Effects of Chain Ends and Chain Entanglement on the Glass Transition Temperature of Polymer Thin Films

Abstract

Monodispersed polystyrene with different molecular weights, M w, from below the entanglement molecular weight, M e (∼20K Da), to well above was studied for effects of chain ends and chain entanglement on the glass transition temperature, T g, of polymer thin films. The relative importance between these two effects is expected to reverse as M w crosses over M e. We found that the T g generally decreases with decreasing film thickness, t, consistent with previous findings. The difference between results from the M w = 13.7K and the 550K samples was negligible, suggesting that entanglement has little significance on the thickness dependence of the T g of polymer under confinement in thin films. On the other hand, the M w dependence of T g was markedly weakened compared to that of the bulk when t was reduced to 15 nm. According to Fox and Flory's theory, our result reveals that some of the chain ends in the film have been segregated to the surface, but they have little effect on the T g of the film. Our findings undermine a surface mobile layer where the surface chain ends localize being the cause for the reduction of T g observed.

Dramatic Reduction of the Effect of Nanoconfinement on the Glass Transition of Polymer Films via Addition of Small-Molecule Diluent

“…1(a). The results obtained in this study for the 5:0 kg=mol PS system are consistent with those of Tsui and Zhang [23] who studied PS films of M w 13:7 and 550 kg=mol and mixtures thereof and found that the nanoconfinement effect is independent of molecular weight for supported PS films. [A very different system, poly(methyl methacrylate) on SiO x substrates treated with hexamethyl disilazane, has reportedly shown [24] suppression of T g -nanoconfinement effects for molecular weights of 5 and 12 kg=mol; further study of the different effects of molecular weight in this system is warranted.…”

Glass Transition Temperature and Isothermal Physical Aging of PMMA Thin Films Incorporated with POSS

“…박막의 두께가 감소함에 따라 무정형 고분자 물질 에서 중요한 유리전이온도(T g )가 변화하는 것으로 알려져 있 으며, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] 이와 관련된 연구가 freestanding 상태 혹은 기질 (substrate)에 코팅된 상태(supported film)에서 spectroscopic ellipsometry, 1-7 X-ray reflectivity, 8 Brillouin scattering, 9 differential scanning calorimetry 10,11 등을 사용하여 지속적으로 진행되고 있다. Freestanding polystyrene (PS, M w =767000) 박막의 경우 두께 변화에 따른 T g 를 측정한 결과 박막의 두께 가 약 70 nm 이하에서 T g 가 감소하기 시작하여 약 30 nm에 서는 70 o C의 T g 감소를 보였고, 2,6 freestanding PMMA 박막 의 경우도 100 nm 이하의 두께에서 T g 가 감소하기 시작하여 약 30 nm 두께에서 20 o C 정도 T g 가 감소한다는 보고가 있다.…”

Anomalous Dynamics of Polymer Films

“…Later, other supported polymer films were also found to demonstrate an increase in T g with decreasing film thickness, such as poly(2-vinyl pyridine) on acidcleaned silicon oxide. 20 A layered model is one of the earliest models proposed to explain these observations, [18][19][20][21][22][23][24][25] and by far the most accepted for the thickness dependence of the T g of polymer films. The model presupposes that the molecular motions near the polymer-air interface are much faster than those in the bulk polymer, which can be due to segregation of chain ends to the surface 24,26 or a reduction in the chain entanglement near the polymer free surface.…”

“…20 A layered model is one of the earliest models proposed to explain these observations, [18][19][20][21][22][23][24][25] and by far the most accepted for the thickness dependence of the T g of polymer films. The model presupposes that the molecular motions near the polymer-air interface are much faster than those in the bulk polymer, which can be due to segregation of chain ends to the surface 24,26 or a reduction in the chain entanglement near the polymer free surface. 24,27 On the other hand, the molecular motions at the polymer-substrate interface can be faster or slower than the polymer motions in bulk depending on whether the polymer-substrate interactions are sufficiently weak or strong, respectively.…”