Reversible Polymeric Gels and Related Systems - American Chemical

Chapter 2

Viscoelastic Behavior of Ultra High Molecular Weight Polyethylene Pseudogels 1

2

Β. Chung and A. E. Zachariades 1

Research Division, Gen Corporation, 2990 Gilchrist Road, Akron, OH 44305 Almaden Research Center, IBM, San Jose, CA 95120

Downloaded by FUDAN UNIV on March 16, 2017 | http://pubs.acs.org Publication Date: September 14, 1987 | doi: 10.1021/bk-1987-0350.ch002

2

The viscoelastic behavior of ultra-high molecular weight polyethylene (UHMWPE) gel-like system in paraffin oil (2-5% w/w concentration) has been studied by rheological and rheo-optical characterization techniques and was found to be significantly different from the true gels of covalent bonded molecular networks. The dynamic moduli of the UHMWPE gel-like systems exhibit shear history, temperature and frequency dependence and a remarkable hysteresis effect during thermal cycling in which the original modulus value cannot be recovered.

Ultra-high molecular weight polyethylene (UHMWPE) "gels" have been processed into high modulus/strength fibers by dissolving and spinning at elevated temperatures ( ~ 2 0 0 ° C ) and subsequently quenching the spun fiber, removing the solvent and stretching to some high draw ratio (1-3). Whereas the effects of processing and preparation conditions on the mechanical properties of spun-drawn fibers have been investigated to a considerable extent (1-4). the viscoelastic behavior of the U H M W P E "gels" have been examined recently (5). Rheo-optical and Theological studies with UHMWPE/paraffin oil (2-5% w/w) systems indicate that these are not true gels like the covalent bonded molecular networks, and their viscoelastic properties, e.g., dynamic moduli (G* and G"), are frequency and temperature dependent. Various diverse systems qualify as gels if one assumes that in these systems the common features are the solid-like behavior and the presence of a continuous structure of macroscopic nature (6/7). For the purpose of the discussion in this paper, we describe a gel as a colloidal system comprised of a dispersed component and a dispersion medium both of which the junction points are formed by covalent bonds, secondary valence bonds, or long range attractive forces that cause association between segments of polymer chains or formation of crystalline regions which have essentially infinite life time (8). Systems such as the concentrated solution of the U H M W P E in paraffin oil (2-8% w/w) contain a three-dimensional molecular network in which the junction points are produced by secondary valence bonds which cause crystalline regions and by physical entanglements of different life times. Entanglements that are trapped between crystallites have, like the crystallites, essentially infinite life times. 0097-6156/87/0350-0022$06.00/0 © 1987 American Chemical Society

Russo; Reversible Polymeric Gels and Related Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2.

C H U N G A N D ZACHARIADES

23

Polyethylene Pseudogels

Entanglements that are not trapped have transistory existence (and will be referred to as "temporary" entanglements). Since the dynamic moduli of these systems depend on the frequency, temperature and shear history of the system, we shall refer to such system as a gel-like or pseudo-gel in contrast to a true gel. A schematic diagram of the pseudo-gel structure is shown in Figure 1. Experimental Sample Preparation. The U H M W P E used in this study was a HiFax 1900 (Himont, Inc.) with an average molecular weight of 2-8 χ 10 . The U H M W P E was added to paraffin oil to concentrations from 2 to 5% w/w. To avoid polymer degradation at high temperatures, the polymer was stabilized with approximately 0.5% of BHT antioxidant. The mixture was stirred at 150° C. A solution was obtained that was clear until it was cooled to about 120°C when it became opaque and transformed into a gel-like or pseudo-gel.


6

Rheo-Optical Observations. The rheo-optical observations were made with a WILD stereoscope equipped with a custom made rotary optical stage using polarized light. The rotary optical stage described in detail elsewhere (9) was used for both the steady state shear and dynamic test conditions. Rheological Properties Measurements. The viscoelastic behavior of the U H M W P E gel-like systems was studied using the Rheometric Mechanical Spectrometer (RMS 705). A cone and plate fixture (radius: 1.25 cm; cone angle: 9.85 χ 10 radian) was used for the dynamic frequency sweep, and the steady state shear rate sweep measurements. In order to minimize the error caused by gap thickness change during the temperature sweep, the parallel plates fixture (radius: 1.25 cm; gap: 1.5 mm) was used for the dynamic temperature sweep measurements. -2

