kind of wood may possess different structural types with different ratios of components. The diversity of wood in the cell type, shape, size and other parameters results in the existence of various types of wood anatomical structures. Moreover, the presence of other wood components (extractives, resins, stilbenes, flavonoids, etc.) significantly influences its properties and causes changes in wood colour, odour, density and durability of wood (Friedrich 2018). The combination of all the above-mentioned components has a significant influence on the interactions between wood and other components. This is especially important for adhesion between phases in wood polymer composites (WPC) (Yáñez-Pacios and Martín-Martínez 2018; Hao et al. 2021). WPCs are materials composed of a polymer matrix and wood powder/fiber, which act as the reinforcement. Usually, commonly available thermoplastics such as polyvinyl chloride (PVC) (Petchwattana et al. 2012), polyethylene (PE) (Kajaks et al. 2018) and polypropylene (PP) (Kuka et al. 2020) are used. Taking into account the significant differences between the chemical structure and polarity of wood and polymeric matrices, there is a noticeable problem with the adhesion between both phases. In most cases, the limited adhesion could be improved by the implementation of mostly expensive wood surface treatment or by the addition of specially designed coupling agents (Moghadamzadeh et al. 2011). Another approach may be the development of new types of matrices for WPC manufacturing. In the scientific literature, we can find reports that show the possibility of using matrices consisting of polyhydroxyalkanoates (PHAs) (Chan et al. 2020), polylactide (PLA) (Liu et al. 2018), polyamide (PA) (Hirsch and Theumer 2022) or polyurethane (PU) (Olszewski et al. 2023a) matrices and others.
Jubinville et al. (2022) studied the influence of thermo-mechanical recycling processes on the structure and physicochemical properties of PLA and maleic anhydride-modified PLA. Materials were manufactured using unmodified wood flour (WF) and WF modified by base-catalyzed reaction with dodecenylsuccinic anhydride. Modification of WF was conducted to reduce the polarity of wood and improve compatibility between components. After the recycling process, the obtained materials retained their properties, but the materials showed increased crystallinity and tensile modulus. The increase in crystallinity was assigned to the nucleating effect of wood flour in PLA-based composites. Moreover, modification of both phases increased the compatibility between components but did not significantly affect tensile strength and modulus of the components. The authors suggest that wood modification has a higher influence on hydrothermal resistance than subjection to maleation.
Chan et al. (2019) investigated the influence of the natural weathering process of biodegradable composites composed of wood flour (WF) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). The composites were manufactured with 0, 20 and 50 wt% wood contents. The authors compared manufactured composites to materials composed of PLA and PE. The authors noticed that UV exposure caused bleaching of all investigated materials but had an insignificant influence on mechanical properties. The weathering tests showed that over the 12 months, pristine PHBV and PHBV/20 wt% WF showed stable mechanical properties. In this case, the wood particles were encapsulated by the PHBV matrix and acted as a barrier. On the other hand, Young’s modulus and tensile strength of all weathered composites, which contained 50 wt% WF, decreased after 12-months of weathering. The authors stated that the decrease in mechanical properties may be caused by the worsened wood-polymer interactions, the introduction of defects into the material and on the surface by fungal attack and enzymatic degradation.
For this reason, an extremely important issue for the development of WPC is understanding its nano- and micro-structure. It is necessary to use advanced research techniques, which will allow for a detailed description of the structure, nano- and micro- scale properties and adhesion. One of the available tools to study this may be atomic force microscopy (AFM) set to PeakForce quantitative nanomechanical property mapping (PeakForce QNM) (Ren et al. 2015, 2019).
Advertisement
This technique is based on PeakForce tapping mode, which is based on the intermittent contact of the AFM probe with the sample. At the same time, the probe tip is moved along the scanning area. The measurement allows for simultaneous surface mapping and determination of nanomechanical properties, which include adhesion and modulus between the tip and the investigated surface. This allows for a comparison of the nanomechanical properties at each point of area, and a distinction of phases, which allows for a deeper understanding of the behaviour of the structurally complex material (Cano et al. 2017). This can be a very interesting concept in the case of polyurethane-wood composites (PU-WC), which are composed of two phases that show strong interactions with each other, and they have significantly different mechanical properties.
