The ordering and crystallization of polymer chains is one of the most impontant research topics in condensed matter physics[1-2].The proess of polymercrystalizationis the tansfomation of its molecular chain from random curlto perfectlyordered structure.However,due to the unique long-chain structure of polymers,they undergo muli-level orderedtransfomations during the crystalization process.These transformations can be used to control the physical properties of polymer materials.However,due to the multi-level transfomations in the crystallization process of polymer materials,their crystal structure in three-dimensional space is very complex,such as multiphase structure,polymorphism,multiple crystal thicknesses,multiple crystal defects,etc.These structures cannot bethoroughly analyzed by conventional means,which limits the further development of polymer crystallzation theory.The use of ulta-thin film technology can determine the dinection of polymer chains,which simplifies thiscomplex process and enables a more in-depth analysis and understanding of polymer crystalization theory.

Ultra-thin polymerfilms(thickness<100 nm)are curenty a hot topic in the field of polymer physics.They are widely used in liquid crystal displays,sensors,data storage,antireflective coatings and other fields.Ultra-thin film technology is to spin-coat a dilute polymer solution onto the surface of a solid substate.After the phase transition,a nano-scale film is fomed,which is in a quasi-two-dimensional space restricted in one dimension.If the oaxis is perpendicular to the plane where the ab-ais is located in the polymer unit cell structure,the direction of the polymer chain in the ultathin fim can be confirmed,as shown in Figure 1.As can be seen from Figure 1,the polymer can grow into a lamella structure with the molecular chain direction perpendicularto the substate(Fat-on) or parallel to the substrate (Edge-on)[3-11].      

In the process of preparing ultra-thin films, the crystallization process caused by external conditions the influence of process and structure is very obvious. The main external conditions include: rotation Coating effect; wetting effect with the solid substrate during solution spreading the dewetting effect between the polymer solution and the substrate during the solvent evaporation process and the solution concentration. 

Figure 1: Dsc Heating Curves of Oriented and Non-Oriented Samples

Figure 2: Drawing of Droplets on Solid Surface (Surface Tension of Liquid- Solid-Gas Equilibrium Contact Surface)

Spin coating is also called orientation effect, which is the use of forces to make the polymer molecular chains, chain segments or microcrystals orderly aranged inthe orientation direction[12].For one-dimensional confined (perpendicular to the solid substrate)polymer film(100-1000nm)or ultra-thin fim(1-100nm)system, since the polymer can only diffuse or move in the direction parallel to the solid substrate, the orientation in the horizontal direction can have a significant effect on the polymer film or ultra-thin film.On the other hand, when the solution is spin-coated onto the surface ofthe solid substate, there is a wetting effect, and when the poly mer solution is dewetting during the solvent evaporation process,it is necessary to consider the wetting or deweting of the polymer and the mica substrate.For the conwenience of analysis, Young et al. derived the wetting problem ofthe polymersolution on the mica substrate through the equilibrium formula(1)[13-14]:

γ_s=γ_sl+γ_lvcosθ

(1)

In formula (1): γ_s—solid surface energy; γ_sl—solid-liquid interfacial tension; γ_lv—liquid surface energy. 

It is well known that the contact angle can be used to charactenizo the suface properties of a solid,that is,the suface tension of the solid [13-14]. When a droplet is in equilibrium on a solid surface, it forms a certain angle with the solid surface,which is the wetting angle y,as shown in Figure2 [13].

For polymer ultra-thin film systems, formula (1) can be optimized into formula (2):

S=γ_s-(γ_c+γ_cs)

(2)

In formula (2): S—spreading coefficient, γ_s—surface energy of solid substrate, γ_c— Ultra-thin film surface energy, γ_cs—interface energy between coating and substrate.

If S is a positive value,the coating will wet on the substate and the film can maintain this state.If S is a negative value,the coating will gather into a protuding sphere on the substate,that is,dewetting,and the contact angle between the coating and the substrate will be much greater than 0 [15].

