Abstract:
An extensional rheometer comprises a rotatable primary wind up drum and one or more secondary rotatable windup drums wherein a sample is attached to the primary windup drum and each secondary windup drum. Counter rotation of the primary windup drum and each secondary drum causes each affixed sample to stretch until rupture. The load response on each primary and secondary windup drum set caused by a stretching sample is measured with a load sensing device. Environmental control may be provided for testing samples under different conditions.

Description:
RELATED APPLICATIONS 
     This application relates to U.S. application Ser. No. 09/849,934 entitled Dual Windup Extensional Rheometer by Martin Sentmanat and having a common assignee with the present invention. 
    
    
     TECHNICAL FIELD 
     The invention relates to a rheometer or rheometer attachment which is used to measure the viscosity and stress relaxation of polymers, elastomers, and rubber compounds in simple extension. More specifically, the present invention relates to the utilization of a dual windup drum method to characterize the extensional flow behavior of one or more material samples simultaneously. 
     BACKGROUND ART 
     Joachim Meissner, in the review article “Polymer Melt Elongation-Methods, Results, and Recent Developments” in Polymer Engineering and Science, April 1987, Vol. 27, No. 8, pp. 537-546 describes different extensional rheometers that have been developed in the prior art. Meissner is also the author of several patents on the subject including U.S. Pat. No. 3,640,127, dated Feb. 8, 1972, German 2,138,504, dated Aug. 2. 1971, German 2,243,816, dated Sep. 7, 1972 and U.K. 1,287,367. 
     Extensional rheometer designs by Cogswell, Vinogradov, and later Muinstedt had in common that one end of the polymer fiber or filament that was used for testing was fixed to a load cell/indicator, while the other end was stretched by mechanical means to a finite maximum elongation. Accordingly, these rheometers operated with a non-uniform extensional rate throughout the sample particularly near the clamped ends of the fiber. Meissner overcame these difficulties with his dual rotary clamp design in which rotary clamps stretched the fiber at either end over a fixed gauged length. See, for example, “Rotary Clamp in Uniaxial and Biaxial Extensional Rheometry of Polymer Melts” by J Meissner, et al., Journal of Rheology, Vol. 25, pp. 1-28 (1981) and “Development of a Universal Extensional Rheometer for the Uniaxial Extension of Polymer Melts”, by J Meissner, Transactions of the Society of Rheology, Vol. 16, No. 3, pp. 405-420 (1972). In a further development of this type of rheometer, in order to improve the transfer of the circumferential speed of the clamps to the local speed of the sample at the location of clamping (strain rate lag), two rotary clamps in the prior art devices were replaced by Meissner and Hostettler as illustrated in “A New Elongational Rheometer for Polymer Melts and other Highly Viscoelastic Liquids”, Rheological Acta, Vol. 33, pp. 1-21 (1994) with matched/grooved, metal conveyor belts. With this design, however, a measurement was limited to a single rotation of the clamps corresponding to a Hencky strain of seven, and the maximum extensional rate was limited to 1/s (a reciprocal second). The extensional viscosity was determined from the force required to deform the fiber, which was measured by the deflection of leaf springs supporting one set of rotating clamps. However, as has been reported in the literature by Erik Wassler in “Determination of true extensional viscosities with a Meissner-type rheometer (RME)”, Proceedings of the 15 th  Annual Meeting of the Polymer Processing Society, Paper 200 (1999), there can be large deviations between the nominal and the true extensional strain with this type of extensional rheometer due to sample slippage between the rotating clamps. 
     Other techniques used to measure extensional viscosity involved winding one end of a fiber around a drum and measuring the resultant stretching force at the other fixed end of the fiber, as described in an article by R. W. Connelly, et al., “Local Stretch History of a Fixed-End-Constant-Length-Polymer-Melt Stretching Experiment,” J. Rheol., Vol. 23, pp. 651-662 (1979). Like the earlier designs, this method imparts a non-uniform extensional deformation to the free gauge length of the stretched fiber, particularly at the fixed end of the fiber that can lead to a false material rupture condition during extension. 
