Effect of sorbitol nucleator on
nonlinear visco-elasticity in polypropylene
H. Uematsu, M. Sugimoto, T. Taniguchi and K. Koyama
Department of Polymer Science and Engineering Yamagata University

Abstract
We investigated effects of the network of sorbitol nucleator on non-linear visco-elasticity of thepolypropylene (PP). We used Bis(p-methylbenzylidene)sorbitol (MDBS) as a nucleator in this study. We
blended MDBS of 0.5wt% (MDBS0.5) and PP at melting state (210 °C ). From the data of linear
visco-elasticity, it was found that the blend sample (PP/MDBS0.5) exhibited a physical gel behavior at
170 °C . On the other hand, the PP/MDBS0.5 did not show a physical gel behavior at 210 °C . Theseresults mean that the network structure of MDBS depends on temperature. In order to clarify the role ofthe network structure, we investigated a nonlinear visco-elasticity at 170 °C . First, we carried out steadyshear viscosity measurements. In the measurements, we observed stress overshoots and found in the rangeof strain rate less than 0.1 s-1 that the stress at the steady state does not depend on strain rate. Secondly, weperformed measurements of uniaxial elongational viscosity in the blend sample. The blend PP/MDBS0.5
showed a non-linearity in viscosity at a lower strain rate less than 0.1 s-1. From the relation between stressand strain rate of PP/MDBS0.5 we estimated a critical stress where the network was broken.

Introduction
Nucleator agent has been effectively used to enhance the crystallization of crystalline polymer. Amongothers sorbitol nucleator is usually used in polypropylene (PP) as a nucleation agent. It is reported1) thatcrystallization temperature (Tc) of PP and crystallization speed are increased by addition of sorbitol
nucleator. However the effect of sorbitol nucleator is not only an enhancement of crystallization but also achange of linear visco-elasticity due to a network formed by sorbitol nucleator in molten polymer.2) We
focus on network structures formed by sorbitol nucleators and study the effect of sorbitol nucleator on
non-linear visco-elasticity in PP.

Experimental-MaterialsAs a nucleator we selected Bis(p-methylbenzyliden)sorbitol(MDBS) provided by Shinnihonrika Corporation. Fig.1 shows
the chemical structure of MDBS. The matrix polymer is isotactic
polypropylene provided by Chisso petrochemical Corporation.
-Coordination of sampleWe mixed MDBS(0.5wt%) and PP by a mixing machine(Toyoseiki corp.). Mixing temperature is 210oC, Screws
revolution speed is 50rpm and mixing time is 5 minutes. Test
specimens for dynamic visco-elasticity were shaped by a
hot press (Toyoseiki corp.) at 230oC. Hereafter the blend
sample is referred as PPLMD05.
-AnalysisBoth dynamic visco-elasticity and uniaxial elongational
viscosity measurement were performed using ARES (TAinstruments). History of temperature is shown in Figure 2.

Figure 1 Chemical Structure of Sorbitol
nucleator used in this study

Figure 2

History of temperature used in this study

Results and Discussion-Linear visco-elasticityFig.3 shows linear visco-elasticities of the blend sample andneat-PP. This experiment was performed after the history oftemperature shown in Fig.2. The result that a gel behavior isappeared indicates that network structures are formed at170 °C .

-Uniaxial elongational viscosityWe measured uniaxial elongational viscosities after the historyof temperature shown in Fig.2. In Fig.4, solid line anddotted-line stand for 3η*(t)’s that are evaluated by using dataof dynamic visco-elasticity of PPLMD05 and neat-PP,respectively. The result shows that the elongational viscosity ofPPLMD05 is in good agreement with 3η∗(t) of neat-PP in ahigher strain rate than 0.5 s -1, but not in agreement with 3η*(t)of neat-PP in a lower strain rate than 0.1 s -1. These results imply
that there is a critical stress where the behaviors of elongationalviscosity can be divided into two cases.

Figure 3 Storage modulus(G') and loss
modulus(G") plotted as a function ofangular frequency, ω .

-Critical stressTo investigate the effect of network formed by sorbitol
nucleator on elongational viscosity, we focused on arelationship between strain rate and maximum stress that is theelongational stress just before the break of the sample inuniaxial elongational flow. On the assumption that maximum
stress indicates a break point of network structures in uniaxial
elongational flow, we plotted the maximum stresses as afunction of strain rate in Fig.5. When the strain rate is fasterthan 0.5s -1, the maximum stress of PPLMD05 agrees to that ofneat-PP. On the other hand, when the strain rate is lower than
0.1s -1, the maximum stresses of PPLMD05 are almost constant.
At the elongational stress higher than about 104Pa, a network
structure of MDBS is broken and elongational stress of
PPLMD05 approaches to that of neat-PP. On the other hand,when the elongational stress is lower than about 104Pa, anetwork structure of MDBS is not broken and elongational
stress of PPLMD05 is increased by elongated networks formedby sorbitol nucleator.

Figure 4
Uniaxial elongational viscosity
plotted as a function of time.

Figure 5 The maximum of stressplotted as functions of strain rate.

Conclusion
From the measurement of non-linear visco-elasticity, we found that non-linearity in viscosity depends onthe critical stress ( ≈ 104Pa) at which the network formed by sorbitol nucleator is broken.

Reference1) R-D. Mair, C. Wang, R. Yhomann, C. Friedrich, R. Mulhaupt. “Self Organization and Nucleation ofI-PP by Sugar Acetals” Proceedings of annual Meeting Polymer Processing Society PPS 16 inNetherlands, 20002) S. Yamasaki, Y. Ohashi, H. Tsutsumi, and K. Tsujii. “The Aggregated Higher-Structure of1,3:2,4-Di-o-benzylidene-D-sorbitol in Organic Gels”, Bull. Chem. Soc. Jpn., 68, 146-151 (1995)