Surface roughness of supercooled polymer melts.

Area of Science:

• Materials Science
• Polymer Physics
• Surface Science

Background:

• Understanding the surface properties of supercooled polymers is crucial for predicting their behavior.
• The relationship between temperature, surface roughness, and surface tension in polymers requires further investigation.

Purpose of the Study:

• To investigate the surface roughness of supercooled glass-forming polymers using in situ x-ray reflectivity.
• To determine if surface roughness is affected by temperature and the glass transition.
• To explore the potential of x-ray reflectivity for measuring surface tension in viscous liquids.

Main Methods:

• In situ x-ray reflectivity measurements were performed on supercooled glass-forming polymers.
• Experiments covered a temperature range from 190 K to 330 K.
• Data analysis utilized the capillary wave model and viscoelastic theory.

Main Results:

• Experimentally determined root-mean-square (rms) roughness adhered to the capillary wave model for a liquid/vapor interface throughout the temperature range.
• An expression for surface roughness below the bulk glass transition temperature (T(G) ≈ 200 K) was derived from viscoelastic theory.
• No evidence of "frozen-in" surface roughness was observed experimentally.

Conclusions:

• The surface roughness of supercooled polymers is well-described by the capillary wave model, even below the glass transition.
• Surface fluctuations do not freeze in upon cooling below T(G).
• X-ray reflectivity provides a precise method for determining the surface tension of highly viscous liquids in the supercooled state.