Temporal Evolution of Optical Gradients during Drying in Cast Polymer

Aug 19, 2013 - ... was tracked using confocal microscopy and particle image velocimetry. ... of drying evidenced by specially developed low field nucl...
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Temporal Evolution of Optical Gradients during Drying in Cast Polymer Solutions Orcun Yucel, Emre Unsal, and M. Cakmak* Polymer Engineering Department, The University of Akron, 250 S. Forge St., Akron, Ohio 44325-0301, United States S Supporting Information *

ABSTRACT: The development of optical anisotropy gradient as a result of solvent evaporation for poly(amide−imide) (PAI) solution in dimethylacetamide (DMAc) was investigated for the first time. Experiments were carried out using real-time optical measurement with spectral birefringence technique coupled with off-line optical techniques such as the Abbe refractometer and optical compensator method. We have shown that drying process induces temporal evolution of nonuniform out-of-plane birefringence profile through the thickness direction while in-plane birefringence remained zero. The highest birefringence was observed at the substrate−solution interface at early stages of drying. Beyond a critical time, the formation of highly oriented layer was observed at the air−solution interface. This oriented layer progresses through the thickness direction as the solvent concentration is disproportionately reduced in these regions. Abbe refractometer results confirmed the anisotropy is preserved at longer drying times, the air−solution interface birefringence becoming higher compared to the substrate−solution interface. Overall, observations obtained by real-time measurement system agreed with off-line measurements.

1. INTRODUCTION

nuclear magnetic resonance (NMR) methods with Carr− Purcell−Meiboom−Gill (CPMG) pulse sequence.9−12 The development of concentration gradient leads to skinning. This skinning phenomenon stems from rapid evaporation of solvent near the air surface coupled with slow diffusion of solvent molecules from interior to the top surface. This occurs by formation of a skin layer at air−polymer coating interface at severe drying conditions.13 Skinning acts as a barrier leading to entrapment of the residual solvent inside the dried film. Trapping skinning effect becomes less evident for thicker coatings as the larger amount of solvent molecules remaining in the film suppresses the formation of the skin as they steadily diffuse to the surface from interior.14 The extent of bulk averaged optical anisotropy in solvent cast polymer films after drying was reported previously.15,16 It was found that level of birefringence was controlled by drying conditions, surface tension of the substrate and boiling point of the solvent chosen. Several theoretical studies carried out to develop proper models to predict out-of-plane birefringence generation.17−19 The generated birefringence was attributed to the residual drying stresses produced during the film casting and drying process along with other parameters such as polymer and solvent type, film thickness, drying rate, and molecular weight. Prest and Luca also discussed the effect of coating thickness, polymer concentration for plasticized systems, and molecular weight on birefringence using principal refractive index measurements.20

The solution casting process is increasingly being used in the production of functional films including optical film retarders, electrically conductive transparent films, and flexible photovoltaic devices. Doctor blade coating and slot die casting of polymer solutions on substrates gained significant importance in film industry due to uniform thickness distribution for rollto-roll continuous film manufacturing.1 Solution casting in general is a complex process in which solvent evaporation triggers multiple physical changes simultaneously. These physical changes include evaporation-induced weight loss and the accompanying reduction in thickness while the polymers undergo reorientation with their primary chain axes oriented in the plane of the film.2 During the drying process diffusion of the solvent occurs from substrate−coating interface to air−coating interface which proceeds through the thickness direction of the wet coating. This results in development of a gradient in physical parameters such as concentration and refractive index.3−5 The concentration profile of poly(vinyl alcohol) (PVA) solution in water with tracer polystyrene (PS) fluorescent particles was tracked using confocal microscopy and particle image velocimetry.6 It showed evaporation driven flow inside the polymer solution toward the substrate with resulting concentration gradient at different drying temperatures. A number of theoretical and experimental studies have been carried out on the drying behavior of glassy polymers and reported skin layer formation during drying.7,8 As a result of this phenomenon, the PVA solutions was found to develop crystallized skin in the early stages of drying evidenced by specially developed low field © 2013 American Chemical Society

Received: June 12, 2013 Revised: August 2, 2013 Published: August 19, 2013 7112

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address the existence of this optical anisotropy and its gradient through the thickness for PAI solution in DMAc by real time tracking of refractive indices on both substrate and air surfaces and their anisotropy by polarized Abbe refractometry and their gradient by rapid microtomy coupled with the optical retardation method.