     There remains a need to measure extensional viscosity and stress relaxation of one or multiple polymers, elastomers, and rubber compounds in uniaxial extension simultaneously. Steps to overcome the latter limitations were disclosed in PCT Publication No. WO00/28321 entitled Dual Windup Extensional Rheometer by Martin Sentmanat and having a common assignee with the present invention. Setting out to improve upon the shortcomings of sample slippage and the non-uniform deformations encountered with other extensional rheometer designs, Sentmanat in PCT Publication No. WO00/28321 described an apparatus in which both ends of a material sample are wound around a set of mechanically coupled counter-rotating drums housed in a torque armature. Upon stretching the sample, the extensional resistance of the material sample hinders drum rotation, and the extensional flow behavior of the sample material can be characterized by monitoring the torque on the torque armature required to rotate the windup drums at a fixed rate of rotation. Like the earlier designs, the rheometer described in WO00/28321 is only capable of assessing a single sample material at a time. In addition, because the master and slave drums of the device described in WO00/28321 are both mounted on bearings within the torque armature, friction from the bearings due to the rotation of the master and slave drums contribute to the measured signal during an experiment. 
     There remains a need to provide a rheometer that can measure a plurality of samples at one time and measures the samples with torque signals that do not include friction from bearings supporting the drums. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, there is disclosed an extensional rheometer apparatus for measuring the extensional flow response of samples of material, such as a low modulus solid sample. The rheometer comprises a primary windup drum mounted to a power drive device for rotating the primary windup drum; a secondary windup drum rotatably mounted in proximity to the primary windup drum; means interconnecting the primary windup drum to the secondary windup drum whereby rotation of the primary windup drum by the power drive device causes the rotation of the secondary windup drum; and a load sensing device for measuring the response of the extensional flow of a low modulus solid sample secured to the primary windup drum and the secondary windup drum. 
     Further, according to the present invention, the primary and secondary windup drums are preferably in substantially parallel alignment. Further, the means for interconnecting the primary and secondary windup drums are first and second gears individually attached to the primary and secondary windup drums and intermeshed such that the primary and secondary windup drums are counter rotating and cause the primary and secondary windup drums to rotate at the same speed. 
     Also, according to the present invention, the load sensing device is attached to the secondary windup drum for supporting the secondary windup drum. In an alternative embodiment, the load sensing means is attached to the primary windup drum driving means. 
     According to another embodiment of the present invention, an extensional rheometer apparatus for measuring the extensional flow response of samples of material, such as a plurality of low modulus solid samples, comprises a primary windup drum mounted to a power drive device for rotating the primary windup drum; a plurality of secondary windup drums rotatably mounted in proximity to the primary windup drum; means interconnecting the primary windup drum to the plurality of secondary windup drums whereby rotation of the primary windup drum by the power drive device causes the rotation of the secondary windup drums; and a load sensing device attached to each of the secondary windup drums for supporting each of the secondary windup drums and measuring the response to the extensional flow of low modulus solid samples secured to the primary windup drum and each of the plurality of secondary windup drums. 
     Further, according to the latter embodiment of the present invention, the primary and plurality of secondary windup drums are in substantially parallel alignment. Also, the means for interconnecting the primary and plurality of secondary windup drums are first and second gears individually attached to the primary and plurality of secondary windup drums and intermeshed such that the primary and plurality of secondary windup drums are counter rotating and rotate at the same speed. 
     According to another embodiment of the present invention, an extensional rheometer apparatus for measuring the extensional flow response of samples of material, such as a low modulus solid sample, comprises a primary windup drum mounted to a primary power drive device for rotating the primary windup drum; a secondary windup drum rotatably mounted to a secondary power drive device for rotating the secondary windup drum in proximity to the primary windup drum; and a load sensing device attached to the secondary power drive device for supporting the secondary windup drum and measuring the extensional flow response of a low modulus solid sample secured to the primary windup drum and the secondary windup drum. The load sensing device is secured at one end to a support member and at the other end to the secondary power drive device. 
