Solid state extrusion of vinylidene fluoride (VF2)/vinylidene trifluoride (VF3) copolymers. II: Structure development

Solid State Extrusion of Vinylidene Fluoride (V F2) / V i n y I i d e n e Tr if I u or i d e (V F3)

Copolymers. I I : Structure Develop men t J. S. LEE and M. CAKMAK*

Institute of Polymer Engineering College of Polymer Science and Polymer Engineering

University of Akron Akron. Ohfo 44325-0301

Copolymers of vinylidene fluoride (W2) and vinyltrifluoride (VF3) exhibit Curie transition temperatures well below their melting points. Above these endothennlc transitions, they soften and this behavior helps in their solid state extrudability. In this paper, the effects of extrusion speed, temperature, and draw ratio on structure development in 60/40 and 72/28 VF2/W3 copolymer compositions are presented. With the increase of extrusion draw ratio the Curie transition temperature of the extrudates decreased and meltlng temperature increased. This behavior suggested that the chains in the crystalline regions contain higher levels of conformational defects while overall crystallinity is increased. Unoriented polymers were optically opaque and extrudates were found to be transparent as a result of breakdown on the superstructural level which decreases the scattering effects in the visible wavelength range. The micro beam WAXS studies on the samples taken from the entrance of the dies revealed that the unoriented core is surrounded by alternating unoriented and oriented layers close to the core. The remainder of the skin layers are found to be oriented with local symmetry axes and main chain orientation being parallel to the die wall surface. The regions that are found to be oriented were also found to be optically translucent and unoriented regions were optically opaque. This structure turns uniformly transparent-and thus oriented-as the polymer enters the die. The existence of layered structure suggests that highly localized yielding occurs during early states of deformation at the converging entrance region of the die. Examlnation of the radial structural variation in extrudates with micro beam X-ray diffraction technique revealed that the local symmetry axes are tilted away from the extrusion direction and this tilt angle reduces at the sample macro-symmetry axis at the core of the samples. This indicated that the tilted structure developed at the converging entrance region is partially preserved through the die. While the orientation of local symmetry axes varies from skin to core in the extrudates, the orientation of chains with respect to these local symmetry axes remains relatively unaffected.

INTRODUCTION

n recent years, a number of publications have I appeared on the solid state extrusion of polymers (1-9). In this process. the deformation of polymers takes place at temperatures well below their typical melt processing temperatures. For semicrystalline polymers these processing temperatures are situated between the crystalline relaxation and melting temperatures. Because of the low processing tem-

*To whom correspondence should be addressed.

perature, the extrudates exhibit increases in strength and modulus as well as optical clarity. The latter occurs due to reduction in the crystallite sizes to below the wavelength of visible light thereby reducing the scattering affect (10).

There have been variants of this process, most significant of which is split billet technique (1 1-13). In this process the polymer is coextruded with the longitudinally cut carrler polymer such as HDPE. This carrier polymer acts as a processing aid during the extrusion process. Poly(ethy1ene terephthal- ate) (PET) (11) and poly ethylene oxide (PEO)/poly

1570 POLYMER ENGINEERING AND SCIENCE, MID-DECEMBER 7993, Vol. 33, NO. 23

Solid State Extrusion of VF2/ VF3 Copolymers. I1

(methylmethacrylate) (PMMA) blends ( 12) have been successfully coextruded by inserting the melt cast film into cylindrical PE billets that had been split longitudinally. To obtain biaxial orientation in the processed parts, recently a novel solid state technique with hydrostatic pressure was also developed for poly propylene ( 14- 16). Aharoni and Sibilia ( 17) have reviewed the literature on the solid state extrudability of semi-crystalline polymers up to 1979.

A number of polymers have been successfully solid state extruded (17). these are PE (18-201, PP (18, 21, 22), PVF, (18). poly(4-methyl pentene-1) (P4MP 1) (23). poly (ethylenechlorotrifluoro-ethylene) (24), PTFE (2% nylon (6.a7.26) and POM (26) and polycarbonate (PCX27). According to Aharoni and Sibilia (171, the reduction in modulus above the crystalline relaxation temperature (some times called a, temperature) is believed to allow the solid state extrusion of semicrystalline polymers.

Weeks, et aL (9) stated that upon solid state extrusion a significant chain unfolding in the extrusion direction takes place. This resulted in the increases in long period observed by small angle X-ray scattering (SAXS) and increases in melting temperature. Desper, et aL (28) measured the orientation function relative to the filament axis and values as high as +0.996 f 0.002 was obtained indicating that virtually all the crystalline PE chain axes are aligned parallel to the extrusion direction.

