Ideal glass transitions in thin films: An energy landscape perspective

The effect of confinement on the glass transition temperature Tg of polymeric glass formers with different side chain stiffness is investigated by coarse-grained molecular dynamics simulations. We find that polymer with stiffer side groups exhibits much more pronounced Tg variation in confinement compared to that with relatively flexible side groups, in good agreement with experiments. Our string analysis demonstrates that the polymer species dependence of dynamics can be described by an Adam-Gibbs like relation between the size of cooperatively rearranging regions and relaxation time. However, the primary effect of changing side-group stiffness is to alter the activation barrier for rearrangement, rather than string size. We clarify that free-surface perturbation is the primary factor in determining the magnitude of Tg variation for polymers in confinement: It is more significant for polymers having higher Tg and results in much more pronounced reduction of surface Tg and then the overall Tg of the polymers.

Section: Introductionmentioning

confidence: 99%

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

Xie

Huang

2015

“…Recently, thermodynamic models of the glass transition in confined polymers were proposed to describe the filmthickness dependence of T g in free-standing thin films. [41][42][43] They are based on the assumption of uniform segment density across the film thickness and, hence, do not take into account the possible non-uniform distribution of chain ends and/or strongly interacting chain moieties, such as phenyl groups of PS chains.…”

Section: Introductionmentioning

confidence: 99%

Interfacial and topological effects on the glass transition in free-standing polystyrene films

Lyulin

Балабаев

Baljon

et al. 2017

United-atom molecular-dynamics computer simulations of atactic polystyrene (PS) were performed for the bulk and free-standing films of 2 nm-20 nm thickness, for both linear and cyclic polymers comprised of 80 monomers. Simulated volumetric glass-transition temperatures (T) show a strong dependence on the film thickness below 10 nm. The glass-transition temperature of linear PS is 13% lower than that of the bulk for 2.5 nm-thick films, as compared to less than 1% lower for 20 nm films. Our studies reveal that the fraction of the chain-end groups is larger in the interfacial layer with its outermost region approximately 1 nm below the surface than it is in the bulk. The enhanced population of the end groups is expected to result in a more mobile interfacial layer and the consequent dependence of T on the film thickness. In addition, the simulations show an enrichment of backbone aliphatic carbons and concomitant deficit of phenyl aromatic carbons in the interfacial film layer. This deficit would weaken the strong phenyl-phenyl aromatic (π-π) interactions and, hence, lead to a lower film-averaged T in thin films, as compared to the bulk sample. To investigate the relative importance of the two possible mechanisms (increased chain ends at the surface or weakened π-π interactions in the interfacial region), the data for linear PS are compared with those for cyclic PS. For the cyclic PS, the reduction of the glass-transition temperature is also significant in thin films, albeit not as much as for linear PS. Moreover, the deficit of phenyl carbons in the film interface is comparable to that observed for linear PS. Therefore, chain-end effects alone cannot explain the observed pronounced T dependence on the thickness of thin PS films; the weakened phenyl-phenyl interactions in the interfacial region seems to be an important cause as well.

“…Such an inference is indeed consistent with the T g observations in the context of both supported films and small MW free standing films. 16 Not surprisingly, many of the theoretical models 22,24,[40][41][42] proposed to rationalize the T g of polymer thin films also do not invoke any "polymeric" aspect of the material (albeit, exceptions do exist 43,44 ). However, since there are outlying, unexplained MW dependencies in polymeric materials, and the fact that the range of influence of surfaces seems to be much larger than that may be expected from the static structure factor, motivates reexamination of the structural features of polymers and their MW dependencies from alternative perspectives.…”

Section: Introductionmentioning

confidence: 99%

Molecular mass dependence of point-to-set correlation length scale in polymers

Hanson¹,

Pryamitsyn²,

Ganesan³

2012

Self Cite

We use a recently proposed metric, termed the point-to-set correlation functions, to probe the molecular weight dependence of the relevant static length scales in glass-forming oligomeric chain liquids of 4, 5, 8, and 10 repeat units. In agreement with the results for simple, monatomic fluids, we find that static length scales of the oligomers increase monotonically when the temperature is lowered towards the glass transition temperature of the fluid. More interestingly, the static length scale increases with increasing chain length. Within the bounds of error in our simulations, the static length scale appears to scale as the radius of gyration of the oligomer, but with a prefactor, which is much larger than unity and which grows with the temperature. The preceding behavior contrasts with the length scales extracted from the radial distribution function of the oligomer system, which is practically independent of the chain length.