In situ formation of poly(ethylene naphthalate) microfibrils in polyethylene and polypropylene during extrusion

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Composites: Part A 39 (2008) 662–676 www.elsevier.com/locate/compositesa

In situ formation of poly(ethylene naphthalate) microfibrils in polyethylene and polypropylene during extrusion Ka Lok Leung b

a,*

, Allan Easteal a, Debes Bhattacharyya

b

a Centre for Advanced Composite Materials, Chemistry Department, University of Auckland, New Zealand Centre for Advanced Composite Materials, Mechanical Engineering Department, School of Engineering, University of Auckland, New Zealand

Received 15 March 2007; received in revised form 6 September 2007; accepted 14 September 2007

Abstract Tensile properties, morphology and the relationship of microfibrils to the extrusion die diameter for poly(ethylene naphthalate) (PEN)/polyethylene (PE) and PEN/polypropylene (PP) blends were investigated. Scanning electron micrographs of the blends revealed that the fibril morphology was developed during extrusion through the die. ‘‘Skin-core’’ morphology was observed and the morphology (size and form for the dispersed phase) was in turn influential on the tensile properties. After drawing followed by annealing of the fibrillized blends, up to 100% increase in the tensile modulus was observed and the tensile strength was increased by up to one order of magnitude. Polarized Raman spectroscopy was used to follow the change in orientation of the blend components during microfibrillization.  2007 Elsevier Ltd. All rights reserved. Keywords: A. Discontinuous reinforcement; E. Extrusion

1. Introduction Fibre reinforced composites provide a high stiffness/ strength to weight ratio. Microfibrillization has emerged in recent years as a new technique for forming efficient in situ reinforcements. It conventionally involves three stages: (i) blending of two immiscible homopolymers by extrusion, (ii) fibrillization and (iii) isotropization. Fibrillization deforms the dispersed phase into fibrillar form and orients the polymer chains. The subsequent isotropization process results in a composite structure in which the matrix melts and returns to an isotropic state while the highly oriented microfibrils are preserved [1–4]. The present work focuses on in situ fibril formation of PEN in PEN/PE and PEN/PP blends by extrusion, and the variation of morphology with changing die diameter. Chapleau and Favis [5] blended polycarbonate (PC) (as the minor phase) with polypropylene (PP) with capillary

*

Corresponding author. Tel.: +64 21 366 023. E-mail address: [email protected] (K.L. Leung).

1359-835X/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2007.09.010

dies of different lengths. They pointed out that intensive extensional deformation took place in the converging region of the die as the molten material enters the capillary zone. At low concentration of PC (5 wt%), they attributed droplet morphology to capillary instabilities in the elongated particles at the capillary entrance. At higher PC concentration (20 wt%), a sufficient amount of PC would allow fibril formation by coalescence. Tsebrenko and co-workers also observed fibril formation of polyoxymethylene in a copolyamide [6]. The authors came to same conclusion as Chapleau and Favis that fibrillization occurred at the entrance of the capillary zone. Fibrillar morphology would result in the extrudate as long as the time of residence is shorter than the time required for breaking up of the fibre-like domains in the melt. Rumscheidt and Mason [7] and Taylor [8] observed the modes of droplet break-up based on Newtonian fluids in uniform shear and extensional flow. The condition at which bursting occurs is described by the critical capillary number which is proportional to the shear rate, droplet diameter, viscosity of the matrix and inversely proportional

