Recent progressive use of advanced atomic force microscopy in polymer science: A review

Atomic force microscopy (AFM) has been extensively used for the nanoscale characterization of polymeric materials. The coupling of AFM with infrared spectroscope (AFM-IR) provides another advantage to the chemical analyses and thus helps to shed light upon the study of polymers. In this perspective paper, we review recent progress in the use of AFM-IR in polymer science. We describe first the principle of AFM-IR and the recent improvements to enhance its resolution. We discuss then the last progress in the use of AFM-IR as a super-resolution correlated scanned-probe IR spectroscopy for chemical characterization of polymer materials dealing with polymer composites, polymer blends, multilayers and biopolymers. To highlight the advantages of AFM-IR, we report here several results in studying crystallization of both miscible and immiscible blends as well as polymer aging. Then, we demonstrate how this novel technique can be used to determine phase separation, spherulitic structure and crystallization mechanisms at nanoscales, which have never been achieved before. The review also discusses future trends in the use of AFMIR in polymer materials, especially in polymer thin film investigation.


Introduction
FT-IR is one of the most powerful spectroscopic techniques for the characterization and identification of numerous materials, especially for the polymers, biomaterials and life science [1][2][3][4][5].
It is a final feature that provides many of AFM's most interesting opportunities to study polymer crystallization, as it is now possible to observe crystal melting, crystal growth and lamellar-scale reorganizations within crystals, seeing how structure evolve and local conditions affect kinetics [84]. A graphic of an AFM set-up is displayed in Figure 1. The main element is sensor, a pyramidal tip attached to a 100-400 μm long cantilever, which is in contact with the sample surface. Cantilever and tip are often made of silicon or silicon nitride, because these materials allow relatively low-cost mass production using semi-conductor technology. The geometry of the apex of the tip is representing one of the key parameters determining the resolution. With a tip that ends in a single atom, the highest resolutions can be reached. The cantilever is a system comparable to a contact profilometry, with the three-dimensional displacement of the AFM tip relative to the pattern being received with the aid of piezoelectric crystals. This set-up allows for placing of the AFM tip with an accuracy of approx. 1 nm in x-, y-and z-direction. When the AFM tip is engaging a surface, the cantilever is bending due to repulsive forces between tip and sample. This deflection, proportional to the pressure either being applied to the specimen, is used to represent a topography of the surface directly or as an input for the feedback loop controlling the AFM's z-position [85]. There are several different techniques for quantifying this deflection [86]. However, the most common use of an optical sensor is a laser beam that is reflected from the cantilever backside to a position-sensitive light 4 of 38 detector. Because the laser place on the detector varies in accordance with the degree of bending of the cantilever of the AFM, the latter can be measured accordingly [87].

AFM Scanning/Operation Modes
AFM can be operated under various modes, the choice of suitable operating mode depends on the desired information and types of AFM image. In summary, there are 3 main operation modes of AFM, they are contact mode, non-contact mode and tapping mode. This classification of the AFM operation modes is based on interactions between tip and surface ( Figure 2 & Table 1). More specifically, AFM works in "contact mode" in the presence of constantly repulsive forces or in "noncontact mode" being attractive forces onto the tip. Lastly, AFM may work in tapping mode in the presence of both attractive and repulsive force [88]. For all the operation modes, the images may be reconstructed by recording all interatomic interactions occurring at the end of the tip during the cantilever scanning onto the sample surface [89].

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Static Modes: Contact mode

Contact mode AFM
Contact is mostly named a static mode, while the other authors prefer to refer to the modes by their detection mechanisms [90]. Tapping mode would then be called Amplitude Modulation AFM (AM-AFM).The contact operational mode is the most suitable for flat and rigid surfaces such as crystal, hard polymers and tissue, enabling the highest resolution level. In this mode, it is possible to capture image artifacts associated with a not-flat surface or to a mechanical drift derived from the scanner motion. These artifacts can prevail the topography image over the real morphological features. To intercept these artefacts and to measure how well the desired deflection set point is maintained constant by the feedback system, the error mode can be used [88].

