Vinylidene chloride copolymers are prominent in the barrier plastic packaging industry. These materials display excellent
barrier to the transport of oxygen (and other small molecules) as well as flavor and aroma molecules. However, they suffer
from a propensity to undergo degradative dehydrochlorination at process temperatures. To scavenge hydrogen chloride formed
and prevent its interaction with the metallic components of process equipment, a passive base is usually included as an additive
prior to processing. The base is most often an inorganic oxide or salt. These may negatively impact the properties of the
polymer, particularly as a film. An organic base that could be covalently incorporated into the copolymer might display better
behavior. Accordingly, a series of copolymers containing low levels of 4-vinylpyridine (0.05–3 mole%) have been prepared,
characterized, and examined by thermogravimetry to assess thermal stability. In all cases, polymers containing 4-vinylpyridine
units are less stable than the polymer containing none of this comonomer. Clearly, the pyridine moiety is a sufficiently strong
base to promote E2 elimination of hydrogen chloride to generate dichlormethylene units in the mainchain from which thermal
degradation may be initiated.
dynamic methods for estimating activation energies have been developed. This
development has arisen largely as a matter of convenience and the desire to
minimize analysis time. While these methods generally afford values which
are somewhat similar, the agreement among values from various methods is never
outstanding. Further, the values obtained are often, at best, only approximations
of the values obtained by the traditional isothermal approach. To better ascertain
the utility of dynamic methods for the determination of activation energies,
the activation energy for the thermal degradation of a standard vinylidene
chloride/methyl acrylate (five-mole percent) copolymer has been generated
by a variety of methods. The degradation of this polymer is an ideal reaction
for evaluation of the various methods. At modest temperatures (<200C),
the only reaction that contributes to mass loss is the first order evolution
of hydrogen chloride, i.e., there is only one significant reaction occurring
and it is not impacted by competing processes. The best values (most reproducible;
best correspondence to values obtained by titrimetry and other methods) are
those obtained by plotting the natural logarithm of rate constants obtained
at various temperatures vs. the reciprocal of the Kelvin temperature. Various
dynamic methods yield values which are less reproducible and which approximate
these values to a greater or lesser degree. In no case is the agreement good.
Two series of styrene monomers, one with phosphorus-containing moieties as substituents and the other with substituents containing both phosphorus and nitrogen, have been prepared, characterized, and converted to oligomers. The oligomers contain, in the one case, phosphorus and, in the other, phosphorus and nitrogen. This provides the opportunity to not only assess the impact of the presence of phosphorus on the combustion characteristics of the oligomers but to determine whether or not this impact is enhanced by the presence of nitrogen. The level of residue from thermogravimetry and heat release rate during combustion suggest that the presence of nitrogen may have a small positive impact on the effectiveness of phosphorus flame retardants.
General purpose poly(styrene) is a large
volume commodity polymer widely used in a range of applications. For many
of these the presence of an additive to impart some flammability resistance
is required. Most commonly, brominated aromatics are used for this purpose.
As the polymer undergoes combustion these compounds decompose to generate
bromine atoms and/or hydrogen bromide which escape to the gas phase and trap
flame propagating radicals. While these species are effective in inhibiting
flame propagation they present the opportunity for loss of halogen to the
atmosphere. For this reason, the use of these compounds is being limited in
some parts of the world. Phosphorus compounds, on the other had, impart a
flame retarding influence by promoting char formation at the surface of the
burning polymer. This prevents heat feedback to the polymer and consequent
pyrolysis to generate fuel fragments. The combination of both bromine and
phosphorus present in a single compound might generate a superior flame-retarding
additive in that both modes of retardancy might be promoted simultaneously.
Should this be the case smaller amounts of additive might be necessary to
achieve a satisfactory level of flame retardancy. A series of such additives,
brominated aryl phosphates, has been synthesized and fully characterized spectroscopically.
Blends of these additives, at various levels, with poly(styrene) have been
examined by DSC, TG and in the UL-94 flame test. The flammability of the polymer
is dramatically diminished by the presence of the additive.
Certain five-membered dioxaheterocyclic compounds (hetero atoms may be P, Si, S, etc.) contain a strained carbon–carbon bond
which may undergo homolytic thermolysis at modest temperatures to generate a diradical capable of initiating vinyl polymerization.