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to the interfacial tension. For extensional flow field, at low viscosity ratio (approximately less than 0.2) the ends of the particles drew out into sharp points from which fragments of dispersed phase were released. When the viscosity ratio was greater than 0.2 the droplet was pulled out into a long thread which eventually disintegrated into fine droplets. The formation of in situ liquid crystalline polymer (LCP) fibrils in the processing of self-reinforcing thermoplastic polymer/LCP in situ composites is closely related to the in situ fibril formation of PEN in PP or PE in this study. A wide range of LCP with different thermoplastics matrices (including polypropylene, polyamide 6 (nylon 6), polycarbonate, polyethylene terephthalate, polybutylene terephthalate and poly(etheretherketone)) has been produced and studied with conventional processing methods such as extrusion or injection moulding. A comprehensive review on different aspects of thermoplastic/LCP composites has been written by Tjong [9]. It has been found in the case of LCP/thermoplastic that fibrils of LCP were formed in situ during melt processing. In elongational flow, the velocity gradient in the flow direction causes the mesogenic units (rigid rod structures in LCP molecular chains) to align along the flow field. Peuvrel and Navard [10] confirmed rheo-optically with the flow of a lyotropic LCP (regular arrangement of the LCP is induced by changing the concentration of the LCP in a solvent) around an obstacle that elongational flow is more effective than shear flow for LCP orientation. Turek and Simon [11] studied the effect of the following parameters for the extrusion of a LCP: extrusion die length to diameter ratio (ranging from 0.5 to 133), temperature, extrusion pressure and extension rate of extrudate. The maximum tensile properties were observed with the specimens extruded with a short die with L/D equal to 2.5. It turned out that a longer die and high extrusion temperatures would enhance the relaxation of the orientation developed in the die. The deformation rate has less effect on the morphology on tensile properties except at lower temperatures where the material is not fully molten. The microfibrils in the as-extruded blends provided little enhancement in mechanical properties but drawing after extrusion was carried out to orient the PEN chains in order to maximize the reinforcing effects of the fibrils. Tensile properties, morphology and orientation of the polymer chain were investigated in this study. 2. Production and treatment of the blends 2.1. Extrusion Poly(ethylene naphthalate) (PEN; Teijin Teonex 8065s), polypropylene (PP; Montell JE 6100) and polyethylene (PE; Cotene 9042) were vacuum dried at 160 C (PEN) and 100 C (PP and PE) for approximately 10 h. The raw materials were extruded using an Axon BX-18 single screw

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extruder (with an Axon Gateway venting screw; screw diameter = 18 mm; screw length to diameter ratio = 30; screw centre height = 255 mm) to produce PEN/PE and PEN/PP blends. The temperatures in the screw barrel segments were 280, 300, 300 and 300 C; the die temperature was 280 C. Four LDPE samples with melt flow indices (MFI) = 2, 4, 8 and 20 were kindly supplied by J R Courtenay (N. Z.) Ltd. The LDPEs were vacuum dried at 100 C and were extruded with PEN under the same conditions as described above to show the effect of matrix viscosity and extrusion speed on the morphology of the blends. The viscosity ratios of PEN to the LDPEs are shown in Fig. 1. At low shear rate, the viscosities of the LDPE ranged over one order of magnitude. At high shear rate, shear thinning of PEN and LDPE caused the viscosities to approach each other, and the viscosity ratio approached unity. The viscosity of the PE used for the microfibrillization study is also shown in Fig. 1. A set of extrusion dies available at our research centre were used. The die geometry is shown in Fig. 2. A set of samples was extruded with different diameter (0.3, 1.0, 1.5 and 3 mm) at the cylindrical section to investigate the effect of die diameters. The length to diameter ratio of the cylindrical section remained at two. 2.2. Microfibrillization The extruded blends were cold drawn at room temperature or zone drawn with a tensile tester equipped with a zone-heater. The zone-heater moved downward, starting from the top (moving) grip of the tensile tester, applying local heating to the sample, while the upper grip of the tensile tester moved upward thereby stretching the sample. Isotropization of the PEN/PP and PEN/PE blends was carried out by clamping the drawn sample at constant length at 180 C or 130 C for 10 h. The extension rate on the crosshead was 5 mm min 1. 3. Experimental 3.1. Scanning electron microscopy (SEM) In order to reveal the PEN fibrils, a modified version of soxhlet extraction published [12] was adopted to remove PP or PE using hot xylene. This modified soxhlet extractor differs from a conventional soxhlet extractor in the sense that the sample was treated by hot solvent and surrounded by hot solvent vapour. The residue from soxhlet extraction was inspected with a field-emission scanning electron microscope (Philips XL30). Since the arrangement of PEN fibrils may have been disturbed after the matrices were extracted, the blends were also fractured under cryogenic conditions and inspected using SEM without further treatment to gain insight into the arrangement of PEN fibrils in the asextruded blends.

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Viscosity Ratio (PEN:LDPE)

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Fig. 1. Viscosity ratio of PEN to different polyethylenes at (a) lower shear rates and (b) higher shear rates.

3.2. Tensile tests

Fig. 2. Geometry of the extrusion die used.