Noncontact mode
In non-contact mode, the oscillating probe is generally influenced by the close surface, so producing a frequency shift in the resonant frequency, due to Van der Waals attractive forces. Hence, the signals recorded in non-contact mode are related to the variation between cantilever resonance frequency and free oscillation of the system, giving a preliminary estimation of atomic tip-sample interaction forces intensities [85].
Tapping and noncontact are called dynamic modes, as the cantilever is oscillated in tapping and noncontact modes. Typically, this is done by adding an extra piezoelectric element that oscillates up and down at somewhere between 5-400 kHz to the cantilever holder. Noncontact AFM, unlike the other AFM techniques can obtain true atomic resolution images [90].

Tapping mode AFM
Lastly, in the case of tapping or intermittent contact AFM (IC-AFM), probe excitation externally occurs, and the amplitude and cantilever phase may be monitored in the proximity of the resonance frequency. With respect to the other operation modes, tapping mode has been employed successfully for the topographic characterization of cell-loaded surfaces, due to the application of lower forces and high-resolution imaging [89].
The main difference between tapping mode and noncontact mode is in tapping mode; the tip of the probe actually touches the sample and moves completely away from the sample in each oscillation cycle. In NC-AFM, the cantilever stays close to the sample all the times and has a much smaller oscillation amplitude. NC-AFM (noncontact AFM) is more sensitive to small oscillations of the cantilever, so may be operated in close contact (almost touching) [90].
Noncontact AFM, unlike the other AFM techniques can obtain true atomic resolution images [89].
AM-AFM is typically used for tapping mode -where the tip actually taps the sample during each oscillation. This is often the most stable mode to use in air, and so is currently more commonly used than either noncontact or contact modes for most applications. The main difference between tapping mode and noncontact mode is that in tapping mode, the tip of the probe actually touches the sample, and moves completely away from the sample in each oscillation cycle. In NC-AFM, the cantilever stays close to the sample all the times, and has a much smaller oscillation amplitude. NC-AFM (noncontact AFM) is more sensitive to small oscillations of the cantilever, so may be operated in close contact (almost touching) [90]. 6 of 38 AM-AFM is typically used for tapping mode -where the tip actually taps the sample during each oscillation. This is often the most stable mode to use in air, and so is currently more commonly used than either noncontact or contact modes for most applications. The main difference between tapping mode and noncontact mode is that in tapping mode, the tip of the probe actually touches the sample, and moves completely away from the sample in each oscillation cycle. In NC-AFM, the cantilever stays close to the sample all the times, and has a much smaller oscillation amplitude. NC-AFM (noncontact AFM) is more sensitive to small oscillations of the cantilever, so may be operated in close contact (almost touching).AM-AFM is typically used for tapping mode -where the tip actually taps the sample during each oscillation. This is often the most stable mode to use in air, and so is currently more commonly used than either noncontact or contact modes for most applications [90].

Phase imaging
The contrast of an atomic force micrograph generally depends on the mechanical properties of the surface and the probe, for instance, the adhesiveness and elasticity. To produce a sharp image of the surface topography, the sample material has to be relatively rigid compared to the probe. As the material becomes softer, the image obtained will become more influenced by the elastic properties of the surface. In extreme cases, the tip-surface interaction can cause damage or displacement of the sample [34].
Phase imaging, a method closely associated with IC-AFM, enables surface properties to be observed beyond pure topography. Once scanning in IC-AFM, the damping of the oscillation outcomes in a loss of oscillation energy occurred in the cantilever due to energy transfer to the sample.
This results in a phase change as well as the lower amplitude of the cantilever oscillation. Since this phase change is a feature of the sample material's energy absorbency, it is idiosyncratic for rigid or soft materials or, more generally, low and high-energy absorbency materials(see Figure 3) [91]. https://parksystems.com/park-spm-modes/91-standard-imaging-mode/221-phase-imaging-phase-detection-microscopy-pdm.
It results in a phase shift of the cantilever oscillation as well as the lower amplitude. As this phase shift is a function of energy absorbency of the sample material, it is idiosyncratic for stiff or soft materials [92], or more precisely, materials with high and low energy absorbency [91]. This allows analyzing the distribution of different materials or phases within the sample, for example, phase separation within lipids [93] or drug distribution within nanoparticles [94].

