Influences of melt-draw ratio and annealing on the crystalline structure and orientation of poly(4-methyl-1-pentene) casting films

Abstract

Poly(4-methyl-1-pentene) casting films with row-nucleated lamellar structure were extruded through a slit die followed by stretching using a chill roll. The influences of melt draw ratio, annealing temperature, annealing time and supplied strain level on the crystalline morphology and orientation of the poly(4-methyl-1-pentene) casting films were carefully investigated by means of scanning electron microscopy, differential scanning calorimetry, Fourier transform infrared spectrometry and X-ray diffraction. It was found that the crystalline morphology and the degree of orientation were greatly influenced by the melt draw ratio. In addition, both the changes of crystallinity and orientation degree exhibited similar tendencies (firstly increasing and then decreasing) upon increasing the free annealing temperature and time. The use of an appropriate temperature and time meant that molecular chains of the amorphous region could rearrange into a crystal lattice and defects could be removed, while an excessive annealing temperature and time could lead to partial imperfect crystals melting and recrystallizing or even disorientation phenomena. In contrast to the free annealing method, it was surprisingly discovered that annealing under supplied strain was really helpful in improving the crystalline structure and orientation of casting films.

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Introduction

Poly(4-methyl-1-pentene), the trade name of which is TPX, is an important semi-crystalline polyolefin material with many outstanding properties.^1,2 Compared with conventional polyolefin materials, TPX exhibits a high degree of transparency, favorable heat resistance properties and excellent permeability, and has been widely used in various industrial fields, such as in fibers^3,4 and in microporous membranes.^5–7 With the advantages of environmental protection and lower costs, the melt extrusion/annealing/uniaxial stretching (MAUS) method (dry process) is widely used to prepare microporous membranes with a row nucleated lamellar structure.^8,9 Normally, the main issues in this process are the generation of an appropriate initial row nucleated structure and subsequent heat treatment.¹⁰ As we know, extrusion leads to planar extensional flow along the machine direction (MD), thus promoting crystallization and uniaxial orientation behavior with respect to the MD axis. The properties of casting films are greatly influenced by the initial crystalline morphology.^11–13 Annealing plays an important role in improving performances by eliminating the internal stress and defects of products. After processing, a portion of the polymer chains in the crystalline and amorphous regions could adjust their energy state to the equilibrium state as the mobility of chains increases during the annealing procedure.

To date, there has been a lot of research^7,14–19 studying the fabrication process of microporous membranes. A precursor film with a lamellar morphology is believed to be obtained by crystallization of the chains during melt stretching, which in the literature is called stress induced crystallization.^14,20 Some papers^21–23 reveal that the elongation induced morphology is strongly dependent on the effective stretching flow expressed in terms of the melt draw ratio. With the increase of the melt draw ratio, the crystalline orientation and crystalline morphology transform from spherulite to parallel lamellae perpendicular to the extrusion direction.²³ Zhi-tian Ding et al.²⁴ studied the effect of annealing on the structure and deformation mechanism of isotactic PP film with a row-nucleated lamellar structure, and found that the recrystallized lamellar structure attributed to annealing improved the slipping resistance ability of original lamellar structure during stretching, which explains the unique tensile property and the microstructure changes of the annealed films.

There is no doubt that the crystalline morphology of precursor films is profoundly influenced by intensive stretching (elongation) of flow fields during the extrusion stage. Moreover, the development of an oriented lamellar structure during annealing often has an important effect on the properties or performance of the annealed films in their next usage or application.²⁵ Therefore, quantitative analysis about the relationship between structural changes (crystallinity, orientation) and accurate annealing conditions (temperature, time and etc.) is necessary and important. In this work, the poly(4-methyl-1-pentene) was selected as the raw material for our research. The effect of the melt draw ratio on the crystalline morphology and orientation structure was studied in detail. Compared with the abundant research in the literature on temperature and time during the annealing stage, studies of annealing under an external force are relatively scarce. Aside from Seyed H. Tabatabaei et al.²⁶ and Ta-Hua Yu,¹⁴ few papers have been published that mention the effect of annealing under an external force, and both of their conclusions were that this annealing method has no benefit on the crystalline morphology and orientation. Therefore, a sceptical attitude towards the above results has still been shown because they did not give too much experimental data or detailed explanation for the results. This motivated us to conduct an intensive study on this problem, as well as the relationship between the strain level and crystallinity and orientation changes of the TPX films. Finally, different results were obtained from the experimental data, which indicated that annealing under strain was helpful to improve the crystalline structure and degree of orientation.