Results Study of the Crystalline Regions. According to our morphological studies, the crystalline structure of U H M W P E pseudo-gel was different under different sample preparations. For example, spherulites and lamellar single crystal stacks were observed when the pseudo-gel was prepared under quiescent conditions, shish-kebab crystals under stirring conditions, and a mixture of single and shish-kebab crystals under uncontrolled conditions (10). Rheological observations of the U H M W P E pseudo-gels of different concentrations under oscillatory shear conditions at different temperatures showed that these systems exhibit considerable drawability at temperatures above ambient. The deformation of the crystalline phase of the gel-like system is not reversible and, as shown in the sequence of photographs Figure 2, for a pseudo-gel of 4% concentration, it was greater when the sample was sheared under the same oscillatory conditions at higher temperatures. The displaced crystals of the U H M W P E pseudo-gel showed remarkable dimensional stability after shear cessation and removal of any compression load in the optical rotary stage. Both the fresh gel and the gels after dynamic measurements were examined by X-ray. As shown in Figure 3, there are two intensity maxima peaks at 20 angles about 21.5 and 24 degree; also, these two crystal peaks become sharper after the dynamic measurements. Furthermore, the DSC analysis showed that the 4% fresh


REVERSIBLE POLYMERIC GELS AND RELATED SYSTEMS


24

X A

Β

: :

crystals - essentially infinite life time "trapped" entanglements between crystals - essentially infinite life time "temporary" entanglements - transitory existence

Figure 1. Schematic diagram of the U H M W P E pseudo-gel structure.



Polyethylene Pseudogeb


2.

Figure 2. Deformability of 4% w/w UHMWPE pseudo-gel sample under oscillatory shear force at different temperatures: (a) 25 °C, (b) 30 C , and (c) 40 °C. e


25

26

REVERSIBLE POLYMERIC GELS A N D RELATED SYSTEMS

gel has a AHm of 8.42 J/g and a Tm at about 116°C; after the dynamic frequency sweep, the AHm increased to 15 J/g but the Tm remained the same. Both the X-ray and DSC results suggest that the crystallinity of pseudo-gels increases under the hydrodynamic shearing force. Study of Viscoelastic Behavior. The dynamic storage shear modulus, G\ and loss modulus, G", of the U H M W P E pseudo-gels were measured at different temperatures in the frequency range from 10" sec to 10 sec . A plot of the G versus the oscillation frequency is shown for the U H M W P E pseudo-gel of 4% concentration in Figure 4. The G decreased in the low frequency range and approached a plateau in the intermediate frequency range, and then increased rapidly as the frequency increased above 10 sec . When the sample was tested for a number of consecutive runs, the G' decreased in each run and approached to an equilibrium value, i.e., the plateau region at a low frequency. Similar is the loss modulus (G") variation with frequency but, as shown in Figure 5, is less sensitive to the shear history of the sample. Furthermore, as shown in Figures 6 and 7, the shear history dependence of the viscoelastic properties, G and G " , was less pronounced at lower temperatures, e.g., at 0 ° C . A similar behavior was observed for the other U H M W P E gel concentrations. Dynamic temperature sweep measurements carried out at different frequencies from - 2 0 ° C to about 1 0 0 ° C indicate that the G' of the U H M W P E pseudo-gels decreased significantly with increasing temperature. As shown in the Arrhenius plot of Figure 8, the variation is not monotonie and a transition was observed which corresponds to the change of the slope. Similar was the variation of the loss modulus with temperature. Most important, the shear modulus dependence on temperature was not reversible. As indicated in Figure 8, for the U H M W P E pseudo-gel of 3% concentration, the G value at any particular temperature is higher in the heating part of thermal cycle between -20 ° C and 1 0 0 ° C at a frequency of 100 sec . The storage modulus assumes a somewhat lower value on cooling at the beginning of the cooling cycle; on further cooling, it decreased to a minimum and then increased as the temperature decreased further but never recovered its original value (at the beginning of the heating cycle). As shown in Figure 9, similar was the variation of G" with temperature; however, the hysteresis phenomenon was less pronounced. The viscoelastic response of the U H M W P E pseudo-gels depends also on the solution concentration. As shown in Figure 10, the G' increased with concentration at frequency 50 sec over the temperature range from - 2 0 ° C to 100°C. This is probably due to the higher entanglement density and/or higher crystallinity in the higher concentration solution. Steady state shear viscosity measurements indicate a power-law type relation for the variation of the shear viscosity with shear rate even in the lower shear rate range between 10" to 1 sec . The results at higher shear rates were questionable due to the slip between the sample and cone and plate fixtures. 2