Force spectroscopy
AFM also can be used to quantify forces between the tip and the surface of the sample, thereby producing curves of force-distance [95].A sort of forces can be detected. These consist of Coulomb forces, van der Waals forces, electrostatic forces or particular forces between ligand-receptor pairs [96][97][98][99][100]. As mentioned above, AFM's theory is based on determining the forces between the tip of the AFM and the surface of the sample. Therefore, this force purpose can not only be used in a response loop for imaging, but also as an pointer for the power of interaction between sample and tip. The advantage of Atomic Force Spectroscopy (AFS) over the other techniques such as the stylus profile meter is the probe's high lateral positioning, enabling the sample's behavior to be observed at nano-scale resolution and high force sensitivity, enabling forces to be recorded down to the pN range [101].

Cantilever-tip systems
A key factor for high-resolution imaging is the AFM tip, particularly, its geometry, radius and chemical composition. The AFM tip, in particular its structure, radius and chemical composition, is a key factor for high-resolution imaging. The key limiting factor with respect to an AFM's overall resolution is the tip apex radius. Highlights in the tip's size range are distortedly and extended, highlights that are smaller may not be apparent at all. [102]. For very high (i.e., atomic) resolution, special probes so-called 'supertips' are commercially available. These are also well suited for lateral force measurements or point force measurements (Figure 4a). AFM tips are typically fabricated from materials like silicon or silicon nitride. However, diamond, carbon or single crystals of minerals have been employed for select special applications ( Figure 4b). Microfabrication etching technique is the most commonly used method for preparing such samples. The cantilever / sample systems are etched using this method from oxidized silicon wafers using photographic masks to describe the cantilever's form. This leads to tips with a radius below 30 nm. Super tips can be generated at the end of pyramidal Si3N4 tips through the regulated growing of carbon filaments [103]. The tips developed by these techniques will be further modified surface yielding well-defined sensor systems at the nano-resolution to characterize the mechanical, elastic and chemical properties of samples. For example, nano-sensors can be produced by covalent binding of molecules such as lipids, DNA, proteins to the surface of the sample based on a wide range of chemical reactions [104].

