In situ AFM study of near‐surface crystallization in PET and PEN

Abstract

The surface crystallization behavior of poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN) spincoated thin films was compared by means of atomic force microscopy (AFM) with an in situ heating stage. As the films were heated up stepwise, characteristic surface crystals appeared at a crystallization temperature (T c ) in the near-surface region which is about 15 8C under the bulk T c , and were replaced by bulk crystals when the temperature was increased to the bulk T c . In the case of films whose thickness is less than 70 nm (PET) and 60 nm (PEN), significant increases in the bulk T c were observed. Scanning force microscopy (SFM) force-distance curve measurements showed that the glass transition temperature (T g ) of the near-surface region of PET and PEN were 22.0 and 26.6 8C below their bulk T g (obtained by DSC). After the onset of surface crystallization, edge-on and flat-on crystals appeared at the free surface of PET and PEN thin films, whose morphologies are very different to those of the bulk crystals. Although the same general behavior was observed for both polyesters, there are significant differences both the influence of the surface and substrate on the transition temperatures, and in morphology of the surface crystals. These phenomena are discussed in terms of the differences in the mobility of polymer chains near the surface.

Synthesis of statistical PET/PEN random block copolymers and their crystallizability in the bulk and at the surface

“…As a result, the observed crystallization temperature at the polymer surface ( T cS ) is decreased, resulting in preferential crystallization at the surface under suitable process conditions. As we reported previously, the surface molecules of PET and PEN films crystallize at lower temperatures than the bulk, exhibiting characteristic lamellar crystalline morphologies with, for films this thick, typically an edge‐on morphology.…”

“…Table shows that, for all compositions, the surface crystallization temperature T cS , determined by AFM observation of relatively thick (436–680 nm) copolymer films, is about 60–90 °C below of the bulk crystallization temperature T cB measured by DSC, meaning that the enhanced local molecular mobility at the surface is observed in PET/PEN copolymers, showing the same trend of crystallizability with composition. The heating rates to induce the cold crystallization in this study were different between the DSC and AFM measurements, however, previous studies on various polymers, including our previous articles on PET and PEN, have shown that the T gS is lower than the T gB , and that there are distinct morphological differences observed at the onset of surface crystallization, and the subsequent crystallization that occurs at T cB , hence it is reasonable to infer that the crystallization temperature differences between T cB by DSC and T cS by AFM in these copolymer materials are also as a result of the depressed T gS .…”

Annealing for the improvement of the capabilities of parylene C as electret

“…This phenomenon occurs in a temperature range not exceeding 15°C before the bulk crystallization and it is exacerbated in the case of relatively thick films that exceed a few 100 nm. Indeed, thick films prevent the influence of the substrate, which restricts the molecular mobility in the bulk layer of the polymer at the bottom interface . This will contribute to the increase of the average surface roughness as can be seen in Figure .…”

Crystalline morphologies at the surface of PET/PEN random copolymer films

“…A full description of this method is given in Shinotsuka and Assender. [24] 2.5 | Force-distance curve analysis…”

“…The force-distance behavior from AFM curves has been used by multiple groups for the determination of a change in properties, associated with T g . [18,[24][25][26][27][28] For measurements reported here, the T g at the near surface was determined from the observed change in the snap-off displacement vs temperature. [18,24,25] The snap-off displacement at each temperature was taken as the mean of 30 measurements from different locations on the sample.…”