-1

-1

1

1


-1

1

1

-1

-1

1

-1

Discussion U H M W P E gels have been studied mainly for their processability into high modulus/strength products. The viscoelastic behavior of this polymer/solvent system, as well as others which are capable of forming a gel-like state at some suitable concentration, has received little attention. Previous studies have been carried out by Ferry et al. (6,7) with gel systems of cellulose derivatives and polyvinychloride



Polyethylene Pseudogeh

27


2.

1

Figure 4. The variation of tit? dynamic storage modulus (G ) with oscillation frequency at 2 5 ° C for U H M W P E pseudo-gel (4% w/w); · first run, ο second run, Δ third run, • fourth run.



REVERSIBLE POLYMERIC GELS A N D R E L A T E D SYSTEMS

Log ω (sec ) -1

Figure 5. The variation of the dynamic loss modulus (G") with oscillation frequency at 25 C for UHMWPE pseudo-gel (4% w/w); #, first run; O , second run; Δ , third run; • , fourth run. e

Figure 6. The variation of the dynamic storage modulus (G') with oscillation frequency at 0 C for UHMWPE pseudo-gel (4% w/w); #, first run; • , second run; V, third run; • , fourth run. e



2.

CHUNG AND ZACHARIADES

Figure 7.


29

The variation of the dynamic loss modulus (G") with oscillation

frequency at 0 ° C for U H M W P E pseudo-gel (4% w/w); · first run, • second run, V third run, A fourth run.

0.0028

0.0032

0.0036

0.0040

1/TO/K)

Figure 8.

The variation of dynamic storage modulus (G') of U H M W P E 1

pseudo-gel (3% w/w) with terr perature at ω = 1 0 0 sec" ; · 1 eating cycle, ο cooling cycle.


30

REVERSIBLE POLYMERIC GELS A N D RELATED SYSTEMS

7


2

1

1

1

1

Γ

5

3 1—1

Figure 9.

1

1—ι

« Ο .0028

«

Ι 0.0032 0.0036 1/T (1/K)

0.0040

The variation of the dynamic loss modulus (G") of U H M W P E -1

pseudo-gel (3% w/w) with ten perature at ω = 1 0 0 sec ; · heating cycle, ο cooling cycle.

π

6

Γ

r-. ο ° ° ο

• • • • •

Λ

#

° ο

· . 4 %

Λ

3% " • ,

g 5

9. 4\

J

L 40

80

Temperature (°C)

Figure 10. The variation of the dynamic storage modulus (G') of U H M W P E pseudo-gels of different concentration (w/w) with temperature at ω = 5 0 sec . -1



2.