The principle of AFM-IR
The principle of AFM-IR [52] is to couple the AFM in contact mode (or taping mode) with a pulsed tunable laser is shown in Figure 5. In the first-generation AFM-IR, a sample is irradiated near field infrared radiations or laser through an infrared-transparent prism (ZnSe). When the laser wavelength is tuned on the absorption bands of the sample, it generates a photo-thermal temperature rise in the absorbing regions of the sample and then, causes a rapid thermal expansion pulsed under the AFM tip (very short, about several tens of microseconds) and creates an exciting resonant oscillation of the AFM cantilever. Therefore, each time, the pulsed light is absorbed and heats the sample; the cantilever oscillates at its resonance frequency leading to absorption spectra that are to those obtained by traditional FTIR spectroscopy. Each absorption peak corresponds to excitation of a specific molecular resonance and the pattern of peaks, i.e. the absorption spectrum acts as a unique chemical fingerprint of the nanoscale region of the sample as shown in Figure 5. by a tunable, pulsed IR-laser (total internal reflection geometry). The probe consists of a standard AFM-lever and detection system (Adopted from: https://www.azonano.com/article.aspx?ArticleID=4509).
Although, the AFM allows analyzing samples with a spatial resolution order of 50-100 nm, its resolution is mainly determined by the thickness of samples. Because the thermal expansion is threedimensional and thus to achieve the spatial resolution of a few hundred nanometers, the thickness of a sample film should be in closed thickness order. In some cases, it will be impossible to prepare the sample with such a thin film (under 1µ m). For example, in the metal sector or silicon materials, it is 9 of 38 very hard to prepare a thin film under 1µ m by using microtome. To overcome this drawback, Baden et al. [105] has adopted a new approach by using the Focused Ion beams (FIB) processing for thin sectioning sample instead of microtome. Although this method may lead to several deteriorations of the substrates due to the ion bombardments attacks and local thermal heating. However, the damages on the studied polymer surface (polyimide) by ion beam irradiation were negligible and the chemical structure of polyimide before and after treatment with FIB remains mostly unchanged. This kind of information is very interesting because it helps to overcome the drawbacks of this new technique at least with polyimide. The analysis of ultrafine thin needs higher sensibility equipment. For this purpose, a new resonance enhanced method to increase the sensibility of AFM-IR by using the Lorentz Contact Resonance (LCR) imaging mode is proposed.
With this improvement, it is possible to perform nanoscale IR spectroscopy on extremely thin films to even 5 nm thick. In other words, with the new devices, AFM-IR (NanoIR2s, nanoIR3) could able to investigate single polymer lamellae, self-assembled monolayers and biological membranes.
Recently, Ni et al. [106] developed also a new setup based on time-resolved broadband Nano-IR using antenna-based near-field spectroscopy. In this approach, AFM tip was illuminated by a focused IR probe beam (Figure 6a  The sensibility of AFM-IR can be also improved by using quantum cascade laser (QCL) [44,108] and tuned the repetition rate of the QCL to match one of the contact modes resonance frequencies of the AFM cantilever, demonstrating a spatial resolution better than 50 nm [108]. Subsequently, they Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 5 March 2020 doi:10.20944/preprints202003.0082.v1 further used a sharp gold-coated AFM tip and a gold-coated substrate with top-down illumination configuration, achieving the sensitive measurement of self-assembled monolayers and a spatial resolution of about 25 nm [44]. This technique, now referred to as resonance enhanced AFM-IR (RE-AFM-IR), allows performing more sensitive measurements compared to the original AFM-IR.
Recently, Dazzi et al. introduced the tapping of AFM-IR as a complement to AFM-IR in the contact mode, extending the application of AFM-IR to the soft or loosely adhesive samples such as polymeric nanoparticles and liposomes that are challenging for investigating, using AFM-IR in the contact mode [53,109,110]. In tapping AFM-IR, the tip off the AFM cantilever oscillates in tapping mode as traditional AFM. This method does not lead to the damage or removal of the sample when the tip scans across the surface of soft samples as seen in the AFM in contact mode. The spatial resolution of tapping AFM-IR with a heterodyne detection scheme on the soft samples was achieved to 10 nm [53,109]. The principle of the tapping AFM-IR is based on heterodyne AFM [111][112][113] where the heterodyne detection scheme is achieved by setting the repetition rate of the laser to the difference between the first and the second Eigen modes of the AFM cantilever, tapping oscillation at the first mode and measuring at the second mode [53,109]. The amplitude of the second mode is proved to be proportional to the absorbance [109] , thus the spectra obtained by the tapping AFM-IR correspond to traditional FT-IR spectra in terms of peak position and intensity.

