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Rheological and rheo-optical properties of high molecular weight syndiotactic and atactic polyvinylalcohol solutions.

INTRODUCTION

Vinyl polymers with different substituents may have different molecular arrangements in space. A polymer in which configurations of monomer units are more or less random is called atactic. A polymer consisting of monomer units that are alternate regularly is called syndiotactic, and a polymer built up of monomer units having configurations all regular is called isotactic. These strereoregular configurations have been achieved through stereospecific polymerization of preordered monomer by adopting a specially designed catalyst (1,2). The so-called tacticity is known to have a profound influence on the physical properties (3-7). For example, an isotactic polypropylene (PP) gives a rigid plastic product, whereas an atactic PP gives a flexible waxlike one (8).

Recently, Lyoo and Ha (6, 7, 9) found that a polyvinylpivalate (PVPi) of high syndiotactic diad content developed a well-oriented microfibrillar structure during saponification, and gave rise to a fibrous PVA product. On the other hand, saponifying a polyvinylacetate (PVAc) of low syndiotactic diad content did not lead to any fibrillar formation. This suggests that tacticity may have a significant influence on rheological properties as well as mechanical properties. Up to now, however, little experimental results have been disclosed to elucidate the effect of tacticity on the rheological and rheo-optical properties. This study investigates the effect of syndiotactic diad content of polymer on the rheological and rheo-optical properties of solutions of PVA in DMSO.

EXPERIMENTAL

Materials

Linear high molecular weight (PVA)s were obtained by saponifying two different precursors, PVPi and PVAc. The precursors were prepared by using a low temperature ultraviolet-initiated bulk polymerization to obtain assigned stereoregularity (7, 10). Since PVPi possesses high degrees of syndiotactic diad content (about 63%), PVA prepared from PVPi is denoted as syndiotactic PVA. On the other hand, PVA prepared from PVAc (syndiotactic diad content of about 53%) is denoted as atactic PVA. The characteristic features of the PVA samples obtained by saponifying two different precursors are listed in Table 1.
Table 1. Characteristics of PVA Prepared.

Pn(a) S-diad (%)(a) D.S. (%)(a)

3100 52.7 99.9
3900 52.2 99.9
4500 52.9 99.9
4800 52.6 99.9
6100 52.4 99.9
6300 63.5 99.9
9800 63.1 99.9
12,300 63.8 99.9
14,000 63.2 99.9
15,500 63.5 99.9

a [P.sub.n], S-diad, and D.S. are, respectively, the number-average
degree of polymerization, syndiotactic diad content, and degree of
saponification of PVA.




Measurements of Rheological and Rheo-Optical Properties

The shear and dynamic rheological properties of 2 g/dl solutions of PVA in DMSO were measured at 30 [degrees] C. For a more precise measurement of rheological properties of low-viscosity PVA/DMSO solutions, a Rheometrics fluid spectrometer RFS II was utilized. The minimum sensitivity and sampling time of RFS II were reported to be 2 x [10.sup.-3] gcm and 5 msec, respectively. To keep the shear history of PVA solutions constant during rheological measurements, a cone-and-plate type flow cell with diameter of 25 mm and deformation degree of 0.02 rad was adopted. It is worth mentioning that the time scale of experiment sometimes has some effect on the rheological properties of the syndiotactic PVA. When the rheological properties of syndiotactic PVA solutions were measured at time interval of 45 min, the data was reproducible.

The [Delta][n.sub.f] of solutions of PVA in DMSO was measured with a phase-modulated flow birefringence (PMFB) apparatus at 30 [degrees] C. PMFB technique can measure time-dependent birefringence and dichroism of polymer solutions, polymer melts, and colloidal dispersions with high sensitivity (11). The PMFB apparatus was composed of He-Ne laser source, polarizer, photo-elastic modulator, two sets of quarter wave plates, analyzer, and photo-detector having relative optical orientation as schematically shown in Fig. 1. In the rheo-optical measurements a rotational parallel-plate geometry was adopted. Figure 2 illustrates flow geometry during [Delta][n.sub.f]/measurements. [Delta][n.sub.f] was calculated by Eq 1:

[Delta][n.sub.f] = [Delta][prime][Lambda]/2[Pi]d (1)

in which, [Delta][prime], [Lambda], and d are phase retardance, wavelength of light, and thickness of solution, respectively (12).