Polyethylene (PE) is by far the most extensively studied material. The flow of partially crystalline PE was first observed by Porter and Johnson (29) in 1963. Thereafter, the extrusion of transparent and high modulus HDPE was initiated by Southern and Porter (30) and by Takayanagi, et aL (31) indepen- dently. Southern and Porter (30) crystallized commer- cial HDPE under combined effect of orientation and pressure in the capillary entrance region of an In- stron capillary rheometer at various temperatures and obtained strands with an extended chain structure and/or a chain folded structure with relatively long fold period deduced from increases in melting temperature. In their studies, the presence of row nucleated crystalline morphologies were suggested.

Keller and coworkers (32-37) also studied the extrusion behavior of PE in the temperature range of 140 to 145°C. From their results, they concluded that to obtain high modulus, complete chain extension need not be required and it can be generated by lamellar crystals provided 1) the lamellae are all parallel, 2) the plane normals are along the tensile direction, 3) the straight chain segments within the lamellae are also parallel to the tensile direction, hence perpendicular to the lamellar surfaces, 4) the lateral extension of the lamellae in all directions is large compared to the lamellar thickness, 5) there exist strong contact between consecutive lamellae piles, and 6) there is interlocking between lamellae in lateral contact. They analyzed the crystal orientation by WAXS, the super structure of the extrudate by SAXS, and surface of the extrudate by electron micro-

scope (1). It was found that the crystal c-axis orientation in extrudates was very high along the extrusion direction and the extrudate obtained with high degree of deformation was highly transparent and dimen- sionally stable at elevated temperature!. The long period was found to be in the 200 to 300A range and it increased with increasing extrusion temperature. The increase in density of the extrudate was attributed to the increase of crystallinity. They classifled the overall morphology as “interlocking shishke babs.” The extrudates of the higher extrusion ratio and those obtained at the higher extrusion temperatures showed higher melting temperature and heat of fusion. These effects were explained as the extrusion conducted at the high temperatures and pressure resulted in the superstructures in which the crystals were larger and ordered than the extrudates obtained at lower temperatures and pressures. The surface of extrudates becomes rough as the extrusion ratio increases, and the extrudates at extrusion ratio of 6.3 showed the shark skin-like surface. At higher extrusion ratio, cracks were also produced on the extrudate surface. Also the extrudates showed a slight tendency to swell and this increased with increasing extrusion rate. It was found that the effect of molecular weight on the extrusion pressure in solid state extrusion was small. When the polyethylene is blended with n-paraf€in, the extrusion pressure was observed to decrease as a result of the lubrication effect of the low molecular weight species.

Porter, et aL, have made extensive studies on the influence of extrusion temperature and extrusion ratio mechanical, and morphological properties of the resultant fibers (38, 39). Kojima, et aL (39, 40). determined the thermal properties, orientation and morphology, and tensile properties of solid state extruded HDPE. Higher molecular weight samples exhibited higher tensile strengths while modulus remained relatively unaffected by the changes in the molecular weight. Their results indicate that the tensile strength of fibers increases with increasing extrusion ratio and approaches a limiting value while Young’s modulus continuously increases with increasing extrusion ratio.

A continuous, steady state extrusion method has been developed for production of highly oriented HDPE with draw ratios over 10 (1 8). The influence of molecular weight of the starting PE has been studied by Perkins, et aL (41). Kolbeck and Uhlmann (42) carried out a study on the solid state extrusion of PP, PVF,, and HDPE. HDPE was extruded in continuous lengths with extrusion ratio of up to 25 below melting temperature, while continuous lengths of both PP and PVF, could not be obtained below their melting temperatures.

Keller and coworkers observed as early as 1974 that normally unextrudable, ultra high molecular weight PE either compression molded or solvent cast film drew smoothly in the molten state in the temperature range of 145 to 155°C well below the usual melt processing temperatures of 160°C and higher.

POLYMER ENGINEERING AND SCIENCE, MID-DECEMBER 1993, Vol. 33, NO. 23 1571

J. S . Lee and M . Cakmak

Recently they (43, 44) identified an unusual fea- ture of melt flow behavior in high molecular weight (between 4 x lo5 and lo6) PE and interpreted the observed behavior to a new mesophase of increased fluidity having hexagonal crystalline phase.

In this paper, we present our results on structure development in solid state processing of VF2/ VF, 72/28 and 60/40 mol% copolymers. These polymers exhibit a mechanical softening above their Curie transitions. This allows them to be solid state deformed well below their melting temperatures.

EXPERIMENTAL PROCEDURES

Materials

VF2/VF3 60/40 mol% were obtained from Atochem, Inc. of France and the VF2/VF3 72/28 mol% experimental grade copolymer was obtained from Solvay and Cie of Belgium. These are given in Table 1.

