The effect of pressure pulses on crystallization of isotactic polypropylene: a dilatometric study

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100 1000 10000 0.0 0.5 1.0  V (-) time (s)

THE EFFECT OF PRESSURE PULSES ON CRYSTALLIZATION OF i-PP:

A DILATOMETRIC STUDY

Susanna Formenti

1

_{, Andrea Brivio}

1

_{, Dario Cavallo}

2

_{, Francesco Briatico Vangosa}

1

1 _{PolyEngLab, Department CMIC, Politecnico di Milano}

2 _{Department of Chemistry and Industrial Chemistry, University of Genova}

Step-like pressure changes during polymer solidification are often encountered in melt processing (e.g. injection moulding). Similar to the effect of shear flow pulses, short-term pressurizations can accelerate crystallization process due to the formation of additional nucleation precursors.

We investigate effect of hydrostatic pressure pulses on the crystallization kinetics of isotactic polypropylene by means of high pressure dilatometry and in-situ X-ray diffraction to assess specific volume/cristallinity changes during isothermal

crystallization upon pressure pulses.

‘short-term’ pressure protocol

Experimental

Results

i-PP pellets Mw = 365 kg/mol Mw/Mn = 5.4 Tg ≈ - 10 °C

Confining fluid dilatometry

L. Zhang, M. van Drongelen, G.C. Alfonso, G.W.M. Peters, EPJ, 71(2015) 185-195 E.M. Troisi, S. Formenti, F. Briatico-Vangosa, D. Cavallo, G.W.M. Peters, EPJ, 85 (2016) 553-563 P. Zoller, Y.A. Fakhreddine, Thermochimica Acta, 238 (1994) 397-415 H. Ito, Y. Tsutsumi, K. Minagawa, J. Takimoto, K. Koyama, Colloid Polym Sci, 273 (1995) 811-815

References

measurement cell hydraulic circuit heating system bellows LVDT sample confining fluid

Material

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     0 0 v v t t v v space filling t_1/2=t(=0.5) 0 10000 20000 30000 40000 0.05 0.10 0.15 P=0 (no pulse) P=10 MPa P=20 MPa P=30 MPa P=40 MPa P=50 MPa P=60 MPa  V (cm 3 /g) time (s) 0 10 20 30 40 50 60 0.0 0.5 1.0 P_C=10 MPa P_C=30 MPa t 1/2 / t NO P ULS E 1/2 (-) P (MPa)

Assuming three–dimensional heterogeneous nucleation: relative nucleation density





 











nopulse Ppulse 1 2 nopulse Ppulse 1 2 n

t

N

t

Avrami theory

 





   3  1/2 1 exp 0.5 n t KNG t 0 10 20 30 40 50 60 1 10 dilatometry WAXD POM N P pul s e / N no pul s e (-) P (MPa) P=20 MPa No pulse P=40 MPa

Conclusions

0 20 40 60 80 100 120 140 160 180 200 _T m T_C T C , T m (°C) P (MPa)

• Enhancement of crystallization kinetics proportional to magnitude of pressure pulse • Pulse magnitude have no effect on final amount of crystallinity

• Acceleration is linked to increase of number of active nuclei after the short term pressurization, as confirmed by ex-situ optical microscopy observations

number of active nuclei increases

P increases

In-situ X-ray diffraction

nucleation separated from spherulite growth t pressurization << t crystallization pressurization WAXD detector barrel slit flow cell piston diamond window ceramic ring pressure transducer synchrotron radiation SAXS

vacuum chamber detector SAXS

4 3 2 r*_P pulse r*_P C G* P_C nucl

eation free energy,

 G cluster size, r P_C P_pulse G* P pulse 1 1 min P_C=10,30 MPa T_C=145,150° C 220° C 10<P<60 MPa time 1 2 3 4