Thermal Analysis (nano-TA)
The other significant feature of AFM about its attitude to be coupled with thermal analysis to better characterize thermal properties of heterogeneous samples in different regions. Also recognized as nano-thermal analysis (nano-TA), this groundbreaking AFM-related method enables a local thermometric analysis [114]. The main advantage of testing nano-scale thermal analyzes is the detection of changes in the polymer crystal state in terms of glass transition temperature (Tg) and melting temperature (Tm), without affecting on its mechanical properties. This is achieved at the end of an AFM probe by using a special tip. During the measurements, the cantilever is heated while the probe is moved toward the specimen until its expansion due to the cantilever is local heating ( Figure 6). The sample swelling pushes the probe up and causes a rise in the vertical bending of the cantilever. Therefore, this deflection is assessed applying AFM photo-detector device based on an AFM standard. The specimen undergoes a change in internal temperature due to the transition temperature, being softer than the initial state; as a result, the cantilever bending decreases and the sample is forced to induce elastic or plastic deformation [114].
The product thermal transition can be determined by measuring the deflection of the cantilever.
Furthermore, through this technique the sample's transition temperature can be positively correlated with the bulk measurements of the transition temperature obtainable with macroscopic thermal approach (i.e., thermomechanical and calorimetric analysis). Contrary to the bulk techniques, the nano-TA enables micro-and nano-scale analysis of local thermal properties of a sample. Therefore, this method will be certainly be used to identify the thermal behavior of specific polymers and/or biomaterials. The cantilever is typically made of silicon with different doses of dopants, whereas the probe is a standard etched silicon probe, which allows examinating high spatial resolution in both tapping and contact modes. The cantilever can reach very high temperatures in this configuration because silicon doping allows it to operate with high currents. It can therefore be argued that this technique is suitable for various polymers at wide range of temperatures [115]. Bozec and Odlyha reported using a thermal probe at the end of an AFM cantilever to analyze the denaturation of collagen fibrils [116].
Nguyen Tri et al. [117] reported in a recent publication on the nanoscale characterization of the structure of milkweed fibers that have special properties due to their superlight weight, natural super hydrophobic and hollow properties. In combination with high-resolution SEM, AFM-IR investigates the morphology and chemical composition of milkweed fibers at the nanoscale in order to provide a better understanding of their structure, especially on the fiber surface [118]. AFM-IR provide several tens of nanometer resolution surface mapping. AFM-IR's main limitation is the laser source near the infrared region (900-4000 cm -1 ). It is not possible to detect bonding signals between polymer and metals below 900 cm -1 [118].

Application of AFM-IR in crystallization of polymer blends and polymer science
With numerous potential advantages by combining a nanoscale morphological characterization and chemical composition identification, the AFM-IR is a very useful tool for chemical characterization of a variety of materials, especially for structure-property relationship. We describe here the latest progress in the application of this technique to identify, verify the miscibility, interaction, aging and crystallization of polymer as well as related phenomena, which cannot be done with traditional AFM [107] and the information of these analyst are limited in the literature.

AFM-IR in crystallization of polymer blends
For blend materials, the understanding direct determination of the miscibility, the phase separation, the crystallization mechanism and the chemical interactions of components in a formulation is crucial to control their morphology and properties. However, its remain key challenges due to the lack of correlated imaging tool at sub nanometer and chemical characterization.
The first application of AFM-IR in polymer blends that should be mentioned was the work of Prater specifically, for pure DEX, the intensity of the 1350 cm −1 band was often higher than that of the 1280 cm −1 band, while for PVP, the opposite tendency was observed. The AFM-IR results showed that the large discrete domains correspond to DEX-rich phases, while the continuous phase is rich in PVP, with a good agreement between the topographical and the chemical images.
In the petroleum and natural gas industry, the use of hydraulic fracturing technique to crack rock is one of the most popular methods to release oil or natural gas by using sealing materials capable of performing under ultra-high temperature high pressure (Ultra HTHP) [124]. These micro, nanostructure and the chemical properties of the blend is the key factor to improve their design and their process development93. AFM-IR spectra helped chemical identification of each pure polymer component in the blend and thus can help to optimize the blending conditions [124]. That means AFM-IR spectroscopy and imaging provides information to control the convergence of processing conditions and thus to obtain a desirable mixing to better performance of the end-use polymer [90].
Recently, the miscibility of a polymer blend based on telaprevir with three different polymers, an amorphous solid dispersions drug, was evaluated using nanoscale infrared spectroscopy, thermal analysis, and Lorentz contact resonance measurements [126]. Author indicated that, the AFM-IR is very useful to characterize the microstructure of solid dispersion as a function of polymer nature and to determine phase separation. The AFM-IR results show that drug-rich phase was found to form discrete domains of various sizes depending on the system, ranging from below 50 nm to a few hundred nanometers, whereas the continuous phase was polymer-rich [126]. The AFM-IR has used to identify the nature of spherulitic and polymeric separation in the blend.

Effect of film thickness
The glass transition temperature (Tg) of polymer films decreases with decreasing film thickness [129,130], which could cause the thin polymer films begin to relax at a temperature far below the value for bulk metric [130]. Among all surface analysis techniques, AFM has the highest vertical resolution of less than 0.1 nm for most commercial systems [133].
To follow crystallization and melting of most polymers, it is necessary to have control over sample temperature. The development of stable controlled temperature accessories to AFMs has allowed a wide range of polymers to be imaged during crystallization at temperatures from 0 °C to 250 °C [81,[134][135][136]. Environmental control, with the use of both vacuum and inert gas atmospheres [136,137], has allowed polymers that are susceptible to oxidation and hydrolysis to be studied at high temperature.
Following crystallization in many polymers with AFM is limited to temperatures close to Tg or Tm, where crystal growth is slow. Recently a novel high speed scanning AFM was developed, Video FM, which allows images to be collected at video rate [137,138].

Effect of blend composition
AFM-IR is now being used in extensive applications in polymer science. A driving factor is the number and diversity of polymeric materials that incorporate micro-and nanoscale structure to

AFM-IR in studying crystallization of miscible blends
In the miscible crystallizable blends, the determination of the polymer distribution becomes more complex because it cannot do with morphological tools. The examination of the polymer distribution in miscible blends, particularly at the sub-micrometer level, is a technically unfeasible task despite the recent advances in several microscopies and X-ray techniques [139][140][141][142]. AFM-IR has been used to investigate the crystallization behavior and the diffusion in crystalline/crystalline blends of polyethylene glycol (PEG) with poly(3-hydroxybutyrate) (PHB). The addition of PEG helps to improve the biodegradation of PHB and extends their applications. However, the presence of PEG will perturb the crystallization of the PHB. The polymers form a miscible blend, the PHB with the higher melting temperature crystallizes first and the lower-Tm PEG component is trapped between the lamellae of the high-Tm component in a two-step crystallization process [143].
To investigate the distribution of each polymer in the spherulitic structure, they take advantage that PHB and PEG have different IR characteristics: PHB exhibits a strong absorption band at 1720 cm -1 , assigned to the stretching vibration of the carbonyl groups, whereas the IR spectrum of PEG does not exhibit this peak. In contrast, a strong band at 1095 cm -1 , which is assigned to the stretching vibration of the C-OH groups, is observed in the PEG spectrum but is negligible in the PHB spectrum.  AFM-IR spectrum of each pure component is first measured to determine the characteristic peak of each polymer. In this case, PCL exhibits a strong peak at about 1725-1728 cm -1 due to the presence of carbonyl group, while this peak is absent in the PEG [127].

Other applications of AFM-IR in polymer science
With numerous potential advantages by combining a nanoscale morphological characterization and chemical composition identification, the AFM-IR is a very useful tool for chemical characterization of a variety of materials, especially for structure-property relationship.
Some the latest progress in the application of this technique to identify, verify the miscibility, interaction and aging as well as related phenomena which cannot be done with traditional AFM are described here [60].

