Thursday 1:30 p.m. Room 1315 March 9, 2017 Chemistry Building

Dr. Didier R. Long Laboratory of Polymers and Advanced Materials

CNRS/Solvay, France        Experiments  show  that  glassy  polymers  submitted  to  an  applied  stress  undergo  yield  at  deformations  of  a  few  percent  and  stresses  of  some  10  MPa,  followed  by  a  slow  drop  in  stress  under  plastic  deformation  corresponding  to  the  strain-­‐softening  regime.  Yield  behavior  is  often  described  by  using  the  phenomenological  Eyring  model,  according   to  which   stress   biases   the  motion.   This  model   requires   the   introduction   of   a   so-­‐called   activation  volume  of  order  typically  1  nm3  without  clear  interpretation.  Upon  increasing  the  deformation,  some  polymers  of  high  molecular  weight  display  an  increase  of  stress  in  the  large  amplitude  regime  of  deformation.  The  typical  slope  of  stress  versus  strain  in  this  regime,  GR,  is  of  order  107  –  108  Pa  well  below  Tg.  Classical  theories  involving  the  entropic  response  of  the  rubbery  network  cannot  explain  such  a  high  value.  GR  is  also  found  to  increase  upon  cooling.      

Regarding  the  onset  of  plastic  flow  and  the  stress  softening  regime,  we  propose  that  the  elastic  energy  stored  in  the  volume  x3  of  dynamical  heterogeneities  effectively  reduces  the  free  energy  barriers  present  for  internal  relaxation.  It  allows  for  calculating  yield  stresses  of  order  a  few  10  MPa  which  are  consistent  as  compared  to  experimental   data   without   additional   adjustable   parameters   than   the   scale   of   dynamical   heterogeneities  measured  and  theoretically  calculated  in  other  contexts.  Regarding  the  strain  hardening  regime,  and  following  Chen  and  Schweizer,  Phys.  Rev.  Lett.  2009,  we  assume  that  local  deformation  induces  a  reduction  of  mobility  at  the   scale   of   dynamical   heterogeneities,   by   orienting  monomers   in   the   drawing   direction.  We   assume   that  consequent  strengthening  of  monomer-­‐monomer  interactions  results  in  a  local  increase  of  the  glass  transition  temperature.      

The  model   is   then  solved   in  3D  with  a  spatial   resolution  corresponding  to  the  scale  of  a   few  nanometers  of  dynamical   heterogeneities.   Simulation   results   are   in   agreement  with   experimental   data,   such   as   the   elastic  modulus  the  yield  stress  and  the  yield  behavior  (strain  softening),  and  the  strain  hardening  regime  (GR∼10  MPa)  with  its  temperature  dependence,  and  its  dependence  on  reticulation  density.    Ref:  Luca  Conca  PhD  thesis,  Lyon,  2016  

Strain hardening of glassy polymers : theory and simulation