RESULTS AND DISCUSSION

The degree of saponification is expected to influence the rheological properties of PVA because a difference in chemical composition may change the physical properties of polymer molecules in a solvent. Figure 3 shows plots of dynamic rheological parameters of 2 g/dl syndiotactic PVA solutions in DMSO against frequency ([Omega]) at 30 [degrees] C at two different degrees of saponification. The degree of saponification seems to have little effect on the rheological properties except that higher G[double prime] is observed at lower degree of saponification. Viewed in detail, however, one can see that there are crossover points in the curves of complex viscosity ([[Eta].sup.*]) and G[prime], indicating that as the degree of saponification is increased, the extent of shear thinning is increased, and the extent of increasing G[prime] with increasing [Omega] is decreased. This may imply that syndiotactic PVA with a higher degree of saponification develops molecular orientation more easily by shear.

Figures 4 and 5 compare behavior of [[Eta].sup.*] of 2 g/dl solutions of atactic and syndiotactic (PVA)s in DMSO at 30 [degrees] C at different number-average degree of saponifications ([P.sub.n]). The syndiotactic diad content in syndiotactic PVA is [approximately]63-64%. It is worth mentioning that over the range of frequencies examined, atactic PVA solutions exhibit almost Newtonian flow behavior, whereas syndiotactic PVA solutions exhibit pseudo-plastic flow behavior at identical polymer concentrations. In addition, the viscosity of syndiotactic PVA solutions is generally much higher than that of atactic PVA solutions at the same polymer concentration of 2 g/dl. During dissolution of 2 g/dl of polymers in DMSO, one can observe that syndiotactlc PVA yields a gel-like state while the atactic PVA gives only a very dilute solution. Formation of a gel-like state at such a low polymer concentration in the case of syndiotactic PVA is very surprising because the difference in the molecular weight of the two polymers is negligible (the atactic PVA with [P.sub.n] of 6100 in [ILLUSTRATION FOR FIGURE 4 OMITTED] may be compared with the syndiotactic PVA with [P.sub.n] of 6300 in [ILLUSTRATION FOR FIGURE 5 OMITTED]).

Newtonian flow behavior of atactic PVA may be attributed partly to lower molecular weight of polymer at the polymer concentration. In reality, [[Eta].sup.*] of atactic PVA is extremely low as seen in Fig. 4. Shear viscosity of the polymer solutions was also measured, and viscosity curves are shown in Fig. 6. This Figure also shows that atactic PVA solutions give rise to almost Newtonian flow behavior. On the other hand, syndiotactic PVA solutions show non-Newtonian viscosity behavior and thus do not give a lower Newtonian flow region over the frequency range observed. As well recognized, the disappearance of a lower Newtonian flow region is indicative of heterogeneity of the system, which is frequently encountered in inhomogeneous systems such as inorganic-filled polymer systems, block copolymers, or liquid-crystalline polymers.

Considering the molecular structure of polymer, it is evident that PVA with higher syndiotactic diad content possesses a more stereoregular chain structure. Consequently, the polymer chains of PVA with higher syndiotactic diad content may be more readily stiffened by more effective intermolecular interactions between hydroxyl groups in a DMSO solution. Thus, syndiotactic PVA may form a mesophase, which includes internal orders or internal structures. In accordance with this proposition, a time decay effect is observed for syndiotactic PVA when shear rate is decreased stepwise, as shown in Fig. 7. On the other hand, atactic PVA exhibits little time effect when the shear rate is decreased stepwise, as shown in Fig. 8. The decrease of viscosity with shearing time shown in Fig. 7 may result from a breakdown of internal structures with shearing time. On this structural concept, Bingham shear-thinning behavior of syndiotactic PVA may be accounted for.