     According to yet another embodiment of the present invention, an extensional rheometer apparatus for measuring the extensional flow response of samples of material, such as a of low modulus solid samples comprises a primary windup drum mounted to a primary power drive device for rotating the primary windup drum; a plurality of secondary windup drums rotatably mounted to a plurality of secondary power drive devices for individually rotating the secondary windup drums in proximity to the primary windup drum; and a plurality of load sensing devices, each attached to one of the plurality of secondary power drive devices for supporting the plurality of secondary windup drums and measuring the extensional flow response of low modulus solid samples secured to the primary windup drum and each of the secondary windup drums. Each of the plurality of load sensing devices are secured at one end to a support member and at the other end to one of the plurality of secondary power drive devices. 
     According to yet another embodiment of the present invention, a method for measuring the extensional flow response of a material, such as a low modulus solid sample comprising the steps of: rotating a primary windup drum with a power drive device; rotating a secondary windup drum in proximity to the primary windup drum; and supporting the secondary windup drum and measuring the extensional flow response of a low modulus solid sample secured to the primary windup drum and the secondary windup drum with a load sensing device. The method includes the steps of rotating a plurality of secondary windup drums in proximity to the primary windup drum; and supporting the plurality of secondary windup drums and measuring the extensional flow response of a plurality of low modulus solid samples secured to the primary windup drum and the plurality of secondary windup drums with a plurality of load sensing devices each attached to the plurality of secondary windup drums. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates a perspective view of a first embodiment of the rheometer apparatus of the present invention including a primary windup drum in parallel relation to a secondary windup drum illustrated as mounted in a cutaway section of a mounting bracket; 
     FIG. 2 illustrates a perspective view of the secondary windup drum shown in FIG. 1 secured in a mounting bracket attached to a vertical support frame via a load sensing device; 
     FIG. 3 is a cross sectional view through line  3 — 3  of FIG. 2 showing the secondary windup drum in a mounting bracket attached to a vertical support frame via a load sensing device; 
     FIG. 4 illustrates a perspective view of an alternative embodiment of the rheometer apparatus of the invention of FIG. 1 illustrating a primary windup drum in parallel relation to a plurality of secondary windup drums; 
     FIG. 5 illustrates a side view of a second embodiment of the present invention including a primary rotating drum with an independent power supply in parallel relation to a secondary rotating drum with an independent power supply; 
     FIG. 6 illustrates a side view of a second embodiment of the present invention including a primary rotating drum with an independent power supply in parallel relation to a plurality of secondary rotating drums each with an independent power supply; and 
     FIG. 7 is a graphic illustration of the top view of the primary and secondary drums as a sample is stretched. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference now to FIGS. 1-3, there is illustrated a first embodiment of a rheometer apparatus  10 . The rheometer apparatus  10  has a primary windup drum  12  connected at one end  12   a  to one end of a primary drive shaft  14 . The opposite end of the drive shaft  14  is mounted to a drive motor  16 , such as a conventional electrically powered motor. The primary windup drum  12  has a primary drive gear  18 , typically a fine toothed, spur gear, on the circumferential outward surface  20  of the drum. 
     In the illustrated embodiment, primary windup drum  12  is illustrated as being in direct alignment with drive shaft  14 . Those skilled in the art will recognize that this alignment is not necessary for operation of the apparatus, but is preferred to make construction easier and simplify the calculations of torque. 
     A secondary windup drum  22  is disposed in parallel relation to the primary windup drum  12 . The secondary windup drum  22  is supported by a support frame  24  as seen in FIGS. 1,  2  and  3 . The outward ends  22   a ,  22   b  of the secondary drum  22  can include bearings  26   a  and  26   b  respectively, to provide low frictional rotation between the secondary drum and the support frame  24 . While the support frame  24  is illustrated with a square configuration to completely enclose the secondary drum  22 , it is within the terms of the present invention to form the support frame with other shapes as long as they can carry out the function as described hereinafter. The secondary winding drum  22  has a secondary drive gear  28 , typically a fine toothed, spur gear, disposed about the circumferential surface  30  of the secondary drum. The primary and secondary windup drums  12  and  22  are disposed so that the primary and secondary gears  18  and  28  intermesh so that the turning of the primary gear causes the turning of the secondary gear. 