Experimental

Material and precursor film preparation

An extrusion grade of TPX (RT18 XB), melt flow rate 8 g/10 min (250 °C, 3.8 kg), purchased from Mitsui Chemicals America, Inc. was used. The schematic diagram for the processing of precursor films is presented in Fig. 1. The precursor films were extruded through a single screw extruder (D = 20 mm, L/D = 25, where D is the diameter of the screw and L is the length of the screw) equipped with a slit die 1.2 mm thick. The temperature of the extruder along the barrel from the hopper to the die was 180 °C, 245 °C, 245 °C, 250 °C, and the rotation speed of the screw was 15 rpm. To improve cooling, an air knife was installed to supply compressed gas to the film surface at the exit of the die. The air temperature was kept at room temperature (about 25 °C) and the cooling rate was controlled through a cooling air blower. There has been a published paper²⁷ discussing the influence of the air flow rate on the crystalline orientation, morphology, mechanical and tear properties of the PP casting films. They concluded that the use of a low air cooling rate contributed significantly to the perfection of the crystalline phase, while a further increase of air cooling did not noticeably affect the crystal structure. The row-nucleated lamellar crystalline structures of precursor films would be formed by applying an elongation stress and intensive cooling on the film surface at the exit of the die.²⁴ The degree of melt extension was described as the draw ratio (DR), which is the ratio of die thickness to precursor films. Precursor films of DR values from 15 to 55 were prepared, whose thicknesses varied from 80 μm to 22 μm. The samples used for thermal annealing and subsequent characterization were obtained from the middle of the precursor films.

Fig. 1 The schematic diagram for processing precursor films.

Thermal annealing

The precursor films of DR = 40 were selected to be free annealed in a temperature-controlled chamber for 30 min at temperatures of 110, 170, 190, 200 and 210 °C, respectively. The annealing times employed were 5, 10, 20, 30 and 40 min, under the consideration of feasible industrial processing times. When annealing under a specified level of extension, the levels of strain were 3, 6, 9, 12 and 15 percent, respectively. The specific levels of strain utilized for the present study are, in part, based upon the previous work of Yu on HDPE films.¹⁴ The effect of the external force during annealing is important to the industrial scale process, where a certain amount of strain is applied to keep the film taut while it proceeds through the annealing oven in a continuous process. The level of strain may alter the film morphology and crystalline orientation prior to stretching, which in turn can affect the microporosity and the morphology of the microporous film.

Film characterization

Scanning electron microscopy (SEM). Observations of the surface morphology of the etched films were carried out using a SEM instrument (model JSM-5900LV, JEOL Inc., Japan) operating at 20 kV. All samples were coated with a thin layer of gold before the tests.

Differential scanning calorimetry (DSC). The crystallization behavior of the specimens before and after annealing were analyzed using a TA Q20 differential scanning calorimeter under a nitrogen atmosphere. The samples were heated from 40 °C to 260 °C at a heating rate of 20 °C min⁻¹. The degree of crystallinity was calculated from enthalpy change values obtained in the heating curve, supposing a heat of fusion of 62.16 J g⁻¹ for fully crystalline poly(4-methyl-1-pentene).

Orientation degree characterization. For measurements of the orientation degree, a Nicolet 6700 Fourier transform infrared spectrometer (FT-IR) instrument from Thermo Electron Corp. was used. The beam was polarized through a Spectra-Tech zinc selenide wire grid polarizer from Thermo Electron Corp. The measurement is based on the absorption of infrared light at certain frequencies corresponding to the vibration modes of atomic groups present within the molecule. In addition, if a specific vibration is attributed to a specific phase, the orientation within that phase can be determined.^8,18,26,28 If the films are oriented, the absorption of plane-polarized radiation by a vibration in two orthogonal directions, specifically parallel and perpendicular to a reference axis machine direction (MD), should be unequal. The ratio of these two absorption values is defined as the dichroic ratio, D. The value of D can be obtained from the equation D = A_∥/A_⊥, where A_∥ is the absorption parallel and A_⊥ is the absorption perpendicular to MD. For uniaxial orientation, the dichroic ratio is related to Hermans’ orientation function (f_H) by the relationship where D₀ = 2 cot² α, and the value of α is the angle between the chromophore transition moment and the chain axis.²⁹ For TPX, a vibration specific to a single phase is not known. He and Porter,³⁰ however, used the dichroism of the 918 cm⁻¹ band, a rocking mode vibration from two methyl groups,³¹ to follow the sample orientation as a function of extension. We have also utilized this absorption band in this study.

