Small Angle X-ray Scattering (SAXS)

GISAXS Application in Polymer Physics - Master Thesis Content

Chapter 4

4.2.1 Small angle X-ray scattering (SAXS)

The small angle X-ray scattering (SAXS) is a small angle scattering (SAS) technique where the X-rays are scattered inhomogeneously in the nanometer scale at very small angle. Typically, the scattering angle for SAXS is 0.1 ~ 5°. The shape, size and structure of macromolecules can be probed by the scattered X-rays in this angle range. Generally, the SAXS technique can probe the structural information of macromolecules with a dimension between 5 and 25 nm. Thus it is a quite powerful technique for the investigation of soft matters like polymers. Depending on the distance between sample and detector, if it is very near that means the scattering angle is quite large, this technique is called wide angle X-ray scattering (WAXS) [36].

Figure 4-1: Geometry of small angle X-ray scattering technique.

4.2.1.1 Ganesha SAXS instrument at TUM

A SAXS instrument consists of an X-ray source, a collimation system, a sample, a beam-stop and a detection system. The collimation system is used for narrowing the beam and defining the zero-angle position. And the beam-stop is positioned before the detector to stop the direct beam and protect the detector. Besides, by covering the direct beam, it also improves the relatively weak scattering signal from the sample and makes the contrast better. For an in-house sample instrument, the X-ray path and the sample should be in vacuum to decrease the background scattering by the air. In this thesis, the SAXS experiments are performed with a Ganesha SAXS instrument in the SAXS lab of Chair of Functional Materials at Technical University of Munich.

Figure 4-2: The Ganesha SAXS instrument in SAXS lab of Chair of Functional Materials at TUM.

The Ganesha SAXS instrument consists of X-ray source (49.8 keV, 0.59 mA) and controller (Genix3D), motor drived sample stage, flying chamber (1.5 m), beam-stops, Pilatus 300k detector (pixel size 172 µm x 172 µm), vacuum pump and software controlling system. The sample to detector distance is 1050 mm and the radiation wavelength λ = 0.154 nm. To start the experiment, the chamber should be vented first and then the sample should be placed correctly on the stage. Move the sample stage to check if it can be freely positioned using the commands in the SAXS terminal and Graphical User Interface (GUI). Then evacuate the chamber until the vacuum reaches 10-2 mbar, move the sample stage out of the beam path and configure the sample stage distance, the SAXS beam-stop, the detector distance. Now move beam-stop to the direct beam position and then move the stage back to the beam path to check the sample center. After everything done correctly, the SAXS instrument is ready for SAXS measurement.

4.2.1.2 Temperature and humidity controlling cell design

For precisely controlling of the temperature and humidity conditions of the thermoresponsive system, a temperature and humidity controlling cell was designed and constructed.

Figure 4-3: Schematic design of the humidity and temperature controlling cell.

Figure 4-4: Assembled humidity and temperature controlling cell in Ganesha SAXS instrument.

4.2.1.3 Ratio of characteristic peak positions for ordered structures in SAXS 1D profiles

The 2D SAXS pattern can be achieved and open in the SAXS GUI software, and there is also automatically processing function to process the 2D SAXS pattern by averaging the intensity of scattering ring and plot the intensity versus q value in a 1D plot. The ratio of scattering peak relative positions on q-scale gives the information of the possible structure.

Ordered Structure	Relative q-scale peak position ratio
Cubic Lamellar Hexagonal	1 : sqrt(2) : sqrt(3) : 2 : sqrt(5) …… 1 : 2 : 3 : 4 : 5…… 1 : sqrt(3) : 2 : sqrt(7) : 3 ……

Table 4-1: Relative q-scale peak position ratio for cubic, lamellar and hexagonal ordered structures.

Chapter 5

Characterizations of PS-b-PNIPAM DBC / Iron Oxide Hybrid Nanocomposites

In chapter 5, the investigation results of PS-b-PNIPAM DBC / iron oxide hybrid nanocomposites are presented. The PS-b-PNIPAM DBC / iron salt hybrid nanocomposites were investigated by static SAXS with a linkam heating stage. And the PS-b-PNIPAM DBC /iron oxide thin films were investigated by static GISAXS measurements at Elettra. The bare PS-b-PNIPAM thin film was investigated by sputtering GISAXS measurements at DESY. The first stage exploring experiments for thermoresponsive PS-b-PNIPAM DBC bulk and thin film were probed by using ex-situ and in-situ SAXS measurements with designed temperature and humidity controlling cell. The superparamagnetic properties were found for the PS-b-PNIPAM DBC / iron oxide hybrid nanocomposites by SQUID measurements.