     As shown in FIGS. 1-3, the support frame  24  is affixed to a load sensing device  34  such as a piezoelectric load cell or a strain-gage force transducer via a support member  36 . In turn the load sensing device  34  is securely attached to a support member  32 . 
     All of the windup drums referenced herein are typically but not limited to axisymmetric cylinders of the same diameter. In the case of non-axisymmetric cylinders, a non-constant rate of drum rotation would need to be employed in order to maintain a constant rate of extensional deformation with regards to a true strain deformation, also referred to as a Hencky strain. 
     Each of the windup drums  12  and  22  have associated therewith a securing means (not shown), such as a sample cradling pin or other clamping mechanism, as shown and described in WO00/28321, which is incorporated in its entirety by reference herein, to attach a filament to the drums as required to carry out the measurements desired. It can be appreciated in viewing FIG. 1 that the drive motor  16  turns the drive shaft  14  which in turn rotates the primary winding drum  12  and its attached primary drive gear  18  in a clockwise direction as indicated by arrow  44 . Gear  18 , in turn, turns gear  28  in a counter-clockwise direction as indicated by arrow  46 , which causes rotation of the secondary windup drum  22 . The rotation of the primary and secondary drums  12  and  22  stretches the sample material  50  of a polymer, elastomer, or compound. The resistance provided by the stretched sample  50  to the turning of the secondary windup drum  22  imparts a force on the support frame  24  via the bearings  26   a  and  26   b , which is then imparted through the support member  36  into the load sensing device  34 , the latter being fixedly attached to support member  32 . The force imparted into the load sensing device  34  tends to move the load sensing device in a direction that follows the rotation of the primary windup drum  12 . The tendency of the support structure  24  to turn in the motion of the primary drum rotation (even though it cannot turn because it is secured to support member  32 ) creates a load on the support member  36  and load sensing device  34  that can be measured. The apparatus in each of the embodiments of the present invention is designed so that support member  36  does not actually move, but a load on support member  36  activates the load sensing device which, through a closed feedback loop in the apparatus, develops a current which tends to counteract the load imposed on support member  36  by the secondary windup drum  22  in the support frame  24 , and the current required to counteract this load is measured, thereby measuring the load generated. Such force rebalance systems are well known to those skilled in the art. Other techniques of measuring loads are known to those skilled in the art, and such other techniques can be used with the apparatus of the invention. 
     In the operation of the apparatus  10  of the first embodiment of the invention, the ends of a prepared sample  50  are first secured to the windup drums  12  and  22  by a securing means. In the case of constant radius windup drums, constant rotation of the windup drums  12  and  22  imparts a constant, uniform extensional deformation rate to the unsupported length of the prepared sample  50 . The zone of deformation, or stretch, for the material sample is defined by the tangent line spanning the windup drum pair. The extensional deformation of the material sample offers a resistance to elongation (related to the extensional flow properties of the material) which in turn offers a resistant load on the secondary drum that is resolved with the associated sensing means. Thus by measuring the resulting force that is perpendicular to the primary axis of deformation on the secondary drum  22 , the extensional stress, or viscosity, of the associated material being deformed can be determined for a given rate of extensional deformation. 
     In the special case for which the drive motor  16  has incorporated therein a means of resolving the resistant torque on the motor for a given rate of rotation, the resultant torque imparted on the primary windup drum  12  may be resolved to determine the resistance to elongation of the sample. In any case, the extensional deformation of the stretched sample  50  offers a resistance to deformation that is related to the extensional viscosity of the sample, which in turn offers a resistance to the primary drum rotation in the form of a resulting torque on the drive motor  16 . By measuring the resultant torque transmitted to the drive motor  16 , the extensional viscosity of the prepared sample  50  may be calculated for a given rate of extensional deformation. 
     While not shown, it is within the scope of the present invention to place the apparatus  10  within an environmental chamber, that can be used to heat or cool the sample as desired so that extensional flow properties of a material may be calculated as a function of extensional deformation rate and temperature. The environmental chamber is designed to measure rheology of samples from −70 degrees Centigrade (C) to 300 degrees Centigrade. Measurements at lower temperatures are designed to measure extensional rheology as it relates to Tg (glass transition) of the sample and the extensional flow of the materials at higher temperatures is related to the melts/viscosity of the sample. The environmental chamber can be in the form of an evan or an oil bath or any other means known to those skilled in the art for controlling the physical state of a sample. 