AFM-IR in identification of polymer blends
For blend materials, the understanding direct determination of the miscibility, the phase separation, the crystallization mechanism and the chemical interactions of components in a formulation is crucial to control their morphology and properties. However, there remain many key challenges due to the lack of correlated imaging tool at sub nanometer and chemical characterization.
The first application of AFM-IR in polymer blends that should be mentioned was the work of Prater et al. [148]. respectively [120].
The characterization and the distribution of polymer in ABS/PC is performed by recording the nano-IR spectra in the CH stretching region between 3200 and 2800 cm -1 . The spectra is scaled to the IR band at 3028 cm -1 , which is due to an aromatic C-H stretching vibration of polystyrene (PS).
The IR intensity of CH3 stretching band at 2960 cm -1 assigned to PC and CH2-stretching band at 2932 cm -1 assigned to polystyrene (PS) exhibit significant changes when the AFM cantilever was scanning on sample surface and thus can provide chemical composition of ABS/PC at the nanoscale [120].
Felts et al. [121,122]  depends on feature size and thus the cantilever response amplitude decreases as the polymer feature size decreases. In the case of PS and PDDT, although AFM-IR spectra show a low signal-to-noise, due to artifacts of laser power drift, the authors can still distinguish between nanostructures of PE, PS, and PDDT [121,122].
The determination of miscibility of amorphous components in polymer blends used in pharmaceutical systems was also investigated by AFM-IR. Van Eerdenbrugh et al [67,123] used a standard atomic force microscopy (AFM) measurements combined with nanoscale mid-infrared (mid-IR) spectroscopy to evaluate the miscibility of an equimolar binary blend between poly-(vinylpyrrolidone) (PVP) with dextran or maltodextrin (DEX). By analyzing IR absorption peaks at 1280 and 1350 cm −1 , it is possible to distinguish between rich-DEX or rich-PVP domains. More specifically, for pure DEX, the intensity of the 1350 cm −1 band was often higher than that of the 1280 cm −1 band, while for PVP, the opposite tendency was observed. The AFM-IR results showed that the large discrete domains correspond to DEX-rich phases, while the continuous phase is rich in PVP, with a good agreement between the topographical and the chemical images.
In the petroleum and natural gas industry, the use of hydraulic fracturing technique to crack rock is one of the most popular methods to release oil or natural gas by using sealing materials capable of performing under ultra-high temperature high pressure (Ultra HTHP) [124]. These  [124].That means AFM-IR spectroscopy and imaging provides information to control the convergence of processing conditions and thus to obtain a desirable mixing to better performance of the end-use polymer [124] Hitesh et al. [66] et al. investigated the miscibility of the drug-polymer amorphous solid dispersion (ASDs) of itraconazole (ITZ) and hydroxypropylmethylcellulose (HPMC) blend using the AFM-IR and fluorescence. They indicated that both AFM-IR and fluorescence imaging were able to provide information about the microstructure of the ASDs, prepared by solvent evaporation and the phase separation occurred at the submicron scale. When the films were heated, it was observed that the ASD components underwent mixing. Authors affirmed that the AFM-IR are promising for the characterization of miscibility and microstructure in drug-polymer systems.
The phase behavior of blends of a pair of partially miscible polymers, poly(vinylpyrolidone) (PVPK90) and hydroxypropyl methylcellulose acetate succinate (HPMCAS) to fabricate of nanofibers for tunable drug delivery and nanoscale phase separation in electrospun blend fibers was confirmed by interpreting of AFM-IR data [125].
Recently, the miscibility of a polymer blend based on telaprevir with three different polymers, an amorphous solid dispersions drug, was evaluated using nanoscale infrared spectroscopy, thermal analysis, and Lorentz contact resonance measurements [126]. Author indicated that, the AFM-IR is very useful to characterize the microstructure of solid dispersion as a function of polymer nature and to determine phase separation. The AFM-IR results show that drug-rich phase was found to form discrete domains of various sizes depending on the system, ranging from below 50 nm to a few hundred nanometers, whereas the continuous phase was polymer-rich [126].
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 5 March 2020 doi:10.20944/preprints202003.0082.v1 The AFM-IR has used to identify the nature of spherulitic and polymeric separation in the blend. Figure 10 show an example of AFM image (Figure 10a) and AFM-IR image at the 1720 cm -1 band (Figure 10b) of the equimolar blend between polycaprolactone (PCL) and polyethylene glycol (PEG). The image of the carbonyl band at 1720 cm -1 is very useful to identify the distribution of PCL in the blend [93]. For composite materials, the chemical characterization in the interfacial region between polymer matrix and reinforcements is very important to better understanding the final properties and the failure mechanisms during the service lifetime of polymeric materials. The latter may help formulation converge more efficiently on optimal processing conditions. AFM-IR has proven to be a useful tool for this kind of analysis processing conditions. AFM-IR has shown to be a useful tool for this kind of analysis.