Specifically, when a polymer ultra-thin film is on a solid substrate,it tends to reduce free energy by reducing the area exposed to the environment. If the surface tensionyc of the polymeris lower than the surface tensionys of the substrate, the fim remains stable and the polymer will not dewet, when γ_c is higher than γ_s, the fim wil reduce the surface energy by generating holes. At this time, the film is unstable and deweting ocours.

However, if the polymer ultra-thin fim undergoes a phase change during the wetting and deweting prooess, the entire process becomes more complicated. That is, if the polymer ultra-thin film crystallizes during the weting process, the dewetting phenomenon may disappear, and if the polymer ultra-thin flm is stil in a random coil after wetting, dewetting is likely to occur. In other words, in the process of forming the morphology of the polymer ultra-thin fim, crystallization, weting and dewetting are three competing processes, and the final morphology is affected by these three processes.

In order to simplily this complex proCess, the crystallization rate can be accelerated by using polyethylene glyool-polycaprolactone block copolymer (PEG-b-PCL), which is easier to crystallze. At the same time, the orientation of the ulta-thin film sample by horizontal rotation can significanty accelerate and prolong the wetting process of the sample,and reduce the oocumence of the dewetting processto a certain extent. This is used as a model to analyze the effects of the crystallization process, wetting process and dewetting process on the final sample morphology, aiming to provide some valuable experimental data in the field of thin film crystallization.

Experimenta Section

Raw Materials

• Methylene chloride,Analytically pure,Tianjin Comeo Chemical Reagent Co., Ltd.
• Polyethylene glycol-polycaprolactone block copolymer (PEG-b-PCL)
• The molecular weights of PEG and PCL are 5000 and 45000 respectively, and each segment is Subquantity distribution is less than 1.1,Jinan Daigang Biotechnology Co., Ltd.

Instruments

• Atomic force microscope (AFM), CSPM-5500, Guangzhou City Former Nano Instruments Co., Ltd
• Differential scanning calorimeter (DSC), Q20, American TA Instruments Company
• Desktop glue leveling machine, KW-4A, Institute of Microelectronics, Chinese Academy of Sciences
• Analytical balance FA22048, Shanghai Jingke Analytical Instrument Co., Ltd

Sample Preparation

First, a dichloromethane solution of 0.5 mol/ml PEG-b-PCL was prepared, and then a polymer ultrathin film was prepared on the mica substrate at different spin coating rates, with each spin coating time being 180 s.

Performance Testing and Characterization

• DSC Analysis: The film sample was peeled off by ultrasonic and collected. About 5 mg was weighed and placed in a crucible. Under N2 protection, the temperature was raised from 20°C to 80°C at a heating rate of 10°C/min
• AFM Analysis: The maximum vertical distance of the instrument is 2.0μm, and the maximum diameter and height of the sample stage that can accommodate the sample are both 20mm; the solid substrates for scanning tests are all mica sheets;Use AFM tapping mold Formula, scanning range 40.0μm×40.0μm, image pixel resolution 512 pix×512 pix, and scanning rate 1Hz.Using tapping type single crystal Silicon probe (Nano Devices RTESP type), the probe's resonance frequency range is 9~17kHz, typical value is 13 kHz, elastic parameter is 40 N/m, and tip curvature is less than 10 nm.During the experiment, the probe fluctuation map, height map and phase map of the sample were simultaneously recorded

RESULTS

Effect of Orientation on the Crystal Morphology of Peg-B-Pcl Films

Figure 3 shows the effect of orientation and non-orientation on the crystal structure of PCL-b-PEG ultra-thin film.

