In situ capabilities of Small Angle X-ray Scattering

Abstract Small Angle X-ray Scattering (SAXS) is an ideal characterization tool to explore nanoscale systems. In order to investigate nanostructural changes of materials under realistic sample environments, it is essential to equip SAXS with diverse in situ capabilities based on the corresponding requirements. In this paper, we highlight the representative experimental setups and corresponding applications of five widely used in situ capabilities: temperature, pressure, stretching, flow-through, and electric field. Additionally, we also briefly introduce other four in situ techniques including humidity, high-throughput, rheology, and magnetic field.

In many fields, it is vital to observe nanostructural changes of materials in the natural or real conditions, which motivates both synchrotron and lab SAXS to develop in situ capabilities. For instance, the microscopic changes of size, shape and morphology of nanoparticles with the variation of temperature can be observed via in situ SAXS [30][31][32][33]. In situ SAXS is also widely utilized to investigate the microscopic structural evolution of alloy or polymer during uniaxial or biaxial stretching, which can improve the material performances via phase transition [34][35][36]. Another common usage of in situ SAXS is to study the mechanisms and kinetics of nanoparticle nucleation and growth via flow-through device [37,38].
Depending on the specific application, the time resolution of in situ SAXS can be adjusted accordingly. In the in situ SAXS-stress-strain experiment from Romo-Uribe's group [39], the SAXS patterns were recorded with time resolution of 10 s due to the relatively slow deformation rate of 5 mm/min. But fast nanoparticle synthesis can occur in a time span of milliseconds to a few seconds which requires relatively much higher time resolution [40]. Graceffa et al. developed a new approach which is capable of achieving a time resolution of 100 µs for in situ SAXS with continuousflow mixer [41].
As it is not practical to cover all the in situ techniques of SAXS in one article, we pick and review nine commonly utilized in situ capabilities of SAXS. The experimental setups of these capabilities are briefly described and the results of some state-of-the-art applications based on these setups are reviewed and discussed.

Temperature
The properties and behaviors of nanomaterials, such as the particle size/shape and morphology [42,43], nanoparticle aggregation [44], are strongly dependent on synthesis temperature. For instance, the study on synthesis of gold nanoparticles (NPs) from Hatakeyama et al. indicates that the diameter of the most abundant NPs grows gradually from around 2 nm to 8 nm when the temperature is increased from 20 to 60 ∘ C, and the anisotropy increases with temperature increase although NPs generated at 20 ∘ C are spherical [30]. The SAXS data obtained by Ingham et al. revealed that the heating can induce the aggregation of the nanocrystals due to the melt and desorption of the capping ligands [31].
For many alloys, the heating and cooling rate can greatly influence the crystal grain size and phase composition, which ultimately determine the properties of the alloys. For example, Kenel et al. applied in situ SAXS to study early precipitation in Al-Cu-Mg alloys and found that the formation of η phase precipitates increases with the decrease of cooling rate [32]. Time-resolved SAXS study carried out by Deschamps et al. showed that increasing the heating rate can significantly decrease the precipitate density due to the influence of heating rate on nucleation rate [33].
The significant influence of temperature on the properties of nanoparticle materials makes the temperature adjustment an essential capability of in situ SAXS. Temperature control of the sample in SAXS can be achieved by heating or cooling.

Laser-driven heating
Utilizing laser-based heating and rapid cooling, Kenel et al. developed an approach of in situ SAXS combined with XRD to study alloy behavior and its influence on microstructures and properties after additive manufacturing (AM). The schematic experimental setup is shown in Figure 1. They applied two diode lasers to heat the samples. The laser spot size is~0.2 mm 2 . To heat different locations of the sample or follow sample movement, the complete setup can be translated in three dimensions. Pyrometer is used to measure the temperature of the specimen, based on which the laser can be adjusted and controlled. The samples were heated to solutionizing temperature at 474 ∘ C and held for 7 min to homogenize the microstructure. The authors obtained the Kratky plot as a function of temperature shown in Figure 2. At relatively lower temperature, the peak appearing at low q values indicates the presence of large-scale scatterers [32].