In our previous studies (Olszewski et al. 2023a), we developed a manufacturing method of catalyst-free PU-WCs with high wood content and validated the influence of BP on the structure and properties of these materials. In this study, we determined the micro- and nano-structure of PU-WCs with different additions of BP (from 0 to 80%). Moreover, we focuse on the examination of mechanical properties at the nanoscale and interactions between two phases. For this reason, five sets of PU-WCs were tested by PeakForce quantitative nanomechanical property mapping (PeakForce QNM) and scanning electron microscopy (SEM) of samples prepared using a microtome. The research conducted allowed for the characterisation of nano- and micro-structure of materials, interactions between phases of composites and properties on the micro-scale.
This research, for the first time, provides the detailed nanoscale characterisation of interfacial interactions within polyurethane–wood composites (PU-WCs), particularly modified with BPs. While previous studies have focused mainly on mechanical or thermal properties, this work expands the current state of the art by describing interfacial interaction phenomena in the BP content function. Through the combination of AFM, PeakForce QNM, and SEM, this study quantitatively links changes in interphase thickness and adhesion. The observed change in interphase width and adhesion strength gives valuable insight into compatibility between wooden filler and bio-based PU matrices. These findings not only fill a knowledge gap regarding the interfacial structure–property relationship in PU-WCs but also are a step toward designing next-generation PU-WCs with tailored properties.
Materials and methods
Materials
Pine wood shavings (Pinus Sylvestris) were obtained from a local sawmill (Gdańsk, Poland). Wood shavings were obtained during the cutting of wood logs. The wood was divided by size and then dried in an oven at a temperature of 100 °C for 24 h. Petrochemical polyols Rokopol® M6000 (polyoxyalkylene triol based on glycerol, LOH = 28 mg KOH/g), and Rokopol® RF551 (polyether sorbitol polyol, LOH = 420 mg KOH/g) were supplied by PCC Rokita S.A (Brzeg Dolny, Poland). Polymeric methylene diphenyl diisocyanate (pMDI, 31% free NCO groups) was supplied by Borsodchem (Kazincbarcika, Hungary). TEGOSTAB B 8465 (non-hydrolysable polyether-modified polydimethylsiloxane (PDMS) copolymer) purchased from Evonik Industries AG (Essen, Germany) was used as a surfactant.
Manufacturing of PU-WCs
The BP was synthesized according to our previous work (Olszewski et al. 2022, 2023b). To eliminate impurities and remove wood shavings particles larger than 2 mm, pine wood shavings underwent segregation via sieving followed by a drying process at 100 °C for 24 h. All PU-WCs were manufactured using a single-step method employing a two-component system. This system involved blending Component A (Rokopol® RF551, Rokopol® M6000, BP, and a surfactant) with Component B (polymeric methylene diphenyl diisocyanate (pMDI). The isocyanate index (INCO - NCO/OH ratio during synthesis) was set to 1. Notably, no catalyst was added during PU-WCs manufacturing. The dried pine wood shavings were thoroughly mixed with the prepared substance for 10 min using a planetary mixer to ensure uniformity. The resulting material was placed into a steel mold and then hot pressed at 100 °C for 15 min. Then the samples were transferred to a cold press and pressed for 5 min to retain shape and facilitate cooling. Five PU-WCs containing up to 80% BP were successfully fabricated (ρ = 0.78 ± 0.02 g/cm3). Samples are presented in Fig. 1. The composition of PU-WCs can be found in Table 1. PU-WCs were named as PU-WCXX%/BP, where ‘XX’ represents the content of the BP. PU-WC samples are presented in Fig. 1.
