New Experimental Evidence for Thermodynamic Links to the Kinetic Fragility of Glass-Forming Polymers

Abstract

Providing a thermodynamic basis to kinetic fragility has been pursued for decades. This objective is particularly challenging for glass-forming polymers because of the difficulty in gauging configurational excess entropy ΔS ex . Herein, we report that enthalpy hysteresis ΔH R determined from a well-defined cooling and subsequent heating cycle of heat capacity curves bears a proportional relationship with αrelated excess enthalpy at the glass transition temperature (T g ). Correlations of fragility (m e ) with ΔH R and heat capacity jump ΔC P at T g are explored in a broad range of polymers, including monodispersed polystyrene with a molecular weight of 580−2 400 000, homopolymers with a fragility ranging from 50 to 200 and T g ranging from 200 to >400 K, and copolymers and polymer/small-molecule mixtures with different compositions and varied intermolecular strengths. Surprisingly, all of the presented data follow m e = 0.75ΔC P •T g /ΔH R + 15, a formula that can be theoretically derived from the Adam−Gibbs equation if the prefactor τ 0 is given at 10 −13 s. These experimental results not only provide a possible routine for estimating the α-related excess entropy of noncrystallizable polymers but also validate the thermodynamic link to the kinetic fragility of glass-forming polymers.

Nanophase Separation-Induced Anomalous Enthalpy Hysteresis in Poly(n-alkyl methacrylate)s

“…The light grey points are the data measured in the previous work. 15 All experimental data followed the red dotted line, which represents eq 3, demonstrating that m does bear a proportional relationship with ΔC p •T g /ΔH R . However, for poly(n-alkyl methylate)s in this work (shown as red full circles in Figure 3d), most data deviated greatly from the red dotted line.…”

“…45 While the anomalous reduction in ΔC p has been rationalized by Donth et al 30 based on the fact that the temperature range between T g and the onset point of the bulk α relaxation (T onset ) decreases 1 in this work, and grey points refer to experimental data redrawn from our previous work. 15 The red dotted line represents the predicted value according to eq 3. Data outside and inside the blanket were determined by DSC and dielectric relaxation (τ α = 100 s), respectively.…”

“…Data from the two measurements were clearly very close. 15,51,52 b Data were estimated in accordance with ΔH R = 0.05T g •ΔS ex,α (T g ), 15 where ΔS ex,α (T g ) could be found from ref 6. with increasing the sidechain length, accounting for the reduction in ΔH R is rather difficult. Notably, when the number of carbon atom further increased to 10 (PnDMA), the hysteresis loop enlarged again with a wider temperature range.…”

Mediating the slow dynamics of polyacrylates by small molecule-bridged hydrogen bonds

“…33,[37][38][39][40][41][42][43][44][45] In contrast to van der Waals and covalent interactions, small molecules with two or more functional groups act as crosslinkers and can form a transient HB network throughout the polymer matrix. 46,47 Recently, the impact of the small molecule-bridged HB network on the segmental dynamics of the polymer matrix has been extensively explored, 37,[47][48][49][50] and many stage results were achieved in terms of changes in non-exponential 46,[51][52][53] and non-Arrhenius [54][55][56] characteristics of cooperative a-relaxations. However, up to now, little is known about the changes in slow dynamics of such HB-driven hybrids.…”

Methods for Creation and Linear Elastic Response Analysis of Packings of Semi-flexible Soft Polymer Chains

“…The success of this simplified model in furthering our understanding of glasses and jamming cannot be overstated [1][2][3][4][5][6][7][8]. However, many glassy systems are not comprised of soft spheres, but polymer chains that interact in more complex ways [9][10][11][12].…”