Solid State Extrusion Instron Capillary Rheometer Table Top model with

a 5000 lb, (22240 N) compression load cell was used to extrude the polymers. The barrel diameter was 0.95 em. Three different dies of fxed length and vary- ing diameter were used. The designs of the dies were presented in an earlier publication (45). The copolymers were melted at 220°C in the barrel. During the melting and cooling process the copolymers were maintained under pressure with a die plug and once the polymer is equilibrated at the desired processing temperature the plug was removed and extrusion started. The extrusion temperatures were varied from below the respected Curie transition temperatures of the copolymers to 5 or 10°C above their melting temperatures.

Differential Scanning Calorimetry (DSC)

After letting the extrudates equilibrate at room temperature for at least a week, Differential Scanning Calorimetry (DSC) was performed using Du Pont 9900 Thermal Analyzer. Before each DSC scan, the cell was calibrated with 10 mg of Indium at the same heating rate as the actual scans. Unless otherwise specifled all the scans were performed at lO"C/min. Appropri- ate corrections were applied to the data to obtain the heats of transitions.

Microbeam WAXS The radial variation of structure in selected extru-

dates as well as in the sample removed from the entrance region of the capillary was studied with a

matrixing microbeam WAXS (MMBX) camera built in our laboratories. With the built-in precision sample X-Y translation stage, WAXS patterns at various locations of the sample can be obtained without d i s mounting. This enables one to study the structural gradient development with high precision. This camera was mounted on a 12 KW Rigaku rotating anode generator and Nickel foil filter was used to obtain CuKa radiation. The beam size of 100 pm was used to take WAXS patterns at various locations from the surface to the core of the specimen.

With Rigaku horizontal difractometer also, the equatorial scans of the extrudates were obtained in reflection mode. The X-rays were monochromatized to CuKa radiation with the help of the graphite monochromator.

Small Angle X-ray Scattering (SAXS)

The experiments on the extrudates obtained under a variety of conditions were performed using the 10 meter SAXS facility at the National Center for Small Angle Scattering Research (NCSASR) at the Oak Ridge National Laboratory (46). The camera consists of a R i g a Rotaflex RU-200 12 KW rotating X-ray source, pinhole collimation, a two dimensional position sen- sitive proportional counter, and a mini-computer system for data acquisition and analysis. All measurements were performed at room temperature with Ni filtered CuK, X-ray radiation ( A = 0.154 nm) being operated 40 KV and 100 mA with sample-to-detector distance of 2.1 m, giving maximum scattering angles of 20 = 44.44 mrad horizontally and vertically. Scattering patterns are presented in the form of a contour plot with scattering angle 2 0 = 0 mrad at the center. The contour levels are increased by a factor of 2".

RESULTS AND DISCUSSION

Thermal Properties and Density

DSC of Solid State Esmudates

Figures 1 and 2 shows the DSC traces of VF2/ VF, 72/28 and 60/40 mol% solid state extruded at different temperatures at the cross head speed of 0.381 mm/min with the capillary diameter of 1.91 mm. The DSC trace of a compression molded sheet is included as a reference.

In can be seen from the DSC scans that the melting temperatures of the solid state extrudates are higher than those of the compression molded sheets. As the extrusion temperature approaches the melting temperature, the melting temperature of the produced extrudates approaches the melting temperature of

Table 1. Specification of Copolymers.

VF, mol% M G P C MGPC Density Resins Supplier (%I ( ~ / m o ~ ~ x i o ~ ) (g /mob x 105) (g/cc)

VF, /VF, Solvay and Cie. 72 1.44 4.38 1.895 Copolymer Elf Atochem 60 2.18 6.8 1.892


Solid State Extrusion of VF2/ VF3 Copolymers. IZ

155

150

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Rig. 1 . The DSC traces of VF2/ W3 7 2 / 2 8 ml% solid state extruded at different temperatures at the cross head speed of 0.381 mm/ min with the capillary diameter of 1.91 nun.

II 211 40 60 00 160 120 1 4 n i i u 100 r L

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Flg. 2. The DSC traces of W2/ W3 60/ 40 ml% solid state extruded at different temperatures at the cross head speed of 0.381 mm/ min with the capillary diameter of 1.91 nun

compression molded sheet, as shown more clearly in Figs. 3 and 4. As observed in the DSC scans, all solid state extruded copolymers exhibit broad melting peaks as compared to that of the compression molded sample. This may be a result of increased crystallite size distribution in the extrudates. Southern and Porter (30) also observed melting temperature increases in the solid state extruded HDPE and attributed such observation to the formation of extended chain structure. Weeks, et aL (9). attributed the increase in the melting temperature of solid state extrudates to significant chain unfolding in the extrusion direction accompanied by increased crystallite sizes. Aharoni and Sibilia (10) attributed this increase in melting temperature to the increased de- grees of crystalline perfection. Most probably, the latter two mechanisms are occurring simultaneously in the actual process.