The as-extruded, drawn and annealed PEN/PP and PEN/PP samples in the form of the extrudate cords were tested using an Instron tensile tester (model 5567) for their tensile properties. Maximum stress before fracture was determined with different crosshead speeds such that the fracture occurred within 0.5–5 min after the test had

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started (ASTM D-638). The tensile modulus was estimated as a 0.2% secant modulus between 0.05% and 0.25% strain and was tested at 5 mm min 1 and calculated from crosshead extension.

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to the electric field vector. Change in intensity of certain bands after rotating by 90 indicates molecular orientation in the two blend components. 4. Results and discussion

3.3. Polarized Raman spectroscopy 4.1. Morphology of the as-extruded blends Polarized Raman spectroscopy determines the orientation of polymer by the changes in band intensities due to anisotropic interactions of the vibrational modes with the polarized excitation beam. A Renishaw system 1000 Raman spectrometer with Leitz microscope was used. A Renishaw solid-state diode laser with wavelength 785 nm and 26 mW power was used as an excitation source. The incident excitation laser beam on the Raman microscope was polarized. Each specimen was placed on the sample stage such that the drawing direction was parallel or perpendicular to the fluctuation of electric field of the laser. Two spectra were taken, one at each direction (parallel or perpendicular). The Raman effect from a molecule is stimulated by the electric field of the incident beam. The directions of change of polarizability of vibrational modes which are oriented in the same direction as the electric field vector of the incident beam will give rise to bands with higher intensity than if they were less oriented with respect

Extrusion of PEN with PP or PE through a 1-mm extrusion die resulted in fibrillar morphology. Inspection of cryoscopic fracture surfaces of the as-extruded blends showed that there was a size distribution of the microfibrils. As shown in Fig. 3, the microfibrils were embedded in the matrix. Moreover, removal of the matrix by modified soxhlet extraction confirms that the microfibrils extend from the fracture surface a significant distance into the matrix (Fig. 4a–e). It should be noted that after modified soxhlet extraction, sample handling and preparation for SEM, the microfibrils observed using SEM do not necessarily reflect the original arrangement in the blend. The size of the microfibrils ranged from 0.5 to 2 lm across the fracture surfaces, fibrils in the skin region being finer than those in the core region. The variation in fibril size with respect to position on the fracture surface is attributed to variation in shear rate in the die. Higher shear rate near

Fig. 3. Micrographs of cryoscopic fracture surfaces of as-extruded 30 wt% PEN/70 wt% PE (a) and (b) and 30 wt% PEN/70 wt% PP (c) and (d) blends. Images (a) and (c) show the core regions and images (b) and (d) show the skin regions.

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Fig. 4. Micrographs of PEN extracts from PEN/PE (a)–(c) and PEN/PP (d)–(f) blends (PEN concentrations: top row: 10 wt%; middle row: 30 wt%; bottom row: 50 wt%.).

the capillary die wall produced finer fibrils due to greater shear deformation. The sizes of these fibrils were approximately the same in both matrices (PE and PP) up to at least 30 wt% PEN. From the images of the fracture surfaces, it can be seen that the microfibrils tend to become thicker at higher PEN content. The main difference in morphology between the two matrices was that coalescence occurred at a lower concentration of PEN in PP matrix. In the case of PP matrix, fibrils appeared to have coalesced to form branched fibrils

at 30 wt% PEN. For the blend with equal proportions by weight of PEN and PP, there was sufficient PEN to form a continuous phase (Fig. 4f). In the case of PE matrix, individual fibrils were observed up to at least 50 wt% PEN (Fig. 4c). 4.2. Influence of matrix viscosity on in situ fibre formation A brief study was performed on blends of PEN and LDPE to see the effect of matrix viscosity and extrusion

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speed. The four different LDPE matrices showed a trend in the ease of in situ fibril formation with respect to matrix viscosity and concentration. The difference in interfacial tension due to LDPE of different viscosity was expected to be small in this case. The interfacial tension depended on the surface tension of the components that were in contact. Different viscosities of the LDPEs were essentially caused by different molecular weight and surface tension is only a very weak function of molecular weight for long chains [13]. Therefore, the interfacial tension in turn was not expected to be very different for the different LDPE matrices used. Initially, 30 wt% PEN blends were produced. High viscosity matrix (MFI = 2) was clearly detrimental to the formation of fibrils during extrusion (Fig. 5a). Some long fibrils were formed but short, elongated or irregular particles were also present in significant amount. However, long fibrils with uniform size distribution were found in the

