Solid-state Grafting Polymerization Of Styrene In Polypropylene Granules And Effects Of Aggregation State Of The Matrix On The Reactions

Grafting polymerization of vinyl monomers with polyolefin is one of the most important ways to modify the properties of polyolefin materials. Modification of polypropylene (PP) by radical initiated solid-state grafting polymerization has been studied for many years. Because only the PP segments in the amorphous phase can be attacked by radicals to form the graft chains, the chain structure of grafting product will be changed when the crystallinity, lamella size and its distribution are changed. In this work, the phase morphology of nascent spherical PP granules was regulated by isothermal annealing treatment, and the annealed PP granules were then used as the matrix for grafting polymerization of styrene in solid state. Effects of annealing treatment on the grafting polymerization and structure of the graft copolymer were systematically studied. The products were found to have long chain branching structure, which markedly changed the rheological properties of PP. A mechanism model of the grafting polymerization was proposed to explain the effect of annealing treatment on the grafting copolymerization and the formation of long chain branched structures.Firstly, nascent PP granules were annealed respectively at 150°C, 130°C and 110°C for 12 hours at each temperature. DSC analysis results showed that the annealing treatment strongly influenced the phase morphology. As annealing temperature increased, the crystallinity of PP became higher and the lamellae thickness distribution became narrower. However, analysis results of SEM, WAXD, and mercury porosimeter demonstrated that the granule's topological morphology, crystal form and porosity were almost intact after the annealing treatment. Annealing treatment only resulted in partial melting and recrystallizion of PP phase. The influence of annealing treatment at 150°C is most remarkable.Grafting copolymerization of styrene with annealed PP or nascent PP was performed, respectively, using tert-butyl perbenzoate (TBPB) as a radical initiator. The grafting degree, amount of gel, and degree of PP degradation were investigated. Experimental results demonstrate that factors such as annealing treatment, initiator concentration, amount of styrene and reaction temperature all showed strong effects on the grafting reaction. As the initiator concentration was increased, the content of gel formed at 130°C continuously increased when annealed PP was used as the matrix. Whereas, the content of gel based on nascent PP was firstly increased and then leveled off. When the initiator concentration was lower than 1%, the grafting product based on annealed PP matrix had lower gel content than that of nascent PP. At reaction temperature of 120°C, the difference in gel content was much remarkable between samples of nascent PP and annealed PP. Gel was formed in graft polymer from nascent PP at 1.0% TBPB feed, while there was no gel formed in graft polymer from annealed PP until 2.0% TBPB .With the increase of reaction temperature, the gel content, grafting degree and grafting efficiency all gradually increased. When reaction temperature was lower than 120°C, no gel was formed in the final products, but the grafting degree and grafting efficiency were also much lower than the samples prepared at 120°C and 130°C. With the increase of styrene concentration, the gel content and grafting degree increased at first and then decreased. However, the grafting degree and grafting efficiency of the products from annealed PP were slightly depressed at low initiator feed. Molecular weight analysis showed that significant degradation of the polymer chains also took place when either annealed PP or nascent PP were used as the matrix. The degradation reaction was depressed by using annealed PP as matrix, and (or) the introduction of styrene. In the presence of styrene, molecular weight of a part of polymer chains increased, implying that chain build-up or grafting reaction happened. DSC analysis demonstrated that there was differences in branch density and average length of graft chains between products from nascent PP and annealed PP. Grafting products based on nascent PP have higher branch density but shorter graft chains as compared with those based on annealed PP.Subsequently, a series of styrene grafting copolymerizations were conducted at low initiator concentration to prevent gel formation in the final products. Several kinds of analysis methods, such as temperature gradient extraction fractionation (TGEF), gel permeation chromatography (GPC), rheological properties measurement (ARES), differential scanning calorimetry (DSC) and infrared spectra (IR) were utilized to characterize the grafted samples. The analysis results demonstrate that long chain branching structure was formed in the grafting products. Styrene took part in the formation of long chain branches and played an important role. The formation of long chain branches was affected by factors such as reaction temperature, feed ratio of initiator, styrene feed and reaction time. Increases in reaction temperature and initiator feed were beneficial to the formation of long chain branches. During the process of grafting reaction, long chain branched structures were not formed until enough styrene was polymerizaed. It was also found that long chain branches can be formed at rather low initiator feed in the presence of styrene. In this work, we also investigated grafting copolymerization of styrene in nascent PP matrix. Experimental results exhibited that long chain branched structure could also be formed using this matrix, but polymer degradation was much more serious comparing with that of annealed PP. In addition, a series of graft copolymer samples were synthesized in larger scale, and their mechanical properties were measured. It was found that the mechanical properties of the grafting products were not deteriorated after the reaction, and in some cases even improved as compared to the unmodified PP. Grafting products based on annealed PP showed better mechanical properties than those of nascent PP. The foaming properties were also improved after the grafting eopolymerization, which evidenced the enhancement of melt strength.Under similar conditions as the styrene grafting copolymerization, styrene-co-MAH grafting copolymerization in annealed PP granules was also studied to introduce SMA branches into PP chains. Experimental results showed that long chain branches could also been formed when the MAH feed ratio was lower than 10% at 0.52% initiator and 64% of styrene feeds. The introduction of MAH was unfavorable to the formation of long chain branches. As the feed ratio of MAH increased, the effects of long chain branches in the graft copolymer declined. The introduction of MAH also made the degradation much serious.Finally, according to the experimental results and basic concepts of radical initiated grafting polymerization, a simplified mechanism model was proposed to elucidate the effect of PP annealing treatment on grafting degree, formation of gel and polymer chain degradation. The mechanism of long chain branches formation in the grafting processes was also proposed.