     Referring to FIG. 4, there is illustrated a second alternative embodiment of the present invention wherein rheometer system  58  is provided with a single primary drive shaft  60  that is designed to operate a plurality of secondary winding drums  62 ,  64 ,  66 ,  68  ( 62 - 68 ). The primary drive shaft  60  includes a plurality of spaced primary drive gears  70   a ,  70   b ,  70   c ,  70   d . As in the first embodiment shown in FIGS. 1-3, the primary drum  60  is operated by a drive motor  72 , which can be operationally connected to the primary drum  60  by any means such as a drive shaft  74 . Each of the secondary drums  62 - 68  have secondary gears  76   a ,  76   b ,  76   c ,  76   d , respectively, which mesh with the primary gears  70   a - 70   d , respectively. As discussed with respect to the first embodiment, each of the secondary drums  62 - 68  is mounted within a support frame  80   a ,  80   b ,  80   c ,  80   d  ( 80   a - 80   d ), which are secured to a support member  82  via load sensing devices  84   a ,  84   b ,  84   c ,  84   d  ( 84   a - 84   d ), respectively, via support members  86   a ,  86   b ,  86   c ,  86   d  ( 86   a - 86   d ), respectively. 
     In operation of the second embodiment, as shown in FIG. 4, a plurality of prepared samples  90   a ,  90   b ,  90   c ,  90   d  (referred collectively as  90  herein), are secured to the primary and secondary windup drums  60  and  62 - 68 , respectively. While four secondary drums  62 - 68  are illustrated, it is within the terms of the invention to have more or fewer secondary drums as desired. Further, while it is illustrated that all of the primary and secondary drums  60  and  62 - 68 , respectively, are loaded with the prepared samples  90 , it is also within the terms of the invention to operate the system  58  with any number of the secondary windup drums  62 - 68  as desired. In the case where constant radius windup drums are used, the constant rotation of the primary and secondary windup drums  60  and  62 - 68  imparts a constant, uniform extensional deformation rate to the unsupported pre-gauge length of the prepared samples  60 . The extensional deformation of the stretched samples  60  offers a resistance to deformation which is related to the extensional viscosity of the samples, which in turn offers a resistance to the rotation of the secondary drums  62 - 68  in the form of a resulting load on the load sensing devices  84   a - 84   d . By measuring the resulting force on the load sensing devices  84   a - 84   d , the extensional viscosity of each of the material samples  90   a - 90   d  respectively, can be calculated for a given rate of extensional deformation and temperature. Again, as with the first embodiment, it is within the scope of the present invention to place the system  58  within an environmental chamber (not shown), which can be used to heat or cool the sample as desired. 
     Referring to FIG. 5, there is illustrated a third embodiment of the invention. The rheometer apparatus  100 , as shown in FIG. 5, includes a primary windup drum  102  connected at one end to a drive shaft  104 . The drive shaft  104  itself is mounted to a drive motor  106 , such as a conventional, electrically powered motor of the type described with regards to the first embodiment of the invention. 
     A secondary windup drum  110  is disposed in parallel to the primary windup drum  102 . The secondary windup drum  110  is supported on a drive shaft  112 , which is attached at an opposite end to a drive motor  114  of a desired type, such as electrically powered motor  106 . The motor  114  is affixed to a load sensing device  116  (compare load sensing device  34 ), via a support member  118 . The load sensing device  116  is secured to a support member  120 . Each of the windup drums  102  and  110  have associated therewith securing means such as disclosed in WO00/28321, to attach a material sample  122  of the type sample described herein before as required to carry out the measurements desired. Similar to the first embodiment, the motors  106  and  114  are operated to turn the drive shafts,  104  and  112 , respectively, and the primary and secondary winding drums  102 ,  110 , respectively, in opposite directions. For example, the primary drum  102  can turn in a clockwise direction while the secondary drum  110  can turn in a counter-clockwise direction as shown by the arrows. The rotation of the primary and secondary drums  102  and  110 , respectively, causes the sample  122  of a polymer, elastomer or compound to stretch. The resistance provided by the stretched sample  122  to the turning of the secondary windup drum  110  imparts a force on the motor  114 , which is then imparted through the support member  118  into the load sensing device  116 , the latter being fixedly attached to support member  120 . The load on motor  114  that is transmitted through support member  118  may be resolved by load sensing device  116  with respect to support member  120  as a force tending to bring secondary drum  110  towards primary drum  102  or as a resistant torque tending to hinder the rotation of secondary drum  110 . This force or torque response can be measured from the load sensing device  116  by conventional means. 
