dynamic mechanical behavior of continuous multilayer composites

Dynamic Mechanical Behavior of Continuous Multilayer Composites

BRYAN GREGORY, ANNE HILTNER, and ERIC BAER

Department of Macromolecular Science Case Western Reserve University

Cleveland, Ohio 441 06

and

J. IM

The Dow Chemical Company Materials Science and Engineered Products

Midland, Michigan 48640

The dispersion processes in multilayer laminate composites of styrene acrylonitrile (SAN) and polycarbonate (PC) were studied utilizing the torsion pendulum. A third damping peak with a log decrement intensity of approximately one was observed at a temperature intermediate to the damping peaks corresponding to the T,’s of the two constituent phases. Variations of numerous material and experimental parameters such as composition ratio, orientation, thermal history, thermal cycling, number of layers, and layer thickness, as well as overall changes in the composition of the phases had no effect on the observance of a third peak. Only the disruption of the continuous layer structure effectively eliminated this novel transition. The origin of this transition was explained by assuming appropriate temperature dependencies for the controlling viscoelastic parameters in such a continuous layer composite.

INTRODUCTION n this paper we describe and analyze the in- I ternal friction processes of coextruded multi-

layer laminate composites ( 1-7). Polycarbonate (PC) and polystyrene-acrylonitrile (SAN) which are known to be nearly immiscible (8) were used as the layer components. Extensive studies of transitional processes on both immiscible bi- phasic polymer blends and miscible systems have been previously reported (8-14). In addi- tion, block systems of varying morphologies have also been examined.

In the immiscible systems such as in PC/SAN blends, two separate loss peaks are always observed at temperatures around the Tg’s of the parent phases. Paul and coworkers (8) observed a shifting of loss peaks toward one another in blends of PC/SAN of various composition ratios. Geil and Kunori (9) have observed similar shifts in the Tg loss peaks of PS/PC blends. In addi- tion, they observed a composition independent transition at 130°C and attribute this peak to partial miscibility that led to intermixing of the two phases.

Similar observations in styrene-butadiene block systems indicating the shift of loss peaks has again been attributed to the intermixing of the parent phases with subsequent creation of a third phase (10-141. Kawai and coworkers have shown that by tapering of the parent chains in polyisoprene and polystyrene block systems, solubilization of the parent phase occurs producing a single dispersive peak with a small shoulder. In light of the previous studies that have demonstrated a shifting of the parent Tg’s toward one another, Kawai has approximated the limit of solubility by achieving a single dispersive peak (14). However, the classic example of a completely miscible blend over all composition ratios is the polystyrene and poly(2,2’ dimethyl-l,4-phenylene oxide) system (17, 18). Blends made from these components show only a single composition dependent Tg peak.

The objectives of this paper were to investigate the internal friction processes in structures with continuous layers. I t was felt that this type of novel composite structure would

568 POLYMER ENGINEERING AND SCIENCE, APRIL, 1987, Vol. 27, No. 8


provide a deeper insight into the effect of mor- phology or geometric configuration on the dispersion processes in two component systems. In particular, our goal was to clarify the under- standing of the origin of the third dispersion peak which had previously been observed in similar systems at intermediate temperatures between the Tg's of the respective blend components.

EXPERIMENTAL All of the composite materials that were in-

vestigated in this paper were prepared at the Dow Chemical Company. Most of this study was focused on continuous PC/SAN multilayer composites that covered a wide range of composition ratios. These materials were prepared in sheets of 49 and 193 alternating layers, and in all instances PC was at the outer layers. The controls were sheets of pure PC and SAN that had been prepared utilizing the same multilayer coextrusion process. No residual layer structure could be observed in these sheets. Composi- tional changes were accomplished by varying the layer thicknesses of the two phases relative to one another while nearly maintaining a constant overall sheet thickness.

Although the impetus of the study was to investigate the many layered composites only, it became necessary to extend our investiga- tions to systems of only three layers. Such three layer specimens were sandwiched together by compression molding.

The internal friction studies were performed on an inverse mounted pendulum from temperatures of 25 to 150°C at a frequency of approximately 1 Hz and a heating rate of approximately 0.5"C/min. Test samples were cut into rectangular strips 6.5 cm long, 0.6 cm wide and approximately 0.06 cm thick. In order to obtain functional width to thickness ratios individual layers were milled away using a shop lathe.

Measurement of internal friction in all of the multilayer composition ratios resulted in the observance of a new 'loss' peak ( T t ) . Figure 1 represents typical behavior of all the composites studied with the torsion pendulum at 1 Hz. Both the log decrement us. temperature and log modulus us. temperature are shown for the 49 layer 40/60 by volume PC/SAN composition. Loss peaks occur at 1 12 and 128°C. Also the log decrement intensity starts rising precipitously at 145°C. The peak at 112°C corresponds to the SAN Tg and the damping rise at 145°C corresponds to the PC Tg. The large, new intermediate peak at 128°C was of unknown origin.