The drying-induced refractive index increase of cyclo-olefin polymer (COP) and polystyrene (PS) films was correlated with density along with film thickness decrease.21 Refractive index change at different temperatures and concentrations of PVA in PVA/PMMA blend was also reported.22 The addition of zinc oxide (ZnO) nanoparticles on spin-coated and bulk polymerized ZnO/PMMA composite showed increased refractive indices with increased nanoparticle concentration.23 Poly(propylene glycol) (PPG)−salt complexes showed decrease in the refractive index value due to density change of the polymer−salt composite.24 Okajima and Koizumi first reported the application of Abbe refractometer to the measurement of principal refractive indices of the dried polymer films.25 This technique can be employed for refractive index measurements in machine direction (MD), transverse direction (TD), and normal direction (ND) as described by Samuels.26 It was shown the refractometer measures the refractive indices at the surface facing the refracting prism.27,28 This technique was used to obtain the refractive indices of each layer in two-component laminates29,30 and principal refractive indices in biaxially stretched poly(ethylene terephthalate) (PET) films.31 It was shown to be useful for direct measurement of surface anisotropic refractive indices on inside as well as outside surfaces of the stretch-blow molded PET bottles as this process imparts significant through thickness deformation (thus orientation) gradient as a result of radial deformation.32 A number of noncontact techniques can be utilized in order to measure birefringence using single-wavelength,33,34 doublewavelength,35−37 or multiwavelength methods.38−40 Among these, the spectral birefringence technique is unique as it can measure the rapid changes on the order milliseconds in the whole visible range (400−700 nm).41,42 Recently, an improved version of spectral birefringence technique for online tracking of birefringence measurements during drying of solution cast polymer coatings was introduced by our group.43 The custombuilt drying equipment can measure real-time in-plane and outof-plane birefringences, weight, thickness, and surface temperature changes during the drying of polymer solutions. The inplane (Δn12) and out-of-plane birefringence (Δn31) are calculated using the equations44 Δn12 =

Δn31 =

R0 = nMD − n TD dm 1 dm

(

R0 − Rθ 1 − sin 2 θ n

2

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(amide−imide) (PAI or L8) 10 wt % solution in DMAc was provided by Akron Polymer Systems (APS). Before solution casting process, PAI solutions were mixed further using Thinky Planetary Centrifugal Vacuum Mixer ARV-310 (rotation + revolution) for improved dissolution, uniformity, and degassing. Refractive index values of PAI and DMAc are 1.688 and 1.435 at 633 nm, respectively. 2.2. Optical Measurements. Principal refractive index measurements on both surfaces were performed by Bellingham + Stanley limited 60/HR Abbe refractometer with eyepiece polarizer. To track the refractive indices against the substrate surface, we directly cast the solution on Abbe refractometer prism and determine the in- and outof-plane refractive indices (MD, ND, and TD) as a function of time with eyepiece polarizer without the use of additional immersion contact liquid. These measurements were carried out using a white light source with a band-pass filter (633 nm) to generate the monochromatic light. In order to track the refractive indices of the solution−air surface, we cast the solution of same thickness (1 mm) onto a PET film substrate and periodically measured this surface by placing a freshly cut sample from large cast film against the measuring prism of the Abbe refractometer with an immersion solvent (refractive index of 1.72).26 Since the solution gels very rapidly (on the order of tens of minutes), it has sufficient structural integrity for this measurement. The birefringence profile through the thickness was measured by a Leitz Laborlux 12 POL S cross-polarized microscope using a 30 order Berek compensator. A large film from PAI solution was cast on a PET substrate film surface (Figure 1, step 1). 1 mm wide slice was cut using

(1) sin 2 θ n2

1/2

)