31


assuming a cross-linked molecular network structure in which the cross-linking was provided by the crystalline regions in the gel. However, in their studies, they did not elaborate on the effects of temperature and shear history on the stability of the gels under hydrodynamic shear forces. This subject is important particularly in view of recent observations of polymer/solvent systems which may have different crystalline morphologies of considerable fraction that may vary significantly with the preparation and processing conditions (10). The gel-like structure of the UHMWPE/paraffin oil system, as shown in Figure 1, associates with the presence of the crystals which act as permanent cross-links at temperatures below their melting point, the trapped entanglements between the crystals and the temporary entanglements which have a short lifetime in comparison to the other two types of junction points. The effect of the entanglements with short lifetime on the viscoelastic behavior of the U H M W P E pseudo-gels is shown in Figure 4 by the initial decrease of G in the low frequency region, and by the overall decrease of the plateau G' in each consecutive run. The effect of shear history becomes less pronounced when the temperature is decreased because of the reduced mobility of the molecular chains. The plateau region in Figure 4 is due to the presence of the crystals and the trapped entanglements between them. However, as shown in Figure 4, this is not a true equilibrium state because the G diminishes in each consecutive run. The fact that the equilibrium region extends into the lower frequency region after each run indicates a tendency of the system to approach a pseudo-equilibrium state. Although this pseudo-equilibrium plateau of G' is similar to what is observed in a covalent cross-linked gel, e.g., elastomeric gel, it depends strongly on the shear history, temperature, and gel concentration. As shown in Figure 8, the G of the U H M W P E pseudo-gel decreases with increasing temperature. The behavior is contrary to the behavior of a cross-linked gel system, in which the G' usually does not change with temperature. This presumably associates with the reduced rigidity of the crystals and simultaneous loss of the trapped entanglements which will disentangle and behave like the temporary entanglements as temperature increases under the hydrodynamic shear force. This is documented by the enhanced drawabilty of the crystals in photos in Figure 2. This behavior is more like the viscoelastic behavior of a polymer melt, however, the G decrease is less drastic as the temperature decreases. The hysteresis phenomenon during the thermal cycle between -20° to 100° C can be explained by considering the relative effects of the crystals, trapped entanglements, and temporary entanglements on the G at the different temperature regions of the thermal cycle. The initial sharp decrease in G' up to about -10° C is due to the increasing mobility of the temporary entanglements, whereas the subsequent monotonie decrease with increasing temperature is mainly due to the reduction also by the rheo-optical observations. The sharp G' decrease above 60 ° C associates with the deformability of the crystals which enhances further the reduction of trapped entanglements. Thus, the U H M W P E pseudo-gel has fewer trapped entanglements at the end of heating cycle. For most polymer systems, the G usually increases during cooling due to the decreasing chain mobility; but the U H M W P E pseudo-gel behaves quite different. On cooling, as shown in Figure 9, the G' of this system initially tends to change with temperature along the path of the heating cycle. However, at the high temperature range, the number of trapped entanglements continues to decrease, and apparently offsets the cooling effect which increases the G'. As a result, the G soon assumes lower values. The sharp decrease of G at lower temperatures reflects the disentanglement effect of trapped 1

1

1

1

!

1

1

1


32

REVERSIBLE POLYMERIC GELS AND RELATED SYSTEMS

entanglements while the crystals are still deformable. As the temperature decreases below 40° C the crystals become less deformable and a larger fraction of entanglements remain trapped. This process alone will prevent the further decrease of the G value. However, at this lower temperature range chain mobility is considerably hindered and, therefore, the storage modulus increases as the temperature decreases to -20 °C. Since some of the trapped entanglements were lost permanently during the thermal cycle the original G value cannot be recovered. !

f


Conclusion Rheo-optical and rheological strudies of concentrated solutions of UHMWPE/paraffin oil (2-5% w/w concentration) systems indicate that these are pseudo-gels unlike the covalent bonded molecular networks which exhibit a true gel behavior. The viscoelastic properties of the UHMWPE/paraffin oil pseudo-gels, i.e., dynamic shear moduli G and G " are frequency and temperature dependent. Furthermore, the dynamic shear moduli depend on the concentration and shear history of the pseudo-gel and exhibit a hysteresis behavior during thermal cycling in which the original modulus value cannot be recovered. 1

Acknowledgments The authors would like to thank Mr. R. Siemens and Miss G . Lim for their technical assistance in the DSC and X-ray measurements. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Smith, P.; Lemstra, P. J. J. Mater. Sci. 1980, 15, 505. Smith, P.; Lemstra, P. J.; Booij, H. C. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 877. Smook, J.; Flinterman, M.; Pennings, A. J. Polym. Bulletin 1980, 2, 775. Barham, P. J. Polymer 1982, 23, 112. Chung, B.; Zachariades, A. E. ACS Polymer Preprints 1986, Vol. 5, 195. Nizomiya, K.; Ferry, J. D.; J. Polym. Sci. 1967, Part A-2, Vol. 5, 195. Birnboim, M. H.; Ferry, J. D. J. Appl. Phys. 1961, Vol. 32, No. 11, 235. Smith, T. L. Mechanical Properties of Gels and Thixotropic Substances, Inst. of Phys. Handbook, Margo Hill Book Co., New York, 1963. Zachariades, A. E.; Logan, J. A. Polym. Eng. Sci. 1983, 15, 797. Zachariades, A. E. J. Appl. Polym. Sci., 1986, 32, 4277.

RECEIVED February 27, 1987


Reversible Polymeric Gels and Related Systems - American Chemical

Recommend Documents