Formation of stacked single crystals by controlling the evaporation of solvent from a thin film solution
To form large-sized single crystals of polymer, it is necessary to achieve a low nucleation density and to grow crystals slowly. Such can be realized by using weakly supersaturated solutions. In principle, polymer solutions can become supersaturated by means of three approaches: by decreasing the temperature of the solution, by increasing the concentration, or by adding a poor solvent into a homogeneous solution with a good solvent. Unlike conjugated polymers, which have a strong tendency for crystallization, the comparatively weaker interactions between PEG chains lead to a rather low nucleation probability and crystallization rate. Therefore, we realized a setup that allowed for an efficient control of the polymer concentration. The schematic of the experimental setup is presented in Fig. 1b. In this setup, solvent molecules are evaporating from the solution and are effusing through the window of the confining box, generating a homogeneous solvent vapor pressure inside the sealed box, which eventually decreased to zero. Evaporation leads also to an increase of the polymer concentration in the deposited solution thin film on the substrate. The rate of solvent loss from the deposited film of the dilute solution is proportional to the area that is exposed to a non-saturated vapor atmosphere. In this regard, for a system with fixed positions and constant temperature, the evaporation rate and thus the evaporation time\({ t}_{\text{e}\text{v}\text{a}\text{p}\text{o}\text{r}\text{a}\text{t}\text{i}\text{o}\text{n}}\) (the time required to evaporate all solvent molecules from the film) is only controlled by the size of the opening within the window.
We compared the morphologies of a film before and after evaporation, as shown in Fig. 1c. Once the polymer solution was deposited on the substrate, a rather smooth surface of film was observed via an optical microscope. After total evaporation of solvent (evaporation time ≈ 20 min), we found that a lot of square-shaped structures had emerged on the substrate. Square-shaped platelets represent classical patterns of PEG single crystals.17,30,31 From the changes in the corresponding grey value plots of the intensity emitted from the AIE groups, we followed the crystallization process in the thin film solution. The number density of polymers at locations close to the edge of substrate was higher, probably due to the so-called “coffee-ring effect”, resulting in an accumulation of PEG aggregates close to the edge of the substrate. For the main part of the sample, the polymer concentration within the solution film increased slowly during solvent evaporation, leading to the formation of well-ordered single crystals that all showed the same square-shape pattern of almost identical size.
At a given temperature for evaporation, we prepared a series of PEG single crystals by varying the evaporation time from \({t}_{\text{e}\text{v}\text{a}\text{p}\text{o}\text{r}\text{a}\text{t}\text{i}\text{o}\text{n}}\)= 10 to 40 min. Representative optical micrographs taken from the center of the dry films are shown in Fig. 2a. Using ImageJ software, we statistically analyzed the size and shape of the single crystals observed in optical micrographs (20 images for each experimental condition). From the histograms (Fig. 2b) and further analyses (Fig. 2c), it is evident that variations in evaporation time/rate had a significant influence on the morphology of crystals. As the evaporation time was increasing, the number density of crystal (\({N}_{\text{c}\text{r}\text{y}\text{s}\text{t}\text{a}\text{l}}\)) was decreasing from 80 to 1.5 (×10− 5 per µm2), whereas the mean lateral length of crystal was monotonically increasing from about 15 µm to 100 µm. Subsequently, based on the results of crystal density and crystal length, we calculated the total area of crystals grown in a film. Assuming that the conversion of polymer chains from the dilute solution to single crystal mainly depends mainly on supersaturation and thus is similar for all evaporation times/rates, the decrease in surface area that was occupied by crystals (decreasing from 36–16%) may indicate the formation of stacked single crystals, i.e., lateral growth of crystal lamellae was competing with the formation of stacks of lamellae. In conclusion, we could efficiently control the density and size of PEG single crystal by varying evaporation time of solvent, with the maximum length of crystals reaching up to more than 100 µm.
Optical features derived from the AIE-groups of PEG-TPE single crystal templates and their temperature response
In addition to the study on crystallization kinetics, we also characterized the optical properties of the square-shaped single crystals of PEG. Corresponding optical micrographs are presented in Fig. 3. The gray value derived from the bright field micrographs changed from the edge to the center of the crystals, indicating variations of thickness, which are consistent with the representative results of AFM measurements (height mode). Interestingly, the thickness of some crystals (up to 1.4 µm) was much higher than the thickness of a single PEG lamella (about 10 nm, Fig. S1), indicating the formation of three dimensional stacks composed of more than 100 layers of lamellae.32 Unlike usual PEG single crystals, due to the terminal TPE group in the polymer chains, the obtained PEG-TPE single crystals featured the unique possibility of characterization via photoluminescence. In the corresponding emission image, the blue color was derived from the aggregated TPE groups. The distribution of TPE groups within a mono-lamellar PEG crystal is expected to be uniform. Accordingly, we can attribute variations in the intensity of the blue color to variations in the thickness of crystals. In fact, as end groups in polymer chains, TPE-groups were rejected from the crystalline core of lamellae during crystallization of PEG, resulting in a sandwich-like structure that consisted of TPE/PEG/TPE. The confinement of TPE-groups to the fold surface of lamellar crystals introduced the features of emission from single crystals, whereas, the amorphous PEG-TPE chains in solution or non-crystalline parts of the sample showed no emission. Bright field optical microscopy and AFM have limited ability to access crystalline structures directly as they are often surrounded by amorphous chains. For instance, both the crystalline core of lamellae and amorphous overlayers contribute to the thickness of stacks determined by AFM. Alternatively, as the TPE-groups are only fluorescing when confined to the crystal fold surface, we could use this emission signal to focus on crystalline structures rather than the amorphous surrounding.