Figure 3: The Central Region of Afm Height Images of Peg-B-Pcl Isothermal Crystallization Samples

As can be seen from Figure 3(a), various morphologies appear in the non-oriented sample film: flat dendritic aggregation structure (shown in the box in the figure), which is the flat-on polymer formed during the wetting process. Platelet, the round droplet-like substance (shown as a circle in the picture) is an amorphous aggregate formed after dewetting of the sample, indicating that some PEG-b-PCL molecules have not yet completed crystallization after experiencing wetting and dewetting. The linear aggregates are edge-on lamellar structures of polymers. In general, under non-orientation, the crystal structure and perfection of the film samples are very heterogeneous.

As can be seen from Figure 3(b), the color of the oriented sample film morphology is relatively uniform, indicating that the corresponding lamellar thickness is relatively uniform and the crystal structure is relatively similar; and there are more round holes formed due to dewetting, indicating that the crystallization process occurs in the dewetting stage.

According to the Gibbs free energy equation [16-17], the free energy of the crystallization process is shown in formula (3):

∆ G = ∆ H-Tm-∆ S

(3)

When the sample is not oriented and when a phase change occurs during the orientation process, its entropy change can be expressed as:

∆ S₁ =S_C-S_A

(4)

∆ S₂ =S_C-S_O

(5)

In formulas (3) to (5): ΔG is the Gibbs free energy; ΔH is the enthalpy change; Tm is the melting temperature; ΔS is the entropy change; ΔS1 is the entropy change when not oriented, ΔS₂ is the entropy change during the orientation process, S_C is the entropy when crystallized, S_A is the entropy when amorphous, and S_O is the entropy when oriented.

Figure 4: Dsc Heating Curves of Oriented and Non-Oriented Samples

Figure 5: The Central Region of Afm Height Images of Peg-B-Pcl Ultrathin Film Lamellae Prepared at Different Rotation Rate

Table 1: The Pore Size of Peg-B-Pcl Ultrathin Film Lamellae Prepared at Different Rotation Rate

Figure 6: Section Thickness of Peg-B-Pcl Ultrathin Film Lamellae at Different Rotation Rate

Figure 7: The Relationship Between the Lamellar Thickness of Peg-B-Pcl and the Rotation Rate

The Rotation Rate/ (R­.Min ^-1)

That is, ΔG₁ >ΔG₂

(9)

In formulas (7) to (9): ΔG₁ is the Gibbs free energy when not oriented; ΔG₂ is the Gibbs free energy when oriented.

It can be seen that the use of orientation can reduce the free energy of the crystallization phase transition of the system to a certain extent, making the crystallization process more likely to occur. From the above experiments, it can be seen that the crystals formed by unoriented samples in a restricted environment are extremely imperfect; while the morphology of samples prepared after orientation is uniform, indicating that orientation is conducive to crystallization.

Effect of Orientation on the Crystal Structureof Peg-B-Pcl

Figure 4 shows the DSC heating curve of PEG-b-PCL sample. As can be seen from Figure 4, the peak shapes of the heating curves of the oriented and non-oriented samples are relatively similar, but the curve of the oriented sample obviously shifts to the right, indicating that the melting point of the oriented sample is higher, that is, the crystal is more perfect. On the other hand, the melting endotherm of the non-oriented sample occurs at around 40°C, while the oriented sample does not appear until around 45°C. This also shows that orientation is conducive to the formation of uniform and perfect crystals.

Effect of Orientation Speed on Thecrystal Morphology of Peg-B-Pcl   Ultrathin Film

Figure 5 shows the comparison of AFM height morphology images in the same area of PEG-b-PCL ultra-thin film crystals prepared with different orientation speeds. The same area selects the central area of rotation, that is, the linear velocity in the central area does not change with the rotation rate, so the central area can be understood as being only affected by the rotation rate. It can be seen from Figure 5 that when the rotation rate is lower than 5000 r/min, the ultra-thin film crystal has circular holes formed by dewetting, and as the rotation rate increases, the dewetting pore diameter becomes larger and larger. , increased from 2.2 μm to 4.1μm (the pore diameter in Table 1 is the average value obtained by measuring 20 sets of data), and the boundary size of the dewetting pores became narrower and narrower. When the orientation speed is 5000 r/min, the dewetting holes almost disappear and dendrites grow. When the orientation speed is 8000 r/min, dendritic crystals with neat and tightly arranged peripheries grow. This shows that the morphology of ultra-thin films can be controlled through orientation, and polymer ultra-thin film crystals with different morphologies can be prepared.