Joule heating
Heat can also be generated electrically by the joule heating effect. Andreasen et al. designed an in situ cell covering temperature from around 300 K to 870 K to study size and morphology changes of nanoparticles by SAXS. The in situ cell is schematically presented in Figure 3. The heater el- ement contains a cylinder made of Macor® glass ceramic, the working temperature of which can be as high as 1070 K. The heater wire (kanthal D) with a resistance of 1.35 µΩm is wound in the 0.5 mm deep, 1 mm steep helical groove on the ceramic cylinder. A NiCr-Ni thermocouple placed 1mm below the sample volume is applied to measure the sample temperature. The temperature at the central of the heating element can reach at least 870 K. This in situ cell is mounted on the JUSIFA beam line at HASYLAB. The authors acquired the in situ SAXS data of the calcination of a hydrozincite powder heated from 350 K to 570 K at a rate of about 8 data sets per hour. Assuming a model of polydisperse spherical particles, the derived radiuses of gyration for the uncalcined and calcined sample are 118.5 ± 0.8 Å and 81.5 ± 3.1 Å, respectively [46]. The above module is a self-designed heating element. Commercial devices can also be directly used. For instance, Anton Paar DHS 1100 heating plate and chamber is applied to investigate a new mechanism for mesostructure formation of ordered mesoporous carbons (OMC): thermally induced self-assembly. During the process of thermopolymerization, DHS 1100 heating plate is used to provide a temperature between 90 and 180 ∘ C until strong reflections are observed in the in situ GISAXS. After that, DHS 1100 heating chamber in nitrogen atmosphere heats the samples up to 1100 ∘ C, the defined limit of this heating element. The obtained in situ SAXS results indicate that the structure formation of OMC systems occurs during the thermopolymerization step, but not the process of evaporation. The authors also found that the rate of structure formation is strongly dependent on the thermopolymerization temperature and the block copolymer template. Total loss of structure does not occur even when the sample is heated up to 1100 ∘ C [47]. The cooling rate achieved based on this structure can be as high as 1.5·10 4 K/s [48]. The quench experiment of a commercial aluminum alloy indicates that at faster cooling rates, the clusters size is smaller while the cluster density is larger. It was also found that heterogeneous precipitates are formed more pronouncedly at lower cooling rates between 400 ∘ C and 200 ∘ C [32].

Cooling by Peltier effect
Albouy et al. used a commercial device (CT160, Deben company) equipped with a Peltier element to study selfassembly of a copolymer by freezing its aqueous solution. The lowest temperature achieved in this article is −7 ∘ C. The in situ SAXS experiments performed at the SWING beamline of the French SOLEIT show that micelles (aggregates of P123 molecules) can form at temperatures lower than the initial value of the CMT (critical micellar temperature), which indicates that it is not always true that selfassembly in aqueous solution of an object with weak interactions cannot take place below the CMT [50].

Pressure
Similar to temperature, pressure as another thermodynamic variable also plays an important role in synthesis of nanoparticles or supramolecular assembly of (bio)macromolecules. Unlike heating or cooling, pressure compression and decompression in a sample can be achieved with the same rate in either direction and the pressure propagates through the sample homogeneously without forming gradients. Analogous to T-jumps, pjumps (pressurizing or depressurizing) can be conducted in both directions with the speed of sound. Soft condensed matter often undergoes thermotropic phase-transitions with temperature changes, same phase-transitions can also occur by changing pressure (barotropic phase transition) [51,52]. That way two-dimensional phase diagrams as a function of temperature and pressure can be obtained as Figure 5 shows.
SAXS can be also used to follow the nanostructural changes of the sample with pressure in real-time. But requirements for pressure cells with in situ X-ray scattering techniques (SAXS, XRD) are not as simple as for Tstudies. First, the cell has to be built to withstand high pressure. Second, the cell must have windows sufficiently transparent for the X-rays used. Diamond is the typical material choice for X-ray windows at high pressure due to its low electron density (Carbon: Z = 6), high hardness and toughness. The transmission for X-rays at different energies (wavelengths) for a diamond of thickness 0.5 mm is shown in Figure 6 (right). As elaborated in the following paragraphs, pressure cells for X-rays may consist of two diamonds, on which mechanical force is applied to attain a pressure of 7.5 Mbar. Or two diamond windows are integrated in a hydrostatic pressure cell in which pressure is generated by a transmitting fluid with low compressibility (up to 5 kbar).

