The heat resistant properties of polyvinyl chloride (PVC) are very poor

so that PVC resin can not be used in many cases．In order to enhance the heat resistant properties of PVC

N-phenyl maleimide (PMI) and／or acrylonitrile (AN) monomer units were introduced into the PVC chain to prepare heat resistant VC-PMI copolymer and VC-PMI-AN terpolymer by suspension polymerization．The Vicat softening point (VSP)

glass transition temperature (Tg)

dynamical mechanical behavior and morphological structure of heat resistant VC-PMI copolymer and VC-PMI-AN terpolymer were studied in detail．The results showed that the VSP and T of the heat resistant PVC increased with the content of PMI or AN．When the content of the heat resistant monomer units was high than a critical value

the increasing rate of the heat resistant properties was changed

two tanδ peaks appeared

and a dual phase continuity structure

named as “stiff chain frame work”

was formed．On the basis of these experimental phenomena

a new concept that the “stiff chain frame network” can increase the heat resisrant properties of the heat resistant PVC was presented．A heat resistant mechanism was discussed and its physical model was proposed also．The results of dynamic mechanical analysis revealed that the polymers had only one tanδ peak in most cases

however

two tanδ peaks appeared with increasing of the content of AN．The miscibility of the heat resistant agent (VC-PMI copolymer or VC-PMI-AN terpolymer) and PVC will influence the formation of the “stiff chain frame network”.