On the other hand, the time when crystallization occurs can be inferred from Figure 5. When the orientation speed is low (less than 5000 r/min), the ultrathin film crystal morphology has holes, indicating that the polymer crystallization occurs in the dewetting stage; while when the orientation speed is high (more than 5000 r/min), the crystal If there are no holes in the morphology, the crystallization occurs in the wetting stage, as shown in Table 1.

Effect of Orientation on Thickness of Ultrathin Film Lamellae

According to the Gibbs-Thomson equation, the melting point of polymer crystals has a linear relationship with the reciprocal of the thickness of the lamellae (1/l) [18-19], so the melting point of the polymer crystal can be directly expressed by the thickness of the lamellae, that is It reflects the mechanical properties of thermal polymers through the thickness of lamellae. Figure 6 shows the section thickness after cutting the marked area in Figure 5, and the result is shown in Figure 7 (the abscissa X in Figure 6 is the length of the section line).

It can be seen from Figure 7 that as the orientation speed increases, the thickness of PEG-b-PCL lamellae shows an increasing trend，When the orientation speed is less than 1500 r/min, the thickness of the lamellae increases significantly with the increase of the rotation speed. When the orientation speed exceeds 2000 r/min, the thickness of the lamellae shows a slowly increasing trend. When the rotation speed is 300 r/min, the thickness of the lamellae increases slowly. 11.7 nm increased to 17.0 nm at a rotation speed of 8000 r/min, an increase of 45.3%. Polymer ultrathin film crystallization belongs to a one-dimensional restricted environment, which is equivalent to poor molecular mobility. As the orientation speed increases, the molecules increase their mobility along the orientation direction, so crystalline phase transitions are more likely to occur. This may be due to the fact that when the orientation speed is low, in an ultra-thin-limited environment, the molecules have poorer mobility. During orientation, the mobility of molecules is increased, making it easier to crystallize, and the formed crystals are more perfect; the higher the orientation speed, the more stable the thermodynamic properties of the resulting polymer crystal.

• As the orientation rotation speed increases, the thickness of the lamellae gradually increases, from 11.7 nm to 17.0 nm, an increase of 45.3%
• As the orientation speed increases, the dewetting pore size continues to increase until the holes disappear, indicating that the time for crystallization gradually advances from the dewetting stage to the wetting stage
• As the orientation speed increases, the morphology of the lamellae gradually changes from porous lamellae to dendritic crystals

In summary, the ultrathin film crystal structure and morphology can be controlled by adjusting the orientation speed.

This study discovered that orientation promotes the crystallization of PEG-b-PCL ultrathin films, which is in line with the view of Chimplee [2] et al. that orientation encourages the ordered arrangement of molecular chains and is conducive to crystallization. Regarding the influence of orientation speed on crystal morphology, the findings are consistent with those of Cusick [10] et al, indicating that the crystalline forms change with the preparation conditions. The lamellar thickness increases as the orientation speeds rise, which is consistent with the research results of Butt H J [18] et al. This finding validates the regulatory effect of orientation on the thermodynamic properties of crystallization. However, this study solely utilized PEG-b-PCL materials, and one should exercise caution when extrapolating the conclusions to other polymer systems. The range of orientation speeds was restricted, and the crystallization behaviors at higher or lower speeds still await exploration. No in - depth characterization or modulation of the surface properties of the mica substrate was carried out. Therefore, further research on the influence of substrate factors on the crystallization process is required. Additionally, the absence of measurement and analysis of kinetic parameters during crystallization necessitates in-depth studies to comprehensively comprehend the crystallization mechanism.

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