EFFECT OF PHYSICAL CROSSLINKING OF PVC/DOP GEL ON SMALLAMPLITUDE OSCILLATORY SHEAR
M. Sugimoto*, T. Suzuki, S. K. Sukumaran, K. Koyama
Graduate School of Science and Engineering, Yamagata University, Japan – sugimoto@yz.yamagata-u.ac.jp
Abstract - Dynamic viscoelasticity of PVC/di-octyl phthalate (DOP) contained 20 wt% of PVC was measured forseveral PVCs which had different molecular weight at various temperatures. The equilibrium modulus, Ge, of various
PVC/DOP gels showed the temperature dependencies. However, at a constant relative distance from the gel point, thefrequency dependencies of Ge for the different molecular weight sample were almost identical. Assuming classicalrubber elasticity, these results suggest that the structure of the gel essentially does not depend on the molecular weightof PVC, but only on the relative distance.
Keywords: Physical crosslinking, PVC, Rheology

Introduction
Poly(vinyl chloride)/di-octyl phthalate (DOP) gels with
PVC content of 20 wt.% was prepared by a solvent
evaporation method. The dynamic viscoelsticity wasmeasured at various temperatures. The gel exhibited atypical sol–gel transition behavior with elevating
temperature. At a critical temperature the PVC gelshowed the sol-gel transition behavior: thecharacteristic power-law dependence of storage andloss modulus, G’ and G”, on the angular frequency ω,G '(ω ) = G"(ω ) / tan(nπ ) ∝ ω n(1)tan δ = tan(nπ / 2)

(2)

It is reported that PVC gel is classified as randomly
crosslinked networks of flexible PVC strands and thecrystal domains play an important role as crosslinking
junctions. The physical properties of PVC gels arestrongly influenced by the network structure. In fact,
the PVC gel shows a pronounced increase of transient
elongational viscosity over a critical strain underconstant strain rate below the critical sol-gel transition
temperature Tgel 1,2. This results from the network
structure and cannot be expected behavior for linear
polymers. When increasing temperature, the PVC gel
still shows the steep increase of the elongational
viscosity. Over Tgel it does not show, however, any
strain hardening. This change is attributed to thechange of network structure. Below Tgel, the gradual
melting of the PVC crystallites takes place withelevating temperature depending on the melting
temperature of the crystal domain. Above Tgel, a
densely connected network throughout the wholesystem disappears. The melting behavior of PVC
crystals worked as crosslinking junction in the physicalgel dominates the rheological properties. The
temperature, concentration of polymer, solvent andmolecular weight may affect the network structure,
thereby a change of the rheological behaviors. So far
the study on the crosslinking structure has not been
fully understood, however. Kakiuchi et al. 3 examinedthe equilibrium modulus Ge of PVC/ DOP gels interms of molecular weight and concentration of PVC.
They found that the equilibrium modulus normalized
by concentration, temperature and molecular weight
was universally dependent on the relative distancefrom the gelation point in terms of PVC concentration.
This suggests that the mesh size of crosslinked networkin the gels near gelation point is universally determinedby the relative distance and molecular weight.
In this study we investigated the gel structure ofPVC/DOP gels with constant polymer concentration.
The equilibrium modulus and the elongational
viscosity were analyzed in terms of the relative
distance from Tgel.
Experimental
PVC/DOP gels having 20 wt% of PVC was used. The
suspension polymerized PVC was supplied from
Mitsubishi Chemical Coorporation. Weight-averagemolecular weights are 61.9, 102 and 173 kg/mol anddenoted as PVC6, 10, 17, respectively. The molecularweight distributions are approximately 2 for all thesamples. Prior to the sample preparation, the PVC
sample was purified through precipitation from atetrahydrofuran(THF)/methanolmixture.
For
preparation of the PVC/DOP gel, prescribed amountsof the purified PVC samples and DOP were dissolved
in THF (70% in weight), and then THF was allowed tothoroughly evaporate from a shallow container (glassdish) at room temperature. During a long evaporation
period (>3 weeks), the PVC/DOP gel waswellstabilized/equilibrated. Thus, no detectable change
was observed for the rheological data obtained fromrepeated measurements with intervals of 1 and 2months. The reproducibility of the data reported in thispaper was confirmed in this way.
The dynamic oscillation measurements and uniaxial
elongational tests were carried out by rotationalrheometer (Ares, TA instruments).
Results and Discussion
The dynamic viscoelastic measurements of PVC6, 10,17 gels with DOP were carried out at varioustemperatures. We determined Tgel by tan δ vs T plots at
various frequencies. All curves converged on a certain

Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany)values at Tgel. At this temperature the relationships ofEq. 1 and 2 were valid. For the gels of various
molecular weight at various temperatures, G’ wasshown in Fig. 1. Here we defined the relative distancefrom the Tgel;

510-1

PVC17 ω=10 rads
PVC17 ω=1
PVC10 ω=10PVC10 ω=1
PVC6 ω=10PVC6 ω=14

α = (T − Tgel ) / Tgel

10

where c, R and Mx are concentration, gas constant andmolecular weight of crosslinking strand. We apply thisconcept to PVC/DOP gels. Fig. 2 shows theequilibrium modulus determined from 1 and 10 rads-1
plotted against α. According to Eq. 4 the molecularweight of crosslinking strand of our gel system can bedemonstrated by cRT/Ge as shown in Fig. 3. Thissuggests that the molecular weight of crosslinking
junction which is effective for elasticity of gel
decreases with increasing α and depends only on αirrespective of M of PVC. We should note that cRT/Geis smaller than Mw of PVC in some cases. For example
at α = 0.1, 2-5 chains for PVC6, 10, 17 systems at leastassociate each other and form independent network
strand which can sustain elasticity of the gel.
5100.010.1
α=|T-T |/Tgel

410310PVC17 ω=10210

PVC17 ω=1PVC10 ω=10PVC10 ω=1PVC6 ω=10PVC6 ω=11100.010.1
α=|T-T |/T
gel

4α=0.25

10G' / Pa

α=0.15α=0.1
310α=0.25
PVC17PVC10PVC6α=0.15

α=0.1

PVC17PVC10PVC6PVC17PVC10
PVC6210-1

10011010ω / rad s

210-1

Figure 1 Storage modulus G’ of PVC-6, 10, 17 plottedagainst angular frequency at the relative distance fromthe Tgel, α = (T − Tgel ) / Tgel .

1gelFigure 2 Equilibrium modulus Ge of PVC-6, 10, 17
plotted against relative distance from Tgel at angularfrequency of 1 and 10 rads-1.

-1

(4)

210-1

Ge = cRT / M x

310cRTG / kg mol

At the same α (0.25, 0.15, 0.1) G’ of various molecularweight showed frequency-independency and thosewere similar values. As decreasing α (approaching toTgel) G’ decreased. G’ at α = 0.05 scatter of the data
was observed. It can be said that this is due to theexperimental error since the physical crosslinking
junction became temperature sensitive by thefluctuation at the temperature close to Tgel.
The equilibrium modulus Ge of chemicallycrosslinked network polymer can be described asfollows;

Ge / Pa(3)1gel

Figure 3 Molecular weight between crosslinking
points of PVC-6, 10, 17 plotted against relative
distance from Tgel at angular frequency of 1 and 10
rads-1.
Conclusions
We studied the rheological behavior of PVC/DOPgels having same concentration but different molecularweight. G’ of the PVC gels shows similar G’ ascomparing at same relative distance from Tgel. When
we compared Ge as a function of α, Ge increased withan increase with α. The molecular weight of gel
network strand based on Eq.4 decreased withincreasing α. This suggests that gel network strand canbe formed by at least 2-5 molecular chains associateeach other to sustain elasticity of the physical gel.

Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany)References
1. Aoki, Y.; Hirayama, K.; Kikuchi, K.; Sugimoto,M.; Koyama, K., Rheol. Acta 49, 1071 2010.
2. Sugimoto, M.; Hida, H.; Taniguchi, T.; Koyama,K.; Aoki, Y., Rheol. Acta 46, 957 2007.
3. Kakiuchi, M.; Aoki, Y.; Watanabe, H.; Osaki, K.,
Macromolecules 34, 2987 2001.

Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany)