Acid effect, grafting

Figure 1. Role of dose rate in the synergistic effect of divinyl-benzene and sulfuric acid on grafting of styrene on polypropylene film in methanol dose rate of 4.1 x 104rad/hr to total dose of 2.4 x lO rad (A) styrene-methanol, (o) styrene-sulfuric acid (0.2M). ( ) styrene-methanol-divinylbenzene (1% v/v), and (o) styrene-methanol -di vinyl benzene (1% v/v)-sulfuric acid (0.2M). dose...

Acid Effects in the Radiation Grafting of Monomers to Polymers, Particularly Polyethylene... 

The inclusion of mineral acid in the grafting solution has recently been shown to increase the radiation copolymerisation yield, particularly when styrene is grafted to trunk polymers like wool (3) and cellulose (4) i.e. polymers which readily swell in polar solvents such as methanol. This acid effect is important since for many copolymerisation reactions, relatively low radiation doses are required to yield finite graft. The process is particularly valuable for monomers and/or polymers that are either radiation sensitive or require high doses of radiation to achieve the required graft. 

A theory for this acid effect has been developed essentially from the wool and cellulose work (3,4). Recently, in a brief communication, we reported analogous acid enhancement effects in the radiation grafting of monomers such as styrene in methanol to nonpolar synthetic backbone polymers like polypropylene and polyethylene (5). In the present work, detailed studies of this acid enhancement effect are discussed for the radiation grafting of styrene in various solvents to polyethylene. The results are fundamentally important since most of the experiments reported here have been performed in solvents such as the low molecular weight alcohols which, unlike cellulose and wool systems, do not swell polyethylene. 

TABLE I. Acid Effect in Radiation Grafting to Polyethylene using Styrene in Low Molecular Weight Alcohols.a... 

TABLE II. Acid Effect Styrene in in the Radiation Isomeric l Grafting to Polyethylene using Butanols.a ... 

TABLE VI. Acid Effect in Radiation Grafting using Styrene in DMF, DMSO, Acetone, Dioxan, Chloroform and Hexane.0 ... 

The parameters which predominantly influence the acid effect in radiation grafting of styrene monomer to polyethylene film are the structure of solvent, the concentration of monomer and the dose rate. Because these three variables are inter-related, it is difficult to predict, a priori, the conditions required to yield an optimum in grafting. In this respect the type of solvent used is particularly important. 

The Acid Effect. The possible mechanistic role of hydrogen atoms in the current radiation grafting work becomes even more significant when acid is used as an additive to enhance the copolymerisation. At the concentrations utilised, acid should not affect essentially the physical properties of the system such as precipitation of the polystyrene grafted chains or the swelling of the polyethylene. Instead the acid effect may be attributed to the radiation chemical properties of the system. Thus Baxendale and Mellows (15) showed that the addition of acid to methanol increased G(H2) considerably. The precursors of this additional hydrogen were considered to be H atoms from thermalised electron capture reactions, typified in Equation 5. 

Further work (10) with acid effects in the radiolysis of binary mixtures such as benzene-methanol and pyridine-methanol indicates that the acid phenomenon is more complicated than the simple H atom model originally developed ( ). These more recent experiments (10) show that whilst increased hydrogen atom yields in the presence of acid enhance the overall grafting yield, other mechanisms also contribute to this acid effect. Thus the acid stability of intermediate radicals (I-III) and also analogous species involving the trunk polymer are important. With radicals (I-III), at low styrene concentrations in methanol, these intermediates (MR-) will predominantly react with other available... 

Acid Effect in Grafting With Isomeric Alcohols. With the isomeric propanols and butanols, some unexpected results were obtained both in the presence and absence of acid (Table II). 

Armstrong, and Rutherford have reported extensive studies on the vapor phase grafting of vinyl monomers to cellulosic fibers both mutual and pre-irradiation methods have been used (97, 120). Again, water or another swelling agent was found to be necessary for effective grafting to rayon and cotton for all the monomers studied. In the case of cellulose acetate water was helpful but not necessary except for styrene. Acetic acid and methanol vapors were also found to be effective promotors of vapor phase grafting to cotton and cellulose acetate fibers. 

Acid Effect in UV- and Radiation-Induced Grafting of Styrene to Cellulose... 

Recently inclusion of mineral acid in the grafting solution was found to enhance the grafting yields for a typical system such as the copolymerisation of styrene in methanol to cellulose in the presence of gamma rays (13). The acid effect now appears... 

In the present work, further mechanistic data for the acid effect in both UV and gamma radiation systems are reported, particularly molecular weight studies on the species present in the grafting solutions. The significance of these acid effects in possible commercial applications of the resulting copolymers of cellulose is also briefly mentioned. 

It is thus possible to investigate the conditions under which common intermediates may exist in both grafting and homopolymerisation processes. The results in the following Tables I -VIII thus concern acid effects in the radiation homopolymerisation of styrene in dioxan. 

---