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Critical Phenomenon During Drying of Semiaromatic, Transparent and Soluble Polyimide Cast Films: Real-Time Observation of Birefringence and Other Integrated Parameters Yuji Eguchi, Emre Unsal, and Miko Cakmak* Institute of Polymer Engineering, University of Akron, Akron, Ohio 44325-0301, United States ABSTRACT: A newly developed real-time multifunctional monitoring system was used to track in-plane and out-of-plane birefringence, weight, thickness and surface temperature of semiaromatic, transparent, and soluble polyimide cast films during drying. At a critical point during drying, out-of-plane birefringence develops rapidly when the cast film undergoes vitrification. The rapid development of out-of-plane birefringence is attributed mainly to polymer chain orientation caused primarily by in-plane confined drying induced by thickness reduction and secondarily by shrinkage stress development in the process of vitrification. Shrinkage stress and its magnitude depend on the details of freezing-in process as dictated by the coating and drying variables. Some drying solutions also found to develop a small negative in-plane birefringence with the higher refractive index in the direction transverse to the air flow direction and it is attributed primarily to the residual stress development. In addition, the influence of solvent type on this phenomenon was studied using series of polar organic solvents. typically polyimide chains exhibit and make them soluble.12−14 When cycloaliphatic containing structures are introduced into the backbone, the resulting polymers exhibit Tg over 300 °C, good transparency and good solubility in certain solvents.15−18 Currently, there is considerable interest in this type of polyimide to be used as substrate films for flexible displays, films for solar cells and a range of other flexible electronic devices. Many of these applications require a precise control over transparency and birefringence which is crucial for producing optical retarder films for liquid crystal displays. Recently, our group developed a real-time monitoring system19,20 to observe the drying and orientation behavior of solution cast films. This apparatus has the capability to measure real-time surface temperature, weight, and thickness change and for the first time, both in-plane and out-of-plane birefringence simultaneously.20−34 In this paper, we present the real time drying behavior of semiaromatic, transparent and soluble polyimide cast films under a range of conditions using this instrumented drying system. The origins and mechanisms of real-time development of coupled stress, in-plane and out-of-plane birefringences discussed through experimental observations for the first time. It is shown that these parameters can be altered and controlled precisely by changing drying temperature, drying speed, and solution compositions without the need of further postprocessing techniques.

1. INTRODUCTION Solution casting is a versatile technique which is used in many industrial applications such as functional coating,1,2 membrane production,3−6 and films.7−9 It is particularly becoming relevant in multifunctional film manufacturing for emerging flexible electronics industry. In this technique, polymers are dissolved in a solvent and applied on a substrate by a casting blade or a die coating followed by evaporation of solvent to form thin films. When the film is dry enough to be handled, it is peeled off from the substrate to transfer it to the next stage of the process. Final properties of the films can be further tailored by additional post processing operations such as stretching and controlling the final drying conditions. It is, therefore, very critical to understand the physical and chemical changes that occurs at the initial drying state as they directly and indirectly affect the behavior and properties of the films downstream of the processing sequence. Polyimides are one of the most important products processed by solution casting method. They have excellent thermal stability, mechanical property, insulation performance and chemical resistance. Typical well-known aromatic polyimides such as PMDA-ODA and BPDA-PDA10,11 are not soluble in common organic solvents, due to their chain stiffness and chain packing behavior. Typically their precursor poly(amic acid)s are synthesized first and then solution cast into a gel film that is followed by thermal or chemical cyclodehydration to convert them into polyimide form. There are also fully imidized soluble polyimides that typically contain aliphatic and halogen chemical groups. The introduction of flexible linkages and bulky substituents to polyimide chain architecture disturb the charge-transfer complex, while © 2013 American Chemical Society

Received: June 12, 2013 Revised: August 8, 2013 Published: September 10, 2013 7488

dx.doi.org/10.1021/ma401209j | Macromolecules 2013, 46, 7488−7501

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Figure 1. (a) Experimental apparatus utilized in this study. Airflow direction is represented with dashed red arrows. The tunnel is made of aluminum which is thermally insulated. (b) Close-up view of the sample positions and real-time measurement sensors. Only the sample platform is covered by glass material to allow optical measurement. for retardation measurements (for 0 and 45°), and optical sensors are mounted on a horizontal breadboard above the tunnel. Optical system setup and theoretical background for spectral birefringence technique used in this study has been reported and verification of the data presented in details earlier.31−34 In order to perform optical retardation measurements, the top and bottom surfaces of the tunnel at the sample platform is covered with glass windows. Surface temperature of cast film is monitored by four pyrometers [no. 2 in Figure 1b] mounted through the holes on the glass plate above the sample stage. Sensing position of each data in sample coating area is depicted in Figure 2 (only two pyrometer positions are shown for clarity of picture, named as Temp(up) and Temp(down)). Weight data is obtained by measuring the total weight change of glass substrate and cast film, therefore this data shows averaged value of overall coating area of 3 in. × 7 in. Retardation is measured at the center, with both light beams going through a 1 in. diameter clear window on carrier glass plate. The thickness is measured both at a location next to this center (by middle laser) and upstream of the sample (by front laser). Real time thickness data is further verified by measuring the final dried film thickness following each test by a precision micrometer. All devices are connected to a computer to