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other three matrices, with MFIs of 4, 8 and 20, such as those showing in Fig. 5b. The diameter of the fibrils ranged from 2 to 5 lm. To further investigate the effect of matrix viscosity, a blend with smaller PEN content (10 wt%) was used. At MFI = 4, mainly spherical droplets mixed with a few thick rods were observed (Fig. 6a). As MFI increased to 8, the morphology resembled a fibrillar structure, however, the diameter along one fibril was not uniform (Fig. 6b). In addition, breakage of the fibrils appeared to be common. At MFI = 20, uniform fibril formation was observed with the diameter of fibrils ranging from 2 to 5 lm, similar to what was shown in Fig. 5b. The above SEM observations demonstrated that low melt viscosity (high MFI) matrices were more beneficial for in situ formation. It was also shown that high concentration of the dispersed phase tended to encourage fibrillar formation. At 30 wt%, the effect of concentration had

Fig. 5. PEN extracted from 30 wt% PEN/70 wt% LDPE blends extruded at 40 rpm. (a) LDPE MFI = 2 and (b) LDPE MFI = 20.

Fig. 6. PEN extracted from 10 wt% PEN/90 wt% LDPE blends extruded at 40 rpm. (a) LDPE MFI = 4 and (b) LDPE MFI = 8.

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masked the effect of matrix viscosity. The blends with LDPE4 and LDPE8 were able to form uniform fibrils at 30 wt% PEN but not at 10 wt% PEN. Wu [14] established a relationship between viscoelastic droplet break-up and viscosity ratio (viscosity of dispersed phase/viscosity of matrix). Observations in this study were consistent with Wu’s finding that the ease of elongated droplet break-up decreases with increasing viscosity ratio i.e., lower matrix viscosity. For example, the elongated droplet may survive higher shear stress without breaking up. 4.3. Influence of extrusion speed on In situ fibre formation Fig. 7. PEN extracted from 30 wt% PEN/70 wt% LDPE (MFI = 20) blends extruded at 80 rpm.

Increasing the extrusion screw speed increases the variation in shear rate at the die. This would lead to increase in

Extrusion Die Diameter = 1 mm surface

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Fig. 8. Particle size distributions across the cryoscopic fracture surface of 10PEN/PE blend extruded through (a) 1, (b) 1.5, (c) 3 mm dies and (d) a diagram used to clarify the terminologies used in the graphs a, b and c.

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Fig. 8 (continued)

disturbance on the fibrillar domains in the die causing the droplets to burst more easily [15,16]. Comparison of Fig. 5b where the 30 wt% PEN blend was extruded at 40 rpm with Fig. 7 where the same blend was extruded at 80 rpm, shows immediately that fine fibrillar morphology was destroyed by higher screw speed. The matrix used in Fig. 7 was LDPE with MFI = 20. A mixture of droplets and fibrils were found. The uniformity of the fibrils was significantly worse than those in Fig. 5b. 4.4. Influence of die diameter on the morphology and tensile properties The deeper beneath the surface of the blend (towards the centre) the thicker the microfibrils became. Fig. 8 shows the distribution in thickness of the microfibrils in a 10 wt% PEN/PE blend extruded through a 1 mm die. A fibril-to-droplet morphology transition occurred at 200–500 lm below the surface when the die diameter was 1.5 mm or 3 mm, as shown in Fig. 9 which is an

extrudate cross-section which was extruded through a 1.5 mm die. Fibrils were observed near the surface, but only droplets were observed at the centre. In the transition region, fibrils and droplets of similar diameter co-existed. Decreasing the die diameter to 0.3 mm reduced the diameter of the fibrils in the centre of the blend and no droplets were observed (Fig. 10). Under the same temperature settings and screw speeds for extrusion, choosing different die size varies the shear stress acting on the molten blends during extrusion. The above examples demonstrated the results of varying the shear stress by changing the extrusion die as well as the effect of variation of shear stress across the cross-section of the die. Similar to a Poiseuille flow through a capillary, the shear rate on the material is greatest at the wall of the capillary section and is smallest at the centre. Higher shear rate at the die wall produced finer fibrils due to greater shear deformation. Such skin-core morphology is regularly observed in injection moulding of LCP-fibril/thermoplastic composites, where highly oriented, fibrillar LCP phase was

