Dynamic mechanical analyses of SBS block copolymers containing tackifiers.

In previous work of this series (ref. 1), dynamic mechanical measurements were performed with styrene-butadienestyrene (SBS) block copolymers, Kraton D-1101 and D-1102. The isochronal data were obtained from -130 to 85 [degrees] C in the tensile mode at 1 Hz and from 60 to 160 [degrees] C in the shear mode at 1 rad/s. The isothermal measurements were also performed at 60, 90, 120, 140 and 160 [degrees] C in the frequency range of 0.0316 to 100 rad/s. The test specimens had been compression molded from the melts. The results suggested that the two polymers had quite different morphology, although the styrene content and the diblock content were about the same for both polymers. D-1101, which had 1.5 times higher molecular weight, had 3~5 times higher rubbery modulus, compared to D-1102.

The connectivity of the polystyrene domains (ref. 2) as expressed by the ratio of dynamic tensile modulus, E', to dynamic shear modulus, G', were about the same for both polymers.

The glass-rubber transition temperatures, [T.sub.g], of the rubber phase and those of the styrene domains were the same for both polymers and they occurred at about the same temperature of corresponding pure polymers.

The domain disruption temperature, [T.sub.dd], which was the beginning of the intermixing of two domains (ref 3), was observed with both polymers. However, [T.sub.dd] of D-1102 was lower than that of D-1101. Also, the magnitude of Tan [delta] at [T.sub.dd] was much higher for D-1102 than for D-1101.

In this work, two tackifiers are added to the above block-copolymers to examine their effects on rubbery modulus, connectivity of the styrene domains, [T.sub.g.]s of rubber and styrene phases, and [T.sub.dd]s. One of the tackifiers is chosen to have its [T.sub.g] above room temperature and the other below. Dynamic mechanical measurements were used as before.

Experimental

Samples and specimen preparation

Samples are SBS block copolymers, Kraton D-1101 and D-1102 (Shell Chemical), and tackifiers are W-95 and W-10 (Goodyear). They are aliphatic C-5 petroleum hydrocarbon resins. The characteristics of the copolymers and the tackifiers are listed in tables 1 and 2. The formulations are described in table 3.

Table 1 - gel permeation chromatograph data
of Kraton D-1101 and [D-1102.sup.a]

Sample  Styrene  Mw [x10.sup.5]  Mn [x10.sup.5]  Mw/Mn  Triblock
        content                                         /diblock

D-1101     31         1.51             1.35       1.12    86/14
D-1102     28         1.02             0.91       1.12    86/14

a - relative values based on polystyrene calibration
standards

Table 2 - [T.sub.g]s of tackifiers, W-95 and W-10

                          W-95            W-10

[T.sub.g'] [degrees] C    47.6           -36.9

Table 3 - formulation, weight percent

Polymer/tackifier  Blend ratio
D-1101/W 95        90/10, 80/20
D-1101/W 10        90/10, 80/20, 70/30, 50/50
D-1102/W 95        90/10, 80/20, 70/30
D-1102/W 10        90/10, 80/20, 70/30

The antioxidant was added as one part per hundred
weight of the mixture.

The ranges of concentration are limited, because further addition of W-95 makes the specimen too stiff and that of W-10 too flexible to handle.

Mixing was performed at 150~155 [degrees] C under a nitrogen purge in a 280-ml internal mixer with internal mixer type rotors at 20~50 rpm. The SBS copolymer was first mixed with an antioxidant, W-100. After about five minutes of mixing the tackifier was slowly added until completion. Mixing time was typically 20~30 minutes.

Test specimens were compression-molded at 150 [degrees] C for 20 minutes in a 40-ton compression molding press.

Test methods

Dynamic mechanical analyzer (DMA)

DMA, Du Pont 983, tensile mode with standard horizontal sample holder, was used at a heating rate of 3 [degrees] C/min from -130 to 85 [degrees] C at a frequency of 1 Hz and at an amplitude of 0.20 mm.