5.1 Structural Characterizations of PS-b-PNIPAM DBC / Iron Oxide Hybrid Bulky Films: SAXS and SEM Study

5.1.1 Block structure

The block structures of the PS-b-PNIPAM DBC / iron oxide hybrid nanocomposites are investigated by small angle X-ray scattering technique (SAXS). A series of PS-b-PNIPAM DBC/iron oxide hybrid nanocomposites with different [Fe]/[NIPAM] molecular ratio from 0 to 0.5 are prepared according to the method described in chapter 3. The thermal post-treatment of bulk samples composed of iron salt and PS-b-PNIPAM DBC would result in not only forming nanostructured materials but also decomposing the iron salt into iron oxide. This has been previously approved for iron salt containing PS-b-PMMA DBC [38]. The SAXS experiments were performed with the Ganesha in-house SAXS instrument. Each SAXS profile was acquired for one hour.

Figure 5-1: (a) SAXS profile of iron salt/ PS-b-PNIPAM nanocomposites with [Fe]/[NIPAM] molecular ratio increasing from 0 to 0.5. (b) Lamellar distance D versus [Fe]/[NIPAM] molecular ratio extracting from the fitting results of SAXS profiles in (a) by Scatter software.

From fitting the SAXS profiles, it is revealed that the PS-b-PNIPAM DBC is forming lamellar structure in which the q/q* ratio is 1, 2 and 3. The inter-lamellar distance of the lamellar structure can be obtained from the position of the first and most intense scattering peak using the formula q=2π/d as well as from the fitting of the SAXS profile. At small initial incorporation of iron salt for [Fe]/[NIPAM] ratio sample, it can be easily overserved that there is a strong shift to a small q value. The shift of main first scattering peak corresponds to an increasing of lamellar distance from 26 nm to 31.4 nm upon salt incorporation.

It is assumed that the iron ions reside in the PNIPAM chain due to its polarity, forming a complex with the polar groups on the PNIPAM chain. The phenomenon for this lamellar expansion can be interpreted by the order-disorder transition (ODT) theory, which is consist of three parameters, χ-the incompatibility of A and B block, N-degree of polymerization and fA-volume fraction of A component. According to the Semenov Method, the lamellar distance d is determined by a balance between interfacial energy and the energy of stretching the blocks of the copolymer, and the given formula is (5-1): [39]

Where a is the size of a monomer. Thus the increasing of lamellar distance is due to the increasing of the incompatibility χ of PS and PNIPAM block. This is also confirmed by observing pronounce tertiary peak in the SAXS profile at [Fe]/[NIPAM] = 0.01 while in the SAXS profile of pure PS-b-NIPAM the lamellar structure is less ordered (showing only two scattering peaks).

In addition, with the increasing of [Fe]/[NIPAM] molecular ratio, the pronounced lamellar structure starts to disappear upon further increase of [Fe]/[NIPAM] ratio > 0.01. At [Fe]/[NIPAM] ratio ≥0.1, the lamellar structure is completely vanished and the structure is mainly ill-defined.

5.1.2 Domain Orientation

The lamellar domain orientation of the PS-b-PNIPAM hybrid nanocomposites seems to depend on the iron oxide content inside the polymer. In figure 5-2 (a) the SAXS 2D patterns of iron oxide/ PS-b-PNIPAM hybrid nanocomposites for different [Fe]/[NIPAM] ratio are presented. And in figure 5-2 (b) the intensity of the secondary scattering ring are plotted using Scatter software.

Figure 5-2: SAXS 2D profiles of iron salt / PS-b-PNIPAM DBC nanocomposites with [Fe]/[NIPAM] molecular ratio of 0, 0.01, 0.03, 0.05 and 0.1 corresponding to figure a) to e)

Apparently the orientation of the lamellar domain can be observed in figure 5-2 and figure 5-3, there is no preferential orientation for the bare DBC while the lamellar domain starts to be preferentially oriented upon adding iron salt. At high [Fe]/[NIPAM] ratio both the lamellar structure and the orientation are vanishing thus there is no ordered structure anymore. The initial increase of preferential orientation indicates the possible accommodation of iron oxide within a specific domain, in this case the PNIPAM block. At high iron salt concentration, the formation of ill-defined structure indicates the loss of high selectivity of iron salt to one block. At [Fe]/[NIPAM] ≥ 0.1, the PNIPAM chain is no longer able to fulfill the coordination requirement of the Fe ions.