Fig. 1
Digital images of PU-WC composites after the manufacturing process (from top PU-WC0%/BP; PU-WC20%/BP,PU-WC40%/BP, PU-WC60%/BP, and PU-WC80%/BP)
Table 1
Chemical composition of PU-WCs composites with different wt% of BP
Material code | Mass of substates [g] | ||||||
|---|---|---|---|---|---|---|---|
Rokopol RF551 | Rokopol M6000 | Bio-based polyol (BP) | pMDI | Pine sawdust | Tegostab B8465 | ||
PU-WC0%/BP | 16.16 | 4.17 | 0 | 18.01 | 60.00 | 1.17 | |
PU-WC20%/BP | 11.77 | 3.92 | 3.92 | 19.21 | |||
PU-WC40%/BP | 7.42 | 3.71 | 7.42 | 20.28 | |||
PU-WC60%/BP | 3.52 | 3.52 | 10.55 | 21.24 | |||
PU-WC80%/BP | 0 | 3.35 | 13.38 | 22.11 | |||
Methods
AFM was used to study the morphology of PU-WC composites. The measurement was carried out using Multimode 8 from Bruker (Santa Barbara, California, United States) equipped with Nanoscope V. Cross-sections of PU-WC composites were analysed in tapping mode using TESP-V2 tip with a resonance frequency of ~ 320 Hz, a cantilever of 123 μm long and a tip radius of 10 nm.
PeakForce quantitative nanomechanical property mapping (PeakForce QNM) was conducted according to research made by Cano et al. (Cano et al. 2017) to analyse the mechanical characteristics of PU-WC composites at the nanoscale. Height, adhesion, and modulus PeakForce QNM images were acquired using a Dimension Icon atomic force microscope manufactured by Bruker (Santa Barbara, California, United States). The measurements were conducted in PeakForce mode under standard ambient conditions. A RTESPA-150 silicon tip with a nominal radius of 10 nm, a nominal spring constant of ~ 5 N/m and a resonance frequency of 150 kHz was utilised for the experiments. The measurements were carried out with a calibrated deflection sensitivity. The precise spring constant of the tip was determined using the Thermal Tune function, and the tip radius was calibrated using a neat PU matrix. To prepare the transverse cross-sectional surfaces of each investigated PU-WC composite, an ultramicrotome Leica Ultracut R equipped with a diamond blade was used. First, the sample material was cut to the appropriate size using a steel blade. Then, the selected surface of the sample is prepared for the AFM measurement using an ultramicrotome. The measurement of the thickness and depth of the interface was performed 15 times for each material. To determine both parameters, the surface profile was measured perpendicular to the interface boundary. The thickness of the interface was defined as the width of the peak corresponding to the interface region, and the depth as the difference between the lowest point of the peak and its edges.
The structure of PU-WC samples prepared using an ultramicrotome was determined using scanning electron microscopy (SEM). The test was performed using a FlexSEM 1000 II scanning electron microscope (Hitachi, Tokyo, Japan). The electron beam accelerating voltage was 10 kV. To improve the image resolution, the samples were coated with a 10 nm gold layer using a 108 Auto Sputter Coater from Cressington Scientific Instruments (Watford, UK).
Results and discussion
AFM of PU-WC interphase
The nano- and micro-structure of PU-WC composites was characterised by AFM in tapping mode. Results of this test are presented in Fig. 2; Table 2. Depending on the cutting plane of the material, a variety of material structures can be observed. Presented structures include areas illustrating the interaction of the PU matrix with filler cut lengthwise or crosswise. AFM phase images allow us to observe that the polyurethane matrix penetrates the part of the wood cells and strongly binds to the wood surface. This indicates a very good adhesion between the synthesised polyurethane and the filler. The phases of each composite are easy to distinguish from each other. The AFM height images show a slight difference between the structure of interphases in each composite. It can be noted that the difference between the topography of PU-WC surfaces, analysed by means of AFM height images, is higher for the composites with higher addition of BP. This is reflected in R (roughness) factors, which increased especially for composites with at least 40 wt% BP content. Rq increases from 6.3 nm to 20.3 nm and Ra from 4.8 nm to 16.4 nm. This may suggest an increase in the degree of phase separation (Santiago et al. 2014; Jung et al. 2022). Moreover, the addition of the BP is directly connected with an increase in isocyanate content, which is responsible for the formation of hard segments. As the hard segment content increases, the hard domain structure becomes more pronounced, which increases phase separation and surface roughness (Jung et al. 2023).