We determined the changes in emission intensity from TPE-groups when confined to PEG crystals upon increasing temperature by heating square-shaped single crystals on a hot stage heated at a rate of 5°C/min accompanied by in-situ imaging using photoluminescence microscopy (Fig. 3b). During the heating process, the emission intensity of a dry film containing many single crystals was first decreasing and then completely lost at around 55°C, i.e., a temperature that was close to the melting temperature of PEG crystals. These significant changes in emission intensity revealed the increasing mobility of TPE-groups upon increasing temperature.33,34 In the melt, the aromatic rotors with respect to the olefinic stators could rotate around the single bond axes, leading to no emission. However, in a crystal, the emission of TPE was initiated due to the synergistic effects of restricted intramolecular rotation. Here, it should be noted that the morphology of square-shaped single crystal (observed under an optical microscope or by AFM) barely changed during the employed heating process from 5°C to 55°C. The square-shape morphology was retained even at 55°C. Theoretically, it is possible to enhance the lamellar thickness by annealing the crystals at a relatively high temperature but below the melting point. In the present case, we did not observe significant thickening which may be the consequence of insufficient experimental time of minutes allowed for chain movements.
Microregional heat treatment on single crystal templates
Increasing the temperature and thereby providing thermal energy is the most important parameter that drives melting of polymer crystals. Typically, there are three methods of heat transfer for a polymer film: a) heat conduction by physical contact, e.g., hot stage heating, b) convection by macroscopic movement of a fluid, e.g., hot air heating, c) radiation by infrared (IR) light, visible light, or another form of electromagnetic radiation that can be absorbed by the molecules. As mentioned in the previous section, generating microregional morphological structures within polymer single crystals is difficult to achieve by methods which heat the crystal uniformly like conduction or convection. However, as we show in this study, by using irradiation through a confocal microscope we can locally heat micrometer scale small regions within single crystals and stacks of uniquely oriented single crystals.
The schematic of the experimental setup used in this study is presented in Fig. 4a. Two incident beams can reach the sample through independent light pathways. A laser beam with the wavelength of 405 nm was used to excite TPE and generate photoluminescence, which provided information on the morphology of PEG crystals. Unlike standard optical microscopy which collects information of the whole sample simultaneously by a CCD detector, a confocal microscope utilizes a spatial pinhole to collect the signal only from a layer (plane) of the sample, which is within the focal plane and excludes signals from out-of-focus regions. Accordingly, photoluminescence intensity can be collected individually for each pixel (image resolution: ca. 0.2 µm/pixel) as a function of the position of the focal plane. Typically, two-photon excitation confocal microscopy is employed for experiments that require deep penetration into living tissue or intact animal specimens. In our study, we adapted this approach for the study of local melting of polymer crystals based on a local heating strategy by continuous irradiation with red light. A laser beam with a wavelength of 800 nm was focused on a small region within a polymer crystal, leading to local excitation of vibrational modes of the polymers. The polymer chains absorbed some of the energy of the photons emitted by the laser and heated up the sample locally.
The heated area (black circle in Fig. 4b with a diameter of 15 µm) was detected at a frame rate of 0.125 second/frame. Thus, we obtained 8 times per second information on each pixel exposed to red light (800 nm) as a function of time, with the total exposure time marked as\({ t}_{\text{e}\text{x}\text{p}\text{o}\text{s}\text{u}\text{r}\text{e}}\). In addition, the emission signal of the whole crystal was collected by a CCD camera in real time through the light pathway of the excitation laser (405 nm). As shown by confocal microscopy images taken as a function of elapsed time, in the middle of the emission image a “hole was drilled” in the single crystal. Surprisingly, the emission intensity was not monotonically decreasing in time through uniform heating by constant irradiation with red light. The temporal evolution of the emission intensity (values integrated over all pixels within the encircled area) as a function of\({ t}_{\text{e}\text{x}\text{p}\text{o}\text{s}\text{u}\text{r}\text{e}}\) is shown in Fig. 4c. When heated at a rate of 5°C/min, we found that the emission intensity first decreased gradually (for\({ t}_{\text{e}\text{x}\text{p}\text{o}\text{s}\text{u}\text{r}\text{e}}\)from 0 to 25 s), then rose to a maximum peak at ca. 50 s, and eventually dropped to almost zero. Considering the corresponding temperature profile that was recorded by an infrared sensor during such heat treatment, the crystalline structures within the irradiated area were completely molten at\({ t}_{\text{e}\text{x}\text{p}\text{o}\text{s}\text{u}\text{r}\text{e}}\)= 90 s. But which factors caused an increase in emission intensity during the heating process? Since the emission of TPE is not favored at high temperatures, the increased fluorescence intensity must come from stronger confinement of the TPE-groups, probably caused by recrystallization within stacked single crystal. We assume that a competition of melting by heating and (local) recrystallization was taking place within the heated area before complete melting of the crystal (labelled as non-equilibrium region in the plot for\({ t}_{\text{e}\text{x}\text{p}\text{o}\text{s}\text{u}\text{r}\text{e}}<90 \text{s}\)) In Fig. 4b, it is also observed that the crystalline structures around the heated area showed a stronger emission intensity, indicating increased confinement of the TPE-groups due to a locally increased lamellar thickness of the stacked crystal at the periphery of the irradiated region. Please note that, in contrast to heating of the whole crystal as shown in Fig. 3c, the locally molten region of the crystal in Figs. 4b,c was always surrounded by a crystalline domain, which probably acted as an epitaxial surface for recrystallization.