5.1.3 Thermal Stability

The in situ SAXS measurements were performed on both thermally treated bare DBC and metal oxide/hybrid materials. The samples were stabilized for 1 hour at each temperature for SAXS measurement. The SAXS measurement results are shown in figure 5-4, and there is no peak position shift observed, which means the structures of both bare PS-b-PNIPAM DBC and bare PS-b-PNIPAM DBC at dry state are thermal stable until 175oC.

Figure 5-4: The SAXS profiles of the a) PS-b-PNIPAM and b) PS-b-PNIPAM bulk sample annealed at 130oC for 48 hours, then heated from 25 oC to 175 oC in the steps of 10 oC. The SAXS profiles are shifted along the y-axis for clarity purposes.

The thermal stability of the PS-b-PNIPAM DBC /iron oxide hybrid nanocomposites with [Fe]/[NIPAM]=0.01 that prior-annealed at 130oC for 48 hours, is also studied by SAXS measurements at different temperatures. The results presented in figure 5-5 show that the lamellar structure of PS-b-PNIPAM DBC/iron oxide hybrid materials does not modify even at high temperatures, indicating thermally stable nanocomposites up to 175oC.

5.1.4 Surface Morphology

The surface morphology of the diblock copolymer films can be probed by scanning electron microscopy (SEM). The fitting results of the SAXS profile show that the structure of PS-b-PNIPAM is lamellar and for PS-b-PNIPAM is mixing of lamellar and cylinder structure. The SEM results show the block structure of PS-b-PNIPAM in a thin film format confirms the SAXS results. The film samples for both PS-b-PNIPAM and PS-b-PNIPAM bare DBCs are prepared according to the spin-coating method described in chapter 3 with 45 mg/ml DBC concentration and acid cleaned silicon substrates.

Figure 5-6 shows that the structure of PS-b-PNIPAM DBC film is mostly parallel lamellar structure and for PS-b-PNIPAM DBC is lamellar mixed with vertical cylinder structure. These films were prepared by spin coating a DBC solution at concentration of 45 mg/mL. For low solution concentration of about 10 mg/mL, the as-prepared PS-b-PNIPAM DBC thin film shows mainly vertical cylinder structure (See figure A-1 in appendix).

5.2 Thermoresponsive Behavior of PS-b-PNIPAM DBC Bulky Films: In-situ and Ex-situ SAXS Study

In the liquid systems, the thermoresponsive behavior of PNIPAM homopolymer and BCs have been extensively investigated [2-5]. While the thermoresponsive behavior of nanostructured PNIPAM based BCs as a bulky material is barely reported. The possible water vapor swelling of PNIPAM domains in nanostructured PS-b-PNIPAM DBC systems opens the pathway to easily manipulate the morphology of these BCs.

5.2.1 In-situ SAXS study of PS-b-PNIPAM DBC bulky film

Here, in-situ SAXS investigation of free-standing PS-b-PNIPAM DBC bulky samples upon water vapor exposure is presented. The PNIPAM dominated PS-b-PNIPAM DBC means large volume fraction of PNIPAM block and referred to using underlined PNIPAM block. The free standing PS-b-PNIPAM bulky sample is prepared according to the method described in chapter 3, and it is mounted in the temperature and humidity controlling cell as described in chapter 4.

This in-situ SAXS experiment aims to observe the thermoresponsive behavior/morphology of PS-b-PNIPAM DBC, during swelling at low temperature below LCST and deswelling at high temperature above LCST, by controlling both the temperature and relative humidity (r.H). Since the relative humidity is not an independent parameter from temperature, it is calculated by the ratio of the partial pressure of water vapor (pH2O) in the mixture to the equilibrium vapor pressure of water over a flat surface of pure water (p*H2O) at a given temperature. Thus when the temperature is increased, the relative humidity decreases due to the calculation method while the absolute humidity (mg/L) keeps the same. The relative humidity has an important effect on the swelling and deswelling of PS-b-PNIPAM, thus the way to control the relative humidity at different temperature is further controlled by increasing or decreasing water content in the cell using water injection or bubbling. Each SAXS measurement is a scan of half an hour.

Figure 5-7: Evolution of in-situ 1D SAXS profiles of bare PS-b-PNIPAM DBC during swelling/de-swelling process. The red vertical line indicates the initial peak positon.

Figure 5-8: Peak fitting results of 1D SAXS profiles for q position and FWHM and its evolution versus time during three temperature cycles.

