^{1}, Zhen Wang

^{1}, Fengmei Su

^{1}, Weiqing Zhou

^{1}, Nan Tian

^{1}, Xiangyang Li

^{1}and Liangbin Li

^{1,a)}

### Abstract

The influence of nonquiescent relaxation of isotactic polypropylene on flow-induced crystallization (FIC) is investigated by a combination of particle tracking velocimeter (PTV) with a cone-plate shearing geometry and synchrotron radiation microbeam wide-angle X-ray diffraction (SR-μWAXD), which is aimed to correlate real flow profile and distribution of crystal orientation. With PTV technique, we observed that flow remains homogeneous during shear, while postshear movement and delayed fracture take place after a step strain when large shear rates and strains were imposed. Delayed fracture slices samples into several layers which move either forward or backward after the cessation of shear imposed externally. SR-μWAXD measurements reveal that the layers moving forward keep high crystal orientations while the layers moving backward show low orientations, which gives an inhomogeneous distribution of crystal orientation across the thickness of sheared samples. The correlation between moving direction and crystal orientation indicates that delayed fracture stems from the interplay between inertia and elastic retraction. The nonquiescent and inhomogeneous chain relaxation due to delayed fracture affects FIC nonuniformly and introduces complication to correlate crystallization behavior with apparent flow parameters.

This work was supported by the National Natural Science Foundation of China (51033004, 51120135002, 51227801), the Fund for One Hundred Talent Scientist of CAS, 973 program of MOST (2010CB934504). The research was also in part supported by “the Fundamental Research Funds for the Central Universities.” The authors would like to thank Professor Shi-Qing Wang (Akron) and Professor Gerrit Peters (TUE) for fruitful discussions. SR-μWAXD experiment is carried out in Shanghai Synchrotron Radiation Facility (SSRF).

I. INTRODUCTION

II. EXPERIMENTAL SECTION

A. Materials

B. Measurements

C. Experimental details

III. RESULTS

IV. DISCUSSION

V. CONCLUSION

### Key Topics

- Crystallization
- 27.0
- Crystal orientation
- 22.0
- Polymers
- 19.0
- Elasticity
- 15.0
- Fracture mechanics
- 14.0

##### B01D9/00

## Figures

Schematic drawings of (a) the cone-plate rheometer with PTV capability and (b) the sliced sample for SR-μWAXD measurement.

Schematic drawings of (a) the cone-plate rheometer with PTV capability and (b) the sliced sample for SR-μWAXD measurement.

A graphical representation of the testing protocol: (a) The iPP sample with tracer particles was heated up to 220 °C with a rate of 8.5 °C/min. (b) The sample was kept in 220 °C for 20 min to eliminate thermal history. (c) The sample was cooled to 144 °C with a rate of 4.5 °C/min and (d) subjected to shear experiment at a given shear rate and strain. (e) The isothermal crystallization of the sheared sample was kept in 144 °C for an hour and then (f) was cooled to room temperature naturally.

A graphical representation of the testing protocol: (a) The iPP sample with tracer particles was heated up to 220 °C with a rate of 8.5 °C/min. (b) The sample was kept in 220 °C for 20 min to eliminate thermal history. (c) The sample was cooled to 144 °C with a rate of 4.5 °C/min and (d) subjected to shear experiment at a given shear rate and strain. (e) The isothermal crystallization of the sheared sample was kept in 144 °C for an hour and then (f) was cooled to room temperature naturally.

(a) and (b) Stress-strain curves, (c) and (d) velocity profiles of tracer particles during shear, (e) and (f) displacement of tracer particles after shear cessation of supercooled melts. The samples were sheared at a fixed apparent strain of with different strain rate in (a), (c), and (e), and fixed apparent strain rate of with different strains in (b), (d), and (f).

(a) and (b) Stress-strain curves, (c) and (d) velocity profiles of tracer particles during shear, (e) and (f) displacement of tracer particles after shear cessation of supercooled melts. The samples were sheared at a fixed apparent strain of with different strain rate in (a), (c), and (e), and fixed apparent strain rate of with different strains in (b), (d), and (f).

The average orientation of iPP crystal measured with 2D SAXS in samples sheared with (a) a constant strain of 10 but different strain rates and (b) a constant strain rate of 20.6 s−1 but different strain.

The average orientation of iPP crystal measured with 2D SAXS in samples sheared with (a) a constant strain of 10 but different strain rates and (b) a constant strain rate of 20.6 s−1 but different strain.