= n TD − nND (2)

where dm is the real-time thickness value, R0 is the measured 0° retardation, Rθ is the retardation value measured at θ° (where θ = 45° in our case), and n̅ is the average refractive index of the polymer coatings in three principal refractive index directions, i.e., nMD, nND, and nTD. Because of the existence of the refractive index gradient due to fast evaporation rate of the solvent at coating−air interface of the drying polymer coating, the use of average refractive index throughout the thickness direction may be in question in determining the real time out-of-plane birefringence using spectral birefringence method that uses eq 2. We will address this issue in this paper as well. Evaporation of the solvent molecules occurs from the air surface that results in gradient in refractive index through the thickness of the coating. In this paper, we experimentally

Figure 1. Solution casting and sample preparation procedure for retardation calculation using the compensator method.

a special cutting tool that keeps two fresh razor blades parallel during cutting as shown in Figure 1 (step 2), and retardation measurements were carried out within 5 min of cutting by laying down the sample in ND−TD plane to minimize changes (Figure 1, steps 3 and 4). For each time interval, we made freshly cut slices from the large cast film to reduce solvent losses from cut surfaces for through thickness optical retardation and birefringence calculation. In this cutting the substrate PET layer was kept attached to eliminate the possibility of evaporation from substrate surface during measurements. Validation of birefrin7113

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gence of the final dried PAI films was performed using an optical bench apparatus with a 7 order Babinet compensator. 2.3. Real-Time Weight, Thickness, Temperature, and Birefringence Measurements. Solution-cast films were dried in the chamber of real-time measurement system described in detail elsewhere.43 This system measures weight, thickness, and in- and outof-plane birefringences of cast solution on a glass substrate with controlled air speed and temperature. Dried films peeled from glass substrate and final thicknesses were measured using Mutitoyo CUA154 μm. Dimensions of cast films were 3 in. by 7 in., and retardation and birefringence values were calculated in the center portion using 633 nm light. Real-time weight measurements incorporate both global and local weight change while the former was directly acquired from precision balance and latter was calculated using both real-time thickness data and density of the coatings. The solutions were dried at room temperature with no air speed. Relative humidity (RH) within the chamber was not controlled and varied between 20% and 25% RH.

Measured refractive indices for two principal directions (MD = TD and ND) and resulting birefringences are shown in Figures 2b and 2c for air and substrate interfaces of the PAI coating (inset drawings show low-energy conformational state for generic PAI single repeating unit (Supporting Information S.3)). Measurements on air−liquid and substrate−liquid surfaces indicate that the solution is optically isotropic at early stages. Beyond about 4 h at air−liquid interface and 6 h at substrate−liquid interface, refractive indices start to increase with parallel development of optical anisotropy with the inplane (MD) refractive indices becoming higher than out-ofplane (ND) direction refractive indices. This was attributed to the solvent evaporation induced planar orientation of highly aromatic and rigid PAI chains resulting in a higher refractive index value in MD rather than ND (Supporting Information S.1). Highly oriented layer development started at the liquid− air interface and progressed through the liquid−substrate interface. Refractive index values in TD and MD were measured to be similar using a polarized Abbe refractometer. Totally dried films also exhibited slightly higher out-of-plane birefringence (measure of Δn31 = Δn23). 3.2. Birefringence Calculations through the Compensator Technique. In order to capture the development of through thickness anisotropy at early stages of drying, we use optical compensation technique on samples cut and laid flat as graphically illustrated in steps 2, 3, and 4 in Figure 1. The photographs shown in Figure 3 are taken with sample oriented in ND−TD plane with substrate attached. The optical

3. RESULTS AND DISCUSSION 3.1. Refractive Index Measurements through Abbe Refractometer. Evaporation of solvent during drying of polymer coatings leads to formation of a concentration gradient as surface regions are depleted of solvent quickly (Figure 2a).

