Influence of Cohesive Energy and Chain Stiffness on Polymer Glass Formation

Abstract

The generalized entropy theory is applied to assess the joint influence of the microscopic cohesive energy and chain stiffness on glass formation in polymer melts using a minimal model containing a single bending energy and a single (monomer averaged) nearest neighbor van der Waals energy. The analysis focuses on the combined impact of the microscopic cohesive energy and chain stiffness on the magnitudes of the isobaric fragility parameter mP and the glass transition temperature Tg. The computations imply that polymers with rigid structures and weak nearest neighbor interactions are the most fragile, while Tg becomes larger when the chains are stiffer and/or nearest neighbor interactions are stronger. Two simple fitting formulas summarize the computations describing the dependence of mP and Tg on the microscopic cohesive and bending energies. The consideration of the combined influence of the microscopic cohesive and bending energies leads to the identification of some important design concepts, such as iso-fragility and iso-Tg lines, where, for instance, isofragility lines are contours with constant mP but variable Tg. Several thermodynamic properties are found to remain invariant along the iso-fragility lines, while no special characteristics are detected along the iso-Tg lines. Our analysis supports the widely held view that fragility provides more fundamental insight for the description of glass formation than Tg.

Energy Renormalization for Coarse‐Graining Polymers with Different Fragilities: Predictions from the Generalized Entropy Theory

“…Figure a shows the Angell plot of logτ α for varying E b . Evidently, the T dependence of τ α becomes stronger for larger E b , which is consistent with previous calculations based on the GET31–33 as well as experimental37,38 and simulation studies 39. Interestingly, the polymer melt with E b / k B = 0 K is predicted to be a quiet “strong” glass former based on the GET.…”

“…The GET prediction also shows that T g becomes elevated with increasing either E b or ε. However, these previous studies mainly focus on polymer models with E b /k B ≫ 0 K. [31][32][33] By comparing the behavior of a polymer melt with E b /k B = 0 K to that of the melt with E b /k B = 800 K, our recent work revealed certain qualitative differences in the dependence of the fragility and characteristic temperatures of glass formation on the cohesive energy strength as well as the temperature variation of the configurational entropy s c (which will be discussed next along with Figure 2). [34] Nevertheless, we have not investigated polymer glass formation when E b is systematically varied from zero to finite values.…”

“…Building upon the recent theoretical and modeling developments, in the present study, we put our ER strategy to the test of coarse‐graining polymers with different glass “fragility,” a key parameter that measures the sensitivity of the structural relaxation time to temperature changes and that is related to molecular packing “frustration.”29,30 Previous studies have shown that the GET is able to harness basic molecular parameters, such as chain flexibility, monomer size and cohesive interaction strength, to predict key properties associated with glass formation, such as relaxation time and characteristic temperatures and fragility of glass formation 23,31. Using analytic calculations based on the GET, the present study aims to systematically investigate how the bending rigidity parameter E b influences the GF dynamics and fragility of polymers and their CG analogs based on the recently proposed ER method ( Figure ).…”

“…[29,30] Previous studies have shown that the GET is able to harness basic molecular parameters, such as chain flexibility, monomer size and cohesive interaction strength, to predict key properties associated with glass formation, such as relaxation time and characteristic temperatures and fragility of glass formation. [23,31] Using analytic calculations based on the GET, the present study aims to systematically investigate how the bending rigidity parameter E b influences the GF dynamics and fragility of polymers and their CG analogs based on the recently proposed ER method (Figure 1). We further apply the GET to predict the ER factors at different degrees of coarse-graining over a wide temperature range.…”

The glass transition of polymers with different side-chain stiffness confined in free-standing thin films

“…12 and 13, we can see that the magnitude of free surface effect mainly depends on the relaxation process in bulk region, which is closely related to the microscopic cohesive energy and chain stiffness for polymeric glass formers, which can be reflected by the value of T g . 60 Moreover, we have noted in Sec. III B that the activation free energy of rearrangement in various bulk glass formers tracks those of the glass transition.…”