2. EXPERIMENTAL SECTION 2.1. Measurement System. Figure 1a shows the experimental apparatus designed and constructed for this study. The environment is controlled by the air heater/blower [no. 1 in Figure 1a] that is attached to the upstream part of airflow tunnel [no. 2 in Figure 1a] constructed with three vertical baffles inside to control the air flow [no. 3 in Figure 1a]. The sample platform is located in the middle of the tunnel and connected to the reverse tapered downstream part [no. 4 in Figure 1a]. The solutions are cast on glass substrate and placed on the stage that sits on an electronic balance [no. 3 in Figure 1b] making a firm contact through four legs. Film thickness is monitored by three laser displacement sensors through a glass window from the top [no. 4 in Figure 1b]. Two of three displacement sensors monitor the location of the surface via the reflected laser, without penetrating inside the film (one looking in the downstream and another one looking very close to birefringence measurement zone in the middle). The third laser sensor continuously measures the glass substrate surface location. Real-time thickness is calculated from the difference between the two values. Two light sources [light paths are indicated as no. 1a and no. 1b in Figure 1b] and incident side polarizer are located underneath the stage 7489


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was made. This then allowed estimation of the real time average refractive indices. 2.3. Materials. 2.3.1. Reagents. Hydrogenated pyromellitic anhydride (HPMDA) was purchased from Iwatani gas chemical. 4,4′-Oxydianiline (ODA) was purchased from JFE chemical. γButyrolactone (GBL) was purchased from Wako Chemical. Toluene and N,N-dimethylformamide (DMF) were purchased from Fischer Scientific. Cyclopentanone (CP), N,N-Dimetylacetamide (DMAc), Nmethyl-2-pyrrolidone (NMP), and pyridine were purchased from Sigma-Aldrich. All reagents and solvents were used as received without further purification. 2.3.2. Soluble Polyimide. Soluble polyimide was synthesized by reacting HPMDA and ODA in GBL/toluene system using pyridine as a catalyst at 130−170 °C for 6 h. Reaction scheme is depicted in Figure 3. In this procedure, slightly excess HPMDA was used to control the molecular weight. Then solution was poured into excess amount of acetone to precipitate the polyimide followed by washing and filtration. White powder was obtained after drying under vacuum at 200 °C overnight. Mn, Mw, and PDI of this polyimide were 40 000, 88 000, and 2.2, respectively, estimated by GPC (Tosoh 8020 GPC system, equipped with two “TSK-GEL alfa-M” column in tandem) with polystyrene standard (at 40 °C, 1.0 mL/min flow rate and NMP as eluent with 10 mM LiBr). The reported Mn, Mw, and PDI values are apparent quantities. For more accurate results, this data need to be coupled with light scattering or a universal calibration is required. The glass transition temperature resulting polyimide was found to be 337 °C through DSC (TA Instruments, DSC 2920 Modulated DSC) at under nitrogen atmosphere at a 10 °C/min heating rate.

Figure 2. Sensing position of each data in sample coating area. Colors correspond to data line color in Figure 4.

enable real-time measurement, monitoring and recording through custom designed Labview data acquisition software. A typical experiment is carried out as follows: The desired ambient temperature and airflow rate are set at the hot air generator and the instrument is turned on and allowed to come to equilibrium for at least 20 min. The polymer solution is then cast to a desired wet thickness in the range of 25 μm to 1 mm on the glass substrate with an automated coater equipped with a 3 in. wide doctor blade. Immediately after casting, glass substrate with polymer coating on top is placed on the sample stage and the data recording is started. 2.2. Spectral Birefringence Technique. In this study, we utilized spectral birefringence technique to evaluate through thickness averaged in-plane and out-of-plane birefringence change during the course of drying. In this technique, in-plane birefringence (Δn12) and out-of-plane birefringence (Δn23) are calculated by eq 1 and 2,35 respectively.

Δn12(avg ) =

R 0(t , avg ) d(t )

⎡ 1 ⎢ R 0(t , avg ) − Rϕ(t , avg ) 1 − Δn23(avg ) = − ⎢ sin 2 ϕ d(t ) ⎢ n̅ 2 ⎣

3. RESULTS 3.1. Overview of Drying Behavior. Figure 4 displays temporal changes in all the monitored parameters using the real time measurement system. Figure 4b depicts the expanded view of Figure 4a in the range 1000−2400 s. In this experiment, 15 wt % polyimide/DMF solution was cast onto glass substrate with a blade gap of 500 μm. The substrate with cast solution immediately placed on to the sample holder inside the machine in which the air temperature and velocity are maintained constant at 60 °C and 0.5 m/s, respectively. Our apparatus has several built-in sensors strategically located over the sample along and across the air flow direction to capture the twodimensional variation on the sample. The projections of each sensor on the sample are shown in Figure 2. We divide the temporal evolution of drying into three stages: Stage 1. Start Test. Temperature Jump at Upstream Position. Following the rapid insertion of glass substrate coated with the polymer solution, its temperature rises quickly and begins to level off. During this period weight decreases rapidly as the solvent evaporation takes place. First, the thickness decrease becomes slower and then this decrease accelerates before starting to level off at longer times. The thickness is measured by laser displacement sensor that has a small beam diameter and thus measures local thickness behavior. During the initial stages of drying, the sample remains isotropic as both in-plane (Δn12), and out-of-plane (Δn23) birefringences are zero. Here, 1 = air flow direction and casting direction, 2 = transverse to air flow direction, and 3 = thickness direction.