To analyze the evolution of the phase transition in figure 5-7, the peak positions are fitted and the results are given in figure 5-8 with temperature and humidity conditions indicated. The impacts of relative humidity and temperature are discussed below respectively.

a) Influence of relative humidity

The relative humidity dominates the water absorption process of the PS-b-PNIPAM system. Due to the amide group in NIPAM molecule functioning as hydrophilic group, the water can be either bonded chemically with NIPAM molecule by hydrogen bond or physically absorbed. As the increasing of water amount inside the polymerized NIPAM molecules, the molecules will expand to certain level then the expansion will stop and the system will reach an equilibrium state because of the balance of elastic shrinking force and water absorbing expansion.

There are three temperature cycles for this in-situ SAXS experiment and for each cycle, as seen in figure 5-7 and figure 5-8, during the first temperature cycle the q value decreases gradually at the beginning with temperature keeping constantly at 20oC, which means the smoothly swelling of the PS-b-PNIPAM free standing film.

After about 7 h the relative humidity is decreased at a very low level (r. H=5%), it shows the deswelling of the system (figure 5-8), as indicated by reverse shift of the main characteristic q peak to higher valuesdue to the decreasing of relative humidity. Interestingly, we can bring the system to almost its initial state as indicated by reaching the same initial q value. Three hours later, the relative humidity is back ramped up to 90% then the system is spontaneously re-swelled again indicating a fast response to the humidity environment at 20oC.

In the second temperature cycle (Figure 5-8), the high relative humidity (r.H=90%) is kept for longer time and the results show a systematic decrease of the q values upon swelling of the system till a phase transition process occurs. Finally, in the third cycle, the humidity is operated at an intermediate level (r.H=70%) between 48 and 50 hours, and the swelling curve gives the same responsive behavior corresponding to the humidity change, but at a slower rate as indicated from the slopes of the time-dependent q values. Our experiment proves high sensitivity of the responsive block to the environment humidity level. This behavior has rarely investigated for the thermal responsive polymer films, where temperature is only switched at certain humidity level to investigate the swelling/deswelling behavior.

The most important thing is that the high relative humidity seems to initiate a phase transition from lamellar mixed cylinder structure to cylinder structure. This can be observed in all of the three cycles in figure 5-8 and it means the thermoresponsive behavior of the PS-b-PNIPAM shows reversible behavior and the free standing film itself is quite stable. The phase transition occurs when the main peak referring to lamellar mixed cylinder structure disappears and two new peaks arise referring to new cylinder structure. Due to the expansion of PNIPAM block in the high relative humidity while PS block keeping rigid, the volume ratio of PNIPAM block increases, which gives new block structure as cylinder.

Figure 5-9: Schematic of block structure evolution versus time for PS-b-PNIPAM in high relative humidity.

a) Influence of temperature

The temperature plays a very import role during the thermoresponsive behavior of PS-b-PNIPAM. As is shown in figure 5-8, in the first swelling cycle, a phase transition occurs, the system is kept for several hours, then the temperature is increased from 20oC to 40oC. Due to temperature jump, the PNIPAM block dramatically shrinks and deswell most of the physically absorbed water. This is indicated by the dramatic shift of the q values (within 30 min measurement time) to its initial value. A second phase transition induced by temperature jump is indicated in concomitant with the fast de-swelling behavior of the system.

Equilibrating the system at 40oC for few hours, the q value is slightly increased until the film is under equilibrium. This oscillation behavior of the system is related to the relaxation of the chain upon water removal. This behavior is previously observed by Q. Zhong et al [40] on homopolymer hydrogel systems. In the second and third cycle, the free standing film shows similar thermoresponsive behavior indicating mechanically stable film. The stabilities of the mechanical structure and thermoresponsive property are significantly important for some applications such as sensors and actuators.

5.2.2 In-situ SAXS study of PS-b-PNIPAM DBC bulky film

The PS-b-PNIPAM is PS dominated diblock copolymer DBC which means the PNIPAM is in the PS matrix. Using the static SAXS and SEM investigation, the lamellar structure of the PS-b-PNIPAM DBC is proved. In principle the PNIPAM block should be able to expand under a high relative humidity circumstances, but since both chain ends are confined by the PS block (glassy rigid block), the swelling behaviour of this DBC should be significantly limited. The swelling/deswelling behaviour of this DBC with major PS block compared with PS-b-PNIPAM DBC with a major PNIPAM block is an interesting topic to study.