Since AFM is considered a valuable technique for miscibility detection and analysis of composite interface mechanics, a deeper analysis of AFM phase images indicated a significant difference in interphase thickness. The interphase thickness decreases from 441.5 ± 25.2 nm, 226.2 ± 33.7 nm to 94.3 ± 16.6 nm for PU-WC0%/BP, PU-WC40%/BP, and PU-WC80%/BP, respectively. This indicates a decrease in the interactions between phases in the PU-WCs, which limits adhesion between components. It results in disturbance of the soft and hard segments structure and microphase separation, especially near interfaces. The heterogeneity of the matrix can reduce the continuity of the interphase, making it prone to microcracks or decohesion under load. This is caused by the complex chemical composition of BP [26], resulting in a less uniform and less densely cross-linked PU network. It should also be noted that the increased molecular mass of BP and the presence of branched structures cause the presence of steric hindrances, which limit the arrangement of polymer chains along the surface of the filler. Relatively less-branched structures derived from petrochemical polyols can arrange themselves more freely along the filler. In contrast, branched BP structures limit contact of the main polymer chain with the filler and reduce interactions. Ultimately, the thickness of the interface can be related to the stress transfer efficiency from the PU matrix phase to the wooden filler, which evidently affects the final mechanical properties of the sample (Huang et al. 2018).
Moreover, the biologically differentiated interfacial structure of wood may influence the wood-PU matrix interactions. (Frybort et al. 2014) showed that different wood structures are characterised by various surface polarities. Based on this, the internal structure of wood influences the quality of interactions. In the four cases (samples with up to 60% BP content), interactions on the polyurethane-cell wall surface are presented. For PU-WC80%/BP, extracellular cell wall-polyurethane interactions are analysed. Looking at the influence of the BP content, a continuous decrease in interphase thickness can be observed with the addition of BP. It suggests a determinant influence of the polymer matrix composition on interfacial adhesion. On the other hand, no significant correlation between the interphase depth of each PU-WC was noticed.
Fig. 2
AFM images of PU-WC composites, a 3D height, b phase, c height horizontal section. The observed structures represent wood cells filled with PU matrix and alternating layers of PU and wood. The white line on the phase image indicates the approximate position where the height profile was taken
Table 2
Characteristics of interphase and roughness of PU-WCs
Sample | PU-WC0%/BP | PU-WC20%/BP | PU-WC40%/BP | PU-WC/60%BP | PU-WC80%/BP | Wood |
|---|---|---|---|---|---|---|
Interphase depth [nm] | 54.7 ± 5.3 | 61.0 ± 6.7 | 82.2 ± 5.7 | 63.8 ± 7.1 | 64.2 ± 9.5 | – |
Interphase thickness [nm] | 441.5 ± 25.2 | 336.5 ± 43.1 | 226.2 ± 33.7 | 194.4 ± 21.9 | 94.3 ± 16.6 | – |
Rq [nm] | 6.3 | 4.3 | 16.3 | 16.9 | 20.3 | 3.4 |
Ra [nm] | 4.9 | 3.1 | 11.8 | 13.4 | 16.4 | 2.9 |
Rmax [nm] | 83.9 | 56.3 | 123.0 | 124.4 | 224.7 | 19.9 |
PeakForce quantitative nanomechanical property mapping (PeakForce QNM) of PU matrix
Figure 3 shows the results of PeakForce QNM for PU-WCs. The QNM adhesion images reveal the existence of two phases with different adhesion forces between the tip and the investigated surface, the dark (low-adhesion) spherical phase separated by a thin continuous net of a brighter (high-adhesion) phase. It can be considered that the tested sample can be considered as a microphase mixture (Li et al. 2020). The brighter domains may be assigned to relatively high-rigidity isocyanate-rich domains due to the expected lower energy dissipation under tip-sample interaction and lower tip-sample adhesion. The darker structures were assigned to polyol-rich regions (Lan and Haugstad 2011). It can be noticed that, depending on the amount of the BP, the presented structure did not change significantly. On the other hand, there is a considerable decrease in adhesion force with an increase in BP addition. The maximal adhesion force decreases from 2.8 nN for PU-WC0%/BP to less than 1.0 nN for PU-WC80%Bio. It is due to the addition of bio-based polyol with other chemical structures and different functionality at the surface. It may reduce polar interactions and hydrogen-bonding with the AFM tip, lowering pull-off forces. It may also be influenced by reduced miscibility with isocyanates, leading to stronger phase segregation between soft (polyol-rich) and hard (urethane/isocyanate-rich) domains (Król and Król 2012).