Figure 3. Cross-polarized optical micrographs with 30 order Berek compensator inserted at 45° orientation captured during drying of PAI−DMAc solution at different drying times ((a) air surface, (s) substrate surface). ΔDol = high orientation layer.

paths in these films are the same, and color profiles shown in the pictures represent retardation gradient from air to substrate surface. These gradients were quantified and are shown in Figure 4 at a series of time intervals. The cast solution started to solidify (self-supporting) after 40 min of drying; therefore, samples were collected after this drying time. At the earlier stages of drying the birefringence is slightly higher at the substrate−solution interface, which we attribute to shear induced planar orientation of rigid PAI chains near the substrate surface. Starting from 140 min of drying, a

Figure 2. Optical measurements through Abbe refractometer. (a) Temporal evolution of principal refractive indices during drying of neat PAI−DMAc solution (air interface change (b), substrate interface change (c)). Inset: molecular drawings depict planar low-energy conformation state for single PAI repeating unit. 7114

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Figure 4. Calculated birefringence values from optical micrographs using the compensator method at different drying times (T = time in minutes, gray color regions depicts unmeasured birefringence values due to solid-rich layer growth (ΔDol)).

rapid increase in the air−solution interface was observed, followed by the formation of a highly oriented layer at 200 min at the air−liquid interface as shown in Figure 3 as depicted. The high value of retardation at this layer, caused by orientation of PAI chains, prevented reading of the retardation as they went out of range of the 30 order Berek compensator (indicated as shaded area as “oriented layer” in Figure 4). After 200 min, the highest birefringence occurred at the air−solution interface that progressively increases as the solvent concentration is disproportionately reduced in these regions. These observations are in good agreement with the Abbe refractometer results at the air−solution interface. Growth rate of the distinct highly oriented layer thickness at air−solution interface directly measured from pictures shown in Figure 3 for the first 360 min of drying. Figure 5a shows the increase in ratio of this highly oriented layer thickness to the real-time total thickness (ΔDol/dm) from 24% to 40% as the highly oriented layer development proceeds toward the substrate (Figure 3). As shown in Figure 4, before oriented layer was observed, birefringence profiles were integrated through thickness direction to obtain birefringence values for each drying time (Supporting Information S.2). In order to extract MD refractive index data from calculated birefringence values, a constant ND refractive index (nND) was assumed during drying. Using eq 3, average refractive index (nAVG) values were calculated. We have shown that nAVG values calculated from the compensator method are in good agreement with air interface and substrate interface polarized Abbe refractometer readings (Figure 5b). 2n + nND nAVG = MD (3) 3

Figure 5. Off-line measured refractive index and calculated birefringence values along with solid layer thickness growth. (a) Growth of solidified layer at air/substrate-coating interface (% overall coating thickness). (b) Average refractive index comparison calculated from off-line techniques incorporating the compensator method and Abbe refractometer. (c) Birefringence comparison between refractometer readings and the compensator method during drying of neat PAI/DMAc solution.

group43 was used to investigate the drying behavior of PAI− DMAc solution by tracking solution weight, thickness, and inand out-of-plane birefringences using the spectral birefringence method where two beams of polarized white light are transmitted at 0° and 45° to normal direction of cast solution sampling the optical anisotropy gradient through the thickness. This technique, thus, determines average anisotropy development (measured by Δn12 and Δn23) of polymer coatings using eq 2. This equation requires a bulk average of the refractive index of the polymer coating at a particular drying time which is estimated linearly through the concentration change (eq 4). n ̅ = nsolidχsolid + nsolventχsolvent (4)

Similarly, Figure 5c depicts the comparison of the compensation method (with integrated birefringence values) along with polarized Abbe refractometer (air and substrate interfaces) during the initial drying period for PAI−DMAc coating. Birefringence values calculated through compensator readings located in between air interface and substrate interface refractometer readings as expected. 3.3. Drying Data through Real-Time Measurement System. A real-time measurement system developed by our

Using the above equation, real-time thickness averaged anisotropy development of PAI solution was monitored during drying (Figure 6a). Out-of-plane birefringence (Δn23) increased rapidly at a critical drying time where total coating thickness leveled off. Throughout the drying process the in-plane birefringence (Δn12) remains zero, indicating in-plane isotropy 7115