X-ray diffraction (XRD). All samples were characterized using a DX-100 automatic X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å) at a generator voltage of 40 kV and a generator current of 40 mA operating at a step size of 0.06° from 5° to 25°.

Two-dimensional wide angle X-ray diffraction (2D-WAXD) is based on the diffraction of a monochromatic X-ray beam by the crystallographic planes (hkl) of the polymer crystalline phase. Using a pole figure accessory, the intensity of the diffracted radiation for a given hkl plane is measured as the sample is rotated through all possible spherical angles with respect to the beam. This gives the probability distribution of the orientation of the normal to the hkl plane with respect to the directions of the sample. The orientation function utilized was Hermans’ orientation function (f_H), where θ is the angle between the chain or the specific unit cell axis and a chosen reference axis, which is the MD in our case.

Results and discussion

Effects of melt draw ratios on crystalline morphology and orientation structure of casting films

In order to characterize the crystal structure of TPX, the XRD profiles of casting films prepared at different DR values were measured and are shown in Fig. 2. In the present report, TPX displays five crystalline modifications with X-ray diffraction profiles. According to Daniel³² and Tao,³³ the most stable TPX crystalline structure of form I could be obtained through cooling from the molten state and is evidenced by the reflections at 2θ = 9.5°, 13.4°, 16.6°, 18.5°, 20.6° and 21.5°, which belong to the (200), (220), (212), (321), (113) and (322/203) crystal plane diffraction peaks, respectively. All the above six diffraction peaks of the precursor films appear at a relatively lower DR. With the increase of DR, the 16.6°, 18.5° and 20.6° diffraction peaks, which belong to the (212), (321) and (113) crystal planes, gradually weaken. At a relatively higher DR, the above three crystal plane peaks even disappear under the effect of high speed stretching. It could be confirmed that the crystal form of TPX casting films is not changed under the effect of the traction-roller. The weakening and even disappearance of the above three diffraction peaks might be due to the increasing crystalline orientation along the flow direction. The growth of the (212), (321) and (113) crystal planes was gradually restrained because of uniaxial stretching, which resulted in the diffraction peaks not being presented in our results. What is more, the crystallinity also improved due to the effect of stress induced crystallization. Therefore, it could be qualitatively concluded that a higher draw ratio is beneficial in forming a more orderly crystal structure.^22,23,34

Fig. 2 XRD profiles of casting films prepared at different DR values.

To better characterize the crystalline morphology under different DR values, the etched surface morphology of SEM photographs is presented in Fig. 3. It was apparent that some twisted lamellae and even randomly arranged lamellae could be seen clearly under lower DR values (15 and 20), whereas with the increase of DR values, regular lamellae structures arranged perpendicular to the machine direction were exhibited in our results. The degree of orderly crystalline material gradually increased with the increase of DR values. It was again well established that the external stress conditions has a profound influence on the crystalline morphology of casting films: lower stress results in less oriented and twisted lamellae structures, and a higher stress produces highly oriented lamellae structures.

Fig. 3 SEM photographs of casting films prepared at different DR values (the flow direction is vertical).

To detect the changes of the crystalline structure, precursor films with different melt draw ratios were tested by DSC, and the results are shown in Fig. 4. As the DR increased, the position of the main peak of precursor films was almost unchanged with the melting point at about 232 °C, suggesting that the crystal lamellar thickness did not change much with the increasing draw ratio. The significant changes were that the low-temperature shoulder peak on the left of the main melting peak gradually weakened or disappeared with the increasing draw ratio. We believed the shoulder peak at lower temperature reveals crystals with a smaller crystal thickness. Looking carefully at the temperature range of the melting processes with various DR values allows us to find that the temperature range for melting is almost independent of the DR values. Furthermore, the FWHM (full width at half maximum) of the melting peaks with a lower DR value is much narrower than peaks with higher DR values. These results suggest that the crystals with lower DR values have more uniform crystal thickness, revealing a more “perfect crystal”.