Mechanical spectrometer

Rheometrics model RMS-800 was used with 25 mm diameter parallel plates. A silicone gum was used to double-check the calibration of the instrument. Strain amplitude of 0.1 and 0.3% were used for D-1101 and D-1102, respectively.

The isochronal measurements were performed at 1 rad/s from 80 to 140 [degrees] C.

The isothermal measurements were performed at 60, 90, 120, 140 and 160 [degrees] C in the frequency range of 0.0316 to 100 rad/s. The measurements were made from the lowest frequency to the highest, and then reversed to the lowest frequency. The agreements of the lowest frequency data were examined to ascertain that the degradation of the material did not occur during the measurement. The measurements were duplicated with fresh specimens.

Results and discussion

Figures 1 and 2 are DMA data presented as the temperature dependence of tan [delta] for the block-copolymers and their mixtures with tackifiers. The peak temperatures, the [T.sub.g]s of polybutadiene blocks, are listed in table 4 as [T.sub.g1].

[TABULAR DATA OMITTED]

When the data are plotted for W-95 systems, 1/[T.sub.g1], is linear with the weight fraction of the tackifier, obeying Fox equation, (ref. 4) figure 3. However, for W-10 systems the Fox equation is obeyed only to 20% tackifier and at the higher concentration 1/[T.sub.g1], deviates significantly lower; ([T.sub.g1] deviates higher).

At 50% tackifier the [T.sub.g] of the mixture is even higher than that of the pure tackifier. This indicates that some polystyrene block is solubilized into the rubber-tackifier phase.

The tan [delta] in the rubbery region for W-10 systems increase with the concentration of the tackifier, figure 1. There are two effects: one is the dilution of the block copolymer, thereby decreasing the number of physical crosslinks. The other is the increased flexibility of the rubber phase caused by the presence of the liquid tackifier. On the other hand, the tan [delta] in the rubbery region for W-95 systems is hardly affected by the presence of the tackifier, figure 2. This is because the above said two effects are cancelling each other; whereas the tackifier causes the dilution of the polystyrene domains, it increases the stiffness of the rubbery phase because of the high [T.sib.g] of the W-95 tackifier.

Figures 4 to 7 are tan [delta] measured in shear for the higher temperature range. With D-1101 copolymer systems, both the glass transition temperature, [T.sub.g2] and the domain disruption temperature [T.sub.dd] are observed. However, with D-1102 systems only [T.sub.dd] can be clearly seen, although a presence of [T.sub.g2] is also implied from the shape of the curves. The values of [T.sub.g2] and [T.sub.dd], whenever observed, are listed in table 4.

The [T.sub.dd]s of D-1101 tackifier systems can not be clearly determined from the figures. However, they don't appear to be lowered significantly by the presence of the tackifiers. If we limit the concentration range to 80/20 mixture, the tackifier increases the magnitude of tan [delta] at [T.sub.dd] only slightly, figures 4 and 5. These facts imply that the tackifiers are not significantly promoting the solubilization of the butadiene and styrene blocks.

For D-1101/W-10 system at concentration higher than 70/30 the tackifier has a significant effect on increasing tan [delta] at [T.sub.dd], figure 5. This is because the tackifier is a low [T.sub.g] liquid. It may be promoting the solubilization of the two blocks also, as the tan [delta] curves are seen to shift towards the lower temperature in the range between [T.sub.g2] and [T.sub.dd].

Comparing figures 4 and 6, tan [delta] values at [T.sub.dd] of D-1102 system are much higher than those of D-1101 system. This was as interpreted in the previous article (ref 1) to mean that D-1102 has a larger amount of the mixed phase of butadiene and styrene block.

Comparing figures 6 and 7, the magnitudes of tan [delta] at [T.sub.dd] are about the same between W-95 and W-10 systems. However, tan [delta] curves move significantly to the lower temperature in figure 7 and not as much in figure 6, indicating that the tackifier, W-10, is more effective than W-95 in promoting the solubilization of the two blocks.