In our study, the higher melting temperatures observed in the extrudates appears to be due to crys-

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145' I 110 120 130 140 150 160

1.91 mm DIE 150

145 110 120 130 140 150 160

160 I

145 110 120 130 140 150 160

EXTRUSION TEMPERATURE ('C)

Rig. 3. Melting temperatures at various solid state extrusion temperatures for W2/ W3 72/ 28 ml%: (a) 1.27 nun diameter capillary, ( b ) 1.91 mm diameter capillary, and (c ) 3.18 mm diameter capdhy.

VF2NF3 60/40 MOL% I I I I I

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110 120 130 140 150 160

............................... 150 1.91 mm DIE

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110 120 130 140 150 160 EXTRUSION TEMPERATURE ('C)

Flg. 4. Melting temperatures at various solid state extrusion temperatures for W2/W3 60/40 ml%: (a) 1.27 nun diameter capillary, ( b ) 1.91 mm diameter capillmy, and ( c ) 3.18 mm diameter capillary.

POLYMER ENGINEERING AND SCIENCE, MID-DECEMBER 1993, YO/. 33, NO. 23 1573

J. S . Lee and M . Cakmak

tallization under high pressure before the actual extension. Figure 5 shows the DSC traces of the samples that were crystallized under pressure follow- ing the same procedures as the actual extrusion and removed from the barrel of the Instron machine without being extruded. All samples crystallized under pressure exhibit higher melting points as compared to that of the compression molded sheet. This is in accord with the results of Bhateja and Pae (47) who performed similar experiments.

Heat of Fusion Figures 6 and 7 show the observed DSC heat of

fusion for solid state extruded VF2/VF3 72/28 and 60/40 mol% copolymers. Although there is a fair amount of scatter in the data, one can still deduce that the solid state extrudates have higher crystallinities than the compression molded sheets and as the extrusion temperature increases crystallinity decreases. As the extrusion temperature approaches the melting temperature, crystallinity of the extrudates approaches that of compression molded sheet. The solid state extruded VF2/VF3 72/28 and 60/ 40mol% have crystallinities in the range of 70% (compared to 60% for the compression molded sheet) and 45% (compared to 33% for the compression molded sheet), respectively, based on the heat of fusion data. The heat of fusion values of the crystalline regions of these copolymers were determined and to be published elsewhere (48).

Density of Solid State Exbudates Results of the density measurements for solid state

extrudates of VF2/VF3 72/28 and VF2/VF3 60/ 40 mol% with 1.9 1 mm capillary are shown in Figs. 8 and 9.

The heat of fusion results correlate quite well with density measurements where the solid state extrudates exhibit higher densities than compression molded sheets. Imada, et aL (11, have also observed the increase of density in solid state extrusion.

CRYSTALLIZED UNDER

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k 20 - 3.81 mwmm w I

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Q. 6. The observed DSC heat of fusion for solid state extruded VF''/W3 72/28 ml%: (a) 1.27 mm diameter capillary, ( b ) 1.91 mm diameter capillary, and ( c ) 3.18 mm diameter capillary.

VF2NF3 60/40 MOL% 35 I I e

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r-------- . . . . . . . . - 7 - I . 1 EXTRUSION TEMPERATURE ('C) 20 4 0 60 80 100 I20 I40 160 100 200

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Fig. 5. Comparisons of DSC traces of compression molded and pressure crystallized VFz / VF3 60/40 mol% copolymers at various temperatures. diameter capiUary.

Fig. 7. The observed DSC heat of fusion for solid state extruded 60/40 mol% copolymers: (a) 1.27 mm diameter capillary, ( b ) 1 . 9 1 mm diameter capillary, and (c) 3.18 mm

1574 POLYMER ENGINEERING AND SCIENCE, MID-DECEMBER 1993, Vol. 33, No. 23

Solid State Extrusion of VF2/ VF3 Copolymers. 11

DENSITY SOLID STATE EXTRUSION 1.91 mm DIE

VF2NF3 72/28 MOL% 2.00 I , 1 ,

DENSITY SOLID STATE EXTRUSION 1.91 mm DIE

VF2NF3 72/28 MOL%

r 1.27 mmlmin 8 3.81 mmlmin

0 0.381 mrmrnin r 1.27 mmlmin 8 3.81 mmlmin

1.80 ' I 120 130 140 150 160

TEMPERATURE ('C)

Rg. 8. 'Ik densities of solid state extrudates with 1.91 nun caprUary for W2/ VF' 72/28 mi% at dizerent extrusion tern peratures measured by flotation method.

DENSITY SOLID STATE DCTRUSION 1.91 mm DIE

VF2NF3 W/40 MOL%

0 0.381 mmlmin r 1.27 mmlmin

3.81 mmlmin

1.80 L I 110 120 130 140 150 160

TEMPERATURE ('C)

Rg. 9. The densities of solid state extwdates with 1.91 mm capiUary for W2/ W3 60/40 mi% at dig'erent extrusion tern peratures measured by flotation method.

