Measuring many properties of the Parylene monomer is impossible via conventional techniques due to its high reactivity toward polymerization. But aside from this particular reactivity, its physical properties should otherwise be unexceptional.  When physical property information is required to understand its behavior in the Parylene deposition process or in applications (e.g. vapor pressure, heat of adsorption), our only recourse is through analogy with similar but stable compounds.

Relative Growth Rates of the Three Most Popular Parylenes vs. Temperature

Will it get through?  Or, more properly, how long will it be kept out?  Small molecules wander rather easily through plastics - the smaller they are, the easier they penetrate. Often cited as barrier quality evidence is Permeability data.  Permeability data are readily available because they are easy to measure with precision. Permeability is the steady state flux of a permeant through a planar sheet or film.  In order to evaluate the performance of a barrier layer, however, you actually need Diffusivity, which is the constant of proportionality in Fick's Law of Diffusion (relating diffusion flux with the concentration gradient of permeant):

Once we have a value for D, we are in a position to tell how long, t,  a given layer of thickness L will keep out a permeant.  Solutions of Fick's Law for the planar sheet geometry show that appreciable amounts of permeant start to arrive at the backside of a barrier at the dimensionless time Dt/L² ~ 0.1.  (t ~ 0.1 L²/D)

D is available experimentally from the non-steady state portion of the Permeability experiment, but can be measured neither as conveniently nor as precisely as Permeability.  Permeability P is the product of Diffusivity D and Solubility S, where Solubility is the concentration of permeant in the polymer in equilibrium with a particular pressure of gaseous permeant. So, if S is known, we have an alternative means of obtaining D.  In the low pressure limit, S is the Henry's Law coefficient.  

In 1978, a mechanism was proposed for the steady state growth of Parylene films, in which the importance of diffusion was emphasized.  In those days, most uses for Parylene were for thicker layers.  As a circuit board coating, a half mil (12,500 Angstroms) of Parylene was already a only small fraction of the thickness of conventional coatings (3-5 mils). Most of the coating at that thickness was grown under steady state conditions, so there was little motivation at that time to investigate the early stages of growth..  But today, uses for and reports of much thinner coatings abound, one claiming the reproducible preparation of a 30 Angstrom layer.  We need to understand better how these thinner coatings  might differ from thicker ones.

The chemistry involved in Parylene VDP is simple: one reaction (initiation) which generates new polymer chains, and a second reaction (propagation), much more rapid, which extends the length of existing chains. The 1978 mechanism left the question of the order of the initiation reaction open, although it seemed even at that time reasonable that the initiation reaction was third order in monomer. In the interim, experimental evidence has surfaced to establish for Parylenes N and C, at least, that the initiation reaction is in fact third order.

The early stages of the mechanism of Parylene VDP depends on whether the substrate is permeable or impermeable to monomer.