Fig. 4 DSC curves of casting films prepared at different DR values.

As shown in Fig. 5, there is a difference in absorbance between the parallel and perpendicular spectroscopy at 918 cm⁻¹ in infrared polarization spectra. By following the formula mentioned above, we chose 918 cm⁻¹ as representative of the crystalline area and calculated the crystalline orientation degree of casting films prepared at different DR values. According to the degree of crystallinity and the crystalline orientation values shown in Table 1, although the melting point has no obvious change with the increasing melt draw ratio, the crystallinity and crystalline orientation increased gradually and become stable until a DR value of 40 was reached. It could be concluded that the films with higher DR values have a higher crystallinity and orientation degree.

Fig. 5 The FTIR spectrum of TPX casting film parallel (0°) and perpendicular (90°) to the extrusion direction.

Table 1 Degree of crystallinity (X_c) and crystalline orientation (f_c) of casting films prepared at different DR values

DR	15	17	20	24	27	30	34	40	48	55
Xc (%)	61.79	62.19	62.61	63.34	63.92	64.25	64.85	65.72	65.74	64.92
fc	0.286	0.285	0.296	0.290	0.300	0.308	0.341	0.363	0.394	0.393

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Effects of free annealing on crystalline morphology and orientation structure of casting films

Free annealing of precursor films of DR = 40 was performed at different temperatures for 30 min in a temperature-controlled chamber. After the DSC test, the main melting peak position showed nearly no apparent change with the increase in annealing temperature, and no endotherm plateau was observed on the left side of the main melting peak after the heat treatment (Fig. 6). If a crystal population with a different morphology or secondary crystals are formed during annealing, the endotherm plateau should be found in the heat flow curve for the melting process.^35,36 Therefore, we could come to the conclusion that no new crystal morphology was formed in our results. In addition, the lack of obvious change of the melting point indicated that the lamellar thickness was hardly any changed at all, which does not match very well to the thickening of lamellae after annealing reported by other researchers.^24,37 This unusual phenomenon was probably owing to that the small changes of melting point were covered up by experimental and test error.

Fig. 6 DSC curves of casting films annealed at different temperatures.

The crystallinity and orientation changes with the increase of the annealing temperature are presented in Fig. 7; both curves show a similar trend. In the first stage, the movement ability of TPX molecular chains was enhanced significantly during the heightening of the temperature, which is helpful to promote the arrangement of the molecular chains of the amorphous region into the lattice, resulting in further crystallization and the rise in crystallinity. When the temperature exceeded 200 °C, the high annealing temperature also lead to molecular chain disentanglement and the melting of the metastable crystalline structure (the appearance of the molten bimodal peaks of the DSC curve of Fig. 6). Similarly, a more regular and orderly chain arrangement resulted from the movement of the polymer segments, providing an explanation of the orientation improvement. So 200 °C was selected as an optimal temperature during the heat treatment stage.

Fig. 7 Changes of the crystallinity (X_c) and the orientation with the increase of annealing temperature.

Precursor films of DR = 40 annealed at 200 °C for various times were characterized by DSC and FT-IR methods. From the changes presented in Fig. 8, it was obvious that the rapid increases of X_c and f_c with annealing time mainly took place in 20 min and reduced gradually after that. After the first 20 min, the X_c and f_c values of the annealed films were 6% and 13% higher than the precursor films, which verified the positive effect of small-scale structural modification on the crystalline structure during the annealing stage.

Fig. 8 Changes of the crystallinity (X_c) and the crystalline orientation with the increase of annealing time.

Through research into the free annealing process, there is no doubt that the annealing procedure will partially remove the defects in the crystalline phase and finally improve the lamellae orientation and uniformity. Nevertheless, it should be noted that overhigh temperatures and overlong times used during annealing might deteriorate the initial lamellar structure through local partial melting and recrystallization, which coordinates with the preceding results of Fig. 7 and 8.