In Fig. 2, the log decrement rises sharply for the SAN control at 105°C and similarly the log decrement for the PC control climbs steeply at 145°C. The shift of the SAN damping peak from 105°C in the control to a value of 112°C in the composite may be a consequence of partial miscibility at the interface. Paul (8) has suggested that such miscibility creates a shift in the Tg's

I 1 Id' . CI I=

E" g 1.0 i 0 n 6 0 -I

0.10 ' A

A A

A

0.01 90 110 130 150 50 7 0

Temperature PC] Fig. I . Log decrement and log modulus us. temperature

for the 49 layer 40/60 by volume PC/SAN composition ratio.

0.01 50 70 90 110 130 150

Temperature pc] Fig. 2. Log decrement and log modulus us. temperature of the PC and SAN controls for the 49 layer system.

of both phases toward one another. Superposi- tion of the two curves cannot give rise to the intermediate peak observed in the multilayer composite shown in Fig. 1.

Both controls as well as a number of compo- sitions in the 49 layer system were studied using a differential scanning calorimeter (DSC). The DSC traces for the composite materials yielded similar results, with discontinuities at the appropriate Tg positions of the constituent phases. However, no discontinuities were observed at intermediate temperatures, which contrasts with the torsion pendulum results. This particular experiment was performed numerous times at different DSC sensitivity ranges and with different PC/SAN composition ratios. Always the results were the same, only the Tg's of the PC and SAN were observed.

To determine the origin of the unknown peak, a number of experimental parameters were var- ied. In Table 1, peak intensities for the SAN Tg

POLYMER ENGINEERING AND SCIENCE, APRIL, 7987, Vol. 27, No. 8 569

B. Gregory, A. Hiltner, E. Baer, and J . lm

Table 1. Transition Temperatures and Intensities for a Wide Variety of Composition Ratios in Both the 49 and 193 Layer

Systems.

nealing had no effect on both peak intensities and the positions of damping temperature maxima. The results for the specimens annealed at

L c

E E 1.0: 0 8 0 6 0 -I

0.1 0

SAN Peak Intermediate Peak

(PC/SAN) Position Intensity Position Intensity Composition

:A

65/35 54/46 40160 27/73

77/23 66/34

40160 55/45

49 Layers 112.5 0.39 112.0 0.51 112.0 1.01 112.0 1.35

193 Layers 112.5 0.16 112.5 0.37 112.5 0.55 112.5 0.75

129.5 1 .oo 129.0 1.05 126.5 1 .oo 128.5 0.91

130.0 0.72 132.0 0.80 129.0 0.82 128.0 0.92

and Ti are shown for various composition ratios in both the 49 and 193 layer system. In the case of 49 layers, changes in the composition ratio had no effect on the positions of either peak. The intensities for the intermediate peak remained virtually constant over all composition ratios while, as expected, the intensities of the SAN peak increased with increasing volume percent of SAN.

Tests performed with approximately the same composition ratios in the 193 layer system yielded similar results as is also shown in Table 1. As before the temperature of the damping maxima remained constant. Also the intensity of the SAN Tg peak again increased with increasing volume percent of SAN. However, with the 193 layer system, the intermediate peak increased slightly with increasing volume percent of SAN. Note that the average intensity of the intermediate peaks is somewhat lower than that of the 49 layer system. Changes in the composition ratio, the number of layers, and the layer thickness had also virtually no effect on the position of the intermediate peak.

To test for possible orientation effects brought about by processing, samples from the same multilayer sheet and thus the same composition ratio were cut at different angles to the processing direction. In this series of experiments specimens were cut at angles of 0, 45, and 90" to the processing direction. Again no change was observed in the position or the intensities of the peaks.

Next, we examined whether TL was associated with residual stresses that may have resulted during the cooling stage of processing. Samples were cut from the same (77/23) multilayer sheet with identical widths and thicknesses. These specimens were annealed at 112, 130, and 137°C for 24 h. The temperatures were chosen because 112°C occurs slightly above the observed Tg of SAN while 130 and 137°C were above Ti. Temperatures above the Tg of PC were not chosen to avoid the possibility of disrupting the layered structure of the composite. The results for the specimen annealed for 24 h at 137°C is shown in Fig. 3. As can be seen, an-

the two lower temperatures exhibited similar results.

In a follow-up study, two different samples of the composition ratio 40/60 were again annealed at 137°C over a 24 h period in a vacuum oven. One of the specimens were annealed taut while the other specimen was annealed free of tension. Again no measurable effects were observed. In a further study, a number of samples were cycled through the heating and cooling phases of the experiment (1 6). Figure 4 shows data taken during the heating and cooling phases. Again, no changes were observed in peak intensities or positions for any of the loss peaks.

To obtain some insight into the origin of TL an experiment with a disrupted layered structure

Unannealed A Annealed A

0.01 90 110 130 150 50 70

Temperature ['c] Fig. 3. Log decrement and log modulus us. temperature for the 72/23 by volume PCISAN composition ratio annealed at 137°C for 24 h compared with an unannealed specimen.