     In operation of the rheometer apparatus  100  of the third embodiment of the invention, the ends of the prepared sample  122  are secured to the primary and secondary drums  102  and  110 . Constant rotation of constant radius windup drums  102 ,  110  imparts a constant, uniform extensional deformation rate to the unsupported pre-gauge length of the prepared sample  122 . The extensional deformation of the stretched sample  122  offers a resistance to deformation which is related to the extensional viscosity of the sample, which in turns offers a resistance to the drum  110  rotation in the form of a resulting load on the load sensing device  116 . By measuring the resulting load on the load sensing device  116 , the extensional viscosity of the material sample can be calculated for a given extensional deformation rate and temperature. 
     Since the windup drums  102 ,  110  in the device  100  described in the embodiment of FIG. 5 of the present invention can be mounted directly on the driving means  106 ,  114 , respectively, the use of bearings for the windup drums is not required and thus the frictional contribution from the drum rotation is obviated. Furthermore, since the primary and secondary drums  102 ,  110 , respectively have unique drive systems, the frictional contribution from any mechanical drive coupling (i.e. intermeshing gears) on the measured signal may also be obviated. 
     Material sample strips/fibers  122  are prepared and secured against a set of mated primary and secondary windup drums that comprise an extensional rheometer system  100 . The rheometer may be comprised of either a single cell or multiple cells in which multiple samples may be characterized simultaneously. The drive means for the primary drum may be common to each to the multiple cells whereas each secondary drum and drive means are unique to a cell. The secondary drum of each cell rotates counter to the rotation of the primary drum and has associated with it a sensing means for resolving the load on the secondary drum. Rotation of the primary and secondary windup drums imparts a uniform extensional deformation to the secured material sample associated with each rheometer cell. The zone of deformation, or stretch, for each material sample is defined by the tangent line spanning each windup drum pair. The extensional deformation of the material sample offers a resistance to elongation (related to the extensional flow properties of the material) which in turn offers a resistant load on the secondary drum that is resolved with the associated sensing means. Thus by measuring the resulting load on each secondary drum, the extensional stress, or viscosity, or each associated material being deformed can be determined for a given rate of extensional deformation. Each cell may be accommodated within an environmental chamber such that the extensional flow properties of materials may be characterized with respect to temperature. 
     Referring to FIG. 6, there is shown an alternative embodiment of a rheometer system  128  incorporating the invention shown in FIG.  5 . In the embodiment of FIG. 6, there is a single primary winding drum  130  designed to operate in conjunction with a plurality of secondary winding drums  132 ,  134 ,  136 . As in the embodiment shown in FIG. 5, the primary winding drum  130  is operated by a drive motor  138  (compare  106 ) connected to the primary drum  130  by means such as a drive shaft  140 . Each of the secondary winding drums  132 ,  134 ,  136  has a separate drive motor  142 ,  144 ,  146 , respectively, (compare  114 ) adapted to rotate their respective secondary winding drum through a drive shaft  146   a ,  146   b ,  146   c , respectively. As in the embodiment shown in FIG. 5, each of the drive motors  142 ,  144 ,  146 , which act to support and turn the secondary winding drums  132 ,  134 ,  136 , respectively, is secured to a load sensing device  148   a ,  148   b ,  148   c  (collectively known as  148 ), respectively, by a support member  150   a ,  150   b ,  150   c , respectively. Each of the load sensing devices  148   a ,  148   b ,  148 , (compare  116  in FIG. 5) and are secured to a support member  152 . Each of the secondary winding drums  132 ,  134 ,  136  operate independently of each other so that any or all of the plurality of secondary winding drums can be used at the same time or at different times. As shown in FIG. 6, the ends of the prepared samples  160   a ,  160   b ,  160   c  are secured to the primary and secondary drums  130  and  132 - 136 , respectively, by a securing