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Fig. 9. Variation in morphology across a cryoscopic fracture surface of a 10 wt% PEN/PE blend extruded through a 1.5 mm die. Distance from the surface of the extrudate: (a) 20 lm (magnification = 10 000·); (b) 200 lm (magnification = 5000·); (c) 1000 lm (magnification = 2500).

of fibrils which provided a strengthening effect as shown in Fig. 11. Two important trends were observed. First, the strength increased with increasing PEN content. Secondly, the tensile strength increased significantly with decreasing die diameter. 4.5. Morphology after drawing

Fig. 10. Core region of 10 wt%PEN/90 wt%PE blend extruded through a 0.3 mm die.

observed in the skin region, but much larger, deformed droplets were found in the core region [17,18]. The tensile properties should be influenced as a result of a change in morphology. Die diameter 61 mm led to formation of in situ fibrils in the core region. As a result, the proportion of fibrils present was greater than if the core region contained droplets. Blends produced in die with smaller diameter were stronger due to greater proportion

The PEN fibrils of PEN/PP and PEN/PE blends after the drawing stage became finer than those of the asextruded blends. The morphology of the as-extruded PEN/polyolefin blends differ from other MFC systems which contained spherical PET prior to drawing, such as those observed by Li and co-workers [19]. The diameters of the fibrils were 0.2–0.5 lm (compared to 0.5–2 lm in the as-extruded blends). The sizes of the microfibrils were roughly estimated from the SEM images by randomly selecting twenty fibrils in each blend to obtain the graph in Fig. 12. Reduction in size of the fibrils appeared to be insensitive to drawing temperature. The drawing stage served to reduce the diameter of the fibrils by elongating them without causing any fracture during stretching. This should increase in the effectiveness of reinforcement due to increased matrix–fibril contact area. In addition, drawing should increase the degree of

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Fig. 11. Variation of tensile strength with PEN content and die size.

orientation of the PEN polymer chains, thereby utilizing the intrinsically high strength covalent bonds to strengthen the material.

blends (0.5–0.8 GPa). Improvement in strength and modulus were offset by significant reduction in elongation at break.

4.6. Tensile properties of microfibrillized blends – PEN/PP

4.7. Tensile properties of microfibrillized blends – PEN/PE

The tensile strengths of the drawn PEN/PP blends were increased remarkably but offset by significantly lower elongation at break. Within the level of uncertainties, the maximum stress of the samples that were drawn at the same temperature did not change with PEN content (Fig. 13a). This indicates that the polypropylene matrix was oriented and contributing to strength. The strength varied with drawing temperature from 160 MPa (drawn at room temperature), to 220 MPa (drawn at 100 C), to 320 MPa (drawn at 140 C). The maximum stress increased with increasing drawing temperature because the degree of chain alignment was likely to be higher if drawing was done closer to the glass transition temperature. After isotropization, the tensile strengths of the PEN/PP blends that were fibrillized at room temperature and at 100 C were almost identical. The annealed blends that were drawn at 140 C had strengths that were almost twice those of the samples drawn at lower temperatures, indicating that stronger PEN reinforcing fibrils were produced by stretching at 140 C. The blends followed the rule of mixture closely (Fig. 13b). For instance, the slope for the series that were drawn at 140 C extrapolates to the strength of stretched PEN and neat PP. This also implied that the matrix has recovered its isotropic state before fibrillization – a composite-like structure was present. The tensile modulus values of the drawn and annealed blends ranged from 1.2 to 1.9 GPa and were insensitive to PEN content, about 100% higher than the as-extruded

The strength of the drawn PEN/PE blends increased to 100–200 MPa. PEN/PE blends that were drawn at room temperature had similar strengths which appeared to be independent of PEN content. The strength of the blends drawn at 100 C or 120 C showed a linear relationship, extrapolating to the strength of PEN drawn at 140 C (Fig. 14a). Tensile strength of the drawn blends increased with increasing PEN content, which meant that the drawn PEN microfibrils were stronger than the drawn matrix. Higher concentration of PEN reinforcing fibrils, would therefore lead to the higher strength of the blend (in the drawing direction). (Note that PEN could not be drawn to the same elongation without fracture if it was drawn at 120 C at the same drawing rate; therefore data for drawn PEN at 120 C were not available.) Drawn PEN/PE blends that were annealed at 130 C retained similar strengths as before annealing (Fig. 14b). The strength of the annealed blend could not be extrapolated to the strength of neat PE. The temperature used might not have been sufficiently high to induce complete isotropization but higher annealing temperature would cause the samples to break during annealing. However, it was observed that as the drawing temperature increased, the strength of the annealed blends could be extrapolated closer and closer to that of PEN drawn at 120 C, which is consistent with increased drawing temperature inducing stronger reinforcement. The tensile modulus values of the drawn blends and partially isotropized blends were in the