(1) sin 2 ϕ n̅ 2

⎤ ⎥ ⎥ ⎥ ⎦

(2) Here R0(t,avg) and Rϕ(t,avg) are temporally varying thickness averaged retardation values for 0° (normal direction) and ϕ angle (45° in this study), d(t) is the thickness of cast film at time t and n̅ represents the average refractive index value of the cast film. Both inplane and out of plane retardation values were corrected for the retardations caused by the windows by separately calibrating the system without the sample and holder and subtracting the changes as the retardations are additive. In order to validate the measurement accuracy, the select solidified samples were checked by optical compensator method using a Leitz Laborlux 12 Pol microscope equipped with a 30λ Berek compensator . It should be noted that the measured retardations represent an average optical anisotropy along their respected optical paths through the temporally changing film and its optical anisotropy gradient in the thickness direction. Temporal variation of average refractive index was also determined through separate calibration involving a series of solutions of varying solvent concentration. Using the real time local weight measurement using the laser thickness monitoring system, the temporal variation of average solvent concentration at the location where the optical measurement

Figure 3. Reaction scheme for soluble semiaromatic polyimide utilized in this study. 7490


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The polymer used in this research exhibits positive intrinsic birefringence. That is, intrinsic refractive index in chain direction is higher than that of transverse direction. This helps us in understanding the details of optical anisotropy development in these films. The development of out-of-plane birefringence at a series of temperatures is shown in Figure 5b. The use of higher drying temperature starts the development of out of plane birefringence earlier but decreases the final value of birefringence at which it reaches the plateau. Since the solution drying rate depends on the temperature, we decided to correlate birefringence with actual solid content to remove the differences in solvent content at different times at these temperatures as shown in Figure 5c. Out-of-plane birefringence starts to increase when drying front reaches the center position. Although, at this point, downstream side of drying front still has higher amount of solvent thus a whole coating area is not uniform. This poses an interpretation difficulty when we discuss certain property against the solvent content especially when drying rate is high. For that reason, it is appropriate that differences in birefringence are discussed above about 75% of solid content where drying rate has fallen considerably. When we consider 80 and 100 °C data at the same level, it is found again that drying at lower temperature leads to higher birefringence for the same level of solid content. In-plane birefringence, shown in Figure 5d, also exhibits small but measurable changes during drying. The sign of inplane birefringence becomes negative as the drying temperature is raised indicating that the refractive index in the transverse direction becomes higher than in the air flow (also original casting direction) direction. The start of this in-plane birefringence temporally coincides with the rapid development of out-of-plane birefringence shown earlier. Drying at higher temperatures lead to higher in-plane birefringence, and that is exact opposite of what has been observed in out-of-plane birefringence behavior. However, magnitude of this anisotropy is considerably smaller than out-of-plane birefringence. Figure 6a shows the effect of air velocity on % solvent remaining in the film. At higher airflow rate, solvent content decreases faster and plateaus at the levels observed at low airflow rate. The onset of increase in out-of-plane birefringence (Figure 6b) occurs earlier, exhibits higher value at longer times and final value reaches the plateau much faster at higher air velocities. The same trend is confirmed in Figure 6c, where the data are plotted vs % solid. Small but measurable negative inplane birefringence is also observed as shown in Figure 6d. Increasing air velocity shortens the onset of this development. As shown in Figure 7a−d, the initial cast solution thickness plays significant role in evaporation rate and optical anisotropy development in these films. Decrease of casting thickness lead to faster development of out of plane birefringence. The out-ofplane birefringence of thinner sample develops much faster than that of thicker one, even when we map this data vs percent solid as shown in Figure 7c. In-plane birefringence of thinner samples actually show slightly positive birefringence meaning the refractive index in the flow direction (both casting and air flow direction) is slightly higher than the transverse direction indicating the possible increased dominance of shear induced orientation that occurred during casting on this birefringence at early stages (see the data for 250 μm wet cast film in Figure 7d). 3.3. Solvent Effect. The most important solvent characteristics that influence the coating and drying behavior are solubility parameter and boiling point. These and other

Figure 4. Typical drying test result of semiaromatic polyimide cast film from 15% DMF solution. Part b is a magnification of part a used to depict the trend of each data in fast changing period. Casting and drying conditions are as follows: Blade gap 500 μm, Air blower set temperature 60 °C, Airflow rate 0.5 m/s.

Stage 2. Temperature Jump at Upstream Position. Temperature Jump at Downstream Position. As the cast solution continues to dry, a front, characterized by rapid local thickness reduction, develops upstream and moves downstream direction on the sample. When it reaches the Temp (up) position in Figure 2, the temperature at this position increases (in Figure 4). As the drying front travels through the center area where the birefringences are measured, the out-of-plane birefringence increases drastically while in-plane birefringence remains constant near 0 at this location. Thickness data nearly levels off at this stage. When the drying front passes through the center area and reaches Temp (down) position, temperature of this position also increases rapidly. These observations point to substantial end of evaporative cooling leading to quick increase in temperature following passage of this front. Stage 3. Temperature Jump at Downstream Position. End Test. The drying front proceeds downstream edge of the coating while the temperatures remain nearly constant, and the weight steadily decreases at a much slower pace while out-ofplane birefringence continues to increase very slowly until the end of the test. 3.2. Influence of Drying Conditions. Figure 5a shows the effect of temperature on the temporal changes in the solvent weight fraction in the films. As expected, the increase of drying temperature leads to faster drying at the start of the experiment. At intermediate time scales, we observe crossover in these curves (see bold arrow in the figure). This indicates that increasing drying temperature does not necessarily accelerate long-term drying but it creates a solid skin layer at solution−air interface quicker. This, in turn, acts as a barrier to diffusion of the remaining solvent to the top surface slowing down the evaporation. 7491


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Figure 5. (a) Temporal development of solvent content at various drying temperatures. Other drying conditions are as follows: solvent, DMF; concentration, 15%; blade gap, 500 μm; airflow rate, 0.5 m/s. (b) Temporal development out-of-plane birefringence of DMF system. Data represent the same samples shown in part a. (c) Correlation between percent solid in cast film and out-of-plane birefringence of PI/DMF system dried at different temperatures. (d) Development of in-plane birefringence of DMF system. Data represent the same samples shown in part a . The scale of the Y axis is 1/10 of part b.