Mechanisms of polymer movement during microregional heat treatment
In order to follow the detailed morphological changes during red light heat treatment, we chose to heat a large ring-belt area on a crystal. The key benefit of using a ring belt rather than a circular disk is that the illuminated region is bounded by crystalline regions on both sides, facilitating the observation of polymer movements in both directions. As demonstrated in Fig. 5a, the chosen crystal exhibited only weak but rather uniform emission at the initial exposure time\({t}_{0}\). As the irradiation time increased, the temperature of the heated area gradually increased. After a certain induction time, a few small spots showing strong emission were “nucleated” within the ring belt (\({t}_{1}\)) which were increasing in size and intensity over time (\({t}_{2}\)). Interestingly, starting at an exposure time\({ t}_{3}\), a dark region appeared in the lower left corner within the ring-belt area. This dark region became clearer and larger (\({t}_{4}\)), accompanied by another dark region appearing in the upper right corner. Because the dark regions were non-emissive, they indicated locally molten regions within the crystal. Eventually, these two dark regions fused (\({t}_{5}\)), resulting finally in a perfect printing of irradiated ring-belt structure on the PEG single crystal (\({t}_{6}\)). The presented phenomenon and the resulting changes in patterns may seem to be complicated. However, if we carefully consider that temperature was the only variable, we may relate the observed changes in a predictable manner to the movements of polymer chains.
In Fig. 5b, we propose possible mechanisms causing polymer movement during microregional heat treatment. As shown in the schematic representations of cross-sectional images, the observed process can be summarized in four main steps. During step 1, red light interacted with the crystal, leading to an increase in temperature. Once the temperature was high enough to allow for movements of individual chains, amorphous and mobile polymers on top of the single crystal started to form multi-lamellar crystal (step 2) (lamellar thickening may not be prominent during the short time of heating). Since the total number of confined TPE groups was increasing, the emission intensity was also increasing. Concerning the position of dark “nucleation” site, local variations in crystal thickness may introduce spatially distributed regions differing in melting temperature. During step 3, when the temperature of the heated area became higher than the (local) melting temperature of the crystal, the crystalline polymers in this area melted. Due to the presence of a temperature gradient at the interface between the heated ring-belt area and the surrounding, the adjacent still crystalline regions bounding the molten areas absorbed the molten “free” chains, which led to the formation of “high crystalline walls” around the heated area. Eventually, the heated area may contain empty “holes”, possibly amplified in size by capillary forces causing the melt to dewet the heated area. We applied AFM measurements to confirm the structures of “walls” and “holes” on crystal template (as shown in Fig. 5c). By adjusting the power of the irradiation light source, the heating rate, and\({ t}_{\text{e}\text{x}\text{p}\text{o}\text{s}\text{u}\text{r}\text{e}}\), we could even control the maximum height of the wall. With increasing power of the irradiation light source, we found that the time required to generate a “hole” was decreasing, accompanied by an increase in emission intensity of the surrounding “wall” (see Fig. S4).
Outlook
In 1932, in his pioneering work, the physicist Ukichiro Nakaya classified snow crystals into about 40 categories of morphology and summarized his results in the form of a diagram.35 Based on his diagram, one can infer the weather conditions in the upper air by observing snow crystal morphologies on the ground. In this sense, Nakaya was often referring to snow crystals as "letters from the sky". As one of the most common single crystals in nature, snowflakes inspired many researchers to study crystallization.36,37 Based on our results in the microregional heat treatment, we are able to literally write or print letters on single crystal of polymers. As demonstrated in Fig. 6, we prepared an alphabet on single crystals of PEG-TPE. This “writing technology” can be applied to store/read information from a very small area (micrometers) with good spatial resolution. More importantly, well-designed 3D single crystals can act as a micro-container for chemical reactions, drug release, cell culture, and so on. In a future study, we plan to enhance the precision of temperature adjustment in order to obtain precise control over polymer nucleation and crystal growth.