Figure 8 shows storage modulus, G' of D-1101/W-10 system as a function of temperature. The modulus does not decrease monotonously with the increase of temperature. It dips first and increases, going over the maximum. This type of the behavior was observed before with an SIS copolymer containing a tackifier. The dip disappeared when the polymer was annealed at the higher temperature (ref 5). Because the phenomenon occurs just, above [T.sub.g] of polystyrene, some polystyrene in the mixed phase may be separating out to increase the styrene-domain concentration. The phenomenon is observed with D-1101/W-95 also, but somewhat smaller dips. With D-1102/W-95 and D-1102/W-10, the dips are observed in some case but in general less noticeable, because [T.sub.g2] and [Tsub.dd] are very close to each other.

Examples of the isothermal measurements are shown in figure 9 for D-1102/W-95 and /W-10 systems with G' as a function of frequency. The large drops of the moduli and change of the shapes at [T.sub.g] and [T.sub.dd] are indicated. The shifting of the curves along with frequency axis and small adjustments in the modulus axis result in crude master curves of storage, G' and loss, G" modulus. They cover the rubbery modulus at the higher frequencies and the flow region at the lower frequencies. Small upturn at the lowest frequencies indicate the presence of domain morphology. However, the master curve is only semi-quantitative, because the time-temperature superposition does not work. The presentation is highly condensed in log-scale, thereby masking the non-superposibility. Because of the heterogenous nature of the systems and each phase having its own temperature-dependence, the time-temperature superposition is not expected. Similar observation is made with all four systems of the polymer-tackifier combination.

Some of the values of the ratio, E'/G'at 80 [degrees] C are shown in table 5. These are measures of the connectivity of polystyrene domains (ref. 2). The connectivity tends to decrease with the tackifier concentration as it may be expected. However, the values are somewhat irregular, implying that the connectivity may not be exactly reproducible morphology.

Table 5 - polystyrene domain connectivity, E'/G'

Polymer/tackifier   Blend ratio         E'/G'

D-1101                     100            62
W-95                     90/10            47
W-95                     80/20            13
W-10                     90/10            26
W-10                     80/20            28
W-10                     70/30            11

Component            Wt. % of polymer   E'/G'
D-1102                            100     78
W-95                            90/10     16
W-95                            80/20      9
W-10                            90/10     29
W-10                            80/20     38
W-10                            70/30     28

Conclusion

Dynamic mechanical analyses of the systems of SBS block copolymers D-1101 and D-1102, containing tackifiers revealed the following; the tackifier, W-95, having [T.sub.g] above room temperature does not affect [T.sub.g] of styrene domain; nor does it affect the styrene domain disruption with temperature, [T.sub.dd]. It does not mix with styrene block until two blocks begin to mix with each other at [T.sub.dd].

The tackifier, W-10, having [T.sub.g] below room temperature dissolves a part of styrene block below the styrene [T.sub.g]. The [T.sub.g] of styrene domain becomes somewhat lower at the high concentration, 50/50, of the tackifier. At above [T.sub.g] of the styrene block the tackifier promotes the solubilization of the styrene and butadiene blocks.

The tackifiers used for this study are aliphatic C-5 hydrocarbon resins. They are usually considered to have a limited compatibility with the butadiene block. However, only one [T.sub.g] was observed for the butadiene-tackifier phase, indicating the miscibility. This observation contradicts the conventional wisdom and customary explanation of these phenomena. It is hoped that this observation will be applied to practical problems of formulating advanced adhesives and lead to improved products in the marketplace.

References

[1.] N. Nakajima Rubber Chem Technol, 69, 73 (1996). [2.] G. Kraus, K.W. Rollmann and J.O. Gardner, J. Polymer Sci., Polym. Phys. Ed, 10, 2061 (1972). [3.] M.F. Tse. S.J. Adhesion Sci. Technol 3, 551 (1989). [4.] T.G. Fox, Bull. Am Phys. Soc. 1, 123 (1965). [5.] N. Nakajima, R. Babrowicz and E.R. Harrell, J. Appl. Polym. Sci, 44, 1437 (1992).