Effects of annealing under external force on crystalline morphology and orientation structure of casting films

Compared with the abundant literature on free annealing procedures,^24,26,37 studies of annealing under extension are relatively rarely reported. However, the special annealing procedure might alter the crystalline structure of precursor films under the effect of an external force, which is of great significance for the subsequent stage of micropore formation. Therefore, this work was fully conducted in our experiments.

After the DSC testing of precursor films annealed under a specific strain level (0, 3, 6, 9, 12 and 15%), no obvious change of the melting peak position was found, revealing that the lamellae thickness have not been apparently altered in this process. Nonetheless, according to the degree of crystallinity variation which is presented in Fig. 9, the degree of crystallinity was improved significantly whilst increasing the strain level, indicating a great dependency between crystallinity and strain levels from the graph. This is somewhat due to the fact that an appropriate external force is helpful in motivating molecular chains to straighten and stretch under the supplied strain levels, and then attain the aim of promoting the rearrangement of molecular chains in the amorphous zone into the crystal lattice and boosting the degree of crystallinity. However, when the strain level exceeds 9%, the excessive tensile force may bring about fracture and even structural breaking in crystalline phase, resulting in the dropping off of crystallinity.

Fig. 9 Changes of the crystallinity (X_c) under increasing strain levels.

To better understand the effect of the external force on the orientation of crystals, sequences of 2D-WAXD patterns annealed at various strain levels are presented in Fig. 10. It could be observed clearly that strong reflection of the (200) plane was seen in the diffraction pattern, suggesting an obvious orientation phenomenon. Under the 9% strain level of annealing, the reflections of the (200) plane were stronger than others, indicating better orientation under this condition.

Fig. 10 2D-WAXD patterns of annealing under different strain levels.

To study the 2D-WAXD patterns in detail, the (200) reflections were analyzed using FIT-2D software and plotted against the azimuthal angle from 0° to 360° (as shown in Fig. 11). With the augmentation of the supplied strain level, we could see an increase of the diffraction intensity of the (200) plane from 0% to 9% and then a decrease from 9% to 15%. Furthermore, the corresponding diffraction intensity of the (200) plane of the 9% level sample was more conspicuous than for other samples, implying a higher degree of orientation of annealing under a 9% strain level.

Fig. 11 The intensity of the (200) plane at different azimuthal angles.

The values of the order parameters at various strain levels, which can give a half-quantitative comparison of the degree of lamellae orientation among different samples, are shown in Fig. 12. It could be seen obviously that annealing under an external force improves the orientation of casting films significantly. Moreover, compared with free annealing, there was a 70% increase for the degree of orientation at the 9% strain level. This can be greatly attributed to the effect of external force induced orientation. The molecular chains become easier to arrange into a crystal lattice orderly under the supplied force, which resulted in a more uniform oriented lamellae structure. However, consistent with the tendency of the crystallinity change, the degree of orientation started to descend when the strain reaches over 9%. This seems due to the fact that the excessive force might lead to lamellae distortion or even destruction, finally resulting in the drop in orientation.

Fig. 12 The orientation of the (200) plane at different strain levels.

Conclusions

In this study, TPX melt was extruded through a slit die followed by stretching using a chill roll. The influence of melt draw ratio, annealing temperature, time and supplied strain level on the crystalline morphology and orientation of casting films was carefully investigated. These above experiments revealed that the morphological character and the degree of orientation have obvious draw ratio dependence. A row nucleated lamellar structure perpendicular to the machine direction was gradually formed under higher stress conditions. In addition, both the crystallinity and orientation degree changes exhibited a similar tendency (firstly increasing and then decreasing) with the increasing free annealing temperature and time. This was somewhat due to the fact that the use of an appropriate temperature and time greatly contributes to the molecular chains of the amorphous region rearranging into a crystal lattice and removing defects, while an excessive temperature and time might lead to partially imperfect crystals melting and recrystallizing or even disorientation phenomena. With regards to annealing under a specified strain level, we found that in contrast to the free annealing method, supplying an external force during the annealing procedure was really helpful in the improvement of the crystalline structure and orientation.

Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Contract No. 51273219, 51573106 and 51421061), the National Key Basic Research Program of China (973 Program, No. 2012CB025902), the Foundation of State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2014-3-12) and the Fundamental Research Funds for the Central Universities (No. 2013SCU04A03).

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