At a strain rate of 20.6 s−1 and a strain of 0.5. (a) Representative 2D SR-μWAXD patterns of iPP at different locations across the shearing gap (the numbers in the top right corner of the patterns are the distances from the bottom plate) (b) The peak positions of (040) diffraction along azimuthal angle.

At a strain rate of 20.6 s−1 and a strain of 0.5. (a) Representative 2D SR-μWAXD patterns of iPP at different locations across the shearing gap (the numbers in the top right corner of the patterns are the distances from the bottom plate) (b) The peak positions of (040) diffraction along azimuthal angle.

(a) Representative 2D SR-μWAXD patterns of iPP at different locations across the shearing gap (the numbers in the top right corner of the patterns are the distances from the bottom plate) at a strain rate of 20.6 s−1 and a strain of 10.7. (b) Distributions of crystal orientation across the shearing gap in blue dotted-line. The displacement of tracer particles after shear is also plotted for the convenience of correlation (ΔX red open square-line).

(a) Representative 2D SR-μWAXD patterns of iPP at different locations across the shearing gap (the numbers in the top right corner of the patterns are the distances from the bottom plate) at a strain rate of 20.6 s−1 and a strain of 10.7. (b) Distributions of crystal orientation across the shearing gap in blue dotted-line. The displacement of tracer particles after shear is also plotted for the convenience of correlation (ΔX red open square-line).

(a) Representative 2D SR-μWAXD patterns of iPP at different locations across the shearing gap (the numbers in the top right corner of the patterns are the distances from the bottom plate) at a strain rate of 20.6 s−1 and a strain of 2.7. (b) Distributions of crystal orientation across the shearing gap in blue dotted-line. The displacement of tracer particles after shear is also plotted for the convenience of correlation (ΔX red open square-line).

(a) Representative 2D SR-μWAXD patterns of iPP at different locations across the shearing gap (the numbers in the top right corner of the patterns are the distances from the bottom plate) at a strain rate of 20.6 s−1 and a strain of 2.7. (b) Distributions of crystal orientation across the shearing gap in blue dotted-line. The displacement of tracer particles after shear is also plotted for the convenience of correlation (ΔX red open square-line).

Representative 2D SR-μWAXD patterns of iPP at different locations across the shearing gap (the numbers in the top right corner of the patterns are the distances from the bottom plate) at a strain rate of 20.6 s−1 and a strain of 27.7 from samples with (a) and without (b) tracer particles, respectively. (c) Distributions of crystal orientation across the shearing gap in blue lines present samples with and without tracer particles, respectively. The displacement of tracer particles after shear is also plotted for the convenience of correlation (ΔX red open square-line).

Representative 2D SR-μWAXD patterns of iPP at different locations across the shearing gap (the numbers in the top right corner of the patterns are the distances from the bottom plate) at a strain rate of 20.6 s−1 and a strain of 27.7 from samples with (a) and without (b) tracer particles, respectively. (c) Distributions of crystal orientation across the shearing gap in blue lines present samples with and without tracer particles, respectively. The displacement of tracer particles after shear is also plotted for the convenience of correlation (ΔX red open square-line).

(a) Representative 2D SR-μWAXD patterns of iPP at different locations across the shearing gap (the numbers in the top right corner of the patterns are the distances from the bottom plate) at a strain rate of 15.4 s−1 and a strain of 10. (b) Distributions of crystal orientation across the shearing gap in blue dotted-line. The displacement of tracer particles after shear is also plotted for the convenience of correlation (ΔX red open square-line).

(a) Representative 2D SR-μWAXD patterns of iPP at different locations across the shearing gap (the numbers in the top right corner of the patterns are the distances from the bottom plate) at a strain rate of 15.4 s−1 and a strain of 10. (b) Distributions of crystal orientation across the shearing gap in blue dotted-line. The displacement of tracer particles after shear is also plotted for the convenience of correlation (ΔX red open square-line).

A schematic molecular illustration on shear induced crystallization. (a) The displacement of tracer particles after shear with an image of PTV. (b) During the shear, the molecular chains with a relative high orientation. (c) After shear cessation, the residual flow helps molecular chains keep their orientation in the upper one while the backflow makes a strong retraction in the bottom one.

A schematic molecular illustration on shear induced crystallization. (a) The displacement of tracer particles after shear with an image of PTV. (b) During the shear, the molecular chains with a relative high orientation. (c) After shear cessation, the residual flow helps molecular chains keep their orientation in the upper one while the backflow makes a strong retraction in the bottom one.

## Tables

Apparent and real rheological parameters.

Apparent and real rheological parameters.

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