I suspected that there would be no concrete directive in the new books and I have not been able to find anything in the General Instruction. However there is an instruction from the US Bishops' Committee on the Liturgy, "Care and Cleansing of Altar Linens". It says that
"The material of purificators should be absorbent and easily laundered. The purificator should never be made of paper or any other disposable material."So it does not say definitely that polyester-cotton may not be used.
Nevertheless, having occasionally been given a polyester-cotton purificator when visiting a Church or shrine, I would say that they are generally not "absorbent" to the required degree. Whereas a linen or cotton cloth will properly absorb all the remnants of the ablution in the chalice, a polyester-cotton will leave smears and droplets. Furthermore, polyester-cotton is generally seen as cheaper and inferior than linen or pure cotton so it would surely be preferable to use the latter for the service of the altar.
Besides this, I wonder what the motive is. Linen or cotton purificators do need to be ironed and if ironed damp or steam-ironed, will turn out very well and look suitably pristine for their liturgical use. But this is a job that takes some time. The idea may be that with many purificators being used (for Holy Communion under both kinds, using several chalices, perhaps?) there can be some labour-saving by not having to iron ones made of polyester-cotton. This would be a pity in my view.
This is where we get to a pastoral point. Many of us are busy and look for the quickest way to do a job. It is easy to forget that in any parish there are some people whose problem is the opposite. Retired, perhaps widowed, a little isolated, their days can seem long and a simple but significant task in the service of the altar and the worship of God can be a welcome additional activity in the week. Something that I have learnt over the years as a parish priest is that we should not always presume that we are imposing on people by looking for volunteers for such tasks, nor that we are being helpful by eliminating the work.
Incidentally, the instruction mentioned above helpfully reminds sacristans that when washing purificators and corporals, they should first be rinsed with water and the water poured into the sacrarium. The "Sacrarium" is a drain designed to run to the earth, rather than into the common sewer. If your Church does not have a sacrarium, it is appropriate to rinse the corporals and purificators in a bowl and then pour the water onto the earth (a flower bed or lawn, for example.)