1. Introduction
Recently optical films were developed for applications in healthcare products, optical devices, electronics, and packaging. These films were also demanded on high functionality, thinner, and high engineering properties such as mechanical, heat resistance, and chemical resistance. The molecular orientation of these optical films is designed and controlled to enhance their properties.
As a unique characteristic, polymers offer the potential to control molecular orientation by controlled stretching. One important aspect of stretched films involves improving mechanical properties by altering the orientation of molecular chains in the film plane and controlling planar orientation based on a balance between properties in machine and transverse directions. We can evaluate this behavior by examining optical retardation during film stretching.
Stretching speeds in film processing have increased for polypropylene (PP), polyethylene terephthalate (PET) and other polymers. For industrial processes, stretching speeds now range up to 400 to 500 m/min. However, such fast stretching can generate crystallization and other sudden structure formation, narrowing the suitable processing window for stretching. Industry response to date has been based on trial and error. The conditions point to the need for on-line structure evaluations under fast stretching conditions as close to possible as the actual industrial processes in which the stretching is being carried out.
The stress optical rule (SOR), or the linear relationship between birefringence and stress, is often associated with development of higher order structures. Ryu et al. [
1] have shown that SOR does not hold at low temperatures close to
Tg or at high strain rates due to the contributions of glassy stress and photo-elastic constant, typically at room temperature. Hassan et al. [
2,
3,
4] discuss on-line measurements of birefringence and the development of higher order structures in investigations of mechano-optical behavior of PET films during stretching, reporting that SOR continues to apply up to the onset of strain hardening. Studies evaluating the control of strain-induced crystallinity in PET films with different deformation modes, including linear (generally assumed as the standard), exponential, logarithmic, and sigmoidal profile [
5], indicate differences in crystallinity behavior. Focusing on fast strain rates, Mahendrasingam et al. [
6] investigated conditions associated with draw ratio and temperatures for PET film stretching in which oriented crystallization can occur, evaluating relaxation rates and finding that strain-induced crystallinity decreases below a critical threshold. Polymer films other than PET films, including polystyrene and low-density polyethylene, have also been studied with on-line birefringence measurements during stretching, including detailed studies of physical properties that correlate with orientation and crystallinity [
7].
Kikutani et al. [
8] examined the retardation of PET running filaments at higher strain rates of up to 1 m/s in investigations of the relationship between birefringence and stress/temperature. Studies of PP film deformation behavior under fast stretching by Tokihisa et al. [
9] focus on uniformity of thickness as another important quality of stretched films. Hong and White [
10] investigated the birefringence of cyclo olefin polymer (COP) filaments under melt spinning and their mechanical properties.
While few reported studies have examined the behavior of films under high stretching rates, such investigations will expand our understanding of the conditions under which SOR holds. For the purposes of this study, to measure the retardation of polymer films with fast stretching processes, we designed equipment that would allow polymer film stretching at speeds of up to 60 m/min; research to date has been limited to 25 m/min. Our study examined the effects on the retardation of cyclic olefin copolymers (COC) of stretching speeds, stretching temperatures, and stretching modes, including uniaxial free width/constrained width and simultaneous biaxial stretching.
2. Experiment
2.1. Materials
We examined COC (TOPAS® 8007F-04) in this study. COC is amorphous, transparent copolymers based on cyclic olefins and linear olefins. Typical properties of COC includes; low density, high transparency, low water absorption, high rigidity, etc. Density is 1.02 g/cm3. The glass transition temperature (Tg) of used COC is 78 °C. We applied the single screw extrusion process to produce cast film measuring 250 µm in thickness. The extruder used was equipped with a screw size of φ20 mm L/D = 25, to which a coat hanger die measuring 150 mm in width was attached. The chill rolls were placed close to the die to reduce the neck-in phenomenon. The temperature was set to 240 °C for the extruder and die and to 80 °C for the chill rolls. The take-up speed was set to 1 m/min.
2.2. Stretching Process
Figure 1 shows a schematic diagram of the film stretcher (Toyo Seiki Seisaku-syo, Ltd., Tokyo, Japan, EX10-S6).
The COC film sample was cut into 90 mm × 90 mm square for stretching process. The film stretcher is equipped with clamping grips, and a load cell is installed on each side of the machine direction (MD) and transverse direction (TD) for stress-strain measurements. We calculated the sectional area during stretching by measuring the thickness of the stretched film at different draw ratio and calculated true stress by dividing the stretching force by sectional area.
The samples were moved to the clamping position in the chamber by a pneumatic device. The chamber was controlled to settle to equilibrium temperatures of 90 °C and 105 °C. Stretching was performed after 3 min of preheating. The sample was stretched at 1, 10, and 60 m/min until draw ratio reaches 3.0. Stretching modes performed include free and constrained uniaxial stretching and simultaneous biaxial stretching. After stretching, the film was immediately air-cooled to room temperature and removed from the chamber. Variability on the quality of stretched film such as thickness uniformity, bowing phenomenon, etc., was carefully considered in this experiment, especially at temperature near Tg. The experiment was designed to be reproducible avoiding unnecessary variability by monitoring the temperature distribution in the chamber including clamping grips, to make sure that temperature is in equilibrium.
2.3. Retardation Measurement
We deployed a high-speed camera (Photron Limited, Tokyo, Japan, FASTCAM SA5) on the stretcher for real-time retardation measurements at high stretching speeds (
Figure 1b). Monochromatic light at 520 nm wavelength was emitted from beneath the chamber and directed up toward the film, with a polarizing film interposed along the path. Using a high-speed camera capable of graphically recording in-plane distribution of retardation by using designated software (Photron Limited, Tokyo, Japan, PhotronFrameWork, Rev.1.0.1.7), we observed retardation at the center of the film.
For off-line measurement, we examined the films after stretching using a polarizing microscope (Olympus Corporation, Tokyo, Japan, BX51-P). The relationship between birefringence Δ
n and retardation
R is given by:
where
d is thickness and
nMD and
nTD are refractive indices in MD and TD, respectively. Stress optical constant (SOC) is given by:
where σ is the true stress.
2.4. Thermal Analysis
2.4.1. Differential Scanning Calorimetry (DSC)
We evaluated glass transition temperature (Tg) of COC by DSC (TA Instruments, New Castle, DE, USA, Q200) at 30–130 °C at a heating rate of 10 °C/min on the first heating.
2.4.2. Dynamic Mechanical Analysis (DMA)
To examine the effects of fast stretching on Tg and modulus, we measured viscoelasticity of the stretched films using a dynamic mechanical analyzer (TA Instruments, New Castle, DE, USA, RSA III) at 40–120 °C at a heating rate of 2 °C/min.
2.4.3. Thermal Mechanical Analysis (TMA)
We measured dimensional changes using a thermal mechanical analyzer (TA Instruments, New Castle, DE, USA, Q400 TMA) at 30–110 °C, at a heating rate of 5 °C/min, and 0.02 N of applied force.
2.5. Mechanical Property Measurements
After preparing the specimen according to DIN 53504-S3, we used a tensile testing machine (Toyo Seiki Seisaku-syo, Ltd., Tokyo, Japan, Strograph VG) to perform engineering stress-strain measurements of the stretched films at room temperature. The strain rate was 5 mm/min. Engineering stress was calculated from the thickness of the stretched films.