Diamond-anvil cell (DAC)
A DAC consists of two opposing specially cut diamonds enclosed by a gasket and the two diamond anvils (0.2 -0.5 carats) with a sample compressed between the polished diamond tips of small area [56]. Pressure can be monitored by a reference material with known pressure behavior like ruby and its fluorescence. The pressure is applied uniaxially and transformed into uniform hydrostatic pressure via a pressure-transmitting medium (enclosed by a gasket and the two diamond anvils) such as noble gases, hydrogen or a mixture of alcohols. Typically, a pressure of 1-2 Mbar can be reached with a maximum up to 7.5 Mbar [57]. Due to the relatively large thickness of the diamonds and consequently low transmission of X-rays, SAXS or XRD in situ measurements can only be carried out at synchrotron sources by selecting a short wavelength. Samples investigated by DACs are usually solid powders or crystals, but can also be liquid crystalline samples [58].

Hydrostatic pressure cells
The hydrostatic high pressure cell itself is machined out of stainless steel with cube dimensions of 3 x 2 x 2 cm and has two disc-shaped diamond windows with a diameter of 4 mm and a thickness of 0.75 mm each, serving as the X- Figure 5: p/T phase-diagram of the phospholipid DOPE (Di-oleoylphosphatidy-ethanolamine) in excess water. Within the shown range (0 to 2.5 kbar and −20 ∘ C to 80 ∘ C) it exhibits two lamellar phases (gel and fluid) and a two-dimensional hexagonal phase [53] ray entrance and exit windows, respectively [59]. The cell is connected to a motor-driven spindle press using water as the pressure transmitting medium. The system can be operated in automated pressure or temperature scans. The sample itself is contained in a flexible polymer-tube, closed on both ends by a teflon-piston, which is placed within the high-pressure cell. Besides slow pressure scans, fast pjumps (<5 ms) triggered by pneumatic pressure valves, separating two reservoirs of different pressures can be applied, and the nanostructural changes in the sample can be followed by time-resolved SAXS [60,61]. The sample in the cell can be pressurized up to moderate 3.5 kbar and above and its angular range encompasses 0 ∘ -20 ∘ (2θ). Changing temperature in the cell can be achieved either by a circulating flow of water through copper plates or by Peltier elements between which the cell is sandwiched. The application range is widely spread from studying phase diagrams of lyotropic or thermotropic liquid crystalline systems, proteins, lipoproteins or polymers and their barotropic phase transitions [62][63][64][65][66][67]. The pressure cell can also be utilized in experiments with super-critical CO 2 [68] and in the grazing incidence mode (GISAXS) for oriented and aligned lipid systems on solid supports [69].

Pressurized gas: super-critical CO 2 as with unique solvent properties
The supercritical state for carbon dioxide is experimentally easy to achieve (pc = 73.8 bar and Tc = 31.1 ∘ C) and thus can be used for in situ high-pressure SAXS measurements [68,70]. A block copolymers formed by a CO 2 -phobic and a CO 2 -philic portion like polyvinyl acetate-b-perfluoro octyl acrylate (PVAc-b-PFOA) forms at low pressure below the critical value, micelle-like aggregates. At relatively high pressures, CO 2 becomes a good solvent for both portions, inducing the destruction of such aggregates and giving fully solvated random coil chains. By In situ SAXS measurements these nanostructural changes can be observed in Tscans (at constant CO 2 pressure) or at P-scans (at constant CO 2 temperature) [68].

