Isothermal uniaxial compression of Amorphous polymer sample

hello all,

I am performing a uniaxial Compression of Amorphous polymer using periodic boundary condition, my polymer sample in completely isotropic in the beginning, but as compression proceeds the sample’s volume increases, I am not getting the point of this affect, is periodic boundary condition creating problem?

hello all,
I am performing a uniaxial Compression of Amorphous polymer using periodic
boundary condition, my polymer sample in completely isotropic in the
beginning, but as compression proceeds the sample's volume increases, I am
not getting the point of this affect, is periodic boundary condition
creating problem?

why *should* the volume remain constant?
and why should that be connected to the use of PBC?

keep in mind that you are looking a samples that are small enough to
have finite size effects which would be negligible only for
macroscopic samples. without PBC you could not even accurately
determine the volume (how do you determine the volume of a single
atom?).

axel.

Enclosing the simulation box under shrink boundary condition could be a better option.

Enclosing the simulation box under shrink boundary condition could be a
better option.

why that?

this would change the system studied from a sample of a macroscopic
object to a section thin film. ...and you could not even _tell_ what
the volume for that sample was anymore. you'd only know the size of
the enclosing box.

axel.

hello axel,

and thanks for reply,

my point is when we compress any system the volume should decrease or remain constant, if poisson’s ratio is 0.5 for isotropic system. but in my case initially my system is isotropic and with compression i am getting the increase in volume. i am asking about the reason of getting that increase in volume of system during compression which is not obvious.

hi prakhar,
I also read it from manual but I am agree with axel, and this would not going to work. but thanks for reply.

hello axel,
and thanks for reply,

my point is when we compress any system the volume should decrease or remain

it would decrease for as long as you are in the elastic regime, but as
soon as you get a plastic deformation, you may force a system that is
tightly packed into a less favorable packing with lower density. this
would be more pronounced due to finite size effects.

axel.

thanks axel, ya i got it, thanks for reply.