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Figure 7. Comparison of birefringence values from real-time measurement system and Abbe refractometer (open symbols depicts air interface refractometer measurements where closed symbols depicts substrate interface measurements).

refractive index (eq 4). The birefringence profile shown with blue curve in Figure 7 represents arithmetic average of air− solution interface and substrate−solution interface refractive indices acquired from Abbe refractometer readings. Both realtime birefringence data located in between air interface and substrate interface refractometer readings with only 5% error. We have shown that birefringence data calculated by real-time measurement system represent reasonable thickness averaged anisotropy during drying. Overall, the arithmetic average agreed with the integrated average where the integrated average was accurately evaluated at early stages of drying. As shown in Figure 8, a molecular model was also developed to represent the physical changes occur during drying to match

Figure 6. Drying data from real-time measurement system. (a) Realtime drying data for PAI−DMAc solution with initial set wet thickness of 1 mm with 0.2 m/s air speed at 25% RH. (b) Comparison of realtime birefringence development for different initial set thicknesses of PAI−DMAc coatings.

maintained. Beyond this critical point, solid wt % continued to increase due to diffusion of remaining solvent within the bulk. The initial wet thickness effect on real-time anisotropy development was also investigated using this technique by varying cast thicknesses of 170 μm, 350 μm, 650 μm, and 1 mm (Figure 6b). Out-of-plane birefringence development occurred earlier for thin samples as expected from increased depletion rate of solvent content. Birefringence values were relatively smaller as initial coating thickness increased. This is in agreement with literature.2,15,16,20 3.4. Comparison of Optical Techniques. Optical measurements were compared with the online spectral birefringence method that uses average refractive index values for the bulk of the polymer film (calculated real time through concentration change as the weight is continuously monitored). Figure 7 shows the comparison of birefringence values measured by spectral birefringence method with the Abbe refractometer for 650 μm initial cast polymer solution. Open and closed symbols represent the birefringence values at the air−solution interface and substrate−solution interface, respectively. In order the check the validity of eq 2, two different birefringence calculations were performed. The birefringence profile shown with black curve in Figure 7 represents birefringence values calculated from percent solid averaged

Figure 8. Molecular model representing the PAI chain orientation during drying.

birefringence data depicted in Figure 7. First the system is at isotropic state (step 1) for 0−200 min of drying. It was followed by partial planar orientation of PAI chains between 200 and 600 min of drying (step 2), and finally after drying (>600 min) total planar orientation of PAI chains was observed (step 3).

4. CONCLUSIONS We have performed a comprehensive investigation on the formation of solvent evaporation induced optical anisotropy gradient during drying of the PAI−DMAc solution. To the best of our knowledge, application of either the Abbe refractometer or compensator method was not reported previously for investigation of any wet polymer coating during drying for birefringence gradient determination through the thickness of 7116

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the film. Abbe refractometer readings revealed that birefringence development occurred earlier at air−solution interface in comparison with substrate−solution interface. The evolution of optical anisotropy gradient and development of highly oriented layer were investigated for the first time using the compensator method in the first 360 min of drying. It was observed that casting procedure introduced higher birefringence on the substrate−liquid interface at earlier stages. As solvent evaporates, this profile was reversed and resulted in higher birefringence on air−solution interface. After a critical time, we observed the formation of a distinct highly oriented layer. In addition, observations from real-time measurement system and off-line measurements showed good agreement. This will allow us to use the real-time measurement system to quantitatively study the thickness averaged optical anisotropy in drying films and coatings.

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ASSOCIATED CONTENT

S Supporting Information *

Supplementary birefringence profile correlation with solid weight percent data, low-energy conformational state calculation, and experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for funding from the Lockheed Martin Corporation and the State of Ohio under grant “High Performance Films with Gas Barrier Properties for HAAs”.

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ABBREVIATIONS PAI, poly(amide−imide); DMAc, dimethylacetamide; n, corresponding refractive index; Dol, highly oriented layer thickness; dm, total coating thickness; χ, associated weight percent.

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REFERENCES

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