Pressurized gas: CO 2 gas sorption in coal
A laboratory SAXS gas cell was used in the investigation of adsorption and swelling behavior of coal to determine the feasibility of CO 2 sequestration. CO 2 storage in appropriate geological reservoirs is considered as one possibility to decrease the amount of CO 2 in the atmosphere. The research project COALSWAD (www.coalswad.eu) dealt with The picture to the right shows the actual cell used at the SAXS-beamline at ELETTRA with the entrance window detached. On the right and lefts sides of the cell the high-pressure tubings for the pressure transmitting fluid (water) can be connected the storage of CO 2 in coal seams. It focused on a comprehensive investigation of the adsorption behavior of coal and its kinetics by taking into consideration the influence of swelling properties. Results obtained by in situ SAXS measurements of gas sorption by different coals (up to 50 bar) show that with increasing coalification grade of coal, the degree of swelling decreases and the CO 2 adsorption capacity increases [72].

Pressure generated by piezo-stack actuator
The newest development for time-resolved in situ solution SAXS studies of kinetic processes induced by sub-ms hydrostatic pressure jumps are based on a high-force piezostack actuator, with which the volume of the sample can be dynamically compressed and the pressure can reach up to 1 kbar, using transparent diamond windows and an easy-

Stretching
Besides being a typical way to evaluate mechanical properties, uniaxial or biaxial stretching is also widely applied in investigations of microscopic structural evolution of materials such as polymer [34,35,[74][75][76] and alloy [77,78], and for the improvement of material performances via phase  [73] transition [34][35][36]. In situ SAXS and wide angle x-ray scattering (WAXS) are usually combined to monitor the structural changes at lattice scale and nanoscale, respectively. The information attained at lattice scale can be used to identify the transition of crystalline phases [35], while the nanoscale information facilitates judging the appearance or extension of cavities (voids) [77,79,80]. Below we will review some In situ SAXS studies on the stretching-induced structural evolution of polymer and alloy in details.
A representative example of research on tensile deformation behavior of polymer is from Romo-Uribe's group. They applied uniaxial stretching on linear low-density polyethylene (LLDPE) and studied its nanostructure evolution by time-resolved synchrotron SAXS at the Advanced Polymer Beamline (X27C) in Brookhaven National Laboratory. Figure 11 depicts the experimental setup: a tabletop  [39] stretching apparatus from Instron Inc. model 4410 is utilized to stretch the sample symmetrically, ensuring the xray beam focus on the same spot on the sample during the deformation process. The stretching is performed at a constant speed of 5 mm/min with a 40 mm clamp-toclamp distance. A MAR CCD of 512 × 512 pixels is applied to record SAXS patterns with time resolution of 10 s. The obtained SAXS patterns indicates that the initial LLDPE has a lamellar morphology with a long period of 21.5 nm. Flow-induced crystallization results in the increase of the long period during the initial deformation process. The lamellae are deformed toward the stretching direction between first and second yield points. In the second yield region (SYR), a gradual rotation and thinning of the offmeridional scattering reveals that a shear process is destroying the lamellae. Further deformation in SYR incurs a melting and recrystallization process in the SYR [39].
By applying SAXS/WAXS as well, Wang et al. investigated structural evolution in Grade 91 steel during tensile deformation at room temperature (RT) and 650 ∘ C. Grade 91 steel is an alloy containing M 23 C 6 and MX precipitates, which provide the material with its high temperature creep strength. The experimental setup is schematically shown in Figure 12. The SAXS patterns were being recorded every 24 s when the alloy specimen was being continuously stretched at a constant rate of 1 µm/s, much slower than the deformation speed of polymer mentioned above. The In situ SAXS/WAXS was performed at Beamline 1-ID of Advanced Photon Source at Argonne National Laboratory, with a beam energy of 70 keV and beam size of 50µm × 50µm. The authors applied two ion chambers to measure the X-ray intensities before and after the sample, which is used to calculate the specimen transmission. The SAXS re- Figure 12: Scheme for simultaneous WAXS/SAXS at 1-ID beamline at APS [81] sults indicates that for both RT and 650 ∘ C, the SAXS intensity does not change significantly during early deformation but begins to increase after necking occurred. The difference is that at RT, the void volume grows immediately after necking occurs at about 8.6% strain, while voids has a significant increase only after around 15% strain for temperature 650 ∘ C. The local stress concentration at the precipitate interface is alleviated by the high mobility of dislocations and thus delay void nucleation to a degree at 650 ∘ C.

