Online structure characterisation during deformation and fracture of polymers

To investigate the correlation between mechanical properties and structural changes in polymer materials a special tensile machine was built, which enables online x-ray scattering during deformation at elevated temperatures.

Deformation and fracture are pinpointed at a well-defined position in the middle of a waisted specimen, the local strain is measured optically.

The local structure is estimated by (scanning) SAXS and WAXS. To get sufficient intensity resp. rates the measurements were performed mainly at synchrotron radiation sources.

The application of the method should be demonstrated for the deformation of isotactic Polypropylene (iPP) at room temperature and at elevated temperature.

Mobile remote-controlled experimental set-up for online deformation and structure characterization by synchrotron x-ray scattering

CDF - Cord Distribution Function

The Cord distribution function (CDF) is generated by particle-ghost autocorrelation of gradient vectors (see figure above), it is used to evaluate SAXS pattern giving model-free information about characteristic correlation lengths within the specimen.

Positive values specify dimensions of homogeneous phases, negative values characterise distances between similar entities.

Deformation at room temperature

Orientation of crystallites and break-down of crystalline order, estimated by WAXS.

WAXS pattern during deformation of iPP at room temperature

During deformation the lamellae become oriented and fragmented, cavity-formation – first perpendicular to the stretching direction, finally in stretching direction.

SAXS patterns and surface plot of the CDF’s (lin scale and for two strains also log scale) at different strains. The stretching (fibre) direction is indicated in the surface plots by arrows.

Deformation at 130 °C

Orientation of crystallites with deformation during yielding

Deformation of compression moulded iPP at 130 °C: The lamellae become highly oriented.

The structure model

a. Lamellae with some tie-molecules

b. Elastic shear-deformation of lamellae under small load and reorientation with respect to the load

c. Fracture of lamellae into smaller blocks due to local stress concentration caused by tie molecules

d. Stretched aligned, but not re-crystallized chains between the bocks during stretching at higher temperatures

e. Some of the fibrillar arranged molecules crystallize, final stage in the case of hot stretched PP

f. Further dissolution of the blocks creating more extended chains at room temperature

g. Finally there are several strands of extended chains, not crystallized, with some amorphous regions in between, final stage in the case of cold stretched PP