(722u) Stress-Induced Crystallization of Poly(Trimethylene Terephthalate) Fibers by Molecular Dynamic Simulations

We have used atomistic molecular dynamic simulations to study isothermal crystallization of poly(trimethylene terephthalate) (PTT) at various temperatures and various degree of external stresses. While extensive experimental and theoretical efforts are made to verify and explain the molecular mechanism in the growth of nuclei in polymeric materials, the primary stage of polymer crystallization is less well understood. Our atomistic reveal the formation and growth of highly oriented but loosely packed clusters (nucleus precursors) upon quenching a melted semi-rigid polymer. This work we force on the stress-induced crystallization phenomena which happened on the industrial process of the fiber spinning. While such a process has been used to increase crystallinity of polymer fibers since 1950s, the molecular motion of the polymer and the mechanism of the crystallization are still unclear.

Our simulations show that the structural order of polymer is affected both by the drawing speed and the crystallization temperature. At all temperature, oriented structure can be created, and the size of oriented domain increases upon drawing. Beyond about a strain of 300% large voids and highly oriented structures are observed. Furthermore, the size of the oriented domains decreases with the drawing speeds. The amount of the oriented structure is a function of temperature at lower drawing speed (3.2 m/s). However, temperature effects are less prominent at high drawing speeds (> 32 m/s). Detailed analysis also performed on the two feature torsions (Φ₁ COCC and Φ₂ OCCC) in the PTT backbone (O-CH2-CH2-CH2-O). It is found that the trans-to-gauche ratio in Φ₁ increases rapidly (from 3 to 6) at lower drawing speed and higher temperatures; however, the increase is less significant at high drawing speed and low temperatures. In contrast, the trans-to-gauche ratio for Φ₂ seems to be insensitive to processing conditions (varies from 0.6 to 1.3 regardless of drawing speeds). Before drawing, the most populated conformations in the backbone torsions are t-t-t-t (Φ₁-Φ₂-Φ₂-Φ₁) and/or t-g-g-t. During draw process, the t-t-t-t conformation increases. Our results are in agreement with some experimental observations, and disagree with others.