Variation of G[prime] of 2 g/dl solutions of atactic and syndiotactic (PVA)s in DMSO with frequency at 30 [degrees] C is compared in Figs. 9 and 10. On an empirical basis, the slope of the plots of G[prime] against frequency for homogeneous isotropic polymer solutions is reported to be 2 (13). As shown in Fig. 9, the slopes of atactic PVA solutions approach [approximately]2, which coincides with the empirically established value. On the other hand, it is interesting to see that the slopes of syndiotactic PVA solutions are in the range of 0.2 to 0.4, as shown in Fig. 10. As one may imagine, in the case of the mesophase, the slope would be reduced because it has many microdomains, which may be easily oriented even by low shear. Further, the orientation relaxation is much slower than in an isotropic solutions. Thus, the degree of increasing energy dissipation is increased with increasing frequency. In consequence, with syndiotactic PVA, which may form a mesophase, much smaller slopes are observed in the G[prime]-frequency plot as compared in Figs. 9 and 10. Hence, these Figures demonstrate that the phase of syndiotactic PVA solutions in DMSO is heterogeneous rather than homogeneous, much like a mesophase.

It has also been reported from experimental results that the slope of plots of first normal stress difference ([N.sub.1]) against shear rate is 2 for homogeneous isotropic polymer solutions, but becomes 0.6 to 0.7 for liquid crystalline polymers in the nematic phase (13, 14). Considering that [N.sub.1] and G[prime] are measures of elasticity and that shear rate and frequency are input processing parameters representing the degree of shearing, the qualitative agreement between two plots may indicate a similarity in the phase morphology of the two systems, viz., heterogeneity.

The G[prime] of 2 g/dl atactic and syndiotactic PVA solutions in DMSO is plotted against G[double prime] in Figs. 11 and 12, respectively. According to Han and John (15), G[prime] is related to G[double prime] by Eq 2:

[Mathematical Expression Omitted] (2)

in which [Mathematical Expression Omitted], [Rho] is density, R is the gas constant, T is temperature, and [M.sub.e] is entanglement molecular weight. This equation has been derived from molecular viscoelasticity theory for entangled flexible homopolymers rather than from an experimental basis. Equation 2 is proposed to be well adapted in the terminal region. As shown in Fig. 11, atactic PVA solutions follow this theoretical prediction, viz., the slope on the plot of G[prime] against G[double prime] is in the vicinity of 2 irrespective of molecular weight. A negligible effect of molecular weight on the slope implies that the phase morphologies of atactic PVA solutions are almost identical, suggesting that atactic PVA solutions are homogeneous.

It has been reported that heterogeneous polymeric systems such as thermotropic liquid-crystalline polymers in nematic phase decrease the slope to below 2 on the plot of G[prime] against G[double prime], and give rise to an inflection point in the curve (16, 17). Unlike atactic PVA solutions in Fig. 11, syndiotactic PVA solutions exhibit slopes less than 2, as shown in Fig. 12. Similar to the nematic phase of thermotropic liquid-crystalline polymers, syndiotactic PVA solutions show an inflection point on the plot of G[prime] against G[double prime] particularly for syndiotactic (PVA)s with [P.sub.n] of higher than 9800. The inflection point at a certain G[double prime] means that the ratio of viscous dissipation to elastic storage is abruptly increased above the G[double prime] (or alternatively, frequency). This may be attributable to the fact some heterogeneous domains with internal order in the mesophase are oriented to a great extent at a high frequency range.

Compared with atactic PVA, syndiotactic PVA has very long relaxation times. A calculation by Wissbrun and Griffin equation (18) reveals that relaxation time of atactic PVA solutions ranges from [10.sup.-3] to 2 x [10.sup.-2] sec, whereas that of syndiotactic PVA solutions ranges from 7 x [10.sup.-3] to 1 sec, over the frequency range of 1 to 100 rad/sec. Taking relaxation times into consideration, it may be said that in the case of syndiotactic PVA, the extent of deformation may be accumulated at high frequencies because the rate of deformation may exceed the time scale of relaxation. Hence, the viscous dissipation would prevail over the elastic storage.