AFM-IR in biopolymers and multilayers
Nowadays technique or method like AFM-IR to analyze morphology, properties and structure of these materials is attracting the attention of the scientific community.
Mayet C et al. [162] investigated the production of bacterial polyhydroxybutyrate in Rhodobacter capsulatus using transmission electron microscopy (TEM) and AFM-IR. They concluded that AFM-IR was more useful and takes less time to prepare the sample compared to TEM. The Samples can be ready for a detailed analysis after 1 day of preparation instead of 2-3 weeks for TEM analysis. The other advantages of AFM-IR relates to its capacity to obtain chemical images and can directly identify the presence of the PHB in blends by analyzing AFM mapping image at the 1740 cm −1 band, assigned to carbonyl group, while the TEM images were not able to do that. AFM-IR was also used to study the interface of a laminated multilayer film based on Ethylene Acrylic Acid (EAA) copolymer/Polyamide (nylon) blend. Compared to traditional FTIR having a special resolution of around 10 μm, the use of AFM-IR shows clearly better resolution at the interface. It was interesting to show that there was a very good correlation between IR spectra of polymer measured by bulk traditional FTIR measurements and AFM-IR [60].
Gong L et al [64] used Recently, Kelchtermans et al. [72] have used AFM-IR to characterize polyethylene-polyamide (PE-PA) multilayer films. Authors indicated that a two-mm-thick barrier layer between two polyamide layers near the center of the multilayer film consists of an ethylene-vinyl alcohol copolymer and this contention was proven by mechanical stiffness and thermal property measurements. They also confirmed that in thin tie layers, even less than several micrometers thick, the assignment of the components in the laminate was readily obtained.

AFM in studying polymer aging
During the service lifetime the performance of polymers can deteriorate through the aging process due to environmental factors such as temperature, UV radiation and humidity [145,147,[163][164][165]. This alteration can lead to the decommissioning of these products. The aging of polymer material is manifested in physical and chemical degradations, a slow and irreversible process. The Morsch's group have shown several works on the use of AFM-IR to investigate the aging processes of several coatings and polymers [161], [167][168][169][170][171]. AFM-IR has been used to analyze the chemical composition of aging products on the surface of a composite based on epoxy resin and silicon rubber. They used the electrical discharge to attack the composite surface and then investigate its effect on the chemical composition of the composite in the surface by both ATR-IR and AFM-IR.
In many cases, ATR-IR gives similar results in aged and non-aged samples, meaning that the aging process has no effect on the chemical structure of the composites. However, AFM-IR can provide more resolved spectra and nanoscale chemical mapping image of oxidative products (interfacial tracks and buries channels). The AFM-IF helps to confirm that the silicon rubber remains stable under these electrical attacks.
In another publication, they used both ATR-IR and AFM-IR to investigate the degradation of oil paint used in the Picasso and Mondrian paintings [170]. The ancient painting made from linseed oil with inorganic titanium white pigment particles was analyzed to identify the degradation of these materials with time. The characterization of oxidative products on the painting surface is found to be very useful for better understanding the degradation mechanisms and thus can be said to be the best method for better preserving these artworks [170].
The aging mechanism of a PET fiber under accelerated aging conditions is also reported [145]. Norrish type II mechanisms [172,173].
The degradation of PET via the Norrish type I mechanism leads to the formation of ketone radicals and to alkyl or aryl radicals around the ester linkage groups by splitting the molecule near the carbonyl bond; in turn, the Norrish type II mechanism involves an intramolecular reaction via a cyclic six-membered transition species and results in carboxylic end groups and alkenes. Although there are several studies on the accelerated weathering degradation of PET in thin films [174,175], the degradation of PET fibers in a complex structure as filtration membranes has not been studied so far. Furthermore, due to limitations inherent in the characterization techniques used, the degradation studies are usually carried out on a macroscopic scale and the mechanism by which PET deteriorates upon accelerated weathering, especially on its surface at the nanoscale, is still an open question. AFM-IR is used to follow the aging process of PET fiber.
By analyzing at the nanoscale of the chemical composition and the distribution of functional groups appearing on the aged sample surface ( Figure.  His field of expertise concerns environmental engineering and especially the study of physical and chemical treatments in relation with traces compounds. His research focuses mainly on processes involving plasma and photocatalysis oxidation of micropollution and / or processes involving a transfer to a solid phase. With an h-index of 19, he published more than 65 Q1 ranking publications, 2 patents. He is Editor to Elsevier Book on "air remediation" and has two book-chapters and presented numerous communications at international meetings (more than 70). He is associate editor (Springer journals), guest editor and regular reviewer for many international peer reviewed journals.