Curie ?)-ansition Temperature The Curie transition behavior seems to be mainly

influenced by the actual solid state extrusion process conditions as comparisons between Figs. 2 and 5 indicates. Pressure crystallization in VFz/VF, 60/ 40 mol% results in different amounts of ordered and disordered phases depending on the crystallization temperatures as shown in Fig. 5. Solid state extrudates show not only similar Curie transition peak shape but also consistent trend with the change in the solid state extrusion temperature. First, peak broadening is apparent for the low temperature extrudates and second, at low extrusion temperatures, Curie transition temperatures are below that of the compression molded sheet. As the extrusion temperature is increased, a distinct secondary peak begins to appear around 75 to 80°C. The lower temperature peak also increase in magnitude and shifts slightly to higher temperature as the extrusion temperature is increased as shown in Fig. 10. bvinger, et Q1. (491, investigated the crystalline forms of VF2/ VF, 52/48 copolymer and concluded that melt solidi-

110 120 130 140 150 160

60 ' , J 110 120 130 140 150 160

T I 0 1;o lA0 1;o I;? lA0 EXTRUSION TEMPERATURE ( C)

Rg. 10. The Cwie transition temperatures at various solid state extrusion ternperatzres for W2/W3 60/40 mi%: (a) 1.27 nun diameter capillary, (b) 1 . 9 1 nun diameter caplllary, and (c) 3.18 nun diameter capillary.

fled samples consisted of a mixture of two disordered phases, one trans planar and the other 3, helical. The all trans phase could be obtained by drawing which causes the transformation of the disordered mixture of phases into a well-ordered planar zigzag phase. The trans planar zigzag phase is reported to go through the Curie transition at higher temperatures. Thus solid state extrusion of VF2/VF3 60/40 mol% at low extrusion temperature leads to broad en- dothermic Curie transition peaks with peak maximum located at low temperature, indicating the extrudates consist of crystalline phase with mostly disordered 3, helical, kink bonds or TG and chain conformations. As extrusion temperature increases, this causes the appearance of secondary higher Curie transition peak. It appears that when sufficient thermal energy is provided the packing of the chains in the crystalline regions improve (or less crystalline distortions are introduced when the processing temperature is increased) and as a result the higher temperature peak of the Curie transition is due to all trans. The entrance region of the capillary used in this study has a tapered converging design. This imparts a deformation field that is a combination of elongation and shear to the polymer. This deformation apparently converts the conformation of chains to trans zigzag conformation that exhibits higher Curie transition temperature. Solid state extrusion at low temperatures tends to induce 3, helical or TG and T 6 conformation possibly due to shear field that

POLYMER ENGlNEERlNG AND SCIENCE, MID-DECEMBER 1993, YO/. 33, NO. 23 1575

J. S. Lee and M. CaJcmak

8

tends to introduce defect structure. Increase in the lower Curie transition peak position with increasing extrusion temperature is probably due to the decrease of disordered regions thus inducing more ordered 3, helical, kink bonds or TG and TG conformation and to the increase of all trans chain conformations. Lovinger, et al (50) among others concluded that for W2/VF3 copolymers with 65/35, 73/27, and 78/ 22 mol%, the structures at low temperature are well ordered and analogous to that of /3 crystalline phase of FVF2 i.e. trans planar. However, the Curie transition of low temperature solid state extrudates go through a broad Curie transition well below that of the compression molded sheets. This is again due to prevalent 3 helical or analogous disordered trans planar conformations in the crystalline region. As the extrusion temperature increases the deformation field induce all trans conformations resulting in higher Curie transition temperature. Increase in the Curie transition temperatures with increase in the extrusion temperatures, shown in Fig. 1 1, is due not only to higher order in the disordered chains in the crystalline region at the higher extrusion temperatures but also to increase of chains having trans planar conformations induced at higher extrusion temperatures.

Heat of Curie Transition Figures 12 and 13 show the heats of Curie transi-

tions us. the extrusion temperatures for VF2/VF3

VF2NF3 72/28 MOL% D L : 120 2 1

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110 120 130 140 150 160 0 L l#120, I -

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Rg. 1 1 . The Curie transition temperatures at various solid state extrusion temperatures for VF2/VF3 7 2 / 2 8 ml%: (a) 1.27 nun diameter capillary, (b ) 1 . 9 1 nun diameter capillary, and ( c ) 3.18 nun diameter capillary.

VF2NF3 72/28 MOL% 1 I 1

ir; L I .., 110 120 130 140 150 160

1

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Fig. 12. Heats of Curie transition for VF2/ W3 7 2 / 2 8 mol% at various extrusion temperatures: (a) 1.27 mm diameter capillary, ( b ) 1 .91 nun diameter capillary, and ( c ) 3.18 nun diarneter capillary.