Flow-through
In situ SAXS equipped with flow-through device is widely used to exploit the rapid formation of self-assembled structures, and study mechanisms and kinetics of nanoparticle nucleation and growth. As the parameters of the sample can be adjusted in the reaction reservoir [38] or other storage containers in the flow path [37] (not directly in the small capillary tube), the researchers can readily investigate the structural changes at different conditions such as solution pH, temperature, and concentration [82,83].
There are typically two types of flow-through devices: stopped-flow [37,[84][85][86] and continuous-flow [38,82,87]. The stopped-flow techniques with SAXS have been applied in kinetic studies for about two decades, but with limited time resolution (~0.5 ms) and relatively higher risk of radiation damage. By reducing the dead time (time between the reaction is triggered and the first point is collected), the continuous-flow device can significantly diminish the risk of radiation damage and improve the time resolution to~100 µs [41,88]. Below we wish to discuss briefly some applications based on these two techniques, respectively.

Stopped-flow
A representative example of the application of stoppedflow technique in kinetics of nucleation and growth is from Tobler's work [37]. The authors mimicked the natural systems of silica polymerisation and silica nanoparticle formation to study the mechanism of silica nanoparticle nucleation and growth under the conditions of high temperature (230 ∘ C) and long residence time (2.5 h).
The experimental setup is schematically shown in Figure 13. The silica solution with concentration of 640 or 960 ppm in a 10 L storage bottle is pumped into a hightemperature oven by a HPLC pump. With 2.5 h residence time in the oven, the silica is completely depolymerized under the high temperature of 230 ∘ C, which is achieved by steel coil. After leaving the oven, the monomeric silica species enters the backpressure regulator (BPR), where the solution temperature drops rapidly to 80 ∘ C within 1 min. The polymerization and subsequent nucleation and growth of silica nanoparticle occurs during the rapid cooling process. To study these processes, the silica solution passes through a flow through quartz capillary which is connected to the outlet of BPR. The distance between BPR and capillary can be adjusted to achieve specific temperature (30 ∘ C for SAXS). The scattered X-ray signal of the solution sample in the capillary is monitored by SAXS detector.
The SAXS data collection is conducted every 5 min up to 3 h. The time-resolved SAXS patterns reveal that for lower concentration (660 ppm), nanoparticle with a radius of approximately 1.5 nm is observed after 60 min and the plateau of about 3.5 nm is estimated to be achieved at about 6 h. The concentration increase to 960 ppm can significantly speed up the appearance of nanoparticles, which occurs in only 10 min. It only takes 3 hours to reach the plateau of 3.5 nm. The near-Gaussian shape of the distance distribution function indicates monodisperse and spherical nanoparticles. The occurrence of some aggregates or polydispersity leads to the slight skew to the right at higher particle radius.

Continuous-flow
Yi et al. applied SAXS with continuous-flow device to investigate the growth mechanism of mesoporous silica nanoparticles (MSNs) at different solution temperature and pH [38]. The setup to monitor the dynamic growth of MSNs is schematically shown in Figure 14. The MSNs are fabricated in a 200 mL beaker, which is connected to the capillary tube by a peristaltic pump. The flow rate between the reaction reservoir and capillary tube is 0.5 mL/s. The dead time is estimated to be about 28 s based on the flow rate between the reaction reservoir and capillary tube (0.5 mL/s), pipe length (2.8 m), and pipe diameter (2.5 mm). Thus, the authors collected SAXS patterns for 2 s every 28 s. At 30 ∘ C, it took 150 min to collect about 300 patterns; while the total data acquisition at 95 ∘ C consumed only around 1 h as MSNs reaches steady state quicker at this temperature. The SAXS results suggested that increasing temperature accelerates the growth rate of MSNs as the higher temperature drives the molecule movement faster and thus escalates the contact frequency. The higher pH value also leads to the faster silica growth as the increase of hydrolysis rate of TEOS (phase transformation from liquid to solid), which is the determinant for the silica growth.