Note in Fig. 12 that in the case of syndiotactic PVA the slope of the curves is greatly dependent on the molecular weight of the polymer. Further, the inflection point is more clearly observed in the polymer with higher molecular weight, which suggests that syndiotactic PVA with higher molecular weight produces more preordered domains in the mesophase. However, syndiotactic PVA with low molecular weight may produce fewer preordered domains in the solution at identical polymer concentration probably because of weaker intermolecular interactions.

To compare the sensitivity of molecular orientation to shear rate, the Any of 2 g/dl solutions of atactic and syndiotactic (PVA)s in DMSO is plotted against shear rate in Figs. 13 and 14, respectively. To observe [Delta][n.sub.f] behavior of two polymers over a wider range of shear rates, we scaled the x-axis (shear rate) at the interval of 5 [sec.sup.-1]. Further, since our principal interest was to observe differences in [Delta][n.sub.f] behavior of atactic and syndiotactic (PVA)s in the high shear rate range, we left out data at shear rates lower than 5 [sec.sup.-1] in which two polymers exhibit similar behaviors in [Delta][n.sub.f]. In the shear rate range of 0 to 5 [sec.sup.-1], in fact, both atactic and syndiotactic (PVA)s show an abrupt increase in [Delta][n.sub.f] from zero to a positive value with increasing shear rate. On account of the abrupt increase of Any with shear rate at low shear rates, it seems in Figs. 13 and 14 that extrapolation of [Delta][n.sub.f] data to zero shear rate does not go to zero. In addition to an abrupt increase of [Delta][n.sub.f], scaling of x- and y- axes is also responsible for it. At shear rate zero, in reality, all the curves fall on the value of [Delta][n.sub.f] of zero.

A careful comparison of Figs. 13 and 14 reveals two things worth noting. Firstly, the [Delta][n.sub.f] of syndiotactic PVA solutions is increased with increasing shear rate, as shown in Fig. 14, while increasing shear rate brings about only a slight increase of [Delta][n.sub.f] in the case of atactic PVA solutions, as shown in Fig. 13. This indicates that in the case of atactic PVA, [Delta][n.sub.f] is almost saturated at shear rates lower than 5 [sec.sup.-1] at which a strong flow is developed, whereas [Delta][n.sub.f] of syndiotactic PVA is not completely saturated even at shear rates higher than 5 [sec.sup.-1]. This is ascribable to the fact that syndiotactic PVA is expected to form a mesophase having much longer relaxation times than atactic PVA. The calculated relaxation times of syndiotactic PVA are much longer than those of atactic PVA aforementioned. Second, for both atactic and syndiotactic (PVA)s, a polymer with a higher molecular weight produces a greater value of [Delta][n.sub.f]. The continuous increase in [Delta][n.sub.f] with increasing shear rate is not typical for ordinary flexible polymer solutions. These rheo-optical results by PMFB qualitatively agree with rheological results by RFS II. These facts suggest that a more rigid chain syndiotactic PVA develops a molecular orientation more easily by shear than atactic PVA. Taking heterogeneity resulting from mesophase formation suggested by rheological responses into consideration, all the aforementioned facts suggest that solutions of syndiotactic PVA in DMSO produce a mesophase containing preorganized internal structures that can be readily oriented even by low shear.

CONCLUSION

In PVA solutions in DMSO, the content of the syndiotactic diad of PVA had a fundamental effect on rheological and rheo-optical properties. The atactic PVA produced only an isotropic phase over the polymer concentration range. On the other hand, the syndiotactic PVA showed rheological responses typical of heterogeneous systems, which indicates that it formed a mesophase through internal ordering similar to liquid crystalline polymer systems. This implies that an order-disorder transition may exist at a critical syndiotactic diad content. The mesophase readily develops an oriented structure even by low shear, which lead to in-situ formation of fibrillar structures during saponifying syndiotactic PVPi, which had been experimentally observed. However, considering that the difference in syndiotactic content between atactic and syndiotactic (PVA)s observed is only [approximately]10%, a chemorheology tracing the real system seems to be necessary to elucidate reasons responsible for differences in rheological and rheo-optical responses.

REFERENCES

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Author:Lyoo, Won Seok; Kim, Byoung Chul; Ha, Wan Shik
Publication:Polymer Engineering and Science
Date:Jul 1, 1997
Words:3046
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