VF2NF3 60/40 MOL%

12

7 ' I 110 120 130 140 150 160

14

11 1.91 rnm DIE

9 8 7 110 120 130 140 150 160

1 8 1 3.18 mm DIE

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Fig. 13. Heats of Curie transition for VF2/ W3 60/40 ml% at various extrusion temperatures: (a) 1.27 nun diameter capillary, ( b ) 1 .91 nun diameter capillary, and ( c ) 3.18 nun diameter capillary.


Solid State Extrusion of VF2/ W3 Copolymers. Zl

72/28 and 60/40 mol%, respectively. Due to dWicul- ties in baseline determination and superimposition of melting region with the Curie transiting in W2p3 72/28 mol%, scatter of data is large. However, the trend is evident that, for the smaller diameter capillaries where large deformation occurs, the heats of Curie transition for the solid state extrudates are lower than that of the compression molded sheets while they are about the same as that of the compression molded sheets for the 3.18 mm diameter capillaries. This is probably due to higher disorder introduced for the smaller diameter capillaries where the polymer experienced larger deformations during solid state extrusion.

Effects of Extrusion Ratios

The effects of the extrusion ratios are shown in Figs. 14 to 18. The most striking effect of the extrusion ratio is observed in the melting temperatures of the solid state extrudates of both compositions and in the Curie transition temperatures of the W 2 p 3 72/28 mol% solid state extrudates. Looking at Figs. 14 and 15, the extrusion ratio of 9 (3.18 mm diameter capillary) shows the lowest melting temperatures with the same composition and same extrusion rate, while the extrusion ratios of 25 (1.91 mm diameter capillary) and 56.25 (1.27 mm diameter capillary) show similar values. Nakamura, et aL (21). reported that the isotactic PP solid state extrudates of the higher extrusion ratio showed higher melting tem-

VF2NF3 72/28 MOL% 1601 1 I

CROSSHEAD SPEED 0.381 mrdmin 0 1.27mmDIE a 1.91 mm DIE V 3 . 1 8 m D I E

140 110 120 130 140 150 160

CROSSHEAD SPEED 0.381 mrdmin 0 1.27mmDIE a 1.91 mm DIE V 3 . 1 8 m D I E

140 110 120 130 140 150 160

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CROSSHEAD SPEED 1.27 mrdmin 145

140 110 120 130 140 150 160

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i3g. 14. The melting temperatures at various solid state extrusion temperatures for VF2/ VF3 72/28 mol%: (a) 0.381 mm/min cross head speed (b ) 1.27 mm/min cross head speed and (c ) 3.18 mm/ min cross head speed.

VF2NF3 60/40 MOL% p 1651 I - I

2 150 t 1 145

- 165 e

I 100 110 120 130 140 150 160

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100 110 120 130 140 150 160 EXTRUSION TEMPERATURE ('C)

Fig. 15. The melting temperatures at various solid state extrusion temperatures for VF2/VF3 60/40 mol%: (a) 0.381 mm/min cross head speed, ( b ) 1.27 mm/min cross head speed and (c ) 3.18 mm/ min cross head speed.

E VF2/VF3 72/28 MOL%

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110 120 130 140 150 160 EXTRUSION TEMPERATURE CC)

Fig. 16. lXe Curie transition temperatures at various solid state extrusion temperatures for VF2 / W3 72/28 mob: (a) 0.381 mm/min cross head speed, (b ) 1.27 mm/min cross head speed, and ( c) 3.18 mm/ min cross head speed.

POLYMER ENGINEERING AND SCIENCE, MID-DECEMBER 1993, Vol. 33, NO. 23 1577

J. S. Lee and M. CaJcrnak

VF2NF3 60/40 MOL%

75 0

CROSSHEAD SPEED 0.381 mmlmin 0 1.27mm DIE e 1.91 mmDIE

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100 120 140

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50 ' 100 120 140

CROSSHEAD SPEED 3.81 mmlmin 6o t 55 t i 50 ' 1

100 120 140 160 EXTRUSION TEMPERATURE ('C)

Fig. 17. The Curie transition temperatures at various solid state extrusion temperatures for VF2/ VF, 60/40 mol%: (a) 0.381 mm/ rnin cross head speed, ( b ) 1.27 mm/ rnin cross head speed, and ( c ) 3.18 mm/ rnin cross head speed.

perature. The same behavior is observed for the copolymers.

The Curie transition temperatures for the VF2/VF3 72/28 mol%, on the other hand, shows the opposite behavior, as shown in Fig. 16. The extrusion ratio of 56.25 shows the lowest Curie transition temperatures. This behavior is expected from the DSC results of the slowly cooled sheets, where the higher melting temperatures of the slowly cooled sheets are accompanied by the lower Curie transition temperatures. The lower Curie transition temperature is due to the higher defect content and with the higher defect content the lateral crystallite size increases. In the pseudohexagonal unit cell, the interchain packing improves (51) due to the decrease of anisotropy. Improved interchain packing is expected to yield higher melting temperatures which may also be due to lamellae thickening. Such behavior is clearer in the VF2/VF, 72/28 than 60/40 mol%, since the compression molded 72/28 mol% sheets have trans planar chain conformation in the crystals, while the compression molded 60/40 mol% sheets have already 3, helical, kink bonds or TG and TG conformations in the crystals. This is what is being observed (9).