sample-air interface at early stages. Other solvent systems show similar trend, however, DMF system has slightly lower out-ofplane birefringence value compare to other three solvent systems. In-plane birefringence was also observed as shown in Figure 8d, where CP actually showed slightly positive birefringence development signifying the preservation of original orientation developed during shearing that occurred during casting. 3.4. Effect of Refractive Index on Out-of-Plane Birefringence. As mentioned above, in-plane and out-ofplane birefringence are calculated with eqs 1 and 2. Equation 2 requires the knowledge of real-time average refractive index value of the drying solution. Since the refractive index and thickness are changing concurrently, it is very difficult to determine the refractive index through a nonintrusive/ destructive method for these systems. In addition, evaporation of the solvent from the air−solution interface creates a concentration gradient through the thickness direction of the system. This results in the refractive index to become anisotropic and takes the second order tensor form that varies primarily in thickness direction. In this study, our experiments designed so that, the thickness averaged optical anisotropies are obtained. The real time retardations in eqs 1 and 2 represent position averaged values. Furthermore, refractive index used in this study also depends on temperature and wavelength, and during drying process it further changes with the of PI/solvent

properties of solvents used in this study are presented in Table 1. The solubility parameter (SP*) values of these solvents are very close (ranging from 10.4 to 12.6 cal1/2 cm−3/2), so the plasticizing power of the solvent is affected mostly by the differences in the boiling points of these solvents. Figure 8a shows the percent solvent versus time for all the solvents used in this study. As expected, the low boiling point solvent such as CP, DMF (dimethylformamide) and DMAc (dimethylacetamide) exhibit much faster solvent weight reduction as compared to high boiling point solvents such as NMP (Nmethyl pyrollidone) or GBL (γ-butyl lactone) . The decrease of solvent content in the CP system is initially the fastest because of its lowest boiling point. However, its reduction rapidly slows down and exhibits leveling off tendency at higher solvent content than those solvents with higher boiling points. Figure 8b shows the development of out-of-plane birefringence of each solvent system. CP system has the fastest increase in out-ofplane birefringence and reaches the largest value, however the differences between these solvent systems diminish at long times. Figure 8c shows the correlation between percent solid in cast film and out-of-plane birefringence in different solvent systems. Only CP with lowest boiling point and solubility parameter has very different behavior in these results. Out-ofplane birefringence starts to develop at very early stages while the film is still containing considerable amount of solvent suggesting rapid formation of solidified and oriented skin at 7492


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Figure 6. (a) Temporal development of solvent content at various airflow rates. Other drying conditions are as follows: solvent, DMF; concentration, 15%; blade gap, 500 μm; temperature, 80 °C. (b) Development curves of out-of-plane birefringence of DMF system dried at various airflow rates. Data represent the same samples shown in part a. (c) Correlation between percent solid in cast film and out-of-plane birefringence of PI/DMF system dried at different airflow rates. (d) Temporal development of in-plane birefringence of DMF system dried at various airflow rates. Data represent the same sample shown in part a.

ratio in cast film. In order to assess its effect on the out-of-plane birefringence Δn23 calculations, we prepared a series of solutions and films containing systematically varying solvent concentration and measured refractive indices using an Abbe refractometer at 25 °C at 589.3 nm of wavelength (Na D-line). As shown in Figure 9, the refractive index decreases linearly with the increase of solvent content in the film. Since we are monitoring the real-time, local thickness and total weight of the sample during drying, the average solvent content at the point where the optical measurements are made in cast film can be calculated as the initial concentration of the solid in the solution is known. Using these data, the average refractive index of the drying film was estimated and used to perform the outof-plane birefringence calculations taking its temporal variation into account. We also considered the wavelength dependence of refractive index. In the spectral birefringence technique, we use white light and this allows us to measure all the retardations in the 500 and 700 nm range. We typically pick 546 nm (green) to represent the birefringence values, since most compensators (off-line retardation measurement devices) are calibrated to this wavelength. Thus, we checked the difference of birefringence data measured at 546 and 589 nm and found there was no discernible difference between the two wavelengths. Similar result is reported about refractive index of PMDA-ODA polyimide measured at 589 and 633 nm,36 in which only a

slight difference observed between two conditions. Furthermore, temperature effect was also verified. Thermo-optic coefficients of some aromatic polyimides were reported earlier.37 According to that study, PMDA-ODA has −94 ppm/K of average thermo-optic value. Semiaromatic polyimide utilized in this study has similar structure to PMDA-ODA, so we calculated the refractive index of our polyimide at high temperature using the same thermo-optic value as PMDAODA. The obtained refractive index is 1.609 at 100 °C as compared to 1.616 at 25 °C which is measured. For drying, the same consideration should also be paid to the solvent. Typical organic solvents exhibit about −450 ppm/K as the thermo-optic coefficient. Using this value, we estimate the refractive index of DMF at 100 °C as 1.398, for instance, against 1.432 at 25 °C which is measured. Now we can calculate refractive index change at 25 and 100 °C. Figure 10 shows the out-of-plane birefringence change of PI/DMF cast film dried at 100 °C. The birefringence is computed with two changing refractive indices of 25 °C (triangle plot) and 100 °C (line). Changing refractive indices were calculated with measured solvent content in cast film. No significant difference between the uncorrected and corrected data sets is seen, which means even at high temperatures, we can use the refractive index values that are calculated with the refractive indices of PI and solvent measured at 25 °C. 7493


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Figure 7. (a) Effect of casting thickness on temporal development of solvent content. Drying conditions are as follows: solvent, DMF; concentration, 15%, temperature, 80 °C; airflow rate, 0.5 m/s. (b) Temporal development out-of-plane birefringence of DMF system. Data represent the same samples shown in part a. (c) Correlation between percent solid in cast film and out-of-plane birefringence of PI/DMF system of different cast (wet) thickness samples. (d) Temporal evolution of in-plane birefringence of different initial cast(wet) thickness samples. Data represent the same sample shown in part a (thickness values of films before and after the drying are as follows: blade gap 250−22 μm; blade gap 380−33 μm; blade gap 500−42 μm.