I Id' ld" * * ***

].d

A A A A A

A

A A

Heating 0.01 i 90 110 Coaling 130 o 150

50 70

Temperature ['c] Fig. 4. Log decrement and log modulus us. temperature during both the heating and cooling phases of testing.



was performed at the Dow laboratories. A sam- ple of composition ratio 54/46 from one of the multilayer sheets was heated above the Tg’s of both phases, stirred up using a thin metal wire, and hot pressed into a thin sheet. In this manner the flat planar structure of the layers was disrupted. Surprisingly, Fig. 5 shows the intermediate peak has been eliminated. The data suggests that Ti results from the continuous nature of the multilayer composite structure.

Next, a simplification of the system was pur- sued by reducing to three the number of layers. The study of three layer specimens was initi- ated because earlier studies suggested that Tt was insensitive to the number of layers. Sub- sequently, a three layer specimen approximately 20 mils thick, and comprised of a layer of SAN sandwiched between two layers of PC was prepared by compression molding. The layer thickness ratio of this specimen was approximately 45/10/45. The results again showed a Ti maximum and the SAN Tg peak had disappeared since evidently all of the SAN was involved with the Tt process.

Other multilayer experiments were performed with different constituent polymers. A polycarbonate/polyethylene terephthalate specimen was investigated and as might have been expected an intermediate peak was observed. Similarly testing a polycarbonate/acrylonitrile butadiene-styrene system showed a Ti at 126°C. Hot pressing of these composites with disrupted layer structure again eliminated the Tt peak.

DISCUSSION During these internal friction studies a third

loss peak at 129°C was observed between the loss peaks corresponding to the Tg’s of the SAN and the PC phases. The peak was found to be insensitive to a number of experimental param-

3 -41 0 39 77 114 150 186

TEMPERATURE

Fig. 5. Log decrement and log modulus us. temperature for a 54/46 b y volume composition ratio that had lost flatness of layers.

eters that included: composition ratio, number of layers, orientation, annealing, and thermal cycling. Only the disruption of the planar structure of the layers effectively eliminated Ti. If the third peak arises from the two continuous phases, a mechanism which incorporates the viscoelastic nature of each of the phases might be operative. Nielson (1 5) has discussed the in- teraction of continuous phases in this manner and the existence of the intermediate peak in multilayer composites can be interpreted using his approach. The issue of continuity has previously been addressed by Takayanagi (1 9) who in his elementary composite models has clearly indicated that observable dispersion peaks of multiphase composites can only be observed in continuous structures that have parallel config- urations.

The damping characteristics of the composites can be reduced to the interactions of two material parameters, the viscosity (q) and the shear modulus (G) of the respective phases. The logarithmic decrement of any viscoelastic system is known to show the following dependency: Log. dec. cc q/G. Both q and G are temperature dependent and both will decrease in a structure dependent characteristic manner with increasing temperature.

At temperatures below the Tg of SAN both QSAN and Gcomp decrease with increasing temperature. A s the Tg of SAN is approached, Gcomp starts decreasing more rapidly than q s A N thus causing a significant rise in the log decrement. However, a maximum in the log decrement is reached at T g S A N . This is because the temperature dependency of the q s A N changes signifi- cantly at and above TgSAN. That is, q s A N now decreases more rapidly than Gcomp thus causing the maximum in the log decrement curve which indicates Tg has now been reached.

This mechanistic argument is now applied to explain the occurrence of Ti. As Ti is approached, the log decrement will rise again since, as is shown in Fig. 6, Gcomp is assumed to decrease more rapidly than 7 s ~ ~ . At Tt there again is an inversion of the temperature dependency of both variables beyond which q s A N

decreases more rapidly than Gcomp. At temperatures beyond Ti both ~ S A N and GSAN

become negligible. The modulus of the system can now be approximated by GPC while the viscosity is determined by qpc. The interactions of these two variables proceed with temperature dependencies analogous to the variables ~ S A N

and Gcomp that were described above. Accord- ingly the previous discussion can be extended to explain the maximum that occurs in the log decrement curve at Tgpc.

CONCLUSIONS 1 . A novel transition in layered composites

of PC and SAN was observed with a log decrement intensity of one at a temperature between Tg’s of the constituent phases.


B. Gregory, A. Hiltner, E. Baer, and J . Im

Tmmporature

Tomparatura

Ftg. 6. Schemattc dlagram for construction of tnterme- dtate relaxatton peak.

2. This transition was shown to be insensitive to changes in composition ratio, thermal history, thermal cycling, number of layers, orientation, and also to changes in the composition of the constituent phases.

3. The origin of the transition was explained by assuming relative temperature dependencies of the controlling viscoelastic parameters over the appropriate temperature regimes.

ACKNOWLEDGMENTS The authors would like to thank the Dow

Chemical Company for the generous financial support of this work. We would also like to thank John Rudd who prepared and tested some of the samples.

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dynamic mechanical behavior of continuous multilayer composites

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