Electric field
External electric field is extensively applied in the field of nanoparticle deposition to assist in the spontaneous selfassembly of nanocomponents, which is capable of accelerating aggregation of nanoparticles [89], promoting formation of high nanoparticle density [90,91], and modulating the size and distribution of nanoparticles [92]. Buttard et al. investigated the deposition of gold colloidal nanoparticle on Silicon surface and found that the curve of Au nanoparticle density versus voltage has a threshold value, at which the density has a sharp increase of two orders of magnitude [93]. Kumar et al. used spray pyrolysis technique to deposit gold nanoparticles on glass substrate. The Glancing incidence angle X-ray diffraction studies indicated that the higher voltage produces smaller and more uniform gold nanoparticles [92]. Qu et al. applied the electric field of 10-40 kV to obtain high-density FeNi nanoparticle films. The electrodeposited films possess stronger in-plane magnetic anisotropy field and higher saturation magnetization simultaneously, which are the two key factors to increase ferromagnetic resonance frequency in a wider GHz range [90,94].
Block copolymer (BCP) is another prominent material which can be formed via the process of molecular selfassembly. But the self-assembly in the absence of external fields normally creates short range order in the BCP. The application of controlled electric field is a promising approach to induce the alignment of microdomains to generate long range order in the BCP [95,96]. BCP may be blended with nanoparticles to achieve desired material properties. For instance, simulations from Yan et al. indicate that the presence of nanoparticles can impact the orientation dynamics and morphology of the lamellar microstructure in the hybrid materials under electric field [97]. Experimental from Bae et al. found that mixed microdomain orientations in BCP films under external electric field are promoted in the presence of gold nanoparticle [98].
Either direct current (DC) electric field or alternating current (AC) electric field can be applied in In situ SAXS based on the effects one would like to create upon the nanomaterials. For instance, Park et al. compared the microstructural evolution of PP/caly nanocomposites under AC and DC field. They found that AC field results in an exfoliated structure (or layer-stacking destruction) by the breakup of the charge balance while DC field leads to the alignment of silicate layers via dielectrophoretic motion [99]. Below some applications of DC and AC field will be reviewed, respectively.

DC electric field
Böker and Schmidt et al. studied the mechanisms of microdomain alignment in BCP solutions under dc electric field via In situ SAXS. The experimental setup is schematically shown in Figure 15. A dc voltage between 0.3 and 3.8 kV is applied across a home-built capacitor with gold electrodes, which creates a homogeneous electric field perpendicular to the direction of X-ray beam. The authors performed In situ SAXS experiment at the ID02 and ID2A Figure 15: Experimental setup for In situ SAXS [96] beamlines at the European Synchrotron Radiation Facility. The results revealed that the process of grain boundary migration dominates in weakly segregating systems, while the rotation of entire grains is the predominant process in strongly segregated systems. They also found that the initial degree of order in BCP system can influence the microscopic mechanism greatly: in a highly ordered lamellar system, a possible pathway to reorientation is nucleation and growth of domains; in a less ordered system, grain rotation becomes an alternative pathway to realize reorientation [95,96].
Kim et al. applied in situ synchrotron SAXS to investigate electromechanical strain responses of a poly and a maleic anhydride grafted poly dielectric elastomer gels [100]. Figure 16 shows a schematic presentation of the experimental setup. The direction of X-ray beam is perpendicular to the sample film surface and parallel to the external electric field. The authors applied a high-voltage power amplifier (Trek 10/10B) to amplify the output amplitude of the function generator (Agilent 33250A) by a factor of 1000. The adjustable high voltage (0-12 kV) is then delivered to the gel film. The synchrotron SAXS patterns of the SEBS gel during actuation is shown in Figure 17. The broader reflection peak in the absence of electric field (0 kV) indicates that the gel film has disordered micelle structure. As the electric field increases, the reflection peak is shifted to lower q ranges, which reveals the decrease of the average d-spacing in nanostructure dimension.