If substituents contain flame-retarding moieties this represents a convenient method for imparting flame retrdancy to a polymeric
material. Of particular interest has been 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane. The thermal degradation of this compound
has been studied using 13C NMR spectroscopy. This may conveniently be done by monitoring the intensity of the signal for the benzylic carbon atom as
a function of time and temperature. A simple transformation is the conversion of the cyclic compound to the linear polymer.
The thermal decomposition characteristics of representatives of three classes of organoplatinum compounds have been examined
by thermogravimetry. Substituted salicylato(1,2-diaminocyclohexane)platinum(II) compounds undergo thermal decomposition by
sequential loss of first the salicylato ligand and then the amine ligand to afford a residue corresponding to the platinum
content of the compound. The thermal decomposition of N-arylsalicylaldimino(1,2-diaminocyclohexane)platinum(II) nitrate is
more complex, but is also characterized by two major weight losses. Thermal decomposition ofbis-(2-thiophenecarboxylato)platinum(II) is characterized by ligand fragmentation to generate a residual mass corresponding to
the platinum content of the compound.
The thermal stability of a commercial triaryl phosphate hydraulic fluid has been assessed using thermogravimetry and pyrolysis. This material is a mixture of triaryl phosphates containing a predominance of triphenyl phosphate. It is volatile at higher temperatures. At temperatures below its boiling point, in the presence of air, it slowly decomposes to evolve phenolic fragments.
The thermal polymerization of styrene is a long-known
and well-practiced phenomena. While the mechanism of the thermal initiation
event has been the subject of several investigations, it is not yet well understood.
In an attempt to gain further insight as to the details of possible initiation
from styrene dimer, analogous stable cycloadducts (maleic anhydride, tetracyanoethylene)
of 1- and 2-vinylnaphthalene have been synthesized, fully characterized spectroscopically,
and subjected to thermal decomposition. In the main, the major thermal event
observed for these styrene dimer mimics is retro cycloaddition. This process
is characterized by an activation enthalpy of approximately 30 kcal mol–1.
Aminor process which accompanies the major reaction is the homolysis of a
carbon–hydrogen bond to generate a carbon radical which may be trapped
as a stable adduct of the 2,2,6,6-tetramethylpiperinyloxy (TEMPO) radical.
multivalent platinum drug based on a poly(amidoamine) [PAMAM] dendrimer (generation
4.5, carboxylate surface) has been synthesized and fully characterized using
a variety of spectroscopic, chromatographic and thermal methods. Treatment
of the dendrimer with an aqueous solution containing an excess diaquo(cis-1,2-diaminocyclohexane)platinum(II) produces
a conjugate containing approximately forty (diaminocyclohexane)platinum(II)
moieties at the surface of the dendrimer. This material undergoes smooth two-stage
thermal decomposition to provide residual platinum oxide reflecting the platinum
loading in the drug.
The thermal degradation characteristics of head-to-head poly(styrene) [HHPS] should provide insight with respect to the impact
of head-to-head placement on the thermal stability of traditional atactic head-to-tail polymer [HTPS]. The synthesis of head-to-head
poly(styrene) must be accomplished indirectly. The head-to-head polymer is most satisfactorily obtained by dissolving metal
reduction of poly(2,3-diphenyl-1,3-butadiene) [PDBD] generated by radical polymerization of the corresponding diene monomer.
Full saturation of the polymer mainchain requires several iterations of the reduction procedure. Since the decomposition of
poly(2,3-diphenyl-1,3-butadiene) is prominent at 374C and that for head-to-head poly(styrene) is similarly facile at 406C,
it seemed feasible that TG of partially hydrogenated PDBD might be utilized as a convenient means of monitoring the extent
of hydrogenation. This has been demonstrated for various levels of unsaturation remaining - from approximately 90 to less
than 10%. Within this range the peak areas from the DTG plots of the partially hydrogenated polymer provide a good reflection
of the ratio of unsaturated to saturated units in the polymer. Even low levels of unsaturation in the polymer may be detected
by the asymmetry of the decomposition peak for the polymer.