Table 1. Properties of Solvents Used in This Study16 bp (°C) mp (°C) density (g/cm3) SP valuea (cal1/2 cm−3/2) a

CP

DMF

DMAc

GBL

NMP

131 −58 0.95 10.4

153 −61 0.94 12.1

165 −20 0.94 10.8

203 −44 1.12 12.6

204 −24 1.03 11.3

match the temperature of air passing by it. When the solvent concentration becomes low enough to cause the Tg of the film rise to the set drying temperature, the material at the surface undergoes vitrification first. The boiling point of the solvent used also play and important role in this process. The lower the boiling point and/or solubility of solvent the faster the solid skin formation. Solvent diffusion rate and all the parameters that it depends on, including temperature, glass transition temperature of the polymer play a significant role in this through thickness concentration and optical anisotropy gradient formation. The origin of the out-of-plane birefringence in dried film has been discussed in detail in literature. For example, Prest and Luca, using their post drying off-line measurements, concluded that chain orientation responsible for out-of-plane birefringence reflects both stress supported by entanglement network and alignment of local segments, and it is a function of the difference between Tg of coating and the drying temperature as well.38,39 Greener et al. summarized their discussion in the study on birefringence control strategy that the out-of-plane birefringence is closely associated with residual stress produced by casting process.40−42 Coburn et al. concluded their discussion about the birefringence of aromatic polyimide film that the birefringence is a combination result from molecular

Solubility parameter value.

4. DISCUSSION 4.1. Overview of Drying Behavior. During drying, the most significant changes take place in stage 2 (Figure 4) where rapid out-of-plane birefringence increase was observed for the first time. What occurs at this point? In stage 2, the onset of out-of-plane birefringence is between the onset of rapid increase of temperature at two spatial locations along the sample, Temp (up) and Temp (down). Therefore, we expect that the rapid increase of surface temperature arises at center area when out-of-plane birefringence increases. At this location the solvent concentration at surface becomes very low; hence the evaporative cooling is no longer effective as it was keeping the temperature from rising to the set air temperature. Consequently, the surface temperature increases rapidly to 7494


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Figure 8. (a) Effect of solvent type on drying behavior. Drying conditions are as follows: concentration, 15%; blade gap, 500 μm; temperature, 80 °C; airflow rate, 0.5 m/s. (b) Development of out-of-plane birefringence in different solvent systems. Data represent the same sample shown in part a. (c) Correlation between percent solid in cast film and out-of-plane birefringence in different solvent systems. (d) Development of in-plane birefringence in different solvent systems. Data represent the same sample shown in part a.

Figure 9. Relationship between solvent content and refractive index at 25 °C. Measurement was performed by Abbe refractometer using Na D-line (589.3 nm).

Figure 10. Effect of refractive index values on out-of-plane birefringence. Drying test was performed at 100 °C with PI/DMF system.

orientation and film stress.36 As seen in above, there are two primary causes for the development of out-of-plane birefringence: stress development and preferential chain orientation in the plane. Thus, we tried to estimate film stress with our apparatus for further discussion.

4.2. Stress Evaluation. When polymer solution is cast on a substrate and dried, the volume of cast film shrinks as solvent evaporates. If surface energy of the substrate is high enough to keep the shape of coating area, film collapses only in thickness direction toward the substrate and in-plane shrinkage is prevented. As a result, this constrained shrinkage generates 7495


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the stress in the film plane. This real-time stress development of polymer coating is reported by Francis et al.43−46 In that study, it was revealed that, stress development is suppressed before solidification because of low modulus of the coating and fast relaxation of stress. After solidification, stress develops rapidly. This stress development curve shape resembles that of our outof-plane birefringence curve. The stress of cast film adhered to the substrate can be calculated by eq 3. σ=

Ests 2 (1 − νs)6rtc

(3)

Here, σ is a film stress, Es is Young’s modulus of substrate, ts is a thickness of substrate, νs is Poisson’s ratio of substrate, tc is a thickness of cast film, and r is a radius of the curvature approximated by eq 4. r=

L2 2d

Figure 11. Comparison of stress and out-of-plane birefringence development. Casting conditions are as follows: GBL, 15% solution; 1 mmt borosilicate glass substrate; blade gap, 500 μm; temperature, 80 °C; airflow rate, 0.5 m/s.