As the properties of polyurethanes strictly depend on their microstructure, a decrease in adhesion force could be directly connected with the mechanical properties of manufactured composites. In our previous research (Olszewski et al. 2024), properties of the presented PU-WCs were tested inter alia by mechanical tests, dynamic mechanical analysis (DMA), which demonstrated a decrease of flexural modulus from 1750 MPa to 780 MPa for PU-WC0%/BP and PU-WC80%/BP, respectively. Additionally, the flexural strength decreased from over 25 MPa to 10 MPa for the same samples. This could also be confirmation of crosslinking density reduction due to the introduction of BP with a more complex and secondary hydroxyl group, which caused weakening PU matrix reactivity and interactions between the phases of composites. Limited adhesion has led to deterioration of the mechanical properties and accelerated process of crack propagation through the material, which is increasingly noticeable as the BP content increases. Putting together these results, the different polyol chain compositions (BP ratio) lead to the modification of the type and intensity of interactions between both domains. This interaction contributes to the mechanical properties of PU-WCs. As the size of each domain is substantial for the properties of PU materials, the average diameter of darker domains was measured by ImageJ software on at least 20 objects and is presented in Table 3. The average size varies from 26.7 ± 2.7 nm for PU-WC40%/BP to 32.0 ± 4.8 nm for PU-WC0%/BP, thus the addition of BP has only a minor influence on the phase size. Therefore, it can be concluded that in the case of PU-WC composites, interactions between domains determine the properties of the resulting materials.
Additionally, the PeakForce QNM allows for the determination of the elastic modulus of each observed phase. Despite the decrease in adhesion, the modulus of domains increased. It has been noted that areas (polyol-rich) with lower adhesion also have a lower modulus value. Surprisingly, the addition of BPs causes a local increase in Young’s modulus at the nanoscale. This result is not consistent with the macroscopic DMA study (Olszewski et al. 2024), which showed a decrease in Young’s modulus with increasing BP content. A possible explanation for this phenomenon is measurement methodology, as the PeakForce QNM measures the properties of a small slice of the matrix, but the DMA and mechanical tests measure material as a macroscopic sample. Additionally, observed decrease in adhesion and interactions between matrix and filler may cause deteriorated load transfer between the matrix and filler, which may have resulted in a decrease in Young’s modulus and mechanical strength.
Fig. 3
500 × 500 nm QNM images of matrix in PU-WC, a height, b adhesion, c modulus, d modulus horizontal section. The brighter domains on adhesion images represent isocyanate-rich domains, and the darker structures can be assigned to polyol-rich regions. The grey line on the modulus image indicates the approximate position where the modulus profile was taken
Table 3
Comparison of darker domain (polyol-rich domains) size
Sample | PU-WC0%/BP | PU-WC20%/BP | PU-WC40%/BP | PU-WC60%/BP | PU-WC80% /BP |
|---|---|---|---|---|---|
Phase size [nm] | 32.0 ± 4.8 | 28.5 ± 4.4 | 26.7 ± 2.7 | 29.8 ± 3.6 | 26.9 ± 2.5 |
Scanning electron microscopy (SEM)
To confirm the results of AFM and broaden the analysis of PU-WC microstructure, SEM was conducted. It is crucial to mention that transverse cross-sectional surfaces were prepared using an ultramicrotome to provide flat and smooth surfaces, which represent the structure of the composite. The microstructure of selected materials is presented in Fig. 4.
Conducted SEM analysis does not focus on a 10 × 10 μm sample area like AFM, but concentrates on analysing a larger area of the sample. The examination was performed at a magnification of 1.5 k to confirm the correct identification of structures previously described in the AFM and to describe the presence of other structures (including defects) throughout the entire cross-section of the material. SEM confirmed that the structure is repeatable on a larger scale, and similar areas can be found in many other places on the sample. Observed structures are in line with AFM results, but also provide additional image details not visible in a smaller-scale analysis. Analysing the obtained structures, the wood phase and PU phase can be distinguished. The structure of the wood was disturbed as a result of its processing (wood cutting), the mixing process with polyurethane, the hot pressing, and the stresses caused by the growth of PU in the mould (Reinprecht 2016). Moreover, depending on the shape of the wood phase and the degree of wood structure rearrangement, three structures can be distinguished.