AC electric field
Paineau and Dozov et al. applied in situ SAXS to study the influence of ac electric field on the orientational degrees of the freedom of the clay platelets in both isotropic and nematic phases. Figure 18 shows the design of capillary   [100] holder and the in situ SAXS experimental setup. A highfrequency (700 kHz) electric field, with voltage ranging from 0 to 400 V, is applied on the sample by a pair of aluminum foil electrodes. The electrodes, rings of foil, are in direct contact with the capillary outer wall. A cushion of soft foam is used to protect the capillary tube. A highly uniform electric field, parallel to the capillary axis, can be created in the whole interelectrode area and strong up to 100 V/mm rms can be achieved in the suspension. The capillary cell was then mounted onto the experimental table of the Swing beamline. The SAXS patterns indicated that the electric field can readily align the nematic phase of beidellite clay suspensions but has no influence on the gels. The authors also observed strong field-induced orientational order in the isotropic phase of both clay suspensions [101,102].
Dozov's group used the same electric setup shown in Figure 18 to investigate the alignment of chitin nanorods under an ac electric field via in situ SAXS. The obtained SAXS patterns and the effective order parameter versus (c) Order parameter S inferred from I = f(ψ) traces as a function of duration of the electric field application. The two last data points were taken more than 80 s after the field was switched off. The dashed line is a guide to the eye [98] electric field duration are displayed in Figure 19. The quick respond (at 5 s) of the anistoropic samples upon the AC electric field proves the alignment of the chitin nanoparticls along the field direction. The chitin nanorod stayed aligned after the filed is switched off (Figure 19c).

Other capabilities
In addition to the five widely applied capabilities discussed above, other in situ techniques that we would like to cover in this review paper include humidity, highthroughput, rheology, and magnetic field.

Humidity
The in situ control of humidity has been applied in studies of membranes via neutron or X-ray scattering for more than two decades [103][104][105][106][107][108], among which polymer electrolyte membrane (PEM) is one of the most intensively explored membranes. For instance, Jackson et al. reported a design of a novel in situ humidity chamber and simultaneously measured morphology and conductivity of a PEM under precise humidity control (Relative humidity range: 20%-95%, Relative humidity precision: 2%) [104]. Another example is from Cruz's group, who constructed an in situ environmental chamber that provides a humidity control with range of 0%-95% and precision of 1.5% at 30 ∘ C. The authors investigated the transient morphologies of Nafion cast film and Nafion nanofiber (a PEM) as a function of relative humidity [105].

High-throughput
With the rapidly growing beamline demand and everincreasing number of samples at different solution conditions, high-throughput technique in SAXS has been developed in recent years to improve the efficiency of synchrotron beamline [109][110][111][112][113][114][115][116][117]. This capability is becoming extremely valuable in the biological field as the growth of sequence and genomics data significantly outpaces structural results [115]. For example, Martel et al. developed an automated high-throughput solution X-ray scattering data acquisition system for structural studies of protein at Stanford Synchrotron Radiation Lightsource (SSRL). They designed and constructed a compact and light sample changer for beamline 4-2 at SSRL, which can automatically deliver the sample aliquot, clean and dry the capillary, and collect the scattering data [113]. Franke et al. developed a modular automated SAXS data acquisition and analysis system, which was implemented and tested at the BioSAXS beamlines of European Molecular Biology Laboratory (EMBL). The authors employed the three-fold integrated networking environment (TINE) to provide reliable device communication. The sample loading, primary data reduction and further processing can be remotely con-trolled by software commands. Automation is conducted not only in sequence but also in parallel by taking into account the interdependencies between commands [112].

Rheology
In situ Rheology coupled with SAXS is commonly applied to investigate shear induced structure orientation and crystallization [118][119][120][121][122][123][124][125][126][127][128][129]. Most of these studies concentrated on the initial formation of crystallization precursor structures (i.e. shish-kebabs), which dictates the final morphology of the polymers [130]. As an archetypal example, Chen et al. studied the crystallization of isotactic polypropylene (iPP) under shear flow and Carbon Nanotubes (CNTs) via in situ SAXS and WAXD. The authors applied commercial Linkam CSS-450 high-temperature shearing stage at the Advanced Polymers Beamline in the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). The shear flow field and thermal history of the polymer samples can be precisely controlled via this stage. It was found that the crystallization rate of sheared CNTs/iPP nanocomposites is around 40 times as that of quiescently crystallized pure iPP. Ma et al. also utilized a modified Linkam CSS450 (quartz windows replaced by diamond windows for X-ray characterizations) as the flow device to explore the flow-induced shish formation in semicrystalline polymers. Their results indicate that shish are formed for a critical strain of 100 with shear rates ranging from 25 to 200 s −1 .