means. As described in the embodiment shown in FIG. 5, the rotation of the primary and secondary drums  130  and  132 - 136 , respectively, cause the sample of a polymer elastomer compound to stretch. The resistance provided by the stretched samples  160   a ,  160   b ,  160   c  to the turning of the secondary windup drums  132 - 136 , respectively, imparts a force on the motors  142 ,  144 ,  145 , which in turn is imparted through the support members  150   a ,  150   b ,  150   c , respectively, into the load sensing devices  148   a ,  148   b ,  148   c , respectively. The latter load sensing devices  148   a ,  148   b ,  148   c  are attached to the support member  152  so that the force imparted into each of the load sensing devices moves the load sensing device and creates a torque with respect to the support member  152 . The torque can be measured from the load sensing devices  148   a ,  148   b ,  148   c  by conventional means. The primary motor  138  and the secondary motors  142 ,  144 ,  145  are operated to provide constant speed of rotation of the primary and secondary winding drums  130  and  132 - 136 , respectively, to impart a constant, uniform extensional deformation rate to the unsupported pre-gauge length of the prepared samples  160   a ,  160   b ,  160   c . The zone of deformation, or stretch, for each material sample is defined by the tangent line spanning each windup drum pair. The extensional deformation of the material sample offers a resistance to elongation (related to the extensional flow properties of the material) which in turn offers a resistant load on the secondary drum that is resolved with the associated load sensing device. Thus by measuring the resulting load that is parallel to the primary axis of deformation on each secondary drum, the extensional stress, or viscosity, or each associated material being deformed can be determined for a given rate of extensional deformation. The extensional deformation of the material samples  160   a ,  160   b ,  160   c  offers a resistance to deformation which is related to the extensional viscosity of the samples. This resistance, in turn, offers a resistance to the secondary drum rotation in the form of a resulting load on the load sensing devices  148   a ,  148   b ,  148   c . By measuring the resulting loads on each of the load sensing devices  148   a ,  148   b ,  148   c , the extensional viscosity of the material samples  160   a ,  160   b ,  160   c  can be calculated for a given extensional deformation rate and temperature. 
     While three secondary winding drums  132 ,  134 ,  136  are shown in FIG. 6, it is within the terms of the present invention to use any number of secondary winding drums as desired. Also, the rheometer system  128  shown in FIG. 6 can be placed in an environmental chamber, as described hereinbefore. 
     Since the primary windup drum  130  and the secondary windup drums  132 ,  134 ,  136  of the extensional rheometer device  128  described with respect to the embodiment of FIG. 6 of the present invention can be mounted directly on the driving means  138  and  142 , 144  and  145 , respectively, the use of shaft bearings for the windup drums is not required and thus the frictional contribution from the drum rotation is obviated. Furthermore, since the primary and secondary drums  130  and  132 , 134 , 136 , respectively, have unique drive systems, the frictional contribution from any mechanical drive coupling (i.e. intermeshing gears) on the measured signal may also be obviated. 
     Material sample strips/fibers  160   a ,  160   b ,  160   c  are prepared and secured against a set of mated primary and secondary windup drums  130  and  132 , 134 , 136 , respectively, that comprise the extensional rheometer system  128 . The rheometer system may be comprised of either a single set of drums, as shown in FIG. 5 or multiple drums, as shown in FIG.  6 . In the latter embodiment, multiple samples may be characterized simultaneously. 
     The invention is further illustrated with reference to the following example. 
     EXAMPLE 1 
     The apparatus  10  shown in FIGS. 1-3 is used for illustrative purposes in this example. 
     Both ends of an uncured polymer filament  50  are secured by the sample securing clamps of the equal diameter, primary and secondary windup drums  12 , 22 , respectively, of the extensional rheometer  10 . A motor  16  rotating at a fixed rotational rate drives the primary windup drum  12  and a fine toothed, spur gear  18  on the same shaft  14 . This spur gear  18  intermeshes with a similar spur gear  28  on the secondary windup drum  22 . 
     Since both spur gears  18  and  28  are similar, motion of the primary drum  12  drives an equal but opposite rotation of the secondary drum  22 . The secondary drum  22  is affixed with precision bearings  26   a ,  26   b  to the frame support  24 . The constant rotational speed (Ω) of the windup drums  12  and  22  of equal radius (R) imparts a constant, uniform extensional true strain rate, or Hencky strain rate ({grave over (∈)}) to the unsupported length (L) of the sample  50  such that: 
     
