Flow And Relaxation At Surfaces Of Polymer Glasses

Surface dynamics of polymer glasses is one of the most important scientific issues in polymer physics.The mobility of the molecules at glassy polymer surface is enhanced,and the relaxation of the surface molecules is significantly faster than the bulk.And this enhancement will decay apart from the surface,reflecting the mobility gradient at glassy polymer surface.When this mobility gradient couples to the polymeric long chains at surface,the polymer there exhibits a unique surface flow behavior.In the previous work,our group developed a method to characterize the nano-creep of glassy polymer by using liquid droplets to induce a deformation at surface of polymer films.Therefore,we use the evolution of the deformation height and nano-indentation depth as a probe to study the linear rheology and relaxation of polymer glasses.We found that there is a stepwise activation of chain flow at the surface of glassy polymer;And the effect of side groups on surface dynamics is also elucidated.The main results were listed as follows:(1)The deformation on glassy polymer surface perturbed by a droplet is well-studied.It is found that a line force γlsinθ(γl is the surface tension of the droplet;θ is the contact angle)would induce a nano-scale deformation at polymer surface,called wetting ridge.Meanwhile,the formation of the wetting ridge is accompanied by nano-indentation on both sides.The nano-indentation originates from the inhomogeneous flow of polymer chains in the film dominated by the finite film thickness effect,while it would be magnified by the Laplace pressure on the inner side of the ridge.The evolution of the wetting ridge height(h)and nano-indentation depth(d)at polymer surface are controlled by the creep modulus relaxation,reflecting the flow and creep of polymer surface.(2)The flow of polymer at surface was investigated by tracking the evolution of the wetting ridge height and nano-indentation depth.It is found that the growth of h and d with time on the polystyrene(PS)surface is complex and two plateau periods are seen when the ridge keeps invariant: the first plateau represents the rubbery elasticity of the chains in the outermost layer while the second plateau reflects the stepwise activation of chain flow at surface.Due to the mobility gradient of segments,polymer chains in different depths at surface show different terminal relaxation times.Thus,the chains with largest mobility in outermost layer firstly reptate and begin to flow,and then the deeper and slower chains flow in sequence,until the surfacial chains that partly trapped in the glassy bulk and the flow ceases,resulting in the second plateau.The nano-indentation depth(Dmax)of the second plateau represents the critical dimension of a layer where the polymer chains could reptate to flow(fluidized layer),while a rubbery layer is defined where the polymer chains partly trapped into the glassy bulk without the ability to flow.Thus,the surface flow region includes a fluidized layer and a rubbery layer where only local segment relaxation is accessible.With the molecule weight increasing,the proportion of chains trapped in the bulk increases in the surface region,causing a thicker rubbery layer and a thinner fluidized layer.Thus,the thickness of the fluidized layer increases with decreasing molecular weight.Once the chain size is larger than the length scale of the mobility gradient,the fluidized layer would disappear and polymer surface become a permanent rubbery state.This work elucidates the gradient activation mechanism of flow and relaxation at glassy polymer surface.(3)The glass transition and surface molecule motion of polystyrene film with different side-groups were studied by ellipsometry and nano-indentation experiment.The surface mobility and the side group could be then correlated.The study found that the magnitude of glass transition temperature(Tg)reduction of different polymeric thin films show a sequence as: poly(4-tert-butylstyrene)(Pt BS)> poly(4-methylstyrene)(P4MS)>PS > poly(α-methyl styrene)(PαMS),indicating that large side-group would enhance the nano-confinement of the film.The larger side-group,the stronger confinement effect.In addition,Pt BS has a thicker fluidized layer and lower surface activation energy than PS.Moreover,the increase of the thermal expansion of Pt BS thin film shows that the bulky tert-butyl group would disrupt the intermolecular interactions between the phenyl groups at the surface and increase the free volume there,enhancing surface molecular mobility and resulting in a more pronounced nanoconfinement effect and enhanced surface dynamics.These results benefit the understanding and tuning the molecular motion at glassy polymer surface.