DSC heat of fusion and heat of Curie transition results do not seem to depend on the extrusion ratios. On the other hand, Nakamura, et a1 (21). reported that the isotactic PP extrudates of higher extrusion ratio showed the larger heat of fusion.

(bl Fig. 18. The WAXS film patterns of the VF2/vF, 60/ 40 mol%: (a) extruded at 155°C with 1.91 mm diameter capillary at the cross head speed of 3.81 mm/min (melt extrusion), and ( b ) extruded at 1 15°C with 1.91 mm diameter capillary at the cross head speed of 3.81 mm/min (solid state extrusion).

WAXS F'%lm Patterns Figure I8 shows the WAXS film patterns of the melt

extruded and the solid state extruded samples. Extrusion direction is vertical and the incident X-ray is perpendicular to the samples.

Orientation is evident in the solid state extruded sample. Faint (001) reflections are located in the meridian while ( 1 10x200) combination reflections are concentrated in the equator. There is a fair amount of azimuthal spread of intensity that indicates low degree of crystalline orientation. In addition, the peaks are fairly broad indicating the presence of imperfections in the crystalline lattice and/or smaller crystallite sizes. To q u a n t a this, we obtained WAXS scans from the 72/28 composition processed at different temperatures with two different extrusion speeds and determined the width at half maximum (WHM) of the (1 10X200) combination peaks. This is shown in Fig. 19. At low temperatures the peaks are rather broad and as the processing temperature is increased the WHM values decrease. In addition, samples extruded at higher speeds exhibit larger WHM values indicating the introduction of higher distortions and/or lower crystallite sizes with higher extrusion speeds.

1578 POLYMER ENGINEERING AND SCIENCE, MID-DECEMBER 7993, Yo/. 33, No. 23

Solid State Extrusion of VF2/ VF3 Copolymers. I1

Analysis of Capillary Entrance Region To identlfy the basic deformation mechanism

through the die, we stopped the extrusion of a sample with 60/40 VF2/VF3 composition and removed the piece at the die entrance region and sliced a segment through the symmetry axis using a diamond saw. This sample is shown in Fig. 20. The transmission optical micrograph of the sectioned sample exhibits opaque core region which gradually reduces in width

1.2

1.1

iE : 1.0 t 3 0

0.9

5

s $ 0.8 1

0.7

1.27mm DIE

0 1.27mWmin

0 3.8lmWmin

\ \ \ ‘0

0.6

TEMPERATURE(C)

Fig. 19. Width at half maximum for ( 1 lOX200) peaks as a function of extrusion speed and extrusion temperature.

as the material enters the die. In addition alternating light and opaque regions surround this opaque core. To investigate the structure of these regions we have taken a series of WAXS patterns using a micro camera and the positions where these WAXS patterns were taken are indicated by ink dots on the photomi- crographs. These are numbered 1 thru 19 from right to left surface of the specimen. Near the skin, the local symmetry axis is tilted to the right and high orientation is observed. The WAXS patterns taken at the opaque regions show no preferential orientation indicating they are composed of undeformed original material. The alternating nature of these opaque unoriented and light oriented regions suggests local yielding has occurred. It should be noted that the local symmetry axes near the surfaces tilt to follow surface orientation of the converging entrance region of the die.

Figure 21 shows the micro WAXS patterns taken on slices cut through the axis of the specimens of 72/28. In the sample extruded with smaller die, the structure gradient is clearly visible in these patterns. The local symmetry axes near the skin are tilted away from the axial direction and the angle between the local symmetry axis (the symmetry axis shown in X-ray pattern) and the extrudate axis gradually disappears towards the center of the sample. The level of chain orientation with respect to these local symmetry axes does not change appreciably. The presence of this structural gradient in the extrudate indicates that structural gradient developed at

Flg. 20. Optical transmission photomicrograph on a 60/ 40 sample cut outfrom the capillary entrance region through the symmetry plane and the micro WAXS patterns on indicated locations (# 1 on the right surface).

POLYMER ENGINEERING AND SCIENCE, MID-DECEMBER 1993, YO/. 33, NO. 23 1579

J. S. Lee and M. CQkmak

EXTRUSKN OIRECTKN

EDGE 1 oopm 200pm

700pm (center) 600pm 400pm

EXTRUSION DIRECTION

$ RADIAL DIRECTION

600um

200pm

800pm

400pm

1000pm

- 0 -

' . d , I .