(4)

Here, d is a displacement measured by laser, L is a distance from center to displacement measuring point. There are several assumptions for eq 3. These are as follows: substrate and coating behave elastically with identical elastic moduli, the substrate is thicker than the coating, and the developed stress is isotropic in the plane. About the first assumption, the elastic behavior of the film can be achieved when the film is fully vitrified. Since majority of evaporation is taking place at the air−film interface, that creates heterogeneity in the thickness direction with a solid skin on air−film interface and viscoelastic fluid at the solution-substrate interface. However, the evaporation also takes place at the edges of the film, which is much thinner due to spreading of the solution following the casting procedure. Because of lower thickness on the edges, the film solidifies much quicker and will be the driving force for the stress development at the early stages of the drying. Theoretical background and more details can be found elsewhere.43−48 We usually use a thick glass substrate to prevent the system from curving caused by film stress and monitor the displacement of glass surface at blank position in Figure 2. In order to evaluate the film stress, a thin glass substrate was utilized and displacement d was measured. Properties of borosilicate glass substrate are shown in Table 2.

stress development arises before onset of birefringence. If coating area is small enough, two drastic changes should occur at the same time. The wave pattern beyond 10000 s in stress data is due to laser interference as film thickness reduces. We also measured photoelastic constant of dried film which contains about 25% of solvent GBL by stretching test and found the value was 6.0 × 10−11Pa−1 at 25 °C and 1.3 × 10−10Pa−1 at 130 °C. Even though 1.3 × 10−10Pa−1 is applied to ordinary stress optical rule expressed by eq 5 with stress value in Figure 11, we can find that calculated Cσ is much smaller than out-of-plane birefringence Δn23 in drying test as shown in Figure 12. Δn = Cσ

(5)

Table 2. Properties of Borosilicate Glass Substrate Utilized for Stress Calculation21 Young’s modulus Poisson’s ratio thickness of substrate distance from center to displacement measuring point

Es νs ts L

63 kN/mm2 0.2 1 mm 63 mm

Figure 12. Comparison between drying test result Δn23 and calculated Cσ value. Photoelastic constant C is 1.3 × 10−10Pa−1 measured by stretching test. σ is a stress shown in Figure 11.

The calculated stress for PI/GBL (15 wt % solution) system is shown in Figure 11 with out-of-plane birefringence. Figure 8d shows the development of small negative in-plane birefringence change for PI/GBL solution. However, this anisotropy is very small and it is neglected for the calculations and the system is assumed to be in isotropic stage in the plane. Figure 11 shows, stress grows slowly from the start of experiments. However birefringence does not develop at all. The drastic slope change of stress corresponds to the development of drying front at the upstream edge of the cast film. As this stress data reflects the whole stress of coating area, it can be understood that onset of

Equation 5 represents the simplified stress optical rule which only takes into account the glassy region behavior. More rigorously in a drying system, where the transition from rubbery to glassy behavior exists, one could also use the modified stress optical rule, which has been developed by Inoue et al.49−52 This result indicates that contribution of film stress generated after vitrification on out-of-plane birefringence is not dominant. Considering the fact that out-of-plane birefringence does not 7496


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During drying, characteristic solvent entrapment by skin formation and consequent large solvent concentration gradient in thickness direction was observed at high drying temperatures. When a cast film is dried under high drying rate conditions, either through high temperature or high airflow rate, it is often reported that the amount of residual solvent becomes higher than that obtained at low drying rate.55−57 This is attributed to trapping as a result of solid skin formation. When the skin layer is formed at the surface by vitrification, it decreases the rate of diffusion of solvent significantly; therefore more solvent is trapped inside the inner layers than in low drying rate condition. The crossover of 100 or 80 °C curve and 60 °C curve in Figure 5a clearly points to this entrapment phenomenon. However, similar behavior is not visible in Figure 6a, which investigates the air speed effect on drying PI/DMF system. The graph shows, % solvent remaining in the film is the same for all air flow rates used in the experiment. This concludes that the selected airflow rates did not cause the formation of trapping skinning. The high boiling point of the DMF significantly slows and even prevents the formation of the skin layer. The skin formation may occur if the air flow rate is sufficiently high in such sytems. As shown in Figure 13, 100 and 80 °C data show positive 5 nm of retardation value from very early stage and this value is

develop at all before drastic change, that should correspond to vitrification, it can be presumed that out-of-plane birefringence mostly arise just before vitrification. Possible out-of-plane birefringence development process is as follows: When enough solvent exists in the vicinity of a polymer chain, it remains in isotropic state by fast relaxation even if slight stress is developed due to shrinkage associated with solvent evaporation. As drying proceeds, the films densify and modulus of cast film increases. At this point, polymer chain axes begin to undergo in-plane orientation caused by thickness direction reduction effectively causing polymer chains to undergo uniaxial compression in thickness direction. After a short period with additional evaporation, polymer chain is frozen-in reaching the final orientation level. Since the polyimide used in this study has very high Tg, this chain orientation and freezing-in process should occur in a very short period. Therefore, out-of-plane birefringence increases drastically at the vitrification point. 4.3. Casting Condition Dependence and Solvent Effect. As we demonstrated, primarily solvent boiling point and secondarily its solubility parameter are key material parameters governing the solidification of polymer films. This is reflected in spatiotemporal variation of solvent concentration in the films and its direct impact on the solvent diffusion through the thickness direction. Increasing temperature leads to lower out-of-plane birefringence, Figure 5, parts b and c. This is a result of slower freezing-in process at higher temperature that leads to chains taking more relaxed conformation prior to vitrification. High temperature drying leads to low residual stress despite fast solvent evaporation experienced by the material.43−46 Drying at higher temperature causes the surface layers to attain higher glass transition temperature53,54 as they are depleted of solvents rapidly. This effect also contributes to lower out-of-plane birefringence as the interior of the samples have longer time to relax before the vitrification sets in as these films possess large solvent concentration gradient in thickness direction. This is evident in the crossover of data observed in Figure 5, parts a and b. Airflow rate can change drying rate at the same drying temperature; therefore, the difference between three development curves of out-of-plane birefringence in Figure 6, parts b and c, reflects only the drying rate effect without the temperature effect. Faster evaporation leads to fast freezing in process leading to slightly higher out-of-plane birefringence. The latter films also exhibit higher residual stresses. The magnitude of in-plane birefringence is much lower than out of plane birefringence. In order to explore the reasons, we investigated a number of factors. For example, drying proceeds faster at high air flow speeds. This is confirmed by laser displacement sensor at Thickness (up) position in Figure 2. Since the drying front develops along the air flow direction traveling from upstream, it defines the development of solidification regions. As the drying front propagates, the stress development is highly anisotropic: The stresses in the flow direction relax more readily as the material just downstream of the solidification front is still in solution/rubbery state while the materials in transverse direction solidify readily leading to more stresses developed in the transverse direction resulting in negative in-plane birefringence. Therefore, we attribute the low levels of in-plane birefringence development to stress development during vitrification. Since the total thickness is used to calculate the in-plane birefringence (eq 1), these small values indicate the developed skin layer due to vitrification process maybe very thin.