Magnetic field
Similar to the role that electric field plays in BCP, magnetic field is also intensively applied to produce highly oriented microstructures in BCP, which eventually determine the characteristics of materials [131][132][133][134][135][136][137]. For instance, Gopinadha et al. performed in situ SAXS studies of a liquid crystalline BCP under an external magnetic field, which is provided by superconducting magnet manufactured by AMI. The work shows that there is no obvious effect on order-disorder transition temperature (T ODT ) up to field strength of 6 T [134]. High-intensity fields (typically >4 T) restricts the wide application of magnetic field in directed self-assembly. But Gopinadha's group later utilized simple permanent magnets with a low-intensity field as small as 0.2 T to successfully produce oriented mesophases with the addition of the large grain size of labile mesogens [131].

Summary and outlook
In this article, the in situ capabilities of temperature, pressure, stretching, flow-through, and electric field for SAXS were highlighted. (1) Temperature has significant impact on the properties of nanoparticle materials. Two main heating approaches extensively applied in SAXS: laser-driven heating and Joule heating, were presented. Then cooling by conduction and Peltier effect were introduced, respectively. (2) Similarly, pressure (hydrostatic pressure to be precise) can be applied to soft matter, which nanostructure is responsive to changes of compressibility thus undergoing barotropic phase transitions or even denaturation of proteins. Alternatively, the pressure variation of the solvent CO 2 (gaseous, liquid or supercritical) can drastically alter the properties of the solute like dissolved amphiphilic polymers. The nanostructural changes can be followed by SAXS, providing a suitable sample-cell, which can cope with high pressure and that is also transparent to X-rays.
(3) Uniaxial or biaxial stretching with SAXS was applied to explore structural evolution of materials at nanoscale. Stretching-induced microscopic changes of polymer and alloy were discussed. (4) In situ SAXS equipped with flowthrough devices can be used to study the formation of selfassembled structures and kinetics of nanoparticle nucleation. Some applications of stopped-flow and continuousflow, which are two typical flow-through devices, were provided. (5) Electric field is capable of assisting in the spontaneous self-assembly of nanocomponents and accelerating the aggregation of nanoparticles. SAXS can be combined with either direct current (DC) or alternating current (AC) electric field based on the effects one would like to create upon the nanomaterials. Besides the above five representative capabilities, the talk about other four in situ techniques including humidity, high-throughput, rheology, and magnetic field, was also briefly presented.
In more complicated sample environments, in situ SAXS equipped with a single capability is not sufficient to study the behaviors and properties of nanomaterials. Thus incorporating two or more in situ capabilities has become a trend for SAXS. An example where both temperature and stretching capabilities are combined is from Zhang et al., who developed a radiated materials experimental module to enable in situ studies of radioactive specimen subject to thermo-mechanical loading at the Advanced Photon Source [138]. But SAXS has its intrinsic limitation as an analytical tool to study materials with relatively large sizes (~1-100 nm). Thus it is necessary to combine it with other techniques to study multi-scale systems. Extending SAXS simultaneously to wide-angle X-ray scattering (WAXS) is a very natural and simple choice and this has been carried out in many applications [35,36,76,139,140]. In addition, more and more non X-ray based techniques are used as complementary methods such as Fourier-transform IR spectroscopy, Raman scattering [141], UV-Vis absorption spectroscopy, etc. These combinations of techniques significantly enrich the information obtained from in situ experiments.
Acknowledgement: This material is based upon work supported by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Generation 3 Concentrated Solar Power (CSP) Systems award number DE-EE0008380. This work was partially supported by Nuclear Regulatory Commission under the contract NRC-HQ-84-15-G-0018. We would like to express our great appreciation to Dr. Fallon Laliberte for her contribution to stretching capability in this paper.