Molecular Weight Dependence of Fragility in Polymers
Molecular Weight Dependence of Fragility in Polymers
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DOI：
10.1021/ma900666z
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发表时间：
2009-09-08
期刊：
影响因子：
5.5
通讯作者：
Roessler, E. A.
中科院分区：
文献类型：
--
作者：
Abou Elfadl, A.;Herrmann, A.;Roessler, E. A.
In a recent contribution we reinvestigated the molecular weight (M in g/mol) dependence of the glass transition temperature Tg for a series of polystyrenes (PS), polydimethylsiloxanes (PDMS), and polybutadienes (PB), as determined from dielectric spectroscopy. 1 A broad range of M was studied including the low M limit. The data have been found to be compatible with assuming a discontinuous change of Tg (M), with two kinks at M1 and M2. Tentatively, we have argued that M1 signals the crossover from simple liquid to Rouse dynamics, and M2 signals the onset of entanglement dynamics. Yet, the statement has been afterward challenged. 2 Discontinuous behavior has also been found when analyzing fragility m (M) as well as T0 (M) from the Vogel-Fulcher-Tammann (VFT) equation (cf. eq 1). In agreement with other reports, 3, 4 m (M) follows Tg (M) for polymers.Here we will show that, although Tg as well as m may change significantly with M, the ratio m/Tg= F is constant above M1 for a series of the same polymer. Thus, fragility m appears not to be an appropriate quantity to be discussed for polymers at least. Given this, we will demonstrate that above M1 the segmental correlation time can be expressed by an M independent function τR= f (T-Tg). We mention that this scaling has been tested before and may be motivated within free volume ideas. 3, 5, 6 The quantity F is the magnitude of the slope of lg τR (T) at Tg, that is, F=-d lg τR/dT| T= Tg (cf. eq 1). In Figure 1 we display the results for F (M), including now also data for polyisoprene (PI). The quantity F turns out to be M independent above a certain M though different for the various polymers. This is quite remarkable because, for example, in the case of PS, Tg changes by about 111K in the range where F is constant. At low M deviations from F= const are discernible most prominently for PS for which F strongly increases with further decreasing M. This may indicate the crossover to an elementary liquid, that is, a nonpolymeric system.