Figure 13. Temperature dependence of 0° retardation data in DMF system. Plus value of retardation means the polymer chain orientation is along the airflow direction. Data represent the same sample shown in Figure 5a.

maintained before the observation of dramatic change. Here, the positive value of retardation means refractive index in casting direction which is the same as air flow direction is higher than the transverse direction. This is more pronounced in relatively low boiling point solvent systems of CP or DMAc, but was not observed in higher boiling point solvent system of GBL or NMP under the same drying condition and it is attributed to the preservation of shear induced orientation of the polymer chains during casting as the fast evaporating solvents preserve part of this and slow evaporating solvents lead to its relaxation Internal stress or birefringence is influenced by the type of solvents used (Table 1) as well as drying conditions. This influence is considered as a plasticization effect in the freezingin process.58 The % solvent in three low boiling point solvents: 7497


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lowest SP value respectively as shown in Table 1 meaning the interaction between these solvents and solute polymer should be smaller than amide solvents. GBL has high boiling point and slow drying rate, almost the same value as NMP, however the plasticizing effect is smaller than NMP hence polymer chains orient more while stress is relaxed less. In CP system, we see an anomalous behavior in not only Figure 14 but also in Figure 8c. The out-of-plane birefringence of this system starts to develop around 65% of solvent content in cast film whereas around 40% in other systems. In CP, we did not see clear drying front during drying test in this system due to fast evaporation, the drying nucleated randomly throughout the film sometimes leading to wrinkles and irregularities as shown in Figure 16.

CP, DMF and DMAc decrease much faster than two high boiling point solvents, GBL and NMP as shown in Figure 8a. The difference between the three lower boiling point solvents and the two higher boiling point solvents appear much larger than expected from the differences in boiling points alone. Once the rate of solvent loss slows down, we observe crossover in Figure 8. To further clarify, the effects of solvents, birefringence data are replotted against normalized time as shown in Figure 14. This normalized time is obtained by

Figure 14. Out-of-plane birefringence development against normalized time. Normalization is performed by selecting the critical time value from the derivatives of % solvent in cast film vs time data (please see the next graph, Figure 15).

Figure 16. Surface irregularity of cast film from CP/polyimide solution.

dividing the test time by the time value where the rate of change reaches close to zero. First and second derivatives (red circles and blue triangles in the graph respectively) of % solvent vs time data are presented in Figure 15 for the CP system

Although this polyimide can be dissolved by CP at 15% concentration by heating, the solution turns to unclear gel-like solid next day at room temperature. This indicates that CP is relatively poor solvent for this polyimide and plasticizing effect is lower than other solvents. Furthermore, this solvent has lower boiling point, therefore, drying of this system proceeds rapidly leading to vitrification at shorter times. 4.4. Drying Process and Out-of-Plane Birefringence Development. To explain drying process and out-of-plane birefringence development in this study, we drew a model depicting side view of the drying film shown in Figure 17. The shown glassy and rubbery regions are not up-to-scale. The formation of skin layer is exaggerated for ease of explanation. The skin formation could not be traced visibly (through a camera or pictures of surface), but is indirectly evident through surface temperature change in Figure 4b. For out-of-plane birefringence calculations, the thickness of the skin layer is assumed to be very thin compared to the overall thickness of the sample and the anisotropy introduced by this later is ignored. Immediately following casting, the out-of-plane birefringence of the cast film remains zero, indicating the casting procedure produced isotropic samples (a). At this stage, the drying and thickness reduction is dominated by uniform solvent evaporation through the sample (b). However, after a short period of time, a drying front develops as characterized by rapid thickness reduction at the front laser thickness detector (c). Drying proceeds and glassy region appears at the upstream edge, stress increases quickly and the front laser does not change after the drying front passes through point c. However, at this point, birefringence at center position does not change at

Figure 15. First (red circles) and second (blue trinangles) derivatives of % solvent vs time (s) graph (black solid line). Dashed line shows the time value selected for the normalization for Figure 14.

(same procedure is applied for the other solvent systems). The time value selected is marked with the vertical dashed line for the calculations. In the data represented in Figure 14, we can find the three amide solvents show quite similar behavior, however CP and GBL are very different. The lower out-ofplane birefringence of three amide solvents indicates plasticizing effect to slow the freezing-in process. Solubility parameter (SP) value supports this result; GBL and CP have highest and 7498


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Figure 17. Drying model representing front (red triangles)/middle (black circles) thickness change and out-of-plane birefringence (blue squares) development under airflow condition. The figures are not up-to-scale. The drying front picture is exaggerated for ease of explanation.