       
         {grave over (∈)}=2 ΩR/L   
       
     
     as illustrated graphically in FIG.  7 . 
     The extension of the sample  50  offers a resistance to deformation due to the extensional viscosity η E  (t) of the material, which in turn offers a resistance to the drum rotation in the form of torque T E . The extensional viscosity of the material can be expressed in the following relationship: 
     
       
         η E ( t )=σ E ( t )/{grave over (∈)}= F   E ( t )/ A ( t )/{grave over (∈)} 
       
     
     where σ E  (t) is the instantaneous extensional stress in the unsupported sample, F E  (t) is the instantaneous force required to stretch the unsupported sample, and A(t) is the instantaneous cross-sectional area of unsupported sample. The resultant torque acting on the drums  12  and  22  may then be expressed as: 
     
       
           T   E ( t )= F   E ( t ) 2   R   
       
     
     Both of these expressions may be combined to yield: 
     
       
         η E ( t )= T   E ( t )/(2 R{grave over (∈)}A ( t )) 
       
     
     By measuring either the resultant torque on the drive drum or the load monitored by the sensing device  34 , the extensional viscosity of the material sample may be calculated for a given extensional deformation rate and temperature. 
     T E  can be resolved by a summation of torques about the axis of rotation of the primary windup drum, point  0  from FIG.  7 . During stretch, the resistance of the sample to extend imparts a torque on the gear teeth which in turn imparts a resultant torque, T R , on the system that is borne by the frame support  24 , the support member  36 , and the load sensing device  34  assembly. Since the bearings  26   a ,  26   b  and intermeshing gears  18 ,  28  also offer resistance to  30  rotation, a summation of torques yields: 
     
       
         Σ T   0 =0 =T   R   −T   E   −T   Gears   −T   Bearings   =T   R   −T   E   −T   Friction   
       
     
     Thus, the above expression for η E  (t) can be rewritten as: 
      η E ( t )=( T   R ( t )− T   Friction )/(2 R{grave over (∈)}A ( t )) 
     where T R (t) is the instantaneous resultant torque on the system, and T Friction  is the torque losses from the bearings and gears which can be determined from calibration. The instantaneous resultant torque, T R (t), may then be resolved by monitoring the instantaneous torque on the primary windup drum motor  16  or by monitoring the instantaneous force on the load sensing device  34  and multiplying by the appropriate moment arm, L, about the axis of rotation of the primary windup drum, point  0 , as illustrated in FIG.  7 . 
     Now for a sample in simple extension, A(t) can be expressed as: 
     
       
           A ( t )= A   O  exp (−∈) 
       
     
     where A O  is the original cross-sectional area prior to sample extension, and E is the true strain in simple extension. For a constant true strain rate of deformation in simple extension, A(t) can be rewritten as: 
     
       
           A ( t )= A   O  exp (−{grave over ( 531  )} t ) 
       
     
     where {grave over (∈)} is the constant true strain rate of deformation in simple extension. Substituting the initial expression for {grave over (∈)}, A(t) can be rewritten as: 
     
       
           A ( t )= A   O  exp (−2 ΩR t/L ) 
       
     
     Since Ω=d(θ(t))/dt where θ(t) is the angular rotation of the primary windup drum  12  as a function of time, then for a constant rotational drum speed, Ω may be expressed as: 
     
       
         Ω=(θ 2 −θ 1 )/(t 2 −t 1 ) 
       
     
     If it is assumed that θ 1 =0 at t 1 =0 and that a constant rotational speed is achieved instantaneously then the expression for Ω simplifies to: 
     
       
         Ω=θ 2/t2 =θ( t )/ t   
       
     
     Assuming no-slip of the fiber on the drum, the above expression can be substituted into the expression for A(t) and the following can be obtained: 
     
       
           A ( t )= A   O  exp(− 2θ(   t ) R/L ) 
       
     
     Thus, the resulting expression for the instantaneous cross-sectional area of a sample is only a function of the angular rotation of the primary windup drum at a given time, t. Beyond the realm of validity of the aforementioned assumptions, however, more rigorous empirical methods for determining instantaneous fiber cross-sectional area should be applied and are well known to those skilled in the art. 
     Note that each windup drum  12  and  22  can be threaded to allow for fiber alignment and multiple drum rotations to allow for very large true strains as described in Ser. No. WO00/28321. In doing so, however, the increased extensional deformation per drum revolution must be accounted for in the expression for extensional deformation rate, {grave over (∈)}. In addition, a non-circumferential force component must be accounted for in the torque measurement, T R (t). 
     While the invention has been specifically illustrated and described, those skilled in the art will recognize that the invention may be variously modified and practiced without departing from the concepts of the invention. The scope of the invention is limited only by the following claims.