The PS-b-PNIPAM bulk sample is prepared according to the method described in chapter 3 with mica window as the solution casting substrate, and it is positioned in the temperature and humidity controlling cell as described in chapter 4. The in-situ SAXS study was performed for 30 hours with each SAXS measurement scans of half an hour.

Figure 5-10: Schematic of block structure evolution versus time for PS-b-PNIPAM in high relative humidity.

Figure 5-11: Evolution of in-situ 1D SAXS profiles for bare PS-b-PNIPAM DBC during swelling/deswelling process. The blue vertical line indicates the initial peak position.

Figure 5-12: Peak fitting results of 1D SAXS profiles for q position and FWHM and its evolution versus time during three temperature cycles.

The in-situ SAXS results for PS-b-PNIPAM bulky film show that it undergoes very limited volume expansion but much slower than PS-b-PNIPAM bulky film. During the first temperature cycle, due to the controlling of the humidity is based on the absolute humidity, the relative humidity drops when the temperature increases. Both of the increasing temperature and decreasing of relative humidity contribute to the deswelling of PNIPAM block, thus the q value goes back to near the initial value. And there is no phase transition during the expansion, which is because of the confinement of the rigid glassy PS block. During the second cycle and third half cycle, the behavior of the film is similar to that of the first cycle. This also confirms that the PS-b-PNIPAM DBC bulky film is stable through thermoresponsive behavior. In principle, our study proves the assumption that the PS-b-PNIPAM DBC with major glassy PS block forms a rigid matrix limiting the flexibility of the PNIPAM block to freely expand upon swelling, indicating a very limited responsive behavior compared with PS-b-PNIPAM DBC with PNIPAM major block. Similarly, this has been recently observed by Q. Zhong et al [40] where it is observed using neutron reflectivity technique that PS layer within nanoscale multilayered PS/PNIPAM stack structures hinders further water absorption in buried layers.

5.2.3 Ex-situ SAXS study of PS-b-PNIPAM DBC bulky film

During the in-situ SAXS experiments, both the PS-b-PNIPAM free standing film and PS-b-PNIPAM bulk sample didn’t reach a final equilibrium states at high relative humidity. Thus the ex-situ SAXS experiments were done for exploring how far the peak can shift in a quasi-final equilibrium state (1-2 week swelling experiment).

A small plastic sample box with cap is used and a small reservoir made by bottle cap is built inside for generating of vapor atmosphere. Then the prepared DBC bulk samples are put in and the water or water/THF mixed solution are injected in the reservoir. The sample boxes are closed and sealed with taps and stored in a stable place for 1 ~ 2 weeks.

• PS-b-PNIPAM DBC bulky film: ex-situ SAXS study

In figure 6-17, the ex-situ SAXS experiment results show that for PS-b-PNIPAM bulk sample in the water vapor, the shift of main peak q position stops after 2 weeks. But for the PS-b-PNIPAM bulk sample in the water:THF=1:1 mixed vapor, the main peak goes far from the original position and the peak also becomes more broad. This happens because both PS and PNIPAM have good solubility in THF, thus the THF vapor enables PS matrix to be more flexible which allows PNIPAM to expand more. But at the same time THF has the mixing effect on PS and PNIPAM block, thus the microphase separation process is affected and weak segregation process is observed as indicated from the peak broadening.

Figure 5-14: Schematic of block structure evolution versus time for PS-b-PNIPAM in water/THF=1:1 mixed vapor.

• PS-b-PNIPAM DBC bulky film: ex-situ SAXS study

Figure 5-15: a) SAXS 1D profiles of PS-b-PNIPAM DBC bulk sample in water vapor for 2 weeks. b) Peak fitting results of the SAXS profiles in figure a).

The ex-situ SAXS experiments for PS-b-PNIPAM DBC bulk sample were performed with the same set-up as PS-b-PNIPAM, the sample is prepared as a free standing film. As shown in figure 5-15 (a), the swelling of PNIPAM brings the block structure to a phase transition process (as previously observed in the in-situ SAXS experiment, figure 5-8) and after two weeks the new phase peaks are still slightly shifting and growing. The equilibrium state seems need even long time to achieve as it can be estimated in according to figure 5-15 (b). This is generally an interesting behavior that the film adapts more and more water content but this can be also coupled with a possible deterioration of the free-standing film upon long water vapor exposure. It has been previously observed by Q. Zhong et al [41] that thermoresponsive hydrogel thin film upon swelling may de-wet upon longer time exposure to high-humidity environment. The robustness of the free-standing film upon very long time high humidity exposure to prepare integer and stable devices can be further examined.

Poster at DPG 2016