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APPENDIX: VALIDATION OF THE OPTICAL MEASUREMENTS The validity of the measurement method is verified by a series of experiments and measurements (also see our earlier publication on instrumentation20). For these experiments, four polypropylene samples (Borclean HB311BF) with different machine direction orientation values (MDO 5.7, 6.2, 6.7 and 7.2) were produced by a single screw extruder melt casting line and Godet rolls. The retardation values of these four samples were measured in the machine direction (MD) which is also stretching direction in the melt casting line. In addition, these four samples were stacked in groups of two and three samples, and the retardation measurements were also taken as stacks. The samples that were used in this experiment is listed in the table below (Table 3). The 0° and 45° retardation values were measured in our spectral birefringence system and these values are compared with Gaertner optical bench polariscope (Model L305; Gaertner Scientific Co.) equipped with a 7 order Babinet compensator (GSC No. 617-F). This system is calibrated for 565 nm wavelength, so the spectral retardation measurements were also recorded in this wavelength for more accurate comparison. The two values measured for the samples are shown in the Figure 18. Retardation values measured in the spectral system are slightly higher (∼50 nm on average) compared with the benchtop system. There is an excellent agreement between the two measurement methods. The in-plane and out-of-plane birefringence values were calculated using eq 1 and 2. Figure 19 shows the comparison of these values in the two systems. There is a 5% error between the two measurement methods, which might be introduced due to the glass windows that are located above and below the sample compartment in the spectral system.

all because it is still in solution state. Also the thickness values measured by middle laser continue to decrease as solvent evaporation still dominates at this portion of the film. When drying front reaches the center area where birefringence is measured, out-of-plane birefringence develops rapidly and a sharp decrease is observed at middle laser thickness as drying front passes through point d. After the drying front passed through the center position, further evaporation of small amount of solvent causes the development of shrinkage stress; hence, birefringence continues to develop at a much slower pace, and the middle thickness almost stays constant (e−g). If the solvent reduction occurs fast, the residual stresses developed primarily in the transverse direction (perpendicular to air flow direction) leading to negative in-plane birefringence development.

5. CONCLUSIONS Multisensor real-time monitoring system revealed mechanisms of drying in semiaromatic soluble polyimide cast film. As drying proceeds by solvent evaporation, cast film goes through vitrification. This leads to rapid out-of-plane birefringence development accompanied by surface temperature rise. This out-of-plane birefringence development is mainly due to chain orientation caused by collapse of polymer chains in the plane of the film and secondarily by elastic component caused by the shrinkage stress developed during vitrification. Drying condition dependence and solvent type results indicate that slow freezing-in conditions lead to lower out-of-plane birefringence due to relaxation of polymer chains. A much lower magnitude in-plane birefringence development accompanies the out of plane birefringence development. This is attributed primarily to anisotropic elastic residual stresses developed during vitrification. 7499


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Table 3. List of Samples Used for Verification Experimentsa sample number sample sample sample sample sample sample sample sample sample sample sample sample sample sample

I II III IV V VI VII VIII IX X XI XII XIII XIV

stacked samples MDO MDO MDO MDO MDO MDO MDO MDO MDO MDO MDO MDO MDO MDO

5.7 6.2 6.7 7.2 5.7 and 6.2 5.7 and 6.7 5.7 and 7.2 6.2 and 6.7 6.2 and 7.2 6.7 and 7.2 5.7, 6.2, and 5.7, 6.7, and 5.7, 6.2, and 6.2, 6.7, and

calculations, this gradient has to be considered as the polymer film dries. To calculate the error introduced by our assumptions for out-of-plane birefringence, we used the sample I (in Table 3) with measured 0° and 45° retardation values and constant thickness and varied the refractive index values from 1.5 to 1.7. The change in the birefringence is shown in Figure 20 as a

refractive index

6.7 7.2 7.2 7.2

1.450 1.475 1.500 1.525 1.463 1.475 1.488 1.488 1.500 1.513 1.475 1.483 1.500 1.492

a

Refractive index values are not real and just assigned to create a gradient for the stacked samples.

Figure 20. Calculated out-of-plane birefringence values as a function of refractive index using Stein’s equations (eq 2).

function of refractive index. For the same retardation and thickness values, the change in the birefringence was only 10% for the wide range of refractive index change (1.5 to 1.7). In a realistic experiment the change and the gradient through the thickness in the refractive index will be smaller. The out-ofplane birefringence difference is shown in Figure 21.

Figure 18. Retardation comparison between spectral system and bench-top system.

Figure 21. Percent out-of-plane birefringence difference calculated by constant and averaged refractive index values. The sample number on the graph represents the samples in Table 3. Constant refractive index is selected as 1.49. Figure 19. Birefringence comparison between spectral system and bench-top system.

These experiments show that the retardation measurements are in very good agreement with the values measured with the benchtop system. The main functionality of our equipment, which is to detect the isotropy to anisotropy change on the solution cast/drying process is satisfied. On the other hand, due to complexity of drying process and the formation of concentration and refractive index gradient, birefringence calculations are effected with certain error values. Although

Another error analysis has to be done on the Stein’s equations, especially in eq 2. Solution drying creates a gradient in the concentration during the process, which also contributes to a refractive index gradient in the sample. For the calculations in this paper, we simply used the averaged refractive index values and ignored this gradient effect, especially through the thickness direction. For more accurate measurements and 7500


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the error is present on the final results, the magnitude of this error is smaller than 10%.

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AUTHOR INFORMATION

Corresponding Author

*(M.C.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Sekisui Chemical Co., Ltd in Japan for the financial support of this project. The development of real time measurement system was funded by the Third Frontier Program of the State of Ohio.

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