The first structure is presented in Fig. 4a, c,f. and represents samples where wood cells remain untouched or were slightly compressed. In these structures, the interior of wood cells is mostly filled with PU and the structure is locally ordered. The second structure presented in Fig. 4d and e is composed of partially collapsed wood cells filled with PU. In areas where the integrity of the structure is interrupted, the PU matrix surrounds wood particles and connects them. It provides the integrity of the whole composite. The third structure is composed of wood, whose primary structure was completely shredded during processing. An example of this structure can be noticed in Fig. 4.b. where fully-collapsed cells, and compressed and corrugated structures can be observed. In this case, it is extremely difficult to distinguish individual cells, which creates the impression of a strongly disordered and random structure. The ruptured wood cells are connected by the PU matrix. Additionally, selected regions of the structure resemble the structure of compressed and densified wood (Sandberg et al. 2013; Frey et al. 2018).
Beyond the observed structures, various material defects can be noticed. The most important of the defects is the presence of voids in the material, which are mainly caused by local foaming of the material due to the reaction between water and isocyanate, and voids between the filler that have not been filled by PU. Selected defects may result from the process of sample preparation, which is composed of preliminary trimming and surface preparation using an ultramicrotome. On selected structured diamond blade marks can be noticed (Fig. 4e) (Witzke et al. 2022). In Fig. 4f. Delamination at the interface between the two phases of the material can be noticed. Observed delamination suggests a limitation of interphase adhesion, but this effect could be disrupted by wood deformation during the sample preparation process, which caused defect formation. Moreover, minor cracks in the wood structure and between wood cells can be noticed and can be attributed to the sample preparation process. Image quality can be improved by using cryo-ultramicrotomy to increase polymer stiffness and reduce smearing and deformation of phases, and by using a focused ion beam (FIB).
Fig. 4
Microstructure of PU-WCs prepared with an ultramicrotome, a, b PU-WC0%/BP, c, d PU-WC0%/BP, e, f PU-WC0%/BP. The observed structures are characterized by various wood cells compression degree which are filled and/or covered with a polymer matrix
Conclusion
Incorporating BPs synthesised by biomass liquefaction into PU matrices reduces their environmental impact but significantly modifies interaction mechanisms on the nano- and micro-scale. Indicated interactions between the domains and phases of composites have a significant impact on the final material properties. Our results demonstrate that a strong adhesion between composite phases, which depends on the amount of BP, is observed. The interphase thickness decreases from 441.5 ± 25.2 nm for PU-WC0%/BP to 94.3 ± 16.6 nm for PU-WC80%/BP. A significant difference between interphase thickness may indicate a decrease in the interactions between domains in the PU-WCs, which influence the macroscopic properties of PU-WC. It is caused by the increased molecular mass of BP and the presence of branched structures, which act as steric hindrances which limit the arrangement of polymer chains along the surface of the filler.
PeakForce QNM confirmed phase separation between existing domains. Adhesion mapping revealed the existence of two phases with different adhesion forces, where the brighter domains may be assigned to relatively high-rigidity isocyanate-rich domains and the darker ones to polyol-rich domains. The maximal adhesion force decreases from 2.8 nN for PU-WC0%/BP to around 1 nN for PU-WC80%BP.
Additionally, the increase in the degree of phase separation is also connected with an increase in material roughness for samples with at least 40 wt% BP content. Rq increases from 6.3 nm to 20.3 nm and Ra from 4.8 nm to 16.4 nm. The presented results highlight the importance of phase interactions and structural characteristics in determining PU-WC performance. We believe that the proper selection of PU components may lead to the optimisation of PU-WC properties and increase the utility of the designed materials. The increase of interfacial interactions may be triggered by purification of BPs, introduction of sustainable compatibilisers or by implementation of other isocyanates, especially those synthesised using bio-based derivatives.
Acknowledgements
This work was supported by the National Science Centre (NCN, Poland) in the frame of the UMO 2021/43/B/ST8/02640 project - Solvothermal liquefaction as a pro-ecological method of wood-like waste management and UMO 2023/49/N/ST11/01890 project Manufacturing and properties of polyurethane-wood composites (PU-WC) using bio-polyols from the biomass liquefaction process.
Declarations
Conflict of interest
The authors declare no conflict of interest or competing interests.
Ethical approval
Not applicable.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.