How does contraction of polyethylene strip depend upon the heat it receives?

Answer to a related question detailing why some polymers contract

The change in dimensions of a material because of temperature change is determined by the Thermal expansion coefficient. In case of contraction, it will be a negative number. This coefficient can be defined for changes in volume, area, or length. For your question, length seems to be the preferred choice.

$alpha_L = frac{1}{L}frac{dL}{dT}$

Here $alpha_L$ is the coefficient, $L$ is length and $T$ is temperature.

If $alpha_L$ doesn't change much in the ranges of $T$ that you are working in, and neither does $L$, the above equation can be simplified to

$Delta L = L.alpha_L . Delta T $

The value of $alpha_L$ for a material may be found in a list in a material engineering handbook. A quick internet search provided me with these results in $10^{-6} m/(m K)$ at $25ºC$

Polyethylene (PE) $ = 200$

Polyethylene (PE) - High Molecular Weight $ = 108$

If you want to calculate how much $T$ will change by the amount of heat energy it receives, you will need to use the material's Heat Capacity