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Fig. 12. Average fibril diameters in (a) PEN/PP; (b) PEN/PE blends (the first two digits in the labels in the x axes show the weight percentage of PEN. ‘‘400%’’ means 400% elongation. ‘‘RT’’, ‘‘100 C’’ and ‘‘120 C’’ indicates the drawing temperature where RT = room temperature.

5. Raman spectroscopy

of the polarization direction of the incident beam. All the Raman vibrational modes in these blends were equally intense in both the parallel and perpendicular spectra. Chain alignments of the blend components after drawing were shown by the following Raman bands:

The isotropic states of the as-extruded PEN/PP and PEN/PE blends were indicated by the fact that radiation scattered from the as-extruded blends were independent

1. PEN – naphthalene ring vibrations (1393, 1483 and 1636 cm 1) which were more intense in the parallel spectra [20].

range of 0.7–2.0 GPa, which were 2–10 times higher compared to the tensile modulus values of the as-extruded blends (0.2–0.3 GPa).

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Fig. 13. Tensile strength of (a) drawn PEN/PP; (b) microfibrilized PEN/PP blends compared to the as-extruded blends.

2. PP – decrease in the intensity ratio of 842:809 bands in the parallel spectra after drawing [21,22]. In addition, after drawing, the stress-sensitive band at 1167 cm 1 was more intense in the parallel spectra [23]. 3. PE – C–C symmetric band at 1127 cm 1 was more intense in the parallel spectra [24].

and perpendicular spectra (Fig. 15). On the other hand, the annealing temperature used was not sufficient to convert the drawn PE matrix back to isotropic as the 1127 band intensity remained different between the parallel and perpendicular spectrum (Fig. 16). 6. Conclusion

After annealing, PEN remained oriented in both blends to give rise to band intensity differences between the parallel and perpendicular spectra. For the matrices, on one hand, PP returned to its isotropic state as the 1167, 842 and 809 cm 1 bands became equally intense in the parallel

The PEN/PE and PEN/PP blend systems discussed above exhibited fibrillar morphology in contrast to other systems that contained discrete spheres of PET dispersed phase prior to microfibrillization. That said, the possibility

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Fig. 14. Tensile strength of (a) drawn PEN/PP; (b) microfibrilized PEN/PE blends compared to the as-extruded blends.

of fibril morphology of as-extruded blends should not be ruled out for blends with PET as the dispersed phase. Morphology in the final microfibrillized product is essentially the same, but the morphology development is subtly different in the intermediate steps. Formation of in situ fibrils in a binary blend during extrusion should be influenced and determined by processing conditions such as screw geometry, die geometry, temperature and screw speed; and material properties such as matrix to dispersed phase viscosity ratio and interfacial tension. Further studies on the influences of these factors on the morphology are in progress. The present work has demonstrated the influence of die diameter on the morphology. A fibrelike dispersed phase will reinforce a matrix polymer but

droplet or ellipsoidal particles with small aspect ratio will not. Fibrillization at different temperatures demonstrated that drawing carried out at close to the glass transition temperature of the fibrillar phase maximizes chain alignment. However further increase in temperature beyond the glass transition is likely to reduce the strength of the fibril phase because isotropization of the fibril phase may occur, albeit slowly at temperatures below the melting point, thus disrupting chain alignment. Data from both annealed PEN/PP and PEN/PE confirmed that high drawing temperature (close to the glass transition temperature) was required to maximize the strength. From the case of PEN/PP, it can be seen that

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Fig. 15. Raman spectra of 30 wt% PEN/70 wt% PP that was drawn to 400% strain at 140 C followed by annealing at 180 C. The spectra were taken such that the drawing direction was parallel and perpendicular to the incident beam polarization.

Fig. 16. Raman spectra of 30 wt% PEN/70 wt% PE that was drawn to 400% strain at 120 C followed by annealing at 130 C. The spectra were taken such that the drawing direction was parallel and perpendicular to the incident beam polarization.

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