ENTER I : I , EDGE

EXTRUSION DIRECTION

4 RADIAL DIRECTION

Flg. 21. Micro WAXS pattern taken on 72/28 sample extruded with (a) 1.27 mm die ( b ) 3.18 mm die.

1580 POLYMER ENGINEERING AND SCIENCE, MID-DECEMBER 1993, Yo/. 33, NO. 23

Solid State Extrusion of VF2/ V F 3 Copolymers. II

the entrance of the converging die is partly preserved in the extrudate. Figure 21b shows the micro WAXS patterns taken on a sample extruded with larger die and the structural gradient is clearly observable in these patterns. The chains near the skin are tilted away from the axis of the extrudate and they become parallel to the axis of the extrudate at the center of the cylindrical sample.

Small Angle X-ray Scattering (SAXS)

SAXS iso-intensity contour plots of solid state extruded VF,/vF, 60/40 mol% with 1.91 mm diameter capillary at two extrusion temperatures are presented in Fig. 22. Extrusion directions were vertical and the incident X-ray beam was perpendicular to the sample. Figure 23 shows meridional the intensity profiles (vertical intensity variations given in Fig. 22). The intensity from the compression molded sheet is included as the reference.

From 9. 22, the 19ng periods are calculated to be - 4 1 OA and - 230A for the compression molded sheet and for the solid state extruded sample, respectively. We also observed similar behavior in samples extruded with different extrusion temperatures. This behavior is opposite of the behavior observed in other polymers where the researchers have observed a steady increase of long periods with solid state extrusion. They attributed this to the breakup of the cry- stallites and unfolding thereby causing the formation of extended chain structure. Here on the other hand we observe smaller long periods in the extrudates. There may be a number of reasons for this behavior. One possibility is the introduction of crystalline chain defects under the deformation field. Another possibil-

ity is break up of the crystalline lamellae during solid state extrusion.

Structural Development in Solid State Extrusion

On the basis of the experimental results, structural developments in solid state extruded copolymers are presented schematically in Fig. 24. The rod-like crys- tallites orient along the extrusion direction with the crystalline chain axis aligning roughly parallel to the extrusion direction with some populations tilted away from this direction and the Crystalline lamellae are stacked along the extrusion direction as evi- denced by SAXS. Within the crystalline region the chains contain large amount of defects.

SOLID STATE EXTRUSION VF2NF3 60/40 MOL%

QlWC

o '0.381 mmlmin a : 1.27 mmlmik v :3.81 mm/min

o_ v I 12.7 mmlmib > 1000

r

W I-

4 100 -

W 1 0 - z 5

1 - W U

0.1 -

0.01 -0.2 -0.1 0.0 0.1 0.2

KAPPA

Fig. 23. The intensity us. scattering vector, kappa plot for the solid state extruded VF2/ VF3 60/ 40 mol% at 140°C at various cross head speeds.

1 2O0C CHS1.27 rnrnhin CHS3.81 rnrnlrnin CHS12.7 rnrnlrnin

140°C CHS1.27 rndrnin CHS3.81 rnrnlrnin CHS=12.7 rnrnlrnin

I! 1- Fig. 22. The SAXS iso-intensity contour plots of solid state extruded VF2/ VF3 mol% with 1.91 nun diameter capiuary at two extrusion temperatures.

Fig. 24. Idealized model structure in solid state extruded VF2/ VF3 copolymers.

POLYMER ENGINEERING AND SCIENCE, MID-DECEMBER 1993, Vol. 33, No. 23 1581

J. S. Lee and M. Cakmak

CONCLUSIONS

Solid state extrudates are translucent and possess higher densities, higher melting temperatures, and lower Curie transition temperatures than the compression molded sheets. Lower Curie transition temperatures in the solid state extrudates were related to the disordered crystalline chain conformations that arise because of the severe deformations introduced during the solid state extrusion. Solid state extrudates have higher crystallinities and lower heat of Curie transition than the compression molded sheets. Higher extrusion ratios yielded extrudates with higher melting temperatures, but with lower Curie transition temperatures. These observations are consistent with the incorporation of higher levels of defect in the crystalline regions at higher extrusion ratios.

WAXS analysis of the capillary entrance region indicates that core region remains unoriented near the entrance and is surrounded by alternating unoriented and oriented layers. Near the surface the chains are highly oriented and the local symmetry axis fol- lows the geometry of the entrance wall. In the solid state extrudates, the crystalline chain axis oriented roughly along the extrusion direction with some population of chains slightly tilted away from the extrusion direction. And the crystalline lamellae are stacked parallel to the extrusion direction.

Finally, a composite idealized model describing the structural development in solid state extruded VF2/VF3 copolymers is presented.

ACKNOWLEDGMENTS

This research was partially funded by the National Science Foundation (Grant #DMC-8858303). We would like to thank Mr. S. Mahan t for taking some of the micro WAXS patterns.

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